A solar photothermo-catalytic combined process for the VOCs combustion and the subsequent CO2 valorization using noble metal-free catalysts

We here investigated a solar photothermo-catalytic combined process where the toluene (as volatile organic compound model) was first oxidized to CO2 which was subsequently converted into solar fuels in a second reactor. For this aim two noble-metal free catalytic systems were used, namely MnOx-ZrO2, which gave the best results for the toluene removal, and brookite TiO2-CeO2 mixed oxides which were the most efficient in the subsequent CO2 conversion. This latter reaction was further improved by the addition of small amounts of copper-based materials on brookite-ceria, acting as co-catalysts. The key point for both the examined reactions was the synergism between the thermocatalytic and the photocatalytic mechanisms that enhanced the energy saving and the catalytic performance. In particular, with the MnOx-5 wt%ZrO2 catalyst 90 % of toluene was converted into CO2 at 180 °C (gas hourly space velocity (GHSV) 3.9 × 10−3 moltoluene h−1 gcat-1), about 50 °C less than in the bare thermocatalytic tests and the obtained CO2 was converted into CO (17.6 µmol/gcat∙h) and CH4 (7.9 µmol/gcat∙h) with the CuOx/TiO2-3 %wt CeO2 sample at 120 °C. The photothermo-catalytic activity was about 4 times higher than solar photocatalytic activity and greater than the thermocatalytic one that required a temperature above the 200 °C. The proposed approach is promising to obtain solar fuels starting from toxic pollutants.