Resonance intensity interior

The 8-phase chromium resonance in chromia-alumina has been observed by several workers. It is dependent upon the alumina support since it does not appear in samples of chromia-silica, although it is observed in the case of chromia-silica-alumina. The isolated Cr + ions could conceivably be situated either in the bulk of the support, or on its surface. There is some evidence that both situations prevail. Nuclear magnetic resonance studies to be discussed presently (121) indicate that for impregnated chromia-alumina catalysts calcined at 500 the majority of these ions are on the surface. However, when such catalysts are heated above 600°, the 8 phase is greatly enhanced because of diffusion of chromium ions from the -phase into interior sites in the alumina lattice. The same situation arises with coprecipitated chromia-alumina catalysts (34). The S-phase resonance intensity of coprecipitated chromia-alumina was found to increase with calcination temperature, indicating an increasing three-dimensional dispersion of Cr + ions. In general, the S-phase resonance dominates the ESR spectra of chromia-alumina catalysts at low chromium concentrations, and therefore it... 

A set of experiments aimed at enhancing the core crystalline resonances over the crystalline surface chain resonances was also undertaken. The approaches, which utilized and T-j relaxation behavior, were predicated on the existence of greater molecular mobility (enhanced relaxation rates) for chains at the crystal surface relative to the crystallite interior. Strong proton-proton spin exchange during T relaxation blurs somewhat the differences between surface chain and interior chain relaxation nevertheless, this approach has the advantage that the CP spectra, taken as a function of the decay, have undistorted Intensities over dimensions of monomer units. On the other hand, relaxation is much more a function of the individual carbons spin-exchange is weak. Therefore one... 

---