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Catalytic Properties and Characterization of LaPd3 Intermetallic Compound
C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactioity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
863
CATALYTIC PROPERTIES AND CHARACTERIZATION OF LaPd3 INTERMETALLIC COMPOUND
K.S. SIM', L. HILAIRE, F. LE NORMAND, R. TOUROUDE Laboratoire de Catalyse et Chimie des Surfaces, U.A. 423 du CNRS, 4 rue Blaise Pascal, 67070 Strasbourg (France) V. PAUL-BONCOUR,A. PERCHERON-GUEGAN
Laboratoire de Chimie Mdtallur ique des Terres Rares, U.A. 209 du CNRS, 1 Place A. Briand, 92195 Meudon (France!
r
ABSTRACT The catalytic behaviour of LaPd3 has been investigated in but-l-ene h drogenation and isomerization. The activity increased and the isomerization se ectivity decreased linearly as a function of the duration of the pretreatment catalyst (H2 ,300OC) to reach constant values after 15 hours.The evolution of catalytic activity and selectivit is due to the pro ressive decomposition of *Pd3 islands into La203 and Pd, whicl were initially founi on the surface of catalyst jointly to lanthanum oxide and hydroxide. That was deduced from photoelectron spectroscopy analysis. INTRODUCTION Recently, rare earth and transition metal intermetallic compounds have been used as catalysts in reaction involving the activation and transfer of hydrogen such as hydrogenation of olefins [l , 21 and other catalytic reactions [3, 41. However, the nature and the structural changes of the catalysts occuring under the catalytic reaction conditions Is a permanent question. X-ray diffraction studies carried out by others workers have led to the hypothesis that metal -rare earth oxide catalysts are formed in situ [5,6]. In this paper, we report on the catalytic properties of LaPdg compound pretreated by thermal reduction. Additional studies of electron microscopy and X-ray photoelectron spectroscopy before and after the catalytic reaction or after the pretreatment have been done.
'Permanent address: Korea Institute of Energy and Resources, P.O. Box 5, Daedeok, Science Town, Daejeon, Chungnam, SOUTH KOREA.
864
EXPERIMENTAL METHODS Preparation LaPd3 was prepared by induction melting of the pure components under vacuum [7]. Its stoichiometry and homogeneity were verified by metallographic examination and microprobe analysis. The sample studied in the catalytic reaction was ground mechanically in argon atmosphere and sieved in order to produce powder with a grain size of less than 36pm. Characterization (i) X-rav Diffraction ( X u. Single plate and powder was analysed by XRD using an adapted Debye-Scherrer camera and CU-K radiation. The lattice spacings were derived from the diffraction patterns using the Nelson-Riley extrapolation fonction. (ii) Electron Microscopy. The bulk analysis of LaPd3 powder before and after catalytic use was performed by STEM (Scanning Transmission Electron Microscopy) and EDAX (Energy Dispersive Analysis by X-ray). liii) X-rav p h o t o m r o n S p e c t r o w ( ) ( P a The surface analysis of LaPd3 was performed with the pellets prepared in argon atmosphere, in the initial state and after the treatment (H2,3OO0C), in situ, within the chamber of the spectrometer.
..
Activitv and Selewitv measurements 0) Gas-Dhase hvdro' . This reaction was carried out in a flow reactor at 760 Torr total pressure over the temperature range, -37.5 to 30°C, with 8.2 Torr but1-ene partial pressure. Catalyst loading was about 40 mg. The selectivity was measured by the ratio of isomenzed to isomenzed plus hydrogenated products. * . Gas chromatographic analysis was effected using a 5m (ii) Product long x 3x10s rn i.d. column packed with 30% dimethyl sulfolane on fire brick. RESULTS and DISCUSSION -0Dert ies
..
..
. This evolution was tested as 0) Fvolution of the c a W actlvla function of the number of successive runs. In each run, LaPd3 was pretreated at 300% for 1 hour in a purified hydrogen flow at 760 Torr (50 ml/min) and the catalytic reaction was then performed. From the results presented in Fig.1 a significant linear increase in activity and a linear decrease in selectivity were observed during the first few runs. After ca.10 runs the catalytic activity and selectivity were stabilized . If we take another loading of LaPd3 and pretreat directly more than 15 hours with H2 at 300°C, we obtain the same activity and selectivity than after 15 funs.
s-
0
0 0 a
-r
0
40
20
hours
Fig. 1. Total activity (r) and isomerization selectivity (S) as a function of the duration of H2-3000Cpretreatment (hours). * The activity and selectivity in the hydrogenation and (ii) m e t i c studtes. isomeriration of but-l-ene obtained with various conditions are summarized in Table 1. From these results the apparent activation energy was approximately 7.3 KcaVmol, r = k PH20.5PB0.1 and the reaction orders lead to the equation where r is the initial rate of hydrogenation only, and PH2 and PBthe initial pressures of hydrogen and but-1-ene respectively. TABLE 1
No
Temp. ("c)
PH2 For)
PB Fan)
L-1 L-2 L-3 L-4 L-5 L-6 L-7
0 -18 -37.5 0
752
8.2 "
0 0 0
a
"
757 747 226 38
2.9 13 8.2 11
Activity Selectivity ( molekg. cata) total hydrogenation
("w
3.7 1.8 0.45 2.8 3.8 2.6 1.8
1.6 0.72 0.18 1.3 1.6 0.94 0.44
58 60 59 53 60 64 75
866
Characterization of LaPG (i) X-rav diffraction measy rements. The XRD results indicated that for the initial LaPd3 there was a good agreement with the LaPd3 compound, but after the catalytic tests other compounds such as palladium and La203 were observed with LaPd,. It is clear that there was a transformation of the intermetallic compound after pretreatment. (ii) Electron microscopv measurements. Fig.2 and 3 show some representative STEM and EDAX results for the initial catalyst and after catalytic tests. Initially STEM examination showed a rather good homogeneity in the dispersion of rare earth and transition metal. However after the catalytic tests, we observed an inhomogeneity of the sample, in good agreement with the results of XRD analysis. For example in the region b of the figure 3,there is a strong enrichment in lanthanum .
Pd
2
la
4
6
8
kev
Figure 2. STEM and EDAX analysis of initial sample, LaPds
867
Figure 3. STEM and EDAX analysis of LaPd3 sample after catalytic tests.
(iii) XPS rneasureme nts. XPS spectra of LaPd3 were taken for the initial sample (a), after treatment during 4,hours (b) and 12 hours (c). The Pd/La value of surface composition , shown in Fig.4, indicated an enrichment of lanthanum initially and the progress of this enrichment after the treatments (H2, 300°C).The spectra and binding energies for Pd 3d, La 3d 5/2 and 0 1s are presented in Fig.5 and Table 2. The 0 1s peak of the initial sample showed a main peak at 531.7eV, characteristic of OH- anion [8]. After treatments the secondary peak increases at 529.7eV which is characteristic of 0 2 - anion [9]. After the second treatment (c), all peaks were shifted towards higher binding energies, due to a charging effect, and the Pd signal was very weak and broadened (full width at half maximum 1.8 eV in sample (a) compared to 2.4eV in sample (c). In the latter case, the binding energies and the shape of Pd peak were similar to those of a 0.2% Pd/La203 sample studied by Fleisch et al [12], using the same reference energy (C 1s, 284,6eV). It is clear that after 12 hours treatment, the LaPd3 surface was identical to the surface of a Pd/La203catalyst.
868
T'ABLE 2
1
0
Compound LaPd3 (a! LaPd3 (by LaPd3 (C
1s
529.7 529.4
531-7 531.9 531.0
3d5l2
Pd 3d312
335.1 335.4 334.9
340.3 340.6 340.2
La MI2 834.9 834.7 834.2
Ref.
838.8 838.9 838.3
10 335.2 340.4 Pd metal 835.9 839.6 8-9 532.1 La(OH)3 834.7 839.2 8-9 La203 530.1 832.9 837.0 335.75 341.05 10-11 LaPd3 834.3 12 334.9 3.2%Pd/La203 1- referred to Fermi level. 2- the binding ener ies are corrected from charging effects, referred to the same C l s ~ ~ o ~ as 0.2% P ~ / L lef.12).
Figure 4 Pd/La ratio (XPS intensities corrected from cross sections and escape depths differences) as a function of the duration of in situ H2-300"C treatment.
Pd
La
3d
.. .: ... ...
3d 5/2
Is
0
,.----
I,
,<-.
.
._ .: . -..
. . . .
:
.
.
.
a: ;*A
b'
I
... .. . . ..> .....
,.
..
I..
.
s
,<-. z I <.-
b--
' d
m d C
336
ev
342
.
..
!
:.
c.::
:
e?x;:
; "..\ ,
i
:
;
; + ,
..;-. .. .. ._ .. --:. . L;'
.:
,I
. : . . ... .
'-_
a,-'
j
b c
1
ev
839
,L
a/./,
,; '=-..& ..
,k.
. ' ,<:
1
:
.
,
/-
'
834
, ;i
'---.
'
*&X
*.* .
i
:
. .-.
? d 4 . .
i; II
. . .:-.. . .; .. .-.- ,:. ... . .. ..-., r.
..
j
i
I
\.
i
.
:.
.
'i.
/,;
528.5
I
ev
\ ,
x
-. *-.
I
I
\-
-\ I
.\\
1
533.5
Figure 5 XPS spectra of Pd, La and 0 elements: a) initial sample, b) after 4 hours-H2-3000C,c) after 12 hours-H*-300°C. 1) corrected from charging effects.
869
ufects of heat treatment. Due to the great affinity of La for O2and the easy decomposition of LaPd3 (at least on the surface), 0 2 is present from the start and forms mainly La(OH)3 together with some LapO3. We found that heat treatment (300°C under hydrogen) led the catalyst to decompose progressively with the duration of the treatment. favouring the segregation of La towards the surface to form more La203, while the hydroxide decomposed. The initial surface of LaPd3 catalyst , which was already 5 1 6 times more enriched with lanthanum than expected from the stoichiometry, became more and more enriched with lanthanum. Meanwhile the amount of carbon and oxygen present on the surface did not change, which means that the increase of activity as a function of the time of pretreatment was not due to a decontamination process.
lm
Model of LaPd? Cata We can imagine the surface of initial LaPd3 with islands of LaPd3 and La(OH), as follows:
5-qT;=+y7-pd OH OH OH I
OH OH OH
- ---::+La
0 0 0 . 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 .-Lap%
With heat treatment these islands of LaPd3 decompose progressively into Pd metal and lanthanum oxide. So after 1 hour of treatment these islands of LaPd3 are still present, therefore the catalytic activity is very weak and the selectivity is high. After 12 hours of treatment these islands are entirely decomposed and the surface of LaPd, became like palladium particles dispersed on lanthanum oxide. The presence of Pd particles exhibits the augmentation of activity and approach the same selectivity as palladium metal.
CONCLUSION The catalytic results obtained for LaPd3 catalyst showed that the activity inscreased and the selectivity diminished linearly with the number of runs, each run consisting of a pretreatment (H2, 1h, 300°C) followed by catalytic hydrogenation reaction. After ca. 10515 runs or 15 hours of H2.3000C pretreatment the catalyst
became stable. The selectivity in isomerization for the stabilized LaPd3 was characteristic of palladium metal. This so-treated LaPd3 catalyst has the surface like palladium
870
particles dispersed on lanthanum oxide, and the catalytic behaviour is as the classical palladium catalyst deposited on inert support. In the initial state, prior to the extensive decomposition of the intermetallic compound, an electron transfer from the rare earth to palladium may be responsible for the lower activity and higher selectivity as compared to palladium particles deposited on oxide. Such a conclusion is supported by the catalytic behaviour of other palladium-rare earths intermetallic compounds [131. REFERENCES
F. Le Normand, P. Girard, L. Hilaire, M.F. Ravet, G. Krill and G. Maire J. Catal. 89, 1 (1984) V.T. Coon. T. Takeshita. W.E. Wallace and R.S. Craia, -. J. Phvs. Chem.. 8p,17 (1976) C.A. Luengo, A.L. Cabrera, H.B. Mckay and M.B. Maple, J. Catal. 47,1 (1 977). &-l.diaue. A. Percheron-GuBaan. J.C. Achard and F. Tasset. J. Less. CommGn Metals 24, 1 (1980). H.C. Siegmann, L. Schlapbach and C.R. Brundle, Phys. Rev. Lett. 972 (1978.) L. Sc'hlapbach, A. Seiler, H.C. Siegmann, T.V. Waldkirch, P. Zucker and C.R. Brundle, Int. J. Hydrogen Energ 5, 21 (1979). 3550 (1982). 10 F.U. Hillebrecht and J.C. Fuggle, Phys. ev. B 2179 (1983). 11 F.U. Hillebrecht and J.C. Fuggle, Phys. Rev. B 12 T.H. Fleisch, R.F. Hicks and A.T. Bell, J. Catal. fl,398 (1984). 13 K.S.Sim, PhD thesis, Strasbourg (1988) .
M
.
a,
A a, a,
863
CATALYTIC PROPERTIES AND CHARACTERIZATION OF LaPd3 INTERMETALLIC COMPOUND
K.S. SIM', L. HILAIRE, F. LE NORMAND, R. TOUROUDE Laboratoire de Catalyse et Chimie des Surfaces, U.A. 423 du CNRS, 4 rue Blaise Pascal, 67070 Strasbourg (France) V. PAUL-BONCOUR,A. PERCHERON-GUEGAN
Laboratoire de Chimie Mdtallur ique des Terres Rares, U.A. 209 du CNRS, 1 Place A. Briand, 92195 Meudon (France!
r
ABSTRACT The catalytic behaviour of LaPd3 has been investigated in but-l-ene h drogenation and isomerization. The activity increased and the isomerization se ectivity decreased linearly as a function of the duration of the pretreatment catalyst (H2 ,300OC) to reach constant values after 15 hours.The evolution of catalytic activity and selectivit is due to the pro ressive decomposition of *Pd3 islands into La203 and Pd, whicl were initially founi on the surface of catalyst jointly to lanthanum oxide and hydroxide. That was deduced from photoelectron spectroscopy analysis. INTRODUCTION Recently, rare earth and transition metal intermetallic compounds have been used as catalysts in reaction involving the activation and transfer of hydrogen such as hydrogenation of olefins [l , 21 and other catalytic reactions [3, 41. However, the nature and the structural changes of the catalysts occuring under the catalytic reaction conditions Is a permanent question. X-ray diffraction studies carried out by others workers have led to the hypothesis that metal -rare earth oxide catalysts are formed in situ [5,6]. In this paper, we report on the catalytic properties of LaPdg compound pretreated by thermal reduction. Additional studies of electron microscopy and X-ray photoelectron spectroscopy before and after the catalytic reaction or after the pretreatment have been done.
'Permanent address: Korea Institute of Energy and Resources, P.O. Box 5, Daedeok, Science Town, Daejeon, Chungnam, SOUTH KOREA.
864
EXPERIMENTAL METHODS Preparation LaPd3 was prepared by induction melting of the pure components under vacuum [7]. Its stoichiometry and homogeneity were verified by metallographic examination and microprobe analysis. The sample studied in the catalytic reaction was ground mechanically in argon atmosphere and sieved in order to produce powder with a grain size of less than 36pm. Characterization (i) X-rav Diffraction ( X u. Single plate and powder was analysed by XRD using an adapted Debye-Scherrer camera and CU-K radiation. The lattice spacings were derived from the diffraction patterns using the Nelson-Riley extrapolation fonction. (ii) Electron Microscopy. The bulk analysis of LaPd3 powder before and after catalytic use was performed by STEM (Scanning Transmission Electron Microscopy) and EDAX (Energy Dispersive Analysis by X-ray). liii) X-rav p h o t o m r o n S p e c t r o w ( ) ( P a The surface analysis of LaPd3 was performed with the pellets prepared in argon atmosphere, in the initial state and after the treatment (H2,3OO0C), in situ, within the chamber of the spectrometer.
..
Activitv and Selewitv measurements 0) Gas-Dhase hvdro' . This reaction was carried out in a flow reactor at 760 Torr total pressure over the temperature range, -37.5 to 30°C, with 8.2 Torr but1-ene partial pressure. Catalyst loading was about 40 mg. The selectivity was measured by the ratio of isomenzed to isomenzed plus hydrogenated products. * . Gas chromatographic analysis was effected using a 5m (ii) Product long x 3x10s rn i.d. column packed with 30% dimethyl sulfolane on fire brick. RESULTS and DISCUSSION -0Dert ies
..
..
. This evolution was tested as 0) Fvolution of the c a W actlvla function of the number of successive runs. In each run, LaPd3 was pretreated at 300% for 1 hour in a purified hydrogen flow at 760 Torr (50 ml/min) and the catalytic reaction was then performed. From the results presented in Fig.1 a significant linear increase in activity and a linear decrease in selectivity were observed during the first few runs. After ca.10 runs the catalytic activity and selectivity were stabilized . If we take another loading of LaPd3 and pretreat directly more than 15 hours with H2 at 300°C, we obtain the same activity and selectivity than after 15 funs.
s-
0
0 0 a
-r
0
40
20
hours
Fig. 1. Total activity (r) and isomerization selectivity (S) as a function of the duration of H2-3000Cpretreatment (hours). * The activity and selectivity in the hydrogenation and (ii) m e t i c studtes. isomeriration of but-l-ene obtained with various conditions are summarized in Table 1. From these results the apparent activation energy was approximately 7.3 KcaVmol, r = k PH20.5PB0.1 and the reaction orders lead to the equation where r is the initial rate of hydrogenation only, and PH2 and PBthe initial pressures of hydrogen and but-1-ene respectively. TABLE 1
No
Temp. ("c)
PH2 For)
PB Fan)
L-1 L-2 L-3 L-4 L-5 L-6 L-7
0 -18 -37.5 0
752
8.2 "
0 0 0
a
"
757 747 226 38
2.9 13 8.2 11
Activity Selectivity ( molekg. cata) total hydrogenation
("w
3.7 1.8 0.45 2.8 3.8 2.6 1.8
1.6 0.72 0.18 1.3 1.6 0.94 0.44
58 60 59 53 60 64 75
866
Characterization of LaPG (i) X-rav diffraction measy rements. The XRD results indicated that for the initial LaPd3 there was a good agreement with the LaPd3 compound, but after the catalytic tests other compounds such as palladium and La203 were observed with LaPd,. It is clear that there was a transformation of the intermetallic compound after pretreatment. (ii) Electron microscopv measurements. Fig.2 and 3 show some representative STEM and EDAX results for the initial catalyst and after catalytic tests. Initially STEM examination showed a rather good homogeneity in the dispersion of rare earth and transition metal. However after the catalytic tests, we observed an inhomogeneity of the sample, in good agreement with the results of XRD analysis. For example in the region b of the figure 3,there is a strong enrichment in lanthanum .
Pd
2
la
4
6
8
kev
Figure 2. STEM and EDAX analysis of initial sample, LaPds
867
Figure 3. STEM and EDAX analysis of LaPd3 sample after catalytic tests.
(iii) XPS rneasureme nts. XPS spectra of LaPd3 were taken for the initial sample (a), after treatment during 4,hours (b) and 12 hours (c). The Pd/La value of surface composition , shown in Fig.4, indicated an enrichment of lanthanum initially and the progress of this enrichment after the treatments (H2, 300°C).The spectra and binding energies for Pd 3d, La 3d 5/2 and 0 1s are presented in Fig.5 and Table 2. The 0 1s peak of the initial sample showed a main peak at 531.7eV, characteristic of OH- anion [8]. After treatments the secondary peak increases at 529.7eV which is characteristic of 0 2 - anion [9]. After the second treatment (c), all peaks were shifted towards higher binding energies, due to a charging effect, and the Pd signal was very weak and broadened (full width at half maximum 1.8 eV in sample (a) compared to 2.4eV in sample (c). In the latter case, the binding energies and the shape of Pd peak were similar to those of a 0.2% Pd/La203 sample studied by Fleisch et al [12], using the same reference energy (C 1s, 284,6eV). It is clear that after 12 hours treatment, the LaPd3 surface was identical to the surface of a Pd/La203catalyst.
868
T'ABLE 2
1
0
Compound LaPd3 (a! LaPd3 (by LaPd3 (C
1s
529.7 529.4
531-7 531.9 531.0
3d5l2
Pd 3d312
335.1 335.4 334.9
340.3 340.6 340.2
La MI2 834.9 834.7 834.2
Ref.
838.8 838.9 838.3
10 335.2 340.4 Pd metal 835.9 839.6 8-9 532.1 La(OH)3 834.7 839.2 8-9 La203 530.1 832.9 837.0 335.75 341.05 10-11 LaPd3 834.3 12 334.9 3.2%Pd/La203 1- referred to Fermi level. 2- the binding ener ies are corrected from charging effects, referred to the same C l s ~ ~ o ~ as 0.2% P ~ / L lef.12).
Figure 4 Pd/La ratio (XPS intensities corrected from cross sections and escape depths differences) as a function of the duration of in situ H2-300"C treatment.
Pd
La
3d
.. .: ... ...
3d 5/2
Is
0
,.----
I,
,<-.
.
._ .: . -..
. . . .
:
.
.
.
a: ;*A
b'
I
... .. . . ..> .....
,.
..
I..
.
s
,<-. z I <.-
b--
' d
m d C
336
ev
342
.
..
!
:.
c.::
:
e?x;:
; "..\ ,
i
:
;
; + ,
..;-. .. .. ._ .. --:. . L;'
.:
,I
. : . . ... .
'-_
a,-'
j
b c
1
ev
839
,L
a/./,
,; '=-..& ..
,k.
. ' ,<:
1
:
.
,
/-
'
834
, ;i
'---.
'
*&X
*.* .
i
:
. .-.
? d 4 . .
i; II
. . .:-.. . .; .. .-.- ,:. ... . .. ..-., r.
..
j
i
I
\.
i
.
:.
.
'i.
/,;
528.5
I
ev
\ ,
x
-. *-.
I
I
\-
-\ I
.\\
1
533.5
Figure 5 XPS spectra of Pd, La and 0 elements: a) initial sample, b) after 4 hours-H2-3000C,c) after 12 hours-H*-300°C. 1) corrected from charging effects.
869
ufects of heat treatment. Due to the great affinity of La for O2and the easy decomposition of LaPd3 (at least on the surface), 0 2 is present from the start and forms mainly La(OH)3 together with some LapO3. We found that heat treatment (300°C under hydrogen) led the catalyst to decompose progressively with the duration of the treatment. favouring the segregation of La towards the surface to form more La203, while the hydroxide decomposed. The initial surface of LaPd3 catalyst , which was already 5 1 6 times more enriched with lanthanum than expected from the stoichiometry, became more and more enriched with lanthanum. Meanwhile the amount of carbon and oxygen present on the surface did not change, which means that the increase of activity as a function of the time of pretreatment was not due to a decontamination process.
lm
Model of LaPd? Cata We can imagine the surface of initial LaPd3 with islands of LaPd3 and La(OH), as follows:
5-qT;=+y7-pd OH OH OH I
OH OH OH
- ---::+La
0 0 0 . 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 .-Lap%
With heat treatment these islands of LaPd3 decompose progressively into Pd metal and lanthanum oxide. So after 1 hour of treatment these islands of LaPd3 are still present, therefore the catalytic activity is very weak and the selectivity is high. After 12 hours of treatment these islands are entirely decomposed and the surface of LaPd, became like palladium particles dispersed on lanthanum oxide. The presence of Pd particles exhibits the augmentation of activity and approach the same selectivity as palladium metal.
CONCLUSION The catalytic results obtained for LaPd3 catalyst showed that the activity inscreased and the selectivity diminished linearly with the number of runs, each run consisting of a pretreatment (H2, 1h, 300°C) followed by catalytic hydrogenation reaction. After ca. 10515 runs or 15 hours of H2.3000C pretreatment the catalyst
became stable. The selectivity in isomerization for the stabilized LaPd3 was characteristic of palladium metal. This so-treated LaPd3 catalyst has the surface like palladium
870
particles dispersed on lanthanum oxide, and the catalytic behaviour is as the classical palladium catalyst deposited on inert support. In the initial state, prior to the extensive decomposition of the intermetallic compound, an electron transfer from the rare earth to palladium may be responsible for the lower activity and higher selectivity as compared to palladium particles deposited on oxide. Such a conclusion is supported by the catalytic behaviour of other palladium-rare earths intermetallic compounds [131. REFERENCES
F. Le Normand, P. Girard, L. Hilaire, M.F. Ravet, G. Krill and G. Maire J. Catal. 89, 1 (1984) V.T. Coon. T. Takeshita. W.E. Wallace and R.S. Craia, -. J. Phvs. Chem.. 8p,17 (1976) C.A. Luengo, A.L. Cabrera, H.B. Mckay and M.B. Maple, J. Catal. 47,1 (1 977). &-l.diaue. A. Percheron-GuBaan. J.C. Achard and F. Tasset. J. Less. CommGn Metals 24, 1 (1980). H.C. Siegmann, L. Schlapbach and C.R. Brundle, Phys. Rev. Lett. 972 (1978.) L. Sc'hlapbach, A. Seiler, H.C. Siegmann, T.V. Waldkirch, P. Zucker and C.R. Brundle, Int. J. Hydrogen Energ 5, 21 (1979). 3550 (1982). 10 F.U. Hillebrecht and J.C. Fuggle, Phys. ev. B 2179 (1983). 11 F.U. Hillebrecht and J.C. Fuggle, Phys. Rev. B 12 T.H. Fleisch, R.F. Hicks and A.T. Bell, J. Catal. fl,398 (1984). 13 K.S.Sim, PhD thesis, Strasbourg (1988) .
M
.
a,
A a, a,