Aluminas structural data

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. 

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. 

Fig.l shows the pore size distributions of y-alumina, titania (anatase) and tetragonal zirconia (after calcination at 450°C for 3 hours). The pore structure data of these three adsorbent samples are compared in Table 2. The pore structure data were obtained from nitrogen adsorption isotherms measured by Micromeritics ASAP-2000 adsorption porosimeter. As shown in Fig.l, the pore size distributions of these materials are rather narrow, with an average pore diameter of about 3 nm. Such narrow size distribution and nanoscale average pore size are determined by the primary crystallite particles. The particles of the... 

FARRINGTON Both structural and NMR results indicate that sodium ions occupy non-equivalent sites within the beta alumina conduction-p1ane. It is unclear exactly how this non-equivalency is manifest on the microscopic level whether at unit cells or in larger domains. Similar non-equivalency does not seem to be the case in alumina. I submit that detailed models of interfacial behaviour in g-alumina must take into consideration these structural data presently available despite their ambiguities. Complete site equivalency and ion mobility should not be assumed for g-alumina. 

Because of the comparable ionic radii of Cr " and Al ions, a coordination geometry of the CrOg entity similar to the sixfold coordinated Al-sites is expected. Therefore, we can make use of structural data of the alumina polyhedra for the AOM calculations. Although we have not adopted the e j parameters to the very different metal-oxygen distances, which vary between 183.5 pm and 198.9 pm at the mirror site [ 104], an acceptable fit of the spectroscopically determined transition energies could be obtained. Table 6 compares the experimental level scheme with the term energies calculated by using AOM parameters, which are... 

There are, of course, many more ceramics available than those listed here alumina is available in many densities, silicon carbide in many qualities. As before, the structure-insensitive properties (density, modulus and melting point) depend little on quality -they do not vary by more than 10%. But the structure-sensitive properties (fracture toughness, modulus of rupture and some thermal properties including expansion) are much more variable. For these, it is essential to consult manufacturers data sheets or conduct your own tests. 

Ciguatoxin. The toxin was isolated from moray eels and purified to crystals by Scheuer s group (1). Structural determination of the toxin by x-ray or NMR analyses was unsuccessful due to the unsuitability of the crystals and due to the extremely small amount of the sample. The toxin was presumed to have a molecular formula of C Hg NO from HRFAB-MS data (MH+, 1111.5570) and to have six hydroxyls, five methyls, and five double bonds in the molecule (2). The number of unsaturations (18 including the five double bonds) and the abundance of oxygen atoms in the molecule point to a polyether nature of the toxin. The toxin, or a closely related toxin if not identical, is believed to be the principal toxin in ciguatera. Ciguatoxin was separable on an alumina column into two interconvertible entities presumably differing only in polarity (J). 

From results on interatomic distances derived from analysis of EXAFS data, one can draw some conclusions about the structure of platinum-iridium clusters (13,17). If the clusters were truly homogeneous, the interatomic distance characteristic of the platinum EXAFS should be identical to that characteristic of the iridium EXAFS. When we analyze EXAFS data on the clusters, however, we do not find this simple result. We find in general that the distances are not equal. The data indicate that the clusters are not homogeneous in other words,the environments about the platinum and iridium are different. We conclude that the platinum concentrates at the surface or boundary of the clusters. In the case of very highly dispersed platinum-iridium clusters on alumina, the clusters may well have "raft-like" two dimensional structures, with platinum... 

After the calcination step, experimental data (XRD, 29 i maSNMR) show that a zeolite with the silicalite structure has been formed. 29 i MASNMR indicates for the zeolite material a Si/Al ratio depending on the sample prepared it has been observed that both the natures of the silicon source and of the alumina supports may originate these fluctuations. 

The adsorption mechanism in chromatography on alumina differs from that on silica gel because of the structural differences between these adsorbents. Relationships between the values of solutes and the adsorption data for the mobile phase components on sihca gel G and alumina G have been investigated by Rozylo [64,65]. The theoretical and experimental results obtained by the relation 2 = /( 1) show a good agreement for the two adsorbents. 

The structure of alumina on NiAl(l 1 0) was the subject of a surface X-ray diffraction study by Stierle et al. [46]. The model derived by Stierle et al. from the analysis of the X-ray diffraction data was based on a strongly distorted double layer of hexagonal oxygen ions, where the Al ions are hosted with equal probability on octahedral- and tetrahedral-coordinated sites the resulting film structure was closely related to bulk k-A1203. An attractive feature of Stierle s model was that it provided a natural explanation of the domain structure of the alumina overlayer, which is induced by a periodic row matching between film and substrate lattices. However, as pointed out recently by Kresse et al. [47], this structure model has two bonds with... 

The single crystal results are compared in Fig. 2 with three sets of data taken from Ref. 13 for nickel supported on alumina, a high surface area catalyst. This comparison shows extraordinary similarities in kinetic data taken under nearly identical conditions. Thus, for the Hj-CO reaction over nickel, there is no significant variation in the specific reaction rates or the activation energy as the catalyst changes from small metal particles to bulk single crystals. These data provide convincing evidence that the methanation reaction rate is indeed structure insensitive on nickel catalysts. 

Mitrovic and Knezic (1979) also prepared ultrafiltration and reverse osmosis membranes by this technique. Their membranes were etched in 5% oxalic acid. The membranes had pores of the order of 100 nm, but only about 1.5 nm in the residual barrier layer (layer AB in Figure 2.15). The pores in the barrier layer were unstable in water and the permeability decreased during the experiments. Complete dehydration of alumina or phase transformation to a-alumina was necessary to stabilize the pore structure. The resulting membranes were found unsuitable for reverse osmosis but suitable for ultrafiltration after removing the barrier layer. Beside reverse osmosis and ultrafiltration measurements, some gas permeability data have also been reported on this type of membranes (Itaya et al. 1984). The water flux through a 50/im thick membrane is about 0.2mL/cm -h with a N2 flow about 6cmVcm -min-bar. The gas transport through the membrane was due to Knudsen diffusion mechanism, which is inversely proportional to the square root of molecular mass. 

The dehydration of cis,alumina surfaces, could readily explain the observed data ... 

Ruthenium catalysts, supported on a commercial alumina (surface area 155 m have been prepared using two different precursors RUCI3 and Ru(acac)3 [172,173]. Ultrasound is used during the reduction step performed with hydrazine or formaldehyde at 70 °C. The ultrasonic power (30 W cm ) was chosen to minimise the destructive effects on the support (loss of morphological structure, change of phase). Palladium catalysts have been supported both on alumina and on active carbon [174,175]. Tab. 3.6 lists the dispersion data provided by hydrogen chemisorption measurements of a series of Pd catalysts supported on alumina. is the ratio between the surface atoms accessible to the chemisorbed probe gas (Hj) and the total number of catalytic atoms on the support. An increase in the dispersion value is observed in all the sonicated samples but the effect is more pronounced for low metal loading. 

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