Selective photoelectrocatalytic CO2 reduction to ethanol using nanotubular oxides grown on metastable Ti-Cu alloy

The photoelectrochemical conversion of CO2 to C2 products such as ethanol is a topic of much current interest and relevance for decreasing the amount of greenhouse gases in the atmosphere. Here, the photoelectrocatalytic selectivity for the CO2 reduction reaction on Ti-O-Cu nanotubes grown on bimetallic alloys (Ti-xCu; x = 0.5, 5.5 and 10 at. %) was investigated. The product selectivity in the CO2 reduction reaction was found to depend on the effects of composition and distribution of the oxidant species along the nanotubes. This behavior was coordinated by the substrate phases obtained from Ti-Cu alloys subjected to different heat treatments (annealed followed by rapid cooling - quenched (Q) or annealed with controlled cooling (A)). A nanotubular (Nt) oxide layer was grown on these substrates and the influence of metastable (quenched) or equilibrium (annealed) phases was related to the diffusion of the copper species. The two types of electrodes were compared in terms of their relative proclivity toward CO2 photoreduction. Methanol was produced as the dominant reduction product on Nt/Ti10Cu (A). On the other hand, Nt/Ti-10Cu (Q) produced a respectable yield and selectivity for ethanol, reaching 2.32 mmol/L of ethanol. In a broader vein, this study demonstrates the importance of surface defects in an active heterogeneous photocatalyst and provides insights into the key parameters for the precursor design, selection, and optimal performance of materials for the CO2 reduction reaction leading to C2 compounds.