On the adsorption of sulfate on low Miller index Rh(hkl) electrodes: an in situ vibrational analysis

Journal of the Brazilian Chemical Society

Moraes, Ines R. de¹  Nart, Francisco C.¹  Universidade de São Paulo, São Carlos, Brazil¹ 

关键词: electrochemical interfaces;    reflection spectroscopy;    adsorption;   

DOI  :  10.1590/S0103-50532001000200003

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

Adsorption of anions is of central interest in electrochemistry, especially of those which are used as supporting electrolytes in electrochemical systems. The sulfate species is the most studied adsorbed anion on electrodes, since sulfuric acid is one of the most commonly used supporting electrolytes, especially in the study of single crystal electrodes. Furthermore, the adsorbed anions may play a role on the kinetics of electrochemical oxidation reactions.The adsorption of sulfate and other oxyanions experimented a renewed interest with the introduction of auxiliary techniques in electrochemistry, such as in situ FTIR spectroscopy1, radioactive labeling2 and in situ scanning tunneling microscopy (STM)3-5.Vibrational studies of sulfate adsorbed on single crystal metal electrodes are reported for Pt(111)6-9, Pt(100)10,11, Pt(110)12, Au(111)3,13, Au(100) and (110)14 and Rh(111)15,16. Single crystal surfaces constitute interesting model systems for the study of adsorbed sulfate, especially using in situ FTIR, since it permits to establish some correlation between the surface crystallography and the observed vibrational features. In spite of some controversy in the literature, the vibrational characterization of adsorbed sulfate on different surfaces helps the understanding of the structure and bonding of sulfate ions to the electrode surface. Additional studies on new systems will help to clarify controversial points in the interpretation of the spectra.The common characteristic for adsorbed sulfate on Pt(111)6-9 and Au(111)3,13 is that the spectroscopic features do not depend on the solution pH, i.e. it is insensitive to the presence of bisulfate or sulfate in solution. On the other hand, adsorbed sulfate on other surfaces, like Pt(100)10,11, Pt(110)12 and Au(110)14 produces different vibrational features depending on the pH. These results reveal the role of the surface topography on the structure and the nature of adsorbed sulfate.While the adsorption of sulfate on Pt and Au single crystal is well documented, data of sulfate ion adsorbed on Rh single crystal electrodes are scarce. Shyngaya et al.15,16, in a study with different M(111) electrodes, reported vibrational spectroscopic data for adsorbed sulfate species on Rh(111) electrodes. A quite remarkable similarity with the bands observed for Pt(111) is observed. Zelenay et al.17 reported cyclic voltammetry and radioactive labeling of adsorbed sulfate on Rh(111) single crystals and polycrystalline Rh electrodes. They found that the total coverage was 40 per cent of the total theoretical maximum coverage on Rh(111) and that the adsorbed sulfate is more stable on Rh(111) than on the other low Miller index surfaces of Rh.STM data on sulfate adsorbed on Rh(111) has been reported recently4, showing that the adsorbed sulfate forms astructure, similar to that found for Au(111)3 and Pt(111)5. It is noteworthy that the surface coverage of 0.2 suggested by this structure is the same for Au(111), Pt(111) and Rh(111) and it is in good agreement with other techniques, such as radiotracer assay15.To our knowledge, the adsorption of sulfate on Rh(100) and Rh(110) has not been reported so far. The study of the adsorption of sulfate on those surfaces is also important in order to accomplish a more general view of the vibrational characteristics of adsorbed sulfate species on electrochemical interfaces. More specifically, we are interested in surfaces which do not match very well a three-fold coordination for the adsorbed sulfate.In the present work we report vibrational data of adsorbed sulfate adlayers on the low Miller index of Rh single crystal surfaces. We will bring into focus previously published data on sulfate adsorbed on Pt(111) and Au(111) to compare with our data of Rh(111). Our objective is to understand the surface chemistry of adsorbed sulfate using in situ vibrational spectroscopy.  ExperimentalAll solutions were prepared with Merck Suprapur reagents and Millipore Milli-Q purified water.The low Miller index Rh(hkl) single crystal (99.999%) was of a cylindrical form with a diameter of 10 mm and obtained from Metal Crystal & Oxides (Cambridge, UK). The final preparation of the rhodium single crystal consisted of annealing the Rh single crystal in gas flame for about 2-3s at a temperature of about 1300oC, cooling in a stream of Ar+H2 and rapid-quenching in ultrapure water. The crystal was transferred rapidly to the spectroelectrochemical cell protected by a drop of water to avoid contamination.Before each experiment the electrode was cycled electrochemically in the base electrolyte in the potential range 0.05-0.8 V to control the quality of the crystal surface and the solution. In order to avoid disordering of the crystal surface, the maximum potential used was 0.8 V, a potential prior the rhodium oxidation.A BOMEM DA-8 spectrometer equipped with a L-N2 cooled MCT detector was used for the in situ experiments. The spectroelectrochemical cell was made of PTFEä with a CaF2 transparent flat window placed at the bottom of the cell. A Pt ring was used as the auxiliary electrode and a Pd mesh loaded with hydrogen (Pd/H2) was used as a reference electrode. All potentials are referred to the Pd/H2 electrode.The procedure followed for the accumulation of the spectra was already described in detail in previous publications10. Typically 10000 scans were collected for each potential. Since 10000 scans take a long time, the spectra were collected by stepping ten times the potential between a reference and a sample potential and 1000 scans were collected at the reference and sample potential. Such number of scans was necessary taking into account the small amount of adsorbed ions at the electrode surface and the reduced intensity of the reflection-absorption bands due to the use of a flat window.  ResultsEffect of solution acidityWhen analyzing the spectra obtained in situ, it is necessary to take into account that the spectra reported correspond to the ratio of two spectra obtained at two different electrode potentials (the sample potential and the reference potential). The reference potential is taken where no sulfate adsorption occurs. The spectral information coming from the adsorbed ions will be coupled with the spectral information coming from the ions depleted from the solution upon adsorption. This happens because the thin layer of solution held between the electrode and the transparent window can be considered practically uncoupled from the bulk solution. No fast diffusion of the ions to the thin solution layer will occur in the timescale of the experiment. Therefore, positive-going bands will correspond to species depleted from the optical path and the negative-going bands, to species formed at the sample potential.The spectra of adsorbed sulfate at pH 3 and 0.2 at the maximum coverage, where SO42- and HSO4- predominate, respectively18 are shown for Rh(111) (Figure 1) and for Rh(100) (Figure 2). For the sulfate adsorbed from a solution containing SO42- ions, three bands can be clearly distinguished (Figure 1a). One positive-going band is centered at 1100 cm-1 and two negative-going bands are located at 1247 cm-1 and 960 cm-1. These bands are in good agreement with those reported for sulfate adsorbed on Rh(111) in sulfuric acid solutions16. As already discussed in previous publications6, the positive feature at 1100 cm-1 is due to the dissolved SO42-, which presents only one active stretching vibration located at this wavenumber. The two negative features correspond to the S-O stretching vibrations of adsorbed sulfate. The solution species spectra were checked using s-polarized light (data not shown) and only the positive-going band at 1100 cm-1 was observed, corroborating the proposal that this band corresponds to the depletion of the sulfate in solution, since the s-polarized light is active only to solution species.1 The spectrum for the adsorbed sulfate on Rh(100) (Figure 2a) does not display very clearly the band at 960 cm-1, since for this electrode the signal to noise ratio is too low to allow the distinction of this weak band.      For solutions of pH 0.2 (Figures 1 and 2), again, two negative-going bands can be distinguished at 1239 and 960 cm-1, for the two electrodes. These bands are at approximately the same positions of those observed for pH 3. As already observed for adsorbed sulfate on Pt(111)6-9 and Au(111),3,13 the vibrational feature of sulfate adsorbed on Rh(111) is independent of the presence of sulfate or bisulfate in solution. Very likely, adsorbed bisulfate species from very acid solutions undergoes complete dissociation when adsorbed on these surfaces. Therefore, only SO42- is adsorbed on Rh(111). For sulfate ions adsorbed on Rh(100) electrodes the band intensity is smaller than on Rh(111) electrodes and therefore, the band center is observed at lower energy. For the very acidic solution, bisulfate in solution has a stretching mode at 1200 cm-1, thus partially overlapping with the band for the adsorbed species, which is centered at 1240 cm-1. The band center is therefore disturbed by the positive going band and probably the actual value is somewhat different of that observed.Effect of the surface geometryThe spectra of adsorbed sulfate at maximum coverage (0.3 V) on Rh(111), Rh(100) and Rh(110) are shown in Figure 3. For sulfate species adsorbed on Rh(100) and Rh(110), the spectra are qualitatively very similar to that of Rh(111). However, the integrated band intensities are, for the sulfate adsorbed on Rh(100) and Rh(110), ca. 1/3 of the intensity for sulfate adsorbed on Rh(111). The band half width for the adsorbed sulfate at saturation coverage on Rh(100) and Rh(110) is 57 cm-1, while for Rh(111) the band half width is 42 cm-1. A broader band means that for Rh(100) and Rh(110) the sulfate adlayer is not as well organized as in the

【 授权许可】

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