흡착제의 등온흡착곡선과 표면에너지 분포 측정

이현송*, 김영순**, 변헌수**

한국과학기술원 화학공학과*, 여수대학교 화학공학과**

Measurements of Adsorption Isotherms and Energy Distributions on Adsorbent Surfaces

Hyun-Song Lee*, Young-Soon Kim**, Hun-Soo Byun**

Dept. of Chem. Eng., KAIST* ;Dept. of Chem. Eng., Yosu Nat. Univ.**

Introduction

Here we propose a new experimental protocol that can provide information about the energetic heterogeneities of surfaces. Consider the adsorption of two gases, one strongly adsorbed, the other weakly adsorbed, and an adsorbent whose surface is energetically heterogeneous. For purposes of illustration, we assume that the strongly adsorbing gas is carbon dioxide and the weakly adsorbing gas is argon. However, the method described below is independent of the gases chosen. The only limitation is that there must be a reasonable difference, a factor of two or three in the adsorption energies. It should be noted that a factor of two in interaction energy results in a factor of about seven difference in Henry's constants and a factor of three difference in interaction energy results in a factor of twenty difference in Henry's constants.

The experiments begin with measurement of the Henry's constant of the weakly adsorbing gas, argon. The nature of adsorption processes is such that argon molecules adsorb first on the most active (energetic) sites, and therefore, the Henry's constant characterizes the most energetic sites. After desorption of the argon, we expose the adsorbent to a low concentration (partial pressure) of the strongly adsorbing gas, carbon dioxide. The carbon dioxide also will adsorb on the most active sites. These sites therefore are blocked against subsequent argon adsorption. If we again introduce a small quantity of argon, it will not displace the carbon dioxide to any appreciable extent, and so the argon will adsorb on the next most active sites. In effect, we are measuring the Henry's constant of argon on that portion of the surface that still is exposed i.e., not covered by carbon dioxide. If the activity, partial pressure of carbon dioxide is increased again, it will continue to adsorb preferentially on the most active sites available. Hence, this process can be repeated with successively higher concentrations of carbon dioxide to cover more and more of the surface and hence more and more of the most active sites. The result is that each time argon is introduced, it probes only those sites not covered by the carbon dioxide molecules. Analysis of the variation of argon's Henry's constant as a function of temperature and carbon dioxide coverage will allow one to determine the surface energy distribution.

Experimental Procedures

There are several ways that the procedure outlined above can be implemented. Here, we discuss two of these briefly and then provide additional details about the method we propose to use. One way that this protocol can be implemented is similar in many respects to the measurements performed in a conventional BET apparatus. In a BET apparatus, two gas streams are mixed and passed though the adsorbent. One is a carrier gas (usually helium) and the other is the adsorbate gas being studied. For convenience, the adsorbent usually is cooled to the boiling point of liquid nitrogen; however, adsorption can be measured at other temperatures if a cryostat is used. After passing the gases through the adsorbent for a time that is sufficient to attain equilibrium, the adsorbent material is heated and the amount of adsorbate that desorbs is measured, usually using a TCD (thermal conductivity detector). For the experiments proposed here, two changes must be made to the procedures and apparatus. The first is that one must mix two adsorbate gases (argon and carbon dioxide in the example above) with the carrier gas. The second difference is that we must be able to separate the signals for the two adsorbate gases following their desorption. This can be accomplished either by using selective detectors or by separating the strongly and weakly adsorbing gases in a chromatographic column after they desorb, but before they reach the detector. In fact, some chromatographic separation will occur naturally in the adsorbent itself. As noted above, the only differences between this experiment and a conventional BET experiment are the use of three gas streams and a means of detecting both adsorbate gases.

A second experiment that can be used to obtain these data uses a gas chromatograph directly. Gas chromatography has been used extensively to measure adsorption isotherms for pure gases [1-9]. The procedure to determine the adsorption isotherm from the experimental chromatogram relies on the fact that rate a solute moves through the chromatographic column depends on the partition coefficient at the column conditions. In these experiments, the chromatographic column is packed with the adsorbent material to be studied and helium is used as the carrier gas. In the traditional experiment, adsorbate gas is injected into the carrier-gas stream and, as discussed below, the asymmetry of the chromatographic peak is used to determine the adsorption isotherm. In principle, the entire adsorption isotherm can be obtained from a single chromatogram; however, in practice, several injections of different sizes usually are made. This helps to minimize errors and also allows one to determine whether the assumptions made in the theoretical analysis of the data are valid. However, if all that is sought is the Henry's constant, the analysis is much simpler. In this case, a small sample of adsorbate is injected and the Henry's constant can be calculated directly from the average retention time in the column.

Here, we propose to use a mixed carrier gas stream (helium and carbon dioxide) of various concentrations. We will inject small amounts of argon, and use the argon retention time to determine the Henry's constant for adsorption on that fraction of the surface not covered by carbon dioxide. By increasing the amount of carbon dioxide in the carrier-gas stream, one can adsorb more and more carbon dioxide, thereby changing the energy of the sites left exposed for adsorption by argon.

Discussion

Two groups of measurements at two temperatures are necessary to determine the energy distribution:

a) adsorption isotherm for the strongly adsorbed component;

b) Henry's constant for the weakly adsorbed component as a function of the adsorbed amount of the strongly adsorbed component.

We have performed the first group of experiments demonstrating a possibility to determine adsorption isotherm by measuring Henry's constant, H, of one component (say, A-component) in the presence of various concentrations of the second component (B-component). A proper choice of A and B components gives an opportunity to block some sites on the surface of the adsorbent by B-molecules and measure Henry's constant for A-molecules on the free part of the surface. The temperature dependence of H gives the energy of adsorption for parts of surface with different adsorption affinities, which is the information about energetic inhomogeneity of the surface.

As an example, we used argon as an A component and carbon dioxide as a B component. Experimental set-up is shown in Figure 1. Injection of a small amount of argon at different concentrations of carbon dioxide in the carrier gas gives argon retention times and Henry's constants for argon adsorbed on sites not blocked by carbon dioxide. Retention time for argon as a function of the carbon dioxide content in the binary carrier gas (carbon dioxide + helium) gives the adsorption isotherm for carbon dioxide.

Let x be a fraction of the surface free of carbon dioxide. Also, q1 is the amount of argon adsorbed on the free surface, and q2 is the amount of argon adsorbed between or on top of adsorbed molecules of carbon dioxide. Then, the total amount of argon adsorbed, aAr, is:

aAr = q1 x + q2 (1 - x)	(1)

Equation (1) allows us to determine x if q1 and q2 are known:

[eqn: x={q_Ar - q_2} over {q_1 - q_2}]	(2)

The value of q1 can be determined from measurement without carbon dioxide. The value of q2 can be measures when carbon dioxide saturates the surface.

From equation (2), it follows that

[eqn: theta_CO_2 = 1-x = {q_1 - q_Ar} over {q_1 - q_2}]	(3)

In equation (3), the value of 1 - x is the adsorbed amount of carbon dioxide. So, it allows one to determine adsorption isotherm for carbon dioxide by measuring adsorption of argon as a function of the concentration of carbon dioxide, C, in gas phase.

Figure 2 shows experimental data on retention volumes of argon and hydrogen in the column filled with molecular sieve. These retention volumes are determined as functions of the carbon dioxide concentration in the carrier gas at T = 100 ℃. These data indicate that the presence of carbon dioxide in the carrier gas changes significantly the adsorbed amount of argon, and even for hydrogen carbon dioxide changes the retention time.

Figure 3 gives the adsorption isotherm for carbon dioxide in molecular sieve obtained from equation (3) by using argon data and by using retention times of hydrogen.

Reference

[1] J. N. Wilson, J. Amer. Chem. Soc., 62, 1583 (1940).

[2] D. De Vault, J. Amer. Chem. Soc., 65, 532 (1943).

[3] E. Glueckauf, Nature, 156, 748 (1945); 160, 301 (1947).

[4] Yu. S. Nikitin and R. S. Petrova, In Experimental Methods in Adsorption and Molecular Chromatography, Moscow State University, Moscow, 1990.

[5] F. G. Helfferich, Journal of Chromatography A, 768, 169 (1997).

[6] A. M. Katti and G. A. Guiochon, Advances in Chromatography 31, 1 (1992).

[7] Y. Guillaume and C. Guinchard, Chromatographia 39, 438 (1994).

[8] Q. Wang, C. Zhu, and B. Yan, Journal of Chromatography A, 541, 13 (1990).

[9] T. C. Gerbino and G. Castello, Journal of Chromatography A, 537, 305 (1991).

Figure 1. Experimental set-up to measure Henry's constants of argon and hydrogen at various contents of carbon dioxide.

Figure 3. Adsorption isotherm for carbon dioxide on molecular sieve5A determined from Henry's constants of argon and hydrogen in the presence of carbon dioxide.

Figure 2. Retention volumes (cc) of argon and hydrogen in column with molecular sieve 5A as functions of the partial pressure of carbon dioxide (psi) at T = 373 K.