Experimental and kinetic modeling study of the effect of NO and SO2 on the oxidation of CO?H2 mixtures

The effect of NO and SO₂ on the oxidation of a CO? H₂ mixture was studied in a jet‐stirred reactor at atmospheric pressure and for various equivalence ratios (0.1, 1, and 2) and initial concentrations of NO and SO₂ (0–5000 ppm). The experiments were performed at fixed residence time and variable temperature ranging from 800 to 1400 K. Additional experiments were conducted in a laminar flow reactor on the effect of SO₂ on CO? H₂ oxidation in the same temperature range for stoichiometric and reducing conditions. It was demonstrated that in fuel‐lean conditions, the addition of NO increases the oxidation of the CO? H₂ mixture below 1000 K and has no significant effect at higher temperatures, whereas the addition of SO₂ has a small inhibiting effect. Under stoichiometric and fuel‐rich conditions, both NO and SO₂ inhibit the oxidation of the CO? H₂ mixture. The results show that a CO? H₂ mixture has a limited NO reduction potential in the investigated temperature range and rule out a significant conversion of HNO to NH through reactions like HNO + CO ?? NH + CO₂ or HNO + H₂ ?? NH + H₂O. The chain terminating effect of SO₂ under stoichiometric and reducing conditions was found to be much more pronounced than previously reported under flow reactor conditions and the present results support a high rate constant for the H + SO₂ + M ?? HOSO + M reaction. The reactor experiments were used to validate a comprehensive kinetic reaction mechanism also used to simulate the reduction of NO by natural gas blends and pure C₁ to C₄ hydrocarbons. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 564–575, 2003     

Experimental and Kinetic Modeling Study of C2H2 Oxidation at High Pressure

A detailed chemical kinetic model for oxidation of acetylene at intermediate temperatures and high pressure has been developed and evaluated experimentally. The rate coefficients for the reactions of C₂H₂ with HO₂ and O₂ were investigated, based on the recent analysis of the potential energy diagram for C₂H₃ + O₂ by Goldsmith et al. and on new ab initio calculations, respectively. The C₂H₂ + HO₂ reaction involves nine pressure‐ and temperature‐dependent product channels, with formation of triplet CHCHO being dominant under most conditions. The barrier to reaction for C₂H₂ + O₂ was found to be more than 50 kcal mol^?1 and predictions of the initiation temperature were not sensitive to this reaction. Experiments were conducted with C₂H₂/O₂ mixtures highly diluted in N₂ in a high‐pressure flow reactor at 600–900 K and 60 bar, varying the reaction stoichiometry from very lean to fuel‐rich conditions. Model predictions were generally in satisfactory agreement with the experimental data. Under the investigated conditions, the oxidation pathways for C₂H₂ are more complex than those prevailing at higher temperatures and lower pressures. Acetylene is mostly consumed by recombination with H to form vinyl (reducing conditions) or with OH to form a CHCHOH adduct (stoichiometric to lean conditions). Both C₂H₃ and CHCHOH then react primarily with O₂. The CHCHOH + O₂ reaction leads to formation of significant amounts of glyoxal (OCHCHO) and formic acid (HOCHO), and the oxidation chemistry of these intermediates is important for the overall reaction.     

Kinetic diagrams of Ln2O2SO4 phase transformations in a H2 flow (Ln = La, Pr, Nd, Sm)

Kinetic diagrams of Ln₂O₂SO₄ (Ln = La, Pr, Nd, Sm) systems reduction in a H₂ flow are plotted for the first time in temperature-duration of treatment coordinates in which there are five areas of phase states. The temperatures of formation are established for products of the Ln₂O₂SO₄ + 4H₂ = Ln₂O₂S + 4H₂O reaction in the temperature range of 880–900 K and products of the Ln₂O₂SO₄ + H₂ = Ln₂O₃ + SO₂+ H₂O reaction in the temperature range of 1090–1220 K. The ranges of the temperature of formation of the homo-geneous Ln₂O₂S phase were found to decrease: 880–1220, 900–1200, 900–1180, and 900–1090 K in the sequence La-Pr-Nd-Sm.     

Fast selective oxidation of alcohols under solvent-free conditions

A combination of acetic anhydride, H₂SO₄-nano silica, wet-SiO₂ (60 %), and K₂Cr₂O₇ as a new oxidizing system for the selective oxidation of different types of alcohols to the corresponding aldehydes and ketones at room temperature under solvent-free conditions is introduced. Mild reaction conditions, high yields of the products, short reaction time, no further oxidation to the corresponding carboxylic acid, and easy work-up make this new system a useful method for oxidizing alcohols.     

High-temperature FeS-FeS2 solid-state transitions: Reactions of solid mackinawite with gaseous H2S

Phase transitions and reactions of non-oxidized and surface-oxidized mackinawite (FeS) in helium and H₂S gas were investigated by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT). DFT was used to obtain optimized structures of the iron-sulfur phases mackinawite, hexagonal pyrrhotite, greigite, marcasite and pyrite and to determine the thermochemical properties of reactions of mackinawite with H₂S to these phases. The phase transitions of mackinawite to hexagonal pyrrhotite are endothermic, while reactions to greigite, marcasite and pyrite are exothermic. The experiments show that non-oxidized mackinawite converts into hexagonal pyrrhotite (Fe₉S₁₀ first and then Fe₇S₈) in He and also in H₂S but at a lower temperature. No further reactions can be observed under these conditions. In the case of surface-oxidized mackinawite, the extent of surface oxidation determines the course and the final product of the reaction with H₂S. If the extent of surface oxidation is low, only Fe²⁺ is oxidized to Fe³⁺. Under these conditions mackinawite converts into the mixed-valence thiospinel compound greigite. In case of pronounced surface oxidation all surface Fe centers are oxidized to the Fe³⁺ state and S²⁻ is oxidized to SO₄²⁻. Oxidation of sulfur is a prerequisite for the formation of pyrite.     

Ethylene pyrolysis and oxidation: A kinetic modeling study

Ethylene oxidation and pyrolysis was modeled using a comprehensive kinetic reaction mechanism. This mechanism is an updated version of one developed earlier. It includes the most recent findings concerning the kinetics of the reactions involved in the oxidation of ethylene. The proposed mechanism was tested against ethylene oxidation experimental data (molecular species concentration profiles) obtained in jet stirred reactors (1–10 atm, 880–1253 K), ignition delay times measured in shock tubes (0.2–12 atm, 1058–2200 K) and ethylene pyrolysis data in shock tube (2–6 atm, 1700–2200 K). The general prediction of concentration profiles of minor species formed during ethylene oxidation is improved in the present model by using more accurate kinetic data for several reactions (principally: HO₂ + HO₂ → H₂O₂ + O₂, C₂H₄ + OH → C₂H₃ + H₂O, C₂H₂ + OH → Products, C₂H₃ → C₂H₂ + H).     

Selective oxidation of hydrogen sulfide to ammonium thiosulfate and sulfur over vanadium-bismuth oxide catalysts

The selective oxidation of hydrogen sulfide containing excess water and ammonia was studied over vanadium-bismuth mixed oxide catalysts. The investigation was focused on understanding the complex reaction steps and the roles of each metal oxide. Therefore, supported V₂O₅/TiO₂, V-Bi-O/TiO₂ catalysts and a mechanical mixture of V₂O₅ + Bi₂O₃ were tested in the reaction. Ammonia reacted either with H₂S or SO₂, produced from the oxidation of H₂S. Water vapor promoted the reaction of ammonia and SO₂. Strong synergistic phenomena in catalytic activity were observed for the mechanically mixed catalyst of V₂O₅ and Bi₂O₃. V-Bi-O/TiO₂ catalyst showed very high H₂S conversion without any considerable emission of SO₂. Temperature-programmed studies (TPR and TPO), XRD and Raman analyses revealed that the high catalytic performance of V-BiO/TiO₂ catalyst originated from the high redox capacity of the bismuth vanadate phase.     

Oxidation and Destruction of Polyvinyl Alcohol under the Combined Action of Ozone–Oxygen Mixture and Hydrogen Peroxide

The oxidative transformations of a polyvinyl alcohol in aqueous solutions are studied under the simultaneous action of the two oxidizing agents, an ozone–oxygen mixture and a hydrogen peroxide. Effective parameters a and b, which characterize the first and second channels of carboxyl group accumulation, respectively, grow linearly upon an increase in the initial concentration of H₂O₂. After the temperature dependence of a and b parameters (331–363 K) in a PVA + O₃ + O₂ + H₂O₂ + H₂O reaction system is studied, the parameters of the activation of COOH group accumulation are found (where PVA is a polyvinyl alcohol). New data on the effect process conditions (length of oxidation, temperature, and hydrogen peroxide concentration) have on the degree of destructive transformations of polyvinyl alcohol in the investigated reaction system are obtained.     

Kinetics and mechanism for the reactions of H atoms with CH3SH and C2H5SH

A kinetic study of the reactions of H atoms with CH₃SH and C₂H₅SH has been carried out at 298 K by the discharge flow technique with EPR and mass spectrometric analysis of the species. The pressure was 1 torr. It was found: k₁ = (2.20 ± 0.20) × 10^?12 for the reaction H + CH₃SH (1) and k₂ = (2.40 ± 0.16) × 10^?12 for the reaction H + C₂H₅SH (2). Units are cm³ molecule^?1 s^?1. A mass spectrometric analysis of the reaction products and a computer simulation of the reacting systems have shown that reaction (1) proceeds through two mechanisms leading to the formation of CH₃S + H₂ (1a) and CH₃ + H₂S (1b).     

Characterization of kmno4/h2so4-oxidized polyethylene surfaces by means of ESCA andca adsorption

The chemical constitution of KMnO₄/H₂SO₄-oxidized (sulfated) polyethylene (PE) surfaces has been explored by means of ESCA utilizing chemical shifts, peak intensity data, and chemical tagging reactions. In addition, the adsorption of Ca²⁺ ions on sulfated PE was quantified by radiotracer measurements. Comparisons have been made with KClO₃/H₂SO₄ and K₂Cr₂O₇/H₂SO₄ as oxidizing agents and through replacing polyethylene by polystyrene. According to the present study the main chemical groups on a

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. In the oxidation grooves, dissociable −COOH groups are also present in appreciable amounts. By annealing, a smoother and more homogeneous sulfated surface is produced, the composition of which strongly depends upon the state of the ionic groups at the heat treatment.     

High temperature H2S selective oxidation on a copper-substituted hexaaluminate catalyst: A facile process for treating low concentration acid gas

H₂ S selective catalytic oxidation technology is a prospective way for the treatment of low concentration acid gas with simple process operation and low investment.However,undesirable results such as large formation of SO₂ and catalyst deactivation inevitably occur,due to the temperature rise of fixed reaction bed caused by the exothermic reaction.Catalyst with high activity in wide operating temperature window,especially in high temperature range,is urgently needed.In this...     

Studies on the oxidation mechanism of H2S based on direct examination of the key reactions

By conducting an excimer laser photolysis (193 and 248 nm) behind shock waves, three elementary reactions important in the oxidation of H₂S have been examined, where, H, O, and S atoms have been monitored by the atomic resonance absorption spectrometry. For HS + O₂ → products (1), the rate constants evaluated by numerical simulations are summarized as: k₁ = 3.1 × 10⁻¹¹exp|-75 kJ mol⁻¹/RT| cm³molecule⁻¹s⁻¹ (T = 1400-1850 K) with an uncertainty factor of about 2. Direct measurements of the rate constants for S + O2 → SO + O (2), and SO + O₂ → SO₂ + O (3) yield k₂ = (2.5 ± 0.6) × 10⁻¹¹ exp|-(15.3 ± 2.5) kJ mol⁻¹/RT| cm³molecule⁻¹s⁻¹ (T = 980-1610 K) and, k₃ = (1.7 ± 0.9) × 10⁻¹² exp|-(34 ± 11) kJ mol⁻¹/RT| cm³molecule⁻¹s⁻¹ (T = 1130-1640 K), respectively. By summarizing these data together with the recent experimental results on the H(SINGLE BOND)S(SINGLE BOND)O reaction systems, a new kinetic model for the H₂S oxidation process is constructed. It is found that this simple reaction scheme is consistent with the experimental result on the induction time of SO₂ formation obtained by Bradley and Dobson. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 57–66, 1997.     

Kinetic modeling of the CO/H2O/O2/NO/SO2 system: Implications for high‐pressure fall‐off in the SO2 + O(+M) = SO3(+M) reaction

Flow reactor experiments were performed to study moist CO oxidation in the presence of trace quantities of NO (0–400 ppm) and SO₂ (0–1300 ppm) at pressures and temperatures ranging from 0.5–10.0 atm and 950–1040 K, respectively. Reaction profile measurements of CO, CO₂, O₂, NO, NO₂, SO₂, and temperature were used to further develop and validate a detailed chemical kinetic reaction mechanism in a manner consistent with previous studies of the CO/H₂/O₂/NO_X and CO/H₂O/N₂O systems. In particular, the experimental data indicate that the spin‐forbidden dissociation‐recombination reaction between SO₂ and O‐atoms is in the fall‐off regime at pressures above 1 atm. The inclusion of a pressure‐dependent rate constant for this reaction, using a high‐pressure limit determined from modeling the consumption of SO₂ in a N₂O/SO₂/N₂ mixture at 10.0 atm and 1000 K, brings model predictions into much better agreement with experimentally measured CO profiles over the entire pressure range. Kinetic coupling of NO_X and SO_X chemistry via the radical pool significantly reduces the ability of SO₂ to inhibit oxidative processes. Measurements of SO₂ indicate fractional conversions of SO₂ to SO₃ on the order of a few percent, in good agreement with previous measurements at atmospheric pressure. Modeling results suggest that, at low pressures, SO₃ formation occurs primarily through SO₂ + O(+M) = SO₃(+M), but at higher pressures where the fractional conversion of NO to NO₂ increases, SO₃ formation via SO₂ + NO₂ = SO₃ + NO becomes important. For the conditions explored in this study, the primary consumption pathways for SO₃ appear to be SO₃ + HO₂ = HOSO₂ + O₂ and SO₃ + H = SO₂ + OH. Further study of these reactions would increase the confidence with which model predictions of SO₃ can be viewed. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 317–339, 2000     

Utilisation des mélanges H2O+H2SO4+SO3 en tant que solvants: I. Étude du ferrocène et de quelques hydrocarbures polynucléaires dans les mélanges H2O+H2SO4. Comparaison des domaines d'électroactivité

The variation of the electroactivity range with composition of water+ sulphuric acid mixtures has been studied. It is shown that ferrocene provides a reference potential independent of solvent at low H₂SO₄ contents while polynuclear hydrocarbons fulfill the same function at high H₂SO₄ contents. Water+H₂SO₄ mixtures become more oxidizing (reduction limit) and less reducing (oxidation limit) when the H₂SO₄ concentration increases.     

An FTIR study of the mechanism for the reaction HO + CH3SCH3

Product studies were made using the Fourier transform infrared method in the uv (300–400-nm) photolysis of mixtures containing CH₃SCH₃, C₂H₅ONO, and NO in ppm concentrations in 700 torr of O₂–N₂ diluent. Methyl thionitrite, CH₃SNO, arising from the reaction CH₃S + NO, was detected as an intermediate product. In addition, the yields of the major sulfur-containing products SO₂ and CH₃SO₃H coincided with those of the oxidation of the CH₃S radicals generated directly by the photodissociation of CH₃SNO. The formation of CH₃S in the HO-initiated oxidation of CH₃SCH₃ in the presence of NO suggests a reaction scheme involving the H-abstraction reaction HO + CH₃SCH₃ → CH₃SCH₂ + H₂O as the primary step.     

Insight into the reaction mechanism of CH2SH with HO2: A density functional theory study

Density functional theory was adopted in this work to reveal the reaction mechanism of CH₂SH with HO₂. Reaction rate constants were computed from 200 to 2000 K using the transition state theory combined with Wigner and Eckart tunneling correction. Moreover, localized orbital locator, atoms in molecules and Mayer bond order analyses were used to study the essence of chemical bonding evolution. Eleven singlet paths and three triplet ones are located on the potential surface (PES). The results show that the main products on the singlet PES are ¹CH₂S and H₂O₂, whereas on the triplet PES they are CH₃SH + ³O₂, which are coincident with the similar reaction of CH₃S and HO₂. This conclusion is also supported by rate constant calculation results. Interestingly, all the possible paths are involved in the hydrogen transfer. The results have provided underlying insights to the analogous reactions and further experimental studies.     

DTA studies on preparation of MnSO4 by reacting pyrite and pyrolusite at high temperatures

The preparation of MnSO₄ by reacting pyrolusite at high temperatures with SO₂ generated from pyrite was followed by DTA, and the process conditions were optimized to fix the minimum time and temperature of reaction required to obtain the maximum yield of pure MnSO₄ from stoichiometric amounts of reactants in a natural draught of air. The presence of MnO and Fe₃O₄ in the reaction products, detected by DTA, indicates that the SO₂ is initially oxidized to SO₃ by reducing MnO₂, Mn₂O₃ and Fe₂O₃ to MnO and Fe₃O₄. SO₃ finally attacks MnO to form MnSO₄. When an intimate stoichiometric blend of pyrite and pyrolusite is heated at temperatures ranging from 873 K to 973 K for 3 hrs, about 93% of the Mn is converted to ironfree MnSO₄.     

Oxidation of some catecholamines by sodium N-chloro-p-toluenesulfonamide in acid medium: A kinetic and mechanistic approach

The kinetics of the oxidation of five catecholamines viz., dopamine (A), L-dopa (B), methyldopa (C), epinephrine (D) and norepinephrine (E) by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT) in presence of HClO₄ was studied at 30±0.1 °C. The five reactions followed identical kinetics with a first-order dependence on [CAT] _o , fractional-order in [substrate] _o , and inverse fractional-order in [H⁺]. Under comparable experimental conditions, the rate of oxidation of catecholamines increases in the order D>E>A>B>C. The variation of ionic strength of the medium and the addition of p-toluenesulfonamide or halide ions had no significant effect on the reaction rate. The rate increased with decreasing dielectric constant of the medium. The solvent isotope effect was studied using D₂O. A Michaelis-Menten type mechanism has been suggested to explain the results. Equilibrium and decomposition constants for CAT-catecholamine complexes have been evaluated. CH₃C₆H₄SO₂NHCl of the oxidant has been postulated as the reactive oxidizing species and oxidation products were identified. An isokinetic relationship is observed with β=361 K, indicating that enthalpy factors control the reaction rate. The mechanism proposed and the derived rate law are consistent with the observed kinetics.     

Photolysis of,and the reactions of H-atoms with,t-butanethiol

A detailed investigation of the photolysis of t-C₄HgSH has been carried out in the absence and presence of the inert gas, C₂H₆. A mechanssm consisting of three primaRy photochemical steps: t-C₄H₉SH → t-C₄H₉S + H (1), t-C₄H₉SH → t-C₄H₉ + SH (2), t-C₄H₉SH → i-C₄H₈ + H₂S (3), six hot and seven thermal reaction steps, adequately explains all the experimental observations. As in the case of hot H* atoms, both the H-atom abstraction H + t-C₄H₉SH → H₂ + t-C₄H₉S (7), and the SH-displacement reactions, H + t-C₄H₉SH → H₂S + i-C₄H₈ (8) occur with thermalized H-atoms. The Arrhenius expression of the rate constant ratio, k₇/k₈ for the latter reactions has been determined over the temperature range 25-14° C to be: ln(k₇/k₈) = (0.3 ± 0.1) + (420 ± 80)/RT.     

Experimental and kinetic modeling study of the effect of sulfur dioxide on the mutual sensitization of the oxidation of nitric oxide and methane

New experimental results were obtained for the mutual sensitization of the oxidation of NO and methane in a fused silica jet‐stirred reactor operating at 10⁵ Pa, over the temperature range 800–1150 K. The effect of the addition of sulfur dioxide was studied. Probe sampling followed by online FTIR analyses and off‐line GC‐TCD/FID analyses allowed the measurement of concentration profiles for the reactants, stable intermediates, and final products. A detailed chemical kinetic modeling of the present experiments was performed. An overall reasonable agreement between the present data and modeling was obtained. According to the present modeling, the mutual sensitization of the oxidation of methane and NO proceeds via the NO to NO₂ conversion by HO₂ and CH₃O₂. The conversion of NO to NO₂ by CH₃O₂ is more important at low temperatures (800 K) than at higher temperatures (850–900 K) where the production of NO₂ is mostly due to the reaction of NO with HO₂. The NO to NO₂ conversion is favored by the production of the HO₂ and CH₃O₂ radicals yielded from the oxidation of the fuel. The production of OH resulting from the oxidation of NO accelerates the oxidation of the fuel: NO + HO₂ → OH+ NO₂ followed by OH + CH₄→ CH₃. In the lower temperature range of this study, the reaction further proceeds via CH₃ + O₂→ CH₃O₂; CH₃O₂+ NO → CH₃O + NO₂. At higher temperatures, the production of CH₃O involves NO₂: CH₃+ NO₂→ CH₃O. This sequence of reactions is followed by CH₃O → CH₂O + H; CH₂O +OH → HCO; HCO + O₂ → HO₂ and H + O₂ → HO₂ → CH₂O + H; CH₂O +OH → HCO; HCO + O₂ → HO₂ and H + O₂ → HO₂. The data and the modeling show that unexpectedly, SO₂ has no measurable effect on the kinetics of the mutual sensitization of the oxidation of NO and methane in the present conditions, whereas it frequently acts as an inhibitor in combustion. This result was rationalized via a detailed kinetic analysis indicating that the inhibiting effect of SO₂ via the sequence of reactions SO₂+H → HOSO, HOSO+O₂ → SO₂+HO₂, equivalent to H+O₂?HO₂, is balanced by the reaction promoting step NO+HO₂ → NO₂+OH. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 406–413, 2005