Theoretical Study on Photocatalytic CO2 Reduction to CO and CH4 over M(II)2M(III/IV)-Layered Double Hydroxides

Photocatalytic CO2 reduction (CO2PR) is of great significance for high-density solar energy storage and carbon balance, and layered double hydroxides (LDHs) as CO2PR catalysts have attracted much attention. In this work, the electronic properties such as the band structure, density of states and band edge placement of 10 MII2MIII/IV-NO3-LDHs (MII = Mg2+, Co2+, Ni2+, Zn2+; MIII = Al3+, In3+, Cr3+, Fe3+; MIV = Ti4+), thermodynamic mechanism, and thermodynamically favorable products of CO2PR, especially CO and CH4, over these LDHs are investigated by using Hubbard-corrected density functional theory. The calculation results indicate that all LDHs except the Mg2Al- and Mg2In-LDHs are responsive to visible light, and all the LDHs have enough driving force to undergo CO2PR to products except the Ni2Al- and Ni2Fe-LDHs. For Mg- and Zn-containing LDHs, the COOH pathway is favorable, while the Ni2Ti- and Co2Al-LDHs prefer the HCOO pathway. According to the minimum-free-energy reaction pathway, for Co2Fe-LDH, CO2PR prefers to generate HCOOH, and the potential-determining step is HCOO* → HCOOH*. For the Mg2Al-, Mg2In-, Co2Al-, Ni2Ti-, Zn2Al-, Zn2Cr-, and Zn2Ti-NO3-LDHs, CH4 is the favorable final product via HCOOH, HCHO, or CH3OH intermediates. The formation of CO mainly goes through the COOH pathway. The relationship between the effective driving force of CO2PR to CH4 or CO and the adsorption energy of the reactant CO2 or the valence band maximum (VBM) of LDHs reveals that the Mg2In-LDH and Mg2Al-LDH, which have a moderate adsorption and VBM favor CO2PR to CH4 and CO the most, respectively. On the Co2Fe-LDH (110), CO2 prefers to form CH4 than that of the Co2Fe-LDH (003), which indicates that the crystal faces also affect favorable products of CO2PR. This work provides useful information of the photocatalytic performance of LDH materials for CO2PR and is helpful for the design of highly efficient CO2PR catalysts.