The major objective of this research project was to reach a microscopic understanding of the structure, function and dynamics of V-Mo-(W) mixed oxides for the partial oxidation of acrolein to acrylic acid. Different model catalysts (from binary and ternary vanadium molybdenum oxides up to quaternary oxides with additional tungsten) were prepared via a solid state preparation route and hydrochemical preparation of precursors by spray-drying or crystallisation with subsequent calcination. The phase composition was investigated ex situ by XRD and HR-TEM. Solid state prepared samples are characterised by crystalline phases associated to suitable phase diagrams. Samples prepared from crystallised and spray-dried precursors show crystalline phases which are not part of the phase diagram. Amorphous or nanocrystalline structures are only found in tungsten doped samples. The kinetics of the partial oxidation as well as the catalysts' structure have been studied in situ by XAS, XRD, temperature programmed reaction and reduction as well as by a transient isotopic tracing technique (SSITKA). The reduction and re-oxidation kinetics of the bulk phase have been evaluated by XAS. A direct influence not only of the catalysts' composition but also of the preparation route is shown. Altogether correlations are drawn between structure, oxygen dynamics and the catalytic performance in terms of activity, selectivity and long-term stability. A model for the solid state behaviour under reaction conditions has been developed. Furthermore, isotope exchange experiments provided a closer image of the mechanism of the selective acrolein oxidation. Based on the in situ characterisation in combination with micro kinetic modelling a detailed reaction model which describes the oxygen exchange and the processes at the catalyst more precisely is discussed.
Oxidation of acrolein to acrylic acid on Mo/V/W-mixed oxide catalysts has been studied by means of the isotopic exchange method steady-state isotopic transient kinetic analysis (SSITKA). As a result of these isotopic exchanges performed with 16 O 2 and 18 O 2 , some new mechanistic details could be obtained. Acrolein exchanges its oxygen with oxygen of the mixed oxide catalyst. Additionally, the kinetics of the different isotopologues is significantly influenced by oxygen exchange reactions between the surface and the bulk sites. From the SSITKA results, an extended reaction model has been deduced. Thus, the experimentally gained data of the individual isotopologue kinetics could be verified by modeling.
Mittels der Isotopenaustauschmethode SSITKA (steady‐state isotopic‐transient kinetic analysis) ergaben sich Rückschlüsse zum Mechanismus der Partialoxidation von Acrolein zu Acrylsäure an Mo/V/W‐Mischoxidkatalysatoren. Der Isotopenaustausch erfolgte mit den beiden Sauerstoffisotopen 16O2 und 18O2. Dabei findet am Katalysator ein Austausch des Carbonylsauerstoffs im Acrolein mit Festkörpersauerstoff statt. Zusätzlich wird die Kinetik der unterschiedlichen Isotopologenspezies durch eine Austauschreaktion von Sauerstoff zwischen belegten und unbelegten Zentren an der Oberfläche und in der Volumenphase des Katalysators signifikant beeinflusst. Gestützt durch einen erweiterten Modellvorschlag zum Mechanismus konnten über Simulationen die experimentell gewonnenen Isotopologenkinetiken verifiziert werden.
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