456
J . Phys. Chem. 1984, 88, 456-464
A Surface Spectroscopic Study of Sulfided Molybdena-Alumina Catalysts Chung Ping Li and David M. Hercules* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: May 12, 1983)
X-ray photoelectron spectroscopy (ESCA), low-energy ion-scattering spectroscopy (ISS), and X-ray diffraction have been used to characterize a series of sulfided and reduced MoO3/y-AI2O3catalysts. X-ray diffraction has shown the presence of small MoS2 crystallites on a catalyst prepared by impregnation of ammonium thiomolybdate in y-alumina with subsequent calcination. ESCA and ISS have identified the presence of both MoS2 and Mo(V) on a MoO,/y-Al,O, catalyst sulfided with 15% H2S/H2. The octahedrally and tetrahedrally coordinated molybdenum on an oxidic Mo03/y-A120,precursor catalyst show different sulfidation behavior. At low Mo concentrations, tetrahedrally coordinated molybdenum is reduced mostly to Mo(V) and only partially sulfided to MoS2. Increasing the Moo3 loading above 8% MOO, caused formation of excess octahedrally coordinated molybdenum which leads to a significant increase of MoS, on the sulfided catalyst. After monolayer coverage has been reached, formation of AI2(Mo0,)3 and MOO, result in prominent MoS2 formation and a small amount of Mo(V) on the sulfided catalyst. The effect of calcination temperature has been studied; it was found that decreased calcination temperatures favor formation of MoS2 on sulfided MoO3/y-A1,O3 catalysts.
Introduction The surface structure of Mo03/yA1203hydrodesulfurization catalysts has been studied by many workers using both physical and chemical techniques.'-l0 Despite differing interpretations, the interactions between molybdenum and alumina and the surface species formed on oxidic MoO3/y-AI20, catalysts are thought to be important factors affecting desulfurization activity. According to Zingg et al.,s isolated Mo(V1) in tetrahedral sites is formed at low molybdenum concentrations (-4 wt % MOO,). Increasing the MOO, content to 8 wt % MOO,, produces polymeric molybdate in octahedral sites after the tetrahedral lattice sites of y-A1203have been preferentially filled. Increasing the Mo surface coverage above a monolayer (-20 wt % Moo3) results in formation of distinct AI,(MoO,), and Moo3 bulk phases. This model is not only consistent with that proposed by Medema et al.) and Giordano et aL2 but also correlated well with the reduction behavior of MoO,/y-A1203 catalysts. The surface structure of sulfided catalysts is directly related to hydrodesulfurization activity. Preliminary studies of sulfided M0O3/y-Al2O3catalysts indicated that MoS2 is produced during sulfidation.11-14However, the nature of the MoS2 species present on the sulfided catalysts was not clear because of inconsistencies reported in the S/Mo r a t i ~ . ' ~ Molybdenum J~ oxysulfide species were proposed as active sulfiding agents based on IR studies.' A partially sulfided MoO3/Al,O3 catalyst is too active to resist reaction with residual oxygen in unpurified nitrogen16 and, for a highly dispersed catalyst such as MoO3/y-AI2O3,the active phases are located on the surface. Thus, it is necessary to utilize surface techniques without exposure to the atmosphere to monitor the surface state of reactive sulfided catalysts. (1) P. C. H. Mitchell and F. Trifiro, J . Cafal.,33, 350 (1974). (2) N. Giordano, J. C. J. Bart, A. Vaghi, A. Castellan, and G. Martinotti, J . Catal., 36, 81 (1975). (3) J. Medema, C. Van Stam, V. H. J. deBeer, A. J. A. Konings, and D. C. Konningsberger, J . Catal., 53, 386 (1978). (4) F. E. Massoth, J. Card., 36, 164 (1975). ( 5 ) T. A. Patterson, J. C. Carver, D. E. Leyden, and D. M. Hercules, J. Phys. Chem., 80, 1900 (1976). (6) K. S. Seshadri and L. Petrakis, J. Phys. Chem., 74, 4102 (1970). (7) F. R. Brown and L. E. Makovsky, Appl. Spectrosc., 31, 44 (1977). (8) D. S. Zingg, L. E. Makovsky, R. E. Tischer, F. R. Brown, and D. M. Hercules, J . Phys. Chem., 84, 2898 (1 980). (9) F. Delannay, P. Gajardo, P. Grange, and B. Delmon, J. Chem. SOC., Faraday Trans. I , 76, 988 (1980). (10) Y . Okamoto, H. Tomioka, Y. Katoh, T . Imanaka, and S. Teranishi, J . Phys. Chem., 84, 1833 (1980). (1 1) C. P. Cheng and G. L. Schrader, J . Caral., 60, 276 (1979). (12) P. Ratnasamy, L. Rodrique, and A. J. Leonard, J . Phys. Chem., 77, 2242 (1973). (13) S. S. Pollack, L. E. Makovsky, and F. R. Brown, J. Card., 59, 452 (1979). (14) K. S. Chuang and F. E. Massoth, J . Catal., 64, 332 (1980). (15) F. E. Massoth and C. L. Kibby, J . C a r d , 47, 300 (1977). (16) F. E . Massoth, Adu. Catal., 27, 265 (1978).
0022-3654/84/2088-0456$01.50/0
ESCA can determine molybdenum oxidation states and can monitor changes in oxidation state distribution as a function of chemical treatment. Some structural information can be inferred by comparing binding energies, peak widths, and the chemical behavior of catalysts with those of known compounds. ISS provides information about the composition of the first monolayer because of its high surface sensitivity (3 A). Although ESCA studies of sulfided catalysts have been reported by many res e a r c h e r ~ , ~ ~identification ~J~J* of the active molybdenum species was based mostly on direct comparison of binding energies and relative intensity ratios with those of standard compounds. For example, Okamoto et al., using S/Mo intensity ratios of sulfided catalysts, proposed formation of an intermediate surface MoS complex.1° However, only by resolving the Mo oxidation state distribution on sulfided catalysts can one correlate intensity ratio measurements with the nature of the molybdenum intermediate. In such cases, reliable and consistent methods for computer curve resolution are necessary to obtain convincing conclusions. In the present study, a nonlinear least-squares curve-fitting program named GAMET is combined with spectra of standard compounds to investigate the distribution of Mo species on sulfided catalysts. Statistical goodness of fit, reasonable peak width, and the number of components were used as criteria to judge the computer-fitting results.
Experimental Section Instrumentation. X-ray diffraction (XRD) data were obtained with a Diano recording diffractometer equipped with a graphite monochromator and a copper X-ray tube, operated at 50 kV and 25 mA. The scan rate was 0.4°/min (in 20), from 20 = 10'-80'. The approximate crystallite dimensions of MoS, in the plane (L,) and perpendicular to the plane of the layer (L,) were determined from the width at half-maximum of the (100) and (002) diffraction lines, respectively. Reference MoS2crystallites measured under identical conditions as the catalysts had widths at half-maximum of 0.24' for the (002) and 0.43' for the (100) diffraction lines, respectively. Examination of the MoS, reference crystallites by electron microscopy showed an average particle size greater than 1000 A; therefore the instrumental broadening correction was applied by using these values. An AEI ES200A electron spectrometer with an AEI DSlOO data system was used to record ESCA spectra. The spectrometer was equipped with an aluminum anode (A1 K a , 1486.6 eV) operated at 12 kV and 22 mA. The base pressure of the spectrometer was typically below torr, The digital data obtained were processed with an Apple I1 plus microcomputer which was linked (17) M. Breysse, B. A. Bennett, and D. Chadwick, J . Catal., 71, 430 (1981). (18) G. C. Stevens and T. Edmonds, J . Less Common Metals, 54, 521 (1 977).
1984 American Chemical Society
Study of Sulfided Mo03/A120, Catalysts
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 457
to a DEC-10 computer for curve-fitting computations. OverS 2P lapping peaks were resolved with a modified nonlinear least-squares fitting routine (GAMET) which was described p r e v i o ~ s l y . ~ ~ ~ ~ ~ ~ ~ Binding energies of standard compounds were determined by referencing to the C 1s line at 284.6 eV, which was checked against the Au 4f7/2line at 83.8 eV. Binding energies reported are averages of at least three separate measurements; they were reproducible to f0.15 eV. The A1 2p peak of the support (74.5 eV) served as the reference line for catalyst samples. This value was referenced to vapor-deposited gold. The ESCA probe system used allowed transport of samples from the reaction chamber to the spectrometer without exposure to air. The probe and reaction chamber have been described e l s e ~ h e r e . ~ , ~ ~ Samples were pressed into rectangular pellets (6 X 50 mm) at a pressure of 5000 psi for all ESCA and ISS analyses. ISS spectra were recorded on a 3M Model 525 ISS spectrometer with a cylindrical mirror analyzer (CMA) and channel electron multiplier (CEM). The inlet system was modified to allow rapid entry of the sealable probes. The instrument was operated with a primary 4He ion beam of 2 keV and a current density of 7 nA/cm2 over a rastered area of 1.5 cm2. A base pressure below torr was typical before backfilling the chamber with the 5X scattering gas. Analyses were carried out at a pressure of 5 X 10” torr of 4He. Spectra were obtained in the analog mode with a total scan time of 8 min. I I I I I I 236 228 220 170 162 154 Sample Preparation. Catalyst samples were prepared from 7-Al,03 (Harshaw Co., Cleveland, OH) having a BET surface Binding Energy ( e V ) area of 196 m2/g. The support was dried overnight at 110 OC. Figure 1. ESCA spectra of Mo 3d, S 2s, and S 2p for a 15 wt % An appropriate amount of (NH4)6M07024-4H20 (Fisher Scientific, Mo0,/y-A1203,catalyst sulfided for various times. Sulfiding conditions Pittsburgh, PA) was dissolved in distilled water and diluted to were 350 ‘C with 15% H2S/H,: (A) 15 min, (B) 30 min, (C) 60 min, a predetermined volume. (The volume necessary to fill the catalyst (D)MoS2. pores was 0.60 cm3/g.) Half of the ammonium paramolybdate solution was added to the support and mixed; the remaining half indicates that surface charging exists on the sulfided catalysts, was then added, and the solution was mixed well and allowed to as was reported previously for oxidic catalysts.* The shoulder on stand for 0.5 h in a covered beaker. The mixture was dried the mMo 3d peak at 225.9 eV is the S 2s line. Another shoulder overnight at 100 OC and calcined for the specific temperature and at -234.8 eV for partially sulfided catalysts is due to residual time in an air muffle furnace. Mo(V), rather than to the S 2s line of sulfate (S04-),21since the Samples of supported (16 wt % MoS,) molybdenum sulfide S 2p region shows no sulfate peak. For prolonged sulfiding, the were prepared by thermal decomposition of ammonium thio234.8-eV shoulder decreased, accompanied by an increase of the molybdate, impregnated on alumina, and calcined at 750 OC for peak at 228.9 eV indicating further sulfidation at longer sulfidation 8 h under a flow of hydrogen (99.999%).20 High-purity motimes. However, the 234.8-eV shoulder was still observed after lybdenum sulfide free from surface oxygen contamination was 4 h of sulfiding. prepared by reacting molybdenum wire and sulfur at 1000 OC Broadening of metal photoelectron lines on a sulfided catalyst in a sealed silica tube. The sample was handled in an inert has been attributed to the presence of more than one metal oxatmosphere to prevent pick-up of water or oxidation of the surface. idation state.30 However, comparison of spectra obtained in the Sample preparation of pure MoS2 for ESCA measurements was present study with those of sulfided catalysts indicates that carried out with a sealable probe loaded in a drybox. No evidence broadening of the Mo 3d5,2 lines of sulfided catalysts is due to for lower oxidation states of Mo was seen by ESCA; absence of surface charging. For example, the Mo 3d spectrum of MoS, oxygen peaks in both ESCA and ISS spectra also confirmed the supported on y-Al2O3 prepared by decomposition of thiomolybdate purity of MoS,. shows no shoulder at 234.8 eV. This indicates that MoS, is the Catalyst Sulfdation and Reduction. Sulfidation of catalysts only molybdenum species on this catalyst. Also, the broad Mo was carried out at 350 OC in a flow (50 mL/min) of 15% H2S/H2 3d5/2 peak (2.1 eV) observed on sulfided catalysts cannot be (Airco) for a specified reaction time, and quenched in a flow (200 deconvoluted effectively into different molybdenum oxidation mL/min) of dry He (99.999%) for 1 h before being sealed inside states. By comparison unsupported MoS, synthesized either from the reaction chamber.19 Reduction was carried out under similar elemental Mo and S or from decomposition of ammonium thioconditions, but with H2 (99.999%) and Ar (99.999%) as a purging molybdate did not show comparable charging. gas. X-ray Diffraction Studies. X-ray diffraction (XRD) is particularly useful to identify specific bulk phases on catalysts. Results and Discussion Unfortunately, X-ray diffraction is limited by moderate detection ESCA Binding Energies. The binding energy of the Mo 3d5/, limits and line broadening effects for amorphous surface species peak of all oxidic catalysts is 232.5 eV, which is the same as for on supported catalysts.16 On the other hand, X-ray diffraction bulk MOO,, indicating the presence of only Mo(V1) on the oxidic line broadening can be a useful method to estimate crystallite sizes catalyst. Typical ESCA spectra of the Mo 3d5 2,312 and S 2p in the range of 50-1000 Figure 2 shows the X-ray diffraction regions for a 15 wt % Mo03/y-A1203catalyst sulkded at 350 OC patterns of MoS,, MoS2/Al2O3,and sulfided Mo/y-Al,O, catby 15% H2S/H2for different times are shown in Figure 1. The alysts. Although these spectra were obtained for catalysts exposed binding energies of Mo 3d5/, for all sulfided catalysts are 228.9 to the atmosphere, MoS, (100) and (002) diffraction lines can eV, which is close to MoS2 (Figure ID). However, slight broadening of the Mo 3d5/2 peak (-2.0 eV vs. 1.5 eV for MoS,)
-
(19) K. T. Ng and D. M. Hercules, J . Yhys. Chem., 80, 2094 (1976). (20) C. J. Wright, C. Sampson, D. Fraser, R. B. Moyes, P. B. Wells, and C. Riekel, J . Chem. Soc., Faraday Trans. 1 , 76, 1585 (1980).
(21) R. M. Friedman, R. I. DeClerck-Grimee, and J. J. Fripiat, J . Electron Spectrosc., 5, 437 (1974). (22) H . P. Klug and L. E. Alexander, “X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials”, 2nd ed, Wiley, New York, 1974, p 688.
Li and Hercules
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A
8b.
(OOa)
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60"
40"
30"
lb
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2e Figure 2. X-ray diffraction of reference compounds and sulfided and reoxidized MoO,/Al2O3catalysts: (A) MoSz prepared from decomposition of (NH4),MoS4,(B) MoS2/A1203prepared from decomposition of (NH4)2MoS4solution impregnated A1203at 750 OC,(C) sulfided MoO,/Al,O,, (D) oxidized MoS2/AI2O3,(E) reoxidized sulfided MoOg/A1203.
TABLE I: Crystallite Size Estimates of MoS, from X-ray Line Broadeninga crystalline Bobsd site, A sample 1. MoS, (from Mo and S) 2. MoS, (from decomposed "(4 ),MoS,) 3. MoS,/y-Al,O, (from decomposition of (",),MoS4Iy-A1,O3) 4. sulfided MoO,/y-Al,O,
L,
002
100
0.24 0.86
0.43 0.92
1000 85
La
198
1.10
1.34
65
127
NP(?)
NP(1)
a Crystallite size was estimated by Scherrer equation, details are given in the text. NP denotes not present.
be detected in the MoS2/y-A1203sample. However, no distinct MoS2 phase was observed on sulfided molybdena catalysts. This could be due either to oxidation of MoS, during XRD analysis or because only amorphous MoS, exists on the sulfided catalyst. An estimate of MoSz crystallite size was obtained by using the Scherrer equation, to approximate the crystallite dimensions of MoS, in the layer (L,) which is parallel to the plane and the layer (L,) which is perpendicular to the plane. These results are shown in Table I. Decomposition of ammonium thiomolybdate on alumina or in the bulk resulted in small crystallites of MoS2. Although X-ray diffraction does not provide evidence for the presence of MoS2 on the sulfided catalysts, Delmon et al. showed the presence of MoS2 crystallites on sulfided catalysts sulfided Also, at 400 OC, 15% H2S/Ar by analytical electron micros~opy.~ Cheng and Schrader,z3in their "in situ" Raman studies of sulfided molybdena-alumina, observed MoS, after only 15 min of sulfidation at 350 OC. Intensity Ratio Measurements. ESCA S 2p/Mo 3d intensity ratios have been used to investigate the stoichiometry of molybdenum on sulfided Mo03/A1203catalysts. Table I1 lists the ESCA S 2p/Mo 3d intensity ratios for a 15 wt % Mo03/y-A1203catalyst sulfided at 350 "C for different times. The contribution of S 2s to the Mo 3d envelope was estimated by using the S 2p/S 2s ratio of WS2, and removing the S 2s band from the total Mo 3d envelope (23) C. P. Cheng and G. L. Schrader, Appl. Spectrosc., 34, 146 (1980).
TABLE 11: ESCA and ISS Intensity Ratios of Sulfided and Reduced 15 wt % ' MoO,/y-Al,O, Catalysts for Various Reaction Times ESCAa ISSb sulfidation S 2p/ Mo 3d/ -time.min M o 3d A12o SiMo Mo/Al OiAl 15 30 60 120 240 MoS,
0.25 0.32 0.36 0.40 0.41 0.49
1.34 1.42 1.33 1.39 1.34
1.19 1.43 1.68 1.75 1.79 2.24
2.81 2.76 2.87 2.92 2.65
0.95 0.92 0.74 0.77 0.59
3.04 3.31 4.4.5 6.32 5.47 4.69
1.52 1.68 1.90 2.62 2.27 1.87
reduction time, min 0 15 30 60 120 240 a
1.37 1.46 1.72
2.13 1.57 1.43
Experimentally measured peak area ratios were reproducible
Experimentally measured peak intensity ratios were to k5%. reproducible to t8%. by subtraction. (S 2s intensities were also estimated by the curve-fitting method; the agreement between two methods was better than =t5%.) Increased sulfidation time results in further sulfur uptake after 2 h sulfidation. To characterize the stoichiometry of sulfided molydbena-alumina catalysts, we plot the S/Mo atomic ratio vs. sulfidation time in Figure 3; the S 2p/Mo 3d intensity ratio of pure MoS2 was used as a standard for S/Mo atomic ratio estimates. Prolonged sulfidation produces an S/Mo atomic ratio of 1.62 f 0.04, not 2.00 as would be expected for MoS2. A similar result is obtained from ISS S/Mo peak intensity measurements, also shown in Figure 3 and Table 11. Because ISS is inherently sensitive to the top atomic layer, the consistency between the ISS and ESCA measurements indicates that migration of Mo below the first few surface layers is not significant. This is also confirmed by the relative constant Mo/Al intensity ratio for sulfided catalysts measured by ESCA, as shown by Table 11. It is clear from these data that at least part of the molybdenum must exist in some form
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 459
Study of Sulfided Mo03/A1203Catalysts
0
r
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Sulfidation Time ( r n i n ) Figure 3. S/Mo ESCA ( 0 )and ISS (X) atomic ratios of a 15 wt % Mo03/A120,catalyst as a function of sulfiding time. Atomic ratios were obtained by using MoSz as a standard: S/Mo = 0.49 (ESCA) and 2.24 (ISS). Dotted line is the atomic ratio S/Mo for MoS2. other than MoSz on a sulfided catalyst surface. Molybdenum Oxidation State Distribution. So that the nature of the sulfided catalysts can be probed further, resolution of the Mo 3d envelope by a reliable and consistent method is essential. To obtain appropriate parameters for resolution of the MO 3d envelopes, we assumed MoSz and MOO, to be analogous to the Mo species on the sulfided M003/7-A1203catalysts. Spectra from a mixture of MoS, and MOO, were examined with the cornputer-fitting routine (GAMET). In this program, linear background subtraction was used and the alumina X-ray Kc19 satellites were i n c l ~ d e d . ~ The ~ ? ~full ~ width at half-maximum (fwhm) and binding energy values obtained for the MoS2/Mo03 mixture corresponded closely to those of pure MoS2 and MOO,. A physical mixture of MoS, and a 15 wt % Mo03/yA1203catalyst showed effects of charging. A shift of the Mo 3d5/2 binding energy of Mo(V1) from 233.8 eV (bulk MOO,, including 1.3 eV due to charging on Moo3) to 235.1 eV (including 2.5 eV due to charging on the catalyst) for the catalysts was observed, as was broadening of the Mo 3d5/, peak from 1.8 eV for MOO, to 2.8 eV for the catalysts. Because the number of component peaks in a composite profile and their half-widths are two of the most critical variables in computer curve fitting,26it is important that the number of peaks should not be determined entirely by statistical criteria.27 Therefore, curve fitting of the Mo envelope of the sulfided catalysts was done with two different sets of parameters: (Mo(VI), Mo(V), and Mo(1V) with fwhm’s of 2.8,2.8, and 2.0 eV, respectively; and Mo(V1) and Mo(1V) with fwhm’s of 4.2 and 2.0 eV, respectively). Because fitting of the Mo envelope with only Mo(1V) and Mo(V1) results in an unreasonable Mo(V1) peak width (4.2 eV), the best computer fit of the Mo 3d envelope requires that Mo(V) (with an Mo 3d5,, binding energy at 231.6 eV) be added as a third component.28 On the other hand, the Mo envelope of a catalyst after 60 min of sulfiding could be fitted with Mo(1V) alone. However, poor weighted variance and incorrect background on the high binding energy side suggested the presence of Mo(V). Fitting this envelope with three sets of molybdenum peaks was also tried; however, it could not achieve a reasonable binding energy for Mo(V1) and a better weighted variance than fitting with only Mo(V) and Mo(IV).~* Based on the above approach, a series of molybdenum 3d envelopes for different sulfidation times were fitted with the ap(24) A. Proctor and P. M. A. Sherwood, Anal. Chem., 54, 13 (1982). (25) A. Proctor and P. M. A. Sherwood, J . Electron Spectiox., 27, 39 11982).
\ - ~ - - I
(26) W. F. Maddams, Appl. Spectrosc., 34, 245 (1981). (27) B. J. Duke and T. C. Gibb, J . Chem. SOC.A , 1478 (1967). (28) C. P. Li and D.M. Hercules, 21st Spring Symposium of the Pittsburgh Catalysis Society, May 1982.
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276
~
232
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224
Binding Energy ieVl Figure 4. Curve fitting of Mo 3d and S 2s spectra of a sulfided 15 wt % Mo03/A120,catalyst as a function of sulfiding time: (A) 15 min, (B) 30 min, ( C ) 60 min, (D) 120 min.
propriate number of components and peak widths to obtain the best statistical values. The results are shown in Figure 4. According to the Mo 3d binding energies, the molybdenum species on the sulfided catalyst can be assigned as Mo(IV) of MoS2 and Mo(V). The change of molybdenum oxidation state distribution is plotted as a function of sulfidation time in Figure 5. Formation of Mo(IV) and Mo(V) by sulfidation at 350 “C reaches a steady state after about 60 min. It is important to note that conversion of Mo(V1) into Mo(V) and Mo(IV) must result from two different Mo(V1) species initially present on the catalyst, without formation of an intermediate Mo(V) state during sulfidation. This behavior is in distinct contrast to the reduction of comparable oxidic catalysts using hydrogen, where Mo(V) is clearly an intermediate in the production of M o ( I V ) . ~ * ~ Nature of the Molybdenum Species. A more quantitative evaluation of sulfidation can be done by plotting the S/Mo(IV) atomic ratio as a function of sulfidation time as ’shown in Figure 6. The overall S/Mo atomic ratio is also presented for comparison. Evidence for the formation of MoSz on sulfided catalysts is indicated by agreement between the S/Mo(IV) atomic ratio of sulfided catalysts and that of the MoS, standard. Sulfidation of a 4 wt % Mo03/y-A1,03 catalyst showed similar results to the 15 wt % catalyst. It was found that Mo(V1) in a 4 wt % catalyst
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The Journal of Physical Chemistry, Vol. 88, No. 3, 1984
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Sulfidation Time (rnin) Figure 5. Distribution of molybdenum oxidation states as a function of sulfiding time. 15 wt % MoO3/AI2O3catalyst sulfided at 350 "C with 15% HIS/HZ: (m) Mo(V1); (0)Mo(V); ( 0 )Mo(IV).
2'5
t
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Sulfidation Time
inn
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I
I mini
Figure 6. ESCA S/Mo and S/Mo(IV) atomic ratios of a sulfided 15 wt % MoO3/A1,O3catalyst as a function of sulfiding time. Mo(IV) con-
centration was determined from curve fitting. Dotted line is the atomic S/Mo ratio for MoS2. decreased more slowly during sulfidation than for a 15 wt % catalyst. The Mo(V) and Mo(IV) concentrations gradually increased to a constant amount after 2 h of sulfidation. The S/ Mo(1V) intensity ratio of a sulfided 4 wt % Mo0,/y-A1203 also indicated the presence of stoichiometric MoS,, but at a lower amount than for a sulfided 15 wt % Mo03/y-A1203catalyst. The overall S/Mo ratio also was lower. Another interesting feature was observed for both the 4 and 15 wt % sulfided catalysts. The surface conductivity increased for the 15 wt % catalyst during sulfiding, but not for the 4 wt % catalyst. Oxidic molybdena-alumina catalysts are electrical insulators if the molybdena is well-dispersed on the alumina support. As the molybdenum content is increased, formation of an octahedral molybdenum network reduces charge buildup on the catalyst surface. This was indicated by increased surface conductivity and decreased peak broadening in the oxidic catalysts.' Surface charging is usually on the order of 3-5 eV for insulators. The spectrometer work function was measured by referencing to the Pd Fermi level; a value of 2.5 eV was obtained. It is assumed that the spectrometer work function is constant for binding energy measurements. A plot of surface charging plus spectrometer work function vs. sulfiding time is shown in Figure 7 for the 4% and 15% catalysts. The increase in surface conductivity (decrease in surface charging) on the 15 wt % catalyst correlates with formation
Time ( r n i n )
Figure 7. Surface conductivity expressed as ESCA sample charging + work function of a sulfided catalyst as a function of sulfiding time: (m) 4 W t % MoO,/AI203; (0) 15 wt % MoO,/A1203.
of a MoSz network from the polymeric octahedrally coordinated Mo(V1) species on the oxidic catalyst. Amorphous MoS, would have higher conductivity than isolated crystallites of MoS, because of its graphite-like hexagonal layer structure and high electron mobility along the a axis.29 Figure 5 indicates that a sulfided 15 wt % Mo03/A1,03 catalyst has 78% of the Mo in the form of MoS,. This amount of MoS, corresponds to about halfmonolayer coverage, based on a hypothetical monolayer of 20 wt % Moo3. This coverage of MoS2 is clearly enough to form a conductive network on the catalyst surface. To the contrary, the 4 wt % catalyst has only about half of its molybdenum present as MoS2, which corresponds to about 10% of a monolayer. Thus, it is not surprising that isolated MoS, crystallites on the 4 wt % catalyst retain the charging effect even after sulfiding. A series of molybdena-alumina catalysts having Moo3 contents from 2 to 30 wt % were treated with the same conditions of sulfidation as were used for the 15% catalyst (15% H,S/H,, 350 "C, 120 min). Figure 8 shows representative Mo 3d and S 2s spectra of catalysts having different MOO, loadings. There is no doubt that increasing the MOO, content leads to increased amounts of MoS,, and lower relative amounts of Mo(V) on a 20 wt % MOO, catalyst (Figure 8D). The S 2s/Mo(IV) intensity ratios indicate stoichiometric MoS2 formation on all sulfided catalysts, regardless of MOO, content. Variation in the Mo/A1 intensity ratio for Mo(IV) and Mo(V) on the sulfided catalysts as a function of MOO, content is shown in Figure 9. ISS results for the oxidic catalysts8 showed that the octahedrally coordinated Mo(V1) drastically increased after the tetrahedral holes of the alumina lattice were filled, above 8 wt % MOO,. An increase in the slope of the Mo(IV)/Al curve occurs at -8% MOO, in Figure 9, which correlates well with the results for the oxidic catalysts and indicates the high susceptibility of octahedral Mo(V1) to sulfiding. The Mo(IV)/Al curve in Figure 9 levels off above 20 wt % MOO,. This effect cannot be attributed to formation of a constant amount of octahedral Mo(V1) at 20% since the opposite effect is observed in the oxidic catalysts. Rather, this behavior correlates with previous s t u d i e ~which ~ ~ ' showed ~~~~ that a monolayer of Mo(V1) is formed at 20% Moo3 and further increases in MOO, resulting in the formation of bulk Moo3.* Sulfidation of Moo3crystallites will result in production of small MoS, crystallites. Variations in the Mo/A1 intensity ratios for the oxidic and sulfided catalysts as a function of MOO, loading show similar trends. For both cases, the Mo/A1 intensity ratio increased linearly with molybdenum concentration and leveled (29) A. J. Jacobson, R. R. Chianelli, and M. S. Whittingham in "Third International Conference on the Chemistry and Uses of Molybdenum", Climax Molybdenum Co., 1979, p 209. (30) Y.Okamoto, H. Tomioka, T. Imanaka, and S. Teranishi, J . Cutui., 66, 93 (1980).
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 461
Sulfided Mo03/A1203Catalysts
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M~+‘CMI A MO+’
wt% MOOJ Figure 9. Distribution of molybdenum oxidation states on sulfided M00,/A1~0, catalysts as a function of MOO, content. Sulfiding was with 15% H2S/H2 at 350 “C for 2 h: (A) Mo(V); ( 0 )Mo(1V).
at 500 “C, the highly stabilized tetrahedral Mo(V1) showed reduction only to Mo(V). Characteristics of Molybdenum Species. The tetrahedral Mo(V1) species on the oxidic catalyst can be either monomeric or dimeric.* Although monomeric and dimeric tetrahedral Mo(V1) may have similar reduction behavior at high temperatures (500 “C), they could react differently during sulfidation at lower temperatures (350 “C). Strong interaction between Mo(V1) and alumina is expected for monomeric tetrahedral Mo(V1) (1) because only one type of surface oxygen (“1”) between Mo and A1 is present, and all oxygens are tightly bound both to A1 and Mo. In the case of a dimeric tetrahedral Mo(V1) species (2) this is not the case.
I
3
236
232
I
218
214
Binding Energy cevl
1
L
The surface layer bonded oxygen (“2”) linking two monomeric Figure 8. Curve fitting of Mo 3d and S 2s spectra of sulfided Moo3/ Mo(V1) atoms is only indirectly bonded to Al, and should be more A1203catalysts. Sulfiding conditions were 350 “C with 15 wt % H2S/H2 susceptible to sulfur attack, resulting in slow production of MoS2. for 2 h. Percentage MOO,: (A) 2 wt %, (B) 6 wt %, (C) 15 wt %, (D) On the contrary, monomeric tetrahedral Mo(V1) is highly sta20 wt %. bilized by the alumina lattice and resists sulfiding. Confirmation for the proposed molybdenum species can be off above monolayer coverage. This type of behavior for Mo/AI obtained from ESCA and ISS studies of reduced molybdena vs. M o o 3 concentration implies that a conversion of bulk MOO, catalysts. Figure 10 shows 4Hef ISS spectra for a 15 wt % to MoS2 occurs during sulfidation. Then, the constant intensity M o 0 3 / ~ - A 1 2 0catalyst 3 after various treatments. Reduction of ratio observed at 20% MOO, must result from growth of larger this catalyst was carried out under the same conditions that were crystallites at the expense of additional MoS2 nucleation sites. used for sulfiding (350 “C, 60 min), except that only H2 was used. Although X-ray diffraction did not observe the presence of bulk M a , , crystallites have been confirmed by Raman s p e c t r o ~ c o p y ~ * ~ ~The ISS spectrum of MoS2 is shown for comparison. It can be seen that the Mo/A1 ISS intensity ratios do not vary significantly and analytical electron microscopyg on similar catalysts. after sulfidation (also refer to Table 11). However, the oxygen It is important to note that Mo(V) on the sulfided catlysts peak is obscured by substitution of the surface capping oxygen probably results from reduction of tetrahedral Mo(VI), thus exby sulfur during sulfiding. plaining why it behaves differently from octahedral Mo(V1). A Figure 11 shows that the increase in the Mo/A1 ISS intensity linear increase in Mo(V) is seen in Figure 9 until a break occurs ratio with increased reduction time is much greater than for around 15% MOO,; then Mo(V) remains constant. The gradual ESCA. The Mo/AI intensity ratio of the reduced catalyst is about increase in Mo(V)/Al compared to that of Mo(IV)/AI on the twice that of the oxidic catalyst after 1 h of reduction. A decrease sulfided catalysts correlates with the idea that the tetrahedral holes in the Mo/AI intensity ratio was observed for longer reduction of the alumina lattice are preferentially filled with Mo(V1) in the times, but a higher Mo/A1 intensity ratio was observed after 4 oxidic state. Studies of reduced molybdena catalysts revealed h of reduction at 350 OC than for the oxidic catalyst. different reduction behavior for tetrahedral and octahedral Mo(V1) To determine if the change in Mo/A1 intensity ratio is due to a change of molybdenum dispersion at the lower reduction tem(31) G . L. Schrader and C. P. Cheng, J . Curd., 80, 369 (1983). perature, one must examine the ISS spectra of the reduced catalyst
462
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984
Li and Hercules
4He ISS 0
Mo
AI S
n
Binding Energy Figure 12. Curve fitting of Mo 3d and S 2s spectra of sulfided and reduced 15%MoO3/AI20,catalysts: (A) sulfided in 15% H2S/H2 at 350 OC for 1 h, (B) reduced in H2 at 350 OC for 1 h. The percentages of Mo(V) for (A) and (B) are 25% and 26%, respectively.
0.s
0.4
1:o
0.8
E/Eo Figure 10. ISS spectra of a 15 wt % MoO3/Al2Ojcatalyst subjected to different treatments: (A) oxidic, (B) reduced at 350 OC in H2 for 1 h, (C) sulfided 15 wt % MoO3/AI2O3at 350 OC with 15% H2S/H2 for 1 h, (D) MoS2.
7.0 1
I
I
I
I
1
A
in detail. Close scrutiny of Figure 10 reveals low-intensity inelastric scattering peaks from adsorbed gases in the ISS spectra of sulfided and reduced catalysts. Thus it is assumed that adsorbed gas broadening of Mo or A1 peaks is negligible. Since the reproducibility of these experiments is better than *8%, then the large increase in the Mo/A1 ISS intensity ratio is probably caused by a decrease in aluminum intensity rather than an increase in molybdenum intensity, as is evident from the figure. To determine the chemical state distribution of molybdenum produced by reduction a t 350 "C, we show the ESCA spectrum for a 15 wt % MOO, catalyst after 1 h of reduction in Figure 12B. The sulfided catalyst is included (Figure 12A) for comparison. From Figure 12B, one can see readily that reduction of the catalyst at 350 "C produces a mixture of Mo(V) and Mo(V1). The Mo(V) state from reduction is probably similar to the Mo(V) produced during sulfidation. Hall et used IR to monitor the surface hydroxyl band of alumina, and to investigate the reduced state of molybdenum on the Mo0,/A1203 catalysts during reduction. Yao33 used temperature-programmed reduction to study Mo03/A1203catalyst for similar treatments. The conclusion of both was that initial hdyrogen uptake at low temperature (- 340 "C) does not affect the two epitaxial oxygens on m o l y b d e n ~ m , ~ ~ but probably the uptake of hydrogen occurs between the alumina surface and molybdenum capping oxygens producing mainly Mo(V):
X
0
1
I
I
I
I
0
60
I20
I80
240
I
Reaction Time ( m i n ) Figure 11. ESCA and ISS intensity ratios of sulfided and reduced 15 wt 7% MoO3/AI2O3catalysts as a function of reaction time. Sulfided catalysts (350 OC with 15% H2S/H2): ( 0 )ESCA; (X) ISS. Reduced catalysts (350 OC with H2): (0)ESCA; (W) ISS.
In the present study, ESCA showed that the oxidation states of Mo produced at 350 'C are Mo(V1) and Mo(V), which is consistent with results from the infrared studies.32 Hydrogen uptake between the alumina surface and the molybdenum capping oxygen could cause shielding of the aluminum atoms toward scattering of 4He+ and decrease the AI peak intensity in ISS. This (32) W. S. Millman, M. Crespin, A. C. Cirillo, and W. K. Hall, J . Cutul., 60, 404 (1979). (33) H. C. Yao, J . Cuful.,70,440 (1981).
Study of Sulfided Mo03/A1203Catalysts
The Journal of Physical Chemistry, Vol. 88. No. 3, 1984 463
attenuation in scattering from aluminum ions on reduced catalysts could be attributed to a combination of atom shadowing, and enhanced 4He+ neutralization by the hydrated surface oxygen anions, which has been observed for several binary o ~ i d e s . ~ ~ , ~ ~ However, the reduction of molybdenum at 350 OC would result in an appreciable amount of Mo(V) (Figure 12), which has a larger ionic size and smaller neutralization probability than Mo(V1) on the oxidic catalysts. The changes of molybdenum chemical state on the reduced catalysts, therefore, enhance the scattering of 4He+ from molybdenum ions due to a decrease of atom shadowing neutralization. The net effect would be an increase of the ISS Mo/Al intensity ratio and dominant Mo(V) on the catalyst reduced at low temperature (350 "C). The Mo 3d/A1 2p ESCA intensity ratio also shows an increase but to a lesser extent. However, the higher surface sensitivity of ISS will show this shielding effect (a combination of shadowing and ion neutralization effects) much more significantly. By comparing sulfided and reduced catalysts, one can conclude that Mo(V) on sulfided catalysts results from reduction of tetrahedral Mo(V1) on the oxidic catalyst. It has been suggested that the binding energy of Mo(V) (231.6 eV) is consistent with Mo coordinated both to 0 and S, such as Mo02S2and Mo03S.36 Raman studies also suggest formation of M002S2 and M o 0 3 S by comparison with solution analogues." However, these results were obtained only for sulfided catalysts that had been exposed to air or which were treated at lower sulfidation temperatures (- 150 "C) and shorter time.31 In contrast, Clausen et al. analyzed the fine structure of the Mo K absorption edge of sulfided Mo03/A1203catalysts and failed to observe a low-energy shoulder due to the 1s 4d transition typical for oxymolybdenum compounds. Similarly, by using Fourier filtered data for estimating the first coordination shell of well-crystallized MoS2 and sulfided Mo03/A1203catalysts, they concluded that molybdenum oxysulfide is not present on sulfided Mo03/A1203 catalyst^.^' The ESCA and ISS S/Mo intensity ratio measurements from the present study are consistent with the results of Clausen et al. A sulfided 15 wt % catalyst has an S / M o atomic ratio of 1.62 0.04. Since curve-fitting results showed (Figure 5) about 25% molybdenum species other than MoS,, if a second molybdenum species like MoOzSzor Mo03S were present, the S / M o atomic ratio should be 2.00 or 1.75, respectively. Therefore, the agreement between ESCA and EXAFS measurements suggests that molybdenum oxysulfides are not present on sulfided Mo03/y-A1203 catalysts. It is important to understand the resistance of tetrahedral Mo(V1) toward sulfidation, to elucidate the difference in behavior Binding Energy between tetrahedral and octahedral Mo(V1). Figure 13 shows Figure 13. Curve fitting of Mo 3d and S 2s spectra of 15 wt % that when an oxidic catalyst is leached with a 3% NH40H solution, Mo03/A1,0, catalysts after leaching with 3% NH40H: (A) sulfided in and sulfided, less Mo(1V) is produced than for the unleached 15% H,S/H, a t 350 'C for 2 h, (B) leached and sulfided, (C) leached catalyst. The broadened Mo(1V) peak in Figure 13B is consistent and reduced at 500 O C for 12 h. with the high dispersion of Mo(1V) (MoS,) on alumina at low MOO, content (for example, 6 wt % M o o 3 in Figure 8B). Only catalyst. Thus, one may conclude that the Mo(1V) in a sulfided octahedral Mo(V1) is leached from the catalyst with ",OH, leached catalyst results from molybdenum species other than and all Mo(V1) remaining on the leached catalyst is tetrahedral octahedral Mo(V1). Based on the ESCA and ISS results for the Mo(V1) which is reduced only to Mo(V) as shown in Figure 13C. reduced catalysts, and the ESCA data for the sulfided leached The ESCA Mo/A1 intensity ratio of a leached catalyst is 0.66 catalysts, we tentatively assign the molybdenum species which which corresponds to 45% Mo (given that the oxidic catalyst has is sulfidable but not reducible at 350 "C to the dimeric tetrahedral Mo/Al = 1.44) remaining in the leached catalyst. This value is Mo(V1) species 2 discussed earlier. close to the relative amount of Mo(V) in the reduced catalyst In summary, these results indicate that octahedral Mo(V1) and (Table IV). A determination of molybdenum concentration in dimeric tetrahedral Mo(V1) are active in the sulfidation reaction. the leached 15 wt % catalyst by atomic absorption showed that Monomeric tetrahedral Mo(V1) is in an isolated, stabilized teta leached catalyst has 7.1 wt % M o o 3 concentration, indicating rahedral environment which is able to resist sulfidation. that 48% Mo remains in the leached catalyst. This represents Effect of Calcination Temperature. Calcination temperature a very consistent result obtained from the different analytical is an important variable which affects the speciation of M o o 3 / techniques. Therefore, a sulfided, leached catalyst cannot have y-A1203 catalysts. Table I11 shows the effect of calcination MoS, formed from octahedral Mo(V1). However, Figure 13B temperature on the sulfidation behavior of a 15 wt % M o 0 3 / y shows a significant amount of MoSz in the leached and sulfided A1203catalyst. For a catalyst calcined at 300 "C 88.5% of the molybdenum is sulfided to MoS2, showing that the octahedral (34) R. C. McCune, J . Vucuum Sci. Technol.,18, 700 (1981). Mo(V1) species is favored by low temperature calcination. In(35) G. C. Nelson, J. Vacuum Sci. Technol.,15, 702 (1978). creasing the calcination temperature to 700 OC results in a de(36) R. A. Walton, J . Cutul., 44, 335 (1976). crease of Mo(1V) on the sulfided catalyst. A constant Mo/A1 (37) B. S. Clausen, H. Topsoe, R. Candia, J. Villadsen, B. Lengeler, J. Als-Neilson, and F. Chirstensen, J . Phys. Chem.,85, 3868 (1981). intensity ratio of 1.34 f 0.04 was observed for all calcination
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464
J . Phys. Chem. 1984, 88, 464-469
TABLE 111: Effect of Calcination Temperature on Sulfided 15 wt 5 M o O , / ~ - A l , O ,Catalysts
1.46 1.34 500 1.44 600 1.33 700 1.37 av 1.39 i 0.05 a Varying calcination temperature with constant calcination time (16 h). Obtained by nonlinear least-squares fitting of Mo 3d envelope after reduction in H, at 500 "C for 12 h.
to the one for the sulfided catalyst, Le., increased calcination temperature favors formation of the tetrahedral Mo(V1) species. The difference between the sulfided and reduced catalysts is that a larger amount of Mo(V) was observed for reduced catalysts than that for sulfided catalyst. This is consistent with the idea that dimeric tetrahedral Mo(V1) is sulfided to Mo(1V) and monomeric tetrahedral Mo(V1) yields Mo(V), resulting in less Mo(V) for sulfided catalysts. The Mo/A1 intensity ratio for sulfided and reduced catalyst is constant regardless of calcination temperature and reaction time as shown in Tables I11 and IV and Figure 1 1. Constant Mo/Al intensity ratio would be expected if molybdenum does not change its dispersion during reduction or sulfidation. This indicates that MoS2 on a sulfided 15 wt % catalyst retains the same structural character present in the oxidic precursor catalyst; Le., Mo does not sinter to form large MoSz crystallites, which would cause a decrease of the Mo/A1 intensity ratio for both ESCA and ISS. The higher surface conductivity of a sulfided 15 wt % catalyst relative to the oxidic catalyst (Figure 7) is consistent with the result that a network of small MoS2 crystallites is produced on the catalyst surface during sulfidation. It is interesting to note that variation of the Mo/A1 intensity ratio is observed for the 350 O C reduced catalyst (Table 11) but not for the 500 O C reduced catalyst. This result implies that the change in the Mo/A1 intensity ratio at 350 "C reduction is not due to a change in molybdenum dispersion or to migration of Mo into the alumina lattice. It is probably due to changes in the shielding of A1 and Mo on the catalyst reduced at 350 "C as discussed earlier.
temperatures, indicating that diffusion of molybdenum into the alumina lattice and formation of bulk molybdate is negligible. Mobility of molybdenum species on the surface, however, favors the formation of tetrahedral Mo(V1). Table IV shows the effect of calcination temperature on a 15 wt % Mo0,/yA1203 catalyst reduced at 500 OC. The amount of Mo(V) shows a trend similar
Acknowledgment. This work was supported in part by the United States Department of Energy under Grant No. DEAC02-79ER 10485, A000. We thank Dr. Andrew Proctor for help with the computer curve fitting and Dr. Marwan Houalla for helpful discussion. Registry No. MOO,, 1313-27-5; A1203,1344-28-1.
-
temp,u OC 300 400 500 600 700
7i MOW)^ 11.5 17.6 25.3 29.8 30.4
7% Mo(IV)~
Mo/Al
-
88.5 82.4
1.30 1.28 74.7 1.39 70.2 1.35 69.6 1.37 av 1.34 i 0.04 a Varying calcination temperature at constant calcination time (16 h). Obtained by nonlinear least-squares fitting of Mo 3d and S 2s envelopes after sulfiding in 15%H,S/H, at 350 "C for 2 h. TABLE IV: Effect of Calcination Temperature on Reduced 15 wt '/o MoO,/yAl,O, Catalysts temp,a
"c
300
400
94 hlo(V)b
% Mo(IV)b
43.8 44.9 46.9 52.0 63.1
56.2 55.1 53.1 48.0 36.9
Mo/A1
Electron Spectroscopic Studies of Oxygen and Carbon Dioxide Adsorbed on Metal Surfaces' P. Vishnu Kamath and C. N. R. Rao* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-56001 2, India (Received: May 17, 1983)
Adsorption of oxygen on Ni, Cu, Pd, Ag, and Au surfaces has been investigated by employing UV and X-ray photoelectron spectrscopy as well as electron energy loss spectroscopy (EELS). Molecularly chemisorbed (singlet) oxygen is found on Ni, Cu, Ag, and Au surfaces showing features such as stabilization of the rB* orbital, destabilization of the .nuorbital, higher O(1s) binding energy than the atomic species, and a band 2-3 eV below the Fermi level due to metal d-O(2p)u* interaction. 0-0 and metal-oxygen stretching frequencies have been observed in EELS. Physical adsorption of O2 is found to occur on Pd and Ni surfaces, only at high exposures in the latter case. Physical adsorption and multilayer condensation of CO, on metal surfaces are distinguished by characteristic relaxation shifts in UPS as well as O(1s) binding energies. Adsorption of CO on a Ni surface covered with presorbed atomic oxygen gives rise to C02.
Introduction Photoelectron spectroscopy has been effectively employed in recent years to study adsorption of molecules on metal surfaces. These studies show how molecules such as carbon monoxide2 and methanol3 adsorb both molecularly and dissociatively on metals. Earlier electron spectroscopic studies4s5had shown that oxygen (1) Contribution No. 216 from the Solid State and Structural Chemistry Unit. (2) Jagannathan, K.; Srinivasan, A.; Hegde, M. S.; Rao, C. N. R. Surf. Sci. 1980, 99, 309. (3) Yashonath, S.; Basu, P. K.; Srinivasan, A,; Hegde, M. S.;Rao, C. N. R. Proc.-Indian Acad. Sci., Sect. A 1982, 91, 101.
0022-3654/84/2088-0464$01.50/0
is adsorbed molecularly on Pt and Ag surfaces at low temperatures. Preliminary studies from this laboratory6 revealed the same to be true on Au surfaces. Considerations based on the heats of dissociative adsorption6 showed that molecular adsorption of oxygen may be unique to these three noble metals. We have investigated the adsorption of oxygen on Cu, Ni, Pd, Ag, and Au by UV photoelectron spectroscopy (UPS) to determine whether molecular adsorption occurs on these surfaces. It was also of (4) Backx, C.; DeGroot, C. P. M.; Biloen, P. Surf. Sci. 1981, 104, 300. (5) Gland, J. L.; Sexton, B. A. Surf. Sci. 1980, 95, 587.
(6) Rao, C. N. R.; Kamath, P. V.; Yashonath, S. Chem. Phys. Lett. 1982, 88, 13.
0 1984 American Chemical Society
