Mediated electrochemical reduction of oxygen to hydrogen peroxide via a surface-confined naphthoquinone reagent and the mediated electrochemical reduction of a naphthoquinone redox reagent anchored to high surface area oxides

Derivatives of 1,4-naphthoquinone, 2-chloro-3-((2-(dimethylpropylammonio)ethyl)amino)-1,4-naphthoquinone bromide, Ia, and 2-chloro-3-((2-(dimethyl(3-(trimethoxysilyl)propyl)ammonio)ethyl)amino)-1,4-naphthoquinone bromide, Ib, have been synthesized and used as solution and surface-bound catalysts, respectively, for the electrochemical and photoelectrochemical reduction of O/sub 2/ to H/sub 2/O/sub 2/. The surface derivatizing reagent Ib having the -Si(OCH/sub 3/)/sub 3/ functionality can be used to functionalize a variety of surfaces including electrode materials and high surface area oxides. The surface reagent, (Q/QH/sub 2/)/sub surf/, has the same E/sup 0/' as Ia in solution, approx. -0.4 V vs. SCE at pH 7. The (QH/sub 2/)/sub surf/ reacts with O/sub 2/ in aqueous electrolyte at pH 7 with a rate constant > 10/sup 5/ M/sup -1/s/sup -1/ to form H/sub 2/O/sub 2/ and (Q)/sub surf/. High surface area oxides functionalized with Ib yield (M/sub x/O/sub y/)-(Q) that can be electrochemically reduced to (M/sub x/O/sub y/)-(QH/sub 2/) via mediation by a low concentration of Ia in solution. The (M/sub x/O/sub y/)-(QH/sub 2/) can be isolated from the electrolyte solution by filtration and reacted with O/sub 2//H/sub 2/O to yield up to 0.1 M H/sub 2/O/sub 2/ in H/sub 2/O free of electrolyte. Study of the reduction of Ia at rotating-W-disk electrodes derivatized with Ib shows that the redox equilibration of the solution quinone and surface quinone is rapid. Reduction of (Q)/sub surf/ or Ia at visible light illuminated p-WS/sub 2/ can be effected at an electrode potential approx. 0.8 V more positive than at a metallic electode. The overall energetics are such that light can be used to effect the uphill formation of H/sub 2/O/sub 2/ via the quinone-mediated reduction of O/sub 2/. The onset of O/sub 2/ reduction is up to 0.6 V more positive than E/sup 0/'(O/sub 2//H/sub 2/O/sub 2/). The sustained photoassisted reduction of O/sub 2/ to H/sub 2/O/sub 2/ has been demonstrated. 15 references, 8 figures, 2 tables.