MgO crystals

FIG. 6 Examples of adsorption isotherms for the Ar/MgO system measured on different samples of MgO crystals. (From Ref. 61.)... 

Figure5.9. Hypothetical particle of cubic MgO, exhibiting three crystallographic surfaces. The (100) and (110) surfaces are nonpolar, implying that Mg and O ions are present in equal numbers, but the polar (111) surface can be terminated either by Mg cations or O anions. In practice, MgO crystals predominantly show (100) terminated surfaces. 

Figure 1. High resolution Electron Micrograph of a cubic MgO crystal viewed in [100] direction showing square net of 22 fringes and apparent bending of atom planes at edges. Courtesy of Dr. T. Tanji.

Figure 3. Microdiffraction patterns obtained with an electron beam of diameter about 1o8 from particles of Ru and Au on a MgO support, (a) MgO crystal, (b) Ru crystal, is2 in diameter, on MgO. (c) Au crystal, 20A in diameter, on MgO.

Conversion electron Mossbauer spectroscopy (CEMS) measurements with back scattering geometry have the merit that spectra can be obtained from a sample with much less isotope content compared with transmission measurements. Another merit is that a sample, deposited on a thick substrate, could be measured, and that because of the limited escape depth of the conversion electrons, depth-selective surface studies are possible. The CEMS technique was found to be best applicable to specimens of 10-100 pg Au cm, i.e., about two orders of magnitudes thinner than required for measurements in transmission mode [443]. This way (1) very thin films of gold alloys, as well as laser- and in beam-modified surfaces in the submicrometers range of depth [443], and (2) metallic gold precipitates in implanted MgO crystals [444] were investigated. 

In the context of the discovery of amines and oxygen-containing organic compounds, the question arises as to whether the presence of atomic carbon in olivine or MgO crystals could have led to the formation of amino acids. Knobel et al. (1984) reported the detection of amino acids in liquid extracts of the reaction mixtures at the 1983 ISSOL conference yields were, however, extremely low, the total yield being 1.5-3.0 x 10-7 g per gram of MgO. These results were the subject of considerable attention in the media. 

The Fs+ color center in the surface of an MgO crystal is an oxygen vacancy plus ... 

A noteworthy line of research is the application of TEM on models for supported catalysts. Figure 7.6 shows a side view of Au particles (diameters <6 nm) on top of MgO crystals, taken from the work of Giorgio et al. [16]. The picture beautifully shows the shape of the particles together with the lattice fringes characteristic of certain orientations of the particles and the support. In addition, the authors obtained... 

Explain why MgO crystallizes in the rock salt (NaCl) structure whereas BeO crystallizes in the wurtzite structure. 

What is surface reconstruction and why is it difficult to prepare MgO (110) and (111) surfaces although MgO crystals having (111) faces occur naturally ... 

TABLE A2 Thermochemical Data of Selected Chemical Compounds Magnesium oxide (MgO), crystal-liquid, molecular weight = 40.3044... 

The equilibrium concentration for a Schottky disorder can be found in a similar manner. Recall that a Schottky defect is a cation-anion defect pair. For example, the migration of an MgO molecule to the surface in an MgO crystal can be described as follows ... 

At the end of the reactivity measurements the sample is transferred back to the preparation chamber where a carbon film is deposited on the sample. This carbon layer has a double use. First, it protects the clusters against a further evolution (coalescence, restructuring, oxidation during air exposure) and second, in the case of MgO crystals it will serve as a thin support for the metal particles (after floating in acidic solution) for subsequent TEM characterization [12]. 

Volume Changes of Solids in Different Atmospheres. The fact that small MgO crystals prepared in vacuo and in air have different lattice parameters means that a crystal prepared in vacuo expands when brought into air. 

Until a few years back, the crystal shape of solid materials was of academic curiosity only and shape was not considered to have an effect on the chemical properties and reactivities of a material. However, recent studies clearly indicate that the shape of nanocrystals does indeed affect the chemistry. For example, it has been shown that 4 nm nanocrystalline MgO particles adsorb six molecules of S02 per nm2 at room temperature and 20 Torr pressure.30 However microcrystalline MgO adsorbs only 2 molecules of S02 per nm2 under similar conditions. Similarly, the nanocrystalline aerogel prepared, AP-MgO material adsorbs four times as much C02 as the microcrystals. There are not only differences in the amounts of gaseous molecules adsorbed on these surfaces, but also the mode of surface binding can also be different. S02 binds more predominantly as a monodentate species on the AP-MgO crystal but favors a bidentate geometry on conventionally prepared, CP-MgO microcrystals. Clearly, these results indicate that the shape and size of the crystals affect the adsorptive properties of the MgO surfaces. The high reactivities of the... 

Fig. 9.3. Models of MgO crystal (a) polyhedralshaped, aerogel prepared AP-MgO nan crystals (b) hexagonal-shaped, conventionally prepared CP-MgO microcrystals and (c) cube-shaped commercial polycrystalline MgO. (Reprinted with permission from Klabunde, K.J. et al Chem. Eur. J 2001, 7, 2505-2510. Fig. 2, by permission ofWiley-VCH, Publishers.)...

K. S. Kim and R. J. Clifton, Dislocation Motion in MgO Crystals Under Plate Impact, Journal of Material Science, 19, 1428-1438 (1984). 

Because of the extreme heterogeneity of 0 basic sites present on the surfaces of high-surface-area MgO crystals, Spoto et al. (26) used the simplest cluster model able to account for the strongly basic character of the 0 sites responsible for the CO oligomerization reactions on MgO. The cluster model consists of a bare, unconstrained neutral Na O Na" cluster. Of course, the limitations of this trivial cluster are evident relative to the more realistic structures adopted by Lu et al. (175), which incorporated three neutral (MgO) ( = 4, 6, and 8) three-dimensional clusters containing Of , O4J, Mgsc, and centers as reactive sites for the... 

As an example of a pseudopotential simulation of boundary conditions for clusters representing some ionic compounds, let us consider the MgO case [79], which has been considered previously [80], [81] within a bare cluster model. The MgO crystal (the rock-salt lattice) was simulated by a cluster MggOg (see Fig. 3) placed in several embeddings comprised of a different number of pseudoatoms possessing the effective potentials and placed at the lattice sites. 

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