Topics in Current Chemistry (2019) 377:11
1 3
the IRE. In the setup shown in Fig. 1, the IRE is made of zinc selenide because of its
wide transmittance permitting investigation in a broad range of spectral vibrations,
and good chemical resistance. The collimated IR light reflects of the ZnSe-solution
interface, refocuses at the point F″, and travels to the IR detector inside the FTIR
instrument upon reflection from the second gold mirror. Taking the average indices
of refraction for ZnSe and solution (water) of 2.4 and 1.3, respectively, the incidence
angle at the ZnSe/water interface can be set to 35°, i.e., just about 2° higher than
θ c [59]. This angle of incidence corresponds to the maximum of the electric field
strength of ZnSe/water/Pt three-phase system for the incident radiation that matches
the frequency of linearly bonded carbon monoxide (CO L ) at Pt surface, ~ 2050 cm
−1
[58]. For the radius r of the IRE hemisphere of 1.27 cm, the distances between two
mirrors and the height of the flat surface of ZnSe from the incident IR beam path
can be calculated (2b = 5.86 cm and h = 4.18 cm).
While Otto configuration has the advantage of being able to accept a variety of
different surfaces including flat poly- and single-crystals as well as rough surfaces
composed of a catalyst layer deposited onto a flat surface of an inactive material,
another setup is used if the electrochemical experiment involves gas evolution. In
Kretschmann configuration, the working electrode is deposited as a thin film (tens
of nanometers) over the flat side of IRE, and the solution side is semi-infinite, thus
allowing the volatile products to escape [60]. The presence of the thin metal film
was shown to enhance the IR absorption bands by one or two orders of magnitude,
although the nature of the enhancement is still being debated [61–63]. Indeed, the
Fresnel equations for three-phase stratified medium show that the electric field
strength, associated with dissipation of the electric energy within the phase, can
be orders of magnitude higher in Kretschmann configuration as opposed to that in
Otto configuration. However, the enhancement occurs in a very narrow range of
incidence angles and for very thin metal films only [64, 65]. The setup depicted in
Fig. 1 can be used for both configurations, but the optimization of the setup is critical, especially for the Kretschmann configuration where a slight misalignment of the
accessory could result in many times lower absorption intensity [58, 66].
In a typical IR spectroelectrochemical experiment, one usually chooses the reference potential at which the considered reaction does not occur, whereas the sample
potentials are selected to cover the potential range in which the reaction proceeds.
For studies occurring at anodic potentials, the reference potential is held sufficiently
negative. For instance, in sulfate/bisulfate adsorption at Pt electrodes the reference
potential is usually kept in hydrogen adsorption region or just positive to it, whereas
the sample potentials are chosen to extend over more positive potentials [67–69].
The sample and reference potentials in oxidation reactions described below are
taken in a similar manner.
Nicolet Nexus 670 FT-IR spectrometer equipped with a liquid nitrogen-cooled
MCT detector was used in the studies. One-hundred and twenty-eight interferograms
with 8 cm
−1
resolution were collected at each potential. The reference spectrum was
collected at the potential before the onset of ethanol oxidation, i.e., at 0.05 V vs.
RHE, and sample spectra were collected at increasingly higher potentials; the resulting spectra are given as absorbance. The working electrodes were pressed against
the IRE to create a solution layer with the thickness of ~ 1 µm. Traces of oxygen in
Reprinted from the journal
6
1 3
the IRE. In the setup shown in Fig. 1, the IRE is made of zinc selenide because of its
wide transmittance permitting investigation in a broad range of spectral vibrations,
and good chemical resistance. The collimated IR light reflects of the ZnSe-solution
interface, refocuses at the point F″, and travels to the IR detector inside the FTIR
instrument upon reflection from the second gold mirror. Taking the average indices
of refraction for ZnSe and solution (water) of 2.4 and 1.3, respectively, the incidence
angle at the ZnSe/water interface can be set to 35°, i.e., just about 2° higher than
θ c [59]. This angle of incidence corresponds to the maximum of the electric field
strength of ZnSe/water/Pt three-phase system for the incident radiation that matches
the frequency of linearly bonded carbon monoxide (CO L ) at Pt surface, ~ 2050 cm
−1
[58]. For the radius r of the IRE hemisphere of 1.27 cm, the distances between two
mirrors and the height of the flat surface of ZnSe from the incident IR beam path
can be calculated (2b = 5.86 cm and h = 4.18 cm).
While Otto configuration has the advantage of being able to accept a variety of
different surfaces including flat poly- and single-crystals as well as rough surfaces
composed of a catalyst layer deposited onto a flat surface of an inactive material,
another setup is used if the electrochemical experiment involves gas evolution. In
Kretschmann configuration, the working electrode is deposited as a thin film (tens
of nanometers) over the flat side of IRE, and the solution side is semi-infinite, thus
allowing the volatile products to escape [60]. The presence of the thin metal film
was shown to enhance the IR absorption bands by one or two orders of magnitude,
although the nature of the enhancement is still being debated [61–63]. Indeed, the
Fresnel equations for three-phase stratified medium show that the electric field
strength, associated with dissipation of the electric energy within the phase, can
be orders of magnitude higher in Kretschmann configuration as opposed to that in
Otto configuration. However, the enhancement occurs in a very narrow range of
incidence angles and for very thin metal films only [64, 65]. The setup depicted in
Fig. 1 can be used for both configurations, but the optimization of the setup is critical, especially for the Kretschmann configuration where a slight misalignment of the
accessory could result in many times lower absorption intensity [58, 66].
In a typical IR spectroelectrochemical experiment, one usually chooses the reference potential at which the considered reaction does not occur, whereas the sample
potentials are selected to cover the potential range in which the reaction proceeds.
For studies occurring at anodic potentials, the reference potential is held sufficiently
negative. For instance, in sulfate/bisulfate adsorption at Pt electrodes the reference
potential is usually kept in hydrogen adsorption region or just positive to it, whereas
the sample potentials are chosen to extend over more positive potentials [67–69].
The sample and reference potentials in oxidation reactions described below are
taken in a similar manner.
Nicolet Nexus 670 FT-IR spectrometer equipped with a liquid nitrogen-cooled
MCT detector was used in the studies. One-hundred and twenty-eight interferograms
with 8 cm
−1
resolution were collected at each potential. The reference spectrum was
collected at the potential before the onset of ethanol oxidation, i.e., at 0.05 V vs.
RHE, and sample spectra were collected at increasingly higher potentials; the resulting spectra are given as absorbance. The working electrodes were pressed against
the IRE to create a solution layer with the thickness of ~ 1 µm. Traces of oxygen in
Reprinted from the journal
6