Sio2 hydrodesulphurization catalysts: an xps and esr characterization of sulfided used catalysts

347

Applied Catalysis, 3 (1982) 347-376 Elsevier Scientific Publishing Company, Amsterdam -Printed

in The Netherlands

STRUCTURE

AND CoMo/Si02

AND CATALYTIC

CATALYSTS:

ACTIVITY

OF CoMo/y-A1203

AN XPS AND ESR CHARACTERIZATION

A. MATHIEUX,

P. GAJARDOa,

Groupe de Physico-chimie

OF SULFIDED

HYDRODESULPHURIZATION

USED CATALYSTS

P. GRANGE and B. DELMON Mine'rale et de Catalyse,

Place Croix du Sud 1, B-1348

Louvain-la-Neuve,

Universit.6 Catholique

de Louvain,

Belgium.

adeceased

(Received

14 April

1981, accepted

1 May 1982)

ABSTRACT A series of sulfided Co, MO and CoMo supported on y-Al 0 and SiO2 catalysts has been investigated, after they had been used as cataly $S t for 8 h at 305°C and 30 bar pressure in contact with a mixture of thiophene, cyclohexene, cyclohexane and hydrogen. The catalytic activity in the hydrodesulphurization (HDS) of thiophene and hydrogenation of cyclohexene was measured. The sulphided and used catalysts were investigated by X-ray diffraction, diffuse reflectance spectroscopy, XPS and ESR. Samples were transferred from the catalytic reactor to the XPS and ESR equipments under controlled argon atmosphere. Several cobalt and molybdenum surface species were identified and their functions in HDS and hydrogenation reactions were evaluated. Cobalt is in its full sulphided state only if molybdenum is present in the catalyst. The high HDS and hydrogenation performance of CoMo supported on A1203 is basically due to a strong CoMo-support interaction; this leads to a high dispersion of the non-sulphided species (precursor) and, in consequence, of the active sulphided species.

INTRODUCTION In industrial

plant,

or, most probably, marked

changes

sulphur

if reaction

in the catalysts

reaction of MoS2

temperature (synergetic

the greater

catalytically encountered

conditions, catalyst

are altered.

drastically

depends

effect

catalysts

flow reactor.

With results

a direct and simple correlation structure

the amount of

ratio and also on the

the higher the H2 pressure

a considerable

under conditions catalyst,

namely

obtained between

which

number of published had not worked

very different

in

in such non-representative catalytic

may lead to wrong conclusions

0

from those

in steady state conditions

behaviour

and

about the nature of the

sites.

0166-9834/82/0000-0000/$02.75

may undergo

effect of cobalt on the HDS activity

have dealt with catalysts

had worked

work in a partially

catalysts

For instance,

on the H2 pressure;

[3]. However,

by the real industrial

surface

catalysts

on the H2S/H2

[1,2]. Also, the promotor

or which

a high pressure

CoMo/Y-A1203

state. These sulphided

conditions

depends

effect)

the promotor

papers on sulphided

active

hydrotreating

fully sulphided

1982 Elsevier Scientific Publishing Company

348

The present

paper is concerned

sulfided

catalysts

pressure

of 30 bar.

having worked

from the reactor

the specific

role of y-Al203

sulfided

on the catalyst

to cobalt and molybdenum

forms of the oxidic

previously

investigated

nature of the species

CoMo/y-A1203

two catalysts

to be an "inert"

[4, 61. The catalysts

and CoMo/Si02

present

in used sulfided

catalytic

catalysts

activities

are the

series of catalysts

[4,5,7 - IO]. In this paper, an attempt

with HDS and hydrogenation

EXPERIMENTAL

is considered

during

Also, to estimate

characteristics,

Silica

under a

from air contamination

to the XPS or ESR apparatus.

on SiO2 have been studied.

carrier with respect

of CoMo/y-A1203

in a flow reactor

The samples were protected

the transfer

supported

with XPS and ESR studies in HDS reaction

to identify

the

and their relationship

will be made.

METHODS

Catalysts The preparation CoMo/y-A1203

and characteristics

and CoMo/Si02

catalysts

elsewhere

[4, 81. All catalysts

supported

phases

of the oxidic

used in the present

supported

(Co3O4 + Mo03),

namely,

on y-A1203

study have been described

contain

the same amount of

The SiO2 supported

were sulfided

cedure estabiished (this procedure

prior to their activity

from a sr;udy performed

enabled

The preparation

flow quartz

reactor,

was 600°C and 4OO"C,

catalysts

a sulfidation

respectively,

on unsupported

oxides to Cogs8 and

form of the catalysts

of Ar-H2S

was carried

(15% H2S). The reaction

for the alumina

of the reactor was increased

in order to avoid the sublimation

temperature

on the surface,

the samples were subsequently

designated

Catalytic

value of the atomic

activity

Apparatus. automatic

evacuated

under outgassing.

In the (IOO'C h-')

of Mo03. Sulfidation adsorbed

Torr) for 2 h and

catalysts

have been

and silica series respectively,

ratio Co/(Co + MO) being indicated

by r.

measurements

Catalytic

high pressure

powder were deposited

activity

s 1.5 cm, for alumina

measurements

"Catatest"

in the middle

of 2.2 cm2 cross-section

for removing

(lOa

Sulfided

as "Al-S-r" and "Si-S-r" for the alumina

the approximate

progressively

lasted 6 h. To desorb the sulfur physically

then cooled at room temperature

out in a

temperature

and silica catalysts.

in the interval 4OO"C-6OO"C, at the ultimate

to a pro-

Co304 and Moo3 [ll, 121

of the unsupported

of the sulfide

using a mixture

test according

former case, the temperature

transfer

proport-

12.2 f 0.1 wt % (Co304 + Mo03) with r = 0.36 and 1.00.

All catalysts

MoS2).

of the sulfided

14.8 + 0.6 wt %, with variable

ions [r = Co/(Co + MO)] of the active elements. contain

precursors

apparatus

part of a continuous

and 85.5 cm length.

and silica catalysts,

the catalyst

from the reactor,

to the XPS apparatus

were performed (GeomCcanique).

Catalyst

or to the ESR tubes,

steel

Two grams of catalyst

fixed bed flow reactor

bed height was z 0.8 cm and

respectively. without

in a stainless

The special

box used

air contamination,

and for

has

previously

been described

349

c131. Charge feed, experimental a mixture

of cyclohexene

of thiophene

was added;

all catalysts, 30 bar.

conditions

the alumina

and silica

zation of thiophene

Analysis

was measured

space velocities

series,

(L.H.S.V.)

respectively.

of the catalysts

and hydrogenation

For of

were 34 and 18 h-' for

The liquid and gas phase products

were studied,

At reaction

temperature

namely

hydrodesulphuri-

(9 h on stream),

was performed

the reactor was allowed

of catalytic

will be defined

feed was changed

by the conversion

Hydrogenation

of cyclohexene

had reached

to the Ar feed

It was then isolated

off the inlet and outlet

activities.

started.

analysis

At the end of the experiment

to cool. When the temperature

to room temperature.

of 10 atm of Ar by turning

to hydrogen

and the reaction

every hour and qualitative

150°C (after g 30 min) the hydrogen-hydrocarbon and the reactor was cooled

of the reactor

(305"C), the argon was changed feed was established

of the gas phase was made after 7 h of reaction.

Expression

ratio was 600.

of cyclohexene.

IO-15 min, the hydrocarbon

of the liquid products

pressure

(liquid)

5,000 ppm

at 305°C under a pressure

Pure argon was fed into the reactor as the temperature

was increased. and, after

activity

The reactant was

by gas chromatography.

The two main functions

Procedure.

analysis.

(70 % wt) to which

H2 (gas, NTP) / hydrocarbon

the catalytic

The liquid hourly

were analysed

and product

(30 % wt) and cyclohexane

reactor

under a valves.

and HDS catalytic

and thiophene,

activities

respectively,

on

2 g of catalyst.

Physico-chemical

measurements

Surface area, X-ray Diffraction copy (DRS). The BET specific measured tests.

volumetrically.

alumina

reference

were analysed

DRS measurements

Reflectance

Spectros-

by N2 adsorption,

by XRD after the catalytic

with a Co target

(28 kV, 12 mA) with

(200-800 nm) of catalysts

(only the

out using a Beckman ACTA IV spectrophotometer.

support

pellets were obtained

of the catalysts

(XRD) and Diffuse

area was determinated

PW 1010 equipped

series) were carried

pellet of the catalyst

surface

The catalysts

A Philips-Norelco

a Fe filter, was used.

Analysis

(y-A1203) was used as reference. by powder

pressing

with air could not be avoided

C;talyst

at 1.7 ton cm

A

and

. The contact

in these physico-chemical

measure-

ments. XPS. (i) Apparatus a Vacuum Generator

and binding

ESCA 2.

improve the signal to noise ratio. employed spectra obtain

for analysis

energy

A Tracer

(BE) reference.

Northern Conditions

The XPS equipment

NS 560 signal averager of spectra

recording

are the same as those used in a previous

work

was

was used to and the method [7].

The

of the Cls, Si2s, A12s, S2s, S2p, Mojd and Co2p levels were recorded. reproducible

recording

was used.

results,

a strict

standardization

Thus, it was expected

to minimize

of the order and time of the effect of residual

To

350 gas deposition

on the surface of the catalyst

Spectra of catalyst was employed

were typically

on the XPS signal

during

14-18 h.

intensities.

Most of the time

to improve the signal to noise ratio of the CO

The electrical

and Mojd levels. 2P of the samples, which was periodically controll-

charge accumulation

ed, was low in these sulfided Preliminary indicated

recorded

measurements,

catalysts. in which Au was used as reference

(AURA 7,2 = 82.8 eV),

that the BE of the Ols and Alps lines were independent

composition.

Hence, the Ols line was taken as internal

value of 530.3 eV.

The advantages

AIEs have been discussed

reference,

of the use of internal

by Ogilvie et al [14].

of the catalyst using a BE

references

The accuracy

like Ols or

in the determin-

ation of the BE of single peaks was f 0.1 eV. X.P.S.

(ii) Deconvolution

by computer

(or "mirror") by gaussian

of composite

using a peak synthesis method.

lines.

out according

sequence: spectrum.

ii)

Background

substraction

This operation

was performed

that the XPS peaks are described

of the spectra

of the original

Smoothing

ground varied

In both cases it is assumed

The deconvolution

to the following

i)

spectra.

program or by hand using the peak symmetry

from a smoothed

linearly with the BE.

line.

by both methods

It was assumed

This assumption

was carried

that this back-

has been justified

elsewhere

c71. iii) Peak deconvolution.

In the method of deconvolution approximate

positions

at half maximum straints

(FWHM) must be initially

are indicated

in the subsections

the S2p, Mogd and Co2p peaks. interactive

optimization

after smoothing

be identified,

either

i)

included

intensities

and full width

in the input data.

corresponding

The deconvolution

by peak symmetry

and background

These con-

to the deconvolution

of the spectrum

of

is based on

only applies

subtraction,

if, from the original

a half of a gaussian

from the left or right side of the spectrum.

ution by the peak symmetry method general

the number of peaks, their

of the XPS parameters.

The method of deconvolution spectrum,

by peak synthesis,

in the BE scale, their relative

is usually

performed

according

can

The deconvol-

to the following

sequence:

The presence

is sought.

of a half gaussian

line in one of the sides of the spectrum

If it does exist, the other half of the gaussian

peak is drawn by

symmetry. ii)

This gaussian

of the resulting subtracted peaks cannot

peak is subtracted

curve

is described

from the spectrum.

from the envelope.

by a gaussian,

This operation

If one of the sides

a new peak is again built and

is repeated

until further

gaussian

be found.

With the above-mentioned of a half of a gaussian

imposed constraints

peak in the smoothed

the error of the deconvolution

and that relative

and background

by peak symmetry

to the presence

subtracted

is determinated

spectrum,

by the quality

of

351

the fitting between experimental very small differences definition

between

of the S2p, S2s - pi03~ and c02~

When necessary,

additional

gaussian

line. This general

of the background

deconvolution

and theoretical

curves were observed:

calculations,

peaks.

In this work, only

they basically

procedure

depend

on the

has been applied

to the

levels of all used catalysts.

specific

to each level, have been done.

In Figure 1 we present examples of original spectra of S2p, S2s - Flojd and c02~ levels

(al, bl and cl respectively).

(a and b) and Al-S-O.75 smoothing

These examples

(c) catalysts.

(al, bl, cl respectively)

deconvolution

procedure

correspond

The deconvolution

is presented

to used Al-S-O.50

of these spectra,

in Figure 2.

after

The details

of the

for each level are the following:

spectrum (Figure 2a). This spectrum was deconvoluted into three separate II (a) s2P I it has been assumed that and S2p '*I). In the deconvolution, peaks (S s2P ITP' the S2p and S2p 'I* peaks are separated by 3.2 eV (the S2p doublet appeared as one single

peak in our equipment).

between

theoretical

assuming

different

because

S2p

II

This value corresponds

and experimental BE distances

III

and S2p

below) and therefore,

separating

are attributed

the peaks. This assumption

to species

S

II 2P

and S2p

III

i)

The original

all catalysts.

spectrum

(bl) after background

to the general

taken from the S2s - Mo3d composite account

and S2p111),

the standard

of the corresponding mined by making ' species

s2P ording

S2p/S2s

of the 2p level. The other

to their relative

BE distances

molybdenum, ii)

of spectrum is presented

In all catalysts,

rum b3 had a gaussian to the half gaussian

(Sppl,

S

into

II

distaz:e

;BE)

of the three sulfur species were deter-

(S2s11 and S2s

and intensities

III

deduced

III

to the

) were drawn acc-

from the deconvolut-

peaks were then

subtracted

part of the spectrum,

to observe

from

coming only from

that the left side of the spect-

shape. Hence, one should assume

that this side corresponded

of a Mo3d 5,2 peak of the molybdenum

to the following

corresponding

hypotheses

by 3.2 eV and

c)

species

to this species,

and constraints:

lines have the same full width at half maximum lines are separated

was made taking

region

in b3.

it was possible

lowest BE. The F?o~~ doublet, according

in the S

species

2b. The remaining

shows

peaks in

at 225.4 eV (Szsl) correspond

ion of the 2p level. The S2s * , s2sII and S2s the envelope

of the

the S2s peaks were thus first

ratio and ChPe relative

intensity

peak located

support-

(b2) clearly

by gaussian

This operation

detected

levels. The positions

the gaussian

(see

Catalysts

of the deconvolution

subtraction

procedure,

spectra.

the number of sulfur species

is justified

with y-A1203

composition.

of S2s species which were well described According

deviation when

peaks.

- Mo3d spectrum (Figure 2b). The details (b) s2s 110~~ and S2s peaks were the following:

the contribution

associated

do not depend on the catalyst

ed on Si02 do not exhibit

to the minimum

in all series of catalysts,

curves.

(FWHM);

a) b)

the ratio of their

having the

has been constructed the Mo

3d 5/2 and M"3d 312 the maxima of these

intensities

(MOBS 5/2:

352

n

m

353

bl

.b2

16(10

P

l7M BE

H

b4 ,nIv:in 2210

FIGURE 2

Examples

226.0

of deconvolution

axis of Figure b indicate

2360

of the S2p(a, Al-S-0.50),

and Co2p 3,2 (c, Al-S-0.75)

Al-S-0.50)

, 231.0

spectra.

the BE values

S2s and Mojd (b,

Dotted vertical

of oxidic

lines on the BE

MO in oxidation

states

IV,

V

and VI (taken from [50]).

is 1.54. The values of 3.2 eV and 1.54 have been determined experimentM"3d 3/2) ally with pure MoD3. This doublet, labelled as Mo3z, is presented in b4 as a solid line. It corresponds The dotted

to a molybdenum

line is the remaining

species denoted

part of the spectrum

hereafter

resulting

as MO,.

after subtraction

of the Mosjd doublet. spectrum (Figure 2~). The deconvolution of this level was done by ('I "2~ 3/2 the peak synthesis method, according to hypotheses and constraints proposed by Declerck-GrimGe method,

et al [15] and further

this level, after smoothing

developed

by us [16]. According

and background

to this

is deconvoluted

subtraction

into three peaks: i)

A narrow peak, Co,, exhibiting

sulfided ii)

diamagnetic

A broader

cobalt

species,

peak, Co,, attributed

iii) A satellite The constraint

This peak, given by

has no satellite. to high spin cobalt

peak, Co, sat, corresponding

species.

to the Car peak.

imposed ias that (FWHM of Co,) / (FWHM of Co,) was equal to

0.74. These hypotheses chemical

the lowest BE value.

properties

and constraint

of the system,

based on XPS measurements

had permitted

the successful

and on the physicodeconvolution

of

354 the Co2p level of similar catalysts X.P.S.

(iii)

Quantitative

has been taken as line intensity. been calculated obtained

considering

normalized

each element

the response

signal

intensity

of molybdenum.

peaks having a satellite, the "2~ 3/2 The calibration

procedure

the calculation,

the sum of of the

species corresponding

to

was the sum of the intensity

of

model

compounds

as that used during the other measurements. were CoMo04, taking

CoA1204,

presence

influence

and

their stability

in the catalyst.

such as transformation

and the possible

using The

Na4(SiMo,2040)

into account

and their possible

side problems,

during analysis

under We

of the surface

of the matrix

of the solid

intensity. of the calculations

surface compositions

for the quantitative

are presented

we designate

intensities,

in the results section

as defined

were performed

The magnetic

corresponding

to the total molybdenum

to individual

field was modulated

of the analysis.

or cobalt

species. using a dual sample

at 100 kHz using an incident microwave

of the X-band

"Strong Pitch Varian" was used as a standard.

the depth of the resonant

on quantitative

on a Varian El2 spectrometer

power of 20 mW. The first derivative

tube (3 mm inner diameter)

analysis

above, of the Alps. S2p, Co2p 3,2 and Mojd lines.

The index s or r corresponds

ESR. Analyses

chemical

as IAl, Is, ICoT. IMoT, IMos, Ices and ICor the

The index T refers to intensities

cavity.

The thus

to the number of ions of

was done by employing

were chosen

of XPS analysis

The details

species.

intensity

used for calibration

thus to diminish

normalized

have

lines.

coso4. These model compounds

Hereafter,

for each element

of the apparatus.

To facilitate

of the spectrometer

the same experimental model substances

on the signal

coefficient

is proportional

the case of cobalt

In

the signal

and satellite

compounds

The area under each gaussian

intensities

of Mogd 5,2 and Mogd 3,2 peaks was taken for the calculation

signal intensity

the conditions

analysis.

The signal

"seen" by the apparatus.

the intensities

expected

[163.

chemical

(9.5 GHz) spectra were recorded.

With our set up, the sample quartz

was filled with the catalysts

to a height greater

than

cavity.

RESULTS Composition,

surface

area, XRD and DRS

Active phase content, areas of the sulfided were determined

atomic

catalysts

on the oxidic

(Co304 + Mo03) were expressed

composition

ratio

are reported

precursor

r = Co/(Co + MO) and BET surface

in Table 1. The Co and MO contents

form of the catalysts.

conventionally

by considering

denum were present as Co304 and Mo03, respectively.

Active

phase contents

that cobalt and molyb-

All sulfided

catalysts

are of

black colour. X-ray diffraction peaks, except

spectra,

recorded

after catalytic

in the case of the Al-S-l.00

catalyst,

test, did not show any

where

lines at d = 2.98 and

355 TABLE

1

Cobalt and molybdenum

contents,

atomic composition

r = Co/(Co + MO) and BET

surface areas of catalysts Co + MO contentlwt

Catalyst

%

Co/(Co

+ MO)

Surface area/m'

Al-S-O.00

14.80

0.00

114

Al-S-O.05

14.42

0.05

138

Al-S-O.25

14.54

0.27

120

Al-S-O.35

14.68

0.38

111

Al-S-O.50

15.61

0.52

115

Al-S-O.75

15.98

0.75

117

Al-S-l.00

14.46

1.00

105

Si-S-O.36

12.30

0.36

125

Si-S-1.00

12.10

1.00

115

1.75 1, corresponding The principal structure

in Al-S-l.00

Co(II),

between

catalyst.

8 h reaction,

The DRS method

of Co in the CoMo/A1203

indicate

of CoA1204

the absence

to CoA1204.

In the other samples,

impossible,

because

Catalytic

the presence

absorption

of a CoA1204-like

is a good tool to identify

catalysts

[7,18]. Measurements,

where molybdenum

of tetrahedral

made on Al-S-l.00

of the local environment

a DRS study is

to those of CoA1204

[19].

activity

Quantitative

analysis

supported

catalysts,

products:

I-butene,

of the gas phase carried

after 7 h of reaction, cis-2-butene,

and traces of isobutane.

Butadiene

activities,

respectively,

The activities

trans-2-butene,

butane,

propane,

Analogously,

of the Si-S-O.00

The dependence

supported

with respect

silica supported activities,

we present, and Si-S-O.36

of catalytic catalysts

measured

propene,

ethane

of the HDS and hydrogenation catalysts

to the activity

during

in Figure

the first 8 h.

of the catalyst

end of the first hour, et/c, x 100 (cl and ct are the conversions of reaction).

and silica

of the following

was not detected.

of the alumina

are expressed

out with alumina

showed the presence

In Figures 3 and 4 we report the evolution

activity

after

of Co corresponding

is present,

bands of MO'S2 superpose

the

[17]. A broad triple band

500 and 700 nm, due to ligand field transition

is characteristic

-1

peaks of CogS8, were observed.

aim of the DRS study was to detect

local configuration situated

to the principal

g

at the

at 1 and t hours

5, the evolution

of catalytic

catalysts.

activities

on atomic composition

is shown respectively

after 8 h, correspond

in Figures

in alumina

and

6 and 7. The reported

to steady state values.

356

160

Thiophene

I

conversion

Al-S-100

401 1

FIGURE 3

Variation

5

of the HDS activity

150

Cyclohexene

9

h

t

of Al-S-r catalysts

with time on stream.

I hydrogenation

I

I

I

AI-S-025

AI-S-100

301

h

t FIGURE 4

Variation

of the hydrogenation

I

9

5

1

I

activity

of Al-S-r catalysts

with time

on stream.

-XPS Spectra and BE. The original

smoothed

spectra of levels S2o. Mo3d - S2s and

corresponding to the series of catalysts supported on A1203 are presented co2py in Figures 8, 9 and 10 respectively. The BE of sulfur, molybdenum (MO,) and cobalt (Co,) species of catalysts Figure

11 presents

and unsupported

sulfides

are presented

the Si2s - S2p line of the Si-S-O.36

in Table 2.

catalyst.

It is worth-

357 120

8 I u*

80

40

0

FIGURE 5

Variation

for Si-S-O.00

FIGURE 6

Conversion

activities

with time on stream

catalysts.

I

I

Q5Cl

0

13

10

hours

t

of the HDS and hydrogenation

and Si-S-O.36

01

7

4

1

1 A Co

l

MO

vs. atomic

composition

of alumina

vs. atomic

composition

of silica

supported

catalysts

after

8 h of reaction.

FIGURE 7

Conversion

supported

catalysts

after

8 h of reaction. while noticing

the absence

Quantitative

analysis.

for the evaluation [4,5,7,21,23-261. dispersed

XPS signal

of the dispersion The intensity

on the surface

III

of the Sppl' and S2p intensities of active

peaks in this spectrum. have been successfully

phase on the surface

of an XPS signal of an element

of the carrier

(forming a monolayer)

employed

of supports

x, homogeneously is described

by the

358

AI-S-Q25

AI-S-Q35

FIGURE 8 S2p spectra of the Al-S-r series (binding energies in eV are indicated on the curves). following relationship [27]: (1 )

where 0 x(y) is the photoelectron cross section of element x for level y, Ax (y) is the mean free path of the corresponding photoelectron, nx is the concentration of element x on the surface of the carrier and k is a proportionality constant which includes factors such as detector efficiency, surface contamination, X-ray fl ux , etc., Using equation (1) and the appropriate calibration of the XPS apparatus for each element (see experimental methods), one may estimate the surface atomic fraction

359

AI-S-Q00

AI-S-005

AI-S-025

AI-S-035

/ /

Al-S-Q50

Al-S-Q75

I 775

I 780

I 786 BE

spectra of the Al-S-r

FIGURE 9

M0Sd-s2~

indicated

on the curves).

FIGURE

Copp S/2 spectra

10

indicated

=

Ix/$,,

+ ICo +

such as surface

An estimation

in eV are

roughness,

had a precision

The IAl/(IAl

error

etc., may partially

in our quantitative

has not

of the above-mentioned

Under these circumstances,

for similar

invalidate

analyses

of all XPS signals, when

data directly

Several

of dispersion.

that various

for the analyses.

et al. [25] have reported,

intensities

measurements

the intensities

of the intensity

(2) is only approximate.

contamination,

it is expected

degree

are followed

that variations

by equation

of the experimental

to a similar

procedures

(2)

(1) for absolute

been done. Nevertheless,

Okamoto

(binding energies

I#&

of dispersion

the use of equation

affect

series

in eV are

"seen" by the spectrometer:

The evaluation factors

of the Al-S-r

(binding energies

on the curves).

of the catalyst

Ix

series

(ev)

reflect

variations

catalysts,

factors

identical one may admit

of dispersion.

that their relative

XPS

of f 5%.

+ ICoT + Il,ioT),IMoT/(IAl

+ ICoT + IH~T) and ICoT/(IAl

+ ICoT + &T)

ratios give indications

about the degree of dispersion

on the T-Al203

These ratios are plotted

The dotted and cobalt Al-S-l.00

surface.

of cobalt and molybdenum

against

Co/(Co+ MO) in Figure

lines correspond

to the values which should be obtained

species

the same degree of dispersion

retained

12.

if molybdenum

as in Al-S-O.00

and

respectively.

To estimate the lS'(lMoT 'COS'(~A~

the degree of sulfidation

+ *COT

of our used catalysts,

we have plotted

) ratio (Figure 13) and the IMos/(IAl + IMoT + ICoT) and

+ 'MOT + 'COT

) ratios (Figure 14) against Co/(Co + MO).

-ESR The ESR spectra of the catalysts Three different

supported

signals are observed.

Signal

on y-Al203 I, present

are presented

in Figure

in all catalysts

15.

containing

TABLE 2 Binding energies species,

(BE) of c02~ 3,2, MOBS 5,2 and S2p levels of Co,, MO, and sulphur

and spin-orbit

Solid

"2~

splitting

3/21eV

(AE)

of the Co

M"3dMz/2'eV

co,

I s2P

S

Al-S-O.00

2P

160.5

227.2

level (Cos). AE/eV

S2IT /eV

III

s2P

s2P

coS

164.7

168.0

-

227.4

160.3

-

168.0

-

Al-S-O.25

777.2

227.6

160.5

164.5

167.8

15.2

Al-S-O.35

776.7

227.8

160.7

165.1

168.2

15.2

Al-S-O.50

777.1

227.6

160.4

165.1

168.4

15.2

Al-S-O.75

776.6

227.3

160.4

165.1

168.4

15.2

Al-S-l.00

776.6

160.4

164.6

167.9

15.4

160.6

-

160.7

-

Al-S-O.05

MOSER

227.8

"9'8 b

776.8

aFrom references bFrom reference molybdenum,

[20,21] [223

has a peak-to-peak

a g-value centered Signal

III

Al-S-O.75

15.1

at 2.0028.

width of 65 G and a g-value of 1.93. Signal It is visible

in all catalysts

except

is a broader signal observed only in cobalt rich catalysts, and Al-S-1.00;

its peak-to-peak

g = 2.14. In the case of the Al-S-l.00 to 2mW, otherwise

the "saturation"

The ESR spectrum

of catalyst

width

phenomenon

Si-S-O.00

namely

is 208 G and it corresponds

catalysts,

the microwave

II has

in Al-S-1.00. in

to

power was reduced

took place.

(Figure

16a) exhibits

a signal similar

to

361

A 1 lA_ Si2,

Si -S 436

z 8 l-4 5

145

150

155

160

166

170

eV

BE FIGURE

11

XPS spectra of Si2s and Spp lines of catalyst

signal

I found in the alumina

series.

This signal

but the shape is different;

we designate

presents

a "triple"

signal V, centered

Si-S-O.36

exhibits

rich alumina

signal, signal

supported

(Figure

III

catalysts.

it: this latter can be identified

Si-S-O.36

has the same g-value

it as signal

IV.

This catalyst

at g = 2.0028

16b), already

(1.93), also

(Figure 16a). Catalyst

observed

in the case of cobalt

A small signal, at g = 1.97, is superimposed with signal

IV observed

in catalyst

on

Si-S-0.00.

DISCUSSION Catalytic

activity

The results effect

reported

is the different supported

become

different

of HDS activities

with the reaction

manner

of evolution

catalysts

consequently,

time (Figures

and produce

(CoMo). Changes of activity of sulfur,

benzothiophene.

3 and 4). The HDS activities

with reaction

on the

may be associated

and presulfided,

with

our pure MO and

they have a poor hydro-

a low amount of H2S. Their surfaces,

these catalysts

situation

catalysts

time (Figure 3). The

In fact, because

to H2 rich atmosphere.

An inverse

fact to be noticed

in CoMo and in Co or MO

of both groups of catalysts

have been prereduced

activity

are exposed

decrease.

elimination

observed

of their surfaces.

the initial period of reaction, further

of a synergetic

CoMo on the one hand, and Co or MO supported

conditioning

desulphurization

the existence

[283. The interesting

higher and lower, respectively.

a progressive Co supported

of CoMo/A1203

evolution

catalysts

of fresh sulfided other,

in Figure 6 corroborate

in the HDS activity

prevails

of fresh sulfided

have been observed

Under such circumstances,

during

lose sulfur and their activities in catalysts

CoMo/A1203,

with high HDS activity

accompanied

by an

by Gissy et al [29] in the HDS of

362

FIGURE

12

Relative

intensities

of the Al, MOT and COT XPS signals

The effect of the support on the activity of the HDS and hydrogenation (Figure 5), whereas

activities

in Al-S-0.00,

is revealed

of MO supported

in HDS. These behaviours

may be explained

considerable

difference

of molybdenum

During catalyst Al203 [6,30].

preparation,

with H2 [8,31,32]

molybdenum

into account

with respect

forms a stable monolayer

at temperatures

does not react so extensively

it forms Moo3 and behaves

as unsupported

drop

is low in hydrogenation

taking

is that it may be only partially

and H2/2S [33] respectively,

500°C). Molybdenum ation,

of reactivity

(A consequence

by a conspicuous

on Si02 (Si-S-0.00)

the decay of activities

and moderate

(Al-S-r series).

the

to the carrier. on the surface of

reduced and sulfided as high as 400-

with Si02; during catalyst

prepar-

Moo3 when it is reduced with H2

[4,5,10,341. CoMo catalysts

keep or increase their HDS activity

3 and 5). In contrast, their hydrogenation

(except Al-S-0.25),

activity.

The different

with reaction

the same catalysts behaviour

time (Figures

undergo a decay of

of catalysts

in HDS and

363

FIGURE

13

Dependence

of the Is/(IMoT

+ ICoT ) ratio on catalyst

of the relative

intensities

composition

(Al-S-r series).

FIGURE

14

catalyst

Dependence

composition

hydrogenation different

and decrease concept, several

strongly

suggests

that these reactions

take place separately

sites. During the initial period,

of activities

for HDS and hydrogenation

one for HDS and another [20,35-451.

also take place on unsupported

[46,47].

hydrogenolysis,

hydrogenation

condit-

to increase The dual site

has been proposed

by

keep in mind that the two types of reactions

catalysts

the basal planes of MoS2 catalyze planes of MoS2 promote

respectively.

one for hydrogenation,

One must

on two

due to reaction

and death of both types of sites should occur leading

authors

Co and MO on

(Al-S-r series).

types of active

ions, genesis

of the sulphided

reactions.

In

Stevens

et al [20] suggest

whereas

the edge of structural

supported

that

catalysts,particularly

364

FIGURE

15

ESR spectra of Al-S-r catalysts.

by the corresponding

in CoMo/Al2OS,

the number of the different

due to the influence will help elucidate active phases

Signals

I,

II

and III

are indicated

number.

of the support.

types of active sites may be modified

XPS and ESR observations

the state of their surfaces

in the HDS of thiophene

of the used catalysts

and make suggestions

and hydrogenation

about of

of cyclohexene.

XPS Identification catalysts,

of surface

may be attributed

species, to sulfur

(i) Sulphur. in sulfides

BE with the BE of S2p in MoS2 and Co9SB [20-221. the presence

of Co9SB, detected

in Al-S-l.00

The S2p peak, present

in all

because of the similarity

This assignment

of its

is supported

(by XRD), and MoS2, previously

ed by analytical electron microscopy on the same used catalysts [91. II Peaks S2p and S2p "I, present in all alumina supported catalysts,

by

observ-

are absent

365

FIGURE 16

ESR spectra

are indicated

in Si-S-O.00

of Si-S-O.00

by the corresponding

and Si-S-0.36.

(a) and Si-S-O.36

Hence, we are led to assume

from species which are able to be adsorbed the presence to suppose adsorbed

of H2S and thiophene

that S2p

II

and Spp

III

on the one hand, and H2S-Na2S

in the reaction

S2p

more, the adsorption by other authors absence

II

system,

and S2p

[49-521.

assignment

of S2p

in our catalysts) experiment s2P CoMo/A1203 obtained

catalysts.

in both cases assignation

A possible catalysts

of peaks S2p

arises

II

leave unexplained

the

is also able to

(the same as that used

set for the accumulation in the analysis

The same negative

with pure thiophene introduced

The XPS of the of the

result was

at room temperature. by H2S or thiophene

in our XPS measurements.

Consequently,

an

and S2p 'I1 must be sought.

of the XPS differences

from the different

Further-

has been observed

to give an unambiguous

as those employed

that the amount of sulfur

explanation

in BE we measured

at 305°C under 10 bar overnight.

No S2p signal was observed.

is undetectable

would

on Si02, which

the pure A1203

with the XPS apparatus

when the A1203 was impregnated

alternative

on A1203 or catalysts

supported

, we treated

using the same conditions

Hence, one may conclude on A1203

from the differences

this interpretation

with a H2/H2S mixture

was performed

spectrum,

However,

III

in the thiophene-Na2S,

, which were 4.4 and 7.6 eV, respectively.

in catalysts

and Spp

logical

and H2S. respectively,

of BE of sulfur

and H2S at 250°C [53]. In an attempt

II

In view of

it seemed

on the other, are 2.5 and 5-8 eV, respective-

of H2S and thiophene

of these species

adsorb thiophene

I

IV and V

that these peaks originated

atmosphere,

arose from thiophene

ly [48]. These values are not too different (Table 2) between

III,

on A1203, but not on SiO2.

Indeed, the differences

in A1203.

(b). Signals

numbers.

catalytic

between

properties

Si02 and A1203 supported of these supports.

The

366 cracking

and isomerizing

silica [54]. Hence, a alumina

before

properties

reasonable

of alumina

hypothesis

it is desulfurized,

yielding

are much stronger

than those of

is that some thiophene a carbon deposit

is cracked

containing

species such as C-S-H and C-S-S-C which could be the origin of signals Szp

on

some sulfur S

III

II

and

2P

* Identification

Mogd spectrum

of surface

species,

MoS2 (Table 2). The component catalysts presence

(ii) Molybdenum.

(Figure 2b) has given a Mo3: doublet

containing

Mozd appears

MO. This assignment

in deconvolutions

[9]. The small differences inherent

The position

in BE values

taken from reference

on the BE axis of Figure 2b (bottom). exhibit

its position subtraction

to Mo(VI),

The molybdenum

in the BE axis, the dotted

sulfided molybdenum

in the VI,

to detect any molybdenum

electron

microscopy

due to the small

MO(V) and Mo(IV)

[50], are shown as vertical ions located

higher BE than in a sulfide environment

of the MoS2 contribution,

of all

by the

of the spectra.

of the Mojd 5,2 line corresponding

oxidic surroundings,

oundings

by analytical

(Table 2) are probably

to the decomposition

of the to BE of

oftheMo3dlevel

of Maid to MoS2 is supported

of MoS2 in these used catalysts,detected

uncertainties

The deconvolution

having a BE similar

line remaining

V and IV oxidation

in oxidic

[55]. Therefore,

in

lines surr-

due to

in b4 (Figure 2), after

may be attributed

to unsulfided

and semi-

In no case was it possible

states.

ion, in oxidic or sulfide

dotted

surrounding,

with oxidation

state lower than IV. Declerck-Grime'e et al. [15,56] found a "shoulder"

on the high energy

side of

the Mojd 3,2 level, which was attributed

to the presence

The authors

could not arise from unsulfided

species

estimated

because

that this shoulder

it was situated

In view of our more complete presence

of MO ions in oxygen

the visible

thiophene Patterson complete

at a BE which did not correspond

interpretation

have reported

HDS on oxidic

sulfided

[23,25,33,50],

by other authors

effect of A1203.

Hence,

surroundings

of alumina

it is possible

and attached

Mo(V1) oxidic monolayer

only represents

et al. [26], also using

of CoMo/A1203

catalysts

was not

at 400°C.

to MoS2 under the sulfiding

observed

supported

may be interpreted to consider

conditions

catalysts

employed

Identification

in

like ours, also

by the stablizing

the Mo(IV) and MO(V) species,

to the surface of A1203, as remainders

of the

created

after

on the surface of the catalyst

calcination.

deconvoluted

the

for 100 min. On the other hand,

to a H2S mixture

sulfiding

to Moo2 or Mo03.

one cannot exclude

the shoulder

Okamoto

molybdenum

of such species after having conducted

catalysts

et al. [50] found that the sulfidation after 10 h of exposure

Free Moo3 is easily

original

actually

spectrum.

the presence

CoMo/A1203

this work. The incomplete

oxidic

of the spectra,

surroundings;

part of a more complicated

XPS techniques,

of MO(V) ions in MoS2.

of surface species, (iii) Cobalt. The Co spectrum has been 2P into Co,, Car and Co, sat peaks. The assignment of these peaks was

in

367 made on the basis of XPS-ESR reported

elsewhere

The assignment

shape and spin-orbit

Cogs8

of Co, was carried

splitting

and used Co/Al203

(AE)

Co2p sulfur deficient

CogSB because

the existence

Cogs8 in Co/Al203

out taking

into account

of the shape of the XPS peaks

in our oxidic

to those of

to CoA1204

or

(typical of high spin

ESR signal corresponding

of CoA1204

Co, CoA1204

the BE, peak

of the 2p level, which are similar

of a strong

and the presence

and CoMo/Al203 to CogS8,

(r = 1.0) and to pseudo

in CogS8. The Car and Co, sat peaks were ascribed

observed

Co species),

of sulfided

to that study, Co, is ascribed

Car sat to sulfur deficient

and its satellite (r = 0.35).

studies

[16]. According

to sulfur deficient

CoMo/A1203

catalyst

precursors. These assignments Table 2 indicates similar;

unambiguous

assignment

the absence

and typical

cobalt will be denoted

CogS8-like

Quantitative

to Coo in our used catalysts

exhibit

species.

to CoA1204

or sulfur deficient

Cogs8

ive exploitation

discussed

together

of the XPS results.

of the XPS results are considerable

of the distribution

is especially similar

useful

catalysts

etc.,) are nearly cancelled

the outer

It

The difficulties [58]. However,

elements

in identical

in the catalyst

these XPS

series,

of a quantitatit has been

one can obtain

on the catalyst.

ways.

an

The method

study of a series of

Under these circumstances,

of the surface,

contamination,

thus facilitating

the inter-

IcoT/(IAl

+ ICoT + IMoT) and IAl/(IAl

+ ICoT +

of the concentration

layers of the catalyst

Co/(Co + MO) (Figure

present

(paramagnetic),

these

of results.

The Ip,oT/(IA1 + ICoT + IbT),

strong

form of

To decide whether

of the method,

(such as roughness

IFloT) ratios are representative

catalysts

form of cobalt ex-

this sulfided

is to make a comparative

and treated

the errors due to the sample

pretation

of the various

if the purpose

prepared

But.

(see later)

with ESR and DRS data.

shown [7,26] that, in spite of the limitations evaluation

due

the Co, and Car sat peaks and their XPS

of high spin cobalt

interpretation

an

species.

are typical

results will be further

However,

on the use of XPS is not possible

to Co in CogS8; hereafter,

similar

of all catalysts

peaks correspond

Co species.

the Co, peak to Cogs8 or to some sulfided

XPS properties

The spectra

in this work.

of Co2p in Coo and Cogs8 [23,26,33,57].

of any ESR signal ascribable

characteristics

of diamagnetic

of Co based entirely

of BE and AE values

us to assign

hibiting

studied

in Cogs8 and Co, in our catalysts are quite 2P splitting values and shape of the Co, peak (FWHM) in

are also similar

to the similarity

in the series of catalysts

that the BE of Co

the AE spin-orbit

all catalysts

allows

are also valid

precursor

in the surface

(compare the ICoT/(IA1

is worthwhile

to the corresponding

plots for the same

form [7]. As in the oxide form, there

dispersion + ICoT

ions in

[7,27]. The plots of these ratios vs.

12) are quite similar

in their oxide

increase

surface

of these respective

of the cobalt

is a

species when molybdenum

+ I,,,oT ) values at r = 1.00 and r = 0.75).

to notice that the atomic

fraction

of cobalt

"seen" by XPS,

is

) in the sulfided catalyst with r = 1.00 is higher than in 'CoT'(IAl + *MoT + ICoT the same catalyst in its oxidic precursor form, namely 0.07 and 0.03, respectively [73, suggesting

that sulfiding

on the bare v-A1203

surface.

signal for r < 0.5 is more overall

decrease

Co into v-A1203 the formation

important

taking

during

the curve is situated

of cobalt.

electron

microcrystallites crystallization

this statement.

in all catalysts

containing

to explain

in the Mo/A1203

reported

preparation,

Figure

theShigh

from the strong bringing

dispersion

Co-MO-Al

vents a full crystallization sulfidation

[25,26],

on the surface

all catalysts:

strongly

of cobalt

forms,

into account.

of the spectra are consider-

as these observed

in Figure

species

structure

and molybdenum

12.

basically

during

reduction

In

results

form and

[7]. This interaction

about a high dispersion

of catalysts.

the dotted

line would forming

Figure

pre-

[S] and

of the active

for unsupported

of unsulfided attached

represent

catalysts

species

a deficit

of sulfur

to A1203, e.g. the Mo(IV), of a deficiency

in

the IS/(IMoT + ICoT) value if all

sulfur deficit

[20,223),

cobalt and molybdenum,

Other evidence

13 indicates

Cogs8 and MoS2. The sulfur deficit

not only in terms of an intrinsic

(as is reported

surroundings.

when the total

taking place in the oxide precursor

of the bilayer

thus bringing

Co and MO ions were sulfided explained,

of Co and MO,

of the catalyst.

Degree of sulfidation

existence

components

are taken

of cobalt and molybdenum

interaction

about the formation

This applies

role in HDS reactions,

and IMo ), shows the same tendencies

our opihion,

of the

to a large extent after sulfiding

conditions.

14, where only the sulfide

and

et al. [23] and

or not) as well as when only the sulfided

to play the active

of

The partial

and reaction

by Dufresne

that the high dispersion

is retained

at 600°C and use for 8 h in the reaction

which appears

The presence

by

catalysts.

in this work demonstrate

catalyst

performed

the drop of the Ma/Al and the increase

and CoMo/A1203

of the

taking place

molybdenum.

sulfidation

of MoS2, has been also mentioned

as

due to

by the destruction

of molybdenum

of the same catalysts

taking place during

line which

of molybdenum

[9] corroborates

of molybdenum,

during

can be explained

by an agglomeration

step. Analysis

amount of Co and MO (sulfided

ed (Ice

of

about

placed above the

microscopy

Okamoto et al. [25,26]

Actually,

of the

retain the same dispersion

was observed

leading to formation

achieved

bringing

nearly on the dotted

the same line was clearly

The difference

accompanied

the reduction-sulfiding

Results

as a consequence

an increase of the degree of spreading

monolayer,

CO/MO ratios

of cobalt

of the cobalt

of catalysts,

to the case where this element would

line, showing

analytical

dispersion

intensity

This effect may be due to a migration

place during the preparation

In oxidic precursors,

in Al-S-0.00.

the presence

enhance

of CoA1204.

would correspond

molybdenum

reaction

than would be expected

of the cobalt content.

In the case of molybdenum,

dotted

and catalytic

The drop in the relative

can be

of the sulfided

phases

but it may also deal with the

for instance

CoA1204 and molybdenum

MO(V) and Mo(V1)

species

in oxide

in sulfur have been presented

by other

369 authors

[25,26,49].

Okamoto

et al. [25,26]

related

zation effect of the MO phase by the formation observed

on silica supported

on alumina vacancies

supported

catalysts

due to the deeper

views expressed remote control

catalysts

by the formation

by one of us [41-433

support

one explanation

several

effects

of higher amounts

induced by cobalt.

effect was

in explaining

the "synergy

at the exclusion

of sulfur

of the other ones.

ion

to the

by contact"

of these sites. Our present

come into play for decreasing

the effect

This corresponds

by Cogs8 on the number of reduced

MoS2 and on the extent of reduction

to the stabili-

No similar

[59]. Nag et al. [49] explained

reduction

effect exerted

this deficiency

of CoA1204.

by a

active

sites on

results

cannot

It is very likely that

the sulfur content.

-ESR The interpretation able ESR literature Signal

I.

sulfided

of the ESR signalswill on sulfided

This signal,

catalysts

when similar

catalysts

number of studies [60,61,62-651).

located

[60-621.

at g = 1.93 has been observed

It has been assigned

has been published

[60,63],

the avail-

concerning

by other authors

to oxo-MO(V);

exists

A

(see for example provided

it is stabili-

but it does not appear

like A1203,

in

it is also observed

at high temperature.

this signal

to remark that this species

carrier

into account

and our own results.

are reduced with H2 or outgassed

Suffice

zed by an active

be done taking

CoMo systems

in unsupported

MoS2 and Moo3 [60,62,64]. Signal

A single signal very similar

II.

to signal

by Dudzik et al. [66] (see also the discussion [67])in an ESR study of sulfur signal to polyatomic thought

sulfur

impregnated

biradicals;

to depend on the pore structure

and on the recording

temperature,

ic sulfur atom chains. O2 was present

Lo Jacono

in the system.

that our signal

in polyatomic

Signal

This broad signal

III.

containing element. which

cobalt.

containing (observed

into account

the triple

the Dudzik's

that it corresponds

this was

on the support

et al. [60], who attributed

it must be assigned

Because

II.

ions, Koningsetal. and NiMolA1203)

located on MoS2 and WS2 crystals. Co/A1203

Depending

signal became a triple one.

(AH = 208 G) at g = 2.14

to our signal

also in NiW/A1203

on sulfided

of these biradicals

of the support.

[65] found a signal

only promotor

in reference

assigned

A similar

it to paramagnet-

signal only if

results

and the fact

to paramagnetic

sul-

sulfur radicals.

Consequently,

Koningsetal.

is similar

the stability

II is single, we conclude

fur present

The authors

et al. [67] observed

Taking

15) has been reported

of the communication

zeolites.

the single

triple band has been found by Seshadri

II (Figure

and Co/Si02

appears

only in catalysts

to a species

in Cow/Al203

containing

this signal was not observed assumed

from paramagnetic

centers

have not reported

the spectra

in their paper [65]. The discrepancy (and therefore

in samples

that this broad signal

originated

In fact, the authors

studies with regard to the presence

this

(g = 2.09, AH = 320 G)

with the origin)

between of signal

both III

370 may be due to different

pretreatment

conditions

Lo Jacono et al. [61] found a similar CoMo/y-A1203.

These authors

suggest

Co(I) or by a low spin Co(I1)

(T < 140 K), in unsupported to our signal

in CoMo2S4).

et al. C68.691

MoS2-Co9S8

discussions

Cogs8

with Hz, which brings about a partial

was caused by a cobalt

species

XPS measurements

surface

species

on Al-S-l.00

of CogS8) which

(as corresponding

the dramatic

disappearance

indicates

of the Co, species

(Figure

on the determination in the catalysts

Because

compositions

strength

of signal

observed

species

of signal

this phenomenon

is not supported

appears at r = 0.50, whereas

is

from 1.00

of the amount

signal

role

is absent

III

present

is observed

Hagenbach

the maximum

remark

of the corresponding

composition,

depends on the preparation catalysts.

using unsupported we cannot

and suggest the dependence

and composition

is a reduced

surface

area developed

[41-431,

[41-433.

species on not only on

by CogS8.

in supported

explaining

the reduced

MoS2 +

at the moment

observation

and cannot be comparable

effect

[44].

signal should be dependent,

In the frame of the hypothesis

by a remote control

of cobalt

in presence

conditions

but also on the surface method

shows

of one of us concerning

is that, if the Car species

CogS8, the intensity

work which

in this work at r = 1.00 and dis-

et al. [68,69],

sites on reaction

of both In our

temperatures.

at r = 0.20. Although

in the frame of an hypothesis

of the number of HDS active

solubility

on the support.

by recent spectroscopic

in the y-A1203,even

de-

form [73. Lo

due to the presence

to an increasing

and calcination III

such as CoA1204,

(III)

these discrepancies , we can make a general

an explanation

the catalyst

a diminution

is hardly dissolved

for typical catalyst

The general

with an increase

in their oxide precursor

when they are simultaneously

this explanation

fully explain

on the

15) al-

in this work

when r decreases

III

shows

sulfur content low (Figure

fact observed

state of cobalt.

as in catalysts

They attributed

ions in the alumina,

Cogs8 mixtures,

signal

CogS8, but to other high Co(I1)

Jacono et al. [613 also observed

The maximum

is relatively

by

analysis

with r < 0.50, XPS peaks Co, and Car sat can no longer be ascribed

to sulfur deficient

that molybdenum

less sulfur than

is justified

14). Hence, one may infer that MO plays a decisive

of the sulfided

tected when r = 0.05-0.50,

of molybdenum.

having

that the corresponding

takes place concomitantly

that both

namely a cobalt associat-

to a species

The significant

of

It was concluded

(where the X-ray diffraction

of this strong

to 0.50; this phenomenon

supported

reduction.

to the Co, species)

that

from the interactions

on CogS8. This hypothesis

catalyst

though no Co is trapped as CoA1204.

contact"

resulting

of Cogs8 and corresponding

i.e. a reduced

opinion,

measurements

This latter signal may be similar

III and the XPS Car peak have the same origin,

ed with the surface

surface

Also, a broad signal at

[163, we were led to the simple hypothesis

III

the presence

be caused either by

in low temperature

catalysts.

signal

CogS8,

used.

(H2S/H2, 400°C)

III.

In our previous

ESR signal

in sulfided

that this signal might

(as cobalt

g = 2.17 was found by Hagenbach

of the samples

signal

the "synergy

surface

This

and unby

species would

be

formed

by some over-reduction

of the surface of Co9S8, when the catalyst

verge of going beyond the maximum the cobalt

content

are too reducing. formation

The proposed mechanism

of the highest concentration

"normal" working

conditions

ration should be observed, because

a catastrophic

is increased the maximum

beyond

The differences

sulfides

could explain in parallel

allows

for the

species for r = 1.00 in

that the high Car concent-

near the maximum

activity,

This would

reconcile

and unsupported

the facts that

catalysts

is ob-

with respect

supported

catalysts

to signal

III:

Al-S-O-35

with unsupported

the presence

of signal

with its presence

illustrat-

exhibits

does not. The SiO2 supported

of the interaction

behave essentially

are well

Si-S-O.36

catalysts CoMo-Si02

[44]. These have been

making

as a mere mixture

that cobalt and

of Co9S8 + MoS2. This

III in r = 0.36 catalysts

on unsupported

catalysts

catalysts

supported

on SiO2,

at r = 0.20 [68,69].

IV. A signal similar to signal IV was observed when the same catalyst,

Signal

its oxidic

form, was reduced at 400°C by H2 [4]. This signal was also found

duced molybdates

namely MO(V). is located Signal

[62,65]

that signal

in the same symmetry,

they correspond

have mentioned

as signal

(denoted

Si-S-0.00,

is situated

signal V to paramagnetic

catalysts,

is no evidence

located

II

in

as signal

molybdenum

I,

II,

III,

and tungsten [62,65], may

(g = 1.985), III and IV to paramagnetic

centers

and MO(V) ions should be

to attribute

signal

our signals

V (Figure

to Konings'

to these

16a), detected signal

ESR proof with which

of catalysts

suggesting

of the catalyst

in

II and IV. to assign our

on the edges of MoS2 crystals.

that if there are Mo(II1)

on the surface

observed

in spite of the fact that the presence

However,

in the XPS spectrum

Hence, one may conclude

their concentration

environment.

by Koningsetal.

by the authors

we have no additional centers

of signals

in which Mo(III)

at g values corresponding

Beside this similarity,

Mo(II1).

Signal

We have not found clear ESR evidence

in our Al203 supported

tetrahedral

and unsupported

I (see above).

of MoS2 on them is quite probable.

to

IV have the same origin,

by the authors

I, reported

(g = 1.995) of Konings' work were ascribed

species

in re-

It was ascribed

of 2.036, 2.0028 and 1.994. Koningsetal.

state. Signal

to our signal

in

in both types of catalysts,

in a distorted V an ensemble

to g-values

located at the edges of MoS2 crystals, involved.

[62,70-731.

I and signal

(Si02 and y-A1203)

in the sulfided

well correspond

namely

5 ESR signals,

IV and V) in supported catalysts

on silica

Abdo et al. [73] state that this MO(V),

V. We have designated

Si-S-0.00;

supported

and in Moo3

MO(V). We must thus conclude

there

HDS catalysts

III)

because

conditions

occurs when the Co concentration

in supported

its counterpart

by the weakness

molybdenum

[42,43].

SiO2 and y-A1203

observed

appear to have many analogies explained

value

of the signal

between

whereas

catalysts,

of the activity

either

or working

compositions.

ed by the differences

III,

for supported

of Co, (signal

on unsupported

decrease

intensity

activity,

the pretreatment

[41]. But it also predicts

the optimal

served for different

signal

hydrodesulphurization

is too high or because

is on the

Moreover,

the presence

species

of

in the catalysts,

is low and undetectable

by XPS.

372

The absence of Mo(III) in similar sulfided catalysts had been previously mentioned in work based on ESR [60J and XPS measurements [50,74J. An alternative assignment of signal V may be made assuming that the signal appearing at g = 2.0028 and signal II of the alumina supported catalysts have the same origin, namely paramagnetic sulfur in polyatomic sulfur radicals (see discussing of signal II). This assumption i~ quite reasonable in view of the fact that alumina and silica have been treated in the same way before ESR analysis. The small signal at g = 2.036 is situated at the same g-value as a signal observed by Peacock et al. [70J in Mo and Bi-Mo catalyst supported on silica; these authors attributed it to carbon (for carbon, g is usually 2.003). The signal at g = 1.99 may be considered as the result of the superposition of signals coming from Mo(V) and paramagnetic sulfur. These ESR results show how essential it is to withdraw the catalysts from the hiah pressure reactor without air contamination if valuable information about used catalysts is to be obtained. For example, if contamination by O2 should take place to a significant degree, signal III would be destroyed [61J and the triple signal mentioned by Lo Jacono [67J should appear in the ESR spectrum of CoMo/y-A1 203 catalysts. Also, it is worthwhile remarking that no signal due to cobalt metal was detected in any catalyst, which corroborates the results found by XPS. Surface structures and relations with catalytic properties The preceding interpretation of experimental data indicated the presence of various cobalt and molybdenum surface species in sulfided catalysts having worked for 8 h. Such a complex systemproducesamultifunctional catalyst. Reactions such as HDS, hydrodenitrogenation, hydrogenation, hydrodeoxygenation, isomerization and cracking take place on their surface under hydrotreating conditions. Having performed an approximate individualization of surface structures, it is worthwhile to make now an attempt to indicate the specific catalytic role of each structure in the above mentioned hydrotreating reactions, particularly in HDS thiophene and hydrogenation of cyclohexene. CoA1 204-like species. Its presence in sulfided catalysts is indirectly infered from the presence of an XPS signal corresponding to a high spin cobalt species (Cor)' not associated with the ESR signal III. It is present in all catalysts with atomic composition 0.05 < r < 0.75. This species is practically inactive for hydrodesulphurization and hydrogenation [75J. The formation of this inactive compound during the calcination stage can be considered as a ransom which must be paid for obtaining good cobalt dispersion [7J. This compound is not reduced nor sulfided with H2 and H2/H 2S, respectively, at temperatures as high as 50QoC [23,75J. Surface cobalt in sulfur deficient Co~ This species is observed in catalysts with r ~ 0.75. It is characterized by the association of an XPS signal corresponding to COr with ESR signal III. The catalytic activity of CoMo/y-A1 203 in HDS had been

373 previously

attributed

and discussion

to the species

of communication

CogS8. Considering

with sulfur deficient properties

of the catalysts,

HDS and hydrogenation the lowest reduced

in addition time

species,

The origin

MO(V) and Mo(IV).

detected

in HDS of thiophene,

4 and 5 suggest

of Si-S-O.00

dramatically

the hydrogenation

[4,5,34].

activity

In addition,

of catalyst

has

if a

Al-S-1.00,

rather than decrease

with

Lombard0

are active

assumed

that unsulfided

behaves

in

of cyclohexene.

at a higher activity

of Al-S-O.00

to CoMo/y-A1203

noticing

where the decay of activity

that Mo(IV)

by various

and MO(V) supported

of olefins.

in HDS catalysts

are

that the

3 and 5). The low hydrogenation

has been reported

sites in the hydrogenation

with

that these species

It is worthwhile

(Figures

the

In other

as in unsupported

behaviour

in HDS of thiophene,

Mo species

that these

observed

the same time.

one may assume

et al. [77] indicated

alumina

differences

is stabilized

this particular

are comparable

relative

effect

indicating

in the hydrogenation

of Al-S-O.00

of cyclohexene.

is not observed

molybdenum

to the stabilizing

in this work

by 93% during

MO(V) and Mo(VI),

and Si-S-O.00

of CoMo/Si02

[54,65,76].

in

which

decays only by 21% after 5 h reaction,

in which molybdenum

in the hydrogenation

same phenomenon

evidence

drops

Hence, matching

of Mo(IV),

of Al-S-O.00

increase,

but significant

of Al-S-O.00

level than that of Si-S-0.00,

activity

the activity

that they are involved

Indeed, while the activity

the presence

activities

of these oxided or semisulfided

is no experimental

Figures

involved

of this species.

by XPS and ESR, have been attributed

There

are active

catalysts

content

species of Co9S8 were active,

species

words,

that the catalytic

Cogs8 is very low. Al-S-1.00,

3 and 4).

of the support.

activity

[69]

our XPS and ESR results and the catalytic

to being high, should presumably

(Figures

Mo(VI),

has the highest

(reference

[67]). This signal has been identified

we must estimate

of sulfur deficient

activity,

surface

III

giving the ESR signal

in reference

authors on

De Beer et al. [76]

are active

for olefin

hydrog-

enation. E,.

This compound

in the catalysts. and operation

seems to be the only fully sulfided

We have already

under high pressure

of the catalyst

produces

separate

expressed

the opinion

conditions,

compound

[9,28] that, after sulfiding

the oxide bilayer

crystallites

of molybdenum

in the precursor

of MoS2 and cobalt

in the sulfided

state. Studies

performed

MoS2 exhibits association

with unsupported

some activity

with cobalt which

in both reactions

(Figures

and will not be discussed -Co9S, or Co&,-like unambiguously

detected

of Co2p peak) suggest

catalysts

in hydrogenation

have permitted

and HDS reactions,

gives CoMo/A1203

outstanding

confirmation

catalytic

properties

3, 4 and 6). This topic has been amply debated further

species

[28,44]

in this paper.

(without

sulfur deficit).

by X-ray diffraction the presence

that

but it is its

in Al-S-1.00.

of this cobalt

species

The Cogs8 structure XPS results

was

(BE and shape

on A1203 supported

374 catalysts. present

It can be reduced

on unsupported

and hydrogenation Co9S8-like catalytic

catalysts

activities

Hence,

13 and 14, yielding species

cobalt and molybdenum during their sulfiding

but it is the association

is not

of CoMo/A1203

of cobalt and molybdenum a great number of contacts

on the surface

of the catalyst.

precursor

catalysts,

its origin

in the strong

also observed

a too extensive

interaction

by Okamoto

of the active

between

excellent is given,

in our

species

as shown

sulfided

molyb-

The high dispersion

process.

in the oxidic

provoked

of

is preserved

This property

of

et al. [26], in our opinion,

of CoMo/A1203

drop of dispersion

sulfided

form of CoMo/v-A1203

and use in the hydrotreating

some HDS

of Cogs8 and/or

sulfide which gives CoMo/A1203

on the oxidic

CoMo/A1203

the formation

that Co9S8 per se exhibits

the high HDS activity

by the high dispersion

denum and cobalt

indicate

[28,44],

species with molybdenum properties.

in Figures

prevents

Co9S8 when molybdenum

in the catalyst.

Studies

opinion,

to a sulfur deficient

bilayer,

by crystallization

has

which

during

phase.

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