Solid kinetic studies

The common problems with those metallomicelles may be summarized as follows (1) Most of these complexes were prepared in situ and often were not isolated. Hence, the intended structures of the metallomicelles in solution or in the solid state were not verified. (2) The metal complexes in solution were not identified or characterized in rigorous thermodynamic senses by potentiometric pH titration, etc. The complexation constants and possible species distribution at various pH s were totally unknown. (3) Possible catalytically active species L-Mn+—OH were not identified by means of the thermodynamic pvalues. Those described were all obtained merely in kinetics. (4) The product (phosphate anion) inhibition was not determined. Accordingly, it often was not clear whether it was catalytic or not. (5) Often, the substrates studied were limited. (6) The kinetics was complex, probably as a result of the existence of various species in solution. Thus, in most of the cases only pseudo-first-order rates (e.g., with excess metal complexes) were given. No solid kinetic studies combined with thermodynamic studies have been presented. It is thus impossible to compare the catalytic efficiency of these metallomicelles with that of the natural system. Besides, different... 

H NMR spectroscopy frequently has been used in kinetic studies, for example, in the isomerization of 2,4,6-triphenyl-4//-thiopyran 56 (R= Ph) to its 2//-isomer 60 (R = H, 81JHC1517). l25Te NMR spectra were also measured for 4//-teluropyran 77 and related compounds (88MI1). Oxo-enol tautomerism of 4-hydroxy-2//-thiopyrans llOa-c in the solid state as well as in CDC13 solution was successfully studied by l3C NMR [86JCS(P2) 1887]. 

The development and ready availability of reliable and accurate electronic microbalances [33,122—128] have led to their wide application in kinetic studies of the decomposition of solids. Certain of the disad-... 

It is important to distinguish clearly between the surface area of a decomposing solid [i.e. aggregate external boundaries of both reactant and product(s)] measured by adsorption methods and the effective area of the active reaction interface which, in most systems, is an internal structure. The area of the contact zone is of fundamental significance in kinetic studies since its determination would allow the Arrhenius pre-exponential term to be expressed in dimensions of area"1 (as in catalysis). This parameter is, however, inaccessible to direct measurement. Estimates from microscopy cannot identify all those regions which participate in reaction or ascertain the effective roughness factor of observed interfaces. Preferential dissolution of either reactant or product in a suitable solvent prior to area measurement may result in sintering [286]. The problems of identify-... 

Hisatsune and co-workers [290—299] have made extensive kinetic studies of the decomposition of various ions in alkali halide discs. Widths and frequencies of IR absorption bands are an indication of the extent to which a reactant ion forms a solid solution with the matrix halide. Sodium acetate was much less soluble in KBr than in KI but the activation energy for acetate breakdown in the latter matrix was the larger [297]. Shifts in frequency, indicating changes in symmetry, have been reported for oxalate [294] and formate [300] ions dispersed in KBr. 

The properties of barrier layers, oxides in particular, and the kinetic characteristics of diffusion-controlled reactions have been extensively investigated, notably in the field of metal oxidation [31,38]. The concepts developed in these studies are undoubtedly capable of modification and application to kinetic studies of reactions between solids where the rate is determined by reactant diffusion across a barrier layer. 

The above rate equations were originally largely developed from studies of gas—solid reactions and assume that particles of the solid reactant are completely covered by a coherent layer of product. Various applications of these models to kinetic studies of solid—solid interactions have been given. 

Rate equations which have found application in kinetic studies of solid phase reactions a... 

Reports of kinetic studies do not always include an explicit statement as to whether or not the reactant melted during reaction or, indeed, if this possibility was investigated or even considered (cf. p. 1). This aspect of behaviour is important in assessing the mechanistic implications of any data since reactions in a homogeneous melt, perhaps a eutectic, usually proceed more rapidly than in a crystalline solid. It is accepted that the detection of partial or localized melting can be experimentally difficult, but, in the absence of relevant information, it is frequently impossible to decide whether a reported reaction proceeds in the solid phase. 

The dissociation of Ag20 in oxygen was an early (1905) example (Lewis [640]) of a kinetic study of a solid state reaction and interest in... 

Hajek et al. [173] have reported a detailed kinetic study of the solid phase decomposition of the ammonium salts of terephthalic and iso-phthalic acids in an inert-gas fluidized bed (373—473 K). Simultaneous release of both NH3 molecules occurred in the diammonium salts, without dehydration or amide formation. Reactant crystallites maintained their external shape and size during decomposition, the rate obeying the contracting volume equation [eqn. (7), n = 3]. For reaction at 423 K of material having particle sizes 0.25—0.40 mm, the rate coefficients for decompositions of diammonium terephthalate, monoammonium tere-phthalate and diammonium isophthalate were in the ratio 7.4 1.0 134 and values of E (in the same sequence) were 87,108 and 99 kJ mole-1. 

References to a number of other kinetic studies of the decomposition of Ni(HC02)2 have been given [375]. Erofe evet al. [1026] observed that doping altered the rate of reaction of this solid and, from conductivity data, concluded that the initial step involves electron transfer (HCOO- - HCOO +e-). Fox et al. [118], using particles of homogeneous size, showed that both the reaction rate and the shape of a time curves were sensitive to the mean particle diameter. However, since the reported measurements refer to reactions at different temperatures, it is at least possible that some part of the effects described could be temperature effects. Decomposition of nickel formate in oxygen [60] yielded NiO and C02 only the shapes of the a—time curves were comparable in some respects with those for reaction in vacuum and E = 160 15 kJ mole-1. Criado et al. [1031] used the Prout—Tompkins equation [eqn. (9)] in a non-isothermal kinetic analysis of nickel formate decomposition and obtained E = 100 4 kJ mole-1. 

There have been comparatively few kinetic studies of the decompositions of solid malonates [1103]. The sodium and potassium salts apparently melt and non-isothermal measurements indicate second-order rate processes with high values of E (962 125 and 385 84 kJ mole-1, respectively). The reaction of barium malonate apparently did not involve melting and, from the third-order behaviour, E = 481 125 kJ mole-1. 

Many of the most detailed kinetic studies of the reactions of these solids have been concerned with mixtures containing a common ion, (A,B)X. 

Johnson and Gallagher [410] showed that, in finely divided powder mixtures, Li2C03 and Fe203 react significantly below the usual temperature of carbonate dissociation, so that C02 evolution can be used in kinetic studies of the solid state reaction... 

Kutty and Murthy [1159] have made a kinetic study of the solid—solid reaction between tricalcium phosphate and urea nitrate, a process of possible technological importance. A reduction in particle size, notably of Ca3(P04)2, increased the rate of reaction in powder mixtures and also changed the kinetic characteristics (318—338 K). Reaction in relatively coarse material (between —180 and +200 mesh) obeyed the parabolic... 

The simplest solid—solid reactions are those involving two solid reactants and a single barrier product phase. The principles used in interpreting the results of kinetic studies on such systems, and which have been described above, can be modified for application to more complex systems. Many of these complex systems have been resolved into a series of interconnected binary reactions and some of the more fully characterized examples have already been mentioned. While certain of these rate processes are of considerable technological importance, e.g. to the cement industry [1], the difficulties of investigation are such that few quantitative kinetic studies have been attempted. Attention has more frequently been restricted to the qualitative identifications of intermediate and product phases, or, at best, empirical rate measurements for technological purposes. 

Ferro, G., et at, Infrared Kinetic Study of Ultrathin CiC Buffer Layers Grown by Reactive CVD, Thin Solid Films, 278(l-2) 22-27(1996)... 

In the wetting and dewetting kinetics studies described earlier, the solid substrate was a flat and smooth surface. However, the sohd deformation due to the action of the vertical component of the hquid surface tension may be expected to act in any geometry. For example, viscoelastic braking is involved in the sliding of a liquid drop on a tilted rubber track [32],... 

A Kinetic Study on the Solid State Grafting of Maleic Anhydride onto Isotactic Polypropylene in Supercritical CO2... 

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