CATALYSIS UNDER TRANSIENT CONDITIONS study

Our main motivation to develop the specific transient technique of wavefront analysis, presented in detail in (21, 22, 5), was to make feasible the direct separation and direct measurements of individual relaxation steps. As we will show this objective is feasible, because the elements of this technique correspond to integral (therefore amplified) effects of the initial rate, the initial acceleration and the differential accumulative effect. Unfortunately the implication of the space coordinate makes the general mathematical analysis of the transient responses cumbersome, particularly if one has to take into account the axial dispersion effects. But we will show that the mathematical analysis of the fastest wavefront which only will be considered here, is straight forward, because it is limited to ordinary differential equations dispersion effects are important only for large residence times of wavefronts in the system, i.e. for slow waves. We naturally recognize that this technique requires an additional experimental and theoretical effort, but we believe that it is an effective technique for the study of catalysis under technical operating conditions, where the micro- as well as the macrorelaxations above mentioned are equally important. 

In situ spectroscopic measurements of a catalytic system provide a considerable opportunity to determine the chemical species present under reactive conditions. FTIR and NMR have been the two most frequently used in situ spectroscopic methods (see Chapters 2 and 3). They have been successfully used to identify labile, non-isolatable transient species believed to be involved in the catalytic product formation. Furthermore, efforts have been made to use this information in order to obtain more detailed kinetics, by decoupling induction, product formation, and deactivation. Thus, in situ spectroscopic techniques have the potential for considerably advancing mechanistic studies in homogeneous catalysis. 

With the inclusion of more sophisticated techniques such as those mentioned above and others, NMR is an excellent quantitative tool for structural catalyst characterization. On the other hand, the question of how this information can be used to understand catalytic mechanisms and to design more potent catalysts often remains unresolved by these studies. Perhaps this is so because to date most NMR applications have sought to correlate catalytic activities with specific structural features present either in the catalyst or on its surface under room temperature conditions. In future studies there should be increasing emphasis on catalyst characterization under operation conditions in situ, including the search for transient adsorbates and reactive intermediates. In fact, such studies are now emerging in other fields of catalysis [90-92]. 

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