Thermochemical model in kinetics of heterogeneous reactions: 100-year jubilee

The milestones in the development of the thermochemical model (TM) in kinetics of heterogeneous reactions over 100 years after publication in 1913 of the Langmuir equation, $ J\; = \;MP/\sqrt{2\pi MRT} $ , were considered. However, the systematic development of TM was begun only in the early 1980s. Among the milestones of this latter period, which passed with the participation of the author, the most important were: the extension of Langmuir equation to dissociative vaporization of compounds and identification of two modes of vaporization (equimolar and isobaric); the discovery of the mechanism of congruent dissociative vaporization; the application of the third-law method to determination of the E parameters (“activation energies” in the Arrhenius equation) and formulation of theoretical and experimental evidence for the absence of activation effect in decomposition reactions. During this period, the new theory was applied to the interpretation of many unsolved problems in heterogeneous kinetics. In the majority of cases, the interpretation is reliably supported by agreement between theory and experiment. The traditional kinetic theories based on the activation effect (the classical Arrhenius theory, as well as the collision and transition state theories) appeared fruitless in this respect.     

Atmospheric pressure microwave assisted heterogeneous catalytic reactions

The purpose of the study was to investigate microwave selective heating phenomena and their impact on heterogeneous chemical reactions. We also present a tool which will help microwave chemists to answer to such questions as "My reaction yields 90% after 7 days at reflux; is it possible to obtain the same yield after a few minutes under microwaves?" and to have an approximation of their reactions when conducted under microwaves with different heterogeneous procedures. This model predicting reaction kinetics and yields under microwave heating is based on the Arrhenius equation, in agreement with experimental data and procedures.     

Catalytic oxidation of hydrogen on platinum

Using the thermochemical approach to interpret the kinetics of heterogeneous reactions and the mechanism of congruent dissociative decomposition of solids developed in the 1980s and (re)analyzing the experimental data available in the literature over the last 90 years, a novel mechanism for the catalytic oxidation of H₂ by PtO₂ is proposed. In place of the conventional Langmuir–Hinshelwood and Eley–Rideal adsorption reaction mechanisms, our model is based on the reactions: PtO₂(s) + 2H₂ ? Pt(g) + 2H₂O and Pt(g) + O₂ ? PtO₂(g) → PtO₂(s). The first reaction determines the kinetics of H₂ oxidation and the second determines the kinetics of restoration of the PtO₂ layer. Thermochemical consideration of kinetic features of this model enables (for first time in the history of this reaction) the enthalpy and equilibrium constants for H₂ oxidation on platinum to be calculated. The results are in good agreement with experimental data. In addition, the proposed mechanism explains the origin of the surface-retexturing effect, the impact of autocatalysis, the influence of H₂O vapor on oxidation rate, and the three-fold variation of the Arrhenius E parameter with temperature. This all convincingly demonstrates the value of the thermochemical approach in interpreting heterogeneous reactions.     

Alternative reagents for chemical noise reduction in liquid chromatography-mass spectrometry using selective ion-molecule reactions

Reduction of ionic chemical background noise based on selective gas-phase reactions with chosen neutral reagents has been proven to be a very promising approach in liquid chromatography—mass spectrometry (LC-MS). In this study further investigations on alternative reagents including the disulfides (dimethyl disulfide, diethyl disulfide, methyl propyl disulfide), dimethyl trisulfide, ethylene oxide, and butadiene monoxide, for example, have been carried out. Tandem mass spectrometric studies of ion/molecule reactions indicate that—besides dimethyl disulfide—ethylene oxide and butadiene monoxide also exhibit very efficient reactions with background ions. Furthermore, it is confirmed that the reactions are very selective according to the test with some analyte ions. In contrast to its rapid reactions with background ions, ethylene oxide does not react, or reacts much less, with these analytes. Therefore, it can be used as an alternative reagent for noise reduction. Although reactions of the other tested neutral reagents with background ions are evaluated, they are generally not suitable as reagents for this purpose because of lack of reactivity or dramatic ion losses during reactions.     

Impact of the gaseous environment on the kinetics of solid-state decompositions

The main purpose of this study is to review the current state of the problem of the impact of gaseous environment on the kinetics of solid-state decompositions. Three different theoretical approaches to the interpretation of the decomposition kinetics have been considered. As it follows from the literature published over the past 80 years, the Arrhenius and Knudsen–Langmuir approaches based on the assumption of two different reaction mechanisms (congruent and incongruent) could not solve the problem. At the same time, successes in the application of the thermochemical approach that is based on the assumption of a unitary congruent dissociative vaporization mechanism with condensation of oversaturated vapor remain unnoticed by the TA community. Taking into account this situation, the author has outlined the key points of the thermochemical kinetics in a compact but rigorous and complete form once more. The revised kinetic equations for the different modes of decomposition, several important interrelations between the kinetic parameters, and, finally, the results in the interpretation or reappraisal of the main effects related to the impact of gaseous environment on the kinetics have been considered. In the framework of the thermochemical approach, the problem being discussed may be considered nowadays practically resolved.     

Thermochemical approach to solid-state decomposition reactions against the background of traditional theories

The novel thermochemical and traditional Arrhenius approaches to solid-state decomposition reactions have been evaluated from the standpoints of the number and the validity of the assumptions introduced in the theories, and achievements obtained over the last decade in the frameworks of both approaches. As it follows from the analysis, in both respects, the thermochemical approach is preferable. The so-called ‘controversial’ problem of the use of thermodynamic concepts and thermochemical data for the quantitative evaluation of decomposition kinetics in the thermochemical approach has also been discussed.     

Reversed-phase ion-pair liquid chromatographic method for determination of reaction equilibria involving ionic species: exemplification of the method using ligand substitution reactions of ethylenediaminetetraacetatochromium(III) ion with acetate and phosphate ions

A reversed-phase ion-pair liquid chromatographic method is presented for the determination of reaction equilibria involving ionic species of the same charge sign as reactant and product compounds. It has been demonstrated that ion-exchange chromatography or reversed-phase ion-pair chromatography is a useful tool for the determination of equilibrium constants of chemical reactions involving ionic species such as metal complexation reactions. Previous work with these methods has been based on the assumption that the limiting retention factors of the reactant and product species are constant independent of concentration of the chemical species (X) in the mobile phase, which reacts with the analyte compound. However, when all the reactant and product species are ions of the same charge sign as that of the species X, it is virtually impossible to apply these methods to the equilibrium constant determination because the retention factors of both the reactant and product species may depend on the concentration of X. In this study, an alternative approach was developed that estimates the limiting retention factors of ionic species from the dependence of the retention factor on the ionic strength of the mobile phase. Ligand substitution reactions of ethylenediaminetetraacetatochromium(III) ion with acetate and phosphate ions were used as model reactions to test this method. The equilibrium constants determined by this method are in good agreement with those obtained by a UV-visible spectrophotometric method.     

Interpretation of atomization mechanisms in electrothermal atomic absorption spectrometry by analysis of the absolute rates of the processes

A series of studies carried out in the author's laboratory during the period 1980–1985 and devoted to development of the theory of atomization in electrothermal atomic absorption spectrometry is summarized. The Hertz-Langmuir model of vaporization is used to interpret the Arrhenius equation and the physical meaning of its parameters, namely, the activation energy E_a and the pre-exponential factor A. Two oxide atomization modes, equimolar and isobaric, differing in the values of E_a and A, have been found to exist. A comparison of the calculated and experimental values of the parameters in the Arrhenius equation has permitted the mechanism of atomization in graphite furnaces to be established for compounds of 25 elements. The effect of oxygen or, alternatively, hydrogen addition to the sheath gas on the atomization of oxides is interpreted on quantitative grounds.     

Theoretical analysis of atom formation-time curves for the HGA-74 furnace—II: Evaluation of the atomisation mechanisms Mn,Cr and Pb

In this study, a novel approach was used in the evaluation of the atomisation mechanisms of Mn, Cr and Pb. The atomisation process was considered as first order kinetics. The effect of the heating rates, chemical form of analyte, gas flow and analyte mass on the atomisation mechanisms of these elements were investigated. The major pathways leading to gaseous atoms have been found to be the thermal dissociation of the metal oxide and reduction of the metal oxide followed by the vaporisation of free metal.     

"Turn-on" fluorescent sensing with "reactive" probes

Chemical probes are valuable tools for the investigation of biochemical processes, diagnosis of disease markers, detection of hazardous compounds, and other purposes. Therefore, the development of chemical probes continues to grow through various approaches with different disciplines and design strategies. Fluorescent probes have received much attention because they are sensitive and easy-to-operate, in general. To realize desired selectivity toward a given analyte, the recognition site of a fluorescent probe is designed in such a way to maximize the binding interactions, usually through weak molecular forces such as hydrogen bonding, toward the analyte over other competing ones. In addition to such a supramolecular approach, the development of fluorescent probes that sense analytes through chemical reactions has witnessed its usefulness for achieving high selectivity, in many cases, superior to that obtainable by the supramolecular approach. Creative incorporations of the reactive groups to latent fluorophores have provided novel chemical probes for various analytes. In this feature article, we overview the recent progress in the development of turn-on fluorescent probes that are operating through chemical reactions triggered by target analytes. Various chemical reactions have been implemented in the development of many reactive probes with very high selectivity and sensitivity toward target analytes. A major emphasis has been focused on the type of chemical reactions utilized, with the hope that further explorations can be made with new chemical reactions to develop reactive probes useful for various applications.     

Toward a general theory of heterogeneous reactions

The thermochemical approach, using the quasi-equilibrium methodology of kinetic analysis and the concepts of equimolar and isobaric reaction modes (regimes), has been used to prove the common mechanism of the representative heterogeneous reactions: decompositions of CaCO₃ and of Ag₂O, reduction of NiO by H_2, and the catalytic oxidation of CO and H₂ on PtO₂. All these rate processes have been analyzed using the congruent dissociative vaporization of the participating solid reactants or catalysts and are described by similar mechanistic schemes. The possible role of atomic oxygen evolution in oxidation catalysis is discussed. Quantitative proofs of the common mechanism of these reactions were provided by agreement between calculated reaction enthalpies and the Arrhenius E parameters, the retardation effect of gaseous products on the reaction rates, and the accordance between experimental and theoretical ratios of isobaric to equimolar values of the E parameter. In conclusion, milestones in the history of the thermochemical approach during the last century are discussed.     

Real time monitoring of accelerated chemical reactions by ultrasonication‐assisted spray ionization mass spectrometry

Ultrasonication has been used to accelerate chemical reactions. It would be ideal if ultrasonication‐assisted chemical reactions could be monitored by suitable detection tools such as mass spectrometry in real time. It would be helpful to clarify reaction intermediates/products and to have a better understanding of reaction mechanism. In this work, we developed a system for ultrasonication‐assisted spray ionization mass spectrometry (UASI–MS) with an ~1.7 MHz ultrasonic transducer to monitor chemical reactions in real time. We demonstrated that simply depositing a sample solution on the MHz‐based ultrasonic transducer, which was placed in front of the orifice of a mass spectrometer, the analyte signals can be readily detected by the mass spectrometer. Singly and multiply charged ions from small and large molecules, respectively, can be observed in the UASI mass spectra. Furthermore, the ultrasonic transducer used in the UASI setup accelerates the chemical reactions while being monitored via UASI–MS. The feasibility of using this approach for real‐time acceleration/monitoring of chemical reactions was demonstrated. The reactions of Girard T reagent and hydroxylamine with steroids were used as the model reactions. Upon the deposition of reactant solutions on the ultrasonic transducer, the intermediate/product ions are readily generated and instantaneously monitored using MS within 1 s. Additionally, we also showed the possibility of using this reactive UASI–MS approach to assist the confirmation of trace steroids from complex urine samples by monitoring the generation of the product ions. Copyright © 2014 John Wiley & Sons, Ltd.     

Tungsten atomizer--theory of atomization mechanism of some volatile analytes

Several types of chemical reactions may participate in the evolution of free atoms in a tungsten furnace. Reactions may take place either in the homogeneous or heterogeneous phase. The assumed reactions may be classified into four types according to the phases in which they take place. Reactions occurring in the gaseous phase, i.e. in the inner volume of the furnace, are kinetically more significant. However, for atomization of easily volatile analytes heterogeneous reaction between gaseous compounds and between condensed salts of analytes and the solid surface of the furnace become significant. With regards to the reaction mechanisms during drying, pyrolysis and atomization of nitrates of volatile analytes, three basic types of chemical reactions may be assumed. Free atoms of analytes arise by evaporation of the elementary form of analytes at atomization temperature, where the particular analyte in its elementary form is produced by direct reduction of analyte nitrate by tungsten or by hydrogen at higher temperatures. Precursory reactions of atom formation are reduction reactions which occur between analyte nitrates and tungsten, between analyte nitrates and hydrogen, as well as reactions of thermal dissociation of relevant nitrates. The importance of particular types of precursory reactions for formation of metallic analytes or their oxides is documented by dependence of Gibbs energy values of particular reactions on the temperature.     

在非平衡态热力学的基础上探索建立催化理的新途径

平衡态热力学一直被认作多相催化理论的基石之一。但是,它并不能概括工作中的催化剂的状态和行为,这主要是这里还发生一些非平衡过程。催化体系常常处于非平衡状态之下,而非平衡态条件下体系状态和行为,同时取决于体系的动力学和热力学。联系动力学和热力学最一般的关系式并非原来的De Donder不等式：Ar≥0,而是新的De Donde方程ln r^-/r^-=A/RT。同时发生的总反应之间的热力学耦合对总反应的作用只是形式上的,远不及催化反应链中各基元步骤之间在动力学上的耦合那么重要。通过在动力学方程中引入反应亲和力（热力学位）得到的动力学-热力学结合近似分析,可以用来分析非平衡态的催化反应和催化剂状态。非平衡态热力学在建立多相催化理论中,较之原来的平衡态热力学将能提供更能采纳的和更有意义的物理化学背景。     

Investigation of chemical effects on the performance of flow-through dialysis applied to the determination of ionic species

In this paper, the influence of chemical variables on the mass transfer kinetics of ionic species under dynamic conditions in flow-through sandwich-type dialysers is thoroughly investigated. Although the driving force of the mass transport is the existence of a concentration gradient between the two phases separated by a semi-permeable membrane, it has been demonstrated that the chemical composition of both donor and acceptor solutions in terms of concentration and kind of ionic compounds has a significant influence on the mass transfer efficiency. The Donnan effect on passive dialysis and the fast migration of ions concomitantly present with the target species improved the transfer of the analyte ion in the membrane separation process. Thus, for the determination of low molecular weight anions, the addition of cationic species with high transport index, such as oxonium ion, to the donor stream, or multicharged ions (e.g. Al³⁺) to the recipient stream, enhanced the dialysis yields more than 62% with respect to the use of water as acceptor and sample medium.As a consequence of the dependence of the dialysis rate on the composition of the sample matrix, different diffusate concentrations were encountered for the same input concentration of analyte when prepared in different electrolytic media. In order to balance the chemical potential on the donor side, the ionic strength for both standards and sample solutions should be carefully adjusted via incorporation of a modifier stream in the flow manifold (e.g. 1.0 mol l⁻¹ KNO₃ or 0.5 mol l⁻¹ H₂SO₄) as demonstrated in the bulk of the text. Appropriate buffering of the recipient solution was equally effective. Furthermore, these strategies were found suitable to overcome the lack of linearity observed by several researchers in in-line dialytic processes at low concentrations of ionic species caused by polar interactions with the membrane surface.Chloride was selected as a model of target species for assessing the effect of chemical variables on the mass transfer rate in flow-through parallel-plate dialyser units. The spectrophotometric detection scheme for chloride, implemented in a secondary flow configuration, is based on the displacement reaction of thiocyanate from the corresponding mercury salt in the presence of iron(III).     

Kinetic parameters of decomposition of some selenites

Studying the kinetics of isothermal decomposition of thirteen selenites at isothermal heating, the values of activation energy E of the process, pre-exponential factor A in Arrhenius equation and changes of entropy for the formation of the activated complex of the reagent were calculated. Direct dependence between the thermal stability of the selenites and their cation radii on their 'hardness' or 'softness' was found. The dependence was interpreted in the terms of the generalized perturbation theory of chemical reactivity. Kinetic compensation effect was observed only for the selenites, which thermally decompose by the same mechanism.     

燃烧反应机理构建的双参数速率常数方法

采用Arrhenius方程的双参数形式描述反应的速率常数对温度的依赖关系, 解决了三参数(A, n, E)过拟合造成的复杂燃烧机理参数缺乏通用性等问题. 在不改变物种数和基元反应数条件下, 将UCSD核心机理进行双参数处理, 并应用于小分子体系的动力学模拟, 得到的模拟结果与三参数机理基本相符. 双参数机理恢复了Arrhenius活化能的物理意义, 可实现机理的参数比较和迁移, 缩小了机理整体优化的变量空间, 为燃烧机理参数的统一奠定了基础.     

Dynamic chromatography: A stochastic approach

During the chromatographic separation process, analyte reactions are often observed leading to band broadening and/or elution of peak clusters. For many different chemical compounds the reaction can be reduced to a simple isomerisation kinetic scheme where elution is the result of adsorption–desorption on the surface stationary phase coupled with a flipping two-level reaction system. In this paper, the chromatographic peak shape for a reacting analyte is calculated in frequency domain when the reaction follows a simple reversible first order scheme. Both reaction and dynamic chromatographic systems have been considered. The derived solutions are expressed in closed form in the Fourier domain. Several limit solutions obtained under conditions of very slow and moderately fast kinetics are exploited. The effects of both kinetics rate constants and retention time on the chromatographic peak shape are singled out.