6048
J. Phys. Chem. 1993,97, 60486053
Local Site Symmetry of Dispersed Molybdenum Oxide Catalysts: XANES at the Mo Lsj-Edges Simon R. Bare,'J Gary E. Mitchell,* Joseph J. Maj,? G. Edwin Vrieland,? and John L. Glands Catalysis Laboratory, Central Research & Development, 1776 Building, The Dow Chemical Company, Midland, Michigan 48674, Analytical Sciences, Michigan Research & Development, 1897 Building, The Dow Chemical Company, Midland, Michigan 49667, and Department of Chemistry, University of Michigan, Ann Arbor, Michigan 481 09 Received: February 4, 1993; In Final Form: March 31, 1993
The local site symmetry of nanodispersed molybdenum oxide phases have been determined for the first time using X-ray absorption near edge spectroscopy (XANES) at the Mo L2,3-edges. Local site symmetries varying between pure tetrahedral coordination through mixed symmetries to pure octahedral coordination were determined for species present at concentrations above and below a submonolayer. X-ray absorption near-edge spectra at the Mo L2,3-edgeswere obtained for a series of supported molybdate catalysts and Mo6+ reference materials with a range of oxygen coordination types. Basic ligand field concepts can be used to interpret the spectra. Tetrahedral molybdates display d-orbital splitting in the 1.8-2.0-eV range, whereas octahedrally coordinated molybdates display split peaks in the 3.0-4.5-eV range. The direct comparison between a complete set of molybdenum reference compounds and dispersed magnesium oxide-supported molybdate catalysts has been used to determine the detailed symmetries of the dispersed molybdenum phases that are present as amorphous, crystalline, and mixed phases.
Introduction Supported and unsupported molybdenum oxides and molybdates are versatile heterogeneouscatalysts. They are utilized for processes ranging from hydrodesulfurization of crude oil to the partial oxidation of propylene to acrolein. As an example of a supported molybdate dispersed on a high surface area support, we have chosen to study molybdenum oxide supported on magnesium oxide. There are numerous examplesin the literature of the utility of this particular catalyst. For example, it has been shown to be selective in the partial oxidation of acrolein to acrylic acid,' the oxydehydrogenationof ethyl benzene to styrene: and the oxydehydrogenation of butane to butene and b~tadiene.~ Understanding the structure-function relationshipsof molybdate catalysts will help in optimizing practical catalyst systems and expand their use to other new industrial processes. Many spectroscopies can be used to characterize the atomic structure of these catalysts, but currently no single technique can provide definitive information concerning all the critical aspects of a supported oxide catalyst structure. Therefore, groups of complementary techniquesare generallyused to characterize the most important aspects of these complex materials. In this communication we highlight the first application of X-ray absorption near-edge spectroscopy (XANES) at the molybdenum L2,3-edges to characterize a supported molybdena catalyst. The unambiguous local site symmetry information determined by XANES is combined with the structural information from a recent laser Raman spectroscopy study4 to give a detailed understanding of the structure of this supported catalyst system. The molybdenum oxide supportedon magnesium oxide catalyst has recently been the subjectof an in situ laser Raman spectroscopy s t ~ d y The . ~ number and frequency of the observed vibrational modes was utilized to suggest the structure of the various species. Distortion from perfect tetrahedral and octahedral symmetry of the molybdate group prevents unambiguous determination of the structure from band position alone. Nevertheless, LRS showed that following calcination at 600 OC in dry air the molybdena is highly dispersed for loadings less than the monolayer coverage of approximately I5 wt %. For low weight loadings, below 5 wt
' Central Research & Development, The Dow Chemical Co. 8
Michigan Research & Development, The Dow Chemical Co. University of Michigan.
% Moo3, the molybdena was present a s isolated species. At intermediate weight loadings, in the 10-20 wt % Moo3 range,
polymolybdena species were observed. At coverages exceeding a monolayer, the laser Raman spectrum was dominated by crystalline MgM004.~ Except for the crystalline magnesium molybdate, however, the local symmetry around the molybdenum atom could not be determined confidently. The XANES data allow a determination of the local symmetry of each of these species. Moreover, the structure of the amorphous dispersed molybdate phases in the presence of the crystalline phases are identified. There are only a few studies in the literature that effectively utilize the XANES at the Mo K-edge to characterize supported catalyst^.^,^ The magnitude of the 1s 4d pre-edge transition has been used to determine the symmetry around the Mo atom. This transition is symmetry forbidden in a strictly octahedral field but is allowed in a tetrahedral field. The octahedral symmetry selection rule breaks down as soon as there is no longer a center of inversion symmetry in the complex, i.e., as soon as the complex deviates from pure octahedral symmetry. Therefore, in supported molybdena catalysts, where the molybdena is usually in a distorted configuration, it is not straightforward to determine the extent of octahedral versus tetrahedral coordination using only the pre-edge feature. Using the splitting between the XANES peaks at the molybdenum L2,3-edgeswe show here that the local symmetry around the Mo center can be determined by comparison with spectra of standard molybdates with known structure. Molybdenum L2,3-edgeXANES studies on materials of biological interest have been reported.'-1° XANES at the Mo L2.3 levels offers some unique advantages over spectroscopy at the K-edge at 20.0 keV.g At the K-edge the effective resolution of approximately 10 eV, resulting from the natural line width of the core hole and the resolution of the monochromator, hinders detailed structural characterization. An improved resolution of approximately 0.5 eV can be easily obtained at the L-edges (2.5 keV) which greatly facilitates structural characterization. In addition, XANES at the L2,3 levels probe orbitals of d character which are the primary orbitals involved in bonding while transitions at the K-edge are dominated by final states of p character which are not involved directly in bonding.
0022-3654/93/2091-6048%04.00/0 0 1993 American Chemical Society
-
Dispersed Molybdenum Oxide Catalysts
The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 6049
Hedman et ala7were the first to publish spectra at the Mo L2,3-edge. For three Mo6+ compounds they showed that the variable splitting in the Mo L2,3-edgesreflected the ligand field splitting of the d-orbitals.7 A smaller splitting at the L2,j-edges was observed for a tetrahedral compound MOO^^-) compared to two octahedrally coordinated compounds (Moo3 and MoO2(acac)~). The most intense component of the split peak is at higher energy for the tetrahedral MOO^^-) compound, whereas for octahedral coordination the most intense component occurs at lower energy. Both the splitting and relative intensities can be rationalizedusing simple ligand field concepts. In a tetrahedral field, the splitting of the d-orbitals is smaller than the splitting inanoctahedralfield (e, t2versu~t2~,e~). Thenumberofavailable orbitals is also consistent with the relative intensities observed (e:2and t2:3, tzg:3ande,:2).7 In this paperweextend thisempirical method to a study of supported molybdenum oxide catalysts and provide a detailed comparison with an extensive list of Mo6+ molybdate reference compounds. The reader is cautioned, however, that while the structures of the supported molybdate catalysts determined in this study are those of the possible active phases in molybdenum partial oxidation catalysts, they are only the precursors of the active catalysts used in hydrodenitrogenation and hydrodesulfurization.
Experimental Section The data were recorded at the National Synchrotron Light Source, Brookhaven National Laboratory, on beam line X19A. The monochromator was a NSLS boomerang-type flat crystal monochromator with Si( 111) crystals. The beam line is ultra high vacuum up to the final 0.01 in. thick Be exit window in the hutch. The slit widthof the monochromatorwas 3 mm, estimated to give an energy resolution at the Mo L-edges of 0.5 eV. The photon flux at 2.5 keV has been estimated to be 1Olophotons s-l with 100 mA ring current at 2.5 GeV." The harmonic content was reduced by detuning the monochromator crystals by approximately 90%. The X-ray absorption edges were measured as fluorescence yield excitation spectra using a Stern-HealdLytledetector withargon as thedetector gas.12J3 Theloionization chamber was placed as close to the beam pipe as feasible, and the entire beam path from the beam pipe to the detector window was flushed with helium gas to minimize absorption. Prolene windows (4 pm thick) were used on the l o chamber and sample box. Spectra were recorded at room temperature. The catalyst samples were prepared as described previ~usly.~ In brief, ammonium heptamolybdate solution was used to impregnatehigh surface area ultrapure magnesiumoxide. Weight loadings of 1.5, 3.0,5.0, 10.0, 15.0,20.0,30.0, and 40.0% Moo3 catalysts were prepared (weight percent Moo3 defined as % Mo03/(MgO + Moo3)). After addition of the ammonium heptamolybdate, the resulting slurries were dried overnight at 80 OC. The catalysts were then heated from room temperature to 600 OC at 1.5 deg/min in flowingair, and held at this temperature for 2 h. After cooling to 400 OC in the furnace the catalysts were transferred to a dry nitrogen purged glovebox to cool to room temperature. Prior laser Raman studies on these same catalysts have shown that the structure of the molybdate supported on the MgO is sensitive to moisture.4 Therefore, in order to prevent water adsorption on the catalysts they were stored in a glovebox before loading into custom designed sample holders for XANES measurements. These custom-designed sample holders held the catalyst powder (0.1-0.2 g) in an O-ring sealed enclosure which had thin Prolene windows to minimize absorption of the soft X-ray photons.14 The reference compounds Na2Mo04 (Climax Molybdenum Co.), PbMo04, Moo3, (NH&Mo207, CaMo04 (Johnson Matthey), MgMo04 (Atomergic), and (NH4)6M07024-4H20(Aldrich) were verified for purity by X-ray diffraction and used as received. Ba2CaMoO6 was synthesized for this study.ls
1
'
~
~
~
1
'
~
'
'
lMo L-edge XANES of Magnesium Molybdate]
25M)
2550
2600
2650
PHOTON ENERGY (em
Figure 1. General appearanceof the Mo L2.3-edge fluorescencespectrum of magnesium molybdate, MgMoO4. The edges are assigned, and the inset shows the assignement of the white line features.
The monochromatorwas calibrated by setting the f i t inflection point of the Mo L3-edge of a clean Mo foil to 2520.0eV. However, chemical shifts have not been used in interpreting the data since the Mo in all of the reference compounds and supported catalysts is in oxidation state +6. The fluorescencespectra of the reference materials and the highly loaded catalysts were recorded in the so-called thick limit, and therefore the white lines do not reflect the true intensity due to self-absorption effects.I6 The XANES spectra are normalized by setting the intensity of the first peak in the white line to unity.
Results and Discussion Reference Compounds. The general appearance of the fluorescence excitation spectrum of the Mo L2,3-edgesof the reference compound magnesium molybdate, MgMoO4, is shown in Figure 1. The L3- and L2-edges, which result from transitions from the 2p3p and 2~112initial states, respectively, show a large "whiteline" feature. This white-line arises from atomic-like electric dipole transitions. For the L3- and L2-edges shown here, the white lines are assigned to the dipole allowed 2p 4d transition. In addition, splitting of the white lines are clearly seen. This reflects resolution of the ligand field splitting of the final state d-orbitals (vide infra). Figures 2 and 3 summarize the L3- and L2-edge data, respectively,of the Mo6+reference compounds that were studied. The XANES spectra in Figures 2 and 3 are arranged so that the tetrahedrally coordinated Mo compounds are in the lower half of the figure, and the octahedrally coordinated molybdenum compounds are in the upper half of the figure. Figures 2a and 3a show the normalized XANES spectra, and Figures 2b and 3b show the secondderivativesof the spectra, whichserveto highlight differences between peak positions and allow for more accurate determination of the splitting between the peaks. For each edge, the second derivative curves clearly show the ligand field splitting of the 4d final state. For those Mo compounds where the Mo is tetrahedrally coordinated to four oxygen atoms, Na2Mo04,I7, PbMoO4,I8 and MgMoO4,19 the splitting is smaller than the splitting in the octahedrally coordinated materials, C O M O O ~ , ~ ~ M003,~l(NH&M07024,~~ and Ba2CaM006.~~The values of the splitting of the d-orbitals are given in Table I. These data are in good agreement with the initial observations reported by Hedman et al.7 for the two symmetry types. Our data give a range of 1.8-2.4 eV for the d-orbital splitting of tetrahedral compounds, and 3.1-4.5 eV for octahedral compounds. Hedman et al. report 1.8 and 2.8 eV, re~pectively.~The splitting of 1.82.1 eV for the tetrahedral compounds can be compared to the
-
1
~
Bare et al.
6050 The Journal of Physical Chemistry, Vol. 97, No. 22, 1993
I""""'""'""'""'"'1 IMolybdenum Lz edge1 I
c
PHOTON ENERGY (eV) I
"
"
I
"
"
I
"
"
I
"
"
I
Molybdenum LZedge
Molybdenum Laedge
PbMoO,
W
P NapoO,
PHOTON ENERGY (eW
NaPoO,
I
100
105
, V , V I , ,, 110
,
, 115
, , ,
,I 120
PHOTON ENERGY (eV)
Figure 2. M o L3-edge spectra (a, top) and their second derivatives (b, bottom) of a series of Mo reference compounds.
Figure 3. M o L2-edge spectra (a, top) and their second derivatives (b, bottom) of a series of Mo reference compounds.
calculated values for e+ splitting of the tetrahedral MOO^^- ion of 1.6924 and 1.80 eV.25 The unit cell of the compound diammonium dimolybdate, (NH&Mo207, has molybdenum in both distorted tetrahedral and distorted octahedral environments,26 and both types of symmetry can be clearly distinguished in the XANES second derivative (Figures 2b and 3b). In addition to the splittingof the d-orbitals, the relative intensity ratios of these white lines are consistent with predictions from simple ligand field theory. That is, for octahedrally coordinated Mo, the first peak is more intense than the second peak, and vice versa for the tetrahedral compounds (Figures 2a and 3a). Among
the tetrahedral compounds, those with the lowest d orbital splitting (1.8 eV) are those where the Mo is almost perfectly tetrahedrally coordinated (PbMoOd and NasMo04). The octahedrally coordinated Mo compounds with the least distortion is Ba*CaMoO6, which exhibits the largest splitting (4.5 eV). The magnitude of the splitting should reflect the ability of the ligand to split the energy levels. In this case the ligand remains constant, and so the shifts may be reflecting a change in the covalency of the molybdenum. There is also a consistent difference in the relative intensity of the L3 and L2 white line absorption edges (compare Figures 2a
Dispersed Molybdenum Oxide Catalysts
The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 6051
TABLE I symmetry tetrahedral tetrahedral tetrahedral tetrahedral and octahedral octahedral octahedral octahedral octahedral
d-orbital splitting, eV
I
1
IMolybdenum Ledgel
2.1
1.9 1.8
2.4,3.1 3.3 4.0 3.6
4.5
and 3a). For each compound the ratio of the first to second white line is lower at the L3- than at the L2-edge. This phenomenon has been observed previously for a series of MwFe-S compounds.* MoOs/MgO Catalysts. The Effect of Moo3 Loading. The Mo L3-edge XANES of a series of MoO3/MgO catalysts as a function of weight loading of Moo3following calcination at 600 OC in dry air are shown in Figure 4. Figure 4a shows the absorption spectra and Figure 4b the second derivatives. For weight loadings up to 10 wt 8 Moo3, the XANES spectra are quite similar. Two minima are observed in the second-derivative curves (Figure 4b). The splitting between the peaks is 3.2 eV, indicating that thecoordination around the Mo atom is octahedral for loadings between 1.5 and 10 wt 8. At loadings of Moo3 above 10 wt 8,there is asymmetry to the low-energy side of the second peak in the absorption spectra. The changes are highlighted in the second derivatives (Figure 4b). A second minima is evident, with a splitting of 2.3 eV, indicating the presence of a tetrahedrally coordinated species. Thus, for low and intermediate weight loadings of Moo3, all the Mo is octahedrally coordinated, whereas, at high loadings, there is both octahedral and tetrahedral molybdate present. The L2-edge data exhibit the same features as the L3-edgedata and are therefore not shown. However, consistent with the reference compounds, the ratio of the first to second peak is lower for the L3-edgespectra relative to L2-edge spectra. The local site symmetry determined by XANES can now be combined with the vibrational data from Raman.4 At intermediate weight loadings, 5.0-15.0 wt 8 Moo3, an octahedrally coordinated molybdena species is the only species observed by XANES in this weight loading range. The Raman spectrum is dominated by a polymolybdate species. Therefore, the XANES and Raman data taken together indicate that in this intermediate weight loading range the molybdena species present on the magnesium oxide support is entirely an octahedrally coordinated polymolybdena species. At high weight loadings, from the Mo L-edge XANES data presented in Figures 4a and b, both octahedral and tetrahedral molybdate species are identified at 20 wt 8 Moo3 and greater. The relative amount of tetrahedral molybdate increases with increasing loading, but even at 40 wt 8 Moo3octahedral species are present. In the 20-40 wt 8 Moo3 loading range, the Raman spectra show poorly resolved bands in the Mo-0 stretching region so no detailed structural information could be determined. However, by 40 wt 8 Moo3 the LRS spectra were dominated by crystallinemagnesium molybdate, MgMo04.4 The tetrahedral molybdate observed by XANES in this loading range is therfore assigned to the presence of the crystalline magnesium molybdate phase. However, whereas the Raman spectroscopywasdominated by the crystalline component which has a much higher scattering cross section, the XANES can still detect the presence of octahedrally coordinated molybdate in addition to the tetrahedral species. The first observation of the MgMo04 phase (tetrahedral) coincides with saturation of the monolayer capacity of the MgO support .4 At low weight loadings of Moo3 (
