S. Strbac

Electrocatalysis of oxygen on single crystal gold electrodes

Abstract Oxide formation and oxygen reduction on gold have been investigated with single crystal electrode surfaces of various orientations. Both reactions show sensitivity to the surface orientation, presence of steps, their density and orientation. The initial stages of oxide formation can be correlated with calculated relative surface energy. Anions exert a pronounced effect on oxide formation. The activity for oxygen reduction can be correlated with the same surface property except for Au(100). Structural effect from this particular plane outweighs the electronic effects. No correlation for alkaline solutions was found. Rotating ring-disc measurements show no hydrogen peroxide generation Au(100) in the presence of almost neutral AuOH −(1 − λ) on the surface. This makes it the most active electrocatalyst for oxygen reduction in alkaline solutions. At more negative potentials (no AuOH on the surface) a series mechanism up to the peroxide stage is operative. The Au(311) plane also shows a high activity dependent on AuOH −(1 − λ) . Data for both reactions clearly demonstrate that each crystallographic orientation gives an electrode surface with distinct electrochemical properties.

The electrocatalytic properties of the oxides of noble metals in the electrooxidation of methanol and formic acid

The electrooxidation of 0.25 M methanol as well as of 0.25 M formic acid was examined on oxides formed on the Au( 111) electrode surface in 0.5 M H 2 SO 4 and 0.5 M NaHCO 3 at a sweep rate of 0.5 mV/s. In the case of methanol, the Au( 111) surface in 0.5 M H 2 SO 4 must be covered by oxides in order to catalyse its oxidation significantly. The current of formic acid oxidation also appears in the potential region before the oxide formation, but the catalytic activity is apparently deeper in the potential region of oxide formation. In 0.5 M NaHCO 3 the inhibition of methanol oxidation and the apparent catalytic activity of Au oxides in the case of formic acid oxidation are observed. The methanol electrooxidation on polycrystalline silver electrode was observed only in the potential region where the surface was covered by oxides in 0.1 M NaOH and at a sweep rate of 0.1 mV/s. This reaction was used to measure the concentration of methanol in the methanol-formaldehyde mixture in the same electrolyte.

Oxide formation on gold single crystal stepped surfaces

Abstract Oxide formation and anion adsorption have been studied on gold single crystal stepped surfaces. Most of the measurements were performed in sulphuric and some in perchloric acid solutions by cyclic voltammetry and by double layer capacity measurements. A pronounced structural sensitivity of both oxide formation and anion adsorption was found. The effects of the terrace orientation, step orientation and step density are reflected in voltammetry and the double layer capacity measurements. A systematic dependence of both processes on step density was found and their interrelation is discussed. The results contradict some gas-phase data reporting no critical role of steps in oxygen chemisorption on gold. A correlation between the relative surface energy of gold single crystals and the peak potentials of the initial stage of oxide formation was found.

Effect of temperature on the methanol oxidation at supported Pt and PtRu catalysts in alkaline solution

Abstract The effect of temperature on the kinetics of methanol oxidation on supported 47.5 wt% Pt, 54 and 33.5 wt% Pt/Ru catalysts was studied in 0.1 M NaOH at 295, 313 and 333 K using thin-film rotating disk electrode (RDE) method. The catalysts were characterized by ex situ STM prior to the voltammetric studies. It was found that the activity of those catalysts for methanol oxidation was determined by a delicate balance between the surface coverage by CO ad and by OH ad species. Significantly faster kinetics at higher temperatures clearly indicates that methanol oxidation at the catalysts studied is highly activated process. The highest effect of temperature is obtained at the least active Pt catalyst which has the highest activation energy. On the contrary, the smallest temperature effect is detected on the most active Pt 3 Ru 2 catalyst having the lowest activation energy. These phenomena have been explained by the important role of the adsorption energies of both reactive intermediates (CO ad and OH ad ).

Electrochemical indication of surface reconstruction of (100), (311) and (111) gold faces in alkaline solutions

The specific adsorption of OH− anion and the changes in the surface atomic structures of (100), (311) and (111) gold faces with charge density are discussed on the basis of cyclic voltammograms and differential capacity—potential curves obtained, in dilute NaOH solutions.

Oxygen reduction on the Au (311) electrode surface in alkaline electrolyte

Oxygen reduction was studied for the first time using a single crystal electrode in a rotating disc-ring arrangement. The Au (311) surface shows a complex behaviour, with a very high activity in certain potential regions. The first electron transfer is rate determining in the region of 4-electron reduction. As with Au (100), a 4 e− reduction changes into a 2 e− process, which reverts back to a 4 e−reaction at very negative potentials. Based on a general reaction scheme of O2 reduction, a map of the operating potential dependent reaction pathways was constructed. Nearly 60% of the mass flux of O2 undergoes a direct reduction to OH− in the region of mixed control. The high activity of Au (311) was ascribed to a high step density and AuOH present on its surface.

Sorption of zinc by novel pH-sensitive hydrogels based on chitosan, itaconic acid and methacrylic acid

Novel pH-sensitive hydrogels based on chitosan, itaconic acid and methacrylic acid were applied as adsorbents for the removal of Zn(2+) ions from aqueous solution. In batch tests, the influence of solution pH, contact time, initial metal ion concentration and temperature was examined. The sorption was found pH dependent, pH 5.5 being the optimum value. The adsorption process was well described by the pseudo-second order kinetic. The hydrogels were characterized by spectral (Fourier transform infrared-FTIR) and structural (SEM/EDX and atomic force microscopy-AFM) analyses. The surface topography changes were observed by atomic force microscopy, while the changes in surface composition were detected using phase imaging AFM. The negative values of free energy and enthalpy indicated that the adsorption is spontaneous and exothermic one. The best fitting isotherms were Langmuir and Redlich-Peterson and it was found that both linear and nonlinear methods were appropriate for obtaining the isotherm parameters. However, the increase of temperature leads to higher adsorption capacity, since swelling degree increased with temperature.

Oxygen reduction on Au(111) and vicinal Au(332) faces : a rotating disc and disc-ring study

The kinetics of oxygen reduction on the Au(111) surface in acidic solution has been re-examined by means of a rotating disc electrode technique. Its vicinal Au(332) = Au[6(111) × (111)] surface has been chosen to examine the role of a small density of steps as active sites. The increase of the average surface energy, caused by the presence of steps on the Au(332) face, causes a slight shift of the half-wave potential to a more positive value, compared to the Au(111). The reaction on Au(111) is kinetically hindered in the whole potential region up to hydrogen evolution. The presence of steps on the Au(332) face apparently causes the kinetic limitations to be lifted. The kinetics of oxygen reduction on the Au(332) face in alkaline solution has been studied by means of a rotating disc-ring method. The mechanism of the reaction has been found the simplest among all the faces of Au investigated so far. In the kinetic and mixed control regions the reduction proceeds through the simplest 2e-series pathway. At large overpotentials, a 4e-reduction occurs, whose extent depends on the potential, with a surface diffusion of HO−2a as a rate determining step. No splitting of the OO bond has been found. A map of the reaction pathways for the whole potential range of oxygen reduction has been constructed.

The structure, growth and reactivity of electrodeposited Ru/Au(111) surfaces

In order to elucidate electronic effects on the oxidation of CO on small Ru clusters, we investigated this reaction on well defined Ru:Au(111) model systems via parallel in-situ STM studies of the structure and electrochemical deposition of Ru on Au(111) in H2SO4 solution and cyclic voltammetry of CO monolayer oxidation on these surfaces. The Ru deposit consists of nanoscale islands, which coalesce with increasing coverage. The Ru saturation coverage depends on the deposition potential, resulting in Ru submonolayer (\0.1 V), (defective) monolayer (] 0.1 V), and multilayer films (B 0.1 V). At potentials \ 0.6 V irreversible formation of Ru oxide:hydroxide species is observed, which can be partly reduced in the range 0.4 to 0.0 V. CO stripping commences at :0.1 V and occurs over a broad potential range. From the stripping charge a local CO coverage on the Ru monolayer islands of 0.7 ML was estimated. The observed influence of the morphology of the Ru deposit on the CO stripping voltammetry is explained by (local) variations in the CO adsorption energy due to electronic modifications of the Ru film.

Surface morphology and the response of piezoelectric gas sensors

The surface morphology of gold electrodes on piezoelectric quartz crystals was characterized by scanning electron microscopy and scanning tunnelling microscopy. Sensors with various coating materials as polymers, liquid crystal and nitrocompounds have been employed to demonstrate the importance of surface properties in determining the response of the device. The sensors were evaluated for detection of organophosphorus and nitroaromatic vapours in air.