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JOURNAL OF CATALYSIS 131 , 385--393 (1991)

Structure and

Activity of NiCo Mo/SiO2

Hydrodesu lfu rization Catalysts

J O R G E L A I N E , J O A Q U ~N L . B R I T O , A N D F R A N C I SC O S E V E R I N O

Laboratorio de Fisico-Quimica de Superficies lnstituto Venezolano de Investigaciones Cient~cas

Apt. 21827 Caracas 1020-A Venezuela

Received December 11, 1990; revi sed May 11, 1991

A se r i e s o f N i - C o- M o hydrode su l fu r i z a t i on (HDS) c a t a l ys t s suppor t e d on s il i ca wa s p re pa re d

va ry i ng t he r at i o r = Co/ (Co + Ni ) be t we e n 0 a nd 1 , ma i n t a i n i ng Mo a nd (Co + Ni ) c onc e n t ra t i o ns

c ons t a n t . Ca l c i na t i on t e mpe ra t u re s e mpl oye d we re 500 a nd 600C. The p rom ot e r (Co a nd / or Ni )

wa s p re se n t i n t he p re c ursor a s a hydra t e d mol ybda t e pha se t ha t supp re s se d bo t h a nhyd rous MoO3

c rys t a ll i t e g rowt h a nd de c re a se i n su r fa ce a re a r e su l t i ng f rom s i n t e r ing o f t he suppor t . Bo t h phe nom -

e na oc c ur re d i n t he nonp romo t e d c a t a l ys t a f t e r ca l c in i ng a t 600C, p roba b l y by a de hy droxy l a t i on

proc e s s r e su l t i ng f rom d i s l i nk i ng o f po l ymol ybda t e s f rom t he s i li c a suppor t . A c or re l a t i on be t we e n

TPR a nd d i spe r s i on g i ve n by XRD wa s e s t a b l i she d , c onf irmi ng t ha t t he d i spe r s i on o f mol y bde nu m

on s i l i c a wa s l owe r t ha n t ha t on t he a l umi na s p re v i ous l y s t ud i e d . Ac c ord i ng t o t he p ropose d

corre la t ion, sul f ida t ion caused an increase in di spers ion in a l l the ca ta lys t s . The presence of

a nhydrous MoO3 wa s he l d r e spons i b l e fo r t he fo rma t i on o f a morphous MoO2 obse rve d by TPR

a f t e r su l f i da t i on . The i nc re a se i n d i spe r s i on a f t e r su l f i da t i on wa s a c c ompa ni e d by a r e ma rka b l e

i nc re a se i n t he i n i ti a l HDS a c t i v i ty o f bo t h non prom ot e d a nd p rom ot e d c a t a l ys t s . Howe v e r , t he

nonp romo t e d sa mpl e show e d a p ronou nc e d i n i t ia l de a c t i va t i on dur i ng use . Comp a re d wi t h a no t h e r

s i mil a r, bu t suppor t e d on a l umi na , s e r ie s o f c at a l ys t s , a mi n i m um i ns t e a d o f a ma xi mum s t e a dy-

stat e HD S act iv i ty w as fo und fo r an inte rme diat e valu e of r . 1991 AcademicPress, Inc.

I N T R O D U C T I O N

The role of promote rs (Co or Ni) in HDS

catalyst s has been the objective of consider-

able debate. Most approaches have advo-

cated structural aspects (1-5). The two main

theories assume that the active catalyst state

is either a single phase, i.e., the promoter

incorporated in

MoS 2

(6), or biphasic, i.e.,

C o 8 S 9 + M o S 2 ( 7 ) . The edge plane in M o S 2

crystallites has been suggested to be more

active than the basal plane (8, 9), and the

HDS activ ity has been related to the number

of promoter atoms located at the edges 10).

The promotion seems to be associated with

a donation of electrons from the promoter to

Mo 11). Other authors 12-14) have pointed

out that the promote r can exhibit a high ac-

tivity even by itself, and thus the synergistic

effect of the promoter on Mo can be inter-

preted in terms of additivity of the

Mo S 2

and

385

the promoter activities. Other evidences

have shown promoter effects related to inhi-

bition of carbon deposition

15-17)

and to

maintainance of the dispersion of molybde-

num sulfide

15).

However, few works

18,

19) have been devoted to comparing Co and

Ni as promoters of Mo catalysts.

The simultaneous presence of Co and Ni

has been found to produce optima in the

ratio r = Co/(Co + Ni) for higher HDS

activity in series of molybdenum-based cat-

alysts supported on alumina 20, 22). The

optimum was found to be influenced by the

method of catalyst preparation 20), in par-

ticular by the calcination temperature 21),

i.e., r varied from ca. 0.3 to 0.7 when in-

creasing calcination temperature from 500

to 600C.

Since the presence of optima r-values has

been proven with two aluminas of different

origin 20), we were tempted to prepare sire-

0021-9517/91 $3.00

Copyright 1991 by AcademicPress, Inc.

All rights of reproduction n any form reserved.

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386 LAINE, BRITO, AND SEVERINO

ilar series o f NiCoMo catalysts using a dif-

ferent support, silica, in order to investigate

the dependency of the dual effect on the

type of support. We were also aiming to

complete more comprehensive information

regarding the differences between Co and

Ni as promoters of HDS catalysts.

E X P E R I M E N T A L

Catalysts were prepared by a two-step im-

pregnation method. The support employed

was a high-purity silica gel: Kiessegel 60

from Merck, 35-70 mesh, and surface area

of 560 mZ/g. This was first impregnated with

enough ammonium heptamolybdate in

aqueous solution to obtain a concentration

of 10 wt% MOO3. Volume of impregnating

solution was slightly larger than the solid

volume. Impregnation was carried out at

70C stirring until the excess solvent was

evaporated. The impregnated sample was

dried in a forced convection oven at 120C,

and then calcined in a muffle furnace in-

creasing temperature at 100C/h to 500C,

at which it was left overnight. Portions of

the resulting molybdate catalyst were coim-

pregnated with cobalt and (or) nickel in

aqueous solutions of the nitrates using the

appropriate concentrat ion to obtain 3.0 wt%

PO (P = Co + Ni). Impregnat ions were

carried out as in the first step.

X-ray diffraction (XRD) spectra were car-

ried out using CuKo~ radiation, Ni filter, and

40 kV. XRD signal broadening/sharpening

was taken as a comparative measure for dis-

persion of the supported catalysts. Surface

areas were measured employing a sorpto-

meter apparatus using nitrogen adsorption

at - 196C.

Temperature-programmed reduction

(TPR) of the catalysts was carried out as

described elsewhere

23).

Briefly, the appa-

ratus consisted o f a flow system connected

to a thermal conductivity cell to follow

changes in the composition of the reducing

gas (10 ml/min H 2 plus 40 ml/min N2). The

sample (100 mg) was placed into a tubular

reactor which was heated at a constant rate

(20C/min). Water produced was removed

with a liquid nitrogen trap at the outlet of

the reactor. Thus, the observed signal was

related only to

H 2

consumption. Presulfided

samples (see procedure below) were cooled

down in a stream of He before TPR.

The HDS activity of the catalyst was re-

ferred to thiophene conversion, which was

measured in an atmospheric pressure con-

tinuous flow reactor connecte d to an on-line

gas chromatograph. The reaction conditions

were 100 ml/min

H2,

0.05 mol thiophene/

mol H2, and 400C. Activity measurements

on both presulfided (1 g) and nonpresulfided

(2 g) catalyst samples were performed. For

catalyst sulfiding, the sample was first

treated at 300C in H 2 for 1 h before H2S

admission and then sulfided with pure HzS

at 400C for 2 h. This procedure has been

found

24)

to be an optimum presulfidation

procedure. For activity tests on nonpresul-

tided samples, prior to reaction the catalyst

was pretreated at 400C in a helium flow for

l h .

R E S U L T S

Surface Areas and Colors

The nonimpregnated silica, having origi-

nally 560 mZ/g, suffered a decrease in sur-

face area to 380 mZ/g as a result o f calcining

at either 500 or 600C.

Surface area did not vary significantly

with either r-value or calcination tempera-

ture (Tc) in promoted ca talysts (Fig. 1).

However, an intermediate r-value seemed

to give a minimum surface area.

In the case of the nonpromoted catalyst

(r = 0/0 shown at the right-hand side of Fig.

1), surface area of the sample calcined at

500C was about the same value as that of

the calcined support; however, there was a

remarkable decrease after calcining at

600C.

Colors of calcined catalysts were as fol-

lows: pale green for r = 0/0, yellow for r =

0 turning to dark beige for intermediate r-

values, and violet for Co-rich samples.

These colors did not change significantly for

the two Tc employed.

The yellow tone of sample with r = 0 is

characteristic of hydrated nickel molybdate

25),

a phase different from anhydrous

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NiCo-Mo/SiO2 HDS CATALYSTS 387

0~2 L I

0.4 0,6 0~,8

f

,5

] _ _

0 / 0

30C

N

E

hi

r r 2 0 0

W

n-

~ Ioo

Fie. 1. Surface areas. Calcination at: , 500C; A,

600C.

nickel molybdate (green) . I t is not possible

to reach an ana logous conclus ion in the case

of sample with r = 1, as bot h anhyd rous and

hydra ted coba l t mo lybda te s a r e known to

have a similar violet tone.

X-Ray Diffraction

Suppor t ed mo lybdenum ox ide p roduced

smal l and broad peaks in the sample wi th

r = 0/0 calcined at 500C (Fig. 2A). How-

ever , i t increased i ts crystal l ini ty to form

well-def ined MoO3 at Tc = 600C (Fig. 2B).

The small peak at 20 = 27.4 , whic h is pres-

en t in most o f the prom oted samples and in

the 0/0 sam ples calci ned at 500C and 600C,

has been found to be the most in tense band

in MoO3 prepared by a co nt inuous prec ip i ta-

t ion method 26), as opp os ed to the signal at

20 = 25.8 , whic h cha racter iz es MoO3 with

preferential crystal grow th in the (010) direc-

t ion. The higher crystal l ini ty o f MoO3 in the

0/0 sample calcined at 600C is parallelled

by a signif icant dev elo pm ent of the 25.8

peak , bu t such an ef fec t i s absen t in the case

o f p romoted samp les .

The increase in Tc a lso produced more

def in i t ion of the broad background peak

arou nd 20 = 22 corres pond ing to the si l ica

suppor t . Again , th is e f fec t seem ed to occur

only in the nonpromoted sample .

In addit ion to the 20 = 27.4 peak , the

promoted samples showed a la rger s igna l

aroun d 20 = 27 , assigned to the hydr ate d

phase : PM oO 4 .n H2 0 (P = C o o r Ni ,

JC PD S po w de r diffract ion f ile N-26-477 for

Co and 13-128 for Ni) . This signal was

sharper at higher r -values ( r = 0.6-1) .

After sulf iding, XRD spectra (not shown)

d id no t show peaks as in Fig . 2 bu t sh ow ed

almost the same s i lica backg round spec t ru m

in all cases, suggest ing an increase in disper-

sion and/or deter iorat ion of the crystal l ine

order by sulf iding. Nevertheless, a very

broad, not well-def ined, small signal around

20 = 14, probably corresponding to dis-

persed and/or def lec ted M o S 2 , w a s ob-

served .

Temperature-Programmed Reduction

Figures 3 and 4 show T PR spec t ra o f ca l-

cined and sulf ided catalysts respectively.

Oxide samples showed a two-peak spec-

trum (Fig. 3) , but t ransformed to a three-

peak spec t rum when increas ing Tc f rom 500

to 600C in the nonpromoted sample. This

may b e re la ted to the format ion of a highly

crystalline MoO3 after calcining at 600C, as

de tec ted by XRD (Fig . 2B) . The f i r s t peak

(575C) may be assigned to dispersed poly-

mo lybd ates l inked to the si l ica surface. Con-

f irming this, S eye dm onir et al. 27) found by

FTIR tha t po lymolybda te s a r e r educed in

H 2

previously to f ree MOO3. Accordingly,

the seco nd peak (655C) in the non pro mo ted

sample calcined at 600C can be as signed to

bulk M oO 3 . The peak loca ted a t the h ighest

tempera ture i s most l ike ly due to poor ly

crys ta ll ine MoO2 genera ted f rom the reduc-

t ion of the M oO3 phases .

In the case of p rom oted samples the

shapes o f TPR spec t ra a re s imi la r , and agree

wi th tha t o f the 0 /0 ca ta lys t ca lc ined a t

500C, except for detai ls in the sharpness

and tempera tu res o f peaks . This suggests

tha t mixed phases in p romoted samples

have reducibil i ty behavior similar to that of

the po lymolybda te s and amorphous MoO3

present in 0/0 calcined at 500C.

After sulfiding, the spec trum of sam ple 0/

0 calcined at 500C was transformed almost

exc lus ive ly to a un ique , lower- tempera ture

signal (Fig. 4A), which may be assigned to

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388 LAIN E, BRITO, AND SEVER INO

f

0.0 - -

I I

I I

f

O .

[

:

4 0 3 0 20 I 0

2e

I

i 0 II I

J

j

I 0 r =O/O

t A /

I ' ~ .

h ~

' r

I

4 0 30 2'0 I0

2 0

FIG. 2. XRD spec t ra of the oxide ca ta lys ts . Ca lc ina t io n a t : (A) 500C; (B) 600C. Phases : , MoO3; V,

PM oO 4" nH 20 ( P = N i o r Co) .

wel l -d ispersed molybdenum su l f ide . How-

ever , a high-tem perature signal, prob abl y an

oxid ic phase ( see Discuss ion) , wa s de tec ted

after sulfiding sample 0/0 calcined at 600C

(Fig. 4B), suggest ing that the appearance of

such ox id ic phase (probably MoOz) im-

paired the sulf iding process.

Sul f ided promoted samples showed two

close low- termpera ture peaks in Ni-con-

ra in ing ca ta lys ts , bu t apparen t ly showed

only one in the Co-p romo ted sample . Also ,

Ni-r ich samples displayed signals with

broadening and assymmetry la rger than

those of the Co-r ich samples .

In both oxidic (Fig. 3) and sulfided cata-

lysts (Fig. 4) there was a decrease in the

tem pera ture of the f irst pea k ( i .e . , an in-

crease in ini t ia l reducibil i ty) as r tended to

0.0. Ho we ve r , the sample with r = 1.0

seem s to hav e had init ial reducibil i ty similar

to the nonpromoted sample .

Catalytic Activity

Figu re 5 shows H DS ac t iv i ty behav io r o f

some selected samples calcined at 500C.

The sulf ided catalysts (Fig. 5a) showed an

init ial high act ivi ty fol lowed by a progres-

sive deactivat ion. The ini t ia l deactivat ion

was s ign i f ican t ly more pronounced in the

case of the nonprom oted sample , a l though

long-term deactivat ion (af ter about 2 h) ap-

pears to be the same.

Nonpresu l f ided samples (Fig. 5b) show ed

a rap id increase fo l lowed by a decrease in

th iophene convers ion ( i . e . , a peak in the

activi ty behavior) . This is mo st l ikely due to

rap id th iophene consumpt ion to accompl ish

catalyst sulf idat ion. Therefore, in this case

the obse rve d ac t iv ity decay may no t neces -

sar ily be a t rue deac t iva t ion bu t p r oba bly an

effect related to the differences in kinetics

be tween su l f ida t ion and ca ta ly t ic reac t ion .

However , wh i l e the nonp romoted samp le

fo l lowed a s lower and more pron ounc ed ac-

t iv ity decay af te r the peak (probably a t rue

deac t iva t ion) the promoted ca ta lys ts

showed more s tab le ac t iv i t ies , and even

slight act ivi ty increases, more clear ly no-

t iced in the high nickel content samples. A

simi lar behavior was observed for samples

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NiCo-Mo/SiO2 HDS CATALYSTS 389

A B

5 7 5 ~ .

? .

5 1 0 ~

5 2 0 - ~ 5 4 0 / "

I'~/z6o

Ni o 7 7 5 /

570-~-hl 775

jA ? 7

"-"

Ii

8 3 o 5 8 0 - -

o o

6 5 5

5 ~ 5 0 0 / 0

Co

555 /

Co+ Ni

7 6 0

0.0

0 . 2

~ / ~ X , ~ 0 . 4

7 7 0

7 B ~ O 0 , 6

7 9 ~ 0 0 . 8

L O

ILL-- I / I t

~ c ~ 8 ~

o o

o o

FIG. 3 . TPR spec t ra of the oxide ca ta lys t s . Ca lc ina-

tion at: (A) 500C; (B) 600C.

ever, it is not clear that a small decrease in

surface area, as shown in Fig. 1, could be

the origin for the minimum in activity, since

the areas are much in excess of a Mo mono-

layer.

D I S C U S S I O N

Results on surface area and XRD (Figs. 1

and 2) suggest that molybdenum is captured

by the promoter, through formation of a pro-

moter molybdate hydrate, so that two dele-

terious effects, crystal growth by stacking

of

M o O 3 ( 01 0 )

planes and sintering of the

silica support, are supressed at 600C.

Coincidentially, the presence of Ni pro-

moter has been shown

25)

to inhibit sin-

tering resulting from the interaction of Mo

with alumina during calcination. In that

case, the fo rmation of the sinterization prod-

uct, A 1 2 ( M o O 4 ) 3 , w a s detected.

Taking into account the relatively high

surface area of the silica gel employed in the

present work as support (560 m2/g), it may

be advanced that it was highly hydroxyl-

A B

calcined at 600C (not shown), and for other

HDS catalysts studied previously 15, 19,

28).

Figure 6 shows that catalyst sulfidation

produced a large increase in steady-sta te 205~.

HDS activity, except in the case of the non- 200~

promoted samples. In general, the re was not N~Mo-I)

a sign ifcant effect of the calcination temper- a Ls~4_2J

ature on the activity of promoted catalysts.

It is also seen that the Ni-promoted catalyst 2E5

was more active than the Co-promoted one, J

either with or without presulfiding. 2q5 --

In contrast to previous works

20-22)

us- 2ao~

ing NiCo-Mo/A120 3 tha t report opt imum r- _.~

values producing maximum steady-state

HDS activities , Figure 6 shows minimum coMoJ

activities for the present NiCo-Mo/S iO 2

catalyst s. Note tha t Fig. I showed a similar

trend in surface area suggesting that the ob-

served minima in thiophene conversion

could be merely a surface area effect. How-

2 2 5 8 9 0

0 / 0

Co

C04Ni

1 8 0 ~ / k ^ ~ 2 9 0

- - 2 8 0 0 . 0

o . ,

22 0 / \ 27 5

220 3 ~ \ ~ 0.6

0.8

ii I I i i I ~ ~ i i ~1 L i I I /i I I~

a,

8g o 8 o 8 o

Fro. 4 . TPR spec t ra of the presul f ided ca ta lys t s .

Calcinat ion at : (A) 500C; (B) 600C.

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390 LAINE, BRITO, AND SEVERINO

I 0 0

a

8 0

6O

Z 40 '~ 'X ~ X~ X ............_X~

0

~0:

X X~X~x

z o

i E I I b

80

',x

_0o

0 I I ~ I

I 2

4

hr

F I G . 5. T y p i c a l a c t i v i t y b e h a v i o r ( 4 00 C ). ( a ) w i t h p r e s u l f i d a t i o n ( 1 -g s a m p l e s ) ; ( b ) n o n p r e s u l f i d i n g

( 2 - g s a m p l e s ) . , r = 0 / 0 ; ( 2 ) , r = 0 . 0 ; A , r = 1 .0 .

ated. Sintering may proceed following dehy-

droxylation, by the collapse of the skeletal

structure left after water evolution. Accord-

ingly, if we assume that the Mo species were

linked to the support by hydroxyl groups,

sintering in the sample with r = 0/0 most

likely occurred as a result of dehydroxyl-

ation assisted by a dislinking of molybde-

num from the support after calcination at

600C. Such phenomena probably involved

decomposition of MoO2(OH)2 27) or of

complex Mo-Si hydroxiphases to generate

free MoO 3 and water vapor.

The presence of hydrate d MoO 3 sup-

ported on silica has been suggested earlier

29, 30). This is apparently transformed to

well-crystallized anhydrous MoO 3 after

calcining at 600C the 0/0 sample (Fig. 2).

This is not the case of the promoted cata-

lysts, where the hydrated state appears to be

stabilized, within the range of Tc employed,

through the formation of the hydrate d pro-

moter molybdate.

The anhydrous promoter molybdate is

known not to be a good precursor phase for

HDS

31, 32),

and in contrast with present

results, it has been detected elsewhere in

samples prepared using another silica as

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N i C o - M o / S i O 2 H D S C A T A L Y S T S 3 91

g

w

>

Z

O

w

Z

w

1-

0_

0

-7

70L PRESULFIDED

~o , \ .

50

4O

50

20

I0

0

0

NON PRESULFIDED

t

0 2 r I

0.4 0.6 018

r

0/0

FIG. 6. Steady-state catalytic activities. Calcined at:

OO, 500C; AA, 600C. With presulfidation (1-g sam-

pies): solid symbols; nonpresulfiding (2-g samples):

open symbols .

suppor t

32).

However , t he su r f ace a r ea o f

that support was relat ively low (179 m2/g)

in compar ison wi th the s i l ica employed in

present work (560 m2/g), suggesting less

poss ib i l i ty o f in te rac t ion of the spec ies

formed dur ing ca lc ina t ion wi th sur face hy-

droxyl g roups to fo rm the hydra ted phase .

As a d i f fe rence wi th the anhydrous phase ,

hydra ted n icke l molybdate has been re -

por ted to be an effec t ive precur sor phase for

high act ivi ty sulf ided cataly st

33).

This was

attr ibuted to i ts easier sulf idabil i ty in com-

par ison wi th the more re f rac tory anhydrous

nickel molybdate. The easier sulf idabil i ty of

the hydra ted s ta te i s p robably assoc ia ted

wi th the posi t ive e f fec t o f water vapor on

sulf idabil i ty reported ear l ier 34). Accord -

ing ly , the presence of the promoter in the

presen t ser ies o f ca ta lys ts seem s to he lp in

at taining a proper sulf idat ion, and hence

high catalyt ic act ivi ty , through formation of

a more su l f idab le-hydra ted phase .

The appea rance o f a TPR peak a ss igned

to an oxidic phase difficult to sulfide (Fig.

4B, peak at 890C in sample 0/0) is also in

acco rdance wi th the p roposed enhancemen t

of sul f ida t ion ef fec ted by the promo ter . This

pha se is prob ably MOO2, form ed during sul-

f idat ion from the anhydrous, well-crystal-

lized M o O 3 25, 34). However , MoO2 was

not c lear ly se en by XR D in su l f ided samples ,

suggest ing that i t is amorphous, and proba-

b ly a "po rou s" MoO2 as r epo r t ed by

Arno ldy

et al.

earlier (35).

The easier sulf idabil i ty brought by the

p resence o f the p romote r can p robab ly a l so

be assoc ia ted wi th the more rap id th iophene

convers ion increase obs erve d when s ta r ting

with ini t ia l nonpresulf ided catalysts (Fig.

5b).

A compar iso n of p resen t TPR resu l t s wi th

prev ious ones 23, 26) using the sam e exper -

imental condit ions indicates the fol lowing

order in f irst peak temperature: alumina-

supported MOO3, 480C; si l ica-supported

MoO3 (present work) , 575C; and unsup-

ported MOO3, 725C. This sequence sug-

gests tha t a re la t ion may ex is t be tween the

tempe ra ture of the f ir st TPR pea k and the

d ispers ion of the cor responding phase ; i . e .,

lower tempera ture impl ies be t te r d isper -

sion. Confirming this relation, the pre-

v iously s tud ied ser ies suppo r ted o n a lumina

20, 21)

did no t respond to XRD as the pres-

ent ser ies, even though the lat ter had larger

sur face a rea . A poss ib le conse quen ce of th is

i s the fac t tha t the presen t ser ies o f ca ta lys ts

has act ivi t ies representing reaction rates

about four times smal le r than those o b ta ined

prev iously for the ser ies suppor ted on a lu-

mina 21).

A n o t h e r c o r r e s p o n d e n c e b e t w e e n T P R

and XRD can be es tab l i shed in the sample

with r = 0/0, where the marked sharpening

of XRD signal result ing from increasing Tc

from 500 to 600C (Fig. 2) cannot be associ-

a ted wi th a d isp lacem ent o f the f i rs t TP R

peak (575C, Fig. 3A) to a higher tempera-

ture, b ut with a spl i t t ing into two p eaks (575

and 655C, Fig. 3B). Acco rdingly , a general

ru le could be formula ted s ta t ing tha t TPR

peak- tempera ture d isp lacement a re the re -

su lt o f changes in d ispers ion , and tha t ne w

peaks a re ass igned to new phas es ; there fore ,

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392 LAIN E, BRITO, AND SEVE RINO

the ap pea ranc e of the pea k at 655C is at tr ib-

uted to a different phase, one more crystal-

l ine and less connected to the suppor t

(" f re e" MOO3) formed f rom the po lym olyb-

da tes as ind ica ted above .

Mo reover , a compar ison o f the peak tem-

pera ture a t t r ibu ted to MoO 2 be tween r e -

duced (750-780C, Fig. 3) and sulfided

(890-900C, Fig. 4) samples with r = 0/0

suggests a more d ispersed MoO 2 in the first

c a s e .

The co r re l a tion be t wee n TPR and d i spe r -

s ion can a lso be es tab l i shed in p romoted

ca ta lys ts : the decrease or increase in f i r s t

peak t empera tu r e in TPR o f p romoted ox ide

samp les (Fig. 3) is acco mp anie d by a respec-

t ive broadening or sharpening of the XR D

signal , corresponding to the hydrated pro-

moter molybdate phase (Fig. 2) , i .e . , sam-

ples with r = 0.0 and 0.4 calcined at 500C

showed the broadest XRD signals a t about

20 = 27 and h ence the low est T PR signals

(510-530C). These lat ter increased to

535-540C, with a corresponding sharpen-

ing in XRD signals after calcining at 600C.

On the other hand, th e sample with r = 1.0

had bo th inva r i ab le TPR peak t empera tu r e

(580C) and invariable XRD signal sharp-

ness fo r bo th Tc. The sample with r = 0.6

sh ow ed a corre latio n similar to r = 1.0. It

can also be seen that samples with high r-

va lues (r = 0 .6-1 .0) have bo th sharper XRD

signals and larger f irst peak temperature

than sam ples with low r-values (r = 0.4-0.0)

for both To.

After sulf iding, there was pract ical ly no

response to XRD suggest ing a highly dis-

persed s ta te . This cor re la tes wi th the very

low temp era ture of the f i rs t peak in the TPR

spectra around 200C (Fig. 4) . Note that

well-crystal l ized sulf ide compounds give

TP R spec t ra wi th muc h h igher tempera ture :

for example , pure MoS2, NiS, and Ni3S 2

produce T PR spec t r a wi th peak t empera tu r e

well over 500C

36).

Regarding the act ivi ty results , the larger

ex ten t o f deac t iva t ion dur ing use in HDS

of nonp romoted samp les a s compared wi th

promoted catalysts (Fig. 5) reinforces previ-

ous repor ts tha t have a t t r ibu ted to the pro-

moter a ro le assoc ia ted wi th pro tec t ing the

catalyst against ini t ia l deactivat ion

15-17).

The refer red pro tec t ion seems to be accom-

plished better by Ni than by Co (Fig. 5) .

I ndeed, NiM o ca ta ly s t s have been found to

inh ib i t carbon deposi t ion be t te r than CoMo

18),

probab ly becau se o f a be t te r hyd roge -

na t ion funct ion of Ni .

The absence o f a max imum in HD S ac tiv -

i ty for an intermediate r -value (Fig. 6) sug-

gests tha t a lumina is a nece ssary com pon ent

for the occ ur rence of the dual p romot ing

effect ( i .e . , the maximum) reported ear l ier

20-22). The s t ronger in te rac t ion of the sup-

por ted spec ies wi th a lumina as compared

with si l ica, referred to elsewhere

37),

is

proba bly an explana t ion for a be t te r d isper -

sant role ci ted abov e for alumina. This mus t

be t aken in to acco un t i f t he two o ppos ed

dual e f fec ts observed , i . e . , the maximum

and minimum, are to be invest igate d fur ther .

C O N C L U S I O N S

The promoter dua l e f fec t (Co + Ni) on

HDS ac t iv i ty can be s t rongly in f luenced by

the type of suppor t . Thus , a posi t ive or a

negat ive e f fec t occurs when us ing a lumina

20, 21)

or si l ica ( this work) respectively.

The prom oter (Ni o r Co) may ha ve a posi t ive

ro le in each of the s teps of the c a ta lys t h is-

tory: inhibit ing sinter ing o f the s upp ort a nd

MoO3 crystal growth during calcination;

provid ing an appropr ia te p recursor phase

( the hydrate) for catalyst sulf idat ion; and

inhibit ing ini tia l catalyst de activ at ion during

use in HD S reac t ion . Nick el -prom oted ca ta-

lys t can prov ide a more d isp ersed and reduc-

ib l e hyd ra te p r ecu r so r phase and may be

less prone to ini tia l deac tivat io n than cobalt-

p romoted ca ta lys t .

A C K N O W L E D G M E N T S

Suppor t f rom t he Conse j o Na c i ona i de Inve s t i ga c i -

ones Cient i f i cas y Tecnologicas i s gra te ful ly acknowl-

e dge d . We a l so t ha nk O swa l do Ca r i a s fo r ob t a i n i ng t he

XRD spe c t ra .

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NiCo-Mo/SiO2 HDS CATALYS TS 393

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