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Al2O3 and on “organometallic cluster derived” catalysts; effect of the metal particle size
Surface Science 106 (1981) 4&471 North-Holland Publishing Company
ISOMERIZATION
OF ‘C
“ORGANOMETALLIC
LABELED IIEXANES ON Pt/A&
CLUSTER DERIVED”
EFFECT OF THE METAL PARTICLE
F. GARIN,
0. ZAHRAA,
C. CROUZET,
AND ON
CATALYSTS;
SIZE
J.L. SCHMITT and G. MAIRE
Laboratoirede Catalyse, Vniversitl Louis Pasteur, 4 rue Blake Pascal, F-67000 Strasbourg,France Received 8 September 1980
Skeletal isomerization of 2-methylpentane (2-MP) and hydrogenolysis of methylcyclopentane (MCP) have been studied over a series of Pt/AlzOj and “cluster derived” (CD) catalysts of high dispersions. 13C labeling allowed estimation of the relative contributions of cyclic mechanisms in isomerization. High resolution electron microscopy was used jointly to determine the metal particle size distributions. In the isomerization of 2-MP to 3-MP the relative amount of selective cyclic mechanisms increases with the percentage of surface or length particles (d c ld A) whatever the catalyst used. In the isomerization of 2-MP to n-hexane for catalysts having the same mean size of particles, the CD catalysts show differences in selectivity compared to the classical Pt/AlzOz catalysts. The observed particle size effect could be explained by changes in electronic or geometrical factors, especially it is proposed that the electronic rather than the geometrical one should be considered in the case of the “organometallic cluster derived” catalysts.
1. Introduction
A major problem in heterogeneous catalysis is characterization of the active sites. A possible solution, especially for metals, is to find a correlation between particle size and some catalytic properties. Much work has been done on the effect of metal particle size either on selectivity or on specific rates for various catalytic reactions. The selectivity approach to the particle size effects resulted in systematic studies of the isomerization of hexanes on metal films and supported catalysts sintered or calcined [14]. Isomerization of 13C labeled hexanes and hydrogenolysis of methylcyclopentane are very sensitive chemical probes to study metal catalysts. Applied successfully to investigate particle size effects they have been used in the present work to study Pt/A1203 and “organometallic cluster derived” catalysts.
2. Experimental
The catalysts studied have been obtained by two different modes. A first series of X%Pt/A1203 catalysts with various Pt loading (X% = 0.2, 2.25, 4.1, 7.1 in weight) has*been prepared by impregnation of three batches of commercial Woelm alumina with different 0039-6028/81/0000-0000/$02.50
@ North-Holland
Publishing Company
467
F. Garin et al. I Isomerizationof 13C labeled hexanes
superficial OH content, A, A’ and B’. B &,,, is an alumina A’ which has been calcined at 600°C in air during 210 h. A second series of Pt catalysts has been obtained from Chini’s clusters [5], [Pt3(C0)3p2(C0)3]i(n = 2, 3, 4, -5) deposited on B’ alumina, previously degased, exposed to air at room temperature. In all cases reduction was performed at 100°C with a hydrogen flow rate of lOml/min and completed at 200°C for 48 h. The synthesis of the carbon-13 labeled hydrocarbons used as reactants, the differential reactor and the procedure for catalytic experiments have been described elsewhere [3,6]. The characterization of the catalysts have been obtained by chemisorption measurements and by transmission electron microscopy (TEM). dH = 8.5/a is related to hydrogen chemisorption measurements and is conventionally calculated from the dispersion (I by using the relationship a X dH = 8.5 which holds when assuming cubic crystallites [7]. The dispersion of platinum was ascertained by using a Philips EM300G TEM and the extractive replica technique. Platinum particles having a diameter as low as 5 8, could be detected.
3. Results and discussion Two labeled hexanes, 2-methylpentane-2-13C and 2-methylpentane-4i3C, allowed us to distinguish between cyclic and bond shift mechanisms in two isomerization reactions: 2-MP + 3-MP, 2-MP + n-H respectively (fig. 1). Besides the two test reactions, hydrogenolysis of methylcyclopentane was also investigated, since in this reaction the ratio 3-MP/n-H allows simply to estimate the contributions of selective and non-selective mechanisms (fig. 2). The experiments were done at 255 2 5°C for classical Pt/A1203 catalysts and at 270 + 5°C for “organometallic cluster derived” catalysts under 101 kPa of hydrogen for isomerization of hexanes and at 220°C for MCP hydrogenolysis. Some of the more significant catalytic results are reported in table 1 as function of the various types of catalysts, the mean particle size, the ratios a = H/R between chemisorbed hydrogen atoms and platinum atoms. dH = 8.5/a determined by hydrogen chemisorption and d, by TEM are in good agreement. The activity measured, at constant space velocity, by the total conversion (Yr in moles is higher for the Pt/A1203 catalysts than for the CD catalysts. Moreover, the reaction temperatures for isomerization of hexanes are lower for the former catalysts as seen in
[d
I)
a
Fig. 1. Isomerization of ‘3C-labeled hexanes.
-+-
‘/2
‘12
468
F. Garin et al. / Isomerization of ‘“C labeled hexanes
Fig. 2. Hydrogenolysis
of methylcyclopentane.
table 1 and the selectivity remains unchanged. But it is worthwhile catalysts lead to a lowering of the rate of coke deposition resulting stability as shown in table 2 [a]. On classical Pt/Al,O, catalysts injections one may observe a deactivation lowering %q. Recently, shown that the percentage of cyclic mechanisms for isomerization
to notice that the CD in an increase of the after 5 hydrocarbon Gault et al. [9] have remains constant on
Table 1 Isomerization of ‘3C-labeled hexanes and hydrogenolysis of MCP Catalysts (%Pt/AIZ03)
7.1, BZIO~ 4.1. Bzloh 2.25, Ae 5.5, B’
a = H/Pt
dH
CT,,
(*)
ZM
c”C)
254 254 275 275
0.35 0.55 0.04 -
24 15 212
36 17 100 20
% (YT (mole)
Sela
%CMb
18 11
56 55
16 30 44 44
90 86 92 92
0.7 0.4 0.7 0.57
57
51
90
0.62
50 41.5
62 67
97 -
0.4 0.73
41
80
82
0.7
_
83
100
0.42
(CD, n = 4) 0.56, B’ (CD, n = 5) 2.5, klo h 1.1, B’
275
0.43
20
23
3.3
254 275
0.7 0.48
12 18
15 17
15 3.3
(CD, n = 2) 0.4, B’
275
0.54
16
15
1.5
(CD, n = 3) 0.2, A
254
1
a Selectivity, IQ
isomers/q
8.5
20
_
(X 100).
bLC&/\T-“ CLCLW d MCP hydrogenolysis,
//vv
’ Catalyst previously reduced at 200°C sintered at 400°C in Hz
% CM’
MCpd
F. Garin et al.
I Isomerizationof “C labeled hexanes
469
Table 2 Isomerization of 2-MP on a (CD, n = 3) catalyst obtained from Chini’s clusters [5] Catalyst
Injection No.
0.4% Pt (CD, n = 3)
1 70
7.4 7.3
Selectivity, Cg isomerslaT (x 100)
3-MP
n-H
MCP
37 40
38 37
46 46
16 17
particles larger than 10 A, which shows that the sites responsible for the bond shift and the cyclic mechanism are topographically similar, i.e. both involve or do not involve edge atoms. Since extremely dispersed catalysts are very active for isomerization they believed that both types of isomerization sites include edge atoms. Moreover, for Gault [lo] the limiting size of 10 8, and not 20 8, below which an enhancement of the cyclic mechanism is observed, seems to favour (over the Bs-sites model) any model involving polytetrahedra as that of Hoare and Pal [ll] or pseudocrystals with Dsh or icosahedral symmetry [12]. Table 1 shows that & varies in the range 15-35 A for very different catalysts except for the sintered one whereas the percentage of cyclic mechanism for isomerization of 2-MP to 3-MP is highly affected changing from 16 to 83%. Focusing our attention on the catalytic behaviour when very small particles lower than 108, are present, and assuming that the aggregates in the range 5-10 8, are responsible for the cyclic mechanism we should obtain a correlation between the percentage of CM versus the percentage of surface particle sizes. In the fig. 3 are plotted the percentage of cyclic mechanism for the reactions 2-MP-2-i3C+ 3-MP-3-13Cand 2-MP-4-13C+ n-H-3-i3C versus the percentages of surface particle sizes 5 < %nidf/hidf s 10 A and the length particle sizes 0 d % &/8ni S 10 A respectively. The following observations have to be underlined: (i) the selective cyclic mechanism contribution in the reaction 2-MP+3-MP increases whatever the catalysts used, with the amount of surface particle sizes or the length particle sizes. Only particles in the range of 5 to 108, seem to be the unique sites responsible for this type of isomerization. The correlation between the %CM and the percentage of surface particle sizes fits much better than the curve %CM = f(%&ni) s 10 A. (ii) Th e non-selective cyclic mechanism in the reaction 2-MP+ n-H shows two distinct phenomena. On classical Pt/A1203 catalysts where the ratio r = 3-MP/n-H in MCP hydrogenolysis is equal to 0.4 (table 1) the cyclic contribution increases with the amount of length particle sizes in the range of 0 to 10 A. On the other hand the CD catalysts lead to a decrease of the percentage of CM, and moreover the ratio r is higher: 0.57, 0.62 and 0.7 for n = 4, n = 5 and n = 3. r decreases with the %CM. (iii) The same feature is observed when plotting the % of CM = f(% nidf/Xnidf) in the range 5 to 10 A. The isomerization of 2-MP+ 3-MP via a cyclic mechanism involving in the intermediate species the rupture of a disecondary carbon-carbon bond is independent for both classical and CD catalysts. On the other hand for isomerization of 2-MP+ n-H the rupture of a secondary-tertiary C-C bond is highly dependent on the catalyst preparation. On classical supported Pt/A1203 catalysts, rh,d= 0.4, we observe a pure non-selective
F. Garin el al. i Isomerization of 13C labeled hexanes
470
%C:M
. A
0
0
Fig 3. Percentage of cyclic mechanisms versus the percentages of surface and length particle sizes. 2-MP-2-“C+ 3MP-3-“C: % length particle sizes, 0 (classical catalysts), 0 (CD catalysts); % surface particle sizes, A (classical catalysts), v (CD catalysts). 2-MP-4-t3’C+n-H-3-t3C: % length particle sizes, 0 (classical catalysts), n (CD catalysts); % surface particle sizes, 0 (classical catalysts), + (CD catalysts), 0 Euro-Pt catalyst.
F. Garin et al. / Isomerizarionof ‘3C labeled hexanes
471
hydrogenolysis and the relative contribution of this mechanism increases with the amount of length or surface particle sizes. On CD catalysts the ratio is higher than 0.4; it may suggest a competition between two types of hydrogenolysis: a purely non-selective one and an allylic mechanism [lo]. In view of these results one may be tempted to attribute the observed catalytic differences to the nature of the active sites. For the two reactions of isomerization studied 2-MP+ 3-MP and 2-MP+n-H, it appears that the % of cyclic mechanism is always high and does not change much (80 to 100%) for isomerization of 2-MP+ n-H and for particle sizes in the range 0 to 10 8, (the length particle sizes varying between 20 to 80%). As proposed by Gault [3] the observed particle size effect in the case of platinum catalysts could be accounted for by changes either in electronic properties or a geometrical factor which could become more important than the electronic properties for particles in the range 20-30 A. The particle size effects observed for the isomerization of 2-MP + 3-MP, especially the sharp increase of the percentage of the cyclic mechanism on the catalysts with particles smaller than 10 A, confirm the previous results [9]. However it is very interesting to notice the differences in selectivity for the reaction 2-MP + n-H for catalysts having the same mean size of particle but prepared from inorganic clusters. On the other hand, experimental evidences seem to indicate that electronic rather than geometrical factors should be considered.
References [l] Y. Barron, G. Maire, D. Cornet, J.M. MuUer and F.G. Gault, J. Catalysis 2 (1963) 152; 5 (1966) 428. [2] J.R. Anderson, R.J. McDonald and Y. Shimoyama, J. Catalysis 20 (1971) 147. [3] J.M. Dart&es, A. Chambellan, S. Corolleur, F.G. Gaul& A. Renouprez, B. Moraweck, P. Bosch-Giral and G. Dalmai-Imelik, NOW. J. Chim. 3 (1979) 591. [4] T.A. Darling and R.L. Moss, J. Catalysis 5 (1966) 111. [5] P. Chini, G. Longoni and V.G. Albone, Advan. Organometal. Chem. 14 (1976) 285. [6] F. Garin and F.G. Gault, J. Am. Chem. Sot. 97 (1975) 4466. [7] G. Dalmai-Imelik, G. Leclercq and I. Mutin, J. Microsc. Electron. 20 (1974) 123. [8] 0. Zahraa, These d’Etat, Strasbourg (1980). [9] F.G. Gault, F. Garin and G. Maire, in: Growth and Properties of Metal Clusters, Ed. J. Bourdon (Elsevier, Amsterdam, 1980) p. 451. [lo] F.G. Gault, Gazz. Chim. Ital. 109 (1979) 255. [ll] M.R. Hoare and P. Pal, Nature Phys. Sci. 236 (1972) 35; J. Crystal Growth 17 (1972) 77. [12] J.J. Burton, Catalysis Rev. Sci. Eng. 9 (1974) 209.
ISOMERIZATION
OF ‘C
“ORGANOMETALLIC
LABELED IIEXANES ON Pt/A&
CLUSTER DERIVED”
EFFECT OF THE METAL PARTICLE
F. GARIN,
0. ZAHRAA,
C. CROUZET,
AND ON
CATALYSTS;
SIZE
J.L. SCHMITT and G. MAIRE
Laboratoirede Catalyse, Vniversitl Louis Pasteur, 4 rue Blake Pascal, F-67000 Strasbourg,France Received 8 September 1980
Skeletal isomerization of 2-methylpentane (2-MP) and hydrogenolysis of methylcyclopentane (MCP) have been studied over a series of Pt/AlzOj and “cluster derived” (CD) catalysts of high dispersions. 13C labeling allowed estimation of the relative contributions of cyclic mechanisms in isomerization. High resolution electron microscopy was used jointly to determine the metal particle size distributions. In the isomerization of 2-MP to 3-MP the relative amount of selective cyclic mechanisms increases with the percentage of surface or length particles (d c ld A) whatever the catalyst used. In the isomerization of 2-MP to n-hexane for catalysts having the same mean size of particles, the CD catalysts show differences in selectivity compared to the classical Pt/AlzOz catalysts. The observed particle size effect could be explained by changes in electronic or geometrical factors, especially it is proposed that the electronic rather than the geometrical one should be considered in the case of the “organometallic cluster derived” catalysts.
1. Introduction
A major problem in heterogeneous catalysis is characterization of the active sites. A possible solution, especially for metals, is to find a correlation between particle size and some catalytic properties. Much work has been done on the effect of metal particle size either on selectivity or on specific rates for various catalytic reactions. The selectivity approach to the particle size effects resulted in systematic studies of the isomerization of hexanes on metal films and supported catalysts sintered or calcined [14]. Isomerization of 13C labeled hexanes and hydrogenolysis of methylcyclopentane are very sensitive chemical probes to study metal catalysts. Applied successfully to investigate particle size effects they have been used in the present work to study Pt/A1203 and “organometallic cluster derived” catalysts.
2. Experimental
The catalysts studied have been obtained by two different modes. A first series of X%Pt/A1203 catalysts with various Pt loading (X% = 0.2, 2.25, 4.1, 7.1 in weight) has*been prepared by impregnation of three batches of commercial Woelm alumina with different 0039-6028/81/0000-0000/$02.50
@ North-Holland
Publishing Company
467
F. Garin et al. I Isomerizationof 13C labeled hexanes
superficial OH content, A, A’ and B’. B &,,, is an alumina A’ which has been calcined at 600°C in air during 210 h. A second series of Pt catalysts has been obtained from Chini’s clusters [5], [Pt3(C0)3p2(C0)3]i(n = 2, 3, 4, -5) deposited on B’ alumina, previously degased, exposed to air at room temperature. In all cases reduction was performed at 100°C with a hydrogen flow rate of lOml/min and completed at 200°C for 48 h. The synthesis of the carbon-13 labeled hydrocarbons used as reactants, the differential reactor and the procedure for catalytic experiments have been described elsewhere [3,6]. The characterization of the catalysts have been obtained by chemisorption measurements and by transmission electron microscopy (TEM). dH = 8.5/a is related to hydrogen chemisorption measurements and is conventionally calculated from the dispersion (I by using the relationship a X dH = 8.5 which holds when assuming cubic crystallites [7]. The dispersion of platinum was ascertained by using a Philips EM300G TEM and the extractive replica technique. Platinum particles having a diameter as low as 5 8, could be detected.
3. Results and discussion Two labeled hexanes, 2-methylpentane-2-13C and 2-methylpentane-4i3C, allowed us to distinguish between cyclic and bond shift mechanisms in two isomerization reactions: 2-MP + 3-MP, 2-MP + n-H respectively (fig. 1). Besides the two test reactions, hydrogenolysis of methylcyclopentane was also investigated, since in this reaction the ratio 3-MP/n-H allows simply to estimate the contributions of selective and non-selective mechanisms (fig. 2). The experiments were done at 255 2 5°C for classical Pt/A1203 catalysts and at 270 + 5°C for “organometallic cluster derived” catalysts under 101 kPa of hydrogen for isomerization of hexanes and at 220°C for MCP hydrogenolysis. Some of the more significant catalytic results are reported in table 1 as function of the various types of catalysts, the mean particle size, the ratios a = H/R between chemisorbed hydrogen atoms and platinum atoms. dH = 8.5/a determined by hydrogen chemisorption and d, by TEM are in good agreement. The activity measured, at constant space velocity, by the total conversion (Yr in moles is higher for the Pt/A1203 catalysts than for the CD catalysts. Moreover, the reaction temperatures for isomerization of hexanes are lower for the former catalysts as seen in
[d
I)
a
Fig. 1. Isomerization of ‘3C-labeled hexanes.
-+-
‘/2
‘12
468
F. Garin et al. / Isomerization of ‘“C labeled hexanes
Fig. 2. Hydrogenolysis
of methylcyclopentane.
table 1 and the selectivity remains unchanged. But it is worthwhile catalysts lead to a lowering of the rate of coke deposition resulting stability as shown in table 2 [a]. On classical Pt/Al,O, catalysts injections one may observe a deactivation lowering %q. Recently, shown that the percentage of cyclic mechanisms for isomerization
to notice that the CD in an increase of the after 5 hydrocarbon Gault et al. [9] have remains constant on
Table 1 Isomerization of ‘3C-labeled hexanes and hydrogenolysis of MCP Catalysts (%Pt/AIZ03)
7.1, BZIO~ 4.1. Bzloh 2.25, Ae 5.5, B’
a = H/Pt
dH
CT,,
(*)
ZM
c”C)
254 254 275 275
0.35 0.55 0.04 -
24 15 212
36 17 100 20
% (YT (mole)
Sela
%CMb
18 11
56 55
16 30 44 44
90 86 92 92
0.7 0.4 0.7 0.57
57
51
90
0.62
50 41.5
62 67
97 -
0.4 0.73
41
80
82
0.7
_
83
100
0.42
(CD, n = 4) 0.56, B’ (CD, n = 5) 2.5, klo h 1.1, B’
275
0.43
20
23
3.3
254 275
0.7 0.48
12 18
15 17
15 3.3
(CD, n = 2) 0.4, B’
275
0.54
16
15
1.5
(CD, n = 3) 0.2, A
254
1
a Selectivity, IQ
isomers/q
8.5
20
_
(X 100).
bLC&/\T-“ CLCLW d MCP hydrogenolysis,
//vv
’ Catalyst previously reduced at 200°C sintered at 400°C in Hz
% CM’
MCpd
F. Garin et al.
I Isomerizationof “C labeled hexanes
469
Table 2 Isomerization of 2-MP on a (CD, n = 3) catalyst obtained from Chini’s clusters [5] Catalyst
Injection No.
0.4% Pt (CD, n = 3)
1 70
7.4 7.3
Selectivity, Cg isomerslaT (x 100)
3-MP
n-H
MCP
37 40
38 37
46 46
16 17
particles larger than 10 A, which shows that the sites responsible for the bond shift and the cyclic mechanism are topographically similar, i.e. both involve or do not involve edge atoms. Since extremely dispersed catalysts are very active for isomerization they believed that both types of isomerization sites include edge atoms. Moreover, for Gault [lo] the limiting size of 10 8, and not 20 8, below which an enhancement of the cyclic mechanism is observed, seems to favour (over the Bs-sites model) any model involving polytetrahedra as that of Hoare and Pal [ll] or pseudocrystals with Dsh or icosahedral symmetry [12]. Table 1 shows that & varies in the range 15-35 A for very different catalysts except for the sintered one whereas the percentage of cyclic mechanism for isomerization of 2-MP to 3-MP is highly affected changing from 16 to 83%. Focusing our attention on the catalytic behaviour when very small particles lower than 108, are present, and assuming that the aggregates in the range 5-10 8, are responsible for the cyclic mechanism we should obtain a correlation between the percentage of CM versus the percentage of surface particle sizes. In the fig. 3 are plotted the percentage of cyclic mechanism for the reactions 2-MP-2-i3C+ 3-MP-3-13Cand 2-MP-4-13C+ n-H-3-i3C versus the percentages of surface particle sizes 5 < %nidf/hidf s 10 A and the length particle sizes 0 d % &/8ni S 10 A respectively. The following observations have to be underlined: (i) the selective cyclic mechanism contribution in the reaction 2-MP+3-MP increases whatever the catalysts used, with the amount of surface particle sizes or the length particle sizes. Only particles in the range of 5 to 108, seem to be the unique sites responsible for this type of isomerization. The correlation between the %CM and the percentage of surface particle sizes fits much better than the curve %CM = f(%&ni) s 10 A. (ii) Th e non-selective cyclic mechanism in the reaction 2-MP+ n-H shows two distinct phenomena. On classical Pt/A1203 catalysts where the ratio r = 3-MP/n-H in MCP hydrogenolysis is equal to 0.4 (table 1) the cyclic contribution increases with the amount of length particle sizes in the range of 0 to 10 A. On the other hand the CD catalysts lead to a decrease of the percentage of CM, and moreover the ratio r is higher: 0.57, 0.62 and 0.7 for n = 4, n = 5 and n = 3. r decreases with the %CM. (iii) The same feature is observed when plotting the % of CM = f(% nidf/Xnidf) in the range 5 to 10 A. The isomerization of 2-MP+ 3-MP via a cyclic mechanism involving in the intermediate species the rupture of a disecondary carbon-carbon bond is independent for both classical and CD catalysts. On the other hand for isomerization of 2-MP+ n-H the rupture of a secondary-tertiary C-C bond is highly dependent on the catalyst preparation. On classical supported Pt/A1203 catalysts, rh,d= 0.4, we observe a pure non-selective
F. Garin el al. i Isomerization of 13C labeled hexanes
470
%C:M
. A
0
0
Fig 3. Percentage of cyclic mechanisms versus the percentages of surface and length particle sizes. 2-MP-2-“C+ 3MP-3-“C: % length particle sizes, 0 (classical catalysts), 0 (CD catalysts); % surface particle sizes, A (classical catalysts), v (CD catalysts). 2-MP-4-t3’C+n-H-3-t3C: % length particle sizes, 0 (classical catalysts), n (CD catalysts); % surface particle sizes, 0 (classical catalysts), + (CD catalysts), 0 Euro-Pt catalyst.
F. Garin et al. / Isomerizarionof ‘3C labeled hexanes
471
hydrogenolysis and the relative contribution of this mechanism increases with the amount of length or surface particle sizes. On CD catalysts the ratio is higher than 0.4; it may suggest a competition between two types of hydrogenolysis: a purely non-selective one and an allylic mechanism [lo]. In view of these results one may be tempted to attribute the observed catalytic differences to the nature of the active sites. For the two reactions of isomerization studied 2-MP+ 3-MP and 2-MP+n-H, it appears that the % of cyclic mechanism is always high and does not change much (80 to 100%) for isomerization of 2-MP+ n-H and for particle sizes in the range 0 to 10 8, (the length particle sizes varying between 20 to 80%). As proposed by Gault [3] the observed particle size effect in the case of platinum catalysts could be accounted for by changes either in electronic properties or a geometrical factor which could become more important than the electronic properties for particles in the range 20-30 A. The particle size effects observed for the isomerization of 2-MP + 3-MP, especially the sharp increase of the percentage of the cyclic mechanism on the catalysts with particles smaller than 10 A, confirm the previous results [9]. However it is very interesting to notice the differences in selectivity for the reaction 2-MP + n-H for catalysts having the same mean size of particle but prepared from inorganic clusters. On the other hand, experimental evidences seem to indicate that electronic rather than geometrical factors should be considered.
References [l] Y. Barron, G. Maire, D. Cornet, J.M. MuUer and F.G. Gault, J. Catalysis 2 (1963) 152; 5 (1966) 428. [2] J.R. Anderson, R.J. McDonald and Y. Shimoyama, J. Catalysis 20 (1971) 147. [3] J.M. Dart&es, A. Chambellan, S. Corolleur, F.G. Gaul& A. Renouprez, B. Moraweck, P. Bosch-Giral and G. Dalmai-Imelik, NOW. J. Chim. 3 (1979) 591. [4] T.A. Darling and R.L. Moss, J. Catalysis 5 (1966) 111. [5] P. Chini, G. Longoni and V.G. Albone, Advan. Organometal. Chem. 14 (1976) 285. [6] F. Garin and F.G. Gault, J. Am. Chem. Sot. 97 (1975) 4466. [7] G. Dalmai-Imelik, G. Leclercq and I. Mutin, J. Microsc. Electron. 20 (1974) 123. [8] 0. Zahraa, These d’Etat, Strasbourg (1980). [9] F.G. Gault, F. Garin and G. Maire, in: Growth and Properties of Metal Clusters, Ed. J. Bourdon (Elsevier, Amsterdam, 1980) p. 451. [lo] F.G. Gault, Gazz. Chim. Ital. 109 (1979) 255. [ll] M.R. Hoare and P. Pal, Nature Phys. Sci. 236 (1972) 35; J. Crystal Growth 17 (1972) 77. [12] J.J. Burton, Catalysis Rev. Sci. Eng. 9 (1974) 209.