Characterization of rhodium species in Na-X zeolite by electron spin resonance and electron spin echo modulation analyses

Abstract

Rh(II) species are formed in X zeolite by heating in flowing oxygen. The various species formed after activation at various temperatures, rehydration, and adsorption of various molecules are characterized by electron spin resonance (ESR) and electron spin echo modulation (ESEM) experiments. Two dominant species, termed A and C, are studied in detail. Species A is generated at lower activation temperature than is species C. At intermediate activation temperatures the total Rh spin concentration decreases which is attributed to Rh dimer formation. At higher activation temperatures the total Rh spin concentration increase is consistent with dimer decomposition. Species A is suggested to be located in the ..beta..-cage and species C in the hexagonal prisms of the zeolite X structure based on interaction distances with adsorbates and on the effect of O/sub 2/ on broadening the ESR lines. Rather large distances to water, methanol, and other adsorbates are obtained from the ESEM measurements which suggest that these Rh species are not directly coordinated to the adsorbates. Complete rehydration generates other Rh species which show shorter distances to water characteristic of direct coordination. 33 references, 8 figures, 4 tables.