Energy & Fuels 2008, 22, 1439–1443
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Articles Water Gas Shift Catalysis Using Iron Aerogels Doped with Palladium by the Gas-Phase Incorporation Method Sumit Bali,† Gregory C. Turpin,† Richard D. Ernst,† Ronald J. Pugmire,† Vivek Singh,‡ Mohindar S. Seehra,‡ and Edward M. Eyring*,† Department of Chemistry, UniVersity of Utah, Salt Lake City, Utah 84112, and Department of Physics, West Virginia UniVersity, Morgantown, West Virginia 26506 ReceiVed NoVember 16, 2007. ReVised Manuscript ReceiVed February 14, 2008
Iron aerogels possessing high surface areas (∼422 m2 g-1) were synthesized and characterized. Catalytic activity of iron aerogels doped with 1% and 2% palladium (Pd) by weight in the water gas shift (WGS) reaction was investigated. Iron aerogels were synthesized using propylene oxide as the proton scavenger for the initiation of hydrolysis and polycondensation of iron(III) chloride hexahydrate precursor. Palladium was doped onto iron aerogels by the gas-phase incorporation (GPI) technique using (η3-allyl)(η5-cyclopentadienyl)palladium as the volatile organometallic 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 iron aerogels showed good WGS activity and conversion of CO compared to undoped iron aerogels. The WGS activity of 2% Pd iron aerogel was higher by 50% than that of 1% Pd incorporated iron aerogel.
1. Introduction Hydrogen is being projected as a promising and clean source of energy for use in fuel cell systems1 for generation of power as well as for use in the transportation sector. One hindrance to everyday application of the fuel cell technology is the need for development of a sustainable source of pure hydrogen for fuel cell applications. Hydrogen is primarily obtained from steam reforming or dry reforming (in presence of carbon dioxide) of natural gas2,3 as well as steam reforming of methanol,4–6 gasoline,7–9 and diesel oil.8 Hydrogen obtained by reforming typically contains carbon monoxide in the range of 5–20%. This CO impurity can poison the anode catalyst in the fuel cell system with a resulting loss of performance. The water gas shift (WGS) reaction of eq 1 offers a useful way for generating hydrogen from carbon monoxide * To whom correspondence should be addressed. Telephone: +1 801 581 8658. Fax: +1 801 581 8433. E-mail:
[email protected]. † University of Utah. ‡ West Virginia University. (1) Morgenstern, D. A.; Fornango, J. P. Energy Fuels 2005, 19, 1708– 1716. (2) Hennings, U.; Reimert, R. Appl. Catal., A.: Gen. 2007, 325, 41–49. (3) Seo, J. G.; Youn, M. H.; Song, I. K. J. Power Sources 2007, 168, 251–257. (4) Patel, S.; Pant, K. K. Fuel Process. Technol. 2007, 88, 825–832. (5) Barthos, R.; Solymosi, F. J. Catal. 2007, 249, 289–299. (6) Chen, G.; Li, S.; Li, H.; Jiao, F.; Yuan, Q. Catal. Today 2007, 125, 87–102. (7) Murata, K.; Saito, M.; Inaba, M.; Takahara, I. Appl. Catal., B: EnViron. 2007, 70, 509–514. (8) Inyong, K.; Joongmyeon, B.; Gyujong, B. J. Power Sources 2006, 163, 538–546. (9) Ferrandon, M.; Krause, T. Appl. Catal., A: Gen. 2006, 311, 135– 145.
CO + H2O h CO2 + H2
∆ H ) -41.1 kJ mol-1
(1)
The WGS reaction is facilitated by a variety of catalyst systems including catalysts based on copper10 and zinc11 as well as those based on iron and chromium.12 Activity patterns for the WGS reaction over precious metal catalysts have lately received considerable attention as they provide efficient conversions even at low CO concentrations.13–18 Au-incorporated Fe2O3 catalyst has been studied for the WGS activity.19,20 In the presence of Au particles, the Fe2O3 support is readily reduced and this contributes to the catalyst performance in the WGS reaction. The WGS activity has also been shown to increase because of Fe-Pd interactions and it has been observed by Zhao that (10) Ko, J. B.; Bae, C. M.; Jung, Y. S.; Kim, D. H. Catal. Lett. 2005, 105, 157–161. (11) Ayastuy, J. L.; Gutierrez-Ortiz, M. A.; Gonzalez-Marcos, J. A.; Aranzabal, A.; Gonzalez- Velasco, J. R. Ind. Eng. Chem. Res. 2005, 44, 41–50. (12) Lei, Y.; Cant, N. W.; Trimm, D. L. J. Catal. 2006, 239, 227–236. (13) Fu, Q.; Weber, A.; Flytzani-Stephanopoulos, M. Catal. Lett. 2001, 77, 87–95. (14) Lei, Y.; Cant, N. W.; Trimm, D. L. Catal. Lett. 2005, 103, 133– 136. (15) Andreeva, D.; Ivanov, I.; Ilieva, L.; Sobczak, J. W.; Avdeev, G.; Petrov, K Top. Catal. 2007, 44, 173–182. (16) Andreeva, D.; Ivanov, I.; Ilieva, L.; Abrashev, M. V. Appl. Catal., A: Gen. 2006, 302, 127–132. (17) Kim, C. H.; Thompson, L. T. J. Catal. 2006, 244, 248–250. (18) 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. 23rd Annu. Int. Pittsburgh Coal Conf. 2006, 23.2/1–23.2/13. (19) Aeijelts Averink Silberova, B.; Mul, G.; Makkee, M.; Moulijn, J. A. J. Catal. 2006, 243, 171–182. (20) Hua, J.; Zheng, Q.; Zheng, Y.; Wei, K.; Lin, X. Catal. Lett. 2005, 102, 99–108.
10.1021/ef700691z CCC: $40.75 2008 American Chemical Society Published on Web 03/28/2008
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addition of Fe2O3 to a Pd metal promoted ceria catalyst substantially increases the WGS activity probably due to formation of a Fe-Pd alloy system.21 The stable form of iron under WGS conditions in the absence of Pd is Fe3O4, and it has been reported21 that formation of Fe-Pd alloy is a driving force for the reduction of iron oxide. With regard to potential supports for the WGS reaction, aerogel applications in chemistry have become widespread.22,23 Aerogels are obtained by supercritical drying of synthetic wet gels. Aerogels have high surface areas and pore volumes with considerably lower densities compared to precipitated catalysts. High surface areas and pore volumes of aerogels have resulted in their use in catalyst applications both as such and as supports for incorporation of other metal species. Iron oxide wet gels are synthesized by a simple room temperature sol–gel method using an epoxide as the gelation agent of the Fe(III) precursor.24–26 Supercritical carbon dioxide drying of these wet gels yields strong monoliths of iron oxide aerogels which retain the shape of the wet gels. Iron aerogels have high surface areas with remarkably low densities.25 The mechanical properties of iron aerogels have been studied by Gash et al.25 who report that the iron aerogels are comparatively stiffer than the alumina and silica aerogels of comparable densities. Gash et al.25 have reported that β-FeOOH aerogels can be converted into R-Fe2O3 aerogels by sintering the monolithic cylinders of β-FeOOH at temperatures as high as 515 °C without shattering the monolithic cylinders. Owing to the high mechanical stability and stiffness, iron aerogels can be handled easily and even machined to some extent thus making them good materials for catalysis applications both as such as well as support for impregnation of other metal species. Here we report the synthesis and WGS activity of 1% and 2% Pd supported onto iron aerogels. The Pd has been supported onto the iron aerogels by using facile and economical gas-phase incorporation (GPI) method.18 Water gas shift catalytic activities have been evaluated in a six-channel fixed-bed reactor at atmospheric pressure and reaction temperatures ranging from 150 to 350 °C. 2. Experimental Section FeCl3 · 6H2O (Sigma) and ethanol (Aaper, absolute 200 proof) were used as received. The palladium precursor (η3-allyl)(η5cyclopentadienyl)palladium was synthesized by literature methods.28,29 Propylene oxide (Aldrich) was passed through an alumina column before being used for synthesis of alcogels of iron oxide. 2.1. Synthesis of Iron Oxide Gels. Iron aerogel was synthesized by a slight modification of the reported procedure24 using propylene oxide as the proton scavenger for the initiation of hydrolysis and polycondensation. FeCl3 · 6H2O was used as the Fe(III) precursor for the synthesis of the aerogels. A typical synthesis begins with the dissolution of FeCl3 · 6H2O (0.43 g, 1.5 mmol) in 3.5 mL of ethanol and mixing with water (0.1 mL). Propylene oxide (0.8 g, (21) Zhao, S.; Gorte, R. J. Catal. Lett. 2004, 92, 75–80. (22) Pierre, A. C.; Pajonk, G. M. Chem. ReV. 2002, 102, 4243–4265. (23) Husing, N.; Schubert, U. Angew. Chem., Int. Ed. 1998, 37, 22–45. (24) Long, J. W.; Logan, M. S.; Rhodes, C. P.; Carpenter, E. E.; Stroud, R. M.; Rolison, D. R. J. Am. Chem. Soc. 2004, 126, 16879–16889. (25) Gash, A. E.; Satcher, J. H., Jr.; Simpson, R. L. Chem. Mater. 2003, 15, 3268–3275. (26) Gash, A. E.; Tillotson, T. M.; Satcher, J. H., Jr.; Poco, J. F.; Hrubesh, L. W.; Simpson, R. L. Chem. Mater. 2001, 13, 999–1007. (27) Dunn, B. C.; Cole, P.; Covington, D.; Webster, M. C.; Pugmire, R. J.; Ernst, R. D.; Eyring, E. M.; Shah, N.; Huffman, G. P. Appl. Catal. 2005, 278, 233–238. (28) Ernst, R. D.; Eyring, E. M.; Turpin, G. C.; Dunn, B. C. U.S. Patent Application, 2006. (29) Tatsuno, Y.; Yoshida, T.; Otsuka, S. Inorg. Synth. 1990, 28, 342– 345.
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Figure 1. Schematic representation of supercritical drying procedure. Table 1. BET Surface Areas of Iron Aerogels at Different Calcination Temperatures calcination temp (°C)
surface area of iron aerogel (m2/g)
as prepared (uncalcined) 260 ( 0.5 300 ( 0.5 350 ( 0.5 450 ( 0.5
422 ( 8 402 ( 8 293 ( 6 247 ( 5 82 ( 2
Table 2. BET Surface Areas of 1% and 2% Pd-Doped Iron Aerogels catalyst
calcination temp (°C)
surface area (m2/g)
FePd(1%)uncal FePd(1%)cal FePd(2%)uncal FePd(2%)cal
as prepared (uncalcined) 260 ( 0.5 as prepared (uncalcined) 260 ( 0.5
409 ( 8 363 ( 7 405 ( 8 348 ( 7
1.3 mmol) was added to this solution in one portion with stirring. (The molar ratio of propylene oxide to Fe(III) precursor was 7.5: 1.) After stirring for ca. 5 min, the solution was transferred to a 15 mL plastic cylindrical tube which was sealed with Seal-View film. Dark red colored monoliths of iron oxide gel were obtained within 10 min. The gels were allowed to age for 24 h and were then transferred into beakers containing ethanol (35 mL). Several solvent exchanges were performed starting with anhydrous ethanol (4 × 35 mL) and finally with acetone (8 × 35 mL) to get the acetonefilled wet gels of iron oxide. 2.2. Processing of Iron Oxide Gels. Supercritical drying of the gels under CO2 yielded dark red monoliths of iron aerogels.27 The schematic diagram for supercritical drying of iron aerogels is shown in Figure 1. In the supercritical drying procedure, the acetone-filled wet gels were immersed in a known quantity (250 mL) of acetone and placed in a stainless steel (SS316) autoclave. The autoclave was equipped with inlet valve I1 and venting valves V1 and V2 to control pressure within safe limits and a hot water circulator bath to circulate hot water around the autoclave to increase the temperature required for supercritical CO2 conditions. The autoclave was filled with CO2 up to a pressure of ∼600 psi through I1. After this, I1 was closed and the acetone was vented as a liquid from the venting valve V1. The autoclave was then filled with liquid CO2 up to a pressure of ∼1600 psi. To replace the acetone and fill the pores of the wet iron oxide gels with liquid CO2, exchange was carried out by keeping the inlet valve I1 and the venting valve V2 open to vent the acetone with CO2 while continuously filling in fresh liquid CO2. During this exchange the pressure was maintained at ∼1600 psi. The exchange process was carried out for ∼4 h until
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Energy & Fuels, Vol. 22, No. 3, 2008 1441 Scheme 1. Synthesis of 1% and 2% Pd by Weight Incorporated Iron Aerogels
the effluent was substantially free of acetone. After this, the inlet valve I1 was closed and the temperature of the autoclave was increased beyond the critical point of CO2 (Tc ) 31 °C; Pc ) 1100 psi) to obtain supercritical CO2. During this the pressure of the autoclave increased rapidly and it was controlled using the venting valve V2 to maintain a pressure of ∼1600 psi. After keeping the autoclave at the supercritical conditions for 1 h, the pressure of the autoclave was reduced slowly by venting CO2 through V2, while maintaining the temperature above the critical temperature CO2 to avoid the liquid–vapor interface. Dark red monoliths of iron oxide aerogels which retained the shape of the wet gels were obtained after complete venting of CO2. For calcined samples the thermal treatment of iron aerogels was carried out in static ambient air. The aerogels were heat treated in a glass dish in a programmable furnace by ramping at 1 °C/min, holding at the predetermined temperature of calcination for 4 h and subsequently cooling to ambient temperature. (See Tables 1 and 2 and Scheme 1 for calcination temperatures.) 2.3. Gas-Phase Incorporation of Palladium onto Iron Aerogels. Palladium was supported onto the iron aerogel by gas-phase incorporation. The palladium compound (η3-allyl)(η5-cyclopentadienyl)palladium28,29 was the precursor. The iron aerogels were crushed and sieved for size (45–100 mesh) and were dried under vacuum (∼10-4 mm Hg) for 1 h. An appropriate amount of Pd precursor, (η3-allyl)(η5-cyclopentadienyl)palladium (red solid), was mixed with the iron aerogel under a N2 atmosphere. The gradual disappearance of the Pd precursor was not readily visible because of the dark red color of the gel. The mixture was allowed to mix overnight to ensure homogeneity and complete incorporation. The 1% and 2% Pd doped onto iron aerogels were synthesized using the sequence of steps shown in Scheme 1. The various catalysts were identified according to their thermal treatments as shown in Scheme 1. 2.4. Catalyst Characterization. 2.4.1. Surface Area Analysis. To evaluate the pore structure of the synthesized iron aerogel and 1% and 2% Pd doped onto iron aerogel, nitrogen adsorption isotherms were determined using Micromeritics Chemisorb instrument (Model 2720). The isotherms were used to calculate the BET specific surface area according to the BJH method. The measurements were carried out under a nitrogen flow rate of 12 mL/min for all the samples. The samples were prepared before the BET surface area measurements by degassing the sample under nitrogen flowing at a rate of 10 mL/min at elevated temperatures. For the samples that were not previously calcined at higher temperatures, the degas temperature was 100 °C for a time of 30 min. For the samples that had been calcined at elevated temperatures, the degas temperature was 250 °C for 30 min. The surface area for iron aerogels was measured for as-prepared as well as for samples calcined at different temperatures. 2.4.2. Temperature-Programmed Reduction. The reduction of the iron aerogels and 1% and 2% Pd doped onto iron aerogels were evaluated by H2 temperature-programmed reduction (TPR) using
Micromeritics instrument (Model 2720). Temperature-programmed reduction was carried out on iron aerogel as well as 1% Pd and 2% Pd doped iron aerogels using an H2 flow rate of 50 mL/min with the temperature programmed to increase at a rate of 10 °C/ min from room temperature to 900 °C. The TPR measurements were carried out on calcined samples (calcined at 260 °C) for both undoped iron aerogel as well as 1% and 2% Pd doped onto iron aerogels. 2.4.3. X-ray Diffraction (XRD) Measurements. XRD measurements at room temperature were made both before and after the WGS reaction to characterize the catalyst before and after undergoing WGS reaction conditions using a Rigaku D-Max diffractometer with Cu KR radiation of wavelength 0.154 185 nm. 2.5. Catalyst Evaluation for WGS Activity. The iron aerogel and the 1% and 2% Pd doped onto iron aerogels synthesized by the reaction steps shown in Scheme 1 were evaluated for WGS activity using a six-channel fixed-bed reactor.30 The reactors were charged with 100 mg of fresh catalysts held in place with Whatman QMA quartz fiber filters. The thermocouple was located inside the catalyst bed for temperature monitoring and control. The catalysts were tested for the WGS shift reaction 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 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).19
3. Results and Discussion The iron oxide wet gels were synthesized using propylene epoxide under ambient conditions. Supercritical drying of these wet iron oxide gels yielded iron oxide aerogels which retained the shape and size of the wet gels. The monolithic iron aerogels can be easily crushed with your fingers. The synthesized iron aerogels were calcined in air at various temperatures to study the effect of temperature on the surface areas of the synthesized iron aerogels. The measured BET surface areas of iron aerogels at different calcination temperatures are shown in Table 1. It was observed that as-prepared iron aerogel shows high surface area of 422 ( 8 m2/g. As the temperature of calcination is increased, the surface area decreases gradually and at a calcination temperature of 450 °C the surface area obtained is only 82 ( 2 m2/g, indicating that the aerogel practically collapses at this temperature. The reduction of surface areas of iron aerogel on calcination in air can be attributed to the loss of pore volumes of calcined aerogels as compared to the wet (30) Dunn, B. C.; Kim, D. J.; Webster, M.; Gasser, J.; Turpin, G. C.; Ernst, R. D.; Eyring, E. M. 19th Annual Technical Meeting CFFS C1 Chemistry, Roanoke, WV, July 31-Aug 3, 2005.
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Figure 2. H2 temperature-programmed reduction of iron aerogel calcined at 260 °C and with 1% and 2% Pd doped iron aerogels calcined at 260 °C (TCD ) thermal conductivity detector; a.u. ) arbitrary units).
gels. The BET surface areas of Pd-incorporated iron aerogel catalysts were also measured for the as-prepared and calcined samples obtained according to Scheme 1. The resulting surface areas of the Pd-incorporated iron aerogel, shown in Table 2, are smaller in comparison to undoped iron aerogels possibly due to Pd deposition on the surface of aerogels. The surface areas of Pd-incorporated iron aerogels also decrease on calcination. This can be attributed to loss of pore volumes on heating. It has been observed by Long et al.24 that the larger pores are preserved during the calcination procedures, while for the smaller pores in the 2–60 nm range, the loss in pore volume is nonspecific in comparison to wet gels. Hence, decrease in surface areas of as-prepared iron aerogels as well as iron aerogels doped with Pd is observed on calcination. The temperature-programmed reduction (TPR) results for the undoped iron aerogel (calcined at 260 °C) as well as 1% and 2% Pd doped onto iron aerogels (calcined at 260 °C) are shown in Figure 2. Iron aerogel consumes hydrogen at 370 °C corresponding to conversion of Fe2O3 to Fe3O4 and finally at 653 °C for further reduction of Fe3O4 to FeO and of FeO to metallic Fe. In the TPR of 1% Pd onto iron aerogel, a first hydrogen consumption peak appears at a low temperature of 100 °C. The first hydrogen consumption peak for the 2% Pd loaded onto iron aerogel is observed at 120 °C. The lowtemperature peaks in the TPR of both 1% and 2% Pd loaded onto iron aerogels can be attributed to the reduction of iron that is in direct contact with Pd. The direct Fe-Pd interaction causes the reduction temperature to shift to considerably lower values for both 1% and 2% Pd loaded onto iron aerogel. The shift in the hydrogen consumption peaks to lower temperatures in the TPR of 1% and 2% Pd doped onto iron aerogels (calcined at 260 °C) compared to the undoped iron aerogel indicates the increased reducibility of the Pd-doped aerogels compared to undoped iron aerogels. The lowering of reduction temperatures in TPR measurements has been noted previously in a Pd-Fe/C system.31 The authors speculated that their experimental observation may arise from noble metal (Pd) catalysis which significantly enhances the reduction of iron oxide. A decrease in reduction temperature has also been observed in Pd-Fe and Pd-Cu systems.32,33 The TPR peaks arising from reduction of various iron oxide phases do not appear as sharp and distinct signals but instead are merged together hence precluding the (31) Golubina, E. V.; Lokteva, E. S.; Lunin, V. V.; Telegina, N. S.; Stakheev, A. Yu.; Tundo, P. Appl. Catal., A 2006, 302, 32–41. (32) Pinna, F.; Selva, M.; Signoretto, M.; Strukul, G.; Boccuzzi, F.; Benedetti, A.; Canton, P.; Fagherazzi, G. J. Catal. 1994, 150, 356–367. (33) Lietz, G.; Nimz, M.; Volter, J. Appl. Catal. 1988, 45, 71–82.
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Figure 3. XRD patterns of calcined as well as uncalcined samples of iron aerogels and 1% and 2% Pd doped onto iron aerogels.
Figure 4. XRD spectrum of 2% Pd incorporated iron aerogels after the water gas shift reaction.
quantitative calculation of the hydrogen uptake for sequential reduction of iron oxides under these experimental conditions. The various chemical phases in the samples were evaluated using XRD measurements. The XRD patterns for the iron aerogels and 1% and 2% Pd doped onto iron aerogels are shown in Figure 3. The XRD measurements were carried out on both as-prepared as well as calcined samples for iron aerogels as well as Pd doped onto iron aerogels. In the XRD spectrum of iron aerogel, there was a strong indication of the presence of Fe3O4 and FeOOH resulting from the iron aerogel in both the as-prepared as well as the calcined samples. In the case of Pd doped onto iron aerogels, at least one of the lines matches with the XRD of Pd. Because of the low concentration of Pd, the observed line is weak. The XRD measurements were also carried out on the 2% Pd doped onto iron aerogel after using it as a catalyst in the WGS reaction to determine the possible phases and thus the changes in the catalyst after the WGS reaction. The XRD pattern of spent 2% Pd iron aerogel catalysts is shown in Figure 4. The phases that were detected in the spent catalyst were γ-Fe2O3 with an average particle size ∼20 nm and also possibly Pd with an observed average particle size ∼10 nm. It has been shown that γ-Fe2O3 is a defect form of magnetite Fe3O4 which is obtained when FeOOH is heated in a reducing atmosphere.34 (34) Ibrahim, M. M.; Edwards, G.; Seehra, M. S.; Ganguly, B.; Huffman, G. P. J. Appl. Phys. 1994, 75, 5873–5875.
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resulting in enhanced WGS activities of Fe-Pd systems. The authors21 have speculated that enhanced water gas shift activity was observed as a result of specific interactions between iron and palladium in catalyst systems that contained both Fe and Pd. In all the experiments, no Fischer–Tropsch (FT) product such as ethane was obtained. On comparing the activities of Pd loaded onto iron aerogel with Pd loaded onto ceria aerogel28 support under the same conditions, it was observed that Pd-loaded Fe aerogels show higher activities at lower temperatures as compared to Pd loaded onto ceria aerogel. 4. Conclusions
Figure 5. Arhenius plot ln(activity(mg CO/g cat. h)) vs 1/T of 1% and 2% Pd doped onto iron aerogels in the WGS reaction.
The iron aerogels as well as 1% and 2% Pd doped onto iron aerogels, which were synthesized by reactions shown in Scheme 1, were tested for WGS activity in a six-channel fixed bed reactor within the 150-350 °C range at atmospheric pressure. The activities of Pd doped onto iron aerogels was compared to undoped iron aerogels. The 1% and 2% Pd doped onto iron aerogel catalysts show WGS activity at lower temperatures. The 1% Pd doped onto iron aerogel FePd(1%)uncal showed an activity of 0.34 g CO/g catalyst h at 350 °C. The 2% Pd doped onto iron aerogel was also evaluated for WGS activity both as prepared and after calcination at 260 °C. The Pd-loaded iron aerogel FePd(2%)uncal showed an even greater activity of 0.51 g CO/g catalyst h at 350 °C. The activities of all the catalysts at different temperatures are shown in the Figure 5 Arrhenius plot. The initial predominant phase of iron is Fe2O3 which is gradually converted to the catalytically active Fe3O4 phase as the catalysts are heated in reducing atmosphere under WGS conditions. The increase in activity of the Pd loaded iron aerogels can also be attributed to formation of an Fe-Pd alloy system. The formation of the alloy has been reported21 to provide a thermodynamic driving force for reduction of iron oxide in such systems thereby
Owing to their high mechanical strength as compared to alumina and silica aerogel, the iron aerogels are promising materials for catalysis applications both as such and as support for impregnation of other metal species. Palladium-doped iron aerogels also have high surface areas that are comparable to iron aerogels. The 1% and 2% by weight Pd-doped iron aerogels were found to show increased activity at lower temperatures toward WGS reaction as compared to undoped iron aerogel. The activity for the Pd-loaded iron aerogels may be attributable to the formation of a Fe-Pd alloy system which enhances the WGS activity. The synthesized iron aerogels are also potential catalyst materials for Fischer–Tropsch (FT) synthesis of hydrocarbons from syngas as suggested by reviewers. The use of iron aerogels and iron aerogels impregnated with promoter metals such as potassium for FT synthesis is an area worth exploring especially for syngas derived from coal gasification. The syngas from coal gasification is lean in H2 and hence WGS activity combined with FT activity of iron aerogel catalyst systems could be particularly advantageous for adjusting the ratio of CO to H2 in the feed. Acknowledgment. The authors are grateful for funding from the U.S. Department of Energy, Office of Fossil Fuel Energy, under contract No. DE-FC26-05NT42456. EF700691Z
