High temperature ammonia treatment of pitch

Fuel Processing Technology 119 (2014) 211–217

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High temperature ammonia treatment of pitch particulates and fibers for nitrogen enriched microporous carbons Urszula Świetlik, Bartosz Grzyb, Kamila Torchała, Grażyna Gryglewicz, Jacek Machnikowski ⁎ Division of Polymer and Carbonaceous Materials, Faculty of Chemistry, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland

a r t i c l e

i n f o

Article history: Received 24 July 2013 Received in revised form 8 November 2013 Accepted 17 November 2013 Available online 6 December 2013 Keywords: Pitch Particulate Fiber Ammonia activation Microporous carbon

a b s t r a c t One step pyrolysis/activation of fine powder and fiber from anthracene oil-based pitch in ammonia atmosphere was successfully applied for producing microporous nitrogen enriched carbons. In the process, the preoxidized precursor was heat-treated in ammonia flow at 2 K/min to the final temperature in the range of 300–900 °C with 0.5 h soak. The intensive reaction of pitch powder with ammonia at 750–800 °C results in N uptake of ~ 10 wt.% and generation of micropores of mean width 0.53–0.67 nm and very narrow pore size distribution (PSD). XPS analysis shows mostly pyridinic nitrogen. Not only increasing process temperature induces enhancement of micropore surface area (Smicro) to 900 m2 g−1 but also widening PSD and reduction of nitrogen content. Similar behavior is observed on the ammonia activation of pitch fibers. The advantage of using fibrous precursor is a higher nitrogen uptake (12 wt.%) which can be combined with Smicro of 880 m2 g−1 and narrow PSD around 0.8 nm. The ammonia activation of stabilized pitch fibers seems to be a feasible route for producing nitrogen enriched microporous activated carbon fibers of narrow PSD and controllable pore width. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Nitrogen is one of a few elements which can be closely associated with the graphene layers constituting carbon material, both as a substituent for carbon and a functional group attached at the edges. The presence of nitrogen atom modifies the electronic structure of graphene layer, thus inducing polarity and basicity of the material surface. In consequence, nitrogen enriched porous carbons exhibit improved performance when used as adsorbent [1–4], catalyst or catalyst support [5–7] and electrode of electric capacitor [8–10]. Recently published review paper [11] proves the continuous interest in this category of porous carbons. The treatment of activated carbon with ammonia at elevated temperature (amination) has been considered as relatively ready and efficient way for producing nitrogen enriched porous carbons. When the process is conducted below 500 °C, ammonia reacts with acidic oxygen surface groups to create mostly thermally unstable functionalities of amide, imide and lactam types [12]. In that case the preoxidation of activated carbon [3,11] or using NH3/air [11–14] or NH3/steam [15] mixtures has been proposed to enhance the amount of nitrogen bound. However, at a higher temperature etching of carbon material by ammonia can occur. As a result nitrogen atoms are being substituted for carbon in graphene layer to form pyrrole, pyridine, pyridone and quaternary functionalities [4,16].

⁎ Corresponding author. Tel./fax: +48 71 3206203. E-mail address: [email protected] (J. Machnikowski). 0378-3820/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2013.11.009

The amount of nitrogen incorporated using amination of activated carbon is moderate, usually it varies between 1.5 and 5 wt.%. More promising seems to be one step process in ammonia atmosphere covering organic precursor pyrolysis and residue activation. The direct ammonia treatments of phenol formaldehyde resin-based fibers [16] and resorcinol-formaldehyde cryogel [17] were reported to give, under appropriate conditions, a porous material with about 8 wt.% of nitrogen and surface area of 1400 and 1000 m2/g, respectively. The present work is an attempt to produce pitch-based nitrogen enriched microporous carbons in one step consisting of pyrolysis followed by residue activation in ammonia atmosphere. The effect of the treatment temperature was monitored by evaluation of the weight loss, nitrogen content and speciation and porosity development (N2 adsorption at 77 K) in the resultant material. The reactions contributing to the nitrogen uptake and porosity generation are proposed based on the analysis of solid residue and gas evolved. The temperature/time conditions are optimized in regard to the nitrogen content and the porosity development. The starting material for the study was high softening point and free from particulate matter isotropic pitch supplied by Industrial Quimica del Nalon, S.A. (IQNSA), Spain. The process developed by IQNSA consists basically in the oxidative thermal condensation of anthracene oil followed by heat-treatment/distillation of the reaction product [18,19]. The latter processing step can be adjusted to produce pitch with softening point of about 250 °C. In general, the interest in high softening point isotropic pitch is in the use as a potential precursor of general purpose carbon fibers, including activated carbon fibers. Basically we study here the behavior of fine

U. Świetlik et al. / Fuel Processing Technology 119 (2014) 211–217

212

11

0.16

10

0.14

Weight loss [%]

9 60 8 50 7 40 6 30 5 20

Weight loss

Pore volume [cm3/g]

Nitrogen content

70

Nitrogen content [wt%]

80

4 3

500

600

700

800

0.10 0.08 0.06 0.04

0.00

0 400

0.12

0.02

10

300

N-ACP/70 N-ACP/75 N-ACP/85 N-ACP/90

0.5

900

1.0

1.5

2.0

Pore width [nm]

Amination temperature [oC] Fig. 1. Variation of weight loss and nitrogen content with increasing amination temperature of AOPox powder.

Fig. 3. Pore size distribution of activated carbons prepared by amination of AOPox powder determined from N2 adsorption isotherms at 77 K using a QSDFT analysis.

pitch powder (b100 μm) during ammonia treatment in the temperature range of 300–900 °C, but the study is intended as a step to producing nitrogen enriched activated carbon fibers (N-ACF). To validate the process for fibrous precursor, the treatment of fibers spun from the pitch and stabilized using appropriate procedure was also performed at selected conditions. Direct nitrogen enrichment during pitch fiber heat-treatment could be of practical meaning.

2. Experimental

(a) 250

N-ACP/90 N-ACP/85

VN2(STP) [cm3/g]

200

N-ACP/80 150

N-ACP/75 100

N-ACP/70

2.1. Materials The isotropic anthracene oil-based pitch (AOP) used as starting material for the study is characterized by lack of ash and particulate matter and highly aromatic structure. Nitrogen and oxygen contents amount to 1.4 and 0.8 wt.%, respectively, and the softening point is 247 °C (Mettler) [20]. For amination the pitch was used in the form of fine particles and fibers. The powdered sample of pitch was prepared by grinding to pass the sieve of 100 μm and subsequent oxidation with air to cross-link the aromatic molecules by oxygen and change the thermoplastic character of pitch into thermosetting. The oxidation was performed according to the following heating program: heating rate of 1 K/min with a soak for 1 h at 180, 200, 220, 240 °C and for 2 h at 270 °C. The treatment resulted in the increase in oxygen content in AOPox to 8.8 wt.%. The fibers were used as received from the Instituto Nacional del Carbon in Oviedo, Spain. Freshly spun pitch fibers were stabilized by air oxidation in a multi-step process including slow heating to 270 °C [20].

50

N-ACP/65

2.2. Ammonia treatment

0 0.0

0.2

0.4

0.6

0.8

1.0

p/p0

(b)

250

N-ACP/90

VN2(STP) [cm3/g]

200

N-ACP/85

The reaction with ammonia was performed in a horizontal tube furnace. The sample, placed in a quartz boat, was heated at 2 K/min in ammonia gas flow of 9 dm3/h to the final temperature in the range of 300–900 °C with 0.5–2 h soak. The reaction products were labeled: N-ACP/30, N-ACP/40 … N-ACP/90 and N-ACF/70, N-ACF/80 for particle and fiber materials, respectively, typically treated at a given temperature for 0.5 h. The number giving time in hours was added when a longer soaking time was used.

N-ACP/80 150 Table 1 Porosity parameters of carbons produced by amination of AOPox powder at different temperatures.

N-ACP/75

100

N-ACP/70

50

N-ACP/65 0 0.00

0.02

0.04

0.06

0.08

0.10

p/p0 Fig. 2. N2 adsorption isotherms at 77 K of carbons produced by amination of AOPox powder measured at p/p0 from 10−7 to 0.98 (a) and from 10−7 to 0.1 (b).

Sample

SBET m2 g−1

VT cm3 g−1

VDR cm3 g−1

L0 Nm

VDR/VT

VDRCO2 cm3 g−1

N-ACP/60 N-ACP/65 N-ACP/70 N-ACP/75 N-ACP/80 N-ACP/85 N-ACP/90

n.d. 125 217 402 631 746 883

n.d. 0.044 0.094 0.176 0.264 0.327 0.382

n.d. 0.021 0.052 0.158 0.255 0.298 0.352

n.d. n.d. 0.77 0.53 0.67 0.83 0.98

n.d. 0.477 0.553 0.898 0.956 0.911 0.921

0.159 0.204 0.189 0.206 0.227 0.132 0.171


XPS experiments were done with a VSW spectrometer using Al K α radiation as excitation source (200 W, 10 keV). The possible contributions of five nitrogen forms with following binding energies have been considered in deconvoluting the complex N1s band of XPS spectra: 398.7 ± 0.3 eV — pyridinic nitrogen (N-6), 399.7 ± 0.2 eV — amide, imide, lactam and nitrile groups (N-C), 400.3 ± 0.3 eV — pyrrolic and pyridonic (N-5), 401.4 ± 0.5 eV — quaternary (N-Q) and 402–405 eV — pyridine-N-oxide (N-X).

1000

Smicro

Surface area [m2/g]

800

213

S0

600

400

3. Results and discussion

200

0 300

400

500

600

700

800

900

1000

Amination temperature [oC] Fig. 4. Variation of micropore (Smicro) and ultramicropore (S0) surface areas with increasing amination temperature of AOPox powder.

2.3. Characterization of ammonia treated carbonaceous materials The elemental composition (CHNS) was determined using a Vario EL analyzer. The oxygen content was analyzed directly using a Carlo Erba analyzer. The particle size distribution was measured in water by Malvern Mastersizer 2000 (laser 632 nm) with a drop of surfactant (Rocafenol N8) added to improve particle wettability. Scanning electron microscopy (JEOL, JSM 5800 LV) was used to assess the morphology of particles and fibers. The structural arrangement of heat-treated in ammonia and nitrogen samples was assessed using X-ray diffraction (Rigaku powder diffractometer Ultima IV). The structural parameters calculated from the spectra included the interlayer distance d002 and crystallite height Lc. The porous texture of the activated materials was characterized by nitrogen adsorption at 77 K (ASAP 2020, Micromeritics, p/p0 from 10−7 to 0.98) in Sol–Gel Nanotechnology Materials Laboratory of Lower Silesian Centre for Advanced Technologies in Wrocław. The adsorption data were used to determine the total pore volume VT, the specific surface area SBET and the microporosity development. The micropore volume VDR and the average micropore width L0 were computed from the application of the Dubinin–Radushkevich and Stoeckli equations [21], respectively, to the adsorption data in the p/p0 range of 2 × 10−5–0.05. The contribution of micropore volume to the total pore volume was assessed by the VDR/VT ratio. The QSDFT analysis [22] was applied to the N2 adsorption isotherms obtained using ASAP to determine pore size distribution (PSD) and micropore surface area (Smicro). CO 2 adsorption at 273 K was measured using NOVA 2200 (Quantachrome) apparatus to characterize porosity in the range of ultramicropores (b0.7 nm). The volume (VDR CO2) and surface area (S0) of the narrow micropores were calculated from the isotherms.

3.1. Mechanism of nitrogen binding and porosity developing in reaction with ammonia The behavior of oxidized pitch (AOPox) particles during heating in ammonia atmosphere was assessed based on the characteristics of a series of carbonaceous materials prepared by the treatment at different temperatures in the range of 300–900 °C and 0.5 h soak. Fig. 1 shows the variation of weight loss and nitrogen content in the residue with amination temperature of AOPox. Two temperature regions can be distinguished in the weight loss profile. Heating up to 700 °C induces rather moderate volatile evolution, above the temperature the weight loss profile becomes more steep. Characteristic of thermal behavior of different origin pitches is the intensive volatile evolution on heating between 350 and 600 °C with a little evolution at higher temperature [23]. The thermogravimetric analysis of AOP used in the study shows that pyrolysis residue yield is about 65 wt.% at 600 °C and decreases to 62 wt.% at 1000 °C [20]. Cross-linking of pitch constituents by oxidation modifies essentially the thermal behavior of the pitch. The volatile evolution in the pyrolysis stage (350–500 °C) is strongly suppressed, hence the carbon yield increases. Residue yield on heat-treatment of AOPox in nitrogen between 700 and 900 °C amounts to about 80 wt.%. At 700 °C the extent of volatile evolution is therefore comparable to that occurring on the treatment with ammonia (18.7 wt.%). Increasing the temperature of treatment with ammonia above 700 °C results in an enhanced weight loss which can be attributed to the gasification of carbonaceous residue. At 750 °C the weight loss amounts to 23.2 wt.% compared to 19.9 wt.% on carbonization under nitrogen flow, however at 900 °C the respective values are 76 and 21 wt.%. The low temperature treatment in ammonia results in a moderate increase in the nitrogen content, from 1.4 to 3.5 wt.% at 400 °C, it can be attributed to the reaction with oxygen groups. The intense nitrogen uptake starts at about 650 °C to reach a maximum, of about 10 wt.%, at 750 °C. Further increasing the reaction temperature leads to a decrease in N content as an effect of the preferential gasification of nitrogen sites [24]. The comparison of N2 adsorption isotherms at 77 K shows that increasing amination temperature of AOPox powder induces an enhancement of nitrogen adsorption capacity at low relative pressure, indicating a distinct progress in the microporosity development (Fig. 2a). High resolution isotherms under 0.1 p/p0 (Fig. 2b) and pore width

Table 2 Relative concentration of nitrogen forms for the AOPox powder treated with ammonia at different temperatures.

N-ACP/30 N-ACP/50 N-ACP/70 N-ACP/75 N-ACP/80 N-ACP/85 N-ACP/90

N6 398.7 ± 0.3 eV

NC 399.8 ± 0.2 eV

N5 400.3 ± 0.2 eV

NQ 401.4 ± 0.3 eV

NX 402.6 ± 0.3 eV

59 45 47 46 47 53 48

23 16 0 0 0 0 0

18 24 27 28 32 26 28

0 15 11 13 14 14 18

0 0 15 13 7 7 6


214

N-ACP/30

Intensity, a.u.

N-ACP/50

N-ACP/75

N-ACP/90

405

400

395

Binding energy, eV Fig. 5. N1s XPS spectra of nitrogen enriched activated carbons produced by ammonia treatment of AOPox powder at 300, 500, 750 and 900 °C.

distribution determined by applying QSDFT analysis (Fig. 3) reveal two different stages in the porous texture evolution. Intensive nitrogen binding with increasing amination temperature between 700 and 750 °C is associated with the narrow micropore generation. As a result, N-ACP750 is characterized by exceptionally narrow PSD around 0.6 nm. Combining the uniform porosity and the enhanced surface functionality can be of interest for gas separation based on adsorption/diffusion mechanism, e.g. for the mixture CO2/N2 or C3H6/N2 [25]. A further increase in the treatment temperature induces rather widening of pores. QSDFT profiles demonstrate slight decrease of the major peak and an emerging hump corresponding to micropores of size up to 1.5 nm. Detailed porous texture analyses based on N2 and CO2 adsorption isotherms of resultant carbons are given in Table 1. The evolutions of micropore surface area, Smicro (QSDFT) and ultramicropore surface

area S0 (CO2 adsorption) with increasing severity of the treatment are presented in Fig. 3. The porosity of materials heat-treated in ammonia up to 600 °C is found to be restricted to very narrow micropores (ultramicropores) accessible to CO2 at 273 K only. Heat-treatment between 650 and 900 °C develops gradually both the micro- and ultramicropores, but the latter disappear in part above 800 °C (Fig. 4). Specific of ammonia etching is the predominance of narrow micropores in the texture of resultant carbons. Mean micropore width varies from a minimum, 0.53 nm in N-ACP750, to about 1 nm in N-ACP900 produced with 74% weight loss. Representative N1s XPS spectra for the low (N-ACP/30) and high (N-ACP/75) temperatures of amination (Fig. 5) suggest some differences in types and distributions of occurring nitrogen forms. The relative contributions of different functionalities calculated based on the deconvoluted bands are given in Table 2. Characteristic of the spectra of materials aminated between 300 and 500 °C is the presence of peak centered at 399.7 ± 0.3 eV, which corresponds to amide, imide, nitrile and lactam groups N-C. These groups constitute only 16–23% of the total nitrogen, the major part occurring as a substituent for carbon in the ring system. It should be noted, however, that starting AOPox contains 1.5 wt.% of nitrogen originated from anthracene oil constituents comprising pyridine and pyrrole rings. It means that only 50–60% of nitrogen that is measured for the low temperature treatment products comes from amination. It seems likely, that the reaction with acidic groups is limited solely to the outer surface of particles. XPS spectra of carbons treated at 700 °C or at a higher temperature show that practically all nitrogen is located in the ring system. Most abundant pyridinic-N constitutes about half of nitrogen. N bonded to three carbon atoms (quaternary-N) amounts to about 13% and its proportion increases to 18% on the treatment at 900 °C. Strong bands from pyrrolic/pyridonic groups (N-5, 400.3 ± 0.3 eV) and pyridine N-oxides (N-X, 403–405 eV) deserve some comments. A considerable contribution of nitrogen associated with oxygen is supported by high oxygen content. It varies from 4.7 wt.% in N-ACP/70 to 2.9 wt.% in N-ACP/90, i.e. along with nitrogen content. The study of thermal evolution of nitrogen functionalities [26] shows that pyridones and pyridine N-oxides are converted already under mild pyrolysis conditions to pyridinic-N. The treatment at 700–800 °C should persist the most resistive carbonyl and furane oxygen functionalities [27]. The reductive atmosphere of amination accelerates the oxygen removal. It means that majority of oxygen determined by elemental analysis has been trapped during manipulations with aminated samples. The exceptional sensitivity of nitrogen sites to air and moisture, even at ambient temperature, has already been reported [23,24]. The expansion of five-membered pyrrolic rings to pyridinic is reported to start above 600 °C, to be completed at about 800 °C [26]. Despite some controversies about thermal stability of pyrrolic ring [28] we conclude that the intense reaction of carbon with ammonia at 750–800 °C leads primarily to the formation of N bonded to two carbon atoms as pyridinic-N.

8

N-ACP/90

7

Volume [%]

6

AOPox

5 4 3 2 1 0 0.1

1

10

100

400

Particle Size (µm) Fig. 6. Particle size distributions of AOPox powder and nitrogen enriched activated carbon N-ACP/90 produced by amination at 900 °C.


215

Fig. 7. SEM micrographs of nitrogen enriched activated carbon N-ACP/90 produced by amination of AOPox powder.

The mechanism of high temperature amination involves the decomposition of ammonia to free radicals such as NH2, NH and atomic H which starts above 600 °C [29]. The reactive species can contribute to the substitution of carbon atoms by nitrogen in the carbon network or to the gasification. The possible reactions are summarized as (1) and (2), respectively. CS þ 2NH3 →ð\N\ÞS þ CH4 þ H2 þ 1=2N2

ð1Þ

CS þ 2NH3 →CH4 þ H2 þ N2

ð2Þ

Fig. 1 shows that the balance between the reactions is controlled by the process temperature. Heating at 700–800 °C favors nitrogen incorporation but increasing the temperature to 850–900 °C shifts the process into etching of carbon and associated porosity generation. Indeed, methane has been identified as a major gaseous product of the reaction occurring at 900 °C by gas chromatography. The treatment in ammonia, even under the most severe conditions, does not induce any noticeable changes to the particle size distribution of the starting AOPox (Fig. 6), however scanning electron microscopy (Fig. 7) reveals pitting of some particle surface, in contrast to the conventional activation with carbon dioxide. Comparison of XRD patterns for materials produced by the treatment of AOPox at 750 °C in ammonia and nitrogen (Fig. 8) shows that substitution of nitrogen in carbon rings results in distinct worsening of structural ordering. The most significant is an increase in the interlayer distance d002 from 0.360 to 0.372 nm, which can be related, at least in part, to the distortion of constituting graphene layers. The crystallite height Lc decreases from 0.98 to 0.86 nm. 4000

ACP/75

Intensity [a.u.]

3000

2000

1000

0 20

30

3.2. Ammonia treatment of pitch-based fibers for nitrogen enriched activated carbon fibers (N-ACF) To check the feasibility of the process developed for powder sample to fibrous precursor, air-oxidized pitch fibers were ammonia treated under selected temperature/time conditions using the same procedure. Characteristics of fibers treated at 800 °C for 0.5 and 1 h (N-ACF/80/0.5 and N-ACF/80/1) and at 850 °C and 900 °C for 0.5 h (N-ACF/85/0.5 and N-ACF/90/0.5, respectively) are given in Table 4. The results confirm, in general, the trends reported for powder materials. All ACF are microporous materials with the maximum nitrogen content of 12 wt.% in ACF/80-0.5. Increasing the severity of treatment leads to more porous but poorer in nitrogen fibers. However, the data suggest somewhat higher reactivity with ammonia of fiber compared Table 3 Properties of selected nitrogen enriched activated carbons produced by amination of AOPox powder.

N-ACP/75

10

The apparent contradiction between the nitrogen incorporation and the porosity development on the reaction with ammonia is a limitation in developing rich nitrogen and highly porous carbons. Compromise temperature/time conditions have to be found to produce a material of satisfactory properties. The results presented here show that 750–800 °C is an optimal temperature of anthracene oil pitch amination as regards the nitrogen uptake, however, the porous texture is at this stage unsatisfactory. Two different ways of increasing porosity development of material produced at 800 °C (N-ACP/80) have been tested. First it was increasing the reaction time at 800 °C to 2 h, second applying the post-activation with CO2. The activation was performed at 950 °C to the burnoff of about 10 (N-ACP/80-10) and 20 (N-ACP/80-20) wt.%. Table 3 gives the characteristics of the materials obtained. As anticipated, increasing the soaking time at 800 °C results in both a larger pore volume and a larger surface area as well as in wider micropores. It is associated with a moderate decrease in nitrogen content, from 9.9 to 7.9 wt.%. At a rough estimation the effect of longer soaking is equivalent to that of elevated temperature. The post-activation seems to be not applicable to the nitrogen enriched carbons. A drastic drop in nitrogen content occurs already on the mild activation while the porosity enhancement is limited.

40

50

60

70

80

90

2 theta Fig. 8. XRD patterns of AOPox heat-treated at 750 °C under nitrogen (ACP/75) and in ammonia (N-ACP/75).

Sample

N wt.%

SBET m2/g

VT cm3/g

VDR cm3/g

L0 nm

VDR/VT

N-ACP/80 N-ACP/80/2 N-ACP/80-10 N-ACP/80-20

9.89 7.93 4.67 3.77

631 816 725 811

0.264 0.315 0.283 0.340

0.255 0.297 0.266 0.304

0.67 1.03 0.84 1.07

0.956 0.942 0.940 0.894


216

Table 4 Properties of nitrogen enriched activated carbon fibers produced by amination at various conditions of oxidized AOP fibers. Sample

Burnoff wt.%

N wt.%

SBET m2 g−1

VT cm3 g−1

VDR cm3 g−1

L0 nm

VDR/VT

Smicro m2 g−1

N-ACF/80/0.5 N-ACF/80/1 N-ACF/85/0.5 N-ACF/90/0.5

39 46 57 85

12.28 9.79 8.01 n.d.

744 806 891 1023

0.300 0.325 0.377 0.437

0.300 0.325 0.352 0.369

0.81 0.88 0.94 1.08

1.00 1.00 0.92 0.84

876 939 959 1024

to powder. The comparison of N-ACF/80 and N-ACP/80 reveals a larger micropore surface area (Smicro 876 vs.770 m2 g−1), wider pores (L0 0.81 vs. 0.67 nm) as well as higher nitrogen content (12.28 vs. 9.89 wt.%) in the activated fibers. It means that milder conditions are needed to achieve a predetermined porosity development and more nitrogen is retained in the structure. QSDFT analysis of N2 adsorption isotherms shows narrow PSD in N-ACF/80 (Fig. 9) and widening of pores as the ammonization temperature increases. The superior behavior of fibrous form can be attributed to a larger surface accessible to reacting gas and rather uniform diameter of 20–30 μm, in contrast to the particulate material covering a wide range of diameters, from very fine to more than 100 μm with a maximum centered at 40 μm (Fig. 6). SEM examination reveals (Fig. 10) that the fibrous morphology is perfectly preserved after ammonia treatment at 900 °C, however, common features are surface pitting of many fibers.

0.20

Pore volume [cm3/g]

0.18

N-ACF/80-0.5 N-ACF/85-0.5 N-ACF/90-0.5

0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0.5

1.0

1.5

2.0

Pore width [nm] Fig. 9. Pore size distribution of active carbon fibers prepared by amination of oxidized AOP fibers, determined from N2 adsorption isotherms at 77 K using a QSDFT analysis.

4. Conclusions The direct ammonia treatment of oxidized anthracene oil-based pitch particulates and fibers at elevated temperature has been studied to produce microporous carbons of high content of nitrogen substituted for carbon in graphene layers. During amination of fine pitch powder the intense reaction starts at 700 °C to reach the maximum N uptake of ~10 wt.% at 750–800 °C. The XPS analysis shows mostly pyridinic nitrogen. The resultant materials are characterized by the micropore surface area Smicro of 500 and 780 m2 g−1, respectively, and very narrow distribution of pores with the widths of 0.53 and 0.67 nm. Extensive etching of char with increasing treatment temperature to 900 °C results in widening of pores to about 1 nm, together with a limited increase in the pore volume and the surface area, but nitrogen content is reduced to 6.3 wt.%. The ammonia treatment of stabilized pitch fibers reveals similar behavior with some advantages which can be attributed to the fibrous form. Activated carbon fibers having nitrogen content as high as 12 wt.% and narrow PSD with the micropore width about 0.8 nm could be produced using amination at 800 °C for 0.5 h. The SEM examination of N-ACF reveals perfectly preserved fibrous morphology but extensive pitting of the surface. The ammonia activation seems to be especially suitable as a route for producing nitrogen enriched activated carbon fibers with the narrow micropore size distribution from stabilized isotropic pitch fibers. Combining the uniform porosity and the enhanced surface functionality can be of interest for the gas separation based on the adsorption/ diffusion mechanism. The apparent contradiction between nitrogen incorporation and porosity development causes, however, that temperature/ time conditions have to be carefully optimized to get N-activated carbon fibers of satisfactory properties. Acknowledgments The research leading to these results has received funding from the European Union's Research Fund for Coal and Steel (RFCS) research programme under grant agreement no. RFCR-CT-2009-0004.

Fig. 10. SEM micrographs of nitrogen enriched activated carbon fibers N-ACF/85-0.5 produced by amination of oxidized AOP fibers.


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