Metastable Structures in Cluster Catalysis from First-Principles: Structural Ensemble in Reaction Conditions and Metastability Triggered Reactivity

Abstract

Reactivity studies on catalytic transition metal clusters are usually performed on a single global minimum structure. With the example of a Pt cluster under a pressure of hydrogen, we show from first-principle calculations that low energy metastable structures of the cluster can play a major role for catalytic reactivity and that hence consideration of the global minimum structure alone can severely underestimate the activity. The catalyst is fluxional with an ensemble of metastable structures energetically accessible at reaction conditions. A modified genetic algorithm is proposed to comprehensively search for the low energy metastable ensemble (LEME) structures instead of merely the global minimum structure. In order to reduce the computational cost of density functional calculations, a high dimensional neural network potential is employed to accelerate the exploration. The presence and influence of LEME structures during catalysis is discussed by the example of H covered Pt clusters for two reactions of major importance: hydrogen evolution reaction and methane activation. The results demonstrate that although the number of accessible metastable structures is reduced under reaction condition for Pt clusters, these metastable structures can exhibit high activity and dominate the observed activity due to their unique electronic or structural properties. This underlines the necessity of thoroughly exploring the LEME structures in catalysis simulations. The approach enables one to systematically address the impact of isomers in catalysis studies, taking into account the high adsorbate coverage induced by reaction conditions.

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

“…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

“…Just the global minima of alumina‐deposited Pt 7 and Pt 7 B have similar activities. Another recent purely theoretical study by Sun and Sautet investigated the hydrogen evolution reaction and methane activation on the Pt 13 cluster covered by H atoms (gas‐phase Pt 13 H 26 ). The second lowest energy structure, rather than the global minimum, was shown to have the highest activity toward methane activation and dominate the catalytic performance despite being a minority species in the population (Figure ).…”

Global Optimization of Adsorbate Covered Supported Cluster Catalysts: The Case of Pt7H10CH3 on α‐Al2O3

“…Constructing such “hot” phase diagrams for catalytic systems, in order to elucidate the structures that emerge within the state of the catalyst at temperatures of interest and remove the focus on the GM alone, has been introduced by Zhang, Hermans, and Alexandrova . It is necessary because often, higher energy structures in the ensemble are responsible for the catalysis . Since many structures are close in energy in the studied case, many of them would be populated and constitute the state of the catalytic system at finite temperatures.…”

“…In 2015, a computational study performed by Pacchioni et al indicates that, at high hydrogen coverage, hydrogen atoms can transfer from Ru 10 cluster to surface, where the supports are anatase TiO 2 and tetragonal ZrO 2 (101) surfaces, but the complete global optimization was not used in the study . Recently, Sun and Sautet proposed a modified genetic algorithm for global optimization of gas phase Pt 13 cluster with high hydrogen coverage, which shows the importance of considering low‐energy metastable cluster isomers in reactions …”