Carbon 41 (2003) 473–478
On the nature of surface acid sites of chlorinated activated
carbons
´ F. Perez-Cadenas,
´
´
Agustın
Francisco J. Maldonado-Hodar,
Carlos Moreno-Castilla*
´
´
, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain
Departamento de Quımica
Inorganica
Received 7 September 2002; accepted 4 October 2002
Abstract
Two activated carbons containing different amounts of chlorine were obtained by chlorination of an activated carbon
prepared from olive stones. Variations in surface physics and chemistry of the samples were studied by N 2 and CO 2
adsorption, mercury porosimetry, TPD, XPS, pH PZC measurements, and by testing their behaviour as catalysts in the
decomposition reaction of isopropanol. Our results indicate that chlorination of activated carbon increases its Lewis acidity
¨
but decreases its Bronsted
acidity, which can be explained by the resonance effect introduced into the aromatic rings of
graphene layers by the chlorine atoms covalently bound to their edges. This resonance effect could also explain the changes
observed in the thermal stability of C–Cl and C–O bonds.
 2002 Elsevier Science Ltd. All rights reserved.
Keywords: A. Activated carbon; B. Chemical treatment; C. Temperature programmed desorption; D. Catalytic properties; Surface properties
1. Introduction
Chlorine surface complexes can be easily bound to
carbon surfaces by treatment with chlorine at moderate
temperatures [1]. Although their existence has long been
known [1], they have not been studied as extensively as
other surface complexes or functionalities present on
carbon surfaces, such as oxygen surface complexes.
The introduction of chlorine surface complexes modifies
the surface chemistry of carbons. With regard to their
acid–base character, the acidic properties of carbons have
recently been shown [2,3] to increase after chlorination.
This was attributed to the inductive effect of chlorine on
surface oxygen functional groups, an effect that
strengthens the very weak acidic oxide sites.
However, when chlorine is bound to the edge of
graphene layers, the situation is more complex, because
there are two opposing inductive and resonance effects
responsible for the withdrawal and release of electrons,
respectively [4]. The present study aimed to elucidate the
*Corresponding author. Tel.: 134-958-243-323; fax: 134958-248-526.
E-mail address: [email protected] (C. Moreno-Castilla).
influence of these two effects on the surface acidity of
chlorinated activated carbons.
2. Experimental
An activated carbon was prepared from olive stones by
carbonization of the raw material at 1123 K for 15 min in
N 2 flow (300 cm 3 / min), and subsequent activation at 1123
K for 4 h in CO 2 flow (300 cm 3 / min) to 22% wt. burn-off.
The activated carbon so obtained is referred to hereafter as
activated carbon H.
Activated carbon H was chlorinated at either 473 or 673
K using a chlorine (Cl 2 ) flow obtained from the reaction of
potassium permanganate with hydrochloric acid (reagent
grades). Prior to the chlorination, the carbon H was treated
at 573 K in nitrogen (N 2 ) flow for 30 min. The temperature was then set at either 473 or 673 K and the N 2 flow
was switched to Cl 2 flow, which was maintained for 30
min. After this treatment, the Cl 2 flow was switched back
to N 2 flow and the chlorinated samples were heated at 673
K for 45 min. The samples were then cooled to room
temperature. Samples prepared at 473 and 673 K are
referred to hereafter as H1 and H2 samples, respectively.
0008-6223 / 02 / $ – see front matter  2002 Elsevier Science Ltd. All rights reserved.
PII: S0008-6223( 02 )00353-6
´
et al. / Carbon 41 (2003) 473–478
A.F. Perez-Cadenas
474
The chlorine content of the samples was determined
following a method described elsewhere [2].
Samples were characterised by the physical adsorption
of N 2 and CO 2 at 77 and 273 K, respectively, and by
mercury porosimetry. The surface chemistry of the original
and chlorinated activated carbons was characterised by
temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and pH at the point of zero
charge (pH PZC ) measurements, and by studying the decomposition reaction of isopropanol when carbon samples
were used as catalysts. Some of these characterisation
techniques were also applied after the treatment of samples
in wet air at 623 K for 6 h with a flow of 60 cm 3 / min. The
wet air was obtained by saturation of air with water at 298
K. After this treatment, the samples were cooled to room
temperature in dry air. This treatment was carried out in
order to increase the oxygen content of the samples.
TPD experiments were carried out by heating the
samples to 1273 K in He flow at a heating rate of 20 K
min 21 and using a Thermocube model quadrupole mass
spectrometer (Balzers) to record the amount of evolved
gases as a function of temperature as noted above [5]. In
the case of chlorine evolution, the mass / charge ratios, m /z,
tracked by mass spectrometry were 35.05, 36.05, 37.06
and 38.06. The only chlorine species generated were those
corresponding to m /z ratios of 35.05 and 37.06.
XPS measurements were made with an Escalab 200R
system (VG Scientific Co.) with Mg Ka source (hn 5
1253.6 eV) and hemispherical electron analyzer. Prior to
the analysis, the samples were pre-treated in wet air at 623
K. Once the pre-treatment was finished, the sample was
cooled to room temperature under He flow, evacuated at
high vacuum and then introduced into the analysis
chamber. A base pressure of 10 29 Torr was maintained
during data acquisition. Survey and multi-region spectra
were recorded at C 1s , O 1s and Cl 2p photoelectron peaks.
Each spectral region of photoelectron interest was scanned
several times to obtained good signal-to-noise ratios. The
spectra obtained after background signal correction were
fitted to Lorentzian and Gaussian curves in order to obtain
the number of components, the position of each peak and
the peak areas.
Surface acidity of the activated carbons was determined
by measurement of their pH PZC and testing them in the
decomposition reaction of isopropanol. The pH PZC of the
activated carbons [6] was obtained by mixing 1 g of
carbon with 20 cm 3 of CO 2 -free distilled water. The slurry
was kept in a plastic bottle and shaken periodically for 2
days until the pH had stabilised, and the final pH of the
slurry was recorded as the pH PZC of the solid.
Original and chlorinated activated carbons were also
used as catalysts in the decomposition reaction of isopropanol using a plug-flow micro-reactor working at
atmospheric pressure with He as carrier gas. The experimental procedure is described in detail elsewhere [7].
Prior to the catalytic activity measurements, the samples
were heat-treated, with a heating-rate of 5 K min 21 either
in He flow at 393 K for 1 h or in wet air at 623 K for 6 h.
Analysis of the reaction products was carried out by
on-line gas chromatography. Propene was the only reaction
product obtained and its production rate was calculated
using Eq. (1):
FISO ? C
r p 5 ]]
W
(1)
where FISO is the isopropanol flow through the catalyst in
mol per min, C is the fraction of isopropanol converted to
propene and W is the weight of the sample in grams. The
reaction was carried out in the temperature range between
403 and 443 K. The Arrhenius equation was applied to the
activity of the catalysts in the above temperature range in
order to obtain the apparent activation energy for each
catalyst.
3. Results
Surface areas and pore characteristics of the original and
chlorinated activated carbons are compiled in Table 1
together with their ash and chlorine contents. SN 2 in Table
1 was obtained by applying the BET equation to the N 2
adsorption isotherm at 77 K. Wo and L are the micropore
volume and mean width of the micropores, respectively,
and were obtained by applying the DR equation to the CO 2
adsorption isotherm at 273 K. L was obtained from the
characteristic adsorption energy [8] by means of Eq. (2)
L (nm) 5 10.8 /(Eo –11.4 kJ / mol)
(2)
Smi in Table 1 is the surface area of the micropores and
was obtained from the equation Smi (m 2 / g)52000 Wo
(cm 3 / g) / L (nm), assuming slit-shaped micropores. Finally,
Table 1
Surface characteristics of the original and chlorinated activated carbons
Sample
Ash
(wt. %)
Cl
(wt. %)
SN 2
(m 2 g 21 )
Wo
(cm 3 g 21 )
L
(nm)
Smi
(m 2 g 21 )
V2
(cm 3 g 21 )
V3
(cm 3 g 21 )
H
H1
H2
0.4
0.4
0.4
0.0
8.4
11.0
691
543
581
0.288
0.265
n.d.
0.994
1.015
n.d.
579
522
n.d.
0.133
0.082
0.123
0.792
0.448
0.645
n.d., not determined.
´
et al. / Carbon 41 (2003) 473–478
A.F. Perez-Cadenas
V2 and V3 , obtained by mercury porosimetry, are the
volumes of pores with diameters of 3.6–50 nm and .50
nm, respectively.
Sample H2 had a higher Cl content than sample H1 as a
consequence of the higher temperature used for its chlorination treatment. In sample H, SN 2 was higher than Smi ,
due to the heterogeneity of the microporosity and the
presence of wide micropores. However, SN 2 was similar to
Smi in sample H1. This indicates that the chlorination
treatment closed some of the micropores, which in turn
increased their mean width, L. Pore volumes V2 and V3 also
decreased with the chlorination treatment, with a higher
decrease in sample H1 than in sample H2, despite the
lower Cl content of H1. This could be due to the different
temperatures used in the chlorination treatment of these
two samples.
Table 2 lists the results of the TPD experiments up to
1273 K in He flow and the pH PZC values of the samples.
Chlorination of activated carbon H led to a net decrease in
its oxygen content, as determined from the amount of CO
and CO 2 evolved during TPD. This suggests a reduction of
oxygen surface complexes, which were probably substituted by chlorine atoms [9]. However and despite the
decrease in oxygen content, the pH PZC decreased with an
increase in the chlorine content of the samples.
Hydrogen evolution from samples H1 and H2 was not
detected during the TPD runs up to 1273 K. Hydrogen
evolution was observed in an earlier study [3], and is due
to a chlorine fixation mechanism of carbons that involves
the substitution of hydrogen by chlorine. On the other
hand, chlorine evolution was observed above 850 K,
indicating that chlorine species were not physisorbed.
The CO desorption profiles of the original and chlorinated carbons are depicted in Fig. 1, showing that the
maximum CO desorption temperature decreased when the
chlorine content of the carbon increased. Thus, the thermal
stability of the CO-evolving groups decreased with increasing chlorination treatment severity.
3.1. Effect of treatment in wet air at 623 K on the
surface chemistry of the original and chlorinated
activated carbons
Original and chlorinated activated carbons were again
characterised by XPS, TPD and pH PZC measurements after
Table 2
Amounts of CO and CO 2 evolved up 1273 K and pH PZC of the
original and chlorinated activated carbons
Sample
pH PZC
CO
(mmol g 21 )
CO 2
(mmol g 21 )
O TPD
(% wt.)
H
H1
H2
9.89
6.50
6.09
827
696
797
194
104
117
1.94
1.45
1.65
475
Fig. 1. CO desorption profile of samples: H (앳), H1 (h) and H2
(n).
their wet-air pre-treatment at 623 K for 6 h. XPS patterns
of the C 1s , O 1s , and Cl 2p regions for sample H1 are
depicted in Fig. 2 as an illustration. Fig. 2 also includes the
curve-fitted spectra. The binding energies of Cl 2p 3 / 2 and
Cl 2p 1 / 2 at 200.5 and 202.1 eV, respectively, indicate that
chlorine atoms were covalently bonded to sp 2 carbon, as
reported for chlorinated organic compounds [9]. The
following functions [10] were considered (with binding
energies) in the curve-fitting of the C 1s spectrum: aromatic
and aliphatic carbon (284.9 eV), single C–O bonds (286.2
eV), double C=O bonds (288.0 eV), carboxyl groups
(289.1 eV) and carbonate groups (290.6 eV). The peak at
286.2 eV in the C 1s spectral region can also be assigned to
C–Cl bonds [9]. Thus, Fig. 3 shows a linear relationship
between the percentage of Cl XPS and the area of the peak
at a binding energy of 286.2 eV.
In the case of the O 1s spectrum, only the double C=O
bonds at 532 eV and the single C–O bonds at 533.9 eV
were considered. The percentages of the different oxygen
complexes considered are shown in Table 3. In all
samples, oxygen surface complexes with single C–O
bonds predominated, with a slightly increased percentage
in samples with higher chlorine content.
Surface oxygen (O XPS ) and chlorine (Cl XPS ) contents
were obtained from the above XP spectra. These data and
TPD and pH PZC measurements are shown in Table 4.
During the wet-air treatment there was a large increase
in the oxygen content, primarily due to an increase in the
number of CO-evolving groups. The final oxygen content
was slightly higher for the original than for the chlorinated
carbon samples, because the chlorinated samples had a
similar capability to fix CO 2 -evolving groups but a lesser
capability than the original samples to fix CO-evolving
groups. The fixation of the surface oxygen groups caused a
decrease in the pH PZC of all samples.
As observed in the samples before the wet-air treatment,
the CO-evolving groups were also thermally less stable
after it. Furthermore, after the wet-air treatment, the
thermal stability of the chlorine bonded to the carbon
surface increased, with chlorine evolving at higher tem-
476
´
et al. / Carbon 41 (2003) 473–478
A.F. Perez-Cadenas
Fig. 3. Relationship between surface chlorine content and area of
XPS peak at binding energy of 286.2 eV: H (m), H1 (s) and H2
(j).
picted in Fig. 5 and show that the propene production
activity, r p , linearly decreased with an increase in the
chlorine content of the carbon. Table 5 shows the apparent
activation energy between 403 and 443 K, which also
decreased when the chlorine content increased.
4. Discussion
It is well known that XPS gives information about the
composition of the most external surface of carbon particles. Thus, comparing XPS results with those of bulk
Table 3
Binding energies (eV) of C 1s , O 1s and Cl 2p 3 / 2 core-level spectra
and percentage of oxygen complexes (in parentheses) for original
and chlorinated activated carbons after their pretreatment in wet
air at 623 K
Fig. 2. Curve-fitted C 1s (a), O 1s (b) and Cl 2p (c) core-level spectra
for sample H1 treated in wet air at 623 K.
peratures than those required before the wet-air treatment,
as illustrated in Fig. 4 for samples H1 and H2.
When the original and chlorinated activated carbons
were pre-treated at 393 K in He flow, they showed no
activity in the decomposition reaction of isopropanol
within the temperature range between 403 and 443 K.
However, they were active when the pre-treatment was
carried out in wet air at 623 K. The only product obtained
was propene, so that the activated carbons used can be
considered as dehydration catalysts. The results are de-
Sample
C 1s (eV)
H
284.9
286.2
287.2
288.6
290.2
291.4
(69)
(20)
(4)
(3)
(3)
(2)
532.1
533.9
(42)
(58)
n.d.
H1
284.9
286.3
287.5
288.6
290.3
291.6
(64)
(25)
(3)
(3)
(3)
(2)
532.0
533.9
(40)
(60)
200.5
H2
284.9
286.2
287.4
288.7
290.2
291.4
(63)
(27)
(5)
(2)
(2)
(1)
531.9
533.9
(37)
(63)
200.5
n.d., not detected.
O 1s (eV)
Cl 2p 3 / 2 (eV)
´
et al. / Carbon 41 (2003) 473–478
A.F. Perez-Cadenas
477
Table 4
Amounts of CO and CO 2 evolved up to 1273 K, oxygen and chlorine contents and pH PZC of the original and chlorinated activated carbons
after their pretreatment in wet air at 623 K
Sample
pH PZC
CO
(mmol g 21 )
CO 2
(mmol g 21 )
O TPD
(% wt.)
O XPS
(% wt.)
Cl XPS
(% wt.)
H
H1
H2
7.81
5.72
5.49
2429
2101
2182
298
323
338
4.84
4.39
4.57
5.37
6.66
6.47
–
Fig. 4. Cl desorption profiles: original sample (h), after treatment
in wet air at 623 K (앳). The m /z ratios monitored were 35.05 and
37.06.
Fig. 5. Relationship between the activity to obtain propene, r p ,
and the chlorine content of the activated carbons at various
reaction temperatures.
8.69
11.58
analysis can give an idea of the homogeneity of the
distribution between the internal and external surface of
the carbon particles. Results shown in Tables 1 and 4
indicate that the total chlorine and the Cl XPS contents were
similar, so that chlorine is homogeneously distributed. In
contrast, the O XPS content was always higher than the
O TPD content (Table 4). This fact suggests that the surface
oxygen complexes were, therefore, primarily fixed on the
more external surface of the samples during the treatment
in wet air.
The surface acidity of the samples, determined by their
pH PZC , increased after the chlorination treatment. This
observation is in line with findings by MacDonald et al.
[3], who attributed this to the inductive effect of chemisorbed chlorine and suggested that very weak acidic oxides
sites are thus strengthened, contributing to the pH PZC
observed in mass titration experiments. According to this
explanation, the chlorine chemisorbed on the carbon
¨
surface increases the Bronsted
acidity of weak acidic oxide
sites. If this were the case, the production rate of propene
from isopropanol would increase with increasing chlorine
content of the carbons, because the dehydration of this
¨
alcohol is catalyzed by Bronsted-type
acid sites [4].
However, the opposite effect was observed in our experiments, which revealed a linear decrease in propene generation activity with an increase in the chlorine content of
the samples.
It is known [4] that when chlorine is bound to aryl
compounds, electrons can be both withdrawn by and
released from the chlorine through inductive and resonance
effects, respectively. The two effects tend to oppose each
other and are important in controlling the reactivity of the
aryl halides.
When the resonance effect is considered in a simple
model molecule, such as chlorobenzene, a delocalization of
electrons through the aromatic ring can be observed,
according to the following canonical structures.
Table 5
Apparent activation energy between 403 and 443 K for the
dehydration of isopropanol to propene
Sample
EP (kJ mol 21 )
H
H1
H2
93.164.1
83.363.1
80.662.9
Experimental evidence supporting this mechanism in-
478
´
et al. / Carbon 41 (2003) 473–478
A.F. Perez-Cadenas
cludes the unusually short carbon–chlorine bond and its
unusually small dipole moment [4]. Thus, the resonance
effect produces a positive partial charge on the halogen,
with the consequent delocalization of negative charge
through the aromatic ring.
In the present experiment, XPS showed that Cl was
bound to C sp 2 , so that the chlorine was localised at the
edges of the graphene layers. The resonance effect would,
therefore, be enhanced relative to that of chlorobenzene
because of the larger number of contributing aromatic
rings, so that the positive partial charge on the chlorine
would be more stabilised.
From an acid–base standpoint, the resonance effect
gives the chlorine a Lewis-type acid character. This would
explain why the pH PZC decreases when the chlorine
content increases, because this kind of pH measurement
gives the total acidity of the carbon, of both Lewis- and
¨
Bronsted-type.
In addition, the release of electrons and their delocaliza¨
tion on the aromatic rings would weaken the Bronsted
acidity of the acidic oxide sites, because their conjugated
¨
bases would be less stabilised. This decrease in Bronsted
acidity with an increase in chlorine content could also
influence our experimental results. In fact, these resonance
effects would explain why the increase in thermal stability
of C–Cl bonds occurred in parallel to a decrease in the
thermal stability of C–O bonds.
Our results thus indicate that the increase in Lewis
acidity of activated carbons after chlorination is greater
¨
than the decrease in Bronsted
acidity of the acidic oxide
sites, so that chlorination decreases the pH PZC of samples.
In conclusion, the surface acidity of chlorinated activated carbons is more influenced by the resonance effects
of chlorine atoms than by their inductive effects.
5. Conclusions
After chlorination treatment, chlorine is bonded covalently to C sp 2 and distributed homogeneously between
the external and internal surfaces of carbon particles.
Chlorination of activated carbons decreases both their
pH PZC and their catalytic activity towards the dehydration
of isopropanol to propene. These results suggest that
chlorination increases the Lewis acidity of activated car¨
bons but decreases the Bronsted
acidity of surface oxygen
complexes. Variations in both types of acid surface site are
explained by resonance effects rather than by inductive
effects. Resonance effects may also explain the decrease in
thermal stability of the oxygen surface complexes that
occurs with increases in the thermal stability of the C–Cl
bonds.
Acknowledgements
The research described in this work was supported by
DGESIC, Project No. PB97-0831. The authors gratefully
acknowledge Professor J.L.G. Fierro from the Instituto de
´
´
Catalisis
y Petroleoquımica,
Madrid, for providing the XP
spectra.
References
[1] Puri BR. Chemistry and physics of carbon, vol. 6. New York:
Marcel Dekker, 1970, pp. 191–282.
[2] Evans MJB, Halliop E, Liang S, MacDonald JAF. The effect
of chlorination on surface properties of activated carbon.
Carbon 1998;36(11):1677–82.
[3] MacDonald JAF, Evans MJB, Liang S, Meech SE, Norman
PR, Pears L. Chlorine and oxygen on the carbon surface.
Carbon 2000;38:1825–30.
[4] Morrison RT, Boyd RN. Organic chemistry. Boston: Allyn
and Bacon, 1968.
´ MA, Utrera-Hidalgo E, Rivera-Utrilla J,
[5] Ferro-Garcıa
Moreno-Castilla C, Joly JP. Regeneration of activated carbons exhausted with chlorophenols. Carbon 1993;31(6):857–
63.
[6] Leon y Leon CA, Radovic LR. Chemistry and physics of
carbon, vol. 24. New York: Marcel Dekker, 1994, pp. 213–
310.
´ F, Parejo-Perez
´
[7] Moreno-Castilla C, Carrasco-Marın
C,
´
´ MV. Dehydration of methanol to dimethyl
Lopez-Ramon
ether catalyzed by oxidized activated carbons with varying
surface acidic character. Carbon 2001;39:869–75.
´
´ MV, Stoeckli F, Moreno-Castilla C, Carrasco[8] Lopez-Ramon
´ F. Specific and nonspecific interactions between
Marın
methanol and ethanol and active carbons. Langmuir
2000;16:5967–72.
[9] Papirer E, Lacroix R, Donnet JB, Nanse G, Fioux P. XPS
study of the halogenation of carbon black–Part 2. Chlorination. Carbon 1995;33:63–72.
[10] Desimoni E, Casella GI, Salvi AM. XPS / XAES study of
carbon fibres during thermal annealing under UHV conditions. Carbon 1992;30(4):521–6.