Letters to the Editor / Carbon 44 (2006) 2330–2356
continuous change of G-FWHM at the d002 value of
0.3370 nm.
The intensity ratio ID/IG is known to be sensitive to the
laser excitation wavelength [7–9]. The full width at the half
maximum intensity of the G band G-FWHM would also
be dependent on excitation wavelength. The relationships
found in this study could be extended to the further work
on the dependence of G-FWHM and ID/IG on excitation
wavelength.
References
[1] Tuinstra F, Koenig JL. Raman spectrum of graphite. J Chem Phys
1970;53(3):1126–31.
[2] Knight DS, White WB. Characterization of diamond films by Raman
spectroscopy. J Mater Res 1989;4(2):385–93.
[3] Katagiri G. Raman spectroscopy of graphite and carbon materials and
its recent application. Tanso 1996;175:304–13 [in Japanese].
2335
[4] Inagaki M. New carbons-control of structure and functions. Elsevier;
2000.
[5] Hishiyama Y, Igarashi K, Fujii H, Kaneda T, Koidesawa T,
Shimazawa Y, et al. Graphitization behavior of Kapton-derived
carbon film related to structure, microtexture and transport properties.
Carbon 1997;35(5):657–8.
[6] Hishiyama Y, Kaburagi Y, Inagaki M. Characterization of structure
and microtexture of carbon materials by magnetoresistance technique.
In: Thrower PA, editor. Chemistry and physics of carbon, vol.
23. New York: Marcel Dekker; 1991. p. 1–68.
[7] Vindano RP, Fischbach DB, Willis LJ, Loehr TM. Observation of
Raman band shifting with excitation wavelength for carbons and
graphite. Solid State Commun 1981;39:341–4.
[8] Mathews MJ, Pimenta MA, Dresselhaus G, Dresselhaus MS, Endo M.
Origin of dispersive effects of the Raman D band in carbon materials.
Phys Rev B 1999;59:R6585–9.
[9] Barros EB, Demir NS, Souza Filho AG, Mendes Filho J, Jorio A,
Drreselhaus G, et al. Raman spectrum of graphite foams. Phys Rev B
2005;71:165422-1–5.
About the endothermic nature of the adsorption of the herbicide
diuron from aqueous solutions on activated carbon fiber
M.A. Fontecha-Cámara a, M.V. López-Ramón a, M.A. Álvarez-Merino a,
C. Moreno-Castilla b,*
a
Departamento de Quı́mica Inorgánica y Orgánica, Facultad de Ciencias Experimentales, Universidad de Jaén, 23071 Jaén, Spain
b
Departamento de Quı́mica Inorgánica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain
Received 4 April 2006; accepted 16 May 2006
Keywords: Carbon fibers; Adsorption; Adsorption properties
Temperature is an important factor that influences
adsorption of organic compounds from aqueous solutions
onto carbon materials. The expected effect of temperature
on adsorption isotherms is a decrease in adsorption with
increasing temperature, since adsorption is a spontaneous
process. However, different examples have been reported
of an increase in the amount adsorbed with a rise in the
temperature [1–7]. This is opposite to the effect of temperature on the physical adsorption of a single component
(e.g., a gas on a solid), because adsorption from solution
involves at least two components, a solute and solvent.
Thus, adsorption from solution is not only governed by
*
Corresponding author.
E-mail address: [email protected] (C. Moreno-Castilla).
0008-6223/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2006.05.031
adsorbent–adsorbate interactions but also by adsorbate–
solvent and adsorbent–solvent interactions [1].
The herbicide diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea) is widely used in agriculture. The diuron molecule is
related to that of 3,4-dichloroaniline (3,4-DCA), with the
difference that the latter does not possess the urea type
chain. The similarity between these molecules is the reason
they were selected for this investigation into the effects of
temperature on their adsorption from aqueous solutions.
The activated carbon fiber (ACF) used was supplied by
Kynol Europe. Its BET surface area was 1709 m2/g and its
mean micropore volume and width were 0.734 cm3/g and
1.69 nm, respectively, both calculated from DR equation
applied to N2 adsorption isotherm. The mesopore volume
obtained from N2 adsorption isotherm was 0.017 cm3/g.
The oxygen content of the ACF, obtained by elemental
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Letters to the Editor / Carbon 44 (2006) 2330–2356
analysis, was 2.9% and its pH at the point of zero charge
was 7.0. Adsorption isotherms were determined between
15 and 35 °C, using 0.05 g carbon/200 mL diuron or 3,4DCA solutions. Solutions were buffered at pH 7 with a
potassium phosphate monobasic/sodium phosphate dibasic buffer and were 0.01 M in KCl. Suspensions were
mechanically shaken at the selected temperature. Although
equilibrium time was reached very quickly, within about
2 h, equilibrium concentrations were determined after
24 h by ultraviolet spectrophotometry.
Table 1
Characteristics of diuron and 3,4-dichloroaniline
Compound
Diuron
3,4-Dichloroaniline
Solubility in water at 25 °C (mg/L)
Dipolar moment (Debyes)
Molecular area (nm2 molecule1)
42
7.55
0.75
338
5.55
0.53
Characteristics of the adsorbates are given in Table 1.
Diuron is more insoluble in water and has a larger dipolar
moment versus 3,4-DCA and is therefore expected to be
more highly adsorbed on a hydrophobic carbon surface
both by hydrophobic and Van der Waals interactions. In
addition, the electron density in the aromatic ring of diuron
is higher than in that of 3,4-DCA, which would also increase dispersion interactions. Potentiometric titrations
showed that both diuron and 3,4-DCA were neutral at
pH 7. Therefore, adsorbate–adsorbent interactions were
non-electrostatic under the experimental conditions used
to determine adsorption isotherms [8]. Molecular areas
shown in Table 1 were obtained from structures in gas
phase, represented in the case of diuron by planar structure
(a) in Fig. 1.
Molecular dimensions of the two adsorbates differed.
Dos Santos et al. [9], applying high level ab initio calculations, demonstrated that diuron can have four different
Fig. 1. Molecular structure of (a) diuron, (b) diuron–(H2O)2 and (c) 3,4-dichloroaniline.
Cl,
H,
O,
N,
C.
Letters to the Editor / Carbon 44 (2006) 2330–2356
conformations in gas phase. Conformations are classified
as trans or cis according to the relative position of N–H
and C@O groups in the H–N–C@O peptide group, with
the two trans conformations being planar. The most stable
structure is one of the trans conformations, depicted in
Fig. 1 with its dimensions. The above authors also studied
the effect of the formation of hydrogen bonds between
water and diuron and concluded that the formation of
hydrogen bonds affects the geometry of the most stable
conformation. Thus, the phenyl ring and urea group are
twisted by 31°, resulting in a loss of planarity and an increase in height, as shown in Fig. 1. In addition, the dipolar
moment of diuron decreased. However, the 3,4-DCA molecule is planar (Fig. 1) and the change in its structure after
hydration would not be as drastic as that observed in diuron. The dimensions and molecular area of 3,4-DCA molecule are smaller than those of diuron molecule (Table 1).
Adsorption isotherms of diuron and 3,4-DCA on ACF
at different temperatures are depicted in Figs. 2 and 3,
respectively. Diuron adsorption capacity, Xm, obtained
from the Langmuir equation increased from 345 mg/g at
15 °C to 909 mg/g at 35 °C. The value of BXm, where B
is the Langmuir constant, increased from 476 to 3336 L/
g. This parameter is related to adsorbate–adsorbent interactions, and results obtained indicate that these interactions markedly increase with higher temperature. The
surface area of ACF occupied by diuron molecules at
X (mg g-1)
1000
500
0
0
3
6
C (mg L-1)
9
12
Fig. 2. Adsorption isotherms of diuron on ACF at 15 °C (), 25 °C (n)
and 35 °C (h).
X (mg g-1)
800
400
0
0
5
10
15
C (mg L-1)
20
25
Fig. 3. Adsorption isotherms of 3,4-dichloroaniline on ACF at 15 °C ()
and 35 °C (h).
2337
35 °C was calculated by using the molecular area of diuron
given in Table 1. The surface area value obtained was
1760 m2/g, which is similar to the SBET value of ACF. This
result indicates that diuron is adsorbed on the entire ACF
surface in planar form at 35 °C. Conversely, 3,4-DCA uptake slightly decreased when the adsorption temperature
increased from 15 to 35 °C. Values of Xm and BXm decreased from 741 mg/L and 1667 L/g to 720 mg/L and
769 L/g, respectively. Variation in the BXm value indicated
a decrease in adsorbent–adsorbate interactions with
increasing temperature. At 15 °C, the surface area of
ACF occupied by 3,4-DCA was 1469 m2/g, which represents 86 % of the BET surface area of ACF.
Results obtained can be explained as follows. The rise in
adsorption temperature weakens the hydrogen bonds
formed among water molecules and between water molecules and solute or adsorbent [6] and also increases the pore
diffusion [2,4]. Therefore, the increase in temperature favors dehydration of the diuron molecules, which makes
them more planar and gives them a larger dipolar moment.
The increase in planarity gives diuron molecules greater access to the microporosity of the ACF, while the increase in
dipolar moment leads to enhanced adsorbent–adsorbate
interactions. In addition, the adsorption potential within
the micropores increases the planarity of diuron molecules,
so that at 35 °C the entire carbon surface is covered by a
monolayer of diuron molecules, as described above. As a
result of this process, diuron adsorption is apparently
endothermic due to the endothermicity of diuron molecule
dehydration. Conversely, the 3,4-DCA adsorption process
is exothermic, since hydration of the 3,4-DCA molecules
in solution does not have such a large effect on their size
and polarity because they lack the urea type chain, which
is twisted after hydration.
On the other hand, it is usual to estimate the heat of
adsorption by applying the van’t Hoff law to the adsorption isotherms. This law can be applied when normal
adsorption behavior is observed in relation to temperature
but should not be applied when the adsorption is apparently endothermic, when it will give positive enthalpy values. In this case, the overall process involves both an
endothermic process (e.g., diuron dehydration in the present work) and an exothermic process (adsorption), with the
former predominating over the latter.
Results obtained in this study show the importance of
temperature in the adsorption of organic compounds from
aqueous solutions onto carbon materials. The formation of
hydrogen bonds between solute and solvent, favored at low
temperatures, can modify the shape and size of solute molecules such that they cannot access the micropores of the
adsorbent. In addition, temperature variations may possibly affect the equilibrium among different conformers when
a solute has various molecular conformations, which could
affect solute uptake on the adsorbent. In conclusion, we believe that studies of adsorption from solutions should include the effect of temperature on adsorption. This is a
relevant issue, since water temperatures can vary in nature
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Letters to the Editor / Carbon 44 (2006) 2330–2356
from 30 °C in the tropics to almost 0 °C in cold regions,
while the temperature range of industrial wastewaters is
even wider.
Acknowledgements
The authors are grateful to MEC, FEDER and Junta de
Andalucı́a projects CTQ2004-07783-C02-02 and RNM
547, for financial support.
References
[1] Corkill JM, Goodman JF, Tate JR. Adsorption of non-ionic surfaceactive agents at the graphon/solutions interface. Trans Faraday Soc
1966;62:979–86.
[2] Costa E, Calleja G, Marijuán L. Comparative adsorption of phenol,
p-nitrophenol and p-hydroxybenzoic acid on activated carbon. Adsorp
Sci Technol 1988;5:213–28.
[3] Terzyk AP, Rychlicki G. The influence of activated carbon surface
chemical composition on the adsorption of acetaminophen (paracetamol) in vitro. The temperature dependence of adsorption at the
neutral pH. Colloid Surf A 2000;163:135–50.
[4] Garcı́a-Araya JF, Beltrán FJ, Álvarez P, Masa FJ. Activated carbon
adsorption of some phenolic compounds present in agroindustrial
wastewater. Adsorption 2003;9:107–15.
[5] Mishra SK, Kanungo SB, Rajeev. Adsorption of sodium dodecyl
benzenesulfonate onto coal. J Colloid Interface Sci 2003;267:42–8.
[6] Terzyk AP. Molecular properties and intermolecular forces-factors
balancing the effect of carbon surface chemistry in adsorption of
organics from dilute aqueous solutions. J Colloid Interface Sci 2004;
275:9–29.
[7] Mohan D, Singh KP, Sinha S, Gosh D. Removal of pyridine
derivatives from aqueous solution by activated carbons developed
from agricultural waste materials. Carbon 2005;43:1680–93.
[8] Moreno-Castilla C. Adsorption of organic molecules from aqueous
solutions on carbon materials. Carbon 2004;42:83–94.
[9] Dos Santos HF, O’Malley PJ, De Almeida WB. Gas phase and water
solution conformational analysis of the herbicide diuron (DCMU): an
ab initio study. Theor Chem Acc 1998;99:301–11.
Catalytic creation of channels in the surface layers of highly
oriented pyrolytic graphite by cobalt nanoparticles
S. Konishi, W. Sugimoto, Y. Murakami, Y. Takasu
*
Department of Fine Materials Engineering, Shinshu University, 3-15-1 Tokida, Ueda 386-8567, Japan
Received 20 December 2005; accepted 8 May 2006
Available online 5 June 2006
Keywords: Highly oriented graphite; Etching; Activation; Gasification; Scanning electron microscopy
Many investigations have been conducted concerning
the catalytic influence of metals on the gasification of carbon under various conditions [1–12]. The channeling of
highly oriented pyrolytic graphite (HOPG) by various
metal and oxide particles has been found in atmospheres
of oxygen [1,11], carbon dioxide and hydrogen [5–8,10]
and steam [9]. Tamai and his co-worker [2] have investigated a series of catalytic behavior of a number of transition metals on the reaction between carbon and hydrogen.
They concluded that the catalytic gasification of carbon
occurred by the dissociation of molecular hydrogen on the
metal particle followed by migration of atomic hydrogen
to the carbon and subsequent reaction to form methane.
*
Corresponding author. Tel.: +81 268 21 5451; fax: +81 268 22 9048.
E-mail address: [email protected] (Y. Takasu).
0008-6223/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2006.05.003
They also found that the particles create channels across
the graphite basal planes, the majority of which were oriented parallel to the h1 1 2 0i directions. McKee [5] found
that the reaction between graphite and hydrogen was
strongly catalyzed by iron, cobalt and nickel; however, control of the channeling has not yet been successful. The channeling phenomenon provides not only visual information
on the hydrogenolysis mechanism of graphite by metal particles, but also provides in-sight into the potential design of
novel nano-textures on the carbon materials. In order to develop a technique to control the channeling on the graphite
surface, such as the orientation, depth and width, we must
more precisely clarify the channeling phenomenon.
We now report the investigation of the catalytic channeling of HOPG with hydrogen by cobalt nanoparticles
mainly by high-resolution scanning electron microscopy
(HRSEM) and atomic force microscopy (AFM). The