Interaction of Olfins with oxygen adsorbed on ZnO and CuO

Abstract

The selectivity in the catalytic oxidation of olefins may originate from the difference in the adsorbed state of reactants, especially that of oxygen. In this work, the adsorbed species of oxygen on ZnO and $Cu_{2}O$ were identified and their reactivities in the oxidation of CO, $C_{2}H_{4}$ and $C_{3}H_{6}$ were studied by epr spectroscopy and temperature programmed desorption-mass spectrometric technique. The oxygen species $O^-$ on ZnO was identified as a symmetric signal with g=2.014 by comparing the epr spectra of oxygen adsorbed at various temperature on ZnO, and by studying the variation of the spectrum during the illumination by light having the band gap energy at $-196^\circ C$. The signal increased with increasing adsorption temperature and decreased when the sample previously treated with $O_2$ at $350^\circ C$ was illuminated. Cutting off the light and heating the sample to $25^\circ C$ also resulted in a decrease of the signal intensity. Reillumination of the sample at $-196^\circ C$ increased the signal intensity but cutting off the light and heating to $25^\circ C$ further decreased it. The shape of the signal was invariant in $O_2$ ambient at $-196^\circ C$ or in the treatment with $O_2$ enriched with 20\% $^{17}O_2$ while its width of maximum slope was increased with increasing recording temperature of epr spectrum. These observations suggest that the signal is due to the trapped hole in its regular lattice or the hole generated from its transformation of $O^-$ in an oxygen vacancy to $O^{2-}$ in the lattice (Td symmetry). The adsorbed species $O^-$ interacted with CO or $C_2$ and $C_3$ olefins at $25^\circ C$ and produced broader or not well resolved signal with g=2.014 showing hyperfine for structure when treated with $^{17}O_2$. The signals completely disappeared on heating the system to about $200^\circ C$. These results were interpreted in terms of the formation of adducts between $O^-$ and the reactants at $25^\circ C$. In the therm...