Highly Efficient Hydrogenation of Nitroarenes by N-Doped Carbon-Supported Cobalt Single-Atom Catalyst in Ethanol/Water Mixed Solvent.

The cobalt single atoms supported on N-doped carbon (Co SAs/NC) were prepared by the direct pyrolysis of metal-organic framework (MOF) precursor. Compared with Co nanoparticle (NP) catalysts prepared by impregnation, the Co SAs/NC catalyst showed a much better performance in the selective hydrogenation of nitrobenzene, giving a specific activity 5.4 and 32.0 times higher than those of Co NPs supported on N-doped carbon and active carbon, respectively. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray absorption fine structure (XAFS), X-ray photoelectron spectra (XPS), N2 adsorption-desorption isotherms, and X-ray diffraction (XRD) characterizations revealed that the atomically dispersed Co species and doped nitrogen atoms in carbon support contributed to the high activity of Co SAs/NC. The reaction rate and aniline selectivity were further remarkably enhanced by introducing water into the reaction system. By adding 30% water to the solvent of ethanol, the activity was increased from 43.2 to 76.8 h-1 and the aniline selectivity was increased from 62.8 to 99.1%. Combining experimental results and density functional theory (DFT) calculations evidenced that the promoting effect of water addition was attributed to its differentiating ability in the competitive adsorption between nitrobenzene and aniline in the hydrogenation reaction. This catalytic system was highly efficient for the selective hydrogenation of various nitroarenes.