Understanding the interaction between transition metal doping and ligand atoms of ZnS and ZnO monolayers to promote the CO2 reduction reaction

Single-atom catalysts (SACs) obtained by doping transition metal (TM) atoms into stable monolayers are a promising way to improve the CO2 reduction reaction (CRR) performance. In this work, we theoretically investigated the effect of ligand atoms around the doped TM (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) in ZnO and ZnS for promoting the CRR performance. We found that the ligand atoms around the TM can influence its oxidation state and the electronic properties of the SACs, thus affecting their CRR activity. Due to the smaller charge transfer between the TM and substrate for TM–ZnS compared to TM–ZnO, the TM binding is weaker for the former. In addition, the more negatively charged oxygen ligand atoms in TM–ZnO interact with reaction intermediates, resulting in CRR products with less electron transfer. Pristine ZnS and ZnO monolayers can produce HCOOH but require a high limiting potential (UL) of about −1.2 V. Doping with TMs can reduce UL compared to the pristine surface. At the same time, the ligand can alter the preferred CRR pathway and product selectivity. We found that Mn–ZnS is selective to the CH4 product with a UL of only −0.29 V, which is a nearly 1 V improvement in the UL compared to ZnS.