Surface Noble Metal Concentration on Ceria as a Key Descriptor for Efficient Catalytic CO Oxidation
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Abstract
During the CO oxidation over metallic Pt clusters and Pt nanoparticles in Pt/CeO2 catalysts, we found that the Pt surface concentration is a key descriptor for the reaction rate. By increasing the surface noble metal concentration (SNMC) of a Pt/CeO2 catalyst by a factor of similar to 4, while keeping the weight hourly space velocity constant, the ignition temperature of CO oxidation was decreased by similar to 200 degrees C in the as-prepared state. Moreover, the stability was enhanced at higher SNMC. Complementary characterization and theoretical calculations unraveled that the origin of this improved reaction rate at higher Pt surface concentrations can be traced back to the formation of larger oxidized Pt-clusters and the SNMC-dependent aggregation rate of highly dispersed Pt species. The Pt diffusion barriers for cluster formation were found to decrease with increasing SNMC, promoting more facile agglomeration of active, metallic Pt particles. In contrast, when Pt particle formation was forced with a reductive pretreatment, the influence of the SNMC was temporarily diminished, and all catalysts showed a similar CO oxidation activity. The work shows the general relevance of the proximity influence in the formation and stabilization of active centers in heterogeneous catalysis.
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