buffer gas if present, and on the temperature and collision energy of the reactant
molecules.
A representative experimental set-up may be described in the following way [31].
The cluster anions are produced by a sputter source and are extracted into a
helium-filled quadrupole, where they are cooled to room temperature and collimated to the axis of the ion optics. A second quadrupole selects a single cluster
size. The cluster ions are then transferred with a quadrupole ion guide into an
octopole ion trap, which can be continuously cooled from T T ¼ 350 to 20 K. The
potentials on the entrance and exit lenses can be switched in order to fill the trap,
store the ions in the trap, and finally extract product ions for mass analysis.
Through collisions with a buffer gas (helium or argon) in the trap, the clusters are
thermalized within a few milliseconds. Different reactants, can be added to the ion
trap under known partial pressures p R . Typically the number of reactant molecules
is orders of magnitude higher than the number of clusters and thus it can be taken
as constant. After reaction time t R , the charged products are extracted from the
trap and are analyzed by a quadrupole mass spectrometer as a function of t R , p R ,
and T T .
With such experimental techniques the interaction of reactants, e.g. CO and O 2
molecules, or even complete catalytic cycles have been observed on small Au n
À
anions [33, 34]. By varying the reaction temperatures and measuring the kinetics
of the processes, the energetics of complete reaction mechanisms could be obtained and compared with ab initio simulations [32, 35]. The reactivity of Au 2
À
toward O 2 or CO, as well as the coadsorption of both molecules, revealed an interesting and unexpected selectivity. O 2 only reacts with even-n Au n
À clusters, while
odd-n clusters and Au 16
À are inert. In addition, the reactive clusters adsorb just one
O 2 molecule. The relative reactivities correlate well with measured electron affinities of neutral clusters, suggesting a mechanism involving the adsorption of O 2 as
a one-electron acceptor [36, 37]. The reactivity of gold cluster anions with CO also
shows a distinct size-selectivity, although less pronounced than that with O 2 [38].
In this reaction, the most reactive cluster sizes are Au 11 , Au 15 , and Au 19 . Most reactive cluster sizes readily adsorb several CO molecules in the saturation limit. The
full geometric coverage, e.g. one CO per surface gold atom, is, however, never
reached. On real catalysts and single crystals competitive adsorption is usually observed, meaning that in many examples CO poisons the active catalysts. This is not
true for small Au n
À clusters. In contrast, for distinct cluster sizes cooperative adsorption is observed, where the presence of preadsorbates enhances the adsorption
probability of subsequent adsorbates. As examples, for Au 3
À preadsorbed CO enables the adsorption probability of O 2 , Au 2
À is inert for the adsorption of CO at
around room temperature, the presence of preadsorbed O 2 , however, makes the
adsorption possible at this temperature [33]. A similar effect is observed for Au 10
À ,
where O 2 enhances the adsorption of CO by a factor of more than 5. Finally, Au 6
À
reveals even an enhanced cooperative adsorption with increasing numbers of adsorbate molecules [34].
Let us now turn our attention to Au 2
À where a full catalytic cycle could unambiguously be observed by measuring the kinetics of the process at different
17 Nanocatalysis
556
molecules.
A representative experimental set-up may be described in the following way [31].
The cluster anions are produced by a sputter source and are extracted into a
helium-filled quadrupole, where they are cooled to room temperature and collimated to the axis of the ion optics. A second quadrupole selects a single cluster
size. The cluster ions are then transferred with a quadrupole ion guide into an
octopole ion trap, which can be continuously cooled from T T ¼ 350 to 20 K. The
potentials on the entrance and exit lenses can be switched in order to fill the trap,
store the ions in the trap, and finally extract product ions for mass analysis.
Through collisions with a buffer gas (helium or argon) in the trap, the clusters are
thermalized within a few milliseconds. Different reactants, can be added to the ion
trap under known partial pressures p R . Typically the number of reactant molecules
is orders of magnitude higher than the number of clusters and thus it can be taken
as constant. After reaction time t R , the charged products are extracted from the
trap and are analyzed by a quadrupole mass spectrometer as a function of t R , p R ,
and T T .
With such experimental techniques the interaction of reactants, e.g. CO and O 2
molecules, or even complete catalytic cycles have been observed on small Au n
À
anions [33, 34]. By varying the reaction temperatures and measuring the kinetics
of the processes, the energetics of complete reaction mechanisms could be obtained and compared with ab initio simulations [32, 35]. The reactivity of Au 2
À
toward O 2 or CO, as well as the coadsorption of both molecules, revealed an interesting and unexpected selectivity. O 2 only reacts with even-n Au n
À clusters, while
odd-n clusters and Au 16
À are inert. In addition, the reactive clusters adsorb just one
O 2 molecule. The relative reactivities correlate well with measured electron affinities of neutral clusters, suggesting a mechanism involving the adsorption of O 2 as
a one-electron acceptor [36, 37]. The reactivity of gold cluster anions with CO also
shows a distinct size-selectivity, although less pronounced than that with O 2 [38].
In this reaction, the most reactive cluster sizes are Au 11 , Au 15 , and Au 19 . Most reactive cluster sizes readily adsorb several CO molecules in the saturation limit. The
full geometric coverage, e.g. one CO per surface gold atom, is, however, never
reached. On real catalysts and single crystals competitive adsorption is usually observed, meaning that in many examples CO poisons the active catalysts. This is not
true for small Au n
À clusters. In contrast, for distinct cluster sizes cooperative adsorption is observed, where the presence of preadsorbates enhances the adsorption
probability of subsequent adsorbates. As examples, for Au 3
À preadsorbed CO enables the adsorption probability of O 2 , Au 2
À is inert for the adsorption of CO at
around room temperature, the presence of preadsorbed O 2 , however, makes the
adsorption possible at this temperature [33]. A similar effect is observed for Au 10
À ,
where O 2 enhances the adsorption of CO by a factor of more than 5. Finally, Au 6
À
reveals even an enhanced cooperative adsorption with increasing numbers of adsorbate molecules [34].
Let us now turn our attention to Au 2
À where a full catalytic cycle could unambiguously be observed by measuring the kinetics of the process at different
17 Nanocatalysis
556