Heterogeneous chemical pathway

Reduction of ozone is greatly enhanced over the poles by a combination of extremely low temperatures, decreased transport and mixing, and the presence of polar stratospheric clouds that provide heterogeneous chemical pathways for the regeneration of atomic chlorine. The resulting rate of O3 destruction is much greater than the rate at which it can be naturally replenished. 

It should also be mentioned that there are many chemical pathways through which SO2 in the purge air of the exposure tool can be oxidized into sulfates, including homogenous processes that take place in the gas phase and in liquid droplets or heterogeneous processes that take place on the surface of particles or droplets. 

Heterogeneous chemical reactions proceeding at the interface between the solid catalyst surface and the adjacent gas-phase are central to many chemical engineering applications. The catalyst basically provides an alternate reaction pathway by lowering the activation energy for the reaction as shown in Fig.2.1. The surface intermediates can react among themselves... 

Semba, K. (2000). Multiple output pathways of the basal forebrain organization, chemical heterogeneity, and roles in vigilance. Behav. Brain Res. 115, 117-41. 

Hess s law of heat summation the scientific law stating that the enthalpy change of a physical or chemical process depends only on the beginning conditions (reactants) and the end conditions (products) and is independent of the pathway of the reaction or the number of intermediate steps in the reaction (5.3) heterogeneous catalyst a catalyst that exists in a phase that is different from the phase of the catalyzed reaction (6.4)... 

Of hundreds of theoretically possible pathways, the list can be trimmed to four using linear sweep voltammetry (LSV) and chemical arguments [22]. The LSV method is an exceptionally powerful one for analyzing electrochemical processes [24-27]. From LSV studies, it was concluded that a single heterogeneous electron transfer precedes the rate-determining step, cyclization is first order in substrate, and that proton transfer occurs before or in the rate-determining step. The candidates include (a) e-c-P-d-p (radical anion closure). 

Obviously, the heterogeneous character of electrochemical process can in some cases lead to essential differences between electrode and homogeneous reaction pathways. Therefore, eventually it needs to verify the results by studying of reactions in homogeneous media. In other words, the problem of correcmess of electrochemical modeling should be analyzed for each reaction anew and at the same time be checked chemically, that is, in the pure liquid-phase conditions. 

At this point it is necessary to consider the mechanism of electron-transfer luminescence in solutions which cannot involve ion-radical annihilation because both cation and anion of the fluorescer are not formed. Such emission can be achieved by treating anion radicals with chemical oxidants or electrochemically under conditions where the corresponding cation cannot be produced, and it may also be achieved by electrochemical reduction of cations without producing the corresponding anion. In addition to triplets, three types of processes and pathways have been proposed to help explain why such emission occurs. These may be described as (7) impurities, (2) ion-radical aggregates, and (5) heterogeneous electron transfer. It is evident63 that impurities,... 

The chemistry of electrochemical reaction mechanisms is the most hampered and therefore most in need of catalytic acceleration. Therefore, we understand that electrochemical catalysis does not, in principle, differ much fundamentally and mechanistically from chemical catalysis. In addition, apart from the fact that charge-transfer rates and electrosorption equilibria do depend exponentially on electrode potential—a fact that has no comparable counterpart in chemical heterogeneous catalysis—in many cases electrocatalysis and catalysis of electrochemical and chemical oxidation or reduction processes follow very similar if not the same pathways. For instance as electrochemical hydrogen oxidation and generation is coupled to the chemical splitting of the H2 molecule or its formation from adsorbed hydrogen atoms, respectively, electrocatalysts for cathodic hydrogen evolution—... 

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