Catalysis by Alkaline Earth Metal Oxides. III. X-Ray Photoelectron Spectroscopic Study of Catalytically Active MgO, CaO, and BaO Surfaces

Abstract

Abstract X-Ray photoelectron spectroscopy was used to reveal the surface states of MgO, CaO, and BaO oxides, which are catalytically active for ethylene hydrogenation. The catalytic activity of CaO exposed to H2O changed with an increase in the temperature of evacuation in a way similar to that of Ca(OH)2 decomposed by heat treatment. The photoelectron spectrum of CaO evacuated at 1223 K showed a peak of O Is at 529.6 eV, while those of Ca 2p3⁄2 were at 346.3 eV and Ca 2p1⁄2 at 349.8 eV. The adsorption of H2O at room temperature gave rise to an additional peak of O Is at 532.4 eV and one of Ca 2p3⁄2 at 348.6 eV, which were attributed to a Ca–O–H structure. With an increase in the temperature of evacuation from 553 to 1053 K, at which the catalytic activity attained a maximum, the latter O Is peak shifted to the lower binding-energy side by 0.4 eV, and the corresponding Ca 2p peak became broader. These spectral variations were ascribable to the formation of O− species. Evacuation above 1273 K destroyed both the catalytic activity and characteristic peaks. BaO provided three O Is peaks, at 528.5, 530.6, and 532.1 eV, after evacuation at 1373 K; these peaks were assigned to lattice oxygen of the oxide, O−, and OH− species respectively. The Ba 4d spectra showed correspondingly superposed peaks. The XPS peak due to O− was observed only when BaO was evacuated at temperatures high enough to generate the catalytic activity. Such spectral features were less pronounced in the case of MgO. The structural analysis of CaO showed that the coordinatively unsaturated cations also play a role in the catalytic hydrogenation. On the basis of these findings, it was concluded that a combined structure of O− and the cations is responsible for the hydrogenation activity.