Surface and Interface Coordination Chemistry Learned from Model Heterogeneous Metal Nanocatalysts: From Atomically Dispersed Catalysts to Atomically Precise Clusters

The surface and interface coordination structures of heterogeneous metal catalysts are crucial to their catalytic performance. However, the complicated surface and interface structures of heterogeneous catalysts make it challenging to identify the molecular-level structure of their active sites and thus precisely control their performance. To address this challenge, atomically dispersed metal catalysts (ADMCs) and ligand-protected atomically precise metal clusters (APMCs) have been emerging as two important classes of model heterogeneous catalysts in recent years, helping to build bridge between homogeneous and heterogeneous catalysis. This review illustrates how the surface and interface coordination chemistry of these two types of model catalysts determines the catalytic performance from multiple dimensions. The section of ADMCs starts with the local coordination structure of metal sites at the metal-support interface, and then focuses on the effects of coordinating atoms, including their basicity and hardness/softness. Studies are also summarized to discuss the cooperativity achieved by dual metal sites and remote effects. In the section of APMCs, the roles of surface ligands and supports in determining the catalytic activity, selectivity, and stability of APMCs are illustrated. Finally, some personal perspectives on the further development of surface coordination and interface chemistry for model heterogeneous metal catalysts are presented.

Single-atom site catalysts for environmental remediation: Recent advances

Single-atom site catalysts (SACs) can maximize the utilization of active metal species and provide an attractive way to regulate the activity and selectivity of catalytic reactions. The adjustable coordination configuration and atomic structure of SACs enable them to be an ideal candidate for revealing reaction mechanisms in various catalytic processes. The minimum use of metals and relatively tight anchoring of the metal atoms significantly reduce leaching and environmental risks. Additionally, the unique physicochemical properties of single atom sites endow SACs with superior activity in various catalytic processes for environmental remediation (ER). Generally, SACs are burgeoning and promising materials in the application of ER. However, a systematic and critical review on the mechanism and broad application of SACs-based ER is lacking. Herein, we review emerging studies applying SACs for different ERs, such as eliminating organic pollutants in water, removing volatile organic compounds, purifying automobile exhaust, and others (hydrodefluorination and disinfection). We have summarized the synthesis, characterization, reaction mechanism and structural-function relationship of SACs in ER. In addition, the perspectives and challenges of SACs for ER are also analyzed. We expect that this review can provide constructive inspiration for discoveries and applications of SACs in environmental catalysis in the future.

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.