Pretreatment of CoIAI,O,
and Mo/AI,O,
Catalysts
The Journal of Physical Chemistry, Vol. 82, No. 8, 1978 885
one hand, and aqueous solutions on the other hand in the sense discussed above. In the entire paper we have stressed the case of uery dilute solutions. In fact this is the most common case dealt with in practical examples. However, it is possible to extend the conclusion of this paper to more concentrated solutions in which case care must be exercised to specify precisely which process is referred to. As an example, if we have a mixture of N , water molecules and N , solute molecules, we can still talk about the HI between one pair of solute molecules in this system. Here the relevant change in the structure of the water is induced by the process of bringing this pair of solute molecules from infinite separation to a close distance. This SCIS must be clearly distinguished from the overall change in the structure from pure water to the mixture under consideration. With this comment in mind we can extend all the conclusions of this paper to more concentrated solutions. However, for all practical purpose it is sufficient to use the standard thermodynamic quantities defined in the limit of dilute solutions.
Molecular Theory”, Plenum Press, New York, N.Y., 1974. R. A. Horne, Ed., “Water and Aqueous Solutions, Structure, Thermodynamics and Transport Processes”,Wlley-Interscience, New York, N.Y., 1972. F. Franks, Ed., ”Water, A Comprehensive Treatise”, Volumes 11, 111, and IV, Plenum Press, New York, N.Y., 1973-1975. H. S. Frank and M. W. Evans, J . Chem. Phys., 13, 507 (1945). G. Nemethy and H. A. Scheraga, J. Phys. Chem., 66, 1773 (1962). W. Kauzmann, Adv. Protein Chem., 14, 1 (1959). A. Ben-Naim, J . Phys. Chem., 79, 1268 (1975). (a) S. Marcelja, D. J. Mitchell, B. W. Nlnham, and M. J. Sculley, J . Chem. Soc., Faraday Trans. 1 , 73, 630 (1977). (b) A similar claim has recently been presented by D. Oakenfull and D. W. Fenwlck, Austr. J . Chem., 30, 741 (1977). In this paper it Is stated that the concluslon of ref 1 is “not entirely correct” and, continuing, that “It is, in any case, hard to see that the major part of the free energy of hydrophobic Interaction could be derived other than from a rearrangement of water molecules”. In another place in that paper il is stated (wtthout proof or any supporting evidence) that “Hydrophobic interaction is the effect of a net increase in the structure of the solvent”. In ref 1 and 2 the present author presented some qualitative arguments showing that the process associated with hydrophobic Interaction causes a net decrease In the structure of water. In the present paper It Is shown that whatever the change in the structure of water, this change has no effect on the strength of the hydrophobic Interaction. T. L. Hill, “Introduction to Statistical Thermodynamlcs”, AddisonWesley, Reading, Mass., 1960. A. Ben-Naim, J. Chem. Phys., 63, 2064 (1975). H. S. Frank and A. S. Quist, J . Chem. Phys., 34, 604 (1961). V. A. Mikhailov, Zh. Strukt. Khim., 2 , 677 (1961). A. Ben-Naim, Isr. J . Chem., 2 , 278 (1964). A. Ben-Naim, J . Phys. Chem., 69, 3240 (1965). R. Lumry and S. Rajender, Biopolymers, 9, 1125 (1970). R. A. Robinson and R. H. Stokes, “Electrolyte Solutlons”, Butterworths, London, 1954. B. E. Conway and J. O’McBockris in “Modern Aspects of Electrochemistry”, Vol. I,J. O’McBockrls and B. E. Conway, Ed., Butterworths, London, 1954, p 47. D. Eisenberg and W. Kauzmann, “The Structure and Properties of Water”, Oxford University Press, London, 1969. See, for example, F. Franks, ref 4, Vol. IV, Chapter I.
Acknowledgment. The author is very grateful to Dr. S. MarEelja for sending a preprint of his paper (ref 9) before its publication, Revision of the paper was made while the author was a visiting professor at the H. C. Qlrsted Institute, University of Copenhagen, Denmark. The hospitality of the members of the institute during this period is gratefully acknowledged.
References and Notes (1) A. Ben-Naim, Biopolymers, 14, 1337 (1975). (2) A. Ben-Naim, “Water and Aqueous Solutions, An Introduction to a
Influence of Reducing and Sulfiding Treatments on Co/AI2O3 and Mo/A1203 Catalysts. An X-Ray Photoelectron Spectroscopy Study R. I. Declerck-Grimee, P. Caneson,* R.
M. Friedman,+and J. J. Fripiat
Groupe de Physico-Chimie Minerale et de Catalyse, Universit.4 Cathoiique de Louvain, 1, Place Crolxdu-Sud, B 1348 Louvain-la-Neuve, Belgium (Received October 1, 1976; Revised Manuscrlpt Received February 6, 1978) Publication cost assisted by Universit6 Cathoiique de Louvain
X-ray photoelectron spectroscopy (XPS) has been used to investigate the phase transformations induced by various pretreatments of Co/A1203and Mo/A1203. Upon sulfiding by H2-H2S,Co2+is partly reduced to Coo while the Mo6+species is completely transformed into MoS2. The behavior of molybdenum is similar to that observed in Co-Mo/A1203hydrodesulfurization catalysts. In contrast, cobalt gives Cooin Co/A1203systems whereas Cogss is observed for HDS catalysts. A nonreducible alumina bound Co is also observed in both the presence and absence of molybdenum. In the oxide form, molybdenum species are in strong interaction with the support but this interaction does not exist after sulfiding.
Introduction Hydrodesulfurization and hydrotreatment have become more and more important in the petroleum industry. These processes currently use catalysts consisting of molybdenum deposited on alumina together with a pro* Present address: C.N.R.S.-C.R.S.O.C.I.lB,rue de la FBrollerie, 45045 Orleans Cedex, France.
Permanent address: Corporate Research Department, Monsanto, 800 N. Lindberg Boulevard, St. Louis, Mo. 63166. 0022-365417812082-0885$01 .OO/O
motor: cobalt or nickel. Various hypotheses have been put forward to explain the role of promotors on the active pha~e.l-~ As manufactured hydrodesulfurization catalysts are in the oxide form. Under working conditions (in the presence of H2 and H2Sin the gas phase), the initial oxides are not stable, and the first step of the reaction (usually conducted under special conditions) is a reduction with partial or total sulfiding of the ~ a t a l y s t .For ~ ~ a~ better understanding of the reaction mechanism of these catalysts, it is important to know the exact nature of the new 0 1978 American Chemical Society
886
The Journal of Physical Chemistry, Vol. 82, No.
TABLE I: Principal Characteristics of the
C o I Al,O,
Catalysts Studied
Catalyst co-1 Co-2a CO-2b Co-3 Co-4 Mo-1 Mo-2 Mo-3
Calcination temp, O C 500 500 700 500 500 500 500 500
Canesson et al.
8, 1978
wt % active phase
Surface area, ma g-’
2.48 4.41 4.41 7.17 13.92 8.95 15.11 21.67
208 199
4.41 %
190 169
surface species formed during this activation stage. In an earlier study, it has been shown that the transformations induced by various reducing or reducingsulfiding treatments on commercial hydrodesulfurization catalysts are very complex.6 From this study it was difficult to conclude to what extent the promoting agent interacts with the active phase. In order to further elucidate this point, it was useful to perform the same kind of study on the related, but more simple, Mo/A1203and Co/A1203systems. As in the previous study, the phase modifications were monitored by XPS since this technique is highly sensitive for specific elemental surface species. Experimental Section (1) Samples. Increasing amounts of cobalt oxide or molybdenum oxide were deposited on y-alumina (210 m2 g-’) powder (supplied by Labofina) by minimum solution impregnation, using cobalt nitrate or ammonium paramolybdate solutions. After drying and calcining in air for 16 h at 500 or 700 “C, the samples have the characteristics summarized in Table I. (2) Treatments. XPS measurements were made on the samples either in their oxide form or after one of the following treatments: (a) reducing with pure H2at 400 “C; (b) reducing-sulfiding with a mixture 85% H2S at 400 “C; (c) same treatment as b, followed by reaction with pure H2 at 400 “C. All the gases had a purity 199.95%. Hydrogen was obtained from Air Liquide and the H2-H2S mixture from Air Products. They were used as received, without any further purification. All these pretreatments under atmospheric pressure were carried out in a stainless steel reactor independent from the ESCA equipment; 400 mg of the ground sample (grain diameter 10.2 mm) was put in a 1/4-in. diameter stainless steel tube connected with two 1/4-1/8-in. reducing swagelok unions to two “quick connects double end shut off” valves which allowed isolation of the cell containing the catalyst and to avoid any contact of it with air or moisture. Two independent gas circuits were used. The gas flows were adjusted at 5 cm3 m i d with a Nupro SS 45 valve and measured with a rotameter. For all treatments, the temperature was linearly raised from ambiant to 400 “C in 24 h under a flow of the desired reactant gas. After further reaction for 20 h at 400 “C, the sample was cooled under a flow of gas. These time conditions were shown to be necessary in order to obtain a steady state for the surface composition of the catalysts. (3) XPS Measurements. The XPS measurements were performed using a commercial Vacuum Generators ESCA 2 system. The aluminum anode (hv = 1486.6 eV) was powered at 500 W (10 kV, 50 mA). An airtight glove box, connected to the sample preparation chamber of the machine, permitted introduction of the sample under a dry nitrogen atmosphere (oxygen content less than 0.5%). The tube with powder inside was
t-
Flgure 1. XPS profiles of the various treatments.
Co 2p,,,
llne of Co-Pa samples after
open by the two swagelok unions and then the catalyst was rapidly sprinkled on a double-sided adhesive tape previously outgassed at Torr in the sample preparation chamber of the machine. The overall operation did not take more than 5 min. A Tracor Northern NS 560 signal averager was used to improve the signal-to-noise ratio. In each measurement, an energy range of 20 eV was scanned. This energy interval was separated into 254 channels; measurements were made by repeated scanning of the interval with cumulated time per channel typically equal to 1 s for C 1s and A1 2s lines, 10 s for Mo 3d3/2,5/2and S 2p, and 110 s for the Co 2p region. Peaks were smoothed manually and decomposed by the “mirror” technique when asymmetrical. Intensity values are the planimetered surface areas. For correcting the binding energies from the charging effect, several methods are possible. Gold evaporation has been used for some catalysts;6in order to eliminate possible interference between the A1 2p peak issuing from the support and the 4f7/2,5 peaks issuing from gold excited by the Ka3v4x ray of the anode, the A1 2s peak has been chosen as a reference for the determination of binding energies. The binding energy value of this level has been arbitrary fixed at 118.0 eV. It has been shown that the precision in binding energy value determinations is improved with this methods7 Results (1)Co/A1203Samples. Figure 1 illustrates the effect of the various treatments on the Co 2p,p level of the Co-2a sample (4.41% COO). In the oxide form, the Co 2p3 line is situated at 780.2 eV; a satellite peak, situated at 484.9 eV, is associated with the main peak. This satellite is well known for the oxides of cobalt. It is a consequence of a shake-up phenomenon and characteristic of Co2+ions in a high spin state.8 The initial line will be called Co-I and its satellite peak Co-1’. A reducing treatment under H2 induces a shift of 0.7 eV of the main Co 2p3/, line (Go-I) toward higher binding energy and shoulder appears near 776.4 eV on the low binding energy side of the Go-I peak. The new line cor-
Pretreatment of Co/AI,O, Oxide
and Mo/AI,O,
Catalysts
The Journal of Physical Chemistry, Vol. 82, No. 8, 1978 887
H2IH2S
Mo 3 d levels
MOC O/ A I 2 0 3
MO/A1203
Moo3
I
I
I 230.5eV
2 2 7.5 CV 2 3 2 .IcV
Figure 3. XPS profiles of the Mo 3d line for various samples in oxide form and after suifidation.
776.4.V
Figure 2. Influence of the total amount of cobalt on the Go 2p3,, line.
TABLE 11: Intensity Ratios between the Main (Co-I) and the Satellite (Co-1') Co 2p3,2Lines for the Co/Al,O, Catalysts Studied Catalyst co-1
Co-2a CO-2b Co-3 Co-4
Calcination temp, " C
Int (Co-I)/ int(Co-1')
500 500 700 500 500
2 2.38 2.17 2.70 4.76
responding to the shoulder is noted Co-F. The reducing treatment brings about an increase of intensity of the satellite peak Co-I' with respect to that of the main peak Co-I; the intensity ratio between Co-I' and Co-I peaks increases about 15% after reduction. After treatment with H2-H2S, the shoulder Co-F near 776.4 eV increases in intensity and appears as the main peak in the spectrum, whereas an additional treatment by pure H2results in a shift of this Co-F line by 0.7 eV toward higher binding energy. The fact that the Co-F line appears as the main peak after sulfiding allows a good determination in the binding energy of this level. Figure 2 (left-hand part) shows the spectra of the Co 2p3/, levels for catalysts containing increasing amounts of deposited cobalt oxide. In the oxide form, as observed in Table 11, the intensity ratio and the distance between the main Co-I and the satellite Co-I' lines increases when the total amount of deposited cobalt increases. After sulfiding (treatment b) the intensity of the second line at 776.4 eV (Co-F) also depends on both the total quantity of cobalt and the calcination temperature (Figure 2, right-hand part). After this treatment, a very low intensity S 2p line situated at 159.8 eV is present for all the samples. (2) Mo/A1203 Catalysts. In the oxide form, the Mo 3d3/2,6/2 doublet lines are situated at 234.9 and 232.0 eV; the resolution of the doublet is poorer than for bulk MoOg
(Figure 3). After sulfiding by H2-H2S, the molybdenum lines shift about 4 eV to lower binding energies. S 2s line appears as a shoulder on the lower binding6energyside of the Mo 3d512line. Another shoulder is also observable on the higher binding energy side of the Mo 3d312line; we shall discuss this line below.
Discussion (1)Co/A1203.The Co 2p3/2 line of Cog+ions is situated on the lower binding energy side very close to that of CO~+,~JO but the Co3+ions do not have any satellite lines. This fact allows a differentiation between Co2+and Co3+. In the oxide form, the progressive increase of the principal line (Co-I) with respect to the satellite (Co-1') with increasing cobalt content can be understood as corresponding to the formation of Co3+ions. These Co3+ions probably belong to a Co304 phase on the surface, as has been previously observed,ll rather than to Co2O3.lo The formation of Co3+ ions is further supported by the observations made after a treatment by H2 of sample Co-2a. This treatment brings about a shift of the main Co 2p3/2 peak to slightly higher binding energy and the ratio between the satellite and the main line intensities increases. These effects are best explained by the reduction of Co3+ to Co2+ions rather than by a shift of the Co 2p3/2 line12 since it has been shown that a C 1s reference is inadequate for this kind of ~ata1ysts.l~ It seems that the shoulder observed at 776.4 eV (Co-F) can be attributed to species reduced to a lower valency state than 2+. The binding energy value is close to that observed for Coo of a metallic-cobalt foil and reduction to metallic cobalt has been already observed for supported cobalt on alumina.ll This reduced species, therefore, corresponds to superficial metallic cobalt. A sulfiding treatment by an H2-H2S mixture (treatment b) results in greater formation of Co-F with respect to Co-I species than H2 reduction alone (treatment a). Sulfiding and reduction of CoMo/A1203commercial HDS catalysts also results in a splitting of the XPS cobalt spectrum into two lines,6 but in the latter case, the Co-F peak is at 778.7 eV. This value corresponds to the binding energy for cobalt in pure C O ~ S The & ~ difference ~ between the value observed in the present work in Co/A1203(776.4 eV) and in CoMo/A1203catalysts after sulfiding treatment indicates
888
The Journal of Physical Chemisfry, Vol. 82, No. 8, 1978
that the presence of molybdenum is a necessary condition for the sulfiding of cobalt. This interpretation is supported by direct studies of the reducing and sulfiding of unsupported species.15 In the absence of molybdenum, treatment by H2-H2S leads to a superficial metallic cobalt species in higher quantity than a simple reducing treatment by Hz. Figure 2 shows the influence of the sulfiding treatment when the total amount of cobalt increases. The reduction to the Co-F species is more significant for high cobalt content. Nevertheless, the reduction is never complete and for the same sample (Co-2, 4.41% Co) a preliminary calcination at 700 "C instead of 500 "C leads to a drastic decrease in the amount of reducible cobalt (Figure 2). Under the conditions used for the various treatments, a portion of the Co2+ions is not sensitive to H2 or H,-H,S; this part corresponds most probably to CoZt ions incorporated into tetrahedral sites of alumina, forming a cobalt pseudo-aluminate. When the calcination temperature is higher a larger proportion of cobalt ions diffuse into the alumina lattice; the residual cobalt ions remain on the surface carrier as COOor Co304. When the cobalt content increases, the amount of unbound cobalt increases, explaining the increase of Co3' (main Co-I line) in the oxide form; only this "free" cobalt, which is in weak interaction with the support, can be reduced by Hz or H2-H2S,to yield the Co-F line. (2) Mo-A1203 Samples. In the oxidic form, the binding energy of the Mo 3d levels is situated between those of pure Mo0316 and of molybdenum oxide in HDS catalysts.6J6 The shift of the binding energy toward higher values of the Mo 3d levels in CoMo/A1203catalysts can be explained by the existence of an electron transfer from Moo3 deposited on alumina to the carrier.17 The position observed in the present work for Mo/Alz03 samples suggests that electron transfer is less intense in the absence of cobalt. The Mo 3d doublet resolution is superior for CoMo/ A1203catalysts than for Mo/A1203. This fact might be explained by the preferential occupancy of tetrahedral sites by cobalt ions in HDS catalysts, thus molybdenum ions could occupy only octahedral sites. If only pure molybdenum oxide is present on the carrier, the Mo6' ions would occupy both tetrahedral and octahedral sites, leading to a possible broadening of the XPS spectrum. Extended x-ray absorption fine structure yields information on the local environment about individual elemental components of complex noncrystalline materials. Studies on the oxide form of alumina supported catalysts have shown an increase in the coordination number for molybdenum as a result of the presence of cobalt.18 After sulfiding, no residual line corresponding to molybdenum oxides, either Moo3 or Mooz, remains, even with the Mo-3 sample containing 21.7 wt. % Moo3. The shift of 4 eV to lower binding energy is a consequence of the reduction and sulfiding of Moo3 to MoS2;the Mo 3d levels are exactly at the same binding energy values as for pure MoS2,sulfided Mo/A1203,and sulfided CoMo/A120p
Canesson et al.
This indicates that, in contrast to molybdenum in the oxide form, there is no, or only a weak, interaction of molybdenum in the sulfided form with the carrier. This also suggests cobalt does not bring about any appreciable difference in the steady state of molybdenum after sulfiding. The shoulder in the high binding energy side of the Mo 3d3j2level cannot be attributed to any molybdenum species in an oxygen environment, since the binding energy value does not correspond to any of those reported for Moo2 and Moop The position of the shoulder indicates molybdenum ions in a higher valence state than in MoS2. As in the case of unsupported MoS2 containing a small amount of cobalt,lg this shoulder could be explained by the formation of small amounts of Mo5+ions in a sulfur environment, possibly oxysulfide species or even the S 2s line from a sulfate impurity.
Conclusion From the above results, it can be concluded that the presence of cobalt does not affect the behavior of molybdenum ions during the activation of HDS catalysts. This is not the case for cobalt ions; in presence of molybdenum, sulfiding transforms part of Co2+ions into Cogs8 whereas for cobalt alone deposited on alumina the same treatment leads to a reduction to Cooof some of the cobalt ions. Acknowledgment. The authors thank Professor B. Delmon for helpful discussions and the revision of the manuscript. One of us (R.I.D.G.) thanks I.R.S.I.A. for financial support. The provision of catalysts samples by Labofina is greatly appreciated. R.M.F. thanks F. W. Lytle for discussion of his EXAFS results prior to publication. References and Notes (1) G. C. A. Schuit and B. C. Gates, AIChEJ., 19, 417 (1973). (2) A. L. Farragher and P. Cossee, Cafal., Proc. Inf. Congr. 5fh, 1972, 1301 (1973). (3) B. Delmon and P. Grange, J. Less Common Met., 36, 353 (1974). (4) J. M. Zabala, P. Grange, and B. Delmon, C.R. Acad. Sci. Paris, Ser. C , 279, 725 (1974). (5) G. Hagenbach, Ph. Courty, and B. Delmon, J. Cafal., 31, 264 (1973). (6) R. M. Friedman, R. I. Declerck-Grimee, and J. J. Fripiat, J . Elecfron Specfrosc., 5, 437 (1974). (7) J. L. Ogilvie and A. Wolberg, Appl. Specfrosc., 26, 401 (1972). (8) D. C. Frost, A. Ishltanl, and C. A. McDowell, Mol. Phys., 24, 861 (1972). (9) J. Grimblot, A. D'Huysser, J. P. Bonnelle, and J. P. Beaufils, J. €/ecfron Specfrosc., 6, 71 (1975). (10)Y. Okamoto, H. Nakano, T. Imanaka, and S.Teranishi, Bull. Chem. SOC. Jpn., 48, 1163 (1975). I1 1) J. Grimblot and J. P. Bonnelle, J. Electron SDectrosc.. 8,437 (1976). (12j K. Hirokawa, F. Honda, and M. Oku, J . Electron. Spectrosc., 6, 333 (1975). (13) T. A. Patterson, J. C. Carver, D. E. Leyden, and D. M. Hercules, J . Phys. Chem., 80, 1700 (1976). (14)R. I. Declerck-Grimee,P. Canesson, R. M. Friedman, and J. J. Friplat, J . Phys. Chem., following article in this issue. (15) P. Canesson, B. Delmon, G. Delvaux, P. Grange, and J. M. Zabala, 6th International Congress on Catalysis, London, 1976,paper B. 32. (16) J. S. Brinen, J . Electron Spectrosc., 5,377 (1974). (17) A. Wolberg, J. L. Ogilvie, and J. F. Roth, J. Catal., 19, 85 (1970). (18) F. W. Lytle, private communicatlon. (19) P. Canesson and P. Grange, C.R. Acad. Sci. Paris, Ser. C , 281,
758 (1975).
