CO2 Conversion to Methanol by Hydrogen Species on n-Type Oxide Semiconductors

An n-type amorphous indium-based oxide semiconductor, a-InGaZnO_x (a-IGZO), was found to be a promising catalyst for CO₂ hydrogenation to methanol. The oxide obtained from the mixed-hydroxide gel proved to be a unique n-type semiconductor material with a large surface area of more than 100 m²/g and a high carrier electron concentration of approximately 10¹⁸/cm³. Incorporating a metal/semiconductor junction with 5 wt % Pd significantly enhanced catalytic performance, achieving a reaction rate more than 20 times higher and a methanol selectivity exceeding 90 mol %. Compared to ZnO and Ga₂O₃ in terms of electronic properties, the superior performance of the indium-based oxides was attributed to their high carrier electron concentration and a conduction band minimum (CBM) positioned near the universal hydrogen charge transition energy level [UHE: ε_H(H⁺/H^–]. Temperature-programmed desorption mass spectrometry (TPD-MS) analyses indicated that the a-IGZO had an unusually high hydrogen adsorption capacity for an oxide material. The introduction of Pd further enhanced hydrogen adsorption in indium-based oxides; this enhancement was not observed in ZnO and Ga₂O₃, which have low carrier electron concentrations. In situ transmittance Fourier transform-Infrared (FT-IR) spectroscopy of Pd/a-IGZO to probe free-electron absorption revealed that hydrogen dissociating on Pd subsequently spilled over to the oxide, where it acted as a shallow donor, increasing the carrier electron concentration. Hard X-ray photoelectron spectroscopy (HAXPES), which is surface and bulk-sensitive, showed that the valence states of the In³⁺, Ga³⁺ and Zn²⁺ remain unchanged after H₂ annealing, even in the presence of Pd nanoparticles. We propose a mechanism in which hydrogen donors and carrier electrons near the UHE promote the formation of both negatively and positively charged hydrogen species on the oxide, enabling the selective conversion of CO₂ to methanol.