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 (γ-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⁺(β-signal) or isolated octahedral Cr³⁺ complexes, and a signal with a positive lobe in the region g= 3.5–5.5 (δ-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.