Mössbauer Spectroscopy Studies of the State of Heterogeneous Catalysts during Catalysis
The magnetic state of a metallic catalyst for the ammonia synthesis and the electronic state of oxide catalysts for the propylene and methanol oxidations were investigated using Mössbauer spectroscopy during the respective catalytic reactions. The experiments were carried out in cells which served simultaneously as catalytic reactors and units for Mössbauer spectroscopy — “Mössbauer catalytic reactors.”
The metallic catalyst for the ammonia synthesis consisted of small iron particles (ca. 5 nm in size) supported on MgO. The superparamagnetic relaxation frequency for these particles was found to be sensitive to both hydrogen chemisorption on the particles and to treatments of the particles which were shown to change their catalytic properties. These results can be understood in terms of Néel’s phenomenological theory of magnetic-surface anisotropy; in addition, surface iron atoms with seven nearest neighbors seem to be particularly active for the ammonia synthesis.
For the soft oxidation of propylene and methanol, cobalt molybdate containing minor amounts of iron and nonstoichiometric iron molybdates were used as catalysts. In both cases, the iron initially entered the catalyst structure mainly in the trivalent state. During both catalytic processes, however, additional, partially reduced forms of iron were detected, and these forms disappeared upon termination of the reaction. These electronic states of iron can be treated in terms of vacancy models.
KeywordsMossbauer Spectroscopy Heterogeneous Catalyst Methanol Oxidation Propylene Oxidation Ammonia Synthesis
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