Carbon structures obtained by the disproportionation of carbon monoxide over nickel catalysts

Article abstract of DOI:10.1016/S0025-5408(98)00016-6

The disproportionation of CO over 18% Ni/MgO and 18% Ni/AlPO₄ catalysts has been studied at different temperatures. The nature of carbonaceous deposits over metal particles after the reaction has been examined by transmission electron microscopy. Carbon nanotubes are obtained by the disproportionation of CO over Ni/MgO at 700 °C.

Full text of DOI:10.1016/S0025-5408(98)00016-6

Materials Research Bulletin, Vol. 33, No. 4, pp. 663–667, 1998
Copyright © 1998 Elsevier Science Ltd
Printed in the USA. All rights reserved
0025-5408/98 $19.00 ϩ .00
PII S0025-5408(98)00016-6
CARBON STRUCTURES OBTAINED BY THE DISPROPORTIONATION OF
CARBON MONOXIDE OVER NICKEL CATALYSTS
A. Govindaraj1*, R. Sen2, A.K. Santra1, and B.V. Nagaraju3
1CSIR Centre of Excellence in Chemistry, Indian Institute of Science,
Bangalore 560012, India
2Materials Research Centre, Indian Institute of Science, Bangalore 560012, India
3Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur,
Bangalore 560064, India
(Refereed)
(Received May 3, 1997; Accepted May 15, 1997)
ABSTRACT
The disproportionation of CO over 18% Ni/MgO and 18% Ni/AlPO4 catalysts
has been studied at different temperatures. The nature of carbonaceous de-
posits over metal particles after the reaction has been examined by transmis-
sion electron microscopy. Carbon nanotubes are obtained by the
disproportionation of CO over Ni/MgO at 700°C. © 1998 Elsevier Science Ltd
KEYWORDS: A. nanostructures, C. electron microscopy, D. catalytic prop-
erties
INTRODUCTION
The disproportionation of CO by the Boudouard reaction (2CO 3 CO2 ϩ C) carried out in
the presence of metals has been reported to yield carbon nanotubes. Several years ago, Audier
et al. [1] reported filamenteous catalytic carbon in the form of simple tubes when the
disproportionation of CO was carried out over Fe–Co and Fe–Ni alloys. These tubes
contained metal particles at one end. In the light of more recent literature on carbon
nanotubes [2], it would appear that Audier et al. had actually obtained carbon nanotubes with
the metal particles at the tips. Martra et al. [3] studied the disproportionation of CO over
Ni/MgO and found that the carbon produced by the Boudouard reaction at 200°C does not
*To whom correspondence should be addressed.
663

664
A. GOVINDARAJ et al.
Vol. 33, No. 4
FIG. 1
TEM image of carbon coated Ni particles obtained by the disproportionation of CO at 400°C
on 18% Ni/MgO.
appear on electron micrographs, suggesting that, under such mild conditions, it is very well
dispersed to form carbide-like species. They found that at 400°C, CO disproportionation on
Ni/MgO gives rise to Ni particles coated with graphitic carbon, as revealed by high-
resolution electron microscopy. The disproportionation of CO over nanoparticles of Mo
supported on fumed Al2O3 has recently been shown to give single-walled carbon nanotubes
[4]. The disproportionation of CO at 460°C over an Ni/Si2–Al2O3 catalyst gives rise to
nanotubes or carbon filaments, depending on whether the reaction is carried out in the
FIG. 2
TEM image of nanotubes obtained by the disproportionation of CO at 700°C on 18%
Ni/MgO.

Vol. 33, No. 4
NICKEL CATALYSTS
665
FIG. 3
TEM images of carbon-coated Ni particles on 18% Ni/AlPO4 catalyst wherein the dispro-
portionation of CO was carried out at (a) 400°C and (b) 600°C.
presence of H2 [5]. Therefore, we considered it useful to carry out a comparative study of the
disproportionation of CO over supported Ni catalysts such as Ni/MgO and Ni/AlPO4.
EXPERIMENTAL
AlPO4 was prepared from an aqueous solution of Al(NO3)3⅐9H2O and H3PO4 (85%) by
precipitation at 0°C with the dropwise addition of aqueous ammonia at a controlled pH. The
final pH of the solution after complete precipitation was maintained at 6.8. After filtration, the
product was washed several times with distilled water and propan-2-ol, dried at 200°C for
12 h, and then calcined at 800°C for 8 h. The powder X-ray diffraction pattern of the prepared
AlPO4 matched that reported in the literature [6]. High-purity MgO (E-Merck, India) was
used for the catalyst preparation.
Ni/MgO and Ni/AlPO4 catalysts were prepared by impregnating MgO or AlPO4 with an
appropriate amount of nickel acetylacetonate in a methanol solution and then evaporating the
solvent with a rotavapor. The catalysts were calcined at 400°C for 16 h in air. The loading
of nickel was 18%.
The disproportionation of CO was carried out in a horizontal tubular furnace. A weighed
quantity (100 mg) of the catalyst was placed in a quartz tube located inside the furnace. The

666
A. GOVINDARAJ et al.
Vol. 33, No. 4
FIG. 4
(a) TEM image of a carbon-coated Ni particle obtained by the disproportionation of CO with
5% H2 over Ni/AlPO4 at 700°C. (b) High-resolution electron microscopic image of the same
particle showing the graphitic nature of the carbon coating.
catalysts were reduced in H2 flow at 700°C for 1 h prior to the disproportionation step. CO
disproportionation was carried out over 18% Ni/MgO and 18% Ni/AlPO4 at appropriate
temperatures. The flow rate of the gases was monitored by unit mass flow controllers.
The products of CO disproportionation were dispersed in acetone and then loaded onto
holey carbon copper grids and observed with a transmission electron microscope (Jeol
JEM3010) operating at 300 kV.
RESULTS AND DISCUSSION
The disproportionation of CO at 400°C on Ni/MgO gives Ni particles with graphitic coatings.
In Figure 1, we show a typical transmission electron microscopy (TEM) image of a nickel
particle covered by graphitic carbon layers. These Ni particles reside at the edges of the MgO
crystal surfaces and have a nearly spherical morphology. When CO disproportionation was
carried out at 700°C over Ni/MgO, we obtained a high yield of carbon nanotubes. In Figure
2, we show a typical TEM image revealing the presence of a large number of nanotubes
produced by this method. The Ni particles are found at the tip of the nanotubes and act as the
catalyst for nanotube formation. Some of the Ni particles seem to have come out from their
MgO support.

Vol. 33, No. 4
NICKEL CATALYSTS
667
Figure 3a shows a TEM image of a carbon-coated Ni particle obtained by the dispropor-
tionation of CO at 400°C over the Ni/AlPO4 catalysts. The disproportionation of CO on
Ni/AlPO4 at a higher temperature (600°C) also yielded graphitic carbon-coated metal par-
ticles, as shown in Figure 3b. The coverage of carbon was higher at higher temperatures, as
observed by Martra et al. [3].
The disproportionation of CO on Ni/AlPO4 in the presence of a small percentage of
hydrogen also yielded graphitic carbon-coated metal particles. In Figure 4a, we show a
typical TEM image of a Ni particle coated with carbon obtained by the disproportionation of
CO with 5% H2 at 700°C over Ni/AlPO4 catalyst. Figure 4b shows a high-resolution electron
microscopic image of the same carbon-coated nickel particle, clearly revealing the graphitic
nature of the coating.
The formation of carbon nanotubes is considered to be associated with the movement of
the metal particles [7], the movement increasing with increasing temperature as well as gas
flow rate. Therefore, it is understandable that we obtained nanotubes by the disproportion-
ation of CO over the Ni/MgO catalyst only at higher temperatures. We observed nanotubes
only with Ni/MgO and not with the Ni/AlPO4, possibly because the nickel particles are less
strongly embedded in the former.
ACKNOWLEDGMENT
We thank Prof. C.N.R. Rao for suggesting the problem and for guidance.
REFERENCES
1. M. Audier, A. Oberlin and M. Coulon, J. Cryst. Growth 55, 549 (1981).
2. S. Iijima, Nature 354, 56 (1991).
3. G. Martra, L. Marchese, F. Arena, A. Parmaliana, and S. Coluccia, Topics in Catalysis 1, 63
(1994).
4. H. Dai, A.G. Rinzler, P. Nikolaev, A. Thess, D.T. Colbert, and R.E. Smalley, Chem. Phys. Lett.
260, 471 (1996).
5. J. Jiao and S. Seraphin, Chem. Phys. Lett. 249, 92 (1996).
6. J.M. Campelo, A. Garcia, D. Luna, and J.M. Marinas, J. Catal. 111, 106 (1988).
7. R.T.K. Baker and P.S. Harris, in Chemistry and Physics of Carbon, ed. P.L. Walker and P.A.
Thrower, Vol. 14, p. 83, Marcel Dekker (1978).