Aqueous Solution-Gel Synthesis of CuNb2O6 and Cu3Nb2O8 and Exploring Their Potential as Precatalysts for Photothermal CO2 Hydrogenation.

Copper niobate metal oxides have gained significant attention in the field of CO2 reduction because of their great potential to address current energy and environmental challenges. Conventional solid-state synthesis methods generally suffer from poor cation homogeneity, whereas sol-gel processes facilitate molecular-scale mixing of precursors and enhanced compositional uniformity. However, many conventional sol-gel routes employ harmful or nonsustainable complexing agents, limiting their sustainability and scalability. Here, we report a facile citrate-complex aqueous solution-gel route for the controlled synthesis of CuNb2O6 and Cu3Nb2O8 phases. Citric acid functions as a chelating and polymerizing agent, facilitating the homogeneous complexation of Cu and Nb cations in aqueous solution while simultaneously promoting molecular-level mixing prior to gelation. The used precursor solution chemistry enables thermal decomposition and crystallization, leading to the reproducible formation of well-defined copper-niobate phases. The phase formation and purity of the target phases were confirmed using in situ high-temperature X-ray diffraction (HT-XRD), Raman spectroscopy, scanning electron microscopy (SEM), and UV-vis spectroscopy. The synthesized materials were further evaluated as precatalysts for photothermal CO2 hydrogenation, where CO was identified as the sole reaction product.