Physicochemical characterization of the interaction between cobalt

Physicochemical studies on a series of cobalt-molybdenum supported on silica catalysts containing the same amount of oxide phase with an atomic ratio ...
0 downloads 0 Views 1MB Size
Download PDF
Interaction between Co-Mo Oxide and S O p

The Journal of Physical Chemistry, Vol. 83, No. 13, 1979

1771

Physicochemical Characterization of the Interaction between Cobalt Molybdenum Oxide and Silicon Dioxide. 1. Influence of the Cobalt-Molybdenum Ratio P. Gajardo, P. Grange, and

B. Delmon”

Universit6 Cathoiique de Louvaln, Groupe de Physico-Chimie Minerale et de Catalyse, 1348 Louvain-la-Neuve, Belgium (Received December 15, 1978) Publication cosb assisted by Services de la Programmation de la Politique Scientifique of the Ministry of Scientific Policy of Belgium

Physicochemical studies on a series of cobalt-molybdenum supported on silica catalysts containing the same amount of oxide phase with an atomic ratio r = Co/(Co + Mo) ranging from 0 to 1 and the phases obtained after reduction by hydrogen allow us a better understanding of the structure of these solids. X-ray diffraction, DRS, and XPS measurements of the oxide phase, the study of the reduction by hydrogen by use of gravimetric measurements, and ESR and DRS measurements carried out on the reduced phase show that weak interaction of the molybdenum with the silica (as compared with those on alumina) strongly modify the nature of the species presented on the support. It has been shown that cobalt and molybdenum almost stoichiometricallyform CoMo04 (a and b phases). Furthermore when Moo3 and Co304are in excess they cover the CoMo04crystals forming a geod structure. In conclusion, it is proposed that, when the interactions of the molybdenum with the support are weak, the CoMo “bilayer” precursor of the active sites is not easily formed. Consequently the strong interaction between cobalt and molybdenum leads to the formation of bulk CoMo04,compound which does not form, after activation, the active phase in HDS catalysts.

1. Introduction

In a previous investigation1y2on oxidic Co-Mo/r-A1203 hydrodesulfurization catalysts, we concluded that a strong interaction takes place between cobalt and molybdenum oxides and the A1203 surface. This interaction brings about the formation of a Co-Mo “bilayer” adsorbed on the 7-A1203surface; the formation of the bilayer corresponds to a decrease of the hydrogen reducibility of the deposited oxides. It was suggested that the formation of the Co-Mo “bilayer” has its origin in the strong interaction between molybdenum oxide and A1,0,, which brings about the formation of a Mo(V1) oxidic monolayer, which, in turn, helps the dispersion of cobalt and keeps it attached on the surface. The consequence of this interpretation was that, if an “inert” material is used as support (where the Mo(VI)-surface interaction is lower), such a Co-Mo “double layer” should not be easily formed and, correlatively, other associations and, in particular, bulk cobalt molybdate, could be formed. The goal of this work was to check this hypothesis. A series of oxidic Co-Mo catalysts supported on SiOz have been studied, and the results have been compared with those obtained on Co-Mo/y-Alz03 catalysts. The same approach has been utilized; viz., we used a series of oxidic Co-Mo/SiOz catalysts where the total active phase content (Co304+ Moo3) was kept constant but where the atomic Co/(Co Mo) ratio was changed. The reduction by hydrogen of these catalysts was investigated by using gravimetric, ESR, and diffuse reflectance techniques. The study was completed by analyses of the oxidic catalysts with several physicochemical methods, including specific surface area, X-ray analysis, and XPS.

+

2. Experimental Methods

2.1. Catalysts and Unsupported Compounds. 2.1.1. Support. SiOz was a Rh6ne Progil Silica (Spherosil XOA 200) of 179 m2 8-l surface and pore volume of 1.2 mL g-l. The mean diameter of the particles was 170 A. 2.1.2. Catalysts. Cobalt and molybdenum were deposited successively on SiOz by using the pore volume impregnation method. Molybdenum was deposited first.

Molybdenum was impregnated from an ammonium paramolybdate (APM) solution in water. In the pore volume impregnation method, the amount of water, where the salts are dissolved, corresponds to pore volume of the added support, During the impregnation, the catalysts were continuously stirred. The solid obtained a t “incipient wettness” was dried a t 110 “C (1 h) and then heated subsequently for 1 h a t 200 “C and a t 300 “C for 1 h, in order to decompose the APM. Finally the catalyst was calcined a t 500 “C for 24 h. Cobalt was impregnated on the calcined Mo/SiOz by the same procedure by using a water solution of cobalt nitrate. 2.1.3. Unsupported Compounds. APM, C O ( N O ~ ) ~ . 6H20, and Na2Mo04.2H20used in the preparation of the catalysts as well as for the physicochemical measurements were of commercial type (Merck GR). The preparation methods of Moo3, Co304,and cobalt molybdate (a-CoMo04and b-CoMo04) have been reported e1sewhere.l 2.2. Catalyst Composition, Surface Area, and X - r a y Analysis. Catalyst composition was determined by the atomic absorption method (Varian Techtron PTY, Ltd). Catalysts were dissolved in H F + H2S04solution. B E T surface area was determined volumetrically by Nz adsorption. X-ray analysis was performed in a Philips BV 1008 apparatus by using cobalt as anthicatod and iron as filter. 2.3. X P S . XPS measurements were carried out in a Vacuum Generator ESCA2 apparatus. A Tracor Northern NS 560 signal averager was used to improve the signalto-noise ratio. The samples were dusted onto a doublesided-adhesive tape and introduced into the preparation chamber for a few minutes at room temperature and under lo* torr vacuum before introduction into the measurement chamber. The method and conditions of spectra recording are the same as those employed in our previous work.2 The lines of the Cls, Ols, Si?,, M03d, and Cozps,2levels were recorded. The A u ~ line ~ ~ was , ~ used as reference (binding energy (BE) = 82.8 eV). In order to perform a quantitative analysis, spectra were recorded before gold evaporation, keeping a strict standardization in the order and in the time of recording.

0022-3654/79/2083-1771$01.00/00 1979 American Chemical Society

1772

The Journal

of Physical ChemWy, Vol. 83, No. 13, 1979

The decomposition of the spectra and the calculation of the signal intensities were done on the basis of the following assumptions: (a) the shape of lines is Gaussian, (b) the background due to inelastically scattered electrons is assumed to vary linearly with BE. The line intensity was the area under the Gaussian peak. The M03d level was not decomposed, because the spectrum was very badly resolved. The Cozp3 level was decomposed into the main peak and one saiellite; this decomposition is adequate, in view of the shape of the line. The corresponding intensities are noted I, (principal) and Z, (satellite). In the case of cobalt oxides, the ratio of their intensities (for instance expressed as Zp/(Zp + I,)) has permitted the estimate of the presence of diamagnetic Co(II1) and high-spin C O ( I I ) . ~ ~ ~ The intensities of the Si,,, Cozps/2, and MOad h e (Zsi,,,Z, and ZM,, respectively) have been calculated by considering the normalized area of the XPS peak and the response coefficient of the apparatus, determined experimentally for each element. The sum of the partial intensities of the main CoZpaj2 peak and of its satellite and of M03dbj2and M03d i z was taken as line intensities of cobalt (Ic,) and respectively. In order to evaluate the molybdenum (IMo), degree of spreading (dispersion) of cobalt and molybdenum on the Si02surface, we shall use the atomic fraction “seen by XPS”. This magnitude is defined, in the case of cobalt for instance, by the formula Ic0/(Zsi + IC, + IMJ.Analogous expressions will be used in the case of Mo and Si. In the interpretation of data, it will be considered that intensities of Si,,, C O ~ , ~and , ~ , M03d lines are similarly affected by “external factors” as for example the surface contamination. The advantages and characteristics of such an analysis have been previously reported in the case of CoMo/y-A1203 catalysts.2 2.4. Gravimetric Measurements. We utilized the same apparatus and procedure as those employed in the study of oxidic Co-Mo/y-A1203 cata1ysts.l We used a McBain balance (5 L) connected to a standard vacuum line (loy5 torr) and to a device permitting the introduction of gases into the system. The quartz reactor, an all-glass magnetic pump, and a trap cooled by liquid nitrogen formed a loop permitting a continuous recirculation of the gases in the system and the elimination of the water produced during the reaction by condensation in the trap. The samples (200-300 mg) were degasified overnight under vacuum torr) at 500 O C prior to the experiments. Hydrogen reduction was performed at 400 “C and 1 atm of hydrogen pressure (99.99 purity). In order to estimate the quantity of water readsorbed during the reaction, samples were degasified overnight under vacuum torr) at 500 “C after each experiment and the final weight was recorded. It was observed that the amount of water remaining adsorbed after the reduction experiments was very low and might be neglected. The extent of reduction a is defined by the ratio a = reduced oxidelinitial oxide. In the calculation of a, we assumed that cobalt and molybdenum oxides occur as Co304and Moo3 in the initial states of the catalyst and that the ultimate reduction state is Coo and Moo. 2.5. E S R and Diffuse Reflectance Spectroscopy (DRS). In order to examine the catalysts at various reduction times both by the ESR and DRS techniques, a reactor comprising a DRS quartz cell and ESR quartz tube was designed (Figure l). 2.5.1. Procedure. Prior to hydrogen reduction, catalysts were evacuated (10-4-10-6 torr) first at room temperature and subsequently at 415 “C, for 15 and 20 min, respectively. In order to eliminate the remaining adsorbed water,

P. Gajardo, P. Grange, and B. Delmon

Figure 1. Reactor for simuttaneous DRS and ESR measurements utilized in reduction studies of catalysts by hydrogen: (a) catalyst bed, (b) Pyrex porous septum, (c) grease-free stopcock.

helium (60 mL mi&) was allowed to pass through the catalyst bed for -2 h at 415 “C. The sample was then evacuated and hydrogen (60 mL min-l) was introduced into the reactor. After the desired reduction time, the hydrogen flow was stopped and the reactor was rapidly withdrawn from the oven. The reactor was turned upside down, allowing the catalyst to fill the cold ESR and DRS cells. The corresponding spectra could then be recorded. It was subsequently possible to continue the hydrogen reduction by allowing the catalyst to fall back on the porous septum, returning rapidly the reactor to the oven at 400 “C, and restoring the hydrogen flow. The operations were repeated a t various reduction times. 2.5.2. ESR Apparatus. Analysis were performed on a Varian E12 spectrometer with a dual sample cavity. The magnetic field was modulated at 100 kHz, and using an incident microwave power of 20 mW we recorded the first derivative of the X-band (9.5 GHz) spectra. “Strong Pitch Varian” was used as standard. With our setup, the sample quartz tube (3 mm i.d.) was filled with the catalysts to a height greater than the depth of the resonant cavity. The product of the signal amplitude and the square of the peak-to-peak width was taken as line intensity. This method of calculation is adequate if the shape of the line spectra does not change with the degree of reduction. 2.5.3. DRS. Diffuse reflectance spectra (DRS) were recorded in a Beckman Acta M IV spectrophotometer (200-800and 800-2000 nm). In order to record the spectra, both catalyst and reference sample were put in separated quartz DRS cells (Figure 1). The reference was the same SiOz used as catalyst support. 3. Results 3.1. Catalyst Composition, Surface Area, and X - r a y Analysis. We report in Table I the principal physicochemical characteristics of the catalysts. Each catalyst is designated by a symbol indicating the support Si @ioz) followed by the value of the ratio r = Co/(Co + Mo) (atomic). The losses after degassing of catalyst (performed prior to reduction experiment) are similar in all catalysts. The X-ray patterns of the catalysts and unsupported compounds are presented in Figure 2. Compounds presenting X-ray patterns similar to patterns of model substances were found in all catalysts except in Si 0.27,

The Journal of Physical Chemistry, Vol. 83, No. 13, 7979

Interactlon between Co-Mo Oxide and S102

1773

TABLE I: Physicochemical Characteristics of Catalysts % (wt) of oxide phase in X-ray analysisb xx catalyst - Co/(Co surface loss MOO, t t Mo area, m 2 of wt a-Co b-Co catalyst color Moo, Co,O, Co,O, (atomic) g-’ (wt % ) a Moo, Co,O, MOO, MOO, 6.0 yes 11.6 0.0 11.6 0.00 122 Si-0.00 bluish 5.6 yes no no no 11.7 0.3 12.0 0.05 110 Si-0.05 bluish 5.0 no no no no 0.27 108 9.5 1.9 11.4 Si-0.27 gray-violet 4.6 ? no no yes 122 2.9 12.3 0.36 Si-0.36 9.4 gray-violet Si-0.49 gray 8.1 4.3 12.4 0.49 117 4.6 ? no ? Yes Si-0.75 black 4.7 7.8 12.5 0.75 119 5.1 no yes no yes 0.0 12.1 12.1 1.00 110 5.5 Yes Si-1.00 black A question mark indicates that a Loss of weight of oxide catalysts after degassing overnight under vacuum at 500 “C. the presence of referred compound in the catalyst is possible. SI-100

SI-100

s1-a~

b - C0M004

a - CoMoO4

I 18 Figure

I

I

44 40

I

i

36

32

l(2e), 28 24

X-ray diffraction patterns of catalysts and model compounds.

where no diffraction peaks were observed. The results are summarized in Table I. 3.2. Diffuse Reflectance Spectra of Catalysts in Their Oxidic Form. The spectra of nonreduced catalysts and model compounds are reported in Figure 3 (200-800 nm) and Figure 4 (800-1920 nm). 3.3. XPS. The binding energies of C02p3,2, Siep,and 01, are reported in Table 11, for catalysts and model compounds. The 01, band of Si-0.00 is slightly asymmetrical and was decomposed into two components. One is ascribed to the oxygen of the support and the second to the oxygen of MOO,; the justification of such an assignation has been presented elsewhere.* We have also reported in Table I1 the Ip/(Ip+ I,) ratio, which characterizes the relative importance of the principal (Ip)and satellite (I,) peaks of C02PS,**

I 300

I

I 500

I

\

I\ I 700 ~ > ~ m

Figure 3. Diffuse reflectance spectra of oxide catalysts and model compounds.

In order to estimate the degree of spreading (dispersion) of cobalt and molybdenum on the SiOz surface, we have plotted in Figure 5 the atomic fraction of the various ions “seen by XPS”, against the atomic composition of the catalysts. 3.4. Reduction Experiments Performed by the Gravimetric Method. Reduction isotherms measured a t 400 “C are presented in the Figure 6. A considerable portion of the oxides is reduced rapidly (spontaneously) (during the first 3 min of reaction) in the catalysts rich in cobalt. The degree of reduction a , measured after 10 h of reduction, is plotted against catalyst composition r = Co/(Co + Mo) in Figure 7 (curve a). A conspicuous fact is the

1774

The Journal of Physical Chemistry, Vol. 83,

No. 13, 1979

P. Gajardo, P. Grange, and B. Delmon

co Co

+

Mo

Flgure 5. Dispersion of Si, Co, and Mo on the catalyst surface. The dotted line corresponds to the theoretical I s , / ( I s , I, iIC,) value which would be observed if the cobalt and molybdenum oxides retain the same state of dispersion as they have in Si-0.00 and S-1.00.

+

I

0

c

1

1) c0304

1 I I I I I 1120 1440 1760 nm I

Figure 4. Diffuse reflectance spectra of oxide catalysts and model compounds.

TABLE 11: Binding Energy (BE) of CO,,~,~,Si,,, and O,, Levels and I,/(I, t I,) RatioC

0.5

C%,2

solids

Ipl(Ip BE CO,~,,,, BE Si,,, + Isla eV eV

BE O,,, eV

531.7 (2.9) Si-0.05 undted undted 529.6 (2.9) Si-0.27 undted undted 532.0 (3.3) Si-0.36b undted undted 531.7 (3.4) 531.9 Si-0.49 0.55 780.1 (3.8) (3.5) 531.7 Si-0.75 0.75 779.5 (3.6) (3.7) 531.7 Si-1.00 0.70 779.2 (3.4) (3.6) a-CoMoO, 0.57 779.7 529.1 (3.0) CO@, 0.82 778.3 528.8 (2.5) a I , and Is are the intensities of C O , , ~ ,and ~ satellite peaks. Data taken from ref 4. FWHM are given in parentheses. “Undted” means that the referred values were not determined because of weakness of the signal intensities. Si-O.OOb

101.7 (3.3) 102.3 (3.2) 102.3 (3.2) 102.3 (3.2) 102.5 (4.0) 102.4 (3.7) 102.5 (3.6)

decrease of reducibility in the middle range of compositions. In the same figure, we have plotted ac0 against Co/(Co Mo). ace, which is proportional to the amount of Co304 present in the catalyst, will be defined and examined in the Discussion section. 3.5. Simultaneous E S R and DRS Study of Partially Reduced Catalysts. The reduction of Si-0.00 and Si-0.36

+

I 0 0

6

12

t

hours

Figure 6. Reduction isotherms (400 “C)of catalysts. I

I

02 00

025

I / 050

075

10

co

C o + Mo

Figure 7. Reducibilities of catalysts at 400 O C vs. catalyst composition: (a) reduction 01 after 10 h, (b) raw reduction cyco corresponding to instantaneous reduction of catalyst. Dotted line corresponds to the theoretical value.

was further investigated by ESR and DRS by using the procedure described in Experimental Methods. The ESR

The Journal of Physical Chemistry, Vol. 83,No. 13, 1979

Interaction between Co-Mo Oxide and Si02

JAk J*

1775

without treatment

degasified at 415°C

Q, V

Flgure 8. ESR signal of reduced Si-0.00 (a, signal I) and Si-0.36 (b, signal 11).

C

crl L

4 a

a

1 ) 1 1 1 1 1

51-036

!oo

I

I

b

I

0,

\

400

600

nm

8oc

b treatment

1 \

without treatment

*\

1

8

s

n L

\ \

degas.at 415°C

4

n

a

144 min

n

J / / T m i -

i 200

_c

1

504 mir

i

I 400

I

I 600

I

nm

I 80C

Flgure 10. Diffuse reflectance spectra of the Si-0.00 catalyst at various reduction times (fred.).

1 1 1 1 1 1 1 1 100 1120 1440 1760 nm Figure 11. Diffuse reflectance spectra of Si-0.36 catalyst at various reduction times ( fred,).

results are reported in Figure 8. In both catalysts, an ESR signal centered at g = 1.936 (signal I) was detected. In the case of Si-0.36 a broad signal (signal 11) is superposed on signal I. In Figure 9 we have plotted the intensity of signal I (arbitrary units) vs. reduction time. The intensity of signal I was normalized to 1g of Moo3. The experimental points corresponding to Si-0.36 have not been reported for times higher than 78 min, because signal I becomes strongly deformed by the superposition of signal 11. The diffuse reflectance spectra recorded at the same reduction times as ESR spectra are presented in Figures 10 and 11for Si-0.00 and Si-0.36, respectively. The Mo(V1)

1776

The Journal of Physical Chemistry, Vol. 83, No. 13, 1979

absorption band (250-375 nm) disappears earlier in Si-0.36 than in Si-0.00. 4. Discussion 4.1. Characteristics of the Oxidic Catalyst. 4.1.1.

X - r a y Data. One conspicuous fact concerning the physicochemical characteristics of the catalysts (Table I) is the presence on the support of compounds identical with usual bulk substances, namely, Moo3, Co304, and bCoMo04. The presence of these compounds, which are not observed in “industrial” Co-Mo/y-A1203 catalysts, reveals a notable difference of SiOz as catalyst support. X-ray analysis only shows the presence of the b form of CoMoO,. However, electron diffraction indicates the simultaneous existence of both a and b CoMoO, phases in the case of Si-0.36.5 The absence of any diffraction peak with Si-0.27 (Figure 2) suggests that the deposited phases are amorphous or form crystallites that are too small. Moo3 and CoMo04 are detected at r = 0.05 and r = 0.36, respectively. One might conclude that the situation is similar with Si-0.27, namely, that it contains Moo3 and CoMo04 but that the corresponding crystallites are very small. The present catalyst series, in both oxided and sulfided states, exhibits a remarkable “synergetic” maximum at r = 0.27 in the plot raw conversion of hydrodesulfurization of thiophene vs. r.6 This maximum of activity can probably be correlated with the higher dispersion of the supported oxides. 4.1.2. H 2 0 Adsorption. There are no significant variations of the loss of weight due to degassing at 500 “C (Table I). In a previous work, on oxidic Co-Mo/A1203,1 it was noted that the loss of weight reached a minimum value for catalysts composition where the dispersion of active phase was maximum. In our experiments, water is the only product likely to adsorb to a large extent on supports such as A1203and SO2. In the case of A1203,a correlation between the amount of adsorbed water and the fraction of the support surface which was not covered by the deposited oxides was found. The almost identical values of the loss of weight reported in Table I suggest that the extent of coverage of the SiOz does not depend on the composition of the deposited species. 4.1.3. DRS. The diffuse reflectance spectra of our samples must be interpreted by using the DRS data obtained with the model compound, the results of X-ray diffraction, and the literature data. The absorption band of the catalyst containing only molybdenum (with a maximum situated at -300 nm, Figure 3) can be ascribed to octahedral Mo(V1) [(charge transfer) 02-s Mo(VI)] in agreement with data reported by several authors.’-1° According to Praliaud6 and Castellan et several compounds containing Mo(VI), such as Moo3, silicomolybdic acid, and polymolybdate, may be present on Si02 The amount of each of them on the catalyst depends on the molybdenum content, preparation method, and calcination temperature. Comparing the diffuse reflectance spectra of Si-0.00 and model compounds not containing cobalt (Figure 3), we find it is possible to observe that the spectrum of Si-0.00 corresponds to the spectrum of APM rather than to the spectra of Moo3 and Na2Mo04.2H20.Therefore, we may ascribe the band at -300 nm of Si-0.00 to Mo(V1) located in an oxide surrounding similar to the polymolybdate one, which is consistent with data reported by Castellan et alelo Owing to the fact that diffuse reflectance spectra are not well resolved, one may speculate that, in the band Si-0.00, the bands of tetrahedral Mo(V1) (as in NazMo04*HzO)and octahedral Mo(V1) coming from Moo3 are slightly superposed. Both types of Mo(V1) could be present in the

P. Gajardo, P. Grange, and B. Delmon

catalyst in small amounts. Actually, the maximum of the Si-0.00 band is slightly displaced to the low nanometer side, which could be due to the presence of tetrahedral Mo(V1). On the other hand, the Si-0.00 band is larger than the APM band, suggesting the superposition of Moo3 band (we must recall that the Moo3 phase was detected by X-ray diffraction in this catalyst, Table I). Nevertheless, the contribution of free Moo3 species on the Si-0.00 spectrum seems to be modest. Indeed, we have reported elsewhere,ll that the absorption band of Si-0.00 catalyst has the same shape and is situated at the same position as Mo/SiOz catalysts with low molybdenum content (2.8 wt % of Moo3), where no Moo3 crystals were detected. Figure 3 shows that the addition of cobalt brings about a broadening of the Mo(V1) band. Taking into account that a-CoMo04and b-CoMo04 exhibit broad absorption bands at 325 and 415 nm and considering that both CoMo04 phases are present in the catalyst, we may conclude that the broadening of the band (with a shift of the maximum to -310 nm) is due to the superposition of the absorption bands of cobalt molybdates and polymolybdates like species. The band at 475-650 nm (Figure 3) has been ascribed to ligand field transition of Co(I1) in both the a- and b-CoMoO, c o m p ~ u n d s . ~The J ~ spectra of Si-0.27, Si-0.36, and b-CoMo0, are very similar; this fact indicates that CoMo04 is present mainly in its b form (as indicated by X-ray diffraction). This is in agreement with results obtained by de Beer et al.13on similar CoMo/Si02 catalysts. The relative intensity of the band due to CoMoO, species is highest in the catalysts with Co/(Co Mo) = 0.27 (compare absorption intensity of the band at 475-650 nm with that of Mo(V1) at 250-375 nm). On the other hand, X-ray diffraction (Figure 2) indicates that the relative intensity of CoMoO, diffraction peaks is highest at Co/(Co + Mo) = 0.49 (which is quite logical, as this corresponds to the atomic composition of CoMoO,), while no diffraction peaks are observed at Co/(Co + Mo) = 0.27. This apparent contradiction could be attributed to a deformation of the DRS band in the 300-310- and 475650-nm ranges by bands of octahedral Co(II1) coming out at -400 and 700-900 nm.237,13This deformation would increase with increasing the Co content, this apparently would diminish the contribution of CoMo04 for Si-0.49 and even suppress it for Si-0.75. Another possible explanation would be based on our interpretation of the X-ray diffraction data, namely, that the species in Si-0.27 are so dispersed that they do not diffract. Hence, the total surface area of CoMoO, exposed to DRS analysis is maximum at this composition and therefore, the absorption band due to CoMoO, (475-650 nm) is highest in Si-0.27. Various cobalt species have absorption band in the 800-1920-nm range, namely, octahedral Co(III), tetrahedral Co(II), and octahedral Co(I1) in oxide surroundThe first two ions are typical of Co304 and possess strong absorption bands which cover the weaker band of octahedral Co(I1): typical of CoMo04. Therefore it is not surprising that the absorption bands due to Co304clearly appear in all Catalysts with r 2 35 (Figure 4). The CoMo04 band only comes out clearly in Si-0.27, where the typical band of c0304 situated at 800-950 nm has disappeared. 4.1.4. XPS. 4.1.4.1. Binding Energy and Identification of Species. The binding energy values of Si, and 01,levels are similar in all catalysts (except in Si-0.00) suggesting that no significant variations of active phase-Si02 interactions take place when the amount of cobalt increases.

+

Interaction between Co-Mo Oxide and SiOp

The case of Si-0.00 is special; the 01,line is asymmetrical and can be decomposed into two peaks. One having a binding energy value similar to the binding energy of the 01,level of other catalysts and a second one about 2.1 eV lower. One of them is attributed to the oxygen of support and the second to the oxygen of Moo3. This result and additional ones, concerning series of Mo03/SiOz catalysts with various Moo3 loading, are examined el~ewhere.~ The BE and the I,/(I I,) values of Si-0.49 and aCoMoO, are quite simirar (Table 11), confirming the presence of cobalt molybdate on the surface of Si-0.49 catalyst. On the other hand, the low BE values and high I /(Ip+ I,) ratio observed in Si-0.75 and Si-1.00 suggest tkat these catalysts contain a high concentration of diamagnetic Co(III), thus indicating the presence of Co304. Nevertheless, values of these catalysts and of pure C0304 are different (Table 11). This is quite surprising, because c0304 is believed to be the only species present in Co/SiOz catalysts15and our forthcoming discussion of the hydrogen reduction experiments will conclude that Co304is present for r I0.49. The fact that the amount of Co(II1) observed is lower than the expected one, may be explained by a slight reduction of Co(II1) ions to Co(I1) in the XPS apparatus; this effect has been observed in the case of unsupported solids.16 In addition to that, it is not excluded that a small contribution to the CoZP line may come out from small quantities of high-spin to(I1) stabilized by SiOz. In the Discussion section concerning hydrogen reduction, we shall refer again to the presence of such Co(I1) species in the catalysts. 4.1.4.2. Quantitative Exploitation of X P S Results. In order to discuss the semiquantitative XPS results reported in Figure 5, we shall recall the salient features about the structure of Mo/SiOz and CoMo/Si02 catalysts, examined elsewherell and confirmed in this work: (i) Mo(V1) is adsorbed on the SiOz surface forming a Mo(V1) oxide monolayer. (ii) Mo(V1) is located in a polymolybdate-like symmetry in the monolayer. (iii) In general, Mo(V1) interacts weakly with the Si02 surface, which brings about an early formation of free MOO,; nevertheless due to a high heterogeneity of the SiOz surface, there is a small amount of Mo(V1) strongly attached to the surface. (iv) The addition of cobalt into the Mo/SiOz provokes the formation of CoMo04 (mainly in its b form). In this case, cobalt reacts with the Mo(V1) weakly adsorbed on the surface. (v) It is not excluded that a minor part of added cobalt stays attached to the Mo(V1) strongly adsorbed on SiOz, forming a small amount of “bilayer” structure as it was envisaged in CoMo/y-Alz03catalysts. Results of Figure 5 will be discussed considering this picture about the structure of catalysts and using the same approach utilized in previous work,2 where we have examined this method and its limitations. Figure 5a indicates that the Si atomic fraction “seen by XPS” [Isi/(Isi I M , IC,)] is nearly identical in all the catalysts of the series, except at r = 0.75, where it is depressed. The dotted line corresponds to the theoretical Isi/(Isi + IM,+ IC,) value which would be observed if the cobalt and molybdenum oxides retain the same state of dispersion as they have in Si-0.00 and Si-1.00. The depression at Si-0.75 may be interpreted in terms of an increase of the coverage of the SiOz surface by Mo and Co. Figure 5b indicates that the addition of cobalt provokes an increase of the spreading of molybdenum on the catalyst surface. The experimental line is situated above the dotted line corresponding to the spreading of molybdenum observed in Si-0.00, The increase of molybdenum spreading

+

+

+

The Journal of Physical Chemlstry, Vol. 83,No. 13, 1979

1777

when cobalt is added may be explained by the formation of small crystallites of CoMo04 and/or by a coverage of these crystallites by Moos layers up to r = 0.36 yielding a geode structure. These phenomena may bring about a real increase of the molybdenum dispersion on the surface and consequently a relative increase of the M03d XPS signal. The absence of any diffraction peaks and the presence of a prominent absorption band due to CoMo04 at r = 0.27 (Figure 3) are consistent with the first hypothesis. The existence of a geode structure is demonstrated by two separate experimental observations: (i) The abrupt extinction of the CoZp3,signal at r = 0.36, in spite of the significant amount of 60 present in the catalyst, suggests that cobalt is covered by molybdenum (cobalt does not migrate into the SiOz at 500 “C). (ii) A t compositions at which the XPS signal of cobalt is very weak, a prominent CoMo04absorption band comes out, revealing the presence of such a compound in the bulk of the grains (DRS receives information from much deeper layers of the solid than XPS). Finally, as the active phase content is constant in all catalyst series, an apparent increase of the molybdenum dispersion must be observed if the decrease of the overall Mo (Figure 5b, from left to right side) is achieved at the expense of bulklike MOO, present in the catalyst. Concerning cobalt, an enhancement of spreading is also observed when molybdenum is present in small amounts (compare Si-0.75 and Si-100, Figure 5b), but the cobalt signal abruptly decays for r C 0.75. The increase of the cobalt spreading at r = 0.75 may be interpreted in terms of a coverage of formed CoMo04 crystallites by Co304,as was supposed to take place with Moo3 at low r (see above). The similitude of the BE and Ip/(Ip+ I,) values in Si-0.75 and Si-1.00 supports this hypothesis. In this case, one could argue against such a geode structure, remarking the absence of CoMo04 bands in the DRS spectrum of the Si-0.75 catalyst (Figure 3). Nevertheless, it must be emphasized that the absence of the weak octahedral Co(I1) band of CoMo04 can be due to a covering of it by the strong tetrahedral Co(I1) band’ of Co304. Moreover, the presence of b-CoMoO, in Si-0.75, detected by X-ray analysis (Table I), supports the hypothesis of a geode structure. The presence of a small amount of Co-Mo bilayer structure associated with Mo(V1) strongly adsorbed on SiOz could contribute to enhance the dispersion of cobalt on the catalyst surface, as was observed in CoMo/Alz03. However, because such a structure should be low extended on SiOz, its participation in the overall Co spreading might be modest. The similar values of BE and Ip/(I, I,) for Si-0.49 and CoMoO, (Table 11) suggest that at this composition ( h o s t stoichiometric for CoMo04)such geode structure does not exist. 4.2. Gravimetric Measurements of Hydrogen Reduction. In a previous study of the reduction of aluminasupported CoMo catalyst, we showed that the reduction of Co304is extremely rapid at 400 OC and that the reduction of CoMo04 is much s1ower.l Taking into account that Co304is present in catalyst with r > 0.49 (Table I), in a first approximation we can attribute the instantaneous initial reduction of our catalysts to Co304(Figure 6) and estimate the amount of Co304from the initial degree of reduction (taking place during the first 3 min of reaction). In order to evaluate the proportion of cobalt which is present as free Co304in each catalyst, we have defined a new quantity, ac,, by the ratio given in eq 1 where (0)inet

+

=

~4(0)~~~~/~3(c0)tot~~

(1)

1778

The Journal of Physical Chemistry, Vol. 83, No. 13, 1979

is the amount of oxygen atoms [mol of O/g of catalyst] which instantaneously reacts with H2 and (Co)totalis the total amount of cobalt present in the catalyst [mol of Co/g of catalyst]. In Figure 7 (curve b) is plotted the instantaneously reduced Co, ace, against catalyst composition. Figure 7 indicates that the quantity of free Co304increases linearly with the amount of cobalt from about 0 a t r N 0.42. In this figure, the dotted right line is the theoretical line obtained on the assumption that cobalt reacts stoichiometrically with molybdenum to form CoMo04 and that cobalt in excess is reduced as in Si-1.00. The experimental points are situated above the dotted line; this suggests that molybdenum does not combine with the maximum amount of cobalt (forming CoMoO,) and that free Co304is more abundant. The apparent incomplete reduction observed with Si-1.00 (ace N 0.92) could be linked to the presence of the small amount of Co(I1) ions strongly attached to the SiOz surface. The presence of such Co(I1) species in Co/Si02 with low cobalt content has been mentioned by Tomlinson et al.17using magnetic measurements (up to 0.5 wt % of Co), by Brotikovskii et al.ls using DRS (up to 1.6 wt % of Co), and by Taniguchi et al.19using both magnetic and DRS techniques (up to 1.2 wt % Co). Brotikovskii et al. indicated that adsorbed Co(I1) is located in a tetrahedral oxide surrounding, whereas Taniguchi et al. suggest that silica gel surface might stabilize the cobalt as high-spin Co(I1) in both tetrahedral and octahedral coordinations. The depression of the reducibility of catalysts containing simultaneously cobalt and molybdenum, when compared with those containing only cobalt or molybdenum (Figure 7, curve a), may be explained by the formation of CoMoO,. The presence of which, in the b and a forms, is established in our previous measurement^.^ According to literature and our own measurements, the various oxides should be placed in the following order of decreasing reducibility:1,20v21Co304 > a-CoMo04 > bCoMo04 1 Moo3. However, we must keep in mind that the reducibility of CoMo04 may be very sensitive to pretreatments,21 which may have consequences on the order of the series, specially between Moo3 and b-CoMo04. Thus, the decrease of reducibility can be explained without involving any specific interaction between deposited species and the support, as was suggested in the case of Mo/SiOz.l1 In previous work on reduction of CoMo/AlZO3oxide catalysts,’ a similar but more strong depression of reducibilities was observed in the middle composition range (r = 0.25-0.75). It was suggested later,2 that the formation of a Co-Mo “bilayer” (in which Co strongly interacts with Mo) is the cause of such a depression. Similarly and admitting the presence of a small amount of Co-Mo bilayer structure in CoMo/Si02 catalysts (see XPS discussion), we may propose, in principle, that the depression of curve a, Figure 7, could be, in small degree, also due to the presence of an incipient Co-Mo bilayer in Si02. 4.3. “ I n situ” DRS and ESR Measurements. ESR is a powerful tool for investigating the interaction between molybdenum and various supports (A1203,S O z , silicaalumina, etc.). The measurements which we shall discuss now should thus help us study possible interactions with the support. DRS should, in principle, complement ESR for this purpose. 4.3.1. ESR. Mo(V) and Mo(II1) ions are the sole molybdenum ions which give ESR signals at room temperature.22 Signal I appears alone in catalysts containing only molybdenum.

P. Gajardo, P. Grange, and B. Delmon

Taking into account its shape (Figure 8a) and its g value (centered at 1.936), we must ascribe signal I to M O W . This signal has already been found in molybdenumcontaining catalysts supported on various oxides such as A1203,Si02, A1203-Si02, and MgO. The origin of this signal has been the subject of comprehensive s t ~ d i e s . ~Several ~ - ~ ~authors2s25 have proposed that MOW) occupies a square-pyramidal environment of CdUsymmetry, giving gII and gI factors of 1.940-1.865 and 1.959-1.872, respectively. On the other hand, Abdo et alaz6 have suggested a tetrahedral distorted environment for this Mo(V). According to these authors, this type of symmetry would be probable in Mo/SiOz as, in this case, MOW)ions may substitute silicon ions situated in tetrahedral environment. If this hypothesis is correct, MOW) might be located in a tetragonally distorted tetrahedron in our catalysts. Additional arguments could possibly be brought by our own measurements in DRS, although the sensitivity of this latter method is much less than that of ESR. Our diffuse reflectance spectra indicate that in oxide catalysts, Mo(V1) is situated in an octahedral polymolybdate-like surrounding (see above discussion of DRS results on oxide catalysts). Therefore, provided strong rearrangements do not take place during the first stage of reduction, MOW) would be in a square-pyramidal environment. However, our DRS results do not exclude the presence of a small amount of Mo(V1) in tetrahedral arrangement in the oxide catalyst. Several forms of Mo(V1) in different symmetries have been reported to be present simultaneously on Si02.8p10 Besides, Asmolov and Kry10v~~ have indicated that the symmetry of Mo(V) ions may change during the reduction of molybdenum supported catalysts; thus Td,C4u,and O4 symmetries would be possible. Results reported by Praliaud8 support this hypothesis. Thus it does not seem possible in the present stage of our knowledge to give an inequivocal picture of the coordination of MOW). Our spectra indicate that no measurable quantities of Mo(II1) are formed during the reduction of our catalysts. Actually, in the case of Si-0.00the shape of signal I remains constant and no new signal appears at high reduction time. Seshadri and Petrakis22suggest that the Mo(II1) signal would be centered at g N 1.96; no such signal appears in our spectrum. Besides, these authors did not observe any Mo(II1) signal in molybdenum supported on various oxides. Let us now examine the variation of signal I with time. Figure 9 indicates that intensity of signal I goes through a prominent maximum when reduction time increases, this maximum being situated at the very beginning of reduction. This effect is observed in both Si-0.00 and Si-0.36 catalysts. This type of evolution has been observed by other investigators in the case of Mo(V1) oxide supported on A1z0325828and on Si02.29730In studies performed by the XPS technique, on both type of supported catalysts, it was found that the amount of Mo(1V) increases and Mo(V1) diminishes with the time, suggesting that Mo(V) is an intermediate in the Mo(V1) reduction. In the case of A1203, the appearance of Mo(V) is attributed to a strong interaction with the support; bulk Moo3, during reduction, does not exhibit any Mo(V) signal.31 The conspicuous fact in our results is that Si02may stabilize MOW)even after 10 h of reduction at 400 “C. The low amount of Mo(V) observed in Si-0.36 may be explained as a consequence of the formation of CoMo04, which probably brings about a partial detachment of Mo(V1) in interaction with SiOz surface. We have no

Interaction between Co-Mo Oxide and SO2

evidence that CoMo04 gives a Mo(V) signal on reduction. The detachment of Mo(V1) in interaction with Si02when cobalt is present is not the only possible explanation of the weakness of the Mo(V) signal. Another explanation can be given in terms of a catalytic action of metallic cobalt on the reduction of molybdenum. An enhancing effect of Ni, Co, and Pt on the reduction of molybdenum oxide supported on A1203and A1203-Si02has been observed by Masson et al.;31p32 the addition of these metal ions provokes a decrease of the amount of Mo(V). The broad signal I1 (Figure 8b), which is observed only in Si-0.36 catalyst, can be assigned to metallic cobalt, which is expected to form easily upon reduction of Co304and CoMo04. Actually, signal I1 becomes very strong when reduction time increases. An identical signal has been ascribed, by Lo Jacono et al.,3, to metallic cobalt, in studies on the reduction of Co/y-A1203. 4.3.2. DRS. ESR is not sensitive to Mo(V1) and Mo(1V); therefore DRS is a good complement because it is able to detect molybdenum ions in other valence states situated in defined symmetries. The diffuse reflectance spectra of the Si-0.00 catalyst indicates that the Mo(V1) absorption band at -300 nm (typical of oxide catalyst) diminishes with the reduction time and almost disappears after 144 min of reduction (Figure 10). The progressive smoothing out of the absorption line is due to the formation of Mo(V) and Mo(V1) species in various symmetries presenting several absorption bands in the 400-1000-nm range.8*27,34 In the case of Si-0.36 catalyst the smoothing of the bands is already completed after 48 min of reduction (Figure lla). On the other hand, the absorption band of the unreduced catalyst situated in the 800-1920-nm range (Figure l l b ) and assigned to Co(1II) in the Co304surroundings disappears in the first 18 min of reduction. These results show a promoting effect of cobalt on the reduction of Mo(VI), which may be due either to a catalytic action of cobalt on the reduction of molybdenum oxide (see ESR discussion) or to the fact that CoMo04 is reduced faster than Mo(V1) oxide in Mo/Si02. However, the last hypothesis might be less probable, because the b-CoMo04 form predominates in Si-0.36 and this form is difficult to reduce. 5. Conclusions We may summarize the following salient conclusions in the present work concerning the structure and behavior in H2 reduction of CoMo/Si02 catalysts: (i) Molybdenum occurs as MOO, but seems to interact with the SiOz surface forming a polymolybdate structure. (ii) Cobalt and molybdenum almost stoichiometrically form CoMo04 (a and b phases) in CoMo/Si02; the cobalt in excess of the stoichiometric quantity forms mainly Co304. It is not excluded that a small part of cobalt and molvbdenum form an incipient Co-Mo bilayer strongly attached to Si&. (iii) and ‘0304‘Over the CoMo04 crysta1s forming a geode structure in molybdenum and cobalt catalysts,

The Journal of Physlcal’Chemistry, Vol. 83, No.

73, 7979 1779

respectively. (iv) A considerably higher dispersion of the deposited species at r = 0.27 is probably the origin of the high hydrodesulfurization activity of this catalyst in the sulfided state. (v) It is confirmed that cobalt catalyzes the reduction of Mo(V1) and MOW) ions to Mo(1V) which seems to be the lowest oxidation state of Mo; MOW)is an intermediate ion in the reduction of Mo(IV). Acknowledgment. Authors gratefully acknowledge financial support from the “Services de la Programmation de la Politique Scientifique” in the frame of the “Actions Concertges Interuniversitaires 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 publicatlon. (2) Gajardo, P.; Grange, P.; Delmon, B., submitted for publicatlon. (3) Grimblot, J.; Bonnelle, J. P. J . Electron Spectrosc. Relat. Phenom. 1976, 9,449. (4) Gajardo, P.; Pirotte, D.; Defosse, C.; Grange, P.; Delmon, B., J. Ekctron Spectrosc. Relat. Phenom. I n press. ( 5 ) Delannay, F.; Gajardo, P.; Grange, P. J. Microsc. Spectrosc. Electron. 1978, 3 , 411. (6) Gajardo, P.; Declerck-GrlmBe, R. J.; Delvaux, G.; Olodo, P.; Zabala, J. M.; Canesson, P.; Grange, P.; Delmon, 8. J . Less-Common Met. 1977, 54,311. (7) Ashley, J. H.;Mitchell, P. C. H. J . Chem. Soc. A 1966, 2821. (8) Prallaud, H. J. Less-Common Met. 1977, 54,387. (9) Asmolov, G. M.; Krylov, 0. V. Kinet. Katal. 1970, 7 7 , 1028. (10) Castellan, A.; Bart, J. C. J.; Vaghi, A.; Giordano, N. J. Catal. 1976, 42, 162. (11) Gajardo, P.; Pirotte, D.; Grange, P.; Delmon, B., J . Phys. Chem. Following article In this issue. (12) Lipsch, J. M. J. G.; Schuit, G. C. A. J. J. Catal. 1969, 75, 163. (13) de Beer, V. H. J.; Van der Aalst, M. J. M.; Machiels, C. I.; Schuit, G. C. A. J . Catal. 1976, 43, 78. (14) Lo Jacono, M.; Cimino, A.; Schuit, G. C. A. Gaz. Chim. Ital. 1973, 103, 1281. (15) Pope, D.; Walker, D. S.;Whalley, L.; Moss, R. L. J . Catal. 1973, 31, 335. (16) Grimblot, J.; D’Huydsser, A.; Bonnelle, J. P.; Beaufils, J. P. J. Ektron Spectrosc. Relat. Phenom. 1975, 6 , 71. (17) Tomiinson, J. R.; Keeling, R. 0.; Rymer, G. T.; Bridges, J. M. Actes Congr. Int. Catal. 2nd1961, 1831. (18) Brotikovskii, 0.I.; Shvets, V. A,; Kazanzkii, V. B. Klnet. Katal. 1972, 73, 1342. (19) Taniguchi, K.; Nakajima, M.; Yoshda, S.;Tarama, K. Bull. Inst. Chem. Res., Kyoto Uiniv. 1971, 49,212. (20) Cimino, A.; de Angelis, 8. A. J . Catal. 1975, 36, 11. (21) Haber, J. J. Less Comnlon Met. 1977, 54, 243. (22) Seshadri, K. S.;Petrakis, L. J . Phys. Chem. 1970, 74, 4102. (23) Dufaux, M.; Che, M.; Naccache, C. J. Chim. Phys. 1970, 67, 527. (24) Burlamacchi, L.; Martini, G.; Ferroni, E. Chem. Phys. Lett. 1971, 9 ,420. (25) Seshadri, K. S.;Petrakis, L. J. Catal. 1973, 30, 195. (26) Abdo, S.;Lo Jacono, M.; Clarkson, R. B.; Hall, W. K., J. Catal. 1975, 36, 330. (27) Asmolov, G. N.; Krylov, 0. V. Kinet. Katal. 1972, 73, 188. (28) Patterson, T. A.; Carver, J. C.; Leyden, D. E.; Hercules, D. M. J. Phys. Chem. 1976, 80, 1700. (29) Peacock, J. M.; Sharp, M. J.; Parker, A. J.; Ashmore, P. G.; Hockey, J. A. J. Catal. 1969, 75,379. (30) Ward, M. B.; Lin, M. J.; Lunsford, J. H. J . Catal. 1977, 50, 306. (31) Masson, J.; Nechtschein, J. Bull. Soc. Chim. Fr. 1968, 70,3933. (32) Masson, J.; Delmon, B.; Nechtschein, J. C. R. &bd. Seances Acad. Sci. Ser. C 1968. 266. 428. (33) LOJacono, M.; Verbeek, J. L.; Schuit, G. C. A. J . Catal. 1973, 29, 463. (34) Giordano, N.; Castellan, A,; Bart, J. C. J.; Vaghl, A.; Campadelli, F. J. Catal. 1975, 37, 204.