11576
J. Phys. Chem. 1994, 98, 11576-11581
Observation of the Structural Change in the Nb Sites during Ethanol Dehydration on a SiO2-Attached Nb Dimer Catalyst by EXAFS Nobuyuki Ichikuni?and Yasuhiro Iwasawa* Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received: May 10, 1994; In Final Form: July 25, 1994@
The Nb dimers attached on Si02 were prepared by using the dimeric complex [Nb(yS-C5Hs)H-~-(y5,y1CS&)]~as precursor. The surface Nb structure was characterized by EXAFS and XANES to possess the monomer-pair structure without Nb-Nb bonding. It was suggested that the catalytic ethanol dehydration on the monomer-pair catalyst proceeds in conjunction with the structure change of the Nb sites involving a formation-break cycle of Nb-Nb bonds, that is, a reversible monomer-dimer transformation. The catalytic performance is compared with that of the Nb dimers on Si02 previously reported.
Introduction The attached metal catalysts which are prepared by reactions between suitable organometallic compounds and inorganic oxides, followed by various chemical treatments depending on what kind of active surface structures should be demanded, can provide novel information on the genesis of solid catalysis and a new way of developing efficient catalysts.1-6 The use of such catalysts which possess chemically and structurally controlled surfaces may also bring about information on catalytic mechanisms involving dynamic structure change of active sites on an atomic scale.6-8 Conventional impregnation niobium catalysts and bulk niobium oxide have been thought to be relatively inert and studied largely from the viewpoints of support effects on metal catalysis like a strong metal-support interaction (SMSI)9-1s or acidic properties at low temperatures below 400 K.16-18 However, the attached Nb monomer catalysts prepared from Nb(v3-C3H5)4 and inorganic oxides like Si02 and Ti02 have been demonstrated to be the first active niobium sample for catalytic ethanol dehydrogenati~n.~.'~ Recently, Nb oxide and its mixed oxide catalysts have been extensively studied. One-atomic-layer niobia on Si02 was prepared by the reaction of Nb(OCzH5)5 with the Si02 surface.20-22 EXAFS revealed that the surface Nb oxide layers are somewhat distorted by a structural mismatch and a strong Nb-0-Si interaction, and the distortion should be released by the creation of the coordinatively unsaturated Nb sites, resulting in Lewis acidic catalysis for the intra-dehydration of ethanol?0-22 More recently, the surface Nb oxide phase which was prepared by using Nb ethoxide or Nb oxalate as a precursor was characterized by means of in situ RamanZ3and FT-IR,243" where highly distorted NbO6 octahedral structures were present. Datka et al. characterized oxide-supported Nb2O5 catalysts prepared by use of an aqueous solution of Nb oxalateloxalic acid by FTIR spectra of chemisorbed pyridine and concluded that the concentration of Lewis acid sites and Bronsted acid sites depends on the surface Nb coverage.26 Burke and KO prepared a series of composite oxides containing NbzOs and Si02 by using Nb ethoxide and examined the relationship between acidity and structure around the Nb atom. They proposed that the tetra-
* To whom correspondence should be addressed. +Present address: Department of Applied Chemistry, Faculty of Engineering, Chiba University, Yayoi-cho, Inage-ku, Chiba 263, Japan. Abstract published in Advance ACS Abstracts, October 1, 1994. @
0022-365419412098-11576$04.5010
hedral Nb species with a Nb=O bond shows a strong Lewis acidity and the highly distorted octahedral species has a moderately strong Lewis a~idity.2~ The additive effects of Nb oxides have also been studied in oxidative dehydrogenation of ethane and propane. Desponds et al. reported that the oxidative dehydrogenation of ethane on the Mo-V-mixed oxide was considerably enhanced by Nb addition.28 In examining the oxidative dehydrogenation of propane over the series of oxides, such as lanthana, magnesia, zirconia, and titania, Smits et al. found that Nb2O5 gave the best performance of the series with a selectivity of 76% to propene and a conversion of propane of 2% at a temperature of 859 K.29 Ross et al. have demonstrated that Nb2O5 bulk and Nb2Os-modified catalysts, when used in high-temperature oxidation processes, can exhibit high selectivities in the oxidative dehydrogenations compared with previous oxide catalysts without N b 2 0 ~ . ~ ' The catalytic reaction mechanism has been studied in the most detail on the attached Nb monomer catalyst for ethanol dehydrogenation. Ethanol adsorbs on the Nb-0 site to form Nb-OH and Nb-OCzHs groups, but this species was too stable under vacuum and it did not decompose at the reaction temperatures 400-523 K. The species is decomposed to ethene and water (dehydration) only above 600 K. However, in the presence of a second ethanol adsorbed on the Nb site, the Nb ethoxide species is readily dehydrogenated to form acetaldehyde and hydrogen, where the switchover of reaction path from dehydration to dehydrogenation was induced by the coexistence of the second ethanol molecule.8 This finding of a new reaction mechanism tempted us to examine the catalytic effect of the second Nb atom instead of the second ethanol molecule in ethanol r e a ~ t i o n . ~ l We - ~ ~have prepared Nb dimers on Si02 by use of [Nb(q5-C5H5)H-p-(q5,q1-C5H4)12 as a precursor. It has been demonstrated that acidhase catalytic properties can be controlled by the nucleation of active sites from one Nb atom to two Nb atoms; that is, on the Nb dimer catalyst the dehydration of ethanol selectively proceeded, in contrast to the dehydrogenation of ethanol on the Nb monomer catalyst.33In the dimer catalysis it is expected that the catalytic properties are changed by the Nb-Nb distance of the dimers and the local structure around the Nb atom and hence modified by cooperation of two adjacent Nb sites. To examine this viewpoint, we prepared various Nb dimers on four different Si02, which were characterized by EXAFS as illustrated in Figure 1. The Nb dimers attached on 200-300 0 1994 American Chemical Society
SiOz-Attached Nb Dimer Catalyst ; 0.170 nm
,
0.303nm
I I I
J. Phys. Chem., Vol. 98, No. 44, 1994 11577 I 0.308nm
0.180 nm
I
I I I
1l111111111111111111ll
TiO2 Figure 2. Structure of the TiOz-attached Nb monomer catalyst.
f0.334nm
7 NbSI: 0.330 nm
Nb2 (4)
Figure 1. Structure models of four different Nb dimers on Si02 determined by EXAFS (0-Sir is not shown for Nbz(4)).
m2/g of Si02 (Nbz(1) and Nbz(2)) possessed the dimeric Nb structures bridged with an oxygen atom (Nb-Nb: 0.303-0.308 nm), while on 500 m2/g of Si02 the monomer pairs (Nb~(3)) were created, showing no Nb-Nb bond. In contrast, the dimer units were suggested to be coupled to form a tetrameric Nb structure on 50 m2/g of Si02 (Nbz(4)). It was found that the ratio of the rate of diethyl ether (inter-dehydration) to the rate of ethene (intra-dehydration) changed among these catalysts, whereas no difference in the selectivity was observed with the corresponding conventional impregnation Nb catalysts on which the catalytic selectivity was not ~ontrollable.~~ Moreover, the rate constant for the intra-dehydration of the first adsorbed ethanol was suppressed due to chemical interaction between two adjacent Nb sites when the second ethanol molecule adsorbed on the adjacent Nb atom in a dimer unit. It was suggested that the Nbz(3) possesses a Nb monomerpair structure on SiOz, as shown in Figure l.34 However, this may be strange from the catalytic viewpoint, because the Nb monomers on Si02 and Ti02 have been demonstrated to be active and selective for the dehydrogenation of ethanol, not for the dehydration.*J9 The local structure around the Nb atom on Ti02 is similar to the Nbz(3) structure, as shown in Figure 2. Therefore, it seemed that the Nbz(3) catalyst should have produced acetaldehyde (dehydrogenation product) as a main product in the ethanol reaction by the self-assisted dehydrogenation mechanism.* Although the static structure of Nbz(3) resembles the Nb monomer/TiO2, the catalysis of Nbz(3) was entirely different from that of Nb monomermi02. To clarify this problem in relation to the genesis of catalysis, we examined the structure of the Nb sites in the working state by means of EXAFS. The preliminary results were briefly reported in the previous paper.34 In this article, we report the structural change in the Nb sites during the catalytic reaction on Nb2(3) characterized by detailed EXAFS analysis and the kinetics for ethanol dehydration on Nbz(3). These are also discussed in comparison with the data for other Nb dimer catalysts. Experimental Section Catalyst Preparation. The dimeric Nb complex [Nb(q5C5H5)H-p-(q5,q1-C5&)]2 was synthesized from NbCpzC12 (Aldrich) with NaH (Aldrich) as in the l i t e r a t ~ r e . ~Because ~ . ~ ~ the
complex was sensitive to moisture and air, it was handled and kept under vacuum or high-purity Ar gas (99.9999%, Takachiho Trading Co., Ltd.). The silica-attached Nb dimer catalyst (Nb2(3)) was prepared by the reaction of a toluene solution of the dimeric Nb complex with OH groups of the Si02 surface at 313 K, followed by decantation and washing twice by toluene to remove the unreacted complex and by evacuation to remove the residual solvent. After the above attachment processes were completed, the sample was reduced with hydrogen at 823 K. The treated sample was slowly exposed to a low pressure of oxygen at room temperature and then oxidized with oxygen at 773 K to convert it to the oxidized form.32,33Si02 (Fuji-Davison micro bead silica gel 4B) with a surface area of 500 m2/g was used as the support. Nb loading was regulated to be 1.1 wt % in the present study. EXAFSKANES Spectroscopy. The Nb K-edge X-ray absorption spectra at 70 K were obtained in a transmission mode at the EXAFS facilities installed on the BL-1OB line of the Photon Factory in the National Laboratory for High Energy Physics (KEK-PF) (Proposal No. 88-020). The synchrotron radiation for EXAFS measurements was operated at a ring energy of 2.5 GeV with a storage positron current of 150-250 mA and monochromatized by a channel cut Si (311) crystal. Energy calibration was performed using the Nb foil K-edge inflection point at the edge (18 986.9 eV). Ion chambers filled with Ar N2 (50%/50%)and Ar were used for X-ray detection of IO (before sample) and I (after sample), respectively. A glass cell sealed with two Kapton windows was used to measure the X-ray absorption spectra. The cell length was chosen as 20 or 10 mm for the total absorption coefficient not to exceed 3. The cell was connected to a Pyrex U-shaped tube in a closed circulating system in which the catalysts were treated and catalytic reactions were conducted, so that the sample transfer was performed without contacting air. The oscillation part of the absorption as a function of the X-ray photon energy was extracted as described el~ewhere.~' Normalization of the EXAFS data was carried out by fitting the background absorption coefficient around the energy region by ca. 50 eV higher than absorption edge with the smoothed absorption coefficient of an isolated atom (Victoreen equation, C d 3 -I- Dd4). Fourier transformation (IT) of k3-weighted normalized EXAFS data was performed over the 30-130 nm-' range to obtain the radial distribution function. The Fourier-filtered data were analyzed by the least-squares curve-fitting method with eqs 1 and 2.37,38
+
k, = Jk2 - 2mehEoi/ti2
(2)
where N,, rj, a,, AEo,, and me represent the coordination number, bond distance, and Debye-Waller factor of the jth shell, the difference between the model compound and experimental threshold energies, and the mass of the electron, respectively.
Ichikuni and Iwasawa
11578 J. Phys. Chem., Vol. 98, No. 44, 1994
Fj and 4j are amplitude and phase shift functions, respectively. Precise structural characterization of surface species by EXAFS requires the use of reference materials in the data analysis.39 Then the curve-fitting analysis was performed by using empirical parameters extracted from Nb foil, LaNb04, and NbSiz for Nb-Nb, Nb-0, and Nb-Si bonds, respectively. Data analyses were conducted using the Program EXAFS2N with the HITAC M-682H computer system at The University of Tokyo. Catalytic Ethanol Dehydration. Catalytic ethanol dehydration reactions were carried out in a closed circulating system equipped with a gas chromatograph (Shimadzu GC-8A). The amount of catalyst was ca. 0.2 g for a typical reaction. The catalyst was treated with 13.3 kPa of oxygen at 773 K for 1 h, followed by evacuation at the same temperature for 30 min in situ before use as a catalyst. Ethanol mako Pure Chemical Co., Ltd.) was purified by freeze-thaw cycles before use. Reaction products were separated by a 2 m DOS column (Gasukuro Kogyo Inc.) at 338 K and detected by TCD.
-4.01 3
" "5 ' " "7 I
9 1 1 1 3
r10.1 nm
k 1 10 nm''
3.2
g ?Y
-3.2
3
5
7 9 1 1 1 3
Results and Discussion Structure of the Catalysts. Figure 3 shows the k3-weighted Nb K-edge EXAFS oscillation (I-a) and its associated Fourier transforms (I-b) for the Nb dimer catalyst (Nb2(3)). The
shoulder at around 53 nm-' in the oscillation has been demonstrated to be due to the Nb-Si bond.20-22*34,40 Since the shoulder appears at ca. 53 nm-', as shown in Figure 3 (I-a), the Nb atoms seem to be chemically attached to the Si02 surface. The chemically attaching reaction of the dimeric Nb precursor with the OH groups of the Si02 surface was also evidenced by the evolution of H2 upon supporting and by the decrease in the intensity of the Y(OH) peak.33 In order to obtain definite structural information around the Nb atom in the attached Nb species (3), we performed the inverse Fourier transformation and its curve fitting by using the empirical parameters. Curve-fitting analyses were performed as follows. The first peak at around 0.15 nm in the Fourier transform (Figure 3 (I-b)) is straightforwardly assigned to the Nb-0 bond, but the analysis by one-wave fitting (Nb-0) never fit the observed curve, as shown in Figure 3 (11-a). Next we tried the two-wave fitting (Nb-0 and Nb-0), resulting in some improvement, but the amplitude and interval of oscillation were still not reproduced, as shown in Figure 3 (11-b). Then, we performed the three-waves analysis (Nb-0, Nb-0, and Nb-0) and obtained the best-fitting in Figure 3 (11-c). Besides the f i s t peak, the second peak around 0.3 nm is observed in the Fourier transform (Figure 3 (I-b)). The peak is relatively small, but the curve-fitting analysis for this peak was performed by assuming the Nb-Nb bond in Figure 3 (11-d) because the dimeric complex was used as a precursor for the catalyst. The one-wave analysis (Nb-Nb) did not agree with the observed oscillation (Figure 3 (11-d)), where there existed a difference between calculated and observed curves in both lower and higher k regions. Moreover, Rf (R-factor) defined by eq 3 as a measure of fitting was as large as 9.3%.
Alternatively, the one-wave fitting by assuming the Nb-Si bond was performed because of the existence of the shoulder around 53 nm-' in the oscillation (Figure 3 (I-a)), which suggests the presence of a Nb-Si bond.20,22*34The calculated curve reproduced the experimental one with an Rf of 4.1% (Figure 3
I
3
6
9
k 110 nm"
1
2
3
e
J
9
12
k I10 n m - '
Figure 3. EXAFS spectra and analyses for the attached Nb dimer
catalyst(3): I-a, k3-weighted oscillation; I-b, its associated Fourier transform; 11-a, one-wave (Nb-0); 11-b, two-wave (Nb-0, Nb-0); and 11-c, three-wave (Nb-0, Nb-0, Nb-0) fitting in the Fourier filtering range 0.10-0.23 nm; 11-4 one-wave (Nb-Nb) and 11-e, onewave (Nb-Si) fitting in the Fourier filtering range 0.25-0.35 nm; 111, four-wave (Nb-0, Nb-0, Nb-0, Nb-Si) fitting in the Fourier Filtering range 0.10-0.35 nm. (11-e)). When the Nb-Nb bond was added to the analysis, the Rf was not improved (3.5%) and the coordination number of the added Nb-Nb bond was estimated to be smaller than 0.1. Furthermore, the doublet feature at 90-100 nm-' in the k3x(k) oscillation can also be used as a fingerprint for the existence of the Nb-Nb bond in the Nb-Si02 systems,34but it was not for Nbz(3), suggesting that there is no Nb-Nb interaction in Nb2(3). Thus, there is no evidence for the existence of Nb-Nb bonding in the Nb2(3) structure. Finally, four-wave fitting (Nb-0, Nb-0, Nb-0, and Nb-Si) was achieved over the whole range of two peaks in the Fourier transform (0.10-0.35 nm), as shown in Figure 3 (III). The experimental oscillation was nearly completely reproduced by this fitting analysis. The best-fit result is listed in Table 1. The coordination number of the Nb=O bond (0.173 nm) demonstrates that there is about one Nb=O double bond around the Nb atom. The coordination numbers of the Nb-0 bond (0.197 nm) and the Nb-0 bond (0.228 nm) were determined as 2.6 and 0.7, respectively. The sum of the coordination numbers for the two Nb-0 single bonds is approximately 3. The XPS binding energies of Nb 3d levels were almost the same as those reported for Nb5+ monomers on Thus, it is suggested that Nb atoms are located on 3-fold sites of the Si02 surface in a tridentate form. The Nb-Si bond has been observed with the Nb/SiOz systems with the Nb-O(surface) bond shorter than 0.2 nm.8920-22331-34,40 It is therefore supposed that the observed Nb-Si bond at 0.336 nm is associated with the Nb-0 bond at 0.197 nm and not with the longer Nb-0 bond at 0.228 nm. Thus, the coordination number (1.7) of the Nb-Si bond is much smaller than the total number of 3 for the Nb-0 bonds. From these results the Nbz(3) structure may be pictured as in Figure 1.
Si02-Attached Nb Dimer Catalyst
J. Pkys. Ckem., Val. 98, No. 44, 1994 11579
TABLE 1: Curve-Fitting Results for (1) the Oxidized Form of the Nb Dimer Catalyst, (2) after Ethanol Admission at 523 K and (3) at the End of the Reaction condition bond Plnm AEocleV DWd/nm 1 Nb=O 0.173 f 0.003 0.5 f 0.4 9.0 0.008 Nb-0 0.197 f 0.002 2.6 f 0.4 5.0 0.005 Nb-0 0.228 f 0.003 0.7 f 0.2 9.0 0.004 Nb-Si 0.336 f 0.003 1.7 f 0.3 5.9 0.003 2 Nb-0 0.198 f 0.002 3.2 f 0.3 9.0 0.006 Nb-0 0.220 f 0.003 1.4 f 0.3 9.0 0.005 Nb-Nb 0.308 f 0.002 1.0 f 0.2 8.0 0.005 Nb-Si 0.326 f 0.003 2.0 f 0.3 1.7 0.003 3 Nb=O 0.176 f 0.003 0.5 f 0.4 9.0 0.008 Nb-0 0.195 f 0.002 2.6 f 0.4 2.0 0.005 Nb-0 0.230 f 0.003 0.7 f 0.2 9.0 0.004 Nb-Si 0.339 f 0.003 2.0 f 0.3 8.0 0.003
5.1
a
-5.1L 3 5
7 9 1 1 1 3
k I10 n m - ’
Bond distance. Coordination number. Difference between model compound and experimental threshold energies. Debye-Waller factor; filtering range 0.10-0.35 nm.
r / O . l nm 2.9 n
?Y
-2.9 3
7 9 1 1 1 3
k / 10 nm-’ Figure 5. EXAFS spectra of Nbz(3) after ethanol admission: (a) k3weighted oscillation; (b) Fourier transform; and (c) curve fittings (solid line, obs; broken line, calc).
a 0.0‘ 18960
5
I
I
19000
19040
Photon energy I eV
-
Figure 4. Nb K-edge XANES spectra for the attached Nb dimer catalysts; the arrow represents the 1s 4d transition peak: (a) Nbz(11, (b) Nb2(2), (c) NbzO), and (d) Nb~(4).
-
The distinct peak due to the 1s 4d transition was observed for Nb2(3) in the Nb K-edge XANES spectra, as shown in Figure 4. The pre-edge peak definitely appears when Nb is located in tetrahedral or distorted tetrahedral symmetry, which implies that Nb-0 double bonds are included in Nbz(3). This result is compatible with the EXAFS result in Table 1 and supports the structure in Figure 1. The definite pre-edge peak in the XANES spectra was also observed with the catalysts Nbz(1) and Nb2(2), but not for Nbz(4). Structural Change of Nb Sites in Catalytic Reaction. N b 2 (3) catalyzed the dehydration of ethanol with a selectivity of 91% at 523 K. The ratio of the rate of diethyl ether(interdehydration) to the rate of ethene (intra-dehydration) was 0.93. The selective dehydration of ethanol on Nb2(3) with the local Nb monomer structure is strange because the Nb monomers on Si02 and Ti02 behave as a dehydrogenation catalyst, as already m e n t i ~ n e d . ~Therefore, .~~ the structural change of the Nb sites in the course of catalytic ethanol dehydration was examined by means of EXAFS. A large amount of H20 was formed at the initial stage of reaction. The amount of produced water at 3 min at 523 K and P(C2H50H) = 3.3 kPa was approximately equal to the Nb dimer unit. After the rapid formation of water, the water was steadily produced, the amount of which corresponded to the amount of the dehydration products (ethene and diethyl ether). Immediately after the initial evolution of water, the catalyst was
quenched by cooling it to room temperature, accompanied by evacuation of the gas phase. The EXAFS data for the sample thus obtained are shown in Figure 5 . The oscillation is much different from Figure 3 (I-a), rather resembling that for the oxygen-bridged Nb dimer catalysts Nbz(1) and N b ~ ( 2 ) .The ~~ EXAFS data were analyzed by the curve-fitting method as mentioned above. The best-fitting curve is shown in Figure 5c, and the bond distances and the coordination numbers are listed in Table 1. The Nb=O double bond disappeared, and a Nb-Nb bond of 0.308 nm with a coordination number of 1.0 newly appeared. The reduction of the 1s 4d transition peak height in the XANES spectrum upon ethanol adsorption was observed, also supporting the disappearance of the Nb-0 bond. Ethanol adsorbs on the Nb=O sites, producing Nb-OH and Nb-0C2H5.8p33334 When the Nb-OH species thus formed are located close to each other, the dehydration from two adjacent Nb-OH species can readily take place at 523 K. As a result, Nb-0-Nb bonds are formed to make a dimeric structure, as shown in Figure 6. The Nb-bridge 0 bond was observed at 0.220 nm and the Nb-Nb length was 0.308 nm, both being similar to those for Nb2(2).34340The coordination number for Nb-0 of 0.198 nm was determined to be 3.2, which is larger by 0.6 than 2.6 for the original catalyst. This suggests that an additional Nb-0 bond at 0.198 nm (Nb-OC2H5) was formed upon ethanol adsorption, which agrees with the results of IR and XANES. The IR spectra at this stage showed the v(CH) peaks of Nb-OCzH5, but no v(0H) peak of Nb-OH.8 Accordingly, the structural change by ethanol adsorption at 523 K accompanied by H20 evolution is illustrated in Figure 6. When the Nb dimers fully adsorbed with ethanol in Figure 6 were decomposed, ethene and diethyl ether were evolved at 523 K. After the formation of ethene and diethyl ether was completed, the EXAFS spectrum of the sample was measured.
-
11580 J. Phys. Chem., Vol. 98, No. 44, 1994 0.228nm
Ichikuni and Iwasawa
v73nm R
k,= 0.0039 min"
Nb-Nb: 0.308 nm'
Figure 6. Reaction scheme for ethanol dehydration on Nb2(3).
TABLE 2: Rate Constants for Ethanol Dehydration at 523 K rate ~onstandlO-~ min-I
U
b
3.1 3.9
2.8 2.5
2.1 1.9
a Determined from each elementary step. Determined in steadystate reaction conditions: see text.
The best-fit result of the EXAFS data is listed in Table 1. The Nb=O bond was regenerated, and the Nb-Nb bond disappeared again. It is evident from Table 1 that the original structure of the catalyst was reproduced. The adsorption of ethanol on Nbz(3) was given by the Langmuir isotherm, indicating that there is almost no effect of the first adsorbed ethanol on the adsorption of the second ethanol on the Nb dimers. Thus, the amounts of species [SlEtl] and [SoEtz] were determined by using the adsorption isotherm of ethanol at 523 K, where [SoEtz] and [SlEtl] were almost 1 and 0, respectively, at the pressure of ethanol of 3.7 Wa. Here, [SlEtl] and [&Et21 represent the amount of the dimer sites on which an ethanol molecule adsorbs and the amount of the dimer sites on which two ethanol molecules adsorb, respectively. Since the rates of diethyl ether formation and ethene formation are expressed as eqs 4 and 5 , v(diethy1 ether) = k,[SoEtJ v(ethene) = k2[SlEtl]
+ 2k,[S,,Eh]
(4) (5)
respectively, the rate constants were statistically determined from the rates at various ethanol pressures. These values were collected under catalytic reaction conditions, which are listed in Table 2 (row b). To examine whether these rate constants correspond to those of each elementary step, the rate constants in each step in Figure 6 were determined as follows. The pressure of ethanol was chosen as 0.3 or 4.0 W a at 523 K to regulate the amount of [SlEtl] and [SoEtzI to be unity at 523
K, respectively. After the catalyst was exposed to the ethanol vapor for a while, the gas phase was evacuated and the formation of ethene and diethyl ether was monitored. The rate constants thus determined under the stoichiometric reaction conditions are listed in Table 2 (row a). Both values listed in Table 2 are in good agreement. Consequently, the reaction scheme for the ethanol dehydration on Nbz(3) is shown in Figure 6. In the scheme, at first the catalyst shows no Nb-Nb bond, as proved by EXAFS. A first ethanol adsorbs on the Nb=O double bond to produce Nb-OH and Nb-OCzHs. When a second ethanol adsorbs on the adjacent Nb-0 double bond to form another Nb-OH and Nb-OCzHs before the first ethanol is decomposed to ethene and water, the dehydration occurs from the two Nb-OH to form the Nb-0-Nb bridge, resulting in a dimeric structure, as indicated by EXAFS. When the ethoxy ligands are dehydrated unimolecularly or bimolecularly to form ethene and diethyl ether, the dimeric structure is converted to the original monomer pairs. It is suggested that the dehydration of ethanol on the catalyst proceeds in conjunction with a dynamic structural change in the Nb sites involving formationbreaking of Nb-0-Nb bonds. The Nb-O(surface) bond distance was almost unchanged under the working conditions (0.197 and 0.198 nm), whereas the Nb-Si length largely changed from 0.336 nm for the monomeric structure to 0.326 nm for the dimeric structure by ethanol adsorption, as illustrated in Figure 6. At the same time the longest Nb-O(surface) bond was shortened from 0.228 to 0.220 nm upon ethanol adsorption as in Figure 6. These results suggest that the bond angles of Nb-0,-Si, and 0,-Nb-0, (subscript: surface) became narrower by a local rearrangement involving the movement of the Nb atom to the longest 0 atom and a deviation of the Si sites at the surface. An appropriate flexibility of surface structures may assist the dynamic change of active Nb structures during the catalysis, which may be a role of the support surface in selective catalysis. It may be necessary to further crosscheck these important reversible local-structure changes at the interface by other spectroscopies. Conclusions (1) The attached Nb dimer catalyst (Nbz(3)) was prepared by using the dimeric complex [Nb(175-CsHs)H-~-(y5,171-Cs~)lz and SiOz. (2) Although Nb2(3) had no Nb-Nb bond (monomer pairs) under the static conditions, the Nb-Nb bond at 0.308 nm appeared under the catalytic reaction conditions and disappeared when the adsorbed ethanol was consumed by the reaction. (3) The catalytic ethanol dehydration on Nbz(3) proceeds in conjunction with a dynamic structural change of a formationbreak cycle of Nb-Nb bonds. (4)A local structure rearrangement at the Nb-0-Si interface on Si02 is suggested to occur in the course of the catalytic reaction. References and Notes (1) Yermakov, Yu. I.; Kuznetsov, B. N.; Zakharov, V. A. Catalysis by Supported Complexes; Elsevier: Amsterdam, 1981. (2) Iwasawa, Y., Ed. Tailored Metal Catalysis; Reidel: Dordrecht, 1986. (3) Iwasawa, Y. Adv. Catal. 1987, 35, 187. (4) Gates, B. C., Guczi, L., Knozinger, H., Eds. Metal Clusters in Catalysis; Studies of Surfure Science Catalysis; Elsevier: Amsterdam, 1986; Vol. 29. ( 5 ) Hertly, F. R. Supported Metal Complexes;Reidel: Dordrecht, 1985. (6) Iwasawa, Y. Catal. Today 1993, 18, 21. (7) Iwasawa, Y.; Asakura, K.; Ishii, H.; Kuroda, H. Z. Phys. Chem. 1985, 144, 105. (8) Nishimura, M.; Asakura, K.; Iwasawa, Y. Proc. 9th Znt. Congr. Catal. 1988, W , 1842.
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