J. Phys. Chem. 1982, 86, 5136-5144
5136
Catalysis by Supported Clusters. Characterization of Highly Dispersed Zerovalent Iron Covered with Dissocfated CO Obtained by Thermal Decomposition of [HFe,(CO),,]and Fe,(CO),, Supported on Magnesia F. Hugues, J. A. Dalmon, P. Busslere, A. K. Smlth, J. M. Basset; Institut de Recherches sur la Catalyse, CNRS, 69626 Vilieurbanne, Cedex, France
and D. Olivier Laboratoire de Chimie des Solides, Universit6 de Paris VI, 75230 Paris, France (Received: February IO, 1982; In Final Form: July 23, 7982)
Thermal decomposition of the supported clusters [ HFe3(CO)11]-/Mg0and Fe3(CO)12/Mg0results in heterogeneous catalysts which are selective for low molecular weight olefins in the Fischer-Tropsch synthesis. Characterization of the metal particles resulting from such decomposition has been carried out by using IR, ferromagnetic resonance (FMR),Mossbauer, magnetism, and gas-phase analysis. The results depend on the state of dehydration of the support: on highly hydrated magnesia (MgO 25), [HFe3(CO)11]-Mg+undergoes an electrophilic attack by surface protons with formation of a significant amount of Fez+and H2 which makes difficult the characterization of zerovalent iron. On highly dehydrated magnesia (MgO 400), thermal decomposition of the supported Fe3(C0112is irreversible and the zerovalent iron is not oxidized by surface protons. During the decarbonylation process the iron agglomerates into a superparamagnetic particle of zerovalent iron covered with dissociated CO.
Introduction Chemisorption of Fe(C0)5or Fe3(C0)12on basic supports such as alumina or magnesia leads to the formation of the anionic hydrido cluster [HFe3(C0)JM+ (M = Al, Mg) as well as other anionic species.' Thermal decomposition of such supported clusters gives heterogeneous catalysts which are highly selective for low molecular weight olefins, in the Fischer-Tropsch synthesis.2 Such catalysts are also selective for the homologation of ethylene to propylene and of propylene to n- and i s ~ b u t e n e s . ~ We report in this paper the state of iron and the particle size of zerovalent iron resulting from the thermal decomposition of Fe3(C0)12adsorbed on magnesia. The results indicate that the starting cluster Fe3(C0)12decomposes to a very small particle containing about 100 Fe atoms covered with dissociated C0.4 Decomposition of supported iron carbonyls were already suspected to produce extremely well-dispersed metal particles by electron microscopy (Fe(CO),/H-Y ~ e o l i t e or ) ~ by chemisorption measurements (Fe3(CO)12/alumina).6 However, due to the (1) F. Hugues, A. Smith, Y. Ben Taarit, J. M. Basset, D. Commereuc, and Y. Chauvin, J. Chem. Soc., Chem. Commun., 68 (1980); F. Hugues, A. K. Smith, Y. Ben Taarit, and J. M. Basset, J. Am. Chem. Soc., in press. (2) D. Commereuc, Y. Chauvin, and F. Hugues, J.Chem. Soc., Chem. Commun., 154 (1980). (3) D. Commereuc, Y. Chauvin, F. Hugues, J. M. Basset, and D. Olivier, J. Chem. Soc., Chem. Commun., 154 (1980); F. Hugues, B. Besson, P. Bussiere, J. A. Dalmon, J. M. Basset, and D. Olivier, Nouu. J. Chim., 5, 207 (1981). (4) So far molecular iron clusters are extremely good models for CO dissociation and hydrogenation on Fe surfaces. However, and to our knowledge, only elementary steps for CO hydrogenation have been shown. See, for example: M. Tachikawa and E. L. Muetterties, J. Am. Chem. Soc., 102,4541 (1980); M. Bend, J. M. Williams, M. Tachikawa, and E. Muetterties, ibid., 102, 4542 (1980); J. S. Bradley, G. B. Ansel, and W. E. Hill, ibid., 101, 7417 (1979); C. E. Summer, P. E. Riley, R. E. Davis, and R. Pettit, ibid., 102,1752 (1980); K. Whitmire and D. F. Shiver, ibid., 102, 1456 (1980). The Fischer-Tropsch reaction has been carried out recently with supported iron clusters. See, for example, G. B. McVicker and M. A. Vannice, J . Catal., 63, 25 (1980). (5) J. B. Nagy, M. Van Eenoo, and E. Derouane, J. Catal., 38, 230 (1979).
difficulty of characterization of supported iron, the agreement between various physical methods such as infrared spectroscopy, ferromagnetic resonance, magnetic measurements, Mossbauer spectroscopy, and electron microscopy was considered as of primary importance. Experimental Section Magnesia Support. The magnesia support (96 m2 g-'I was obtained by decomposition, under high vacuum at 400 "C of high-purity Mg(OH12. It was there subjected to several cycles of oxygen treatment (500 torr; 0.5 h) and vacuum treatment torr; 14 h) at the same temperature. This procedure allowed the support to be degassed and dehydrated, and reduced the amount of carbonate present. The supports obtained in this way could be rehydrated by introducing water vapor (18 torr at 25 " C ) for 2 h, and then treatment under vacuum (IO4 torr) at a given temperature (25 or 400 "C) allowing the degree of hydration to be varied in a controlled manner. Such support contained a very small amount of Fe3+ (less than 0.1%) which gave a weak ESR signal at g = 4.4. Analysis of the Gas Phase. The thermal decomposition studies were carried out in the equipment depicted in Figure 1. The solvent first was carefully dehydrated over zeolite previously treated at 250 "C under vacuum and then was degassed by repeated freezing (tube A). The solvent was condensed, through a break-seal, into tube B, which contained a known amount of cluster. The solution of the cluster was then introduced into tube C having been previously degassed or thermally treated a t a given temperature. After the solution was stirred for 6 h, the solvent was removed under vacuum and tube C was isolated from tube B by sealing under vacuum. The gas phase was then analyzed by removing a sample from vessel F which was isolated from tube C by a stopcock so as to avoid any contamination by traces of air which might be present in (6) A. Brenner, J. Chem. Soc., Chem. Commun., 251 (1979);A. Brenner and D. A. Jucul, Inorg. Chem., 18, 2836 (1979).
0022-3654/82/2086-5 136$01.25/0 0 1982 American Chemical Society
Catalysis by Supported Clusters
The Journal of Physical Chemistry, Vol. 86, No. 26, 1982 5137
0
%
y.: :.: :: .;.:..250 "C) CO is consumed, and the parallel formation of H2 and COz is observed. Again this is due to the water-gas shift reaction taking place between the gaseous CO and the adsorbed water present on the surface as hydroxyl groups. ESR and Ferromagnetic Resonance Studies. The infrared spectroscopic studies and the gas analyses discussed
5140
The Journal of Physical Chemisrfy, Vol. 86, No. 26, 1982
Hugues et
al.
Flgure 6. ESR spectra of Fe,(CO),,/MgO 25 decomposed under vacuum at 120 "C: (a) spectrum obtained at 25 "C; (b) spectrum obtained at -196 "C.
above indicate that Fe,(CO),,/MgO is thermally decomposed at about 120 "C and that this decomposition leads to a mixture of zerovalent and oxidized iron species. ESR and ferromagnetic resonance studies were carried out to try to characterize these iron species. No ESR signals were observed for the initial species adsorbed on a hydrated magnesium oxide, [HFe,(CO)ll]-/MgO 25, except for very weak signals due to the presence of impurities (Fe3+)on the support. This is in agreement with the diamagneticcharacter of [HFe,(CO),,](ref 10) and indicates the absence of decomposition of the iron cluster during adsorption. After decomposition of [HFe3(CO)11]-Mg025 at 120 "C for 14 h under vacuum, the ESR spectrum shown in Figure 6 is obtained. The spectrum consists of a very broad resonance (AH = 3150 G) at -196 "C, which is considerably sharpened when the spectrum is obtained at 25 "C (AH = 220 G). From the spectrum obtained at 25 "C, a value for the g factor may be obtained, g25 = 2.08. The broad band and its very significant decrease in width as the temperature is increased from -196 to 25 "C are characteristic of ferromagnetic compounds.l' This result therefore prompted us to study the ferromagnetic behavior of Fe,(CO),,/MgO. The initial sample studied was Fe3(C0)12on a dehydrated magnesium oxide, Fe3(CO)12/Mg0400. This shows only a very weak signal at g = 4.4, corresponding to paramagnetic Fe3+which is present as an impurity on the support. After decomposition for 14 h at 130 "C under vacuum, the ferromagnetic resonance spectrum of the sample gives a very broad signal when obtained at -196 "C, but the width decreases as the temperature is raised to 150 "C (Figure 7a). At 25 "C, AH = 1050 and g = 2062. As noted earlier, the decrease in bandwidth with increasing temperature is a characteristic of ferromagnetic compounds, which can be either zerovalent iron or a ferromagnetic iron oxide such as magnetite, Fe304. The spectrum obtained at -196 "C shows no signal due to paramagnetic Fe3+. In addition, after oxidation of the sample at 120 "C by activated O2 (see Experimental Section), the spectrum obtained at 25 "C (Figure 7 c ) has significantly changed from the initial spectrum. The signal obtained after this oxidation (g = 200) is characteristic of a species containing paramagnetic Fe3+.12This indicates that the initial ferromagnetic species is zerovalent iron. A complete study of the ferromagneticresonance spectra has provided further information concerning the iron particles obtained, by comparing our results with those previously obtained with samples of iron and nickel.'l For example, the absence of any variation in the detection current during the resonance indicates that the sue of the iron particles is less than 100 A, as has been previously observed with various sizes of nickel particles." Also, the decrease in intensity of the signal as the sample
Figure 7. Ferromagnetic resonance spectra of Fe,(CO),,/MgO 400 decomposed at 130 "C under vacuum (sample A): (a) sample A, spectra obtained at -196, 25, and 150 "C; (b) after decomposition of sample A under vacuum at 200 "C, spectrum obtained at 25 "C; (c) after oxidation of sample A by 0,' at 130 "C, spectrum obtained at 25 "C.
t
intensity of FMR signal
, 0
100
200
"Ct
Flgure 8. Thermomagnetic curve for Fe,(CO),,/MgO 400.
temperature is raised from 50 to 150 "C (Figure 8) indicates superparamagnetic behavior of the iron parti~1es.l~ Such behavior has been observed in ferromagnetic resonance studies of iron particles with diameters less than 35 A.14 The value of the g factor (g = 2.062 at 25 "C), which is less than the value for bulk iron (g = 2.12-2.17),15 and the anisotropy of the spectra again indicate that the iron particles obtained have a very small diameter. In general, magnetic anisotropy can be due to a crystalline anisotropy, to a structural anisotropy, or to magnetostriction. However, it has been shown that, in the case of iron, magnetostriction is negligible compared to the other forms of anisotropy, and structural anisotropy does not vary with temperature. Crystalline anisotropy decreases as the temperature increase^,'^ so that the observed anisotropy appears to be essentially of this type. Its origin may be
The Journal of Physical Chemistry, Vol. 86, No. 26, 1982 5141
Catalysis by Supported Clusters
2 5°C
t
intensity of FMR signal
-196°C
0 Figure
I
Figure 9. Ferromagnetic resonance spectra of Fe,(CO),,/MgO 25 decomposed at 120 "C under vacuum (sample B): (a) sample B spectra obtained at 25 and -196 "C; (b) after oxidation of sample B by 02*at 120 "C, spectra obtained at 25 and -196 "C.
found in the crystalline anisotropy of the surface atoms of the particle, indicating that the surface-to-volumeratio of this particle is high, again illustrating the very small size of the iron particles. In the case of nickel, a comparable anisotropy of the surface atoms corresponds to a particle diameter less than 20 The curves obtained at low temperature (between -196 and 25 OC) all pass through an isosbestic point. This isosbestic point is shown either from the derivative signals, by superimposing the spectra, or from the double derivative signals,by taking the minimum of the curve obtained. This second method was used as it is more accurate, particularly in our case where the width of the bands is considerable. The existence of an isosbestic point, by analogy with work carried out on nickel particles,16 indicates a homodispersionof the metal particles. In addition, this point corresponds to the value of the g factor at low temperature. If the sample is heated to a higher temperature, the spectra change. Below 200 "C no change in the form or intensity of the ferromagneticresonance signal is observed. However, above 200 "C, the intensity of the signal increases (Figure 8), the width of the band decreases (AH= 250 G at 25 "C), and the asymmetry disappears (Figure 7b). This decrease in bandwidth and loss of anisotropy is due to an increase in the size of the iron particles, by sintering, which takes place above 200 "C. For Fe3(C0)12adsorbed on a hydrated magnesium oxide, the decomposition under vacuum at 120 "C of the initial surface species, [HFe3(CO)J/Mg0 25, gives a sample whose ferromagnetic resonance spectrum is shown in Figure 9. Again, the spectra obtained at -196 "C (AH= 3350 G ) and at 25 "C (AH= 250 G, g = 2.065) are char-
100
200
300
"c
-
IO. Thermomagnetic curve for Fe3(C0),2/Mg0 25.
acteristic of zerovalent iron particles. The thermomagnetic curve for this sample (Figure 10) also shows the superparamagnetic behavior of these particles. However, for this sample, the narrower bandwidth (250 G) and the symmetry of the spectrum obtained at 25 "C (Figure 9a) indicate a rather larger particle size than that obtained from the decomposition of Fe3(CO)12/Mg0400. In addition, the paramagnetic resonance at g = 4.4, attributed to Fe3+,and which is not observed for Fe3(CO)12/Mg0400, indicates that partial oxidation of the iron takes place during decomposition. The iron particles are completely oxidized by 02*at 120 "C to a ferromagnetic species (Figure 9b) characterized by g = 200, corresponding to Fe3+in an oxide of the type Fe30,. As in the case of the dehydrated magnesium oxide, the decomposition of Fe3(CO)12/Mg0 25 at temperatures above 200 "C leads to an increase in the intensity of the ferromagneticresonance signal (Figure lo), and a decrease in the bandwidth, indicating an increase in the size of the iron particles. Mossbauer Spectroscopy and Magnetic Measurements. The previous investigations (gas analysis and ferromagnetic resonance spectroscopy) have shown that the decomposition of Fe3(C0)12on dehydrated magnesium oxide takes place without oxidation of the metal by the hydroxyl groups on the support and leads to very small iron particles, which cannot be observed by electron microscopy. We therefore extended this study by using Mossbauer spectroscopy and magnetic measurements in order to determine the exact nature of the metallic phases present after decomposition and to measure the size of the iron particles. These investigations have been carried out by using Fe3(CO),/Mg0 400 decomposed under vacuum for 14 h at three different temperatures, 130,200, and 300 "C. The Mossbauer spectra obtained after the decomposition of Fe3(CO)12/Mg0400 at 130 "C and then at 200 "C are shown in Figures 11 and 12, respectively. The spectra were run at 25 "C. Both spectra show an absorption peak at 0.3 mm s-l containing a shoulder on the positive velocity side, and a weaker absorption peak at 2.3 mm s-l. On its own, the peak at 2.3 mm s-l cannot be assigned and, therefore, is assumed to be part of a quadrupole doublet of which the major part of the second line is hidden under the peak at 0.3 mm s-l. The spectrum is in good agreement with this assumption, which leads to the spectral parameters given in Table 11. The peaks situated a t IS = 0.32 mm s-l and IS = 0.34 mm s-l in the two samples can only be assigned to metallic iron. The fact that a single line is observed for FeO instead
Hugues et al.
5142 The Journal of Physical Chemistry, Vol. 86, No. 26, 7982
//
d,
25c
2.035
-2
-1
0
1
2
3
mm/s
Mossbauer spectrum obtained after decomposition Fe,(CO),,/MgO 400 at 130 O C . Figure 11.
of
Yz
o
20
H( k O e L
Flgure 13.
,v
1.254
?
l
o
1
2
3
mm/s
Mossbauer spectrum obtained after decompositlon Fe,(CO),,/MgO 400 at 200 O C . Flgure 12.
of
TABLE 11: Spectral Parameters 1 3 0 "C
IS, mm
s-l
W,m m
s"
0.32 0.58
QS, mm s-l
0.00
SP, %
36 FeO
species
200 " C
1.52
0.34
0.16 1.54 64
0.66
Fez+
0.00 50 FeO
1.51 0.74 1.31 50 Fez+
of the sextet usually obtained shows that, as already determined, the iron is in the form of superparamagnetic particles1'J8 surely smaller than 50 A. The quadrupole doublet is attributed to Fe2+,present as a separate phase or surface species since these ions inserted into MgO in substitutional positions would give rise to a single line instead of a doublet as a result of a dynamical Jahn-Teller effect.18 Nothing more can be said about the structure corresponding to these Fez+ions because it could involve surface species or a highly disperse phase as well, with rather ill-known effects on IS and QS values. The relative contents in metallic iron and Fe2+cannot be precisely determined, mainly due to the uncertainty on the Lamb-Mossbauer factors f. Taking into account the line widths results in approximately equal amounts of
metal and ionic form in the two samples. Magnetic measurements were carried out on different samples: support MgO 400 and Fe,(CO),,/MgO 400 before decomposition and after decomposition at 130,200, and 300 OC. Figure 13 shows the results as obtained with the electromagnet at 77 and 300 K. The support alone (plot a) shows a negative susceptibility, corresponding to the diamagnetism of the support. The supported cluster Fe,(CO),,/MgO 400 before decomposition (plot b) shows behavior very similar to that of the support alone, in agreement with our earlier finding that the initial diamagnetic cluster does not decompose upon adsorption. After decomposition of the cluster at 130 OC, the magnetization curves are strongly modified. The plots obtained at 300 and 77 K, after correction for the magnetization of the support (plot d), suggest the paramagnetic character of the surface species. Mossbauer spectroscopy has shown that these surface species are FeO and Fe2+,and the linearity of the magnetization plots observed at 77 K in magnetic fields of up to 21 kOe suggests that the iron in the metallic state is very highly dispersed. Under these conditions, it is not possible to calculate the saturation magnetization and difficult to obtain the size of the iron particles. For this, it was necessary to carry out the magnetic measurements at higher field strengths and at lower temperatures as obtained in the supercondutivecoil. The results of this study are given in Figures 14 and 15. Again we see that the magnetization curve for the Fe,(CO),,/MgO 400 sample before decomposition (Figure 14b) is similar to that of MgO 400 alone (plot a), in agreement with the magnetic measurements carried out at 25 "C. The positive magnetization observed at low temperature for these samples could be due to some paramagnetic impurities of the support. Thermal decomposition at 130, 200, or 300 "C leads to a large increase in magnetization. The magnetization curves obtained after subtraction of the contribution due to the support are given in Figure 15. These curves are characteristic of a ferromagnetic species.
The Journal of Physical Chemistry, Vol. 86, No. 26, 1982 5143
Catalysis by Supported Clusters
TABLE 111: Magnetic Measurements at 4 K on
Az
Fe,(CO),,/MgO 400 Decomposed at 130, 200, and 300 "C
M,, emu cgs m , mg mlm,, 7% D,, A D,, it D = (D, t D,)/2, A
130°C
200°C
300°C
1.16 4.7 45 17 k 2 622 12 k 2
1.36 5.5 52 16 7 12
0.65 2.6 25 21 6 14
M,: magnetization at saturation (emu cgs/g of catalyst), m : mass of ferromagnetic iron. m,: total mass of iron (10.5 mg). m/m,: % iron in zerovalent state. D, : average diameter of large particles. D , : average diameter of small particles. D : surface average diameter of all particles,
Flgure 14.
0'
20
40
60
HkOem
Flgure 15.
The absence of a paramagnetic contribution due to the Fez+ species, shown to be present by Mossbauer spectroscopy, is notable. Most iron oxides have a Nee1 temperature close to room temperaturesJ5and therefore have a magnetic susceptibility of almost zero at about 0 K. We may therefore assume that the Fe2+species is in an antiferromagnetic state at the temperature (4 K) at which these measurements are made. This accounts for the magnetization curves obtained which are due only to the ferromagnetic contribution of FeO. If we consider the magnetization curves c and d shown in Figure 15, we see that saturation is attained at high field strength. In addition, a very weak remanent magnetization exists (about 2% of the magnetization at saturation). This indicates that the particle size is not greater than a critical value (of the order of 50 A). Under these conditions it is possible to calculate the amount of metallic iron corresponding to the saturation magnetization and to obtain a value for the particle size. The results of these calculations are given in Table 111. The results for the percentage of iron in the zerovalent state are in good agreement with those estimated from Mossbauer spectroscopy. The particle size determination indicates a fairly narrow range of size distribution, with a mean diameter of about 12 A. This mean particle size is the same for decomposition at 130 and 200 "C. If the decomposition is carried out at 300 "C, we note a marked decrease in the amount of FeO and an increase in the mean particle size to 14 A. This suggests that, at this temper-
ature, partial oxidation of the iron to an antiferromagnetic species occurs and that some sintering occurs to give a slightly larger particle size. We note that the formation of oxidized iron species during the decomposition of Fe(CO),, Fe2(C0)9,and Fe3(C0)12adsorbed on dehydrated alumina has been reported.6 Determination of Quantity of Carbon Deposited during the Thermal Decomposition of Fe3(CO)lz/Mg0400. It was mentioned earlier that carbon is deposited during the thermal decomposition of Fe3(CO)lz/Mg0 25 and Fe3(CO),,/MgO 400,indicating that dissociation of CO takes place in the presence of FeO particles. We therefore investigated whether carbon deposition occurs in the decomposition temperature range 130-200 "C for a sample of Fe,(CO),,/MgO 400. Microanalysis showed that a significant quantity of carbon is indeed deposited at these low temperatures, but the amount could not be determined accurately due to the presence of adsorbed COz.24 We therefore used another method based on the hydrogenation of carbon to methane, followed by a gas-chromatographic determination of the methane. Fe,(CO),,/MgO 400 was decomposed at 200 "C under vacuum for 14 h. Any carbon formed was converted to methane (90%), with some higher saturated hydrocarbons (ethane, propane, butane), by two successive treatments with hydrogen. The amount of carbon conrtained in the hydrocarbons formed was found to be 1.7 f 0.2 mol of C/mol of cluster. This result, together with the observation that approximately50% of the iron is oxidized to Fez+ during the decomposition, suggests that CO dissociation occurs in the following way:
CO dissociation on iron is known to take place even at 25 "C9 and it has been shown that this dissociation leads to a species of the type FepC, and a species of the type Fe=O (where the iron is divalent).
Discussion Thermal decomposition of molecular iron cluster carbonyls supported on magnesia depends strongly on the pretreatment temperature of the support. On magnesia 25, Fe3(C0)lzis chemisorbed mainly as [HFe3(CO)11]-Mg+. Thermal decomposition of [HFe,(CO),,]- produces evolution of HP. The mechanism of such protonation is not clear since it is well-known that in solution electrophilic attack on [HFe,(CO),,]- usually occurs at the oxygen lone pair of a bridging ~ a r b o n y l . ~ ~ ~ ~ ~ ~ ~ ~ Such an hypothesis would be in agreement with the reported case of methane formation by protonation of
5144
The Journal of Physical Chemistty, Vol. 86, No. 26, 1982
[Fe4(C0)13]2-described recently by Shriver: 22 Fe,EZCO +H+ Fe4C -H20
-
H+
CHI
+ Fez++ iron carbonyls
However, in our case methane formation was not observed a t the temperature where decarbonylation of the cluster was observed. It is therefore more likely that an oxidative addition of MgOH groups occurs a t the metal-metal bond of [HFe3(CO)ll]- with breakage of the clusters cage. Such oxidative addition could occur twice a t the same metal center and lead after reductive elimination of H2to the formation of Fez+as it occurs with O S ~ ( C Oon ) ~alumina: ~ 23
However in magnesia, Fe2+ could occupy interstitial positions in the magnesia lattice. It is probably the reason that this Fe2+is not easily reduced by H2. This Fez+ is probably responsible for the water-gas shift reaction which occurs between surface water and the gaseous CO which is produced during the decarbonylation of the cluster. Besides the oxidation of FeO to Fe2+,agglomeration of a fraction of FeO to highly dispersed metal particles covered with CO occurs. The complexity of such a decomposition process is therefore an obstacle to the characterization of the particle of iron on magnesia 25. On magnesia 400 the very low concentration of reactive OH groups makes the decomposition process of supported
Hugues et al.
Fe3(C0)12more simple. First the clusters are chemisorbed mainly as Fe3(C0)12with some [HFe,(CO),J and possibly Fe(C0)4(C02Mg).1Irreversible decarbonylation occurs at ca. 130 “C without release of H2,that is, without oxidation of low-valent Fe by surface protons. The small amount of H2 evolved confirms also the small amount of [HFe,(CO)ll]- Mg+ initially present. This decarbonylation produces zerovalent iron as well as Fe2+as determined by Mossbauer spectroscopy. According to FMR measurements this zerovalent iron exhibits superparamagnetic properties, which is confirmed by magnetic measurements. Besides, magnetic measurements indicate that about 50 % of iron is present as zerovalent iron and 50% as Fe2+. The average size of such superparamagnetic particles was found to be 12 A with a rather narrow distribution of 6-17 A. The remaining Fez+exhibits an antiferromagnetic behavior and can be detected only by Mossbauer spectroscopy. This Fez+probably arises from CO dissociation on zerovalent iron since the amount of gaseous H2 is too small to account for it. Besides the formation of CH4 and higher hydrocarbons after treatment of the decarbonylated cluster under H2 indicates that dissociation of CO had occurred on this “particle” of iron with formation of surface C and Fe2+02-species. The highly dispersed particle would be covered with dissociated CO. Consequently all the techniques that have been used to characterize the final state of iron after thermal decomposition of Fe3(C0)12adsorbed on magnesia 400 are in agreement with the following: Fe is present as a 12-A cluster of superparamagnetic zerovalent iron covered with dissociated CO. This clearly indicates that the nuclearity of the initial cluster is not preserved after thermal decarbonylation. The real nuclearity of the new “cluster” is around 100. It seems therefore that the high selectivity for low molecular weight olefins in the Fischer-Tropsch synthesis observed with supported Fe clusters may be attributed to such particles. It seems also that the appearance of catalytic properties with Fe clusters in the Fischer-Tropsch synthesis will be related with aggregation of molecular clusters to small metal particles for which the metallic frame is more stable than with real molecular clusters.
