Heterogeneous catalysis is at the heart of the chemical industry, and thus, a molecular-level understanding of heterogeneous catalysts is a prime focus of catalysis research. For a long time, the single most thermodynamically stable structure of the catalytic interface attained under the reaction conditions had been envisioned as the reactive phase. However, some catalytic interfaces continue to undergo structural dynamics in the steady state, triggered by high temperatures, pressures, and reagents activity. Among particularly dynamic interfaces are supported (sub)nanometallic clusters. We show that this dynamic fluxionality causes the supported clusters to populate many distinct structural and stoichiometric states under catalytic conditions, turning the catalyst into an evolving statistical ensemble of many structures. Every member in the ensemble contributes to the properties of the catalyst differently, in proportion to its probability of being populated. The compositions and dynamics of the ensemble leads to size-specific catalytic activities and the famous “every atom counts” phenomenon. The adsorbates affect catalyst structures, and so the ensemble changesin every reaction intermediate. The most stable member of the ensemble dominates the thermodynamic properties of the corresponding intermediate, whereas the kinetics can be determined by more active but less populated metastable catalyst states, and that suggests that many earlier studies might have overlooked the actual active sites. Theory began addressing the kinetic accessibility of highly-active metastable states in reaction conditions, the dynamic coupling of cluster motion to the reaction, and the degree to which the system operates at or off equilibrium. The fact that the per-site and per-species resolution is needed for the description and prediction of catalyst properties gives theory the central role in catalysis research, as most experiments provide ensemble-average information and cannot detect the crucial minority species that may be responsible for the catalytic activity.

Dynamic ensembles of metastable states drive the catalysis on size-selected supported clusters

ACS Fall 2020 Virtual Meeting

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