Characterization of Activated Carbon Fibers by CO2 Adsorption

The adsorption of CO2 up to pressures of 4 MPa has been studied using two series of activated carbon fibers (ACFs) covering a wide range of burn-off...
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Characterization of Activated Carbon Fibers by CO2 Adsorption D. Cazorla-Amoro´s, J. Alcan˜iz-Monge, and A. Linares-Solano* Departamento de Quı´mica Inorga´ nica, Universidad de Alicante, Alicante, Spain Received January 4, 1996. In Final Form: March 11, 1996X The adsorption of CO2 up to pressures of 4 MPa has been studied using two series of activated carbon fibers (ACFs) covering a wide range of burn-off. The relative fugacities covered in these experiments range from 3 × 10-4 to 0.76. Additionally, N2 adsorption at 77 K and CO2 adsorption at 273 and 298 K at subatmospheric pressures have been carried out. The experiments performed at high pressures allow us to compare both adsorptives at similar ranges of adsorption potential. The results obtained led to the following conclusions: (i) CO2 adsorption at 273 K at subatmospheric pressures is a suitable technique to characterize the narrow microporosity of the ACF. (ii) The use of N2 to characterize the narrow microporosity is not appropriate because its adsorption is limited by the existence of diffusional restrictions in this type of porosity. (iii) CO2 at 273 K (or 298 K) is an adsorptive that behaves quite similarly to N2 at 77 K at comparable relative pressure ranges; thus, CO2 adsorbs in the super-microporosity range (pore size: 0.7-2 nm) at 298 K if pressures of about 4 MPa are used.

1. Introduction Physical adsorption of gases is the most employed technique for the characterization of porous solids.1-3 Different adsorptives, like N2, CO2, Ar, He, CH4, benzene, nonane, etc., can be used for this purpose.1-7 Among the gases, N2 at 77 K is the more used and, usually, has a special status of recommended adsorptive.8 The advantage of N2 adsorption is that it covers relative pressures from 10-8 to 1, which results in adsorption in the whole range of porosity. The main disadvantage of N2 adsorption at 77 K is that when it is used for the characterization of microporous solids, diffusional problems of the molecules inside the narrow porosity range (size < 0.7 nm) occur.4 To overcome this problem, the use of other adsorptives has been proposed. CO2 adsorption, either at 273 or 298 K,4,9 and He adsorption at 4.2 K5,6 are two alternatives to N2 adsorption for the assessment of the narrow microporosity. He adsorption at 4.2 K has been proposed5,6 as a promising method for the accurate determination of microporosity. In this way, important differences between He and N2 adsorptions have been observed in activated carbon fibers containing an important contribution of narrow and necked porosity. He adsorption at 4.2 K requires lower equilibrium times, and the amount adsorbed is higher than in the case of N2 at 77 K. However, in spite of the interesting results obtained, the experimental conditions used (adsorption at 4.2 K) X

Abstract published in Advance ACS Abstracts, May 1, 1996.

(1) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Science and Porosity; Academic Press: New York, 1982. (2) Lowell, S.; Shields, J. E. Powder, Surface Area and Porosity, 3rd ed.; Chapman and Hall: New York, 1991. (3) Characterization of Porous Solids III; Rouquerol, J., et al., Eds.; Elsevier Science Publishers B. V.: Amsterdam, 1994. (4) Rodriguez-Reinoso, F.; Linares-Solano, A. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1988; Vol. 21, p 1. (5) Kaneko, K.; Setoyama, N.; Suzuki, T. In Characterization of Porous Solids III; Rouquerol, J., et al., Eds.; Elsevier Science Publishers B.V.: Amsterdam, 1994; p 593. (6) Setoyama, N.; Ruike, M.; Kasu, T.; Suzuki, T.; Kaneko, K. Langmuir 1993, 9, 2612. (7) Sosin, K. A.; Quinn, D. F. J. Porous Mater. 1995, 1, 111. (8) Rouquerol, J.; et al. In Characterization of Porous Solids III; Rouquerol, J., et al., Eds.; Elsevier Science Publishers B.V.: Amsterdam, 1994; p 1. (9) Garrido, J.; Linares-Solano, A.; Martı´n-Martı´nez, J. M.; MolinaSabio, M.; Rodrı´guez-Reinoso, F.; Torregrosa, R. Langmuir 1987, 3, 76.

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make this technique not so available as CO2, adsorption. In the case of CO2 adsorption, though the critical dimension of the CO2 molecule is similar to that of N2, the higher temperature of adsorption used for CO2 results in a larger kinetic energy of the molecules able to enter into the narrow porosity. In this way, CO2 adsorption has been proposed by us as a good complementary technique for the analysis of the porous texture, as it could be used to assess the narrow microporosity (size < 0.7 nm), where N2 adsorption can be kinetically retricted.4,9,10 A confirmation of the reliability of the method for microporous materials requires the comparison of both N2 and CO2 adsorptions at comparable relative pressures. To achieve this comparison, there are two possibilities: the use of high pressures for CO2 or low pressures for N2. The analysis of microporous materials requires performing CO2 adsorption at high pressures because N2 adsorption at 77 K has diffusional limitations. This type of comparison of both adsorptives has not been reported in the literature, mainly due to the important lack of adsorption data at pressures higher than 0.1 MPa. One exception to the studies of adsorption at high pressures is that of Agarwal and Schwarz.11 The authors analyzed highpressure adsorption of different gases (methane, ethane, propane, carbon dioxide, and nitrogen) at different temperatures on one activated carbon sample to obtain temperature independent characteristic curves for each adsorptive. However, they do not compare the results of the different adsorptives. After this work, Koresh et al.12 studied the high-pressure adsorption of N2, CO2, O2, and He at 308 K on ultra-microporous carbon and compared the results with water adsorption at room temperature. These authors discuss the different accessibility of the adsorptives used without further analysis of the isotherms obtained. Recently, DeGance et al.13,14 analyzed multicomponent high-pressure adsorption and focused on the interpretation of adsorption data by the use of equations of state (eos). (10) Salinas-Martı´nez de Lecea, C.; Linares-Solano, A.; Rodrı´guezReinoso, F.; Sepu´lveda-Escribano, A. In Characterization of Porous Solids I; Unger, K. K., et al., Eds.; Elsevier Science Publishers B.V.: Amsterdam, 1988; p 173. (11) Agarwal, R. K.; Schwarz, J. A. Carbon 1988, 26, 873. (12) Koresh, J. E.; Kim, T. H.; Koros, W. J. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1537. (13) DeGance, A. E. Fluid Phase Equilib. 1992, 78, 99. (14) DeGance, A. E.; Morgan, W. D.; Yee, D. Fluid Phase Equilib. 1993, 82, 215.

© 1996 American Chemical Society

Characterization of Carbon Fibers by CO2 Adsorption

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Table 1. DR Micropore Volumes of the ACF (cm3/g)c sample

V(N2) (77 K)

V(CO2) (273 K)

V(CO2)1pa (298 K)

V(CO2)hpb (298 K)

CFC14 CFC30 CFC40 CFC47 CFC54

0.23 0.41 0.61 0.79 0.85

0.33 0.65 0.65 0.77 0.75

0.33 0.55 0.60 0.77 0.74

0.32 0.48 0.62 0.73 0.86

CFS15 CFS25 CFS50 CFS60

0.35 0.46 0.65 0.69

0.50 0.48 0.48 0.51

0.46 0.48 0.48 0.48

0.43 0.48 0.66 0.69

a lp: CO adsorption at subatmospheric pressures at 298 K. b hp: 2 CO2 adsorption at high pressures at 298 K. c CO2 density at 273 K ) 1.023 g/cm3 (ref 4). CO2 density at 298 K ) 0.85 g/cm3.

According to all this, the main objective of this work is to strengthen the validity of CO2 adsorption for characterizing the narrow microporosity of carbonaceous materials. To reach this aim, the following aspects have been analyzed: (i) CO2 adsorption at high and subatmospheric pressures (This covers the scarce data available on CO2 adsorption at high pressures and allows us to compare the adsorption of this gas with that of N2 at 77 K in a comparable range of relative pressures.) and (ii) N2 adsorption at 77 K from low relative pressures (10-7) to show the problems of the use of N2 adsorption at 77 K. This study has been done with activated carbon fibers because they are essentially microporous materials,6,15 with slit-shaped pores and a quite uniform pore size distribution.16,17 Thus, these materials have simpler structures than ordinary granulated activated carbon.6

Figure 1. High-pressure CO2 adsorption isotherms at 298 K plotted versus pressure and relative fugacity for series CFC (a) and series CFS (b).

2. Experimental Section

3. Results and Discussion

Two series of activated carbon fibers (ACFs) obtained from CO2 (series CFC) and steam (series CFS) activations have been used in this study. The mechanical properties and porosity of these materials have already been analyzed.15 The nomenclature of each sample includes the burn-off degree. Table 1 includes the porous texture of the ACF studied. It contains the volume of micropores calculated from the application of the DubininRadushkevich (DR) equation to the N2 adsorption at 77 K and CO2 adsorption at 273 K. The isotherms were measured in an Autosorb-6 apparatus. CO2 adsorption isotherms at 298 K and at high pressures have been measured in a DMT high-pressure microbalance (Sartorius 4406) connected to a computer for data acquisition. The balance is equipped with a pressure indicator and a thermocouple mounted in the sample housing as well as with a rotary pump. The maximum pressure reached is 4 MPa. The experimental results have been corrected for buoyancy effects11 related to the displacement of gas by the sample, sample holder, adsorbed phase, and pan. The corrections due to the sample holder and pan were obtained with a blank experiment carried out with the sample holder empty. The buoyancy due to the sample, which results in an apparent loss of weight, was estimated as the product of the skeletal volume of the sample and gas density. The buoyancy effect related to the adsorbed phase was corrected to give the absolute adsorption isotherms.13 The high-pressure CO2 adsorption isotherms have been measured at least twice, obtaining a high reproducibility (of about 1%) within the error of the balance. Additionally, CO2 adsorption at 298 K and N2 adsorption at 77 K up to 0.1 MPa have also been performed with Autosorb-6 and Omnisorp equipment, respectively, to cover lower relative pressures.

3.1. High-Pressure CO2 Adsorption Isotherms at 298 K. Figure 1 presents the high-pressure CO2 adsorption isotherms obtained for the samples studied. The isotherms are plotted versus relative fugacity (see lower abscissa axis) and total pressure (see upper abscissa axis). The relative fugacity used in the isotherms is the fugacity divided by the saturation fugacity for CO2 at 298 K (the saturation fugacity, estimated from data compiled in ref 18, is 4.2 MPa). Each isotherm contains the experiment obtained at subatmospheric pressures, performed in the Autosorb-6 apparatus, that covers relative fugacities from 3 × 10-4 to 0.024 and the isotherm done up to 4 MPa in the gravimetric system (f/fs from 0.024 to 0.76). Thus, the range of relative fugacities (f/fs) covered in these experiments varies from 3 × 10-4 to 0.76. It is important to note, by its relevance in the content of the paper, that there is good continuation in measurements performed at both subatmospheric and high pressures, respectively, in spite of the different experimental systems used (volumetric for the low-pressure adsorption and gravimetric for the high-pressure adsorption). CO2 adsorption isotherms plotted as shown in Figure 1 can be compared with those obtained from N2 adsorption at 77 K previously described,15 because the range of relative fugacities covered is similar. As an example, N2 adsorption isotherms of the CFC series are presented in Figure 2 for a better comparison. Both CO2 and N2 adsorption isotherms obtained for the different samples are of type I according to the IUPAC classification.19 The evolution of the isotherms with burn-off is similar for both adsor-

(15) Alcan˜iz-Monge, J.; Cazorla-Amoro´s, D.; Linares-Solano, A.; Yoshida, S.; Oya, A. Carbon 1994, 32, 1277. (16) Kaneko, K.; Shimizu, K.; Suzuki, T. J. Chem. Phys. 1992, 97, 8705. (17) Matsumoto, A.; Kaneko, K.; Ramsey, J. D. F. In Fundamentals of Adsorption IV; Suzuki, M., Ed.; Kodansha: Tokyo, 1993.

(18) Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. In The Properties of gases and liquids, 3rd ed.; Carberry, J. J., et al., Eds.; McGraw-Hill: New York, 1977. (19) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603.

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Figure 2. N2 adsorption isotherms at 77 K for series CFC.

bates. In fact, several common features can be noted from these experiments: (i) The adsorption capacity increases with burn-off, and (ii) as burn-off increases, the knee of the isotherm widens, showing an increase in microporosity distribution. These results indicate that, due to the range of relative fugacities covered in the high-pressure CO2 adsorption isotherms, this molecule is also adsorbed in the super-microporosity range (pore size 0.7 mm < φ < 2 nm). 3.2. Micropore Volumes from Low- and HighPressure Adsorptions. The porous texture of these activated carbon fibers was previously characterized by N2 and CO2 adsorptions at 77 and 273 K, respectively, up to 0.1 MPA.15 From these experiments and using the DR equation, the volumes of narrow microporosity (CO2 volume) and total microporosity (N2 volume) were calculated (Table 1, columns 2 and 3). The densities used for liquid N2 at 77 K and adsorbed CO2 at 273 K were, respectively, 0.808 and 1.023 g/mL.4 In summary, the samples are essentially microporous, with a negligible volume of mesopores (mesoporosity of size larger than 7.5 nm is only observed in samples with high burn-off). Samples with burn-off lower than about 40% have a DR N2 volume lower than the DR CO2 one, indicating the existence of narrow microporosity where N2 adsorption has diffusional limitations. The ACFs with higher burnoff have some amount of super-microporosity, as reflected by the larger value of the DR N2 volume compared to the DR CO2 volume. To determine micropore volumes from high-pressure CO2 adsorption measured at 298 K, it is necessary to know the density of adsorbed CO2 in the micropores at this temperature. There is a considerable uncertainty in the value of this density, especially at 298 K. The values of density that can be used at this temperature fall within the range of 0.7-1.03 g/mL. The first value is the density of liquid CO2, and the second is the density of adsorbed CO2 according to Dubinin’s approach.20 It must be noted that the corresponding range of densities for CO2 adsorption at 273 K is narrower (0.924-1.07 g/mL) and, hence, the error in the calculation of micropore volume will be lower at this temperature. In the study of Garrido et al.,9 the use of the density of liquid CO2 at 298 K gave micropore volumes higher than those obtained from CO2 adsorption at 273 K and, moreover, the characteristic curve obtained at 298 K was parallel and above that at 273 K. The density of CO2 at 298 K was calculated to have a unique characteristic curve and similar micropore volumes to those obtained from CO2 adsorption at 273 K. From these results, the calculated density was 0.97 g/mL. Following a similar approach, the density of CO2 at 298 K has been calculated from the comparison of CO2 (20) Dubinin, M. M. Chem. Rev. 1960, 60, 235.

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adsorption at both 273 and 298 K up to subatmospheric pressures. The density that gives, for the samples of this study, a better fit of the characteristic curves at both temperatures and similar micropore volumes is 0.85 g/mL. This value is between the density of liquid CO2 at 298 K (0.71 g/mL) and that for the gas adsorbed at the same temperature (1.03 g/mL). Table 1 includes the micropore volumes calculated with this density. The different value of density obtained shows the difficulty in determining micropore volumes from CO2 adsorption at 298 K, as the density seems to be highly dependent on the nature of the activated carbon. That also confirms the advantage of using 273 K instead of 298 K, as proposed elsewhere.9 Table 1 also contains the micropore volumes calculated through the application of the DR equation to the highpressure CO2 adsorption experiments. Comparing these results with those obtained from N2 and CO2 adsorptions up to subatmospheric pressures at 77 and 273 K, respectively, we can observe two interesting aspects: (i) For those samples in which nitrogen volume is lower than the CO2 one (which indicates that N2 adsorption has diffusional problems), the micropore volume obtained from high-pressure CO2 adsorption agrees with the CO2 one. (ii) For those samples in which the nitrogen volume adsorbed is higher than the CO2 one (which indicates that the sample has some super-microporosity), the micropore volume obtained from high-pressure CO2 adsorption agrees with the N2 one. These observations suggest that, due to the range of relative pressures used by highpressure CO2 adsorption, this technique is sensitive to the narrow microporosity (where the access of CO2 is not kinetically restricted) and to the super-microporosity (i.e., CO2 also fills the super-microporosity if high pressures are used). These results are highly interesting from a point of view of the fundamentals of CO2 adsorption. In fact, it can be concluded that CO2 behaves as N2 if a similar range of relative fugacities is compared for both adsorptives. 3.3. Characteristic Curves for N2 and CO2 Adsorptions. The characteristic curves presented in the following discussion have been obtained by applying the DR equation20 (eq 1) to the different adsorption measurements performed:

( (

))

p0 V K ) exp - 2 RT ln V0 p β

2

(1)

where V is the volume adsorbed at a pressure p, V0 is the micropore volume, K is a constant dependent on the pore structure, and β is the affinity coefficient that is characteristic of the adsorptive. The term (RT ln(p0/p))2 is usually named A2. The characteristic curves obtained for N2 correspond to the experiments performed with an Omnisorp apparatus that cover relative pressures from 10-7 to 1. The affinity coefficient used in this case is 0.33.21 The characteristic curves for CO2 adsorption contain the isotherms obtained up to subatmospheric pressures (with an Autosorb apparatus) and up to high pressures (with a high-pressure balance). The affinity coefficient for CO2 has been calculated to have coincident characteristic curves for CO2 and N2 adsorptions in those samples where the adsorption of N2 is not kinetically restricted (samples CFC40, CFC47, CFC54, CFS50, and CFS60). From this approach, the affinity coefficient calculated for CO2 is 0.35, a value similar to that proposed by Dubinin.21 (21) Dubinin, M. M. In Chemistry and Physics of Carbon; Walker, P. L., Ed.; Marcel Dekker: New York, 1966; Vol. 2, p 1.

Characterization of Carbon Fibers by CO2 Adsorption

Figure 3. Characteristic curves for sample CFC14: (9) CO2 at 298 K, subatmospheric pressures; (2) CO2 at 298 K, high pressures; (4) N2 at 77 K.

Figure 4. Characteristic curves for sample CFC40: (9) CO2 at 298 K, subatmospheric pressures: (2) CO2 at 298 K, high pressures; (4) N2 at 77 K.

Figure 5. Characteristic curves for sample CFS50: (9) CO2 at 298 K, subatmospheric pressures; (2) CO2 at 298 K, high pressures; (4) N2 at 77 K.

Figures 3-5 include three examples of characteristic curves obtained for the samples CFC14, CFC40, and CFS50 (plots of ln V versus (A/β)2). These samples cover the different types of porosity found for the ACFs studied. Sample CFC14 has a quite narrow porosity, and N2 adsorption has important diffusional problems (see Table 1). Sample CFC40 has similar N2 and CO2 micropore volumes (i.e., the porosity has widened considerably compared to that of sample CFC14). Finally, the porosity of sample CFS50 is well developed and it contains some amount of super-microporosity. There are several relevant points that must be emphasized from Figures 3-5. In all the cases, the overlap-

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Figure 6. Characteristic curves for samples CFC14, CFC40, and CFS50 in the (A/β)2 range of 200-600 (kJ/mol)2.

ping and continuation of the CO2 characteristic curves obtained at low and high pressures are very good. Sample CFC14, which contains a narrow porosity, gives a CO2 characteristic curve with the same slope at high and low values of (A/β)2, which shows that the porosity of this sample is very homogeneous. Moreover, the characteristic curve for N2 adsorption remains always below that for CO2, in agreement with the kinetically restricted adsorption for N2 in this sample. With increasing burn-off, the characteristic curve does not have a unique slope and deviates upward. This reflects the development of porosity and the widening of the pore size distribution. This is clearly observed in Figure 5, which corresponds to sample CFS50. In this case (see the zone between 0 and 300 (kJ/mol)2 in Figure 5), the characteristic curve for N2 adsorption is very similar to the one for CO2 obtained at high pressures, indicating that CO2 also fills the supermicroporosity that exists in this sample (see Table 1). Finally, the characteristic curves for N2 adsorption show in all the samples a large deviation with respect to the one for CO2 for values of (A/β)2 higher than about 300 (kJ/mol)2. In this zone, the volume of N2 adsorbed by the sample is lower than the volume of CO2 and decreases with increasing (A/β)2. The adsorption potential, (A/β)2, at which this deviation finishes depends on the burn-off of the sample, as is clearly shown in Figure 6. So, with increasing burn-off, the recovery of the curve occurs at higher (A/β)2. This deviation, which happens at low relative pressures of N2 (lower than 10-5 for sample CFS50 and lower than 10-4 for sample CFC14), shows that N2 adsorption in the narrow microporosity range is influenced by diffusional limitations. With increasing burn-off, the porosity widens and the relative pressure at which N2 adsorption can enter into the porosity decreases. These experimental results are important for their relevance in the use of N2 adsorption in the characterization of porosity. As a consequence of the diffusional limitations, N2 adsorption at 77 K cannot be used to determine the micropore volume at the narrowest porosity, which makes necessary the use of other adsorptives to analyze this range of porosity. This study shows that CO2 adsorption at 273 K at subatmospheric pressures is very convenient for this purpose. Furthermore, these results confirm previous results4 in which the use of N2 adsorption, complemented with CO2 adsorption, is proposed as an adequated procedure to determine the porosity of activated carbons. So, CO2 adsorption at 273 K can be used to measure the narrowest microporosity, and N2 at 77 K, to analyze the widest porosity.

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4. Conclusions The use of high-pressure CO2 adsorption confirms that CO2 adsorption at subatmospheric pressures can be used to calculate the volume of the narrow microporosity and that it is a convenient technique to complement the characterization of porosity through N2 adsorption. This avoids the use of N2 at very low pressures. It is more convenient to perform CO2 adsorption at 273 K than at 298 K due to the uncertainty of the density of adsorbed CO2 at this second temperature. CO2 adsorbs in the supermicroporosity range at 298 K when CO2 pressures of about

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4 MPa are used. The adsorption of N2 at 77 K is limited by diffusional problems occurring in the narrow pores. For this reason, N2 adsorption cannot be used to characterize this range of porosity, which can be estimated by CO2 adsorption. Acknowledgment. The authors thank the DGICYT (Project PB93-0945) and OCICARBON (Project C-23-353) for financial support and IBERDROLA for the Thesis grant of J.A.M. LA960022S