G Model
CATTOD-9033; No. of Pages7
ARTICLE IN PRESS
R.P. Rocha et al. / Catalysis Today xxx (2014) xxx–xxx
2
oxidation of the organic compounds occurs both in the liquid phase
homogeneous reaction) and on the catalyst surface; (ii) free rad-
programmed desorption (TPD), carried out in an Altamira Instru-
ments AMI-200 apparatus. The pH at the point of zero charge was
measured by analysis of the pH change of NaCl solutions of differ-
ent initial values of pH when exposed to a sample of the prepared
materials. Elemental analysis was performed on a Carlo Erba instru-
ment, model EA 1108. Additional details can be found in reference
[36]. The XPS analysis of the CX samples after oxidation reactions
was performed using a Kratos AXIS Ultra HSA, with VISION soft-
ware for data acquisition and CASAXPS software for data analysis.
The analysis was carried out with a monochromatic Al K␣ X-ray
source (1486.7 eV), operating at 15 kV (90 W), in FAT mode (Fixed
Analyser Transmission), with a pass energy of 40 eV for regions of
interest and 80 eV for survey.
(
ical species are involved in the mechanism; (iii) basic carbons are
generally the best catalysts. In particular, correlations between the
catalytic activity and the surface chemistry of the carbon materi-
als were established [15,24–26], showing that conversions increase
with the pH at the point of zero charge (pHPZC).
The presence of surface nitrogen groups is also extremely
important for the catalytic applications of carbon materials. The
additional electrons provided by nitrogen increase surface basic-
ity. Moreover, electrons may be transferred from the surface to
adsorbed oxygen, leading to the formation of highly reactive radi-
cals in the presence of water [27]. Nitrogen groups can be formed
by carbonization of nitrogen-containing precursors, or by reaction
of the carbon material with a suitable nitrogen compound.
Various types of carbon materials with incorporated nitrogen
2.3. Experimental procedure
(
carbon foams and fibers, carbon nanotubes and nanofibers, and
Oxalic acid was used as a model compound due to its refractory
activated carbons) have been tested in CWAO [14,28–30] and in
catalytic ozonation [31,32]. Comparison of reported results is diffi-
cult, due to the wide differences in morphology, textural properties
and amounts of impurities in these materials (for instance, acti-
vated carbons may contain significant amounts of transition metals,
which may act as catalysts in AOPs). A more systematic approach
is required in order to assess the effect of nitrogen on the per-
formance of carbon materials in these AOPs. Carbon xerogels are
interesting materials for this purpose, as they offer the possibility
of tuning their textural properties by adequate selection of the syn-
thesis parameters [33], and can be easily doped with nitrogen by
incorporation of a nitrogen precursor during the synthesis [34,35].
Moreover, the synthesis does not involve contact with any transi-
tion metals.
Therefore, N-doped carbon xerogels were synthesized from
resorcinol, formaldehyde and melamine or urea as nitrogen
sources. In this way, carbon materials free from any transition metal
contamination and with different nitrogen contents were obtained
and subsequently tested as catalysts in the oxidation of oxalic acid
by CWAO and ozonation.
nature to direct oxidation by dissolved molecular ozone, and to
its common presence as an accumulated final product in several
processes of oxidation of organic pollutants [37].
Ozonation experiments were performed at room temperature
and pressure in a stirred semi-batch tank reactor. A volume of
−1
7
00 mL of solution was used, consisting of a 90 mg L
solution
of oxalic acid prepared with milliQ ultrapure water. Ozone gen-
erated from pure oxygen using a BMT 802X ozone generator was
bubbled into the bulk of the solution using a diffusor (total flow
3
−1
−3
rate = 150 cm min ; ozone concentration = 50 g m ). Ozone in
the gas phase was monitored using a BMT 964 ozone analyser.
The powdered catalysts (100 mg, particle size between 0.1 and
0
.3 mm) were introduced into the solution before starting gas flow
admission and kept in suspension by stirring at 200 rpm. Blank
no catalyst) and adsorption (no ozone) tests were also performed.
In the adsorption experiment, pure oxygen was kept flowing to
maintain the mixing conditions of the ozonation tests. The ozona-
tion reactions were carried out at the natural pH of the oxalic
acid solution (around 3), no buffer being added. The homogeneous
decomposition of ozone into hydroxyl radicals is not expected at
this pH [37].
(
2
. Experimental
CWAO experiments with oxalic acid were performed in a 160 mL
3
16-SS high pressure batch reactor housing a glass liner (Parr
2
.1. Materials
Instruments, USA Mod. 4564). 50 mg of catalyst with a particle size
between 0.2 and 0.3 mm were added to 75 mL of a 1000 mg L
solution and placed into the reactor. The reactor was flushed with
pure nitrogen till complete removal of oxygen, pressurized with
5
ature (140 C) under continuous stirring at 500 rpm in order to
ensure proper mass transfer of oxygen in the liquid phase [12,38].
When the desired temperature was reached, pure air was injected
to obtain a total pressure of 40 bar inside the reactor (correspond-
ing to 7 bar of oxygen partial pressure), this being considered time
zero for the reaction. In addition, non-catalytic wet air oxidation
−
1
A series of nitrogen-doped carbon xerogels was prepared as
described elsewhere [36]. In brief, a gel was prepared using resor-
cinol and formaldehyde, together with a nitrogen precursor, either
urea or melamine. Before gelation, the pH of the solution was
adjusted to the desired value (5.3, 6.0, or 6.9). Afterwards, the
gels were carbonized at different temperatures under nitrogen
bar of nitrogen and then pre-heated up to the desired temper-
◦
◦
flow (500, 700 or 900 C). The materials prepared with urea were
named CXU, and those prepared with melamine were named CXM,
followed by the pH of the solution prior to gelation, and the car-
bonization temperature.
The carbon xerogel samples selected for the present work were
CXM 6.9 700, CXM 6.9 900, and CXU 6.9 700, since they have rel-
atively similar textural properties and different types and amounts
of nitrogen surface groups. In addition, a carbon xerogel sample
without nitrogen was prepared from resorcinol and formaldehyde
(WAO) and adsorption experiments (absence of oxygen, 40 bar of
N ) were carried out.
2
Reutilization tests were carried out with the CX sample show-
ing the best performance, in order to evaluate the stability of the
catalyst in both processes. The catalyst was recovered at the end of
◦
each run, washed with distilled water and dried at 110 C. It was
◦
at pH 6.9, and carbonized at 500 C (CX 6.9 500).
then tested for a second time using fresh oxalic acid solutions. The
same procedure was repeated before testing the sample in a third
run. In order to obtain additional mechanistic information, both
CWAO and catalytic ozonation experiments were performed with
the best CX sample in the presence of the radical scavenger tert-
butanol (t-BuOH) with a concentration ten times higher than oxalic
2.2. Catalyst characterization
The textural characterization of the materials was based on the
◦
surface oxygenated groups were determined by temperature
Reproducibility tests showed relative errors lower than ± 5%.
Please cite this article in press as: R.P. Rocha, et al., Nitrogen-doped carbon xerogels as catalysts for advanced oxidation processes, Catal.