Single-atom catalysts (SACs) often exhibit dynamic responses to the reaction and pre-treatment conditions that affect their activity. The lack of understanding of these behaviors hinders the development of effective, stable SACs, and also makes understanding the active sites and reaction mechanisms rather difficult. In this presentation, we combine kinetic, microscopic (STEM), spectroscopic (XAS and IR), and theoretical (DFT) studies, to elucidate how the chemical environment of metal sites on Pt/TiO2 and Pd/TiO2 (both anatase) SACs evolve under catalytically relevant conditions, and how it affects their activity for the reverse water-gas shift (rWGS) reaction, i.e., CO2 hydrogenation to CO. Reducing fresh Pt/TiO2 SAC with H2 sinters Pt single atoms into ~1 nm, amorphous, flat Pt rafts with similar activity. Re-oxidizing the Pt rafts re-disperses them into single atoms, in a new, less O-coordinated chemical environment that makes them more accessible, and thus 5 times more active than fresh Pt single atoms. Similarly, the same type of rafts are also formed on Pd/TiO2 SAC after H2 reduction at 400 °C, but are significantly more active than the fresh SAC, because the support reduction creates electron-rich active Pd sites with partially cleaved Pd−O interface. Meanwhile, although the Pd rafts are also re-dispersed into single atoms by re-oxidation, the re-dispersed single atoms have identical chemical environment and thus activity with fresh Pd single atoms. In contrast, CO formed during the rWGS facilitate the sintering of Pd into ~5 nm, stable, crystalline nanoparticles, deactivating the SAC. This work demonstrates a clear picture of how the structural evolution of two SACs enhances their catalytic efficiency, with the contrast of the two emphasizing that the chemical environment of highly dispersed metal species, instead of their nuclearity, could ultimately determine the activity.

Dynamic chemical environment of TiO2-supported single metal atoms determining their activity for the reverse water-gas shift (RWGS) reaction

ACS Spring 2022

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