Nanocrystals of MgO and CaO have been prepared by a modified aerogel/hypercritical drying/dehydration method. For nanocrystalline MgO (AP-MgO) 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 crystallites, while CP-samples (conventional preparation) are larger, hexagonal platelets and cubes);(2) residual surface OH (AP-samples have less acidic OH, which are more isolated from each other; (3) acid gas adsorption (AP-samples adsorb more SO 2 and CO 2 at low pressures and room temperature and prefer monodentate rather than bidentate adsorption modes, but at higher pressures CP-samples adsorb more SO 2 and HCl apparently 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 surface 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 CCl 4 . The data are conclusive that the nanocrystals are more reactive than the microcrystals, and this is mainly attributed to morphological differences, including defects. However, intrinsic electronic effects due purely to "smallness" cannot be ruled out.

Surface adsorptive properties of nanoscale MgO particles have been compared with more conventional samples. Morphologically the nanoparticles (autoclave prepared = AP-MgO) are unique and very different from the conventional samples (conventionally prepared = CP-MgO), and AP-MgO possesses more defects, edge and corner sites, higher surface area and more higher index surfaces. The number of residual surface −OH groups/nm2 is similar for both types of samples. Differences in adsorptivity of SO2 and CO2 at relatively low pressure (20 Torr) were determined by gravimetric means. Much larger quantities were adsorbed by AP-MgO. This process of chemisorption was dynamic, and oxygen scrambling occurred when SO2 and Mg18O nanoparticles were in contact. These results, complying with FTIR studies, are rationalized as due to higher intrinsic surface reactivity coupled with higher concentrations of lower coordination ions on the nanoparticle. Pressure studies showed, however, that as 100 Torr of SO2 or CO2 was reached, the CP-MgO samples exhibited higher adsorptive capacities. Quantitative determinations of SO2(CO2) loading indicate that this difference can be attributed to multilayered physisorption on CP-MgO, which with its flatter, extended planes, can apparently form more ordered multilayered structures and thus physically adsorb more SO2 (or CO2). In the case of SO3, large amounts of surface sulfates were detected by FTIR. Overall, our results indicate that nanoparticles of MgO possess a unique surface chemistry and their high surface reactivity coupled with a high surface area allowed them to approach the goal of being stoichiometric chemical reagents.

Adsorption of HCl, HBr, NO and SO 3 on nanoscale MgO (autoclave prepared ) AP-MgO) and microscale MgO (conventionally prepared ) CP-MgO) has been studied. The higher surface area of AP-MgO allows a higher capacity of these gases to be adsorbed/mol MgO. However, at pressures of 100 Torr or higher, the amounts adsorbed/nm 2 for HX and SO 3 are larger on the microcrystals. This is explained as due to the formation of ordered multilayers of adsorbate on the more perfect crystals of CP-MgO (adsorption on flatter, more extended planes). In the case of NO, the different surface chemistry of AP-MgO vs CP-MgO is again demonstrated. In this case, AP-MgO adsorbed more NO/nm 2 , and NO 2 , N 2 , and N 2 O were formed on the surface. The high surface area and unusual surface reactivity of nanoscale MgO allows it to be considered as a new type of adsorbent as well as a near-stoichiometric chemical reagent.

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