Clay minerals and their gallery guests: an ab initio investigation into their interactions.

Clay minerals are ubiquitous and readily accessible in the natural environment and consequently have become an essential ingredient in the development of Western Society. Their structural properties are responsible for many of their uses, their layered-leaf composition enables the absorption of water and other solutes, for example. In this thesis, the focus of interest lies primarily in the chemical properties of the clay minerals, which is due to the large surface areas of varying atomistic environments comprising the mineral layers. Clay minerals offer a challenge to the electronic structure modeller as their atomistic composition is non-exact, consequently a number of constraints are automatically applied during the modelling process, the first being the choice of composition of the model. There are currently few examples of density functional theory studies using planewaves and the pseudopotential approximation, and the available experimental data is not necessarily directly applicable to theoretical data due in part, to the inexactness of the clay mineral composition. Consequently, in the studies presented in this thesis, as much time has been spent in considering the modelling methods as on the results obtained and the implication of these in the modelling environment chosen. This thesis records investigations into the decarboxylation of a fatty acid into an alkane and CO$_{2}$ with the modelling of a catalytic environment of an aluminium-bearing clay mineral; the identification of a transition state of this reaction pathway using lattice dynamics and finally, the mechanism of reduction within iron-bearing clay minerals

Off-the-shelf DFT-DISPersion methods : Are they now “on-trend” for organic molecular crystals?

Organic molecular crystals contain long-range dispersion interactions that can be challenging for solid-state methods such as density functional theory (DFT) to capture, and in some industrial sectors are overlooked in favor of classical methods to calculate atomistic properties. Hence, this publication addresses the critical question of whether dispersion corrected DFT calculations for organic crystals can reproduce the structural and energetic trends seen from experiment, i.e., whether the calculations can now be said to be truly “on-trend.” In this work, we assess the performance of three of the latest dispersion-corrected DFT methods, in calculating the long-range, dispersion energy: the pairwise methods of D3(0) and D3(BJ) and the many-body dispersion method, MBD@rsSCS. We calculate the energetics and optimized structures of two homologous series of organic molecular crystals, namely, carboxylic acids and amino acids. We also use a classical force field method (using COMPASS II) and compare all results to experimental data where possible. The mean absolute error in lattice energies is 9.59 and 343.85 kJ/mol (COMPASS II), 10.17 and 16.23 kJ/mol (MBD@rsSCS), 10.57 and 18.76 kJ/mol [D3(0)], and 8.52 and 14.66 kJ/mol [D3(BJ)] for the carboxylic acids and amino acids, respectively. MBD@rsSCS produces structural and energetic trends that most closely match experimental trends, performing the most consistently across the two series and competing favorably with COMPASS II

A perspective on using experiment and theory to identify design principles in dye-sensitized solar cells

Dye-sensitized solar cells (DSCs) have been the subject of wide-ranging studies for many years because of their potential for large-scale manufacturing using roll-to-roll processing allied to their use of earth abundant raw materials. Two main challenges exist for DSC devices to achieve this goal; uplifting device efficiency from the 12 to 14% currently achieved for laboratory-scale ‘hero’ cells and replacement of the widely-used liquid electrolytes which can limit device lifetimes. To increase device efficiency requires optimized dye injection and regeneration, most likely from multiple dyes while replacement of liquid electrolytes requires solid charge transporters (most likely hole transport materials – HTMs). While theoretical and experimental work have both been widely applied to different aspects of DSC research, these approaches are most effective when working in tandem. In this context, this perspective paper considers the key parameters which influence electron transfer processes in DSC devices using one or more dye molecules and how modelling and experimental approaches can work together to optimize electron injection and dye regeneration. This paper provides a perspective that theory and experiment are best used in tandem to study DSC device

DFT+U investigation of the catalytic properties of ferruginous clay

The formation of fossil oil within clay minerals i.e., mineral-catalyzed decarboxylation, is a mechanism awaiting a thorough chemical explanation. To contribute to such an explanation, the study presented here investigates this mechanism at the level of first-principles, electronic structure computations, employing density functional theory (DFT plus Hubbard-U), planewaves, pseudopotentials, and periodic cells of two types of ferruginous clay minerals, specifically two types of nontronite [Fe2 (Si,Al)4O10(OH)2]. The formation of the fossil oil is modeled as a decarboxylation pathway, converting the fatty acid propionic acid, C2H5COOH to an alkane, C2H6 and the intermediate stages along this conversion pathway are represented by five configurations of interlayer species within the clay minerals. In this study, we test both the effect of the presence of iron on the theoretical stages of decarboxylation, together with the effect of two different density functionals: with and without strong correlations of the d-orbital electrons of iron. We have found that inclusion of the d-orbital electron correlations in the guise of a Hubbard parameter results in the introduction of three new intermediate configurations (one of which is potentially a new transition state), alters the location of the occupied Fermi level orbitals, and changes the band gaps of the clay mineral/interlayer species composites, all of which serves to inform the chemical interpretation of mineral-catalyzed decarboxylation

Role of Clay Minerals in Oil-Forming Reactions

Mineral-catalyzed decarboxylation reactions are important in both crude oil formation and, increasingly, biofuel production. In this study we examined decarboxylation reactions of a model fatty acid, propionic acid, C2H5COOH, to an alkane, C2H6, in a model of pyrophillite with an isomorphic substitution of aluminum in the tetrahedral layer. We model a postulated reaction mechanism (Almon, W. R.; Johns, W. D. 7th International Meeting on Organic Geochemistry 1975, Vol. 7) to ascertain the role of Al substitution and a counterion in decarboxylation reactions. We employ a periodic cell, planewave, ab initio DFT computation to examine the total energies and the frontier orbitals of different model sets, including the effects of charge on the reaction, the effect of Al substitution, and the role of Na counterions. The results show that an uncharged system with a sodium counterion is most feasible for catalyzing the decarboxylation reaction in an Al-substituted pyrophillite and, also, that analysis of the orbitals is a better indicator of a reaction than charge alone

An ab initio characterization of the electronic structure of LaCo x Fe 1- x O 3 for x ≤ 0.5

Solid oxide fuel cells are an important class of energy conversion devices in the search to replace fossil fuels. Their electrodes’ materials mostly belong to the perovskite family, which in their versatile composition are numerous; here we focus on the perovskite LaCoxFe1−xO3 and examine its electronic structure for x ≤ 0.5 using density functional theory with a plane wave basis and pseudopotentials. The resulting lattice parameters show good agreement with experiment, and the Mulliken and Bader charges show that iron and cobalt mostly remain as Fe3+ and Co3+ throughout an increasing Co:Fe ratio. The charge and spin magnitudes of oxygen ions is determined by their local, cation neighbors with the largest charge and spin magnitudes found on oxygen ions sandwiched between two Fe ions. Density of states and partial density of states analyses reveal that increasing the ratio of Co to Fe in oxygen stoichiometric materials decreases their relative, semi‐conducting nature toward insulating, by virtue of the decrease in the number of (conducting) O–Fe–O bonds and the increase in (insulating) O–Co–O bonds. The appearance of an intermediate spin state of Co and examination of its PDOS confirms the hypothesis that Co–O, d–p hybridization is a necessary factor for its occurrence

An ab initio characterization of the electronic structure of LaCo x_{x} Fe 1−x_{1- x} O 3_{3} for x ≤ 0.5 (Phys. Status Solidi B 9/2016)

Simple perovskite-type materials have the general chemical formula ABO3. While that already offers a broad variability in choosing A and B, additional tuning of the properties can be achieved by partial substitution of the A and/or B site. As such, this class of materials has found widespread use in chemical and electrochemical applications, e.g. in catalytic conversions, electrolysers and fuel cells. Calculating the changes in lattice parameters and electronic structure (oxidation and spin states, density of states) caused by the A and/or B site substitutions, deepens the understanding and enables prediction of the associated experimental trends, thereby increasing the potential applicability of these perovskite-type materials. Geatches et al. (pp. 1673–1687) have focused on the elaboration of the electronic structure of LaCoxFe1-xO3 with varying ratios of Fe/Co, revealing that increasing the ratio of Co to Fe changes their rather semiconducting nature toward insulating. This behaviour can be related to the decrease in the number of (conducting) O–Fe–O bonds and the increase in (insulating) O–Co–O bonds

Understanding cationic polymer adsorption on mineral surfaces: kaolinite in cement aggregates

We present a joint experimental and theoretical investigation into the adsorption of polycationic quaternary ammonium polymers on the clay mineral kaolinite. Within the cement and concrete manufacturing industries such polymers are used to improve the final product by inerting the adsorption capacity of the clay minerals for more expensive additives. The adsorption of the presently used polymer (FL22) was compared with both a similar variant, but without a hydroxyl group (Fl22mod) and uncharged polyvinyl alcohol (PVA). Experimental results show that adsorption of FL22 is higher than that of FL22mod at both pH 6 and at pH > 10 and that the adsorption of PVA is the highest. Theoretical density functional theory (DFT) results and simplified models consisting of the basal surfaces of kaolinite, with monomers of FL22, FL22mod and PVA gave monomer coverage per unit surface area of kaolinite, a comparison of the configurations of the relaxed models, formation energies and Mulliken charges. These results show that the polycationic polymers interact with the basal surfaces of kaolinite electrostatically, explaining the high affinity of these polymers for kaolinite surfaces in the experimental results. The hydroxyl groups of FL22 and PVA form hydrogen bonds with the basal surfaces of kaolinite in conditions of pH 6. The joint experimental and theoretical results suggest that, due to the presence of the hydroxyl group, the conformation of FL22 changes under pH, where at neutral pH it lies relatively flat to the kaolinite surfaces, but at higher pH, conformational changes of the polymer occur, thereby increasing the adsorbed quantity of FL2