Structures of molybdenum silica catalysts reduced by ethanol, and their relations to catalytic oxidation reactions: I. Structure changes of Mo/SiO 2 with gradual reduction by ethanol

Abstract

Structural changes of silica supported molybdenum catalysts accompanied with reduction by ethanol were investigated with EXAFS and UV–VIS spectroscopies. Catalysts made from four different precursors were investigated, and it was demonstrated that the dependence on preparation method was small. Catalysts prepared by the reaction between surface hydroxide groups and organometallic complexes are called `fixed catalysts' and some researchers assert that fixed Mo/SiO 2 have uniform and specific molecular-like active sites, which remains controversial. It was found that what had been proposed as characteristics of the molecular-like active sites of the fixed catalysts were merely prejudiced interpretation of a few aspects in the structural changes common to Mo/SiO 2 in general. At least two distinctive states existed in the course of reduction. One has IVCT (intervalence charge transfer) absorption band and corresponds to shallow reductions. The other has 2.6 Å Mo–Mo bond and average oxidation state of it was estimated to be +5. Structure similar to the latter was found also on alumina supported Mo catalysts and can be considered thermodynamically stable.