Heterogeneous catalysis electron spectroscopy study

A researcher in the field of heterogeneous catalysis, alongside the important studies of catalysts chemical properties (i.e., properties at a molecular level), inevitably encounters problems determining the catalyst structure at a supramolecular (textural) level. A powerful combination of physical and chemical methods (numerous variants x-ray diffraction (XRD), IR, nuclear magnetic resonance (NMR), XPS, EXAFS, ESR, Raman of Moessbauer spectroscopy, etc. and achievements of modem analytical chemistry) may be used to study the catalysts chemical and phase molecular structure. At the same time, characterizations of texture as a fairytale Cinderella fulfill the routine and very frequently senseless work, usually limited (obviously in our modem transcription) with electron microscopy, formal estimation of a surface area by a BET method, and eventually with porosimetry without any thorough insight. 

The author discusses selected examples of Auger electron spectroscopy applications to the study of the role played by sulfur adsorption in the field of heterogeneous catalysis and its implication for the metallurgical problems of segregation, surface self-diffusion and temper embrittlement. 

For such studies, both electrochemical and nonelectrochemical experimental techniques have been developed. Several of them are outlined here electrosorption methods, surface electron spectroscopies, and isotopic-mass spectrometric techniques, linking electrocatalysis to conventional heterogeneous catalysis. The spectroscopic and isotopic methods have been recently applied to a limited number of simple electrocatalytic systems. The exciting results that these methods have provided demonstrate their power for future electrode reaction studies. 

Application of the Mossbauer effect, which is essentially a bulk phenomenon, to the study of surfaces has received significant attention in recent years. The usefulness of this technique lies in its ability to determine the electronic environment and symmetry of the surface nucleus, and it offers a method of investigation that is clearly complementary to other physical methods for the characterization of solid surfaces. Mossbauer spectroscopy has the attractive advantage that it may be used at a variety of pressures and can be applied to the study of heterogeneous catalysis and adsorption processes to probe the nature of the solid surface and its electronic modification when holding adsorbed species. 

The structure and reactivity of ethylene chemisorbed on transition-metal surfaces are of fimdamental importance in surface science and heterogeneous catalysis. HREELS has been foremost among the surface characterization techniques employed in fact, the first vibrational spectroscopic study of ethylene chemisorbed on Pt(lll) was carried out with electron energy-loss spectroscopy (EELS) almost a decade before IRAS was employed. ... 

The use of synchrotron based in situ x-ray absorption spectroscopy (XAS) for the study of catalysis, both heterogeneous and electrocatalysis has matured over the last decade with simultaneous efforts in the United Sates, European Union and Japan. Some recent exemplification of the state of the art can be obtained in the following references, " and an extensive database of literature on its application to catalysis can be accessed electronically (www.exafs.chem.msu.su/ papers). Detailed aspects on application of the technique and methodology used for data analysis has been recently published. " ... 

As early research on oscillatory reactions in heterogeneous catalysis began, little attention was given to the state of the catalyst surface. These first studies recorded the reaction rate by analysis of the product concentrations (see, e.g.. Refs. 3,81) or by measurement of catalyst temperatures 3,162). Later, however, attempts were also made to monitor the catalyst surface during the oscillations, first by measurement of the work function 81), and later by methods such as infrared (IR) spectroscopy 108) and low-energy electron diffraction (LEED) for HV oscillations 245). Table III lists methods employed to study oscillations. 

In spite of the close theoretical relationship between EPR and NMR spectroscopy, EPR has only very narrow applications. The primary reason for this is that the EPR phenomenon is spectroscopically silent unless there are unpaired electrons. Most biological macromolecules are closed shell molecules and contain no unpaired electrons. Therefore, EPR is of little real value for biological macromolecular structure characterisation. The only exception to this rule is that certain prosthetic groups in proteins may contain redox active metal centres/clusters that have transient or even permanent unpaired electrons (see Chapter 4). These metal centres/ clusters can be studied by EPR spectroscopy in order to demonstrate the presence of unpaired electrons. Thereafter, EPR data may then be used to derive the relative structural arrangements of metals within centres or clusters, and to assign putative distributions of redox states should there be any obvious redox heterogeneity. EPR is also useful to detect transient or even metastable radical formation during bio catalysis (see Chapter 8). 

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