Effect of pre-oxidation on the development of porosity in activated carbons from petroleum coke

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palladium. The exact composition of the prepared nanocomposites is thus determined. The prepared Pdnano/CNT and Pdnano/GN are tested for potential application as electrochemical catalysts for reduction by oxygen. Fig. 3a–c compares the activities of Pdnano/SWCNT, SWCNT, Pdnano/MWCNT, MWCNT, Pdnano/GN and GN in reduction by oxygen revealed by electrochemical CV measurement. A comparison with carbon nanomaterials reveals the high activity of Pd nanoparticles decorated on carbon nanomaterials. The typical curve in Fig. 3a shows the high activity of Pdnano/SWCNT. A comparison with the catalyst-free SWCNT indicates that the Pdnano/SWCNT has higher activity from 0.5 V in the oxygen-saturated acid solution. Notably, a comparison with the Pdnano/MWCNT prepared by the electrochemical method [4] demonstrates a positive voltage of 0.7 V in oxygen reduction when Pdnano/MWCNT is prepared, as presented in Fig. 3b, indicating that the Pdnano/MWCNT formed by this method has high activity and potential for application to energy storage. In conclusion, a new and simple in situ approach was developed to prepare and Pd nanoparticles and attach them to the inert wall of not only GNs but also CNTs without pre-oxidation by the self-regulated reduction of SC14S. As supported by CV measurements, newly prepared Pdnano/CNT and GN have high activity in oxygen reduction. The positive reduction peak of Pdnano/MWCNT is measured from 0.7 V. The unique catalytic properties are of great interest. This method for synthesizing Pdnano/ CNT and GN yields extraordinary physical/chemical features that are useful in advanced applications.

References [1] Guo DJ, Li HL. Electrochemical synthesis of Pd nanoparticles on functional MWNT surfaces. Electrochem Commun 2004;6:999– 1003. [2] Wildgoose GG, Banks CE, Compton RG. Metal nanoparticles and related materials supported on carbon nanotubes: methods and applications. Small 2006;2(2):182–93. [3] Lin Y, Cui X, Ye X. Electrocatalytic reactivity for oxygen reduction of palladium-modified carbon nanotubes synthesized in supercritical fluid. Electrochem Commun 2005;7:267–74. [4] Britto PJ, Santhanam KSV, Rubio A, Alonso JA, Ajayan PM. Improved charge transfer at carbon nanotube electrodes. Adv Mater 1999;11(2):154–7. [5] Kim BK, Park N, Na PS, So HM, Kim JJ, Kim H, et al. The effect of metal cluster coatings on carbon nanotubes. Nanotechnology 2006;17(2):496–500. [6] Guo DJ, Li HL. High dispersion and electrocatalytic properties of palladium nanoparticles on single-walled carbon nanotubes. J Colloid Interface Sci 2005;286:274–9. [7] Ang LM, Hor TSA, Xu GQ, Tung CH, Zhao S, Wang JLS. Electroless plating of metals onto carbon nanotubes activated by a single-step activation method. Chem Mater 1999;11:2115–8. [8] Lee CL, Wan CC, Wang YY. Synthesis of metal nanoparticles via self-regulated reduction by an alcohol surfactant. Adv Funct Mater 2001;11(5):344–7. [9] Lee CL, Huang YC, Kuo LC, Oung JC, Wu FC. Preparation and characterization of Pd/Ag and Pd/Ag/Au nanosponges with network nanowires and their high electroactivitivites toward oxygen reduction. Nanotechnology 2006;17(9):2390–5. [10] Lee CL, Ju YC, Chou PT, Huang YC, Kuo LC, Oung JC. Preparation of Pt nanoparticles on carbon nanotubes and graphite nanofibers via self-regulated reduction of surfactants and their application as electrochemical catalyst. Electrochem Commun 2005;7:453–8.

Effect of pre-oxidation on the development of porosity in activated carbons from petroleum coke Chunlan Lu, Shaoping Xu *, Mei Wang, Ligang Wei, Shuqin Liu, Changhou Liu State Key Laboratory of Fine Chemicals, Department of Chemical Engineering, Dalian University of Technology, 158 Zhongshan Road, Dalian, Liaoning 116012, PR China Received 10 April 2006; accepted 3 October 2006 Available online 30 October 2006

Pre-oxidation of carbonaceous raw material by either air or oxidizing solutions before activation is an important step for obtaining high performance adsorbent materials [1–5]. In our previous work [6], it has been proved that the surface functional groups of petroleum coke (PC) such as C–O–C, C–O–H and alkyl groups play an important role in chemical activation and contribute to the porosity *

Corresponding author. Tel.: +86 411 88993837; fax: +86 411 83683467. E-mail address: [email protected] (S. Xu). 0008-6223/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2006.10.003

development of the resulted activated carbon (AC). In this work ACs were produced by KOH activation from pre-oxidized PCs. The influence of pre-oxidization of the PC by H2O2 on the porosity development is studied to reveal the courses of the surface chemistry and the texture. A green PC and the sample PC500 pre-carbonized at 500 C [6] were used as raw materials. They were oxidized by 20% H2O2 aqueous solution and denoted as PC-HO and PC500-HO, respectively. The oxidized sample was physically mixed with KOH in a mass ratio of 1:3. The mixture was carbonized in an argon flow of 200 cm3 min1 from

Letters to the Editor / Carbon 45 (2007) 203–228

% Transmittance

PC500 - HO PC 500 PC

500

1000

1500

2000

2500

3000

3500

4000

Wavenumbers (cm-1)

Fig. 1. FTIR spectra of the original and the treated PCs.

Table 1 Microcrystalline structural parameters of the ACs and their precursors Sample

Lc (nm)

d002 (nm)

Sample

Lc (nm)

d002 (nm)

PC PC-HO PC500 PC500-HO

2.26 2.18 1.37 1.41

0.345 0.348 0.347 0.348

AC AC–HO AC500 AC500-HO

0.51 –a 0.69 0.62

0.365 –a 0.369 0.376

a

– can not be calculated.

room temperature to 400 C at a heating rate of 8 C min1, then from 400 to 730 C at a heating rate of 4 C min1, and finally held at 730 C for 1 h to produce the AC sample. The samples activated from PC, PC-HO, PC500, PC500-HO were denoted as AC, AC–HO, AC500, and AC500-HO, respectively. The evolved gases during the chemical activation process were analyzed by a GC-920 chromatography. The carbonaceous samples were characterized by using Fourier transform infrared (FTIR), X-ray diffraction (XRD) and BET techniques [6]. In Fig. 1, the FTIR spectrum of PC presented characteristic bands of –CH, –CH2, –CH3, C–O–C, C–O, O–H and C@C. The intensities of these bands for sample PC500 de-

H2 evolution (cm3g-1min-1)

50

PC-HO/KOH PC/KOH PC

40

30

20

10

0 100

200

300

400

500

600

Temperature(ºC)

700

800

creased significantly, indicating a reduction in the number of the IR-observable groups. For the H2O2 oxidized samples, the most important features are the intensity reduction of IR bands at around 2900 cm1 assigned to alkyl groups and an appearance of IR band around 1700 cm1 assigned to the C@O stretching vibration. The PC-HO sample showed a stronger intensity of C@O stretching vibration at 1700 cm1 than the PC500-HO. These FTIR results show that the pre-oxidation promoted the formation of surface oxygen complexes and decreased the alkyl groups of the cokes at the same time. Conversion of alkyl groups to the oxygen complexes may occur during the oxidization process. The interlayer spacing (d002) and the dimension Lc perpendicular to the basal plane of the graphitic microcrystallines in the samples were determined from XRD patterns (not shown) and listed in Table 1. PC-HO and PC500HO samples gave slightly larger d002 than PC and PC500, suggesting that H2O2 may lead to addition of oxygen to the graphitic microcrystallines. Except for AC–HO sample, all other three activated samples showed smaller Lc and larger d002 values than their precursors. Compared with the sample AC500, AC500-HO showed a decrease of Lc from 0.69 to 0.62 nm and an increase of d002 from 0.369 to 0.376 nm. Lc and d002 of the sample AC–HO cannot be calculated due to the lack of 0 0 2 peak in the XRD spectrum. The absence of 0 0 2 peak indicates the graphitic microcrystallines in AC–HO were amorphous or completely decomposed during activation. Therefore, it can be concluded that the pre-oxidation treatment of the PC contributes to the chemical activation. Hydrogen can be produced during the activation from C/KOH reaction [7] and PC pyrolysis. Fig. 2 shows Table 2 Porous structure parameters of ACs prepared by various pre-treatments Sample

Yield (%)

SBET (m2 g1)

Vt (cm3 g1)

rp (nm)

AC AC500 AC–HO AC500-HO

61.4 76.5 40.5 68.7

1763 586 2744 1282

1.113 0.407 1.649 0.863

1.26 1.39 1.20 1.35

1.2

CH4 evolution (cm3g-1min-1)

PC - HO

207

PC-HO/KOH PC/KOH PC

1.0 0.8 0.6 0.4 0.2 0.0 300

400

500

600

Temperature(ºC)

Fig. 2. Hydrogen (a) and methane (b) evolution at different reaction temperatures.

700

208

Letters to the Editor / Carbon 45 (2007) 203–228 O

H 3C HO

O

O OH

oxidization O

HO

O

H 2O 2

O

O

CH 3 O

OH

I

OH

KOH

activation

II

O

K

K

O O

H 3C

activation O

KO H

K O

CH 2

H 3C

K

O

CH 3 K

O

K O

IV

III

K

Fig. 3. Schematic mechanism of chemical activation.

hydrogen and methane evolution profiles along with temperature increase. The starting and peak temperatures of hydrogen and methane evolution for PC–HO/KOH were lower than those of PC/KOH, indicating that the pre-oxidation treatment by H2O2 increased the reactivity of the PC. Methane comes mainly from the pyrolysis of the alkyl groups of the coke material. As shown in Fig. 2b, the amount of methane evolution of PC–HO/KOH was lower than that of PC/KOH, which is in agreement with the decrease of alkyl groups after oxidization. Compared with the PC, the peaks of hydrogen and methane evolution profiles for PC/KOH and PC–HO/KOH moved towards lower temperature due to the pre-oxidation and interaction with KOH. In addition, both PC/KOH and PC–HO/KOH presented additional high temperature peaks of methane evolution profile between 550 and 600 C, which is resulted from destruction of the aromatic ring in the presence of hydrogen during the activation. Results listed in Table 2 indicate that pre-carbonization of the precursor at 500 C dramatically decreased the BET surface area and total pore volume Vt (estimated from liquid volume of nitrogen adsorbed at relative pressure P/P0 = 0.95) of the AC sample, while the oxidation of the precursor with H2O2 increased the BET surface area and the pore volume significantly. The average pore radiuses rp of the carbons from the oxidized precursors are smaller than that from non-oxidized samples. Obviously, the pre-oxidation treatment of the precursor plays an important role in the porosity development of the ACs. Based on the results above, Fig. 3 is proposed to illustrate the chemical activation mechanism for the pre-oxi-

dized PC in chemical activation with KOH. Model I shows the aromatic carbon layer in the original PC characterized of C–OH, C–O–C, C@C, –CH, –CH2 and –CH3, which has been confirmed by FTIR analysis. It has been reported that oxidation of the carbon increases the concentration of carbonyl surface groups [8]. After the PC is treated with H2O2, the alkyl groups and C–OH are oxidized to C@C as shown in model II. During the chemical activation process, the surface oxygen groups are proposed to act as ‘active sites’ and react with KOH to produce the intermediate C–O–K groups shown as model III. When the resulted groups continue to react with KOH, the aromatic carbon layers are disintegrated into various fragments, such as model IV and methane. The model I–IV structures are highly speculated, but the disintegration process is evidenced by the small crystallite thickness and short range ordering from XRD analysis as well as the methane evolution from GC analysis. Consequently the porous structure develops under the experimental conditions employed in this work. Considering the hydrogen and methane evolution as shown in Fig. 2 as well as the known carbon–potassium hydroxide reaction [7,9], additional reactions may happen as follows: –COK þ KOH þ H2 O ! K2 CO3 þ 3=2H2 –CO2 K þ KOH ! K2 CO3 þ 1=2H2 –CHCO2 K þ H2 þ KOH ! K2 CO3 þ CH4 It is indicated that the diffusion of KOH to the active sites is an important step of the activation process, the more active sites and the wider of the d002 of the graphitic microcrystalline in the precursor, the easier the activation. As a result, the pre-oxidation of the precursors to combine the oxygen-containing groups into the graphitic microcrystalline provides a way to produce ACs with high surface area by chemical activation at a relative lower KOH/C ratio. References [1] Go´mez-de-Salazar C, Sepu´lveda-Escribano A, Rodrı´guez-Reinoso F. Preparation of carbon molecular sieves by controlled oxidation treatments. Carbon 2000;38(13):1889–92. [2] Daulan C, Lyubchik SB, Rouzaud JN, Be´guin F. Influence of anthracite pretreatment in the preparation of activated carbons. Fuel 1998;77(6):495–502. [3] Parra JB, Pis JJ, De Sousa JC, Pajares Jesu´s A, Bansal RC. Effect of coal preoxidation on the development of microporosity in activated carbons. Carbon 1996;34(6):783–7. [4] Lyubchik SB, Benoit R, Be´guin F. Influence of chemical modification of anthracite on the porosity of the resulting activated carbons. Carbon 2002;40(8):1287–94. [5] Serrano-Talavera B, Mun˜oz-Guillena MJ, Linares-Solano A, SalinasMartı´nez de Lecea C. Activated carbons from spanish Coals. 3. Preoxidation effect on anthracite activation. Energ Fuel 1997;11(4): 785–91. [6] Lu CL, Xu SP, Gan YX, Liu SQ, Liu CH. Effect of pre-carbonization of petroleum cokes on chemical activation process with KOH. Carbon 2005;43(11):2295–301. [7] Lillo-Ro´denas MA, Cazorla-Amoro´s D, Linares-Solano A. Understanding chemical reactions between carbons and NaOH and KOH:

Letters to the Editor / Carbon 45 (2007) 203–228 An insight into the chemical activation mechanism. Carbon 2003;41(2): 267–75. ´ rfa˜o JJM. Modification [8] Figueiredo JL, Pereira MFR, Freitas MMA, O of the surface chemistry of activated carbons. Carbon 1999;37(9): 1379–89.

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[9] Yamashita Y, Ouchi K. Influence of alkali on the carbonization process: I. Carbonization of 3,5-dimethylphenol-formaldehyde resin with NaOH. Carbon 1982;20(1):41–5.

Preparation of a carbon with a 2 nm pore size and of a carbon with a bi-modal pore size distribution Takahiro Morishita, Kaori Ishihara, Masaya Kato, Michio Inagaki

*

Faculty of Engineering, Aichi Institute of Technology, Yakusa, Toyota 470-0392, Japan Received 21 June 2006; accepted 29 September 2006 Available online 24 October 2006

In our previous papers [1–5], a simple heat treatment of mixtures of thermoplastic precursors with MgO precursors in an inert atmosphere was reported to give carbons with a high surface area without any stabilization and activation processes, and was proposed to be a novel process for the preparation of porous carbons. Most of the carbons obtained by this process contained a large amount of mesopores, whose size was governed by the size of MgO particles formed by the pyrolysis of their precursor at a temperature below 250 C. Mg acetate gave either a broad pore size distribution in the mesopore region or about a 12 nm pore size depending on the mixing procedure of Mg acetate with the carbon precursor, poly(vinyl alcohol) (PVA) [3]. From MgO citrate, a large number of mesopores with 5 nm size were obtained [5]. Here, we report that Mg gluconate Mg(C11H22O14) (reagent grade) gave carbons with a sharp pore size distribution at around 2 nm and demonstrate that carbon with a bi-modal pore size distribution could be prepared by mixing different MgO precursors. The experimental procedure to prepare the carbons was exactly as reported in our previous papers [1–5]. The mixing ratio of MgO gluconate with PVA was defined on the basis of the mass of MgO expected to be formed from the precursor and PVA itself. 1. Preparation of carbon with a 2 nm pores size Fig. 1a shows changes in surface areas, total surface area Stotal, surface area due to mesopores Smeso. and that due to micropores Smicro. determined by the BJH method, with MgO/PVA mixing ratio in Mg gluconate/PVA mixtures. For comparison, the results on Mg citrate/PVA mixtures are shown in Fig. 1b. Mg gluconate alone (MgO/

*

Corresponding author. Fax: +81 565 48 0076. E-mail address: [email protected] (M. Inagaki).

0008-6223/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2006.09.032

PVA = 10/0) gave a carbon residue after 900 C carbonization, as did Mg citrate. The pore structure of these two carbons, however, was quite different. The carbon derived from Mg gluconate had a relatively high Smicro. of about 900 m2/g (micropore volume of about 0.58 mL/g) and Smeso. of about 440 m2/g (mesopore volume of about 0.24 mL/g), though that from Mg citrate was mesoporous, with Smicro. being about 300 m2/g but Smeso. reaching 1600 m2/g (mesopore volume of about 2.2 mL/g). The carbon yield from Mg gluconate was about 26 mass%, almost the same with Mg citrate [5], because the two precursors have the same carbon content. Mixing Mg gluconate with PVA changed the relative proportion of mesopores to micropores in the resultant carbon (Fig. 1a). With increasing PVA content (decreasing MgO/PVA ratio) Smicro. decreases and Smeso. increases, in contrast with Mg citrate/PVA mixtures where Smeso. is predominant (Fig. 1b). At the mixing ratio of 7/3, Smicro. almost equals with Smeso. (about 680 m2/g). The pore size distribution in the mesopore region is shown in Fig. 2. The carbons prepared from Mg gluconate alone (MgO/PVA = 10/0), show a sharp maximum at around 2 nm (Fig. 2a), in contrast to about 5 nm from Mg citrate. In the Mg gluconate/PVA system, the carbon obtained from the mixture with a MgO/PVA ratio of 7/3 has higher population of pores with 2–4 nm size, which is caused by the formation of a large number of mesopores. With increasing PVA content in the mixture (decreasing MgO/PVA ratio) the population of 2 nm pores decreases gradually. In the Mg citrate/PVA system, the population of pores around 5 nm does not change up to a MgO/ PVA ratio of 5/5 and then the size becomes larger, accompanying the decrease in maximum population with decreasing MgO/PVA ratio. The crystallite size of MgO calculated from full width at half maximum intensity of the 220 X-ray diffraction line of MgO was about 4 nm for the carbons prepared in the Mg