M. I. Borzenko

Platinization assisted by Keggin-type heteropolytungstates

Abstract Chronoamperometry and cyclic voltammetry (CV) were used to address the platinum deposition kinetics on glassy carbon and polycrystalline Pt supports from H 2 PtCl 6 -containing solutions with and without additives of different Keggin-type heteropoly acids (HPAs) of tungsten. The effect of addition of these polyoxometallates manifested itself in the pronounced disturbance of instantaneous nucleation and it can be interpreted in terms of strong HPA adsorption on certain nucleation centers followed by the modification of their activity. HPA additives decreased the current efficiency for Pt deposition despite the fact that the reduction of HPA additives was thermodynamically forbidden at the potentials chosen for electrodeposition. The latter fact can be explained as selective inhibition of the intermediate Pt II reduction by tungstates. CV of Pt deposits in sulfuric acid solutions indicated a noticeable increase in the deposit roughness if P- and Si-containing HPA were added in the course of platinization. More complex behavior was found for B -dodecatungstate that most likely originated from its tendency to undergo codeposition with Pt. Ex situ scanning tunneling microscopy (STM) images demonstrate more pronounced globular structure and smaller crystal sizes for HPA-affected deposits as compared with usual platinized platinum. These features can be explained by HPA effects on deposition kinetics assumed on the basis of electrochemical data and demonstrate that HPA can act in template-like manner.

Reduction of an Ensemble of Chloride Aquacomplexes of Platinum(II): An Analysis in the Framework of Phenomenological Approach

A general approach is offered to a phenomenological analysis of experimental data on the kinetics of an electrode reaction complicated by chemical stages. The approach is based on a consideration of the way the process rate depends on the supporting-electrolyte concentration at a constant electrode charge. The possibilities of the approach are illustrated by the example of the reduction of chloride complexes of Pt(II) in the presence of chloride, indifferent (in the sense of inner-sphere substitution), and mixed supporting electrolytes. The performed analysis generalizes the Frumkin–Petrii relationship, which determines the average charge of the single reactant in the solution bulk. A system may contain an ensemble of reactants. In this case, in addition to the bulk properties of the system, similar relationships take into account the interface structure and the balance between partial rate constants for parallel reactions involving reactants of different chargeness.

Co-adsorbtion of Cu and Keggin type polytungstates on polycrystalline Pt: interplay of atomic and molecular UPD

Second harmonic generation (SHG), electrochemical quartz microbalance (EQCM), and cyclic voltammetry are applied to clarify the structure and properties of Cu adlayers formed in the presence of Keggin polytungstate anions. For 0.02-10 mM CuSO4 solutions, no pronounced suppression of underpotential copper deposition (Cu UPD) by 0.1-10 mM H3PW12O40 (PW12) or H4SiW12O40 (SiW12) is observed in electrochemical experiments. Moreover, coadsorption with polyanions results in an increase of charge in the Cu UPD region. EQCM data demonstrate high surface coverage with polytungstate in the overall potential range and their pronounced co-adsorption with Cu2+ cations under open circuit. The unusual potential dependence of EQCM response of polytungstates is discovered and discussed in terms of anion interactions with adsorbed hydrogen. The SHG response of Cu UPD demonstrates a non-linear dependence on Cu surface coverage, which is interpreted in terms of discontinuous submonolayers consisting of 2D Cu islands. The additives of PW12 or SiW12 decrease copper SHG response at low and high CuSO4 concentrations, with minor effect for a mid range of concentrations. In all mixed solutions, the potential dependence of the SHG response remains typical for Cu UPD, not for polytungstates. SHG transients measured under potential step mode demonstrate that the initial non-steady-state SHG behavior of the adlayer is more close to the behavior of polytungstates, but typical copper features appear at longer wavelength. These facts favor the hypothesis of Cu adatom penetration through anionic adlayers and formation of a metal submonolayer at the vacant areas between large quasi-spherical polyanions, with subsequent transformation into a Pt/Cu/polytungstate layered structure.

Speciation in strongly acidic metastable solutions of oxotungstate(vi) compounds

The ionic composition of long-lived metastable aqueous solutions of oxo compounds of tungsten(IV) with pH ≤ 1 was studied by spectrophotometry. The nature of some isopolytung-states formed in these solutions in the presence of various inorganic acids (HCl, HClO4, HNO3, H2SO4, and H2SeO4) was established.

Determining the integral dissolution rate of two-phase alloys in cryolite-alumina melts

Electron microscopy and x-ray diffraction analysis are used in studying Cu-Fe and Ni-Fe-Al alloys upon their anodic polarization in a cryolite-alumina melt, thus, clarifying the regularities of their dissolution and the formation of oxide coatings. Based on the results obtained, a technique of determining the integral corrosion rate of the alloys is proposed.