Electron spin resonance study of rearrangements in the co-ordination of Cr5+ and V4+ complexes due to chemisorption

Publisher Website
Google Scholar
Abstract
The surface chemistry of Cr5+ complexes in chromia/silica catalysts for the polymerization of ethene is investigated. To this end not only the chromia/silica system is studied, but also the related combinations chromia/alumina, vanadia/silica and vanadia/alumina. Electron spin resonance spectra are used to follow the rearrangements in the co-ordination of the Cr5+ and V4+ ions, caused by contact with a number of different gases of vapours: H2O, air, CO, C2H4, HCI and NH3. It is found that the atomic arrangement at the surface of some of the systems studied is very flexible and easily influenced by, for instance, contact at room temperature with H2O, HCl or NH3. It is suggested that such a flexibility—encountered especially in chromia/silica—is a prerequisite for the catalytic activity. The active species in the fresh catalyst is Cr5+ in tetrahedral co-ordination. A mechanism is proposed according to which in the initiation step of the polymerization the co-ordination number is increased from four to five. An empty sixth co-ordination site may then be available to accommodate ethene monomers.

This publication has 0 references indexed in Scilit: