48
reflectivity are caused by the UPD adlayers, in particular on substrates that exhibit
large electroreflectance signals, viz., Au, Ag, and Cu. The optical properties of submonolayers and monolayers of metals were found to be markedly different from the
properties of bulk phases. Application of synchrotron radiation-based x-ray absorption techniques, X-ray absorption near-edge structure spectroscopy (XANES) measurements of Pb on Pt show that in the underpotential deposition region, the Pb
species are essentially neutral Pb atoms [33]. There was no evidence of Pb interaction with oxygenated species in the UPD region.
5.3.1 Oxidation of Small Organic Molecules on Surfaces
Modified by UPD Adlayers
Various catalytic effects are caused by monolayers of Pb, Bi, Tl, Cd, Sn, Sb, Se, and
Ge on the oxidation of organic molecules that are potential fuels for fuel cells, viz.,
HCOOH, CH 3 OH, HCHO, CH 3 CHO, CO, glucose and other monosaccharides, ethylene glycol and some aliphatic alcohols on Pt, as well as on some other platinum
metals (see reviews of previous work in references [26]).
Slow rates are characteristic of the oxidation of these organic molecules even
with the best catalysts because of the generation of strongly bonded intermediates –
poisoning species – that reduce their activity. Adsorption, dehydrogenation, and
subsequent oxidation of adsorbed hydrogen of small organic molecules leave
strongly bonded fragments on the surface. Their oxidations require very positive
potentials. The UPD metal adatoms can reduce the poisoning effects by several
mechanisms, thus causing the catalytic effects. There is a consensus regarding the
mechanism of the oxidation of formic acid on Pt, which involves a dual path mechanism [40] with direct oxidation to CO 2 and an indirect mechanism through a blocking intermediate (see Sect. 6.6).
The mechanism of the catalytic action of metal adlayers in the oxidation of
organic molecules has been interpreted by a “third-body” effect, the bifunctional
catalyst mechanism and electronic effect.
A third-body effect, which is equivalent to an “ensemble” effect in heterogeneous catalysis, is based on the role of metal adatoms in blocking the surface sites
for a side reaction that generates poisoning species or in blocking the adsorption of
the inhibiting species, which requires more than one surface site for adsorption. The
reaction in the main pathway can proceed at the unoccupied sites that now form
smaller ensembles.
In bifunctional mechanism, the surface is considered to have two types of sites
that have distinct roles in the reaction. The substrate, such as Pt, breaks the bonds in
organic molecules upon adsorption, while the adatom can adsorb oxygen- containing
species that can oxidize strongly adsorbed intermediates such as CO.
5 Important Electrosorption Reactions
reflectivity are caused by the UPD adlayers, in particular on substrates that exhibit
large electroreflectance signals, viz., Au, Ag, and Cu. The optical properties of submonolayers and monolayers of metals were found to be markedly different from the
properties of bulk phases. Application of synchrotron radiation-based x-ray absorption techniques, X-ray absorption near-edge structure spectroscopy (XANES) measurements of Pb on Pt show that in the underpotential deposition region, the Pb
species are essentially neutral Pb atoms [33]. There was no evidence of Pb interaction with oxygenated species in the UPD region.
5.3.1 Oxidation of Small Organic Molecules on Surfaces
Modified by UPD Adlayers
Various catalytic effects are caused by monolayers of Pb, Bi, Tl, Cd, Sn, Sb, Se, and
Ge on the oxidation of organic molecules that are potential fuels for fuel cells, viz.,
HCOOH, CH 3 OH, HCHO, CH 3 CHO, CO, glucose and other monosaccharides, ethylene glycol and some aliphatic alcohols on Pt, as well as on some other platinum
metals (see reviews of previous work in references [26]).
Slow rates are characteristic of the oxidation of these organic molecules even
with the best catalysts because of the generation of strongly bonded intermediates –
poisoning species – that reduce their activity. Adsorption, dehydrogenation, and
subsequent oxidation of adsorbed hydrogen of small organic molecules leave
strongly bonded fragments on the surface. Their oxidations require very positive
potentials. The UPD metal adatoms can reduce the poisoning effects by several
mechanisms, thus causing the catalytic effects. There is a consensus regarding the
mechanism of the oxidation of formic acid on Pt, which involves a dual path mechanism [40] with direct oxidation to CO 2 and an indirect mechanism through a blocking intermediate (see Sect. 6.6).
The mechanism of the catalytic action of metal adlayers in the oxidation of
organic molecules has been interpreted by a “third-body” effect, the bifunctional
catalyst mechanism and electronic effect.
A third-body effect, which is equivalent to an “ensemble” effect in heterogeneous catalysis, is based on the role of metal adatoms in blocking the surface sites
for a side reaction that generates poisoning species or in blocking the adsorption of
the inhibiting species, which requires more than one surface site for adsorption. The
reaction in the main pathway can proceed at the unoccupied sites that now form
smaller ensembles.
In bifunctional mechanism, the surface is considered to have two types of sites
that have distinct roles in the reaction. The substrate, such as Pt, breaks the bonds in
organic molecules upon adsorption, while the adatom can adsorb oxygen- containing
species that can oxidize strongly adsorbed intermediates such as CO.
5 Important Electrosorption Reactions