It is demonstrated that physisorption of CO molecules on a MgO (100) surface, supporting epitaxially grown Pd particles, acts as a precursor state for CO chemisorption on the particles. The adsorption rate of CO on the Pd particles is measured, at zero coverage by a molecular beam technique, as a function of the substrate temperature and for different particle sizes. A kinetic model describing the adsorption, desorption, diffusion, and capture by the clusters of CO molecules is given. Comparison with the experimental data gives the adsorption probability: 0.5±0.05 and the saddle energy for surface diffusion: 0.25±0.05 eV of CO molecules on MgO.

1.
J. B.
Taylor
and
I.
Langmuir
,
Phys. Rev.
44
,
423
(
1933
).
2.
P.
Kisliuk
,
J. Phys. Chem. Solids
3
,
95
(
1957
);
P.
Kisliuk
,
5
,
78
(
1958
).,
J. Phys. Chem. Solids
3.
D. A.
King
and
M. G.
Wells
,
Proc. R. Soc. London Ser. A
339
,
245
(
1974
).
4.
S. P.
Singh-Boparai
,
M.
Bowker
, and
D. A.
King
,
Surf. Sci.
53
,
55
(
1975
).
5.
P.
Alnot
and
D. A.
King
,
Surf. Sci.
126
,
359
(
1983
).
6.
D. A.
King
,
Surf. Sci.
64
,
43
(
1977
).
7.
A.
Cassuto
and
D. A.
King
,
Surf. Sci.
102
,
388
(
1981
).
8.
M. Bowker and P. D. A. Pudney (unpublished).
9.
S.
Ladas
,
H.
Poppa
, and
M.
Boudart
,
Surf. Sci.
102
,
151
(
1981
).
10.
E.
Gillet
,
S.
Channakhone
, and
V.
Matolin
,
J. Catal.
97
,
437
(
1986
).
11.
C. R.
Henry
and
H.
Poppa
,
J. Vac. Sci. Technol. A
6
,
1113
(
1988
).
12.
C. R. Henry, Ph.D thesis, University of Marseille (1983).
13.
C. R.
Henry
and
C.
Chapon
,
Surf. Sci.
156
,
952
(
1985
).
14.
V.
Matolin
and
E.
Gillet
,
Surf. Sci.
166
,
L
115
(
1986
).
15.
F.
Rumpf
,
H.
Poppa
, and
M.
Boudart
,
Langmuir
4
,
722
(
1988
).
16.
C. R.
Henry
,
Surf. Sci.
223
,
519
(
1989
).
17.
C. R. Henry, C. Chapon, and C. Duriez, in The Physics and Chemistry of Small Clusters, edited by P. Jena, B. K. Rao, and S. N. Khanna, NATO ASI Series (Plenum, New York, 1987), Vol. 158, p. 795.
18.
C. Duriez, C. R. Henry, and C. Chapon, Surf. Sci. (in press).
19.
C.
Chapon
,
C. R.
Henry
, and
A.
Chemam
,
Surf. Sci.
162
,
747
(
1985
).
20.
C. R. Henry, C. Chapon, C. Duriez, and S. Giorgio, Surf. Sci. (in press).
21.
C.
Duriez
,
C.
Chapon
,
C. R.
Henry
, and
J. M.
Rickard
,
Surf. Sci.
230
,
123
(
1990
).
22.
For a detailed description of the techniques and a complete characterization of the particles see Ref. 20.
23.
S.
Furuyama
,
H.
Fujii
,
M.
Kawamura
, and
T.
Morimoto
,
J. Phys. Chem.
82
,
1028
(
1978
).
24.
G. A.
Colbourn
and
W. C.
Mackrodt
,
Surf. Sci.
143
,
391
(
1984
).
25.
C.
Pisani
,
R.
Dovesi
,
R.
Nada
, and
S.
Tamiro
,
Surf. Sci.
216
,
489
(
1989
).
26.
We have taken into account for the actual angular distribution in the measurement of 10−7
27.
W. K.
Burton
,
N.
Cabrera
, and
F. C.
Frank
,
Philos. Trans. R. Soc. London Ser. A
243
,
299
(
1951
).
28.
K. J.
Routledge
and
M. J.
Stowell
,
Thin Solid Films
6
,
397
(
1970
).
29.
M. J.
Stowell
,
Philos. Mag.
26
,
349
(
1972
).
30.
R. Gomer, in Surface Mobilities on Solid Materials, edited by Vu Tienh Binh, (NATO ASI series, 1983), Vol. 86, p. 127.
31.
R.
Dovesi
,
R.
Orlando
,
F.
Ricca
, and
C.
Roetti
,
Surf. Sci.
186
,
267
(
1987
).
32.
After the writing of this paper new calculations on the CO adsorption on (100)MgO, have been published [
A.
Lakhlifi
and
C.
Girardet
,
Surf. Sci.
241
,
400
(
1991
);
A.
Lakhlifi
and
C.
Girardet
,
J. Chem. Phys.
94
,
688
(
1991
)]. These calculations, based on semiempirical potentials, give an adsorption energy of 0.14 eV and a diffusion energy of 0.017 eV for an isolated CO molecule on MgO. These results are in disagreement both with ab initio calculations (Refs. 24 and 25) and with the present measurements. The origin of this discrepancy is not yet understood. It could be the inadequacy of some of the parameters used in the potential, or a more fundamental reason. For example, ab initio calculations of Colbourn et al., (Ref. 24) have shown that the contributions of the nearest neighbors (of the MgO surface), in the binding energy of a CO molecule, are not additive.
This content is only available via PDF.
You do not currently have access to this content.