W-Doped TiO₂ for photothermocatalytic CO₂ reduction

TiO₂ is one of the most widely used photocatalysts and photothermocatalysts. Tailoring their structure and electronic properties is crucial for the design of high-performance TiO₂ catalysts. Herein, we report a strategy to significantly enhance the performance of TiO₂ in the photothermocatalytic reduction of CO₂ by doping high crystalline nano-TiO₂ with tungsten. A variety of tungsten doping concentrations ranging from 2% to 10% were tested and they all showed enhanced catalytic activities. The 4% W-doped TiO₂ exhibited the highest activity, which was 3.5 times greater than that of the undoped TiO₂ reference. Structural characterization of these W-doped TiO₂ catalysts indicated that W was successfully doped into the TiO₂ lattice at relatively low dopant concentration. Synchrotron X-ray absorption spectroscopy at both the W L₃- and Ti K-edges was further used to provide insight into the local structure and bonding properties of the catalysts. It was found that the replacement of Ti with W led to the formation of Ti vacancies in order to maintain the charge neutrality. Consequently, dangling oxygen and oxygen vacancies were produced that acted as catalytically active sites for the CO₂ reduction. As the W doping concentration increased from 2% to 4%, more such active sites were generated which thus resulted in the enhancement of the catalytic activity. When the W doping concentration was further increased to 10%, the extra W species that cannot replace the Ti in the lattice aggregated to form WO₃. Due to the lower conduction band of WO₃, the catalytic O sites were deactivated and CO₂ reduction was inhibited. This work presents a useful strategy for the development of highly efficient catalysts for CO₂ reduction as well as new insights into the catalytic mechanism in cation-doped TiO₂ photothermocatalysis.