Regulating Cu atom orbital state on self-built photogate catalyst for improving HCOOH selectivity of CO2 reduction

Abstract

While single atom site catalysts receive tremendous attention for CO2 reduction, the understanding of orbital coupling interaction between metal-atom and CO2 molecular remains challenging and rare toward CO2 activation and selectivity of reduction products. Herein, binuclear-site Zn/Cu-porphyrin covalent organic framework (ZnCu-COF) photogate catalyst was synthesized, with characterizations of intraskeletal photogate molecular device (PMD). The ZnCu-COF showed the HCOOH selectivity of 89.1%, which was superior than Zn-COF and CuCOF counterparts. The in-depth experimental and theoretical analyses reveal that Cu atom centers can enrich the electrons of Zn photocenters via directional and stepwise electron diffusion, which remarkably affects the orbital energy of Cu-3d, further deciding the electron state re-distribution of Cu-3d (dxy, dyz, dzx, dz2, and dx2_y2 ) orbitals. By a strong orbital coupling strength between Cu-3dz2 and C-2pz, Cu atom centers promote CO2 activation and nucleophilic reaction of the unsaturated carbon of *COOH intermediate, which clearly accounts for the different HCOOH and CO selectivity in COFs. This work opens critical perspective to the rational engineering, catalytic mechanism, and application of PMD-derived atomic-site catalysts.