Selective photocatalytic CO2 reduction over Zn-based layered double hydroxides containing tri or tetravalent metals

Photocatalytic CO2 reduction holds promise as a future technology for the manufacture of fuels and commodity chemicals. However, factors controlling product selectivity remain poorly understood. Herein, we compared the performance of a homologous series of Zn-based layered double hydroxide (ZnM-LDH) photocatalysts for CO2 reduction. By varying the trivalent or tetravalent metal cations in the ZnM-LDH photocatalysts (M = Ti4+, Fe3+, Co3+, Ga3+, Al3+), the product selectivity of the reaction could be precisely controlled. ZnTi-LDH afforded CH4 as the main reduction product; ZnFe-LDH and ZnCo-LDH yielded H2 exclusively from water splitting; whilst ZnGa-LDH and ZnAl-LDH generated CO. In-situ diffuse reflectance infrared measurements, valence band XPS and density function theory calculations were applied to rationalize the CO2 reduction selectivities of the different ZnM-LDH photocatalysts. The analyses revealed that the d-band center (εd) position of the M3+ or M4+ cations controlled the adsorption strength of CO2 and thus the selectivity to carbon-containing products or H2. Cations with d-band centers relatively close to the Fermi level (Ti4+, Ga3+ and Al3+) adsorbed CO2 strongly yielding CH4 or CO, whereas metal cations with d-band centers further from the Fermi level (Fe3+ and Co3+) adsorbed CO2 poorly, thereby yielding H2 only (from water splitting). Our findings clarify the role of trivalent and tetravalent metal cations in LDH photocatalysts for the selective CO2 reduction, paving new ways for the development of improved LDH photocatalyst with high selectivities to specific products.