INTRODUCTION
Raman spectroscopy had been used extensively to characterize the structural
features of carbonaceous matter since Tuinstra and Koenig
(1970) first correlated Raman bands to structural parameters measured from
XRD for polycrystalline graphite. Raman spectroscopy had also been applied to
study the structural features of coal-derived products (Friedel
and Carlson, 1972; Wang et al., 1990). In
these studies, the Raman spectral characteristics, mainly those of graphite
(G) and defect bands (D) were used to investigate the coal structure and its
correlation to other characteristics, e.g., the coal rank and the ‘graphite’
crystalline size parameters (Guedes et al., 2010).
Lasers in visible range, such as the Ar+ laser at 488 and 515 nm
and the He-Ne laser at 623 nm, were often used, which easily excite strong fluorescence
emission, distorting Raman spectra and resulting in difficulties in structural
quantification (Schwan et al., 1996).
Little work had been done using a NIR laser (1064 nm) to study coal/char structures.
The Raman spectra of highly disordered carbonaceous materials such as amorphous
carbon or coal chars differ considerably from that of ‘ideal’ polycrystalline
graphite. Normally there was a large overlap of Graphite (G) and defect (D)
band in disordered carbon material and more structural information was hidden
in the ‘overlap’. The Raman spectroscopy of clay minerals had received
less attention due to the weakness of the Raman scattered signal, the photo-degradation
of the sample and the occurrence of fluorescence which swamped the signal. The
use of FT Raman spectroscopy offered the advantages of reduced fluorescence,
improved signal to noise by co-adding of scans and the longer wavelength of
light reduced sample degradation.
The application of infrared and Raman spectra to the study of intractable carbonaceous
material had produced information valuable to researchers involved studies of
the structure of coal. Spectral frequencies were principally assignable to functional
groups but some of the important spectral features, not assignable to groups,
had been involved in considerable conjecture concerning proper assignments (Jelicka
et al., 2006; Li et al., 2006). In
present study, the authors had obtained Raman results on two Indian Coals that
showed two sharp lines in each sample in the range 1575-1620 and 1350-1400 cm-1
region. Other weak lines found in region were also assigned. The change of intensity
of the spectral line with leaching was also investigated.
MATERIALS AND METHODS
Experiment was carried out during 2009-2010 at Department of Physics Christ University, Bangalore, India. Coal samples were obtained from two different sources; sub-bituminous coal from Godavari coal field and high volatile bituminous coal from Korba coal field by random picking. The samples, in as received condition, is powdered and dried in a dessicator to remove the absorbed water. Samples were leached with Concentrated HF (40, 30, 20 and 10%) for 24 h and washed with enormous amount of distilled water. The slurry was filtered and dried at about 80°C for removing the absorbed water and allowed to cool slowly in dessicator. The spectral measurements were carried out at Sree-Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram, Kerala, India.
Raman Spectroscopy
The Raman and infrared spectra were obtained using a Bruker RFS 100/S series
FTIR spectrometer equipped with a Raman accessory. This comprised a Nd-YAG laser
operating at a wavelength of 1064 nm and a Raman sampling compartment incorporating
180 degree optics. For analysis, about 20 g of the sample was crushed in to
fine powder of about 5 μm in size. The Raman detector was a highly sensitive
standard Ge detector and was operated at room temperature. Under these conditions
Raman shifts were observed in the spectral range 1800-400 cm-1. Raman
spectra were obtained directly keeping the powdered sample in the incident beam
50 scan mode with a resolution of 4 cm-1. Raman spectra were collected
as single beam spectra and were not corrected for instrumental effects. A laser
power of 200 mW was used. This power was low enough to prevent damage to the
minerals, but was sufficient to produce quality spectra.
RESULTS AND DISCUSSION
Low Wavenumber Region (400-1200 cm-1)
The FT-Raman of coal in the <1200 cm-1 region is shown in
Fig. 1 and 2; the coal minerals were characterized
by very intense bands in the 200 to 1200 cm-1 region. These bands
have been identified using conventional dispersive Raman spectroscopy (Johnson
et al., 1986).
|
| Fig. 1: |
FT Raman Spectra of sub-bituminous coal (400-1200 cm-1)
G1-Virgin sample, GH-HF leached sample |
|
| Fig. 2: |
FT Raman spectra of high volatile bituminous coal (400-1200
cm-1) K1-Virgin sample, KH-HF leached sample |
Frost et al. (1993) assigned the same band
to clay minerals in his FT Raman spectroscopy study on Kaolinite, Dickite and
Halloysite. This part of the spectral region is very much sample dependent.
The band had been attributed to Si-O-Si stretch (Ishii et
al., 1967). The band at 570 cm-1 was changed to lower wavelength
of 561 cm-1 with HF leaching. The band at 602 cm-1 was
also lowered with leaching. Intense bands observed at 569-640 cm-1
could also correspond to ring breathing vibration. With HF leaching, the intensity
of this band is slightly decreased in sub-bituminous coal where as in the case
of bituminous coal the intensity was increased. In the present study authors
assigned this band in sub-bituminous coal due to the combined effect of minerals
and ring breathing vibration, where as in the bituminous coal this was due to
ring breathing. This absorption increased in bituminous coal with leaching.
Bands at 704, 752 and 785 are attributed for Kaolinite; 703, 744 and 792 for
Halloysite and 744 and 794 cm-1 for Dickite to the Al-OH vibrations
of surface hydroxyls. These bands were resolved in the Raman spectra but with
less intensity. There were small peaks for Kaolinite and for Halloysite. It
was again related to symmetry reduction. The in-plain bending vibrations of
the surface hydroxyls in the kaolinite minerals lie at 936 cm-1 and
inner hydroxyl at 915 cm-1. These bands are strongly infrared active
but are very weak in the Raman. These bands were very weak with HF leaching
in both sub-bituminous and high volatile bituminous sample. Jelicka
et al. (2006) observed the ring-plane bending modes of COO- at 811
and 773 cm-1 and 792 and 737 cm-1 for the COOH bending
modes in mellitic acid. Present study also showed absorption spikes in region
which was more pronounced in sub-bitumionus coal than bituminous coal. With
HF leaching intensity of this band decreased in both the samples. This was due
to the removal of oxygenated functional group with leaching.
Kaolinites being a dioctahedral layer of silicates as opposed to tri octahedral possess lower effective symmetry thus imposing distortions on the tetrahedral sheet. Thus the intense infrared bands at 1014, 1036 and 1108 cm-1 for kaolinite were attributed to the perpendicular Si-O vibrations. These bands were weak in Raman but are observable as small spikes with maximum intensity at 1090 cm-1.
Schwan et al. (1996) assigned the band at 1180
cm-1 to sp3- rich structures in amorphous carbon films.
Nemanich and Solin (1979) assigned the bands at 1170
cm-1 to hexagonal diamond, nano-crystalline diamond or sp3-
rich carbon structure. In the present study, a band at 1180 cm-1
represents the sp2-sp3 carbonaceous structures in Indian
coal. This structure was comparatively weaker in virgin coals, but with HF leaching
intensity of this band increased marginally.
Li et al. (2006) in their studies on Victorian
coal assigned the band at 960, 1230 and 1060 cm-1 to the contribution
from ether and benzene resulted/accompanied structure respectively. In the present
study these bands had weak intensity in the spectrum of both bituminous and
sub-bituminous coal. The vibrational frequency below 900 cm-1 of
coal had not yet been explored and there is no general agreement about the origin
of the features observed at about 400-500 and 700-800 cm-1 in amorphous
carbon-based systems like coal. The 460 and 720 cm-1 features showed
a different behaviour with chemical leaching; the 460 cm-1 peak became
stronger with HF leaching in both samples. Conversely, the 720 cm-1
feature did not show any appreciable change with leaching.
1300-1800 cm-1 Region
The FT-Raman of coal in the 1300-1700 cm-1 region is shown in
Fig. 3 and 4. This region consists of so
many peaks. Wang et al. (1990) investigated the
structures of highly ordered carbonaceous materials with exciting laser in the
visible range and identified the G band at 1585 cm-1 and D-band at
1350 cm-1. Li et al. (2006) had been
found that the D band position was dependent on the excitation laser wavelength
(λo). i.e., the position of D-band will move from 1360 cm-1
at 488 nm to 1285-1327 cm-1 at the wavelength of 1064 nm. The Raman
spectra of the coal samples do had two very broad bands at around 1580-1605
and 1300-1350 cm-1 (Fig. 3, 4)
at the position of G and D bands as stated above. This G band may be due to
presence of graphite crystalline structures or due to the aromatic ring breathing
of alkene (C = C). In the present study the sub-bituminous coal had two well
defined absorption at 1585 and 1605 cm-1. With HF leaching this doublet
was disappeared to a singlet. In the bituminous coal there was only a narrow
band observed at 1605 cm-1 but with HF leaching this band was splitted
in to two defined spikes with more intensity.
|
| Fig. 3: |
FT Raman spectra of bituminous coal (1300-1800 cm-1)
K-Virgin sample, KH-HF leached sample |
|
| Fig. 4: |
FT Raman spectra of sub-bituminous (1300-1800 cm-1)
G1-Virgin sample GF-HF leached sample |
This indicated that with increasing rank the graphite band becomes narrower.
Bituminous coal and sub-bituminous coal are showing different result in this
region. Schwan et al. (1996) assigned the D-band
at 1355 cm-1 to benzene or condensed benzene rings in amorphous carbon.
Li et al. (2006) suggested that aromatics with
a ring size of no less than 6 fused benzene rings gave peaks close to the D-band
(Fig. 3, 4). Aromatics having 6 or more
fused benzene rings but less than in graphite, will contribute to the observed
D band in the Raman spectrum. D band will disappear as an infinite sized aromatic
network such as the single graphite structure is formed (Tuinstra
and Koenig, 1970; Li et al., 2006). In the
present study, the sample is showing weak but defined absorption in this region.
This showed the presence of fused benzene rings in the sample. This absorption
was stronger for bituminous coal than sub-bituminous coal. This indicated that
aromaticity increased with rank of the coal. With leaching, the absorption of
this band marginally increased.
Very recent studies conducted by Guedes et al. (2010)
reported narrowing of G band with increase in rank of coal. They also reported
shift of D-band to lower wave number with increase of rank of coal. In the present
study, authors observed narrowing of G-band in bituminous coal compared to sub-bituminous
coal. But surprisingly with HF leaching, the doublet observed in sub-bituminous
coal reduced to a singlet where as in bituminous coal, this band became in to
two defined peak at 1580 and 1605 cm-1along with increase of intensity.
Nemanich and Solin (1979) assigned a peak at 1490 cm-1
as semi-circle ring stretch or condensed benzene rings or contribution from
the phonon density of states in finite crystal of graphite. Li
et al. (2006) assigned three bands at 1540, 1465 and 1380 cm-1
to GR band (standing for G right) VR band (standing for
valley right) and VL band (standing for Valley left) respectively
for the Victorian brown coal. Jelicka et al. (2006)
assigned the band at 1550 cm-1 to the asymmetric stretch of the
COO- units and the bands at 1468 and 1386 cm-1 to the symmetric modes
from the same COO- groups. They assigned the band at 1676 cm-1 due
to the acid COOH group C=O function. In the present study, Indian sub-bituminous
coal had moderate absorption at 1540 and 1465 cm-1 compared to bituminous
coal. These bands could represent aromatic ring systems typically found in amorphous
carbon materials. A significant difference between the Raman spectra of sub-bituminous
coal and bituminous coal was seen in this region. It was more prominent in sub-bituminous
coal than bituminous coal. In sub-bituminous coal peaks correspond to COOH groups
were present and with HF leaching its intensity was reduced drastically. Presences
of oxygenated functional groups are normally more in low rank coals. In bituminous
coal, may of this band did not show any change with leaching. The spikes observed
in the bituminous coal were mainly due to graphite structure than carbonyl groups.
Li et al. (2006) assigned band at 1700 cm-1
to carbonyl (C=O) structure in the coal. This band was moderate in sub-bituminous
coal in the present study. Starsinic et al. (1984)
assigned the band at 1695 cm-1 to carbonyl groups, possibly ketones.
The relatively strong band near 1695 cm-1 was attributed predominantly
to carboxylic acid and a relatively weak band at 1670 cm-1 was assigned
to ketonic structures. In the spectrum, sub-bituminous coal was having higher
absorption than bituminous coal in the carboxylic region. It was clear that
the samples having higher oxygen content have intense absorption band at 1695
cm-1 compared to samples having lower oxygen content.
CONCLUSIONS
FT Raman spectroscopy study of coal revealed the presence of more oxygenated functional groups along with graphite structure. Intense band for kaolinite was found at 601 and 569 cm-1. The band had also been attributed to carbonyl group. The band at 570 cm-1 was changed to lower wavelength of 561 cm-1 with HF leaching. The band at 602 cm-1 was also lowered with leaching. The bands at 704, 752 and 785 were assigned for kaolinite; 703, 744 and 792 for halloysite and 744 and 794 cm-1 for dickite to the Al-OH vibrations of surface hydroxyls. These bands were resolved in the Raman spectra but of less intensity. Thus the intense infrared bands at 1014, 1030 and 1090 cm-1 for kaolinite were attributed to the perpendicular Si-O vibrations. These bands were weak in Raman but were observable as small spikes with maximum intensity at 1090 cm-1. The Raman spectra of sub-bitumionus coal sample had two very broad bands at around 1580-1602 cm-1 and this was due to the presence of Graphite structure and the doublet disappeared to a singlet of almost the same intensity with leaching. Where as in the bituminous coal these two bands were originally weak but become well defined absorption spikes with HF leaching. This indicated that with increase of coal rank the graphite band becomes weaker but with leaching the absorption becomes stronger. There are small spikes observed at 1300-1350 cm-1 due to Defect bands. This band was comparatively stronger in bituminous coal than sub-bituminous coal and showing a shift to lower wavenumber and intensity with leaching. The D-band at 1355 cm-1 was due to benzene or condensed benzene rings in amorphous carbon. Indian sub-bituminous coal had moderate absorption at 1520 and 1500 cm-1 compared to bituminous coal. In the spectrum, sub-bituminous coal was having higher absorption than bituminous coal in the carboxylic region. It was clear that the samples having higher oxygen content had intense absorption band at 1695 cm-1 compared to samples having lower oxygen content.
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