Palladium electrodeposits: Dependence of structure and sorption properties on the deposition potential

A series of Pd electrodeposits (edPd) on Pt substrates is prepared at deposition potentials of -0.05 to 0.55 V with respect to a reversible hydrogen electrode in 0.5 M H₂SO₄. Their nanostructure is characterized by scanning tunneling microscopy. The size distribution of particles is estimated, and dependences of its maximum and half width on the deposition potential are determined. A comparative coulometric study of adsorption of copper and oxygen on edPd shows that real surface areas determined from these data substantially differ. The average size of particles for edPd, estimated within the model of equal-size spheres, is shown to be incorrect. The assumption that particles in the deposits essentially coalesce is substantiated. It is shown that the equilibrium hydrogen content in thea and β hydrides is anomalously high for the deposits whose growth was accompanied by deep hydrogenation of Pd. At a given effective pressure, for the α-phase, this value is always substantially higher as compared with less defective materials. In the β-phase, the hydrogen concentration can be either lower or higher     

Solid state features of electrocrystallized tungstate films

X-ray diffraction, scanning electron microscopy, and infra-red spectroscopy are applied to study the evolution of films electrodeposited from acidic tungstate solutions. Structural inhomogeneity is found to be responsible for the difference in rechargeability of films of different thickness. Voltammetric responses demonstrate pronounced sensitivity to the nature of crystalline phases, thus throwing light on the defects of the lattice features of nonstoichiometric W(V)–W(VI) oxides. One crystalline phase observed in the films under study and attributed to the layered nonstoichiometric oxohydroxide was never reported for oxotungstate films fabricated by other techniques. This phase is believed to be special to electrocrystallized films and to keep some structural features of dissolved isopolytungstate molecular precursors.Dedicated to Prof. G.Horanyi with the highest respect to his experimental talents, remarkable papers and important contributions in various fields of modern electrochemistry.     

Formation of Rechargeable Films on Platinum in Sulfuric Acid Solutions of Isopolytungstates

Two types of redox processes, with the formation of (a) dissolved reduced forms of tungstates and (b) solid products, occur on a platinum electrode in metastable long-lived true solutions of isopolytungstates (pH 0.5) in the hydrogen adsorption region. The solid products accumulate on the platinum surface as crystalline films capable of reversible recharging. Basic parameters of the films are determined using cyclic voltammetry, x-ray diffractometry, and scanning tunneling microscopy. Based on a spectrophotometric analysis of solutions, redox conversions, observed in the hydrogen range of potentials, are referred to dissolved hepta- and decatungstates present in the system.     

Electrooxidation of Organic Substances on Platinum Modified with Oxotungstate Adlayers

The behavior of a platinized platinum electrode (roughness factor 500) in 0.5 M H₂SO₄ containing Na₃PW₁₂O₄₀ (PW₁₂) is studied by cyclic voltammetry. It is established that the effect of the PW₁₂ additive on the adsorption of hydrogen and oxygen is similar to the influence exerted by specifically adsorbed anions: a predominant displacement of strongly-bound hydrogen and a heavy suppression of oxygen adsorption are observed. It is shown that the regularities of oxidation of methanol, ethylene glycol, and formic acid at platinized platinum undergoe substantial alteration in the presence of PW₁₂. In the region of potentials E _r = 0.6–0.8 V, where the limiting stage of these processes is the adsorption of organic molecules, the oxidation rates substantially decrease. The run of steady-state polarization curves in the region E _r = 0.40–0.55 V depends on the direction of potential change and the extreme value of E _r. With E _r varied from 0.6–0.8 V in the direction of lower values in the interval E _r < 0.45–0.50 V, the oxidation processes accelerate as compared with the rates in the absence of PW₁₂. The acceleration occurs if conditions are created for a preliminary adsorption of the tungstate.     

Platinum electrodeposits on glassy carbon: The formation mechanism, morphology, and adsorption properties

The deposition of platinum on glassy carbon (GC) is studied by chronoamperometry. Basic tendencies of the formation of aggregate platinum particles on the oxidized carbon surface are established. These include a primary instantaneous nucleation of platinum under diffusion control and the beginning of a secondary nucleation prior to filling primary active centers. The deposit morphology is examined byex situ methods of scannng electron microscopy (SEM), transmission electron microscopy (TEM), and scanning tunneling microscopy (STM). A globular structure of platinum, formed by crystallites 3–5 nm in size, is revealed. A comparison of the STM, SEM, and TEM data demonstrates a high information value and accuracy of STM in studies of disperse materials in both nanometer and submicron ranges. Various coulometry techniques intended for the determination of the true surface area of deposited platinum are compared. The most informative techniques are the voltammetry of desorption of copper adatoms and chemisorbed carbon monoxide at, respectively, low and high platinum contents. Differences in the formation kinetics and properties of aggregate particles in Pt/GC and Pt/Pt are found, specifically, smaller Pt/GC crystallites and higher degrees of their concrescence (screening)     

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.