An Aza-Fused pi-Conjugated Microporous Framework Catalyzes the Production of Hydrogen Peroxide

Abstract

[EN] In order to produce hydrogen peroxide in small-scale electrochemical plants, selective catalysts for the oxygen reduction reaction (ORR) toward the desired species are required. Here, we report about the synthesis, characterization, ORR electrochemical behavior, and reaction mechanism of an aza-fused pi-conjugated microporous polymer, which presents high selectivity toward hydrogen peroxide. It was synthesized by polycondensation of 1,2,4,5-benzenetetramine tetrahydrochloride and triquinoyl octahydrate. A cobalt-modified version of the material was also prepared by a simple postsynthesis treatment with a Co(II) salt. The characterization of the material is consistent with the formation of a conductive robust porous covalent laminar polyaza structure. The ORR properties of these catalysts were investigated using rotating disk and rotating disk ring arrangements. The results indicate that hydrogen peroxide is almost exclusively produced at very low overpotentials on these materials. Density functional theory calculations provide key elements to understand the reaction mechanism. It is found that, at the relevant potential for the reaction, half of the nitrogen atoms of the material would be hydrogenated. This hydrogenation process would destabilize some carbon atoms in the lattice and would provide segregated charge. On the destabilized carbon atoms, molecular oxygen would be chemisorbed with the aid of charge transferred from the hydrogenated nitrogen atoms and solvation effects. Due to the low destabilization of the carbon sites, the resulting molecular oxygen chemisorbed state, which would have the characteristics of a superoxide species, would be only slightly stable, promoting the formation of hydrogen peroxide.