Assessing thermal and nonthermal contributions during CO2 hydrogenation over ruthenium catalysts: effects of the illumination conditions and the nature of the support

Photothermal catalysis offers a promising approach for the clean production of carbon-neutral chemicals from CO2 through reactions with hydrogen as a renewable energy carrier. While the combined action of photons and heat from solar radiation can drive catalytic reactions, the interactions involved are very complex, depend on the catalyst composition, and often remain uncertain. Herein, we assessed the thermal and nonthermal contributions to the overall activities of a series of Ru catalysts during the photothermal hydrogenation of CO2. TiO2 (anatase/rutile mixture), anatase, ZrO2, CeO2, and SiO2 were used as supports for Ru nanoparticles (2 wt%) that were deposited using an amino-acid-assisted method. Ru@TiO2 and Ru@ZrO2 presented the best catalytic performance at relatively low reaction temperatures (220-250 °C), whereas Ru@CeO2 was the most active catalyst above 300 °C. The catalysts were tested under direct and indirect illumination conditions to assess their thermal and nonthermal contributions to the overall production of methane, with a nonthermal contribution of 60-75 % observed at the highest applied irradiance (2.2 W·cm-²). Ru@ZrO2 registered the highest nonthermal CH4 production, which is tentatively ascribable to the participation of photo-generated electrons in the catalytic reaction and the light-induced formation of oxygen vacancies. The selected catalysts were also tested under concentrated-sunlight conditions in outdoor experiments, with a maximum methane production of 200 mmolCH4·gcat-¹·h-¹ achieved over Ru@ZrO2, which resulted in 31 % CO2 conversion and 92 % selectivity for methane in a continuous flow reactor at a space velocity of 1500 mLSTP·g-¹·min-¹.