Pyrolytic graphite ring electrode

It has been recently demonstrated that the simplest of the cobalt porphyrins (Co porphine) adsorbed on a pyrolytic graphite electrode is also an efficient electrocatalyst for reduction of 02 into 1120.376 The catalytic activity was attributed to the spontaneous aggregation of the complex on the electrode surface to produce a structure in which the cobalt-cobalt separation is small enough to bridge and activate 02 molecules. The stability of the catalyst is quite poor and largely improved by using porphyrin rings with mew-substitu-tion.377-380 Flowever, as the size of the mew-substituents increases the four-electron reduction efficiency decreases. 

Electrochemical oxidation of adenine and hydroxyadenines in aqueous solution at pH 3-11.2 using a pyrolytic graphite electrode gave after extended oxidation, a diimine species which undergoes a series of reactions to give various ring-opened products <9UCS(P2)1369> (see Section 7.11.5.2.4). 

Cyclic voltammetric data at a pyrolytic graphite electrode of 2,6-dimercaptopurine show three anodic peaks. The first and second correspond to a disulfide formation from the 6- and 2-mercapto groups, respectively, whereas the third peak is due to an oxidation to purine-2,6-disulfonic acid. Some disulfinic acid was obtained on preparative oxidation [476]. Oxidation of 6-thioguanine gives at low potential the disulfide, whereas oxidation at more positive potentials involves both the sulfur group and the purine ring [477]. 

Figure 3.9 (A) Dynamic polarization curves obtained using the pyrolytic graphite (PC) disk (A = 0.46 cm2) of a Pt PG RRDE recorded in 1.5 mM Co (111) (cyclam) in 0.5 M HCIO4 aqueous solutions,forw= 100,400,900 and 1600 rpm, where SSCE stands for a sodium chloride saturated calomel electrode. The corresponding ring currents obtained for Ering = 0.6 V are shown in the lower panel. (B) Plots of fJjTjc and /ring versus CO1 2.

Figure 7.5 (A) Oxygen reduction and hydrogen peroxide oxidation on a bare pyrolytic graphite disk-Pt ring electrode (B) Oxygen reduction and hydrogen peroxide oxidation on an oxidized pyrolytic graphite disk-Pt ring electrode in Oa-saturated 0.1 M KOH and (C) Proposed ORR mechanism. Potential scan rate 5 mV s ring potential 1.34 V vs 1 M NaOH//Hg/HgO. Reprinted with permission from Ref. 24.

R = NH2) and 6-thioxanthine (R = OH) at sufficiently positive potentials to oxidize the purine ring in 0.5 M phosphate buffer (pH 7.0) at pyrolytic graphite electrode. The pathway explains formation of the identified product (VII) 5-guanidino-hydantoin (R = NH2) allantoin (R = OH). 

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