Decomposition Channels Research Articles

The ammonium nitrate and its mechanism of decomposition in the gas phase: a theoretical study and a DFT benchmark

The decomposition mechanism of ammonium nitrate in the gas phase was investigated and fully characterized by means of CBS-QB3 calculations. Five reaction channels were identified, leading to the formation of products (N2, H2O, O2, OH, HNO, NO3) found in the experimental works. The identified mechanism well underlines the origin of the chemical hazard of ammonium nitrate which is related to the exothermicity of the lowest decomposition channels. Furthermore, the high barrier to overcome in the rate determining step well explained the fact that the reaction is not usually spontaneous and requires a significant external stimulus for its onset. An accurate DFT benchmark study was then conducted to determine the most suitable exchange-correlation functional to accurately describe the reaction profile both in terms of structures and thermochemistry. This evaluation supports the use of the M06-2X functional as the best option for the study of ammonium nitrate decomposition and related reactions. Indeed, this level of theory provided the lowest deviations with respect to CBS-QB3 reference values, outperforming functionals especially developed for reaction kinetics.

Thermal stability of the N10 compound with extended nitrogen chain

1,1′-Azobis(tetrazole) (N10) as a potential eco-friendly material, has the highest nitrogen content among the azo-stabilized poly-nitrogen compounds. The detailed potential energy surface of N10 was thoroughly investigated using M06-2X, MP2 and CCSD(T) calculations, and the main decomposition pathways were calculated by canonical transition state theory modeling. Amongst all the channels studied, ring opening of the N10 compound, followed by N2 elimination to form the linear molecule Im8 is predicted to be the primary decomposition channel. The linear species formed (Im8) is characterized by a successive four-nitrogen atom chain stabilized by two terminal HNC groups. Its thorough decomposition reaction is strongly exothermic with a barrier height of 67.5 kcal mol−1. TST calculations were performed to probe the influence of temperature on the rate coefficients at 1 atm. Based on the thermal decomposition mechanism of N10, the novel species N12 and N14 were explored. These longer nitrogen chain compounds having higher nitrogen content than N10 are strongly expected to improve performance of high energy density materials. It is found that the predicted structures could exist at room temperature by comparison of their energetic barriers with the corresponding primary N2-elimination reaction of N10.

Thermal decomposition of 2-phenylethanol: A computational study on mechanism

Quantum mechanical calculations for the thermal decomposition of 2-phenylethanol have been performed using the CBS-QB3 method. Based on the potential energy surfaces at the CBS-QB3 level of theory, the preferred reaction channel for the thermal decomposition of 2-phenylethanol was the six-membered cyclic rearrangement reaction and the dehydration reaction to form styrene and H2O. Further quantum chemical calculations of the subsequent reactions followed by the six-membered cyclic rearrange reaction of 2-phenylethanol were carried out and it was revealed that the barrier height for the ring opening reaction was the lowest among all of the other subsequent reactions.

Dynamics of the reactions of O(1D) with CD3OH and CH3OD studied with time-resolved Fourier-transform IR spectroscopy

We investigated the reactivity of O((1)D) towards two types of hydrogen atoms in CH(3)OH. The reaction was initiated on irradiation of a flowing mixture of O(3) and CD(3)OH or CH(3)OD at 248 nm. Relative vibration-rotational populations of OH and OD (1 ≤ v ≤ 4) states were determined from their infrared emission recorded with a step-scan time-resolved Fourier-transform spectrometer. In O((1)D) + CD(3)OH, the rotational distribution of OD is nearly Boltzmann, whereas that of OH is bimodal; the product ratio [OH]/[OD] is 1.56 ± 0.36. In O((1)D) + CH(3)OD, the rotational distribution of OH is nearly Boltzmann, whereas that of OD is bimodal; the product ratio [OH]/[OD] is 0.59 ± 0.14. Quantum-chemical calculations of the potential energy and microcanonical rate coefficients of various channels indicate that the abstraction channels are unimportant and O((1)D) inserts into the C-H and O-H bonds of CH(3)OH to form HOCH(2)OH and CH(3)OOH, respectively. The observed three channels of OH are consistent with those produced via decomposition of the newly formed OH or the original OH moiety in HOCH(2)OH or decomposition of CH(3)OOH. The former decomposition channel of HOCH(2)OH produces vibrationally more excited OH because of incomplete intramolecular vibrational relaxation, and decomposition of CH(3)COOH produces OH with greater rotational excitation, likely due to a large torque angle during dissociation. The predicted [OH]/[OD] ratios are 1.31 and 0.61 for O((1)D) + CD(3)OH and CH(3)OD, respectively, at collision energy of 26 kJ mol(-1), in satisfactory agreement with the experimental results. These predicted product ratios vary weakly with collision energy.

The decomposition of aromatic hydrocarbons during coal pyrolysis in hydrogen plasma: A density functional theory study

Coal pyrolysis in hydrogen plasma has been proposed to undergo two steps. Volatiles such as aromatic hydrocarbons vaporize from coal and subsequently decompose to produce acetylene and hydrogen. We employed a density functional theory (DFT) to investigate the decomposition pathway of benzene, a model aromatic hydrocarbon, for understanding the coal pyrolysis in hydrogen plasma. The results indicate that there are two low-energy decomposition channels. Active hydrogen atoms in the plasma play an important role in the initiation of benzene decomposition, which leads to the formation of c-C6H5 particle and hydrogen molecule. The c-C6H5 could further decompose to yield acetylene, hydrogen and carbon soot, which is more favorable based on its lower activation energies. The decomposition makes the primary contribution to acetylene formation, and the dehydrogenation results in the additional hydrogen gas and serious coking. The active hydrogen atoms in plasma can remarkably lessen the energy barriers required for the reactions.

Redox properties of [Fe2(SC6H5)2(NO)4]: an experimental study and quantum chemical modeling

Reduction of the complex [Fe2(SC6H5)2(NO)4] in an aprotic solvent was studied by cyclic voltammetry in a wide range of potential scan rates. It was established that transfer of the first electron is reversible and the redox potential of this reaction was determined. Further reduction of the complex is irreversible because the product of attachment of the second electron is unstable and partially decomposes during the characteristic time of potential scan. The molecular and electronic structures of mono- and dianion of the complex as well as its theoretical redox potential value were calculated using the density functional theory methods with the local (BP86) and hybrid (B3LYP) functionals. The former functional better describes the geometry of the complex while the latter gives a better insight into its electronic structure. The extra negative charge is delocalized over NO groups, phenyl ligands, and iron atoms. The calculated redox potentials of one-electron reduction of the complexes are close to the experimental values obtained by analyzing cyclic voltammograms. Attachment of the second electron opens the decomposition channel of the complex, which is also consistent with experimental data.

Thermal Decomposition of 1-Pentanol and Its Isomers: A Theoretical Study

Pentanol is one of the promising "next generation" alcohol fuels with high energy density and low hygroscopicity. In the present work, dominant reaction channels of thermal decomposition of three isomers of pentanol: 1-pentanol, 2-methyl-1-butanol, and 3-methyl-1-butanol were investigated by CBS-QB3 calculations. Subsequently, the temperature- and pressure-dependent rate constants for these channels were computed by RRKM/master equation simulations. The difference between the thermal decomposition behavior of pentanol and butanol were discussed, while butanol as another potential alternative alcohol fuel has been extensively studied both experimentally and theoretically. Rate constants of barrierless bond dissociation reactions of pentanol isomers were treated by the variational transition state theory. The comparison between various channels revealed that the entropies of variational transition states significantly impact the rate constants of pentanol decomposition reactions. This work provides sound quality kinetic data for major decomposition channels of three pentanol isomers in the temperature range of 800-2000 K with pressure varying from 7.6 to 7.6 × 10(4) Torr, which might be valuable for developing detailed kinetic models for pentanol combustion.

Photodissociation of pyrrole-ammonia clusters below 218 nm: Quenching of statistical decomposition pathways under clustering conditions

The excited state hydrogen transfer (ESHT) reaction in pyrrole-ammonia clusters (PyH·(NH(3))(n), n = 2-5) at excitation wavelengths below 218 nm down to 199 nm, has been studied using a combination of velocity map imaging and non-resonant detection of the NH(4)(NH(3))(n-1) products. Special care has been taken to avoid evaporation of solvent molecules from the excited clusters by controlling the intensity of both the excitation and probing lasers. The high resolution translational energy distributions obtained are analyzed on the base of an impulsive mechanism for the hydrogen transfer, which mimics the direct N-H bond dissociation of the bare pyrrole. In spite of the low dissociation wavelengths attained (~200 nm) no evidence of hydrogen-loss statistical dynamics has been observed. The effects of clustering of pyrrole with ammonia molecules on the possible statistical decomposition channels of the bare pyrrole are discussed.

How are nematode communities affected during a conversion from conventional to organic farming in southern French vineyards?

The rate of conversion from conventional vineyards to organic farming practices is increasing. Organic farming improves some soil properties, although some organic practices have negative effects on soils. The objective of this work was to study the long-term effects of organic farming through the use of soil nematodes as bio-indicators of soil processes. Our experimentation was conducted in a commercial vineyard where plots belonged to two types of viticulture: conventional viticulture and organic viticulture (for 7, 11 and 17 years). The nematode community structure and nematode indices were determined. The main result was that organic practices increased soil nematode density. An increase in the available resources, as measured by a higher enrichment index (EI), led to an increase in the microbial feeder density and mainly opportunistic fungal-feeding nematodes. A greater density of plant-feeding nematodes was attributed to the presence of a grass cover. The functioning of the soil was shifted with the decomposition channel of the soil organic matter becoming more fungal than bacterial. Even though changes were observed in the nematode community structure following the conversion, the maturity index (MI), the plant-parasitic index (PPI) and the structure index (SI) remained constant. Consequently, the organic practices did not improve the soil food web length or complexity even though the biological activity, as measured by microbial biomass and total nematode density, increased.

Multi-species measurements in 1-butanol pyrolysis behind reflected shock waves

The kinetics of 1-butanol pyrolysis were investigated by measuring multi-species time histories using shock tube/laser absorption methods. Species time histories of OH, H2O, C2H4, CO, and CH4 were measured behind reflected shock waves using UV and IR laser absorption during the high-temperature decomposition of 1% 1-butanol/argon mixtures. Initial reflected shock temperatures and pressures for these experiments covered 1250–1650K and 1.3–1.9atm. Measured OH and H2O time histories are in good agreement with previous experimental studies; measured C2H4, CO, and CH4 time histories are the first reported for this fuel in shock tube experiments.Production pathways and sensitivities for the measured species are analyzed using the recent Sarathy et al. (2012) [37] detailed mechanism. Simulations using this mechanism underpredict H2O, OH, and C2H4 mole fractions, overpredict CH4 mole fractions, and significantly underpredict CO mole fractions at early times. As discussed in past papers and confirmed in this study, the branching ratios of H abstraction rates from 1-butanol, which are not precisely known, can significantly affect H2O time history simulations. These simulations show that H2O is produced primarily through H-atom abstraction from 1-butanol by OH, and therefore H2O time histories are extremely sensitive to 1-butanol decomposition channels that contribute to the OH radical pool. Simulations also show that more C2H4 would be produced by faster decomposition of 1-butanol through several channels that also affect H2O production. Finally, simulations show that CO time histories are strongly sensitive to 1-butanol decomposition into nC3H7 and CH2OH, especially at early times. Evidence is presented that indicates this decomposition pathway is too slow in the simulations by a factor of three to five at conditions of the current study.

Dissociation of dimethyl ether at high temperatures

The decomposition of dimethyl ether (CH3OCH3) has been investigated behind incident shock waves in a diaphragmless shock tube using laser schlieren densitometry, LS (T=1500–2450K, P=57±4, 125±5 and 253±12Torr). The LS density gradient profiles were simulated and excellent agreement was found between the simulations and experimental profiles. Rate coefficients for CH3OCH3→CH3O+CH3 were obtained. They showed strong fall-off, and at the lower end of the experimental temperature range are close to the low pressure limit. First order rate coefficient expressions were determined over 1500<T<2450K. k57Torr=(3.10±1.0)×1079T−19.03exp(−54417/T)s−1, k125Torr=(1.12±0.3)×1083T−19.94exp(−55554/T)s−1and k253Torr=(1.02±0.3)×1073T−17.09exp(−51500/T)s−1. The effect of a roaming channel for decomposition of dimethyl ether was assessed and the best agreement was obtained with 1% dissociation of DME via the roaming path.

Theoretical Study of the Thermal Decomposition of the 5-Methyl-2-furanylmethyl Radical

The thermal decomposition of the 5-methyl-2-furanylmethyl radical (R(1)), the most important primary radical formed during the combustion and thermal decomposition of 2,5-dimethylfuran (a promising next-generation biofuel), was studied using CBS-QB3 calculations and master equation (ME)/RRKM modeling. Because very little information is available in the literature, the detailed potential energy surface (PES) was investigated thoroughly. Only the main pathways, having a kinetic influence on the decomposition of R(1), were retained in the final ME/RRKM model. Among all the channels studied, the ring-opening of the 5-methyl-2-furanylmethyl radical, followed by ring enlargement to form cyclohexadienone molecules is predicted to be the easiest decomposition channel of R(1). The C(6) cyclic species formed can undergo unimolecular reactions to yield phenol and to a lesser extent cyclopentadiene and CO. Our calculations predict that these species are important products formed during the pyrolysis of 2,5-dimethylfuran (DMF). Other channels involved in the decomposition of R(1) lead directly to the formation of linear and cyclic unsaturated C(5) species and constitute an additional source of cyclopentadiene and CO. High-pressure limit rate constants were computed as well as thermochemical properties for important species. ME/RRKM analysis was performed to probe the influence of pressure on the rate coefficients and pressure dependent rate coefficients were proposed for pressures and temperatures ranging, respectively, from 10(-2) bar to 10 bar and 1000 to 2000 K.

Hydrogen production by methanol decomposition using gliding arc gas discharge

AbstractDirect decomposition of methanol has been investigated using gliding arc gas discharge (GRD) at atmospheric pressure. Depending on the experimental conditions of Ar flow rate, methanol concentration, the electrode gap, input voltage and vaporization room temperature (VRT), different conversions are achieved ranging from 51.0% to 81.7%. Interestingly, the selectivity to the production of hydrogen and carbon monoxide is kept almost constant under all the experimental conditions. The formation of little methane and C2Hx as a byproduct, and trace quantity of carbon dioxide are detected. The reaction channels of methanol decomposition induced by GRD plasma is proposed in detail.

Experimental and Theoretical Study on Pyrolysis of Isopsoralen

The pyrolysis of isopsoralen was studied by synchrotron vacuum ultraviolet photoionization mass spectrometry at low pressure. The pyrolysis products were detected at different photon energies, the ratios of products to precursor were measured at various pyrolysis temperatures. The experimental results demonstrate that the main pyrolysis products are primary CO and sequential CO elimination products (C10H6O2 and C9H6O). The decomposition channels of isopsoralen were also studied by the density functional theory, then rate constants for competing pathways were calculated by the transition state theory. The dominant decomposition channels of isopsoralen and the molecular structures for corresponding products were identified by combined experimental and theoretical studies.

Theoretical Studies on the Unimolecular Decomposition of Propanediols and Glycerol

Polyols, a typical type of alcohol containing multiple hydroxyl groups, are being regarded as a new generation of a green energy platform. In this paper, the decomposition mechanisms for three polyol molecules, i.e., 1,2-propanediol, 1,3-propanediol, and glycerol, have been investigated by quantum chemistry calculations. The potential energy surfaces of propanediols and glycerol have been built by the QCISD(T) and CBS-QB3 methods, respectively. For the three molecules studied, the H(2)O-elimination and C-C bond dissociation reactions show great importance among all of the unimolecular decomposition channels. Rate constant calculations further demonstrate that the H(2)O-elimination reactions are predominant at low temperature and pressure, whereas the direct C-C bond dissociation reactions prevail at high temperature and pressure. The temperature and pressure dependence of calculated rate constants was demonstrated by the fitted Arrhenius equations. This work aims to better understand the thermal decomposition process of polyols and provide useful thermochemical and kinetic data for kinetic modeling of polyols-derived fuel combustion.

Response of soil nematode community structure and diversity to long‐term land use in the black soil region in China

AbstractSoil nematodes are sensitive to environmental changes and are used widely as indicators of soil conditions. The community structure and diversity of soil nematodes were studied in different long‐term land use regimes in the black soil area in Northeast China. The land use regimes were maintained for 22 years, and included crop land (CL), grass land (GL) and bare land (BL). Soil samples were taken throughout the growing season, and nematodes were extracted and identified. A total of 39 nematode genera with relative abundance over 0.1 % were identified. Heterodera was the dominant genus in CL; Boleodorus was the dominant genus in GL, and Boleodorus, Eucephalobus and Filenchus were the dominant genera in BL. Land use had a significant effect on abundance of all soil nematode tropic groups and ecological indices. Sampling time had an effect on soil nematode abundance, but only on three of the eight nematode ecological indices MI (maturity index of free‐living nematode), CI (channel index) and EI (enrichment index). SR (species richness index) was highest in GL where plant species diversity was also high. The CI was the highest in BL among three land uses, which means the soil food web dominated, with fungal decomposition channels in BL. Soil nematode community structure and diversity was shown to be an effective and informative tool for analyzing ecological aspects of land use in black soil regions. The data are inconclusive as to whether the effect of land use on soil nematode parameters is direct, or indirect via inducing changes in soil physicochemical properties.

Computational study on the thermal decomposition and isomerization of the CH3OCF2O• radical

The geometries of the reactant, products, and transition states involved in the decomposition pathways of the CH3OCF2O• radical formed during the photooxidation of CH3OCHF2 (HFE-152a) have been optimized and characterized at the DFT-B3LYP level of theory using the 6–311G(d,p) basis set. Single-point energy calculations have been made at the G2M (CC,MP2) level of theory. Out of the four prominent decomposition channels considered, the β-C–O bond scission is found to be the dominant path involving a barrier height of 9.78 kcal mol–1 (1 cal = 4.184 J). The thermal rate constant for the above decomposition pathway is evaluated using canonical transition state theory (CTST) and was found to be 5.27 × 104 s–1 at 298 K and 1 atm (1 atm = 101.325 kPa).

An experimental and kinetic modeling study of cyclohexane pyrolysis at low pressure

The pyrolysis of cyclohexane at low pressure (40mbar) was studied in a plug flow reactor from 950 to 1520K by synchrotron VUV photoionization mass spectrometry. More than 30 species were identified by measurement of photoionization efficiency (PIE) spectra, including some radicals like methyl, propargyl, allyl and cyclopentadienyl radicals, and stable products (e.g., 1-hexene, benzene and some aromatics). Among all the products, 1-hexene is formed at the lowest temperature, indicating that the isomerization of cyclohexane to 1-hexene is the dominant initial decomposition channel under the condition of our experiment. We built a kinetic model including 148 species and 557 reactions to simulate the experimental results. The model satisfactorily reproduced the mole fraction profiles of most pyrolysis products. The rate of production (ROP) analysis at 1360 and 1520K shows that cyclohexane is consumed mainly through two reaction sequences: cyclohexane→1-hexene→allyl radical+n-propyl radical, and cyclohexane→cyclohexyl radical→hex-5-en-1-yl radical that further decomposes to 1,3-butadiene via hex-1-en-3-yl and but-3-en-1-yl radicals. Besides the stepwise dehydrogenation of cyclohexane, C3+C3 channels, i.e. C3H3+C3H3 and C3H3+aC3H5 also have important contribution to benzene formation. The simulation reveals that C3H3+C3H3=phenyl+H reaction is the key step for other aromatics formation, i.e. toluene, phenylacetylene, styrene, ethylbenzene and indene in this work.

N2 emission-channel change in NO reduction over stepped Pd(211) by angle-resolved desorption

A sharp change in the N2 emission channel from N2O(a)→N2(g)+O(a) to N(a)+N(a)→N2(g) has been found at around 500K in a steady-state NO+D2 reaction over stepped Pd(211)=[(S)3(111)×(100)] by means of angle-resolved desorption. The desorbing N2 is highly collimated at around 30° off normal toward the step-down direction below about 500K due to the intermediate N2O decomposition, whereas, above 500K, the near normally directed desorption due to the recombination of N(a) is relatively enhanced. The N2O decomposition channel is promoted when the reaction is carried out with hydrogen (deuterium) and the channel change is accelerated by quick changes of the amounts of surface hydrogen and oxygen (or NO(a)) into the opposite directions, and enhanced nitrogen removal as ammonia on the resultant hydrogen-rich surface. In the steady-state NO+CO reaction, the N2 emission channel gradually changes above 500K toward recombination. A model for the off-normal N2 emission is briefly described.

Binding and structure of tetramers in the scaling limit

The momentum-space structure of the Faddeev-Yakubovsky (FY)components of weakly-bound tetramers is investigated at the unitary limit using a renormalized zero-range two-body interaction. The results, obtained by considering a given trimer level with binding energy $B_3$, provide further support to a universal scaling function relating the binding energies of two successive tetramer states. The correlated scaling between the tetramer energies comes from the sensitivity of the four-boson system to a short-range four-body scale. Each excited $N-$th tetramer energy $B_4^{(N)}$ moves as the short-range four-body scale changes, while the trimer properties are kept fixed, with the next excited tetramer $B_4^{(N+1)}$ emerging from the atom-trimer threshold for a universal ratio $B_4^{(N)}/B_3 = B_4^ {(N)}/B_4^{(N+1)} \simeq 4.6$, which does not depend on $N$. We show that both channels of the FY decomposition [atom-trimer ($K-$type) and dimer-dimer ($H-$type)] present high momentum tails, which reflect the short-range four-body scale. We also found that the $H-$channel is favored over $K-$channel at low momentum when the four-body momentum scale largely overcomes the three-body one.