Insight into mechanism of divalent metal cations with different d-bands classification in layered double hydroxides for light-driven CO2 reduction

By varying divalent-metal cations (M = Mg2+, Ni2+, Cu2+, Zn2+) with different d-bands classification, MAl-LDHs were prepared and inspected for solar-driven CO2 conversion. The CO and CH4 yields over these LDHs followed an order of: NiAl-LDHs > CuAl-LDHs > ZnAl-LDHs > MgAl-LDHs. Coupled with multiple characterizations, the activity distinction was revealed from three crucial perspectives: (i) light harvesting, (ii) charge separation and transfer, (iii) surface reactions. The results affirmed that when d orbital was partially occupied with active electrons, both activation of CO2 and excitation of electrons were much easier to achieve. In-situ DRIFTS manifested that NiAl-LDHs and CuAl-LDHs exhibited more active reaction with CO2, which may be related to the upward shifts of the d-band center determined by Valence-band XPS. Particularly, these LDHs exhibited various catalytic stabilities, which was explained by Pearson׳s hard and soft acid–base (HSAB) principle for the first time in this field.