46
3 Metal Cluster Reacting with Oxygen
formation of V n O 2 and the production of V atom is the main reaction pathways for
V n reacting with O 2, that is, V n + O 2 → V n−1 O 2 + V .
3.2 Oxygen Addition
In addition to oxygen etching effect, [26–28] the reactivities involving oxygen addition onto metal clusters have also been well illustrated in the past 20 years [29–31].
To some extent, the addition of molecular oxygen to metal clusters is actually the
initial reaction channel (followed by etching effect, etc.) for most metal clusters,
such as the aforementioned vanadium cluster neutrals. The addition of oxygen is
often associated with an oxidation process, i.e., loss of electrons, charge distribution
rearrangement and orbital hybridization. Also, the oxygen addition/adsorption could
follow the principle of increasing valence of metals but bear dramatic selectivity
depending on cluster sizes, bonding energies and electronic behaviors. For example,
selective anionic Au n
− with an odd number of electrons (i.e., even number of Au
atoms) showed significant O 2 uptake, whereas some other clusters either showed very
weak reactivity or did not exhibit any propensity for reactions with oxygen [32–34]. In
comparison, neutral and cationic gold clusters were found to be inert toward oxygen
with rare exceptions (e.g., Au 10
+ ) [30]. Considering an oxygen double bond, there
is much larger electron density within an oxygen molecule than the molecular outer
end, allowing for relatively strong electron-withdrawing ability of the oxygen atom.
Therefore, noble metal clusters with additional unpaired electrons are favorable for
such oxygen-addition reactivity.
In addition to the investigations revealing the oxygen-addition reactivity of Au
clusters, there were also a few reports showing similarity of copper clusters in reacting
with oxygen [35]. The study by T. H. Lee and K. M. Ervin demonstrated that the
addition of molecular oxygen through “Cu n
−
+ O 2 → Cu n O 2
− ” is a primary reaction
channel for most copper cluster anions [29]. Further reactions were also observed
for small Cu n
− clusters (n = 2–5) allowing for the formation of a subsequent product
Cu n O 4
− . Further, the effective bimolecular rate coefficients for the reactions of Cu n
− ,
Ag n
− and Au n
− with oxygen have been measured at a certain buffer gas pressure,
as shown in Fig. 3.7. Other than oxygen addition, collision-induced dissociation
(or fragmentation) of coinage metal clusters was also included in such gas-phase
reactions [36].
Oxygen-addition was also found to dominate the reactivity of cationic Co n
+ clusters [37]. The cationic Co n
+ (n = 2–9) clusters displayed a high reactivity toward
O 2 by taking successive oxidation pathways. It is notable that the oxygen addition
on Co n
+ clusters does not follow a direct attachment; instead, the primary reaction
mainly results in a replacement of a Co atom by an O 2 molecule. Also, the formed
oxide clusters allow for successive reactions toward oxygen, which resembles the
etching effect observed for anionic Co n
− as discussed above. Whereas, there is huge
difference between cationic and anionic cobalt clusters, probably because the latter
3 Metal Cluster Reacting with Oxygen
formation of V n O 2 and the production of V atom is the main reaction pathways for
V n reacting with O 2, that is, V n + O 2 → V n−1 O 2 + V .
3.2 Oxygen Addition
In addition to oxygen etching effect, [26–28] the reactivities involving oxygen addition onto metal clusters have also been well illustrated in the past 20 years [29–31].
To some extent, the addition of molecular oxygen to metal clusters is actually the
initial reaction channel (followed by etching effect, etc.) for most metal clusters,
such as the aforementioned vanadium cluster neutrals. The addition of oxygen is
often associated with an oxidation process, i.e., loss of electrons, charge distribution
rearrangement and orbital hybridization. Also, the oxygen addition/adsorption could
follow the principle of increasing valence of metals but bear dramatic selectivity
depending on cluster sizes, bonding energies and electronic behaviors. For example,
selective anionic Au n
− with an odd number of electrons (i.e., even number of Au
atoms) showed significant O 2 uptake, whereas some other clusters either showed very
weak reactivity or did not exhibit any propensity for reactions with oxygen [32–34]. In
comparison, neutral and cationic gold clusters were found to be inert toward oxygen
with rare exceptions (e.g., Au 10
+ ) [30]. Considering an oxygen double bond, there
is much larger electron density within an oxygen molecule than the molecular outer
end, allowing for relatively strong electron-withdrawing ability of the oxygen atom.
Therefore, noble metal clusters with additional unpaired electrons are favorable for
such oxygen-addition reactivity.
In addition to the investigations revealing the oxygen-addition reactivity of Au
clusters, there were also a few reports showing similarity of copper clusters in reacting
with oxygen [35]. The study by T. H. Lee and K. M. Ervin demonstrated that the
addition of molecular oxygen through “Cu n
−
+ O 2 → Cu n O 2
− ” is a primary reaction
channel for most copper cluster anions [29]. Further reactions were also observed
for small Cu n
− clusters (n = 2–5) allowing for the formation of a subsequent product
Cu n O 4
− . Further, the effective bimolecular rate coefficients for the reactions of Cu n
− ,
Ag n
− and Au n
− with oxygen have been measured at a certain buffer gas pressure,
as shown in Fig. 3.7. Other than oxygen addition, collision-induced dissociation
(or fragmentation) of coinage metal clusters was also included in such gas-phase
reactions [36].
Oxygen-addition was also found to dominate the reactivity of cationic Co n
+ clusters [37]. The cationic Co n
+ (n = 2–9) clusters displayed a high reactivity toward
O 2 by taking successive oxidation pathways. It is notable that the oxygen addition
on Co n
+ clusters does not follow a direct attachment; instead, the primary reaction
mainly results in a replacement of a Co atom by an O 2 molecule. Also, the formed
oxide clusters allow for successive reactions toward oxygen, which resembles the
etching effect observed for anionic Co n
− as discussed above. Whereas, there is huge
difference between cationic and anionic cobalt clusters, probably because the latter