| Several structurally modified water-soluble metalloporphyrins have been synthesized and characterized. The electrochemistry of these metalloporphyrins also have been studied in the absence and presence of oxygen. The E(DEGREES)('') of Fe(2-TMPyP) is 0.17 V more positive than that of Fe(4-TMPyP). Their cobalt analogues, Co(2-TMPyP) and Co(4-TMPyP), have 0.13 V difference in redox potential. The redox potentials of the iron porphyrins were in the order of Fe(2-TMPyP) > Fe(4-TMPyP) > Fe(3-TMPyP) and paralleled the non-metallated forms in nitrogen purged 0.1 N H(,2)SO(,4) solution. The normalized cyclic voltammetric peak currents for the catalyzed reduction of oxygen as a function of catalyst concentration were about equivalent for Fe(4-TMPyP) and Fe(3-TMPyP) whereas, for Fe(2-TMPyP) the peaks were considerably lower.;Cobalt porphyrins have low heterogeneous electron-transfer rate due to their slow outer-sphere self-exchange rate. The addition of thiocyanate ligand can enhance the heterogeneous electron-transfer rate. Spectroelectrochemistry of Co(III)(4-TMPyP)(SCN) indicates that the thiocyanate ligand is displaced when the Co(III) is reduced. It is suggested that thiocyanate ligation is responsible for the enhancement of the rate of heterogeneous electron-transfer by changing the pathway of electron transfer from outer-sphere to inner-sphere.;The kinetic rates for O(,2) reduction catalyzed by Co(4-TPyP) and Co(2-TAPP) were 1.0 x 10('6) M('-1)s('-1) and 4.2 x 10('6) M('-1)s('-1) respectively determined from the Koutecky-Levich plots. Electrolysis of O(,2) reduction catalyzed by adsorbed Co(4-TPyP) exhibited some deactivation/reactivation phenomenum, in which the activity of Co(4-TPyP) could be restored by open circuit. The deactivation rate was lower in the presence of SCN('-) and was also lower for Co(2-TAPP) than Co(4-TPyP). A detailed EC mechanism including the formation of peroxy-CoP intermediate was proposed.;Another iron porphyrin, Fe(2-TAPP), appeared to strongly adsorb on a glassy carbon surface to catalyze an overall four electron reduction of oxygen. The E(DEGREES)('') of the adsorbed Fe(2-TAPP) is more positive than that of the bulk Fe(2-TAPP) by 0.05 V in 0.1 N H(,2)SO(,4) solution. Moreover, the pKa of the adsorbed Fe(2-TAPP) is shifted by more than 2 units compared with that of the bulk catalyst. The E(,p,cat) for O(,2) reduction is more positive by 0.06 V than the E(DEGREES)('') of the adsorbed catalyst. |
