Recent Developments Towards Commercialization of Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFC) are receiving an increasing level of attention for both small- and large-scale applications due their high fuel flexibility compared to other types of fuel cells. This article reviews recent work in SOFC materials research, with an emphasis on anodes that can directly utilize hydrocarbon fuels

The Activity of Fe-Pd Alloys for the Water-Gas Shift Reaction

The role of Fe promoters has been investigated on Pd/ceria, Pt/ceria and Rh/ceria catalysts for the water-gas shift (WGS) reaction in 25 Torr of CO and H2O, under differential reaction conditions. While no enhancement was observed with Pt and Rh, the activity of Pd/ceria increased by as much as an order of magnitude upon the addition of an optimal amount of Fe. Similarly, the addition of 1 wt% Pd to an Fe2O3 catalyst increased the WGS rate at 453 K by a factor of 10 over that measured on Fe2O3 alone, while the addition of Pt or Rh to Fe2O3 had no effect on rates. The amount of Fe that was necessary to optimize the rates increased with Pd loading but was independent of the order in which Fe and Pd were added to the ceria. Increased WGS activity was also observed upon the addition of Fe to Pd supported on Ce0.5Zr0.5O2. XRD measurements, performed after running the catalyst under WGS conditions, show the formation of a Fe-Pd alloy, even though similar measurements on an Fe/ceria catalyst showed that Fe3O4 was the stable phase for Fe in the absence of Pd. Possible implications of these results on the development of new WGS catalysts are discussed

The Effect of Oxide Dopants in Ceria on \u3cem\u3en\u3c/em\u3e-Butane Oxidation

Solid solutions of CeO2 with Yb2O3, Y2O3, Sm2O3, Gd2O3, La2O3, Nb2O5, Ta2O5, and Pr6O11 were prepared by sol-gel methods. All of the samples exhibited a single, cubic phase in x-ray diffraction and the lattice parameters were consistent with formation of the solid solutions. Surprisingly, all of the mixed oxides exhibited a much lower catalytic activity for n-butane oxidation than that of pure CeO2 at 650 K, with Sm0.2Ce0.8O1.9, Gd0.2Ce0.8O1.9, La0.2Ce0.8O1.9, Nb0.1Ce0.9O2.05, and Ta0.1Ce0.9O2.05 showing rates less than 10-2 that of pure ceria. Lower dopant levels with Sm+3 and Gd+3 affected the rates proportionately less. The implications of these results for use of ceria as an oxidation catalyst are discussed

A Comparison of Ceria and Sm-Doped Ceria for Hydrocarbon Oxidation Reactions

The oxidation of methane, ethane, propane, and n-butane has been studied over CeO2 and Ce0.8Sm0.2Ox (SDC) catalysts. The rates for methane and ethane were found to be indistinguishable over the two catalysts, while the rates for propane and n-butane were much higher on ceria compared to SDC. The difference between n-butane oxidation over ceria and SDC is shown to result from a low-temperature rate process on ceria that is not present on SDC. Measurements using CD4 and C4D10 show that both low- and high-temperature rates exhibit a similar kinetic-isotope effect; however, the low-temperature process is half-order in O2, while the high-temperature process is zeroth-order. Pulse studies demonstrate that oxygen from the bulk becomes accessible for reaction at approximately the same temperature as that at which the high-temperature rate process becomes important. The implications of these results for understanding the effect of doping on reactions over ceria, and for characterization of oxides using standard test reactions, are discussed

The Effect of Fe and Other Promoters on the Activity of Pd/Ceria for the Water-Gas Shift Reaction

The effect of adding oxide promoters, at monolayer coverages, to Pd/ceria catalysts was studied for the water-gas-shift reaction in 25 torr each of H2O and CO under differential conditions. Oxides of Fe, Tb, Gd, Y, Sn, Sm, Pr, Eu, Bi, Cr, V, Pb, and Mo were added to ceria by wet impregnation of aqueous salts and calcined to 873 K prior to addition of 1 wt% Pd. While most of the oxides had a negligible effect on the water-gas-shift rates, rates on the Fe-promoted sample were 8 times higher than on the unpromoted Pd/ceria at 473 K, while rates on the Mo- and Pb-promoted cerias were significantly lower than on pure Pd/ceria. By varying the Fe loading, it was found that the maximum catalytic activity was achieved at the monolayer coverage. Addition of Fe in bulk ceria through sol-gel and preparation of Fe2O3-CeO2 physical mixtures had no effect on the catalytic activity

Characterization of SO\u3csub\u3e2\u3c/sub\u3e-Poisoned Ceria-Zirconia Mixed Oxides

CeO2, ZrO2, and a series of CexZr1-xO2 catalysts with 1 wt% Pd were exposed to fixed exposures of SO2 under oxidizing environments and then characterized by FTIR, pulse-reactor studies with CO and O2, and temperature-programmed desorption (TPD). For exposures above 473 K, sulfates were formed on all of the materials; however, the results are consistent with the formation of bulk sulfates on CeO2 and only surface sulfates on ZrO2. For the mixed oxides, the quantity of sulfates formed at 673 K increased linearly with the Ce content. In TPD, the sulfates on ZrO2 were stable to higher temperatures than those formed on CeO2, which decomposed in a well-defined peak between 900 and 1050 K. The sulfates on both oxides were reduced by CO above 750 K. Even though XRD patterns for the mixed oxide were significantly different from that of the physical mixture, the TPD and pulse-reactor results were similar to what would be expected for physical mixtures of CeO2 and ZrO2, suggesting that sulfate species are associated with individual metal cations. Finally, pulse-reactor studies with CO and O2 at 873 K show that the sulfates can be reversibly reduced and oxidized on both CeO2 and ZrO2, so that sulfur poisoning gives rise to an apparent increase in oxygen storage, demonstrating that this method is not acceptable for measurement of this quantity

A Mechanistic Study of Sulfur Poisoning of the Water-Gas-Shift Reaction Over Pd/Ceria

The effect of sulfur on the water-gas-shift (WGS) activity of Pd/ceria catalysts has been studied using steady-state rate measurements, pulse-reactor studies, and FTIR. After exposing Pd/ceria to SO2 at 673 K in an oxidizing environment, the WGS rates dropped to a value close to that observed on Pd/alumina. Both pulse-reactor and FTIR measurements showed that cerium sulfates can be readily reduced by CO and re-oxidized by O2 at 723 K; however, unlike reduced ceria, the Ce2O2S formed by reduction of the sulfates cannot be re-oxidized by H2O or CO2. The implications of these measurements for understanding oxygen-storage capacity (OSC) of three-way catalysts are discussed

Novel SOFC anodes for the direct electrochemical oxidation of hydrocarbons

Recent developments in solid-oxide fuel cells (SOFC) that electrochemically oxidize hydrocarbon fuels to produce electrical power without first reforming them to H2 are described. First, the operating principles of SOFCs are reviewed, along with a description of state-of-the-art SOFC designs. This is followed by a discussion of the concepts and procedures used in the synthesis of direct-oxidation fuel cells with anodes based on composites of Cu, ceria, and yttria-stabilized zirconia. The discussion focuses on how heterogeneous catalysis has an important role to play in the development of SOFCs that directly oxidize hydrocarbon fuels

The Acylation of Propene by Acetic Acid over H-[Fe]ZSM-5 and H-[Al]ZSM-5

We have studied the vapor-phase reaction of acetic acid with propene over H-[Al]ZSM-5 and H-[Fe]ZSM-5 molecular sieves between 425 and 600 K, for acetic acid partial pressures of 25 to 50 Torr, with propene:acetic acid ratios between 1 and 4. Reasonable conversions were obtained above 450 K. Both H-[Fe]ZSM-5 and H-[Al]ZSM-5 showed high activities at 450 K, with selectivities to propyl acetates of ~90% using a propene:acetic acid ratio of 2 and ketones formed by dehydration of the acetates making up most of the remaining products. The reaction rate was completely stable at 450 K for at least 20 h, and the product distribution was insensitive to the reactant feed ratio for a propene:acetic acid ratio above 2. On H-[Fe]ZSM-5, dehydration of the acetates became increasingly important at higher temperatures, with the selectivity to ketones approaching 80% at 525 K. At still higher temperatures, reaction pathways that did not involve propyl acetates became important. The high selectivity for forming propyl acetates and the absence of olefin oligomerization is believed to result from having the acid sites occupied (saturated) by acetic acid. Finally, the acid-site density measured on H-[Fe]ZSM-5 after weeks of catalyst testing remained the same as the initial site density, showing that this material has sufficient stability for fine-chemicals synthesis

An investigation of NO\u3csub\u3ex \u3c/sub\u3estorage on Pt–BaO–Al\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e

A series of samples containing 5-wt% or 20-wt% BaO on γ-Al2O3 with different loadings of Pt were prepared and examined for their NO2 adsorption properties using temperature programmed desorption (TPD), temperature programmed reduction (TPR), and x-ray diffraction (XRD). For calcination at 873 K or above, BaO/Al2O3 formed BaAl2O4. While carbonates were found to be unstable on the aluminate phase, NO2 reacted with the aluminate to form bulk Ba(NO3)2 and Al2O3, even at room temperature. With BaO/Al2O3, reaction to form the nitrate required slightly higher temperatures because of the need to displace CO2; however, pulsing NO2 over pure Ba(CO3) showed rapid reaction to form CO2 and NO in the gas phase, along with Ba(NO3)2, at 673 K. The decomposition temperature for Ba(NO3)2 shifted by more than 100 degrees when TPD was carried out in vacuum rather than in a carrier gas, showing that re-equilibration with the gas phase is important in the decomposition process. The addition of Pt had a minimal effect on the thermal stability of the nitrates but was essential for the reduction of the nitrate in H2. Since a relatively small amount of Pt was sufficient to cause the complete reduction of the Ba(NO3)2 phase at temperatures below 400 K, it appears that the nitrates must be extremely mobile within the Ba-containing phase. Finally, trapping studies of NO2 at 573 K, with or without 10% CO2 in the gas phase, showed no measurable difference between BaO/Al2O3 and BaAl2O4, with or without CO2