Volkmar M. Schmidt

Electrochemical surface reactions of intermediates formed in the oxidative ethanol adsorption on porous Pt and PtRu

The oxidative ethanol adsorption and electrochemical surface reactions of adsorbed intermediates were studied on electrodeposited Pt, Ru, Pt0.92Ru0.08 and Pt0.85Ru0.15 in 1 M HClO4. A flow cell procedure was performed in order to separate surface reactions of intermediates formed in the oxidative ethanol adsorption from those reactions for ethanol in the bulk electrolyte solution. Oxidation and reduction reactions of adsorbed species were studied by potential-controlled electrodesorption spectrometry. No adsorbate formation was observed on a pure Ru electrode. In all cases, CO2 was the sole product formed in the oxidative electrodesorption of the adsorbates. Using 12CH312CH2OH and 12CH313CH2OH, the oxidation reaction pathways of each C-atom to CO2 were followed. On the other hand, methane and ethane were detected during the reductive electrodesorption of adlayer species. The onset potential for these reduction products shifts to more negative potentials as the Ru content of the alloy increases. The methane to ethane yield ratio decreases in the sequence Pt>Pt0.92Ru0.08>Pt0.85Ru0.15. A critical interpretation of the experimental data leads to a tentative yield of different adsorbed species as function of the Ru surface concentration.

Quartz crystal microbalance and probe beam deflection studies of poly(1-hydroxyphenazine) modified electrodes

Abstract Quartz crystal microbalance (QCM) and probe beam deflection (PBD) were used to evaluate the exchange of mobile species (ions and solvent) during the redox reaction in poly(1-hydroxyphenazine) in aqueous acidic media. Both techniques indicate clearly two separate ion exchange steps with successive uptake and expulsion of ions and solvent molecules. The PBD and QCM results are consistent with a redox mechanism where protons are inserted in the first reduction step and anions are expelled in the second reduction step. During oxidation the process is inverted. At fast scan rates or potential jump experiments, however, QCM indicates a transient insertion of excess anions in the first oxidation step with a corresponding successive ion pair expulsion. Anion exchange at the more negative potentials involves a counter flux of solvent. Mass balance shows that the water content of the film is higher in its reduced state. The different processes are rationalized on the basis of electron/ion transfer coupled with protonation/deprotonation reactions and a potential pH diagram for the stability domains of the various states of the polymer is suggested.

Adsorption and oxidation of acetylene and ethylene on gold electrodes

Abstract A comparative study was made on the reactivity of acetylene and ethylene in acids (0.05 M H 2 SO 4 and 0.1 M HClO 4 ) on polycrystalline Au electrodes. On-line mass spectrometry (DEMS) and in-situ Fourier Transform IR spectroscopy were used to identify reaction products. It was found that the interaction of the reactant with the electrode | electrolyte interface depends strongly on the electronic structure of the hydrocarbon. Acetylene forms a chemisorbed intermediate in a wide potential range above the potential of zero charge. Only CO 2 was observed during oxidation of both adsorbed and dissolved acetylene. On the other hand, ethylene interacts more weakly with the Au surface and no strongly bonded adsorbate was found. Acetaldehyde, acetic acid and CO 2 were detected as the main oxidation products of dissolved ethylene.

Electrochemical reactions of ethene on polycrystalline Au electrodes in acid solution studied by differential electrochemical mass spectrometry and isotope labelling

The electro-oxidation and electroreduction of ethene on polycrystalline Au were studied by means of differential electrochemical mass spectrometry (DEMS). It was found that the electroreduction of ethene in 0.1 M HClO4 + H2O yielded ethane whereas in DClO4 + D2O deuterated ethane C2H4D2 was detetected. The electro-oxidation of C2H4 was accompanied by a large signal for mz = 44 in the mass spectrometer owing to the formation of both CO2 and acetaldehyde. Using C2D4 in H2O + HClO4 the production of CO2 and ethanal was distinguished. Experiments with C2H4 in DClO4 + D2O demonstrated that all hydrogen atoms in ethanal belong to the initial C2H4 molecule, thus indicating an internal rearrangement during ethene electro-oxidation.

On the adsorption of acetylene on gold in acid media studied by DEMS

One of the first studies concermng the mteractlon of acetylene with noble metal electrodes was made m the slxtles by Bockrls et al on polycrystallme Pt [l] They found a coulomblc efficiency for CO, production from acetylene oxldatlon of 100% In analysmg stationary current-potential curves the rate-determining step was proposed between the adsorbed acetylene molecules and adsorbed water However, a correspondmg study on gold has shown that acetylene was only partially oxldlzed to CO, with a coulomblc efficiency of about 60% [2] Chemical analySIS and IR spectra of the reaction products have mdlcated the absence of CHO and C=O species The most probable by-product seemed to be a polymer structure havmg conjugated (s=C bonds and -OH groups [2] Different results were reported by Cwlkhnskl et al [3,4] who have found partial oxldatlon products such as CH,COOH, CHOCHO, CH,OHCHO and HCHO formed after the anodlc oxldatlon of aqueous solutions of acetylene They proposed a stable Au-C,H, complex as an intermediate mvolvmg a preferential corrosion of the gold anode The structure of adsorbed acetylene on gold electrodes was studied by Weaver et al [5-71 using surface enhanced Raman spectroscopy (SERS) Multiple bands were observed m the triple bond region These frequencies were assigned to u-r alkyne-metal complexes m which the acetylene molecule lies flat on the surface Recently, it has been shown that the reactivity and adsorbate formation on gold electrodes depends

Electro-oxidation of propene on gold in acid solution studied by DEMS and FTIRS

Abstract The oxidation of propene in aqueous HClO 4 solution was studied on polycrystalline Au electrodes using differential electrochemical mass spectrometry (DEMS) and in-situ Fourier transform IR spectroscopy (FTIRS). Experimental results show that the interaction of propene with the electrode|electrolyte interface is characterized by a weak adsorption since no chemisorbed intermediates could be found. Different reaction pathways were observed during the electro-oxidation of propene. Acetone was identified as the main product of the intact molecule chain. The detection of ethanal and carbon dioxide indicates fission of a CC bond in a reaction pathway parallel to propene electro-oxidation.