Atomic Catalytic Processes and Single-Atom Catalysts

Supported single-atom catalysts with atomically dispersed metal atoms anchored on supports have attracted increasing research interest [1-15]. Attention stems not only from the structural simplicity of the uniformed active sites of atomically dispersed catalysts, but also from the idea that these materials bridge the gap between homogenous and heterogeneous catalysis. Therefore, there is a growing body of literature reporting the syntheses of various isolated site metal catalysts for reactions, particularly those based on Earth-scarce metals such as Pt [1-5], Ag [6], Au [7-9], Pd[10-12], and Ir [13-15]. However, due to the high surface energies of isolated noble metal sites, atomically dispersed atoms on the oxide support tend to sinter to form large particles, resulting in a loss of active surface area and the instability of supported singe-atom catalysts [16-18]. Driven by the long-standing interest in the development of thermally stable catalysts, there is an increasing demand in synthesizing sintering-resistant noble metal single-atom catalysts during synthetic and catalytic processes.

In recent years, the noble metal-based catalysts are shown to be of great importance due to their use in many technological applications, with the highest demand coming from automotive three-way catalytic convertors and diesel oxidation convertors [19,20]. Singe-atom heterogeneous catalysts may offer potential applications in the automotive emission abatement, but they should meet the demand of high-temperature thermal durability that is needed during the periodic regeneration of catalytic soot filters and operation under high engine loads. For example, a temperature as high as oC is used for the accelerated testing of diesel oxidation convertors. Several synthetic strategies for thermally stable supported single-atom catalysts have emerged [1-15]. Among them, the strategy of using voids or undercoordinated defects on supports to anchor atomically dispersed metal sites has elicited attention due to its high efficiency, low cost, and scale-up flexibility [21]. In most cases, the supports for the single-atom heterogeneous catalysts should be deliberately chosen [22], especially for materials that must be catalytically and mechanically stable at high reaction temperatures. Although thermally stable single-atom catalysts would be highly describable for practical applications, preparation of such materials has proven to be challenging.

Previous experimental and theoretical studies real that tin oxide (TO) with a rutile structure is the most stable crystalline phase, but the pure TO has limited thermal stability. Doping with a rare earth element, such as antimony (Sb), can be used to create defects and provide heteroatom and to stabilize TO particles against sintering at high temperatures [23-25]. We present here a high-temperature self-assembly method to synthesize thermally stable Ag single-atom catalysts on commercially available antimony-doped tin oxide (ATO) at a temperature as high as oC. The processes that produce atomically dispersed Ag adatoms by using supported Ag crystals as precursor at high-temperature aging is visualized by X-ray diffraction (XRD) and site-specific probe molecule infrared (IR) spectroscopy with CO. The single-atom structure of the Ag adatoms can be evidenced by extended X-ray absorption fine structure (EXAFS) and the cationic nature of atomically dispersed Ag is revealed by x-ray absorption near-edge spectroscopy (XANES). Ag single-atom catalyst is synthesized over commercial ATO support at temperature up to oC in air. CO oxidation test reveals that the stable single-atom Ag-on-ATO catalyst possesses excellent catalytic activity distinct from conventional instable Ag heterogeneous nanocatalysts. The thermal stability, as well as the uniform active site structure of this Ag single-atom catalyst, suggests that the atomically dispersed Ag-on-ATO catalyst is an excellent single-atom model system that matched the catalytic performance for commercial prospects under ultra high exhaust temperatures.