MgO , structure

These results suggest that oxidation state is not solely responsible for catalyst deactivation but that other factors such as V location and mobility may play an important role. Basic alkaline earth oxide passivators such as MgO, admixed to the catalyst, interact strongly with vanadium during the regeneration period. Although the oxidation state of vanadium is essentially unaffected, MgO structurally modifies V as evidenced by a unique X-ray absorption spectrum. 

Fig. 5. (a) Effect of sintering on particle dimensions. (Ill) facelets are omitted, (b) (Top) Representation of MgO structure reconstructed (111) facelet with tricoordinated oxygen (middle) terraced surface with steps and edges, where ions are tetracoordinated (bottom) (001) flat surface with pentacoordinated species (Mg2+ ions are white and O2 ions are gray). 

Triantafillou, N. D., Purnell, S. K., Papile, G X., Chang, I.-R., and Gates, B. C., A Family of rhenium subcarbonyls on MgO Structural characterization by extendedX-ray absorption fine structure spectroscopy. Langmuir 10, 4077 (1994). 

Our support precursor having a Mg Al molar ratio of about 3 1 shows an x-ray diffraction pattern typical of hydrotalcite (see Figure la) (10). After calcination at 873 K the resulting diffraction pattern exhibits diffuse peaks corresponding to MgO (Figure lb). No evidence for separate crystalline aluminum phases was found so A1 cations probably remain closely associated with or dissolved in the MgO structure. However, it is possible that amorphous alumina phases, not detectable by x-ray diffraction, may be present in our mixed oxide. 

It has been proposed that the slit-shaped micropores may be regarded as spaces between (111) planes in the freshly formed MgO structure. Thus, if the width of each micropore was equivalent to four (111) planes, its width would be 0.96 nm, which is quite close to the hydraulic values. It is not surprising to find pore widening to have occurred as the decomposition neared completion. 

When impregnated with Ce(N03)3 solution and calcined the resulting catalysts are extremely active towards SOx abatement (Table I, Figure 4). The solid solution spinel catalyst is nearly 25% more active than the stoichiometric spinel catalyst. This can be explained by assuming that -MgO- structural fragments of the spinel are the chemisorption active sites (1). There are more active sites in Mg2Al20c than in MgA1 04. We also see that the catalyst prepared by co-precipitation method is 4 times more active than the catalyst prepared by the thermal co-condensation method. In Zone D we see that the sulfated catalyst is very effectively reduced by H. Nearly 70% of the absorbed species is reduced within 2 min. of reduction. 

Fig. 2.22. Electronic (optical) absorption spectrum of octahedrally coordinated Fe-+ ions in the periclase (MgO) structure (after Goto et al., 1980 reproduced with the publisher s permission).

Chang, K. J., and M. O. Cohen (1984b). High-pressure behaviour of MgO structural and electronic properties. Phys. Rev. B30, 4774-81. 

Fig. 193. Relation between MgO structure and a-AhOg, (a) MgO structure (b) AljOg structure.

Alexeev OS, Kim D-W, Gates BC (2000) Partially decarbonylated tetrairidium clusters on MgO Structural characterization and catalysis of toluene hydrogenation. J Mol Catal A Chem 162 67... 

The decomposition of magnesium hydroxide has been evaluated as a process of nucleation and growth of MgO crystals within the brucite matrix. The formation of a defect layer of hydroxide structure, which suddenly recrystallizes to the cubic MgO structure when the fracture stress is exceeded in the defect layer, has been postulated (Freund et al., 1975 Guilliat et al., 1970 Garn et al., 1978 Lpnvik, 1978). The MgO crystals formed have an expanded cubic lattice that upon increasing calcination temperature gradually decreases in size until the equilibrium unit cell dimension is reached. 

Because of their basicity, hydrotalcites stored in air become carbonated, then neutral. They can be decarbonated by treatment above 673 K, and a mixed oxide of MgO structure is then obtained which has basic properties. The basic strength can be estimated from the temperature of decarbonation, and it is then observed that this temperature is affected by the presence of chlorine on the surface, even in trace amounts [11]. This temperature is shifted by ca 30 K towards high temperatures when CP has been fully exchanged by carbonate. Because chlorine is normally present at trace levels only, these anions must have a long-range influence and reduce the basicity of the stronger basic sites. 

Calcined Hydrotalcite. In most catalytic applications, hydrotalcite materials are used after calcination at around 773 K to form mixed oxides (16,17). The Mg0-Al203 mixed oxides have periclase (MgO) structure and are highly basic with some acidic sites. They are active for aldol condensation, hydrogen-transfer reactions, and many other reactions. From the catalytic activity of a calcined Mg-Al hydrotalcite for the Knoevenagel condensation of benzaldehyde with various activated methylenic compounds with different values, it was concluded that the calcined hydrotalcite had basic sites with H- up to 16.5, most of them being in the range of 10.7 < H < 13.3 (21). 

As already stated, several slip systems have been proven for the MgO structure, and so dislocation movement is relatively easy and varied. Such... 

---