Indium-doped TiO₂ nanoparticles for photocatalytic CO₂ reduction with H₂O vapors to CH₄

Indium (In)-doped titanium dioxide (TiO₂) nanoparticles were synthesized using a controlled sol-gel method. The structures and properties of the catalysts were characterized by XRD, FE-SEM, TEM, XPS, BET, UV-vis and PL spectroscopy. Indium, present over the TiO₂ in metal state, inhibited crystal growth and produced anatase phase of mesoporous TiO₂ nanoparticles. Doping In in TiO₂ also increased the surface area and enlarged the band gap. The photocatalytic activities of In-doped TiO₂ nanoparticles were considerably improved for CO₂ reduction with H₂O vapors in a cell type photoreactor. CO was observed as the main product over TiO₂, but doping In in TiO₂ remarkably increased the CH₄ yield. CH₄ production rate over 10wt.% In-doped TiO ₂ was 7.9-fold higher than the bare TiO₂ at 100°C and CO₂/H₂O ratio of 1.43. In addition, C₁-₃ higher hydrocarbons namely C₂H₄, C₂H₆, C₃H₆ and C₃H₈ were detected in the product mixture. The enhanced photoactivity in mesoporous In-doped TiO₂ nanoparticles can be attributed to interfacial transfer of photogenerated charges, which led to effective charge separation and inhibited recombination of photogenerated electron-hole (e^-/h⁺) pairs. Langmuir-Hinshelwood model, developed to investigate reaction rate parameters, fitted well with the experimental data.