Role of Nitrogen Doping on the Performance of Carbon Nanotube Catalysts: A Catalytic Wet Peroxide Oxidation Application

Article abstract of DOI:10.1002/cctc.201600123

Four magnetic carbon nanotube (CNT) samples (undoped, completely N-doped, and two selectively N-doped) were synthesized by chemical vapor deposition. The materials were tested in the catalytic wet peroxide oxidation (CWPO) of highly concentrated 4-nitrophenol solutions (4-NP, 5 g L^?1). Relatively mild operating conditions were considered (atmospheric pressure, T=50 °C, pH 3), using a catalyst load of 2.5 g L^?1 and the stoichiometric amount of H₂O₂ needed for the complete mineralization of 4-NP. N-doping was identified to influence considerably the CWPO performance of the materials. In particular, undoped CNTs, with a moderate hydrophobicity, favor the controllable and efficient decomposition of H₂O₂ into highly reactive hydroxyl radicals (HO^.), thus showing high catalytic activity for 4-NP degradation. On the other hand, the completely N-doped catalyst, fully hydrophilic, favors a quick decomposition of H₂O₂ into nonreactive O₂ and H₂O species. The selectively N-doped amphiphilic catalysts, that is, hybrid structures containing undoped sections followed by N-doped ones, provided intermediate results, namely, a higher N content favored H₂O₂ decomposition towards nonreactive H₂O and O₂ species, whereas a lower N content resulted in the formation of HO^., increasing 4-NP mineralization. Catalyst stability and reusability were also investigated by consecutive CWPO runs.

Full text of DOI:10.1002/cctc.201600123

DOI: 10.1002/cctc.201600123
Full Papers
Role of Nitrogen Doping on the Performance of Carbon
Nanotube Catalysts: A Catalytic Wet Peroxide Oxidation
Application
Maria Martin-Martinez,[a] Rui S. Ribeiro,[a] Bruno F. Machado,[b] Philippe Serp,[b]
Sergio Morales-Torres,[c] Adriµn M. T. Silva,[c] JosØ L. Figueiredo,[c] Joaquim L. Faria,[c] and
Helder T. Gomes*[a]
C
Four magnetic carbon nanotube (CNT) samples (undoped,
completely N-doped, and two selectively N-doped) were syn-
thesized by chemical vapor deposition. The materials were
tested in the catalytic wet peroxide oxidation (CWPO) of highly
concentrated 4-nitrophenol solutions (4-NP, 5 gLÀ1). Relatively
mild operating conditions were considered (atmospheric pres-
sure, T=508C, pH 3), using a catalyst load of 2.5 gLÀ1 and the
stoichiometric amount of H2O2 needed for the complete miner-
alization of 4-NP. N-doping was identified to influence consid-
erably the CWPO performance of the materials. In particular,
undoped CNTs, with a moderate hydrophobicity, favor the con-
trollable and efficient decomposition of H2O2 into highly reac-
tive hydroxyl radicals (HO ), thus showing high catalytic activity
for 4-NP degradation. On the other hand, the completely N-
doped catalyst, fully hydrophilic, favors a quick decomposition
of H2O2 into nonreactive O2 and H2O species. The selectively N-
doped amphiphilic catalysts, that is, hybrid structures contain-
ing undoped sections followed by N-doped ones, provided in-
termediate results, namely, a higher N content favored H2O2
decomposition towards nonreactive H2O and O2 species,
C
whereas a lower N content resulted in the formation of HO , in-
creasing 4-NP mineralization. Catalyst stability and reusability
were also investigated by consecutive CWPO runs.
Introduction
During the past decade, the development and use of different
nanostructured porous carbon materials in catalysis have in-
creased in importance, with these carbon materials used as
supports or as catalysts on their own.[1] Carbon nanotubes
(CNTs) are particularly suitable for liquid-phase reactions,
owing to their high external surface area.[2] In addition, they
exhibit high mechanical resistance, superior electronic proper-
ties, relatively high thermal stability under oxidizing conditions,
and tunable surface chemistry, making them highly versatile
for several applications.[2] Moreover, the presence of nitrogen
in the structure of CNTs increases their polarity and improves
their reactivity towards different molecules.[3] In this sense, am-
phiphilic materials produced by selective N-doping are gaining
interest for several applications.[4,5] Pioneering works have re-
ported the synthesis of magnetic amphiphilic N-doped CNTs
from a Fe/g-Al2O3 catalyst and their successful application in
different processes, such as adsorption of polyaromatic hydro-
carbons,[3] chemical oxidation of alcohols,[3,6] and transesterifi-
cation of soybean oil.[3] Moreover, it is known that the presence
of N groups on the CNT surface highly enhances their activity
for hydrogen peroxide (H2O2) decomposition.[7,8]
[a] Dr. M. Martin-Martinez, R. S. Ribeiro, Dr. H. T. Gomes
Laboratory of Separation and Reaction Engineering
- Laboratory of Catalysis and Materials (LSRE-LCM)
Departamento de Tecnologia Química e Biológica
Escola Superior de Tecnologia e Gest¼o
Instituto PolitØcnico de BraganÅa
Campus de Santa Apolónia, 5300-253 BraganÅa (Portugal)
E-mail: htgomes@ipb.pt
Advanced oxidation processes, primarily based on the action
[b] Dr. B. F. Machado, Dr. P. Serp
Laboratoire de Chimie de Coordination UPR CNRS 8241
composante ENSIACET
C
of hydroxyl radicals (HO ) to oxidize organic pollutants, are re-
garded as promising solutions for the treatment of aqueous ef-
fluents from pharmaceutical, petrochemical, dye, or paper in-
dustries, among others, containing recalcitrant and nonbiode-
gradable compounds[9,10] difficult to remove by conventional
biological processes, mainly if present at high concentrations
(1–10 gLÀ1).[11] Among them, catalytic wet peroxide oxidation
UniversitØ de Toulouse
UPS-INP-LCC 4 allØ Emile Monso BP 44362
31030 Toulouse Cedex 4 (France)
[c] Dr. S. Morales-Torres, Dr. A. M. T. Silva, Dr. J. L. Figueiredo, Dr. J. L. Faria
Laboratory of Separation and Reaction Engineering
- Laboratory of Catalysis and Materials (LSRE-LCM)
Departamento de Engenharia Química, Faculdade de Engenharia
Universidade do Porto
C
(CWPO) uses H2O2 as a source of HO and a suitable catalyst for
the degradation of the organic species under relatively mild
conditions (0.1–0.2 MPa and 20–1308C).[12,13]
H2O2 easily decomposes into oxygen and water [Eq. (1)], re-
leasing heat. Nevertheless, for the CWPO process, it is impor-
Rua Dr. Roberto Frias s/n, 4200-465 Porto (Portugal)
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
cctc.201600123.
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C
tant to decompose selectively H2O2 into highly reactive HO .
Results and Discussion
The reaction mechanisms describing H2O2 decomposition are
well accepted and reported in the literature, including the for-
Carbon nanotubes characterization
C
mation of HO with the participation of a suitable catalyst, as
shown in Equations (2–5) (in which [AS] refers to an adequate
active site at the surface of the catalyst, for example, a surface-
reducing active site or a transition-metal particle, among
others).[14–20] However, radical recombination may also occur
[Eqs. (6–8)], affecting the efficiency of the CWPO process.[15,21,22]
The transmission electron microscopy (TEM) results reported in
previous studies,[3,6,37] included in Table 1, reveal that the intro-
duction of nitrogen in the carbon structure induces an increase
of the tube diameter and a decrease in the interstitial space
between individual CNTs. Undoped E30 consists of very regular
2 H2O2 ! 2 H2O þ O2
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
ð6Þ
ð7Þ
ð8Þ
Table 1. Textural properties (average tube diameter and interstitial spac-
ing) of the CNTs, extracted from TEM micrographs and N2 adsorption–
desorption isotherms.
Cþ
C
C
À
H2O2 þ ½ASŠ ! ½AS Š þ OH þ HO
Catalyst
Tube
diameter
[nm][a]
Interstitial
space
[nm][a]
SBET
Vpore
Vmeso
Cþ
þ
H2O2 þ ½AS Š ! ½ASŠ þ H þ HOO
(Æ5)
(Æ0.01)
(Æ0.01)
[m2 gÀ1
]
[cm3 gÀ1
]
[cm3 gÀ1
]
E30
8
9
n/a
14
30.0
21.9
n/a
275
214
196
212
1.92
1.49
1.31
1.11
1.72
1.26
1.22
0.74
C
C
H2O2 þ HO ! HOO þ H2O
E10A20
E3A27
A30
C
C
H2O2 þ HOO ! HO þ H2O þ O2
14.8
[a] Data from [6]. n/a: not available.
C
C
HO þ HO ! H2O2
C
C
HOO þ HOO ! H2O2 þ O2
multiwalled CNTs with an average external diameter of approx-
imately 8 nm and Fe particles encapsulated in the structure
(formed during the synthesis by catalytic chemical vapor depo-
sition, CVD, through reduction of the iron oxide using H2 pro-
duced during the decomposition of the carbon sources at high
temperature, and that remained in the CNT structure after the
C
C
HO þ HOO ! H2O þ O2
The classical heterogeneous catalysts employed in CWPO
consist of an active phase, mainly Fe or Cu, immobilized on
the surface of a porous support (activated carbon, silica, pil-
lared clays, or zeolites, among others). However, these catalysts
usually suffer from severe deactivation resulting from leaching
of the metallic phase after a few hours of operation.[23–27] Re-
cently, metal-free carbon-based materials[28] were found to be
promising catalysts for CWPO, showing high activity and stabil-
ity and the ability to ensure an efficient H2O2 usage, that is, the
purification step with H2SO4—see the Experimental Section).
À1 [3]
The yield of the CVD process was 4.8 gC gcat
.
On the other
hand, the completely N-doped A30, generated in a considera-
ble lower yield (1.1 gC gcatÀ1),[3] is characterized by a “bamboo-
like” structure with larger diameters (ꢀ14 nm). The selectively
N-doped materials are hybrid structures containing both un-
doped and N-doped sections with intermediate diameters
(Supporting Information, Figure S1).
C
selective formation of HO and further effective reaction with
Analysis of the N2 adsorption–desorption isotherms obtained
with the different CNTs (Figure S2; Table 1) reveals the highest
value of specific surface area (SBET) for the undoped material
(E30), consistent with the lower tube diameter and hollow
nature of its structure. It should be noted the area available
inside E30 (assuming some opened tubes), contrary to the
doped CNTs where the bamboo-type structure prevents a sig-
nificant adsorption inside the tubes. With regard to the N-
doped nanotubes, despite their larger tube diameter, the SBET
of the completely N-doped material (A30) is similar to that ob-
tained for the selectively N-doped CNTs (E10A20 and E3A27),
which might be explained by the smaller interstitial space
shown by A30. The mesopore and total pore volumes also de-
crease as the nitrogen content in the CNT increases, which
could be explained by a more efficient packing of the CNTs
caused by an excess hydrogen bonding between functional
groups, leading to thicker bundles.[38,39] In all cases, the nitro-
gen uptake dramatically increased at high relative pressures
leading to a soft hysteresis loop, which may be associated to
pollutant molecules.[14,29–31]
In previous studies,[32,33] the behavior of different commercial
CNTs in the CWPO process was analyzed. These materials were
found to be highly effective for the removal of different phe-
nolic compounds, even without added metal species. As, to
the best of our knowledge, amphiphilic CNTs have never been
reported in CWPO, the present work aims to assess the role of
N doping in CNTs if employed in CWPO. In addition, the pres-
ence of reminiscent Fe in their structures, resulting from the
synthesis procedure, will make them very attractive for CWPO
applications coupled with in situ magnetic separation systems
for catalyst recovery. For this purpose, three different N-doped
CNTs (with varying doping levels) and one undoped CNT
sample were synthesized and characterized. A highly concen-
trated (5 gLÀ1) 4-nitrophenol (4-NP) solution was considered as
a model system to simulate high-loaded wastewaters, chal-
lenging conditions normally not considered in CWPO
studies.[34–36]
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capillary condensation in the mesoporous structure of the CNT
bundles.[40,41] By comparing the obtained isotherms, a signifi-
cant difference in the shape of the hysteresis loop and a shift
towards lower relative pressures are recorded for the com-
pletely N-doped A30 material, indicating a less uniform pore
size distribution in this sample as compared to the other CNTs.
The N-doped CNT structure is known to show several defects
and the presence of nitrogen in the graphitic network induces
a curvature of the tube graphitic layers.[42] XRD, thermogravi-
metric, and Raman analysis performed previously[3,6,37] support
the well-organized carbon structure of E30, as opposed to the
more defective and reactive carbon present in A30, because
a significant increase of defects occurs if increasing the N con-
tent. Briefly, Raman spectra showed the presence of one first-
order peak at approximately 1560 cmÀ1 (G-band) and three
second-order features at approximately 1330 cmÀ1 (D-band),
2700 cmÀ1 (G-band), and 2940 cmÀ1 (combined D/G-bands).
The higher the ratio between D and G band intensities (ID/IG),
the more defective is the structure under study. This parameter
decreases from 0.8 in A30 to 0.6 in E30. Concomitantly, XRD
analysis showed a slight decrease of d(002) spacing with the ni-
trogen content, from 3.43 Š in E30 to 3.36 Š in A30.
face groups increases with the N content, affecting the type of
surface chemical functionalities. As the undoped and the selec-
tively N-doped CNTs (E30 and E10A20) present intermediate
amounts of carbonyl/carboxylic acid groups (C=O) and phenol/
ether groups (CÀO), the materials with high N content (E3A27
and A30) present carbonyl/carboxylic acid groups in their sur-
face. This is in accordance with the potentiometric titration
curves (Figure S5), because the completely N-doped sample
exhibits a broadening of the buffering area, showing the high-
est amount of surface acidic groups (Table 3). On the other
hand, in spite of its small concentration, the total Fe on the
CNTs also seems to increase with the presence of N (Table 2),
in accordance with the remaining bulk Fe content determined
in a previous study, which decreases from approximately 5%
in the N-doped A30 to approximately 2% in the undoped
E30.[6]
Table 3. Concentration of surface acidic groups and contact angles deter-
mined for buckypapers prepared with different CNTs.
Catalyst
Acidic groups
Contact angle
[8]
[mmolgÀ1
]
X-ray photoelectron spectroscopy (XPS) results (Table 2) con-
firm the low amount of nitrogen-containing surface groups on
the N-doped materials, because these are mainly building
blocks for the internal NÀC bonds. Nevertheless, as reported
before,[3] the deconvolution of the N1s region of the XPS spec-
tra (Figure S3) shows the presence of pyridinic nitrogen (N6),
pyrrolic nitrogen (N5), quaternary nitrogen (NQ), and nitrogen
oxides (N-oxides) in the N-doped materials, strongly affecting
the CNT reactivity by their electron-donating properties. Pyri-
dinic nitrogen, present on the edge of graphite planes bonded
to two carbon atoms, has a well-known basic character, slightly
more basic than that shown by the pyrrolic nitrogen. Both
structures contain a basic nitrogen atom with a lone pair of
electrons to donate. On the other hand, the positive charge of
the quaternary nitrogen, in which nitrogen is within a graphite
plane and bonded to three carbon atoms, favors the electron
transfer reactions in the CNT surface. The peak related to nitro-
gen oxides is also present in the O1s region of the N-doped
materials (Table 2; Figure S4). The O1s deconvolution reveals
the presence of oxygen-containing surface groups in the CNTs
as well, which could be formed during the CNT purification
step with H2SO4. Although present in small amounts, the elec-
tron-withdrawing properties of these surface acidic oxygen-
containing groups may also affect the reactivity of the CNTs.
As observed in Table 2, the amount of oxygen-containing sur-
E30
0.146
60Æ2
47Æ2
16Æ2
11Æ1
E10A20
E3A27
A30
0.162
n.d.
0.337
n.d.: not determined.
As explained above, both the undoped E30 and the com-
pletely N-doped A30 materials contain a single structure, con-
sisting of very regular multiwalled CNTs in E30 and “bamboo-
like” CNTs in A30. The thermogravimetric (TGA) curve (Figure 1)
of the undoped E30 sample shows a maximum weight loss at
562.58C, which corresponds to the typical combustion temper-
ature of multiwalled CNTs.[43] On the other hand, the complete-
ly N-doped A30 sample presented its maximum loss at a lower
Table 2. Surface atomic composition (by XPS) of the CNTs.
Catalyst
C
[%]
O
[%]
N
[%]
Fe
[%]
N1s [%]
NQ
O1s [%]
CÀO
N6
N5
N-ox.
C=O
N-ox.
E30
99.0
98.9
94.9
91.8
0.85
0.75
1.6
–
0.05
0.06
0.2
–
37
33
31
–
17
6
–
26
27
28
–
20
34
34
46
46
58
64
54
51
29
29
–
3
13
7
E10A20
E3A27
A30
0.29
3.4
5.3
2.7
0.3
7
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hydrophobicity (contact angle of 608), and the N-doped cata-
lysts are highly hydrophilic, in particular the completely N-
doped A30 (contact angle of 118). The hydrophilic properties
and the N doping were found to strongly influence the activity
and effectiveness of the CNTs if used as catalysts in the CWPO
of 4-NP, as shown in the next section.
Catalytic activity
Screening experiments were performed to evaluate the per-
formance of the synthesized CNTs during the CWPO of 4-NP.
The conversions (X) of 4-NP, total organic carbon (TOC), and
H2O2 achieved with each catalyst, as well as the concentration
of Fe leached into the solution, after 24 h of reaction, are
given in Figure 2. The removal of 4-NP in the absence of H2O2
is also shown in order to differentiate between pure adsorp-
tion and CWPO.
Figure 2. Removal of 4-NP from aqueous solution by adsorption and by
CWPO; TOC removal, H2O2 decomposition, Fe leaching (right y axis) and
hH obtained in the CWPO experiments; results after 24 h.
2 O2
As observed, the contribution of adsorption is negligible in
all cases and decreases with increasing N content, presumably
as a result of a weaker interaction with the N-doped surface.
This enables the direct comparison of the different CNTs as cat-
alysts in the CWPO experiments. All catalysts remarkably de-
compose H2O2, but with rather different catalytic activity. As
expected, H2O2 consumption increases in the CNTs with higher
amount of nitrogen,[7,8] because the presence of electron do-
nating groups on these materials favors the decomposition of
H2O2. However, contrary to expectations, 4-NP removal de-
creases with increasing H2O2 consumption; 100% of the pollu-
tant was removed after 24 h with the undoped material (E30),
whereas only a 9% removal was observed with the completely
N-doped sample (A30). This singular behavior may be ex-
plained by the different physicochemical properties of the syn-
thesized CNTs. In a previous study, it has been reported the
ability of several carbon materials with different structural and
Figure 1. Weight loss by TGA of the CNTs (black) and their derivatives (gray).
temperature (539.28C), coherent with a more defective struc-
ture and the presence of oxygen- and nitrogen-containing sur-
face groups. As pointed out by the TEM results, the TGA
curves of the selectively N-doped CNTs confirm the presence
of both structures in these materials, because two negative
peaks situated at approximately 500 and 6008C may be clearly
identified in the weight-loss derivative (DTG curve), corre-
sponding to N-doped and undoped sections, respectively.
Finally, the water contact-angle measurements obtained for
the buckypapers composed of different CNTs (Table 3) seem to
be correlated with the amount of oxygen- and nitrogen-con-
taining functional groups incorporated during the CNT synthe-
sis. In this context, the undoped material E30 has a moderate
C
chemical properties to selectively decompose H2O2 into HO
radicals, revealing that the presence of basic groups in the
[14]
C
carbon materials increases the yield of HO formation. On
the other hand, the hydrophilic N-doped catalysts have
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a strong affinity towards polar molecules such as H2O2. The in-
sertion of N atoms in the CNT structure lowers the electron
work function value of the carbonaceous surface, decreasing
In addition, the higher Fe leaching observed with the N-
doped catalysts is in agreement with the higher content of Fe
on the surface of the N-doped catalysts, and to the favorable
conditions found at their surface for fast and inefficient H2O2
decomposition, resulting in the oxidation of the Fe particles
and consequent leaching into the solution. Despite the higher
amounts of leached Fe in the reactions with the N-doped cata-
lysts, this factor does not lead to an increased 4-NP removal,
discarding the homogeneous contribution promoted by Fe
species in the solution. This further emphasizes the influence
of N-doping on the efficiency of the CWPO process, as
explained previously, by the different surface interactions
between CNTs, H2O2, and 4-NP.
the band gap, thus producing a higher electron mobility and
[44]
C
into HO radicals,
enhancing the decomposition of H2O2
which ultimately result in the formation of nonreactive O2 and
H2O species during the CWPO experiments. As explained in
the Introduction, the formation of nonreactive species occurs
in a two-step mechanism: first, H2O2 decomposes into reactive
radicals, and second these radicals react between themselves,
recombining into nonreactive species. Therefore, with the ex-
ception of E10A20, very low 4-NP removal is reached by
CWPO with the N-doped catalysts. On the other hand, the un-
doped E30 sample, which is moderately hydrophobic, has
a modest affinity towards H2O2, resulting in a controllable and
As explained above, the presence of electron-donating
groups substantially favors the decomposition of H2O2. By
studying the time evolution of 4-NP and H2O2 concentrations
obtained with each catalyst during the CWPO runs (Figure 3), it
can be observed that E30 and E10A20 decompose H2O2 at
a moderate rate, but the highly and completely N-doped ma-
terials (E3A27 and A30, respectively) promote a sharp decom-
position of this oxidizing agent (also visually observed by the
intense formation of gas bubbles and white smog in the reac-
tor, pointing to the formation of O2 and H2O by recombination
of the radicals).
C
more efficient H2O2 decomposition into HO , the first step in
the CWPO process. These radicals further react with 4-NP ad-
sorbed in the close vicinity of the active sites, resulting in a
remarkable removal of 4-NP.
The different affinity of the CNTs for the reactant molecules
(H2O2 and 4-NP) also affects the mineralization level obtained,
the TOC conversion decreasing if using the CNTs with higher
amount of nitrogen. Owing to a more efficient H2O2 decompo-
C
sition into HO , the undoped E30 catalyst converts approxi-
mately 60% of the TOC content after 24 h. On the other hand,
the inefficient decomposition of H2O2 into nonreactive species
with the completely N-doped A30 hinders the reaction (TOC
conversion of only approximately 18%). This higher efficiency
in the H2O2 decomposition with E30 is highlighted if analyzing
the TOC removal per unit of H2O2 decomposed (hH O ). In this
To understand better the different behavior of the catalysts,
the decomposition of H2O2 was studied without the presence
of the pollutant. In Figure 4, the H2O2 concentration is shown
as a function of time in the experiments conducted with and
2
2
case, an efficiency of approximately 100% is observed, sug-
gesting that all the decomposed H2O2 is effectively used in the
mineralization of 4-NP. In contrast, only approximately 20% of
the decomposed H2O2 was used in the mineralization of the
pollutant with the highly N-doped E3A27 material and the
completely N-doped A30 sample.
Both the increase of structural defects in carbon materi-
als[45,46] and presence of Fe at their surface[33,47] have been
shown to increase the efficiency of CWPO processes, mainly re-
lated to increased H2O2 consumption. Nevertheless, if the se-
quence of the 4-NP removal obtained by CWPO in the pres-
ence of each catalyst (E30>E10A20>E3A27>A30) is directly
compared to that of the amount of surface Fe and defects in
the structure of the catalysts (A30>E3A27>E10A20>E30), an
opposite tendency is observed. Therefore, surface Fe and struc-
tural defects cannot explain the results shown in Figure 2, sug-
gesting that N-doping is a determining factor influencing the
CNTs activity. The different textural properties observed for the
CNTs could also have some influence on the catalyst activity. In
particular, E30 is the most active material, with the highest SBET
,
mesopore (Vmeso), and total pore (Vpore) volumes, which could
favor the accessibility of H2O2 to the active sites as well as 4-
NP adsorption. However, these values are, in general, similar
for all the samples, and the differences are only significant for
E30. Therefore, it is possible to conclude that the influence of
the N doping surpasses that of the textural properties of the
CNTs.
Figure 3. Concentration evolution with time (h) for 4-NP (top, solid symbols)
and H2O2 (bottom, open symbols) during the CWPO experiments (normal-
ized by the corresponding initial concentrations).
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catalysts: E30, E10A20, and A30. As observed in Figure 2, com-
plete TOC conversion was never achieved with any of the cata-
lysts, suggesting the existence of organic intermediates and/or
byproducts at the end of the reaction. Accordingly, the time-
evolution of the different compounds that were identified
during the CWPO of 4-NP with E30, E10A20, and A30, is shown
in Figure 5–7, respectively. As expected, the evolution of the
identified compounds is quite different with each catalyst.
Figure 5. Concentration (mgLÀ1) evolution with time (h) for the different
compounds resulting from the CWPO of 4-NP using E30.
Figure 4. Concentration evolution with time (h) for H2O2 in experiments with
(solid symbols) and without (open symbols) pollutant, using E30, E10A20,
and A30.
The first difference is related with the total concentration of
intermediates and/or by-products obtained. The fast H2O2 con-
sumption compared to 4-NP adsorption/decomposition, if
using A30, limits the CWPO process. Therefore, small amounts
of these compounds were detected with A30. Specifically, in-
termediate compounds were mainly formed during the first
2 h of reaction, during which H2O2 was still present in the solu-
tion. As described in a previous study,[45] the electrophilic addi-
without 4-NP with three representative catalysts: the undoped
E30 catalyst, the selectively N-doped E10A20, and the com-
pletely N-doped A30. It is observed that the decomposition of
H2O2 in the absence of 4-NP is faster in all cases, which may be
explained by the absence of adsorption competition between
H2O2 and 4-NP molecules on the surface of the catalyst. A
noteworthy large difference is found in the catalyst E10A20
with and without 4-NP, revealing that the competition be-
tween H2O2 and 4-NP for the catalyst surface hindered the
strong affinity of H2O2 for the CNTs, resulting in a gradual
decomposition of this reactant during the CWPO of 4-NP.
C
tion of HO to the aromatic ring of 4-NP favors the formation
of 4-nitrocatechol, which was by far the main aromatic com-
pound formed with all the catalysts. However, secondary reac-
tions also occur: 1) the denitration of the 4-NP molecule lead-
ing to the formation of hydroquinone, which may be further
oxidized to 1,4-benzoquinone;[45,48] 2) the electrophilic HO ad-
C
dition at the ortho position leading to the formation of cate-
chol. This is consistent with the aromatic intermediates ob-
served with the undoped E30 catalyst (Figure 5). In this case,
the concentrations observed follow the sequence: 4-nitrocate-
chol> > >hydroquinone>1,4-benzoquinoneꢀcatechol. On
Reaction mechanism
To correlate the CWPO reaction mechanism with the different
physicochemical properties of the synthesized CNTs, a more
detailed study was performed with the three representative
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Figure 6. Concentration (mgLÀ1) evolution with time (h) for the different
compounds resulting from the CWPO of 4-NP using E10A20.
Figure 7. Concentration (mgLÀ1) evolution with time (h) for the different
compounds resulting from the CWPO of 4-NP using A30.
the other hand, with the completely N-doped A30 catalyst
(Figure 7), the reaction medium was remarkably oxidizing as
a result of the strong affinity between the catalysts and H2O2,
and 1,4-benzoquinone was rapidly obtained. After about 2 h of
reaction, H2O2 was completely consumed and 1,4-benzoqui-
none was immediately reduced back to hydroquinone,
showing an atypical behavior in CWPO reactions.
All the findings reported so far contribute to understand fur-
ther the 4-NP CWPO mechanism if applying catalysts with dif-
ferent properties, as represented in Figure 8. The hydrophilic
N-doped surfaces strongly attract the H2O2 molecules, which
are quickly decomposed into radicals on the active sites with
N-containing functionalities, owing to their electron-donating
properties. The organic pollutant molecules cannot compete
with H2O2 for the catalyst surface. Consequently, in a second
As reported in previous studies,[32,45] HO radicals further
C
attack these aromatic intermediates leading to the ring open-
ing, and yielding low-molecular-weight carboxylic acids. With
the undoped E30 catalyst (Figure 5), the aromatic intermedi-
ates were almost completely converted after 24 h of reaction
and high concentrations of carboxylic acids were obtained,
causing a dramatic decrease in the solution pH at the end of
the reaction (dropping from 3 to 1.8). With the N-doped A30
sample (Figure 7), as explained above, the reaction environ-
ment in the first hours of reaction helped to highly oxidize 4-
NP, forming considerable amounts of acetic acid, which de-
creased again when the reaction medium switch to a nonoxid-
izing environment. The E10A20 catalyst behaved like A30
during the first minutes of reaction (Figure 6), with large
amounts of acetic acid formed. However, after that, owing to
the competition between H2O2 and 4-NP adsorption on the
catalyst surface, the remaining H2O2 decomposed gradually
C
and efficiently into HO , switching to the undoped material-like
Figure 8. Proposed mechanism for the CWPO of 4-NP in N-doped hydro-
behavior.
philic and undoped catalyst surfaces.
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stage, the radicals formed in large amounts do not react with
the pollutant, but between themselves, recombining into non-
reactive O2 and H2O. The more hydrophobic undoped surfaces
attract both H2O2 and pollutant molecules. H2O2 is decom-
posed into radicals, which immediately attack the pollutant
molecules adsorbed in the close vicinity, leading to their effi-
cient mineralization. The hydrophilic N-doped sections in the
hybrid structure strongly attract the H2O2 molecules. However,
in this case, the pollutant molecule can approach the undoped
sections and reach the catalyst surface, reacting with the
formed radicals. This mechanistic insight puts into evidence
amphiphilic materials as promising catalysts for CWPO.
The higher or lower hydrophilicity of the catalyst is directly
affected by the N content. Then, if representing the 4-NP con-
version obtained by CWPO, TOC removal, H2O2 conversion or
hH O efficiency versus the catalyst contact angle (Figure 9), the
Figure 10. Concentration (normalized by the initial value) evolution with
time (h) for 4-NP during three consecutive CWPO experiments using E30.
2
2
higher H2O2 decomposition obtained on the highly hydrophilic
surfaces of the completely N-doped CNTs is evidenced, but the
4-NP and TOC conversions are lower because of the poor effi-
ciency of H2O2 decomposition, which forms nonreactive
species.
approximately 30 h were needed to achieve total pollutant re-
moval (results not shown). With the highly oxidizing reaction
environment, the surface chemistry of E30 may change during
the CWPO, turning it into a more hydrophilic structure, hinder-
ing its use in the consecutive reactions. Additional studies are
needed to understand the catalyst performance and to identify
the reasons for this behavior. Nevertheless, TEM images of the
catalysts after CWPO are shown in Figure S6, and their compar-
ison with the catalysts before CWPO (Figure S1) reveals no par-
ticular changes in the morphology of the materials during
reaction.
Considering the potential of amphiphilic CNTs in CWPO, the
mechanisms involved in the deactivation of these materials are
currently under study for further process improvement, as well
as its possible optimization by intensification of the operating
conditions and the production of more resistant materials
against possible chemical alterations during the CWPO
process.
Figure 9. Conversion of 4-NP, TOC removal, H2O2 decomposition, and hH
2O2
(in %) obtained for the CWPO experiments after 24 h as a function of the
Conclusions
catalyst contact angle (8).
Nitrogen doping of magnetic carbon nanotubes (CNTs) play
a very important role in the catalytic wet peroxide oxidation
(CWPO) performance of 4-nitrophenol (4-NP). Specifically, it af-
fects the mechanism of H2O2 decomposition. The controllable
H2O2 decomposition over hydrophobic undoped surfaces
Catalyst stability and reusability
An essential requirement for the industrial application of cata-
lysts is their stability. To assess the stability and reusability
properties, three consecutive CWPO runs were performed with
the undoped E30 sample, which is the catalyst showing the
best performance. As mentioned in the experimental section,
the catalyst was recovered, washed with distilled water, and
dried after each run, prior to its reuse in a new CWPO experi-
ment. In Figure 10, the time evolution of the 4-NP concentra-
tion is shown during 24 h of reaction for the three consecutive
CWPO cycles. Approximately 100% of the initial pollutant was
removed in the first and second cycles after 24 h of reaction.
In the third run, however, the 4-NP conversion decreased, and
C
favors the formation of reactive HO . On the other hand, the
hydrophilic N-doped surfaces promote the total H2O2 decom-
position into nonreactive species (H2O and O2). The competi-
tion between H2O2 and 4-NP for the catalyst surface in amphi-
philic CNTs makes them promising catalysts for CWPO, because
their combined hydrophilic N-doped sections (with high affini-
ty for polar molecules) with undoped sections (ensuring a con-
trollable and effective H2O2 decomposition) allow them to be
explored in wastewater treatment.
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surface, according to NIST database. Potentiometric titration (Met-
rohm 670 automatic titrator) was used to estimate the concentra-
tion of acidic active sites at the catalysts surface. CNTs (50 mg) dis-
persed in a 25 mL volume of aqueous HCl (0.0070 molLÀ1) and
KNO3 (0.04 molLÀ1) solutions were placed directly into the electro-
Experimental Section
Chemicals
The main reactants involved in the process under study, 4-NP
(98 wt%) and H2O2 (30%, w/v), were purchased from Acros Organ-
ics and Fluka, respectively. Working standard solutions of formic
acid (98 wt.%; Panreac), acetic acid (glacial acetic acid, 99.8 wt%;
Fisher Chemical), oxalic acid, malonic acid, maleic acid, malic acid,
hydroquinone, phenol (all 99 wt.%; Sigma–Aldrich), 1,4-benzoqui-
none (99.5 wt.%; Fluka), catechol (98 wt%; Fluka) and 4-nitrocate-
chol (98 wt.%; Fluka) were prepared and used for calibration in
HPLC. Methanol (HPLC grade, 99.99 wt.%; Fisher Chemical), glacial
acetic acid (HPLC grade, 99.99 wt.%; Fisher Chemical), acetonitrile
(HPLC grade, 99.99 wt%; Fisher Chemical) and sulfuric acid (H2SO4,
96–98 wt%; Riedel-de-Han) were used to prepare the mobile
phases required for HPLC. The reactants used for the determina-
tion of Fe were l-ascorbic acid (99 wt%; Fisher Chemical), o-phe-
nanthroline (99 wt.%; Panreac), glacial acetic acid (HPLC grade,
99.99 wt.%; Fisher Chemical), ammonium acetate (98 wt.%; Prona-
lab) and iron(II) chloride tetrahydrate (99 wt%; Sigma–Aldrich).
Other reactants used were sodium hydroxide (NaOH, 98 wt%; Pan-
reac), hydrochloric acid (HCl, 37 wt.%; Sigma–Aldrich), potassium
nitrate (KNO3, 99 wt.%; Sigma–Aldrich), titanium(IV) oxysulfate
(15 wt.% in dilute sulfuric acid, 99.99%; Sigma–Aldrich), and
sodium sulfite (Na2SO3, 98 wt.%; Sigma–Aldrich). All chemicals
were used as received without further purification. Distilled water
was used throughout the work except for mobile phase prepara-
tion, for which ultrapure water was employed.
chemical cell and titrated with
a CO2-free NaOH solution
(0.0524 molLÀ1). The experimental data were treated according to
the literature.[51,52] TGA analyses were performed by using an Elmer
Diamond TG/DTA thermo balance, heating the sample powders at
108CminÀ1 up to 10008C in air atmosphere (50 cm3 minÀ1). The hy-
drophobicity/hydrophilicity of the CNTs (in the form of buckypa-
pers) was determined by water contact-angle measurements using
an Attension optical tensiometer (model Theta) that allowed image
acquisition and data analysis. The measurements with water were
performed on dry buckypapers at RT using the sessile-drop
method.[38] Each contact angle was measured at least in five differ-
ent locations on the buckypapers to determine the average value.
TEM images were obtained by using a TEM-FEI microscope (Tecnai-
G2-20-FEI 2006) operating at 200 kV. The samples magnetism was
verified qualitatively by using magnets (Figure S7).
CWPO, adsorption, and H2O2 decomposition experiments
The CWPO runs were conducted in a batch reaction system con-
sisting of a 250 mL magnetically stirred (600 rpm) glass reactor,
equipped with a reflux condenser and a sample collection port, im-
mersed in an oil bath with temperature control. In a typical experi-
ment, the reactor was loaded with a 50 mL volume of the 4-NP
aqueous solution (5 gLÀ1) and heated up to 508C. After tempera-
ture stabilization, pH was adjusted to 3 by using H2SO4 and NaOH
solutions, and a calculated volume of H2O2 was incorporated to
the system to reach the stoichiometric concentration needed to
mineralize completely 4-NP. The reaction started with the addition
of the catalyst (0.125 g) corresponding to a catalyst load of
2.5 gLÀ1. During the experiment, samples of the resulting effluent
were collected at different reaction times (typically at 0, 5, 15, 30,
60, 120, 240, 480, and 1440 min) and prepared for analysis, as de-
scribed below. After 24 h of reaction, the catalyst was separated by
filtration (20 mm, Prat Dumas), washed with distilled water and
dried at 608C. A blank experiment, that is, without catalyst, was
performed to assess possible noncatalytic oxidation reactions pro-
moted by H2O2. On the other hand, the adsorption capacity of the
different synthesized CNTs was evaluated by means of pure ad-
sorption experiments, in which the operating conditions used in
the CWPO runs were reproduced, but a volume of distilled water
was incorporated to the system in substitution of H2O2. Finally, to
assess the activity of the catalysts to decompose H2O2 avoiding
pollutant competition, a set of experiments was conducted by in-
troducing distilled water (50 mL) in the reactor instead of the 4-NP
aqueous solution. The experiments were performed in triplicate,
the standard deviation was less than 5% in all cases.
Synthesis of carbon nanotubes
The CNTs were synthesized by a catalytic CVD process in a fluid-
ized-bed reactor, as described elsewhere,[3] using ethylene as
a carbon source and acetonitrile/N2 as a carbon/nitrogen source, at
6508C. The synthesis was conducted in the presence of a Fe/g-
Al2O3 (20 wt.% Fe) catalyst prepared by impregnation and reduced
in situ at 6508C for 30 min. Four samples were produced by feed-
ing to the fluidized bed reactor: 1) ethylene alone for 30 min
(sample E30); 2) ethylene for 10 min, followed by acetonitrile/N2
for 20 min (sample E10A20); 3) ethylene for 3 min, followed by
acetonitrile/N2 for 27 min (sample E3A27); and 4) acetonitrile/N2
alone for 30 min (sample A30). Finally, the synthesized CNTs were
purified under reflux at 1408C, with an aqueous solution of H2SO4
(50 vol.%) for 3 h to facilitate the total dissolution of the alumina
and exposed Fe particles.
Characterization of the carbon nanotubes
N2 adsorption–desorption isotherms (À1968C) were obtained to
characterize the textural properties of the materials (Quantachrome
autosorb-iQ2). The samples were previously outgassed for 12 h at
1208C. The SBET was calculated by the BET equation,[49] and the
Vmeso and Vpore volumes were estimated using the BJH method.[50]
The surface chemical composition of the CNTs was analyzed by
XPS (Kratos AXIS Ultra HSA spectrometer), using MgKa radiation
(1486.7 eV). The elements present and their corresponding concen-
trations were determined by recording general XPS spectra, scan-
ning up to a binding energy (BE) of 1300 eV. The C1s peak
(284.9 eV) was taken as an internal standard to correct the shift in
BE caused by sample charging. The BE of the C1s, N1s, O1s and
Fe2p3/2 core levels and the full width at half maximum values were
used to assess the chemical state of these elements on the catalyst
Analytical methods
4-NP and the aromatic intermediates derived from its oxidation
were identified and quantified by HPLC, adapting the procedure
described elsewhere,[53] using a Jasco system equipped with an
UV/Vis detector (UV-2075 Plus) and a quaternary gradient pump
(PU-2089 Plus) for solvent delivery (1 mLminÀ1). The stationary
phase consisted of a Kromasil 100-5-C18 column (15 cm”4.6 mm;
5 mm particle size) working at RT. As explained above, small ali-
quots were periodically withdrawn from the reactor. An excess of
Na2SO3 was immediately added to consume the residual H2O2 and
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the catalyst was removed by filtration to stop the reaction.[33,36,54]
The mobile phase consisted of an isocratic method of an A:B
(40:60) mixture of 3% acetic acid and 1% acetonitrile in methanol
(A) and 3% acetic acid in ultrapure water (B). The absorbance
wavelength was adjusted to 318 nm for the determination of 4-NP,
and to 277 nm for the aromatic intermediates. The concentration
of carboxylic acids was monitored by a Jasco HPLC system fitted
with an YMC-Triart C18 column (25 cm”4.6 mm; 5 mm particle
size), adapting the procedures reported elsewhere.[55,56] The mobile
phase consisted of an isocratic method of an A:B (95:5) mixture of
1% sulfuric acid in ultrapure water (A) and acetonitrile (B), deliv-
ered to the system at 0.6 mLminÀ1. The UV/Vis detector was set to
210 nm. The TOC content and the concentration of nitrate ions
were determined using a Shimadzu TOC-5000A analyzer and a Met-
rohm 881 Compact Ion Chromatograph equipped with a Metrosep
A Supp 7-250 column, respectively. The concentrations of H2O2 and
Fe in solution were determined by colorimetric methods with
a UV/Vis spectrophotometer (T70 spectrometer, PG Instruments
Ltd.). For the determination of H2O2, a filtered sample was acidified
with a H2SO4 0.5m solution. 0.1 mL of titanium oxysulfate was
added, and the absorbance was measured at 405 nm.[57] On the
other hand, the concentration of Fe species leached at the end of
the experiments was determined by the o-phenanthroline method,
according to ISO 6332,[58] using ascorbic acid as reducing agent,
and measuring the absorbance at 510 nm.
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Received: January 31, 2016
Revised: March 29, 2016
Published online on June 15, 2016
ChemCatChem 2016, 8, 2068 – 2078
www.chemcatchem.org
2078
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim