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.

光催化 CO ₂还原有望成为燃料和商品化学品制造的未来技术。然而，控制产品选择性的因素仍然知之甚少。在此，我们比较了同系列锌基层状双氢氧化物 (ZnM-LDH) 光催化剂对 CO ₂还原的性能。通过改变 ZnM-LDH 光催化剂（M = Ti ⁴⁺、Fe ³⁺、Co ³⁺、Ga ³⁺、Al ³⁺）中的三价或四价金属阳离子，可以精确控制反应的产物选择性。ZnTi-LDH还原产物主要为CH _{4 ；}ZnFe-LDH和ZnCo-LDH产生H ₂完全来自水分解；而 ZnGa-LDH 和 ZnAl-LDH 产生 CO。应用原位漫反射红外测量、价带 XPS 和密度函数理论计算来合理化不同 ZnM-LDH 光催化剂的 CO ₂还原选择性。分析表明， M ³⁺或 M ⁴⁺阳离子的d带中心 ( ε _d ) 位置控制 CO ₂的吸附强度，从而控制对含碳产物或 H ₂的选择性。d带中心相对接近费米能级的阳离子(Ti ⁴⁺ , Ga ³⁺和 Al ³⁺ )强烈吸附 CO _{2产生 CH 4}或 CO，而d带中心远离费米能级的金属阳离子（Fe ³⁺和 Co ³⁺）吸附 CO ₂较差，从而仅产生 H ₂（来自水分裂）。我们的研究结果阐明了三价和四价金属阳离子在 LDH 光催化剂中对选择性 CO ₂还原的作用，为开发对特定产品具有高选择性的改进型 LDH 光催化剂铺平了新途径。