Chemistry Reference
In-Depth Information
(110)
(100)
(110)
FIGURE 7.8
Bulk.unit.cell.of.ceria.(CeO
2
),.along.with.one.of.its.dominant.cleavage.planes.
(110)..The.oxygen.positions.are.shown.in.dark.and.the.metal.ion.positions.are.shown.in.the.
lighter.atomic.positions.shown.
reducing.NO
x
.emissions.and.converting.carbon.monoxide.to.carbon.dioxide..Cerium.
oxide's.superior.catalytic.properties.in.transforming.oxygen-containing.species.are.
the. result. of. its. ability. to. easily. become. nonstoichiometric. in. oxygen. content.. As.
shown.in.Figure.7.8,.cerium.oxide's.(110).surface.is.easily.reduced.since.the.oxygen.
atoms.at.the.surface.are.all.coplanar,.thus.facilitating.a.rapid.diffusion.of.the.oxygen.
atoms.and.creating.oxygen.vacancies.
DFT. calculations. have. shown. that. the. energetic. stability. of. the. cleave. planes.
of. cerium. oxide. follows. the. trend:. (111).>.(110).>.(100).. It. follows. then. that. oxygen.
vacancies.form.more.easily.on.CeO
2
.(110).than.CeO
2
.(111)..However,.the.calculations.
also.demonstrate.that.the.initial.oxygen.vacancy.forms.in.the.surface.atomic.layer.in.
CeO
2
.(110),.and.in.the.subsurface.atomic.layer.in.CeO
2
.(111).
97
Commonly,.ceria.is.used.to.support.noble.metal.nanoparticles,.such.as.platinum,.
palladium,.rhodium,.and.gold.by.altering.the.electronic.structure.of.these.compounds..
Thus,.ceria.is.used.as.a.cocatalyst.in.a.number.of.industrial.chemical.applications.
involving.hydrocarbon.transformations.including.the.water-gas.shift.reaction,.steam.
reforming,. and. the. Fischer-Tropsch. reaction.. Several. reactants. including. hydro-
gen.gas,
98
.water,
99
.methane,
100
.methanol,
101
.and.carbon.monoxide
102,103
.adsorb.very.
strongly.and.exothermically.on.the.stoichiometric,.oxidized,.and.reduced.ceria.sur-
faces.and.dissociate.at.oxygen.vacancies..In.particular,.the.reduced.surfaces.of.ceria.
are.of.tremendous.interest.in.catalysis..DFT.calculations.have.suggested.that.oxygen.
atoms.initially.contained.in.adsorbed.reactants,.such.as.carbon.monoxide,.are.easily.
incorporated.into.reduced.surfaces..For.this.reason,.ceria.has.been.considered.as.a.
potential. component. of. an. integrated. catalyst. system. for. the. conversion. of. carbon.
dioxide.to.liquid.fuels.
Perhaps.one.of.the.most.successful.applications.of.DFT.is.in.computational.cataly-
sis..In.particular,.the.nudged.elastic.band
104
.of.Jónsson.et.al..is.widely.acknowledged.
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