Journal of Natural Gas Chemistry 13(2004)95100

FT-IR Study of Carbon Nanotube Supported Co-Mo Catalysts

Hongyan Shang1, Chenguang Liu1, Fei Wei2

1. State Key Laboratory of Heavy Oil Processing, Key Laboratory of Catalysis, CNPC, University of Petroleum,

Dongying 257061, China; 2. Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

[Manuscript received April 28, 2004; revised May 25, 2004]

Abstract: In this paper, adsorption properties of dibenzothiophene (DBT) on carbon nanotube, carbonnanotube supported oxide state and sulfide state CoMo catalysts are studied by using thermal gravi-metric analysis (TGA) technique and FT-IR spectroscopy. Activated carbon support, -Al2O3 supportand supported CoMo catalysts are also subjected to studies for comparison. It was found that sulfidestate CoMoS/MWCNT, CoMoS/AC and CoMoS/-Al2O3 catalysts adsorbed much more DBT moleculesthan their corresponding oxide state catalysts, as well as their corresponding supports. The chemically

adsorbed DBT aromatic molecules did not undergo decomposition on the surface of supports, supportedoxide state CoMo catalysts and sulfide state CoMo catalysts when out-gassing at 373 K. FT-IR resultsindicated that DBT molecules mainly stand upright on the active sites (acid sites and/or transition activephases) of CoMoS/MWCNT catalyst. However, DBT aromatic molecules mainly lie flat on MWCNT andCoMoO/MWCNT.

Key words: FT-IR, dibenzothiophene, carbon nanotube, adsorption

1. Introduction

Recently, carbon materials including carbon nan-

otube have received increased attention as supports

for catalytic systems [1,2]. Potential advantages in-

clude easy metal recovery, low propensity of coke for-

mation and the nanometer dimensions of carbon nan-

otube. In the system of heterogeneous catalysis, main

catalytic reactions are carried out on the surface of

catalysts, therefore, the adsorption properties, espe-

cially the chemical adsorption of reactants on the sur-

face of catalysts play a very important role in the pro-

cess of catalysis.

Hydrodeseulfurization (HDS) is critical in the pro-

duction of clean oil and Dibenzothiophene (DBT) is

the most common sulfur containing organic molecules

existing in the petroleum-derived feed-stocks which is

hard to be removed from the feed stocks [35]. Farag

[6] studied the adsorption of DBT on carbon sup-

ported Co-Mo catalysts and some surface character-

istics from the adsorption and desorption techniques,

following the concepts of physical adsorption and sur-

face science to get useful information on the dispersion

nature of the catalyst dispersed on the support by the

application of thermal gravimetric technique. Lar-

rubia [7] studied the adsorption of benzothiophene,

dibenzothiophene and 4,6-dimenthylbenzothiophene

on catalytic supports such as alumina, zirconia and

magnesia by IR spectroscopy and the results show

that DBT and 4,6-DBT do not undergo decompo-

sition during the desorption process because of the

stability of the aromatic rings. Up to now, few stud-

ies have been done on the adsorption states of DBT

on carbon nanotube and carbon nanotube supported

Co-Mo catalysts.

In this paper, the adsorption states of DBT aro-

matic molecules on carbon nanotube support, sup-

ported oxide Co-Mo catalyst and sulfide Co-Mo cat-

Corresponding author. Tel/Fax: (0546)8392284; E-mail: catagroupsh@hotmail.com

Foundation of Innovation for Middle-aged and Youth, CNPC (Foundation No.W990411)

96 Hongyan Shang et al./ Journal of Natural Gas Chemistry Vol. 13 No. 2 2004

alyst were studied by thermal gravimetric analysis

technique and IR spectroscopy, respectively. Acti-

vated carbon, -Al2O3 and their corresponding cat-

alysts were also subjected to studies for comparison.

This work on the carbon nanotube supported Co-Mo

catalyst was done with the hope of gaining some in-

sight into the HDS performance (activity, selectivity).

2. Experimental

2.1. Adsorption of pyridine

A kind of multi-walled carbon nanotube

(MWCNT) supplied by Tsinghua University was used

as-received. A pillar activated carbon (AC) offered

by the Beijing Institute of Coal Science was also used

as-received. The samples were put into a desiccator

filled with pyridine vapor for 24 h at room tempera-

ture in a high vacuum system (0.1 mPa).

After adsorption with pyridine, the samples were

subjected to analysis by using FT-IR technique to

characterize the surface acidic property.

2.2. Preparation of Co-Mo catalyst

The supported catalysts with Co/Mo atomic ra-

tio of 0.35 were prepared by pore volume impregna-

tion using aqueous solutions of (NH4)6Mo7O2424H2O

and Co (NO3)26H2O (both A. R.). The Mo phase

was introduced first and dried in air at 383 K for 12

h, then Co precursor was introduced with pore vol-

ume impregnation followed by drying at 383 K for 12

h, finally the bimetallic catalysts were heat-treated

at 773 K for 4 h in the flow of nitrogen. In case of

alumina, the catalyst was calcinated in air at 773 K

for 4 h. The oxide state catalyst was labeled as Co-

MoO/MWCNT, CoMoO/AC and CoMoO/-Al2O3,

respectively.

In the preparation of sulfide state Co-Mo cat-

alysts, sulfide Mo phase was provided by ammo-

nium tetrathiomolybdate (ATTM) [8] followed the

impregnation of Co(NO3)26H2O and dried at 383

K for 24 h, finally heat-treated at 773 K under N2for 4 h. The sulfide catalysts were designated as

CoMoS/MWCNT, CoMoS/AC and CoMoS/-Al2O3,

respectively.

2.3. Adsorption of DBT

The details for adsorption experiment procedure

of DBT can be found in Ref. [7]. The samples were

out-gassed at 373 K for 1 h to remove physisorbed

DBT before making IR and gravimetric analyses.

2.4. Thermal gravimetric analyses (TGA)

The sample after filtration was investigated us-

ing a Du Pont 952 thermo-gravimetric apparatus with

a thermo-balance, equipped with a computer control

unit for the recording of TGA. Before recording the

curves, the sample was treated in the flow of N2 for

1 h at 373 K in order to remove the trace of toluene

and physically adsorbed DBT molecules. Then, the

runs were carried out in a continuous flow of N2 gas

and the TGA curves of samples were recorded from

373 K to 873 K at a rate of 10 K/min for all runs.

2.5. FT-IR analyses

The IR spectra were recorded on a VECTOR33

FT-IR instrument (100 Spectra accumulation, 2 cm1

Resolution), using pressed disks of the pure solid and

catalyst powders combined with KBr.

2.6. BET measurements

The BET surface area of the supports as well as

the catalysts were determined by nitrogen adsorption-

desorption isotherm at 77.35 K in an ASAP2010 ad-

sorption apparatus. The physical properties are listed

in Table 1.

Table 1. Physical properties of supports and Co-Mo catalysts

Sample BET surface area (m2/g) Total pore volume (cm3/g) Average pore diameter (nm)

MWCNT 189.6 0.47 8.9

CoMoO/MWCNT 163.7 0.43 8.1

CoMoS/MWCNT 107.0 0.37 7.8

AC 1006.0 0.37 4.8

CoMoO/AC 860.0 0.31 3.9

CoMoS/AC 920.0 0.34 4.2

-Al2O3 216.0 0.51 7.1

CoMoO/-Al2O3 174.7 0.27 6.1

CoMoS/-Al2O3 193.0 0.50 6.8

Journal of Natural Gas Chemistry Vol. 13 No. 2 2004 97

3. Results and discussion

3.1. Surface acidic property of carbon nan-

otube and AC

The surface acidic properties of samples are mea-

sured on the basis of the IR spectra of the samples

contacted with pyridine. According to the literature

[9], 1540 cm1 adsorption peak represents B acid and

1440 cm1 represents the L acid, whereas 1490 cm1

represents the total amount of B and L acids. In the

IR spectra of carbon nanotube, none but very weak

adsorption peaks can be found around 1540 cm1, in-

dicating that there are only B acid sites on the surface

(see Figure 1).

Figure 1. IR spectra arising from contact of

MWCNT and AC with pyridine

(1) MWCNT, (2) AC

As for AC, there are clear peaks in the range of

1540, 1490 and 1440 cm1, indicating that there are

B acidic sites and L acidic sites on the surface of AC

(see Figure 1). In addition, there is a new adsorption

peak around 1720 cm1, representing the existence of

carbonyl group on the surface of AC.

Similarly, there are strong peaks in the range of

1540, 1490 and 1440 cm1, indicating that there are B

acidic sites and L acidic sites on the surface of -Al2O3(see Figure 2), indicating that the surface acidic prop-

erties are different from the MWCNT support.

Figure 2. IR spectra arising from contact of -

Al2O3 with pyridine

3.2. DBT adsorption properties

Based on the calculated surface area [6] and the

assumption that the DBT molecules stand upright on

the surface of catalyst in a manner of monolayer cov-

erage, the following equation is applied to calculate

the surface area of adsorbed DBT:

A =m1/M 6.02 10

23 66.5 1020

mm1(1)

Where A is the total specific area (m2/g) of ad-

sorbed DBT, M is the molar weight (g/mol) of DBT,

m represents the total weight (mg) of sample after

the adsorption of DBT, m1 represents the amount of

weight loss of DBT during TGA process. 66.51020

is the surface area of DBT per molecule, m2.

The specific surface areas of adsorbed DBT on

supports and catalysts calculated according to for-

mula (1) are listed in Table 2. According to the

BET data, the BET specific surface area of MWCNT

(189.6 m2/g) is much larger than the surface area

(53.56 m2/g) of DBT molecules which are adsorbed on

the MWCNT, it can be deduced that the surface of

CNT is not thoroughly covered by DBT molecules.

It is also the case with CoMoO/MWCNT. As for

CoMoS/MWCNT, its specific surface area coincides

with that of adsorbed DBT molecules. In general,

the sulfide state catalyst adsorbs much more DBT

molecules than the corresponding oxide state cata-

lyst and corresponding support (see Table 2). DBT

molecules can adsorb on two kinds of sites, one is the

acidic sites and another is on the transition metals

with vacant orbits which can accept electrons. The

great increase of DBT adsorption on sulfide state cat-

alysts may be the result of newly produced active sites

(edges of active phases) in the sulfide state catalysts

compared with oxide state catalysts.

98 Hongyan Shang et al./ Journal of Natural Gas Chemistry Vol. 13 No. 2 2004

Table 2. Specif ic surface area of adsorbed DBT

Adsorption ratio Specific surface area of DBT Ratio of surface area covered bySample

of DBT (mg/g) adsorbed on samples (m2/g) adsorbed DBT moleculesa (%)

MWCNT 2.47 53.56 28.2

AC 3.13 67.91 6.75

-Al2O3 2.01 43.59 20.1

CoMoO/MWCNT 2.15 46.69 28.5

CoMoO/AC 7.81 169.63 19.7

CoMoO/-Al2O3 1.71 37.08 21.2

CoMoS/MWCNT 4.97 107.92 100.8

CoMoS/AC 8.11 176.18 19.1

CoMoS/-Al2O3 6.91 150.22 77.8

a Calculated specific surface area of adsorbed DBT molecules (m2/g) / specific surface area of catalysts or supports(m2/g)

AC has the strongest adsorption ability to DBT

molecules among MWCNT, AC and -Al2O3. Al-

though the adsorption ratio is high with AC, the sur-

face area of adsorbed DBT molecules is merely 67.91

m2/g which is very small compared with the BET

surface area of AC (1006 m2/g), far from full cov-

erage even in a manner of flat adsorption, indicat-

ing that large part of surface is uncovered. It is also

the case with the oxide state CoMoO/AC. In case

of -Al2O3 and CoMoO/-Al2O3, the surface area of

DBT molecules adsorbed is merely 43.59 and 38.07

m2/g, respectively. The adsorbed DBT molecules

could not cover the surface of -Al2O3 (216 m2/g) and

CoMoO/-Al2O3 (174.7 m2/g) even in a manner of

monolayer coverage. In contrast, the DBT molecules

adsorbed (155.22 m2/g) on CoMoS/-Al2O3 almost

entirely cover the surface of sulfide catalyst (193

m2/g).

3.3. FT-IR results and adsorption states of

DBT molecules

According to Ref. [7], the analyses of the out-of-

plane deformation modes of DBT aromatic molecules

in the IR spectra could indicate whether the aromatic

molecules stand upright or lie flat on the surface. In

Figure 3, IR spectra concerning the interaction be-

tween DBT and MWCNT and supported catalysts

are presented. Strong bands are evident in the re-

gion 16001400 cm1, in particular at 1400, 1560

cm1 for WMCNT and 1568, 1400 cm1 for Co-

MoO/MWCNT, and 1560, 1461, 1400 cm1 for Co-

MoS/MWCNT, which may be due to the vibrations of

an aromatic ring, indicating that out-gassing at 373

K does not cause the complete desorption of DBT

from the surface, additional clear bands are observed

at 1162, 1221, 1113 cm1, respectively. The aromatic

CH band of DBT is not found at around 3030 cm1.

Figure 3. IR spectra of adsorbed species arising

from the contact of DBT with MWCNT

and supported catalysts

(1) MWCNT, (2) CoMoO/MWCNT, (3) CoMoS/MWCNT

The band at around 3384 cm1 may be the stretch-

ing vibration of coordinated surface hydroxyl OH,

a significant shift from the region 36403610 cm1,

indicating that at least part of DBT molecules inter-

act with surface OH via H-bonding. In addition, the

absence of bands in the region 30002800 cm1 (2969

and 2933 cm1, typical vibrations of aliphatic methyl

and methylene groups) indicates that DBT decompo-

sition does not take place under out-gassing at 373

Journal of Natural Gas Chemistry Vol. 13 No. 2 2004 99

K. At low frequency region, significant shift from 739

cm1 (pure solid DBT) to 731 cm1 for MWCNT and

737 cm1 for CoMoO/MWCNT indicates that DBT

aromatic molecules mainly lie flat on MWCNT and

CoMoO/MWCNT. In case of CoMoS/MWCNT, the

absence of shift (exactly at 739 cm1) suggests that

the DBT aromatic molecules mainly stand upright on

the active sites or surface of CoMoS/MWCT. The IR

spectra of pure solid DBT is also given for reference

(see Figure 4).

Figure 4. IR spectrum of pure solid DBT

In Figure 5, the FT-IR spectra of the adsorbed

species arising from the contact of AC supported cat-

alysts with DBT are presented. Weak bands at 1087,

1558 cm1 for AC, 1110, 1562 cm1 for CoMoO/AC

are characteristics of aromatic vibrations, indicating

the chemisorbed DBT molecules still exist after out-

gassing at 373 K. In case of CoMoS/AC, the intensity

of bands at 1562, 1400 and 1119 cm1 are relatively

stronger than those on AC and CoMoO/AC, indi-

cating the sulfide state catalyst adsorbs more DBT

molecules. The absence of bands in the region 3000

2800 cm1 indicates that DBT molecules will not un-

dergo decomposition during out-gassing at 373 K. The

slight shift of a band from 739 to 740 cm1 shows

that DBT molecules may mainly stand upright on

the surface of CoMoS/AC. The shift from 739 to 737

cm1 for CoMoO/AC suggests that a portion of DBT

molecules possibly lie flat on the surface, and the rest

stands upright on the surface of CoMoO/AC. The

sketch map of adsorption states of DBT molecules on

the active sites of catalysts is shown in Figure 6. The

band at 3396 cm1 in all the three samples may be the

vibration of surface OH (hydroxyl groups) which

interacts with DBT molecules [10], and the band at

1745 cm1 is the contribution of stretching vibration

of carboxyl groups on the edges of the layer plane or

conjugated carbonyl groups (C=O in carboxylic acid

and lactone groups) [11].

Figure 5. IR spectra of adsorbed species arising

from the contact of DBT with AC and

supported catalysts

(1) AC, (2) CoMoO/AC, (3) CoMoS/AC

Figure 6. Adsorption states of DBT on the surface of catalysts

100 Hongyan Shang et al./ Journal of Natural Gas Chemistry Vol. 13 No. 2 2004

In Figure 7, the IR spectra of the adsorbed species

arising from adsorption of DBT with alumina sup-

ported catalysts are presented. Bands at 1489, 1448

and 1402 cm1 for CoMoO/-Al2O3 and at 1490,

1449 and 1402 cm1 for CoMoS/-Al2O3 are typi-

cal of aromatic compound and coincide with those of

the solid DBT (Figure 4), indicating DBT molecules

still can be detected after out-gassing at 373 K. In

case of -Al2O3 support, typical aromatic bands are

not evident. The bands in high frequency region and

in low frequency region are covered and concealed by

two strong and broad bands at around 3448 cm1 and

840 cm1 respectively, so the aliphatic bands in the

region 30002800 cm1 could not be detected. More-

over, the useful information concerning whether DBT

molecules stand upright or lie flat on the surface could

not be obtained.

Figure 7. IR spectra of the adsorbed species aris-

ing from contact of DBT molecules with

-Al2O3 and supported catalysts

(1) -Al2O3, (2) CoMoO/-Al2O3, (3) CoMoS/-Al2O3

4. Conclusions

(1) Sulfide state CoMoS/MWCNT, CoMoS/AC

and CoMoS/-Al2O3 catalysts adsorb much more

DBT molecules than their corresponding oxide state

catalyst, as well as the corresponding supports. AC

has the strongest adsorption ability to DBT molecules

among MWCNT, AC and -Al2O3 support.

(2) Based on fact that vibrations in the region

30002800 cm1 representing methyl and/or methy-

lene groups are not found in all the nine samples, it

is thus concluded that the chemically adsorbed DBT

aromatic molecules undergo no decomposition when

out-gassing at 373 K for all the samples studied.

(3) Based on the fact that the absence of shift

of the band at 739 cm1 (out-of-plane deformation

mode of DBT aromatic molecules), it is suggested

that DBT molecules mainly stand upright on the

surface of CoMoS/MWCNT catalyst. Significant

shift from 739 to 731 cm1 for MWCNT and 737

cm1 for CoMoO/MWCNT indicate that DBT aro-

matic molecules mainly lie flat on MWCNT and Co-

MoO/MWCNT. The slight shift of a band from 739

cm1 to 740 cm1 indicates that DBT molecules

mainly stand upright on the surface of CoMoS/AC,

and that at least a small portion of DBT molecules

lie flat on the surface of CoMoS/AC. The slight shift

from 739 to 737 cm1 for CoMoO/AC suggests that

a portion of DBT molecules possibly lie flat on the

surface, and the rest stands upright on the surface of

CoMoO/AC.

Acknowledgements

The Nano-Material Research Center of Tsinghua

University is gratefully acknowledged for supplying us

with different kinds of carbon nanotubes with high

quality.

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