It has recently been shown that surface-supported nanocluster catalysts in realistic reaction conditions do not follow the conventional models used in catalysis thanks to their dynamic nature. This opens new doors in catalysis by giving more leverage in catalyst design, but also requires a major revision of the understanding of how dynamic heterogeneous catalytic interfaces operate. Major aspects of the new paradigm include the collective action of many catalyst states that form a statistical ensemble in reacting conditions, the catalytic activity and selectivity being driven by rare and metastable catalyst states, reaction thermodynamics and kinetics being controlled by different states of the catalyst, broken scaling relationships, and catalyst dynamic restructuring being an essential part of the reaction mechanism. Hence, one should consider the catalyst state (geometric and electronic) that might not be present in the as-prepared catalyst, and rather would develop in the reaction conditions and would have the desired activity. For example, it has been shown that while Pt7 and Pt8 on alumina have very similar prismatic global minimum geometries, Pt7 at higher temperatures also has access to more active single-layer isomers, which makes Pt7 much more active than Pt8 toward ethylene dehydrogenation reaction. This shows that size-dependent catalytic activity of clusters is not a simple property of single cluster geometry but the average over a statistical ensemble at relevant conditions. In fact, isomers may interconvert across barriers during catalysis meaning that low-energy metastable isomers can play an important role in catalysis. Furthermore, we have recently shown that subnanometer Pt clusters do not follow the scaling relations, which are well-established in heterogenous catalysis. We show that, at the subnano-scale, scaling relationships can be broken through catalyst dynamics due to the highly fluxional behavior of subnanometer clusters.

Surface-supported cluster catalysis: Fluxionality and the importance of ensemble representation

2019 Fall National Meeting

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