Ottavio Lugaresi

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.

Fixed energy X-ray absorption voltammetry.

In this paper, the fixed energy X-ray absorption voltammetry (FEXRAV) is introduced. FEXRAV represents a novel in situ X-ray absorption technique for fast and easy preliminary characterization of electrode materials and consists of recording the absorption coefficient at a fixed energy while varying at will the electrode potential. The energy is chosen close to an X-ray absorption edge, in order to give the maximum contrast between different oxidation states of an element. It follows that any shift from the original oxidation state determines a variation of the absorption coefficient. Although the information given by FEXRAV obviously does not supply the detailed information of X-ray absorption near edge structure (XANES) or extended X-ray absorption fine structure (EXAFS), it allows to quickly map the oxidation states of the element under consideration within the selected potential windows. This leads to the rapid screening of several systems under different experimental conditions (e.g., nature of the electrolyte, potential window) and is preliminary to more deep X-ray absorption spectroscopy (XAS) characterizations, like XANES or EXAFS. In addition, the time-length of the experiment is much shorter than a series of XAS spectra and opens the door to kinetic analysis.

Benzyl Chloride Electroreduction on Ag Cathodes in CH3CN in the Presence of Small Amounts of Water: Evidences of Quantitative Effects on Reaction Rates and Mechanism

In this work, we show our most recent results on the reductive dehalogenation of organic chlorides in acetonitrile + water media on silver electrodes, whose impressive catalytic power toward this reaction have already been extensively demonstrated and discussed. We adopt benzyl chloride as model molecule because of its wide use in previous literature. The results show that the presence of increasing amount of water to pure acetonitrile deeply and quantitatively modifies not only the peak potential (and thus the apparent reaction kinetics) but also the reaction mechanism.

In Situ Dispersive EXAFS in Electrocatalysis: The Investigation of the Local Structure of IrOx in Chronoamperometric Conditions as a Case Study

An in situ study with dispersive EXAFS (Extended X-Ray Absorption Spectroscopy) at the Ir- edge is performed to characterize Electrodeposited Iridium Oxide Films (EIROF) under chronoamperometric conditions. The technique monitors the local chemical environment and electronic structure of iridium during the oxidation of Ir(III) to Ir(IV) with a time resolution of milliseconds. The study is performed in both acidic and basic media. The Fourier transforms of the time-resolved EXAFS signals clearly show that the short-range structure of Ir is similar to that of rutile-type IrO2 and is maintained during the reaction, thus accounting for the flexibility of the structure of the electrode material in accommodating different oxidation states. From a more general point of view, the work demonstrates the capabilities of in situ experiments based on state-of-the-art dispersive EXAFS in clarifying the mechanistic aspects of electrochemical processes.