Nanocrystals as Stoichiometric Reagents with Unique Surface Chemistry

Abstract

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.

Destructive Adsorption of CCl4 over Lanthanum-Based Solids: Linking Activity to Acid−Base Properties

“…Potential materials involve alkaline earth metal oxides, lanthanide oxides, and transition metal oxides, and the process of destructive adsorption of chlorinated hydrocarbons has been studied for all these compounds by Klabunde and co-workers [3][4][5][6][7][8] and several other groups. [9][10][11][12][13][14][15][16] In this reaction, the solid material is chlorinated, while the chlorinated hydrocarbons are oxidized to, e.g., CO 2 .…”

Chemistry of SO 2 and DeSO x Processes on Oxide Nanoparticles

“…Nanoparticles of CaO, MgO, and SrO usually prefer to adopt a nearly perfect or somewhat distorted cubic shape, exposing the (100) face of a rocksalt crystal structure (40,41). Nanoparticles exhibiting (110) and (111) faces are much less common and frequently are not stable at high temperatures.…”

“…Depending on the exact procedure followed for the preparation, nanoparticles of MgO and CaO with polyhedral or hexagonal shapes can be prepared, but they also contain OH groups (40,49).…”

“…Such defects could be of the Frenkel or Schottky type (vacancies), or be manifested as unusual configurations of edges, corners, or crystal planes (40).…”

Synthesis, Spectral and Thermal Studies on Schiff Base Derivatives of Heterobimetallic [Ca(II)-Ti(IV)]-μ-oxoisopropoxide

“…Recently, calcium titanate has also been used as a competent anticorrosion pigment for paints 18 . Apart from their role as precursors for mixed metal oxides, the bimetallic-µ-oxoalkoxides of transition metals have been found to rank among the excellent catalysts for the polymerization of heterocyclic monomers like lactones, oxiranes, thiiranes and epoxides [19][20] .…”