Enhancement of photocatalytic reduction of CO2 to CH4 over TiO2 nanosheets by modifying with sulfuric acid

TiO 2 nanosheets modified with various concentrations of sulfuric acid have been synthesized through hydrothermal treatment at 240   °C followed by heat treatment at 105   °C. Compared with untreated TiO 2 nanosheets, the H 2 SO 4 -modified samples exhibited markedly improved quantum yield (QY), energy returned on energy invested (EROEI), and turnover number (TON) for CO 2 reduction to CH 4 under visible-light irradiation. As supported by physical-chemical characterization, the enhanced photocatalytic activities can be attributed to acidification promoting the formation of hydroxyl groups (Brønsted acidic sites) and oxygen vacancies/Ti 3+ species. Thus, efficient charge separation and transfer to the TiO 2 surface for both CO 2 reduction and the accompanying H 2 O oxidation is facilitated. The highest activity for CO 2 photoreduction to CH 4 was obtained with TiO 2 nanosheets with 0.5   mol   L −1 H 2 SO 4 , with QY, EROEI, and TON of 0.726‰, 0.335‰, and 83.124, respectively. Furthermore, the catalyst maintained stable performance throughout five successive recyclability test runs.