silica catalysts

175

Journal of Molecular Structure, 80 (1982) 175-179 Elsevier Scientific

Publishing Company,

AN INFRARED STUDY OF SPECIES ON PALLADIUM/SILICA

D.I.

Amsterdam

- Printed in The Netherlands

FROM ETHYLENE,

HYDROGEN

AND CARBON MONOXIDE

ADSORBED

CATALYSTS

JAMES+ and N. SHEPPARD

School of Chemical

Sciences,

University

of East Anglia,

Norwich

(Gt. Britain)

ABSTRACT Two different give different

procedures,

infrared

from the adsorption from ethylene dominant

of ethylene,

is probably

hydrogen

preparations.

strong bands are tentatively adsorbed

species

in the accessible

region above

1300 cm

-1

surface

species,

A n-species

On preparation

species

interpreted

(III).

on silica catalysts

different

or carbon monoxide.

the o-diadsorbed

non-associatively

palladium

and to a large measure

is found on both catalyst

species

additional

A and B, for preparing

spectra,

(I)

A the other

On preparation

(II).

B

in terms of a doubly T- bonded

Only preparation

B gives absorptions

which can be attributed

to adsorbed

hydrogen.

-T_CH* H2C

HC7-,-CH

HC -CH 21 12

$d

Pd

(1)

Pd

'Pd

P/d

(II)

(III)

INTRODUCTION The infrared

spectrum

been extensively spectra

substituted surface

adsorbed

and hydrogen,which

The spectra molecules

features

adsorbed

on silica-supported

In this paper we report differences

of room temperature

carbon monoxide results.

of ethylene

studied.

obtained

ethylene,

and the interpretation

particles

has

and also the spectra of adsorbed

aid in the rationalisation

will be discussed,

of the metal

palladium

in the infrared

of the ethylene

procedures

and suggestions

using isotopically

made about the different

that cause the different

spectra.

EXPERIMENTAL Catalyst

A was prepared

200-300 m2g-',

+Present

amorphous

address:

by the traditional

silica with an aqueous

BP Research

0022-2860/82/0000-+000/$02.75

method of impregnating

Centre,

solution

Sunbury-on-Thames,

0 1982 Elsevier Scientific

a high area,

of palladium

Middlesex,

Publishing Company

chloride.

England.

176 The resulting

impregnated

silica was dried at 130°C.

supporting

disc, nominally

absorption

cell.

according

50 mg, was pressed and transferred

in a similar manner, reduced catalyst

for only one hour. adsorption

the only difference

was subjected

Catalyst

by Kember

(ref. 3).

The catalysts

at ca. 400°C

Catalyst

B was prepared

being the length of time which the

to high temperature

A was heated

self-

to an infrared

The disc was then reduced -in situ in hydrogen

to the method described

reduction.

A one inch diameter

evacuation,

ca. 4OO"C, following

in vacua for twelve hours, whereas were cooled to room temperature

catalyst

B

prior to the

experiments.

Transmission

infrared

Fourier transform

spectra were obtained,

in situ, using a Digilab -~

FTS-14

spectrometer.

RESULTS Because of its relative

simplicity,

probe of the metal surface. metals

is well documented;

discusses

carbon monoxide

carbon monoxide

The infrared a recent

adsorption

spectra of adsorbed

is used as a

carbon monoxide

review by Nguyen and Sheppard

adsorption

and relates the observed

on

(ref. 4)

spectra

to the

nature of the metal surface.

Carbon monoxide

adsorption

Carbon monoxide spectrum

adsorption

(ref. 5).

observed,

which increases

increasing

on catalyst

At low coverages

coverage.

A gives the traditionally

in intensity

observed

at 1920 cm -' is

a broad band centred

and shifts to a maximum

of 1990 cm -' with

Also at low coverage

a weak band at 2075 cm-' is observed, -1 as the coverage is which again intensifies and shifts to a maximum of 2100 cm -1 are assigned to bridged species, the The bands below 2000 cm increased. -1 are assigned to linearly bonded dominant species, whilst those above 2000 cm The spectra are consistent

species.

(111) and (110) (ref. 4). spectrum.

The bands assigned

saturate

by Van Hardeveld typically

to this species appear

coverage.

However

in intensity and Hartog

15 A in diameter.

the metal particles be explained catalyst

with adsorption catalyst

In this case it is the linearly

with increasing quickly

However,

catalyst

at low coverages

at low coverages.

For more normal

Similar

to grow

species

spectra were observed

on small metal particles, such as preparation

The spectral

differences

of high index planes present on

A has a high proportion

brought about by lengthy annealing

and continue

to the bridged

Pd catalysts,

of ca. 200 A.

(ref. 4).

(IOO),

different

species which are dominant.

the bands assigned

in terms of a high proportion

B, whereas

bonded

(ref. 6) for adsorption

have diameters

on low index planes,

B gives an entirely

of low index planes

A,

might

Hydrogen

adsorption

There have been few studies of hydrogen exhibits forming

the phenomenon bulk hydride -1

a band at 1850 cm Reflection results

phases

indicate

the B-hydride

(ref. 7).

for hydrogen

absorption

infrared

that hydrogen

phase.

adsorption

of being able to absorb Kavtardze

adsorbed

on palladium.

large quantities and Sokolova

on alumina-supported

(ref. 9) and inelastic

neutron

Palladium

of hydrogen

(ref. 8) observed palladium. scattering

does not adsorb on "clean" palladium,

The e-hydride

(ref. 10)

but only on

phase readily forms when high index planes

are exposed. On catalyst

-1 -1 bands were observed at 1745 cm and 1900 cm , -1 a band at 2052 cm appeared. These results indicate that

6, at low coverages,

and at higher coverages hydrogen

is probably

catalyst

A assignable

planes

adsorbing

on the B-hydride

to adsorbed

hydrogen,

No bands were observed

is thought to contain

on

low index

in predominance.

Experiments

with deuterium

did not give any bands attributable

deuterium,

this is probably

deuterides

as readily as hydrogen

bands were observed generated

during

of the surface SiOH + O2

due to the fact that deuterium

assigned

Ethylene

hydrogen.

the rapid metal-catalysed

silanol

->

groups,

exposure

to deuterium,

The hydrogen was probably of deuterium

with the hydrogen

viz.

->

D+SiOH

exchange

to adsorbed

does not form bulk

After extensive

(ref. 7).

to adsorbed

SiOD+H

!

!

adsorption

Adsorption obtained (w)

phase.

which

of ethylene

(ref. 2).

on catalyst

Bonds assigned

(vCH(s)) and 1525 cm-'

A gives a spectrum

to the n-species

on catalyst

at 2980 cm-'

(m) (vC=C), and those due to the di-o species

2870 cm-' (m) (vCH(s)) and 2780 cm-' (w) (overtone Adsorption

of the type previously

(I) were observed

of fundamental

B also gave bands due to the r-species

(II) at

near 1400 cm-').

(I).

Additional

strong bands at 2940 cm-' (s) (vCH(s)) and 1415 cm-' (vs) (vCZC) were observed which are tentatively The spectrum

assigned

attributed

to the non-associatively

to species

(III)

Iwashita -et al. (ref. 11) for the complex

(CH?CH)CO~(CO)~

is assigned

to a carbon-carbon

mode.

substituted

ethylenes

stretching

adsorbed

is very similar

di-71 species

to that reported

The band at 1415 cm-'

.

To confirm

this isotopically

were used, and the results are given in Table

those of Iwashita -_* et al

(ref. 11) for comparison.

1415 cm-' band is due to a carbon-carbon shift for deuteroethylene

stretching

(C2D4) is probably

1 along with

The results confirm mode.

(III)

by

that the

The lower-than-expected

due to coupling

effects.

178 TABLE

1

A comparison of frequencies of species III from different isotopic species of ethylene adsorbed on palladium catalyst B with those from the analogous compound (CHECH)CO~(CO)~

Adsorbate

Frequency/cm

(C~CH)CO~(C~)~

-1

Frequency ratio*

Frequency/cm-l

Frequency

Cz"4

1415

___

1402.5

(---I

CH2CD2

1395

0.986

1381

(0.985)

C2D4

1400

0.989

1346.5

(0.960)

12CH213CH2

1387

0.980

1379

(0.983)

*Relative

to the value for C2H4: for a hydrogenic

Together

with species

(III)

derived

bands in the region

1700-1900

adsorbed

For the deuterated

hydrogen.

were observed

after prolonged

These observations Intensity suggest

C2H4 (g)

confirm

variations

the following

species

mode the shift would be ca. 0.74

from C2H4 and 12CH213 CH2 on catalyst

cm-' were also observed ethylenes

exposure,

and can be assigned

bands due to adsorbed

along with bands assignable

non-associative

adsorption

of the bands due to species

B,

to

hydrogen

to Si-OD.

of hydrogen.

(I)

and (III)

with time

scheme:

(I)

+ Pd,, -----a

where the +-species

ratio*

(I)

----->

formation

(III)

is the initial step in the formation

of

(III).

SUMMARY AND CONCLUSIONS From the results 1.

Catalyst

A is a typical

200 4 diameter, 2.

Catalyst

reported we conclude catalyst

containing

which have predominantly

B is a novel catalyst

the following. metal particles

of the order of

low index planes exposed.

with metal particles

having predominantly

high index planes exposed. 3.

It is not possible

catalyst

B comprises

crystallites,

to ascertain

from the infrared

small crystallites

i.e. 200 i diameter,

with

spectra alone whether

of ca. 15 fi diameter, "rougher"

surfaces.

or typically

sized

179 ACKNOWLEDGEMENTS We are indebted their many helpful Research

Council

The SRC provided

to the members discussions.

of the surface

spectroscopy

One of us (DIJ) wishes

(SRC) for the funds which enabled a grant for the FTS-14 spectrometer

group at UEA for

to thank the Science

this work to be undertaken. and for general

support of

this work.

REFERENCES 1 2 3 4

A. Lesiunas, M.Sc. Thesis, University of East Anglia, (1974). J.D. Prentice, A. Lesiunas and N. Sheppard, J.C.S. Chem. Comm., (1976)76-77. D. Kember, Ph.D. Thesis, University of East Anglia (1976). T.T. Nguyen and N. Sheppard, in R.E. Hester and R.T.H. Clark (Eds.), Advances in Infrared and Raman Spectroscopy (Vol. 5), Heyden and Sons, London, 1978, pp. 67-148. 5 A. Palazov, C.C. Chang and R.J. Kokes, J. Catalysis, 36(1975)338. 6 R. Van Hardeveld and F. Hartog, in Advances in Catalysis 22(1975)75, Proc. 4th International Congress of Catalysis (Moscow). 7 F.A. Lewis, The Palladium-Hydrogen System, Academic Press, London, 1967. 8 N.K. Kavtaradze and N.P. Sokolova, Russ. J. Phys. Chem., 44 (1970)1485. 9 I. Ratajczkowa, Surface Science, 48(1975)549-560. 10 J. Howard, T.C. Waddington and C.J. Wright, Chem. Phys. Letts., 56(1978)258. 11 Y. Iwashita, F. Tamura and A. Nakamura, Inorganic Chemistry, 8(1969)1179.