NANOCRYSTALS AS STOICHIOMETRIC REAGENTS WITH UNIQUE SURFACE-CHEMISTRY

Nanocrystals of MgO and CaO have been prepared by a modified aerogel/h ypercritical drying/dehydration method. For nanocrystalline MgO (AP-Mg O) surface areas ranged from 250 to 500 m(2)/g, whereas for AP-CaO 100 -160 m(2)/g. These materials have been compared with more conventional (CP) microcrystalline samples of lower surface area with regard to (1 ) morphology (AP-samples (autoclave preparation) are tiny polyhedral c rystallites, while CP-samples (conventional preparation) are larger, h exagonal platelets and cubes); (2) residual surface OH (AP-samples hav e less acidic OH, which are more isolated from each other; (3) acid ga s adsorption (AP-samples adsorb more SO2 and CO2 at low pressures and room temperature and prefer monodentate rather than bidentate adsorpti on modes, but at higher pressures CP-samples adsorb more SO2 and HCl a pparently due to the formation of more well ordered multilayers); (4) destructive adsorption of organophosphorus compounds and chlorocarbons (AP-samples are superior due to higher surface areas and higher surfa ce reactivities), and (5) very thin layers of transition metal oxides on the MgO and CaO nanocrystals that significantly enhance destructive adsorption capacities to the point where [M(x)O(y)]AP-MgO and [M(x)O( y)]AP-CaO become stoichiometric in reaction with CCl4. The data are co nclusive that the nanocrystals are more reactive than the microcrystal s, and this is mainly attributed to morphological differences, includi ng defects. However, intrinsic electronic effects due purely to ''smal lness'' cannot be ruled out.