XAFS Study of Tungsten L1- and L3-Edges - ACS Publications

J. Phys. Chem. C 2008, 112, 6869-6879

6869

XAFS Study of Tungsten L1- and L3-Edges: Structural Analysis of WO3 Species Loaded on TiO2 as a Catalyst for Photo-oxidation of NH3 Seiji Yamazoe, Yutaka Hitomi, Tetsuya Shishido, and Tsunehiro Tanaka* Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Kyoto 615-8510, Japan ReceiVed: NoVember 28, 2007; In Final Form: March 3, 2008

We analyzed the X-ray absorption fine structure (XAFS) spectra of W L1- and L3-edges in order to determine the structure of various W species on TiO2. Two 2p f 5d transitions were observed in the second derivative of the W L3-edge X-ray absorption near-edge structure (XANES) spectra of all samples containing W (WO3, Na2WO4, Cr2WO6, Ba2NiWO6, (NH4)10W12O41‚5H2O, Sc2W3O12, H3PW12O40‚13H2O, and TiO2-loaded WO3 samples). These two transitions can be assigned to 5d orbitals split by the ligand field. The split in the 5d orbital has a linear relationship with the area of pre-edge peak of the W L1-edge XANES spectrum. This linear relationship is supported by density functional theory (DFT) calculations of the octahedral and tetrahedral W models. On the basis of this linear relationship, we estimated the structure of various W species on TiO2. The obtained structure is in accordance with the curve fitting analysis of W L3-edge extended X-ray absorption fine structure (EXAFS). Additionally, we found that the octahedral WO3 species loaded on TiO2 sample exhibits the high activity for the photo-oxidation of NH3 (photo-SCO).

Introduction The structural analysis of W species loaded onto supports has been reported in several publications.1-7 W L1- and L3edge X-ray absorption has often been used in order to provide information on the local symmetry, coordination, and valence of the W species.2,3,8,9 In general, the local structure and the valence of the W species are determined by the following methods. The valence of the W species is determined by the position of the W L1-edge. The local symmetry is determined by the area of pre-edge peak of the W L1-edge XANES, which is due to a 2s f 5d transition. The 2s f 5d transition is a dipoleforbidden transition for regular octahedral symmetry; however, this is partially allowed for a distorted octahedral structure, which gives rise to the absence of an inversion symmetry because the p orbitals are mixed with 5d orbitals. Therefore, a W unit with tetrahedral symmetry exhibits a large pre-edge peak area in W L1 edge XANES. It is well-known that the EXAFS oscillation of the W L3-edge gives information on the local structure of the W species. The coordination number and bond distance of the W species are estimated by curve-fitting analysis of W L3-edge EXAFS. Although the analysis of W L1-edge XANES and L3-edge EXAFS gives the structural information described above, the study of W L3-edge XANES is limited. The L3-edge X-ray absorption white line of transition metals is attributed to electronic transitions from the 2p3/2 orbital to vacant s and d orbitals of the metal. Teo and Lee have reported that the contribution of the p-d transition in the L3-edge absorption white line is ca. 50 times stronger than that of the p-s transition.10 Therefore, the L3-edge X-ray absorption white line reflects the electronic state of the vacant d orbitals of the absorbing atom. In the case of Mo L3-edge X-ray absorption, two white lines are observed. The splitting and area of the two white lines depend on the symmetry of the Mo unit because * Corresponding author: e-mail [email protected], Tel +81-75-383-2559, Fax +81-75-383-2561.

the splitting corresponds to ligand field splitting of the d orbitals.11-14 A Mo unit with tetrahedral symmetry shows a small white line (due to the electron transition from 2p3/2 to e (dx2-y2 and dz2) of 4d) at a lower energy than the larger one (from 2p3/2 to t2 (dxy, dyz, and dzx) of 4d). In the case of an octahedral Mo unit, however, the small white line (electron transfer of 2p3/2 f eg (dx2-y2 and dz2) appears at a higher energy than the larger one (2p3/2 f t2g (dxy, dyz, and dzx)). The splitting of the two white lines of tetrahedral Mo is smaller than that of octahedral Mo. In the case of W L3-edge X-ray absorption, the prominent peak is the white line caused by the 2p f 5d transition. It is expected that W L3-edge XANES might reveal the character of the 5d orbitals in a similar fashion to Mo L3edge XANES. However, to the best of our knowledge, no such relationship between splitting of the white line due to the L3edge of W and the structure of the W species has been described because the white line is not distinctly split as is the case for Mo. The multiple peaks in the white line of the W L3-edge are very likely to correspond to electron transitions from 2p orbitals to the split 5d orbitals of W. Moreover, it is important to clarify the relationship between splitting of the white line and the structure of the W unit in order to obtain structural information on unknown W units. In this study, the W L1- and L3-edge X-ray absorption spectra of Cr2WO6 (D2 symmetry), Ba2NiWO6 (Oh symmetry), WO3 (distorted octahedron), H3PW12O40 (distorted octahedron), (NH4)10W12O41‚5H2O (distorted octahedron), Sc2W3O12 (nearly Td symmetry), and Na2WO4 (Td symmetry) were recorded, and splitting of the W L3-edge white line was carefully evaluated. The split 5d orbitals of several W models (4-, 5- and 6-coordinated W species) were calculated using DFT, and the relationship between the split and the structure of the W species was investigated. Moreover, the W L-edge (L1 and L3) X-ray absorption spectra of several WO3 loaded on TiO2 samples were recorded, and the local structures of the loaded W species were determined by the following two steps. (1) The structure was

10.1021/jp711250f CCC: $40.75 © 2008 American Chemical Society Published on Web 04/10/2008

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estimated from the splitting of the white line of the W L3-edge and the pre-edge peak area of the W L1-edge. (2) Analysis of W L3-edge EXAFS including curve fitting of the W-O range. Additionally, these WO3 loaded on TiO2 samples were applied to the photo-SCO reaction;15,16 the relationship of the structure of W oxide species to the photo-SCO activity was investigated. Experimental Section Preparation of Reference Samples. WO3 and Na2WO4 were purchased from Nakarai Tesque, Ltd. (NH4)10W12O41‚5H2O was purchased from Wako. H3PW12O40‚13H2O (HPA) was kindly supplied by the Nippon Inorganic Colour & Chemical Co., Ltd. Cr2WO6, Ba2NiWO6, and Sc2W3O12 were synthesized by the solid-state reaction described in previous reports.17,18 Cr2WO6: Cr2O3 (purchased from the Koso Chemical Co., Ltd.) and WO3 were mixed and ground. The mixture was calcined at 1423 K for 24 h. Ba2NiWO6:BaCO3, NiCO3 (purchased from Wako), and WO3 were mixed and ground. The mixture was heated at 1373 K for 20 h and, after regrinding, calcined at 1423 K for 12 h. Sc2W3O12:Sc2O3 (purchased from Kanto Chemical Co., Inc.) and WO3 were mixed, ground, and calcined at 1273 K for 5 h, and the calcined compound was then reheated at 1423 K for 24 h after regrinding. Identification of the synthesized Cr2WO6, Ba2NiWO6, and Sc2W3O12 was done by the XRD patterns they produced. Preparation of TiO2 Samples Loaded with Tungsten Oxide. The TiO2 substrates used in this study were ST-01 and were supplied by Ishihara Sangyo Kaisha Ltd. These substrates were in the anatase phase determined from the X-ray diffraction (XRD) pattern. WO3 was loaded onto the TiO2 substrate using the impregnation method, with HPA being used as a precursor for the W species. The substrates were impregnated with an aqueous solution containing the precursor at 353 K for 3 h and concentrated at 353 K. The concentrated samples were dried at 383 K overnight. All the dried samples were calcined in dry air at 573 K for 3 h. W L1,3-Edge X-ray Absorption Spectroscopy. X-ray absorption spectra around the W L1,3-edges were recorded at the BL01B1 beamline of the SPring-8 of the Japan Synchrotron Radiation Research Institute (8 GeV, 100 mA). A Si (111) twocrystal monochromator with an energy resolution of 1.3 eV at the W L3-edge and 1.8 eV at the W L1-edge was used. For W L3-edge XANES of the reference samples, a Si (311) two-crystal monochromator with an energy resolution of 0.64 eV was used. Ion chambers filled with N2(85%)/Ar(15%) and N2(50%)/Ar(50%) were used for the I0 and I detectors, respectively, and the samples were located between these ion chambers. In the case of the fluorescence mode, a Lytle detector filled with Kr was used instead of the I ion chamber. Energy calibration was carried out using a Cu foil. The samples, pretreated with O2 at the temperature at which each sample was calcined, were sealed into polyethylene bags under dry N2. W L1-edge XANES spectra of all reference samples were recorded in the quick scan mode. The other XANES and EXAFS spectra were recorded in the step scan mode. Data reduction using the REX2000 Ver.2.3.3 program (Rigaku) was carried out. The EXAFS analysis was performed as described below. The spectra were extracted by utilizing the cubic spline method and normalized to the edge height. The k3-weighted EXAFS oscillation was Fourier transformed into r space, with the Fourier transformation range between 3.0 and 15 Å-1. Curve-fitting analysis was performed for the first shell (W-O shell) with the range of the W-O shell being between 1.0 and 1.9 Å. The W-O phase and amplitude functions were extracted from WO2, which has a cassiterite structure, by calculation using the FEFF Ver.8.0 program.19

Figure 1. W L3-edge XANES spectra of (a) Ba2NiWO6, (b) Cr2WO6, (c) (NH4)10W12O41‚5H2O, (d) WO3, (e) H3PW12O40‚13H2O, (f) Sc2W3O12, and (g) Na2WO4.

Calculation Method. DFT calculations were performed by B3LYP/LanL2DZ with the Gaussian03 program package20 used for the calculating the splitting of empty d orbitals due to the ligand field. The models used for the 6-, 5-, and 4-coordinated W units were WH6, [WH5]1+, and [WH4]2+, respectively. Energy calculations were performed for the structurally different WH6, [WH5]1+, and [WH4]2+ models, and splitting of the 5d orbitals by the ligand field was estimated. Additionally, the contribution of the 6p orbitals to the 5d orbitals of W was also determined from the calculated molecular orbital coefficients in order to estimate the pre-edge peak area of W L1-edge XANES, which corresponds to the dipole-forbidden 2s f 5d transition. Catalytic Reaction. Selective catalytic oxidation of NH3 in the presence of O2 under UV irradiation (photo-SCO) was carried out in a conventional fixed bed flow system at atmospheric pressure as a catalytic test. Pyrex reactors with a flat facet (50 × 15 × 1 mm3) were filled with catalysts and fixed with quartz wool. Before the reaction, the catalysts were pretreated at 673 K in O2 diluted with Ar at a flow rate of 50 mL min-1. The composition of the reaction gas was NH3: 1000 ppm; O2: 2% (Ar balance). A Perkin-Elmer PE300BF 300 W Xe lamp was used for the light source, and the samples were illuminated from the flat face of the reactor at room temperature. The concentrations of the N2 and N2O produced were determined by two Shimadzu GC-8A TCD gas chromatographs equipped with two kinds of columns (Molecular Sieves and Porpack Q) using Ar as a carrier gas. The amount of NOx (NO and NO2) produced was determined using a Shimadzu NOA7000 NOx analyzer. Results and Discussion Analysis of W L3-Edge XANES of the Reference Compounds. The W L3-edge white line mostly derives from electron transitions from the 2p3/2 state to a vacant 5d state. Figure 1 shows the W L3-edge XANES spectra of the reference samples (Cr2WO6, Ba2NiWO6, WO3, H3PW12O40‚13H2O, (NH4)10W12O41‚ 5H2O, Sc2W3O12, and Na2WO4) with W6+ (d0) ion. Cr2WO6 and Ba2NiWO6, which have nearly octahedral (D2) and octahedral (Oh) symmetry, give two peaks in the white line of the W L3-edge. On the other hand, WO3, H3PW12O40‚13H2O, (NH4)10W12O41‚5H2O, Sc2W3O12, and Na2WO4 produce only one peak in the white line, but the shape depends on the particular structure. WO3, H3PW12O40‚13H2O, and

XAFS Study of Tungsten L1- and L3-Edges

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Figure 2. Second derivatives of W L3-edge XANES spectra of (a) Ba2NiWO6, (b) Cr2WO6, (c) (NH4)10W12O41‚5H2O, (d) WO3, (e) H3PW12O40‚13H2O, (f) Sc2W3O12, and (g) Na2WO4.

(NH4)10W12O41‚5H2O, which have a distorted octahedral structure, show a broad peak with an indistinct top, whereas Sc2W3O12 and Na2WO4, which have tetrahedral W units, have one sharp asymmetrical peak. These differences in the shape of the white line are interpreted as being due to splitting of the W 5d states by the ligand field. The two peaks of Cr2WO6 and Ba2NiWO6 can be attributed to electron transitions from 2p3/2 to split 5d states, which are the t2g and eg orbitals. The structural distortion of the octahedral symmetry generally leads to a small splitting of the 5d states because the declination of the position of the oxygen ligands on an equatorial plane located at the corners of a regular square makes the raised eg orbitals stable. Therefore, the distorted octahedral structures of WO3, H3PW12O40‚13H2O, and (NH4)10W12O41‚5H2O give rise to smaller splits in the white line than Cr2WO6 and Ba2NiWO6. In consequence, one broad peak consisting of two peaks was observed. It is known that the split 5d orbitals of a tetrahedral W unit are smaller than those of an octahedral or distorted octahedral W unit. Thus, the narrow peaks in the white line of Sc2W3O12 and Na2WO4 consist of two electron transitions from 2p3/2 to split 5d states, which are the e and tg orbitals, respectively. In order to clarify the splitting of the final state of the 5d orbitals from the splitting of W L3-edge white line, secondderivative spectra are often used.12,13,21 Figure 2 shows the second derivatives of the W L3-edge XANES spectra of the reference compounds. The split states can be clearly seen, and the energy gap reflects the d orbital splitting directly. Cr2WO6 and Ba2NiWO6, having octahedral WO6 units, have the largest gaps with values of 5.6 and 4.9 eV, respectively. The splitting of WO3, H3PW12O40‚13H2O, and (NH4)10W12O41‚5H2O, which have a distorted octahedral structure, was 4.0, 3.0, and 3.8 eV, respectively. Sc2W3O12 and Na2WO4, having WO4 units, produced a large peak with a shoulder peak at lower energy. This is because the split in the 5d orbitals of the tetrahedral W unit is too small. The energy gaps between the large peak and the shoulder peak for Sc2W3O12 and Na2WO4 are 1.7 and 1.9 eV, respectively. The changes observed in the split values for W structures are very similar to those for Mo oxide. Bare et al. reported that tetrahedrally coordinated molybdates display small d orbital splitting (1.8-2.0 eV), whereas octahedral molybdates show large splitting in the range 3.0-4.5 eV at the Mo L3edge.12 The authors concluded that the results could be interpreted in terms of basic ligand field concepts. The gap between the two peaks in the W L3-edge white line signifies

Figure 3. Deconvolution of W L3-edge XANES spectra of (a) Ba2NiWO6, (b) Cr2WO6, (c) (NH4)10W12O41‚5H2O, (d) WO3, (e) H3PW12O40‚13H2O, (f) Sc2W3O12, and (g) Na2WO4.

splitting of the 5d orbitals by the ligand field, in a similar manner to the case for the Mo L3-edge. X-ray absorption around the W L3-edge consists of electron transitions from 2p3/2 orbitals to vacant 5d orbitals and to the vacuum level. The electron transition to a vacant 5d orbital can be represented by a Lorentz function and that to the vacuum level by an arctangent function. Two Lorentz functions, whose top positions were determined on the basis of the values of the two peaks in the second derivative of each sample, and one arctangent function were used for deconvoluting the W L3-edge XANES spectra of the reference samples. The arctangent function used for all samples was identical because the valence of tungsten in all the samples was six. The results of the deconvolutions are shown in Figure 3. All the spectra could be fitted with two Lorentz functions (peak 1: peak 1 appears at lower energy position; peak 2: peak 2 appears at higher energy position) and one arctangent function. The ratio of the areas under the peaks (peak 1/peak 2) depends on the structure of the tungsten species. For Ba2NiWO6, Cr2WO6, WO3, (NH4)10W12O41‚5H2O, and H3W12O40‚13H2O, which have WO6 units, the value of (peak 1/peak 2) was about 1.5 (Ba2NiWO6: 1.51, Cr2WO6: 1.52, WO3: 1.52, (NH4)10W12O41‚5H2O: 1.49, and H3W12O40‚13H2O: 1.51). In contrast, Na2WO4 and Sc2W3O12, having tetrahedral W species, the ratio was about 0.67 (Na2WO4: 0.66; Sc2W3O12: 0.67). It is known that the X-ray absorption energies as well as the X-ray absorption intensities are different for the split 5d states of both octahedral and tetrahedral tungstates (valence of W: +6). For the octahedral WO6 unit the the t2g orbital appears at a lower energy than the eg orbital, and the X-ray absorption intensity is t2g:eg ) 3:2. On the other hand, X-ray absorption by the tg orbital for the tetrahedral W species is at a higher energy than that by the e


Yamazoe et al.

Figure 4. Ligand field splitting of the d orbitals of the W cation (d0) in 6-coordination: (a) octahedral structure and the bond length of all W-H is 1.90 Å; (b-f) the distance(s) between the H atom(s) and the central cation are changed by 0.1 Å in the direction(s) marked by the arrow(s); (g-i) the Z axis is rotated by 15°.

orbital, and the absorption intensity ratio e:tg is 2:3. Thus, the ratios of peak 1 to peak 2 for all the samples reflect the absorption intensities of the split 5d states. The total 2p f 5d absorption intensity, corresponding to the total area of the combined peaks in the W L3-edge white line, was not the same for all the samples in spite of each having W6+. The total absorption intensity depends on the structure of the W species. For example, the total absorption intensities of Ba2NiWO6 (Oh) and Cr2WO6 (D2) were large (Ba2NiWO6: 36.4 eV; Cr2WO6: 37.5 eV), and those of Na2WO4 and Sc2W3O12, having tetrahedral W species, were small (Na2WO4: 27.0 eV; Sc2W3O12: 30.5 eV). The difference in the total absorption intensity is caused by the mixing of p orbitals into 5d orbitals. The relationship between the total 2p f 5d absorption intensity and the mixing of p orbitals into 5d orbitals is discussed below. The results of the second derivative and deconvolution of the W L3-edge XANES spectra indicate that the X-ray absorption in the W L3-edge white line gives rise to electron transitions from 2p3/2 orbitals to 5d orbitals and that the two peaks in the white line are due to the splitting of the 5d states by the ligand field. Additionally, the split into two peaks is an indicator of the tungsten structure. Calculation of the Energy Gap of the 5d Orbital for Each WHx (x ) 4, 5, and 6) Model Using the DFT Method. It is important to obtain the relationship between the splitting of the

5d orbital and the W geometry in order to estimate the structure of the W species from the energy gap in the split W L3-edge white line. In this section, DFT calculations were carried out for several WHx (x ) 4, 5, and 6) model units in order to calculate the energy gap in the split 5d orbitals for each model and to investigate the correlation between this and the W structure. Figure 4 shows several WH6 models and the calculated split in the 5d orbitals. A series of WH6 models with various W-H lengths on the Z axis are collected in Figure 4a-e. The WH6 model of Figure 4a has octahedral symmetry (Oh), and all the W-H lengths are 1.9 Å. 5d orbitals split into t2g and eg states. When one or two H atoms on the Z axis are 0.1 Å closer to or further from the W atom, splitting of the eg state occurs, as shown in Figure 4b-e, because shortening or elongation of the H-W bond makes the dz2 orbital unstable or stable. The average energy gap between the t2g and eg states showed that the coordination distance of W-H has a small effect on the energy gap in 5d splitting. On the other hand, the WH6 models in Figure 4f,g, where the central W atom moves 0.1 Å in the YZ direction or two H atoms on the Z axis are rotated toward the XY direction, exhibited only small splitting of the t2g and eg states. However, the energy gaps of the t2g and eg states are smaller than those of the basic octahedral WH6 model. Additionally, we made calculations where the H atoms on the X and/or Y axis were



Figure 5. Ligand field splitting of the d orbitals of the W cation (d0) in 4-coordination: (a) tetrahedral structure and the bond length of all W-H is 1.85 Å; (b-e) the distance(s) to the H atom(s) are changed by 0.1 Å in the direction marked by the arrow(s); (d, e) the angle(s) of H-W-H is changed to 140°.

rotated toward the Z axis, shown in Figure 4h,i. Both the splitting of the t2g and eg states and the reduction in the energy gap occur because of the stability of the dx2-y2 orbital and the instability of the dyz and dzx orbitals. These results indicate that the W-H bond length is associated with splitting of the eg state and that the angular distortion from an octahedral structure affects the energy gap between the split 5d orbitals and also, in some instances, the splitting of the t2g and eg states. DFT calculations were also carried out for WH4 models as well as WH6 models, and the results are shown in Figure 5. The WH4 model in Figure 5a has tetrahedral symmetry (Td), and the bond lengths of W-H are all 1.85 Å. 5d orbitals of W consisting of doubly degenerate e states and triply degenerate tg states. The energy gap of the split 5d orbitals is 2.53 eV, and this is smaller than the energy gaps for all WH6 models. Shortening one or two W-H bonds in Figure 5b,c destabilizes the dyz orbital, but has little effect on the energy gap between the e and tg states. Figure 5d shows the WH4 model in which the angle between the H-W-H bonds of model (c) changes from 109.5° to 140.0° with the two bond lengths being 1.85 Å. Opening up the angle makes the dxy orbital unstable and the dzx orbital stable. Therefore, splitting of the tg state becomes spread. The angle of H-W-H has a larger influence on the spreading of the tg states than the W-H bond length. The energy level of the stable dzx orbital is almost midway between the e state and the other tg state. Therefore, the split 5d orbitals would consist of e and tg states, except for dzx. In this case, the energy gap of the split 5d orbital is 3.37 eV, and the large angle leads to a large energy gap. Additionally, as shown in Figure 5e, changing the angle between the other H-W-H bonds, of which the two H-W bond lengths are 1.75 Å, from 109.5° to 140.0° makes the dxy orbital unstable and, in contrast, the dyz orbital becomes stable as well as the dzx orbital. The energy gap in the 5d orbitals of the WH4 model in Figure 5e is expressed by the difference between the average of the energies of the four 5d orbitals (dx2-y2, dz2, dzx, and dyz) and the energy of dxy because the stable dzx and dyz orbitals are close to dx2-y2 and dz2. The energy gap is calculated to be 3.70 eV. These results indicate that the energy gap of the split 5d orbitals depends on the distortion of the H-W-H bond angle from tetrahedral symmetry as is the case for WH6. Some WH5 models were also calculated using the DFT method, and the results are shown in Figure 6. The 5d states of the WH5 model in Figure 6a, having a trigonal-bipyramidal structure, consist of two stable 5d orbitals (dzx and dyz), slightly

unstable 5d orbitals (dxy and dx2-y2), and an unstable dz2 orbital. The energy gap of the split 5d orbitals is expressed by the difference between the average energy of the four orbitals of dzx, dyz, dxy, and dx2-y2 and the energy of the dz2 orbital because the dxy and dx2-y2 levels are close to the dzx and dyz levels. The energy gap is 5.01 eV, and this value is between that for tetrahedral WH4 and octahedral WH6. Shortening one H-W bond length by 0.1 Å of the three H-W bonds on the XY plane has hardly any effect on the 5d states, as shown in Figure 6b. On the other hand, distortion of two H atoms on the Z axis of the trigonal-bipyramidal structure in the X direction (Figure 6c) mainly leads to an unstable dzx orbital and a smaller energy gap (4.16 eV) for the split 5d orbital than that of WH5 (a). The WH5 model in Figure 6d, having a square-pyramidal structure, showed different 5d states from WH5 (a-c). The 5d states for Figure 6d split the t2g states and broaden the eg states, which is similar to those of 6-coordinated WH6. However, the energy gap between the t2g and eg states is smaller than that of the WH6 models. At least, the energy gaps of the 5d states of all the WH5 models have values between those of the WH4 and WH6 models. We also carried out DFT calculations for models of the reference samples in order to compare the results with experimental ones. The geometries of tungsten in the reference samples were obtained from published data.22-28 The calculated 5d states are shown in Figure 7. Ba2NiWO6, which has Oh symmetry, exhibits the same 5d states as the WH6 model in Figure 4a. Cr2WO6, having D2 symmetry, shows similar 5d states to the WH6 model in Figure 4i, although the splits in the t2g and eg states are smaller than in the WH6 model. The structure of WO3 is similar to that in Figure 4f. Therefore, the 5d states are close to those of the WH6 model depicted in Figure 4f. WO3 has a larger distortion of the H-W-H bond angle from Oh symmetry than Cr2WO6, so the energy gaps of WO3 and (NH4)10W12O41‚ 5H2O are smaller than that of Cr2WO6. H3PW12O40‚13H2O has a large distorted octahedral structure, close to a 5-coordinated structure such as that shown in Figure 6d. Therefore, the 5d states are similar to those of the WH5 structure shown in Figure 6d. Na2WO4 and Sc2W3O12 exhibit similar energy gaps for split 5d orbitals to those for WH4 in Figure 5a because Na2WO4 and Sc2W3O12 have a tetrahedral structure. The relationship between the calculated energy gap of the split 5d state and the structure of the tungsten in the reference samples agrees approximately with the results obtained from the split in the W L3-edge white line. This agreement supports the argument that the energy gap


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Figure 6. Ligand field splitting of the d orbitals of the W cation (d0) in 5-coordination: (a) trigonal-bipyramidal structure and bond length of all W-H is 1.90 Å; (b) the distance to the H atom is changed by 0.1 Å in the direction marked by the arrow; (c) the Z axis is rotated by 15°; (d) square-pyramidal structure and bond length of all W-H is 1.90 Å.

Figure 7. Ligand field splitting of the d orbitals of the W cation (d0) in the reference samples.

of the split 5d states for some structures with tungsten species can be estimated by DFT calculations.

Analysis of the Pre-edge Peak Area of L1-Edge XANES. The W L1-edge XANES spectrum provides information on the



Figure 8. W L1-edge XANES spectra of (a) Ba2NiWO6, (b) Cr2WO6, (c) (NH4)10W12O41‚5H2O, (d) WO3, (e) H3PW12O40‚13H2O, (f) Sc2W3O12, and (g) Na2WO4.

electronic state and the geometry of the tungsten species. The electronic state and the geometry are identified by the position of the X-ray absorption edge and the pre-edge peak intensity. The pre-edge peak is mainly attributed to forbidden electron transitions from 2s orbitals to 5d orbitals. A less symmetric tungstate structure (distortion from Oh symmetry) provides a larger pre-edge peak because this forbidden electron transition is allowed by mixing the p orbitals of tungsten and the ligand into empty d orbitals. Figure 8 shows W L1-edge XANES spectra of the reference samples. Ba2NiWO6 and Cr2WO6, having octahedral WO6 units, exhibit small pre-edge peaks, and WO3, (NH4)10W12O41‚5H2O, and H3PW12O40‚13H2O, which have distorted octahedral W species, show moderate pre-edge peaks. In contrast, Na2WO4 and Sc2W3O12 having tetrahedral W units exhibit large pre-edge peaks. Deconvolution of the W L1-edge XANES spectra was carried out with Lorentz functions and an arctangent function in the same way as the extraction of the pre-edge peak from the W L3-edge XANES spectra. The deconvolution results are shown in Figure 9. Although the preedge peaks of WO3, (NH4)10W12O41‚5H2O, H3PW12O40‚13H2O, Na2WO4, and Sc2W3O12 can be fitted with one Lorentz function, two Lorentz functions were used for Cr2WO6 and Ba2NiWO6. Tami et al. have reported that the pre-edge (1s f 3d) features of the Fe K-edge of iron complexes consist of dipole and quadrupole electron transitions.29 The electronic quadrupole intensity is much weaker than the electronic dipole intensity. They reported that the electronic dipole intensity can be explained by the mixing of 4p into 3d orbitals. Because Ba2NiWO6 and Cr2WO6 have no (or little) hybrid of the d and p orbitals, only quadrupole electron transitions can occur. Therefore, the two peaks in the pre-edge of Cr2WO6 and Ba2NiWO6 were assigned to the quadrupole electron transitions from 2s to t2g and eg of the 5d orbitals, respectively. On the other hand, the large pre-edge peaks of Na2WO4 and Sc2W3O12 are mainly due to dipole electron transitions (forbidden electron transitions). In the cases of WO3, (NH4)10W12O41‚5H2O, and H3PW12O40‚ 13H2O, the difference in the WO6 symmetry from Oh would provide the medium pre-edge peak intensity.

Figure 9. Deconvolution of the pre-edge peak of W L1-edge XANES spectra of (a) Ba2NiWO6, (b) Cr2WO6, (c) (NH4)10W12O41‚5H2O, (d) WO3, (e) H3PW12O40‚13H2O, (f) Sc2W3O12, and (g) Na2WO4.

In the analysis of the W L3-edge white line, the total 2p f 5d electron absorption intensities (sum of the areas of peaks 1 and 2 in Figure 3) depends on the structure of the W species. Figure 10 shows a plot of the total area of peaks 1 and 2 in the W L3-edge white line vs the pre-edge peak area of W L1-edge XANES for the reference samples. A good linear correlation is demonstrated. The decrease in the total absorption intensity of tetrahedral W species in the W L3-edge white line is due to the increase in the mixing ratio of p orbitals into 5d orbitals, because p f p electron transitions are forbidden. Relationship between the Splitting of the White Line of L3-Edge XANES and the Pre-edge Peak Area of L1-Edge XANES. The area of pre-edge peak of W L1-edge XANES expresses the mixing ratio of the p orbitals of tungsten itself and the ligands into empty 5d orbitals. The mixing ratio of p orbitals into 5d orbitals depends on the symmetry of the tungsten species. Therefore, the area of pre-edge peak is often used as an indicator to obtain information about the W structure. On the other hand, the energy gap of the two peaks in the W L3edge white line reflects the splitting of the 5d orbitals by the ligand field. This splitting also depends on the structure of the W species. The combination of the area of pre-edge peak of W L1-edge XANES and the energy gap between the peaks in the W L3-edge white line provides more reliable information on the structure of the W species. Figure 11 shows the area of preedge peak of W L1-edge XANES plotted against the energy gap of the split W L3-edge white line for the reference samples. Figure 11 reveals an interesting relationship, in that the area of pre-edge peak of W L1-edge XANES has a linear correlation with the energy gap of the split W L3-edge white line. The octahedral W species appears on the lower right in Figure 11, and the distortion from Oh symmetry to Td symmetry is shifted


Figure 10. Total area of peaks 1 and 2 in the W L3-edge white line plotted against the pre-edge peak area of the W L1-edge XANES for the reference samples.

Yamazoe et al.

Figure 12. Dependency of the mixing ratio of 6p orbitals into 5d orbitals on the energy gap of split 5d orbitals: 9, WH6 models; 2, WH4 models; [, WH5 models; b, reference models.

Figure 11. Dependency of the pre-edge peak area of W L1-edge XANES on the energy gap of split W L3-edge white line for the reference samples (Ba2NiWO6, Cr2WO6, (NH4)10W12O41‚5H2O, WO3, H3PW12O40‚13H2O, Sc2W3O12, and Na2WO4).

to the upper left. WO3, H3PW12O40‚13H2O and (NH4)10W12O41‚ 5H2O have a large distorted octahedral structure because WO3 and H3PW12O40‚13H2O are located between the octahedral WO6 (Cr2WO6 and Ba2NiWO6) and tetrahedral WO4 (Na2WO4 and Sc2W3O12) in Figure 11. We calculated the mixing ratio of empty 6p orbitals into 5d orbitals for all WHx models in Figures 4-7 and investigated the relationship between the mixing ratio and the energy gap of split 5d orbitals using the DFT method. The mixing ratio of 6p orbitals into 5d orbitals for all WHx models is plotted against the energy gap of split 5d states in Figure 12, which clearly shows that the relationship is linear. This relationship calculated by DFT supports the linear correlation between the area of preedge peak and the energy gap of split 5d orbitals obtained from W L1 and L3 XANES, respectively. Moreover, the linear correlation shown in Figure 11 suggests that the structure of the W species can be easily estimated by a combination of the pre-edge peak areas of W L1-edge XANES and the energy gap of the split W L3-edge white line. Analysis of W L1, L3-Edge XANES for TiO2-Loaded Tungstate Samples. The results of the W L1, L3-edge XANES analysis for the reference samples and the DFT calculations revealed that the structure of the tungsten species can be estimated from the linear correlation between the energy gap of the split W L3-edge white line and the area of pre-edge peak of W L1-edge XANES. In this section, we carry out an analysis

Figure 13. W L3-edge XANES spectra of (a) 2, (b) 4, (c) 12, (d) 20, (e) 40, and (f) 80 µmol g-1 H3PW12O40/TiO2 calcined at 573 K.

of W L1, L3-edge XANES for several tungstate-loaded TiO2 samples and estimated the structures of the W species loaded on the TiO2. Figure 13 shows the W L3-edge XANES of several samples. The samples used in this study were x (2, 4, 12, 20, 40, and 80) µmol g-1 H3PW12O40‚13H2O loaded on TiO2 (xHPA/TiO2). The increasing amount of tungstate gives rise to a change in shape of the white line of W L3-edge XANES from a broad peak having a shoulder to a sharp peak. The changing shape is a consequence of the different 5d states caused by structural changes in the tungstate. In order to clarify the energy gap of the split 5d states, the split values of all samples were determined from the second derivative of the W L3-edge XANES white line (Figure 14). All the second derivatives had two peaks due to the split 5d states caused by the ligand field. The W L1-edge XANES spectra the various samples are shown in Figure 15. Deconvolution of W L1-edge XANES was carried out for all spectra, and the pre-edge peak area was determined. In order to estimate the structure of the W, the pre-edge peak area of W L1-edge XANES was plotted against the energy gap of the split W L3-edge white line, as shown in Figure 16. Figure 16, which



Figure 16. Dependency of the pre-edge peak area of W L1-edge XANES on the energy gap of the split W L3-edge white line of (b) reference samples and (2) 2, 4, 12, 20, 40, and 80 µmol g-1 H3PW12O40/ TiO2 calcined at 573 K.

Figure 14. Second derivatives of W L3-edge XANES spectra of (a) 2, (b) 4, (c) 12, (d) 20, (e) 40, and (f) 80 µmol g-1 H3PW12O40/TiO2 calcined at 573 K.

Figure 17. W L3-edge EXAFS oscillations of (a) 2, (b) 4, (c) 12, (d) 20, (e) 40, and (f) 80 µmol g-1 H3PW12O40/TiO2 calcined at 573 K and (g) Na2WO4.

Figure 15. W L1-edge XANES spectra of (a) 2, (b) 4, (c) 12, (d) 20, (e) 40, and (f) 80 µmol g-1 H3PW12O40/TiO2 calcined at 573 K.

includes the results of the reference samples, exhibits a linear correlation for all the samples and strongly supports the results of the DFT calculations. Thus, we were able to estimate the structure of the tungstate from this linear correlation. 2HPA/TiO2 showed a large energy gap for split 5d states and a small pre-edge peak area and these are close to those of Ba2NiWO6 and Cr2WO6. Increasing the amount of HPA decreases the energy gap and increases the area of pre-edge peak gradually. This indicates that the structure of the W species loaded on the TiO2 is a distorted octahedron when the amount of HPA is small

and that there is an increase in the tetrahedral W species as the amount of HPA is increased. Determination of the Structure of Tungstate on TiO2. The linear correlation in Figure 16 suggests that the tungstate structure of 2HPA/TiO2 is a distorted octahedron, and the structure gradually changes to a distorted tetrahedron with increasing HPA. The W L3-edge EXAFS oscillation spectra of x (x ) 2, 4, 12, 20, 40 and 80) HPA/TiO2 calcined at 573 K are shown in Figure 17. Increasing the amount of loaded HPA broadens the EXAFS oscillation. Additionally, the superimposed oscillation spectra of all xHPA/TiO2 samples, as shown in Figure 18, show that the change in oscillation with increasing amounts of HPA exhibits some clear isosbestic points. Although the shape of the EXAFS oscillation of 80HPA/TiO2 is slightly different from that of Na2WO4, the period of the oscillation is similar to that of Na2WO4. These results indicate that the tungstate structure for a small amount of HPA is a distorted octahedron, and increasing the amount of HPA increases the ratio of the tetrahedral tungstate. The FT spectra of W L3-edge EXAFS of the xHPA/TiO2 samples are shown in Figure 19. The peak in


Yamazoe et al.

Figure 18. Superimposed W L3-edge EXAFS oscillations of (a) 2, (b) 4, (c) 12, (d) 20, (e) 40, and (f) 80 µmol g-1 H3PW12O40/TiO2 calcined at 573 K. Figure 20. Dependency of NH3 conversion on the amount of HPA loaded on TiO2. Reaction conditions: NH3, 1000 ppm; O2, 2%; Ar balance; GHSV ) 100 000 h-1; b, NH3 conversion; 9, N2 selectivity.

TABLE 1: Structural Parameters for the W-O Shells of HPA/TiO2a

Figure 19. Fourier transforms of W L3-edge EXAFS oscillations of (a) 2, (b) 4, (c) 12, (d) 20, (e) 40, and (f) 80 µmol g-1 H3PW12O40/ TiO2 calcined at 573 K.

the range 1-2 Å due to the W-O shell and the peak around 3 Å due to W-M (M ) Ti or W) appear in the all the FT spectra. The peak intensity due to the W-M shell gradually decreases with the increasing amount of HPA whereas the position of the peak due to the W-O shell shifts to a shorter W-O bond length. Curve-fitting analysis was carried out for the W-O shell in order to obtain the structural parameters of the tungstate of xHPA/TiO2. All samples can be fitted with two W-O shells whose bond length is different, and the obtained parameters for two W-O shells are shown in Table 1. The coordination number and the bond length of 2HPA/TiO2 are almost identical with those of 4HPA/TiO2. Increasing the amount of HPA gradually decreases the coordination number and the bond length of both the short and long W-O. The changes in the EXAFS oscillation, the FT spectra, and the curve-fitting analysis agree with the result of the linear correlation in Figure 16. On the basis of these results, we conclude that the tungstate structure of 2HPA/ TiO2 is a distorted octahedron and that the amount of distorted tetrahedral W species increases with increasing HPA. Application to the Photo-SCO Reaction. We carried out photo-SCO over xHPA/TiO2 (x ) 0, 2, 4, 12, 20, 40 and 80) catalysts at a gas hourly space velocity (GHSV) of 100 000 h-1. Figure 20 shows the dependency of the NH3 conversion on the amount of HPA loaded on the TiO2. A bare TiO2 catalyst exhibited 28% NH3 conversion and 65% N2 selectivity. NOx (NO and NO2) and N2O were produced as byproducts. The addition of a few HPA to the TiO2 caused the photo-SCO activity and the N2 selectivity to develop and 4HPA/TiO2

sample

CNb

rc/Å

σ2 d/Å2

Re (%)

2HPA/TiO2 (573 K) 4HPA/TiO2 (573 K) 12HPA/TiO2 (573 K) 20HPA/TiO2 (573 K) 40HPA/TiO2 (573 K) 80HPA/TiO2 (573 K)

4.0 2.3 4.1 1.8 3.7 1.7 3.6 1.7 3.4 1.7 2.8 1.6

1.894 1.780 1.896 1.785 1.893 1.771 1.869 1.751 1.854 1.747 1.831 1.748

0.0008 0.0018 0.0027 0.0029 0.0027 0.0020 0.0053 0.0042 0.0065 0.0046 0.0076 0.0046

4.7 6.3 2.7 9.0 5.0 6.8

a R fitting range is 1.0-1.9 Å, and k fitting range is 3.0-14.8 Å-1. Coordination number. c Bond distance. d Debye-Waller. e So-called R factor

b

exhibited 50% NH3 conversion and 81% N2 selectivity. However, further addition of HPA led to a decrease in the photoSCO activity. The change in the photo-SCO activity with the amount of HPA can be explained by the tungstate structure obtained from the XAFS analysis of the W L1 and L3-edges. The distorted octahedral tungstate species mainly present on the 2- and 4HPA/TiO2 samples enhances the photo-SCO activity and the distorted tetrahedral tungstate, whose ratio gradually increases with increasing amounts of HPA, gives a negative effect to the photo-SCO activity. We concluded that the distorted octahedral tungstate, determined by a combination of the linear correlation between the split 5d states and pre-edge peak area, the DFT calculation, and the analysis of W L3-edge EXAFS, promotes the photo-SCO activity. Conclusion A new structural analytical approach of tungsten oxide species in XAFS was described to determine the structure of the loaded W oxide species which is efficient for the photo-SCO. In the XANES spectra of W L3-edge, two bands were observed in all reference samples containing W (WO3, Na2WO4, Cr2WO6, Ba2NiWO6, (NH4)10W12O41‚5H2O, Sc2W3O12, H3PW12O40‚13H2O). The gap energy of the split two bands in W L3-edge white line decreased as the structure of tungsten oxide going from octahedral tungsten surrounding to tetrahedral surrounding. These two bands can be assigned to the electron transitions from 2s orbital to 5d orbitals split by the ligand field. The gap energy of the two bands has a linear relationship with the area of pre-

XAFS Study of Tungsten L1- and L3-Edges edge peak of the W L1-edge XANES spectrum. This linear relationship is supported by DFT calculations of the 4-, 5-, and 6-coordinated W models. Using this linear relationship, we estimated the structure of various WO3 species loaded on TiO2. In the case of the small amount (2-12 µmol g-1 HPA) of WO3 species loaded on TiO2 samples, tungsten oxide species had distorted octahedral structure. Increasing in the loaded WO3 amount let the structure of WO3 close to tetrahedral structure. The estimated structure was in accordance with the curve-fitting analysis of W L3-edge EXAFS. As compared to the results of photo-SCO reaction, the distorted octahedral WO3 species promotes the photo-SCO activity. Acknowledgment. S. Yamazoe is supported by a JSPS Research Fellowship for Young Scientists. We thank Drs. H. Tanida and T. Uruga (Japan Synchrotron Radiation Research Institute, SPring-8) in carrying out the X-ray absorption experiments. The X-ray absorption experiments were performed under the approval of the JASRI (Proposal No. 2007A1805). References and Notes (1) Barton, D. G.; Soled, S. L.; Meitzner, G. D.; Fuentes, G. A.; Iglesia, E. J. Catal. 1999, 181, 57. (2) Hilbrig, F.; Gobel, H. E.; Knozinger, H.; Schmelz, H.; Lengeler, B. J. Phys. Chem. 1991, 95, 6973. (3) Horsley, J. A.; Wachs, I. E.; Brown, J. M.; Via, G. H.; Hardcastle, F. D. J. Phys. Chem. 1987, 91, 4014. (4) Kim, D. S.; Ostromecki, M.; Wachs, I. E. J. Mol. Catal. A 1996, 106, 93. (5) Lebarbier, V.; Clet, G.; Houalla, M. J. Phys. Chem. B 2006, 110, 22608. (6) Vaidyanathan, N.; Hercules, D. M.; Houalla, M. Anal. Bioanal. Chem. 2002, 373, 547. (7) Vuurman, M. A.; Wachs, I. E.; Hirt, A. M. J. Phys. Chem. 1991, 95, 9928. (8) Charton, P.; Gengembre, L.; Armand, P. J. Solid State Chem. 2002, 168, 175. (9) Fernandez-Garcia, V; Martinez-Arias, A.; Fuerte, A.; Conesa, J. C. J. Phys. Chem. B 2005, 109, 6075. (10) Teo, B. K.; Lee, P. A. J. Am. Chem. Soc. 1979, 101, 2815. (11) George, G. N.; Cleland, W. E.; Enemark, J. H.; Smith, B. E.; Kipke, C. A.; Roberts, S. A.; Cramer, S. P. J. Am. Chem. Soc. 1990, 112, 2541.

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XAFS Study of Tungsten L1- and L3-Edges - ACS Publications

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