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The mechanism responsible for the irreversible deactivation of ceria-supported precious metals for the watergas-shift reaction has been investigated through accelerated aging tests. It is shown that deactivation of Pd/ ceria occurs more rapidly at 673 K compared to 523 K when operating with an integral reactor in 25 torr each of CO and H 2 O. By heating a fresh catalyst in H 2 , H 2 O, CO, or CO 2 , it was shown that deactivation occurs due to the presence of CO. Measurements of metal dispersion by CO adsorption and by x-ray diffraction show that deactivation on Pt/ceria and Pd/ceria catalysts in our studies was due to a loss of metal surface area. Finally, water-gas-shift rates on a series Pd/ceria catalysts with ceria crystallite sizes ranging from 7.2 to 40 nm and Pd loadings of either 1 wt% or 6 wt% demonstrated that rates were strictly proportional to the Pd surface area. Based on these observations, ceria-supported precious metals could be active and stable for the watergas-shift reaction if steps are taken to avoid metal particle-size growth. AbstractThe mechanism responsible for the irreversible deactivation of ceria-supported precious metals for the water-gas-shift reaction has been investigated through accelerated aging tests. It is shown that deactivation of Pd/ceria occurs more rapidly at 673 K compared to 523 K when operating with an integral reactor in 25 torr each of CO and H 2 O. By heating a fresh catalyst in H 2 , H 2 O, CO, or CO 2 , it was shown that deactivation occurs due to the presence of CO.Measurements of metal dispersion by CO adsorption and by x-ray diffraction show that deactivation on Pt/ceria and Pd/ceria catalysts in our studies was due to a loss of metal surface area. Finally, water-gas-shift rates on a series Pd/ceria catalysts with ceria crystallite sizes ranging from 7.2 to 40 nm and Pd loadings of either 1 wt% or 6 wt% demonstrated that rates were strictly proportional to the Pd surface area. Based on these observations, ceria-supported precious metals could be active and stable for the water-gas-shift reaction if steps are taken to avoid metal particle-size growth.

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In the industrial production of hydrogen and of synthesis gas the water gas shift reaction represents an essential step for the overall process efficiency of the process by increasing the hydrogen yield and by adjusting the desired ratio of hydrogen and carbon monoxide for a subsequent synthesis step. In addition to that in fuel cell technology it is absolutely necessary to decrease the CO content of the fuel hydrogen due to the limited CO tolerance of present day fuel cell anodes. To be successful applying water gas shift catalysts in hydrogen production for fuel cells especially in automotive systems some major obstacles of the commercial Cu/Zn/Al catalyst systems must be overcome. Major new demands are reduction of volume and weight by two to three orders of magnitude, oxidation resistance and improved tolerance for steam condensation and poisons, here mainly sulfur present in gas feedstock and in gasoline. Precious metal catalyst systems on rare earth metal oxides are most promising and could fulfill the requirements in nonstationary applications were conventional catalysts are not applicable.