Influence of the Surface Acidity of Niobium Catalysts Modified with Phosphate in the Photocatalysis of CO2 for Conversion into Methanol and Acetic Acid

Artificial photosynthesis represents a promising avenue for chemical energy storage, addressing the urgent need to mitigate carbon emissions through emerging technologies. This work investigates how phosphate-induced surface acidity in niobium photocatalysts influences the aqueous photoreduction of CO2 into methanol and acetic acid. We synthesized the TT-Nb2O5 catalyst via the oxidant peroxide method (OPM) using a niobium oxalate precursor, followed by mild thermal treatment at 150 °C and surface modification with varying concentrations of H3PO4. The density of Cas was quantified through indirect potentiometric titration in alkaline aqueous suspensions. Photocatalytic tests were conducted in a quartz reactor saturated with CO2 under UV irradiation (254 nm, 15 W) for 4 h. The catalyst modified with 0.5 mol L−1 H3PO4 (Nb-0.5) exhibited superior performance, achieving high selectivity for acetic acid (59%) and methanol (40%), while HCOOH and CO remained as minor intermediates (0.2%). Notably, the phosphatization process suppressed CO evolution and preserved catalytic activity over four reaction cycles, a significant improvement over non-phosphatized counterparts. Within the evaluated phosphatization conditions, the results suggest that surface acidity and textural accessibility jointly influence conversion efficiency. The specific acidity of 0.99 ± 0.07 mmol g−1 was associated with the highest production rates of 267 µmol g−1 h−1 for acetic acid and 181 µmol g−1 h−1 for methanol. These findings elucidate the critical role of surface acidity modulation in phosphatized niobium systems, offering a strategic pathway for enhancing sustainable artificial photosynthesis.