Electron paramagnetic resonance of heterogeneous chromium catalysts

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

Electron Spin Resonance of High-Spin Cobalt in Microporous Crystalline Cobalt-Containing Aluminophosphates

Four highly crystalline, cobalt-containing microporous aluminophosphates (CoAPO-5, CoAPO-11, CoAPO-44, and CoAPO-46) have been investigated by using liquid He X- and/or Q-band electron spin resonance(ESR) spectroscopy in order to investigate the coordination of high-spin cobalt before and after calcination. The ESR spectra of the four zeolite structures are characterized by an axial signal with an effective g^ ≈ 5.80-5.44 and gII ≈ 2.00. Quantitative temperature dependence measurements of this axial signal in the temperature range 4-30 K reveal a Curie-Weiss behavior for both as-synthesized and calcined samples confirming (a) the ms ) ( 1/2 ground state of magnetically isolated high-spin cobalt and (b) a zero field splitting ¢ > 0 cm-1. Quantitation of the ESR signals indicated that most of the Co2+ is ESR active and that only about 30% of this Co2+ can be oxidized to the ESR-inactive Co3+ after calcination. The spin Hamiltonian parameters of as-synthesized and calcined CoAPO-5 material, as determined by spectrum simulation and the microwave power saturation technique, support the presence of framework Co2+ in a flattened or elongated Four highly crystalline, cobalt-containing microporous aluminophosphates (CoAPO-5, CoAPO-11, CoAPO- 44, and CoAPO-46) have been investigated by using liquid He X- and/or Q-band electron spin resonance (ESR) spectroscopy in order to investigate the coordination of high-spin cobalt before and after calcination. The ESR spectra of the four zeolite structures are characterized by an axial signal with an effective g^ ≈ 5.80-5.44 and gII ≈ 2.00. Quantitative temperature dependence measurements of this axial signal in the temperature range 4-30 K reveal a Curie-Weiss behavior for both as-synthesized and calcined samples confirming (a) the ms ) ( 1/2 ground state of magnetically isolated high-spin cobalt and (b) a zero field splitting ¢ > 0 cm-1. Quantitation of the ESR signals indicated that most of the Co2+ is ESR active and that only about 30% of this Co2+ can be oxidized to the ESR-inactive Co3+ after calcination. The spin Hamiltonian parameters of as-synthesized and calcined CoAPO-5 material, as determined by spectrum simulation and the microwave power saturation technique, support the presence of framework Co2+ in a flattened or elongated tetrahedron (D2d)