Computer modelling of the thermal decomposition of solids
- Authors: De la Croix, Annemarie
- Date: 1996
- Subjects: Solids -- Thermal properties -- Computer simulation
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4302 , http://hdl.handle.net/10962/d1004960 , Solids -- Thermal properties -- Computer simulation
- Description: Decompositions of solids are typically of the form: A(s) ----> B(s) + gases. Symmetry-controlled routes (based on known and hypothetical crystal structures) for transforming the solid reactant into the solid product were devised as possible decomposition pathways. Lattice energies of the reactants, of the postulated transient intermediate structures and of the final solid products were then estimated by crystal modelling procedures. Profiles of lattice energy changes during the proposed decomposition routes were constructed and any energy barriers were compared with experimental activation energies reported for the thermal decompositions. The crystal modelling was performed with the computer program WMIN. Calculation of the lattice energies involved the development of a model potential for the perfect lattice and the evaluation of the interatomic parameters. The potential was based on the Born model of ionic solids using the Buckingham potential (Ø(r)= Ae⁻r/p - C/r⁶) to describe the short-range energy contribution. Empirical fitting was used to establish reliable interatomic energy parameters. The reliability of the interatomic potentials was assessed by calculating crystal structures and lattice energies (which were not included in the fitting). The particular reactions selected for modelling were the decompositions of the alkaline-earth metal (Ca, Sr, Ba) peroxides and carbonates: M0₂(s) ---> MO(s) + ¹/₂0₂(g) MC0₃(s) ---> MO(s) + CO₂(g)The lattice energies calculated for the known structures were in good agreement with reported values, (except for Ba0₂ and BaC0₃) which provided support for the adequacy of the potential model used. Activation energies calculated for the decomposition of the carbonates were in the correct order but hlgher than experimental values, i. e., 422, 422, 465 and 499 kJ mol̄̄⁻¹ compared to the experimental values of 205, 87(?), 222 and 283 kJ mol̄̄⁻¹ for CaC0₃ (calcite), CaC0₃(aragonite), SrC0₃ and BaC0₃. The values calculated for the peroxides (91 and 100 kJ mol⁻¹ compared to the experimental values of 119 and 185 kJ mol⁻¹ for Sr0₂ and Ba0₂ respectively) were less satisfactory but could be a reflection of the poor structural data used for the peroxides. The significance of this approach to the modelling of solid decompositions is discussed.
- Full Text:
- Date Issued: 1996
- Authors: De la Croix, Annemarie
- Date: 1996
- Subjects: Solids -- Thermal properties -- Computer simulation
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4302 , http://hdl.handle.net/10962/d1004960 , Solids -- Thermal properties -- Computer simulation
- Description: Decompositions of solids are typically of the form: A(s) ----> B(s) + gases. Symmetry-controlled routes (based on known and hypothetical crystal structures) for transforming the solid reactant into the solid product were devised as possible decomposition pathways. Lattice energies of the reactants, of the postulated transient intermediate structures and of the final solid products were then estimated by crystal modelling procedures. Profiles of lattice energy changes during the proposed decomposition routes were constructed and any energy barriers were compared with experimental activation energies reported for the thermal decompositions. The crystal modelling was performed with the computer program WMIN. Calculation of the lattice energies involved the development of a model potential for the perfect lattice and the evaluation of the interatomic parameters. The potential was based on the Born model of ionic solids using the Buckingham potential (Ø(r)= Ae⁻r/p - C/r⁶) to describe the short-range energy contribution. Empirical fitting was used to establish reliable interatomic energy parameters. The reliability of the interatomic potentials was assessed by calculating crystal structures and lattice energies (which were not included in the fitting). The particular reactions selected for modelling were the decompositions of the alkaline-earth metal (Ca, Sr, Ba) peroxides and carbonates: M0₂(s) ---> MO(s) + ¹/₂0₂(g) MC0₃(s) ---> MO(s) + CO₂(g)The lattice energies calculated for the known structures were in good agreement with reported values, (except for Ba0₂ and BaC0₃) which provided support for the adequacy of the potential model used. Activation energies calculated for the decomposition of the carbonates were in the correct order but hlgher than experimental values, i. e., 422, 422, 465 and 499 kJ mol̄̄⁻¹ compared to the experimental values of 205, 87(?), 222 and 283 kJ mol̄̄⁻¹ for CaC0₃ (calcite), CaC0₃(aragonite), SrC0₃ and BaC0₃. The values calculated for the peroxides (91 and 100 kJ mol⁻¹ compared to the experimental values of 119 and 185 kJ mol⁻¹ for Sr0₂ and Ba0₂ respectively) were less satisfactory but could be a reflection of the poor structural data used for the peroxides. The significance of this approach to the modelling of solid decompositions is discussed.
- Full Text:
- Date Issued: 1996
Reduction of tungsten oxides with carbon and hydrogen
- Authors: Venables, Dean Stuart
- Date: 1996
- Subjects: Oxidation-reduction reaction , Tungsten , Hydrogen , Carbon
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4342 , http://hdl.handle.net/10962/d1005004 , Oxidation-reduction reaction , Tungsten , Hydrogen , Carbon
- Description: The reductions of WO₃ with hydrogen, with CO, and with carbon, as well as the reduction of WO₃/graphite mixtures with hydrogen, were studied using thermogravimetry, evolved gas analysis, X-ray powder diffraction, and scanning electron microscopy. The intermediate phases W₂₀O₅₈, W₁₈O₄₉ and WO₂, were observed in the reductions. The final product of the reductions with hydrogen and carbon was tungsten, and we was formed in the reduction with CO. The reaction paths in the overall processes were determined. The reactant/product gas ratio had a considerable influence on which reactions took place. The morphology of the sample was characterised at different stages of the reduction. The shape of the WO₃ particles was retained during the reduction. Particle growth was observed in the reduction with hydrogen and was attributed to the formation of WO₂(OH)₂(g). The kinetics of the reductions were investigated , and the reaction mechanisms determined. The reduction of WO₃ with CO was studied from 650 to 900°C, and occurred at a phase boundary with an activation energy of 40 kJ mol⁻¹ . The reduction of WO₂, was studied under the same conditions. The reaction also occurred at a phase boundary and had an activation energy of 62 kJ mol⁻¹. The reduction of WO₃ with carbon was studied from 935 to 1100°C and took place via CO and CO₂. Two stages were observed in the reduction . The first stage, which corresponded approximately to the formation of WO₂ had an activation energy of 66 kJ mol⁻¹ and was limited by diffusion through the porous reacting particles. The second stage was first order and had an activation energy of 40 kJ mol⁻¹. The reduction of WO₃ and WO₃ graphite mixtures with hydrogen were studied from 575 to 975 °C. The reactions were controlled by mass-transfer under the conditions investigated. The addition of carbon increased the rate of the reduction process , but did not affect the phases formed in the system. CO₂ was evolved mainly at the start, and CO mainly at the end of the process.
- Full Text:
- Date Issued: 1996
- Authors: Venables, Dean Stuart
- Date: 1996
- Subjects: Oxidation-reduction reaction , Tungsten , Hydrogen , Carbon
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4342 , http://hdl.handle.net/10962/d1005004 , Oxidation-reduction reaction , Tungsten , Hydrogen , Carbon
- Description: The reductions of WO₃ with hydrogen, with CO, and with carbon, as well as the reduction of WO₃/graphite mixtures with hydrogen, were studied using thermogravimetry, evolved gas analysis, X-ray powder diffraction, and scanning electron microscopy. The intermediate phases W₂₀O₅₈, W₁₈O₄₉ and WO₂, were observed in the reductions. The final product of the reductions with hydrogen and carbon was tungsten, and we was formed in the reduction with CO. The reaction paths in the overall processes were determined. The reactant/product gas ratio had a considerable influence on which reactions took place. The morphology of the sample was characterised at different stages of the reduction. The shape of the WO₃ particles was retained during the reduction. Particle growth was observed in the reduction with hydrogen and was attributed to the formation of WO₂(OH)₂(g). The kinetics of the reductions were investigated , and the reaction mechanisms determined. The reduction of WO₃ with CO was studied from 650 to 900°C, and occurred at a phase boundary with an activation energy of 40 kJ mol⁻¹ . The reduction of WO₂, was studied under the same conditions. The reaction also occurred at a phase boundary and had an activation energy of 62 kJ mol⁻¹. The reduction of WO₃ with carbon was studied from 935 to 1100°C and took place via CO and CO₂. Two stages were observed in the reduction . The first stage, which corresponded approximately to the formation of WO₂ had an activation energy of 66 kJ mol⁻¹ and was limited by diffusion through the porous reacting particles. The second stage was first order and had an activation energy of 40 kJ mol⁻¹. The reduction of WO₃ and WO₃ graphite mixtures with hydrogen were studied from 575 to 975 °C. The reactions were controlled by mass-transfer under the conditions investigated. The addition of carbon increased the rate of the reduction process , but did not affect the phases formed in the system. CO₂ was evolved mainly at the start, and CO mainly at the end of the process.
- Full Text:
- Date Issued: 1996
Structures and thermal behaviour of some monooxalato and dioxalato metal complexes
- Authors: Bacsa, John
- Date: 1996
- Subjects: Oxalates -- Research , Crystallography -- Research , Chemistry, Inorganic -- Research
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4351 , http://hdl.handle.net/10962/d1005016 , Oxalates -- Research , Crystallography -- Research , Chemistry, Inorganic -- Research
- Description: The crystal structure of Ba [Cu(C₂0₄)₂(H₂O)].5H₂O has been determined using single crystal X-ray diffractometry. It crystallises in the triclinic system, space group Pī , with a = 6.539(2) Å, b = 9.211(3) Å, c = 10.928(3) Å, a = 85.42(3)°, β = 79.22(3)° , γ = 80.30(3)°, V = 636.08(8)ų and Z = 2. The structure consists of [Cu(C₂0₄)₂(H₂O)]²⁻ ions weakly bridged by barium ions and water molecules. The copper(II) ions are in a tetragonally elongated square-pyramidal environment with some trigonal distortion. The two oxalate groups occupy the equatorial positions and a water molecule occupies the axial position. The barium ion is surrounded by nine oxygens: five oxygens from water molecules and four oxygens from oxalate groups. The thermal behaviour of Ba [Cu(C₂0₄)₂(H₂O)].5H₂0 in N₂ has been examined using thermogravimetry (TG) and differential scanning calorimetry (DSC). The dehydration starts at relatively low temperatures (~80°C), but continues until the onset of the decomposition (~280°C). The decomposition takes place in two major stages. The mass of the intermediate after the first stage corresponded to the formation of barium oxalate and copper metal and, after the second stage, to the formation of barium carbonate and copper metal. The enthalpy for the dehydration was found to be 311 ±30 kJ mol⁻¹. The overall enthalpy change for the decomposition of Ba[Cu(C₂0₄)₂]in N₂ was estimated from the combined area of the peaks of the DSC curve as -347 kJ mol⁻¹. The kinetics of the thermal dehydration and decomposition were studied using isothermal TG. The dehydration was strongly deceleratory and the α-time curves could be described by the three-dimensional diffusion (D3) model. The values of the activation energy and the pre-exponential factor for the dehydration were 125 ±4 kJ mol⁻¹ and (1.38 ±0.08)x10¹⁵ min⁻¹, respectively. The decomposition was complex, consisting of at least two concurrent processes. The decomposition was analysed in terms of two overlapping deceleratory processes. One process was fast and could be described by the contracting-geometry model with n = 5. The other process was slow and could also be described by the contracting-geometry model , but with n = 2. The values of Eₐ and A were 206 ±23 kJ mol⁻¹ and (2.2 ±O.5)xl0¹⁹min⁻¹, respectively, for the fast process, and 259 ±37 kJ mol⁻¹ and (6.3 ±1.8)x10²³min⁻¹, respectively, for the slow process.The crystal structure of zinc oxalate dihydrate ([Zn(C₂0₄)(H₂O)₂]n) has also been determined by X-ray diffraction methods. It crystallises in the monoclinic system, space group C2/c with a = 11.786(2) Å, b = 5.397(1)Å, c = 9.712(1) Å, B = 126.19(5)°, V = 498.58(8)ų, Z = 4 and R = 0.037 for 435 absorption-corrected independent reflections and 50 parameters. The asymmetric unit consists of half the monomeric unit [Zn(C₂0₄)(H₂O)₂). The structure consists of infinite, linear chains of zinc ions bridged by oxalate groups. The geometry of the coordination polyhedron surrounding the zinc ion is octahedral, with the oxalate oxygens occupying the equatorial positions and water molecules occupying the axial positions.
- Full Text:
- Date Issued: 1996
- Authors: Bacsa, John
- Date: 1996
- Subjects: Oxalates -- Research , Crystallography -- Research , Chemistry, Inorganic -- Research
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4351 , http://hdl.handle.net/10962/d1005016 , Oxalates -- Research , Crystallography -- Research , Chemistry, Inorganic -- Research
- Description: The crystal structure of Ba [Cu(C₂0₄)₂(H₂O)].5H₂O has been determined using single crystal X-ray diffractometry. It crystallises in the triclinic system, space group Pī , with a = 6.539(2) Å, b = 9.211(3) Å, c = 10.928(3) Å, a = 85.42(3)°, β = 79.22(3)° , γ = 80.30(3)°, V = 636.08(8)ų and Z = 2. The structure consists of [Cu(C₂0₄)₂(H₂O)]²⁻ ions weakly bridged by barium ions and water molecules. The copper(II) ions are in a tetragonally elongated square-pyramidal environment with some trigonal distortion. The two oxalate groups occupy the equatorial positions and a water molecule occupies the axial position. The barium ion is surrounded by nine oxygens: five oxygens from water molecules and four oxygens from oxalate groups. The thermal behaviour of Ba [Cu(C₂0₄)₂(H₂O)].5H₂0 in N₂ has been examined using thermogravimetry (TG) and differential scanning calorimetry (DSC). The dehydration starts at relatively low temperatures (~80°C), but continues until the onset of the decomposition (~280°C). The decomposition takes place in two major stages. The mass of the intermediate after the first stage corresponded to the formation of barium oxalate and copper metal and, after the second stage, to the formation of barium carbonate and copper metal. The enthalpy for the dehydration was found to be 311 ±30 kJ mol⁻¹. The overall enthalpy change for the decomposition of Ba[Cu(C₂0₄)₂]in N₂ was estimated from the combined area of the peaks of the DSC curve as -347 kJ mol⁻¹. The kinetics of the thermal dehydration and decomposition were studied using isothermal TG. The dehydration was strongly deceleratory and the α-time curves could be described by the three-dimensional diffusion (D3) model. The values of the activation energy and the pre-exponential factor for the dehydration were 125 ±4 kJ mol⁻¹ and (1.38 ±0.08)x10¹⁵ min⁻¹, respectively. The decomposition was complex, consisting of at least two concurrent processes. The decomposition was analysed in terms of two overlapping deceleratory processes. One process was fast and could be described by the contracting-geometry model with n = 5. The other process was slow and could also be described by the contracting-geometry model , but with n = 2. The values of Eₐ and A were 206 ±23 kJ mol⁻¹ and (2.2 ±O.5)xl0¹⁹min⁻¹, respectively, for the fast process, and 259 ±37 kJ mol⁻¹ and (6.3 ±1.8)x10²³min⁻¹, respectively, for the slow process.The crystal structure of zinc oxalate dihydrate ([Zn(C₂0₄)(H₂O)₂]n) has also been determined by X-ray diffraction methods. It crystallises in the monoclinic system, space group C2/c with a = 11.786(2) Å, b = 5.397(1)Å, c = 9.712(1) Å, B = 126.19(5)°, V = 498.58(8)ų, Z = 4 and R = 0.037 for 435 absorption-corrected independent reflections and 50 parameters. The asymmetric unit consists of half the monomeric unit [Zn(C₂0₄)(H₂O)₂). The structure consists of infinite, linear chains of zinc ions bridged by oxalate groups. The geometry of the coordination polyhedron surrounding the zinc ion is octahedral, with the oxalate oxygens occupying the equatorial positions and water molecules occupying the axial positions.
- Full Text:
- Date Issued: 1996
Synthetic and mechanistic studies of heterocyclic systems
- Authors: Deane, Philip O'Grady
- Date: 1996
- Subjects: Heterocyclic chemistry
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4379 , http://hdl.handle.net/10962/d1005044 , Heterocyclic chemistry
- Description: A series of acrylate esters and selected analogues have been reacted with pyridine-2-,pyridine-3-, and pyridine-4-carboxaldehydes in the presence of diazabicyclo[2,2,2]octane (DARCO) to afford a range of Baylis-Hillman products. The pyridine-2-carboxaldehyde-derived products have been acetylated using acetic anhydride and the kinetics of the thermal cyclisation of the acetylated compounds to indolizines was investigated using proton NMR spectroscopy. The first-order kinetics of the cyclisation has been confirmed and the influence of substituents on the first-order rate constant, kₒbs has been examined. The kinetic data has been shown to be consistent with the previously proposed mechanism in which loss of the acetate group is ratedetermining. Each of the cyclisations was also monitored at three different temperatures permitting evaluation of the activation parameters. The Baylis-Hillman products and related acetylated derivatives were treated at room temperature with sodium methylthiolate; the hydroxy precursors were observed to undergo conjugate addition with a degree of diastereocontrol but the acetylated derivatives favoured an apparent SN¹ displacement of the acetate group.
- Full Text:
- Date Issued: 1996
- Authors: Deane, Philip O'Grady
- Date: 1996
- Subjects: Heterocyclic chemistry
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4379 , http://hdl.handle.net/10962/d1005044 , Heterocyclic chemistry
- Description: A series of acrylate esters and selected analogues have been reacted with pyridine-2-,pyridine-3-, and pyridine-4-carboxaldehydes in the presence of diazabicyclo[2,2,2]octane (DARCO) to afford a range of Baylis-Hillman products. The pyridine-2-carboxaldehyde-derived products have been acetylated using acetic anhydride and the kinetics of the thermal cyclisation of the acetylated compounds to indolizines was investigated using proton NMR spectroscopy. The first-order kinetics of the cyclisation has been confirmed and the influence of substituents on the first-order rate constant, kₒbs has been examined. The kinetic data has been shown to be consistent with the previously proposed mechanism in which loss of the acetate group is ratedetermining. Each of the cyclisations was also monitored at three different temperatures permitting evaluation of the activation parameters. The Baylis-Hillman products and related acetylated derivatives were treated at room temperature with sodium methylthiolate; the hydroxy precursors were observed to undergo conjugate addition with a degree of diastereocontrol but the acetylated derivatives favoured an apparent SN¹ displacement of the acetate group.
- Full Text:
- Date Issued: 1996
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