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ISRN Chemical Engineering
Volume 2012 (2012), Article ID 964936, 21 pages
http://dx.doi.org/10.5402/2012/964936
Review Article

Hydrogen Production by Photoreforming of Renewable Substrates

Dipartimento di Chimica, Università degli Studi di Milano, INSTM Unit Milano-Università and CNR-ISTM, v. C. Golgi 19, 20133 Milano, Italy

Received 23 September 2012; Accepted 11 October 2012

Academic Editors: C.-T. Hsieh, C. Perego, I. Poulios, and A. M. Seayad

Copyright © 2012 Ilenia Rossetti. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

This paper focuses on the application of photocatalysis to hydrogen production from organic substrates. This process, usually called photoreforming, makes use of semiconductors to promote redox reactions, namely, the oxidation of organic molecules and the reduction of H+ to H2. This may be an interesting and fully sustainable way to produce this interesting fuel, provided that materials efficiency becomes sufficient and solar light can be effectively harvested. After a first introduction to the key features of the photoreforming process, the attention will be directed to the needs for materials development correlated to the existing knowledge on reaction mechanisms. Examples are then given on the photoreforming of alcohols, the most studied topic, especially in the case of methanol and carbohydrates. Finally, some examples of more complex but more interesting substrates, such as waste solutions, are proposed.

1. Introduction

H2 production through water splitting (WS) is thermodynamically limited by the high Gibbs-free energy (237 kJ/mol, 1.23 eV), representing a typical uphill reaction. Nevertheless, solar energy storage as H2 by photocatalysis is an intriguing topic, but very low efficiency is reported for direct photocatalytic WS [13]. In order to reduce protons and oxidize oxygen ions, a semiconductor should possess a band gap energy higher than the required energy for such reaction. Moreover, the energy levels of the valence and conduction bands should be compatible with those of the species to be reduced or oxidized. Indeed, the electron, promoted someway in the conduction band, should have much negative potential than the redox couple H+/H2, and the hole left in the valence band should possess higher positive potential than O2/H2O (Figure 1). The band gap energies of many common oxides and sulphides have been recently recalled in [1], together with their performance in direct WS.

964936.fig.001
Figure 1: Basic mechanism of photocatalytic WS over semiconductors with proper band gap.

Since the first report of WS on TiO2 [5], various photocatalysts were developed [1], but the efficiency for WS remains very low [68]. Sacrificial reagents, such as methanol or EDTA, can improve hydrogen productivity, but they are nonrenewable [9]. Compared to thermochemical processes, photocatalytic reforming (PR) is a valid approach to produce H2 under ambient conditions and using sunlight, the cheapest energy source available on earth. PR is also thermodynamically more feasible than WS [10]. After a pioneering study in 1980 [11], the attention was more recently focused mostly on the PR of methanol, the simplest molecule, and a few examples of PR of compounds really obtainable from biomass can be found, for example, ethanol, glycerol [1214], glucose, sucrose [14], starch and wood [11], and sewage sludge [15]. Glucose and cellulose were also processed with homogeneous catalytic processes [16]. Interestingly, biomass-derived substrates, such as alcohols or carbohydrates, may be used for hydrogen production particularly in the case of waste materials (sewage from food, wine, or paper industry) [17], simultaneously helping to clean wastes (without disposal costs) and to produce a highly valued fuel [18]. On the other hand, technology is mature for the conversion of lignocellulosic biomass, not competing with the food and feed chain, into elementary alcohols and carbohydrates.

PR offers a route to H2 that, unlikely thermal reforming or gasification, may be exploited under ambient conditions and, using sunlight, it is particularly attractive for areas of the world where high biomass supply coincides with high sunlight intensity. Furthermore, it is a relatively low tech method and therefore particularly useful for small production facilities and developing countries.

Various reviews have been published till now on WS (see e.g., [1, 1925]), whereas much more limited records can be found relative to PR. In this latter case the attention was predominantly focused on methanol as sacrificial agent and on the development of materials and mechanisms. In this work, besides recalling some of such points, the central topic will be the evaluation of H2 productivity of different materials and substrates to assess process feasibility and to critically compare various solutions.

2. Materials and Mechanisms

The mechanism of PR in aqueous medium is based on photon absorption by a semiconductor, which causes the photopromotion of an electron to the conduction band, with formation of positively charged holes in the valence band. The bandgap energy extension of the semiconductor has to fit the above mentioned requirements on coupling with the redox potential of the hydrogen reduction and oxygen oxidation half reactions. Therefore, it should be in general higher than 1.23 eV. However, if it overcomes 3 eV solar light harvesting becomes minimal. Low efficiency is usually due to light scattering/reflection, poor absorption, and quick electron/hole recombination in the bulk or on the surface of catalyst particles [19] and fast back reaction of O2 and H2 during WS. The addition of small quantities of metals limit charge recombination [26], since excited electrons are captured by the metal and reduced protons. Metals should better have low overpotential for H2. Strong metal support interaction also improves the efficiency of the metal as electron scavenger helping in the decrease of electron/hole recombination in semiconductors photocatalysis [17, 27, 28]. However, the recombination rate increases at high metal loading because the distance between trapping sites in a particle decreases with the amount of metal, so that an optimum exists [29]. It is also worth mentioning the importance of the spill-over effect in the surface process for hydrogen production [17].

On the other hand, holes migrate to the surface where they oxidize the adsorbed organic molecules, so, another key factor to decrease the electron/hole recombination rate is the ability of the organic substrate as hole scavenger, in turn related with geometry and electronic structure upon adsorption [13].

The time scale of the different reaction steps is very different. The photoinduced generation of an electron-hole couple occurs in femtoseconds, their recombination in 10–100 ns. Reduction mediated by electrons needs a much longer time, in the order of ms, whereas the oxidative reaction carried out by holes takes ca. 100 ns, a comparable time scale with respect to recombination. Therefore, electrons need to be preserved for longer time by trapping them into a metal where they can migrate, or by making holes react with a suitable hole scavenger [30, 31]. When a metal (with proper work function) is deposed on a semiconductor, excited electrons from the semiconductor migrate to the metal until the two Fermi levels are aligned. The Schottky barrier [10, 32] formed at the metal and semiconductor interface can serve as an efficient electron trap to prevent photogenerated electron-hole recombination, greatly enhancing the efficiency of the photocatalytic reaction. The metal also serves as an active site for H2 production, in which the trapped photogenerated electrons are transferred to protons to produce H2.

Methanol and other organic compounds may act as sacrificial agents and hole scavengers in photocatalysis, being able to combine with photogenerated valence-band holes more efficiently than water. For instance, glucose is an efficient hole scavenger for titania [10, 3335]. Theoretical studies on glucose adsorption over titania showed distinct intragap states; this explains its hole trapping role in TiO2 [34]. Possible mechanisms for the whole PR process have been proposed, but this complex mechanism still needs validation. In aqueous solution other powerful oxidizing agents may be formed from surface adsorbed water, such as hydroxyl radicals and superoxide anions () which are able to oxidize and mineralize almost all organic pollutants yielding CO2. As a matter of fact, PR is a much less explored route for obtaining H2 from oxygenates, such as alcohols, or from biomass extracts and wastes [13, 36, 37].

The feasibility of PR of different biomass-derived compounds has been proved [38] with alcohols, organic acids, and different carbohydrates, such as ribose, arabinose, glucose, galactose, fructose, and mannose. In all cases, the amounts of excess H2 with respect to simple WS and CO2 produced are in accordance with the stoichiometry of the following reaction:

The rate of H2 production depends strongly on the nature and concentration of the organic substrate and, to a lesser extent, on pH and temperature. For instance, H2 productivity under solar light irradiation as high as ca. 3.75 mol H2/h kg of photocatalyst (0.5% Pt/TiO2). The heat content of H2 produced was higher than that of the substrate, demonstrating efficient storage of solar energy.

Among the photocatalysts, TiO2 is a cheap, widely available and chemically stable, wide bandgap energy semiconductor, hence, the most studied material [39, 40]. Its main properties and photocatalytic application issues have been extensively reviewed recently [41]. However, its main drawback is the scarce harvesting of sunlight, TiO2 absorbing in the UV region, which constitutes not more than 5% of the solar spectrum. The crystalline phase of TiO2 is also believed to be a pivotal factor for H2 production [10, 42, 43]. However, there is not univocal interpretation on the activity of the different crystal phases. Anatase is often considered the most active single phase photocatalyst, though its coupling with rutile may lead to the most interesting results [44]. Indeed, the photogenerated electrons in the anatase crystals may transfer to the conduction band of the rutile crystal and contribute to the reduction of H+. The valence bands of both phases have similar energy and thus contribute to the photooxidation of methanol in the same way. Furthermore, the anatase structure is usually more defective than the rutile one, but oxygen defects are alternatively believed as efficient electron traps, enhancing charge separation [45], or undesired recombination sites [46].

Other materials, such as La doped NaTaO3 [47], Sr2Nb2O7 [48], Sr2Ta2O7 [48], La2Ti2O7 [49], K2La2Ti3O10 [50] have been also proposed for WS, but they share with TiO2 the drawback of absorption only in the UV region, thus limiting solar light harvesting. Another very active photocatalyst is NaTaO3, with NiO as cocatalyst, which however keeps the limitation of absorption in the UV range only [51]. A recent review focused on various materials for photocatalytic applications, where some suggestions on photocatalyst properties are proposed [52].

The effect of crystal or particle size is not univocal, since on one hand quantum confinement of photogenerated charges, revealing for nanometric systems, should enhance activity. Furthermore, increased available surface sites concentration usually improves catalyst activity [5355]. Nevertheless, surface recombination of electron and holes may prevail, thus limiting the overall activity of the sample [56]. Also porosity may be important since holes may migrate to the surface of micro- or mesopores to oxidize an organic substrate, provided that it is able to diffuse inside the pores. The complementary H+ reduction reaction may occur on the external surface, where some metal nanoparticles may be present, drawing the photogenerated electrons. This physical separation of the reaction sites may help to avoid charge recombination, more likely if both species, electrons, and holes, should else migrate towards the external surface [57].

Besides offering a high catalytic activity [58], nanomaterials can provide enhanced performances thanks to a suppression of ohmic losses, usually occurring in bulk semiconductors [59, 60]. In addition, the band gap energy may be in principle tuned with particle size, in order to increase light absorption in the solar spectrum [61]. However, size effects sometimes induce an upward/downward shift of the CB/VB edges, complicating the prediction of the exact band positions [62].

Finally, morphology also showed important, especially when 1D nanostructures have been developed, such as nanotubes and nanowires [6366]. The higher photocatalytic activity of such systems is usually attributed to the high surface area combined with the short diffusion path of the photoexcited species across the bulk to reach the surface, where the reactants are adsorbed [66]. The effect of structure and morphology of semiconductors active for WS has been also recently reviewed [67].

The role of metal ions or oxides as cocatalysts has been enunciated since long time. For instance, bare TiO2 was almost inactive for both WS and PR of alcohols, whereas 1.0 wt% loading of various noble metals, such as Pt, Pd, Au, Rh, Ag, and Ru, improved H2 evolution. The activity scale (better results with Pd and Pt) was correlated with the workfunction of the metal. TiO2 has a large overpotential for H2 evolution. Since the work functions of the noble metals are larger than that of TiO2 [6870], a Schottky barrier can be formed at the metal-TiO2 interface, as introduced above. The presence of such barrier can decrease the recombination of photogenerated electron-hole pairs and prolong their lifetime, so greatly enhancing the photocatalytic activity [71]. Among nonnoble metals NiO was also effective [7275]. Complex activation procedures are however reported for NiO cocatalysts, such as combined redox treatments in order to obtain a Ni/NiO system, able to trap electrons while inhibiting the backward electron transfer [76, 77].

The role of metal loading is not univocal. H2 productivity during WS increased with increasing Pt content up to 1 wt% [10, 78]. When the metal loading was too high, the TiO2 surface was excessively covered, so decreasing light absorption. Furthermore, excessive metal sites may act as recombination centers. Furthermore, the presence of O2 strongly inhibits hydrogen production due to back reaction to H2O. In addition, dissolved oxygen can quickly consume electrons of TiO2 due to its stronger ability to trap electrons compared to H+.

PR reaction mechanism has been proposed for Pt/TiO2 catalysts (Figure 2) [10]. Alternative reaction paths have been proposed for Pd/TiO2 catalysts, where the metal is the main responsible for activity, while photoactivation plays a somehow limited role, leading to activated oxygen species on titania, which may contribute to further oxidation of CO, strongly chemisorbed over Pd particles (Figure 3) [79, 80]. However, the true process must be more complex and relies on the determination of intermediates [10].

964936.fig.002
Figure 2: Proposed alcohols PR mechanism over Pt/TiO2 photocatalysts.
964936.fig.003
Figure 3: Proposed PR mechanism over Pd/TiO2 samples. VO represents an oxygen vacancy.

One of the key points to develop a reliable catalyst for the PR of biomass derived substrates is its ability to harvest solar light, which is only in part constituted by UV radiation, mostly of IR and visible light (ca. 43%). Keeping in mind the requirement on minimum bandgap energy and its coupling with the redox potential of the reactants, different strategies have been considered, such as metal and nonmetal ions doping to narrow bandgap energy, dye-sensitization, development of new photocatalysts through bandgap engineering.

Metals have been sometimes added to improve response of the semiconductor (mainly TiO2 and SrTiO3) [53, 8184] under visible light irradiation. Indeed, transition metal ions may add unoccupied orbitals allowing electron transfer with much lower energy absorption than the undoped compound, as confirmed by the theoretical evaluation of density of states [85].

Different transition metals were proposed to improve visible light absorption and to generate effective charge separation [86, 87]. Doping ions may trap electrons or holes making them available for the reaction but acting as recombination inhibitors, though this point has been not univocally accepted. Attempts to dope TiO2 to improve absorption in the visible region have been also reported [18]. Doping with V, Mn, and Cr effectively decreased the TiO2 bandgap energy from 3.2 to 2.2–2.8 eV. By contrast, addition of Au imparted a pink-violet color due to surface plasmon resonance [88]. It has been demonstrated recently that plasmonic nanostructures of noble metals (mainly Ag and Au) induced interesting applications for photocatalysis, in particular for plasmon-enhanced WS on composite photocatalysts containing semiconductor and plasmonic-metal systems [89].

More exotic catalyst formulation () showed activity for the PR of glucose under visible light, especially at high Bi loading [90]. Sulphides were also used [1, 91], but they showed much less stability than oxides.

Doping has been proposed with anions, which more likely shift the valence band to higher energy (reducing in any case the bandgap energy) [53, 92]. Some examples of anions doping led however to very limited red-shift [14, 18].

Dye-sensitization, thoroughly explored for solar cells, has some limitations for the present application. In principle a dye, able to absorb visible light, promotes an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied one (LUMO) upon photon absorption. The LUMO has higher energy than the conduction band of the semiconductor, so the net effect may be that an electron is pumped in the CB, able to reduce H+ to H2 (Figure 4). On the other hand, the situation is complicated by the need of additional redox couples to oxidize water or any other substrate and to recover the original electronic state of the dye molecule (Figure 5). Furthermore, due to their direct participation in the reaction, many concerns are raised by the stability of the dye molecule for repeated reaction cycles.

964936.fig.004
Figure 4: Dye-sensitized photocatalytic H2 production.
964936.fig.005
Figure 5: Electron transfer paths in dye-sensitized TiO2. 1: dye (S) excitation; 2: electron injection from the excited dye to TiO2 conduction band (CB); 3: electron entrapment into Pt particles; 4: H+ reduction and H2 evolution over the metallic site; 5: regeneration of the oxidized dye by an electron donor (D); 6: electron recombination in the oxidized dye. Readapted from [1].

Finally, the coupling of different semiconductors has been proposed to improve visible light absorption, provided that the band gap energy of one of them is sufficiently small to efficiently harvest solar radiation and that the electron promoted in its conduction band may actually migrate into the conduction band of the other semiconductor [93, 94]. The same concept has been extended to visible light absorbing semiconductors embedded into a lamellar [95] or tubular [96, 97] structures, able to withdraw the photogenerated separated charges (Figure 6).

fig6
Figure 6: Mechanism of charge separation in CdS-TiO2 composite semiconductors (a). Mechanism of photocatalytic H2 evolution on CdS-intercalated layered composites loaded with Pt. Photogenerated electrons in CdS are quickly transferred to host layers through the nanostructure, preventing charge recombination. Readapted from [1].

Bioinspired approaches have been also attempted, as recently reviewed [98]. During the photosynthetic process, light is absorbed over a wide range of energy and charge recombination is effectively prevented by electron transfer. In particular, two proteins, called Photosystem I and II, are involved in these key steps, coadjuvated by other enzymes. By looking at the active sites of such natural catalysts, many researchers took inspiration to prepare metal complexes able to carry out some interesting reactions. For instance, the enzyme hydrogenase, responsible of H+ reduction to H2, has been immobilized over different materials. TiO2 has been functionalized with both hydrogenase and a Ru-based complex acting as sensitizer. The latter indeed absorbs visible light, transferring one electron into the conduction band of TiO2, which is further transferred to the enzyme for H2 production. Of course the cycle should be closed by oxidation of an electron donor species at the Ru complex side [99, 100]. Different ligands have been proposed for Ru, the most common ones being polypyridyl or mixed pyridyl-CO complexes. Other metals have been also proposed, such as Re, Pt, and Ir, showing very high photoactivity for H2 evolution [101103]. Unfortunately most of these systems did not prove stable under irradiation [104] and leaching of the dye was often observed. Interesting complexes have been also reported for Co, active for both water reduction and oxidation [3].

Unfortunately, for most of the catalytic systems proposed poor attention is paid to deactivation phenomena. Due to mild temperature conditions sintering is usually ruled out, but poisoning, often by reaction intermediates, and leaching are very critical points [105].

Details on the different catalyst formulations and their features will be discussed in the next sections.

3. Photoreforming of Alcohols

A possible mechanism for the PR of alcohols on TiO2 involves the oxidation of water molecules by photoinduced holes in the semiconductor. This produces hydroxyl radicals, which abstract an alpha-hydrogen to create a RCH2–OH radical. The radical is further oxidised to an aldehyde [106108].

The effect of different metals and their loading on the reactivity of titania-based catalysts for the PR of methanol has been investigated [109]. The highest catalytic activity was achieved with Pd and Pt, though significant improvement of the photocatalytic activity has been observed also with Ir and Au. High activity has been ascribed to noble metals due to their hard oxidability, and they are likely involved in the first dehydrogenation step of the reaction. In particular, the metal should be reduced by methanol or in general by the organic substrate [2]. An optimal loading exists for Pd, at ca. 0.5 wt%, and even two optimal loadings (two activity maxima) for Au. This has been correlated to the reaction mechanism, studied in detail over Pd/TiO2 catalysts. The metal active sites should be exposed on the surface of the metallic particle, and it should be connected with photoactive sites on titania [2, 109, 110].

The H2 production rate showed dependent on methanol concentration only for diluted solutions, that is, up to the formation of a monolayer of chemisorbed methanol on the catalyst surface. This suggests that the reaction rate follows a Langmuir-type mechanism. Due to instability of methanol adsorbed over Pd, the authors proposed its decomposition and oxidation to form CO, which is strongly chemisorbed over Pd particles and limits the kinetics of the reaction [111, 112]. This part of the reaction is suggested to occur also without irradiation, but in the dark Pd poisoning by CO would stop any further reactive event. Once the catalyst is irradiated with suitable wavelength to promote an electron from the valence band to the conduction band of TiO2, it may form strongly oxidizing species such as O, which induce further CO oxidation to CO2, finally released making a metal site free for further methanol adsorption (new reaction cycle). Of course this implies a bifunctional catalyst where the metal active site and the oxidising one on the titania surface are adjacent. In this view, H2 production occurs only when H2O reoxidises the reduced titania site (Figure 3). Metals with dehydrogenating activity may form acetaldehyde, which is weakly adsorbed, and it may desorb in gas phase [2].

At least for TiO2, the main oxidising species are reported to be free or trapped holes, radicals, , 1O2, H2O2, and O2 [30, 113]. In most cases direct oxidation by holes is simply hypothesised, in others the process is reported to be mediated by other highly oxidizing species, for example, OH radicals, derived by the reaction of photogenerated holes with water molecules [114]. Several studies highlight the presence of both shallowly and deeply trapped holes on the photocatalyst surface [115, 116]. Shallowly trapped holes show reactivity and mobility comparable to free ones, reacting thus very rapidly with chemisorbed species. In this case, oxidation processes might even be competitive with ultrafast charge-trapping events. On the other hand, deeply trapped holes, which exhibit lower oxidizing potentials, preferentially react with more mobile physisorbed substances, and the corresponding reaction rates are lower [105, 117].

The reaction mechanism of methanol PR over Au/TiO2 catalysts likely follows a different mechanism [4]. At first, two maxima for reaction rate were observed when varying metal loading up to 5 wt%. Slow rates at low and high loading were interpreted on the same basis above reported for Pd catalysts, that is, unavailable perimeter of metal particles leading to too scarce interphase between the photoactive species (TiO2) and the metal. The two maxima are located at 0.5 and 2.0 wt% Au, likely corresponding, in the view of the authors, to different morphologies of the metal particles. The proposed reaction mechanism is slightly different than that above outlined for Pd. Indeed methanol dissociatively adsorbs on a free metal site forming an adsorbed methoxy species, which seems stable at room temperature. Then, this species is further oxidized by photogeneration of strongly oxidizing agents (e.g., O) on the surface of titania upon absorption of a photon of sufficient energy (>3.2 eV). The oxidant migrates to the interface of the metal promoting the oxidation of the methoxy group to CO2, which is finally desorbed. This liberates 2 moles of H2 per mol of methanol. Finally, the catalyst is regenerated by water, which acts as an oxidant, refilling titania with its oxygen and making another H2 molecule available (Figure 7). The effect of phase composition and textural properties for Au/TiO2 catalysts has been also investigated [118].

964936.fig.007
Figure 7: Proposed mechanism for methanol PR over Au/TiO2 catalysts. Readapted from [4].

Similar results for methanol PR over Au/TiO2 have been reported by Wu et al. [119], particularly focusing on low CO selectivity. The reaction rate was dependent on methanol concentration up to a certain value, after which the reaction order with respect to the reactant was ca. zero, and the rate dependence was of Langmuir type. Thus, methanol adsorption on catalyst surface showed a pivotal role. This in turn partly explained the effect of pH, since adsorption (of reactant and intermediates) depends on surface charge of the semiconductor. The role of Au particle size was central most of all to address CO selectivity problems, since full CO oxidation may occur at the interface between the metal and the semiconductor. Thus, Au nanoparticles may expose higher contact perimeter for CO conversion. A parallel investigation on CO suppression has been carried out over Pt/TiO2 samples [120]. The effect of Pt loading was considered together with the addition of small amounts of inorganic anions. Focusing on the main route bringing to CO formation, for example, the dehydrogenation of a formic acid intermediate, Pt showed to adsorb on defective sites of TiO2, which act both as recombination sites, decreasing photocatalytic efficiency, and as active sites for CO formation. Similarly, anions were competing for adsorption with formic acid.

In this model the rate determining step is the rate of CO removal [2, 80]. The attribution of the nature of the active oxygen species that removes CO2 from the Pd surface is still uncertain: OH radicals have been observed by electron spin resonance [121]. On the other hand, H2O2 has been observed over Pt/TiO2 catalysts, maybe formed through the dimerization of hydroxyl radicals [122].

In this view, a detailed investigation on the effect of Pd loading over titania has been carried out, evidencing the highest reaction rate for methanol PR 0.5 wt% metal loading [110]. The effect of metal loading has been often interpreted in view of the availability of surface metal sites (increasing with loading) and surface titania exposure, the semiconductor surface being shadowed at too high metal coverage. However, Bowker and coworkers proposed a different explanation, based on the rate determining step of the reaction. As stated above, the latter should be something happening at the interface between the Pd particle and TiO2, if the bifunctional mechanism proposed by this research group is valid. Therefore, for reactions with rate dependence on the active sites located at the interphase, the extension of the metal particle perimeter is the predominant factor influencing activity. A calculation of the dependence of the particle perimeter with respect to its volume (radius) is proposed and correlated to initial reaction rate for methanol PR [110]. The model fits the data as for the curve trend with metal loading, though some refinements are needed to provide accurate numerical fitting.

The mechanism of PR of different alcohols has been also investigated on Pd/TiO2 [79]. The reaction byproducts were widely variable depending on the substrate structure. For instance, methanol gave only H2 and CO2, ethanol and 2-propanol produced also methane as the only major by-product, 1-propanol produced ethane, 1-Butanol and 2-butanol produce propane and ethane, respectively. D-glucose, sucrose and glycerol also showed very good reactivity. The presence of hydrogen atoms in the molecule at the alpha carbon position seems a necessary condition for the reaction. The amount of H2 produced can be estimated by the number of alpha-hydrogen atoms present. In methanol and glycerol reforming all the carbon atoms possess alpha-hydrogen; thus, complete dehydrogenation occurs on Pd, leaving absorbed CO on the surface, and converting all the hydrogen atoms into H2 with a rate for glycerol PR ca. twice than for methanol [13]. As mentioned above [109], the semiconductor plays an active role in the further oxidation of CO to CO2, thus regenerating an active site. Therefore, CO actually acts as hole scavenger according to this mechanism [79].

A general rule has been derived to predict the PR products of a general alcohol [80]:(a)a hydrogen atom in alpha position to the alcoholic function must be present (tertiary alcohols show very poor H2 productivity);(b)alkyl groups attached to alcohols yield the corresponding alkanes;(c)methylene groups are fully oxidised to CO2.

K4Nb6O17 showed the highest activity for H2 evolution from an aqueous methanol solution, with ca. 50% quantum yield at 330 nm [123125]. The addition of various cocatalysts, such as NiO, Au, Pt, and Cs [126131] improved H2 productivity even from pure water.

H2 production with only 10 ppm CO was observed with methanol PR over Pt-TiO2 catalysts with adsorbed sulfate/phosphate ions [120, 132]. Furthermore, a significant H2 production efficiency (120 μmol/min) was obtained with very low methanol concentration without any significant deactivation even after long irradiation time [118].

Ag-doped TiO2 was studied for various applications like sucrose mineralization [133], photocatalytic decomposition of O-cresol [134], degradation of salicylic acid [135], E-coli bacteria degradation [136], and selective NO reduction to N2 and N2O [137]. Ag and AgO were differently added to TiO2 for use in the PR of methanol [138]. A broadband around 450–550 nm was observed corresponding to the surface plasmon resonance of silver particles, and the appearance of this band confirms the presence of metallic silver [139]. The highest H2 productivity was reached with 1 wt% metal loading and activity increased with substrate concentration up to 5%. At lower concentration mass transfer limitations prevailed, whereas when methanol was too abundant the surface was saturated and the reactions at the interface dominated the whole process [140]. When Ag2O/TiO2 and Ag/TiO2 catalysts are irradiated by UV light, titania absorbs the photons and the generated electron-hole pairs are separated by both the metal and the metal oxide. Under solar light the mechanism is different. Silver ions interacting with the surface layers of TiO2 expanding the poor visible response of the latter (bandgap energy 2.88 eV).

A laser-based method for the PR of methanol over WO3 photocatalyst has been proposed. Hydrogen, carbon monoxide, and methane were obtained and no catalyst aging was observed during 15 days [141]. NiO was also tested under the same conditions for the PR of methanol and higher alcohols [142]. It is p-type semiconductor with a bandgap energy of 3.5 eV and the potentials of the valence and conduction band edges at +3.0 and −0.5 V, respectively [143]. Absorption of a photon of suitable energy () causes the transfer of an electron from the valence band of oxygen (2p) to the conduction band of Ni (3d) atom and in turn the reduction of Ni2+ to Ni+. This electron transfer process weakens the Ni–O bond forming an oxygen vacancy. Increasing water concentration with respect to pure methanol allowed to increase the H2/CO ratio and to virtually eliminate the byproduct methane.

Copper oxide may also be effective for methanol PR, because the CuO conduction band is located below that of TiO2, so electron transfer to the conduction band of CuO is possible [144]. The phenomenon becomes important when a part of incident radiation has wavelength in the UV range. The hole transfer from the valance band of TiO2 to that of CuO is also possible [145]. Kawai and Sakata [146, 147] have described that the reaction can proceed either stepwise, involving stable intermediates such aldehydes and acids, whereas a single step reaction was proposed by Chen et al. [148]. The overall process can be divided into three steps. At first WS occurs on the surface of TiO2, but its rate is limited by accumulation of holes and oxydryl radicals. In the second step, the photogenerated holes oxidise methanol to formaldehyde, then further oxidized by both OH radicals and photogenerated holes to produce formic acid. In the third step, formic acid is decarboxylated by the photo-Kolbe reaction to release CO2. H+ ions deprotonated along all the steps transfer to CuO and reduce to hydrogen by action of the photogenerated electrons [145].

High CuO/TiO2 activity for methanol PR was reported by Xu and Sun [149]. The active sites for hydrogen generation were likely located at the interface rather than on the isolated CuO or TiO2 surfaces, since no H2 production was achieved over both the pure oxides. Activity decreased at high CuO loading, partly for the shrink of available CuO-TiO2 interface and for depressed light absorption by TiO2. Methanol adsorption confirmed a rate limiting step, interpreted according to a Freundlich isotherm. Different causes of catalyst deactivation have been taken into account, with Cu reduction and leaching (under acidic conditions) as main problems. Cu doping for TiO2 under visible light irradiation for methanol PR was attempted by Yoong et al. [144].

Sonochemical synthesis of Pd, Pt, and Au doped TiO2 photocatalysts was reported for the PR of ethanol [150]. Pt was the most active catalysts due to its higher dispersion. Noble metal doped samples prepared by flame pyrolysis were also proposed for this reaction [151]. The highest H2 yield was achieved for 1 wt% Pd/Pt-TiO2 catalysts, whereas Rh was too prone to oxidation.

Photodeposition of the cocatalyst is a powerful tool to achieve significantly higher dispersion of the metal phase with respect to impregnation and calcination. Indeed, the latter treatment may induce TiO2 reconstruction over the metallic phase, thus limiting its activity [152].

Flame pyrolysis is a preparation method allowing to obtain single or mixed oxides in nanosized form [153157]. Titania was also prepared for photocatalytic applications. For instance, Au deposition over titania prepared by flame pyrolysis allowed to improve by ca. 30 times the photocatalytic activity for the PR of methanol in liquid phase with respect to the bare titania support and by 50 times with respect to a commercial P25 sample [158]. A gas phase apparatus has been also developed to test the same samples [159]. Methanol conversion appeared rather low, and a CO2/H2 production ratio very much lower than that expected from the stoichiometry was achieved. Some attempts to determine possible byproducts have been made, observing the formation of formaldehyde. However, a reliable quantification of intermediates is very hard with such kind of experimental setting, based on recirculation of the reacting mixture through the starting 20 vol% methanol/water solution. Such solution very likely dissolves reaction products (and also CO2) much more effectively than pure water, preventing their reliable quantification. Other metals were also tested, obtaining the highest H2 productivity with 1 wt% Pt/TiO2 [160].

Gas phase methanol PR was previously proposed by Bowker and coworkers over Au/TiO2 catalysts. They reported ca. 135 μL/gcat min H2 productivity, more than three times higher value than in liquid phase photoreforming [161].

The effect of TiO2 crystalline facets exposition on H2 productivity has been taken into account [39, 41, 162, 163]. The most stable form of the anatase polymorph is a tetragonal bipyramidal structure in which {101} facets are exposed, with a small contribution of {001} facets [164], but the use of HF during the synthesis of the semiconductor allows to form truncated bipyramids [165]. Morphological and crystallographic modifications of TiO2 nanocrystals have been achieved by modulating F concentration and dosage [166]. Indeed, the use of TiF4 as precursor, pure or in mixture, allowed the formation of HF in situ. The latter has been used as a shape directing agent, binding selectively to the {001} facet of anatase and altering the shape of the resulting nanocrystals. The samples were blue colored, due to free conduction band electrons in TiO2. Additional electrons may come from oxygen vacancies or titanium interstitials. In the latter case, irradation produces conduction band electrons which localize on surface Ti atoms, but this coloration is quickly quenched upon exposure to oxygen or other oxidizers [167, 168], except under extremely high photon flux [169]. A broad Vis-NIR-IR absorption band has been observed for such samples, commonly attributed to the excitation of conduction band electrons in blue TiO2 and therefore reflecting the density of states of the conduction band [170]. Fluorine in this case is not substitutionally doped into the TiO2 lattice, but it plays an important role in the formation of oxygen vacancies due to stronger Ti−F bond with respect to Ti−O. No hydrogen was evolved in the absence of Pt or photocatalyst or when samples are illuminated with visible light only ( nm). An induction period of ~1 h was observed, and H2 productivity up to ~2.1 mmol H2 h−1 g−1 was achieved.

Despite the crystal facets exposed, the role of anatase-rutile junctions was investigated by varying calcination temperature of a Degussa P25 sample, originally starting form a 80 : 20 ratio of the two phases, respectively. Alignment at solid-solid interfaces is one of the most important factors for effective interfacial charge transfer and improved photocatalytic activity. The more numerous the anatase-rutile junctions, the higher the photocatalytic activity for hydrogen production [171]. Indeed, phase boundary suppresses hole-electron recombination and the CO concentration during PR of methanol, glycerol, and glucose was the lowest. The change of surface acidity/basicity at the interfacial site should be responsible for the suppression of CO in biomass PR.

Metals, especially noble ones, have been extensively investigated as cocatalysts, in spite of their high cost. A much cheaper alternative has been proposed, using graphite silica, a natural mineral as cocatalyst for TiO2 for the PR of methanol. The physical mixing of the two solids improved the photocatalytic performance with respect to pure TiO2, though a comparison with a typical Pt/TiO2 formulation revealed ca. one eight H2 productivity, only [172]. Therefore, graphite silica was added to a Pt + TiO2 mixture, obtaining an increase of H2 productivity by 150%. However, when adding the mineral to an impregnated Pt/TiO2 sample, the H2 yield halved [173].

SrTiO3 has been shown to possess good structural stability as a host for metals [174], and it has been used in photocatalytic WS for hydrogen production with different hole scavengers (alcohols, Na2SO3, and D-glucose) [175]. The photocatalytic activity was found in the order: MeOH > EtOH > D-glucose > 2-PrOH > Na2SO3. Mesoporous-assembled SrTiO3 nanocrystals exhibited much higher activity than two commercial photocatalysts, that is, SrTiO3 and TiO2.

Even if the bandgap energy of ETS-10, a large pore zeotype titanosilicate (Na2TiSi5O11), is higher than that of TiO2 ( eV with respect to ca. 3.0), its value was cut to 2.25 eV (visible light absorption) by doping with thiourea. This induced C, N, and S ions doping at once, improving the photocatalytic activity for the PR of methanol [176].

The reaction mechanism has been investigated for the PR of ethanol over Pt-based samples. H atoms belonging to adsorbed hydroxyl groups are reduced by electrons cumulating into Pt particles at the Pt-TiO2 interface, while chemisorbed ethoxide species behave as hole traps [151]. Different indications are furnished about the resistance to deactivation during ethanol PR. Indeed on one hand ethanol is one of the easiest substrate to reform, due to its simple structure and limited amount of byproducts. On the other hand poor resistance to deactivation was reported, with heavy deactivation after 20 h due to the formation of several byproducts, such as acetaldehyde, acetone, and 2-butenal, produced by the partial oxidation and UV decomposition of ethanol [177, 178].

TiO2 thin films, functionalized with different metals, were used for ethanol PR [140]. Contrarily to most findings, ethanol showed the most efficient hole scavenger, even better than methanol. The importance of optimizing catalyst exposure in form of films has been highlighted and, interestingly, the quantum yield and the energy conversion efficiency have been provided. A stable photoactive layer based on TiO2 over a porous polymer membrane has been proposed [179]. The PR of ethanol was investigated under UV light irradiation. The influences of Pt loading, coating, and composition of the TiO2 layer and its aging time were determined. Interestingly, the rate of hydrogen production was higher in the liquid-membrane-gas configuration compared to the liquid-membrane-liquid one, opening the way to effective separation of the gaseous products from the reaction environment.

In order to decrease significantly catalyst cost Cu-based catalysts were also developed. CuxO-TiO2 () nanocomposites were tested for ethanol PR [180, 181]. The best performance was achieved with 1 wt% metal loading, due to the main presence of Cu(I) ions, active sites for the transfer of photogenerated electrons, whereas Cu(II) species seem to promote electron-hole recombination. Deactivation was also taken into account [149], due to poisoning by accumulation of intermediates and by reduction to the metallic state.

nanorods were also tested for this application, with a bandgap energy of 2.7 eV, with quantum efficiency as high as 40% without addition of noble metals [182]. Pt-doped SrTiO3 catalysts proved also active under UV and visible light for the PR of methanol and ethanol [175] and mixed La-Ta oxynitrides were proposed as visible-active photocatalysts for hydrogen production from water/ethanol solutions [183].

Ethanol PR occurs through a progressive oxidation coupled with acetal formation, promoted by the presence of surface acidic sites, and accompanied by dehydration and water gas shift reaction [184].

The PR of aqueous solutions of ethanol and glycerol has been performed over CuOx/TiO2 catalysts prepared in different ways [181, 185]. The higher dispersion of the metal oxide achieved by embedding CuO via a microemulsion technique allowed superior activity with respect to classical impregnation. The amount and kind of byproducts depends on the type of catalysts and of substrate. During PR of glycerol some deactivation unfortunately occurred over the most active catalyst, leading to ca. 10–20% loss of activity. Some metal leaching was observed, limited under irradiation, but by far more pronounced under oxidizing and acidic conditions in the absence of light, up to 25% loss of the cocatalyst.

CuO was also coated by RF-sputtering over ZnO, trying to improve the performance of a powdered composite [186]. The former material has very low bandgap energy, and it absorbs in the visible region, whereas ZnO is characterized by much wider bandgap energy. However, the formation of a p-n heterojunction favors electron-hole separation. CuO has been loaded over ZnO using different RF frequency. The lowest RF-power led to high quality CuO-ZnO interfaces (core-shell structure) improving photogenerated charge carrier separation, but completely covering the underlying ZnO surface. By contrast, the highest RF-power improved CuO particles dispersion. The most promising performances were obtained for the sample prepared with an intermediate RF-power, leading to a sufficient fraction of uncovered ZnO available for reactant adsorption and light absorption, together with a higher active area [187]. These samples were successfully used for H2 photogeneration from ethanol/water solutions. However, no complete PR reaction took place, the catalyst mainly produce acetaldehyde through ethanol dehydrogenation, with almost no CO2 formation.

Au/TiO2 catalysts were tested for the PR of ethanol, particularly focusing on the effect of particle size, varying over one order of magnitude [188]. An investigation of intermediates and products has been carried out by mean of infrared spectroscopy and temperature programmed desorption, evidencing a wide variety of species over the catalyst surface. The nano-sized catalyst showed higher H2 production, by one order of magnitude, compared with the micro-sized system, but normalizing over the surface area the productivity was equal. This allows to conclude that while nanostructuring the TiO2 semiconductor enhanced productivity, this effect was merely geometrical and did not depend on changes of intrinsic electronic properties.

The reforming of glycerol may lead to the production of 7 moles of H2 per mole of reformed substrate, and it is particularly attractive since glycerol is the major byproduct of biodiesel production. The huge overproduction of glycerol has a negative impact on the global biodiesel economy, since the processes for the economical valorization of glycerol are still limited [189, 190]. In addition, raw glycerol also contains methanol, water, inorganic salts, free fatty acids, triglycerides, and methyl esters, requiring expensive purification for most applications. Therefore, means for the energetic valorization of this polyalcohol are more than welcome, in light of its increasing availability.

0.5 wt% Pt/TiO2 was tested for glycerol PR in aqueous solution, increasing the H2 production rate by one order of magnitude with respect to pure water [38]. Other examples of glycerol PR over Pd/TiO2 and Au/TiO2 were proposed in [13], though the catalysts were sensitive to UV light only. Visible light was efficiently harvested by Fu and Lu [191], who used heteropolytungstate/TiO2 catalysts. Upon UV irradiation in the presence of an electron donor (glycerol), heteropolytungstate yields dark blue heteropoly blue, absorbing in the visible spectral range. H2 production under visible light was effectively achieved, but a dramatic system deactivation took place, mainly due to the light-induced decomposition of the sensitizer.

Luo et al. [14] proposed Pt-B,N-codoped TiO2 photocatalysts, while Cu/TiO2 catalysts were active for visible light driven PR of glycerol [192]. High activity for the latter catalytic system was ascribed to very high Cu dispersion, also achievable by photodeposition [184, 185]. A significant increase in the absorption at wavelengths shorter than 400 nm can be assigned to the intrinsic bandgap absorption of TiO2 enhanced by CuO [193]. High activity can be attributed to the electron transfer from TiO2 to the quantum-sized CuO. The optimal CuO content in the photocatalyst was found to be 1.3 wt% with H2 production rate of 2061 μmol h−1 g−1, ca. 130 times the production rate of pure titania, and the apparent QE was 13.4%.

Glycerol PR has been investigated over Pd/TiO2 and Au/TiO2 catalysts, achieving ca. double H2 production rate with respect to methanol, approximately according to the reactions stoichiometries [13]. Pd, though in lower loading than Au, proved almost twice as active. The reaction mechanism, proposed by Bowker and coworkers for Pd-based photocatalysts is similar to that above outlined for methanol. The adsorbed substrate is decomposed into CO and H2. CO is further oxidized to CO2 only under irradiation, thanks to the formation of highly oxidizing species over the titania surface, as already discussed for the mechanism of methanol PR.

Glycerol PR has been also investigated over Pt/TiO2 catalysts [12]. Glycerol confirmed an efficient scavenger for oxidizing species (i.e., holes, hydroxyl radicals, etc.) increasing the rate of hydrogen production with respect to direct water splitting. Very dispersed nanocrystallites of Pt played a determinant role in the photocatalytic process, both as electron scavenger and oxidizing catalyst for dark reaction steps. The effect of noble metal concentration was going through a maximum, as described in other references, in this case for 0.1–0.5 wt% loading. Also in this case, the reaction rate first increased with the concentration of the photocatalyst and then decreased for high values due to a shielding effect. Neutral or slightly basic conditions improved the reaction rate. This dependence can be due to the favorable positions of the valence- and conduction-band levels of the semiconductor with respect to those of the redox couples in solution, to the charging behavior of the semiconductor surface, to the speciation of the reactants in liquid phase [12, 194, 195]. A slight increase of temperature (60°C) also proved beneficial to improve reaction rate, likely increasing the rate of the “dark” reaction steps. Many intermediates may also form the most abundant being methanol, which are completely converted to H2 and CO2 after proper reaction time. The kind of oxidizing species was also investigated, particularly focusing on the formation of peroxide species on catalyst surface and in solution [122].

The alcohol may adsorb both in a molecular way or forming alcoxy species, which are more readily oxidized than the H-bonded ones [196, 197]. A correlation between H2 yield and the polarity of alcohols was also proposed over TiO2-based photocatalysts [151].

Polyols (e.g., glycerol or glucose) showed usually higher activity for photoreforming reactions. Surface OH sites showed important since they accept photogenerated holes to form hydroxyl radicals (OH) [30, 198200], one of the principal reactive oxidant in photocatalytic reactions. They also serve as active sites for the adsorption of reactants and intermediates [201]. It was concluded by time-resolved transient absorption spectroscopy [202, 203] that C2–C6 polyols and carbohydrates could scavenge 50–60% of the photogenerated holes of TiO2 in the time lapse of 6 ns. The scavenging efficiency was higher for increasing the number of OH groups. The adsorption of polyol on TiO2 can result in levels inside the TiO2 bandgap, responsible for the fast hole trapping efficiency [35]. By comparing isopropanol, propyleneglycol, and glycerol with increasing number of OH groups in the molecule H2 production increased and alkanes concentration decreased [204]. Furthermore, undercoordinated surface Ti sites can be stabilized by hydroxyl groups, water, or other hydroxylic molecules, such as alcohols, polyols, and carbohydrates [202, 205, 206]. R-OH molecules bind to Ti atoms through their hydroxyl O and release protons [35]. The conversion of surface Ti to octahedral coordination by the chemisorption of polyols has also been experimentally verified by the observation of Ti K-edge XANES spectra [202]. Two types of adsorption have been observed, however, leading to the same surface intermediate. The role of surface hydroxyls is also of hole scavengers, forming very reactive OH radicals which promote substrate oxidation: the higher the number of OH groups in the molecule, the higher the efficiency of the molecule as hole scavenger. The net effect is the breakup of a C–H bond leaving an adsorbed ketyl radical. A detailed reaction mechanism has been proposed for 2-propanol, supported by FT-IR data [207].

4. Photoreforming of Carbohydrates

Due to the current low efficiency of the WS process, also biomass derived substrates, such as carbohydrates, are gaining increasing interest as an abundant, renewable, and clean feedstock for hydrogen production. Furthermore, the food industry gives a huge amount of solid and liquid wastes (classified as 2nd generation biomass) which have to be valorized. The sewage from food industry contains first of all water-soluble carbohydrates. Until now the sewage from food industry are purified using mainly biological methods. Using pollutants as electron donors shows the promise for integration of wastes decomposition and H2 production.

The feasibility of H2 photoproduction from carbohydrates, such as sugars, starches, and/or cellulose, on RuO2/TiO2/Pt catalysts was first attested by Kawai and Sakata [11]. The abundance of hydroxyls in sugars leads to easier activation of the substrate, though the full degradation of their complex structure is much more complex than simple alcohols. Thus, lower productivity is usually observed. The order of reactivity is consequently starch < sucrose < glucose [10]. The PR of glucose and sucrose over B,N-codoped TiO2 has been reported even with visible light [14]. A reaction mechanism has been proposed for Pt/TiO2 [10]. Glucose adsorbs through one of its oxydryls to an under-coordinated Ti(IV) surface site and dissociates leaving a proton and an alkoxide anion. The organic fragment is oxidized by a hole to give a radical, which reacts with another glucose molecule. Further oxidations occur to aldehyde and carboxylic acid derivatives, which are ultimately converted to CO2. Adsorbed H+ can be reduced to hydrogen on Pt sites by using photogenerated electrons.

The photocatalytic reforming of aqueous solutions of glucose over metal doped titania has been recently investigated [10]. A possible mechanism for the process has been also proposed, but the details of this complex mechanism still need a firm validation. Indeed, glucose behaves as an efficient hole scavenger for titania, though, the interpretation of the experimental results were insufficiently explained by a significant mechanism due to the lack of structural information [202]. A detailed mechanism for glucose photo-oxidation on titania has been introduced in [35]. However, only a single adsorption configuration was considered. The adsorption of α-D-glucose on the (101) surface of anatase has been computationally investigated [34]. End-on adsorption via a single hydroxyl group, as well as bridge adsorption via two hydroxyl groups, have been considered, in both molecular and dissociative modes. The Ti–O(glucose) bond length showed rather insensitive to the selected hydroxyl group(s) and to the end-on or bridge adsorption mode. By contrast, the bond length decreased significantly when adsorption occurred dissociatively. Ti ions adsorbing glucose in end-on mode showed exposed outward from the surface, while the opposite seems to occur to oxygen ions involved in bridging adsorption. Molecular adsorption seems more favorable with respect to dissociative one. The highest occupied electronic state is localized almost completely on the glucose molecule and distinct intragap states appear in several cases; this offers an explanation for the experimentally observed hole trapping in photoexcited titania nanoparticles in the presence of glucose.

The effect of various parameters has been evaluated on the PR of mono-, di-, and poly-saccharides [10]. The rate of H2 evolution decreased with increasing molecular complexity, but microwave pretreatment of soluble starch displayed significantly enhanced H2 evolution. H2 evolution increases with increasing glucose amount, though also water takes part in the reaction. However, the reaction rate is dependent on glucose concentration on the catalyst surface rather than on the solution.

The H2 evolution rate during PR of glucose increased with pH with a maximum at pH = 11 with a very complex effect. The pKa of glucose is about 12.3 [208] and when pH ≪ pKa, glucose is mainly in its molecular form, preferentially bonding with surface Ti atoms through its hydroxyl [35]. At higher pH, glucose dissociates and the organic anion captures positive holes more efficiently, enhancing activity. If pH > pKa, glucose is in the negatively charged form, but TiO2 surface is negatively charged too (pH higher than the isoelectric point of TiO2, ca. pH = 6 [209]), limiting adsorption and, thus, H2 evolution rate. Furthermore, different H2 production rate was obtained with the two anomers α- and β-D-glucose. The latter indeed adsorbed in almost planar form over TiO2, leading to lower adsorption [210].

The dependence of the PR activity on glucose concentration showed in line with a Langmuir-type kinetic model and the catalytic activity increased in the order Ag, Ru < Pd, Au, Rh < Pt and was correlated with the metal work functions; Pt shows the highest work function and, thus, it may store more efficiently the photogenerated electrons [10, 105].

NiO was used as cocatalyst for TiO2-SiO2 during PR of glucose [211]. An intense absorption onset in the range of 350–400 nm was found for Ni/TiO2-SiO2, while the support only absorbed UV-light with wavelength lower than 300 nm. Besides effectively decreasing the bandgap extension, Ni addition also allowed to limit CO formation with respect to the bare support.

Among alkaline tantalates, NaTaO3 exhibited the highest activity for H2 production from glucose solution, as well as for the water splitting reactions, whereas LiTaO3 showed the lowest activity. This was ascribed to the lower Ta–O–Ta bond angle of LiTaO3 (143°), with respect to NaTaO3 (163°) and KTaO3 (180°). This inhibits the charge migration [212]. Furthermore, the former has wider bandgap energy and poor absorbance [51]. The Fermi level of NiO, used as cocatalyst for such systems is compatible with the energy level of the conduction band of NaTaO3, but not with that of KTaO3.

Hydrogen production was achieved under visible light by PR of glucose over . It was found that a certain amount of Y doping would promote the transition of BiVO4 from monoclinic to tetragonal phase and finally lead to the formation of tetragonal solid solution. B/Y ratio of 1 : 1 showed the highest activity for hydrogen production. The rate of H2 production decreased after several hours of reaction, but it was recovered after flushing in N2 [90].

Aqueous sucrose solutions were used as model for sugar industry wastewaters and their PR reaction was carried out over metal or anion doped TiO2 and TiO2-SiO2 [18]. The latter semiconductor showed very similar bandgap energy with respect to titania, whereas the addition of metals variously decreased the absorption edge of the photocatalysts. In particular, 1% V/5% TiO2/SiO2 decreased it to 2.6 eV, likely due to the superposition of the TiO2 and interband absorptions. 1% Mn/5% TiO2/SiO2 and 1% Cr/5% TiO2/SiO2 decreased the to 2.8 eV and 2.2 eV, respectively, due to intercalation of doping metals in the TiO2 framework. By contrast, 0.5% Au/5% TiO2/SiO2 exhibited a pink-violet color due to the surface plasmon resonance originating from the collective oscillations of the electrons on the surface of the gold nanoparticles. Visible light is absorbed by gold nanoparticles due to such plasmon resonance, which leads to a photoexcited state of the gold nanoparticles followed by the transfer of the electrons into TiO2 [88]. All the metals strongly improved the photocatalytic activity with respect to the Ti-Si support, with an activity scale V > Au > Mn > Cr. Mn and V ions can trap both electrons and holes [213], whereas Cr ion can only trap one type of charge carrier [18] so its activity is lower. In addition, vanadium has a larger capability compared to the other metal to coordinate and release oxygen species. On the contrary, anion doping of TiO2-SiO2 lead to poorer activity, except for sulphur doping.

An increase of H2 production with respect to WS was achieved by Kondarides et al. [38] by PR of a variety of biomass-derived, including monosaccharides, such as pentoses (ribose, arabinose) and hexoses (glucose, galactose, fructose, mannose), alcohols (methanol, ethanol, propanol, butanol), and organic acids (acetic acid, formic acid). In all cases, H2 and CO2 were produced in accordance to the stoichiometry of the general reforming reaction:

The maximum H2 production rate increased almost linearly with the logarithm of the concentration of the substrate, with temperature (60–80°C) and pH.

5. Photoreforming of Other Substrates

5.1. Waste Solutions

A very interesting approach is to combine H2 production and degradation of organic or inorganic pollutants from wastewaters. Various examples have been reported about water solutions containing various alcohols, for example, 2-propanol [214], acids [215, 216], and aldehydes, such as formaldehyde and acetaldehyde [194, 217].

LaNiO3-based catalysts were active under visible light for the PR of formaldehyde [194], while the degradation of azo-dyes was performed with simultaneous H2 production over Pt-TiO2 [218].

Different examples have been recently reported, such as the treatment of olive mill wastewater over titania, predominantly in the anatase phase, prepared by sol-gel [219]. The rate of H2 production decreased with time and with prolonged exposure to light due to deactivation of photocatalyst and decreasing reactant concentration. The suppression of the organic fraction, expressed as Chemical Oxygen Demand (COD) removal was 83%, demonstrating the effectiveness of this approach [220223]. High H2 productivity was achieved, especially under acidic conditions (pH = 3). The effect of pH is consistent with the point of zero charge of TiO2 (pH = 6.8), which leads to a positively charged surface in acidic medium. This favors the adsorption of organic pollutants onto the TiO2 surface and facilitates their further degradation [216]. Under these conditions the conduction band position is also favorable to promote the reduction of H+ [12]. An optimal catalyst concentration of ca. 2 g/L was also observed [219], beyond which turbidity induces light scattering and poor absorption [224].

A variety of possible organic pollutants in wastewaters (alcohols, acids, and aldehydes) have been photoreformed over Pt/TiO2 under variable reaction conditions, achieving in all cases, except with acetaldehyde, full mineralization according to the reforming reaction above reported. CO2 release in the gas phase was delayed with respect to H2 due to stronger adsorption of CO2 on the photocatalyst surface, to the higher solubility of CO2 in water and/or to mass transfer limitations. It is also likely that the full mineralization of the organic molecule occurs through many consecutive reactions, where the intermediates are slowly oxidized to CO2 while continuously releasing H2 in various reaction steps. The rate of reaction was higher with simpler molecules. The reaction rate confirmed higher under neutral or basic conditions, at relatively high temperature (60–80°C) [194].

The photocatalytic degradation of azo-dyes, such as Acid Orange 7, in aerated aqueous TiO2 suspensions has been taken into account [225227] to achieve full decoloration and the reduction of the chemical oxygen demand of the solution. The reaction was then carried out under anaerobic conditions to explore the possibility of producing hydrogen and to abate dyes such as Acid Orange 7, Basic Blue 41, and Basic Red 46 [218]. Poor photocatalytic activity was achieved with bare TiO2, whereas H2 production increased after addition of Pt, with an apparent activation energy of  kcal/mol, and most of all of dyes, acting as hole scavengers. Decoloration of the solution was observed under UV-vis or visible light irradiation, by cleavage of the dye molecule in the vicinity of the azo-bond forming colorless intermediates which may be subsequently oxidized. The amount of H2 additionally produced with respect to WS was proportional to the amount of dye used.

The photocatalytic H2 production has been investigated in the presence of chloroacetic acids over Pt/TiO2, prepared by photodeposition. Monochloroacetic acid and dichloroacetic acid enhance photocatalytic hydrogen generation, whereas trichloroacetic acid does not. The photocatalytic oxidation of monochloroacetic acid and dichloroacetic acid mainly produces CO2, HCl, and formaldehyde, whereas the photocatalytic oxidation of trichloroacetic acid mainly produces CO2 and HCl [228].

Finally, though it is not strictly an example of biomass based waste stream, a process of simultaneous hydrogen production and H2S removal under visible light irradiation has been investigated over bulk CdS decorated with nanoparticles of TiO2 [229].

5.2. Methane Photo-Steam Reforming

The photo-steam reforming of methane [230, 231] has been studied only marginally due to limitations in the possible feed compositions at low temperature. Furthermore, gas phase applications for H2 photocatalytic production are much more limited than liquid phase examples. A detailed investigation on methane photo-steam reforming has been proposed by Shimura et al. [232]. Of course the interest would be the application with biogas. NaTaO3 showed the most interesting sample among different tanatalates, though for suitably high H2 productivity different dopants and cocatalysts were used. In particular, La doping, affecting bandgap energy, and Pt addition, activating methane, were the most effective modifications for NaTaO3. NiO, a good cocatalyst for WS over this catalyst was ineffective for methane photo-steam-reforming. Pt was added by photodeposition, but only a subsequent thermal treatment allowed to reach proper catalyst stability. Some limitations in the CH4/H2O ratio derived both by the need to operate in gas phase and by the formation of adsorbed organic intermediates on catalyst surface rendering it hydrophobic.

6. Conclusions and Future Perspectives

The research on materials and methods for the PR of organic substrates has thrown its basis and demonstrated the feasibility of H2 production by this route. From the existing reports it appears that TiO2-based photocatalysts have been extensively investigated and sound reaction mechanism has been proposed for the PR of very simple molecules. Further details however are needed to clarify the basic step of the reaction, surface interactions between the adsorbent and the adsorbate and the nature of active species. The reaction mechanism derived for the photoreforming of methanol has been roughly extended to other substrates, but much heavier efforts are needed to better understand the behavior of complex molecules, such as carbohydrates.

One of the key points is to reach an effective separation of the photogenerated charges. This seems achievable by combining the action of the organic substrate, acting ad hole scavenger, and a metallic phase drawing the photogenerated electrons and acting as active phase for proton reduction. The data reported mostly refer to UV-absorbing materials. Only few reports focus on photocatalysts active under solar light irradiation, which is a pivotal factor to impart practical relevance to this process.

Finally, the research reports on practically relevant substrates, that is, waste solutions, which enable to couple an environmentally relevant problem (waste disposal) with the need to produce clean fuels, are very limited. The potentiality of application of the PR process in this sense is enormous. The advantage is also the possibility to tune the type of waste to the local needs. However, to really exploit this route, it is of primary importance to focus the attention not only to the photocatalytic activity of the material, but also to the possible deactivation. Catalyst life represents a key problem when complex (possibly poisoning) substrates are used.

References

  1. X. Chen, S. Shen, L. Guo, and S. S. Mao, “Semiconductor-based photocatalytic hydrogen generation,” Chemical Reviews, vol. 110, no. 11, pp. 6503–6570, 2010. View at Publisher · View at Google Scholar · View at Scopus
  2. M. Bowker, “Sustainable hydrogen production by the application of ambient temperature photocatalysis,” Green Chemistry, vol. 13, no. 9, pp. 2235–2246, 2011. View at Publisher · View at Google Scholar
  3. V. Artero, M. Chavarot-Kerlidou, and M. Fontecave, “Splitting water with cobalt,” Angewandte Chemie—International Edition, vol. 50, no. 32, pp. 7238–7266, 2011. View at Publisher · View at Google Scholar · View at Scopus
  4. M. Bowker, L. Millard, J. Greaves, D. James, and J. Soares, “Photocatalysis by Au nanoparticles: reforming of methanol,” Gold Bulletin, vol. 37, no. 3-4, pp. 170–173, 2004. View at Scopus
  5. A. Fujishima and K. Honda, “Electrochemical photolysis of water at a semiconductor electrode,” Nature, vol. 238, no. 5358, pp. 37–38, 1972. View at Publisher · View at Google Scholar · View at Scopus
  6. K. Maeda, K. Teramura, D. Lu et al., “Photocatalyst releasing hydrogen from water,” Nature, vol. 440, no. 7082, p. 295, 2006. View at Publisher · View at Google Scholar · View at Scopus
  7. S. U. M. Khan, M. Al-Shahry, and W. B. Ingler, “Efficient photochemical water splitting by a chemically modified n-TiO2,” Science, vol. 297, no. 5590, pp. 2243–2245, 2002. View at Publisher · View at Google Scholar · View at Scopus
  8. R. Eisenberg, “Rethinking water splitting,” Science, vol. 324, no. 5923, pp. 44–45, 2009. View at Publisher · View at Google Scholar · View at Scopus
  9. A. Galińska and J. Walendziewski, “Photocatalytic water splitting over Pt-TiO2 in the presence of sacrificial reagents,” Energy and Fuels, vol. 19, no. 3, pp. 1143–1147, 2005. View at Publisher · View at Google Scholar · View at Scopus
  10. X. Fu, J. Long, X. Wang et al., “Photocatalytic reforming of biomass: a systematic study of hydrogen evolution from glucose solution,” International Journal of Hydrogen Energy, vol. 33, no. 22, pp. 6484–6491, 2008. View at Publisher · View at Google Scholar · View at Scopus
  11. T. Kawai and T. Sakata, “Conversion of carbohydrate into hydrogen fuel by a photocatalytic process,” Nature, vol. 286, no. 5772, pp. 474–476, 1980. View at Publisher · View at Google Scholar · View at Scopus
  12. V. M. Daskalaki and D. I. Kondarides, “Efficient production of hydrogen by photo-induced reforming of glycerol at ambient conditions,” Catalysis Today, vol. 144, no. 1-2, pp. 75–80, 2009. View at Publisher · View at Google Scholar
  13. M. Bowker, P. R. Davies, and L. Saeed Al-Mazroai, “Photocatalytic reforming of glycerol over gold and palladium as an alternative fuel source,” Catalysis Letters, vol. 128, no. 3-4, pp. 253–255, 2009. View at Publisher · View at Google Scholar
  14. N. Luo, Z. Jiang, H. Shi, F. Cao, T. Xiao, and P. P. Edwards, “Photo-catalytic conversion of oxygenated hydrocarbons to hydrogen over heteroatom-doped TiO2 catalysts,” International Journal of Hydrogen Energy, vol. 34, no. 1, pp. 125–129, 2009. View at Publisher · View at Google Scholar · View at Scopus
  15. T. Kida, G. Guan, N. Yamada, T. Ma, K. Kimura, and A. Yoshida, “Hydrogen production from sewage sludge solubilized in hot-compressed water using photocatalyst under light irradiation,” International Journal of Hydrogen Energy, vol. 29, no. 3, pp. 269–274, 2004. View at Publisher · View at Google Scholar · View at Scopus
  16. N. Taccardi, D. Assenbaum, M. E. M. Berger, A. Bosmann, F. Enzenberger, and R. Wolfel, “Catalytic production of hydrogen from glucose and other carbohydrates under exceptionally mild reaction conditions,” Green Chemistry, vol. 12, no. 7, pp. 1150–1156, 2010. View at Publisher · View at Google Scholar
  17. J. C. Colmenares, A. Magdziarz, M. A. Aramendia et al., “Influence of the strong metal support interaction effect (SMSI) of Pt/TiO2 and Pd/TiO2 systems in the photocatalytic biohydrogen production from glucose solution,” Catalysis Communications, vol. 16, no. 1, pp. 1–6, 2011. View at Publisher · View at Google Scholar
  18. M. Ilie, B. Cojocaru, V. I. Parvulescu, and H. Garcia, “Improving TiO2 activity in photo-production of hydrogen from sugar industry wastewaters,” International Journal of Hydrogen Energy, vol. 36, no. 24, pp. 15509–15518, 2011. View at Publisher · View at Google Scholar
  19. A. Kudo, “Development of photocatalyst materials for water splitting,” International Journal of Hydrogen Energy, vol. 31, no. 2, pp. 197–202, 2006. View at Publisher · View at Google Scholar · View at Scopus
  20. M. Matsuoka, M. Kitano, M. Takeuchi, K. Tsujimaru, M. Anpo, and J. M. Thomas, “Photocatalysis for new energy production. Recent advances in photocatalytic water splitting reactions for hydrogen production,” Catalysis Today, vol. 122, no. 1-2, pp. 51–61, 2007. View at Publisher · View at Google Scholar · View at Scopus
  21. K. Maeda and K. Domen, “New non-oxide photocatalysts designed for overall water splitting under visible light,” Journal of Physical Chemistry C, vol. 111, no. 22, pp. 7851–7861, 2007. View at Publisher · View at Google Scholar · View at Scopus
  22. F. E. Osterloh, “Inorganic materials as catalysts for photochemical splitting of water,” Chemistry of Materials, vol. 20, no. 1, pp. 35–54, 2008. View at Publisher · View at Google Scholar
  23. A. Kudo and Y. Miseki, “Heterogeneous photocatalyst materials for water splitting,” Chemical Society Reviews, vol. 38, no. 1, pp. 253–278, 2009. View at Publisher · View at Google Scholar
  24. R. M. N. Yerga, M. Consuelo Álvarez Galván, F. del Valle, J. A. V. de la Mano, and J. L. Fierro, “Water splitting on semiconductor catalysts under visiblelight irradiation,” ChemSusChem, vol. 2, no. 6, pp. 471–485, 2009. View at Publisher · View at Google Scholar · View at Scopus
  25. M. G. Walter, E. L. Warren, J. R. McKone et al., “Solar water splitting cells,” Chemical Reviews, vol. 110, no. 11, pp. 6446–6473, 2010. View at Publisher · View at Google Scholar · View at Scopus
  26. A. L. Linsebigler, G. Lu, and J. T. Yates, “Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results,” Chemical Reviews, vol. 95, no. 3, pp. 735–758, 1995. View at Scopus
  27. M. A. Aramendía, J. C. Colmenares, A. Marinas et al., “Effect of the redox treatment of Pt/TiO2 system on its photocatalytic behaviour in the gas phase selective photooxidation of propan-2-ol,” Catalysis Today, vol. 128, no. 3-4, pp. 235–244, 2007. View at Publisher · View at Google Scholar · View at Scopus
  28. M. A. Aramendía, V. Borau, J. C. Colmenares et al., “Modification of the photocatalytic activity of Pd/TiO2 and Zn/TiO2 systems through different oxidative and reductive calcination treatments,” Applied Catalysis B, vol. 80, no. 1-2, pp. 88–97, 2008. View at Publisher · View at Google Scholar · View at Scopus
  29. P. Pichat, J. M. Herrmann, J. Disdier, M. N. Mozzanega, and H. Courbon, “Modification of the TiO2 electron density by ion doping or metal deposit and consequences for photoassisted reactions,” Studies in Surface Science and Catalysis, vol. 19, pp. 319–326, 1984. View at Publisher · View at Google Scholar · View at Scopus
  30. M. R. Hoffmann, S. T. Martin, W. Choi, and D. W. Bahnemann, “Environmental applications of semiconductor photocatalysis,” Chemical Reviews, vol. 95, no. 1, pp. 69–96, 1995. View at Scopus
  31. M. de Oliveira Melo and L. Almeida Silva, “Photocatalytic production of hydrogen: an innovative use for biomass derivatives,” The Journal of the Brazilian Chemical Society, vol. 22, no. 8, Article ID 1399, 2011. View at Publisher · View at Google Scholar
  32. S. Chand and S. Bala, “Simulation studies of current transport in metal-insulator-semiconductor Schottky barrier diodes,” Physica B, vol. 390, no. 1-2, pp. 179–184, 2007. View at Publisher · View at Google Scholar · View at Scopus
  33. G. Wu, T. Chen, G. Zhou, X. Zong, and C. Li, “H-2 production with low CO selectivity from photocatalytic reforming of glucose on metal/TiO2 catalysts,” Science in China Series B, vol. 51, no. 2, pp. 97–100, 2008. View at Publisher · View at Google Scholar
  34. G. Balducci, “The adsorption of glucose at the surface of anatase: a computational study,” Chemical Physics Letters, vol. 494, no. 1–3, pp. 54–59, 2010. View at Publisher · View at Google Scholar
  35. M. Du, J. Feng, and S. B. Zhang, “Photo-oxidation of polyhydroxyl molecules on TiO2 surfaces: from hole scavenging to light-induced self-assembly of TiO2-cyclodextrin wires,” Physical Review Letters, vol. 98, no. 6, Article ID 066102, 4 pages, 2007. View at Publisher · View at Google Scholar
  36. D. Barreca, G. Carraro, V. Gombac et al., “Supported metal oxide nanosystems for hydrogen photogeneration: Quo vadis?” Advanced Functional Materials, vol. 21, no. 14, pp. 2611–2623, 2011. View at Publisher · View at Google Scholar · View at Scopus
  37. D. Barreca, P. Fornasiero, A. Gasparotto et al., “CVD Co3O4 nanopyramids: a nano-platform for photo-assisted H2 production,” Chemical Vapor Deposition, vol. 16, no. 10–12, pp. 296–300, 2010. View at Publisher · View at Google Scholar · View at Scopus
  38. D. I. Kondarides, V. M. Daskalaki, A. Patsoura, and X. E. Verykios, “Hydrogen production by photo-induced reforming of biomass components and derivatives at ambient conditions,” Catalysis Letters, vol. 122, no. 1-2, pp. 26–32, 2008. View at Publisher · View at Google Scholar · View at Scopus
  39. A. L. Linsebigler, G. Lu, and J. T. Yates Jr., “Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results,” Chemical Reviews, vol. 95, no. 3, pp. 735–758, 1995. View at Scopus
  40. A. Fujishima, X. Zhang, and D. A. Tryk, “TiO2 photocatalysis and related surface phenomena,” Surface Science Reports, vol. 63, no. 12, pp. 515–582, 2008. View at Publisher · View at Google Scholar
  41. M. A. Henderson, “A surface science perspective on TiO2 photocatalysis,” Surface Science Reports, vol. 66, no. 6-7, pp. 185–297, 2011. View at Publisher · View at Google Scholar
  42. L. Spanhel, H. Weller, and A. Henglein, “Photochemistry of semiconductor colloids. 22. Electron injection from illuminated CdS into attached TiO2 and ZnO particles,” Journal of the American Chemical Society, vol. 109, no. 22, pp. 6632–6635, 1987. View at Scopus
  43. K. E. Karakitsou and X. E. Verykios, “Effects of altervalent cation doping of TiO2 on its performance as a photocatalyst for water cleavage,” Journal of Physical Chemistry, vol. 97, no. 6, pp. 1184–1189, 1993. View at Scopus
  44. Y. K. Kho, A. Iwase, W. Y. Teoh, L. Mädler, A. Kudo, and R. Amal, “Photocatalytic H2 evolution over TiO2 nanoparticles. The synergistic effect of anatase and rutile,” Journal of Physical Chemistry C, vol. 114, no. 6, pp. 2821–2829, 2010. View at Publisher · View at Google Scholar · View at Scopus
  45. J. Shi, J. Chen, Z. Feng et al., “Photoluminescence characteristics of TiO2 and their relationship to the photoassisted reaction of water/methanol mixture,” Journal of Physical Chemistry C, vol. 111, no. 2, pp. 693–699, 2007. View at Publisher · View at Google Scholar
  46. J. Jitputti, S. Pavasupree, Y. Suzuki, and S. Yoshikawa, “Synthesis and photocatalytic activity for water-splitting reaction of nanocrystalline mesoporous titania prepared by hydrothermal method,” Journal of Solid State Chemistry, vol. 180, no. 5, pp. 1743–1749, 2007. View at Publisher · View at Google Scholar · View at Scopus
  47. H. Kato, K. Asakura, and A. Kudo, “Highly efficient water splitting into H2 and O2 over lanthanum-doped NaTaO3 photocatalysts with high crystallinity and surface nanostructure,” Journal of the American Chemical Society, vol. 125, no. 10, pp. 3082–3089, 2003. View at Publisher · View at Google Scholar
  48. A. Kudo, H. Kato, and S. Nakagawa, “Water splitting into H2 and O2 on new Sr2M2O7 (M = Nb and Ta) photocatalysts with layered perovskite structures: factors affecting the photocatalytic activity,” Journal of Physical Chemistry B, vol. 104, no. 3, pp. 571–575, 2000. View at Publisher · View at Google Scholar
  49. H. G. Kim, D. W. Hwang, S. W. Bae, J. H. Jung, and J. S. Lee, “Photocatalytic water splitting over La2Ti2O7 synthesized by the polymerizable complex method,” Catalysis Letters, vol. 91, no. 3-4, pp. 193–198, 2003. View at Publisher · View at Google Scholar
  50. S. Ikeda, M. Hara, J. N. Kondo et al., “Preparation of K2La2Ti3O10 by polymerized complex method and photocatalytic decomposition of water,” Chemistry of Materials, vol. 10, no. 1, pp. 72–77, 1998. View at Scopus
  51. X. Fu, X. Wang, D. Y. C. Leung et al., “Photocatalytic reforming of glucose over La doped alkali tantalate photocatalysts for H2 production,” Catalysis Communications, vol. 12, no. 3, pp. 184–187, 2010. View at Publisher · View at Google Scholar
  52. A. Kubacka, M. Fernàndez-García, and G. Colòn, “Advanced nanoarchitectures for solar photocatalytic applications,” Chemical Reviews, vol. 112, no. 3, pp. 1555–1614, 2012. View at Publisher · View at Google Scholar
  53. X. Chen and S. S. Mao, “Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications,” Chemical Reviews, vol. 107, no. 7, pp. 2891–2959, 2007. View at Publisher · View at Google Scholar
  54. A. Testino, I. R. Bellobono, V. Buscaglia et al., “Optimizing the photocatalytic properties of hydrothermal TiO2 by the control of phase composition and particle morphology. A systematic approach,” Journal of the American Chemical Society, vol. 129, no. 12, pp. 3564–3575, 2007. View at Publisher · View at Google Scholar · View at Scopus
  55. S. Y. Chae, M. K. Park, S. K. Lee, T. Y. Kim, S. K. Kim, and W. I. Lee, “Preparation of size-controlled TiO2 nanoparticles and derivation of optically transparent photocatalytic films,” Chemistry of Materials, vol. 15, no. 17, pp. 3326–3331, 2003. View at Publisher · View at Google Scholar · View at Scopus
  56. Z. Zhang, C. C. Wang, R. Zakaria, and J. Y. Ying, “Role of particle size in nanocrystalline TiOi-based photocatalysts,” Journal of Physical Chemistry B, vol. 102, no. 52, pp. 10871–10878, 1998. View at Scopus
  57. X. Chen, T. Yu, X. Fan et al., “Enhanced activity of mesoporous Nb2O5 for photocatalytic hydrogen production,” Applied Surface Science, vol. 253, no. 20, pp. 8500–8506, 2007. View at Publisher · View at Google Scholar
  58. R. van de Krol, Y. Liang, and J. Schoonman, “Solar hydrogen production with nanostructured metal oxides,” Journal of Materials Chemistry, vol. 18, no. 20, pp. 2311–2320, 2008. View at Publisher · View at Google Scholar · View at Scopus
  59. N. T. Hahn, H. Ye, D. W. Flaherty, A. J. Bard, and C. B. Mullins, “Reactive ballistic deposition of α-Fe2O3 thin films for photoelectrochemical water oxidation,” ACS Nano, vol. 4, no. 4, pp. 1977–1986, 2010. View at Publisher · View at Google Scholar · View at Scopus
  60. V. M. Arakelyan, V. M. Aroutiounian, G. E. Shahnazaryan, and E. A. Khachaturyan, “Thin film n-titanium oxide photoanodes for photoelectrochemical production of hydrogen,” Renewable Energy, vol. 33, no. 2, pp. 299–303, 2008. View at Publisher · View at Google Scholar · View at Scopus
  61. Y. Li and J. Z. Zhang, “Hydrogen generation from photoelectrochemical water splitting based on nanomaterials,” Laser and Photonics Reviews, vol. 4, no. 4, pp. 517–528, 2010. View at Publisher · View at Google Scholar
  62. M. K. I. Senevirathna, P. K. D. D. P. Pitigala, and K. Tennakone, “Water photoreduction with Cu2O quantum dots on TiO2 nano-particles,” Journal of Photochemistry and Photobiology A, vol. 171, no. 3, pp. 257–259, 2005. View at Publisher · View at Google Scholar · View at Scopus
  63. Z. Jiang, F. Yang, N. Luo et al., “Solvothermal synthesis of N-doped TiO2 nanotubes for visible-light-responsive photocatalysis,” Chemical Communications, no. 47, pp. 6372–6374, 2008. View at Publisher · View at Google Scholar · View at Scopus
  64. S. Palmas, A. M. Polcaro, J. R. Ruiz, A. da Pozzo, M. Mascia, and A. Vacca, “TiO2 photoanodes for electrically enhanced water splitting,” International Journal of Hydrogen Energy, vol. 35, no. 13, pp. 6561–6570, 2010. View at Publisher · View at Google Scholar · View at Scopus
  65. Y. Li, Y. Hu, S. Peng, G. Lu, and S. Li, “Synthesis of CdS nanorods by an ethylenediamine assisted hydrothermal method for photocatalytic hydrogen evolution,” Journal of Physical Chemistry C, vol. 113, no. 21, pp. 9352–9358, 2009. View at Publisher · View at Google Scholar
  66. Y. Li, T. Sasaki, Y. Shimizu, and N. Koshizaki, “Hexagonal-close-packed, hierarchical amorphous TiO2 nanocolumn arrays: transferability, enhanced photocatalytic activity, and superamphiphilicity without UV irradiation,” Journal of the American Chemical Society, vol. 130, no. 44, pp. 14755–14762, 2008. View at Publisher · View at Google Scholar · View at Scopus
  67. Y. Lin, G. Yuan, R. Liu, S. Zhou, S. W. Sheehan, and D. Wang, “Semiconductor nanostructure-based photoelectrochemical water splitting: a brief review,” Chemical Physics Letters, vol. 507, no. 4–6, pp. 209–215, 2011. View at Publisher · View at Google Scholar
  68. H. B. Michaelson, “The work function of the elements and its periodicity,” Journal of Applied Physics, vol. 48, no. 11, pp. 4729–4733, 1977. View at Publisher · View at Google Scholar · View at Scopus
  69. T. T. Y. Tan, C. K. Yip, D. Beydoun, and R. Amal, “Effects of nano-Ag particles loading on TiO2 photocatalytic reduction of selenate ions,” Chemical Engineering Journal, vol. 95, no. 1, pp. 179–186, 2003. View at Publisher · View at Google Scholar · View at Scopus
  70. D. E. Eastman, “Photoelectric work functions of transition, rare-earth, and noble metals,” Physical Review B, vol. 2, no. 1, pp. 1–2, 1970. View at Publisher · View at Google Scholar
  71. W. X. Dai, X. X. Wang, P. Liu, Y. M. Xu, G. S. Li, and X. Z. Fu, “Effects of electron transfer between TiO2 films and conducting substrates on the photocatalytic oxidation of organic pollutants,” Journal of Physical Chemistry B, vol. 110, no. 27, pp. 13470–13476, 2006. View at Publisher · View at Google Scholar
  72. K. Domen, S. Naito, T. Onishi, K. Tamaru, and M. Soma, “Study of the photocatalytic decomposition of water vapor over a NiO–SrTiO3 catalyst,” Journal of Physical Chemistry, vol. 86, no. 18, pp. 3657–3661, 1982. View at Scopus
  73. B. Zielińska, E. Borowiak-Palen, and R. J. Kalenzuk, “Preparation and characterization of lithium niobate as a novel photocatalyst in hydrogen generation,” Journal of Physics and Chemistry of Solids, vol. 69, no. 1, pp. 236–242, 2008. View at Publisher · View at Google Scholar · View at Scopus
  74. H. Lin, Y. Chen, and Y. Chen, “Water splitting reaction on NiO/InVO4 under visible light irradiation,” International Journal of Hydrogen Energy, vol. 32, no. 1, pp. 86–92, 2007. View at Publisher · View at Google Scholar
  75. T. Sreethawong, Y. Suzuki, and S. Yoshikawa, “Photocatalytic evolution of hydrogen over mesoporous TiO2 supported NiO photocatalyst prepared by single-step sol-gel process with surfactant template,” International Journal of Hydrogen Energy, vol. 30, no. 10, pp. 1053–1062, 2005. View at Publisher · View at Google Scholar · View at Scopus
  76. Y. Miseki, H. Kato, and A. Kudo, “Water splitting into H2 and O2 over Cs2Nb4O11 photocatalyst,” Chemistry Letters, vol. 34, no. 1, pp. 54–55, 2005. View at Publisher · View at Google Scholar · View at Scopus
  77. Z. Zou, J. Ye, K. Sayama, and H. Arakawa, “Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst,” Nature, vol. 414, no. 6864, pp. 625–627, 2001. View at Publisher · View at Google Scholar · View at Scopus
  78. B. Ohtani, K. Iwai, S. I. Nishimoto, and S. Sato, “Role of platinum deposits on titanium(IV) oxide particles: structural and kinetic analyses of photocatalytic reaction in aqueous alcohol and amino acid solutions,” Journal of Physical Chemistry B, vol. 101, no. 17, pp. 3349–3359, 1997. View at Scopus
  79. H. Bahruji, M. Bowker, P. R. Davies et al., “Sustainable H2 gas production by photocatalysis,” Journal of Photochemistry and Photobiology A, vol. 216, no. 2–4, pp. 115–118, 2010. View at Publisher · View at Google Scholar · View at Scopus
  80. H. Bahruji, M. Bowker, P. R. Davies, and F. Pedrono, “New insights into the mechanism of photocatalytic reforming on Pd/TiO2,” Applied Catalysis B, vol. 107, no. 1-2, pp. 205–209, 2011. View at Publisher · View at Google Scholar · View at Scopus
  81. D. Y. C. Leung, X. L. Fu, C. F. Wang et al., “Hydrogen production over titania-based photocatalysts,” ChemSusChem, vol. 3, no. 6, pp. 681–694, 2010. View at Publisher · View at Google Scholar · View at Scopus
  82. H. Kato and A. Kudo, “Visible-light-response and photocatalytic activities of TiO2 and SrTiO3 photocatalysts codoped with antimony and chromium,” Journal of Physical Chemistry B, vol. 106, no. 19, pp. 5029–5034, 2002. View at Publisher · View at Google Scholar
  83. A. Kudo and M. Sekizawa, “Photocatalytic H2 evolution under visible light irradiation on Zn1-xCuxS solid solution,” Catalysis Letters, vol. 58, no. 4, pp. 241–243, 1999. View at Publisher · View at Google Scholar
  84. A. Kudo and M. Sekizawa, “Photocatalytic H2 evolution under visible light irradiation on Ni-doped ZnS photocatalyst,” Chemical Communications, no. 15, pp. 1371–1372, 2000. View at Publisher · View at Google Scholar
  85. T. Umebayashi, T. Yamaki, H. Itoh, and K. Asai, “Analysis of electronic structures of 3D transition metal-doped TiO2 based on band calculations,” Journal of Physics and Chemistry of Solids, vol. 63, no. 10, pp. 1909–1920, 2002. View at Publisher · View at Google Scholar · View at Scopus
  86. S. Rengaraj and X. Z. Li, “Enhanced photocatalytic activity of TiO2 by doping with Ag for degradation of 2,4,6-trichlorophenol in aqueous suspension,” Journal of Molecular Catalysis A, vol. 243, no. 1, pp. 60–67, 2006. View at Publisher · View at Google Scholar
  87. M. Anpo and M. Takeuchi, “The design and development of highly reactive titanium oxide photocatalysts operating under visible light irradiation,” Journal of Catalysis, vol. 216, no. 1-2, pp. 505–516, 2003. View at Publisher · View at Google Scholar · View at Scopus
  88. Y. Tian and T. Tatsuma, “Mechanisms and applications of plasmon-induced charge separation at TiO2 films loaded with gold nanoparticles,” Journal of the American Chemical Society, vol. 127, no. 20, pp. 7632–7637, 2005. View at Publisher · View at Google Scholar · View at Scopus
  89. S. Linic, P. Christopher, and D. B. Ingram, “Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy,” Nature Materials, vol. 10, no. 12, pp. 911–921, 2011. View at Publisher · View at Google Scholar
  90. D. Jing, M. Liu, J. Shi, W. Tang, and L. Guo, “Hydrogen production under visible light by photocatalytic reforming of glucose over an oxide solid solution photocatalyst,” Catalysis Communications, vol. 12, no. 4, pp. 264–267, 2010. View at Publisher · View at Google Scholar
  91. R. M. Navarro, M. C. Sanchez-Sanchez, M. C. Alvarez-Galvan, F. del Valle, and J. L. G. Fierro, “Hydrogen production from renewable sources: biomass and photocatalytic opportunities,” Energy and Environmental Science, vol. 2, no. 1, pp. 35–54, 2009. View at Publisher · View at Google Scholar
  92. K. Maeda, K. Teramura, T. Takata et al., “Overall water splitting on (Ga1-xZnx)(N1-xOx) solid solution photocatalyst: relationship between physical properties and photocatalytic activity,” Journal of Physical Chemistry B, vol. 109, no. 43, pp. 20504–20510, 2005. View at Publisher · View at Google Scholar · View at Scopus
  93. K. Shimizu, H. Murayama, A. Nagai, and A. Shimada, “Degradation of hydrophobic organic pollutants by titania pillared fluorine mica as a substrate specific photocatalyst,” Applied Catalysis B, vol. 55, no. 2, pp. 141–148, 2005. View at Publisher · View at Google Scholar
  94. S. Liu, J. Yang, and J. Choy, “Microporous SiO2–TiO2 nanosols pillared montmorillonite for photocatalytic decomposition of methyl orange,” Journal of Photochemistry and Photobiology A, vol. 179, no. 1-2, pp. 75–80, 2006. View at Publisher · View at Google Scholar
  95. W. Shangguan, “Hydrogen evolution from water splitting on nanocomposite photocatalysts,” Science and Technology of Advanced Materials, vol. 8, no. 1-2, article 76, 2007. View at Publisher · View at Google Scholar
  96. C. Li, J. Yuan, B. Han, L. Jiang, and W. Shangguan, “TiO2 nanotubes incorporated with CdS for photocatalytic hydrogen production from splitting water under visible light irradiation,” International Journal of Hydrogen Energy, vol. 35, no. 13, pp. 7073–7079, 2010. View at Publisher · View at Google Scholar · View at Scopus
  97. Y. Zhang, Y. Wang, W. Yan, T. Li, S. Li, and Y. R. Hu, “Synthesis of Cr2O3/TNTs nanocomposite and its photocatalytic hydrogen generation under visible light irradiation,” Applied Surface Science, vol. 255, no. 23, pp. 9508–9511, 2009. View at Publisher · View at Google Scholar
  98. E. S. Andreiadis, M. Chavarot-Kerlidou, M. Fontecave, and V. Artero, “Artificial photosynthesis: from molecular catalysts for light-driven water splitting to photoelectrochemical cells,” Photochemistry and Photobiology, vol. 87, no. 5, pp. 946–964, 2011. View at Publisher · View at Google Scholar · View at Scopus
  99. E. Reisner, J. C. Fontecilla-Camps, and F. A. Armstrong, “Catalytic electrochemistry of a [NiFeSe]-hydrogenase on TiO2 and demonstration of its suitability for visible-light driven H2 production,” Chemical Communications, no. 5, pp. 550–552, 2009. View at Publisher · View at Google Scholar · View at Scopus
  100. E. Reisner, D. J. Powell, C. Cavazza, J. C. Fontecilla-Camps, and F. A. Armstrong, “Chemoenzymatic synthesis of GD3 oligosaccharides and other disialyl glycans containing natural and non-natural sialic acids,” Journal of the American Chemical Society, vol. 131, no. 51, pp. 18467–18477, 2009. View at Publisher · View at Google Scholar
  101. J. I. Goldsmith, W. R. Hudson, M. S. Lowry, T. H. Anderson, and S. Bernhard, “Discovery and high-throughput screening of heteroleptic iridium complexes for photoinduced hydrogen production,” Journal of the American Chemical Society, vol. 127, no. 20, pp. 7502–7510, 2005. View at Publisher · View at Google Scholar · View at Scopus
  102. P. Zhang, M. Wang, Y. Na, X. Li, Y. Jiang, and L. Sun, “Homogeneous photocatalytic production of hydrogen from water by a bioinspired [Fe2S2] catalyst with high turnover numbers,” Dalton Transactions, vol. 39, no. 5, pp. 1204–1206, 2010. View at Publisher · View at Google Scholar · View at Scopus
  103. A. Fihri, V. Artero, A. Pereira, and M. Fontecave, “Efficient H2-producing photocatalytic systems based on cyclometalated iridium- and tricarbonylrhenium-diimine photosensitizers and cobaloxime catalysts,” Dalton Transactions, no. 41, pp. 5567–5569, 2008. View at Publisher · View at Google Scholar · View at Scopus
  104. W. Gao, J. Sun, T. Akermark et al., “Attachment of a hydrogen-bonding carboxylate side chain to an [FeFe]-hydrogenase model complex: influence on the catalytic mechanism,” Chemistry—A European Journal, vol. 16, no. 8, pp. 2537–2546, 2010. View at Publisher · View at Google Scholar
  105. M. Cargnello, A. Gasparotto, V. Gombac, T. Montini, D. Barreca, and P. Fornasiero, “Photocatalytic H2 and added-value by-products—the role of metal oxide systems in their synthesis from oxygenates,” European Journal of Inorganic Chemistry, vol. 2011, no. 28, pp. 4309–4323, 2011. View at Publisher · View at Google Scholar
  106. L. Sun and J. R. Bolton, “Determination of the quantum yield for the photochemical generation of hydroxyl radicals in TiO2 suspensions,” Journal of Physical Chemistry, vol. 100, no. 10, pp. 4127–4134, 1996. View at Publisher · View at Google Scholar
  107. D. Bahnemann, A. Henglein, and L. Spanhel, “Detection of the intermediates of colloidal TiO2-catalysed photoreactions,” Faraday Discussions of the Chemical Society, vol. 78, pp. 151–163, 1984. View at Publisher · View at Google Scholar · View at Scopus
  108. C. Y. Wang, J. Rabani, D. W. Bahnemann, and J. K. Dohrmann, “Photonic efficiency and quantum yield of formaldehyde formation from methanol in the presence of various TiO2 photocatalysts,” Journal of Photochemistry and Photobiology A, vol. 148, no. 1–3, pp. 169–176, 2002. View at Publisher · View at Google Scholar
  109. L. S. Al-Mazroai, M. Bowker, P. Davies et al., “The photocatalytic reforming of methanol,” Catalysis Today, vol. 122, no. 1-2, pp. 46–50, 2007. View at Publisher · View at Google Scholar · View at Scopus
  110. M. Bowker, D. James, P. Stone et al., “Catalysis at the metal-support interface: exemplified by the photocatalytic reforming of methanol on Pd/TiO2,” Journal of Catalysis, vol. 217, no. 2, pp. 427–433, 2003. View at Publisher · View at Google Scholar · View at Scopus
  111. R. P. Holroyd and M. Bowker, “Molecular beam studies of alcohol (C1–C3) adsorption and reaction with oxygen pre-covered Pd(110),” Surface Science, vol. 377–379, pp. 786–790, 1997. View at Scopus
  112. A. Dickinson, D. James, N. Perkins, T. Cassidy, and M. Bowker, “The photocatalytic reforming of methanol,” Journal of Molecular Catalysis A, vol. 146, no. 1-2, pp. 211–221, 1999. View at Publisher · View at Google Scholar · View at Scopus
  113. A. Fujishima, T. N. Rao, and D. A. Tryk, “Titanium dioxide photocatalysis,” Journal of Photochemistry and Photobiology C, vol. 1, no. 1, pp. 1–21, 2000. View at Publisher · View at Google Scholar
  114. J. Marugán, D. Hufschmidt, M. J. López-Muñoz, V. Selzer, and D. Bahnemann, “Photonic efficiency for methanol photooxidation and hydroxyl radical generation on silica-supported TiO2 photocatalysts,” Applied Catalysis B, vol. 62, no. 3-4, pp. 201–207, 2006. View at Publisher · View at Google Scholar · View at Scopus
  115. D. W. Bahnemann, M. Hilgendorff, and R. Memming, “Charge carrier dynamics at TiO2 particles: reactivity of free and trapped holes,” Journal of Physical Chemistry B, vol. 101, no. 21, pp. 4265–4275, 1997. View at Scopus
  116. N. Serpone, D. Lawless, and R. Khairutdinov, “Size effects on the photophysical properties of colloidal anatase TiO2 particles: size quantization or direct transitions in this indirect semiconductor?” Journal of Physical Chemistry, vol. 99, no. 45, pp. 16646–16654, 1995. View at Scopus
  117. Y. Tamaki, A. Furube, M. Murai, K. Hara, R. Katoh, and M. Tachiya, “Direct observation of reactive trapped holes in TiO2 undergoing photocatalytic oxidation of adsorbed alcohols: evaluation of the reaction rates and yields,” Journal of the American Chemical Society, vol. 128, no. 2, pp. 416–417, 2006. View at Publisher · View at Google Scholar · View at Scopus
  118. O. Rosseler, M. V. Shankar, M. K. L. Du, L. Schmidlin, N. Keller, and V. Keller, “Solar light photocatalytic hydrogen production from water over Pt and Au/TiO2(anatase/rutile) photocatalysts: influence of noble metal and porogen promotion,” Journal of Catalysis, vol. 269, no. 1, pp. 179–190, 2010. View at Publisher · View at Google Scholar · View at Scopus
  119. G. Wu, T. Chen, W. Su et al., “Conversion of steel mill waste into nanoscale zerovalent iron (nZVI) particles for hydrogen generation via metal-steam reforming,” International Journal of Hydrogen Energy, vol. 33, no. 4, pp. 1243–1242, 2008. View at Publisher · View at Google Scholar
  120. G. Wu, T. Chen, X. Zong et al., “Suppressing CO formation by anion adsorption and Pt deposition on TiO2 in H2 production from photocatalytic reforming of methanol,” Journal of Catalysis, vol. 253, no. 1, pp. 225–227, 2008. View at Publisher · View at Google Scholar
  121. A. J. Bard, “Design of semiconductor photoelectrochemical systems for solar energy conversion,” Journal of Physical Chemistry, vol. 86, no. 2, pp. 172–177, 1982. View at Publisher · View at Google Scholar
  122. V. M. Daskalaki, P. Panagiotopoulou, and D. I. Kondarides, “Production of peroxide species in Pt/TiO2 suspensions under conditions of photocatalytic water splitting and glycerol photoreforming,” Chemical Engineering Journal, vol. 170, no. 2-3, pp. 433–439, 2011. View at Publisher · View at Google Scholar · View at Scopus
  123. K. Domen, A. Kudo, M. Shibata, A. Tanaka, K. I. Maruya, and T. Onishi, “Novel photocatalysts, ion-exchanged K4Nb6O17, with a layer structure,” Journal of the Chemical Society, Chemical Communications, no. 23, pp. 1706–1707, 1986. View at Scopus
  124. K. Domen, A. Kudo, A. Shinozaki, A. Tanaka, K. I. Maruya, and T. Onishi, “Photodecomposition of water and hydrogen evolution from aqueous methanol solution over novel niobate photocatalysts,” Journal of the Chemical Society, Chemical Communications, no. 4, pp. 356–357, 1986. View at Publisher · View at Google Scholar · View at Scopus
  125. A. Kudo, A. Tanaka, K. Domen, K. I. Maruya, K. I. Aika, and T. Onishi, “Photocatalytic decomposition of water over NiOK4Nb6O17 catalyst,” Journal of Catalysis, vol. 111, no. 1, pp. 67–76, 1988. View at Scopus
  126. S. Ikeda, A. Tanaka, K. Shinohara et al., “Effect of the particle size for photocatalytic decomposition of water on Ni-loaded K4Nb6O17,” Microporous Materials, vol. 9, no. 5-6, pp. 253–258, 1997. View at Scopus
  127. K. Sayama, A. Tanaka, K. Domen, K. Maruya, and T. Onishi, “Improvement of nickel-loaded K4Nb6O17 photocatalyst for the decomposition of H2O,” Catalysis Letters, vol. 4, no. 3, pp. 217–222, 1990. View at Publisher · View at Google Scholar · View at Scopus
  128. A. Iwase, H. Kato, and A. Kudo, “Nanosized Au particles as an efficient cocatalyst for photocatalytic overall water splitting,” Catalysis Letters, vol. 108, no. 1-2, pp. 7–10, 2006. View at Publisher · View at Google Scholar
  129. K. Sayama, A. Tanaka, K. Domen, K. Maruya, and T. Onishi, “Photocatalytic decomposition of water over platinum-intercalated K4Nb6O17,” Journal of Physical Chemistry, vol. 95, no. 3, pp. 1345–1348, 1991. View at Scopus
  130. K. Sayama, K. Yase, H. Arakawa et al., “Photocatalytic activity and reaction mechanism of Pt-intercalated K4Nb6O17 catalyst on the water splitting in carbonate salt aqueous solution,” Journal of Photochemistry and Photobiology A, vol. 114, no. 2, pp. 125–135, 1998. View at Scopus
  131. K. H. Chung and D. C. Park, “Photocatalytic decomposition of water over cesium-loaded potassium niobate photocatalysts,” Journal of Molecular Catalysis A, vol. 129, no. 1, pp. 53–59, 1998. View at Publisher · View at Google Scholar
  132. T. A. Kandiel, R. Dillert, L. Robben, and D. W. Bahnemann, “Photonic efficiency and mechanism of photocatalytic molecular hydrogen production over platinized titanium dioxide from aqueous methanol solutions,” Catalysis Today, vol. 161, no. 1, pp. 196–201, 2011. View at Publisher · View at Google Scholar · View at Scopus
  133. A. Vamathevan, R. Amal, and D. Beydown, “Photocatalytic oxidation of organics in water using pure and silver-modified titanium dioxide particles,” Journal of Photochemistry and Photobiology A, vol. 148, no. 1–3, pp. 233–245, 2002. View at Publisher · View at Google Scholar
  134. Y. L. Kuo, H. W. Chen, and Y. Ku, “Analysis of silver particles incorporated on TiO2 coatings for the photodecomposition of o-cresol,” Thin Solid Films, vol. 515, no. 7-8, pp. 3461–3468, 2007. View at Publisher · View at Google Scholar · View at Scopus
  135. W. Lee, H. S. Shen, K. Dwight, and A. Wold, “Effect of silver on the photocatalytic activity of TiO2,” Journal of Solid State Chemistry, vol. 106, no. 2, pp. 288–294, 1993. View at Publisher · View at Google Scholar · View at Scopus
  136. M. P. Reddy, A. Venugopal, and M. Subrahmanyam, “Hydroxyapatite-supported Ag–TiO2 as Escherichia coli disinfection photocatalyst,” Water Research, vol. 41, no. 2, pp. 379–386, 2007. View at Publisher · View at Google Scholar · View at Scopus
  137. K. Shiba, H. Hinode, and M. Wakihara, “Catalytic reduction of nitric monoxide by ethene over Ag/TiO2 in the presence of excess oxygen,” Reaction Kinetics and Catalysis Letters, vol. 64, no. 2, pp. 281–288, 1998.
  138. K. Lalitha, J. K. Reddy, M. V. P. Sharma, V. D. Kumari, and M. Subrahmanyam, “Continuous hydrogen production activity over finely dispersed Ag2O/TiO2 catalysts from methanol : water mixtures under solar irradiation: a structure-activity correlation,” International Journal of Hydrogen Energy, vol. 35, no. 9, pp. 3991–4001, 2010. View at Publisher · View at Google Scholar
  139. J. Du, J. Zhang, Z. Liu, B. Han, T. Jiang, and Y. Huang, “Controlled synthesis of Ag/TiO2 core-shell nano wires with smooth and bristled surfaces via a one-step solution route,” Langmuir, vol. 22, no. 3, pp. 1307–1312, 2006. View at Publisher · View at Google Scholar · View at Scopus
  140. N. Strataki, V. Bekiari, D. I. Kondarides, and P. Liasnos, “Hydrogen production by photocatalytic alcohol reforming employing highly efficient nanocrystalline titania films,” Applied Catalysis B, vol. 77, no. 1-2, pp. 184–189, 2007. View at Publisher · View at Google Scholar
  141. M. A. Gondal, A. Hameed, and Z. H. Yamani, “Hydrogen generation by laser transformation of methanol using n-type WO3 semiconductor catalyst,” Journal of Molecular Catalysis A, vol. 222, no. 1-2, pp. 259–264, 2004. View at Publisher · View at Google Scholar
  142. A. Hameed and M. A. Gondal, “Production of hydrogen-rich syngas using p-type NiO catalyst: a laser-based photocatalytic approach,” Journal of Molecular Catalysis A, vol. 233, no. 1-2, pp. 35–41, 2005. View at Publisher · View at Google Scholar
  143. F. Di Quarto, C. Sunseri, S. Piazza, and M. C. Romano, “Semiempirical correlation between optical band gap values of oxides and the difference of electronegativity of the elements. Its importance for a quantitative use of photocurrent spectroscopy in corrosion studies,” Journal of Physical Chemistry B, vol. 101, no. 14, pp. 2519–2525, 1997. View at Scopus
  144. L. S. Yoong, F. K. Chong, and B. K. Dutta, “Development of copper-doped TiO2 photocatalyst for hydrogen production under visible light,” Energy, vol. 34, no. 10, pp. 1652–1661, 2009. View at Publisher · View at Google Scholar · View at Scopus
  145. T. Miwa, S. Kaneco, H. Katsumata et al., “Photocatalytic hydrogen production from aqueous methanol solution with CuO/Al2O3/TiO2 nanocomposite,” International Journal of Hydrogen Energy, vol. 35, no. 13, pp. 6554–6560, 2010. View at Publisher · View at Google Scholar · View at Scopus
  146. T. Kawai and T. Sakata, “Photocatalytic hydrogen production from liquid methanol and water,” Chemical Communications, no. 15, pp. 694–695, 1980. View at Publisher · View at Google Scholar
  147. T. Kawai and T. Sakata, “Photocatalytic hydrogen production from water by the decomposition of poly-vinylchloride, protein, algae dead insects and excrement,” Chemistry Letters, vol. 10, no. 1, pp. 81–84, 1981.
  148. J. Chen, D. F. Ollis, W. H. Rulkens, and H. Bruning, “Photocatalyzed oxidation of alcohols and organochlorides in the presence of native TiO2 and metallized TiO2 suspensions. Part (II): photocatalytic mechanisms,” Water Research, vol. 33, no. 3, pp. 669–676, 1999. View at Publisher · View at Google Scholar · View at Scopus
  149. S. Xu and D. D. Sun, “Significant improvement of photocatalytic hydrogen generation rate over TiO2 with deposited CuO,” International Journal of Hydrogen Energy, vol. 34, no. 15, pp. 6096–6104, 2009. View at Publisher · View at Google Scholar
  150. Y. Mizukoshi, Y. Makise, T. Shuto et al., “Immobilization of noble metal nanoparticles on the surface of TiO2 by the sonochemical method: photocatalytic production of hydrogen from an aqueous solution of ethanol,” Ultrasonics Sonochemistry, vol. 14, no. 3, pp. 387–392, 2007. View at Publisher · View at Google Scholar · View at Scopus
  151. Y. Z. Yang, C. H. Chang, and H. Idriss, “Photo-catalytic production of hydrogen form ethanol over M/TiO2 catalysts (M = Pd, Pt or Rh),” Applied Catalysis B, vol. 67, no. 3-4, pp. 217–222, 2006. View at Publisher · View at Google Scholar
  152. S. Meyer, S. Saborowski, and B. Schäfer, “Photocatalytic reforming of methanol by spatially separated Pd particles on special TiO2 layers,” ChemPhysChem, vol. 7, no. 3, pp. 572–574, 2006. View at Publisher · View at Google Scholar · View at Scopus
  153. I. Rossetti, L. Fabbrini, N. Ballarini et al., “V2O5-SiO2 systems prepared by flame pyrolysis as catalysts for the oxidative dehydrogenation of propane,” Journal of Catalysis, vol. 256, no. 1, pp. 45–61, 2008. View at Publisher · View at Google Scholar · View at Scopus
  154. I. Rossetti, L. Fabbrini, N. Ballarini et al., “V–Al–O catalysts prepared by flame pyrolysis for the oxidative dehydrogenation of propane to propylene,” Catalysis Today, vol. 141, no. 3-4, pp. 271–281, 2009. View at Publisher · View at Google Scholar · View at Scopus
  155. L. Mädler and S. E. Pratsinis, “Bismuth oxide nanoparticles by flame spray pyrolysis,” Journal of the American Ceramic Society, vol. 85, no. 7, pp. 1713–1718, 2002. View at Scopus
  156. R. Jossen, W. J. Stark, L. Mädler, and S. E. Pratsinis, “Effect of precursor and solvent on particle homogeneity and morphology during spray flame synthesis of nanoparticles,” Chemie-Ingenieur-Technik, vol. 75, no. 8, pp. 1129–1130, 2003. View at Publisher · View at Google Scholar · View at Scopus
  157. L. Mädler, H. K. Kammler, R. Mueller, and S. E. Pratsinis, “Controlled synthesis of nanostructured particles by flame spray pyrolysis,” Journal of Aerosol Science, vol. 33, no. 2, pp. 369–389, 2002. View at Publisher · View at Google Scholar · View at Scopus
  158. G. L. Chiarello, E. Selli, and L. Forni, “Photocatalytic hydrogen production over flame spray pyrolysis-synthesised TiO2 and Au/TiO2,” Applied Catalysis B, vol. 84, no. 1-2, pp. 332–339, 2008. View at Publisher · View at Google Scholar
  159. G. L. Chiarello, L. Forni, and E. Selli, “Photocatalytic hydrogen production by liquid- and gas-phase reforming of CH3OH over flame-made TiO2 and Au/TiO2,” Catalysis Today, vol. 144, no. 1-2, pp. 69–74, 2009. View at Publisher · View at Google Scholar · View at Scopus
  160. G. L. Chiarello, M. H. Aguirre, and E. Selli, “Hydrogen production by photocatalytic steam reforming of methanol on noble metal-modified TiO2,” Journal of Catalysis, vol. 273, no. 2, pp. 182–190, 2010. View at Publisher · View at Google Scholar · View at Scopus
  161. J. Greaves, L. Al-Mazroai, A. Nuhu, P. Davies, and M. Bowker, “Photocatalytic methanol reforming on Au/TiO2 for hydrogen production,” Gold Bulletin, vol. 39, no. 4, pp. 216–219, 2006. View at Scopus
  162. U. Diebold, “The surface science of titanium dioxide,” Surface Science Reports, vol. 48, no. 5–8, pp. 53–229, 2003. View at Publisher · View at Google Scholar
  163. T. L. Thompson and J. T. Yates Jr., “Surface science studies of the photoactivation of TiO2—new photochemical processes,” Chemical Reviews, vol. 106, no. 10, pp. 4428–4453, 2006. View at Publisher · View at Google Scholar
  164. M. Lazzeri, A. Vittadini, and A. Selloni, “Structure and energetics of stoichiometric TiO2 anatase surfaces,” Physical Review B, vol. 63, no. 15, Article ID 155409, 9 pages, 2001. View at Scopus
  165. H. G. Yang, C. H. Sun, S. Z. Qiao et al., “Anatase TiO2 single crystals with a large percentage of reactive facets,” Nature, vol. 453, no. 7195, pp. 638–641, 2008. View at Publisher · View at Google Scholar · View at Scopus
  166. T. R. Gordon, M. Cargnello, T. Paik et al., “Nonaqueous synthesis of TiO2 nanocrystals using TiF4 to engineer morphology, oxygen vacancy concentration, and photocatalytic activity,” Journal of the American Chemical Society, vol. 134, no. 15, pp. 6751–6761, 2012. View at Publisher · View at Google Scholar
  167. R. F. Howe and M. Gratzel, “EPR observation of trapped electrons in colloidal titanium dioxide,” Journal of Physical Chemistry, vol. 89, no. 21, pp. 4495–4499, 1985. View at Publisher · View at Google Scholar
  168. R. F. Howe and M. Gratzel, “EPR study of hydrated anatase under UV irradiation,” Journal of Physical Chemistry, vol. 91, no. 14, pp. 3906–3909, 1987. View at Publisher · View at Google Scholar
  169. T. Torimoto, R. J. Fox, and M. A. Fox, “Photoelectrochemical doping of TiO2 particles and the effect of charge carrier density on the photocatalytic activity of microporous semiconductor electrode films,” Journal of the Electrochemical Society, vol. 143, no. 11, pp. 3712–3717, 1996. View at Scopus
  170. T. Berger, M. Sterrer, O. Diwald et al., “Light-induced charge separation in anatase TiO2 particles,” Journal of Physical Chemistry B, vol. 109, no. 13, pp. 6061–6068, 2005. View at Publisher · View at Google Scholar · View at Scopus
  171. Q. Xu, Y. Ma, J. Zhang, X. Wang, Z. Feng, and C. Li, “Enhancing hydrogen production activity and suppressing CO formation from photocatalytic biomass reforming on Pt/TiO2 by optimizing anatase-rutile phase structure,” Journal of Catalysis, vol. 278, no. 2, pp. 329–335, 2011. View at Publisher · View at Google Scholar
  172. M. Ikeda, Y. Kusumoto, S. Somekawa, P. Ngweniform, and B. Ahmmad, “Effect of graphite silica on TiO2 photocatalysis in hydrogen production from water-methanol solution,” Journal of Photochemistry and Photobiology A, vol. 184, no. 3, pp. 306–312, 2006. View at Publisher · View at Google Scholar · View at Scopus
  173. M. Ikeda, Y. Kusumoto, Y. Yakushijin, S. Somekawa, P. Ngweniform, and B. Ahmmad, “Hybridized synergy effect among TiO2, Pt and graphite silica on photocatalytic hydrogen production from water-methanol solution,” Catalysis Communications, vol. 8, no. 12, pp. 1943–1946, 2007. View at Publisher · View at Google Scholar · View at Scopus
  174. T. Ohno, T. Tsubota, Y. Nakamura, and K. Sayama, “Preparation of S, C cation-codoped SrTiO3 and its photocatalytic activity under visible light,” Applied Catalysis A, vol. 288, no. 1-2, pp. 74–79, 2005. View at Publisher · View at Google Scholar
  175. T. Puangpetch, T. Sreethawong, S. Yoshikawa, and S. Chavadej, “Hydrogen production from photocatalytic water splitting over mesoporous-assembled SrTiO3 nanocrystal-based photocatalysts,” Journal of Molecular Catalysis A, vol. 312, no. 1-2, pp. 97–106, 2009. View at Publisher · View at Google Scholar · View at Scopus
  176. M. V. Shankar and J. Ye, “Inorganic alkaline-sols as precursors for rapid synthesis of ETS-10 microporous titanosilicates and their photocatalytic reforming of methanol under visible-light irradiation,” Catalysis Communications, vol. 11, no. 4, pp. 261–265, 2009. View at Publisher · View at Google Scholar
  177. M. Antoniadou, D. I. Kondarides, and P. Lianos, “Photooxidation products of ethanol during photoelectrochemical operation using a nanocrystalline titania anode and a two compartment chemically biased cell,” Catalysis Letters, vol. 129, no. 3-4, pp. 344–349, 2009. View at Publisher · View at Google Scholar · View at Scopus
  178. M. Kaneko, J. Nemoto, H. Ueno et al., “Photoelectrochemical reaction of biomass and bio-related compounds with nanoporous TiO2 film photoanode and O2-reducing cathode,” Electrochemistry Communications, vol. 8, no. 2, pp. 336–340, 2006. View at Publisher · View at Google Scholar · View at Scopus
  179. D. E. Tsydenov, V. N. Parmon, and A. V. Vorontsov, “Toward the design of asymmetric photocatalytic membranes for hydrogen production: preparation of TiO2-based membranes and their properties,” International Journal of Hydrogen Energy, vol. 37, no. 15, pp. 11046–11060, 2012.
  180. Y. Wu, G. Lu, and S. Li, “The role of Cu(I) species for photocatalytic hydrogen generation over CuOx/TiO2,” Catalysis Letters, vol. 133, no. 1-2, pp. 97–105, 2009. View at Publisher · View at Google Scholar
  181. D. Barreca, P. Fornasiero, A. Gasparotto et al., “The potential of supported Cu2O and CuO nanosystems in photocatalytic H2 production,” ChemSusChem, vol. 2, no. 3, pp. 230–233, 2009. View at Publisher · View at Google Scholar · View at Scopus
  182. Y. Wang, Z. Zhang, Y. Zhu et al., “Nanostructured VO2 photocatalysts for hydrogen production,” ACS Nano, vol. 2, no. 7, pp. 1492–1496, 2008. View at Publisher · View at Google Scholar · View at Scopus
  183. M. Liu, W. You, Z. Lei, T. Takata, K. Domen, and C. Li, “Photocatalytic water splitting to hydrogen over a visible light-driven LaTaON2 catalyst,” Chinese Journal of Catalysis, vol. 27, no. 7, pp. 556–558, 2006. View at Publisher · View at Google Scholar
  184. T. Montini, V. Gombac, L. Sordelli et al., “Nanostructured Cu/TiO2 photocatalysts for H2 production from ethanol and glycerol aqueous solutions,” ChemCatChem, vol. 3, no. 3, pp. 574–577, 2011. View at Publisher · View at Google Scholar
  185. V. Gombac, L. Sordelli, T. Montini et al., “CuOx-TiO2 photocatalysts for H2 production from ethanol and glycerol solutions,” Journal of Physical Chemistry A, vol. 114, no. 11, pp. 3916–3925, 2010. View at Publisher · View at Google Scholar · View at Scopus
  186. Z. Liu, H. Bai, S. Xu, and D. D. Sun, “Hierarchical CuO/ZnO “corn-like” architecture for photocatalytic hydrogen generation,” International Journal of Hydrogen Energy, vol. 36, no. 21, pp. 13473–13480, 2011. View at Publisher · View at Google Scholar
  187. Q. Simon, D. Barreca, A. Gasparotto et al., “Vertically oriented CuO/ZnO nanorod arrays: from plasma-assisted synthesis to photocatalytic H2 production,” Journal of Materials Chemistry, vol. 22, no. 23, pp. 11739–11747, 2012. View at Publisher · View at Google Scholar
  188. M. A. Nadeem, M. Murdoch, G. I. N. Waterhouse et al., “Photoreaction of ethanol on Au/TiO2 anatase: comparing the micro to nanoparticle size activities of the support for hydrogen production,” Journal of Photochemistry and Photobiology A, vol. 216, no. 2–4, pp. 250–255, 2010. View at Publisher · View at Google Scholar · View at Scopus
  189. M. Aresta, A. Dibenedetto, F. Nocito, and C. Ferragina, “Valorization of bio-glycerol: new catalytic materials for the synthesis of glycerol carbonate via glycerolysis of urea,” Journal of Catalysis, vol. 268, no. 1, pp. 106–114, 2009. View at Publisher · View at Google Scholar · View at Scopus
  190. A. Behr, J. Eilting, K. Irawadi, J. Leschinski, and F. Lindner, “Improved utilisation of renewable resources: new important derivatives of glycerol,” Green Chemistry, vol. 10, no. 1, pp. 13–30, 2008. View at Publisher · View at Google Scholar · View at Scopus
  191. N. Fu and G. Lu, “Hydrogen evolution over heteropoly blue-sensitized Pt/TiO2 under visible light irradiation,” Catalysis Letters, vol. 127, no. 3-4, pp. 319–322, 2009. View at Publisher · View at Google Scholar
  192. K. Lalitha, G. Sadanandam, V. D. Kumari, M. Subrahmanyam, B. Sreedhar, and N. Y. Hebalkar, “Highly stabilized and finely dispersed Cu2O/TiO2: a promising visible sensitive photocatalyst for continuous production of hydrogen from glycerol:water mixtures,” Journal of Physical Chemistry C, vol. 114, no. 50, pp. 22181–22189, 2010. View at Publisher · View at Google Scholar · View at Scopus
  193. J. Yu, Y. Hai, and M. Jaroniec, “Photocatalytic hydrogen production over CuO-modified titania,” Journal of Colloid and Interface Science, vol. 357, no. 1, pp. 223–228, 2011. View at Publisher · View at Google Scholar
  194. A. Patsoura, D. I. Kondarides, and X. E. Verykios, “Photocatalytic degradation of organic pollutants with simultaneous production of hydrogen,” Catalysis Today, vol. 124, no. 3-4, pp. 94–102, 2007. View at Publisher · View at Google Scholar · View at Scopus
  195. J. M. Herrmann, “Heterogeneous photocatalysis: state of the art and present applications In honor of Pr. R.L. Burwell Jr. (1912–2003), Former Head of Ipatieff Laboratories, Northwestern University, Evanston (Ill),” Topics in Catalysis, vol. 34, no. 1–4, pp. 49–65, 2005. View at Publisher · View at Google Scholar
  196. S. Pilkenton, S. J. Hwang, and D. Raftery, “Ethanol photocatalysis on TiO2-coated optical microfiber, supported monolayer, and powdered catalysts: an in situ NMR study,” Journal of Physical Chemistry B, vol. 103, no. 50, pp. 11152–11160, 1999. View at Scopus
  197. W. Xu and D. Raftery, “Photocatalytic oxidation of 2-propanol on TiO2 powder and TiO2 monolayer catalysts studied by solid-state NMR,” Journal of Physical Chemistry B, vol. 105, no. 19, pp. 4343–4349, 2001. View at Publisher · View at Google Scholar
  198. M. E. Simonsen, Z. Li, and E. G. Søgaard, “Influence of the OH groups on the photocatalytic activity and photoinduced hydrophilicity of microwave assisted sol-gel TiO2 film,” Applied Surface Science, vol. 255, no. 18, pp. 8054–8062, 2009. View at Publisher · View at Google Scholar
  199. O. M. Alfano, M. I. Cabrera, and A. E. Cassano, “Photocatalytic reactions involving hydroxyl radical attack: I. Reaction kinetics formulation with explicit photon absorption effects,” Journal of Catalysis, vol. 172, no. 2, pp. 370–379, 1997. View at Scopus
  200. C. S. Turchi and D. F. Ollis, “Photocatalytic degradation of organic water contaminants: mechanisms involving hydroxyl radical attack,” Journal of Catalysis, vol. 122, no. 1, pp. 178–192, 1990. View at Scopus
  201. A. H. Boonstra and C. A. H. A. Mutsaers, “Relation between the photoadsorption of oxygen and the number of hydroxyl groups on a titanium dioxide surface,” Journal of Physical Chemistry, vol. 79, no. 16, pp. 1694–1698, 1975. View at Scopus
  202. I. A. Shkrob, M. C. Sauer Jr., and D. Gosztola, “Efficient, rapid photooxidation of chemisorbed polyhydroxyl alcohols and carbohydrates by TiO2 nanoparticles in an aqueous solution,” Journal of Physical Chemistry B, vol. 108, no. 33, pp. 12512–12517, 2004. View at Publisher · View at Google Scholar · View at Scopus
  203. I. A. Shkrob and M. C. Sauer, “Hole scavenging and photo-stimulated recombination of electron-hole Pairs in aqueous TiO2 nanoparticles,” Journal of Physical Chemistry B, vol. 108, no. 33, pp. 12497–12511, 2004. View at Publisher · View at Google Scholar
  204. X. Fu, X. Wang, D. Y. C. Leung, Q. Gu, S. Chen, and H. Huang, “Photocatalytic reforming of C3-polyols for H2 production: part (I). Role of their OH groups,” Applied Catalysis B, vol. 106, no. 3-4, pp. 681–688, 2011. View at Publisher · View at Google Scholar
  205. T. Rajh, L. X. Chen, K. Lukas, T. Liu, M. C. Thurnauer, and D. M. Tiede, “Surface restructuring of nanoparticles: an efficient route for ligand-metal oxide crosstalk,” Journal of Physical Chemistry B, vol. 106, no. 41, pp. 10543–10552, 2002. View at Publisher · View at Google Scholar · View at Scopus
  206. W. Xu, D. Raftery, and J. S. Francisco, “Effect of irradiation sources and oxygen concentration on the photocatalytic oxidation of 2-propanol and acetone studied by in situ FTIR,” Journal of Physical Chemistry B, vol. 107, no. 19, pp. 4537–4544, 2003. View at Publisher · View at Google Scholar
  207. Q. Gu, X. Fu, X. Wang, S. Chen, D. Y. C. Leung, and X. Xie, “Photocatalytic reforming of C3-polyols for H2 production: Part II. FTIR study on the adsorption and photocatalytic reforming reaction of 2-propanol on Pt/TiO2,” Applied Catalysis B, vol. 106, no. 3-4, pp. 689–696, 2011. View at Publisher · View at Google Scholar
  208. C. A. Bunton, G. Savelli, and L. Sepulveda, “Role of glucose and related compounds in micellar and nonmicellar nucleophilic reactions,” Journal of Organic Chemistry, vol. 43, no. 10, pp. 1925–1929, 1978. View at Scopus
  209. T. Imae, K. Muto, and S. Ikeda, “The pH dependence of dispersion of TiO2 particles in aqueous surfactant solutions,” Colloid & Polymer Science, vol. 269, no. 1, pp. 43–48, 1991. View at Publisher · View at Google Scholar · View at Scopus
  210. M. Zhou, Y. Li, S. Peng, G. Lu, and S. Li, “Effect of epimerization of d-glucose on photocatalytic hydrogen generation over Pt/TiO2,” Catalysis Communications, vol. 18, pp. 21–25, 2012. View at Publisher · View at Google Scholar
  211. R. M. Mohamed and E. S. Aazam, “H2 production with low CO selectivity from photocatalytic reforming of glucose on Ni/TiO2–SiO2,” Chinese Journal of Catalysis, vol. 33, no. 2-3, pp. 247–253, 2012. View at Publisher · View at Google Scholar
  212. H. Kato and A. Kudo, “Photocatalytic water splitting into H2 and O2 over various tantalate photocatalysts,” Catalysis Today, vol. 78, no. 1–4, pp. 561–569, 2003. View at Publisher · View at Google Scholar
  213. W. Choi, A. Tormin, and M. R. Hoffman, “The role of metal ion dopants in quantum-sized TiO2: correlation between photoreactivity and charge carrier recombination dynamics,” Journal of Physical Chemistry, vol. 98, no. 51, pp. 13669–13679, 1994. View at Publisher · View at Google Scholar
  214. B. Zieliñska, E. Borowiak-Palen, and R. J. Kalenczuk, “Photocatalytic hydrogen generation over alkaline-earth titanates in the presence of electron donors,” International Journal of Hydrogen Energy, vol. 33, no. 7, pp. 1797–1802, 2008. View at Publisher · View at Google Scholar · View at Scopus
  215. S. Shen and L. Guo, “Hydrothermal synthesis, characterization, and photocatalytic performances of Cr incorporated, and Cr and Ti co-incorporated MCM-41 as visible light photocatalysts for water splitting,” Catalysis Today, vol. 129, no. 3-4, pp. 414–420, 2007. View at Publisher · View at Google Scholar
  216. X. J. Zheng, L. F. Wei, Z. H. Zhang et al., “Research on photocatalytic H2 production from acetic acid solution by Pt/TiO2 nanoparticles under UV irradiation,” International Journal of Hydrogen Energy, vol. 34, no. 22, pp. 9033–9041, 2009. View at Publisher · View at Google Scholar · View at Scopus
  217. L. Jia, J. Li, and W. Fang, “Effect of H2/CO2 mixture gas treatment temperature on the activity of LaNiO3 catalyst for hydrogen production from formaldehyde aqueous solution under visible light,” Journal of Alloys and Compounds, vol. 489, no. 2, pp. L13–L16, 2010. View at Publisher · View at Google Scholar
  218. A. Patsoura, D. I. Kondarides, and X. E. Verykios, “Enhancement of photoinduced hydrogen production from irradiated Pt/TiO2 suspensions with simultaneous degradation of azo-dyes,” Applied Catalysis B, vol. 64, no. 3-4, pp. 171–179, 2006. View at Publisher · View at Google Scholar · View at Scopus
  219. M. I. Badawy, M. Y. Ghaly, and M. E. M. Ali, “Photocatalytic hydrogen production over nanostructured mesoporous titania from olive mill wastewater,” Desalination, vol. 267, no. 2-3, pp. 250–255, 2011. View at Publisher · View at Google Scholar · View at Scopus
  220. K. Mogyorosi, I. Dekany, and H. J. Fendler, “Preparation and characterization of clay mineral intercalated titanium dioxide nanoparticles,” Langmuir, vol. 19, no. 7, pp. 2938–2946, 2003. View at Publisher · View at Google Scholar
  221. A. Scalfani, L. Palmisano, and M. Schiavello, “Influence of the preparation methods of titanium dioxide on the photocatalytic degradation of phenol in aqueous dispersion,” Journal of Physical Chemistry, vol. 94, no. 2, pp. 829–832, 1990. View at Publisher · View at Google Scholar
  222. M. Andersson, L. Österlund, S. Ljungstrom, and A. Palmqvist, “Preparation of nanosize anatase and rutile TiO2 by hydrothermal treatment of microemulsions and their activity for photocatalytic wet oxidation of phenol,” Journal of Physical Chemistry B, vol. 106, no. 41, pp. 10674–10679, 2002. View at Publisher · View at Google Scholar
  223. M. Addamo, V. Augugliaro, A. di Paola et al., “Preparation, characterization, and photoactivity of polycrystalline nanostructured TiO2 catalysts,” Journal of Physical Chemistry B, vol. 108, no. 10, pp. 3303–3310, 2004. View at Scopus
  224. C. C. Chen, C. S. Lu, Y. C. Chung, and J. L. Jan, “UV light induced photodegradation of malachite green on TiO2 nanoparticles,” Journal of Hazardous Materials, vol. 141, no. 3, pp. 520–528, 2007. View at Publisher · View at Google Scholar
  225. F. Kiriakidou, D. I. Kondarides, and X. E. Verykios, “The effect of operational parameters and TiO2-doping on the photocatalytic degradation of azo-dyes,” Catalysis Today, vol. 54, no. 1, pp. 119–130, 1999. View at Scopus
  226. M. Stylidi, D. I. Kondarides, and X. E. Verykios, “Pathways of solar light-induced photocatalytic degradation of azo dyes in aqueous TiO2 suspensions,” Applied Catalysis B, vol. 40, no. 4, pp. 271–286, 2003. View at Publisher · View at Google Scholar · View at Scopus
  227. M. Stylidi, D. I. Kondarides, and X. E. Verykios, “Visible light-induced photocatalytic degradation of acid orange 7 in aqueous TiO2 suspensions,” Applied Catalysis B, vol. 47, no. 3, pp. 189–201, 2004. View at Publisher · View at Google Scholar · View at Scopus
  228. Y. Li, Y. Xie, S. Peng, G. Lu, and S. Li, “Photocatalytic hydrogen generation in the presence of chloroacetic acids over Pt/TiO2,” Chemosphere, vol. 63, no. 8, pp. 1312–1318, 2006. View at Publisher · View at Google Scholar · View at Scopus
  229. J. S. Jang, H. G. Kim, H. G. Kim, and J. S. Lee, “Simultaneous hydrogen production and decomposition of H2S dissolved in alkaline water over CdS–TiO2 composite photocatalysts under visible light irradiation,” International Journal of Hydrogen Energy, vol. 32, no. 18, pp. 4786–4791, 2007. View at Publisher · View at Google Scholar
  230. H. Yoshida, S. Kato, K. Hirao, J. I. Nishimoto, and T. Hattori, “Photocatalytic steam reforming of methane over platinum-loaded semiconductors for hydrogen production,” Chemistry Letters, vol. 36, no. 3, pp. 430–431, 2007. View at Publisher · View at Google Scholar · View at Scopus
  231. H. Yoshida, K. Hirao, J. I. Nishimoto et al., “Hydrogen production from methane and water on platinum loaded titanium oxide photocatalysts,” Journal of Physical Chemistry C, vol. 112, no. 14, pp. 5542–5551, 2008. View at Publisher · View at Google Scholar · View at Scopus
  232. K. Shimura, S. Kato, T. Yoshida, H. Itoh, T. Hattori, and H. Yoshida, “Photocatalytic steam reforming of methane over sodium tantalate,” Journal of Physical Chemistry C, vol. 114, no. 8, pp. 3493–3503, 2010. View at Publisher · View at Google Scholar · View at Scopus