Abstrakt
The redox capacity of Pt-CeO2 catalysts for low-temperature CO oxidation has been investigated by means of near-ambient pressure X-ray photoelectron spectroscopy, synchrotron radiation photoelectron spectroscopy, and resonant photoemission spectroscopy. The well-defined model Pt-CeO2 systems containing specific Pt species which differ with respect to the oxidation state, chemical environment, and nuclearity, including atomically dispersed Pt2+ and Pt4+ species, metallic Pt-0 nanoparticles, ultra-small Pt* aggregates, and PtOx clusters were prepared by physical vapor co-deposition of Pt and Ce metals in an oxygen atmosphere onto a CeO2(111) buffer layer on Ru(0001) and subsequent annealing under reducing or oxidizing conditions.
The oxidation states of Pt species and Ce cations were monitored upon CO exposure as a function of temperature. We found that metallic Pt-0 nanoparticles, ultra-small Pt*/PtOx clusters, and Pt4+ species serve as CO adsorption sites at low temperature.
Exclusively, the redox capacity for the low-temperature CO oxidation (below the room temperature) was observed only for the Pt-CeO2 catalyst containing metallic Pt-0 nanoparticles. The corresponding redox pathway is associated with CO spillover and the formation of bidentate carbonate species.
Above 400 K, the redox interaction of CO with model Pt-CeO2 catalysts involves the Mars-van Krevelen mechanism regardless of the nature of the Pt species.