Experimental Characterization of Decomposition Reactions

This chapter describes a runaway scenario. The first section presents a general review of the decomposition reaction characteristics. The second section is devoted to the energy release that defines the consequences of a runaway. The third section deals with triggering conditions of undesired reactions, based on the concept of TMRld. The next section reviews some important aspects for the experimental characterization of decomposition reactions. Finally, the last section gives some examples stemming from industrial practice. 

Since a reaction product catalyses the reaction, the initial concentration of product also has a strong effect on the TMRad. In the case illustrated in (Figure 12.6), an initial conversion of 10% leads to a reduction of the TMRad by a factor of 2. This also has direct implications for process safety the thermal history of the substance, that is, its exposure to temperature for a certain time increases initial product concentration, leading to effects comparable to those illustrated in Figure 12.5. Hence it becomes obvious that substances showing an autocatalytic decomposition are very sensitive to external effects, such as contaminations and previous thermal treatments. This is important for industrial applications as well as during the experimental characterization of such decompositions the sample chosen must be representative of the industrial situation, or several samples must be analysed. 

In many descriptions of electrochemical preparations of organic substances, only the overall current and voltage applied across the cell have been specified. It must be emphasized that this information is generally inadequate for a proper electrochemical specification of the experimental conditions and a characterization of the reaction mechanism. Under constant current conditions, as consumption of the reactant occurs, the potential normally becomes increased (greater polarization) until eventually some new electrode process becomes predominant (see Section 5.1). This may either be decomposition of the solvent or supporting electrolyte or, in some cases, a further reaction with the substrate involved in the electroorganic preparation. In the latter case, it is clear that the preparation will yield more than one principal product. A classical case, first investigated by Haber, is the electroreduction of nitrobenzene referred to above and also the Kolbe reaction. ... 

In a later study by the Schmidt group (27), electron microscopy was used to characterize morphological changes in microspheres (<0.6 cm in diameter) of Pt, Rh, Pd, and Pt-Rh alloy in a number of reaction environments the reactions were ammonia oxidation, ammonia decomposition, and propane oxidation. No other experimental techniques, such as weight-loss measurements, were employed. After prolonged exposure to reaction mixtures of ammonia and air at temperatures less than 727°C, the surfaces of the spheres were reconstructed to favor specific crystal planes. The structure of the facets was found to be a function of the reaction mixture, temperature, and metal (Fig. 13). In the same reaction mixtures, as well as in pure ammonia at higher temperatures... 

Dioxetanes have been the sole subject of several specialized reviews in recent years (Bartlett and Landis, 1979 Horn et al., 1978-79 Adam, 1977 T. Wilson, 1976 Turro et al., 1974a Mumford, 1975). These articles cover with depth which is not possible here such topics as (1) preparation, (2) physical and spectroscopic characterization, (3) experimental techniques, especially for the study of chemiluminescence, (4) mechanisms of decomposition and chemiexcitation, (5) ground state transformations, and (6) reactions involving dioxetanes as postulated intermediates. The interested reader is referred to these articles for details on these specialized topics, and for some interesting historical perspectives. 

Because the amount of decomposition of a well-characterized photochemical reaction depends on the photochemical quantum yield (a constant) and the amount of radiation absorbed by the sample, the intensity of unknown irradiation sources can be determined very accurately by measuring the amount of decomposition, if the quantum yield of the photoreaction is known. Among the large number of well-characterized photoreactions, only a few are suitable for actinometry. Thus, only photoreactions with very simple mechanisms are less sensitive to the experimental conditions of the irradiation. Well-defined experimental conditions and easy monitoring are important requirements for a suitable actinometric system if reproducible results are to be obtained. Examples of acceptable reaction types include photodegradations, photoisomerizations, photooxidation, etc., as discussed later in detail (vide infra). 

Many of the experimental studies of the thermal decompositions of coordination compounds have been restricted to non-isothermal measurements primarily directed towards identification of the occurence of a reaction and the characterization of the major products of this change. The improved sensitivity of experimental methods (notably TG and DSC or DTA) has revealed the chemical complexity of the thermal reactions of solid coordination compounds. Much comparative information concerning the relative reactivities of related materials has been obtained. While... 

Additions of Na20 were equivalent to those of Na202. and Na2C03 was found to decompose completely with the formation of Na20 under the experimental conditions. The process of decomposition is efficiently retarded in concentrated solutions, since C02 is hardly removed from strongly basic media. The basic solutions were characterized by practically stable potentials, and only slightly shifted towards the neutral point of the ionic solvent studied. In contrast, acidic solutions obtained by the addition of NaPO to the pure sulfate melt are extremely unstable dissolution of the acid is accompanied with fast evolution of SO3, and in a short time the e.m.f. value achieves that of the neutral point. The equilibrium constant of the following reaction... 

Co(II) or Cu(II) histidine or imidazole complexes were immobilized in porous matrices (montmorillonite and MCM-41) via two methods (introduction of preformed complex or complex formation within the ion-exchanged host substances). It was found that immobilization in general and the latter method in particular increased catalytic activity and catalyst life time in the decomposition reactions of hydrogen peroxide relative to the matrix-free complexes. The immobilized materials were characterized by experimental and computational methods and the structures of the guest molecules inside the hosts were also investigated. 

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