Computational Investigations of Metal Oxide Surfaces - Metallic Systems: A Quantum Chemist's Perspective - page 207

on.a.model.of.hydroxylated.γ-Al 2 O 3 , 58 .and.alkane.metathesis. 59 .A.recent.review.by.
Raybaud. et. al.. describes. progress. in. DFT. calculations. of. fuel. hydrotreating. and.
hydrosulfurization.catalysts.supported.on.γ-Al 2 O 3 . 60
κ-Al 2 O 3 . is. challenging. to. model. due. to. dificulty. in. determining. structures. of.
complex.and.metastable.materials. 61 .Accordingly,.DFT.studies.employing.this.phase.
have.appeared.only.within.the.past.few.years..Fortrie.et.al..investigated.VO x /κ-Al 2 O 3 .
(001). using. DFT. to. parameterize. Monte. Carlo. simulations. and. employed. a. model.
with.a.much.larger.surface.area.than.could.be.addressed.through.a.straightforward.
application.of.DFT. 62
Current. applications. of. DFT. calculations. to. alumina. are. investigating. novel.
mechanistic. pathways,. low. symmetry. crystal. structures,. and. comparisons. with.
experiments. that. were. not. addressed. until. recent. computational. and. theoretical.
advances.. For. instance,. density. functional. investigations. of. vacancies, 63-65 . interfa-
cial.vacancies, 66 and.charge.defects 67 .are.now.available..Comparisons.with.chemical.
shifts.in.calculated.and.experimental.solid.state.NMR 68 .are.also.now.possible.and.
may. assist. in. creating. models. that. better. approximate. the. electronic. structure. of.
alumina. in. noncrystalline. environments,. such. as. thin. ilms,. which. have. tradition-
ally.been.a.challenge.for.DFT.applications..Additionally,.calculations.of.alumina.for.
features.typically.considered.to.be.associated.with.length.scales.beyond.the.scope.of.
quantum.chemistry,.and.of.interest.in.the.materials.science.and.engineering.realm.
are.being.tackled.via.DFT,.including.investigations.of.mesoporous.materials, 69 .pos-
sible. fatigue. mechanisms, 70 . grain. boundary. structure, 71 . and. ion. segregation. and.
sliding. 72

7.2.2 f e 2 o 3 c alculationS
Hematite.(α-Fe 2 O 3 ).is.the.mineral.form.of.iron.(III).oxide,.which.is.the.most.com-
monly.mined.ore.of.iron.for.industrial.production. 73 .Like.alumina,.hematite.crys-
tallizes.in.the.rhombohedral.system,.and.shares.the.corundum.structure..The.bulk.
structure.of.hematite.has.a.hexagonal.unit.cell.(space.group. R3

_

c ),.with.a.distorted.
close-packed.array.of.oxygen.anions.and.iron.cations.that.occupy.two-thirds.of.the.
octahedral.holes..The.atomic.layer.stacking.sequence.along.the. c .axis.consists.of.six.
stoichiometric. − (Fe − O 3 − Fe) − .repeat.units.per.bulk.unit.cell..The.iron.cations.are.
staggered.along.the. c s .axis.and.displaced.vertically.from.their.ideal.hexagonal.close-
packed.(hcp).positions.centered.between.the.oxygen.layers..Unlike.alumina,.hema-
tite.is.a.bulk.antiferromagnet.with.alternating.spin.layers.in.the.bulk.crystal, 73,74 .so.
quantum.mechanics.methods.are.of.great.interest.in.probing.the.correlation.between.
the.electronic.structure.and.the.experimental.observations.of.surface.reactivity.
While.its.crystals.do.not.cleave,.hematite's.predominant.growth.faces.include.the.
(0001).basal.plane 75 .and.the.(11

_
02).plane,.as.shown.in.Figure.7.3..The.clean.surfaces.
of.Fe 2 O 3 .have.been.well.characterized.as.a.function.of.surface.preparation.using.low-
energy-electron-diffraction.(LEED);.in.these.studies,.the.surface.was.irst.cleaned.
by.Ar + .bombardment,.and.then.annealed.to.determine.the.effect.of.temperature.and.
oxygen.partial.pressure.on.the.surface.structure.and.composition..Several.types.of.
LEED.patterns,. corresponding. to. different.surface. reconstructions,. were. observed.
upon. annealing,. but. these. patterns. were. found. to. depend. on. both. the. O 2 . partial.