El olvido y el perdón

The thesis is organised as follows:

Chapter 2 presents a critical analysis of the relevant available literature on: (i) the chemistry of borate with a primary focus on reactivity, synthesis procedures, properties and, structures of diboron trioxide; and (ii) the catalytic activity, structures, and electronic properties of the thermodynamically stable phase of alumina i.e. α-Al2O3. Basically, Chapter 2 achieves the

following:1) critically summarises the industrial synthesis of boron oxide; 2) discusses the catalytic inhibition properties of B2O3; 3) presents the molecular structure of diboron trioxide

(vitreous and crystalline forms); 4) presents alternative industrial applications of diboron trioxide; 5) summarises the classification of alumina; 6) analyses the acidity and basicity of alumina; 7) surveys the effect of alumina’s hydration on its surface reactivity; 8) highlights the role of various transitional metal oxides, particularly alumina, in the formation of notorious PCDD/Fs, and phenoxy-type EPFR.

Chapter 3 introduces the theoretical background of the computational techniques employed in this study, along with a brief review of the basic concepts of ab initio atomistic thermodynamics and the density function theory (DFT) approaches. Moreover, the experimental approaches and computer codes employed within the scope of this thesis (i.e. DMol3 cod and CRYSTAL14 code) are described briefly.

Chapter 4 offers a computational account of the strong and exothermic interaction of atomic and molecular oxygen with the α(001)B12 surface of boron. We found that physisorbed oxygen interacts weakly with the surface, but the dissociative chemisorption entails

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considerable exothermicity in the range of 2.47 – 3.45 eV/mol, depending on the adsorbed sites of the two oxygen atoms. Nonetheless, rupture of dioxygen on the surface involves a sizable intrinsic reaction barrier of 3.40 eV. Such a high level of energy clearly explains the chemical inertness (i.e. lack of oxidation) of boron at room temperature. However, elevated temperatures encountered in real applications of boron, such as cutting machinery, overcome the high-energy barrier for the dissociative adsorption of molecular oxygen (3.40 eV). A stability T-P phase diagram reveals the spontaneous nature of the substitutional O/α(001)B12 adsorption modes that lead to the formation of diboron trioxide, at temperatures and pressure pertinent to practical applications. The finding of this Chapter conclusively collaborates the experimental observation of the formation of the B2O3 phase from the adsorption of oxygen on boron.

Finally, charge analysis provides an atomic-scale probe for the predicted stability ordering of the considered O/α(001)B12 configurations.

Chapter 5 presents accurate quantum mechanical calculations using the PW1PW hybrid HF/DFT functional of four low-index surfaces of the low-pressure phase of B2O3: (101), (100), (011) and

(001). Bond lengths, bond angles, and net Mulliken charges of the surface atoms are analysed in detail. The total and projected density of states and surface energies are discussed. The occurrence of tetrahedral BO4 units on the lowest energy structures of two of these surfaces is

demonstrated for the first time. The corresponding surface orientations incur larger energies in reference to the two orientations featuring only BO3 units. None of the four investigated lowest

energy structures have dangling bonds, which reasonably relates to the experimentally observed low reactivity of this compound. The findings in this Chapter pave the way for potential interest in future studies regarding the surfaces of amorphous B2O3, as well as on the hydroxylation of

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Chapter 6 focuses on the adsorption and dissociation mechanisms of two hydrogen chalcogenides, namely water (H2O) and hydrogen sulfide (H2S) molecules, over the B2O3 -I (101)

surface. Aided by both experimental diffuse reflectance infrared spectroscopy and computational first-principle techniques, this Chapter confirms the hygroscopic behaviour of diboron trioxide, elucidating the corresponding enthalpic requirements. We show that the diboron trioxide surface exhibits high physiochemical reactivity towards water molecules with an activation energy of 39 kJ/mol dissociative adsorption. Furthermore, desorption of both molecularly adsorbed and dissociated structures of water molecules from the B2O3 -I (101) surface requires activation

energies of 124–127 kJ/mol, in agreement with the experimentally derived isoconversional activation energies for the same process. Our investigation on the other hydrogen-chalcogenide compound, i.e. H2S, reveals that diboron trioxide attracts H2S molecules and forms molecular

adsorption via sp3 hybridisation between the lone pair electron of the H2S and the empty p orbital

of the Bsurf atom without activation barrier. However, the energy barrier required to dissociate

H2S over the B2O3 -I (101) surface appears exceedingly high at 310 kJ/mol. The present insight

resolves the two different behaviours of B2O3 concerning hydrogen chalcogenides reported in the

literature. While acting as a water scavenger to generate dissociated radicals, it exhibits an inhibitor characteristic towards the dissociation of H2S molecules, representing an ideal reactor

wall coating for desired pure gas phase reactions.

Chapter 7 uses first-principle calculations to investigate the activity of the alumina neat α- Al2O3(0001) surface in the formation of phenolic EPFR, under conditions relevant to cooling

zones of combustion systems. We show that the molecular adsorption of phenol on α- Al2O3(0001) entails binding energies in the range of -202–-127 kJ/mol. The dehydroxylated

alumina catalyses the conversion of phenol into its phenolate moiety with a modest activation energy of 48 kJ/mol. Kinetic rate parameters, established over the temperature range of 300 to

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1000 K, confirm the formation of the phenolate as the preferred pathways for the adsorption of phenol on alumina surfaces, corroborating the role of metal oxides deposited on particulate matter in the cooling zone of combustion systems in the generation of environmentally- persistent free radicals.

Chapter 8 presents a computational study of the catalytic role of the hydrated α-Al2O3 (0001)

surface and Si modified Al2O3 surfaces in producing phenolic EPFR. First, we present the

geometric and electronic properties of bulk α-Al2O3. Then, we investigate in detail the

molecular adsorption of phenol over the α-Al2O3 (0001) surface. This is followed by an

investigation of surface-mediated dissociation of phenol over both doped and undoped hydroxylated alumina surfaces. Molecular phenol is found to interact in vertical and flat/title configurations with calculated binding energies of -91 and 136 kJ/mol, respectively. The hydrated alumina surface is active toward the attack phenol molecule and forms a phenoxy moiety, via the H2O elimination mechanism. The Si-α-Al2O3 (0001) substituted surface is

considered for both hydrated and dehydrated alumina systems. The activation energy barrier required to form phenoxy moiety over the Si- Al2O3 (0001) surface was found to be nearly 37%

lower than that of the undoped dehydrated alumina surface.

Chapter 9 analyses dissociative adsorption mechanisms of phenol molecules over Al2O3 and

hydrated Al2O3.nH2O clusters that mimic dehydrated and hydrated alumina structures,

respectively. We show that fission of the phenol’s hydroxyl bond over dehydrated alumina systematically incurs lower energy barriers than that of the hydrated structures. In contrast, a 1,2-water elimination step marks the most feasible channel in the interaction of phenol and hydrated clusters. It is found that the catalytic activity of the alumina surface in producing the phenoxy/phenolate species reversibly correlates with the degree of hydroxyl coverage.

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Desorption of adsorbed phenolates requires sizable desorption energies and is expected to facilitate surface-mediated condensation into dioxin-like moieties.

Finally, Chapter 10 highlights the concluding remarks of this thesis and provides insights for future research.

 Research motivations

 Thesis objectives

 Thesis outline and overview

Chapter 2: Literature Review

 Chemistry of borates: Case of diboron trioxide

 Alumina from fundamentals to applications

 Gap of knowledge

Chapter 3: Research Methodology Chapter 4: Interaction of Oxygen with

α-Rhombohedral Boron (001) Surface  The initial steps governing conversion of

elemental boron into B2O3

Chapter 5: Structure, Stability and (non) reactivity of Low-Index Surfaces of Crystalline B2O3-I

 Inertness of B2O3 and the structure of its crystalline form

Chapter 6: Probing the Chemical Reactivity of the B2O3-I (101) surface: Interaction with H2O and H2S

 Mechanistic hygroscopic effect of B2O3

 Nature of catalytic inhibition by B2O3

Chapter 9: Formation of Environmentally Persistent Free Radicals on α-Al2O3 clusters

 Role of surface acidity on the catalytic activity of alumina in generating EPFR

Chapter 8: Formation of Phenoxy-Type EPFR over Hydrated Pure Alumina and Si-Alumina Surfaces

 Catalytic effect of fully hydrated alumina surface in generating phenoxy-type EPFR

 Role of atomic dopants on the catalytic activity of alumina in generating EPFR

Chapter 7: Formation of Environmentally Persistent Free Radicals on α-Al2O3

 Catalytic effect of dehydrated alumina surface in generating phenoxy-type EPFR

Chapter 10: Conclusions and recommendations Theme I

Theme II B2O3 inhibitors

Al2O3 catalysts

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