and Fourier transform infrared (FTIR) spectroscopies. As an example [48], CO desorbs from the adsorbed Pd atoms at a temperature of about 250 K, which corresponds to a binding energy, E b , of about 0.7 G 0.1 eV. FTIR spectra suggest that at
saturation two different sites for CO adsorption exist on a single Pd atom. The vibrational frequency of the most stable, singly adsorbed CO molecule is 2055 cm
À1 .
Density functional cluster model calculations have been used to model possible
defect sites at the MgO surface where the Pd atoms are likely to be adsorbed. CO/
Pd complexes located at regular or low-coordinated O anions of the surface exhibit
considerably stronger binding energies, E b ¼ 2–2.5 eV, and larger vibrational shifts
than were observed in the experiment. CO/Pd complexes located at oxygen vacancies (F or F
þ centers) are characterized by much smaller binding energies,
E b ¼ 0.5 G 0.2 or 0.7 G 0.2 eV, which are in agreement with the experimental
value. Such comparisons therefore identify adsorption sites of the clusters on the
MgO surface, particularly in the examples shown below these trapping centers are
shown to be oxygen vacancies.
After characterizing this type of nanocatalyst consisting of metal atoms trapped
on oxygen vacancies, simple reactions like acetylene polymerization or CO oxidation can be studied by means of temperature programmed reaction studies and
infrared spectroscopy.
For the cyclotrimerization on Ag, Pd and Rh atoms and the CO oxidation on Pd
atoms the nanocatalysts were exposed at 90 K, using a calibrated molecular beam
doser, to about 1 Langmuir (L) of acetylene or 1 L of
18 O 2 and
13 CO, respectively. In
a temperature programmed reaction (TPR) study, catalytically formed benzene
(C 6 H 6 ), butadiene (C 4 H 6 ) and butene (C 4 H 8 ) or
13 C
18 O
16 O molecules were detected by a mass spectrometer and monitored as a function of temperature. It is
interesting to note that the reactants (
13 CO,
18 O 2 , C 2 H 2 ) are only physisorbed on
the MgO and desorb at temperatures lower than 150 K in all three cases, e.g. before the reaction takes place. In addition, the interactions of the product molecules
with the model catalysts are weak, as described in [48, 49]. The thermal stabilities
of the deposited Pd and Rh atoms were investigated by using Fourier transform
infrared spectroscopy and the probe molecule, CO. It was found that Pd starts to
migrate at around 300 K [48], whereas Rh is stable up to 450 K [50].
Surprisingly, and in contrast to single crystal studies [51], a single Pd atom already catalyzes the cyclotrimerization reaction and the benzene molecule (C 6 H 6 )
desorbs at 300 K (Figure 17.4a). No other product molecules of the polymerization
reaction (e.g. C 4 H 6 , C 4 H 8 ) are observed. Thus this model catalyst is highly selective
for the polymerization reaction. We note that on a clean MgO(100) surface no
products are formed (Figure 17.4a). This surprising result can be rationalized by
studying theoretically a Pd atom adsorbed on different MgO sites represented by a
cluster of ions embedded in an array of point charges. First a Pd atom was adsorbed on a five-coordinated oxygen ion on the MgO(001) terrace, O 5c , the binding
energy is about 1 eV. It was found that indeed the Pd(C 4 H 4 ) complex is formed but
the third acetylene molecule is not bound to the complex and therefore this configuration is catalytically inactive. Other bonding sites for the adsorption of Pd
atoms are considered. On four-coordinated step or three-coordinated corner oxygen
17.3 Chemical Reactions and Catalytic Processes on Free and Supported Clusters 563
saturation two different sites for CO adsorption exist on a single Pd atom. The vibrational frequency of the most stable, singly adsorbed CO molecule is 2055 cm
À1 .
Density functional cluster model calculations have been used to model possible
defect sites at the MgO surface where the Pd atoms are likely to be adsorbed. CO/
Pd complexes located at regular or low-coordinated O anions of the surface exhibit
considerably stronger binding energies, E b ¼ 2–2.5 eV, and larger vibrational shifts
than were observed in the experiment. CO/Pd complexes located at oxygen vacancies (F or F
þ centers) are characterized by much smaller binding energies,
E b ¼ 0.5 G 0.2 or 0.7 G 0.2 eV, which are in agreement with the experimental
value. Such comparisons therefore identify adsorption sites of the clusters on the
MgO surface, particularly in the examples shown below these trapping centers are
shown to be oxygen vacancies.
After characterizing this type of nanocatalyst consisting of metal atoms trapped
on oxygen vacancies, simple reactions like acetylene polymerization or CO oxidation can be studied by means of temperature programmed reaction studies and
infrared spectroscopy.
For the cyclotrimerization on Ag, Pd and Rh atoms and the CO oxidation on Pd
atoms the nanocatalysts were exposed at 90 K, using a calibrated molecular beam
doser, to about 1 Langmuir (L) of acetylene or 1 L of
18 O 2 and
13 CO, respectively. In
a temperature programmed reaction (TPR) study, catalytically formed benzene
(C 6 H 6 ), butadiene (C 4 H 6 ) and butene (C 4 H 8 ) or
13 C
18 O
16 O molecules were detected by a mass spectrometer and monitored as a function of temperature. It is
interesting to note that the reactants (
13 CO,
18 O 2 , C 2 H 2 ) are only physisorbed on
the MgO and desorb at temperatures lower than 150 K in all three cases, e.g. before the reaction takes place. In addition, the interactions of the product molecules
with the model catalysts are weak, as described in [48, 49]. The thermal stabilities
of the deposited Pd and Rh atoms were investigated by using Fourier transform
infrared spectroscopy and the probe molecule, CO. It was found that Pd starts to
migrate at around 300 K [48], whereas Rh is stable up to 450 K [50].
Surprisingly, and in contrast to single crystal studies [51], a single Pd atom already catalyzes the cyclotrimerization reaction and the benzene molecule (C 6 H 6 )
desorbs at 300 K (Figure 17.4a). No other product molecules of the polymerization
reaction (e.g. C 4 H 6 , C 4 H 8 ) are observed. Thus this model catalyst is highly selective
for the polymerization reaction. We note that on a clean MgO(100) surface no
products are formed (Figure 17.4a). This surprising result can be rationalized by
studying theoretically a Pd atom adsorbed on different MgO sites represented by a
cluster of ions embedded in an array of point charges. First a Pd atom was adsorbed on a five-coordinated oxygen ion on the MgO(001) terrace, O 5c , the binding
energy is about 1 eV. It was found that indeed the Pd(C 4 H 4 ) complex is formed but
the third acetylene molecule is not bound to the complex and therefore this configuration is catalytically inactive. Other bonding sites for the adsorption of Pd
atoms are considered. On four-coordinated step or three-coordinated corner oxygen
17.3 Chemical Reactions and Catalytic Processes on Free and Supported Clusters 563