Cobalt single atoms supported on monolithic carbon with a hollow-on-hollow architecture for efficient transfer hydrogenations

Single-atom catalysts (SACs) have received considerable attention in hydrogenation of nitroaromatic compounds to aromatic amines. In order to enhance the exposure of single atoms and overcome the mass transfer limitation, construction of hierarchical porous supports for single atoms is highly desirable. Herein, we report a straightforward method to synthesize Co single-atoms supported on a hollow-on-hollow structured carbon monolith (Co₁/HOHC-M) by pyrolysis of α-cellulose monolith loaded with PS-core@ZnCo-zeolite imidazolate frameworks-shell nanospheres (PS@Zn-ZIFs/α-cellulose). The hollow-on-hollow structure consists of a large hollow void with a diameter of ~ 290 nm (derived from the decomposition of polystyrene (PS) nanospheres) and a thin shell with hollow spherical pores with a diameter of ~ 10 nm (derived from the evaporation of ZnO nanoparticles that are in-situ formed during pyrolysis in the presence of CO₂ from α-cellulose decomposition). Benefitting from the hierarchically porous architecture, the Co₁/HOHC-M exhibits excellent catalytic performance (reaction rate of 421.6 mmol·g_Co⁻¹·h⁻¹) in the transfer hydrogenation of nitrobenzene to aniline, outperforming the powdered sample of Co₁/HCS without the hollow spherical mesopores (reaction rate of 353.8 mmol·g_Co⁻¹·h⁻¹). It is expected that this strategy could be well extended in heterogeneous catalysis, given the wide applications of porous carbon-supported single-atom catalysts.