Electronic Structure of Transition Metal Ion Centers in Zeolites and Zinc-Oxide
Abstract
Zeolites and zinc oxide containing transition -metal ions are two important catalytic materials in today's technology. The present investigation explores the electronic structure of the transition-metal ion centers in these oxides. Diffuse reflectance spectroscopy was used to obtain the optical spectra of a series of dehydrated Co(II)A zeolites. The line shape of the optical spectra was analyzed using the semiclassical model of the dynamic Jahn-Teller effect. For Cu(II) centers in zeolites and ZnO, the electronic structure of these centers was calculated using the scattered-wave X-alpha (SWX-alpha ) method. The optical spectrum of dehydrated Co(II)A zeolite is characterized by three major bands at 7,000 cm ^{-1} (Band I), 15,800 cm ^{-1} (Band II) and 24,500 cm ^{-1} (Band III). The absorbances of these three bands were found to be linearly dependent on the Co(II) concentration in zeolite, which indicated that the Co(II) centers in A type zeolite consisted of single species. According to the ligand field theory, Band I and Band II are characterized by an E to E type transition. The line shapes of these two bands were simulated using the dynamic Jahn-Teller effect model for an E to E transition. The simulated line shapes showed the asymetric feature of the optical spectrum. However, the difference in detail between the simulated and experimental line shape were noticeable. Adsorption of CO in the dehydrated Co(II)A zeolite resulted in a Co(II)-Co species, and the resultant optical spectrum was fitted by a term diagram that was derived from ligand field theory. The best fit indicated that the ligand field strength of CO was weak which was consistent with the results obtained from X-ray diffraction studies. The band structure of ZnO was calculated using the SWX-alpha method in a (Zn _4O_{11}) ^{-14} cluster. After the Watson sphere and Outersphere were reduced down to 80% of the initial radii, the calculated band gap, valence band edge and valence band width were close to the experimental results. The band structure was also calculated for the case in which a copper atom was substituted for one of the zinc atoms. The SWX-alpha studies indicated that the Cu levels were located in the band gaps of the host ZnO and zeolites. (Abstract shortened with permission of author.).
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1988
- Bibcode:
- 1988PhDT.......130L
- Keywords:
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- Physics: Condensed Matter; Physics: Atomic