through the incorporation of impurity atoms, and via manipulation of the strength
of the cluster–support interaction, the degree of charging of the cluster, and by
changing their magnetic properties. In a completely different approach catalytic
reactions can be induced by nanodevices, where the energy of the localized electrons inducing the reaction can be tuned by changing the potential of the device.
An example is the use of an STM tip in close contact with the reactants. In this
chapter we present some studies on model systems for nanocatalysts. With such
systems the complexity of working nanocatalysts can be reduced and in many
cases simple reaction steps can be identified. It has to be pointed out that in this
chapter we use the expression ‘‘catalysis’’ when a chemical reaction is catalyzed by
a nanoscale system. We are aware, however, that none of the systems/methods
presented here have been tested to withstand the harsh conditions of real catalysis.
We feel, however, that such studies are extremely important for developing guiding
principles in nanocatalysis.
17.2
Chemical Reactions on Point Defects of Oxide Surfaces
In a recent review, Pacchioni presents a detailed classification of point defects at
the surfaces of oxide materials [12]. He describes at least four major kinds of irregularities: low-coordinated sites, divacancies, impurity atoms and surface vacancies. The last category includes cation vacancies, usually called V centers, as well as
oxygen vacancies, which are called color centers or F centers (from the German
word for ‘‘color’’: ‘‘Farbe’’). In a broader sense, an F center is defined as an electron
trapped in an anion vacancy and was first introduced to explain chemical- and
radiation-induced coloration of alkali halides. In bivalent oxides, like MgO and
CaO, two types of F centers exist. The vacancy containing two electrons is called
the F center and the one containing only one electron is labelled as the F
þ center.
In both vacancies the electrons are confined by the Madelung potential of the
crystal. Thus, in a first approach, the F/F
þ centers can be described as two/one
electrons moving in quantum box of nanometer length scale; i.e. F/F
þ centers are
systems with highly confined electrons and with our definition they are examples
of nanocatalysts.
The unique chemical activity of F centers has been demonstrated recently. In a
combined theoretical and experimental study, it was shown that the heterolytic decomposition of methanol on MgO thin films is catalyzed by surface oxygen vacancies [13]. In these films the oxygen vacancies are generated by changing the preparation method [14], e.g. the Mg evaporation rate and the oxygen background
pressure. In this way, two kinds of films are prepared: defect-poor (Mg evaporation
rate: 0.3–0.5 ML min
À1 , O 2 background: 5 Á 10
À7 Torr) and defect-rich (Mg evaporation rate: 2–5 ML min
À1 , O 2 background: 10
À6 Torr) films. Both films are annealed to 1200 K and 840 K for 10 min, respectively. Auger electron spectroscopy
(AES) measurements show a one-to-one stoichiometry for magnesium and oxygen
and the absence of any impurity [15]. Typical thicknesses are about 10 monolayers,
17 Nanocatalysis
552
of the cluster–support interaction, the degree of charging of the cluster, and by
changing their magnetic properties. In a completely different approach catalytic
reactions can be induced by nanodevices, where the energy of the localized electrons inducing the reaction can be tuned by changing the potential of the device.
An example is the use of an STM tip in close contact with the reactants. In this
chapter we present some studies on model systems for nanocatalysts. With such
systems the complexity of working nanocatalysts can be reduced and in many
cases simple reaction steps can be identified. It has to be pointed out that in this
chapter we use the expression ‘‘catalysis’’ when a chemical reaction is catalyzed by
a nanoscale system. We are aware, however, that none of the systems/methods
presented here have been tested to withstand the harsh conditions of real catalysis.
We feel, however, that such studies are extremely important for developing guiding
principles in nanocatalysis.
17.2
Chemical Reactions on Point Defects of Oxide Surfaces
In a recent review, Pacchioni presents a detailed classification of point defects at
the surfaces of oxide materials [12]. He describes at least four major kinds of irregularities: low-coordinated sites, divacancies, impurity atoms and surface vacancies. The last category includes cation vacancies, usually called V centers, as well as
oxygen vacancies, which are called color centers or F centers (from the German
word for ‘‘color’’: ‘‘Farbe’’). In a broader sense, an F center is defined as an electron
trapped in an anion vacancy and was first introduced to explain chemical- and
radiation-induced coloration of alkali halides. In bivalent oxides, like MgO and
CaO, two types of F centers exist. The vacancy containing two electrons is called
the F center and the one containing only one electron is labelled as the F
þ center.
In both vacancies the electrons are confined by the Madelung potential of the
crystal. Thus, in a first approach, the F/F
þ centers can be described as two/one
electrons moving in quantum box of nanometer length scale; i.e. F/F
þ centers are
systems with highly confined electrons and with our definition they are examples
of nanocatalysts.
The unique chemical activity of F centers has been demonstrated recently. In a
combined theoretical and experimental study, it was shown that the heterolytic decomposition of methanol on MgO thin films is catalyzed by surface oxygen vacancies [13]. In these films the oxygen vacancies are generated by changing the preparation method [14], e.g. the Mg evaporation rate and the oxygen background
pressure. In this way, two kinds of films are prepared: defect-poor (Mg evaporation
rate: 0.3–0.5 ML min
À1 , O 2 background: 5 Á 10
À7 Torr) and defect-rich (Mg evaporation rate: 2–5 ML min
À1 , O 2 background: 10
À6 Torr) films. Both films are annealed to 1200 K and 840 K for 10 min, respectively. Auger electron spectroscopy
(AES) measurements show a one-to-one stoichiometry for magnesium and oxygen
and the absence of any impurity [15]. Typical thicknesses are about 10 monolayers,
17 Nanocatalysis
552