Utilizing Quantitative in Situ FTIR Spectroscopy to Identify Well-Coordinated Pt Atoms as the Active Site for CO Oxidation on Al₂O₃-Supported Pt Catalysts

Relationships between geometric structures of active metallic sites and areal rates of reaction (structure sensitivity) are extensively studied for supported metal catalysts. For CO oxidation on irreducible oxide-supported Pt catalysts, there still exists a discrepancy regarding structure sensitivity. Theoretical and single-crystal analyses suggest the CO oxidation reaction rate should be highly structure sensitive, whereas measurements on supported Pt catalysts show only minimal structure sensitivity. Here, we used quantitative in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) to investigate the influence of CO oxidation reaction conditions on the fraction of well-coordinated (WC) and under-coordinated (UC) Pt active sites on a series of four α-Al₂O₃-supported Pt catalysts with average Pt sizes ranging from ∼1.4 to 19 nm. Pt nanoparticle surfaces were observed to restructure under CO oxidation reaction conditions, increasing the fraction of UC Pt sites. Reconstruction rendered the fraction of WC and UC sites less dependent on Pt particle size than expected from geometric models. A model, coupling the DRIFTS measurements with previous theoretical calculations, was quantitatively correlated to the measured slight structure sensitivity on the same series of catalysts. Our results bridge the gap between previous studies exploiting theory, single crystals, and supported Pt catalysts by demonstrating that WC Pt atoms are the active site for CO oxidation, but that CO-induced restructuring of Pt nanoparticle surfaces masks the inherent structure sensitivity in particle-size-dependent rate measurements.