Single Atom Platforms on Surfaces: A Step Towards Future Catalysts

Abstract

Single (metal) atoms (SAs) have attracted great attention in virtue of their intriguing physical and chemical properties, representing a fertile playground for fundamental studies of electronic, magnetic and chemical phenomena. Placing SAs on clean, crystalline and atomically flat substrates allows the study of their properties and functions via the whole plethora of surface science tools, offering direct access to their features with ultimate resolution. Moreover, the isolated metal atoms exhibit peculiar properties and tend to behave similarly to reactive centres in solution or gas phase, bridging homogeneous and heterogeneous catalysis. In these regards, atomically dispersed metal catalysts (ADCs) have recently received great attention [1, 2]. However, they suffer some intrinsic bottlenecks such as a poor tunability and limited maximum density.Here we show the successful achievement of single atom platforms (SAPs), where SAs are coordinated to atomically precise organic templates, confined to a two dimensional surface. The templates are obtained via the on-surface synthesis (OSS) of carefully designed molecular precursors, and consist of carbon-based, covalent polymers equipped with side moieties that act as coordination sites. These groups capture metal atoms that are deposited on the substrate in a post-synthetic step, and together form active site with rich potential applications. Conceptually, the obtained SAP offers rich tunability prospects by varying not only the substrate and the SA, but also the organic ligand. Its shape and composition (e.g. heteroatoms, functional groups, strain, etc.) will influence the properties of the SA and enable their fine tuning. Moreover, the versatility of the OSS strategy offers a wide plethora of possible architectures, allowing also the precise modification of the SA density.We have characterized each stage of the SAP preparation via high-resolution scanning tunnelling microscopy (STM) and noncontact atomic force microscopy (nc-AFM) with functionalized tips [3]. The trapping and conversion ability of these well-defined active sites towards CO and CO2 was then studied. Our investigation allows the direct visualization of reactants and binding motifs with unprecedented resolution, and opens new avenues for the study of chemical reactions localized at the active sites of our SAP.[1] Wang, A.; Li, J.; Zhang, T. Heterogeneous Single-Atom Catalysis. Nature Reviews Chemistry 2018, 2 (6), 65–81.[2] Hannagan, R. T.; Giannakakis, G.; Flytzani-Stephanopoulos, M.; Sykes, E. C. H. Single-Atom Alloy Catalysis. Chem. Rev. 2020, 120 (21), 12044–12088.[3] Gross, L.; Mohn, F.; Moll, N.; Liljeroth, P.; Meyer, G. The Chemical Structure of a Molecule Resolved by Atomic Force Microscopy. Science 2009, 325 (5944), 1110–1114. Figure 1. Single atom platform concept. Sketch of surface-adsorbed single atoms (orange dots) and of a single atom platform. Here, the organic part is represented by 1D polymers adsorbed on a surface, and the single atoms decorate the coordination sites (U-shaped motifs). Figure 1