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Electron paramagnetic resonance studies of adsorbed species Pelman, Alan Irwin

Abstract

Electron paramagnetic resonance techniques have been used to investigate the nature and possible effects of adsorption of gaseous species on several adsorbents, in particular several synthetic zeolites, at temperatures from 77°K upwards. Analysis of the spectra obtained has been aided through computer simulation of the various spectra and comparison of these to the actual observed spectra. The molecule chlorine dioxide ( C10₂ ) has been studied in various low temperature matrices but little has been published for C10₂ in the adsorbed state. An attempt was made to find an adsorbent such that an inert matrix might be approximated, to give a base from which to make comparisons. To this end, adsorbents including silica gel, synthetic zeolites 13X, 10X, 4A, 5A, Na-mordenite and H-mordenite were investigated. The results vary between those from silica gel, where spectra yielding EPR parameters similar to other matrices were obtained, to those from 13X where it was evident that two distinct adsorption sites of the C10₂ were present. In the 13X as in the other synthetic zeolites, EPR parameters markedly different from other studies were found and were attributed to the intense electrostatic fields present in these zeolites. Results obtained at room temperature for these adsorbents ranged from C10₂ molecules freely rotating in the cages of the zeolites to other molecules having hindered rotations. Nitrogen dioxide ( NO₂ ) was also investigated with a view to finding similar interactions. Although changes as marked as for C10₂ compared to other studies were not observed, the synthetic zeolite H-mordenite yielded spectra closely approximating those obtained in solid N₂O₄ matrices. It is proposed the NO₂ molecules are caged in the numerous side pockets emanating from the main channels in this zeolite and are effectively isolated from other NO₂ molecules. The resulting spectra are strikingly more resolved than those obtained using other adsorbents and enabled accurate computer simulations to be made. The adsorption of nitric oxide ( NO ) produced an effect not found with the other molecules. A new species was formed from a reaction of the NO with H-mordenite and could not be removed at room temperature, indicating a strong bond to the surface. The new species does not contain nitrogen as identical spectra were obtained from adsorption of ¹⁴NO and ¹⁵NO. Attempts to observe spectra which could be assigned to the difluoroamino radical from adsorption of tetrafluorohydrazine were unsuccessful. The spectra observed were assigned to a species having no hyperfine structure and an anisotropic g tensor.

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