Langmuir 1992,8, 1779-1783
1779
Surface Film Pressure Computations of Organic Vapors on Metal Dopant/Active Carbon Systems M. Afzal,* F. Mahmood, and P. K. Butt Department of Chemistry, Quaid-I-Azam University, Islamabad, Pakistan Received October 18,1991. In Final Form: February 5, 1992 It has been observed that doping of active carbon with metals (Ni, Cu, Cd, Zn) is accompanied by diminution of microporosity due to progressive closure of micropores. A TPR study indicates that the active carbon acta as dispersing agent and supported metals exist on the surface of carbon in the dispersed form. The effect of metal dopanta on the adsorbing behavior of active carbon for acetone and acetaldehyde is also studied as a function of temperature. From adsorption data, surface film pressure and related thermodynamic parameters are calculated and interpreted.
Introduction Heterogeneous catalysis is an important branch of chemistry and is gaining much more importance in recent years.'+ In heterogeneous catalysis, the catalyst is spread on solid support of high surface area which makes the catalyst highly dispersed and active throughout the system of support. Support material can be inert like silica or catalytically active like alumina, zeolites, and active carbons. Active carbons are heterogeneous, catalytic materials with high porosity and so are extensively used as catalysts,lOJl catalyst supports,12-l4and adsorbents.l5J6 The porosity in active carbon does not adopt totally random shapes an'd sizes. These shapes and sizes are associated with the intrinsic properties of the parent material and manufacturing and activation methods. The chemical properties of the active carbon can be modified by adatoms or dopants which have a profound effect on both the reactivity and selectively of the surface in catalytic r e a c t i o n ~ . l ~The - ~ ~dopant/support systems are usually synergic, which exhibit an activity greater than the sum of those of constituents.22 The aim of the present work is to study the state of dispersion of doped metals on the surface of active carbon. The adsorbing behavior of active carbon for organic vapors and the effect of metal dopants are investigated. (1) Cobes, J.; Phillips, J. J. Phys. Chem. 1991, 95, 8776. (2) Goddard, S. A.; Amiridis, M. D.; Rekoske, J. E.; Cardona-Martinez, N.; Dumesic, J. A. J. Catal. 1989, 117, 155. (3) Gatte, R. R.; Phillips, J. Langmuir 1989, 5, 758. (4) Phillips, J.; Gatte, R. R. Thermochim. Acta 1989, 154, 13. (5) Zaera, F.; Gellman,A. J.; Samorjai,G. A. Acc. Chem. Res. 1986,19, 24. (6) Burwell, R. L. J. Langmuir 1986,2, 2. (7) Reguire, G. E. Anal. Chem. 1987,59, 294R. (8) H&culus, D. M. J. Chem. Educ. 1984,61,402. (9) Pines, H. Adu. Catal. 1987, 35, 323. (10) Koresh. J. E.: Sofer. A. Sea Sci. Technol. 1983.18.723. (11) Hassan; M. hi.; Raghavan, N. S.; Ruthven, D. M. Che'm.Eng. Sci. 1987.42. 2037. (12) Grunewald, G. C.; Drago, R. S. J. Mol. Catal. 1990,58, 227. (13) Song, G.; Jiang, 2. Spu. 1987,5, 58. (14) Surmova, S. I.; Kostomarova, M. A.; Petokhov, S. S. Zh.Prikl. Khim. (Leningrida) 1987,60,640. (15) Suda, Y.;Morimoto, T.; Nagao, M. Langmuir 1987, 3, 99. N. S.: Hassan, M. M. Chem. En#. Sci. (16) Ruthven, D. M.: Raahavan, 1986,41,1325. (17) Arai. H.: Uehara. K.: Kinoshita.. S.:. Kunue. T. Ind. Eng. Chem. Rod. Res. Deu.' 1972, 11, 308. (18) Cilense, M.; Banedetti, A. V.; Jafelicci, J. M.; Varela, J. A.; DaCosta, R. A. Electrca Quim. 1984, 9, 23. (19) Berg,R.; Gulfrandsen,A. H.; Neefjes, G. A.Reu.Port. Quim.1977, 19 (1-4), 378. (20) Smisek,M.; Gerny, S. Active Carbon; Elsevier: Amsterdam, 1970. (21) Jacques, J.; Van Bokhoven; Medema, M. Proceedings of Symposium, Universite Neuchatel, Switzerland, 1978. (22) Gerald, G. C.; Russel, S. D. J. Am. Chem. SOC.1991,113, 1639.
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1.
Experimental Section Active carbon was supplied by Merck (catalog no. 2184). It was washed by several cycles of immersion in distilled water until there was no change in the pH. Metal(I1) chlorides used were supplied by Merck with the purities better than 99%. Acetone and acetaldehyde, HPLC grade, were supplied by Merck and used as such. For doping of active carbon, a predetermined amount of metal chloride was magnetically stirred in 200 mL of distilled water and 10g of active carbon was added to the mixture. The mixture was stirred for 8 h at 373 K until a slurry was formed which was then dried under vacuum at 363 K for 3 h. The dried samples were then heated at 735 K for 5 h in a nitrogen atmosphere. A blank carbon sample was also prepared by giving the same treatment except that distilled water used in place of the metal chloride solution. For measurement of the metal concentration, 1 g of the sample was thoroughly stirred with hydrogen perchlorate solution for 4 h a t room temperature. The total amount of the metal in solution was then determined by atomic absorption spectroscopy (Zeiss Co., Model FMD 47). Complete adsorption-desorption isotherms for nitrogen were measured at 77 K using a Quantasorb sorption system and surface area was calculated by the BET method. Pore size distribution was determined from desorption isotherms of nitrogen by the method of Dollimore and Heal.23 The temperature-programmed reduction (TPR) system used for the reduction study was similar to that described by S. KarimJ4 Diffraction patterns for all samples were obtained with a Philips PW 1050diffractometer goniometer. The detector was a xenon proportional counter linked to a PW 4620ratemeter and a channel analyzer. The radiation was nickel-filtered Cu Ka (1.5418A) generated in a Philips PW 1010/80operated at 40 kV and 20mA. The diffractometer was operated with l.Oo diverging and 0.loreceivingslits at a scan rate of 2 deglmin. The continuous trace of X-ray reflection was obtained from the flat surface of the carbon pellets, since this technique was found to give more reproducible results than powder samples. Adsorption data for acetone and acetaldehyde on active carbon and metal-doped carbon (containing 0.07 mol of metal/100 g of carbon) were obtained by using a Cahn lo00 electrobalance attached to a vacuum line. A 0.25-g portion of the dehydrated adsorbent was taken in a glazed silica crucible which was placed in the weighing unit. Prior to pumping, the hang down tube was maintained at a fixed temperature using a water thermostat (accurate to iO.1 OC). The entire system was then evacuated at 10-5mbar for at least 7 h before taking adsorption data. Vapors of the adsorptive were admitted and equilibrium pressure was read from mercury manometer with the aid of cathetometer. Dissolved gases were removed from the adsorptive liquid by several freeze-pump-thaw cycles. (23) Dollimore, D.; Heal, G. R. Appl. Chem. (London) 1964,14,109. (24) Karim, S. Ph.D. Thesis, Brunel University, U.K.
0743-746319212408-1779$03.00/0 0 1992 American Chemical Society
1780 Langmuir, Vol. 8,No. 7, 1992
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Afzal et al.
I
i
-
m
"E
c1
>
7.50
7.75
-
8.00 In(p/V)
0.25
20
r
8.0 0
II = RT/MS Lpm d In P
(1) where m is the amount adsorbed in grams and M is the molecular weight of adsorbate. The numerical computation of II involves, in the first place, the determination of specific surface area S and, secondly, the evaluation of the integral m d In P. According to Schofield,26the two-dimensional behavior of the adsorbed film may be represented by II(SL/NV- SLINV,) = R T (2) where S is the specific surface area, L is the molar volume of the gas at NTP, V is the volume of gas adsorbed a t pressure p, and Vm is the monolayer coverage. Gibb's equation may be written as (3)
(4) In p = In V/(V, - V) + V/(V, - V) + I On expanding the terms as a series and rearranging, we obtain
If
+ ~ ( v / v , ) ~ / z+ ...+ In V, + I
100
doped active carbon containing 0.074 mol of metal per 100 g of active carbon.
Calculation of Film Pressure Surface film pressure, n, was computed from Gibb's adsorption isotherm equation25
In p / v = ZV/V,
EO
( O A )
Figure 2. Pore size distribution in active carbon and nickel-
Figure 1. Plots of V against In @/V).
dII = RTL/SV d In p Combining eqs 2 and 3 and integrating, we get
60
LO
(5)
v
