Electron paramagnetic resonance of heterogeneous chromium catalysts

Abstract

The X- and Q-band EPR spectra of Cr5+ and Cr3+ ions in heterogeneous Cr catalysts have been investigated as a function of the support type and composition (silica, silica–alumina, alumina, mordenite and AlPO-5). A spin-Hamiltonian operator for each of the observed types of resonances is proposed and discussed, while the extensive use of computer-simulated spectra provides detailed information on the nature of supported Cr ions. Mononuclear Cr5+ ions (-signal), with g-values ca. 2, are axial on alumina and silica–alumina, and rhombic on silica and mordenite. Two different X-band EPR signals of Cr3+ ions are present on amorphous and crystalline oxidic surfaces: a broad signal with g around 2 and zero field parameters D and E equal to 0, due to clustered Cr3+(-signal) or isolated octahedral Cr3+ complexes, and a signal with a positive lobe in the region g= 3.5–5.5 (-signal). The latter signal is characterized by high values of D and E and due to the presence of a highly distorted Cr3+ octahedron. The data so obtained allow us to develop a general model of supported Cr5+ and Cr3+ ions on oxidic surfaces.