Crystallization heterogeneous 21 - desorption rates

Macpherson and Unwin (43) developed the theory for dissolution processes at the substrate induced by depleting of electroactive species at the SECM tip. The UME tip can oxidize or reduce the species of interest in solution at the crystal surface. If this species is one of the crystal components, the depletion of its concentration in the solution gap between the tip and substrate induces crystal dissolution. This process produces additional flux of electroactive species to the tip similarly to positive feedback situation discussed in previous sections. Unlike the desorption reaction, where only a small amount of adsorbed species can contribute to the tip current, the dissolution of a macroscopic crystal is not limited by surface diffusion. Accordingly, the developed theory is somewhat similar to that for finite heterogeneous kinetics at the substrate. Several models developed in Ref. 43a-d use different forms of the dissolution rate law applicable to different experimental systems. In general, the rate of the substrate process is (43a) ... 

Heterogeneous catalysis has to deal not only with the catalyzed reaction itself but, in addition, with the complexities of surface properties (different crystal surfaces, different catalytic sites), possible segregation of adsorbates (so-called island formation), contamination or deterioration of catalytic sites, and adsorption and desorption equilibria and rates. Moreover, mass transfer to and from the reaction site is a factor more often than in homogeneous catalysis. In practice, these complications may affect behavior more profoundly than does the kinetics of the surface reaction itself. A practical and balanced kinetic treatment therefore uses simplifications and approximations much more generously than was done in the preceding chapters. Excellent textbooks on the subject are available [G1-G7], so coverage here can remain restricted to a critical overview and indications showing when and how concepts and methods developed in the earlier chapters can be useful. 

---