The addition of platinum-group
metals (PGMs, e.g., Pt) to CeO2 is used in heterogeneous
catalysis to promote the rate of
redox surface reactions. Well-defined model system studies have shown
that PGMs facilitate H2 dissociation, H-spillover onto
CeO2 surfaces, and CeO2 surface reduction. However,
it remains unclear how the heterogeneous structures and interfaces
that exist on powder catalysts influence the mechanistic picture of
PGM-promoted H2 reactions on CeO2 surfaces developed
from model system studies. Here, controlled catalyst synthesis, temperature-programmed
reduction (TPR), in situ infrared spectroscopy (IR), and in situ electron
energy loss spectroscopy (EELS) were used to interrogate the mechanisms
of how Pt nanoclusters and single atoms influence H2 reactions
on high-surface area Pt/CeO2 powder catalysts. TPR showed
that Pt promotes H2 consumption rates on Pt/CeO2 even when Pt exists on a small fraction of CeO2 particles,
suggesting that H-spillover proceeds far from Pt–CeO2 interfaces and across CeO2–CeO2 particle
interfaces. IR and EELS measurements provided evidence that Pt changes
the mechanism of H2 activation and the rate limiting step
for Ce3+, oxygen vacancy, and water formation as compared
to pure CeO2. As a result, higher-saturation surface hydroxyl
coverages can be achieved on Pt/CeO2 compared to pure CeO2. Further, Ce3+ formed by spillover-H from Pt is
heterogeneously distributed and localized at and around interparticle
CeO2–CeO2 boundaries, while activated
H2 on pure CeO2 results in homogeneously distributed
Ce3+. Ce3+ localization at and around CeO2–CeO2 boundaries for Pt/CeO2 is
accompanied by surface reconstruction that enables faster rates of
H2 consumption. This study reconciles the materials gap
between model structures and powder catalysts for H2 reactions
with Pt/CeO2 and highlights how the spatial heterogeneity
of powder catalysts dictates the influence of Pt on H2 reactions
at CeO2 surfaces.