Chemical Kinetics of the Thermal Decomposition of NTO

The kinetics of thermal decomposition of 3‐nitro‐2,4‐dihydro‐3H‐1,2,4‐triazol‐5‐one (NTO) in the temperature interval from 200 °C to 260 °C was investigated using a glass Bourdon gauge. The overall decomposition reaction includes two distinct stages: the fast first‐order decomposition and the subsequent autocatalytic reaction. The importance of the first stage increases with increasing decomposition temperature and decreasing loading density of the Bourdon gauge (m/V). A period of preliminary heating, at a lower temperature, strongly influences the autocatalytic stage when the decomposition is carried out at a higher temperature. In the temperature domain 200–220 °C, the Arrhenius constants of the decomposition reaction are found to be close to the values usually observed for nitrocompounds: E=173 kJ/mol and log₁₀ k≈12.5 (s⁻¹). It is shown that a simple model of NTO decomposition based on an autocatalytic reaction of the m‐th order can describe the course of the decomposition at high temperature but the m number appears to be excessively high, up to 4. A new model of the decomposition is developed, including an initial monomolecular reaction, decomposition of the crystalline substance, and an autocatalytic reaction of NTO dissolved in liquid decomposition products. This model gives the common order of autocatalysis, m=1.     

Thermal decomposition studies on copper-chromium-silver impregnated activated charcoal

Thermal gravimetric analyses (TGA) were used to examine the thermal decomposition kinetics of ASC whetlerite, a copper-chromium-silver impregnated activated charcoal. In the low temperature region (< 490 K) the proposed decomposition reaction involves the reduction of the copper chromate species into a copper chromite form accompanied by the oxidation of the carbon support to carbon dioxide. The first order decomposition rate constant was determined to be k_l = 1.22 × 10³exp(− 4.4 × 10⁴/RT). In the high temperature region (598 K to 678 K) a supported cupric oxide species is proposed to react with the carbon support resulting in the formation of cuprous oxide and carbon dioxide. The first order decomposition rate constant for this reaction was evaluated to be k_h = 6.92 × 10⁻²exp(−3.7 × 10⁴/RT).     

Kinetics of thermal decomposition of hydromagnesite

The thermal decomposition of hydromagnesite has been studied using thermogravimetric analysis. The experiments were conducted in a nitrogen atmosphere with pellets of hydromagnesite in the temperature range 300 to 550°C, and also with powder samples under non-isothermal conditions at a heating rate of 10°C/min. It is found that the reaction proceeds in two stages: dehydration followed by decomposition. The dehydration reaction is controlled by external mass transfer whilst the decomposition reaction is controlled by both external and internal mass transfer. The activation energy for the dehydration reaction was found to be 2.67 × 10⁷ J/kmol. From the non-isothermal analysis the activation energy for the decomposition of magnesium carbonate was found to be 1.62 × 10⁸ J/kmol.     

Treatment of dyehouse waste‐water by supercritical water oxidation: a case study

BACKGROUND: Supercritical water oxidation (SCWO) of dyehouse waste‐water containing several organic pollutants has been studied. The removal of these organic components with unknown proportions is considered in terms of total organic carbon concentration (TOC), with an initial value of 856.9 mg L^?1. Oxidation reactions were performed using diluted hydrogen peroxide. The reaction conditions ranged between temperatures of 400–600 °C and residence times of 8–16 s under 25 MPa of pressure. RESULTS: TOC removal efficiencies using SCWO and hydrothermal decomposition were between 92.0 and 100% and 6.6 and 93.8%, respectively. An overall reaction rate, which consists of hydrothermal decomposition and the oxidation reaction, was determined for the hydrothermal decomposition of the waste‐water with an activation energy of 104.12 ( ± 2.6) kJ mol^?1 and a pre‐exponential factor of 1.59( ± 0.5) × 10⁵ s^?1. The oxidation reaction rate orders for the TOC and the oxidant were 1.169 ( ± 0.3) and 0.075 ( ± 0.04) with activation energies of 18.194 ( ± 1.09) kJ mol^?1, and pre‐exponential factor of 5.181 ( ± 1.3) L^0.244 mmol^?0.244 s^?1 at the 95% confidence level. CONCLUSION: Results demonstrate that the SCWO process decreased TOC content by up to 100% in residence times between 8 and 16 s under various reaction conditions. The treatment efficiency increased remarkably with increasing temperature and the presence of excess oxygen in the reaction medium. Color of the waste‐water was removed completely at temperatures of 450 °C and above. Copyright © 2010 Society of Chemical Industry     

Kinetic Study of Ozone Decay in Homogeneous Phosphate-Buffered Medium

The ozone decomposition reaction is analyzed in a homogeneous reactor through in-situ measurement of the ozone depletion. The experiments were carried out at pHs between 1 to 11 in H₂PO₄^?/HPO₄^2– buffers at constant ionic strength (0.1 M) and between 5 and 35 °C. A kinetic model for ozone decomposition is proposed considering the existence of two chemical subsystems, one accounting for direct ozone decomposition leading to hydrogen peroxide and the second one accounting for the reaction between the hydrogen peroxide with the ozone to give different radical species. The model explains the apparent reaction order respect of the ozone for the entire pH interval. The decomposition kinetics at pH 4.5, 6.1, and 9.0 is analyzed at different ionic strength and the results suggest that the phosphate ions do not act as a hydroxyl radical scavenger in the ozone decomposition mechanism.     

Modification of Ammonia Decomposition Activity of Ruthenium Nanoparticles by N-Doping of CNT Supports

The use of ammonia as a hydrogen vector has the potential to unlock the hydrogen economy. In this context, this paper presents novel insights into improving the ammonia decomposition activity of ruthenium nanoparticles supported on carbon nanotubes (CNT) by nitrogen doping. Our results can be applied to develop more active systems capable of delivering hydrogen on demand, with a view to move towards the low temperature target of less than 150?°C. Herein we demonstrate that nitrogen doping of the CNT support enhances the activity of ruthenium nanoparticles for the low temperature ammonia decomposition with turnover frequency numbers at 400?°C of 6200 LH₂ mol_Ru ^?1 h^?1, higher than the corresponding value of unmodified CNT supports under the same conditions (4400 LH₂ mol_Ru ^?1 h^??1), despite presenting similar ruthenium particle sizes. However, when the nitrogen doping process is carried out with cetyltrimethylammonium bromide (CTAB) to enhance the dispersion of CNTs, the catalyst becomes virtually inactive despite the small ruthenium particle size, likely due to interference of CTAB, weakening the metal–support interaction. Our results demonstrate that the low temperature ammonia decomposition activity of ruthenium can be enhanced by nitrogen doping of the CNT support due to simultaneously increasing the support’s conductivity and basicity, electronically modifying the ruthenium active sites and promoting a strong metal–support interaction.     

Application of thermogravimetric analysis to the thermal decomposition of polybutadiene

Thermogravimetric analysis (TGA) has been used extensively to determine the thermal stability of polymers. The present study indicates that the isothermal decomposition of polybutadienes (PBDs) is significantly different from that in the heat mode. The isothermal decomposition of PBDs is an exothermic reaction occurring at 350°–375°C. This decomposition is shown to be rapid and temperature specific. It appears to be related to the cyclization reaction reported previously by several investigators. Decomposition of PBDs in the heat mode (10°C/min) occurs at 447°–461°C. This is about 100°C higher than that observed in the isothermal mode. Further TGA experiments indicate that a period of slow heating stabilizes PBD and can eliminate the exothermic decomposition at about 360°C. This stabilization appears to be related to the ease with which both 1,2- and 1,4-PBDs thermally crosslink. Heating 1,4-PBD for 6 min at 270°C gives rise to 92% gel. 1,2-PBD is shown to crosslink more extensively. It is shown that polymers which do not thermally crosslink or cyclize, such as polystyrene, decompose similarly in the two modes of heating.     

Kinetics of oxidation of food wastes with H2O2 in supercritical water

In this study, supercritical water oxidation (SCWO) of carrots and beef suet was carried out in a batch reactor system with an H₂O₂ oxidant, at a temperature between 400 and 450°C and reaction times from 10 s to 10 min. The results showed that the oxidative decomposition of carrots and beef suet proceeded rapidly and a high total organic carbon (TOC) decomposition of up to 97.5% was obtained within 3 min at 420°C for carrots and within 5 min at 450°C for beef suet when there was a sufficient supply of oxygen. It was also found that the oxidation reaction for both carrots and beef suet might be separated into a fast reaction at the early stage and a slow reaction at the later stage. In the later stage following the early stage reaction, acetic acid, which is a fairly stable product of the early stage reaction, is the reactant and the rate of overall oxidation reaction for complete decomposition is dominated by the later stage reaction. Global kinetic analysis based on the model described above showed that the early stage oxidative reaction of beef suet could be considered as a first-order reaction with respect to the concentration of organic carbon. The activation energy was 37.3 kJ mol⁻¹. Oxidation of acetic acid could also be expressed as a first-order reaction, and the activation energy was 106.5 kJ mol⁻¹. The early stage oxidation reaction of carrots was too fast to be analyzed. On the basis of intermediate products identified, reaction pathways were discussed. For carrots, polysaccharides may first be hydrolyzed to glucose and then oxidation of the glucose may take place. For beef suet, glyceride is first hydrolyzed to glycerin and carboxylic acids corresponding to the components of glyceride, followed by consecutive reactions for oxidative decomposition.     

Modification of the Standard Neutral Ozone Decomposition Model

The modified Staehelin, Buhler, and Hoigné model for aqueous ozone decomposition was tested over a wide range of hydroxyl radical scavenger concentrations at a pH of 7.1–7.2. Results from these experiments showed that the modified model appeared to underpredict the residual ozone concentration and overpredict the residual hydroxyl radical probe compound, tetrachloroethylene, concentration. The modified Staehelin, Buhler, and Hoigné model was recalibrated and two rate constants, the rate constant of the initiation reaction of ozone decomposition of hydroxide ion and the rate constant of the promotion reaction of ozone decomposition by hydroxyl radical, were reestimated. The new estimates of these rate constants are 1.8 × 10² M^?1s^?1 (initiation reaction) and 2 × 10⁸ M^?1s^?1 (promotion reaction), while the values estimated by Staehelin, Buhler, and Hoigné for these rate constants are 70 M^?1s^?1 (initiation reaction) and 2 × 10⁹ M^?1s^?1 (promotion reaction). The recalibrated-modified model was tested and validated by conducting experiments at different pH values and hydroxyl radical scavenger concentrations. Also, the effect of phosphate buffer as a hydroxyl radical scavenger was investigated at phosphate buffer concentrations of 10 mM and 1 mM.     

Kinetics of the iron(III)-catalysed oxidation of oxalate by permanganate

The method of reaction times has been used to investigate the kinetics and mechanism of the iron-catalysed reaction between permanganate and oxalic acid. The rate-controlling step is the decomposition of the dioxalatoiron(III) ion. The reaction rate is independent of the initial permanganate concentration below 0.0015 N but is proportional to the initial concentration above 0.002 N. The rate equation for the reaction at 24°c is dx/dt= ?(9.9 × 10^?3 a+ 1.8 × 10^?5[Fe³⁺]^1/3).[H₂C₂O₄]^2/3 where a is the initial permanganate concentration.     

Characterisation of the Major Synthetic Products of the Reactions Between Butanone and Hydrogen Peroxide

The reactions between butanone and hydrogen peroxide, both catalysed and un‐catalysed, were investigated and spectral and sensitiveness data reported. The major product of the un‐catalysed reaction, 2‐hydroxy,2‐hydroperoxybutane, displayed a Figure of Insensitiveness (F of I) of 10, Temperature of Ignition (T of I) of 132 °C, and initiated when 128 N of frictional force or an electrostatic discharge (ESD) of 4.5 J was applied. Differential scanning calorimetric analyses revealed an onset of decomposition at 128 °C, peak maximum of 140 °C, and decomposition energy of 203 J g⁻¹. The major product of the cooled (5 °C) acid catalysed reaction between butanone and hydrogen peroxide, 2,2′‐dihydroperoxy‐2,2′‐dibutyl peroxide, displayed a F of I of<10, T of I of 110 °C and initiated upon application of 5 N of friction or a 0.45 J ESD. Calorimetry showed a melt at 38.3 °C, an onset of exothermic decomposition at 127 °C and the evolution of 1292 J g⁻¹. The major product of the raised temperature (20 °C) acid catalysed synthesis, 1,4,7‐trimethyl‐1,4,7‐triethyl‐1,4,7‐cyclononatriperoxane, displayed F of I of<10 and initiated upon application of 5 N of friction or a 0.45 J ESD. Calorimetry revealed an onset to melting at 28.9 °C, an onset to thermal decomposition at 128 °C, and decomposition energy of 1438 J g⁻¹.     

Pyrolysis of methane in a single pulse shock tube

The thermal decomposition of methane has been studied in a chemical shock tube at pressures up to 20 atm over a temperature range of between 1750 and 2700 K and for reaction times up to 2.5 ms. Attention is drawn to some of the experimental features of the shock tube and to the fact that reaction temperatures were measured. Optimum conditions for the production of acetylene from methane are suggested, and the relatively small effect of pressure on the acetylene yields is noted. Values for activation energy (93.6 kcal/mol) and frequency factor (3.8 × 10¹³ s^?1 for methane decomposition are given. The experimental results obtained are discussed in connection with suggested mechanisms of decomposition. In the discussion attention is drawn to the difficulty of predicting acetylene yields arising from the incomplete understanding of the mechanism of acetylene decomposition and of “carbon” formation under the conditions employed.     

The conversion of polysaccharides to hydrogen gas. Part II: The catalytic decomposition of formaldehyde in the aqueous phase

The aqueous phase decomposition of formaldehyde, to hydrogen gas, catalysed by platinum—copper chromite, has been carried out in the temperature range 20–60°C, at a solution pH of 12. The production of hydrogen was favoured by intermediate temperatures (40–50°C) and an activation energy of 22.2 kJ mol^?1 (5.3 kcal mol^?1) was recorded. The rate of reaction was first order with respect to OH^? ion concentration at low alkali concentrations and was first order with respect to HCHO concentration at all concentrations. At high alkali concentrations the reaction should become zero order with respect to OH^? ion concentration, but initial rates actually decrease under these conditions having passed through a maximum. The rate of reaction was directly proportional to catalyst weight at low catalyst loading, but the relationship became non-linear at high catalyst loadings. Conversions of formaldehyde to hydrogen gas were substantially less than theoretical. The decomposition reaction has to compete with a number of side reactions such as polymerization of formaldehyde at low temperatures (<40°C) and at higher temperatures with the Cannizzano reaction, aldol condensation, and possibly formaldehyde hydrogenation to methanol. In addition hydrogen loss may occur due to copper chromite reduction. A reaction mechanism is proposed involving a surface formate intermediate.     

Vapor pressure and hysteresis in the graphite-bromine system

Vapor pressure-composition diagram of the graphite-bromine system shows continuous changes in composition between the definite stages. It also shows a large hysteresis in the process of bromination-debromination. X-ray studies show that an intermediate compound at a composition between definite stages possesses a structure composed of microdomains of either of the two stages. The vapor pressure of the initiation of decomposition of C₈Br has practically the same value as that of the initiation of bromination of C₁₂Br, which is believed to give the equilibrium decomposition pressure of C₈Br. The decomposition pressure measurements at temperatures between 15 and 27°C gave the standard heat of reaction ?10·2 kcal·mol^?1 and the standard entropy change of reaction ?29·9 cal·K^?1 mol^?1. The structural transformation and the hysteresis phenomena in the graphite-bromine system are interpreted assuming that the compound possesses a conglomerate structure of microdomains.     

Quantitative studies by differential scanning calorimetry of the reaction between hydrogen peroxide and lignocellulose

Heat of reaction and kinetic parameters were determined by differential scanning calorimetry for decomposition of hydrogen peroxide, reaction of hydrogen peroxide with lignocellulosic materials, glucose and pinitol, and for the reaction of the same materials with produced or introduced oxygen. The heat of decomposition of hydrogen peroxide obtained in N₂ (720 cal/g H₂O₂) was in fair agreement with literature data, considering the different temperature and pressure conditions. The heats of reaction of hydrogen peroxide and lignocelluloses were higher when determined in N₂ (1670–2500 cal/g H₂O₂) than in O₂ (1450–2020 cal/g H₂O₂) atmosphere. The activation energy for decomposition of hydrogen peroxide amounted to 20.3 kcal/mol in N₂ and 15.9 kcal/mol in O₂ with frequency factors of 5.7 × 10⁹ and 3.7 × 10⁷ min^?1, respectively. The activation energies for the reaction of hydrogen peroxide and lignocellulosic materials tested were similar and not influenced by the atmospheric composition, ranging overall between 19.7 and 22.4 kcal/mol. The corresponding frequency factors ranged between 2.77 × 10⁹ and 2.23 × 10¹¹.     

The inhibitory effect of water on the Co2+ and Cu2+ catalyzed decomposition of methyl linoleate hydroperoxides

The inhibitory effect of water on the decomposition of methyl linoleate hydroperoxides (MLHP) catalyzed by Co²⁺ and Cu²⁺ was studied in a model system using proton nuclear magnetic resonance (NMR) spectroscopy. MLHP were prepared by photoxidation and purified by chromatographic methods. Proton NMR spectroscopy was used to measure reaction rates by monitoring changes in the intensity of the OOH signal. The rate constant of the reaction was obtained by plotting the natural logarithm of MLHP concentrationvs time. In the first part of the study, no transition metals were added to the model system, so that the effect of water could be attributed to the interaction between water and MLHP only. The rate constant of the reaction (K) was found inversely proportional to the concentration of water. There was a downfield chemical shift of both hydroperoxide and water peaks in the NMR spectra when water was added. As temperature increased to 40°C, the difference in K between the systems with 0% and 2% water disappeared. It is proposed that the hydroperoxides were solvated with water which retarded their decomposition. When Co²⁺ was added to the model system, K decreased as the concentration of water increased from 0% to 1.5%. As temperature increased from 18°C to 40°C, differences between the K for 0% and 2% water disappeared. A similar phenomenon was observed in reactions catalyzed with Cu²⁺. These findings would support a mechanism in which the protective effect of water involves both the solvation of OOH and hydration of the metal catalyst. Based on a paper presented at the Symposium on Metals and Lipid Oxidation, held at the AOCS Annual Meeting in Baltimore, MD, Apirl 1990.     

Experimental Aspects of Mechanistic Studies on Aqueous Ozone Decomposition in Alkaline Solution

Ozone decomposition in aqueous solution was studied by the stopped - flow method over the pH 10.4 - 13.2 range at 25 ± 0.1 °C and I = 0.5 M NaClO₄. At 260 nm the molar absorptivity of aqueous ozone was determined to be 3135 ± 22 M^?1cm^?1. It was shown that various experimental factors may significantly alter the course of the reaction. Even small amounts of H₂O₂ absorbed by the plastic parts of the stopped-flow instrumént can affect the kinetic features of the reaction for an extended period of time. Under strictly controlled experimental conditions sufficiently reproducible data could be obtained for the decomposition. The data were evaluated by comparing experimental and simulated kinetic traces. A detailed kinetic model was developed which is able to predict the decay and life-time of ozone as well as the formation and decomposition of the ozonide ion radical (O₃ ^?) over the pH 10.4 - 13.2 range.     

Unsaturated 2‐oxazoline end capping of liquid crystalline polyester by reactive processing and in a solution

End capping of liquid crystalline poly(ethylene terephthalate‐co‐oxybenzoate) with a bifunctional 2‐oxazoline derivative, 2‐(4‐allyloxyphenyl)‐2‐oxazoline, has been performed in melt under the condition of reactive processing and in a solution. The reaction in melt is very fast and, despite some modifier evaporation, it is completed in 2 min at 230°C. The product is a polyester containing unsaturated end groups bonded via esteramide linkage. The presence of unsaturation was proved by ¹³C‐NMR spectroscopy. An increase in temperature and prolongation of the processing time gives raise to thermal‐induced reactions on the unsaturated end groups, resulting in an increase of the glass transition temperature. Depending on the processing temperature decomposition, propagation and crosslinking occur in different extent and influence polymer properties. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1047–1053, 1999     

Nitrous oxide decomposition and reduction over copper catalysts supported on various types of carbonaceous materials

Copper supported on three different allotropic forms of carbon materials have been prepared and evaluated as catalysts for the N₂O decomposition and reduction reactions. It was found that all the catalysts underwent severe deactivation during the N₂O decomposition reaction due to the gasification of carbon substrates. This behavior was particularly evident when activated carbon was used as the support medium. The chemical identity of the active entity involved in the carbon gasification process is believed to consist of a mixture of Cu⁺ and Cu²⁺ species and, according to the well established mechanism, the reaction proceeds in such a manner so that the surface of the catalyst undergoes a redox cycle at the gas/solid carbon interface. The introduction of CO into the system was shown to result not only in an enhancement in the activity of the desired N₂O decomposition reaction, but also served to inhibit the deleterious carbon gasification process. In addition, this procedure stabilized the copper particles in the metallic state, which is the active species responsible for the dissociation of N₂O. Copper dispersed on a diamond substrate appeared to attain the highest activity for the N₂O reduction reaction, a feature that is associated with the ability of the metal to undergo a wetting and spreading action on the support surface, possibly resulting in an epitaxial relationship between the two components.     

Analytic Solution of Diffusion‐Reaction in Spherical Porous Catalyst

The model for coupled diffusion and nt^h‐order reaction in a spherical catalyst pellet, which is a strong nonlinear differential equation with boundary values, is solved using the Adomian decomposition method. The explicit solutions for concentration profiles and effectiveness factors, for the model equation, are obtained and compared with the solutions obtained by a finite difference numerical method. For reaction orders less than 1.0, the Adomian polynomial series solution with 4 terms gives comparable results to those obtained with a finite difference numerical procedure. For reaction orders greater than 1.0, series solutions with more than 6 terms, are required for good accuracy.