Covalent and Strong Metal-Support Interactions for Robust Single-Atom Catalysts.

ConspectusOver the past decade, single-atom catalysis has emerged as a cutting-edge research frontier in the field of catalysis, because of its distinctive attributes, such as optimal metal utilization, atomically precise active sites, and unique geometric/electronic configurations. However, practical applications of single-atom catalysts (SACs) remain challenging, due to their limited stability arising from the aggregation and sintering of single atoms which possess higher formation free energy, compared to nanoparticles (NPs). Consequently, the development of thermally stable SACs is critical in fundamental studies and large-scale industrial catalysis. Currently, developing thermally stable SACs, especially under reaction conditions, remains a long-standing challenge.Metal-support interactions (MSIs) play a critical role in determining the stability and catalytic performance of supported catalysts. Among them, a strong metal-support interaction (SMSI) has been particularly significant for stabilizing supported metal species. Inspired by the role of MSIs in supported nanocatalysts, we investigated such interactions in SACs over a decade ago, leading to the discovery of covalent MSIs (CMSIs) and classical SMSI in the domain of SACs. This Account provides an overview of these two types of MSIs and summarizes their applications in the development of highly active and thermally stable SACs.We began by introducing the concept of CMSI, defined as the covalent bonding interaction between single metal atoms and the surface atoms of supports. This interaction has been instrumental in developing thermally stable SACs for scalable production and modulating their catalytic properties. Unlike traditional stabilization mechanisms that rely on surface defect sites, limited in their number and stability, CMSI stabilizes single atoms through surface lattice atoms (e.g., oxygen, carbon, etc.) in supports via covalent orbital interactions, thus significantly increasing the metal density and loading in single-atom dispersion. Notably, CMSI can also be induced on supports that do not intrinsically exhibit such an interaction by doping them with materials capable of forming CMSI, offering a universal method for fabricating high-density thermally stable SACs over diverse supports. Additionally, a simple water treatment can modulate CMSI, enhancing the reactivity by subtly tuning the local coordination environment of metal atoms to weaken the covalent bonding between single atoms and the surface atoms of supports. This approach effectively balances the tradeoff between high stability and high activity, optimizing the catalytic performance of SACs.On the other hand, while SMSI has been extensively studied in nanocatalysts for more than 40 years, its applicability to SACs remained unexplored. We recently demonstrated the identification of SMSI in Pt₁/TiO₂ SACs. It has been found that SACs can exhibit SMSI at significantly higher reduction temperatures. This interaction not only enhances the stability of SACs but also enables the selective encapsulation of coexisting metal NPs while keeping single atoms exposed, thereby offering a useful strategy for precisely tuning the reaction selectivity.In summary, MSIs in SACs have demonstrated significant value in both fundamental research and industrial applications. This Account concludes by highlighting current challenges and opportunities related to CMSI and SMSI in SACs, providing insights for guiding the future design and commercialization of high-efficiency, scalable, and robust SACs.