Facile Preparation of Nitrogen-Doped Carbon Spheres with Wrinkled Cage-Supported Single-Atom Copper Catalysts for Selective Oxidation of Glycerol to Formic Acid

Selective conversion of the surplus glycerol (GLY) to a hydrogen storage material, formic acid (FA), in heterogeneous catalytic systems is still challenging in the catalysis community. Herein, nitrogen-doped carbon spheres with wrinkled cage-supported single-atom Cu catalysts (Cu/NCSWCs) were prepared by wet ball milling of 5,5′-diamino-3,3′-bis(1H-1,2,4-triazole) (DABT) and copper nitrate with hydrothermal carbon-encapsulated dendritic fibrous nanosilica (DFNS) as the hard template in methanol. After the removal of DFNS, structural characterization revealed that single copper atoms with four nitrogen coordination were successfully immobilized on NCSWCs (CuN4/NCSWCs), which is attributed to the fact that DABT could act as the chelating agent and nitrogen source. Furthermore, due to their unique CuN4 structures and large meso/macropore volumes derived from their wrinkled cages, CuN4/NCSWCs exhibited the highest catalytic activity and selectivity in oxidation of GLY to FA using H2O2 as the oxidant. In contrast, only C3 and C2 products were produced when O2 was used as the oxidant. Electron paramagnetic resonance measurements indicated that H2O2 and O2 can be activated to generate ·OH and ·O2– over CuN4/NCSWCs, respectively. Experimental studies and density functional theory calculations revealed that ·OH adsorbed on CuN4 structures reacted with GLY to produce FA through consecutive steps involving dehydrogenation, oxidation, and oxidative cleavage. Moreover, CuN4/NCSWCs also exhibited high catalytic stability during eight consecutive catalytic runs.