Title: Understanding Proton-Coupled Electron Transfer on Polyoxometalates

Abstract:

Catalysis is essential to efficient and selective chemical transformations, and advancing catalytic performance is critical to meeting the challenges of global demand and sustainability while supporting our quality of life. Copious efforts are underway in pursuit of enhanced and innovative catalytic capabilities; however, development efforts still face 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 systemically developed to generalizable framework.

Fundamental to many industrially relevant chemistries are the dynamics of proton, electron, and hydrogen atom transfers, and metal oxides are a key class of catalytic materials with potent versatile abilities to influence the activities of these reactions. Their transfer behaviors are anchored in their thermodynamics, a key descriptor for which is the Bond Dissociation Free Energy, specifically 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 of these chemistries as well as their 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.

The principal goal of this proposal is to develop foundational knowledge of the factors influencing BDFE(O–H) and its component proton/electron transfers, known as Proton-Coupled Electron Transfer (PCET). The specific aims of this proposal are to: (1) understand how BDFE(O–H) is affected by varying surface coverage, including H atom reduction and POM ligation, (2) elucidate POM morphological effects on BDFE(O–H), and (3) quantify POM compositional effects on BDFE(O–H). Our initial efforts to these ends have employed Density Functional Theory (DFT) on select polyoxovanadates and polyoxotungstates to study the effects of (i) POM H-atom reduction (surface coverage), (ii) differences due to POM ligation, (iii) the role of ensemble behavior in reconciling DFT and experimental results, (iv) POM morphology, and (v) key relationships between BDFE(O–H) and the atomic charges of relevant oxygen binding sites. Building on the understanding gained from these works, we may now methodically expand our consideration across predominant POM framework metals (M = V, Nb, Mo, W). 

In aim 1 we focus on how BDFE(O–H)_x change with increasing degrees of surface coverage (i.e. POM reduction by H atom equivalents; x = 0 to 6 for Lindqvist POMs having 6 metal atoms). Our initial studies found stark differences between the surface coverage trends for W and V POMs. Additionally, we reveal the importance of considering ensemble effects for integrating DFT and experimental results. In aim 2, we investigate the BDFE(O–H) for different site geometries of homometallic POMs with varying morphology, ranging from the 6 metal atom octahedral Lindqvist structure, to the 3 times larger and approx. ellipsoidal Well-Dawson POM (D_4h point group). Initial work with polyoxotungstates across this range (specifically W₆O₁₉^-2, W₁₀O₃₂^-4, SiW₁₂O₄₀^-4, and P₂W₁₈O₆₂^-6) revealed substantial BDFE(O–H) differentiation with changing site geometries, related to their electronic structure. In aim 3 we plan to quantify the effects on BDFE(O–H) that arise from changing POM metal composition. Specifically, we plan to investigate BDFE(O–H) for different homometallic POMs (M = V, Nb, Mo, W), and the effects of single metal atom substitution. In addition to quantifying the changing composition and dopant effects, we hypothesize that investigating framework-dopant sets (e.g. Mo₆O₁₉^-2, MoW₅O₁₉^-2, and W₆O₁₉^-2) may allow us to postulate on and perhaps model the behavior of intermediate compositions.

Successful completion of these aims will further advance understanding of proton/electron and hydrogen atom transfer energetics on metal oxides, revealing structure-function relationships encompassing surface coverage, differing metal oxide morphology, and composition. Linear scaling relationships will be developed for these aspects and their interconnection. The combined studies will further support generalizable understanding of fundamental transfer behaviors on metal oxides, enable model investigation of these systems with greater accuracy and efficiency, and support future catalyst development efforts.

Dissertation Chair:

Dr. Giannis Mpourmpakis
Chemical and Petroleum Engineering, University of Pittsburgh

School of Chemical Engineering, National Technical University of Athens

Dissertation Committee:

Dr. John Keith
Chemical and Petroleum Engineering, University of Pittsburgh

Dr. Peng Liu
Department of Chemistry, University of Pittsburgh

Dr. James McKone
Chemical and Petroleum Engineering, University of Pittsburgh