Shawn Decker

Nanocrystalline Metal Oxides as Unique Chemical Reagents/Sorbents

A new family of porous inorganic solids based on nanocrystalline metal oxides is discussed. These materials, made up of 4-7 nm MgO, CaO, Al2O3, ZnO, and others, exhibit unparalleled destructive adsorption properties for acid gases, polar organics, and even chemical/biological warfare agents. These unique sorption properties are due to nanocrystal shape, polar surfaces, and high surface areas. Free-flowing powders or consolidated pellets are effective, and pore structure can be controlled by consolidation pressures. Chemical properties can be adjusted by choice of metal oxide as well as by incorporating other oxides as monolayer films.

Unexpected Fe local order in iron oxide coated nanocrystalline magnesium oxides with exceptional reactivities against environmental toxins.

Mg oxide nanoparticles are very reactive materials used to mitigate atmospheric pollution and to sequester polluting molecules. Using Fe K-edge XAFS, we have studied the structure of iron oxide-coated MgO nanoparticles before and after reaction with CCl4. Before reaction, the local structure around Fe is totally different from that in iron oxide coatings on SrO and CaO nanoparticles, although these coated materials were prepared in the same way. In SrO and CaO, the iron oxide coating has been shown to be well separated from the bulk of the nanoparticle, whereas in MgO, Fe was found to mix with MgO. After reaction with CCl4, Fe-Cl bonds can be detected when the coated nanoparticle is saturated. Such Fe-Cl EXAFS signals have not been observed in previously studied nanoparticles.

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

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→ [FeClx]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 O2-/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-FeC1x layer is mobile and dynamic, allowing continual O2-/C1- exchange deeper into the nanoparticle. Morphological studies were aided by Atomic Force Microscopy experiments, which will also be discussed.