Elementary reaction

Abstract: The generation by combustion processes of airborne species of current health concern such as polycyclic aromatic hydrocarbons (PAH) and soot particles necessitates a detailed understanding of chemical reaction pathways responsible for their formation. The present review discusses a general scheme of PAH formation and sequential growth of PAH by reactions with stable and radical species, including single-ring aromatics, other PAH and acetylene, followed by the nucleation or inception of small soot particles, soot growth by coagulation and mass addition from gas phase species, and carbonization of the particulate material. Experimental and theoretical tools which have allowed the achievement of deeper insight into the corresponding chemical processes are presented. The significant roles of propargyl (C3H3) and cyclopentadienyl (C5H5) radicals in the formation of first aromatic rings in combustion of aliphatic fuels are discussed. Detailed kinetic modeling of well-defined combustion systems, such as premixed flames, for which sufficient experimental data for a quantitative understanding are available, is of increasing importance. Reliable thermodynamic and kinetic property data are also required for meaningful conclusions, and computational techniques for their determination are presented. Routes of ongoing and future research leading to more detailed experimental data as well as computational approaches for the exploration of elementary reaction steps and the description of systems of increasing complexity are discussed.

Abstract: A number of elementary reactions at metal surfaces show a linear Bronsted–Evans–Polanyi relation between the activation energy and the reaction energy, and reactions belonging to the same class even follow the same relation. We investigate the implications of this finding on the kinetics of surface-catalyzed chemical processes. We focus in particular on the variation in the activity from one metal to the next. By analyzing a number of simple microkinetic models we show that the reaction rate under given reaction conditions shows a maximum as a function of the dissociative adsorption energy of the key reactant, and that for most conditions this maximum is in the same range of reaction energies. We also provide a database of chemisorption energies calculated using density-functional theory for a number of simple gas molecules on 13 different transition metals. An important part of the analysis consists of developing a general framework for analyzing the maximum rate. We use these concepts to rationalize trends in the catalytic activity of a number of metals for the methanation process.

Abstract: Reactions and reaction rates reactions with a simple kinetic form reversible and concurrent reactions consecutive reactions - the steady state and other approximations consecutive mechanisms - intermediates and numerical solutions deduction of reaction mechanisms transition state theory and microscopic reversibility chain reactions and oscillating reactions reactions in solution extrakinetic probes of mechanism reactions at extreme rates.

Abstract: Classical reaction kinetics has been found to be unsatisfactory when the reactants are spatially constrained on the microscopic level by either walls, phase boundaries, or force fields. Recently discovered theories of heterogeneous reaction kinetics have dramatic consequences, such as fractal orders for elementary reactions, self-ordering and self-unmixing of reactants, and rate coefficients with temporal "memories." The new theories were needed to explain the results of experiments and supercomputer simulations of reactions that were confined to low dimensions or fractal dimensions or both. Among the practical examples of "fractal-like kinetics" are chemical reactions in pores of membranes, excitation trapping in molecular aggregates, exciton fusion in composite materials, and charge recombination in colloids and clouds.