Surface Noble Metal Concentration on Ceria as a Key Descriptor for Efficient Catalytic CO Oxidation

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 ∼4, while keeping the weight hourly space velocity
constant, the ignition temperature of CO oxidation was decreased by
∼200 °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.