(710g) Elucidating the Evolution of Pd Single-Atoms on Anatase TiO2 during CO2 Hydrogenation

Interests in single-atom catalysts (SACs) have observed rapid increases recently. As a result of the instability and high mobility of single-atoms (SAs), the structure of SACs and the chemical environment of metal centers are often dynamic under reaction conditions, limiting their application scopes and challenging mechanistic investigations. Nonetheless, limited understanding has been obtained in the important area and systematic insights are desired. Here, we present detailed studies that elucidate the evolution of Pd SAs supported on anatase TiO₂ during high-temperature CO₂ hydrogenation, a highly enticing reaction for carbon mitigation and chemical production. We discovered that under the reaction conditions, low-surface-density Pd₁O₃ SAs (Figure a) on TiO₂ evolve in two pathways: forming a more active species, Pd_active, and sintering into large metallic particles. The former leads to the in situ increase in the reaction rate over ~ 30 h (Figure b), has an activation barrier of ~121 kJ/mol, and is facilitated by H₂ in the reaction stream (Figure b). In contrast, the latter is induced by CO produced by the reaction, resulting in catalyst deactivation (Figure b). The in situ Pd_active formation is unique to Pd/TiO₂, as other noble metal SAs (Pt, Rh, or Ru) and supports (CeO₂ or SiO₂) do not exhibit the same behavior (Figure c). The exact structure of Pd_active is under thorough investigation with in situ spectroscopy and theory, and ex situ XAS reveal that they can be completely oxidized into bulk-like PdO upon air exposure, while metallic particles created by CO treatment cannot (Figure a). This presentation depicts a complete and clear picture of the dynamic behaviors of TiO₂-supported Pd SAs under CO₂ hydrogenation and relevant reducing conditions. These insights will benefit both the understanding of hydrogenation mechanisms over SAs and the design of more stable, versatile SACs.