40% for n b 4; so only a few collisions would be required for complete conversion.
From this it was concluded that at near room temperatures the gas-phase metal
cluster anions are better catalysts than the supported catalysts used in current
technology for automotive catalytic converters. These need to be heated to high
temperatures: on platinum surfaces, temperatures of 400–500 K are typically required for oxidation of CO [45].
Both examples revealed unique and size-dependent catalytic properties of very
small metal clusters. As mentioned in the introduction, gas phase clusters will
probably never be important in real catalysis, however, such small clusters can be
stabilized on support materials so they may become relevant for future applications.
17.3.2
Chemical Reactions and Catalytic Cycles on Supported Clusters
17.3.2.1 Single Atoms on Oxide Surfaces
F centers can act as nanocatalysts, however few reactions are catalyzed by oxygen
vacancies because only two confined electrons at the same energy level are present.
In order to change these two parameters (number of confined electrons and energy
levels) without modifying the size of the quantum box, the F center can be decorated with metal atoms to produce a third kind of nanocatalyst: supported atoms
on oxide surfaces.
The metal atoms are produced by a recently developed high-frequency laser
evaporation source [15]. The positively charged ions are guided by home-built
ion optics through differentially pumped vacuum chambers and are size-selected
by a quadrupole mass spectrometer (Extrel Merlin System; mass limits: 1000,
4000, 9000 u). In these experiments it is important to deposit only 0.5–0.8% of a
monolayer of atoms (1 ML ¼ 2.25 Â 10
15 clusters cm
À2 ) at 90 K with low kinetic
energy in order to land them isolated on the surface and to prevent agglomeration
on the defect-rich MgO films. The presence of isolated atoms is confirmed experimentally and theoretically. Experimentally, nickel atoms, dimers and trimers are
used as it is well known that they form stable metal carbonyls. This carbonyl formation of small deposited Ni n (n ¼ 1–3) was studied by exposing the deposited
clusters to carbon monoxide. Mass spectrometry experiments showed that the nuclearity of the formed Ni n carbonyls (n ¼ 1–3) is not changed. The absence of, for
example, Ni 2 (CO) x and Ni 3 (CO) x after deposition of Ni atoms directly excludes
agglomeration, whereas the absence of Ni(CO) 4 after depositing Ni 2 excludes
fragmentation of the cluster, relevant for the experiments described in the next
section [46]. Second, Monte Carlo simulations revealed that under such experimental conditions, e.g. cluster flux (@10
9 cm
À1 ), cluster density (@10
13 cm
À1 ), and
defect density (@5 Á 10
13 cm
À1 ) on the MgO(100) films less than 10% of the atoms
coalesce when migrating to the trapping centers [47].
To characterize the model catalysts in detail it is, however, necessary to identify
the adsorption sites of the clusters. One possibility is to study the adsorption of
probe molecules on the deposited clusters by means of thermal desorption (TDS)
17 Nanocatalysis
562
From this it was concluded that at near room temperatures the gas-phase metal
cluster anions are better catalysts than the supported catalysts used in current
technology for automotive catalytic converters. These need to be heated to high
temperatures: on platinum surfaces, temperatures of 400–500 K are typically required for oxidation of CO [45].
Both examples revealed unique and size-dependent catalytic properties of very
small metal clusters. As mentioned in the introduction, gas phase clusters will
probably never be important in real catalysis, however, such small clusters can be
stabilized on support materials so they may become relevant for future applications.
17.3.2
Chemical Reactions and Catalytic Cycles on Supported Clusters
17.3.2.1 Single Atoms on Oxide Surfaces
F centers can act as nanocatalysts, however few reactions are catalyzed by oxygen
vacancies because only two confined electrons at the same energy level are present.
In order to change these two parameters (number of confined electrons and energy
levels) without modifying the size of the quantum box, the F center can be decorated with metal atoms to produce a third kind of nanocatalyst: supported atoms
on oxide surfaces.
The metal atoms are produced by a recently developed high-frequency laser
evaporation source [15]. The positively charged ions are guided by home-built
ion optics through differentially pumped vacuum chambers and are size-selected
by a quadrupole mass spectrometer (Extrel Merlin System; mass limits: 1000,
4000, 9000 u). In these experiments it is important to deposit only 0.5–0.8% of a
monolayer of atoms (1 ML ¼ 2.25 Â 10
15 clusters cm
À2 ) at 90 K with low kinetic
energy in order to land them isolated on the surface and to prevent agglomeration
on the defect-rich MgO films. The presence of isolated atoms is confirmed experimentally and theoretically. Experimentally, nickel atoms, dimers and trimers are
used as it is well known that they form stable metal carbonyls. This carbonyl formation of small deposited Ni n (n ¼ 1–3) was studied by exposing the deposited
clusters to carbon monoxide. Mass spectrometry experiments showed that the nuclearity of the formed Ni n carbonyls (n ¼ 1–3) is not changed. The absence of, for
example, Ni 2 (CO) x and Ni 3 (CO) x after deposition of Ni atoms directly excludes
agglomeration, whereas the absence of Ni(CO) 4 after depositing Ni 2 excludes
fragmentation of the cluster, relevant for the experiments described in the next
section [46]. Second, Monte Carlo simulations revealed that under such experimental conditions, e.g. cluster flux (@10
9 cm
À1 ), cluster density (@10
13 cm
À1 ), and
defect density (@5 Á 10
13 cm
À1 ) on the MgO(100) films less than 10% of the atoms
coalesce when migrating to the trapping centers [47].
To characterize the model catalysts in detail it is, however, necessary to identify
the adsorption sites of the clusters. One possibility is to study the adsorption of
probe molecules on the deposited clusters by means of thermal desorption (TDS)
17 Nanocatalysis
562