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Stabilization effect of oxide supports on the decomposition of iron-ruthenium carbonyl clusters
Surface Science 156 (1985) 995-1002 North-Holland, Amsterdam
995
STABILIZATION EFFECT OF OXIDE SUPPORTS ON Tm DECOMPOSITION OF IRON-R~HENIU~ CARBONYL CLUSTERS Istvan B&SZGRMENYI, and L&z16 GUCZI *
Sandor
DOBOS,
Karoly
LAZAR,
Zoltan
SCHAY
Institute of Isotopes of the Hungarian Academy of Sciences, P. 0. Box 77, H - I.725Budapest, Hungary Received
13 July 1984; accepted
for publication
I8 September
1984
and HaFeRu,(CO),, deposited on Cab-0-Sil and AlaO, have been Ru,(C%, FeaRu(CO),, investigated. Infrared and Mbssbauer spectroscopy revealed a strong interaction between a molecular cluster and alumina which leads to the disintegration of the cluster framework up to monoruthenium subcarbonyl species and iron oxide. Temperature-programmed decomposition showed carbon retained in aiumina-supported catalyst whereas with Cab-0-Sil the ligands from the adsorbed cluster could be recovered. After temperature-programmed decomposition in hydrogen and vacuum. CO adsorption resulted in four IR bands at about 2138, 2075, 2020 and 2043/2050 cm-‘. From these data the formation of highly dispersed bimetallic particles on alumina is discussed.
1. Introduction Molecular bimetallic carbonyl clusters have been used to prepare supported metal catalysts [l-4]. It was assumed that the original metal framework remains intact and in this way bimetallic catalysts of high dispersion and well-defined structure can be prepared. Most recently it was observed that in some cases due to the strong support-cluster interaction the original metal framework is disturbed in the very first steps of the catalyst preparation [2,5]. In H,FeOs3(CO),, supported on silica gel the Fe-OS bond cleaved to form iron and silica grafted tri-osmium species [6]. The same was observed for RuOs, cluster where Ru’, RuZL and OS’+ -carbonyl species were produced on alumina [2]. In a study of the interaction of iron-ruthenium carbonyl clusters with Al,O, we observed [5] that during 12 h evacuation after the impregnation of Al,O, with n-hexane solution of the carbonyl clusters the iron-ruthenium bond is already ruptured at room temperature. Since sometimes in high-temperature reduction bimetallic particles are * To whom correspondence
should be addressed.
0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
996
I. Biirriirmknyi et al. / Decomposition
of Fe-Ru
carbonyi clusters
formed depending on the nature of the bimetallic cluster and the support, our goal was to study the stabilization effect of the support on the very fine metal or bimetallic particles formed and its relevance to CO adsorption.
2. Experimental For the preparation of the catalysts Degussa Al,O, C and Cab-O&l HS 5 (Cabot Co., Mass.) previously evacuated at 573 K overnight, were imprecated by hexane solution of the carbonyl clusters Fe,(CO),,, Ru,(CO),,, Fe,Ru(CO),, and H,FeRu3(C0)i3. The total metal load determined by the XRF method was around 1 wt%. Infrared spectra were measured on a DIGILAB FTS-20 C Fourier transform spectrometer equipped with a Nova-3 computer. A Pyrex infrared cell with a quartz tube heater was used. The catalysts were stored in different atmospheres before pressing into a 25 mm diameter wafer. The cell could be evacuated to 5 x 10e2 Pa. Mossbauer measurements were carried out in a cell which allowed in situ heat treatments in different atmospheres. For the measurements 57Fe-enriched clusters were used. In some cases an in situ impregnation apparatus connected to a mass spectrometer (Kratos MS 10 C 2) was applied to measure CO balance during impregnation and subsequent decomposition. Details of the experimental techniques are given in ref. [7].
3. Results 3. I. Infrared measurements Previously we showed [5] that the iron-containing clusters readily react with Al,O, and there is practically no difference in the IR spectra of the supported bimetallic clusters. All spectra are similar to those of Al,Oj-supported Ru,(CO),, but the CO stretching vibration mode characteristic of the CO-iron system cannot be observed. IR spectra of Al,O,-supported Ru,(CO),, stored overnight in static He and CO, in flowing He and in vacuum are given in fig. 1. For reference, spectra of the cluster in n-hexane and supported on Cab-0-Sil are also included. Fig. 1 clearly shows that under static conditions both in He and in CO the structure of the Al,O,- and Cab-0-Sil-supported cluster is similar. On the other hand, under dynamic conditions in flowing He or in vacuum, Ru(CO), subcarbonyls characterized by bands at 2138, 2070, 2050, 2000 and 1980 cm-i are formed VI.
I. Btiszijrmhyi et al. / Decomposition of Fe-Ru carbonyl clusters
997
When the spectra of Ru,(CO),, having been in contact for 12 h with the support, are compared with those supported on Cab-0-Sil, the band broadening points to the coexistence of various subcarbonyl fragments superimposed to the original cluster framework. If the sample is evacuated, there is a quick process in which the original Ru,(CO),, is further decomposed into a structure similar to that obtained under dynamic condition. This is shown in fig. 2. It is of interest to note that in this process the original Ru,(CO),, is not sublimed as was proved in separate experiments. 3.2. Miissbauer measurements Simultaneously with the IR measurements information was collected from iron by Mossbauer spectroscopy. In agreement with the IR spectra the decomposition of the bimetallic cluster was indicated by the appearance of oxidized iron in the Mossbauer spectra (see fig. 3 curves a and c for Fe,Ru(CO),, and H,FeRu,(CO),,, respectively). During impregnation iron is oxidized to Fe’+ for both clusters as indicated by Mossbauer parameters peak for the (IS = 0.35 mm s-‘, QS = 1.1 mm s-t ). The unresolved
2100 209 Moo 1950 cm-l Fig. 1. IR spectra of Ru,(CO),,/A120, stored for 12 h after impregnation from n-hexane in: (a) vacuum; (b) He flow; (c) static He; (d) static CO; (e) vacuum but Cab-O-Sit supported; (f) unsupported Ruj(C0)t2 in n-hexane. Fig. 2. IR spectra of Ru,(CO),,/AI,03 stored for 12 h in static He and evacuated for (a) 0.05 h; (b) 0.5 h; (c) 1 h; (d) 2 h; (e) 5 h; (f) 12 h; (g) 24 h; (h) 28 h; (i) stored in air for 168 h.
998
I. B&z@rm&yi
et ai. / Decomposition of Fe-Ru
velocity
/
carbonyl dusters
mms‘l
Fig. 3. Miissbauer spectra at 300 K of A120,-supported clusters: (a) F~,Ru(CO)~~ after impregafter nation; (b) F~,Ru(CO),~ after decomposition at 660 K in H,; (c) H,FeRu,(CO),, impregnation; (c) H,FeRu,(CO),, after decomposition at 640 K in H,.
Fe,Ru(CO),,cluster again shows iron in different environments. Reduction in H, flow at 640-660 K results in complete reduction of Fe3+ to Fe2+ (IS = 1.07 mm s-‘, QS = 2.04 mm s-r). There was no magnetic splitting in the spectra at low temperatures, proving the very small dimensions of the particles containing iron ions. 3.3. Temperature-programmed
decomposition
Decomposition in situ in a closed system after impregnation resulted in a fair carbon balance. On Cab-0-Sil-supported clusters the decomposition starts at about 350 K and most of the CO leaves the clusters below 450 K. There is
I. Bijsziirmhtyi
999
et al. / Decomposition of Fe-Ru carbonyl clusiers
molC0 molcluster 98-
IV
l6-
Id0
200
30
T/Y
molC0 molcluster
III II
llIO9al65432b)
l100 Fig. 4. In situ decomposition Cab-O-S&supported clusters; RQCO),z.
200 after impregnation: upper graphs (I) Fe&O),,; (II) Fe,Ru(CO),,;
--z
TPC
Al,O,-supported, lower graphs (III) H2FeRu3(CO),,; (IV)
I. Biisriirmknyr et al. / Decomposition of Fe-Ru carbonyl clusters
1000
an additional CO evolution between 480 and 550 K (see fig. 4). On Al,O,-supported clusters CO evolution is already observed during impregnation [5], and by increasing the temperature, CO is continuously leaving the clusters. Fe,(CO),, seems to be the least stable and H,FeRu,(C0),3 is the most stable on A1203. Due to initial CO loss during impregnation only 7-9 moles of CO are recovered from 1 mole cluster. 3.4. CO adsorption For characterization of bimetallic catalysts formed after the decomposition of bimetallic molecular clusters in vacuum or in hydrogen, the infrared spectra of the adsorbed CO have been studied as shown in fig. 5. CO adsorption measured at 1.2 kPa is characterized by four bands. Three of them can be
2m2000cm’
2
Fig. 5. IR spectra of adsorbed CO on 700 K decomposed clusters: (I) Cab-O-S&supported, decomposed in vacuum; (II) Cab-O&l-supported, decomposed in H,; (III) Al,O+upported, decomposed in vacuum; (IV) Al,O,-supported, decomposed in H,; (a) Fe,Ru(CO),,; (b) H,FeRu,(CO),,; (c) Ru,(CO),,.nTo whom correspondence should be addressed.
I. Biisziirmhtyi et al. / Decomposition of Fe-Ru carbonyl clusters
1001
found in more or less distorted form in all spectra: at about 2140, 2075 and 2020 cm-’ and these are similar to those found for bimetallic catalysts prepared from the mixture of Fe,(CO),, and Ru3(CO)i2 [9]. In addition to these bands, on Cab-0-Sil-supported samples a band at 2043 cm-’ can be observed and on Al,O,-supported sample this band is shifted significantly to higher wavenumber, 2050 cm-‘. No bands can be observed at 1870 cm-’ which is characteristic of bridged bonded CO on ruthenium [lo]. The most characteristic is the band structure of CO adsorbed on Ru/Cab-0-%1/H, which resembles the sample prepared from RuCl, [lo] deposited on SiO, with a particle size of 5 nm. Alumina-supported mono- and bimetallic clusters after hydrogen treatment show a similar band structure, but the band is much broader than its counterpart on Cab-0-Sil. In the vacuum-decomposed samples both for Cab-O-Sil- and Al,O,-supported clusters, the main feature is the appearance of a three-band structure (at 2140, 2075 and 2020 cm-‘). Two of these bands are assigned to CO adsorbed on oxidized ruthenium [11,12] whereas the band at 2020 cm- ’ has not been identified. For Cab-O-Sil-supported samples decomposed in vacuum, the band at 2050 cm-’ also appeared.
4. Discussion IR spectra of Cab-O-Siland Al,O,-supported clusters proved marked differences even after impregnation [5]. On Cab-0-Sil the original structure is retained whereas on Al,O, the clusters decompose because of the strong support-cluster interaction. Mossbauer and IR spectra prove that decomposition leads to the formation of Fe3+ and Ru(CO), subcarbonyls, respectively, in which Ru is also in an oxidized state. During evacuation this oxidation continues, and simultaneously IR spectra show a one-step disappearance of Ru,(CO),, without formation of subcarbonyls. During reduction in hydrogen an interaction between iron and ruthenium develops, Fe3+ is reduced to Fe*+ of high dispersion. The high dispersion of alumina-supported bimetallic cluster has been proven by low-temperature Mossbauer measurements [5] as well as in catalytic CO + H, reaction [13]. In order to correlate the catalytic structure and the IR bands developed at CO adsorption, the following should be considered. When the molecular clusters are decomposed in vacuum, the IR bands which can be assigned to CO adsorbed on ruthenium in strongly oxidic environment [12,14] are predominant. Since no hydrogen is present only small proportion of metallic ruthenium characterized by the linear CO stretching frequency at 2050 cm-’ is formed. The shift of this band as compared to that observed for Ru/Cab-0-Sil/H, is probably due to the difference in electron density on the metallic particle causing a stronger CO bond. When ruthenium is not stabilized by either iron oxide or by alumina, large particles are formed and the CO band structure
1002
I. Bijsziirmhtyi
et al. / Decomposition
of Fe-Ru
carbonyl clusters
resembles that observed for a supported ruthenium catalyst [lo]. On aluminasupported catalysts, in the presence of hydrogen the constituent metals are stabilized in the proximity of each other by the Al,O, matrix and probably small bimetallic particles are present. The high dispersion results in an undefined structure and a broadening of the CO adsorption band. An additional factor, namely, the presence of carbon should not be neglected and the specific properties of bimetallic particles are probably a result of the two factors; the alumina and surface carbon together. In summary, strong interaction between bimetallic cluster and Al 203 causes not only the decomposition of the cluster framework at impregnation but the stabilization of the metal in oxidized form by the oxide matrix as well and we infer that the presence of carbon contributes to the unique behaviour of alumina-supported iron-ruthenium particles.
References [l] J.R. Anderson and D.E. Mainvaring. J. Catalysis 35 (1974) 162. [2] J.R. Budge, B.F. Liicke. J.P. Scott and B.C. Gates, in: Proc. 8th Intern. Congr. on Catalysis. Vol. 5 (Verlag Chemie. Weinheim, 1984) p. 89. [3] R. Hemmerich. W. Keim and M. Riiper. J. Chem. Sot. Chem. Commun. (1983) 428. [4] L. Guczi. Z. Schay, K. Lb&. A. Vizi and L. Markb, Surface Sci. 106 (1981) 516. [5] I. BaszGrmtnyi, S. Dobos. L. Guczi, L. Mark& K. LBzPr, W.M. Reiff, Z. Schay. L. Takacs and A. Vizi-Orosz, in: Proc. 8th Intern. Congr. on Catalysis, Vol. 5 (Verlag Chemie, Weinheim. 1984) p. 183. [6] A. Chaplin. 389.
M. Leconte,
J.M. Basset. S.G. Shore and W.-I. Hsu, J. Mol. Catalysis
21 (1983)
[7] K. LBzBr, K. Matusek, J. Mink, S. Dobos, L. Guczi. A. Vizi-Orosz. L. Markh and W.M. Reiff. J. Catalysis 87 (1984) 163. [8] S. Dobos. I. Biiszarmenyi. V. Silberer, L. Guczi and J. Mink, Acta Chim. Inorg. 96 (1985) L13. [9] Z. Schay. K. LUu. J. Mink and L. Guczi, J. Catalysis 87 (1984) 179. [lo] G. Xienxian. X. Qin, L. Yongxue, J. Dai and Y. Pinliang, in: Proc. 8th Intern. Congr. on Catalysis, Vol. 4 (Verlag Chemie, Weinheim, 1984) p. 599. [ll] P. Ramamoorthy and R.D. Gonzales, J. Catalysis 59 (1979) 130. [12] A.A. Davidov and A.T. Bell, J. Catalysis 49 (1977) 332. (13) Z. Schay. I. Bogyay and L. Guczi, Acta Chim. Hung., in press. [14] K. Tamaru, in: Proc. 7th Intern. Congr. on Catalysis, Part A (Kodansha/Elsevier, Tokyo/ Amsterdam. 1981) p. 47.
995
STABILIZATION EFFECT OF OXIDE SUPPORTS ON Tm DECOMPOSITION OF IRON-R~HENIU~ CARBONYL CLUSTERS Istvan B&SZGRMENYI, and L&z16 GUCZI *
Sandor
DOBOS,
Karoly
LAZAR,
Zoltan
SCHAY
Institute of Isotopes of the Hungarian Academy of Sciences, P. 0. Box 77, H - I.725Budapest, Hungary Received
13 July 1984; accepted
for publication
I8 September
1984
and HaFeRu,(CO),, deposited on Cab-0-Sil and AlaO, have been Ru,(C%, FeaRu(CO),, investigated. Infrared and Mbssbauer spectroscopy revealed a strong interaction between a molecular cluster and alumina which leads to the disintegration of the cluster framework up to monoruthenium subcarbonyl species and iron oxide. Temperature-programmed decomposition showed carbon retained in aiumina-supported catalyst whereas with Cab-0-Sil the ligands from the adsorbed cluster could be recovered. After temperature-programmed decomposition in hydrogen and vacuum. CO adsorption resulted in four IR bands at about 2138, 2075, 2020 and 2043/2050 cm-‘. From these data the formation of highly dispersed bimetallic particles on alumina is discussed.
1. Introduction Molecular bimetallic carbonyl clusters have been used to prepare supported metal catalysts [l-4]. It was assumed that the original metal framework remains intact and in this way bimetallic catalysts of high dispersion and well-defined structure can be prepared. Most recently it was observed that in some cases due to the strong support-cluster interaction the original metal framework is disturbed in the very first steps of the catalyst preparation [2,5]. In H,FeOs3(CO),, supported on silica gel the Fe-OS bond cleaved to form iron and silica grafted tri-osmium species [6]. The same was observed for RuOs, cluster where Ru’, RuZL and OS’+ -carbonyl species were produced on alumina [2]. In a study of the interaction of iron-ruthenium carbonyl clusters with Al,O, we observed [5] that during 12 h evacuation after the impregnation of Al,O, with n-hexane solution of the carbonyl clusters the iron-ruthenium bond is already ruptured at room temperature. Since sometimes in high-temperature reduction bimetallic particles are * To whom correspondence
should be addressed.
0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
996
I. Biirriirmknyi et al. / Decomposition
of Fe-Ru
carbonyi clusters
formed depending on the nature of the bimetallic cluster and the support, our goal was to study the stabilization effect of the support on the very fine metal or bimetallic particles formed and its relevance to CO adsorption.
2. Experimental For the preparation of the catalysts Degussa Al,O, C and Cab-O&l HS 5 (Cabot Co., Mass.) previously evacuated at 573 K overnight, were imprecated by hexane solution of the carbonyl clusters Fe,(CO),,, Ru,(CO),,, Fe,Ru(CO),, and H,FeRu3(C0)i3. The total metal load determined by the XRF method was around 1 wt%. Infrared spectra were measured on a DIGILAB FTS-20 C Fourier transform spectrometer equipped with a Nova-3 computer. A Pyrex infrared cell with a quartz tube heater was used. The catalysts were stored in different atmospheres before pressing into a 25 mm diameter wafer. The cell could be evacuated to 5 x 10e2 Pa. Mossbauer measurements were carried out in a cell which allowed in situ heat treatments in different atmospheres. For the measurements 57Fe-enriched clusters were used. In some cases an in situ impregnation apparatus connected to a mass spectrometer (Kratos MS 10 C 2) was applied to measure CO balance during impregnation and subsequent decomposition. Details of the experimental techniques are given in ref. [7].
3. Results 3. I. Infrared measurements Previously we showed [5] that the iron-containing clusters readily react with Al,O, and there is practically no difference in the IR spectra of the supported bimetallic clusters. All spectra are similar to those of Al,Oj-supported Ru,(CO),, but the CO stretching vibration mode characteristic of the CO-iron system cannot be observed. IR spectra of Al,O,-supported Ru,(CO),, stored overnight in static He and CO, in flowing He and in vacuum are given in fig. 1. For reference, spectra of the cluster in n-hexane and supported on Cab-0-Sil are also included. Fig. 1 clearly shows that under static conditions both in He and in CO the structure of the Al,O,- and Cab-0-Sil-supported cluster is similar. On the other hand, under dynamic conditions in flowing He or in vacuum, Ru(CO), subcarbonyls characterized by bands at 2138, 2070, 2050, 2000 and 1980 cm-i are formed VI.
I. Btiszijrmhyi et al. / Decomposition of Fe-Ru carbonyl clusters
997
When the spectra of Ru,(CO),, having been in contact for 12 h with the support, are compared with those supported on Cab-0-Sil, the band broadening points to the coexistence of various subcarbonyl fragments superimposed to the original cluster framework. If the sample is evacuated, there is a quick process in which the original Ru,(CO),, is further decomposed into a structure similar to that obtained under dynamic condition. This is shown in fig. 2. It is of interest to note that in this process the original Ru,(CO),, is not sublimed as was proved in separate experiments. 3.2. Miissbauer measurements Simultaneously with the IR measurements information was collected from iron by Mossbauer spectroscopy. In agreement with the IR spectra the decomposition of the bimetallic cluster was indicated by the appearance of oxidized iron in the Mossbauer spectra (see fig. 3 curves a and c for Fe,Ru(CO),, and H,FeRu,(CO),,, respectively). During impregnation iron is oxidized to Fe’+ for both clusters as indicated by Mossbauer parameters peak for the (IS = 0.35 mm s-‘, QS = 1.1 mm s-t ). The unresolved
2100 209 Moo 1950 cm-l Fig. 1. IR spectra of Ru,(CO),,/A120, stored for 12 h after impregnation from n-hexane in: (a) vacuum; (b) He flow; (c) static He; (d) static CO; (e) vacuum but Cab-O-Sit supported; (f) unsupported Ruj(C0)t2 in n-hexane. Fig. 2. IR spectra of Ru,(CO),,/AI,03 stored for 12 h in static He and evacuated for (a) 0.05 h; (b) 0.5 h; (c) 1 h; (d) 2 h; (e) 5 h; (f) 12 h; (g) 24 h; (h) 28 h; (i) stored in air for 168 h.
998
I. B&z@rm&yi
et ai. / Decomposition of Fe-Ru
velocity
/
carbonyl dusters
mms‘l
Fig. 3. Miissbauer spectra at 300 K of A120,-supported clusters: (a) F~,Ru(CO)~~ after impregafter nation; (b) F~,Ru(CO),~ after decomposition at 660 K in H,; (c) H,FeRu,(CO),, impregnation; (c) H,FeRu,(CO),, after decomposition at 640 K in H,.
Fe,Ru(CO),,cluster again shows iron in different environments. Reduction in H, flow at 640-660 K results in complete reduction of Fe3+ to Fe2+ (IS = 1.07 mm s-‘, QS = 2.04 mm s-r). There was no magnetic splitting in the spectra at low temperatures, proving the very small dimensions of the particles containing iron ions. 3.3. Temperature-programmed
decomposition
Decomposition in situ in a closed system after impregnation resulted in a fair carbon balance. On Cab-0-Sil-supported clusters the decomposition starts at about 350 K and most of the CO leaves the clusters below 450 K. There is
I. Bijsziirmhtyi
999
et al. / Decomposition of Fe-Ru carbonyl clusiers
molC0 molcluster 98-
IV
l6-
Id0
200
30
T/Y
molC0 molcluster
III II
llIO9al65432b)
l100 Fig. 4. In situ decomposition Cab-O-S&supported clusters; RQCO),z.
200 after impregnation: upper graphs (I) Fe&O),,; (II) Fe,Ru(CO),,;
--z
TPC
Al,O,-supported, lower graphs (III) H2FeRu3(CO),,; (IV)
I. Biisriirmknyr et al. / Decomposition of Fe-Ru carbonyl clusters
1000
an additional CO evolution between 480 and 550 K (see fig. 4). On Al,O,-supported clusters CO evolution is already observed during impregnation [5], and by increasing the temperature, CO is continuously leaving the clusters. Fe,(CO),, seems to be the least stable and H,FeRu,(C0),3 is the most stable on A1203. Due to initial CO loss during impregnation only 7-9 moles of CO are recovered from 1 mole cluster. 3.4. CO adsorption For characterization of bimetallic catalysts formed after the decomposition of bimetallic molecular clusters in vacuum or in hydrogen, the infrared spectra of the adsorbed CO have been studied as shown in fig. 5. CO adsorption measured at 1.2 kPa is characterized by four bands. Three of them can be
2m2000cm’
2
Fig. 5. IR spectra of adsorbed CO on 700 K decomposed clusters: (I) Cab-O-S&supported, decomposed in vacuum; (II) Cab-O&l-supported, decomposed in H,; (III) Al,O+upported, decomposed in vacuum; (IV) Al,O,-supported, decomposed in H,; (a) Fe,Ru(CO),,; (b) H,FeRu,(CO),,; (c) Ru,(CO),,.nTo whom correspondence should be addressed.
I. Biisziirmhtyi et al. / Decomposition of Fe-Ru carbonyl clusters
1001
found in more or less distorted form in all spectra: at about 2140, 2075 and 2020 cm-’ and these are similar to those found for bimetallic catalysts prepared from the mixture of Fe,(CO),, and Ru3(CO)i2 [9]. In addition to these bands, on Cab-0-Sil-supported samples a band at 2043 cm-’ can be observed and on Al,O,-supported sample this band is shifted significantly to higher wavenumber, 2050 cm-‘. No bands can be observed at 1870 cm-’ which is characteristic of bridged bonded CO on ruthenium [lo]. The most characteristic is the band structure of CO adsorbed on Ru/Cab-0-%1/H, which resembles the sample prepared from RuCl, [lo] deposited on SiO, with a particle size of 5 nm. Alumina-supported mono- and bimetallic clusters after hydrogen treatment show a similar band structure, but the band is much broader than its counterpart on Cab-0-Sil. In the vacuum-decomposed samples both for Cab-O-Sil- and Al,O,-supported clusters, the main feature is the appearance of a three-band structure (at 2140, 2075 and 2020 cm-‘). Two of these bands are assigned to CO adsorbed on oxidized ruthenium [11,12] whereas the band at 2020 cm- ’ has not been identified. For Cab-O-Sil-supported samples decomposed in vacuum, the band at 2050 cm-’ also appeared.
4. Discussion IR spectra of Cab-O-Siland Al,O,-supported clusters proved marked differences even after impregnation [5]. On Cab-0-Sil the original structure is retained whereas on Al,O, the clusters decompose because of the strong support-cluster interaction. Mossbauer and IR spectra prove that decomposition leads to the formation of Fe3+ and Ru(CO), subcarbonyls, respectively, in which Ru is also in an oxidized state. During evacuation this oxidation continues, and simultaneously IR spectra show a one-step disappearance of Ru,(CO),, without formation of subcarbonyls. During reduction in hydrogen an interaction between iron and ruthenium develops, Fe3+ is reduced to Fe*+ of high dispersion. The high dispersion of alumina-supported bimetallic cluster has been proven by low-temperature Mossbauer measurements [5] as well as in catalytic CO + H, reaction [13]. In order to correlate the catalytic structure and the IR bands developed at CO adsorption, the following should be considered. When the molecular clusters are decomposed in vacuum, the IR bands which can be assigned to CO adsorbed on ruthenium in strongly oxidic environment [12,14] are predominant. Since no hydrogen is present only small proportion of metallic ruthenium characterized by the linear CO stretching frequency at 2050 cm-’ is formed. The shift of this band as compared to that observed for Ru/Cab-0-Sil/H, is probably due to the difference in electron density on the metallic particle causing a stronger CO bond. When ruthenium is not stabilized by either iron oxide or by alumina, large particles are formed and the CO band structure
1002
I. Bijsziirmhtyi
et al. / Decomposition
of Fe-Ru
carbonyl clusters
resembles that observed for a supported ruthenium catalyst [lo]. On aluminasupported catalysts, in the presence of hydrogen the constituent metals are stabilized in the proximity of each other by the Al,O, matrix and probably small bimetallic particles are present. The high dispersion results in an undefined structure and a broadening of the CO adsorption band. An additional factor, namely, the presence of carbon should not be neglected and the specific properties of bimetallic particles are probably a result of the two factors; the alumina and surface carbon together. In summary, strong interaction between bimetallic cluster and Al 203 causes not only the decomposition of the cluster framework at impregnation but the stabilization of the metal in oxidized form by the oxide matrix as well and we infer that the presence of carbon contributes to the unique behaviour of alumina-supported iron-ruthenium particles.
References [l] J.R. Anderson and D.E. Mainvaring. J. Catalysis 35 (1974) 162. [2] J.R. Budge, B.F. Liicke. J.P. Scott and B.C. Gates, in: Proc. 8th Intern. Congr. on Catalysis. Vol. 5 (Verlag Chemie. Weinheim, 1984) p. 89. [3] R. Hemmerich. W. Keim and M. Riiper. J. Chem. Sot. Chem. Commun. (1983) 428. [4] L. Guczi. Z. Schay, K. Lb&. A. Vizi and L. Markb, Surface Sci. 106 (1981) 516. [5] I. BaszGrmtnyi, S. Dobos. L. Guczi, L. Mark& K. LBzPr, W.M. Reiff, Z. Schay. L. Takacs and A. Vizi-Orosz, in: Proc. 8th Intern. Congr. on Catalysis, Vol. 5 (Verlag Chemie, Weinheim. 1984) p. 183. [6] A. Chaplin. 389.
M. Leconte,
J.M. Basset. S.G. Shore and W.-I. Hsu, J. Mol. Catalysis
21 (1983)
[7] K. LBzBr, K. Matusek, J. Mink, S. Dobos, L. Guczi. A. Vizi-Orosz. L. Markh and W.M. Reiff. J. Catalysis 87 (1984) 163. [8] S. Dobos. I. Biiszarmenyi. V. Silberer, L. Guczi and J. Mink, Acta Chim. Inorg. 96 (1985) L13. [9] Z. Schay. K. LUu. J. Mink and L. Guczi, J. Catalysis 87 (1984) 179. [lo] G. Xienxian. X. Qin, L. Yongxue, J. Dai and Y. Pinliang, in: Proc. 8th Intern. Congr. on Catalysis, Vol. 4 (Verlag Chemie, Weinheim, 1984) p. 599. [ll] P. Ramamoorthy and R.D. Gonzales, J. Catalysis 59 (1979) 130. [12] A.A. Davidov and A.T. Bell, J. Catalysis 49 (1977) 332. (13) Z. Schay. I. Bogyay and L. Guczi, Acta Chim. Hung., in press. [14] K. Tamaru, in: Proc. 7th Intern. Congr. on Catalysis, Part A (Kodansha/Elsevier, Tokyo/ Amsterdam. 1981) p. 47.