of strong intensity at 830–950 cm
À1 when isolated, and to an intense band at
1030 cm
À1 in the chemisorbed complex shown in Figure 17.1b. Therefore the stable species is assigned to H atoms incorporated into oxygen vacancies.
In conclusion, this combined experimental and theoretical study of methanol
adsorbed on MgO films with different defect density allows one to better identify
the surface sites responsible for the MgO reactivity. On the inert terrace sites only
physisorption is observed. Molecular chemisorption, activation and heterolytic dissociation occur on irregular sites. The low-coordinated MgaO pairs of ions located
at edges and steps can lead to strongly activated and even dissociated methanol
molecules. Adsorption of CH 3 Oa and H
þ fragments seems to be preferred over
dissociation into CH 3
þ and OH
À units. All these species are stable on the surface
for temperatures up to 350 K and account for the TDS spectra of the defect-poor
films. On defect-rich films (F centers) the OaH bond is selectively dissociated, resulting in the observed desorption of H 2 at high temperature. Thus, these oxygen
vacancy centers, called F centers, act as nanocatalysts.
17.3
Chemical Reactions and Catalytic Processes on Free and Supported Clusters
17.3.1
Catalytic Processes on Free Metal Clusters
Free clusters are ideal model systems to probe the influence of their intrinsic, sizedependent properties on the catalytic activity due to the lack of any support interactions. Free clusters are prepared from cluster sources [20] and only very
low densities are obtained. They are highly unstable under normal conditions and,
even under UHV conditions, exothermal catalytic reactions may lead to fragmentation without the presence of a buffer gas. Thus, free clusters may not become
relevant for industrial applications. Nevertheless, they are important vehicles to
gain a fundamental understanding of nanocatalysis.
The following method is often used for producing free clusters. The material of
interest is evaporated by means of intense laser pulses [21], electrical discharges
[22] or highly energetic inert ions [23]. A buffer gas thermalizes the ultra-hot
plasma or supersaturated atomic gas produced. The mixture expands through a
nozzle into the vacuum upon supersonic expansion. By this process, the clusters
formed are cooled to cryogenic temperatures and generate well-defined molecular
beams of neutral and charged clusters. The reactivity of the clusters can then be
studied by various experimental techniques, including fast flow reactor kinetics in
the post-vaporization expansion region of a laser evaporation source [24, 25], ion
flow tube reactor kinetics of ionic clusters [26, 27], ion cyclotron resonance [28, 29],
guided-ion-beam [30], and ion-trap experiments [31–33]. Which of these techniques is applied depends on the charge state of the reactants (neutral, cationic,
anionic), on whether the clusters are size-selected before the reaction zone, on
single or multiple collisions of the clusters with the reactants, on the pressure of a
17.3 Chemical Reactions and Catalytic Processes on Free and Supported Clusters 555
À1 when isolated, and to an intense band at
1030 cm
À1 in the chemisorbed complex shown in Figure 17.1b. Therefore the stable species is assigned to H atoms incorporated into oxygen vacancies.
In conclusion, this combined experimental and theoretical study of methanol
adsorbed on MgO films with different defect density allows one to better identify
the surface sites responsible for the MgO reactivity. On the inert terrace sites only
physisorption is observed. Molecular chemisorption, activation and heterolytic dissociation occur on irregular sites. The low-coordinated MgaO pairs of ions located
at edges and steps can lead to strongly activated and even dissociated methanol
molecules. Adsorption of CH 3 Oa and H
þ fragments seems to be preferred over
dissociation into CH 3
þ and OH
À units. All these species are stable on the surface
for temperatures up to 350 K and account for the TDS spectra of the defect-poor
films. On defect-rich films (F centers) the OaH bond is selectively dissociated, resulting in the observed desorption of H 2 at high temperature. Thus, these oxygen
vacancy centers, called F centers, act as nanocatalysts.
17.3
Chemical Reactions and Catalytic Processes on Free and Supported Clusters
17.3.1
Catalytic Processes on Free Metal Clusters
Free clusters are ideal model systems to probe the influence of their intrinsic, sizedependent properties on the catalytic activity due to the lack of any support interactions. Free clusters are prepared from cluster sources [20] and only very
low densities are obtained. They are highly unstable under normal conditions and,
even under UHV conditions, exothermal catalytic reactions may lead to fragmentation without the presence of a buffer gas. Thus, free clusters may not become
relevant for industrial applications. Nevertheless, they are important vehicles to
gain a fundamental understanding of nanocatalysis.
The following method is often used for producing free clusters. The material of
interest is evaporated by means of intense laser pulses [21], electrical discharges
[22] or highly energetic inert ions [23]. A buffer gas thermalizes the ultra-hot
plasma or supersaturated atomic gas produced. The mixture expands through a
nozzle into the vacuum upon supersonic expansion. By this process, the clusters
formed are cooled to cryogenic temperatures and generate well-defined molecular
beams of neutral and charged clusters. The reactivity of the clusters can then be
studied by various experimental techniques, including fast flow reactor kinetics in
the post-vaporization expansion region of a laser evaporation source [24, 25], ion
flow tube reactor kinetics of ionic clusters [26, 27], ion cyclotron resonance [28, 29],
guided-ion-beam [30], and ion-trap experiments [31–33]. Which of these techniques is applied depends on the charge state of the reactants (neutral, cationic,
anionic), on whether the clusters are size-selected before the reaction zone, on
single or multiple collisions of the clusters with the reactants, on the pressure of a
17.3 Chemical Reactions and Catalytic Processes on Free and Supported Clusters 555