Kinetics surface analysis techniques

Recently, the steady-state reaction kinetics of CO oxidation at high pressure over Ru , Rh " , Pt, Pd, and Ir single crystals have been studied in our laboratory. These studies have convincingly demonstrated the applicability and advantages of model single crystal studies, which combine UHV surface analysis techniques with high pressure kinetic measurements, in the elucidation of reaction mechanisms over supported catalysts. 

It should be noted that this quantifleation approach assumes that the sample is homogeneous in depth. If this were the case however, the use of a surface analysis technique would not be Justifled. The approximation involved is applied in the absence of other information that can be used to describe the depth distribution of the elements. In particular, if a surface contamination layer (for example, atmospheric hydrocarbons) is present on the sample this will influence the intensity of the peaks to an extent which depends on the energy of the pho-toclcctron (through the dependence of X on the kinetic energy) and thus the clement. 

Photoelectron spectroscopy provides a direct measure of the filled density of states of a solid. The kinetic energy distribution of the electrons that are emitted via the photoelectric effect when a sample is exposed to a monochromatic ultraviolet (UV) or x-ray beam yields a photoelectron spectrum. Photoelectron spectroscopy not only provides the atomic composition, but also information concerning the chemical environment of the atoms in the near-surface region. Thus, it is probably the most popular and useful surface analysis technique. There are a number of forms of photoelectron spectroscopy in common use. 

X-ray photoelectron spectroscopy (XPS). This is a surface analysis technique whereby X-ray radiation excites the core electrons of surface atoms, and such electrons are emitted each with a characteristic kinetic energy which is analyzed in the spectrometer to provide a spectrum as seen in Figure 4.29. There is overlap of some bands which requite some deconvolution before assignments are made of peaks to structures. 

A unique pilot plant/minlreactor/surface analysis system has been designed and put Into operation. This system represents the closest encounter reported In the literature to date between "real world" catalysis and-surface analytical techniques. It allows In depth studies of reaction kinetics and reaction mechanisms and their correlation with catalyst surface properties. 

X-Ray Photoelectron Spectroscopy (XPS). This technique is also known as electron spectroscopy for chemical analysis (ESCA), and as this name implies, it is a surface analytical technique. At present it is probably the most versatile and generally applicable surface spectroscopic technique. It is called XPS because of the type of beam used to study the interfacial region, that is, X-rays. These X-rays consist of monochromatic radiation—radiation of a given energy—emitted by a metal target bombarded by an electron beam of several kiloelectron volts of kinetic energy... 

Applicability of Surface Compositional Analysis Techniques for the Study of the Kinetics of Hydride Formation... 

The four sections above followed the typical workflow of screening approaches of the early days. In recent years, it became evident that additional steps have to be added on top of the workflow. One such step is the analysis of the true kinetics and the interplay of its elemental reactions by modem surface-science techniques. 

Photoelectron spectroscopy of valence and core electrons in solids has been useful in the study of the surface properties of transition metals and other solid-phase materials. When photoelectron spectroscopy is performed on a solid sample, an additional step that must be considered is the escape of the resultant photoelectron from the bulk. The analysis can only be performed as deep as the electrons can escape from the bulk and then be detected. The escape depth is dependent upon the inelastic mean free path of the electrons, determined by electron-electron and electron-phonon collisions, which varies with photoelectron kinetic energy. The depth that can be probed is on the order of about 5-50 A, which makes this spectroscopy actually a surface-sensitive technique rather than a probe of the bulk properties of a material. Because photoelectron spectroscopy only probes such a thin layer, analysis of bulk materials, absorbed molecules, or thin films must be performed in ultrahigh vacuum (<10 torr) to prevent interference from contaminants that may adhere to the surface. 

In chemical reaction kinetics, isotope-labelled reactants are frequently employed to follow a reaction pathway and to determine the reaction mechanism (see Chapter 7.6). The isotopic tracer technique is a useful tool in catalyst surface analysis, because it enables determination of whether the adsorbed species present on the surface during the reaction are by-products or reaction intermediates. One of the adsorbed species is labelled by an isotope atom and its rate of disappearance is followed by surface spectroscopy. Simultaneously, its rate of appearance in the product molecule is followed by mass spectrometry. When both rates are identical, it can be concluded that the observed adsorbed species is the reaction intermediate. 

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