Ultralow-loading single-atom cobalt on graphitic carbon nitrogen with robust Co-N pairs for aerobic cyclohexane oxidation
Nano Research(2022)
摘要
Selectively aerobic oxidation of cyclohexane using the homogeneous catalysts is an industrially important process, suffering from the difficult catalyst separation, low conversion and selectivity. Herein, a series of single-atom Co catalysts, possessing the ultralow Co loading of below 1.0 wt.‰, supported on mesoporous graphitic carbon nitrogen (Co/g-C 3 N 4 - w ) were prepared by a simple adsorption method and applied into the cyclohexane oxidation. Characterization results demonstrate that the confinement effect of the voids in g-C 3 N 4 facilitates the formation of the single-atom Co, which is stabilized by bonding with the pyridinic nitrogen of g-C 3 N 4 and accompanied by the electron transfer from Co to g-C 3 N 4 . The catalytic performance presents an increasing trend with the increment of the Co loading, and the superior value with the conversion of 23.8% and selectivity of 95.6% is obtained over Co/g-C 3 N 4 -0.9. Kinetic study, density functional theory (DFT) calculations, and characterizations reveal that the decreased activation energy of cyclohexane oxidation over Co/g-C 3 N 4 - w can be attributed to the favorable dissociation activation of O 2 molecules and decomposition of cyclohexylhydroperoxide (CHHP) intermediate on the coordination unsaturated single-atom Co as well as the enhanced adsorption of cyclohexane on the electron-rich g-C 3 N 4 surface, boosting the cyclohexane oxidation following the surface catalytic mechanism. Distinctively, robust Co-N structures and hydrophobic nature of g-C 3 N 4 contribute to the high stability of Co/g-C 3 N 4 -0.9 for cyclohexane oxidation.
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关键词
single-atom Co, carbon nitride, cyclohexane, catalytic oxidation
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