Alkali-promoted indium oxide as a highly active and selective catalyst for 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. 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.