Crystalline Structure Transformation of Carbon Anodes during

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Energy & Fuels 2008, 22, 1902–1910

Crystalline Structure Transformation of Carbon Anodes during Gasification Kien N. Tran,† Adam J. Berkovich,‡ Alan Tomsett,‡ and Suresh K. Bhatia*,† DiVision of Chemical Engineering, The UniVersity of Queensland, St. Lucia, QLD 4072, Australia, and Rio Tinto Alcan Technology, Thomastown, VIC 3074, Australia ReceiVed December 5, 2007. ReVised Manuscript ReceiVed February 13, 2008

The crystalline structure transformation of five carbon anodes during gasification in air and carbon dioxide was studied using quantitative X-ray diffraction (XRD) analysis and high-resolution transmission electron microscopy (HRTEM). XRD analysis and HRTEM observations confirmed that anodes have a highly ordered graphitic structure. The examination of partially gasified samples indicated that crystalline structure transformation occurred in two stages during gasification. The first stage involved the consumption of disorganized carbon matter in the initial 15% conversion. Oxygen was found to be more reactive toward disorganized carbon at this stage of the gasification process compared to carbon dioxide. Following this stage, as more carbon was consumed, especially with the removal of smaller crystallites, it was found that the crystalline structure became more ordered with increasing conversion levels. This is due to the merging of neighboring crystallites, required to maintain the minimum energy configuration. In addition, the interaction between the pitch and the coke components was found to be strongly linked to the initial coke structure. “Stress graphitization” occurred at the pitch-coke interface, which helps to enhance the structural development of the anodes.

1. Introduction To understand the carbon gasification process and ultimately to develop a model to assess and predict the intrinsic reaction rate of the carbon material, it is important to study the internal structural evolution during the gasification process. The crystalline structures of carbons and chars are generally disordered in nature and comprised of a distribution of small crystallites. During gasification, these small crystallites are consumed, which leads to changes in both the pore structure as well as the crystalline structure. The changes in pore structure due to gasification in air can be significantly different compared to changes occurring due to gasification in CO2.1,2 Changes in the crystalline structure due to gasification by these two agents can be similar.3 Compounding these complexities, the variation in the intrinsic reaction rate with conversion, normalized using surface area, can also vary widely between different materials, even for the same gasifying agent. It is expected that changes in the crystallite structure will be directly reflected in changes in the active parts of the carbon, because the active sites are generally considered to reside mainly at the edge sites of the crystallites. Therefore, more reliable information regarding the gasification process can be obtained from studying the changes in the crystalline structure during gasification. The evolution of the carbon structure during gasification can be studied using various techniques, among which X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) provide the most direct analysis of the crystalline structure. Both techniques are complimentary to each * To whom correspondence should be addressed. Telephone: +61-73365-4263. Fax: +61-7-3365-4199. E-mail: [email protected]. † The University of Queensland. ‡ Rio Tinto Alcan Technology. (1) Feng, B.; Bhatia, S. K. Carbon 2003, 41, 507–523. (2) Salatino, P.; Senneca, O.; Masi, S. Carbon 1998, 36, 443–452. (3) Feng, B.; Bhatia, S. K. Energy Fuels 2003, 17, 744–754.

other. Feng et al.3 studied the changes in the crystalline structure of raw and demineralized coal chars due to gasification in air and in CO2 by using XRD and HRTEM techniques. They observed a decrease in ordering for the raw coal chars with gasification but an increase in ordering for the demineralized coal char during gasification. They attributed the decrease in ordering to the catalytic effect of inorganic impurities on the gasification process. In a similar study, Lu et al.4 reported that chars showed an apparent increase in structural ordering during combustion. Sharma et al.5 studied the microtextural changes of chars during low temperature gasification by using a quantitative HRTEM technique. They found that the ordering of the crystalline structure can decrease, remain the same, or increase, depending on the initial distribution of the crystallite size of coal chars as well as the presence of inorganic impurities. At high temperature, the change in the crystalline structure can also occur over very small time scales. Davis et al.6 combusted pulverized coals in a laboratory entrained flow reactor and noted an increase in structural ordering with conversion. This ordering is seen to occur over a time scale comparable to that of the combustion process itself, which is on the order of 100 ms at a particle temperature of 1800 K and an oxygen concentration of 12% mol. Carbon anodes are a type of carbon-carbon composite made from a combination of calcined petroleum coke and coal tar pitch.7 These anodes generally possess low porosity and a partially graphitized carbon structure. Hence, the reactivity of carbon anodes is much lower than that of other more porous (4) Lu, L.; Kong, C.; Sahajwalla, V.; Harris, D. Fuel 2002, 81, 1215– 1225. (5) Sharma, A.; Kadooka, H.; Kyotani, T.; Tomita, A. Energy Fuels 2002, 16, 54–61. (6) Davis, K. A.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. J. Combust. Flame 1995, 100, 31–40. (7) Marsh, H.; Heintz, E. A.; Rodriguez-Reinoso, F. Introduction to Carbon Technologies; University of Alicante: Alicante, Spain, 1997.

10.1021/ef700732p CCC: $40.75  2008 American Chemical Society Published on Web 04/05/2008

Structure Transformation of Carbon Anodes

Energy & Fuels, Vol. 22, No. 3, 2008 1903

Table 1. Composition and Physical Properties of Carbon Anodes element (wt %) sample

C

O

N

S

BET area (m2/g)

HSC-1000(1025) HSC-1100(1100) HSC-1225(1200) LSC-1125(1100) LSC-1220(1200)

95.0 95.3 95.7 96.5 97.0

1.2 1.0 0.7 1.0 0.5

1.3 1.2 1.0 1.1 1.1

2.5 2.5 2.6 1.4 1.4

9.75 5.87 2.21 4.46 1.43

carbons such as coal chars. Despite their low reactivity, the loss through gasification at the high temperature operational conditions in the aluminum reduction cells is still largely about 15%, and it is considered excessive by the aluminum industry. Despite the vast industrial literature dedicated to understanding the factors influencing the reactivity of carbon anodes,8 there are few studies focusing on understanding the crystalline structure evolution of carbon anodes during gasification. Due to its low porosity, the carbon consumption of anodes should be strongly influenced by its crystalline structure, especially during the gasification process. This paper seeks to understand this important carbon consumption process. This study was carried out using carbon anodes made from two different types of petroleum coke and baked at different temperatures. The crystalline structure evolution was assessed using a combination of helium pycnometry, quantitative XRD analysis, and HRTEM fringe lattice imaging. 2. Experimental Section 2.1. Materials. Five carbon composite samples simulating anodes made from a mixture of calcined petroleum coke and coal tar pitch were used in this study. There were two types of petroleum coke utilized in this process: high sulfur cokes (HSCs), which contain approximately 3.2 wt % S, and low sulfur cokes (LSCs), which contain approximately 1.7 wt % S. These petroleum cokes were calcined at several temperatures in the range between 1000 and 1300 °C. The cokes were combined with coal tar pitch in a 70/30 wt % mixture of coke/pitch by using a laboratory scale mixer. The green paste mixture was then baked in crucibles packed within a bed of petroleum coke. The baking program was as follows: heat from room temperature to 650 °C at 50 °C/min, hold at this temperature for 6 h to allow the pitch to carbonize, ramp the temperature up to the final baking temperature (1025–1200 °C) at 100 °C/min, and hold at this final temperature for 12 h to allow the structure of the composite to fully develop. These samples were prepared at the research laboratory of the Rio Tinto Alcan Technology laboratory. The sample identification is assigned as follows: coke type-calcination temperature (baking temperature). For example, HSC-1000(1025) stands for a sample made using high sulfur coke calcined at 1000 °C and baked at 1025 °C. The samples selected for this study are as follows: HSC-1000(1025), HSC1100(1100),HSC-1225(1200),LSC-1125(1100),andLSC-1220(1200). The aim was to compare the influences of different petroleum coke types and calcination levels on the structural development of the anodes at various baking levels. The chemical compositions and BET (Brunauer, Emmett, and Teller) surface areas of these samples are summarized in Table 1. 2.2. Sample Preparation. The samples (particle size 180–212 µm) were gasified in a hinged tube furnance (Linberg/Blue Model HTF55347C) under either an air or CO2 atmosphere. The samples, placed in a small quartz boat (4–15 g), were first heated to the reaction temperature (540 °C for air and 940 °C for CO2) under a nitrogen atmosphere. The sample was then kept at this temperature for about 15 min to allow the sample bed to reach the reaction (8) Fischer, W. K.; Mannweiler, U.; Keller, F.; Perruchoud, R. C.; Buhler, U. Anodes for the Aluminum Industry; R&D Carbon Ltd.: Sierre, Switzerland, 1995.

temperature. The oxidizing gas was then introduced, and the reaction was allowed to take place until the desired conversion was reached. The gas flow rate used in the experiment was 500 cm3/min. Once a set conversion point was reached, the oxidizing gas was switched back to nitrogen, and the furnace was allowed to cool to room temperature. The cooled samples were transferred to a desiccator for storage. All anode types were gasified to at least five different conversion levels in the range of 5–90%. 2.3. Helium Density. The helium densities of the partially oxidized anode samples were measured using a Micromeritics AccuPyc 1340 helium pycnometer. 2.4. X-ray Diffraction (XRD) Analysis. The XRD analysis was conducted with a diffractometer (PANalytical XPERT-PRO) using a Cu KR target (λ ) 0.15406 nm). Measurements were made in a step scan mode (0.1o/step) over the 2θ range of 10–120o. A preset scan time of 30 s was used for each step. Quantitative analysis was carried out using two methods. The first method is applied according to the conventional Scherrer equation: Lc )

0.89λ Bc cos(θc)

La )

1.84λ Ba cos(θa)

(1)

where λ is the wavelength of the radiation used. Bc and Ba are the full width at half-maximum height of the (002) and (110) peaks, respectively; θc and θa are the corresponding scattering angles. In addition, the parallelism indicator (R), defined as the ratio of the peak height to the background height of the 002 peak, was determined directly from the XRD patterns. This parallelism indicator represents the probability that parallel stacked graphene sheets occurred in the crystalline structure. The larger the value of R, the greater the proportion of layers within the crystallites having more than one layer.9 The second method is one that was originally developed by Alexander and Sommer10 to analyze the structure of carbon black. The method was based on the established XRD theory of Franklin,12 which assumes that the disorganized carbon distorts the peaks, leading to asymmetry. The fraction of disorganized carbon, D, was determined from the degree of asymmetry, and the resolved symmetric bands were then interpreted to determine the mean crystallite parameters such as the interlayer spacing d002, the crystallite height Lc, the crystallite length La, and the apparent crystallite stack distribution pn. The quantitative analysis procedure is outlined as follows: Since the observed intensity curve can be affected by many random factors, such as sample packing, it must be converted to an appropriate form for quantitative analysis. The measured intensity curve is converted to atomic units by using the techniques described by Klug and Alexander.13 The coherent scattering factors of Berghuis et al.14 and the incoherent scattering factors of Keating and Vineyard15 are used in the conversion process. The asymmetric (002) line profile is converted to the symmetrical form I′ according to the following equation:17 I′ )

(

)( )

s2 I-D 0.0606 1 - D

(2)

where s ) 2 sin θ/λ, and D is the fraction of disorganized carbon. The optimum value of D is that which makes the I′ profile most (9) Dahn, J. R.; Xing, W.; Gao, Y. Carbon 1997, 35, 825–830. (10) Alexander, L. E.; Sommer, E. C. J. Phys. Chem. 1956, 60, 1646– 1649. (11) Lu, L.; Sahajwalla, V.; Kong, C.; Harris, D. Carbon 2001, 39, 1821– 1833. (12) Franklin, R. E. Acta Crystallogr. 1950, 3, 107–121. (13) Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials; Wiley: New York, 1974. (14) Berghuis, J.; Haanappel, I. M.; Potters, M.; Loopstra, B. O.; MacGillavry, C. H.; Veenendaal, A. L. Acta Crystallogr. 1955, 8, 478–483. (15) Keating, D. T.; Vineyard, G. H. Acta Crystallogr. 1956, 9, 895– 896. (16) Franklin, R. E. Proc. R. Soc. London 1951, A209, 196–219. (17) Pollack, S. S.; Alexander, L. E. J. Chem. Eng. Data 1960, 5, 88–93.

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Tran et al.

Figure 1. Variation of the real density with carbon conversion: (a) for gasification in 1 atm air at 540 °C; (b) for gasification in 1 atm CO2 at 940 °C.

symmetrical. The interlayer spacing d002 is computed from the position of the 002 peak according to d002 ) 1/smax

(3)

The mean crystallite height Lc is given by Lc ) Med002

(4)

where the effective number of layers, Me, per parallel-layer group is Me )

∑p n

(5)

n

where pn is the fraction of aromatic carbon contained in groups of n parallel layers. These parameters were determined from the fitting of the symmetrical I′ profile. The mean crystallite length, La, can be calculated using the Warren equation for the height of an (hk) profile as applied by Franklin.16 The Warren equation is expressed as follows:13 Ihk )

mFg2

( ) La

2√2πNAa √π

1⁄2

s-3⁄2F(a),

a ) √πLa(s - smax) < 3 (6)

where m is the two-dimensional multiplicity, Fg2 is the structure factor of the unit cell in a single layer, N is the number of atoms in the unit cell, Aa is the area of the unit cell. For the (110) band, m ) 12, Fg2 ) 4, N ) 2, and Aa ) 0.525 nm.12 F(a) is a function of a and has been tabulated by Warren.18 By using the tabulated relationship, La is computed from the positions at the maximum and half of the maximum of the (110) band, s1 and s2, and the corresponding a values, a1 and a2, by using the following expression: La )

(a1 - a2)

(7) √π(s1 - s2) 2.5. High-Resolution Transmission Electron Microscopy (HRTEM). HRTEM observations of the samples were performed on a TECHNAI F30 FEG-TEM operating at 300 kV. The sample was first lightly ground in ethanol, and the floating fraction was dispersed on a standard copper grid with supporting membrane, for observation. The transmission electron microscope was equipped with a computerized imaging system (Gatan image filter). In all cases, submicron size particles were first examined at moderate magnification to find the wedge-shaped particles that are optically thin at the edge, and their diffraction patterns were taken. A number of such regions were then imaged at high magnification (×160 000). Several pictures were taken for each sample from different locations to get a general view.

3. Results and Discussion 3.1. Real Density Development. A real density measurement by the helium pycnometry technique provides a simple and quick (18) Warren, B. E. Phys. ReV. 1941, 59, 693–698.

way to study changes in solid structure. This is because helium can penetrate the submicropore range down to a size of about 3 Å. Figure 1 shows the development of the real density of anodes with increasing carbon conversion as measured using helium pycnometry. For the carbon-air reaction, the real density increased sharply during the first 10% conversion and then remained relatively constant to almost 70% conversion, beyond which a small continual increase occurred during the final conversion range. In contrast, for the carbon-CO2 reaction, the real density shows a consistently smaller increasing rate throughout the conversion range. The solid structure of carbon anodes is a composite containing the structures of petroleum coke and coked pitch (pitch-coke). From a gasification point of view, the pitch-coke matrix is known to be the more reactive and generally gasifies first.19 As the pitch-coke matrix is gasified, part of the petroleum coke structure is exposed to gasification. With increasing gasification conversion, the most reactive part of the coke structure will be consumed first, leaving only the most stable constituent at the highest conversion. This is supported by the observation in Figure 1 showing the real density approaching the density of graphite (2.26 g/cm3) at the highest conversion. The difference in the rate of increase of the real density between air and CO2 gasification is attributed to the difference in the rates of attack on the carbon structure. Feng and Bhatia1 and Salatino et al.2 have reported similar results with coal chars gasified in air and in CO2. Salatino et al.2 attributed the differences in the observed gasification patterns to the combination of uneven surface reactivity and the different mechanisms of intraparticle diffusion of either gasifying agents. 3.2. Change in Structural Reordering Based on the Scherrer Method. The XRD patterns were first normalized to atomic units for analysis. Figure 2 shows the typical reduced intensity curves for sample HSC-1100 (1100) gasified in 1 atm air at 540 °C. There are four visible peaks in all curves: (002), (100), (004), and (110). The presence of a sharp peak (002) as well as the (004) peak indicates that the anodes used in this study have a highly ordered structure. It is difficult to qualitatively distinguish the changes in structure based on visual examination of the diffraction patterns due to this highly ordered nature of the anode structure. The quantitative analysis method must be employed to accurately measure the changes in the structure during gasification. The parallelism indicator, R, provides a direct gauge of the proportion of planes in the crystallites having more than one layer. Figure 3 shows the variation in the R value with conversion during gasification in air and in CO2. The initial R value for all samples lies within the range of 9–18. It is much (19) Fischer, W. K.; Perruchoud, R. C. Light Met. 1986, 575–580.


Figure 2. Reduced intensity curves of sample HSC-1000(1025) gasified to various conversions in air at 540 °C.

higher than coal char, which has an R value in the range of 2–5.3 The value of R is seen to increase with conversion for all cases except for sample HSC-1225(1200) gasified in air, for which a mildly decreasing trend is observed. A simple explanation for the increase in the R value is that the crystalline structure becomes more ordered due to the removal of amorphous carbon and disorganized matter as gasification proceeds to higher conversion. It is unclear why sample HSC-1225(1200) did not follow the general trend. This may be due to experimental errors. The changes in the crystallite dimensions, Lc and La, are shown in Figure 4. The anode has crystallites of very similar width (La ) ∼3 nm) while the crystallite height (Lc ) ∼1.8–3.0 nm) increases with heat treatment temperature, as expected. The trend in the variation of the Lc value with conversion mirrors that of the R value. This is expected, as the R value reflects the proportion of crystallites having more than one layer. On the other hand, the La value shows a different trend with conversion than the Lc value. For most cases, the La value increases in the first 10% conversion and thereafter decreases with increasing


conversion, although samples HSC-1100(1100) and HSC1225(1200) did not show a decreasing trend in the La value at higher conversion. During the initial 10% conversion, amorphous carbon and disorganized matter, particularly those of the pitch-coke constituent, may be expected to be gasified first. This leads to an improved structure having a large proportion of the more organized petroleum coke fraction. The improved structure was confirmed with the observation of the increase in the real density and the La value in this conversion range. With a further increase in gasification, parts of the crystallites are consumed. In particular, the graphene layers are gasified predominantly from the edges, leading to a decrease in the La value. The reason for the increase in the Lc value is not clear. Lu et al.4 reported a similar observation of the Lc value variation with conversion. They attributed it to the preferential consumption of small crystallites and the merging of neighboring crystallites during gasification. 3.3. Change in Structural Reordering Based on the Alexander and Sommer Method. The changes in the fraction of disorganized matter (D) and mean crystallite dimensions (Lc and La) of anodes due to gasification are shown in Figures 5 and 6, respectively. The anodes have a substantial amount of disorganized carbon matter, as indicated by the initial D values in the range of 0.35–0.40. LSC anodes have a smaller D value than that of HSC anodes. The difference in heteroatom contents may contribute to the difference in the D values. It is wellknown that heteroatoms can form cross-link bridges between graphene layers and distort the orientation of the crystallites.20 The D value decreases at a relatively faster rate for gasification in air than in CO2. This trend is closely related to the trend observed in the development of real density with conversion. The changes in the crystallite dimensions, Lc and La, are very similar to the trends observed based on the Scherrer equation method. The only difference between the two quantitative methods is that the Lc and La values based on the Alexander and Sommer method are smaller than those obtained from the Scherrer equation. This is expected, since the Scherrer equation was developed for crystallites that have no lattice strain or distortion in their structures. This is not true for a carbon anode because it contains a substantial amount of heteroatoms forming cross-linking bridges between the crystallites.20 The Lc value increases mildly in all cases except for sample HSC-1225(1200) gasified in air. This pattern is identical to the Scherrer’s result. For gasification in CO2, the La value increases during the initial 10% conversion and then decreases at higher conversion. This is also consistent with the results obtained from the Scherrer equations. However, for gasification in air, the La values increase excessively at higher conversion. This pattern is more pronounced for the HSC anodes. Such an excessive increase may be due to different oxidation mechanisms by different reactants with carbon. Oxygen may prefer to attack the most reactive parts of the carbon structure, such as disorganized carbons and smaller crystallites, leaving only the least reactive parts at the highest conversion. The least reactive parts of the carbon structure would be large crystallites with high Lc and La values. This suggests that anodes with nonuniform structure may experience higher structural degradation from reaction with oxygen due to the preferential removal of the weakest parts of the structure during the initial stages of the oxidation process. 3.4. HRTEM Observations. The HRTEM fringe lattice images of sample HSC1100(1100) when fresh and when partly (20) Monthioux, M.; Oberlin, M.; Oberlin, A.; Bourat, X. Carbon 1982, 20, 167–176.


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Figure 3. Variation of parallelism indicator, R, with carbon conversion: (a) for gasification in 1 atm air at 540 °C; (b) for gasification in 1 atm CO2 at 940 °C.

Figure 4. Variation of crystallite size parameters, Lc and La, as determined using the Scherrer equations with carbon conversion: (a) Lc for gasification in 1 atm air at 540 °C; (b) Lc for gasification in 1 atm CO2 at 940 °C; (c) La for gasification in 1 atm air at 540 °C; (d) La for gasification in 1 atm CO2 at 940 °C.

Figure 5. Variation of the fraction of disorganized carbon, D, with carbon conversion: (a) for gasification in 1 atm air at 540 °C; (b) for gasification in 1 atm CO2 at 940 °C.

gasified at low, medium, and high conversions in air and in CO2 are shown in Figure 7. The image of the fresh sample clearly shows that the carbon anode has a partially graphitized structure. The graphene layers can be identified easily, with an average crystallite size of 3–6 layers orientated parallel to each other. This is in agreement with the XRD results. Similar to

the XRD pattern profiles (Figure 2), it is very difficult to determine the subtle changes in structural ordering of the carbon anodes based on these TEM images. A quantitative method such as that proposed by Sharma et al.21 can be employed to determine the small changes occurring among different samples. With an increase in conversion, however, the structure seems



Figure 6. Variation of the crystallite size parameters, Lc and La, as determined using the Alexander and Sommer method with carbon conversion: (a) Lc for gasification in 1 atm air at 540 °C; (b) Lc for gasification in 1 atm CO2 at 940 °C; (c) La for gasification in 1 atm air at 540 °C; (d) La for gasification in 1 atm CO2 at 940 °C.

Figure 7. HRTEM fringe lattice images of sample HSC-1100(1100) for conversion of (a) 0%; (b) 10.3% in air; (c, e) 44.0% and 77.5% in CO2, respectively; and (d, f) 51.8% and 81.5% in air, respectively.

to become more ordered, and the crystallite size increases. There is little difference in the degree of structural transformation observed between gasification in air and in CO2 based on these HRTEM images. 3.5. Gasification Process in Air and CO2. Based on the above observations, a conceptual model of the gasification (21) Sharma, A.; Kyotani, T.; Tomita, A. Fuel 1999, 78, 1203–1212.

process has been developed. Prior to gasification, the carbon anode has a graphitic-like carbon structure. It is mainly comprised of small crystallites tightly packed within an amorphous carbon phase. These crystallites may also interlink between each other via bonding through heteroatoms. The pyrolysis products evolved from the pitch during the carbonization stage could also contribute the majority of the amorphous carbon phase. During the initial 15% conversion, the amorphous carbons as well as the fused edge atoms are removed preferentially, and thus, a sharp increase in the real density and the crystallite width, and a sharp decrease in the disorganized carbon fraction is observed. This sudden change in solid structure is more pronounced in the air gasification process. In our previous publication on the air reactivity of petroleum coke,22 we observed similar behavior and were able to reconcile the reactivity of the cokes after the initial changes in structure were taken into account. Beyond 15% conversion, the crystalline structure tends to become more ordered as gasification proceeds. There are two views frequently proposed for the increase in structural ordering in the later stages of gasification. One explanation is that highly ordered portions are developed during gasification due to thermal annealing and/or a catalytic graphitization effect. This is unlikely to happen with the carbon anode used in this study because the gasification temperatures are lower than the heat treatment temperatures. Additionally, the heat treatment time is significantly longer than the gasification time. The other explanation is that the material initially contained a wide distribution of crystallite sizes. Among these, the smallest crystallites are gasified preferentially, leaving only the larger and less reactive crystallites at later conversion levels. In this study, our results suggest that carbon anodes generally follow the second route of crystalline structure transformation. The distribution of stacking number for sample HSC-1100(1100) at various conversion levels after gasification in air and in CO2 is shown in Figure (22) Tran, K. N.; Bhatia, S. K.; Tomsett, A. Ind. Eng. Chem. Res. 2007, 46, 3265–3274.


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Figure 8. Distribution of stacking number for sample HSC-1100(1100) at various conversion levels after gasification in air at 540 °C and CO2 at 940 °C. Me is the mean stacking size.

8. It can be seen that the fraction of crystallites with a smaller stacking number decreases with conversion, while the fraction of crystallites with a larger stacking number slightly increases with conversion. The most notable changes occurred with the stacking fractions of 3 and 4. For a stacking size of 3, the fraction decreased from >30% to 20% during the gasification process. On the other hand, for a stacking size of 4, the fraction increased from about 5% to over 10% in the same gasification period. Such changes reflect the preferential oxidation of the graphene layers associated with smaller crystallites. With the removal of some graphene layers, the neighboring crystallites merged and formed slightly larger crystallites. Another observation in this study is the different rate of attack by oxygen and CO2 on the structure of the anodes. Based on the real density and disorganized carbon fraction results, it is clearly evident that oxygen attacks the crystalline structure much more aggressively than does CO2. The difference in structural modification by oxygen and CO2 has been widely investigated in pore structure studies1,2 but seldom studied for the crystalline structure. Recently, Chen and Yang23 and Frankcombe et al.24 performed molecular ab initio calculations for the gas-carbon reactions of oxygen and CO2. They concluded that oxygen preferentially attacks the edge sites of the crystallite. Chen and Yang23 found that the equilibrium constant of dissociative chemisorption of oxygen on the edge carbon is several orders of magnitude higher than that of CO2. As a consequence, an off-plane epoxy oxygen intermediate is formed in addition to (23) Chen, N.; Yang, R. T. J. Phys. Chem. A 1998, 102, 6348–6356. (24) Frankcombe, T. J.; Bhatia, S. K.; Smith, S. C. Carbon 2002, 40, 2341–2349.

Table 2. Average Reactivity Index of the Coke and Carbon Anodes air reactivity [×103 g/(g min)] sample HSC-1225 HSC-1000(1025) HSC-1000(1200) HSC-1225(1200)

CO2 reactivity [×103 g/(g min)]

600 °C 540 °C 470 °C 1000 °C 940 °C 870 °C 32.6 21.8 16.0 13.4

7.39 4.60 3.24 2.93

0.90 0.50 0.33 0.31

1.41 1.73 1.30 1.28

0.45 0.55 0.51 0.40

0.14 0.12 0.12 0.11

semiquinone and carbonyl intemerdiates. The off-plane epoxide destabilized the adjacent C-C bond, allowing a CO intermediate to dissociate at a much lower activation energy (58 kcal/mol) compared to a higher value of 80 kcal/mol required for CO2 gasification. In addition, Frankcombe et al.24 found that oxygen atoms chemisorbed on interior sites of the graphene planes contributed little to the gasification process. From Table 2, the air reactivity of the anodes at 540 °C is about 1 order of magnitude greater than the CO2 reactivity at 940 °C. Such differences in reactivity agree well with the findings of ab initio calculations. With a higher reaction rate, oxygen prefers to consume disorganized carbons and smaller crystallites, as these reactive parts are more accessible to reaction during the initial stages. In contrast, CO2 reacts slowly, leading to less variation in texture due to reaction at different scales in the carbon structure. The difference in structural evolution due to a different reactant is more pronounced at higher reaction temperatures because the reaction rate is a strong function of temperature. Both types of gasification processes will, however, cause the crystalline structure to become more ordered from the removal



Figure 9. Comparison between anode gasification and coke gasification: (a, c) comparison between anode data and coke data for gasification in air; (b, d) comparison between anode data and coke data for gasification in CO2. The crystallite structure parameters were determined using the Scherrer equations.

of the most reactive parts and the merging of neighboring crystallites during the carbon consumption process. 3.6. Comparison Between the Anode and Coke. Since the samples used in this study comprised a significant amount of petroleum coke, it is expected that the coke would have a strong influence on the overall gasification behavior. Figure 9 shows the comparison of the crystallite size development between sample HSC-1225 (1200) and its coke constituent HSC-1225. The gasification of the cokes has been studied in an earlier publication.22 The coke structure developed differently compared to that of the anode sample in air gasification, but the coke has similar development compared to that of the anode sample in CO2 gasification. In air gasification, the coke shows an increasing trend of the Lc value with conversion, while the anode shows a slight decreasing trend of the Lc value with conversion. In contrast, the La value of the coke decreases quickly from 0 to 5% conversion and remains relatively constant for the remaining conversion range, while the La value of the anode increases quickly from 0 to 40% and remains constant thereafter. In CO2 gasification, the Lc and La values of the parent coke are nearly the same as those of the anode throughout the gasification process. The differences in structural development observed in air gasification highlighted the influence of the pitch-coke component on the overall structure of the anode. Without the pitch-coke, the coke structure simply gasified in two steps, as outlined in the previous sections. The amorphous carbon and smaller crystallites are quickly gasified in the first 10% conversions, and then the larger crystallites are slowly gasified in the remaining conversion range. This explains the large increase in the Lc value and the decrease in the La value in the first 5% conversion and the relatively constant values of Lc and La over the remaining conversion range. In the anode, the coke particles interacted with the pitch during the heat treatment period to form a composite with a wide range of reactivity depending on the initial properties of the coke. For anode sample HSC-1225(1200), it is expected that the composite would have a high oxidation resistance

Table 3. Ratio of the Anode Structural Parameters to the Parent Coke sample

helium density

Lc

La

HSC-1000(1025) HSC-1000(1200) HSC-1100(1100) HSC-1225(1200) LSC-1125(1100) LSC-1220(1200)

1.03 1.07 0.99 0.98 1.00 0.97

1.20 1.64 1.07 1.09 1.03 1.03

1.31 1.38 1.04 1.12 1.29 1.24

compared to the case of anodes made using coke with lower calcination temperature. However, it was found that the reactivity of the anodes made from coke with low calcination temperature was comparable with the reactivity of anodes made from coke with high calcination temperature, as shown by the results summarized in Table 2. A similar observation was reported in a recent industrial study carried out by Samanos and Dreyer.25 They proposed that a better pitch-coke contact was achieved for coke with a low calcination temperature, leading to a more homogeneous composite mixture upon heat treatment. The resultant anode would have a more uniform structure compared to that of the anode made from coke with a high calcination temperature. From Figure 9a, it would appear that the structure of anode sample HSC-1225(1200) was not uniformly developed after heat treatment due to weak pitch-coke contact. The decreasing trend of the Lc value with conversion indicated that a structure with a wide distribution of crystallite sizes was gasified. Further taking into account the reactivity measurements shown in Table 2, the findings in this study supported the hypothesis proposed by Samanos and Dreyer.25 Table 3 shows the ratios between the structural parameters of the anodes and the parent cokes. These ratios suggested that the structure of the anodes is strongly linked to the initial coke structure. Anodes made using low temperature heat-treated coke (1000 °C) display a better developed structure compared to that of the parent coke, but anodes made with medium to high (25) Samanos, B.; Dreyer, C. Light Met. 2001, 681–688.


temperature heat-treated cokes (>1100 °C) have a similar structure to that of the parent cokes. One possible explanation for the improvement in structure in the case of low temperature heat-treated coke is that “stress graphitization” occurred within the confined spaces at the pitch-coke interface. RodriguezMirasol et al.26 found the interaction between the carbon fiber and filler pitch is synergistic when the composite was made with fiber having an underdeveloped structure, but the interaction between the pitch and fiber became additive for the composite made with fiber having a fully developed structure. They attributed the difference in structural development between different composites to the “stress” caused by the shrinking of the microstructure of the underdeveloped carbon fiber with increasing heat treatment. The underdeveloped carbon fiber naturally undergoes structural realignment and becomes more ordered with increasing heat treatment. As a result, the pitch component, which filled the pores inside the fiber, experienced additional “stress” and formed a stronger bond with the fiber at the interface. Similarly, the coke with the lowest heat treatment temperature has an underdeveloped structure initially and becomes more ordered and compact with increasing heat treatment. The effect of stress graphitization is further enhanced with increasing heat treatment temperature, as illustrated by a significant increase of the Lc value between samples HSC1000(1025) and HSC-1000(1200). The significance of this finding is that it is possible to make a high oxidation resistant carbon-carbon composite (anode) by using a low heat treatment temperature carbon precursor, contrary to the current industrial practice of utilizing a high heat treatment temperature carbon precursor. 4. Conclusions The transformation of the crystalline structure of several carbon anode samples with carbon conversion in air and in CO2

Tran et al.

has been studied by quantitative XRD analysis and HRTEM. It was found that the crystalline structure transformed in two stages. The first stage involved the removal of the disorganized carbon fraction. Oxygen and CO2 preferentially attack different reactive sites of the crystalline structure. The results based on helium density measurement and quantitative XRD analysis showed that the most reactive parts of the crystalline structure such as amorphous carbons and carbon atoms at the aliphatic side chains are quickly gasified in air. In contrast, CO2 gasified at a relatively equal rate on all parts of the crystalline structure. With further progress of gasification, the crystallite width became smaller due to preferential consumption of carbon from the edge sites of the graphene layers. As smaller crystallites are gasified, the neighboring crystallites merge to form a more ordered structure reflected by the increase in crystallite height with conversion. The interaction between the pitch and the coke in the anode composite was found to be strongly linked to the initial coke structure. The stress caused by shrinkage of the microstructure with increased heat treatment induced enhanced graphitization at the pitch-petroleum coke interface. Anodes made with low temperature heat-treated coke have a better developed structure than that of the parent coke, but anodes made with high temperature heat-treated coke have a similar structure compared to that of the parent cokes. Acknowledgment. The financial support for this research provided by the Australian Research Council and by Rio Tinto Alcan Technology is gratefully acknowledged. We thank Dr. John Barry for help with the HRTEM analysis. EF700732P (26) Rodriguez-Mirasol, J.; Thrower, P. A.; Radovic, L. R. Carbon 1995, 33, 545–554.

Crystalline Structure Transformation of Carbon Anodes during

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