D. Stoychev

Electroless deposition of Pt on Ti : catalytic activity for the hydrogen evolution reaction

Abstract The deposition of platinum on titanium at open-circuit potential from an aqueous 0.1 M HClO 4 solution containing K 2 PtCl 6 was studied. On freshly polished titanium, spontaneous deposition of platinum takes place through a displacement reaction between Ti(0) and dissolved Pt(IV). At very short times of deposition, fine dispersions of platinum crystals were formed on titanium. At longer deposition times, the size of the platinum crystals increases whereas their number decreases due to coalescence and agglomeration processes. On titanium covered by a thin oxide film, the rate of deposition is very low and photocatalytic deposition of Pt on the TiO 2 surface occurs simultaneously with the displacement reaction. The hydrogen evolution reaction (her) was used as a probe reaction for testing both the catalytic activity and deposition conditions. There is evidence that the catalytic activity of the Pt crystals for the her increases with decreasing crystal size.

Electroless deposition of Pt on Ti: Part II. Catalytic activity for oxygen reduction

Abstract The reduction of oxygen on electroless deposited platinum on a freshly polished titanium electrode was studied in aqueous 0.1 M HClO 4 and 0.2 M NaOH solutions, utilising the rotating-disc electrode technique. TEM studies show that at short deposition times Pt is highly dispersed on titanium. At longer deposition times the degree of dispersion decreases due to coalescence and agglomeration processes and larger clusters are formed. The kinetics of oxygen reduction were affected substantially by the time of Pt deposition on Ti at the open-circuit potential. A kinetic analysis of the current–potential curves was made in the kinetically controlled region in the form of Tafel plots and in the mixed kinetic-diffusion control region in the form of Koutecky–Levich plots. The catalytic activity of the Pt crystals decreases with increasing deposition time. The results were interpreted in terms of the effect of the Pt crystal size. The larger the Pt particles the lower the catalytic activity. The role of titanium oxide formation and the electronic effects were also discussed.

Chemical composition and corrosion resistance of passive chromate films formed on stainless steels 316 L and 1.4301

Abstract The chemical composition and corrosion behavior of the natural and formed by chemical treatment in chromium-containing solution passive films on 316 L and 1.4301 stainless steel surfaces have been investigated by means of X-ray photoelectron spectroscopy and electrochemical in situ method of anodic polarization curves. It have been established that the oxide films formed by the chemical treatment have different chemical composition (Cr-enriched), color and reduced corrosion resistance compared to the natural passive films on both steels. The results have shown that the lower part of the oxide layer represents a uniform modified passive film and the upper one is a porous Cr3+-enriched film with an island-like structure. The latter permits a direct contact of the solution with the modified passive film which controls the corrosion resistance by the dissolution of the Fe3+ oxides present in the lower thinner layer.

Electrochemical growth of thin La2O3 films on oxide and metal surfaces

This work presents a new electrochemical method (i.e. new electrolyte composition and electrolysis regime) for direct formation of La2O3 on ZrO2 and other oxide and metal substrates from nonaqueous electrolytes, data on the kinetics of the electrochemical processes and scanning electron microscopy (SEM) and XPS studies of the morphology, structure, dispersion and chemical composition of La2O3.

Electrocrystallisation of metal catalysts: nucleation and growth of platinum on a titanium electrode

Abstract Electrodeposition of noble metals that can be used as catalysts of chemical and electrochemical reactions is a process of major importance both from a practical and a scientific point of view. This is particularly true for the very initial stage of the phase transition comprising nucleation and growth phenomena leading to the formation of a highly dispersed metal phase with a strongly developed active surface area. The paper contains information on the nucleation and growth stage of platinum electrodeposition on an oxidised titanium working electrode. Cathodic current transients are recorded under potentiostatic conditions and are analysed in terms of the theory of progressive nucleation with overlap of diffusion zones and limited number of active sites. Data are obtained on the diffusion coefficient D of the platinum ions, on the nucleation rate constant A and on the number N 0 of active sites on the titanium surface.

Characterization of the surface structure and composition of stainless steel 316 L after electrochemical roughening

SUMMARY The changes in morphology, structure and chemical composition of the surface of stainless steel 316 L roughened by an electrochemical treatment have been studied by means of profilometry, SEM and XPS methods. The steel was subjected to a two-stage electrochemical anodic treatment based on the use of electrolytes containing an organic acid and a mixture of inorganic acids, respectively. The anodic treatment caused an essential change in surface morphology and structure, this resulting in an increase of the averaged surface depth Rz from 0.28 to 2.58 μm. XPS depth profiling has shown that the iron and nickel concentrations in the surface layer of the roughened steel are slightly reduced.