Abstract
Paramagnetic Rh species generated in RhNa-Y and RhCa-Y zeolites after various treatments were characterized by using electron spin resonance (ESR) and electron spin-echo modulation (ESEM) spectroscopies. Activation in flowing oxygen at 500/sup 0/C ..beta..-hydrogen a considerable amount of Rh(II) located in site I in the hexagonal prism of the zeolite structure for 3 wt % Rh in RhNa-Y zeolite. Samples of 1 wt % Rh in RhNa-Y and RhCa-Y did not show any paramagnetic signals. Adsorption of various adsorbates such as water, ammonia, methanol, carbon monoxide, and oxygen on activated samples induced a considerable increase in the ESR intensities. Adsorption of oxygen and carbon monoxide yields the corresponding adducts which are located in the ..cap alpha..-cage of the zeolite structure. Hydration generated a species which is coordinated to three water molecules. Adsorption of methanol on RhNa-Y generated a species H2 which is also formed after reduction of RhNa-Y with H/sub 2/, suggesting that the methanol molecule undergoes a reaction to generate products which further reduce Rh(III) species in the ..beta..-cage of the zeolite structure to Rh(II). No significant differences were observed between RhNa-Y and RhCa-Y except for the formation of different Rh(II) species after methanol adsorption in RhCa-Y and themore » generation of a larger amount of Rh(II) in site I in RhNa-Y. These results are compared to previously obtained data in RhNa-X and RhCa-X to account for the effect of the cocations and the Si/Al ratio on the generation of Rh(II) species in zeolites.« less
Published Version
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