Antonio Rodes

Electrochemical characterisation of platinum-palladium nanoparticles prepared in a water-in-oil microemulsion

Abstract A detailed electrochemical characterisation of platinum-palladium nanoparticles prepared by reduction with hydrazine of H 2 PtCl 6 and K 2 PdCl 4 in a water-in-oil microemulsion of water/polyethylenglycol-dodecylether (BRIJ ® 30)/ n -heptane is reported. XPS, AAS and UV–vis experiments were carried out to determine the atomic composition of the bimetallic nanoparticles obtained in the whole composition range. Cyclic voltammograms of clean nanoparticle alloys were obtained after a controlled decontamination procedure of their surfaces. CO adsorption–oxidation and dissociative adsorption of formic acid and methanol were used as test reactions to check the electrocatalytic behaviour of the bimetallic nanoparticles. FTIRS experiments of adsorbed CO were also carried out giving information on the relative amount of linearly and bridge-bonded CO, which is known to depend on the surface distribution of the two elements. Experimental results show how the electrochemical adsorption behaviour and electrocatalytic properties of the particles are under the control of their composition and, in a case studied in this work, of their size.

Determination of the potentials of zero total charge of Pt(100) stepped surfaces in the [] zone. Effect of the step density and anion adsorption

Abstract The electrochemical behavior of Pt(S)[ n (100)×(111)] electrodes is examined in perchloric acid solutions in the absence and in the presence of sodium acetate. Charge densities associated with adsorption processes at step and terrace sites have been analyzed as a function of the step density. The results obtained from CO displacement experiments allow the estimation of, in a first approximation, the corresponding potentials of zero total charge (pztc). The values of the pztc diminish with the step density. In the case of acetate containing solution, the specific adsorption of acetate anions was monitored by means of Fourier transform infrared (FTIR) spectroscopy. The potential and the step density dependence of the corresponding infrared adsorption bands have been found to be in agreement with the electrochemical results.

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.

Anion adsorption on Pd–Pt(111) electrodes in sulphuric acid solution

Anion adsorption at palladium-covered Pt(111) electrodes has been studied in sulphuric acid solutions using cyclic voltammetry, charge displacement and in situ FTIR experiments. The infrared spectra obtained for partially covered Pt(111) substrates show distinct bands for adsorbed (bi)sulphate on platinum and palladium surface atoms with frequencies similar to those observed for the bare and fully covered Pt(111) electrode surfaces, respectively. This observation is in agreement with the epitaxial growth of palladium on Pt(111). Increasing the palladium coverage in the (sub)monolayer range is shown to shift (bi)sulphate adsorption towards less positive potentials, whereas the total amount of adsorbed anions on the ensemble of palladium and platinum domains is approximately constant. Combination of CO displacement and voltammetric curves allows the determination of the potential of zero total charge (pztc) as a function of the palladium coverage. The pztc shifts from 0.33 to 0.235 V when the palladium coverage on the Pt(111) is increased up to a monolayer. The oxidation of the CO adlayer resulting from the charge displacement experiments has also been studied. The oxidation of adsorbed CO on palladium-covered Pt(111) electrodes takes place at higher potentials than on Pt(111). The pztc values have been used to calculate the contribution from anion adsorption to the charge obtained from the integration of the CO stripping voltammograms. Coverage values calculated for the CO adlayers generated during the CO displacement experiments at the palladium-covered Pt(111) electrodes diminish as the palladium coverage increases.

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.

Potential of zero total charge of palladium modified Pt(111) electrodes in perchloric acid solutions

The electrochemical behaviour of palladium-modified Pt(111) electrodes in perchloric acid solutions has been studied by means of cyclic voltammetry and CO charge displacement. From the results of the charge displacement experiments, values of the potentials of zero total charge (pztc) have been determined and their dependence on the palladium coverage has been studied in solutions without specifically adsorbing anions. These pztc values are compared to those obtained in sulfuric acid solutions. Whilst in the presence of bisulfate anions the pztc decreases significantly as a consequence of the deposited palladium, in the case of perchloric acid solutions the pztc remains constant within experimental error. This constancy has been explained by means of a compensation between the displacement of the potential of zero free charge toward less positive potentials and the increase of hydrogen coverage on the palladium-covered electrode surfaces. The influence of the concentration of specifically adsorbed (bi)sulfate anions at constant pH has been pointed out. The oxidation of the CO adlayer resulting from the charge displacement has also been studied. Despite the apparently easier OH adsorption, the oxidation of adsorbed CO takes place at higher potentials on the modified surfaces. CO coverage values, calculated from knowledge of the pztc value and the CO stripping charge, diminish as the amount of palladium increases.

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.