CO2 photoreduction with H2O vapor by porous MgO–TiO2 microspheres: effects of surface MgO dispersion and CO2 adsorption–desorption dynamics

Abstract

Photocatalytic reduction of CO₂ with H₂O vapor for CO production at a temperature of 150 °C was studied using porous MgO–TiO₂ microspheres as the photocatalysts with the benefits of improved CO₂ adsorption by incorporating MgO and enhanced products/intermediates desorption at a higher temperature. The MgO–TiO₂ microspheres were fabricated by two methods: (1) a one-step spray pyrolysis method using TiO₂ (P25) nanoparticles dispersed in Mg(NO₃)₂ solution as the precursors (Mg/Ti-SP), and (2) spray pyrolysis synthesis of pure TiO₂ (P25) microspheres first and then wet-impregnation with MgO (Mg/Ti-WI). The two material synthesis methods led to different MgO dispersion on the TiO₂ surface. For Mg/Ti-SP, the strong aggregation of MgO nanoparticles caused a rough surface of the MgO–TiO₂ microsphere; while for Mg/Ti-WI, MgO was more uniformly deposited leading to a much smoother surface of the microsphere. The surface dispersion of MgO was found to significantly affect the performance of MgO–TiO₂ in CO₂ photoreduction. At the same MgO concentration, Mg/Ti-SP had more than two times higher activity than Mg/Ti-WI, and most importantly, little deactivation of the catalyst was observed on Mg/Ti-SP while Mg/Ti-WI started to deactivate after 1 to 2 h when the reactor was operating in a continuous flow mode. The ease of photo-induced electron transfer to the catalyst surface may have contributed to the superb activity of Mg/Ti-SP samples. The optimum MgO concentration was found to be 5% for both types of materials. Besides the dispersion of MgO, we also found that the CO₂ adsorption–desorption dynamics strongly influenced the CO₂ photoreduction. The results from in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed two advantages of Mg/Ti-SP over Mg/Ti-WI: (1) more abundant bicarbonates (important intermediates for CO production) on the surface and (2) easier desorption/transformation of intermediates.