10152
J. Phys. Chem. 1993,97, 10152-10157
Chemistry of Surface Tungsten Species on W03/A1203 Composite Oxides under Aqueous Conditions Cristian Contescu,t Jacek JagieUo,: and J. A. Schward Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, New York 13244- 1 1 90 Received: April 29, 1993; In Final Form: July 22, 1993'
The method of potentiometric titration in conjunction with an improved procedure for smoothing the experimental data and deconvoluting the proton binding isotherms was used to characterize the proton binding processes at the oxide/solution interface for Al2O3, wo3 and a series of W03/A1203 composite oxides. A similar procedure was applied to reference tungsten compounds (BaW04, ammonium metatungstate) for which the buffering capacity due to proton consumption in hydrolysis equilibria was characterized. The results showed that the reactivity towards protons of WOs/A1203 composites with low W03 loading is much closer to that of the alumina support, while for high WO3 loading it is better described by the aqueous chemistry of tungstate/ isopolytungstate solutions rather than of crystalline W03. At surface densities close or above monolayer, supported tungsten species showed a definite tendency to form polymeric surface aggregates in solutions with pH < 6 and more extensively below pH 4. The consequences of these phenomena on the equilibrium adsorption of cationic activators (Ni, Co) on preformed WO3/Al203 supports are discussed.
Introduction Many recent studies demonstrated the Occurrence of twodimensional oxide overlayers in catalysts prepared by dispersing metal oxides such as Cr03, Moo3, WO3, Re207, or VZOSover a primary oxide substrate (A1203,Ti02, SiOz) having a high specific surface area. The conclusions of structural characterization of these materials by different investigation techniques (such as Raman spectroscopy, EXAFS/XANES, and solid-state NMR) were summarized recently by Deoand Wachs.2 In general, these studies showed that the nature of the surface metal oxide species is dependent on the specific oxide support, surface coverage, extent of surface hydration, and calcination temperature. The degree of hydration changed reversibly the molecular structure of the overlayer oxide. For all the elements comprising the dispersed oxides above, the solution chemistry is dominated by hydrolytic formation of polyoxoanions, which is pH and concentrationdependent. Under ambient conditions, i.e., in hydrated form, it was proposed2v3that the acidic/basic properties of the host oxide determined the molecular structure of the two-dimensionaloxideoverlayers. More specifically, it was assumed that the stable molecular structure of the dispersed oxide under normal storageconditionscorresponds to the molecular species which prevail in solution equilibria at the pH corresponding to the net surface pH of the mixed support at its point of zero charge (PZC).This criterion was successfully used2J to predict the molecular structure of surface oxide species derived from vanadium, rhenium, chromium, molybdenum, and tungsten on various supports under ambient conditions, in good agreement with previous spectroscopic studies. We have shown4.5 that impregnation of Co or Ni on W03/ A1203 supports, followed by suitable thermal treatment, leads to the formation of stable and reproducible interaction species between the adsorbed cations and the dispersed tungsten oxide layer. However, the interaction mechanism between the positively charged ions in solution and the tungsten species which might be f Permanentaddress: Instituteof Physical Chemistry,RomanianAcademy, Spl. Independentei 202, Bucharest 77208, Romania, 8 Permanent address: Institute of Energochemistry of Coal and Physicochemistryof Sorbents,Universityof Miningand Metallurgy, 30-059 Krakow, Poland. To whom correspondence should be addressed. Abstract published in Advance ACS Abstracts. September 1, 1993.
present on the solid surfacewas not completely deciphered, because the molecular structure of the tungsten overlayer under impregnation conditions was not known. The objective of this communication is to reexamine the chemistry of surface tungsten species supported on alumina under aqueous conditions and determine their relationship to the pH of impregnation solutions such as those that would be used for mounting a second active metal. To accomplish this objective, the method of potentiometric titration in conjunction with an improved procedure for smoothing the experimental data and deconvoluting the isotherm was used to measure proton binding isotherms at the oxide/solution interface. The proton binding data for W03/A1203 samples and thederived proton affinity spectra were thencompared with those of reference tungsten compounds (WO3, BaWO,, ammonium metatungstate solution) for which the characteristic proton consumption/release equilibria were known.
Experimental Section Materials. Alumina used in this study wasy-Al203 (American Cyanamid) with a BET surface area of 150mz/g and pore volume of 0.48 cm3/g. Before use, the I/ls-in. extrudates were ground to obtain grains of 40-80 mesh, which were then calcined at 875 K for 24 h in air. The reference materials with tungsten initially present in tetrahedral and octahedralcoordination were solid BaW0, (purity 98%, from Alfa Division) and, respectively, solid W03 (purity 99.7%. from Alfa Division) and ammonium metatungstate, (NH&H2W120~5H20,0btainedfrom GTESylvania. Tungsten oxide is a nonporous powder (average particle size 0.04 mm) with a BET surface area of about 1 m2/g. The material was calcined at 875 K for 24 h before use. Barium tungstate is an insoluble solid powder. Ammonium metatungstate (AMT) was the precursor used to prepare the WO3/Al2O, composites. They were prepared by the incipient wetness method, using the required amounts ofalumina and ammonium metatungstate to obtain 2.595, 12%, and 30% W 0 3 loadings. After drying at 380 K overnight, the composites were calcined at 875 K for 24 h. All samples were stored in a drybox purged with nitrogen. Metbods. The experimentalprocedure used for potentiometric titration was described elsewhere.6 We used a 665 Dosimat
0022-36S4/93/2097-10152$04.00/0 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, NO. 39, 1993 10153
Chemistry of Surface Tungsten Species (Metrohm) microburette (fO.OO1 mL), a thermostated titration vessel, and a digital Fisher Accumet Model 805 MP pH meter (fO.O1 pH units) equipped with a combination glass electrode (Coming). All experimentswere done under a protective nitrogen atmosphere and at constant temperature (298 K) and ionic strength (0.01 N, NaNO3). The solid samples were titrated after several hours of equilibration with the inert electrolyte solution (initial volume, VO). The procedure consisted of adding small increments (V,) of either NaOH or H N 0 3 (normality Nt = 0.1 N) to the well-agitated suspension and reading the equilibrium pH at regular time intervals. The proton consumption function was then calculated as
Hm,,= vo(Ca-C&+ Vt:Nt-(Vo+ Vt)([HJ;-[OHld (1) from the analytical concentrations of acid or bases, C, and cb, and from the actually measured concentrations of H+ or OH-. The&, functionwas then normalized with respect to the amount of titrated sample (grams of oxide or millimoles of tungstate) and the function Q(pH) obtained this way was transformed into either the proton adsorption isotherm (for solid samples) or the average number of protons bound per tungsten atom (for AMT solutions).
Calculation of Proton A f f i t y Distribution. The concept of the 1 pK/multisites model with a continuousdistribution of proton affinities was presented elsewhere.6 The oxide surface is pictured as composed of nonequivalent oxygen groups that differ by their coordinative configuration (number and type of surrounding cations) and therefore by their acidic-basic properties. In the case of oxide samples, the surface heterogeneity may be so complex that a continuous description, rather than discrete, may apply. The affinity distribution function,f(pK), is defined as the mole fraction of binding sites having the acidity constant in the interval (pK, pK+ dpK). When this model is applied to a complex system of proton binding sites, the total concentration of basic (deprotonated) forms of surface groups is given by
or
where Cris the concentration of sites characterized by a particular acidity constant, K,. Our approach is to rely on the experimental data and to find the distribution function,f(pK), for the heterogeneous population of surface sites by deconvoluting the experimental proton adsorption isotherm. We used the local solution of the integral eq 2b, which has the advantage that it may be applied to data which are measured over a limited experimental window of concentration^.^ We applied the approximate method proposed by Rudzinski and Jagiello (RJ)? which is a special case of the exact local solution derived by Jagiello et al.9 for the case of a Langmuir local isotherm. The local solution is given by the following series: APK) =
ae [--+--d(pH)
a30 3!ln2(10) ~ ( P H ) ~ T4 8% =2
5!ln4(10) a(pH)'
+
...I
(3)
PH=PK
When only one or two terms are retained, this series represents thecondensation approximation (CA)Ioorthe RJ approximation, respectively. The condensation approximation is equivalent with
replacement of the Langmuir type local binding function: (4)
by a step function. In acid-base titration of polyprotic systems, the first derivativeof the overall binding curve shows the tendency of the system at equilibrium to resist a pH change for an incremental addition of a constituent, and it is known as the buffering intensity of the system. The buffering intensity is at a maximum for pH values equal to the p 6 s of particular groups of acidic sites, where inflection points exist on the binding curve. The buffering power may be similarly defined for hydrolizable systems11 and for the solid oxide/electrolyte solution interface.12 Although correspondence is formally made with the result of the condensation approximation for the series in eq 3, this remains a crude approximation for the true distribution of proton binding sites. When polynuclear species are formed upon hydrolysis, as for example '
the derivative of the formation curve (number of protons bound per metal ion) is still a measure of the buffer intensity. However, the pH where the buffer intensity is at a maximum is in a complicated way related to the equilibrium quotient, Qp,q;it is only approximately equal to (1 /p)log QpRand does not necessarily indicate the point of half-transformation. Each hydrolysis species contributes to the buffer intensity, and usually no steps occur in the formation curve, because successive acidity constants are close to each other. In our approach we used the third order derivative (Le., the RJ approximation) in eq 3. This contributes to a better separation of the hydrolysis steps; however the peaks obtained (in the case of AMT solutions) were slightly concentrationdependent. Theoretically the more terms in expansion 3 that are used, the more exact is the result. Practically, however, since higher order derivatives arevery sensitive to experimental errors, it is necessary to truncate this expansion. Our choice of the RJ approximation is based on the fact that when subsequent approximations are compared, the greatest improvement is obtained between the CA and RJ approximation^.^^ Furthermore, it was shown independently that for broader distributions the RJ approximation gives very accurate result^.^ Certainly the application of higher order approximations may improve the results provided the quality of experimental data allows one to calculate correctly higher order derivatives of an adsorption isotherm. To approximate experimental proton binding isotherms and to calculate appropriate derivatives, we applied a procedure of smoothingsplinesdescribed by Reinsch.14 A detailed discussion of the method to find the optimal value of the smoothing parameter for a given set of data so that it smoothes out fluctuations due to the experimental error but retains maximum information about the true shape of the original function was given elsewhere.6
Results and Discussion The proton binding isotherms for the solid samples titrated (pure alumina, tungsten oxide, and WO,/A1203 composites) are shown in Figure 1. The Q(pH) function is a measure of the net excess of bound protons with respect to a reference level. Positive values of Q(pH) indicate binding of protons in excess of this reference level, while negative values indicate release of protons. In titration of solid samples the Q(pH) curves could be converted into absolute surface charge curves, u(pH), provided the surface charge at one point of Q(pH) is known. Customarily, this point is taken at the pH of the apparent PZC, but for heterogeneous surfaces the significance of the PZC should be more carefully considered,because the occurrence of common intersectionpoints
Contescu et al.
10154 The Journal of Physical Chemistry, Vol. 97, No. 39, 1993
-m
0.5
.
o
-BE
-0.5
a
-1
-1.5 2
6
4
0
10
12
PH
Figure 1. Proton binding isotherms measured for suspensions of the solid samples titrated (298 K, 0.1 N NaNO3). 3
2.5
2
g 1.5 Y
r
1
0.5
0 2
3
4
5
6
7
0
9 1 0 1 1
PK Figure 2. Proton affinity distributions for alumina (a) and W03/A1203 composite oxides (b, 2.5% WO3; c, 12% W03; d, 30% WO3) calculated by deconvolutingthe proton binding isotherms shown in Figure 1. The curves were arbitrarily shifted for clarity.
for Q(pH) curves, when observed, does not necessarily mean a zero charge condition.15 Because our analysis of surface heterogeneity does not make reference to any electrostatic model, the knowledge of an "absolute" value of surface charge is not needed. In other words, the 0 function introduced in eq 3 is shifted from the experimentally found Q(pH) function by a constant term which does not change the distribution function because it is a differential quantity. We used as an operational reference in all titrations that charge condition acquired when the sample being studied had been equilibrated with the neutral electrolyte before any addition of strong acid or base. As shown by the alumina curve in Figure 1, this is not far from the apparent PZC of 7.50 determined by either mass titration16or the common intersection method. Reference Materials. Alumina: The proton binding isotherm for alumina in Figure 1 shows the known amphoteric behavior of this oxide: proton adsorption (basic character) in the low pH range and proton release (acid character) in the high pH range. The corresponding proton affinity distribution calculated from the Q(pH) curve is shown in Figure 2. Three categories of surface sites participate in proton reactions between pH 3 and 11. They were previously identified6 with three of the five types of surface hydroxyls predicted by structural models1' on the surface of an alumina spinel and observed in IR spectra of surface hydroxyls in the 3700-3800-~m-~range. Using the Knozinger notation17 for surface hydroxyls, the most basic groups with pK = 9.8-10
are type Ib hydroxyls singly coordinated to octahedral Al ions; the acidic groups with pK = 4.4-4.6 were assigned to bridging hydroxyls between A1 ions in octahedral and tetrahedral positions (type l l a ) , whereas the intermediate groups with pK of about 6.6-6.8 are type l a hydroxyls linked to tetrahedral A1 ions. Occasionally, a peak with pK < 3 was also observed, which was assigned to the very acidic, triply coordinated hydroxyls (type HI).The above assignments are consistent with an independent, semiquantitative model c a l c ~ l a t i o n ~based ~ J ~ on Pauling's electrostatic valence rule, which allowed for quite an exact prediction of proton affinity constants at the oxidesolution interface. Tungsten oxide, tungstates, isopolytungstates: As reference tungsten compounds we used solid samples (tungsten oxide and barium tungstate) as well as solutions (ammonium metatungstate). For AMT solutions, titrations were done after adding known amounts of KOH, which transformed the polytungstate ion into monomeric tungstate. The proton binding isothermof tungstenoxide (Figure 1) shows only proton releasing processes over the entire pH range studied, in agreement with the known acidic character of this oxide. The proton affinity distribution shown in Figure 3 reveals two main groups of surface sites whose protons dissociate at pH 7 and 11; a secondary proton-releasing process takes place at pH 6. The model calculations based on the electrostatic valence rule1*J9can hardly be applied to tungsten oxide, as they were applied for alumina,6J8 because binding in W 0 3 is partially covalent. However, the assignment of the experimental acidity constants to the structural groups on a WO3 surface can still be made. Crystalline W 0 3 possesses a distorted corner-shared octahedral structure.20v21 On its surface and in a water environment,the oxygen atoms occupy either terminal or bridging positions. Inspection of all possible structures reveals that the following octahedra occur: w(06)w(05)ow(o4)o; w(03)03where 0 atoms shown in parentheses are those belonging to W-0-W or W-OH bridges. The corresponding pK values for dissociation of protons from terminal oxygens were evaluated by Tytko and Glemser from a comparison with the strength of mononuclear oxoacids:22 W(04)O(OH)/W(04)O; W(O&OH/W(05)0-
pK = 1-4 PK = 7-10
On the basis of these estimations, we identify the peaks at pH I and 11 in Figure 2 with the weak and very weak acidity of protons linked to W(O6)- and W(O5)O- groups. However, other chemical equilibria, besides the dissociationof surface hydroxyls which naturally exist on the W 0 3surface, may be envisaged. We will return later to this point. Solid BaW04 is composed of slightly distorted W 0 4 tetrahedraa23In an aqueous environment,the terminal W 0 4structures on the BaW04surfaceareconvertedto WO6structures by bonding of two water molecules. An octahedral coordination of hydrated normal tungstate ion was postulated by several authors, based on the fact that many tungstates separate from solution with two molecules of water of crystallization. Our results support the octahedral configuration of the outermost tungstate layers of hydrated BaW04. This is shown by the occurrence of two weak, broad peaks around pH 10.5 and 7 in the deconvoluted proton adsorption isotherm (Figure 3); they probably have the same origin as those observed for crystalline WO3 in the same pH range. A much more intense peak develops at pH 3, and this value corresponds22 to the logarithm of the formation constant
The Journal of Physical Chemistry, Vol. 97, No. 39, 1993 10155
Chemistry of Surface Tungsten Species 3
2.5
2
Y
1.5
c
1
0.5
.,,--.........,,..-
...-.-
,'
0 3
2
4
5
6
7
8
9 1 0 1 1
PK
Figwe 3. Proton affinitydistributionsfor reference tungsten compounds (a, crystallineWO3; b, solid BaW04; c, AMT solution). The curves were arbitrarily shifted for clarity.
r,-.,
9
4a I
~
7 -1j
' . 1 ' . ~ . . 1 1 I. . . 1
~
~
-
AMT-2
PH
-0.5
0
0.5
1 added
1.5
2
2.5
'
Figure 4. Experimental proton binding curves for ammonium metatungstatesolutions at various molar concentrationsof total tungsten (AMT 1,4.8 X 10-2; AMT 2,2.4 X 10-2; AMT 3 and 4, 1.2 X 10-2), different base/total tungsten ratios (AMT 1,0.416;AMT 2,1.083;AMT 3,1.666; AMT 4,2.083) and different temperatures (AMT 1-3,298 K AMT 4, 323 K).
for HWOd-: WOt-
+ H+
c;)
HWO,
pK = 3.4
The product can also be written as an octahedral [WOs(OH)(HZ0)2]- species.22 We performed also several titrations of ammonium metatungstate solutions at various concentrations and with various ratios of initially added base to the total existing tungsten (Figure 4). Above a pH of 8 or 9, only the tetrahedral W042-ion occurs in solution, but as the pH is decreased, the degree of aggregation increases. Because of formation of polynuclear complexes, variations in the tungsten concentration shifted the formation curves, but they retained a similar shape. A characteristic break, which always occurred around pH 6 (at H+/W = 6/7), was caused by the formation of the paratungstate A ion: 7H'
+ 6WO;- * [HW602,(OH)2]S- + 2 H 2 0
On further acidification, a second break at H+/W = 1.5 showed the formation of a new intermediate (pseudo-metatungstate) which transformed slowly and irreversibly into the most stable product, the metatungstate ion, [ H ~ W I ~ O MAlthough ]~. these equilibria were extensively st~died,~zJ3 the structure of transient
~
s p e c k and the mechanism of their aggregationare not completely understood.24 The polycondensation of monomeric WO42- on acidification is a proton consumption process and is easily identified in potentiometric titration. We applied the same analysis to the titration curve measured for ammoniummetatungstate solutions. The result shown in Figure 3 indicates a more complex structure which combines the features found for the crystalline tungsten oxide and barium tungstate. The rapid formation of the paratungstate A ion, [HW&(OH)Z]~, imparts to the solution a high buffer intensity at pH about 6, which can be recognized in the well defined peak of the derivativesof the formation curve. Maxima also occur around pHs of 11 and 7, where the solution contains monomeric tungstate species. These maxima are common to the solid tungsten oxide sample. On the basis of the fact that normal tungstate ions apparently have two molecules of water of hydration even in alkaline solutions, which makes no clear distinction between the tetrahedral and octahedral forms,23 we assume that these maxima have the same origin as those already found for crystalline tungsten oxide. The broad maximum around pH 9 is probably due to the NH4+ ion present in AMT solutions (p&~,+ = 9.5). The less pronounced peak at pH 4 is assigned to the slow proton consumption in formationof the metatungstate ion: ~
~
18H'
~
~
+ 12W0,"o
,
[H,W,,O,]"
+ 8H20
The fact that the W 0 3curve in Figure 3 shows similar features, though much weaker, around pH 7 and 4 indicates that similar aggregation processes may occur on acidification with loosely bound WOb octahedra at the tungsten oxide/water interface. They are not observed with barium tungstate, whose crystalline structure does not allow polycondensation of WO6 octahedra, even though a pseudooctahedral structure may be formed in water from W04" in the outermost layer. SupportMaterials. W03/A1203composite oxides: The proton isotherms for the W03/A1203 composites (Figure 1) indicate an intermediate behavior between the amphoteric character of alumina and the exclusively acidic one of tungsten oxide. The acidity of the composites increases with an increase in tungsten oxide loading, as shown by the decrease of the initial pH of solutions equilibrated before titration. Also, the Q(pH) function shows that proton releasing processes prevail above pH 4 which is the equilibrium pH of composite/electrolyte solutions before any addition of acids or bases. Acidification below pH 4 is accompanied by proton consumption processes (positive values of the Q(pH) function), and their magnitude increases as the WOs loading increases. A simple examination of these curves is not sufficient to ascertain the surface processes responsible in proton reactions. Besides the protonation/deprotonation reactions of surface hydroxyls exposed on the bare alumina surface, surface tungsten species present as a dispersed second phase may undergo pHdependent transformations which are detected in potentiometric titration. Recall that aggregation of monomeric WO, species is a proton consumption process, while disaggregationof polymeric tungstates is accompanied by a proton release. Inspection of the proton affinity distributions (Figure 2) shows a gradual transformation of the surface properties of composites as the tungsten oxide loading increases. The 2.5% WO3 sample retains much of the alumina characteristics, though new features are observed in the intermediate pH range (6-8). On the other hand, the sample with 30% W 0 3 develops sharp peaks at pH 9, 6, and
