Solid decompositions, experimental

Experimental techniques used in the kinetic investigation of solid decompositions (described in Chap. 2) may need modification for the study of rate processes which yield no gaseous products. Measurements... 

Fig 5 The Effect of Reactor Irradiation on the Thermal Decomposition of Colloidal o -Pb Azide. (Ref 110). Fraction of total decomposition a against time. Solid circles experimental points for unirradiated material at 240.9 open circles experimental points for irradiated material at 238.5°C other points are the attempted fits indicated by arrows... 

The ratio of the experimental flow J to. /lllM, the decomposition coefficient, may be useful in assessing the decomposition mechanism. However, account must be taken of formation of the solid product in a metastable state. The formation of this material may be the rate-limiting step. Beruto and Searcy3334 have carried out a study of CaC03 decomposition using these principles. They have also studied the nature of the CaO formed. Thus, the field of solid decomposition kinetics seems to be moving toward a detailed examination of the products formed, with less emphasis on the overall kinetic equations. 

The sigmoid or-time ciuves observed for the decomposition of barium azide are often characteristic of nucleation and growth processes and such a mechanism was here confirmed by microscopic observations [21]. This early study by Wischin (1939) has been described [11] as "one of the most important experimental papers on solid decompositions". Kinetic observations reported include the following ... 

Third, these equations permit the calculation of the absolute rates of a process, a possibility that had been believed unrealizable before their first application in 1981 to the kinetics of solid decomposition [25], The interest in theories of the transition state and of the activated complex was primarily stimulated by the possibility of calculating absolute reaction rates, although the attempts to use them in studies of heterogeneous processes met with only limited success [1, 2]. In contrast, the first comparison of theoretical with experimental values of the A parameters performed within the framework of Langmuir vaporization equations was much more successful [25]. 

FIG. 4 Broad line H NMR spectrum of solid polyethylene and its components obtained by digital decomposition (a) absorption lineshape and (b) derivative lineshape. Solid curves, experimental dash-dot, crystalline short dashes, partially ordered long dashes, amorphous phase. (Adapted from Ref. 10.)... 

For good results, the following experimental conditions must be observed (1) the hydrochloric acid concentration in the final solution should be at least 4M (2) air should be displaced from the titration mixture by adding a little solid sodium hydrogencarbonate (3) the solution must be allowed to stand for at least 5 minutes before the liberated iodine is titrated and (4) constant stirring is essential during the titration to prevent decomposition of the thiosulphate in the strongly acid solution. 

Salts of diazonium ions with certain arenesulfonate ions also have a relatively high stability in the solid state. They are also used for inhibiting the decomposition of diazonium ions in solution. The most recent experimental data (Roller and Zollinger, 1970 Kampar et al., 1977) point to the formation of molecular complexes of the diazonium ions with the arenesulfonates rather than to diazosulfonates (ArN2 —0S02Ar ) as previously thought. For a diazonium ion in acetic acid/water (4 1) solutions of naphthalene derivatives, the complex equilibrium constants are found to increase in the order naphthalene < 1-methylnaphthalene < naphthalene-1-sulfonic acid < 1-naphthylmethanesulfonic acid. The sequence reflects the combined effects of the electron donor properties of these compounds and the Coulomb attraction between the diazonium cation and the sulfonate anions (where present). Arenediazonium salt solutions are also stabilized by crown ethers (see Sec. 11.2). 

The experimental methods used to investigate solid—solid interactions need not, in principle, be any different from those used to study the thermal decomposition of solids. Those methods, however, which rely on the measurement of parameters related to the loss of gaseous product cannot be applied to those solid—solid reactions where no gas is evolved. 

Metal carbonate decompositions proceed to completion in one or more stages which are generally both endothermic and reversible. Kinetic behaviour is sensitive to the pressure and composition of the prevailing atmosphere and, in particular, to the availability and ease of removal of C02. The structure and porosity of the solid product and its relationship with the reactant phase controls the rate of escape of volatile product by inter-and/or intragranular diffusion, so that rapid and effectively complete withdrawal of C02 from the interface may be difficult to achieve experimentally. Similar features have been described for the removal of water from crystalline hydrates and attention has been drawn to comparable aspects of reactions of both types in Garner s review [ 64 ]. 

This account of the kinetics of reactions between (inorganic) solids commences with a consideration of the reactant mixture (Sect. 1), since composition, particle sizes, method of mixing and other pretreatments exert important influences on rate characteristics. Some comments on experimental methods are included here. Section 2 is concerned with reaction mechanisms formulated to account for observed behaviour, including references to rate processes which involve diffusion across a barrier layer. This section also includes a consideration of the application of mechanistic criteria to the classification of the kinetic characteristics of solid-solid reactions. Section 3 surveys rate processes identified as the decomposition of a solid catalyzed by a solid. Section 4 reviews other types of solid + solid reactions, which may be conveniently subdivided further into the classes... 

There have been remarkably few reviews of the chemistry of decompositions and interactions of solids. The present account is specifically concerned with the kinetic characteristics described in the literature for the reactions of many and diverse compounds. Coverage necessarily includes references to a variety of relevant and closely related topics, such as the background theory of the subject, proposed mechanistic interpretations of observations, experimental methods with their shortcomings and errors, etc. In a survey of acceptable length, however, it is clearly impossible to explore in depth all features of all reports concerned with the reactivity and reactions of all solids. We believe that there is a need for separate and more detailed reviews of topics referred to here briefly. The value of individual publications in the field, which continue to appear in a not inconsiderable flow, would undoubtedly be enhanced by their discussion in the widest context. Systematic presentation and constructive comparisons of observations and reports, which are at present widely dispersed, would be expected to produce significant correlations and conclusions. Useful advances in the subject are just as likely to emerge in the form of generalizations discerned in the wealth of published material as from further individual studies of specific systems. Perhaps potential reviewers have been deterred by the combination of the formidable volume and the extensive dispersal of the information now available. 

Numerous and varied conclusions have been stated explicitly or implied in the text of Chaps. 2—5. These include mechanistic deductions, references to inconsistencies or irreconcilable interpretations found in different studies, recommendations for the re-analysis of certain experimental data etc. These will not be repeated in detail in the present chapter, which summarizes only the most significant and general conclusions relating to the kinetics of decompositions and interactions of solids. 

Kevan and colleagues69 studied the products of the radiolysis of solid diaryl sulfones at room temperature, such as p,p -ditolyl, diphenyl sulfone and dibenzothiophene-S,S-dioxide. The products found for the first two were S02 and the diaryl hydrocarbon. For p,p -ditolyl sulfone the S02 yield is linear with dose upto about 13 Mrad, above which it falls off considerably from linearity. The initial yields give G(S02) = 0.05, which is equal within experimental error to the yield of p,p -bitolyl. The only another organic product observed had a smaller yield by a factor of 7, and could not be identified. The authors pointed out that no polymeric product was found in contrast to what is known on benzene radiolysis. The mass balance suggests that a simple decomposition as shown by equation 50 is the net consequence of radiolysis. 

Analysis of thermal decomposition of molecules on hot surfaces of solids is of considerable interest not only for investigation of mechanisms of heterogeneous decomposition of molecules into fragments which interact actively with solid surfaces. It is of importance also for clarifying the role of the chemical nature of a solid in this process. Furthermore, pyrolysis of molecules on hot filaments made of noble metals, tungsten, tantalum, etc., is a convenient experimental method for producing active particles. Note that it allows continuous adjustment of the intensity of the molecular flux by varying the temperature of the filament [8]. 

According to detailed XRD analyses, the two catalyst preparation procedures under study formed solid solutions. The application of sol-gel method led to improved selectivity to olefins in the reaction of propane ODH, compared to the simple procedure of evaporation and decomposition. However, the propane conversion on the sol-gel catalysts was lower at the same experimental conditions, while the catalysts surface area was higher. Moreover, the sol-gel samples presented higher basicity as shown by C02 TPD. It could be explained by a better incorporation of Nd into the AEO lattice, creating cationic vacancies for attaining electroneutrality and thus rendering the nearby oxide anions coordinatively unsaturated and more basic. 

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