Computational mechanistic studies on the transition metal catalyzed activation of carbon dioxide
- Autores: Michael David Higham
- Directores de la Tesis: Nuria Lopez Alonso (dir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2017
- Idioma: español
- Tribunal Calificador de la Tesis: Zbigniew Lodziana (presid.), Maria Besora Bonet (secret.), Jordi Cirera (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencia y Tecnología Química por la Universidad Rovira i Virgili
- Materias:
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- Resumen
Computational techniques are applied to investigate the utility of rutile transition metal oxides as catalysts for processes in halogen chemistry. Ruthenium dioxide and titanium dioxide are investigated as catalysts for both hydrogen chloride and hydrogen bromide oxidation processes, and particular attention is devoted to the relationship between surface structure and composition, and the reaction mechanism. Further investigations focus on the effects of intentional doping of titanium dioxide on the system's performance as a hydrogen halide oxidation catalyst, with attention being focused on the relationship between dopant-induced variations in the electronic structure of the system, and the activity of the system towards essential elementary processes. Finally, the discussion turns to the utility of ruthenium dioxide as catalyst for ethylene oxychlorination, examining the interplay between surface composition and selectivity.
The work presented in the thesis is primarily theoretical in nature, employing computational quantum chemistry techniques to perform calculations which reveal vital chemical insights. Specifically, calculations were performed to obtain optimised geometries for various systems, to calculate energy changes and activation barriers associated with elementary reaction processes, and to obtain Density Of States (DOS) plots providing detailed information on the electronic structure of the system. Additionally, in some parts of the thesis, ab initio themodynamics methods are applied, with computational techniques being used to obtain input data which is then subsequently combined with the relevant experimental data to explore the behaviour of the system under realistic reaction conditions. All of the calculations were performed used Density Functional Theory (DFT) as implemented in the VASP code. The calculations are supported by complementary experimental studies conducted by external collaborators. Specifically, the experimental studies utilised the following techniques: High Resolution Transmission Electron Microscopy (HRTEM), Temporal Analysis of Products (TAP) studies, and Prompt Gamma Activation Analysis (PGAA).
The studies revealed that the extent of halogen uptake by ruthenium dioxide and titanium oxide under hydrogen halide oxidation conditions is key to understanding the reaction mechanism. It was found that whilst titanium dioxide is highly resistant to the replacement of surface oxygen atoms for halogen atoms, quite different behaviour is observed for ruthenium dioxide. In contrast, it was found that moderate chlorine uptake was present under HCl oxidation conditions, and under HBr oxidation conditions, extensive surface bromination takes place, extending to the immediate subsurface layers of the oxide. The high Br uptake induces a significant rearrangement of the surface geometry, differing greatly from the surface structure of pristine ruthenium dioxide. It was proposed that the mechanism of hydrogen bromide oxidation over ruthenium dioxide depends on the bromine uptake and that potentially several different reaction mechanisms might be responsible for HBr oxidation.
The investigations concerning the intentional doping of titanium dioxide found that dopants can allow for the formation of electronic structure defects localised on the Ti atoms arising from charge compensation induced by the dopant atom. Additionally, compound effects of multiple dopants can be used to control the number and nature of the defect states formed, and thus can be used to tune the activity of the material, since defect sites in titanium dioxide serve as highly active sites for reactant adsorption. Specifically, it was found that co-doping of titanium dioxide with ruthenium and bromine results in the formation of a Ru(3+) defect state, with the defect state being preferentially localised on Ru rather than Ti. Studies also investigated the interstitial doping of boron in titanium dioxide; it was found that B doping induced the formation of three electronic structure defect states in titanium dioxide, with two of these defect states being localised on adjacent surface Ti atoms. It was found that this arrangement is highly conducive to oxygen dissociative adsorption, which is typically the most energetically challenging step for HBr oxidation over titanium dioxide, thus providing insights into potential doping strategies.
The studies of ruthenium dioxide as a catalyst for ethylene oxychlorination revealed the importance of surface coverage and composition in determining the selectivity of the reaction. It was found that for certain key elementary processes, high adsorbate coverages are essential to the feasibility of the process in question. Adjacent adsorbates can destabilise key intermediates, essentially diminishing high activation barriers calculated for low coverage conditions. The energetic cost of the destabilisation is compensated by the exothermicity of adsorption of additional species. The results also showed that surface structures can lead to the dimensional confinement of adsorbates, thus constraining selectivity by limiting the reactive neighbours available to a given adsorbate. The studies also explored the relationship between selectivty and the presence of acidic and basic sites on the surface, which were determined to be of importance for selectivity towards certain products. All of the insights gained from the studies contributed to a more in-depth understanding on the requirements for ideal ethylene oxychlorination catalysts.
