How Pt Influences H₂ Reactions on High Surface-Area Pt/CeO₂ Powder Catalyst Surfaces

Abstract

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.