Ind. Eng. Chem. Fundam. 1988, 25, 25-36
25
Recent Basic Research in Hydrodesulfurization Catalysis Henrlk Tops~le,”Bjerne S. Clausen, Nan-Yu Topsae, and Erik Pedersen HaMor T o p s ~ eLaboratories, DK-2800 Lyngby, Denmark
Hydrodesulfurization (HDS) catalysts such as Co-Mo/Al,O, and Ni-Mo/Al,O, are among the most important industrial catalysts. I n spite of their widespread industrial use, very little basic knowledge has been available and the industrial developments have occurred to a large extent as a result of trial and error experiments. The introduction of new approaches and techniques to the problem has recently changed the situation. The active structures (so-called Co-Mo-S or Ni-Mo-S type structures) and active sites have been identified and characterized. This has allowed one to understand the observed catalytic behavior (e.g., the pronounced promotional effect). Insight has also been obtained regarding the parameters governing the formation of the active phase and the important role played by supports (e.g., alumina, silica, and carbon).
culty of performing spectroscopic measurements while the catalytic reaction takes place. Therefore, scientists have been searching for new spectroscopic techniques that avoid these difficulties. One such technique is Mossbauer spectroscopy (Dumesic and Topsae (41),Delgass et al. (34), and Topsrae et al. (103)).In situ studies are conveniently carried out since the radiation used is high-energy y-rays, which easily penetrate a catalyst and its working environment. Mossbauer spectroscopy also has the advantage that structural and chemical information may be obtained about highly dispersed or noncrystalline phases like those encountered in HDS catalysts. The initial Mossbauer spectroscopy measurements of Co-Mo/A1203 catalysts were carried out in 1972-1973 by Topsrae and Boudart (97) by introducing (“doping”) small amounts of 57Feinto such catalysts during the inital impregnation of the alumina carriers. These studies were continued and extended by Topsae and Marup (104) and Topsae et al. (102). The results (Figure 1) showed that sulfiding drastically changes the environment of the Mossbauer atoms and that these are present in surface phases after sulfiding. However, in such doping experiments it is not known to what extent the information obtained on the 57FeMossbauer atoms relates to the Co promoter atoms. Therefore, these investigations were discontinued since later studies by Clausen et al. (27) showed that the Co atoms could be studied directly by doping the catalysts with radioactive 57C0and using socalled Mossbauer emission spectroscopy (MES). The features of MES have been discussed elsewhere (Mrarup et al. (73) and Topsae et al. (101, 102)). The in situ MES technique has allowed one to identify the type of Co phases present in typical sulfided CoMo/Alz03catalysts. Some of the c o species observed were found to correspond to well-known Co structures such as Co in the alumina (Co:A1203)and in Co9S8(10,18,19,20, 27, 73, 95, 100, 101,102, 120). The fraction of Co atoms present in the alumina support was typically observed to be small, but it depends critically on the preparation parameters. For example, Candia et al. (20) observed that the amount of Co:A1203increases with increasing calcination temperature. Also, the amount of Cogsswas found to depend critically on preparation parameters such as the Co loading (Wive1 et al. (120),Breysse et al. ( l o ) ,and Candia et al. (19)),the order of impregnation (Breysse et al. (10)and Candia et al. (20)),and the sulfiding conditions (Candia et al. (18, 19)). Besides the above Co structures, the MES investigations showed that Co may also be present in a MoSz-like cobalt-molybdenum sulfide structure which did not corre-
A. Introduction The public is made increasingly aware of the problems related to sulfur pollution arising, for example, from the burning of coal or fuel oil. In principle, the most attractive way of avoiding such emissions will be to remove the sulfur from the fuel before it is being used. This type of sulfur removal is carried out presently in oil refineries by catalytic hydrodesulfurization (HDS). The industrial development which has occurred in HDS catalysts (e.g., Co-Mo/Alz03 or Ni-Mo/A1203) has to a large extent been based on trial and error type experiments rather than on an understanding of the nature of the active phase and the factors governing its formation. Despite extensive research, it has been very difficult even to establish the form in which the elements are present in the active state of such catalysts. (For reviews of the earlier literature and models see, e.g., ref 35,45,46,49,53,67, 69, 71,82,95,99,116,and 123.) Thus, hydrodesulfurization catalysts give an excellent example of a field where practical implementation has preceded fundamental understanding. The need for industry to change this situation has stimulated very extensive efforts in basic research. These efforts have been quite successful recently such that both structural and catalytic aspects are now relatively well understood. Much of the recent progress can be attributed to the introduction of various novel techniques such as Mossbauer emission spectroscopy (MES) and extended X-ray absorption fine structure (EXAFS). Our improved understanding of HDS catalysts has been obtained by combing the results from these novel techniques both with measurements of catalytic activity and with results obtained from more conventional techniques such as infrared spectroscopy (IR), high-resolution and analytical electron microscopy (HREM and AEM), X-ray photoelectron spectroscopy ( X P S ) ,magnetic susceptibility, and chemisorption measurements. The great advantage of MES and EXAFS in the studies of HDS catalysts has been the possibility of obtaining direct structural and chemical information while the catalysts are in the working state (i.e., in situ studies can be carried out). This has allowed one to assess directly the catalytic implications of the results. We will presently give an overview of the new developments.
B. Nature of Phases in Active Co-Mo/A120, Catalysts The previous lack of detailed knowledge concerning the nature of phases in HDS catalysts is related to both the absence of well-crystallized phases and the general diffi0196-4313/86/1025-0025$01.50/0
0
1986 American Chemical Society
26
Ind. Eng. Chem. Fundam.. Vol. 25. No. 1. 1986
Table I. Comparison of the Structural and Chemical Properties of Co-Mo-S and Co,S, As Determined by a Number of Techniques method Co-Mo-S COP, MES
IS = 0 . 3 5 mm s-' QS = 1 . 0 - 1 . 3
mm s-'
1 doublet
B D = 200 K
XPS MSa
CO,,,,~ BE= 779.0 eV, line symmetric x = 4.0 x emu/g Co (at 3 0 0 K),
EXAFS
temperature dependent structure MoS.-like. Co-S distance 2.27 a, co Gas IO& sulfur CN
AEM
Co at MoS, edges, Co/Mo edges up
IS = 0 . 3 5 mm s-'
ref 101,102
1 1'
mm s - ' doublet IS = 0 . 3 7 mm s-' s,ngle line QS= 0 mm s-' QS = 0 . 3 6
B D = 305 K
C O , ~ BE ~ , =~ 778.4 eV, line symmetric x = 2.1 x emulg Co, temperature independent various backscattering neaks due to the different shells in tGe-Co,S, structure
1
64, 1 0 5 , 1 0 6 28-30
87,105,112
to 0.9 IR Coat MoS, edges covering the M o adsorption sites Magnetic susceptibility.
105, 111
I " ' ' " ~ ' " " ' l
0 s Co IN81
-6 -4 2
0 2
0 Ma
4
......,,.........
Figure 2. Illustration of the structural complexity of typical sulfided Co-Mo/A120B catalysts. Reprinted with permission from ref 98. Copyright 1984 Societes Chimiques Belges.
.. c :, ,.-----4
'.,
.. -5
-4
-2 0 -2 .4 VELOCITY (MM/S)
.6
Figure
1. Massbauer absorption spectra obtained at 300 K of a 57Fe-dopedCc-Mo/AI,O, catalyst after different treatments: (a) after calcination, (b) after room-temperature sulfiding in a H,/H,S mixture, (e) after sulfiding at 350 OC, (d) after erpmure of (e) to H2 a t 420 "C, (e) after exposure of (d) to air at room temperature, and (0after exposure of (e) to hydrogen at room temperature. Reprinted with permission from ref 104. Copyright 1915 Nuclear Information
Service.
spond to a known phase (Clausen et al. (27) and Topsrae et al. (101)). This phase has been termed Co-Mo-S (Topsrae et al. (101)).Similar structures have later been observed in other promoted catalysts based on Mo or W (Topsrae et al. (98, 102)). It has been shown (see section C) that CwMo-S can be regarded as MoS, structures with Co located a t edge positions. Figure 2 illustrates the complexity of an industrial Co-Mo/A1,08 catalysts which may have the promoter atoms simultaneously present in all the different types of structures discussed above. The fact that MES allows one to determine quantitatively the distribution of Co among the different phases is very important since (as discussed in section D) the change in this distribution is the main origin for the observed differences in catalytic behavior. C. Properties of Co-Mo-S a n d Related Structures The recent combination of MES studies with investigations using a variety of other techniques has led to a
more detailed insight into the structural and physicochemical properties of Co-Mo-S. XPS is one of the techniques that has been used extensively to characterize Co-Mo/Al,O, catalysts, and many different cobalt phases have been proposed to be present in such catalysts (Friedman et al. (43),Patterson et al. (7% Okamoto et al. (74), and Brinen and Armstrong (13)). However, recently Alstrup et al. (I) have confirmed the existence of Co-Mo-S by XPS. For example, in unsupported catalysts, Co-Mo-S could be distinguished from Cogss by a Cop binding energy difference (BE) of about 0.5 eV (see T a h e I). For alumina-supported catalysts, measurements of the absolute BE'S with the necessary accuracy are difficult due to accumulation of charge on the samples. However, by comparison of relative binding energies, it was still possible to make the distinction. Also, the analyses of the Cozppeak shapes proved to be helpful in distinguishing between different Co phases (Alstrup et al. (I)). EXAFS has directly confirmed that the Mo atoms in Co-Mo-S have a MoS,-like structure as suggested by MES. The two techniques have the common advantage of giving in situ information on the type of X-ray amorphous or microcrystalline phases which may be present in typical HDS catalysts. In the first EXAFS study of sulfided Co-Mo/Al,O, catalysts Clausen et al. (29) found that the Mo atoms were predominantly present as very small (ca. 10 A) MoS,-like domains or crystallites. Boudart et al. (9)have also employed EXAFS to the study of HDS catalysts, but in contrast to the previous study, they reported a much lower number of sulfur atoms coordinated to molybdenum than in bulk MoS,. The coordination
Ind. Eng. Chem. Fundam., Vol. 25, No. 1, 1986 27 I
a ) Interior
zt
b)
0
50 Crystal thickness
Edge
1’
100 150 (number of MoS2 slabs)
Figure 3. (a) Co concentration measured with the electron beam in interior (basal plane) position for a number of Cc-Mc-S crystals. (b) Co concentration measured with the beam in edge position for the same crystals. Error bars are equal to twice the standard deviations. Reprinted with permission from ref 87. Copyright 1985 Elsevier Science Publishers.
number (CN) was furthermore found to correlate with the HDS activity. However, recently both Boudart et al. (8) and Clausen et al. (30) have confirmed the previous results that the Mo atoms in typical Co-Mo-A1203 catalysts have close to six sulfur atoms in the first coordination shell. EXAFS studies of other sulfided Co-Mo or Ni-Mo catalysts appear to be in agreement with the presence of MoS2-like structures (15,28,56,75, 83). Studies of unsupported model catalysts by XRD and HREM have also confirmed the MoS2-like nature of Co-Mo-S (Candia et al. (16)). The MES results indicate that the structure of Co-Mo-S can be regarded as Co atoms located at the edges of the MoS2crystallites (Topsae et al. (102)).Direct confirmation of this has been obtained by means of AEM (Topsae et al. (105) and Sarensen et al. (87)). Measurements were carried out on large Co-Mo-S crystals (composition verified by MES) whose dimensions were much larger than the size of the electron beam. The results (Figure 3) show that the Co atoms are indeed located a t the edges of the MoS2crystals. Quantitative analysis showed that the Co edge concentration (coverage) may achieve large values (Codge/Modge 0.8). The AEM results do not provide information concerning the detailed nature of the edge sites. In the intercalation model of Farragher and Cossee (42), it was suggested that the promoter atoms occupy intercalation sites at the edges. With respect to this proposal, it should be recalled that IR (Topsae (107) and Topsae and Topsae (108)),XPS (Grimblot et al. (50)),and recent HREM results (Topsae (95) and Pollack et al. (78)) have all shown that single-sheet MoS2 structures may predominate in typical sulfided Co-Mo/A1203 catalysts. Thus, it is clear that the Co sites are not edge intercalation sites since at least two S-Mo-S sheets must be stacked on top of each other in order to have the van der Waals gap necessary for intercalation. Rather, the Co edge atoms appear to be located in the same plane as the Mo atoms, for example, in edge substitutional or interstitial type positions. In agreement with this, Topsae and Topsae (111)observed by use of IR measurements that increasing
-
amounts of Co in the form of Co-Mo-S gave rise to a parallel decrease in the number of exposed Mo edge atoms. The Co EXAFS results (Clausen et al. (25, 28)), which show that the Co-S distance (ca. 2.27 A) is much shorter than the Mo-S distance in MoS2 (2.41 A), indicate that the Co atoms do not occupy perfect edge substitutional Mo positions. Recently, Ledoux et al. (66) have applied 59C0NMR to Si02and C-supported catalysts, and it has been suggested that the Co edge atoms have a nearest coordination sphere of a distorted tetrahedron. I t has been argued by Delmon (35)that there may not be enough sites available to accommodate all the Co atoms observed in the form of Co-Mo-S. However, this can now be understood in view of the EXAFS and AEM results which show that both high edge dispersion of MoS2 and high edge coverages by Co can be achieved. Furthermore, quantitative measurements of the edge coverage in real catalyst systems using different techniques give consistent results (Topsae et al. (112)). It should be stressed that Co-Mo-S is not a stoichiometric phase with a fixed Co:Mo:S stoichiometry. In fact, Co-Mo-S has been observed in samples where the overall Co/Mo ratio varies by many orders of magnitude (Topsae et al. (102)). The properties of Co-Mo-S are found by MES (Topsae et al. (102),Wive1 et al. (120),and Christensen et al. (22))to vary with the local Co concentration of the MoS2edges. For high Co concentrations, where the MoS2 edges will be “saturated”,the properties will most likely be influenced by interactions between neighboring Co atoms at the edges. Evidence for such interactions can be found from magnetic susceptibility measurements which show the presence of weak antiferromagnetism in Co-Mo-S with high Co content (Topsae et al. (105,106)). In the literature, it has often been claimed that support interactions with the alumina are important or even essential for creation of the active phase. However, in view of the structural characteristics of Co-Mo-S, it may be useful to regard the MoS2as the primary support for the catalytically important Co edge atoms and to regard the alumina more as a secondary support (see also section H). It has been observed that Co-Mo-S type structures are a general feature of many promoted Mo and W catalyst systems. For example, Mossbauer spectroscopy studies have revealed the existence of Co-W-S in Co-promoted, alumina-supported W catalysts (Topsae et al. (102))and Fe-Mo-S in both unsupported and alumina-supported Fe-Mo catalysts (see Figure 4 and ref 26,27,73,and 102). Infrared studies of both NO (Topscae and Topsae (111)and Topsae et al. (112))and CO (Bachelier (2) and Bachelier et al. ( 4 , 5 ) )adsorbed on Ni-Mo/A1203 catalysts have also given evidence for the presence of Ni-Mo-S structures in such catalysts. Similarly, Ni-W-S structures in Ni-W/ SiOz catalysts have also been proposed (Yermakov et al. (122) and Shepelin et al. (86)).
D. Origin of Promotion of HDS Activity and Catalytic Significance of Co-Mo-S The characteristic of HDS catalysts that has probably attracted most attention over the years is the strong dependence of the catalytic activity on the concentration of the promoter atoms (see, e.g., ref 35,45,46,49,67,71,82, and 95). It is generally observed that the activity passes through a maximum upon increasing the concentration of the promoter atoms. The extent of promotion achieved, as well as the location of this maximum (or in some cases two maxima), may vary from study to study. By combining MES and activity measurements on a series of catalysts exhibiting different promotional behaviors, we have elucidated the origin of these activity variations.
28
Ind. Eng. Chem. Fundam., Vol. 25, No. 1, 1986
.-fl
J
i ' ..
. :.-
Co-MoA1203 C O M o = 0.30
. . . . .. .. . . .
' . '
\I .e .-*--*-
,**
-; E
0
*.
a
. . . . .. . . ... ..
i
7
Fe- Mo A1203 Fe M o . 0 . 2 9 L
e.
a
Cobalt loading ( m g g catalyst)
Cobalt loading lmg g catalyst)
%'
. *..
. ..
l-_i-_L-._i--L---_l -4
-2
0
2
4
Velocity (mm s) Figure 4. Mossbauer spectra of alumina-supported catalysts obtained in situ at room temperature. In order to compare the MES spectrum of the Co-Mo/Alz03 catalyst with the absorption spectra of the Fe-Mo/Alz03 catalysts, the definition of the positive velocity has been reversed. Reprinted with permission from ref 98. Copyright 1984 Societes Chimiques Belges. Table 11. Dibenzothiophene (DBT) HDS Activities and MES Phase Composition for Co-Mo/A1203 Catalysts Prepared by Different Methods"
DBT convermain phase(s) sion, by MES mol/(s g ) 21 Co-Mo-S 16.3 Co-Mo-S 12 Co-Mo-S +
6
3 1
impregnation method Mo added first Co added first Co added first
6
1
equil ads of Mo Cogs8
wt 70 Mo wt % Co
10 10
3
Cogs8
7.3
"Adopted from ref 10.
Figure 5a-c and Table I1 summarize some of the information obtained. Figure 5a shows an example of a series of Co-Mo/A1203 catalysts with fixed Mo content which exhibits the typical strong synergistic behavior (Wive1et al. (120)). In Figure 5b an example of a series of CoMo/Al,03 catalysts with a different and quite unusual catalytic behavior (Candia et al. (18,19))is presented. For these latter catalysts, the catalytic activity is observed to be quite constant over a wide range of Co/Mo ratios. It can be seen that this lack of synergistic behavior is not due to the fact that no promotion of the HDS activity has taken place. On the contrary, a large promotional effect is present for all the catalysts (compare with Figure 5a). Figure 5c shows the effect on the catalytic activty of changing the calcination temperature for a series of CoMo/A1,0, catalysts with fixed Co/Mo ratio (Candia et al. (20)1.
For all the Catalysts discussed above, the Co phase composition has been obtained by in situ MES. The results have been included in Figure 5. It is evident that a close correspondence between the promotion of the actvity
u 600
Calcination temp ("C)
700
5
10
15
20
25
Amount of Co as Co-Mo-S ( m g g catalyst)
Figure 5. (a-c) Thiophene HDS rate parameters and Co phase distributions for different Co-Mo/A1203 catalysts. (d) Relationship between activity and amount of Co as Co-Mo-S. Adopted from ref 19,20, and 120.
and the amount of Co present as Co-Mo-S is observed in
atl cases in spite of the difference in catalytic behavior. In fact, for each series of Catalysts the promotion of the HDS activity is found to be roughly proportional to the amount of Co present as Co-Mo-S (see Figure 5d). This indicates that from a catalytic point of view Co-Mo-S is the most important species present in such catalysts. In the above studies thiophene hydrodesulfurization was used as the test reaction, but very similar conclusions have been obtained also by Breysse et al. (10) with other HDS reactions. The activity per Co atom present as Co-Mo-S has been observed not to be the same for all types of catalyst systems. This may be explained by differences in the accessibility to the reactants (Candia et al. (16, 17) and Clausen et al. (28)) or to the existence of different types of Co-Mo-S structures (Candia et al. (19,181 and T o p s ~ e et al. (112);see also the discussion in section I)). Unsupported catalysts have been found to exhibit catalytic activity behavior quite similar to that of supported catalysts (16, 17,28,35, 37,42,44,45,51,52,59, 79, 115, 117, 118).
Thus, studies of unsupported catalysts are very relevant, both in their own right and as models for the more elusive supported catalysts. MES studies (11, 16, 17,27,28, 48, 100, 101) have shown that unsupported catalysts may contain Co, both as Co-Mo-S and as Cogsg. The relative abundance of these structures was found to depend critically on the preparation parameters. For example, it was found by Candia et al. (16) that, by using the comaceration method, Cogsswas favored. Formation of Co9S, could be suppressed significantly when a homogeneous sulfide precipitation (HSP) method was employed ( T o p s ~ et e al. (101) and Candia et al. (16)). Thus, the latter type catalysts exhibited the highest HDS activity.
Ind. Eng. Chem. Fundam., Vol. 25, No. 1, 1986 29 Table 111. Comparisons of HDS Activities of MoSz, Cogsg,
A
and Co-Mo-Sg
BET surface
--___---
sample MoSZ cogs8 6.5% Co/A1203 CO-Mo-g
area, m2/g 15.8 17.8
~HDS?
kms, mol/(h
mol/ (g of cat. h) 2.9 x 10-4 4.3 x 10-4 1.7 x 10-4 4.3 x 10-3
mol of surf. atom) 0.9* 1.43c 1.P 53e
nFirst-order rate constant for thiophene HDS at 623 K expressed as moles of thiophene converted. Per Mo surface atom. Per Co surface atom, Per Co surface atom. e X-ray diffraction shows that Co is present as Co-Mo-S (as determined by MES). fData from a Co-Mo/A1203 catalyst with Co/Mo = 0.09. gData from ref 15.
-
I
C
0.25
0.50
0.75
1.00
co
Co.MO
Figure 6. (A) Relative selectivities for hydrogenation of butenes for unsupported Co-Mo catalysts. (B) First-order rate parameters for hydrogenation (k-) and hydrodesulfurization (kms). (C) Amount of Co as Co-Mo-S and Cogss. Adopted from ref 17 and 28.
Figure 6 shows the results from combined MES and activity measurements on unsupported (HSP) Co-Mo catalysts. It is seen that the HDS activity increases by a factor of about 20 upon cobalt promotion. EXAFS results show that the promotion leads only to small changes in the edge dispersion (Clausen et al. (28)), and the BET surface area was quite constant for all these catalysts (Candia et al. (17)). Thus, the promotion is not linked to changes in the dispersion of MoS2but to different numbers of cobalt atoms present as Co-Mo-S. Figure 6A shows that the selectivity toward butane formation (Le., the rate of formation of butane relative to that of the butenes) decreases as the Co/Mo ratio increases in the unsupported catalysts. Similar results have been reported for alumina-supported Co-Mo catalysts (see section G and ref 55 and 68); this behavior, therefore, appears to be a quite general feature of Co-Mo catalysts. The large change in the selectivity is observed (Figure 6B) to be related to a greater promotion of the HDS reaction rate compared to that of the hydrogenation rate of butenes. In fact, it is seen that the unpromoted catalyst has almost the same hydrogenation activity as the promoted catalysts.
E. Catalytic Significance of Other Phases As discussed above, Co-Mo/A1203 catalysts besides Co-Mo-S may contain other Co phases such as Cogs8and Co:A1203. It is also likely that different amounts of unpromoted MoS2 edges will be present. It is known that both Cogs8 and MoS, are moderately active HDS catalysts (15, 35, 37, 44, 77, 96); thus, such phases could play an important catalytic role in certain catalyst systems, although this was not seen to be the case for the systems discussed above. The activity contributions from these phases will depend not only on the abundance (compared to that of Co-Mo-S) of the phases but also on their dis-
persion and maybe their morphology. In order to reveal the possible contribution of cogs8 and MoS2, their specific activities have been measured by Topsm et al. (96) and Candia et al. (15) and compared with that of Co-Mo-S. The results given in Table I11 show that the specific activity of Co-Mo-S per surface atom is much greater than that of both Cogs8and MoS2 Therefore, even if the latter phases are present in large amounts and with a high degree of dispersion, their contribution to the overall catalyst activity will usually be small as is the case for the catalyst systems presented in Figures 5 and 6 (see also section G). Thus, it follows from the above findings that one of the goals in preparing active HDS catalysts is to maximize the amount of Co in the form of Co-Mo-S and to minimize the formation of Co:A1203and C0,S8. F. Precursors to Co-Mo-S in Calcined Co-Mo/A1203 Catalysts From detailed studies of calcined catalysts, one has gained a considerable amount of insight into how the choice of preparation parameters influences the amount of Co which ends up as Co-Mo-S, Cogs8, and Co:A1203 (10, 18, 19,20, 27, 101, 102, 109, 110, 121). For example, by use of IR studies of NO adsorption on calcined catalysts, Topsae and Topme (109) have shown that part of the Co interacts with Mo atoms at the surface of the alumina. This interaction phase was found to be the precursor for the active Co-Mo-S phase structures. Further insight into the nature of the precursor phases has been obtained by means of MES by Wivel et al. (121). A 1:l relationship between the amount of the Co precursor which was shown to have octahedral coordination (Coo,,) and the amount of Co present as Co-Mo-S was observed (Figure 7). The catalytic activity is thus determined by the amount of Cod present in the calcined catalysts. Co atoms have also been found in tetrahedral surroundings, Cotet,inside the alumina. The fraction of these Co species was found to increase with increasing calcination temperature (Candia et al. (20)). This resulted in less active catalysts after typical sulfding since the Co, species is not found to be affected by the sulfiding and, therefore, less Co is available in forming Co-Mo-S. For catalysts with high concentration of Co, a separate Co304phase has also been observed by MES (Wivel et al. (121)). This phase was found to sulfide to Cogss which, as discussed in the previous section, usually has a negligible activity contribution. G . Active Sites in Unpromoted and Promoted Mo Catalysts The chemisorption of different probe molecules has recently been extensively used in order to get information on the active sites in HDS catalysts. For MoS2 samples, as well as alumina-supported Mo catalysts (which EXAFS
30
Ind. Eng. Chem. Fundam., Vol. 25, No. 1, 1986
Table IV. Comparison of O2 and NO Chemisorption Results with HDS Activities of Mo/A1203 Catalystsd kHDSI(I O2 uptake,b NO uptake,c catalyst sulfiding temp, K mol/(g h) 10-~ mol/g io+ mol/g k H D S / [ 0 2 1 , h-* 4.5% Mo/A1203 873 15.4 2.1 7.3 8.6% Mo/A1203 873 33.1 3.6 9.2 11.5% Mo/A1203 873 20.6 2.6 7.9 4.0% Mo/A1203 673 21.1 4.1 8.6% Mo/AlZO, 673 50.2 9.4
kHDS/[NOl, h-'
5.2 5.3
"First-order rate constants based on moles of thiophene converted at 623 K. *The oxygen adsorption was carried out by allowing a 1% 02/He mixture to pass over the sample at 195 K. 'NO adsorption was carried out volumetrically at room temperature. dData from ref 98. 30L
Table V. Comparison of EXAFS Data with Activity Measurements for Mo/A1201 Catalysts Sulfided at 673 K' activity datab
A 4 )/
EXAFS data" edge 2nd shell dispercatalyst CN siond 4.0% M0/A1203 2.2 100 8.6% Mo/A1203 3.2 -90
-
[Moedgel,
IZHYD/ [Modgel.
mol/(h mol of
mol/(h mol of
Moedge)
Moedge)
0.51 0.62
1.16 1.43
IZHDS/
" Data from ref 30. First-order rate constants based on moles of thiophene converted at 623 K. Data from ref 98. % Mo atoms at edges.
-_
~
1
10
20
A
I
30
Co,,, imgig Al,O,i
Figure 7. (A) Absolute amount of Co as Co-Mo-S in the sulfided state plotted as a function of the amount of octahedral Co (Co,,) in the calcined state as determined by MES. The dashed line represents a l:t relation. (B)First-order rate parameters vs. the amount of Co as Co,. Reprinted with permission from ref 121. Copyright 1984 Academic Press.
has shown to contain MoS2 (Clawen et al. (29)),it has been found by Tauster et al. (93) and Bachelier et al. (3) that the HDS activity correlates with the oxygen uptake. The data shown in Table IV confirm these conclusions (Topsere et al. (98)). Oxygen has been observed by Bahl et al. (6) to adsorb on the edges of MoS,, and it was therefore suggested (Tauster et al. (93))that the HDS reaction occurs on the edge planes. Also, for NO adsorption (Suzuki et al. (90))and other reactions (Tanaka and Okuhara (92)), it has been found that the edge planes are the most reactive. NO adsorption experiments on unpromoted Mo/A1203catalysts have shown that the HDS activity is proportional to the amount of NO adsorbed (see Table IV and ref 61). Recently, we have obtained results which indicate that not all the sites (vacancies) along the edges are equally active in HDS or hydrogenation (Topsere et al. (98)). Mo/A1203catalysts with different loadings were studied, and the fraciton of edge sites was estimated from EXAFS measurements of the MoS2particle size. The results which are given in Table V show that the smaller MoS2particle have the lowest HDS and hydrogenation activity per Mo edge atom. As the MoS2 particle size is decreased, an increasing fraction of the atoms along the edges will be "corner atoms" and not true edge atoms. Thus, the results indicate that the true edge atoms are more active for HDS than the corner atoms. This difference may be related to
the possibility that sulfur and thiophene are more weakly bound at a corner than at an edge site. Recently, Massoth et al. (70) have also reported measurements which indicate that sites at edges and corners may have different reactivities. However, their conclusions differ from the present ones. Theoretical models considering the effects of geometrical shape on the activity have appeared recently (Kasztelan et al. (62, 63)). Besides making the distinction between corner and edge sites, one probably also has to take into account that different types of vacancies may exist. ,Recent oxygen chemisorption results on Mo/A1203catalysts by Valyon and Hall (113)suggest that sulfur vacancy pairs (i.e., exposed Mo pairs) are necessary for the dissociative chemisorption of 02.Thus, in view of the correlations observed between oxygen chemisorption and HDS activity (Table IV), it is likely that sulfur vacancy pairs also play an important role in HDS. Therefore, the results shown in Table V may indicate that such vacancy pairs do not have a tendency to involve corner atoms or may be more difficult to form on very small MoS2particles. With respect to the secondary hydrogenation of the butenes to butane, the trends follow that of the HDS reaction. Thus, it appears that the edge Mo atoms have a higher hydrogenation activity than the more coordinative unsaturated corner atoms. Whether this will apply to other hydrogenation reactions remains to be seen. Kinetic studies seem to indicate that aromatic and olefin hydrogenation occurs on different types of sites (Massoth and MuraliDhar (69)). Also, it is expected that coordinative unsaturated corner atoms will readily .rr-bond aromatics. In contrast to the results for unpromoted catalysts, oxygen chemisorption results for promoted catalysts systems show no simple correlation between the catalytic activity and uptake (Burch and Collins (14),Zmierzcak et al. (I%), and Candia et al. (15)). This lack of correlation between the HDS activity and the oxygen uptake has been shown by Candia et al. (15) to be due to the fact that oxygen adsorbs both on the less active unpromoted sites associated with the Mo edge atoms and on the more active sites associated with the promoter atoms present at the edges. Topsae and Topsae (111)found that this difficulty can be avoided by use of NO adsorption combined with infrared measurements since information on the promoted and
Ind. Eng. Chem. Fundam., Vol. 25, No. 1, 1986 31 ’
Co-MoiA1203
al
Ni-Mo /AI203
1.0
2.0
co / Mo N i 1 Mo
COlMO
Figure 8. IR absorbances of the NO absorption bands and activity data for a series of sulfided Co-Mo/Alz03 (a and b) and Ni-Mo/ AlZO8catalysts (c and d). Adopted from ref 15 and 111.
unpromoted edge sites may be obtained simultaneously. In Figure 8 we show in the cases of Co-Mo/A1203 and Ni-Mo/A1203 catalysts the variation in the relative concentration of promoted and unpromoted edge sites as a function of the promoter/molybdenum atomic ratio. In this figure we have also shown the HDS and hydrogenation activities. The results show that, for both HDS and hydrogenation, the promoted sites are the dominating ones. The degree of the promotion of the hydrogenation is found to be less than that of HDS in accordance with earlier findings (Hargreaves and Ross (55), Massoth and Chung (68), and Clausen et al. (28)). It is seen that, for the promoted catalysts, unpromoted Mo edge sites will still be present but their intrinsic activity for both HDS and hydrogenation is smaller than that of the promoted edge sites associated with Co-Mo-S. In view of the above results, the total catalytic activity, kbt, for MoS2 (or WS2) systems having an element X located a t the edges, can to a first approximation be decomposed into separate contributions from promoted and unpromoted edge sites.
+k
~ ~ n (1) ~ Here, kx and kMoare the specific activities for promoted and unpromoted edge sites, respectively. nx and n~~ are the corresponding concentrations of the promoted and unpromoted sites. The total concentration of edge sites is given by (2) nedge = nX + nMo kbt
= kxnx
Equation 1can also be written in terms of the fractional coverage of the edges by promoter a p m s (8, = nX/nedge): k,t = (kx& i- k M o ( 1 &)Inedge (3)
-
Consider two special cases. When the contribution from the promoted sites is the more important (i.e., kx >> kMo), we have k t kxnx = k x e x n e c i g e (4)
Figure 9. (a) Relative ESR intensities VI. Co/Mo ratios of a signal corresponding to g,, = 2.08 and g, = 1.99 measured at 77 K. (b) First-order rate parameter of HDS activity vs. Co/Mo ratio. Reprinted with permission from ref 38. Copyright 1986 Academic Press.
This appears to be the situation for the HDS reaction (and to some extent also hydrogenation) in most Co-Mo or Ni-Mo catalysts as long as Ox is not very small. If, on the other hand, kM0>> kx, we have ktot
k M ~ n M=~ kMo(l
- eX)nedge
(5)
This situation corresponds to X acting as a simple poison which blocks Mo sites. The slight linear decrease in the HDS activity with increasing nx which has been observed in Fe-Mo/A1203 catalysts (Topsrae et al. (98)) indicates that this situation prevails. It has been shown that nitrogen compounds such as pyridine adsorb strongly and partially block Mo edge sites (Topsrae and Topsrae (111)). The observed decrease in activity may thus be described by eq 5. Note that we now have methods available (e.g., IR, MES, and EXAFS) which may provide us with information on and nx. Thus, it may be possible, for a given OX, catalyst system, to get detailed insight into the origin of observed activity variations. This is important since all the above parameters may depend on preparation proce~dures. It has not been decided definitively whether the active HDS sites associated with the Co edge atoms in Co-Mo-S are associated directly with the cobalt atoms (e.g., vacancies associated only with Co) or whether the neighboring Mo edge atoms also play a direct role in the catalysis (e.g., vacancies shared by Co and Mo edge atoms are the active sites (Topsrae (95), Chianelli et al. (21), and Topsrae and Clausen (99))). The MoS2“support” is, of course, necessary for creation of Co-Mo-S and also influences the properties of the Co atoms (see section C). The observation by Derouane et al. (38) of a Mo species having an ESR intensity which can be correlated with the HDS activity (see Figure 9) and the amount of Co-Mo-S gives further evidence for an influence of the neighboring Mo atoms. However, the fact that active catalysts can be prepared without Mo (see section K) leaves open the possibility that
32 Ind. Eng. Chem. Fundam., Vol. 25, NO. 1. 1986
the role of the neighboring Mo atoms to the Co edge atoms is of a more secondary nature. Recently, it has been proposed that S,2- ions, being more reactive than S2-ions, may play a role in HDS catalysis (Goodenough (47). Stiefel (89), and Duchet et al. (40)). H. Role of Alumina a n d Nature of Support Interactions A distinction should be made between two types of support interactions in Co-Mo-S-containing catalysts. The first type deals with the interactions between the Co atoms and the “primary” MoS, support, and the second concerns the interaction of the MoS,-like Co-Mo-S structures with the “secondarv” suuoorts (Le.. alumina. .. carbon, etc.). The MoS, “support” should not be considered as an inert for the c0edge atoms. In fact, evidencefor structural and electronic support interactions is Seen from several observations: (i) the properties of the Co atoms present as Co-Mo-S are very different catalytically, structurally, and chemically from those of the thermodynamically favored cobalt sulfide, Cogsg(see Table I); (ii) the Co atoms are associated only with certain sites in the MoS, structure (i.e., the edge sites); (iii) magnetic measurements on samples exhibiting Co-Mo-S as the only Co-containing phase indicate electronic delocalization involving the Co and the surrounding Mo atoms (Tops= et al. (105,106)). Recent ESR results by Derouane et al. (38) have given more evidence for the existence of special Mo neighbor atoms and for their interaction with the Coatoms. Several results indicate that some direct bonding between the MoS,-like structures and the alumina may be present (Topsoe et al. (98)). For example, MES results showed that the bonding of Co-Mo-S to alumina is stronger than that to carbon. Recent magnetic susceptibility results (Topsee et al. (106)) indicate that the bonding to the alumina support could occur via Mo-&A1 bridges. However, both the MES (Topme et al. (101)) and EXAFS results (Clausen et al. (29)) show that the Concentration of oxidic Mo species is very low under typical reaction conditions. In view of the greater “reactivity” of edge atoms compared to that of basal plane atoms in MoS,, it is likely that bonding to the alumina involves the edge Mo atoms. Thus, the MoS,-like sheets may be bonded perpendicular to the alumina surface. Recent microscopy measurements by Hayden and Dumesic (58) on model systems have given evidence for this. The support interaction may be the reason why single-layer MoS, structures can be prepared and can be observed to he present even after relative high-temperature sulfiding (Candia et al. (18)). The bonds to the support will eventually be broken by increasing the sulfiding temperature, and this leads to changes in the properties of Co-Mo-S (Candia et al. (18, 19)). After high-temperature sulfiding, Co-Mo-S (type 11) has properties which resemble those of the Co-Mo-S structures in systems with weak support interactions (see also section K). One important role of the support interactions in alumina-supported catalysts is to allow the preparation and stabilization of highly dispersed MoS, structures capable of accommodating large amounts of promoter atoms at their edges. It is in this rather indirect way that the alumina support highly active catalysts to be prepared. I. Deactivation Phenomena The discussion of the catalytic properties has centered 80 far mainly on intrinsic or steady-state catalytic activities. However, during industrial use the catalytic activity of HDS catalysts will most often show significant changes
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