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The Journal of Physical Chemistty, Vol. 83,No. 13, 1979
Gajardo
et al.
Physicochemical Characterization of the Interaction between Cobalt Molybdenum Oxide and Silicon Dioxlde. 2. Influence of the Amount of Oxide Phase P. Gajardo, D. Plrotte, P. Grange, and 6. Delmon" Universit6 Cathollque de Louvain, Groupe de fhysico-Chimie Min6raie et de Catalyse, 1348 Louvaln-ia-Neuve, Belgium (Received December 15, 1978) Publication costs assisted by Services de la frogrammation de la foiitique Scientifique of the Ministry of Scientific Policy of BeQium
Silica is a relatively inert support in comparison with alumina. For the same amount of active phase, it has been shown that the nature of the compound present on these two supports is very different in the range of composition from 0 to 1 coba1t:molybdenum atomic ratio. We have determined the influence of the amount of oxide phase present on the support on the structure of Mo/SiOz and CoMo/SiOz catalysts by using X-ray diffraction, DRS measurements, and a gravimetry study of the reduction by hydrogen. In the first series of catalyst, the molybdenum oxide content ranges from 2.8 to 20.6 wt %. In the second one, the amount of active phase (Co304+ MOO,) ranges from 3.7 to 27.7 wt 70. In the latter case the atomic ratio Co/(Co + Mo) is constant and equal to 0.36. When only molybdenum is supported on silica both attached Mo(V1) interacting with SiOp and bulk MOO, are present in proportion which depends on the amount of molybdenum oxide phase. The addition of cobalt on Mo/SiOz catalyst brings about a detachment of molybdenum oxide in interaction with SiOz and formation of CoMo04. Furthermore, there is no evidence of cobalt oxide formation.
1. Introduction
In previous studies1S2performed on Co-Mo/yAl,O3 and Co-Mo/SiOz hydrodesulfurization catalysts, in their oxide precursor form, we have observed a strong interaction between cobalt and molybdenum oxide species. This interaction brings about the formation of a Co-Mo bilayer adsorbed on 7-A1203,in the first case, and the formation of cobalt molybdate on SiOz in the second case. These interactions have the consequence that the hydrogen reducibilities of supported cobalt and molybdenum oxides are substantially reduced. The observed differences between silica and alumina supported catalysts can be attributed to the quite different properties of supports.lS2 Alumina is an active support, with which molybdenum oxide interacts, forming a Mo(V1) monolayer; in contrast, SiOz is a relatively inert support, with which molybdenum only weakly interacts.,~~ In spite of this relative inertness there is experimental evidence that some Mo-Si02 interactions take place. There are principally three observations which indicate this inter~!~ action: a broadening of the XPS MoQdd o ~ b l e t ,the simultaneous existence of various forms of molybdenum on Si02,617and the absence of induction period in the hydrogen reduction curvesa8 In previous works,2P8we have shown that the gravimetric study of Hzreduction may serve as a good tool for the investigation of the structure of oxide supported catalysts, when used in conjunction with other physicochemical methods. The previous work on Co-Mo/Si02 reported measurements on a series of catalysts having a constant total load of oxides [Co304+ Moo3 = 12 f 0.4% (wt)] but varying composition [r = Co/(Co + Mo) from 0.00 to 1.001. In the present one, we have mainly investigated the influence of the total oxide load. Hydrogen reduction measurements, X-ray analysis, and diffuse reflectance spectroscopy (DRS) have been used. The first series of catalysts contained only Mo, in different quantities. This series was used to estimate the Mo03-SiOz interaction and the influence of SiOz on the reduction of the deposited molybdenum oxide species. The influence of cobalt on the structure and reduction behavior of Mo/SiOz was analyzed by using a second series characterized by r = 0.36. In the Discussion section, we shall also recall results 0022-3654/79/2083-1780$01 .OO/O
obtained on the same catalysts of the series by analytical electron microscopy (AEM).g 2. Experimental Methods 2.1. Catalysts and Unsupported Oxides. Mo/Si02 catalysts were prepared by impregnation of SiOz (Rh6ne Progil, 179 m2/gb1, pore volume 1.2 mL g-l, and mean diameter of the particles 170 A) with molybdenum from an ammonium paramolybdate (APM) solution, by using the pore volume method. The resulting solid was dried a t 110 "C (1 h) and then calcined a t 500 "C (24 h). The catalysts of this series were designated as SiMox, where x is the molybdenum content value expressed as weight percentage of Moos in the catalyst. Impregnation of molybdenum was carried out in two steps in the case of catalysts with x > 12.3, otherwise the APM solution became too concentrated, producing a white precipitate. In a two-step impregnation method, 50% molybdenum was deposited in each step. Prior to the second step, the catalyst was dried a t 110 "C (1 h). Co-Mo/ Si02 catalysts were prepared by cobalt impregnation of calcined SiMox catalysts, by using a cobalt nitrate solution and the pore volume method. Drying and calcination were performed as in the SiMox series. These catalysts were designated as SiMoCoy where y is the cobalt and molybdenum content expressed as weight percentage of Moo3 Co304 in the catalyst [let us recall that r = Co/(Co + Mo) = 0.361. Experimental details concerning the pore volume impregnation method, drying, calcination, chemical analysis, materials used, X-ray analysis, BET surface determination, and preparation of unsupported Moo3, Co304, and CoMo04 have been reported elsewhere.'S8 2.2. Mechanical Mixtures of APM and Na2MoO4c?H2O. In order to obtain mixtures of Mo(V1) situated in both octahedral and tetrahedral oxide surroundings, various mixtures of MOO, [octahedral Mo(VI)] and Na2Mo04. 2Hz0 [tetrahedral Mo(VI)] were prepared with [octahedral Mo(VI)]/ [octahedral Mo(V1) + tetrahedral Mo(VI)] atomic ratio ranging from 0.0 to 1.0. 2.3. Pore-Size Distribution. The pore size distribution determination was performed by using the Nz adsorption-desorption isotherms a t liquid nitrogen temperature,
+
0 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 13, 1979
Interaction between Co-Mo Oxide and SiO,
1781
TABLE I: Phase Active Content, Atomic Composition [Co/(Co t M o ) ] , BET Specific Surface, Color and Compound Detected bv X-rav Analvsis in SiMox and SiMoCoN Catalysts'
catalyst
phase active content, wt %
COI (Co + Mo)
SiMo(2.8) SiMo(4.8) SiMo(6.7) SiMo(8.2) SiMo(9.4) SiMo(ll.6) SiMo( 14.8) SiMo( 20.6) SiMoCo(3.7) SiMoCo(8.7 ) SiMoCo(l2.3) SiMoCo( 1 9 . 8 ) SiMoCo( 27.6)
2.8 4.8 6.7 8.2 9.4 11.6 14.8 20.6 3.8 8.7 12.3 19.8 27.6
0 0 0 0 0 0 0 0 0.37 0.35 0.36 0.36 0.36
surface BET. m2g-'
X-ray analysis
145 132 143 125 126 122
118 121 135 129 122 105 97
color
MOO,
CO,O,
b-CoMoO,
bluish bluish bluish bluish bluish bluish light blue light blue light violet gray violet gray gray gray
no no no no yes Yes Yes Yes no no no Yes Yes
no no no no no
no no Yes Yes Yes
' SiMolll.6) and SiMoColl2.3) are the same catalvsts studied in previous work,' where they were denominated as Si-0.00 and Si-0.36, respectively. ' by using a standard volumetric BET system. The surface area occupied by a Nzmolecule was assumed to be 16.2
AZ.
2.4. Diffuse Reflectance Spectroscopy (DRS). Spectra (200-800 nm) of SiMox catalysts were recorded by using a Beckman Acta M IV spectrophotometer. A pellet of the pure catalyst support @io2)was used as reference. Pellets of catalyst were obtained by powder pressing at 1.7 tons/cm2. 2.5. Reduction Apparatus and Procedure. Reduction experiments were carried out gravimetrically, by using the same McBain balance and experimental procedure as employed in the hydrogen reduction investigations on Co-Mo/y-A1203 and Co-Mo/Si02 catalysts.2*8 The McBain balance has a 5-L volume connected to a standard vacuum line ( torr) and to a device permitting the introduction of gases. The quartz reactor is inserted in a circulation loop, comprising an all-glass magnetic pump and liquid nitrogen traps for the condensation of water produced in the reduction. Samples of 200-300 mg were deposited in the McBain quartz bucket and degasified overnight under vacuum at a degassing temperature (DT) of 500 "C. All experiments were performed at atmospheric H2 pressure (99.99% purity) and in isothermal conditions, 400 "C. After reduction, the catalyst was degasified at 500 "C, in order to evaluate the amount of water possibly readsorbed during reduction. Actually, the measurements showed that, in silica supported catalysts, the amount of readsorbed water is very small and can be neglected, if reduction is carried out a t a temperature of 400 OC or higher. The degree of reduction is expressed by a , defined by the following relationship a = (reduced oxide)/ (initial oxide). In the calculation of a, it is assumed that cobalt and molybdenum oxides occur on SiOz as c0304 and MOO,, respectively, and they are reduced to metallic cobalt and molybdenum. The reproducibility of the measurements was better than 4.5%. 3. Results 3.1. Composition, BET Surface Area, and X-ray Analysis. A summary of the results concerning activephase content, atomic ratio r = Co/(Co + Mo), BET specific surface area, catalyst color, and X-ray analysis of both catalyst series are reported in Table I. In both catalyst series a decrease of the surface area is observed when the active-phase load increases. The pore-size distribution for pure SiOz and some SiMoCoy catalysts is
dV
SiMoCo37
Si Mo Co 19.8
il' c
R (A) R (A) Y Figure 1. Pore-size distribution (a-e) and dependence of cumulate pore volume (f, for 50-87 and 87-120 A pores) on y , for pure SO2 and some SiMoCoycatalysts. d VIdR and V(cumu1ated pore volume, e.g., the area under pore-size distribution curves) are expressed in arbitrary units (au).
presented in Figure la-e. The area under the pore-size distribution curves (cumulated pore volume) corresponding to pores of dimensions 50-87 and 87-120 A, respectively, are plotted against y in Figure If. The contribution of 50-87-A and 87-120-A pores to the pore volume is considerable. An abrupt decrease of the curve corresponding to 50-87-A pores is observed in y = 0.0-3.7 range. The X-ray diffraction patterns of some catalysts of the SiMox series and of Moos (model compound) are presented in Figure 2. Catalysts containing a small amount of molybdenum (x I 8.2) do not present any diffraction peak. Catalysts with higher molybdenum content show peaks corresponding to those of Moo3, but the relative intensity of these peaks is different from those observed in unsupported MOO,. In the Figure 3, , we have reported the X-ray diffraction patterns of some catalysts of the SiMoCoy series and also those corresponding to MOO,, Co304,and b-CoMo04. The presence of peaks corresponding to b-CoMo04 is detected in all catalysts with y L 12.3. MOO, was only clearly observed in catalysts having y 2 19.8. The last three columns of Table I summarize the observations on all the catalysts of both series. 3.2. Diffuse Reflectance Spectroscopy (DRS). Spectra of model compounds and catalysts are presented in the Figure 4a. Spectra of mechanical mixtures of Na2Mo04.2H20and MOO, [Mo(VI) in tetrahedral and octahedral oxide environments, respectively] are reported in Figure
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0
2
8
-
0
0
I
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Gajardo et ai.
A 4
0 (D
a
I
\\
SI Mo 94
SiMo148 SI Mo 198
Na2Mo042 H 9
47
43
39
35
MOO,
31
27 23 (28) Figure 2. X-ray diffraction patterns of SiMox catalysts and unsupported MOO,: (a) unsupported MOO,, (b) SiMo(20.6), (c) SiMo(14.8), (d) SiMo(6.7). o Moo3 C03O4
b
b-CoMoOq 0
MOO,
0.00 Na2Mo04.2 H 9
L
I
I
I
I
I
I
nm Flgure 4. (a) Diffuse reflectance spectra of SiMox catalysts and of MooBmechanical mixture, model compounds. In the Na,MoO,.W,O the atomic ratio of octahedral Mo(VI)/(octahedrai Mo(V1) tetrahedral Mo(V1)) is equal to 0.75. (b) Diffuse reflectance spectra of mechanical mixtures of NapMo0,.2H,0 MOO,. The value of the octahedral Mo(VI)/(octahedralMo(V1) tetrahedral Mo(V1)) atomic ratio is indicated on the corresponding curve.
300
500
700
+
+
I
47
43
39
35
31
27 23 (28)
Figure 3. X-ray dlffractlon patterns of SiMoCoy catalysts and of unsupported MOO,, b-CoMo04, and c030,: (a) MOO,, (b) Co304,(c) b-CoMoO,, (d) SlMoCo(27.6), (e) SiMoCo(l9.8), (f) SIMoCo(12.3).
4b. The values of the atomic ratio [octahedral Mo(VI)]/ [octahedral Mo(V1) + tetrahedral Mo(VI)] are indicated on the corresponding curves.
+
+
3.3. Reduction Isotherms. The reduction isotherms of unsupported Moo3, Co304,and CoMo04 are presented in Figure 5 (taken from ref 1). Notice the considerable differences in the reduction behavior of these solids, namely, an important induction time and an S-shaped curve for Moo3 and instantaneous reduction of Co304;the curve corresponding to CoMo04 has an intermediate aspect. In Figures 6 and 7 are presented the reduction isotherms of the SiMox and SiMoCoy catalysts, respectively. No
The Journal of Physical Chemistry, Vol. 83, No. 13, 1979
Interaction between Co-Mo Oxide and SOp
1783
1
I
d
,
51 MoCol
0.42
05
a28
0.14
C 180
0
540
360
t Figure 5. Reduction isotherms of unsupported a-CoMo04 at 400 'C.
720
min
Moo3, Co304,and
0
0
6
3
t
,
9 hours
Si Mo11.61
Figure 7. Reduction isotherms of the SiMoCoy catalysts. T = 400 'C. Numbers on each curve indicate the y value.
t , hours Figure 6. Reduction isotherms of the SiMox catalysts. T = 400 OC. Numbers on each curve indicate the x value.
Figure 8. Dependence of the reduction after given times on the molybdenum content, for the SiMox series ( T = 400 OC). One experimental point at the right (0)indicates the extent of reduction of unsupported MOO, after 10 h.
induction period is observed. Another conspicuous fact is the important difference in the reduction behavior of various catalysts of the SiMox series and less important differences in the SiMoCoy series. The dependence of the reducibility of the SiMox series, on the active phase content x , is summarized in Figure 8. The extent of reduction increases with molybdenum content reaching a constant value or a maximum at x = 14.8, whether it is measured at 3, 6, or 10 h. For comparison, the value for bulk MOO, at 10 h is also given in Figure 8. Other differences in the kinetics of reduction of the various catalysts can be observed in the initial stage of the reaction, for times less than 90 min (Figures 6 and 7). We have calculated, from the isotherm curves, the rates of reduction during this period. These rates are plotted against reduction time in Figures 9 and 10, corresponding to SiMox and SiMoCoy catalysts, respectively. In order to interpret the results, another plot will be necessary. Indeed, we shall have to consider the possible formation of CoMoO, on the SiMoCoy series. Thus, it is
not legitimate to calculate CY on the basis that the starting deposited oxide is Co304[where part of Co is Co(III)], as CoMoO, only contains Co(I1). Figure 11 is a plot of the degree of reduction, expressed as 0 reacted vs. total oxygen, supposing that 0 comes from MooBin the SiMox series and from CoMoO, and MOO, in the SiMoCoy series. Finally, a remark must be made concerning the shape of the curve in Figure 6. The S shape of the SiMo(20.6) curve does not show up in the catalysts with low Mo content (e.g., SiMo(2.8)). This suggests that different mechanisms operate during reduction. In order to make the difference clear, we have plotted in Figure 12, a vs. reduced time t / t , , (where is the time of half reduction) in conformity with what is done when studying unsupported solids.1°
0.481
0
IO? 3
6
9
M
O
( 9~ ) /catalyst ( g )
4. Discussion 4.1. Physicochemical Measurements. 4.1.1 Surface I
Area, Pore-Size Distribution, and X-ray Analysis. The diminution of the specific surface area with increasing loading of the catalysts (Table I) seems to be due to the
The Journal of Physical Chemistry, Vol. 83, No. 13, 1979
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6.8I
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Gajardo et al.
b
a ‘ S i Mo 2.8
I
h
c-
.-‘c
E
1
5.1
v
c
2 u
U
34
Figure 12. Reduction cy vs. f / t O 5 ;tis the reduction time and reduction time at a = 0.5. 1.7
I
I
0
30
90 120 t , minutes
60
Figure 9. Evolution of the reduction rate of the SiMoxcatalysts during the initial reduction period ( T = 400 “C). Numbers indicate the xvalues.
x IO2
-
2.4
7
‘C .-
I
I
I
E
v
. c
U
8 1.6
U
a8
0 30
60 t
,
90 rnn
Figure 10. Evolution of the reduction rate of SiMoCoycatalystsduring the initial reduction period ( T = 400 “C). Numbers indicate the yvalues.
i
,104
Total Oxygen (mol ) / catalyst ( g )
Figure 11. Degree of reduction of SiMox and SiMoCoy catalysts measured after 10 h at 400 O C .
the
obstruction of small pores of the carrier. The pore-size distribution curves (Figure 1) are consistent with this hypothesis. The pore volume of the SiMoCoy catalysts decreases with increasing y; this diminution is considerable when the 50-87 A pores are concerned. For loading below x = 8.2, no Moo3 was detected by X-ray analysis; diffraction peaks corresponding to MOO, come out for x 2 9.4. For the latter catalyst, however, the relative intensities of the diffraction peaks of the catalysts and of free MOO, are different. This fact suggests the existence of Mo-Si02 interaction, leading to the formation of MOO, crystallites on the surface, having shapes much different from that of free MOO,. 4.1.2. Diffuse Reflectance Spectroscopy (DRS). It has been r e p ~ r t e d that ~ , ~ Mo(V1) is located in tetrahedral oxygen coordination in Mo/SiOz catalysts with low molybdenum content. However, other molybdenum species, as dimolybdates, polymolybdates, silicomolybdicacid, and Moo3, in which Mo(V1) is situated in octahedral symmetry, may also be present. The proportion of these different species would depend on molybdenum loading and preparation method (especially calcination temperature~).~>~ Actually, the bands corresponding to Mo(V1) in octahedral oxygen coordination [charge transfer 02S Mo(VI)6J1J2]and to Mo(V1) in tetrahedral coordination are located at only slightly different positions, namely, 300-320 and 250-270 nm, respectively. The interpretation of the spectra is therefore not unambiguous. The measurements reported in Figure 4b were intended to make easier the interpretation of the spectra, by allowing a comparison with the spectra of artificial “mixtures” of octahedral and tetrahedral Mo(V1). Our diffuse reflectance spectra of SiMox catalysts (Figure 4a) clearly show an absorption band at 300 nm, which is almost identical in all catalysts. This suggests that most Mo(V1) is in octahedral coordination. Indeed, the shape of the observed band is similar to the band of mechanical mixtures containing much octahedral Mo(V1) (20.75, Figure 4b). However, this does not exclude that a minor part of Mo(V1) is in tetrahedral coordination. On the other hand, the presence of octahedral Mo(VI), which is suggested by the band a t 300 nm appearing on catalysts with small loading ( x = 2.8), is not corroborated by X-ray diffraction in catalyst with x < 9 (Table I). One might therefore speculate that most of the Mo(V1) is really in octahedral coordination, but not as MOO,; Mo(V1) would form a new surface species similar to polymolybdates; possibly two dimensional as has been proposed by Giordano et al.13 for Mo/A1203catalysts. Indeed, absorption bands of catalysts and APM, reported in Figure 4a, are quite similar. We must also examine the possibility of some silicomolybdic acid forming in our catalysts. Our spectra do not
Interaction between Co-Mo Oxide and 30,
show evidence supporting the presence of such a species, which would correspond to the simultaneous presence of two bands (250-270 and 300-320 nm) and to a yellow color typical of the silicomolybdic a ~ i d . ~ , ~ In conclusion, diffuse reflectance spectra suggest that Mo(V1) is mainly in octahedral oxygen coordination, but probably in part as a surface species, similar to a polymolybdate, and that the presence of tetrahedral Mo(VI), although certainly not important, cannot be completely excluded. 4.2. Reducibilities of SiMox and SiMoCoy Catalysts. Let us first discuss the results obtained on the catalysts containing only Mo. We have already indicated the differences in reduction behavior between catalysts with low and high molybdenum content, when we presented Figures 6, 8, and 9. Let us first examine the reduction behavior of the catalysts with low molybdenum load. These catalysts present a high initial reduction rate and a low degree of reduction at 10 h. These characteristics and the absence of any induction time (Figure 6) could be interpreted as indicating an interaction between molybdenum in the form of a superficial oxide species and Si02 The Mo-Si02 bond would be sufficiently strong to stabilize Mo at intermediate oxidation states [e.g., Mo(V)], which are not observed in unsupported M003.2J4 This might explain the low values of a a t long reduction time (10 h) (Figure 8). The high initial rate of reduction of the catalyst containing low quantities of molybdenum might be interpreted as another consequence of this interaction, namely, an extensive spreading of molybdenum oxide species (the superficial polymolybdate detected in DRS) on the support, with correlatively, an excellent accessibility to attack by hydrogen. Actually, comparing curve a of Figure 12 to similar curves corresponding to various reduction kinetics, (according to the method reported by Delmonl'), we can conclude that SiMo(2.8) is reduced following a formal kinetic of instantaneous nucleation. In summary, the polymolybdate layer on SiOz would be attacked rapidly (without induction period) by H2, but further reduction is not as high as in bulk MOO,, because of the interaction with SiO2 The a vs. t/t0.5plot of Figure 12 was intended to explain the differences between catalysts with low and high Mo loading. There is a clear indication of an accelerating period with catalysts with high loading, this effect is attributed to a rate-limiting nucleation phenomenon." The curve observed a t high Mo loading (Figure 12b) clearly shows that SiMo(20.6) is reduced following two different kinetics. This curve could be thus interpreted as the sum of a contribution of the polymolybdate layer (Figure 12a), discussed above, and of another species being reduced with an induction period. This last species might be the bulklike Moos (free MOO,, as opposed to a polymolybdate layer), which indeed, is observed in X-ray diffraction (Table I). The reduction of unsupported MOO, exhibits a considerable induction period (Figure 5 ) . In principle, the reduction of bulklike MOO, could go further than that of the polymolybdate layer. The tendency of the degree of reduction to increase when x increases (and, consequently, when the proportion of bulklike MOO, increases) corroborates our interpretation. However, a reduction time of 10 h is not enough for a complete reduction of bulk MOO,, because of a low initial reduction rate (Figure 5), which could explain the decreasing of reducibilities a at 10 h for catalysts with x > 15 (Figure 8). Catalysts with an active phase content of x = 10-12 are an intermediate
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situation; viz., they have a relatively high initial reduction rate as well as a relatively high reducibility a at 10 h. The properties of catalyst with low and high molybdenum loading are then superposed on them resulting in the maximum of the curve at 10 h in Figure 8. The addition of cobalt to SiMox catalysts brings about extensive changes in the reduction behavior. Now, the differences in a value at 10 h of reduction from catalyst to catalyst become smaller (Figure 7). The independence of raw reduction a at 10 h on y suggests that all catalysts contain essentially the same species. This species could be a- and b-CoMoO,. Form b was detected by X-ray analysis in catalysts with y > 9.4 and both forms in all catalysts by AEM.g Even if we assume that cobalt quantitatively forms CoMoO, in catalyst with Co/(Co + Mo) 0.36,2 some molybdenum oxide must remain free, presumably forming species much like those observed on the SiMox series, at low or moderate loadings. In this connection, the absence of an induction period (Figure 7) can be easily explained. One may, however, remark that another effect comes into play, namely, the fact that Co accelerates the reduction of molybdenum oxide.14-16 This effect has the consequences (i) that the induction period which would possibly be observed with hypothetical bulklike MOO, would be suppressed and (ii) that the degree of reduction of the polymolybdate layer is increased.2 Thus, MOO, or the polymolybdate layer being reduced to Mo(1V) and the remaining supported species being always the same (i.e., CoMoO,), one should expect that (i) the degree of reduction after a given time is identical (this is what was suggested by Figure 7) and (ii) that the amount of removed 0 varies linearly with loading (this is what is indicated by Figure 11, upper curve). In addition, the curve relative to SiMoCoy catalysts, in Figure 11, is above that of SiMox. 4.3. Hypothesis about the Structure of SiMox and SiMoCoy Oxide Catalysts. Let us first recall the main results of analytical electron microscopy (AEM)9previously obtained on the same series of catalysts. SiMox Series. (i) In all catalysts molybdenum forms a dispersed phase where no crystalline forms can be observed (at a scale corresponding to the direct observation on the screen, namely, 5 nm). (ii) MOO, crystals begin to appear at x = 7. (iii) The concentration of molybdenum in the dispersed phase reaches a maximum for x = 7-12; in the catalysts with x > 1 2 most molybdenum seems to be present as free MOO,. SiMoCoy Series. (i) The addition of cobalt into SiMox catalysts provokes the formation of a- and b-CoMoO,. (ii) Co304is not detected. (iii) The concentration of dispersed molybdenum diminishes when cobalt is present. 4.3.1. Structure of SiMox Catalysts. The above summarized AEM results present some similarity with those obtained on the Mo/-y-A12031system. They strongly suggest that something similar to the Mo(V1) monolayer adsorbed on yA1203and generalized for other supports4 may exist on Si02. Results are lacking in literature for confirming the existence of the hypothetical monolayer and proposing a structure; experimental results are ambiguous, because several species of molybdenum may simultaneously coexist on the catalyst. In addition, the relative quantities of each of them depends on several factors, such as the impregnation method, loading, and calcination temperature. Let us suppose that a molybdenum monolayer indeed exists on Si02 and that it is very similar to that formed on yA1203. One can calculate the maximum molybdenum
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content which can be accomodated as a monolayer on our SiOz support. It would be in the range x CI 21-25, if the area occupied by one Moo3 unit is the same as on 7-A1203, namely, 17-20 A. Then Mo(V1) could be fixed on SiOz by a reaction between two OH- groups of SiOz and MoOz(OH)z(as has been proposed for Mo/A12028);this would give tetrahedral Mo(V1) adsorbed on Si02. This type of symmetry should be observed in catalysts with low molybdenum content6-’ where the monolayer species should be predominant over bulklike Moo3. This is not in contradiction with our findings but, in our catalyst with x = 2.8 (Figure 4a), the presence of tetrahedral Mo(V1) is not evident. In our case, probably due to the utilized preparation method of catalysts, tetrahedral Mo(V1) may exist alone on the surface (and therefore clearly detected by DRS) only a t lower molybdenum loading. At higher molybdenum content, polymolybdate species are formed.6 If Mo(V1) is situated in the monolayer in tetrahedral coordination at low molybdenum loading, the change to polymolybdate, with increasing molybdenum content, would involve a change of symmetry of the molybdenum ions. A tentative explanation may be given in terms of the formation of Movl-O-Mom bonds between adsorbed tetrahedral Mo(VI), eventually yielding polymolybdate-like structures, when the concentration of tetrahedral Mo(V1) becomes considerable. Giordano et al.13 have similar proposed the formation of polymolybdate structures in Mo/Alz03 catalysts. The formation of free Moo3 at very low loadings (at x = 6.7) may be explained by the weakness of the Mo-SiOz interaction (compared to the Mo-A1203 interaction) and possibly a high heterogeneity of the Si02surface. One may assume that the adsorption sites which are able to adsorb oxide Mo(V1) strongly will be occupied early; these sites may participate in a monolayer structure. In contrast, the adsorption sites forming weak links with Mo(V1) will be occupied only after the most active sites are full, Le., at high x values. These sites do not participate in the monolayer structure and at this molybdenum content, Mo(V1) ions tend to form free Moo3 rather than a monolayer. The formation of Moo3 would be energetically more favorable than the formation of a monolayer structure on weak sites. 4.3.2. Structure of SiMoCoy Catalysts. The formation of CoMo04 in SiMoCoy catalysts corroborates the proposed above picture. The addition of cobalt on Mo/Si02 catalysts presumably brings about a detachment of molybdenum oxide in interaction with SiOz and the formation of CoMoOl. This process takes place due to the weakness of the Mo-SiOz bond; the formation of a free compound like CoMo04 is energetically more favorable. This does not occur with 7-A1203,because molybdenum is strongly attached. In this case, with molybdenum keeping attached to the support, cobalt has to adsorb on the Mo(V1) monolayer, forming a Co-Mo bi1ayer.l According to this picture, CoMo/SiO, and CoMo/Alz03catalysts at the same atomic composition r = Co/(Co + Mo) = 0.36 have quite different structures. 5. Conclusions The conclusion of the present work concerning studies
Gajardo et al.
on the CoMo/y-Alz03 and CoMo/SiOz systems is that there is no qualitative difference between them, but only a different order of stability of the various supported species. A conspicuous characteristic of the Mo/ SiOz catalysts is the heterogeneity of their surface, on which small amounts of strong attached Mo(V1) coexist with weakly adsorbed Mo(V1). This fact gives to this kind of catalyst a dual character: (i) one typical of the catalyst in which the Mo(V1) monolayer is very stable (for instance Mo/ Al,03) and responsible for properties such as high initial reduction rate, absence of induction time, and presence of Mo(V) ions; (ii) a second one, connected to weakness of the Mo(V1)-support interaction, which gives to the catalyst properties such as low molybdenum dispersion, easy formation of bulk Moo3, and formation of bulk CoMoO, when cobalt is added. In catalyst with moderate molybdenum content the second character largely predominates, but at low molybdenum loading, the properties of the first type may become important. Although the differences between CoMo/y-AlzOs and CoMo/SiOz systems are essentially quantitative, the consequences with respect to catalytic activity are enormous in hydrodesulfurization. Indeed, the catalytic activity is largely determined by the dispersion of cobalt and molybdenum species in their sulfide forms on the support, which are, in turn, extensively determined by the dispersion of the oxide precursor forms. Thus, the much poorer dispersion of Co on SiOa (as CoMo04) compared to the Co-Mo bilayer in 7-Alz03is sufficient to explain the much poorer activity of the former.
Acknowledgment. The authors gratefully acknowledge financial support from the “Services de Programmation de la Politique Scientifique” in the frame of the “Actions concertees Interuniversitaires de Catalyse”. P.Ga. acknowledges support from C.T.M. during the course of this investigation. References and Notes (1) Gajardo, P.; Grange, P.; Delmon, B., submitted for publication. (2) Gajardo, P.; Grange, P.; Delmon, B., submitted for publication. (3) Pott, G. T.; Stork, W. H. J. “Preparation of Catalysts”, Delmon, B.; Jacobs, P. A.; Ponceiet, G., Eds.; Elsevier Sclentific Publlshing Co.; Amsterdam, 1976; p 537. (4) Fransen, T.; Van Berge, P. C.; Mars, P., ref 3, p 405. (5) Cimino, A.; de Angelis, B. A. J . Catal. 1975, 36, 11. (6) Castellan, A,; Bart, J. C. J.; Vaghl, A.; Giordano, N. J. Catal. 1976, 42, 162. (7) Praliaud, H.; J. Less-Common Met. 1977, 54, 387. (8) Gajardo, P.; Grange, P.; Delmon, B., J. Phys. C b m . Precedingarticle in this issue. (9) Delannay, F.; Gajardo, P.; Grange, P. J. Microsc. Spectrosc. Ekcfron 1978, 3 , 411. (10) Delmon, B., “IntroductionI la Cinetique HBttirogBne”, Technlp Edts., Paris, 1969, p 426. (11) Ashley, J. H.; Mitchell, P. C. H. J . Chem. SOC. A 1988 2821. (12) Asmolov. 0. N.: Krvlov. 0. V.. Kinet. Katal. 1970, 1 1 , 1028. (13) Giordano, N.; Bart,-J. C. J.; Vaghl, A,; Castellan, A,; Martinotti, G. J. Catal. 1975, 36, 81. (14) Masson, J.; Nechtschein, J. Bull. SOC.Chim. Fr. 1968, 10, 3933. (15) Masson, J.; Delmon, B.; Nechtschein, J. C. R . Hebd. Seances Acad. Scl., Ser. C 1968, 266, 428. (16) Zabala, J. M.;Grange, P.; Deimon, B. C. R . Hebd. Seances Acad. Sci., Ser. C 1974, 279, 561. (17) Sonnemans, J.; Mars, P. J . Catal. 1973, 31, 209. (18) Dufaux, M.;Che, M.; Naccache, C. J . Chim. fhys., 1970, 67, 527.
