The structure of ethylene polymerisation catalysts. Part 2.—Electron spin resonance studies of γ-phase chromium oxide

Abstract

Electron spin resonance spectra have shown δ-type and β-type Cr³⁺ resonances, indicating the prevalence of clustered chromium. At calcination temperatures which produce only partial decomposition of supported Cr⁶⁺, a second β-type resonance, β₂, has shown γ-resonance characteristics with evidence for dipole–dipole broadening and exchange narrowing associated with clustered chromium. The occurrence of γ-resonance is related to the extent of chromium oxide aggregation. Both alumina and silica γ-resonances are complex, exchange-narrowed resonances experiencing different degrees of near-neighbour magnetic broadening. It is proposed that γ-resonance arises from the surface of mixed-valency chromium oxide clusters, rejecting the usual model of magnetically isolated, crystal-field-stabilised Cr⁵⁺ species.