Preparation of Pt/MgO catalysts. Influence of the precursor metal salt and solvent used

Hydration and Hydroxylation of MgO in Solution: NMR Identification of Proton-Containing Intermediate Phases

Magnesium oxide (MgO)-engineered barriers used in subsurface applications will be exposed to high concentration brine environments and may form stable intermediate phases that can alter the effectiveness of the barrier. To explore the formation of these secondary intermediate phases, MgO was aged in water and three different brine solutions and characterized with X-ray diffraction (XRD) and ¹H magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. After aging, there is ∼4% molar equivalent of a hydrogen-containing species formed. The ¹H MAS NMR spectra resolved multiple minor phases not visible in XRD, indicating that diverse disordered proton-containing environments are present in addition to crystalline Mg(OH)₂ brucite. Density functional theory (DFT) simulations for the proposed Mg-O-H-, Mg-Cl-O-H-, and Na-O-H-containing phases were performed to index resonances observed in the experimental ¹H MAS NMR spectra. Although the intermediate crystal structures exhibited overlapping ¹H NMR resonances in the spectra, Mg-O-H intermediates were attributed to the growth of resonances in the δ +1.0 to 0.0 ppm region, and Mg-Cl-O-H structures produced the increasing contributions of the δ = +2.5 to 5.0 ppm resonances in the chloride-containing brines. Overall, ¹H NMR analysis of aged MgO indicates the formation of a wide range of possible intermediate structures that cannot be observed or resolved in the XRD analysis.

Tuning surface basic properties of nanocrystalline MgO by controlling the preparation conditions

Different nanocrystalline magnesias were synthesized by precipitation and hydrothermal treatments of aqueous salt solutions in an attempt to tune their surface basicity. CO(2) was chosen as an acidic molecule to probe the basic sites by both temperature-programmed desorption and infrared spectroscopy. All samples were shown to be crystalline, and except that obtained by nitrate decomposition, they all possessed high surface areas. The oxides presented different basic site distributions, evidencing the significant role of the preparation conditions on tuning the surface basicity: while medium-strength basic centers are dominant in the samples prepared by precipitation aging or hydrothermal treatment, the one obtained by precipitation features a roughly equal concentration of medium-strength and strong centers. Infrared spectra revealed that hydrogen carbonate and monodentate and bidentate carbonates were formed in distinct proportion on all oxides. However, the bidentate complexes were shown to have different thermal stabilities; the more stable species are thought to be formed on acid-base pair centers associated with an anionic vacancy. Distinct morphological and structural characteristics were also observed by high-resolution transmission electron microscopy. It was consistently found that the high-surface area samples are formed by aggregates of nanoparticles (2-5 nm) randomly oriented and with a high concentration of structural defects. These findings allowed us to conclude that the surface heterogeneity promoted during synthesis increases the concentration of basic sites and plays an important role in tuning the basicity of the solids.

Structure Analysis of Nanocrystalline MgO Aerogel Prepared by Sol-Gel Method

Wet gel obtained by sol-gel technique was dried in supercritical CO2 to prepare hydrated form of magnesium oxide. Calcination at 723 K under vacuum yielded nanocrystalline MgO aerogel. Structure studies were performed by X-ray diffraction, scanning and transmission electron microcopies. Electron microscopy images reveal rough, unfolded and ramified structure of solid skeleton. Specific surface area SBET was equal to 238 m2/g. X-ray pattern reveals the broadened diffraction lines of periclase, the only crystalline form of magnesium oxide. The gamma crystallite size distribution was determined using FW 5 4 / 5 1 M method proposed by R. Pielaszek. The obtained values of <R> and σ (measure of polydispersity) of particle size parameters are equal to 6.5 nm and 1.8 nm, respectively, whereas the average crystallite size estimated by Williamson-Hall procedure was equal to 6.0 nm. The obtained at Rietveld refinement Rwp, and S fitting parameters equal to 6.62% and 1.77, respectively, seem to be satisfactory due to the nanosize of MgO crystallites and because of the presence of amorphous phase.