Identifying active sites of CoNC/CNT from pyrolysis of molecularly defined complexes for oxidative esterification and hydrogenation reactions

Abstract

As an emerging catalyst based on earth-abundant metals, Co₃O₄ supported on carbon materials, derived from the pyrolysis of metal salts and nitrogen-containing ligands, shows excellent performance in hydrogenation, hydrogenated coupling, oxidative esterification and oxidation of amines. Herein, we unravel the real active sites of this catalyst through a combination of XRD, XPS, HRTEM, EELS and poisoning experiments with sulfur-containing compounds. The oxidative esterification of benzyl alcohol, hydrogenation of nitrobenzene and hydrogenated coupling of nitrobenzene with benzaldehyde were used as probe reactions. By comparing the catalytic performance before and after HCl washing, it was demonstrated that particulate Co₃O₄ has a marginal effect on the catalysis, and the highly dispersed CoN_x on carbon nanotubes created during the pyrolysis is responsible for the activity. It was proposed that cobalt chelate complexes bonded to 2 to 3 nitrogens in the graphene lattice, probably like the pyridinic vacancy, might be responsible for the activity.