Langmuir 1992,8, 1318-1324
1318
Mechanism of Adsorption of Tungstates on the Interface of y-AluminalElectrolyte Solutions L. Karakonstantis, Ch. Kordulis, and A. Lycourghiotis* Department of Chemistry and the Institute of Chemical Engineering and Chemical Processes at High Temperatures, P.O. Box 1239, University Campus, GR-26110, Patras, Greece Received August 20, 1991. In Final Form: January 27, 1992
The mechanism of adsorptionof tungstates on the surface of y-alumina was investigated using adsorption experiments, potentiometric titrations, microelectrophoretic mobility measurements, and electronic spectroscopy. It was found that under our experimentalconditions the deposition of Wm ions takes place via adsorption of the tungstate ions on energetically equivalent sites of the inner Helmholtz plane (IHP) of the electricaldouble layer developed between the impregnatingsolutionand the surfaceof the support. The protonated and neutral surface hydroxyls of the support and not the deprotonated hydroxyls should be mainly responsible for the creation of the adsorption sites. Moreover, between the adsorbed tungstate ions considerable lateral interactions are exerted. Finally, it was inferred that the adsorption constant of monomeric (W04*-) or oligomeric (H,W,O,"- where 0 < x < 5,l< y < 6 , 4 < z < 22)tungstates should be higher compared to the adsorption constants of polymeric tungstates. The application of the 'SternLangmuir-Fowler" equation to our experimentalresults allowed for the determination of the adsorption constant, the energy of lateral interaction, and the surface concentrationof tungstate ions, corresponding to the saturation of the surface, at various pH values. The value of the latter parameter was found to increase with decreasing pH. The experimentalresults of this work showed that in the pH range between 3.0and 4.0supported y-alumina catalystswith extremely high W6+content may be prepared by adsorption. Introduction The deposition of species containing the active ions on the surface of oxidic supports (a/Y-A1203,SiOz,TiOz, etc.) is presumably one of the most critical steps in the preparation of supported catalysts. Numerous studies in the field of catalystpreparation have shownthat deposition follows one or more of the followingroutes: (i) uncontrolled precipitation, mainly taking place during drying following impregnation; (ii) controlled precipitation, taking place during impregnation (deposition-precipitation); (iii) adsorption, taking place during impregnation; (iv) chemical reaction with the neutral surface hydroxyls of the oxidic supports (grafted catalysts). Uncontrolled precipitation prevails when pore volume impregnation is used. As it results in the formation of large supported crystallites and thus gives supported catalysts with poor dispersion, pore volume impregnation followed by drying is only used when large amounts of inexpensive active ions should be mounted on the support surface. Controlled precipitation could be achieved by adding several substances in the impregnating suspension. Relatively small supported crystallites are obtained using deposition-precipitation and this new technique is quite promising. Extremely small supported crystallites are obtained when adsorption or chemical reaction is the predominant deposition process. The former predominates when relatively dilute aqueous electrolyte solutions are used for impregnation and the removal of the aqueous phase takes place through filtration. Chemical reaction predominates when organometallicor carbonylspecies are used to deposit the active ion. Although in both cases high active ion dispersion is obtained, these techniques have serious weaknesses. The procedure required for anchoring is usually quite complicated and the extremely high active surface obtained, being rather unstable, usually diminishes considerably upon calcination. These are presumably the
* To whom all correspondence should be addressed.
mainreasonsfor whichgrafted catalystshavenot yet found industrial applications. The experimental procedure followed in the preparation of catalysts by adsorption is very simple and in this respect is better compared with anchoring. The main disadvantage of adsorption is that it usually results in catalysts with low active surface though the dispersion achieved is high. This result is mainly due to the fact that the density of the adsorption sites of the industrial oxidic supports is generally low, thus limiting the amount of the species deposited. In our laboratory, we have undertaken a long-term research program aiming at finding the appropriate methodologies for the regulation of the concentration of the surface groups considered to be the active adsorption sites of a number of industrially used oxidic supports including 7-A1203,SiOz, and TiOz. In the course of this program it has been prove that not only the traditional pH change but also temperature changes of the impregnating solution or doping the support may be used for the regulation of the density of the surface The assumptionsadopted in the aforementioned studies were that the adsorption of anionic species on the oxide surfaces is predominantly electrostatic and that the protonated (deprotonated) surface hydroxyls were mainly responsible for the creation of the adsorption sites for negative (positive) species. A test of these assumptions required the knowledge of the mechanism of adsorption of species containing the catalytically active ions on the surface of oxidic supports. Unfortunately, this mecha(1)Vordonis, L.; Koutaoukos, P. G.; Lycourghiotis, A.J. Chem. SOC., Chem. Commun. 1984, 1309. (2) Vordonis, L.; Koutsoukos, P. G.; Lycourghiotis, A.J. Catal. 1986, 98, 296.
(3)Vordonis, L.; Koutaoukos, P. G.; Lycourghiotis, A. J.Cutul. 1986,
101. 186. ~.~
(kjkkratopulu, K.;Vordonis,L.;Lycourghiotis,A. J.Chem. Soc.,Faraday Trans 1 1986,82, 3697. ( 5 ) Vordonis, L.; Koutsoukos, P. G.; Lycourghiotis, A.Lungmuir 1986, 2, 281.
(6)Akratopulu, K.; Vordonis,L.; Lycourghiotis,A.J.Chem. Soc.,Faraday Truns 1 1990,86, 3697.
(7) Akratopulu, K.; Vordonis, L.; Lycourghiotis, A.J.Catal. 1988,209,
41.
0743-7463/92/2408-1318$03.00/00 1992 American Chemical Society
Adsorption of Tungstates on r-Al2O3
nism remained rather unclear. Recently, we developed a methodology, based on the mathematical analysis of the adsorption isotherms as well as on the combined use of potentiometric titrations and microelectrophoresis, to elucidate the mechanism of adsorption from electrolyte solutions. This methodology, applied to the adsorption of oxomolybdenum species on the y-alumina surface, allowed the elucidation of the adsorption mechanism.&1° In the present work, using a similar approach, we attempted to elucidate the mechanism of adsorption of tungstate ions on the y-alumina surface. From the basic research point of view, there have been few studies devoted to W03/y-A1203.11-13 The main objectives, in our study, were the following: To demonstrate that the methodology established may be used with some modifications to investigate the adsorption of all species containing catalytically active ions and to the elucidation of the mechanism of adsorption in any catalyticallyimportant system. Moreover, in this study we tried to increase the active surface of the W03/y-Al203 catalysts by increasing the density of adsorption sites on the y-alumina surface and thus the amount of the adsorbed H,WyOzn- species. It should be noted that monomeric tungstate species (WO& are predominant at pH > 8 and low concentration values while oligomeric tungstate species (H,WyOzn-where 0 < x < 5 , 1 < y < 6, 4 < z < 22) at pH < 8 and higher concentration values. Experimental Section Materials. y-Alumina powder, 80-150 mesh, impregnated with bidistilled water, was used as support after drying at 120 O C for 2.5 h and air-calcination at 600 "C for 12 h. The powder used was obtained by crushing Houdry Ho 415 y-alumina ex= 123 m2.g-l, water pore volume 0.45 cm3.g-l). trudates Ammonium tungstate ( ( N H ~ ) I ~ W I ~ O ~ I -99.999 ~ H Z% O ), purchased from Ventron has been used for the preparation of the solutions used in adsorption, potentiometric titrations, and microelectrophoresis experiments. Ammonium nitrate stock solutions were prepared from the respective solid (Riedel de Haen, 99%). Standard nitric acid and potassium hydroxide solutions were prepared from concentrated standards (Merck, Titrisol) by dilution. Adsorption Experiments. Equilibrium adsorption experiments were done at 25.0 f 0.1 O C , ionic strength 0.16 M adjusted with the addition of ammonium nitrate and over a pH range between 3.5 and 10.0. In each experiment a volume of 0.015 dm3 of tungstate solution of various concentrations ranging between 1 X 10-3 and 2.5 X 10-2mol-dm-3was placed in a small polyethylene vial, thermostated in a double-walled, water-jacketed vessel at 25.0 f 0.1 O C . The pH of the solution (pHh) was adjusted to the desired value by adding small amounts of standard nitric acid or potassium hydroxide solutions. pH measurements were performed using a glass/saturated calomel electrode (Metrohm) standardized before use by NBS standard buffer solutions. The amount of HN03or KOH was small enough to avoid any change in the ionic strength of the solution. The selected value of the ionic strength was such that the tungstate solutions were stable at least for 15 days. Preliminary experiments showed that at ionic strength values exceeding 0.16 M, the solutions were not stable at relatively low pH values (pH c 4). (8)Spanos, N.;Vordonis, L.; Kordulis, Ch.;Lycourghiotis, A. J. Catal. 1990,124, 301. (9) Spanos, N.; Vordonis, L.; Kordulis, Ch.;Lycourghiotis, A. J. Catal. 1990,124, 315. (10) Vordonis, L.; Koutsoukos, P. G.; Lycourghiotis, A. Colloids Surf. . 1990, 50, 353. (11) Le Page, J. F. Catalyse de Contact; Techinip.: Paris, 1978; p 441. (12) Salvati, L., Jr.; Makovsky, L. E.; Stencel, J. M.; Brown, F. R.; Hercules, D. M. J. Phys. Chem. 1981,85,3700. (13) Brady, R. L.; Southmayd, D.; Contesku, C.; Zhang, R.; Schwarz, J. A. J. Catal. 1991, 129, 196.
Langmuir, Vol. 8, No. 5, 1992 1319 A 0.001 dm3 sample was withdrawn from the solution, to be used as reference and, next, a quantity of the support equal to 0.05 g was suspended in the solution and the suspension was equilibrated for 20 h at 25.0 f 0.1 O C , under constant stirring. Preliminary experiments showed that adsorption reached equilibrium after 6 h. Following equilibration, the suspension pH was measured (pHfm)and then the suspension was filtered. The filtrate was analyzed spectrophotometrically for total tungsten at 403 nm.14 Equation 1 was used to determine the surface concentration of WvI, r (mo1.m-2)
r = [v(c,- c,)iiws
(1) where C,, C,, V, W ,and S represent respectively the W(V1) total concentration before and after adsorption (moledm-3), the suspension volume (dm3), the weight (g), and the specific surface area of the support (m2*g1). Potentiometric Titrations. Potentiometric titrations were done at 25.0 f 0.1 O C and ionic strength 0.16 M in order to study the influence of the adsorption of tungstate ions on the ability of the support to adsorb H+ions over the entire pH range. Details concerning the setup and the procedure followed have been reported el~ewhere.~J According to this method, the electrolytesolution or suspension was titrated using an acid and the pH was recorded as a function of the titrant volume. The technique allows for the determination of the amount of the hydrogen ions consumed (H+,) for the protonation of the surface deprotonated or neutral hydroxyls (see equilibria 5) of the support as well as in the equilibria taking place in the solution. The following equation, derived by applying simple H+ mass balance, was used H+, = H+, - H',
- H+,
(2) where H+ad,H+,, and H+, represent the amounts of the hydrogen ions added to the solution or suspension, accumulated in the solution or suspension decreasing its pH, and consumed in the equilibrium HzO H+ + OH-, respectively. These quantities may be determined using the following relationships: H+ad= C AV H+, = ( V + AV)CH+ - VCH+,h
(3)
H+, = VCOH_'~~ - (v AV)Co,In the above relationships by C, V, AV, CH+,COW, and COH-,~,, we denote, respectively, the concentration of the titrant (HN03),the volume of the solution or suspension before titration, the titrant volume increment, the concentration of the hydrogen and hydroxyl ions in the solution or suspension after each acid addition, and the corresponding concentration before adding titrant. Microelectrophoresis. Microelectrophoreticmobility measurements were done in order to determine the electrokinetic charge, i.e. the charge at the shear plane of the double layer of the suspended support or catalyst particles, in the presence and in the absence of tungstates, over the entire pH range between 3.0 and 11.0. The measurements were done at 25.0 f 0.1 "C and ionic strength equal to 0.01 M. Full details have been reported elsewhere.* Electronic Spectra of the Tungstate Solutions. The electronicspectra of tungstate solutionsof concentrationsbetween 5X mol~dm-~ were recorded in the range of and 2 X 400-180 nm using a Shimadzu-Bausch & Lomb Spectronic 210 UV spectrophotometer.
Results and Discussion Adsorption versus Precipitation. The present study is based on the assumption that the principal deposition mechanism is adsorption. A first, very strong, evidence favoring this view is the shape of the isotherms achieved (Figure 1). However, in order to further investigate the (14) Marczenko, Z. Spectrophotometric Determination of Elements; Ramsay, C. G., Translation Ed.; John Wiley & Sons: New York, 1970; p 569.
1320 Langmuir, Vol. 8, No. 5, 1992
Karakohstantis et al. SOLID
"
SOLUTION
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8 3 Potentials
c,
a 2
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PI=. I
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2.0
Charge densities
2
/ mol dm-'
Figure 1. Surface concentration of W(V1) as a function of the equilibriumW(V1) concentrationobtained at various pH values of the impregnating solution: T = 25 "C, I = 0.16 M adjusted with NH4NO3. The pH values are indicated. Table I. Values of r,,, Achieved at Constant p H (3.5-4.2). Constant C,/[total surface area of the support particles in the suspension], (2-2.2) X 1od mol m-*, and the Corresponding C, Values C,, mol L-1 uptake (rd,atoms nm-2 5.43 x 10-3 5.368 5.632 6.92 x 10-3 9.56 X lW3
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Figure 2. Simplified representation of the triple layer model.
The OHP and IHP represent respectively the outer and inner Helmholtz planes (ref 9).
5.410
Table 11. Correlation Coefficient (r)and Percentage Standard Errors of the Slope (ob, %) and Intercept (u., %) o f E ~ u a t i o n 4 f o r E iOandE=O t E#O E=O DH r ab, % Un, % r ab. % ab, % 0.9992 1.79 1.18 0.9916 3.56 5.84 4.28 4.23 0.9991 1.89 1.32 0.9930 5.28 3.01 0.9945 2.46 1.22 4.29 1.64 4.54 0.9985 5.10 0.9989 1.81 0.73 0.9506 12.40 6.30 6.20 0.9992 1.66 0.70 0.9843 7.33 3.94 5.92 2.14 6.41 0.9914 2.96 1.95 0.9887 6.71 0.9995 1.72 0.87 0.9946 4.26 2.50 6.92 0.9978 3.01 1.07 3.86 1.42 0.9960 6.93 2.62 7.31 0.9976 2.06 1.30 0.9901 8.55 0.9993 1.69 3.34 0.9749 10.21 90.70 9.95 0.9976 4.04 9.09 0.9909 7.83 86.47
PH Figure 3. Variation of the electrokinetic charge of y-alumina and WO&-A1203 catalyst with the pH of the Suspension at 25 O C : (a) y-alumina, 0.16 mol dm" NHdNOs solution; (b) WO9/ y-AlzO3 catalyst, 0.16 mol dm-3NH4NO3 solution; (c) y-alumina, ammonium tungstate solution C, = 1 X 10-3 Ww mol dm-3, I = 0.16 mol dm-3 NHf103.
validity of this assumption we have determined the value of r at various values of C, or C , but keeping constant the ratio C,/(total surface area of the support particles in the suspension). It is plausible that if precipitation contributed to the whole deposition,the uptake should be increased with C, or . ,C However, as may be seen in Table I the uptake remained practically constant regardless of the initial tungstate concentration, thus corroborating the assumption that under our experimental conditions the deposition takes place by adsorption. Mechanism of Adsorption of Tungstates on yAlumina. The elucidation of the mechanism of adsorption requires investigation on the following pointa: (i) determination of the part of the double layer, developed between the suspended support particles and the impregnating solution, where the adsorbed H,WyOzn-ions are located; (ii)characterizationof the features of adsorption, is it localized or nonlocalized; (iii) existence of lateral interactions exerted between the adsorbed species; (iv) identification of the surface groups responsible for the creation of adsorption sites; (v) the kind of the H,WyOznspecies adsorbed. Part of the Double Layer Where the Adsorbed Species Are Locatad. Qualitative Approach. It ia wellknown that the "triple layer model" (Figure 2) describes the structure of the double layer developed between the
suspended yalumina particles and the impregnating s o l ~ t i o n .By ~ ~ adoption ~ of this model the following possibilities exist concerning the part of the double layer where the adsorbed tungstates are located (i) on the surface; (ii) on the inner Helmholtz plane (IHP); (iii) on the diffuse part of the double layer. Figure 3 illustrates the variation of the electrokinetic charge density of the support suspension in the presence and absence of tungstate ions (curvesc and a, respectively) as well as of a W03/y-A1203catalyst suspension (curve b). It should be noted that the a,+. values refer to the total charge from the surface to the shear plane (Figure 2). It may be observed that the presence of the H,W,O,n- ions in the suspension of the support caused a shift of the electrokinetic charge rendering it negative over the entire pH range studied. Obviously this finding precluded the location of the negative tungstate ions as counterions in the diffuse part of the double layer because in that case the charge from the surface to the shear plane, namely the electrokinetic charge, should be positive. Two possibilities remain, therefore, for the H,WyOznions: (i) they may be located on the surface of the support, (ii)in the IHP. Surface bonds between the tungstate ions and the surface hydroxylsof the support should be formed in case i in a way similar to those formed after calcination in the W03/y&03 catalysta. In such a case similar elec-
I
." 2
4
6
8
10
12
Langmuir, Vol. 8, No.5,1992 1321
Adsorption of Tungstates on y-AlzOs trokinetic curves should be expected. Our experimental results show that this is not the case (compareFigure 3b with Figure 3c) precluding thus the location of the H,WyOzn-ions on the support surface. In conclusion, the microelectrophoretic mobility measurements at various pH values strongly suggest that the H,WyOzn- ions are located on the IHP of the double layer. The location of the tungstate ions on the IHP explains the fact that the electrokinetic charge is negative even at pH values lower than the PZC of the support (5.3) where its surface is positive.2J Part of the Double Layer Where the Adsorbed Species Are Located. Quantitative Approach. The title problem may be quantitatively investigated by analyzing the adsorption isotherms obtained at various pH values (Figure 1). It may be seen that at pH = 8.5 the adsorption isotherm obtained is an S-shaped curve, implying "Langmuir type" adsorption on energetically equivalent adsorption sites but with lateral interactions being exerted between the adsorbed species. Although the S character of the isotherms obtained in the other pH values is not obvious at first glance, statistical analysis presented in the next paragraph confirmed that the curve shape is the same. The analysis of the isotherms should take into account the finding that the adsorbed tungstate ions are located on the IHP and the S character of the isotherms. It is therefore plausible that the subsequent analysis should be based on the following assumptions: (i) The tungstate ions are specificallyadsorbed being located on the IHP. (ii) The adsorbed tungstates are located on energetically equivalent sites. (iii) Lateral interactions are exerted between the adsorbed species and that E is the energy of these interactions. (iv) More than one kind of tungstate ion may in principle be adsorbed. Considering the followingequilibrium for the adsorption H,O(IHP) + H,W,O,"-(b) == H,O(b) + H,W,O,"-(IHP) where IHP and b stand for the species in the inner Helmholtz plane and in the bulk solution, as well as the aforementioned assumptions, the "Stern-Langmuir-Fowler" equation, similar to that derived by de Keizer,lSmay be derived i / r = i/r,
+ i/[r,kc,
exp(Er/r,RT)I
(4)
where
k = Zi[(ai/55.5)exp(-nF@/RT- AGo,,i/RT)I In the above equation ni, q b , and AGo,,i represent, respectively, the charge of the ith kind of the tungstate ion, the potential at the IHP and the contribution to the AGo,a,i of the chemical interactions between the adsorbed tungstates and the support surface. By ai we denote a coefficient relating the total equilibrium concentration with that for the species i. It may shown that eq 4 is also valid even in the case where i = 1 . 8 1 ~Therefore, it cannot be used in order to investigate whether one or more kinds of tungstates are adsorbed on the y-alumina surface. It was found that eq 4 described satisfactorily our experimental results. Typical examples such as those illustrated in Figure 4 confirmed quantitatively the above mentioned findingthat the adsorbed tungstates are located on IHP. Characteristics of Adsorption and Lateral Interactions Exerted between the Adsorbed Species. The excellent fitting of the experimental data achieved by eq 4 demonstratedthat the adsorption is localized in the sense (15) de Keizer, A.; Lyklema, J. J. Colloid. Interface Sci. 1980,75,171.
1.0
I
0.0 L 0
I
I
15
30
1
45 0
10' [C,exp(hr/RT)]-'
I 8
4
/mol-'
dm'
Figure 4. Reciprocal surface concentration of W mas a function of (l/C,) exp(XrlR7'): (a) pH 3.56, T = 25 O C ; (b) pH 4.23, T = 25 O C ; (c) pH 7.31, T = 25 O C ; (d) pH 6.20,T = 25 O C ; (e) pH 5.10, T = 25 O C . The solid lines represent the values calculated using eq 5.
of the location of the adsorbed ions at the IHP and that the tungstate ions are adsorbedon energeticallyequivalent sites. Moreover, the fitting obtained demonstrated that lateral interactions between the adsorbed tungstate ions exist. At this point it might be argued that the S character of the isotherms assumed above is not apparent and presumably these are of the L type described by the simple Langmuir equation derived from eq 4 for E = 0. In order to investigate this point, we calculated the correlation coefficients, as well as the percentage standard errors of the slope (q,,% ) and intercept (ua, % ) for all adsorption isotherms, obtained at various pH values, assuming E # 0 or E = 0. The values obtained are compiled in Table 11. Inspection of this table shows that eq 4 with E # 0 describes better our experimental data, thus corroborating the forementioned assumption. Surface Groups Responsible for the Creation of Adsorption Sites. It is well-known the charging mechanism of the y-alumina particles in electrolyte solutions2p3 may be described by equilibria Ki AlOHC * AlOH + H: - Kz AlOH * AlO-+ H,+ (5) where ZOH:, XOH, and EOrepresent the protonated, neutral, and deprotonated surface hydroxyls. H,+ and Hb+ denote hydrogen ions on the surface of the support and in the bulk solution, respectively. Concerning the mode of deposition of the H,WyOzn-ionson the y-alumina two possibilities, analogous to those reported for the adsorption of the molybdates on y-alumina, exist: (i) deposition on the surface by chemical reaction of the species mentioned with the neutral hydroxyls;1e18 (ii) deposition by adsorption of the tungstate ions on sites created a t the IHP of the doublelayer.l*zl In the previous paragraphs we showed that the first case should be (16) Van Veen, J. A. R.; De Wit, H.; Emeis, C. A.; Hendrike, P. A. J. M.J. Catal. 1987,107,579. (17) Van Veen, J. A. R.; Hendriks, P. A. J. M.Polyhedron 1986,5,75.
(18) Jerriorowski, H.; Knozinger, H. J. Phys. Chem. 1979,83, 1186. (19) Wang, L.; Hall, W. K. J. Catal. 1982, 77, 232. (20) Kasztalan, S.; Grimblot, J.; Bonnelle,J. P.; Payen, E.; Toulhoat, H.; Jacquin, Y.Appl. Catol. 1983, 7, 91. (21) Caceree, C. V.; Fierro, L. G.; Agudo, A. L.; Blanco, M.N.;Thomas, H. J. J. Catal. 1985, 95,501.
1322 Langmuir, Vol. 8,No.5,1992 AlOH
/
Karakonstantis et al.
sites nm-' 4.23
N
'
5 -
-
4 -
c,
6
' 2 '
la,55
01 0.0
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,
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0
2 -
/
9.910
6.0 ,\;9.95
,
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0.5 1.0 1.5 2.0 2.5 A ~ O - or A ~ o H / ~ +sites nm-'
Figure 5. Saturation surface W(V1) concentration achieved at various values of pH as a function of the concentration of the protonated (e),neutral (m), and deprotonated (A)surface hydroxylsregulated by varying the pH of the impregnating solution. The values of pH are indicated.
precluded. In order to investigate which one of the surface groups is mainly responsible for the creation of the adsorption sites in the IHP, we correlated the concentration of the ZOH;, =OH, and ZO-groups calculated at various pH values2-swith the values of rm determined at the same pH from the respective adsorption isotherms (Figure 5). Inspection of Figure 5 showed that the value of rmdecreased with the concentration of the negatively charged groups thus precluding their participation to the creation of the adsorption sites. On the other hand above the pH corresponding to the PZC (5.3) increase in the concentration of both AlOH2+ and AlOH groups caused an increase in the value of rm. At pH values lower than the PZC, rmincreased with the concentration of AlO%+, whereas it decreased with the concentration of the AlOH groups. Careful observation of the I?,, vs AlOH; and I',vs;b;IOH curves suggested that both groups are involved in the creation of the adsorption sites. In fact, at very high pH values a considerable increase of the value of rmas pH decreases was observed, even though the change in the concentration of ZOH; was very small. In the other extreme of the pH range, rm increased, although the concentration of E O H decreased with pH. To confirm this hypothesis we correlated the sum of the concentrationofzOH;andzOHgroupswithI', (Figure 6). From the data presented it may be inferred that both the =OH; and the E O H groups participated in the creation of the adsorption sites. This conclusion was not applicable to the adsorption of oxomolybdenum species on the y-alumina surface, where it has been shown that the positively charged and not the neutral surface hydroxyls were mainly responsible for the creation of the adsorption sites. The Kind of the H,WyOs* Species Adsorbed. An important point to investigate is the kind of the H,WyOznspecies adsorbed on the IHP. According to the literature more than one kind of tungstate ions is present in the impregnating s o l ~ t i o n . ~ The ~ - ~ ~tungstate speciation problem under the experimental conditions at which the adsorption isotherms were obtained was solved by ob(22) Llambias, F. J.;Salvatierra,J.; Bouyssieres, L.; Escudey, M. Appl. Catal. 1990,59, 186. (23) Vermaire, D. C.;Van Berge, P. C. J. Catal. 1989, 116, 309. (24) Mulcahy, F. M.; Fay, M. J.; Proctor, A.; Houalla, M.; Hercules, D. M. J. Catal. 1990, 124, 231.
6.5 7.0 7.5 8.0 AlOH f ZOH+2 / sites nm-'
Figure 6. Saturation surface W(V1) concentration obtained at various pH values as a function of the sum of the concentration of the protonated and neutral surface hydroxyls regulated by varying the pH of the impregnating solution. The values of pH are indicated.
0
/ nm Figure 7. Electronic spectra of tungstate solutions used in the adsorption experiments and of the NazWO, aqueous solution W(V1) used as reference: (a) pH 5.5, T = 25 O C , C, = 2 X mol dm-3 prepared from ammonium tungstate; (b)pH 9.5,T = 25 O C , C, = 2 X 10-3 W(V1) mol dm-3 prepared from ammonium tungstate; (c) pH 5.5, T = 25 OC,C, = 5 X lo-' W(VI) mol dm4 prepared from ammonium tungstate; (d) pH 5.5, T = 25 O C , C, =2x W(V1) mol dm4 prepared from NaZWO,. h
taining the electronic spectra of tungstate solutions at conditions of temperature and pH corresponding to those in which adsorption on y-Al203 was investigated. The spectra of ammonium tungstate solutions as well as of Na2W04 solution used as reference are shown in Figure 7. According to the literature2s126a peak centered at about 210 nm indicates tetrahedral W(V1) in WO*2-ions. The spectrumof NazWO4 isa typical example (curve 7d). Peaks at wavelengths higher than 210 nm are attributed to polymeric tungstates with various degrees of polymerization.26p26An inspection of Figure 7 curves a-c clearly shows that more than one kind of H,WyOzn- species is present in the impregnating solutions under our experimental conditions. A question raised in this point is related with the adsorption constant of the various H,WYOzn-ions present in the impregnating solution. Are these values similar? Is there any preference in the adsorption? Concerning the adsorption of the molybdates on y-alumina some authors claimed that these are transformed into Mood2before adsorption.18 In spite of the fact that it was not (25) Wells, A. F. Structural Inorganic Chemistry, 4th ed.; Clarendon Press: Oxford, 1975; p 430. (26) Tytko,K.H.; Glemser, 0. Adu. Znorg. Chem. Radiochem. 1976, 19, 239.
Langmuir, Vol. 8,No. 5, 1992 1323
Adsorption of Tungstates on yAl203
r 2.00 I M
7
0.0
0.5
1.0 C. 10'
1.5
/
2.0
2.5
mol dm-'
Figure 8. Variation in the pH measured before, pHb, and after, pHf, adsorption with the concentration of the W(V1) in the solution before adsorption.
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8
9
PH Figure 9. Potentiometric titration curves of selected solutions and suspensions: (1) y-AlzOs/NH1N03; (2) NH,NOs; (3) ~-A~~O~/NHINO~-H,W,O,~-; and (4)NH4N03-H,WyOzn-.
possible to be proven experimentally though the change of the pH of the impregnating solution before ( p H d and after (pHf)adsorption, with Mow concentration resulted to the conclusion that the adsorption constant for monomeric molybdates is higher than the adsorption constants of the polymeric molybdatese8The same conclusions may be drawn for the adsorption of the tungstate ions as similar (pHh, pH$ vs C, curves were obtained compared with those obtruned in the case of the molybdates. A typical example is illustrated in Figure 8. The use of potentiometric titrations allowed for the further investigation of the aforementioned question. Figure 9 illustrates the potentiometric curves of selected solutions [NHdNOs (curve 21, NH4N03-H,WyOzn- (curve 4)] and suspensions [y-A1203/NHrNO3 (curve 11, y-A12091NH4N03-H,W,0zn- (curve 311. By substracting curve 2 from curve 1and curve 4 from curve 3, we obtain, respectively, the curves (a and b) illustrated in Figure 10, assuming that the hydrogen consumption due to the hydrolysis of the N&+ ions (NH4++ HzO * NH40H + H+) is not considerably changed because of the presence of the support particles. This assumption is reasonable as preliminary experiments showed the absence of considerable specificadsorptionof the N&+ ions. In fact, similar potentiometric curves were obtained for the y-alumina suspensions in NH4NOs and KN03 aqueous solutions. On the contrary, the fact that a portion of the H,WyOzn- ions is specifically adsorbed on the IHP would, under certain conditions, result in considerable disturbance of the polymerization equilibria [H,WyO," + H+ +H,gWyO,." + H20, where y < y' and z < 2'3 taking place in the solution and consequently in a significant change of the hydrogen
X
3
4
5
6
7
8
PH Figure 10. Amount of hydrogen ions consumedon the yalumina surface as a function of the pH of the suspension in the absence (curve a) and presence (curve b) of the H,WYOzn-ions.
ion uptake for these reactions. Finally, the presence of the H,WyOz"- ions in the IHP is expected to have some influence on the hydrogen ion uptake due to the surface equilibria (5). In view of the above we may denote by H + Aand ~ H+hithe hydrogen ions consumed in the surface equilibria (5) in the absence and presence of the H,WyO," ions, respectively, whereas by H+wi and H+wi we may denote the hydrogen ions consumed in the forementioned tungstate equilibria in the absence and presence of the support, respectively. From the above it may be seen that parta a and b of Figure 10 illustrate the variations of the H+Ai and H+Ai + H+ws - H+wi, respectively, with pH. These figures show that H+h > H+hi + H+wfi- H+wiover the entire pH range. The presence however of the negative ions in the IHP is expected to promote the protonation of the NOH and the formation of additional AlOH+groups necessary to stabilize the H,WyOzn- through ion pair formation. Analogous ion pair formation has been reported in the adsorption of the molybdate ions on the surface of y-alumina and titania.8*27 It is therefore plausible to assume that H+fi > H+h which, in combination with the above mentioned relationship i.e. H+A~ > H+~ii + H+wii- H+wi,yields H+wi > H+wii. Considering the aforementionedtungstate equilibria, it may be easily concluded that in order for the above inequality to be valid, w 0 d 2 - or H,WyOZ"- ions with low x , y, and z values should be selectively adsorbed. Increase of the Amount of W(V1) Deposited through Adsorption by Changing the Solution pH. The values of rm,k, and E determined at various pH values by applying eq 5 to our experimental data are summarized in Table 111. Table I11includes also the values of the percentage loading corresponding to rm. It may be seen that the W(V1) loading increased with decreasing pH in agreement with the mechanism suggested in the present work. At pH values in the range between 3.0 and 4.0, supported y-alumina catalysts with extremely high W(VI) content (=21% WO3) may be prepared by adsorption. Since adsorption is related with high dispersity, the catalysts prepared in the pH range between 3.0 and 4.0 are expected to have high active surfaces and this has been experimentally ~ o n f i r m e d However, . ~ ~ ~ ~ ~a detailed physicochemical study (including the determination of the active surface by various methods) of the oxidic and sulfided WIy-Al203 catalysts prepared by adsorption at various pH values is now in order. Concerning the values of k and E, it may be seen that both varied rather randomnly with pH. This was antic(27) Spanos, N.; Matralis, H. K.; Kordulis, Ch.;Lycourghiotk, A. Submitted for publication.
Karakonstantis et al.
1324 Langmuir, Vol. 8, No. 5, 1992
Table 111. Compilation of the Values of,,'I the Corresponding Values of Percentage Loading as Well as the Values of Adsorption Constants and Energy of Lateral Interactions Determined at Various pH's
3.56 4.23 4.54 5.10 6.20 6.41 6.71 6.92 7.31 8.55 9.95
loading % '
energy of lateral interactions E, kJ mol-' 3.59 f 0.06 5.69 f 0.10 21.22 f 0.37 (1.06f 0.04)X lW
uptake (rm),
RH atomsnm-2
5.34f 0.11 4.26 f 0.07 4.44 f 0.02 3.91 f 0.03 3.21 f 0.06 3.42 f 0.03 3.29 f 0.04 2.82 f 0.04 1.24 f 0.03 0.16 f 0.01
WOdg
adsorption k, constant mol-'
20.18 f 0.42 16.79 f 0.27 17.38 f 0.06 15.62 f 0.11 15.15 f 0.27 13.94 f 0.12 13.48f 0.12 11.38f 0.16 5.55 f 0.13 0.74 f 0.04
(1.28f 0.05)X lW (1.76f 0.07)X 10' (2.60f 0.02)X lW (1.98f 0.03)X 103 (3.91f 0.24) X 103 (2.11f 0.04)X 103 (2.43f 0.08)X 103 (1.93f 0.08)X 103 (9.73i 0.17) X 10' (7.86f 0.40)X 10'
ofcat.
-0.72 f 0.08 -0.90f 0.01
*
6.83 0.02 4.15 f 0.03 1.60 f 0.03 2.34 f 0.02 1.35 0.02 3.12f 0.04 8.72 f 0.26 8.86 f 0.60
*
ipated because both parameters depend on the relative concentration of the various tungstate ions in the impregnatin solution (see for instance the expression defining &) which varies in a rather complicated manner with pH.
Conclusions The following conclusions may be drawn from the present work. (i) The methodology recently reported to investigate the mechanism of adsorption of the molybdates on y-alumina may be applied, with some modifications in the investigation of the adsorption mechanism of any negatively charged species containing the catalyt-
i c d y active ions. (ii) The deposition of W(V1) on the y-alumina surface takes place, through adsorption of the H,WyOzn-ions at 25 "C, at ionic strength 0.16 M in NH4NOa, and over the pH range between 3.0 and 10.0. (iii) The adsorbate species are located on energetically equivalent sites, located at the inner Helmholtz plane of the double layer,developed between the impregnatingsolution and the support surface. (iv) The creation of these sites should be attributed to the protonated and neutral surface hydroxyls of y-alumina and not to the deprotonated hydroxyls. (v) The adsorbed species are located exclusively at the IHP and lateral interactions are exerted between them. (vi) The values of the adsorption constants and of the energy of lateral interactions, determined by applying the 'Stern-Langmuir-Fowler" equation to our results, varied rather randomly with pH and this was attributed to the complicated way the relative concentration of the various tungstate species in the solution depend on pH. (vii)From the change of pH with the solute concentration before and after adsorption, as well as from potentiometric titration curves,it may be suggestedthat the adsorption constant of monomeric tungstate (w04'-) or H,WyOznions with relatively low x , y, and z values should be higher compared to the H,W,Ozn- with relatively high x , y , and z values (polymerictungstate ions). (viii)The Wwloading increases as pH decreases. In the pH range between 3.0 and 4.0supported y-alumina catalysts with extremely high Wvr content may be prepared by adsorption. Registry No. &03,1344-28-l;WO3,1314-35-8; WOd2-,1431152-5;tungstate, 12737-86-9.
