Ph.D. Final Defense - Andreas Towarnicky
Title:
Understanding Proton-Coupled Electron Transfer on Polyoxometalates
Abstract:
Catalysis is essential to efficient chemical reactions, and improving its performance is key to meeting the challenges of global demand and sustainability while supporting our quality of life. Copious efforts pursue enhanced and innovative catalytic capabilities; however, their development still faces myriad complexities with incomplete understanding, slowing progress. Investigation via molecular models promises to unlock deeper understanding of the underlying physics, particularly when theoretical modelling is aligned with experiments and developed to general principles.
The mechanics of proton and electron transfers are essential to many commercially relevant systems and offer fundamental descriptors for chemical behavior, especially the dynamics that arise with Proton-Coupled Electron Transfer (PCET). Metal oxides are an indispensable class of catalytic materials with potent and versatile abilities to influence PCET and other chemical activities. Many phenomena in these systems are rooted in their thermodynamics. Unifying them is the foundational descriptor of Bond Dissociation Free Energy, i.e. the BDFE(O–H), of hydroxide bonds on metal oxides.
PolyOxoMetalates (POMs) are connected polyhedra of individual metal-oxide building blocks, and their atomic specificity is transformative for computational investigation and experimental characterization. Synthetic methods can obtain specific POMs, POM molecular solubilities lend them well to electrochemical investigation, and their precise structure and molecular scale facilitate advanced theoretical investigation at reasonable computational cost.
Our present investigations leverage the molecular precision of POMs, the fundamental nature of PCET behavior, defined description via BDFE(O–H), experimental corroboration, and the insightful power of computational methods to deepen our understanding of structural influences on chemical function and their generalizable principles.
Specifically, we used Density Functional Theory (DFT) to: (1) investigate how BDFE(O–H) is affected by varying surface coverage, including H atom reduction, POM ligation, and ensemble behavior, (2) elucidate POM morphological effects on BDFE(O–H), and (3) quantify compositional effects on BDFE(O–H) for common POM framework metals M = V, Nb, Mo, and W. Our initial studies found a general prevailing trend of decreasing BDFE(O–H) with increasing H surface coverage, and that changes in POM ligation had relatively minor impact on BDFE(O–H), perhaps due to an offsetting phenomenon of thermodynamic compensation. We further reveal the importance of accounting for ensemble effects to better rationalize DFT and experimental results. However, stark differences appeared between the surface coverage trends for hexametalate V and W POMs, indicating that another dynamic becomes operative. To explore morphological effects, we investigated POMs of varying size and shape, from six metal atom octahedral Lindqvist POMs, decametalate and Keggin types, to ellipsoidal 18 metal atom Well-Dawson clusters. We found that the BDFE(O–H) generally increases with larger clusters, and that anisotropic geometries influence POM electronic structure and correspondingly differentiate site-specific H interaction energetics. To elucidate compositional influences on BDFE(O–H), we systemically evaluated an interrelated set of monometallic and single-metal-atom substituted POMs, with predominant POM framework metals M = V, Nb, Mo, and W. We quantified the impact of changing M on BDFE(O–H), and identified how composition influences the localization of electron reduction and combines with morphological, coverage, and ensemble factors. Across all investigations we found strong and important relations to the atomic charges of the O atoms the H interact with, affording a powerful and multifaceted descriptor for BDFE(O–H) from first principles.
Overall, these works help advance understanding of fundamental chemical behaviors and structural relationships, enable model investigation of these systems with greater accuracy and efficiency, and support future catalyst development efforts.
Chair:
Dr. Giannis Mpourmpakis
Department of Chemical and Petroleum Engineering, University of Pittsburgh
School of Chemical Engineering, National Technical University of Athens
Committee Members:
Dr. Susan Fullerton
Department of Chemical and Petroleum Engineering, University of Pittsburgh
Department of Electrical and Computer Engineering, University of Pittsburgh
Dr. John Keith
Department of Chemical and Petroleum Engineering, University of Pittsburgh
Dr. Peng Liu
Department of Chemistry, University of Pittsburgh
Dr. James McKone
Adjunct Professor, Department of Chemical and Petroleum Engineering, University of Pittsburgh
Claros Technologies Inc., Minneapolis, Minnesota