2898
J. Phys. Chem. 1980, 84. 2898-2906
A comparison among the three intermetallics reveals striking differences. Evaluation of the changes in the Ni/M intensity ratios provides information concerning the increase of decrease in surface concentration of one component relative to .the other. For reactions conducted under comparable temperatures and times, the increase in Ni relative to M exhibits the following trend: ThNi5 > UNi5 > ZrNi5 These results are in agreement with an earlier study of ThNi5 and ZrNi, by Auger spectro~copy;~ that is, the superior activity for methanation of ThNi, compared to ZrNi5 may be related to the degree of surface enrichment of the catalytically active nickel species. The deactivation of ZrNi, was also found to result from graphite deposition on the nickel particles during methanation. However, at 300 “C this process does not become significant for reaction times of less than ca. 32 h.lS In a previous study of LaNi5 by Siegmann et al., an increase in the quantity of surface nickel was observed upon hydrogenation.12 LaNi, differs from ThNi, in that the La compound absorbs hydrogenlgwhereas ThNi, does not.20 Hence the nickel enrichment during hydrogenation may originate differently with the T h compound. However, it is clear that there is a strong interaction between metallic nickel and hydrogen since the hydrogen bond is readily cleaved on metallic nickel.21 It is probably this interaction which produces the migration of Ni to the surface out of the mixture of Ni, NiO, and T h o 2 which overlays ThNiS. In addition to the hydrogen interaction, an effect associated with the synthesis gas must also exist. This is presumably an interaction between Ni and carbon monoxide. The combined effects of CO and H2 result in the decomposition of the intermetallic and produce extensive surface enrichment of Ni.
Acknowledgment. This work was supported by the National Science Foundation under grant CHE78-00876.
References and Notes (1) V. T. Coon, T. Takeshita, W. E. Wallace, and R. S.Cralg, J. Phys. Chem.. 80. 1878 11976). (2)A. Elatkr, T. Takeshita, W. E. Wallace, and R. S.Craig, Science, 196,1093 (1977). (3)A. G.Moldovan, A. Elattar, W. E. Wallace, and R. S.Craig, - J. SoM Sfate Chem., 25, 23 (1978). (4) V. T. Coon, W. E. Wallace, and R. S. Craig In “The Rare Earths In Science and Technology”, G. J. McCarthy and J. J. Rhyne, Eds., Plenum Press, New York, 1978. (5) A. Elattar, W. E. Wallace, and R. S.Craig, Adv. Chem. Ser., No. 178, 2 (1979). (6) W. E. Wallace In “Hydrides for Energy Storage”, A. F. Andersen and A. J. Maeland, Eds., Pergamon Press Eimsford, NY, 1978,p 33. (7) A. Elattar, W. E. Wallace, and R. S.Craig in “Hydrides for Energy Storage”, A. F. Andersen and A. J. Maeland, Eds., Pergamon Press, Elmsford, NY, 1978,p 87. (8) V. T. Coon, Ph.D. Thesis, University of Pittsburgh, Pittsburgh, PA, Aug 1977. (9)C. A. Luengo, A. L. Cabrera, H. B. Mackay, and M. B. Maple, J. Catal., 47, l(1977). (10)G. B. Atkinson and L. J. Nicks, J. Cafal., 46, 417 (1977). (11)M, A. Vannice, J. Cafal., 37, 449 and 462 (1975);40, 129 (1975); 44, 152 (1976);50, 228 (1977):56, 236 (1979). (12)H. C. Siegmann, L. Schlapbach, and C. R. Brundle, Phys. Rev. Lett., 40, 972 (1978). (13)W. E. Wallace, R. F. Kalicik, Jr., and H. Imamura, J. Phys. Chem., 83, 1708 (1979). (14) K. T. Ng and D. M. Hercules, J. Phys. Chem., 80, 2094 (1976). (15)T. A. Patterson, J. C. Carver, D. E. Leyden, and D. M Hercules, J. Phys. Chem., 80, 1700 (1976). (16)A. Elattar and W. E. Wallace, submltted to Science. (17)J. H. Scofield, J. Electron Specfrosc. Relaf. phenom., 8, 129 (1976). 118) R. L. Chin et. al.. Unlversitv of Pittsburah. unDublished results. (19j J. H. N. van Vucht, F. A. Khijpers, and k:C. A. M. Brunlng, Phi/ips Res. Rep., 25, 133 (1970). (20)T. Takeshita, W. E. Wallace, and R. S.Craig, Inorg. Chem., 13,2282
(1974). (21)K. S o b , H. Imamura, and S.Ikeda, Nippon KagakuKaishi,9, 1304 (1977).
A Surface Spectroscopic Study of Molybdenum-Alumina Catalysts Using X-ray Photoelectron, Ion-Scattering, and Raman Spectroscopies D. S. Zlngg,+ Leo E. Makovsky,’ R. E. Tlscher,t Fred R. Brown,*$ and David M. Herculest Department of Chemlstry, University of Pinsburgh, Pitfsburgh, Pennsylvania 15260; and United States Department of Energy, Pittsburgh Energy Technology Center, Pittsburgh, Pennsylvania 152 13 (Received: February 1 1, 1980)
X-ray photoelectron spectroscopy (ESCA, XPS), low-energy ion-scatteringspectroscopy (ISS),and laser Raman spectroscopy have been used to characterize a series of Mo03/7-A1203catalysts. Laser Raman spectroscopy has shown the presence of three distinctly different molybdenum species. At low concentrations an interaction species, possibly resulting from the reaction of molybdic acid with surface hydroxyl groups, is found. ESCA and ISS have identified the presence of both tetrahedrally and octahedrally coordinated molybdenum, the latter being present above 4 w t % Moo3. After monolayer coverage has been reached, A12(Mo04)3 is formed and, at higher concentrations, bulk Moos is observed. Below monolayer coverage discrete compound formation by Mo does not occur; geometrical configurations are determined by sites of the y-alumina lattice. The effects of both calcination time and calcination temperature have been studied; it was found that both increased calcination temperature and calcination time favor the formation of A12(Mo04)3.
In roduc ion Hydrodesulfurization is a catalytic process used industrially to remove sulfur from petroleum feedstocks and coal. The commercial catalysts used in this process are generally molybdenum supported on activated alumina t University of Pittsburgh. t Pittsburgh Energy Technology
Center.
0022-3654/80/2084-2898$01 .OO/O
with various promoters such as Co and Ni added to improve catalyst performance. However, the COmmOn constituent Of most of these catalysts is molybdenum. In Order to understand the role of various additives, it is necessary for one first to understand how the molybdenum interacts with the support. Over the last few years much work has been published on the molybdena-alumina system. This work has employed a variety of spectroscopic techniques, 0 1980 American Chemical Society
Surface Spectroscopic Study of Mo/AI Catalysts
including UV-visiblte refle~tance,l-~ ESR,l'-li IR,3v6*14 laser Raman,15-lEand EEICA.19-23 The above work resulted in numerous suggestions about the nature of the molybdenum present on both the oxidic and reduced catalysts. Data obtained by the simultaneous use of several spectroscopic techniques can provide more information than that obtained from a single technique. Therefore, in the present study three complementary spectroscopic techniques (laser Raman spectroscopy (LRS), X-ray photoelectron spectroscopy (ESCA), and ion-scattering spectroscopy (ISS)) were used to study the molybdenum-alumina catalyst system. LRS permits the observation of fundamental vibrations associated with the incorporated molybdenum which are directly related to the structure of the molybdenum species. ESCA allows determination of 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 and chemical behavior of the catalyst with that of known chemical compounds. ISS provides information on the composition of the first few monolayers of the catalyst. Data from the three techniques can be combined and definitive structural information obtained. Both ESCA and ISS are surface-sensitive techniques with sampling depths of approximately 20 and 3 A, respectively. In addition both techniques are capable of detecting less than a monolayer (typically 0.1-1% of a monolayer for ESCA; 0.01-0.1% of a monolayer for ISS). It would not seem thkat LRS would complement either ISS or ESCA since its sampling depth is generally much greater than 20 A, but the LRS sampling depth is dependent on the absorptive properties of the sample. As will be shown, the changes seen by LRS are occurring within the region sampled by ESCA. The surface sensitivity of LRS is difficult to assess but, since ESCA shows most of the molybdenum to be on the surface, LRS is able to detect the presence of truly surface species. Experimental Section The Raman spectra were recorded on a Spex Ramalog spectrometer equipped with holographic gratings. The 5145-A line from a Spectra-Physics Model 165 argon-ion laser was used as the exciting source. The spectral shift width was typically 4 cm-l, and laser source powers of approximately 45 mW, measured at the sample, were used. The samples were pressed into pellets wth KBr as a support. Rotation of the sample to provide noncontinuous irradiation of any given spot on the sample was the method used to avoid sample decomposition. An AEI ES2OO-A electron spectrometer with an AEI DSlOO data system was used to record all ESCA spectra. The spectrometer was equipped with an aluminum anode (A1 K, = 1486.6 eV) operated at 12 kV and 25 mA. The digital data obtained were processed with an H P 2114 A computer. Overlapping peaks were deconvoluted by using a modified non-linear least-squares fitting routine (GAMET).^^ All binding energies are reported as the mean f the standard deviation. Standard compounds are referenced to the C 1s line at 284.4 eV. This was checked by using the Au 4f peaks of vapor-deposited gold at 83.8 eV.27 Catalyst binding eneirgies are referenced to the Al2p peak of the alumina support at 74.0 eV.2s Catalyst binding energies were also clhecked by using the Au 4f peaks of vapor-deposited gold. The ESCA probe system used allowed transport of the sample from the spectrometer to the reaction chamber without exposure to air. The probe and reaction chamber design have been described p r e v i o u ~ l y .Samples ~ ~ ~ ~ ~were
The Journal of Physical Chemlstty, Vol. 84, No. 22, 1980 2899
pressed into pellets and mounted on the probe, allowing reactions to be carried out without exposure to air. The probe permits up to four samples to be treated simultaneously. ISS spectra were recorded on a 3M Model 520 SIMS-ISS spectrometer. The inlet system was modified to allow rapid entry of the sealable probes discussed above. Isotopically pure 3He,4He,W e , and *Ar (Monsanto Research Corp. Mound Laboratories, Miamisburg, OH) were available for use as primary ions. The ISS instrument torr. operates at a base pressure of 5 X Sample Preparation Catalyst samples were prepared from yA1203(Harshaw Chemical Co., Cleveland, OH) having a BET surface area of 196 m2/g. The support was dried overnight at 110 "C. An appropriate amount of (NH4)6M07024*4H20 (Fisher Scientific, Pittsburgh, PA) was dissolved in distilled water and diluted to a predetermined volume (volume necessary to just fill catalyst pores 0.60 cm3/g). Half of the ammonium paramolybdate solution was added to the support and mixed; the remaining half was then added, and the solution was mixed well and allowed to stand for 0.5 h in a covered beaker. The mixture was dried overnight at 110 "C and calcined at the specific temperature for the specified time in an air-purged muffle furnace. Reductions were carried out a t 500 "C in a flow of H2 at a flowrate of 50 mL/min. After the specified reduction time, the reaction was quenched in a flow of dry N2 (200 mL/min).
-
Results and Discussion 1. Oxidic Catalyst. To provide a basis for interpretation of the Raman spectra of the catalyst, we recorded spectra of several reference "standardsyy.These are reproduced in Figure 1. Figure 1A shows the Raman spectrum of (NH4)6M07024-4H20, the catalyst precursor. Upon calcination, the paramolybdate decomposes to Moo3 (Figure lb). Figure ICreproduces the spectrum of the high surface area alumina support; no discrete scattering is apparent. Thus, if paramolybdate is added to the alumina support and no interaction occurs on calcination, the resulting spectrum should be the sum of spectra l b and IC.Coversely, if a different spectrum results, it is indicative of a chemical interaction between molybdenum oxide and the alumina support. One such possible interaction is the solid state reaction of Moo3 and A1203to form A12(M004)3 (Figure IC).The spectra of Moo3 and A12(Mo04)3differ considerably; thus LRS can detect the presence of A12MOO^)^ and/or Moo3. Representative LRS spectra of molybdena-alumina catalysts cohtaining various percentages of Moo3 are shown in Figure 2. Figure 2A shows the Raman spectrum typical of a catalyst containing a low concentration (7%) of Moo3. The broad band centered around 950 cm-' has previously been attributed to a molybdena-alumina support interaction.'&'18 Throughout the rest of this work, the species responsible for the broad band around 950 cm-l will be termed the "interaction species". An increase in the Moo3 content above 20 w t % results in the appearance of bands near 1006 and 380 cm-' (Figure 2B). These frequencies indicate formation of A&(MoO~)~ (Figure Id). A further increase in the Moo3 content (30 wt % Moo3 and above) results in the formation of bulk Moo3 which is evident from comparing Figure 2C and Figure lb. A closer examination of the 1000-cm-l region of Figure 2C (30 w t % Moo3) shows the presence of both the interaction species (950 cm-') and A12(Mo04)3(1006 cm-l) in addition to bulk Moo3 (821 and 998 cm-l). The
2000
The Journal of Physical Chemistty, Vol. 84, No. 22, 1980 l
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'
l
l
l
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800
0
400
A Wovenumber (cm-') Flgure 2. Laser Raman spectra of molybdena-alumina catalysts, calcined at 500 O C for 16 h: (A) 7 wt % Mo03/y-A120 (E) 25 wt % MOO,/y-A1203; (C)30 wt % MoOs/y-AlZOs.
TABLE I: ESCA Bindin Energiesa (eV) for Various Molybdenum Compounds
5
compd
MOO, MOO, Al,(MoO,), Mo (metal) MoS,
Mo 3d312 235.5 232.3 235.6 230.6 231.9
Mo 3d,12 232.4 229.2 232.5 227.4 228.8
2p
0 IS 530.4 530.0 530.4
74.0
All values are * 0.1 eV standard deviation. referenced t o A u 4f,,, = 83.8 eV. a
A WAVENUMBER, cm-' Flgure 1. Laser Raman spectra of reference materials. (a) (NH,),Mo,O2,.4H20; (b) Moo3; (c) y-A1203; (d) AI,(MoO,)~.
appearance of bulk Moo3 at these higher molybdenum loadings is indicative of saturation of those sites which lead to either the interaction species or A12(M00& Though the absolute concentration of bulk Moos in the 30 w t % Moo3 catalyst probably is lower than that of either the interaction species or A&(MO04)3,the Raman spectrum of this catalyst is dominated by that for bulk Moo9. This is evidence of the higher Raman cross section for Moo3 relative to that for the interaction species or A12(Mo04)3. Thus LRS indicates the presence of three distinct molybdenum species: the interaction species, Al2(MoO4)3,and bulk MOO,. The ESCA binding energies for various molybdenum compounds are shown in Table I and are in good agreement with those previously obtained by Patterson et al.l9 As can be seen, ESCA binding energies, alone, cannot distinguish between Moo3 and A&(MoO,)~because the Mo
s 2p
161.6 b
All values
3d binding energies are essentially the same. Figure 3 shows the Mo 3d ESCA spectra of calcined catalysts with varying Moo3 content along with the spectrum of MOO, for comparison. The binding energy of the Mo 3d6/2peak on all catalyst samples is 232.4 eV which is the same as that for bulk Moos (Figure 3D), indicating the presence of only Mo(V1) on the oxidic (calcined) catalyst. The full-width at half-maximum (fwhm) of the Mo 3d doublet on the oxidic catalyst is observed to decrease as a function of increasing MOO, content, approaching the fwhm of bulk MooBat very high loadings of Moo3. Peak broadening has been attributed either to the presence of more than one Mo(V1) species24or to electron transfer reactions between the support and However, closer examination indicates that peak broadening is due to sample charging. Sample charging results from the photoionization of nonconducting materials. The charge buildup due to this photoionization is inhomogeneous across the surface, thus the photoelectrons are escaping from regions of unequal potential. This increases their kinetic energy distribution and hence peak broadening occurs. Table I1 shows how the A1 2p fwhm varies as a function of its absolute binding energy. The absolute binding energy includes contributions from both work
Surface Spectroscopic: Study of Mo/AI Catalysts
The Journal of Physical Chemistry, Vol. 84, No. 22, 1980 2001
A -.A A.
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Fkure 4. Correlation between AI 213 line width and Dercentage of Moos for molybdena-alumina catalysts.'
8
240 236 232 228 224 Binding E n e r g y ( e V )
Flgure 3. ESCA spectra of moiybdena-alumina catalysts, calcined at 500 OC for 16 h: (A) 1 wt % Mo03/y-Ai20B; (B) 11 wt % MoO,/yA1203; (C) 30 wt % MoO3/y-AI2O3; (D)MOO,.
, 233
I
I
229
225
Binding Energy ( e V )
TABLE 11: Correlation of Al 2p fwhm with Sample Charging
Al 2p BE,b
1
Al2p fwhm,a eV 3.1
9
2.9
80.0
wt % MOO,
I
237
eV 80.3
79.8 15 2.7 79.6 20 2.6 79.2 25 2.3 BE denotes binding a Standard deviation is 0.05 i eV. energy. Standard deviation is k0.3 eV.
function and sample charging. Since it is unlikely that the A1203work function is changing, the absolute binding energy is directly related to sample charge and, as can be seen, the increase in the AI 2p fwhm on the oxidic catalyst can be directly correlated with the increase in binding energy. Since neither an electron transfer reaction nor various sites on the1 A1203surface would affect the AI 2p binding energy, and since the A1 2p binding energy is constant when referenced to the Au 4f line, the change in absolute binding energy must be due to sample charging. A plot of the AI 2p fwhm, which was found to be proportional to sample charging, as a function of MOO, content is shown in Figure 4. At low concentrations (1-5 wt % MOO,), the charging is constant with respect to Moos content; however, above 5 wt % MOO, the charging decreases linearly with respect to the amount of Moos until -24 wt % Moos wlhere there is no further change. Thus it can be concluded that broadening of the Mo 3d doublet on the alumina surface is the result of charging, not the presence of more than one species or the electron transfer reaction proposed p r e v i ~ u s l y . ~ ~ ~ ~ ~ 2. Reduction Studies. ESCA was also used to study the effect of reduction at 500 "C in H, on MoO,/y-Al,O, catalysts. In a study by Patterson et al., Mo(V1) was reported to be preslent after 15 h of reduction in H2.In
Flgure 5. Comparison of computer deconvolutions: (+) experimental Mo(VI), (---) MW), Mo(1V). (A) Uslng Patterson's parameters (2.0-eV fwhm) (welghed varlance = 566). (B) Using parameters from this work ( 2 . 8 4 fwhm) (welghed variance = 92.5).
data,(-) computer fit, (-)
-----
the present work the intensity of the peak due to Mo(V1) (232.4 eV) was found to decrease rapidly and was no longer present after 45-min reduction. The difference between our results and those of Patterson et al. results from the use of a model Mo 3d doublet having a fwhm that was too narrow (2.0 eV) in the earlier study. In the present work a more realistic fwhm of 2.8 eV has been used for deconvolution, which allows for broadening due to sample charging. The best computer fits of a Mo 3d envelope, using both Patterson's parameters and the parameters discussed in this work, are shown in Figure 5. As can be seen, the fit using a Mo 3d fwhm of 2.8 eV (Figure 5B) is significantly better (weighted variance = 92.5 vs. 566 for Patterson's), especially in the 231-eV region. An attempt was made to fit three doublets with the more realistic 2.8-eV fwhm but was unsuccessful. The use of a fwhm of 2.8 eV was justified because this is the approximate fwhm of the Mo 3d6p peaks after reduction of a low weight percent MOO,/~-Al,O, catalyst where only one set of doublets was found. Since the absolute binding energy of the A1 2p peak after reduction did not vary as a function of concentration as it did with the oxidic catalyst, it is assumed that the sample charging is constant over the range of concentrations studied and thus the fwhm of the Mo 3d doublet is constant throughout the concentration range. Figure 6 shows the molybdenum oxidation state distribution as a function of reduction time, deduced by using the 2.8-eV fwhm for a 15% Mo03/y-A10, catalyst. A nearly identical result was obtained for the catalyst used by Patterson et al. when the 2.8-eV fwhm was used as
2902
The Journal of Physlcal Chemistty, Vol. 84, No. 22, 1980
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R e d u c t i o n Time ( m i n )
Flgure 6. Mo oxldatlon state as a function of reduction time for the 15 wt % MoO,/y-Ai,O, catalyst. Reduction was at 500 O C in Hz: (A) Mo(V1); (0)Mo(V); (0)Mo(IV).
described above. The Mo(V1) intensity decreases rapidly with time, resulting in a large amount of Mo(V) and some Mo(1V). The Mo(1V) intensity then increases at the expense of the Mo(V) until approximately 600-min reduction time, where no further change in the concentration of either Mo(1V) or MOW)was seen. The attainment of this steady state, in addition to the absence of Mo(VI), indicates that disproportionation mechanisms previously proposedg0 are not occurring and that the Mo(V) and Mo(1V) produced result from reduction of two distinctly different Mo(VI) species. The reduction behavior of both MOO,, in which the Mo(V1) is octahedrally coordinated,g6and &(MOO&, in which the Mo(V1) is tetrahedrally ~oordinated,5~1~' were studied. It was found that octahedral Mo(V1) (MOO,) was reduced completely to Mo(1V) after 12 h of reduction at 500 "C in H2. Conversely, it was found that under the same conditions, tetrahedral Mo(V1) (A12(Mo04)3)was reduced only to Mo(V). Figure 7 shows the catalyst reduction behavior as a function of MOO, content. Each catalyst was reduced for 12 h to assure that reduction of the Mo(V1) is complete and that the steady state seen in Figure 5 was obtained. The most prominent change seen is the increased intensity of the low binding energy peak at 229.2 eV with increasing Moo3 content. The relative concentrations of Mo(IV) and Mo(V) for each catalyst were obtained by deconvolution of the Mo 3d spectral envelope. In order to compare the variations in concentration of the two molybdenum species on catalysts with different Moo3 contents, we took the ratio of the intensity of each species (obtained by deconvolution of the Mo 3d spectral envelope) to the A1 2p intensity. The A1 2p intensity is assumed to be constant throughout the concentration range studied. The variation in concentration of both MOW)and Mo(IV) as a function of total MOO, content is shown in Figure 8. The fact that tetrahedral Mo(V1) is reduced to Mo(V), combined with evidence obtained previously,'$l$l indicates that a tetrahedral environment is necessary for stabilization of Mo(V). This result allows the Mo(V)/Al intensity ratio obtained after reduction to be correlated with the tetrahedral Mo(V1) present before reduction. On the basis of the reduction behavior of MOOS,it can be postulated that the Mo(1V) present on the reduced catalyst surface results from reduction of octahedrally coordinated Mo(V1). This is confirmed by leaching a 30 wt % catalyst, which is known to have MooBpresent (Figure 2C), with a 3% NH40H solution. The spectral changes associated with various treatments are shown in Figure 9. It is clear that upon reduction of the oxidic catalyst there is an increase in intensity at 231.7 eV, indicating the presence of Mo(V). Also, a low binding energy peak at 229.2 eV appears, indicating that some Mo(V1) is reduced to Mo(1V) (Figure 9 parts A and B). When the oxidic catalyst is leached with a 3% NH40H solution, the Mo 3d spectrum shows a broad unresolved band having
2 4 0 236 232 228 224 Binding Energy ( e V )
Flgurr 7. Mo 3d ESCA spectra of reduced MoO3ly-AizO3catalysts. Reduction carried out at 500 O C in H2 for 12 h: (A) 20 wt % MoO3/y-AI2O3; (B)15 wt % MoO3/y-AI2O3; (C)11 w l % MOO,& A1203; (D) 9 wt % MO03/y-A120,; (E) 5 wt % M003/y-A1203.
wt
% MOO,
Flgwr 8. Mo oxldatkn state distribution as a function of Moo, content. Mo(1V). Reduction at 500 'C In H2 for 12 h (0)Mo(V), (0)
ita maximum at 232.4 eV (Figure 9C). This indicates that Moos is the species leading to charge reduction on the catalyst surface (Figure 4) since it is the only species present that would be removed with a NH40H leach.32 The spectrum of the leached, oxidic catalyst after reduction shows a shift from a peak binding energy of 232.4 eV on the oxidic catalyst toward 231.7 eV on the reduced catalyst, indicating some reduction to the MOW)species (Figure 9D). The difference between reduction of the oxidic catalyst and the leached oxidic catalyst is the absence of the low binding energy peak at 229.2 eV in the latter as shown in Figure 9D.Since leaching results in the removal of only MOO, (octahedral Mo(VI)), and the peak due to Mo(1V) after reduction is absent after leaching, it can be concluded that the production of Mo(1V) is the result of reduction of octahedral Mo(V1). Since the MOW)produced by reduction of the catalyst results from tetrahedrally coordinated Mo(VI), and the Mo(1V) from octahedrally coordinated Mo(VI), Figure 8
Surface Spectroscopici Study of
The Journal of Physical Chemistty, Vol. 84, No. 22, 1980 2003
Mo/AI Catalysts
4He ISS
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;239
233
0.40
227
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I
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0.60 0.80
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Binding E n e r g y ( e V ) Flgure 9. Mo 3d ESCA spectra of a 30 wt % Mo03/y-Alz03catalyst: (A) oxidic catalyst; (8) reduced catalyst (H2, 500 O C for 12 h); (C) leached oxidic catalyst (3% ”,OH solution until constant Mo/AI ratio): (D) catalyst leached (siame as C) then reduced (same as B).
indicates the variation in concentration of octahedrally and tetrahedrally Coordinated Mo(V1) as a function of total Moo3 content. Figure 8 shows that at low MOO, concentrations most of t h e molybdenum is tetrahedrally coordinated as has been shown previously.11i12The tetrahedrally coordinated molybdenum increases rapidly until about 7 wt % MOO, where it begins to level off. This initial increase is probably due to formation of a “surface molybdate” as has been proposed p r e ~ i o u s l y , 3the ~ ~pro~~ duction of which levels off due to the decrease of available tetrahedral surface sites. The term “surface molybdate” is used because this species has reduction behavior similar to bulk A12(Mo04)3.However, X-ray diffraction did not indicate the presence of bulk molybdate. The increase in the tetrahedral species above about 17-20 wt % Moo3 is due to formation of bulk (A12(Mo0& the presence of which has been confirmed by LRS (Figure 2B). The octahedral species is first seen in detectable concentrations at 4 wt % MOO, and increases steadily. This is in agreement with results from previous workers who have shown that the octahedral to tetrahedral Mo(VI) ratio increases with increasing MOO, content?^^ In addition, at monolayer coverage (-20 wt % Moo3), the ratio of octahedral Mo(V1) to tetrahedral Mo(V1) is approximately 2:l (corrected for aluminum molybdate) which is the ratio of octahedral to tetrahedral cation sites predicted for y~ 1 ~ 0 ~ . 1 3
One can calculate several “breakpoints” on the curves in Figure 8. For Mo(V), breakpoints occur at -7 and 19% Moo3. Thus one can say that, at -7% Moo3, the tetrahedral holes of the lattice are filled and, at 19%,one is seeing significant dormation of A ~ , ( M o O ~ )This ~ . latter number correlates well with LRS data (20%). Breakpoints are more difficult to calculate for Mo(1V) because of the nature of the curve. I t is clear that a break occurs around 7% Moo3 corresportding to filling of the tetrahedral holes.
-
Figure 10. 4He ISS spectra of Mo03/y-A1203catalysts: (A) 3 wt % MOo$y-A1& (B) 9 wt % bbo~/’&O$ (C) 25 wt % M o O ~ / y A I z O ~
It is also clear that the Mo(1V) curve begins to rise rapidly after about 24% Moo3. An estimate of the breakpoint here corresponds to about 22% MOO,; beyond this percentage, significant Moo3 formation occurs. It is interesting that the breakpoints for A12(Mo04),and Moo3 correlate well with the estimated monolayer percentage, -20 wt % MOO,. The ESCA value for onset of Moo3 formation is lower than that of LRS (22 vs. 30%),probably because of interferences in the Raman spectra by much larger amounts of aluminum molybdate and the interaction species. I t is clear that the octahedral Mo is present in the range 7-22 % , but it is not bulk MOO,. This corresponds to molybdenum atoms filling the octahedral sites of y-alumina. Interestingly, this octahedral species has chemical properties similar to Moo3. On the basis of the above arguments, one may conclude that the “interaction species” observed by Raman spectroscopy is really two species: tetrahedrally and octahedrally coordinated molybdenum bound to alumina lattice sites. Figure 8 shows that at 8 wt % Moo3,the site preference shifts from tetrahedral to octahedral as indicated by the leveling of the Mo(V) curve and the steady increase in the Mo(1V) curve. This change in site preference is confirmed by ISS. Figure 10 shows typical 4HeISS spectra for various Mo03/A1203catalysts. The prominent scattering peaks are for oxygen at E/Eo = 0.410, aluminum at E/Eo = 0.595, and molybdenum at E / E o = 0.865. Above monolayer coverage the added molybdenum is covering the support and thus obscuring the 4He ISS peak due to aluminum, as shown in Figure 1OC. A plot of the Mo/Al ISS intensity ratio vs. Moo3 content is shown in Figure 11. The discontinuity at about 7 wt % Moo3 indicates that at low concentrations ( vacuum > O2 at a lower AV, but O2 > H 2 0 > vacuum at a higher AV. The OSEE of the specimen heat-treated during exposure to oxygen became weaker with increasing temperature. The OSEE vs. Iprelationship at a value of AV also gave a maximum emission (EImBxJ ) at a particular value (Ip,,,); the value of Ipwas expressed in the electric power (W) expended in an electricbulb used for the light; the value of ZP- decreased as the value of AV set at the start of the measurement was increased. The effects of operational parameters such as anode voltage and discriminator level were also examined. The occurrence of a maximum emission in the OSEE vs. AV (or I,) can be explained in terms of the so-called Schottky effect (or external photoelectric effect) on the metal surface of a lower work function due to the adsorbed molecules and alteration in the orientation of an electric dipole formed by the ethanol molecules adsorbed, the latter increasing the work function.
Introduction “Extrinsic” exoelectron emission, which is the term proposed by Ramsey,l results from the interaction of metals with the environment such as adsorption and oxidation; the adsorption of atoms or molecules causes changes of the work function of the metals due to the redistribution of electron density at the surface. The OSEE described in the present paper is associated with 0022-3654/80/2O84-2906$0 1.OO/O
the species adsorbed on the metal surface under relatively high pressure conditions. Previous investigation^^^^ have demonstrated the following: (1) the “extrinsic” OSEE from the real metal surface is strongly influenced not only by the adsorption of oxygen or water vapor but by the interaction with an organic vapor contained in the counter gas; (2) this surface-organic interaction is considered to be hydrogen bonding and to affect the electric field formed 0 1980 American Chemical Society
