Origin of the High CO Oxidation Activity on CeO2 Supported Pt Nanoparticles: Weaker Binding of CO or Facile Oxygen Transfer from the Support?

Pt clusters supported on CeO2 are highly active for low temperature CO oxidation. The enhanced reactivity could be caused by weakening of the CO binding on Pt, allowing adsorbed oxygen to more effectively compete for sites. An alternative explanation is that interfacial sites allow adsorbed CO on Pt to react with lattice oxygen in the ceria. Here we explore the origins of enhanced CO oxidation reactivity on Pt/CeO2 using in-situ/operando infrared and x-ray spectroscopies, microcalorimetry, and reaction kinetics. We show that CO adsorbs strongly (similar to 110-120 kJ/mol) on Pt clusters (similar to 1.5 nm) and Pt is almost fully covered by CO during reaction, indicating that the high activity can be related to reactive interfacial O* species. Using in-situ infrared spectroscopy we show that when the reaction mixture of CO and O-2 stops flowing over the catalyst, the adsorbed CO on Pt is lost since it reacts with interfacial O*, but if this O* is depleted, the CO band does not disappear. The role of CeO2 is not to alter the binding of CO, but rather to enhance the reactivity of the interfacial metal-support lattice oxygens. The reaction mechanism involving the interfacial Pt-CeO2 sites (two-site mechanism) is further confirmed by kinetic measurements which show near zero order dependence on CO and O-2 partial pressures.