Alumina modifications

Furthermore there is a need of detailed information about the formation of metastable alumina modifications during the isothermal oxidation of p-NiAl and about the influence of reactive elements on the. alumina modifications. At temperatures below 900°C and in the initial state the formation of -y-Al203 was reported by Doychak [14] on Zr-doped 0-NiAl and on undoped 0-NiAl by Brumm and Grabke [4]. The initial formation of -Al2Oa on 0-NiAl and its transformation to a-Al203 at temperatures higher than 950°C was reported by Brumm and Grabke [4], Rybicki and Smialek [8] and Doychak [14]. 

The formation and conversion of the metastable alumina modification is also indicated by the time dependent changes of the parabolic rate constant kp. From initially high values corresponding to the fast growth of the metastable alumina modification, the kp values decrease to a lower level corresponding to the slow growth of the a-... 

During the oxidation time of 70 h series of X-ray diffraction patterns were recorded at time intervals of 1 h. The formation of each alumina modification is observed simultaneously and in situ (Fig. 10). The peak intensities of each oxide were determined as a function of time and plotted as iz(t) curves showing the growth of each observed modification. 

Values of the oxidation rate constants kp were determined from the slope of plots of the square of mass gain per area versus time. It is possible to separate the regions for the growth of metastable alumina modifications (I) in the first hours of the experiments and for protective a-alumina growth (II) after longer oxidation times. The... 

Watzig K, Hutzler T, Krell A (2009) Transparent spinel by reactive sintering of different alumina modifications with MgO. CFI-Ceram Forum Int 86 E47-E49... 

Tsyganenko, A.A., Smirnov, K.S., Rzhevskij, A.M., and Mardilovich, P.P. Infrared spectroscopic evidence for the structural OH groups of speinel alumina modifications. Mater. Chem. Phys. 1990,26, 35 6. 

Selective Toluene Disproportionation. Toluene disproportionates over ZSM-5 to benzene and a mixture of xylenes. Unlike this reaction over amorphous sihca—alumina catalyst, ZSM-5 produces a xylene mixture with increased -isomer content compared with the thermodynamic equihbtium. Chemical modification of the zeohte causing the pore diameter to be reduced produces catalysts that achieve almost 100% selectivity to -xylene. This favorable result is explained by the greatly reduced diffusivity of 0- and / -xylene compared with that of the less bulky -isomer. For the same reason, large crystals (3 llm) of ZSM-5 produce a higher ratio of -xyleneitotal xylenes than smaller crystahites (28,57). 

In order to extead appHcatioas of cBN to iaclude machining of medium-hardness steels, modifications of the cBN were iatroduced. An example is the fabrication of siatered cBN tools by the same HP—HT process, but usiag biader and second phase (either metallic or nonmetaUic) such as TiN or TiC to iacrease toughness (171). In regard to phase distribution, cBN tools resemble cemented-carbide or alumina—TiC ceramic tools, but are tougher and have greater chemical stabUity. 

Alumina is used because it is relatively inert and provides the high surface area needed to efftciendy disperse the expensive active catalytic components. However, no one alumina phase possesses the thermal, physical, and chemical properties ideal for the perfect activated coating layer. A great deal of research has been carried out in search of modifications that can make one or more of the alumina crystalline phases more suitable. Eor instance, components such as ceria, baria, lanthana, or 2irconia are added to enhance the thermal characteristics of the alumina. Eigure 6 shows the thermal performance of an alumina-activated coating material. 

Originally, p-tolylsulfonyldiazomethane was prepared by passing an ethereal solution of its precursor, ethyl i 7-nitroso-iV -(j3-toly]8ulfonyI-methyl)carbamate, slowly through a column of alumina, This procedure, which results in yields about 10% higher, is convenient only for small-scale preparations, up to a maximum of 5 g. of p-tolylsulfonyl-diazomethane. The present modification is due to Middelbos, ... 

The various studies of shock-modified powders provide clear indications of the principal characteristics of shock modification. The picture is one in which the powders have been extensively plastically deformed and defect levels are extraordinarily large. The extreme nature of the plastic deformation in these brittle materials is clearly evident in the optical microscopy of spherical alumina [85B01]. In these defect states their solid state reactivities would be expected to achieve values as large as possible in their particular morphologies greatly enhanced solid state reactivity is to be expected. 

Fig. 7.13. The conversion of theta- to alpha-phase alumina was found to be strongly affected by shock modification in work of Beauchamp and co-workers [90B01]. Whereas the unshocked powder showed evidence for an incubation period of 60 min, the shock-modified materials show immediate conversion typical of the presence of shock-formed nuclei.

A conventional FCC unit can be an olefin machine with proper operating conditions and hardware. Catalysts with a low unit cell size and a high silica/alumina ratio favor olefins. Additionally, the addition of ZSM-5, with its lower acid site density and very high framework silica-alumina ratio, converts gasoline into olefins. A high reactor temperature and elimination of the post-riser residence time will also produce more olefins. Mechanical modification of the FCC riser for millisecond cracking has shown potential for maximizing olefin yield. 

Serious research in catalytic reduction of automotive exhaust was begun in 1949 by Eugene Houdry, who developed mufflers for fork lift trucks used in confined spaces such as mines and warehouses (18). One of the supports used was the monolith—porcelain rods covered with films of alumina, on which platinum was deposited. California enacted laws in 1959 and 1960 on air quality and motor vehicle emission standards, which would be operative when at least two devices were developed that could meet the requirements. This gave the impetus for a greater effort in automotive catalysis research (19). Catalyst developments and fleet tests involved the partnership of catalyst manufacturers and muffler manufacturers. Three of these teams were certified by the California Motor Vehicle Pollution Control Board in 1964-65 American Cyanamid and Walker, W. R. Grace and Norris-Thermador, and Universal Oil Products and Arvin. At the same time, Detroit announced that engine modifications by lean carburation and secondary air injection enabled them to meet the California standard without the use of catalysts. This then delayed the use of catalysts in automobiles. 

This procedure for the synthesis of ethyl 3-nitroacrylate is essentially that of Stevens and Emmons.2 Four major changes have been introduced by the submitters rapid introduction of dinitrogen tetroxide no purification of the intermediate nitro iodo ester use of dry, finely powdered sodium acetate for elimination of hydroiodic acid and percolation of the final product through a mat of alumina. With these modifications, the preparation is reproducible and highly efficient (80-90% overall). 

Some advances have been made in the Paal-Knorr synthesis of pyrroles by the condensation of primary amines with 1,4-dicarbonyl species. For instance, a new synthetic route to monosubstituted succinaldehydes allows for the facile preparation of 3-substituted pyrroles <96TL4099>. Additionally, a general method for the synthesis of 1-aminopyiroles has been devised by the condensation of commercially available 2,2,2-trichloroethyl- or 2-(tri-methylsilyl)ethylhydrazine with 1,4-dicarbonyl compounds <96JOCl 180>. A related route to such compounds involves the reaction of a-halohydrazones with p-dicarbonyl compounds <96H(43)1447>. Finally, hexamethyldisilazane (HMDS) can be utilized as the amine component in the Paal-Knorr synthesis in the presence of alumina, and this modification has been employed in the synthesis of tm azaprostacyclin analog <96S1336>. 

Alumina, present in the gamma modification, is the most suitable high surface area support for noble metals. The y-Al203 in washcoats typically has a surface area of 150-175 m g However, at high temperatures y-alumina transforms into the alpha phase, and stabilization to prevent this is essential. Another concern is the diffusion of rhodium into alumina, which calls for the application of diffusion barriers. 

Since spillover phenomena have been most directly sensed through the use of IR in OH-OD exchange [10] (in addition, in the case of reactions of solids, to phase modification), we used this technique to correlate with the catalytic results. One of the expected results of the action of Hjp is the enhancement of the number of Bronsted sites. FTIR analysis of adsorbed pyridine was then used to determine the relative amounts of the various kinds of acidic sites present. Isotopic exchange (OH-OD) experiments, followed by FTIR measurements, were used to obtain direct evidence of the spillover phenomena. This technique has already been successfully used for this purpose in other systems like Pt mixed or supported on silica, alumina or zeolites [10]. Conner et al. [11] and Roland et al. [12], employed FTIR to follow the deuterium spillover in systems where the source and the acceptor of Hjp were physically distinct phases, separated by a distance of several millimeters. In both cases, a gradient of deuterium concentration as a function of the distance to the source was observed and the zone where deuterium was detected extended with time. If spillover phenomena had not been involved, a gradientless exchange should have been observed. 

Modification of Y-AI2O3 by K cation was shown to form a potassium oxides layer. This layer blocks the alumina surface and decreases drastically (> 99%) the catalytic activity with respect to pure Y-AI2O3... 

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