Restructuring of an Ir(210) electrode surface by potential cycling
Summary This study addresses the electrochemical surface faceting and restructuring of Ir(210) single crystal electrodes. Cyclic voltammetry measurements and in situ scanning tunnelling microscopy are used to probe structural changes and variations in the electrochemical behaviour after potential cycling of Ir(210) in 0.1 M H2SO4. Faceted structures are obtained electrochemically as a function of time by cycling at a scanrate of 1 V·s−1 between −0.28 and 0.70 V vs SCE, i.e., between the onset of hydrogen evolution and the surface oxidation regime. The electrochemical behaviour in sulfuric acid solution is compared with that of thermally faceted Ir(210), which shows a sharp characteristic voltammetric peak for (311) facets. Structures similar to thermally-induced faceted Ir(210) are obtained electrochemically, which typically correspond to polyoriented facets at nano-pyramids. These structures grow anisotropically in a preferred direction and reach a height of about 5 nm after 4 h of cycling. The structural changes are reflected in variations of the electrocatalytic activity towards carbon monoxide adlayer oxidation.
Changes in the electrochemical response of polycrystalline platinum electrodes promoted by fast repetitive square wave potential signals
There is a great interest in modifying the properties of solid electrodes to achieve their best performance under definite operating conditions. In this sense, at present the preparation, stabilization and reproducibili ty in the electrochemical response of solid electrode surfaces are subjects of special at tention [ 1--4] . A recent contr ibution in this respect that polycrystalline platinum electrodes immersed in acid electrolytes after being subjected to a fast repetitive triangular potential scanning under carefully selected potential perturbation conditions show remarkable changes in the potent iodynamic I/E profiles at v = 0.1 V s -I corresponding to the Hand O-adatom electroadsorption/electrodesorption potential region with respect to those of the initial polycrystalline material [ 5]. These results suggested that the same changes in the electrochemical response of platinum electrodes could be achieved by means of a fast repetitive square wave potential signal covering the potential range of the Hand O-adatom electroadsorption/electrodesorption. When a polycrystalline platinum electrode immersed in 1 M H2 SO4 at 25 ° C, which has been previously electropolished with ac (50 Hz; 10--20 V) in a slightly acid (HC1) saturated CaC12 solution, is subjected to a repetitive square wave potential signal (RSWPS) between 0.70 and 1.40 V (vs. RHE) at 0.5 kHz during 3 min, the voltammogram run immediately afterwards by applying a symmetrical triangular potential scan (RTPS) at 0.1 V s -1 , in the 0.04--1.50 V range (Fig. 1), approaches closely the characteristics of a Pt (111) single crystal surface after it has been cycled at a low potential sweep rate a few times through the anodic oxide region (Type-I electrode) [6--12] . Thus, the corresponding voltammogram shows the reversible conjugated current peaks of Hadatom electroadsorption/electrodesorption at ca. 0.11 V, a relative increase in height of the second broad anodic current peak associated with the O-elect roadsorption and a shift of the potential of the O-electrodesorption current peak towards more negative values than those found in comparable voltammograms for polycrystalline platinum. An appreciable increase in the electrode active area, as measured through either the H-adatom monolayer charge or the O-monolayer electroreduction charge, is also observed.
Characterization of ceramic composite membrane filters for hot gas cleaning
Abstract Novel composite ceramic membrane filters suitable for hot gas cleaning operations have been prepared from fly ash and titania particles on stainless steel woven mesh substrates. Both membranes had a structure like a solid packed bed with a thickness of less than 100 μm. In the domain of laminar flow, significant differences in the pressure drop between experimental results and those predicted from the Kozeny-Carman relationship were found, particularly for flow through the titania membrane. Membrane filters with extremely small pores such as the titania membrane are greatly affected by slip up to 80% in permeability at elevated temperatures. Efficiencies of filtration of typical coal fired power station exhaust fly ashes were found to be greater than 909% for both titania and ash membranes, whilst there was greater variation in the pressure drops in the ash membrane than in the titania at typical gas flows.