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

An n-type amorphous indium-based oxide semiconductor, a-InGaZnOx (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: εH(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.