Online BET analysis of single-wall carbon nanotube growth and its effect on catalyst reactivation

doi:10.1016/j.carbon.2005.01.013

Carbon

Volume 43, Issue 7, June 2005, Pages 1439-1444

https://doi.org/10.1016/j.carbon.2005.01.013 Get rights and content

Abstract

A system combining pulse chemical vapor deposition (CVD) reaction and cryogenic gas sorption (BET) measurement capabilities was designed to allow the sequential synthesis and online analysis of single-wall carbon nanotubes (SWNTs). Cooling treatment in liquid nitrogen (77 K) during BET measurement was found to be efficient for restoring catalysts when deactivation occurs after carbon deposition. By this treatment, the methane conversion could be enhanced by up to seven times, such as from 5.8% to 42.6% mol. When the temperature changes from 850 °C to 77 K, the metal particles on the tip of nanotubes might contract and be separated from the graphite layer of the nanotubes, leading to more active sites on metal particles being exposed. The single point BET analysis of SWNT has been tested as an efficient method for the rapid online analysis of SWNTs produced by CVD.

Introduction

Single-wall carbon nanotubes (SWNTs) have attracted much attention due to their unique structural, mechanical, and electrical properties [1]. Large scale synthesis of SWNTs is significant for both theoretical research and practical applications. Chemical vapor deposition (CVD) appears to be a promising method to achieve this objective [2]. However, SWNT growth is often terminated after 30 min or less because of a rapid deactivation of the catalysts, as reported in the literature [3], [4]. Here, we report a novel online analysis method for SWNT growth by single point BET surface area measurement, and its significant effect on catalyst reactivation: by a cooling treatment during BET measurement (at 77 K, in liquid nitrogen) after carbon deposition from methane cracking at 850 °C, the methane conversion could be enhanced by as much as seven times. The as-prepared materials contain SWNTs with nodes and bends, and the enhancement of methane conversion after cooling treatment did not increase the amount of carbon impurities when Ar was used instead of N₂ as sorbent for BET analyses, which is significant for SWNT production at a large scale. The online BET analysis has been tested to be efficient for inspecting the process of carbon nanotube (CNT) growth.

Section snippets

Catalyst preparation

MgO supported Fe catalysts were made by impregnating the support materials in salt solutions [5]. In brief, a MgO carrier was prepared by heating alkaline magnesium carbonate at 500 °C. After ultrasonicating the MgO powder in an iron nitrate aqueous solution, the resulting gel was dried at 120 °C, and then ground to a fine power. Finally, the powder was calcined at 650 °C. The loading capacity of Fe(NO₃)₃ · 9H₂O is 0.014 mol per mol MgO.

Apparatus and operation

A reaction system and a desorption chromatographic BET measurement

Catalyst reactivation by cooling treatment and the mechanism

Fig. 2(a) shows the curve of methane conversion during successive circles of reaction and BET measurement. Methane conversion decreased during carbon deposition, and was markedly enhanced after BET measurement. For example, the methane conversion was enhanced from 5.8% to 42.6% after the first BET measurement. This suggested that the cooling treatment at 77 K reactivated the catalyst. It was also observed that the swing range of methane conversion drops when the number of cycles increases,

Conclusions

Cooling treatment at 77 K can efficiently reactivate nanotube CVD catalysts after carbon deposition. This observation can be explained as follows: the binding force between the tubes and the catalysts was destroyed during cooling treatment due to the different thermal expansion coefficients of carbon and metal particles, leading to more active sites on metal particles coming forth. Such a novel method offers the potential to be applied to enhance CNT yield for a wide range of catalysts. In

Acknowledgement

This work is supported by the Chinese National Natural Science Foundation under contract number of 20236020.

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Online BET analysis of single-wall carbon nanotube growth and its effect on catalyst reactivation

Abstract

Introduction

Section snippets

Catalyst preparation

Apparatus and operation

Catalyst reactivation by cooling treatment and the mechanism

Conclusions

Acknowledgement

Carbon

Diam Relat Mater

Carbon

J Phys Chem Solids

Diam Relat Mater

Chem Phys Lett