The comparison of photocatalytic reduction of CO2 on ZnO photocatalysts at slightly acidic and alkaline pH towards the formation of valuable products

Abstract

The photocatalytic reduction of CO2 to carbon monoxide, methane and hydrogen (as a valuable byproduct) in the suspension or solution of various Zn-based forms in an alkaline environment and water, respectively, was investigated. These are the first results showing homogeneous CO2 photoreduction in the presence of Zn-based photocatalysts in an inorganic environment.The ZnO photocatalysts were suspended in the alkaline environment, while in water, they completely dissolved during at least 6 hours of carbonation, forming a transparent solution. The small crystallite size also determines a high zinc solubility in an aqueous solution (97.7 % for 14 nm, 92.2 % for 25 nm, and 68.5 % for crystallites larger than 100 nm). This homogeneous solution showed a slightly lower photoactivity than the heterogeneous suspension because of a lower concentration of crucial bicarbonate HCO3- ions. 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. In addition, the H+ ions generated during the ZnO dissolving in water can first participate in the CO2 photoreduction towards CO and CH4 and, afterwards, involve forming small quantities of hydrogen.