Chemistry Reference
In-Depth Information
to. be. the.
de facto
. method. for. studying. reactive. processes. on. metal. oxide. surface.
catalysts..The.method.seeks.to.ind.the.minimum.energy.path.to.the.saddle.point.that.
separates.the.reactant.and.product.basins.on.the.potential.energy.surface..Assuming.
that.the.reactants.and.products.are.known.beforehand,.a.path.linking.the.two.states.
is.created.with.a.number.of.replicas.of.the.original.system.initially.located.equidis-
tantly.along.this.path.and.connected.by.harmonic.springs..This.path.is.then.relaxed.
to.ind.the.true.minimum.energy.path.by.allowing.the.intermolecular.forces.to.move.
until.the.transverse.forces.(e.g.,.those.perpendicular.to.the.local.tangent).on.the.rep-
licas.vanish..The.nudged.elastic.band.method.has.been.implemented.in.several.plane.
wave.codes.including.Dacapo,
105,106
.NWChem,
107
.PwSCF,
108
.and.VASP.
109,110
7.3.2 e
nvironmental
a
PPlicationS
The. metal. oxide-water. interface. is. of. tremendous. interest. in. the. study. of. envi-
ronmental. reactivity. and. recent. quantum. chemical. calculations. have. proven. to. be.
invaluable.in.determining.the.structural.and.electronic.factors.that.affect.contami-
nant.metal.sorption.at.environmental.interfaces..Of.these.contaminant.metals,.lead
51
.
and.chromium
111
.are.two.elements.that.have.been.recently.characterized.using.DFT.
calculations.
Experimental. XAS. studies. have. shown. that. Pb(II). binds. weakly. to. form. outer-
sphere. complexes. with. α-Al
2
O
3
. (0001),
112
. and. strongly. to. form. inner-sphere. com-
plexes.with.α-Fe
2
O
3
.(0001).
113
.Since.the.two.surfaces.are.isostructural,.the.differences.
in. binding. were. suspected. to. be. the. result. of. differences. in. the. electronic. structure.
between.transition.metal-containing.compounds.and.nontransition.metal-containing.
compounds..Mason.et.al..used.DFT.calculations.to.show.that.in.hematite,.the.Pb-O.
covalent.interactions.are.stabilized.by.the.partially.occupied.Fe.
d
.band,.and.the.addi-
tional.electronic.interaction.between.the.valence.O.
p
,.Fe.
d
,.and.Pb.
s
.and.
p
.orbitals.
51
.
Thus,.the.reactivity.of.α-Fe
2
O
3
.(0001).toward.Pb(II).is.much.higher.than.the.reactivity.
of.α-Al
2
O
3
.(0001)..In.fact,.α-Fe
2
O
3
.(0001).is.more.reactive.toward.Pb(II).than.even.
α-Fe
2
O
3
.(11
_
02),
113
.which.also.forms.inner-sphere.complexes.with.Pb(II).due.to.struc-
tural.differences.and.the.coordination.of.the.reactive.Fe(II).sites.on.both.surfaces.
In. contrast,. Cr(VI). has. been. shown. to. form. only. outer-sphere. adsorption. com-
plexes. on. hydrated. α-Fe
2
O
3
. (11
_
02).
114
. Since. chromium. forms. chromate. complexes.
readily,.the.favorable.thermodynamics.of.hydrogen.bonding.and.ion-dipole.attrac-
tion.between.the.complexes.and.the.surface.hydroxyls,.coupled.with.the.unfavorable.
energetic. overlap. between. the. valence. Cr. orbitals. and. the. O.
p
. and. Fe.
d
. orbitals,.
naturally.lend.themselves.to.the.observation.of.outer-sphere.adsorption.of.Cr(VI).on.
hydrated.hematite,.as.observed.by.Yin.et.al.
111
.Thus,.quantum.chemical.approaches.
are. essential. for. probing. the. effect. of. the. electronic. structure. on. the. reactivity. of.
metal. oxide. surfaces,. particularly. in. the. prediction. of. inner-sphere. and/or. outer-
sphere.adsorption.complexes.of.contaminant.metals.on.soils.and.sediments.
7.3.3 P
hotovoltaicS
Another.promising.use.of.metal.oxides.is.in.thin.ilm.materials.for.multi-junction.
photovoltaic.devices..The.third.generation.of.these.devices.aims.to.improve.on.the.
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