Tomáš Loučka

Adsorption and oxidation of sulphur and of sulphur dioxide at the platinum electrode

Summary The adsorption and oxidation of sulphur and of sulphur dioxide on a smooth platinum electrode have been studied by the potentiodynamic method. The charges consumed in the anodic oxidation of adsorbed sulphur as well as of adsorbed sulphur dioxide are considerably higher than those required for the oxidation of a monolayer of sulphur or of sulphur dioxide, respectively. In the course of the anodic oxidation of the adsorbed sulphur or of sulphur dioxide a special form of oxide is presumably formed which can be reduced only in the potential range where the adsorption of hydrogen takes place. The possibility of the adsorption of sulphur in multilayers is also discussed.

Adsorption and oxidation of organic compounds on a platinum electrode partly covered by adsorbed sulphur

Summary The dependence of the non-stationary currents of formic acid, formaldehyde and methanol on the degree of coverage of the platinum electrode by adsorbed sulphur and the effect of adsorbed sulphur on the electrooxidation of methanol and formaldehyde have been studied. Non-stationary currents of formic acid for θHS

Adsorption and kinetics of oxidation of ascorbic acid at platinum electrodes

Summary As shown by cyclic voltammetry and by polarography with a rotating disc electrode, ascorbic acid is oxidized at platinum electrodes in a two-electron process. There are two reaction paths of the oxidation, one corresponding to an irreversible electrode reaction of the substance without appreciable adsorption with α n a ≈0.4, the other being the oxidation of the adsorbed substance in the potential range of Pt surface oxide formation. The adsorption of ascorbic acid is reversible and non-destructive, attaining its maximum value (68% of sites of hydrogen adsorption) even at low concentrations of ascorbic acid. The process is controlled by kinetics of adsorption proper together with diffusion. One molecule of ascorbic acid occupies two hydrogen adsorption sites.

The adsorption of sulphur and of simple organic substances on platinum electrodes

Summary The adsorptions of hydrogen sulphide, methanol and formic acid on a platinum electrode have been studied. The adsorption of hydrogen sulphide is controlled by diffusion. Experimental results suggest that hydrogen sulphide adsorption proceeds formally in the same way as on a homogeneous electrode surface. The bonds between an adsorbed organic compound and an electrode surface are weaker in the presence of sulphur adsorbed on a platinum electrode surface than those for a surface without adsorbed sulphur. The presence of sulphur adsorbed on the electrode surface increases the value of the adsorption rate constant of formic acid.

The formation and reduction of an oxide layer on a platinum electrode partially covered with adsorbed sulphur

Summary It has been found that the total amount of oxide on a platinum electrode corresponds to a monolayer even under conditions such that adsorbed sulphur is present on the surface of the electrode ( Θ s

Adsorption and oxidation of formaldehyde at the platinum electrode in acid solutions

Summary The adsorption of formaldehyde on a smooth platinum electrode was studied in 1 N H2SO4 at 40°. The kinetics of adsorption follow the Elovich equation and the dependence of the equilibrium coverage on the bulk concentration of formaldehyde can be described by the Temkin isotherm. The maximum coverage of the electrode is attained at the potential, E=+0.2 V vs. HE. The mechanisms of adsorption and of electro-oxidation of formaldehyde were deduced from the adsorption measurements. The chemisorbed species formed by the dehydrogenation of formaldehyde molecules are relatively less active. They act as inhibitors in the anodic oxidation of formaldehyde in the region of potentials, E 〈 +0.6 V.

Accelerated dephosphorylation of adenosine phosphates and related compounds in the presence of nanocrystalline cerium oxide

Recently, cerium oxide has been used in many exciting applications in biology and medicine, where its enzyme-mimetic abilities can be exploited. A more in-depth understanding of its interactions with biologically relevant molecules is highly desirable. It is shown in this study that certain forms of cerium oxide, namely, the nanocrystalline cerium oxide (nanoceria) prepared by the thermal decomposition of cerium carbonate, may exhibit a remarkable ability to promote the dephosphorylation reactions of nucleotides and related compounds – adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP), inosine monophosphate (IMP), cytidine monophosphate (CMP), guanosine monophosphate (GMP), thiamine pyrophosphate (TPP) and thiamine monophosphate (TMP). The dephosphorylation proceeded under relatively mild (ambient) conditions with a rate constant proportional to the concentration of nanoceria and was almost independent of pH. The phosphoester bonds were cleaved through nucleophilic substitution, utilizing the surface hydroxyl groups as nucleophilic agents, whereas the electrophilicity of the phosphorus atoms was enhanced by the Lewis acid effect of the cerium cations. Moreover, more complex biologically relevant molecules may be subjected to degradation in the presence of cerium oxide, namely, β-nicotinamide adenine dinucleotide (NAD). The key characteristics of cerium oxide (surface area, crystallinity) as well as its surface chemistry and dephosphorylation activity are governed by the calcination temperature – the most effective cerium oxides were prepared by annealing at temperatures below ca. 600 °C. Notably, certain commercially available cerium oxides exhibited even higher dephosphorylation activity than the in-house nanoceria.

Potential-pH diagrams for the systems Ir−H2O−Cl−, Pt−H2O−Cl−, and Pd−H2O−Cl− at 25°C

Les diagrammes de Pourbaix des quatre systemes ternaires, mentionnes dans le titre, sont calcules a partir de donnees thermodynamiques, dans une solution aqueuse de chlorure de sodium