Dispersion and stabilization of reactive atoms on the surfaces of alkaline earth metal oxide ultrafine powders.

摘要

Metal oxides, in particular MgO and CaO can be prepared in a continuum of surface areas. As the surface areas get larger, the crystallites naturally get smaller, and crystal shape changes from cubes (low surface area) to hexagonal platelets (moderate surface area) to polyhedrons (high surface area). Shape change and surface area increases work together to provide larger and larger ratios of surface edge-comer (and other defect) sites verses the total surface ions, and larger surface ions/bulk (inner lattice) ions ratio.; The high proportions of the surface-comer and the defect sites allow relatively large amounts of reactive atoms to be adsorbed and “stored” on the surface. Examples include potassium, chlorine, bromine, and iodine on a series of CaO and MgO samples in our research.; Nanocrystalline MgO doped with potassium metal caused the formation of highly basic sites that are capable of alkene isomerization and alkene alkylation, including the conversion of propylene/ethylene mixtures to pentenes and heptenes. The total number of basic sites closely correlates with the estimated number of ions that exist on edges/comers on the polyhedral crystallites. Only a fraction of these sites are basic enough to cat the akylation reactions and these were formed more prevalently on nanocrystalline MgO as compared with microcrystalline MgO. The results represent another example of nanocrystalline materials possessing unique surface chemistry.; Nanocrystals of MgO and CaO adsorb chlorine gas exothermally in large amounts. Dissociative chemisorption takes place when chlorine atoms are trapped on highly basic surface oxide anion sites, probably at edge and corner sites, mainly. These adducts are active for the catalytic chlorination of methane to form chlorinated methanes, and 2,3-dimethylbutane to give monochlorinated products at the tertiary site. No light is required for the reactions, showing that the nanocrystal-Cl₂ adducts behave as trapped Cl atoms, which are intermediate in reactivity between Cl₂ and free Cl atoms.