Formic acid and hydrogen generation from the photocatalytic reduction of CO2 on visible light activated N-TiO2/CeO2/CuO composites

Abstract

Ternary of titania-based composites were prepared in two steps. First, pure and nitrogen-doped titanium dioxide powders were obtained using Pechini´s method. Then, these powders were impregnated with 3% wt. of cerium and copper oxide particles. The X-ray diffraction characterization shows the presence of rutile and anatase phases. The morphological characterizations exhibited some CuO agglomeration that resulted in a limited dispersion. Nitrogen-doped composite materials present better surface area due to the synergy effect of the urea precursor employed in their synthesis. UV–Vis analysis shows a redshift displacement for double and triple composites related to enhancing light absorption. XPS characterization made it possible to identify a mixed-valence state of Ti associated with the presence of oxygen vacancies, which presumable are responsible for the increase of adsorption of interest molecules. N-TiO2/CuO presents the highest formic acid production (33 µmol g−1 min-1) attributed to a better capacity to adsorb CO2 molecules and the formation of a Z-scheme where charges are spatially separated. The addition of CeO2 co-catalyst decreases formic acid performance due to its inherent ability to adsorb CO3−2 and OH ions that could hinder available active sites. On the other hand, in the case of hydrogen production, N-TiO2/CeO2/CuO material exhibited the highest production due to its highest Ti3+/Ti4+ valence state ratio, which presumably increases available active sites for molecular water adsorption and consequently for transformation into H2.