J . Phys. Chem. 1991,95,6973-6978
6973
Thus, in gases with the Treanor distribution of vibrational states, the value of the typical molecular diameter depends on the rotational constant B, and the ratios w , / ~ & ,and T/Tu;at higher temperatures, the diameter can be a strong, increasing function of these two ratios. As an example illustrating the impact of the rotational-vibrational excitation on the molecular diameters we calculated T dependence of the ratio
‘‘20m--7--7
X = (d,,’/dc)2
1.00 “ 1000
3 w00
2000
4000
5000
5
6000
7000
8wO J
Temperature (K)
Figure 1. T dependence of the ratio X (eq 28) in nitrogen, oxygen,
chlorine, and iodine when the low and intermediatevibrational states are in local thermal equilibrium with translational temperature. The ‘bump” on the N2and O2curves are in vicinity of q = 1, and they result from approximation 24.
upcan be taken as the vibrational quantum number corresponding to the minimum of the distribution. If T C To up
= ( E , /Zhcw&,)(T/Tu)
(26)
where Tuis the vibrational temperature. In the same case, but with T > Tu,the quantum number up can be obtained from eq 23 by replacing T with To. Comparison of expressions 26 and 23 indicates that relationship 25 also can be used when the distribution of the vibrational states is the Treanor distribution. In such a case the factor r] is t = (w,2/2w&,)(hc/kTu) (6)
(27)
Treanor, C.; Rich, J.; Rehm, R. J. Chem. Phys. 1968,18, 1798.
(28)
for several gases of homonuclear diatomic molecules with an LTE distribution of low and intermediate vibrational states. Ratio 28 is roughly proportional to the ratio of the “typical” kinetic cross section Qbin “hot” gas to the corresponding cross section Q, in “cold” gas. Examples of T dependence of ratio 28 are given in Figure 1. As can be seen from the figure, the rotational-vibrational excitation of molecules can have a meaningful effect on the collisional properties of gases. This effect is stronger in gases of weakly bound molecules (molecules with low values of the dissociation energy). (The dissociation energies of the molecules considered in Figure 1 are as follows: 9.76 (N2), 5.12 (02), 2.48 (Clz), and 1.54 eV (121.)
The assumptions made during the evaluation of the analytical approximation 25 limit its validity (if accuracy better than a few percent is required) to temperatures T S 9000 (N2), 8000 (02), 6000 (C12), and 3000 K (I2). However, because all diatomic gases are usually well-dissociated and atom-atom, not molecule-molecule, collisions dominate the gas properties at temperatures higher than those temperatures, this limitation is not important.
Acknowledgment. This work was supported by the National Aeronautics and Space Administration, Grant NAGW-1061, by the Air Force Office of Scientific Research, Grant 88-01 19, and the URI Grant 90-0170. Registry NO. 12, 7553-56-2; CIz, 7782-50-5; 02,7782-44-7; NZ, 7727-37-9.
X-ray Absotptlon Spectroscopy Study of the Titania- and Alumlna-Supported Tungsten Oxlde System Frank Hilbrig,”: Herbert E. Cabel,# Helmut Knozinger,*lt Helmut Schmelz,: and Bruno Lengeler*J Institut fur Physikalische Chemie, Universitat Miinchen, Sophienstrasse 1I, 8000 Miinchen 2, Germany, Power Generation Group KWU, Siemens AG, Otto Hahn Ring 6, 8000 Miinchen 83, Germany, Corporate Research and Technology, Siemens AG, Otto Hahn Ring 6, 8000 Miinchen 83, Germany, and Znstitut fiir Festkarperforschung, Forschungszentrum Jiilich, Posrfach I91 3, 5770 Jiilich I , Germany (Received: August 9, 1990; In Final Form: March 28, 1991) Tungsten oxide supported on titania is an important material for the selective catalytic reduction of nitrogen oxides NO,. A structure analysis by means of X-ray absorption spectroscopy (XANES and EXAFS) is reported for materials prepared by spreading of W03in physical mixtures with the support oxide as well as by impregnation from aqueous solution. A comparison is also made with alumina-supported materials. Analysis of the XANES region at the W LIand W L3 edges indicated that tungsten is hexavalent and anchored to the surface as W05 and W04 units, the relative proportion of which increases with loading. When water is absorbed, pseudooctahedrally coordinated species are formed in both cases. The analysis of the EXAFS provides additional support for the existence of these structures, which contain oxo groups W 4 and W 4 W bridges. A tentative structure model is proposed, in which islands of surface tungstate species are formed by branched chains of WO, units. The chains are assumed to be terminated by W04 units, the W05/W04 ratio thus increasing with chain length or island size, which obviously increases with loading.
Introduction Titania-supported tungsten oxide catalysts are very efficient for various acid-catalyzad heterogeneous reactions,’ preferentially Universitit MBnchen. $PowerGeneration Grou KWU, Sicmen8 AG. I Corporate Rcaearch ant!Technology, Siemens AG. I Inititut far FcrtkCperfonchung.
0022-3654/91/2095-6973$02.50/0
for the disproportionation of pr0pene.u For the selective catalytic reduction (SCR) of N O by NH3. WOdTi02 catalysts are of (1) Ai, M.J . Carol. 1977,19, 305. (2) Yamaguchi, T.; Tanaka, Y.; Tanabe, K.J . Coral. 1980, 65, 442.
(3) Yamaguchi, T.; Nakamura, S.;Nagumo, H.In Proceedings of d e International Congre8s on Catalysis, 8th; Vcrlag Chemic: Weinheim, Germany, 1984; Vol. 5, p 579.
CP 1991 American Chemical Society
6974 The Journal of Physical Chemistry, Vol. 95, No. 18, 19'91
interest in the industrial application because of their good activity at high temperature, their thermal stability, and their low oxidation activity for S02.4J The de-NO, activity increases with the addition of low amounts of vanadium oxides by a synergistic effect.$ Despite its importance and in contrast to the W03/ A1203(Si02)system (ref 10 and references therein), only few studies concerning the W03/Ti02system have been reported2"' and the structure of titania-supported tungsten oxide is still unknown. Proposed structures based on laser Raman spectroscopy ( LRS)9Jl are tentative because the frequency of the observed W - 0 stretching mode of the surface tungsten oxide can be related with polyhedral symmetries only in exceptional cases.IO X-ray absorption spectroscopy (XAS)I2-lSis a local probe of the symmetry and coordination around a given site and also provides the valence of the absorbing species. Horsley et a1.I0 first reported W LI near-edge spectra for alumina-supported tungsten oxide. They demonstrated the possibility of distinguishing between tetrahedral and octahedral surface tungsten oxide groups. The conventional method of preparing supported tungsten oxide catalysts is the impregnation of the carrier oxide powder with an aqueous solution of ammonium metatungstate, drying, and subsequent calcination at temperatures around 770 K. An altemative method is based on the property of W03 to spread over the titania as well as over the alumina surface by a temperature treatment in a moist oxygen ~tream.'4~' This solidsolid wetting pro~ess,'~.~' which is comparable to the spreading of MOO, over the surface of alumina,I7*'*results in a highly dispersed tungsten oxide phase. The aim of this work was the structural characterization of Ti02- and AI2O3-supportedtungsten oxide in dependence of the support oxide coverage, the mode of preparation, and the presence or absence of physically and chemically bonded water molecules. By considering the XANES results of Horsley et a1.I0 for the W03/A1203system prepared by the incipient wetness technique, an effort was made to find structural similarities.
Experimental Section The support oxides 7-A1203-Cand Ti02-P25 were Degussa products with N2 BET surface areas of 95 and 50 m2/g, respectively. The Ti02-P25is composed of 75% anatase and 25% rutile. The WO, used for the preparation of the spread samples was also a Degussa product prepared by flame hydrolysis that had an N2 BET surface area of 43 m2/g. Physical mixtures of 3 and 9 wt 8 W03/Ti02 were ground in an agate mortar and calcined at 723 K for 2 h in a moist oxygen stream (50 mL/min;p(H20) = 18 mhr). In the following, these two samples will be called TiW3A and TiW9A. Physical mixtures of 3 and 10 wt 8 WO3/AI2O3were treated in the same way except for a calcination temperature of 1023 K (samples AIW3A and AI W 1OA). (4) Imanari, M.; Watanabc. Y.; Matsuda, S.;Nakajima, F. In Proceedings ofthe 7th International Congress on Catalysis; 1981; p 841. (5) Morikawa, S.; Takahashi, K.; Mogi, J.; Kurita, S. Bull. Chem. Soc. Jpn. 1982, 55, 2254. (6) Satsuma, A.; Hattori, A,; Mizutani, K.; Furuta, A.; Miyamoto, A.; Hattori, T.; Murakami, Y. J. Phys. Chem. 1988,92,6052. (7) Chan, S.S.;Wachs, I. E.; Murrell, L. L.; Wang, L.; Hall, W. K. 1. Phys. Chem. 1984,88, 5831. (8) van Hengstum, A. J. Thesis, Twente University of Technology, Enschede, The Netherlands, 1984. (9) Bond, 0. C.; Flamerz, S.;van Wijk, L. Catal. Today 1987, I , 229. (IO) Honley, J. A.; Wachs, 1. E.; Brown, J. M.;Via, G. H.; Hardcastle, F. D. J. Phys. Chem. 1987, 91,4014. (1 I ) Vermaire. D. C.; van krge, P. C. J . Catal. 1989, 116, 309. (12) Bart, J. C. Adu. Catal. 1986, 34, 203. (13) Lee, P.A.; Citrin, P. H.; Eisenkrger, P.; Kincaid, 8. M. Reo. Mod. Phys. 1981, 53, 769. (14) Koninpskrger, D. C., Prins, R., Eds. X-ray absorption: principles and techniques of EXAFS, SEXAFS and XANES; J. Wiley: New York, 1989. (15) Lengeler, 8. X-ray absorption and reflection in material sciences. Festk6rperprobleme 1989, 29, 53. (16) Hilbrig, F. Dissertation, Univenitkt, Mllnchen, 1989. ( 1 7) Leyrer, J.; Margraf, R.; Taglauer, E.; KnWnger, H. Surj. Sei. 1988, 201, 603. (18) Leyrer, J.; Mey, D.; Knbzinger, H. J . Catal. 1990, 124, 349.
Hilbrig et al. X-ray diffraction (XRD), high-resolution electron microscopy (HREM), laser Raman, and IR spectroscopy ensured that the spreading of W 0 3 over the Ti02 and the AI203 surfaces was complete in all samples and that the supported tungsten oxide is only present as a highly dispersed surface species.I6 The Ti02 was not changed by the temperature treatment whereas the N2 BET surface of 7-A1203decreased from 95 to 85 m2/g because of the transformation of the low content of gibbsitelgin y-AI2O3-C to y-A1203.20 The wet impregnated W03/Ti02 samples with 3 and 7 wt % WO, were prepared by suspending 50 g of Ti02 in an aqueous solution containing the appropriate amounts of ammonium metatungstate (500 mL; pH = 4), stirring for 0.5 h, removing the water under vacuum, drying the resulting paste at 385 K in air overnight, crushing and sieving to a particle size below 0.1 mm, and finally calcining at 723 K in a moist oxygen flow (50 m L / m h p(H20) = 18 mbar) for 12 h (samples TiW3B and TiW7B). The reference compounds used for the XANES and EXAFS analysis were W 0 3 (Merck), W03-H20(Merck), ammonium metatungstate(Starck), CaWO, (Alfa), and Na2W0,. The latter was prepared by dehydration of Na2W04.2H20(Merck) at 423 K21 and identified by LRSZ2and XRD.23 The X-ray absorption experiments at the W Ll and W L3edges were carried out at the beam line E2 of the electron storage ring DORIS I1 (3.7 GeV, 40-100 mA) at Hasylab (DESYHamburg). The experimental setup is described in ref 24. We have used a doublecrystal Si(311) monochromator with an energy resolution of 1.6 eV at the W L3 edge and of 2.3 eV at the W Ll edge. The harmonic content was reduced below 1 part in 10' by detuning the monochromator crystals. The reduction was checked by a NaJ detector, which measures the harmonic content at 3 times the energy of the fundamental. Another test is to look at the K edge of Sb (at 30.49 keV) when an S b film is put into the beam and the energy is swept around 10.16 keV. No Sb edge was observed. This setup was supplied with an in situ ce11I6 designed for simultaneous measurements in the transmission and fluorescence modes between 300 and 750 K in a defined gas atmosphere, The present studies were carried out in the transmission mode using ionization chambers containing N2 in the first chamber (- 10% absorption) and a mixture of N2 and Ar in the second chamber (-95% absorption). The energy calibration was monitored by measuring a W foil or WO, simultaneously with the unknown sample using a third ionization chamber in series. For the XAS experiments the sample powders (TiW3A, TiW9A, TiW3B, TiW7B, AIW3A, AIWlOA) were slightly p d in an aluminum frame (0.5 mm thick) Over an area of 10 X 25 mm2and enveloped in an aluminum foil. For the reference compounds the coverage of the fine ground powder onto a Kapton adhesive tape was sufficient to achieve an optimal sample thickness. We have measured the linear absorption coefficient p ( E ) in the vicinity of the W LIand W L3 edges (XANES) of the samples TiW3A, TiW9A, TiW3B, TiW7B, AIW3A, and AlWlOA at 300 K in air and of the samples TiW3A, TiW9A, TiW3B, and AIW3A at 673 K in O2 (20 mL/min). Additionally, the EXAFS above the W L, edge of the samples TiW3A and TiW9A were measured at 77 K in N2. For the reference compounds, the W LI and W L3 XANES spectra and the W L3 EXAFS spectra were recorded at 77 K in N2. The XANES spectra were normalized by fitting linear functions to the p d g e data and to the data of the EXAFS region, extrapolating both functions to the zero of energy EO, subtracting the pre-edge data function from each point in the experimental spectrum, and dividing by the step height at E@ Eo (19) Powder Diffraction Standard Data File; International Center of Diffraction Data (ICPDS) 29-0041. (20) Powder Diffraction Standard Data File; International Center of Diffraction Data (ICPDS) 16-0394. (21) Kust, R. N . Inorg. Chem. 1967.6, 157. (22) Reudhomme. J.; Tarte, P. Spectrochim. Acta 1972,28A, 69. (23) Okada, K.; Morikawa, H.; Marumo, F.; Iwai, S.Acta Crystallop. 1974,830, 1872. (24) Lengeler, B. Micmhim. Acta ( W e n ) 1987, I , 455. (25) Lytle, F.W.; Sayers, D. E.; Stern, E. A. Phys. Rev.B. 1975,II,4825.
Titania- and Alumina-Supported Tungsten Oxide System
The Journal of Physical Chemistry, Vol. 95, No. 18, 1991 6915
v 1 -
0.5
-
Despite the deviation from the ideal tetrahedral symmetry, in the W L1 edge of CaW04 a pre-edge was seen at the same energy position and with the same intensity as in the W L1edge of Na2W0,. Horsley et a1.I0 did not detect any difference between Na2W04and Al2(WO,),. Hence, a fairly intense pre-edge peak in the W LI is typical for tetrahedral W 0 4 groups. Distortions and variations in W-O distances do not affect the energy position and intensity of these pre-edge peaks. They lead to a loss of fine structure in the high-energy region of the W Lledge. If the tungsten has a distorted octahedral oxygen environment, as in W0333and in ammonium metat~ngstate?~ only a broad shoulder is observed on the low-energy side of the W LI edge. In W03.H2W5the tungsten is also surrounded by an octahedron with four W-0 bonds in the equatorial plane and one short W - 0 and one long W 4 H 2 bond in the axial directions. This stronger deviation from the ideal octahedral symmetry leads to a more pronounced pre-edge structure. A correlation of the oscillator strength of the s-d transition in the Mo K edge of molybdenum oxides” and in the V K edge of vanadium oxides3*with the degree of deviation from the ideal octahedral symmetry was found experimentally. For the molybdenum oxides, Chiu et al.37have defined a so-called NUD parameter as a measure of the extent of distortion of the octahedron of oxygen atoms in the first coordination shell (Mo-O, distances R,) as
I
19080
12100
12120
12140
E
12160
(4
Figure 1. Normalized W L,X A N E S spectra of N a 2 W 0 4 (-), W03-
H20( 0 ) ,and W 0 3 (A). is defined as the point of inflection of the W L1or W L3absorption edge. This procedure results in a normalization of the data to unit step height. The oscillatory part of the absorption beyond the W L3 edge (EXAFS) was extracted from the total absorption and normalized to the jump height according to the procedure described in ref 26. Then the abscissa of the normalized EXAFS x(E) was converted from the photon energy E to the wavenumber k. The data (k = 2-13 A-l) were weighted by k2 and Fourier tran~formed.”,~~.~~
P
Results and Discussion 1. X-ray Absorption Near-EdgeStructure (XANES). The fine structure of an absorption edge is directly related to the local, l-dependent density of final states.27 Band structure calculations based on a known geometric structure allow calculation of the linear absorption coefficient. Without band structure calculations the position and the fine structure of an absorption edge can be used as a “fingerprint” for the changes in the valence and in the local arrangement of the neighboring atoms around the absorber a t 0 m , 2 ~in* ~comparison ~ with reference compounds, which can have different structures.M Figure 1 displays the W L1 edge of the reference compounds Na2W04, W03*H20,and WOO. The characteristics of these spectra are the appearance of pre-edge peaks. A very intense and sharp pre-edge peak is seen in the W LI edge of Na2W04,where the WO, groups have ideal tetrahedral ~ymmetry.2~ In contrast, the pre-edge peaks are less intense for W 0 3 and W03-H20, containing distorted octahedral W 0 6 groups. For W 0 3 only a shoulder on the rising absorption edge is observed. The pre-edge peak in the W LI edge is due to tungsten 2s-5d transitions. These transitions are possible because of a mixing of tungsten d orbitals with oxygen p orbitals.% Only in the case of an ideal octahedral symmetry this transition is strictly dipole forbidden and a quadrupole transition is rarely observed. A distortion of the ideal octahedral symmetry removes the center of inversion allowing s-d transitions. The W 0 4 tetrahedra in the structure of CaW0, have uniform W-O distances but slightly irregular W-0-W angles.,l A12(WO,), contains regular and distorted WO, tetrahedra, the latter with large variations in W-O distances and in W-0-W angles.32 (26)Lengeler, B.;Eiecnberger, P. Phys. Reo. B 1980, 21,4507. (27)Lmgeler, B. X-ray abmrption and reflection in the hard X-ray range, Summer School ’Enrico Fermi”, 1988 Varenna, Italy; Roclei, R., et al., Us.; North Holland: Amsterdam, 1990. (28) Lengeler, B. Z . Phys. B 1985,61. 421. (29)Lengeler, 6. Ado. Morrr. 1990,2,123. Lengeler, B.Phys. BI. 1990, 46, 50. (30)Rao, K.J.; Wong, J.; Weber, M. J. J . Chem. Phys. 1983,78,6228. (31) Polyanrkaya, 7 . M.;Borirov, S. V.; k l o v , N . V. Sou. Phys,Crysrollogr. (Engl. Trans/.)1974,18,719.Zellrin, A,; Templeton, D. H. J. Chem. Phys. 1964,40,501. (32)Craig, D.C.;Stephenson, N . C. Acro Crystallogr. 1968,B24, 1250. (33) Salje, E.; ,Visvanathan, K. Acto Crysrollogr. 1975, A31, 356. Looptra, B. 0.;Rietveld, H. M. Acto Crysrollogr. 1969,825, 1420. (34)Fuchr, J. 2. Naturforch. 1973, 288, 389. (35) Szymaniki, J. T.;Roberts, A. C. Can. MInerol. 1984, 22,681. (36)Kutzler, F. W.; Natdi, C. R.; M i m e r . D. K.;Doniach, S.;Hodgoon. K.0.1.Chrm. Phys. 1980, 73, 3274.
NUD
P
1/pC [(R,- Rm)/Rm] 1-1
R, being the arithmetic mean of the Mo-0 distances. According to the detailed investigation of vanadium the “molecular cage size” effect, which corresponds to the average first V-O distances, is responsible for the intensity of the s-d transition. The reduction of the coordination number increases the oscillator strength. On the other hand, it decreases again if the average bonding distance is increased within one class of polyhedra. Furthermore, for tetrahedral V-O compounds the full width at half-maximum (fwhm) of the pre-edge peaks correlate with the variation in the nearest-neighbor V-O distances.38 For WO,, W03-H20,and ammonium metatungstate the mean deviation from the average W-O distances in the W06 octahedra (NUD values) are 0.0712, 0.0740, and 0.071, where the latter was estimated from the data of PW120a*?9 which is isomorphous to m e t a t ~ n g s t a t e .Although ~~ there are significant differences in the W LI edge of WO, and ammonium metatungstate on the one hand and W03*H20on the other, the NUD parameters are nearly identical. This illustrates that the NUD parameter concept is inadequate for tungsten oxides. Also the molecular cage size effect introduced by Wong et aL3*is not seen, neither for tetrahedral nor for octahedral tungsten oxides. As described above for the tetrahedral reference compounds, the energy position and intensity of the pre-edge peak in the W L1edge are insensitive to the distortion of the W 0 4 tetrahedron, and to the mean W-O distance and its variations. The difference of average W-O distances between WO, and ammonium metatungstate amounts to 0.069 A and between WO,and W0,-H20 to only 0.003 A. The identical pre-edge features of WO, and ammonium metatungstate and the quite different ones of WO, and W03-H20rule out this effect. In summary, on the basis of the fine structure of the W L1edge, tetrahedral and octahedral tungsten oxides can be readily distinguished. Structural details within one polyhedral symmetry, however, do not influence the energy position and the intensity of the pre-edge peak. Because of its pseudooctahedral (WOs OH& structure, W03*H20occupies a special position. Therefore intermediate energies and intensities of pre-edge peaks in W L1 edges of amorphous phases are to be interpreted as mixtures of polyhedral types.
+
(37) Chiu, N.S.;Bauer, S.H.; Johnson, M.F. L. J . Coral. 1984,89,226. (38) Wong, J.; Lytle, F. W.; Mesamer, R. P.; Maylotte, D. H. Phys. Rco. B 1984,30, 5596. (39)Linnett, J. W. J . Chrm. Soc. b d o n 1961, 3796.
Hilbrig et al.
6976 The Journal of Physical Chemistry, Vol. 95, No. 18, 1991
t
f
019080 10220
E
12120 .
12100
12140
12160 5
~
10240
(4
Figure 2. Normalized W L3XANES spectra of Na2W04(-) and W 0 3 (A).
E (ev) Figure 3. Normalized W LIXANES spectra of TiW3A, measured at 300 K in air
(X)
and in situ at 673 K in O2 (-).
TABLE I: Energy (E) and Relative Difference in Peak Height ( A H ) of the Observed Pre-Edge Peak in the W L, Edges’ sample CaWO, WO3sH20 TiW3A TiW3A TiW9A TiW9A TiW3B TiW3B TiW7B AIW3A AIW3A AlWlOA
meas temp, K 77 77 300 673 300 673 300 673 300
300 673 300
AE,”eV +0.2 +2.5 +3.2 +1.1
+2.9 +1.7 +3.0 +l.2 +3.0 +1.8 +1.2 +2.3
AH:
w
*O
-32 -34 -2 1 -32 -29 -32 -22 -32 -30 -16 -32
#Reference: Na2W04, 77 K/N2; 12101.8 eV, 0.68 bExperimental error h0.l eV. CExperimentalerror *2%.
19080
12120
12140
12160
E(e4 Figure 4. Normalized W L, XANES spectra of TiW9A, measured at 300 K in air (X) and in situ at 673 K in O2 (-). p.
We will now discuss the W L3 absorption edges, which are related to empty d electronic states in the tungsten oxide compounds. The shape and fwhm of the “white line” in the W L3 edge portray distinctions between tetrahedral and octahedral tungsten oxides (Figure 2). The resonance line is very sharp with a fwhm of 5.3 eV if the tungsten atom has a tetrahedral oxygen environment (Na2W04,CaWO,). A uniformly broad white line with a fwhm of 8.0 eV is observed in the W L3 edge of W03, W03-H20, and ammonium metatungstate. For ammonium metatungstate a slight d-band splitting of 2 eV occurred in the white line. The features of L2 and L3 absorption edges are often described in terms of the crystal field theory,’M3 although the argumentation in localized molecular orbitals is not entirely appropriate. Nevertheless, the features of the W L3 edge give additional “fingerprints” of structurally defined tungsten oxides. These W L3 fingerprints are less informative than those of the W LIedge but they are advantageous in supporting the interpretation of the latter. Having analyzed the typical features in the W L1and W Lo edges for a number of model compounds, we can start discussing the structure of the highly dispersed tungsten oxide phases according to the fingerprint procedure. Table I lists the energy shifts AE (eV) and relative differences in the height AH (%) of the pre-edge peaks in the W L,edges relative to the intense peak in the W L,edge of Na2W04(12 101.8 eV, 0.68 p ) . In the W LI edges of all TiW samples, TiW3A (Figure 3), TiW9A (Figure 4), TiW3B, and TiW7B at 300 K in ambient atmosphere, the energy shift AE of the pre-edge peaks is +3.0 ( f O . 1 ) eV and the differences in the peak height AH are between -32 and -34%. All these features show a high degree of similarity to that of W03.H20. The same AH values can be found for the samples (40)Balzarotti. A.; Comin, F.; Incoccia, L.; Piaccntini, M.;Mobilio, S.; Savoia. A. Solid Stote Commun. 1980, 35, 145. (41) Fischet, D. W.J . Appl. Phys. 1970, 41, 3561. (42) Srivaetava, U. C.; Nigam, H. L. Coord. Chem. Rev. 1972, 9, 275. (43) Vishnoi, A. D.; Sapre. V. B.; Mande, C. X-Ray Sprdrom. 1988,17, 213.
12100
1
\t
L
f
O
10200
10220
10240
E (4 Figure 5. Normalized W L3XANES spectra of TiW3A, measured at 300 K in air
O
(X)
and in situ at 673 K in O2(-).
1 10200
10220
E
10240
(4
Figure 6. Normalized W L, XANES spectra of TiWgA, measured at 300 K in air ( X ) and in situ at 673 K in O2 (-). AIW3A and AlWlOA at 300 K in air, but the AE values of +1.8 and +2.3 eV, respectively, are smaller. By an in situ treatment at 673 K in flowing O2 the energy position of the pre-edge peak in the W LIedge of the samples with low W 0 3 content, TiW3A (Figure 3), TiW3B, and AIW3A.
Titania- and Alumina-Supported Tungsten Oxide System is shifted to a common position of +1.2 eV. Simultaneously,the AH values become significantly less negative and assume values of -21, -22, and -16%. However, for the sample TiW9A (Figure 4) the changes in AE and AH by the in situ treatment are less pronounced, The pre-edge peak in the W L1 edge of the sample TiW9A shifts by -1.2 eV and the peak height increases only by 3%. The shape and fwhm of the white line in the W L3 edges of TiW3A (Figure 5), TiW9A (Figure 6), TiW3B, TiW7B, AlW3A, and AlWlOA at 300 K in air agree completely with those of the reference compounds containing W 0 6 octahedra (W03, W03.H20, ammonium metatungstate). On the other hand, for in situ samples at 673 K, the shape of the W L3 white lines in TiW3A (Figure 5), TiW3B, and AlW3A resemble closely those of the tetrahedral reference compounds. Their fwhm of the W L3 resonance line is 5.8 eV at 673 K in 0, as compared to 5.3 eV in Na2W04and 8.0 eV in W03. But in TiW9A (Figure 6), the resonance line is 7.1 eV wide at 673 K in 02.Independently of the choice of the support oxide, the mode of preparation, and the WOOcoverage, the W LI and W L, edge features of all investigated air-exposed samples resemble those of WOrH?O. On the W L1prsedge peak intenshes increase heating at 673 K in 02, and their energy positions decrease, and the white lines in the W L, edges become narrower. The differences are only influenced by the W 0 3 coverage and not by the mode of preparation or the nature of the support oxide. At low W 0 3 content these differences are much clearer than for the sample TiW9A, although the overall tendency is similar. But it turns out that, in the samples containing 3 wt % W 0 3 at 673 K in 02,the W L1and W L3 edge features do not coincide with those of the tetrahedral standards. The reason for the observed changes in the W LIand W L3 absorption edges with in situ thermal treatment are attributed to the removal of coordinated water from the sample as a result of which the oxygen coordination number in the first shell around the tungsten sites decreases. Based on their W LIXANES results for wet impregnated W03/A1203samples, Horsley et a1.I0 demonstrated the reversibility of this process by the readsorption of water. High-temperature measurements of samples TiW7A and AlWlOA are not available. We do, however, expect these samples to behave comparably to TiW9A. This assumption is based on the fact that laser Raman and in situ DRIFT studies16 gave identical results for all three materials, namely, TiW7A, AlWlOA, and TiW9A. For Ti02-P25, the monolayer capacity of tungsten oxide is reached at 9.3 wt % W03,* and for r-A1203with an N2 BET surface of 85 m2/g, at approximately 12 wt % W03.44 In this context, the monolayer capacity is defined by the amount of W 0 3 that can spread or disperse over the support oxide surface. According to the fingerprints of the W LI and W L3 edges of the reference compounds and their interpretation as discussed above, we conclude that the structure of tungsten oxide supported on Ti02 or A1203in the absence of adsorbed water consists of W 0 4 and WOs groups, the ratio of which is controlled by the W 0 3coverage. Tungsten is hexavalent. At a coverage below of a monolayer (samples TiW3A, TiW3B, and AlW3A), the W 0 4 units are more abundant, whereas at about 1 monolayer of WO, (sample TiW9A), the WOs groups predominate. With adsorbed water, the tungsten sites are pseudooctahedrally surrounded by oxygen, as in WO3.H2O. It must be emphasized in this context, that the small shoulder in the rising WLI edge was only observed for the slightly distorted octahedral coordinations in W0333 and ammonium metat~ngstate.,~ The higher intensity of this pre-edge structure for W0,-H20 is assumed to be due to its W 0 5 OH2coordination. It is very important to note that the W LIedge of this reference compound is very similar to that of TiW9A (300 K). Pure W03*H20is transformed into WO, at elevated temperatures. When TiW9A is heated to 673 K, the pre-edge peak shifts to lower energy by a small but distinct value, this not being expected for
+
(44) Chan, S.S.;Wachs, 1. Caral. 1985, 92, 1.
E.;Murrell, L. L.;Dispenzierc, N. C., Jr. J .
The Journal of Physical Chemistry, Vol. 95, No. 18, 1991 6977
401' R 9 .r.0 1J
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R,A Figure 8. Radial distribution function around tungsten in TiW9A.
the formation of an undistorted octahedral surface species. We therefore believe that WOs units and pseudooctahedral groups are the most likely surface species predominating in highly loaded materials in the dehydrated and hydrated states, respectively. The existence of such WOs units containing 5-coordinate W6+ sites in dehydrated samples is also supported by the observation of W6+-C0 complexes when CO was adsorbed at 80 K (low-temperature infrared spectroscopy)?s Also, the Raman spectra of the materials studied here are entirely consistent with the conclusions drawn from the X-ray absorption ~pectra.'~ This interpretation given above is in some disagreement with that of Horsley et a1.,I0 who proposed that on alumina in the absence of coordinated water isolated and dimeric tetrahedra were present at a W 0 3 coverage of less than monolayer and a high fraction of distorted W 0 6 octahedra were formed at 1 monolayer of W 0 3 . 2. Extended X-ray Absorption Fine Structure (EXAFS).The Fourier transforms of the EXAFS k2x at the W L3 edge of W 0 3 and of the air-exposed sample TiW9A are shown in Figure 7 and Figure 8. These spectra give the radial distribution function (RDF) of the neighboring coordination shells around a tungsten absorber with noncorrected distances R,. It should be noted that the EXAFS of sample TiW3A, which was also covered by water, is identical with that of sample TiW9A. The first three peaks between R = 0.6-2.5 A appearing in the RDF plot of TiW9A are due to W-O bonds. In comparison to the spectrum of WO,, the peak at R = 3.5 A shows the presence of W-W distances. In a first approximation these distances and their distribution are very similar to those in W 0 3 with W-W distances at 3.73 (hO.1) A.33In addition, the RDF plots of TiW9A and TiW3A show a new peak at R = 2.92 A, which cannot be observed in the RDF spectrum of W 0 3 or in the spectra of the other reference compounds. Kisfaludi et alaa also observed in their Mo K EXAFS study of spread MoO3/Al2O3a new distance at -2.8 A. They suggested that this new distance might be attributed to a Mo-AI distance. In analogy, this new distance at R = 2.92 A may at least partially be related to a W-Ti distance, Le., to the anchoring (45) Hilbrig, F.; Schmelz, H.;Knbzinger, H., to be published. 192. (46) Kisfaludi, G.; L e p r . J.; Knbzinger, H.; Prins, R.J. Caral. 1991, f30,
6978 The Journal of Physical Chemistry, Vol. 95, No. 18, 1991 TABLE II: Curve-Fittinn AnalvrQ of Two W - O S W in TiW9A rjq A Nj $, A2 E& CV 1.76 1.6 -0.OOO4 -0.67 W-01 w-02 1.91 2.0 0.0095 -0.67 .
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Figure 10. Model of titania-supported tungsten oxide. -0.5 5
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Figure 9. Curve fit of the Fourier-filtered EXAFS of two W - 0 shells in TiW9A.
of the tungsten oxide onto the titania surface. On the other hand, this distance can be attributed to the distance of the centers of edge-connected W 0 6 octahedra. It should be mentioned that the Ti02 support oxide is built-up by a mixture of edge- and corner-sharing octahedra. The corrected distances rI and coordination numbers Nd of the first two W-O shells in the EXAFS of TiW9A were estimated by a nonlinear curve-fitting ana1y~is.l~ The two W-O peaks (R = 0.6-1.9 A) were isolated with a window and Fourier backtransformed. The k range used in this procedure was 10.5 A-I. According to the Brillouin equation, the number of independent points N should not exceed the number of fit parameters (four per shell!. With the k and R ranges (1.9 A) applied, Npshould be less than 13. A two-shell fit was performed. The W-O phase shift and back-scattering function including the inelastic loss factor were extracted analogously from the well-separated first peak (R = 0.6-2.0 A) in the RDF plot of CaWO,, which represents the first oxygen shell at rl = 1.788 A and N1= 4." The absolute DebytWaller factor appears to be unchanged and the zero of energy Eo was the first point of inflection in the W L3edge of CaWO, at 10204.0 eV. The results of this curvefitting analysis according to the concept of chemical transferability are given in Table I1 and the excellent quality of the fit itself is shown in Figure 9. The first distance (1.76 A) corresponds to the W 4 bond, which gives rise to a stretching mode at 970 cm-l in the Raman spactrum.I6 The next distances at 1.91 A with a high DebytWaller factor presumably correspond to the W-O bonds in the W U W and W U T i bridges. The coordination numbers are lower than expected on the basis of the XANES results. This probably indicates disorder on a large scale," mostly due to the presence of adsorbed water or due to (47) Tanah, T.; Yamashita, H.; Tsuchitani, R.; Funabiki, T.; Yoshida, S. J . Chrm. Soc., Faraday Tram. 1 191#),84, 2987.
a non-Gaussian distance distribution.q On the basis of the structural characteristics described above, a tentative model for the tungsten oxide spread on the surface of Ti02-P25can be postulated, which is schematically shown in Figure 10. The supported tungsten oxide is thought to be built by islands of branched chains of WOs units and terminal WO, units anchored onto the titania surface via W-0-Ti bonds. The length of these WOs chains and thus the WO4/WOSratio is then determined by the degree of the tungsten oxide coverage. By the adsorption (associative and/or dissociative) of one and two water molecules, respectively, the oxygen coordination increases to six for all surface tungsten sites.
Conclusion The present XANES investigation indicates that the Ti02-P25and 7-Al2O3-C-supported tungsten oxide prepared by the spreading method and by the wet impregnation method consists of tetrahedral WO, and pentahedral WOs groups in the dehydrated state after thermal treatment at 673 K. At a loading below I/, of a monolayer, the W 0 4 units predominate, whereas at 1 monolayer of tungsten oxide on TiO2-P25, prepared by spreading, the WOs groups predominate. We expect the same situation for the samples TiW7B and AIWlOA. On adsorption of water all tungsten sites become octahedrally surrounded by oxygen and water in a structure comparable to that of W0,-H20. This structure seems to be formed independently of the support oxide, the loading, and the preparation. For the spread W0,/Ti02 samples at low and high W 0 3 coverage W-0-W are seen, and W 4 T i bonds may be inferred. The data analysis is based on the comparison of XAS data from supported tungsten oxide with a large number of reference compounds with known structure. Consideration of both L1 and L, edges supports these conclusions, on the basis of which a tentative model for the structure of the supported tungsten oxide phase is developed. Acknowledgment. We thank the Siemens Corp., Munich, for financial support and the Hasylab (DESY Hamburg) for experimental assistance. RegImtry No. W03, 1314-35-8; Ti02, 13463-67-7; H20, 7732-18-5. (48) Eisenberger, P.; Brown,G. S. Soffd Srarr Commun. 1979, 29,481.
