CO, NO, and H-2 adsorption on ceria-supported Pd

We have examined the adsorption of CO, NO, and H-2 on model Pd/ceria catalysts, formed by vapor deposition of Pd metal onto a flat, polycrystalline ceria support, using temperature programmed desorption. For CO adsorption on freshly deposited Pd films, a substantial fraction of the CO reacts to CO2 in TPD, between 450 and 550 K, indicating that oxygen from ceria can react with CO on Pd. Prereduction of the catalyst by heating in CO or H-2 results in TPD curves which are virtually identical to that observed for Pd/alpha-Al2O3(0001), suggesting that electronic interactions between ceria and Pd have a negligible effect on the adsorption properties. For NO, there are differences between Pd/ceria and Pd/alpha-Al2O3(0001). A significantly higher fraction of the NO on Pd/ceria reacts to N-2 and N2O, and prereduction of the Pd/ceria increases the amount of NO that can adsorb. Both of these results are interpreted as indicating that NO can adsorb on reduced ceria sites which are formed when lattice oxygen at the Pd-ceria interface is transferred to the Pd. For H-2, the desorption curves are similar to that observed on Pd/alpha-Al2O3(0001), except that the low-temperature feature assigned to the bulk hydride is less well resolved. If water is formed upon H-2 adsorption, it apparently readsorbs on the ceria. The implications of these results for reactions involving NO and CO on Pd/ceria are discussed. (C) 1996 Academic Press, Inc.