Homogeneous reactions, scanning electrochemical microscopy

An attempt was made to model the interfacial chemistry in microemulsions by using the interface between two immiscible electrolyte solutions [95]. The reaction between the electrochemically generated Co(I) form of vitamin B12 in the aqueous phase and /-DBCH in benzonitrile was probed directly at the interface by using scanning electrochemical microscopy. The kinetics of /-DBCH reduction by vitamin B12 was observed to be more complex at the liquid/liquid interface than in a homogeneous solution [95]. 

Approach curve (SECM) — A current-distance curve recorded as an ultramicro electrode (tip) approaches a surface in scanning electrochemical microscopy. On a surface where the reverse of the tip reaction occurs on the substrate surface the current increases as the distance between the tip and substrate, d, decreases (positive feedback). On an insulator surface the tip current decreases with decreasing d (negative feedback). The approach curve is useful in determining the absolute magnitude of d and can provide information about the heterogeneous electron transfer kinetics on the substrate and the rates of homogeneous reactions that occur in the gap between tip and substrate. 

FIGURE 5.12 Kinetic parameter k as a function of the collection efficiency ( /s//x )- (A) E Cj mechanism K=k tPlD, solid line was computed from Equation 5.85, diamonds are simulated data taken from Ref. [5]. (B) EjCji mechanism k =c° k tP/aD, solid line was computed from Equation 5.88, squares are simulated data taken from Ref. [54]. (Reprinted with permission from Treichel, D.A., Mirkin, M.V., and Bard, A. J., Scanning electrochemical microscopy. 27. Application of a simplified treatment of an irreversible homogeneous reaction following electron transfer to the oxidative dimerization of 4-nitrophenolate in acetonitrile, J. Phys. Chem., 98, 5751-5757, 1994. Copyright 1994 American Chemical Society.)... 

Zhou, F., Unwin, P. R., Bard, A. J. 1992. Scanning electrochemical microscopy. 16. Study of second-order homogeneous chemical reactions via the feedback and generation/coUection modes. J Phys Chem 96 4917 924. 

Treichel, D. A., Mirkin, M. V., Bard, A. J. 1994. Scanning electrochemical microscopy. 27. Application of a simplified treatment of an irreversible homogeneous reaction following electron transfer to the oxidative dimerization of 4-nitrophenolate in acetonitrile. J Phys Chem 98 5751-5757. 

Scanning electrochemical microscopy (SECM) and related ultramicroelectrode (UME) methods have proven powerful for measuring the kinetics of homogeneous reactions coupled to heterogeneous electron transfer. For this type of investigation, the tip and substrate are both electrodes and one can usefully consider the tip/substrate electrode configuration as a variable gap ultrathin layer cell. In essence, the gap thickness determines the diffusional transit time of chemical species between the tip and substrate and hence the range of timescales that can be studied. 

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