Mechanical solid-state reactions

Keywords Reaction mechanism Solid-state reaction Topochemical posmlate... 

The aim of the series is to present the latest fundamental material for research chemists, lecturers and students across the breadth of the subject, reaching into the various applications of theoretical techniques and modelling. The series concentrates on teaching the fundamentals of chemical structure, symmetry, bonding, reactivity, reaction mechanism, solid-state chemistry and applications in molecular modelling. It will emphasize the transfer of theoretical ideas and results to practical situations so as to demonstrate the role of theory in the solution of chemical problems in the laboratory and in industry. 

Classification of solid state reactions according to Boldyrev [100] Initial step Reaction mechanism Examples... 

Additional information concerning the mechanisms of solid—solid interactions has been obtained by many diverse experimental approaches, as the following examples testify adsorptive and catalytic properties of the reactant mixture [1,111], reflectance spectroscopy [420], NMR [421], EPR [347], electromotive force determinations [421], tracer experiments [422], and doping effects [423], This list cannot be comprehensive. Electron probe microanalysis has also been used as an analytical (rather than a kinetic) tool [422,424] for the determination of distributions of elements within the reactant mixture. Infrared analyses have been used [425] for the investigation of the solid state reactions between NH3 and S02 at low temperatures in the presence and in the absence of water. 

It is probably true that many of the descriptions of physical and solid state reaction mechanisms now existing in the literature are only partially correct. It would appear that part of the frontier of knowledge for Chemistry of The Solid State wiU lie in measurement of physical and chemical properties of inorgcuiic compounds as a function of purity. 

We have Investigated the structure of solids In the second chapter and the nature of point defects of the solid in the third chapter. We are now ready to describe how solids react. This will Include the mechanisms Involved when solids form by reaction from constituent compounds. We will also describe some methods of measurement and how one determines extent and rate of the soUd state reaction actually taking place. We will also show how the presence and/or formation of point defects affect reactivity In solid state reactions. They do so, but not In the memner that you might suspect. We will also show how solid state reactions progress, particularly those involving silicates where several different phases appear as a function of both time and relative ratios of reacting components. 

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

In both 4.1.1. and Table 4-1, S, L G refer to solid, liquid and gas, respectively. Note that we have also classified these heterogeneous mechanisms in terms of the same PHYSICAL CHANGES given above for homogeneous transformations. For the most part, the initial material will be a solid while the nature of the final product will vary according to the type of material undergoing solid state reaction. 

In the case where two particles are involved, those that are not in close proximity will not react, whereas those that are close will undergo solid state reaction with ease. This is due to the fact that cations and/or anions from one structure must be transported, or interchange by some mechcinlsm, to the other structure in order to form a completely new compound. Thus, the degree of dispersion and mixing of one reacting solid with emother is important to the overall mechanism of solid state... 

We have already dealt with two of these. Section 2 dealt with formation of a phase boundary while we have just completed Section 4 concerning nuclei growth as related to a phase boundary. We will consider diffusion mechanisms in nuclei and diffusion-controlled solid state reactions at a later part of this chapter. 

We are now ready to consider some soUd state reactions that relate more directly to the real world. These include the tarnishing reaction and Pick s Laws of Diffusion. Both of these scientific areas have been rigorously studied because of their importcmce in revealing how diffusion mechanisms are related to everyday solid state reactions which occur on a daily basis. 

Let us now turn to diffusion in the general case, without worrying about the exact mechanism or the rates of diffusion of the various species. As an example to illustrate how we would analyze a diffusion-limited solid state reaction, we use the general equation describing formation of a compound with spinel (cubic) structure and stoichiometry ... 

In this mechanism where Da 2+ Db3+, transport of external oxygen gas is involved in the overall solid state reaction, accompanied by electronic charge diffusion. 

The overall reaction mechanisms are diffusion-controlled, and the total solid state reaction can be summarized as follows ... 

Although we have covered mechanisms relating to solid state reactions, the formation and growth of nuclei and the rate of their growth in both heterogeneous and homogeneous solids, and the diffusion processes thereby associated, there exist still other processes zifter the particles have formed. These include sequences in particle growth, once the particles have formed. Such sequences include ... 

Thermal solid-state reactions were carried out by keeping a mixture of powdered reactant and reagent at room temperature or elevated temperature, or by mixing with pestle and mortar. In some cases, the solid-state reactions proceed much more efficiently in a water suspension medium or in the presence of a small amount of solvent. Sometimes, a mixture of solid reactant and reagent turns to liquid as the reaction proceeds. All these reactions are called solid-state reactions in this chapter. Solid-state reactions were found to be useful in the study of reaction mechanisms, since it is easy to monitor the reaction by continuous measurement of IR spectra. 

Since solid-state reactions can easily be monitored by continuous measurement of spectra, it is easy to study the mechanism of the reactions. For this purpose, IR spectroscopy is the most useful, because IR spectra can be measured simply as Nujol mulls or directly for any mixture of solid-solid, solid-liquid, or liquid-liquid by using the ATP (attenuated total reflection) method. Some such examples of the mechanistic study are described. 

Until recently the products of all nitrile cyclizations by the Thorpe reaction had been formulated as imines, although the products were found in 1955 to be better written as the enamine structure. In order to verify the reaction mechanism of the Thorpe reaction, the solid-state reaction of 84 and Bu OK was monitored by measurement of IR spectra in Nujol mulls. As the reaction proceeds (Scheme 14), the CN absorption of 84 at 2250 cm" decreases and a new CN absorption of the imine intermediate (87) arises at 2143 cm As 87 is converted into 88 by a proton migration, the CN absorption of 87 at 2143 cm" disappears, and only the CN absorption of 88 at 2189 cm remains finally [13]. 

Solid-state reactions between two phases of constant composition are called heterogeneous solid-state reactions. The mechanism of heterogeneous solid-state reactions... 

Deactivation is a common and important phenomenon in FTS. Deactivation effects of water are recorded on all commonly used supports. The suggested mechanisms include oxidation, sintering and solid state reactions rendering cobalt inactive. 

Alloy crystal and thermal data symbols. A number of tables show, for selected alloys, the highest melting points observed in the systems considered, as well as the mechanism of formation (p = peritectic melting, syn = synthetic reaction, s.s.r. = solid-state reaction, est. = estimated melting point, etc.), the value of the Raynor Index (<1, =1 or>l). The question mark means that no reliable data are available. 

In addition, the results of such reactions have suggested plausible models for the mechanism of abiotic generation of optical activity, including an autocatalytic feedback mechanism (261). The latter involves random development of chiral crystals from achiral starting material, and solid-state reaction leading to products in which one enantiomer is in excess and thus can bias subsequent further crystallization (262). 

All of these syntheses are preceded by mechanical activation of reagents in ball mills and aU of them can be considered when mannfacturing of nanohydride powders is anticipated. However, solid-state reaction rontes are not always well investigated, and can be complex. More often than not, the yields of reactions leading to nanohydrides are not reaching 100% and an amorphons phase, which provides lower storage capacity, is formed. 

---