Tuning Surface Basic Properties of Nanocrystalline MgO by Controlling the Preparation Conditions

Abstract

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.

Reverse micellar based synthesis of ultrafine MgO nanoparticles (8–10nm): Characterization and catalytic properties

“…The exposure of the negatively charged sites increases the basicity. The surface heterogeneity is affected by the synthetic route which plays an important role in the enhancement of the basicity of the solid [25]. It is possibly due to the presence of these defects which explains our catalyst with low surface area (108 m 2 /g) but equivalent activity as the catalyst with high surface area (250 m 2 /g) [22].…”

Relationship between Acid–Base Properties and the Activity of ZrO2‐Based Catalysts for the Cannizzaro Reaction of Pyruvaldehyde to Lactic Acid

“…For calibration and quantitative measurements, pulses of pure CO 2 were analyzed before the main experiment. Experimental curves were mathematically deconvoluted by using multiple Gaussian shaped peaks . Each peak position was set from an initial estimate, and the calculations were conducted based on a nonlinear curve to minimize the deviations of the squares.…”

Influence of the Precipitation Method on Acid–Base‐Catalyzed Reactions over Mg–Zr Mixed Oxides

“…The common solid base MgO has been explored extensively by using many different techniques, including IR spectroscopy of adsorbed molecular probes,16–20 adsorption microcalorimetry,4, 21, 22 temperature‐programmed desorption,1, 23–26 catalytic probe reactions,20, 23, 24, 27 electron microscopy,25, 26, 28, 29 and quantum chemical calculations 18. 30–37 These experimental and theoretical methods have provided a thorough understanding of the surface basicity and its structural origins in MgO.…”