Page 93 - MODERN ASPECTS OF ELECTROCHEMISTRY

Claude LamyAet al.
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as a function of potential. Such information is valuable for comparing the
behavior of different kinds of electrodes.
Reaction products can also be identified by in situ infrared reflectance
spectroscopy (Fourier transform infrared reflectance spectroscopy,
FTIRS) used as single potential alteration infrared reflectance spectros-
copy (SPAIRS). This method is suitable not only for obtaining information
on adsorbed products (see below), but also for observing infrared (IR)
absorption bands due to the products immediately after their formation in
the vicinity of the electrode surface. It is thus easy to follow the production
of CO 2 versus the oxidation potential and to compare the behavior of
different electrocatalysts.
(b) Identication of the adsorbed intermediates
It is only since 1980 that in situ spectroscopic techniques have been
developed to obtain identification of the adsorbed intermediates and hence
of reliable reaction rnechanisms. These new infrared spectroscopic in
31
situ techniques, such as electrochemically modulated infrared reflectance
spectroscopy (EMIRS), which uses a dispersive spectrometer, Fourier
transform infrared reflectance spectroscopy, or a subtractively normalized
interfacial Fourier transform infrared reflectance spectroscopy
(SNIFTIRS), have provided definitive proof for the presence of strongly
adsorbed species (mainly adsorbed carbon monoxide) acting as catalytic
poisons. 30,31 Even though this chapter is not devoted to the description of
in situ infrared techniques, it is useful to briefly note the advantages and
limitations of such spectroscopic methods.
The objective is to identify species present at the electrode/aqueous
electrolyte interface. Owing to the water absorption limitations, the spec-
troscopic signal from the adsorbed layer is very weak and most of the
information contained in the reflected signal arises from the water layer.
However, such problems were overcome by using different approaches. 31
The first was to decrease the thickness of the electrolyte layer. The EMIRS
and SNIFTIRS techniques have been used to extract the useful informa-
tion from the signal. In all cases, signal averaging of spectra was necessary
to improve the signal-to-noise ratio. The improvements made in the
experimental techniques permit the detection of the adsorbed species, not
only on a smooth pure platinum electrode (Fig. 6) or a bulk alloy (such as
a Pt-Ru alloy, Fig. 7), but also in the case of metallic particles dispersed
32
in a carbon powder (Pt-Ru particles deposited on carbon, Fig. 8). The in
situ IR techniques are at present sufficiently powerful to observe such poor