Situ Non-Vibrational Characterization Techniques to Analyse Oxidation Catalysts and Mechanisms

In Situ Non-Vibrational Characterization Techniques to Analyse Oxidation Catalysts and Mechanisms 

Angelika BRUCKNER, Evgenii KONDRATENKO, Vita KONDRATENKO, Jorg RADNIK and Matthias SCHNEIDER  

Catalytic oxidation processes are usually connected with transfer of electrons and changes of structure and valence state of active catalyst components. This chapter presents methods that are especially suitable for monitoring these kinds of changes (UV-vis-DRS, EPR, X-ray scattering, XPS, XAS, TPO, TPR, TPRS, TAP and SSITKA). After a short section on basic principles and experimental details, the potential of each technique is illustrated by selected application examples that include a wide variety of oxidation catalysts such as mixed metal oxides and oxynitrides, zeolites containing transition metal ions, heteropoly acids and supported noble metals. 

While vibrational in situ methods such as Fourier transform infrared (FTIR) and Raman spectroscopy are excellent tools for monitoring the formation of organic intermediates and vibrations of molecular moieties in the catalyst lattice and/or svuface, these techniques usually fail to detect changes of structure and valence states of transition metal ions (TMI), which are very often part of the active sites 

Temperature-prograimned reduction, oxidation and desorption (TPR, TPO, TPD), belong probably to the most widely used in situ techiuques for the characterization of oxidation catalysts and are discussed in more detail in Section 19.4. While TPD (with ammonia as the probe molecule) is frequently used to examine surface acid sites, TPR and TPO (with H2 or O2, respectively) provide information on the redox properties of oxide catalysts being crucial for their performance in catalytic oxidation reactions. Important information on reaction mechanisms can be obtained when the catalysts are heated in the presence of reactants combined with mass spectrometric product analysis. This is called temperature-programmed reaction spectroscopy (TPRS). As far as reaction mechanisms and kinetics are concerned, transient techniques which reflect the response of the catalytic system to a sudden change of reactant are inevitable tools. Two such techiuques, namely the temporal analysis of products (TAP) reactor and steady-state isotopic transient kinetic analysis (SSITKA) will be described in more detail in Section 19.5. 

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