Reduction and oxidation effects on rhodium species in Na-X zeolite studied by electron spin echo and electron spin resonance spectroscopies

Abstract

Reduction and oxidation of Rh-exchanged Na-X zeolite activated in flowing oxygen generated several new paramagnetic Rh(II) species which differ in symmetry and location from the Rh(II) species formed after activation only. The various new species were characterized by electron spin resonance (ESR) and electron spin echo modulation (ESEM) experiments. Reduction of samples, activated at 240-400/sup 0/C, with H/sub 2/ generated a species designated as H1 which has an isotropic g tensor at 77 K and shows a dynamic Jahn-Teller effect. A different species, H2, was formed after reducing samples which were activated at 400-500/sup 0/C. In both cases the ESR signals of the original Rh(II) species formed after activation disappeared after the reduction. On the other hand, oxidation of activated samples by molecular oxygen did not affect any of the original paramagnetic Rh(II) species but produced several additional paramagnetic species. These species did not interact with benzene or ethylene whereas they did respond to the adsorption of water, methanol, or ammonia. 24 references, 16 figures, 2 tables.