Published in Chemical Engineering Journal, Volume 293, 1 June 2016, Pages 102–

111

Carbon nanofibers doped with nitrogen for the continuous catalytic ozonation of organic pollutants

J. Restivoa, E. Garcia-Bordejéb, J.J.M. Órfãoa, M.F.R. Pereiraa

aLaboratório de Catálise e Materiais (LCM), Laboratório Associado LSRE-LCM,

Departamento de Engenharia Química, Faculdade de Engenharia,

Universidade do Porto, Rua Dr. Roberto Frias, 4200-465

bInstituto de Carboquimica (ICB-C.S.I.C.), Miguel Luesma Castán 4, 50018

Zaragoza, Spain

Corresponding author: M. F. R. Pereira, fpereira@fe.up.pt

Abstract

Catalytic ozonation using carbon materials, in particular nanocarbons, has been

appointed as an interesting alternative for the abatement of recalcitrant

emerging organic pollutants. Efforts to achieve more efficient catalysts have

been carried out, including carbon doping with heteroatoms. In this study, the

effect of nitrogen doping of carbon nanofibers in their catalytic activity for the

ozonation of organic pollutants was assessed. For this end, pristine and N-

doped carbon nanofibers were prepared, both in powder and in structured

forms. The former were tested in semi-batch ozonation experiments, while the

latter were used in continuous ozonation experiments. It was observed that the

presence of N-containing functionalities on the surface of the carbon nanofibers

enhances their capability as catalysts for the studied reaction.

Keywords: ozonation, nitrogen-doped nanocarbons, structured catalysts

1. INTRODUCTION

Recent findings of the resistance of harmful products to conventional water

treatments, as well as of the toxicity associated with the degradation products of

these compounds, have led to a growing interest in the development of novel

water treatment technologies that may efficiently remove recalcitrant emerging

organic pollutants [1]. In particular, advanced oxidation processes such as

catalytic ozonation have been shown to be an interesting method for the

abatement of such pollutants [2-7].

Different materials have been used as catalysts for the ozonation process [8]. In

particular, carbon materials such as activated carbons [2,3, 9-11], multi-walled

carbon nanotubes (MWCNT) [12-16], carbon xerogels [17] and carbon

nanofibers (CNF) [18-21] have been shown to have the potential to be effective

solutions.

The modification of the surface properties of carbon materials have been known

to influence their catalytic properties [22, 23]. In particular, the presence of

surface heteroatoms, such as oxygen, sulfur and nitrogen, has been shown to

significantly affect the catalytic activity of carbon materials in catalytic ozonation

[2, 16, 24-29].

The application of structured catalysts, in particular as honeycomb monoliths,

has been appointed as an interesting alternative for the application of carbon

materials as catalysts during the ozonation process [18-21].

In this work, a study of the influence of the presence of N-containing

functionalities on the surface of CNF was carried out. For this end, catalytic

ozonation experiments were performed in semi-batch and continuous operation,

using CNF in powder form and in structured form, respectively. Oxalic acid and

phenol were selected as model compounds, and further ahead atrazine,

metolachlor and nonylphenol were used as model organic micropollutants.

2. METHODS AND MATERIALS

2.1 PREPARATION OF CATALYSTS

2.1.1 POWDER CARBON NANOFIBERS

Carbon nanofibers in powder form were prepared using a 20 % Ni on Al2O3

catalyst, which was previously reduced under H2 at  550 ºC. Carbon growth was

carried out using a C2H6:H2 (50:50) mixture at 600 ºC. Nitrogen functionalities

were introduced on the surface of the carbon nanofibers replacing H2 by NH3

during the growth phase. Both samples (without and with nitrogen) CNF and N-

CNF were purified from the growth catalyst first under NaOH reflux at 80 ºC for

4 h and later under HCl reflux at 100 ºC for 4 h. After purification, less than 1 wt

% residual catalyst remains on the carbon material.

2.1.2 STRUCTURED CARBON NANOFIBERS

A well attached layer of entangled CNFs was grown on the walls of cordierite

monoliths as reported elsewhere [30]. In brief, cordierite monoliths (from

Corning, 22 mm diameter, 60 mm length, 400 cpsi) were washcoated with

alumina by a dip-coating method similar to the sol-gel coating described by

Nijhuis et al. [31]. Nickel was deposited by adsorption from a pH-neutral nickel

solution as described elsewhere [32]. To growth the CNFs, the Ni/alumina

coated monolith was fitted in a quartz reactor by wrapping it in quartz band. The

reduction of the calcined catalyst was carried out in a hydrogen atmosphere at

550 ºC for 120 min (heating rate of 5 ºC/min). The monolith was then heated (5

ºC/min) to 600 ºC. When this temperature was reached, 100 cm3 (STP)/min of a

C2H6:H2 (50:50) gas mixture was fed. For N-doping of grown CNF, H2 was

replaced by NH3 in the gas feed [33]. The CNF growth was allowed to proceed

for 2 hours following up by cooling down under inert atmosphere.

2.2 CHARACTERIZATION OF CATALYSTS

The textural characterization of the prepared carbon nanofibers was carried out

by N2 physisorption at -196 ºC using a Micromeritics ASAP 2020 apparatus,

after outgassing for 4 h at 150 ºC. The pore volume was calculated from the

adsorbed amount at a relative pressure of 0.99. The specific surface area was

calculated by the BET (Brunauer, Emmet and Teller) method in the relative

pressure range 0.01–0.10 following the ASTM-4365 standard.

The nature of the N-containing functionalities introduced on the surface of the

carbon nanofibers was characterized by XPS using a ESCAPlus Omnicrom

equipped with a Mg Kα radiation source to excite the sample. Calibration of the

instrument was performed with Ag 3d5/2 line at 368.27 eV. All measurements

were performed under ultra-high vacuum better than 10-10 torr. Internal

referencing of spectrometer energies was made using the C 1s signal at 284.6

eV. The curve fitting of the spectra was performed using CASA XPS software

after applying a Shirley baseline. For peak deconvolution, the full width at half

maximum (FWHM) was fixed equal for all the peaks and with a maximum value

of 2.5 eV.

2.3 EVALUATION OF CATALYSTS

The prepared catalysts were evaluated in the catalytic ozonation of organic

pollutants using two systems: a semi-batch system where the catalysts were

used in powder form and a continuous ozonation system where the catalysts

were used in their structured form. The powder samples are identified as CNF

and N-CNF (without and with nitrogen, respectively), and the honeycomb

monolith structured samples are identified as HM-CNF and HM-N-CNF (without

and with nitrogen, respectively).

The selected organic pollutants included oxalic acid, phenol, atrazine (ATZ),

metolachlor (MTLC) and nonylphenol (NLP).

The semi-batch ozonation experiments were carried out using a conventional

stirred tank reactor. Ozone was produced from pure oxygen using a BMT 802N

ozone generator, at 50 g m-3 (STP), and introduced into the reactor using a

glass disperser at 150 cm3 (STP) min-1. A volume of 700 mL of solution

containing the pollutants at the desired concentration was used, and the amount

of powder catalyst used, when applicable, was 100 mg. The solution was kept

homogeneously stirred using a magnetic stirrer at 200 rpm.

The continuous ozonation experiments were carried out using a bubble column

containing an internal loop. Ozone was produced from pure oxygen using a

BMT 802N ozone generator, at 50 g m-3 (STP), and introduced into the reactor

using a glass disperser at the bottom of the bubble column at 20 cm3 (STP) min-

1. The solution containing the selected pollutant was fed into the reaction

system using a peristaltic pump at 12 mL min-1, and the internal loop was kept

flowing at all times at 60 mL min-1. The structured catalysts were placed inside

the bubble column, thus operating in a multiphase flow where the gas and liquid

simultaneously contact with the solid phase. The reaction conditions were

optimized to achieve homogeneous axial dispersion of the phases throughout

the channels of the monoliths, and to allow the formation of Taylor flow, which is

known to enhance the performance of such systems due to decreased mass

transfer resistance [34].

The concentration of oxalic acid and other organic acids was followed using an

Elite LaChrom HPLC coupled with a UV-Vis detector. Separation was achieved

using an Alltech OA-1000 chromatography column with a 5 mM H2SO4 mobile

phase and detection was carried out at 200 nm. The concentrations of atrazine,

metolachlor, nonylphenol and phenol were followed using an Elite LaChrom

HPLC coupled with a DAD detector. Separation was achieved using a

Lichrocart C18-RP Puroshper Star chromatography column. For atrazine and

phenol a MeOH:H2O mobile phase was used, and detection was carried out at

222 nm. For metolachlor an ACN:H2O mobile phase was used and detection

was carried out at 196 nm. For nonylphenol an ACN:H2O mobile phase was

used and detection was carried out at 190 nm.

Acute toxicity analyses were performed using an Azure Environment Microtox

apparatus and procedure ISO/DIN 11348-3. The microorganisms used were the

luminescent bacteria Vibrio fischeri from Hach Lange, which is used as

representative of aquatic environments. The bacteria were exposed to samples

after activation and 15 min incubation at 15 ºC, and the decrease in activity as

function of the luminescence was measured after 30 min.

Total organic carbon (TOC), measured with a Shimadzu TOC-5000A apparatus,

was used to assess the mineralization degree.

3. RESULTS AND DISCUSSION

3.1 CHARACTERIZATION OF FRESH CATALYSTS

3.1.1 POWDER CARBON NANOFIBERS

Textural characterization of the pristine and N-doped powder CNF is presented

in Table 1.

Table 1 – Textural characterization of the pristine and N-doped CNF.

CNFBET Area

(m2 g-1)

Pore

Volume

(cm3 g-

1)

Average pore

Size

(nm)

CNF 151 0.39 9.1

N-CNF 318 0.62 6.2

Some differences are observable in the textural properties of pristine and N-

doped CNF. Particularly, the specific surface area calculated by the BET

method showed a significant increase when the N-CNF sample is considered.

An increase in the pore volume was also observed, while the average pore size

decreased.

The N-doped carbon nanofibers were characterized by XPS. The amount of

each nitrogen species identified on the surface of the N-doped CNF are

presented in Table 2.

Table 2 – Relative amounts of nitrogen groups on the surface of N-doped CNF

determined by XPS analysis.

Sample

PyridinicGroups

PyrrolicGroups

QuaternaryGroups

N/CBE

(ev)

RA

(%)

BE

(ev)

RA

(%)

BE

(ev)

RA

(%)

As-grown

N-CNF398.4 69.7 399.8 1.3 400.8 29.0 0.026

Purified N-

CNF398.4 43.6 399.8 25.8 400.8 30.6 0.021

Where: BE – Binding energy; RA – Relative amount

Pyridinic and quaternary nitrogen functionalities were detected by XPS on the

surface of the as-grown N-CNF, together with a small amount of pyrrolic groups.

The analysis carried out after purification with NaOH-HCl showed changes in

the relative amounts of these functionalities. Namely, a much larger relative

amount of pyrrolic groups was identified. The purified N-CNF fibres were used

for the catalytic experiments presented below.

3.1.2 STRUCTURED CARBON NANOFIBERS

The characterization of the structured CNF catalysts has been thoroughly

described elsewhere, for both pristine [30, 35] and N-doped [33] carbon

nanofibers.

For brevity, only the nature and amount of the N-containing functionalities found

on the surface of the HM-N-CNF structured catalyst are reproduced here (Table

3).

Table 3 – Characterization by XPS of nitrogen groups on the surface of N-

doped structured CNF (HM-N-CNF).

Sample

Pyridinicgroups

Pyrrolicgroups

Quaternarygroups

Oxidized N

N/C

BE

(ev)

RA

(%)

BE

(ev)

RA

(%)

BE

(ev)

RA

(%)

BE

(ev)

RA

(%)

HM-N-CNF 398.4 52.6 400.3 14.4 401.2 25.2 404.5 7.8 0.074Where: BE – Binding energy; RA – Relative amount

When compared with the powder N-CNF, a wider relative amount of nitrogen

was found in the structured catalyst. Furthermore, some oxidized nitrogen was

also detected in this sample.

3.2 OZONATION EXPERIMENTS

3.2.1 POWDER CARBON NANOFIBERS

The first approach to the evaluation of the influence of N-doping of CNF on their

performance as catalysts in ozonation was carried out using oxalic acid as a

model compound, in a semi-batch system. The dimensionless removal of oxalic

acid during catalytic ozonation using pristine and N-doped CNF samples is

presented in Figure 1. The result obtained by single ozonation is also included

for comparison purposes.

It is clear from Figure 1 that the addition of CNF (either pristine or N-doped) to

the ozonation system enhanced the removal of oxalic acid from solution. The

catalytic activity of carbon nanomaterials in this reaction is well-known [12, 16,

18-21, 24, 36, 37].

On a more interesting note, similarly to what was reported for multiwalled

carbon nanotubes [38], the N-doped CNF showed better performance in the

removal of oxalic acid than the pristine CNF sample. It has been previously

reported that the introduction of nitrogen-containing functionalities on the

surface of carbon materials might enhance their catalytic activity. Studies

performed using activated carbons have shown that the inclusion of basic

nitrogen functionalities on the surface enhances their catalytic activity in the

ozonation reaction; this effect has been attributed to either an increase in the

rate of surface reactions due to larger availability of free electrons on the

surface, or to the reaction of ozone with pyrrol surface functionalities leading to

the formation of hydroxyl radicals that are released and are able to further react

with the organic pollutants in the liquid bulk [25, 26, 28]. On the other hand,

studies carried out using carbon xerogels [29] and carbon nanotubes [38] have

suggested that the main factor for the improvement of the catalytic activity by

introduction of nitrogen functionalities is the increase in the surface density of

free-electrons.

The results obtained in the catalytic ozonation of oxalic acid support the idea

that the increase in electronic density on the surface, promoted by the N-

functionalities, favours the reduction of ozone, as recently proposed [25, 26, 28,

29, 39]. It is interesting to notice that the activity observed for the CNF samples

is very close to that observed for MWCNT [38], for both pristine and N-doped

samples.

The same samples were also used in the catalytic ozonation of phenol. The

dimensionless concentration of phenol during these experiments is presented in

Figure 2.

As it was observed when MWCNT were used [38], the presence of a catalyst in

the ozonation of phenol does not significantly alter its conversion, likely due to

its fast reaction with molecular ozone [40, 41].

The dimensionless concentration of TOC during ozonation experiments using

CNF catalysts is presented in Figure 3, as a measure of the mineralization

degree. While the mineralization degree was slightly improved with the addition

of the catalysts, there was not much difference between the pristine and the N-

doped CNF.

The concentration of the main primary intermediates formed (benzoquinone and

hydroquinone) was followed (not shown), and it was observed that the presence

of a catalyst did not significantly affect their evolution. Moreover, both

compounds were quickly removed from solution, even in the case of single

ozonation. A much more significant difference was observed in the case of

oxalic acid, which was observed to accumulate in solution throughout the

ozonation process until 3 h (duration of the experiments). The concentration of

oxalic acid during these experiments is presented in Figure 4a, while the final

accumulated TOC values at the end of the reactions, where the contribution of

oxalic acid and of other unidentified organic compounds is discriminated, is

presented in Figure 4b.

It is clear from Figure 4a that the addition of a catalyst to the ozonation system

reduces the amount of oxalic acid accumulated during the ozonation of phenol.

This effect might be attributed either to the enhanced removal of oxalic acid

from solution, and/or hindering of the formation of oxalic acid due to changes in

the reaction pathway.

Observation of Figure 4b indicates that the main contribution for the remaining

TOC after 3 h during experiments with CNF catalysts was the increase in the

released amount of organic intermediates other than oxalic acid, when

compared with the single ozonation experiment. While a slight increase in the

formation of these other organic intermediates was observed in the case of N-

CNF, a conclusive statement on the role of the N-containing functionalities in

this case cannot be formulated. The observed differences in the formation of

oxalic acid and of other organic intermediates for the CNF and N-CNF catalysts

are not sufficiently significant for such a conclusion. The presence of nitrogen

species on the surface of CNF are, however, expected to play a role in the

ozonation of phenol, due to changes in the electronic density on the surface of

the fibres [42]. Moreover, changes in the degradation mechanism of organic

pollutants during catalytic ozonation with N-containing carbon materials have

been previously reported [25, 26, 28].

3.2.2 STRUCTURED CARBON NANOFIBERS

After assessment of the influence of N-doping of CNF in the catalytic

performance for the ozonation process, continuous experiments were carried

out using the structured CNF catalysts.

First, the pristine and the N-doped CNF structured catalysts (samples HM-CNF

and HM-N-CNF, respectively) were tested in the continuous ozonation of oxalic

acid. The removals measured at steady state are presented in Figure 5.

While the removal of oxalic acid was less extensive when the HM-N-CNF

monolith was used, the normalization by the amount of carbon on the structured

catalysts inverts this trend. In fact, the HM-N-CNF structured catalyst allows the

removal at steady state of approximately more 0.05 mM of oxalic acid per gram

of CNF than the other sample. This observation agrees with what was observed

in semi-batch experiments, where the amount of catalyst used was the same for

the experiments with the different samples. Thus, the doping of the CNF with

nitrogen improves the activity of the CNF in the catalytic ozonation of oxalic

acid.

Further experiments were carried out using selected organic micropollutants:

atrazine, metolachlor and nonylphenol. When comparing the removals of the

parent pollutants in the single and catalytic ozonation experiments (not shown),

it is clear that the inclusion of a catalyst does not significantly alter the removal

achieved, due to quick reaction of these pollutants with ozone [9, 19, 21, 43-49].

Furthermore, no significant difference between the removals achieved was

observed when N-doped or pristine CNF structured catalysts were used. In fact,

there is no evidence to support the idea that surface reactions might be

improving the oxidation of these pollutants [12]. In the case of oxalic acid, it is

known that surface reactions are contributing to the oxidation of the pollutant

during ozonation, at similar conditions to those used here [2], and thus it was

possible to conclude about the role of the nitrogen functionalities in the

improvement of the removal rate obtained. The mineralization degree, as

measured by TOC removal, is a better measure of the efficiency of the

catalysts, and is much more closely related to surface reactions, or radical

reactions in the liquid bulk, than the oxidation of the parent pollutant [9, 12, 19,

21]. The mineralization degree of the three organic pollutants, obtained during

continuous ozonation experiments using HM-CNF and HM-N-CNF structured

catalysts, is presented in Figure 6. The mineralization degrees achieved at

steady state in the presence of the CNF and N-CNF structured catalysts were

higher than those observed in the case of the non-catalytic experiments (not

shown). This is attributed to the oxidation of intermediates resistant to direct

ozonation, by either reactions occurring on the surface of the CNF or by

reaction with hydroxyl radicals in the liquid bulk [12, 18, 19, 21]. On the other

hand, only slight differences were observed between the two catalysts; the CNF

sample showing a somewhat improved performance when compared with the

N-doped sample. However, similarly to what was concluded in the experiments

with oxalic acid, the TOC removal normalized by the amount of CNF is higher

when the N-CNF structure is considered. Thus, it is possible to conclude that

the presence of nitrogen on the CNF improves the rate of surface reactions

capable of oxidizing compounds resistant to direct ozonation, or increases the

production of hydroxyl radicals that may react with compounds resistant to

direct ozonation in the solution bulk.

The toxicity of the resulting effluents was also evaluated, using CNF and N-CNF

structured catalysts in the catalytic ozonation of the selected organic

micropollutants. However, since the toxicity is not inherent to a single

parameter, but rather due to the interaction between all the components in the

water sample, it is not possible to normalize the obtained inhibitions levels by

the mass of CNF on the catalyst. Thus, only the absolute values are presented

in Figure 7.

Observing the results of atrazine, it is clear that the application of a catalyst to

the ozonation system results in more toxic effluents [12]. Nevertheless, the HM-

N-CNF sample leads to a less toxic effluent than HM-CNF, probably due to

changes in the outlet distribution of the products of atrazine ozonation, resulting

in a less toxic mixture of components. In the case of metolachlor, the trend

follows closely what was observed in previous experiments [21]. The HM-N-

CNF produced a more toxic effluent than HM-CNF, despite decreasing the

toxicity relatively to the effluent subject to single (i.e., non-catalytic) ozonation.

Finally, nonylphenol did not present significant variations of the toxicity

measured at steady state.

4. CONCLUSIONS

The effect of the introduction of N-containing surface functionalities on carbon

nanofibers was assessed in the catalytic ozonation of organic pollutants. First,

the catalytic activity was evaluated using model pollutants in a semi-batch

system and powder carbon nanofibers. Afterwards, the catalysts were used in

their structured form. The removal of the parent pollutants, the mineralization

degree achieved and the toxicity of the resulting effluents were used as

parameters to assess the performance of the catalysts.

The application of N-doped carbon nanofibers as a structured catalyst

presented a better activity in the catalytic ozonation process. Besides

enhancing the catalytic activity of the carbon materials, the N-containing

functionalities were also observed to lead to changes in the degradation path of

organic micropollutants, which resulted in different toxicity levels in the resulting

effluents.

The role of the nitrogen surface functionalities was suggested to be related to

the enhancement of the rate of reactions occurring on the surface of the carbon

nanofibers, due to an increase in the electronic density. Nevertheless, the role

of reactions occurring in the liquid phase was also observed to be significant, at

least in the changes of the degradation pathways that were observed.

Acknowledgements: This work was co-financed by FCT and FEDER under

Programe PT2020 (Project UID/EQU/50020/2013). CEMUP and Dr. Carlos Sá

are acknowledged for XPS analysis. J. Restivo thanks FCT for research grant

SFRH/BD/95751/2012.

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LIST OF FIGURES

Figure 1 – Dimensionless concentration of oxalic acid during semi-batch

ozonation experiments carried out in the absence or in the presence of pristine

and N-doped carbon nanofibers (C0 =1 mM).

Figure 2 – Dimensionless concentration of phenol during the semi-batch

ozonation experiments carried out in the absence or in the presence of pristine

and N-doped carbon nanofibers (C0 = 0.797 mM ).

Figure 3 – Dimensionless concentration of TOC during the semi-batch

ozonation of phenol experiments carried out in the absence or in the presence

of pristine and N-doped carbon nanofibers (C0 = 0.797 mM).

Figure 4 – Concentration of oxalic acid (a), and dimensionless concentration of

TOC after 180 minutes with the contribution of oxalic acid and of other

unidentified organic compounds (b), during the semi-batch ozonation of phenol

carried out in the absence or in the presence of pristine and N-doped carbon

nanofibers (C0 = 0.797 mM).

Figure 5 – Removal of oxalic acid (Cinlet = 1mM) at steady state during

continuous ozonation experiments using unmodified (HM-CNF) and N-doped

CNF (HM-N-CNF) covered monoliths.

Figure 6 – Removal of TOC at steady state during continuous ozonation

experiments of atrazine (ATZ, Cinlet = 0.046 mM ), metolachlor (MTLC, C inlet =

0.070 mM) and nonylphenol (NLP, Cinlet = 0.027 mM) using unmodified and N-

doped CNF covered monoliths.

Figure 7 – Inhibition of luminescent activity of Vibrio Fischeri bacteria during

Microtox tests, after exposure to samples taken at steady-state during

continuous ozonation experiments of atrazine, metolachlor and nonylphenol

using CNF and N-CNF structured catalysts.

FIGURE 1

FIGURE 2

FIGURE 3

FIGURE 4

FIGURE 5

FIGURE 6

FIGURE 7