Vacant ZnAl-layered double hydroxides photocatalytic CO2 to CH4: A DFT study

Abstract

Photocatalytic reduction of carbon dioxide (CO₂PR) into hydrocarbons is one of the most promising technologies to address global energy demand and environmental issues, and vacancy engineering can significantly enhance the activity of photocatalysts in CO₂PR. Herein, the structure, charge properties of vacant ZnAl-LDHs (hydrogen vacancy: V_H, hydroxyl vacancy: V_OH, zinc vacancy: V_Zn) and the thermodynamic mechanism of CO₂PR over these LDHs are investigated by using density functional theory. The calculation results indicate that the introduction of vacancies can promote the adsorption and activation of reactant CO₂. Compared to perfect ZnAl-LDH, introducing vacancies can reduce the Gibbs free energy barriers for CO₂PR to CO and CH₄. The analysis of the Gibbs free energy barriers of potential-determining step (ΔG_PDS) and HER suggests that V_OH-ZnAl-LDH is the best CO₂PR catalyst. By comparing the adsorption of reactants and intermediate species at different positions on the catalyst, it is found that species adsorption at vacancies is more conducive to the occurrence of catalytic reactions. Regulating the OH concentrations on the surface of LDHs also affects the catalytic activity of the catalyst, and when the concentration is 2/9, it has the lowest Gibbs free energy (0.258 eV) barrier for CO₂PR to CH₄. This work provides theoretical guidance for understanding the photocatalytic performance of LDHs materials containing vacancies for CO₂PR, and provides useful directions for designing and preparing efficient CO₂PR catalysts.