Elementary reaction

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

On the origin of power-law kinetics in carbon oxidation

Abstract: Many experimental studies report high fractional orders for the global carbon/oxygen reaction, including several studies that cover wide ranges in oxygen partial pressure and observe power-law kinetics with a constant apparent order over the entire range. This persistent n th-order behavior is inconsistent with the simple Langmuir kinetic model and is also a challenge for more elaborate multi-step models of elementary reactions on ideal surfaces. The power law form is a simple and attractive rate law, but without a fundamental basis, it will remain empirical and ultimately controversial. The present paper evaluates the effect of surface or site heterogeneity as an explanation for the paradox of persistent power-law behavior. Simple models of intrinsic and induced heterogeneity are used to show that power-law kinetics are indeed expected when desorption or adsorption activation energy distributions are broad. Examination of experimental TPD data shows that desorption activation energy distributions are broad enough for global power-law kinetics to be generally expected for disordered carbons. The particular formulation of Haynes was evaluated as a candidate framework for describing the main features in the carbon oxidation database at temperatures below 1000 K and oxygen pressures above 0.01 bar. The Haynes turnover model predicts persistent power-law behavior and gives a promising description of the absolute reaction orders, activation energies, and near-atmospheric rates for several coal and polymer chars. It also predicts the low reaction order and its weak variation with pressure in the graphitized carbon black data of Tyler and coworkers. The origin of power-law kinetics is discussed in terms of the detailed behavior of three classes of active sites: reactive bare sites, partially covered sites, and stable oxide, and an approximate analytical expression is derived that relates global order features in the desorption activation energy distribution.

Reaction Route Graphs. I. Theory and Algorithm

Abstract: A theory and algorithm for reaction route (RR) network analysis is developed in analogy with electrical networks and is based on the combined use of RR theory, graph theory, and Kirchhoff's laws. The result is a powerful new approach of “RR graphs” that is useful in not only topological representation of complex reactions and mechanisms but, when combined with techniques of electrical network analysis, is able to provide revealing insights into the mechanism as well as the kinetics of the overall reactions involving multiple elementary reaction steps including the effect of topological constraints. Unlike existing graph theory approaches of reaction networks, the approach developed here is suitable for linear as well as nonlinear kinetic mechanisms and for single and multiple overall reactions. The theoretical approach for the case of a single overall reaction involving minimal kinetic mechanisms (unit stoichiometric numbers) is developed in Part I of this series followed by its application to examples of...