selected gold clusters, Au n (n a 20), and strontium-doped gold clusters, SrAu n
(n a 20), supported on defect-poor and defect-rich MgO(100) films [6, 14].
Subsequent to verification that oxidation of CO does not occur on bare MgO
substrate films (with and without defects), the catalyzed combustion of CO by
nanosize gold and strontium-doped gold clusters adsorbed on the two different
MgO films was studied by temperature programmed reaction (TPR) experiments.
Isotopically labeled
18 O 2 and
13 CO were used to disentangle the CO 2 production on
the cluster from an eventual catalytic oxidation of CO involving oxygen atoms from
the MgO substrate. Indeed, the
13 CO molecule is exclusively oxidized by
18 O 2 since
only the
13 C
16 O
18 O isotopomer is detected. Furthermore, the experiments revealed
that the pure gold clusters up to the heptamer are inert for the oxidation of CO and
that the low temperature (T < 250 K) combustion of CO is most effective for Au 8 .
By doping gold clusters with strontium the number of gold atoms in the smallest
catalytically active cluster size is reduced to just three. In addition, these results
revealed that, e.g. Au 8 , the smallest active cluster size, becomes inert when it is
deposited it on defect-poor films. This indicates that substrate defects are important when trying to understand the factors, which govern the chemical properties
of nanoscale, supported clusters (Figure 17.5A).
Let us first turn our attention to the reaction mechanism and role of defect
sites in the case of Au 8 . When first deposited on MgO(100) films with a vanishing number of F centers the catalytic activity observed experimentally during one
heating cycle reveals a very small combustion of CO at temperatures between 150
and 250 K (Figure 17.5A-d). Surprisingly, the CO combustion on Au 8 at 250 K
increases dramatically when depositing the octamer on defect-rich MgO-films
(Figure 17.5A-e). This particular behavior pertains even after several reaction cycles
up to temperatures of 350 K. It indicates that the model catalyst is thermally stable
and the reaction heat of the combustion of CO does not change the cluster’s
structure, an important issue when using Au 8 as a real catalyst. Evaluating the area
of the TPR signal yields the production of about one CO 2 molecule per Au 8 cluster.
To explore the energetics and reaction mechanisms underlying the above observations extensive ab initio simulations were performed [14, 64]. Figure 17.5B summarizes the energetics and atomic configurations of the initial, transition, and final
states proposed for the combustion of CO at 140 K and 250 K catalyzed by Au 8
clusters adsorbed on MgO(F 5c ). Both O 2 and CO adsorb on the Au 8 cluster supported on the MgO(100) surface (with or without F 5c defects). However, the observed single sharply peaked stretching frequency of adsorbed CO with u a ¼ 2102
cm
1 compared to u a ¼ 2143 cm
1 for the free
12 C
16 O molecule, implies a single
adsorption site on the cluster and a rather minor influence of adsorption on the
internal structure and bonding of the molecule. Indeed, it is found that in the optimal adsorption configuration CO binds on-top of an Au atom of the upper triangular facet of the adsorbed Au 8 cluster (see Figures 17.5B-d,f, initial state) with
d(CO) ¼ 1.14 A ˚ (compared to 1.13 A ˚ in the free molecule) and d(AuC) ¼ 1.92 A ˚ . On
the other hand, an O 2 molecule may readily adsorb at several sites, both on the
upper Au, facet (Figure 17.5B-d) and at the interface between the adsorbed gold
cluster and the underlying magnesia surface (Figure 17.5B-f ). In all cases the ad17 Nanocatalysis
568
(n a 20), supported on defect-poor and defect-rich MgO(100) films [6, 14].
Subsequent to verification that oxidation of CO does not occur on bare MgO
substrate films (with and without defects), the catalyzed combustion of CO by
nanosize gold and strontium-doped gold clusters adsorbed on the two different
MgO films was studied by temperature programmed reaction (TPR) experiments.
Isotopically labeled
18 O 2 and
13 CO were used to disentangle the CO 2 production on
the cluster from an eventual catalytic oxidation of CO involving oxygen atoms from
the MgO substrate. Indeed, the
13 CO molecule is exclusively oxidized by
18 O 2 since
only the
13 C
16 O
18 O isotopomer is detected. Furthermore, the experiments revealed
that the pure gold clusters up to the heptamer are inert for the oxidation of CO and
that the low temperature (T < 250 K) combustion of CO is most effective for Au 8 .
By doping gold clusters with strontium the number of gold atoms in the smallest
catalytically active cluster size is reduced to just three. In addition, these results
revealed that, e.g. Au 8 , the smallest active cluster size, becomes inert when it is
deposited it on defect-poor films. This indicates that substrate defects are important when trying to understand the factors, which govern the chemical properties
of nanoscale, supported clusters (Figure 17.5A).
Let us first turn our attention to the reaction mechanism and role of defect
sites in the case of Au 8 . When first deposited on MgO(100) films with a vanishing number of F centers the catalytic activity observed experimentally during one
heating cycle reveals a very small combustion of CO at temperatures between 150
and 250 K (Figure 17.5A-d). Surprisingly, the CO combustion on Au 8 at 250 K
increases dramatically when depositing the octamer on defect-rich MgO-films
(Figure 17.5A-e). This particular behavior pertains even after several reaction cycles
up to temperatures of 350 K. It indicates that the model catalyst is thermally stable
and the reaction heat of the combustion of CO does not change the cluster’s
structure, an important issue when using Au 8 as a real catalyst. Evaluating the area
of the TPR signal yields the production of about one CO 2 molecule per Au 8 cluster.
To explore the energetics and reaction mechanisms underlying the above observations extensive ab initio simulations were performed [14, 64]. Figure 17.5B summarizes the energetics and atomic configurations of the initial, transition, and final
states proposed for the combustion of CO at 140 K and 250 K catalyzed by Au 8
clusters adsorbed on MgO(F 5c ). Both O 2 and CO adsorb on the Au 8 cluster supported on the MgO(100) surface (with or without F 5c defects). However, the observed single sharply peaked stretching frequency of adsorbed CO with u a ¼ 2102
cm
1 compared to u a ¼ 2143 cm
1 for the free
12 C
16 O molecule, implies a single
adsorption site on the cluster and a rather minor influence of adsorption on the
internal structure and bonding of the molecule. Indeed, it is found that in the optimal adsorption configuration CO binds on-top of an Au atom of the upper triangular facet of the adsorbed Au 8 cluster (see Figures 17.5B-d,f, initial state) with
d(CO) ¼ 1.14 A ˚ (compared to 1.13 A ˚ in the free molecule) and d(AuC) ¼ 1.92 A ˚ . On
the other hand, an O 2 molecule may readily adsorb at several sites, both on the
upper Au, facet (Figure 17.5B-d) and at the interface between the adsorbed gold
cluster and the underlying magnesia surface (Figure 17.5B-f ). In all cases the ad17 Nanocatalysis
568