Living Libraries of Intermetallic Superatoms

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Intermetallics of abundant metals of very different properties (M/E) can potentially substitute industrial catalysts based on rare and precious metals. To realize this opportunity, atom-precise/-efficient catalyst design is required at the size regime of  1 nm, where every metal atom ‘counts’. We discovered a novel way to generate ‘living libraries’ of meta-stable, ligated clusters [MaEb](R)n. These intermetallic ‘superatoms’ are molecular counterparts of the solid state M/E materials. The electronic state, e.g. open vs. closed ‘superatom shell’, controls stability and reactivity. The design criteria are metals selection and mixing ratio, nuclearity (size), core structure and ligation. The term ‘living’ highlights the properties of an evolving library, which is defined as a dynamic mixture of clusters, growth species and additives. A living library has typically not reached chemical equilibrium. Its distribution of clusters is highly sensitive to perturbation, which includes the interaction with reactants to be trapped or converted at the cluster surface. The hypothesis is that the libraries are populated with interrelated clusters, transient and highly reactive as well as more accessible but less reactive ones. These portfolios are generated by multivariate sets of initial components and library evolution conditions, which factors relate to networks of nucleation, growth and degradation reactions, to additive association, dissociation, to metal-atom site activation (e.g. by ligand deprotection) and to reactions with small molecules at the cluster surface (H2, CO2, CH4, H2O, O2, N2, NH3 NOx, small hydrocarbons). The central approach of the project is probing (sets of) libraries rather than targeting and isolating specific clusters first and then studying their reactivity. The strategy of parallel testing enables us to explore a wide materials space and it directly addresses chemical complexity. The libraries (Fig. 1) allow for efficient probing elementary reaction steps related to catalytic transformation of small molecules at superatoms. Multi-factor design of experiment methods and library evolution monitoring by mass spectrometry combined with computational modeling will be applied. Risk arises from the subtle control of cluster properties in the non-scalable size regime (size, structure, isomers, fluctuation, M/E disorder, stability). Kinetics and mechanisms cannot be predicted, so far: thus, we have to deal with the high sensitivity of the libraries to components and conditions, leading to a diversity of species to be analyzed. However, the gain arises exactly from this chemical richness and we anticipate a new ‘superatom chemistry for catalysis’.