Environmental Engineering Reference
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Argile, 1992; Paffett et al., 1986]. This decrease is consistent with the formation of
dipoles with negative charge on the metal and positive charge on the adatom, in agree-
ment with the model proposed to explain both the decoration of stepped surfaces and
the results from laser-induced potential transients. Unfortunately, detailed work func-
tion data in this regard are very scarce. Another approach that can be used to indirectly
infer changes in the electronic structure induced by adatom modification is based on
the analysis of vibrational frequencies of coadsorbed species sensitive to the local
electronic state of the surface. In this way, the CO stretching frequency is sensitive
to the ability of the substrate to back-donate electronic density towards the antibonding
2p
orbital. This backdonation results in a weakening of the C;;O bond and decreases
the vibrational frequency. However, this analysis should be performed with caution,
since the frequency of the CO stretching band may depend also on other factors
such as electrode potential and local coverage (through dipole - dipole coupling).
Besides, spectroscopic results will be relevant for the present purpose only if both
adlayers are properly intermixed.
It has been shown that Bi [Chang and Weaver, 1991; Herrero et al., 1995a, d; Lin
et al., 1994], Pb [Hoshi et al., 2000], Sb [Kizhakevariam and Weaver, 1994] and S
[Gracia et al., 2005; Lin et al., 1994] form mixed adlayers with CO. Adsorption of
electropositive adatoms causes a displacement of the CO stretching frequency to
lower values, indicating a reinforcement of the backdonation mechanism. On the
other hand, S causes a displacement of the CO vibrational frequency to higher
values, suggesting that the withdrawal of electronic density caused by the electro-
negative adatom decreases the degree of backdonation. These explanations are in
agreement with quantum chemical calculations performed with model cluster
systems that demonstrate that Bi donates electrons to Pt, causing an increase in the
extent of dp
Pt
!
2p
Co
backdonation, whereas S gains electrons from Pt, decreasing
the extent of backdonation from Pt to CO [Lin et al., 1994].
Coming back to the results obtained with the laser-induced temperature jump
method, a small, but measurable, increase in the PME is observed at low coverages.
This increase in the PME caused by adatom deposition is similar for all studied
adatoms, regardless of their chemical nature. This observation has been explained
by the disruptive effect of adatom deposition on the structure of interfacial water.
Optimization of hydrogen bonding is a key factor determining the structure of the
water network. It is reasonable to propose that hydrogen bonding will stabilize a
configuration that possesses a net orientation of water molecules with the oxygen
towards the metal. The disruption of the water network caused by the adatoms
would facilitate the turnover of the water dipoles to the hydrogen-toward-the-metal
orientation, which would take place at less negative potentials, i.e., shifting the
PME towards positive potentials.
7.6 ADATOMS AND ELECTROCATALYSIS
In general, the effect of surface modifiers on the electrocatalytic activity of a
bimetallic material can be classified intro three main categories [Bligaard and
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