Cobalt Precursor Anion Engineering of CoO/Nb2O5 Photocatalysts for Selective CO2 Reduction to Liquid Fuels

Abstract Modulating metal–support interactions is a relevant strategy for improving the efficiency of CO2 photoreduction. In this work, we investigated the effect of the cobalt precursor on the electronic structure and photocatalytic performance of CoO/Nb2O5 composites prepared from hydrated Nb2O5 modified with salts containing different anions (acetate, nitrate, and sulfate). The results indicate that the nature of the precursor influences the optical properties, oxidation states, and surface chemistry of the materials, reflecting differences in catalytic activity and selectivity. XPS analysis showed the predominant presence of Co2+ and, in the case of Nb/Co-S, the additional formation of Nb4+ species, indicating a modification of the material's electronic structure. The sulfate-derived material (Nb/Co-S) showed superior performance, achieving high yields of ethanol (55.2 μmol g−1) and methanol (49.7 μmol g−1), while pure Nb2O5-H exhibited high selectivity for CO (2163 μmol g−1). These results demonstrate a significant shift in selectivity from gaseous products to highly reduced liquids, an effect that is here linked directly to the anion-dependent formation of Nb4+ sites rather than to differences in cobalt loading, which remained comparable (∼10 wt %) across all composites. Furthermore, Nb/Co-S maintained its activity over three consecutive cycles, indicating good catalytic stability. In contrast, the material obtained from nitrate showed low activity for the formation of liquid products, underscoring that precursor anion choice, and not cobalt content alone, is the parameter governing product distribution in this system.