Computational Design of Clusters for Catalysis

Abstract

When small clusters are studied in chemical physics or physical chemistry, one perhaps thinks of the fundamental aspects of cluster electronic structure, or precision spectroscopy in ultracold molecular beams. However, small clusters are also of interest in catalysis, where the cold ground state or an isolated cluster may not even be the right starting point. Instead, the big question is: What happens to cluster-based catalysts under real conditions of catalysis, such as high temperature and coverage with reagents? Myriads of metastable cluster states become accessible, the entire system is dynamic, and catalysis may be driven by rare sites present only under those conditions. Activity, selectivity, and stability are highly dependent on size, composition, shape, support, and environment. To probe and master cluster catalysis, sophisticated tools are being developed for precision synthesis, operando measurements, and multiscale modeling. This review intends to tell the messy story of clusters in catalysis.

Oxidative Dehydrogenation of Cyclohexane by Cu vs Pd Clusters: Selectivity Control by Specific Cluster Dynamics

“…The catalysts were prepared by depositing size‐selected, originally metallic, ligand‐free Pd and Cu clusters from the gas phase on alumina, titania, or ultrananocrystalline diamond (UNCD) support, with the goal to explore the effect of cluster size and the nature of the support on catalytic activity and selectivity . The catalysts were tested and characterized using a multiprobe in situ approach that combined temperature programmed reaction (TPRx), grazing incidence X‐ray absorption spectroscopy (GIXAS), and grazing incidence small‐angle X‐ray scattering (GISAXS), to find a correlation between catalytic performance and the oxidation state of the metal under realistic reaction conditions of temperature and pressure, while also allowing simultaneous monitoring of the sintering resistance of the clusters . Further details on the experimental methods are given in in Experimental Methods section and in the Supporting Information (SI).…”

“…Therefore, the global minimum alone cannot accurately represent the catalyst. Less stable isomers are likely to be more chemically reactive and therefore cannot be ignored . In Table , we specify the number of thermally‐relevant isomers of supported clusters and the average charges on the metal atoms.…”

Surface‐supported cluster catalysis: Ensembles of metastable states run the show

“…In this section, we discuss the methods and models that are mainly being used in the nanocluster catalysis community, and emphasize why we need to be careful when using some of these models to describe the catalytic properties when the catalytic interface is so dynamic. It is clear that finding the correlation between catalyst morphology and catalytic performance is essential to designing new catalysts with enhanced efficiency . Hence, a major tool is global optimization, already discussed above, to be used for the finding of both the global and the accessible local minima of cluster catalysts.…”

“…Always being a nontrivial task, it is truly complicated for surface‐supported cluster catalysts. High temperatures (~300°C in thermal catalysis), coverage with adsorbates, or the presence of electrolyte and electrochemical potential (in electrocatalysis) significantly affect the structure of both the nanocluster (due to its dynamic nature) and the support (by causing defects) . Given that, it is crucial to develop an accurate, yet computationally practical model to describe the system .…”

A Systematic Study on Bond Activation Energies of NO, N2, and O2 on Hexamers of Eight Transition Metals

“…Metal clusters often exhibit catalytic activities different from bulk. Such nanocatalysts have therefore attracted much attention as candidates of alternatives to conventional heterogeneous catalysts . Density functional theory (DFT) calculations have indicated that existence of low‐coordinated metal atoms makes metal clusters more reactive than solid surfaces in many catalytic bond activation reactions .…”