CO oxidation by Pd supported on CeO₂(100) and CeO₂(111) facets

Pd/CeO₂ is an active component in emission control catalysts for CO oxidation. Nanostructured CeO₂ powders can be prepared in the form of rods exposing predominantly (111) surfaces and cubes exposing (100) surfaces. While differences in the reactivity of Pd supported on these facets of ceria have been reported, the origins of the reactivity differences are not well understood. Pd supported on (111) surfaces of ceria rods exhibits room-temperature CO oxidation activity, while Pd on (100) surface of ceria cubes shows comparable activity at a temperature that is 60 °C higher. Earlier, we established that Pd/CeO₂-rods are active due to a Langmuir-Hinshelwood mechanism involving isolated Pd atoms in the form of Pd₁O and Pd₁O₂ species. Here, we establish using in situ CO IR spectroscopy and density functional theory (DFT) that, in addition to TEM-visible Pd nanoparticles, Pd/CeO₂-cubes also contain isolated Pd species, predominantly in the form of Pd₁O. DFT calculations show that CO oxidation proceeds via a Mars-van Krevelen pathway, which is possible for the (100) surface because of the lower Ce-O binding energy compared to the (111) surface. Overall, the catalytic cycle for CO oxidation on Pd/CeO₂(100) involves a higher free energy barrier than on Pd/CeO₂(111) in keeping with the experimentally observed activity difference. EXAFS measurements show that the active Pd phase in both Pd/CeO₂-rods and Pd/CeO₂-cubes responds dynamically with respect to reducing and oxidizing conditions. The redispersion of Pd in oxidative conditions is more pronounced for Pd/CeO₂-rods and the catalyst is more active after an oxidative treatment.