224
13 Metal Cluster Catalysis
catalytic activity of the monodispersed Pt clusters as the number of catalyzed CO 2
molecules per cluster is displayed in Fig. 13.7B. Note that the catalytic reactivity
shows a local decrease for the cluster Pt 13 which bears unique electronic structures
and symmetry [139].
Through a comparison with the catalytic oxidation of CO on Pt single-crystal
surfaces, the observed overall size-dependent reactivity in such system was rationalized with changes of the cluster structure, together with simple frontier orbital
considerations. Considering the oxidation temperatures on size-selected Pt clusters
are in a similar range as on Pt single-crystal surfaces, the reaction sites were identified
accordingly based on the following two main mechanisms:
i. reaction α on Pt(111) terrace sites,
O 2 + 2CO terrace → 2CO 2
(13.4)
ii. reaction β on Pt(355) stepped sites, including
β 1 , O terrace + CO terrace → CO 2
(13.5)
β 2 , O step + CO step → CO 2
(13.6)
β 3 , O step + CO terrace → CO 2
(13.7)
where the reaction α was also found at 160 K involving hot oxygen atoms [131, 140];
while for the stepped Pt(355) surface, reactions β at 290, 350, and 200 K have also
been identified, respectively [135]. The throughout presence of the β-mechanism
(Fig. 13.7A) indicates dissociation of O 2 on all cluster sizes, but the α-mechanism
was only observed for larger clusters, Pt 15–20 , indicating that oxygen is adsorbed
molecularly in an ionic state only on these finite species.
To further understand this size-dependent catalytic reactivity, the geometric and
electronic structures of each Pt cluster must be taken into consideration. For the
oxygen molecule, (Fig. 13.7C(b)), the electronic states of interest include π u and
σ g (bonding, fully occupied) and π g * (antibonding, half-occupied) orbitals. The
adsorption and dissociation of O 2 likely occur providing there exists a resonance
(energetically and symmetrically) between one of these orbitals and the cluster’s
density of state [141]. Specifically, fragmentation occurs when there is enough backdonation from the clusters into the antibonding π g * state, or a sufficient donation
from the π u or σ g orbital into the cluster. The energies of the fragment orbitals were
calculated to be −9.23 eV, −8.32 eV, and −6.1 eV for π u , σ g , and π g * respectively
[130, 141]. On the cluster side (Fig. 13.7C(a)), the energy of the highest occupied
molecular orbitals (HOMO) of the clusters changes with the cluster size and is
13 Metal Cluster Catalysis
catalytic activity of the monodispersed Pt clusters as the number of catalyzed CO 2
molecules per cluster is displayed in Fig. 13.7B. Note that the catalytic reactivity
shows a local decrease for the cluster Pt 13 which bears unique electronic structures
and symmetry [139].
Through a comparison with the catalytic oxidation of CO on Pt single-crystal
surfaces, the observed overall size-dependent reactivity in such system was rationalized with changes of the cluster structure, together with simple frontier orbital
considerations. Considering the oxidation temperatures on size-selected Pt clusters
are in a similar range as on Pt single-crystal surfaces, the reaction sites were identified
accordingly based on the following two main mechanisms:
i. reaction α on Pt(111) terrace sites,
O 2 + 2CO terrace → 2CO 2
(13.4)
ii. reaction β on Pt(355) stepped sites, including
β 1 , O terrace + CO terrace → CO 2
(13.5)
β 2 , O step + CO step → CO 2
(13.6)
β 3 , O step + CO terrace → CO 2
(13.7)
where the reaction α was also found at 160 K involving hot oxygen atoms [131, 140];
while for the stepped Pt(355) surface, reactions β at 290, 350, and 200 K have also
been identified, respectively [135]. The throughout presence of the β-mechanism
(Fig. 13.7A) indicates dissociation of O 2 on all cluster sizes, but the α-mechanism
was only observed for larger clusters, Pt 15–20 , indicating that oxygen is adsorbed
molecularly in an ionic state only on these finite species.
To further understand this size-dependent catalytic reactivity, the geometric and
electronic structures of each Pt cluster must be taken into consideration. For the
oxygen molecule, (Fig. 13.7C(b)), the electronic states of interest include π u and
σ g (bonding, fully occupied) and π g * (antibonding, half-occupied) orbitals. The
adsorption and dissociation of O 2 likely occur providing there exists a resonance
(energetically and symmetrically) between one of these orbitals and the cluster’s
density of state [141]. Specifically, fragmentation occurs when there is enough backdonation from the clusters into the antibonding π g * state, or a sufficient donation
from the π u or σ g orbital into the cluster. The energies of the fragment orbitals were
calculated to be −9.23 eV, −8.32 eV, and −6.1 eV for π u , σ g , and π g * respectively
[130, 141]. On the cluster side (Fig. 13.7C(a)), the energy of the highest occupied
molecular orbitals (HOMO) of the clusters changes with the cluster size and is