Ru
3
(CO)
12
supported on alumina samples with different
dehydration degrees
E. Escalona Platero
Ã
andF.RuizdePeralta
Departamento de Qu|
¨
mica, Universidad delas Islas Baleares, 07071 Palma de Mallorca, Spain
Received 6 August 1997; accepted 2 September 1997
FTIR spectroscopy was used to characterise Ru
3
(CO)
12
supported by chemical vapour deposition onto several -alumina sam-
ples with different dehydration degrees. The metal carbonyl was found to be molecularly physisorbed onto hydrated alumina; it
undergoes oxidative addition of a surface OH group onto hydroxylated alumina; it forms Lewis type adducts, via a CO group, with
Al
3
ions at the surfaceof dehydroxylated alumina.
Keywords: alumina, trirutheniumdodecacarbonyl, infrared spectroscopy
1. Introduction
The adsorption and reactivity of transition metal car-
bonyls on oxidic surfaces have attracted the attention of
many research groups [1^6]. A number of these studies
are motivated by the attempt to understand the chemical
bonding between the surface of the support and the
metal carbonyl, and to investigate the reactivity of the
surface adduct thus formed.
As recently outlined by Zecchina and Otero Are
an
[4], there are several ways in which transition metal car-
bonyls can interact with oxidic surfaces, and most of
them have their analogues in homogeneous chemistry.
The main interaction modes can be divided into ligand-
centred interactions and metal-centred interactions
[2,4,7,8], and they depend on several parameters: the na-
ture of both metal carbonyl and oxide support; the
degree of dehydroxylation of the support; the method
adopted to deposit the carbonyl on the support (chem-
ical vapour deposition or impregnation); and the nature
of the surrounding atmosphere.
The interaction of Ru
3
(CO)
12
with alumina [9^12]
has been studied mainly by IR spectroscopy, although
EXAFS and temperature-programmed decomposition
have also been used. The interaction is rather complex:
on partially hydroxylated alumina, Ru
3
(CO)
12
can
undergo oxidative addition of a surface OH group with
simultaneous release of two CO ligands [11,12]; slightly
acidic OH groups on the alumina surface can also lead to
several anchored subcarbonylic species. However, no
well-documented reports appear to be available to date
on the interaction between Ru
3
(CO)
12
and highly dehy-
droxylated alumina.
Previous studies on other metal carbonyls supported
on highly dehydroxylated supports indicated the forma-
tion of Lewis-type (donor^acceptor) '-adducts where
the oxygen atom of a CO ligand interacts with a coordi-
natively unsaturated metal ion at the oxide surface
[2,4,8,13]. We report here on a detailed FTIR characteri-
sation of Ru
3
(CO)
12
sublimed onto highly dehydroxy-
lated -alumina in order to investigate the possible
interaction of the metal carbonyl with Lewis acid sites
on the support. Also included in this paper is the FTIR
characterisation of Ru
3
(CO)
12
supported on hydrated
and on hydroxylated alumina. The results on hydroxy-
lated alumina will be compared with those obtained by
other authors [11,12].
2. Experimental
High-purity trirutheniumdodecacarbonyl was sup-
plied by Aldrich (Steinheim) and was used without
further purification. The -Al
2
O
3
sample was obtained
by thermolysis at 873 K of synthetic boehmite and
showed a BET surface area of 222 m
2
g
À1
and a mesopor-
ous texture with a most frequent pore diameter of 4 nm
(although a small contribution from microporosity was
also present). These pore openings allow free diffusion
of Ru
3
(CO)
12
molecules, which have dimensions of
about 1X1 Â 0X9 nm [14].
A quartz cell fitted with NaCl windows, similar to
that described by Boccuzzi et al. [15], allowed in situ acti-
vation of the alumina sample and dosing of the metal
carbonyl by vacuum sublimation at 380 K. This tem-
perature ensured a vapour pressure of Ru
3
(CO)
12
,high
enough for vapour deposition onto the alumina without
causing thermolysis of the metal carbonyl. For activa-
tion, three portions of the alumina sample, in the form of
thin self-supporting wafers, were heated (inside the IR
cell) for 1 h at 273, 873 and 1173 K respectively, under a
dynamic vacuum (residual pressure ` 10
À3
Pa). These
Catalysis Letters 48 (1997) 159^163
159
* To whom correspondence should be addressed.
Ä J.C. Baltzer AG, Science Publishers