The catalytic hydrogenation of benzene over supported metal catalysts.

Applied Catalysis, 30 (1987) 339-352

339

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

THE CATALYTIC

HYDROGENATION

GAS-PHASE

III.

M.C. SCHOENMAKER-STOLK,

Department

J.W. VERWIJS

METAL

CATALYSTS.

OVER SILICA-SUPPORTED

and J.J.F.

Ru-Cu

CATALYSTS

SCHOLTEN*

Delft University

of Technology,

Julianlaan

136,

The Netherlands.

to whom all correspondence

Dept. of Catalysis,

(Received

OVER SUPPORTED

OF BENZENE

Technology,

of Chemical

2628 BL Delft, *Author

OF BENZENE

HYDROGENATION

Central

8 September

should

be addressed:

Laboratories,

DSM, Geleen,

1986, accepted

23 October

also affiliated

with the

The Netherlands.

1986)

ABSTRACT A study has been made of the gas-phase hydrogenation of benzene over silicasupported Ru-Cu catalysts in the temperature range from 300 to 400 K and at a total pressure of 130 kPa. Special attention has been given to the catalytic stability and the activity of the catalysts as compared with monometallic supported ruthenium and copper catalysts, described in previous articles. Via a reductive deposition technique, copper was deposited on silica-supported ruthenium catalysts. Two bimetallic catalysts were prepared, one with about 10% of the ruthenium surface covered by copper and one with about 70% copper coverage. The texture of the catalysts was studied by means of mercury penetration, nitrogen physisorption, hydrogen chemisorption and transmission electron microscopy, whereas surface analysis was performed by XPS/AES. At 11% copper coverage a shift in the copper electron binding energy is observed, pointing to a small electron transfer from ruthenium to copper, but at 68% copper coverage this shift has disappeared. Copper deposition on ruthenium increases the lifetime of the catalysts considerably without seriously affecting the activity per ruthenium surface atom. At low copper coverage the reaction kinetics are very similar to the kinetics found with ruthenium, but at high copper coverage the small size of the ruthenium ensembles influences the kinetics considerably.

INTRODUCTION In previous genation special

articles

of benzene attention

Our experiments

over

silica-supported

to the reaction showed

the hydrogenation

[I,21 we reported

ruthenium

of benzene,

the use of copper

shows perfect

catalytic

though

very active,

adsorbed which

byproducts,

are formed

0166-9834/87/$03.50

copper

exhibits

as a catalyst

from benzene

and the reaction article

catalysts,

giving

catalysts

in

very poor activity.

are formed

and this is mainly

hydro-

of the reaction.

has some advantages

and no byproducts

like cyclohexylcyclohexane

It is the aim of the present

and copper

and the kinetics

to be one of the most active

stability

is unstable,

ruthenium

mechanism

whereas

Nevertheless,

on our study of the gas-phase

as this metal

[Z]. Ruthenium,

due to the formation

of strongly

and 1,4-dicyclohexylcyclohexane, intermediate

to report

cyclohexene

on an investigation

0 1987 Elsevier Science Publishers B.V.

[3]. of the

340 catalytic

performance

as a support

of bimetallic

in the hydrogenation

far, by combining

both metals,

and catalytically

stable

The hydrogenation for many years. palladium,

and Allison catalytic surface

and which

catalysts

activity

with

increasing

proportional surface.

increasing

[5], in studying

platinum-gold,

gold percentage.

to the extent

An increasing

gold coverage

of byproducts.

platinum

and

19 metals like copper and gold. Ponec [4] platinum-gold observed

residual

of the uncovered

apparent

activation

[4]. Furthermore,

did not find a stabilizing

catalysts, a decreasing

Gold accumulates

VIII metals, but the attending

is not simply

is both active

has been investigated

like nickel,

and Bond [6] for the case of palladium-gold,

of these group

on silica

to know in how

which

in the formation

VIII metals,

and Teichner

and palladium with

It is interesting

is inactive

with group

Tournier

and copper

can be obtained

over bimetallic

group

In most cases

of ruthenium

of benzene.

a catalyst

of benzene

are combined

and Hoang-Van,

catalysts

at the

catalytic

activity

part of the platinum energy

is observed

Hoang-Van

et al. in studying

of gold

in the hydrogenation

effect

of

benzene. In the literature copper.

much attention

As found for platinum-gold

in the hydrogenation ratio,

but this

with

increasing

Concerning system

content,

then passes

smoothly

to about

activation

the apparent

Much

on the behaviour

[lO,ll],

of ethane

The mutual an extremely

however,

solubility

of ruthenium

the ruthenium-copper

According

to Christmann

coverages

copper

coverage

of nearly

size effect

coverage [3,15].

with

known.

at the surface

on ruthenium.

is formed, islands

of

bond [II]. At low

but increasing

and only at a coverage

starts. may evoke the so-called

that on small

in the hydrogenation is sterically

are well

than the copper-copper

of copper

copper

It is to be expected

of byproducts

dicyclohexylcyclohexane, be stable.

of ruthenium

reactions

[I21 on the hydro-

is chemisorbed

of multilayers

investigated.

catalytic

is very low: in the phase diagram

of copper

in the formation

one the formation

A partial

formation

results

dispersion

in other

[lS]. However,

bond is stronger

catalyst

has not yet been

by Sinfelt

and copper

et al. [14] copper

a homogeneous

of the nickel-copper when the copper/nickel

of cyclohexane

gap is observed

ruthenium

[9].

increases.

of ruthenium-copper

and the studies

increases

at 25% copper,

of the bimetallic

of benzene

and on the dehydrogenation

wide miscibility

copper

energy

performance

in the hydrogenation

is available

the behaviour

and palladium-gold:

activation

As far as we know, the catalytic

a maximum

around

than that of pure first

zero for pure copper

energy,

that of platinum-gold

activity

At high temperatures, is higher

through

nickel-

of the copper/nickel

that at 425 and 463 K the activity

ruthenium-copper

genolysis

system

the catalytic

an increase

combination

falls

ratio is increased

information

with

copper

the apparent

resembles

decreases

of the nickel-copper

[7,8]. Bond reports

and subsequently

to the bimetallic

is the case only at low temperatures.

600 K, the activity nickel

of benzene

is given

and for palladium-gold,

ruthenium

of benzene,

hindered,

ensemble

ensembles

like cyclohexyl-

so that catalytic

the and 1,4-

behaviour

will

341 From the literature on the activity XPS core level found

binding

of electron

upon deposition which

the impression

of surface

energies

transfer

of copper

is an indication

We conclude

ruthenium

that the influence

in the ruthenium-copper

from copper

to ruthenium

on a clean Ru(0001)

of a slight

that significant

ruthenium-copper

is gained

atoms and vice versa

electron

electronic

system

transfer

From

no evidence

or vice versa

surface

effects

of copper

is very small.

the work function from ruthenium

in the catalytic

was

[15]. However, increases,

to copper

behaviour

[141.

of

are not to be expected.

EXPERIMENTAL Materials supplied

"fur die Spectroscopic",

Benzene, by adding

an excess

tillation

in an ultra-pure

a blanketting

of Drina,

by Merck,

a 9:l Pb/Na alloy

FRG, was further

from Merck,

Pyrex glass apparatus

followed

under oxygen-free

purified

by dis-

nitrogen

as

gas. The last traces

of sulphur-containing components were removed -3 of reduced ruthenium powder (Drijfhout, by slurrying the benzene with 4 g dm 2 -1 The Netherlands), with S(BET) = 6 m g , under dry, oxygen-free nitrogen for 30 min. This leads to a final

concentration

of sulphur-containing

components

of

less than 0.3 ppm C171. Hydrogen, by passing pure,

99.9%

pure, from Hoek Loos, The Netherlands,

it over a Pd/A1203

catalyst

from Hoek Loos, The Netherlands)

a finely

dispersed

gases were dried As support diameter

copper-on-silica

over molecular

material

(BASF, FRG, type R-020). was made oxygen-free

catalyst,

sieves

we used Shell

was purified

from BASF,

Helium

by passing

from oxygen (99.9% it over

FRG, type R-3-111.

Both

type 3A.

silica

spheres,

3 mm. According to the manufacturer 2 -1 = 130 m g .

type CLA 33569,

the average

mean

pore diameter

particle

is 53 nm

and S(BET)

Ruthenium and copper

Catalyst

trichloride

nitrate

was obtained

First two supported

nation)

[18]. After

Next, heated

catalysts,

by means

determination

water

The Netherlands,

The Netherlands.

solutions

were

slowly

of the pore volume

the samples

were dried

point,

was added.

of, respectively, added with

viz. 0.5 wt% Ru-on-silica

of the incipient

up to the caking

equal to this pore volume

trichloride solutions

ruthenium

were prepared

distilled

solution

from J.T. Baker,

by Drijfhout,

preparation

Ru-on-silica,

adding

(spec pure) was supplied

wetness

of the dried

a volume

For the two catalysts

stirring

and 3 wt% (dry impreg-

support

of ruthenium

0.06 M and 0.34 M were

continuous

method

by

trichloride ruthenium

used. These

of the pre-dried

silica.

in air at 400 K for 17 h and then the catalysts

were

to 673 K in a stream of hydrogen (60 cm3 (STP) min-') at a heating rate -1 . The reduction was continued for 3 h at 673 K. Finally the catalysts

of 0.5 K min were cooled

to room temperature

in hydrogen.

Cu2+ concentration in solution (Mx103)

1

time (minl FIGURE

1

The decrease

preparation

of catalyst

Starting prepared placed

of the Cu

from these

under oxygen-free catalyst

ruthenium

nitrogen

surface

the copper

Provided

taken

in the copper

model

1 represents

deposition

almost

linearly

mining

step,

with

physically

distilled adsorbed

(STP) min-') temperature

water,

was slowly

in order

reduction

replaced

and,

by air.

absorption

particles

was

of the copper

ion

that practically

during

the catalysts possible

the reductive

reduction

were

traces

the catalysts

decreases

being the rate detersurface. thoroughly

of copper

were then dried

out in a stream

in order to passivate

all

spectro-

concentration

ions to the ruthenium

to remove

copper-

line at x = 745 nm. Figure

of copper

The catalysts

was carried

at 673 K for 3 h. After in hydrogen

It appeared

to chemisorption

on the support.

for 17 h, after which

decrease

ion concentration

of copper

the

of

pre-chosen

Use was made of a Beckman

had been completed,

with vigorous

in the introduction,

of the ruthenium

solutions.

B).

surface.

B). The fact that the copper

of diffusion

deposition

area is known,

measured

the copper

time points

instead

copper

nitrate

of the copper

(catalyst

through

and as the recipient

surface

up by the surface

(catalyst

the Cu 2+ ions to Cu , the

on the ruthenium

35, monitoring

with

were

3 wt% Ku-on-silica

solution

in this way. As stated

was deposited.

the decrease

copper

nitrate

was bubbled

catalyst

the free-ruthenium

can be realized

of copper

catalysts

the reduced

reduces

from the spectrophotometrically

photometer,

washed

as a reduction

from the solutions

After

hydrogen

atoms are chemisorbed

concentrations copper

acting

ratios

The amount calculated

A). Likewise

kept at 315 K and hydrogen

ruthenium

atoms.

ruthenium-copper

catalysts,

(catalyst

Under these conditions

to-ruthenium

the

spectrophotometrically.

to a 100 cm3 2 x 1o-3 M copper

were

during

0.5 wt% Ru-on-silica catalyst was -5 M copper nitrate solution, 100 cm3 of a 6 x 10

stirring.

copper

in the solution

way. The reduced

filled with

was added

Both solutions

concentration

6, measured

in the following

in a vessel

2+

of hydrogen were

the samples,

nitrate

in air at 400 K

cooled

(60 cm3 to room

the hydrogen

gas

343 Texture

and surface

The pore volume penetration

composition distribution

type 2000, from Carlo Free-metal

surface

chemisorption

Erba,

previously

Leybold

Heraeus,

were

from the extent

from nitrogen

area of a nitrogen

carried

out in a type LHS-10

FRG. A MgKu excitation

source

or prereduced

at 670 K in a 90% Ar + 10% H2 mixture

TEM micrographs Philips

of the reduced

EM 420T apparatus.

methylmethyacrylate slices

of 13 kV and 20 mA. Samples

to the XPS/AES

hydrogen

chamber

The catalysts

were cut with an ultramicrotome

apparatus

from

eV) was applied in vacua

in a preparation

chamber,

mixture.

at

UHV lock.

catalysts

(Ultracut

at 78 K,

pretreated

were crushed

+ 30% butylmethacrylate

1253.6

were

via a valveless

and passivated

isotherms

0.162 nm2.

XPS/AES

(energy

conditions

was connected

of strong

adsorption molecule

the operating

which

of mercury

[I].

areas were calculated

analyses

by means

Italy.

for the cross-sectional

Surface

was determined

from 0.1 to 200 MPa, using a Porosimeter,

areas were determined

as described

BET surface taking

of the silica

in the range of pressures

were taken with a

and embedded After

curing,

E, Reichert)

in a 70% 70 nm thick

with a diamond

cutter.

Hydrogenation

equipment

The performance were tested

of the catalysts

in continuous

article

[I]. The total pressure

studied

in the temperature

stream was analysed and a Hewlett graph

in the gas phase hydrogenation

flow fixed

was kept constant

gas chromatographically,

Packard

equipment

electrometer,

type 5704A.

a flame

The column

W-HP,

from Digital

was

of the product

ionization

detector

in the gas chromato-

of 3 mm;

on Chromosorb

with a microprocessor

in a previous

at 130 kPa and the reaction

applying

had a length of 4 m and an inside diameter

integrated

of benzene

described

range from 290 to 400 K. The composition

30% 1,2,3-tris(2-cyanoethoxy)propane were

bed reactor

it was filled with 80-100

Equipment

mesh. The peaks

Co. USA, type LCI

11/03.

RESULTS

AND DISCUSSION

Catalyst

characterization

Data on the composition The BET surface

of 0.5 wt% of ruthenium by the subsequent catalyst

deposition Mercury

2 in Table

of copper B, with

on S(BET)

is very

penetration

with

showed

with

of the catalysts

was not measurably

are summarized influenced

1; precursor

(see catalyst

3 wt% ruthenium,

but it is to be expected

distribution

accordance

(see column

deposition

B and for catalyst

not measured,

volume

and texture

area of the support

in Table

of catalyst

A), nor

A). For the precursor the BET surface

that the influence

of

area was

of ruthenium

and copper

small. the support

a maximum

the value mentioned

and catalysts

at a pore radius by the supplier.

1.

by the deposition

to have a narrow

of 50 nm (column

pore

3), in

344 TABLE

1

Characterization

of the support,

copper

catalysts.

Column

1

Sample

silica

support

precursor

the ruthenium

precursor

catalysts

2

3

4

5

6

7

S(BET)a

dpb

SHc

DMd

Cu/Rue

Cu/Rusf

and the rutheniur-

8

9 dVS

fromg

fromh

H ads

TEM

115

50

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

4.8

2.6

of

catalyst

A

115

50

100

0.28

0

0

catalyst

A

115

50

n.m.

0.28

0.031

0.11

precursor

2.6

of

catalyst

B

n.m.

n.m.

80

0.224

0

0

6.0

catalyst

B

n.m.

n.m.

n.m.

0.224

0.154

0.618

2.5 2.5

n.m. = not measured. n.a. = not applicable. aBET surface area per gram of catalyst b Mean pore diameter in nm. 'Free-ruthenium surface d Metallic dispersion. eCopper/ruthenium f Copper/ruthenium gVolume-surface h Volume-surface

in the metal

-1

g

.

from hydrogen

calculated

from TEM micrographs,

metal

After

particle

surface

areas

deposition

of copper

size distribution

for catalysts

in mind the immiscibility surface

of ruthenium

distinct

from catalyst coverage

on the precursor

dispersions

with

in nm.

in nm.

4) the metallic

by the total number

was observed

of ruthenium

dispersion,

of metal

catalysts,

the electron

DM,

atoms, no change microscope;

as found for the precursor

in the bulk, we

from the total amount

1). In calculating

per m2 was assumed.

A in that the ruthenium as high.

and copper

on ruthenium

7 in Table

atoms of 1.63 x 10"

6.2 times

chemisorption,

A and 6.

coverage

taken up (see column

a number

(column

atoms divided

1 the same metallic

the copper

the copper

2

calculated

in Table

atoms

in m

mean diameter,

are mentioned

copper

.

ratio on the surface.

catalysts

calculated

-1

mean diameter,

therefore

Bearing

g

ratio.

of surface

was calculated.

2

area per gram of ruthenium

From the free-ruthenium the number

in m

of

this coverage, Catalyst

B is

load is six times as high and

345

FIGURE

2

TEN micrograph

of catalyst

A.

346

FIGURE

3

TEN micrograph

of catalyst

B.

347

In columns 8 and 9 of the table the volume-surface ruthenium

as calculated

crystallites,

chemisorption the values ruthenium

and from the total

found

from electron

catalysts

TEM is much sorption.

A TEM micrograph

which

A is given

was calculated

from this photograph

that the metal

particles

The TEM micrograph of 2.5 nm is arrived

A, which hardly

of catalyt

means

hydrogen

hydrogen surface

2. The $S

chemibeing

value of 2.6 nm

of 500 particles.

distributed

loading

dispersion;

and

chemisorption.

It is seen

over the support.

in Figure

3. From this a dVS value

excluding

is very near to the value

that the higher metal the metal

for the copper

crystallite

at from a sample of 350 particles, This value

with

[1,21, dVS found from

to hydrogen

from a sample

B is presented

are compared

from strong

in Figure

are homogeneously

in clusters.

influenced

calculated

part is inaccessible

of catalyst

of the strong atoms,

articles

to a large part of the metal

bound to the support,

of the

Just as reported

in our previous

than the dVS value

This points

of ruthenium

microscopy.

described

smaller

concentrated

from the extent

number

mean diameters

(by a factor

it only resulted

the particles

found for catalyst

six) for this sample in clustering

of part

of the crystallites.

XPS/AES

surface

XPS/AES

analysis

spectra

of the reduced

show peaks of silicon, of reference extremely

samples.

oxygen

Furthermore,

small amounts

The binding are presented

catalysts

and ruthenium

energies in Table

of sodium

A and B (see Figure in normal

indications and carbon

of electrons

4, catalyst

positions,

equal

are found of the presence

levels

2.

4-

3-

CPSxlO4

260

520

100

1040

k.energy eV FIGURE

4

XPS/AES

spectrum

of the reduced

of

impurities.

from the 2P,,2 and 2P3,2

5-

0

B)

to those

catalyst

B.

1300

of copper

348 TABLE

2

Cu(ZP,,2)

and Cu(ZP3,2

) electron

binding

energies

(in eV), for reduced

catalysts

A and 6. Electron

Catalyst

A

Catalyst

B

Pure copper

level

(reference

cm,

,?I CU(2P3,2) For catalyst energy

952.5

952.5

928.9

932.6

932.6

A, with a copper

shifts

whereas

949.3

for catalyst

electron

binding

follows.

As stated

distribution

coverage

of -3.2 eV (2P,,2 B, with

energy

is absent.

in the introduction, copper

At a coverage

of 0.11 these copper

copper-copper

interaction

(see introduction)

observed copper

coverage

copper-copper present, energy

binding

islands

exists.

Though

interaction

bonding

largely

electron

this is the reason For catalyst

are present

surface. and therefore

of the work function

a slight

energies.

transfer for the

B, with

and now a strong

to the ruthenium

compensates

as

a homogeneous

as "singletons"

From the increase

and apparently

of 0.68, copper

interaction

and activity

In Figure

are given

of reaction

in the legend

0.5 wt% Ru-on-silica cations

of the catalysts

5 the turn-over

as a function

[1,2],

per number Copper

the high

lateral

surface

the energy

shift,

is still

and the

of ruthenium

surface

(0.68,

catalyst

in the hydrogenation

In the presence hydrogenation

of copper

activity,

site are initially

silica.

Comparing

equal

to the initial

results

numbers

striking

publi-

are calculated

facts

emerge:

(0.11, catalyst effect

for

A) and

on the catalytic

of benzene.

activity catalyst

On the other activity

are plotted

conditions

in our previous

B), has a stabilizing

copper

being a metal with

of catalysts

of the same order

the initial

catalyst.

The following

on ruthenium,

of 0.5 wt% Ru-on-silica,

ruthenium

reported

5. The turn-over

both at low coverage

the activities

surface

value

atoms.

of benzene

A and B. The reaction

For the sake of comparison,

also in Figure

on ruthenium,

hydrogenation

for the hydrogenation

and 14 wt% Cu-on-silica,

chemisorbed

activity

numbers

in benzene

time for catalysts

to the figure.

are plotted

at high coverage

b)

occurs,

coverage

on the ruthenium

atoms are present

in this situation

of 0.68, an

can be explained

levels are the same as found for pure copper.

Stability

a)

is present

binding

level) are observed,

on ruthenium

at low copper

atoms

of the electron

this lateral

coverage

This phenomenon

is impossible.

we know that

to copper

lowering

of only 0.11, electron

level) and -3.7 eV (2P3,2

the high copper

shift

of chemisorbed

from ruthenium

on ruthenium

sample)

a very

low

A and B per ruthenium

as that of pure ruthenium-on-

of catalyst

A with

A is 2.6 times

hand, the activity

of the ruthenium

the initial

as active

of catalyst

catalyst.

activity

as monometallic B is nearly

349

I

I

I

I

catalystA -2 c

-6 LnTON (moleculesC6H6. sited.s-'I

-a

FIGURE

5

Turn-over

a function

total pressure pressure

numbers

of reaction

(molecules

C6H6 per ruthenium

-1

In our view the strong gain in catalytic copper

coverage

and its homogeneous

be of the order

by-products

for which

ensembles

the experiments are perfectly

of circa

from ruthenium

possible effect

to copper

ratio at the surface

formed surface higher

in Figure

the ensemle

This hampers

A and its 11%

size will

the formation

of large

and 1,4-dicyclohexylcyclohexane, atoms

are necessary.

5: the catalysts

as, apart

covered

A (ensemble

higher

(see ref. [14]),

We finally as

with copper

strongly

size effect),

ruthenium

level of catalyst

catalyst.

be operative

explanation

adsorbed contrary

transfer

the copper/ruthenium

effect would

step. The following

level of catalyst

activity

from the fact that the electron

is 0.11 and an electronic

of a monometallic activity

the somewhat

is very small

start of the reaction,

on catalyst

is more

by-products

This then means

likely.

cannot

to the situation

A is in fact the activity

at a

be

on the

that the somewhat

level of a really

surface.

Next the question

arises

why the turn-over

2.5 times as low as for catalyst chemisorption

A, with

out at 400 K led to the same conclusions

to ascribe

of at most one atomic

pure ruthenium

2 kPa, helium

for both catalysts

For catalyst

distribution,

atoms.

16 and 24 ruthenium

carried

at 300 K, plotted

A to an electronic

From the very

copper

as 300 K,

stable.

It is hardly

distance

stability

like cyclohexylcyclohexane

that experiments

Temperature

.

size effect.

of 15 to 20 ruthenium

poisoning

remark

to the ensemble

site per second)

of benzene.

37 kPa, benzene, pressure

pressure

66 kPa, flow rate 35 cm3 (STP) min

B has to be ascribed

60

48

36

time for the hydrogenation

105 kPa, hydrogen

I

I

I

I

I

12 24 reactiontime lhrsl

0

of hydrogen

being

number

A. We speculate hampered

found

for catalyst

B is

this to be due to the dissociative

by the very small

size of the ruthenium

350 ensembles

on catalyst

down. According adjacent

TABLE

B, by which

to Shimizu,

ruthenium

hampering

Christmann

the rate of chemisorption

and Ertl

atoms are involved

[I91 ensembles

in dissociative

is slowed

of up to 5-10

hydrogen

chemisorption.

3

Reaction

orders

in the hydrogenation

of benzene

over catalysts

the reference‘catalyst

0.5 wt% Ru-on-silica

velocity

over catalyst

A: 5.6 x IO4 cm3 (STP) rns2 Ru.h-',

catalyst

B: 1.6 x IO4 cm3 (STP) mm2 Ru.h-'.

case by adding

[I]. Total

Total

A and B and over

pressure

pressure

130 kPa, space

space velocity

over

was reached

in each

helium.

Catalyst

Temperature

Order

Order

in C6Hga

in Hpb

/K A

303

-0.2

1.4

A

400

0.3

2.2

B

303

-0.4

0.3

B

400

0.6

1.5

reference

303

0.0

1.2

reference

400

0.2

2.1

aWithin

the range of benzene

pressure b

Within

pressures

from

1.9 to 7.9 kPa and at a hydrogen

of 79 kPa.

the range of hydrogen

pressures

from

13 to B9 kPa and at a benzene

pressure

of 5 kPa.

Kinetics

of the hydrogenation

Reaction in exactly lysts

orders

in benzene

and in hydrogen

the same way as described

[1,23.

mentioned

of benzene

Results

are given

in the legend

were determined

at 303 and at 400 K

for the case of ruthenium

in Table

to the Table.

3. Further

The results

experimental

and copper conditions

for pure Ru-on-silica

cataare

are added

as a reference.

In view of the small pressure of the orders

is relatively

for silica-supported strongly

influenced

catalyst

A.

For catalyst

ranges

of benzene

low. The results

pure ruthenium.

It follows

by the low copper

coverage

B the results

deviate

sample

order

in hydrogen.

A direct explanation

might

be due to hampering

of the dissociative

small

ruthenium

on catalyst

ensembles

and with catalyst

on the ruthenium

are not

component

from the results

A, especially

is not at hand,

B.

the accuracy

A are very near to those

that the kinetics

significantly

with the reference

and of hydrogen

for catalyst

hydrogen

with

of

obtained

respect

to the

but this phenomenon chemisorption

on the very

351

1000/T

[K-l) reference

Ru sample

-4 -8

In k(molecules [PCgHglaX

ln k

C,5Hg.

s-’ ,divided

site“.

by P 1

(pH2]

reference

-fl -l2

-10

I 1000/T

FIGURE

6

over the reference

catalyst

hydrogen

pressure

velocity

over catalyst

over catalyst

(K-l)

plots of benzene

Arrhenius

Ru sample

hydrogenation

0.5 wt% Ru-on-silica.

62 kPa, benzene

pressure

over catalysts Total

A and B and

pressure

6 kPa, helium

130 kPa,

pressure

62 kPa, space

A: 4.5 x IO4 cm3 (STP) mm2 Ru. h-' and space velocity -2 h-1 B: 2.2 x IO4 cm3 (STP) m . pi is the variable order in benzene

and b is the variable

order

in hydrogen.

curve B is measured

over catalyst

reference

All rate constants

catalyst.

Curve A is measured

B and curve

Ru/Si02

over catalyst

is measured

have been calculated

A,

over the

per free ruthenium

site.

In Figure 6 an Arrhenius catalysts that,

A and B; results

plot is presented for the reference

apart from the level of activity

energy

in the case of catalyst

reference

sample.

This

respect

to the gradual

change

On the other the orders

of sample

are included.

of the apparent

over

It is seen

activation

observed

for the

in ref. [I]. We conclude

the behaviour activation

of the reference

hand, the deviation

in benzene

orders,

of the apparent

from the behaviour

hydrogenation

equal to the course

is explained

just as was the case for the reaction

appreciably

catalyst

the course

A is about

last course

for benzene

of catalyst

energy

monometallic

B is appreciable,

that, A with

does not deviate

ruthenium

catalyst.

just as found for

and hydrogen.

CONCLUSIONS An attractive of copper crease

atoms

method

of 1ifetime)of

of activity

of covering

has been developed. the catalyst

per ruthenium

site.

ruthenium

catalysts

with

pre-chosen

In this way a high catalytic is achieved,

without

It is to be recommended

an appreciable to choose

quantities

stability(inchange

a low copper

352 coverage;

higher

coverages

have a beneficial

effect

as well,

but the decrease

in

activityper unit weight of catalystis unnecessarily high and the kinetics start to deviate copper

strongly

from the behaviour

and its influence

ensemble

size effect

According

to copper

in ruthenium-copper of low copper

by Christmann effect

[16] no direct

We arrived

action

of

on the basis of the

at low coverage,

This is in accordance

et al. [14]. A significant

on the catalytic

evidence

performance

of an electron

XPS core level

at the same result,

This may be accounted

interaction

interaction.

can be explained

is found from the copper

catalysts.

coverage.

ruthenium-copper copper

on the kinetics

The stabilizing

theory.

to Helms and Sinfelt

from ruthenium

of ruthenium.

except

for by the strong together

with

influence

energy

for the case

chemisorptive

the absence

with the work function

transfer

binding

of copper-

results

published

of the very weak electronic

is not observed.

ACKNOWLEDGEMENTS We thank Mr. A.P. Central

Laboratories,

analyses.

Pijpers

and Mr. J. Cremers

DSM, Geleen,

The Netherlands)

We also wish to thank Mr. A. Pijpers,

TEM micrographs.

Thanks

Delft University

of Technology,

(Department

for carrying

Chemistry,

out the XPS/AES

from the same department,

are due to Mr. J. Teunisse for assistance

of Physical

and Mr. N. van Westen,

for the from

in the study of the texture

of the

catalysts. The investigations Chemical

Research

the Advancement

were

supported

(partly)

(SON) with financial

of Pure Research

by The Netherlands

aid from The Netherlands

Foundation Organization

for for

(ZWO).

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