1991-j-catal-1991-131-385-libre.pdf
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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.
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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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