Electrochemical study of the heterogeneously catalysed reaction between N,N-dimethyl-p-phenylenediamine and CoIII(NH3)5Cl2+ at monometallic and bimetallic surfaces of silver and gold

The mechanism of the heterogeneously catalysed reaction between N,N-dimethyl-p-phenylenediamine and CoIII(NH3)5CI2+ at silver, gold and silver-on-gold (Ag/Au) discs has been studied by means of electrochemical methods. Both the mixed (or mixture) potentials and the mixture currents were determined by recording the current–potential curves of the reactants. Silver halide, formed during the reaction, was determined by subsequent galvanostatic reduction. The reaction at silver was strongly inhibited by the formation of silver halide whereas the reaction at gold was inhibited by adsorption of the organic compounds as well as by iodide. At Ag/Au discs almost no inhibition occurred. The explanation is that the reduction of the cobalt complex takes place predominantly at the silver surface whereas the simultaneous oxidation of the p-phenylenediamine occurs on the gold. In this way the formation of inhibiting silver halide is suppressed as well as the inhibition caused by the adsorption of p-phenylenediamine.