Highly efficient catalytic hydrogenation of nitrobenzene on cobalt- immobilized nitrogen-doped carbon: A dual-sites synergistic effect between cobalt single atoms and cobalt nanoparticles

Non-precious catalysts are a potential candidate for highly efficient conversion of nitrobenzene to aniline if these catalysts can work under mild conditions. Here, we developed a catalyst (Co (SA+NPs) -C/N) by depositing Co species on nitrogen-doped carbon support with a pyrolysis route under a nitrogen atmosphere. As two forms of Co species, both single atom cobalt (Co (SA) ) and cobalt nanoparticles (Co (NPs) ) were distributed on the surface of the catalyst particles. These two forms of Co species provided a synergistic effect (dual-sites effect) on the hydrogenation of nitrobenzene to aniline. Using water as a solvent, the optimized Co (SA+NPs) -C/N catalyst yielded a full conversion of nitrobenzene into aniline at room temperature within 2.5 min in the presence of NaBH 4 with a molar ratio of –NO 2 : NaBH 4  = 1: 3. The observed catalytic activity of Co (SA+NPs) -C/N was much higher than that of the catalyst with only single atom cobalt (Co (SA) -C/N) or only cobalt nanoparticles (Co (NPs) -C/N). The synergistic dual-sites effect is assigned as that Co nanoparticle sites contribute enough sites for the adsorption of nitrobenzene and its intermediates, while the dispersed Co atom sites boost the dissociation of NaBH 4 molecules to active H atoms. The Co (SA+NPs) -C/N catalyst was able to smoothly work at a wide temperature range of 15-45°C, being highly stable with a recycling number of at least ten. This work not only opens up a new approach for the transfer hydrogenation of nitrobenzene, but also deepens the knowledge on their hydrogenation mechanism.