ELECTRON-PARAMAGNETIC-RESONANCE OF HETEROGENEOUS CHROMIUM CATALYSTS

The X- and Q-band EPR spectra of Cr5+ and Cr3+ ions in heterogeneous C r catalysts have been investigated as a function of the support type a nd composition (silica, silica-alumina, alumina, mordenite and AlPO-5) . A spin-Hamiltonian operator for each of the observed types of resona nces is proposed and discussed, while the extensive use of computer-si mulated spectra provides detailed information on the nature of support ed Cr ions. Mononuclear Cr5+ ions (gamma-signal), with g-values ca. 2, are axial on alumina and silica-alumina, and rhombic on silica and mo rdenite. Two different X-band EPR signals of Cr3+ ions are present on amorphous and crystalline oxidic surfaces: a broad signal with g aroun d 2 and zero field parameters D and E equal to 0, due to clustered Cr3 + (beta-signal) or isolated octahedral Cr3+ complexes, and a signal wi th a positive lobe in the region g = 3.5-5.5 (delta-signal). The latte r signal is characterized by high values of D and E and due to the pre sence of a highly distorted Cr3+ octahedron. The data so obtained allo w us to develop a general model of supported Cr5+ and Cr3+ ions on oxi dic surfaces.