Study of the Water-Gas-Shift Reaction on Pd@CeO2/Al2O3 Core-Shell Catalysts

An alumina-supported, Pd@CeO2, core-shell catalyst having 1 wt % Pd and 9 wt % ceria was characterized for the water-gas-shift (WGS) reaction. Although the catalyst initially exhibited similar WGS rates to that of a conventional Pd/ceria catalyst at 623 K in 25 Torr each of CO and H2O, the Pd@CeO2 catalyst deactivated severely over the period of 1 h. The WGS activity of the Pd@CeO2 could be completely restored by mild oxidation, and oxygen-titration measurements showed that the ceria shell in the Pd@CeO2 catalyst was significantly reduced after being used for the WGS reaction. These observations are in sharp contrast to those found with a conventional Pd/ceria catalyst, for which the ceria remains almost fully oxidized under WGS conditions. CO adsorption measurements, using FTIR at room temperature and CO uptakes at 195 K, indicated that Pd in the oxidized Pd@CeO2 catalyst was accessible to CO, but adsorption was completely suppressed on the reduced catalyst. A model is presented to explain the results, which assumes that cracks and fissures in the oxidized ceria shell allow access to the Pd core but that reduction blocks access, either due to changes in the density of the ceria, which closes the fissures, or to coverage of the metal surface with ceria.