http://nova.newcastle.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:12285 This contribution studies the decomposition of folpet fungicide under oxidative conditions and compares the product species with those of captan fungicide, which is structurally related to folpet. Toxic products arising from folpet comprised carbon disulfide (highest emission factor of 4.9mg g¹ folpet), thiophosgene (14.4), phosgene (34.1), hydrogencyanide(2.6), tetrachloroethylene (111), hexachloroethane (167), and benzonitrile (4.5). Owing to the irrelated molecular structures, folpet emitted similar products to captan but at different yields, under the same experimental conditions. It appears that the availability of easily abstractable H atoms, in the structure of captan but not in that of folpet, defines the product distribution. In conjunction with the quantum chemical calculations, these experimental measurements afford an enhanced explanation of the formation pathways of hazardous decomposition products of these two structurally related fungicides. 2013-01-30T04:12:32.618Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:12434 The drying properties of linseed oil have been known since the 15th century, and exploited in artistic and industrial paints. Unfortunately, in the presence of metal salts, linseed oil applied to cotton fabric may induce self-heating and spontaneous ignition. The present study elucidates the chemical mechanisms which trigger both phenomena. Low-temperature oxidation of linseed oil and its active components, linoleic, linolenic and oleic acids in equimolar O₂/N₂ mixture has been investigated in a plug flow reactor housed inside an oven at an initial temperature between 60 and 100℃, with the liquids impregnated onto the glass wool support. In particular, we have studied the effect of transition metal salts on the oxidation of linseed oil. We identified the gaseous species produced in the oxidation by means of Fourier transform infrared (FTIR) spectroscopy and gas chromatography–mass spectrometry (GC–MS), and quantified them by micro gas chromatography (µGC). FTIR spectroscopy indicated the presence of aldehydes and carboxylic acids, with identification of these species confirmed by GC–MS. We propose that in the presence of a metal catalyst, the oxidation process involves the formation of metal-dioxygen (superoxide) adducts. The catalytic effect of a metal cation depends on its ability to form superoxide. Cobalt(II) is the most effective catalyst among other transition metal salts, followed by manganese(II) and iron(II). In the absence of a catalyst, we found the oil samples to undergo a slower autoxidation process, probably associated with cross-linking or polymerisation together with partial fragmentation to form the observed low molecular weight products. 2013-01-18T00:20:02.373Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:12365 Gas phase reaction of CHF₃ with CH₄, and the effect of additives such as CBrF₃ and CH₃OH on the reaction, have been investigated in the temperature range of 800–1100 K. The major products detected include CH₂double bond; length as m-dashCF₂ and C₂F₄ under all conditions studied. The presence of small amounts of CBrF₃ and CH₃OH enhances the conversion of CH₄ and rate of formation of CH₂double bond; length as m-dashCF₂ significantly, while concomitantly the rate of formation of C₂F4 decreases. The experiments were modeled against a comprehensive reaction mechanism involving 90 species and 790 reactions. Based on the experimental observations and reaction modeling, the crucial steps influencing the conversion of CHF₃ and CH₄, the formation of major and minor products and the promoting effects of CBrF₃ and CH₃OH are fully elucidated. For comparative purposes, the reaction of CHClF₂ with CH₄ under similar conditions is also studied and the results are consistent with the reaction mechanism developed and discussed. 2013-01-08T03:42:11.186Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:12273 This study assessed the emission of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD and PCDF) from captan, a commonly used fungicide, in vapour-phase oxidative pyrolysis under conditions similar to those encountered in fires and burning biomass contaminated or treated with pesticides. The laboratory-scale apparatus consisted of a pesticide vaporiser, a tubular reactor and a product sampling system. The sampling train comprised tandem XAD-2 resin cartridges to trap PCDD/F as well as an activated charcoal tube to capture the organic volatile compounds (VOC). The analyses of PCDD/F were conducted by means of high resolution gas chromatograph (HRGC) – ion trap mass spectrometer (ITMS) and analyses of VOC by HRGC – quadrupole mass spectrometer (QMS). Substantially more PCDF formed than PCDD in the oxidative pyrolysis of captan, with higher yield of total PCDD/F observed at longer residence time. As indicated by the homologue distribution of PCDD/F, only mono to tetra chlorinated congeners were detected in our measurements, with 4-monochlorinated dibenzofuran (4-MCDF) ranking as the most abundant congener. The results of VOC analysis revealed benzene and chlorinated benzenes as important PCDD/F precursors. Combining the experimental measurements and the results of quantum chemical calculations, we established the reaction pathways for formation of PCDD/F from the vapour-phase oxidative pyrolysis of this widely employed fungicide. 2012-12-18T00:43:07.698Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:12272 Phthalimide (PI) and tetrahydrophthalimide (THPI) are two structurally similar compounds extensively used as intermediates for the synthesis of variety of industrial chemicals. This paper investigates the thermal decomposition of PI and THPI under oxygen rich to oxygen lean conditions, quantifying the production of toxicants and explaining their formation pathways. The experiments involved a plug flow reactor followed by silica cartridges, activated charcoal trap and a condenser, with the decomposition products identified and quantified by Fourier transform infrared spectroscopy (FTIR), gas chromatography–mass spectrometry (GC–MS) and micro gas chromatography (μGC). The density functional theory (DFT) calculations served to obtain dissociation energies and reaction pathways, to elucidate the reaction mechanism. The oxidation of PI and THPI produced several toxic nitrogen-containing gases and volatile organic compounds, including hydrogen cyanide, isocyanic acid, nitrogen oxides, benzonitrile, maleimide and tentatively identified benzenemethanimine. The detection of dibenzo-p-dioxin (DD) and dibenzofuran (DF) suggests potential formation of the toxic persistent organic pollutants (POPs) in fires involving PI and THPI, in presence of a chlorine source. The oxidation of THPI produced 2-cyclohexen-1-one, a toxic unsaturated ketone. The results of the present study provide the data for quantitative risk assessments of emissions of toxicants in combustion processes involving PI and THPI. 2012-12-17T23:48:19.751Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:9983 The gas-phase reaction of CH₄ and CHC1F₂ (HCFC-22, R22) has been studied in an alumina tubular reactor at atmospheric pressure and in the temperature range of 673−1073 K. The motivation of the investigation is to assess this process as a potential route for the treatment of CHC1F₂, as well as a technology for the synthesis of CH₂═CF₂ (vinylidene fluoride, VDF). Under the conditions studied, the major products are C₂F₄, CH₂═CF₂, HF, and HCl. Minor products detected include C₂HF₃, C₂H₂, CHF₃, C₂H₃F, C₂H₂F₄, CH₂F₂, C₃F₆, CH₃Cl. A mechanistic interpretation of the results is proposed, including the reactions involved in the initial decomposition of CHClF₂, those contributing to the activation of CH₄ and developing the pathways leading to the formation of product species. The result of changing feed ratio experiments is consistent with the reaction mechanism developed. The introduction of small amounts of O₂ improves the conversion of CH₄ and formation of CH₂═CF₂ markedly. 2012-03-21T04:00:04.158Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:10420 Density functional theory calculations have been performed to study the combined interaction of oxygen and chlorine with the Cu(100) surface. We found the presence of atomic chlorine increases the stability of molecular oxygen adsorption, 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 the clean Cu(100) surface. In addition, chlorine monoxide was generated on the surface when molecular oxygen was adsorbed horizontally into a hollow site immediately adjacent to atomic chlorine. Our 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 subsurface atomic oxygen and enables an oxygen atom to go into the Cu bulk with a small activation energy barrier. 2012-03-15T00:40:03.797Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:10414 A mechanism for the oxidation of ammonia by hypochlorous acid to form nitrogen gas has been developed at the B3LYP/6-31G(d,p) level of theory using the Gaussian 03 software package. The formation of NH₂Cl, NHCl₂, and NCl₃ was studied in the gas phase, with explicit water molecules included to examine the transition state energy in aqueous solution. The inclusion of explicit water molecules in the transition state dramatically reduced the reaction barrier in reactions involving transfer of a hydrogen atom between molecules, effects that were not taken into account through use of a solvation model alone. Three mechanisms were identified for the decomposition of chloramine species to form N₂, involving the combination of two chloramine species to form hydrazine, dichlorohydrazine and tetrachlorohydrazine intermediates. The highest barrier in each pathway was found to be the formation of the hydrazine derivative. 2012-03-14T23:30:03.725Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:9981 The reaction of CHF₃ with CH₄ in the presence of small amounts of CH₃OH (5% of CHF₃ concentration) was investigated in a tubular alumina reactor at temperatures between 900 K and 1150 K. The presence of CH₃OH has a significant influence on both the conversion and selectivity of the reaction. Increasing the CH₃OH concentration enhances the conversion of CHF₃ by a factor of 1−4 times at temperatures between 873 K and 1123 K, although the rate of formation of the major products (CH₂═CF₂) does not increase. A reaction scheme, based on NIST HFC and GRI-Mech mechanisms, is developed to model the reactions. Satisfactory agreement between experimental results and predictions is achieved for the conversion of reactants and formation of products. Based on experimental measurements and modeling results, a detailed reaction mechanism is proposed, and insight into how CH₃OH influences the reaction of CHF₃ with CH₄ is presented. 2012-02-09T02:00:03.861Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:9907 Semiquinones are persistent free environmental radicals formed as important initial products from the decomposition of dihydroxylated benzene isomers. This study develops detailed decomposition pathways for the thermal decomposition of the three isomeric semiquinone radicals. Branching ratios based on the calculated high-pressure limit reaction rate constants predict that p-benzoquinone is a major product from the unimolecular decomposition of the p-semiquinone radical, while the formation of o-benzoquinone from the o-semiquinone radical corresponds to a minor channel. This finding is consistent with the absence of o-benzoquinone from the thermal degradation of the 1,2-dihydroxybenzene isomer and the abundance of p-benzoquinone from the thermal decomposition of 1,4-dihydroxybenzene. Ring contraction/CO elimination is shown to be the dominant sink pathway for the o-semiquinone and m-semiquinone radicals. Thermochemical properties, in terms of enthalpies of formation, entropies, and heat capacities for dihydroxylated benzene isomers, semiquinone radicals, and benzoquinones, are evaluated by quantum chemical calculations. Values of the enthalpies of formation calculated by the B3LYP/GTLarge method show good agreement with those obtained at the G3B3 level of theory. 2012-02-03T03:20:05.790Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:9906 This study develops the reaction pathway map for the unimolecular decomposition of catechol, a model compound for various structural entities present in biomass, coal, and wood. Reaction rate constants at the high-pressure limit are calculated for the various possible initiation channels. It is found that catechol decomposition is initiated dominantly via hydroxyl H migration to a neighboring ortho carbon bearing an H atom. We identify the direct formation of o-benzoquinone to be unimportant at all temperatures, consistent with the absence of this species from experimental measurements. At temperatures higher than 1000 K, water elimination through concerted expulsion of a hydroxyl OH together with an ortho H becomes the most significant channel. Rice−Ramsperger−Kassel−Marcus simulations are performed to establish the branching ratio between these two important channels as a function of temperature and pressure. All unimolecular routes to the reported major experimental products (CO, 1,3-C₄H₆ and cyclo-C₅H₆) are shown to incur large activation barriers. The results presented herein should be instrumental in gaining a better understanding of the decomposition behavior of catechol-related compounds. 2012-02-03T03:00:03.739Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:9905 Alkylated hydroxylated aromatics are major constituents of various types of fuels, including biomass and low-rank coal. In this study, thermochemical parameters are obtained for the various isomeric forms of methylbenzenediol isomers in terms of their enthalpies of formation, entropies, and heat capacities. Isodesmic work reactions are used in quantum chemical computations of the reaction enthalpies for O-H and H₂C-H bond fissions and the formation of phenoxy- and benzyl-type radicals. A reaction potential energy on the singlet-state surface surface is mapped out for the unimolecular decomposition of the 3-methylbenzene-1,2-diol isomer. According to the calculated high pressure-limit reaction rate constants, concerted hydrogen molecule elimination from the methyl group and the hydroxyl group, in addition to intermolecular H migration from the hydroxyl group, dominates the unimolecular decomposition at low to intermediate temperatures (T ≤ 1200 K). At higher temperatures, O-H bond fission and concerted water elimination are expected to become the sole decomposition pathways. 2012-02-03T02:30:05.242Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:9804 A detailed reaction mechanism is developed and used to model experimental data on the pyrolysis of CHF₃ and the non-oxidative gas-phase reaction of CHF₃ with CH₄ in an alumina tube reactor at temperatures between 873 and 1173 K and at atmospheric pressure. It was found that CHF₃ can be converted into C₂F₄ during pyrolysis and CH₂=CF₂ via reaction with CH₄. Other products generated include C₃F₆, CH₂F₂, C₂H₃F, C₂HF₃, C₂H₆, C₂H₂ and CHF₂CHF₂. The rate of CHF₃ decomposition can be expressed as 5.2 x 10¹³[s⁻¹]e−295[kJmol⁻¹]/RT. During the pyrolysis of CHF₃ and in the reaction of CHF₃ with CH₄, the initial steps in the reaction involve the decomposition of CHF₃ and subsequent formation of CF₂ difluorocarbene radical and HF. It is proposed that CH₄ is activated by a series of chain reactions, initiated by H radicals. The NIST HFC and GRI-Mech mechanisms, with minor modifications, are able to obtain satisfactory agreement between modelling results and experimental data. With these modelling analyses, the reactions leading to the formation of major and minor products are fully elucidated. 2012-01-13T03:50:04.116Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:9800 Gas-phase reaction of CHF₃ (HFC 23) with CH₄ in the presence of CBrF₃ (halon 1301) to produce CH₂=CF₂ (VDF) is presented. Experiments were carried out in a plug-flow reactor at temperatures between 873 and 1173 K. Under these conditions, CH₂=CF₂ was a dominant product observed, with CH₂F₂, C₂H₂, CH₂CHF, C₂F₄, C₂H₆ and CHFCF₂ also detected. In the presence of less than 6000 ppm of CBrF₃, the rate of formation of CH₂=CF₂ is significantly enhanced, and a much lower rate of formation of the major by-product, C₂F₄ is observed. Further increasing the proportion of CBrF₃ in the feed resulted in an even higher rate of formation of CH₂=CF₂. Experimental results are fitted very well by a model derived from the NIST and GRI-Mech 3.0 mechanisms, modified to include additional published kinetic data. 2012-01-13T01:10:04.764Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:9799 The gas-phase reaction of a mixture of waste refrigerant gases, namely R22 (CHCIF₂), R12 (CCI₂F₂) and R134a (CH₂FCF₃) with CH₄ has been investigated over the temperature range of 873–1133 K. The investigation was undertaken as an initial assessment of the viability of this process as a treatment option for waste mixtures of hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFC), chlorofluorocarbons (CFCs) and as a potential route for the synthesis of CH₂=CF₂ (VDF). During the reaction, CH₂=CF₂ is observed as the major product formed and a 43% selectivity to CH₂=CF₂ is obtained at 1073 K. A detailed mechanism is developed based on the mechanistic analysis from kinetic modeling, with the initiation reaction involving the formation of CI radicals from CCI₂F₂. Good agreement is achieved between the predictions and experimental results. Based on a mechanistic analysis, a summary of the major reaction pathways is proposed, which is consistent with the experimental observations. 2012-01-13T00:10:03.666Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:9797 The catalytic activity of activated carbon (AC) and activated carbon supported potassium for the decomposition of CHF₃ was investigated at temperatures between 873 and 1173 K and at a space velocity of 4300 h⁻¹. It is found that activated carbon supported potassium shows high and relatively stable activity during the pyrolysis of CHF₃ under the conditions studied. Compared with the gas phase reaction, the conversion of CHF₃ increases by up to 10 times between 873 and 1123 K, with the major products being C₂F₄ and C₃F₆. Selectivities as high as 55% to C₂F₄ and 35% to C₃F₆ are achieved under optimum conditions. The main byproduct HF readily reacts with K₂O in the catalyst, converting the catalyst from K₂O/AC into KF/AC. Selectivity to the major products remains relatively constant following this transformation. 2012-01-12T23:10:05.802Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:9350 First-principles density functional theory and a periodic-slab model have been utilized to investigate the adsorption of a 2-chlorophenol molecule on a CuO(1 1 1) surface with a vacant Cu surface site, namely Cu₂O(1 1 1)–CuCUS. Several vertical and flat orientations have been studied. All of these molecular configurations interact very weakly with the Cu₂O(1 1 1)–CuCUS surface, an observation which also holds for clean copper surfaces and the Cu₂O(1 1 0):CuO surface. Hydroxyl-bond dissociation assisted by the surface was found to be endoergic by 0.42–1.72 eV, depending predominantly on the position of the isolated H on the surface. In addition, the corresponding adsorbed 2-chlorophenoxy moiety was found to be more stable than a vacuum 2-chlorophenoxy radical by about 0.76 eV. Despite these predicted endoergicities, however, we would predict the formation of 2-chlorophenoxy radicals from gaseous 2-chlorophenol over the copper (I) oxide Cu₂O(1 1 1)–CuCUS surface to be a feasible and important process in the formation of PCDD/Fs in the post-flame region where gas-phase routes are negligible. 2011-11-13T23:00:17.272Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:9349 We have generated a set of thermodynamic and kinetic parameters for the reactions involving HO₂ in the very early stages of benzene oxidation at low temperatures using density functional theory (DFT). In particular, we report the rate constants for the reactions of HO₂ with benzene and phenyl. The calculated reaction rate constant for the abstraction of H–C₆H₅ by HO₂ is found to be in good agreement with the limited experimental values. HO₂ addition to benzene is found to be more important than direct abstraction. We show that the reactions of HO₂ with the phenyl radical generate the propagating radical OH in a highly exoergic reaction. The results presented herein should be useful in modeling the oxidation of aromatic compounds at low temperatures. 2011-11-13T23:00:13.523Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:9345 Self-dimerisation of the three isomeric semiquinone radicals is considered herein. Optimised geometries and thermochemical parameters, in terms of heat of formation, entropy, heat capacity and Gibbs free energy of formation are provided for all possible cross coupling products. It is found that self-dimerisation of the three semiquinone radicals is not as thermodynamically favoured as the self-dimerisation of the phenoxy radical. Accordingly, the three isomeric semiquinone radicals can be regarded as less active precursors for the formation of dioxin compounds than phenoxy radicals. 2011-11-13T22:40:19.813Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:9025 Drying oils, especially linseed oil, have been used since the 15th century as a painting medium. Hardening of linseed oil is a complex and little understood process involving autoxidation that has frequently led to the spontaneous ignition of rags doused with linseed oils. In the present study, low temperature oxidation of linseed oil and its active components, linoleic, linolenic and oleic acids in 50 mol% 0₂ has been investigated in a plug flow reactor, with and without an added cobalt catalyst. Glass wool, silanised glass wool and cotton wool have been impregnated with the drying oils and held in the flow reactor at an initial temperature between 60 and 100 °C. Online analysis of products has been performed by micro gas chromatography. Sampling gas analysis involved μGC, FTIR spectroscopy and GC-MS. Reaction has been monitored over a period of several hours. An induction period which depends on the substrate and the initial temperature has been observed prior to rapid increase in the evolution of products of oxidation including CO, C0₂ and H₂0. The ratio of carbon monoxide to carbon dioxide becomes larger as the initial temperature increases suggesting distinct pathways for the evolution of CO and C0₂. FTIR spectroscopy indicated the presence of aldehydes and carboxylic acids, with identification of these species confirmed by GC-MS. Light hydrocarbons, especially ethane and ethylene were quantified by the micro gas chromatography. Cobalt(II) nitrate catalysed the autoxidation reactions. In the absence of a catalyst, the oil samples were found to undergo a slower hardening process, probably associated with cross-linking or polymerisation together with partial fragmentation to form the observed low molecular weight products. 2011-09-20T06:10:02.193Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:8082 This article reports the computational and experimental results of the thermal decomposition of permethrin, a potential source of dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF). We have performed a quantum chemical analysis by applying density functional theory to obtain the decomposition pathways of permethrin and the formation mechanism of dibenzofuran. We have conducted the pyrolysis experiments in a tubular reactor and identified the pyrolysis products to demonstrate the agreement between the experimental measurements and quantum chemical calculations. The initiation of the decomposition of permethrin involves principally the aromatisation of permethrin into 3-phenoxyphenylacetic acid, 2-methylphenyl ester (J) and concomitant loss of 2HCl. This rearrangement is followed by the rupture of the O–CH₂ linkage in J, with a rate constant derived from the quantum chemical results of 1 x 10¹⁵ exp(−68 kcal/mol/RT) s⁻¹ for temperatures between 700 and 1300 K. This is confirmed by finding that the rate constant for unimolecular rearrangement of permethrin into J is 1.2 x 10¹² exp(−53 kcal/mol/RT) s⁻¹ over the same range of temperatures and exceeds the direct fission rate constant at all temperatures up to 850 ± 120 °C as well as by the experimental detection of J prior to the detection of the initial products incorporating diphenyl ether, 1-methyl-3-phenoxybenzene, 3-phenoxybenzaldehyde and 1-chloromethyl-3-phenoxybenzene. As the temperature increases, we observe a rise in secondary products formed directly or indirectly (via phenol/phenoxy) including aromatics (naphthalene), biphenyls (biphenyl, 4-methyl-1,1'-biphenyl) and dibenzofuran (DF). In particular, we discover by means of quantum chemistry a direct route from 2-phenoxyphenoxy to naphthalene. We detect no polychlorinated dibenzo-p-dioxins and dibenzofurans. Unlike the case of oxidative pyrolysis [Tame, N.W., Dlugogorski, B.Z., Kennedy, E.M., 2007b. Formation of dioxins in fires of arsenic-free treated wood: Role of organic preservatives. Environ. Sci. Technol. 41, 6425–6432] where significant yields of both PCDD and PCDF were obtained, under non-oxidative conditions the thermal decomposition of permethrin does not form appreciable amounts of PCDD or PCDF and the presence of oxygen (and/or a sizable radical pool) appears necessary for the formation of dibenzo-p-dioxin itself or PCDD/F from phenol/phenoxy. 2011-07-05T06:00:09.587Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:8041 Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) constitute a group of persistent organic pollutants that form almost inexorably in all thermal and combustion operations. This review focuses on mechanisms that govern their formation, chlorination, dechlorination and destruction. As a consequence of their extreme toxicity and propensity to bioaccumulate, PCDD/Fs have been subjected to much scientific research, designed to understand mechanisms and conditions that govern their emission rates and congener distribution (fingerprints). Consensus of opinions in the literature points to heterogeneous pathways contributing substantially more in the total yield of PCDD/Fs in combustion systems than the gas phase pathway. However, in our view, a great complexity of both homogeneous and heterogeneous routes and uncertainties in many thermochemical and rate parameters enable no conclusive statement about the contribution of each route. Chlorination patterns of precursors appear to play a major role in final congener profiles of PCDD/F emissions. According to the most recent theoretical studies, these congener profiles seem consistent with thermodynamic stabilities of dioxins and furans produced in thermal processes, however, further theoretical investigations at more accurate levels are needed to clarify this matter further. Theoretical studies along with experimental findings reveal that the PCDD/PCDF ratio remains very sensitive to the operating conditions, with pyrolytic conditions favouring the formation of PCDFs. A number of reaction mechanisms has been proposed to answer many of the most intriguing questions about the formation of PCDD/Fs. These mechanisms include models of gaseous and heterogeneous reactions, often inferred from theoretical quantum chemical calculations studies, which incorporate steps responsible for formation, chlorination, dechlorination and destruction of dioxins and furans. The review identifies gaps in our present understanding of the reaction mechanisms and suggests that further progress in the field needs to be facilitated by development of reliable mechanistic models for (i) catalytic pathways, (ii) chlorination/dechlorination reactions including the Deacon reaction and the direct transfer of chlorine from solid surfaces into the aromatic moieties, and (iii) formation of PCDD/Fs from precursors other than chlorophenols, especially pesticides. 2011-07-04T06:20:17.464Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:7987 First-principles density functional theory and a periodic-slab model have been employed to explore the adsorption of a two-chlorophenol molecule on a Cu₂O(110) surface containing surface Cu–O bonds, namely, the Cu₂O(110):CuO surface. The two-chlorophenol molecule is found to interact very weakly with the Cu₂O(110):CuO surface, forming several vertical and flat orientations. These weakly bound states tend to result from interaction between the phenolic hydrogen and an oxygen surface atom. The formation of a two-chlorophenoxy moiety and an isolated hydrogen on the Cu₂O(110):CuO surface from a vacuum two-chlorophenol molecule is determined to have an endothermicity of 8.2 kcal/mol (0.37 eV). The energy required to form a two-chlorophenoxy radical in the gas phase is also found to be much smaller when assisted by the Cu₂O(110):CuO surface than direct breaking of the hydroxyl bond of a free two-chlorophenol molecule. The calculated binding energy of a two-chlorophenoxy radical adsorbed directly onto the Cu₂O(110):CuO surface is −12.5 kcal/mol (0.54 eV). The Cu₂O(110):CuO and Cu(100) surfaces are found to have similar energy barriers for forming a surface-bound two-chlorophenoxy moiety from the adsorption of a two-chlorophenol molecule. 2011-06-28T23:50:05.890Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:6220 The catalytic activity of K₂-xO/AC for the decomposition of CHF₃ was investigated at temperatures between 600-900°C and at a space velocity of 4,300 h⁻¹. It is found that this catalyst shows high and relatively stable activity during the pyrolysis of CHF₃ under the conditions studied. Compared with the gas phase reaction, the conversion of CHF₃ increased by 2-10 times between 600-850°C, with the major products being C₂F₄ and C₃F₆ which are key monomers for the synthesis of Teflon and other polymers. As high as 55% selectivity to C₂FF₄ and 35 % of selectivity to C₃F₆ were achieved under optimum conditions. The main byproduct HF readily reacts with K₂O in the catalyst, changing the catalyst from K₂-xO/AC into KFIAC. Selectivity to the primary products remains relatively constant following this transformation. 2010-05-11T04:20:28.635Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:996 A density functional theory (DFT) study of the reaction of dibenzofuranyl radical with oxygen molecule has been made. The geometries, energies, and vibrational frequencies of the reactant, transition states, intermediates, and products have been calculated at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) level of theory. The initial reaction of dibenzofuran (DBF) with molecular oxygen results in the formation of the 1-dibenzofuranylperoxy radical. The stability of this adduct toward decomposition at low to intermediate temperatures results in it undergoing several possible rearrangements. The lowest energy pathway with a barrier of 24.2 kcal/mol involves a rearrangement to the 1,1-dioxadibenzofuran radical. The next lowest energy pathway involves fission of the O-O linkage whose reaction energy was found to be 37.6 kcal/mol. Transition state theory (TST) calculations indicate that the lowest energy pathway should predominate at temperatures up to about 1200 K. Two other unimolecular reaction pathways with barriers of 45.5 and 91.1 kcal/mol have also been discovered. The latter pathway leads to the formation of a para-quinone (dibenzofuran quinone) which has been detected experimentally in the low-temperature oxidation of DBF [Marquaire, P. M.; Worner, R.; Rambaud, P.; Baronnet, F. Organohalogen Compd. 1999, 40, 519]. Our quantum calculations, however, do not support this latter pathway to quinone formation. Instead, the quinone is most probably formed as a consequence of recombination of the 1-dibenzofuranyloxy radical (produced by peroxy fission) with an O atom in the para position. Each of the unimolecular reaction pathways have been subjected to detailed quantum chemical investigation and transition states and intermediates leading to the final products (principally CO, CO₂, and C₂H₂ with traces of benzofuran and benzene) have been identified. For certain stable intermediates, their possible reactions with molecular oxygen have been further investigated quantum chemically. The present work therefore presents a detailed quantum chemical investigation of the reaction pathways in the low-temperature oxidation mechanism of DBF. Since the dibenzofuran moiety is present in the polychlorinated DBFs, our conclusions should be generally applicable to this family of compounds. 2010-04-27T06:43:09.664Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:1077 The gas-phase reaction of CHF₃ with CH₄ has been studied experimentally and computationally. The motivation behind the study is that reaction of CHF₃ with CH₄ provides a possible route for synthesis of CH₂=CF₂ (C₂H₂F₂). Experiments are carried out in a plug flow, isothermal α-alumina reactor at atmospheric pressure over the temperature range of 973-1173 K. To assist in understanding the reaction mechanism and the role of the reactor material involved in the reaction of CHF₃ with CH₄, the reaction of CHF₃ with CH₄, pyrolysis of CH₄, and pyrolysis of CHCIF₂ have been studied in the presence of α-alumina or α-AIF₃ particles under various conditions. Under all conditions studied for the reaction of CHF₃ and CH₄, the major products are C₂F₄, C₂H₂F₂, and HF. Minor products include C₂H₂, C₂H₄, C₂H₃F, C₂HF₃, C₃F₆, CO₂, and H₂. C₂H₆, CH₂F₂, and CHF₂CHF₂ are detected in trace amounts. The initial step is the gas-phase unimolecular decomposition of CHF₃, producing CF₂ and HF. It is proposed that CF₂ decomposes on the surface of α-alumina, producing F radicals that are responsible for the activation of CH₄. A reaction scheme developed on the basis of the existing NIST HFC and GRI-Mech 3.0 mechanisms is used to model the reaction of CHF₃ with CH₄. Generally satisfactory agreement between experimental and modeling results is obtained on the conversion levels of CHF₃ and CH₄ and rates of formation of major products. Using the software package AURORA, the reaction pathways leading to the formation of major products are elucidated. 2010-04-27T06:06:42.894Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:4438 This study investigates the kinetic parameters of the formation of the chlorophenoxy radical from the 2-chlorophenol molecule, a key precursor to polychlorinated dibenzo-p-dioxins and dibenzofurans (PCCD/F), in unimolecular and bimolecular reactions in the gas phase. The study develops the reaction potential energy surface for the unimolecular decomposition of 2-chlorophenol. The migration of the phenolic hydrogen to the ortho-C hearing the hydrogen atom produces 2-chlorocyclohexa-2,4-dienone through an activation barrier of 73.6 kcal/mol (0 K). This route holds more importance than the direct fission of Cl or the phenolic H. Reaction rate constants for the bimolecular reactions, 2-chlorophenol + X → X-H + 2-chlorophenoxy (X = H, OH, Cl, O₂) are calculated and compared with the available experimental kinetics for the analogous reactions of X with phenol. OH reaction with 2-chlorophenol produces 2-chlorophenoxy by direct abstraction rather than through addition and subsequent water elimination. The results of the present study will find applications in the construction of detailed kinetic models describing the formation of PCDD/F in the gas phase. 2010-04-27T05:32:31.909Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:4437 The atmospheric degradation of dibenzofuran (DF) initiated by OH addition has been studied by using density functional theory (B3LYP method). Site C1 in DF is predicted to be the favored site for OH addition, with a branching ratio of 0.61 to produce a DF-OH(1) adduct. The calculated reaction rate constant for OH addition to DF has been used to predict the atmospheric lifetime of DF to be 0.45 day. Three different modes of attack of O₂(³∑g) on DF-OH(1) have been examined. Abstraction of hydrogen gem to OH in DF-OH(1) by O₂(³∑g) (producing 1-dibenzofuranol I) and dioxygen addition in the three radical sites in cis and trans orientation (relative to the ispo-added OH) of the π-delocalized electron system of DF-OH(1) are feasible under atmospheric conditions. The free energy of activation (at 298.15 K) for the formation of 1-dibenzofuranol is 15.1 kcal/ mol with a free energy change of -36.3 kcal/mol, while the formation of DF-OH(1)-O₂adducts are endergonic by 9.2-21.8 kcal/mol with a 16.3-23.6 kcal/mol free energy of activation. On the basis of the calculated reaction rate constants, the formation of 1-dibenzofuranol is more important than the formation of DF-OH-O₂adducts. The results presented here are a first attempt to gain a better understanding of the atmospheric oxidation of dioxin-like compounds on a precise molecular basis. 2010-04-27T05:32:31.879Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:3500 Density functional theory (DFT) calculations have been used to obtain thermochemical parameters for formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/PCDF) from the oxidation of 2-chlorophenol. Formation mechanisms of PCDD through radical−radical coupling have been investigated in detail. The sequence of 2-chlorophenoxy radical coupling has been studied. The formation of chlorinated bis keto dimers which results from cross coupling of 2-chlorophenoxy at the ortho carbon bearing hydrogen (a known direct route for PCDF formation) passes through a tight transition structure whose barrier is 9.4 kcal/mol (0 K). Three routes for the formation of the most abundant PCDD/PCDF species (viz., 4,6-dichlorodibenzofuran, 4,6-DCDF, and 1-monochlorodibenzo-p-dioxin, 1-MCDD) in oxidation and pyrolysis of 2-chlorophenol are discussed. In the case of 4,6-DCDF, formation through H or HO + keto−keto ⇄ H₂ or H₂O + keto−keto· ⇄ H₂ or H₂O + enol−keto· ⇄ H₂ or H₂O + 4,6-DCDF + HO is shown to be the preferred route. The other two routes proceed via closed shell processes (keto−keto ⇄ enol−keto ⇄ enol−enol ⇄ H₂O + 4,6-DCDF) and (keto−keto ⇄ enol−keto ⇄ (H-,OH-) 4,6-DCDF ⇄ H₂O + 4,6-DCDF). Results indicate that 1-MCDD should be the favored product in 2-chlorophenol pyrolysis in agreement with experimental findings. According to our results, tautomerization (inter-ring hydrogen transfer) and intra-annular displacement of HCl would not be competitive with paths deriving from H abstraction from the phenolic oxygen and the benzene ring followed by displacement of Cl in the formation of dibenzo-p-dioxin (DD) and 1-MCDD. The results presented here will assist in construction of detailed kinetic models to account for the formation of PCDD/PCDF from chlorophenols. 2010-04-27T05:30:24.591Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:3410 A density functional theory (DFT) study was carried out to investigate possible reactions of dibenzofuran (DF) and dibenzo-p-dioxin (DD) in a reducing environment. Reaction energies, barrier heights, and molecular parameters for reactants, intermediates, products, and transition states have been generated for a wide range of possible reactions. It was found that C−O β-scission in DF incurs a very large energy barrier (107 kcal/mol at 0 K), which is just 3 kcal/mol less than the direct H fission from C−H in DF to form dibenzofuranyl radicals. It was found that DF allows direct H addition to C1−C4 and C6−C9 as well as addition of two H atoms from a hydrogen molecule at sites 1 and 9 of DF. A bimolecular reaction of DF with H or H2 is found to have a significantly lower barrier than unimolecular decomposition through C−O β-scission. An explanation for the predominance of polychlorinated dibenzofurans (PCDF) over polychlorinated dibenzo-p-dioxins (PCDD) in municipal waste pyrolysis is presented in the view of the facile conversion of DD into DF through ipso-addition at the four C sites of the two C−O−C central bonds in DD. 2010-04-27T05:01:21.379Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:4449 The gas-phase pyrolysis of n-C₄F₁₀ has been studied experimentally in a plug flow, isothermal α-alumina reactor at atmospheric pressure and various residence times over the temperature range of 873 to 1148 K. The major products are C₂F₆, C₂F₄, C₃F₈, and C₃F₆ (CF₃-CF=CF₂). Minor products include cycio-C₄F₈ (c-C₄F₈) and CO₂. CF₄ is detected in trace amounts at high temperatures. A kinetic reaction scheme involving 20 species and 30 reactions is developed and used to model n-C₄F₁₀ pyrolysis. A generally satisfactory agreement between experimental and modeling results is obtained on the conversion levels of n-C₄F₁₀ and the rates of formation of C₂F₆ and C₃F₈. 2010-04-27T04:58:49.446Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:4445 Reaction of CHF₃ and CH₄ over CaBr₂ was investigated at 400-900 degrees C as a potential route for transforming the highly potent greenhouse gas, CHF₃, into the valuable product CH₂=CF₂. The homogeneous reaction of CHF₃ with CH₄ was also studied to assist in understanding the chemistries involved. Compared to the gas phase reaction, the addition of CaBr₂ as a reactant increases the conversion of CHF₃ and CH₄ significantly at low temperatures while to a lesser extent at higher temperatures. In the absence of CaBr₂, besides the target product, CH₂=CF₂, a large amount Of C₂F₄ forms. On addition of CaBr₂, the rate of formation Of C₂F₄ drops dramatically to near zero, while the rate of formation of CH₂=CF₂ increases considerably at temperatures below 880 degrees C. Experimental and theoretical studies suggest that CHF₃ strongly interacts with CaBr₂, resulting in the fluorination of CaBr₂ to CaF₂, the release of active Br species results in the selective formation of CBrF₃. The subsequent reactions involving Br, methane, and CBrF₃ play a major role inthe observed enhanced yield of CH₂=CF₂. 2010-04-27T04:58:46.844Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:5163 The interaction between a 2-chlorophenol (C₆H₄OHCl) molecule and the Cu(111) surface has been investigated using density functional theory as an initial step in gaining a better understanding of the catalyzed formation of dioxin compounds on a clean copper surface. The 2-chlorophenol molecule is found to form several weakly bonded, horizontally and vertically oriented configurations. Dissociative modes have also been investigated. For the latter, the formation of phenyl and benzyne fragments is found to be more energetically favourable than the formation of 2-chlorophenoxy radicals. 2010-04-27T04:45:00.541Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:5224 Interactions between a 2-chlorophenol molecule and the Cu(100) surface have been analyzed using a first-principles density functional theory (DFT) plane-wave method as the first step in gaining a detailed understanding at the molecular level of the copper catalyzed formation of PCDD/F's (dioxin compounds). While the 2-chlorophenol molecule is found to be only very weakly bound to the Cu(100) surface in a number of vertical and flat-lying configurations, its dissociation into either 2-hydroxyl-phenyl (C₆H₄OH) or benzyne (C₆H₄) results in chemisorption on the surface. The dissociative reaction (2-chlorophenol + Cu-surface → 2-chlorophenoxy + H-Cu-surface), which is an initial and key step in the catalytic process, is also found to be exothermic on two considered sites. The 2-chlorophenoxy radical on the Cu(100) surface transforms into a mono chloro-phenolate and is more stable than the gas-phase species by about 1.74 eV. The most stable configurations occur when the benzyne moiety, hydroxyl group and chlorine atom are all separately chemisorbed on the surface with an overall stability of 0.97 eV. 2010-04-27T04:40:56.878Z ]]>