Langmuir 1987, 3, 699-703 2 is probably a reflection of the similarity in shape of the experimental isotherms in Figure 1. When fitting models to experimental data, it is necessary to ensure that the residuals are not biased, that is, they do not vary systematically with the independent variables. Inspection of the present results shows that for all three methods the residuals are biased, the model tending to overpredict the amount adsorbed at low pressures and to underpredict at high pressures. It is probable that this indicates a limitation of the Langmuir equation as the local isotherm for this system. The choice of the Langmuir equation for the local isotherm implies that the total isotherm has the same limits, that is, tending to Henry’s law at low pressures and to saturation at high pressures. Neither of these limits is observed over the range of pressures for this work. The wide range of q for regularization resulting in qma/qmin = 4 may also be due in part to the bias resulting from the use of the Langmuir kernel. Better results may be obtained in future work either by extending the range of pressures or by choosing a local isotherm whose dependence on pressure is similar to that of the total isotherm.
5. Conclusions Two analytic solutions to the GAI (Sircar’s equation and the condensation approximation) and one numerical me-
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thod (regularization incorporating a new smoothing algorithm) were applied to adsorption of Ar at 77 K on two microporous carbons activated to 28 wt % and 80 w t % burn-off. For both carbons the energy distribution functions obtained from the three methods were similar but the dispersions of the distributions obtained from regularization were higher than those obtained from Sircar’s equation. The distribution functions obtained from the condensation approximation are similar to those obtained from regularization, suggesting that it is a good approximation for adsorption of Ar at 77 K on these carbons. Comparing the two carbons, although PVDC-80 gives a slightly wider dispersion than the PVDC-28 for all three methods, the energy distribution does not change a great deal with burn-off. It is concluded that, although activation from 28 to 80 wt % burn-off increases adsorptive capacity substantially, there is no significant increase in the mean width of micropores.
Acknowledgment. We thank the Science and Engineering Research Council of the UK and the British Gas Corporation for financial support. B.McE. also thanks the American Chemical Society for a travel grant which enabled him to present this paper at the Kiselev Memorial Symposium. Registry No. Ar, 7440-37-1; C, 7440-44-0.
Computer-ControlledVacuum Microbalance Techniques for Surface Area and Porosity Measurements? K. A. Thompson* and E. L. Fuller, Jr. Plant Laboratory, Oak Ridge Y-12 Plant,t Martin Marietta Energy Systems, Inc., Oak Ridge, Tennessee 37831 Received November 24, 1986. I n Final Form: February 24, 1987 A versatile, automated, high-vacuum microbalance system has been constructed and evaluated for obtaining extensive and precise physical adsorption and desorption data. A dedicated minicomputer (LSI 11/23) is used to monitor and control the relevant parameters (temperature, pressure, mass, etc.) so that detailed kinetics and mechanisms can be evaluated. In this manner, each value of the adsorption isotherm is constructed as the composite result of a rapid isochoric pressure change followed by a kinetically controlled isobaric approach to the final steady-state mass at the fixed temperature and pressure of interest. With the aid of the computer, it is often possible to calculate the steady-statemass before equilibrium by monitoring the rate of mass change. However, the system was designed to be as versatile as possible, not depending on any single-orderkinetics but using it to full advantage when required. The test sample used for initial testing was a commercially available silica-supportedalumina catalyst (A1203) for which nitrogen and argon sorption isotherms were acquired. True hysteresis is shown to prevail by virtue of the isobaric kinetic accountability that cannot be determined volumetrically. Other comparisons are given, and the merits of the automated high-vacuum microbalance become quite evident when nonroutine, detailed exploratory studies are required.
Introduction Physisorption measurements are a common and often preferred method of determining surface area and pore-size distributions. Since the advent of the Brunauer-Emmett-Teller (BET) theory of “multilayer adsorption”,’ Presented at the “Kiselev Memorial Symposium”, 60th Colloid and Surface Science Symposium, Atlanta, GA, June 15-18,1986; K. S. W. Sing and R. A. Pierotti, Chairmen. *Operated for the US.Department of Energy by Martin Marietta Energy Systems, Inc., under Contract DE-AC05-840R21400. 0743-7463/87/2403-0699$01.50/0
sorption isotherm analysis for surface area has become commonplace for industrial use. For this application, the absolute value of surface area or porosity is not required and is used only as a relative parameter. In cases in which absolute values are needed, more detailed analyses are required, along with further improvements upon the theory. Such attempts have been made in the past, such as the potential theory,2 as,or t method^.^,^ For further (1) Brunauer, S.; Emmett, P. H.; Teller, E. J.Am. Chem. SOC.1938, 60. 309.
0 1987 American Chemical Society
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F i g u r e 1. Typical sorption isotherm.
development of better theories, accurate and more detailed data are needed. Figure 1shows a schematic sorption isotherm. To better understand physisorption as a function of PIPs (the ratio of the adsorbate gas pressure to the saturation pressure), the isotherm can be broken into different regions. The BET regime of the sorption isotherm lies between 0.05and 0.35P/Ps. This is a fairly flat region of the isotherm where multilayer adsorption begins and from which the surface area may be calculated by using the BET theory. Although this is the most studied area of the isotherm, most of the sorption occurs outside this region. Micropore filling occurs at 20 but 500 A. Macropores are hard to study because they are filled very near the saturation pressure; however, mesopores have been studied in great detail. The process that occurs is capillary condensation. The hysteresis that often exists between the adsorption and desorption branches of the isotherm is due to the structurally different configuration between adsorption and desorption. Examples of the various shapes that the pores can have are “ink bottle”, cylindrical, and lit.^,^ By assuming a particular geometry, a pore-size distribution can be calculated. It is hoped that a more detailed analysis of the mesopore region may lead to better identification of the particular pore shape.lOJ1 This analysis would require (2) Fuller, E. L.,Jr.; Smithwick, R. W., III Surface Area and Porosity: I. Computer Aided Evaluation of Monolayer Capacity from Physical Adsorption Data; work in progress. (3) Sing, K. W. S.In Proceedings of the International Symposium of Surface Determination; Everett, D. H., Ottewill, R. H., Eds.; Butterworths: London, 1970; Vol. 25. (4) L.ippens, B. C.; DeBoer, J. H. J . Catal. 1965, 4, 319. (5) Sing, K. S.W.; Everett, D. H.; Haul, R. A. W.; Moecou, L.;Pierotti, A.; Rouquerol, J.; Siemeniewska, T. Pure Appl. Chem. 1986, 57, 603. (6) Mikhail, R. SH.; Brunauer, S.;Bodor, E. E. J. Colloid Interface Sci. 1968, 26, 45. (7) Gregg, S.J.; Sing, K. S.W. Surf. Colloid. Sci. 1976, 9, 231. (8) Zsiquinondy, A. 2. Anorg. Chem. 1911, 71, 356. (9) Karnaukhov, A. P. In Pore Structure and Properties of Materials; Modry, S., Ed.; Academia Prague 1973; Part 1.
scanning the hysteresis loop several times, requiring a large amount of time for data acquisition, which leads to the need for computer automation. The majority of previous sorption isotherms have been acquired by using the volumetric method, which for industrial use has the advantage of being rugged and relatively inexpensive. The gravimetric method, however, has the advantage of measuring adsorbed mass directly, in addition to gas pressure and temperature, and all independently of each other. This allows direct measurement of the degassed sample weight, which can be as much as 5% less than the original weight. Other advantagesinclude direct accountability of mass during the isotherm acquisition (vs. cumulative errors in the volumetric method) and monitoring possible chemical reactions, such as decomposition. The true versatility of the gravimetric method is realized when the temperature of the sample can be regulated to facilitate controlled reactions in situ, followed by sorption isotherm measurements. A more detailed comparison of the two techniques has previously been made by Fuller et a1.12
Experimental Section A schematic diagram of the apparatus is shown in Figure 2.
It is basically a Cahn RG-UHV electrobalance with a precision of 0.1 pg. All components are compatible with an ultrahigh vacuum. The chamber that houses the balance is stainless steel, with conflat flange connections to other segments, such as the ion pump (150 L/s), sorption pump, rotary vane pump, hang-down tubes (stainless steel or aluminum), gas manifold system, and tare weight manipulator. This allows for a clean system, which is necessary for detailed studies. Figure 2 also shows the configuration of manual and solenoid valves used to control the pressure. T h e system was designed to be as flexible as possible; it allows for four gases (indicated as HP, N B ,Ar, and He), although other gases can be used as well. There is also a cold finger in the gas line to trap impurities, if needed. All gases used in this study are high purity (99.9995 or better), eliminating the need for the cold finger in this study. A vapor pressure thermometer is also installed to determine the temperature of the bath by monitoring the vapor pressure of the condensed adsorbate gas. The pressure manometers are MKS 170 series 1000- and 10-torr sensor heads. For pressures below loF2torr, a Varian broad-range ionization gauge is used. The automation system is shown in Figure 3. The computer is an LSI 11/23 minicomputer with various input/output devices to monitor and control the complete microbalance apparatus. The operating system is RTllFB, dowing foreground and background jobs to run simultaneously. The programming language used wm Fortran, although this application is not restricted to that particular language. It does give the advantage of being compiled and of running faster, which is necessary for the computational parts of the program. Output devices include the monitor, dot matrix printer, a Hewlett-Packard plotter, floppy disk drives, and a Winchester disk drive. This combination allows for storage of the data and documentation of the results. The control of the pressure and temperature is handled through a high current output (HCO) control board, which controls the solenoid valves. By opening the proper valves, the pressure can be increased or decreased in a controlled manner. The automatic pressure control (APC)valve is usually set as a leak valve, allowing the open/close solenoid valves to control the leak rate. The temperature is controlled by opening or closing a solenoid valve for a n appropriate bath liquid, such as liquid nitrogen, and regulating the level within f l mm. (10)Pierotti, R. A., paper presented at the 60th Colloid and Surface Science Symposium, 1986. (11) Fuller, E. L.,Jr.; Condon, J. B.; Eager, M. H.; Jones, L. L. Report Y/DK-264; Martin Marietta Energy Systems, Inc.,Oak Ridge Y-12 Plant, 1981. (12) Fuller, E. L., Jr.; Poulis, J. A.; Czanderna, A. W.; Robens, E. Thernochim. Acta 1979, 29, 315.
Langmuir, Vol. 3, No. 5, 1987 701
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Figure 2. Schematic diagram of the instrument. COMPUTER AUTOMATED MICROBALANCE
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Figure 3. Schematic diagram of the automation system. The temperature and pressure are monitored by feeding the respective voltages (proportionalto the variable parameters being measured) into a Keithley 705 scanner, which routas the variables being measured to a Keithley 192 programmable digital multimeter (DMM). The scanner and DMM are both interfaced with the computer through the IEEE-488 bus. The mass is measured by directing the voltage from the recorder output off the electrobalance controls to a DMM directly, again interfaced with the computer through the IEEE-488bus. By using the scanner, the limit of variables that can be measured is increased to 12, without the need of installing more DMMs.
Results and Discussion The software is a versatile package that allows complete control of the microbalance system. At the present time, initialization includes a series of questions the operator must answer, which includes the time interval between data points, the maximum number of data points for each pressure, a minimum rate change required to jump to the next pressure setting, and a list of pressure settings desired. Other options include liquid bath filling, vapor pressure thermometer use, and immediate termination upon request. Safety precautions are also built in to protect the system from such disasters as the bath supply running empty or a power failure. After the.data have been acquired (or while they are being acquired, since the main software runs in the background mode) data manipulation
Figure 4. Chronology of mass as a function of pressure. can be applied to determine the steady-state mass at each pressure setting and to calculate the BET surface area or other desired quantities. The main purpose of this study is to acquire accurate data; thus, the acquisition process is emphasized. Figure 4 shows the nitrogen uptake as a function of time at 77 K for A1,0,, a commercially available alumina catalyst (pelleted into 0.12541. right cylinders). Each data point was taken at 30-s intervals. A rapid isochoric pressure change was followed by a kinetically controlled isobaric approach to the final steady-state mass for each pressure setting. Due to the large volume of the sample chamber, the adsorbate weight gain is a minor perturbation to the system, and the pressure remains essentially constant without using external means of regulation. Thermal-molecular-floweffects are observed at the low-pressure settings, indicated by the large fluctuations in the mass. At higher pressure settings, the steady-state mass was not reached before the pressure was changed to the next set-
702 Langmuir, Vol. 3, No. 5, 1987
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ting. This is not a limitation of the system but demonstrates the increased efficiency of not waiting for equilibrium if a single-order rate exists. Figure 5 shows the same data plotted in a different fashion, showing the first-order kinetics of the sorption phenomenon. For this particular material, it is not necessary to wait for equilibrium, but the steady-state mass can be calculated by using a least-squares linear fit for the first derivative of the mass (firstorder). Once this fact has been demonstrated, the steady-state mass can be calculated with the precision the operator requires, and the next pressure setting can be obtained. Figure 6 demonstrates the behavior of the rate kinetics as a function of pressure. This behavior has been observed for a variety of bath temperatures and adsorbate gases, indicating that the rate of adsorption or desorption is slower at high pressures. Because of this behavior, longer periods of time are required for equilibrium to be reached as the pressure increases, which shows the advantage of calculating the steady-state mass if possible. The final sorption isotherm containing the adsorption and desorption branches is shown in Figure 7. All the data points are shown demonstrating the kinetically controlled isobaric approach to the final steady-state mass. The least-squares projected steady-state mass is also indicated, and a smooth isotherm is drawn through these points. True permanent hysteresis is seen to exist, since the gra-
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vimetric method is a direct measurement and no processing has occurred to force the low-pressure end of the adsorption and desorption branches to coincide. This direct measurement is also an indication of the purity of the gases, since water would tend to raise the desorption branch above the adsorption branch. A comparison of the gravimetrically determined isotherm with the volumetrically determined isotherm'l shows the P,to be too low. This is due to the sample being slightly warmer than the liquid nitrogen bath due to radiative heat. There are several ways of overcoming the difference between the measured and actual saturation pressure. Another solution is to design the system in such a way as to minimize radiative heat, which has been previously accomplished by using screens13 or by restricting the aperture of the hang-down wire in such a manner as to surround the sample by as much surface area as possible at liquid nitrogen temperature.14 ~~~~
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(13) Robens, E.; Gast, Th.J. Vac. Sci. Technol. 1978, 15, 805.
Langmuir 1987, 3, 703-707 Figure 8 shows the sorption isotherm of argon at 77 K for A1203. Again, the same type of results are seen, with a smaller hysteresis effect. This is due to the saturation pressure of argon as a solid being different from that of a liquid, which is believed to be the form of adsorbed argon on the surface of the material being studied. Another difference in the isotherms is the reduced "knee effect" at low pressure where a monolayer of adsorbate is formed. This is due to the less energetic argon physisorption. Further work is needed to determine the advantages and disadvantages of using the monoatomic argon gas vs. the diatomic quadrupolar nitrogen gas as the adsorbate for sorption isotherms.
Conclusion The feasibility of a computer-controlled microbalance has been proven. Actual data of nitrogen and argon sorption isotherms for A1203show that the system per(14) Thompson, K. A.;Fuller, E. L., Jr., work concerning the development of an inner drop tube that clamps to the outer drop tube, thus surrounding the sample by surfacea at a constant temperature of the bath. Work soon to appear in the literature.
703
forms well and that tedious data acquisition can be accomplished through the computer, freeing the operator from the boring tedium. The software that has been developed is preliminary but versatile, allowing a wide assortment of variables to be adjusted in various ways. For instance, the steady-state mass can be reached if so desired, but time limits can also be imposed that restrict the time expended for each pressure setting, since a least-squares fit of the data has been shown to give the steady-state mass for Al,03. The computer-controlled functions of gaspressure automation, liquid nitrogen bath control, and temperature, pressure, and mass monitoring have been accomplished with a combination of HCO boards, IEEE488 bus, scanner, and DMMs. This combination allows up to 13 variables to be monitored in its present configuration and can be increased to 25 with an additional board in the scanner. In addition, 12 high-current systems (solenoid valves are used at present) can be controlled with expansion capability of 24 available. This allows the system to be expandable upon demand, allowing other possible function to be incorporated, such as high-temperature catalysis, or decomposition reactions to be studied.
Adsorption and Chromatographic Properties of Some Polymeric and Ionic Adsorbentst L. D. Belyakova Institute of Physical Chemistry, USSR Academy of Sciences, Moscow 117915, USSR Received September 10, 1986. I n Final Form: February 26,1987 Modification of the surface of adsorbents changes their adsorption and chromatographicproperties. Polymeric adsorbents obtained by modifying styrene-divinylbenzene,polystyrene, and methacrylate matrices with amines, amine alcohols, and sulfuric acid as well as salt-based ionic adsorbents (chlorides and sulfates of bivalent metals) have been studied. The dependences of retention and adsorption values and thermodynamic characteristics on structural parameters and the chemical nature of the adsorbent surface have been found. The contributions to the total adsorption energy of specific interactions between adsorbates of different types and the adsorbent surface have been evaluated.
Introduction The effect of the chemical nature of the surface on adsorption and chromatographic properties of such widely used adsorbents as silica gels, alumina gels, aluminosilicates, zeolites, carbon black, and porous carbons has been studied in detail with various methods. In this paper we investigated polymeric and salt-based ionic adsorbents. In recent years they have been widely used, particularly in ~hromatography,'-~ since they have important advantages. A change in conditions of synthesis of polymeric sorbents made it possible to obtain porous polymers of different ~ t r u c t u r e s . ~Thermal treatment of salts gives rise to adsorbents with a homogeneous surface.6 Chemical modification of the surface of polymers, a choice of proper salts, and methods of their treatment in the case of ionic adsorbents open up possibilities for a wide change in the chemical nature of the surface of the adsorbents and, hence, in selectivity of the samples toward different com'Presented at the "Kiselev Memorial Symposium", 60th Colloid and Surface Science Symposium, Atlanta, GA, June 15-18,1986; K. S. W.Sing and R. A. Pierotti, Chairmen.
pounds. A combination of surface modification with improved homogeneity of pores and surface and with more adequate shape and packing of the grains enhances both selectivity and separation efficiency in chromatography, thus favoring accumulation of impurities and analysis of complex mixtures. Polymeric adsorbents are also advantageous in that they adsorb adsorbates in the presence of water vapor which is important for many practical applications. In addition, they are easily regenerated by (1)Kiselev, A. V.; Yashin Ya. I. Adsorption Gas and Liquid Chromatography; Khimiya: Moscow, 1979. (2) Sakodynskii,K. I.; Panina, L. I. Polymeric Sorbents for Molecular Chromatography; Nauka: Moscow, 1977. (3) Kiselev, A. V. Intermolecular Interactions in Adsorption and Chromatography; Vysshaya shkola: Moscow, 1986. (4) Belyakova, L. D.; Davankov, V. A.; Kiselev, A. V.; Muttik, G. G.; Tsyurupa, M. P.; Shevchenko, T. I. Kolloidn. Zh. 1978, 40, 1059. Davankov, V. A.; Tsyurupa, M. P. Angew. Makromol. Chem. 1980,91,127. Rosenberg, G.I.; Shabaeva, A. S.; Moryakov, V. S.; Musin, T. G.; Tsyurupa, M. P.; Davankov, V. A. React. Polym., Ion Exch., Sorbents 1983, 1, 175. ( 5 ) Belyakova, L. D.; Kalpakyan, A. M.; Kiselev, A. V. Kolloidn. Zh. 1973,35, 906. Belyakova, L. D.; Kalpakyan, A. M.; Kiselev, A. V. Chromatographia 1974, 7, 14. Belyakova, L.D.;Kalpakyan, A. M. J. Chromatogr. 1974, 91, 699.
0743-7463/81/2403-0703$01.50/00 1987 American Chemical Society
