Atomic spectroscopy empirical analysis

Theoretical models based on first principles, such as Langmuir s adsorption model, help us understand what is happening at the catalyst surface. However, there is (still) no substitute for empirical evidence, and most of the papers published on heterogeneous catalysis include a characterization of surfaces and surface-bound species. Chemists are faced with a plethora of characterization methods, from micrometer-scale particle size measurement, all the way to angstrom-scale atomic force microscopy [77]. Some methods require UHV conditions and room temperature, while others work at 200 bar and 750 °C. Some methods use real industrial catalysts, while others require very clean single-crystal model catalysts. In this book, I will focus on four main areas classic surface characterization methods, temperature-programmed techniques, spectroscopy and microscopy, and analysis of macroscopic properties. For more details on the specific methods see the references in each section, as well as the books by Niemantsverdriet [78] and Thomas [79]. 

Quantitative elemental analysis in electron spectroscopy is similar to that in X-ray spectroscopy. Analysis quantifies the concentrations of chemical elements on a sample surface from the peak intensities of the spectra. In theory, the quantitative relationship between the intensities of electron signals and atomic fractions of elements can be calculated. In practice, for quantification in both XPS and AES, most parameters for calculations are not available. Thus, the following empirical equation is commonly used. 

The application of the above rules to all kinds of compounds has been made in order to exploit the use of infrared spectroscopy as a method of structural analysis. Empirically, it has been observed a correlation between position of certain band maxima and the presence within a molecule of organic functional groups or of particular structural features within the skeleton of molecules (see Table 10.1). This property comes from the fact that each organic functional group corresponds to a collective-type of several atoms. For a given bond, the force constant k (expression 10.3) does not vary significantly from one molecule to another. 

The chemical composition is usually expressed as the empirical formula of the active components, for example, H3 jMo4VP30,. Determination ofthe chemical composition by wet chemical analysis, atomic absorption spectroscopy, or X-ray fluorescence can reveal errors in the production process (e.g., weighing of starting materials), but it gives no information on catalyst performance. 

The realization that electronic interactions play a significant role in polyenes came via the experimental observations by Hudson and Kohler (1972) that the dipole-forbidden 2 Ag state lies below the dipole-allowed PS state. Further extensive studies of polyene oligomers by Kohler confirm the hypothesis that the relaxed energy of the 2 Ag state lies below the relaxed energy of the state. An analysis of oligomer spectroscopy from 6 to 16 carbon atoms suggests the empirical relation (Kohler 1988),... 

Detailed interpretation of mass spectra is beyond the scope of this book. Also, although taken together, combustion analysis and mass spectrometry can give the molecular mass and empirical and molecular formulae, they do not give us any further information about the compound under study, such as the linkage of the atoms within the molecule. How we solve this will be dealt with in Part 2, Spectroscopy (on the CD-ROM). 

Groups of atoms or particular kinds of bonds which differ from groups or bonds in other parts of the molecule in their mass or type, respectively, or in their position in the molecule, display, in a series of similar compoimds, absorption bands which occur in approximately the same part of the spectrum. The frequencies of certain absorptions can be designated as characteristic of the appropriate groups or bonds. This fact, observed empirically and also accounted for theoretically, is the basis of qualitative analysis by means of infrared spectra, which makes it possible to identify certain groups of atoms or even whole molecules of unknown structure. Characteristic frequencies of certain bonds are usually listed in monographs or in work in the literature dealing with infrared and Raman spectroscopy. 

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