Kinetics of solid-state ion exchange

Fig. 78. Description of the kinetics of solid-state ion exchange in the system CuCl/Na-Y through a diffusion model the symbols represent experimental data derived from the measured integrated absorbances of the probe (pyridine), the broken lines represent results of the fitting to the diffusion model (for details, see text after [289], with permission)...

The sequence of solid state ion-exchange ability of different metal chlorides and of different cadmium compounds has not yet been interpreted. Most probably kinetic factors are decisive, because total ion-exchange can... 

Little is known about the thermodynamics, kinetics and the mechanisms of solid-state ion exchange. This might be an interesting field of further research. 

Fig. 5 shows the time dependence of the solid-state ion exchange process. The process has pseudo first order kinetics in the investigated conversion range for both (i) the distribution and crystallinity loss of the CdCl2 salt and (ii) the formation of new Cd,H-Y phase. The rate constant obtained for the decay of... 

Figure 50 shows clearly that most of the Mn cations were introduced during the initial stage of the reaction (i.e., within the first hour). Subsequently, the reaction proceeded rather slowly (compare also the system C0CI2/H-CLIN, vide supra). This kinetic behavior in solid-state ion exchange was confirmed by TPE measurements of HCl evolution [188]. 

Chemical solid state processes are dependent upon the mobility of the individual atomic structure elements. In a solid which is in thermal equilibrium, this mobility is normally attained by the exchange of atoms (ions) with vacant lattice sites (i.e., vacancies). Vacancies are point defects which exist in well defined concentrations in thermal equilibrium, as do other kinds of point defects such as interstitial atoms. We refer to them as irregular structure elements. Kinetic parameters such as rate constants and transport coefficients are thus directly related to the number and kind of irregular structure elements (point defects) or, in more general terms, to atomic disorder. A quantitative kinetic theory therefore requires a quantitative understanding of the behavior of point defects as a function of the (local) thermodynamic parameters of the system (such as T, P, and composition, i.e., the fraction of chemical components). This understanding is provided by statistical thermodynamics and has been cast in a useful form for application to solid state chemical kinetics as the so-called point defect thermodynamics. 

The NO decomposition was studied over Cu-ZSM-5 catalysts prepared by ion exchange and solid state exchange methods. Transient kinetic studies and FTIR measurements indicated the presence of excess oxygen on the surface which is involved in the formation of Cu (0)(NO)(NO2) species. These intermediates play a key role in the NO decomposition. 

Chiral metal centers are usually stable in the solid state at ambient temperature, but behave differently in solution where the epimerization process can even occur at room temperature (—293 K) with half-life (ri/2) less than 24 h (Table 3.1) [104]. Brunner et al. have carried out temperature-dependent kinetic experiments for the epimerization reaction and Cl/I exchange on Ru(II) complexes containing cyclopentadienyl and phosphine ligands (5r , Sc)-/(Rru, 5c)-[CpRu(Chairphos)Q] ((S)-Chairphos = (S)-l,3-bis(diphenylphosphanyl)butane) and c/s-/ira s-[CpRu(Dppm-Me)Q] (Dppm-Me = l,l-bis(diphenylphosphanyl) ethane) [104]. Mechanistic studies on epimerization concluded that the chelate ring size and consequently the bond angle between donor atoms and metal ion (P-Ru-P) in the transition state (16-electron species) determine the rate of... 

Transport of a species through a liquid membrane is superior to solvent extraction (SX) since extraction and stripping are performed in a single unit operation. Also, liquid membrane transport is a non-equilibrium, steady state process which depends upon kinetic factors in contrast to SX which is an equilibrium process. Furthermore, even solvents with low distribution coefficients for Ae desired species may be utilized in LM processes. Although LM systems generally have slower rates than ion-exchange (DC) processes, the latter are particularly sensitive to the presence of suspended solids and other foulants and also must be operated in cycles. 

Roen, L. M., Paik, C. H. and Jarvi, T. D. (2004) Electrocatalytic corrosion of carbon support in PEMFC cathodes. Electrochem. Solid-State Lett. 7, A19-A22 Samec, Z., Trojdnek, A., Langmaier, J. and Samcova, E. (1997) Diffusion coefficients of alkeili metal cations in Nafion from ion-exchange measurements - An advanced kinetic model. J. Electrochem. Soc. 144, 4236-4242... 

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