1818
Ind. Eng. Chem. Res. 1993,32, 1818-1821
Temperature Programmed Sulfiding of Commercial CoO-M003/A1203 Catalysts P. J. Mangnus,+E. K. Poels,f and J. A. Moulijn**s Department of Chemical Engineering, University of Amsterdam Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands
The potential of the temperature programmed sulfiding (TPS) technique is enlightened using commercial type of CoO-M003/A1~03catalysts. A modified TPS procedure is proposed to simulate more closely industrial sulfiding procedures. A relation is found between the amount of sulfidable cobalt ions and the hydrodesulfurization activity of CoO-M003/Al~03 catalysts, calcined between 823 and 873 K and containing a t most 4 w t % COO. Provided (i) typically industrial calcination temperatures are used and (ii) the catalysts do not contain phosphorus, TPS is a direct method to estimate the amount of cobalt which shows hardly any catalytic activity. Introduction Co-Mo/AlzOs catalysts are extremely important in oil refining processes. They are widely used for the removal of sulfur in all kinds of aliphatic and aromatic compounds. These catalysts are manufactured in the oxidic form. Although sulfiding of the oxidic precursor occurs under reaction conditions, it is common practice to presulfide the catalysts under well-controlled conditions. In this way a high and stable activity is obtained, and as aconsequence, the performance of the catalyst does not depend on the feed used. The discovery of the so-called “CoMoS phase” by Topsae and Clausen (1984) and Wivel et al. (1981) was a milestone in hydrotreating research. They observed a linear relation between the amount of the “CoMoS phase” and the hydrodesulfurization (HDS) activity. In addition to the “CoMoS phase”, Co can be present as COBSS, disperse COSspecies, or CoAl204-like structures (Chung and Massoth, 1980; Wivel et al., 1981; Candia et al., 1982;Topsae and Clausen, 1984; Scheffer et al., 1984; Wivel et al., 1984; Arnoldy et al., 1985a-c; Topsrae et al., 1986). The amount of these various species depends on the preparation method, temperature of calcination, Co/ Mo content, and sulfiding temperature (Chung and Massoth, 1980; Wivel et al., 1981; Candia et al., 1982; Scheffer et al., 1984; Wivel et al., 1984; Arnoldy et al., 1985a-c; Topsae et al., 1986). In summary, the oxidic cobalt species can be divided into easily sulfidable and hardly sulfidable species. The group of easily sulfidable compounds consists of microcrystalline COOand C03O4, octahedrally coordinated Co2+ and C O ~ ions, + and the CoMoO phase (the precursor of the CoMoS phase) (Chung and Massoth, 1980; Candia et al., 1982; Scheffer et al., 1984; Arnoldy et al., 1985a-c; Topsrae et al., 1986). The only compound which is hardly sulfidable is tetrahedrally coordinated Co2+. Since Co species present in the “CoMoS phase” are the most active species for hydrotreating reactions (hydrodesulfurization, hydrodenitrogenation, and hydrodeoxygenation), it is most efficient to prepare catalysts with all Co present in this particular phase. Nevertheless, we observed that appreciable amounts of Co in commercial catalysts were not sulfidable during a standard sulfiding procedure and, therefore, cannot be present as a “CoMoS phase”. + Present address: Akzo Chemicals bv, Nieuwendammerkade 1-3, 1002 GE Amsterdam, The Netherlands. Unilever Research Laboratorium Vlaardingen, Olivier van Noortlaan 120, 3133 AT Vlaardingen, The Netherlands. 1 Present address: Faculty of Chemical Technology and Materials Science, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands.
*
Since a large variety of preparation methods and additives are used, it is difficult to generalize this observation. In this paper, we illustrate this observation using aluminasupported CoMo catalysts prepared according to a commercial preparation method without addition of additives (van Leeuwen et al., 1987). The COOand Moo3 weight content of these catalysts falls in the range commercially applied. Wivel et al. (1981) and Topsae and Clausen (1986) showed that, up to a COOcontent of about 3.4 wt % ,only minor amounts of C0304 are present. Since commercial hydrotreating catalysts generally contain at most 4 wt % COO,major part of the Co species is present as a disperse species. On catalysts calcined at temperatures higher than 770 K, the amount of octahedrally surrounded disperse (surface) species decreases due to the diffusion of the Co ions into the alumina support. This results in the formation of CoA1204 spinel structures in which Co is tetrahedrally coordinated. It is reasonable to expect that a direct relation exists between the amount of sulfidable Co species and the HDS activity. The amount of sflidable Co species can be estimated using various techniques like Miissbauer emission spectrometry (Wivelet al., 1981,1984, Topsrae and Clausen, 19861, X-ray photoelectron spectroscopy (Grimblot et al., 19811, magnetic susceptibility measurements (Chiplunker et al., 19811,and temperature programmed reduction measurements (Scheffer et al., 1990). In the present work the sulfiding degree of commercialprototype catalysts is determined directly from the temperature programmed sulfiding (TPS) patterns, and subsequently, it is investigated if TPS can be used to assess the catalytic activity of conventional industrial catalysts. Experimental Section Catalyst Preparation. The Co-Mo/AlzOs catalysts have been prepared accordingto the patent of van Leeuwen et al. (1987). A 7-Alz03support was impregnated with an ammoniacal solution containing Co and Mo. Coz(0H)zCOrnH20 and (NH~)vMo,OZC~HZO were used as precursor salts. The catalysts have been dried and subsequently calcined at 823or 873K. The catalysts are denoted by the amount of metal oxides per gram of catalyst and the calcination temperature; e.g., Co0(3.7)Mo03(12.9)873 contains 3.7 wt % COOand 12.9 wt % Moo3 and has been calcined at 873 K. The catalysts applied are listed in Table I. Sulfiding Apparatus. A detailed description of the temperature programmed sulfiding (TPS)technique can be found elsewhere (Arnoldy et al., 1985a-c). A sulfiding
0888-5885/93/2632-1818~04.00/0 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 9,1993 1819 Table I. Catalysts Applied catalyst code N203-773 MoO3(15.4)-823 C00(2.O)MoO3(8.6)-823 Co0(3.7)MoO3(12.9)-823 C00(3.7)M003(12.9)-873 CoO(3.9)MoO3(12.9)-823 Co0(2.2)MoO3(13.6)-823
I
, .
I
I
,
,
,
,
,
,
,
I
coo
Moo3
(wt%)
(wt%)
0 0 2.0 3.7 3.7 3.9 2.2
15.4 8.6 12.9 12.9 12.9 13.6
0
calcination temp(K) 773 823 823 823 873 823 823
mixture of 3.3% HzS, 28.1% Hz, and 68.6% Ar was used. The flow rate and pressure used were 11X 1o-B mol/s and 1.05 X lo6Pa, respectively. The amount of catalyst varied between 0.08 and 0.14 g. The TPS measurements were carried out according to the following procedure: (i) flushing of the reactor with argon in order to remove air; (ii) sulfiding a t room temperature for 10.8 X 103 s; (iii) raising the reactor temperature with 0.167 K/s up to 673 K; (iv) isothermal sulfiding at 673 K for 36 X lo3 s; (v) raising the reactor temperature with 0.167 K/s up to 1270 K; (vi) isothermal sulfiding at 1270 K for 1.8 X 103s; (vii) cooling down to room temperature. Thiophene HDS. The thiophene HDS experiments were carried out at two sets of different experimental conditions. These two different activity testa are denoted by A and B. Actiuity Test A: This activity test was carried out in the gas phase at lo5Pa, 623 K, and a flow rate of 34 X lo4 mol/s. The reaction mixture consisted of 9.5 vol % thiophene in H2. The gas mixture has been analyzed with an on-line gas chromatograph. By using six reactors simultaneously, all catalysts could be measured in the same run. Prior to the activity test, the catalysts were sulfided at 673 K in situ, in a mixture of 15% H2S in Hz. Subsequently, the reactor was cooled down to 623 K and the thiophene/Hz mixture was led through the reactors. The reaction rate constants (khds) were calculated on the basis of first-order kinetics after 45 X lo3 s on stream. Activity Test B: This activity test was carried out in the liquid phase in a tubular reactor a t 7 X lo6 P a and between 600 and 643 K. The reaction mixture consisted of a light cycle oil containing 1.71% S and 490 ppm N. The H$oil ratio was 350 mL/mL. The reactor was filled with 3.0 g of crushed catalyst particles (0.25-1.0 mm) diluted with 6 g of Sic (80 mesh). The reaction products were analyzed continuously on-line by gas-liquid chromatographic analysis. Preceding the activitytest,the catalysts were sulfided at 673 K with 10% H2S in Hz. A rate constant khda was calculated after (173-295) X lo3 8 under the assumption of first-order kinetics. This rate constant was normalized to that of an arbitrary reference catalyst and multiplied by 100, resulting in a relative weight activity (RWA) for the catalyst under consideration.
Results The TPS patterns of the CoMo/Al catalysts are presented in Figures 1 and 2. Both the H2 and H2S signals of the Co0(2.O)MoO3(8.6)-823 catalyst are depicted in Figure 1. Since the Hz peaks are observed at the same position for all the catalysts investigated, only the HzS signals are shown in Figure 2. The quantitative data are listed in Table 11. Recently, Scheffer et al. (1984) have reported TPS patterns of series of CoO-M003/Al~03 catalysts with a varying Co/Mo ratio which had been calcined at different temperatures. Despite the fact that the sulfiding patterns presented in this paper exhibit sulfiding characteristics comparable to those for the catalysts studied by Scheffer, the patterns are described in detail since the series of
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3
1
-I
,
aoo
ti73
400
'
I
'
1273
1200
temperature (K) isothermal
'i .___t
!isothermal
Figure 1. TPS pattern of Co0(2.O)MoO~(8.O)/A1~0~ (upper curve, H2S signal; lower curve, Hz signal). A peak in positive direction is a H2S or Hz production, and a peak in negative direction represents a H2S or H2 consumption. I
i i
i
~~
.
! !
I
i i
i i i
i
i
298 isothermal
400
I
-
673
a
i
600
1200
-
b
1273
temperature
I Isothermal I
I Isothermal
Figure 2. TPS patterns (H2S signals) of commercial CoO-MoOs/ Al203 catalysts. Only the H2S concentrations are shown. (a) AlzO3773; (b) CoO(O)MoO3(15.4)-823; (c) Co0(2.O)MoO3(8.6)-823; (d) Co0(2.2)MoO3(13.6)-8.23;(e) Co0(3.9)MoO3(12.9)-823;(f)coo(3.7)MoO3(12.9)-823;(9) C00(3.7)MoO3(12.9)-873. Table 11. Quantitative TPS Data.
H2S prod.
H2S upt
Hfi prod.
catalyst code COO MOO3 A B C N203-773 0 0 165 0 0 0 1069 CoO(O)MoO3(15.4)-823 114 0 0 C00(2.2)MoO3(13.6)-823 294 945 99 156 11 C00(3.9)MoO3(12.9)-823 521 857 95 285 51 C00(3.7)MoO3(12.9)-823 494 895 79 214 57 Co0(2.0)MoO3(8.6)-823 267 597 111 178 24 Co0(3.7)Mo03(12.9)-873 494 895 109 334 62 I, All amounts shown are in pmollg. A H2S production between 673 and 850 K. B: H2S uptake between 850 and 1150 K. C: H2S production at about 1240 K.
catalysts described in this paper (i) have been prepared according to a commercially applied preparation method, (ii) contain metal loadings commercially applied, and (iii) have been sulfided according to a modified procedure. For the sake of clarity, the results will be discussed in two different parts.
1820 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 Table 111. Correlation of the TPS Data with the Results of the Activity Tests* co amt of tetrahedral CoMoS RWA catalyst code C o A B C D E F C00(2.2)M003(13.6)-823 294 156 176 138 118 100 100 C00(3.9)M003(12.9)-823 521 285 321 236 200 154 128 C00(3.7)MoO3(12.9)-823 494 214 241 280 253 178 147 C00(2.0)M003(8.6)-823 267 178 200 89 67 88 87 C00(3.7)M003(12.9)-873 494 334 375 160 119 145 72 0 All amounts are listed inpmol/g of catalyst. A amount of cobalt that has been sulfided above 673 K, assumingthat Co-Al204 species are sulfided to COS. B: amount of cobalt sulfided above 673 K assuming that the Co-Al204 species are sulfided into Co&. C: maximum amount of CoMoS;this amount is calculatedby subtracting the amount of Co given in column A from the total amount of cobalt present. D: maximum amount of CoMoS;calculated as in column C,except by subtracting the amount of cobalt reported in column B. E: relative weight activity based on activity test A. F: same as E, except based on activity test B.
Sulfiding Pattern from 298 to 673 K. All catalysts show a HzS uptake at room temperature causing a color change from white (Mo/Al) or blue (CoMo/Al) to black. The amount of HzS consumed up to about 470 K is larger for the cobalt-containing catalysts than for comparable Mo/A1 catalysts. In the temperature region from 430 to 600 K, a HzS production peak appears simultaneously with a Hz consumption peak. The maximum of this peak is shifted to lower temperatures for the cobalt-containing catalysts. Above about 500 K, all catalysts show a broad H2S consumption peak, whereas almost no Hz is consumed. Sulfiding Pattern from 673 to 1270 K. Immediately after the reactor temperature is raised, two HzS production peaks are observed, whereas hardly any HZis consumed. The amount of HzS produced, in this temperature region, is the highest for the bare A1203 support. Between 800 and 1150 K a broad HzS consumption peak is observed only for the cobalt-containing catalysts. This amount varies for the various catalysts. Around 1240 K, a small HzS production peak coupled to a Hz consumption peak is observed for the cobalt-containing catalysts. The results of the activity tests are shown in Table I11 in the form of relative weight activities. Both activity tests resulted in the same activity order for the catalysts except for the catalyst calcined at the highest temperature, which showed a clear difference between the two test methods. To emphasize the commercial character of the catalyts applied, a well-established commercial reference catalyst, nominally containing 5 wt 76 COO and 16.2 wt 76 MoO3, was tested and showed a HDS RWA of 102. Discussion Sulfiding Pattern below 673 K. Below 673 K, the sulfiding patterns of the catalysts applied in this study are similar to those of the model catalysts discussed by Scheffer et al. (1984). Therefore, only the most important conclusions will be mentioned here. (i)The color change of the catalysts at room temperature after sulfiding indicates that a part of the HzSconsumed results in sulfiding of the metal oxides. (ii)The absence of Hz consumption up to 430 K indicates that sulfiding takes place by an 0-S exchange reaction on the Moa+and Co2+ions, without preliminary reduction of the Mo and Co by hydrogen. (iii)The amount of HzS consumed below 470 K increased after addition of cobalt to the Mo/A1catalysts, confirming that a part of the Co ions is sulfided in this temperature regime.
(iv) The Mo6+ ions are reduced by S elimination from the MoO,S, compounds. (v) The elemental S formed according to this reaction step reacts in the temperature region from 430 to 600 K with HZto HzS, catalyzed by the Co ions. Sulfiding above 673 K. Essentially, the structure of the oxidic CoO-MoOdAlz03 catalysts has been elucidated. Dependent on the loadings and temperature of calcination (Arnoldy et al. (1985a)) it is a mixture of (i) monolayer and bilayer molybdate species (Mas+), (ii) Co-Mo-0 species, (iii) Co3+in surface positions or in a crystalline Co3+-A13+oxide, (iv) CoAl~O4-likespecies, and (v) Co2+ surface species (both octahedrally and tetrahedrally coordinated). In the catalysts applied in this study, the dominating phases will most likely be monolayer and bilayer molybdate, Co-Mo-0, and CoAlzO4-like species. These species are sulfidable below 673 K, except the CoAlzOr-likephase. By introducing an isothermal step at 673 K, the commonly applied sulfiding procedure is simulated and the amount of nonsulfidable species can be determined by raising the reactor temperature up to 1270 K. The absence of a significant HzS consumption peak, for the CoO(0)MoOa(15.41-823catalyst, between 673 and 1270 K shows that disperse Moo3 is sulfided nearly 100%after 10 h sulfiding at 673 K. This result agrees with extended X-ray absorption fine structure data which showed no evidence for Mo in oxygen surroundings after typical sulfiding conditions (Clausen et al., 1981). Since a HzS consumption peak is absent for the Mo/A1 catalyst in the hightemperature region, HzS consumption in this area for the CoMo/Al catalysts can, consequently, be attributed to the sulfiding of cobalt species and, most likely, exclusively to the sulfiding of CoAlzO4-like species containing tetrahedrally coordinated Co2+ions in a spinel lattice. This phase is formed due to solid-state diffusion processes during calcination and, most likely partly due to the nature of the catalyst preparation method; more conventional methods result in less CoAl2O4 spinel but also usually in a worse dispersion of the active phase. From thermodynamics it can be calculated that cogs8 crystallites are stable up to 1000 K, whereas COS^+^ (0.04 Ix I 0.13) crystallites are stable between 1000 and 1200 K. Since sulfiding of the tetrahedral Co species takes place in both temperature regions, it is not clear, a priori, whether we should calculate the amount of Co that is sulfided, between 673 and 1270 K, on the basis of formation of c o S ~or + C09S8. ~ Therefore, both reactions are taken into consideration in Table 111. On catalysts which contain up to approximately 4.0 wt 76 COO and which are calcined above 773 K, CoMoO (precursor of CoMoS) and diluted spinel-likecobalt species are assumed to be the dominating Co-containing compounds (Wivel et al., 1984). Since the Co spinel species is sulfided in the high-temperature part, the maximum amount of CoMoS can be calculated from the difference between the amount of cobalt which is sulfided in the high-temperature part and the total amount of cobalt present. The amount of CoMoS calculated in this way is shown in Table 111. Since the CoMoS phase is the most active phase for the HDS reaction, we expect a direct relation between the amount of CoMoS and the activity. In Figure 3, the RWA activities of both activity tests A and B are plotted versus the (calculated) amount of CoMoS present. As a result, a slightly bent curve is obtained for both activity tests. The decrease of the gradient of the curve for the catalysts with the largest amount of CoMoS indicates that the assumption that only two phases are present in these catalysts is too simple. Most likely, part of the Co is present as Co304 microcrystallites (Wivel et al., 1984; Arnoldy et al., 1985a-c), which are sulfidable
Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 1821 in complete activation of the oxidic precursor. Consequently, large amounts of Co are wasted. TPS can be used to acquire an indication of the HDS activity of catalysts calcined above 770 K.
Acknowledgment
-B ---'
A
The authors gratefully acknowledge that part of the work was financed by the Commission of the European Communities under Contract No. JOUF 00494 (JR).
I
o ' " ' ~ ' ~ " ~ " ' ' " ' " " ' ~ ~ ~ ' ~ ' ' " Literature Cited 0 SO 100 150 200 250 300 estimated amount of CoMoS (pnol/g)
Figure 3. Relative weight activity according to activity test A (0) and B (+) as a function of the estimated amount of CoMoS.
below 673 K but are not really active (Scheffer et al., 1984; Wivel et al., 1984). Hence, the calculated amount of CoMoS phase is an upper limit. At this point, it should be mentioned that the amount of CoMoS can only be determined relatively accurately with the TPS technique when the HDS catalysts are calcined above 773 K, since catalysts calcined at lower temperatures contain significant amount of phases, viz., COO,C03O4, and octahedral Co2+surface species, which are sulfidable below 673 K but do not show extensive catalytic activity. It is remarkable that, after the isothermal stage, H2S is produced without a simultaneous H2 consumption. This production cannot be attributed to desorption of physisorbed H2S since this occurs in the low-temperature region. Since this peak is the largest for the bare support (see Table 11),it is concluded that H2S is essentially formed by recombination of S-H groups present on the A1203 support. The sulfide transformation around 1240 K has been ascribed to the conversion of C09Ss to CO& (Scheffer et al., 1984), according to 4c09s8 + 5H2 Ft 9cO,s3
+ 5H2S
(1)
However, for all catalysts studied, the amount of H2S produced is less than that calculated from the stoichiometry of this reaction. Apparently, a part of the Co species is present as disperse species, being stable against sintering, and, as a consequence, against the abovementioned sulfide transformation. Most likely, this cobalt species is stabilized by the A1203support, while the other part of noncrystalline Co species is located at the edges of the sintered MoS2 slabs. It should be noted that the straightforward conclusions in this paper could only be drawn since the catalysts chosen did not contain additives like phosphorus. When P is added to Co-Mo HDS catalysts, TPS patterns are more complicated (Mangnus et al., 1991) due to the formation of cobalt phosphides in the high-temperature region.
Conclusions By introducing an isothermal stage in the TPS technique, the sulfiding degree of hydrotreating catalysts under industrial conditions can be estimated accurately. CoO-M003/A1203 catalysts calcined between 823 and 873 K and containing at most 4 wt '% COOcontain 40-70 '% of nonsulfidable COOspecies. Preparation methods are still open to improvement, since even sulfiding for 36 X 103 s at 673 K did not result
Arnoldy, P.; de Booijs,J. L.; Scheffer, B.; Moulijn, J. A. TemperatureProgrammed SuKding and -Reduction of CoO/A1203Catalysts. J. Catal. 1985a,96,122-138. Arnoldy,P.; Franken, M. C.; Scheffer,B.; Moulijn, J. A. TemperatureProgrammed Reduction of CoO-MoOs/AlzOs catalysts. J. Catal. 1985b,96,381-395. Arnoldy, P.; van den Heijkant, J. A. M.; de Bok, G. D.; Moulijn, J. A. Temperature Programmed Sulfiding of MoOs/AlzOsCatalysts. J. Catal. 1985~, 92,35-55. Candia, R.; Topsrae, N.-Y.; Clausen, B. S.; Wivel, C.; Nevald, R.; Msrup, S.; Topsrae, H. The Influence of Calcination Temperature on Structural and Catalytic Properties of Co-Mo/AlzOs. In Proceedings of the Climax Fourth International Conference on Chemistry and Uses of Molybdenum; Bary, H. F., Mitchell, P. C. H., Eds.; Climax Molybdenum Co.: Ann Arbor, MI, 1982;pp 374383. Chiplunker, P.;Martinez, N. P.; Mitchell, P. C. H. Some Observations on the Structure and Catalytic Properties of Co-Mo/AlzOs and Ni-Mo/AlzOs Hydrodesulfurisation Catalysts. Bull. SOC.Chim. Belg. 1981,90,1319-1330. Chung, K.S.;Massoth, F. E. Studies on Molydena-alumina Catalysts: VIII. Effect of Cobalt on Catalyst Sulfiding. J.Catal. 1980, 64,322-345. Clausen, B. S.; Toparae, H.; Candia, R.; Villardsen, J.; Lengeler, B.; Ala-Nielsen, J.; Christensen, F. Extended X-ray Absorption Fine Structure Study of Co-Mo Hydrodesulfurization Catalysts. J. Phys. Chem. 1981,85,3868. Grimblot, J.; Dufresne, P.; Gengembre, L.; Bonnelle, J.-P. The Sulfided State of Hydrodesulfurisation Catalysts Characterized by XPS. Bull. SOC.Chim. Belg. 1981,90,1261-1269. Mangnus, P. J.; van Langeveld, A. D.; de Beer, V. H. J.; Moulijn, J. A. Influence of Phosphates on the Structure of Sulfided Alumina Supported Cobalt-Molybdenum Catalysts. Appl. Catal. 1991,68, 161-177. Scheffer, B.; de Jonge, J. C. M.; Arnoldy, P.; Moulijn, J. A. Temperature Programmed Sulfiding of CoO/Mo03/yA120s Catalysts. Bull. SOC. Chim. Belg. 1984,93,751-762. Scheffer, B.; Dekker, N.; Mangnus, P. J.; Moulijn, J. A. A Temperature-Programmed Reduction Study of Sulfided Co-Mo/AlzOs Hydrodesulfurisation Catalysts. J. Catal. 1990,121,31-46. Topsse, H.; Clausen, B. S. Importance of Co-Mo-SType Structures in Hydrodesulfurisation. Catal. Rev.-Sci. Eng. 1984,26,395420. Topsrae, H.; Clausen, B. S. Active Sites and Support Effects in Hydrodesulfurization Catalysts. App. Catal. 1986,25,273-293. Topsrae, H.; Clausen, B. S.;Topsrae, N.-Y.; Pedersen, E. Recent Basic Research in Hydrodesulfurization Catalysis. Znd. Eng. Chem. Fundam. 1986,25,25-36. van Leeuwen, W. A.; Poels, E. K.; Staal, L. H.; Verzijl, D. US Patent 4,665,048,1987. Wivel, C.; Candia, R.; Clausen, B. S.; Msrup, S.; Topsrae, H. On the Catalytic Significance of a Co-Mo-S Phase in Co-MoIAlzOs Hydrodesulfurization Catalysts Combined in Situ M6ssbauer Emission Spectroscopy and Activity Studies. J. Catal. 1981,68, 453-463. Wivel,C.; Clausen,B. S.;Candia,R.; Msrup, S.;Topsrae,H. MBasbauer Emission Studies of Calcined Co-Mo/AlzOa Catalysts Catalytic Significance of Co Precursors. J. Catal. 1984,87,497-513. Received for review January 5, 1993 Revised manuscript received May 17,1993 Accepted June 7, 1993
