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Abstract

Platinum supported on ceria can oxidize CO in excess hydrogen selectively (PROX process). In-situ DRIFTS and high-pressure (~1 mbar) XPS experiments were performed to study the mechanism of the PROX reaction on Pt/ceria catalysts. The partial pressure of O2 and/or CO was varied and correlated with induced changes in activity and selectivity as well as to the surface state and species under reaction conditions. Pt-carbonyl species changed rather insignificantly, especially relative to the wide variations of the product pattern with changing feed composition. Furthermore the interconversion of formate and carbonate species was observed. Therefore the changes in the evolution of surface species detected by in-situ DRIFTS cannot explain the observed variation in the CO oxidation activity. On the other hand, high-pressure XPS showed significant modification of the surface state with changing feed composition. Most significantly, oxygen vacancy formation seems to correlate with enhancing CO oxidation activity. At higher vacancy density water desorption was hindered. Highly hydrated ceria with significant vacancy density was found to be beneficial for the PROX process; here surface water blocked Hads oxidation sites. Moreover, lower apparent activation energy of CO oxidation was measured in the PROX reaction on catalysts with more vacancies. The results shown here reinforce the view of catalysts being adaptive to a certain reaction rather than having active sites as prepared. While IR-detectable surface species may only be indicators and/or consequence of this surface change, the formation of the beneficial surface/near-surface state may be the rate limiting factor in several catalytic processes.