Phase separation, solid state reactions

It should be clear by comparing the examples for calcium silicate and barium silicate that one cannot predict how the diffusion-controlled solid state reactions will proceed since they are predicated upon the relative thermodsmamic stability of the compounds formed in each separate phase. 

Care has to be taken when extrapolating kinetic parameters measured under melt-phase conditions for describing the solid-state reaction. The available kinetic data are not free from mass-transfer influences and the effects of proton and metal catalysis are not thoroughly separated. Therefore, the adaptation of kinetic parameters is often carried out by fixing the activation energies and adjusting the pre-exponential factors to the experimental data. 

Many solid-state reactions may be pictured as proceeding in two steps. First a homogeneous process leads to product molecules dissolved in residual parent matrix. Curtin and Paul, in a review on thermal solid-state reactions (6), divide this step into a number of stages First, there is a loosening of the molecules at the reaction site to be, then molecular change (the true reaction), and finally solid-solution formation. When the concentration of the accumulated product exceeds the solubility limit the second step, the decomposition of this solid solution into separate reactant and product phases, occurs. However, in some cases the solubility limit is very low, so that the overall process appears to become simpler ... 

Unfortunately the authors argue that they were performing mechanochemical reactions with mechanical energy input for the salt formation or complexation to occur, rather than just creating the required contacts between reacting crystals. Furthermore, they did not exclude moisture, reported intermediate liquid phases in various cases, and did not separate out any real solid-state reactions that might have been achieved. It is therefore not possible to discuss the results in more detail here. 

When the reactivity of a solid is controlled by the crystal structure, rather than by the chemical constituents of the crystal, the reaction is said to be topochemically controlled. The nature of products obtained in a decomposition reaction is frequently decided by topochemical factors, particularly when the reaction occurs within the solid without separation of a new phase (Thomas, 1974 Manohar, 1974). A topotactic reaction is a solid state reaction where the atomic arrangement in the reactant crystal remains largely unaffected during the course of the reaction, except for changes in dimension in one or more directions. Dehydration of Mo03-2H20 is a typical example of a topotactic reaction ... 

Colourless diacetylene monomer crystals can be polymerized under heat, ultraviolet. X-ray or y-ray irradiation to form single-crystal, highly coloured polyacetylenes. The solid state reaction transforms the entire monomer crystal to polymer crystal without phase separation the polymer forms a solid solution with the monomer over the entire... 

Several reviews deal with the solid-state reactions of simple inorganic salts and of organic compounds.1-8 The essential differences between solid-state reactions and reactions in solution can be ascribed to the fact that solid-state reactions occur within the constraining environment of the crystal lattice. The reactant crystal lattice can control both the kinetic features of a reaction, and the nature of the products. In many solid-phase reactions the separation distances and mutual orientations of reactants in the solid determine the product. Such reactions are said to be topo-chemically controlled.9 Topochemical control of a reaction product is analogous to kinetic control in solution. The product is not necessarily the thermodynamically most stable product available to the system, but is rather the one dictated by the reaction pathway available in the constraining environment of the solid. 

Heterogeneous solid state reactions occur when two phases, A and B, contact and react to form a different product phase C. A and B may be either chemical elements or compounds. We have already introduced this type of solid state reaction in Section 1.3.4. The rate law is parabolic if the reacting system is in local equilibrium and the growth geometry is linear. The characteristic feature of this type of reaction is the fact that the product C separates the reactants A and B and that growth of the product proceeds by transport of A and/or B through the product layer. 

Chapters 6 and 7 dealt with solid state reactions in which the product separates the reactants spatially. For binary (or quasi-binary) systems, reactive growth is the only mode possible for an isothermal heterogeneous solid state reaction if local equilibrium prevails and phase transitions are disregarded. In ternary (and higher) systems, another reactive growth mode can occur. This is the internal reaction mode. The reaction product does not form at the contacting surfaces of the two reactants as discussed in Chapters 6 and 7, but instead forms within the interior of one of the reactants or within a solvent crystal. 

Although there are very few documented examples of solid-state reactions that proceed with high control and that occur by phase-separation mechanisms, it is likely that this will prove to be the most common type of solid-to-solid reaction. A few examples from the literature are described below. 

The photochemical reaction of crystalline /er/-butyl succinimide recently studied by Fu et al. constitutes another interesting and well-documented example of an efficient solid-state reaction that proceeds through mixed crystalline phases and which involves a phase-separation mechanism [133]. 

Each of the four cases delineated [(ci), (cv), (ai), and (av)] is developed individually in such a way that it is self-contained. Each theoretical development is preceded by a formulation of the relevant equations for the defect solid-state reactions [63, 64] occurring at the phase boundaries separating the oxides. That is, balanced chemical equations involving... 

Deactivation of SCR catalysts also occurs by solid-state reactions between the reagents or poisons and the catalytically active surface. The active metal oxides are thus reduced (or over oxidized) to inactive oxidation states. As an example, if, as often presumed, is the active form of vanadium, then the formation of vanadium pentoxide (as a separate phase) would result in a loss of activity. Alternately, vanadium may be reduced to vanadium +3 or less which, again, may be inactive. 

In spite of the topochemical principle, the details of solid-state reactions may be difficult to understand. Whai we think of chemical reactions in solution or in the gas phase, we normally focus attention on the fate of a single molecule and its interaction with one or two immediate neighbors. This kind of simplification is generally not possible when we deal with phase transitions or solid-solid chemical reactions in which phase separation occurs. Even when overall crystal orientation is maintained between initial and final... 

Reactions between two solids are analogous to the oxidation of a metal, because the product of the reaction separates the two reactants. Further reaction is dependent on the transport of material across this barrier. As with oxidation, cracking, porosity and volume mismatch can all help in this. In this section, the case when a coherent layer forms between the two reactants will be considered. The mechanism of the reaction may depend on whether electron transport is possible in the intermediate phase, and the rate of reaction will be controlled by the rate of diffusion of the slowest species. To illustrate the problems encountered a typical solid-state reaction, the formation of oxide spinels, is described. 

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