Kinetics and mechanism in the decomposition of NH3 in a radio-frequency pulse discharge

Studies of time-resolved absorption spectra of transient species in the decomposition of NH₃ by an r.f. pulse discharge together with product analysis showed that the major radical formed was NH at concentrations of the order of 10^–6 mol dm^–3 (10⁵ molec. cm^–3). Possible mechanisms for the formation of the radical during the discharge and its decay following pulse cut-off were tested by computer simulation of the kinetic data. Following zero-order formation with rate coefficient 0.19±0.03 mol dm^–3 s^–1, the decay was second order in NH with rate coefficient 2.1±0.5×10⁹ mol^–1 dm³ s^–1 both for pure NH₃ and where NH₃/rare gas mixtures were investigated. The kinetic data are consistent with NH removal in a nonassociative radical-radical reaction proceeding via a short-lived collision complex, probably 2NH N₂H₂ N₂ + H₂.     

The thermal decomposition of (NH4)[VO(O2)2(NH3)]

The compound, (NH₄)[VO(O₂)₂(NH₃)], thermally decomposes to ammonium metavanadate, which then decomposes to vanadium pentoxide. Using a heating rate of 5 deg·min^–1, the first decomposition step occurs between 74° and 102°C. The transformation degree dependence of the activation energy (-E) is shown to follow a decreasing convex form, indicating that the first decomposition step is a complex reaction with a change in the limiting stage of the reaction. Infrared spectra indicated that the decomposition proceeds via the gradual reduction of the ratio of the (NH₄)₂O to V₂O₅ units from the original 11 ratio in ammonium metavanadate, which may be written as (NH₄)₂O·V₂O₅, to V₂O₅.The assistance of Professor A. M. Heyns (University of Pretoria) and Professor K. L. Range (University of Regensburg) is gratefully acknowledged as well as the financial assistance of the University of Pretoria and the FRD.     

Synthesis of ammonia in high-frequency discharges. II. Synthesis of ammonia in a microwave discharge under various conditions

The synthesis of ammonia from nitrogen-hydrogen plasma prepared using microwave discharge was studied by changing some experimental conditions, such as pressure (260–2600 Pa), power input (30–280 W), and nitrogen-hydrogen mixing ratio [H₂/(N₂+H₂)=0–1.0]. The ammonia yield increased with decreasing pressure and saturated at lower pressures. When the power input and the nitrogen-hydrogen mixing ratio were changed, the maximum yield of ammonia was obtained at the optimum experimental conditions (power input 150W; H₂/(N₂+H₂)0.75). Amounts of NH, H, and H₂ in the plasma also changed by changing the experimental conditions. From the changes in ammonia yield and amounts of NH, H, and H₂ by changing the experimental conditions, it is suggested that ammonia molecules are formed by the reaction of NH radicals not only with hydrogen atoms but also with hydrogen molecules. Otherwise, the formation and the decomposition of ammonia would occur simultaneously.     

Lithium Imide Synergy with 3d Transition‐Metal Nitrides Leading to Unprecedented Catalytic Activities for Ammonia Decomposition

Alkali metals have been widely employed as catalyst promoters; however, the promoting mechanism remains essentially unclear. Li, when in the imide form, is shown to synergize with 3d transition metals or their nitrides TM(N) spreading from Ti to Cu, leading to universal and unprecedentedly high catalytic activities in NH₃ decomposition, among which Li₂NH? MnN has an activity superior to that of the highly active Ru/carbon nanotube catalyst. The catalysis is fulfilled via the two‐step cycle comprising: 1) the reaction of Li₂NH and 3d TM(N) to form ternary nitride of LiTMN and H₂, and 2) the ammoniation of LiTMN to Li₂NH, TM(N) and N₂ resulting in the neat reaction of 2 NH₃?N₂+3 H₂. Li₂NH, as an NH₃ transmitting agent, favors the formation of higher N‐content intermediate (LiTMN), where Li executes inductive effect to stabilize the TM? N bonding and thus alters the reaction energetics.     

The isothermal flash photolysis of hydrazoic acid

The flash photolysis of HN₃ was studied by coordinated time-resolved spectroscopic measurements of HN₃ NH(a¹Δ), NH(X³Σ), NH(c¹π), NH(A³π), NH₂, and N₃ following flash photolysis of mixtures of HN₃ with argon or helium. The primary photolysis is complex, but when the wavelength distribution of the flash is limited to values greater than about 200 nm, the major reactive product is NH(¹Δ), or states which quickly decay to NH(¹Δ). Disappearance of NH(¹Δ) occurs predominantly by the process The process has little, if any, energy of activation, and no detectable dependence on the pressure of inert gas below 1 atm. The rate of formation of NH₂ in its ground vibrational state depends on the inert gas pressure in a way that can be accounted for by vibrational relaxation from initial excited vibrational states. The total amount of NH₂ is roughly comparable with the amount of HN₃ decomposed by primary photolysis. The observed N₃ can be attributed to the NH(¹Δ) + HN₃ reaction, although a smaller amount could also be formed by primary photolysis. The value of k₂ is revised upward from the value given in a preliminary report on the basis of a more careful consideration of the effects of Beer's law failure in absorption measurements involving narrow spectral lines.     

Thermal decomposition of cobalt nitrato compounds: Preparation of anhydrous cobalt(II)nitrate and its characterisation by Infrared and Raman spectra

The thermal decomposition of Co(NO₃)₂·6H₂O (1) as well as that one of NO[Co(NO₃)₃] (Co(NO₃)₂·N₂O₄) (2) was followed by thermogravimetric (TG) measurements, X-ray recording and Raman and IR spectra. The stepwise decomposition reactions of 1 and 2 leading to anhydrous cobalt(II)nitrate (3) were established. In N₂ atmosphere, cobalt oxides are finally formed whereas in H₂/N₂ (10% H₂) cobalt metal is produced. Rapid heating of cobalt(II)nitrate hexahydrate causes melting (formation of a hydrate melt) and therefore side reactions in the hydrate melt by incoupled reactions and evolution/evaporation of different species as, e.g., HNO₃, NO₂, etc. In case of larger amounts in dense packing in the sample container, the formation of oxo(hydoxo)nitrates is possible at higher temperature. For 2, its thermal decomposition to 3 was followed and its decomposition mechanism is proposed.     

Water-catalyzed isomerization of the glycine radical cation. From hydrogen-atom transfer to proton-transport catalysis

The isomerization reactions of the glycine radical cation, from [NH₂CH₂COOH]⁺, I, to [NH₃CHCOOH]⁺, II, or [NH₂CHC(OH)₂]⁺, III, in the presence of a water molecule have been studied theoretically. The water molecule reduces dramatically the energy barriers of the III and IIII tautomerizations owing to a change in the nature of the process. However, the role of the water molecule depends on the kind of isomerization, the catalytic effect being more important for the IIII reaction. As a consequence, the preferred mechanism for the interconversion of glycine radical cation I to the stablest isomer, III, is the direct one-step mechanism instead of the two step (III and IIIII) process found for isolated [NH₂CH₂COOH]⁺. When using ammonia as a solvent molecule, a spontaneous proton-transfer process from [NH₂CH₂COOH]⁺ to NH₃ is observed and so no tautomerization reactions take place. This behavior is the same as that observed in aqueous solution, as has been confirmed by continuum model calculations.Contribution to the Jacopo Tomasi Honorary Issue     

Production of N,H, and NH active species in N2-H2 dc flowing discharges

Densities of N, H, and NH active species have been detected by laser-induced fluorescence (LIF) in N₂-xH₂ dc flowing discharges. A peak value of N atom densities far x = 0.2–0.5% and a plateau value of H atom densities between x = 1% and 90% in post-discharge conditions (0.05 sec, p = 2 torr) has been found. Comparison between LIF measurements of N atoms and the trend of the N₂(B, v = II) population shows that the emission from this state can be used for monitoring N atoms. The NH radical has only been detected inside the discharge region.On leave from Instituto Tecnológico de Aeronáutica, CNPq, Brazil.     

Synthesis of <Emphasis Type="Italic">N</Emphasis>-nitro-<Emphasis Type="Italic">N</Emphasis>′-(trimethylsilyl)carbodiimide

Nitration of N,N′-bis(trimethylsilyl)carbodiimide with N₂O₅ or (NO₂)₂SiF₆ afforded N-nitro-N´-(trimethylsilyl)carbodiimide, the first representative of N-nitro carbodiimides. Its further nitration led to the release of CO₂, which is presumably formed in the course of N,N´-dinitrocarbodiimide decomposition. The reactions of N-nitro-N´-(trimethylsilyl)carbodiimide with nucleophiles take place both at the tri methylsilyl group (for example, with NH₃) to give nitrocyanamide salts and at the carbodiimide C atom (for example, with Et₂NH) to give the corresponding nitroguan idines.     

Synthese und Kristallstruktur von Na10[P4(NH)6N4](NH2)6(NH3)0,5 mit dem adamantanartig aufgebauten Anion [P4(NH)6N4]4−

Synthesis and Crystal Structure of Na₁₀[P₄(NH)₆N₄](NH₂)₆(NH₃)_0.5 with an Adamantane-like Anion [P₄(NH)₆N₄]^4? Crystals of Na₁₀[P₄(NH)₆N₄](NH₂)₆(NH₃)_0.5 were obtained by the reaction of P₃N₅ with NaNH₂ (molar ratio 1:20) within 5 d at 600°C in autoclaves. The following data characterize X-ray investigations: Fm3 m, Z = 8, a = 15.423(2) Å, Z(F) = 261 with F ≥ 3 σ(F) Z(Variables) = 27, R/R_w = 0.086/0.089 The compound contains the hitherto unknown anion [P₄(NH)₆N₄]^4?, which resembles adamantane. The total structure can be described as follows: The centers of gravity of units of [Na₈(NH₂)₆(NH₃)]²⁺ – 8Na⁺ on the corners of a cube, 6NH₂^? on the ones of an inscribed octahedron with NH₃ in the center – follow the motif of a cubic-closest packed arrangement. Units of [Na₁₂(NH₂)₆]⁶⁺ – 12Na⁺ on the corners of a cuboctahedron and 6NH₂^? on the ones of an inscribed octahedron – occupy all octahedral and those of [P₄(NH)₆N₄]^4? all tetrahedral sites.     

Thin Films Deposition from Hexamethyldisiloxane and Hexamethyldisilazane under Dielectric-Barrier Discharge (DBD) Conditions

Hexamethyldisiloxane (HMDSO) and hexamethyldisilazane (HMDSN) were used as organosilicon reagents for PE-CVD of thin films under filamentary barrier-discharge conditions at atmospheric pressure. Efficient discharges were obtained in the region of moderate frequencies (5 kHz). The following mixtures of organosilicon reagents with carrier gas and oxidants or ammonia were investigated: HMDSO+Ar, HMDSO+N₂, HMDSO+O₂+Ar, HMDSO+N₂O+Ar, and HMDSN+NH₃+N₂. Under such conditions HMDSO was converted to produce thin films (10–1000 nm) of silicon oxide, generally containing admixtures of residual organic content (Si—CH_n and Si—H groups). The films deposited from HMDSN+NH₃+N₂ contained silicon, nitrogen and oxygen.     

Modeling the thermal DENOx process in flow reactors. Surface effects and Nitrous Oxide formation

We have investigated the impact of surface reactions such as NH₃ decomposition and radical adsorption on quartz flow reactor data for Thermal DeNO_x using a model that accounts for surface chemistry as well as molecular transport. Our calculations support experimental observations that surface effects are not important for experiments carried out in low surface to volume quartz reactors. The reaction mechanism for Thermal DeNO_x has been revised in order to reflect recent experimental results. Among the important changes are a smaller chain branching ratio for the NH₂ + NO reaction and a shorter NNH lifetime than previously used in modeling. The revised mechanism has been tested against a range of experimental flow reactor data for Thermal DeNO_x with reasonable results. The formation of N₂O in Thermal DeNO_x has been modelled and calculations show good agreement with experimental data. The important reactions in formation and destruction of N₂O have been identified. Our calculations indicate that N₂O is formed primarily from the reaction between NH and NO, even though the NH₂ + NO₂ reaction possibly contributes at lower temperatures. At higher temperatures N₂O concentrations are limited by thermal dissociation of N₂O and by reaction with radicals, primarily OH. © 1994 John Wiley & Sons, Inc.     

Solvent and deuterium isotope effects on the decomposition of ammonium nitrite solutions

The rate of decomposition of NH₄NO₂ solutions, at pH 5–7, equals k[NH₃] [HNO₂]² or k[NH ₄ ⁺ ] [NO ₂ ^– ][HNO₂]. A plausible mechanism involves a ratedetermining attack of N₂O₃, derived from HNO₂, on NH₃. H⁺⁺ and S⁺⁺ are 82 kJ-mol^–1 and –27 J-mol^–1-K^–1, respectively. On partially replacing the solvent water by methanol or ethanol, the change G⁺⁺, coupled with the calculated standard Gibbs energy of transfer of the reactants from water to the mixed solvent indicated that, in the latter, there is a greater destabilization of the transition state compared to that of the reactants. This can be explained by assuming two hydrogen bonds from the same water molecule to the transition state and hence a loss of hydrogen bond energy in the mixed solvent compared to the aqueous solution. The rate constant for the reaction of ND₄NO₂ in D₂O compared to the reaction of NH₄NO₂ in water, gave a composite isotope effect involving two acid-base equilibria, suggested in the proposed mechanism; in addition to primary isotope effects in the equilibrium: 2 HNO₂N₂O₃+H₂O.     

Production of the NH radical from the catalytic decomposition of NH3 on polycrystalline Pt and Fe surfaces at high temperatures

Decomposition of NH₃ on polycrystalline platinum at temperatures >1200 K was found to result in desorption of the NH radical into the gas phase. The gaseous NH radical was detected by means of the laser-induced fluorescence technique. The desorption process is one of several possible steps involving the surface-adsorbed NH radical. NH₂ was not observed to desorb from the catalyst at all temperatures studied. The measured activation energy for NH desorption is 66 ± 3 kcal/mol and appears to be independent of NH₃ or H₂ gas phase concentration over the range studied. Addition of H₂ to NH₃ enhances the yield of NH desorbed; however, addition of D₂ to NH₃ enhances the desorption yield of both ND and NH. A mechanism for NH₃ decomposition is proposed to account for these observations and the results of other studies. A similar but brief study of NH₃ decomposition on polycrystalline Fe surfaces was also made. Similarly NH, but not NH₂, was found to desorb from the Fe surface above 1000 K.     

Synthese und Kristallstruktur von Rb8[P4N6(NH)4](NH2)2 mit dem adamantanartigen Anion [P4N6(NH)4]6−

Synthesis and Crystal Structure of Rb₈[P₄N₆(NH)₄](NH₂)₂ with the Adamantane-like Anion [P₄N₆(NH)₄]^6? RbNH₂ reacts with P₃N₅ (molar ratio 6:1) at 400°C within 5 d to colourless Rb₈[P₄N₆(NH)₄](NH₂)₂. Suitable crystals for a X-ray structure determination were obtained: The compound contains adamantane-like molecular anions [P₄N₆(NH)₄]^6?. Their centres of gravity are arranged in a distorted hexagonal primitive array. All trigonal prisms of this array contain one amide ion. Rubidium ions connect the anions irregularly.     

Ion-molecular reactions of primary radical cations of aliphatic amines in freon matrices

ESR spectra for -irradiated, at –196 °C, solution of Me₂NH, Me₃N, and EtNH₂ in CFCl₃ /0.05÷100% amine/ have been studied. Radical cations Me₂NH^+., Me₃N^+. and EtNH ₂ ^+. were trapped in dilute solutions /less than 1% amine in CFCl₃/. The yields of radical cations decrease and those of neutral radicals /Me₂ N, CH₂NMe₂, Et NH/ correspondingly increase as the amine concentration increases. Radical cations Me₂NH^+. are transformed to Me₂ N as well as Me₃N^+. to C H₂NMe₂ via proton transfer reaction, which is described by the reaction volume model.     

Thermal behaviour of nickel(II) sulphate,nitrate and halide complexes containing ammine and ethylenediamine as ligands

Thermal behaviour of nickel amine complexes containing SO₄ ²⁻, NO₃ ⁻, Cl⁻ and Br⁻ as counter ions and ammonia and ethylenediamine as ligands have been investigated using simultaneous TG/DTA coupled with mass spectroscopy (TG/DTA–MS). Evolved gas analyses detected various transient intermediates during thermal decomposition. The nickel ammonium sulphate complex produces NH, N, S, O and N₂ species. The nickel ammonium nitrate complex generated fragments like N, N₂, NO, O₂, N₂O, NH₂ and NH. The halide complexes produce NH₂, NH, N₂ and H₂ species during decomposition. The ligand ethylenediamine is fragmented as N₂/C₂H₄, NH₃ and H₂. The residue hexaamminenickel(II) sulphate produces NiO with crystallite size 50 nm. Hexaammine and tris(ethylenediamine)nickel(II) nitrate produce NiO in the range 25.5 nm and 23 nm, respectively. The halide complexes produce nano sized metallic nickel (20 nm) as the residue. Among the complexes studied, the nitrate containing complexes undergo simultaneous oxidation and reduction.     

The thermal decomposition of diimide in the gas phase: Kinetics and stoichiometry

The kinetics and stoiehiometry of the decomposition of N₂H₂ and N₂D₂ have been studied as a function of sample size, pressure, and temperature. The reaction follows a single first order kinetic expression over most of its time course. It is suggested that the rate-determining step in the mechanism is a first-order homogeneous gas-phase isomerization of trans-diimide with rate constants:k = 1.8 exp (-4.2 kcal/mol/RT) sec^?1 and k = 1 exp (-4.4 kcal/mol/RT) sec^?1. The detailed mechanism of this isomerization, however, is not evident. At temperatures above room temperature, self-heating has been observed which leads to an initial fast decay. At room temperature the reaction exhibits autocatalysis with the rate increasing as the reaction proceeds. This has been attributed to enhancement by a surface decay process involving adsorbed hydrazine. The only significant products from the decomposition of N₂H₂ are N₂, H₂, and N₂H₄, and the results are interpreted in terms of two parallel reactions: The decomposition of N₂D₂ occurs almost completely by the single reaction giving N₂ + N₂D₄. No azide formation has been detected from either N₂D₂, or N₂D₂, and limits have been put on the yield of ammonia. Extinction coefficients at 365 nm of 3.9 ± 0.2 for N₂H₂ and 3.3 ± 0.1 for N₂D₂ have been measured. Both the rate of decay and the stoichiometry of products show pressure dependence below 150 torr, and this is suggested to be due to direct decomposition of cis-N₂H₂ on the surface.     

Very low-pressure pyrolysis. VII. The decomposition of methylhydrazine, 1,1-dimethylhydrazine, 1,2-dimethylhydrazine,and tetramethylhydrazine. Concerted deamination and dehydrogenation of methylhydrazine

The rate constants (k_uni) for the first-order disappearance of the title molecules have been determined under VLPP conditions. The k_uni are not the rate constants of ultimate interest since they reflect the fact that energy transfer competes with the chemical decomposition. Use of the Rice-Ramsperger-Kassel-(Marcus) [RRK(M)] theory allows the determination of the high-pressure rate constants (k_α), if the mode of decomposition is known. The heats of formation of the radicals NH₂, CH₃NH, and (CH₃)₂N are known. These values should be usable for prediction of the activation energy for N? N bond homolysis in the hydrazines. Measured rate constants for UDMH and TMH bear this out, but the rate constant for MMH does not. This and other evidence lead to the conclusion that MMH decomposes via molecular concerted elimination of NH₃ and H₂ not and by N? N bond scission. The following values are preferred from this work (θ = 2.303RT in kcal/mole). Mode of decomposition is N—N bond scission unless noted otherwise in parenthesis: .     

Energetics and mechanism for hydroxyl radical production from the Pt-catalyzed decomposition of water

The catalytic decomposition of H₂O over a polycrystalline platinum surface has been studied in a low pressure flow system. Hydroxyl radicals were detected in gaseous decomposition products above about 820 K by laser-induced fluorescence at 300 nm. The activation energy for the HO desorption process, HO* → HO(g)+*, was determined to be 30 ± 1 kcal/mol, which is in excellent agreement with the values reported previously for the Pt-catalyzed oxidation of H₂ by O₂ and N₂O. The effects of added H₂, D₂ and O₂ on HO formation were also investigated and the observed data can be satisfactorily accounted for by a mechanism which involves chemically adsorbed H, O and HO species. A similar but brief experiment was carried out for the catalytic decomposition of NH₃ over the same catalyst. However, no NH or NH₂ radicals were detected in gaseous decomposition products up to 1100 K.