Simultaneous Photocatalytic CO2 Reduction and Methylene Blue Degradation over TiO2@(Pt, Au, or Pd)

In this work, the photocatalytic reduction of CO2 was innovatively tested with the simultaneous removal and mineralization of a textile contaminant, methylene blue (MB), which acts as a sacrificial agent. The process was carried out in a flow regime under atmospheric conditions, using a liquid-phase photoreactor under UVA illumination with a duration of 24 h per test. Two commercial TiO2-based photocatalysts, P25 and P90 from Evonik, were used and surface modified through the photodeposition of metallic nanoparticles of Pt, Au, and Pd, as they did not show gas-phase products from CO2 reduction on their own. The optimal pH was 5, the decreasing order of activity by metal was Pt > Au > Pd, and the optimal MB concentration was 20 ppm. The major products were CH4 and H2 in the gas phase. The presence of CH4 was only detected in the presence of a CO2 flow. In the liquid phase, carboxylic acids were also detected in small amounts, and in the test, 100 ppm of MB ethanol was additionally detected. A 100% degradation of MB and 72.5% mineralization was achieved under the conditions of highest CH4 production (20 ppm MB at pH 5 with 4 g·L−1 P25-0.70%Pt).