J.M. Pérez

Voltammetric and in-situ FTIR spectroscopic study of the oxidation of methanol on Pt(hkl) in alkaline media

Abstract An electrochemical and spectroelectrochemical study of the adsorption and oxidation of methanol on platinum single-crystal electrodes was carried out in alkaline media (Na 2 CO 3 and NaOH). The adsorbed species were identified by Fourier transform IR (FTIR) spectroscopy as linearly and multibonded CO in a ratio which depends on the orientation of the electrode surface. From the FTIR spectra obtained during the oxidation of methanol, formate and CO 2 have been identified as the main soluble reaction products in alkaline solution.

On the voltammetric and spectroscopic characterization of nitric oxide adlayers formed from nitrous acid on Pt(h,k,l) and Rh(h,k,l) electrodes

Cyclic voltammetry and in situ FTIR spectroscopy have been employed to characterize NO adlayers at platinum and rhodium single-crystal electrodes. These adlayers, which are generated upon surface decomposition of nitrous acid, give infrared spectra similar to those observed for the same surface at high NO coverages under UHV conditions. The NO stretching frequency turned out to be potential dependent with a significant upward shift as the electrode potential increased. The analysis of the absolute spectra obtained for NO-covered Pt(111) and Rh(111) electrodes shows a linear variation of the band center frequency of linearly bonded NO with slopes of 65 and 20cm−1 V−1, respectively. In addition to this potential dependence of the NO stretching frequency, the spectra obtained with the Rh(111) electrode suggests the existence of a change from bridge to linearly bonded NO as the electrode potential increases. Parallel voltammetric experiments allowed the determination of the surface coverage.

Electrochemical behaviour of aqueous SO2 at Pt electrodes in acidic medium. A voltammetric and in situ Fourier transform IR study Part I. Oxidation of SO2 on Pt electrodes with sulphur-oxygen adsorbed species

Abstract The electrochemical oxidation of aqueous SO2 at platinum electrodes has been studied in an acidic medium by means of cyclic voltammetry and in situ Fourier transform IR (FTIR) spectroscopy. In the potential zone ranging from 0.55 to 1.50 V (vs.RHE), the oxidation reaction of SO2 takes place at a Pt surface initially covered by a sulphur-oxygen adsorbate. In situ FTIR spectroscopy yields a band at 1271 cm−1, which could be assigned to adsorbed SO2. However, (bi)sulphate is detected as the only SO2 oxidation product. The generation of S(VI) species occurs during both the forward and the reverse sweeps. The different voltammetric responses obtained in each case are attributed to the oxidation of SO2 on a Pt electrode with different surface states.

Nitric oxide adsorption at Pt(100) electrode surfaces

Abstract Nitric oxide adlayers formed at Pt(100) electrode surfaces have been characterized in situ by means of electrochemical and FTIR spectroscopic experiments. These adlayers can be reductively stripped from the electrode surface yielding dissolved ammonium as the main reduction product in acidic solution. From the voltammetric charge involved in this process the absolute NO coverage for the saturated adlayers has been calculated to be 0.5. This value agrees with that corresponding to the c(4×2) overlayer found by different authors under UHV conditions. The oxidation of the NO adlayer in acid media proceeds between 0.90 and 1.10 V RHE through the formation of surface species (probably a mixture of nitrite and nitro species), which can be further oxidized to dissolved nitrate anions at higher potentials. The same NO adlayers have been found to oxidize directly to dissolved nitrite anions in alkaline solutions. The potential dependence of the N–O stretching mode, which is shifted towards higher wavenumbers when the electrode potential increases, has been analyzed for the saturated NO adlayer. Linear relations were found both in acidic and neutral solutions with tuning rates of ca. 50 cm −1 V −1 in the potential region between 0.50 and 0.90 V RHE where the saturated NO adlayers are stable. A slow dissolution process has been evidenced at potentials slightly below 0.80 V for the NO adlayer in alkaline solutions.

Electrochemical and in situ FTIR studies of the CO adsorption at palladium and rhodium multilayers deposited on platinum single crystal surfaces. I. Pt(110) substrate

Abstract Multilayers of rhodium and palladium have been deposited on Pt(100) electrodes and subsequently characterized by means of cyclic voltammetry and charge displacement experiments. In addition, the saturated adlayer of CO on Pd Pt (100) and Rh Pt (100) has been studied by both electrochemical and infrared techniques. The voltammetric profile corresponding to these multilayers is due to a coupled adsorption/desorption process involving both anions and hydrogen. The set of different results seems to indicate that the multilayer is epitaxial in the case of palladium, thus behaving as a bulk Pd(100) electrode, whereas it is disordered in the case of rhodium. The spectra for the saturated CO adlayer on Rh Pt (100) and Pd Pt (100) also reflect the presence of the foreign metal deposit and give some indication about the bonding geometry and order of the CO adlayer. The CO saturation coverage for the Pd Pt (100) system is around 0.63.

Electrochemical behaviour of aqueous sulphur dioxide at polycrystalline Pt electrodes in acidic medium. A voltammetric and in-situ FT-IR study Part II. Promoted oxidation of sulphur dioxide. Reduction of sulphur dioxide

The promoted electro-oxidation of aqueous sulphur dioxide at platinum electrodes has been studied in acidic medium with the aid of cyclic voltammetry and in-situ FT-IR spectroscopy. Promotion of SO2 oxidation is achieved when adsorbed SO2 is reduced previously to adsorbed sulphur. On a platinum surface covered by sulphur in this way, an enhancement of SO2 oxidation is attained. Spectroscopic evidence demonstrates that, like oxidation of SO2 in the oxygen adsorption region, soluble S(VI) is the ultimate reaction product of the catalysed SO2 oxidation. The electroreduction of SO2 has been dealt with by using the same techniques. Besides a surface process converting adsorbed SO2 into adsorbed sulphur, bulk SO2 reduces irreversibly giving rise to a diffusion-limited voltammetric peak. The absence of significant IR bands in the potential region at which reduction of bulk SO2 takes place, allows discarding the generation of S-H containing species. A sulphur + polysulphide mixture is suggested as the ultimate product, yet whether this mixture is formed directly in the electron-transfer step or stems from chemical decomposition of sulphur-oxygen short lifetime intermediates is not clear. Reduction of SO2 leads to a progressive accumulation of sulphur on the platinum surface. An excess of adsorbed sulphur negatively affects the kinetic stages of both oxidation and reduction of bulk SO2.

CO adsorption and oxidation on Pt(111) electrodes modified by irreversibly adsorbed arsenic in sulphuric acid medium. Comparison with bismuth-modified electrodes

CO adsorption and stripping on Pt(111) electrodes modified by irreversibly adsorbed bismuth and arsenic were studied in sulphuric acid medium. Coadsorbed bismuth and CO form a mixed adlayer, whereas arsenic tends to desorb in the presence of CO. Both adatoms modify the CO stripping process, resulting in the catalysis of CO oxidation, in which arsenic is more effective than bismuth. Fourier transform IR (FTIR) studies demonstrate that bismuth stabilizes the adsorbed CO, while arsenic shows the opposite effect. In both cases, linear CO is the only species present on the electrode surface at high adatom coverage. The catalysis mechanism is an adatom-mediated oxygen transfer for both cases, with an additional electronic effect when arsenic is on the surface. Using the CO stripping charge after adequate correction, the values of the CO coverage for the different adatom coverages were calculated. The results of the CO coverage agree well with the behaviour shown in the FTIR experiments.

Sulphate adsorption at chemically deposited silver thin film electrodes: time-dependent behaviour as studied by internal reflection step-scan infrared spectroscopy

Abstract Stable silver thin films were chemically deposited on a germanium substrate. Ex-situ STM images showed that these films are formed by grains with a diameter between 20 and 100 nm. The silver films have been used as the working electrode in infrared spectroelectrochemical experiments with an internal reflection (Kretschmann) configuration. The adsorption of sulphate anions in neutral or slightly acidic solutions has been studied. The high intensity of the S–O stretching bands in the absorption spectra allowed the monitoring of the time-dependent behaviour of the adsorption/desorption processes in time-domain step-scan experiments.

FTIRS and electrochemical characterization of NO adlayers on Pt(hkl) generated upon immersion in an acidic solution of nitrite

Nitric oxide adlayers on platinum single crystal electrodes have been studied by means of IR spectroscopy and cyclic voltammetry. These adlayers were generated upon immersion of the sample in an acidic solution of nitrite. The in-situ FTIR spectra obtained for each electrode agree with those previously reported under UHV conditions suggesting that similar NO adlayers could be formed in all cases. At saturation, only one bipolar band was observed for Pt(111) and Pt(100) centered at 1680 and 1630 cm−1, respectively. In the case of non-saturated adlayers, a different behavior was observed for each basal orientation. The NOPt(111) system showed spectra with two bands at 1680 and 1430 cm−1. The spectrum corresponding to Pt(100) presented a single band directly related to that observed under UHV conditions in the whole range of coverages. The nonsaturated NO adlayer on Pt(110) was characterized by a spectrum having a single bipolar feature located at around 1590 cm−1. It has also been shown that the adsorbed NO molecules undergo a surface redox process in the case of Pt(100) and Pt(111). There is some spectroscopic and voltammetric evidence indicating that the oxidized form of the adlayer consists of adsorbed nitrite anions.

Electrochemical oxidation of ethanol on Pt(hkl) basal surfaces in NaOH and Na2CO3 media

Abstract The electrooxidation of ethanol as well as its irreversible adsorption on platinum single crystal electrodes have been studied in NaOH and Na 2 CO 3 media. Pt(110) and Pt(100) electrodes show deactivation for oxidation of bulk ethanol in both media, caused mainly by a structural surface modification of the electrode surface. However, in NaOH solution Pt(lll) exhibits the best behaviour on ethano] oxidation showing the highest current density and the most stable surface. From the SNIFT-IR spectra adsorbed CO is the main poisoning species in the irreversible adsorption of ethanol on Pt(110) and Pt(100); on Pt(111) only a very weak band of adsorbed CO is observed. From the FT-IR spectra obtained during the oxidation of ethanol on Pt(HO), Pt(100) and Pt(111) electrodes, acetate anions have been identified as the main soluble oxidation product.