102
7 The Reactions with Monoxides for Pollution Removal
7.3 Reactivity of CO with Iron Oxides
7.3.1 Anionic Clusters Fe n O m
−
Castleman and Khanna [22] showed a synergistic investigation combining gasphase experiments and theoretical first-principles calculations to study the structure,
stability, and reactivity of Fe n O m
− clusters towards CO. Collision-induced dissociation of these iron oxide species under xenon gas collisions showed that both FeO 3
−
and FeO 2
− are stable building blocks in forming larger iron oxide clusters. Based on
mass-selected experiments, the transfer of oxygen atoms from Fe n O m
− to CO was
seen as the dominant reaction pathway. Further, theoretical calculations demonstrated
that the fragmentation patterns leading to the production of O or FeO n fragments are
governed both by the energetics of the overall process (as well as the number of
intermediate states) and the changes in spin multiplicity [22].
Figure 7.4A shows the anionic iron oxide cluster distribution with both dissociated
and molecular oxygen adsorbed at near thermal energy. The calculated optimized
lowest-energy structures for these anionic FeO 1–4
− and Fe 2 O 2–6
− clusters are shown
in Fig. 7.4B. For iron oxide clusters containing a single Fe atom, oxygen atoms
bind directly to the metal with no molecular oxygen units, where the maximum
coordination number of Fe is four with a tetrahedral form of the FeO 4
− as ground state
structure. In comparison, an isomer of FeO 4
− possessing a molecular oxygen subunit
exhibits a higher energy of 0.78 eV above the ground-state energy. All the Fe–O bond
lengths in FeO 1–4
− are relatively similar (~1.65 Å); but the spin multiplicity changes
from quartet in FeO 1–3
− species to doublet for FeO 4
− . Different from the structures
of FeO 1–4
− , a basic ring structure composed of Fe 2 O 2
− with oxygen bridging each
iron atom is formed for all the clusters Fe 2 O 2–6
− . The species Fe 2 O 3–6
− have their
additional oxygen atoms attached to iron outside the 1.85–1.79 Å within the ring
(i.e., Fe–O–Fe) while 1.65–1.62 Å outside. Optimized calculations showed that the
maximum coordination for the Fe 2 O 6
− cluster is two extra oxygen atoms bounding
directly to each Fe atom. In addition, an anti-ferromagnetic spin coupling was noted
for Fe 2 O 2
− , Fe 2 O 3
− , Fe 2 O 4
− and Fe 2 O 5
− ; however, it was found that the Fe sites in
Fe 2 O 6
− are coupled ferromagnetically [22]. Note that the progression of the exchange
coupling with oxidation coincides with the case of Cr 2 O n clusters [76].
In order to ascertain the stability of these iron oxide clusters, collision induced
dissociation studies were undertaken with inert xenon gas under single (0.09 mTorr)
and multiple (0.2 mTorr) collision conditions. Simultaneously, calculations were
done on the energies required to remove an O, O 2 , O
− , and O 2
− from FeO n
− and
Fe 2 O n
− clusters, as well as the energies for fragmentation pathways leading to the
production of Fe, FeO, FeO 2 and FeO 3 for the Fe 2 O n
− clusters. The results of these
investigations are plotted in Fig. 7.4C, which corresponded to the observed experimental products. It was noted that the loss of O 2 reaches an energetic minimum
for Fe 2 O 6
− , indicating the energy needed to break two Fe–O bonds possibly overcome by the exothermic formation of O 2 (which releases 6.21 eV energy). Such
oxygen recombination was also observed in studies conducted with V 2 O 5
+ [77]. In
7 The Reactions with Monoxides for Pollution Removal
7.3 Reactivity of CO with Iron Oxides
7.3.1 Anionic Clusters Fe n O m
−
Castleman and Khanna [22] showed a synergistic investigation combining gasphase experiments and theoretical first-principles calculations to study the structure,
stability, and reactivity of Fe n O m
− clusters towards CO. Collision-induced dissociation of these iron oxide species under xenon gas collisions showed that both FeO 3
−
and FeO 2
− are stable building blocks in forming larger iron oxide clusters. Based on
mass-selected experiments, the transfer of oxygen atoms from Fe n O m
− to CO was
seen as the dominant reaction pathway. Further, theoretical calculations demonstrated
that the fragmentation patterns leading to the production of O or FeO n fragments are
governed both by the energetics of the overall process (as well as the number of
intermediate states) and the changes in spin multiplicity [22].
Figure 7.4A shows the anionic iron oxide cluster distribution with both dissociated
and molecular oxygen adsorbed at near thermal energy. The calculated optimized
lowest-energy structures for these anionic FeO 1–4
− and Fe 2 O 2–6
− clusters are shown
in Fig. 7.4B. For iron oxide clusters containing a single Fe atom, oxygen atoms
bind directly to the metal with no molecular oxygen units, where the maximum
coordination number of Fe is four with a tetrahedral form of the FeO 4
− as ground state
structure. In comparison, an isomer of FeO 4
− possessing a molecular oxygen subunit
exhibits a higher energy of 0.78 eV above the ground-state energy. All the Fe–O bond
lengths in FeO 1–4
− are relatively similar (~1.65 Å); but the spin multiplicity changes
from quartet in FeO 1–3
− species to doublet for FeO 4
− . Different from the structures
of FeO 1–4
− , a basic ring structure composed of Fe 2 O 2
− with oxygen bridging each
iron atom is formed for all the clusters Fe 2 O 2–6
− . The species Fe 2 O 3–6
− have their
additional oxygen atoms attached to iron outside the 1.85–1.79 Å within the ring
(i.e., Fe–O–Fe) while 1.65–1.62 Å outside. Optimized calculations showed that the
maximum coordination for the Fe 2 O 6
− cluster is two extra oxygen atoms bounding
directly to each Fe atom. In addition, an anti-ferromagnetic spin coupling was noted
for Fe 2 O 2
− , Fe 2 O 3
− , Fe 2 O 4
− and Fe 2 O 5
− ; however, it was found that the Fe sites in
Fe 2 O 6
− are coupled ferromagnetically [22]. Note that the progression of the exchange
coupling with oxidation coincides with the case of Cr 2 O n clusters [76].
In order to ascertain the stability of these iron oxide clusters, collision induced
dissociation studies were undertaken with inert xenon gas under single (0.09 mTorr)
and multiple (0.2 mTorr) collision conditions. Simultaneously, calculations were
done on the energies required to remove an O, O 2 , O
− , and O 2
− from FeO n
− and
Fe 2 O n
− clusters, as well as the energies for fragmentation pathways leading to the
production of Fe, FeO, FeO 2 and FeO 3 for the Fe 2 O n
− clusters. The results of these
investigations are plotted in Fig. 7.4C, which corresponded to the observed experimental products. It was noted that the loss of O 2 reaches an energetic minimum
for Fe 2 O 6
− , indicating the energy needed to break two Fe–O bonds possibly overcome by the exothermic formation of O 2 (which releases 6.21 eV energy). Such
oxygen recombination was also observed in studies conducted with V 2 O 5
+ [77]. In