Tungsten Oxide Monolayer Loaded on Zirconia - American Chemical

The benzaldehyde-ammonia titration (BAT) method clarified that the tungsta impregnated on zirconia formed a monolayer and almost completely covered th...
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J. Phys. Chem. B 1999, 103, 7206-7213

Tungsten Oxide Monolayer Loaded on Zirconia: Determination of Acidity Generated on the Monolayer Norihiro Naito, Naonobu Katada,* and Miki Niwa Department of Materials Science, Faculty of Engineering, Tottori UniVersity, 4-101 Koyama-cho Minami, Tottori 680-8552, Japan ReceiVed: February 23, 1999; In Final Form: June 7, 1999

The benzaldehyde-ammonia titration (BAT) method clarified that the tungsta impregnated on zirconia formed a monolayer and almost completely covered the surface with ca. 4 W nm-2. Generation of acidity on the monolayer was shown by the water vapor treatment method of ammonia TPD (temperature-programmed desorption). Strength of the acid site was calculated according to the theoretical equation to be ca. 130 kJ mol-1 in adsorption heat of ammonia, which does not correspond to superacidity. The generated acid site was active for skeletal isomerization of butane.

Introduction Subsequently to the discovery of superacidity on a sulfateloaded zirconia catalyst,1,2 Hino and Arata found that the tungsten oxide (tungsta) loaded on zirconia showed a high catalytic activity for skeletal isomerization of alkanes.3 Later, the activity was also found on various supports such as tin, iron, and titanium oxides.4 The early studies proposed the superacidity on the tungsta-loaded catalyst on the basis of the Hammet indicator method.3,5 The nature of the tungsta- and sulfate-loaded zirconia is similar; both catalysts change the color of a base indicator with pKa < -13, and both catalysts have the activity for the skeletal isomerization of such an alkane as n-butane. Studies in the past decade seem to indicate consistently the presence of the superacidity on the sulfate-loaded zirconia.6-8 However, the surface structure and the acidic property (acid amount and strength) of the tungsta-loaded zirconia have not been clarified, although several spectroscopic and catalytic data have been published.6,8-18 The following fundamental questions have not been solved for this oxide system: (1) Does the tungsta-loaded zirconia indeed have the superacidity? (2) What type of surface/interface species has the acidity? (3) Does the superacid site catalyze the alkane isomerization? To overcome the difficulty of analyzing the structure of binary oxide, it is promising to measure the spreading of loaded oxide. We have developed a method to distinguish between the surface areas of two oxides on one binary oxide sample. This is the benzaldehyde-ammonia titration (BAT) method using the strong chemisorption of benzaldehyde on basic oxides.19-21 Benzaldehyde is adsorbed on such basic oxides as alumina, titania, tin oxide, and zirconia in high surface concentration, while almost no benzaldehyde is adsorbed on such an acidic oxides as silicon, vanadium, molybdenum, and tungsten oxides. The amount of the adsorbed benzoate anion can be determined precisely from the amount of the produced benzonitrile by the reaction between the adsorbed anion and ammonia. Therefore, it is possible to estimate the area of exposed fraction of the surface of a basic oxide from the amount of adsorbed benzoate * Corresponding author. Phone, +81-857-31-5684; fax, +81-857-315684; E-mail, [email protected].

species (amount of the produced nitrile) on a binary oxide sample. The spreading of the acidic promoter loaded on basic support has been measured on V2O5,20 MoO3,22 SiO2,23-25 and GeO226 loaded on Al2O3, TiO2, ZrO2, and SnO2. In all of these cases, it was shown that the acidic oxide formed a monolayer that almost completely covered the surface upon the strong interaction between the acidic and basic materials, and that the formed monolayer was the active species. Recently the validity of the BAT method was confirmed by the group study carried out by many researchers using various techniques, i.e., X-ray photoelectron spectroscopy (XPS), laser Raman spectroscopy (LRS), electron probe microanalysis (EPMA), and nitrogen monoxide adsorption. These techniques were applied to the same MoO3/Al2O3 samples prepared under various conditions, and they agreed well with the conclusion by the BAT method that the loaded MoO3 formed a monolayer on Al2O3.27 In the present study, this BAT method was applied to the tungsta-loaded zirconia in order to clarify the surface structure. The chemisorption of carbon dioxide has been applied to a tungsta-titania catalyst.28 The chemisorption of carbon monoxide has been applied to tungsta-alumina catalyst to measure the coverage.29 On the other hand, we have studied the temperatureprogrammed desorption (TPD) method of ammonia to measure the acidic property of the solid quickly and easily.30 We have dealt mainly with zeolites in the previous studies. The water vapor treatment after the adsorption of ammonia removed the unnecessary peak that was ascribed to the weakly held ammonia31 at low temperature to simplify the spectrum.32-36 The ammonia or ammonium cation bonded to the acid site was never removed because of the strong basicity of ammonia.35 The desorption of ammonia was found to be controlled by the equilibrium between the gaseous and adsorbed ammonia, i.e., the readsorption freely occurred during the TPD experiments under the experimental conditions widely utilized.31,37 The entropy change with respect to the ammonia desorption was constant on various zeolites,38 and a method to calculate the adsorption heat of ammonia, namely acid strength, from the peak intensity, position, and shape has been proposed.39 The acidic properties of mordenite, ZSM-5,39 β,40 Y,35 and gallo-

10.1021/jp9906381 CCC: $18.00 © 1999 American Chemical Society Published on Web 08/06/1999

Acidity Generated on Tungsten Oxide Monolayer silicate36 have been determined by the improved method of ammonia TPD. On zirconia, a broad TPD spectrum is observed by the conventional method, as shown in the following section. This observation is inconsistent with the inactivity of zirconia for acid-catalyzed reactions such as isomerization of alkane. The activity is created by loading of tungsta. Probably zirconia adsorbs ammonia via weak interactions with the high concentration, and the peak showing the acidity on the tungsta-loaded catalyst may be hindered by the unnecessary peak. In the present study, we applied the water vapor treatment method of TPD to the tungsta-zirconia system in order to determine the acidic property and to relate it to the catalytic activity for the skeletal isomerization of alkane. The TPD spectra were measured by varying the W (sample amount)/F (flow rate of carrier gas) ratio to clarify what controlled the TPD process on these catalysts. Based on the observed results, theoretical analysis of the TPD spectrum was first applied to a WO3/ZrO2 system in order to obtain the acid strength. Experimental Section Catalyst Preparation. To avoid contamination of halogen and sulfur, zirconium oxynitrate [ZrO(NO3)2] was used as the precursor of zirconia. It was solved into a nitric acid solution, and aqueous ammonia was slowly added to precipitate the zirconium hydroxide until the pH reached ca. 11. The obtained solid was washed with water and calcined at 573 K. It was put into an aqueous solution of ammonium tungstate [5(NH4)2O‚ 12WO3‚5H2O], followed by drying the water and calcining at 923 K for 4 h under atmospheric conditions. Structural Characterizations. The total surface area was determined according to the BET equation from the nitrogen adsorption experiments at 77 K with p/p0 ) 0.3. The BAT experiments19 were carried out in a Pyrex reactor (4 mm i.d.) after the pretreatment at 673 K for 1 h in oxygen flow. Benzaldehyde was repeatedly injected at 523 K in a flow of helium, which was purified with a liquid nitrogen trap. The eluted aldehyde was monitored with an FID (flame ionization detector), and the injection was done until no further adsorption of aldehyde was observed. Finally, ammonia was injected at 673 K to desorb the adsorbed material as benzonitrile. The formed nitrile was analyzed by the FID, and the coverage by tungsta on zirconia was calculated from the following equation: Exposure (%) ) 100 × amount of produced nitrile per BET surface area (BN) on the binary oxide sample (molecules nm-2)/ BN on the unmodified zirconia (2.2 molecules nm-2). Coverage (%) ) 100 - exposure (%). Ammonia TPD. The TPD measurements30 were carried out in a quartz cell 1 cm in diameter. The sample was evacuated at 873 K for 1 h, and ammonia (13.3 kPa) was introduced into the cell at 373 K. After 30 min, excess ammonia was evacuated for 30 min. Then, water vapor (ca. 3 kPa, the vapor pressure at room temperature) was introduced into the cell for 30 min at 373 K, followed by evacuation for 30 min. After the introduction of water vapor and the evacuation were repeated again, the adsorbed ammonia was desorbed in helium flow under a reduced pressure with 10 K min-1 of the heating rate from 373 K, and the desorbed ammonia was analyzed by a mass spectrometer (ULVAC, UPM-ST-200P). Although the molecular weight of ammonia is 17, the fragment with m/e ) 16 was used to quantify the ammonia, because the fragment 17 is affected by the desorbed water.30,35 For standard experiments, the sample amount was 0.1 g, the flow rate of helium was 0.044 mmol s-1, and the pressure in the sample cell was 13.3 kPa; hence,

J. Phys. Chem. B, Vol. 103, No. 34, 1999 7207

Figure 1. BET surface area of WO3/ZrO2 calcined at 573 (O and *) and 923 K (b).

the W (sample amount)/F (flow rate of carrier gas) ratio was ca. 12 kg s m-3. To show the influence of the W/F ratio on the peak temperature, the W/F ratio was varied from 1 to 6000 kg s m-3 on several samples. Catalytic Reaction. The skeletal isomerization of n-butane into isobutane (2-methylpropane) was carried out by a conventional pulse method. The catalyst (250 mg) was set into a quartz tube (4 mm i.d.) and pretreated at 873 K for 1 h in 20 cm3 min-1 of flowing helium, which was purified by passing a liquid nitrogen trap. Then, a pulse of n-butane (1.3 cm3, 5.3 × 10-5 mol) was injected into the helium flow at 623 K, and the products were analyzed by a gas chromatography (GC) with a column of VZ-7. Small amount of byproducts, i.e., C1-3 hydrocarbons, were generally detected. The activity was shown by the yield of major product, namely isobutane. IR Spectroscopy. The IR spectrum was recorded from 950 to 4000 cm-1 on the self-supporting disk 10 mm in diameter, molded from 10 mg of the sample in an in situ cell with CaF2 windows. After evacuation at 873 K for 1 h followed by cooling the sample disk for 5 min, the spectrum was recorded. The spectrum of adsorbed ammonia was collected after the introduction of ammonia (13.3 kPa) for 30 min, followed by evacuation for 30 min at 373 K. To show the adsorbed species formed/ diminished by the water vapor treatment, the introduction of water vapor (ca. 3 kPa) and evacuation were twice repeated after the adsorption of ammonia, and the spectrum was collected. On the other hand, vapor of pyridine (ca. 400 Pa, the vapor pressure at room temperature) was introduced into the cell at 373 K for 30 min, and the spectrum of the adsorbed pyridine was recorded after evacuation at 573 K for 30 min. Results Structural Characterizations. Figure 1 shows the change of BET surface area by the loading of tungsten oxide. Zirconia had a high surface area, ca. 250 m2 g-1 at 573 K. To show the effect of the loading of tungsten oxide, the surface areas of the loaded catalysts were measured after calcination at the same temperature, 573 K. The loading decreased the specific surface area shown by the unit of m2 g-cat-1. Since this decrease is affected by the change of weight of the catalyst by loading, the surface area shown by the unit of m2 g-ZrO2-1 is also plotted; it indicates that the loading of tungsta gradually decreased the surface area. The symbol b shows the surface area after calcination at 923 K. The high-temperature calcination seriously decreased the surface area of zirconia into ca. 30 m2 g-1 by thermal sintering.

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Figure 2. Coverage by WO3 on ZrO2 surface measured by BAT method over WO3/ZrO2 calcined at 573 (O) and 923 K (b).

However, the loaded tungsten oxide prevented the particle from sintering, and as a result, ca. 100 m2 g-1 of the surface area was obtained at ca. 20 wt % of WO3. From the measured surface area, the surface concentration of tungsten was calculated as follows: W concentration (W-atom nm-2) ) [WO3 loading (wt %)/ 100]/231.8 (formula weight of WO3) × 6.023 × 1023/[BET surface area (m2 g-1) × 1018]. The WO3 loading, 20%, which provided the maximum surface area, corresponded to ca. 5 W nm-2 of the surface concentration of tungsten. The X-ray diffraction (XRD) showed that the loading of tungsta prevented the transformation of tetragonal to monoclinic zirconia (spectrum not shown). As a result, only the XRD pattern ascribable to tetragonal zirconia was observed on the sample with 6.4 W nm-2 after calcination at 923 K. On the other hand, the formation of free WO3 particle was suggested on the sample with a high concentration of tungsta, 17.3 W nm-2, due to the diffractions ascribable to the monoclinic phase of WO3. The suppression of sintering and phase-transformation of zirconia by the loaded tungsta, and the formation of free WO3 at the high concentration of tungsta, are in agreement with previous studies.3,10,17 Figure 2 shows the coverage measured by the BAT method against the W concentration. On the samples calcined at 573 K, the coverage increased linearly against the W concentration and arrived at ca. 100% with 10 W nm-2. Calcination at 923 K made the coverage higher; the coverage arrived at almost 100% with ca. 4 W nm-2 as shown in Figure 2 (b). Temperature-Programmed Desorption (TPD). Figure 3 shows the TPD spectra on zirconia and WO3/ZrO2 (ca. 5 W nm-2) obtained with a constant W/F ratio (ca. 12 kg s m-3) by the conventional and novel water vapor treatment methods. By the conventional method, in which the temperature was simply raised after the adsorption of ammonia, a large and broad desorption peak of ammonia was observed at 400-600 K on zirconia. The water vapor treatment after the adsorption of ammonia almost completely diminished the peak. In place of the ammonia peak, indeed, the desorption peak of water (m/e ) 18) was observed. The mechanism of replacement is discussed in the following section. On the other hand, on the WO3/ZrO2 catalyst, a peak of ammonia was observed at 400-700 K by the conventional method, and the water vapor treatment changed it a little; the

Naito et al.

Figure 3. TPD spectra obtained by conventional (without water vapor treatment, solid line) and water vapor treatment method (broken line) over ZrO2 (a) and WO3/ZrO2 (b: 5.2 W nm-2).

Figure 4. TPD spectra obtained by water vapor treatment method over ZrO2 (a) and WO3/ZrO2 [(b) 1.5, (c) 3.1, (d) 5.2, (e) 6.4, (f) 8.2, (g) 17.3 W nm-2).

peak maximum shifted slightly to the higher temperature. As a result, creation of a certain type of acid site by the loading of tungsta was clearly indicated. The conventional method showed the peak intensity on the WO3/ZrO2 (b) about five times higher than that on zirconia (a); this can be associated with the BET surface area. On the contrary, the TPD peak after the water vapor treatment was observed only on the tungsta-loaded catalysts. The TPD spectra were recorded by the water vapor treatment method as shown in Figure 4. As shown in Figure 5, the loading of tungsta created the acid site, and the surface concentration of the acid site showed the maximum (ca. 1.3 nm-2) at 6.4 W nm-2 of the tungsten concentration; at the maximum, the number of acid sites was ca. 1/4-1/5 the number of tungsten atoms. Further loading decreased the acidity. On the samples with 5.1 and 5.8 W-atom nm-2, the TPD spectra were recorded with varying of the W/F ratio from 1 to 6000 kg s m-3. The peak maximum temperature was shifted with changing the W/F ratio. To interpret this behavior according to the theory, the relationship between ln 1/Tm - ln A0W/F and 1/Tm, where Tm is peak maximum temperature (K) and A0 is the amount of desorbed ammonia (mol kg-cat-1), is shown in Figure 6. An almost linear relationship was obtained, indicating that the TPD process is controlled by the equilibrium, as explained in the next section.

Acidity Generated on Tungsten Oxide Monolayer

Figure 5. Surface concentration of acid site determined by ammonia TPD with water vapor treatment method (b) and catalytic activity for skeletal isomerization of n-butane to isobutane (2-methylpropane) at 623 K (O). The catalytic activity was shown by [yield of isobutane (mol) × flow rate of carrier gas (m3 h-1)/surface area of catalyst (m2)].

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Figure 7. IR spectrum of ZrO2 evacuated at 873 K (a), followed by adsorption of ammonia and evacuation (b) at 373 K and two repetitions of water vapor treatment and evacuation at 373 K (c).

Figure 6. Plots of ln 1/Tm - ln A0W/F against 1/Tm on WO3/ZrO2 (4, 5.1 and 3, 5.8 W nm-2).

Skeletal Isomerization of Butane. Figure 5 also shows the catalytic activity for the isomerization of butane. A volcanoshaped relationship against the loading was obtained. No activity was observed on zirconia, and the loading of tungsta created the activity. The activity showed the maximum at 6.4 W nm-2, which agreed with the maximum acidity shown by the TPD, and further loading diminished the activity. IR Spectroscopy. The IR spectrum of zirconia evacuated at 873 K (Figure 7 a) showed a peak at 3676 cm-1, assigned to the hydroxyl group. After the adsorption of ammonia, peaks were observed at 3338, 1604, 1445, 1223, and 1174 cm-1, as shown in Figure 7b. As shown below, the peaks at 1604 and 1445 cm-1 are ascribable to the coordinated NH3 molecule and NH4+ cation, respectively.41 The intensities of these peaks were comparable. The water vapor treatment strongly suppressed the peaks at 1604, 1445, and 1174 cm-1, whereas the band at 1223 cm-1 was less affected, as shown in Figure 7c. In addition to the absorptions by zirconia at 3676 cm-1, the loading of tungsta generated a new band at 982 cm-1, as shown in Figure 8a. The ammonia species adsorbed on the tungstaloaded sample (5.2 W nm-2) showed new IR peaks at 3262,

Figure 8. IR spectrum of WO3/ZrO2 (5.2 W nm-2) evacuated at 873 K (a), followed by adsorption of ammonia at and evacuation (b) and two repetitions of water vapor treatment and evacuation at 373 K (c).

3180 and 1460 cm-1, as shown in Figure 8b. The peaks generated at 3344 and 1608 cm-1 were also observed on zirconia at 3338 and 1604 cm-1. A large absorption at 1237 cm-1 appeared in place of the peaks at 1223 (small) and 1174 cm-1 (large) observed on zirconia. The water vapor treatment little changed the peak shape in the stretching region, 3000-4000 cm-1; made the peak at 1433 cm-1 slightly small; and generated a small peak at 1262 cm-1, as shown in Figure 8c. Although these small changes were observed, the spectrum of the ammonia species adsorbed on the tungsta-loaded zirconia was approximately maintained. After the adsorption of pyridine, the peak at 1447 cm-1 ascribable to the pyridine molecule coordinated on Lewis acid site8,41 was observed (data not shown). On the other hand, the WO3/ZrO2 catalyst possessed both Lewis and Brønsted acidity, as shown by the IR band of pyridinium cation8,41 at 1542 cm-1. The spectra were collected on the samples with various tungsta contents; Lewis acidity was monotonically decreased with

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Figure 9. Intensity of peaks due to pyridinium cation bounded to Brønsted acid site (b, 1542 cm-1) and pyridine coordinated to Lewis acid site (O, 1447 cm-1).

loading of tungsta, while Brønsted acidity showed the maximum with 6 W-atom nm-2 of the concentration, as shown in Figure 9. Discussion Monolayer Structure. As shown in Figure 2, tungsten oxide almost completely covered the surface with 10 W nm-2 of the surface concentration at 573 K. This concentration would correspond to the doubly or further accumulated layer of tungsten oxide. Calcination at 923 K enhanced the coverage, and the tungsten oxide covered the surface with ca. 4 W nm-2. The enhancement of coverage, namely the dispersion of the tungsta species on the surface, in the temperature region from 573 to 923 K is in agreement with the reported observation by Raman spectroscopy that the free WO3 phase once formed at 773 K was redispersed into the monolayer by calcination at 1073 K.10 The concentration 4 W nm-2 presumably shows the monolayer coverage, because the W concentration of tungsta monolayer loaded on such a basic metal oxide as alumina has been proposed to be 4 to 7 W nm-2; Xie and Tang estimated the maximum packing density of planar WO3 species as 0.21 g-WO3/100 m2,42 corresponding to 5.5 W-atom nm-2 of the surface concentration, whereas Igresia et al. adopted 3.7 nm-2 10 based on the X-ray photoelectron spectroscopy (XPS) studied on WO3/Al2O343 or 7 nm-2 based on the crystal structure of WO3.18 Other authors also proposed similar surface concentrations of tungsten atoms on the monolayer loaded on metal oxides.44-46 In conclusion, after calcination at 923 K, the monolayer of tungsta completely covered the surface of zirconia. The full coverage by the monolayer on zirconia supports the observations by Igresia et al.; they suggested the monolayer formation based on the change in BET surface area and X-ray diffraction.17 The IR band at 982 cm-1 on the tungsta monolayer (Figure 8a) is in agreement with Raman spectroscopy.10,17 The origin of high coverage is speculated to be the strong interaction between the support and the promoter. We have found the similar monolayer coverage on V2O5,20 MoO3,22 SiO2,23,24 and GeO226 loaded on such a basic oxide as Al2O3, TiO2, SnO2, and ZrO2. Moreover, the other authors also observed the monolayer coverage on WO3/Al2O3 and WO3/TiO2 by means of the adsorption of carbon monoxide29 and dioxide,28 respectively. In these cases, spontaneous monolayer dispersion

Naito et al. is suggested, as proposed by Xie and Tang,42 probably owing to the strong interaction between the basic support and the acidic promoter. The suppression of thermal sintering of zirconia by the loaded tungsta layer was observed, as shown in Figure 1. On the zirconia precalcined at 573 K, it is suggested that the loaded tungsta formed a thermally stable layer like an eggshell to protect the oxide particle from sintering. Simultaneously, the phase-transformation of zirconia from tetragonal into monoclinic was inhibited. The suppression of sintering and phasetransformation are in agreement with the previous studies.3,10 We also found that the silica monolayer loaded on alumina prevented the particle from sintering and phase-transformation at a quite high temperature, 1493 K, and confirmed by means of the BAT method that the loaded monolayer protected the particle from sintering like an eggshell.47 It is suggested that such an acidic oxide promoter as tungsta and silica commonly suppresses the thermal sintering of such a basic oxide as zirconia and alumina via the tight bonding between them, based upon the strong interaction which also induces the formation of an ultrathin layer as described above. With the loading exceeding the monolayer region, the XRD showed the formation of free WO3 crystal, as reported by other authors by means of XRD and Raman spectroscopy.3,10,17 This also supports that the interaction between excess tungsta and the solid surface becomes weak when the surface has been covered by the monolayer. Water Vapor Treatment of TPD. On zirconia, ammonia was strongly adsorbed unless the water vapor treatment was applied, as shown by the large TPD peak (Figure 3a, solid line), in which the desorption of ammonia continued up to a relatively high temperature, ca. 600 K. On the basis of the early studies of IR spectroscopy carried out on ammine complexes48 and solid catalysts,41 the IR bands which were observed on zirconia adsorbing ammonia at 3338, 1604, and 1174 cm-1 (Figure 7b) were assigned to N-H stretching, asymmetric and symmetric deformation of H-N-H, respectively, of the NH3 molecule coordinated to the solid, but not the NH4+ cation. Lack of an NH- anion,48 shown by the absence of absorption at 1510 cm-1, is consistent with the TPD spectrum in which the desorbed species was only NH3 (m/e ) 16 and 17), but not N2 (28) and H2 (2). The peak intensity at 1445 cm-1 ascribable41 to the asymmetric deformation of the NH4+ was similar to that of the NH3 molecule at 1604 cm-1; this indicates the small amount of the NH4+ species based on the molar absorption coefficiency, ca. 1:7 for NH3 and NH4+. Therefore, the major species on zirconia is concluded to be the NH3 molecule adsorbed on the Lewis acid site, namely the Zr4+ cation, or hydrogen-bonded. The former is more reliable, because the desorption temperature was quite high (up to ca. 600 K, as shown above). These IR peaks on zirconia were almost completely diminished by the contact with water vapor, as shown in Figure 7c. The TPD experiments also indicate that most of the adsorbed species was replaced by the water molecule, resulting in the removal of the TPD peak, as shown in Figure 3a. It was also observed on zeolites that the adsorbed ammonia species was removed by the water vapor treatment after the adsorption of ammonia, and, as a result, the unnecessary l-peak was diminished.35,36 The origin of replacement of ammonia by water on zeolite is suggested to be the polarity of the OH bond in the water molecule, which is considered to be stronger than that of NH bond in ammonia based on the higher dipole moment;49 the ammonia hydrogen-bonded to the NH4+ cation50 was selectively replaced by the water molecule to form a stronger

Acidity Generated on Tungsten Oxide Monolayer hydrogen bond.35 However, the present case of the zirconiatungsta system seems different, because the adsorbed species is suggested to be coordinated to the Lewis acid site, as described above. Another explanation is required for the adsorption site on zirconia. Probably the hydration of the surface should completely change the nature of the zirconia surface. It is suggested that only the dehydrated surface of zirconia has Lewis acidity, but the hydrated surface, probably filled with ZrOH groups, has no or quite weak acidity, resulting in the elimination of ammonia, i.e., removal of the TPD peak. On the other hand, the large peaks of NH4+ cation adsorbed on the Brønsted acid site (3180 cm-1 due to the N-H stretching and 1433 cm-1 due to the H-N-H deformation41) were observed on the tungsta-loaded zirconia (Figure 8b). This agrees with the presence of Brønsted acidity shown by the spectrum of adsorbed pyridine (Figure 9). The peaks ascribed to the NH3 molecule coordinated to the Lewis acid site were also observed; the asymmetric deformation was observed at 1608 cm-1 and symmetric deformation was observed at 1237 cm-1. The wavenumber of the latter was higher than that on zirconia, 1174 cm-1, showing the species coordinated not on Zr4+, but on another element, namely W; this indicates the Lewis acidity generated on the loaded tungsta layer. The spectrum of adsorbed pyridine also shows the presence of both Brønsted and Lewis acidity on the sample fully covered by the tungsta monolayer. Both species adsorbed on Brønsted and Lewis acid sites of the tungsta layer were not diminished by the water vapor treatment, as shown in Figure 8c. The TPD spectra (Figure 3b) also indicate that the species adsorbed on the tungsta layer was not removed by the water vapor treatment. In conclusion, the water vapor treatment selectively removed the ammonia species adsorbed on zirconia support. Applying this method to the ammonia TPD clearly demonstrated the creation of acidity, as shown by the change in the TPD spectrum by varying the loading amount of tungsta (Figure 4). Acidic Property. By the measurements of TPD with the water vapor treatment method, the generation of an acid site is clearly concerned with the coverage by monolayer, as shown in Figure 5. The maximum concentration of the acid site was observed at 6 W nm-2, where the tungsta monolayer almost completely covered the surface. As shown by the IR study, Brønsted acidity was generated. The maximum acidity at this tungsten concentration region is generally in agreement with the observations on tungsta loaded on alumina51 and other supports.52 Excess of tungsta decreased the acidity to almost zero with ca. 18 W nm-2. The diminishing of acidity suggests that the inactive layer of tungsta further accumulated on the surface of the active monolayer. The concentration of the acid site (1.3 nm-2) was almost 1/41/ of the concentration of the tungsten atom (6 nm-2) on the 5 monolayer. This suggests the stoichiometric generation of the acid site by a certain type of cluster consisting of several W, Zr, O, and H atoms. We have found similar results on the MoO3/SnO2 system: The water vapor treatment removed the TPD peak on tin oxide, while the peak on MoO3/SnO2 was maintained; on the monolayer of molybdena, the number of acid sites was almost 1/4 the number of molybdenum atoms.53 Because tin and molybdenum have properties similar to zirconium and tungsten, respectively, these results support the present study. Igresia et al. proposed distorted octahedral WOx clusters possessing W-O-W bonds as an active species of WO3/ZrO2 catalyst based on the X-ray absorption and UV-vis spectroscopy.10,17 This is in good agreement with the generation of an

J. Phys. Chem. B, Vol. 103, No. 34, 1999 7211 acid site by a certain type of cluster consisting of multiple W, Zr, O, and H atoms. To determine the acid strength from the TPD spectrum, the TPD measurements were carried out by varying the W/F ratio on the samples which had almost completely been covered by the monolayer. Cvetanovic and Amenomiya classified the TPD experiments into the following three cases: (1) The process is controlled by the kinetics, namely, the activation energy is high. (2) The process is controlled by the equilibrium between gaseous and adsorbed ammonia, namely, readsorption of ammonia freely occurs. (3) The process is controlled by the slow diffusion.54 From the linear relationship between ln1/Tm - ln A0W/F and 1/Tm shown in Figure 6, the present TPD experiments are classified into the second case, as observed on various zeolites.30,38 This is the first classification of ammonia TPD on a nonzeolitic catalyst. Two parameters, ∆H and ∆S, were calculated based on the linear relationship shown in Figure 6 according to the derived equation30

ln Tm - ln

A0W ∆H β(1 - θm)2(∆H - RTm) ) + ln F RTm ∆S P0 exp R

( )

(1)

where R is the gas constant (8.314 J K-1 mol-1), β is the heating rate (K s-1), θm is the coverage of the acid site by ammonia at the peak maximum, P0 is the pressure at standard conditions (1.013 × 105 Pa), ∆H is the adsorption heat of ammonia (J mol-1), and ∆S is the entropy change with respect to the desorption of ammonia (J K-1 mol-1). The adsorption heat ∆H, i.e., acid strength, was determined from the slope to be 128 kJ mol-1 for both samples. On the other hand, the entropy change ∆S was calculated to be 160-165 J K-1 mol-1 for each experimental run. The entropy change ∆S must consist of the terms of phase-transformation and mixing30 as ∆S ) ∆Strans + ∆Smix, and ∆Smix ) -R(ln xNH3 + xHe/xNH3 ln xHe), where ∆Strans and ∆Smix are the terms of phase-transformation and mixing, respectively, and xi means the molar ratio of component i. The term ∆Smix at the peak maximum is calculated from the experimentally observed pressures of ammonia and helium to be 40 to 80, in most cases 60-70 J K-1 mol-1 for the experiments shown in Figure 6. Therefore, the former term is determined to be ca. 90-100 J K-1 mol-1. This value is almost same as those observed on zeolites,30 and close to the entropy change with respect to the vaporization of ammonia (97.2 J K-1 mol-1) and other various liquids (80-110 J K-1 mol-1). These findings indicate that the entropy change with respect to the desorption of ammonia from the WO3/ZrO2 catalyst is mainly determined by the increase of free volume of molecule,30 and the desorption process on the WO3/ZrO2 system is controlled by the same rule as that for zeolites. According to these findings, hereafter we can determine the acid strength of the WO3/ZrO2 catalyst by one point experiment of ammonia TPD. The determined adsorption heat, ca. 130 kJ mol-1, is close to the strength of the acid site generated by the isomorphous substitution of aluminum into the silicate framework in the ZSM-5 zeolite,39 which has never been categorized as a superacid. The material with an acid strength stronger than 100% perchloric or sulfuric acid,55 namely the material whose H0 function is lower than -11.93, is termed superacid.56 Therefore, the adsorption heat is related to the H0 scale of acid strength in solution. It is assumed that the solid acid acts as an acid also in

7212 J. Phys. Chem. B, Vol. 103, No. 34, 1999

Naito et al.

an aqueous solution as

WZ + NH3 (aq) a WZ-NH3

(2)

where WZ is the tungsta-zirconia catalyst. The equilibrium constant of the assumed reaction at 298 K is roughly estimated to be 1.4 × 1016 on the basis of the determined standard enthalpy change (ca. 130 kJ mol-1) and the entropy change (95 J K-1 mol-1) of the reaction

WZ-NH3 a WZ + NH3 (g)

(3)

For such a solid S with H0 < -11.93, the equilibrium constant of the reaction with ammonia at 298 K in aqueous medium

S + NH3 (aq) a S-NH3

where ∆Gf,i0 is the standard Gibbs energy change for formation of component i.57 Since reaction (2) can be expressed as -(3)-(6), the Gibbs energy change, ∆G20, of reaction (2) can be calculated as

∆G20 ) -∆G30 - ∆G60 ) -102 + 9.85) -92 kJ mol-1 (8) Here we assume that the Gibbs energy change of suspension of WZ into aqueous solution is close to that of WZ-NH3; it must be balanced. Therefore, the equilibrium constant K2 of reaction (2) at 298 K is roughly estimated to be

K2 ) exp

(4)

is roughly calculated to be > 1.5 × 1021 (see Appendix). Therefore, it is concluded that the present tungsta-zirconia catalyst with the apparently lower constant, 1.4 × 1016, is not a superacid. Catalytic Activity. The catalytic activity for the skeletal isomerization of n-butane was created by the loading of the tungsta on zirconia, as shown in Figure 5. The maximum activity was found at 6 W nm-2, and further accumulation of tungsta layer diminished the activity. It is therefore considered that the isomerization proceeded on the Brønsted acid site with 130 kJ mol-1 in the adsorption heat of ammonia on the tungsta monolayer. Lack of superacidity on these catalysts suggests that the superacidity is not required for the skeletal isomerization, at least not under these conditions.

Appendix 1. Calculation of the Equilibrium Constant of Reaction (3) on the Tungsta-Zirconia Catalyst from the Determined Thermodynamic Parameters. The enthalpy change of reaction (3), ∆H30, is assumed to be 130 kJ mol-1, and the entropy change ∆S30 is assumed to be 95 J K-1 mol-1. If temperature dependence of ∆H30 and ∆S30 is ignored, we obtain the standard Gibbs energy change, ∆G30, of reaction (3) at 298 K to be

∆G30

)

∆H30

-

T∆S30

-1

) 102 kJ mol

(5)

H0 < pKa

S + B a S-B

∆G60 ) ∆Gf,NH3(aq)0 - ∆Gf,NH3(g)0 ) -26.50 + 16.65 ) -9.85 kJ mol-1 (7)

(9)

where S is a solid superacid with H0 < -11.93 and B is the base indicator with pKa ) -11.93, the equilibrium constant K9 can be drawn as

K9 )

[S-B] >1 [S][B]

(10)

In an aqueous solution, the equilibrium constant K4 of the assumed reaction (4) can be expressed as

K4 )

[S-NH3] [S][NH3]

)

[B][S-NH3] [S-B] [S-B][NH3] [S][B]

(11)

On the other hand, because pKa of the conjugate acid of B is -11.93, and that of ammonia59 is 9.25, we can obtain equations

-log

[H3O+][B]

-log

[H3O+][NH3]

[BH+]

) -11.93

(12)

and

[NH4+]

) 9.25

(13)

) -21.18

(14)

The difference (12) - (13) shows

-log

(6)

the standard Gibbs energy change, ∆G60, is

(8)

where pKa is the acid dissociation constant of the conjugate acid of the used indicator.58 Therefore, in the reaction

With respect to the dissolving of ammonia into an aqueous solution

NH3 (g) + aq a NH3 (aq)

(7)

2. Calculation of the Equilibrium Constant on a Solid Acid with H0 < -11.93. It is regarded that the H0 function of solid acid which can convert the base indicator into the acidic form is

Conclusion 1. Tungsten oxide almost completely covered the surface of zirconia with 5-6 W nm-2 based upon the strong interaction between W and Zr. 2. The improved ammonia TPD method showed the creation of a Brønsted acid site with a strength corresponding to ca. 130 kJ mol-1 of the adsorption heat of ammonia on the tungsta monolayer. 3. The Brønsted acid site on the monolayer was active for skeletal isomerization of butane.

-∆G20 ) 1.38 × 1016 RT

[B][NH4+] [BH+][NH3]

Therefore, the equilibrium constant K15 of the reaction

BH+ (aq) + NH3 (aq) a B (aq) + NH4+ (aq) should be

(15)

Acidity Generated on Tungsten Oxide Monolayer

K15 )

[B][NH4+] [BH+][NH3]

) 1021.18 ) 1.514 × 1021

J. Phys. Chem. B, Vol. 103, No. 34, 1999 7213

(16)

It is then assumed that the equilibrium (15) exists on the surface of solid as well as in aqueous solution. In other words, the equilibrium constant of the reaction

S-B + NH3 (aq) a B (aq) + S-NH3

(17)

is assumed to be

[B][S-NH3]

) K15 ) 1.514 × 1021

[S-B][NH3]

(18)

Substituting (18) into (11) derives

K4 )

[B][S-NH3] [S-B] [S-B][NH3] [S][B]

[S-B] ) 1.514 × 1021 (19) [S][B]

From equations (10) and (19), we obtain

K4 > 1.514 × 1021

(20)

In summary, the solid superacid whose H0 function is lower than -11.93 must have an equilibrium constant higher than 1.514 × 1021 for the assumed reaction with ammonia in an aqueous solution. References and Notes (1) Tanabe, K.; Itoh, M.; Morishige, K.; Hattori, H. In Preparation of Catalysts; Delmon, B, Jacobs, P. A., Poncelet, G., Eds.; Elsevier: Amsterdam, 1976; p 65. (2) Hino, M.; Kobayashi, S.; Arata, K. J. Am. Chem. Soc. 1979, 101, 6439. (3) Hino, M.; Arata, K. J. Chem. Soc., Chem. Commun. 1987, 1259. (4) Hino, M.; Arata, K. Bull. Chem. Soc. Jpn. 1994, 672, 1472. (5) Hino, M.; Arata, K. Chem. Lett. 1989, 971. (6) Arata, K. AdV. Catal. 1990, 37, 165. (7) Lin, C.; Hsu, C. Y. J. Chem. Soc., Chem. Commun. 1992, 1479. (8) Corma, A. Chem. ReV. 1995, 95, 559. (9) Vermaire, D. C.; van Berge, P. C. J. Catal. 1989, 116, 309. (10) Kim, D.-S.; Ostromecki, M.; Wachs, I. E. J. Mol. Catal., A: Chemical 1996, 106, 93. (11) Igresia, E.; Barton, D. G.; Soled, S. L.; Miseo, S.; Baumgartner, J. E.; Gates, W. E.; Fuentes, G. A.; Meitzner, G. D. Stud. Surf. Sci. Catal. 1996, 101, 533. (12) Zhao, B.; Xu, X.; Gao, J.; Fu, Q.; Tang, Y. J. Raman Spectrosc. 1996, 27, 549. (13) Santiesteban, J. G.; Vartuli, J. C.; Han, S.; Bastian, R. D.; Chang, C. D. J. Catal. 1997, 168, 431. (14) Scheithauer, M.; Grasselli, R.; Kno¨zinger, H. Prepr. Pap.sAm. Chem. Soc., DiV. Pet. Chem. 1997, 42, 738. (15) Martin, C.; Martin, I.; Rives, V.; Solana, G.; Loddo, V.; Palmisano, L.; Sclafani, A. J. Mater. Sci. 1997, 32, 6039. (16) Boyse, R. A.; Ko, E. I. J. Catal. 1997, 171, 191. (17) Sohn, J.-R.; Park, M.-Y. Langmuir 1998, 14, 6140. (18) Barton, D. G.; Soled, S. L.; Meitzner, G. D.; Fuentes, G. A.; Iglesia, E. J. Catal. 1999, 181, 57. (19) Niwa, M.; Inagaki, S.; Murakami, Y. J. Phys. Chem. 1985, 89, 3869.

(20) Niwa, M.; Matsuoka, Y.; Murakami, Y. J. Phys. Chem. 1987, 91, 4519. (21) Niwa, M.; Suzuki, K.; Kishida, M.; Murakami, Y. Appl. Catal. 1991, 67, 297. (22) Yamada, H.; Niwa, M.; Murakami, Y. Appl. Catal. 1993, 96, 113. (23) Niwa, M.; Katada, N.; Murakami, Y. J. Phys. Chem. 1990, 94, 6441. (24) Niwa, M.; Katada, N.; Murakami, Y. J. Catal. 1992, 134, 340. (25) Katada, N.; Niwa, M. AdV. Mater., Chem. Vap. Deposition 1996, 2, 125. (26) Katada, N.; Niwa, M.; Sano, M. Catal. Lett. 1995, 32, 131. (27) Okamoto, Y.; Arima, Y.; Nakai, K.; Umeno, S.; Katada, N.; Yoshida, H.; Tanaka, T.; Yamada, M.; Akai, Y.; Segawa, K.; Nishijima, A.; Matsumoto, H.; Niwa, M.; Uchijima, T. Appl. Catal., A 1998, 170, 315. (28) Vaidyanathan, N.; Houalla, M.; Hercules, D. M. Catal. Lett. 1997, 43, 209. (29) Praff, C.; Zurita, M. J. P.; Scott, C.; Patin˜o, P.; Goldwasser, M. R.; Goldwasser, J.; Mulcahy, F. M.; Houalla, M.; Hercules, D. M. Catal. Lett. 1997, 49, 13. (30) Niwa, M.; Katada, N. Catal. SurVeys Jpn. 1997, 1, 215. (31) Niwa, M.; Iwamoto, M.; Segawa, K. Bull. Chem. Soc. Jpn. 1986, 59, 3735. (32) Chester, A. W.; Higgins, J. B.; Kuehl, G. H.; Schlenker, J. L.; Woolery, G. L. In Preprint of the 11th Meeting on the Reference Catalyst of Japan; The Catalysis Society of Japan: Tokyo, 1987; p 22. (33) Bagnasco, G. J. Catal. 1996, 159, 249. (34) Woolery, G. L.; Kuehl, G. H.; Timken, H. C.; Chester, A. W. Zeolites 1997, 19, 288. (35) Igi, H.; Katada, N.; Niwa, M. In Proc. 12th Int. Zeolite Conf. Tracy, M. M. J., Marcus, B. K., Bisher, M. E., Higgins, J. B., Eds.; Materials Research Society: Warrendale, 1998, p 2643. (36) Miyamoto, T.; Katada, N.; Kim, J.-H.; Niwa, M. J. Phys. Chem., B 1998, 102, 6738. (37) Sawa, M.; Niwa, M.; Murakami, Y. Zeolites 1990, 10, 307. (38) Niwa, M.; Katada, N.; Sawa, M.; Murakami, Y. J. Phys. Chem. 1995, 99, 8812. (39) Katada, N.; Igi, H.; Kim, J.-H.; Niwa, M. J. Phys. Chem., B 1997, 101, 5969. (40) Katada, N.; Iijima, S.; Igi, H.; Niwa, M. Stud. Surf. Sci. Catal. 1997, 105, 1227. (41) Kno¨zinger, H. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, 1997; p 707. (42) Xie, Y.-C.; Tang, Y.-Q. AdV. Catal. 1997, 37, 1. (43) Salvatti, L.; Makovski, L. E.; Stencel, J. M.; Brown, F. R.; Hercules, F. D. J. Phys. Chem. 1981, 85, 3700. (44) Vuurman, M. A.; Wachs, I. E. J. Phys. Chem. 1992, 96, 5008. (45) Gru¨nert, W.; Feldhaus, R.; Anders, K.; Shpiro, E. S.; Minachev, K. M. J. Catal. 1995, 154, 201. (46) Karakonstantis, L.; Matralis, H.; Kordulis, C.; Lycourghiotis, A. J. Catal. 1996, 162, 295. (47) Katada, N.; Ishiguro, H.; Muto, K.; Niwa, M. AdV. Mater., Chem. Vap. Deposition 1995, 1, 54. (48) Nakamoto, K. In Infrared and Raman Spectra of Inorganic and Coordination Compounds, 3rd ed.; John Wiley & Sons: New York, 1962; p 197. (49) Lide, D. R. In Handbook of Chemistry and Physics, 73rd ed.; CRC Press: Boca Raton, Ann Arbor, London and Tokyo, 1992; pp 9-44. (50) Earl, W. L.; Fritz, P. O.; Gibson, A. A. V.; Lunsford, J. H. J. Phys. Chem. 1987, 91, 2091. (51) Soled, S. L.; McVicker, G. B.; Murrell, L. L.; Sherman, L. G.; Dispenziere, Jr., N. C.; Hsu, S. L.; Waldman, D. J. Catal. 1988, 111, 286. (52) Wachs, I. E. Catal. Today 1996, 27, 437. (53) Niwa, M.; Igarashi, J. Catal. Today in press. (54) Cvetanovic, R. J.; Amenomiya, Y. AdV. Catal. 1967, 17, 103. (55) Hall, N. F.; Conant, J. B. J. Am. Chem. Soc. 1927, 49, 3047. (56) Gillespie, R. J. Acc. Chem. Res. 1968, 1, 202. (57) Atkins, P. W. In Physical Chemistry, 3rd ed.; Oxford University Press: Oxford, 1986; p 818. (58) Walling, C. J. Am. Chem. Soc. 1950, 72, 1164. (59) Atkins, P. W. In Physical Chemistry, 3rd ed.; Oxford University Press: Oxford, 1986; p 826.