Reactivity of electrogenerated free hydroxyl radicals and activation of dioxygen on boron-doped diamond electrodes

Synthetic boron-doped diamond (BDD) thin film is an electrode material with high chemical and dimensional stability, low background current and a very wide potential window of water stability. Upon anodic polarization, BDD generates hydroxyl radicals that mediate the oxidation processes in the vicinity of the electrode surface. These hydroxyl radials are assumed to be free, i.e., not adsorbed on the electrode surface. Hydroxyl radicals are formed on BDD during water discharge, which is the rate determining step of oxygen evolution reaction. Oxygen evolution on BDD occurs at a high overpotential (over 1 V) with respect to the thermodynamic potential for O2 formation (E0OER = 1.23 V vs. SHE), but very close to the thermodynamic potential of HO• formation (E0HO• = 2.38 V vs. SHE). In the presence of organics, the onset potential of HO• formation changes, which affects the current–potential curves. As a consequence, oxidation of most of organic compounds on BDD results in a shift of the current–potential curves toward lower potentials. An exception is acetic acid, which adsorbs on the electrode surface causing auto-inhibition of its oxidation (oxidation is shifted toward higher potentials). In general, however, oxidation of organics on BDD is a fast reaction, controlled by the mass transport of organics to the anode surface. In the presence of an organic compound, hydroxyl radicals initiate chain reactions resulting in involvement (activation) of molecular oxygen dissolved in aerated aqueous solution. Direct evidence for this process was found during oxidation of acetic acid solution saturated with isotopically labelled 18O2. As a result, C18O2 and C16O18O were evolved proving that molecular oxygen participates in the mineralization of acetic acid. This electrochemically induced activation process is probably initiated by organic free radicals (R•) formed during reaction between organic compounds and hydroxyl radicals. Organic free radicals react further with molecular oxygen to form peroxy radical (RO2•). Organic peroxy radicals are very reactive and can initiate subsequent chain reactions leading, via several possible intermediates, to complete oxidation of organic compound. In fact, similar mechanism has been reported in radiolysis (upon X-ray or γ-ray) of aqueous solutions of acetic acid. This non-faradaic enhancement of the electrooxidation processes opens the possibilities for designing a less energy consuming degradation of organic pollutants on BDD anodes.

Electrocatalysis induced by surface redox activities on conductive metal oxide electrodes

Stéphane Fierro

Iridium dioxide electrodes form part of the dimensionally stable anodes (DSA®) and this electrode material is widely used in many industrial processes namely water electrolysis, metal electro-winning, cathodic protection and electro-organic synthesis due to the high electrochemical activity and stability of this electrode material. IrO2-based electrodes can be prepared using different techniques but the most common is the thermal decomposition of H2IrCl6 precursor solution on an inert substrate like titanium. Within the water stability potential domain, the charging/discharging process is attributed to the slow diffusion of protons within the IrO2 coating together with the electrical double layer capacitance. In fact the valence state of the Ir surface atoms of the coating varies from +IV to +VI in the potential domain between the on-set potentials of H2 and O2 evolution. Concerning the oxygen evolution reaction, direct evidence was found that the IrO2 coating participates actively in the reaction using an electrolyte solution containing isotopically labeled H218O. In fact, measurements of the relative amounts of electrogenerated 16O2 and 18O16O have demonstrated that the hydroxyl radicals coming from water discharge interact strongly with IrO2 resulting in the formation of the higher oxide (IrO3) and the decomposition of that oxide produces oxygen. IrO3 is thus the intermediate involved in the OER on these electrodes. During the oxidation of organic compounds, direct evidence was found by marking the IrO2 electrode with 18O that the higher valence state oxide IrO3 participates effectively also in this process. In fact, the oxidation of a solution of formic acid on a marked IrO2 coating containing 18O has shown that C16O18O is evolved proving that the oxidation of organic compounds occurs on IrO3 with competing side-reaction of oxygen evolution. The low overpotential of the OER allows performing selective electro-oxidation of a wide variety of organic compounds. In fact, the competing side reaction of oxygen evolution 'buffers' the potential around values where the oxidation products are not further oxidized showing that the IrO2 electrode is particularly suited for electro-organic synthesis. However, the current efficiency of the oxidation process remains low due to the competing side reaction of oxygen evolution.

EPFL2010

Anodic oxidation of organics on thermally prepared oxide electrodes

Christos Comninellis, György Foti

Electrochem. oxidn. of orgs. at thermally prepd. dimensionally stable metal oxide anodes (DSA) has been studied. In reactions involving only simple electron transfer, oxide electrodes exhibit an electrocatalytic activity similar to that of noble metal electrodes. In oxidn. reactions of more complex mechanism, oxide anodes show no electrocatalytic activity below the potential of oxygen evolution and oxidn. of orgs. can take place only under conditions of simultaneous oxygen evolution. A simplified mechanism of orgs. oxidn. catalyzed by intermediates of oxygen evolution has been proposed distinguishing between two limiting electrode behaviors: (i) At "non-active" anodes (typically fully oxidized metal oxides) oxidn. of orgs. occurs at a high potential and leads to their combustion through physisorbed hydroxyl radicals. (ii) At "active" electrodes (IrO2, RuO2) the reaction takes place at lower potentials characteristic of the metal oxide and results in a selective oxidn. of the org. compd. at higher oxidized metal oxide surface sites. A kinetic model of orgs. oxidn. competed by oxygen evolution at "active" type anodes is also proposed and confirmed by preparative electrolysis. [on SciFinder (R)]

1997

Electrochemical promotion of Pt catalysts for gas phase reactions

Arnaud Jaccoud

The subject of this work is the study of the electrochemical promotion of catalysis (EPOC), also called non-Faradaic electrochemical modification of catalytic activity (NEMCA). In this phenomenon, application of small currents or potentials on catalysts in contact with solid electrolytes leads to very pronounced strongly non-Faradaic and reversible changes in catalytic activity and selectivity. This work concerns the system composed of platinum film electrodes, deposited on the YSZ electrolyte (ZrO2 doped with 8% Y2O3), in contact with an oxygen containing atmosphere (the O2(g),Pt/YSZ system). The aim of this work was to focus on two points in the phenomenon of EPOC: 1) The nature of the species present on the surface of the catalyst and their impact on the catalytic activity. 2) The phenomenon of permanent promotion of catalysis (P-EPOC), for which the catalytic rate enhancement is not totally reversible, leading to a long lasting activated state. The Pt electrodes were prepared via three different methods (sputtering, thermal decomposition and screen-printing) and, as expected, the resulting microstructure, observed by scanning electron microscopy (SEM), was found to depend greatly on the preparation technique. The electrochemical response of each electrode was characterized by the technique of cyclic voltammetry. Due to the occurrence of different site-related electrochemical reactions, the voltammogram shape was highly influenced by the microstructure of the Pt/YSZ contact. Based on the preliminary studies, the cermet electrode was chosen for additional electrochemical experiments, using chronoamperometry, chronopotentiometry, cyclic voltammetry, steady state polarization and impedance spectroscopy. The results of all these techniques suggest that under anodic polarization, two parallel reactions take place. The first reaction is the oxygen evolution, giving rise to a steady state current under a constant applied potential. The other reaction is the oxygen storage (oxygen injection) in the form of Pt-O species. This second process gives rise to a time dependant current under a constant applied potential, and is therefore responsible for the pseudocapacitive behavior of the electrode. Moreover, linear sweep voltammetry measurements indicated that, by application of an anodic potential, at least three types of Pt-O species were stored, following distinct kinetics. Based on the amount of stored Pt-O and on the corresponding storage kinetics, they were attributed to three different locations on the electrode: 1) at the Pt/YSZ interface, 2) diffusing from the tpb toward the Pt/gas interface, 3) diffusing from the Pt/YSZ interface toward the bulk of the platinum electrode. It has been shown that the Pt film exhibited various activity states toward the catalytic oxidation of ethylene, depending on the oxygen species present at its surface. As evidenced by XPS measurements and in agreement with the thermodynamic data for the given system, the inactive (deactivated) state has been attributed to the formation of surface platinum oxide at T > 550°C. Decreasing the reaction temperature to 525°C caused the slow spontaneous reactivation of the catalyst and several days were necessary to reach the active state, where most of the catalyst was present in its metallic state. The catalyst activity recovery could be accelerated by the application of anodic potential pulses. In agreement with the mechanism of EPOC, this was explained by the oxide destabilizing effect of the anodically produced Oδ- species. Anodic polarization also caused reversible electrochemical promotion of catalysis at unusual high temperature (600°C). In addition, very intriguing catalytic relaxation transients were observed after prolonged anodic polarization times. Indeed, it was found that enhanced catalytic activity could be maintained for up to 10 hours after current interruption. Combination of these latter results, the related literature, the state of the art model of EPOC, and the results obtained by electrochemical techniques led to the proposition of an original model based on the processes of Pt-O storage/release at various locations of the O2(g),Pt/YSZ system. According to this model, anodic polarization produces Oδ- back-spillover species, promoting the catalytic activity, in agreement with the mechanism of EPOC. In parallel, hidden oxygen species are stored at the Pt/YSZ interface and diffuse toward the Pt phase. When the polarization is switched off, these hidden oxygen species diffuse to the gas-exposed surface and cause non-Faradaic promotion, as back-spillover Oδ- do. The large amount of stored charge and its slow diffusion controlled emergence causes the rate enhancement to last for hours.