Sandra Rondinini

Dynamic potential–pH diagrams application to electrocatalysts for water oxidation

The construction and use of “dynamic potential–pH diagrams” (DPPDs), that are intended to extend the usefulness of thermodynamic Pourbaix diagrams to include kinetic considerations is described. As an example, DPPDs are presented for the comparison of electrocatalysts for water oxidation, i.e., the oxygen evolution reaction (OER), an important electrochemical reaction because of its key role in energy conversion devices and biological systems (water electrolyses, photoelectrochemical water splitting, plant photosynthesis). The criteria for obtaining kinetic data are discussed and a 3-D diagram, which shows the heterogeneous electron transfer kinetics of an electrochemical system as a function of pH and applied potential is presented. DPPDs are given for four catalysts: IrO2, Co3O4, Co3O4 electrodeposited in a phosphate medium (Co–Pi) and Pt, allowing a direct comparison of the activity of different electrode materials over a broad range of experimental conditions (pH, potential, current density). In addition, the experimental setup and the factors affecting the accurate collection and presentation of data (e.g., reference electrode system, correction of ohmic drops, bubble formation) are discussed.

Silver as a powerful electrocatalyst for organic halide reduction: The critical role of molecular structure

Abstract The remarkable electrocatalytic properties of silver for organic halide reductions, related to its strong specific interactions with halide ions, and therefore modulated by the surface state and by the nature of the supporting electrolyte, have been shown by us recently. The key role played by the molecular structure is now described together with its effect on the reaction pathway, in terms of not only the intrinsic R⋯X reactivity modification but also of elements connected to the heterogeneous nature of the process, including the accessibility of the leaving group and the possible presence of adsorption auxiliary groups stabilizing the postulated R⋯X⋯Me intermediate. The present discussion is supported by: (a) cyclovoltammetric investigations on a wide set of aliphatic and aromatic halides performed on silver, mercury and glassy carbon; and (b) a systematic program of preparative electroreductions carried out on variously configurated haloadamantanes in different operating conditions. The haloadamantane case, yielding a mixture of adamantane and dimers in a ratio heavily affected by the operating conditions, is very appropriate for an elucidation of the factors favouring monoelectronic dimerisation versus bielectronic halogen replacement by hydrogen.

In Situ Identification of Intermediates of Benzyl Chloride Reduction at a Silver Electrode by SERS Coupled with DFT Calculations

Aiming to deeply understand the electrocatalytic mechanism of silver on reduction of benzyl chloride, we carried out an in situ electrochemical surface-enhanced Raman spectroscopic study to characterize various surface species in different electrode potential regions. A further analysis with DFT calculation reveals that the benzyl radical and its anionic derivate bonded on a silver electrode are the key intermediates, implying that the pathway could drastically differ from the outer sphere concerted electron reduction at inert electrodes.

Electrocatalytic potentialities of silver as a cathode for organic halide reductions

The peculiar halide affinity for silver results in an extraordinary electrocatalytic activity for the reduction of halides (either glycosyl halides or, more generally, aryl and alkyl halides). The most striking features are: (a) a reduction potential shift in the positive direction of about 1000 mV with respect to glassy carbon and 500 mV with respect to mercury; (b) a cage effect, evidenced in previous synthetic work concerning bromosugars, promoted by the halide acting as a bridge between the electrode surface and the reacting substrate, which mainly results in dimerization and/or addition products. The above electrocatalytic effect is here investigated by means of a systematic reactivity study on Ag, Hg and glassy carbon cathodes, with a variety of substrates. The effect of the supporting electrolyte is also analysed in detail, providing a first inspection on specific halide/silver interactions in acetonitrile media.

Observing the oxidation state turnover in heterogeneous iridium-based water oxidation catalysts

In this work the oxidation states assumed by Ir in oxide systems used as heterogeneous catalysts for water oxidation are determined by means of in situ X-ray Absorption Spectroscopy (XAS). Using a highly hydrated iridium oxide film allows the maximum number of Ir sites to be involved in the electrochemical processes occurring at the catalysts during water oxidation (oxygen evolution reaction, OER). X-ray Absorption Near Edge Structure (XANES) spectra clearly indicate the co-existence of Ir(III) and Ir(V) at the electrode potentials where OER occurs. This represents a fundamental step both in the understanding of the water oxidation mechanism catalysed by heterogeneous Ir oxide systems, and in the possible tailoring of electrocatalysts for OER.

Bridging the Gap between Electrochemical and Organometallic Activation: Benzyl Chloride Reduction at Silver Cathodes

Integration of voltammetry, surface-enhanced Raman spectroscopy (SERS), and density functional theory (DFT) has allowed unraveling the mechanistic origin of the exceptional electrocatalytic properties of silver cathodes during the reduction of benzyl chloride. At inert electrodes the initial reduction proceeds through a concerted direct electron transfer yielding a benzyl radical as the first intermediate. Conversely, at silver electrodes it involves an uphill preadsorption of benzyl chloride onto the silver cathode. Reduction of this adduct affords a species tentatively described as a distorted benzyl radical anion stabilized by the silver surface. This transient species rapidly evolves to yield ultimately a benzyl radical bound onto the silver surface, the latter being reduced into a benzyl-silver anionic adduct which eventually dissociates into a free benzyl anion at more negative potentials. Within this framework, the exceptional electrocatalytic properties of silver cathodes stem from the fact that they drastically modify the mechanism of the 2e-reduction pathway through a direct consequence of the electrophilicity of silver cathode surfaces toward organic halides. This mechanism contrasts drastically with any of those tentatively invoked previously, and bridges classical electroreduction mechanisms and oxidative additions similar to those occurring during organometallic homogeneous activation of organic halides by low-valent transition-metal complexes.

Microcrystalline cellulose powders: structure, surface features and water sorption capability

Characterization of an Avicel PH 102 MCC powder through polymerization degree determination, X‐ray diffraction and N2 adsorptions at subcritical temperatures is described. Specific characterizations of MCC/H2O exchange in different environments were performed by means of thermogravimetric weight losses on time/temperature scale and analyses of water adsorption/absorption from saturated vapors at 310 K. The reversibility of MCC/H2O interactions and the influence of different ‘surface area probes’ on the specific surface area are discussed.

Screening of oxygen evolution electrocatalysts by scanning electrochemical microscopy using a shielded tip approach

Oxygen evolution electrocatalysts in acidic media were studied by scanning electrochemical microscopy (SECM) in the substrate generation-tip collection (SG-TC) imaging mode with a 100 microm diam tip. Pure IrO2 and Sn(1-x)Ir(x)O2 combinatorial mixtures were prepared by a sol-gel route to form arrays of electrocatalyst spots. The experimental setup has been developed to optimize screening of electrocatalyst libraries under conditions where the entire array is capable of the oxygen evolution reaction (OER). The activity of individual spots was determined by reducing the interference from the reaction products of neighboring spots diffusing to the tip over the spot of interest. A gold layer deposited on the external wall of the SECM tip was used as a tip shield. In this study the shield was kept at a constant potential to reduce oxygen under mass transfer controlled conditions. The tip shield consumes oxygen coming from the neighbor spots in the array and enables the tip to correctly detect the activity of the spot below the tip. Simulations and experimental results are shown, demonstrating the effectiveness of the tip shield with the SG-TC setup in determining the properties of the composite materials and imaging arrays.

Surface screening effects by specifically adsorbed halide anions in the electrocatalytic reduction of a model organic halide at mono- and polycrystalline silver in acetonitrile

Abstract The silver surface screening effects by specifically adsorbed halide anions in the electrocatalytic reduction of a model organic bromide (acetobromoglucose) have been studied by cyclic voltammetry on controlled mono- and polycrystalline silver surfaces in acetonitrile+0.1 M tetraethylammonium perchlorate medium as a function of the concentration c X of added TEAX (X=Cl, Br, or I, TEA, tetraethylammonium). The reduction peak potentials, E p , are regularly shifted in the negative direction with increasing c X , typically tending to an asymptotic value for c X ≈0.1 M. Several literature models describing adsorption/desorption equilibria have been applied to justify the above experimental E p versus c X trends (being logarithmic in the iodide cases) for the three halides and the four silver surfaces tested.

Glycosyl Halides as Building Blocks for the Electrosynthesis of Glycosides

The importance and variety of glycosides in nature together with the lack of a conclusive synthetic methodology prompted us to study the electrochemical reductive behavior of glycosyl halides to generate suitable intermediates for the glycosidation of both aliphatic and aromatic acceptors. A special case considered here is the almost quantitative synthesis of C-C glycosyl dimers.