Alkali-promoted indium oxide as highly active and selective catalyst for the photo-thermal CO2 hydrogenation

Abstract

Photo-thermal carbon dioxide (CO2) reduction has recently gained significant attention as a strategy to harness solar energy and address environmental challenges. Among other photo-thermal catalysts, indium oxide (In2O3) has emerged as a promising candidate for the CO2 hydrogenation reaction. However, owing to its wide band gap semiconductor nature and relatively low CO2 adsorption capacity, modifications are imperative to facilitate efficient light absorption and CO2 activation. In this study, we report alkali-promoted In2O3 catalysts for efficient and selective photo-thermal CO2 hydrogenation to carbon monoxide (CO). By virtue of the enhanced CO2 adsorption capacity, Cs-promoted In2O3 demonstrated superior catalytic performance with a CO production rate of 28 mmol·g-1·h-1 and 100 % selectivity under full solar spectrum irradiation and without external heating, which is more than 3 times higher compared to that of pure In2O3. Furthermore, the catalyst exhibited exceptional performance under mild pressure conditions, achieving a CO productivity of 74 mmol·g-1·h-1 at 300 °C and 10 bar. Mechanistic studies indicated that non-thermal effects dominate the reaction pathway, particularly at low reaction temperatures and high light intensities, accompanied by minor pure thermal effects. Additional experiments revealed that the in-situ formation of defective sites on the indium oxide catalyst surface under illumination could enhance the light absorption ability, thereby significantly enhancing CO production. Diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) demonstrated that light irradiation could stabilize key reaction intermediates and accelerate the CO2 hydrogenation reaction at low temperatures, ultimately boosting CO production compared to dark conditions.