Sunlight-Powered Reverse Water Gas Shift Reaction Catalysed by Plasmonic Au/TiO 2 Nanocatalysts: Effects of Au Particle Size on the Activity and Selectivity.

This study reports the low temperature and low pressure conversion (up to 160 °C, p = 3.5 bar) of CO 2 and H 2 to CO using plasmonic Au/TiO 2 nanocatalysts and mildly concentrated artificial sunlight as the sole energy source (up to 13.9 kW·m -2 = 13.9 suns). To distinguish between photothermal and non-thermal contributors, we investigated the impact of the Au nanoparticle size and light intensity on the activity and selectivity of the catalyst. A comparative study between P25 TiO 2 -supported Au nanocatalysts of a size of 6 nm and 16 nm displayed a 15 times higher activity for the smaller particles, which can only partially be attributed to the higher Au surface area. Other factors that may play a role are e.g., the electronic contact between Au and TiO 2 and the ratio between plasmonic absorption and scattering. Both catalysts displayed ≥84% selectivity for CO (side product is CH 4 ). Furthermore, we demonstrated that the catalytic activity of Au/TiO 2 increases exponentially with increasing light intensity, which indicated the presence of a photothermal contributor. In dark, however, both Au/TiO 2 catalysts solely produced CH 4 at the same catalyst bed temperature (160 °C). We propose that the difference in selectivity is caused by the promotion of CO desorption through charge transfer of plasmon generated charges (as a non-thermal contributor).