Unveiling the mechanism of selective CO₂ hydrogenation to CO on amorphous black TiO_X in thermal and photo-assisted thermal catalysis: The role of defects in amorphous structure and light irradiation

Developing catalysts capable of efficiently utilizing both visible and near-infrared wavelengths of the solar spectrum for CO₂ hydrogenation has led to growing interest in reduced TiO₂ materials. Achieving efficient long-wavelength solar light harvesting requires a high concentration of oxygen vacancies (O_V). However, extensive O_V formation can lead to atomic rearrangements within TiO_X, causing a dispersion of O_V throughout the material, as opposed to creating localized and distinct O_V sites typical of crystalline TiO_X, which interact directly with reactants. In this study, we synthesized amorphous black TiO_X (AM-TiO_X) catalysts and thoroughly characterized their surface properties, including acidity and the desorption bond strengths of H₂ and CO₂. Density functional theory (DFT) simulations were performed to analyze the hydrogen adsorption profile and structural changes in the material due to O_V formation. We found that hydrogen mobility on the surface is restricted due to strong hydrogen bonding. The CO₂ hydrogenation process was investigated using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), enabling the development of a reaction pathway to elucidate the catalyst’s selectivity towards CO and the effect of light irradiation on product formation rates. Notably, m-HCO₃* formation was favored, with CO and CH₄ production proceeding primarily via the formate pathway. To enhance catalyst stability against oxidation during reaction, the surface was decorated with Ru particles. The findings of this study are relevant to catalysts that leverage extensive Oᵥ formation as a strategy to extend light responsiveness, as well as to the design of catalysts for CO₂ hydrogenation to CO.