equivalent; one CO binds on top and the second adsorbs on the side of the Pd
atom (this top-side geometry is similar to that shown in Figure 17.4bI but without
the O 2 ), and the total binding energy of the two CO molecules is 1.62 eV. To study
the oxidation mechanisms of CO on MgO(F 5c )aPd the system was optimized first
with coadsorbed O 2 and two CO molecules. Two stable geometric arrangements
were found, with the most stable one shown in Figure 17.4bI where the CO molecules bind in a top-side configuration and the O 2 is adsorbed parallel to the surface
on the other side of the Pd atom. The adsorbed O 2 molecular bond is stretched and
activated (1.46 A ˚ compared to the calculated gas-phase value of 1.25 A ˚ ). In addition,
a stable carbonate complex Pd(CO 3 )(CO) was found (Figure 17.4bII), whose binding energy is 4.08 eV larger than the aforementioned Pd(CO) 2 (O 2 ) complex. These
complexes were identified by comparison of calculated and spectroscopically measured CO vibrational frequencies [55]. Two reaction mechanisms are proposed
corresponding to the two CO 2 peaks observed experimentally (Figure 17.4b). At
low temperatures the two relevant precursors are shown in Figures 17.4bI and
17.4bII. Corresponding to the CO 2 desorption peak at 260 K, the following reaction
mechanism is proposed. In a competitive process, CO desorbs or is oxidized upon
heating. The theoretically estimated activation energies of the two processes are
0.89 eV for desorption and 0.84 eV for oxidation. Formation of CO 2 at higher temperatures (corresponding to desorption at around 500 K, Figure 17.4b) involves
decomposition of the Pd(CO 3 )(CO) carbonate complex (Figure 17.4bII). This
mechanism is observed in molecular dynamics simulations where the temperature
is controlled to 500 K by Langevin dynamics.
17.3.2.2 Size-Selected Clusters on Oxide Surfaces
By decorating F centres with atoms, it is possible to increase the number of confined electrons whilst modifying the size of the quantum box only slightly. By using size-selected clusters instead of atoms, the confinement of the valence electrons is subtler and the examples presented below reveal the possibility for tuning
the efficiency and selectivity of chemical reactions atom by atom by simply changing cluster size or doping the cluster with impurity atoms. Furthermore, a comparison of the experimental results with first-principles theoretical simulations
provides insights into the physical factors and microscopic mechanisms that govern nanocatalysis.
The metal clusters are produced by the same high-frequency laser evaporation
source used to produce metal atoms. In this source a cold He pulse (40 K) thermalizes the laser-produced plasma [15]. Subsequent supersonic expansion of the
helium–metal vapor leads to cold clusters with a narrow kinetic energy distribution (E kin a 0.2 eV atom
À1 ). The positively charged clusters guided by ion optics
through differentially pumped vacuum chambers and size-selected by a quadrupole mass spectrometer (Extrel Merlin System; mass limit: 1000, 4000, 9000 u) are
then deposited with low kinetic energy (<0.2 eV atom
À1 ) onto MgO/Mo(100) thin
films, under these conditions the clusters ‘softland’ on the substrate. In fact, when
exploring chemical or catalytic properties of nanoassembled materials consisting
of small monodispersed clusters on surfaces the key challenge is to preserve the
17 Nanocatalysis
566
atom (this top-side geometry is similar to that shown in Figure 17.4bI but without
the O 2 ), and the total binding energy of the two CO molecules is 1.62 eV. To study
the oxidation mechanisms of CO on MgO(F 5c )aPd the system was optimized first
with coadsorbed O 2 and two CO molecules. Two stable geometric arrangements
were found, with the most stable one shown in Figure 17.4bI where the CO molecules bind in a top-side configuration and the O 2 is adsorbed parallel to the surface
on the other side of the Pd atom. The adsorbed O 2 molecular bond is stretched and
activated (1.46 A ˚ compared to the calculated gas-phase value of 1.25 A ˚ ). In addition,
a stable carbonate complex Pd(CO 3 )(CO) was found (Figure 17.4bII), whose binding energy is 4.08 eV larger than the aforementioned Pd(CO) 2 (O 2 ) complex. These
complexes were identified by comparison of calculated and spectroscopically measured CO vibrational frequencies [55]. Two reaction mechanisms are proposed
corresponding to the two CO 2 peaks observed experimentally (Figure 17.4b). At
low temperatures the two relevant precursors are shown in Figures 17.4bI and
17.4bII. Corresponding to the CO 2 desorption peak at 260 K, the following reaction
mechanism is proposed. In a competitive process, CO desorbs or is oxidized upon
heating. The theoretically estimated activation energies of the two processes are
0.89 eV for desorption and 0.84 eV for oxidation. Formation of CO 2 at higher temperatures (corresponding to desorption at around 500 K, Figure 17.4b) involves
decomposition of the Pd(CO 3 )(CO) carbonate complex (Figure 17.4bII). This
mechanism is observed in molecular dynamics simulations where the temperature
is controlled to 500 K by Langevin dynamics.
17.3.2.2 Size-Selected Clusters on Oxide Surfaces
By decorating F centres with atoms, it is possible to increase the number of confined electrons whilst modifying the size of the quantum box only slightly. By using size-selected clusters instead of atoms, the confinement of the valence electrons is subtler and the examples presented below reveal the possibility for tuning
the efficiency and selectivity of chemical reactions atom by atom by simply changing cluster size or doping the cluster with impurity atoms. Furthermore, a comparison of the experimental results with first-principles theoretical simulations
provides insights into the physical factors and microscopic mechanisms that govern nanocatalysis.
The metal clusters are produced by the same high-frequency laser evaporation
source used to produce metal atoms. In this source a cold He pulse (40 K) thermalizes the laser-produced plasma [15]. Subsequent supersonic expansion of the
helium–metal vapor leads to cold clusters with a narrow kinetic energy distribution (E kin a 0.2 eV atom
À1 ). The positively charged clusters guided by ion optics
through differentially pumped vacuum chambers and size-selected by a quadrupole mass spectrometer (Extrel Merlin System; mass limit: 1000, 4000, 9000 u) are
then deposited with low kinetic energy (<0.2 eV atom
À1 ) onto MgO/Mo(100) thin
films, under these conditions the clusters ‘softland’ on the substrate. In fact, when
exploring chemical or catalytic properties of nanoassembled materials consisting
of small monodispersed clusters on surfaces the key challenge is to preserve the
17 Nanocatalysis
566