Z. Nagy

Applications of surface X-ray scattering to electrochemistry problems

Abstract Applications of the synchrotron X-ray scattering technique to electrochemistry problems are briefly reviewed ranging from submonolayer level phenomena, through nanometer size phenomena, to submicron size phenomena; that is, covering the full range of the ‘interphase’ at an electrode surface. The examples include, (i) incipient oxidation/reduction of platinum single crystal surfaces; (ii) submonolayer/monolayer level oxidation/reduction of ruthenium dioxide single crystal surfaces; (iii) copper passivation/depassivation; and (iv) anodic formation of porous silicon and silicon dioxide layers. The design of several X-ray/electrochemical cells is also described.

Chloride Ion Catalysis of the Copper Deposition Reaction

The accelerating effect of anions for electrode reactions has been known for a long time, but it is much less appreciated that these effects can sometimes be caused by traces of anions. The authors have demonstrated that the Cu{sup ++}/Cu{sup +} reaction is strongly catalyzed by trace amounts of chloride ions in the solution. The Cu{sup +}/Cu reaction was found to be unaffected by chloride ions. These experimental results were also substantiated by theoretical calculations. The authors have investigated the electronic coupling for homogeneous electron-transfer reactions that are approximate models for electron transfer in the copper deposition: (1) outer-sphere reaction (water-water bridge), and (2) innersphere reaction (chloride bridge). For Cu{sup ++}/Cu{sup +} electron transfer the authors found increased coupling for the chloride bridge, which they attribute to the closer approach found for this complex compared to the water bridge, while for Cu{sup +}/Cu electron transfer, coupling was not increased for the chloride bridge reaction.

In‐situ x‐ray reflectivity study of incipient oxidation of Pt(111) surface in electrolyte solutions

Electrochemical oxidation causes the lifting of Pt atoms of the surface layer, substantiating a place‐exchange mechanism. Furthermore, for a charge transfer of ≲1.7e−/Pt atom, the flat surface is recovered by reduction, while the surface is irreversibly roughened by more excessive oxidation. Roughening involves only the atoms in the top layer.

Temperature Dependence of the Heterogeneous Ferrous‐Ferric Electron Transfer Reaction Rate: Comparison of Experiment and Theory

This paper describes experiments on the temperature dependence of the rate of the ferrous-ferric electron transfer reaction at a gold electrode and compare them with a detailed molecular dynamics simulation which is used to predict the rate. We find from the experiments that the temperature dependence of the rate has the Arrhenius form over the temperature range from 25 to 275{degrees} C, and that the transfer coefficient is independent of temperature in this range. The molecular dynamics simulations are used to two ways to extract activation energies and transfer coefficients for comparison with experiment. In one of these methods, we assume parabolic dependence of the energies for the product and reactant in a reaction coordinate which is not specified a priori. In the other method, we use a quantum mechanical calculation extrapolated from the very short molecular dynamics time scale to times characteristic of the electron transfer rate. The assumption of parabolic dependence of the energies gives an estimate for the activation energy which is consistent with experiment. The transfer coefficient calculated using this assumption is also consistent with experiment.

ELECTROCHEMICAL AND X-RAY SCATTERING STUDY OF WELL DEFINED RUO2 SINGLE CRYSTAL SURFACES

Abstract Electrochemical and synchrotron surface-X-ray scattering measurements were performed on two low index faces, (110) and (100), of RuO 2 single crystal electrodes in a variety of solutions. The two crystal faces displayed uniquely different cyclic voltammograms and the structural changes associated with cyclic voltammograms were investigated with synchrotron surface-X-ray scattering measurements. In sulfuric acid solution, the cyclic voltammogram of the (100) surface exhibits a reduction signature near the hydrogen evolution potential. The reduction feature was found to be associated with an expansion of the top ruthenium layer approximately along the (110) direction. The same reduction feature was seen much less clearly at the (110) surface. However, the associated displacement of ruthenium atoms is very similar to that of the (100) surface, leading to the conclusion that the oxygen bonds on the surface are elongated by a chemical reaction of the oxygen atoms with hydronium molecules in solution. In NaOH solution, the cyclic voltammogram of the (110) surface indicates two clearly identifiable oxidation features near the oxygen evolution potential. We find that the oxidation features are associated with the clearly identifiable surface-structure changes and these structure models are presented. The structure models and corresponding charge-transfer amounts were consistent with the pH-dependent cyclic voltammograms and the super-nernstian behavior measured by adding phosphoric acid to the solution.

Cell design for in—situ X-ray scattering study of electrodes in transmission geometry

A novel electrochemical cell that permits in—situ X-ray scattering studies in transmission geometry for the examination of electrode/electrolyte interfaces is described. The advantages of this geometry over others used presently are discussed, and initial experimental results are reported on the electrochemical formation of passive oxide film on silver.

The electrochemical oxidation of 4-bromoaniline, 2,4-dibromoaniline, 2,4,6-tribromoaniline and 4-iodoaniline in acetonitrile solution

The electrochemical oxidation of 4-bromo-, 2,4-dibromo-, 2,4,6-tribromo- and 4-iodoanilines was investigated in acetonitrile solution. Based on the experimental results, it can be stated that the Bacon–Adams mechanism (J. Am. Chem. Soc. 90 (1968) 6596) is the main electrochemical oxidation route in acetonitrile solution similar to the 4-chloroanilines. In the cases of 4-bromo- and 2,4-dibromoanilines, the substituent in the para position eliminated in the dimerisation step, following its oxidation to bromine, which quantitatively substitutes the free ortho position of the starting bromoaniline. As a consequence of this, oxidised form of brominated 4-amino-diphenylamines is formed besides brominated anilines. Halogenisation did not take place at the oxidation of 2,4,6-tribromo- and 4-iodoaniline because the rejected halide ion can only be oxidised to elemental halogen (bromine and iodine). In the case of all bromo monomer compounds, the role of ‘head-to-head’ coupling was found negligible (azobenzene type compounds). 4,4′-Diiodo-azobenzene has been observed as by-product in traces at 4-iodoaniline oxidation. The electrochemical oxidation of 4-bromoaniline resulted in the ‘ortho-coupling’ product in a considerable less degree in contrast with 4-chloroaniline. The oxidation products prepared by controlled potential electrolysis (CPC) were isolated and identified by different techniques: GC-ECD (gas chromatography), GC–MS (gas chromatograph–mass spectrometry), and ES-MS (electrospray mass spectrometry). Cyclic voltammograms of the 4-halogenoaniline type substrates and n-tetrabutylammonium halides, have been recorded before and after the CPC electrolysis, and were also used for product identification.

Theory and experiment on the cuprous–cupric electron transfer rate at a copper electrode

We describe results of experiment and theory of the cuprous–cupric electron transfer rate in an aqueous solution at a copper electrode. The methods are similar to those we reported earlier for the ferrous–ferric rate. The comparison strongly suggests that, in marked distinction to the ferrous–ferric case, the electron transfer reaction is adiabatic. The model shows that the activation barrier is dominated by the energy required for the ion to approach the electrode, rather than by the energy required for rearrangement of the solvation shell, also in sharp distinction to the case of the ferric–ferrous electron transfer at a gold electrode. Calculated activation barriers based on this image agree with the experimental results reported here.

The electrochemical oxidation of 4-chloroaniline, 2,4-dichloroaniline and 2,4,6-trichloroaniline in acetonitrile solution

Abstract The electrochemical oxidation of 4-chloro-, 2,4-dichloro- and 2,4,6-trichloroanilines was investigated in acetonitrile solution. Based on the experimental results, it can be stated that the mechanism introduced by J. Bacon and R.N. Adams (J. Am. Chem. Soc. 90 (1968) 6596) describing the voltammetric oxidation of 4-substituted aniline derivatives in acidic aqueous media is the main electrochemical oxidation route in acetonitrile as well. It is shown, that in the cases of 4-chloro- and 2,4-dichloroanilines, the substituent in the para position is not only eliminated at the electrochemically initiated dimerization step, but it was oxidized to chlorine, which substitutes the free ortho position of the starting chloroaniline. As a consequence of that oxidized form of chlorinated 4-amino-diphenylamines is formed also besides chlorinated anilines. 2-Amino-4′,5-dichloro-diphenylamine has been found as a product of oxidation at the oxidation of 4-chloroaniline. This can be explained by dimerization of 4-chloroaniline at the ortho position by the one electron oxidized substrate. In the case of both monomer compounds, the role of ‘head-to-head’ coupling found negligible (azobenzene type compounds). Chlorination did not take place at the oxidation of 2,4,6-trichloroaniline, because the rejected chloride ion can be oxidized only to chlorine. The oxidation products prepared by controlled potential electrolysis (CPC), were isolated and identified by different techniques (GC-ECD, GC-MS and ES-MS: gas chromatography, gas chromatograph-mass spectrometry and electrospray mass spectrometry). Cyclic voltammograms of the chloroaniline type substrates and n -tetrabutylammonium chloride have been recorded before and after the CPC electrolysis, and were also used for product identification.

Catalytic effect of under-potential deposited layers on the ferrous/ferric outer-sphere electrode reaction

The influence of the electrode metal on the kinetics of outer-sphere redox reactions is still an unsettled question with contradictory results being re indicate the complete absence of an effect, while there is also experimental evidence that underpotential deposited (UPD) metal layers can influence th suggested that some of the reported catalytic effect may be experimental artifacts caused by anion adsorption on the electrodes. We have carried out a UPD-metal layers on the rate of the Fe2+/Fe3+ redox reaction on polycrystalline gold and platinum electrodes in perchloric acid solutions rigor with copper, silver, and bismuth UPD layers. Our experimental results indicate clearly that the catalytic effect of UPD-metal layers on outer-sphere re since the phenomenon persisted in rigorously purified solutions. We also suggest a possible theoretical explanation of this phenomenon. In a recent stu at a gold surface to be non-adiabatic, i.e. electronic coupling contributed significantly to the rate. In the work reported here we have investigated, effect observed for this reaction at a gold electrode with a UPD-copper monolayer may be due to changes in electronic coupling. We have found that, whi UPD-copper layer, the observed catalytic effect may still be due to a larger electronic coupling because the hydrated ion approaches closer to the UPD- electronic coupling and the rate of the reaction.