Theoretical insight on why N-vacancy promotes the selective CO<inf>2</inf> reduction to ethanol on NiMn doped graphitic carbon nitride sheets

Abstract

To develop promising dual atom catalysts (DACs) for enhancing valuable C2+ products in CO2 electroreduction (CO2RR), we need a molecular level understanding of the interaction between reaction intermediates, metal atoms, and substrates. NiMn on graphitic carbon nitride (g-C3N4) was experimentally reported to be an efficient CO2RR catalyst. Here, we studied the origin of its activity. We used integrated crystal orbital Hamiltonian population (ICOHP) analysis along the reaction coordinate of the carbon–carbon (C-C) coupling reaction to understand how the electronic structures of NiMn doped on pristine (NiMn@g-C3N4) and N-vacancy graphitic carbon nitride (NiMn@V-g-C3N4) affect the reaction. NiMn@V-g-C3N4 selectively produces ethanol at low limiting potential −0.55 V and a low kinetic barrier (0.78 eV) for *CO+*CHO→*COCHO. At this step, electron donation from the NiMn in the N-vacancy to the adsorbate is essential. Tricoordinated Ni atom at the vacancy site has a stable oxidation state 0 with a fully filled 3d10 configuration, while Mn atom takes +2 oxidation state with a half-filled 3d5 configuration. ICOHP shows that these electronic configurations result in a moderate binding strength of key intermediates near the Ni while facilitating the flexible change in Mn-C to Mn-O binding for producing *COCHO, thus promoting the formation of ethanol.