Ceria−Zirconia High-Temperature Desulfurization Sorbents - Industrial

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Ceria-Zirconia High-Temperature Desulfurization Sorbents Kwang Bok Yi, Elizabeth J. Podlaha, and Douglas P. Harrison* Gordon A. and Mary Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803

Ceria and ceria-zirconia desulfurization sorbents containing up to 20 mol % ZrO2 were prepared using a coprecipitation method. XRD analysis indicated the formation of solid solutions of ZrO2 in CeO2. ZrO2 addition produced larger specific surface area, improved sintering resistance, and increased the reducibility of the ceria. Desulfurization was studied using a laboratory-scale fixedbed quartz reactor. Special precautions were taken to minimize interaction between sub-ppmv concentrations of H2S with stainless steel and eliminate contamination from residual sulfur in the reactor system. The performance of sorbents containing ZrO2 was superior to that of ZrO2free sorbents. Product gas concentrations in the range of 0.1-0.2 ppmv H2S were achieved during the prebreakthrough period over the temperature range of 600-750 °C. In particular, the duration of the sub-ppmv prebreakthrough period lasted as much as twice as long when the sorbent contained 20 mol % ZrO2 compared to ZrO2-free CeO2. Introduction The development of a high-temperature desulfurization sorbent capable of reducing H2S concentrations in coal gas to levels suitable for advanced power generation processes such as the integrated gasification combined cycle (IGCC) has been a major U.S. Department of Energy research objective for a number of years. However, first-generation sorbents developed for these applications are not capable of meeting the new desulfurization levels established by the DOE Vision 21 concept, where sub-ppmv concentrations are required to allow coal gas to be used in fuel cells and other catalytic-based synthesis gas processes. Meng and Kay1 were the first to study the use of ceria as a high-temperature desulfurization sorbent. The solid product is cerium oxysulfide, and the reaction is

2CeO2(s) + H2S(g) + H2(g) f Ce2O2S(s) + 2H2O(g) The H2S concentration was reduced from 1.2% (mol) in the feed gas to 3 ppmv in the product gas at 872 °C and 1 atm. The amount of data reported, however, was quite limited. Li and Flytzani-Stephanopoulos2 studied the desulfurization ability of mixed copper-cerium oxides. Although some evidence of cerium sulfidation was reported, the primary function of ceria was to maintain the active copper in highly dispersed form. Earlier studies in this laboratory3,4 demonstrated that cerium oxide, CeO2, was capable of reducing the H2S concentration from 2500 ppmv in the inlet gas to below 1 ppmv in the product at 700 °C. The sulfidation product, Ce2O2S, could be regenerated directly to elemental sulfur using SO2. There was no apparent deterioration in sorbent performance in an extended test consisting of 25 cycles using 800 °C sulfidation and 600 °C regeneration temperatures. The primary factor limiting the direct application of CeO2 is its high stability. To achieve the low H2S levels quoted above it was necessary to prereduce the CeO2 to CeOn (with 1.5 < n * To whom correspondence should be addressed. E-mail: [email protected].

< 2.0) in an oxygen-free, highly reducing gas prior to sulfidation. Ceria has recently found extensive use in automotive and oxidation catalysis. In the three-way automotive catalyst (TWC) CeO2 helps to regulate the exhaust gas oxygen pressure. During fuel-rich operation CeO2 is reduced to CeOn and the oxygen released assists in the oxidation of CO and hydrocarbons to CO2. Under fuellean conditions CeOn is reoxidized to CeO2 and the removal of oxygen from the exhaust gas assists in the reduction of NOx to N2. In oxidation catalysts the CeO2 is reduced to CeOn by surface-adsorbed species as they are oxidized to CO2, and the CeOn is then reoxidized by oxygen from the gas phase. The addition of ZrO2 to CeO2 has been found to enhance the redox reactions. Colon et al.5 report that the addition of ZrO2 increases the oxygen mobility within the crystal lattice and improves the catalyst thermal stability at 1000 °C. Zamar et al.6 discuss the enhanced oxygen storage and release capacity of CeO2ZrO2 mixtures used for CH4 combustion. ZrO2 is said to promote the formation of oxygen vacancies and increase the mobility of bulk oxygen. The 50% CH4 conversion level was reached at a temperature 130 °C lower using Ce0.8Zr0.2O2 compared to CeO2 alone. Hori et al.7 report an increase in reversibly stored oxygen by a factor of 1.7-2.5 for phase-separated CeO2ZrO2 compared to CeO2 alone and by 3-5 for solid solutions of CeO2-ZrO2. The optimum zirconia concentration was 25 mol %, but performance was relatively insensitive to zirconia loading between 15 and 50 mol %. Trovarelli et al.8 discuss the improved performance of three-way automotive catalysts and state that the addition of ZrO2 enhances the catalytic, textural, redox, and oxygen storage properties of ceria. It may be argued that the positive effects associated with zirconia addition in automotive and oxidation catalysts are the same factors needed to improve the performance of cerium-based desulfurization sorbents. Therefore, the objective of this research was to study the desulfurization ability of ceria-zirconia sorbents. Sorbents were prepared using a coprecipitation method. Desulfurization was tested in a laboratory-scale, fixed-

10.1021/ie050441x CCC: $30.25 © 2005 American Chemical Society Published on Web 08/05/2005

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Figure 1. Reactor system.

bed quartz reactor operated at atmospheric pressure as a function of temperature and reducing strength of the feed gas. The reducing strength was controlled by the addition of varying concentrations of CO2 to a base gas consisting of H2S and H2 in N2. The effect of prereduction was studied by exposing the sorbents to sulfur-free reducing gas prior to introducing H2S. The sorbents were characterized by X-ray diffraction, specific surface area, and reducibility in independent reduction tests using an electrobalance (TGA) reactor. Experimental Section Sorbent Preparation. Sorbents consisting of pure CeO2 (designated Ce100), 90 mol % CeO2-10 mol % ZrO2 (Ce90Zr), and 80% CeO2-20% ZrO2 (Ce80Zr) were prepared by coprecipitation (or precipitation) adopted from a procedure described by Adachi and Masui.9 Desired quantities of Ce(NO3)3‚6H2O and ZrO(NO3)2‚ xH2O were dissolved separately in 200 mL of distilled water at room temperature. Ce(NO3)3‚6H2O was readily soluble, but about 1 h was required to dissolve the ZrO(NO3)2‚xH2O. The dissolved solutions were mixed and stirred for 0.5 h. Precipitation was achieved by the dropwise addition of NH4OH with constant stirring until no more precipitation occurred. The precipitate was filtered, and the filter cake was washed with distilled water. The washed cake was dried overnight in air at 250 °C and calcined at 400 °C in air for 2 h. The calcined particles were then crushed and sieved with 150-300 µm particles used in characterization and reaction tests. Sorbents used for BET surface area characterization were also calcined at 700 °C in air for 4 h to study the thermal stability. Experimental Reactor. Sulfidation tests were carried out in the fixed-bed reactor system shown in Figure 1. The entire reactor system and downstream analytical facilities, except for the H2S feed line, were constructed of quartz, Teflon, or Silcosteel to prevent interaction between low H2S concentrations and stainless steel. The desired feed gas composition was obtained by mixing pure CO2, H2, and N2 with a certified gas mixture containing 5% H2S in N2 (Gas Analytical Services). All gas flow rates were controlled using calibrated mass flow controllers (Porter Instruments). H2S-free gases entered near the bottom of the quartz reactor and were preheated as they flowed upward in

the annular area surrounding the reactor insert. The feed mixture containing H2S entered at the top of the reactor and mixed with the other gases just above the sorbent bed. The combined gases then flowed downward through the sorbent bed and exited to a gas chromatograph for analysis. The sorbent plus Al2O3 diluent was supported on a porous quartz disk that rested on three dimples inside the reactor insert. A thin layer of quartz wool was placed at both the top and bottom of the sorbent bed. The reactor temperature was controlled using a single-zone, split-tube furnace. Product or feed gas was analyzed using a Varian model 3800 gas chromatograph equipped with both pulsed flame photometric (PFPD) and thermal conductivity (TCD) detectors. The PFPD was used for H2S concentrations from sub-ppmv to 6 ppmv and the TCD for concentrations in excess of 100 ppmv. While there was an analytical gap between 6 and 100 ppmv, primary interest was in the less than 6 ppmv concentration range. The ultimate H2S detection limit of the PFPD was approximately 0.1 ppmv. The chromatograph was equipped with dual columns, CP SIL 5 for the PFPD and HAYESEP for the TCD, as well as 10-port and 6-port automatic sampling valves. This sampling arrangement permitted the simultaneous injection of samples into the two columns and the backflushing of H2O reaction product to prevent it from reaching the detectors. During a sulfidation test the reactor product was sampled at 7-min intervals. The flow arrangement shown in Figure 1 also permitted H2S-free gas to be routed past a temperaturecontrolled H2S permeation tube (Valco Instruments) that released 196 ng/min of H2S at 30 °C for PFPD calibration purposes. H2S concentrations larger than 100 ppmv for TCD calibration were prepared by mixing cylinder gases using the mass flow controllers. The reactor system was carefully cleaned following each test by flowing a reducing gas through the system at high temperature for sufficient time that no sulfur was detected in the product. This was necessary so that that trace contamination left from a previous test would not affect the sub-ppmv sulfur levels achieved in the subsequent desulfurization test. Reaction Conditions. The quartz reactor limited the sulfidation tests to 1 atm. Sulfidation temperature was varied between 600 and 750 °C, and the reducing potential of the sulfidation feed gas was controlled by the addition of CO2 at levels between 0% and 1.0% to a feed gas containing 0.25% H2S, 10% H2, and balance N2. The equilibrium oxygen pressure for those feed compositions containing CO2, as calculated using HSC Chemistry,10 varied between about 10-25 bars at 600 °C with 0.25% CO2 and 10-20 bars at 750 °C with 1.0% CO2. Any oxygen pressure in the CO2-free feed gas was due to trace impurities in the gas cylinders. The volumetric feed rate of sulfidation gas was held constant at 72 mL(stp)/min, and 2.0 g of sorbent diluted with either 4.0 or 6.0 g of inert Al2O3 was used in each test. Fresh sorbent and Al2O3 were used in each test, and the sorbent was not regenerated. The sorbent mass was corrected for between 5% and 10% volatiles loss during the analysis of experimental data. Sorbent Characterization. The sorbents were characterized by their specific surface area, their X-ray diffraction spectra, and in terms of their reducibility in an H2S-free reducing gas using an electrobalance reactor (TGA). The specific surface areas of calcined sorbents

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Table 1. Specific Surface Areas of Ceria and Ceria-Zirconia Sorbents sorbent

calcination conditions

specific surface area, m2/g

Ce100

400 °C for 2 h in air 700 °C for 4 h in air 400 °C for 2 h in air 700 °C for 4 h in air 400 °C for 2 h in air 700 °C for 4 h in air

75 35 75 50 85 55

Ce90Zr Ce80Zr

prior to sulfidation were measured using an Omnisorp model 360 BET apparatus. The surface areas of sulfided sorbents could not be measured, as the sulfided product was pyrophoric. XRD spectra of calcined sorbents were collected using a Rigaku MiniFlex X-ray diffractometer using Cu KR radiation. Reducibility was measured using a Cahn Instruments model 2000 TGA with sample weight measured as a function of time, temperature, and reducing gas composition. Note that the electrobalance is not suitable for characterizing the sulfidation reaction, as the conversion of 2CeO2 to Ce2O2S does not result in a change in solid weight.

Figure 2. XRD spectra: (a) CeO2 (library), (b) Ce100, (c) Ce90Zr, (d) Ce80Zr.

Experimental Results Specific Surface Area. BET surface areas were measured following calcination at 400 °C in air for 2 h and following calcination at 700 °C in air for 4 h and are summarized in Table 1. The specific surface areas of Ce100 and Ce90Zr sorbents following 400°C calcination were effectively equal, while the addition of 20% ZrO2 produced a 13% increase in surface area. All three compositions experienced sintering at the more severe calcination conditions, but the presence of ZrO2 clearly moderated the surface area loss. The surface area of Ce80Zr sorbent was almost 60% larger than that of Ce100 sorbent following 700 °C calcination. XRD Analysis. The XRD spectra of the three sorbents are compared to the library spectrum for CeO2 (JCPDS 88-1007) in Figure 2. No peaks other than those associated with CeO2 are present, indicating the formation of a solid solution of ZrO2 in CeO2. The addition of 20% ZrO2 produced a small shift to larger values of 2θ, but there is no apparent difference between the spectra of Ce100 and Ce90Zr sorbents. The peak shift is consistent with results reported in the literature.11 The mean crystal size, calculated from the full-width at halfmaximum intensity (fwhm) of the (111) peak using the Scherrer equation, varied from 12.7 nm for Ce90Zr to 18.8 nm for Ce80Zr. Sorbent Reduction. The degree of sorbent reduction was based on the reaction

Ce1-xZrxO2(s) + (2 - n)H2(g) f Ce1-xZrxOn(s) + (2 - n)H2O(g) with n calculated from measured weight loss using the formula

[

n)2 1-

]

M∆w 32wo

M is the molecular weight of the sorbent (M ) 172.12 for Ce100, 167.23 for Ce90Zr, and 162.34 for Ce80Zr), wo is the initial sorbent mass, and ∆w is the mass loss associated with reduction.

Figure 3. Reduction of ceria and ceria-zirconia sorbents.

Results of sorbent reduction tests are shown in Figure 3, where the value of n is plotted against reduction temperature using a reducing gas containing 50% H2, 3.5% CO2, and balance He. He was substituted for N2 to improve the electrobalance sensitivity. In these tests the sorbent was first heated from room temperature to 800 °C at 10 °C/min in pure He to ensure that no volatile material remained. After cooling to 200 °C under He, the reducing gas was introduced and the sample was heated to 1000 °C at 10 °C/min. Two features are immediately evident from Figure 3. The addition of ZrO2 reduced the temperature at which significant reduction began and also increased the final degree of reduction at 1000 °C. Without ZrO2 the reduction of Ce100 did not become significant until about 700 °C and the overall weight loss at 1000 °C corresponded to a final product composition of CeO1.86. With the addition of ZrO2 reduction began at about 400 °C and the final degree of reduction corresponded to Ce0.9Zr0.1O1.78 and Ce0.8Zr0.2O1.77. All sorbent characterization test results were in qualitative agreement with earlier literature discussions on the effect of ZrO2 addition. Sulfidation Test Results. Sulfidation testing was carried out using the fixed-bed reactor, and the H2S concentration in the reactor product gas was measured as a function of time. The performance of the three sorbents at 750 °C is compared in Figure 4, where the entire H2S breakthrough curve is shown. The reactor

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Figure 4. Complete H2S breakthrough curves.

Figure 5. H2S prebreakthrough curves in the absence of CO2.

feed gas contained no CO2 in these three tests. On the concentration scale of Figure 4 the H2S concentrations are all near zero for the first 350 min of the test, and sulfidation is effectively complete at about 900 min. There appears to be little overall difference in performance of the three sorbents. Results from early (prebreakthrough) portions of similar series of tests using the same feed gas composition but at 700 °C are shown in Figure 5 on the highly expanded concentration scale of 0-1.0 ppmv. In each case the sorbent was successful in reducing the prebreakthrough H2S concentration to the 0.1-0.2 ppmv level. The lower concentration is near the detection limit of the PFPD. The Figure 5 results are plotted versus dimensionless time to correct for the different quantities of CeO2 in the sorbents. Dimensionless time, t*, is defined as the ratio of the actual reaction time to the theoretical time corresponding to complete conversion of ceria to cerium oxysulfide assuming total removal of H2S based on the stoichiometry

2Ce1-xZrxO2(s) + H2S(g) + H2(g) f Ce2(1-x)Zr2xO2S(s) + 2H2O(g) t* is calculated from

t* )

(t)(nsorb) 2(nH2S)

t is the actual reaction time, nH2S is the molar feed rate of H2S, nsorb is the number of moles of sorbent charged

Figure 6. H2S prebreakthrough curves with 0.25% CO2 in the feed gas.

to the reactor, and 2 is the ratio of stoichiometric coefficients between sorbent and H2S. nsorb was corrected for loss of volatile material and the variable sorbent composition. Although breakthrough to 1 ppmv H2S occurred at approximately the same dimensionless time for both the Ce100 and Ce90Zr sorbents, the prebreakthrough period lasted about 33% longer with the Ce80Zr sorbent. The breakthrough value of t* ) 0.44 for Ce80Zr corresponds to just over 300 min of actual reaction time. Increasing the oxygen pressure of the feed gas by the addition of 0.50% CO2 produced the results shown in Figure 6. The duration of the prebreakthrough period was reduced in each case, for example, from t* ) 0.44 with no CO2 to t* ) 0.25 with 0.50% CO2 for Ce80Zr sorbent. However, both Ce90Zr and Ce80Zr sorbents produced prebreakthrough H2S concentrations in the 0.1-0.2 ppmv range, while the prebreakthrough concentration for Ce100 sorbent increased to approximately 0.5 ppmv. The initial peak in the H2S concentration, which is clearly visible in Figure 6 and present to a much lesser extent in Figure 5, is attributed to incomplete reduction of the sorbent during the early stages of the test. Initially the sorbent is in its fully oxidized form and H2S removal is relatively poor. However, the reduction reaction front moves ahead of the sulfidation front because of the large ratio of H2 to H2S in the feed gas. After a short time the H2S contacts reduced sorbent and the level of H2S removal increases. With CO2 present in the feed gas reduction occurs more slowly and to a lesser degree. Hence, the initial H2S peak is magnified. The superior performance of Ce80Zr sorbent compared to Ce90Zr and Ce100 illustrated in Figures 5 and 6 was observed in all test sequences. The effect of sulfidation temperature on the prebreakthrough duration period is illustrated in Figure 7 using Ce80Zr sorbent. There was a slight increase in prebreakthrough H2S concentration with increasing temperature, from about 0.1 ppmv at 600 °C to 0.15 ppmv at 700 °C to 0.2 ppmv at 750 °C. However, the most significant result from Figure 7 is the increase in prebreakthrough time with increasing temperature. The dimensionless time corresponding to breakthrough was about 3 times larger at 750 °C than at 600 °C. Similar increases in breakthrough time were observed with Ce100 and Ce90Zr sorbents, but breakthrough at all temperatures occurred at an earlier time for these two sorbents.

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Figure 7. Effect of sulfidation temperature: Ce80Zr sorbent.

Figure 10. Effect of CO2 concentration: prebreakthrough curves.

for sorbent Ce80Zr at CO2 concentrations of 0%, 0.2%, 0.5%, and 1.0%. Once again, on this concentration scale the H2S concentration appears to be near zero initially, but there is a clear decrease in time at which the H2S concentration remained at that low level. H2S concentrations during the prebreakthrough period of these same tests are presented in Figure 10 on an expanded concentration scale. Prebreakthrough concentrations were in the 0.1-0.2 ppmv H2S for 0%, 0.2%, and 0.5% CO2 but increased to about 0.4 ppmv when the feed gas contained 1.0% CO2. The prebreakthrough H2S concentrations using Ce100 and Ce90Zr sorbents were near those shown in Figure 10 for Ce80Zr sorbent. However, the dimensionless times corresponding to breakthrough were considerably smaller. Figure 8. Effect of prereduction: Ce80Zr sorbent.

Figure 9. Effect of CO2 concentration: complete breakthrough curves.

Prereduction of the sorbents at 700°C in 10% H2, 90% N2 for 4 h produced an increase in the breakthrough time and a decrease in prebreakthrough concentration as illustrated in Figure 8 for sorbent Ce80Zr. The breakthrough time increased by about 25%, from dimensionless time of about 0.44 to 0.53 with prereduction, while the prebreakthrough concentration decreased from about 0.15 ppmv H2S to near zero (below the PFPD detection limit). Prereduction of the other two sorbents also resulted in near zero prebreakthrough concentrations but for shorter values of dimensionless time. The effect of CO2 concentration is illustrated in Figure 9, where the entire breakthrough curves are presented

Conclusions Solid solutions of zirconia in ceria enhance the thermal stability, reducibility, and desulfurization performance of ceria-based high-temperature desulfurization sorbents. The addition of 20 mol % ZrO2 produced a 13% increase in as-prepared sorbent specific surface area and a 60% larger surface area following calcination at 700 °C in air for 4 h. The oxygen losses in reduction experiments at 1000 °C were equivalent to final compositions of CeO1.86 and Ce0.8Zr0.2O1.77 for Ce100 and Ce80Zr, respectively. The desulfurization tests used a laboratory-scale fixed-bed quartz reactor with the concentration of H2S in the product gas measured as a function time over the temperature range of 600-750 °C. The reducing power of the feed gas was controlled by the addition of CO2 to a base gas containing H2S and H2 in N2. Special precautions were taken to eliminate the possibility of interaction between low concentrations of H2S and stainless steel surfaces and ensure that the entire reactor system was free of residual sulfur prior to each new test. All sorbents, with and without ZrO2 addition, successfully reduced the H2S concentration from 0.25% in the feed gas to sub-ppmv levels in the reactor product gas during the prebreakthrough period. Prereduction of the sorbents produced prebreakthrough H2S concentrations below the chromatograph detection limit of about 0.1 ppmv. The primary advantage associated with ZrO2 addition was an increase in the duration of the prebreakthrough period by as much as a factor of 2. Lowering the reducing potential of the sulfidation feed gas by the addition of CO2 reduced the duration of the

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prebreakthrough period, but sub-ppmv H2S concentrations were still achieved during the prebreakthrough period. HSC Chemistry calculations at the most common sulfidation temperature of 700 °C suggest that the reducing potential, as characterized by the oxygen pressure, of the product from a Shell gasifier and the experimental gas containing 1% CO2 are approximately equal. On this basis it may be argued that Ce-Zr sorbents may be capable of reducing the H2S content of Shell gas to sub-ppmv levels but, as shown in Figure 10, for a relatively short period of time. However, the same argument suggests that Ce-Zr sorbents would not be able to reach sub-ppmv levels in less reducing coal gas such as that produced in the Texaco process. Acknowledgment The authors are grateful for the financial support supplied by the U.S. Department of Energy under grant no. DE-FG26-00NT40813. Literature Cited (1) Meng, V.; Kay, D. Gaseous Desulfurization Using Rate Earth Oxide. In High Technology Ceramics; Vincinzini, P., Ed.; Elsevier: Amsterdam 1987. (2) Li, Z.; Flytzani-Stephanopoulos, M. Cu-Cr-O and CuCe-O Regenerable Oxide Sorbents for Hot Gas Desulfurization. Ind. Eng. Chem. Res. 1997, 36, 187. (3) Zeng, Y.; Zhang, S.; Groves, F. R.; Harrison, D. P. HighTemperature Gas Desulfurization with Elemental Sulfur Production. Chem. Eng. Sci. 1999, 54, 3007.

(4) Zeng, Y.; Kaytakoglu, S.; Harrison, D. P. Reduced Cerium Oxide as an Efficient and Durable High-Temperature Desulfurization Sorbent. Chem. Eng. Sci. 2000, 55, 4893. (5) Colon, G.; Pijolat, M.; Valdivieso, F.; Vidal, H.; Kaspar, J.; Daturi, M.; Binet, C.; Lavalley, J.; Baker, R.; Bernal, S. Surface and Structural Characterization of CexZr1-xO2 Mixed Oxides as Potential Three-Way Catalyst Promoters. J. Chem. Soc., Faraday Trans. 1998, 94, 3717. (6) Zamar, F.; Trovarelli, A.; de Leitenburg, C.; Dolcetti, G. CeO2-Based Solid Solutions with the Fluorite Structure as Novel and Effective Catalysts for Methane Combustion. J. Chem. Soc., Chem. Commun. 1995, 9, 965. (7) Hori, C.; Permana, H.; Ng, K.; Brenner, A.; Moore, K.; Rahmoeller, K.; Belton, D. Thermal Stability and Oxygen Storage Properties in a Mixed CeO2-ZrO2 System. Appl. Catal. B, Environ. 1998, 16, 105. (8) Trovarelli, A. Catalytic Properties of Ceria and CeriaContaining Materials. Catal. Rev. Sci. Eng. 1996, 38, 439. (9) Adachi, G.; Masui, T. Synthesis and Modification of CeriaBased Materials. In Catalysis by Ceria and Related Materials; Trovarelli, A., Ed.; Catalytic Science Series; Imperial College Press: London, 2002; Vol. 2, p 15. (10) Roine, A. Outokumpu HSC for Windows-Chemical Reaction Equilibrium Softward with Extensive Thermodynamic Data Base. User’s Guide, Version 3.0; Outokumpu Research Oy: Finland, 1997. (11) Cerium Oxide Based Powders and Suspensions, http://www.fuelcellmaterials.com/cerium_oxide_powders.htm.

Received for review April 12, 2005 Revised manuscript received July 1, 2005 Accepted July 12, 2005 IE050441X