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Please use this identifier to cite or link to this item: http://hdl.handle.net/1959.13/923600

Title
Catalytic mechanism of the Deacon reaction
Author/Creator
Suleiman, Ibrahim
Institution
University of Newcastle. Faculty of Engineering and Built Environment, School of Engineering
Description
Research Doctorate - Doctor of Philosophy (PhD)
Description
In this Thesis, the role of copper in catalysing the HCl oxidation (Deacon) reaction is explored theoretically from first principles as part of a larger project aimed at understanding the formation of toxic and environmentally harmful materials polychlorinated dibenzo–p–dioxins and polychlorinated dibenzofurans. The Thesis deployed both density functional theory (DFT) and ab initio atomistic thermodynamics. The DFT calculations were adopted to identify various gas-gas and gas-copper-surface structures and their energetic stabilities, and predict steps of chemical reactions to explain the interaction of copper with the species involved. Ab initio atomistic thermodynamics method was used to extrapolate the T = 0 K and p = 0 bar DFT results to the experimental T and p conditions by calculating surface free energies for the gas-surfaces stable structures as a function of chlorine or oxygen chemical potentials (at various p and T). This helped to determine the active species for a chemical reaction, which is one of the most important factors in understanding the heterogeneous catalysis. For the gas phase oxidation of hydrogen chloride in the presence of copper (II) chloride it was found that chlorine (Cl₂) is produced by the thermal decomposition of CuCl₂ generating Cu₂Cl₂ which reacts with O₂ (³Σg) to form several intermediates and complexes which further react with hydrogen chloride. The fission of a O-O linkage between two Cu₂Cl₂ moieties was identified as a key step in this process. However, this step possesses a barrier prohibitive to homogeneous gas phase catalysis but one which is expected to be easily overcome on a copper base catalyst. For the gas-surface interactions, the Thesis studies the adsorption of oxygen and chlorine on the low-index Cu(100) and Cu(110) surfaces. For Cu(100) it is found that the presence of atomic chlorine increases the stability of adsorbed molecular oxygen, and that the barrier required to dissociate the oxygen molecule in the presence of chlorine is three times larger than the dissociation barrier of molecular oxygen on clean Cu(100). In addition, chlorine monoxide was generated on the surface when molecular oxygen was adsorbed horizontally into a hollow site immediately adjacent to atomic chlorine. The calculations indicate that while chlorine is easily adsorbed dissociatively on the clean Cu(100) surface, it is stable in the molecular form in the presence of atomic oxygen. The presence of chlorine leads to the production of sub-surface atomic oxygen and enables an oxygen atom to go into the Cu bulk with a small activation energy barrier. Calculations using ab initio atomistic thermodynamics showed that for Cl/Cu(100) and coverages up to 1/2 ML, the 1/2 ML c(2×2)-Cl phase with Cl atoms adsorbed on the hollow site is the most stable structure at room temperature, in agreement with experiment. The 1/9 ML and 1/4 ML structures show some stability at very low pressures of chlorine, and elevated temperatures. At high coverages, the stable structures were found to be only kinetically accessible. The subsurface adsorption of Cl atoms, however, dramatically increases the stability of the 1 ML and 2 ML adsorption configurations providing a possible pathway for the formation of the bulk-chloride surface phases in the kinetic regime. In order to understand the role of copper in catalysing the HCl oxidation (Deacon) reaction, its mechanism on the Cu(100) surface is investigated using DFT. The mechanism of water formation on the Cu(100) surface was found to consist of three major steps; these are: (1) oxygen adsorption and dissociation, (2) OH formation and (3) H₂O formation and desorption. The most preferred pathway for the OH formation step involved the reaction of gaseous HCl molecule with an adsorbed O atom on the surface obeying the Eley-Rideal mechanism. Energetics and transition states were reported for different pathways of each step. It is shown that while water can be formed easily on the Cu(100) surface, Cl₂ is not the result of the direct oxidation of HCl molecules. An ab initio atomistic thermodynamics technique was utilised to study the thermodynamic stability of different O-Cl/Cu(100) structures. It is shown that the high coverages of both Cl and O (1/2 ML-Cl // 1/2 ML-O) are only kinetically accessible. The subsurface adsorption of oxygen, however, was appeared to increase the stability of this structure providing a possible pathway for bulk-like Cu₂Cl₂O formation in the kinetic regime. The formation of the latter structure was found to be a prerequisite for Cl₂ formation and liberation on the Cu(100) surface. For Cl/Cu(110), the 1/2 ML c(2×2)-Cl structure was found to be the most stable for coverages ≤ 1/2 ML at room temperature, in agreement with experiment. However, the predicted short bridge site for the most stable c(2×2)-Cl structure on Cu(110) differs from the experimentally proposed hollow position. Beyond 1/2 ML coverage, the (2×3)-Cl and (1×4)-Cl configurations were predicted to be the most stable for 2/3 ML and 3/4 ML coverage, respectively. As for Cu(100), the high Cl coverage stable structures on Cu(110) were also found to be only kinetically accessible. By incorporating the results from previous work of Cl adsorption on Cu(111), the morphologies of a copper nano-structure terminated by the low-index Cu(100), Cu(110) and Cu(111) surfaces in a chlorine environment were predicted using the Wulff construction. It was determined that the (100) planes become predominant with increasing chlorine concentration resulting in a cubical crystal shape, in agreement with the experimental observations.
Relation
University of Newcastle Research Higher Degree Thesis
Date
2011
Keyword(s)
Deacon reaction;  DFT;  copper chloride;  Cu(100)
Resource Type
thesis
Rights
Copyright 2011 Ibrahim Suleiman
Identifier
http://hdl.handle.net/1959.13/923600
Language
eng
 
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