A comparative study of the thermal reactivities of some transition metal oxalates in selected atmospheres

A comparative investigation has been made of the nonisothermal, solid-state thermal decompositions of the oxalates of six divalent transition metals (cations: manganese, iron, cobalt, nickel, copper and zinc) in alternative flowing atmospheres, inert (N₂, CO₂), reducing (H₂) and oxidizing (air). Derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC) response peak maxima, providing a measure of reaction temperatures, have been used to determine salt reactivities and thus to characterize the factors that control the relative stabilities of this set of chemically related reactants. Two trends were identified. Trend (1): in the inert and reducing atmospheres, the decomposition temperature (salt stability) increased with rise in enthalpy of formation of the divalent transition metal oxide, MO. It is concluded that the rupture of the cation-oxygen (oxalate) bond is the parameter that determines the stability of salts within this set. Trend (2): the diminution of decomposition temperatures from values for reactions in inert/reducing atmosphere to those for reactions in an oxidizing atmosphere increased with the difference in formation enthalpy between MO and the other participating oxide (MO_3/2 or MO_1/2). The change of cation valence tended to promote reaction, most decompositions in O₂ occurred at lower temperatures, but the magnitude of the effect varied considerably within this set of reactants. Observed variations in stoichiometric and kinetic characteristics with reaction conditions are discussed, together with the mechanisms of thermal decompositions of these solid oxalates.This approach to the elucidation of crystolysis reaction mechanisms emphasizes the value of comparative investigations within the group of chemically related reactants. Previous isothermal kinetic studies had been made for each of the reactants selected here. From these, much has been learned about the form of the (isothermal) solid-state yield-time curves, often interpreted to provide information about the geometry of interface development for the individual rate processes. However, identification of the controls of reactivity, reaction initiation (nucleation) and advance (nucleus growth), is much more difficult to characterize and less progress has been made towards elucidation of the interface chemistry. The trends of reactivity changes with salt compositions, identified here, offer a complementary approach to that provided by the study of single salts. Much of the recent literature on thermal decompositions of solids has been concerned with individual reactants, but many results and conclusions are not presented in the widest possible perspective. Comparisons between systematically related reactants are identified here as providing a chemical context for the elucidation of the chemical steps that participate in interface reactions. The article advocates the use of a more chemical approach in investigations of crystolysis (solid-state chemical) reactions.     

Voltammetric response of the interface between single-crystal cerium fluoride and metallic microelectrodes

The processes occurring at the interface between single-crystal fluoride-conducting solid electrolyte CeF₃: Sr²⁺ and metallic microelectrodes of Bi, Sn, Sb, and Ag are studied by the method of cyclic voltammetry with use of traditional silver-silver chloride electrode or solid-phase reference electrode Sn, SnF₂. Responses of solid-phase reactions involving mobile fluoride ions of the solid electrolyte and the microelectrode material are obtained. It is suggested that signals relating to the reduction of fluorinated metals be used for local qualitative assay of metal traces contained in quantities of about a few nanograms on the surface of conducting solids in air.     

Kinetic Relationships between Reactions in the Gas Phase and in Solution

Some recent examples of reactions proceeding both in the gas phase and in solution have been investigated to determine their kinetics and mechanisms. The ratio of the corresponding rate constants, k_G and k_L, of the elementary processes studied has been found to be about unity for unimolecular reactions and between 1 and 10 for bimolecular reactions. The mechanisms, overall rates, rate constants, and activation energies have been determined for the homogeneous gas reaction NOCl + Cl₂O = NO₂Cl + Cl₂ and the reaction NOCl + N₂O₅ = NO₂Cl + 2 NO₂, carried out in C₂F₃Cl₃.     

Melting and thermal decompositions of solids

This analysis of interface phenomena considers the alternative processes that may result from heating a crystal, particularly including thermal decomposition, involving chemical reactions, and melting, involving loss of long-range structural order. Such comparisons are expected to provide insights into the factors that determine and control the different types of thermal changes of solids. The survey also critically reviews some theoretical concepts that are currently used to describe solid-state thermal reactions and which provides relevant background information to models used in a recently proposed theory of melting. Probable reasons for the current lack of progress in characterizing the factors that control chemical changes and mechanisms of thermal reactions in solids are also discussed. It is concluded that some aspects of the macro properties of reaction interfaces in crystal reactions have been adequately described, including geometric representations of interface advance during nucleation and growth processes. In contrast, relatively very little is known about the detailed (micro) processes occurring within these active, advancing interfacial zones: reactant/product contacts during chemical reactions and crystal/melt contacts during fusion. From the patterns of behaviour distinguished, a correlation scheme, based on relative stabilities of crystal structures and components therein, is proposed, which accounts for the four principal types of thermal changes that occur on heating solids: sublimation, decomposition, crystallographic transformation or melting. Identifications of the reasons for these different consequences of heating are expected to contribute towards increasing our understanding of each of the individual processes mentioned and to advance theory of the thermal chemistry of solids, currently enjoying a prolonged quiescent phase.     

Use of the quartz crystal microbalance for kinetic studies of thermal decomposition of solids

A quartz crystal microbalance has been proposed for studies on the temperature dependence of the linear rate of a reaction interface advance in topochemical reactions of the thermal decomposition of solids. A quartz crystal microbalance has been used in investigations of the CuSO₄ · 5H₂O dehydration. The data agree fairly well with those available in the literature. Advantages and disadvantages of the method proposed are discussed.     

Radical Abstraction Reactions with Concerted Fragmentation in the Chain Decay of Nitroalkanes

Reactions of the type X? + HCR₂CH2NO₂ → XH + R₂C=CH₂ + N?O₂ are exothermic, due to the breaking of weak C–N bonds and the formation of energy-intensive C=C bonds. Quantum chemistry calculations of the transition state using the reactions of Et? and EtO? with 2-nitrobutane shows that such reactions can be categorized as one-step, due to the extreme instability of the intermediate nitrobutyl radical toward decay with the formation of N?O₂. Kinetic parameters that allow us to calculate the energy of activation and rate constant of such a reaction from its enthalpy are estimated using a model of intersecting parabolas. Enthalpies, energies of activation, and rate constants are calculated for a series of reactions with the participation of Et?, EtO?, RO?₂, N?O₂ radicals on the one hand and a series of nitroalkanes on the other. A new kinetic scheme of the chain decay of nitroalkanes with the participation of abstraction reactions with concerted fragmentation is proposed on the basis of the obtained data.     

An Exploratory Flow Reactor Study of H2S Oxidation at 30–100 Bar

Hydrogen sulfide oxidation experiments were conducted in O₂/N₂ at high pressure (30 and 100 bar) under oxidizing and stoichiometric conditions. Temperatures ranged from 450 to 925 K, with residence times of 3–20 s. Under stoichiometric conditions, the oxidation of H₂S was initiated at 600 K and almost completed at 900 K. Under oxidizing conditions, the onset temperature for reaction was 500–550 K, depending on pressure and residence time, with full oxidization to SO₂ at 550–600 K. Similar results were obtained in quartz and alumina tubes, indicating little influence of surface chemistry. The data were interpreted in terms of a detailed chemical kinetic model. The rate constants for selected reactions, including SH + O₂ ⇄ SO₂ + H, were determined from ab initio calculations. Modeling predictions generally overpredicted the temperature for onset of reaction. Calculations were sensitive to reactions of the comparatively unreactive SH radical. Under stoichiometric conditions, the oxidation rate was mostly controlled by the SH + SH branching ratio to form H₂S + S (promoting reaction) and HSSH (terminating). Further work is desirable on the SH + SH recombination and on subsequent reactions in the S₂ subset of the mechanism. Under oxidizing conditions, a high O₂ concentration (augmented by the high pressure) causes the termolecular reaction SH + O₂ + O₂ → HSO + O₃ to become the major consumption step for SH, according to the model. Consequently, calculations become very sensitive to the rate constant and product channels for the H₂S + O₃ reaction, which are currently not well established.     

Implications of the HCN → HNC process to high-temperature nitrogen-containing fuel chemistry

We have found in our recent kinetic study of the oxidation of HCN by NO₂ in the temperature range 623–773 K that HNCO and CO₂ are very important early products. The measured kinetic data cannot be accounted for by a “conventional” mechanism involving HCN reactions with NO₂, O, and OH. However, the introduction of the isomerization reaction HCN → HNC, followed by the rapid oxidation of HNC by NO₂, O, and OH, can quantitatively simulate all measured kinetic data. A similar study of the NO₂ + HCN reaction in shock waves at temperatures between 1500 and 2400 K also required the inclusion of HNC reactions in order to quantitatively account for measured product distributions. The effects of the HNC molecule on the high temperature HCN chemistry are discussed in terms of the predicted rate constants for HNC reactions with O and OH employing the BAC-MP4 method. © John Wiley & Sons, Inc.     

Quantum Dynamics of Oxyhydrogen Complex-Forming Reactions for the HO2 and HO3 Systems

Complex-forming reactions widely exist in gas-phase chemical reactions.Various complexforming bimolecular reactions have been investigated and interesting phenomena have been discovered.The complex-forming reactions usually have small or no barrier in the entrance channel, which leads to obvious differences in kinetic and dynamic characteristics compared with direct reactions.Theoretically, quantum state-resolved reaction dynamics can provide the most detailed microscopic dynamic mechanisms and is now feasible for a direct reaction with only one potential barrier.However, it is of great challenge to construct accurate potential energy surfaces and perform accurate quantum dynamics calculations for a complex polyatomic reaction involving deep potential wells and multi-channels.This paper reviews the most recent progress in two prototypical oxyhydrogen complex-forming reaction systems, HO₂ and HO₃, which are significant in combustion, atmospheric, and interstellar chemistry.We will present a brief survey of both computational and experimental work and emphasize on some unsolved problems existing in these systems.     

A barrier‐free atomic radical‐molecule reaction: N (2D) NO2 (2A1) mechanistic study

The reaction of N (²D) radical with NO₂ molecule has been studied theoretically using density functional theory and ab initio quantum chemistry method. Singlet electronic state [N₂O₂] potential energy surfaces (PES) are calculated at the CCSD(T)/aug‐cc‐pVDZ//B3LYP/6‐311+G(d) + ZPE and G3B3 levels of theory. All the involved transition states for generation of (2NO) and (O₂ + N₂) lie much lower than the reactants. Thus, the novel reaction N + NO₂ can proceed effectively even at low temperatures and it is expected to play a role in both combustion and interstellar processes. On the basis of the analysis of the kinetics of all pathways through which the reactions proceed, we expect that the competitive power of reaction pathways may vary with experimental conditions for the title reaction. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

High‐Quality Covalently Grafting Hemoglobin on Gold Electrodes: Characterization,Redox Thermodynamics and Bio‐electrocatalysis

Herein, we report a versatile surface chemistry methodology to covalently immobilize ligands and proteins to self‐assembled monolayers (SAMs) on gold electrode. The strategy is based on two steps: 1) the coupling of soluble azido‐PEG‐amimo ligand with an alkynyl‐terminated monolayer via click reaction and 2) covalent immobilization hemoglobin (Hb) to the amine‐terminated ligand via carbodiimide reaction. Surface‐enhanced Raman scattering spectroscopy (SERS), atomic force microscopy (AFM), reflection absorption infrared spectroscopy (RAIR) and cyclic voltammetry are used to characterize the model interfacial reactions. We also demonstrate the excellent biocompatibility of the interface for Hb immobilization and reliable application of the proposed method for H₂O₂ biosensing. Moreover, the redox thermodynamics of the Fe³⁺/Fe²⁺ couple in Hb is also investigated.     

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.     

Accelerated reactions of amines with carbon dioxide driven by superacid at the microdroplet interface

Microdroplets display distinctive interfacial chemistry, manifested as accelerated reactions relative to those observed for the same reagents in bulk. Carbon dioxide undergoes C–N bond formation reactions with amines at the interface of droplets to form carbamic acids. Electrospray ionization mass spectrometry displays the reaction products in the form of the protonated and deprotonated carbamic acid. Electrosonic spray ionization (ESSI) utilizing carbon dioxide as nebulization gas, confines reaction to the gas–liquid interface where it proceeds much faster than in the bulk. Intriguingly, trace amounts of water accelerate the reaction, presumably by formation of superacid or superbase at the water interface. The suggested mechanism of protonation of CO₂ followed by nucleophilic attack by the amine is analogous to that previously advanced for imidazole formation from carboxylic acids and diamines.

Microdroplets display distinctive interfacial chemistry, manifested as accelerated reactions relative to those observed for the same reagents in bulk.     

Ab initio study on the reaction mechanism of ozone with bromine atom

Ab initio calculations of the potential energy surface (PES) for the Br+O₃ reaction have been performed using the MP2, CCSD(T), and QCISD(T) methods with 6‐31G(d), 6‐311G(d), and 6‐311+G(3df). The reaction begins with a transition state (TS) when the Br atom attacks a terminal oxygen of ozone, producing an intermediate, the bromine trioxide (M), which immediately dissociates to BrO+O₂. The geometry optimizations of the reactants, products, and intermediate and transition states are carried out at the MP2/6‐31G(d) level. The reaction potential barrier is 3.09 kcal/mol at the CCSD(T)/6‐311+G(3df)//MP2 level, which shows that the bromine atom trends intensively to react with the ozone. The comparison of the Br+O₃ reaction with the F+O₃ and Cl+O₃ reactions indicates that the reactions of ozone with the halogen atoms have the similar reaction mechanism. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Interaction Between Fe and Cl2 in the Bed of Graphitized Carbon Black

A possible route to remove Fe catalyst from graphitized carbon black synthesized in Boudouard's reaction is employment of gaseous chlorine in role of carrier. This process was explored by means of DTA method using the equipment designed in the laboratory. Obtained results demonstrate the complexity of processes occurring in systems containing Fe, C and Cl₂. Reactions in the system are highly influenced by the geometry of reacting solids. Process of FeCl₃ intercalation between graphite layers was observed analyzing DTA curves. The amount of Fe in the raw product of Boudouard's reaction was determined during the process of chlorination. Obtained results indicate that Cl₂ stream does not remove all the Fe even at high temperatures and prolonged chlorinating time. This revised version was published online in August 2006 with corrections to the Cover Date.     

Steady state hot atom kinetic theory model calculations. Time dependent rate coefficients for the nonthermal 18F + H2 reaction

This paper briefly summarizes the methodology and results for a new type of hot atom kinetic theory model calculation. The microscopic time dependence for the nuclear recoil ¹⁸F + H₂ reaction is characterized using the steady state kinetic theory of hot atom reactions. The relaxing hot atom laboratory momentum distribution is represented using a local equilibrium model. The calculated results illustrate the nature and significance of quasi steady state reaction rate coefficients and of the dynamical coupling between reactive heating and nonreactive cooling phenomena. Under nuclear recoil conditions the nonthermal ¹⁸F + H₂ reaction, which exhibits a total hot yield of 100%, fails to establish a high temperature steady state.     

A study of the reactions of fluorine with hydrogen and methane in the initiation phase using a miniature tubular reactor

A small tubular reactor having an inner diameter of 1–2 mm andused as the source in a molecular beam apparatus is described in detail. This arrangement allows the study of fast reactions with reaction times smaller than 1 msec. The preexplosive reaction phase between F₂ and H₂ and CH₄, respectively, is investigated to find out the initiation reactions. In the F₂/H₂ reaction, initiation is brought about by heterogeneous generation of F atoms or some other surface reaction. Evidence is also obtained for chain branching reactions. In the F₂/CH₄ case the dominant initiation reaction is the homogeneous reaction CH₄ + F₂ → CH₃ + HF + F. The rate constant for the reaction between 300 and 400 K is 10^12.3±0.3 exp[?47 ± 8 kJ/mol/RT] cm³/mol sec. The analysis of the experimental data also yields the rate constant for the propagation reaction CH₃ + F₂ → CH₃ F + F, which is 10^12.3±0.3 exp[?4.6 ±2.1 kJ/mol/RT] cm³/mol sec.     

Theoretical study of the P-Ylide reaction in the carbon nanotube

Recent studies have shown that the inner phase of carbon nanotubes (CNTs) can not only change the properties of molecules inside the tube, but also enhance or restrain the S_N2 reactions. Thus, the CNTs can be considered a form of solid solvent. In this paper, we study the [2+2] cycloaddition reaction between CH₂O and PH₃CH₂ in the gas phase, benzene solution and inner phase of CNT using the density functional theory (DFT). The results indicate that the inner phase of CNT has little effect on the [2+2] cycloaddition reaction. This can be explained as that while taking the linear arrangement for S_N2 reaction, the reactants do not possess the axial symmetry for the studied [2+2] cycloaddition reaction. Therefore, although the CNT has large axial polarizability, it can exert little influence on the [2+2] cycloaddition reaction. Our studies will be helpful for further understanding of the inner phase chemistry of CNTs.     

The Effect of Ring Size on Reactivity: The Diagnostic Value of ‘Rate Profiles’

The rates of cycloalkyl phenyl sulfide formation of a series of homologous bromocycloalkanes upon treatment with sodium benzenethiolate have been determined to ascertain the effect of ring size on reactivity. The ‘rate profile’, i.e., reaction rate vs. ring size, for these nucleophilic substitutions (S_N2) was determined. A linear free‐energy relationship could be derived from computed hydride affinities of cycloalkanes and rates of typical S_N1 reactions, whereas rates of S_N2 reactions exhibited a strong discrepancy from the seven‐ up to the twelve‐membered rings. This discrepancy was rationalized by a careful examination of the geometry of the intermediates and transition states involved in these reactions.     

Reversible Topotactic Redox Reactions of Solids by Electron/Ion Transfer

Solids having suitable structural and electronic properties are able to form intercalation compounds by reversible redox reactions at room temperature via topotactic electron/ion transfer processes. The host lattices range from inorganic solids with different structural dimensionality to organic molecular solids. Similarly, depending on the host lattice type, the guest species may vary from protons and metal ions to large inorganic and organic molecular ions. The possibilities of a systematic “tailoring” of new stable or metastable compounds, the controlled modification of physical properties of solids, and the technical application of electronic/ionic conductors, provide a wide and attractive field for academic and applied research in an interdisciplinary area that involves solid state chemistry and physics, molecular chemistry, electrochemistry, and interface science.