Role of vapour oversaturation in the thermal decomposition of solids

Elementary thermochemical calculations show that in all cases of formation of solid product in the process of the congruent dissociative vaporization of reactants, the equilibrium partial pressure of the main product greatly exceeds its saturation vapour pressure, and therefore causes the appearance of vapour oversaturation. The oversaturation is responsible for the formation and growth of nuclei, their shape and position, the transfer of condensation energy to the reactant, the existence of induction and acceleration decomposition periods, the reaction localization, the epitaxial/topotaxy effects and the nanocrystal structure of the solid product. Variations in the energy transfer explain an increase of the molar enthalpy with temperature and the decelerating influence of melting on the rate of decomposition.     

Mechanism of the thermal decomposition of nitrates from graphite furnace mass spectrometry studies

A basically new mechanism of the thermal decomposition of solids is proposed to explain the mass spectral observations of gaseous molecules of CoO, CuO, Cu₂O, NiO, PbO and Mg(OH)₂ during the low-temperature decomposition of the anhydrous and hydrated nitrates of these metals. The mechanism consists of two stages: congruent gasification of all reaction products irrespective of their saturated vapor pressure and subsequent condensation of the low-volatility species (oxides and hydroxides). The partial pressures of these species at the appearance temperatures calculated from this theory for the first stage of the process (1–50 mPa) are in agreement with the detection limits of the quadrupole mass spectrometers used in these experiments. The proposed mechanism is supported by other available data obtained by thermal analysis.     

A DFT theoretical analysis of aldehyde condensation pathways onto methyllithium, lithium dimethylamide, and their aggregates

A DFT analysis of the condensation of monomeric methyllithium and lithium dimethylamide (LMA), as well as their homo and hetero dimers, on formaldehyde and acetaldehyde is reported. A stable complex, exhibiting a directional interaction between a lone pair of the oxygen on the aldehyde and a lithium, is first found. At this stage, the aldehyde carbonyl and the Li-X (X = C or N) bonds lie in the same plane. To proceed, the condensation reaction has to go through a transition state that mainly consists of a rotation of the aldehyde plane, placing it perpendicular to the C-C or C-N forming bond. The reaction then leads, in a strongly exothermic final step, to the addition product that is a lithium alcoholate or alpha-amino alcoholate, associating into an hetero-aggregate with the remaining moiety of the initial dimer. From the relative heights of the activation barriers, it appears that, for the heterodimer MeLi-LMA, the formation of the C-N bond should be kinetically favored over the C-C one, while the lithium ethylate resulting from the C-C binding is the thermodynamic product. A decomposition of the activation energy barriers has been carried out in order to determine the physicochemical forces responsible for the variation of the condensation activation barriers with the structure of the final species formed. The results obtained are discussed in relation with corresponding experimental data.     

Coverage dependence of oxygen decomposition and surface diffusion on rhodium 111: a DFT study

A systematic study of oxygen adsorption, decomposition and diffusion on Rh111 and its dependence on coadsorbed oxygen molecules has been performed using density functional theory calculations. First, the bonding strength between metal surface and adsorbed oxygen molecules has been studied as a function of initial oxygen coverage. The bonding strength decreases with increasing oxygen coverage, which points towards a self-inhibition of the adsorption process. The potential energy hypersurface (PES) for the dissociation of oxygen molecules adsorbed on a threefold fcc position perpendicular to the surface was calculated using a combined linear/quadratic synchronous transit method with conjugate gradient refinements. The results indicate that a minor amount of oxygen on the surface enhances the decomposition of further oxygen molecules, while this process is inhibited at higher coverage. Moreover, PES calculations of a single site jump of atomic oxygen on rhodium 111 indicate that the activation energy increases as well with increasing oxygen coverage. All results are discussed with respect to a rhodium based catalytic NOx reduction/decomposition system proposed by Nakatsuji, which decomposes nitrogen oxides in oxygen excess.     

Excited electronic state decomposition of furazan based energetic materials: 3,3'-diamino-4,4'-azoxyfurazan and its model systems, diaminofurazan and furazan

We report the first experimental and theoretical study of gas phase excited electronic state decomposition of a furazan based, high nitrogen content energetic material, 3,3'-diamino-4,4'-azoxyfurazan (DAAF), and its model systems, diaminofurazan (DAF) and furazan (C2H2N2O). DAAF has received major attention as an insensitive high energy explosive; however, the mechanism and dynamics of the decomposition of this material are not clear yet. In order to understand the initial decomposition mechanism of DAAF and those of its model systems, nanosecond energy resolved and femtosecond time resolved spectroscopies and complete active space self-consistent field (CASSCF) calculations have been employed to investigate the excited electronic state decomposition of these materials. The NO molecule is observed as an initial decomposition product from DAAF and its model systems at three UV excitation wavelengths (226, 236, and 248 nm) with a pulse duration of 8 ns. Energies of the three excitation wavelengths coincide with the (0-0), (0-1), and (0-2) vibronic bands of the NO A 2Sigma+<--X 2Pi electronic transition, respectively. A unique excitation wavelength independent dissociation channel is observed for DAAF, which generates the NO product with a rotationally cold (20 K) and a vibrationally hot (1265 K) distribution. On the contrary, excitation wavelength dependent dissociation channels are observed for the model systems, which generate the NO product with both rotationally cold and hot distributions depending on the excitation wavelengths. Potential energy surface calculations at the CASSCF level of theory illustrates that two conical intersections between the excited and ground electronic states are involved in two different excitation wavelength dependent dissociation channels for the model systems. Femtosecond pump-probe experiments at 226 nm reveal that the NO molecule is still the main observed decomposition product from the materials of interest and that the formation dynamics of the NO product is faster than 180 fs. Two additional fragments are observed from furazan with mass of 40 amu (C2H2N) and 28 amu (CH2N) employing femtosecond laser ionization. This observation suggests a five-membered heterocyclic furazan ring opening mechanism with rupture of a CN and a NO bond, yielding NO as a major decomposition product. NH2 is not observed as a secondary decomposition product of DAAF and DAF.     

Fast H-vacancy dynamics during alanate decomposition by anelastic spectroscopy. proposition of a model for Ti-enhanced hydrogen transport

A systematic study of the dehydrogenation process of undoped and of catalyzed NaAlH4 by means of anelastic spectroscopy is presented. Evidence is reported of the formation of a highly mobile species during decomposition, which has been identified in off-stoichiometric AlH6-x units, giving rise to fast H vacancy local dynamics. The formation of such stoichiometry defects starts at temperatures much lower in Ti doped than in undoped samples, and concomitantly with the decomposition reaction. The catalyst atoms decrease the energy barrier to be overcome by H to break the bond, thus enhancing the kinetics of the chemical reactions and decreasing the temperature at which the dehydrogenation processes take place. The experimental data show that not all the hydrogen released by the formula units during the evolution of decomposition evolves out of the sample, but part of it remains in the lattice and migrates on a long-range scale within the sample. We identify, in this H mobilized population, the species which induces the fast tetragonal to monoclinic phase transformation accompanying decomposition. A partial spontaneous thermally activated regression of decomposition has also been observed by aging experiments. A model is proposed which accounts for the action of the Ti catalyst and for the atomistic mechanism of decomposition.     

Synthesis and characterization of silicate polymers

In this work an inorganic polymer is developed based on Elkem microsilica and potassium hydroxide. Using experimental data and the partial charge model a model for the gelation is suggested based on the hydrolysis and condensation reactions occurring during synthesis. In addition the optimal composition of the binder system was determined using compressive strength test and solubility experiments. Based on partial charge calculations and experimental data for the hydroxide concentration and the viscosity obtained in this study it is suggested that the polymerization of the inorganic polymer is determined by the concentration of silica species. It was found that the alkalinity has a crucial effect on the condensation process. The optimal potassium hydroxide concentration used in the synthesis of the inorganic polymer was found to be around 3.5 M, which resulted in a compressive strength of the product in the region of 50 MPa.     

Controlled rate thermal analysis commanded by mass spectrometry for studying the kinetics of thermal decomposition of very stable solids

A quadrupole mass spectrometer coupled to a thermobalance has been used to develop a controlled rate thermal analysis (CRTA) equipment. This instrument maintains constant the partial pressure of the gas generated in a solid–gas reaction during the entire process while recording the weight loss along the process. This system permits to perform the kinetic analysis of the thermal decomposition of very stable compounds with a very low equilibrium pressure in the temperature range at which the reaction takes place. The performance of the equipment was checked by studying the kinetics of the thermal decomposition of BaCO₃ using for the control the CO₂ partial pressure. Both the activation energy and the reaction kinetic model has been calculated. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 184–192, 2002; DOI 10.1002/kin.10042     

用于离子迁移率谱仪的快速热解析进样方法

热解析是用于离子迁移率谱仪(IMS)的液体和固体样品进样的重要手段,但在检测高沸点或易分解有机物时,存在易重新凝结或过热分解的问题.本研究建立了一种与IMS半透膜结合的热解析进样方法,设计了可快速加热的微型热解析装置,通过热子通电发热及热电偶测温实现对样品解析区域的升温控制,提高半透膜的进样效率.以低浓度三聚氰胺溶液为...     

Theory of solid-state decomposition reactions: A historical essay

The history of the formation and development of the theory of solid-state decomposition reactions, based on the mechanism of congruent dissociative vaporization of a solid with simultaneous condensation of the supersaturated vapor of the low-volatility product, is described here in the form of recollections. The review covers a 30-year period (1981-2010), beginning with basic experimental studies in the decomposition process by electrothermal atomic absorption spectrometry (ETAAS) and quadrupole mass spectrometry (QMS) and ending with measurements of the decomposition kinetics by thermogravimetric analysis. Some remarks and recommendations based on the author's participation in two long-term projects, the development of the ETAAS method and that of the theory of solid-state reactions, are formulated in the conclusion.     

Concept of variable activation energy and its validity in nonisothermal kinetics

The concept of variable activation energy in solid-state kinetics under nonisothermal conditions has been suffering from doubt and controversy. Rate equations of nonisothermal kinetics of solid decomposition, which involve the factors of thermodynamics conditions, pressure of gaseous product, structure parameters of solid, and/or extent of conversion, are derived from the models of the interface reaction, the diffusion of gaseous product, and the nuclei growth of the solid product, respectively. The definition of the validity function in the rate equations represents the influence of the factors on the reaction rate. A function of variable activation energy depending on the validity function is also developed. The changing trend and degree of activation energy are extrapolated from the function of variable activation energy and based on the data of nonisothermal thermal decomposition of calcium carbonate. It is shown that the concept of variable activation energy is meaningfully applicable to solid-state reactions under nonisothermal conditions.     

Vapor detection enabled by self-assembled colloidal photonic crystals

Here we report the sensitive and reversible detection of vapors by using self-assembled colloidal photonic crystals. The condensation of various vapors in the interstitials of silica colloidal photonic crystals leads to red-shift and amplitude reduction of optical stop bands. A linear relationship between wavelength shift and vapor partial pressure has been observed for a variety of vapors including ethanol, water, and toluene. Importantly, the sensitivity of colloidal photonic crystal-based vapor detectors can be improved by nearly two orders of magnitude by using a new full-peak analysis technique that takes advantage of the manifest amplitude reduction of optical stop bands during vapor condensation. Optical simulation based on a scalar-wave approximation model shows that the predicted optical responses during vapor condensation in colloidal photonic crystals agree well with experimental results. The condensation of vapors between submicrometer-scale microspheres, a topic that has received little examination, has also been investigated by both experiments and theoretical calculations. Predictions based on a modified Kelvin equation match with the experiments for a wide range of vapor partial pressures.     

Theoretical study of HKrOX (X = F, Cl, Br and I): structure, anharmonic vibrational spectroscopy, stability and bonding

The noble-gas molecules, HKrOX (with X = F, Cl, Br and I), have been investigated by ab initio calculation. Equilibrium geometry, harmonic and anharmonic vibrational frequencies, energies, partial charges are calculated. All HKrOX molecules studied here are bound equilibrium structures with Cs symmetry. The frequency calculation indicates that the H-Kr stretching mode is anharmonic and is very likely to be observed in the experiments. The two-body decomposition reaction is exothermic and lead to products of Kr as well as HOX, while the three-body decomposition reaction is also exothermic with respect to the neutral decomposition products (H + Kr + OX). Moreover, HKrOX is kinetically stable with respect to the decomposition reactions due to the enough high energy barriers, which indicates the possibility to identify these HKrOX compounds in noble-gas matrices. The bonding in HKrOX is studied by QTAIM analysis and the localized molecular orbital energy decomposition analysis (LMO-EDA) method at the MP2 level of theory with a large basis set. The results show that HKrOX is a typical ionic bond, denoted as (HKr)(+)(OX)(-), and the electrostatic interaction between (HKr)(+) and (OX)(-) makes the main contribution to the ionic bond.     

Partial reduction of pyrroles: application to natural product synthesis

The partial reduction of N-Boc pyrroles has been explored giving stereoselective routes to disubstituted pyrrolines in good yields and with excellent diastereoselectivities. A novel methodology has been developed to carry out reductive aldol reactions on 2-substituted N-Boc pyrroles; use of aldehydes under reductive aldol conditions gave the anti aldol product in good selectivity. This chemistry was used as the key transformation in a synthesis of omuralide, which was achieved in 13 steps and 14% overall yield. We also report a methodology for selectively forming either cis or trans 2,5-disubstituted pyrrolines via a partial reduction of an electron-deficient N-Boc pyrrole. The trans pyrroline formed using this route was utilized in the syntheses of the polyhydroxylated pyrrolizidine natural products hyacinthacine A1 and 1-epiaustraline. Further investigation has led to the development of routes to enantiopure substituted pyrroline compounds. This has been achieved via a chiral protonation approach using easily accessible chiral acids, such as ephedrine and oxazolidinones, to quench enolates formed during the partial reduction process. Alternatively, enzymatic desymmetrization of symmetrical diol compounds formed from the partial reduction products of substituted pyrroles is also reported. This leads to formation of both enantiomers of 2,2- and 2,5-disubstituted N-Boc pyrrolines in excellent ee and yields.     

Studies of the thermal degradation of acetaminophen using a conventional HPLC approach and electrospray ionization-mass spectrometry

The thermal degradation of acetaminophen is studied via conventional accelerated aging studies by initially thermally stressing the compound at temperatures between 160 degrees C and 190 degrees C and measuring the rate of decomposition by reversed-phase high-performance liquid chromatography. Rates of decomposition of the compound in the dry state and the activation energy for the process are determined and compared with previously published kinetic and thermodynamic data for the degradation of acetaminophen in solution. In addition, the thermal fragmentation of acetaminophen under electrospray ionization (ESI) conditions using an interface with a heated capillary inlet is studied and the apparent activation energy for this process also is characterized. A comparison of the data shows that acetaminophen is significantly more stable in the dry state than in solution. However, the gas-phase fragmentation of acetaminophen under ESI conditions occurs more readily than either dry- or solution-state degradation. Although the resulting electrospray fragmentation mimics the breakdown product that is formed when the compound undergoes either acid or base catalyzed hydrolysis in aqueous solutions, the mechanism that produces the fragment ion appears to involve a two-step process. Initially, the parent ion forms of the analyte are produced in the spray region of the interface followed by wall-catalyzed decomposition and re-ionization in the heated inlet capillary of the spectrometer.     

Oxygen reduction reaction on polycrystalline gold. Pathways of hydrogen peroxide transformation in the acidic medium

The mechanism of oxygen electroreduction on polycrystalline gold is studied in the acidic medium. Hydrogen peroxide is the main reaction product. However, two potential regions can be singled out in which the oxygen electroreduction reaction proceeds by different pathways. The first region is the potential interval close to the steady-state potential. Here, the oxygen electroreduction virtually completely produces peroxide. The second interval is the potential range of considerable cathodic polarization values. In this case, peroxide can be reduced to water. The low energy of hydrogen peroxide adsorption on gold determines the considerable overpotential of peroxide reduction. It is shown that on the gold electrode surface, the catalytic decomposition of peroxide occurs. The use of the method of electrochemical impedance spectroscopy allows the peculiarities of the oxygen reaction associated with hydrogen peroxide transformations to be revealed. In the acidic medium, the reactions of consecutive reduction of oxygen through the intermediate formation of hydrogen peroxide and the catalytic decomposition of the intermediate product are shown to proceed simultaneously. The ratio of rate constants of electrochemical stages depends on the potential. The chemical decomposition is observed both near the steady-state potential and in the cathodic region where considerable electrochemical reduction of peroxide occurs.     

Potential of gas chromatography in the determination of low-volatile dicarboxylic acids

The analysis of published data shows that the results of the gas chromatographic determination of low-volatile polar aliphatic dicarboxylic acids are rather poorly reproducible. A considerable part of the published values of their retention indices appears to be erroneous. The experimental verification of the capabilities of gas chromatography and gas chromatography–mass spectrometry demonstrates that some of the compounds of this series (for example, glutaric acid) can be determined without decomposition; others are characterized by the formation of products of interaction with solvents (oxalic acid), and in some cases, the only detectable moieties are products of thermal degradation (citric acid).     

Characterization of the surface content,hydrolysis ratio,and condensation degree of polyalkoxysiloxane segregated to the surface of a polyurethane crosslinked film by X‐ray photoelectron spectroscopy

We report, using an electron spectrometer equipped with both monochromatized Al Kα and unmonochromatized Mg Kα sources, the quantitative characterization of the surface content, hydrolysis ratio, and condensation degree of polyalkoxysiloxane segregated to the surface of a polyurethane crosslinked film consisting of acryl polyol, polyisocyanate, and polyalkoxysiloxane. Unmonochromatized Mg Kα X‐ray irradiation extremely accelerated the decomposition of alkoxy groups of polyalkoxysiloxane. The surface content and hydrolysis ratio were determined from C 1s, Si 2p, and N 1s spectral intensities measured with monochromatized Al Kα X rays after decomposition by unmonochromatized Mg Kα X‐ray irradiation. The condensation degree was determined by the kinetic energy of the silicon KLL Auger electron after decomposition. We applied the established characterization method for a polyurethane film containing polyalkoxysiloxane. After 20 days, the surface content of polyalkoxysiloxane was greater than 60 wt %, the hydrolysis ratio ranged from 0.8 to 0.95, and the higher hydrolysis ratio resulted in a larger condensation degree. The hydrophilicity of the film surface became higher as the surface content and hydrolysis ratio increased, and the crack density became higher as the condensation degree increased. A method for characterizing the practical properties of coating film surfaces containing polyalkoxysiloxane was established. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2917–2926, 2002     

Phase diagrams of metal systems with highly volatile components: Simulation and experiments

A critical analysis is performed of existing literature data on the melting diagrams of systems whose components are a low-volatile metal and a high-volatile component. Models for the р–Т phase diagrams of binary systems are developed, and characteristic isobaric sections (partial state diagrams) are presented that correspond to the experimental data for real systems.     

Decomposition of pentaerythritol tetranitrate [C(CH2ONO2)4] following electronic excitation

We report the experimental and theoretical study of the decomposition of gas phase pentaerythritol tetranitrate (PETN) [C(CH(2)ONO(2))(4)] following electronic state excitation. PETN has received major attention as an insensitive, high energy explosive; however, the mechanism and dynamics of the decomposition of this material are not clear yet. The initial decomposition mechanism of PETN is explored with nanosecond energy resolved spectroscopy and quantum chemical theory employing the ONIOM algorithm at the complete active space self-consistent field (CASSCF) level. The nitric oxide (NO) molecule is observed as an initial decomposition product from PETN at three UV excitation wavelengths (226, 236, and 248 nm) with a pulse duration of 8 ns. Energies of the three excitation wavelengths coincide with the (0-0), (0-1), and (0-2) vibronic bands of the NO A (2)Σ(+) ← X (2)Π electronic transition, respectively. A unique excitation wavelength independent dissociation channel is observed for PETN, which generates the NO product with a rotationally cold (~20 K) and a vibrationally hot (~1300 K) distribution. Potential energy surface calculations at the ONIOM(CASSCF:UFF) level of theory illustrate that conical intersections play an important role in the decomposition mechanism. Electronically excited S(1) PETN returns to the ground state through the (S(1)/S(0))(CI) conical intersection, and undergoes a nitro-nitrite isomerization to generate the NO product.