Heterogeneous Photocatalytic Systems Formed by Compound [Zr6O4(OH)4(C6H5COO)8(H2O)8][SiW12O40] in Combination with Inorganic Cocatalysts for the CO2 Reduction to Alcohols in Water

Abstract

The photoreduction of CO₂ to methanol and ethanol is a highly sought-after reaction due to the economic and environmental implications of these products. Both methanol and ethanol are versatile chemical feedstock and renewable fuels. The ionic hybrid compound [Zr₆O₄(OH)₄(C₆H₅COO)₈(H₂O)₈][SiW₁₂O₄₀] (Zr₆W₁₂) provides effective separation of the generated electron-hole pair during exposure to UV radiation through a Z-scheme disposition of the HOMO-LUMO levels of each discrete ionic entity. However, this compound does not promote the CO₂ reduction. In contrast, the incorporation of selected inorganic cocatalysts, such as AgI, Bi₂O₃, CeO₂, CuI, CuO, Cu₂O, In₂O₃, PbO, Sb₂O₃, SnO, TiO₂ or ZnO, to the photocatalytic system can enable the activation and reduction of CO₂, leveraging their electronic properties and interactions with Zr₆W₁₂. Some of these heterogeneous photocatalytic systems perform well for the photoreduction of CO₂ into methanol and/or ethanol in water and without the need of any sacrificial chemical reagent, achieving maximum production levels of 163 μg g⁻¹ h⁻¹ and 144 μg g⁻¹ h⁻¹ for methanol and ethanol, respectively, for the Zr₆W₁₂/CuI photocatalytic mixture. Theoretical calculations have been conducted to determine how the relative disposition of the HOMO/LUMO energy levels of Zr₆W₁₂ and the band structure of the inorganic cocatalysts impact on the CO₂ photocatalytic reduction to alcohols.