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

TiO2 nanosheets modified with various concentrations of sulfuric acid have been synthesized through hydrothermal treatment at 240 degrees C followed by heat treatment at 105 degrees C. Compared with untreated TiO2 nanosheets, the H2SO4-modified samples exhibited markedly improved quantum yield (QY), energy returned on energy invested (EROEI), and turnover number (TON) for CO2 reduction to CH4 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 (Bronsted acidic sites) and oxygen vacancies/Ti3+ species. Thus, efficient charge separation and transfer to the TiO2 surface for both CO2 reduction and the accompanying H2O oxidation is facilitated. The highest activity for CO2 photoreduction to CH4 was obtained with TiO2 nanosheets with 0.5 mol L-1 H2SO4, with QY, EROEI, and TON of 0.726 parts per thousand, 0.335 parts per thousand, and 83.124, respectively. Furthermore, the catalyst maintained stable performance throughout five successive recyclability test runs. (C) 2015 Elsevier B.V. All rights reserved.