Single-atom catalysts (SACs) often exhibit dynamic responses to the reaction and pretreatment environment that affect their activity. The lack of understanding of these behaviors hinders the development of effective, stable SACs, and also makes their investigations rather difficult. Here we present a reduction-oxidation cycle that induces nearly five-fold activity enhancement on Pt/TiO2 SACs for the reverse water-gas shift (rWGS) reaction, i.e., CO2 hydrogenation to CO. We combine microscopy (STEM) and spectroscopy (XAS and IR) studies with kinetic measurements, to convincingly show that the low activity on the fresh SAC is a result of limited accessibility of Pt single atoms due to high Pt−O coordination. The reduction step mobilizes Pt single-atoms, forming small, amorphous, and unstable Pt clusters. The re-oxidation step re-disperses Pt clusters into single atoms, but in a new, less O-coordinated chemical environment that makes them more accessible and, consequently, more active. After the cycle, the SAC exhibits superior rWGS activity to non-atomically-dispersed Pt/TiO2. During the rWGS, the activated Pt/TiO2 SAC experiences slow deactivation, but can be re-activated by mild oxidation, as a result of the interchange between small clusters and single atoms. This work demonstrates a clear picture of how the structural evolution of Pt/TiO2 SACs leads to ultimate catalytic efficiency, offering desired understanding on the rarely explored dynamic chemical environment of supported single metal atoms and its catalytic consequences.

Unlocking the catalytic potential of TiO2-supported Pt single atoms by altering their chemical environment

ACS Fall 2021

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