Quantitative Electrochemical Kinetics Studies of "Microelectrodes": Catalytic Water Reduction By Methyl Viologen/Colloidal Platinum

Article abstract of DOI:10.1021/ja00413a001

The reduction of water by methyl viologen, catalyzed by colloidal platinum, has been extensively studied as a function of pH, radical concentration, and platinum concentration by several independent techniques.The data present a comparative test of a homogeneous kinetic approach and a novel electrochemical approach.Two techiniques were used to monitor the platinum-catalyzed oxidation of methyl viologen radical: conventional and stopped flow mixing experiments, and a novel application of electrocatalytic cyclic voltammetry.Conventional kinetic analysis for homogeneous reactions provides conditional rate constants which are great than first order in platinum.At = 10^-6 M and pH 3.0, the conditional rate constant is k = 1.2*10⁴ L mol^-1 s^-1.In addition, steady state catalysis of hydrogen production analogous to photochemical systems was monitored in a nonphotochemical system, using electrochemically reduced mediators.The methods are critically compared as mechanistic tools.Only data obtained with use of steady state techniques can be quantitatively analyzed by using simple electrochemical theory.The principle findings include the following: (1) a surprising apparent second order rate dependence on colloid concentration is accounted for by electrochemical theory, but not by homogeneous theory; (2) the electrochemical model quantitatively acounts for the shift in pH_1/2 with mediator concentration; and (3) with use of electrogenerated mediator an experimental value for log j_H2⁰ = -3.4 is obtained for colloidal platinum, in good agreement with the bulk electrode.