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CO2 capture capacities of activated carbon fibre-phenolic resin composites
CARBON
4 7 ( 2 0 0 9 ) 2 3 9 6 –2 4 0 5
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
CO2 capture capacities of activated carbon fibre-phenolic resin composites Hui Ana,b, Bo Fenga,*, Shi Sub a
School of Engineering, The University of Queensland, St. Lucia, QLD 4072, Australia CSIRO, P.O. Box 883, Kenmore, QLD 4069, Australia
b
A R T I C L E I N F O
A B S T R A C T
Article history:
The potential of activated carbon fibre-phenolic resin composites for CO2 capture has been
Received 12 December 2008
evaluated in this work. A number of composites were fabricated using different types of
Accepted 20 April 2009
carbon fibre under various conditions. The effect of a range of variables such as the type
Available online 3 May 2009
of carbon fibre, mass ratio of carbon fibre to phenolic resin, activation temperature and duration on the CO2 adsorption capacity was investigated. Activated carbon derived from powdered phenolic resin demonstrates its capability to capture CO2 and it plays a significant role in the low burn-off range. An apparent optimal degree of activation for CO2 adsorption capacity was identified which was coincident with the maximum micropore volume measured by CO2 physical adsorption. Micropore volume by CO2 has been identified as a potential design parameter for the development of activated carbon fibre-phenolic resin composites for CO2 capture. The existence of a cross-over regime is confirmed and lower burn-off samples are found to capture more CO2 at ambient conditions. This is attributed to a narrow microporosity and a large contribution of micropore volume from smaller pores in the microporosity range of the composites. The optimal pore size for CO2 capture becomes smaller when the relative pressure of CO2 goes lower. Ó 2009 Elsevier Ltd. All rights reserved.
1.
Introduction
CO2 removal from fossil fuel fired power station flue gas (i.e. post-combustion CO2 capture) is now an important strategy for the reduction of CO2 emissions in the world. Among all the research efforts in developing techniques for carbon capture and sequestration, CO2 separation and capture cost is still a main focus as it is an expensive part in CO2 capture and sequestration process. Meanwhile, improving the capture efficiency is also very important. For example, amine based scrubbing has been trialed for post-combustion CO2 capture at several pilot-scale plants in the world; however, it suffers from low efficiency, high energy consumption and environmental problems of waste water and sludge. Apart from amine, a lot of other materials have been investigated for
CO2 capture. Table 1 summarises CO2 capture capacities of some sorbents as reported in the literature. Activated carbon fibre materials have attracted a lot of research and studies for volatile organic compounds (VOCs) removal [9–13]. They are made of carbon fibres and normally a carbonisable carbon as the binder. However, very rare studies have been done on these materials for CO2 capture specifically. For instance, a few studies have been conducted for the selectivity of CO2 and CH4 [14,15]. Commercial pitch based active carbon fibres were tested for the separation of CO2 and CH4 and the selective separation of CO2 was demonstrated with selectivity (CO2/CH4) of 2.2. Pitch based activated carbon fibre material was also compared with a commercial coconut shell carbon [8] and a larger uptake of CO2 was demonstrated as shown in Table 1. There are some factors during the
* Corresponding author: Fax: +61 733 654 799. E-mail address: [email protected] (B. Feng). 0008-6223/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.04.029
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Table 1 – CO2 capture capacities of sorbents as reported. Sorbent Monoethanolamine (MEA) [1] Aqueous ammonium [2] Aminated mesoporous silica [3] PEI-impregnated MCM-41 [4] Anthracite activated carbon [5] Lithium silicate [6] 13X zeolite [7] Granular coconut shell carbon [8] Pitch based activated carbon fibre material [8]
activation process of activated carbon fibre materials that can potentially have an effect on the adsorption performance of the sample. Research on the effects of these factors is scarce in the literature. In particular, phenolic resin has been commonly used as the binder. And the mass ratio of carbon fibres to phenolic resin is normally maintained at 3:1. It has to be noted that activated carbons can be produced from various types of phenolic resin. The obtained activated carbons can be microporous [11,16–18] and mesoporous [19–21]. Hence, activated carbon derived from phenolic resin after carbonisation and activation may also play a role in adsorbing CO2. Therefore, in this work, a larger proportion of phenolic resin in the raw materials will be tried. Mass ratio of carbon fibre to phenolic resin will be investigated as a variable. The materials produced are thus referred to as activated carbon fibrephenolic resin composites. An intriguing phenomenon has been observed while using activated carbon fibre materials for VOC capture. Foster et al. [12] reported that a cross-over point is observed with three nbutane adsorption isotherms on activated carbon fibres with different activation degrees. It is found that above the crossover point (at high relative pressures), the amount adsorbed is depended on the pore volume. Namely, the higher burnoff sample captured the greater amount of n-butane. On the other hand, a reverse correlation is observed at the low relative pressures (below the cross-over point). Mangun et al. [11] studied the cross-over regime of a series of alkanes and concluded that the occurrence of cross-over regime is due to a pore size–pore volume effect. At lower relative pressures, low burn-off samples have a larger uptake because of a higher overlap in potential between the pore walls of smaller micropores that binds adsorbate molecules more tightly. At higher relative pressures, the total micropore volume is utilised and the adsorption capacity is a function of burn-off. However, the cross-over regime phenomenon has not been investigated on CO2, although an optimal micropore volume measured by N2 has been reported to be around 0.5 cm3/g for CO2 capture [8,22]. There also exists an optimal pore size by N2 isotherm [8], which is about 0.8 nm using the Dubinin–Radushkevich (DR) method and ca. 1.5–1.6 nm if using the Dubinin–Astakhov (DA) method. It is known that higher burn-offs (defined as the weight loss of the sample in percentage during the activation step) can enlarge micropore volume and micropore size to be in excess of the optimal values for CO2 capture. Furthermore, micropore volume and micropore size appear as two independent parameters. A sample with
Capacity
Capture conditions
0.4 g/g MEA 1.20 g/g NH3 0.07 g/g sorbent 0.09 g/g adsorbent 65.7 mg/g adsorbent 0.35 g/g sorbent 0.14 g/g adsorbent 0.08 g/g adsorbent 0.12 g/g adsorbent
16% CO2, balanced with air 15% CO2, 85% N2 90% CO2, 10% Ar, 20 °C 15% CO2, 4% O2, 81% N2, 75 °C 99.8% CO2, 30 °C 100% CO2, 700 °C 10% CO2, 90% N2, 15 °C 100% CO2, 25 °C 100% CO2, 25 °C
the optimal micropore volume does not necessarily have the optimal micropore size; and vice versa. Therefore, either micropore volume or micropore size by N2 is not sufficient to be a good parameter for the design of activated carbon fibre materials. In this paper, the uniformity levels of the samples are investigated since uniform activation is desired in order that the microstructure of activated carbon fibre-phenolic resin composite can be closely controlled. The effects of the fabrication conditions are explored on the activation level and the adsorption capacity as well. Efforts have also been made to study the cross-over regime with CO2 adsorption on activated carbon fibre-phenolic resin composites. In addition, this work tries to identify a single parameter that can predict a composite’s CO2 adsorption capacity at ambient conditions.
2.
Experiments
2.1.
Experimental process
The experiments were conducted as follows. A number of activated carbon fibre-phenolic resin composites were fabricated using different types of carbon fibre. The detailed fabrication procedure is introduced in Section 2.2. At first, some samples were tested for their uniformity level of activation by CO2 before further investigation was conducted. Then, the effects of some variables during the fabrication were investigated. There are a number of variables that could affect the structure of the final product including the type of carbon fibre, mass ratio of carbon fibre to phenolic resin, drying temperature and environment, heat treatment temperature and duration, activation agent, activation temperature and duration. Based on the experience and preliminary test results, four variables were considered the most important, i.e. the type of carbon fibre, mass ratio of carbon fibre to phenolic resin, activation temperature and duration. Mass ratio reflects the relative proportion of carbon fibres and phenolic resin. The relative importance of each material can be examined by investigating it. In order to identify the most important factors by experiment while minimising the number of samples to be tested, an orthogonal experimental design method was used as discussed in Section 2.3. At last, more samples with a large range of burn-offs were prepared to study their CO2 adsorption capacities at 0 and 25 °C under atmospheric pressure.
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2.2.
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Sample preparation
Activated carbon fibre-phenolic resin composites were made from carbon fibre and powdered phenolic resin. The detailed fabrication procedure can refer to [8,23]. Briefly, three types of carbon fibre were utilised: pitch based carbon fibres, activated pitch based carbon fibres (APitch) and polyacrylonitrile (PAN) carbon fibres. At first, carbon fibres and powdered phenolic resin were mixed in water slurry at a certain mass ratio. Then, the slurry was transferred to a moulding tank and water was vacuum pumped from the bottom of the tank. The sample was subsequently dried and cured to solidify the structure. After that, the resultant product was placed in a three-stage tube furnace and carbonised at 650 °C under N2 flow. The last stage involved activating the sample in CO2 in the temperature range of 800–920 °C. The final sample is in a cylindrical form with maximum dimensions of 3.2 cm in diameter and 10 cm in length. Inside the sample, there are 17 evenly distributed longitudinal channels with a diameter of 0.03 cm.
3.
Results and discussion
3.1.
Preliminary studies
3.1.1.
Confirmation of uniform activation
Four samples were made in order to test their activation uniformity levels. Each sample was evenly cut into three pieces. Each piece was crushed into smaller pieces separately and stored in different containers. Approximately 0.5 g specimens were obtained from each container for microstructure analysis by the TriStar 3000. Samples tested were made from different carbon fibres at different mass ratios of carbon fibres to powdered phenolic resin and activated under different conditions. Burn-off is in the range of 5–14.7%. It is found that BET surface area does not change significantly throughout a sample. For all the samples, the standard deviation is below 5%, which is acceptable. It has also been found that micropore volume changed very little at different locations within a sample.
3.1.2. 2.3.
Orthogonal experimental designs
Orthogonal experimental designs were carried out by ‘‘The Unscrambler’’ (The Unscrambler 9.7, 2007) for experimental plan on studying the effects of the three variables, including mass ratio, activation time and temperature. Carbon fibre type was investigated separately because it is not a continuous variable. The Unscrambler helps generate well-designed experimental plan with a minimum number of experiments to save time and cost. Two experimental designs were built: a screening design and an optimisation design. The screening design aimed to find out the relative importance of each variable. The optimisation design was used in an attempt to identify the optimal conditions to produce activated carbon fibre-phenolic resin composites and also to study the effects of the variables in more detail.
2.4.
Characterisation
The porous structure of the samples was analysed by N2 adsorption at 196 °C and CO2 adsorption at 0 °C on TriStar 3000 (Micrometritics). N2 adsorption at 196 °C, which covers a large range of relative pressures from P/P0 = 0 to 0.98, can be used as a measure of the whole range of micropores. However, due to the very low temperature, N2 suffers from low kinetic energy and cannot enter into ultra-micropores (<0.7 nm). Therefore, CO2 adsorption at 0 °C under sub-atmospheric pressure (which corresponds to a relative pressure of P/P0 < 0.015) is usually used as a complement to account for ultra-microporosity. It has to be noted that some larger micropores are not completely taken into account by CO2 adsorption due to the low relative pressure. The BET surface area SBET, DR micropore volume V N2 and DA average micropore size DN2 were calculated from N2 adsorption isotherms. In the case of CO2 adsorption isotherms at 0 °C, only DR micropore volume V CO2 and DA average micropore size DCO2 were calculated. Additionally, CO2 adsorption isotherms were also obtained at 20 and 25 °C under sub-atmospheric pressure.
Screening of carbon fibres
A series of samples were made utilising three types of carbon fibre to identify the carbon fibre that gives the most satisfactory performance. The results are tabulated in Table 2. Two samples were produced from each type of carbon fibre. Samples 4 and 5 were made from polyacrylonitrile carbon fibres and have the smallest BET surface area and micropore volume compared with other samples. Correspondingly, the amount of CO2 captured by those two samples was also the smallest. Sample 5 for example trapped less than half the CO2 of samples made from pitch based and activated pitch based carbon fibres. It can thus be inferred that polyacrylonitrile carbon fibres are not promising as far as CO2 adsorption is concerned. It has been reported that samples prepared from polyacrylonitrile carbon fibres maintained a low surface area (around 700 m2/g) even at very high burn-off [24]. Another intriguing point to note is that even samples made from activated pitch based carbon fibres exhibit significantly higher BET surface areas than those made from pitch based carbon fibres, they do not show any advantages in CO2 capture. Furthermore, when activated carbon fibres are mixed with a binder, Fuertes et al. [10] observed that there is a loss in surface area, porosity and adsorption capacity for volatile organic gases (VOCs) with respect to the same parameters for nonagglomerated fibres. It is suggested that a fraction of the porosity be plugged by the binder. And it is recommended that non-activated carbon fibres be used to optimise the adsorption properties. Therefore, pitch based carbon fibres were chosen for further investigation.
3.2.
The effects of mass ratio and activation conditions
3.2.1.
Factor screening
Seven samples were involved in the screening design. Three (11, 12 and 13) of them were centre samples, of which the values of the variables were in the middle of the corresponding range. Table 3 shows the fabrication conditions and response data. The mass ratio of pitch based carbon fibres to powdered phenolic resin varied from 1:1 to 3:1. 3:1 was the maximum in
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Table 2 – Experimental results for screening of carbon fibres. Sample
Precursor
Mass ratio (fibre:resin)
Activation Temp. (°C)
1 2 3 4 5 6
APitch APitch Pitch PAN PAN Pitch
4:1 2:1 2:1 2:1 1:1 1:1
850 850 800 800 800 900
Time (h) 1 1 1.5 2 2 1
CO2 adsorbeda
BO
SBET
V N2
(%)
(m2/g)
(cm3/g)
(mmol/g)
14.7 7.4 9.2 4.9 7.6 14
925 819 366 140 245 424
– 0.43 0.21 0.07 0.13 0.22
2.26 2.23 2.23 0.55 0.94 2.28
a The amount of CO2 adsorbed at 20 °C under atmospheric pressure.
order that carbon fibres and powdered phenolic resin could mix well. Burn-off of these samples was from 1.4% to 27.9%. ‘‘Analysis of effects’’ carried out by The Unscrambler shows that none of these three variables is significant compared with others. In other words, each variable has non-negligible influence on the adsorption performance of the samples as other variables. As a result, these three variables would be further investigated in the optimisation design. Another finding is that the mean value of the centre samples is lower than that of the overall design, indicating that optimal conditions are beyond the region studied so far. The upper activation temperature of 900 °C was considered very high. Therefore, the upper limit of activation time was raised to 5.5 h in the following optimisation design to broaden the burn-off range in order to have the optimal conditions included in the region studied.
3.2.2.
The effects of the factors
The optimisation design was carried out in an attempt to identify the optimal conditions by expanding the region investigated by involving more samples fabricated under various conditions. Fifteen samples in total were studied (see Table 4). The upper limit of activation time was extended to 5.5 h. However, the maximum burn-off obtained was only 31.1%. The evident gain of CO2 adsorption capacity with the increase of burn-off (see Fig. 1) demonstrates that the optimisation design failed to identify the optimal conditions. Fig. 1 shows the perfect linear relationship between burn-off and CO2 adsorption capacities of the samples in the optimisation design. Samples with different mass ratios are marked using different symbols. On the other hand, the effects of the factors in more detail can be extracted from this design since more samples were involved. This was conducted by running ‘‘analysis of effects’’. The results show that activation temperature has the most
significant effect on burn-off and activation time less significant (see Fig. 2a). A sample activated under higher temperature could achieve significantly higher burn-off. For instance, a sample activated at 800 °C for 5.5 h obtained a burn-off of ca. 6%. When the activation temperature was raised to 900 °C, the burn-off could achieve a value of 31.1%, which is five times higher (see Fig. 2a). Mass ratio is found to have the least influence on burn-off as shown in Fig. 2b. On the contrary, similar conclusions cannot be made regarding the effects of these factors on CO2 adsorption capacities as can be seen later in this paper that CO2 adsorption capacity does not always increase with burn-off. Nonetheless, the role of activated carbon derived form powdered phenolic resin can be examined by studying the influence of mass ratio, discussed as follows. As shown by the good linear relationship between burn-off and CO2 adsorption capacities in Fig. 1 as well as the relation between mass ratio and burn-off in Fig. 2b, samples with lower mass ratios (or higher contents of phenolic resin) actually captured slightly more CO2 than those with higher mass ratios whilst other fabrication conditions were the same. This demonstrates the existence of microporosity of the activated carbon derived from powdered phenolic resin. It is therefore confirmed that apart from acting as a carbon fibre binder, activated carbon derived from powdered phenolic resin also plays a significant role in adsorbing CO2. Under the tested experimental conditions it is slightly better than the activated carbon fibre used.
3.3. Characterisation analysis of activated carbon fibrephenolic resin composites In the optimisation design, the maximum burn-off achieved was 31.1%. Adsorption capacity increasing with burn-off demonstrates that the design has failed to identify the opti-
Table 3 – Experimental data for the factor screening design. Sample
Mass ratio (fibre:resin)
Activation Temp. (°C)
7 8 9 10 11 12 13
3:1 3:1 1:1 1:1 2:1 2:1 2:1
800 900 800 900 850 850 850
BO Time (h) 0.5 3.5 3.5 0.5 2 2 2
SBET 2
(%)
(m /g)
1.4 27.9 9 9.1 6.3 9.3 8.8
131 709 322 200 341 317 331
NCO2 , 0 °C (mmol/g) 2.63 3.99 2.74 2.62 2.75 2.87 2.84
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Table 4 – Experimental data for the optimisation design. Sample
Mass ratio (fibre:resin)
Activation Temp. (°C)
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
1:1 1:1 3:1 2:1 3:1 1:1 2:1 1:1 3:1 2:1 3:1 2:1 2:1 2:1 2:1
Time (h)
900 850 800 800 850 850 900 800 850 800 900 900 850 850 850
3 0.5 3 0.5 0.5 5.5 0.5 3 5.5 5.5 3 5.5 3 3 3
4.5
NCO2, 0 (mmol/g)
SBET 2
(%)
(m /g)
25.6 4.2 4.2 0.2 2.4 17.8 4.7 5.7 12.9 6.0 18.6 31.1 7.9 11.3 10.3
697 204 291 147 100 587 280 137 521 375 677 998 298 396 451
NCO2 , 0 °C (mmol/g) 4.13 2.64 2.67 2.45 2.42 3.68 2.65 2.75 3.48 2.84 4.05 4.29 2.92 2.96 3.06
to obtain a burn-off at different levels with a maximum at 65.1%. Hence, CO2 adsorption capacities of samples with a large range of burn-offs can be studied.
5.0
4.0
3.3.1.
2
R : 0.94
3.5 3.0
Mass ratio: 1:1 2:1 3:1
2.5 2.0
BO
0
5
10
15
20
25
30
35
Burn-off (%)
Fig. 1 – The relation between burn-off and CO2 adsorption capacities of the samples in the optimisation design.
mal fabrication conditions. However, higher activation temperature, longer activation duration and lower mass ratio have been found beneficial for achieving higher activation degrees. Therefore, more samples with higher burn-offs were made. This was realised by fabricating the samples under the conditions based on the optimisation design results. A higher activation temperature of 920 °C was adopted and the mass ratio was fixed at 1:1. Activation time was varied
Activation time: 0.5 hr Activation time: 5.5 hrs
20 10 0 800
(a) 820
840
860
880
o
Activation temp: 800 C
30
Burn-off (%)
Burn-off (%)
30
Adsorption isotherms
N2 adsorption isotherms of samples with different burn-off levels are shown in Fig. 3, which plots N2 amount adsorbed (nN2) at various relative pressures. It can be seen that nN2 increases as burn-off goes higher. The isotherms are of type I, which indicates the samples are microporous solids. The upward deviation at higher relative pressures shows that samples also contain mesoporosity. Two major observations can be made from this figure. One is that the onset of the adsorption isotherms climbs up on the y-axis until a burn-off of 32% reached. After that, adsorption isotherms all start from the same point. The other observation is that the knee of the isotherms is very sharp at lower burn-offs, an indication of narrow microporosity. However, those samples with higher burn-offs exhibit round knees. These two observations indicate that samples with higher burn-offs experienced a significant widening in the microporosity. Fig. 4 presents CO2 adsorption isotherms at 0 °C for the same samples as shown in Fig. 3. The isotherms are plotted in respect of total pressure with the highest pressure being one atmosphere. The amount of CO2 adsorbed increases as the total pressure goes up. It can be observed that isotherm
Activation temperature (oC)
900
o
Activation temp: 900 C
20 10 0
1.0
(b) 1.5
2. 0
Mass ratio
2.5
3.0
Fig. 2 – The interaction effects of (a) activation temperature and time, and (b) mass ratio and activation temperature on burnoff.
CARBON
25
5.0 4.5
61.3%
20
NCO2, 0 (mmol/g)
nN2 (mmol/g)
45.8% 32.1%
15
25.4% 16.0%
10 7.9%
5 0
3.5 3.0 Activation temperature:
2.0 0.0
0.2
0.4
0.6
0.8
1.0
800-850oC 870-900oC 920oC
of the sample at a burn-off of 32.1% is above those with either higher or lower burn-offs. This shows that the evolution of CO2 adsorption isotherm with burn-off is different from that of N2 adsorption isotherm. In the case of CO2 adsorption, the sample with the highest burn-off fails to capture the largest amount compared with other samples at lower burn-offs.
3.3.2. The relationship between burn-off and adsorption capacity Fig. 5 plots samples’ CO2 adsorption capacities at 0 °C under one atmosphere at various burn-offs. Samples activated under different temperatures are denoted using different marks. It has to be noted that mass ratios of samples activated under the temperature below 920 °C varied from 1:1 to 3:1 and these samples achieved a burn-off up to 31.1%. From this figure, it can be seen that the adsorption capacity (NCO2 , 0) initially increases as burn-off goes up until reaching a burn-off of 32%, after which a levelling off occurs. Data presented in this figure confirms the view obtained from Fig. 4 that higher burn-offs can be detrimental for CO2 adsorption at low relative pressures (P/P0 = 0.029 at 0 °C and 1 atm). Since N2 adsorption isotherm and CO2 adsorption isotherm behave differently with burn-offs, a cross-over region is likely to exist for CO2 adsorption.
5
4
3
2
Burn-off: 7.9% 16% 25.4% 32.1% 45.8% 61.3%
1
0 200
400
0
10
20
30
40
50
60
70
Burn-off (%)
Fig. 3 – N2 adsorption isotherms at -196 °C for samples with different burn-offs.
NCO2, 0 (mmol/g)
4.0
2.5
P/P0
0
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600
800
P (mmHg) Fig. 4 – CO2 adsorption isotherms at 0 °C for samples with different burn-offs.
Fig. 5 – CO2 adsorption capacities at 0 °C at various burnoffs.
To further investigate the effect of burn-off, BET surface area, DR micropore volume and DA micropore size of these samples have been measured by N2 adsorption isotherms. BET surface area, micropore volume and average micropore size are found to increase as burn-off goes higher, which agrees with the literature. It has been reported that at very high relative pressures (P/P0 = 1) the adsorption capacity is only depended on pore volume [10]. Both narrow and wide micropores can be filled. This explains why N2 adsorption isotherms of samples with higher burn-offs are above those with lower burn-offs as shown in Fig. 3. As the activation degree goes up, microporosity develops and results in larger micropore volume, and thus the larger adsorbed amount. Based on the Gurvitsch rule, the adsorbed volume onto one adsorbent will be the same for different adsorbates, provided that the molecules of the adsorbates are not large compared to the pore dimensions. Foster et al. [12] supported this by obtaining a similar micropore volume of a certain adsorbent with a series of VOC. Therefore, CO2 adsorption at P/ P0 = 1 will follow the same trend as that of N2, i.e., CO2 adsorption capacity will increase with the activation level due to the increase of micropore volume. Adsorption capacity at high relative pressures is a simple function of micropore volume, or burn-off. However, as far as CO2 adsorption (at 0 °C and 1 atm) is concerned as shown in Fig. 5, the linear proportionality does not exist between CO2 adsorption capacity and burn-off. A sample with an intermediate burn-off captures the larger amount of CO2 than those with higher burn-offs. This demonstrates that a cross-over region exists for CO2 adsorption on activated carbon fibre-phenolic resin composites. This is because the phenomenon is consistent with the description of ‘‘cross-over regime’’ as reported [11,12]. CO2 adsorption capacity at 0 °C and 1 atm does not go up with the increase of micropore volume because only a fraction of the micropore is occupied at lower relative pressures. Therefore, an excessive micropore volume is detrimental to the adsorption capacity of a sample. Similarly, too high the BET surface area or micropore size does not necessarily result in higher adsorption capacity. The measurement of BET surface area, micropore volume or micropore size by N2 thus cannot be used to design activated carbon fibre-phenolic resin composites. It has to be pointed out that at the burn-off of 32%,
2402
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maximises when the sample has a narrow microporosity and contains a large micropore volume measured by CO2 adsorption isotherm. This thus partially explains why lower mass ratio samples achieved higher CO2 adsorption capacities. The reason is that activated carbon derived from powdered phenolic resin can produce narrow micropores suitable for CO2 adsorption as well. Indeed, Tennison successfully produced phenolic-resin-derived activated carbons which have a very narrow pore size distribution with a mean pore size of 0.8 nm (DR) [18]. Note that this micropore size has been identified as optimal for CO2 capture.
3.3.3.
0.45
5.0
0.40
4.5
0.35 0.30 0.25
Activation temperature: 800-850oC
0.20
870-900oC 920oC
0.15 0
20
40
The effect of adsorption temperature
The effect of adsorption temperature was also investigated. CO2 adsorption isotherms at 25 °C also under sub-atmospheric pressure were obtained and the CO2 adsorption capacities derived are shown in Fig. 7. There is a clear peak at about 25% burn-off, at which an adsorption capacity of 2.9 mmol/g is reached, although samples at 32% burn-offs still exhibit very good adsorption capacities. Noting that the peak value for CO2 adsorption at 0 °C reaches 4.5 mmol/g. The maximum CO2 adsorption capacity of activated carbon fibre-phenolic resin composite has dropped by more than 30% when adsorption temperature increases from 0 to 25 °C. A closer look at the relation between adsorption capacity at 25 °C and V CO2 (Fig. 8a) shows that adsorption capacity at 25 °C still increases proportionally with the increasing V CO2 except a few outliers. It has to be noted that data obtained for the three samples in the two ovals are considered outliers and not taken into account while making the linear regression. However, the optimal pore size has shifted to a smaller size region, which is in the range of 1.45–1.55 nm, compared with the optimal micropore size range for CO2 adsorption at 0 °C.
NCO2, 0 (mmol/g)
VCO2 (cm3/g)
the micropore volume and micropore size are 0.6 cm3/g and 1.6 nm, respectively. In the literature, the optimal micropore volume and micropore size (DA) were previously found to be 0.5 cm3/g and 1.5–1.6 nm, respectively [8,22]. The optimal micropore volume is slightly lower than that observed in this work. The samples were also examined by CO2 adsorption isotherms at 0 °C. DR micropore volume and DA micropore size were obtained. Fig. 6a and b illustrates the relations between burn-off and micropore volume and size by CO2 adsorption isotherms. Micropore volume obtained from CO2 adsorption isotherm behaves quite differently from that obtained from N2 adsorption isotherm. Upon increasing burn-off micropore volume by CO2 initially increases and reaches a peak at 32%, then goes downward as burn-off keeps going further. This trend is similar to that shown by burn-off and adsorption capacity at 0 °C. It is therefore possible that adsorption capacity is closely related to micropore volume measured by CO2 adsorption isotherm. Fig. 6c, which shows the relation between micropore volume by CO2 and adsorption capacity, confirms this. It can be observed that a sample’s adsorption capacity can be evaluated by its micropore volume by CO2. Micropore size by CO2 presented in Fig. 6b behaves similarly as that by N2. It can be read from Fig. 6d that the optimal micropore size by CO2 is in the range of 1.5–1.65 nm, which is in agreement with that obtained from N2 isotherm. Therefore, micropore volume by CO2 is the only factor that is closely related to a sample’s CO2 adsorption capacity at 0 °C under atmospheric pressure. Bear in mind that samples at burn-offs up to 32% have shown a narrow microporosity and N2 adsorption isotherms of samples at higher burn-offs starting from the same point indicates a widened microporosity (see Fig. 3). Combining all the results, it can be concluded that CO2 adsorption capacity at 0 °C under one atmosphere
(a)
2
R : 0.86
4.0 3.5 Activation temperature: 800-850oC
3.0
2.0 0.15
60
870-900oC 920oC
2.5 0.20
0.30
0.35
0.40
0.45
VCO2 (cm3/g)
Burn-off (%) 5.0
19 17
NCO2, 0 (mmol/g)
Activation temperature: 800-850oC
18
DCO2 (A)
0.25
(c)
870-900oC 920oC
16 15 14 13
(b)
12 0
10
20
30
40
Burn-off (%)
50
60
70
4.5 4.0 3.5 Activation temperature:
3.0
800-850oC 870-900oC
2.5
920oC
(d)
2.0 13
14
15
16
17
18
19
DCO2 (A)
Fig. 6 – The relations between burn-off and micropore volume (a) and micropore size (b) by CO2 adsorption isotherms, and the relations between adsorption capacity at 0 °C and micropore volume (c) and micropore size (d).
CARBON
0.8
3.0
V N2 VCO2
2.8
0.6 2.6
V (cm3/g)
NCO2, 25 (mmol/g)
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2.4
0.4
2.2
0.2
Activation temperature: 800-850oC 870-900oC 920oC
2.0
0.0
1.8 0
10
20
30
40
50
60
70
0
10
20
30
Burn-off (%)
Fig. 7 – CO2 adsorption capacities at 25 °C at various burnoffs.
It has to be pointed out that the largest amount (2.9 mmol/ g) of CO2 adsorbed at 25 °C is only slightly greater than that (2.8 mmol/g) obtained by Burchell’s group (See Table 1).
Characteristics of CO2 adsorption isotherms
It is intriguing to investigate the significant difference between micropore volume determined by N2 isotherm and that by CO2 isotherm. Fig. 9 shows micropore volumes by N2 isotherm and by CO2 isotherm for samples at various burn-off levels. It can be clearly seen from this figure that there is proportionality between micropore volume obtained from N2 isotherm and burn-off and a peak when micropore volume by CO2 isotherm is concerned. This disagrees with the results reported by Alcaniz-Monge et al. [25]. In their work, activated carbon fibres were prepared by CO2 and steam activation. For the samples activated by CO2, it is observed that micropore volume obtained from N2 isotherm is on the rise with the increasing burn-off and so is the micropore volume obtained from CO2 isotherm. The reason to this disagreement is not clear yet. In our case, V N2 is smaller than V CO2 at a low burn-off at 8%. This indicates that N2 adsorption has experienced diffusional limitations and there is a large portion of the ultra-micropores. As soon as burn-off reaches 16%, V N2 increases to a level higher than that of V CO2 . It is believed that the portion of ultra-microporosity becomes smaller as the microporosity of the sample develops and more micropores are accessible to N2 molecules. When burn-off goes above 16%, the difference between V N2 and V CO2 becomes
NCO2, 25 (mmol/g)
3.0 2.8
2
R : 0.80
2.2
Activation temperature: 800-850oC
2.0
870-900oC 920oC
1.8 0.15
0.20
0.25
0.30
0.35 3
VCO2 (cm /g)
0.40
60
70
more distinct. As aforementioned, the sample with a burn-off of 32% achieves the highest adsorption capacity at 0 °C. At this burn-off, V CO2 is at the peak while V N2 has an intermediate value. The different features of V N2 and V CO2 can possibly lead to the answer to why a sample adsorbs more CO2 at an intermediate burn-off. There are two possible explanations for the initial increase of V CO2 up to 32%: (1) the opening of restricted micropores, and (2) microporosity development during the activation. It has been observed that average micropore size obtained from either N2 or CO2 isotherm goes up with the increasing burn-off. This was confirmed by Kasaoka et al. [26] by testing the adsorption properties of a series of organic dyes of various molecular dimensions on phenolic fibres with various activation degrees. These researchers observed that the adsorption of larger adsorbates did not take place on low burn-off samples but on high burn-off ones, in which the micropores were expanded such that larger molecules could be admitted. They concluded that the activation process did play a role in increasing micropore width. Consequently, micropore volume increase is predominantly due to the widening of the existing micropores. This explains the relation between V N2 and burn-off. The decrease of V CO2 at higher burn-offs has to be explained by the unique feature of CO2 adsorption isotherm. Benefiting from the higher kinetic energy at 0 °C compared with N2 at 196 °C, CO2 has the access to ultra-micropores (<0.7 nm). Because CO2 adsorption isotherm is conducted under sub-atmospheric pressure, it fails to account for the larger micropores in the micropore range. Therefore, as pore size widens at higher burn-offs,
3.0
(a)
2.6 2.4
50
Fig. 9 – Micropore volume measured by CO2 isotherm and N2 isotherm.
0.45
NCO2, 25 (mmol/g)
3.3.4.
40
Burn-off (%)
2.8
(b)
2.6 2.4 2.2
Activation temperature: 800-850oC
2.0
870-900oC 920oC
1.8 13
14
15
16
17
18
19
DCO2 (A)
Fig. 8 – The relations between adsorption capacity at 25 °C and micropore volume (a) and micropore size (b) by CO2 adsorption isotherm.
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some of the pores go beyond the range that can be measured by CO2 isotherm, a contribution of micropore volume from these pores cannot be taken into account by V CO2 . A decline in V CO2 at higher burn-offs can thus be observed. The sample with the maximum V CO2 captures the most CO2 at 0 °C emphasises the importance of smaller pores within the microporosity range. When the adsorption temperature is lifted to 25 °C, CO2 shows the preference to fill in smaller micropores at 25 °C due to the even lower relative pressure (P/P0 = 0.016). Nonetheless, samples with a large micropore volume by CO2 isotherm still achieve higher adsorption capacities at 25 °C, which indicates the contribution of micropore volume from smaller micropores to these samples is still significant at 25 °C.
3.4.
Discussion
At ambient conditions, a few samples have achieved a CO2 adsorption capacity slightly above 2.8 mmol/g (0.12 g/g) with the maximum at 2.9 mmol/g (0.13 g/g). CO2 adsorption capacities of activated carbon fibre-phenolic resin composites have not been significantly improved considering that Burchell and Judkins [10] reported a similar value of 2.8 mmol/g. However, this work confirms the superiority of activated carbon fibre-phenolic resin composites over a series of sorbents as shown in Table 1. When aminated mesoporous silica, anthracite activated carbon and granular coconut shell carbon reached a CO2 adsorption capacity well below 0.1 g/g sorbent under similar conditions, activated carbon fibre-phenolic resin composites have demonstrated a much higher value of 0.13 g/g adsorbent. Therefore, activated carbon fibre-phenolic resin composites have the potential for high efficiency CO2 capture. Micropore volume by sub-atmospheric CO2 isotherm at 0 °C has been identified as a potential design parameter for the development of activated carbon fibre-phenolic resin composites for CO2 capture at low relative pressures. The reason is that micropore volume obtained from sub-atmospheric CO2 isotherm has the ability to measure the volume of smaller micropores in the microporosity range due to its high kinetic energy at 0 °C and does not take into account of bigger micropores because of the low relative pressure. To improve CO2 adsorption capacities at low relative pressures of activated carbon fibre-phenolic resin composites, efforts have to be made to produce narrow microporosity with a large contribution of micropore volume coming from smaller micropores. Since micropores only develop from existing micropores, it is very important to identify suitable raw materials for the fabrication of activated carbon fibre-phenolic resin composites. Three types of carbon fibre were investigated and pitched based carbon fibres were picked over the other two types. More types of carbon fibre should also be examined to compare with pitched based carbon fibres. Additionally, the capability of activated carbon derived from powdered phenolic resin to adsorb CO2 has been confirmed apart from just binding carbon fibres. Powdered phenolic resin can also produce micropores after carbonisation and activation for the purpose of CO2 adsorption. Therefore, various types of phenolic resin should be investigated in or-
der to identify a type that provides the largest volume of micropores with the right size for CO2 capture at low relative pressures. It has to be borne in mind that lower mass ratio samples achieved higher adsorption capacities than those with higher mass ratios in the low burn-off range. The role of the activated carbon derived from powdered phenolic resin in the high burn-off range has to be investigated. It is also worthwhile to investigate mass ratios lower than 1:1 when powdered phenolic resin has a bigger proportion.
4.
Conclusions
Activated carbon fibre-phenolic resin composites fabricated in our lab do not have the problem of non-uniform activation. The uniformity of some samples tested obtained a standard deviation less than 5%. Higher activation temperature, longer activation time and lower mass ratio can help achieve higher activation degree with activation temperature the most important factor and mass ratio the least. As far as CO2 adsorption capacity is concerned, similar conclusions cannot be made because a linear relationship between burn-off and CO2 adsorption capacity does not exist. However, lower mass ratios are found beneficial for achieving higher CO2 adsorption capacities. This demonstrates that activated carbon derived from powdered phenolic resin also has the capability to capture CO2. The cross-over regime of CO2 adsorption on activated carbon fibre-phenolic resin composites has been confirmed. At lower relative pressures, lower burn-off samples capture larger amount of CO2 than those with higher activation degrees. This is due to a higher overlap in potential between the pore walls of smaller micropores in the low burn-off samples. The existence of the cross-over regime has a practical implication as it demonstrates that the development and design of activated carbon fibre-phenolic resin composites are depended on the relative pressure of CO2. Future work can be made to obtain CO2 adsorption isotherms of activated carbon fibrephenolic resin composites with various activation degrees in a relative pressure range of 0–1. This is to further understand the existence and identify the location of the cross-over regime for CO2 adsorption on activated carbon fibre-phenolic resin composites. Micropore volume by CO2 isotherm has been identified as a potential parameter for the design of activated carbon fibrephenolic resin composites. It measures the micropore volume contributed mostly from smaller micropores in the microporosity range, which are paramount for CO2 capture at low relative pressures. Due to the unique features of CO2 adsorption isotherm at 0 °C under sub-atmospheric pressure, micropore volume by CO2 contains the information of both micropore volume and micropore size. The optimal micropore size is found to decrease as the relative pressure goes down and a stronger overlap of even smaller micropores is desired. CO2 adsorption isotherm at 0 °C under sub-atmospheric pressure is believed to be more appropriate for the characterisation of activated carbon fibre-phenolic resin composites than N2 adsorption isotherm at 196 °C as far as low relative pressure adsorption is concerned.
CARBON
4 7 ( 20 0 9 ) 2 3 9 6–24 0 5
Acknowledgements The authors wish to acknowledge the support from the University of Queensland and CSIRO Energy Transformed, National Research Flagship. Special thanks are due to Dr. Ramesh Thiruvenkatachari and Mr. XinXiang Yu for their contributions to the setup of experimental equipment at the CSIRO QCAT Laboratories.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2009.04.029.
R E F E R E N C E S
[1] Yeh AC, Bai H. Comparison of ammonia and monoethanolamine solvents to reduce CO2 greenhouse gas emissions. Sci Total Environ 1999;228(2–3):121–33. [2] Yeh JT, Resnik KP, Rygle K, Pennline HW. Semi-batch absorption and regeneration studies for CO2 capture by aqueous ammonia. Fuel Process Technol 2005;86(14– 15):1533–46. [3] Knowles GP, Graham JV, Delaney SW, Chaffee AL. Aminopropyl-functionalized mesoporous silicas as CO2 adsorbents. Fuel Process Technol 2005;86(14–15):1435–48. [4] Xu X, Song C, Miller BG, Scaroni AW. Adsorption separation of carbon dioxide from flue gas of natural gas-fired boiler by a novel nanoporous ‘‘molecular basket’’ adsorbent. Fuel Process Technol 2005;86(14–15):1457–72. [5] Maroto-Valer MM, Tang Z, Zhang Y. CO2 capture by activated and impregnated anthracites. Fuel Process Technol 2005;86(14–15):1487–502. [6] Kato M, Nakagawa K, Essaki K, Maezawa Y, Takeda S, Kogo R, et al. Novel CO2 absorbents using lithium-containing oxide. Int J Appl Ceram Technol 2005;2(6):467–75. [7] Merel J, Clausse M, Meunier F. Carbon dioxide capture by indirect thermal swing adsorption using 13X zeolite. Environ Prog 2006;25(4):327–33. [8] Burchell TD, Judkins RR. Passive CO2 removal using a carbon fiber - composite molecular sieve. Energy Convers Manage 1996;37(6–8):947–54. [9] Sullivan PD, Rood MJ, Grevillot G, Wander JD, Hay KJ. Activated carbon fiber cloth electrothermal swing adsorption system. Environ Sci Technol 2004;38(18):4865–77. [10] Fuertes AB, Marban G, Nevskaia DM. Adsorption of volatile organic compounds by means of activated carbon fibre-based monoliths. Carbon 2003;41(1):87–96. [11] Mangun CL, Daley MA, Braatz RD, Economy J. Effect of pore size on adsorption of hydrocarbons in phenolic-based activated carbon fibers. Carbon 1998;36:123–31.
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[12] Foster KL, Fuerman RG, Economy J, Larson SM, Rood MJ. Adsorption characteristics of trace volatile organiccompounds in gas streams onto activated carbon-fibers. Chem Mater 1992;4(5):1068–73. [13] Subrenat AS, Le Cloirec PA. Volatile organic compound (VOC) removal by adsorption onto activated carbon fiber cloth and electrothermal desorption: an industrial application. Chem Eng Commun 2006;193(4):478–86. [14] Burchell TD, Judkins RR, Rogers MR, Williams AM. A novel process and material for the separation of carbon dioxide and hydrogen sulfide gas mixtures. Carbon 1997;35(9):1279–94. [15] Moon S-H, Shim J-W. A novel process for CO2/CH4 gas separation on activated carbon fibers – electric swing adsorption. J Colloid Interface Sci 2006;298(2):523–8. [16] Katsaros FK, Steriotis TA, Stubos AK, Mitropoulos A, Kanellopoulos NK, Tennison S. High pressure gas permeability of microporous carbon membranes. Microporous Mater 1997;8(3–4):171–6. [17] Lenghaus K, Qiao GG, Solomon DH, Gomez C, RodriguezReinoso F, Sepulveda-Escribano A. Controlling carbon microporosity: the structure of carbons obtained from different phenolic resin precursors. Carbon 2002;40(5):743–9. [18] Tennison SR. Phenolic-resin-derived activated carbons. Appl Catal A 1998;173(2):289–311. [19] Nakagawa K, Mukai SR, Tamura K, Tamon H. Mesoporous activated carbons from phenolic resins. Chem Eng Res Des 2007;85(9):1331–7. [20] Yang JB, Ling LC, Liu L, Kang FY, Huang ZH, Wu H. Preparation and properties of phenolic resin-based activated carbon spheres with controlled pore size distribution. Carbon 2002;40(6):911–6. [21] Oya A, Yoshida S, Alcaniz-Monge J, Linares-Solano A. Formation of mesopores in phenolic resin-derived carbon fiber by catalytic activation using cobalt. Carbon 1995;33(8):1085–90. [22] Burchell TD, Klett JW, Weaver CE. A novel carbon fiber based porous carbon monolith. In: Proceedings of the 9th annual conference on fossil energy materials, 1995, Oak Ridge: Oak Ridge National Lab; 1995. p. 447–56. [23] Burchell TD, Weaver CE, Chilcoat BR, Derbyshire F, Jagtoyen M. Activated carbon fibre composite material and method of making. US patent 6030698, 2000. [24] Martin-Gullon I, Andrews R, Jagtoyen M, Derbyshire F. PANbased activated carbon fiber composites for sulfur dioxide conversion: influence of fiber activation method. Fuel 2001;80(7):969–77. [25] Alcaniz-Monge J, Cazorla-Amoros D, Linares-Solano A, Yoshida S, Oya A. Effect of the activating gas on tensile strength and pore structure of pitch-based carbon fibres. Carbon 1994;32(7):1277–83. [26] Kasaoka S, Sakata Y, Tanaka E, Naitoh R. Design of molecularsieve carbon. Studies on the adsorption of various dyes in the liquid phase. Int Chem Eng 1989;29(4):734–42.
4 7 ( 2 0 0 9 ) 2 3 9 6 –2 4 0 5
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
CO2 capture capacities of activated carbon fibre-phenolic resin composites Hui Ana,b, Bo Fenga,*, Shi Sub a
School of Engineering, The University of Queensland, St. Lucia, QLD 4072, Australia CSIRO, P.O. Box 883, Kenmore, QLD 4069, Australia
b
A R T I C L E I N F O
A B S T R A C T
Article history:
The potential of activated carbon fibre-phenolic resin composites for CO2 capture has been
Received 12 December 2008
evaluated in this work. A number of composites were fabricated using different types of
Accepted 20 April 2009
carbon fibre under various conditions. The effect of a range of variables such as the type
Available online 3 May 2009
of carbon fibre, mass ratio of carbon fibre to phenolic resin, activation temperature and duration on the CO2 adsorption capacity was investigated. Activated carbon derived from powdered phenolic resin demonstrates its capability to capture CO2 and it plays a significant role in the low burn-off range. An apparent optimal degree of activation for CO2 adsorption capacity was identified which was coincident with the maximum micropore volume measured by CO2 physical adsorption. Micropore volume by CO2 has been identified as a potential design parameter for the development of activated carbon fibre-phenolic resin composites for CO2 capture. The existence of a cross-over regime is confirmed and lower burn-off samples are found to capture more CO2 at ambient conditions. This is attributed to a narrow microporosity and a large contribution of micropore volume from smaller pores in the microporosity range of the composites. The optimal pore size for CO2 capture becomes smaller when the relative pressure of CO2 goes lower. Ó 2009 Elsevier Ltd. All rights reserved.
1.
Introduction
CO2 removal from fossil fuel fired power station flue gas (i.e. post-combustion CO2 capture) is now an important strategy for the reduction of CO2 emissions in the world. Among all the research efforts in developing techniques for carbon capture and sequestration, CO2 separation and capture cost is still a main focus as it is an expensive part in CO2 capture and sequestration process. Meanwhile, improving the capture efficiency is also very important. For example, amine based scrubbing has been trialed for post-combustion CO2 capture at several pilot-scale plants in the world; however, it suffers from low efficiency, high energy consumption and environmental problems of waste water and sludge. Apart from amine, a lot of other materials have been investigated for
CO2 capture. Table 1 summarises CO2 capture capacities of some sorbents as reported in the literature. Activated carbon fibre materials have attracted a lot of research and studies for volatile organic compounds (VOCs) removal [9–13]. They are made of carbon fibres and normally a carbonisable carbon as the binder. However, very rare studies have been done on these materials for CO2 capture specifically. For instance, a few studies have been conducted for the selectivity of CO2 and CH4 [14,15]. Commercial pitch based active carbon fibres were tested for the separation of CO2 and CH4 and the selective separation of CO2 was demonstrated with selectivity (CO2/CH4) of 2.2. Pitch based activated carbon fibre material was also compared with a commercial coconut shell carbon [8] and a larger uptake of CO2 was demonstrated as shown in Table 1. There are some factors during the
* Corresponding author: Fax: +61 733 654 799. E-mail address: [email protected] (B. Feng). 0008-6223/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.04.029
CARBON
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Table 1 – CO2 capture capacities of sorbents as reported. Sorbent Monoethanolamine (MEA) [1] Aqueous ammonium [2] Aminated mesoporous silica [3] PEI-impregnated MCM-41 [4] Anthracite activated carbon [5] Lithium silicate [6] 13X zeolite [7] Granular coconut shell carbon [8] Pitch based activated carbon fibre material [8]
activation process of activated carbon fibre materials that can potentially have an effect on the adsorption performance of the sample. Research on the effects of these factors is scarce in the literature. In particular, phenolic resin has been commonly used as the binder. And the mass ratio of carbon fibres to phenolic resin is normally maintained at 3:1. It has to be noted that activated carbons can be produced from various types of phenolic resin. The obtained activated carbons can be microporous [11,16–18] and mesoporous [19–21]. Hence, activated carbon derived from phenolic resin after carbonisation and activation may also play a role in adsorbing CO2. Therefore, in this work, a larger proportion of phenolic resin in the raw materials will be tried. Mass ratio of carbon fibre to phenolic resin will be investigated as a variable. The materials produced are thus referred to as activated carbon fibrephenolic resin composites. An intriguing phenomenon has been observed while using activated carbon fibre materials for VOC capture. Foster et al. [12] reported that a cross-over point is observed with three nbutane adsorption isotherms on activated carbon fibres with different activation degrees. It is found that above the crossover point (at high relative pressures), the amount adsorbed is depended on the pore volume. Namely, the higher burnoff sample captured the greater amount of n-butane. On the other hand, a reverse correlation is observed at the low relative pressures (below the cross-over point). Mangun et al. [11] studied the cross-over regime of a series of alkanes and concluded that the occurrence of cross-over regime is due to a pore size–pore volume effect. At lower relative pressures, low burn-off samples have a larger uptake because of a higher overlap in potential between the pore walls of smaller micropores that binds adsorbate molecules more tightly. At higher relative pressures, the total micropore volume is utilised and the adsorption capacity is a function of burn-off. However, the cross-over regime phenomenon has not been investigated on CO2, although an optimal micropore volume measured by N2 has been reported to be around 0.5 cm3/g for CO2 capture [8,22]. There also exists an optimal pore size by N2 isotherm [8], which is about 0.8 nm using the Dubinin–Radushkevich (DR) method and ca. 1.5–1.6 nm if using the Dubinin–Astakhov (DA) method. It is known that higher burn-offs (defined as the weight loss of the sample in percentage during the activation step) can enlarge micropore volume and micropore size to be in excess of the optimal values for CO2 capture. Furthermore, micropore volume and micropore size appear as two independent parameters. A sample with
Capacity
Capture conditions
0.4 g/g MEA 1.20 g/g NH3 0.07 g/g sorbent 0.09 g/g adsorbent 65.7 mg/g adsorbent 0.35 g/g sorbent 0.14 g/g adsorbent 0.08 g/g adsorbent 0.12 g/g adsorbent
16% CO2, balanced with air 15% CO2, 85% N2 90% CO2, 10% Ar, 20 °C 15% CO2, 4% O2, 81% N2, 75 °C 99.8% CO2, 30 °C 100% CO2, 700 °C 10% CO2, 90% N2, 15 °C 100% CO2, 25 °C 100% CO2, 25 °C
the optimal micropore volume does not necessarily have the optimal micropore size; and vice versa. Therefore, either micropore volume or micropore size by N2 is not sufficient to be a good parameter for the design of activated carbon fibre materials. In this paper, the uniformity levels of the samples are investigated since uniform activation is desired in order that the microstructure of activated carbon fibre-phenolic resin composite can be closely controlled. The effects of the fabrication conditions are explored on the activation level and the adsorption capacity as well. Efforts have also been made to study the cross-over regime with CO2 adsorption on activated carbon fibre-phenolic resin composites. In addition, this work tries to identify a single parameter that can predict a composite’s CO2 adsorption capacity at ambient conditions.
2.
Experiments
2.1.
Experimental process
The experiments were conducted as follows. A number of activated carbon fibre-phenolic resin composites were fabricated using different types of carbon fibre. The detailed fabrication procedure is introduced in Section 2.2. At first, some samples were tested for their uniformity level of activation by CO2 before further investigation was conducted. Then, the effects of some variables during the fabrication were investigated. There are a number of variables that could affect the structure of the final product including the type of carbon fibre, mass ratio of carbon fibre to phenolic resin, drying temperature and environment, heat treatment temperature and duration, activation agent, activation temperature and duration. Based on the experience and preliminary test results, four variables were considered the most important, i.e. the type of carbon fibre, mass ratio of carbon fibre to phenolic resin, activation temperature and duration. Mass ratio reflects the relative proportion of carbon fibres and phenolic resin. The relative importance of each material can be examined by investigating it. In order to identify the most important factors by experiment while minimising the number of samples to be tested, an orthogonal experimental design method was used as discussed in Section 2.3. At last, more samples with a large range of burn-offs were prepared to study their CO2 adsorption capacities at 0 and 25 °C under atmospheric pressure.
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2.2.
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Sample preparation
Activated carbon fibre-phenolic resin composites were made from carbon fibre and powdered phenolic resin. The detailed fabrication procedure can refer to [8,23]. Briefly, three types of carbon fibre were utilised: pitch based carbon fibres, activated pitch based carbon fibres (APitch) and polyacrylonitrile (PAN) carbon fibres. At first, carbon fibres and powdered phenolic resin were mixed in water slurry at a certain mass ratio. Then, the slurry was transferred to a moulding tank and water was vacuum pumped from the bottom of the tank. The sample was subsequently dried and cured to solidify the structure. After that, the resultant product was placed in a three-stage tube furnace and carbonised at 650 °C under N2 flow. The last stage involved activating the sample in CO2 in the temperature range of 800–920 °C. The final sample is in a cylindrical form with maximum dimensions of 3.2 cm in diameter and 10 cm in length. Inside the sample, there are 17 evenly distributed longitudinal channels with a diameter of 0.03 cm.
3.
Results and discussion
3.1.
Preliminary studies
3.1.1.
Confirmation of uniform activation
Four samples were made in order to test their activation uniformity levels. Each sample was evenly cut into three pieces. Each piece was crushed into smaller pieces separately and stored in different containers. Approximately 0.5 g specimens were obtained from each container for microstructure analysis by the TriStar 3000. Samples tested were made from different carbon fibres at different mass ratios of carbon fibres to powdered phenolic resin and activated under different conditions. Burn-off is in the range of 5–14.7%. It is found that BET surface area does not change significantly throughout a sample. For all the samples, the standard deviation is below 5%, which is acceptable. It has also been found that micropore volume changed very little at different locations within a sample.
3.1.2. 2.3.
Orthogonal experimental designs
Orthogonal experimental designs were carried out by ‘‘The Unscrambler’’ (The Unscrambler 9.7, 2007) for experimental plan on studying the effects of the three variables, including mass ratio, activation time and temperature. Carbon fibre type was investigated separately because it is not a continuous variable. The Unscrambler helps generate well-designed experimental plan with a minimum number of experiments to save time and cost. Two experimental designs were built: a screening design and an optimisation design. The screening design aimed to find out the relative importance of each variable. The optimisation design was used in an attempt to identify the optimal conditions to produce activated carbon fibre-phenolic resin composites and also to study the effects of the variables in more detail.
2.4.
Characterisation
The porous structure of the samples was analysed by N2 adsorption at 196 °C and CO2 adsorption at 0 °C on TriStar 3000 (Micrometritics). N2 adsorption at 196 °C, which covers a large range of relative pressures from P/P0 = 0 to 0.98, can be used as a measure of the whole range of micropores. However, due to the very low temperature, N2 suffers from low kinetic energy and cannot enter into ultra-micropores (<0.7 nm). Therefore, CO2 adsorption at 0 °C under sub-atmospheric pressure (which corresponds to a relative pressure of P/P0 < 0.015) is usually used as a complement to account for ultra-microporosity. It has to be noted that some larger micropores are not completely taken into account by CO2 adsorption due to the low relative pressure. The BET surface area SBET, DR micropore volume V N2 and DA average micropore size DN2 were calculated from N2 adsorption isotherms. In the case of CO2 adsorption isotherms at 0 °C, only DR micropore volume V CO2 and DA average micropore size DCO2 were calculated. Additionally, CO2 adsorption isotherms were also obtained at 20 and 25 °C under sub-atmospheric pressure.
Screening of carbon fibres
A series of samples were made utilising three types of carbon fibre to identify the carbon fibre that gives the most satisfactory performance. The results are tabulated in Table 2. Two samples were produced from each type of carbon fibre. Samples 4 and 5 were made from polyacrylonitrile carbon fibres and have the smallest BET surface area and micropore volume compared with other samples. Correspondingly, the amount of CO2 captured by those two samples was also the smallest. Sample 5 for example trapped less than half the CO2 of samples made from pitch based and activated pitch based carbon fibres. It can thus be inferred that polyacrylonitrile carbon fibres are not promising as far as CO2 adsorption is concerned. It has been reported that samples prepared from polyacrylonitrile carbon fibres maintained a low surface area (around 700 m2/g) even at very high burn-off [24]. Another intriguing point to note is that even samples made from activated pitch based carbon fibres exhibit significantly higher BET surface areas than those made from pitch based carbon fibres, they do not show any advantages in CO2 capture. Furthermore, when activated carbon fibres are mixed with a binder, Fuertes et al. [10] observed that there is a loss in surface area, porosity and adsorption capacity for volatile organic gases (VOCs) with respect to the same parameters for nonagglomerated fibres. It is suggested that a fraction of the porosity be plugged by the binder. And it is recommended that non-activated carbon fibres be used to optimise the adsorption properties. Therefore, pitch based carbon fibres were chosen for further investigation.
3.2.
The effects of mass ratio and activation conditions
3.2.1.
Factor screening
Seven samples were involved in the screening design. Three (11, 12 and 13) of them were centre samples, of which the values of the variables were in the middle of the corresponding range. Table 3 shows the fabrication conditions and response data. The mass ratio of pitch based carbon fibres to powdered phenolic resin varied from 1:1 to 3:1. 3:1 was the maximum in
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Table 2 – Experimental results for screening of carbon fibres. Sample
Precursor
Mass ratio (fibre:resin)
Activation Temp. (°C)
1 2 3 4 5 6
APitch APitch Pitch PAN PAN Pitch
4:1 2:1 2:1 2:1 1:1 1:1
850 850 800 800 800 900
Time (h) 1 1 1.5 2 2 1
CO2 adsorbeda
BO
SBET
V N2
(%)
(m2/g)
(cm3/g)
(mmol/g)
14.7 7.4 9.2 4.9 7.6 14
925 819 366 140 245 424
– 0.43 0.21 0.07 0.13 0.22
2.26 2.23 2.23 0.55 0.94 2.28
a The amount of CO2 adsorbed at 20 °C under atmospheric pressure.
order that carbon fibres and powdered phenolic resin could mix well. Burn-off of these samples was from 1.4% to 27.9%. ‘‘Analysis of effects’’ carried out by The Unscrambler shows that none of these three variables is significant compared with others. In other words, each variable has non-negligible influence on the adsorption performance of the samples as other variables. As a result, these three variables would be further investigated in the optimisation design. Another finding is that the mean value of the centre samples is lower than that of the overall design, indicating that optimal conditions are beyond the region studied so far. The upper activation temperature of 900 °C was considered very high. Therefore, the upper limit of activation time was raised to 5.5 h in the following optimisation design to broaden the burn-off range in order to have the optimal conditions included in the region studied.
3.2.2.
The effects of the factors
The optimisation design was carried out in an attempt to identify the optimal conditions by expanding the region investigated by involving more samples fabricated under various conditions. Fifteen samples in total were studied (see Table 4). The upper limit of activation time was extended to 5.5 h. However, the maximum burn-off obtained was only 31.1%. The evident gain of CO2 adsorption capacity with the increase of burn-off (see Fig. 1) demonstrates that the optimisation design failed to identify the optimal conditions. Fig. 1 shows the perfect linear relationship between burn-off and CO2 adsorption capacities of the samples in the optimisation design. Samples with different mass ratios are marked using different symbols. On the other hand, the effects of the factors in more detail can be extracted from this design since more samples were involved. This was conducted by running ‘‘analysis of effects’’. The results show that activation temperature has the most
significant effect on burn-off and activation time less significant (see Fig. 2a). A sample activated under higher temperature could achieve significantly higher burn-off. For instance, a sample activated at 800 °C for 5.5 h obtained a burn-off of ca. 6%. When the activation temperature was raised to 900 °C, the burn-off could achieve a value of 31.1%, which is five times higher (see Fig. 2a). Mass ratio is found to have the least influence on burn-off as shown in Fig. 2b. On the contrary, similar conclusions cannot be made regarding the effects of these factors on CO2 adsorption capacities as can be seen later in this paper that CO2 adsorption capacity does not always increase with burn-off. Nonetheless, the role of activated carbon derived form powdered phenolic resin can be examined by studying the influence of mass ratio, discussed as follows. As shown by the good linear relationship between burn-off and CO2 adsorption capacities in Fig. 1 as well as the relation between mass ratio and burn-off in Fig. 2b, samples with lower mass ratios (or higher contents of phenolic resin) actually captured slightly more CO2 than those with higher mass ratios whilst other fabrication conditions were the same. This demonstrates the existence of microporosity of the activated carbon derived from powdered phenolic resin. It is therefore confirmed that apart from acting as a carbon fibre binder, activated carbon derived from powdered phenolic resin also plays a significant role in adsorbing CO2. Under the tested experimental conditions it is slightly better than the activated carbon fibre used.
3.3. Characterisation analysis of activated carbon fibrephenolic resin composites In the optimisation design, the maximum burn-off achieved was 31.1%. Adsorption capacity increasing with burn-off demonstrates that the design has failed to identify the opti-
Table 3 – Experimental data for the factor screening design. Sample
Mass ratio (fibre:resin)
Activation Temp. (°C)
7 8 9 10 11 12 13
3:1 3:1 1:1 1:1 2:1 2:1 2:1
800 900 800 900 850 850 850
BO Time (h) 0.5 3.5 3.5 0.5 2 2 2
SBET 2
(%)
(m /g)
1.4 27.9 9 9.1 6.3 9.3 8.8
131 709 322 200 341 317 331
NCO2 , 0 °C (mmol/g) 2.63 3.99 2.74 2.62 2.75 2.87 2.84
2400
CARBON
4 7 ( 2 0 0 9 ) 2 3 9 6 –2 4 0 5
Table 4 – Experimental data for the optimisation design. Sample
Mass ratio (fibre:resin)
Activation Temp. (°C)
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
1:1 1:1 3:1 2:1 3:1 1:1 2:1 1:1 3:1 2:1 3:1 2:1 2:1 2:1 2:1
Time (h)
900 850 800 800 850 850 900 800 850 800 900 900 850 850 850
3 0.5 3 0.5 0.5 5.5 0.5 3 5.5 5.5 3 5.5 3 3 3
4.5
NCO2, 0 (mmol/g)
SBET 2
(%)
(m /g)
25.6 4.2 4.2 0.2 2.4 17.8 4.7 5.7 12.9 6.0 18.6 31.1 7.9 11.3 10.3
697 204 291 147 100 587 280 137 521 375 677 998 298 396 451
NCO2 , 0 °C (mmol/g) 4.13 2.64 2.67 2.45 2.42 3.68 2.65 2.75 3.48 2.84 4.05 4.29 2.92 2.96 3.06
to obtain a burn-off at different levels with a maximum at 65.1%. Hence, CO2 adsorption capacities of samples with a large range of burn-offs can be studied.
5.0
4.0
3.3.1.
2
R : 0.94
3.5 3.0
Mass ratio: 1:1 2:1 3:1
2.5 2.0
BO
0
5
10
15
20
25
30
35
Burn-off (%)
Fig. 1 – The relation between burn-off and CO2 adsorption capacities of the samples in the optimisation design.
mal fabrication conditions. However, higher activation temperature, longer activation duration and lower mass ratio have been found beneficial for achieving higher activation degrees. Therefore, more samples with higher burn-offs were made. This was realised by fabricating the samples under the conditions based on the optimisation design results. A higher activation temperature of 920 °C was adopted and the mass ratio was fixed at 1:1. Activation time was varied
Activation time: 0.5 hr Activation time: 5.5 hrs
20 10 0 800
(a) 820
840
860
880
o
Activation temp: 800 C
30
Burn-off (%)
Burn-off (%)
30
Adsorption isotherms
N2 adsorption isotherms of samples with different burn-off levels are shown in Fig. 3, which plots N2 amount adsorbed (nN2) at various relative pressures. It can be seen that nN2 increases as burn-off goes higher. The isotherms are of type I, which indicates the samples are microporous solids. The upward deviation at higher relative pressures shows that samples also contain mesoporosity. Two major observations can be made from this figure. One is that the onset of the adsorption isotherms climbs up on the y-axis until a burn-off of 32% reached. After that, adsorption isotherms all start from the same point. The other observation is that the knee of the isotherms is very sharp at lower burn-offs, an indication of narrow microporosity. However, those samples with higher burn-offs exhibit round knees. These two observations indicate that samples with higher burn-offs experienced a significant widening in the microporosity. Fig. 4 presents CO2 adsorption isotherms at 0 °C for the same samples as shown in Fig. 3. The isotherms are plotted in respect of total pressure with the highest pressure being one atmosphere. The amount of CO2 adsorbed increases as the total pressure goes up. It can be observed that isotherm
Activation temperature (oC)
900
o
Activation temp: 900 C
20 10 0
1.0
(b) 1.5
2. 0
Mass ratio
2.5
3.0
Fig. 2 – The interaction effects of (a) activation temperature and time, and (b) mass ratio and activation temperature on burnoff.
CARBON
25
5.0 4.5
61.3%
20
NCO2, 0 (mmol/g)
nN2 (mmol/g)
45.8% 32.1%
15
25.4% 16.0%
10 7.9%
5 0
3.5 3.0 Activation temperature:
2.0 0.0
0.2
0.4
0.6
0.8
1.0
800-850oC 870-900oC 920oC
of the sample at a burn-off of 32.1% is above those with either higher or lower burn-offs. This shows that the evolution of CO2 adsorption isotherm with burn-off is different from that of N2 adsorption isotherm. In the case of CO2 adsorption, the sample with the highest burn-off fails to capture the largest amount compared with other samples at lower burn-offs.
3.3.2. The relationship between burn-off and adsorption capacity Fig. 5 plots samples’ CO2 adsorption capacities at 0 °C under one atmosphere at various burn-offs. Samples activated under different temperatures are denoted using different marks. It has to be noted that mass ratios of samples activated under the temperature below 920 °C varied from 1:1 to 3:1 and these samples achieved a burn-off up to 31.1%. From this figure, it can be seen that the adsorption capacity (NCO2 , 0) initially increases as burn-off goes up until reaching a burn-off of 32%, after which a levelling off occurs. Data presented in this figure confirms the view obtained from Fig. 4 that higher burn-offs can be detrimental for CO2 adsorption at low relative pressures (P/P0 = 0.029 at 0 °C and 1 atm). Since N2 adsorption isotherm and CO2 adsorption isotherm behave differently with burn-offs, a cross-over region is likely to exist for CO2 adsorption.
5
4
3
2
Burn-off: 7.9% 16% 25.4% 32.1% 45.8% 61.3%
1
0 200
400
0
10
20
30
40
50
60
70
Burn-off (%)
Fig. 3 – N2 adsorption isotherms at -196 °C for samples with different burn-offs.
NCO2, 0 (mmol/g)
4.0
2.5
P/P0
0
2401
4 7 ( 20 0 9 ) 2 3 9 6–24 0 5
600
800
P (mmHg) Fig. 4 – CO2 adsorption isotherms at 0 °C for samples with different burn-offs.
Fig. 5 – CO2 adsorption capacities at 0 °C at various burnoffs.
To further investigate the effect of burn-off, BET surface area, DR micropore volume and DA micropore size of these samples have been measured by N2 adsorption isotherms. BET surface area, micropore volume and average micropore size are found to increase as burn-off goes higher, which agrees with the literature. It has been reported that at very high relative pressures (P/P0 = 1) the adsorption capacity is only depended on pore volume [10]. Both narrow and wide micropores can be filled. This explains why N2 adsorption isotherms of samples with higher burn-offs are above those with lower burn-offs as shown in Fig. 3. As the activation degree goes up, microporosity develops and results in larger micropore volume, and thus the larger adsorbed amount. Based on the Gurvitsch rule, the adsorbed volume onto one adsorbent will be the same for different adsorbates, provided that the molecules of the adsorbates are not large compared to the pore dimensions. Foster et al. [12] supported this by obtaining a similar micropore volume of a certain adsorbent with a series of VOC. Therefore, CO2 adsorption at P/ P0 = 1 will follow the same trend as that of N2, i.e., CO2 adsorption capacity will increase with the activation level due to the increase of micropore volume. Adsorption capacity at high relative pressures is a simple function of micropore volume, or burn-off. However, as far as CO2 adsorption (at 0 °C and 1 atm) is concerned as shown in Fig. 5, the linear proportionality does not exist between CO2 adsorption capacity and burn-off. A sample with an intermediate burn-off captures the larger amount of CO2 than those with higher burn-offs. This demonstrates that a cross-over region exists for CO2 adsorption on activated carbon fibre-phenolic resin composites. This is because the phenomenon is consistent with the description of ‘‘cross-over regime’’ as reported [11,12]. CO2 adsorption capacity at 0 °C and 1 atm does not go up with the increase of micropore volume because only a fraction of the micropore is occupied at lower relative pressures. Therefore, an excessive micropore volume is detrimental to the adsorption capacity of a sample. Similarly, too high the BET surface area or micropore size does not necessarily result in higher adsorption capacity. The measurement of BET surface area, micropore volume or micropore size by N2 thus cannot be used to design activated carbon fibre-phenolic resin composites. It has to be pointed out that at the burn-off of 32%,
2402
CARBON
4 7 ( 2 0 0 9 ) 2 3 9 6 –2 4 0 5
maximises when the sample has a narrow microporosity and contains a large micropore volume measured by CO2 adsorption isotherm. This thus partially explains why lower mass ratio samples achieved higher CO2 adsorption capacities. The reason is that activated carbon derived from powdered phenolic resin can produce narrow micropores suitable for CO2 adsorption as well. Indeed, Tennison successfully produced phenolic-resin-derived activated carbons which have a very narrow pore size distribution with a mean pore size of 0.8 nm (DR) [18]. Note that this micropore size has been identified as optimal for CO2 capture.
3.3.3.
0.45
5.0
0.40
4.5
0.35 0.30 0.25
Activation temperature: 800-850oC
0.20
870-900oC 920oC
0.15 0
20
40
The effect of adsorption temperature
The effect of adsorption temperature was also investigated. CO2 adsorption isotherms at 25 °C also under sub-atmospheric pressure were obtained and the CO2 adsorption capacities derived are shown in Fig. 7. There is a clear peak at about 25% burn-off, at which an adsorption capacity of 2.9 mmol/g is reached, although samples at 32% burn-offs still exhibit very good adsorption capacities. Noting that the peak value for CO2 adsorption at 0 °C reaches 4.5 mmol/g. The maximum CO2 adsorption capacity of activated carbon fibre-phenolic resin composite has dropped by more than 30% when adsorption temperature increases from 0 to 25 °C. A closer look at the relation between adsorption capacity at 25 °C and V CO2 (Fig. 8a) shows that adsorption capacity at 25 °C still increases proportionally with the increasing V CO2 except a few outliers. It has to be noted that data obtained for the three samples in the two ovals are considered outliers and not taken into account while making the linear regression. However, the optimal pore size has shifted to a smaller size region, which is in the range of 1.45–1.55 nm, compared with the optimal micropore size range for CO2 adsorption at 0 °C.
NCO2, 0 (mmol/g)
VCO2 (cm3/g)
the micropore volume and micropore size are 0.6 cm3/g and 1.6 nm, respectively. In the literature, the optimal micropore volume and micropore size (DA) were previously found to be 0.5 cm3/g and 1.5–1.6 nm, respectively [8,22]. The optimal micropore volume is slightly lower than that observed in this work. The samples were also examined by CO2 adsorption isotherms at 0 °C. DR micropore volume and DA micropore size were obtained. Fig. 6a and b illustrates the relations between burn-off and micropore volume and size by CO2 adsorption isotherms. Micropore volume obtained from CO2 adsorption isotherm behaves quite differently from that obtained from N2 adsorption isotherm. Upon increasing burn-off micropore volume by CO2 initially increases and reaches a peak at 32%, then goes downward as burn-off keeps going further. This trend is similar to that shown by burn-off and adsorption capacity at 0 °C. It is therefore possible that adsorption capacity is closely related to micropore volume measured by CO2 adsorption isotherm. Fig. 6c, which shows the relation between micropore volume by CO2 and adsorption capacity, confirms this. It can be observed that a sample’s adsorption capacity can be evaluated by its micropore volume by CO2. Micropore size by CO2 presented in Fig. 6b behaves similarly as that by N2. It can be read from Fig. 6d that the optimal micropore size by CO2 is in the range of 1.5–1.65 nm, which is in agreement with that obtained from N2 isotherm. Therefore, micropore volume by CO2 is the only factor that is closely related to a sample’s CO2 adsorption capacity at 0 °C under atmospheric pressure. Bear in mind that samples at burn-offs up to 32% have shown a narrow microporosity and N2 adsorption isotherms of samples at higher burn-offs starting from the same point indicates a widened microporosity (see Fig. 3). Combining all the results, it can be concluded that CO2 adsorption capacity at 0 °C under one atmosphere
(a)
2
R : 0.86
4.0 3.5 Activation temperature: 800-850oC
3.0
2.0 0.15
60
870-900oC 920oC
2.5 0.20
0.30
0.35
0.40
0.45
VCO2 (cm3/g)
Burn-off (%) 5.0
19 17
NCO2, 0 (mmol/g)
Activation temperature: 800-850oC
18
DCO2 (A)
0.25
(c)
870-900oC 920oC
16 15 14 13
(b)
12 0
10
20
30
40
Burn-off (%)
50
60
70
4.5 4.0 3.5 Activation temperature:
3.0
800-850oC 870-900oC
2.5
920oC
(d)
2.0 13
14
15
16
17
18
19
DCO2 (A)
Fig. 6 – The relations between burn-off and micropore volume (a) and micropore size (b) by CO2 adsorption isotherms, and the relations between adsorption capacity at 0 °C and micropore volume (c) and micropore size (d).
CARBON
0.8
3.0
V N2 VCO2
2.8
0.6 2.6
V (cm3/g)
NCO2, 25 (mmol/g)
2403
4 7 ( 20 0 9 ) 2 3 9 6–24 0 5
2.4
0.4
2.2
0.2
Activation temperature: 800-850oC 870-900oC 920oC
2.0
0.0
1.8 0
10
20
30
40
50
60
70
0
10
20
30
Burn-off (%)
Fig. 7 – CO2 adsorption capacities at 25 °C at various burnoffs.
It has to be pointed out that the largest amount (2.9 mmol/ g) of CO2 adsorbed at 25 °C is only slightly greater than that (2.8 mmol/g) obtained by Burchell’s group (See Table 1).
Characteristics of CO2 adsorption isotherms
It is intriguing to investigate the significant difference between micropore volume determined by N2 isotherm and that by CO2 isotherm. Fig. 9 shows micropore volumes by N2 isotherm and by CO2 isotherm for samples at various burn-off levels. It can be clearly seen from this figure that there is proportionality between micropore volume obtained from N2 isotherm and burn-off and a peak when micropore volume by CO2 isotherm is concerned. This disagrees with the results reported by Alcaniz-Monge et al. [25]. In their work, activated carbon fibres were prepared by CO2 and steam activation. For the samples activated by CO2, it is observed that micropore volume obtained from N2 isotherm is on the rise with the increasing burn-off and so is the micropore volume obtained from CO2 isotherm. The reason to this disagreement is not clear yet. In our case, V N2 is smaller than V CO2 at a low burn-off at 8%. This indicates that N2 adsorption has experienced diffusional limitations and there is a large portion of the ultra-micropores. As soon as burn-off reaches 16%, V N2 increases to a level higher than that of V CO2 . It is believed that the portion of ultra-microporosity becomes smaller as the microporosity of the sample develops and more micropores are accessible to N2 molecules. When burn-off goes above 16%, the difference between V N2 and V CO2 becomes
NCO2, 25 (mmol/g)
3.0 2.8
2
R : 0.80
2.2
Activation temperature: 800-850oC
2.0
870-900oC 920oC
1.8 0.15
0.20
0.25
0.30
0.35 3
VCO2 (cm /g)
0.40
60
70
more distinct. As aforementioned, the sample with a burn-off of 32% achieves the highest adsorption capacity at 0 °C. At this burn-off, V CO2 is at the peak while V N2 has an intermediate value. The different features of V N2 and V CO2 can possibly lead to the answer to why a sample adsorbs more CO2 at an intermediate burn-off. There are two possible explanations for the initial increase of V CO2 up to 32%: (1) the opening of restricted micropores, and (2) microporosity development during the activation. It has been observed that average micropore size obtained from either N2 or CO2 isotherm goes up with the increasing burn-off. This was confirmed by Kasaoka et al. [26] by testing the adsorption properties of a series of organic dyes of various molecular dimensions on phenolic fibres with various activation degrees. These researchers observed that the adsorption of larger adsorbates did not take place on low burn-off samples but on high burn-off ones, in which the micropores were expanded such that larger molecules could be admitted. They concluded that the activation process did play a role in increasing micropore width. Consequently, micropore volume increase is predominantly due to the widening of the existing micropores. This explains the relation between V N2 and burn-off. The decrease of V CO2 at higher burn-offs has to be explained by the unique feature of CO2 adsorption isotherm. Benefiting from the higher kinetic energy at 0 °C compared with N2 at 196 °C, CO2 has the access to ultra-micropores (<0.7 nm). Because CO2 adsorption isotherm is conducted under sub-atmospheric pressure, it fails to account for the larger micropores in the micropore range. Therefore, as pore size widens at higher burn-offs,
3.0
(a)
2.6 2.4
50
Fig. 9 – Micropore volume measured by CO2 isotherm and N2 isotherm.
0.45
NCO2, 25 (mmol/g)
3.3.4.
40
Burn-off (%)
2.8
(b)
2.6 2.4 2.2
Activation temperature: 800-850oC
2.0
870-900oC 920oC
1.8 13
14
15
16
17
18
19
DCO2 (A)
Fig. 8 – The relations between adsorption capacity at 25 °C and micropore volume (a) and micropore size (b) by CO2 adsorption isotherm.
2404
CARBON
4 7 ( 2 0 0 9 ) 2 3 9 6 –2 4 0 5
some of the pores go beyond the range that can be measured by CO2 isotherm, a contribution of micropore volume from these pores cannot be taken into account by V CO2 . A decline in V CO2 at higher burn-offs can thus be observed. The sample with the maximum V CO2 captures the most CO2 at 0 °C emphasises the importance of smaller pores within the microporosity range. When the adsorption temperature is lifted to 25 °C, CO2 shows the preference to fill in smaller micropores at 25 °C due to the even lower relative pressure (P/P0 = 0.016). Nonetheless, samples with a large micropore volume by CO2 isotherm still achieve higher adsorption capacities at 25 °C, which indicates the contribution of micropore volume from smaller micropores to these samples is still significant at 25 °C.
3.4.
Discussion
At ambient conditions, a few samples have achieved a CO2 adsorption capacity slightly above 2.8 mmol/g (0.12 g/g) with the maximum at 2.9 mmol/g (0.13 g/g). CO2 adsorption capacities of activated carbon fibre-phenolic resin composites have not been significantly improved considering that Burchell and Judkins [10] reported a similar value of 2.8 mmol/g. However, this work confirms the superiority of activated carbon fibre-phenolic resin composites over a series of sorbents as shown in Table 1. When aminated mesoporous silica, anthracite activated carbon and granular coconut shell carbon reached a CO2 adsorption capacity well below 0.1 g/g sorbent under similar conditions, activated carbon fibre-phenolic resin composites have demonstrated a much higher value of 0.13 g/g adsorbent. Therefore, activated carbon fibre-phenolic resin composites have the potential for high efficiency CO2 capture. Micropore volume by sub-atmospheric CO2 isotherm at 0 °C has been identified as a potential design parameter for the development of activated carbon fibre-phenolic resin composites for CO2 capture at low relative pressures. The reason is that micropore volume obtained from sub-atmospheric CO2 isotherm has the ability to measure the volume of smaller micropores in the microporosity range due to its high kinetic energy at 0 °C and does not take into account of bigger micropores because of the low relative pressure. To improve CO2 adsorption capacities at low relative pressures of activated carbon fibre-phenolic resin composites, efforts have to be made to produce narrow microporosity with a large contribution of micropore volume coming from smaller micropores. Since micropores only develop from existing micropores, it is very important to identify suitable raw materials for the fabrication of activated carbon fibre-phenolic resin composites. Three types of carbon fibre were investigated and pitched based carbon fibres were picked over the other two types. More types of carbon fibre should also be examined to compare with pitched based carbon fibres. Additionally, the capability of activated carbon derived from powdered phenolic resin to adsorb CO2 has been confirmed apart from just binding carbon fibres. Powdered phenolic resin can also produce micropores after carbonisation and activation for the purpose of CO2 adsorption. Therefore, various types of phenolic resin should be investigated in or-
der to identify a type that provides the largest volume of micropores with the right size for CO2 capture at low relative pressures. It has to be borne in mind that lower mass ratio samples achieved higher adsorption capacities than those with higher mass ratios in the low burn-off range. The role of the activated carbon derived from powdered phenolic resin in the high burn-off range has to be investigated. It is also worthwhile to investigate mass ratios lower than 1:1 when powdered phenolic resin has a bigger proportion.
4.
Conclusions
Activated carbon fibre-phenolic resin composites fabricated in our lab do not have the problem of non-uniform activation. The uniformity of some samples tested obtained a standard deviation less than 5%. Higher activation temperature, longer activation time and lower mass ratio can help achieve higher activation degree with activation temperature the most important factor and mass ratio the least. As far as CO2 adsorption capacity is concerned, similar conclusions cannot be made because a linear relationship between burn-off and CO2 adsorption capacity does not exist. However, lower mass ratios are found beneficial for achieving higher CO2 adsorption capacities. This demonstrates that activated carbon derived from powdered phenolic resin also has the capability to capture CO2. The cross-over regime of CO2 adsorption on activated carbon fibre-phenolic resin composites has been confirmed. At lower relative pressures, lower burn-off samples capture larger amount of CO2 than those with higher activation degrees. This is due to a higher overlap in potential between the pore walls of smaller micropores in the low burn-off samples. The existence of the cross-over regime has a practical implication as it demonstrates that the development and design of activated carbon fibre-phenolic resin composites are depended on the relative pressure of CO2. Future work can be made to obtain CO2 adsorption isotherms of activated carbon fibrephenolic resin composites with various activation degrees in a relative pressure range of 0–1. This is to further understand the existence and identify the location of the cross-over regime for CO2 adsorption on activated carbon fibre-phenolic resin composites. Micropore volume by CO2 isotherm has been identified as a potential parameter for the design of activated carbon fibrephenolic resin composites. It measures the micropore volume contributed mostly from smaller micropores in the microporosity range, which are paramount for CO2 capture at low relative pressures. Due to the unique features of CO2 adsorption isotherm at 0 °C under sub-atmospheric pressure, micropore volume by CO2 contains the information of both micropore volume and micropore size. The optimal micropore size is found to decrease as the relative pressure goes down and a stronger overlap of even smaller micropores is desired. CO2 adsorption isotherm at 0 °C under sub-atmospheric pressure is believed to be more appropriate for the characterisation of activated carbon fibre-phenolic resin composites than N2 adsorption isotherm at 196 °C as far as low relative pressure adsorption is concerned.
CARBON
4 7 ( 20 0 9 ) 2 3 9 6–24 0 5
Acknowledgements The authors wish to acknowledge the support from the University of Queensland and CSIRO Energy Transformed, National Research Flagship. Special thanks are due to Dr. Ramesh Thiruvenkatachari and Mr. XinXiang Yu for their contributions to the setup of experimental equipment at the CSIRO QCAT Laboratories.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2009.04.029.
R E F E R E N C E S
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