TiO2-Supported Single-Atom Catalysts for Photocatalytic Reactions

Titania (TIO2) has been among the most widely investigated and used metal oxides over the past years, as it has various functional applications. Extensive research into TiO2 and industrial interest in this material have been triggered by its high abundance, excellent corrosion resistance, and low cost. To improve the activity of TiO2 in heterogeneous catalytic reactions, noble metals are used to accelerate the reactions. However, in the case of nanoparticles supported on TiO2, the active sites are usually limited to the peripheral sites of the noble metal particles or at the interface between the particle and the support. Thus, highly dispersed single metal atoms are desired for the effective utilization of precious noble metals. The study of oxide-supported isolated atoms, the so-called single-atom catalysts (SACs), was pioneered by Zhang's group. The high dispersion of precious noble metals results helps reduce the cost associated with catalyst preparation. Because of the presence of active centers as single atoms, the deactivation of metal atoms during the reaction, e.g., by coking for large agglomerates, is retarded. The unique coordination environment of the noble metal center provides special sites for the reaction, consequently increasing the selectivity of the reaction, including the enantioselectivity and stereoselectivity. Hence, supported SACs can bridge homogenous and heterogeneous reactions in solution as they provide selective reaction sites and are recyclable. Moreover, owing to the high site homogeneity of the isolated metal atoms, SACs are ideal models for establishing the structure-activity relationships. The present review provides an overview of recent works on the synthesis, characterization, and photocatalytic applications of SACs (Pt-1, Pd-1, Ir-1, Rh-1, Cu-1, Ru-1) supported on TiO2. The preparation of single atoms on TiO2 includes the creation of surface defective sites, surface modification, stabilization by high-temperature shockwave treatment, and metal-ligand self-assembly. Conventional characterization methods are categorized as microscopic imaging and spectroscopic methods, such as aberration-corrected scanning transmission electron microscopy (STEM), scanning tunneling microscopy (STM), extended X-ray absorption fine structure analysis (EXAFS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). We attempted to address the critical factors that lead to the stabilization of single-metal atoms on TiO2, and elucidate the mechanism underlying the photocatalytic hydrogen evolution and CO2 reduction. Although many fascinating applications of TiO2-supported SACs in photocatalysis could only be addressed superficially and in a referencing manner, we hope to provide interested readers with guidelines based on the wide literature, and more specifically, to provide a comprehensive overview of TiO2-supported SACs.