Light and temperature effect on CO desorption during photothermal CO2 hydrogenation over Pt/Al2O3

Visible light illumination has been widely applied to promote activity, selectivity and stability of traditional thermal catalysts. Nevertheless, the role of light irradiation during catalytic reactions is not well understood. In this work, Pt/TiO2 and Pt/Al2O3 prepared by wet impregnation were used for photothermal CO2 hydrogenation. They showed a similar photothermal effect. Hence, operando diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations were conducted on Pt/Al2O3 to reveal more insight into the mechanism. The results indicated that CO desorption from Pt sites including step sites (Ptstep) or/and terrace site (Ptterrace) is an important step during CO2 hydrogenation to free the active Pt sites. Notably, light illumination and temperature affected CO desorption in different ways. The calculated adsorption energy of CO on Ptstep and Ptterrace sites was -1.24 and -1.43 eV, respectively. Hence, CO is stronger bound to Ptstep sites. During heating process in the dark, CO preferentially desorbs from Ptterrace site. However, the additional light irradiation facilitates transfer of CO from Ptstep to Ptterrace sites and its subsequent desorption from the Ptterrace sites, thus promoting CO2 hydrogenation. This work provides further understanding of photothermal catalysis as a mechanism of promotion of the thermal reaction.