Traditional Zno-Based Heterogeneous vs Novel Homogeneous Co2 Photoreduction Towards the Formation of Valuable Products

The photocatalytic reduction of CO2 to hydrogen, carbon monoxide, and methane in the suspension of various ZnO forms in an alkaline environment and water was investigated. The ZnO photocatalysts were suspended in the alkaline environment, while in water, they completely dissolved during at least 6 hours of water carbonation, forming a transparent solution. The homogeneous solution showed a slightly lower photoactivity than the heterogeneous suspension. In both cases, the selectivity gives the order H2>CO>CH4. It was suggested that the photoactive centres in the dissolved agglomerates are the –Zn–O–Zn– hydrous bridges, which can use bicarbonate HCO3- ions as a transporter of CO2. These are the first results showing homogeneous CO2 photoreduction in an inorganic environment. Additionally, the small crystallite size determines a high zinc recovery rate in an aqueous solution (97.7% for 14 nm, 92.2% for 25 nm, and 68.5% for crystallites larger than 100 nm).