Phase changes, solid state reactions

Catalytic molecular surface species may undergo drastic changes in their structure in the presence of reactants. For example, polymeric clusters may transform into highly distorted monomeric species. A crystalline phase may become mobile at its Tammann temperature, as shown by Raman spectroscopy, and it may spread over oxide supports driven by the reduction of the overall surface free energy. Reactive environments trigger many structural transformations, exemplified by particle sintering, dispersion of bulk phases, segregation of surface species into bulk phases, and solid-state reactions between supported oxides and supports. 

Our first step will be to delineate known solid state reaction mechanisms. Mechanisms Relating to Solid State Reactions Phase changes... 

In heterogeneous solid-state reactions where the composition of both solid reactants does not change, the electrode s eqnilibrinm potential depends only on the nature of the two phases, not on their relative amonnts. Hence, dnring the reaction the potential does not change. It also remains constant when the cnrrent is interrupted after partial reduction or oxidation. 

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 ... 

As described above, most solid-state reactions are heterogeneous, in the sense that reactant and product are in different solid phases. In many of these, product crystals first appear as nuclei that grow at the expense of the parent crystal. On the other hand, there are some solid-state reactions that are not accompanied by a phase change and for which, therefore, analogy with a solid-state transformation is not plausible. Such reactions are of particular interest in several respects They make possible conversion of a single crystal of reactant to a single crystal of product they enable study, for example by X-ray diffraction, of the structures of the parent and product molecules as functions of the degree of conversion in more or less constant environments and one can elucidate from them the constraints that the parent crystal imposes both on the reaction pathway and on the conformation of the product. It is in connection with the latter that this subject is of particular interest in the present context. This class of processes has been discussed by Thomas (183). 

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 ... 

Boundaries between solids transmit shear stress, particularly if they are coherent or semicoherent. Therefore, the strain energy density near boundaries changes over the course of solid state reactions. Misfit dislocation networks connected with moving boundaries also change with time. They alter the transport properties at and near the interface. Even if we neglect all this, boundaries between heterogeneous phases are sites of a discontinuous structural change, which may occur cooperatively or by individual thermally activated steps. 

These experimental methods give information similar to thermogravimetry. Changes in the gas phase surrounding a sample are probed when the gaseous reaction partner is liberated or consumed during the solid state reaction. This can be done either at... 

Boehmite Based. Catalysts. Hedvall (24) has discussed the formation of cobalt aluminate from CoO and AI2O3. He has shown that a relatively fast solid state reaction takes place when the alumina undergoes a phase change, viz. y-Al203 ->rt-A Og. This phenomenon is known as the Hedvall effect. 

Although spillover species may, in other cases, promote solid-state reactions leading to a more complicated change of activity or selectivity, we shall focus on the clearly positive effects they exert in many (probably most) of the selective oxidation reactions, by preventing phase reduction. 

As far as macroscopic crystals are concerned, the equilibrium properties will not be noticeably affected by a change in the environment, but the interaction of adsorbed molecules with the surface may trigger a phase change which is initiated in the surface. Hence, even for macroscopic crystals, the environment may influence rate phenomena such as solid-state reactions. 

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