Chemistry Reference
In-Depth Information
bond.to.the.undercoordinated.layer-1.O.anions.and.the.layer-2.Fe.cations,.giving.a.
surface. termination. based. on.
O
2
−
O
2
−
Fe
2
−
O
2
−
Fe
2
−
O
2
−
R,. where. the. bold. atoms.
represent.those.above.the.stoichiometric.termination..On.the.other.hand,.the.partial.
occupancy.of.the.layer-2.Fe.cations,.giving.a.surface.termination.based.on.O
2
−
X
−
O
2
−
Fe
2
−
O
2
−
R,. has. also. been. observed. at. room. temperature. using. crystal. trunca-
tion. rod. x-ray. diffraction.. Both. surface. terminations. have. been. observed. at. room.
temperature,.but.the.surface.preparation.procedures.differed.slightly..Lo.et.al..used.
DFT.calculations.and.ab.initio.thermodynamics.to.show.that.while.the.(H
2
O)
2
−
X
−
(HO)
2
−
Fe
2
−
O
2
−
R.termination.is.the.most.energetically.stable.at.room.temperature,.
the.
(HO)
2
−
(HO)
2
−
Fe
2
−
O
2
−
Fe
2
−
O
2
−
R. termination. is. more. stable. above. 450.K.
85
.
These. results. suggest. that. the. prevalent. surface. termination. is. highly. sensitive. to.
temperature..One.plausible.explanation.is.that.molecular.water.desorption.and.sur-
face.reconstruction.occur.at.high.temperatures,.thus.increasing.the.occupancy.of.the.
layer-2.Fe.cations.in.the.(H
2
O)
2
−
X
−
(HO)
2
−
Fe
2
−
O
2
−
R.surface.termination.to.obtain.
(HO)
2
−
Fe
2
−
O
2
−
Fe
2
−
O
2
−
R..Upon.annealing.under.high.partial.pressures.of.water,.
the.surface.may.be.hydroxylated,.thus.giving.the.
(HO)
2
−
(HO)
2
−
Fe
2
−
O
2
−
Fe
2
−
O
2
−
R.
termination..However,.the.results.based.on.ab.initio.thermodynamics.calculations.on.
the.hydroxylated.surface.at.room.temperature.suggest.that.if.the.kinetic.barriers.to.
rearrangement.could.be.overcome,.then.the.annealed.surface.should.reorganize.back.
to.the.(H
2
O)
2
−
X
−
(HO)
2
−
Fe
2
−
O
2
−
R.termination.
The. electronic. structure. of. alumina. and. hematite. surfaces. has. been. character-
ized. with. unprecedented. accuracy. at. the. molecular. scale. using. DFT. calculations.
and. ab. initio. thermodynamics.. The. computed. structural. models,. including. atomic.
layer. relaxations. and. surface. reconstructions,. are. in. close. agreement. with. experi-
mental.data.on.the.surfaces..The.quantum.mechanical.approach.for.modeling.metal.
oxide.surfaces.thus.facilitates.future.explorations.of.chemical.reactivity.for.diverse.
applications.including.chemical.catalysis,.environmental.remediation,.and.alterna-
tive.energy.production..An.overview.of.several.key.areas.where.these.applications.
will.contribute.is.discussed.briely.in.the.following.section.
7.3 CURRENT APPLICATIONS OF DFT TO METAL OXIDE SURFACES
Metal.oxides.are.widely.used.in.a.diverse.and.fascinating.array.of.industrial.appli-
cations.. For. instance,. metal. oxides. are. used. to. passivate. metal. surfaces. against.
.corrosion,.function.as.gas.sensors.for.pollution.monitoring,.and.control.and.serve.as.
electrolytes.in.solid.oxide.fuel.cells.
93-96
.In.this.section,.several.examples.will.be.pre-
sented,.where.quantum.chemical.methods.have.been.widely.used.to.study.the.struc-
ture.and.reactivity.of.metal.oxide.surfaces..These.examples.will.include.the.chemical.
catalysis.for.hydrocarbon.transformations,.the.environmental.control.of.contaminant.
fate.and.transport,.and.materials.engineering.for.solar.energy.conversion.
7.3.1 c
atalySiS
As.mentioned.in.Section.7.1,.metal.oxides.are.often.used.as.supports.for.noble.metal.
catalyst.particles,.and.also.on.their.own..For.instance,.cerium.oxide.(CeO
2
).is.increas-
ingly.being.used.as.a.catalyst.and.as.a.catalyst.support.in.automotive.applications.for.
Search WWH ::
Custom Search