J. Phys. Chem. 1995, 99, 327-331
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Use of XPS Valence Bands To Infer Structural Information about the Molybdenum Phase in Carbon-Supported Molybdenum Catalysts Sonia Rondon, Andrew Proctor, Marwan Houalla, and David M. Hercules* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received: July 15, 1994@
X-ray photoelectron spectroscopy (XPS)valence band spectra were used to infer information about the structure of molybdates having different symmetries. Standard compounds such as sodium molybdate, ammonium dimolybdate, and ammonium heptamolybdate were used as references. It has been verified that XPS valence band measurements can be used to determine the Mo symmetry in physical mixtures of Mo compounds and carbon as well as in Mo/C catalysts. This information was obtained without subtraction of the carbon contribution to the overall valence band envelope. The results show that for Mo/carbon catalysts prepared by equilibrium adsorption at pH 2 and 5 the major Mo surface species is octahedrally coordinated.
Introduction Supported molybdenum catalysts in their oxidic form have been characterized by a wide variety of spectroscopic techniques.'-* It has been verified that the oxidic catalyst generally contains tetrahedral monomeric and octahedral polymeric Mo species. The distribution of these species depends on preparation parameters such as the pH of the impregnating solution and the nature of the support. XPS valence band measurements have emerged as a useful technique to determine subtle chemical and structural changes in a surface. This is due to the well-known sensitivity of the valence band electrons to chemical bonding or chemical changes. Xie and Sherwoodg-13 showed that valence band measurements can be used for monitoring carbon surface chemistry after different treatments (electrochemical, thermal, etc.). Similarly, XPS valence bands have been used to determine the chemical structure of metal oxides. Sherwood et al.14,15 also reported that the valence bands for some metal oxides can be predicted by theoretical X a calculations. This provided a valuable tool for the interpretation of valence band spectra. In addition, results reported by Sherwood16 indicated that the XPS valence band region of molybdenum trioxide, where Mo is octahedrally coordinated, differs from that of sodium molybdate, where Mo is tetrahedrally coordinated. This suggested that the valence band of a Mo compound is sensitive to structural changes, and therefore it can be used to distinguish Mo compounds with different symmetry. Recently, Fiedor et al.17 used X P S valence band measurements to obtain complementary structural information about the Mo phases of standard molybdenum compounds and MolA1203 catalysts. They showed that the valence band region of the Mo phase can be extracted from the valence band of a catalyst where molybdenum is the minor component. Additionally, they stated that more definitive results could be obtained if the contribution of the valence band from the support to the overall valence band is not significant. This is expected for carbon, which is known to have a low valence band photoelectron cross section for A1 K a radiation.ls The objective of the present work was to determine from XPS
* To whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, November 15, 1994.
valence band measurements the symmetry of Mo species in Mo/ activated carbon catalysts prepared by equilibrium adsorption.
Experimental Section Materials and Catalysts. A commercial carbon (Kansai and Coke Chemical Co., Ltd.) was used as support. The manufacturerlg reports its surface area (N2 BET) as 3150 m2/g and a low ash yield of 0.25%. It was oxidized prior to use by heat treatment in air at 350 "C for 16 h. The isoelectric point (IEP) of the treated carbon was measured by the method described by Mulcahy et An acidic IEP value of 3.1 was obtained. This acidic nature of the carbon surface is associated with the presence of carboxylic and phenolic groups.21 Ammonium heptamolybdate (AHM, (NH&Mo7024) and sodium molybdate (SM, NazMoOd, supplied by Fisher, and ammonium dimolybdate (ADM, (NH4)2M0207), supplied by Alfa, were used as standard compounds. Mo atoms have different symmetries in these compounds. Mo is octahedrally coordinated in ammonium heptamolybdate and tetrahedrally coordinated in sodium molybdate. The structure of ammonium dimolybdate has been described by Mitchell and co-workers.22 The compound contains infinite chains of (M0.207)~-ions which comprise equal numbers of distorted octahedral (MoO6) and tetrahedral (Mo04) units.22 The Mo/carbon catalysts were prepared by equilibrium adsorption of Mo on activated carbon from a 0.005 M solution of ammonium heptamolybdate at pH 2 and 5 . This was followed by drying at 120 "C for 16 h. Details of the preparation method can be found in ref 6. Sodium molybdate-carbon, ammonium molybdate-carbon, and ammonium dimolybdate-carbon physical mixtures were prepared. The Mo content in these physical mixtures was selected to obtain XPS intensity ratios Z M ~3d/zC lS comparable to those of the carbon catalysts studied in this work. X P S Measurements. The X P S spectra (C Is, 0 Is, Mo 3d, and valence bands) were acquired with a Leybold Heraeus LHlO spectrometer equipped with an aluminum X-ray source (Ehv = 1486.6 eV) and a hemispherical analyzer operated at a constant pass energy of 100 eV. The X-ray source was operated at 240 W (12 kV, 20 mA), and the pressure in the analysis chamber was maintained at Torr during data acquisition. The XPS
0022-365419512099-0327$09.00/0 0 1995 American Chemical Society
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data were collected and subsequently analyzed on an IBM PC compatible. The binding energy values were referenced to the C 1s peak at 284.6 eV. Physical mixtures and the catalysts were pressed (6000 psi) and mounted on the specimen holder using double-side adhesive tape. Data Analysis. A. DifSerence Spectra. Differences between the XPS valence band spectra containing contributions from the Mo phase and the carbon support and those of the carbon support were carried out. Thus, the resulting envelopes include only the contribution from the Mo phase valence band. The procedure adopted was similar to that described by Fiedor et
I
I
I
SM
ADM
AHM
a1.17
Prior to caliration, each spectrum was interpolated to a common abscissa grid (binding energy window). Subsequently, before subtraction the two valence spectra were aligned appropriately, so that corresponding points represent equivalent binding energies. The physical mixtures exhibit no problems associated with differential charging. Consequently, the C 1s peak (284.6 eV) was used as an internal standard for binding energy calibration for the Mo/C catalysts as well as for the physical mixtures. Once aligned, the two spectra were “area n ~ r m a l i z e d ” .The ~~ normalization constant (NC) was determined as the fraction of the spectrum (C Mo) which is solely due to the C spectrum. NC is defined as
+
NC = (I,
Is
for mixtures or Mo/C catalysts)/(Z,
Is for
C) (1)
where IC lS is the area of the C 1s peak. B. Derivative Spectra. Second derivatives of the valence band spectra were determined in order to analyze small changes in peak envelopes.23 The data were smoothed using a modified version of the method developed by Savitzky and G01ay.~~ The same filter width was used for each spectrum so that any distortion introduced by the process should be similar. C . Cross-Correlation. Cross-correlation is a mathematical technique that allows one to determine the similarity between a pair of spectra. For two discrete periodic functions a(k) and b(k) the correlation of a(k) by b(k) is defined as25 N- 1 -
k-0
where N is the number of data points in one period of the discrete functions. Equation 2 refers to correlation of the functions a(k) and b(x) at zero displacement (z = 0). If these functions are two spectra, the obtained cross-correlation value can be thought of as a measure of the identical features shared by the two s p e ~ t r a . ~ ~A, ~ large ’ value of the cross-correlation function at t = 0 indicates a high degree of similarity between the two spectra considered. In this work we will cross-correlate the second derivative of an experimental spectrum with a series of computer composite spectra. The first step of cross correlation of a series of spectra includes the formation of a data matrix, [D]. The columns of [D] indicate the individual n spectra, and its rows contain information about the spectral intensities at identical energy increments. Next, [D] must be pretreated by subtracting the mean intensity value and normalizing each spectrum. The normalization factor is the square root of the inverse of the sum of the squares of the intensity values. Finally, the correlation matrix is a diagonally symmetric matrix obtained by multiplying [D] by its transpose. High similarity between an experimental
I
I
30
20 10 Binding Energy/eV
I
Carbon
c
Figure 1. XPS valence band spectra of standard compounds and
carbon. spectrum and any of the theoretical spectra is indicated by correlation values close to unity.
Results and Discussion
Standard Compounds. Figure 1 shows the valence band regions of sodium molybdate, ammonium dimolybdate, ammonium heptamolybdate, and carbon. The valence bands of the molybdates look similar to those of some oxide^.^*,*^ In general, the spectra for oxides can be divided in two groups of peaks or sub-bands. The first, located at about 22 eV, corresponds to electron emission from the 2s level of oxygen. Another sub-band, located below 12 eV, includes a big contribution from 0 2p nonbonding orbitals and a contribution from the Mo-0 molecular orbitals. The carbon spectrum displayed in Figure 1 resembles the valence band of some PAN fibers studied by Xie and Sherwood.9 It shows intense C 2s and 0 2s features at 18 and 28 eV, respectively. The features below 18 eV are due to superposition of the C 2p and 0 2p bands. It is well-known that the surface carbon atoms show strong tendency to chemisorb oxygen and give rise to oxygen surface compound^.^^^^^ However, the concentration of oxygen on the carbon surface is much lower than on the surface of molybdates. Therefore, as can be inferred from Figure 1, the contribution of the 0 2p features to the XPS valence band of carbon is small. At this point, one can state that the valence band of molybdate-carbon physical mixtures should show clearly the contribution of the Mo compound valence band in the outer valence band region. Figure 2, A and B, shows respectively the outer valence band region and the inverted second derivative for the standard compounds: sodium molybdate (tetrahedrally coordinated Mo), ammonium heptamolybdate (octahedrally coordinated Mo), and ammonium dimolybdate (octahedral and tetrahedral Mo). It can be seen that the valence band of sodium molybdate consists of two partially resolved peaks, while that of ammonium heptamolybdate consists of one broad peak. In addition, note that the envelope of the dimolybdate shows intermediate characteristics. Thus, the valence band of molybdates is sensitive to structural changes. Therefore, the XPS valence band region can be used to monitor the symmetry of different types of molybdates. Note that, as illustrated by Figure 2b, the second-derivative spectra are more informative for analysis of subtle changes in peak envelopes. It can be seen that the second-derivative spectra
Carbon-Supported Molybdenum Catalysts
J. Phys. Chem., Vol. 99,No. 1, 1995 329
0
I
I
SM
SM+C (35% Mo)
ADM
AOM + C (20% Mo)
AHM + C (15% MO)
AHM
Blndlng Ensrgy/eV
Blndlng EnergyieV
Figure 2. (A) Valence band region of the standard compounds. (B) The corresponding second derivative.
,
I
%Od.
Correlation Values 0.783
1
0
1
,
20 10 Blnding Energy/eV
Carbon
\, (
Figure 4. Valence band regions of carbon and molybdate-carbon physical mixtures.
0.797 0.810 SM
&
0.823
(3SX S M +Ma) C
0.836
ADM
0.847 0.854 0.856 0.850 0.833 0.801
1.000 I
I
1 0 8 8 4 Binding Energy/eV
2
Figure 3. Second-derivative valence band composite spectra crosscorrelated with the second-derivative valence spectra of ammonium dimolybdate (bottom). The correlation parameters are shown for each spectrum.
of the dimolybdate show an envelope which can be attributed to a mixture of the two different symmetries involved. On the basis of this result, we attempted to obtain a semiquantitative estimate of the octahedrdtetrahedral composition for the ammonium dimolybdate used in this work. In order to do this, a series of computer composite spectra were generated. Figure 3 shows the series of valence band second-derivative composite spectra for different ratios of octahedral and tetrahedral components. The first spectrum is for the sodium molybdate secondderivative valence band (100% tetrahedral Mo), and the second spectrum from the bottom is for the ammonium heptamolybdate second-derivative valence band (100% octahedral Mo). Between these two are the valence band second-derivative composite spectra with increasing octahedral character. The bottom valence band second-derivative spectrum is that of ammonium dimolybdate. This last spectrum was cross-correlated with the composite spectra. The highest degree of correlation was found between the second-derivative valence band of the dimolybdate and those of composites containing between 60% and 80% of octahedral Mo. This result does not agree with the theoretical value (50%). This may be explained in terms of partial surface polymerization of the dimolybdate due to exposure of the sample to ambient conditions. Physical Mixtures. Three physical mixtures containing 35 wt % Mo as sodium molybdate, 15 wt % Mo as ammonium
Binding EnergyleV
Bindlnp EnergyleV
Figure 5. (A) Valence band regions of Mo standard compounds and Mo-C physical mixtures of the standard compounds with carbon. (B) Second derivative of the valence band spectra shown in (A).
heptamolybdate, and 20 wt % Mo as ammonium dimolybdate were prepared. These physical mixtures showed X P S Mo 3dC 1s intensity ratios comparable to those of the Mo/carbon catalysts prepared for this work. Figure 4 shows the valence band regions of the physical mixtures and the carbon support. These spectra clearly show the presence of 0 2s features from the carbon support (at 27 eV) and from the Mo phase (at 22 eV). Note that the 0 2s signal from the Mo phase is much more intense. Additionally, the features below 12 eV observed in the physical mixtures should be mainly associated with the Mo compounds. Figure 5A shows the outer X P S valence band region obtained from the overall valence band spectra of physical mixtures and standard compounds. The valence band region of the ammonium heptamolybdate-carbon physical mixture resembles that of the standard compound, but that of the sodium molybdate-carbon physical mixture does not clearly show the two components typical of tetrahedral symmetry. However, conclusive information can be obtained by analyzing Figure 5B. This figure compares the second derivatives of the physical mixture spectra to those of standard compounds. It can be seen that the derivative spectra of the mixtures resemble those of the corresponding standard compounds. Therefore, the secondderivative spectra can be used successfully to verify the presence of tetrahedral or octahedral species. Figure 6A shows the outer valence band envelope of the physical mixtures and standard compounds obtained following subtraction of the carbon
330 J. Phys. Chem., Vol. 99, No. I, 1995
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SM
MoX: Catnlysl pn 5
SMtC MolC Catalyst PH 2
&?SA Mo) ADM
SM
ADM + C (20% MO)
ADM AHM AHM + C
AHM
( 15% Mo)
Binding EnergylsV
Binding EnsrgyIeV
Binding EnergyleV
Figure 6. (A) Valence band spectra of standard Mo compounds and the Mo phase valance band spectra obtained from the physical mixtures using the spectral subtraction procedure. (B) Second derivative of the valence band spectra shown in (A).
Binding EnergykV
Figure 8. (A) Valence band spectra for the Mo/C catalsts following subtraction of the carbon contribution and for the standard compounds. (B) Second derivatives of the spectra shown in (A).
species are expected to be present in agreement with the valence band results. 40lC Cnlaiysl PH5
40lC Calalyrl PH 2
SM
ADM
AHM
Blndlng EnergyleV
Binding EnergyleV
Figure 7. (A) Valence band spectra for Mo/C catalysts prepared by equilibrium adsorption at pH 2 and 5 and for the standard Mo compounds. (B) Second derivatives of the valence band regions shown in (A).
contribution. The subtracted spectra demonstrate the similarity between each spectrum and its corresponding standard compound. Thus, the XPS valence band of the Mo compound can be extracted from that of a mixture. The structural information obtained from the second derivative of the subtracted spectra (Figure 6B) is comparable to that obtained from the overall spectra (Figure 5B). Summarizing, for physical mixtures of carbon and standard Mo compounds, it is not necessary to actually subtract the carbon contribution from the valence band in order to obtain structural information about the Mo phase. MoKarbon Catalysts. Figures 7A and 8A show the valence band spectra of the dried catalysts (the original spectra and the spectra after subtraction of the carbon contribution) and the spectra of the standard compounds. It can be seen that the spectra of both catalysts resemble the spectrum of the octahedral standard compound. A more definitive conclusion can be obtained from Figures 7b and 8b where the second-derivative spectra of each catalyst are compared to those of the three standard compounds. These results are consistent with those of Kim et a1.; who studied the structure of molybdenum oxyanions on different supports by Raman spectroscopy. They verified that the nature of the Mo species adsorbed on a given support is dependent on the IEP of the support surface; oxide supports with low IEP contain only polymeric octahedral species. Note that the IEP of the carbon support used in this work is 3.1. Thus, polymeric
Conclusion It has been confirmed that the XPS valence band region for Mo compounds such as ammonium heptamolybdate, ammonium dimolybdate, and sodium molybdate is sensitive to structural changes. XPS valence band measurements can be used to monitor the Mo symmetry in physical mixtures of pure molybdenum compounds and carbon and Mo/carbon catalysts. The results showed that it is not necessary to subtract the contribution of the carbon envelope to get structural information about the Mo phase. For Mo/carbon catalysts prepared by the equilibrium adsorption method at pH 2 and 5 , the valence band envelope showed that the major species adsorbed on the carbon surface are octhedrally coordinated.
Acknowledgment. We acknowledge financial support for this work from the Department of Energy, Grant DE-FGOZ 87ER13781. Sonia Rondon is grateful to INTEVEP S.A. for a graduate fellowship. References and Notes (1) Wang, L.; Hall, K. J. Catal. 1980, 66, 251. (2) Van Veen, J. A. R.; De Wit, H.; Emeis, C. E.; Hendriks, P. A. J. M. J . Catal. 1987, 107, 579. (3) Schrader, G. L.; Cheng, C. P. J. Phys. Chem. 1983, 87, 3675. (4) Medema, J.; Van Stam, C.; De Beer, V. H. J.; Konings, A. J. A.; Koningsberger, D. C. J. Catal. 1978, 53, 386. ( 5 ) Kim, D. S.; Segawa, K.; Soeya, T.; Wachs, I. J . Catal. 1992, 136, 539. (6) Wang, L.; Hall, K. J . Catal. 1982, 77, 232. (7) Clausen, B. S.; Lengeler, B.; Topsoe, H. Polyedron 1986,5, 199. (8) Chiu, N. S.; Bauer, S. H.; Johnson, M. F. L. J . Catal. 1984, 89, 226. (9) Xie, Y.; Sherwood, P. M. A. Chem. Mater. 1989, 1, 427. (10) Xie, Y.; Sherwood, P. M. A. Appl. Spectrosc. 1990, 44, 1621. (11) Xie, Y.; Shewood, P. M. A. Appl. Spectrosc. 1990, 44, 797. (12) Xie, Y.; Sherwood, P. M. A. Chem. Mater. 1991, 3, 164. (13) Xie, Y.; Wang, T.; Franklin, 0.; Sherwood, P. M. A. Appl. Spectrosc. 1992, 46, 645. (14) Sherwood, P. M. A. Phys. Rev. 1990,41, 10151. (15) Welsh, I. D.; Sherwood, P. M. A. Phys. Rev. 1989, 40, 6386. (16) Sherwood, P. M. A. Unpublished results. (17) Fiedor, J. N.; Proctor, A.; Houalla, M.; Sherwood, P. M. A.; Mulcahy, F. M.; Hercules, D. M. J . Phys. Chem. 1992, 96, 10967. (18) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976,8, 129. (19) Otowa, T. U.S. Patent 5,064,805, 1991. (20) Mulcahy, F. M.; Houalla, M.; Hercules, D. M. J. Catal. 1987, 106, 210.
Carbon-SupportedMolybdenum Catalysts (21) Solar, J. M.; Leon y Leon, C. A.; Osseo-Asare, K.; Radovic, L. R. Carbon 1990,28,369. (22) Armour, A. W.; Drew, M. G. B. Mitchell, P. C. H. J. Chem. Soc., Dalton Trans. 1975,14, 1493. (23) Proctor, A.; Shenvood, P. M. A. Anal. Chem. 1980,52,2315. (24) Proctor, A.; Shenvood, P. M. A. Anal. Chem. 1982,54, 13. (25) Hoffmann, D. P.; Proctor, A.; Hercules, D. M. A d . Chem. 1989, 61, 898. (26) Horlick, G.Anal. Chem. 1973,45,319.
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