Iron−Ceria Aerogels Doped with Palladium as Water−Gas Shift

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Iron-Ceria Aerogels Doped with Palladium as Water-Gas Shift Catalysts for the Production of Hydrogen Sumit Bali,† Frank E. Huggins,‡ Richard D. Ernst,† Ronald J. Pugmire,† Gerald P. Huffman,‡ and Edward M. Eyring*,† Department of Chemistry, UniVersity of Utah, Salt Lake City, Utah 84112, and Consortium for Fossil Fuel Science and Department of Chemical and Materials Engineering, UniVersity of Kentucky, Lexington, Kentucky 40506

Mixed 4.5% iron oxide-95.5% cerium oxide aerogels doped with 1% and 2% palladium (Pd) by weight have been synthesized, and their activities for the catalysis of water-gas shift (WGS) reaction have been determined. The aerogels were synthesized using propylene oxide as the proton scavenger for the initiation of hydrolysis and polycondensation of a homogeneous alcoholic solution of cerium(III) chloride heptahydrate and iron(III) chloride hexahydrate precursor. Palladium was doped onto some of these materials by gasphase incorporation (GPI) using (η3-allyl)(η5-cyclopentadienyl)palladium as the volatile Pd precursor. Water-gas shift catalytic activities were evaluated in a six-channel fixed-bed reactor at atmospheric pressure and reaction temperatures ranging from 150 to 350 °C. Both 1% and 2% Pd-doped 4.5% iron oxide-95.5% cerium oxide aerogels showed WGS activities that increased significantly from 150 to 350 °C. The activities of 1% Pd-doped 4.5% iron oxide-95.5% cerium oxide aerogels were also compared with that of the 1% Pd-doped ceria aerogel without iron. The WGS activity of 1% Pd on 4.5% iron oxide-95.5% cerium oxide aerogels is substantially higher (∼5 times) than the activity of 1% Pd-doped ceria aerogel without iron. The gas-phase incorporation results in a better Pd dispersion. Ceria aerogel provides a nonrigid structure wherein iron is not significantly incorporated inside the matrix, thereby resulting in better contact between the Fe and Pd and thus enhancing the WGS activity. Further, neither Fe nor Pd is reduced during the ceria-aerogelcatalyzed WGS reaction. This behavior contrasts with that noted for other Fe-based WGS catalysts, in which the original ferric oxide is typically reduced to a nonstoichiometric magnetite form. 1. Introduction Ever-increasing demands for energy and uncertainty in supplies and prices of conventional fossil fuels have generated widespread interest in alternative energy systems. Among such sources, hydrogen is being evaluated as an alternative source of energy for fuel cells used to generate power and also for transportation. The water-gas shift (WGS) reaction (eq 1) is an effective way of producing hydrogen fuel CO + H2O h CO2 + H2

∆H ) -41.1 kJ mol-1

(1) The WGS reaction is primarily catalyzed by conventional Cr-Fe-based catalysts1 at temperatures above 350 °C, as well as Cu-Zn-based catalyst systems2 at temperatures below 250 °C. Ceria-based catalysts are considered to be good prospects for better water-gas shift activities because of the ability of ceria to undergo rapid reduction/oxidation cycles and because of its high oxygen storage capacity (OSC). As a result of this versatility, ceria-based catalysts combined with metals such as copper,3-5 gold,6,7 platinum,8,9 palladium,10 and so on, have been extensively studied for the WGS reaction owing to their higher activities. Further, it was reported by Zhao and Gorte11 that addition of Fe2O3 to a Pd-metal-promoted ceria catalyst increases the WGS activity many times, whereas addition of Fe2O3 to other ceria-precious-metal-based systems such as * To whom correspondence should be addressed. Tel.: +1 801 581 8658. Fax: +1 801 581 8433. Email: [email protected]. † University of Utah. ‡ University of Kentucky.

Ce-Pt and Ce-Rh has no effect on the activities. The substantial increase in the activity of ceria-Pd catalysts has been attributed to formation of a Fe-Pd alloy system.11 It has been reported11 that formation of the Fe-Pd alloy is a driving force for the reduction of iron oxide. With regard to potential supports for these applications, substantial effort has been directed toward the use of high-surface-area aerogels.12,13 These supports can be obtained from wet gels by using supercritical drying technique. In addition to their high surface areas, aerogels have considerably higher pore volumes with much lower densities than precipitated catalysts, thus making them ideal for applications in catalysis. Aerogels also have through-connected pore networks that can accommodate the rapid diffusive transport of gas-phase reactants and products.14,15 The combination of high surface area and rapid gas fluxes through the structure makes aerogels particularly attractive for heterogeneous catalysis. Their high surface areas and pore volumes have resulted in their use in applications both as catalysts and as supports for the incorporation of other metal species. Herein, we report the synthesis and WGS activity of Pd supported on ceria aerogels and mixed 4.5% iron oxide-cerium oxide aerogels. The aerogels were synthesized by supercritical CO2 drying of wet gels of ceria and 5% iron-doped ceria wet gels. The Pd was doped onto the ceria aerogels and iron-ceria aerogels by a facile and economical gas-phase incorporation (GPI) method.16 Water-gas shift activities were evaluated in a fixed-bed reactor at atmospheric pressure and reaction temperatures ranging from 150 to 350 °C. The activities of Pd-doped iron-ceria mixed aerogels were also compared with those of Pd-doped ceria aerogels without iron.

10.1021/ie901543w  2010 American Chemical Society Published on Web 01/08/2010

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2. Experimental Section FeCl3 · 6H2O (Sigma), CeCl3 · 7H2O, and anhydrous ethanol (Pharmco-AAPER 200 proof) were used as received. Propylene oxide (Aldrich) was passed through an alumina column (to remove the peroxides) before being used for the synthesis of wet gels of iron oxide. 2.1. Synthesis of Iron-Cerium Mixed Wet Gels. Mixed 4.5% iron oxide-95.5% cerium oxide wet gels were synthesized by a slight modification of the reported procedure17 using propylene oxide as the proton scavenger for the initiation of hydrolysis and polycondensation. CeCl3 · 7H2O was used as the Ce(III) precursor, and FeCl3 · 6H2O was used as the Fe(III) precursor for the synthesis of the mixed wet gels. In a typical synthesis, CeCl3 · 7H2O (2.98 g, 7.9 mmol) was dissolved in 14 mL of anhydrous ethanol. To this solution was added FeCl3 · 6H2O (0.20 g, 0.74 mmol). The solution was stirred until complete dissolution occurred to give a clear greenish-yellow solution. Propylene oxide (11 mL, passed freshly through an alumina column) was then added to this solution in one portion with vigorous stirring. After being stirred for ca. 7 min, the solution was transferred to 15 mL plastic cylindrical tubes that were then sealed with Seal-View. Light brown monoliths of mixed iron-ceria oxide gel were obtained within 10 min. The gels were allowed to age for 24 h under ambient conditions. 2.2. Processing of Iron-Cerium Mixed Oxide Wet Gels. The aged monoliths of mixed iron-cerium oxide gels were transferred into 50 mL cylindrical tubes containing anhydrous 2-propanol. Several solvent exchanges were performed starting with anhydrous 2-propanol (2 × 35 mL) and finally with acetone (8 × 35 mL) to obtain the acetone-filled wet gels. The relatively high miscibility with CO2 of acetone, as compared to that of 2-propanol, facilitated the supercritical drying of the wet gels to get aerogels. 2.3. Synthesis of Iron-Ceria Mixed Oxide Aerogels. The supercritical CO2 drying of the iron-ceria mixed oxide wet gels resulted in iron-ceria mixed oxide aerogels that retained the shape of the wet gels. In a typical supercritical CO2 drying procedure, which has been described previously,18 the wet acetone-filled gels were immersed in ∼250 mL of acetone and placed in a fabricated stainless steel (SS316) autoclave equipped with an inlet and venting valves to maintain the pressure within safe limits and a circulating hot water bath to increase the autoclave temperature to that required for supercritical CO2 conditions. The detailed procedure and a schematic diagram for supercritical CO2 drying have been reported earlier.18 The autoclave was filled with liquid CO2, and the acetone was vented as a liquid. The liquid CO2 and acetone were exchanged at a pressure of ∼1600 psi for 4 h until the effluent was substantially free of acetone. After this exchange, the valve was closed, and the temperature of the autoclave was increased above the critical point of CO2 (Tc ) 31 °C, Pc ) 7.4 MPa) to obtain supercritical CO2. After 1 h of static supercritical conditions, the CO2 was vented slowly over a period of 1 h to give monoliths of iron oxide mixed cerium oxide aerogels that retained the shape of the wet gels. 2.4. Gas-Phase Incorporation of Palladium on Iron-Ceria Oxide Aerogels. Palladium was deposited onto the iron-ceria mixed oxide aerogels by the gas-phase incorporation (GPI) technique. The compound (η3-allyl)(η5-cyclopentadienyl)palladium18,19 was the precursor. The iron-ceria mixed oxide aerogels were crushed and sieved for size (45-100 mesh) and dried under a vacuum (∼10-4 mm Hg) for 1 h at room temperature. An appropriate amount of metal precursor, (η3allyl)(η5-cyclopentadienyl)palladium (red solid; added in an

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amount sufficient to yield 1% and 2% Pd by weight), was mixed with the iron-ceria mixed oxide aerogels under a nitrogen atmosphere. The mixture was allowed to mix overnight in a rotary evaporator at atmospheric pressure to ensure homogeneity and complete incorporation. 2.5. Catalyst Characterization. 2.5.1. Surface Area Analysis. To evaluate the surface area of the synthesized 4.5% iron oxide-95.5% cerium oxide aerogel and its 1% and 2% Pddoped counterparts, nitrogen adsorption isotherms were determined using a Micromeritics Chemisorb instrument (model 2720). The isotherms were used to calculate the BrunauerEmmett-Teller (BET) specific surface area according to the Barrett-Joyner-Halenda (BJH) method. The measurements were carried out under a nitrogen flow (12 mL/min) for all samples. The samples were prepared before the BET surface area measurements by degassing the samples under a nitrogen flow of 10 mL/min at 100 °C. 2.5.2. X-ray Absorption Fine Structure (XAFS) Spectroscopy. XAFS spectra were collected at the K absorption edges of iron (7112 eV) and Pd (24350 eV) and at the LIII absorption edge of cerium (5723 eV) at beamline X-18B of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory, Upton, NY. After the WGS reaction, the spent catalysts from the reactor were transferred to glass vials that were then closed tightly. For XAFS spectroscopy, the catalyst samples, both as-prepared and spent catalysts (after the WGS reaction), were transferred to a small bag prepared from 6-µm-thick polypropylene for suspension in the X-ray beam. The sample was exposed to air during the transfer from the vials and during the analysis. However, there was no significant variation in consecutive scans of the samples to indicate any change in oxidation state as a result of exposure of the sample to air and the X-ray beam. Experimental samples consisted of powders held in the X-ray beam either in polypropylene baggies for fluorescence measurements (Fe, Pd) or in lightly smeared samples on tape for transmission measurements (Ce). A passivated implanted planar silicon (PIPS) detector was used to collect the fluorescent radiation. X-ray spectra were collected from as much as 200 eV below the appropriate absorption edge to more than 800 eV above the edge. Metallic foils of Fe, Pd, and Cr (for the Ce LIII edge) were used to calibrate the zero point of energy of the XAFS spectra. The XAFS spectra were divided into separate X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions and analyzed using the program SIXPack.20 Owing to the relatively poor signal/noise ratio, the EXAFS regions of the Ce spectra could not be profitably analyzed. However, the iron and palladium EXAFS regions were sufficiently robust that they could be mathematically manipulated by first separation of the EXAFS oscillations from the absorption step and conversion to a reciprocal space (k space) representation (χ spectrum). A Fourier transform was then applied to the k3-weighted χ spectrum to generate the radial structure function (RSF). 2.6. Catalyst Evaluation for WGS Activity. The Pd-doped 4.5% iron oxide-95.5% cerium oxide aerogels were evaluated for WGS activity using a six-channel fixed-bed reactor.21 In a given run, the reactors were charged with 100 mg of fresh catalyst, held in place with a Whatman QMA quartz fiber filter. The thermocouple was located inside the catalyst bed for temperature monitoring and control. The catalysts were tested for their WGS activities within the 150-350 °C range at atmospheric pressure. The average run time for each temperature was 12 h. Nitrogen, CO, and water were passed continuously

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Table 1. BET Surface Areas of the Catalysts catalyst

surface area (m2/g)

Fe-Ce aerogel 1% Pd-doped Fe-Ce aerogel 2% Pd-doped Fe-Ce aerogel

192 ( 8 151 ( 7 102 ( 8

at 1.412, 0.221, and 0.435 mmol/min, respectively. Hydrogen, CO2, and unreacted CO and H2O were analyzed by gas chromatography on a capillary column (HP-PLOT Q, 0.53-mm i.d. and 30-m length) using a thermal conductivity detector (TCD). 3. Results and Discussion The measured BET surface areas of the synthesized catalysts are given in Table 1. The 1% and 2% Pd on iron-ceria mixed oxide aerogels exhibit high surface areas of 151 and 102 m2 g-1, respectively. The surface area of undoped iron-ceria mixed aerogel was 192 m2/g. The surface areas of the Pd incorporated iron-ceria mixed aerogels are smaller in comparison to undoped iron-ceria mixed aerogels possibly due to Pd deposition on the surface of aerogels. The Ce XANES spectra of the two as-prepared iron-ceria mixed oxide aerogels and 2% Pd-doped iron-ceria mixed oxide aerogel samples differ significantly from that of the iron-ceria mixed oxide aerogel after the water-gas shift (WGS) reaction. The Ce LIII XANES spectra of the as-prepared undoped 4.5% iron oxide-95.5% cerium oxide aerogels (Ce-1A) and the 2% Pd-doped 4.5% iron oxide-95.5% cerium oxide aerogels (Ce2A), as well as the 2% Pd-doped 4.5% iron oxide-95.5% cerium oxide aerogels after the WGS reaction (Ce-2R), are shown in Figure 1a. After the WGS reaction the Ce XANES spectrum is very similar to that of CeO2. However, the enhanced intensity of the peak at lower energy at about 5726 eV indicates that the Ce in the as-prepared iron-ceria mixed oxide aerogel contains a significant fraction of Ce3+. This difference is accentuated by the difference XANES spectra shown in Figure 1b. These observations indicate that the ceria aerogels are not entirely Ce4+ in their as-prepared states. Rather, significant fractions of Ce remain as Ce3+ following preparation. However, during the WGS reaction, this Ce3+ component is oxidized to Ce4+ as the aerogel structure transforms to that of CeO2. Palladium XAFS spectra for the Pd-iron-ceria mixed oxide

aerogels before and after WGS reaction are shown in Figure 2. Corresponding spectra of Pd metal are also shown for comparison. The spectra show little change as a result of the WGS reaction, except that the Pd-O peak in the RSF spectrum is significantly more intense after reaction and the peaks in the Pd XANES spectrum are somewhat better defined. However, there appears to be no significant metallic Pd formed during the WGS reaction. This result for Pd in ceria aerogels contrasts with that noted previously18 for Pd in iron aerogels, in which the palladium oxide present in the original iron aerogel was partially reduced to metallic Pd during the WGS reaction. The aerogels were also investigated by iron XAFS spectroscopy. The iron XANES and EXAFS/RSF spectra for as-prepared undoped 4.5% iron oxide-95.5% cerium oxide aerogels (Ce1A) and 2% Pd-doped 4.5% iron oxide-95.5% cerium oxide aerogels (Ce-2A), as well as 2% Pd-doped 4.5% iron oxide-95.5% cerium oxide aerogels after WGS reaction (Ce-2R), are shown in Figure 3. The differences between the iron XANES and EXAFS/RSF spectra of the two as-prepared ceria aerogels are very minor, and it can be concluded that the spectra are essentially the same. The spectrum of the reacted sample is also similar to those of the as-prepared samples, although close inspection reveals subtle differences in the intensity of the preedge peak at about 7114 eV and in the shape of the main XANES peak at about 7135 eV. However, iron in all three samples appears to be exclusively ferric, and the lack of a second peak in the EXAFS/RSF spectra indicates that there is no significant long-range order in these samples, consistent with the amorphous nature of the aerogels. Even if the ceria aerogel collapsed to bulk CeO2 during the WGS reaction, it would appear that the iron is not significantly incorporated into the ceria matrix, at least not in any systematic manner. Furthermore, there is no compelling evidence for significant reduction of the iron as a result of exposure to WGS reaction conditions. The activities of the 1% and 2% Pd-doped iron-ceria mixed oxide aerogels were compared with those of the undoped iron-ceria mixed oxide aerogels and also Pd-doped ceria aerogels without iron. The activities of all of the catalysts at different temperatures are shown in an Arrhenius plot in Figure 4. A plot of the percent CO conversion versus temperature for all of the catalysts is shown in Figure 5.

Figure 1. Ce LIII XANES spectra of the as-prepared 4.5% iron oxide-95.5% cerium oxide aerogel (Ce-1A) and 2% Pd on 4.5% iron oxide-95.5% cerium oxide aerogel (Ce-2A), as well as 2% Pd on the 4.5% iron oxide-95.5% cerium oxide aerogel after WGS reaction (Ce-2R). (b) Ce XANES difference spectra obtained by subtracting the Ce XANES spectrum of sample Ce-2R from that of sample Ce-2A. The enhanced peak at about 5.726 eV indicates the presence of Ce3+ in the as-prepared aerogels.

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Figure 2. Pd XANES and EXAFS/RSF spectra of the as-prepared 2% Pd on 4.5% iron oxide-95.5% cerium oxide aerogel, 2% Pd on 4.5% iron oxide-95.5% cerium oxide aerogel after WGS reaction, and metallic palladium.

Figure 3. Fe XANES and EXAFS/RSF spectra of 4.5% iron oxide-95.5% cerium oxide aerogel (Ce-1A) and 2% Pd on 4.5% iron oxide-95.5% cerium oxide aerogel (Ce-2A), as well as 2% Pd on 4.5% iron oxide-95.5% cerium oxide aerogel after WGS reaction (Ce-2R).

Figure 4. Arrhenius plot of ln R (micromoles of CO per gram of catalyst per second) vs 1/T for 1% and 2% Pd doped onto iron-ceria mixed aerogels and 1% Pd doped onto ceria aerogel (without iron) in the WGS reaction.

The undoped iron-ceria mixed oxide aerogels exhibit no activity toward the WGS reaction at all temperatures. However,

Figure 5. CO conversion (%) versus temperature (°C) for various catalysts.

the 1% and 2% Pd-doped iron-ceria mixed oxide aerogels show WGS activity at low temperatures, and the activity increases as the temperature increases from 150 to 350 °C. The activity

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of 1% Pd-doped iron-ceria mixed oxide aerogels was also compared with that of 1% Pd-doped ceria aerogel without iron. It can be observed that the WGS activity of Pd on iron-doped cerium aerogel is substantially higher than the activity of Pddoped ceria aerogel without iron. The activity increased by almost a factor of 5 for 1% Pd doped on iron-doped ceria aerogel as compared to 1% Pd doped on ceria aerogel (without iron). The activation energies calculated from the Arrhenius plot of ln R vs 1/T were found to be 81.6 ( 8.0, 70.2 ( 7.0, and 68.2 ( 2.0 kJ/mol for 1% Pd on ceria aerogel, 1% Pd on iron-ceria mixed oxide aerogel, and 2% Pd on iron-ceria mixed oxide aerogel, respectively. Such enhancements in activities for bimetallic catalysts have been shown to be a result of the formation of the metal alloys (e.g., Ni Cu alloys in reforming of CO2),22 the combination of a reduced metal and a metal oxide (e.g., Cu and ZnO in syngas for DME synthesis),23 or the interaction of two metal oxides (e.g., molybdenum and bismuth oxides in the oxidation of butadiene to furan).24 Furthermore the enhancement of WGS activity by Fe-Pd bimetallic systems has also been shown to depend on the synthesis procedure of the bimetallic catalyst, and contact between Fe and Pd has been reported25 to be detrimental to activity. It has also been reported25 that no enhancement in the WGS activity is observed for catalysts prepared by the sol-gel technique (Fe present in the bulk of the ceria) and for catalysts prepared by physical mixing of iron oxide and cerium oxide followed by wet impregnation of palladium. Based on the iron EXAFS/RSF spectra, it was concluded in the present study that the iron is not significantly incorporated into the nonrigid ceria aerogel matrix. The Pd and Fe are found exclusively as Pd2+ and Fe3+, respectively, in the as-prepared aerogel samples. Even after exposure to WGS reaction conditions, the Fe and Pd spectra indicate that the oxidation states of these elements are not significantly altered by the reaction conditions and that they exist as Pd2+ and Fe3+. Use of the gas-phase incorporation technique to dope palladium provides better dispersion of Pd on the aerogel. This fact, combined with the presence of iron in nonrigid ceria, possibly contributes to a better interaction between palladium and iron, resulting in enhanced WGS activity of Pd-doped iron-ceria mixed oxide aerogels in comparison to Pd-doped ceria aerogels. 4. Conclusions Mixed 4.5% iron oxide-95.5% cerium oxide aerogels having high surface areas were synthesized and doped with 1% and 2% by weight of Pd using the versatile gas-phase incorporation technique. The water-gas shift activities of 1% and 2% Pddoped iron-ceria mixed oxide aerogels were found to increase with increasing temperature from 150 to 350 °C. Further, the activity of 1% Pd-doped 4.5% iron oxide-95.5% cerium oxide was almost 5 times greater than the activity of 1% Pd-loaded cerium oxide aerogels without iron. Furthermore, both Fe and Pd show no signs of reduction during the ceria-aerogel-catalyzed WGS reaction, consistent with Ce becoming fully oxidized during the reaction. This behavior contrasts with that noted for Fe-based catalysts in the WGS reaction, in which the original ferric oxide is typically reduced to a nonstoichiometric magnetite.11 Acknowledgment The authors are grateful for funding from the U.S. Department of Energy, Office of Fossil Fuel Energy, under Contract DEFC26-05NT42456. The authors acknowledge the generous

contribution to this study of Dr. N. Marinkovic of the University of Delaware and the Synchrotron Catalyst Consortium who acquired the Ce XAFS data. The XAFS measurements at beamline X-18B of the National Synchrotron Light Source, Brookhaven National Laboratory, were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-98CH10886. Literature Cited (1) Martos, C.; Dufour, J.; Ruiz, A. Synthesis of Fe3O4-based Catalysts for the High-Temperature Water Gas Shift Reaction. Int. J. Hydrogen Energy 2009, 34, 4475. (2) Ayastuy, J. L.; Gutierrez-Ortiz, M. A.; Gonzalez-Marcos, J. A.; Aranzabal, A.; Gonzalez-Velasco, J. R. Kinetics of the Low-Temperature WGS Reaction over a CuO/ZnO/Al2O3 Catalyst. Ind. Eng. Chem. Res. 2005, 44, 41. (3) Pradhan, S.; Satyanarayana Reddy, A.; Devi, R. N.; Chilukuri, S. Copper-based Catalysts for Water Gas Shift Reaction: Influence of Support on Their Catalytic Activity. Catal. Today 2009, 141, 72. (4) Kumar, P.; Idem, R. A Comparative Study of Copper-Promoted Water-Gas-Shift (WGS) Catalysts. Energy Fuels 2007, 21, 522. (5) Zerva, C.; Philippopoulos, C. J. Ceria Catalysts for Water Gas Shift Reaction: Influence of Preparation Method on their Activity. Appl. Catal. B 2006, 67, 105. (6) Andreeva, D.; Ivanov, I.; Ilieva, L.; Abrashev, M. V.; Zanella, R.; Sobczak, J. W.; Lisowski, W.; Kantcheva, M.; Avdeev, G.; Petrov, K. Gold Catalysts Supported on Ceria Doped by Rare Earth Metals for Water Gas Shift Reaction: Influence of the Preparation Method. Appl. Catal. A 2009, 357, 159. (7) Burch, R. Gold Catalysts for Pure Hydrogen Production in the Water-Gas Shift Reaction: Activity, Structure and Reaction Mechanism. Phys. Chem. Chem. Phys. 2006, 8, 5483. (8) Germani, G.; Schuurman, Y. Water-Gas Shift Reaction Kinetics over µ-Structured Pt/CeO2/Al2O3 Catalysts. Reactor Kinetics. React. Kinet. Catal. 2006, 52, 1808. (9) Kim, T. Y.; Park, E. D.; Lee, H. C.; Lee, D.; Lee, K. H. WaterGas Shift Reaction over Supported Pt-CeOx Catalysts. Appl. Catal. B: EnViron. 2009, 90, 45. (10) Bakhmutsky, K.; Zhou, G.; Timothy, S.; Gorte, R. J. The WaterGas-Shift Reaction on Pd/Ceria-Praseodymia: The Effect of Redox Thermodynamics. Catal. Lett. 2009, 129, 61. (11) Zhao, S.; Gorte, R. J. The Activity of Fe-Pd Alloys for the WaterGas Shift Reaction. Catal. Lett. 2004, 92, 75. (12) Pierre, A. C.; Pajonk, G. M. Chemistry of Aerogels and Their Applications. Chem. ReV. 2002, 102, 4243. (13) Husing, N.; Schubert, U. AerogelssAiry Materials: Chemistry, Structure, and Properties. Angew. Chem., Int. Ed. 1998, 37, 22. (14) Wallace, J. M.; Rice, J. K.; Pietron, J. J.; Stroud, R. M.; Long, J. W.; Rolison, D. R. Silica Nanoarchitectures Incorporating Self-Organized Protein Superstructures with Gas-Phase Bioactivity. Nano Lett. 2003, 3, 1463. (15) Leventis, N.; Elder, I. A.; Rolison, D. R.; Anderson, M. L.; Merzbacher, C. I. Durable Modification of Silica Aerogel Monoliths with Fluorescent 2,7-Diazapyrenium Moieties Sensing Oxygen Near the Speed of Open-Air Diffusion. Chem. Mater. 1999, 11, 2837. (16) Turpin, G. C.; Dunn, B. C.; Fillerup, E.; Shi, Y.; Dutta, P.; Singh, V.; Seehra, M.; Pugmire, R. J.; Eyring, E. M.; Ernst, R. D. Proc. Annu. Int. Pittsburgh Coal Conf. 2006, 23, 23.2/1. (17) Laberty-Robert, C.; Long, J. W.; Lucas, E. M.; Pettigrew, K. A.; Stroud, R. M.; Doescher, M. S.; Rolison, D. R. Sol-Gel-Derived Ceria Nanoarchitectures: Synthesis, Characterization, and Electrical Properties. Chem. Mater. 2006, 18, 50. (18) Bali, S.; Turpin, G. C.; Ernst, R. D.; Pugmire, R. J.; Singh, V.; Seehra, M. S.; Eyring, E. M. Water Gas Shift Catalysis Using Iron Aerogels Doped with Palladium by the Gas-Phase Incorporation Method. Energy Fuels 2008, 22, 1439. (19) Tatsuno, Y.; Yoshida, T.; Otsuka, S. Inorganic Syntheses: Reagents for Transition Metal Complex and Organometallic Syntheses; Wiley: New York, 1990; Vol. 28, p 342. (20) Webb., S. M. SIXPack: A Graphical User Interface for XAS Analysis Using IFEFFIT. Phys. Scripta 2005, T115, 1011. (21) Dunn, B. C.; Kim, D. J.; Webster, M.; Gasser, J.; Turpin, G. C.; Ernst, R. D.; Eyring, E. M. Presented at the 19th Annual Technical Meeting CFFS C1 Chemistry, Roanoke, WV, Jul 31-Aug 3, 2005.

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ReceiVed for reView October 1, 2009 ReVised manuscript receiVed November 11, 2009 Accepted December 19, 2009 IE901543W