Scanning electrochemical microscopy. 16. Study of second-order homogeneous chemical reactions via the feedback and generation/collection modes

Abstract

The application of the scanning electrochemical microscope feedback and generation/collection (G/C) modes in the measurement of second-order homogeneous reactions of electrogenerated species (Le., the ErCzi mechanism) is considered, with particular emphasis on dimerization. Two G/C modes are assessed: (i) tip generation/substrate collection (TG/SC) and (ii) substrate generation/tip collection (SG/TC). The TG/SC mode is shown to be preferable for kinetic studies in terms of the higher collection efficiencies attainable under steady-state conditions. A numerical treatment of the feedback and TG/SC problem, which relates the tip (feedback) and substrate (collection) currents to the tip-substrate separation, the electrode radii, and dimerization rate constant, is developed, and an extensive set of calculated steady-state feedback and collection characteristics is presented that allow construction of appropriate working curves. The theoretical results suggest that fast dimerization rate constants, up to 4 X lo8 M-I s-l (defined in terms of the rate of loss of the monomer), should be accessible to measurement in the steady-state TG/SC mode. The application of the proposed methodology is demonstrated through studies of the reductive coupling of both dimethyl fumarate (DF) and fumaronitrile (FN) in N,N-dimethylformamide. Good agreement between theory and experiment is displayed over a wide range of concentrations, yielding mean values for the dimerization rate constants of 170 M-l s-l (DF) and 2.0 X lo5 M-' s-l (FN).