Electrochemical reduction of oxygen and hydrogen peroxide catalyzed by a surface copper(II)–2,4,6-tris(2-piridil)-1,3,5-triazine complex adsorbed on a graphite electrode

A graphite electrode irreversibly adsorbed by 2,4,6-tris(2-piridil)-1,3,5-triazine (abbreviated as TPT) was examined by cyclic voltammetry. The adsorbed TPT exhibited two irreversible reduction waves in the potential range of −0.7 and −1.0 V (versus SCE). Upon strong adsorption, TPT can serve as a coordination ligand for copper ions to form a surface complex. Its three adjacent nitrogen positions provide strong affinity to the metal ions and bond copper(II) to an electrode surface. A 1:1 coordination between Cu(II) or Cu(I) and the TPT ligand to form [Cu(II)(TPT)]2+ or [Cu(I)(TPT)]+ is the predominant process, evidenced by spectrophotometry, surface cyclic voltammetry, and coordinated structural feasibility of Cu(II)/Cu(I)–TPT complexes. The predominant copper(II)–TPT surface complex shows a reversible redox wave, which is identified as one-electron process of [Cu(II)(TPT)]2+ ↔ [Cu(I)(TPT)]+. The electrode adsorbed by [Cu(II)(TPT)]2+ complex showed electrocatalytic activity towards oxygen and/or hydrogen peroxide reductions. The catalyzed reduction of oxygen and hydrogen peroxide were identified as four-electron and two-electron process to form water. It is suggested that the possible electrocatalytic reductions were due to an inner-sphere mechanism, which involved a coordination between substrate (O2 or H2O2) and [Cu(I)(TPT)]+. The reduction kinetics were also investigated by a rotating disk electrode method.