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

Abstract

ABSTRACT:The electrochemical production of hydrogen peroxide can be implemented in small-scale plants under "on demand" approach. For that, selective catalysts for the oxygen reduction reaction (ORR) towards the desired species are required. Here, we report about the synthesis, characterization, ORR electrochemical behavior and reaction mechanism of an aza-fused π-conjugated microporous polymer, which presents high selectivity towards hydrogen peroxide. It was synthesized by polycondensation of 1,2,4,5-benzenetetramine tetrahydrochloride and triquinoyl octahydrate. A cobaltmodified version of the material was also prepared by a simple post-synthesis treatment with a Co(II) salt. The characterization of the material is consistent with the formation of a conductive robust porous covalent laminar poly-aza 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 overpotential values 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 provides 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 has the characteristics of a superoxide species, would be only slightly stable and would promote the formation of hydrogen peroxide.

Dynamic Covalent Synthesis of Crystalline Porous Graphitic Frameworks

“…However, to date, the ideal PGF-1 that exhibits a high level of structural precision has not been realized as prior synthetic attempts only resulted in amorphous crosslinked microporous polymers. [13][14][15][16] The great challenge of synthesizing crystalline PGF-1 lies in the lack of effective chemical tools to covalently assemble annulated aromatic units into periodic 2D sheets. Retrosynthetically, the formation of PGF-1 can be conceived by stitching the two monomeric units: C2-symmetric 1,2,4,5-benzenetetramine (BTA) tetrahydrochloride and C3-symmetric hexaketocyclohexane (HKH) octahydrate ( Figure 1).…”

Solar‐Driven Production of Hydrogen Peroxide from Water and Dioxygen

“…Among them, [Co II (Ch)] exhibits the highest catalytic reactivity with as mall overpotential for selectivet wo-electron/two-proton reduction of O 2 to produce H 2 O 2 .Ametal-free aza-fused p-conjugated microporous polymer (Aza-CMP) also acts agood electrocatalyst for the selective two-electron/two-protonr eductiono fO 2 to H 2 O 2 with an overpotentialo fb elow 100 mV. [156] There have been an umber of reports on electrocatalysts for selective two-electron/twoproton reduction of O 2 to produce H 2 O 2 against four-electron/ four-proton reduction of O 2 to produce H 2 O. Thus, solar-driven production of H 2 O 2 from H 2 Oa nd O 2 hasb een made possible by the combination of the catalytic four-electron/four-proton oxidation of H 2 Ot oO 2 and the photocatalytic two-electron/ two-proton reduction of O 2 to H 2 O 2 .T he maximum concentration of H 2 O 2 (61 mm)w as attained by using at wo-compartment cell with the FeO(OH)/BiVO 4 /FTO photoanode and the [Co II (Ch)]/MWCNT/GC cathode under simulated sun illumination.…”

“…[156] In addition, the current efficiency for H 2 O 2 production with Aza-CMP is about 70 %, while the selectivity is above 80 %a ss hown in Figure 13, which demonstrates the excellent performance of the metal-freec atalyst as compared with Aza-CMP modified with Co II (Aza-CMP@Co). [156] At an appliedp otential of 0.60 V, which corresponds to an overpotential of À0.08 V, the measured currentd ensity at 1600 rpm is À1.17 mA cm À2 ,w hich is much larger than the value [À0.3 mA cm À2 with similars electivity for H 2 O 2 production (ca. 75 %)] of best-performing Pt-Hg alloy at this potential value.…”

“…Representation of the PtHg 4 (110) [155] An aza-fused p-conjugated microporous polymer (Aza-CMP) was synthesized by the polycondensation reactionw ith water formationb etween 1,2,4,5-benzenetetramine tetrahydrochloride and triquinoyl octahydrate ( Figure 12). [156] Metal-free Aza-CMP was reported to act as an excellent electrocatalyst for the two-electron/two-protonr eduction of O 2 to produce H 2 O 2 , when the onset potential is 0.65 Vand the overpotential for effective currents in the reduction process of O 2 to H 2 O 2 is below 100 mV. [156] In addition, the current efficiency for H 2 O 2 production with Aza-CMP is about 70 %, while the selectivity is above 80 %a ss hown in Figure 13, which demonstrates the excellent performance of the metal-freec atalyst as compared with Aza-CMP modified with Co II (Aza-CMP@Co).…”

“…[156] Metal-free Aza-CMP was reported to act as an excellent electrocatalyst for the two-electron/two-protonr eduction of O 2 to produce H 2 O 2 , when the onset potential is 0.65 Vand the overpotential for effective currents in the reduction process of O 2 to H 2 O 2 is below 100 mV. [156] In addition, the current efficiency for H 2 O 2 production with Aza-CMP is about 70 %, while the selectivity is above 80 %a ss hown in Figure 13, which demonstrates the excellent performance of the metal-freec atalyst as compared with Aza-CMP modified with Co II (Aza-CMP@Co). [156] At an appliedp otential of 0.60 V, which corresponds to an overpotential of À0.08 V, the measured currentd ensity at 1600 rpm is À1.17 mA cm À2 ,w hich is much larger than the value [À0.3 mA cm À2 with similars electivity for H 2 O 2 production (ca.…”