MgO , preparation

EXAFS characterization of supported PtRu/MgO prepared from a molecular precursor and organometallic mixture... 

Table 1 Coordination number (N) and bond distance (R) of cluster core of crystalline A determined by XRD [3] and EXAFS data of PtRu/MgO prepared from cluster A and acac precursors after ligand removal...

Edge Shell Cluster A PtRu/MgO prepared from cluster A PtRu/MgO prepared from acac ... 

Fig. 3 A shows the effluent NH3 concentration observed for Ru/MgO as a function of reaction temperature for three different Pn, / Phj / Paf ratios at 20 bar total pressure. It is obvious that the reaction orders for N2 and H2 have opposite signs. Fig. 3B illustrates that the reaction orders for N2 and H2 partly compensate each other in the kineticaliy controlled temperature regime. Hence an increase in total pressure with a constant Pnj / Phj 1/3 ratio does not lead to a significant increase in conversion at lower temperatures. For the plication of alkali-promoted Ru catalysts under industrial synthesis conditions, it is necessary to find a compromise between kinetics and thermodynamics by increasing the Pn, / Phj ratio. The optimum observed for Cs-Ru/MgO prepared from CS2CO3 at 50 bar is at about Pnj / Phj 40 / 60 [15]. The high NH3 concentration of about 8 % obtained with 0.138 g catalyst using a total flow of 100 Nml/min clearly shows that Ru catalysts have indeed the potential to replace Fe-based catalysts in industrial synthesis [15].

The interpretation of the C.E. by a superimposition of reactions occurring at different active surface centers is compatible with the fact that many multicomponent catalysts exhibit a C.E. but no C.E. is found when very pure substances have been subjected to different thermal pretreatments (17). This implies the possibility that many active centers are due to impurities and that their numbers may change with the pretreatment of the catalyst, e.g., by means of aggregation, volatilization, etc. As an illustration, data for the decomposition of N2O on MgO, prepared from synthetic and from natural magnesites, and data for the para-ortho hydrogen conversion on pure metals and on alloys are presented in Tables II and III. 

However the catalyst can be easily reactivated in flow of o2/Ar at 350°C (compare runs 2 and 2.1, Table 1) using the procedure reported in the previous section. Catalytic activity can be restored also by a thermal treatment in flow of He (350°C, 15 h), and this suggests that strongly adsorbed produts could be responsible for catalyst deactivation. The amount of 4-hexen-3-one converted depends on the nature of the catalyst precursor and on its thermal pretreatment. Thus, over a non activated commercial Mgo (obtained by thermal decomposition of MgC03, surface area 17 m2/g), 0.5 moles of 4-hexen-3-one/mole Mgo are converted, while when the same Mgo was activated at 350°C (surface area 34 m2/g), 2 moles of 4-hexen-3-one/mole MgO are converted. Over a high surface area Mgo (prepared by thermal decomposition of Mg(OH)2r surface area 281 m2/g) up to 5 moles of 4-hexen-3-one/mole Mgo can be converted. Conversion of 4-hexen-3-one depends also on reaction temperature 250°C is found to be the best one, since both at higher and lower temperatures side reaction are favoured (runs 2.2 and 2.3, Table 1). Since different oxides were employed, the product distributions reported in Table 1 were measured in stationary conditions after 1 hour of reaction. 

In a detailed academic study, the application potential of Li-doped MgO, prepared via different synthetic routes and with different loadings of Li, was investigated [11], Catalysts were prepared via decomposition of single source precursors, wet impregnation, precipitation, and mixed milling. The materials were sieved to a particle size < 200 pm to avoid mass transfer limitations. Certain preparations (e.g. precipitation) result in very fluffy materials in such cases, the catalysts were pressed in advance of sieving. 

A few moments thought about the nature of the surface of an oxide leads to the conclusion that the surface oxide ion should have quite different properties than the bulk lattice ions. For example, consider a simple cubic oxide such as MO with a sodium chloride structure where each ion is sixfold coordinated if this is cleaved along a <100) plane, then the coordination of the ions in this plane is reduced from six- to fivefold. This new surface will not be ideal, and ions of still lower coordination will also be present where higher index planes are exposed at the surface. However, for MgO prepared by thermal decomposition of the hydroxide or carbonate, evidence from electron microscopy (130) indicates that these have high index planes that... 

Fig. 21. Arrhenius plots of methanol and methane formation over the pyrolyzed catalyst on MgO prepared from Rh4(CO)12 0.11 g (0.15 mmol) and MgO20g, 0.75% Rh wt. dispersion. The reactor volume was 320 ml CO H2 = 200 Torr 450 Torr (33). [Reprinted with permission from Bull. Chem. Soc. Jpn. 51, 2268 (1978). Copyright 1978 Chemical Society, Japan.]...

A small crystal of MgO prepared in vacuo behaves as if it were under a hydrostatic pressure generated by the changed internuclear distances of its surface film, which would decrease its lattice parameters. As the sum of all stresses in a solid must be zero, the skin must be in a state of tension. 

Vilches, 1984). The deleterious effect of water vapour was attributed to the formation of surface layers of Mg(OH)2. This conclusion was consistent with the inferior quality (i.e. non-stepwise character) of isotherms determined on MgO prepared by die thermal decomposition of Mg(OH)2. 

Table 10.14. Comparison of BET areas derived from isotherms of nitrogen and cyclohexane on samples of MgO prepared by thermal decomposition of oxalate (Mikhail et al,. 1971).

Figure 21.9 TEM of MgO prepared by burning magnesium in air and collecting the ribbon residue. (Reproduced from [41] with permission).

Figure 21.12 High index mean planes exhibited by MgO prepared by the decomposition of magnesium basic carbonate precursor. (Reproduced from [43] with permission).

Sample Preparation The supports employed in this study were y-Al 0 (Davison, SMR 7-5913), Si02 (Davison, Grade 952), ZnO (New Jersey Zinc, CO-064), Ti02 ( g lssa, P25), and MgO (prepared by precipitation and decomposition of magnesium hydroxide, as described elsewhere (24)). The preparation of these samples was presented in... 

Water and Methanol The interaction of H20 with the (100) surface of MgO prepared by polishing and ion-bombarded single-crystal surfaces has been studied using X-ray photoelectron spectroscopy (XPS) both before and after annealing at 900 K (Onishi et al., 1987). It was observed that at room temperature water did not adsorb on the annealed surfaces. However, ion-bombarded surfaces did display features of OH-, presumably due to the creation of surface defect sites allowing water adsorption. At 200 K, both the polished and ion-bombarded surfaces showed evidence of OH- after exposure to H20, the ion-bombarded surface being covered with a monolayer of OH- (Peng and Barteau, 1990, 1991). 

I) that the mutual distances of the ions in the outer layers of LiF are the same as in the crystal. Ever since Bom calculated the theoretical figures for the surface energies of the various crystal faces of polar salts (3), all theoretical approaches agree that the mutual distance between the outer layers should be smaller than the distance between the layers in the interior of the lattice. The predicted degree of this contraction on the surface depends on which repulsion law is used and whether or not the polarization and van der Waals forces are incorporated in the calculations. Recently a publication of the work of the late Dr. Nicolson (4) revealed that small cubes of MgO, prepared in vacuo and having a particle size of roughly 500 A., show a contraction of about 0.05 %, with respect to the normal crystal parameter. 

The other common method of MgO preparation is the calcination of megnesium carbonates, oxalates and other organic salts. MgO prepared using as a starting point megnesium nitrate, acetate or... 

Basic metal oxides are active in diene hydrogenation to mono-olefins. MgO prepared from Mg(0H)2 by calcination at 1373K is highly selective in butadiene hydrogenation to cis-2-butene. A-isopropenyl-methyl-cyclohexene in the... 

Przepidrski et al. reported the competitive uptake of SO2 and CO2 on the porons carbon materials containing CaO and MgO, prepared by carbonization of poly(ethylene terephthalate) mixed with a natural dolomite. The as-prepared porous carbon was examined as a sorbent for simultaneous removal of CO2 and SO2 from air in dry conditions and in a presence of humidity, at temperatures ranging from 20 to 70 °C. The attained results clearly confirmed the crncial effect of water on the amounts of gases removed from air streams and the removal mechanisms. The... 

FIGURE 10.5 Uigh-iesolution transmission electron micrograph of [OsjC(CO),4] on MgO, prepared as shown in Figure 10.4. This cluster was present with some osmium carbonyl clusters containing three and four Os atoms. ... 

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