(52e) Spectroscopic Signatures and Reactivity of CO Adsorbed to Atomically Dispersed Pt Atoms, Pt Oxide Clusters, and Metallic Pt Clusters on Anatase TiO2

Due to their broad application in industrial chemical conversion processes and low abundance in the earths crust, significant efforts have been dedicated to developing efficient approaches for utilizing Pt-group metals as catalysts. The primary approach for optimizing Pt-group metal utilization in catalysis is synthesizing metal active sites that exhibit 100% dispersion - all metal atoms exposed to catalytic environment. However, there are significant challenges associated with the controlled synthesis and structural characterization of perfectly dispersed (this could be single metal atoms on the support, or sub-nanometer metal clusters) Pt-group metal species, which has led to a variety of inconsistent conclusions about their reactivity.

In this talk we will discuss a rigorous comparison of the reactivity of single Pt atoms, sub-nanometer PtO_x clusters, and sub-nanometer Pt⁰ clusters supported on anatase TiO₂ for CO adsorption and catalytic CO oxidation. Control over the synthesized structure was achieved through the use of strong electrostatic adsorption synthesis protocols on extremely high surface area TiO₂ (290 m²/g) at dilute synthesis conditions and varied Pt weight loadings (0.025-1%), combined with controlled oxidative or reductive pre-treatments. Using correlated IR spectroscopy and scanning transmission electron microscopy (STEM) we identify characteristics signatures of CO bound to cationic single Pt atoms, PtO_x clusters and Pt⁰ clusters. Interestingly, it was identified that while single Pt atoms on TiO₂ and PtO_x clusters both exhibit similar electronic and local coordinative environments, they exhibit drastic variation in CO adsorption energies, with single Pt atoms adsorbing CO weakly and PtO_x clusters adsorbing CO even more strongly than Pt⁰ clusters. Using the correlated STEM-IR characterization we compare the CO oxidation reactivity of single Pt atoms and metallic clusters under conditions of strict kinetic control where we find that both types of active sites exhibit an identical reaction mechanism, while single Pt atoms are more active on Pt mass and Pt surface area bases. We discuss the ramification of these findings in the context of designing stable catalysts with optimal Pt-group metal utilization.