Interaction of Ni(CO)4, Fe(CO)5, Co2(CO)8 With MgO and Formation of Very Small Metallic Clusters.

J. BOURDON (Editor) Growth and Properties of Metal Clusters, pp. 165-174 © 1980 Elsevier Scientific Publishing Company - Printed in The Netherlands

165

INTERACTION OF Ni(CO)4' Fe(CO)S, C02(CO)8 WITH MgO AND FORMATION OF VERY SMALL METALLIC CLUSTERS. E. GUGLIELMINOTTI, A. ZECCHINA, F. BOCCUZZI and E. BORELLO Istituto di Chimica Fisica, Universita di Torino, Torino (ITALY).

INTRODUCTION The I.R. spectra of CO adsorbed on supported or evaporated metal particles show very broad bands in the 2100 - 1700 cm- l, one order of magnitude broader than those of well defined carbonyl compounds. This fact is likely due to the large dispersion of the particle dimensions normally obtained by these methods (20 - 200 A). In fact particles of different sizes likely expose adsorbing sites of different nature, i.e. exhibit a large overall surface heterogeneity. Thus the I.R. spectra of CO adsorbed on dispersed metals allow an approximate description of the system in terms of few broad types of surface structures (i.e. linear and bridged forms). In order to make the particle size dispersion more uniform and to reduce the dimensions, attempts have been made in recent years (ref. 1) to obtain the dispersed metal by decarbonylation of suitable metal carbonyl compounds deposited by impregnation on high area solids. The state of the final products however (dimensions, size dispersion, oxidation state) depends in a complicated way on the impregnation and decarbonylation conditions and on the hydroxyl content of supporting surface. For these reasons in the present paper a method is described by which the impregnation of a high area support is carried out in conditions of total surface dehydration, absence of solvents and atmospheric gases. EXPERIMENTAL High area MgO samples (~200 m2/g) have been obtained directly in the cells by decomposing under high vacuum at 523 K high purity Mg(OH)2 as described in a previous paper (ref. 2). Total surface dehydration is achieved by subsequent outgassing at 1073 K under high vacuum conditions. The pellet is then cooled down to room temperature under vacuum and the volatile Ni(CO)4, Fe(CO)5 and C02(CO)8 carbonyls allowed to contact the surface. After each dose the spectrum of adsorbed species (both I.R. and UV-VIS-Near Infrared diffuse reflectance) was carried out with a Beckman IR 12 and DK-2 spectrometers respectively. RESULTS The I.R. spectrum of increasing amounts of Ni(CO)4 adsorbed on MgO is shown in fig. la (2150 - 1850 cm-l range) and fig. lb (1600 - 1000 cm-l range). It must be noticed that, due to the instability of Ni(CO)4' CO is always present in the

166

gaseous phase of this compound. After Ni(CO)4 chemisorption bands are observed at ~2100 (shoulder), 2085, 2072, 2025, 2010, 1990, 1975, 1930, 1915 cm-l (fig. la) and at 1490 1462 and at 1060 - 1030 cm- l (fig. lb). Some minor component are observed in fig. lb which will not be described in detail. We only recall that they (together with the shoulder at ~2100 cm- l) are due to CO adsorbed on MgO as discussed in detail in a previous paper (ref.: b

_., -" _., -" -"

0I-+-.lL-_~-+-~---U
2100

2000

cm- 1

1400

_ _.l--_+-......I

1200

1000

Fig. la,b.--Increasing amounts of Ni(CO)4 adsorbed on MgO. --- Sample outgassed 30' at R.T. By removal of the CO phase, the bands at 2085, 2072, 2025, 1990, 1975 and at 1490 - 62 and 1060 - 30 cm-l disappear while bands at 2010, 1965 and 1930 - 1915 cm- l grow in intensity (fig. 1, curve ---). Reexposure to CO gas reverses the process as shown in detail in fig. 2. This figure refers to a sample onto which a smaller amount of Ni(CO)4 has been chemisorbed, as this allowsa better inspection of the behaviour of the bands in the high frequency range. From Fig. 1 and 2 the following conclusions can be drawn: i) species responsible of the bands at 2085, 2072, 2025, 1990, 1975 and at 1490 - 62, 1060 - 30 cm- l are stable only in the presence of CO in the gas phase; ii)species absorbing at 2010, 1965 and 1930 - 1915 cm- l are favoured by low CO pressures; iii) interconversion between these species can be obtained by varying the CO pressure and this process is definitely activated as shown by its time dependence (see Fig. 2 --- and -.-.). Degassing in high vacuum at 523 K causes the complete disappearance of all the carbonyl bands. The reexposure at this stage to CO gas is shown in Fig. 3.

167

100

Fig. 2. Ni(CO)4 adsorbed and outgassed 3D' at R.T. --- Immediately after contact with 10 torr CO -·-·After 2 days.

0,1..-+--_ _+-_ _--1.........., 2100

2000

cm- 1

1900

100'.------------------,

..

-" -" -" -"

.......... ..--

OL-_I-----l'-----l-_----L.--'--I-_---'-_--'

2100

Fig. 3. - .. - .. - Background after Ni(CO)4 adsorption at R.T. and 2h desorption at 523

K.

After contact with CO: ( 0,5, ~2, -- 4 and _ _ 10 torr).

168

For low CO pressures and short contact time a broad structureless band centered at 203 cm- l with a large tail on the low frequency side is observed, similar to what observed for CO adsorbed on finely divided Nickel obtained in conventional ways (ref. 3). By incre ing the CO pressure and or the contact time, narrow bands develope which are identical to those previously illustrated in fig. 1 - 2. The first bandsto appear are those at 2010 - 1930 cm- 1, whereas those at 2085, 2025, 1990 cm- l are formed in the final stages of rea tion. Experiments similar to those illustrated in figs. 1 - 3 have been carried out in a cm- 1 reflectance cell and the results are illustrated in Fig. 4. The band at ~25.000 in Fig. 4a is formed upon contacting the surface with a Ni(CO)4 dose roughly similar to that originating the most intense spectrum of Fig. 1.

1

a

400

nm

500 600 800 1000

400

500 600 800 1000

b

Fig. 4. - .. - .. - MgO background a) - After contact with Ni (CO)4 After outgassing at R. T.:· -- 20", -.- l' ,-e-- 5', ...... 30'. b) --- After outgassing 1 h at 523 K ~ 10 torr CO adsorbed and spectrum recorded after 2 days. By outgassing at room temperature (R.T.), the intensity of the band at 25.000 cm- 1 increases; at the same time a broad absorption at ~15.000 cm- 1 gradually developes. Reexposure to CO at this stage nearly restores the initial spectrum. The effect of outgassing at 523 K is shown in fig. 4b. The band at 25.000 cm- 1 is drastically weakened, while a continuous, extremely broad and strong absorption in the 20.000 - 5000 cm- 1 range becomes the most important feature of the spectrum. Reexposure to CO after this treatment partially destroys the absorption in the 20.000 5000 cm- 1 range and restores the band at 25.000 cm- 1 (fig. 4b ~). Fe(CO)5' The adsorption of Fe(CO)5 is illustrated in Fig. 5. Medium strong bands appear at 2080, 2020, 1995, 1967, 1930, 1900, 1875, 1845 cm- 1 (fig. 5a) and at 1525 - 1481 and 1065 (broad) cm- 1 (Fig. 5b) which are not weakened by R.T. outgassing.

16£

100,------------r.--------=o------. b a

- --, ..-- .. - .. _.. _.. ..

.....

I

,, \

'", , I -, , , \ \. , (~ , \11

1'''',

I

/'

I (l'1

\/V', I' \I ., r,j ..

1\ ',"';/ \('" .r' ~!

\

.

0L....-_ _-f-_~-42100 2000

Fig. 5a,b.

v

_ _--1-_--1-_

1900

1600

_+_---J--_+_--L--...J

1400 cm- 1

1200

1000

--- Fe(CO)5 adsorbed on MgO --- After outgassing lh at 353 K -·-·After 20h contact with 40 torr CO.

Outgassing at 353 K destroys the bands at 2080,2020, 1967, 1930, and at 1525 - 1481, 1065 cm- l, so showing that they are due to the same or very similar surface complexes. Moreover two bands at 1325 and 1660 cm- l (partially shown in Fig. 5b) are formed which are due to carbonate-like species. Exposure to CO only partially restores the initial spectrum, whereas the bands due to carbonate-like groups are not modified. Complete decarbonylation is achieved at 473 K. Exposure to CO after this treatment gives origin only to very weak carbonyl bands, so showing that the decarbonylation is irreversible. In this case parallel experiments have been carried out in the reflectance cell. The spectra, not reported for the sake of brevity, can be summarized as follows. After exposure to Fe(CO)5 a weak band at ~20.000 cm- l is formed; in the following outgassing step at 353 - 473 K a broad intense absorption at 25.000 - 10.000 cm- l is formed which is not destroyed by successive exposure to CO gas. This fact confirms that decarbonylation at 353 - 473 K is an irreversible process. CozifQls. By exposure of MgO to C02(CO)8 the bands illustrated in Fig. 6 are formed. Contemporarily in the low frequency range, several bands are observed which are due to CO adsorbed on MgO. The effect of R.T. outgassing is to change the relative intensity of the high frequency components in an extremely complicated way. Exposure to CO after the R.T. outgassing step only partially restores the initial spectrum.

170

100r------------------,

-"

_ .. -··-~··.=----I

---- ---..

..

-.

20L..-+-----l_--t_----l-_-+-_--J...._-+-_-' 1800 1900 2000 2100 -t em

Fig. 6. Co 2(CO)8 adsorbed on MgO at increasing amounts. DISCUSSION Ni(CO)4. The LR. bands described in Figs. 1 - 3 are not due to weakly adsorbed molecular Ni(C~)4' being their frequencies definitely different from that of the Ni(CO)4 in the gas phase (2057 cm- l). We consider the bands reaching their maximum intensity in presence of CO gas (2085, 2072, 2025, 1990, 1975 and 1490-62, 1060-30 cm- l) to be associated to carbonylic structures Nix(CO)y (A species) with large y values. Bands whose intensity grows when CO is pumped off and the previous species depleted (2010, 1965, 1930 - 15 cm- l)" are associated with subcarbonylic structures (B species) where the CO/Ni ratio is smaller. The CO rich and the CO poor species A and B can be converted the ones into the others by admission or removal of CO at R.T. As shown by Fig. 4, B species are associated with electronic transition in the 25.000 - 15.000 cm- l range, which is the typical range of the cr + cr· transition in Ni-Ni bonds. In particular the band at 25.000 cm~l which grows upon short evacuation times,exactly corresponds to the frequency values found for dintkel or trintkel clusters obtained in cryogenic matri ces (ref. 4). As a consequence B species can be described in terms of low nuclearity clusters compounQS where 2, On the contrary species A, which do not show any electronic transition at frequencies lower than 30.000 cm~l should have a mononuclear nature (x = 1). The transformation (upon CO abstraction) of the A species into the B ones can be descrtbed as an aggregation process where the loss of CO ligands is accompanied by interaction of two sUbcarbonylic moietysand formation of Ni-Ni bond.

x,

171

As far as the structure of A and B species is concerned the following considerations can be made. CO rich mononuclear A species give rise to at least six bands in the l.R. (2085, 2025, 1990; 1975, 1490 - 62, 1060 - 30 cm- l) two of them falling at extremely low frequencies for simple carbonylic or subcarbonylic species. originated Uncommon structures must be invoked like the prototype one by the strong interaction of a Ni(CO)4 molecule with a Mg++O-- surface ionic pair. As this structure alone cannot justify all six l.R. bands, other. species with similar structure must be at . + the same time present on the surface (for instance Mg Ni(CO)3)' Ni(CO)2 etc. sUbcarbonylic species interacting with other suitably placed c.U.S. Mg 2+ and 02- ions). On the basis of the presented data it is impossible to give a more detailed assignment. B species are characterized by bands at 2010, 1965, 1930, 1915 cm- l. Two of them (2010 - 1930 cm- l) show a constant intensity ratio and hence are due to the same surface complex (B l), whereas the others, which disappear last upon prolonged outgassing at 473 523 K, belong to other two different species (B 2 and B3)' Bl complexes, responsible of absorption at ~25.000 cm- l are thought to be the smallest nuclearity complexes Ni2(CO)y formed during the agglomeration process induced by the CO removal. On the basis of the observed frequencies, linear CO (absorbing at 2010 cm- l) and bridged CO (absorbing at 1930 cm- l) are li~ly present in constant ratio, as for example in the following structure:

~

OC -

Ni~

----" 'Ni - CO 'C/

~

B2 complexes are evidenced upon further CO outgassing by the band at 1965 cm- l and by the broad absorption at 20.000 - 15.000 cm- l. Therefore it is inferred that they are originated by decarbonylation of Bl species, the remaining CO groups being in bridging position. The frequency values (1915 cm- l) of the CO stretching in the B3 species indicates that a tricentric bond like CO is probably present and as a consequence the B3 species

J\

' ~ --1 N' N1

are thought to be aggregation products containing more than 2 metal atoms. The observed frequencies and structures of B2 and B3 complexes are very similar to those found by Hulse and Moskovits (ref. 5) for low-nuclearity Ni - CO clusters formed in cryogenic matrices. After complete decarbonylation at higher temperatures, the samples exhibit a dark grey eolour, which is an indication of further aggregation and formation of very small Ni crystallites. Reexposure to CO (Fig. 3) initially gives rise to weak and very broad bands typical of CO ad~orbed on supported or evaporated Ni crystallites (ref. 3). By increasing the CO pressure, the bands due to Bl' B2' B3 species appear again, followed, for higher CO pressures and contact times, by the bands of the A mononuclearspecies. As

172

this process is accompanied by the partial destruction of the extremely broad absorption in all the visible region (Fig. 4b) likely associated with electronic transition of very small metal particles, it is concluded that CO adsorption causes the disgregation of Ni crystallites, leading ultimately to adsorbed mono and dinuclear complexes. Fe(CO)5' The spectrum obtained after Fe(CO)5 adsorption is extremely complicated and very different with respect to that of the gaseous carbonyl (YC=O at 2035 and 2013 cm- l): as a consequence a detailed assignment is at the present time impossible. As in the Ni(CO)4 case, surface species can be roughly divided into two groups A and B depending on their behaviour towards CO. A species (bands at 2080, 2020, 1967, 1930 cm- l) tend to disappear upon outgassing at 353 K and, like in the Ni(CO)4 case, are characterized by very unusual bands in the low frequency region (1525 - 81, 1065 cm- l broad) and the absence of any electronic transition at frequencies lower than 25.000 cm- l. Hence, as in the previous case, their assignment to mononuclear carbonylic or sUbcarbonylic Fe(CO)y species interacting with MgO surface is strongly favoured. B species (bands at 1995, 1900, 1875, 1845 cm- l) are the most resistent to outgassing and can be partially converted into the A ones by successive CO adsorption (Fig. 5a: _._): for example bands at 1845 cm- l, probably due to CO bridged groups, decrease upon CO adsorption. Hence their assignment to Fex(CO)y' complexes were x ~ 2 is strongly favoured. Moreover, the formation of carbonate groups after outgassing at 353 K and the strong weakening of carbonyls bands show that a prevailing oxidation process due to carbonyl groups disproportionation is beginning at this temperature, becoming faster at 473 K. In fact, reexposure to CO after a 473 Koutgassing gives rise to very weak carbonyl bands: in this case oxidized Fe2+ ion can diffuse from the surface into the MgO bulk, the final product at higher temperatures being a FeO/MgO solid solution. C02(CO)8' Due to the larger experimental difficulties, only few data are available at the present time. However some considerations can be made, i.e.: i) by interaction of C02(CO)8 with MgO, CO ligands are abstracted by the MgO matrix leading to adsorbed CO and to several carbonylic species whose relative concentration depends on the CO pressure; ii) the CO abstraction is accompanied by surface migration and agglomeration which can be partly reversed by exposure to CO; iii) no low-frequency bands similar to those observed for Ni(CO)4 and Fe(CO)5 are observed, in agreement with their assignment to mononuclear species. CONCLUSIONS We demonstrate that adsorption of simple carbonyls at R.T. in controlled condition of vacuum and support treatment can produce new polymetallic carbonyls with very few Me atoms. But at higher temperatures (353 - 523 K) upon CO loss, the new carbonyl clusters can agglomerate (mainly in the Ni case) and/or the metal is oxidized by CO dispropo.rtionation (mainly in the Fe case). The experiments therefore demonstrate that it is very difficult to maintain highly dispersed metal clusters on MgO support due to strong interaction between adsorbed carbonyl and the MgO basic surface.

173

REFERENCES

2 3 4 5

a) R.F. Howe, D.E. Davidson and D.A. Whan, Trans. Faraday Soc.,1 (1973) 1967. ~) J.R. Anderson, P.S. Elmes, R.F. Howe and D.E. Mainwaring, J. Catalysis, 50 (1977) 508. c) D. Ba11ivet Tkatchenko and G. Couduriez, 1norg. Chern. 18 (1979) 558. E. Gug1ie1minotti, S. Co1uccia, E. Garrone, L. Cerruti and A. Zecchina, Faraday Trans. I, 75 (1979) 96. a) A.M. Bradshow and J. Pritchard, Surface Sci., 17 (1969) 372. b) C.E. O'Neill and D.J. Yates, J. Phys. Chern., 65 (1961) 901. M. Moskovits and J.H. Hulse, J. Chern. Phys., 66 (1977) 3988. J.H. Hulse and M. Moskovits, Surface Sci., 57 (1976) 125.

174 DISCUSSION E. GUGLIELMINOTTI, A. ZECCHINA, F. BOCCUZI, E. BORELLO Basset - By adsorption of Fe(CO)S on alumina, magnesia, lanthanum oxide, zinc oxide, we observed a general behaviour : nucleophilic attack of OH- groups on coordinated CO givin~ rise to the formation of anionic hydride species Fe(CO)5 + M-OH ..,. [H Fe 3(CO)II]

-

[M-O]

+

Did you try to identify such anionic systems Zecchina - We used MgO outgassed in vacuo at 800 0 C and in these conditions no -OH groups are present on the surface (no -OH stretching bands are detected by IR spectroscopy). Indeed, the 800 0 C outgassing temperature has been chosen in order to avoid the complicatio you have mentioned. Bonzel - What is the assignment of the low frequency modes of the deposited Ni carbonyl ( cies I, frequencies at about 1500 and 1050 em-I) ? Can these still be viewed as CO stretc vibrations ? The vibrational analysis of CO chemisorbed on stepped Ni(III) surface (Erley, Wagner and in Surface Science \979) gives rise to a band at 1520 cm- I which was identied as a CO stretching vibration due to molecules adsorbed at step sites. Such a low CO stretching fr quency indicates severe bond weakening and hence a high probability for these CO molecule to dissociate. Zecchina - The bands at '- 1500 and 1050 cm-\ have a constant intensity ratio at all cover so they belong to the same (more than diatomic) surface species. This observation rules 0 a possible interpretation of the-1500cm-\ band in terms of one CO bonded to a special si The bands at -1500 cm- 1 and 1050 cm- I are associated to the carbonyl stretching bands at -2080 and -2030 em-I. We are so dealing with a surface species characterized by at least four IR active modes in the 2100 - 1000 em-I. The assignment can be only tentative and th, structure CO C0"-N1i/'CO

I

O..... C-... o-

Mg+

is favoured, as I said before. Alternative choices are possible so that a structure like

cannot be ruled out.