Abstracts Category : Other

Realistic Modeling of the Catalytic Heterogeneous Interface

by Mai-Anh Ha

At the catalytic interface, catalysts are represented by an ensemble of cluster isomers or surfaces, each of which may contribute to the systems stability, activity, selectivity, and resistance to sintering or poisons by reaction byproducts. I use ab-initio methods in conjunction with statistical-mechanical arguments to predict physico-chemical properties and guide development of premiering catalysts. Specifically, I utilize the work-horse of computation, density functional theory (DFT) calculations, to characterize systems of interest. Statistical-mechanical arguments such as Boltzmann-populations allow us to understand the role of relevant cluster isomers, surfaces, and reagents within the high temperature regime of real catalysis. My theoretical work applies this ensemble perspective of heterogeneous catalysis to diverse systems, from supported subnano-clusters to surfaces such as defective anatase for photocatalysis and Pt-Ni for fuel cells.Supported metal clusters often display non-monotonic behavior in which a cluster of a specific n size may be especially active. My theoretical work at UCLA explores the high activity and tunability of supported clusters by investigating the system as an ensemble. Three of my four first author papers represent fundamental, surface science work into tuning supported clusters. In particular, the well-known Pt-Pd resistance to sintering i.e. agglomeration of the clusters to bulk inertness was explained successfully through this ensemble consideration. 1:1 ratios of Pt:Pd featured more accessible isomers than their pure and mixed counterparts, resulting in an entropic contribution to chemical stability. In collaboration with experiment, we identified the highly active Pt7 for ethylene dehydrogenation, each negatively charged cluster able to adsorb and activate a maximum of 3 ethylenes. Pt7s high activity over that of a similar cluster size, Pt8, resulted from Pt7s fluxionality, the ensemble is able to access isomer geometries with more exposed Pt sites. Selective de(hydrogenation) forms the basis for fossil fuel refinery through cracking of hydrocarbons3 and, due to its endothermic nature, can act as a self-cooling mechanism for jet engines. It also represents a tractable process for testing the tunability of cluster size and dopant effects. Successive de(hydrogenation) often results in deactivation as catalyst sites are blocked by carbon (coking). We therefore tempered that high activity and predilection towards coke formation by doping Pt7 with the electropositive boron, which sustained activity during successive reaction cycles by adsorbing and activating ~1 ethylene.The ensemble perspective may also be extended to surfaces by accounting for non-equivalent defect sites, the local minima of reaction reagents and their subsequent products, or the distribution of facets present at the interface. The anatase surface remains ubiquitous in the field of catalysis for its unique photoactivity and reactivity, specifically, for CO2 reduction and water-splitting. CO2 reduction is an