The determination of the structure of heterogeneous catalytic systems is a key aspect for a detailed understanding of the nature of active sites and for the rational design of efficient catalysts. However, the situation for the catalytic active sites is complicated and the determination of their structure is not straightforward. Indeed, catalysts are not static but dynamic, fluxional, metastable and they strongly evolve under reaction conditions, creating new active sites, not present for the as prepared catalysts. The best active sites are not found on the most stable structure of the catalysts, but require first its restructuring. The lecture will present two examples, based on quantum chemical calculations.
In the first part we will focus on the modelling of small Pt clusters (Pt7 to Pt13) under hydrogen pressure and on their reactivity for alkane dehydrogenation [1,2]. The approach combines Density Functional Theory, high-dimensional Neural Networks and evolutionary techniques. It also includes grand canonical global optimization to study variable amount of adsorbed hydrogen and novel constrained global optimization methods to determine the structure of the cluster active site that provides the lowest barrier for the reaction. We show, with methane activation on supported Pt clusters and by an explicit sampling of cluster configurations at the transition state, that important restructuring is required to reach the most active transition state. The capability of the cluster to reconstruct, simultaneously with the C-H dissociation, is a key aspect for catalytic activity [2]. The approach will be extended to Cu4Ox clusters on amorphous alumina, showing that the irregular nature of the support is another parameter to generate diverse sets of supported clusters, and hence diverse catalytic reactivity [3].
The second part will deal with bimetallic catalysts, that are widely investigated. We will show how metastable structures at bimetallic surfaces evolve in time and in reaction conditions from microscopy and machine-learning molecular dynamics [4,6]. In return we will present how the different obtained active metal ensembles control the catalytic reactivity.
References
- Geng Sun, Anastassia N. Alexandrova, and Philippe Sautet, Pt8 cluster on alumina under a pressure of hydrogen: Support-dependent reconstruction from first-principles global optimization, J. Chem. Phys. 2019, 151, 194703
- Geng Sun, Jack T. Fuller, Anastassia N. Alexandrova, and Philippe Sautet, Global Activity Search Uncovers Reaction Induced Concomitant Catalyst Restructuring for Alkane Dissociation on Model Pt Catalysts, ACS Catalysis 2021 11, 1877-1885.
3.Geng Sun, Anastassia N. Alexandrova, Philippe Sautet, Structural Rearrangements of Subnanometer Cu Oxide Clusters Govern Catalytic Oxidation, ACS Catal. 2020, 10, 5309−5317
- Matthijs A. van Spronsen, Kaining Duanmu, Christopher R. O’Connor, Tobias Egle, Heath Kersell, Judit Oliver-Meseguer, Miquel B. Salmeron, Robert J. Madix, Philippe Sautet, Cynthia M. Friend, Dynamics of Surface Alloys: Rearrangement of Pd/Ag(111) Induced by CO and O2, J. Phys. Chem. C 2019, 123, 8312−8323
- Jin Soo Lim, Jonathan Vandermause, Matthijs A. van Spronsen, Albert Musaelian, Yu Xie, Lixin Sun, Christopher R. O’Connor, Tobias Egle, Nicola Molinari, Jacob Florian, Kaining Duanmu, Robert J. Madix, Philippe Sautet, Cynthia M. Friend, and Boris Kozinsky, Evolution of Metastable Structures at Bimetallic Surfaces from Microscopy and Machine-Learning Molecular Dynamics, J. Am. Chem. Soc. 2020, 142, 15907−15916.
- Jessi E. S. van der Hoeven, Hio Tong Ngan, Austin Taylor, Nathaniel M. Eagan, Joanna Aizenberg, Philippe Sautet, Robert J. Madix, and Cynthia M. Friend, Entropic Control of HD Exchange Rates over Dilute Pd-in-Au Alloy Nanoparticle Catalysts, ACS Catal. 2021, 11, 6971−6981