The specific surface area of solids and the surface area occupied by the active phase (metal or oxide) on a support are parameters of the utmost importance in adsorption and catalysis. For the determination of the former, the BET equation is universally established. For the evaluation of the latter the works of selective chemisorption, initiated by Emmett and Brunauer (I ), for metals and by Bridges et al. (2) and Weller et al. (3), for oxides have come to represent important contributions. Some of the classical models of adsorption have also been used for evaluation of specific surfaces (Langmuir equation) (4) and the dispersion of supported metals or oxides (Freundlich equation) (5). Both of them are applicable to physisorption as well as chemisorption processes

García Fierro, José Luis

González Tejuca, L.

Martín-Luengo, María Ángeles

Pajares, J. A.

Tascón, J. M. D.

Journal of Catalysis

Digital.CSIC

JOURNAL OF CATALYSIS 71, 201-204 (1981) NOTES The Freundlich Model of Adsorption for Calculation of Specific Surface Areas INTRODUCTION The specific surface area of solids and the surface area occupied by the active phase (metal or oxide) on a support are parame- ters of the utmost importance in adsorption and catalysis. For the determination of the former, the BET equation is universally established. For the evaluation of the latter the works of selective chemisorption, initi- ated by Emmett and Brunauer (I ), for metals and by Bridges et al. (2) and Weller et al. (3), for oxides have come to represent important contributions. Some of the clas- sical models of adsorption have also been used for evaluation of specific surfaces (Langmuir equation) (4) and the dispersion of supported metals or oxides (Freundlich equation) (5). Both of them are applicable to physisorption as well as chemisorption processes. Although the Freundlich equation is not more theoretically justified than the Langmuir equation (the basic concept of localized, monolayer adsorption is assumed for both) the former is more widely obeyed. According to the statistical derivation, and assuming that the adsorption sites are dis- tributed exponentially with respect to the energy of adsorption, the Freundlich equa- tion becomes: lgc*=lgc~+~~lg1I~+~~lgP. (1) m C, is the adsorption at pressure P; q,,, has the physical meaning of a heat of adsorp- tion at coverage 0 = 0.37; ~1~ is a constant. This equation requires that a family of isotherms at different temperatures cross at a common point at a pressure P = l/a,, where the monolayer coverage is reached. The application of the Clausius-Clapeyron equation to (1) leads to an expression in which the heat of adsorption decreases exponentially with coverage. Rudnitsky and Alexeyev (6) have re- cently elaborated equations for adsorption isotherms on adsorbents with heteroge- neous surface, assuming the same exponen- tial distribution for adsorption sites men- tioned before. Those authors found that at high equilibrium pressures, the theoretical isotherms, lg 8 vs lg P bend toward the pressure axis. In any case, the simpler Freundlich equation (1) for which the lg CA vs lg P plots are straight lines, accounts fairly well for the physical behavior of the adsorption systems studied here. Otto et nl. (5) have extensively used Eq. (1) for the study of supported metals (Pt) and oxides (Cr,O,, Fe203, NiO, CuO). These authors found that NO adsorption at monolayer coverage, as calculated by ex- trapolation of the linear lg CA vs lg P plots, is equal to the number of surface metal atoms calculated from crystallographic data (nonsupported catalysts) or by chemical analysis (supported catalysts at concentra- tions below l%, where all the active phase is supposed to be on the surface). By as- suming that the adsorption for supported metals or oxides at higher concentrations is similar to that on nonsupported catalysts, the fraction of the support surface occupied by the active phase can be calculated. In this work, experimental data of COZ adsorption on the perovskite-type oxide LaFeO, are given. Equilibrium data of ad- sorption of N20 (7), N, (8), CO (9), O2 (10), and COZ (II) on oxides, obtained 201 0021-9517/81/090201-04!$02.00/0 Copyright @ 1981 by Academic Press. Inc. All rights of reproduction m any form reserved. 202 NOTES previously in this laboratory, are also ana- lyzed. These results provide new informa- tion on the applicability of the Freundlich equation for the evaluation of the adsorp- tion monolayer and of specific surface areas. EXPERIMENTAL The LaFeO, sample was obtained by amorphous precursor decomposition as described by Delmon et al. (12). Briefly, a concentrated solution of metal nitrates and citric acid was evaporated at 343 K. The precipitated amorphous precursor was de- composed by heating at 923 K for 4 hr in air atmosphere. After this treatment a single perovskite phase was obtained as verified by thermogravimetric analysis and X-ray diffraction. The cylindrical pellets prepared by pressing the resultant powder at moder- ate pressure (2 x lo6 N rn+) were gently crushed in an agate mortar and particle sizes between 0.8 and 1.2 mm were taken for use. A more detailed account of the preparation method and also of the textural properties of perovskites obtained by this procedure can be found elsewhere (13 ). The adsorption measurements were car- ried out in a conventional volumetric Pyrex glass high-vacuum system with a calibrated Schaevitz P-502-0004 pressure transducer. Temperatures were kept within kO.5 K. Between successive experiments, the sam- ple (2 g) was outgassed by means of a thermal treatment at 1.3 x lop4 N rnp2 and 773 K for 15 hr. RESULTS AND DISCUSSION In Fig. 1, equilibrium data, lg C, vs lg P, of adsorption of CO2 on the perovskite-type oxide LaFeO, at 432, 480, and 531 K are given. As it can be observed, the Freundlich equation is obeyed. The cross- ing point of the straight lines, as calculated by the least-square method, yields an ad- sorption for the monolayer of 8.6 x 1018 molec me2 corresponding to a pressure of 3.4 x lo4 atm (1 atm = 1.01 x 105 N m-“). PressurelmmHg FIG. 1. Linear plots of Freundlich equation for COz adsorption on LaFeO,. parallel and the pressure calculated for cov- erage equal to 1 is rather high. Isotherms of this type were also observed by Otto and Shelef (5) for adsorption of NO on oxidized Cr203 supported on alumina. As illustra- tion, experimental results for N2 adsorption on Sc203 between 113 and 323 K, from a previous work (8), are also given in Fig. 2. An adsorption for monolayer coverage of 6 x lo’* molec m-2 at a pressure of 26 atmo- spheres was found. The materials used in adsorption and catalysis, generally, have a heterogeneous surface. Representative examples of this are perovskite-type oxides whose stoi- chiometry strongly depends on pretreat- ment conditions. At the temperature used for their preparation (13), nonstoichio- metric compounds with excess (LaFeO,,,) or defect (LaCoO,-,) of oxygen, giving place to transition ions with different oxida- tion states, are formed (14). On the other FIG. 2. Linear plots of Freundlich equation for N, adsorption on SczOI. (Reproduced with permission from Ref. (811. In this case the lines show a course quite \ , NOTES 203 TABLE 1 Values of Specific Surface Area as Calculated by the BET Method and from Freundlich Monolayer Adsorbent Adsorptive Adsorptive’s Temperature Surface area (m2 g-l) Reference cross section of isotherms (A* molec-l)n (K) BET Freundlich method* monolayer LaFeO, co2 17 432, 480, 531 15.7 13.7 This work LaCoO, co2 17 623, 673 15.5 15.0 (II) LaCoO, 0, 14.1 423, 473, 523, 573, 623 15.5 17.0 (10) SC203 N* 16.2 113, 195, 273, 323 11.8 11.5 (8) SC*03 co 16.8 77, 142, 195 16.4 16.5 (9) SC203 W 20.4 195, 243, 273, 373, 573 12.5 15.3 (7) 4 1 A2 = 10-20 m2. * Adsorption of N, at 77 K; 16.2 AZ for N2 cross section. hand, the breadth of the ir bands observed after adsorption of CO (15) and CO, (I I) on LaCoO,, due to mono- and bidentate car- bonates of different kinds, also reflects sur- face heterogeneity. In the adsorption sys- tems included in this work (Table 1) an exponential decrease of the adsorption heat with coverage was experimentally ob- served, even at low temperatures (113 K for N,/Sc,O, (8); 194 K for N,O/Sc,O, (7)). On the basis of the satisfactory fits obtained for Eq. (l), it can be concluded that the Freundlich model is the one that best de- scribes the physical picture of the adsorp- tion systems reported here. In Table 1, values of specific surface areas (column 5) calculated by the BET method (N2 adsorption at 77 K; 16.2 AZ for Nz cross section), and also values obtained from the Freundlich monolayer (column 6) for the system CO,/LaFeO,, are given. Data calculated from previous tiorks (7- 1 I ) are also included. For Nz, CO, 02, and COz, cross sections (column 3) calculated by Emmett (16) using the density of the adsorbate in the liquid state were taken. For NzO, the cross section given by Living- stone (17) was used. As it can be observed, the values obtained from both methods are similar. The largest discrepancy was found for the N,O/Sc,O, system (22%). The equi- librium data were obtained, in this case, below 130 N me2 and, consequently, low coverages were reached. In order to obtain reliable data, it is advisable to analyze isotherms belonging to a descending branch of the isobar (see the work of Otto et al. (5)). An ascending nonequilibrium branch may yield inaccurate results. The Freundlich equation may have an important potential application in sup- ported oxides for which the active phase was found to be dispersed on the support as a bidimensional layer, when the tempem- tures used in the catalyst preparation are moderately high (18). However, to estab- lish, unambiguously, the scope of validity of Eq. (1) and its usefulness for evaluation of the adsorption monolayer and of specific surface areas, more experimental work will be needed. 1. 2. 3. 4. 5. Emmett, P. and Brunauer, S., J. Amer. Chem. Sot. 59,310, 1553 (1937);J. Amer. Chem. Sot. 62, 1732 (1940). Bridges, J. M., McIver, D. S., and Tobin, H. H., “Actes 2eme Congr. Intern. Catalysis, 1960” (Technip, Ed.), Vol. 2, p. 2161, 1961. Parekh, B. S., and Weller, S. W., J. Canal. 37, 100 (1977); Garcia Fierro, J. L., Mendioroz, S., Pa- jares, J. A., and Weller, S. W., J. Cutal. 65, 263 ( 1980). Thomas, J. M., and Thomas, W. J. “Introduction to the Principles of Heterogeneous Catalysis,” Academic Press, London, 1967. Otto, K., and Shelef, M., J. Curd. 14, 226 (1969); REFERENCES NOTES 6. 7. 8. 9. IO. II. 12. 13. 18, 184 (1970); 29, 138 (1973). Gandhi, H. S., and Shelef, M., J. Caral. 24, 241 (1972); 28, 1 (1973). Rudnitsky, L. A., and Alexeyev, A. M., J. Catal. 37, 232 (1975). Gontilez de Prado, J. E., Moreno, J., and Pajares, J. A., An. Quim. 71, 477 (1975). Tascon, J. M. D., Gonzalez Tejuca, L., Garcia Fierro, J. L., and Pajares, J. A.,An. Quim. Sup. 1, 70 (1978). Gonzalez de Prado, J. E., Gonzalez Tejuca, L., Pajares, J. A., and Soria, J., Act. Cienr. Venez. 24, 189 (1973). Tascon, J. M. D., and Gonzalez Tejuca, L., Z. Phys. Chem. Neue Forge 121, 79 (1980). Tascon, J. M. D., and Gonzalez Tejuca, L., J. Chem. Sot. Faraday Trans. Z 77, 591 (1981). Courty, P. H., Ajot, H., Marcilly, C. H., and Delmon, B., Powder Technol. 7, 21 (1973). Tascon, J. M. D., Mendioroz, S., and Gonzalez Tejuca, L., Z. Phys. Chem. Neue Forge 124, 109 (1981). 14. Voorhoeve, R. J. H., Remeika, J. P., and Trimble, Received November 5, 1980; revised December SO, L. E., Ann. N. Y. Acad. Sci. 272, 3 (1976). 1980 15. 16. 17. 18. Tascon, J. M. D., and Gonzalez Tejuca, L., Z. Phys. Chem. Neue Folge 121, 63 (1980). Emmett, P. H., “Catalysis” (P. H. Emmett, Ed.), Vol. I. Reinhold, New York, 1954. Livingstone, H. K., J. Colloid. Sci. 4, 447 (1949). Roozeboom, F., Fransen, T., Mars, P., and Gel- lings, P. J., Z. Anorg. Al/g. Chem. 449, 25 (1979). MA. A. MARTIN J. M. D. TASC~N J. L. GARCIA FIERRO J. A. PAJARES L. GONZ&LEZ TEJUCA Znstituto de Catdlisis y Petroleoquimica, cszc, Serrano 119, Madrid 6, Spain 

The Freundlich model of adsorption for calculation of specific surface areaS

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