Microwave-assisted synthesis of M/TiO₂/C (M=Ni, Cu, Ni–Cu) photocatalysts for CO₂ reduction: structural evolution and photocatalytic properties

This study presents the synthesis of a TiO 2 -based composite material with transition metal (Ni, Cu) nanoparticles using microwave radiation. The obtained materials were characterised using X-ray powder diffraction, and the size of the nanoparticles was determined using the Scherrer equation. The photocatalytic activity of the synthesised composites was studied in reaction of CO 2 reduction to CO and CH 4 under the visible light with a wavelength of 400 nm. Microwave treatment of a mixture of TiO 2 with transition metal salts (Ni, Cu) and graphite was founded to decrease a photocatalytic activity in CO 2 reduction reaction, while a mechanical mixture of TiO 2 and graphite, not subjected to microwave treatment, demonstrated increased catalytic activity compared to unmodified TiO 2 Evonik P25. The decrease in catalytic activity of the case of microwave-treated samples is associated with an irreversible phase transition of the photoactive anatase phase into the catalytically inert rutile phase and formation of TiO 2−x phases. This process is induced by overheating during microwave synthesis, where graphite (C g ) acts as an effective microwave absorber and a reducing agent for Ti 4+ cations in TiO 2 . The obtained results are interesting for the development of efficient TiO 2 -based photocatalysts for CO 2 reduction.