108
7 The Reactions with Monoxides for Pollution Removal
product Fe 2 O 3
+ ; nevertheless, the detection of minor product species such as Fe 2 O 2
+
and Fe 2 O 3 CO
+ in the reaction of Fe 2 O 5
+ with CO still revealed the occurrence of a
sequential oxidation process of CO. As shown in Fig. 7.8d, in the initial reaction step,
a CO molecule attaches to the O atom bonded to Fe with the lowest coordination, with
a binding energy at 3.26 eV; following that, an intermediate with CO 2 emanation and
generating a Fe 2 O 4
+ cluster requires 1.23 eV energy. Note the structural difference of
this intermediate of Fe 2 O 4
+ in a high-energy state where the two external O atoms are
binding to one Fe atom, which differs from the ground state Fe 2 O 4
+ and give rise to
two subsequent reaction channels: (i) releases O 2 and proceeds through a transition
state 1.82 eV higher in energy, resulting an energy gain of 1.35 eV to generate the
Fe 2 O 2
+ cluster; (ii) further attaches CO onto another O atom generating the Fe 2 O 3
+
species after CO 2 release, and then Fe 2 O 3
+ allows association with additional CO to
form Fe 2 O 3 CO
+ .
7.3.3 Neutral Clusters Fe n O m Reacting with CO
In addition to these cationic and anionic cluster ions, small neutral iron oxide clusters
were also demonstrated to react with carbon monoxide in a comparable pathway.
Xue et al. [79] reported the reactions of small neutral iron oxide clusters (FeO 1–3 and
Fe 2 O 4,5 ) with carbon monoxide by a joint experimental and theoretical approach.
Neutral Fe n O m clusters were generated by reaction of laser-ablation-generated iron
plasma with O 2 in a supersonic expansion and were reacted with CO in a fast flow
reactor. Detection of these neutral clusters was done through 118 nm VUV laser
ionization together with time-of-flight mass spectrometry, as shown in Fig. 7.9.
Akin to the above results on Fe n O m cluster ions, it was demonstrated that FeO 2
and FeO 3 neutral clusters are also reactive toward CO, while Fe 2 O 4 , Fe 2 O 5 , and FeO
are relatively less reactive. Furthermore, FeO 2 was noted to bear a higher reactivity
than FeO 3 with CO. The reaction cross section σ or first order rate constant (k 1 ) in
the fast flow reactor was estimated simply by using the equation:
I gas = I He exp(−σ nl) = I He ex p(−k 1 nt)
(7.1)
where I gas and I He are signal magnitudes of the clusters after reaction with the reactant
gas CO (pure He involved as buffer gas); n is the molecular density of reactant gas
which is simply estimated with the ideal gas law P = nkT in which P is the pressure,
k and T are the Boltzmann constant and the gas temperature; [79] l is the effective
path length of the reactor, while t is the reaction time that can be estimated as Δt
= l/υ (υ is the cluster beam velocity, e.g., 1 km/s in the mentioned study). Based on
Eq. (7.1), it was evaluated that the independent quantities of cross section for “FeO 2
+ CO” and “FeO 3 + CO” was ~3 × 10
−17 cm
2 and 1 × 10
−17 cm
2 respectively.
The experimental observations were supported through density functional theory
(DFT) calculations. The reaction pathways with negative or very slight overall
7 The Reactions with Monoxides for Pollution Removal
product Fe 2 O 3
+ ; nevertheless, the detection of minor product species such as Fe 2 O 2
+
and Fe 2 O 3 CO
+ in the reaction of Fe 2 O 5
+ with CO still revealed the occurrence of a
sequential oxidation process of CO. As shown in Fig. 7.8d, in the initial reaction step,
a CO molecule attaches to the O atom bonded to Fe with the lowest coordination, with
a binding energy at 3.26 eV; following that, an intermediate with CO 2 emanation and
generating a Fe 2 O 4
+ cluster requires 1.23 eV energy. Note the structural difference of
this intermediate of Fe 2 O 4
+ in a high-energy state where the two external O atoms are
binding to one Fe atom, which differs from the ground state Fe 2 O 4
+ and give rise to
two subsequent reaction channels: (i) releases O 2 and proceeds through a transition
state 1.82 eV higher in energy, resulting an energy gain of 1.35 eV to generate the
Fe 2 O 2
+ cluster; (ii) further attaches CO onto another O atom generating the Fe 2 O 3
+
species after CO 2 release, and then Fe 2 O 3
+ allows association with additional CO to
form Fe 2 O 3 CO
+ .
7.3.3 Neutral Clusters Fe n O m Reacting with CO
In addition to these cationic and anionic cluster ions, small neutral iron oxide clusters
were also demonstrated to react with carbon monoxide in a comparable pathway.
Xue et al. [79] reported the reactions of small neutral iron oxide clusters (FeO 1–3 and
Fe 2 O 4,5 ) with carbon monoxide by a joint experimental and theoretical approach.
Neutral Fe n O m clusters were generated by reaction of laser-ablation-generated iron
plasma with O 2 in a supersonic expansion and were reacted with CO in a fast flow
reactor. Detection of these neutral clusters was done through 118 nm VUV laser
ionization together with time-of-flight mass spectrometry, as shown in Fig. 7.9.
Akin to the above results on Fe n O m cluster ions, it was demonstrated that FeO 2
and FeO 3 neutral clusters are also reactive toward CO, while Fe 2 O 4 , Fe 2 O 5 , and FeO
are relatively less reactive. Furthermore, FeO 2 was noted to bear a higher reactivity
than FeO 3 with CO. The reaction cross section σ or first order rate constant (k 1 ) in
the fast flow reactor was estimated simply by using the equation:
I gas = I He exp(−σ nl) = I He ex p(−k 1 nt)
(7.1)
where I gas and I He are signal magnitudes of the clusters after reaction with the reactant
gas CO (pure He involved as buffer gas); n is the molecular density of reactant gas
which is simply estimated with the ideal gas law P = nkT in which P is the pressure,
k and T are the Boltzmann constant and the gas temperature; [79] l is the effective
path length of the reactor, while t is the reaction time that can be estimated as Δt
= l/υ (υ is the cluster beam velocity, e.g., 1 km/s in the mentioned study). Based on
Eq. (7.1), it was evaluated that the independent quantities of cross section for “FeO 2
+ CO” and “FeO 3 + CO” was ~3 × 10
−17 cm
2 and 1 × 10
−17 cm
2 respectively.
The experimental observations were supported through density functional theory
(DFT) calculations. The reaction pathways with negative or very slight overall