3798
J. Phys. Chem. 1981, 85,3798-3805
X-Ray Photoelectron Spectroscopic Characterization of Mo03/Si0, Hydrodesulfurization Catalysts Yasuaki Okamoto, * Toshlnobu Imanaka, and Shiichlro Teranishi Department of Chernlcal Engineering, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Receivecl: January 12, 198 1; In Final Form: August 6, 198 I )
An X-ray photoelectron spectroscopic study of Mo03/SiOzhydrodesulfurization catalysts was carried out to understand their physicochemical and catalytic behavior and to compare the results with a previous study of Mo03/A1203catalysts. With the oxidic Mo03/Si02catalysts, it was found that Mo was well dispersed to form monolayers below 2 wt % Moo3 content (ca. 0.4 X 10l8Mo atoms/m2), compared to ca. 13 w t % Moo3 (4 X 10l8 Mo atoms/m2) for alumina-supported catalysts. Multilayered Mo species and microcrystalline Moos developed as the Moo3 content increased to ca. 9 wt %. Above 13 w t % Moo3,crystalline Moo3was detected by XRD. At high Moo3 content, no Mo species interacting strongly with Si02was observed by XPS. When sulfided, Mo03/Si02catalysts were characterized by more vigorom surface segregations and subsequent sinterings of Mo and by lower degrees of Mo sulfidation, compared to those for Mo03/A1203catalysts, except for the extremely low Moo3 content catalysts. It was found that the extent of the sulfidation of Mo was restricted within the formation of double layers of a two-dimensional MoSzlayer over Moo2 at atmospheric pressure and 400 “C. The discrepancies in the physicochemical and catalytic features between Si02-and A1203-supported catalysts are well understood in terms of the strength of Mo-support interactions and resultant Mo species on sulfidation. Differential charging effects in the XPS measurements are pointed out to be very important for the interpretations of the XPS results on Mo03/Si02 catalysts.
Introduction T h e physicochemical characterization of hydrodesulfurization (HDS) catalysts has been carried out by many workers using various techniques including X-ray photoelectron spectroscopy (XPS).l However, a great part of such efforts was devoted to the characterization of alumina-supported Mo and Co-Mo catalysts because of practical considerations. Only a few papers reported results on silica-supported HDS catalysts. Silica-supported Mo and Co-Mo catalysts show much lower activity for the HDS of thiophene than alumina-supported catalyst^^-^ because of worse dispersions of active species and/or because of the formation of relatively inactive phases such as CoMoOl. Investigation of less-active catalysts will provide some insight into understanding the reasons that alumina-supported catalysts are active for HDS reactions. The active phase-support interactions would explain the distinction between SiOz- and A1203-supportedcatalysts in the reactions. The oxidic states of Mo and Co-Mo catalysts supported on silica have recently been studied by utilizing various techniques including XPS, diffuse reflectance spectroscopy, Raman spectroscopy, and X-ray diffraction analysisS5-l1 However, the behaviors of Mo and Co upon sul(1)Massoth, F.E.Adu. Catal. 1978,27,265. (2)Ahuja, S. P.; Derrien, M. L.; LePage, J. F. Ind. Eng. Chem. Prod. Res. Deu. 1970,9,272. (3)Ueda, H.; Todo, N.; Kurita, M. “First International Conference on the Chemistry and Uses of Molybdenum”; Climax Molybdenum Co. Ltd.: London, 1973;p 204. (4) deBeer, V. H. J.; Van der Aalst, M. J. M.; Machiels, C. J.; Schuit, G. C. A. J . Catal. 1976,43,78. Tomioka, H.; Imanaka, T.; Teranishi, S. Prepr. Int. (5)Okamoto, Y.; Congr. Catal., 7th 1980,A43. (6)Castellan, A.; Bart, J. C. J.; Vaghi, A.; Giordano, N. J. Catal. 1976, 42, 162. (7)Gajardo, P.; Pirotte, D.; Defosse, C.; Grange, P.; Delmon, B. J . Electron Spectrosc. Relat. Phenom. 1979,17,121. (8)Gajardo, P.;Grange, P.; Delmon, B. J. Phys. Chem. 1979,83,1171. (9) Gajardo, P.; Pirotte, D.; Grange, P.; Delmon, B. J. Phys. Chem. 1979,83,1780. 0022-3654/81/2085-3798$01.25/0
fidation of the catalysts are not fully understood. In this paper, we report the XPS results for oxidic and sulfided Mo03/SiOz catalysts, revealing the extent of dispersion of Mo, the degree of Mo sulfidation, the dynamic behavior of Mo during sflidation and reaction, and the activity and selectivity of the catalyst for the HDS of thiophene as a function of the surface state of the catalyst. These results are compared with those for the Mo03/A1203catalysts studied in our group by using XPS.I2-l4 As for the XPS study of oxidic Mo03/Si02 catalysts, Gajardo et ala7reported an increase in the binding energy of the Si(2p) level with an increase in the loading amount of MOO,. They considered this to be evidence for relatively strong Mo03-Si02 interactions. However, it does not seem to be conclusive. Therefore, one of our objectives in this study is to examine precisely the interactions between Si02 and oxidic or sulfided Mo. Experimental Methods Catalysts. Mo03/Si02 catalysts were prepared by impregnating SiOz (Nakarai Chemical Ltd., 60-200 mesh) with aqueous solutions of ammonium paramolybdate. The pore volume and the mean pore diameter of the SiOz used here were 0.30 mL/g and 3.3 nm, respectively, as measured by a N2-adsorption technique. The water was evaporated to dryness a t 90 “C while being stirred, followed by drying at 110 OC for 16 h and calcining at 550 OC for 5 h in air. The supported amount of Moo3ranged from 1.5 to 23 w t % MOO,. The catalysts were used in powders as prepared. The BET surface areas of the catalysts (N2adsorption at 77 K) are listed in Table I. (10)Jeziorowski, H.; Knozinger, H.; Grange, P.; Gajardo, P. J.Phys. Chem. 1980,84,1825. (11)Cheng, C. P.; Schrader, G. L. J. Catal. 1979,60,276. (12)Okamoto, Y.;Tomioka, H.; Imanaka, T.; Teranishi, S. Chem. Lett. 1979,381. (13)Okamoto, Y.;Tomioka, H.; Katoh, Y.; Imanaka, T.; Teranishi, S. J . Phys. Chem. 1980,84,1833. (14)Okamoto, Y.;Tomioka, H.; Imanaka, T.; Teranishi, S. J. Catal. 1980,66,93.
0 1981 American Chemical Society
The Journal of Physical Chemistiy, Vol. 85,No. 25, 198 1 3799
Mo03/Si02 HydrodesulfurizationCatalysts
TABLE I: Binding Energies (BE)' and Full Widths at Half-Maximum (fwhm) of t h e Mo(3d), O(ls), and C(1s) Levels Catalysts Calcined at 550 "C for 5 h
for t h e MoOJSiO, -
MOO,, wt % 0 1.5 4.8 9.1 13.0 16.7 23.1 9.3b
7.OC
surface area, m'/g of catalyst 384 29 5 292 282
101 97 84
Mo(3d)
OUS)
BE
W S )
of Mo(3dS,,), eV
fwhm, eV
BE, eV
fwhm, eV
BE, eV
fwhm, eV
232.6 232.9 233.1 232.3 232.2 231.9 233.2 231.8
5.9 5.8 5.6 5.5 5.4 5.4 5.3 5.2
532.7 532.8 532.6 532.7 532.4 532.6 532.4 532.7 532.6
2.7 2.5 2.8 2.7 2.8 2.8 3.3 2.7 2.8
285.0 284.9 284.9 285.3 285.2 284.9 284.6 284.7 284.5
2.9 3.2 3.0 2.9 3.4 3.5 3.8 3.0 3.6
' Referenced to the Si(2p)level (103.3 eV).
MoOJSiO, ( 2 3 wt %)catalyst extracted with H,O for 10 days.
Mechanical mixture of MOO, and SiO,.
XPS Measurements. X-ray photoelectron spectra were measured at room temperature on a Hitachi 507 photoelectron spectrometer using A1 Kcq2 radiation (10 kV, 50 mA). The performance and the calibration of the spectrometer have been described e1~ewhere.l~The catalyst sample was mounted on double-sided adhesive tape and evacuated at room temperature to ca. 1 X torr (1torr = 133 N/m2) in the pretreatment chamber by using a sorption pump. Subsequently, the sample was transferred to an analyzer chamber for the measurements of X-ray photoelectron spectra (base pressure during the measurements, ca. l X lo-' torr). Binding energies were referenced to the Si(2p) level (103.3 eV) for the catalyst suppok, Si02,as an internal standard. Its value was originally determined for the pure Si02 by using the C(1s) level (285.0 eV) due to contaminant carbon as a reference. The reproducibility of the binding-energy value thus determined was within f0.2 eV. The X-ray photoelectron spectra were analyzed in terms of the relative peak area intensities, full widths a t half-maximum (fwhm), and chemical shifts of the Mo(3d), O(ls), C(ls), S(2p), and Si(2p) levels. In order to avoid possible photoreductions of Mo during XPS measurements, we measured the Mo(3d) level first. Hydrodesulfurization of Thiophene. The HDS of thiophene was carried out over the Mo03/Si02catalysts (ca. 0.15 g) at 400 "C and atmospheric pressure by using a conventional fixed-bed flow reactor in a differential mode. The reaction was undertaken after (a) prereducing with H2 (50 mL/min) or (b) presulfiding with CS2/H2(1/9; H2, 27 mL/min) at 400 "C for 1h. A mixture of thiophenelbenzene was fed by a microfeeder into a stream of hydrogen (thiophene/benzene/H2, 1/03/18; H2,50 mL/ min). The reaction reached steady state after ca. 3 h for the prereduced catalysts and after ca. 2 h for the presulfided ones. The reaction gas was analyzed by gas chromatography. The conversion of thiophene was determined by using benzene as an internal standard, since no hydrogenation of benzene was detected under our reaction conditions. The X-ray photoelectron spectra of sulfided Mo03/Si02 catalysts were measured without exposing the catalyst sample, which had reached steady-state activity, to air or moisture by using a N2-filled glovebox attached directly to the pretreatment chamber of the spectrometer. In these procedures, no oxidized sulfur was detected by XPS. The S(2p)/Mo(3d) peak area intensity ratios were converted to S/Mo atomic ratios by using atomic sensitivities ob(15) Okamoto, Y.; Nitta, Y.; Imanaka, T.; Teranishi, S. J.Chern. Soc., Faraday Trans. 1 1979, 75, 2027.
0 IS 2i0
235
225 2'20 Binding Energy (eV)
230
Mo3d
Flgure 1. X-ray photoelectron spectra of the Mo(3d) and O(1s) levels for fresh and used Mo03/Si02catalysts: (a) fresh 1.5 wt %, (b) fresh 9.1 wt %, (c) fresh 23.1 wt %, (d) presulfided 1.5 wt %, (e) presulfided 9.1 wt %, (f) presulfided 23.1 wt %, and (9) prereduced 23.1 wt % MoO,/SiO, catalysts.
tained from MoS2 and CoS04 which were ground in an N2 atmosphere (S(2p)/Mo(3d) = 0.22 for S/Mo = 1). The detailed procedures have been described in the previous paper.13 Results and Discussion XPS Characterization of Oxidic M o 0 3 / S i 0 2Catalysts. Shown in Figure 1 are the X-ray photoelectron spectra of the MO(3d) and O(ls) levels for the Mo03/Si02catalysts calcined at 550 "C for 5 h. The resolution of the Mo(3d) doublet improved with an increase in the loading amount of Moo3, approaching that for unsupported Moo3 However, even in the lowest level of MOO, (1.5 w t %), the resolution was not so poor as that reported for Mo03/Si02' and W03/Si02 catalysts.16 Gajardo et al.' ascribed the unresolved peaks to the existence of several Mo species, whereas Kerkhof et a1.I6 ascribed them to differential chargings of the W03/Si02 catalysts. (16) Kerkhof, F. P. J. M.; Moulijn, J. A.; Heeres, A. J. Electron Spectrosc. Relat. Phenorn. 1978, 14, 453.
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The Journal of Physical Chemistry, Vol. 85, No. 25, 1981
TABLE 11: Absolute Binding Energies (BE) of the Si(2p) and Mo(3d,,,) Levels and the fwhm of the Si(2p) Level for the Oxidic MoO,/SiO, Catalvsts
MOO,, wt % 0 1.5 4.8 9.1 13.0 16.7 23.1
BE fwhm of Si(2p) of Si(2p) level, level, eV eV 110.2 110.3 110.3 109.7 109.7 109.7 109.7
3.1 3.0 3.1 2.9 2.9 3.0 3.0
BE of Mo(3d5,,) level, eV 239.6 240.0 239.5 238.8 238.6 238.3
2 VI
.!? 240
6 239 G?
'=cz 2 3 8,I .-m
:\
M o 3d5/2
;111 I
0
- -
-
109-
Table I summarizes the XPS data for the oxidic MoO3/SiOZcatalysts. The binding energies of the Mo(3d5,2), O(ls), and C(1s) levels are given together with their fwhm values. The binding energy of the Si(2s) level was 154.3 f 0.1 eV for all of the catalysts. The prominent features in Table I are as follows: (1) the binding energy of the Mo(3d5/,) level increases slightly up to 9 w t %, followed by a steep decrease at 13 wt % and a subsequent slow decrease; (2) the fwhm of the Mo(3d) band decreases gradually with an increase in the loading amount of MOO,, approaching 5.2 eV for unsupported MOO,; (3) the binding energy and the fwhm of the O(1s) level are invariant, taking into consideration the superposition of the O(1s) spectrum due to supported Mo oxide which is explicitly observed at a high level of supported MOO, (Figure 1); and (4) the fwhm of the C(1s) level due to contaminant carbon decreases with an increase in the loading amount of MOO, up to 9 w t %, followed by a sudden increase at over 13 wt % MOO,. Before identifying the Mo species on the Mo03/SiOz catalysts on the basis of the above observations, it is necessary to examine the effects of differential chargings" on the binding energy of the Mo(3d) level. In Table 11, the absolute binding energies of the Mo(3d5,,) and Si(2p) bands and the fwhm values of the Si(2p) level are summarized, since these values are considered to provide a more definitive criterion for the distinction between the differential charging effects and the formation of interaction species than the corrected binding energies in Table I. The absolute binding energies of the Si(2p) and Mo(3d5 2 ) levels are illustrated in Figure 2 as functions of the Mod3 content in the catalysts. The absolute binding energy of the Si(2p) level was invariant below 5 wt % MOO, but decreased stepwise by 0.6 eV between 5 and 9 wt % and then remained constant with a further increase in the MOO, content. However, the absolute binding energy of the Mo(3d5 2) level showed a maximum at 5 wt 5% . On the basis of the behavior of the absolute binding energies of the Si(2p) and Mo(3dgI2) bands, we can divide the Mo03/SiOz catalysts into three distinct groups: the catalysts containing 9 wt % MOO,. As for the Mo03/SiOz catalysts containing >9 wt % MOO,, the absolute binding energy of the Si(2p) level was constant, while that of the Mo(3d) level decreased gradually. This is obviously ascribable to differential chargings between supported molybdenum oxide species and the support, SiOz, since the absolute binding energies of both levels must change simultaneously if certain interaction species are to be progressively formed with an increase in the MOO, content. The above assignment is supported by (17) (a) Brinen, J. S. J.Electron Spectrosc. Relat. Phenom. 1974,5, 377. (b) Brinen, J. S. Acc. Chem. Res. 1976, 9, 86.
(18)Abeles, B.; Sheng, P.; Coutta, M. D.; Arie, Y. Adu. Phys. 1975,24, 407.
The Journal of Physical Chemistry, Vol. 85, No. 25, 1981 3801
Mo03/SiOpHydrodesulfurizationCatalysts
racy of the fwhm determinations in spite of the line broadenings of the Mo(3d) level. Therefore, it is considered that, in this composition range, certain Mo species interacting with SiOzare formed. This is further supported by the high resistivities against reduction and sulfidation of Mo and by the low mobility of Mo during these treatments, as described below. The broadenings of the Mo(3d) level, however, may be partly induced by differential chargings of distinct Mo species across the catalyst surface. This might be evidenced by the broadened C(1s) level (Table I). According to Castellan et a1.,6 Mo03/SiOz catalysts calcined at 500 "C contain several Mo species: silicomolybdic acid, (di)molybdate in tetrahedral coordinations, polymolybdates, and free Moo3, the fractions of these species being dependent on the MOO, content in the catalyst. At low Moo3 levels, Mo03/SiOz catalysts consist predominantly of the former two species. The binding energy of the Mo(3d6 2) level for Naz(SiMo12040)was reported by Swartz and k e r ~ u l e st.Q ' ~ be lower by 1.4 eV than that for Moo3. On the other hand, the Mo(3d6,,) binding energies for well-characterized molybdate species in tetrahedral coordinations in MoO3/Al2O3and Mo03/Si02 catalysts are close to that for M003.13,20Accordingly, it could be surmised that, in the low MOO, content catalysts, the coexistence of silicomolybdic acid and (di)molybdate species results in a larger fwhm value and a slightly lower binding energy of the Mo(3d) level than those for unsupported Moo3. In the medium concentration range of Moo3 (5-9 wt %), it is safe to say that microcrystals of MOO, (< ca. 5 nm) begin to appear at ca. 5 wt % and develop with an increase in the Moo3 content. The formation of semiconducting MOOSmicrocrystals would enhance the electric conductivity of the catalyst surface and, in turn, reduce the catalyst chargings, this resulting in the lower absolute binding energies of both Mo(3d) and Si(2p) bands. In summary, the XPS results in Tables I and I1 show that, at the Moo3 content of 5 wt % , followed by crystallization of Moo3 with the Moo3 content. The differential chargings are troublesome in the XPS measurements of heterogeneous catalysts. However, close analyses of these phenomena would provide some important clues in the surface characterization of the catalysts. Dispersion of Mo over Si02 To examine the extent of dispersion of Mo over Si02,we calculated the peak area levels, since intensities of the Mo(3d) (IMo) and Si(2p) (Isi) the I M ~ratio /I~ is ~ sensitive to the degree of Mo dispersion over the c a t a l y ~ t . ~ J ~ As , ~ l in the previous study on Mo03/A1203catalysts,13 the following equation was used to evaluate the degree of Mo dispersion: exp(-d/h0)1 k($) Isi 1 O[l exp(-d/Xsi)] -
-
-
(1)
where 8 denotes the coverage of Mo over the catalyst surface and d the thickness of the molybdenum phase. IM: and Is: represent I M o and Isi for infinitely thick samples. AMo and Xsi are the escape depths of the photoelectrons corresponding to the Mo(3d) and Si(2p) levels, respectively. We consider here two extreme cases; a monolayer distri(19) Swartz, W. E.; Hercules, D. M. Anal. Chem. 1971, 43, 1774. (20) Iwasawa, Y.; Ogasawara, S. J. Chem. Soc., Faraday Trans. I 1979, 75, 1465. (21) Angevine, P. J.; Vartuli, J. C.; Delgass, W. N. Proc. Int. Congr. Catal., 6th, 1976 1977, 611.
3 1 P
Figure 3. Correlation between the IM/Ia ratio and N , / S , (a)for Mo03/Si02catalysts. The dashed line shows the correlation expected from MoO3/AI2O3catalyst^.'^ The dotted line indicates a pseudohyexpected from eq 3. perbolic dependence of Zm/Is, on NM/S,
bution of Mo (AMo, Xsi >> d ) and a thick multilayer of Mo (d >> XM~, XsJ. When Mo is dispersed well forming a uniform monolayer, eq 1 can be simplified to eq 2,l3tZ1
a = NM~/SBET
N M=~~ M ~ ~ ~ S B E T where uMo and asi are the effective cross sections of the Mo(3d) and Si(2p) photoelectrons that will reach the detector. nMoand nsi are the number density of Mo in surface molybdenum species and that of Si in SiOz, respectively. NMo (atoms/g) is the number of atoms of the supported Mo and SBET (m2/g) the support surface area, taken as the BET area per gram of the catalyst. It was demon~trated'~ that eq 2 was valid for MoO3/Al2O3catalysts which form a monolayer up to ca. 4 X 1OI8 Mo atoms/m2. When Mo forms large crystalline MOO, over the catalyst (d >> AM^, ha), eq 3 can be derived. Equation 3 predicts (3)
a pseudohyperbolic dependence of the I M o / I s i ratio on a if d does not change so much as a. Equation 1and, consequently, eq 2 and 3 are derived by assuming that Mo is evenly dispersed on a semiinfinite support. Therefore, they may be oversimplified for porous carriers such as SiOz. More detailed treatments have been published by Kerkhof and MoulijnZ2and FungZ3for these systems. However, it was shown13~21 that eq 1and 2 described semiquantitatively the extent of dispersion of supported compounds. Figure 3 shows the dependence of the I M 0 / I s i ratio on the surface concentration of Mo, a. The I M O / I s i ratio increased with a at a low a range (C9 wt % Moo3) and leveled off (9-13 w t % Moo3),followed by a considerable increase at a high CY region (>13 wt % Moo3). In Figure 3, the previous correlation (dashed line) for Mo03/Alz03 (22) Kerkhof, F. P. J. M.; Moulijn, J. A. J. Phys. Chem. 1979,83,1612. (23) Fung, S. C. J . Catal. 1979, 58, 454.
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Okamoto et al.
The Journal of Physical Chemistty, Vol. 85, No. 25, 1981
TABLE 111: Binding Energies (BE)= and Full Widths a t Half-Maximum (fwhm) of t h e Mo(3d), O ( l s ) , S(2p), and C(1s) Levels, Degree of Mo Sulfidation, and Z ~ ~ / lRatios si for MoO,/SiO, Catalysts After t h e Hydrodesulfurization of Thiophene a t 400 "C and Atmospheric Pressure Mo(3d)
MOO,, wt %
pretreatmentb
1.5
H
4.8
S H S
9.1
H
S 13.0
H
S 16.7
H
S 23.1
H
S
BE of Mo(3d5,,), eV 229.0 229.0 228.9 228.9 228.4 228.7 227.2 227.9 227.8 227.6 227.7 227.1 223.9
O(lS) fwhm, eV
BE, eV
fwhm, eV
7.6 7.7 7.5 7.0 6.9 6.5 6.6 6.7 5.5 6.9 5.5
532.7 532.7 532.8 532.6 532.6 532.4 532.6 532.4 532.3 532.6 unde
2.5 2.5 2.9 2.7 2.5 2.7 3.5 2.8 3.8 2.8 5.1
6.7
532.7
2.6
S(2P) BE, eV 161.9 161.6 161.9 161.8 161.4 161.8 160.3 160.6 160.6 160.3 160.6 160.6 156.6
COS) BE, eV
SIMoC
IMo
d
284.4 284.8 284.6 284.7 284.4 284.7 283.6 284.6 283.7 283.8 283.7
0.06 0.13 0.22 0.39 0.62 0.72 0.84 0.90 0.93 0.86
1.05 1.05 1.46 1.38 2.08 1.24 9.22 2.21 19.8 1.37 28.2
unde
0.71
1.21
0.88
Referenced t o the Si(2p) level (103.3 eV). H: prereduced with H,. S: presulfided with CS,/H, at 400 "C for 1 h. Atomic ratio obtained from the S(2p) and Mo(3d) levels. Normalized to fresh catalyst. e Undetermined because of poor resolutions. a
catalysts calcined at 550 "C for 5 h13 is also shown for comparison. I A ~(intensity of the Al(2s) level) for Mo03/A1203catalysts was converted to Isiby using the cross sections of the Al(2s) and Si(2p) levels published by S ~ o f i e l dand ~ ~ nAl and nsi for A1203and Si02. XAl was assumed to be the same as Xsi, since both XPS lines have very close kinetic energies. In the low a region, the IMO/Isi ratios are close to the linear line for Mo03/A1203catalysts which corresponds to the monomolecular dispersion of M0,13 but they deviate to lower values with increasing a.25 Consequently, the extent of dispersion can be approximated by eq 1, indicating that Mo is well dispersed to form a monolayer up to ca. 0.4 X 10l8Mo atoms/m2 (ca. 2 wt % Moo3) and that the fraction of multilayered Mo species increases with the loading amount of Moo3 up to 9 wt %, where crystalline Moo3 was not yet detected by X-ray diffraction analysis. According to Castellan et al.? this low a region corresponds to the progressive formation of polymolybdate at the expense of (di)molybdate and silicomolybdate species. Our XPS results support these transformations in the surface species with the loading level of Moo3 from the viewpoints of the dispersion of Mo over SiOz,the XPS binding energy of the Mo(3d) band (Table I), and the differential chargings of the catalysts (Table 11). Mo08/A1203catalysts showed a monolayer structure up , 1 ~value being much larger to ca. 4 X l0ls Mo a t o m ~ / m ~this than that for the Mo03/Si02 catalysts (0.4 X 10l8 Mo atoms/m2). This difference results from the differences in the Mo-support interactions, this probably being due to larger numbers of anionic surface hydroxyl groups on A1203than on Si02.26 The anionic hydroxyl groups facilitate the following surface reaction:27
I
SI
(24) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976,8, 129. (25) The same conclusion can be obtained when the analysis by Kerkhof and MoulijnZ2is applied, since the parameter 2 proposed by them was very close in the A1203 and SiOz systems (1.12 for the Moos A120313 and 1.06 for the MoOS/SiOz). (26j Yamagata, N.; Owada, Y.; Okazaki, S.; Tanabe, K. J. Catal. 1977, 47, 358.
As for the high a region, where crystalline Moo3 was detected by X-ray diffraction analysis, it is apparent that ratio vs. a relation is approximately described the IMo/Isi by eq 3; that is, IMo/Isi shows a pseudohyperbolic dependence on a, as shown in Figure 3 by a dotted line (d >> AMo or Xsi = 1.5 nm). This observation is consistent with the results by Castellan et a1.6 that the fraction of free Moos begins to increase from -13 wt % Moo3 loading at the expense of polymolybdates. The extent of differential chargings in XPS measurements can be surmised to reflect the strength of the interaction between surface species and the support, particularly when the support is an insulator and surface species are conductors or semiconductors. At low MooB loadings (9 wt %), significant charging effects were observed. These results conform to the degree of Mo dispersion over Si02 In the case of A1203-supported catalysts,13the binding energy of the M0(3d5/2)level (232.9 eV) did not change with the supported amount of Moo3, except for the 23 wt % Moo3 catalyst calcined at 700 OC in which a slight indication of differential chargings was observed (Mo(3d5,,), 232.6 eV) with concomitant detection of crystalline Moo3 by X-ray diffraction analysis. X P S Characterization of Sulfided M o 0 3 / S i 0 2 Catalysts. Typical Mo(3d) and O(1s) spectra are shown in Figure 1for sulfided catalysts. Table 111shows the binding energies of the Mo(3d5i2),O(ls), S(2p), and C(ls) levels and their fwhm values for the Mo03/Si02 catalysts sulfided during the presulfidation with CS2/H2and the hydrodesulfwization of thiophene. X-ray diffraction analysis indicated the presence of Moo2for the catalysts containing more than 13 w t % Moo3. However, no MoS2 phase was detected in all of the catalysts examined here under our reaction conditions. The Mo(3dbi2)binding energies were close to that for unsupported MoS2 (228.9 eV; fwhm, 5.4 eV) for the low Moo3 content catalysts (
