Abstract
The addition of platinum-group metals (PGMs, e.g., Pt) to CeO(2) is used in heterogeneous catalysis to promote the rate of redox surface reactions. Well-defined model system studies have shown that PGMs facilitate H(2) dissociation, H-spillover onto CeO(2) surfaces, and CeO(2) surface reduction. However, it remains unclear how the heterogeneous structures and interfaces that exist on powder catalysts influence the mechanistic picture of PGM-promoted H(2) reactions on CeO(2) 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 H(2) reactions on high-surface area Pt/CeO(2) powder catalysts. TPR showed that Pt promotes H(2) consumption rates on Pt/CeO(2) even when Pt exists on a small fraction of CeO(2) particles, suggesting that H-spillover proceeds far from Pt-CeO(2) interfaces and across CeO(2)-CeO(2) particle interfaces. IR and EELS measurements provided evidence that Pt changes the mechanism of H(2) activation and the rate limiting step for Ce(3+), oxygen vacancy, and water formation as compared to pure CeO(2). As a result, higher-saturation surface hydroxyl coverages can be achieved on Pt/CeO(2) compared to pure CeO(2). Further, Ce(3+) formed by spillover-H from Pt is heterogeneously distributed and localized at and around interparticle CeO(2)-CeO(2) boundaries, while activated H(2) on pure CeO(2) results in homogeneously distributed Ce(3+). Ce(3+) localization at and around CeO(2)-CeO(2) boundaries for Pt/CeO(2) is accompanied by surface reconstruction that enables faster rates of H(2) consumption. This study reconciles the materials gap between model structures and powder catalysts for H(2) reactions with Pt/CeO(2) and highlights how the spatial heterogeneity of powder catalysts dictates the influence of Pt on H(2) reactions at CeO(2) surfaces.