3-alumina structure

Figure 5. Schematic representation of the ft -alumina structure. The aluminum (green) and oxygen (red) ions form spinel blocks which are separated from each other by oxygen bridges. The mobile sodium ions (blue) are located in the layer. The unit cell is indicated.

The ionic conductivity for various ions in the piP"-alumina structure along the conduction planes shows a maximum for an optimum size of the ions. It should be neither too small nor too big to fit the available pathways in the lattice [8]. 

The P —alumina structures are remarkable not only for their ionic conductivities but also for the versatility in isomorphous replacement. There is little of the structure of (Na2S)l+Jt 11A1203 which cannot be substituted, at least in part, by an alternative ion. MgO and Li20 are preferred additives to P —alumina in order to obtain good ionic conduction with no electronic contribution. 

Fig. 2.17 A scheme of /3-alumina structure. The gaps between blocks of /3-phase A1203 function as bridging layers sparsely populated by oxygen (O) and sodium ( ) ions.

The beta-alumina structures show a strong resemblance to the spinel structure. They are layered structures in which densely packed blocks with spinel-like structure alternate with open conduction planes containing the mobile Na ions. The and /S" structures differ in the detailed stacking arrangement of the spinel blocks and conduction planes. Fig. 2.9. 

The beta-alumina structures (j8,j8"-alumina and the analogous gallates) can be prepared as H30 or NH4 derivatives by ion exchange and some of these are good proton conductors at temperatures up to 200-400 °C, until they decompose by loss of H2O/NH3. 

Framework (skeleton) structures of oxides have been identified for fast ion conduction of Na" and other ions (Goodenough et al., 1976). One-, two- or three-dimensional space is interconnected by large bottlenecks in these oxide hosts. While the tungsten bronze and j8-alumina structures contain one- and two-dimensional interstitial space, the hexagonal framework of NaZr2(P04)3 has a three-dimensional... 

Effect of the Transition Metal Ions - Hexaaluminate materials, 1 >19, including transition metal ions in the structure (M = Mn, Fe, Cr, Co, Ni) were prepared both via the alkoxide15 and the coprecipitation route.23,24,25 For all the compositions investigated, monophasic samples with layered-alumina structure and surface area in the range 10-15 m2/g were obtained upon calcination at 1300 °C. 

These data indicate that sintering resistance is related not only to the formation of a layered-alumina structure. It appears that a critical Ba content is required for the effective suppression of crystal growth along the c axis of crystallites. Indeed, in BaAli4022, sintering proceeded even after the formation of Ba-P-Al203. 

Effective preparation methods of hexaaluminates for catalytic applications, such as the hydrolysis of alkoxides and the co-precipitation in aqueous medium, ensure high interspersion of the constituents in the precursor. This allows the formation of single phase materials with layered-alumina structure at reasonably low temperature (1100-1200 °C) and with high surface area. The hydrolysis of alkoxides was extensively studied and used for the industrial scale-up in the production of catalysts in the monolith shape. However, the co-precipitation in aqueous medium has much potential in view of the possible commercialization of these materials due to its simplicity and low cost. 

The most obvious choice to determine phases that may be present in the molybdena catalyst is XRD. Matching of diffraction lines obtained for the catalyst with those of pure bulk compounds gives unequivocal identification of phases present. This is one of the few techniques that yields positive results. The absence of matching diffraction lines, however, is not proof that the phase in question is not present in the catalyst. The XRD technique is limited to particle sizes of above approximately 40 A for oxides or sulfides, lower sized particles giving no discernible pattern over that of the broad alumina pattern. Thus, the presence of a highly dispersed phase, either as small crystallites or as a surface compound of several layers thickness will not be detected. Also, if the phase is highly disordered (amorphous), a sharp pattern will not be obtained, although some broad structure above that of the alumina may be detected. It is a moot point as to whether such a case is considered as a separate phase or a perturbation of the alumina structure. Ratnasamy et al. (11) have examined their CoMo/Al catalyst from the latter point of view, with particular emphasis on the effect of calcination temperature. 

In this study, a novel Monolith alumina structure was of interest as a base (or a carrier) material for Co-Mo-Alumina catalysts. The specific interest centered around assessing the suitability of the catalyst prepared by impregnating the novel alumina support with Co and Mo for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of a relatively high boiling stock. The Monolith catalyst was also tested on a low boiling coal-derived liquid. 

Figure 2. Monolith alumina structure top, Monolith alumina segments bottom, cross section of a monolith alumina segment...

The crystal structure of BAM is shown in fig. 34. It has the ft -alumina structure with lattice parameters a = 5.6275 k,b = 22.658 A (fyi et al., 1986). There are layers consisting of Ba2+ and O2- ions between the spinel blocks and Eu2+ is generally believed to stay within these Ba-0 layers. The positions of BR, aBR, and mO sites within this Ba-0 layer are shown in fig. 34. The BR site is the substitutional site of Ba and the other two sites are interstitial sites. 

Pt-Sn-Alumina Structure. No single model will adequately describe the above catalyst characterization data and the published data that has not been included because of space limitations. The relative distribution of both the Pt and Sn species depend upon a number of factors such as surface area of the support, calcination and/or reduction temperature, Sn/Pt ratio, etc. Furthermore, it appears that the "co-impregnated" and "co-precipitated" catalysts are so different that their structure should be considered separately. 

Various schemes have been proposed for the classification of the different alumina structures (Lippens and Steggerda, 1970). One approach was to focus attention on the temperatures at which they are formed, but it is perhaps more logical to look for differences in the oxide lattice. On this basis, one can distinguish broadly between the a-series with hexagonal close-packed lattices (i.e. ABAB...) and the y-series with cubic close-packed lattices (i.e. ABCABC...). Furthermore, there is little doubt that both y- and j/-A1203 have a spinel (MgAl204) type of lattice. The unit cell of spinel is made up of 32 cubic close-packed O2" ions and therefore 21.33 Al3+ ions have to be distributed between a total of 24 possible cationic sites. Differences between the individual members of the y-series are likely to be due to disorder of the lattice and in the distribution of the cations between octahedral and tetrahedral interstices. 

Hydrothermal treatment causes much more intensive transformation of alumina structure in comparison to thermal treatment without water steam. It is especially important in the case of alumina used in hydrothermal conditions, e.g. in membrane separation technology and washcoats in car catalytic converters. 

Strobel, R., Stark, W.J., Madler, L., Pratsinis, S.E., and Baiker, A., Flame-made platinum/alumina structural properties and catalytic behaviour in enantioselective... 

FIGURE 6.7 Scanning electronic microscopy images of inorganic membrane porous structures (a) asymmetric alumina (b) asymmetric carbon structure (c) homogeneous alumina structure and (d) homogeneous glass structure. 

The p>ore arrangement on the alumina surface is random (Fig. 3b). Selfordering of porous alumina appeared in the bottom parts (Fig. 3c,d). Ordering is enhanced with increasing of porous alumina thickness. The important feature of porous aluminium formed at high current densities is the formation of a nanotubular alumina structure. We have found that such structures were observed at forming current densities more than 100 mA/cm. Such forming... 

Figure 49 Possible arrangements of alkali-metal ions in the p-alumina structure, (a) and (b) show two alternative cation distributions, (c) and (d) show possible domain walls between these alternatives. Alkali-metal atoms are represented by filled circles and oxygen atoms by open circles...

Since Li is much smaller than Na+ it might diffuse more readily imo the alumina structure, leaving more surface sites intact causing less deactivation. 

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