Density Functional Calculations of Metal Clusters: Structure, Dynamics, and Reactivity - Metallic Systems: A Quantum Chemist's Perspective - page 332

Figure. 9.35. displays. the. calculated. reaction. proile. for. the. reaction. between.
FeO 2 + . and. CO.. Oxygen. atoms. are. given. in. red,. carbon. atoms. in. black,. and. iron.
atoms. in. blue.. Transition. states. are. indicated. by. structural. formulas. in. brackets..
The.leading.superscript.on.the.structural.formulas.indicates.the.multiplicity.of.the.
.corresponding.PES..The.reaction.proile.in.Figure.9.35.explains.the.experimentally.
observed.product.formation.of.FeO + .and.FeCO + ..The.reaction.is.initialized.by.the.
attachment.of.CO.to. FeO 2 + ,.forming.the.intermediate. OCFeO 2 + ..This.process.occurs.
without.barrier,.i.e.,.spontaneously..As.is.well.known.from.metal.carbonyls,.the.CO.
binds.over.the.carbon.to.the.iron.atom..The. OCFeO 2 + .intermediate.can.further.react.
over.two.different.reaction.channels..Along.the.energetically.higher.reaction.chan-
nel,.a.second.intermediate.with.a.preformed.O 2 .unit.is.obtained..The.correspond-
ing. transition. state. lies. 0.04.eV. above. the. FeO 2 + . reference. energy.. This. high-lying.
transition.state.is.critical.for.the.formation.of.FeCO + ,.which.is.obtained.by.oxygen.
release.from.the.previous.intermediate..The.oxygen.release.also.results.in.a.change.
of.the.multiplicity.of.the.PES.of.the.iron.system,.from.the.doublet.to.the.quartet..
As. Figure. 9.35. shows,. the. formation. of. FeCO + . from. FeO 2 + . and. CO. under. oxygen.
release. is. overall. exothermic.. However,. the. high-lying. transition. state. limits. the.
FeCO + .production..This.is.consistent.with.experiment.where.the.FeCO + .production.
is.observed.after.FeO + .
In. fact,. the. second. reaction. path. from. the. OCFeO 2 + . intermediate. leads. to. FeO + .
over. a. low. energy. transition. state.. The. product. on. the. doublet. energy. surface. is. a.
linear. molecule. with. a. preformed. CO 2 . unit.. As. the. low-energy. transition. state. in.
Figure.9.35.shows,.the.preformed.CO 2 .unit.is.produced.by.the.insertion.of.an.oxygen.
atom.into.the.iron.carbonyl.bond..The.resulting.product.can.be.further.stabilized.by.
spin.lips.from.the.doublet.to.the.sextet.surface..The.CO 2 .is.inally.released.in.order.
to.form.the.experimentally.observed.FeO + .product..Thus,.the.oxidation.of.CO.to.CO 2 .
is.performed.by.the.formal.reaction

+ +

FeO CO + → FeO CO . +

.

2 2

In.Figure.9.36,.the.calculated.reaction.proile.for. Fe O
2

+ .with.CO.is.depicted..As.in.
the.previous.case,.the.initial.attachment.of.CO.to.the.cationic.iron.oxide.cluster.is.
without.barrier..However,.in.this.case,.the.CO.attaches.not.directly.to.the.iron.but.
rather. to. an. external. oxygen. atom.. As. a. result,. the. formation. of. a. CO 2 . subunit. is.
immediate.. The. 4 Fe 2 O 5 CO + . intermediate. releases. CO 2 .. Beyond. the. 4

5

+
Fe O . prod-
uct.of.this.elimination,.the.reaction.proile.splits..The.higher.energy.reaction.path.
leads,.by.oxygen.elimination,.to. 4

2 4

+
Fe O ..The.formation.of.the.O 2 .subunit.inside.the.

2 2

+ . on. the. quartet.
energy.surface..The.spin.lip.to.the.doublet.surface.occurs.with.the.release.of.oxy-
gen.and.the.formation.of.the.inal.product.of.this.reaction.branch,. 2

Fe O
2

+ . system. proceeds. over. the. depicted. transition. state. O Fe O
2 4 2 2

+
Fe O ..The.low.
energy.reaction.path.is.initialized.by.a.second.CO.attachment,.now.to.the. 4

2 2

+
Fe O .
intermediate..Again,.CO.attaches.to.an.external.oxygen.atom..The.corresponding.
CO 2 .release.then.forms.the. 4

2 4

+
Fe O .intermediate.that.by.itself.serves.again.as.a.CO.
adsorbate.forming. 2 Fe 2 O 3 CO + ..This.species.releases.CO 2 .and,.thus,.forms. 2

2 3

+
Fe O .
through. a. second. reaction. channel.. As. Figure. 9.36. shows,. no. oxygen. insertion.

2 2