Year: 2020
Type: dissertation
Abstract: Exploitation of mechanical, physical and chemical properties of reactive solid-liquid interfaces experienced fast growth in recent years due to the rapid increase of available systems, computational power and experimental instrumentation. Despite this, acquiring detailed information about molecular interactions involving complex interfaces is still a major challenge. The aim of this thesis is to give a detailed insight into the atomic description of two different solid-liquid interfaces. Specifically, quantum chemical methodology is applied to the water-gas shift reaction (WGSR) occurring in the supported ionic liquid phase (SILP), and classical molecular dynamics simulations are applied in a study of a model system for the process of biomineralization. Water-gas shift reaction is conversion of water and carbon monoxide to hydrogen and carbon dioxide. It is of high industrial importance, and it usually follows the standard reaction in hydrogen production called steam reforming. However, typical implementations of the WGSR involve catalysts that operate at high temperatures, which, coupled with the exothermic nature of the reaction, lead to lower conversion. This problem was recently addressed by conducting WGSR in thin films of ionic liquid. In this case, the reaction is catalyzed by ruthenium complexes effective at low temperatures, yielding hydrogen of very high purity. However, the optimization of this process could not be achieved so far because of the lack of understanding of the underlying chemistry. In this thesis the equilibrium composition of catalytic complexes in such a reactor, elucidation of the mechanistic details of reaction with said catalytic complexes, and identifying molecular additives that enhance the reaction turnover, has been achieved, hence providing the necessary framework for further development of WGS SILP systems. Specifically, using density functional theory (DFT) to interpret infra-red spectroscopy data from our collaborators, we were able to determine the chemical species emerging in the operating WGS SILP reactor, among which we found the anionic [RuCl3(CO)3]-. We showed that this monomer is the predominant catalytic complex, as opposed to the initially hypothesized dimer precursor. This prompted a theoretical study of the reaction mechanism, which was performed using the highly accurate coupled-cluster theory methods.We demonstrated that the reaction can work in acidic conditions, in contrast to the standard base mechanisms, leading to the formation of HCl as an intermediate preceding the rate limiting step. The latter involves the formation of a six-membered ring catalytic complex containing water, which is a transition structure to a hydride. From the hydride a hydrogen complex forms at a cost that is significantly lower than previously envisaged. This finding was confirmed in primary isotope experiments. The involvement of HCl prompted the use of transition metal chlorides as additives to enhance the WGS turnover rate. Indeed, 20% improvement was obtained with neutral and anionic copper complexes, which were characterized using DFT vibrational analysis of experimental IR spectra. The mechanism for this activity is yet to be elucidated, however, the current results suggest that the copper complexes act as CO shuttles facilitating the exchange at the reactive center, due to stronger CO binding to the ruthenium based catalyst. These results point to an exciting opportunity for further optimization of the underlying catalytic process, the full understanding of which emerge only using theoretical approaches. As such, the results presented in this thesis provide a substantially sharper picture of the most efficient application of the WGSR, simultaneously contributing to the foundation for the future commercial exploitation of the SILP technology. While in the first part of the thesis we focused on a system where the reactivity takes place in the liquid phase, leaving the solid phase intact, in the second part we study the process of biomineralization in which the solid phase is itself affected by the interface activity. During the formation of minerals, growth occurs under the influence of biomolecules, resulting in uncommon morphologies and improved mechanical properties of the solid. Motivated by the fact that proteins rich in aspartate have particularly strong effect on the inorganic crystal growth, our model system is constructed from simple aspartate derivatives binding to calcite. We perform fully atomistic molecular dynamics simulations with the goal of determining the free energies of binding of biomolecules to the ideal model of a calcite surface, and to a more realistic model containing corners and edges. Using this more complex surface is instrumental to recovering the binding energies measured by our collaborators, and also to show that the stereochemical blocking efficiency is not a monotonic function of molecular weight of the aspartate derivative. This agreement with experiments warrants confidence in the correctness of our model system, and allows us to proceed with the analysis of molecular details of the interactions occurring at the interface. Besides explaining the non monotonous behavior of the blocking efficiency by exploring the main binding modes, we were able to demonstrate the importance of the N-terminus, and the solvation of the carboxylates for the overall interaction. This contribution not only elucidates some important characteristics of the biological calcite formation, but also it points out to the importance of dealing with defects and realistic morphologies of the solid phase in studies of reactive interfaces. The two systems in question allowed us to demonstrate the power of the state-of-the-art computational methods in complex problems regarding the contact of the solid and the liquid phase. In the process we have established a reproducible methodology on multiple scales using well founded quantum and classical physical models, setting a path for the future studies on similar systems.
Author: Stepić, Robert 1990-
Language: English
Subfield: Physical and Theoretical Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7071677629