Nanoscale Metal Oxides as Destructive Adsorbents. New Surface Chemistry and Environmental Applications

Abstract

An aerogel procedure combined with hypercritical drying has yielded magnesium oxide and calcium oxide in ultrahigh surface area forms. These nanoparticles of MgO and CaO possess intrinsically higher surface reactivities, and serve as destructive adsorbents for a variety of toxic substances, including organophosphorus compounds, and chlorocarbons. They also serve to adsorb large amounts of gases very strongly, such as CO₂, SO₂, SO₃, and HX. A second generation of even more effective destructive adsorbents has been prepared by depositing a monolayer of transition metal oxide on the MgO or CaO nanoparticles, for example [Fe₂O₃]MgO, [NiO]CaO, [ZnO]MgO, and others. As a test reaction CCl₄ + [Fe₂O₃]MgO→ [FeCl_x]MgCl₂ + CO₂ was employed. This gassolid reaction was facilitated and enhanced by two things: (1) Unusual morphology of nanoscale MgO, probably because of exposure of {111} crystal faces and high concentrations of edge sites and defect sites, and (2) the presence of the thin layer of Fe₂O₃ (or other transition metal oxide), which allows a catalytic O^2-/Cl^- solid state ion/ion exchange to take place. The reaction proceeded to almost stoichiometric proportions when Fe₂O₃ was present, which indicates that the surface Fe₂O₃-FeC1_x layer is mobile and dynamic, allowing continual O^2-/C1^- exchange deeper into the nanoparticle. Morphological studies were aided by Atomic Force Microscopy experiments, which will also be discussed.