X-ray Absorption Spectroscopy Studies of Electrochemical ... The spectroscopic methods cited above provide

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  • X-ray Absorption Spectroscopy Studies

    of Electrochemical Processes

    by

    Ornella Smila-Castro

    Supervisor: Prof. Trevor Rayment

    A thesis submitted to The University of Birmingham for the degree of

    DOCTOR OF PHILOSOPHY

    School of Chemistry

    University of Birmingham

    December 2010

  • University of Birmingham Research Archive

    e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder.

  • Acknowledgements

    I would like to extend my gratitude to the following people for their help, guidance and

    encouragement over the course of my research.

    First, to Prof. Trevor Rayment for providing with the opportunity to work under his

    supervision, and for his constant optimism for facing the challenges of this study. Beyond this

    work, he has been a very caring person. I am looking forward to another debate on French vs.

    British puddings (although, is there any debate...). It has been a pleasure and an honour,

    Trevor.

    Then, my gratitude goes to Prof. William Geiger from the University of Vermont. My time in

    his laboratory brought me knowledge and guidance in inorganic electrochemistry. I would

    like to thank him as well for his precious help beyond my visit.

    I would also like to thank Dr. Sarah Horswell for helping me in times where I really needed it.

    Members of staff of the synchrotron radiation sources that I’ve had the chance to work at

    during this work: staff at the SRS, Daresbury, Swiss Light Source and Diamond Light Source.

    In particular to Dr. Messaoud Harfouche (Swiss Light Source), Dr. Andy Dent (Diamond

    Light Source), and Dr. Giannantonio Cibin (Diamond Light Source).

    This work would not have been possible without the immense efforts of the members of the

    Workshops. Particular appreciation is given to Tony Rothin (mechanical) and Andy Tanner

    (electrical) for the construction of the spectroelectrochemical cells and associated parts, often

    in unreasonably short timescales!

    The EPSRC for financial support.

    My friends that have been here all the way through, and the ones I met here in Birmingham

    for their support and encouragement. They’ll know who they are.

    I would like now to finish this page in my own mother-tongue language:

    Mille mercis á mes parents Chantal et Maurice, à mes trois soeurs Audrey, Odélia, et

    Alexandra pour leur soutien continu durant ces quatre années d’études.

  • Abstract

    Electron transfer is a key part of many chemical, biological and physical processes, that is

    commonly studied by electrochemical methods, which give insight into reaction mechanisms

    but no structural information. It is necessary to combine electroanalysis with another

    technique to gain essential knowledge of metal-ligand bond length and oxidation states. X-ray

    absorption spectroscopy (XAS) can provide these data for species in dilute solution and, if

    combined with electrochemistry, could potentially provide powerful insight into electron

    transfer reactions.

    This dissertation describes the development and application of techniques for the study of

    electrochemical intermediates by XAS.

    Chapters 2 and 3 introduce the theory and practice of electrochemistry and spectroscopy with

    emphasis on XAS. Chapter 4 describes the development of variable-temperature

    spectroelectrolysis cells for the study of electrochemical intermediates. In Chapter 5, the

    electrochemical behaviour of Cp * Rh(CO)2, is investigated as an organometallic compound

    representative of the redox chemistry studied in this thesis. Chapter 6 describes a new

    approach to the study of electrochemical intermediates in which a miniature electrolysis cell is

    combined with a microdispenser so that electrochemical intermediates can be generated and

    then dispensed, quenched at low temperature prior to study by XAS. Chapter 7 contains final

    conclusions.

  • Table of Contents

    Chapter 1: Introduction

    Introduction 2

    References 5

    Chapter 2: Electrochemical theory

    2.1 Introduction 8

    2.2 the Electrical double Layer 8

    2.3 The electrochemical cell and the three-electrode system 14

    2.4 Interfacial kinetics 17

    2.5 Mass transport 22

    2.6 Experimental considerations 28

    References 35

    Chapter 3: X-ray Absorption Spectroscopy

    3.1 Introduction 38

    3.2 The principle of interaction of X-rays with matter 38

    3.3 The EXAFS formula 43

    3.4 Data analysis 53

    3.5 Synchrotron Radiation Sources 56

    References 65

    Chapter 4: Low temperature studies of Ruthenocene

    4.1 Introduction 70

    4.2 Systems under study 71

    4.3 Designs of X-ray spectroelectrochemical cells 81

    4.4 Experiments at the SRS 94

    4.5 Spectroelectrochemical cell 2 110

    References 130

  • Chapter 5: Electrochemical behaviour of Cp * Rh(CO)2

    5.1 Introduction 134

    5.2 Experimental 135

    5.3 Voltammetry studies of Cp * Rh(CO)2 137

    5.4 Conclusions for studies at 298 K 144

    5.5 Voltammetry studies of Cp * Rh(CO)2 at low temperature 145

    5.6 Exhaustive Electrolysis 150

    5.7 In situ IR Spectroelectrochemistry 160

    5.8 NMR Spectroscopy 164

    5.9 Conclusions 167

    References 168

    Chapter 6: Development of a microdispenser for XAS

    6.1 Introduction 171

    6.2 Understanding the microdispenser 173

    6.3 Electrochemical characterisation of the microdispenser