J. Phys. Chem. 1982,86,3079-3089
3079
ARTICLES Surface Spectroscopic Characterization of Cobalt-Molybdenum-Alumina Catalysts Roland L. Chln and Davld M. Hercules' Depatm"et of Chemistry, UnlversiW of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: September 1. 1981; I n Final Form: January 7. 1982)
The structural characteristics of Co/Mo/Alz03 hydrodesulfurization catalysts have been investigated with a variety of surface spectroscopictechniques: ESCA, ISS, and photoacoustic spectroscopy (PAS). All catalysts were prepared by sequential impregnation of Mo and Co into 7-Al,03 with the Mo concentration held constant at 15% Moos by weight as the cobalt concentration was varied from 1to 9% COO. In all catalysts interaction between Co and the support was observed which rendered a portion of the Co inert toward Hzreduction and H2Ssulfidation. This interaction species (Co-t) is dominant for catalysts of low cobalt loading (Le., 1%COO). As the cobalt concentration is increased a new phase is formed characterized by interactions between Co and Mo. This species (Co-M) is produced up to a concentration of 7% COO. Above this level, Co304is formed. From ISS it was determined that the Co ions were situated beneath the Mo ions as Co-t and the Co-Mo interaction species. The presence of cobalt was found to have no effect on the reduction or sulfidation of Mo.
Introduction Hydrodesulfurization catalysts have been the subject of intense research because of industrial and environmental concerns. Typical catalysts contain Mo as the active catalyst; and often Co is present as a promoter. In general, 7-A1203is used as a support to provide high dispersion for the metals. Many techniques have been employed to investigate these systems, despite this fact the nature of the surface species on the catalysts is still poorly understood as evidenced by the conflicting interpretations reported in the literature. It is widely accepted that the dispersion of Mo on the support is very high and that it probably involves monolayer-type coverage. Several models have been proposed to describe the structure of this monolayer, but its exact identity is still in I t is known that the geometry of the Mo ions with respect to the support is affected by metal concentration and calcination temperature! Increasing the Mo concentration up to monolayer coverage (- 1520% Moo3) favors octahedral coordination whereas an increase in calcination temperature favors tetrahedral coordination. The addition of Co as a promoter complicates structural analysis of the catalyst. As in the C0/Alz03system, there is a strong interaction between the metal and support which renders a portion of the cobalt chemically inert, but the fate of the remaining Co is uncertain. The formation of Co304has been demonstrated for catalysts containing more than 1.5 atom 9% Co on alumina." However, CoMoo4 has also been detected independently of the formation of Co304.819 Hence, the exact identity of the Co (1)F. E.Maseoth, J. Catal., 36, 164 (1975). (2)G. C. A. Schuitt and B. C. Gates, AZChE J.,19,417 (1973). (3)J. M. J. G. Lipsch and G. C. A. Schuitt, J. Catal., 15, 179 (1969). (4)D.S.Zingg, L. Makovsky, F. R. Brown, and D. M. Hercules, J. Phys. Chem., 89,2898 (1980). (5)J. H.Ashley and P. C. H. Mitchell, J. Chem. Soc. A, 2821 (1968). (6)R. 1. DeClerck-Grimee, P. Canesson, R. M. Friedman, and J. J. Fripiat, J. Phys. Chem., 82, 885 (1978). (7) M. LoJacono, A. Cimino, and G . C. A. Schuitt, Gazz. Chim. Ital., 103, 1281 (1973). 0022-3654/82/2086-3079$01.25/0
species on the surface of the oxidic catalyst is still a matter of debate. Although a thorough understanding of the chemical composition of the oxidic catalyst is important, it is the catalyst in the sulfided form which is active in the hydrodesulfurization process. Again, controversy exists as to the exact chemical make up of the sulfided catalyst. It is fairly well established that MoSz is present but the mechanism for its formation is still in question. In addition, the chemical nature of the Co species has produced conflicting results. Sulfiding of the cobalt has not been firmly established.
Experimental Section Instrumentation. All ESCA spectra were taken on an AEI ES2OOA electron spectrometer interfaced to an AEI DSlOO data system. Non-monochromatized A1 Ka!radiation (1486.6 eV) was employed as the excitation source. The digital data obtained were processed with an H P 2114A computer and deconvolution of overlapping peaks was performed with a Dupont 310 curve resolver. The ES2OOA was operated 12 kV and 22 mA and at a pressure torr or lower. This is a typical background of 1 X pressure for catalyst samples that have been reacted. Instrumental base pressure is below torr. The probe system used allowed transfer of the sample from the reaction chamber to the spectrometer without exposure to air. The probe and reaction chamber design have been described previously.lOJ1 The A1 2p line (74.5 eV) of the support was used as an internal reference for determination of binding energies. ISS spectra were recorded on a 3M Model 525 spectrometer which employs a cylindrical mirror analyzer (CMA) to measure the energy of the backscattered ions. 4He ions were used exclusively with a primary ion energy (8) J. Medema, C. van Stam, V. H. J. deBeer, A. J. A. Konings, and D. C. Koningsberger, J. Catal., 53,386 (1978). (9)C. P. Cheng and G. L. Schrader, J. Catal., 60,276 (1979). (10)K. T.Ng and D. M. Hercules, J. Phys. Chem., 80, 2094 (1976). (11)T.A. Patterson, J. C. Carver, D. C. Leyden, and D. M. Hercules, J.Phys. Chem., 80, 1700 (1976).
0 1982 American Chemical Society
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The Journal of Physical Chemistry, Vol. 86, No. 16, 1982
TABLE 11: ESCA Binding Energies of Oxidic Catalysts and Reference Compoundsa
TABLE I : Atomic Absorption Analyses of Catalysts Studieda nominal wt % coo per 1 5 % MoO,/Al,O, 0 1 3
5 7 9
BE, eV catalyst designation MA-15 CMA 1 1 5 CMA 3 1 5 CMA 5 1 5 CMA 7 1 5 CMA 9 1 5
actual wt 96
coo 0 1.0 2.8 4.8 6.7 8.9
MOO, 15.6 14.9 16.5 14.2 14.2 14.5
Standard error of difference is 0.1% for Co and M o determinations. a
of 2 keV. The base pressure of the instrument was ca. 5 X lo* torr before backfilling with the noble gas. Spectra were obtained with a noble gas pressure of 1.8 X lo4 torr. Primary ion current density was 5 X A/cm2. The E / E o region of 0.3 to 1 was analyzed with a scan time of ca. 3.5 min. Photoacoustic analysis was performed at the University of Denver on an in-house instrument. Optical spectra in the visible region were obtained with a xenon arc lamp operated at 300 W. Modulation of the excitating radiation was accomplished with a mechanical chopper operating at 45 Hz. Air was used exclusively as the coupling gas. Normalization of data was performed against the PAS spectrum of carbon black. Sample Preparation. Catalysts were prepared by incipient wetness from 7-A1203(Harshaw Chemical Co.) having a BET surface area of 190 m2/g. The support was ground with a steel sample mill, sieved through a 100 mesh screen, and dried overnight at 110 "C. An appropriate volume of an aqueous (NH4)6M0702409H20 solution was added to the support and mixed well. The mixture was dried overnight at 110 "C and calcined in air at 500 "C for 16 h in a muffle furnace. For those samples containing cobalt, impregnation with an aqueous Co(N03)2solution was performed on samples previously impregnated with Mo. This mixture was dried overnight and calcined in air for 16 h at 500 "C. All samples contained 15% MOO, by weight and a varying quantity of COO by weight. The actual percentages of Co and Mo for the samples were determined by atomic absorption and are given in Table I. In addition, each sample is designated with an abbreviation to describe ita composition and the concentration of metal(s) present. For example, MA-15 refers to a Mo/A1203catalyst containing nominally 15% Moo3 by weight, whereas CMA 915 refers to a Co/ Mo/A120, catalyst containing nominally 9% COOand 15% MOO, by weight. Reduction and Sulfidation. All catalyst samples were pressed into rectangular pellets (6 X 15 mm) under 5000 psi before any gas-phase reaction. So that we could confirm that the pelleting process did not induce chemical changes in the catalyst, a sample dusted onto double-sided tape was compared with a sample pressed as a pellet. No differences were observed between the two samples in terms of binding energies or intensity ratios. Hence, it was assumed that the original powdered catalyst was of the same chemical composition as the pressed catalyst. The samples were mounted on the sample probe and secured in place by stainless steel clips. The reaction furnace was heated to the desired temperature and allowed to equilibrate under a flow of hydrogen. The sample probe was inserted into the reaction chamber in the sealed poeition and allowed to reach reaction temperature (30-min hold period). For the reduction studies, pure H2 (99.999%)
CMA 1 1 5 CMA 3 1 5 CMA 5 1 5 CMA 7 1 5 CMA 915 MA 15 CoAI,O, c030,
CoMoO, Co(metal) MOO, Mo(metal)
(s) (s) (s) (s) (s)
782.0 781.9 781.9 781.9 781.6
787.1 787.0 787.1 787.2 786.9
232.8 232.1 232.7 232.8 232.7 232.8
235.9 235.8 235.8 235.9 235.8 235.9
782.1 780.7 781.2 778.1
broad (s) 787.4 ( s ) 232.8 781.6 (el)
235.9
232.7 227.3
235.9 230.5
787.1 (s)
531.4 531.4
531.3 531.4 531.4 531.3 531.4 530.8 530.7
All binding energies were measured with a precision of t 0 . 1 5 eV. (s) denotes shake-up and (el) electron energy loss.
at a flow rate of 50 mL/min was employed. The reaction was allowed to proceed for the desired length of time. Quenching of the reaction was accomplished by stopping the flow of H2, flushing the chamber with a high flow of nitrogen (ca. 200 mL/min), and cooling of the sample. Sulfidation was accomplished with 15% H2S/H2. The procedure was the same as that used for reduction.
Results ESCA Analysis. The ESCA binding energies of the catalysts studied and of relevant reference compounds are tabulated in Table 11. It is evident that within experimental error the binding energies of the Co 2 ~ 3 1 2lines of all catalysts are essentially equal with the exception of CMA 915. These values correspond closely to those of C0A120,. It should also be noted that the binding energies of the Mo 3d512,312doublet are constant throughout the series of catalysts studied and correspond exactly to the binding energies of MOO, (Le., M O T Although the observed binding energies for the catalysts are very close to those of bulk reference compounds this does not necessarily mean that these compounds are present on the surface of the catalyst. For example, the binding energy of Mo6+remains constant, independent of the particular compound (compare BE'S of CoMoO., and MOO,). The Moos concentration of all catalysts included in this study was held constant at nominal 15% Moo3 by weight as the COOconcentration was varied. Calculations based on a Mo content of 15% MOO, by weight reveal that approximately 7% COO would be needed to produce a 1:l stoichiometric Co-Mo compound. From Table I1 it is observed that the binding energy of the Co 2 ~ 3 1 2level of CMA 915 is slightly lower than for all other catalysts (i.e., lower COOcontent). The difference in binding energies between CMA 915 and the other catalysts indicates a difference in chemical composition. Based on the calculations given above it is evident that a new Co species is being formed on the surface of the catalyst as the Co loading is increased from 7 to 9% COO. This species could possibly be Co304or CoMoO, since the formation of either compound would cause a shift of the Co 2pSl2peak maximum toward lower binding energies. Figure 1 shows the Co 2p3 ESCA spectra of cobalt reference compounds and C h A 315 (used as a representative example of the CMA catalysts). It is observed that, accompanying the principal core line, satellite structures are also present at 5-6-eV higher binding energy. This structure has been assigned to a monopole chargetransfer process (shake-up) from 0 2p Co 3d and is
-
Characterization of Co/Mo/Al,03 Catalysts
The Journal of Physical Chemistty, Vol. 86, No. 16, 1982 3081
400
600
800
Wavelength ( n m ) Binding Energy ( e V 1
Flgure 1. ESCA spectra of Co 2p3,* core level of reference compounds and CMA 315: (a) capo,, (b) CoMo04, (c) CoAI,O,, (d) CMA 315.
characteristic for photoionization of cobaltous ions (Co2+).12 The shake-up peak for Co304is much lower in intensity than for either CoMo04or CoA1204. Co304contains two distinct cobalt ions, Co2+and Co3+in the ratio of 1:2, and the ESCA spectrum is a convolution of lines from these two species. Since Co3+shows only weak satellite structure,13 the convolution of peaks due to Co2+and Co3+ would show a lower intensity in the satellite peak than for pure Co2+. In addition to the presence of shake-up peaks, spin-orbit splitting of the Co 2p3/2 and 2p1i2levels is affected by the oxidation state and can be used as a diagnostic tool. Okamoto et al.14 have determined the spin-orbit splitting (AE)of paramagnetic cobaltous compounds to be ca. 16 eV whereas AE for diamagnetic cobaltic compounds and cobalt metal was ca. 15 eV. For the CMA catalysts AE’s were calculated from the energy difference between the Co 2 ~ 3 1 2and 2plI2 peaks. The value determined for all catalysts was 16.1 eV which is in agreement with that expeded for cobaltous compounds. CMA 915 was slightly lower (15.8 eV). The ESCA results provide good evidence for the existence of Co2+on the surface of the catalyst. The exact geometry of the cobalt ions with respect to neighboring atoms would be of great value for elucidating the particular cobalt species. This information may be obtained by evaluation of the energy difference between the principal photopeak and the satellite peak. Oku et d.13have shown that satellite splitting for cobaltous compounds is affected by coordination of Co2+with oxygen atoms. The splitting in the Co Zp3,2 level was observed to be 5.3 eV for tetrahedral coordination of Co2+and 6.2 eV for octahedrally coordinated Co2+. The examples used by Oku to demonstrate this effect were the spinel compounds CoCr204 (12) T. J. Chuang, C. R. Brundle, and D. W. Rice, Surf. Sci., 59,413 (1976). (13) M.Oku and K. Hirokawa, J. Electron Spectrosc., 8,475 (1976). (14) Y. Okamoto, H.Nakano,T. Imanaka, and 5. Teranishi, Bull. Chem. SOC. Jpn., 48, 1163 (1975).
Flgwe 2. Photoacoustic spectra of cobalt reference compounds: (a) CoAI,O,, (b) CoMoO,, (c) c030,.
(tetrahedral) and CoFe204(octahedral). Determination of the satellite splitting for the reference compounds used in this study shows that CoMo04 (octahedral) has a value of 6.2 eV and C0A1204 (tetrahedral) has a value of 5.1 eV. These values are in good agreement with the results of Oku. For catalyst CMA 315, the satellite splitting of 5.1 eV suggests that a majority of the cobalt is tetrahedrally coordinated. It is also evident that the ESCA spectrum of CMA 315 is nearly identical with that of C0A1204(compare Figure 1, c and d), although results to be presented later will show that CoA1204is not the major Co species on the surface of the catalyst. For CMA 915 a small increase in the satellite splitting would be expected if CoMo04 is present. This increase in satellite splitting was not observed. On the basis of data to be presented later, it was determined that Co304is formed and not C0M004on the surface of CMA 915. From Figure 1 it is apparent that the satellite structure of Co304is much weaker than for the pure cobaltous compounds (e.g., CoMoo4or CoA1204). In addition, the concentration of Co304 on CMA 915 was determined to be low relative to other Co species. Hence changes in satellite splitting may not be detected. Photoacoustic Analysis. The photoacoustic (PAS) spectra of cobalt reference compounds are shown in Figure 2. C0A1204has a broad band centered at -600 nm. This band has fine structure consisting of peaks at ca. 545,580, and 620 nm, characteristic of Co2+in a tetrahedral environment. These bands are absent in CoMo04, although a peak at 520 nm is clearly detectable as well as broad structure extending out to ca. 700 nm. The origin of the bands in CoMo04 has been assigned to Co2+in an octahedral c~ordination.’~The PAS spectra obtained for CoA1204and CoMo04 are in good agreement with those obtained by Rosencwaig.16 The spectrum of Co304 is rather featureless and does not show the triple bands of tetrahedral cobalt as would be expected. This spectrum is characteristic of signal saturation which occurs when the (15) G. N. Asmolov and 0. V. Krylov, Kinet. Catal., 12, 403 (1971). (16)A. Fkmencwaig, ‘Optoacoustic Spectroscopy and Detaction”,Y. H. Pao, Ed., Academic Preas, New York, 1977, p 193.
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Mo
0
E
__
0.4
0.6
0.8
1.u
Energy ( E / E , )
400
600
800
Wavelength ( n m ) Figure 3. PAS spectra of oxidic CMA catalysts: (a) CMA 315,(b) CMA 715,(c) CMA 915.
sample acts as a blackbody absorber. PAS spectra of CMA catalysts are shown in Figure 3. For CMA 315, the triple bands corresponding to tetrahedral cobalt are clearly evident between 500 and 700 nm. In addition, strong absorption is also apparent at ca. 400 nm. It has been established by diffuse reflectance spectroscopy that absorption by Mo6+in an octahedral environment has a maximum a t ca. 400 nm.17-20 Although Co3+(octahedral) is also known to absorb in this region, it has been shown by ESCA to be essentially absent from the surface of CMA 315 catalyst. Hence, any absorption contribution by Co3+(octahedral) would be overshadowed by the more intense absorption of Mas+ (octahedral). The PAS spectrum of CMA 715 (Figure 3b) is very similar to that of CMA 315. However, degradation in the quality of the spectrum is obvious relative to CMA 315. The triple bands of tetrahedral cobalt are less well defiied and, in general, a decrease in the signal-to-noise ratio is observed. The poorer quality of the spectrum suggests that signal saturation conditions are being approached or that masking of the bands of tetrahedral cobalt by a stronger absorber is occurring. This will result upon the formation of a new cobalt species having a very high absorption coefficient (e.g., Co304). The effect of such a compound would be to suppress the peaks due to the characteristic absorption by other species present. Because of the high absorption coefficient of Co304,its presence in even small quantities would cause a general degradation of the observed PAS spectrum. It is clearly evident that this situation has occurred in the spectrum of CMA 915 (Figure 3c) * ISS Analysis. The formation of CoMo04 has been suggested by several worker^*^^ to occur on the surface of (17) P.Gajardo, P.Grange, and B. Delmon, J.Catal., 63,201 (1980). (18)N. P.Martinez, P. C. H. Mitchell, and P. Chiplunker, J. Less Common Met., 54, 333 (1977). (19) H. Praliaud, J. Less Common Met., 64, 387 (1977). (20)M.Giordano, J. C. H. Bart,A. Vaghi, A. Castellan, and G.Martinotti, J. Catal., 36,81 (1975).
Flgve 4. ISS spectra of CoMoO, as a function of time. The incident ion beam was ‘He at an energy of 2 keV and a partial pressure of 1.8 x 1 0 - ~torr. Scan time was ca. 3.5 min: (a) initial scan, i.e., t o = 0 min, t , = 3.5 mln; (b) t o = 3.5 min, t , = 7.0 mln; (c) t o = 7.0 min, t , = 10.5 min. Current density = 5 X lo-* A/cm2.
CMA catalysts. Its formation was found to be influenced by the method of preparation and the Co and Mo loadings. For example, coimpregnation of Co and Mo produces CoMo04 on the surface of the catalyst whereas sequential impregnation does not. In addition, CoMo04 formation is also enhanced when the metal loadings of Co and Mo are high (i.e., 10% COOand 30% MOO& In order to better ascertain the surface characteristics of CoMo04 and the CMA catalysts, we performed low-energy ISS measurements. The ISS spectra obtained for bulk CoMo04 (Climax Molybdenum Co.) are shown in Figure 4. It is apparent that the sample is contaminated with NaCl as indicated by scattering peaks due to Na and C1. Spectrum a is the initial scan of the sample. The data acquisition time was approximately 3.5 min for each spectrum. Spectra b and c represent successive scans of the sample. It can be seen that the Co/Mo intensity ratio decreases with time, Le., as the sputtering process is allowed to proceed an increase in the quantity of Mo exposed to the surface occurs relative to cobalt. The surface characteristics of the CMA catalyst are very different from those of CoMoOl. This situation is illustrated for CMA 715 in Figure 5. The other catalysts studied also showed similar behavior. It is observed that initially very little cobalt is present on the surface of the catalyst. As the sample is sputtered a growth in the Co peak relative to Mo peak takes place with time. This behavior is contrary to bulk CoMoOl where a growth in the Mo peak relative to Co is observed with longer sputter times. Reduction Studies. It is often possible to detect one species in the presence of others by taking advantage of changes in ESCA spectra caused by differences in chemical reactivity. Referring to Table I1 the lower BE of the Co 2~~~~ l i e of CMA 915 indicates a detectable difference in surface composition relative to the other catalysts. The dissimilarity in the BE of CMA 915 may result from an additional surface species on the catalyst. However, it is difficult to verify this postulate from ESCA spectra alone, because it is not possible to resolve the spectral envelope into individual components. However, from the PAS results and X-ray diffraction analysis it was determined that
Characterization of Co/Mo/Al,03 Catalysts
The Journal of Physical Chemistry, Vol. 86,No. 16, 1982 3083
0
A
0.4
Co 2p
level 'i2
AI
I
co
I\
0.6
0.8
Energy ( E / E ,
)
I
Flgue 5. ISS spectra of catalyst CMA 715 as a function of time. The inddent km beam was ?-le at an energy of 2 keV and a padl presswe of 1.8 X lo6 torr. Scan time was ca. 3.5 min: (a) inltial scan, i.e., t o = 0 mln, t , = 3.5 min; (b) to = 3.5 min, t , = 7.0 min; (c) t o = 7.0 min, t , = 10.5 min. Current density = 5 X lo-' A/cm2.
Co304is formed in detectable quantities on the surface of CMA 915. For CMA 715 only trace amounts of Co304are present and no Co304is detected for cobalt catalysts having lower loadings. The ease with which Co304is reduced to metallic cobalt was used as a means of demonstrating the differences in surface composition between CMA 915 and the other catalysts. Figure 6 shows the ESCA spectra of the Co 2pSl2level and Mo 3d512,3/2 doublet for CMA catalysts after hydrogen reduction at 500 "C for 6 h. In the cobalt spectra, the line which intersects the peak at 779 eV denotes metallic cobalt. For the molybdenum spectra, lines are drawn only as references to indicate relative peak positions and do not correspond to specific oxidation states. From Figure 6 it is apparent that the quantity of reducible cobalt increases significantly as the cobalt loading is increased from 7 to 9% COOat fixed Mo concentration (15% Moo3). In CMA 715 only 12% of the Co is reducible whereas in CMA 915 30% is reducible. The enhanced reducibility of CMA 915 is due to the presence of c0304 on the surface of CMA 915 which is present in much smaller amounts on other catalysts. By comparison CMA 115 shows no reducible cobalt present. The reduction behavior of CMA 515 and 715 are essentially the same as for CMA 315 (10% reducible Co) in Figure 6. It should be noted that under identical reduction conditions, bulk C0A1204is not reducible while bulk CoMo04 is easily reduced almost entirely to metallic cobalt as shown in Figure 6d. The spectrum of the Mo 3d5/2,3/2 doublet (Figure 6) for the reduced CMA catalysts is a convolution of peaks contributed by Mow and Mo4+(ref 4) due to the reduction of Mo6+ in the oxidic catalyst. The spectra for all three samples show the same degree of reduction. By deconvolving the Mo 3d envelope into the contributions due to Mob+and Mo4+,we determined the relative percentages of the 5+ and 4+ states to be 40 and 60% (f5%), respectively, for all catalysts studied. Catalyst MA-15 under
1
6
I
I
I
I
700
I
I
,
I
700
I
L
I
,
,
I
,
240
I
I
232
I
I
I
I
224
Binding Energy ( e V )
Figure 6. ESCA spectra of the Co 2p312 and Mo 3d~,2,3/2levels in reduced CMA catalysts. All samples were reduced in pure H2 for 6 h at 500 OC: (a) CMA 915, (b) CMA 715, (c) CMA 315, (d) CoMoO,.
TABLE 111: ESCA Binding Energies of Sulfided Catalysts and Reference Compoundsa
BE, eV Mo Mo sample co 2P312 3d512 3d3,, 0 1s S 2p CMA 115 (778.4),b 782.0 228.5 231.7 531.3 161.7 CMA 315 778.4, 781.9 228.6 231.7 531.4 161.6 CMA 915 778.3 228.6 231.7 531.3 161.7 MA 15 228.6 231.8 531.4 161.7 MoS, 228.6 231.8 161.7 778.3 161.7 COA Co(meta1) 778.1 a All values were measured with a precision of kO.15 eV. Binding energy assignment is uncertain due to low intensity of peak.
the same reduction conditions produces a Mo spectrum which is qualitatively the same as the Mo spectrum of Figure 6, Le., the relative percentages of Mo5+to Mo4+are ca. 40 and 60%, respectively. These results are in agreement with those found previously.4 It should also be noted that no elemental molybdenum is detected. The reduction behavior of Mo in CoMo04 is quite different, however. The distribution of lower oxidation states is clearly different, and deconvolution of the spectrum reveals the presence of elemental Mo. Sulfidation Studies. Table I11 lists the binding energies of completely sulfided CMA catalysts and relevant reference compounds. All CMA catalysts in the fully sulfided state produced nearly identical binding energies although the relative intensities of the peaks differ. Comparison of the binding energies of bulk Cogsgand metallic cobalt shows only a small shift between the two. In addition, the
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The Journal of Wysical Chemistty, Vol. 86, No. 16, 1982
TABLE IV: ESCA Intensity Ratios of Sulfided Catalystsa S 2p/Mo S 2p/Mo sample 3dsn 3/2 sample 3dbiz.w~b CMA 115 0.47 CMA 715 0.69 CMA 315 0.53 CMA 915 0.79 CMA 515 0.61 MA 15 0.46 All samples were sulfided for 1 h in 15%H,S/H, at All values measured with a relative standard 250 "C. deviation of + 10%.
Chin and Hercules
Mo S
3d's ond 2 s Level
In
-
c
C
a
E
e-
c
5
r
L .-
VI
.d '
(a 1
C
W
L
C Y
W
.-c
0
W
a
l
I
I
I
1
I
l
I
I
2 40 232 224 Binding Energy ( e V )
Flgure 8. ESCA spectra of Mo 3d doublet and S 2s region of sulflded catalysts. All samples were reacted with a 15% H,S/H, mixture for 1 h a t 250 OC: (a) MA-15, (b) CMA 315, (c) CMA 515, (d) CMA 715, (e) CMA 915.
l
l
l
l
l
l
l
l
l
790 784 778 Binding Energy (eV i
Figure 7. ESCA spectra of Co 2p,, region of sulfided CMA catalysts. All samples were reacted wlth a 15% H,S/H, mlxture for 1 h at 250 'C: (a) CMA 315, (b) CMA 515, (c) CMA 715, (d) CMA 915.
peak shapes of the Co 2p,/, lines are the same in both cases. Hence, distinction between the two is not possible by comparison of ESCA binding energies and/or peak shapes. The S/Mo ESCA intensity ratios of partially sulfided CMA catalysts and MA-15 are tabulated in Table IV. All samples were sulfided in a 15% H2S/H2mixture for 1 h at 250 "C. So that differences in the S/Mo intensity ratios among the catalysts could be detected it was necessary to partially sulfide the catalysts. In the fully sulfided state the majority of the sulfur signal would originate from MoS2 and the contribution due to the cobalt sulfide (if present) would be overshadowed particularly for catalysts having low cobalt loadings. The ESCA spectra of the Co 2~312and Mo 3d5 2,312 lines of the partially sulfided catalysts are shown in kigures 7 and 8, respectively. Figure 8 also contains the S 2s peak (-226 eV) which overlaps with the Mo 3d envelope. From Table IV it is clear that the S/Mo intensity ratio increases with increasing cobalt content. Note that in all samples studied, the Mo concentration is fixed at 15% MooBby weight. Furthermore, the intensity of the Co peak at ca. 778 eV increases as a function of the cobalt content. This is shown in Figure 7. The Mo spectra in Figure 8 indicate that sflidation is not complete and the observed spectrum is a convolution of different chemical states. It is apparent, however, that the Mo spectra of all catalysts are virtually identical. Differences do exist in the intensities of the S
2s peak at ca. 225 eV. An increase in the intensity of this peak is observed as the Co content increases. It is clear from Figure 8 that Co has little or no effect on the extent of Mo sulfidation. Thus, the increase in the S/Mo intensity must be due to sulfiding of the cobalt since a constant S/Mo intensity ratio would result if there was no sulfur uptake by cobalt. This is further substantiated by the increase in intensity of the S 2s peak as the cobalt concentration is increased. Cobalt reference compounds were sulfided and compared to CMA catalysts. CoMo04was sulfided for 1 h at 400 "C and CoA1204was sulfided for 9 h at 400 "C. After sulfidation the cobalt of CoMo04 is predominantly in a reduced state. A small peak is seen at ca. 781 eV and is associated with the principal peak; it corresponds to an electron energy loss peak. The Mo spectrum confirms the presence of Cogsgby an intense S 2s peak at ca. 225 eV. The position of the Mo 3d doublet correlates with the binding energy of MoS2. In contrast, sulfidation of CoAZO4 at 400 "C for 4 h produced no change in the ESCA spectrum. Sulfidation for 9 h produced a very weak peak at 778.4 eV. The sulfur uptake as a function of sulfiding time has been established by ESCA for CMA 315. The results are presented in Figure 9. The plot indicates that rapid sulfur uptake occurs immediately on exposure of the catalyst to H2S as measured by the rate of change in the S/Mo intensity ratio. After reaction for 60 min, the rate of change of the S/Mo intensity ratio decreases. The results of this study are in good agreement with previous results obtained on catalysts corresponding to MA-4 and MA-1tiZ1 However, in that study no net sulfur uptake was observed after 60 min of sulfiding. This suggests that the slow increase in the S/Mo intensity ratios for CMA catalysts may be the result of sulfur uptake by the cobalt. If this postulate is (21) D. S.Zingg, Ph.D. Thesis, University of Pittsburgh, 1979.
Characterization of Co/Mo/A1,03 Catalysts
The Journal of Physlcal Chemistry, Vol. 86, No. 16, 1982 3085
0.6 -
n
-
If
r"
m \ 0.41:
Y
OO
60
I20
I80
il
1 240
Sulfiding Time ( m i n ) Flgure 9. Sulfur uptake of CMA 315 as a function sulfiding time. Reactions were carried out in 15% H,S/H, at 400 OC.
correct, the additional sulfur uptake should be reflected in the Co ESCA spectra. The Co 2p3/2 ESCA spectra of CMA 315 are shown in Figure 10 after sulfidation in 15% H2H/H2for 1 , 4 , and 14 h a t 400 "C. It is clear that after 1 h of reaction, maximum sulfidation of Co is not reached as evidenced by the increase of the Co peak at ca. 778 eV after an additional 3 h of reaction (4 h total). Increasing the reaction time from 4 to 14 h results in a small but detectable increase in the intensity of the peak corresponding to C G B . Thus, it appears that virtually complete sulfidation of the catalyst is accomplished after 4 h of reaction. I t is also important to note that a majority of the cobalt is sulfidable, unlike the cobalt in CoA1204. As stated previously sulfidation of the Mo species is essentially complete after 1 h at 400 "C.
Discussion The nature of the species present on the surfaces of CMA catalysts is controlled by many variables, particularly the method of preparation. This factor more than any other is probably responsible for the contradicting interpretations reported in the literature. For example, the species formed on catalysts prepared by incipient wetness will be determined by the order in which the Co and Mo are i m ~ r e g n a t e d . ~ The catalysts in the present study were prepared by impregnation of Mo to incipient wetness followed by calcining and subsequent impregnation of Co. From the ESCA results (Table I1 and Figure 1) it is clear that the dominant Co species is tetrahedrally coordinated to oxygen. These resulb are substantiated by the PAS studies (Figures 2 and 3). The existence of Co2+tetrahedrally coordinated as the dominant Co species precludes the presence of bulk-phase CoMoO, (Co2+ octahedrally coordinated as the major component on the catalyst. Under certain circumstances, the formation of CoMo04 has been found to occur on CMA catalysts. For example, at high Co and Mo loadings where bulk oxides (cos04 and MOO,) are likely to be formed, solid-state reaction between the two oxides may produce a CoMoOl phase? In addition, simultaneous impregnation of Co and Mo produces C O M O O ~In. ~the present study the quantity of Mo on the catalysts (15% MOO,) is below monolayer coverage (ca. 20% MOO,); as demonstrated previously, Moos formation does not O C C W . ~ However, it is quite possible that a small amount of octahedrally coordinated Co2+is present which causes formation of CoM004.17 I t is generally accepted, however, that for the
I
I
l
I
I
I
I
1
I
I
790 782 7 74 Binding Energy ( e V ) Flgure 10. ESCA spectra of Co 2p,,, level for CMA 315 after suifidation for various times in 15% H,S/H, at 400 OC: (a) 1 h, (b) 4 h, (c) 14 h.
preparation method and concentrations of Co and Mo employed in this study formation of large quantities of CoMo04 is ~ n l i k e l y . ~ v ~Because * ~ ~ - ~ the ~ photoelectron peaks and optical absorption bands of octahedral Co2+ overlap with those of tetrahedral Co2+,the presence of a small quantity of CoMoOl could go undetected. Based on the binding energy data of Table 111, it is difficult to define the oxidation state of cobalt on the sulfided catalysts. A binding energy of 778.4 eV corresponds closely to that of Cogs8 (778.3 eV). However, formation of Cogs8 on sulfided CMA catalysts has not gained widespread acceptance and is presently the subject of controversy. For example, Brinen and Armstr~ng~ have suggested from ESCA data that the cobalt present on fully sulfided catalysts is in the metallic state. In direct contrast Declerck-Grimee et a1.6 determined that Cogs8is formed based on the difference in binding energies between metallic cobalt and Cogs8,which they determined to be 2.3 eV. However, this conclusion must be considered tenuous since an energy separation between metallic cobalt and Cogs8of 2.3 eV is much larger than all other values reported in the literature. For example, an energy separation of only 0.2 eV is observed in the present study (see Table 111). The discrepancy is probably due to error in binding energy referencing. Nevertheless, the exact identity of the cobalt species on the sulfided catalyst is still controversial. The chemical state of Mo is much less ambiguous. The binding energies of the Mo 3d5,,,,, doublet of the sulfided CMA catalysts are consistent with that of bulk MoS* This assignment is generally accepted and has been confirmed by a variety of t e c h n i q u e ~ . ~ s ~ ~ - ~ ~ (22)H.Ueda and N. Todo, J. Catal., 21, 281 (1972). (23)P.Ratnasamy and H. Knozinger, J. Catal., 54, 155 (1978). (24)V.H.deBeer and G. C. A. Schuitt, "Preparationof Catalysts",B. Delmon, P. A. Jacobs, and G. Poncelet, Ed., Elsevier, New York, 1976, p 343. (25)J. S. Brinen and W. D. Armstrong, J. Catal., 54, 57 (1978). (26)K.S.Chung and F. E. Massoth, J. Catal., 64, 320 (1980).
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The Journal of Physlcal Chemistry, Vol. 86, No. 16, 1982
The exact identity(ies) of the Cc2+tetrahedral species present on the oxidic CMA catalysts may be derived from comparison with the C0/A1203system.z6a It is apparent that an interaction species resulting from the diffusion of Co2+into tetrahedral interstices of the support (Co-t) is formed. This species was found in the earlier study to be chemically inert toward hydrogen reduction and accounts in part for the difficulty in reduction of the Co species on the catalyst. Co-t was determined to be chemically and physicglly analogous to C0A1204and hence, CoA1204was used to model this species. However, it was found that Co-t is not the only Co species present on the CMA catalysts. For all catalysts studied (except CMA 115) a measurable quantity of reducible cobalt is evident on the catalyst upon hydrogen reduction. Under the same reaction conditions CoA1204is nonreducible. The nonreducibility of the cobalt on CMA 115 is thus attributable to a high percentage (relative to total cobalt) of Co-t. The sulfidation experiments on the CMA catalysts further illustrate the nonreactivity of Co-t and establish that it is not the major cobalt species present. It is clear that the reactivity of CoA1204toward H2S sulfidation is low. Similar results were also obtained for CMA 115 where a high percentage of Co-t is present. Unlike C0A1204,CMA 315-915 are highly reactive to H$. (Figure 7, c and d). For the completely sulfided CMA 315 catalyst (Figure 1Oc) the percentage of nonsulfidable cobalt constitutes approximately 35% of the total cobalt or ca. 1% COOby weight. For catalysts having higher cobalt loadings, the percentage of the total cobalt existing as Co-t decreases, and the absolute amount remains constant at about 1%. Orientation of the Co ions relative to Mo ions may be deduced from the ISS studies. The initial scan of the surface of CMA catalysts indicates a near absence of Co. With additional sputtering of the surface an increase in the Co signal is observed relative to Mo (Figure 5). This behavior is exactly contrary to that of CoMo04 where an increase in the Mo signal relative to Co is observed (Figure 4). This observation provides further support for the absence (or low concentration) of CoMo04 on the oxidic catalyst. The ISS results suggest that the Co ions are situated beneath the Mo ions and that the dispersion of Mo is high. This was based on comparing the Mo/Al peak height intensity ratios of MA-15 and the CMA catalysts. Only slight decreases in Mo dispersion are observed by the addition of up to 7% COO. Above 7% COO(i.e., CMA 915) a large decrease in surface Mo concentration results presumably due to scattering from surface Co304 which "shadows" the underlying Mo. From the earlier study of Co/A1203~ a t a l y s t sit, ~was ~ observed by ISS that scattering from tetrahedrally coordinated Co ions (Co-t) was suppressed relative to octahedrally coordinated Co ions (Co-0). This was presumably due to greater shielding of Co-t from the ion beam due to its tetrahedral configuration. An analogous situation is seen in the CMA catalysts where the cobalt scattering signal is very weak relative to scattering from other atoms. This observation is in support of the ESCA data and PAS data which indicate that the majority of Co ions are tetrahedrally coordinated. Recently, Delmon et al.30 have studied the surface structure of Co/Mo/A1203 catalysts using ISS and have (27)Y.Okamoto, H. Tomioka, Y. Katoh, T. Imanaka, and S.Teranishi, J . Phyu. Chem., 84,1833 (1980). (28)S. S. Pollack, L. E. Makovsky, and F. R. Brown, J. Catal., 59,452 (1979). (29)R. L.Chin and D. M. Hercules, J. Phyu. Chem., 86, 360 (1982). (30)F. Delannay, E.N. Haeussler, and B. Delmon, J. Catal., 66,469 (1980).
Chin and Hercules
reached the same conclusions as those presented here. However, the agreement between the two studies may be fortuitous because Delmon et al. used high current densities, 5 X lo* A/cm2. Under such conditions (comparable to dynamic SIMS) the catalyst surface layer would be destroyed in a few seconds. In a previous study from this laboratory4 it was determined that the surface characteristics of Mo/Al203 catalysts depend on Mo loading and calcination temperature. It is well established for the catalysts employed in this study that at 15% Moos the Mo is dispersed as a monolayer with both tetrahedrally and octahedrally coordinated Mo6+p r e ~ e n t . ' The ~ ~ approximate ~ ~ ~ ~ ~ ~percentages ~ ~ of octahedral Mo (Mo-o) and tetrahedral Mo (Mo-t) are 60 and 40%, re~pectively.~ It was also determined that reduction of Mo6+to Mo4+was due to Mo-o and that Mo-t was reducible only to the 5+ state. Hydrogen reduction experiments and H2Ssflidation experiments indicate that the presence of cobalt has little or no effect on the reducibility of the Mo species present on CMA catalysts. This holds true independent of the cobalt loading. This observation is in agreement with the recent results of Chung and Massoth.26 Because the Mo 3d spectra are identical for reduced and sulfided MA-15 and the CMA catalysts (Figure 6) the mechanism for reaction with H2 and H2S must be the same for both systems. Since the addition of Co does not alter the reduction behavior of the Mo species, it can be concluded that the percentages of Mo-o and Mo-t in the CMA catalysts must be the same as in MA-15. This observation coupled with the ISS results suggests that the Mo monolayer remains intact in the presence of cobalt. It is evident that in many ways the characteristics of CMA catalysts resemble a combination of Co/Alz03and Mo/A1203catalysts acting independently. For example, it was concluded that metal-support interactions produce Co-t which is present as a subsurface constituent. The quantity of Co-t formed is fairly low (ca. 1% COOtotal) as in the Co/Alz03catalysts. Thus, it is presumed that the factors which govern its formation are the same as in the Co/A1203system (e.g., metal loading and calcination temperature). In regard to the Mo species present on CMA catalysts, monolayer dispersion is clearly evident as in unpromoted Mo/A120, catalysts. This conclusion is based on the ISS results which indicate a high Mo dispersion at all cobalt loadings up to 7% COO. In addition reduction experiments have shown that the distribution of Mo-o and Mo-t are identical in both CMA and MA-15 catalysts as evidenced by similar reduction behavior of the Mo species. Although similarities exist among CMA, C0/Alz03, and Mo/A1203 catalysts, differences are also present. For C0/A120, catalysts, the distribution of Co species depends on metal loading at a fixed calcination temperature. At low metal loadings the interaction species designated Co-t and Co-o predominate. As the Co concentration increases c0304 segregation occurs and is detectable for catalysts with as little as 2.5% COOby X-ray diffraction (Le., detection limit of the instrument employed). However, in CMA catalysts a c0@4 phase is not observed until ca. 7% COO. Thus, it is apparent that a cobalt species not found in the C0/A1203catalysts is formed on CMA catalysts and that this species must be related to the presence of Mo. Since it was established that metal-support interactions were very similar in both CMA and C0/A1203catalysts and that the interaction species (Co-t) constitutes ca. 1%COO by weight of the total cobalt, the formation of this new (31)M. Ldacono. J. L. Verbeek, and G. C. A. Schuitt, J.Catal., 29, 463 (1973). (32)F.E.Massoth, Adu. Catal., 27, 265 (1978).
Characterization of Co/Mo/A120, Catalysts
species must occur for cobalt concentrations between 1and 7% COO. From ESCA and PAS results, it was determined that tetrahedrally coordinated Co2+constitutes the major cobalt species up to 7% COO. This is in part due to Co-t and also due to this new Co species which will be designated Co-M. Co-M is presumably a species formed by the interaction of Co and Mo since it is observed only in the CMA system. The important point is that it has tetrahedral coordination. The chemical and physical nature of Co-M may be deduced by combining information from the various spectroscopic techniques. It is quite clear that Co-M involves tetrahedrally coordinated Co2+ since it was shown by ESCA and PAS to be the dominant Co species up to 7% COO. As shown by ISS, these Co ions occupy sites below the Mo monolayer. However, this species is unlike Co-t and can be differentiated from Co-t because of differences in reactivity to Hzand Ha.Since the nonreactivity of Co-t arises because of a strong interaction between Co ions and the support, Co-M must interact with -pAlz03to a much lesser extent and it is doubtful that this interaction involves diffusion of ions into lattice sites of the support. It is also clear that Co-M does not resemble CoMo04in which Co2+ions are octahedrally coordinated. No evidence could be found to suggest the presence of octahedrally coordinated cobalt, although the presence of small quantities of this species cannot be excluded. In addition, the relative surface concentrations of Co and Mo were much different between CoMo04 and CMA catalysts as shown by ISS analysis. Calculations based on a catalyst with a Mo concentration of 15% MOO, by weight indicate that approximately 7% COO is needed to produce a 1:l stoichiometric Co-Mo compound. This calculation neglects the formation of Co-t which is formed only in small quantities. Since a new Co phase appears above 7% COOin the catalysts studied, it is reasonable to conclude that Co-M involves a nearly 1:l stoichiometry between Co and Mo ions. This implies that the manner in which Co-M is situated below the Mo monolayer is not totally random but involves a definite site preference. The Mo monolayer is visualized as being made up of Mo-o and Mo-t. The Co-M structure may be envisioned as a two-dimensional layer of cobalt ions lying between the y-Alz03surface and the Mo monolayer. The cobalt ions are in tetrahedral coordination with oxygen atoms from both the support and the "molybdate" ions. An independent study of Co/Mo/Al,O, catalysts in the same range of concentrations has been carried out by Makovsky et ala3 This study used primarily data obtained from laser Raman spectroscopy (LRS) and X-ray diffraction (XRD), supported by ESCA and ISS. The catalysts studied by Makovsky et al. were prepared similarly to the catalysts used in our work, except that the alumina was dried at 500 "C before impregnation; the alumina was then impregnated and calcined a t 500 "C. The catalysts in our work were impregnated, dried at 110 "C, and then calcined at 500 "C. Results from the two studies are in agreement except for one important difference. Makovsky et al. present both XRD and LRS data which indicate the presence of CoMoOl on their catalysts at loadings as low as 2% COO (15% MOO,); the amount of CoMo04 increases as the Co content of the catalysts increases. To the contrary, our data do not detect a separate CoMo04 phase. Although one can rationalize the LRS data as not requiring long-range order (and thus consistent with Co-M), the XRD data do support strongly a separate (33) L. E.Makovsky, J. M. Stencel, F. R. Brown, R. E. Tischer, and S. S. Pollack, J . Phys. Chem., submitted for publication.
The Journal of Physical Chemistry, Vol. 86, No. 16, 1982 3087
TABLE V: Composition of CMA Catalysts Expressed aa the Atom Ratio Co/Co+Mo ( r )
catalysts
r
catalvsts
r
CMA 115 CMA 315 CMA 515
0.11 0.25 0.39
CMA 715 CMA 915
0.48 0.54
CoMo04 phase. Thus, it appears that the method of catalyst preparation is very crucial. It seems astonishing that a difference in drying conditions can have such a profound effect on catalyst composition. Confirmation of this explanation is currently under investigation. Recently a model describing the interaction of Co and Mo was proposed by Gajardo, Grange, and Delmon (GGD).l' Employing ESCA and diffuse reflectance spectroscopy (DRS) they postulated the existence of five different species on the surfaces of CMA catalysts: Mo in polymolybdate bulklike MOO,, Mo monolayer, a Co-Mo bilayer, Co304,and Co-t. The formation of these species was found to depend on the loadings of both Go and Mo. In order to make comparisons between the catalysts employed in this study and those of GGD, we will designate the catalysts by a value r where r is the atom ratio of Co to the total metal content, Le., Co/Co+Mo. This nomenclature is used by GGD. The r values of the catalysts employed in this study are tabulated in Table V. Before comparisons can be made between the present study and that of GGD several comments are in order. The calcination temperature of the catalysts studied by GGD was not stated. The calcination temperature will have a great effect on the species formed and their relative amounts. Whereas the Mo concentration was held constant at 15% MOO, for all catalysts in the present study, the total metal content (i.e., Co + Mo) was held constant at 15% in GGD's catalysts. The consequence of employing a constant total metal content is that, as the concentration of one component is varied, the other will in turn vary. It has been shown that the distribution of Mo species is greatly affected by changes in Mo ~ o n t e n t .Hence, ~ it is expected that the surface characteristics of the catalyst will vary as a function of both Co and Mo, whereas in the present study, changes in surface composition will be directly related to changes in the Co content. The model of GGD postulated formation of the five species mentioned above. They concluded that the dispersion of cobalt was very high. As a consequence of this high dispersion, there is an enhancement in the formation of Co-t. The Co-Mo bilayer is visualized as forming by the interaction of Co ions with the Mo monolayer and that it is produced on the Co-t. In this model the cobalt is situated above the Mo monolayer and involved octahedrally coordinated Co3+. The model proposed in the present study and that proposed by GGD are in conflict on several points. Although it is agreed the Co-Mo interactions produce a dual layer structure consisting of the Mo monolayer and Co ions, the location of the Co ions relative to Mo is not in agreement. Additionally, differences exist as to the extent of Co-t formation and the oxidation state of the Co ions comprising the dual layer structure. Estimate of the Co dispersion by GGD was based on measurement of ESCA intensity ratios. While this is a legitimate method of comparing relative dispersions, good quality ESCA spectra (i.e., good signal-to-noise)are needed to obtain accurate intensity ratios. As stated by GGD, contamination due to carbon was a problem in obtaining spectra presumably due to poor vacuum conditions of the preparation chamber (lo* torr). Consequently, data ac-
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quisition time was limited to a minimum needed to obtain decomposable spectra. This may introduce errors in the measured intensities. Although increases in the Co/Al+Mo+Co intensity ratios were observed as the cobalt content was increased, this is expected since the quantity of Co present is increasing. Comparisons by GGD showed that the measured Co intensity ratios for CMA catalysts with r 1 0.25 was greater than for a Co/A1203catalyst of 15% Co304although the total cobalt is less for the CMA catalysts. This supports the proposal that Co forms a two-dimensional layered structure since a majority of the Co ions will be detectable by ESCA whereas the formation of Co304 three-dimensional clusters on the Co/A1203 catalyst will render only a fraction of the cobalt susceptible to detection. However, it is not possible to infer the location of the c o ions based on these data. Measurement of relative dispersion by ISS is a better method because the inherent surface sensitivity of ISS is much greater than that of ESCA. Thus, their proposal that Co ions occupy the top position in the dual layer structure is open to serious question. In regard to the oxidation state of Co, the results which GGD obtained by ESCA and DRS were not in agreement, a situation acknowledged by the authors. They were unable to detect Co3+ by ESCA which was thought to be present based on DRS data. The absence of a Co3+signal in the ESCA spectra was explained by the reduction of Co3+to Co2+under exposure to the X-ray beam. Although this reduction has been shown to occur for certain bulk compounds, there is no evidence that this occurs on catalysts. In fact, in the previous study Co3+was detected on C0/A1203catalysts.3o Moreover, there was no indication of photoreduction in their analysis of bulk Co304nor in their catalysts of high cobalt loading where Co304is a major component. Thus, it is unlikely that photoreduction is occurring on the surface of their catalysts. However, small quantities of Co3+may be present which are not in quantities detectable by ESCA. Finally, the extent of Co-t formation as proposed by GGD is not in agreement with the present results. Their conclusion that Co-t formation is extensive was derived from ESCA and DRS results. It should be noted that positive identification of Co-t by the methods they used is not possible. Both techniques only provide information about the oxidation state and coordination of the Co ions. However, as shown in the present study, the Co-Mo interaction species (Co-M) is similar to Co-t with respect to oxidation state and coordination. Distinction between Co-t and Co-M can be based only on differences in chemical reactivity of these two species. Since GGD did not perform combined ESCA-reduction studies on their catalysts, it is very likely that Co-M was mistaken for Co-t. Although the conclusions of GGD and the present study differ in some respects, similarities in results should also be noted. It was observed that the formation of Co304did not occur in the catalysts of GGD until r became greater than 0.50, Le., the Co/Mo atom ratio exceeds unity. From Table I this corresponds to a cobalt concentration of greater than 7% COO(CMA 715) of the present study. For CMA 715 a small amount of Co304 is observed on the catalyst whereas increasing the cobalt concentration to CMA 915 produces Co304in readily detectable quantities. It is worthy to note that the formation of c0304 appears to depend only on the value of r and not on the absolute concentration of Co and Mo since they differed in the two studies. This is true provided that monolayer coverage of Mo is not exceeded. Above monolayer coverage (ca. 20% Moo3) bulklike CoMo04 is formed from the solid-state
Chin and Hercules
reaction of Moo3 and C O ~ O From ~ . ~ these results, the postulate that the dual layer structure comprised of the Mo monolayer and Co2+in a nearly 1:l stoichiometry is supported. The observation of intense absorption bands due to Co2+in tetrahedral coordination by DRS agrees well with the ESCA and PAS results presented above. More importantly, the existence of a dual layer Co-Mo structure is in mutual agreement. Recent investigations by Chung and M a s s ~ t hon ~~,~~ Co/A1203and CMA catalysts are in agreement with the results presented here and in a previous It was observed by these workers that Co304formation on Co/ A1203catalysts occurred at relatively low levels of cobalt (
