Nanoscale Metal Oxide Particles/Clusters as Chemical Reagents. Unique Surface Chemistry on Magnesium Oxide As Shown by Enhanced Adsorption of Acid Gases (Sulfur Dioxide and Carbon Dioxide) and Pressure Dependence

摘要

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 groupsm~2 is similar for both types of samples. Differences in adsorptivity of SO_2 and CO_2 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 SO_2 and Mg~(18)O 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 SO_2 or CO_2 was reached, the CP-MgO samples exhibited higher adsorptive capacities. Quantitative determinations of SO_2(CO_2) 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 SO_2 (or CO_2). In the case of SO_3, 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.