INTERACTION OF PLATINUM AND CERIA PROBED BY TRANSMISSION ELECTRON-MICROSCOPY AND CATALYTIC REACTIVITY

The beneficial effects of ceria on automotive exhaust catalysis have been amply documented in the literature, but the mechanisms for ceria promotion are still being debated. Contact between the metal and ceria could lead either to electronic or geometric perturbations in the surface of the metal particles, or atom transfer (such as of oxygen) between ceria and the metal may be responsible for the enhanced oxidation activity. In this study, we have probed the nature of surface modifications in the metal by measuring catalyst activity in reactions such as the hydrogenolysis of n-butane and carbon monoxide hydrogenation. The activity in the probe reactions provides a measure of the interaction between the metal and ceria. Transmission electron microscopy (TEM) has been used to provide direct information on the nature of the metal-ceria contacts. We find that metal-ceria interaction leads to suppressed hydrogenolysis activity after high-temperature reduction (773 K), but the loss in activity can be reversed after oxidation at 773 K. The presence of ceria also leads to an enhancement in methanation activity which, unlike hydrogenolysis, is unaffected by the high-temperature pretreatments (oxidation or reduction). These results in conjunction with the TEM data have been used to propose a model for the beneficial effects of ceria in automotive catalysts.