Formation of hydrogen peroxide by electrochemical reduction of molecular Oxygen using luminol chemiluminescence

Formation of hydrogen peroxide by electrochemical reduction of molecular oxygen was examined by measuring luminol chemiluminescence and absorption spectrum using flow-injection method. Ferryl porphyrin is widely accepted as responsible species to stimulate the emission in hydrogen peroxide / iron porphyrin / luminol system. Emission was observed under the cathodic potentials (0.05 V at pH 2.0 and -0.3V at pH 11.0) by the electrochemical reduction of aerated electrolytes solution but no emission was observed at anodic potentials. Iron porphyrin solution was added at the down stream of the working electrode and was essential for the emission. Removal of dissolved molecular oxygen resulted in the decrease of emission intensity by more than 70%. In order to examine the life time of reduced active species, delay tubes were introduced between working electrode and FeIII TMPyP inlet. Experimental results suggested the active species were stable for quite a long period. The emission was quenched considerably ( 90%) when hydroperoxy catalase was added at the down stream of the working electrode whereas Superoxide dismutase (SOD) had little effect and mannitol had no effect. The spectra at reduction potential under aerated condition were shifted to the longer wavelength ( 430nm) compared to the original spectrum of FeIII TMPyP (422nm), indicating that the ferryl species were mixed to some extent. These observations lead to the conclusion that hydrogen peroxide was produced first by electrochemical reduction of molecular oxygen which then converted FeIII TMPyP into O=FeIV TMPyP to activate luminol. Comparing emission intensities with the reference experiments, the current efficiencies for the formation of hydrogen peroxide were estimated as about 30-65% in all over the pH range used.