Unveiling Metal Single Atom Migration Dynamics: Insights into Support-Induced Stability

Supported single metal atoms have gained significant attention in heterogeneous catalysis for their efficient utilization of noble metals, leading to considerable cost savings.However, most supports for single atom (SA) incorporation exhibit non-uniform physical dimensions and random surface arrangements [1].The effects of support characteristics, such as size, shape, exposed facets, and grain boundaries on the structure and stability of SAs, remain largely unexplored.Closed gas cell systems for in situ scanning transmission electron microscopy (STEM) present a promising approach to investigate the dynamic evolution of subnanometric metal species under various gaseous conditions, up to atmospheric pressure and 1000C [2].Nonetheless, the inherent instability of single atoms under the electron beam has limited the direct observation of their structural dynamics.Consequently, two critical questions for single-atom catalysts (SACs) remain unresolved: how the support structure influences single atom stability, and how to directly observe the dynamics of single atoms using in situ electron microscopy.In this study, two types of Pt SACs (with nominal loading of 0.025 -0.05 wt%) supported by commercially purchased TiO 2 nanoparticle and lab-synthesized TiO 2 with the shape of truncated bipyramid were achieved (referred to as Pt/TiO 2 -commercial and Pt/TiO 2 -truncated bipyramid), as depicted in Figure 1a-b.The TiO 2 -commercial (average diameter: 10 nm) exhibits randomly oriented support facets, resulting in heterogenous Pt-O coordination environments.In contrast, the TiO 2 -truncated bipyramid has a larger average size of 20 nm with specific facet exposure of ( 101), which promotes a homogeneous coordination with Pt SAs.Comparing Pt SA dynamics on these two different TiO 2 supports within the in situ gas cell would deepen our understanding on the influence of support structure (size, shape, exposed facet) on SA stability.Figure 2 shows high-angle annular dark field (HAADF) STEM images of Pt SAs on the surface of TiO 2 -commercial and TiO 2 -truncated bipyramid before and after reducing treatment by 760 torr of 5%H 2 /Ar at 450 o C. To minimize beam effects on the SA structure, the sample was exposed to electron beam only during image acquisition, utilizing small probe currents and minimal exposure times.As shown in Figure 2a-b, after 30 min of exposure to H 2 , the Pt SA migrated from one TiO 2 -commercial nanoparticle to the interphase between two nanoparticles.Both the small size and random orientation of TiO 2 -commercial nanoparticles result in abundant interphases rich in defects and dislocations, providing favorable locations for Pt SA migration.In comparison, due to the large particle size of TiO 2 -truncated bipyramid and the high migration barrier between facets, the Pt SA located on TiO 2 -truncated bipyramid was more prone to migrate within the (101) surface.Additionally, these differences in Pt SA migration behavior, location, and structure under the high reducing temperature of 450 o C have also endowed catalysts with varied performance towards CO adsorption (Figure 1c-d).The direct observation of single atoms' (SAs) migration dynamics on the surfaces of both homogeneous and heterogeneous oxides in this study underscores the profound impact of support structure on the stability and local structure of SAs under reducing treatments.These findings significantly contribute to our understanding of how the physical characteristics of supports, such as size, shape, and specific facet exposure, influence the catalytic behavior and performance of single-atom catalysts.Through a comparative analysis of Pt single-atom catalysts supported by TiO 2 nanoparticles with different morphologies, this research highlights the critical role of support design in catalyst stability and efficiency.Ultimately, this work paves the way for further exploration of structure-property relationships and catalyst structure optimization, aiming to enhance performance under realistic working conditions and achieve more efficient and cost-effective catalytic processes [3].