J. Phys. Chem. B 2002, 106, 1913-1920
1913
Structural Characterization of Tungsten Phosphide (WP) Hydrotreating Catalysts by X-ray Absorption Spectroscopy and Nuclear Magnetic Resonance Spectroscopy S. T. Oyama,*,† P. Clark,†,⊥ X. Wang,† T. Shido,‡ Y. Iwasawa,‡ S. Hayashi,§ J. M. Ramallo-Lo´ pez,| and F. G. Requejo*,| Department of Chemical Engineering (0211), Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia 24061, Department of Chemistry, School of Science, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Institute for Materials and Chemical Process, National Institute of AdVanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, and Departamento de Fı´sica, Fac. de Ciencias Exactas, UniVersidad Nacional de la Plata and IFLP, CONICET, CC/67-1900 La Plata, Argentina ReceiVed: September 24, 2001
This paper presents a detailed discussion of the structure of tungsten phosphide and its relationship to simpler crystalline forms. The structure (space group Pbnm) is based on distorted hexagonal prisms with the phosphorus atoms forming P-P chains. Measurements by 31P magic angle spinning nuclear magnetic resonance show that there is only one type of phosphorus with a Knight shift of 255 ppm from 85% H3PO4, which is located in a highly anisotropic environment. Numerical simulations of extended X-ray absorption fine structure data at the W L3-edge give good agreement with crystallographic data. The W-P distance in WP was found to be 0.246 nm compared to the value of 0.247 nm obtained by X-ray diffraction. The catalytic activity of tungsten phosphide in the hydrotreating of a simulated petroleum feed is reported and compared to those of other tungsten compounds on an areal basis. Bulk (WP) and supported tungsten phosphide (WP/SiO2) are found to be more active in HDS and HDN than tungsten carbide (WC), tungsten nitride (W2N), tungsten sulfide (WS2), and more active in HDN than a commercial Ni-Mo-S/Al2O3 catalyst. The enhanced activity may be related to a considerably reduced white line area in the X-ray absorption edge of WP and WP/SiO2 compared to W metal, which makes their electronic structure resemble those of the highly active elements, Os and Ir, to the right in the periodic table.
Introduction Tungsten phosphide (WP) has been recently reported1 as a member of a highly active group of new catalysts for hydroprocessing, the transition metal phosphides. These catalysts constitute a broad class of materials, which include MoP,2-5 CoP and Ni2P.5,6 Work in this area is timely, as there is currently a pressing need to develop new types of catalysts that are more active than sulfides in order to meet stringent new specifications for sulfur content in transportation fuels.7,8 In this paper we report for the first time characterization of WP by extended and near-edge X-ray absorption fine structure (EXAFS and NEXAFS) and 31P magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). Tungsten phosphide has the manganese phosphide (MnP) structure (strukturbericht designation B31, space group Pbnm). The X-ray analysis was first reported by Scho¨nberg9 and later refined by Gue´rin et al.10 The structures of hexagonal WC (Figure 1a) and NiAs (Figure 1b) are related to that of WP (Figure 1c). All these structures are formed from different combinations of trigonal prisms, with the nonmetal atoms * Corresponding authors:
[email protected];
[email protected]. † Virginia Polytechnic Institute and State University. ‡ The University of Tokyo. § National Institute of Advanced Industrial Science and Technology (AIST). | Universidad Nacional de la Plata and IFLP. ⊥ Present address: Luna Innovations, 2851 Commerce St., Blacksburg, Virginia 24073.
Figure 1. Crystal Structure in Phosphides. (a) WC structure characteristic of MoP. (b) NiAs structure characteristic of VP. (c) MnP structure characteristic of WP.
occupying interstitial positions within those prisms. WP also has prisms, but in the actual structure they are highly distorted, and the nonmetal atoms (P) within are linked in chains. Examples of phosphides that adopt the WC, NiAs, and MnP structures are MoP, VP, and WP, respectively. A thorough review of the structure and its relationship with the spectroscopic measurements will be covered in the discussion. Tungsten phosphide is a good catalyst for hydrodesulfurization and hydrodenitrogenation. Its activity is better than that of other tungsten compounds such as the nitride, carbide, and sulfide. The activity may be related to its electronic structure, which NEXAFS indicates to be electron-rich in comparison to the metal.
10.1021/jp0136056 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/30/2002
1914 J. Phys. Chem. B, Vol. 106, No. 8, 2002 Experimental Section Bulk tungsten phosphide was prepared by the temperatureprogrammed reduction (TPR) in hydrogen (650 µmol s-1 g-1) of an amorphous tungsten phosphate precursor of approximate composition WPO6. In the program the temperature was raised from the ambient at a heating rate of 0.0167 K s-1 to 938 K, at which point it was held for 2 h, before quenching in He flow. A detailed description of the preparation is reported elsewhere.1,11 Briefly, the progress of the reaction was monitored by following the mass 18 (H2O) signal in the effluent with a mass spectrometer (Ametek/Dycor MA100). The phosphate precursor was prepared by dissolving stoichiometric quantities of ammonium metatungstate (NH4)6W12O39‚xH2O (Aldrich) and ammonium phosphate (NH4)2HPO4 (Aldrich, 99%) in distilled water, evaporating to dryness, and calcining in air at 773 K for 6 h. Silica-supported tungsten phosphide was prepared in a similar manner using TPR in hydrogen. The precursor in this case was obtained by impregnating a solution of the tungstate and phosphate compounds onto a silica support (Cabosil L-90). The moist paste was calcined in air at 773 K for 6 h, ground and mixed, then pressed, broken, and sieved between 650 and 1180 µm screens to form pelletized supported phosphate materials. The molar loading of tungsten monophosphide was 1.16 mmol each of W, and P per gram of silica, leading to 20 wt % WP/ SiO2. The samples were passivated in O2/He streams with progressively higher oxygen concentrations starting with 0.1 mol %, and were rereduced prior to activity measurement. For the NMR and EXAFS measurements the freshly prepared samples were sealed (by glassblowing) into glass cells without passivation or exposure to the atmosphere. The EXAFS cells were provided with Kapton windows for transmission of X-rays. Tungsten sulfide (WS2), carbide (WC), and nitride (W2N) were prepared as described earlier.1 Briefly, WS2 was prepared by the decomposition of ammonium tetrathiotungstate, (NH4)2WS4 (Aldrich, 99.9+ %), in a 10% H2S/H2 mixture at 678 K. WC and W2N were prepared by the temperature-programmed reaction of WO3 with 20% CH4/H2 and 100% NH3 streams, respectively, by methods described in the literature.12,13 The surface areas of these samples were measured in a volumetric nitrogen adsorption apparatus (Micromeritics 2000). A commercial Ni-Mo-S/Al2O3 (Shell 324) sulfided in situ with 10% H2S/H2 at 723 K for 3 h was employed for comparisons of reactivity. Catalytic activity was measured in an upflow, three-phase, packed bed reactor at 3.1 MPa (450 psig) and 643 K (370 °C) with a H2 flow rate of 100 µmol s-1 (150 cm3 NTP min-1). The feed liquid consisted of 3000 wppm S as dibenzothiophene (Aldrich, 99%), 2000 wppm N as quinoline (Aldrich, 99%), 500 wppm O as benzofuran (Aldrich, 99%), 20% tetralin (Aldrich 99%), and balance tetradecane (Fisher, 99%), and was delivered at 5 cm3 h-1. X-ray diffraction patterns (XRD) of the passivated materials were measured with a powder diffractometer (Scintag XDS2000) using Ni-filtered CuKR (λ ) 0.1541 nm) radiation. 31P NMR spectra were acquired at room temperature (RT) on Bruker MSL300 and Chemagnetics CMX-300 spectrometers operating at 121.50 and 121.62 MHz, respectively. The ordinary singlepulse sequence was used. For the measurements with the Chemagnetic spectrometer, the freshly prepared samples were placed in glass ampules sized to fit snugly within 5 mm diameter rotors, while for the Bruker spectrometer determinations the samples were loaded directly in the rotors in a glovebox. 31P NMR spectra for a static sample were also measured with a
Oyama et al. Bruker ASX200 spectrometer operating at 81.01 MHz. The spin-echo pulse sequence was used, and the latter half of the echo signal was acquired. The pulse interval between the two pulses was set at 40 µs. Chemical shifts were referenced to 85% H3PO4. Extended X-ray absorption fine structure (EXAFS) and nearedge X-ray absorption fine structure (NEXAFS) spectra of the W L3-edge (10207 eV) were recorded in transmission mode using a Si(220) double-crystal monochromator with a slit aperture of 1 mm at the XAS beamline of the LNLS (National Synchrotron Light Laboratory) in Campinas, Brasil. EXAFS Data Processing. The fine structure oscillations of each spectrum were isolated using the AUTOBK14 program. The background function was approximated using a spline with knots evenly spaced in photoelectron wavenumber. The values of the spline at the knot energies were found by optimizing the Fourier transform of the isolated χ(k) spectrum, which was obtained from the following formula:
χ(k) )
µ(E) - µ0(E) ∆µ
(1)
where µ0(E) is the background spline and ∆µ is the edge step. The abscissa was converted from energy to photoelectron wavenumber k by the relation
k ) p-1x2m(E - E0)
(2)
The obtained χ(k) was then Fourier transformed over a specified k range. The spline was adjusted so that this experimentally derived Fourier transform best matched the one obtained from theoretical calculation using the FEFF7 program.15 As an ab initio calculation, FEFF uses a list of atomic coordinates in a cluster and physical information about the system, such as type of absorbing atom and excited core-level for its calculation. In our case, the list of atomic coordinates was simplified using ATOMS16 which generates the required coordinates starting from a crystallographic description of the WP system taken from the work of Gue´rin et al.10 The data were then analyzed using the FEFFIT17 program. In FEFFIT, the fitting model was expressed as a sum over all scattering geometries or simple paths j. Each individual path was described by an appropriate theoretical fitting standard from FEFF. The fitting standards were modified according to the EXAFS equation18
χF(k) ) Im
∑j
NjS02Fj(k) 2 2 ei[2kRj+Φj(k)]e-(2k σj ) 2 2kRj
(3)
This equation expresses the total calculated χF(k) for a particle central atom as a sum over all scattering geometries about that atom. The effective scattering amplitude Fj(k) and phase shifts Φj(k) for each path were calculated using FEFF. For each path j, FEFFIT can modify the path distance Rj, the mean-square displacement (or Debye-Waller factor) σ2j around Rj, and the amplitude NjS20 in some manner specified by the fitting model. In our case, an isotropic relaxation was assumed to calculate the Rj. This reduced the number of fitting parameters considerably. Nj was set to the probability of occurrence of the path and accounted for the average coordination number of the shell. The passive electron reduction factor S20 depended only on the absorbing atom and was determined from a reference compound (WO3). FEFFIT also allows a shift in E0, the energy reference for each path, which modifies the effective photoelectron wavenumber by eq 2 and accounts for errors in the FEFF model.
Characterization of WP by XAFS and NMR Spectroscopy
J. Phys. Chem. B, Vol. 106, No. 8, 2002 1915
Figure 2. X-ray diffraction pattern of bulk WP. (a) Synthesized bulk sample. (b) Powder diffraction file reference.
FEFFIT allows fitting of multiple data sets and the construction of different fitting models and uses eq 3 to express the different fitting parameters. By fitting the data sets of our two samples simultaneously we significantly reduced the number of variables that must be determined in our fits, thus improving the statistical significance of the results. Several of the XAFS parameters are treated similarly for both fits in order to reduce the number of free parameters. We discuss this briefly. One amplitude factor A is fitted for the four W-P paths, and one is fitted for each W-W for each sample, giving a total of four parameters. The average coordination numbers of each shell Nj is determined from this value and from the S20 ) 0.89 value obtained from WO3, being Nj ) djA/S20, where dj is the theoretical degeneracy of the path obtained from the literature.10 As the four P shells and the three W shells have similar distances between them, only two parameters are fitted for each sample, one related to the change in the distances for the W-P paths and one for the W-W paths, considering an isotropic relaxation of distances. This results in four more parameters. The same assumption is used for the Debye-Waller factors. Only two parameters are fitted for the E0 values, as they can be considered to being dependent only on the type of atoms of the path, one for the W-P paths and one for the W-W paths, both being equal in the two WP samples. This gives a total of 14 free parameters. The number of independent data points for our EXAFS spectra are given from the so-called Nyquist theorem19 and the EXAFS specific modifications thereof described by Stern.20,36 This means that our fits are limited by systematic errors in the model calculations. Results The synthesis of the unsupported WP and the supported WP/ SiO2 samples was carried out by temperature programmed reduction of phosphate precursors. Figure 2 shows the X-ray diffraction pattern of the resulting samples. Comparison is made with a standard pattern from the powder diffraction files21 (PDF 29-1364). Figure 3 shows the 31P MAS and static NMR spectra of the unsupported WP obtained at different spinning rates. The spectra show a complex pattern of spinning sidebands (SSBs) around one resonance at 255 ppm (arrow) that did not shift with sample rotation. Another small resonance at 0 ppm was also invariant with spinning speed. The SSBs were analyzed and Table 1 summarizes the shift tensor results for the peak at 255 ppm. For the EXAFS analysis of the WP catalyst samples, WO3 was used as reference for the determination of the passive electron reduction factor S20. Phase shifts and effective scattering amplitudes are calculated from the WP structure using FEFF.
Figure 3. 31P MAS and static NMR spectra of bulk WP at different sample rotation frequencies indicated in the figure. Larmor frequencies were 121.50 MHz for (a) and (b) and 81.01 MHz for (c).
TABLE 1: NMR Parameters for WP (δiso ) 255 ppm)
a
spinning rate/Hz
δanis/ppm
ηc
4000a 5000a 6000a 7000a 7380b 8000a 8240b averaged value
379 ( 35 396 ( 44 394 ( 11 399 ( 27 399 ( 22 389 ( 27 401 ( 16 394 ( 8c
0.41 ( 0.09 0.53 ( 0.11 0.40 ( 0.04 0.42 ( 0.10 0.43 ( 0.08 0.38 ( 0.09 0.35 ( 0.06 0.42 ( 0.06c
At 121.61 MHz. b At 121.50 MHz. c Errors are standard deviations.
Figure 4. Tungsten L3-edge EXAFS of WO3 reference. (a) Plot of k versus χ(k). (b) The corresponding Fourier transform.
EXAFS experiments were performed in order to determine the local ordering characteristics at W sites. Figure 4a shows the W L3-edge EXAFS (χ(k) versus k) spectra of WO3 used as reference. The corresponding k3-weighted Fourier transform of the EXAFS spectrum is also shown (Figure 4b). The first peak, which is produced by the six oxygens first-neighbors of each W site, is used to obtain the parameter S20 of W.22 In Figures 5a
1916 J. Phys. Chem. B, Vol. 106, No. 8, 2002
Oyama et al. TABLE 2: Bond Distances from Crystallographic Data for WP (from ref 10) shell
coordination number N
distance/nm
W-P W-P W-P W-P W-W W-W W-W P-P
1 2 2 1 2 2 2 2
0.24704 0.24945 0.25054 0.25369 0.28595 0.29661 0.3248 0.2777
TABLE 3: Fitted Values for the First Four P Shells Surrounding W in WPa
Figure 5. Tungsten L3-edge EXAFS spectra of WP. (a) Plot of k versus χ(k). (b) The corresponding Fourier transform (0) and the fitted EXAFS function for the first four P shells surrounding W (solid curve). Fitting parameters are shown in Table 3.
shell
coordination number Nj
distance Rj/nmb
σ2/×10-5 nm2 c
E0/eVc
W-P W-P W-P W-P W-W W-W W-W
0.9(2) 1.8(2) 1.8(2) 0.9(2) 2.3(8) 2.3(8) 2.3(8)
0.246 (1) 0.247(1) 0.250(1) 0.252(1) 0.28(1) 0.29(1) 0.32(1)
1.8(5) 1.8(5) 1.8(5) 1.8(5) 5(3) 5(3) 5(3)
9.5(4) 9.5(4) 9.5(4) 9.5(4) 5(2) 5(2) 5(2)
a Values in parentheses represent the errors (() in the last significant digit. b An isotropic relaxation of distances was assumed. c These parameters were constrained equal for all shells.
TABLE 4: Fitted Values for the First Four P Shells Surrounding W in WP/SiO2a shell
coordination number Nj
distance Rj/nmb
σ2/×10-5 nm2 c
E0/eVc
W-P W-P W-P W-P W-W W-W W-W
0.6(2) 1.2(2) 1.2(2) 0.6(2) 0.4(2) 0.4(2) 0.4(2)
0.2438 (4) 0.2452(4) 0.2472(4) 0.2503(4) 0.28(1) 0.29(1) 0.32(1)
5.6(5) 5.6(5) 5.6(5) 5.6(5) 3(2) 3(2) 3(2)
9.5(4) 9.5(4) 9.5(4) 9.5(4) 5(2) 5(2) 5(2)
a Values in parentheses represent the errors (() in the last significant digit. b An isotropic relaxation of distances was assumed. c These parameters were constrained equal for all shells.
Figure 6. Tungsten L3-edge EXAFS spectra of WP/SiO2. (a) Plot of k versus χ(k). (b) The corresponding Fourier transform (0) and the fitted EXAFS function for the first four P shells surrounding W (solid curve). Fitting parameters are shown in Table 4.
and 6a, the experimental W L3-edge EXAFS functions (χ (k)) of the two samples (unsupported WP and SiO2-supported WP) are plotted against the photoelectron wave vector (k) after a smooth background decay has been subtracted from the data above the absorption edge using the AUTOBK program. The corresponding Fourier transforms (k3-weighted) of the EXAFS spectra and curve fitting analyses of both samples are also shown (Figure 5b and 6b). Both samples have one large peak around 0.2 nm (before phase correction) derived from W-P bonds. Table 2 shows the distances and coordination numbers for the various bond types in bulk WP obtained from the work of Gue´rin et al.10 Tables 3 and 4 show the fitted distance for the four P shells. It is seen that the distances are around 0.25 nm
Figure 7. Comparison of the Fourier transform of the W L3-edge EXAFS spectra for WP calculated using FEFF (solid curve), measured for bulk WP (dashed line), and for WP/SiO2 (dotted line).
and are very similar to those reported for WP (Table 2). A small contraction is observed in WP/SiO2. Figure 7 compares the intensities of the Fourier transforms. The radial distribution function for WP obtained with FEFF at 0 K and for S20 ) 1 is also shown. The order in intensity of the peak at 0.2 nm is FEFF > unsupported WP > WP/SiO2. This behavior is reflected in the average coordination number and Debye-Waller factors fitted for each sample. A higher intensity of the Fourier transform found by FEFF is obtained because of the low temperature of calculation and the value of S20.
Characterization of WP by XAFS and NMR Spectroscopy
Figure 8. NEXAFS spectra around the W L3 absorption edge for W foil (0), commercial WP (solid line), synthesized WP (dotted line), and WP/SiO2 (dashed line). A detail of the edge region is shown in the insert.
Figure 9. Comparison of areal rates in HDN and HDS for various W compounds to a commercial Ni-Mo-S/Al2O3 catalyst.
Figure 8 compares NEXAFS spectra at the W L3-edge of unsupported WP and supported WP/SiO2 to W foil. The area of the postedge peak (white line) increases in the order WP/ SiO2 < WP < W. Figure 9 shows the areal activity in HDS and HDN of several tungsten compounds. It is seen that WP has higher activity in these reactions than WS2, W2N, and WC, and higher HDN than a commercial Ni-Mo-S/Al2O3 catalyst. Discussion The XRD patterns of the unsupported WP and the supported WP/SiO2 samples clearly show the presence of tungsten phosphide (Figure 2). The diffraction lines of no other compounds are evident, except, of course, in the case of the SiO2containing sample which shows a broad feature at low angle due to the amorphous SiO2 support. The structure of WP is orthorhombic with unit cell parameters of ao ) 0.5731 nm, bo ) 0.3248 nm, and co ) 0.6227 nm.10 The structure of WP is complex, but may be understood by comparison to the hexagonal WC and NiAs structures, which are built up from trigonal prismatic units.23,24 Schematic diagrams of the conventional cells can be used to show the relationships (Figure 1). In the WC structure (Figure 1a) the nonmetal atoms (C) are beneath each other in adjacent vertical prisms, but in NiAs (Figure 1b) the nonmetal atoms (As) are displaced laterally one-half a lattice spacing. In the MnP structure, which is adopted by WP (Figure 1c), a metal arrangement similar to that in NiAs exists, except that now the nonmetal atoms (P) are linked to form chains. The orthorhombic unit cell of WP (Figure 10a) contains four W atoms on opposing cell faces, two W atoms in the interior, and four P atoms also in the interior. Extension of the unit cell by 20% along all coordinates (Figure 10b) shows the complexity
J. Phys. Chem. B, Vol. 106, No. 8, 2002 1917
Figure 10. Structure of WP. (a) Unit cell. (b) Atoms in the unit cell plus 20%. (c) Relationship between unit cell and trigonal prisms in the conventional cell.
alluded to earlier. Elimination of some of the atoms to leave a conventional cell with two prisms (Figure 10c) shows the relationship between the unit cell (Figure 10a) and the prismatic structure (Figure 1c). Two additional P atoms have been included to show how the P-P chains are formed (Figure 10c). The top four W atoms and bottom four W atoms (Figure 10c) form prisms (dashed lines), but the middle four W atoms are drawn closer to each other and away from the interstitial P atoms. In the ideal case, those middle four W atoms would lie at the midpoint of the vertical dashed lines. This distortion accounts for the complex structure of WP (Figure 10b), whose relationship to the ideal MnP structure (Figure 1c) can hardly be discerned in the actual structure. Despite the distortions, the individual phosphorus atoms are found in identical environments within the trigonal prisms. In an analysis of the electronic structures of the NiAs and MnP phases,25 Tremel et al. concluded that the former was preferred for d1 and d2 electron counts and the latter for d3 to d6 electron counts. The adoption of the MnP structure by WP is then expected from its d6 electronic structure. Slices parallel to the crystal faces of the WP unit cell can be used to calculate metal atom areal densities (Figure 11). Examination of the unit cell (Figure 10a) reveals that there are two atoms each on the ab, ac, and bc unit cell faces. This gives values of 1.07 × 1015 atoms cm-2 for the ab plane, 0.560 × 1015 atoms cm-2 for the ac plane, 0.989 × 1015 atoms cm-2 for the bc plane, to give an average of 0.873 × 1015 atoms cm-2. This corresponds to the expected26 order for the number of surface metal atoms, 1 × 1015 atoms cm-2. From this, the average metal atom concentration can be calculated as 14.5 µmol m-2 and the surface area per metal atom as 0.116 nm2. These values differ slightly from previously reported quantities,1 which were based on an erroneous number of surface atoms for the ab plane. The NMR spectra of the samples at different spinning speeds (Figure 3) show a complex pattern due to spinning sidebands (SSBs) around one resonance that defines an isotropic shift, δiso ) 255 ppm. This resonance appears at high frequency (downfield) compared to the normal range of phosphorus resonances, 250 ppm for PMeF2 to -461 ppm for P4.27 In semiconductors the resonance is at low frequency, -142 ppm for GaP and -148 ppm for InP.28 In metallic substances such as WP, interactions of the resonant nucleus with conduction electrons cause a shift to high frequency known as a Knight shift. This shift does not generally provide chemical information about the direction of electron transfer in the material. To our knowledge there has been only one previous determination of the Knight shift for WP, and this was without using magic angle spinning.29 The value reported for the Knight shift in that case was 150 ppm with an error of about 50 ppm.
1918 J. Phys. Chem. B, Vol. 106, No. 8, 2002
Oyama et al.
Figure 11. Surface structures in WP. (a) Unit cell. (b) Projection along the a-b axis. (c) Projection along the a-c axis. (d) Projection along the b-c axis.
The NMR spectra (Figure 3) also show another feature at 0 ppm, which does not shift with sample rotation frequency. The position of this resonance is typical of that of phosphate groups (PO43-),30 for example -4 ppm for PMo12O40 and -32 ppm for AlPO4, 14.0 ppm for Na3PO431, and 0 ppm for H3PO4. The resonance does not have spinning sidebands because the species is symmetric.31 This resonance is due to a small amount of oxide impurity on the surface of the sample, and its intensity varies sample to sample. The XRD pattern does not show any bulk oxide, and as will be discussed, the EXAFS results do not show bulk oxide either. Thus, the oxide impurity must be a minority phase. NMR is demonstrated to be a sensitive technique for detecting such species. The significant finding is the presence of only one isotropic resonance in the phosphide region, which shows that the phosphorus atoms in the WP structure reside in identical environments, as expected from the crystal structure. Although only one type of phosphorus atom is present in the structure, its location is off-center in the trigonal prisms and its environment is anisotropic. The SSBs in 31P MAS NMR spectra are produced by anisotropic shift interactions. This is confirmed by the facts that the total signal spread is independent of Larmor frequency when expressed in ppm units and that the signal is easily split into many sidebands by slow sample rotation. The SSB intensity was analyzed using a software program written in Fortran and working on an NEC PC9801 series personal computer32 based on a least-squares procedure33 and formula34 reported in the literature. Three principal components, δ11, δ22, and δ33, in the shift tensor are defined to satisfy the condition |δ33 - δiso| g |δ11 - δiso| g |δ22 - δiso|, where δiso is the isotropic shift, i.e., δiso ) (δ11 + δ22 + δ33)/3. The magnitude of the shift anisotropy is δanis ) δ33 - (δ11 + δ22)/2, and the asymmetry factor is ηC ) (δ22 - δ11)/(δ33 - δiso) (0 e ηC e 1). These calculated quantities are reported in Table 1, and the obtained averaged values are δanis ) 394 ( 8 ppm and ηC ) 0.42 ( 0.06. Figures 5 and 6 show the W L3-edge EXAFS data for bulk WP and supported WP/SiO2. Tables 3 and 4 summarize the results of the analysis of the first four P shells around the W atoms. For the WP reference (Table 3) there is close agreement between the experimentally determined distances and coordination numbers and the corresponding values obtained by X-ray diffraction (Table 2). The results for the unsupported WP show
that its structure is that of WP as reported in the literature. The total coordination number of P is close to the expected value of 6 while the distances are well within the error limits. The disorder reflected in the Debye-Waller factor is consistent with the temperature of the measurement (RT). For the supported WP/SiO2 sample, smaller average coordination factors are found (Table 4). When small clusters are examined by EXAFS, the average coordination number is smaller than the one observed in the bulk because of the higher proportion of surface atoms. This result is dependent on the size and shape of the cluster. The low coordination numbers found for the four P shells in WP/SiO2 indicate high dispersion of the phosphide particles on the support due to their small dimensions. Such results have been found for other W compounds dispersed on different supports such as W/C 35 and W/SiO2.36 For the supported WP/SiO2, it is also found that the distances determined by EXAFS are smaller than in the bulk reference. A small average contraction of 0.85% is found in the W-P distances. Contraction of distances in small particles is common. For example it has been reported that smaller distances than in the bulk are found in small clusters of Pt.37,38 The origin of these bond contractions in the Pt clusters is uncertain and has been deemed to be intrinsic to the clusters, but could be associated with surface tension forces. Also, earlier theoretical work on small copper particles39 using the Hartree-Fock Slater LCAO method has clearly demonstrated that the Cu-Cu distance in Cu particles becomes shorter than in the bulk metal. There is also an increase in the Debye-Waller factors. This is related not only to the inherent disorder of the small particles but also to the presence of the support. An increase of 1 order of magnitude in the Debye-Waller factors has been found between small metal particles isolated and supported on SiO2.40 The reason for this increase is the broad distribution in the distances between the atoms in the clusters and the support, causing an exponential decay in the observed oscillations. This reinforces our conclusion about the small dimensions of the phosphide particles dispersed on the SiO2 support. The analysis of the W-W shells shows a trend similar to that observed for the W-P shells. The average coordination number of all shells in WP is similar to that expected for the bulk material while the dispersion is even more evident in the
Characterization of WP by XAFS and NMR Spectroscopy SiO2 supported sample, as expected for more distant shells. Because of the errors in the parameters, no analysis can be made regarding the variation of the distances, although they seem to agree with the theoretical values. The FEFF simulation of the WP radial distribution function (Figure 7) shows excellent agreement between the calculated and experimental results. Both the peak positions and the intensity pattern are reproduced accurately. The Fourier transforms of the WP, and particularly the supported WP/SiO2, show lower intensity than the FEFF results because of the effect of the Debye-Waller factor. The thermal broadening is particularly strong in the WP/SiO2 sample because of the small size of the particles. The results here strongly indicate that the WP phase in the synthesized sample is essentially pure. The Fourier transforms of both samples show that no W-O bonds are present as no peak at 0.14 nm is observed. This indicates the absence of oxygen in the bulk. The phosphate signal present in the NMR spectra is due to a minority species on the surface of the samples. The NEXAFS spectra show differences in the electronic properties between supported and bulk WP. The absorption edge positions of a sample, defined as the major inflection point in the edge, are a function of the oxidation state of the element and shift to a higher energy as the oxidation state of the absorbing element becomes greater than zero.41 Contrary to the observations for other phosphides, such as MoP,4 no shifts in the absorption edge positions are seen between bulk and supported samples, indicating that they have the same oxidation state. The absorption edge for the two samples compares well with the value of 10198 eV, which is assigned as the characteristic absorption edge of bulk W. This agreement suggests that the W is in a zerovalent state in the phosphide samples. At the WL3-edge the allowed bound-state to bound-state transition from 2p3/2 to vacant d states of the absorbing atom gives rise to a resonance peak (white line) above the edge due to vacancies in the 5d level of W. The line shape of the resonance contains information on the type of final state involved in the transition. Lytle et al.42 have shown that increases in the resonance peak intensity with respect to pure element absorbers correlates with the ionicity estimated from the number of d electrons removed from the element by formation of chemical bonds. Lytle43 has also shown that the L3 resonance peak areas for the third-row transition metals increased from Au to Ta, which is also the sequence for increased electron vacancies in the d band. Meitzner et al.44 have found similar results for Re, Os, Ir, Pt, and Au. They also observed that the resonance for the metal in a highly dispersed state was only slightly more intense than it was for the bulk form of the metal, indicating a small electron deficiency of the highly dispersed form relative to the bulk. In our case differences in the white line intensity are seen between unsupported and supported WP, both being smaller than that for W foil. This indicates a lower density of unoccupied states above the Fermi energy and suggests a greater electron availability in the d band for the WP samples than in the metal. The most surprising result is the W L3-edge NEXAFS spectrum for WP/SiO2, which shows practically no white line. This is indicating an even higher electron availability close to the Fermi energy in this sample than in bulk WP, which is contrary to that found by Meitzner et al. for highly dispersed metals. It has been shown that support interactions have a small effect on white line intensities,45 so this is unlikely to be the reason for the observed change in its
J. Phys. Chem. B, Vol. 106, No. 8, 2002 1919 intensity. In addition, no W-O bonds have been found by EXAFS, which should be evident in a strong support-W interaction. The areal rates of WP and WP/SiO2 reported in Figure 9 show better performance than other tungsten compounds in hydrotreating. Comparison to a commercial Ni-Mo-S/Al2O3 sample shows that they are good catalysts for HDN and moderately good catalysts for HDS. The surface area of the WP is only 10 m2g-1, while that of the supported sulfide is 160 m2g-1, so the comparison here is given only as an indication of relative areal activity. The commercial sulfide sample is likely to be highly optimized with most of its surface area active, so that the comparison is reasonable. Clearly, on a weight basis the high surface area sulfide will be more active. The areal rates of the WP based on BET surface area at 643 K and 3.1 MPa are 3.1 × 10-9 mol m-2 s-1 for HDN and 2.4 × 10-9 mol m-2 s-1 for HDS (Figure 9). The corresponding conversions are 58% for HDN and 67% for HDS.1 Using the average surface atom site density of 0.873 × 1015 atoms cm-2 calculated earlier, this gives turnover rates for the WP of 2.1 × 10-4 s-1 for HDN and 1.7 × 10-4 s-1 for HDS. The corresponding areal rates of WP/SiO2 are 3.4 × 10-9 mol m-2 s-1 for HDN and 7.4 × 10-10 mol m-2 s-1 for HDS.11 The respective turnover rates are 2.3 × 10-4 s-1 for HDN and 5.1 × 10-5 s-1 for HDS. For the Ni-Mo-S/Al2O3 catalyst the areal rates are 2.0 × 10-9 mol m-2 s-1 for HDN and 2.7 × 10-9 mol m-2 s-1 for HDS, and the conversions are 38% for HDN and 79% for HDS. On a molecular basis, the rate on WP is equivalent to 1.9 × 1015 molecules m-2 s-1 for HDN and 1.4 × 1015 molecules m-2 s-1 for HDS. The HDN rate is about 40 times smaller than that reported recently by Stinner et al.5 of 8.1 × 1016 molecules m-2 s-1 for the HDN of o-propylaniline at 643 K and 3 MPa on a WP of 1 m2g-1. The HDN rate is smaller in the present study because of the use of quinoline, in which the nitrogen is embedded in an aromatic ring, rather than aniline. Also the present study is carried out in the presence of dibenzothiophene, which is highly inhibiting of HDN. Taking into consideration these factors and the errors associated with low surface area materials, the difference is reasonable. The enhanced activity of WP and WP/SiO2 may be related to their electronic structure as indicated by the decrease in the white line area in its NEXAFS spectrum. Decreases in white line area are observed in moving from tungsten toward the right in the periodic table toward osmium, iridium, and platinum.46 Osmium and iridium are among the most active metals for hydrotreating47 and it may be that in forming WP the behavior of tungsten approaches that of these elements. Conclusions A bulk WP was characterized by solid-state nuclear magnetic resonance and X-ray absorption spectroscopy. The results obtained by these methods were consistent with the findings obtained by X-ray diffraction reported in the literature. Only one type of phosphorus atom was found in an anisotropic environment with a characteristic Knight shift of 255 ppm from 85% H3PO4. The nuclear magnetic resonance measurements were able to discern the presence of a small amount of oxide impurity with a chemical shift close to 0 ppm. The closest W-P distance in WP was found to be 0.246 nm, in close agreement with the value of 0.247 nm obtained by X-ray diffraction. For a supported WP/SiO2 sample, this distance decreased to 0.244 nm. The WP catalyst had good activity in HDN and HDS of a model hydrocarbon mixture, with areal HDN activity superior
1920 J. Phys. Chem. B, Vol. 106, No. 8, 2002 to that of a commercial Ni-Mo-S/Al2O3 sample. The WP/ SiO2 catalyst had even higher HDN activity, but lower HDS rate. The activity may be related to the electronic structure of WP, which NEXAFS indicates has a smaller white line area than the metal and hence a greater electron availability in the d band. Acknowledgment. We acknowledge support from the U.S. Department of Energy, Office of Basic Energy Sciences through Grant No. DE-FG02-963414669, from CONICET, Argentina through grant PEI-0132/98, and use of the XAS beamline at the LNLS - National Synchrotron Light Laboratory in Campinas, Brasil under project XAS #592/99. References and Notes (1) Clark, P; Li, W.; Oyama, S. T. J. Catal. 2001, 200, 140. (2) Li, W.; Dhandapani, B.; Oyama, S. T. Chem. Lett. 1998, 207. (3) Stinner, C.; Prins, R.; Weber, T. J. Catal. 2000, 191, 438. (4) Oyama, S. T.; Clark, P.; Teixeira da Silva, V. L. S.; Lede, E. J.; Requejo, F. G. J. Phys. Chem. B 2001, 105, 4961. (5) Stinner, C.; Prins, R.; Weber, Th. J. Catal. 2001, 202, 187. (6) Robinson, W. R. A. M.; van Gestel, J. N. M.; Kora´nyi, T. I.; Eijsbouts, S.; van der Kraan, A. M.; van Veen, J. A. R.; de Beer, V. H. J. J. Catal. 1996, 161, 539. (7) Knudsen, K. G.; Cooper, B. H.; Topsøe, H. Appl. Catal. A: Gen. 1999, 189, 205. (8) Shafi, R.; Hutchings, G. J. Catal. Today 2000, 59, 423. (9) Scho¨nberg, N. Acta Chem. Scand. 1954, 8, 226. (10) Gue´rin, R.; Sergent, M.; Prigent, J. Mater. Res. Bull. 1975, 10, 957. (11) Wang, X.; Clark, P.; Oyama, S. T. J. Catal. 2002, in press. (12) Ramanathan, S.; Oyama, S. T. J. Phys. Chem. 1995, 99, 16365. (13) Lucy, T.; St. Clair, T.; Oyama, S. T. J. Mater. Res. 1998, 13, 2321. (14) Newville, M.; Livins, P.; Yacoby, Y.; Rehr, J. J.; Stern, E. A. Phys ReV. B 1993, 47, 14126. (15) Zabinsky, S. I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C.; Eller, M. J. Phys. ReV. B 1995, 52, 2995. (16) Ravel, B. J. Synchrotron Rad. 2001, 8, 314. (17) Stern, E. A.; Newville, M.; Ravel, B.; Yacoby, Y.; Haskel, D. Physica B 1995, 208-209, 117. (18) Stern, E. A.; Heald, S. M. in Handbook of Synchrotron Radiation; Koch, E. E., Ed.; North-Holland: New York, 1983; Chap. 10, pp 9951014.
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