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Catalysis Today 249 (2015) 59–62

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Carbonized polyacrylonitrile fibers for the catalytic ozonation of oxalic acid

Alexandra G. Gonc alves ∗, Jéssica Moreira, Juliana P.S. Sousa, José L. Figueiredo,Manuel F.R. Pereira, José J.M. ÓrfãoLaboratory of Catalysisand Materials– Associate Laboratory LSRE/LCM,Faculty of Engineering, University of Porto, RuaDr.Roberto Frias,4200-465Porto,

Portugal

a r t i c l e i n f o

 Article history:

Received 13 August 2014Received in revised form17 December 2014Accepted 28 December 2014Available online 25 January 2015

Keywords:

Catalytic ozonationPolyacrylonitrile fibersNitrogen-containing groupsSurface chemistryOxalic acid

a b s t r a c t

Textile polyacrylonitrile (PAN) fibers, which contain nitrogen, were carbonized at different temperaturesand activated during 10 h in order to assess their performance as ozonation catalysts in the oxalic aciddegradation The fiber carbonized at 800 ◦C shows the best performance due to its basic character and,mainly, the high concentration of  nitrogen surface groups, particularly N-pyridinic (N6) groups. Thecarbonization of PAN fibers is a simple and effective method to prepare materials with good catalyticactivity in ozonation processes

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Catalytic ozonation has emerged as a powerful treatment of water pollutants, even for refractory organic compounds. Carbonmaterials,such as activated carbon [1–15], carbonxerogels[16] andcarbon nanotubes [17–22], have shown to be promising materialsas ozonation catalysts. Some studies have reported that their text-ural properties and, mainly, the chemical surface properties playan important role in their performance as ozonation catalysts. Inthe case of the surface chemistry, it is well known that basic car-bon materials favor the formation of hydroxyl radicals, which arespecies capable to quickly react with organic pollutants in solution[2,18]. Furthermore, the basicity of the carbon materials favors theadsorption of those organic compounds and their oxidation by sur-face reactions [2,4,18]. The basic character of the carbon materialsis originated by a high density of  electrons on the basal planes.Therefore, this basicity could be promoted removing oxygenatedelectron-withdrawing groups and/or introducing some convenientfunctionalgroups,suchas different nitrogenatedgroups (pyridines,

∗ Corresponding author: Tel.: +351 225 084871; fax: +351 225 081449.E-mail addresses: agg@fe.up.pt (A.G. Goncalves), ext12035@fe.up.pt(J. Moreira),

 juliana.sousa@fe.up.pt (J.P.S.Sousa), jlfig@fe.up.pt(J.L.Figueiredo), fpereira@fe.up.pt(M.F.R. Pereira), jjmo@fe.up.pt (J.J.M. Órfão).

pyridones and pyrroles), which contribute to the increase of theelectronic density on the material surface [23].

Nitrogen-containing groups can be introduced afterpreparationof carbon materials or during the material synthesis by severaltreatments. Among them, nitric acid oxidation, reaction of sur-face carboxyl groups with diamine compounds, and treatmentat high temperatures with ammonia, ammonia-air, or ammonia-steam mixtures are commonly used [24]. Various types of carbonmaterials with incorporated nitrogen (carbon xerogels [25], carbonnanotubes [26] and activated carbons [27,28]) have been tested incatalytic ozonation. These studies have reported that the presenceof nitrogenated groupson the surface of carbonmaterialsimprovestheir catalytic performance.

Adequate carbon materials having nitrogenated groups canbe synthesized by carbonization of nitrogen-containing precur-sors, like polymers with nitrogen in their structure. This is thecase of polyacrylonitrile (PAN) fibers, produced by polymeriza-tion from acrylonitrile and vinyl acetate monomers, which containhigh amounts of nitrogen [29]. In this approach, chemical treat-ments forthe introduction of nitrogenated groupsare not required,decreasing the cost and time consumed for the synthesis of carbonmaterials enriched with nitrogen.

Therefore, this work aims to carbonize textile polyacrylonitrilefibers at different temperatures and expose then to 10h of activa-tion in order to test them as catalysts in the ozonation of oxalic

http://dx.doi.org/10.1016/j.cattod.2014.12.0450920-5861/© 2015Elsevier B.V. All rights reserved.

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60  A.G. Goncalves et al. / Catalysis Today 249 (2015) 59–62

acid, which is a common final oxidation product of several organicpollutants and is refractory to single ozonation.

2. Experimental

 2.1. Preparation of materials

The materials to be used as ozonation catalysts were prepared

from textile PAN fibers (FISIVON) supplied by FISIPE, Portugal,which were subsequently knitted in the facilities of the Techno-logical Centre for the Textile and Clothing Industries of Portugal(CITEVE).

Pre-treatment of the fibers was carried out by heating 5 g of theoriginal fibers up to 300 ◦C in a tubular reactor. A heating rate of 1 ◦Cmin−1 under a constant air/N2 flow of 85cm3 min−1 was used,and the final temperature was maintained for 2 h. The fibers werethen carbonized by raising the temperature at 5 ◦Cmin−1 upto thedesiredtemperature(800,850or900◦C),whichwasmaintainedfor1 h [30]. The study of the effect of the fiber activation in its perfor-mance as ozonation catalyst was carried out using the carbonizedsamplethat presented the highest catalytic activity (i.e. carbonizedat800 ◦C).Thus, furtheractivation wasobtained by raising the tem-perature again at 15 ◦Cmin−1 to 800 ◦C, but then switching the N2

flow to a CO2  flow for a specific time [29]. The reference given toeach sample indicates carbon fiber (FI) as well as the respectivecarbonization temperature (800–900◦C) and activation time (0 or10h).

 2.2. Characterization

The textural characterization of the materials, namely theBrunauer–Emmett–Teller (BET) surface area, was based on the N2adsorptionisotherms,determinedat−196 ◦CwithaQuantachromeNOVA 4200e apparatus.

The surface chemistry of the prepared samples was character-ized by temperature programmed desorption (TPD), in order toquantify the oxygenated groups [31], and by X-ray photoelectron

spectroscopy (XPS). TPD analyses were performed in an AMI-200(Altamira Instruments) apparatus. Helium was used as carrier gas(25cm3 min−1) and the temperature was programmed from roomtemperature to 1100◦Cataheatingrateof5 ◦Cmin−1 The amountsof CO and CO2   released from the samples were monitored witha Dymaxion mass spectrometer (Ametek Process Instruments).XPS analyses were performed in a Kratos AXIS Ultra HSA usinga monochromatic Al K X-ray source (1486.7 eV), operating at15kV (90W), in FAT mode (Fixed Analyser Transmission), witha pass energy of 40eV for regions of interest and 80eV for sur-vey.

Elemental analysis was performed in a Carlo Erba instrument,model EA 1108.

 2.3. Kinetic experiments

The ozonation experiments were carried out in a laboratoryscale reactor (ca. 1L) equipped with agitation and a circulation

 jacket. Ozone was producedfrompureoxygeninaBMT 802X ozonegenerator. The concentration of ozone in the gas phase was mon-itored with a BMT 964 ozone analyzer. Ozone leaving the reactorwas removed in a series of gas washing bottles filled with potas-sium iodide (KI) solution.In each ozonationexperiment the reactorwas filled with 700mL of a solution containing 1 mM of oxalicacid, at natural pH (approximately 3). Oxalic acid was purchasedfrom Sigma–Aldrich. All solutions were prepared with ultrapurewater with a resistivity of 18.2 m cm at room temperature. Incatalytic ozonation experiments, 100mg of catalyst was intro-

duced in the reactor. The experiments were performed at constant

 Table 1

Textural properties of the prepared samples.

Sample S BET (m2 g−1) V pore (cm3 g−1) S  /=   pore (m2 g−1)

FI 800 ◦C 0 h <0.5 – –FI 800 ◦C 10 h 117 0.050 20FI 850 ◦C 0 h <0.5 – –FI 900 ◦C 0 h <0.5 – –

gas flow rate (150cm3 min−1) and constant inlet ozone concen-tration (50 g m−3). The stirring rate was maintained constant at200rpm, in order to keep the reactor content perfectly mixed. Forcomparative purposes, adsorption experiments were performedin the same system, under identical conditions. All experimentswere performed at room pressure and temperature. Samples foranalysis were collected at selected times using a syringe andcentrifuged. The experiments were carried out in duplicate andthe maximum relative deviation obtained was 1%. The analyticalmeasurements were also performed in duplicate with a maxi-mum relative error of ±0.5%. Concentration of oxalic acid wasfollowed by HPLC, according to the procedure described in refer-ence [18].

3. Results and discussion

 3.1. Characterization of catalysts

The textural properties of the materials are shown in Table 1.The carbonized fibers have negligible surface areas regardless of the temperature used. However, activation has a strong impact onthe textural properties. In fact, the activated sample presents a BETsurface area of 117 m2 g−1.

XPS analysis was performed for all prepared materials in orderto obtain additional information about the nature of the N bond-ing. According to the literature [30,32–34], the N1s XPS spectracan be decomposed in four peaks (1) at about 398eV, attributed to

pyridinic-N (N6); (2) between 400.0 and 400.9eV, correspondingto pyrrolic-N or pyridone-N (N5); (3) in the range 401.4–401.7 eV,attributed to quaternary nitrogen (N-Q); and (4) between 402 and403eV, related to certain forms of oxidized nitrogen (N-X). How-ever, in the XPS N1sspectra of all prepared samples only twopeakswere found, as can be observed in Fig.1. These peaks are associatedto the presence of pyridinic groups (N6) and quaternary nitrogen(N-Q)onthefiberssurface.Thenitrogencontentsobtainedfromthedeconvolution of the XPS N1s spectra and from elemental analysisare presented in Table 2.

The original fiber has a large amount of nitrogen (24%) [30]. Asexpected, increasingthe carbonization temperature,the bulknitro-gen amount (obtained from elemental analysis) decreases, as wellas the total nitrogenated surface. Further activation also leads to a

reduction of both bulk and surface nitrogen.The amounts ofCO and CO2 released, obtained by integration of the areas under TPD profiles, the ratio CO/CO2  and the mass per-centage of oxygen (mO%) on thesurface of thecarbon materials areshown in Table 2. For all the samples, the amount of CO-releasinggroups is larger than the amount of groups which decompose intoCO2, indicating their basic character, which is in agreement withtherespective pHpzc values (between 7.2 and 8.5 for FI 800 ◦C 10 hand FI 850 ◦C 0 h, respectively), as reported in theliterature [30]. Asthe temperatures used in the preparation of samples are higher orequal to 800 ◦C, the oxygenated surface groups present are mainlyneutral and basic groups like carbonyls/quinones [31], which havea positive effect on the ozonation process [2,4,18]. The increase of carbonization temperatures or the activation lead to a decrease inthe amount of CO-releasing groups.

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B. E. (eV)410

 

405 

400 

395

FI_850ºC_0h   I  n   t  e  n  s   i   t  y 

   (  a .  u .

   )

B. E. (eV)410 405 400 395

FI_900ºC_0h   I  n   t  e  n  s   i   t  y 

   (  a .  u .

   )

410 405 400 395

B. E. (eV)

FI_800ºC_0h

   I  n   t  e  n  s   i   t  y 

   (  a .  u .   )

410 405 400 395B. E. (eV)

FI_800ºC_10h   I  n   t  e  n  s   i   t  y 

   (  a .  u .

   )

Fig. 1. N1s XPSspectra forthe prepared samples.

 Table 2

Chemical characterization of the prepared samples.

Sample   N6 N-Q Ntotal, XPS   (%, m/m) Ntotal, EA   (%, m/m) CO (mol g−1) CO2 (molg−1) CO/CO2   mO%

B.E.(eV) (%, m/m) B.E. ( eV) (%, m /m)

FI 800 ◦C 0 h 398.6 4.6 400.9 4.6 9.2 13.7±0.3 4687 174 26.9 15.3FI 800 ◦C 10 h 398.5 2.3 401.2 3.4 5.7 10.3±0.2 1009 92 11.0 3.4FI 850 ◦C 0 h 398.6 1.9 401.3 4.3 6.2 12.5±0.3 3400 138 24.7 11.1FI 900 ◦C 0 h 398.8 2.6 401.1 3.3 5.9 10.8±0.2 1359 87 15.7 4.5

 3.2. Kinetic experiments

In the present study, the ozonation of oxalic acid, at the naturalpH(approximately3), inthe presence ofthe carbonized oractivatedfibers was investigated. This carboxylic acid was selected becauseit has been identified among the most common oxidation prod-

ucts from organic pollutants degradation and is refractory to singleozonation in references [4,18,35]. The kinetic results are depictedin Fig. 2. Besides the degradation profile, the oxalic acid removalafter 360 min of reaction was the value used to compare the cata-lysts. Ozonation of oxalic acid leads directly to mineralization, i.e.there is no formation of organic intermediates.

The adsorption capacity of the samples towards oxalic acid wasdetermined. The results indicated that adsorption on the prepared

Fig. 2. Evolution of the dimensionless concentration of oxalic acid at natural pH(∼3) during single ozonation, ozonation catalyzed by the prepared samples and

adsorption (C 0 =1 mM, catalyst=0.14gL −1

).

catalysts does not contribute to the removal of oxalic acid (seeFig. 2), and can therefore be neglected. Thus, it can be consideredthat the removal of oxalic acid in the presence of ozone and theprepared samples mainly occurs by catalytic ozonation.

The influence of carbonization temperature in the catalyticactivity of the carbonized PAN fibers for ozonation of oxalic acid

can be evaluated by comparison of the catalytic performance of the followingsamples: FI 800◦C 0 h, F I 850 ◦C 0 h and F I 900 ◦C 0 h(see Fig. 2). It can be verified that the activity of these materials isnot directly related to the temperature of carbonization, since theworst performance was obtained with the fiber carbonized at theintermediate temperature (850◦C). The non-activated materialshave similar textural properties, since the surface area is negligi-ble for all of them, but different performances in the oxidation of oxalic acid. In this context, the presence of pyridinic groups (N6)seems to decisively influences the catalysis: the higher the con-centration of N6 groups on the surface of carbonized samples, thebetter is its performance. However, this correlation is not strong(see Fig. 3) probably because the basic oxygen-containing sur-face groups present also play a role in the catalytic mechanism

[2,4,18]. Nevertheless, the presence of pyridinic groups (N6) oncarbonized fibers contributes positively to the oxalic acid removal,since they are Lewis bases inducing basicity at the carbon surfaceby increasing the electron density [23,36] and, therefore, increas-ing the number of active sites for ozonation. In order to assess theinfluence of the activation in the performance of the materials, thekinetic results obtained with FI 800◦C 0 h and FI 800 ◦C 10h werecompared. Fig. 2 shows that the activated sample (FI 800 ◦C 10 h )presents better performance for oxalic acid removal than the non-activated sample (FI 800◦C 0 h) in thebeginning ofreaction, whichmay be mainly justified by its high specific surface area, whereoxalic acid and molecular ozone can adsorb and react. However,this trend is inverted after 120min and, thus, the non-activatedsample leads to a faster removal of oxalic acid from the solution.

These results indicate that the specific surface area of the studied

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1.5  2.0 2.5 3.0 3.5 4.0 4.5  5.0

75

80

85

90

95

100

   X  o  x  a   l   i  c  a  c   i   d

   (   %   )

N6 groups (%)

R = 0.8906

Fig. 3. Oxalic acid conversion after360 min of reactionvs. N6content ofcarbonizedsamples (FI 800◦C 0h , F I 850 ◦C 0h and FI 900 ◦C 0h ).

materials has not the most important role in the ozonation of oxalic acid, suggesting again that the reaction is mainly favored bythe presence of nitrogenated groups and basic oxygenated surface

groups (CO-releasing groups).

4. Conclusions

PAN fibers containing high amounts of nitrogen were car-bonized at different temperatures and further activated during10h. The prepared samples were tested as catalysts in the oxalicacid degradation by catalytic ozonation.

The influence of the surface chemistry was evidenced by com-parison of carbonized fibers,for which thesurface area is negligible.The presence of nitrogenated groups on the surface, particularlypyridinic (N6) groups, favors the oxidation of oxalic acid.

The activation of the carbonized fibers, which significantlyincreased their surface area, did not improve the performance,

stressing the important role of nitrogenated and basic oxigenatedsurface groups on the reaction mechanism.Thefibercarbonizedat800◦Cpresentedthebestperformancein

the removal of oxalic acid due to itsbasic character and the relativehigh concentration of pyridinic (N6) groups on the surface.

The carbonization of PAN fibers is a simple and effective methodto prepare materials with good catalytic performance in ozonationprocesses.

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