Probing the redox capacity of Pt–CeO2 model catalyst for low-temperature CO oxidation

Abstract

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 Pt⁰ nanoparticles, ultra-small Pt* aggregates, and PtO_x 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 Pt⁰ nanoparticles, ultra-small Pt*/PtO_x 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 Pt⁰ 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.