Atomically precise control in the design of low-nuclearity supported metal catalysts

Sharon Mitchell, Javier Perez-Ramirez
2021
Nanostructured catalysts incorporating supported metal atoms or small clusters of defined size and chemical composition attract considerable attention because of their potential to maximize resource efficiency. When optimally assembled, all the metal nuclei can participate in the catalytic cycle with properties tailored to deliver high specific activity and stable performance. Over the past decade, both the number and diversity of reported systems have exploded as researchers attempt to control
more » ... the nanostructure with increasing atomic precision. Nonetheless, spatially resolving the architecture and properties of supported low-nuclearity catalysts remains challenging using existing analytical methods. After identifying general structural features of this advanced family of catalytic materials, including the composition, nuclearity, coordination environment, and location, as well as dynamic effects in reactive environments, our contribution critically examines progress in their control and understanding. State-of-the-art experimental and theoretical approaches for the characterization are explored, addressing strengths and limitations through recent case studies. Finally, we outline directions for future work that will cross frontiers in the design of catalytic materials, which will be indispensable for developing high-performing new architectures for sustainable technologies. Accordingly, the development of high-performance catalysts based on supported metal atoms or clusters requires an ability to precisely control the assembly and structure of active sites and relate this to their associated reactivity patterns. Success relies on the availability of techniques that can discriminate several fundamental properties. However, the spatial resolution of these tiny species at the atomic scale still poses an incredible challenge. The fact that practically relevant host materials typically have irregular three-dimensional morphologies and exhibit non-uniform surface structures and compositions compounds the difficulty because of the high resulting polydispersity of coordination sites. There is also growing awareness that small metal entities often contain chemically distinct atoms, for example, p-block elements or halogens. These atoms may come from the carrier or the chemisorption of simple ligands and determine their stability and associated properties. For this reason, most fundamental insights about the effects of nuclearity on structureperformance relationships derive from the study of simplified systems such as clusters in the gas phase 12 or anchored on single-crystal surfaces 6 . Besides, under reaction conditions, for example at elevated temperature, pressure, potential, and in the presence of distinct chemical species, various structural isomers of low-nuclearity species may coexist and interconvert from one to another, a phenomenon referred to as fluxionality 13 . The potential for emerging low-nuclearity catalysts to surpass the performance of conventionally supported metal nanoparticles together with the additional complexity for their structure elucidation calls for a critical analysis of our understanding of their controlled assembly and properties. In this review, we explore key characteristics in the design of catalytic solids integrating well-defined low-nuclearity metal species, concentrating on sizes of less than ten atoms (Fig. 1 ). After introducing the principal synthetic approaches for their stabilization, we examine the scope of characterization tools for discriminating the nuclearity, composition, geometry, and local environment of metal ensembles of distinct size, comparing experimental and theoretical insights. We also survey evidence of structural dynamics and
doi:10.3929/ethz-b-000505976 fatcat:eitge7boefcx7pzqsmfrh3zo6a