Ruthenium Single Atomic Sites Surrounding the Support Pit with Exceptional Photocatalytic Activity.

Single-metal atomic sites and vacancies can accelerate the transfer of photogenerated electrons and enhance photocatalytic performance in photocatalysis. In this study, a series of nickel hydroxide nanoboards (Ni(OH) x NBs) with different loadings of single-atomic Ru sites (w-SA-Ru/Ni(OH) x ) were synthesized via a photoreduction strategy. In such catalysts, single-atomic Ru sites are anchored to the vacancies surrounding the pits. Notably, the SA-Ru/Ni(OH) x with 0.60 wt % Ru loading (0.60-SA-Ru/Ni(OH) x ) exhibits the highest catalytic performance (27.6 mmol g -1  h -1 ) during the photocatalytic reduction of CO 2 (CO 2 RR). Either superfluous (0.64 wt %, 18.9 mmol g -1  h -1 ; 3.35 wt %, 9.4 mmol -1  h -1 ) or scarce (0.06 wt %, 15.8 mmol g -1  h -1 ; 0.29 wt %, 21.95 mmol g -1  h -1 ; 0.58 wt %, 23.4 mmol g -1  h -1 ) of Ru sites have negative effect on its catalytic properties. Density functional theory (DFT) calculations combined with experimental results revealed that CO 2 can be adsorbed in the pits; single-atomic Ru sites can help with the conversion of as-adsorbed CO 2 and lower the energy of *COOH formation accelerating the reaction; the excessive single-atomic Ru sites occupy vacancies that retard the completion of CO 2 RR.