Identification of Stable Species Formed Under CO Adsorption and Oxidation on Alumina-Supported Single Pt Atoms: Why Nanoparticles Are More Active

Single-atom catalysis is attractive in the context of sustainable chemistry, but single-atom catalysts (SACs) are not always more active than corresponding clusters or nanoparticles. This is the case, inter alia, of CO oxidation on Pt/γ-Al2O3, an archetypal catalytic system where SACs are poorly active. In the present work, combining diffuse reflectance infrared spectroscopy experiments and density functional theory calculations, we identify the stable species formed on a Pt/γ-Al2O3 SAC compared to its nanocatalyst counterpart. Formates predominantly occupy the alumina support sites, while oxidized Pt1 species can stabilize carbonyl, carbonate, and bicarbonate species, depending on the temperature regime. Coadsorption of carbonyl and carbonate moieties on the same platinum atom is found likely, based on both experimental and thermodynamic arguments. Unlike the mild adsorption of CO on Pt clusters, allowing for efficient CO oxidation, carbonyl and carbonate species exhibit high stability on the single Pt atoms, which can explain the low activity of the SAC.