ELECTROCATALYSIS OF ANODIC OXYGEN-TRANSFER REACTIONS - OXIDATION OF PHENOL AND BENZENE AT BISMUTH-DOPED LEAD DIOXIDE ELECTRODES IN ACIDIC SOLUTIONS

The electrocatalytic activities are compared for pure beta-lead dioxide electrodes (PbO2) and bismuth-doped lead dioxide electrodes (Bi-PbO2) for oxidation of dimethylsulfoxide (DMSO), phenol, and benzene in acidic media. A correlation is observed between the bismuth content of Bi-PbO2 films on Au disk electrodes and the apparent heterogeneous rate constant for conversion of DMSO to DMSO(2). The oxidation of phenol occurs at a transport-limited rate for both the PbO2 and Bi-PbO2 film electrodes; however, the Bi-PbO2 electrodes exhibit significantly greater stability against surface fouling by polymeric films. This is a direct benefit of the greater electrocatalytic activity of Bi-PbO2 electrodes for oxidation of phenol beyond the phenolic radical. A ring-disk experiment for benzene oxidation at a Bi-PbO2 disk demonstrates that o-benzoquinone and p-benzoquinone (n(aff)=6 eq mol(-1)) are produced at the disk in the ratio ca. 0.25. However, the total ring current for reduction of these products from the disk reaction is significantly below that predicted from the disk current, and it must be concluded that a significant amount of electroinactive species is produced at the disk. This production is speculated to occur with n(aff)>6 eq mol(-1) and probably corresponds to a ring-opening reaction with production of one or more organic acids. The product solution from exhaustive electrolysis of benzene contains p-benzoquinone as the only electroactive product; hence, the o-benzoquinone is concluded to undergo further oxidation at the Bi-PbO2 electrode to one or more electroinactive products.