The Role of Effective Rate COnstants in Interfacial Kinetics

摘要

A brief excerpt of a more comprehensive quantitative theoretical analysis on the role of effective rate constants in interfacial processes is given. Chemical incorporation, tracer incorposration and steady state electrical experiments are considered. It is shown that besides experimental differences there are mechanistic and conceptual differences between the three effective rate ocnstants. The treatment based on irreversible thermodynamics and chemical kinetics shows similarities and differences with respect to the analogous situation in the bulk. As special cases adsorpton and transfer limited kinetics are cosidered. Having related the K-values to the exchange rates of the rate determining step and to the chemical capacitance outsidethe boundary zone, the characteristic dependencies on controllign parameters and also the correlaton with diffusion coefficients can be derived. This is done for electron-rich materials such as SOFC cathodes. A further point which is briefly mentioned in this contect isthe fact that flux constriction effects may lead to apparent surface rate constants. The role of space charges is briefly discussed for the case ofgrain boundary kinetics[4]. In electron-poor materials such as SrTiO_3(Fe_2O_3) or ZrO_2(Y_2O_3) addtional mechanistic differences should occur, since in the tracer case mechanisms are possible at the surfaces which do not have to involve electrons directly. Here also idscrepancies with respect to electricla experiments are predicted. SOme experimental results obtained with SrTiO_3 are discussed in this context.