Benzene adsorption on microporous activated carbons

Benzene adsorption on microporous activated carbons

LETTERS TO THE EDITOR Benzene adsorption on microporous activated carbons (Received 17 October 198%; accepted in revisedform 6 December 1988) Ke...

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LETTERS TO THE EDITOR

Benzene

adsorption

on microporous

activated

carbons

(Received 17 October 198%; accepted in revisedform 6 December 1988) Key Words - Adsorption,

activated carbons, benzene, microporous

Benzene is used frequently as the reference adsorbate to characterize the microporous structure of activated carbons [l]. In this paper, we report gravimetric measurements of benzene adsorption isotherms for two The commercial microporous activated carbons. Jaroniec-Choma (JC) isotherm equation [2] was used to describe these experimental isotherms and to evaluate the parameters that characterize the structural and energetic heterogeneities of the activated carbons studied. Vapor benzene adsorption isotherms were measured on BPL and PA activated carbons at 293 K by a gravimetric method. The BPL carbon was obtained from Calgon Carbon Corporation in Pittsburgh, Penna.; whereas the PA activated carbon was provided by the Barnebey-Cheney Company in Columbus, Ohio. Both activated carbons were designed for use in vapor phase applications. Presented in Fig. 1 are the benzene

Relative 0.2

Pressure, 0.4

0.6

adsorption isotherms on these activated carbons for the complete range of relative pressures. For low and moderate relative pressures, the adsorbed amount of

p/p, 0.8

structure

1.0

benzene is plotted against the natural logarithm of the relative pressure; however, for high relative pressures, this quantity is plotted against the relative pressure. The measured benzene isotherms intersect in the region of low relative pressures: below the intersection point, the isotherm curve for the PA carbon lies above the isotherm curve for the BPL carbon; however, above the intersection point, the amount of benzene adsorbed on the BPL carbon is greater than that for the PA carbon. It follows from Fig. 1 that the benzene isotherms for both activated carbons possess hysteresis loops, which begin at a relative pressure of about p/pa = 0.18. The loop for the PA activated carbon has an HCshape according to the new classification of hysteresis loops, which was recommended recently by the International Union of Pure and Applied Chemistry (IUPAC) [3,4]. The loop for the BPL activated carbon resembles the H3-type loop more than the HCtype loop. The shapes of hysteresis loops are associated with specific pore structures [3]. Types H3 and H4 were obtained with adsorbents possessing Z!

slit-shaped

pores [4].

107 1 8

t

-10

-8

Relative

-6

-4

Pressure,

-2

27:3-J

PA

l

BPL

0

ln(p/p,)

O__ 0.0

Fig. 1. Amount of benzene adsorbed (a3 for BPL and PA activated carbons at 293 K plotted as a function of the natural logarithm of the relative pressures p/p0 in the region of low and moderate pressures, and as a function of the.relative pressure p/p0 in the region of high pressures. CAR

0

485

-l

0.5

Standard

1.0

1.5

2.0

Adsorption,

2.5 as

Fig. 2. Amount of benzene adsorbed (at) for BPL and PA activated carbons at 293 K plotted as a function of the standard adsorption as.

486

Letters to the Editor

The benzene adsorption isotherms shown in Fig. 1 were compared with the standard benzene isotherm measured on a nonporous graphitized carbon black at 293 K [5]. The us-plots for both activated carbons are shown in Fig. 2; as denotes the standard adsorption, which is equal to the ratio of the amount adsorbed on a nonporous reference solid to the amount adsorbed on this same solid at the relative pressure p/p0 = 0.4. Theze as-plots permit evaluation of the maximum amount pi adsorbed in the micropores and the mesopore specl ic surface area Sme for both activated carbons; the values of pi and Sme are given in Table I.

Table 1 Parameters of the c+plots for benzene adsorbed onBPL and PA activated carbons $Fbe

Mi~ropo~ capacity hi (mmole/g)

Me~po~=esZ~~ me

area

Tabie 2 Parameters n and q of eqn. (1) for benzene adsorbed on BPL and PA activated carbons and derivative parameters* that characterise the microporous strclcture of these carbons BPL-carbon

Parameter*

PA-carbon

Micropore capacity pi (mmolelq)

4.22

3.22

Parameter q (kJ/molej2

852

962

Parameter n

1.62

1.27

xm-Value (nm)

0.60

0.51

Average value, 2 (nm)

0.63

0.55

Disperson, 0, (nm)

0.20

0.19

BPL

4.33

go

Am-value &J/mole)

11.7

13.2

PA

3.09

20

Average value, A (M/mole)

lg.8

22.2

Dispersion, cr.4

13.1

16.3

Smi (m2M

690

590

Total surface area St (m2/g)

770

610

The parameters that characterize the microporous structures of the BPL and PA activated carbons were calculated according to the JC isotherm equation [2]: n+l 4

ami = a”. ml r q+A2 1 Here ami is the A = RT ln(pdp) are parameters distribution [5]. total adsorbed equation:

(1)

amount adsorbed in the micropores, is the adsorption potential, and q and n of a gamma-type micropore-size The amount ami is extracted. from the amount at by using the following (21

where qs is the standard adsorbed amount expressed in moles per unit surface area, and Sme is the mesopore specific surface area of the adsorbent studied. In a previous paper, [5] we published equations that define the micropore-size distribution function J(x) (where x is the half-width of the slit-like micropores), the adsorption otential distribution function X(A), the average values 1 and %, and the dispersions (JA and 0,. Each of these qu~tities is associated with eqn. (1) and may be evaluated by means of the parameters n and q [5I. Table 2 contains the parameters n and q for the BPL and PA activated carbons. These parameters were used to calculate the micropore-size distribution functions J(x) plotted in Fig. 3, and the adsorption potential distribution X(A) plotted in Fig. 4. The micropore-size dis~bution functions for both activated carbons are similar; in Fig. 3, the J(x)-function for the PA carbon is slightly sharper with a peak at a slightly smaller micropore dimension than that for the BPL carbon. The average value X and the x,,,-vaIue that produces the maximum of the J(x)function for the PA carbon are smaIler than those for the BPL carbon; however, the values of the dispersion ox in Table 2 are essentially the same. Because the PA carbon contains smaller micropores than the BPL carbon, the maximum of the adsorption potential distribution function X(A) for the

Micropore surface area

PA carbon is shifted relative to that for the BPL carbon in the direction of higher values of the adsorption potential. Table 2 shows that the Am-value for the PA carbon is greater than that for the BPL carbon; the values of A for the PA and BPL carbons have this same relationship. Also, smaller micropores in the PA activated carbon are a source of a greater energetic heterogeneity of this carbon in comparison to that for the BPL carbon; therefore, the dispersion (TA for the PA carbon is greater than that for the BPL carbon, as listed in Table 2.

0.0

0.5

Micropore

1.0

Dimension,

1.5

x (nm)

Fig.3. Micropore-size distribution function J(x) for BPL and PA activated carbons.

Letters to the Editor

c

s

0.05

Acknowledgement - This work was supported in part by

the National Science Foundation. Department of Physics Kent State University Kent, OH 44242

M. JARONIEC’ R. MADEY x. LU J. CHOMA

0.02

Institute of Chemistry WAT, 00908 Warsaw POLAND

0.01

*Permanent address: Institute of Chemistry, MCS University, 20031Lublin. POLAND

‘;j

0.04

2 2

0.03

x $ 3 Z L c, r/l ;;j

4x7

0.00 f

REFERENCES

i

0

Adsorption

15

30

Potential,

1. M.M. Dubinin, Progress Surface Membrane Sci., 9, 1 (1975). 2. M. Jaroniec and J. Choma, Mater. Chem. Phys., 15, 521 (1985). 3. K.S.W. Sing, D. H. Everett, R.A.W. Haul, L. Moscou, R. A. Pierotti, I. Rouquerol and T. Siemieniewska, Pure Applied Chem., 57, 603

45

A(kJ/mole)

Fig. 4. Adsorption potential distribution function X(A) for BPL and PA activated carbons.

(1985).

In a previous paper [6], we discussed an equation for calculating the geometric surface area of the micropores, Smi; according to this equation, we need onlv the A-value to evaluate S,;. Table 2 contains the S,&alues for both activated c&l&s. The total specific surface area St may be calculated by summing the values of Sme and S,i. Comparison of the !&-values for both activated carbons shows that the BPL carbon has a greater surface area than the PA carbon, which means that its adsorption capacity is greater than that for the PA carbon; on the other hand, at low relative pressures the adsorption isotherm for the PA carbon lies above that for the BPL carbon (cf., Fig. 1) because the PA carbon possesses smaller micropores and shows a greater energetic heterogeneity than the BPL carbon.

The use of surface signal enhancement of carbon

4. S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area, and Porosity, Academic Press, New York, 1982. 5. M. Jaroniec, R. Madey, X. Lu and J. Choma, Lungmuir, 4, 911 (1988). 6. M. Jaroniec and R. Madey, J. Phys. Chem., 92, 3986 (1988).

techniques fibre

in x-ray surfaces

photoelectronic

studies

(Received 13 January 1989; accepted 30 January 1989) Key Words - Carbon fibers, XPS, surface treatment

There have been many studies of the chemical changes induced by oxidative treatments of carbon fibre surfaces with the attempt to understand the nature of fibre/matrix interaction in carbon fibre/epoxy composites [e.g. 1 - 51. Most of the more successful studies utilized x-ray photoelectron spectroscopy for chemical groups identification. However, there are many conflicting reports on the amount and type of oxygen and nitrogen functionality induced by similar surface treatments. This arises becaise most cotinnercially successful treatments only alter the first few atomic lavers of the fibre itself and signals from these functional g&ups are swamped by the graphitic backbone in the carbon 1s spectrum. It will be shown here, that by using the small spot and small

acceptance angle features of the PHI 5400 X-ray photoelectron spectrometer signals from these surface features can be enhanced dramatically. (This technique is well established for surface signal enhancement for flat metal and polymer samples but has not been exploited for fibre surface studies.) The sampling depth, d, of the XPS experiment is governed mainly by the inelastic mean free path, k , of Fhe photoelectrbn in a solid sample. It can safely be assumed that 95% of the signal arises from the fist 3 A of the sample (which is approximately 60 A when the kinetic energy of the photoelectrons is between 500 and 1OOOeV[6]). The fibres themselves are approximately 5-7 pm in diameter. When these are arranged so that the