Topics in Current Chemistry (2019) 377:11
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since proven that Pt-Rh can be highly active catalyst for ethanol oxidation in various
form, but the activity strongly depends on the substrate and synthesis [26–30].
In this review we describe the synthesis, characterization, electrochemical activity, and optimization of ternary PtRh/SnO 2 catalysts. Attempts to reduce the cost of
the catalyst by replacing Rh with alternative co-catalysts are illustrated, in which
it was shown that Ir in ternary catalysts could also be used to split the C–C bond
in ethanol [31]. We also underline particularly exciting results with ethanol oxidation to CO 2 on a Pt monolayer under tensile strain, as reported by Li et al. [32].
The emphasis is given to in situ characterization of the electrocatalysts by IR spectroscopy and X-ray absorption spectroscopy in an attempt to show how the combination of these spectroscopies, electrochemical techniques, and theoretical studies
can enhance the understanding of the ethanol electrooxidation. The examples shown
here hold great promise that impediments to practical direct alcohol fuel cells can be
overcome.
2 In Situ Characterization of New Catalysts for Ethanol Oxidation
2.1 In Situ FTIR Experimental Details
Infrared (IR) spectroscopy has proven to be one of the most useful techniques for
studying electrode/electrolyte interface at a molecular level. It provides information on the species involved in adsorption/desorption, surface bonding, orientation,
coordination, etc., and has substantially advanced our knowledge on various electrochemical systems, including both organic and inorganic chemical reactions [33–38].
In many absorption spectroscopy techniques, the intensity I of the transmitted signal through an absorbing medium is compared to the intensity of the incident radiation I 0 [33, 34]. In IR spectroscopy, the two intensities are related to the
extinction coefficient ε, thickness of the absorption layer b, and the concentration
c by Beer’s law, as I = I 0 exp(− εbc). The spectrum is usually presented as the variation of absorbance A = – ln (I/I 0 ) on the wavelength of the radiation λ, or wavenumber = 1/λ, which is related to frequency ν, as = 1/λ = ν/c. In infrared reflection–absorption spectroscopic (IRRAS) experiments described below, instead of
absorbance, one usually presents subtractively normalized change in reflectivity,
− ∆R/R = − (R s − R 0 )/R 0 , where the reflection signals R 0 and R s are taken at the reference and sample potential E r and E s , respectively [34–43]. For small differences in
R s and R 0 , – ∆R/R is proportional to the absorbance [39]. However, unlike the regular transmission experiment in which absorbance peaks are always positive-going,
the bands in the in situ IR spectroelectrochemical experiment can be both positiveand negative-going, representing the gain and the loss of a species at the sample
potential relative to that at the reference, respectively [44–54]. Because the change
in the band intensities taken at two potentials is usually of the order of a fraction of
a percent, a sufficient number of scans (usually over a hundred) must be coadded to
overcome statistical noise. Furthermore, care must be taken to minimize the changes
in the reflectivity induced by the factors other than potential, e.g., changes in the air
Reprinted from the journal
4
1 3
since proven that Pt-Rh can be highly active catalyst for ethanol oxidation in various
form, but the activity strongly depends on the substrate and synthesis [26–30].
In this review we describe the synthesis, characterization, electrochemical activity, and optimization of ternary PtRh/SnO 2 catalysts. Attempts to reduce the cost of
the catalyst by replacing Rh with alternative co-catalysts are illustrated, in which
it was shown that Ir in ternary catalysts could also be used to split the C–C bond
in ethanol [31]. We also underline particularly exciting results with ethanol oxidation to CO 2 on a Pt monolayer under tensile strain, as reported by Li et al. [32].
The emphasis is given to in situ characterization of the electrocatalysts by IR spectroscopy and X-ray absorption spectroscopy in an attempt to show how the combination of these spectroscopies, electrochemical techniques, and theoretical studies
can enhance the understanding of the ethanol electrooxidation. The examples shown
here hold great promise that impediments to practical direct alcohol fuel cells can be
overcome.
2 In Situ Characterization of New Catalysts for Ethanol Oxidation
2.1 In Situ FTIR Experimental Details
Infrared (IR) spectroscopy has proven to be one of the most useful techniques for
studying electrode/electrolyte interface at a molecular level. It provides information on the species involved in adsorption/desorption, surface bonding, orientation,
coordination, etc., and has substantially advanced our knowledge on various electrochemical systems, including both organic and inorganic chemical reactions [33–38].
In many absorption spectroscopy techniques, the intensity I of the transmitted signal through an absorbing medium is compared to the intensity of the incident radiation I 0 [33, 34]. In IR spectroscopy, the two intensities are related to the
extinction coefficient ε, thickness of the absorption layer b, and the concentration
c by Beer’s law, as I = I 0 exp(− εbc). The spectrum is usually presented as the variation of absorbance A = – ln (I/I 0 ) on the wavelength of the radiation λ, or wavenumber = 1/λ, which is related to frequency ν, as = 1/λ = ν/c. In infrared reflection–absorption spectroscopic (IRRAS) experiments described below, instead of
absorbance, one usually presents subtractively normalized change in reflectivity,
− ∆R/R = − (R s − R 0 )/R 0 , where the reflection signals R 0 and R s are taken at the reference and sample potential E r and E s , respectively [34–43]. For small differences in
R s and R 0 , – ∆R/R is proportional to the absorbance [39]. However, unlike the regular transmission experiment in which absorbance peaks are always positive-going,
the bands in the in situ IR spectroelectrochemical experiment can be both positiveand negative-going, representing the gain and the loss of a species at the sample
potential relative to that at the reference, respectively [44–54]. Because the change
in the band intensities taken at two potentials is usually of the order of a fraction of
a percent, a sufficient number of scans (usually over a hundred) must be coadded to
overcome statistical noise. Furthermore, care must be taken to minimize the changes
in the reflectivity induced by the factors other than potential, e.g., changes in the air
Reprinted from the journal
4