Dennis C. Johnson

Electroanalysis, Electrocatalysis, Environmental remediation


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Research Interests

Professor Johnson's research program is focused on the study of electrocatalytic phenomena and their applications for amperometric detection in liquid chromatography and capillary electrophoresis, and for the electrolytic degradation of toxic organic chemical wastes. These applications are anodic (oxidative) and they occur with transfer of oxygen atoms from H2O in the solvent phase to the oxidation product(s).

Virtually all organic compounds are predicted, on the basis of thermodynamic data, to be easily oxidized to their elemental oxides (CO2, etc.) at conventional anode materials (Au, Pt, GC) in aqueous and mixed solvents. However, the requisite anodic O-transfer reactions are virtually always characterized by very slow heterogeneous kinetics. The premise of Johnson's research is that these reactions can be catalyzed when the electrode surface is capable of participating within the O-transfer mechanisms. The reactive intermediate oxygen species is believed to correspond to adsorbed hydroxyl radicals (·OH) generated by anodic discharge of H2O. Also viewed as important in these mechanisms is the adsorption of reactant species at the anode surface.

At Au and Pt electrodes, the reactive surface oxygen (AuOH and PtOH) is generated as the intermediate product of the anodic formation of inert surface oxides (AuO and PtO) following the application of a positive potential step. These reactive oxygen species are short-lived (< ca. 1 s) and, therefore, activity for O-transfer reactions is quickly attenuated. However, the inert oxides are easily and quickly reduced back to the pristine metals by a negative potential pulse. Therefore, multistep waveforms applied at frequencies of 0.5-5 Hz can be applied to achieve electrocatalytic anodic detection of numerous polar aliphatic compounds. Compounds detected include alcohols, alditols and carbohydrates; amines, amino alcohols, and amino acids; and organic thiols, thioethers, and disulfides. Applications of this pulsed electrochemical technology are becoming widespread in liquid chromatography.

The second important aspect of Johnson's research program is the development of new anode materials that exhibit specific activity for anodic O-transfer reactions under constant applied potential. In general, these new materials are electrically conductive mixed-metal oxides, as illustrated by the examples of lead dioxide films doped with Fe(III). It is believed that the mixed-metal oxide electrodes can be useful for both amperometric detection in liquid chromatography as well as for electrolytic degradation of organic chemical wastes. As an example of the later application, a recent work has demonstrated that 4-chlorophenol is degraded to CO2 and ClO4-. Johnson's group is especially interested in the study of the mechanisms of these degradation processes.