No abstract

Here we report a scalable and template-free production strategy in the synthesis of a mesoporous calcium carbonate, which undergoes self-assembled nanostructure formation through a temperature-induced aggregation and polymorphic transformation of the colloids. The specific surface area and pore size distribution of resulting mesoporous calcium carbonate are clearly different depending on the aging temperature. The specific surface area and average pore size for aging temperature of 293 K are 207.3±9.8 m 2 /g and 8.8±0.6 nm, respectively, and 65.1±10.1 m 2 /g and 19.9±2.6 nm at 473 K. Additionally, we apply the mesoporous calcium carbonate powder to formaldehyde vapor adsorbent. We measure the adsorbed amount of gaseous formaldehyde and find that the vaterite-rich powder has larger adsorption per unit area than the calcite-rich one.

The equilibrium in the isotope exchange reaction between 235U-enriched UF5 nanoparticles and natural UF6 gas was experimentally investigated. From the relationship between the isotopic fraction of UF6 gas at equilibrium and the specific surface area of the UF5 nanoparticles, it was experimentally demonstrated for the first time that all UF5 molecules on the outermost layer of the particles participated in the reaction until the isotopic fraction of UF5 on the outermost layer became equal to that of the UF6 gas. A fast rate process during the early period of the reaction and a subsequent slow rate process were quantitatively observed. These facts contradict the previously reported assumption not supported by experimental evidence that the UF5 molecules in the underlying layer of the particle participate in the reaction. A new model based on our experimental findings was proposed. Our model suggests that there are two kinds of molecules on the outermost layer with different reactivities and all UF5 molecules on the outermost layer contribute to the reaction. The rate equations were derived from the proposed model and analytically solved. The time dependency of the isotopic fraction of the UF6 gas and UF5 particles was expressed by three kinetic parameters such as the rate constant of the high reactivity molecule, ks, the rate constant of the low reactivity molecule, ks′, and the fraction of high reactivity molecules on the outermost layer, ξ. Our model reproduced well all the experimental data determined in the present study with ks=5.5×10−20 cm3/(s molecules), ks′=4.5×10−22 cm3/(s molecules), and ξ=0.078.

Calcium oxides with a specific surface area between 4.5 m 2 g-1 and 62.5 m 2 g-1 were obtained by calcination of scallop shells, following by methanol-assisted dry nano-grinding. Three distinct phases are formed on the surface of these catalysts during nano-grinding: calcium methoxide, calcium hydroxide, and calcium oxide. The effects of specific surface area and active surface phase composition on the catalytic activity of calcium oxide during methanolysis of soybean oil were investigated. The properties of the calcium oxide before, during, and after methanol assisted dry nano-grinding were studied by XRD, FTIR, and nitrogen gas adsorption based on the BET method. The ground calcium oxides were found to be effective in catalyzing the methanolysis of soybean oil, with the optimal catalyst producing a 72.3% ester yield after 20 mins of reaction. The improvements in rate of reaction were attributed to the rapid formation of calcium diglyceroxide during the initial stages of methanolysis. A combination high specific surface area and effective active phases on the surface of the calcium oxide catalysts is correlated with reductions in mass transfer limitations in the early steps of the reaction, indicated by the rapid formation of calcium diglyceroxide.

Mechanically stirring the precursor colloidal dispersion can control the polymorphs of mesoporous calcium carbonate.