J. Phys. Chem. 1989, 93, 6501-6506 Finally, we note that it is possible to kinetically monitor the consecutive addition of ligands to metal clusters. We have investigated the addition of a second N2 molecule to Nb3N2,Nb5N2, Nb6N2, and Nb8N2 and have found that this secondary reaction proceeds much more slOwly than the first addition Of an N2 for Nb3, Nb5, and Nb6 but proceeds more rapidly for Nb8. Acknowledgment. We are grateful for support of this research from the Science under Grant 8521050. Registry NO. H2, 1333-74-0; N2, 7727-37-9; C2H6, 74-84-0; V3C, 12070-1 1-0; V4C, 121674-52-0; VSC, 121674-53-1; V&, 12316-57-3; V7C, 55892-24-5; V&, 121674-54-2; VgC, 121674-55-3; VI&, 12167456-4; VjiC, 121674-57-5; VIZC, 121674-58-6; V,,C, 121674-59-7; VIdC,
6501
121674-60-0; V&, 121674-61-1; VI&, 121674-62-2; Vi7C, 12167463-3; vi&, 121674-64-4; V& 121674-65-5; v30, 64764-10-9; v40, 12306-38-6; VsO, 121674-40-6; V.50, 39350-97-5; V70, 121674-41-7; v@, 54651-68-2; v@, 12509-78-3; VloO, 121674-42-8; V l l O , 12167443-9; V120, 121674-44-0; Vl,O, 121674-45-1; VI40, 121674-46-2; VI50, 121674-47-3; V,,O, 121674-48-4; V,,O, 121674-49-5; V,,O, 12167450-8; VI@, 121674-51-9; T a 3 0 , 112510-43-7; T a 4 0 , 12059-92-6; T a 5 0 , 112510-44-8; Tab03 1 2 2 0 2 - 0 5 4 Ta@, 112510-45-9; TasO, 112510-46-0; T a 9 0 , 112510-47-1; TaioO, 112510-48-2; T a l l O , 112510-49-3; T a 1 2 0 , 71767-52-7; Tal@, 121674-22-4; T a i 4 0 , 121674-23-5; Tal@, 12167424-6; T a 1 6 0 , 121674-25-7; T a i 7 0 , 121674-26-8; TaI8O, 121674-27-9; T a i 9 0 , 121674-28-0; Ta200, 121674-29-1; T a 2 1 0 , 121674-30-4; Ta2,0, 121674-31-5; Ta230, 121674-32-6; Ta,,O, 121674-33-7; T a 2 5 0 , 121674-34-8; Ta2,0, 121674-35-9; T a 2 7 0 , 121674-36-0; T a 2 8 0 , 121674-37-1; Ta290, 121674-38-2; Ta300, 121674-39-3.
Genesis and Characterization by Laser Raman Spectroscopy and High-Resolution Electron Microscopy of Supported MoS, Crystallites Edmond Payen,+,$Slavik Kasztelan,t*s Sabine Houssenbay,+ Raymond Szymanski,g and Jean Grimblot*,+ Laboratoire de Catalyse HPtCrogPne et HomogZne, U.A. C.N.R.S.402, UniversitP des Sciences et Techniques de Lille Flandres- Artois, 59655 Villeneuve D'Ascq Cedex, France, L.A.S.I.R.,L.P. 2641, UniversitP des Sciences et Techniques de Lille Flandres-Artois, 59655 Villeneuve D'Ascq Cedex, France, and Institut Francais du PPtrole, 1 et 4, Avenue de Bois-PrPau, B.P. 311, 92506 Rueil-Malmaison Cedex, France (Received: January 18, 1989)
Sulfidation of Mo/y-Alz03based hydrotreating catalyst is a key step to activate the supported phase. In situ laser Raman spectroscopy (LRS) appears as a suitable technique for identifying intermediate species and characterization of the final state, Le., MoS, crystallites supported on A1203. A stepwise sulfiding procedure was used in varying different parameters, Le., hydration state of the oxide precursor, composition of the sulfiding mixture Hz/H2Sor N2/H2S,temperature, and time duration. The important role of these experimental conditions on the detection of transient species during sulfidation has been pointed out. Different intermediates such as oxysulfides, sulfido compounds, and MoS3 appear to be the dominant ones, whose existence depends on the previously mentioned parameters. The crystallite size of the MoSz, a function of the Mo loading and determined by HREM, influences the position and width of the -385-cm-' line.
Introduction The sulfidation of the Mo03/y-A1203or WO3/y-Al2O3hydrotreating catalysts, which are generally associated with a promoter, is the necessary step, usually performed directly in the refinery plant, to get the active working form of the catalysts. This sulfidation step produces supported MoS2 or WS2 particles which have been identified by several techniques.' However, a good knowledge of the nature and reactivity of the intermediate species formed during the complex transformation of the oxomolybdate entities into MoSz on the support surface is of considerable interest in order to optimize the catalytic properties of this active MoS2 phase. In situ LRS appears a suitable technique for such an investigation as it permits to detect both oxo and sulfido species. To date a large amount of results has been obtained on the oxide precursor2 whereas sulfidation has been less studied. Schrader and Cheng3 observed different surface species during the sulfidation of MoO3/y-AI2O3catalyst, Le., oxysulfides, a reduced phase and MoS2. Cobalt present as a promoter favors reduction of the oxomolybdate phase but leads to a more difficult s ~ l f i d a t i o n . ~ Since the work of Arnoldy et aL5 it appears that water, present in the hydrated precursor, influences the sulfidation rate. In a previous study6 we reported LRS observations on W 0 3 supported catalysts evolution when changing the nature of the sulfiding mixture, temperature, and time of reaction as well as
* To whom correspondence should be addressed.
' Laboratoire de Catalyse HBtBroggne et HomogLne. 'L.A.s.I.R. 5 Institut FranGais du Petrole.
0022-3654/89/2093-6501$0l.50/0
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- -
the effect of oxide hydration. The following scheme was proposed: oxysulfide WS3 WS2. supported oxotungstate In this work, to complement this previous investigation, the nature of the intermediates formed during the sulfidation steps of molybdate/A1203based catalysts has been investigated by using in situ laser Raman spectroscopy (LRS). Several samples of different Mo loadings have been studied to monitor the effect of this parameter on the size of the MoS2 slabs after sulfidation. Indeed, both LRS and HREM (high-resolution electron microscopy) allow us to observe features of the MoSz final product.
Experimental Section Catalysts and Pretreatment. The catalysts studied in this work have been prepared by the pore filling method using y-A1203 extrudate (surface area, 238 m2.g-'; pore volume 0.6 g ~ m - and ~) heptamolybdate solutions. The solid was then dried at 380 K (1) Massoth, F. E. Advunces in Cutalysis; Academic Press: New York, 1978; Vol. 25, p 265. Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemisfry of Catalytic Processes; McGraw-Hill: New York, 1979; Chapter 5. (2) (a) Payen, E.; Kasztelan, S.; Grimblot, J.; Bonnelle, J. P. J . Rumun Spectrosc. 1986, 17, 233. (b) Stencel, J. M.; Makowsky, L. E.; Diehl, J. R.; Sarkus, T. A. J . Roman Spectrosc. 1984, 15, 282. (c) Jeziorowski, H.; Knozinger, H. J . Phys. Chem. 1979,83, 1166 and references therein for earlier literature. (d) Zing, D. S.; Makowski, L. E.; Tisher, R. E.; Brown, F. R.; Hercules, D. M. J . Phys. Chem. 1980, 84, 2898. (3) Schrader, G. L.; Cheng, C. P. J. Catul. 1983, 80, 369. (4) Schrader, G. L.; Cheng, C. P. J. Cutal. 1984, 85, 488. (5) Arnoldy, P.; Van Den Heijkant, J. A. M.; De Bok, G. D.; Moulijn, J. A. J. Cutal. 1985, 92, 3 5 . (6) Payen, E.; Kasztelan, S.; Grimblot, J.; Bonnelle, J. P. Catul. Today 1988,4, 57. Payen, E.; Kasztelan, S.; Grimblot, J. J . Mol. Strucf. 1988, 71, 174.
0 1989 American Chemical Society
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The Journal of Physical Chemistry, Vol. 93, No. 17, I989
Payen et al. 407
I
385
I
I,
3851
I
840
IL
Figure 1. Laser Raman spectra of dehydrated 14 Mo/A1203 sulfided by a H2/H2S mixture (10%): (a) oxide; (b) 330 K, 5 min; (c) 470 K 1 h; (d) MoS3 from ref. 8; (e) 620 K, 3 h.
overnight and calcined in air at 770 K for 2 h. The Mo loading is indicated in the catalyst designation; e.g., 14 Mo represents a sample with 14 wt % of Moo3. The sulfidation was carried out in situ directly on the sample as received (hydrated sample) or after calcination under dry oxygen at 700 K for 3 h (dehydrated sample). After the sample was cooled down to room temperature the sulfidation was performed by raising the temperature at a rate of 5 Kmin-’ to the desired final temperature under a flow of the sulfiding gas mixture. Laser Raman Measurements. The spectra have been recorded on a Raman microprobe MOLE (JOBIN-YVON) equipped with a pretreatment cell allowing in situ measurements.’ The 488.0-nm line of an Ar’ laser was used at a power at the sample of about 1 mW. Photon counting detection and data processing were employed to improve the sensitivity and signal to noise ratio. The spectra were recorded systematically with the sample maintained at room temperature under the reactive sulfiding mixture. The spectral slit width was typically 4 cm-I and reported peaks are accurate to &2 cm-’. High-Resolution Electron Microscopy. After the LRS study, the powder was dispersed in hexane for the preparation of the electron microscope grid. The instrument was a JEOL 100 CX microscope with a side entry double tilt stage which provides instrument resolution over a continuous range of periodicities down to 3 A.
Figure 2. Laser Raman spectra of hydrated 14 Mo/A1203 sulfided by a H2/H2S mixture (10%): (a) oxide; (b) 330 K, 5 min; (c) 470 K, 1 h; (d) state c H 2 0 ; (e) 620 K, 3 h.
+
I
*
-~
200
300
400
600 M c m - 1
500
Figure 3. Laser Raman spectrum of the HR306 catalyst (Procatalyse Co Mo catalyst) sulfided by H2/H2S (10%) at 590 K, 1 min. The spectrum is recorded when the sample is maintained at 420 K and the line down shift of MoS, is partly due to a temperature effect. For bulk MoS,, at the same temperature, the two main lines are observed at 404 and 380 cm-l.
Results Sulfdation of the 14 Mo/Al2O3Sample. On the oxide precursor, before interaction with the sulfiding mixture, no bulk MOO, (sharp line at 820 and 995 cm-’) has been detected. In Figures 1 and 2 are reported the LR spectra showing the evolution of this sample in the dehydrated or hydrated forms upon progressive reaction with the H2/H2S (IO%) mixture. The lines at 1005 cm-l (Figure l a ) or at 952 cm-l (Figure 2a) are characteristic of the two-dimensional polymolybdate in the dehydrated or hydrated form, respectively.2a After a short exposure time to the H2/H2Smixture at 330 K (5 min) drastic changes are observed
(Figures l b and 2b). The bands in the 800-1050-~m-~range are no more detected whereas several new lines on a broad band appear below 600 cm-]. Although the noise level is high, detection of these peaks is reproducible. When the sulfidation is carried out at 470 K (1 h), on the dehydrated phase, broad lines quite similar to MoS3 are obtained8 (compare parts c and d of Figure 1). In fact, this intermediate is formed on both hydrated or dehydrated samples. However, further transformation into MoS2 is faster when H 2 0 is introduced in the reactive mixture (compare parts c and d of Figure 2). Therefore the MoS, intermediate phase is more difficult to observe on the hydrated sample. In similar LRS experiments Schrader et al.3 observed also an intermediate state with lines below 550 cm-’ with in addition and contrary to our results an intense line at -440 cm-’. This was attributed to a reduced or to an oxysulfide species. We have been able to observe this intermediate on an industrial CoO/MoO,/ y-A1203catalyst (HR 306 from Procatalyse) which has a Mo
(7) Payen, E.; Dhamelincourt, M. C.; Dhamelincourt, P.; Grimblot, J.; Bonnelle, J. P. Appl. Spectrosc. 1982, 36, 30.
( 8 ) Bhattacharya, R. N ; Lee, C. Y.; Pollak, F. H.; Schleich, D. M. J. Noncryst. Solids 1987, 91, 235
~
~~
~~
~
The Journal of Physical Chemistry, Vol. 93, No. 17, 1989 6503
Characterization of Supported MoS2 Crystallites 407
A407
c"c,
JV1
I
200
/
600
400
800
\
1000
Ji?
Figure 4. Laser Raman spectra of hydrated 14 Mo/A1203 sulfided by a N2/H2S mixture (10%): (a) 330 K, 5 min; (b) 470 K, 1 h; (c) 620 K, 3 h. 407
590 I
200
400
600
760
800
io00
\icL-l
Figure 5. Laser Raman spectra of dehydrated 14 Mo/AI2O3sulfided by a N,/H2S mixture (10%): (a) 330 K, 5 min; (b) 470 K, 1 h; (c) 620 K, 3 h.
loading of 14 wt % of M o o 3 (Figure 3) under the following experimental conditions. The catalyst, previously heated at 420 K under dry N2 for 5 min, is exposed for 10 min to the H2/H2S mixture. Before the LR spectrum was recorded, the cell was purged with N2. This emphasizes the important role of experimental conditions to detect transient species during sulfidation of those catalysts. In Figures 4 and 5 are presented typical spectra when the N2/H2S (10%) mixture is used instead of the classical H2/H2S reactive mixture. On the hydrated sample (Figure 4), similar intermediate species as those observed in Figure 2 are obtained. However, the oxide precursor is still observed after reaction at 500 K during 1 h. On the dehydrated sample (Figure 5), no apparent reaction takes place when the N2/H2S mixture is admitted during 5 min at 330 K. Reaction at 500 K for 1 h leads to formation of another intermediate species (lines a t 760, 590, 440, 340 cm-I) present simultaneously with MoSz (-407, -385 cm-I). When more drastic sulfidation conditions are used (Le., 620 K and 3 h). whatever the mixture commsition. all the s w t r a show intense lines at 405-407 cm-' and 3 8 b 3 8 5 cm-' (Figures le, 2e, 4c, 5c) which are characteristic of MoS,.
Figure 6. Laser Raman spectra of sulfided Mo/Al,03 with various Mo loadings (620 K, 4 h). The spectral slit widths are 4 cm-I and the reported peak is accurate to =k1 cm-I. (a) MoS,; (b) 2%; (c) 4%; (d) 8%; (e) 14%; (f) 8%, previously dehydrated.
Effect of the Mo Loading. For the different Mo-loaded catalysts, we have not attempted to analyze all the intermediate species observed during the sulfidation steps of the 14 Mo/A1203 catalyst. The attention is mainly focused on the spectral characteristics of the final sulfided product, i.e., MoS2, obtained at typically 620 K. By comparison with bulk MoS,, line broadening and small shifts are detected on the lower wavenumber line (Figure 6). On the other hand, some of these sulfided samples have been observed by transmission electron microscopy (see typical micrographs in Figure 7). The black lines are the lattice images from MoS2 with a lamellar structure one or two layers thick. The observed spacing of about 6.1 8,is consistent with the interplane spacing of bulk MoS,.~ These lines come from crystals viewed (9) Dickinson, R. G.; Pauling, L. J . Am. Chem. SOC.1923, 45, 1466.
6504 The Journal of Physical Chemistry, Vol. 93, No. 17, 1989
\
10
nm
Payen et al. obtained during the adsorption procedure is readily transformed even after mild sulfidation. The lines observed in the 500-55O-cm-' region do not correspond to the presence of MoV1O$.+2- oxysulfide species by comparison with vibrational data obtained on such ions.I2 In addition, ESR measurements indicate existence of Mov species after this sulfidation step, in agreement with recent results.13 Such high wavenumbers for sulfided species are sometimes observed in compounds like CH3CSH4MoS(p-S)2for which the line at 530 cm-' is assigned to the MoV= S(t) terminal b ~ n d . ' ~ . But ' ~ this peak position corresponds also to the stretching vibration of terminal or bridged sulfido group (S;-).l2 More confident is the comparison between our LRS results with the characteristic lines of Mo sulfido anions, such as [ Mo2(S2)J2-, [ M O O ~ S , ( S ~ ) ~Mo2S82-, ~-], with both MolV and MoV valence state^.'^*'^.'^ The absence of line in the 800-1000-~m-~range indicates that these "sulfido species" have no Mo = O(t) groups. However, these supported sulfide moieties do contain an S-H group, as its characteristic line at 590 cm-I is always observed." Such an SH group has already been identified in bulk sulfido compound^.'^*'^ The main Raman lines may then be correlated to v s s (530, 550, 520 cm-I), vMO$ (450 cm-I), and vMO-s [320, 360,380,280 cm-'1 vibration of Mo$,, cores.18 The 450-cm-' line could be also assigned to a p-sulfido bridge like M O - S H . ' ~ * ' ~ Let us note also, to confirm our interpretation, that such sulfido complexes have recently been identified as a reaction product of ammonium tetrathiomolybdate aging.20 Cheng and Schrader3 assigned the 440-cm-I Raman line to a Mo-S-Mo vibration of reduced entities or to a MoV' oxysulfide species. On W-based catalysts, we also observed transient oxysulfides. On increasing the sulfidation temperature, their Raman characteristics are slightly changing. This probably indicates a transition to more sulfided groups, possibly sulfido-tungsten complexes. The difference between the W- and Mo-based catalysts during sulfidation clearly underlines the higher reactivity to H2S of the supported oxomolybdate species by comparison with the oxotungstate species. It is therefore assumed that the Mo-oxysulfide intermediates are not easily detected due to their high reactivity with H2S. It is also informative to note that the chemistry of MoVsulfido complexes is well developed,21whereas little is known about the corresponding W vcompounds. The second intermediate, -MoS3-, can be formed by internal redox processes which are well-known in bulk Mo sulfido compounds.22 They can be observed by thermal or photochemical activation. For example, transformation of bulk ammonium tetrathiomolybdate into MoS3 upon heating involves metal reduction (MoV1 MoV)and ligand oxidation (2S2- 7SZ2-). On alumina, the transformation of the Mo sulfido intermediate
-
Figure 7. Electron micrograph of sulfided Mo/Al2O3 catalysts (620 K, 4 h): (a, bottom) 8% Mo; (b, middle and top) 14% Mo.
"edge on", the average length of which was determined from measurements of more than hundred crystallites. The length seems to increase with the Mo loading. On the 14 Mo sample, HREM permits observation of a heterogeneity with large amorphous grains covered by Mus2 fringes as those described by Sanders and Pratt" for bulk NiMo sulfides. They can be due to a small excess of molybdenum which becomes separated from the support upon sul fida tion.
Discussion Mechanism of Sulfidation: Nature of the Intermediate Species. H2/H+ Surjiding Mixture. The two-dimensional polymolybdate phase, previously described as supported heptamolybdate species," (IO) Sanders, J. V.; Pratt, K. C. J . Carol. 1981, 67, 331
(1 1) Payen, E.; Kasztelan, S.;Grimblot, J. J . Phys. Chem. 1981,91,6642. Payen. E.; Kasttelan. S.;Grimblot, J.; Bonnelle, J. P. Polyhedron 1986, 5, 157. ( 1 2) (a) Muller, A.; Weinstock, N.; Schulze, H. Spcrruchim. Acru 1972, 28A. 1075. (b) Muller, A.; Krebs, B.; Kebabgioglu, R.; Stocburger, M.; Glemser, 0. Specrrochim. Acto 1968,24A, 183 1. ( 1 3) Oliver, S. W.; Smith, T. D.; Pilbrown, J. R.; Pratt, K. C.; Christov, V. J . Carol. 1988, I l l . 88. (14) Rakowski-Dubois, M.; Dubois. D. L.; Van Der Veer, M. C.; Haltiwanger, R. C. Inorg. Chem. 1981, 20, 3064. Draganjac, M.; Simhon. S.; Chen, L. T.; Kantzidis, M.; Baenziger, N. C.; Coucouvanis, D. Inorg. Chem. 1982, 21, 3321. ( 15) Muller, A.; Sabyasachi, S.; Battacharyya. R. G.; Pohl, S.; Dartmann, M. Angew. Chem., Int. Ed. Engl. 1978, 17, 7. (16) (a) Muller, A.; Jostes, R.; Jacgermann, W.; Battacharryya, R. G. Inorg. Chim. Acta 1980,41,259. (b) Muller, A.; Nolte, Wulf Otto; Krebs, B. Angew. Chem., Int. Ed. Engl. 1978, 17, 279. (17) (a) Koestner, R. J.; Salmeron, M.; Kollin, E. B.; Gland, J. L. Chem. Phys. Leu. 1%. 125, 134. (b) Gland, J. L.; Kollin, E. B.; Zaera, F. hngmuir 1988.4, 118. (18) Koniger-Ahlborn, V. E.; Schulze, H.; Muller, A. 2.Anorg. Allg. Chem. 1977.5-428. (19) 011, V. R.; Sureter, D. S.;Schulze, F. A. lnorg. Chem. 19'77, IO,2538. (20) Chandrasekaran, J.; Ansari, M. A.; Sarkar, Sabayasachi J . LessCommon Metals 1987, L23, 134. (21) Muller, A.; Diemann, E.; Jostes, R.; Bogge, H. Angew. Chem., Inr. Ed. Engl. 1981, 20, 934. (22) Muller, A. Polyhedron 1986, 5, I , 323.
Characterization of Supported MoS, Crystallites compound into supported MoS3 is accompanied by a loss of sulfur. This was observed in the thermoprogrammed sulfidation measurements of Arnoldy et aL5 in the same temperature range (330-470 K) with, in addition, detection of H2 consumption and H2S removal. Bulk or supported MoS3, not very stable, can be easily transformed by the laser beam into MoS2. H 2 0 in the sulfidation mixture accelerates the MoS2 formation (see Figure 2, c and d). N , / H 8 Sulfiding Mixture. Sulfidation of a hydrated catalyst proceeds by the same intermediate species (sulfido compounds and MaS3). However, it appears that H2speeds up the sulfidation, since under the same conditions, the oxide features of the precursor are no longer observed when the H2/H2Smixture is used. The sulfidation mechanism of a dehydrated catalyst by N2/H2S appears quite different. The LR spectra may be correlated to a reduced oxysulfide species whose lines at 340,440, 590, 760 cm-’ correspond respectively to Mo-S deformation and Mo-S, S-H, and A1-0 stretching mode^.'^-^^ These experiments pointed out the concomitant role of H, and H 2 0 in the sulfiding mechanism. Description of the Final M0S2 Phase. After sulfidation at 620 K or at higher temperatures and whatever the experimental conditions, MoS, is the only detected phase that has been characterized by both LRS (Figure 2e) or HREM (Figure 7). No definitive conclusion can be established concerning the possible bonds of these MoS, crystallites with the support as the corresponding Raman lines are weak.24 Nevertheless, presence of a broad band at about 760 cm-’ (Figures l e and 2e) could be assigned to vibrations of such bonds.23 For the catalytic applications of these sulfided systems, a precise description of the active MoS2 phase is of crucial importance as dispersion or crystal size may considerably affect the activity. In a geometrical modeling of A1203-supported MoS, compared with experimental catalytic results obtained with hydrogenation or hydrodesulfurization reactions, it was shown that the mean crystallite size or the mean number of Mo atoms in one elemental MoS, platelet is increasing when the Mo loading increases.25 For such two-dimensional small MoS, crystallites, the effects of both the molybdenum loading and the sulfidation conditions on the layer stacking were not considered. In light of this modeling, it is therefore interesting to examine the vibrational results obtained on the different Mo-loaded catalysts and compare them with the direct HREM observation. Shift and broadening of LR line of such sulfided catalysts have been already discussed in terms of disordered structure.26 However, for highly disordered (amorphous) compounds the LR spectrum consists mainly of broad bands which are the result of the Raman active vibrational density of states function:’ covering a range of complex Mo-S stretching, bending, and deformation motions. Such observations have been presented by Chang et a1.28 for amorphous MoS2 with bands in 300-500- and 200-cm-l spectral range. The shift and broadening of the LR line at 380 cm-l could be due to the presence of broad band in the same spectral range whose origin could be due to the existence of amorphous and/or very small crystallite of M O S ~ . , ~HREM micrographs reveal the presence of well-defined small crystallites whereas similar LRS features have been observed for poorly crystallized M O S ~In . ~the present work these LRS features, whatever their origin, can refer either to the mean crystallite size in the “a” direction30since these lines correspond to a Mo-S stretching in a MoS2 layer. The Ezs mode at -32 cm-l corresponding to a vibrational mode between layers cannot be resolved from the Rayleigh line. For small MoS,
The Journal of Physical Chemistry, Vol. 93, No. 17, 1989 6505
12
%Moo3
Figure 8. Shift AU and line width r (fwhm) as a function of the Mo loading. ( I ) Hydrated precursor sulfided at 620 K, 4 h (X). (2) Features of the 385 cm-I line of dehydrated precursor sulfided at 620 K, 4 h: fwhm (0);Au (A). nm 4.
4
8
12 % M & O ~
Figure 9. MoSz crystallite average length versus the Mo loading.
particles with dimensions in the nanometer range (see Figure 7) and by comparison with LRS experiments on other small particles of semiconductor materials3I it appears useful to describe Raman line-shape evolution with two-parameters: the frequency shift w with respect to the bulk compound, and, the full width at halfminimum of this line (fwhm). The two parameters relative to the ~ 3 8 0 - c m - ’line are plotted versus the Mo loading in Figure 8. By comparison of the HREM results reported in Figure 9 with the LRS results reported in Figure 8 it appears that the main effect responsible for the MoS2 Raman line shift and broadening is a crystallite size effect due to lateral growth of the MoS2 particle when the Mo loading increases. This result is directly in relation with the results and discussion we have previously mentioned.2s The 8 Mo sample which was dehydrated before sulfidation does not follow this general trend. LRS appears therefore as a sensitive technique to detect size evolutions of MoS, crystallites supported on a carrier as a function of various parameters such as conditions of sulfidation or Mo loading. In particular it appears that hydration of the oxide precursor has a real effect on the size of the MoS, final phase. This behavior may explain the observation that H,O addition to the catalyst before sulfidation improves the sulfidation rate.s The role of H2 is not clear. It appears that H2 speeds up the sulfidation process but H2 is not necessary to obtain MoS2. The LRS line parameters seem to indicate, in the case of a sulfidation by N,/H,S, presence of MoS2 crystallites of larger size. Conclusion The nature of the surface transient and final species generated during the sulfidation of molybdate-based catalysts has been investigated by in situ LRS. The following scheme can be proposed: d e hydrated
supported
(23) McMillan, P.; Piriou, B. J . Noncryst. Solids 1983, 55, 221. (24) Cheng, C. P.; Schrader, G. L. J . Catal. 1979, 60, 276. (25) Kasztelan, S.; Toulhoat, H.; Grimblot, J.; Bonnelle, J. P. Appl. Catal. 1984, 13, 121. (26) Chien, F. 2.;Moss, S . C.; Liang, K. S . ; Chianelli, R. R. J . Phys. 1981, 42, C4, 273. (27) Shuker, R.; Gammon, R. W. Phys. Rev. Lett. 1970, 25, 4, 222. (28) Chang, C. H.; Chan, S . S. J. Catal. 1980, 72, 139. (29) Fauchet, P. M.; Campbell, 1. H. Crit. Reu. Solid State Mater. Sci. 1988, 79, 14, suppl. 1. (30) Wopenka, B.; Pasteris, J. D. Microbeam Anal. 1988, 196.
E
4
N2/H2SW reduced
oxysulfides
MS2 M s3
supported
hydrated
oxometallates
(3.1) Campbell, I . H.; Fauchet, P. M. International Conference on the Physics of Semiconductors, Stockholm, August 1986.
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J. Phys. Chem. 1989, 93, 6506-651 1
Here M = Mo but other results6 are indicative of a similar behavior with oxotungstate supported on alumina. However, there exist some differences between these two systems. The transformation oxysulfide sulfido compounds seems rather difficult when M is the tungsten. The MoSz final product has been characterized by both HREM
-
and LRS. A correlation exists to indicate that the Raman lines characteristic of MoSz are sensitive to their crystal size. Dehydration of a sample before sulfidation noticeably affects the size of the supported MoS, slabs. Registry No. Mo, 7439-98-7; H,S, 7783-06-4; MoS,, 1317-33-5.
Molecular Motlon of Benzene, n -Hexane, and Cyclohexane in Potassium Zeolite L Studied by Deuterium NMR B. G. Silbernagel,* A. R. Garcia, J. M. Newsam, and R. Hulme Exxon Research and Engineering Company, Route 22 East, Annandale. New Jersey 08801 (Received: January 18, 1989; In Final Form: April 14, 1989)
The molecular motions of perdeuterated benzene, n-hexane, and cyclohexane sorbed at loading levels of 1 molecule per channel lobe (or, equivalently, per unit cell) in potassium zeolite L have been examined by ' H nuclear magnetic resonance (NMR) for 100 K 5 T I350 K. Benzene (C6D6)gives a broad signal with, for T < -150 K, a quadrupolar splitting of half of the static value, interpreted in terms of rapid reorientation in the molecular plane. This mode of motion is consistent with the location of benzene in capping positions above channel wall site potassium cations observed at 78 K by powder neutron diffraction. For temperatures above -250 K, a less broad component (interpreted as indicating activated benzene site hopping) develops. Sorbed n-hexane (C&) also shows no evidence for isotropic motion. Two distinct spectral components, associated respectively with the methyl and methylene deuterons, are observed. The magnitudes of the ZHquadrupolar coupling strengths can be interpreted solely in terms of torsional rotations about the carbon-carbon bonds. The temperature dependence of the 2Hspin-lattice relaxation processes suggests an activation energy of -2 kcal mol-' for these C-C bond rotations, consistent with earlier studies of bond rotation in alkanes. Sorbed cyclohexane (C6D12)shows a transition in the dynamical behavior (on the time scale of the 'H NMR experiment) in the vicinity of 280 K. Close to 280 K, a narrow signal is observed consistent with effectively isotropic reorientations, but both above and below 280 K there is a significant residual quadrupole interaction. The narrow component decreases rapidly with temperature, becoming unobservable below 230 K.
Introduction The mobilities of hydrocarbons sorbed within zeolites can be probed by several techniques.' Such measurements can generally yield intracrystalline diffusion coefficients but provide little information about the character of the hydrocarbon motion on a molecular level. In deuterium (2H) NMR experiments, molecular motions that are faster than the time scale of the N M R measurement (- 10" s) lead to reductions in the effective quadrupolar coupling constants.* Analysis of these effects provides information about the nature of the molecular motion. Measurements on aromatic hydrocarbons in zeolite ZSM-5,37 on benzene in zeolite X,s*7*8 and on dimethyl ether,9 NH,+ groups,1° water and methanol," and methylaminesI2 in zeolite rho have already demonstrated the applicability of these types of measurement to the study of sorbed phases within zeolites. Measurements on similar species in other constrained environments have also been reported.I3-l6 ( I ) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; Wiley-Interscience: New York, 1984. (2) Townes, C. H.; Dailey, B. P. J. Chem. Phys. 1952, 20, 35-40. (3) Eckman, R.; Vega, A. J. J. Am. Chem. Soc. 1983, 105, 4841-4842. (4) Eckman, R. R.; Vega, A. J. J. Phys. Chem. 1986, 90, 4679-4683. (5) Zibrowius, B.;Caro, J.; Pfeifer, H. In Preprints of Workshop III on Adsorption in Microporous Adsorbents; Academy of Sciences GDR: Berlin, 1987; pp 20-28. (6) Kustanovich, I.; Fraenkel, D.; Luz, Z.; Vega, S.; Zimmermann, H. J . Phys. Chem. 1988, 92, 4134-4141. (7) Zibrowius, B.;Caro, J.; Pfeifer, H. J. Chem. Soc., Faraday Trans. 2 1988,84, 2347-2356. (8) Hasha, D. L.;Miner, V. W.; Garces, J. M.; Rocke, S . C. In Catalyst Characterization Science; Deviney, M. L., Gland, J. L.,Eds.; ACS Symposium Series 288; American Chemical Society: Washington, DC, 1985; pp 485-497. (9) Luz, Z.; Vega, A. J. J. Phys. Chem. 1986, 90, 4903-4905. (IO) Vega, A. J.; Luz, Z. J . Phys. Chem. 1987, 91, 365-373. ( 1 1 ) Luz, Z.; Vega, A. J. J. Phys. Chem. 1987, 91, 374-382. (12) Vega, A. J.; Luz, Z. Zeolites 1988, 8, 19-26.
0022-3654/89/2093-6506$01.50/0
In the present paper, we describe 2H NMR experiments on three perdeuterated c6 hydrocarbons, benzene, n-hexane, and cyclohexane, sorbed at loading levels of 1 molecule per channel lobe (or, equivalently, per unit cell) within potassium zeolite L. These molecules differ significantly in their shapes and internal degrees of freedom and give rise to very different types of motion when sorbed within the zeolite L host. Deuterium NMR Properties The resonance properties of the 2H nucleus are dominated by the interaction of the zHnuclear quadrupolar moment with the electric field gradients in its vicinity. These electric field inhomogeneities at the site of a 2H nucleus in a 4 - D bond arise from the positive charge of the neighboring carbon nucleus and the negative charge of the electrons in the bond itself.2 In the NMR experiment, the three spin states of the Z = 1 ZHnucleus are split by the static magnetic field,17 and the quadrupole interaction causes an additional shift of these spin energy states (which reflects the angular dependence given in eq 1 below). For a powder sample in which individual grains have an arbitrary orientation with respect to the applied magnetic field, the weighted average of the orientations yields a characteristic 'horned" spectrum (see, e.g., Figure 1). Improved abilities to calculate the electronic properties of molecules enable the quadrupolar interaction strengths to be calculated to an accuracy of -2%.18 In addition, quadrupolar (13) Silbernagel, B. G.; Gamble, F. R. J. Chem. Phys. 1976, 65, 1914-1919. (14) McDaniel, P. L.; Barbara, T. M.; Jonas, J. J. Phys. Chem. 1988, 92, 626-630. (15) Boddenberg, B.; Grosse, R. Z . Naturforsch. 1987, 42a, 272-274. (16) Heyes, S. R.; Clayden, N. J.; Dobson, C. M.; Green, M. L. H.; Wiseman, P. J. J. Chem. SOC.,Chem. Commun. 1987, 1560-1562. (17) Abragam, A. The Principles of Nuclear Magnetism; Oxford University Press: Oxford, UK, 1961; Chapter 6.
0 1989 American Chemical Society
