Ruthenium—platinum bimetallic catalysts supported on silica: Characterization and study of benzene hydrogenation and CO methanation

69

Applied Clztalysis,28 (1986) 69-79 Elsevier Science Publishers B.V., Amsterdam

RUTHENIUM-PLATINUM

BIMETALLIC

- Printed

CATALYSTS

in The Netherlands

SUPPORTED

STUDY OF BENZENE

HYDROGENATION

D.K. CHAKRABARTY'

, K. Mohan RAO, N. SUNDARARAMAN

Solid State Bombay

I

Laboratory,

ON SILICA:

CHARACTERIZATION

AND

AND CO METHANATION

Chemistry

Department,

and Kalpana

Indian

CHANDAVAR

Institute

of Technology,

400 076, India.

To whom correspondence

(Received

3 January

should

be addressed.

1986, accepted

9 September

1986)

ABSTRACT Ru-Pt/Si02 bimetallic catalysts with varying Ru:Pt ratio have been prepared and studied with the aim to establish if they contain coclusters or isolated ruthenium and platinum particles. X-ray diffraction studies show that individual crystallites of ruthenium and platinum are present and no coclusters are formed. Metal dispersion has been determined by hydrogen chemisorption and surface composition of the catalysts has been obtained from XPS. It was found that preoxidation of the catalysts prior to reduction is essential for good platinum dispersion. The experimental turnover number (TN) for benzene hydrogenation on the bimetallic catalysts agrees very well with that of the weighted average on the individual metal catalysts and this may be taken as a kinetic evidence for the absence of coclusters. Carbon monoxide methanation activity of the bimetallic catalyst is quite similar to that of the supported platinum catalyst.

INTRODUCTION Supported

ruthenium

carbon monoxide

shows an unusually

111. According

to one view,

genation

of absorbed

however,

is the view that CO undergoes

thus deposited

demonstrated

CO through

reacts

CO on ruthenium

explained

by Miura

supported

nearly

three-fold

affect

the chemisorption

Platinum catalyst. adsorber supported

However,

and showed

in the presence

Ru-Pt catalyst

the coadsorption

that CO adsorption

0 1986 Elsevier

carbon

formation

anomaly

was

of CO and H2 on increases

of H2, however,

does not

on a surface

is a poor methanation

of platinum

Publishers

that is a good

CO d issociatively

From this point of view,

Science

of

out that although

on ruthenium

and hence

that can adsorb

attractive.

and the carbon

and platinum.

CO dissociatively presence

hydro-

chemisorption

This apparent

of HT. The presence

and ruthenium

of

has been convincingly

of CO than cobalt,

on cobalt.

of CO on cobalt

simultaneous

of hydrogen

0166-9834/86/$03.50

is higher

by direct

chemisorption

[3-51. That dissociative

methanation

methanation

[I,,?]. More convincing,

[6]. Rabo et al. [3] pointed

towards

does not adsorb

occurs

intermediate

a dissociative

et al. [7] who studied metals

towards

the reaction

step for methanation

and Bell

activity

due to CO disproportionation

various

an oxygenated

to give methane

is an essential

by Ekerdt

Ru has a higher

high activity

B.V.

makes

Gonzalez

a

and co-

70 workers

have studied

important support

Ru-Pt/SiO2

catalysts

to find out if coclusters

or if they form separate

(XRD) evidence

for the formation

[ll] in this system that they studied mmole/g)

and in these

is only indirect

is correct

oxygen

species

loading

work

studies

of ruthenium were carried

hydrogen

chemisorption.

reaction.

is to prepare

Ru-Pt/Si02

techniques

of benzene

Since this is known to be a 'facile'

rate is not dependent on the Ru-Pt/Si02

on the metal

particle

catalysts

Careful

composition

dispersion

of

composition

from

as a test

in the sense that its

[IZ], the turnover

will be the sum of those on Ru/Si02 This may be taken as an additional

for the absence

of coclusters.

methanation

low metal

was determined

has been studied

reaction

size

with

was obtained

catalysts

in a pulsed-flow

were obtained

X-ray diffraction

are formed.

been studied

(0.3

of coclusters

stoichiometry

if no coclusters

Finally,

loading

so as to find out if co-

Surface

(XPS) and metal

Hydrogenation

the catalysts

of these catalysts

have been formed.

spectroscopy

et al.

of coclusters.

out to check this point.

photoelectron

However,

[9]. Even if the surface

the formation

them by various

by Miura

of lower metal

different

on the

X-ray diffraction

for the formation

by CO assuming

and platinum

by X-ray

were

composition

at the Ru and Pt sites

it does not establish

and characterise

clusters

the evidence

titration

The aim of the present

mmole/g).

for CO methanation

catalysts,

are formed

Direct

has been obtained

(1.2

loading

of CO [7-11-j. It is

and platinum

clusters.

of coclusters

[9,10]. The surface

based on adsorbed the adsorbed

of ruthenium

monometallic

at a high metal

extensively

for methanation

number

(TN)

and Pt/SiO2 kinetic

evidence

of CO on these catalysts

has

microreactor.

EXPERIMENTAL Materials Ruthenium

trichloride

The support purity

was Davison

(IOLAR)

purified

hydrogen

by passing

(specific

supplied

several

after

acid were obtained

traps.

drying.

surface

area 280 m2/g).

by Indian Oxygen Ultrahigh

purity

All other chemicals

from Johnson

Matthey. High

Ltd., were further helium was directly

used were of AR grade.

and procedure

Catalysts prepared

grade 979 silica and oxygen

through

used from the cylinder

Apparatus

and chloroplatinic

with

by adding

a thin paste.

total metal aqueous

loading

solutions

This was dried on a waterbath

hours. The dry mass was preoxidised in a stream

0.1 mmole

of hydrogen

of metal

per gram of silica

of RuC13 and H2PtC15 with continous

in flowing

oxygen

to the support stirring

were

to form

for several

at 573 K and then reduced

at 773 K for 4 h. The catalysts

with Ru:Pt

ratio

l:O, 1:

0.25,

I:1 and 0:l were designated

A, 6, C and D respectively.

A catalyst

with

Ru-Pt

ratio

by direct

any oxygen

pre-

treatment

I:1 was also prepared

(sample

E).

reduction

without

71 XRD of the samples CuKcl radiation rates

to obtain

Scherrer's

were

recorded

at scanning

on a Philips

diffractometer

rates k", 1' and 1" per minute

the best result.

The particle

(PW 1140) using

and a variety

size (d) was determined

of count

using

equation.

KX

d=

where

(I)

cos (e)

0

6 is the corrected

The value Metal

line width

dispersion

and metal

volumetric

Torr pressure,

cooled

equilibrium.

for 20 minutes

The first measurement

gave total

The difference

the purpose

of obtaining

metal

ruthenium.

not give accurate

has been found

to obtain

dispersion. adsorbed

hydrogen

of ruthenium

[15] as CO may have both multiple

with

the metal

adsorbed

data. The ratio H/MS where

to be much

has been mentioned adsorption

metal

platinum

have also been

the ratio CO/MS

Unsuitability

[13]

there-

MS is surface

[14]. Attempts

assuming

as well as bridged

for

particle

will,

used for supported

lower.

dispersion

hydrogen

by Yang and Goodwin

of hydrogen

from CO chemisorption

manner.

the reversible

of chemisorbed

varies

it was

out in a similar

the second

ratio has been widely

at 50 Torr allowing

was complete,

It was shown

The total amount

were found

for the determination

while

by H2 chemisorption at 450°C at 1Cls4

introduced

was carried

to hold good for ruthenium

dispersion

1.0, but the results

was

adsorption

adsorption

dispersion

atom was taken as 1.0. This and also

After

were degassed

was taken as the amount

of reversibly

size of supported

made

relation

size were also determined

The samples

and again adsorption

adsorption.

that the amount

particle

apparatus.

to 25°C and then hydrogen

2 hours to attain

evacuated

fore,

using Warren's

of K = 0.9 has been used in all calculations.

using a static

about

obtained

to be

of CO chemisorption by Dalla Betta on the ruthenium

surface. XPS studies Binding

energy

were carried correction

out on a VG Scientific

due to charging

due to pump oil contamination were consistent

within

The XPS spectra Ru:Pt atomic

Ru lx=

ESCA-3

was done

and also by taking

Mark

by comparing

Si(2s)

II spectrometer. the C(ls)

peak

peak at 154 eV. The results

0.2 eV.

were resolved

ratio on the surface

into Gaussians was determined

and peak positions

were ascertained.

by using the relationship.

IRu ~Pt(Ert)"2 (3) IPt u~JER~)"*

72 Where

I, P and E

section

stand for the integrated

and kinetic

Pt (4f7,2) tables

energy

The cross sections

photoelectrons.

photoionisation

respectively

cross

for Ru (3d5,2)

were obtained

and

from Scofield's

[16].

A glass flow reactor

was used to study

The reactor

has been described

held within

f O.l"C using a liquid

passing

hydrogen

specially online

with

and Pines

Analysis

elsewhere

by pulsing

fittings

closed

between

was

calibration

containing

were

analysed

The reaction

pulsed-flow

benzene by an

was studied

microcatalytic

to that described

in situ by hydrogen

GC using CTR-I

with a standard

steel

in design

reduced

be by

3-10 per cent.

in a stainless similar

could

was controlled

glass vessel

W column.

hydrogenation.

temperature

and the products

0.5 ml of CO in a stream

was done by an online

after calibrating

[17]. The reactor

20M on chromosorb

K and total conversion

swage-lok

of benzene

bath. The flow of benzene

a thermostated

of CO was studied

[18]. The catalyst

was studied

the kinetics

for this. The reactants

10% carbowax

343-383

Methanation reactor

gas through

designed

GC using

between

TABLE

peak area,

of photoelectrons

of hydrogen

column

by Steingaszer

at 400°C. flowing

and hot filament

mixture

supplied

Methanation

at 25 ml/min. detector

by Altech

Corpn.

1

Metal dispersion

and particle

size of Ru-Pt/Si02

catalysts.

Dispersion(%) Composition

from chemisorption

of

Particle

size (nm)

Ru:Pt H2 chemisorption

co

H2

XRD Ru

1:O

24.0

7.1

1:0.25

35.0

9.4

3.00

l:la

16.8

8.0

5.83

large

large

I:1

18.6

5.23

14.8

12.3

0:l

16.4

6.8

aSample

prepared

RESULTS

AND DISCUSSION

Metal particle sorptions

by direct

by hydrogen

ruthenium

crystallite

size estimated an average

by

reduction

as determined

lower

Inadequacy

size has been pointed

XRD is much larger

in Table

results

It should

and CO chemi1. It can be

as compared

of CO adsorption

out by Dalla

than obtained

atom.

pre-oxidation.

by hydrogen

are presented

gave consistently

area of 8.17 A"'/surface

-

8.1

in Hz at 500°C without

line broadening

chemisorption.

12.4

6.91

size and dispersion

and from X-ray

seen that CO chemisorption obtained

3.45

Pt

Betta

for determining [IS]. The particle

by Hp adsorption be noted

to that

assuming

that XRD gives

the

73

(c 1

(d) I

I

I

48

45

42

39

F IGURE 1

XRD patterns

I

36

-

28

(0.25);

I

I

of the silica

supported

catalysts:

(c) Ru (1) Pt (I); (d) Pt. (Total loading

(a) Ru, (b) Ru (I) Pt

is 0.1 Imole

of metal/g

of

support).

estimate

of only those particles

diffraction, dispersion

that are large enough

but does not account

the Imethod of H2 adsorption

XRD patterns

of the various

Ru-Pt/Si02

It can be seen that the individual are retained

in the bimetallic

planes

clusters

(coclusters). loading,

platinum

which

and there

contains

is no indication

that the larger

particles

of Pt-Ru alloy

were

are presented

(TN)

1.

and platinum

to the results

of 1.2 mole/g,

the

in the XRD of Ru-Pt,iSi02, but a new to the fomation

however, separate

of formation studied

number

in Figure

of ruthenium

that at a Imetal loading

Our XRD results,

the catalyst

for obtaining

turnover

be preferred.

planes

they attributed

out here that Diaz et al. [I91 who

sisted

Hence,

This is in contrast

of Ru and Pt are not shown

line (2~) = SIG) appeared

mole/g

should catalysts

diffraction

catalysts.

et al. [II], who observed

individual

to be seen by X-ray

particles.

data that can be Imeaningfully used for calculating

of a reaction,

of Miura

for the mailer

clearly

show that even at 0.1

crystallites

of ruthenium

of coclusters.

Pt-Ru/A1203

those of platinum

of bimetallic

catalysts

and only

and that only 8.7“ of the particles

by TEM concluded

smaller were

and

It imay be pointed

particles

alloyed.

con-

74 TABLE 2 XPS binding

energies

for the various

catalysts.

Sample composition

Binding

(Ru:Pt) Bulk

Surface

1:o

energies

Rui3d5,2)

U (Is)

280.7

532.1

(ev) PW7,2)

282.4 1:0.25

1:0.33

280.1

532.0

71.1

532.3

71.3

530.3

71.2

281.4 283.1 I:1

1:0.67

279.5 281.1

l:la

1:0.54

281.0

531.9

aAfter treating

280 BINDING FIGURE

2

Pt(0.25),

Ru(3d)

with a I:3 mixture

285

70

290

ENERGY

BINDING

(‘3’)

XPS spectra

(C) Ru(l)Pt(l),

of CO and H,, at 400°C

of the silica

(F) sample

supported

C after treatment

for one hour.

7s

80

ENERGY (eV) catalysts:

_f

(A) Ru, (B) Ru(l)

with CO + H2 mixture

at

400°C.

FIGURE 3

Pt(4f)

(c) Ru(l)Pt(l), (F) sample

XPS spectra

of the silica

(E) Ru(l) pt(1)

C after treatment

prepared

with

supported

by reduction

CO + H2 at 400°C.

catalysts:

(6) Ru(l)

tiith H2 without

Pt(U.25),

preoxidation,

75

I

I

230

235

BINDING FIGURE with

4

O(ls)

XPS spectra:

ENERGY

(ev)

(C) Ru(l)Pt(l)

-

catalyst

before

and (F) after

treatment

CO + H2 at 400°C.

The results curve

of XPS studies

resolution

spectrum

are presented

(Figure

This is because Ru(3d3,2).

superimposed

by Miura

Hence,

and Gonzalez

in which

a core of ruthenium

dispersed Although

to be correct

catalysts. is coated

as can be seen from absence it showed

convincingly the dispersion

that preoxidation

of platinum

that are discussed

prior

3) is

enrichment

they

proposed

a model

is much more

for sample

E (Figure

was absent.

to reduction

of

[II]. This does

ruthenium

signal

to the

3).

This

is essential

for

on silica.

been seen from the XRD results

did not form coclusters.

is contrary

a crust of platinum

the Pt(4f7,2)

(Figure

intensity

on the surface,

surface this,

In any case,

of Pt XPS peaks

an Ru(3dg,2 ) signal,

demonstrates

It has already

with

for our samples.

ratio

This

a large

To describe

again

Ru(4s)

have been used. The results

ruthenium.

[9] who reported

region of the

(from pump oil) on the

Ru:Pt

and Pt(4f7,2) with

after

of the 3d,,, and 3d3,2 peaks.

value because

for obtaining

energies

at the Ru(3d)

of C(ls) peak

of the surface

for their Ru-Pt/Si02

binding

of Pt (4f7,2) and Pt (4f5,2)

peak areas of Ru(3dg,2)

platinum

not appear

2. Looking

from the expected

on Pt (4fg,2).

show a small enrichment report

ratio

The various

in the intensity

of the superimposition

deviation

the integrated

in Table

2) we see reversal

The intensity

shows a minor

are revealing.

The results

in the following

of benzene section

that ruthenium hydrogenation

also lead

and platinum on these catalysts

to the same conclusion.

TABLE

3

pH2 and pB are the partial

4.238 x w5 Ru(0.75) Pt(O.Z5)lSiO2

pressures

pH21*25

pH2“'

0.1

-2 1.146

5.273

ohs.

respectively.

0.493

-

-

cal,

-1 -1 s )

383 K

site

3.79 x rdQ.545

-

-

ca-l.

of benzene

TN

and benzene

4.8 x w2

II.2 x la-2

:.I31 x 10

ohs.

343 K

(molecules

of hydrogen

pB-"*12

pB

1.064 x IV4

PUSiD

‘2

&=

’

2 187 x lo-* pY2‘6

2

Rl&%iO

at 343 K

hydrogenation.

Kinetics

data for benzene

Catalyst

Kinetic

15.7

15.2

19.1

Ea Kcai /tit01

Preexponential

4.88

5.58

x 10

a

x vJ8

1.14 x IO'0

factor A molecule -1 -1 site s

77

If

no caclusters

different

are formed,

When the catalyst

C is treated

the following

changes

in intensity

indicating

is somewhat

Pt(4f& O(ls)

the surface

from the bulk composition.

spectra

carbon

reduced.

I:3 mixture

hydroxy

The C(ls) signal

on the surface.

is clearly

change

split

to be much

with our XPS results.

of CO and Hz at 400°C for 2 h,

in the XPS spectra.

The most drastic

of some surface

is not expected

is in fair agreement

deposition

(Figure 4), which

to the formation

with

are noticed

composition

This

increases

The intensity

is, however,

into a doublet,

of the

noticed

most

likely

in the due

species.

-T(K) 383373363353343

4.8 v

2.5

3.0

$1) FIGURE

5

Arrhenius

plots far the hydrogenation

Ru, (B) Ru(l)Pt(0.25),

Benzene

on the catalysts:

(A)

(0) Pt.

hydrogenation

Kinetic

studies

in the activity limitations. catalysts Turnover

of benzene

were conducted

vs. temperature

Such behaviour

[20,21]. numbers

on the surface

between

curve occurred

has been observed

The results

of the kinetic

(TN) were obtained

from hydrogen

343-383

by knowing

chemisorption.

of Ru and Pt atoms on the surface

K. At high temperature

which

a maximum

is not due to thermodynamic

earlier

on several

studies

are summarised

the total number

supported in Table

of metal

3.

atoms

In order to know the separate number

for the Ru-Pt/Si02

catalysts,

the information

on Ru/Pt ratio on the surface

as obtained

sorption data that gave the total a metal: H ratio 1:l). This from the TN values

number

enabled

accepted

reaction rate is independent between the experimental

and calculated evidence

alloying of the metals,

it is most

Figure 5 shows the Arrhenius is the more active at higher

TN values

of nonformation

plots.

catalyst

catalysts.

numbers

(assuming

The agreement (Table

i.e., the

the good agreement

on the bimetallic of coclusters.

In the low temperatures

catalysts

Had there been

the catalytic

and ruthenium

catalysts

are quite good

is facile,

size,

to affect

573

623

673

TEMPERATURE(K) Conversion

and Pt/Si02

turnover

particle

likely

on the surface

’

activity.

of our experiment,

is the least active.

This

trend

temperature.

523

FIGURE 6.

atoms

with chemi-

the TN on the bimetallic

that this reaction

of the metal

may be taken as kinetic

is reversed

Ru/Si02

and the estimated

3). Since it is generally

platinum

of metal

to estimate

of the individual

between the experimental

from XPS has been combined

of CO to methane

723 -

at various

temperaturesQRu;@Pt;

A

Ru(1)

Pt(l). Methanation

of CO

The results of methanation catalyst shows a much the activity

conversion

of the Ru-Pt/SiO*

Although we did not attempt qualitatively

of carbon monoxide

higher

similar

that the ruthenium

is very similar

to calculate

is shown

the turnover

crystallites

6. Ru/Si02

or Ru-Pt/Si02.

to that of the platinum

to that of Miura and Gonzalez

and platinum

in Figure

to CH4 than Pt/Si02

number, [9]. If

are acting

Also

catalysts.

this result

is

it is considered

independently

as catalysts,

79 a much

higher

It appears

the ruthenium neither

than the observed

that the presence sites,

be explained

a point

conversion

of platinum that needs

by assuming

is expected inhibits

further

cocluster

for the bimetallic

the methanation

investigation.

catalyst.

of CO even at

The results

can

formation.

CONCLUSIONS Ruthenium, silica

platinum

and ruthenium-platinum

have been prepared.

of Ru-Pt coclusters.

This

Careful

hydrogenation,

very close

to the bulk composition.

inhibits

the methanation

XRD studies

is also supported

of benzene

bimetallic

XPS results

showed

catalysts

the absence

by the results

indicate

The presence

of platinum

on

of the formation

of the kinetic

that the surface

of CO at the ruthenium

supported

study

composition

is

on the surface

sites.

ACKNOWLEDGEMENT The authors Science

gratefully

and Technology,

acknowledge

a research

New Delhi which

helped

grant

them

from the Department

in carrying

of

out the present

work.

REFERENCES

1 2 ; Z 7 a 9 IO 11 12 13 14 15 16 17 ia 19 20 21

D.F. Ollis and M.A. Vannice, J. Catal., 37 (1975) 449. M.A. Vannice, J. Catal., 37 (1975) 462. J.A. Rabo, A.P. Risch and J.L. Poutsma, J. Catal., 53 (1978) 295. G.G. Low and A.T. Bell, J. Catal., 57 (1979) 397. R.A. Dalla Betta and M. Shelef, J. Catal., 49 (1977) 383. J.G. Ekerdt and A.T. Bell, J. Catal., 58 (1979) 170. H. Miura, M.L. McLaughlin and R.D.Gonzalez, J. Catal., 79 (1983) 227. P. Ramamoorthy and R.D. Gonzalez, J. Catal., 58 (1979) 88. H. Miura and R.D. Gonzalez, J. Catal., 74 (1982) 216. H. Miura and R.D. Gonzalez, I. and E.C. Prod. Res. Dev ., 21 (1982) 274. H. Miura, T. Suzuki, Y. Ushikubo, K. Sugiyama, T. Matsuda and R.D. Gonzalez, J. Catal., 85 (1984) 331. M. Boudart in 'Adv. Catalysis', ~01.20, Acad. Press, New York, 1969, p.153. C. Yang and J.G. Goodwin, J. Catal., 78 (1982) 182. J.G. Goodwin, Jr., J. Catal., 68 (1981) 227. R.A. Dalla Betta, J. Catal., 34 (1974) 57. J. Scofield, J. Electronspectrosc., 8 (1976) 129. S.P. Sivanand, R.S. Singh and D.K. Chakrabarty, Proc. Ind. Acad. Sci., (Chem. Sci.,), 92 (1983) 227. P. Steingaszer and H. Pines, J. Catal., 5 (1966) 356. G. Diaz, F. Garin and G. Maire, J. Catal., 82 (1983) 13. J.M. Orozco and G. Sebb, Appl. Catal., 6 (1983) 67. K. Yoon and M.A. Vannice, J. Catal., 82 (1983) 457,