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Photocatalytic Ethanol Oxidative Dehydrogenation over Pt/TiO2: Effect of the Addition of Blue Phosphors
Ethanol oxidative dehydrogenation over Pt/TiO2 photocatalyst, in the presence and absence of blue phosphors, was performed. The catalyst was prepared by photodeposition of Pt on sulphated TiO2. This material was tested in a gas-solid photocatalytic fluidized bed reactor at high illumination efficiency. The effect of the addition of blue phosphors into the fluidized bed has been evaluated. The synthesized catalysts were extensively characterized by different techniques. Pt/TiO2 with a loading of 0.5 wt% of Pt appeared to be an active photocatalyst in the selective partial oxidation of ethanol to acetaldehyde improving its activity and selectivity compared to pure TiO2. In the same way, a notable enhancement of ethanol conversion in the presence of the blue phosphors has been obtained. The blue phosphors produced an increase in the level of ethanol conversion over the Pt/TiO2 catalyst, keeping at the same time the high selectivity to acetaldehyde.
The selective oxidation of alcohols is one of the most important reactions in organic chemistry. The conversion of primary alcohols to aldehydes is very important for the synthesis of fine chemicals such as fragrances or food additives [1–6]. There is a great industrial interest to convert low alcohols into useful organic intermediates or products, that is, ethene, diethyl ether, or acetaldehyde . Ethanol serves as a feedstock for acetaldehyde production by oxidative dehydrogenation over pure or mixed oxide catalysts [8–10]. Acetaldehyde is an important intermediary in organic syntheses, and this compound is normally obtained by catalytic ethanol partial oxidation [11, 12]. The catalytic oxidation of ethanol has generally been investigated in order to develop catalysts that maximize products, such as acetaldehyde or acetic acid, and minimize production of deep oxidation reactions .
Different supported metals, such as Mo, Fe, Co, Ni, Cu, Ag, V, and Au, has been studied in the current literature for the reaction of partial oxidation of ethanol by thermal catalysis [11–18]. It is important to note that the selective oxidation of alcohols to either aldehydes or acid in the presence of a noble metal catalyst is of simple work-up procedure, shows a wide applicability to various alcohols, and is an attractive, environment-friendly process [19, 20].
On the other hand, heterogeneous photocatalysis using semiconductor oxides has demonstrated to be very effective in the oxidation of different organic compounds. The heterogeneous photocatalysis based on TiO2 is an interesting alternative because this oxide is an effective, photostable, reusable, inexpensive, nontoxic, and easily available catalyst . In the past, the majority of the research in the field of photocatalysis was focused on the use of TiO2 photocatalysts for the purification of water or gas atmospheres from environmental contaminants [22, 23]. However, partial photocatalytic oxidation in the gas phase has recently attracted great interest due to the high potential of this technique in green chemistry . Metal deposition on TiO2 has been intensively studied as a means of reducing electron/hole recombination and enhancing efficiencies of TiO2 in the photocatalytic degradation of organic compounds . Pt deposited on TiO2 has been reported to improve [26–31], be detrimental [29, 32], or have negligible effects  on photocatalysis depending on many different factors.
Chen et al.  studied the photocatalytic partial oxidation of ethanol over TiO2 and Pt/TiO2 and found that the selectivity to acetaldehyde (which is more easily oxidized) was enhanced with the platinization as opposed to the higher formation of acetic acid with bare TiO2. These authors explained that loaded on TiO2, Pt can accelerate the oxygen reduction process occurring at the cathodic area, thereby diminishing the electron accumulation on the surface of TiO2 particles. This would accelerate the oxidation rate of alcohols, which, in fact, is controlled by the cathodic reduction of oxygen [32, 33]. This process has been often observed not only for alcohols, but also for other organic compounds. Vorontsov and Dubovitskaya  reported also up to a two times increase in the rate of ethanol photooxidation at various Pt loadings.
In previous papers regarding the oxidative dehydrogenation of organic compounds using fluidized bed photoreactors [17, 35–37], it has been evinced that UV light does not penetrate in the reactor core volume, since it is mostly absorbed by the catalyst circulating near the irradiated reactor windows within few millimetres . To improve photon transfer in the reactor core volume, the reactor thickness has to be reduced, and, moreover, it is possible to mix the photocatalyst with emitting phosphorescent particles as light carriers, known generally as phosphors. Recently, some of us have reported that the addition of phosphors into a fluidized bed has, therefore, resulted in about doubling of the catalytic surface totally irradiated , allowing a considerably increase of the photocatalytic activity. In this paper, the selective partial oxidation of ethanol to acetaldehyde over Pt/TiO2 photocatalyst was studied. The photocatalytic reactions were carried out in a gas-solid photocatalytic fluidized bed reactor. The study of the effect of the addition of blue phosphors into the fluidized bed was also attempted.
2.1. Synthesis Procedure
TiO2 used as starting material was prepared by the hydrolysis of titanium tetraisopropoxide (Aldrich, 97%) in isopropanol solution (1.6 M) by the slow addition of distilled water (volume ratio isopropanol/water 1 : 1). Afterward, the generated precipitate was filtered and dried at 110°C overnight. The powders thus obtained were then sulphated by immersion in 1 M sulphuric acid solution for 1 h and calcinated at 650°C for 2 h. Sulphate treatment was carried out for two reasons. On one hand, previous results have shown that sulphate pretreatment stabilizes anatase phase up to high temperatures and protects surface area against sintering . On the other hand, at the calcination temperature of 650°C, the elimination of sulphate groups promotes the creation of high number of oxygen vacancies, which have been reported as preferential sites for Pt adsorption .
Photodeposition of platinum was performed over the calcined TiO2 powder using hexachloroplatinic acid (H2PtCl6, Aldrich 99.9%) as metal precursor. Under an inert atmosphere (N2), a suspension of TiO2 in distilled water containing isopropanol (Merck 99.8%) which acts as sacrificial donor was prepared. Then, the appropriate amount of H2PtCl6 to obtain a nominal platinum loading of 0.5% weight total to TiO2 was added. Final pH of the suspensions was 3. Photodeposition of platinum was then performed by illuminating the suspension during 120 min with an Osram Ultra-Vitalux lamp (300 W) with a sun-like radiation spectrum and a main emission line in the UVA range at 365 nm. The intensity of the lamp was 140 W/m2. After photodeposition, the powders were recovered by filtration and dried at 110°C overnight.
Blue phosphors (model RL-UV-B-Y; Excitation Wavelength: 365 nm; Emission Wavelength: 440 nm; particles diameter: 5–10 μm) were provided by DB Chemic. A wide characterization of this material was carried out, and these results are collected in a recent article , evidencing that the host crystal structure of the phosphors used in this work is ZnS.
2.2. Characterization Techniques
Crystalline phase composition and degree of crystallinity of the samples were estimated by X-ray diffraction (XRD). XRD patterns were obtained on a Siemens D-501 diffractometer with Ni filter and graphite monochromator using Cu Kα radiation. Crystallite sizes were calculated from the line broadening of the main X-ray diffraction peaks by using the Scherrer equation. Peaks were fitted by using a Voigt function.
Laser Raman spectra of catalyst were obtained at room temperature with a Dispersive MicroRaman (Invia, Renishaw), equipped with 785 nm diode-laser, in the range 100–2500 cm−1 Raman shift.
Light absorption properties of the samples were studied by UV-Vis spectroscopy. The UV-Vis DRS spectra were recorded by a Perkin Elmer spectrometer Lambda 35. Band-gaps values were calculated from the corresponding Kubelka-Munk functions, , which are proportional to the absorption of radiation, by plotting against .
BET surface area and porosity measurements were carried out by N2 adsorption at 77 K using a Micromeritics ASAP 2010 instrument.
Chemical composition and total platinum content of the samples were determined by X-ray fluorescence spectrometry (XRF) in a Panalytical Axios sequential spectrophotometer equipped with a rhodium tube as the source of radiation. XRF measurements were performed onto pressed pellets (sample included in 10 wt% of wax).
Thermogravimetric analysis (TG-DTG) of the samples was carried out in air flow with a thermobalance (SDT Q600, TA Instruments), in the range 20°C–1000°C at heating rate of 10°C/min.
Field emission SEM images were obtained in a Hitachi S-4800 microscope. The samples were dispersed in ethanol using an ultrasonicator and dropped on a carbon grid. The platinum particle sizes were evaluated by TEM, in a microscope Philips CM 200.
X-ray photoelectron spectroscopy (XPS) studies were carried out on a Leybold-Heraeus LHS-10 spectrometer, working with constant pass energy of 50 eV. The spectrometer main chamber, working at a pressure Torr, is equipped with an EA-200 MCD hemispherical electron analyser with a dual X-ray source working with Al Kα ( eV) at 120 W and 30 mA. C 1 s signal (284.6 eV) was used as internal energy reference in all the experiments. Samples were outgassed in the prechamber of the instrument at 150°C up to a pressure Torr to remove chemisorbed water.
2.3. Photocatalytic Tests
Photocatalytic tests were carried out with a feeding 30 L/h (STP), at ethanol concentration in the range 0.1–2 vol.%, in helium flow with oxygen/ethanol ratio of 2. Temperature and pressure reaction were 60°C and 1 atm, respectively. Oxygen and helium were fed from cylinders, helium being the carrier gas for ethanol vaporized from a temperature controlled saturator. Different concentrations of ethanol in the reaction feed were obtained by changing temperature and He flow through the saturator. The gas flow rates were measured and regulated by mass flow controllers (Brooks Instrument).
The fluidized bed reactor used in this work was designed for working with a gas flow rate in the range 20–70 L/h (STP) with a Sauter average diameter in the particles size range 50–100 μm, assuring optimal fluidization [17, 35]. It was a two-dimensional reactor with 40 mm × 6 mm cross-section, 230 mm height pyrex-glass walls, and a bronze filter (mean pores size 5 μm) to provide a uniform distribution of fed gas. In order to decrease the amount of transported particles, an expanding section (50 mm × 50 mm cross-section at the top) and a cyclone specifically designed  are located on the top and at the outlet of the reactor, respectively. The reactor was illuminated by two UVA-LEDs modules (80 × 50 mm) positioned in front of the reactor pyrex windows (light intensity: 90 mW/cm2). Each module consisted of 40 UV-LEDs emitting at 365 nm (provided by Nichia Corporation). With these illumination conditions, the light pathlength in the photoreactor was about 2 mm. The catalytic bed was composed by 1.2 g of catalyst diluted with 20 g of glass spheres (grain size: 70–110 μm) (Lampugnani Sandblasting HI-TECH).
To optimize the composition of fluidizable solid mixture, photocatalytic tests were also carried out in the presence of blue phosphors. For these tests, the catalytic bed was composed by 1.2 g of catalyst and 1.8 g of blue phosphors diluted with 20 g of glass spheres. In this way, phosphors were fluidized with the catalyst, excited by external UVA-LEDs, and emitted their stored energy in the proximity to the catalyst.
Hereon, the catalytic systems are denoted as Pt/TiO2 and Pt/TiO2 + Blue P for the setups without and with phosphors, respectively.
Concentrations of inlet reactants and outlet products were measured by an online mass detector (MS) (Quantra Fourier transform ion cyclotron resonance mass spectrometer, Siemens) and a continuous CO-CO2 NDIR analyzer (Uras 10, Hartmann and Braun).
Preliminary tests were carried out to check the amount of solid particles elutriated from the reactor by fluidizing the powders for several hours. Elutriation was negligible, confirmed also by the stability of catalytic activity during irradiation time in the photocatalytic tests. In addition, experimental tests to check the fluidization properties were realized, both in the presence and absence of phosphors. These tests showed that the expansion of fluidized bed was the same in both cases, evidencing that the fluidization regime did not change in the presence of phosphors.
3. Results and Discussion
3.1. Characterization of the Catalysts
The XRD patterns (Figure 1) and Raman spectra (Figure 2) for sulphated TiO2 and Pt/TiO2 photocatalyst showed that anatase is the only crystalline phase present in these samples. The stabilization of anatase phase by sulphate pretreatment of the TiO2 can be noticed here, as no traces of rutile were found even after the high calcination temperature used (650°C) [38, 40]. No peaks corresponding to platinum was detected by XRD due to the low loading and high dispersion of metal present in the Pt/TiO2 catalyst. Anatase crystallite sizes of the samples were estimated by the Scherrer equation, and the values are presented in Table 1. As it can be observed, the addition of platinum did not induce any significant change in the anatase crystallite size of TiO2 (20-21 nm).
Figure 3 shows the UV-Vis DRS spectra for Pt/TiO2 catalyst and the starting material (sulphated TiO2). No significant differences between the spectra analyzed are visible for wavelengths below 400 nm in which the characteristic sharp absorption threshold of TiO2 around 350 nm can be observed. For Pt/TiO2 catalyst, in the range between 400 and 600 nm, there is the presence of a very broad absorption band probably due to an interaction between platinum surface species and titanium dioxide. Figure 3 shows also the UV-Vis DRS spectrum of blue phosphors; this sample has absorption at wavelength of 320 nm and band gap energy of 3.1 eV.
Differences in the absorption properties of the two catalysts are markedly evinced by plotting versus (Figure 4) and correspond to a decrease in band gap energies, from 3.4 eV for TiO2 to 2.8 eV for Pt/TiO2 sample. This last value implies that the activation energy can be provided by the energy related to the wavelength emission of the selected phosphors (Figure 5).
BET surface area values for the analyzed samples are given in Table 1. As it can be observed, a little increase in the TiO2 value was produced by the process of platinum photodeposition.
Total amount of platinum in the Pt/TiO2 photocatalyst was calculated by XRF, being 0.41%. This value is close to the nominal content (0.5%) indicating a high yield for the photodeposition process. XRF analysis also showed the presence of sulphur from the sulphate pretreatment even though the amount of this element was low corresponding just to a 0.39%.
Hydroxyls surface densities were evaluated by TG-DTG losses in the temperature range of 180°C–300°C. The analysis revealed that the hydroxyls groups on the surface of the TiO2 increased with the deposition of Pt. The TG-DTG results (Table 1) also showed that remaining sulphur content on TiO2 decreased with the addition of platinum, probably due to the synthesis procedure of the Pt/TiO2 catalyst.
The samples were studied by SEM and TEM to obtain information about Pt particle size and dispersion. A selected SEM micrograph for Pt/TiO2 catalyst is presented in Figure 6. As it can be clearly observed, platinum particles are quite heterogeneously distributed over the oxide surface with a poor dispersion, and the metal deposits present different sizes. TEM was used to estimate the Pt particle size distribution. One selected micrograph is shown in Figure 7 together with a representation of the metal particle size distribution estimated by counting and measuring particles in a high number of micrographs taken on different areas of the sample. As we can see, Pt particles present a heterogeneous distribution, with the highest number of particles (ca. 35%) being in the range of particle size of 5-6 nm.
XPS studies for TiO2 and Pt/TiO2 catalyst were also carried out, and a summary of the obtained results is shown in Table 2. For the platinized sample, it was especially useful to analyze the Pt 4f core level (4f7/2 and 4f5/2) (Figure 8). The region corresponding to Pt peaks may be deconvoluted into two components: one corresponding to Pt0 at binding energy of 74.5 eV and an other assigned to a partially oxidised Pt(δ+) at binding energy of 71.1 eV . In this way, it is possible to make an estimation of the fraction of Pt in the metallic state (Pt0) and in the oxidized state (Pt(2+)/Pt(4+)). The deconvolution of the peaks as achieved by using the program UNIFIT 2009  and the results are presented in Table 2 and in Figure 8. As it can be seen, the major part of platinum (ca. 70%) is present on the sample as metallic platinum (Pt0), while about the 30% of the metal was not totally reduced, and it was still present as oxidized forms (Pt(δ+)). The oxidation state of Pt particles on the TiO2 is one of the most important parameters influencing the improvement of the photocatalytic activity of TiO2 according to many previous reported results [43, 44], being Pt0 the state of the metal that appears to provide the highest enhancement of activity.
Regarding the analysis of the O 1 s region, a peak located at a binding energy of eV was registered in both samples, corresponding to lattice oxygen in TiO2, with a broad shoulder at higher binding energies ascribed to surface hydroxyl groups. This later shoulder was more pronounced for the Pt/TiO2 sample, indicating a higher degree of hydroxylation, in agreement with TG-DTG results. On the other hand, the XPS Ti 2p core level spectra were similar for TiO2 and Pt/TiO2 without significant changes in the binding energy of the peaks ( eV), corresponding to Ti4+ as the main component.
From XPS data, O/Ti ratios were calculated for sulphated TiO2 and Pt/TiO2 samples. For the pure sulphated TiO2, O/Ti value is 1.70, lower than the stoichiometric value (O/Ti = 2), indicating the presence of oxygen vacancies on the surface of this oxide. It has been reported that sulphated TiO2 presents a lower O/Ti ratio that nonsulphated TiO2, which indicates that the amount of oxygen vacancies is higher in the former samples . For Pt/TiO2 the O/Ti ratio is higher (O/Ti = 1.88), suggesting that the oxygen vacancies are partially annihilated during the photodeposition process.
3.2. Photocatalytic Tests
The evaluation of the photocatalytic activity was carried out by following the reaction of ethanol oxidative dehydrogenation in gas phase. All the photocatalytic tests started feeding the reaction gaseous mixture to the reactor in the dark, until the outlet ethanol concentration reached the equilibrium value, taken as initial value. Therefore, UVA-LEDs were switched on after the establishment of the dark adsorption equilibrium of ethanol on the catalyst surface. No reaction products were observed during or after the ethanol dark adsorption at 60°C, indicating that no selective ethanol oxidation occurs by thermal catalysis in the used operating conditions .
The photocatalytic activity of the pure (TiO2) and platinized oxide (Pt/TiO2) in the presence and absence of blue phosphors was evaluated for the reaction of ethanol dehydrogenation at a concentration of 0.2 vol%. Ethanol photocatalytic conversion on Pt/TiO2 as a function of the ethanol inlet concentration was also investigated. Results are presented in Figure 9. As it can be seen, the photocatalytic activity of TiO2 is slightly improved by the photodeposition of Pt. Thus, the ethanol conversion increased with the addition of platinum reaching a maximum of about 69% at 0.2 vol% of initial ethanol concentration. At higher ethanol concentration values, conversion progressively decreased going down to 7% for 2 vol% of ethanol. According to MS analysis, acetaldehyde was the main product together with low amounts of CO2, ethylene and crotonaldehyde as byproducts which were also detected.
The effect of metal deposition on the photocatalytic activity of TiO2 has been widely studied [25–30]. Noble metal nanoparticles deposited on the TiO2 surface are known to act as effective traps for photogenerated electrons due to the formation of a Schottky barrier at the metal-semiconductor contact. These electrons improve the rate of oxygen reduction and inhibit the electron-hole recombination even though the improvement of activity in this work has not been as accused as in other reported literature, being strongly depending on the considered substrate [27–31].
As it is also shown in Figure 9, the addition of blue phosphors notably increased the activity of the Pt/TiO2 catalyst for all the ethanol concentrations studied. In the same manner then for the tests without phosphors, total ethanol conversion decreased with the increase of ethanol concentration; however, as it has been said, for all ethanol concentrations analyzed, an important improvement of the photocatalytic activity of Pt/TiO2 catalyst with the addition of phosphors into the fluidized bed can be observed. This is due to the phosphors exploited as light carriers inside the photocatalytic core bed, giving a shorter optical pathlength to the radiation. Moreover, the photoactivity was enhanced, because the suitable phosphors introduced into the system were able to transform 365 nm radiation coming from UV-LEDs into 440 nm emission and able to photoexcited the fraction of photocatalyst in the core reactor volume, otherwise screened by the photocatalyst itself as previously found for V2O5/TiO2 .
Blank tests were also carried out under irradiation with the reactor loaded only with phosphors and glass spheres without catalyst showing only negligible ethanol consumption and acetaldehyde production. In this test, the ethanol conversion was less than 2%; therefore, the presence of the specific photocatalyst was necessary for the reaction .
In a same way, a blank test with an inlet ethanol concentration of 0.2 vol%, using only sulphated pure TiO2, phosphors, and glass spheres, was carried out, and we have found an ethanol conversion of 60%. The conversion is lower than that obtained without phosphors, because the phosphors act as screening for UV light decreasing the percentage of TiO2 effectively irradiated. This result underlines that Pt species are crucial to activate the catalysts at emission wavelength of phosphors.
With an inlet ethanol concentration of 0.2 vol%, the conversion levels for sulphated pure TiO2, Pt/TiO2, and Pt/TiO2 + Blue phosphors corresponded to values of 69%, 72%, and 84%, respectively. At higher initial inlet concentration, the increase in ethanol conversion in the presence of blue phosphors clearly evidenced the photon transfer limitations in the absence of phosphors that are overcame with Pt/TiO2 photocatalyst and able to catch the light emission carried by phosphors.
Selectivity trend for the different systems as a function of ethanol concentration is shown in Figure 10. It can be noticed that addition of Pt to the TiO2 remarkably increased the selectivity to acetaldehyde at the studied ethanol concentration of 0.2 vol%. On the other hand, the selectivity to acetaldehyde obtained with the Pt/TiO2 photocatalyst is very similar for the different ethanol concentrations evaluated, and in any case, it is notably higher than the selectivity obtained with the pristine TiO2.
These results suggest that the reaction mechanism for ethanol conversion follows different pathways when using pure or platinized TiO2, as it has been already reported by Siemon et al. for different substrates over platinized and nonplatinized commercial TiO2 . A hypothesis on the action of Pt in the reaction mechanism could be related to the already suggested formation of acetaldehydes radicals on TiO2. In fact, considering that in TiO2, ethanol is adsorbed as ethoxy specie and the formation of OH radicals under UV irradiation by reaction of hydroxyls with positive holes, the abstraction of hydrogen by the adsorbed ethoxy species results in adsorbed acetaldehyde radicals . Acetaldehyde radical could be transformed into adsorbed acetaldehyde by electron withdrawing by Pt nanoparticles then favouring the desorption of the product.
In the presence of blue phosphors, the main reaction product was also acetaldehyde, and the selectivity to this compound was also much higher than the one obtained with the pure TiO2. The acetaldehyde selectivity of Pt/TiO2 catalyst with or without the addition of blue phosphors is very similar, increasing slightly with the ethanol concentration, with values ranging 90%–95%.
The ethanol oxidative dehydrogenation in gas phase using sulphated TiO2 and Pt/TiO2 as catalysts has been studied. An efficient photocatalyst, active and selective to acetaldehyde in the ethanol dehydrogenation, can be obtained by modification of TiO2 by photodeposition of platinum nanoparticles. We can observe that the photocatalytic activity of TiO2 and selectivity to acetaldehyde can be improved by photodeposition of platinum. In the same way, the effect of the addition of blue phosphors to photocatalytic bed was evaluated. Experimental data evidenced that the presence of phosphors allowed an improving in the photocatalytic activity of Pt/TiO2, limited by the photon transfer, because a notable increase of ethanol conversion was observed.
This research was financed by the Spanish Ministerio Ciencia e Innovación (Project no. CTQ2008-05961-CO2-01) and Junta de Andalucía (Excellence Project no. P06-FQM-1406). J. J. Murcia thanks CSIC for the concession of a JAE grant and for financing the short stay no. 2011ESTCSIC-6717. The authors would like also to thank Lampugnani Sandblasting HI-TECH for the glass spheres utilized in this work.
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