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Professor Anastassia Alexandrova
Tuesday, October 22, 2019, 11:00am - 12:00pm
 

Anastassia Alexandrova“Dynamic Catalytic Interfaces: Ensembles of Multiple Metastable States Break the Rules of Catalysis”

I will show that dynamic catalytic interfaces exhibit a great structural fluxionality in conditions of catalysis, and thus need to be viewed (and modeled) as dynamic ensembles of many states. This leads to beautiful and bewildering properties of such interfaces. Multitude of metastable states dictates the catalytic activity, selectivity, and durability. Every intermediate of the catalyzed reaction is bound to a different ensemble-state of the catalyst. For every step of the reaction, the most stable catalyst isomer may not be the most catalytically active, and thus thermodynamics and kinetics are governed by different states of the catalyst. This then leads to an inescapable break-down of scaling relations, which normally preclude catalyst optimization beyond a certain fundamental limit. Phase diagrams of dynamic catalytic interfaces define phases as ensembles of many states of different structures and also stoichiometries (not a normal way to think of phase diagrams). There are also areas on phase diagrams that have no dominant composition in the ensemble, and those appear to correspond to conditions of highest catalyst activity found in experiment, leading to the view of stoichiometric flexibility as a reaction driving force.I will show that dynamic catalytic interfaces exhibit a great structural fluxionality in conditions of catalysis, and thus need to be viewed (and modeled) as dynamic ensembles of many states. This leads to beautiful and bewildering properties of such interfaces. Multitude of metastable states dictates the catalytic activity, selectivity, and durability. Every intermediate of the catalyzed reaction is bound to a different ensemble-state of the catalyst. For every step of the reaction, the most stable catalyst isomer may not be the most catalytically active, and thus thermodynamics and kinetics are governed by different states of the catalyst. This then leads to an inescapable break-down of scaling relations, which normally preclude catalyst optimization beyond a certain fundamental limit. Phase diagrams of dynamic catalytic interfaces define phases as ensembles of many states of different structures and also stoichiometries (not a normal way to think of phase diagrams). There are also areas on phase diagrams that have no dominant composition in the ensemble, and those appear to correspond to conditions of highest catalyst activity found in experiment, leading to the view of stoichiometric flexibility as a reaction driving force.

This new paradigm of dynamic catalytic interfaces will be illustrated using several catalytic systems as examples. I will discuss semiconductor surfaces decorated with small Pt clusters used for hydrocarbon dehydrogenation for endothermic cooling of hypersonic jets. We will see how the described rules are essential to explain and predict cluster-size dependence of activity, selectivity, and deactivation propensity, in agreement with experiment. Pt clusters on graphene will be used to illustrate the break-down of scaling relations in oxygen reduction reaction. I will also present some results for oxidative dehydrogenation on surface-deposited clusters of Cu, Pd, and also hexagonal boron nitride. Most of this work is done in close collaboration with experiment, particularly operando spectroscopy when possible.

~Coffee/tea will be served prior to lecture~

Location CCB Auditorium (1303)
Hosted by Professor Sagar Khare