Effectiveness factor of isopropanol oxidation on IrO2 based electrodes of different loading

The influence of IrO2 loading on the effectiveness factor Et of the electrochemical oxidation of isopropanol was investigated A model has been proposed based on three main reactions electrochemical IrO2 oxidation to IrO3 chemical oxidation of the organic compound via IrO3 and O-2 evolution via decomposition of IrO3 It has been found that the relative effectiveness factor E-f for the electrochemical oxidation of IrO2 to IrO3 is loading independent contrary to the chemical reaction which decreases with increasing IrO2 loading (C) 2010 Elsevier Ltd All rights reserved

Effectiveness factor of thin-layer IrO2 electrocatalyst

Erika Herrera Calderon

Dimensionally Stable Anodes (DSA®) are electrodes composed of a metal support and an oxide coating (catalyst). Particularly in this thesis, the Ti/IrO2 electrode was used for the study of the fraction of the surface which participates effectively in the investigated reactions (effectiveness factor, Ef ). Four reactions with different kinetics have been investigated: (a) Fe3+/Fe2+ (fast reaction), (b) O2 and (c) Cl2 evolutions (slow reactions), and (d) isopropanol oxidation (complex reaction involving redox catalysis). This study has allowed us to apply for the first time in electrocatalysis the term, effectiveness factor, Ef , used before in other fields like in heavy metals recovery with 3D electrodes and in heterogeneous catalysis. On the other hand, in order to achieve the main objective of this thesis, an analytical and a qualitative approach was proposed. Furthermore, the Ti/IrO2 electrodes were characterized electrochemically using the cyclic voltammetry (CV) technique in which the voltammetric charge was measured at different potential scan rates, potential windows, and electrolyte temperatures using various Ti/IrO2 loadings. From the voltammetric charge, two contributions were found. The first contribution is related to the double layer capacitance and the second with the redox couples present on the electrode surface. The first reaction investigated in this work was the Fe3+/Fe2+ redox couple. In the first part of the investigation of this reaction, the CV technique was used; however, some issues related to the uncompensated electrolyte resistance were encountered. Therefore the effectiveness factor for this particular case was not possible to obtain. On the other hand, using rotating Ti/IrO2 disk electrodes (RDE) for the same reaction, the effectiveness factor, Ef , has been estimated, however, this value is much lower than the predicted one. The other investigated reactions in this work were O2 and Cl2 evolutions. The investigated kinetics of both reactions showed that the evolution of chlorine is much faster than that of oxygen. The effectiveness factor, Ef , for the oxygen evolution was close to the unity for all IrO2 loadings, contrary to the chlorine reaction that decreased with the loading. This behaviour was predicted according to what is given in the analytical approach. The last reaction investigated in this work was the oxidation of isopropanol. In this case a model was proposed based on three main reactions: Electrochemical IrO2 oxidation to IrO3, chemical oxidation of isopropanol via IrO3, and chemical decomposition of IrO3 to IrO2. The effectiveness factor, for the electrochemical reaction seemed not to correlate with the loading, whereas for the chemical one, it diminished with the loading.

EPFL2008

Control of electron transfer kinetics at boron-doped diamond electrodes by specific surface modification

Ilaria Duo

Diamond films with different levels of doping have been the subject of applications and fundamental research in electrochemistry, opening up a new branch known as the electrochemistry of diamond. The electrochemical properties of diamond mostly depend on the operating conditions during the deposition of the film and on the treatment of the surface. The absence so far of a standard procedure in the production and treatment of diamond films has created a wide range of diamond quality and properties. This work would try to clarify some aspects of the electrochemical activity of boron-doped diamond electrodes in light of the results obtained in our and other laboratories. Boron-doped diamond (BDD) electrodes, both as-grown and polarised anodically under different conditions, were prepared in order to study the chemical and electrochemical changes of diamond and clarify the rule played by the surface-state density. Many different treatments were employed, but only the most relevant results will be presented in this work, viz., those obtained with four kinds of surface treatment: as-grown (BDDag), mildly polarised (BDDmild), strongly polarised in perchloric acid (BDDsevere), and strongly polarised in a sulphuric acid-acetic acid mixture (BDDAcOH). Charge transfer processes at the electrode surface were studied by steady-state (polarisation) and dynamic (cyclic voltammetry) potential step methods. Simple electron transfer processes such as the outer-sphere redox system ferri/ferrocyanide (Fe(CN)6III/II) and complex charge transfer reactions such as the inner-sphere 1,4-benzoquinone/hydroquinone (Q/H2Q) redox reaction were chosen to test the electrochemical properties of the electrodes. The properties of the diamond electrodes were found to undergo strong modification as a function of surface treatment. The reaction rate constants decreased significantly upon anodic polarisation. Important drops in the carrier surface concentration and in true surface area led to hindrance of electron transfer at the electrode surface. The concept of non-diamond (sp2) impurities as charge transfer mediators on a diamond surface was suggested as an explanation for the electrochemical behaviour of diamond electrodes after surface oxidation treatment. To verify this concept, graphite particles were deposited on mildly polarised diamond electrodes (BDDmild) so as to prepare diamond-graphite composite electrodes (BDD-g), and their properties were compared with those of as-grown (BDDag) diamond and carbon electrodes. The sp2 coverage on BDD was estimated by cyclic voltammetry. A strong analogy existed between as-grown diamond electrodes and diamond-graphite composite electrodes. In fact, by depositing graphite particles on diamond after its deactivation by anodic polarisation is was possible to restore the initial properties of the as-grown diamond electrode. In the potential region of electrolyte decomposition, sp2 was eliminated from the BDD-graphite composite surface by oxidation to CO2. BDD itself behaved as an "non-active" electrode. In this case the oxidation of organic compounds was mediated by hydroxyl radicals formed during water discharge, without any participation of the electrode surface. The behaviour of "non-active" diamond in organic oxidation processes occurring in the potential region of water discharge was described in greater detail in the chapter about electrochemical oxidation of ethylenediaminetetraacetic acid (EDTA). The theoretical model developed for organic oxidation at "non-active" anodes was used to predict results for the oxidation of EDTA at boron-doped diamond electrodes. The very good fit between experimental results and theoretical values confirmed that EDTA oxidation at diamond in the potential region of electrolyte decomposition was a fast reaction involving free electrogenerated hydroxyl radical intermediates formed at the anode during water discharge. Nanoparticles of IrO2 were deposited by thermal decomposition on boron-doped diamond electrodes (BDD-IrO2) in order to improve and better understand their electrocatalytic activity toward redox processes and the oxygen evolution reaction (OER). Electrodes with different IrO2 loadings were prepared and their electrochemical properties tested. The reaction rate constants of redox processes increased by several orders of magnitude, the overpotential of the oxygen evolution reaction decreased by about one volt. The IrO2 deposition completely altered the electrochemical properties of diamond from an "non-active" to an "active" material. The high electrocatalytic activity of BDD-IrO2 electrodes toward the OER was also confirmed in an indirect way by the observation that the oxidation of organic substances and the production of peroxodisulphate are inhibited. The high degree of dispersion of the IrO2 particles led to a high electrocatalytic activity, even at extremely low IrO2 loadings. On the basis of experimental results a phenomenological model was proposed. Boron-doped diamond was defined as a degenerated semiconductor material because of the quite high charge density on its surface due to the boron doping (> 1021 boron atoms cm-3), to the non diamond (sp2) carbon and in general to surface species. This distribution of charges on the surface promoted processes of electron transfer across the interface diamond/electrolyte. Deposition of particles on the diamond surface (graphite or IrO2) increased further this surface charge density until reaching a perfectly metallic behaviour. Since charge transfer processes occurred on the deposited particles at lower overpotential than on diamond sites, surface modified diamond electrodes showed the electrochemical properties of the deposited material.

EPFL2003

Chemical and electrochemical promotion of supported rhodium catalyst

Olena Baranova

The chemical and electrochemical promotion of highly dispersed nanofilm Rh catalysts (dispersion: about 10 %, film thickness: 40 nm) has been investigated for the first time. To this end Rh metal was sputter-deposited, either on a purely ionic conductor (8 % Y2O3-stabilized ZrO2) or on a mixed ionic-electronic conductor (TiO2), the latter being a highly dispersed layer of TiO2 (4 µm) deposited on YSZ. These catalysts are designated as Rh/YSZ and Rh/TiO2/YSZ, respectively. It was established analytically that both in the Rh/YSZ and in the Rh/TiO2/YSZ system, the catalyst films have a nanoparticle-size grain structure. The Rh supported on titania is rather porous, exhibiting a higher dispersion and surface area than Rh on YSZ. Both after reduction (H2, T=400 °C) and after oxidation (O2, T=400 °C), Rh supported on TiO2 was found to be in a more highly reduced state than Rh on YSZ. After reducing treatment, the Rh/TiO2/YSZ samples contain a larger amount of weakly bonded oxygen, which can be attributed to oxygen "backspillover" from the TiO2. A major feature of this research was the electrochemical characterization of the oxygen/Rh/solid electrolyte three-phase boundary by steady-state polarization measurements and by impedance spectroscopy. These are powerful techniques for extracting experimental trends and details that are useful for an understanding of the electrochemical promotion principles. It was shown that the exchange current densities at the Rh/solid electrolyte interface are lower on account of the TiO2 layer. The exchange current densities are more than twice lower at Rh/TiO2(4 µm)/YSZ than at Rh/YSZ, demonstrating that the former interface is much more polarizable. The mechanism of oxygen exchange occurring close to equilibrium (O2/O2- couple) was investigated for the first time at Rh catalyst electrodes interfaced with solid electrolyte. The processes of cathodic oxygen reduction and anodic oxygen evolution are not symmetric, but they are similar in the two systems, Rh/YSZ and Rh/TiO2(4 µm)/YSZ. The cathodic process consists of three steps: dissociative adsorption of oxygen at the gas-exposed Rh surface, atomic oxygen diffusion to the electrochemical reaction sites (ERS), and a two-electron transfer to this oxygen on the ERS. The cathodic process is limited by interfacial diffusion of oxygen atoms from the gas-exposed metal surface to the ERS. The anodic process includes two steps: two-electron transfer reaction, which is the rate-determining step, and oxygen desorption to the gas phase. With data obtained from impedance spectroscopy at the equilibrium potential, it was possible to confirm the reaction scheme proposed. It was demonstrated that Rh nanofilm catalysts interfaced with YSZ or TiO2/YSZ can be electrochemically promoted for the reaction of ethylene oxidation. Small anodic currents cause periodic oscillations in catalytic rate and potential of the Rh/YSZ catalyst, while at Rh/TiO2/YSZ they give rise to a stable and reversible rate enhancement by up to a factor 80. The increase in ethylene oxidation rate is up to 2000 times larger than the electrochemical rate, I/2F, of O2- oxidation. The pronounced electrochemical promotion behavior that has been observed is due to the anodically controlled migration of O2- species from the electrolyte to the Rh/gas interface. At the Rh surface, these species destabilize the formation of rhodium surface oxide (Rh2O3). The existence of backspillover oxygen species has been confirmed by impedance measurements under positive applied potential. Another significant result for heterogeneous catalysis is the finding that thick as well as thin films of Rh/TiO2/YSZ catalyst are open to chemical promotion of ethylene oxidation and partial methane oxidation, ie, they offer a higher catalytic activity and stability than the Rh/YSZ catalysts. The modification of catalytic activity observed for Rh/TiO2/YSZ was attributed to either a "long-range" electronic-type SMSI mechanism or to a self-driven electrochemical promotion mechanism. In both cases, the ultimate cause of promotion are different work functions of catalyst and support. Equilibration of the work functions of two solids in contact induces surface charging, a migration of O2- ions to the catalyst/gas interface, and a weakening of the Rh-O chemisorptive bonds. It facilitates reduction of oxidized surface sites. Another important achievement of the present work was that of exploring and confirming the possibilities of current-assisted activation of Rh/TiO2/YSZ catalysts. In partial methane oxidation using close to stoichiometric gas compositions (CH4 : O2 = 2 : 1) at 550 °C, the inactive (oxidized) Rh/TiO2/YSZ catalyst was successfully activated by applied currents, either positive or negative. This phenomenon is an example of "permanent" electrochemical promotion furnishing a permanent rate enhancement ratio of γ = 11. The activation by negative currents is explained in terms of an electrochemical reduction of rhodium surface oxide, while the activation by positive currents can be explained by the mechanism of electrochemical promotion.

EPFL2005