Photocatalytic Reduction of CO2 to Fuels by Modified TiO2 Nanoparticles and Nanotubes

Photocatalytic reduction of CO₂ to fuels (CO, methane, methanol, etc) by sunlight is potentially a promising sustainable energy technology that not only reduces greenhouse gas emissions but also produces renewable fuels. However, the low CO₂-to-fuel conversion efficiency has impeded the development of this technology.  TiO₂ has been widely used as a photocatalyst due to its low cost, high stability, and environmental benignness.  In this work, material innovations have been made to TiO₂ nanostructures to improve the CO₂ conversion efficiency by increasing the surface area, inhibiting photoinduced electron-hole recombination, and enhancing visible light utilization.  We have synthesized high-surface-area TiO₂ nanoparticles (TiNPs) and TiO₂ nanotubes (TiNTs) through a hydrothermal process in a Teflon-lined autoclave. The TiNPs and TiNTs are modified by metal and nonmetal dopings to enhance charge separation and narrow the band gap. The catalysts are well characterized by UV-vis, XRD, SEM, TEM, XPS, and BET analysis. The experiments of photocatalytic CO₂ reduction with water vapor using the prepared catalysts are conducted under visible light, UV-visible, and simulated sunlight, respectively. The product yield and selectivity are measured by a gas chromatograph (GC) equipped with thermal conductivity detector (TCD) and flame ionization detector (FID). The effects of the dopings and nanostructures as well as the reaction mechanisms are discussed in this study.