Insight into the Roles of Metal Loading on CO₂ Photocatalytic Reduction Behaviors of TiO₂

The photocatalytic reduction of carbon dioxide (CO₂) into value-added chemicals is considered to be a green and sustainable technology, and has recently gained considerable research interest. In this work, titanium dioxide (TiO₂) supported Pt, Pd, Ni, and Cu catalysts were synthesized by photodeposition. The formation of various metal species on an anatase TiO₂ surface, after ultraviolet (UV) light irradiation, was investigated insightfully by the X-ray absorption near edge structure (XANES) technique. CO₂ reduction under UV-light irradiation at an ambient pressure was demonstrated. To gain an insight into the charge recombination rate during reduction, the catalysts were carefully investigated by the intensity modulated photocurrent spectroscopy (IMPS) and photoluminescence spectroscopy (PL). The catalytic behaviors of the catalysts were investigated by density functional theory using the self-consistent Hubbard U-correction (DFT+U) approach. In addition, Mott-Schottky measurement was employed to study the effect of energy band alignment of metal-semiconductor on CO₂ photoreduction. Heterojunction formed at Pt-, Pd-, Ni-, and Cu-TiO₂ interface has crucial roles on the charge recombination and the catalytic behaviors. Furthermore, it was found that Pt-TiO₂ provides the highest methanol yield of 17.85 µmol/g_cat/h, and CO as a minor product. According to the IMPS data, Pt-TiO₂ has the best charge transfer ability, with the mean electron transit time of 4.513 µs. We believe that this extensive study on the junction between TiO₂ could provide a profound understanding of catalytic behaviors, which will pave the way for rational designs of novel catalysts with improved photocatalytic performance for CO₂ reduction.