The effect of passivation on the activity of sulfided Mo and Co-Mo hydrodesulphurization catalysts

345

Catalysis Today, 10 (1991) 345-352 Ekevier Science Publishers B.V.. Amsterdam

THE EFFECT OF PASSIVATION HYDRODESULPHURIZATION

ON THE ACTIVITY

OF SULFIDED

MO AND CO-MO

CATALYSTS

V.M. Browne,S.P.A.LouwersandR.Prins Technisch Chemisches Laboratorium, Eidgenijssische Technische Hochschule, 8092 Zurich, Switzerland.

SUMMARY Exposum to air at room temperature (passivation) caused a substantial increase in the hydrodesulphurization activity of a sulphided Co-Ma/Al 03 catalyst. EXAFS measurements showed that passivation did not change the structure of the c% sites. Because of this, and because a Co-MO/C catalyst did not show any increase after passivation, it is concluded that a breakage of Moo-Al interactions and a concurrent transition of Co-Mo-S type I into CoMo-S type Il is responsible for the activity increase. This suggestion is supported by the fact that a Co-Mo(NTA)&03 catalyst, which contains large amounts of Co-MO-S type II, showed a relatively small activity increase. INTRODUCTION Because of their widespread use, much research has been performed on hydrotreating catalysts (refs. l-3). and methods for improving their activity have been sought. It has been reported, that the hydrodesulphurization

activity of a sulpbided Co-Mo/Al203 catalyst increased when the

catalyst was exposed to air at mom temperature (ref. 4). For unpromoted Mo/A1203 catalysts such an increase in activity has not been observed, although there are indications that the intrinsic activity of the active sites is raised (ref. 5). Several causes of this effect have been suggested. It might be that oxygen roughens the edges of the MoSz particles, thereby increasing the edge area and the number of active sites (ref. 4). Another explanation is that oxygen changes the electronic properties of MoS2 (ref. 6). At this moment, however, it is not clear why the activity is raised. In this report the results of studies undertaken to elucidate the cause of this increase are presented. EXPERIMENTAL Catalvst preparation Both unpromoted and cobalt promoted molybdenum catalyst were pmpared by

Catalyst

wt%Mo

wt%Co

with

MoW20

respectively (Merck p.a) and

CoMo/C

7.00 6.82 4.33 7.31

1.25 0.80 1.35

pore volume impregnation (Condea

Chemie,

Co(NO&.6H20

of y-Al203

233 m2g-l)

aqueouS solutions (NH4)aMo,024.4H20

TABLE 1. Compositions of the catalysts.

of

(Johnson

co-Mc&03 Co-Mo(NTN/LU203

Matthey,

A.R.). After each impregnation, the catalyst was dried (120 ‘C, 12 hours) and calcined (400 ‘C, 0920~5361/91/$03.50

0 1991 Eleevier Science Publishers B.V. All rights reserved.

346

2 hours), A carbon supported catalyst was made using the same procedm,

but in this instance no

Cal&a&m was applied The carboxt support used was Lonza HSAG-300 (300 m2 g-l). Finally, a special Co-Mo/AlzO, catalyst was made by the recipe given by Van Veen et al. (ref. 7). In this recipe nitrilotriacetic acid (NTA) was used as a complexing agent. This catalyst will be denoted Co-Mo(NTA~/Al~O~~ It has been shown (ref. 7) that, for catalysts made according to this recipe, all the Co is in the so-called ‘Co-No-S’ structure (ref. 8). Compositions of all c&alysts am given in Table 1.

The catalytic activities of the catalysts for thiophene HDS were measured in a micro flow reactor at atmospheric pressure. Catalyst samples were sulphided in a flow of 60 cm3 min-* of 10 % H$ in HP During sulphidation, the temperature was raised at a rate of 6 ‘C min-l to 400 lC, except during the first sulphidation of the Co-Mo&TA)/Al203

catalyst, when the temperature was

raised at a much lower rate of 1 ‘C min-I, in order to avoid too early decompositon of the Co-MO-NTA complex. The temperature was kept at 400 ‘C far 2 hours. Next, a gas mixture, consisting of 3 % thiophene in Hz, was introduced into the reactor at a flow of 60 cm3 min-l. Reaction products were analysed by on line gas chromatography. The activity of the catalyst samples was expressed as kms, a first order rate constant, After several hours of reaction the catalyst samples were cooled down to room temperature under a flow of He. At this point, the reactor was opened and the catalyst exposed to air for a predetermined

period (passivationJ. After passivation, the reactor was closed, the catalyst

resulphided and its activity remeasured as described above. measEXAFS measurements of the Co-Mo/AlzO3 catalyst were carried out on station 7.1 at the SRS in Daresbury, England. This catalyst was measured at three different stages. Fimtly after the first sulphidation, secondly after a passivation of 1 week, and finally after a second sulphidation. The sulphidation was carried out in an in-situ cell (ref. 9) with 10 % H$

in Hz at a flow rate of

60 cms min-l under atmospheric pressum. The temperatum was linearly increased t&m room temperature to 400 ‘C (6 ‘C min-l) and kept at this tempera-

for 1 hour. Themafter, the catalyst

was purged with He for 30 minu& at 400 ‘C and cooled to room temperature under flowing He. After this pretreatment, the in-situ cell was closed, cooled down to liquid nitrogen temperatures, and the EXAFS spectrum was measured. In the case of measumm ents on the passivated catalyst, the pmtn?atment consisted of flushing with He for 30 m.im&s at room temperature. Phase shifts and backscattering amplitudes from reference compounds were used to analyse the EXAEB c~~~ti~.

For the Co-S EXAFS signals the Co-S coordhution in CoS2 was use& for

the Co-Co contributions the Ni-Ni Wtion

in NiO was W,

and for the CO-MOcontribu-

tions the Ni-Mo coo&nation in ((C,jH5)4P)2NiWS,& was taken. Phase shift and backscattering amplitude for the Co-O contribution were obtained from the tables of M&ale (ref. 10).

347

The Co-Mo/Al,03 catalyst underwent several psssivation cycles. The rate cons&t kmS as a function of time for a series of pas&&on 1 nightwpown

to air

reactiin time (min)

u

cycles is shown in Fig. la. In each cycle the catalyst

no&h

time (min)

Fig. 1. HDS rate constant for a Co-Mo/A120~ catalyst. S= sulphided catal , SOS= one additional passivation cle, S(OS)n= n additional passlvation cles. For reasons 0r”’ cla&y several cycles am omitted. 7 a) ane night pas&a&m time per cycle.r b) one week passivation time per cycle.

wss exposed to

air overnight

(12-15 hrs.). It c!axt be seen that kms

shows a dramatic increase

when

the catalyst

is passivated

several times. In Fig. lb similar activity data are plotted for the same catalyst. In this case, however, the passivation time was increased to 1 week per cycle. These data show that when the passivatkm time is increased,

the jump

in kms

is

greater. The activity data from all catalysti are summarized in Fig. 2. InthisFigurethesteadystaterate constant

(i.e.

kms

after

400

Fig. 2 HDS rate constant at 400 minutes reaction time for all catalysts. The passivation time (one night or one week)isgiveninparentheses,

minutes reaction time) is plotted as a funtion

of the number of passivation cycles. For the unpromoted catalyst, Mo/Al&

a

passivation time of 1 week was chosen, because it was felt that this catalyst might oxidize more slowly than the Co’promoted one.

348

It is clear from Fig. 2 that for the Mo/Al& Pas&a&n

catalyst no increase in kmS

is observed.

only led to small changes in the catalytic activity of this catalyst. Similarly, exposure

of the CO-MO/C catalyst to air had no effect on km@ After three cycles the activity was still approximately the same as the initial activity. Nowever, the catalytic activity of this carbon supported catalyst is relatively low. For other carbon supported CO-MO catalysts much higher activities per Co atom have been found, higher than for comparable alumina supported catalysts (refs. 7,11,12). Probably this is due to a more pronounced Cosulphide formation in our catalyst. This would lead to less Co atoms in the active Co-Ma-S structure, and consequently to a lower activity, The Co-Mo(NTA&%l~O3 catalyst was also subjected to passivation studies. As can he seen fmm Fig. 2 them is a small iuitisl increase in activity, but after the fii

cycle no further significant

changes in activity were observed. The total increase is considerably smaller than with the Co-Mo/Alz03 catalyst. The G3Mo/A1203(1 week) catalyst has reached its maximum activity after 5 cycles Imerestingly, this fmal activity is aurally

equal to the activity of the C~MO~A)/A~~~

catalyst,

which might indicate that after passivation the two catalysts are similar. on the Co-MO/A&&G&&& The results of the EXAFS data analysis for the Co_Mo/Al,O3 catalyst are given in Table 2 and Fig. 3. In the sulphided state (i.e. catalysts S and SOS) no Co-Co contribution at 2.5 A, typical for Co&$, is found. This shows that both after the fit,

and after the second sulphidation, all Co is in

the Co-Mo-S structure. The data are in accordance with pmvious studies on CO-MO and Ni-Mo catalysts, where it was shown that the promoter atom was 5 or 6 fold coo&rutted with sulphur and was situated at the edge of the MoS2 particles in the plane of the MO atoms (mfs. 13,14). TABLE 2. Structural parameters of the Co-O, Co-S and Co-MO coordinations for catalyst Co-Me/Al Catalyst S= sulphided catalyst, Catalyst SO= Sulphided catalyst, passivated for 1 week, c!?ata yst SOS= Passivated and resulphided catalyst. The qxxdination numbers have been corrected for the photoelectron mean free path dependenCy (A= 5 A). Catalyst

:o SOS

Co-0 creation N x

17

Es e

2.0

0.0072

1.6

2.02

second Co-S coordination N 1; H :0 SOS

1.8 1.5

2.24 2.25

0.0002 -0.0004

Eo eV -9.4 -9.4

first Co-S c~ation N 1; x

eV

4.4 2.0 4.5

4.9 5.7 4.1

2.20 2.23 2.20

CO-MO~nati~ N f 0.9 0.3 1.0

2.86 2.85

-0.0002 0.0058 0.0054

B -0.0004 0.0050 0.0056

EO

Eo eV -10.2 -11.5

349

In the passivated state Co appears to have lost some of its sulphur neighbours, and taken up oxygen neighbours. It must be kept in mind, however, that a I&Kale reference was used for the Co-O contribution

Since M&ale refemnces do not take into account inelastic effects or multi-

electron processes, the Co-O backscattering amplitude can contain large emxs, and consequently

Figure 3. Comparison of the k3 Fourier transformed Co EXAFS data of catalyst CoMo/Al~O3 and the corresponding calculated Fourier transforn~ (a) Sulphided catalyst. (bl Catalyst, after 1 week passivation. (cl Passivated and resulphided catalyst the

inaccuracy in the

coordination numbers may be substantial.

Not only can the Co-O

coordination number be determined with limited accuracy, but because all contributions

are

overlapping, the accuracy with which the other cooxdination numbers can be determined is also reduced. This said, however, the data do seem to indicate that Co is partly oxidized, and has lost some of its neighbours. The CO-MOcontribution in particular seems to be decreased. Interestingly, Van der fiaan et al. observed that for a &-MO/C catalyst with an extremely low Co/h40 atomic ratio (CoiMo= 0.0095) no oxidation of the Co occurs, even after exposure to ambient air for 23 days (ref. 15). In other studies, however, oxidation of Co and MO has been observed (tefs. 16-18). Especially the Co atoms seemed to be sensitive to oxidation (ref. 16). Also 02 chemisorption studies on Ni-MO/C, similar to Co-MO/C, show that the amount of chemisorbed 02 increases dramatically with the Ni/Mo ratio, thus indicating that in these catalytsts Ni and Co in these are oxidized (ref. 19). At present we have no explanation for this discrepancy. The most important conclusion that can be derived from the EXAFS measurements is that the local environment of the Co atoms is the same for both the sulphided (S) and the resulphided (SOS) catalyst. The sites in the neighbourhood of the cobalt atoms can be expected to be the most active, since they profit most from cobalt’s promotional effect. It has even been suggested that the cobalt atoms themselves are the active sites, MoSz merely serving as a (secondary) support (ref. 20). The fact that the local environm ent of the Co atoms is the same for catalysts S and SOS therefore implies that the active sites in these catalysts are the same. In addition the data suggest that the number of active sites has not changed after one passivation cycle, since in both samples, S and SOS, all cobalt was found to be in the Co-Mo-S structure.

350 DISCUSSION Several explanations for the observed increase in activity can be considered, De Heer et al. suggested that oxygen breaks up the MoSz particles on the Al203 support During resulphidation, the Co might be redistdbuted on the enlarged MoS2 edge, thereby preventing the recombination of the particles (ref. 4). This is, however, not in agreement with the observations of Schrader and Cheng, who found that the MoSz particles in Co-Mo/Alfls

and Mo/Alfl,

catalysts am relatively

stable to air exposure (refs. 21,22). Also other studies have shown, that although exposure to air partly oxidizes the edge of the MoSz particles in a Co-Mo/Al&

catalyst, the bulk of the MoS2 is

not changed (ref. 181. Another explanation that has been presented for the increase in activity, is that it is due to a change in the active sites (tefs. 5,61. This explanation is, however, not supported by our EXAFS results, which show that the Co en vironment, and therefore the environment of the most active sites, is the same before and after passivation and subsequent sulphidation. A third possibility is that the passivation treatment induces a transformation of a Co-MoS type I into CO-MO-Stype II struw.

Theselabels were originally introduced by Candia et al., when

they observed that the activity of a Co_Mo/Alz0~ catalyst increased when the sulphidation temperature was raised (ref. 23). The CO-MO-S structum after high temperatuna sulphiding was labelled type II, whereas the CO-MO-S structure resulting

from sulphidation

at normal

temperatures was labelled type I. Candia et al. also suggested that Co-Mo-S type I is attached to the support by MoO-Al bonds, while the type II structure is fully sulphided, and has no such linkages (ref. 241. In this model the breakage of these linkages is necessary for the formation of the more active, CO-MO-Stype II structure. Also Scheffer et al. have suggested that a decrease in the MO-support interactions can lead to a higher activity. They observed a increase in the intrinsic activity of a Co-Mo/AlzO, catalyst, upon increasing the calcination temperature, thereby breaking Mo-support bonds (ref. 25). It has been proved that the Co-Mo-S stn~ctum in alumina and carbon supported catalysts is the same (refs. 26-28). Therefom, our observation that the Co-Mo/Al,O, catalyst shows an increase in activity upon passivation, while the CO-MO/Ccatalyst does not, suggests that support interactions are important for this passivation effect. This also implies that passivation indeed causes a transformation of CO-MO-S type I into Co-Mo-S type II. This suggestion is further supported by the fact that the Co-Mo(NTA)/AlzO3 catalyst, which contains a large proportion of Co-MO-S type II (ref. 7), only shows a small increase in activity upon passivation. Them are several ways in which a passivation

treatment

might be able to break

Mo-O-Al bonds. One possibility could be the formation of S042- upon oxidation of the edges of the Co-Mo-S structure. It has previously been shown that sulphate is formed by passivation (ref. 16). Once formed sulphate could act as a mcdifier in a similar manner as phosphate, which has been mported to decrease the interaction between molybdenum and the alumina support (refs. 29,30), and incmase the specific activity of the CO-MO-S structute (mf. 17). Although not much work has been done on the effect of sulphate on the activity of hydrotreating catalysts, there are studies that show a decrease in binding energy of H$ on Al203 upon adding sulphate to the alumina (refi 311, indicating a ehaae in the surfaea urenerbesl Qbvieusl~, a ahain the surfasa

properties

of the supporthas an

influene

on the MO-support interactions. In another study

employing XPS it was shown that the Ma/Al intensity ratio of a oxidic Mo/Al&

catalyst

decteasedonaddingsulphatetothesuppart(ref.32),iadicatingaincreaseinthesizeoftheMo03 suuctum, and a decmase of the Mo-support interactions. It must be noted, however, that in all of these studies sulphate was added to the oxidic catalyst before MO had been impregnated In the study being repot&d in this report the situation is somewhat different: we have a sulphided catalyst, to which sulphate is added by oxidation of the MoS;! edges. ‘Ihe results of the previous studies are, therefore, not necessarily applicable to our catalysts. A second way by which support interactions could be broken might be indicated by our EXAFS results on the passivated Co-Mo/AlzOs catalyst. As already mentioned, the CO-MO coordination number in the passivated state seems to be lower than in the sulphided catalyst. This could mean that a number of cobalt atoms are removed from the MoSz edge, forming a cobalt oxide or -oxysulphide.

Alternatively

it could mean that the disorder in the local cobalt

environment is increased, since a higher amount of disorder might lead to either a higher DebyeWaller factor, or a lower coordination number (ref. 33). Electron micmscopy on unsupported MoSz has shown, that oxygen corrodes the edges, leading to a toughening of the MoSz edge (ref. 34). Although in this work oxygen was added at high temperatutes, this could also indicate an increase in disorder upon oxygen exposure. Whichever of these two possibilities is the case (i.e. whether cobalt is leaving the MoSz or whether them is an increase in disorder) the cobalt atoms would be able to change their position on the MoSz edge. They could in this way move towards Mo-O-Al bonds and help in the sulphidation of Mo-O-Al linkages. In this respect it is noteworthy, that Ni, which has similar properties as Co, can improve the sulphidation of W in Ni-W/A+& catalysts (ref. 351. CONCLUSIONS When a sulphided Co-Mo/AlzO, catalyst is vsed

to air at room temperature, an increase in

the activity for the HDS of thiophene is observed. EXAFS measurements showed that this increase is not caused by a change in the nature or the amount of active sites, since the Co environment was the same for a sulphided catalyst and for a catalyst, that had been subjected to an additional passivation and (second) sulphidation. Also all Co is shown to be in the Co-Mo-S structure. No activity increase is seen with passivation of Mo/A1203, &-MO/C and CoMo(NTA)/Al&, which leads to the conclusion that this passivation effect is caused by a weakening of Mo-support interactions, possibly leading to a transformation of a Co-Mo-S type I into Co-Mo-S type II structure. What causes the Mo-support interactions to be weakened is at present not clear. It might be that sulphate, formed during the passivation treatment, is responsible. Another possibility is a relocation of the Co atoms along the MoS2 edge during passivation. The relocated Co atoms might then help sulphide Moo-Al

linkages during the second sulphidation. However, more research

needs to be done in order to fully understand this phenomenon, and this research will be carried out in the near future.

362 ACKNOWLEDGEMENT We thank Prof. Dr. D.C. Koningsberger and coworkers from the Eindhoven University of Technology, The Netherlands,for the use of their equipment and data analysis program. REFERENCES 1 R. Prins, V.HJ. de Beer and G.A. Somorjai, Catal. Rev. Sci. En ., 31(1989) 1. P. Ratnasamy and S. Sivasanker, Catal. Rev. Sci. Eng.. 22 (1980$ 401. P. Gr e, Catal. Rev. Sci. Eng., 21 (1980) 135. 4 V.HJ.y e Beer, C. Bevelander, T.H.M. van Sint Fiet. P.G.AJ. Werter and C.H. Amberg, j. Catal., 43 (1976) 68. F.E. Massoth, C.-S. Kim and Jian-W. Cui, Appl. Cat& 58 (1990) 199. S. Kolboe and C.H. Amber& Can. J. Chem.. 44 (1966) 2623. 7 J.A.R van Veen, E. Gerke&i, A.M. van de; K&n and A. Knoester, J. Chem, Sot. Chem.Commun., (1987) 1684. H. Topsrae.B.S. Clausen and S. &p, Hyperfine Interactions, 27 (1980) 231. ; F.W.H. Kampers, T.MJ. Maas, J. van Gmndelle, J. Brinkgreve and D.C. Koningsberger, Rev. Sci. Instr., 60 (1989) 2635. 10 A.G. McKale, B.W. Veal, A.P. Paul&as, S.-K. Chan and G.S. Knapp, J. Am. Chem. Sot., 110 (1988) 3763. J.P.R. Vissers, V.HJ. de Beer, R. Prins, J. Chem. Sot., Far. trans. I. 83 (1987) 2145. :a J.P.R. V&em. J. Bachelier, HJ.M. ten Does&ate, J.C. Duchet, V.HJ. de Beer, and R Prins in “Pnx. 8th InternationalCongress on Catalysis, Berlin, 1984”, H-387. 13 S.M.A.M. Bouwens, D.C. Konhgsberger, V.HJ. de Beer, S.P.A. Louwers and R. Prins, Catalysis Letters, 5 (1990) 273. 14 S.M.A.M. Bouwens, J.A.R van Veen, D.C. Koningslxrger, V.HJ. de Beer and R. Ptins, J. Ph . Chem., 95 (1991) 123. 15 M. Wys J. Craje, E. Gerkema, V.HJ. de Beer and A.M. van der Kraan, Hyperfine Interactions, 37(1990)1795. J.S. Brinen and W.D. Armstrong, J. Catal.. 54 (1978) 57. :: J.kRvanVeen,E.Gerkema,A.M.vanderIGaan.P.AJ.M.Hendriks. and H. Beens, 6 be published. 18 T.G. Parham and R.P. Merrill, J. Catal., 85 (1984) 295. 19 S.M.A.M. Bouwens, N. Barthe-Zahir, V.HJ. de Beer and R. Prins, to be Dublished. J.P.R. Vissers. V.Hj. de Beer and R~Pains,J. Chem. Sot.. Faraday Tra&. I, 83 (1987) 2145. 4! G.L. Schrader and C.P. Cheng, J. Catal., 80 (1983) 369. 22 G.L. Schrader and C.P. Cheng, J. Catal.. 85 (1984) 488. 23 R. Candia, H. Tops@e and B.S. Clausen, Proc. 9th Iberoamerican Symposium on Catalysis, Lisbon, Portugal, July 1984. 24 R. Candia, 0. &mnsen, J. Vi&&en, N. Tops=, B.S. Clausen and H. Topssx, Bull. Sot. Chim. Belg., 93 (1984) 763. 25 B. Scheffer, EM. van Oers, P. Amoldy, V.HJ. de Beer, and J.A. Moulijn, Appt. Catal. 25 (1986) 303. 26 S.M.A.M. Bouwens, R. Prins, V.HJ. de Beer and D.C. Koningsberger, J. Phys. Chem., 94 (1990) 3711. 27 B.S. Clausen, H. Tops@e, R Candia, J. Villadsen, B. Lengeler, J. Als-Nielsen and F. Christensen,J. Phys. Chem., 85 (1981) 3868. S.P.A. Louwem and R Prins, Div. of Petrol. Chem., Am. Chem. Sot., 35 (1990) 211. z: K. Gishti, A. IannibelIo, S. Marengo, G. Morelli and P. Tittarelli, Appl. Catal., 12 (1984) 381. 30 JAR. van Veen, P.AJ.M. Hendriks, R.R And&a, EJ.G.M. Romers and A.E. Wilson, J. Phys. Chem., 94 (1990) 5282. 31 Y. Okamoto, M. Oh-ham, A. Maezawa, T. Imanaka and S. Teranishi, J. Phys. Chem., 90 (1986) 23%. 32 Y. Okamoto and T. Imanaka, Proc. Int. Symp. on Acid-Base Catalysis,Sa , Japan, November 1988 (eds. K. Tanabe, H. Hattori,T. Yamaguchi and T. Tanaka y . P. Eisenberger and G.S. Brown, Solid State Cormnun. 29 (1979) 481. g C.B. Roxlo, M. Daage, D.P. Leta, KS. Liig, S. Rice, A.F. Ruppert and R.R Chianelli, Solid State Ionics, 22 (1986) 97. 35 B. Scheffer, PJ. Mangnus and J.A. Moulijn, J. Catal., 121 (1990) 18.

z

2