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 (ε_d) 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.