Carbon nanotubes as catalysts for wet peroxide oxidation: The effect of surface chemistry

Article abstract of DOI:10.1016/j.cattod.2019.03.014

Three magnetic carbon nanotube (CNT) samples, named A30 (N-doped), E30 (undoped) and E10A20 (selectively N-doped), synthesized by catalytic chemical vapor deposition, were modified by introducing oxygenated surface groups (oxidation with HNO₃, samples CNT-N), and by heat treatment at 800 °C for the removal of surface functionalities (samples CNT-HT). Both treatments lead to higher specific surface areas. The acid treatment results in more acidic surfaces, with higher amounts of oxygenated species being introduced on N-doped surfaces. Heat-treated samples are less hydrophilic than those treated with nitric acid, heat treatment leading to neutral or basic surfaces, only N-quaternary and N-pyridinic species being found by XPS on N-doped surfaces. These materials were tested in the catalytic wet peroxide oxidation (CWPO) of highly concentrated 4-nitrophenol solutions (4-NP, 5 g L^?1) at atmospheric pressure, T = 50 °C and 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. The high temperature treatment enhanced significantly the activity of the CNTs towards CWPO, evaluated in terms of 4-NP and total organic carbon conversion, due to the increased hydrophobicity of their surface. In particular, E30HT and E10A20HT were able to remove ca. 100% of 4-NP after 8 h of operation. On the other hand, by treating the CNTs with HNO₃, the activity of the less hydrophilic samples decreased upon increasing the concentration of surface oxygen-containing functionalities, whilst the reactivity generated inside the opened nanotubes improved the activity of the highly hydrophilic A30 N.

Full text of DOI:10.1016/j.cattod.2019.03.014

Accepted Manuscript
Title: Carbon nanotubes as catalysts for wet peroxide
oxidation: The effect of surface chemistry
Authors: Maria Martin-Martinez, Bruno F. Machado, Philippe
Serp, Sergio Morales-Torres, Adri a´ n M.T. Silva, Jos e´ L.
Figueiredo, Joaquim L. Faria, Helder T. Gomes
PII:
S0920-5861(18)31662-6
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2019.03.014
CATTOD 12036
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
31 October 2018
9 February 2019
6 March 2019
Please cite this article as: Martin-Martinez M, Machado BF, Serp P, Morales-Torres
S, Silva AMT, Figueiredo JL, Faria JL, Gomes HT, Carbon nanotubes as catalysts
for wet peroxide oxidation: The effect of surface chemistry, Catalysis Today (2019),
https://doi.org/10.1016/j.cattod.2019.03.014
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Carbon nanotubes as catalysts for wet peroxide oxidation: The effect
of surface chemistry
a,e
b
c
d
Maria Martin-Martinez , Bruno F. Machado , Philippe Serp , Sergio Morales-Torres ,
b
b
b
a,b,*
Adrián M.T. Silva , José L. Figueiredo , Joaquim L. Faria , Helder T. Gomes
aCentro de Investigação de Montanha (CIMO), Instituto Politécnico de Bragança, Campus de Santa
Apolónia, 5300-253 Bragança, Portugal.
bLaboratory of Separation and Reaction Engineering - Laboratory of Catalysis and Materials (LSRE-
LCM), Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, Rua Dr.
Roberto Frias s/n, 4200-465 Porto, Portugal.
cLaboratoire de Chimie de Coordination UPR CNRS 8241, composante ENSIACET, Université de
Toulouse, UPS-INP-LCC 4 allé Emile Monso BP 44362, 31030 Toulouse Cedex 4, France.
dCarbon Materials Research Group, Department of Inorganic Chemistry, Faculty of Sciences, University
of Granada, Campus Fuentenueva s/n, 18071 Granada, Spain.
eDepartamento de Ingeniería Química, Facultad de Ciencias, Universidad Autónoma de Madrid,
Cantoblanco, 28049 Madrid, Spain.
*Corresponding author. Tel.: +351 273 303 110; Fax: +351 273 313 051.
E-mail address: htgomes@ipb.pt
GRAPHICAL ABSTRACT
1

O2
HO
1.0
0.8
0.6
0.4
0.2
H O
2 2
H O
2
HYDROPHILIC
HO
H O
2
0.0
2
HO
0
5
10
15
20
4
-NP
time /h
HO
-NP
4
HIGHLIGHTS





CNTs with different surface properties were synthesized by chemical vapor deposition.

Hydrophobic surfaces promote the controllable production of HO radicals.
Highly hydrophilic surfaces promote the formation of non-reactive species.
Oxidation with nitric acid produces open hydrophilic carbon nanotubes.
The new reactivity generated inside open nanotubes favours CWPO.
2

Abstract
Three magnetic carbon nanotube (CNT) samples, named A30 (N-doped), E30 (undoped)
and E10A20 (selectively N-doped), synthesized by catalytic chemical vapor deposition,
were modified by introducing oxygenated surface groups (oxidation with HNO
CNT-N), and by heat treatment at 800 °C for the removal of surface functionalities
samples CNT-HT). Both treatments lead to higher specific surface areas. The acid
3
, samples
(
treatment results in more acidic surfaces, with higher amounts of oxygenated species
being introduced on N-doped surfaces. Heat-treated samples are less hydrophilic than
those treated with nitric acid, and leads to neutral or basic surfaces, only N-quaternary
and N-pyridinic species being found by XPS on N-doped surfaces. These materials were
tested in the catalytic wet peroxide oxidation (CWPO) of highly concentrated 4-
-1
nitrophenol solutions (4-NP, 5 g L ) at atmospheric pressure, T = 50 ºC and pH = 3, using
-1
a catalyst load of 2.5 g L and the stoichiometric amount of H
2
2
O needed for the complete
mineralization of 4-NP. The high temperature treatment enhanced significantly the
activity of the CNTs towards CWPO, evaluated in terms of 4-NP and total organic carbon
conversion, due to the increased hydrophobicity of their surface. In particular, E30HT
and E10A20HT were able to remove ca. 100% of 4-NP after 8 h of operation. On the
other hand, by treating the CNTs with HNO , the activity of the less hydrophilic samples
3
decreased upon increasing the concentration of surface oxygen-containing functionalities,
whilst the reactivity generated inside the opened nanotubes improved the activity of the
highly hydrophilic A30N.
Keywords: Carbon nanotubes; Surface modification; CWPO; 4-nitrophenol.
3

1
. Introduction
Carbon materials, such as activated carbon, carbon gels, graphite, graphene, and carbon
nanofibers and nanotubes (CNTs), have been extensively used in different heterogeneous
catalytic processes [1-5]. Among them, CNTs are particularly suitable for liquid-phase
reactions (as catalysts or catalyst supports) in different environmental processes, owing
to their particular mechanical, electronic, and thermal properties [6, 7]. CNTs can be
synthesized by different procedures, such as arc discharge [8], laser ablation [9], and
plasma torch [10], but catalytic chemical vapor deposition (CVD) has been the most
widely used method for their production, including at the industrial scale [11]. During
catalytic CVD, a carbon-containing precursor (methane, acetylene, ethylene, benzene,
etc.) first decomposes on a metallic surface (Fe, Co, Ni), generally in the presence of
hydrogen. Then, the carbon atoms diffuse on or in the metallic phase and precipitate to
produce a CNT [1,12] Due to the generally high synthesis temperatures, and reductive
synthesis atmosphere, the surface of these materials commonly presents low amounts of
heteroatoms (e.g., O), rendering them mostly inert [13, 14]. One way to increase their
reactivity is to introduce surface groups through thermal, chemical, and physical
treatments. This allows the nature and concentration of surface functional groups to be
fine-tuned to specific applications [15]. For example, oxidation treatments with nitric
acid, hydrogen peroxide, or ozone introduce oxygen-containing surface groups such as
carboxylic acids and anhydrides, lactones, phenols, carbonyls, and quinones, with
different acidic/basic properties [16, 17]. Ammonia, urea, and melamine [18, 19] are
commonly used for the incorporation of nitrogen-containing surface groups (e.g., ex situ
at high temperatures), which are known to contribute to the basicity of CNTs [20, 21].
Nevertheless, nitrogen atoms can also be introduced during the in situ CVD growth of
CNTs, resulting in materials with new properties such as increased polarity and electrical
conductivity, improved CNTs dispersion in liquid media, higher reactivity towards
different molecules, and lower toxicity [22-26]. Moreover, it is known that, due to their
electron donating capacity, the presence of N-groups on the CNT surface enhances their

activity towards hydrogen peroxide (H
2
O
2
) decomposition into HO radicals, the
preliminary step for several catalytic processes [27, 28]. The acidity of CNTs can also be
increased by oxidation with sulfuric acid, due to the introduction of sulfonic acid and thiol
groups [29-31]. In order to control the CNT surface chemistry, certain functional groups
can be removed following a thermal treatment. Working at specific temperatures under
4

inert atmosphere, it is possible to remove selectively different types of surface groups [17,
2, 33].
3
Catalytic wet peroxide oxidation (CWPO) uses H
2
2
O as a source of hydroxyl radicals

(
HO ) for the oxidation of organic species. This process has been successfully used for
the treatment of aqueous effluents containing recalcitrant and non-biodegradable
compounds from pharmaceutical and petrochemical industries, among others, under
relatively mild operation conditions [34, 35]. These types of compounds are difficult to
remove by conventional biological processes, especially when present at high
-1
concentrations (1-10 g L ). Among organic pollutants, phenol and its derivatives have
received increasing attention in the last years due to their high toxicity. Some of the most
toxic members of the phenol family are chloro- and nitro-phenols, commonly used as
pesticides and antibacterials. The classical heterogeneous catalysts used in CWPO consist
of an active phase, mainly Fe, supported on a porous material such as activated carbon,
alumina, or zeolites. However, they usually suffer from severe deactivation due to
leaching of the metallic phase into the reaction medium, with further steps being needed
in the overall treatment process to remove or recover the metals from the treated
wastewaters [36-39]. In addition, rather than attacking the pollutant molecules, hydroxyl
radicals can also recombine, originating non-reactive species like H
order to overcome this limitation, the properties of the catalysts for CWPO can be
improved through surface chemical modification. Basic functionalities enhance H
decomposition. Basic oxygen-containing functionalities (e.g., chromene, pyrone and
quinones) are electron donating species, promoting the decomposition of H
to HO
35, 40-42]. Moreover, basic nitrogen-containing functionalities have been related with
increased rates of H decomposition [27]. On the other hand, acidic oxygen-containing
functionalities withdraw electrons from the catalyst surface, hindering their use in H
attack. Therefore, the presence of acidic functionalities may limit the CWPO process [43,
4].
2
O and O
2
[25]. In
2
O
2
2
O
2
[
2
O
2
2
O
2
4
Instead of supporting metallic particles on CNTs, in this work, a Fe/-Al
2
3
O catalyst has
been used for the synthesis of N-doped and undoped CNTs by CVD, enabling the
incorporation of Fe magnetic particles within the CNT structure. After inducing different
surface properties to the synthesized CNTs by oxidation with nitric acid and thermal
treatment at 800 °C, the materials have been tested as catalysts for the CWPO of 4-
5

-1
nitrophenol (4-NP) solutions. High pollutant concentrations (5 g L ) were used to
simulate highly loaded wastewaters.
2
2
4
. Materials and Methods
.1. Chemicals
-NP (98 wt.%) was purchased from Acros Organics. H
2
O
2
(30%, w/v), 1,4-
benzoquinone (C
NO , 98 wt.%) were obtained from Fluka. Formic acid (CH
phenanthroline (C12 , 99 wt.%) and sodium hydroxide (NaOH, 98 wt.%) were
acquired from Panreac. Acetic acid (CH COOH, glacial acetic acid), acetonitrile
CH CN, HPLC grade, 99.99 wt.%), methanol (CH OH, HPLC grade, 99.99 wt.%) and
L-ascorbic acid (C , 99 wt.%) were purchased from Fisher Chemical. Hydroquinone
, 99 wt.%), maleic acid (C , 99 wt.%), malic acid (C , 99 wt.%),
malonic acid (C , 99 wt.%), oxalic acid (C , 99 wt.%), phenol (C OH, 99
wt.%), iron(II) chloride tetrahydrate (FeCl ·4H O, 99 wt.%), hydrochloric acid (HCl, 37
wt.%), potassium nitrate (KNO , 99 wt.%), titanium(IV) oxysulphate (TiO 15 wt.% in
dilute sulphuric acid, 99.99%) and sodium sulphite (Na SO , 98 wt.%) were obtained
from Sigma-Aldrich. Sulphuric acid (H SO , 96-98 wt.%) and nitric acid (HNO , 65
wt.%) were acquired from Riedel-de-Haën and ammonium acetate (CH COONH , 98
6
H
4
O
2
, 99.5 wt.%), catechol (C
6
H
6
O
2
, 98 wt.%) and 4-nitrocatechol
(C
6
H
5
4
2
O
2
, 98 wt.%), o-
H
8
N
2
3
(
3
3
6
H
8
O
6
(C
6
H
6
O
2
4
H
4
O
4
4
H
6
O
5
3
H
4
O
4
H
2 2
O
4
6
H
5
2
2
3
2
2
3
2
4
3
3
4
wt.%) was obtained from Pronalab. All chemicals were used as received without further
purification. Distilled water was used throughout the work, except for mobile phase
preparation, where ultrapure water was employed.
2
.2. Synthesis and modification of carbon nanotubes
The original CNTs were synthesized by a CVD process in a fluidized bed reactor, as
described elsewhere [24]. The CNTs were grown with a Fe/-Al (20 wt.% Fe) catalyst
using ethylene as carbon source and acetonitrile as carbon/nitrogen source. The Fe/-
Al catalyst was prepared by impregnation, using a FeCl ·4H O solution, and activated
by reduction with H at 650 °C for 30 min, as described by Purceno et al. [24]. Three
2
O
3
2
O
3
2
2
2
different samples were produced by feeding to the fluidized bed reactor: (i) ethylene alone
for 30 min (sample E30); (ii) ethylene for 10 min, followed by acetonitrile for 20 min
(E10A20); and (iii) acetonitrile alone for 30 min (A30). The synthesized CNTs were
6

purified under reflux at 140 °C, with an aqueous solution of H
2
SO (50 vol.%) for 3 h to
4
facilitate the total dissolution of both the alumina and exposed Fe particles. Each sample
then underwent two different treatments (thermal and chemical), resulting in the
production of six additional materials. The thermal treatment was carried out to remove
surface functionalities; it consisted on heating the CNT samples under N
2
atmosphere
3
-1
(
(
100 cm min ) at 120 °C for 60 min, followed by heating at 400 °C (60 min), 600 °C
-1
60 min) and 800 °C (240 min), in a vertical furnace (2 °C min heating ramp between
each temperature). On the other hand, the chemical treatment consisted in the oxidation
of the original CNT samples with nitric acid to incorporate surface O-containing
functionalities, adapting the experimental procedure described elsewhere [19]. A 20 g L-
1
-1
mixture containing CNTs and nitric acid (7 mol L ) was maintained at boiling
temperature for 3 h in a 250 mL round-bottom flask. The obtained oxidized solids were
thoroughly washed with distilled water until neutrality of the rinsing waters, and dried in
an oven at 110 °C for 24 h. The samples were named as the original CNT, (E30, E10A20,
and A30), followed by “HT” for the heat-treated samples or “N” for the nitric acid-treated
ones.
2
.3. Characterization
The porous structure of the materials was characterized using N
2
adsorption-desorption
at –196 °C (Tristar II 3020, Micromeritics). The samples were previously outgassed for
-3
1
2 h at 150 °C at a residual pressure of 10 Torr (VacPrep 061, Micromeritics). The
apparent surface areas (SBET) were calculated by the BET equation [45] and total pore
volumes (Vpore) were estimated using the BJH method [46].
Transmission Electron Microscopy (TEM) images were obtained using a JEOL 1011
microscope operating at 100 kV. Average CNT diameters were calculated from statistical
distributions from TEM images.
Elemental analyses were performed on a Vario MICRO Cube system (C, H, N and S),
and on a rapid OXY cube analyser (O), from Elementar; C, H, N and S contents were
determined by combustion of the sample at 1050 °C, whereas O content was determined
by pyrolysis at 1450 °C. The results correspond to the average of three independent
measurements, using a daily calibration with a standard compound.
The chemical surface composition of the CNTs was analyzed by X-ray Photoelectron
Spectroscopy (XPS, Kratos AXIS Ultra HSA spectrometer), using Mg-Kα radiation
7

(
1486.7 eV). The elements present and their corresponding concentrations were
determined by recording general XPS spectra, scanning up to a binding energy (BE) of
300 eV. The C1s peak (284.6 eV) was taken as an internal standard to correct the shift
1
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 at the catalyst surface, according to the NIST database.
Temperature programmed desorption (TPD) analyses were performed by heating 0.1 g of
-1
the carbon sample, placed in a quartz tube, defining a heating ramp of 5 °C min from
3
-1
room temperature up to 1000 °C under N
2
atmosphere (1000 cm min ). The oxygenated
[47], and their
surface groups present in each sample decompose as CO and/or CO
2
resulting concentrations were monitored during the thermal analysis using a SIEMENS
Ultramat 22 gas analyzer. Peak deconvolution of the TPD profiles (multiple Gaussian
functions) was carried out using Origin Pro 9.0 software, according to the literature [47].
The pH of the point of zero charge (pHPZC) was determined by pH drift tests adapting the
procedure described elsewhere [48]. Briefly, five NaCl (0.01 M) solutions were prepared
as electrolyte with varying initial pH (range 2–11, using HCl and NaOH 0.1 M solutions).
0
.05 g of carbon sample was added to 20 mL of each NaCl solution. The equilibrium pH
of each suspension was measured after 48 h under stirring (200 rpm) at room temperature.
The pHPZC value was determined by intercepting the curve ‘final pH vs initial pH’ with
the straight line ‘initial pH = final pH’ [49].
The hydrophobicity/hydrophilicity of the CNTs (in the form of buckypapers) was
determined by water contact angle measurements using an Attension optical tensiometer
(model Theta) that allowed image acquisition and data analysis. The measurements were
performed on dry buckypapers at room temperature using the sessile drop method [50].
For each sample, the contact angle was measured using at least five different locations on
the buckypapers in order to determine the average value.
The magnetism of the samples was assessed qualitatively using magnets.
2
.4. 4-Nitrophenol removal experiments
The CWPO runs were carried out in a batch reaction system described elsewhere [25],
consisting of a 250 mL, magnetically stirred (600 rpm), glass reactor immersed in an oil
bath with temperature control, equipped with a reflux condenser and a sample collection
8

port. In a typical experiment, the reactor was loaded with 50 mL of a highly concentrated
-1
4
-NP aqueous solution (5 g L ) and heated to 50 °C. The pH was adjusted to 3.0 using
SO and NaOH 0.1 M solutions, and the stoichiometric concentration of H
necessary to mineralize completely the 4-NP, was added to the system. The reaction
H
2
4
2
2
O ,
-1
started with the addition of 2.5 g L of catalyst. During the experiment, samples of the
resulting effluent were collected at different reaction times and prepared for analysis, as
described in subsection 2.5. A blank experiment, i.e. without catalyst, was carried out to
assess possible non-catalytic oxidation reactions promoted by H
capacity of the catalysts was evaluated by means of pure adsorption experiments using
distilled water instead of H . The experiments were performed in triplicate, and the
standard deviation was lower than 5% in all cases.
2
O
2
. The adsorption
2
O
2
2
.5. Analytical methods
4
-NP and the aromatic intermediates were identified and quantified as described
elsewhere [25], using a Jasco HPLC system equipped with an UV-VIS detector (UV-
075 Plus), a quaternary gradient pump (PU-2089 Plus) for solvent delivery and a
2
Kromasil 100-5-C18 column (15 cm × 4.6 mm; 5 µm particle size) working at room
temperature. Briefly, small aliquots were periodically withdrawn from the reactor,
typically at 0, 5, 15, 30, 60, 120, 240, 480, and 1440 min. In order to stop the reaction, an
excess of Na
2
SO
3
was immediately added to consume the residual H
2
2
O [51, 52] and the
-1
catalyst was removed by filtration. The mobile phase (1 mL min ) 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 using a Jasco HPLC
system fitted with an YMC - Triart C18 column (25 cm × 4.6 mm; 5 µm particle size)
-1
[
25]. The mobile phase (0.6 mL min ) consisted of an isocratic method of an A:B (95:5)
mixture of 1% sulphuric acid in ultrapure water (A) and acetonitrile (B). The UV/VIS
detector was set to 210 nm. The TOC content was determined using a Shimadzu TOC-L
analyzer. Additionally, two colorimetric methods were used to determine the
concentrations of H
spectrometer, PG Instruments Ltd.). For the determination of H
acidified with a H SO 0.5 M solution. 0.1 mL of titanium oxysulfate was added, and the
2
O
2
and Fe in solution by means of UV/VIS spectrophotometry (T70
2
O
2
, a filtered sample was
2
4
absorbance was measured at 405 nm [53]. The concentration of Fe species was
9

determined by the o-phenanthroline method, according to ISO 6332 [54], using ascorbic
acid as reducing agent, and measuring the absorbance at 510 nm.
3
. Results and Discussion
3
.1. Carbon nanotubes characterization
The N
II according to the IUPAC classification, which are typical of non-porous materials. The
hysteresis loop present at high P/P values is associated with capillary condensation
2
adsorption-desorption isotherms of the CNTs (Figure 1) can be classified as type
0
occurring in large mesopores, as the spaces between agglomerated CNTs [55]. The SBET
and Vpore values are summarized in Table 1. As previously reported [25], the original E30
material consists of very regular multi-walled CNTs, the N-doped A30 is characterized
by a “bamboo-like” structure, and the selectively N-doped E10A20 contains sections of
both doped and undoped structures (Figure S1, Supplementary Material). Both treatments
(
thermal treatment under inert atmosphere, and oxidation with HNO ) lead to higher SBET
3
values, which could be related to CNT endcaps opening during the treatments, and a
thinning effect of the nanotube walls, as shown in the TEM images (Figure 2). This effect
is especially important for the E30N sample, which presented the highest SBET value.
Since the other two types of CNTs present a partial (E10A20) or complete (A30)
“bamboo-like” structure, the endcaps opening effect is less significant, with the increase
in surface area being limited by the existence of small interconnected compartments on
the N-doped sections.
FIGURE 1
TABLE 1
FIGURE 2
TEM images (Figure 2) show that the regular E30 nanotubes heat-treated at 800 ºC
(E30HT) present an average external diameter of ca. 9.6 nm, whilst those oxidized with
nitric acid (E30N) show a diameter of ca. 8.5 nm. Although these values are relatively
similar, the smaller diameter of E30N could indicate a thinning effect of the nanotube
walls. In fact, at higher magnification, it was possible to confirm that some nanotubes
were damaged by the nitric acid treatment (Figure S2a, Supplementary Material),
occasionally presenting small openings. This effect could be enhanced by existing defects
10

located throughout the walls of the nanotube, which act as nucleation sites for the
oxidation and corresponding sidewall etching. Hence, the textural properties, such as
S
BET, might be significantly impacted, since the access to the interior of some nanotubes
would be possible. Interestingly, this could explain why SBET of E30 increased from 275
2
-1
to 652 m g after the nitric acid treatment. With regard to the nanotubes prepared entirely
from acetonitrile (A30-series), a slightly larger external diameter than that found for the
undoped tubes is observed (12.6 nm for A30HT, and 11.7 nm for A30N). However, a
similar thinning effect as that described before for the E30-series was observed again for
the nanotubes that have undergone chemical treatment. Finally, although to a smaller
extent (compared to E30 and A30), E10A20N also presented a smaller external diameter
(9.8 nm) than E10A20HT (10.5 nm), confirming a thinning effect from the nitric acid
treatment of these materials. It should be noted that, when the nanotubes were initially
grown from ethylene (E10A20), the diameter of the N-doped portion was smaller than
that observed for the sample grown from only acetonitrile (A30), indicating that these
hybrid materials contain two different sections in the same CNT [31] (highlighted in
Figure S2b in Supplementary Material), as opposed to an independent and separate
growth of each section. The TEM images also show the presence of encapsulated metal
nanoparticles. They are formed during the catalytic CVD synthesis by in situ reduction
of the iron oxide catalyst at high temperature, and remain in the CNT structure even after
the purification step with 50 vol.% H
2
SO , conferring magnetic properties to these
4
materials (shown in Figure S3 in Supplementary Material).
The elemental composition (wt.%) of the synthesized CNTs is given in Table 1. All
samples are mainly composed of carbon, the hydrogen contents being negligible in all
cases. Compared to E30-series, there is a decrease in carbon content for the N-doped
materials (E10A20 and A30-series), with a corresponding increase of the nitrogen
content. Nitrogen and oxygen contents decreased upon heating the CNTs at 800 °C, due
to the removal of surface functional groups. On the other hand, the oxygen content
increased significantly after the nitric acid treatment, due to the incorporation of
oxygenated surface groups. The remaining material consisted mainly of Fe particles from
the Fe/Al
2
3
O catalyst used in the synthesis of the CNTs, as observed by TEM, which
decreases after the modifications, especially after the nitric acid treatment of the N-doped
materials.
11

From the XPS results (Table 2), it can be observed that the surface carbon content of the
different CNTs is higher than 95 at.%, with the exception of the A30 samples, which is
lower due to the high concentration of N and O heteroatoms (ca. 93 and 88 at.% C for the
heat-treated and nitric acid-treated samples, respectively). Given that E30 is prepared
from ethylene, there is no nitrogen present, and only a small concentration of oxygen is
detected (0.7 at.%). Nevertheless, after the nitric acid functionalization, the concentration
of both oxygen and nitrogen increased to 3.1 and 0.2 at.%, respectively. Regarding the
other samples, the amount of oxygen-containing surface groups increased with the N
content, affecting the type of surface functionalities. It has been shown that ex situ N-
doping (i.e., high-temperature thermal treatment under e.g., ammonia) of carbon materials
is significantly improved if its surface is functionalized with oxygen species [56].
Interestingly, in this study, we have observed that the oxidation step is much more
effective when carried out on N-containing surfaces, based on the higher amount of
oxygenated species introduced on A30 compared to E30 (3.1 at.% for E30N and 6.4 at.%
for A30N). Given that oxidation first occurs on highly reactive sites [16], the introduction
of heteroatoms on the carbon surface is energetically more favorable on defects than on
the pristine/undoped surface. This supports the concept that defect-containing surfaces
are necessary to introduce high amounts of heteroatoms. Hence, N-containing species can
be seen as defects, with theoretical calculations predicting a localization of the unpaired
electrons around the nitrogen-defect in the hetero-nanotubes [57], changing the reactivity
of the material. Comparing the heat-treated samples with those treated with nitric acid,
the amount of nitrogen of the latter is higher. There are two main reasons for this behavior:
(i) the nitric acid treatment introduces a small amount of N-containing species on the
surface (e.g., 0 at.% for E30HT and 0.2 at.% for E30N), and (ii) for the heat-treated
samples, the treatment at 800 °C removes part of the N-containing groups produced in
situ during the CVD synthesis. Hence, only the more stable functionalities, such as N-
quaternary and N-pyridinic, remain, whilst N-oxides are mostly removed (Table 2). Small
amounts of iron detected by XPS appear only in the N-containing samples, regardless of
the treatment (heat-treated or acid-treated), since its concentration in E30 was negligible.
Due to the synthesis process, the yield of CNT grown with acetonitrile is significantly
lower than that obtained with ethylene [24], leading to higher metal concentrations in the
final material. Moreover, given the bamboo-like structure of the N-doped CNTs (Figure
2
), Fe nanoparticles are more easily encapsulated, preventing their removal. This is
confirmed by the ash percentages determined by elemental analysis, with N-doped CNTs
12

showing higher values than the E30-series. The decrease in Fe content in the oxidized
samples can be explained by the additional dissolution of Fe by nitric acid, perhaps
through the opening of some more reactive CNT endcaps during the functionalization
step. Nevertheless, given the magnetic properties of the nanoparticles encapsulated inside
the CNTs (both undoped and doped, despite the higher concentration in the latter),
catalyst separation after the catalytic reaction can be easily achieved using a magnet.
TABLE 2
The C1s spectra of the CNTs show the presence of four different carbon bonds. The peak
2
at BE ∼284.6 eV is attributed to the C=C bonds corresponding to aromatic sp structures,
3
whereas that at ∼285.2 eV is assigned to C–C (sp carbon species). The peak at ∼287.5
eV is typically assigned to C–O, including epoxy (–O–) and hydroxyl (O–H) groups,
while the peak at ∼291.2 eV might be attributed to C=O, i.e. corresponding to carbonyl
(–C=O), carboxyl (–COOH), and carboxylate (–COOR) groups [58]. On the other hand,
the high-resolution O1s spectra of the CNTs were deconvoluted into three components
namely, C=O (either in carbonyl or in carboxyl groups), C–O (assigned to oxygen in
phenol/ether groups), and the last component that could correspond to N-oxides or
chemisorbed water. Finally, deconvolution of the N1s region of the XPS spectra (Table
2
) shows the presence of pyridinic-N (N6), pyrrolic-N (N5), quaternary-N (NQ), and N-
oxides (N-ox) for the N-doped materials. As can be seen, no significant differences are
observed when comparing the deconvoluted C1s and O1s peaks, with the exception of
E30HT, which has no peak associated to N-oxides (this material has no N in its structure)
and presents higher concentration of phenol/ether instead. Larger differences can be
observed in the peaks deconvoluted from the N1s region. As explained above, only the
most stable groups remain in the materials surface after the heat treatment (NQ and N6),
while the acid-treated samples present higher N-ox concentrations.
The decomposition of the different groups on the CNT surface produced the TPD profiles
shown in Figure 3. According to the procedure described elsewhere [47, 59], the peaks
appearing in the CO
2
-TPD profiles can be assigned to carboxylic acids (peaks below 450
°
C), carboxylic anhydrides (between 350-600 °C) and lactones (above 600 °C), whilst the
peaks in the CO-TPD profiles are related to carboxylic anhydrides (between 350-600 °C),
phenols/ethers (500-800 °C) and carbonyls/quinones (850-950 °C). The difference
between the thermally treated and the oxidized samples is demonstrated by the profile
13

shape for each group of materials, as well as the amount of CO and CO
the analyses (Table 3). The incorporation of oxygenated surface groups during nitric acid
treatment leads to the release of large amounts of CO and CO , whilst the removal of
2
released during
2
surface functionalities during the thermal treatment results in much lower amounts of
these gases. This effect is clearly observed by comparing the amounts of the different
functional groups obtained from deconvolution of TPD profiles (Table 3). The CNTs
treated at 800 °C show a large signal related to the presence of carbonyl and/or quinone
groups, but lower amounts of the other groups. In particular, carboxylic acids, anhydrides
and lactones almost disappear in E30HT. The larger amounts of surface functional groups
observed in A30HT are due to the larger concentrations present in the original A30
catalyst. On the other hand, the profiles of the samples treated with nitric acid show the
presence of large amounts of carboxylic acids and anhydrides. With the exception of the
carbonyl/quinone peak, which seems to be preferentially present on the undoped samples,
all other surface groups are present in higher concentrations on the N-doped materials.
Moreover, the sharp peaks observed at 850 C in the thermally treated samples can be
attributed to the presence of Fe. These sharp peaks are not found in the oxidized samples,
since, as was observed by XPS, the treatment with nitric acid removes the remaining Fe.
FIGURE 3
TABLE 3
The pHPZC of the modified CNTs (Table 3) is consistent with the results of both XPS and
TPD analyses, since the heat treatment of the materials under inert atmosphere at 800 °C
leads to neutral or basic surfaces, while oxidation with nitric acid results in materials with
a more acidic surface.
The incorporation of oxygen- and nitrogen-containing functionalities onto the CNTs
modifies their hydrophilic properties, which were determined from water contact angle
measurements obtained from the corresponding buckypapers. In Figure 4, it can be
observed that the heat-treated CNTs were less hydrophilic than those treated with nitric
acid, which should be ascribed to the increased surface content of oxygen and nitrogen
after oxidation, as shown by XPS and TPD. It is noteworthy the low contact angle of
A30HT (i.e., 18 ± 2 °), which should be due to the 2.3 at.% OXPS and 4.1 at.% NXPS even
after the thermal treatment at 800 °C. In general, the contact angles decreased as follows:
E30 > E10A20 > A30, regardless of whether the samples were heat-treated or oxidized.
14

Thus, the E30HT sample presented a moderate hydrophobicity, with the highest contact
angle (i.e., 59 ± 2 °), while the A30N sample can be considered highly hydrophilic, with
a contact angle as low as 9 ± 1 °.
FIGURE 4
3
.2. Catalytic performance in CWPO experiments
In a previous study, the role of N-doping in CNTs used in the CWPO process was studied
25]. The aim of the current work is to study the activity of these CNTs after different
[
surface modifications, in which there will be a competition between the effect caused by
the N-doping, and the effect caused by the treatment (heat treatment or oxidation).
Screening experiments were carried out to assess the performances of the different CNT
samples during the CWPO of highly concentrated 4-NP solutions. The removal of 4-NP
by CWPO and the decomposition of H
2
2
O observed during 24-hour experiments are
shown in Figure 5. The curves corresponding to the non-modified CNTs from a previous
work [25] have been included for comparison purposes (represented in grey).
FIGURE 5
It is expected that the adsorption of 4-NP, a weak acid, is favored on materials with a
basic surface. Using the heat-treated materials, with a pHPZC significantly higher than the
working pH (3 in all cases), 4-NP partially dissociates to its anionic form, thus favoring
the electrostatic interaction between adsorbent and adsorptive. On the other hand, it has
been reported that the adsorption of molecules such as 4-NP on hydrophilic surfaces (e.g.,
HNO treated samples) is lower due to the formation of water molecule clusters on the
3
carbon porous structure, resulting in pore blockages (solvent effect) [60]. Due to this
phenomenon, some pores may be inaccessible to the formed 4-NP-water aggregates.
Hence, thermally-treated samples are expected to show a much better behavior in the
CWPO of 4-NP than oxidized ones.
As observed in Figure 5, the catalysts synthesized from the completely N-doped sample
(
A30HT and A30N) promote a fast decomposition of H
removal. For sample A30, the limited CWPO performance was attributed to the fast
decomposition of H into non-reactive species (H O and O ) promoted by the N-doped
hydrophilic surfaces [25]. Nevertheless, the conversion of 4-NP is significantly improved
2
O , resulting in a poor 4-NP
2
2
O
2
2
2
15

for the surface-modified samples (heat-treated and nitric acid-treated), despite the fast
decomposition observed in both cases. As observed for A30, both A30HT and
especially A30N have hydrophilic character, and contain large concentrations of electron
donating groups (as evidenced by XPS), explaining the fast decomposition of H . The
H
2
O
2
2
O
2
activity improvement observed for A30N, when compared to A30, could be related to the

reactivity generated inside the opened nanotubes, allowing HO radicals to react with the
4
-NP molecules adsorbed in the close proximity, before the radicals disappear by
scavenging effects [43].
With regard to the catalysts derived from E30, E30HT decomposed H
2
2
O at a moderate
rate, resulting in the complete conversion of 4-NP. The moderate hydrophobicity of E30
has been reported to favor the controllable and efficient decomposition of H
2
2
O into

highly reactive HO radicals [25]. The same moderate hydrophobicity has been found for

E30HT, favoring the decomposition of H
2
O
2
into HO radicals, thus showing that E30HT
has a high catalytic activity for the 4-NP degradation. As will be seen below (Table 4), in
comparison with the other catalysts, the efficiency of the H when using E30HT is
2
O
2
clearly superior. On the other hand, when treated with nitric acid (increased concentration
of oxygen functionalities, viz. lactones, phenols, carboxylic acids and anhydrides,
increasing the catalyst surface hydrophilicity - a contact angle similar to that of A30HT
was determined, ca. 18°) this sample (E30N) promoted a faster H
2
O
2
decomposition,
with
which slowed down the CWPO of 4-NP. Despite the fast decomposition of H
2
O
2
2
-1
E30N, the higher specific surface area of this material (650 m g ) favored the reaction

between the HO radicals and the 4-NP molecules, allowing the reaction to occur in the
internal cylindrical mesopores (confinement effect).
TABLE 4
As already reported, the selectively N-doped E10A20 sample, consisting of hybrid
structures containing both undoped and N-doped sections, has an intermediate behavior,
showing an intermediate H
treatment (E10A20HT) improved the 4-NP conversion, showing a behavior similar to
E30 and E30HT, removing 99% of the pollutant after 8 h. Nevertheless, the HNO
treatment (E10A20N) hindered the H decomposition and, consequently, the 4-NP
2
2
O decomposition rate and 4-NP removal [25]. The thermal
3
2
O
2
removal (conversion of 60%). It is well accepted that basic active sites are responsible for

the generation of HO radicals at the carbon surface, which include basic surface groups
16

such as quinones, chromenes, and pyrones, as well as delocalized  electrons on carbon
basal planes [40, 61].
In general, materials with higher microporosities and surface densities of available
electrons yield higher H
basic carbon materials lead to higher H
exhibit a limited activity in the decomposition of H
surface acidic groups in E10A20N, demonstrated by XPS and TPD analyses, as well as
the pHPZC shown by this material, explains its decreased ability to decompose H O .
2
O
2
decomposition rates [41]. Several authors have reported that
decomposition rates, whilst acidic carbons
[40, 42, 62, 63]. The presence of
2
O
2
2
O
2
2
2
Despite this more controllable decomposition, the same acidity hinders the CWPO
reaction, since, as explained before, the adsorption of the 4-NP molecules on the
material’s hydrophilic surface is hindered.
Table 4 presents the conversions (X) of 4-NP, total organic carbon (TOC), and H
achieved for each catalyst, as well as the concentration of Fe leached into the solution,
the efficiency of H consumption (휂 ), defined as the TOC removal (푋푇푂퐶 in %)
per unit of H decomposed (푋
2
O
2
2
O
2
퐻 푂
2
2
2
O
2
in %), and the reaction byproducts obtained after 24
퐻 푂
2
2
h of reaction. As observed in Figure 5, the removal of surface functionalities by heating
the CNT samples at 800 °C improved their activity towards CWPO, evaluated in terms
of 4-NP and TOC conversion. In particular, E30HT and E10A20HT were able to remove
ca. 100% of 4-NP. Despite the high conversion obtained with E30N, 14% corresponds to
pure adsorption of the pollutant molecules on the catalyst surface. In general, TOC
conversion is higher with the thermally treated materials. Moreover, the highly efficient

decomposition of H
2
O
2
into HO with E30HT results in a TOC conversion of ca. 60%,
-1
-1
with ~3 mgC L of aromatic byproducts and 1 gC L of organic acids being detected
after 24 h. Figure 6 shows the time-evolution of the different compounds identified during
the CWPO of 4-NP for E30HT; the concentrations of the aromatic byproducts follow the
sequence: 4-nitrocatechol > hydroquinone > 1,4-benzoquinone > catechol, which is
consistent with the reaction mechanisms published for this reaction [32, 64-66]. These
intermediates disappear almost completely after 24 h, increasing the concentration of low-
molecular-weight carboxylic acids (mainly malonic and malic acids). E10A20HT shows
an activity similar to E30HT. Despite that, the efficiency of the H
2
2
O consumption
decreased by half, with the Fe leaching into the solution possibly contributing to the
activity. This significant Fe leaching was also observed with A30HT. However, in this
case, the fast and inefficient decomposition of H
2
2
O slowed down the reaction, resulting
17

in lower TOC conversion and byproducts concentration. Regarding the CNTs treated with
HNO , as already observed for the heat-treated ones, when the N content increased (E30N
E10A20N < A30N), the efficiency of H consumption decreased and so did the TOC
3
<
2
O
2
conversion.
FIGURE 6
Conclusions
The surface chemistry of carbon nanotubes (CNTs) with different N-doping and
hydrophilicity grade (E30, undoped and moderately hydrophobic; A30, N-doped and
hydrophilic; E10A20, selectively N-doped, with both hydrophilic and hydrophobic
sections) plays an important role in their performance during the catalytic wet peroxide
-1
oxidation (CWPO) of highly concentrated solutions of 4-nitrophenol (4-NP, 5 g L ). The
removal of surface functionalities by heat treatment of the CNTs under inert atmosphere
at 800 °C produced the samples CNT-HT, for which the interaction between the 4-NP
molecules and the CNT surface is favored. Among the CNT-HT samples, the more
hydrophobic materials (E30HT and E10A20HT) promote a controllable H
2
O
2

decomposition into HO radicals, being able to remove ca. 100% of 4-NP after 8 h of
operation. On the contrary, the hydrophilic A30HT promotes the fast H
2
2
O decomposition
into non-reactive species, resulting in a poor pollutant removal. The oxidation of the
CNTs by nitric acid produced hydrophilic samples (CNT-N) with opened ends, in which
the decomposition of H
2
O
2
is fast. Nevertheless, the reactivity generated inside the

opened nanotubes favored the reaction between the HO radicals and the 4-NP molecules
in the internal cylindrical mesopores, allowing the reaction to occur before the radicals
disappear by scavenging effects.
Acknowledgments
This work is a result of project “AIProcMat@N2020 - Advanced Industrial Processes and
Materials for a Sustainable Northern Region of Portugal 2020”, with the reference
NORTE-01-0145-FEDER-000006, supported by NORTE 2020, under the Portugal 2020
18

Partnership Agreement, through the ERDF and of Project Associate Laboratory LSRE-
LCM - UID/EQU/50020/2019 - funded by national funds through FCT/MCTES
(PIDDAC). M. Martín Martínez acknowledges the postdoc grant 2017-T2/AMB-5668
from Comunidad de Madrid, Programme “Atracción de talento investigador”. Machado
acknowledges the exploratory project under FCT Investigator Programme (ref.
IF/00301/2015) with the financial support of FCT/MCTES through national funds
(PIDDAC). S. Morales Torres acknowledges the financial support from University of
Granada (Reincorporación Plan Propio).
References
[
1] P. Serp, J. L. Figueiredo, Editors., Carbon Materials For Catalysis, John Wiley &
Sons, Inc 2009.
[
[
[
[
[
[
2] F. Rodríguez-Reinoso, Carbon 36 (1998) 159-175.
3] P. Serp, M. Corrias, P. Kalck, Appl. Catal. A 253 (2003) 337-358.
4] C. Moreno-Castilla, F. J. Maldonado-Hódar, Carbon 43 (2005) 455-465.
5] B. F. Machado, P. Serp, Catal. Sci. Technol. 2 (2012) 54-75.
6] V. N. Popov, Mater. Sci. Eng., R 43 (2004) 61-102.
7] G. Ovejero, J. L. Sotelo, M. D. Romero, A. Rodríguez, M. A. Ocaña, G. Rodríguez,
J. García, Ind. Eng. Chem. Res. 45 (2006) 2206-2212.
[
[
8] T. W. Ebbesen, P. M. Ajayan, Nature 358 (1992) 220.
9] T. Guo, P. Nikolaev, A. G. Rinzler, D. Tomanek, D. T. Colbert, R. E. Smalley, J.
Phys. Chem. 99 (1995) 10694-10697.
[
[
10]O. Smiljanic, B. L. Stansfield, J. Dodelet, A. Serventi, S. Désilets, Chem. Phys. Lett.
3
56 (2002) 189-193.
11]M. José‐ Yacamán, M. Miki‐ Yoshida, L. Rendón, J. G. Santiesteban, Appl. Phys.
Lett. 62 (1993) 202-204.
[
[
[
12] M. Kulmar and Y. Ando, J. Nanosci. Nanotechnol. 10 (2010) 3739-3758.
13] S. Mallakpour and S. Soltanian, RSC Adv. 6 (2016) 109916-109935.
14] A.K. Deb, C.C. Chusuei, Aqueous solution surface chemistry of carbon nanotubes,
in: Satoru Suzuki (Ed.), Physical and Chemistry Properties of Carbon Nanotubes,
Intech Open, 2013, pp. 275-293.
[
15]R. P. Rocha, O. S. Soares, J. L. Figueiredo, M. F. Pereira, C 2 (2016) 1-18.
19

[
[
[
[
[
16]I. Gerber, M. Oubenali, R. Bacsa, J. Durand, A. Gonçalves, M. F. R. Pereira, F.
Jolibois, L. Perrin, R. Poteau, P. Serp, Chem. Eur. J. 17 (2011) 11467-11477.
17]R. P. Rocha, A. M. T. Silva, S. M. M. Romero, M. F. R. Pereira, J. L. Figueiredo,
Appl. Catal. B 147 (2014) 314-321.
18]E. N. Nxumalo, P. J. Letsoalo, L. M. Cele, N. J. Coville, J. Organomet. Chem. 695
(2010) 2596-2602.
19]R. P. Rocha, J. P. S. Sousa, A. M. T. Silva, M. F. R. Pereira, J. L. Figueiredo, Appl.
Catal. B 104 (2011) 330-336.
20]H. P. Boehm, Catalytic Properties of Nitrogen‐ Containing Carbons, in: P. Serp, J.
L. Figueiredo (Eds.), Carbon Materials for Catalysis, John Wiley & Sons, Inc., 2008,
pp. 219-265.
[
[
21]J. L. Figueiredo, M. F. R. Pereira, Catal. Today 150 (2010) 2-7.
22]O. Stephan, P. M. Ajayan, C. Colliex, P. Redlich, J. M. Lambert, P. Bernier, P. Lefin,
Science 266 (1994) 1683.
[
[
23]P. Ayala, R. Arenal, M. Rümmeli, A. Rubio, T. Pichler, Carbon 48 (2010) 575-586.
24]A. D. Purceno, B. F. Machado, A. P. C. Teixeira, T. V. Medeiros, A. Benyounes, J.
Beausoleil, H. C. Menezes, Z. L. Cardeal, R. M. Lago, P. Serp, Nanoscale 7 (2015)
2
94-300.
[
[
[
[
25]M. Martin-Martinez, R. S. Ribeiro, B. F. Machado, P. Serp, S. Morales-Torres, A.
M. T. Silva, J. L. Figueiredo, J. L. Faria, H. T. Gomes, ChemCatChem 8 (2016) 2068-
2
078.
26]A.L. Mihalchik, W. Ding, D.W. Porter, C. McLoughlin, D. Schwegler-Berry, J.D.
Sisler, A.B. Stefaniak, B.N. Snyder-Talkington, R. Cruz-Silva, M. Terrones, S.
Tsuruoka, M. Endo, V. Castranova, Y. Qian, Toxicology 333 (2015) 25-36.
27]K. V. Voitko, R. L. D. Whitby, V. M. Gun’ko, O. M. Bakalinska, M. T. Kartel, K.
Laszlo, A. B. Cundy, S. V. Mikhalovsky, J. Colloid Interface Sci. 361 (2011) 129-
1
36.
28]K. Voitko, A. Tóth, E. Demianenko, G. Dobos, B. Berke, O. Bakalinska, A.
Grebenyuk, E. Tombácz, V. Kuts, Y. Tarasenko, M. Kartel, K. László, J. Colloid
Interface Sci. 437 (2015) 283-290.
[
[
29]F. Peng, L. Zhang, H. Wang, P. Lv, H. Yu, Carbon 43 (2005) 2405-2408.
30]H. T. Gomes, S. M. Miranda, M. J. Sampaio, J. L. Figueiredo, A. M. T. Silva, J. L.
Faria, Appl. Catal. B 106 (2011) 390-397.
[
[
31]A. Benyounes, F. Ouanji, S. Louisia, M. Ziyad, P. Serp, M. Kacimi, Catal. Today
3
01 (2018) 183-190.
32]M. Martin-Martinez, M. F. F. Barreiro, A. M. T. Silva, J. L. Figueiredo, J. L. Faria,
H. T. Gomes, Appl. Catal. B 219 (2017) 372-378.
20

[
[
[
33]M. Martin-Martinez, S. Álvarez-Torrellas, J. García, A. M. T. Silva, J. L. Faria, H.
T. Gomes, Catal. Today 313 (2018) 20-25.
34]L. F. Liotta, M. Gruttadauria, G. Di Carlo, G. Perrini, V. Librando, J. Hazard. Mater.
1
62 (2009) 588-606.
35]R. S. Ribeiro, A. M. T. Silva, J. L. Figueiredo, J. L. Faria, H. T. Gomes, Appl. Catal.
B 187 (2016) 428-460.
[
[
[
36]E. V. Rokhina, J. Virkutyte, Crit. Rev. Environ. Sci. Technol. 41 (2010) 125-167.
37]G. Calleja, J.A. Melero, F. Martinez, R. Molina, Water Res. 39 (2005) 1741-1750.
38]A. Rey, M. Faraldos, J.A. Casas, J.A. Zazo, A. Bahamonde, J.J. Rodriguez, Appl.
Catal. B 86 (2009) 69-77.
[
[
[
[
[
[
39]A. Onda, Y. Suzuki, S. Takemasa, K. Kajiyoshi, K. Yanagisawa, J. Mater. Sci. 43
(2008) 4230-4235.
40]R. S. Ribeiro, A. M. T. Silva, J. L. Figueiredo, J. L. Faria, H. T. Gomes, Carbon 62
(2013) 97-108.
41]A. Rey, J. A. Zazo, J. A. Casas, A. Bahamonde, J. J. Rodriguez, Appl. Catal. A 402
(2011) 146-155.
42]V. P. Santos, M. F. R. Pereira, P. C. C. Faria, J. J. M. Órfão, J. Hazard. Mater. 162
(2009) 736-742.
43]M. Soria-Sánchez, E. Castillejos-López, A. Maroto-Valiente, M. F. R. Pereira, J. J.
M. Órfão, A. Guerrero-Ruiz, Appl. Catal. B 121-122 (2012) 182-189.
44] R.S. Ribeiro, N.A. Fathy, A.A. Attia, A.M.T. Silva, J.L. Faria, H.T. Gomes, Chem.
Eng. J. 195-196 (2012) 112-121.
[
[
[
45]S. Brunauer, P. H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309-319.
46]E. P. Barrett, L. G. Joyner, P. P. Halenda, J. Am. Chem. Soc. 73 (1951) 373-80.
47]J. L. Figueiredo, M. F. R. Pereira, M. M. A. Freitas, J. J. M. Órfão, Carbon 37 (1999)
1
379-1389.
[
[
48]G. Newcombe, R. Hayes, M. Drikas, Colloids Surf. A 78 (1993) 65-71.
49]J. J. M. Órfão, A. I. M. Silva, J. C. V. Pereira, S. A. Barata, I. M. Fonseca, P. C. C.
Faria, M. F. R. Pereira, J. Colloid Interface Sci. 296 (2006) 480-489.
[
[
[
50]S. Morales-Torres, T. L. S. Silva, L. Pastrana-Martinez, A. T. S. C. Brandao, J. L.
Figueiredo, A. M. T. Silva, Phys. Chem. Chem. Phys. 16 (2014) 12237-12250.
51]F. Duarte, F. J. Maldonado-Hódar, L. M. Madeira, Appl. Catal. B 103 (2011) 109-
1
15.
52]J. H. Ramirez, C. A. Costa, L. M. Madeira, G. Mata, M. A. Vicente, M. L. Rojas-
Cervantes, A. J. López-Peinado, R. M. Martín-Aranda, Appl. Catal. B 71 (2007) 44-
5
6.
21

[
53]W. C. Ketchie, Y. Fang, M. S. Wong, M. Murayama, R. J. Davis, J. Catal. 250 (2007)
4-101.
9
[
[
54]ISO 6332:1988, 1988.
55]K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J. Rouquerol,
T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603-619.
[
56]P. Serp, B. F. Machado, Doped Nanostructured Carbon Materials as Catalysts, in: P.
Serp, B. F. Machado (Eds.), Nanostructured Carbon Materials for Catalysis, RSC,
2
015, pp. 268-311.
[
[
[
57]A. H. Nevidomskyy, G. Csányi, M. C. Payne, Phys. Rev. Lett. 91 (2003) 105502.
58]D. Luo, G. Zhang, J. Liu, X. Sun, J. Phys. Chem. C 115 (2011) 11327-11335.
59]J. L. Figueiredo, M. F. R. Pereira, M. M. A. Freitas, J. J. M. Órfão, Ind. Eng. Chem.
Res. 46 (2007) 4110-4115.
[
[
60]A. Morsli, A. Benhamou, J. Basly, M. Baudu, Z. Derriche, RSC Adv. 5 (2015)
4
1631-41638.
61]C. A. Leon y Leon, J. M. Solar, V. Calemma, L. R. Radovic, Carbon 30 (1992) 797-
8
11.
[
[
62]F. Lücking, H. Köser, M. Jank, A. Ritter, Water Res. 32 (1998) 2607-2614.
63]L. B. Khalil, B. S. Girgis, T. A. Tawfik, J. Chem. Technol. Biotechnol. 76 (2001)
1
132-1140.
[
[
[
64]R. S. Ribeiro, A. M. T. Silva, L. M. Pastrana-Martínez, J. L. Figueiredo, J. L. Faria,
H. T. Gomes, Catal. Today 249 (2015) 204-212.
65]R. S. Ribeiro, A. M. T. Silva, P. B. Tavares, J. L. Figueiredo, J. L. Faria, H. T. Gomes,
Catal. Today 280 (2017) 184-191.
66]R. S. Ribeiro, A. M. T. Silva, J. L. Figueiredo, J. L. Faria, H. T. Gomes, Catal. Today
2
96 (2017) 66-75.
22

TABLES
Table 1. Composition (wt.%) of CNTs as determined by elemental analysis, and textural
properties calculated from N
2
adsorption-desorption isotherms.
C
%)
H
N
(%)
-
-
-
3.0
1.9
2.7
5.0
3.8
4.8
O
(%)
1.6
0.7
5.4
3.1
2.2
6.4
7.3
5.4
11.9
Ash
(%)
1.5
1.1
1.5
3.2
2.8
1.5
4.7
3.9
2.3
S
BET (5)
Vpore (0.01)
3 -1
2
-1
(
(%)
0.3
0.2
0.4
0.5
0.4
0.5
0.8
0.6
0.8
(m g )
(cm g )
[a]
[a]
E30
E30HT
E30N
E10A20
E10A20HT
E10A20N
A30
96.7
98.0
92.6
90.1
92.8
88.8
82.2
86.3
80.1
275
1.92
300
652
1.63
1.91
[a]
[a]
214
1.49
273
274
1.46
1.22
[a]
[a]
212
1.11
A30HT
307
291
0.94
0.68
A30N
data from [25]
[a]
23

Table 2. Surface atomic composition, species percentage and corresponding binding energies (in brackets, eV) of the CNTs obtained by XPS.
C
O
N
Fe
C1s (%)
O1s (%)
N1s (%)
C=C
49
C–C
30
C–O
C=O
10
C=O
29
C-O
N-ox
N6
N5
NQ
N-ox
(
%)
(%) (%) (%)
E30HT
E10A20HT
A30HT
99.3
0.7
1.0
2.3
3.1
3.6
6.4
0.0
0.8
4.1
0.2
1.2
5.5
0.0
0.2
0.6
0.0
0.1
0.3
11
71
-
-
-
-
-
-
(
(
(
(
(
(
284.6) (285.2) (287.7) (291.2) (530.4) (532.6)
46 32 12 10 54 37
284.6) (285.2) (287.5) (291.2) (530.6) (532.7) (534.9) (398.7)
46 32 13 49 38 13 42
98.0
93.0
96.7
95.1
87.8
9
46
47
7
(401.2) (404.0)
35 14
9
9
284.6) (285.4) (287.8) (291.2) (530.6) (532.2) (533.8) (398.5) (400.4) (401.1) (404.1)
E30N
48
284.6) (285.2) (287.5) (291.1) (531.6) (533.3) (535.6)
55 19 15 11 48 47
284.6) (285.5) (287.3) (291.1) (531.4) (533.3) (536.0) (398.9)
45 31 15 51 44 35
28
13
11
46
49
5
-
-
-
-
-
E10A20N
A30N
5
36
34
30
(401.1) (405.1)
33 24
9
5
8
284.6) (285.4) (287.9) (291.3) (531.1) (532.9) (535.5) (399.5) (400.4) (402.0) (405.7)
2
4

Table 3. Surface oxygenated functionalities by TPD analyses, and pHPZC of the original and modified CNTs.
CO CO
2
carboxylic
lactone
anhydride phenol/ether carbonyl/quinone
-
-
Sample
(µmol g
(µmol g
pHPZC
-1
-1
-1
-1
-1
(
µmol g ) (µmol g ) (µmol g )
(µmol g )
(µmol g )
1
1
)
)
E30
A30
364
864
930
427
699
1416
921
49
61
5
25
55
261
256
264
40
62
15
17
59
238
236
243
40
200
16
102
552
348
401
424
340
318
233
219
6.9
510
173
306
519
691
714
1056
605
141
189
186
637
831
982
6.3
8.1
7.8
7.2
2.2
2.2
2.0
E30HT
E10A20HT
A30HT
E30N
E10A20N
A30N
30
132
192
216
501
1075
1472
2
5

Table 4. Removal of 4-NP from aqueous solution by adsorption (Ads.) and CWPO (X4-NP); TOC removal (XTOC), H
2
O
2
decomposition 푋
), efficiency of
퐻 푂
2
2
H O
2 2
consumption (휂
), Fe leaching, and reaction byproducts resulting from the CWPO experiments; results after 24 h.
2
퐻 푂
2
Fe aromatics[a]
[b]
Ads.
X
4-NP
푋
X
TOC
휂
acids
퐻 푂
퐻 푂
2
2
2
2
-1
(mgC L-1)
(mgC L-1)
(
%)
(%)
(%)
(mg L )
(
%)
E30
6
7
100
88
9
54
67
93
59
99
99
90
42
54
44
18
59
48
22
54
13
2
100
66
19
100
48
22
60
31
2
1.35
2.61
3.89
1.46
4.56
4.38
0.54
1.42
0.15
33.1
355.2
21.0
2,169.5
1,325.7
359.0
E10A20
A30
1
E30HT
E10A20HT
A30HT
E30N
8
100
99
35
99
60
48
3.4
1,076.6
786.9
7
140.2
155.0
36.8
4
166.0
14
7
1,480.4
220.3
E10A20N
A30N
460.7
73.9
8
100
74.5
[a]
[b]
Sum of mgC in hydroquinone, 1,4-benzoquinone, catechol and 4-nitrocatechol. Sum of mgC in oxalic acid, formic acid, malic acid, malonic acid, acetic acid
and maleic acid
2
6

FIGURE CAPTIONS
Figure 1. N
Figure 2. TEM micrographs of the modified CNTs.
Figure 3. TPD profiles of the modified CNTs: CO
2
adsorption-desorption isotherms of the modified CNTs, obtained at -196 °C.
2
(top) and CO (bottom).
Figure 4. Contact angle measurements determined for buckypapers prepared with different CNTs:
Heat-treated (a-c) and nitric acid-treated (d-f).
Figure 5. Concentration evolution of 4-NP (top) and H
experiments (normalized by the initial concentrations).
2
2
O (bottom) during the CWPO
-1
Figure 6. Concentration (mg L ) evolution with time (h) for the different compounds resulting
from the CWPO of 4-NP using E30HT.
27

FIGURE 1
1400
1200
1000
E30HT
E30N
E10A20HT
E10A20N
A30HT
A30N
800
600
400
200
0
0.0
0.5
1.0 0.0
0.5
1.0 0.0
0.5
1.0
P/Po
P/Po
P/Po
28

FIGURE 2
E30HT
E10A20HT
A30HT
E30N
E10A20N
A30N
29