J. Phys. Chem. B 2001, 105, 4961-4966
4961
XAFS Characterization of Highly Active Alumina-Supported Molybdenum Phosphide Catalysts (MoP/Al2O3) for Hydrotreating S. T. Oyama,*,† P. Clark,† V. L. S. Teixeira da Silva,‡ E. J. Lede,§ and F. G. Requejo§,| Department of Chemical Engineering (0211), Virginia Polytechnic Institute & State UniVersity, Blacksburg, Virginia 24061, Departamento de Engenharia Quimica, Instituto Militar de Engenharia, Rio de Janeiro, RJ, 22290-270 Brasil, and Departamento de Fisica, UniVersidad Nacional de la Plata and IFILP (CONICET), CC/67-1900 La Plata, Argentina ReceiVed: December 14, 2000; In Final Form: March 30, 2001
Alumina-supported molybdenum phosphide (MoP/Al2O3) catalysts were prepared by temperature-programmed reduction of supported phosphate precursors in H2 at 0.083 K s-1 to 1123 K. The catalysts had excellent activity in the simultaneous hydrodenitrogenation (HDN) of quinoline and hydrodesulfurization (HDS) of dibenzothiophene at realistic conditions [P ) 3.1 MPa (450 psig) and T ) 643 K (370 °C)]. The catalysts surpassed the performance of a sulfided commercial Ni-Mo-S/Al2O3 catalyst in both HDN and HDS. X-ray absorption fine structure (XAFS) analysis of the supported samples indicated the presence of dispersed species retaining metallic bonding with a characteristic Mo-P distance of 0.243 nm but showing considerable attenuation in Mo-Mo linkages.
Introduction Interest in the development of novel catalysts for hydroprocessing has been spurred by the need to meet the more stringent environmental regulations that have recently been enacted throughout the world. This has led to the study of a variety of compositions that differ from standard sulfide catalysts. Basically, the approach has been to explore (a) new supports, (b) noble metal catalysts, (c) zeolite-containing combinations, and (d) new compositions. In this work, we report on the preparation and catalytic activity of a unique composition, molybdenum phosphide (MoP), which differs substantially from other materials. The compound was first reported by our group1 as a moderately active catalytic material for the simultaneous hydrodenitrogenation (HDN) of quinoline and hydrodesulfurization (HDS) of dibenzothiophene. A subsequent study of MoP by the group of Prins2 found enhanced activity for the HDN of propylaniline, with activity that was 6 times higher per surface metal atom than that of a supported molybdenum sulfide catalyst. The objective of this work is to confirm the enhanced activity and to explore the possibility of supporting the catalyst on alumina. The phoshorus promotion of commercial molybdenum and tungsten sulfide catalysts is quite common and is summarized in a recent review by Iwamoto and Grimblot.3 Techniques such as X-ray photoelectron spectroscopy,4-6 infrared spectroscopy,7 and nuclear magnetic resonance spectroscopy8 have established that the phosphorus in these catalysts is in the form of phosphate. The phosphorus in transition metal phosphides is in reduced form, with the bonding ranging from ionic (P3- character) to metallic or covalent (zero valent).9 Thus, the MoP is distinctly * To whom correspondence should be addressed. E-mail:
[email protected]. † Virginia Polytechnic Institute & State University. ‡ Instituto Militar de Engenharia. § Universidad Nacional de la Plata and IFILP (CONICET). | Present address: Mail stop 66/200, Material Science Division, LBNL, Berkeley, CA 94702.
different chemically from traditional phosphorus-promoted molybdenum sulfide catalysts. Experimental Section Catalysts were prepared by incipient wetness impregnation of an alumina support (Degussa, Aluminumoxid C) using equimolar Mo and P solutions formed from ammonium paramolybdate tetrahydrate, (NH4)6Mo7O24‚4H2O (Aldrich, 99%), and ammonium phosphate, (NH4)2HPO4 (Aldrich, 99%). Quantities were adjusted to obtain loadings of MoP of 6.8 wt % and 13 wt % (0.54 and 1.04 mmol g-1, respectively). The impregnated supports were calcined in air at 773 K for 6 h, and the resulting solids were subjected to temperature-programmed reduction in flowing hydrogen (Airco, 99.999%) at a heating rate of 0.0833 K s-1 (5 °C min-1) and a flow rate of 98 µmol g-1 s-1. The final temperature of 1123 K was held for 2 h, and then the samples were cooled to room temperature under helium flow (Airco, 99.999%) and passivated sequentially with 0.1% O2/He, 0.5% O2/He, and slow exposure to air. Prior to use as catalysts or characterization, the samples were rereduced at 723 K in hydrogen. A reference bulk MoP sample was prepared by reduction of a molybdenum phosphate precursor1 of estimated formula MoPO5.5. Briefly, the phosphate was formed from ammonium molybdate and ammonium phosphate by calcination in air at 773 K for 6 h, and the phosphide was made by reduction of the phosphate at 923 K. The chemisorption of CO was measured by a flow technique using calibrated pulses (5.6 µmol) of CO in a He stream (65 µmol s-1, 100 cm3 min-1). Measurements on spent catalysts were obtained after removal of samples from the reactor, washing in hexane, and rereduction at 723 K. BET surface areas were measured in a volumetric N2 adsorption apparatus (Micromeritics 2000). X-ray diffraction patterns were collected on a powder diffractometer (Scintag XDS-2000) using Ni-filtered Cu KR (λ ) 0.1541 nm) radiation.
10.1021/jp004500q CCC: $20.00 © 2001 American Chemical Society Published on Web 05/05/2001
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Oyama et al.
Extended X-ray absorption fine structure (EXAFS) and nearedge X-ray absorption fine structure (NEXAFS) spectra of the Mo K edge (20 keV) were recorded in transmission mode using a Si(220) double-crystal monochromator with a slit aperture of 0.5 mm at the SAX beamline of the LNLS (National Synchrotron Light Laboratory) in Campinas, Brasil. All samples were freshly prepared and transferred at reduced hydrogen pressure to glass cells with Kapton windows, which were sealed (by glassblowing) without exposure of the samples to air. Determinations on these samples are referred to in this paper as in situ measurements. NEXAFS spectra at the P K edge were measured on samples exposed to air, because the energy range did not allow for the use of Kapton windows. Measurements were made at the SXS beamline using a monochromator with an InSb(111) crystal with a slit aperture of 2.5 mm. EXAFS data analysis followed standard procedures using the software package WinXAS97 v1.2.10 A Victoreen function was employed for background subtraction in the entire spectral range. The edge jump was normalized to one, and the energy threshold of the absorption spectrum was positioned at the first inflection point in the Mo K edge at 20 000 eV, and this value was used to convert the absorption spectrum from k space into r space. A fifth-order polynomial, representing the atomic absorption (in the postedge region), was finally subtracted to obtain the EXAFS oscillations. To determine the structural parameters, interatomic distances (R), coordination numbes (N), Debye-Waller factors (σ2), and threshold energy differences (E0), a nonlinear least-squares curve fitting in R space was performed for Mo-P and Mo-Mo bonding pairs according to the EXAFS formula
χ(k) )
∑j
NjS02Fj(k) ′′ 4 2 2 e(-2k σj )e(-2Rj/λ)e(2/3σj k ) kRj2
[
4 sin 2kRj + φj(k) - σj′k3 3
]
where the backscattering amplitude F(k) and phase φ(k) were taken from references (see below). Coordination numbers N, distances R, and disorder parameters σ were free running parameters in the refinement, as well as, optionally, the 3rd and 4th cumulants σ′ and σ′′, respectively. The latter two are utilized when higher asymmetry in the neighborhood of the central atom are expected, but were not employed here. An amplitude reduction factor S02 and the photoelectron mean free path λ can be obtained from theoretical calculations using FEFF7 code.11 The EXAFS spectra of the selected shells in the bulk MoP sample were fit assuming plane waves and single scattering using theoretical phase shifts and amplitude functions (generated by FEFF 7.0) for Mo-P and Mo-Mo bonds. Fits for the supported MoP/Al2O3 catalysts employed corresponding experimental values (obtained from the measurement of bulk MoP). The FEFF simulation to obtain phase shifts and amplitude functions were refined by varying S02 and the Debye-Waller factors. The activity of the catalysts in the hydrotreatment of a model petroleum liquid was measured in an upflow, three-phase fixedbed reactor operated at 3.1 MPa (450 psig) and 643 K (370 °C), as described earlier.12 The feed, delivered at a flow rate of 0.0014 cm3 s-1 (5.0 cm3 h-1), contained 2000 wppm of N (quinoline, Aldrich, 99%), 3000 of wppm S (dibenzothiophene, Aldrich, 99%), 500 of wppm O (benzofuran, Aldrich, 99%), 20% aromatics (tetralin, Aldrich, 99%), and the balance aliphatics (tetradecane, Fisher, 99%). Analysis was by gas chro-
Figure 1. Crystal structure of MoP.
Figure 2. X-ray diffraction patterns of MoP catalysts and reference.
TABLE 1: Physical Characteristics of MoP/Al2O3 Catalysts sample
surface areaa
CO uptakeb
CO uptakec
Al2O3 6.8% MoP/Al2O3 13% MoP/Al2O3
91 75 71
0 26 36
20 31
a
Fresh, m2g-1. b Fresh, µmol g-1. c Spent, µmol g-1.
matography. Comparison of activity was made to a commercial Ni-Mo-S/Al2O3 catalyst (Shell 324) sulfided in the reactor with 10% H2S/H2 (100 µmol s-1, 150 cm3 min-1) at 678 K. Results and Discussion MoP has a hexagonal WC type structure with space group P6hm2 and bulk lattice parameters ao ) 322 pm and co ) 319 pm.13 The structure is depicted in Figure 1. X-ray diffraction (XRD) patterns of the supported samples and the bulk Al2O3 and MoP references are shown in Figure 2. The 6.8% MoP/ Al2O3 predominantly shows the peaks of the alumina support, but the 13% MoP/Al2O3 presents distinctive peaks due to MoP. The chemisorption properties and surface areas of the catalysts are summarized in Table 1. The results of the catalytic hydroprocessing test are presented in Table 2. Hydrodesulfurization (HDS) refers to the conversion of dibenzothiophene to biphenyl, hydronitrogenation (HDN) refers to the removal of nitrogen from quinoline, and hydrogenation (HYD) refers to the conversion of quinoline to saturated N-containing hydrocarbons. The major products analyzed for were 1,2,3,4-tetrahydroquinoline, 5,6,7,8-tetrahydroquinoline, ortho-propylaniline, propylbenzene, propylcyclohexane, ethyl-
XAFS Characterization of MoP/Al2O3 for Hydrotreating
J. Phys. Chem. B, Vol. 105, No. 21, 2001 4963
TABLE 2: Hydroprocessing Performance of MoP Catalysts sample
% HDN
% HYD
% HDS
Al2O3a 6.8% MoP/Al2O3b 13% MoP/Al2O3b MoPc Ni-Mo-S/Al2O3c
2.4 54 52 54 22
32 31 33 34 29
1.1 51 57 24 54
a 2 g loaded. b 70 µmol of CO uptake sites loaded in reactor. Corrected to 70 µmol of sites using the first-order formula ln(1 - X2) ) S2/S1 ln(1 - X1), where X ) conversion and S ) chemisorption quantity.
c
benzene, ethylcyclohexane, bibenzyl, and cyclohexylbenzene, which accounted for over 95% of all products. The comparison of the catalysts is carried out in a rigorous manner in order to allow for an assessment of their intrinsic activity. Thus, the reported conversions are based on quantities of catalyst loaded in the reactor corresponding to 70 µmol of sites as measured by CO uptake (MoP) or O2 uptake (Ni-Mo-S). For the Al2O3 support, which did not exhibit CO uptake, an amount of 1.9 g was used, corresponding to the quantity of 13% MoP/Al2O3 utilized. It can be seen that the activity of Al2O3 for HDN and HDS was minimal, as expected for a blank run, but that its hydrogenation activity was moderate. This HYD conversion of above 30% was similar to that obtained with the other catalysts and corresponds to that expected for thermal equilibrium. The MoP/Al2O3 catalysts show very similar behavior for HDN and HDS, which suggests that the natures of their sites are similar. This is surprising because the 6.8% sample does not show the presence of appreciable crystalline MoP whereas the 13% sample does. The crystalline MoP phase in the 13% sample probably coexists with dispersed molybdenum phosphide species, which constitute the main active phase. This accounts for the higher CO uptake compared to that of the 6.8% sample and the close catalytic performance. Compared to bulk MoP, the supported catalysts show similar HDN conversions but substantially higher HDS activities. Despite the similarity in composition, there appear to be fundamental differences between the supported and unsupported catalysts. A significant result is the finding of higher HDN conversions for the supported MoP/Al2O3 catalysts compared to the commercial Ni-Mo-S/Al2O3 sample without loss of HDS activity. In fact, the 13% MoP/Al2O3 catalyst is higher in both HDN and HDS. In terms of first-order kinetics, the difference in HDN conversions corresponds to a factor of 3.0 in reaction rate constant and confirms the findings of the group of Prins.2 From the conversions reported in Table 2, it can be calculated that the turnover rates in HDN and HDS for the 6.8% MoP/Al2O3 catalyst are 1.2 × 10-3 and 7.2 × 10-4 s-1, respectively. The corresponding quantities for the 13% MoP/Al2O3 sample are 1.1 × 10-3 and 8.0 × 10-4 s-1, and for the Ni-Mo-S/Al2O3 catalyst, they are 4.7 × 10-4 and 7.7 × 10-4 s-1. The turnover rates are based on sites titrated by CO for the phosphides and O atoms for the sulfide. The supported phosphide catalysts display stable activity with no sign of deactivation for over 90 h in HDN (Figure 3). The HDS and HYD activities were similarly stable. The chemical state of the phosphorus in unsupported MoP was probed using NEXAFS spectroscopy under non-in situ conditions (Figure 4). A comparison was made of the P K-edge signals of unsupported MoP, the precursor MoPO5.5, and a reference AlPO4 compound. As expected, for the phosphate compounds, the P edge position appears at high energy, 2152.2 and 2152.5 eV, respectively, because the phosphorus is in a
Figure 3. Time course of HDN.
Figure 4. NEXAFS spectra at the P K edge for air-exposed AlPO4, MoPO6, and MoP.
high oxidation state. The spectrum of MoP shows a main peak at 2143.1 eV and a second feature at ∼2152 eV. The main peak is due to the phosphide and duly appears at low energy because the phosphorus is reduced. The weaker signal at higher energy is likely to be due to surface oxidation of the bulk molybdenum phosphide, which had to be exposed to air for the NEXAFS measurements. The chemical state of the molybdenum was also probed using NEXAFS spectroscopy but at the Mo K edge (Figure 5). These measurements were taken at in situ reduced conditions using samples in Kapton cells. The figure compares the near-edge features of the bulk and supported molybdenum phosphides and compares them to two references, Mo metal foil and MoO3. The respective edge positions from the inflection point of the spectra (not shown) are 19 999.6 eV for Mo foil, 20 000.7 eV for bulk MoP, 20 004.0 eV for 6.8% MoP/Al2O3, and 20 007.7 eV for MoO3. The value reported for MoO3 is that of the main peak, not the preedge, which is due to a 1s(Mo) to 4d(Mo) + 2p(O) transition not present in the other materials. The edge position for MoP is close to that of the Mo metal foil reference, indicating that the bonding is metallic, as expected. The edge position for the supported MoP/Al2O3 sample is displaced 3.3 eV to higher energy but not as much as that of MoO3, which
4964 J. Phys. Chem. B, Vol. 105, No. 21, 2001
Oyama et al. TABLE 3: Summary of Structural Parameters for Different Mo Coordination Spheres referencea bonding 1 2
Mo-P Mo-Mo
3 4 5
Mo-P Mo-Mo Mo-P
coordination numberb coordinationc 6 2 6 6 12 18
5.9 7.4 d d d
distanceb (nm)
distancec (nm)
0.246 0.321 0.323 0.406 0.455 0.516-0.519
0.245 0.318 0.389e 0.448e 0.548e
a See Figure 5. b Reference 12. c Present work. d These parameters cannot be fitted within low statistical errors. e Estimated (not fitted) parameters obtained from phase shifts calculated from the first and second neighbors.
Figure 5. NEXAFS spectra at the Mo K edge under in situ conditions for Mo foil, MoP bulk, MoP/Al2O3, and MoO3. Inset shows an expanded view of the edge region.
appears 7.0 eV higher in energy. The shift in the edge position indicates that the Mo in MoP/Al2O3 is in a higher oxidation state than that in bulk MoP. This is probably due to interactions of the Mo with the Al2O3 via Mo-O-Al linkages or to retention of unreduced oxygen in the sample. Interaction with the support is likely because of the high dispersion of the sample, although we cannot rule out retention of small amounts of oxygen despite the high temperatures of reduction (1123 K). As will be seen from the EXAFS results, no Mo-O distances were observed, indicating that the number of these bonds is very small. Investigations of the structure of the catalysts were undertaken using EXAFS spectroscopy at the Mo K edge, particularly, as the 6.8% MoP/Al2O3 sample did not show a distinctive XRD pattern. These measurements were also taken under in situ reduced conditions using Kapton cells. First, a reference bulk MoP sample was examined (Figure 6). Comparison was made
to the results of a theoretical calculation for a 0.5-nm cluster using FEFF 7.0 code. The input to the code consisted solely of the cluster size, atomic positions, and types of atoms, which in this case were obtained from crystallographic data from the literature.13 There is excellent agreement between the spectra, indicating that the FEFF simulation is able to account for the structure of bulk MoP well. The numerical results are summarized in Table 3, which again show very good agreement in coordination numbers and distances between the experimental determination for bulk MoP and the crystallographic data. The most accurate values are for those of the first (Mo-P) and second (Mo-Mo) nearest neighbors, which were obtained by fitting the EXAFS data. The rest of the values are estimates obtained by assuming the same phase shifts for the more distant Mo-P and Mo-Mo bonds as for the nearest neighbors. A comparison of the bulk MoP to the experimental 6.8% MoP/Al2O3 EXAFS data is presented in Figure 7, with the left panels showing the raw EXAFS oscillations, the center panels the Fourier transforms (FTs), and the right panels the backtransforms of the peaks centered at ∼0.2 nm. In the present case, the k3-weighted oscillation was Fourier transformed within the limits 24.5-145 nm-1. The coordinations of the different
Figure 6. EXAFS patterns at the Mo K edge for bulk MoP under in situ conditions (a) Theoretical results for bulk MoP using FEFF 7.0 with a 0.5-nm-radius cluster. (b) Experimental EXAFS oscillations of bulk MoP. On the left are the EXAFS oscillations, and on the right are the corresponding Fourier transforms. Vertical dotted lines indicate different Mo coordination spheres (see Table 1) taken from ref 13.
XAFS Characterization of MoP/Al2O3 for Hydrotreating
J. Phys. Chem. B, Vol. 105, No. 21, 2001 4965
Figure 7. EXAFS patterns at the Mo K edge for bulk and supported MoP/Al2O3 under in situ conditions. (a) Theoretical bulk MoP generated by FEFF 7.0, (b) experimental bulk MoP, and (c) fresh 6.8% MoP/Al2O3 catalyst. The first column shows the EXAFS oscillations. The second column gives the respective Fourier transforms, with insets indicating real and imaginary components. The last column indicates the corresponding filtered oscillation for the main peak of the Fourier spectra; insets here indicate the experimental data and the fitted function. Vertical dotted lines show the transformed window (k ranges from 31.15 to 123.85 nm-1).
shells were isolated from the FT-filtered oscillation between adjacent minima (i.e., zero values) for the imaginary component (see dotted spectra in Figure 7) and were then fitted in r space. This was done in order to minimize contributions from other different shells. The selected windows for the first Mo-P shell was in the region (not corrected for phase shift) r ) 0.1700.240 nm, whereas that for the Mo-Mo first coordination shell was in the region r ) 0.247-0.324 nm (see vertical dotted lines in Figure 7). Again the simulated (Figure 7a) and experimental (Figure 7b) results for bulk MoP agree well. The Fourier transforms show two prominent features corresponding to Mo-P and MoMo distances, as were also shown in Figure 6. For the supported MoP/Al2O3 sample (Figure 7c), the middle panel indicates retention of the Mo-P peak at close to 0.2 nm but a substantial attenuation of the Mo-Mo peak. This is consistent with the high dispersion of the MoP phase in the supported sample. According to our NEXAFS Mo K-edge data, recorded under the same conditions as the present EXAFS measurements, no peaks in the Fourier data corresponding to Mo-O shells (roughly at 0.21 nm) are noticeable, suggesting once again that species with Mo-O bonding are minimal in the catalysts. The experimental EXAFS parameters for bulk MoP and 6.8% MoP/Al2O3 are summarized in Table 4. As can be seen, the coordination number, N, around P is close to the expected value of 6 for bulk MoP but is lowered to 4.6 in the supported sample. The bond distances, R, are very close, though, perhaps showing
TABLE 4: Comparison of Bulk MoP and Supported MoP/Al2O3 Parametersa sample bulk MoP MoP/Al2O3 a
〈N〉 5.9 (5 × 10-4) 4.6 (5 × 10-4)
R (nm)
σ2 (nm-2)
E0 (eV)
0.244 (4 × 10-7) 0.243 (5 × 10-7)
7 × 10 (5 × 10-5) 1 × 10-1 (5 × 10-4)
4.2 (1 × 10-3) -4.2 (1 × 10-3)
-2
Standard deviations of least-squares-fitted values are in parentheses.
a minute contraction in the 6.8% MoP/Al2O3 sample. The Debye-Waller factor, σ2, is substantially larger in the supported sample. As seen from the coordination number, there are still a fair number of Mo atoms surrounding the P centers, so that the attenuation of the Mo-Mo peak can be ascribed to thermal vibrations and to likely disorder in the dispersed structure. Commercial catalysts, including Shell 324, are well-known to contain phosphorus promoters. It is well-established, though, that the chemical state of the phosphorus in these catalysts is in phosphate, not phosphide, form. This is known from the preparation conditions of the materials, which includes hightemperature calcination of precursors (∼723 K) and lowtemperature sulfidation (∼623 K). Studies by various physical techniques have also conclusively shown the presence of phosphate. For example, for supported Co and Co-Mo catalysts, X-ray photoelectron spectroscopy showed that the P 2p binding energy was 133.8 eV, which is typical for phosphates, and was unchanged in the sulfided catalysts.6 Nuclear magnetic resonance
4966 J. Phys. Chem. B, Vol. 105, No. 21, 2001 measurements8 on supported Mo and Ni-Mo catalysts showed that the 31P resonance appeared at -12.4 and -12 ppm, respectively, for sulfided samples, close to the values found on oxidized and sulfided PO4/Al2O3 of -18.5 and -17.6 ppm. This was entirely different from the value on MoP of +213 ppm.14 In summary, the material reported here, molybdenum phosphide, is uniquely different from past materials. The surface phase in the alumina-supported material remains a phosphide with characteristic Mo-P bonds. The surface species is highly dispersed, and probably disordered, and shows a considerable reduction in Mo-Mo linkages. The phosphorus is reduced, but the Mo might be slightly oxidized through the formation of MoO-Al bonds with the support. Its catalytic properties are substantially different from those of bulk MoP, in particular showing considerably enhanced HDS activity while retaining high HDN levels. The supported MoP exhibits better performance than a commercial sulfided Ni-Mo-S/Al2O3 catalyst (Shell 324) on an equal-site basis. Conclusions An alumina-supported molybdenum phosphide catalyst was successfully prepared by the temperature-programmed reduction of molybdenum phosphate precursors. The catalyst had high activities in the HDN of quinoline and HDS of dibenzothiophene under conditions of high pressure and high temperature, resembling commercial operation. XAFS analysis of the supported phase indicated that it was a phosphide with retention of Mo-P bonds of length 0.243 nm, similar to the bulk, but highly dispersed and disordered, with a greatly reduced number of Mo-Mo bonds.
Oyama et al. Acknowledgment. We acknowledge support from the U.S. Department of Energy, Office of Basic Energy Sciences, through Grant DE-FG02-963414669 and from CONICET, Argentina, through Grant PEI-0132/98, as well as use of the XAS and SXS beamlines at the LNLS (National Synchrotron Light Laboratory) in Campinas, Brasil, under Projects XAS #592/99 and SXS # 588/99, respectively. We also thank J.M. Ramallo-Lo´pez for assistance in taking data at the SXS beamline. References and Notes (1) Li, W.; Dhandapani, B.; Oyama, S. T. Chem. Lett. 1998, 207. (2) Stinner, C.; Prins, R.; Weber, T. J. Catal. 2000, 191, 438. (3) Iwamoto, R.; Grimblot, J. AdV. Catal. 1999, 44, 417. (4) Ramı´rez de Agudelo, M. M.; Morales, A. Proc. 9th Int. Cong. Catal. 1988, 1, 42. (5) Moralez, A.; Prada-Silvy, R.; Leon, V. Stud. Surf. Sci. Catal. 1993, 75, 1899. (6) Bouwens, S. M. A. M.; van der Kraan, A. M.; de Beer, V. H. J.; Prins, R. J. Catal. 1991, 128, 559. (7) Van Veen, J. A. R.; Hendriks, P. A. J. M.; Andre´a, R. R.; Romers, E. J. G. M.; Wilson, A. E. J. Phys. Chem. 1990, 94, 5282. (8) Startsev, A. N.; Klimov, O. V.; Kalinkin, A. V.; Mastikhin, V. M. Kin. Katal. 1994, 35, 552. (9) Aronsson, B.; Lundstro¨m, T.; Rundqvist, S. Borides, Silicides and Phosphides; John Wiley & Sons: New York, 1965. (10) WinXAS 97, version 1.2, 1998. T. Ressler, J. Physique IV 1997, 7, C2-269. (See also the WinXAS Home page at http://www.winxas.de.) (11) Zabinsky, S. I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C.; Eller, M. J. Phys. ReV. B 1995, 52, 2995. (12) Ramanathan, S.; Oyama, S. T. J. Phys. Chem. 1995, 99, 16365. (13) Rundqvist, S.; Lundstrom, T. Acta Chem. Scand. 1963, 17, 37. (14) Wang, X.; Oyama, S. T. Manuscript in preparation.
