Reactant-product interface

If, as is generally assumed, the rate of linear advance of the reactant-product interface is constant, the radius, r, at time t, of a nucleus generated at time tr, is... 

When reaction is absent from certain crystallographic surfaces, the formulation of rate equations based on geometric considerations proceeds exactly as outlined above, but includes only the advance of interfaces into the bulk of the reactant particle from those crystallographic surfaces upon which the coherent reactant/product contact is initially established. When reaction occurs only at the edges of a disc or plate-like particle... 

The dehydration reactions of Cu(HC02)2 4 H20 [213] and of Mn(CH02)2 2 H20 [91,212] are characterized by the rapid initial production of a complete and coherent reactant—product interface at... 

Metal salts of carboxylic acids obviously possess some organic character, but decompositions of these substances can be considered in the present context. Many metal carboxylates decompose at a reactant—product interface and their nucleation and growth processes are similar to the behav-... 

The activation energy found for the decomposition of an individual oxalate ion in a KBr matrix (270 15 kJ mole-1) [292,294] is regarded as the energy requirement for C—C bond rupture. The generally lower values of E observed for many oxalates ( 165—175 kJ mole-1) are attributed to the facilitation of reaction at the reactant—product interface. 

Chemists use a special notation to specify the structure of electrode compartments in a galvanic cell. The two electrodes in the Daniell cell, for instance, are denoted Zn(s) Zn2+(aq) and Cu2+(aq) Cu(s). Each vertical line represents an interface between phases—in this case, between solid metal and ions in solution in the order reactant product. 

The overall formation mechanism of PS must involve the fundamental electrochemical reactions in three essential aspects 1. nature of reactions, reactants, products, intermediates, number of steps, and their sequences, 2. nature and rate of charge transport in the different phases at silicon/electrolyte interface, 3. spatial and temporal distributions of reactions and the cause of such distributions. The first and second aspects, which governs the properties of a uniform and flat surface and do not involve geometric factors, have been characterized in previous Sections and the major characteristics are summarized in Table 5. This Section deals with the third aspect, that is, spatial and temporal... 

The last part of the polarization curve is dominated by mass-transfer limitations (i.e., concentration overpotential). These limitations arise from conditions wherein the necessary reactants (products) cannot reach (leave) the electrocatalytic site. Thus, for fuel cells, these limitations arise either from diffusive resistances that do not allow hydrogen and oxygen to reach the sites or from conductive resistances that do not allow protons or electrons to reach or leave the sites. For general models, a limiting current density can be used to describe the mass-transport limitations. For this review, the limiting current density is defined as the current density at which a reactant concentration becomes zero at the diffusion medium/catalyst layer interface. 

Under conditions leading to a porous shell of magnetite, the kinetic curve displayed an induction period corresponding to formation of nuclei and the subsequent reaction followed the cube root law. Diffusion of the reducing gas to the reactant/ product interface took place readily with a porous product. Whether chemical or diffusion control predominated depended on reaction conditions. With small crystals... 

It should be noted that over the past 50 years studies of solid decomposition kinetics have progressed from the application of equations which were originally derived for gases to more and more detailed studies on molecular events occurring at the reactant-product interface. This point has been emphasized by Boldyrev28 who, with his co-workers, has amassed a large body of evidence that shows the importance of defect structure on kinetics. This structure may be affected by radiation or by mechanical forces, such as grinding, which affect both the surface and the internal structure of crystals. 

The factor ki depends on the experimental conditions. For ICI reactions in constant conditions, r keeps decreasing during the whole reaction. Figure 1(b) suggests that the area of the interface between product (black) and reactant (white) steadily decreases. This is not the case for NCI reactions, where r, first increases... 

The designers of SOFCs have to face the new developments posed by the circulators of Figures A.l and A.2. The letters SOFC denote a vigorous, diverse and expanding family of fuel cells based on the idea that at high temperature the thermal oscillations of the ions at the electrolyte/reactant/product interfaces lead to vigorous exchange... 

Banfield J. F. and Barker W. W. (1994) Direct observation of reactant—product interfaces formed in natural weathering of exsolved, defective amphibole to smectite evidence for episodic, isovolumetric reactions involving structural inheritance. Geochim. Cosmochim. Acta 58, 1419-1429. 

The activity, stability, and tolerance of supported platinum-based anode and cathode electrocatalysts in PEM fuel cells clearly depend on a large number of parameters including particle-size distribution, morphology, composition, operating potential, and temperature. Combining what is known of the surface chemical reactivity of reactants, products, and intermediates at well-characterized surfaces with studies correlating electrochemical behavior of simple and modified platinum and platinum alloy surfaces can lead to a better understanding of the electrocatalysis. Steps, defects, and alloyed components clearly influence reactivity at both gas-solid and gas-liquid interfaces and will understandably influence the electrocatalytic activity. 

For exothermic solid-solid reactions involving no gases, reaction should proceed to completion provided that reactant/product interfaces maintain physical contact and that diffusion of reactants through the solid product does not become too slow. 

In principle, if it were possible to control accurately the ratio of the partial pressures, / (02) V(/7(CC2), the solid reaction product could be predetermined, but in practice the prevailing pressures at the reactant/product interface may differ appreciably from the controlled values. 

Nucleation can be defined as "the initial establishment of a new and discrete product particle within the solid reactant" [9], Two types of chemical change may be involved (i) the chemical transformation of one or more constituents (e.g., ions or molecules) of the reactant into the constituents of the product and (ii) recrystalhzation of reacted material to the lattice structure of the product phase. The result of nucleation, perhaps requiring several steps, is the generation of the active reactant/product interface at which reaction occurs preferentially during its subsequent advance into unchanged reactant as the nucleus grows. 

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