Targeted H2O activation to manipulate the selective photocatalytic reduction of CO2 to CH3OH over carbon nitride-supported cobalt sulfide

Abstract

The selective photocatalytic reduction of CO₂ by H₂O to methanol is a desirable solution for solar energy storage with the production of abundant chemicals. However, this is a formidable challenge because the conversion of CO₂ to CH₃OH is a 6-electron transfer reaction that needs 4 protons, while other products requiring less electrons and protons, such as CO, HCOOH, and HCHO, are much easier to form than CH₃OH, leading to lower selectivity for the production CH₃OH. Herein, we propose a novel concept of targeting activated H₂O to provide ample protons without the generation of strong oxidative free radicals for the highly selective photocatalytic reduction of CO₂ to CH₃OH with H₂O. Carbon nitride (CN)-supported cobalt sulfide (CS) was fabricated as the desired prototype photocatalyst (CS/CN). Cobalt sulfide was confirmed to promote the generation of protons without the formation of strong oxidative free radicals (such as ˙OH and ˙O₂⁻) by significantly weakening the overpotential of the H₂O oxidation half-reaction. This targeted H₂O activation contributed to the continuous coupling of multi-protons/electrons and CO₂ to form the key intermediates CHO* and CH₃O*, which were necessary for the generation of CH₃OH. Accordingly, the CH₃OH selectivity by the optimized CS/CN (87.2%) photocatalyst was 2.3 times higher than that of CN (38.6%) with an increase in the CH₃OH production rate from 22.0 μmol g⁻¹ h⁻¹ to 97.3 μmol g⁻¹ h⁻¹. This work provides an elegant solution to achieve the highly selective photocatalytic conversion of CO₂ to CH₃OH by modulating the H₂O activation process.