Oxidation behavior of furan-resin-derived carbon alloyed with Ta or Ti

Carbon 40 (2002) 1949–1955

Oxidation behavior of furan-resin-derived carbon alloyed with Ta or Ti Yasuhiro Tanabe a , *, Masao Utsunomiya a , Manabu Ishibashi a , Takashi Kyotani b , Yutaka Kaburagi c , Eiichi Yasuda a a

Center for Materials Design, Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226 -8503, Japan b Institute for Chemical Reaction Science, Tohoku University, 2 -1 -1 Katahira, Aoba-ku, Sendai 980 -8577, Japan c Faculty of Engineering, Musashi Institute of Technology, 1 -28 -1 Tamazutsumi, Setagaya-ku, Tokyo 158 -8557, Japan Received 25 July 2001; accepted 4 January 2002

Abstract The effect of Ti, Nb and Ta on the anti-oxidation of furan-resin-derived carbons was investigated by thermogravimetric analysis combined with gas / mass spectroscopy, Hall coefficient and magnetoresistance measurements, transmission electron microscopy, and X-ray diffraction analysis. Hall coefficient and magnetoresistance measurements revealed that the electronic properties of carbons with the above metal elements, and thus their crystallinity, are similar to that of neat carbon. The oxidation rates of the carbons with a small amount of Ti or Ta are decreased up to 1000 8C compared to that of neat carbon. The number of working active sites in carbons with Ta or Ti is smaller than that in carbons with Nb or neat carbon. Ta or Ti terminates some active sites in the carbons, or somehow modifies the structures of the active sites and retards the formation of oxygen–carbon intermediate products (surface complexes) on the materials. Thus, the oxidation rate of the carbons is decreased.  2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Glass-like carbon; B. Oxidation; C. Thermal analysis (DTA and TGA); D. Activation energy

1. Introduction We have previously reported that carbons derived from furan resin with a small amount of Ta 2 O 5 added show anti-oxidation properties, while carbons with Nb 2 O 5 addition show no such property [1]. The addition of Ta 2 O 5 to a matrix precursor resin led to the maintenance of a high shear strength in carbon-fiber-reinforced carbon composites, even if they were heat-treated at high temperatures [2]. However, the anti-oxidation mechanism caused by Ta 2 O 5 has not been clarified. In this study, the oxidation behavior was analyzed in order to evaluate the effect of additives on the antioxidation properties of resin-derived carbons. Working active sites (WASs) were estimated using the transient *Corresponding author. Fax: 181-45-924-5345. E-mail address: [email protected] (Y. Tanabe).

kinetics (TK) method, and changes of the sites by the additives were investigated.

2. Experimental procedure

2.1. Sample preparation Fulfuryl-alcohol condensate (Hitafuran302; Hitachi Chemical: hereafter denoted as furan precursor) was used as the raw material. The following were used as additives: Ta 2 O 5 powder with a diameter of 0.5 mm and 99.9% purity (High Purity Chemical), Ta(OC 2 H 5 ) 5 with 99.999% purity (High Purity Chemical), Nb 2 O 5 powder of 1 mm diameter and 99.9% purity (High Purity Chemical), Nb(OC 2 H 5 ) 5 with 99.999% purity (High Purity Chemical), and TiO 2 powder of 1 to 2 mm diameter and 99.9% purity (High Purity Chemical).

0008-6223 / 02 / $ – see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 02 )00028-3

Y. Tanabe et al. / Carbon 40 (2002) 1949 – 1955

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Each of the above additives was added to the furan precursor together with the hardening agent p-toluenesulfonic acid at 0.3 mass% with respect to the precursor, and mixed with a horn-type ultrasonication apparatus (Type 450 Sonifier; Branson) for homogeneous mixing and pore removal. The process was carried out in a dry N 2 atmosphere. The mixed precursors were hardened in an oven at 50 8C for 2 days, and cured for 6 h at 160 8C. Cured resins were heat-treated at 1000 8C for 30 min in an inert atmosphere at a heating rate of 0.25 8C / min, and then heat-treated at 2500 8C for 30 min in an inert atmosphere. They were then machined to produce specimens of dimensions 33330.5 mm 3 : 100 mm was removed from the original surfaces that had been the outer faces during heat treatment to eliminate well-graphitized surface layers [3]. The notation for 2500 8C heat-treated specimens is given in Table 1. The first number indicates the mass% of the additive with respect to the resin precursor; the following two letters indicate the added element, and ET indicates that the additive was in the form of an ethoxide. The added mass% of Ta(OC 2 H 5 ) 5 and Nb(OC 2 H 5 ) 5 was the mass% after conversion to Ta 2 O 5 and Nb 2 O 5 , respectively.

2.2. Infrared spectroscopy The structures of the Ta(OC 2 H 5 ) 5 - and Nb(OC 2 H 5 ) 5 added resins cured at 160 8C were analyzed as KBr pellets using a diffuse reflectance FT-IR spectrometer (FTIR-8000 and DRS-8000; Shimadzu).

2.3. X-ray diffraction analysis Crystallite size and layer spacing were measured using the corresponding 002 diffraction peak of hexagonal carbon [4]. When the obtained profiles appeared to overlap, they were separated with a Pearson VII type function. The compositions of the specimens were determined, and the crystallite sizes of the products, TaC, TiC and NbC, in the carbons were estimated using the Scherrer equation. Table 1 Notation for the specimens used in this study Additive

Contents with respect to furan precursor (mass%) 0

None Ta 2 O 5 Ta(OC 2 H 5 ) 5 Nb 2 O 5 Nb(OC 2 H5) 5 TiO 2

0.5

5

0.5TA 0.5TA-ET 0.5NB 0.5NB-ET 0.5TI

5TA 5TA-ET – – –

Neat carbon

The contents of ethoxides were equivalent to those after conversion to their oxides.

2.4. Magnetoresistance and Hall coefficient Magnetoresistance is very sensitive to the structure of carbon materials; thus it is a useful parameter to investigate the change / difference in crystallinity of glass-like carbons [5]. Magnetoresistance and the Hall coefficient were measured at liquid nitrogen temperature in magnetic fields up to 1 T using a conventional DC method.

2.5. Thermogravimetric analysis Thermogravimetric analysis was carried out under isothermal conditions of 700 to 1000 8C to evaluate the oxidation behavior of the carbons. The specimens were heated to a certain temperature in flowing Ar, kept for 1 h at that temperature, and then the Ar flow was changed to a dried air (O 2 , 21%; N 2 , 79%) flow of 200 ml / min to measure the oxidation weight loss. Oxidation rates were calculated as follows: r ox 5 2 (dX / dt) /(1 2 X), X 5 1 2 W/W0

(1)

where W is the sample weight (g), t is time (s), W0 is the initial weight of the sample (g), X is the weight loss ratio, dX / dt is the weight loss rate (s 21 ), and r ox is the oxidation rate (s 21 ).

2.6. WAS (working active site) analysis by the TK (transient kinetics) method If the oxidation of carbon materials is controlled by the desorption of CO and CO 2 from surface complexes of the materials, a detailed analysis of the desorption behavior will be quite important. Therefore, WAS was analyzed by the TK method. WAS corresponds to the number of carbon atoms, per unit total number of carbon atoms, that actually react / bond to oxygen at the oxidation temperature. The specimens were heated to 727 8C in flowing Ar atmosphere, kept at that temperature for 30 min, and then the Ar flow was changed to an O 2 flow at 100 ml / min. After the specimens showed a certain weight loss, the O 2 flow was immediately changed to an Ar flow. Using a gas / mass spectrometer (QP5050; Shimadzu), it was confirmed that Ar (O 2 ) gas could be replaced by O 2 (Ar) gas in a few seconds: the gas inlet port, which consisted of a four-way valve, and the sampling port were as close to the sample holder as possible. The reason why the TK method was carried out at 727 8C is described in Section 3.6. Desorbed gases, CO and CO 2 , were detected quantitatively by the mass spectrometer. The desorption rate vs. time profiles were curve-fitted. From the integral of the profiles, the number of working active sites (WASs) (g / g) was calculated [6,7]. The relationships between the above parameters can be expressed as follows: r(CO 1 CO 2 ) 5 ([CO] ? F /Ws 1 [CO 2 ] ? F /Ws )

(2)

Y. Tanabe et al. / Carbon 40 (2002) 1949 – 1955

r(CO) 5 [CO] ? F /Ws

(3)

r(CO 2 ) 5 [CO 2 ] ? F /Ws

(4)

E

(5)

WAS 5 12 r(CO 1 CO 2 ) dt

where F is the gas flow rate (mol / s), Ws is the sample weight (g), r(CO1CO 2 ) is the total desorption rate of CO and CO 2 (mol g 21 s 21 ), [CO] and [CO 2 ] are the concentrations of CO and CO 2 (mol / mol) in the analyzing gas, r(CO) and r(CO 2 ) are the desorption rates of CO and CO 2 (s 21 ) and t is time (s); time zero was when oxygen gas started to flow.

3. Results and discussion

3.1. IR spectroscopy In resins mixed with ethoxide cured at 160 8C, only peaks assigned to furan rings were detected. The basic structure of the furan resin was maintained in the above specimens after curing. Alkoxides did not destroy the basic structure of the furan resin, even if 5 mass% was added to the furan precursor.

3.2. Microstructure Specimens mixed with the horn-type ultrasonic apparatus showed no clear small pores with optical micrography; the added powder was distributed homogeneously compared with those mixed by a ball mill [8]. The curing reaction was accelerated by cavities produced by the ultrasonic apparatus [9]. Optical microscopic observation revealed that no agglomerated oxide powder was present in all oxide-added specimens. The bulk densities of neat carbon, 0.5TA, 0.5TA-ET, 0.5NB, 0.5NB-ET and 0.5TI ranged from 1.43 to 1.45 g / cm 3 . However, those of 5TA and 5TA-ET ranged from 1.28 to 1.29 g / cm 3 . The density became lower upon increasing the amount of additives. Reaction between the additives and carbon by heat treatment retarded the densification, i.e. the reaction gases retarded it. Fig. 1 shows a TEM image of 0.5TA-ET after 30 mass% oxidation. Particles of TaC of ca. 20 nm diameter, observed as dots in the figure, were dispersed uniformly in the specimen. These particles did not produce a dense layer that would serve as a protection layer against oxidation.

3.3. X-ray diffraction Table 2 shows the crystallite size, Lc( 002 ) , and the

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Fig. 1. TEM image of the interior of 0.5TA-ET after 30 mass% oxidation at 700 8C.

inter-layer spacing, d(002), of the broad and glassy / amorphous (B) and turbostratic (T) components [10] and their ratios, which are the T / B area ratios in the profiles. The T component was observed for 5TA, 0.5NB and 0.5TI. However, for the other specimens, no T component was observed. For 0.5TA, the T component was not observed because the powder content was lower than for 0.5TI and 0.5NB. Since TiO 2 acts as a graphitization catalyst [11], 0.5TI showed a larger T component than 0.5NB. This appearance of the T component indicates that catalytic graphitization had started in the specimens. For 0.5NB, 0.5NB-ET and 0.5TI, only NbC or TiC was detected by XRD. In all the Ta-containing specimens, TaC and Ta 2 O 5 were detected even if the specimens were heat-treated at 2500 8C in an inert atmosphere. Ta 2 O 5 was stable in the carbons at high temperature. The mean crystallite size of TaC in 5TA-ET and 0.5TA-ET was 20 nm, and that of NbC in 0.5NB-ET was 40 nm, which indicates that tiny particles were dispersed in the specimen prepared from Ta(OC 2 H 5 ) 5 or Nb(OC 2 H 5 ) 5 mixed precursors. The TEM image in Fig. 1 also reveals the presence of tiny particles.

Table 2 XRD parameters of the specimens used in this study Specimen

Lc( 002 ) -B (nm)

d ( 002 ) -B (nm)

Lc( 002 ) -T (nm)

d ( 002 ) -T (nm)

T/B (%)

Neat carbon 0.5TA 5TA 0.5TA-ET 5TA-ET 0.5NB 0.5NB-ET 0.5TI

2.6 2.6 2.5 2.3 2.1 2.8 2.6 2.7

0.3468 0.3451 0.3498 0.3458 0.3459 0.3450 0.3459 0.3471

– – 17.8 – – 21.0 – 10.0

– – 0.3396 – – 0.3413 – 0.3387

0 0 16 0 0 1 0 4

Lc( 002 ) -B, crystallite size along the c-axis of the broad component; Lc( 002 ) -T, crystallite size along the c-axis of the turbostratic component; d ( 002 ) -B, interlayer spacing of the broad component; d ( 002 ) -T, interlayer spacing of the turbostratic component. –: Not detected.

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3.4. Magnetoresistance and Hall coefficient Magnetoresistance in glass-like carbons, even when heat-treated at high temperatures, for example a glass-like carbon heat-treated at 3000 8C, shows negative values that decrease with increasing magnetic field [12]. All specimens in the present study showed negative magnetoresistance that decreased with increasing magnetic field. This phenomenon was attributed to the two-dimensional weak localization of carriers. The Hall coefficients for all specimens were positive and independent of the magnetic field. The Hall coefficients were ca. 50 to 60310 28 m 3 / C, and no distinct differences could be detected among the specimens. The hole carrier densities in neat carbon and in carbons with Nb ranged from 1.13 to 1.53310 26 m 23 , and those in carbons with Ta and Ti ranged from 1.04 to 1.60310 26 m 23 . Although the Fermi levels were different among the specimens judging from the hole density, systematic differences could not be found between Nbcontaining carbon or neat carbon and Ta- or Ti-containing carbons. The crystallographic structures of the carbons in all specimens were considered to be similar and all the carbons were categorized as typical glass-like carbon.

3.5. TG analysis The residual weights after complete oxidation of 5TAET, 0.5TA-ET and 0.5NB-ET were the same as those of specimens with their respective oxide additive. The weight loss of specimens as a function of time is shown in Fig. 2. Weight loss rates of the specimens are shown in Fig. 3. It was confirmed from preliminary tests that the weight loss rates of neat carbon and 0.5TA are the same when using larger specimens with dimensions 33930.5 mm 3 . Below 800 8C, the weight loss rates of 0.5NB and 0.5NBET were higher than that of neat carbon above a few mass% weight loss, while those of Ta-containing specimens and 0.5TI were lower than that of neat carbon. At 900 and 1000 8C, although the weight loss rates of 0.5NB and 0.5NB-ET were lower than that of neat carbon, they were higher than those of Ta-containing specimens and 0.5TI. These data indicate that Ta-containing specimens and 0.5TI exhibit anti-oxidation properties in this temperature region. The weight loss rates of 0.5NB and 0.5NB-ET increased below 900 8C, while those of other specimens decreased in the same region. Regardless of the temperature, the weight loss rates were almost constant for all specimens at around 30 mass% weight loss, except for that of 0.5NB at 700 8C: the weight loss rate of 0.5NB at 700 8C became almost constant above 30 mass%, as shown in Fig. 3. All of the specimens had a few micropores and a very limited surface area, because they were glass-like carbons, as described above. During early oxidation weight loss, specimens could be oxidized linearly from the surface. It has been reported that the thickness of a commercially

Fig. 2. Weight loss as a function of time.

available glassy carbon specimen heat-treated at 3000 8C decreased linearly with increasing burn-off weight [13]. In this case, the second derivative of X with respect to time, d 2 X / dt 2 , is proportional to time. Neat carbon, 0.5NB and 0.5NB-ET appeared to have a linear relationship during

Y. Tanabe et al. / Carbon 40 (2002) 1949 – 1955

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positive correlation with weight loss up to more than 50 mass%. Since the weight loss rates of all specimens at 30 mass% weight loss were almost constant, the oxidation rates were adopted for calculating the apparent activation energy. The Arrhenius plots are shown in Fig. 4. The apparent activation energies below 800 8C were not very different and ranged from 183 to 203 kJ / mol. Even though the reported activation energy of graphite oxidized with air or oxygen at around 600 to 800 8C ranges from 175 to 263 kJ / mol [14], below 800 8C the oxidation of the specimens was reaction controlled, not diffusion controlled. For glassy carbon heat-treated at 3000 8C an activation energy of 264612 kJ / mol was reported in the oxidation temperature region of 750 to 850 8C [13]. For glassy carbon, the activation energy seemed to increase with heat treatment temperature [13,15,16]. Above 800 8C for 0.5NB and 0.5NB-ET, and above 850 8C for neat carbon, the apparent activation energies were lower and in the range 133 to 113 kJ / mol. However, for Ta-containing specimens and 0.5TI, the activation energies were the same as those below 800 8C. For neat carbon, 0.5NB and 0.5NB-ET, the oxidation reaction mechanism changed from surface reaction controlled to diffusion controlled above 800 or 850 8C.

3.6. WAS measurement and oxidation mechanism From the discussion in Section 3.5, experiments using the TK method were carried out at 727 8C, because the control mechanism of oxidation for all specimens in this temperature range was the same: surface reaction (either adsorption or desorption) control. CO and CO 2 are directly released from surface com-

Fig. 3. Weight loss rate as a function of weight loss.

early oxidation weight loss, while the other specimens did not show such a relationship. In the oxidation of Tacontaining specimens and 0.5TI, the weight loss rates, dX / dt, were not proportional to either (1 2 X), according to the volume reaction model, (1 2 X)2 / 3 , of the grain model, or (1 2 X) ? ln(1 2 X), of the random pore model [6]; therefore, another model should be considered. At a weight loss above a few mass%, the oxidation rates, 2 (dX / dt) /(1 2 X), of all specimens appeared to exhibit a linear relationship with the weight loss ratio, even though 0.5NB and 0.5NB-ET at 700 and 800 8C and neat carbon at 900 8C had an inflection point. The oxidation rates of neat carbon, 0.5NB and 0.5NB-ET exhibited a strong

Fig. 4. Arrhenius plot for the oxidation rate at 30 mass% oxidation weight loss.

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Y. Tanabe et al. / Carbon 40 (2002) 1949 – 1955

plexes, and are not by-products [17]. CO 2 was the most predominantly desorbed gas, and the amount of CO was about one-seventh to one-tenth that of CO 2 . Typical curves for the desorption rates as a function of time are shown in Fig. 5 in the case of 30 mass% weight loss. When the carbons were oxidized at about 900 K (627 8C), CO 2 was released from lactone or acid anhydride and CO was released from carbonyl or carboxyl surface complexes [18]. The oxidation rate, r ox (s 21 ), at 727 8C is plotted as a function of WAS in the case of 30 mass% weight loss in Fig. 6. If the desorption of CO 2 and CO from surface complexes controls the oxidation, r ox will be proportional to WAS as a constant (the reaction rate constant) because the reaction rate constant, k 5 r ox / WAS (s 21 ), is, in general, the same when the carbons are made of the same raw materials [19]. In the CO 2 gasification of 10 chars, the difference in gasification rate is expressed by the difference in the number of WAS, and not by rate constants. The number of WAS is related to the carbon structure of the chars, i.e. the crystallite size [20]. However, r ox did not show a linear relationship with WAS for these materials,

Fig. 6. Oxidation rate at 727 8C as a function of working active site (WAS) in the case of 30 mass% weight loss.

even though the precursor resin was the same. Moreover, the crystallite size of the major component, the B component, along the c-direction was similar among the specimens used in this study, as shown in Table 2. Upon oxidation, the specimens in this study, which were glasslike carbons, showed very different behavior, as in the case of chars [20]. Fig. 7 shows the relationship between desorption rate and oxidation rate at 727 8C in the case of 30 mass% weight loss. For neat carbon, 0.5NB and 0.5NB-ET, oxidation was controlled by the desorption of carbon oxide gases, CO 2 and CO, because the desorption rate was similar to the oxidation rate. However, for Ta-containing specimens and 0.5TI, the desorption rate was higher than

Fig. 5. Desorption rate of CO and CO 2 , r(CO) and r(CO 2 ), in neat carbon and 0.5TA-ET at 727 8C as a function of time.

Fig. 7. Total desorption rate, r(CO1CO 2 ), as a function of oxidation rate, r ox , at 727 8C in the case of 30 mass% weight loss.

Y. Tanabe et al. / Carbon 40 (2002) 1949 – 1955

the oxidation rate. Although the details need to be clarified, another mechanism(s) may be considered. Judging from the decrease in the oxidation rates at lower oxidation weight loss, we should focus on the formation processes of surface complexes in order to evaluate the controlling mechanism. Detailed analyses of WAS and the total desorption rate were carried out for neat carbon and 0.5TA-ET at ca. 10 to 30 mass% oxidation weight loss at 727 8C. WAS and the total desorption rate were not influenced very much by the weight loss. These results reveal that the difference in oxidation behavior between the two specimens is not a particular phenomenon occurring at a certain weight loss and that the anti-oxidation properties were due to the intrinsic properties of the Ta-containing specimens. The above results indicate that Ta and Ti reduce the affinity of some active sites for oxygen adsorption or modify the structures of the sites, thereby decreasing the oxidation rates. The formation of surface complexes may control the oxidation of these carbons, and this requires further study.

[2]

[3]

[4]

[5]

[6]

[7]

[8] [9]

4. Conclusions Furan-resin-derived carbons with Ta, Ti and Nb, in the oxide or ethoxide form, were prepared. The furan-ring structure was retained even with the addition of 5 mass% ethoxide and the structure of the carbons with metal elements was the same as that of neat carbon. The oxidation studies revealed that, in carbon with Ta or TiO 2 , no dense layers acting as protective layers against oxidation were formed. This indicates that the anti-oxidation behavior was due to the intrinsic properties of the carbons. Analysis of WAS revealed that Ta and Ti inactivated some active sites of the carbons effectively, or they modified the structure of the active sites, thereby decreasing the oxidation rates of the carbons.

Acknowledgements The authors thank Dr. Takashi Nishizawa of Kobe Steel Ltd, and Dr. K. Hoshi of Toyo Tanso Co., Ltd for extensive discussions. This work was partly supported by a Grant-in-Aid for Scientific Research on Priority Area, Carbon Alloys.

[10] [11]

[12]

[13] [14]

[15]

[16]

[17]

[18]

[19]

References [1] Yasuda E, Park S, Akatsu T, Tanabe Y. Development of self-mending type oxidation protection of furan-resin-derived

[20]

1955

carbon by Ta-compounds addition. J Mater Sci Lett 1994;13:378–80. Tanabe Y, Hotta Y, Akatsu T, Yamada S, Yasuda E. Shear strength of C / C composites containing inorganic compound effective for anti-oxidation. J Mater Sci Lett 1997;16:557–9. Tanabe Y, Yamanaka J, Hoshi K, Migita H, Yasuda E. Surface graphitization of furan-resin-derived carbon. Carbon 2001;39:2343–9. The 117th Meeting of JSPS, Measuring procedure for lattice constant and crystallite size of artificial graphites. In: Introduction to carbon materials. Tokyo: Carbon Society of Japan, Kagaku Gizyutsu Sha, 1973:117–42, 184–92 (in Japanese). Hishiyama Y. Electronic properties of carbon. In: Introduction to carbon materials, revised version, Tokyo: Carbon Society of Japan, 1984, pp. 41–53, in Japanese. Kyotani Y. Chasing in gasification behavior. In: Handbook for experiment and analysis of coal, Tokyo: Japan Institute of Energy, 1997, pp. 135–8, in Japanese. Zhuang Q, Kyotani T, Tomita A. Dynamics of surface oxygen complexes during carbon gasification with oxygen. Energy Fuels 1995;9:630–4. Hoshi K, Tanabe Y, Yasuda E. Effect of sonication on curing process in furan resin. TANSO 1997;1997:199–201. Price GJ. Ultrasonically enhanced polymer synthesis. Ultrason Sonochem 1996;3:S229–38. Inagaki M, Kamiya K. On multi-phase graphitization. TANSO 1972;1972(71):135–48, in Japanese. Zaldivar RJ, Rellick GS. Some observations on stress graphitization in carbon–carbon composites. Carbon 1991;29:1155–63. Yoshida A, Kaburagi Y, Hishiyama Y. Microstructure and magnetoresistance of glass-like carbon. Carbon 1991;29:1107–11. Rodriguez-Reinoso F, Waker Jr. PL. Reaction of glassy carbon with oxygen. Carbon 1975;13:7–10. Walker Jr. PL, Shelef M, Anderson RA. Catalysis of carbon gasification. In: Chemistry and physics of carbon, vol. 4, New York: Marcel Dekker, 1968, pp. 287–383; Smith IW. The intrinsic reactivity of carbon to oxygen. Fuel 1978;57:409–14. Perez-Mendoza M, Domingo-Garcia M, Lopez-Garzon FJ. Modification produced by O 2 and CO 2 plasma treatments on a glassy carbon: comparison with molecular gases. Carbon 1999;37:1463–74. Domingo-Garcia M, Lopez-Garzon FJ, Perez-Mendoza M. Modification produced by O 2 plasma treatments on a mesoporous glassy carbon. Carbon 2000;38:555–63. Walker Jr. PL, Vastola FJ, Hart PJ, editors, Fundamentals of gas–surface interactions, New York: Academic Press, 1967, pp. 307–17. Zhuang Q, Kyotani T, Tomita A. DRIFT and TK / TPD analyses of surface oxygen complexes formed during carbon gasification. Energy Fuels 1994;8:714–8. Radovic LR, Jiang H, Lizzio AA. A transient kinetics study of char gasification in carbon dioxide and oxygen. Energy Fuels 1991;5:68–74. Adschri T, Nozaki T, Furusawa T, Zhu Z. Characterization of coal char gasification rate. AIChE J 1991;37(6):897–904.