In Situ Dispersion of Pd on TiO2 During Reverse Water-Gas Shift Reaction: Formation of Atomically Dispersed Pd.

Expanding the application scope of single-atom catalysts (SACs) to high-temperature hydrogenation requires materials that thermodynamically favor metal atom isolation over cluster formation under these conditions. Here, we show that Pd can be predominantly dispersed as isolated atoms onto TiO 2 during the reverse water-gas shift (rWGS) reaction at 400 °C. Achieving atomic dispersion requires an artificial increase of the absolute TiO 2 surface area by an order of magnitude and can be accomplished by physically mixing a precatalyst (Pd/TiO 2 ) with neat TiO 2 prior to the rWGS reaction. The in situ dispersion of Pd was reflected through a continuous increase of rWGS activity over 92 h and supported by kinetic analysis, infrared spectroscopy, scanning transmission electron microscopy, and X-ray absorption spectroscopy. The thermodynamic stability of Pd under high-temperature rWGS conditions is associated with Pd-Ti coordination (enthalpic stabilization), which seemingly manifests upon O-vacancy formation, and the artificial increase in TiO 2 surface area (entropic stabilization). In addition to this, steady-state isotopic transient kinetic analysis suggests an upper limit of two for the number of oxide-supported atoms required to catalyze the WGS reaction. Our approach to noble metal SACs synthesis can be generally applied and provides a platform to study the reactivity of heterolytically-activated hydrogen toward unsaturated functional groups.