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Cobalt Oxide Supported γ-Alumina Catalyst with Very High Active Surface Area Prepared by Equilibrium Deposition Filtration John Vakros, Christos Kordulis, and Alexis Lycourghiotis* Department of Chemistry University of Patras and Institute of Chemical Engineering and High-Temperature Chemical Processes (ICE/HT), P.O. Box 1414, GR-265 00, Patras, Hellas, Greece Received January 8, 2001. In Final Form: September 6, 2001 The equilibrium deposition filtration technique was properly developed in order to allow the deposition, via adsorption, of a relatively large amount of the Co2+ ions on the γ-alumina surface. This, in turn, permitted the preparation of a Co2+/γ-Al2O3 catalyst with a very high active surface as compared to that achieved by preparing the corresponding catalyst using the conventional pore volume impregnation technique. In the so-prepared EDF catalyst, the joint use of nitrogen adsorption, X-ray photoelectron spectroscopy, temperature programmed reduction, and diffuse reflectance spectroscopy showed that the γ-alumina surface is mainly covered by very well dispersed Co2+ oxo-species with octahedral symmetry.
Introduction The achievement of a very high active surface is the principal target in the preparation of the supported catalysts. The following simple equation relates this parameter with the amount and the dispersity of the active species mounted on the support surface:
active surface ) K(amount of the active species) (dispersity of the active species) (1) where K represents a constant. It is obvious that increase in the active surface should be attempted by increasing both parameters on the righthand side of eq 1. The increase in the amount of the active species can be rather easily obtained by simply increasing its amount in the impregnation solution when dry or wet impregnation is used for the preparation of a supported catalyst. However, using the above-mentioned techniques, the deposition of the active species takes place by uncontrolled precipitation in the step of drying, which follows impregnation, resulting thus to rather large supported crystallites and therefore to relatively low dispersity of the active species. Moreover, in some cases the relatively large supported crystallites cause a partial closing of the pores of the support, which brings about a decrease in the specific surface area of the supported catalyst prepared. In the contrast to the above, extremely small supported crystallites may be obtained by applying the equilibrium deposition filtration technique (EDF), otherwise called equilibrium adsorption. This is due to the deposition mechanism followed. In fact, in this case the deposition takes place through adsorption or surface reaction of the species to be deposited on (or with) the reception sites formed on the support surface. The deposition occurs during the equilibration of the suspension containing the species of the active ion and the support particles. Although EDF results to high dispersity, certain limitations exist concerning the amount of the supported active ion achieved when this technique is used. These are related to the * Corresponding author. Fax:
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nature of the reception sites rendering necessary a brief mention on this nature. On the surfaces of oxidic supports, like γ-Al2O3, SiO2, or TiO2, which are the most important carriers used in catalysis, the reception sites are actually their deprotonated, neutral, or protonated surface hydroxyls. Extensive studies carried out by our group in the past decades (e.g., refs 1-5) have shown that the protonated surface hydroxyls are responsible for the adsorption of the negatively charged species, e.g., MoO42-, whereas the deprotonated ones are responsible for the adsorption of positively charged species, e.g., Co2+ ions. Finally the neutral surface hydroxyls react with the negative species to be deposited, e.g., WO42-. Although the above findings were closely related with “the one site/two pK model” postulated to describe the support surface in electrolyte solutions, they indicated, irrespectively of the model postulated, that the concentration of the reception sites for a given species may be too low at several pH values, limiting thus the amount of the active ion deposited by adsorption or surface reaction on the support surface. Three independents methodologies have been developed by our group, namely, the change of the impregnation pH, the change of the impregnation temperature, and the doping of the surface carrier by several ions, e.g., Li+ and F-, which allowed the regulation and thus the maximization of the reception sites developed on the surface of the oxidic supports (e.g., ref 6). Therefore, the simultaneous achievement of high values for both the dispersity and the amount of the supported active ion allowed the preparation, using EDF, of supported catalysts with relatively high active surface and thus high activity. (1) Lycourghiotis, A. Studies in Surface Science and Catalysis, Preparation of Catalysts VI; Poncelet, G., Martens, J., Delmon, B., Jacobs, P. A., Grange, P., Eds.; Elsevier: Amsterdam, 1994; Vol. 91, p 95. (2) Matralis, H. K.; Bourikas, K.; Papadopoulou, Ch.; Kordulis, Ch.; Lycourghiotis, A. Polish J. Appl. Chem. 1997, XLI, z. 4, 275. (3) Bourikas, K.; Matralis, H. K.; Kordulis, Ch.; Lycourghiotis, A. J. Phys. Chem. 1996, 100, 11711. (4) Vordonis, L.; Spanos, N.; Koutsoukos, P. G.; Lycourghiotis, A. Langmuir 1992, 8, 1736. (5) Spanos, N.; Lycourghiotis, A. J. Chem. Soc., Faraday Trans. 1993, 89, 4101. (6) Lycourghiotis, A. Acidity and Basicity of Solids; Fraissard, J., Petrakis, L., Eds.; NATO ASI series, Kluwer Academic Publishers: London, 1994; p 415.
10.1021/la010040w CCC: $22.00 © 2002 American Chemical Society Published on Web 12/27/2001
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However, there are extremely important catalytic systems where the regulation of the reception sites, obtained using the aforementioned methodologies, was proved to be not sufficient to allow an actual increase in the amount of the supported active ion. In those cases EDF failed to provide supported catalysts with high active surfaces though the dispersity achieved was quite high. Typical examples are the depositions of Co2+ and Ni2+ on the γ-alumina surface using EDF.4,5 With the main intent to prepare CoO/γ-Al2O3 catalysts with very active surfaces, we attempt in this work to modify the EDF procedure in order to maximize the amount of the Co2+ ions deposited on the support surface through adsorption, which guarantees high dispersity for the supported ion. Theoretical Considerations
K6
To suggest the modification of the EDF procedure mentioned above, it is necessary to examine the surface of the support particles in the impregnating suspension as well as the possible equilibria responsible for the Co2+ deposition on the γ-alumina surface. According to the “multisite/one pK model”, which is the most realistic one, for describing the surface of the oxidic supports in electrolyte solutions, the charging of the surface of γ-alumina may be described by the following equilibria.7-14 K1
(AlohOH)0.5- + H+ 798 (AlohOH2)0.5+ K2
(AlohOAltd)0.75- + H+ 798 (AlohOHAltd)0.25+ K3
(AltdOH)0.25- + H+ 798 (AltdOH2)0.75+ K4
((Aloh)3O)0.5- + H+ 798 ((Aloh)3OH)0.5+ K5
((Aloh)2OH) + H+ 798 ((Aloh)2OH2)1+
surface groups.4,5 Although this finding was based on the approximate “one site/two pK model” and not to the equilibria (1)-(5), it seems to us plausible to assume that this is generally valid. This assumption is necessary because the exact Co2+ deposition mechanism on the γ-alumina surface based on the “multisite/one pK model” has not been elucidated. In view of the above one may assume that the hydrated Co2+ ions are adsorbed on the negatively charged surface groups illustrated in the left-hand side of the equilibria (1)-(4) resulting in the formation of Co2+ “surface” species, e.g., [(AlohOAltd)0.42-...Co((H2O)5)1.67+]. Although the Co2+ adsorption mechanism is not yet fully clarified,15-20 the following tentative equilibria could be proposed on the base of the multisite/one pK model
(1)
(AlohOH)0.5- + (Co(H2O)6)2+ 798 [(AlohOH)0.17-...Co((H2O)5)1.67+] + H2O (6) K7
(AlohOAltd)0.75- + Co((H2O)6)2+ 798 [(AlohOAltd)0.42-...Co((H2O)5)1.67+] + H2O (7) K8
(AltdOH)0.25- + (Co(H2O)6)2+ 798 [(AltdOH)0.08+...Co((H2O)5)1.67+] + H2O (8) K9
(2) (3) (4) (5)
In the above equilibria by Aloh and Altd we symbolize the Al3+ surface ions in octahedral and tetrahedral symmetry, respectively, whereas by O and H we denote the surface oxygen and hydrogen involved in the five types of surface hydroxyls and by H+ the hydrogen ions in the electrolyte solution. Finally, the numbers in the exponents indicate the charge of the surface oxygen calculated on the basis of the pure ionic model for the γ-Al2O3 and using the values of the Pauling’s valence for the Aloh3+ and Altd3+. Concerning the equilibria responsible for the deposition of the Co2+ ions on the γ-alumina surface previous studies have shown that the experimentally achieved adsorption isotherms and microelectrophoresis results are sufficiently explained assuming that the deposition takes place exclusively by adsorption of the Co2+ on negatively charged (7) Hiemstra, T.; Van Riemsdijk, W. H.; Bolt, G. H. J. Colloid Interface Sci. 1989, 133, 91. (8) Hiemstra, T.; De Wit, J. C. M.; Van Riemsdijk, W. H. J. Colloid Interface Sci. 1989, 133, 105. (9) Hiemstra, T.; Van Riemsdijk, W. H. Colloids Surfaces 1991, 59, 7. (10) Hiemstra, T.; Venema, P.; Van Riemsdijk, W. H. J. Colloid Interface Sci. 1996, 184, 680. (11) Contescu, C.; Popa, V.; Miller, J. B.; Ko, E. I.; Schwarz, J. A. J. Catal. 1995, 157, 244. (12) Contescu, C.; Contescu, A.; Schramm, C.; Sato, R.; Schwarz, J. A. J. Colloid Interface Sci. 1994, 165, 66. (13) Contescu, C.; Contescu, A.; Schwarz, J. A. J. Phys. Chem. 1994, 98, 4327. (14) Contescu, C.; Popa, V.; Schwarz, J. A. J. Colloid Interface Sci. 1996, 180, 149.
((Aloh)3O)0.5+ + (Co(H2O)6)2+ 798 [((Aloh)3O)0.17-...Co((H2O)5)1.67+] + H2O (9) Inspection of the above equilibria shows that upon Co2+ adsorption the calculated charge of the surface oxygen approaches zero. It should be noted that according to the literature this trend is energetically favorable.21 The resulting charge of the surface oxygen does not permit exclusion of the Co2+ adsorption on more than one neighboring surface hydroxyl. However, taking into account the concentration of the surface hydroxyls and the maximum amount of the deposited Co2+ by adsorption we are inclined to believe that the Co2+ adsorption takes place following the above-mentioned equilibria. It is obvious that in order to increase the extent of the adsorption it is necessary to displace the equilibria (1)(4) to the left by increasing the pH of the impregnating solution. Unfortunately, increasing the pH above a critical value (pH ≈ 7.8) causes the precipitation of Co2+-hydroxy species [Co(NO3)OH]. Therefore, the dispersity of the deposited Co2+ is drastically decreased. Therefore we attempted deposition by adsorption at pH very near, but not exceeding, 7.8. Upon the Co2+ adsorption, the concentrations of the left-hand side surface groups of the equilibria (1)-(4) is decreased. Consequently these equilibria are shifted to the left, resulting in the increase of the concentration of the H+ ions. Therefore, the system is expected to reach equilibrium at a lower pH where a certain amount of cobalt has been adsorbed. (15) Tamura, H.; Katayama, N.; Furuichi, R. J. Colloid Interface Sci. 1997, 195, 192. (16) Towle, S. N.; Bargar, J. R.; Brown, G. E., Jr.; Parks, G. A. J. Colloid Interface Sci. 1999, 217, 312. (17) Tewari, P. H.; Lee, W. Can. J. Chem. 1972, 50, 1642. (18) Tewari, P. H.; Lee, W. J. Colloid Interface Sci. 1975, 52, 77. (19) Huang, C. P.; Lin, Y. T. Adsorption from aqueous solutions; Tewari, P. H., Ed.; Plenum Press: New York, 1981; p 21. (20) Davis, J. A.; Leckie, J. O. J. Colloid Interface Sci. 1978, 67, 90. (21) Bargar, J. R.; Brown, G. E., Jr.; Parks, G. A. Geochim. Cosmochim Acta 1997, 61, 2639.
Co2+ Deposition on γ-Al2O3 by EDF
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Table 1. Code Numbers of Several Co2+/γ-Al2O3 Catalysts Prepared by EDF with Their Content in Co2+, the Initial Co2+ Concentration in the Impregnating Solution, the Amount of the OH-s Ions, and the Impregnation Time initial Co2+ code % preparation in impregnation impregnation no. Co2+ a method OH-s, Mb solution, M time 1 2 3 4 5 6 7 8 9 10d 11d
1.18 1.06 2.20 7.90 10.60 9.80 9.04 7.87 9.44 7.08 9.32
EDF EDF EDF EDF EDF EDF EDF PVIc PVI EDF EDF
1.16 x 107 1.36 x 107 1.27 x 106 5.12 x 106 1. 6 x 106 1.3 x 106 5.55 x 106
0.02 0.02 0.02 0.02 0.07 0.1 0.02
1 day 1 day 1 day 1 week 1 week 1 week 1 week
1.5 x 105 1.9 x 105
0.02 0.02
1 week 1 week
a Grams of cobalt per 100 g of catalyst. b See text. c PVI: pore volume impregnation. d In these specimens the total amount of the base has been added to the impregnating suspension at once, resulting to the formation of the Co(OH)NO3 precipitate. This precipitate was progressively dissolved during the 1-week equilibration.
The above considerations explain the observation that during adsorption pH decreases. It is obvious that in order to increase the adsorbed amount of cobalt it is necessary to neutralize the H+ ions released from the equilibria (1)(4). This may be obtained by modifying properly the typical EDF procedure. Experimental Section (i) Modification of the Typical EDF Procedure and Preparation of Co2+/γ-Al2O3 Catalysts.The typical EDF procedure1,2 followed for the deposition of Co2+ ions on the γ-alumina surface involves the following steps: (i) preparation of an aqueous solution of Co(NO3)2 (Merck analytical grade) of a given concentration (0.02 M) corresponding to the plateau of the Co2+ adsorption isotherms, (ii) impregnation by this solution of the γ-alumina powder (Houdry Ho415, 90 < d
