Elsevier

Journal of Catalysis

Volume 225, Issue 2, 25 July 2004, Pages 453-465
Journal of Catalysis

Synthesis of uniform diameter single-wall carbon nanotubes in Co-MCM-41: effects of the catalyst prereduction and nanotube growth temperatures

https://doi.org/10.1016/j.jcat.2004.04.022Get rights and content

Abstract

Catalyst pretreatment and CO disproportionation reaction conditions were observed to strongly affect the diameter uniformity of single-wall carbon nanotubes (SWNT) grown on Co-MCM-41 catalysts. The prereduction and CO disproportionation reaction temperatures were varied systematically while the carbon loading and the SWNT diameter uniformity were monitored by TGA, Raman spectroscopy, and TEM. The state of the catalyst during prereduction and the size of the cobalt clusters formed during the SWNT growth process were monitored by in situ XANES during the prereduction of the Co-MCM-41, and ex situ EXAFS of catalyst samples was performed after carbon deposition. These experiments allow development of correlations between the SWNT quality and the state of the catalyst. Control of the cobalt cluster size in the Co-MCM-41 catalyst is critical to the SWNT diameter control. The size of the cobalt cluster changes with both the prereduction and the SWNT synthesis temperatures. SWNT with a very narrow diameter distribution can be grown in Co-MCM-41 by controlling both the prereduction and the reaction temperatures.

Introduction

Many recent experimental studies have been dedicated to the synthesis of single wall carbon nanotubes (SWNT) due to their special electronic and mechanical properties [1], [2], [3]. SWNT growth techniques explored so far produce either a broad distribution of tube diameters [4] or SWNT of a narrow diameter distribution at a certain average diameter size imposed by the process used [5], [6]. The electronic properties of the SWNT depend on their diameter and chirality. Most electronic applications require SWNT of uniform electronic properties. This requirement may be fulfilled by a synthesis process growing SWNT of uniform diameter and structure. We have recently developed a catalytic system that allows engineered diameter control of the SWNT produced within ±0.05 nm [7]. Mean diameter can be varied over the range from 0.5 to 1 nm. The cobalt catalytic component is incorporated into the pore wall of the MCM-41 mesoporous molecular sieve by isomorphous silicon substitution. During the SWNT growth process, cobalt is reduced and nucleates into metallic clusters that initiate the growth of the carbon nanotubes.

Our previous investigations of SWNT growth by carbon monoxide disproportionation showed SWNT yield, selectivity, and diameter uniformity to be strongly influenced by both catalyst pretreatment and the SWNT growth reaction conditions. Similar observations were previously reported by Resasco's research group working with a different catalytic system consisting of a silica-supported bimetallic cobalt–molybdenum particles [8], [9], [10]. The sensitivity of SWNT diameter distribution to growth temperature was also addressed by many other researchers [11], [12], [13], [14]. The purpose of the present contribution is to investigate the influence of the process variables on the selectivity and the diameter uniformity of SWNT produced in a monometallic Co-MCM-41 catalyst and how these are affected by the state of the catalyst. This is a first critical step toward the optimization of the synthesis conditions. The prereduction and reaction temperatures were varied systematically while monitoring the carbon yield and selectivity, and the diameter uniformity of the SWNT produced. The states of the cobalt catalyst in the MCM-41 material at different stages of pretreatment and reaction were investigated by extended X-ray absorption fine structure (EXAFS), and X-ray absorption near-edge structure (XANES) was carried out in situ during the prereduction step. The ultimate goal is to understand the correlation between the state of the cobalt catalyst and the quality of the SWNT produced.

Section snippets

Catalyst

A Co-MCM-41 catalyst with 1 wt% cobalt loading (analyzed by inductively coupled plasma (ICP) at Galbraith Laboratories, Inc.) was prepared following the procedure described in Ref. [15] using a hexadecyltrimethylammonium bromide (C16H33(CH3)2NBr) as templating material. All reagents used were research grade; the purity of the Cab-O-Sil fumed silica source was 99.8% with chlorine as the only identified impurity (78 ppm). Therefore, cobalt is of very high purity with respect to metals present in

Results and discussion

The experiments were designed to systematically follow the influence of the pretreatment and synthesis temperatures on the overall carbon yield, and the selectivity and diameter uniformity of the SWNT produced. Cobalt states were investigated before and after reaction and in situ during reduction at two temperatures by EXAFS and XANES. We will further discuss the effect of each investigated process variable in separate sections below.

Conclusions

A systematic investigation of the influence of the catalyst pretreatment and nanotube synthesis temperature on the characteristics of the SWNT diameter distribution produced in Co-MCM-41 was carried out. The results obtained by TGA, Raman spectroscopy, and HR-TEM, combined with EXAFS and XANES spectroscopy suggest that the diameter distribution and quality of SWNT in Co-MCM-41 can be engineered by controlling cobalt cluster size through the hydrogen prereduction and the reaction temperature.

Acknowledgements

We thank DARPA-DSO for the financial support for this project. Partial support for the synthesis of the MCM-41 catalysts and the use of the National Synchrotron Light Source at Brookhaven National Laboratory was obtained from DoE-BES. We are grateful to Sergei Bachilo and Bruce Weisman at Rice University for the fluorescence measurement.

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