title: Using theoretical chemistry to model the redox properties of polyoxometalates and their potential as ammonia synthesis catalysts
Creators: name: Thompson, Jake A.
Publisher: name: University of Glasgow
Year: 2024

description: Polyoxometalates (POMs) have attracted significant interest owing to their structural diversity, redox stability, and functionality at the nanoscale. These structures have attracted significant interest as catalysts in fields including but not limited to: (i) water oxidation; (ii) carbon dioxide reduction; (iii) and nitrogen activation. In this thesis, we provide an in-depth investigation into modelling the structural and electronic behaviours in Keggin-based POMs by means of Density Functional Theory (DFT) calculations, validating with experimental testing. This PhD systematically studies the influence of Hartree-Fock (HF) exchange and the explicitly located counterions on reproducing experimental redox potentials and chemical shifts. Additionally, the present work explores the potential of mono-substituted Keggin POMs as ammonia synthesis catalysts and rationalising their catalytic behaviour using their structural stability and electronic characteristics.

Chapter 1 provides a general introduction to POMs, before focusing on lacunary derivatives which comprise the bulk of our research. In Chapter 2, we provide an overview of quantum mechanics by first introducing the time-independent Schrödinger equation before outlining the fundamentals of DFT, briefly discussing the importance of exchange-correlation functionals and basis sets. This chapter concludes by discussing modern quantum chemical calculations used to model redox and spectroscopic properties in POMs. Chapter 3 details all experimental work carried out during this Ph.D.

In Chapter 4, DFT calculations have been employed to systematically explore the influence of the exchange-correlation functional for Mn(III/II), Fe(III/II), and Co(III/II), and Ru(III/II) redox couples present in K5[PW11M(H2O)O39]. Early work employed reduction energies (REs) using fully anionic systems coupled with implicit solvent models to reproduce experimental potentials. This assumption has proven useful for Keggin and Wells-Dawson anions, as the reduction process involves adding a single electron to non-bonding orbitals, hence, the entropic and vibrational contributions to ∆G are negligible. In this work, [PW11M(H2O)O39]q− systems have been modelled using an COSMO solvation model coupled with explicitly located counterions to render the system charge neutral. The incorporation of K+ counterions induced a positive shifting in potentials of &gt; 500 mV which proved beneficial where M = Mn(III/II)/Co(III/II). By contrast, locating counterions were to the detriment for M = Fe(III/II)/Ru(III/II) due to over-stabilisation of the ion-pairs. Generally, exceeding 25 % Hartree–Fock (HF) exchange is not recommended for K5[PW11M(H2O)O39] systems due to their tendency to over exaggerate the proximity of the ion-pairing. Further work exploring the nature of the cation-POM pairing is crucial in obtaining a reasonable agreement between theoretical and experimental results.

In Chapter 5, we have systematically studied the accuracy of exchange-correlation functionals and applied basis sets for replicating experimental chemical shifts in Keggin, Na3[PW12O40] and their lacunary clusters: Na7[PW11O39], Na8H[A–PW9O34], and Na8H[B–PW9O34]. In this work, we have provided an analysis of the geometric and electronic factors controlling experimental chemical shifts across several computational methods. Early work attempted to model chemical shifts using fully anionic systems with implicit solvent models. Anionic systems modelling using implicit solvation models were highly sensitive to P-O distance in which HF exchange from 15 % (B3LYP*) to 50 % (BH&amp;H), caused chemical shifts to alter by up to ca. 8 ppm for [PW12O40]3−, despite only a minor change to P-O of ca. 0.03 Å. These approach typically benefited from hybrid exchange-correlation functionals as opposed to GGA-based methods. Later, the work explicitly located counterions (X = Li+, Na+, K+) to render the system charge neutral, approximating the solvent environment by means of an COSMO solvation model. Herein, explicitly located counterions generally induced deshielding of resonance signals with respect to their anionic systems. Such signals were progressively shifted upfield as a function of HF exchange, for example,-10.27 (15 % B3LYP*) to-18.55 ppm (50 % BH&amp;H) in Li3[PW12O40]3− salts and-10.80 (15 % B3LYP*) to-19.04 ppm (50 % BH&amp;H) in Na3[PW12O40]. Unfortunately, obtaining reliable chemical shifts for [B–PW9O34]9− proved challenging and relies on linear scaling to correct for errors attributed to direct electrostatic interactions between the located counterions and central tetrahedron. Optimal results were accomplished using the PBE/TZP//PBE0/TZP level of theory achieving a MAE and MSE of 4.0 ppm.

In Chapter 6, DFT calculations have been employed to systematically assess the accuracy of various x-c functionals in reproducing experimental redox properties in X5[PW11M(H2O)O39] salts; where X= Li, Na or K and M = Mn(III/II), Fe(III/II) or Co(III/II). Herein, increasing contributions to HF-exchange coincided with positive shifting of computed potentials, U0Calc, attributed to the overstabilisation of the ion-pairing. In some instances, strong sensitivity towards HF exchange was observed, for example, Li5[PW11Co(H2O)O39] salts ranged by &gt; 1000 mV shifting from 0 to 50 % exchange. The effect of the applied basis set, restricted to the GGA-PBE functional was explored. By increasing the size of the basis set (TZP → TZ2P → QZ4P) negative shifting by 200 mV was induced. The challenge in attaining accurate potentials is effectively controlling the proximity of the ion-pairing. Herein, we propose an economical route by explicitly specifying the heteroatom– counterion, dP−X distances at discrete intervals (6- 10 Å) as opposed to necessitating CPU-expensive optimisations with hybrid x-c functionals. Expansion of the solvation shell (from the optimised state to 8 Å) reduced closed-contact electrostatic interactions in the ion-pairing, resulting in a systematic (negative) shifting of computed potentials. Furthermore, we employed polynomial relations to precisely determine explicitly located heteroatom– counterion distances enabling the minimization of computed error (U0Error → 0 V).

Chapter 7 provides a pioneering study exploring the efficacy of mono-transition-metal-substituted polyoxotungstates, K5[PW11M(H2O)O39] (M = M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II)) as heterogeneous ammonia catalysts. The significance of the metal composition was shown to be significant given the range in synthesis rates of 12.37 µmol h−1 g−1. The most active catalyst was K5[PW11Mn(H2O)O39] reporting a rate of 20.09 µmol h−1 g−1, whilst K5[PW11Zn(H2O)O39] only produced 7.72 µmol h−1 g−1. Structural studies on the post-reaction catalysts revealed the disappearance of the two bands attributed to the asymmetric stretching of the P–O bond upon heating. The breakdown of υ(P–O) vibrations for K5[PW11M(H2O)O39]; M = Ni, Cu, and Zn suggests comparatively poorer thermal stability, closely correlating with the temperatures of decomposition shown previous work. XRD patterns for K5[PW11M(H2O)O39]; M = Ni, Cu, and Zn suggest the crystal structure has decomposed forming phosphotungstate bronze (PO2)2(WO3)24 crystals.