Flexible Intervention of Polyoxometalate Support on the Electronegativity of Single Atoms to Enhance Catalytic Activity

The construction of single-atom catalysts (SACs) using polyoxometalates (POMs) as supports has attracted significant attention. Specifically, POMs possess the unique ability to reversibly accept and donate electrons; yet, the potential benefits of this distinctive characteristic on the activity of single atoms have remained unexplored. In this study, we employ density functional theory (DFT) calculations to investigate the synthesis of CH₃COOH from CO, CH₄, and H₂O catalyzed by POM-supported SAC M1/POM (M = Pt, Rh, Ru, Pd, Co), aiming to gain a more comprehensive understanding of the role of POM support in SACs. Our proposed mechanism first involves CH₄ activation for producing •CH₃ and, at the same time, the catalytic intermediate [M1/POM]^-. Then, CO and •CH₃ are sequentially adsorbed on the single-atom site of [M1/POM]^- and coupled to form COCH₃. Finally, as H₂O attacks CH₃CO, CH₃COOH is formed and released. The poorer activity of M1/POM (M = Rh, Ru, Pd, Co) compared with that of Pt1/POM is attributed to the low matching degree in the frontier molecular orbital energy between [M1/POM]^- and CO/•CH₃, which results in the inaccessibility of CO and •CH₃ adsorptions, thus hindering the subsequent CH₃COOH formation. Throughout the reaction process, the POM support promotes dynamic switching of single atoms between electron-rich and electron-poor states, leveraging its reversible electron transfer capability. The electron-deficient species •CH₃ adsorption and H₂O attack are enabled by the regular access of single atoms to electron-saturated state, while timely switching of single atoms to electron-deficient states facilitates CO adsorption and CH₃ attack.