Single-atom catalysts (SACs) often exhibit dynamic responses to the reaction and pre-treatment conditions, which significantly affect their performance. The lack of understanding of these behaviors hinders the development of effective, stable SACs, and makes understanding the active sites and reaction mechanisms rather difficult. Here, we presents the dynamic structural responses of Pd/TiO2-anatase SAC (Pd1/TiO2) to various atmospheres related to the reverse water-gas shift (rWGS) reaction, which determine its rWGS activity. Combining reaction kinetics, in-situ characterization, and theoretical modelling, we demonstrate at ≥ 350 °C, the bulk reduction of TiO2 by H2 leads to electron-rich Pd sites with partially cleaved Pd−O interface, which exhibit high intrinsic rWGS activity through the carboxyl pathway. Such highly active Pd environment is reversed by oxidation, which when performed at 400 °C, also promotes TiO2 reduction and thus the active Pd site formation by removing acid layers adsorbed from ambient atmosphere. Meanwhile, under H2, some single Pd atoms (Pd1) sinter into disordered, ~1 nm clusters (Pdn), which are re-dispersed into Pd1 by 400 °C oxidation. In contrast, under CO, all Pd1 aggregate into crystalline, ~5 nm nanoparticles (PdNP), deactivating Pd1/TiO2. During the rWGS, H2 and CO, and thus the two Pd1 evolution pathways co-exist. The H2-activation prevails, leading to the rate increases with time-on-stream, a unique phenomenon not observed with other metals or oxide supports. This work clearly unveils how the structure of a SAC evolves under conditions relevant to catalysis and how such dynamics can create highly active metal sites in modified chemical environment. The fundamental understanding is valuable to designing more effective and stable SACs.

Dynamic chemical environment of TiO2-supported single Pd atoms determining their activity for the reverse water-gas Shift (rWGS) reaction

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