Location and geometry of Rh species in X zeolites studied by electron spin resonance and electron spin echo spectroscopies: the effect of Ca/sup 2 +/ vs. Na/sup +/ cocations
Rh(II) species are formed in Ca-X zeolite by heating in flowing oxygen and subsequent adsorption of various molecules. These species are characterized by electron spin resonance (ESR) and electron spin echo modulation (ESEM) experiments. The concentration of the Rh(II) species after activation is less than 1%. Adsorption of water, ammonia, CO, and O/sub 2/ considerably increased the spin concentration. The increase in the Rh(II) concentration is due to dissociation of Rh(II) dimers located in the ..cap alpha..- and /or ..beta..-cages of the zeolite structure. Hydration generated species I, which is coordinated to three water molecules and is located in sites I' or II' in the ..beta..-cage or site II in the ..cap alpha..-cage. Adsorption of oxygen and CO generated the corresponding adducts which are located in the ..cap alpha..-cage. The oxygen adduct reacts with water and with methanol and one of the species formed is located in site II' or site II where it is coordinated to two hydroxyl groups and to framework oxygens. Adsorption of methanol without oxygen generates species C, located in site I in the hexagonal prism with no indication of reaction with methanol. The Rh(II) species formed in Ca-X zeolite are compared to Rh(II) species generated in Na-X zeolite, and the effect of exchanging Na/sup +/ by Ca/sup 2 +/ on the nature of the Rh(II) species is discussed.
- Research Organization:
- Univ. of Houston, TX
- OSTI ID:
- 7006053
- Journal Information:
- J. Phys. Chem.; (United States), Vol. 90:22
- Country of Publication:
- United States
- Language:
- English
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