Nanoparticle Catalysis in Microemulsions: Oxidation ofN,N-Dimethyl-p-phenylenediamine by Cobalt(III) Pentaammine Chloride Catalyzed by Colloidal Palladium in Water/AOT/n-Heptane Microemulsions

The kinetics have been studied of the reaction between p -Me 2 NC 6 H 4 NH 2 (DMPPD) and Co(NH 3 ) 5 Cl 2+, a process of relevance in color photography, in a water/AOT/ n -heptane microemulsion catalyzed by nanoparticles of colloidal palladium. The rate of formation of the colored N, N -dimethyl- p -semiquinonediimine (S + ) was found to be first order in Co(NH 3 ) 5 Cl 2+ and almost zero order in DMPPD and to increase linearly with the concentration of palladium. Studies over the 10−45 °C temperature range gave activation energies which increased markedly as the temperature fell. Voltammograms of the oxidation of DMPPD and the reduction of Co(NH 3 ) 5 Cl 2+ using a rotating Pd electrode were carried out to investigate the catalytic mechanism. Evidence was obtained of DMPPD adsorption on the metal surface which indicated that the rate-determining step in the catalysis was diffusion of Co(NH 3 ) 5 Cl 2+ through a layer of DMPPD adsorbed on the palladium particles in the water pools.