Ind. Eng. Chem. Fundam. 1981, 20, 1-5
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Structure of Nickel-Molybdenum Hydrodesulfurization Catalysts Jorge Lalne and Kerry C. Pratl* Department of Chemical Englneering, Imperlal College, London SW7 2BY. England
X-ray diffraction (XRD) was used to investigate the structure of the oxidic and sulfided forms of the Ni-Mo-AI2O3 hydrodesulfurization catalyst. The oxidic phase is suggested to be composed of an intimate dispersion of Ni and Mo oxides in the form of a hydrated solid solution (xNi0.Mo03.yH20).The acthre phase in the operating catalyst is thought to consist of thin layers of highly dispersed, defected MoS2 covering the support. Ni is thought to reduce the formation of AI,(MoO,),, which is difficult to sulfide, probably by the formation of a nickel spinel.
Introduction Catalytic processes for the removal of sulfur from crude oils have been in operation for many years. More recently, however, as a result of the impending depletion of petroleum deposits, existing catalysts are being called upon to desulfurize increasingly heavier feedstocks and to provide the basis of catalytic coal-to-oil conversion processes. In spite of the intensive research occasioned by these new applications, the chemistry of catalytic hydrodesulfurization and the structure of the operating catalyst are still incompletely understood. Most of the work has been devoted to the Co-Mo system, while somewhat less information is available for the Ni-Mo catalyst. The Ni-Mo system is generally agreed to be less active for hydrodesulfurization than Co-Mo, though it appears to have better hydrorefining properties (Weisser and Landa, 1973). Generally speaking, these catalysts are manufactured by impregnation of a support by solutions containing the active metal ions. After drying and calcining, the catalyst precursors are present in their oxidic form. The structure of the oxidic form influences the final catalyst and has been the subject of a number of investigations (MonB, 1976; Lditau et al., 1976). One picture of the oxidic form involves a layer structure, in which Mas+ is incorporated in an epitaxial monolayer of Moo3 (Schuit and Gates, 1973; Massoth, 1975). An additional layer of oxygen ions (termed the "capping layer") is located on top of the monolayer to maintain charge nuetrality. Some of the promoter (Co or Ni) remains on the surface as an oxide while a portion penetrates the bulk in coordinations analogous to the respective spinels (Co is tetrahedral and Ni octahedral) (MonB, 1976). Formation of the nickel spinel appears more facile than cobalt (Lo Jacono et al., 1973). Alternatively, the possibility of the formation of small crystallites of molybdenum and promoter oxides on the surface cannot be discarded. In such a case, the degree of interaction of Mo and Co(Ni) with the support would be expected to be somewhat less than in the monolayer model. Activation of the catalyst involves the process of sulfidation, and three models have been advanced to describe the operating catalyst. The first is simply an extension of the monolayer theory outlined above, in which the oxygen ions of the capping layer are replaced by sulfur ions (Schuit and Gates, 1973; Massoth, 1975). In the second, the intercalation model, promoter ions are supposed to be intercalated at the edges of layers of MoS2, giving rise to sites consisting of exposed Mo3+ions (de Beer and Schuit,
* CSIRO, Division of Materials Science, Catalysis and Surface Science Laboratory, University of Melbourne, Parkville, Vic. 3052, Australia. 0196-4313/81/1020-0001$01.00/0
Table I. Key t o X-ray Diffraction Spectra phase
file no.a
MOO, (orthorhombic) MOO, (hexagonal) MOO, AlAMoO,), MoS, NiMoO, NiMoO,.H,O xNiO.MoO,.yH,O
5-508 21-569 5-452 23-764 6-97 18-897 13-128 12-348
Powder Diffraction File, 1976.
1976). The third proposal, the synergistic model (Hagenbach et al., 19731, conceives of a synergistic effect, arising from contact of the sulfides of promoter and molybdenum, presumably as a result of electron transfer. It is worth observing that only in the case of the monolayer model is the support considered to play an active role in the catalytic chemistry. In this investigation, X-ray diffraction was applied to the oxidic and sulfide forms of Ni-Mo catalyst of varying compositions and different pretreatment conditions, in order to determine the phases present at various stages of the catalyst preparation. Experimental Section (i) Preparation of Catalysts. Supported catalysts were prepared by co-impregnation (at 80 "C) of y-alumina (supplied by BDH, surface area 114 m2/g, size range 150-300 mesh), with solutions containing ammonium molybdate and nickel nitrate, in amounts calculated to give the composition required. The slurry containing the catalyst was then evaporated to dryness at 120 "C over 15 h. Calcinations were carried out at the required temperature under a flow of dry air for 5 h. When required, presulfiding was carried out under pure H2S at indicated temperatures and times. Following this, the samples were purged with helium for 15 min and stored in airtight containers to await reaction testing. (ii) Surface area Determinations. These were made using the BET method, by gravimetric measurement of adsorbed nitrogen at liquid nitrogen temperature. (iii) X-ray Diffraction Analysis. A conventional powder diffractometer was employed to determine crystalline phases, using 30 kV Cu K a radiation and a nickel filter. Instrumental broadening was determined using a TaSz foil and was less than 0.3' over the range of diffraction angles employed. The keys used to identify the peaks in the figures accompanying this paper are given in Table I. In the text that follows, the composition of the catalyst will be referred to as y / x (= % NiO/% MoOJ. The percentages refer to the total weight of the supported 0 1981 American Chemical Society
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Ind. Eng. Chem. Fundam., Vol. 20, No. 1, 1981
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catalyst. Further experimental details have been given elsewhere (Laine, 1977).
Samples of unsupported NiMo04-H20(yellow) (Alfa Chemicals) were calcined at 300,500, and 700 "C (Laine and Pratt, 1979). The corresponding XRD traces are shown in Figure 2. The sample calcined at 300 "C displayed a similar pattern to the uncalcined material and indicated the presence of NiMo04-H20. However, the calcined sample developed a yellow-brown color. During outgassing of this material (at 250 "C) prior to surface area measurement, a significant loss of weight was observed (10.5%for the uncalcined sample, and 7.1% for that calcined a t 300 "C). Nevertheless, the measured surface areas were not too dissimilar: 46 m2/g (uncalcined) and 40 m2/g (calcined). It appears that calcining at 300 "C serves only to eliminate some water without substantially altering the crystalline arrangement. Hence the brown tone appearing in the sample calcined at 300 OC is apparently a result of the lower water content. For the samples calcined at 500 and 700 "C, two other nickel molybdate phases were detected: NiMoOl (p-phase Plyasova et al., 1973a) and the solid solution xNiO. Mo03.yH20. The peaks represented diagrammatically by lines in Figure 2 were quite broad for both the uncalcined sample and the sample calcined at 300 "C, but they became very sharp for the sample calcined at 700 "C. This suggests that contraction of a poorly crystalline structure (the hydrated phase) has occurred during the elimination of water. This hypothesis is substantiated by the values of the surface areas determined: 40 m2/g, and 9 m2/g for the samples calcined a t 300 and 700 "C, respectively. The presence of the second hydrated phase, even after calcination at 700 "C, demonstrates strong water retention by nickel molybdate and confirms the existence of solid solution formation between NiO and MOO, (Plyasova et al., 197313). The value of x should be unity in this case. The weight ratio Ni/Mo in the supported catalyst of composition 5/10 is essentially the same as in NiMo04. This fact, in addition to the observations above, leads to
Figure 1. XRD traces of catalysts 5 / X calcined at 500 'C: catalyst 5/15; B, catalyst 5/35; C, catalyst 5/60.
Results and Discussion The XRD traces for catalysts calcined at 500 "C, containing a fixed Ni composition (5% NiO) and variable Mo composition are shown in Figure 1. Signals characteristic of NiMo04 and two modifications of Moos begin to appear for MOO, concentrations higher than 15%. It should be noted, however, that compositions of MOO, higher than about 15% are not employed commercially and are likely to contain bulk crystallites of MOO, on the surface. Our calculations (Laine, 1977) show that for the support employed in this work, a monolayer of MOO, is formed at about 12% w/w Moop Nevertheless, it is likely that the phases detected at the higher concentrations are also present in the catalyst containing 15% or less Moo3 The sample containing 10% MooBshowed neither Ni nor Mo oxide signals. This is not to suggest that these phases do not exist, but is most probably a result of the signals being indistinguishable from the baseline noise level (indicated by a dashed line) at the concentrations involved. The colors of these samples also give an indication of the phases present. The samples containing the high concentrations of Mo (5/35 and 5/60) as expected, show a pale green color, corresponding to the color of the pure unsupported Moo3 The sample of composition 5/15 had a green-brown tone, implying the presence of NiMo04 (green), and NiMo04. H20 (yellow). The catalyst 5/10 had a yellow-brown color, suggestive of the pure hydrated nickel molybdate. Thus, although no X-ray signals for NiMo04.H20 were detected for this catalyst, it seems possible that this phase may be present in small concentrations, or in small or imperfect crystallites, a t the surface of the support. These speculations are not intended to be definitive, but they do provide some guidelines as to the phases which may be present.
Ind. Eng. Chem. Fundam., Vol. 20, No. 1, 1981
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Figure 4. Detail of XRD peak at around 67 O C : A, alumina as received; B, alumina calcined at 800 "C; C, catalyst 0135 calcined at 800 O C ; D, catalyst 1010 calcined at 800 O C . Figure 3. XRD traces of various catalysts A, catalyst 0135 calcined at 500 "C; B, catalyst 0135 calcined at 800 "C; C, catalyst 5/35 calcined at 600 "C.
the suggestion that the structure of oxidic precursor in the low concentration supported catalysts calcined at 500 "C (for which no XRD signals were detectable) is a dispersion of semi-amorphous Ni and Mo oxides in the hydrated state, with an atomic arrangement similar to xNiO. Mo03.yH20 (or NiMo04-yH20for the case of catalyst 5/10). The optimum calcination temperature for the supported 5/ 10 catalyst is found to be approximately 500 "C (Laine et al., 1979a). Figure 3A shows that the sample 0/35 (Le., containing no nickel) exhibits the same Moo3 signals as the sample 5/35 at a calcination temperature of 500 "C (Figure 1B). However, the relative intensities of some of the peaks for the sample 0/35 are lower than for the 5/35 catalyst. At 800 "C (Figure 3B), all the Mo reacted with the support to form A12(Mo04)3.It appears likely then, that in the case of 0/35 catalyst calcined at 500 'C, a small (undetectable) amount of Mo reacts with the support to form a "surface A12(M004)3(',this reaction being inhibited by the presence of Ni. Further support is given to this suggestion by examining the relative peak intensities between Figures 1B and 3A in the region of 28 = 23.4'. Whereas the intense Moos peaks at 25.8 and 27.6' are substantially reduced when passing from the 5/35 to the 0135 sample, the peak at 23.4' apparently remains with the same intensity in both samples. However, this region (ca. 23.4') also covers the location of the main peaks of A12 MOO^)^, and thus we suggest that this peak in the 0/35 sample represents the overlap of the main peaks of Al2(MoO4)3with the peak of Moo3 at 23.4', giving further credence to the idea of the formation of the surface &(M004)3 in the absence of Ni at 500 "C. Particle shape effects might also produce such results, but this seems relatively unlikely in the
supported catalyst. The presence of A ~ , ( M o O ~in) ~unpromoted Mo03-A1203catalyst was also reported by de Beer et al. (1976a). In the sample 0/35, it might be expected that the A12(Mo04)3is associated with the bulk alumina of the support, whereas the Moo3is composed of independent crystallites. At lower Mo concentrations, less bulk Moo3 should be found, and at concentrations around 10-20% Moo3, Mo probably forms mainly a surface A12(M004)p At a calcination temperature of 600 "C, (see Figure 3C), Al2(MoO4I3is clearly identified, together with Moo3 and NiMo04 in the nickel containing catalyst. We conclude therefore, that one effect of the Ni is to retard the formation of A12(Mo04)3at temperatures below 600 "C. Considering that the hydrodesulfurization activity of the catalyst is reflected in its sulfur content (Laine et al., 1979b; Stork et al., 1974; Tanatarov et al., 1972), it is to be expected that the presence of &2(M004)3 would lower the activity of the catalyst, as a result of the low sulfidability of this phase (Laine, 1977). An examination of the crystal structure of some Mo compounds shows that Mo is tetrahedrally coordinated in Al2(M004)3 (Stork et al., 19741, and octahedrally in Moos (Wyckoff, 1965). Both coordinations have been reported in the literature (Ashley and Mitchell, 1969; Lipsch and Schuit, 1969) as apparent contradictions. However, the results above suggest that both could be present, depending upon which compounds are formed, which in turn, is most likely dictated by the method of preparation of the particular catalyst. In order to investigate the behaviour of the alumina support, the strong y-alumina peak at 28 = 67' (corresponding to the 440 plane) was analyzed in some detail by fixing the sample at various angles in this range and noting the count frequency on the scintillation counter of the XRD apparatus. The results are shown in Figure 4. The
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Ind. Eng. Chem. Fundam., Vol. 20,No. 1 , 1981
Table 11. Surface Areas of Catalysts Calcined at 800 "C cat. compn, BET surface % NiO/MoO, area, m'/g 0/10 5/10 510 010
12 28 60
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crystal size can be related inversely to the half-height breadth by the Scherrer equation (Klug and Alexander, 1974). Comparing Figures 4B and C, we see that alumina peak is considerably narrower in the Mo-containing catalyst for the samples calcined at 800 "C. This suggests that Mo promotes sintering of the support, and this is confirmed by examining the values of surface area given in Table 11. This phenomenon has been previously reported in the literature (e.g., McKinley, 1957; Ratnasamy et al., 1974; Srinivasan et al., 1979). The surface area data for the 5/0 catalyst show that Ni also causes sintering of the alumina to some extent, indicating an increase in the particle size. However, comparison of Figures 4B and D shows a slight broadening of the alumina peak in the nickel-containing catalyst. If this broadening is attributed to a decrease in particle size, then a conflict arises, which requires closer examination. One possibility is that the apparent broadening is really a result of the overlapping of some new peaks with the alumina peak in question. The presence of two intense peaks very close to one another around that particular angular location is a characteristic of the Ni spinel of formula 2Ni0.9Al2O3(Powder Diffraction File, 1976). The two peaks are in fact located at 28 = 66.3' and 28 = 66.8'. Unfortunately, the presence of this spinel would be difficult to detect at lower calcination temperatures (e.g., 500 "C) more representative of the conditions of manufacture of commercial HDS catalysts, because of extensive broadening of the peaks, and the coincidence of all the other main peaks of both alumina and the spinel. Diffraction peaks can also be broadened through defect contributions. Under ideal conditions, the two forms of peak broadening (defect and particle size) can be distinguished by examining the angular dependence of the broadening. Unfortunately, this was not possible in this investigation. However, it does raise the possibility that the alumina in the nickel-containing catalyst is more defected than in the original alumina. In fact, such defect broadening, as a result of FeA1204defects in the ammonia synthesis catalyst, has been observed (Ludwiczek et al., 1978). Ni would be present as a point defect in the spinel, and this might lead to some broadening. However, it seems unlikely that the rigid alumina lattice could be strained sufficiently to result in line broadening. Some strain may occur at the interface of the "surface Ni spinel" and the bulk alumina. Evidence for the formation of Ni spinels has also appeared in several other investigations (Mond, 1976; Lafitau et al., 1976). It has been suggested (Schultz, 1972) that under certain conditions, the relaxation of the elastic fields about defects can lead to a stable equilibrium particle size. Thus it seems possible that at low temperatures (e.g., less than 500 "C), a "surface" Ni spinel may be formed, as well as the A12(Mo04)3.Competition between the formation of these phases, or perhaps stabilization of particle sizes, as a result of elastic field relaxation about defects, may be among the reasons for the observed stabilization of surface area in the nickel-molybdenum catalyst. However, the exact origin of the line broadening requires further study and could provide valuable infor-
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+20 Figure 5. XRD traces of 5/35 catalyst sulfided at different temperatures and times: A, 400 " C , H2S for 3 h; B, 400 "C,H2S for 1 h; C, 500 "C, H2S for 3 h; D, 500 "C, H2S for 1 h.
mation relating to the role of Ni in the catalyst. Figure 5 shows the XRD traces for the sulfided catalyst of composition 5135. The peak located at about 28 = 14" corresponds to the 002 plane of MoS2. This is the plane in the direction of the layers. There is evidence of a low intensity broad peak at about 28 = 33.5', which may correspond to the 101 plane of MoS2. No other signals of MoS2 were detected, and the peak at 28 = 14" is very broad. This suggests that the number of layers constituting the MoS2 crystals is quite small. This is confirmed by recent transmission electron microscope studies (Pratt and Sanders, 1980) of similar catalysts which showed the number of layers in the MoS2 structure to be often only 1 or 2, and never more than 10. Again, the broadening could be attributed to defects, and calculations show that the breadth of the fourth-order peak (correspondingto the 004 plane) should be only marginally greater than the 28 = 14" peaks; hence it should be visible. Its absence suggests defect broadening. However, the 28 = 14' peak is by far the most intense of the MoS2 spectrum, and the intensity of the fourth-order peak is so low that it would not be seen in our work. It is possible, however, that considerable defecting occurs as the electron microscope studies (Pratt and Sanders, 1980) showed the MoS2 to present as highly dispersed, distorted "books" of MoS2 containing only 1 to 10 layers. Recently, Delannay et al. (1979) have observed dislocations in the flakes of MoS2and reported a marked decrease in the c parameter of the MoS2 at low promoter concentration. However, electron microprobe analysis was not able to detect any contamination of the MoS2 phase by the promoter atoms, within the sensitivity of their technique (0.13% atomic ratio, promoter/Mo). The structure of the MoSz crystallised at high temperature (lo00 "C) observed by Delannay et al. (1979) bears a striking similarity to the "rag" structure for MoS2 recently proposed by Chianelli et al. (1979). It is likely that considerable strain exists in such structures, which would account for the highly broadened X-ray diffraction pattern. Signals corresponding to Moo2were also detected. This may be expected in view of the mechanism of sulfidation of bulk Moo3 proposed by Sotani (1975) wherein the sulfidation would proceed according to
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Ind. Eng. Chem. Fundam., Vol. 20, No. 1, 1981 MoS~
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Simultaneous reduction of Moo3to Moo2occurs, together with sulfidation, and the Mooz so produced is difficult to sulfide. Sulfided samples of low loadings 6/10) did not show any peaks corresponding to sulfides. This is probably a consequence of the low concentrations and small crystal sizes. At these lower concentrations, sulfiding was found to be easier (de Beer et al., 1976b),perhaps because Moo2 is not formed as in the case of the bulk oxide described above. X-ray analysis of sulfided samples of unsupported nickel molybdate and molybdenum trioxide showed the formation of broad peaks at 26’ = 14’. Analysis of commercial MoS2 (B.D.H.) resulted in a set of sharp peaks, with a very intense signal at 28 = 14”. This suggests relatively large, perfect crystals, which adopted a preferred orientation in the direction of the layers in the diffraction apparatus. Testing of these samples (Laine, 1977) showed that both the sulfided nickel molybdate, and molybdenum trioxide were active for thiophene desulfurization, while the commercial MoS2 had no detectable activity. The observations above seem to suggest that the MoS2 is present as small, defected crystals, only a few layers thick. Whether the defective structure is a result of point defects due to Ni atoms, strain induced defects, or both, is not clear from this work. It is interesting to note that Wentrcek and Wise (1978) in experiments with a single crystal of MoS2attribute the increasing activity of the MoSz as Co is added, to the increase in hole carrier density, as a result of defect conductivity induced by Co. Recently, Laine et al. (1979b) have demonstrated that the concept was consistent with the reactivity behavior of supported Ni-Mo catalysts. A detailed study of the angular dependence of line broadening in these catalyst systems may well provide some confirmation of the speculations above. Conclusions For the catalysts investigated containing a fixed NiO concentration of 5% and a calcination temperature of 500 “C, nickel molybdate and two crystallographic modifications of Moo3 were detected for Moo3 concentrations on the catalyst higher than about 15%. For the concentrations of Moo3 used in industrial catalysts (12-E%), the structure of the oxidic phase is suggested to be a dispersion of Ni and Mo oxides, intimately associated, spread over the surface of the support. This dispersion may be in the form of a hydrated solid solution, xNi0.Mo03.yH20. It was found that the presence of Ni to some extent prevents the interaction of Mo with the support, which would lead to the formation of Alz(Mo04)3.This phase, detected in the absence of Ni, at a calcination temperature of 500 “C, and at 600 “C in the presence of Ni, is thought to afford less hydrodesulfurization activity, since it is
5
difficult to sulfide. In addition, the formation of A12is accompanied by a significant increase in crystallite size, suggesting that Mo promotes sintering of the support during calcination. The structure of the sulfided catalyst appears to consist of highly dispersed defected layers of crystalline MoSz covering the support. This covering is relatively thin (probably less than 10 layers). The exact nature of the defects is not clear from this work. They may arise from strains induced by buckling of the layers of MoS2or from the introduction of Ni (by intercalation between layers or by replacement of Mo) to form point defects. The role of such defects in the production of positive hole carriers in Co-Mo catalysts has been highlighted by Wentrcek and Wise (1978) and would be consistent with the observations above on the Ni-Mo system.
Literature Cited Ashley, J.; Mitchell, P. C. H. J . Chem. SOC. A 1060, 2730. Chianelll, R. R.; Prestridge, E. B.; Pecoraro, T. A,; De Neufvllle, J. P. Sclence 1070. 203l4385).1105. de Beer,’ V. h.’J.; van der Aalst, M. J. M.; Machlels, C. J.; Schuit. G. C. A. J . Catal. 10768, 43, 78. de Beer, V. H. J.; Bevelander, C.; van Slnt Fiet, T. H. M.; Werter, P. 0.A. J.; Amberg. C. H. J. Catal. 1078b, 43,68. de Beer, V. H. J.; Schuit. 0. C. A. “Preparation of Catalysts”, Delmon, 6.; Jacobs, P. A.; Poncelet, G., Ed.; Elsevler: Amsterdam 1976;p 343. Deiannay, F.; Thakur, D. S.; Delmon, 8. J. Less-Common Met. 1070, 63,
265. Hagenbach, 0.; Courty, Ph.; Delmon, 8. J. Catal. 1073, 37, 264. Klug, J.; Alexander, L. “X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials”, Wlley: New York, 1974. Lafitau, H.; Neel, E.; Clement, J. C. “Preparation of Catalysts”, Delmon, 6.; Jacobs, P. A.; Poncelet, 0..Ed.; Elsevler: Amsterdam, 1976;p 393. Laine, J. Ph.D. Thesis, Imperial College, London, 1977. Laine, J.; Pratt, K. C. React. Klnet. Catal. Left. 1070, 70(2),207. Lalne, J.; Pratt, K. C.; Trlmm, D. L. J. Chem. Tech. Blotechnol. 1070, 29,
397. Lalne, J.; Pratt, K. C.; Trlmm. D. L. I d . Eng. Chem. Prod. Des. Dev. 1070,
78,329. Llpsch, J.; Schult, G. C. A. J. Catal. 1069, 75, 163. Lo Jacono, M.; Cimlno, A.; Schuit, G. C. A. Gazz. Chlm. Ita/. 1073, 703, 1281
Ludiizek, H.; Preislnger, A.; Flscher, A.; Hosemann, R.; Schonfeid, A.; Vogel, w. J. Catal. 1078, 57,326. McKinley, J. 8. “Catalysis”, Vol. V Emmett, P. H., Ed.; Reinhold: New York,
1957:D 405. Mon6, R: ‘Preparation of Catalysts”, Delmon, B.; Jacobs, P. A,; Poncelet, G., Ed.; Elsevier: Amsterdam, 1976;p 381. Plyasova. L. M.; Ivanchenko, I.Yu.; Andrushkevlch, M. M.; Buyanov, R. A,; Itenberg, I. Sh.; Khtsmova, G. A.; Karakchiev, L. G.; Kustova, 0. N.; Stepanov, 0.A.; Tsalllngol’d, A. L.; Plllpenko, F. s. Kinet. Catal. (Engl. Transl.) 1973a, 14, 882. Plyasova, L. M.; Andrushkevlch, M. M.; Buyanov, R. A,; Itenberg. I. Sh. Klmt. Catal. (Engl. Transl.) 1073b, 74. 1190. Powder Diffraction File. (Alphabetical Listing). Joint Committee on Powder Dlfferaction Standards, Swarlhmore, Pa. 1976. Pratt, K. C.; Sanders, J. V. J . Catal. 1080, in press. Ratnaswamy, P.; Mehrota, R. P.; Ramaswamy, A. V. J. Catal. 1074, 32. 83. Schuit, G. C. A.; Gates, B. C. AIChEJ. 1073, 79, 417. Schuitz, J. J. Catal. 1972, 27,64. Sotanl, N. Bull. Chem. SOC. Jpn. 1075, 48, 1820. Srinlvasan, R.; Liu, H. C.; Weller, S. W. J. Catel. 1070, 57,87. Stork, W. K J.; Coolegen, J. G. F.; Pott, G. T. J. Catal. 1074, 32, 497. Tanatarov, N. A.; Fashutdlnov, R. A.; Levinter, M. E.; Akhmetov, I. 0.Int. Chem. Eng. 1072, 72,85. Weisser, 0.;Landa, S. “Sulphide Catalysts, Their Propertles and Applications”, Pergamon: Prague, 1973. Wentrcek, P. R.; Wise, H. J. Catal. 1078, 57,80. Wyckoff, R. “Crystal Structures”, Interscience: New York, :965.
Received for review September 25, 1978 Accepted August 22, 1980
