Confinement Effect of Carbon Nanotubes: Copper Nanoparticles Filled Carbon Nanotubes for Hydrogenation of Methyl Acetate

Copper nanoparticles filled carbon nanotubes (CNTs) with varied tube diameters were prepared via a convenient sonication-assisted impregnation approach. CNTs and Cu nanoparticles filled CNTs were thoroughly investigated using transmission electron microscopy, Raman spectroscopy, N2 adsorption, X-ray diffraction, and H2 temperature programmed reduction. Confinement effects of CNTs, originated from its diameter and microstructure, on the filled Cu nanoparticles were discovered. For CNTs with smaller tube diameter, a consequential strong autoreduction for the confined Cu nanoparticles was observed, which showed a downward trend with the increasing anneal temperature. Methyl acetate (MeOAc) hydrogenation forming methanol and ethanol was chosen as model reaction to further explore the influence of CNTs confinement effects on the catalytic performance. In the case of catalysts derived from CNTs with smaller inner diameter (4–10 nm), their catalytic performance were improved after the CNTs heat treatment under Ar atmosphere and the optimum treatment temperature was 973 K. On the contrary, the CNTs catalysts with larger inner diameter (20–30 nm) exhibited reduced catalytic activity after the same heat treatment. In addition, the selectivity to ethanol from MeOAc was also found dependent on CNTs tube diameter. Furthermore, the Cu nanoparticles loaded outside of CNTs catalyst were prepared and compared with the Cu nanoparticles filled inside CNTs catalyst, and the latter exhibited higher catalytic activity because of the confinement effects of CNTs. All in all, when using CNTs as support materials, the likely synergistic effects created by CNTs diameter, the pretreatment on CNTs, and the nanocatalyst loaded inside or outside CNTs are responsible for the catalytic activity of the prepared catalysts. These findings propose an in-depth insight into the confinement of CNTs to the filled metal catalysts, and thus inspire the development of novel catalyst with beneficial catalytic performance.