48
3 Metal Cluster Reacting with Oxygen
with metal clusters leading to the formation of superoxo state (O 2
−• ) and peroxo
state (O 2
2− ) complexes due to chemisorption and charge transfer [41]. For example,
Klacar et al. [42] examined the reactivity of oxygen with Ag n clusters containing up
to 9 atoms and found that the molecular oxygen preferred a dissociation mode for its
adsorption on larger sized clusters (e.g., containing more than 5 Ag atoms), where
the activation of O–O bond was initiated at a superoxo state.
There could be coexistence of superoxo and peroxo states, but a transition from
the former to the latter determines the O–O bond activation within a cluster-oxygen
reaction process [44]. For this, Wang, Zeng and coworkers [43] reported an in-depth
study of O 2 chemisorption on even-sized Au n
– clusters (Fig. 3.8). They demonstrated
spectroscopic and electronic evidences of the transition from superoxo to peroxo
chemisorption for Au 8
– . It was noted that both superoxo and peroxo states coexisted
in the cluster beam of O 2 Au 8
– , and the superoxo form involves a single O-Au bond
(η
1 –O 2 ) while the peroxo form involves two O–Au bonds (η
2 –O 2 ). Also the superoxo
O 2 Au 8
– exhibited low binding energy within the O 2 -induced PES feature, while the
peroxo O 2 Au 8
– displayed sharper and higher binding energy feature. It was also
noted that, although both two-dimensional (2D) and 3D isomers of Au 12
– coexist in
the cluster beam, O 2 prefers the peroxo binding with the 3D isomer of Au 12
– [43].
Superoxo and peroxo states have also been found as important chemical processes
associated with other metal cluster reactions [34, 45].
3.4 Cluster Odd–Even Alternation
As has been shown above, with sufficient oxygen flowing, a dramatic loss of Al
cluster signal is readily discerned for most species especially even-numbered clusters
(i.e., odd number of electrons), but odd-numbered clusters (i.e., even number of
electrons) could survive in the rich-pressure condition displaying slightly enlarged
mass abundance. This odd/even alternation was also seen in other experiments where
most of the even-atom clusters reacted away in sharp contrast to the odd, indicative
of a paired electron effect [46, 47]. Taking a glance over the studies of metal cluster
in reacting with oxygen, there are actually abundant investigations showing the odd–
even alternation [25, 28, 32, 48–58]. For example, among the Au n
– cluster reactions
with O 2 , molecular oxygen addition was found to be the main pathway for the evensized clusters, while the odd-sized clusters were inert toward O 2 [29, 32, 34]. Such
even–odd alternation correlates with a similar pattern in the electron affinities of
Au n clusters, validating that electron transfer between Au n
– and O 2 dominates their
reactivity [32, 43]. Experimental evidences have also been obtained via photoelectron
spectroscopy, revealing that even-sized gold clusters favour O 2 chemisorption by
noting the distinguishable O–O vibrational fingerprints [59, 60].
Bernhardt et al. [53, 61] reported the reactivity of anionic silver clusters Ag n
−
with O 2 , as shown in Fig. 3.9. It was demonstrated that, among the Ag n
− (n = 1–11)
clusters they studied, the even-atom anions (i.e., odd number of valence electrons)
were more reactive than the odd-atom cluster anions for the reaction of the first O 2 .
3 Metal Cluster Reacting with Oxygen
with metal clusters leading to the formation of superoxo state (O 2
−• ) and peroxo
state (O 2
2− ) complexes due to chemisorption and charge transfer [41]. For example,
Klacar et al. [42] examined the reactivity of oxygen with Ag n clusters containing up
to 9 atoms and found that the molecular oxygen preferred a dissociation mode for its
adsorption on larger sized clusters (e.g., containing more than 5 Ag atoms), where
the activation of O–O bond was initiated at a superoxo state.
There could be coexistence of superoxo and peroxo states, but a transition from
the former to the latter determines the O–O bond activation within a cluster-oxygen
reaction process [44]. For this, Wang, Zeng and coworkers [43] reported an in-depth
study of O 2 chemisorption on even-sized Au n
– clusters (Fig. 3.8). They demonstrated
spectroscopic and electronic evidences of the transition from superoxo to peroxo
chemisorption for Au 8
– . It was noted that both superoxo and peroxo states coexisted
in the cluster beam of O 2 Au 8
– , and the superoxo form involves a single O-Au bond
(η
1 –O 2 ) while the peroxo form involves two O–Au bonds (η
2 –O 2 ). Also the superoxo
O 2 Au 8
– exhibited low binding energy within the O 2 -induced PES feature, while the
peroxo O 2 Au 8
– displayed sharper and higher binding energy feature. It was also
noted that, although both two-dimensional (2D) and 3D isomers of Au 12
– coexist in
the cluster beam, O 2 prefers the peroxo binding with the 3D isomer of Au 12
– [43].
Superoxo and peroxo states have also been found as important chemical processes
associated with other metal cluster reactions [34, 45].
3.4 Cluster Odd–Even Alternation
As has been shown above, with sufficient oxygen flowing, a dramatic loss of Al
cluster signal is readily discerned for most species especially even-numbered clusters
(i.e., odd number of electrons), but odd-numbered clusters (i.e., even number of
electrons) could survive in the rich-pressure condition displaying slightly enlarged
mass abundance. This odd/even alternation was also seen in other experiments where
most of the even-atom clusters reacted away in sharp contrast to the odd, indicative
of a paired electron effect [46, 47]. Taking a glance over the studies of metal cluster
in reacting with oxygen, there are actually abundant investigations showing the odd–
even alternation [25, 28, 32, 48–58]. For example, among the Au n
– cluster reactions
with O 2 , molecular oxygen addition was found to be the main pathway for the evensized clusters, while the odd-sized clusters were inert toward O 2 [29, 32, 34]. Such
even–odd alternation correlates with a similar pattern in the electron affinities of
Au n clusters, validating that electron transfer between Au n
– and O 2 dominates their
reactivity [32, 43]. Experimental evidences have also been obtained via photoelectron
spectroscopy, revealing that even-sized gold clusters favour O 2 chemisorption by
noting the distinguishable O–O vibrational fingerprints [59, 60].
Bernhardt et al. [53, 61] reported the reactivity of anionic silver clusters Ag n
−
with O 2 , as shown in Fig. 3.9. It was demonstrated that, among the Ag n
− (n = 1–11)
clusters they studied, the even-atom anions (i.e., odd number of valence electrons)
were more reactive than the odd-atom cluster anions for the reaction of the first O 2 .