Ru-Decorated Ti Nanostructures for the Quantification and Mechanistic Understanding of Non-Thermal Effects in Photothermal CO2 Methanation

Gas-phase photothermal reactions using metallic catalysts have attracted increasing attention recently due to their potential for reducing activation energy and improving reaction selectivity. However, many challenges remain, including the validation of non-thermal contributions with accurate temperature measurement, strategies to enhance the non-thermal contribution, and its microscopic understanding. Here we propose using metallic nanostructures instead of metal oxide powder as catalyst support to eliminate the challenge in temperature measurement and further enhance the non-thermal contribution. We choose CO2 methanation using Ru on Ti nanostructures to demonstrate this idea. Ru-decorated Ti nanostructures are fabricated using two-step pulsed laser irradiation on Ti foil. Several-fold enhancements in CH4 production rate are observed under 1 sun illumination in the 150-350 ℃ temperature range. More than a 100-fold enhancement in reaction rate is achieved with white light alone as both the thermal and photon sources. The non-thermal contribution is further confirmed by the observations that only blue light (445 nm) accelerates the reaction rate while green (525 nm) and red (665 nm) light have no effect. The wavelength-dependent enhancement and atomic-scale Ru catalysts lead us to conclude that the non-thermal contribution is due to the direct photo-excitation of CO2 adsorbed on the Ru catalysts, and this excitation is further enhanced by the near-filed of Ti nanostructures. The idea of nanostructured metallic support and our experimental approach can be applied to other photothermal reactions to achieve high activity and mechanistic understanding.