carbon composite materials in the presence of potassium and calcium acetates

Carbon 43 (2005) 333–344 www.elsevier.com/locate/carbon

Catalytic oxidation of carbon/carbon composite materials in the presence of potassium and calcium acetates Xianxian Wu *, Ljubisa R. Radovic Department of Energy and Geo-Environmental Engineering, The Pennsylvania State University, University Park, PA 16802, USA Received 23 February 2004; accepted 23 September 2004

Abstract The catalytic effects of potassium acetate (KAC) and calcium acetate (CaAC) on the oxidation of carbon/carbon composites (C/C composites) used in aircraft brake system have been characterized. Potassium exhibited a very strong catalytic effect on the oxidation of the selected carbon samples, including C/C composite blocks impregnated with aqueous KAC solution and graphite powder physically mixed with KAC powder. The initial amount of catalyst loading and the pre-treatment in inert gas were found to affect its catalytic effectiveness. Impregnated calcium was also a good catalyst for the oxidation of C/C composites, but its effectiveness is much lower than that of potassium and is much less sensitive to catalyst loading amount and pre-treatment. Calcium acetate physically mixed with graphite powder only showed a slight catalytic effect. The experimental results suggested that the interfacial contact between catalyst and carbon is the key factor determining catalytic effectiveness, in agreement with previous studies using porous carbon materials. Due to its unique wetting ability and mobility on the carbon surface, potassium can form and maintain such contact with carbon and is, therefore, more effective in the C–O2 reaction than calcium. The formation and development of such contact, which can also be affected by catalyst loading and pre-treatment process, can explain well the influence of these experimental conditions on the catalytic effect of potassium. The decreasing trend of reactivity with increasing burn-off in calcium-catalyzed oxidation is a result of interfacial contact loss because calcium does not have the necessary mobility to maintain such contact during reaction.
2004 Elsevier Ltd. All rights reserved. Keywords: A. Carbon/carbon composites, Graphite; B. Impregnation, Oxidation; D. Catalytic properties, Reactivity

1. Introduction Oxidation at more than 400 C is an inherent limitation for the applications of many carbon materials. This reaction can be significantly catalyzed by some metals, metal oxides or salts. A very small amount of such catalysts can produce a remarkable increase in carbon oxidation rate [1–3]. Since the presence of such catalysts is *

Corresponding author. Present address: Carbon Materials and Technology Group, Oak Ridge National Laboratory, 1 Bethel Valley Road, P.O. Box 2008, MS 6087, Oak Ridge, TN 37831-6087, USA. Tel.: +1 865 576 6690; fax: +1 865 576 8424. E-mail address: [email protected] (X. Wu). 0008-6223/$ - see front matter
2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.09.025

sometimes unavoidable, catalytic carbon oxidation is a persistent problem for material applications desiring low carbon loss. An outstanding example is that of C/ C composites used as aircraft brakes: the use of acetate salts as a more environmentally benign airport runway deicer [4], instead of the traditional urea-based materials, has led to more rapid brake wear because of carbon oxidation catalyzed by potassium [5,6]. In order to improve the resistance of carbon brake materials to catalytic oxidation, it is necessary to investigate in detail the catalytic oxidation of these materials and to understand the fundamental reasons. Extensive studies have been carried out in the past several decades on the gasification/oxidation of different

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carbons for the purpose of enhancing their reactivity. Those studies, mainly driven by interest in catalytic coal gasification, have reported notable advances in understanding the catalyzed C–O2 reaction, as well as quantifying the catalytic effects of alkali or alkaline earth metals. Potassium and calcium salts are known to be very effective catalysts for carbon oxidation [1,2]. The catalytic effects of potassium on the C–O2 reactivity of graphite [7–10], C/C composites [5,6], activated carbon [10–13], carbon black [9,10,14–16], cellulosic chars [17,18], and de-mineralized coal chars [10,19] have been investigated. Potassium compounds were loaded either by powder mixing [7,14,15,17] or by aqueous solution impregnation [8–13,18]. For porous carbons, when the amount of solution used was not greater than the pore volume of the carbon, incipient wetness impregnation techniques [20] were also used. In all cases, no matter what catalyst addition method was used, significant catalytic effects were observed. Calcium compounds are also active catalysts for carbon oxidation, as long as their dispersion on the carbon surface is good. It has been argued that the dispersion before the onset of reaction is the controlling factor in catalytic oxidation [21–23], and that the mobility of calcium species on carbon surface during reaction is quite limited [24]. Experiments showed that calcium compounds are capable of enhancing the oxidation rate of de-mineralized lignite chars [21–23,25,26], coal chars [27–31], cellulosic chars [32], soot [33], and graphite [34–37]. Experiments also showed that the loading method of catalyst affects its activity to a large extent [27,28,30]. Compared to CaCO3 precipitation and calcium acetate impregnation, calcium ion exchange gave the most active catalyst for calcium-loaded char oxidation. In the case of carbons with very low porosity [28,30,33–36], a pre-oxidation process was always conducted before ion exchange in order to increase the concentration of ion-exchangeable sites. A mild oxidizing agent, such as oxygen at low temperature [28,30,33] or dilute nitric acid solution [34,35] are most often used. While the knowledge and understanding of catalytic carbon oxidation obtained from those previous studies are valuable and important, a careful characterization of the catalytic oxidation of C/C composites is still necessary because of the use of different materials and for different purposes. Much of the previous research was done on high-surface-area, less-ordered and more porous carbons than of interest here. Because the desired objectives in those studies were rapid oxidation and high catalytic activity, excellent catalyst dispersion was always desirable. The measurements used to meet the pre-requisite conditions include relatively high catalyst loading (e.g., in the case of potassium, up to 50.0 wt% in powder mixture) and a carefully controlled catalyst loading procedure (e.g., in the case of calcium,

pre-oxidation and ion exchange). The catalytic effects reported in those studies, therefore, represent typically the highest activities of the catalysts used. In this study, the desired objective is a lower reactivity or, in other words, the chemical inhibition of carbon oxidation. The materials used here are highly non-porous carbons, C/C composites and a high purity graphite powder. The selected catalyst loading method is meant to simulate the actual conditions experienced by these materials on airport runways. Under these conditions, the activity of the catalysts may not be optimized. Some factors overshadowed in previous catalytic oxidation studies may thus become dominant. The objectives of this study were, therefore, to characterize the catalytic effect of potassium or calcium compounds on the oxidation of C/C composites, to understand those factors critical for catalytic effect, and to explore their significance in the oxidation inhibition of C/C composites.

2. Experimental A commercial C/C composite aircraft brake (CARBENIX(r) 4000 (C4N), courtesy of Aircraft Landing Systems, Honeywell, Inc.) and a high purity graphite powder (Ultra superior, <200 mesh, Johnson Matthey, designated as GP) were used in this study. Their properties are shown in Table 1. The C/C composite was cut into (6 ± 1) · (6 ± 1) · (2 ± 1) mm3 pieces with a diamond saw. Before impregnation with catalyst or the oxidation test, the cut samples were carefully washed with de-ionized water. The catalysts were loaded by placing the samples into an aqueous solution of potassium acetate (>99.0%, EM Science) or calcium acetate (99%, Aldrich Chemical Company, Inc.). This was followed by drying the soaked samples overnight in an oven at 383 K. The amount of catalyst deposited on the surface was controlled by varying the solution

Table 1 Characterization of carbon samples Property

C4N

GP

Carbon fiber content Density (g/cm3, He) Surface area (m2/g, N2 BET, 77 K) Purity


27% PAN fibers 1.99 0.22

– 2.13 10.90

<0.05 wt% K, Ca, Pb, Cu, Mg, Pa

99.9999%b

3.437 25.88 115

3.367 26.42 357

XRD parameters ˚ d0 0 2, A 0 0 2 peak position (2h) ˚ Lc, A a b

Determined by emission spectroscopy. According to the provider.

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3. Results 3.1. Catalytic effects The TPO profiles of uncatalyzed and catalyzed oxidation of C4N and graphite samples are shown in Figs. 1 and 2. The processes were terminated when the concentration of CO2 reached 500 ppm. The burn-off levels after the TPO test were generally around 2.0%. The TPO observations indicate that both potassium and calcium exhibit catalytic effects. If here we select the temperature at which the reactivity reaches 6.0 · 106 s1 as a comparison criterion, then the relevant temperature values are indications of catalyst effectiveness. As shown in Fig. 1, the calcium-impregnated sample (designated as C4NC, where the last C represents CaAC impregnation) is seen to have a lower characteristic temperature than its non-impregnated counterpart (74 K lower), while the potassium-impregnated sample (designated as C4NK, the last K representing KAC impregnation) has the lowest characteristic temperature. Compared to sample C4N, this temperature is approximately 180 K lower. In the case of graphite, physically mixed CaAC (designated as GPC) only has a slight catalytic effect (26 K difference in characteristic temperature). Physically mixed KAC (designated as GPK), on the other hand, displays almost the same strong catalytic effect as the impregnated potassium on the composite sample (175 K difference in the characteristic temperature between samples GP and GPK). The peaks appearing at <700 K in the TPO profiles of catalyst-impregnated samples represent the decomposition of the acetates. It is seen that CaAC decomposes at around 660 K in sample C4NC and at approximately 675 K in samples GPC. In the case of KAC, the decomposition temperatures are approximately 580 K in sample C4NK and approximately 640 K in sample GPK. The above results indicate

9.0E-6 8.0E-6 7.0E-6 -1

Reaction Rate (s )

concentration and determined by the weight difference before and after impregnation. In general, after impregnation the catalyst covered the sample homogeneously when its amount was not too high. The loading is expressed as weight percent of KAC or CaAC. In the case of graphite powder, the catalysts were introduced by physically mixing the powder with the 3.0 wt% acetates, the typical catalyst loading used for most experiments. The crystalline properties of the samples were characterized by X-ray diffraction (XRD). A Scintag-pad V theta/2 theta goniometer with Cu normal focus X-rays (Scintag Inc.) was used. The power supply was at 30 mA and 35 kV. Peaks were obtained by step scanning at a rate of 0.04 2h/min. XRD results indicate that the (0 0 2) peak of C4N sample is at lower angle and broader than that of the graphite samples. This indicates that its average crystallite height (Lc, the FWHM from SP-7 curve fitting of (0 0 2) peaks) is smaller than that of graphite, as listed in Table 1. The graphite samples have resolved (1 0 0) and (1 0 1) peaks, a signature of highly oriented 3-dimensional structure. In C4N sample, the (1 0 0) and (1 0 1) peaks are merged into a broad (1 0) peak, which indicates random-layer 2-dimensional order in the composites [38]. The commercial transient kinetics apparatus (SSITKA, Altamira Instruments, Inc.) with a ‘‘Sensorlab’’ mass spectrometer (VG Quadrupoles) was used for relatively high rate isothermal oxidation. Another reactor, the furnace part of a TGA system, with NDIR analyzers (Beckman, Model 864) for continuous monitoring of CO and CO2 was used for temperature programmed oxidation (TPO), low-rate isothermal oxidation and activation energy measurements. The reactivity of the samples (defined as R ¼  w10  dw g/g/s, where dw is the weight variation dt dt rate, and w0 is the initial weight of the sample) was calculated from the release rates of CO and CO2 gaseous products. In order to investigate the effect of sample pretreatment in inert gas on catalyst effectiveness, two different experimental procedures were adopted: (1) In the ‘‘no pre-treatment’’ procedure, the samples were heated in UHP Ar or N2 at 10 C/min to reaction temperature, and then kept in inert gas for 2–4 min to allow the system to achieve thermal equilibrium before the introduction of O2. This is referred to as 0 min in the text below. It is the default procedure used for experimental observations, except in cases indicated as ‘‘pre-treatment’’. (2) In the ‘‘pre-treatment’’ procedure, the samples were heated to the isothermal reaction temperature and held for hours in Ar, and then underwent oxidation. In the isothermal experiments, the gas flow rate was 50 cm3/min in the SSITKA system, and 155 or 300 cm3/min in the NDIR system.

335

6.0E-6

c

5.0E-6

b

a

4.0E-6 3.0E-6 2.0E-6 1.0E-6 0.0E+0 550

600

650

700

750

800

850

Temperature (K)

Fig. 1. TPO profiles of C/C composite samples: (a) C4N, (b) C4NC, 1.28 wt% CaAC, (c) C4NK, 3.10 wt% KAC.

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X. Wu, L.R. Radovic / Carbon 43 (2005) 333–344 9.0E-6

1.1E-5

8.0E-6

1.0E-5

b

a

-1

c

Reaction Rate (s )

-1

Reaction Rate (s )

7.0E-6 6.0E-6 5.0E-6 4.0E-6 3.0E-6

a

9.0E-6

b

8.0E-6 7.0E-6 6.0E-6

2.0E-6

5.0E-6

1.0E-6 0.0E+0 550

600

650

700

750

800

850

900

4.0E-6

950

c 0

0.1

0.2

0.3

0.4

0.5

0.6

Burn-off

Temperature (K)

Fig. 2. TPO profiles of graphite powder: (a) GP, (b) GPC, 3.0 wt% CaAC, (c) GPK, 3.0 wt% KAC.

Fig. 4. Burn-off profiles of graphite powder in 1 atm O2: (a) GP at 893 K, (b) GPC, 3.0 wt% CaAC at 873 K, (c) GPK, 3.0 wt% KAC at 713 K.

that KAC decomposes more easily than CaAC on the carbon surface, and aqueous solution impregnation lowers the decomposition temperature more than physical mixing does.

monotonic decrease in the measured burn-off range, while that of calcium-catalyzed graphite sample shows a rapid decrease in the initial stages (up to about 12.0% burn-off) and then a slight but monotonic increase. This indicates that the reactivity increase with burn-off (the distinctive profile of uncatalyzed oxidation due to the well documented increase in reactive site concentration [39–41]) becomes more and more predominant compared to the catalytic effect of calcium, and calcium continually loses its catalytic effect as carbon burn-off proceeds. The potassium-catalyzed oxidation displays a distinct burn-off profile which is very different from that of its uncatalyzed or calcium-catalyzed counterparts. There is a pronounced maximum at low burn-off: Catalyst activation is dominant at very low burn-off, but catalyst de-activation occurs at higher burn-off.

3.2. Burn-off profiles The influence of potassium and calcium species on carbon reactivity as a function of conversion was determined by isothermal oxidation in the same reaction rate range as their uncatalyzed counterparts for a relatively long time (up to 2 days). The temperature differences are themselves good indications of catalytic effectiveness. Figs. 3 and 4 show the characteristic reactivity profiles of composite and graphite samples as determined in the NDIR system. For both samples, uncatalyzed carbon oxidation exhibits a monotonic rise in specific reactivity with burn-off. Calcium-catalyzed oxidation shows evidence of continuous catalyst de-activation: The burn-off profile of sample C4NC shows a

3.3. Effect of catalyst loading Fig. 5 shows the reactivity of sample C4NK (no pretreatment) in the presence of different amounts of potas-

7.0E-6

2.4E-5 2.0E-5 -1

5.0E-6

c 4.0E-6 3.0E-6

b 2.0E-6 1.0E-6

Reaction Rate (s )

a

-1

Reaction Rate (s )

6.0E-6

1.6E-5 1.2E-5 0.46 wt% KAC 0.89 wt% KAC 1.37 wt% KAC 1.92 wt% KAC 4.38 wt% KAC

8.0E-6 4.0E-6 0.0E+0

0

0.1

0.2

0.3

0.4

Burn-off

Fig. 3. Burn-off profiles of C4N composite samples in 1 atm O2: (a) C4N at 843 K, (b) C4NC, 0.69 wt% CaAC at 773 K, (c) C4NK, 2.00 wt% KAC at 653 K.

0

0.1

0.2

0.3

0.4

Burn-off

Fig. 5. Effect of catalyst loading on the reactivity of C4NK samples at 693 K (1 atm O2, 50 cm3/min, no pre-treatment in Ar before oxidation).

X. Wu, L.R. Radovic / Carbon 43 (2005) 333–344

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Table 2 Effect of potassium catalyst loading at 713 K on the reactivity of C4NK samples (1 atm O2, 50 cm3/min) Induction period before runaway reaction (s)

0.61 0.76 1.26 2.35 3.70 3.78

Ignition Ignition Ignition 2.74 · 105 1.29 · 105 2.39 · 105

1680 1290 2580 41.1% Burn-off in 6 h 21.6% Burn-off in 6 h 18.0% Burn-off in 2 h (at 733 K)

Carbon samples with high surface area

-1

Maximum reactivity (Rmax, s1)

Reactivity (s )

KAC loading (wt%)

0.0

sium. When KAC loading was increased from 0.46 to 1.37 wt%, the reactivity at 693 K increased. But when it was further increased, there was a decrease in reactivity. As shown in Table 2, the same trends were observed at a temperature 20 K higher. At this temperature, sample C4NK with a lower KAC loading (0.61–1.26 wt%) ignited 20–40 min after the introduction of O2, while at 733 K a high-loading sample (3.78 wt% KAC) did not ignite. (Ignition is due to the highly exothermic character of the C–O2 reaction: the sample ignites when the dissipation of heat cannot keep up with heat generation.) Immediate ignition upon O2 introduction indicates that the initial reactivity is very high. A long induction period (i.e., 40 min), on the other hand, indicates that the catalytic effectiveness of the active potassium species and/or the amount of active species increases during this period. In Table 2, the duration of this induction period is seen to be a function of catalyst loading. An intermediate loading (0.76 wt%) shows the shortest induction period which indicates its highest catalytic effect. A higher initial catalyst loading results in a delay of ignition or no ignition at all. The dependence of catalytic activity on potassium loading is illustrated in a diagram of maximum reaction rate vs. catalyst loading, as shown in Fig. 6 (based on the data at 693 K). For comparison, the qualitative diagram for carbons with high surface area [7,42–44] is also shown in the same figure. For both non-porous materials used here and porous carbons used in previous studies, catalytic activity increases with increasing loading rapidly until approximately 1.0 wt%. At higher loading, the reactivity of porous carbons still increases with increasing loading up to approximately 50.0 wt% [7], while that of C/C composites decreases and becomes almost constant above a saturation point (
2.0 wt%). Due to the low surface area of C/C composites, the saturation point presumably corresponds to a loading at which the catalyst encapsulates the sample. An interesting phenomenon in calcium-catalyzed oxidation is the initial high catalytic activity. This is thought to be governed by the effective catalyst/carbon interfacial contact achieved during catalyst impregna-

C/C composite sample

1.0

2.0

3.0

4.0

5.0

Percent of K catalyst Fig. 6. Variation of catalytic effect with potassium catalyst loading.

tion. (The rate of the catalyzed reaction is assumed to be proportional to the number of reactive sites at the catalyst/carbon interface, as discussed below.) A straightforward inference is that a higher loading below the saturation level would result in better interfacial contact which would lead to higher initial catalytic activity. Experimental observations do confirm this trend. As seen in Table 3, initial reactivity is lower at low loading than at high loading. But the high-loading samples exhibit a steeper de-activation profile at the beginning of oxidation. Reactivity is seen to decrease to almost the same level after approximately 1.0% burn-off (corresponding to about 1 h of reaction). Subsequently, the de-activation trends are similar for both high- and low-calcium loading samples, and the total burn-off levels differ only slightly after more than 2 days (38.6% at 1.49% CaAC vs. 34.9% at 0.72% CaAC). The observation of ignition coincided with the above results (details not shown here): higher calcium loading results in higher initial reactivity. At higher calcium loading, the sample immediately ignited at 833 K, while the low-loading sample only reached ignition at 863 K. 3.4. Effect of pre-treatment period in inert gas The reactivities of samples containing more than 3.0% KAC (a typical uptake from saturated solution)

Table 3 Effect of calcium catalyst loading at 768 K on the reactivity of C4NC samples (1 atm O2, 155 cm3/min) CaAC loading (wt%)

Initial reactivity (s1)

Reactivity at 1.0% burn off (s1)

Total burn-off (%)

0.37 0.72 1.49

4.62 · 106 6.64 · 106 7.16 · 106

4.10 · 106 4.31 · 106 4.88 · 106

35.2 in 50 h 34.9 in 48 h 38.6 in 48 h

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-11 0 hours, 3.70 wt% KAC 2 hours, 3.37 wt% KAC 4.25 hours, 3.29 wt% KAC 6 hours , 3.60 wt% KAC 10 hours, 3.99 wt% KAC

-1

Reaction Rate (s )

4.0E-5

-11.5 -12 -1

lnR (s )

3.0E-5

2.0E-5

-12.5 -13

a

c

b

-13.5

1.0E-5

-14 0.0E+0

0

0.1

0.2 Burn-off

0.3

0.4

-14.5 0.0011

0.0013

0.0015

0.0017

-1

Fig. 8. Arrhenius plots for catalytic oxidation of C4N composite samples in 1 atm O2: (a) C4N, (b) C4NC, 0.69 wt% CaAC, and (c) C4NK, 2.00 wt% KAC.

-11.3

-1

vs. the duration of sample pre-treatment in Ar are shown in Fig. 7. The sample with no pre-treatment exhibited the lowest reactivity, while the sample with an intermediate pre-treatment time (4.25 h) displayed the highest reactivity. A longer period (such as 6 or 10 h) decreased the reactivity slightly, but it is still higher than when there was no pre-treatment. It is seen that the profile at intermediate pre-treatment time also has the highest initial reactivity as well as the sharpest increase at low burn-off, which corresponds to the range in which catalyst activation is important. This is attributed to a strong initial catalytic effect induced during an optimum pre-treatment period and to a rapid subsequent development of this effect. On the other hand, the lowest curve (corresponding to no pre-treatment) only shows a slight increase in 6 h. Obviously, in the absence of pre-treatment KAC shows the lowest initial catalytic effect and the slowest development of its activity. Pretreatment of calcium-loaded C4N samples in Ar does not show any such effect on their activity; instead, a lower initial reactivity is observed upon pre-treatment. This initial difference disappeared very quickly and there was no significant difference in the total burn-off after 6 h. This is a characteristic result for calcium-catalyzed oxidation: the magnitude of the catalytic effect mainly depends on the interfacial contact established during impregnation by the catalyst precursor. There is no activation process observed in calcium-catalyzed oxidation.

1/T (K )

Ln (R) (s )

Fig. 7. Effect of pre-treatment time (in Ar) on the reactivity of C4NK samples at 713 K (1 atm O2, 50 cm3/min, oxidation for 6 h).

-11.8

-12.3

-12.8

-13.3 0.00105

a

b

0.00115

c

0.00125

0.00135

0.00145

0.00155

-1

1/T (k )

Fig. 9. Arrhenius plots for catalytic oxidation of graphite powder in 1 atm O2: (a) GP, (b) GPC, 3.0 wt% CaAC, and (c) GPK, 3.0 wt% KAC.

Table 4 Activation energy of catalytic carbon oxidation Samples

Ea (kJ/mol)

Pre-exponential factor

Measured temperature range (K)

Measured burn off (%)

3.5. Dependence of catalytic oxidation on temperature and O2 partial pressure

C4N C4NC C4NK GP GPC GPK

170.2 141.2 116.4 190.5 169.7 102.8

1.28 · 105 1.99 · 104 8.68 · 103 7.82 · 105 1.14 · 105 1.91 · 102

798–888 703–773 613–663 863–913 853–903 673–743

2.1–5.3 3.7–5.8 12.5–14.9 7.5–11.5 18.3–20.6 10.0–12.0

The influence of temperature on the reactivity of composite and graphite samples was determined when the reactivity was less than 105 s1. The Arrhenius plots are shown in Figs. 8 and 9. The intercept reflects the contributions from reactants concentrations (the concentration of active sites and oxygen pressure). Because 1 atm O2 was used, the measured numbers are equal to a constant A, which has the same unit as the

rate coefficient k. Table 4 summarizes the relevant parameters obtained. The Ea value for graphite samples is in reasonable agreement with data reported in the literature: 196.0–243.0 kJ/mol [45,46], which verifies chemical reaction control. Composite samples show a slightly lower Ea, which is in agreement with the well known, but as yet unexplained, fact that more highly ordered car-

X. Wu, L.R. Radovic / Carbon 43 (2005) 333–344

3.6. CO/CO2 ratio Fig. 10 shows the CO/CO2 ratio as a function of carbon burn-off under isothermal conditions. The presence of catalysts is seen to reduce the CO/CO2 ratio, an often observed trend reported by many other investigators [2,16,41]. The ratio for samples C4N (curve (a)) and C4NK (curve (c)) remains essentially constant over the measured burn-off range, while that of samples C4NC (curve (b)) and GPK (curve (f)) shows an increasing trend. The latter is a symptom of catalyst de-activation [2]. After a certain burn-off level, sample GPC (curve (e)) displays the same decreasing trend as sample GP (curve (d)). This observation also reflects a rapid loss Table 5 Reaction order (n) for oxidation of C/C composites Sample

C4N

C4NC

C4NK

n

0.40

0.40

0.51

0.5

0.4

CO/CO2 ratio

bons have higher activation energies of oxidation [47]. In addition, Chang and Rusnak [48] reported 181.0 kJ/ mol for heat-treated and ground carbon composites. Ehrburger et al. [49] measured a value of 178.0 kJ/mol for graphitized C/C composites blocks (10 · 5 · 2 mm). McKee [50] reported that the Ea of C/C composites is in the range 147.0–189.0 kJ/mol. The values for the composite samples studied here show very good agreement with these observations, and this further confirms that oxidation kinetics was not affected by diffusion (for which the reported Ea was, for example, 14.0–16.0 kJ/ mol [46]). For both composite and graphite samples, the two catalysts are seen to reduce both the Ea and the preexponential factor, in agreement with similar trends abundantly documented in the literature [1,2,46]. This is the well known ‘‘compensation effect’’ in heterogeneous catalysis [51]. In addition, the Ea values listed in Table 4 are also very close to the ranges reported in the literature [14,15,28,29,37]. A comparison of Ea values shows that potassium has a stronger catalytic effect than calcium. The dependence of catalytic activity on partial pressure was determined in the range of 10–100 vol% O2. The measurements were performed in the same reactivity ranges to ensure a Zone I reaction [46] and in narrow burn-off ranges to minimize the effect of burn-off. (The corresponding temperature ranges and burn-off levels are listed in Table 4.) The relevant data can be described well by a power-law rate equation: R(s1) = k(PO2)n. The measured reaction orders n for the composite samples are listed in Table 5. It is seen that the order with respect to oxygen concentration is approximately 0.4 for uncatalyzed and calcium-catalyzed oxidation, while it is approximately 0.5 for potassium-catalyzed oxidation.

339

d

0.3

a 0.2

e f

0.1

b c

0

0

0.1

0.2

0.3

0.4

0.5

Burn-off

Fig. 10. Dependence of the CO/CO2 ratio on the burn-off level of C4N composites and graphite powder (in 1 atm O2): (a) C4N at 843 K, (b) C4NC, 0.69 wt% CaAC at 773 K, (c) C4NK, 2.00 wt% KAC at 653 K, (d) GP at 893 K, (e) GPC, with 3.0 wt% CaAC at 893 K, (f) GPK, with 3.0 wt% KAC at 713 K.

of catalytic activity of calcium when CaAC is physically mixed with graphite powder (in agreement with the results of isothermal oxidation in Fig. 4). As carbon burn-off proceeds, the non-catalytic reaction becomes dominant.

4. Discussion 4.1. Catalytic mechanisms The catalysts presumably affect the various stages of carbon oxidation in different ways. For potassium- and calcium-catalyzed oxidation, it is believed [2,7,52–54] that the catalytically active species, which can undergo an oxidation/reduction (redox) cycle, act primarily as more effective adsorption and dissociation agents for the gaseous reactant than carbon itself, and they then transfer the adsorbed oxygen to the carbon. According to the reaction sequence anticipated by McKee [53], this process can be represented as 2S + O2 ! 2S(O)

ð1Þ

S(O) + C ! Cf (O) + S

ð2Þ

Here S represents an active catalytic site, i.e., a reduced site (e.g., a substoichiometric oxide) on the catalyst surface or at the catalyst/carbon interface, on which dissociative adsorption of O2 is much easier (and thus presumably faster) than on the carbon sites; and Cf represents a free reactive site on the carbon surface. In addition to enhanced adsorption, the key to effective catalysis is seen to be the transfer of surface oxygen from

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the catalyst to the carbon, which essentially depends on the formation of good interfacial contact between catalyst and carbon. Furthermore, as oxygen accumulates on the carbon surface, the desorption of CO and/or CO2 is enhanced by virtue of the so called ‘‘induced heterogeneity’’ effect [55]: The activation energy for desorption is thought to decrease as surface coverage increases. The main experimental observations in this study can be explained well by the above general mechanism. Due to the largely non-porous and graphitic C/C composite samples, it is not a surprise that catalyst effectiveness, especially for the case of calcium, is very sensitive to the interfacial contact, because this is a ‘‘lumped’’ parameter which can affect the adsorption and transfer of oxygen as well as the desorption of intermediate complexes. Because all measurements of this study were done in a low reactivity range (generally 105 s1 or lower), the measured reaction order should reflect the combined effects of chemisorption, oxygen migration and surface complex desorption. The higher the order, the larger the contribution of O2 chemisorption to the reaction rate. Compared to calcium, potassium has the ability to maintain good interfacial contact with carbon, which is beneficial for oxygen transfer and results in the increase of the contribution of oxygen adsorption by potassium species at relatively lower temperature (almost 200 C lower than in uncatalyzed oxidation). This explains why the reaction orders for potassium-catalyzed oxidation are the highest when compared to both uncatalyzed and calcium-catalyzed oxidation. 4.2. Effectiveness of catalysts It is well known that the catalytic effect of calcium in carbon oxidation does not appear to be as significant as that of potassium [2]. The difference is seen to depend here on the material used and catalyst loading method: compared to potassium, which significantly reduced the characteristic temperature regardless of the material and method, calcium catalysis is more effective for the composite sample than for graphite sample. Presumably, mixing solid catalyst particles with graphite powder vs. obtaining catalysts upon the decomposition of impregnated acetate precursors results in different catalytic activities because of different resultant catalyst surface areas and different degrees of interfacial (catalyst/ support) contact. But potassium obviously is an exception to this general rule because the active potassium species has the ability to re-disperse itself quite readily during oxidation; therefore potassium-catalyzed oxidation behavior is often independent of the catalyst preparation method [56]. This is a consequence of the lower melting point and higher mobility of potassium salts and oxides. It is well known that melting of the catalyst phase and subsequent wetting of the carbon substrate facilitates the dispersal of the catalyst and thus

enhances its activity [57]. Indeed, the presence of an intermediate wetting state has been argued to be important for a catalyst to establish good contact with graphite edge sites [58] and thus facilitate the necessary oxygen transfer. Experiments showed [59] that potassium compounds have very good wetting ability on the carbon surface, e.g., they exhibit a very small contact angle with graphite (e.g., 18 for the carbonate, <50 for the oxides formed upon thermal decomposition of KNO3). On one hand, the decomposition and melting of the acetates at low temperature gives the impregnated catalysts a chance to form such good contact. On the other hand, the potassium species formed upon acetate decomposition also have the ability to wet and migrate on the carbon surface [6,7,16], because those species also can melt and migrate at relatively low temperatures. In catalytic carbon oxidation, significant mobility and thus potential spreading of the catalyst occurs at the Tammann temperature [60], approximately half the bulk melting point (in Kelvin). Compared to calcium, potassium compounds obviously have much lower mobility temperatures (e.g., 653 K for KO2 vs. 3173 K for CaO). The results obtained here with non-porous carbons are exactly analogous to those reported for the microporous coal chars and activated carbons. The initial catalytic activity of calcium is thought to be governed by the interfacial contact achieved during catalyst impregnation. The loss of catalytic activity, on the other hand, is also related to the surface properties of the carbon materials. As proved by many studies [21–23,25–33], a strong catalytic effect of calcium is only obtained for those materials where calcium is initially highly dispersed e.g., by ion exchange onto a pre-oxidized carbon or a low-rank coal precursor. Thus, for example, achieving high calcium dispersion on a coal char can be difficult when the pre-existing ion-exchangeable groups (e.g., carboxylic groups) are destroyed during coal de-volatilization [28]. The now standard method to solve this problem is to treat the char (or activated carbon) with a mild oxidizing agent such as oxygen at low temperature [27,28,33] or dilute nitric acid solution [34,35] to re-create these active sites for calcium exchange. Unlike potassium compounds, calcium oxide or salts cannot re-disperse on the carbon surface during reaction with O2 because of their higher melting point. It has been reported [29] that the reactivity increase for a CaCO3-loaded char, if based on Ca/C atomic ratio, is in agreement with the approximately linear relationship between calcium loading and reactivity. This implies that surface complexes can form at the interface, but that no further calcium dispersion can take place during reaction. As oxidation proceeds, this interfacial contact is lost because of limited catalyst mobility, and catalyst de-activation occurs. The insignificant catalytic effect of calcium on the reactivity of graphite powder also gives evidence of the importance of interfacial contact. Be-

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cause CaAC was physically mixed with the powder, catalyst/support interaction is much poorer than in the case of aqueous impregnation. This also resulted in rapid deactivation with carbon burn-off; indeed, after a certain level of burn-off, the uncatalyzed oxidation signature profile is seen to become dominant (curve (b) in Fig. 4). Catalyst loading should certainly have a strong influence on catalytic activity: If catalyst dispersion and interfacial contact can be maintained as the amount of catalyst increases, catalytic activity should be directly proportional to catalyst loading [16,61,62]. In the case of highly porous materials, such as activated carbon, it was found that changes in calcium loading affect catalytic effectiveness to a greater extent than changes in potassium content [63]. Our experimental observations coincide with such results, even though apparently they are contradictory, especially regarding the effect of calcium loading. Due to the non-porous character and very low concentration of surface functional groups, the contribution of ion exchange to catalyst loading on our samples is negligible. Furthermore, these amounts, even at the lowest loadings used, are much greater than the highest possible ion-exchange loadings. As mentioned in Section 3.3, catalytic activity of ion-exchanged or impregnated calcium typically exhibits a saturation point, beyond which higher loading does not significantly increase the reactivity [64]. This saturation point often coincides with the Ca2+ exchange capacity of the carbon. The loadings used in this study (which were guided by practical considerations) are surely far above the saturation point, so there is no significant influence on catalytic activity. In the case of potassium, even though the loadings are above the saturation point, the wetting ability and mobility of potassium species formed by carbon-assisted reduction of salts ensure the maintenance of good interfacial contact, which results in the activation of the catalyst during pre-treatment and the absence of significant de-activation during the burn-off process.

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enhanced decomposition or reduction occurs at low temperatures only when the carbonate is in good contact with the carbon [42,66]. Indeed, decomposition or reduction can be greatly increased by intimate contact with the carbon substrate [2,7,16,52,61]. As shown in Fig. 5 and Table 2, there is an optimum potassium loading which can result in the highest catalytic effect. This is schematically represented in Fig. 11. We hypothesize that this amount corresponds to the most favorable coverage of the carbon surface by KAC or its derivative compounds (Fig. 11a). When loading is lower, an additional amount of catalyst could be both chemically activated (i.e., carbothermally reduced to the active state) and well dispersed (by maintaining high interfacial contact area), thus resulting in increased catalytic activity. When loading is higher, a decrease in catalytic activity is due either to inability of the carbon to reduce the catalyst to its catalytically active state (Fig. 11b) or to inability of the catalyst to preserve the high interfacial contact area (Fig. 11c); in the extreme case, which is much more readily seen for calcium than for potassium, carbon encapsulation by an impervious catalyst layer results in complete loss of catalytic activity. In addition, Fig. 7 shows that the catalytic activity of potassium is sensitive to the thermal history of the sample, because the reactivity depends on the extent of catalyst reduction prior to introducing the reaction gases [54]. A moderate pre-treatment results in the highest initial reactivity and fastest catalyst activation. In Fig. 11, case (a) represents the highest catalytic activity as a combined result of excellent interfacial contact, complete catalyst reduction and high O2 accessibility [61]. This case corresponds to the maximum reactivity of Figs. 5 and 7, and is responsible for the auto-ignition of C4NK samples (Table 2). In case (b) only a fraction of the catalyst is reduced to its active state; even though O2 accessibility is still high, catalytic activity can thus reach only an intermediate level. The reactivity of ‘‘no pre-treatment’’ C4NK samples is of this type. In case (c), catalyst loading is excessive, the catalyst can only be partially reduced, interfacial contact is poor, and perhaps most importantly O2 accessibility (through discontinuities in the catalyst layer) is low, thus resulting in the lowest catalytic activity. In this study, catalyst loading above the saturation point actually belongs to this case, because that catalyst amount can cover the surface of the block samples and form an almost impervious layer.

4.3. Interfacial contact between carbon and catalyst As discussed in Section 4.2, the interfacial contact is known to be the key reactivity-determining factor [16,52,62,63,65]. The contact area increases as catalyst particle size decreases or as its dispersion increases. It has been concluded that catalytic activity of alkali carbonates requires carbonate decomposition, and carbon-

Inactive phase

Carbon layer

Active phase

Catalyst particles

a

b

Catalyst layer

c

Fig. 11. Schematic representation of the catalyst/carbon interface: (a) highest catalyst activity (at low-to-intermediate catalyst loading), (b) intermediate catalyst activity (at low-to-intermediate catalyst loading), (c) low catalyst activity (at intermediate-to-high catalyst loading).

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4.4. Approaches to oxidation inhibition From the standpoint of a simplified reaction mechanism, uncatalyzed oxidation occurs as follows: oxygen molecules are chemisorbed at active sites to form carbon–oxygen surface complexes, and at the right temperature, these complexes dissociate to CO or CO2. In this case, a suitable solution to oxidation protection is to lower the concentration of active sites. Catalytic oxidation, on the other hand, involves a ‘‘redox’’ cycle of the intermediate active species. The overall effect of the cycle is the oxygen transfer to the carbon. So in order to improve the resistance of carbon to catalytic oxidation (without using a coating layer which in friction applications is of limited usefulness), there are two general approaches: intrinsic inhibition of carbon oxidation and catalyst de-activation. Minimizing the number of active sites in carbon material itself by enhancing graphitization and/or adding chemical inhibitors would be two approaches to intrinsic inhibition. If only graphitization is used, the catalytic effect of potassium and calcium on high-purity single crystal graphite provides an obvious upper limit which can be reached by this method: The concentration of active sites is the lowest and the initial interfacial contact cannot be good when physical mixing in the solid state is used. In the case of calcium, it is indeed seen that the catalytic effect is not too high, but potassium still turns out to be an effective catalyst even in this extreme situation. This leaves adding inhibitors to improve intrinsic resistance of carbon materials a better choice, which has been explored elsewhere [67]. In a composite material, the fibers and the matrix are expected to have different resistances to oxidation. For example, it has been observed [68] that PAN carbon fibers are oxidized much more rapidly than the CVD matrix. A combined use of in situ SEM and XRD techniques has shown that potassium strongly catalyzes the preferential oxidation of carbon fibers in a CVD composite [6]. Our own SEM observations of fresh composite samples and partially oxidized samples support these findings. PAN carbon fibers in sample C4N were oxidized first, which resulted in the formation of holes or residual graphitized sheathes that once surrounded the fibers [69]. To improve the intrinsic resistance of the composites, the first consideration should therefore be the relatively weaker components. Improvement of oxidation resistance of carbon fibers will certainly improve the overall resistance of the composites. Another consideration could be the optimization of porosity of the fibers because this can improve the oxidation resistance of C/C composites by promoting fiber-matrix adhesion and enhancing carbon graphitization [70]. For catalyst de-activation experienced by airport runway deicing agents, approaches based on analysis of the oxygen transfer mechanism could include any method

for minimizing the interfacial contact area, preventing the catalyst reduction or interfering with the redox cycle, destroying the active catalyst species, and scavenging the catalyst by promoting its conversion to an inactive glass/ glaze. The results of this study arguably shed light on strategies to minimize interfacial contact: Because it has been demonstrated that this contact is so important for catalytic activity, any approach that can reduce it will result in lower catalytic activity. This is a more effective strategy in the case of calcium than potassium. Because potassium has the ability to maintain good interfacial contact with the carbon surface, an initially poor contact can be improved by its mobility and wetting ability during reaction. Protection of C/C composites from potassium-catalyzed oxidation remains a very challenging task.

5. Conclusions The catalytic effectiveness of potassium and calcium on the oxidation of non-porous carbon materials has been characterized. Both potassium and calcium are effective in C/C composite oxidation, but potassium has a much stronger effect. Very different burn-off profiles were observed for these two catalysts: in contrast to calcium the effectiveness of potassium was insensitive to catalyst loading method but sensitive to catalyst loading amount and the pre-treatment in inert gas. The establishment and maintenance of interfacial contact is the fundamental reason behind these observations. Due to better wetting ability and higher mobility, potassium is more effective as C–O2 reaction catalyst than calcium. Because calcium does not have the ability to increase or maintain such contact, its effectiveness was quite sensitive to the catalyst loading method and it decreased continuously with carbon burn-off. The appearance of a maximum reactivity during the initial stage of potassium-catalyzed oxidation is noticeable, and it is dangerous for practical oxidation protection.

Acknowledgments This work was supported by the Carbon Research Center at the Energy Institute of the Pennsylvania State University (F. Rusinko, Jr. Director). We thank T. Walker of Honeywell Inc. for providing samples of C/ C composites.

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