Nitrogen-doped TiO₂ for promoting CO₂ activation towards methanol synthesis over Cu-based catalyst

Creating surface oxygen vacancy (O_v) is key to enrich catalytically active metal-oxide interfaces to promote CO₂ activation and thus facilitating methanol synthesis via CO₂ hydrogenation. This study shows that nitrogen (N) doping of TiO₂ could achieve this, and the bimetallic CuGa supported on N doped TiO₂ (CuGa/TiO₂-N_0.5) attained the improved methanol yield compared to the undoped counterpart (CuGa/TiO₂, 1.2 g g_Cu⁻¹ h⁻¹ vs. 0.75 g g_Cu⁻¹ h⁻¹ at 300 °C). The combined characterization, such as high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray absorption spectroscopy (XAS), and CO/H₂ temperature-programmed reduction (CO/H₂-TPR), reveals that incorporation of N species in crystalline rutile TiO₂ promoted O_v formation, being beneficial to promote CO₂ activation. High-pressure in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) combining steady-state isotope transient kinetic analysis (SSITKA, with H₂/D₂ isotopic exchange) was conducted, and the results suggest that the hydrogenation of formates bound on the supported Cu species is the rate-limiting step, while in situ DRIFTS experiments with CO₂-H₂ switching specifically demonstrate that the formation of Cu-bound formates on CuGa/TiO₂-N_0.5 is facilitated by the improved CO₂ interaction affinity induced by N doping. In addition, relevant findings by in situ DRIFTS-MS studies (temperature-programmed surface reaction and CO₂-CO switching) also suggest that, during CO₂ hydrogenation, H₂ and CO (by-product) could possibly react with surface weak oxygen species to regenerate weak surface O_V maintaining the catalyst's activity.