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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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