NMR of hydrogen adsorbed on carbon nanotubes

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Abstract

Hydrogen gas is introduced to multi-walled carbon nanotubes and 1H nuclear magnetic resonances are measured as the functions of hydrogen gas pressure and temperature. The resonance frequency is shifted in inverse proportion to temperature due to the super-paramagnetic Fe catalysts that remained in the nanotubes. By the differences in relaxation times, the signal from hydrogen adsorbed on carbon nanotubes is differentiated from that of gas phase hydrogen.

Introduction

Since the discovery of carbon nanotubes there have been explosive interests in using them for hydrogen storage purposes [1]. The curved surface and large surface area of carbon nanotubes make the material particularly attractive [2]. Much of the theoretical and experimental efforts are devoted to find out where the hydrogen goes and which form it takes [3], [4], [5].

Nuclear magnetic resonance (NMR) is a valuable tool to examine the hydrogen adsorbed on the surface [6] or confined in a cage [7]. It is especially so since NMR is sensitive not only to the spin states but also to the rotational states of hydrogen molecules.

Section snippets

Experimental

We use the multi-walled carbon nanotube obtained from a local company.1 Its outer diameter and length are about 50nm and >60μm, respectively. About 1mg of the powder form

Results and discussions

At room temperature, the NMR signal intensity increases with the gas pressure, however, the spectrum does not change in its position nor width.

When temperature is changed, the resonance spectra changes as shown in Fig. 1. The sample was sealed in a can with 3atm of hydrogen gas at room temperature. As the temperature is lowered the resonance peak shifts to higher frequencies. The shift is inversely proportional to absolute temperature, indicating a paramagnetic shift (Fig. 2). The intensity of

Acknowledgements

This work was supported in part by KOSEF through a Korea–Japan collaborative research program and the BK21 project of the Ministry of Education, Korea.

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