Heterogeneous reactions, scanning electrochemical microscopy

Scanning Electrochemical Microscopy Another way in which it can be confirmed that oxidative and reductive pho-tocatalytic reactions simultaneously occur on Ti02 particles is to set up a simple heterogeneous model electrode system. 

Borgwarth, K., and J. Heinze. 2001. Heterogeneous electron transfer reactions. In Scanning electrochemical microscopy, eds. A.J. Bard and M.V. Mirkin, 201. New York Marcel Dekker. 

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. 

Fernandez, J. L., C. Hurth, and A. J. Bard. 2005. Scanning electrochemical microscopy 54. Application to the study of heterogeneous catalytic reactions—Hydrogen peroxide decomposition. J. Phys. Chem. B 109 9532-9539. 

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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