Environmental Engineering Reference
In-Depth Information
Norskov, 2007; Jarvi and Stuve, 1998; Motoo and Watanabe, 1980; Parsons and
VanderNoot, 1988]:
†
Change in the Electronic Properties of the Substrate. The new electronic prop-
erties alter the adsorption energy and intramolecular bond energy in adsorbed
reactants and intermediates. Besides the ligand effect that occurs when an
atom changes the electronic state of neighboring atoms, foreign metal adatoms
can also induce or suffer strain effects, when either the adatoms or the host
metal atoms are forced to adopt positions different from the equilibrium position
in the bulk materials.
†
Ensemble or Third-Body Effects. These effects concern the selective blockage of
a particular adsorption site by adatom deposition. This can be advantageous when
the reaction mechanism contains parallel paths that can be affected differently by
blocking particular sites. In some cases, the undesired reaction needs more than
one free adjacent site (ensemble), and can be inhibited by blocking particular sites
without decreasing the reactivity of the surface for the catalyzed reaction.
†
Bifunctional Catalysts. The adatoms provide suitable adsorption sites for a
second reactant necessary for the reaction to proceed, while the main reactant
still adsorbs on the free sites of the substrate.
A group of reactions extensively studied in electrocatalysis, because of their great inter-
est in the context of fuel cell technology, involve oxidation of small oxygenated organic
compounds. Most of these reactions proceed through a dual-path mechanism. This
means that at least two parallel pathways exists: one leading to the formation of a poi-
soning intermediate and another producing a stable and soluble oxidized compound,
ideally CO
2
. The main poison identified spectroscopically is CO. Adatom modification
can affect each of these pathways independently, the effect normally sought being
inhibition of the poisoning reaction and enhancement of the direct oxidation reaction.
The simplest reactions in this group are oxidations of molecules containing only
one carbon (C
1
molecules), and, among these, the two most widely studied reactions
are methanol and formic acid oxidation. The interest in the latter arises not only from
its possible application as a noncontaminant fuel but also because this simple reaction
has been considered as a prototype among the oxidation of C
1
molecules. Surface
modifications with sp
n
elements have greater effects on the electrocatalysis of
formic acid [Clavilier et al., 1989b, c; Fern´ndez-Vega et al., 1989, 1991] than on
methanol oxidation [Chang et al., 1992]. Electrocatalysis for the oxidation of CO
has also been studied, as this molecule constitutes the main poison in the oxidation
of small organic molecules.
7.6.1 Electrocatalysis of CO Oxidation
CO forms intermixed adlayers with most of the p-block adatoms. CO oxidation from
mixed adlayers with Bi [Chang and Weaver, 1991; Herrero et al., 1995a, d], As
[Herrero et al., 1995d], Sb [Kizhakevariam and Weaver, 1994], Se [Herrero et al.,
1996], and Te [Herrero et al., 1996] on Pt(111) has been studied. The formation of
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