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The electron paramagnetic resonance ahsorption of ehroniia supported on alumina has been investigated a t X-band (9.5 khfcps.) and K-band (23.9 kMcps.). Spectra of chroinia-alumina reduced in hydrogen a t 500" show two distinct phases: a dispersed (6) phase whic,h predominates a t low concentrations of chromium and a bulk ( B ) phase which is prevalent at, the higher concentrations. 1 he temperature dependence of the intensity of these phases indicates the &phase consists of rather isolated Cr+3 ions not coupled electronically and the p-phtHe consists of clusters of Cr+3 ions with strong exchange roupling of 3d electrons. Intensity calibrations of e.p.r. spectra have been made and these indicate all the chromium present in the rcduced catalysh to be in the trivalent state.A relatively sharp resonance absorption appears upon oxidation of reduced chromia-alumina with oxygen. A spin-Hamiltonian for each of the observed types of resonances is proposed and discussed.Absolute amounts of 6-and @-phase chromium also arc given.

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