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Advances in Materials Science and Engineering
Volume 2010, Article ID 681574, 15 pages
http://dx.doi.org/10.1155/2010/681574
Research Article

A Phenomenological Study on the Synergistic Role of Precious Metals and the Support in the Steam Reforming of Logistic Fuels on Monometal Supported Catalysts

Chemical Engineering Department, The University of Toledo, 2801 W. Bancroft St., Toledo, OH 43606-3390, USA

Received 30 January 2010; Accepted 8 May 2010

Academic Editor: Markku Leskela

Copyright © 2010 Abdul-Majeed Azad and Desikan Sundararajan. 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

Clean power source utilizing vast logistic fuel reserves (jet fuels, diesel, and coal) would be the main driver in the 21st century for high efficiency. Fuel processors are required to convert these fuels into hydrogen-rich reformate for extended periods in the presence of sulfur, and deliver hydrogen with little or no sulfur to the fuel cell stack. However, the jet and other logistic fuels are invariably sulfur-laden. Sulfur poisons and deactivates the reforming catalyst and therefore, to facilitate continuous uninterrupted operation of logistic fuel processors, robust sulfur-tolerant catalysts ought to be developed. New noble metal-supported ceria-based sulfur-tolerant nanocatalysts were developed and thoroughly characterized. In this paper, the performance of single metal-supported catalysts in the steam-reforming of kerosene, with 260 ppm sulfur is highlighted. It was found that ruthenium-based formulation provided an excellent balance between hydrogen production and stability towards sulfur, while palladium-based catalyst exhibited rapid and steady deactivation due to the highest propensity to sulfur poisoning. The rhodium supported system was found to be most attractive in terms of high hydrogen yield and long-term stability. A mechanistic correlation between the role of the nature of the precious metal and the support for generating clean desulfurized -rich reformate is discussed.

1. Introduction

Our energy infrastructure dictates continued and increased use of fossil fuels for the next several years and maybe decades. The use of domestic resources, such as coal, is especially attractive. Clean power source utilizing vast reserves of other logistic fuels (JP-8, Jet-A, kerosene and diesel) would be the main driver for high efficiency fuel cells. Hydrocarbon fuels such as diesel and jet fuel have logistics (well-established distribution network) and safety advantages for military applications compared to compressed hydrogen. Energy densities of both diesel and jet fuel JP-8 (11.8 and 11.1 kWh/Kg resp.) are much higher compared to lower hydrocarbons (natural gas). Furthermore, Diesel is a petroleum derived product that is a complex mixture of paraffins (n-, iso-, and cycloparaffins) and ring chained aromatic compounds (squaline, tetralin, and methyl naphthalenes). Reforming of diesel to produce hydrogen for solid oxide fuel cells (SOFCs) has great advantages, as the hydrogen density of diesel is very high. The reformate gas contains mainly CO, CO2, H2, and trace amount of H2S. The sulfur products can be removed by sulfur sorbents. A direct application of this technology is by installing SOFC-based auxiliary power unit (APU) in trucks to provide energy instead of relying on running diesel engine. Similarly, fuel cells running on jet fuel reformate are attractive to NASA for its uninhabited aerial vehicle and low emission alternative power missions. The US military has interest in fuel cells due to their lightweight, zero emission and low acoustic signatures in strategic zones. Jet fuels, such as Jet-A and JP-8 are already on board in many applications. If they can be reformed and used in SOFCs military missions can be made more effective. Hydrogen had been touted to be the ideal energy carrier of the future well over a century ago by Verne [1].

However, the infrastructure for fuel cell-based H2 economy is years or even decades away. Therefore, hydrocarbon fuels must first be converted into a hydrogen-rich stream. The catalysts used for reforming heavy hydrocarbons are poisoned by sulfur invariably present in logistic fuels as organosulfur, up to 0.3–4% (3000–40,000 wppm) [2]; the level of sulfur in coal varies between 2.3% and 4.5% depending upon the quality of the bituminous coal [3]. During reforming process, this sulfur is also likely to get transformed into H2S; if not removed effectively, H2S (<1 ppm) deactivates the SOFC anode rapidly [4]. Thus, fuel processors are required to convert logistic fuels into hydrogen-rich reformate for extended periods in the presence of sulfur, and deliver hydrogen with little or no sulfur to the fuel cell stack. In order to facilitate continuous uninterrupted operation of logistic fuel processors, robust sulfur-tolerant catalysts ought to be developed.

In the light of the recent quest for cleaner and greener energy, the use of hydrogen-rich reformates of logistic fuels such as Avgas (aviation gasoline), JP-5, JP-8, and Jet-A as the feed for PEMFCs and/or SOFCs for NASA and the Department of Defense, is attractive. Given the energy crisis in the US and especially in Southeast Asia in terms of its ever-rising demand, fuel cells running on clean hydrogen feeds derived from abundant logistic fuels, are attractive options, provided a strategy for effective desulfurization and sustained reforming via robust sulfur-tolerant catalysts is in place.

Much work has been done to better understand the sulfur poisoning of Ni- and Rh-based reforming catalysts [5, 6]. In presence of CeO2, Ru-supported Al2O3 catalyst showed high sulfur resistance in the steam reforming of kerosene [7]; sulfur tolerance increased from 0.1 to 50 ppm. Nevertheless, a highly active sulfur-tolerant catalyst for steam reforming of logistic fuels has not yet been developed.

Even though the key active metals used in the reforming catalysts are Pt, Pd, and Rh, no systematic study exists to compare their performance in the steam reforming of a given logistic fuel under identical experimental conditions. Nor is there a study where the definitive role of one precious metal over the other has been elucidated. Therefore, systematic investigation was undertaken to understand the role of precious metals in imparting sulfur-tolerance to the nanoscale ceria-based reforming catalysts in the process of hydrogen generation via steam reforming. The active metal dispersion in these sulfur-tolerant catalyst formulations was varied between 1 and 1.33 wt.%. The formulations contained either a single precious metal or a bimetallic or a tri-metallic combination. These formulations were assessed in terms of their sulfur-tolerance, phase integrity and the quality of reformate from kerosene (JP-8 surrogate) fuel at temperatures typically employed in reforming.

The rationale behind the choice of nanoscale ceria as support material in this work is as follows. While alumina (Al2O3) is a common support for a number of catalysts in oil refining due to its high thermal and chemical stability, there a great deal of rationale for using ceria supports in the present work. Ceria is commercially used in catalytic converters as a support for three-way catalyst for reducing harmful emissions from automobiles [8]. It is well known for its high oxygen storage capacity (OSC) and easy reducibility due to its facile equilibrium; CeO2-based materials are quite active and stable both in reducing and oxidizing atmospheres in high-temperature regimes compared to other conventional support systems [912]. This, in conjunction with a preparative technique that provides nanoscale particles, results in a high-surface-area catalyst with the possibility of high activity for the reforming of sulfur-laden fuels. When used as a support for precious metal(s), it results in the transfer of oxygen to the supported metals while the support is then reoxidized by the water present in the steam reforming reaction. Increased OSC leads to increase in oxygen vacancies which contribute to increased reducibility of ceria as well. Although, CeO2 is more active as a reforming catalyst than Ce2O3, it can act beneficially towards sulfur tolerance as well, as will be shown later.

Thermodynamically, in a reducing environment Ce2O3 has higher sulfidation equilibrium constant than CeO2, suggesting a possibility of formation of either cerium oxysulfide (partial sulfidation) or cerium sesquisulfide (complete sulfidation). This mechanism might help mitigate sulfur-mediated poisoning and deactivation of ceria-based catalysts in the long run. This sacrificial role may also allow precious metal(s) to remain active longer, resulting in better catalyst life. Furthermore, H2S adsorption on ceria is partially reversible. The thermodynamics of the CeO2-H2S reaction, however, do not allow the reduction of to below 200 ppm at about ; even at a temperature of , the H2S level cannot be reduced to below 100 ppm. This requires ceria to be in a reduced form (CeOn; ) to achieve the removal of sulfur to desired levels. Upon reduction, CeO2 goes to Ce2O3, thereby creating oxygen ion vacancies, and thus sulfidation of ceria occurs rapidly. In the light of these characteristics, ceria was used as a support matrix for the development of sulfur-tolerant catalytic formulations [1315].

This communication is the first in a series that highlights the performance of single metal-supported nanocatalysts (active metal loading: 1 wt.%) in the steam reforming of kerosene (a JP-8 surrogate) in terms of high (75%) hydrogen yield and excellent sulfur-tolerance over extended period of time on stream. A unique feature of these formulations was the unusually low levels of carbon monoxide, carbon dioxide and methane in reformate. A plausible mechanistic correlation between the role of the precious metals and the behavior of the catalyst formulations in terms of the above-mentioned features is also discussed.

2. Experimental Details

Nanoscale support material consisting of 10 mole % gadolinia (Gd2O3)-doped CeO2—hereafter referred to as GDC—was synthesized via ammonia precipitation using appropriate water soluble precursors for cerium and gadolinium (from Alfa-Aesar, MA) followed by hydrothermal treatment of the basic slurry thus obtained, in an autoclave at , for 2 h. The resulting material was dried overnight and systematic structural and morphological characterizations were carried out to follow the crystallization and phase evolution pathways as a function of calcination temperature-time profile.

The catalysts were formulated by dispersing precious metals onto the GDC support material, using their respective water soluble precursors: ruthenium (III) chloride (99.999%), palladium (II) nitrate (99.95%), and rhodium (III) nitrate hexahydrate (99.999%), all from Alfa-Aesar, MA. For this purpose, aqueous slurry of GDC and the precious metal precursors was homogenized in an evaporator for 4–6 h and the thickened mass was dried overnight in a vacuum oven at . The dried cake consisting of soft agglomerates was crushed using mortar and pestle, and pulverized in a ball mill using 2-propanol and 5-mm diameter zirconia media. The ball-milled slurry was dried again under ambient conditions and sieved through 325 mesh screen. Each formulation was calcined in air at for 2 h. These formulations contain, on metal weight basis, 1% of Ru (ruthenium), Pd (palladium) and (Rh) rhodium each and will be referred to as such.

The catalysts were characterized by a host of techniques with respect to their crystallinity, active metal dispersion, structural and morphological features. Temperature programmed reduction (TPR) of the as-prepared reforming catalysts was conducted using the Micromeritics AutoChem 2910 (Norcross, GA). TPR analysis was used to identify reduction temperatures of the noble metal active sites in the catalyst as well as their hydrogen uptake volume. This technique was used to optimize a systematic activation protocol for each catalyst prior to sulfidation and reforming experiments.

Independent sulfidation experiments were carried out by introducing 1000 ppm H2S-balance nitrogen stream over a bed of 1 g of powder catalyst at for 4 and 24 h. X-ray powder diffraction (XRD) signatures on the as-prepared and the postsulfided sample were collected on a Philips diffractometer (PW 3050/60 X’pert Pro), using monochromatic CuKα1 radiations () and Ni filter. The total amount of sulfur retained by the catalysts was quantified by wet chemical techniques by NSL Analytical, Cleveland, OH.

Steam-reforming experiments were carried out on powder samples (1 g) using kerosene as the fuel. LECO test established the sulfur content of this kerosene sample to be 260 ppm as determined by Paragon Laboratories, MI. The packed-bed reactor used here was a quartz tube, 19-in long and 1-in diameter. The catalyst was packed in a stainless steel filter with size pore from Swagelok. The filter containing the catalyst was placed in the center of the reactor and plugged with loosely packed quartz wool. The quartz reactor was then placed in a 1-in. ID split tube furnace equipped with a PID controller.

Water and fuel (kerosene) were pumped into the reactor with high precision HPLC pumps. Argon was used as a sweep gas for the reforming reaction. The steam to carbon ratio (S/C) was maintained at 3. The flow rates of water and fuel were adjusted to maintain a constant space velocity in the reactor at . The feed stream containing sweep gas, water, and fuel was first introduced into a preheater (vaporizer furnace) operated at to ensure complete vaporization of the feed. All reforming reactions were carried out at . The product stream coming out of the reactor was drawn into a condenser to separate the liquid and gas products. The liquid fraction consisted mainly of water and unreacted fuel with traces of other liquid hydrocarbons. The composition of the gas reformate mixture was analyzed via Gas Chromatograph (GC). Shimadzu GC, Model 2010 equipped with pulsed discharge helium ionization detector (PDHID) and flame photometric detector (FPD) was used to analyze the gas products. The FPD on the GC was facile in detecting sulfur species in the reformate stream. The duration of the experiment was determined by the stability and activity exhibited by the catalysts on stream. The hydrogen yield was defined as where Fi is the moles of species i produced per min, is the stoichiometric coefficient of species i, and Fj,0 is the molar flow rate of the reactant in moles/min.

The reforming tests were conducted over extended period of time and terminated as per a predetermined criterion of arbitrarily selected hydrogen yield reducing to 20% of its highest level or the sulfur level reaching 50 ppm (as H2S) in the exit stream. Concentration of other gases was also monitored simultaneously as a function of time. The GC was calibrated with respect to hydrogen, carbon monoxide, carbon dioxide, and methane—the major constituents in a typical reformate—over a wide range of concentrations. GC was also calibrated with respect to H2S concentration between zero and 1000 ppm.

Systematic microstructural analysis was performed by scanning electron microscopy (SEM-Phillips XL 30 FEG) and transmission electron microscopy (TEM-JEOL 3011). Both these techniques are equipped with the capability of performing energy dispersive X-ray spectroscopy (EDS) as well. The SEM is equipped with Rutherford’s back scattering electron mapping (RBSE) as well. The chemical state analysis of the active components in the postreformed samples was performed by using X-ray Photoelectron Spectroscopy on the Kratos Axis Ultra XPS.

3. Results and Discussion

3.1. Structural and Microstructural Artifacts of the Precious Metal-Supported Nanocatalysts

The morphological features of the as-prepared catalysts are shown in the TEM images of Figure 1; clearly all formulations possess nanofeatures. The contrasting light and dark particle contours in these images belong to the GDC support and the noble metals, respectively. This further suggests that despite low concentration, the dispersion of the catalytic metals is highly uniform throughout the matrix. Thus, the preparatory technique utilized for making them is quite effective in yielding uniform distribution of the precious metals throughout the support. The nanofeatures observed in the TEM images are also corroborated by the rather broad diffraction peaks in the XRD patterns of the powders calcined at for 2 h, shown in Figure 2.

fig1
Figure 1: TEM images of 1 wt% Ru (a), Pd (b), and Rh (c), supported GDC matrices. Scale bar: 5 nm.
681574.fig.002
Figure 2: XRD signatures of virgin monometal supported catalysts (calcined: /2 h).

The gross XRD patterns are typical of those for 10 mole% gadolina-doped ceria (GDC) used in this work [16]. Peaks for the precious meals are not discernable due to their low loading (1 wt.%), which is below the detection limit of the XRD technique. Estimation of the crystallite size using Scherrer’s equation, yielded values between 5 and 7 nm, which is in excellent agreement with those seen in the TEM images. The nanofeatures of these formulations ensure large surface area and effective noble metal dispersion. This lowers the loading and keeps the manufacturing cost low without compromising the performance.

3.2. Propensity to Sulfur Tolerance

With logistic fuels, sulfur tolerance of the reforming catalyst is of prime importance, as recent studies have shown irreversible sulfur poisoning to be the main cause of their deactivation [4]. Thus, it is imperative that the catalysts intended for the reforming of sulfur-laden fuels be evaluated for their sulfur tolerance first. Therefore, the catalysts synthesized in this work were first examined for their sulfur tolerance and phase integrity under conditions of sulfur exposures for extended period at the temperature employed for reforming (). Packed beds of the catalyst powders (1 g) were exposed to stream containing 1000 ppm H2S in nitrogen for 4 and 24 h, and the postsulfided samples were analyzed for sulfur pick-up. The results of 24 h soak are presented in Figure 3.

681574.fig.003
Figure 3: Quantification of sulfur pick-up by the formulations developed in this work.

The amount of sulfur sorbed by these formulations showed an interesting pattern. First, as expected, it is proportional to the soak-time. Second, in terms of the precious metal, the sulfidation susceptibility follows the pattern: Ru Rh Pd. This, in terms of reforming of a sulfur-laden fuel, means that the highest and the lowest sulfur-tolerance would be exhibited by Ru- and Pd-supported formulations, respectively. This prediction would be evaluated in the light of steam-reforming data reported later in the paper.

Figure 4 shows the TEM images of the postsulfided catalysts while their XRD patterns are shown in Figure 5.

fig4
Figure 4: TEM images of sulfided Ru (a), Pd (b), and Rh (c); bar: 5 nm.
681574.fig.005
Figure 5: XRD patterns of the catalysts sulfided for 24 h at in 1000 ppm H2S/bal. N2.

Clearly, there is evidence of grain growth in the samples sulfided for longer (24 h) duration; the particles have grown to about 10–12 nm in size, a fact which is corroborated by the peak narrowing seen in the XRD signatures. The peak sharpening is an indication of systematic crystallinity enhancement as a function of time at high temperature. In this case, the peak narrowing is an artifact of grain growth and crystallite size enhancement upon sulfidation. However, the crystallite texture is maintained; the crystallite size calculation and TEM images suggest that the grain growth in the sulfided samples is only nominal, indicating no significant surface area reduction occurred. It can also be seen that no new phases could be discerned, suggesting phase integrity was preserved under sulfidation at high temperatures. Moreover, the TEM images indicate some agglomeration as well, upon sulfidation. The obvious question is: does this grain growth and agglomeration adversely affect the reforming performance of these catalysts? The answer is no, as will be seen from the data reported in the following section.

3.3. Steam-Reforming Characteristics

As stated previously, all kerosene steam-reforming experiments were carried out at and atmospheric pressure (~14.7 psi) keeping the steam to carbon ratio constant at 3.0. The experiments were terminated when the hydrogen yield reached about 20% or the sulfur (as H2S) peaked to about 50 ppm in the reformate. Figure 6 compares the reformate compositions resulting from the steam reforming of kerosene on three monometal supported reforming catalysts, as a function of time on-stream.

fig6
Figure 6: Composition of kerosene reformates using: (a) Ru, (b) Pd, and (c) Rh.

The performance trend in term of hydrogen yield in these catalysts is characteristic of the precious metal on them, as seen from the data summarized in Table 1.

tab1
Table 1: Performance summary of the monometal-supported reforming catalysts.

Since the exact composition of the fuel (kerosene) is not precisely known, thermodynamic calculations were conducted on a close surrogate, which comprised of a mixture of 70% n-hexadecane and 30% toluene. This choice was based on similar assumptions used by others in the literature [17]. Equilibrium conversions were estimated by Invensys Pro/II Simulation Software Version 8.3 [18, 19]. The conditions chosen for simulation were identical to those employed in the reforming experiments: , , molar feed ratio of and GHSV of . Assuming complete conversion of the fuel, the equilibrium concentrations thus obtained for H2, CO, CO2, and CH4 were 78.34, 9.86, 11.27 and 0.53%, respectively. These simulation results are similar to those reported in the literature for n-hexadecane reforming [18, 20]. As can be seen, the reformate composition shown for the three calayst systems in Figure 6 are in good agreement with the equilibrium data obtained from simulation. The hydrogen content in the reformate stream was slightly lower than the equilibrium conversion. It may be recalled that traces of unreacted liquid fuel was seen in the condensate. This clearly suggests that fuel conversion was not complete as presumed in the thermodynamic calculations by the simulation technique.

Since kerosene contains 260 ppm of sulfur, it is imperative to understand the difference in performance of these catalysts in terms of their behavior towards sulfur as well. For that purpose, the sulfur level (as H2S) in the exit stream was also monitored whose concentration as a function of the progress of the reaction is shown in Figure 7.

681574.fig.007
Figure 7: Trend in H2S concentration build-up in the exit stream with time on-stream.

Several interesting aspects emerge. First, the steady-state hydrogen concentration in the reformate is ~70% in all but one case (viz., Pd-based catalyst) and stays that way for more than 50 h continuously. The Pd-based catalyst appears to deactivate quickly from the very beginning of the reforming process, as evidenced by the steadily declining level of hydrogen in the reformate, suggesting that in the case of Pd-supported GDC, the catalyst poisoning had begun very early on. The sulfur level in the exit drops to 2.03 ppm at the end of 11 h on-stream, beyond which it shows steady increase with a concomitant decline in hydrogen concentration. The characteristic “tilted L-shaped” locus of sulfur concentration with time is indicative of rapid and quantitative sulfur-poisoning of Pd-based catalyst. In the light of Figure 3, it can be inferred that the formation of thermodynamically stable PdS phase began from the very start. Therefore, at the first glance, it would appear that Pd-supported formulation is not a preferred reforming catalyst for a logistic fuel such as kerosene sulfur-laden to the extent of 260 ppm sulfur.

Nevertheless, the Pd-supported catalyst still continues to generate hydrogen at a reasonable albeit steadily diminishing rate. The distinct difference in slope seen in Figure 6(b) for hydrogen concentration is likely due to two somewhat different deactivation mechanisms. It is evident that the initial far quicker surface-dominated deactivation is followed by bulk sulfidation of the active metal in the later stages. The latter is diffusion-controlled and hence somewhat slower; at a given temperature ( in this case) it would have a finite value of the limiting rate constant.

Therefore, as seen from Figure 8, the rather high (84%) hydrogen yield calculated as per (1), in the early stage of steam reforming is due to the exceptionally high activity of the fresh Pd-supported catalyst. However, after about 4 h on-stream, it is believed that the surface sulfidation of palladium (hence more rapid decline in the yield) is completed and changes to bulk sulfidation (diffusion controlled). Thus, for the remainder of the duration on-stream, the deactivation becomes more monotonic.

681574.fig.008
Figure 8: Comparison of hydrogen yield in kerosene reformates using different catalysts.

In contrast, the performance of the Ru- and Rh-based catalysts is superior, both in terms of hydrogen yield and on-stream stability over extended period. The steady-state hydrogen yield is about 80% or higher for about 51–55 h in both the cases. It takes another 15 h for the amount of hydrogen to fall below the predetermined level of 20%. A comparison between Figures 7 and 8 brings out an interesting correlation between hydrogen and sulfur level in the kerosene reformate. The initial 3–5 h on-stream represent unsteady-state during which the sulfur level drops precipitously and then attains a low value which remains unaltered for about 50 h. This trend is reflected in the fluctuating hydrogen yield in the initial period, followed by a corresponding flat region for the next 50 h or so, marking the attainment of steady-state with respect to hydrogen generation as well.

In the light of Figures 7 and 8, certain distinct behavioral features of Ru and Rh-based catalysts become evident. For example, in the case of Ru-supported GDC, the hydrogen production in the early stages of reforming is somewhat lower (~71.5%) which increases to about 79% once the catalyst stabilizes and steady-state is achieved. This is in direct correspondence to the drastic drop in sulfur level during the first few hours, leveling off to about 3 ppm H2S (kerosene originally contains 260 ppm sulfur) for the remainder of the steady-state duration. A similar trend is seen in the case of Rh-supported GDC. However, in this case, the hydrogen level drops from ~87% in the early part of reforming to about 79% in the stead-state regime. Correspondingly, sulfur in the reformate levels off around 15 ppm. In both the cases, the decline in hydrogen yield beyond 54 h on-stream signals the catalytic deactivation which is corroborated by the concomitant increase in sulfur level in the reformate. With regard to steady-state hydrogen yield, the reformation capability of Ru- and Rh-based catalysts is almost identical. However, their sulfur-tolerance characteristics are different. In the case of Pd-supported catalyst, the quicker reduction and attainment of lower amount of residual sulfur in the reformate indicate better reforming capability together with superior sulfur tolerance. Moreover, as the hydrogen yield begins to decline probably due to sulfur-poisoning, the sulfur concentration in the exit stream increases rapidly and attains a new steady-state value of 50 ppm H2S. This is in contrast to Rh-based where the steady-state sulfur concentration in the reformate is somewhat higher (~15 ppm) than in Ru-based (~3 ppm) and increases steadily, once the catalyst deactivation has set in. Thus, Ru-based formulation appears to be more sulfur-tolerant than the Rh-based analog in the case of kerosene reforming.

This distinctive behavior of these formulations with regard to their tolerance to sulfur in a real logistic fuel such as kerosene has been observed perhaps for the first time. This prompts us to suggest the following phenomenological mechanism operative in the two cases.

In early stages, the hydrogen produced during steam-reforming acts as a hydrodesulfurization (HDS) precursor converting the organosulfur into H2S [2123].

Thiophene is used in the above example for illustrative purpose. Credence to this mechanism is derived from Figure 7, where H2S is detected from the very beginning of the reaction.

The hydrogen sulfide thus formed then partially reacts with nanoscale precious metals. In the case of palladium, the following reaction can be envisaged. In the case of ruthenium, it binds sulfur as ruthenium sulfide, RuSx [24]: In the case of Ru-supported GDC catalyst, the sulfidation could be viewed as a two-stage process, accounting for the two steady-states observed in Figure 7. Ruthenium compounds exhibit at least eight oxidation states, but the +2, +3, and +4 are the most common. One would expect these to be the most prevalent ones under conditions existing in reforming.

One could anticipate the occurrence of one or all of the following reactions.

The transition of the atomic ratio of ruthenium to sulfur from 1 : 1 (in RuS) to 1 : 1.5 (in Ru2S3) to 1 : 2 (in RuS2) as outlined in the above scheme is substantiated by the observation made by Pecoraro and Chianelli [22] that the sulfides of ruthenium exhibit exceptional HDS activity. A similar conclusion was arrived at by Ishiguro et al. [25] who examined the evolution of ruthenium sulfide nanoclusters on titania support.

On the other hand, as can be seen from Figure 7, the drop in sulfur concentration in the case of Rh-supported GDC catalyst is somewhat slower (~6 h) compared to ~3 h in the case of Ru-supported GDC catalyst. This could be ascribed to the single-step process, where the most stable rhodium sulfide, Rh2S3 is partially formed [26]. At this juncture it is relevant to ask the question: is the sulfidation of the precious metals as outlined above feasible as the key mechanism, responsible for the observed sulfur tolerance? The answer is no; while this scheme is feasible, it cannot possibly be the primary mechanism. Were this true, the sustained high hydrogen yields over long period as recorded in Figure 8 would not be possible. It stands to reason that the outstanding performance of the formulations developed in this work is borne out of the fact that there is another mechanism for sulfur resistance.

It is well-known that ceria itself is capable of forming an oxysulfide [2730]. Therefore, in reducing environments, due cognizance should be given to the highly favorable thermodynamics of CeO2-H2-H2S system. where = the reaction fraction of each species at the steady-state.

At , is computed to be 0.862; the value of for reaction (7b) and (7c) is also calculated and is found to be 0.797 and 0.769, respectively, at the same temperature.

The equilibrium concentration of various species calculated for reactions represented in (7a)–(7c) is compared with the experimentally obtained data (Figure 15) in Table 2.

tab2
Table 2: Steady-state concentration (%) of the reactants and products of methanation process.

Understandably, the scheme proposed in (7c) is likely to be realistically closer to the conditions prevailing in the reformate than by (7a) or (7b). Consequently, as seen from Table 2, there is an excellent agreement (within1%) between the theoretically predicted concentration of various components in the mixture and that determined experimentally, for reaction (7c). These results clearly explain and corroborate the rather high methane concentration and lower levels of CO and CO2 found in the kerosene reformates.

In the light of the foregoing discussion, several multifunctional aspects of the catalysts developed in this work become evident. The nanostructured monometal-supported ceria formulations are capable of steam-reforming sulfur-laden (260 ppm) kerosene, resulting in very high hydrogen yield. Some key features of the sulfidation and reforming experiments of the monometal-supported formulations are as follows:(a)Ruthenium-based formulation provided an excellent balance in hydrogen production and sulfur stability. (b)Palladium-based formulation sorbed highest amounts of sulfur and also exhibited rapid and steady deactivation, thus suggesting that palladium has the highest propensity to sulfur poisoning.(c)The rhodium-based formulation is most effective in hydrogen generation in terms of hydrogen yield and long-term stability.(d)Ceria support plays a crucial role in rendering the catalyst sulfur tolerant and allowing the generation of hydrogen in high yield.

The on-stream long-term stability of these catalysts is due to their superior sulfur-tolerance without the need of a priori desulfurization stage; in the case of a real fuel such as kerosene which is a surrogate of JP-8, this aspect has hitherto not been examined or reported in the literature. Fukunaga et al. [42] have discussed the performance of a combined Ni-based desulfurizer and a Ru-supported alumina catalyst in the steam reforming of kerosene containing 48–64 ppm sulfur at . Even with the sulfur level reduced to about 0.05 ppm by desulfurization, the hydrogen level was ~70% with CO between 12–15% and CO2 between 15–18% in the reformate. It can be reasonably argued that with pre-reforming desulfurization, the catalysts developed in this work would perform even better.

The catalysts are also capable of in-situ methanation of carbon monoxide and carbon dioxide in the presence of hydrogen—a feature also not reported in the literature with regard to fuel reforming. This aspect is unique and relevant in the context of eliminating CO and CO2, thereby obviating the need of separate and elaborate stages for CO removal and CO2 sequestration and/or capture. Fukunaga et al. [42] had employed a water-gas-shift reaction to remove CO from the stream.

The observed behavior of in-situ methanation is common to all the three formulations irrespective of the type of precious metal supported on the GDC matrix. Thus, it can be concluded that this is an artifact of the novel combination of the precious metal(s) with doped ceria support, both being endowed with nanofeatures. A recent study on methanation of carbon oxides used a catalyst that contains 9–16% Ru on -Al2O3 [43]. Methanation of CO in hydrogen rich gas mixture was also reported using catalysts containing 10% of several metals (Pt, Pd, Co, Ni, and Ru) over SiO2 support [44]. Some studies using 7–10% NiO on TiO2 and -Al2O3 for the methanation of carbon oxides in hydrogen-rich stream catalysts have also been reported [4547].

As can be seen, these studies employ very high loading of Ni and other noble metals, all of which are quite expensive (in addition to Ni being prone to carburization), whereas the catalysts developed in this research contain 1% of the noble metal and are capable of performing in-situ methanation in addition to generating hydrogen in high yield.

4. Conclusion

A new family of high performance monometal-supported ceria nanocatalysts has been developed and systematically characterized with respect to their structural, microstructural and sulfur-tolerance aspects. Steam-reforming of kerosene at and atmospheric pressure with steam-to-carbon (S/C) ratio equal to 3.0 on these formulations produces hydrogen-rich reformates over long durations before deactivation sets in, which is rather slow and monotonic than sudden and precipitous. The difference in the behavior of various formulations is explained in terms of the inherent ability of the ceria support to pick-up sulfur in the stream. This together with the sulfidation propensity of the precious metals dispersed on ceria is the reason behind excellent performance of these formulations. The catalyst deactivation appears mainly due to the sulfur poisoning rather than carburization (coke formation) based on the absence of noticeable level of elemental carbon in high resolution microscopic images in the postreformed samples. A mechanistic scheme for the phenomenon of in-situ methanation is also proposed which successfully explains the experimental data.

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