NMR Study of Preferential Endohedral Adsorption of Methanol in Multiwalled Carbon Nanotubes
journal contributionposted on 12.04.2012, 00:00 by Xin Liu, Xiulian Pan, Wanling Shen, Pengju Ren, Xiuwen Han, Xinhe Bao
Adsorption of 13C-enriched methanol in multiwalled carbon nanotubes (MWNTs) was systematically studied under well-controlled environment by 13C MAS NMR. The results of variable-pressure experiments indicate that the endohedral adsorption of methanol in carbon nanotubes (CNTs) is preferential over the exohedral adsorption. The exohedral adsorption does not occur until the endohedral adsorption saturates. The 13C chemical shift of endohedral methanol exhibits a large upfield shift due to the strong spatial diamagnetic shielding effect induced by the delocalized electrons of nanotubes under the influence of external magnetic field. The exohedral methanol is also shielded, but to a lesser extent. DFT calculations support these experimental results. The 13C spin–lattice relaxation times, T1, of endohedral and exohedral methanol were also measured, and they have different vapor pressure dependence at room temperature. Furthermore, variable-temperature experiments suggest that methanol molecules inside CNTs may form a layered structure at low temperature with one layer close to the wall and the second layer near the center of the nanotubes.
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Preferential Endohedral AdsorptionMWNTCNT13 C chemical shiftdiamagnetic shielding effectendohedral methanol exhibitsexohedral methanolvapor pressure dependenceMultiwalled Carbon NanotubesAdsorptionendohedral adsorption saturatesDFT calculations supportexohedral adsorptionmultiwalled carbon nanotubes13 C MAS NMR