Carbon 41 (2003) 387–395
Adsorption of p-nitrophenol on an activated carbon with
different oxidations
´ b , J. Rivera-Utrilla b , J.P. Joly a , *
S. Haydar a , M.A. Ferro-Garcıa
a
Laboratoire d’ Application de la Chimie a` l’ Environnement, UMR 5634 CNRS-Universite´ Claude Bernard Lyon 1,
43 Bd. du Onze Novembre 1918, 69622 Villeurbanne cedex, France
b
´
´
, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain
Departamento de Quımica
Inorganica
Received 31 May 2002; accepted 22 August 2002
Abstract
An activated carbon prepared from olive stones has been modified through oxidation by nitric acid or sodium
hypochlorite. These treatments introduced large amounts of oxygen groups, which were characterized by mass-spectrometry,
temperature-programmed desorption (DTP–MS). Both CO 2 - and CO-evolving groups were created by these oxidation
treatments. A part of these oxidized samples was then outgassed under vacuum up to 823 K in order to remove most of the
CO 2 -evolving groups from their surface. Oxidized samples have a smaller surface area than the original sample. The
subsequent partial outgassing increases the surface area which, however, does not reach the value it had before oxidation.
p-Nitrophenol (PNP) adsorption isotherms from aqueous solutions were determined at 298 K for the original, oxidized, and
partly outgassed samples. The results confirm the presence of an intermediate plateau at low equilibrium PNP concentration
(at about 10 mg / l). The relative effects of textural versus surface chemistry on PNP uptakes are then discussed. The presence
of CO-evolving groups showed no influence on PNP uptakes. The conclusion is that models in which carbonylic groups are
basic adsorption sites for substituted phenols can be ruled out for the entire isotherm of PNP obtained with the original
carbon. These models are also unlikely for PNP adsorption on oxidized and partly outgassed samples.
 2002 Elsevier Science Ltd. All rights reserved.
Keywords: A. Activated carbon; B. Oxidation; C. Adsorption
1. Introduction
The adsorption of phenol and substituted phenols from
aqueous solution on activated carbons has been intensively
investigated for decades. However, this subject remains
highly controversial as described in a recent review by
Radovic et al. [1]. There is a general agreement that
textural characteristics and surface chemistry of activated
carbons play a fundamental role in substituted phenol
adsorption but discrepancies appear when an in-depth
description of this phenomenon is attempted.
A general concept in adsorption is that the larger the
specific surface area of the solid, the larger is the maximum adsorption capacity. Nevertheless, with microporous
activated carbons, a part of the surface can be inaccessible
*Corresponding author. Tel.: 133-472-448-333; fax: 133472-448-114.
E-mail address: [email protected] (J.P. Joly).
to the adsorbate, so that a relationship between surface area
and adsorption capacity is generally not observed. Conversely, some narrow pores, if accessible, may favor a
strong physical interaction between the adsorbate molecules and the surface of carbon. The influence of textural
properties of carbons in substituted phenols’ adsorption,
and in particular the favorable role of small micropores,
has been recognized by numerous authors up to recently
[2–9].
Among many other subjects discussed in the literature,
the nature of the adsorption sites for substituted phenols is
a major source of discrepancies as largely reported in Ref.
[1]. Many authors consider electron-rich regions located in
graphene layers interacting with the p electrons of the
aromatic ring of phenols (p–p argument [1]), while
numerous other authors arrived at or supported the conclusion that some basic carbonyl surface groups form a bond
with the aromatic ring of phenol (donor–acceptor argument [1]). These two assumptions developed independent-
0008-6223 / 02 / $ – see front matter  2002 Elsevier Science Ltd. All rights reserved.
PII: S0008-6223( 02 )00344-5
388
S. Haydar et al. / Carbon 41 (2003) 387–395
ly since they were, respectively, introduced by Coughlin
and Ezra [10] on the one hand and by Mattson et al. [11]
on the other hand. Apart from these hypotheses, superficial
carboxylic groups, which are able to form a hydrogen bond
with the OH function of phenols, are other possible
adsorption sites [12].
Indeed, the experimental discrimination between these
adsorption models is not easy because the introduction of
oxygen groups in an activated carbon may concomitantly
modify most of its textural and chemical properties. For
instance, a classical treatment of carbon with nitric acid not
only introduces both acidic (i.e. carboxylic) and basic (i.e.
carbonyl) oxygen groups at the surface of carbon, but also
may modify the nature of preexisting oxygen groups [11].
In addition, even if carried out under mild conditions to
avoid a destruction of the porous structure of carbon,
carboxylic groups created by nitric treatment may block
micropore entrances and consequently prevent the access
of phenol molecules to adsorption sites [12–14].
This paper presents a study of p-nitrophenol (PNP)
adsorption from aqueous solutions on a variously oxidized
activated carbon. The aim of this study was to get
additional information on the influence of oxygen species
on the adsorption of substituted phenols.
2. Experimental
An activated carbon, denoted AC, was prepared from
olive stones as described below. The raw vegetal material
was washed in sulfuric acid to eliminate most inorganic
elements and then washed with distilled water up to sulfate
elimination. The resulting material was carbonized at 1273
K for 1 h under a nitrogen flow. The activation was then
carried out in a stream of carbon dioxide at 1113 K for 16
h. Carbon AC was then used in the form of grains, the size
of which were in the range 1–1.4 mm.
In order to introduce surface oxygen groups, carbon AC
was separated into two parts. The first part was oxidized
with nitric acid, the second one by sodium hypochlorite.
The resulting materials were respectively termed AC-N
and AC-Cl.
Nitric oxidation was carried out as follows: 80 cm 3 of a
concentrated HNO 3 solution (15 M) was slowly poured on
4 g of AC located in a vessel provided with a magnetic
stirrer and cooled in an ice bath. Carbon AC was then left
in contact with nitric acid at ambient temperature for 24 h.
The vessel contents were diluted with water and filtered.
The carbon was washed with distilled water until neutral
pH and separated by filtration. It was finally dried under
vacuum at 323 K for one night.
Hypochlorite oxidation treatment has been proposed in
early studies for carbon oxidation [15,16]. In this study, it
was carried out by contacting 4 g of AC with a NaOCl
solution (20 volumes) in conditions similar to those used
for nitric oxidation. At the end of this operation, hypochlo-
rite ions in excess in the solution were destroyed by
chlorhydric acid (0.1 M; 70 cm 3 ). The carbon was then
washed, separated by filtration and dried by the same
procedures as described above.
A part of the carbons AC-N and AC-Cl underwent an
additional treatment in order to remove a part of the
initially present oxygen groups. This treatment consisted of
heating the carbon under vacuum up to 823 K by TPD.
The resulting materials are termed AC-N-D and AC-Cl-D,
respectively.
Temperature programmed desorption experiments followed by a mass spectrometer (TPD–MS) were performed
under vacuum (10 26 –10 23 mmHg or 0.0001–0.1 Pa) at a
heating rate of 5 K / min. The apparatus has been described
elsewhere [17]. CO 2 and CO desorption fluxes were
followed by monitoring the heights of the mass peaks
m /e 5 44 and m /e 5 28 amu, respectively. Ionization
voltage was fixed to 25 eV instead of the conventional 70
eV in order to minimize CO 2 fractionation within the MS
source and, hence, make negligible the contribution of CO 2
to the mass peak m /e 5 28 amu.
BET specific surface areas of carbons were determined
by nitrogen adsorption at 77 K. Micropore volumes Vp
were obtained by Dubinin–Radushkevich transforms of
isotherms. Pore size distributions were determined using a
mercury porosimeter (maximum pressure 4200 kg / cm 2 )
allowing the mercury penetration in pores with a size
larger than 3.7 nm. This technique provides the external
specific surface area Sext that corresponds to pores with a
size larger than 3.7 nm, the volume V1 of pores with a size
ranging from 3.7 to 50 nm and the volume V2 of pores
larger than 50 nm (macropores).
The adsorption of p-nitrophenol (PNP) was carried out
by contacting 100 mg of carbon and 100 ml of variously
concentrated aqueous solutions of PNP in tightly closed
glass flasks. Carbon samples were weighed after they were
kept at 383 K under air for one night. Flasks were shaken
at 298 K in a thermostated bath for 1 week. PNP solutions
were then separated by sedimentation and their concentrations were evaluated by UV absorption at l 5317 nm.
No buffer was used in PNP solutions to avoid a possible
influence of foreign ions on the adsorption measurements.
The free pH of the solutions during adsorption was in the
range 5–6.
3. Results
Table 1 shows the elemental analysis of the samples of
activated carbon. The oxygen content was not obtained by
difference but by an analytical method. This method
consisted of a complete pyrolysis of the sample at 1353 K
under nitrogen. The products of this pyrolysis were
transformed into CO by flowing through an active carbon
bed at 1393 K. Carbon monoxide was finally evaluated by
means of a specific IR detector. The sum of the per-
S. Haydar et al. / Carbon 41 (2003) 387–395
Table 1
Results of elemental analysis of carbons
AC
AC-N
AC-N-D
AC-Cl
AC-Cl-D
C%
H%
N%
O%
Sum (%)
96.79
86.54
92.96
86.5
92.72
0.51
0.77
0.53
0.82
0.57
0.3
0.57
0.34
0.3
0.3
0.6
11.72
4.58
10.06
4.22
98.2
99.6
98.41
97.68
97.81
centages is always around 98.5%. The missing 1.5% is
very likely due to the presence of impurities in carbons,
which were not considered in this analysis. The nitrogen
content remains 0.3% except for the sample treated with
nitric acid that shows a higher content. This is not
surprising because it has been noted that nitrate or nitro
surface groups are present in small amounts on the surface
after such a treatment [18]. The oxygen content of carbon
AC as prepared is low, i.e. 0.6%, while carbons oxidized
with nitric acid or sodium hypochlorite present the highest
oxygen contents, i.e. 11.72% and 10.06%, respectively.
389
Oxidized carbons partly outgassed present intermediate
oxygen contents.
Table 2 gives textural data obtained from nitrogen
isotherms (type I) on the one hand, and from mercury
porosimetry on the other hand. The BET surface area of
carbon AC decreases on oxidation by HNO 3 or NaOCl. It
is partly restored by the outgassing following the oxidation
but it does not reach its original value. DR micropore
volume Vp follows the same trends. The external surface
area Sext is lower after each subsequent carbon treatment.
The same trends are also followed by V1 , the volume of
pores the size of which is in the range 3.7–50 nm. Volume
V2 , corresponding to pores larger than 50 nm, decreases
also on subsequent treatments. This volume is strongly
affected by the treatment with NaOCl.
Fig. 1 shows the TPD profiles of CO 2 from the various
samples studied. It is seen that CO 2 desorbs between 350
and 1000 K. As expected, oxidation of carbon by HNO 3 or
NaOCl dramatically increases the amount of CO 2 -evolving
oxygen groups. However, CO 2 TPD profiles are not
proportional for samples AC-N and AC-Cl. This means
Table 2
Textural characteristics obtained from N 2 adsorption at 77 K (columns 2 and 3) and mercury porosity (columns 4–6)
AC
AC-N
AC-N-D
AC-Cl
AC-Cl-D
Surface area
(m 2 / g)
Vp
(cm 3 / g)
V1
(cm 3 / g)
V2
(cm 3 / g)
Ext. surface
area (m 2 / g)
1696
1559
1603
1350
1453
0.76
0.68
0.69
0.59
0.66
0.426
0.352
0.293
0.308
–
0.139
0.109
0.093
0.004
–
215
176
113
155
–
Fig. 1. CO 2 TPD profiles; d, carbon AC; j, carbon AC-N; h, carbon AC-N-D; m, carbon AC-Cl; ^, carbon AC-Cl-D.
S. Haydar et al. / Carbon 41 (2003) 387–395
390
Fig. 2. CO TPD profiles; d, carbon AC; j, carbon AC-N; h, carbon AC-N-D; m, carbon AC-Cl; ^, carbon AC-Cl-D.
that the oxygen entities, or their populations, are not
identical on both samples. Outgassing of samples AC-N
and AC-Cl up to 823 K strongly reduces the amounts of
CO 2 -evolving groups.
Fig. 2 shows the TPD profiles of CO from the various
samples studied. It is seen that CO-evolving oxygen
groups are not completely decomposed at the final temperature of the TPD runs, i.e. at 1123 K. The amount of
these groups also strongly increases during the oxidation of
carbon AC by HNO 3 or NaOCl. The partial outgassing of
samples AC-N and AC-Cl up to 823 K not only decreased
the amount of CO desorbed up to this temperature, as
expected, but also increased the amount of CO desorbed in
the range 823–1123 K for carbon AC-N and in the range
823–1050 K for carbon AC-Cl. This means that, apart
from the decomposition of CO-evolving entities at lower
temperatures during the partial outgassing, there are some
transformations among these entities. Such transformations
have been observed previously [19].
Table 3 provides quantitative results obtained by integration of the TPD profiles shown in Figs. 3 and 4.
Oxidized carbons AC-N and AC-Cl desorbed more CO
than CO 2 . After outgassing (samples AC-N-D and AC-Cl-
D) the ratios CO / CO 2 are strongly increased. The amounts
of atomic oxygen evaluated from the TPD profiles of CO
and CO 2 are always smaller than those found by elemental
analysis. This is mainly due to the contribution of adsorbed
water to elemental analysis results.
Fig. 3 shows the PNP adsorption isotherms obtained
with carbon AC as prepared, oxidized with nitric acid
(sample AC-N), and partly outgassed after this oxidation
(sample AC-N-D). It is seen that, for concentrations
ranging from 1 to 140 mg / l, the isotherm for the carbon
AC-N is clearly located under that for carbon AC. The
isotherm for carbon AC-N-D is located in between. As
shown in Fig. 4, exactly the same behavior is observed
when carbon AC is oxidized with sodium hypochlorite.
4. Discussion
4.1. Intermediate plateau
The shape of isotherms shown in Figs. 5 and 6 suggests
the existence of an intermediate plateau at PNP concentrations close to 10 mg / l, at least for oxidized samples
Table 3
Comparison of the amounts of carbon oxides and of elemental oxygen calculated from TPD and from elemental analysis
Sample
AC
AC-N
AC-N-D
AC-Cl
AC-Cl-D
TPD
Elem. analysis
CO 2 (mmol / g)
CO (mmol / g)
CO / CO 2
O (mmol / g)
O (mmol / g)
42
1140
219
636
231
92
1867
1112
1533
977
2.19
1.64
5.08
2.41
4.23
176
4147
1550
2805
1439
375
7325
2863
6288
2638
S. Haydar et al. / Carbon 41 (2003) 387–395
391
Fig. 3. PNP isotherms; d, carbon AC; j, carbon AC-N; h, carbon AC-N-D.
Fig. 4. PNP isotherms; d, carbon AC; m, carbon AC-Cl; ^, carbon AC-Cl-D.
Fig. 5. PNP uptakes per unit surface of carbon; d, carbon AC; j, carbon AC-N; h, carbon AC-N-D; m, carbon AC-Cl; ^, carbon
AC-Cl-D.
392
S. Haydar et al. / Carbon 41 (2003) 387–395
Fig. 6. PNP uptakes per unit surface of carbon in the lower equilibrium concentrations region; (a) d, carbon AC; j, carbon AC-N; h,
carbon AC-N-D; (b) d, carbon AC; m, carbon AC-Cl; ^, carbon AC-Cl-D.
AC-N and AC-Cl. This confirms the early finding of
Mattson et al. [11] who showed that PNP isotherms on an
activated carbon indeed exhibit such a plateau. In a recent
work by Finqueneisel et al. [20] about PNP adsorption on
carbons from lignites of relatively low surface area (300–
400 m 2 / g), this plateau is also visible in some isotherms
though the authors did not mention it. The intermediate
plateau, the existence of which is more firmly established
in the case of phenol [10,21–23], has been only rarely
mentioned in the case of PNP because it is less marked for
this compound than for phenol [11]. It is also seen in PNP
isotherms shown in Figs. 5 and 6 that the intermediate
plateau appears more clearly for oxidized samples than for
the original and the partly outgassed samples. This observation may be compared to that made by Nevskaia et al.
[23] in the adsorption isotherms of phenol which exhibit
this plateau only for their sample oxidized with nitric acid.
This confirms the qualitative similarity between the isotherms of PNP and those of phenol.
General interpretations for the existence of two plateaus
in solute isotherms have been proposed by Giles [21]:
firstly, the formation a double layer, the saturation of each
layer corresponding to a plateau, secondly, a different
orientation, i.e. parallel or perpendicular to the surface, of
adsorbed molecules and, thirdly, the adsorption on different types of sites. Mattson et al. [11] have followed this
third interpretation, proposing that the lower plateau is due
to the adsorption of PNP on carbonyl groups (according to
the donor–acceptor argument) while the higher plateau
results from an adsorption of PNP on graphene basal
planes (in agreement with the p–p argument). More
recently, Nevskaia et al. [23] also assumed that the donor–
acceptor complex mechanism is operating in the case of
phenol adsorption on HNO 3 -treated activated carbons.
Besides, these authors intended to explain the presence of
the two plateaus as follows: the lower concentration part of
the isotherm corresponds to a displacement of water from
carbon surfaces free of functional groups while the higher
S. Haydar et al. / Carbon 41 (2003) 387–395
concentration part results from a displacement of water
bonded to oxygen functional groups. These different
assumptions show the interest of studying the low concentration part of the isotherms. Despite numerous efforts
already dedicated to understand the adsorption of substituted phenol on carbons, more work is needed to clarify
the behavior of isotherms at these concentrations.
4.2. Textural versus chemical effects
4.2.1. Higher part of isotherms
The role of the chemical properties of carbon surfaces
has been evidenced by numerous authors in the case of
phenol adsorption, which has been much more investigated
than that of PNP adsorption. In the present work, the
qualitative parallelism between the amounts of PNP adsorbed under a given solution concentration and the
surface area of carbon samples is striking. Such a parallelism is also evident when the microporous volume Vp of
samples is considered. In an attempt to assess the respective role of surface chemistry and textural properties, we
divided the amounts of PNP adsorbed by the BET surface
areas. The resulting isotherms are presented in Fig. 5. It is
seen that all isotherm points move toward a single curve
except those for sample AC-N. The same behavior is
observed when the amounts of adsorbed PNP are divided
by the microporous volume Vp , this volume being virtually
proportional to the surface area.
The decrease in adsorption capacity, expressed per
surface unit of the sample after a humid oxidation treatment, has been often observed in the case of phenol
[24,25]. The comparison in Fig. 5 of the results we
obtained for the original carbon AC and the HNO 3 -treated
carbon AC-N show that the adsorption of PNP follows the
same trend. A partial outgassing of the HNO 3 -treated
sample restores the initial capacity per unit surface. This
behavior is in favor of the p–p argument. Thus, the
treatment by HNO 3 strongly increases the number of
CO 2 -evolving groups that withdraw electrons from the
graphene layers, and consequently decreases the number of
adsorption sites for PNP. On the other hand, the partly
outgassed sample AC-N-D has lost a large part of the
CO 2 -evolving groups but contains about 12 times more
CO-evolving groups than the original carbon AC (see
Table 3). Nevertheless, these oxygen groups have virtually
no effect on the PNP uptakes. This result disagrees with
the expected consequences of the donor–acceptor argument.
Results for samples AC-Cl and AC-Cl-D depicted in
Fig. 4 show virtually no effect of the oxygen groups on
PNP uptakes. This is in part understandable, in the frame
of the p–p argument, because the oxidation by NaOCl
created less CO 2 -evolving groups than the oxidation by
HNO 3 (see Table 3). It is also possible that the oxygen
groups created by the oxidation by NaOCl, which are
different to those created by the nitric treatment as shown
393
by TPD, have an electron withdrawal effect less important
than those created by HNO 3 . Anyway, as in the case of
nitric acid treatment, no favorable effect of CO-evolving
groups is observed for carbon AC-Cl-D, either.
4.2.2. Lower part of isotherms
Fig. 6a shows the PNP isotherms, expressed per unit
surface, for samples AC, AC-N and AC-N-D, at equilibrium concentrations lower than 10 mg / l. Again, PNP
uptakes support the p–p argument, since they decrease
when CO 2 -evolving groups are massively introduced in
carbon and then increase without reaching their original
values when an important part of CO 2 -evolving groups are
removed by the partial outgassing.
Isotherms for carbon AC-Cl and AC-Cl-D are shown in
Fig. 6b. Despite the dispersion of the experimental points,
it is seen that the effect of oxygen groups on PNP uptakes
is much less important than for carbons AC-N and AC-ND. The possible reasons for this property are the same for
low and high concentrations and they have been given
earlier. Besides, it is clear from Fig. 6 that no enhancement
of the adsorption with the strong increase of CO-evolving
groups for carbons AC-N-D and AC-Cl-D are observed in
the lower concentration part of the isotherms. Therefore,
our results do not support the donor–acceptor argument for
the lower part of the PNP isotherms, either.
4.3. Adsorption sites
The donor–acceptor argument implies the presence of a
sufficient amount of basic oxygen group for binding the
aromatic cycle of substituted phenols. According to a
recent review by Boehm [26], candidates for such basic
centers are carbonyl groups, in particular when these
groups are engaged in a polycyclic pyrone-like structure.
In Fig. 3, it is seen that the maximum PNP adsorption
capacity at high concentration is roughly 500 mg / g, i.e.
3600 mmol / g. This amount is about 40 times higher than
that of CO-evolving groups reported in Table 3. This
means that the donor–acceptor model may be definitely
ruled out for the entire isotherm for carbon AC. The p–p
argument seems then relevant to PNP adsorption on a
non-oxidized carbon. This conclusion is similar to that
reached by Nevskaia et al. [23] in the case of phenol.
The intermediate plateaus observed in the frames included in Figs. 5 and 6 correspond roughly to PNP uptakes
of 250 mg / g of carbon, i.e. 1800 mmol of PNP per gram.
Considering the donor–acceptor argument, the same
amount of basic oxygen groups would be required to
adsorb PNP molecules. This is not verified for carbon AC
as stressed before. There is also a lack of CO-evolving
groups for carbons AC-N-D and AC-Cl-D. The amount of
CO-evolving groups for carbons AC-N and AC-Cl-D
indeed reach 1867 and 1553 mmol / g. Thus, in these cases,
it is not possible to rule out the role of carbonyl groups as
adsorption sites for PNP on the single basis of the possible
394
S. Haydar et al. / Carbon 41 (2003) 387–395
amount of CO-evolving groups. However, it is likely that
these last figures remain insufficient because carbonyl
functional groups are only a part of CO-evolving groups
among esters, phenols and anhydride species for instance
[27]. Anyway, the qualitative arguments developed in
Section 4.2 showed that it is unlikely that carbonyl groups
are the adsorption sites for PNP.
The pH of the solutions was left free during adsorption
experiments and therefore these solutions were slightly
acidic (pH 5–6). Consequently, PNP that has a pKa equal
to 7.13 was virtually not ionized in the solutions used.
Then, it is expected that an electrostatic interaction does
not play a significant role in PNP adsorption under our
experimental conditions. At this point, the present discussion allows us to conclude that, definitely ruled out for
carbon AC because of the evident lack of adsorption sites,
the donor–acceptor argument is quite unlikely for oxidized
and partly outgassed carbons for the whole PNP isotherms.
Franz et al. [12] recently arrived at a similar conclusion
for the adsorption of aqueous and organic solutions of
phenol, nitrobenzene, aniline and benzoic acid. The present
study deals with PNP possessing the strong deactivating
group NO 2 attached to the aromatic ring. A priori, one
could have thought that the presence of this group is in
favor of the formation of a donor–acceptor bond formation
between surface carboxylic groups and the electron-poor
aromatic ring of PNP. Our results show that, even in this
case, the donor–acceptor mechanism is not operating.
There are other important differences between the study of
Franz et al. [12] and the present study. Thus, in the work
described by these authors, carbon samples were oxidized
with oxygen while they were oxidized with HNO 3 or
NaOCl in the present work. In addition, oxidized carbons
were only partly outgassed in the present study while the
original carbon was virtually completely outgassed in the
work of Franz et al. Finally, the concentration range we
used goes to lower concentrations, allowing the apparition
of the intermediate plateau in PNP isotherms. Thus, results
presented here complete those found by Franz et al.
Our results also show that the presence of carboxylic
groups, which thermally decompose into CO 2 , inhibits
PNP adsorption. This observation shows that H-bonding of
PNP molecules to these groups does not play any predominant role in the aqueous adsorption of this adsorbate.
This is likely due to the strong competition of water versus
PNP molecules to form such a bond [12].
5. Conclusion
The determination of p-nitrophenol adsorption isotherms
from aqueous solutions on a carbon variously oxidized led
us to definitely rule out the donor–acceptor argument for
carbon AC because of the evident lack of carbonyl
adsorption sites. No relationship between the amounts of
CO-evolving groups and the PNP adsorption capacity
being observed for oxidized carbons, the donor–acceptor
argument may be rejected in this case too. Consequently,
our findings are in favor of an adsorption mechanism
involving an interaction between some basic groups located in graphene layers and the aromatic ring of PNP, in
agreement with the early proposal of Coughlin et al. [10]:
the negative role of superficial CO 2 -yielding groups on
PNP adsorption is attributed to the withdrawal of electrons
from these adsorption sites by these groups.
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