Kinetic Isolation between Turnovers on Au18 Nanoclusters: Formic Acid Decomposition One Molecule at a Time

Subnanometric materials have found applications in a wide variety of fields, due to their extraordinary and complex properties. Among these applications, accelerating (catalysing) chemical reactions of interest in the energy and environmental fields has attracted significant focus. Thus, this collaborative study between the labs of Manos Mavrikakis (University of Wisconsin-Madison) and Michail Stamatakis (UCL), focuses on catalytic formic acid decomposition on subnanometric gold clusters, specifically Au18. Formic acid (HCOOH) is a clean, safe, and renewable hydrogen storage material, which can be converted efficiently to hydrogen on gold catalysts. However, the active site and reaction mechanism remain unclear. In this respect, a puzzling experimental observation is that when using deuterated formic acid as the reactant (HCOOD or DCOOH), HD is the only produced hydrogen species; no H2 or D2 is observed.

First principles based kinetic Monte Carlo modelling performed by Dr Benjamin W. J. Chen, who was partially supported by a TYC Junior Research Fellowship, elucidate the intricacies of the catalytic function of Au18 towards HCOOH decomposition. The model suggests that the active site consists of a triangular ensemble of three atoms, each possessing a coordination number of 5. Even though there are two such active site ensembles on the Au18 cluster, their occupations are strongly correlated because of strong, stabilising interactions between pairs of open-shell adsorbates mediated by the superatomic nature of the cluster. Since the occupation of the active sites blocks the dissociation of additional HCOOH molecules, there is kinetic isolation between turnovers; therefore, only one HCOOH molecule can dissociate on the cluster at a time. These results are in excellent agreement with the experimentally observed selectivity of Au catalysts toward HD during decomposition of HCOOD and DCOOH, thereby providing a compelling explanation for this puzzling phenomenon.

Hence, this work sheds light on how the unique electronic properties of subnanometric clusters can be used to design subnanometric, quasi-molecular catalysts with high activity and selectivity. Advancing our understanding of such complex catalytic materials opens up avenues for using them towards cleaner and more sustainable technological applications.

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