Andrzej Wieckowski

Aqueous Electrochemical Synthesis of YBa2Cu3 O 7 − x Superconductors

This paper examines aqueous solutions containing yttrium, barium, and copper ions for the feasibility of obtaining superconducting films (YBa{sub 2}Cu{sub 3}O{sub 7{minus}x}) via electrochemical methods. The research considered co-precipitation of all three ions simultaneously as hydroxides. After high-temperature treatment, the films showed superconducting characteristics. However, low and broad transition temperatures were observed when measuring the magnetic susceptibility temperature dependence.

The Role of Surface Defects in CO Oxidation, Methanol Oxidation, and Oxygen Reduction on Pt ( 111 )

Some surface reactions of interest to electrocatalysis in alkaline media are promoted by crystalline defects, while others occur preferentially on defect-free terraces. Different forms of structure sensitivity, and the underlying causes of this structure sensitivity, have been examined using several fuel-cell-relevant surface reactions in alkaline media as model reactions. Oxidation of CO serves as a model for defect favored reactions, while reduction of oxygen serves as a model for terrace favored reactions. More complicated reactions, such as methanol oxidation, can be interpreted as containing multiple steps that are either defect favored or terrace favored. The role of defects in each of these reactions was interpreted in terms of geometric and electronic effects, with different types of defects kink type and step type showing different effects for the different electrocatalytic processes. CO oxidation is promoted by both step-type and kink-type defects, as a result of electronic structure, but methanol dehydrogenation is promoted only by step-type defects, as a result of geometric structure.

Chapter 12 – Resonance Elastic and Inelastic X-ray Scattering Processes for In-situ Investigation of Electrochemical Interfaces

Publisher Summary nThis chapter discusses the resonance elastic and inelastic x-ray scattering processes for the in-situ investigation of electrochemical interfaces. X-ray-in and x-ray-out methods with high-energy and high-brilliance x-rays are some important techniques that can be used for the study of electrochemical interfaces. The most widely used technique is x-ray absorption spectroscopy (XAS). An advantage of the x-ray techniques is that they are highly versatile. They have been powerful in case of both single-crystal electrode interfaces and nano-structured electrochemical interfaces. In addition, they serve as promising tools for the surfaces of nanoparticles. This chapter elaborates on the potential new x-ray applications beyond ordinary XAS. Most techniques are possible only because of third-generation synchrotron x-ray sources. The scattering probability of x-rays at an atom can be enhanced when the photon energy is close to an absorption edge of the atom. This is similar to classical resonance phenomena. The discreteness of electronic energy levels in atoms or molecules is inherently quantum mechanical in nature. A quantum mechanical description of the resonance phenomena has also been reviewed in the chapter.

Coverage Dependence of CO Surface Diffusion on Pt Nanoparticles - an EC-NMR Study

We have studied the effects of CO coverage on surface diffusion rates of CO adsorbed on nanoparticle Pt catalysts in sulfuric acid media by using 13 C electrochemical nuclear magnetic resonance spectroscopy (EC-NMR) in the temperature range 253 - 293 K. For CO coverage from θ = 1.0 to 0.36, the diffusion coefficients follow Arrhenius behavior and both activation energy ( E d ) and pre-exponential factor ( D co ) show CO coverage dependence. Ed increases from 6.0 to 8.4 kcal/mol and DCO varies from 1.1 X 10 -8 to 3.7 X 10 -6 cm 2 /s when the coverage is increased from θ = 0.36 to θ = 1.0. On the Pt catalyst surface at partial CO coverage, our data strongly support the free site hopping model of adsorbed CO as the major surface diffusion mechanism, unlike the situation found with a fully CO covered surface where CO exchange between different surface sites is believed to be the major diffusion mechanism. Our results also indicate that the contributions of lateral repulsive interactions exert a stronger influence on the diffusive motion than does the nature of the surface structure. When the diffusion coefficient was estimated from CO stripping measurements by using an electrochemical modeling protocol, the estimated diffusion coefficients were a few orders of magnitude larger than those obtained from the EC-NMR experiments. Overall these results are important for improving our understanding of electrochemical surface dynamics of molecules at interfaces, and may help facilitate better control of fuel cell reactions where the presence of surface CO plays a crucial role in controlling the reaction rates.