J . Phys. Chem. 1987,91, 1503-1508
1503
Surface Analysis of Alumina-Supported MOO,, NiO, and NiO-MOO, by Low Energy Ion Scattering Spectroscopy S. Kasztelan,* J. Grimblot, and J. P. Bonnelle Laboratoire de Catalyse H8t5rogZne et Homogsne, U.A. C.N.R.S.No. 402, UniversitZ des Sciences et Techniques de Lille Flandres-Artois, 59655 Villeneuve d'Ascq CZdex, France (Received: July 10, 1986; In Final Form: November 4, 1986)
A series of alumina-supported Moo3, NiO, and NiO-Mo03 catalysts with varying amount of Moo3 and/or NiO has been examined by low energy ion scattering spectroscopy. The evolution of the surface structure of a MoO3/AI2O3sample during the preparation and after the addition of the promoter (cobalt or nickel), as well as the effect of the calcination temperature, have been studied. The results, when compared with previous studies by X-ray photoelectron and laser Raman spectroscopy, confirm the description of the supported species as small oxomolybdenum entities, well dispersed and occupying only a small fraction of the support surface. A strong shielding effect of the Mo element by the promoter Co or Ni has been detected; this gives direct evidence of a chemical interaction between the promoter and the oxomolybdenum species. A description of this association as an isopolymolybdate salt of Co or Ni is proposed.
Introduction
TABLE I: Nomenclature and Composition of the Studied Samplesa
Angle-resolved low energy ion scattering spectroscopy (LEISS or ISS) is becoming a mature technique for the structural description of the adsorption of simple molecules on well-defined surfaces of single crystals. For practical surfaces, such as those of powdered catalytic materials, the angle-resolved facilities seem to be of little value whereas the information obtained from a simple LEISS investigation or by a depth profile analysis is of great interest. This interest results from the technique's high sensitivity to the top layer of the surface even on rough or high-surface-area materials. In the past 10 years, a significant effort has been made to investigate by LEISS the surface composition of heterogeneous catalysts belonging to the family of hydrotreating catalysts based on oxide phases such as Moo3, W03, COO, NiO, or a combination of them supported on a1~mina.I-I~ The characterization of these solids in the oxidic precursor, or in the reduced or sulfided form, has been largely improved by the use of surface spectroscopic technique^.'^ Among them, X-ray photoelectron spectroscopy (XPS) and laser Raman spectroscopy (LRS) have been shown to be c ~ m p l e m e n t a r y . ~ ~The ~ ~recent *'~ development of ISS gives a stimulating opportunity to improve the knowledge of these catalytic materials due to the recognized high surface sensitivity of this technique." In particular, it has been found by ISS that the Moo3, COO, and NiO phases are well ~~~
(1) Wu, M.; Chin, R. L.; Hercules, D. M. Spectrosc. Lett. 1978, 11,615. (2) Wu, M.; Hercules, D. M. J . Phys. Chem. 1979, 83, 2003. (3) Delannay, F.; Haeussler, E.; Delmon, B. J . Catal. 1980, 66, 469. (4) Knozinger, H.; Jeriorowski, H.; Taglauer, E. Proc. In!. Congr. Catal., 7th 1980, 604. ( 5 ) Z i n g , D. S.; Makowski, L. E.; Tisher, R. E.; Brown, F. R.; Hercules, D. M.J . Phys. Chem. 1980, 84, 2898. (6) Salvatti, L.; Makowski, L.; Stencel, J. M.; Brown, F. R. Hercules, D. M. J. Phys. Chem. 1981,85, 3700. (7) Canosa-Rodrigo, B.; Jeziorowski, H.; KnBzinger, H.; Wang, X.Zh.; Taglauer, E. Bull. SOC.Chim. Eelg. 1981, 90, 1339. (8) Chin, R. L.; Hercules, D. M. J . Phys. Chem. 1982, 86, 360. (9) Chin, R. L.; Hercules, D. M. J . Phys. Chem. 1982, 86, 3079. (10) Abart, J.; Delgado, E.; Ertl, G.; Jeziorowski, H.; Knozinger, H.; Thiele, N.; Wang, X. Zh.; Taglauer, E. Appl. Catal. 1982, 2, 155. (11) Houalla, M.; Kibby, C. L.; Petrakis, L.; Hercules, D. M. J. Catal. 1983, 83, 50. (12) Jeziorowski, H.; Knozinger, H.; Taglauet, E.; Vogdt, C. J . Catal. 1983, 80, 286. (13) Kasztelan, S.;Grimblot, J.; Bonnelle, J. P. J . Chim. Phys. 1983, 80, 793. (14) Brinen, J. S.; DAvignon, D. A.; Meyen, E. A,; Deng, P. T. Behnken, D. W. Surf. Interface Anal. 1984, 6, 295. (15) Massoth, F. E. A h . Catal. 1978, 27, 265. (16) Grimblot, J.; Payen, E. In Surface Properties and Catalysis by Nonmetals, Bonnelle, J. P.,Delmon, B., Derouane, E.G., Eds.; Riedel: New York, 1983,; p 139. (17) Smith, D. P. Surf. Sci. 1971, 25, 171.
metal content
metal content, wt % MOO,
symbol
3 4.5 7 8 15 16.9 21.7 30 14 13.2 14
Nil Nil Ni, Ni4 Ni5 Ni, NiMo, NiMo, NiMo, N iM 0, * CoMolo*
wt % MOO,
wt % NiO (COO) 0.5 1
14 14 14 12.9 12.9
3.6 7.2 15 22 1 3.6 7.2 3.2 3.2
"The support used is A1203230 m2 g-I, 0.6 cm3 g-I or (*) 246 m2 0.53 cm3 g-l.
g-l,
dispersed on the support surface.1s2,8Incorporation of the cobalt and nickel ions into the alumina support, at high temperature, has also been clearly evidenced even in the presence of M o o 3 supported phase in the NiO-MOO, or more recently with W 0 3 in Ni0-W03/A1203 catalysts.18 However, the location of the promoter (Co or Ni) relative to the supported oxomolybdenum phase is still a matter of debate. A Co-Mo bilayer has been postulated by Delannay et al.3 and by Chin and Herc ~ l e s .Similarly, ~ Horrell et al.I8 discussed their results in terms of a Ni-W bilayer while Knozinger et al.79109'2proposed an incorporation of the Ni promoter into the molybdenum layer. Previously, we have shownI3 on Ni0-Mo03/A1203 catalysts that some experimental parameters such as the primary ion energy and contamination effects lead to difficulties in the interpretation of the promoter position from depth profile measurements. However, it was established that identical procedures and experimental conditions allow useful comparisons of the ISS results obtained on different series of samples. The potential of the ISS technique to give precise information on the repartition of the oxide phases supported on 7-Al2O3 and on the position of the promoter relative to the oxomolybdenum species will be emphasized in this paper. Experimental Section
Samples Preparation. Composition and nomenclature of the The different studied samples are reported in Table I. Mo03/A1203and NiO/A1203 samples (Mol-9, Moll, Nil+) were prepared by a pore filling impregnation of y-alumina extrudates (18) Horrell, B.; Cocke, D. L.; Sparrow, G.; Murray, J. J. Caral. 1985, 95, 309.
0022-3654/87/2091-1503$01.50/00 1987 American Chemical Society
1504 The Journal of Physical Chemistry, Vol. 91, No. 6, 1987
with an ammonium heptamolybdate (AHM) solution or with a nickel nitrate solution. The wet samples were dried at 110 OC overnight and calcined at 500 OC for 2 h. The Mol, sample has been calcined at different temperatures ranging from 100 to 900 “C. The NiMo1-, samples were prepared by starting with the M o sample ~ (intermediate calcination at 350 OC, 2 h) and were impregnated with a nickel nitrate solution. After the samples were dried, the final calcination was done at 500 “ C for 2 h. Preparation by equilibrium adsorption in a fluidized bed column, described elsewhere,I9 was used for the Molo sample. The pH of the impregnating A H M solution was adjusted to 2, causing heptamolybdate to be the major species in solution. The subsequent drying, calcination, and promoter addition (CoMolo-NiMalo) steps were carried out in a classical way as already reported. The symbols “d” for dried and “c” for calcined are used in order to distinguish these samples in their different preparation states. ZSS Measurements. The samples, finely ground in a mortar, were deposited on a gold foil as a thin skin obtained by vaporization of a suspension of the powder in isopropyl alcohol. Such a deposition method avoids the pressing of the sample into a thin wafer. The sample holder was then introduced into the preparation chamber of a Leybold-Heraeus LHS- 10 spectrometer, outgassed at 150 O C for several hours, maintained under vacuum overnight, and then transferred to the analysis chamber (base pressure 5 X lo-” Torr). The ISS measurements were performed when the analysis chamber vacuum level was better than 5 X Torr. This long degassing procedure was necessary to remove most of the contaminants (adsorbed water and carbon or hydrogenated carbon from isopropyl alcohol).” The gold of the sample holder was not detected during the analysis. 4He+ions with an incident energy of 1000 eV were used. This energy was chosen in order to obtain a low sputtering rate with good spectral resolution. For the study of the calcination temperature on the Mol sample, different ion energies (500-1000-2000 eV) have also been used. A surface area of 4 mm2 (2 x 2) was rastered and the ion density was estimated at 1 pA/cm2 for an emission current of 1 mA at 500 eV. The broad band of secondary ions and inelastic scattering was attenuated by using a flooding electron beam.
Results and Discussion The Mo03/yA1,03System. In Figure l a , the intensity ratios (ZMo/I,,JISs are plotted as a function of the Mo loading of the Mol-9 series of samples. From the Mo loading in wt % Moo3, the number of Mo atoms per nm2 of surface area occupied (LMo) can be deduced by assuming a complete Mo repartition on the yA120, support. As an XPS and LRS analysis of this set of samples has been previously published,” we also plotted in Figure values. (The similarity between the ISS and l a the (ZMo/IAJxB XPS ratio values is purely fortuitous as the physical processes of each technique are completely different.) In Figure lb, other and (ZMo/Zo)Isare also reported. The intensity ratios (Zo/ZN)Is ISS data obtained at the beginning of the etching process (etching time N 0) by the 4He+ ion beam at 1000 eV will be discussed first. In Figure l a , variations of (Zhl0/ZAJ~ss and (ZM,,/IA~)X~S as a function of the molybdenum loading are very similar. These ratios increase linearly up to 20 wt % MOO, (which corresponds to ca. 4.5 Mo atoms nm-*) and then reach a plateau for higher loading. For the XPS results it was considered that the A1 intensity (IAI)= is a constant parameter for the different samples of this series due to the relatively large photoelectron escape depth of the A1 2p line (kinetic energy = 1410 eV). Therefore the linear (ZMo/IN)xps increase was interpreted by the increasing amount of small oxomolybdenum species well dispersed on the support.20 At loadings higher than 20 wt % MOO, large particules or aggregates, identified as “free MOO, crystallites” by LRS,zoare growing on AI2O3. The loading corresponding to the break between the straight line (19) Kasztelan, S.;Grimblot, J.; Bonnelle, J. P.: Payen, E.: Toulhoat, H.; Jacquin, Y . Appl. Catal. 1983, 7, 91. (20) Dufresne, P.; Payen, E.; Grimblot, J.: Bonnelle, J. P. J . Phys. Chem.
1981, 85, 2344.
Kasztelan et al. Wt %
10
Moo3 20
30
b
/
/
2
4
6
8
At.Mo.nm-2
Figure 1. Variation of the ISS intensity ratio (4He+,1000 eV) vs. the Mo loading on A1203(samples Mol-Mo9). (a) (IMo/IAI)Iss at different time of sputtering: (W) initial spectra, ( 0 )10 min, (A)120 min. (b) (0) ( I o / I A J I S s and (A) ( I M o / I ~ ) l sfor s the initial spectra. Dotted line: (fMo/IAl)XB taken from ref 20.
and the plateau was considered to be the limit of a “molybdenum monolayer”. Conversely, the high sensitivity to the top layer of the ISS technique should preclude consideration of the A1 intensity as a constant parameter when the Mo loading changes because the supported oxomolybdenum species potentially have a shadowing effect on the elements of the support surface. Such an effect is often important and is largely used to characterize surface and adsorbate positions. However, in a study of oxygen adsorption on nickel, Taglauer and HeilandZ1reported that the intensity ratio (Zo/ZNi)Iss variations as a function of the oxygen coverage gave, within the experimental uncertainties, a straight line for low oxygen coverages. In that case, they concluded that only a small part of the substrate surface was effectively hidden by the adsorbed species. Although the ISS intensity variations are expected to be more complicated when an oxide is deposited on another oxide, to a first approximation the different intensity ratios can be calculated as a function of the molybdenum loading LMoof the investigated samples following the same approach developed by Taglauer and Heilandz1for the 0-Ni system. These calculations are described in the Appendix and shows that the straight line observed for low Mo loadings corresponds to the following relationship (eq 9 of the Appendix): ( K = constant) (ZM0/ZA1)Iss = KLM, This formula is obtained by considering that for Mo loading less than 20 wt % Moo3 (or less than 4.5 Mo at nm-2) the effective surface area occupied by the deposited species is small compared to the total surface area of the support. In the equations developed in the Appendix, a factor r = NM,/LM, was introduced to take into account the fact that a fraction of the Mo atoms deposited on the support could not be (21) Taglauer, E.; Heiland, W. Surf. Sci. 1975, 47, 234.
Alumina Supported Moo3, NiO, and NiO-Mo03 detected by the ion beam. This is the case when the Mo phase is not perfectly spread out over the Alz03 surface but exists as aggregates or large species. It is now well accepted that the supported phase of MoO3/Al2O3catalysts in the “monolayer range” is composed of either monomer or polymer oxomolybdenum species. More precisely it has been proposed that on calcined samples (Le., 500 O C for 2 h for the series we are investigating) the monomer MOO^^- anion is the major species at low loading, up to ca. 1 Mo at nm-2, whereas the heptamer MqOz4&is present, in interaction with the support, for loadings between 1 and ca. 4.5 Mo nm-2.1922-24This upper limit can be related to the presence on A l z 0 3of a defined number of sites which interact with the anions present in solution during the impregnation step. This number was estimated to 0.6 to 0.7 sites per nmz of alumina when incipient wetness impregnation is used or 0.35 sites nm-z when the equilibrium adsorption method is carried out.2S Such a low surface density of oxomolybdenum species in the calcined state with a rather small cross-sectional area of the heptamolybdate anion, estimated between 15 and 20 A2 (the peripheral oxygens are not considered), implies an effective coverage of 10 to 15% of the overall surface by the Mo oxo species. This coverage is consistent with the approximations we used in the Appendix. The observed straight line (Figure l a ) for LMobelow 4.5 MO atoms nm-2 implies also that the coefficient r is (almost) the same whatever the nature (monomer or hepamer) of the dominant species present on the support. It has been proposed that the heptamer is adsorbed on y-Alz03as a bilayer with 4 Mo ions lying on the A1203plane and the 3 other Mo ions sitting on the top.26 This species most likely remains fixed on the surface through Mo-0-AI bonds. Such entities are so small so that even if the geometry of such a chemical edifice is not greatly perturbed by the support, a geometric projection on it shows all seven Mo ions. Consequently, it appears that the ISS experiment, within the experimental conditions we used (He+ at 1000 eV, a relatively high energy), does not allow one to distinguish the different types (environment) of Mo ions and we assume that r = 1 whenever LMois below 4.5 Mo at nm-2. The observation in Figure l b of a straight line for the variations of the (lo/Zal)rss ratio vs. the molybdenum loading up to 4.5 Mo at nm-z further supports the preceeding conclusion. These large variations are related to eq 10 of the Appendix and indicate that some oxygen atoms associated to the molybdenum species adsorbed on the support are detected by ISS. This was not the case by XPSzobecause of the very large contribution to the oxygen XPS intensity of the oxygen belonging to the support. The effect of the etching time on the (ZMo/lAI)Iss intensity ratio vs. the molybdenum loading can be observed in Figure la. The spectra have been recorded after 10 and 120 min of erosion corresponding to roughly 0.5-1 and 4-8 monolayers removed, respectively, on the basis of a 15-30 min per monolayer sputtering rate as a rough estimate.’* This process is likely nonhomogeneous for rough surfaces as sputter rates depend on the angle of incidence and preferential sputtering can occur. However, when using the same experimental conditions to compare similar samples (Le., the same support therefore the same roughness) these effects can be expected to not interfere in a first approximation. The results of Figure l a indeed show that this is the case as the shape of the repartition curve is kept when the etching time increases whereas a decrease of the intensity ratio occurs as molybdenum is removed gradually. The NiO/AlZO3System. The Nild series of samples, investigated in this paper by ISS, was previously characterized by XPS and LRS.*O The main conclusion reached was that the nickel species belong to a well-dispersed bidimensional Spinel-like (22) Wang, L.;Hall, W. K. J . Catal. 1982, 77, 232. (23) Kasztelan, S. Thesis, Lille, 1984. (24) Kasztelan, S.; Payen, E.; Toulhoat, H.; Grimblot, J.; Bonnelle, J. P. Polyhedron 1986, 5 , 157. (25) Meunier, G.; Mocaer, B.; Kasztelan, S.;Le Coustumer, L. R.; Grimblot, J.; Bonnelle, J. P. Appl. Catal. 1986, 21, 329. (26) Hall, W. K. Proceedings of the Fourth Conference on Chemistry and Uses of Molybdenum, Barry, H. F., Mitchell, P. C. H., Eds.; 1982, p 224.
The Journal of Physical Chemistry, Vol. 91, No. 6, 1987
1505
Wt% NiO
5
0
IO
15
20
b
At.Ni.nm-2 Figure 2. Variation of the ISS intensity ratio (4Het, 1000 eV) vs. the N i loading on A1203(samples Nil-Ni6). (a) (Z”i/IAI)Iss at different time of sputtering: (m) initial spectra, ( 0 ) 10 min, (A) 200 min. (b) (0) (Io/IM)Iss and (A) (INi/Io)Issfor the initial spectra. Dotted line: (INi/IAI)xps taken from ref 20.
“NiA1204layer” on y-A1203up to 7 wt % NiO (or ca. 2.5 Ni at nm-2). In this surface aluminate the nickel ions are ocoupying both tetrahedral and octahedral sites (the Ni 2p binding energy is slightly different in these two environments). For higher nickel loadings, formation of bulk NiO was suspected. The (ZNJZAJISs ratio variations vs. the nickel loading (in wt % NiO or in number of nickel atoms nm-2, LNi) in Figure 2a appear very similar to those found previously by XPS.zo Of particular interest is the initial linear part starting from the origin of the axis and the break point corresponding, for both methods, to the same Ni loading (2.5 Ni at nm-2). These ISS results confirm that, up to 7 wt % NiO, the nickel ions are well dispersed on/in the top layer of the alumina support with no substantial shadowing effect on the aluminum ions of Alz03. Hence eq 9 of the Appendix in which LMois replaced by LNi is adequate to account for the results. In Figure 2a, for each sample three different results, as a function of the sputtering time, have been reported. As already noted for the Mo03/A1203series, the initial information about the nickel repartition is preserved despite a noticeable surface erosion. More surprising is the increase of the (ZNi/ZAJIss ratios after an erosion time of 10 min compared to the initial values. Such a profile with a maximum has been already reported for NiO,I3 C O O - M O O ~or , ~Ni0-W0318 catalysts (all supported on AlZO3). In these two latter supported oxide systems, a bilayer has been or could be suggested to explain these results. However, with such a profile existing for the NiO/AlZO3catalysts here and in ref 13, another interpretation have to be found. Tentatively we can propose that the nickel ions, located in tetrahedral or octahedral sites of a NiAl2O4 surface spinel, are partly hidden to the ion beam by A1 or 0 surrounding species. In Figure 2b, the (Zo/ZAl)Iss intensity ratio variations against the nickel loading reach very rapidly a plateau at a value about 15% higher than in pure alumina. These observations suggest that (1) the shielding of the aluminum atoms by Ni is very small, in agreement with the results of Figure 2a, and (2) the surface oxygen density is not significantly modified when nickel is added (which is the converse to the adsorption of molybdenum oxo species as
1506 The Journal of Physical Chemistry, Vol. 91, No. 6. 1987 1.5 b
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3 1 2 At NI nm-2 Figure 3. Variation of the ISS intensity ratio (4He+, 1000 eV) vs. the Ni loading on 14 wt 5'5 MoO3/Al2O3(samples NiMol-NiMo3). (a) (INi/IAI)Lss at different time of sputtering: (m) initial spectra, ( 0 )10 min, (A)200 min. (b) (A) (IMO/IAI)ISS~ (x) (&/IAI)Iss, and (0) (INi/IMo)lsS for the initial spectra. Dotted line: ( I N i / I A I ) x p s taken from ref 20. noticed previously). Evidently this difference in the O/metal ratios reflects a strong difference in the nature of each supported phase: the oxomolybdenum species can be really considered as deposited on the alumina surface whereas the nickel ions are incorporated in the surface vacant sites.20 The Ni0-Mo03/A1203 System. The N ~ M O ,Series. -~ This set of sample, with a constant Mo loading (14 wt % Moo3) and varying Ni content, has been previously studied by XPS and LRS.20 In Figure 3a, the (ZN,/ZAI)Iss ratios, measured after different sputtering times, are reported as a function of the nickel loading. The values of the slopes of the straight lines obtained at low Ni loading are very close to those measured on the NiO/y-A1203 catalysts with 0.26, 0.2, and 0.05 for the initial spectra and after 10 and 200 min of sputtering, respectively. However, a difference can be noted for the break point position: 2.5 Ni atoms nm-2 in the NiO/y-A1203 samples and 1.0 Ni atoms nm-2 in this NiMo series. By XPS, no special modification for that loading was noted. In Figure 3b, other typical intensity ratios are reported for the first ISS spectra (etching time r)r! 0). Within the experimental uncertainties it appears that the relative Mo intensity is not modified by the addition of nickel. Therefore, the and (ZMo/Zo)Iss are constant all along the other ratios (Zo/ZAI)Iss set of samples. In the previous XPS-LRS investigation it was proposed that, at low Ni loading (NiMo, and NiMo, samples), the Ni and Mo elements are well dispersed on the support as independent phases whereas for the third sample (NiMo,), the LRS analysis suggested the presence of a mixed Ni-Mo phase. The ISS results presented in Figure 3, a and b, are in good agreement with this interpretation. For NiMo, and NiMo2 the presence of Mo species on the support does not modify the nickel interaction with the free remaining A1203 surface. Taking into account the descriptions of the M003/A1203 and NiO/A1203 catalysts, we propose to schematically describe the NiMo, or NiMo2 samples in Scheme I. In the NiMo3 sample, the excess nickel is located differently and probably interacts with the oxomolybdenum species. Interestingly, such a scheme can be viewed as a bilayer systems as the Ni ions in the surface aluminate are less accessible than the molybdenum species. Comparison of Mol, with the NiMoloand CoMol0Samples. For these samples, the characteristic ISS ratios were measured
021;*I 0
Dry
\ ;
Calc
I
Dry
Ni,Co Mo I Calc
Figure 4. Evolution of the ISS intensity ratio (He*, 1000 eV) vs. the preparation step of the samples. Centred symbols: first deposition of Mo. Black and open symbols: addition of Co and nickel, respectively. (0) (IM~/IAI)Iss, (0) (IN~(~)/IAI)Iss, Dotted line and (A)are for UMJIAJXPS taken from ref 19.
after each step of preparation in order to investigate more precisely the repartition and the interaction of the promoter with the oxomolybdenum species during the genesis of these catalysts. The (ZMo/ZAI)Iss or xps ratios increase slightly when the dried Molo sample is calcined (transition MolM to Malo in Figure 4). The origin of such a modification can be the removal of some adsorbed species such as water and the counterion NH4+ present during the impregnation of the heptamolybdate salt. Indeed recent LRS r e s ~ l t s have ~ ~ Jestablished ~ ~ ~ ~ that the calcination provokes dehydration of the supported oxomolybdenum species. In conjunction with a stronger interaction with the support, the dehydrationhydration process has also been shown to be reversible.2s It is therefore conceivable that the deposited Mo atoms are less shielded after such a treatment, in which case the Mo intensity would be enhanced. After the addition of the Co or Ni nitrate solution to the calcined Mol,, samples and drying, a net decrease of the (IMo/ ratio is observed whereas the XPS variation is far less important. This effect can be due to a large shadowing effect on Mo by the promoter ions (Ni2+,Cd+) rather than the modification of the Mo repartition on the support. After the final calcination at 500 O C , the Mo intensity (ISS) increases again but does not reach the value obtained before the promoter impregnation. During the transformation (dried to calcined) the Co or N i intensity ratios slightly decrease. These later observations can be rationalized by three simultaneous events: (1) a dehydroxylation of the oxomolybdenum species, as already noted; (2) the presence (27) Stencel, J. M.; Makowski, L. E.; Sarkus, T. A,; De Vries, J.; Thomas, R.; Moulijn, J. A. J . Card. 1984, 90, 314. (28) Payen, E.; Kasztelan, S.;Grimblot, J.; Bonnelle, J. P. J . Raman Specirosc. 1986, 17, 233.
Alumina Supported MOO,, NiO, and NiO-MOO,
The Journal of Physical Chemistry, Vol. 91 No. 6, 1987 1507 ~
SCHEME I1
.................... .................... of the promoter in close contact with the Mo adsorbed species, which provokes a real shadowing effect of Mo by Co or Ni and; (3) a migration, at that calcination temperature, of a fraction of the promoter ions from the oxomolybdenum species to the free surface of the support (decrease of Zco and IN,). Of particular importance is the strong evidence of an interaction between the promoter and the molybdate species (event 2 ) which can be depicted with Scheme I1 (same symbols as Scheme I). Evidently this scheme describes a limit case and a distribution of Ni ions between ions interacting with the molybdate species and ions interacting with the support surface octahedral and tetrahedral sites is more reasonable. The reasons the two Ni0-Mo03/yA1,03 series of samples (NiMo1-, and NiMolo) have different distribution of Ni ions are not obvious at the moment. However, both the support and the preparation methods used were not exactly the same and such preparation parameters may influence the final architecture of these catalysts. The direct interaction between the promoter and the oxomolybdenum species evidenced by the results of Figure 4 deserves more comments. It is well-known that the oxomolybdenum species, proposed to be mainly a heptamolybdate strongly bound to the s ~ p p o r t ,shows ~ ~ , some ~ ~ Bronsted acidity, detected, for example, by the presence in the Raman spectra of bands characteristic of the pyridinium ion after pyridine adsorption on the calcined sample.20 Such protons are potentially exchangeable with some cations, in particular during the second impregnation step with the promoter solution. Preparation of a large number of inorganic or organic salt of heteropolyoxometalates is really carried out by taking advantage of this exchange p r ~ p e r t y . ~ ’ Therefore by analogy with the heteropoly compound properties we suggest that the interaction of Ni or Co with the supported oxomolybdenum species results in the formation of an isopolymolybdate salt of Co or Ni adsorbed on yA1203. The Effect of the Calcination Temperature. The influence of the temperature of calcination has been studied first on the Moll sample (14 wt % MOO,) and the ISS results are reported in Figure 5. Whatever the ISS ion energy used, the trend is to an increase and (Zo/ZAI)Iss ratios when the temperature of the (ZMo/ZAI)Iss of calcination of the sample increases. At high calcination temperatures, it is likely that decomposition of the isopolymolybdate structure occurs simultaneously with a modification of the y-Al,03 structure. By LRS it was shown that formation of A~,(MoO,)~ was detected at temperatures as low as 700 O C Z 4 In the lowtemperature range, the (ZMo/ZAI)Iss increase could result from decomposition of the adsorbed salt (NH, is evolved) and water desorption or dehydroxylation. More interesting is the comparison of the effect of the final calcination temperature on the ISS ratios of Mo03/A1203and Ni0-MoO3/AI2O3 samples. A study of the latter has been performed by Knozinger et a1.,10,12 and the intensity ratio, deduced from their spectra, are reported in Figure 6 along with those we obtained on the M o l l sample. Such a comparison is reasonable as the experimental conditions of both studies are similar. When the temperature of calcination of the N ~ O - M O O ~ / A ~ sample ~O, is increased, the Mo intensity, very low at the beginning due to the shielding effect of the nickel ions on the Mo ions, increases and almost reaches the level obtained on the sample without Ni after calcination at 600 OC. Simultaneously, the nickel signal intensity decreases sharply after calcination at 450-500 OC,an effect attributed to diffusion of Ni in the surface sites of the alumina These results can be rationalized by migration of the promoter ions away from the isopolymolybdate salt species to form a surface aluminate. Schematically, this temperature (29) Pope, M. J. Heteropoly and Isopolyoxometalates; Springer-Verlag: Berlin, 1983.
13
I
i 0
200
400 600 800 T“C
Figure 5. Effect of the calcination temperature on the ISS intensity ratio of the 14 wt % Mo0,/A120,: (0) (IO/fAl)[ss and (0)(IM~/IAI)Iss for 4He+at 500 eV, (A) (IM~/IAJ~ss at 1000 eV, ( X ) Z(M&~,I/S at 2000
eV, ( 0 ) (IMOIIAJXPS.
J
,
300
,
,
500
,
,
700
T‘C Figure 6. Effect of the calcination temperature on the ISS intensity ratio (4He+,500 eV) of a 14 wt % MOO, and 3% wt Ni0-12 wt % MOO,/ A120, samples with (0) (ZM~/ZAJISS taken from Figure 5 and (X) (INr/IAI)ISS,(0)(IMo/IAJ[ss deduced from the spectra reported in ref 12.
effect transforms the structure depicted by Scheme I1 gradually into that of Scheme I in the 500-600 “ C range, which is the usual temperature at which the NiO-MoO,/y-Al,O, commercial or laboratory made catalysts are prepared. Conclusion The results presented in this work reinforce a large number of conclusions already reported and demonstrate the usefulness of the ISS technique to provide interesting information on heterogeneous catalysts. In particular, the description of the Moo3/ yAl2O3 system as being formed by small oxomolybdenum patches occupying only a small fraction of the alumina surface is confirmed. However, the ISS results do not permit us to distinguish isolated molybdates from adsorbed heptamolybdate or polymolybdate species. The promoter molybdate interaction, likely by the formation of a supported isopolymolybdate salt, is evidenced by a strong shielding of Mo by the promoter (Co or Ni). Such a structure can be destroyed when the final calcination temperature of the Ni0-MoO3/y-AlZO3 samples reaches 500 O C or more. A different “history” in the preparation sequences may also preclude the formation of this entity.
1508 The Journal of Physical Chemistry, Vol, 91, No. 6, 1987
Kasztelan et al.
Acknowledgment. We are indebted to the Institut FranGais du PEtrole for financial support. The improvement of the manuscript by the referee’s comments has been appreciated.
and the oxygen atoms associated with the Mo phase. Hence
Appendix Taglauer et aLZ1developed quantitative relationships to interpret the ISS intensity measurements of adsorbed layers on metal surfaces. We used a similar approach to investigate supported oxides, MOO, or NiO on A1203. For a surface layer of pure alumina with NAI aluminum atoms exposed per surface area unit, the L E E S ZAl intensity is given by IAI = Io C A I (~ PAJNAITAIR AQ (1)
with the shielding effect of Mo ions on the support oxygen ions and p the O/Mo atomic ratio of the adsorbed or supported phase. In the preceding formula, NMois the number of Mo atoms per surface area unit effectively detected (unshielded) by the ion beam. This term is in fact the Mo loading LMo(number of supported Mo atoms per surface area unit) if all the Mo ions of the supported phase contribute to the ISS signal. On the other hand, when the supported phase is composed of crystallites in which bulk Mo atoms are not detectable by the incident ion beam, we can write
where Io is the primary ion current density; gAIis the differential scattering cross section; PAIis the neutralization probability; TAI is an instrumental factor taking into account the analyzer transmission and the detector sensitivity; R and Ai2 are a rugosity factor and the solid angle of the scattered ions. As we will consider series of samples examined under similar experimental conditions, eq 1 can be simplified as
AI
= ~AINAI
(2)
where iAl is a constant and NAI is the number of A1 atoms effectively detected within the experimental conditions. Similarly, the intensity signal of the element Mo belonging to a surface phase supported on the A1203surface previously considered will be IMO
=
iMnNMo
(3)
with NMo the surface density of the Mo atoms effectively detected by the incident beam. Evidently, such a supported phase can hide in part the support surface and therefore perturb the ZAl term. This shielding effect, a(MpAI), is a complex term which combines effective geometrical effects and modifications of the neutralization probability. Hence, when the Mo phase is present on A1203, ZAl can be written as [AI
= ~ A I ( N-A~I ( M ~ - A I ) N M O )
(4)
For the oxygen element, the LEISS intensity can be schematically decomposed in two contributions: the oxygen atoms belonging to A120, less those shielded by the supported Mo phase,
Io = ioWo - ~ ( M ~ - O ) N +M PNMA O
NMo= rLMo
with r
