Elementary reaction

About: Elementary reaction is a research topic. Over the lifetime, 2972 publications have been published within this topic receiving 76110 citations.

The transition state method

Abstract: According to our present notions, the theory of reaction rates involves three steps. First, one should know the behaviour of all molecules present in the system during the reaction, how they will move, and which products they will yield when colliding with definite velocities, etc. Practically, this amounts in most cases to the construction of the energy surface for the reacting system. Professor Eyring told us about the results which can be obtained by the application of quantum mechanics to molecular systems for this part of the theory. The second step in the theory I would call the statistical part. It endeavours to solve the problem of the rate of elementary reactions. Assuming only the material on the left side of a chemical equation to be present in a vessel, and the molecules of these to have the Maxwell-Boltzman energy and velocity distribution, one wants to know how many molecules corresponding to the right side of the equation will be formed in unit time. The elementary properties of the molecules are supposed to be known in this second step and one wants to express the reaction rate of elementary reactions in terms of these. The present paper will be devoted entirely to this second step. The third step is the consideration of the co-operation of the various elementary reactions, which may occur beside and must occur after each other in order to complete a real reaction. In especially favourable cases there is only one important chain of reactions leading to the final products and this has one link which is so much slower than all the others, that it is made responsible for the observed rate. The others are then assumed to be so much faster that one has practically equilibrium between the two sides of their chemical equations.

Gas-phase catalysis by atomic and cluster metal ions: the ultimate single-site catalysts.

Abstract: Gas-phase experiments with state-of-the-art techniques of mass spectrometry provide detailed insights into numerous elementary processes. The focus of this Review is on elementary reactions of ions that achieve complete catalytic cycles under thermal conditions. The examples chosen cover aspects of catalysis pertinent to areas as diverse as atmospheric chemistry and surface chemistry. We describe how transfer of oxygen atoms, bond activation, and coupling of fragments can be mediated by atomic or cluster metal ions. In some cases truly unexpected analogies of the idealized gas-phase ion catalysis can be drawn with related chemical transformations in solution or the solid state, and so improve our understanding of the intrinsic operation of a practical catalyst at a strictly molecular level.

A review of catalytic partial oxidation of methane to synthesis gas with emphasis on reaction mechanisms over transition metal catalysts

Abstract: Catalytic partial oxidation of methane has been reviewed with an emphasis on the reaction mechanisms over transition metal catalysts. The thermodynamics and aspects related to heat and mass transport is also evaluated, and an extensive table on research contributions to methane partial oxidation over transition metal catalysts in the literature is provided. Presented are both theoretical and experimental evidence pointing to inherent differences in the reaction mechanism over transition metals. These differences are related to methane dissociation, binding site preferences, the stability of OH surface species, surface residence times of active species and contributions from lattice oxygen atoms and support species. Methane dissociation requires a reduced metal surface, but at elevated temperatures oxides of active species may be reduced by direct interaction with methane or from the reaction with H, H2, C or CO. The comparison of elementary reaction steps on Pt and Rh illustrates that a key factor to produce hydrogen as a primary product is a high activation energy barrier to the formation of OH. Another essential property for the formation of H2 and CO as primary products is a low surface coverage of intermediates, such that the probability of O–H, OH–H and CO–O interactions are reduced. The local concentrations of reactants and products change rapidly through the catalyst bed. This influences the reaction mechanisms, but the product composition is typically close to equilibrated at the bed exit temperature.

The Catalytic Oxidative Coupling of Methane

Abstract: One of the great challenges in the field of heterogeneous catalysis is the conversion of methane to more useful chemicals and fuels. A chemical of particular importance is ethene, which can be obtained by the oxidative coupling of methane. In this reaction CH4 is first oxidatively converted into C2H6, and then into C2H4. The fundamental aspects of the problem involve both a heterogeneous component, which includes the activation of CH4 on a metal oxide surface, and a homogeneous gas-phase component, which includes free-radical chemistry. Ethane is produced mainly by the coupling of the surface-generated CH radicals in the gas phase. The yield of C2H4 and C2H6 is limited by secondary reactions of CH radicals with the surface and by the further oxidation of C2H4, both on the catalyst surface and in the gas phase. Currently, the best catalysts provide 20% CH4 conversion with 80% combined C2H4 and C2H6 selectivity in a single pass through the reactor. Less is known about the nature of the active centers than about the reaction mechanism; however, reactive oxygen ions are apparently required for the activation of CH4 on certain catalysts. There is spectroscopic evidence for surface O− or O ions. In addition to the oxidative coupling of CH4, cross-coupling reactions, such as between methane and toluene to produce styrene, have been investigated. Many of the same catalysts are effective, and the cross-coupling reaction also appears to involve surface-generated radicals. Although a technological process has not been developed, extensive research has resulted in a reasonable understanding of the elementary reactions that occur during the oxidative coupling of methane.

Rate Coefficients in the C/H/O System

Abstract: This chapter is a critical survey of reaction rate coefficient data important in describing high-temperature combustion of H2, CO, and small hydrocarbons up to C4. A recommended reaction mechanism and rate coefficient set is presented. The approximate temperature range for this mechanism is from 1200 to 2500 K, which therefore excludes detailed consideration of cool flames, low-temperature ignition, or reactions of organic peroxides or peroxy radicals. Low-temperature rate-coefficient data are presented, however, when they contribute to defining or understanding high-temperature rate coefficients. Because our current knowledge of reaction kinetics is incomplete, this mechanism is inadequate for very fuel-rich conditions (see Warnatz et al., 1982). For the most part, reactions are considered only when their rates may be important for modeling combustion processes. This criterion eliminates considering many reactions among minor species present at concentrations so low that reactions of these species cannot play an essential part in combustion processes. The philosophy in evaluating the rate-coefficient data was to be selective rather than exhaustive: Recent results obtained with experimental methods capable of measuring isolated elementary reaction rate parameters directly were preferred, while results obtained using computer simulations of complex reacting systems were considered only when sensitivity to a particular elementary reaction was demonstrated or when direct measurements are not available. Theoretical results were not considered.