Surface chemical bond experimental techniques

Abstract X-ray spectroscopy provides a number of experimental techniques that give an atom-specific projection of the electronic structure. When applied to surface adsorbates in combination with theoretical density functional spectrum simulations, it becomes an extremely powerful tool to analyze in detail the surface chemical bond. This is of great relevance to heterogeneous catalysis as discussed in depth for a number of example systems taken from the five categories of bonding types (i) atomic radical, (ii) diatomics with unsaturated n-systems (Blyholder model), (iii) unsaturated hydrocarbons (Dewar-Chatt-Duncanson model), (iv) lone-pair interactions, and (v) saturated hydrocarbons (physisorption). 

Some experimental techniques [e.g., low-energy electron diffraction (LEED)-surface crystallography] can detect the structural changes that occur on both sides of the surface chemical bond. However, most currently used techniques are only capable of detecting the structural changes that occur on the adsorbate side (e.g., infrared spectroscopy) or on the substrate side (e.g., electron microscopy). As a result, we often gain only incomplete information about the surface chemical bond, leading to a one-sided molecule-centric or surface-centric view of the adsorbate-surface compound that is produced. 

In this section we discuss briefly a few important experimental techniques used to obtain information about surface electronic structure. The four techniques that we describe have been selected because of their use to interpret features of surface chemical bonds. In the following section, we show that some of the commonly accepted interpretations are oversimplified and give misleading information. More importantly, we also present correct interpretations based on the analysis of the results of cluster calculations. In the present section, we will describe the physical properties that the techniques that we discuss are used to measure for a detailed discussion of the measurement techniques themselves, see, for example. Woodruff and Delchar. ... 

The surface area and the dimensions and volume of the pores can be determined in many ways. A convenient method is based on measurement of the capacity for adsorption. The experimental techniques do not differ from those used for chemisorption (see Section 3.6.3). The fundamental difference between physi.sorption and chemisorption is that in chemisorption chemical bonds are formed, and, as a consequence, the number of specific sites is measured, whereas in physisorption the bonds are weak so that non-chemical properties, in particular the surface area, are determined. 

Inspired by these Surface Science studies at the gas-solid interface, the field of electrochemical Surface Science ( Surface Electrochemistry ) has developed similar conceptual and experimental approaches to characterize electrochemical surface processes on the molecular level. Single-crystal electrode surfaces inside liquid electrolytes provide electrochemical interfaces of well-controlled structure and composition [2-9]. In addition, novel in situ surface characterization techniques, such as optical spectroscopies, X-ray scattering, and local probe imaging techniques, have become available and helped to understand electrochemical interfaces at the atomic or molecular level [10-18]. Today, Surface electrochemistry represents an important field of research that has recognized the study of chemical bonding at electrochemical interfaces as the basis for an understanding of structure-reactivity relationships and mechanistic reaction pathways. 

Several earlier review articles are relevant to our subject. Slichter reviews the work done in his laboratory [16], most of it concerned with atoms or molecules adsorbed on the metal clusters, and the experimental techniques used in such studies [17]. Duncan s review [9] pays special attention to the C NMR of adsorbed CO. Most recently, one of us has given a rather detailed review of the held, in particular on metal NMR of supported metal catalysts [18]. While the topics and examples discussed in this chapter will inevitably have some overlap with these previous reviews, particular emphasis is directed toward highlighting the ability of metal NMR to access the iff-LDOS at both metal surfaces and molecular adsorbates. The iff-LDOS is an attractive concept, in that it contains information on both a spatial (local) and energy (electronic excitations) scale. It can bridge the conceptual gap between localized chemical descriptors (e.g., the active site or the surface bond) and the delocalized descriptors of condensed matter physics (e.g., the band structure of the metal surfaces). 

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