Insight into the Effect of the d-Orbital Energy of Copper Ions in Metal–Organic Frameworks on the Selectivity of Electroreduction of CO2 to CH4

It is of great significance to reveal the influence of small differences in the coordination environments of metal ions in the catalytic active centers on the selectivity of products in the electrocatalytic CO2 reduction reaction (CO2RR). Here, two types of metal–organic frameworks (MOFs) based on square-pyramidal CuO5 and square-planar CuO4 nodes, respectively, are compared in regard to their performances in electrocatalytic CO2RR. The MOF (Cu-DBC, H8DBC = dibenzo-[g,p]chrysene-2,3,6,7,10,11,14,15-octaol) constructed by CuO5 nodes and the catechol-derived ligands exhibit high performance for the electrocatalytic reduction of CO2 to CH4 with a Faradaic efficiency of 56% and a current density of 11.4 mA cm–2 at −1.4 V vs RHE. In comparison, two other MOFs, Cu-HHTP (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) and Cu-THQ (THQ = tetrahydroxy-1,4-quinone), constructed by CuO4 and catechol-derived ligands, exhibit that CO is the sole reduced product. Theoretical calculations and Cu L-edge adsorption spectroscopy revealed that the energy levels of metal d-orbitals (dz2, dxz, and dyz) in the square-pyramidal CuO5 site are elevated compared to those in the square-planar CuO4 site. As a result, the CuO5 active sites can strongly adsorb *CO intermediates and hence facilitate the hydrogenation of *CO into *CHO, which is beneficial for yielding CH4 instead of CO. This work will be helpful to understand the mechanism of copper-based catalysts for the electrocatalytic reduction of CO2 to hydrocarbons.