Importance of micropore size distribution on adsorption at low adsorbate concentrations

Letters to the Editor / Carbon 41 (2003) CO1–846

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Importance of micropore size distribution on adsorption at low adsorbate concentrations ´ Antonio B. Fuertes Teresa A. Centeno*, Gregorio Marban, ´ ( C.S.I.C.), Apartado 73, 33080, Oviedo, Spain Instituto Nacional del Carbon Received 2 November 2002; accepted 5 December 2002 Keywords: A. Carbon fibers; B. Activation; D. Adsorption properties, Immersion enthalpy, Microporosity

Different authors [1,2] observed an unusual crossover in n-butane adsorption capacity of activated carbon fibers (ACFs) at low relative pressures. Foster et al. [1] concluded that above the crossover point (around 5000 ppm), the adsorption capacity was directly related to the specific surface area of the ACFs, whereas for lower n-butane concentrations this trend was reversed. More recently, Magnun et al. [2] reported that the crossover point shifted to lower concentrations with increasing boiling points of the adsorbates. The crossover phenomenon can be explained by the higher adsorption energies (up to three times) in the micropores than on the same nonporous surface, due to overlapping of the adsorption potentials of opposite walls in micropores [3]. At very low relative pressures only the finest micropores are involved in the adsorption process, whereas an increase in the relative pressure will lead to the filling of wider micropores. As a consequence, carbonaceous materials with relatively low equivalent BET specific areas, ascribed to uniform and narrow micropores, show a higher adsorption capacity at low relative pressures than samples with wider micropores. The existence of a crossover point has important implications for the practical use of ACFs. Thus, ACFs with moderate degrees of activation around 20% could be more appropriate for adsorbing at very low adsorbate concentrations (i.e. ,100 ppm), than ACFs with higher activation degrees. In previous studies [1,2], the crossover phenomenon was described mainly in qualitative terms and there was a lack of knowledge of the microporous structure of the adsorbents used. The main purpose of this work is to analyse the correlation between the crossover phenomenon and the micropore size of the ACFs. Five activated Nomex–carbon fiber monoliths (ACFMs) with burn-offs (b.o.) between 6 and 40 wt.% were prepared by gasification with steam (25 vol.% in N 2 at 700 8C) [4]. In order to attain uniformity in the micropore sizes, the gasification rates, expressed as the weight loss fraction per time unit, were below 2 wt.% h 21 . The carbons prepared

*Corresponding author. Tel.: 134-985-119-090; fax: 134985-297-662. E-mail address: [email protected] (T.A. Centeno).

in the present study will be referred herein as ACFM followed by the activation degree. For a comparative purpose, a commercial granular activated carbon (Norit, RB-3) specially designed for removal of low concentrations of contaminants was included in this study. Adsorption of n-butane (concentrations from 100 to 10 6 ppm) was carried out at 30 8C in a gravimetric system (CI Electronics). The adsorption isotherms were analyzed by applying the DR equation [5], which yielded the micropore volume (Wo ) and the characteristic energy (Eo ). The average micropore width, Lo , was calculated by the equation Lo (nm)510.8 / [Eo (kJ / mol)211.4]. The microporous surface area was deduced from Smi (m 2 / g 21 )5 2000 Wo (cm 3 / g) /Lo (nm) [5]. BET-specific surface areas were determined by N 2 physisorption at 2196 8C in a volumetric equipment (ASAP 2010). The micropore size distribution of the adsorbents was characterised by measurements of the enthalpy of immersion [5] at 25 8C, using molecular probes of different critical sizes: benzene (C 6 H 6 , 0.41 nm), cyclohexane (C 6 H 8 , 0.54 nm), carbon tetrachloride (CCl 4 , 0.63 nm), 1,5,9-cyclododecatriene (CDDT, 0.76 nm), tetraisopropyl-o-titanate (TIPOT, 1.05 nm), tetrabutyl-o-titanate (TBOT, 1.3 nm) and tri-2,4xylylphosphate (TXP, 1.5 nm). The characteristics of porous structure and enthalpies of immersion are reported in Table 1. The variation of the structural characteristics of the carbons with the degree of burn-off is summarized in Table 1. It is interesting to note that ACFMs fit the general pattern of Lo versus Wo described recently by Stoeckli et al. [6]. Fig. 1 shows that the uptake of n-butane directly depends on the pore volume of the ACFMs for a butane concentration as high as 10 6 ppm. Thus, the amount adsorbed increases with burn-off and the highest adsorption capacity is achieved by the sample activated to 40% (ACFM-40) which presents 0.46 cm 3 g 21 of pore volume accessible to n-butane and an equivalent BET-specific surface area of 1306 m 2 g 21 . This dependence of adsorption capacity on the pore volume of the adsorbent is confirmed by the results obtained for sample RB-3. This material shows a much higher adsorption capacity (4.65 mmol g 21 ) than that observed for ACFM-30 (3.81 mmol

0008-6223 / 02 / $ – see front matter  2002 Elsevier Science Ltd. All rights reserved. doi:10.1016 / S0008-6223(02)00430-X

Letters to the Editor / Carbon 41 (2003) CO1–846

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Table 1 Characteristics of porous structure and enthalpies of immersion into different liquids Sample

ACFM-6 ACFM-12 ACFM-21 ACFM-30 ACFM-40 RB-3 (pelletised a b

N 2 (2196 8C)

2 Di H (J g 21 )

n-Butane (30 8C) a

b

SBET (m 2 g 21 )

W0 (cm 3 g 21 )

Eo Lo Smi (kJ mol 21 ) (nm) (m 2 g 21 )

Sm (m 2 g 21 )

C6H6 C6H8 CCl 4 (0.41 nm) (0.54 nm) (0.63 nm)

CDDT TIPOT TBOT TXP (0.76 nm) (1.05 nm) (1.3 nm) (1.5 nm)

–

0.21 0.26 0.32 0.38 0.46 0.47

33.7 32.3 26.8 25.4 23.0 20.4

591 704 901 1070 1294 1323

114.0 120.0 134.6 140.5 – 124.2

– 3.5 10.2 18.0 – 72.2

810 973 1222 1306 1220 AC)

0.48 875 0.52 1000 0.70 914 0.77 987 0.93 980 1.2 783

8.2 53.2 98.1 107.2 – 83.5

12.6 44.1 100.6 125.6 – 93.8

– – – – – 100.3

– – – – – 17.6

– – – 16.1 – –

From the equation Smi (m 2 / g) 5 2000 Wo (cm 3 / g) /L (nm) (Ref. [5]). Monolayer equivalent of Wo .

g 21 ), although both have BET-surface areas around 1220 m 2 g 21 . However, as shown in Table 1, RB-3 has a larger pore volume accessible to n-butane than the latter. At sufficiently low n-butane concentrations, in the range 100–1500 ppm, this tendency is reversed and a maximum in butane adsorption is observed for the material activated to 21% b.o. with a pore volume of 0.32 cm 3 g 21 . This maximum is significantly less pronounced as the n-butane concentration in the stream increases. In the case of a concentration of 100 ppm, the carbon activated to 40% b.o.

presents an adsorption capacity even lower than for the carbon only activated to a weight loss of 6%. A similar pattern is observed for sample RB-3. The n-butane adsorption capacity of RB3 is only 0.79 mmol g 21 , in spite of its micropore volume is twice that of ACFM-6. At this stage, it seems obvious that, at low adsorbate concentrations, the adsorption capacity is not only determined by the pore volume but could depend also on the micropore size distribution of the adsorbent, which plays a significant role in the adsorption process. Immersion calorimetry provides a means to assess rapidly the micropore size distribution of carbon material or, at least its accessibility if ‘‘gate effects’’ are present. The micropore volume Wo (Lc ) filled by a liquid of critical molecular dimensions, Lc , can be calculated by using the expression [5] Wo (Lc ) 5 (D i H(Lc ) 2 h i Se )2Vm /b Eo (1 1 a T )p 1 / 2

Fig. 1. Modification of n-butane adsorption uptake with activation degree at different concentrations. (adsorption temperature: 30 8C).

where D i H(Lc ) is the experimental enthalpy of immersion of the carbon into the liquid with a dimension Lc , a and Vm are the thermal expansion coefficient and the molar volume of the liquid and h i is the specific enthalpy of wetting of the external (non-microporous) surface Se of the carbon. Stoeckli et al. [7] have shown that, for carbons with small external surface areas, the ratio between the limiting volumes filled by liquids of variable molecular dimensions and the micropore volume accessible to a small molecule used as reference can be estimated with a good approximation from the enthalpies of immersion alone. This approach provides a picture of the micropore size distribution and its evolution as the activation progresses. Fig. 2 shows the profiles of the relative micropore volumes Wo (Lc ) /Wo (benzene) obtained for the ACFMs and RB-3. In this work, benzene (0.41 nm) was taken as reference since n-butane (0.43 nm) probes the same micropore volume. It is seen that the smaller microporosity tends to disappear and the average micropore width of the ACFMs becomes wider as the activation degree increases. However, the ACFMs are essentially microporous and the

Letters to the Editor / Carbon 41 (2003) CO1–846

Fig. 2. Ratio between the micropore volume Wo (Lc ) accessible to liquids with different critical dimensions and the total micropore volume Wo (C 6 H 6 ) for ACFMs activated to different burn-offs and commercial activated carbon RB-3.

majority of the pores of these materials are below 0.8 nm, even for an activation degree of 30%. Thus, the pore system of samples ACFM-12, ACFM-21 and ACFM-30 accommodate only 4, 9 and 15% of liquid CDDT. Fig. 2 also illustrates that carbon RB-3 presents a much wider micropore size distribution than ACFMs. Furthermore, the agreement of the average micropore size Lo derived from enthalpies of immersion with that resulting from the characteristic energies Eo of the n-butane adsorption isotherm (Table 1), indicates that the profiles reflect the real microporosity of ACFMs and RB-3 and no gate effects are present in the micropores system of these carbons. The micropore size distribution [5] of the carbons has

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been estimated from the percentage of total pore volume filled by the different probes and the micropore volume accessible to n-butane determined from the isotherm (Fig. 3). Taking into account the microporosity characteristics of the ACFMs of the present work, the adsorption of nbutane, at low n-butane concentrations, takes place in pores below 1 nm, where the equivalent of 1–3 layers of adsorbate can be accommodated. In the case of carbons ACFM-6 and ACFM-12, where the average micropore width is around 0.5 nm, the micropore surface area is larger than the monolayer equivalent of Wo (see Table 1) due to the presence of a single layer facing the two walls of the micropore. Owing to the proximity of the walls, the adsorption energy is higher than in larger pores [8]. The relatively high n-butane adsorption capacity of ACFM-6, at low butane concentrations, appears related to the existence of a large volume of narrow pores between 0.4 and 0.5 nm where a primary filling occurs. For an activation degree of 12%, the micropore volume increases, whereas the widening of the micropores takes place very gently (Fig. 3). As a result, the n-butane adsorption capacity of the adsorbent is increased. For sample ACFM-21, Lo 5 0.7 nm and the value of Smi is 914 m 2 / g and the monolayer equivalent of W0 corresponds to 901 m 2 / g. These data confirm that there are approximately two layers of n-butane in the micropores of these materials. The maximum in adsorption capacity observed for ACFM-21, at low n-butane concentrations, is explained by its larger volume of pores with adequate diameter to enhance the interactions between the butane molecules and the pore walls. As activation increases to 30% b.o., the development of the pores with a size between 0.63 and 0.74 nm takes place but the volume

Fig. 3. Micropore size distribution in carbons ACFMs and RB-3.

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Letters to the Editor / Carbon 41 (2003) CO1–846

associated with pores between 0.41 and 0.63 nm is notably reduced. As a result, although the total pore volume increases, a substantial fraction of the adsorption capacity is lost. Finally, in the case of sample RB-3, the average micropore width is 1.2 nm and Smi 5 783 m 2 / g instead of 1323 m 2 / g obtained from the equivalent monolayer, which indicates the presence of approximately 3–4 layers in the micropores of this carbon. The low adsorption capacity shown by RB-3, at low n-butane concentrations, is related to the limited number of pores involved in primary filling process existence (smaller pores) and a greater number of larger pores than those displayed by ACFMs. In summary, at low adsorbate concentrations a maximum in adsorption capacity results from the balance between the adsorption potential and the micropore volume of the adsorbent. That means that the factor which determines the adsorption capacity at low concentrations is its micropore size distribution and, more specifically, the ratio between the diameter of the molecule to be retained and the micropores size of the adsorbent. This is particularly important for the use of carbons in the adsorption of contaminants at low concentrations since the design of

microporous structure of the adsorbent is required to suit the size of the molecules to be removed. On the other hand, at high adsorbate concentrations, the entire micropore system is involved in the adsorption process and the adsorption capacity is directly related to the total pore volume Wo .

References [1] Foster KL, Fuerman RG, Economy J, Larson SM, Rood MJ. Chem Mater 1992;4:1068–73. [2] Magnun CL, Daley MA, Braatz RD. Economy J Carbon 1998;36:123–31. [3] Dubinin MM. Carbon 1989;27:457–67. ´ [4] Marban G, Fuertes AB, Nevskaia DM. Carbon 2000;38:2167–70. [5] Stoeckli F. In: Patrick J, editor, Porosity in carbons, London: Edward Arnold, 1995, Chapter 3. [6] Stoeckli F, Daguerre E, Guillot A. Carbon 1999;37:2075–7. [7] Stoeckli F, Centeno TA. Carbon 1997;35:1097–100. [8] Stoeckli F, Huguenin D, Greppi AJ. Chem Soc Faraday Trans 1993;89:2055–8.

Carbon nanotubes containing iron and molybdenum particles as a catalyst for methane decomposition Weizhong Qian*, Tang Liu, Fei Wei, Zhanwen Wang, Hao Yu Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Received 18 November 2002; accepted 6 December 2002 Keywords: A. Carbon nanotubes; B. Catalyst; Chemical vapor deposition; C. Transmission electron microscopy

Carbon nanotubes (CNTs) have been intensively studied since their discovery in 1991 [1]. So far, they show potential application as a field emission material [2], quantum wires [3], etc. Also, due to their nanometer hollow shell, they can be used as a reactor for some reactions [4], which is not easily performed under normal operation conditions. The key to this lies in the confinement of the hollow shell, which gives rise to the nanometer effect [4]. Here we show another interesting confinement effect of the carbon layer on the metal particles at the tip of CNTs, which produces high activity and high thermal stability for methane decomposition. This provides new hope for the relatively difficult methane decomposition system requiring high thermal stability of the metal supported catalyst. *Corresponding author. Fax: 186-10-6277-2051. E-mail address: [email protected] (W. Qian).

CNTs are prepared by the methods described in Ref. [5]. Generally, the outer diameter of CNTs is 3–25 nm, the inner diameter 2–5 nm (Fig. 1) and the length up to several tens of micrometers. The specific surface area is 300 m 2 / g, and the average pore diameter is 7.5 nm. The metal particles are observed at the tips of the CNTs, and the atomic ratio of iron to molybdenum is about 20–30 as determined by EDS. From TGA analysis, the content of metal particles in the CNTs is about 10%. To test the activity of such CNTs containing metal particles for methane decomposition, the CNT samples (25 g) are packed into a reactor, reduced by hydrogen (823 K, 2 h) and used to decompose methane (nitrogen, 1000 ml / min; methane, 400 ml / min) in a fluidized bed reactor [6]. Temperature is increased from 823 to 1123 K at a rate of 100 K / h. Online gas chromatography is used to detect the gas products. The weight gain of the CNT products is about 20 g. Meanwhile, a blank experiment was also done

0008-6223 / 02 / $ – see front matter  2002 Elsevier Science Ltd. All rights reserved. doi:10.1016 / S0008-6223(02)00422-0