Gamma alumina

Polyphosphazene Alumina (gamma) Alumina (alpha) Glass Ziiconia... 

Vapor decomposition techniques are the most promising for producing single-crystal particles. In the case of alumina, gamma alumina particles are... 

To add surface area, the supports are uniformly coated with a slurry of gamma-alumina and recalcined under moderate conditions. The wash coat acts to accept the active metals, typically low levels of platinum and palladium, in a conventional impregnation process. In the United States in passenger car apphcations the spherical catalyst is used almost exclusively, and methods have been developed to replace the catalyst without removing the converter shell when vehicle inspection reveals that emission standards are not met. 

Support-phase changes or loss of surface area are, of course, irreversible, and replacement of the catalyst may be appropriate. Catalyst damage may take the form of phase changes to the alumina support from gamma to theta or alpha phase. The last is catalyticaky inert because of insignificant surface area. Theta alumina has a low surface area (< 100 /g) relative to gamma alumina (180 m /g) and has poor halogen retention. 

The tendency for high surface area gamma-alumina to siater and lose that cmcial area duriag high temperature operatioa is retarded by the intimate additioa of several perceat of cerium oxide. The mechanism is stiU under debate but may iavolve a surface LN—aluminate species on the alumina. 

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. 

Alumina is not widely used in modem HPLC [48]. Porous gamma alumina is prepared by dehydration and thermal treatment of crystalline bayerite [8,49]. It is available in several types with pore diameters from 6-lS nm, surface areas 70-250 m /g and pore volumes 0.2-0.3 ml/g. After conditioning with acid or base its apparent surface pH can be adjusted between pH 3-9. The alumina surface is more heterogeneous than silica containing both hydroxyl... 

Alumina is an amphoteric catalyst, which can difficult to characterize via chemical and physic methods. The transformation of cyclopentanol/cyclohexanone mixture allows us to estimate at the same time the acid-base properties of aluminas. From this transformation, it was shown that aluminas can be classified into two families only basic aluminas, such as theta, which were more basic than MgO, and acido-basic aluminas, eta, gamma and delta, which possess an acidic character less pronounced than dealuminated HMOR zeolite... 

From RCE experiments at 1500 rpm and 158 g/1 loading of particles in suspension, 4.4 wt% of gamma alumina particle was incorporated, which is more than three times the previously reported maximum amount of gamma alumina incorporation of 1.45 wt% [37], These results will be discussed further in terms of the process variables. 

Perhaps even more noteworthy is the effect of crystallographic phase. While one phase of a specific composition may readily incorporate from a particular bath composition, another phase of the same composition may incorporate to a much lower extent or not at all. For instance, in the alumina particle system, the alpha phase has been found to readily incorporate from an acidic copper bath while the gamma phase incorporates at less than one tenth the amount of alpha, if at all, as shown in Table 1 [2, 11, 27, 31, 33],... 

X-ray diffraction conducted on the codeposited powder revealed that the deposit obtained from a suspension of gamma alumina, which had been partially converted to the alpha phase, contained both phases of alumina. Whereas, the powder codeposited from a suspension having a 50 50 mixture of alpha to gamma alumina powder, consisted only of the alpha phase. Using a parallel plate electrode configuration, Chen et al. [31] concluded that only alpha alumina can be codeposited. Chen also observed a difference in codeposition with copper when using two different phases of the titanium oxide particle system rutile readily codeposited but anatase titania did not... 

The work by Chen et al. also resulted in no measurable incorporation with particle diameters of 0.05 and 0.02 pm gamma alumina in copper [31]. When the 0.02 pm gamma particles were calcined to obtain a mix of gamma and alpha alumina, codeposition increased to 2.9 vol.%, under the same codeposition conditions. Furthermore, when the 0.02 pm gamma powders were completely converted to the alpha phase of alumina, incorporation rose to 3.3 vol.% [31]-... 

In 1983, Buelens et al. conducted studies using Linde-manufactured 0.05 pm gamma alumina powder [30]. This work using a RDE resulted in incorporation levels of 0.035 and 0.0175 wt%, i.e. no significant incorporation. Work published in 1987 by the same group resulted in codeposition of 1.45, 0.55 and 0.30 wt% [37]. Comparison of this latter work to the work from 1983 shows codeposition increased by more than an order of magnitude. While the absolute amounts may be questionable in accuracy, the relative increase is significant. Neither of the RDE studies [30, 37] involved x-ray diffraction verification of the Linde powders, presumably since Chen et al. [31] had published such verification. 

X-ray diffraction of the Buehler-supplied gamma alumina powder used in a RCE study showed the powder to be actually a combination of alpha and gamma phase alumina [28], However, the relatively high particle incorporation obtained with this alpha-gamma alumina cannot be attributed to the partial alpha crystallographic character of the powder, since the pure alpha alumina codeposited to a lesser extent. 

Cobalt nitrate hexahydrate (97%) and D-mannitol (99%) were obtained from Alfa Aesar and used as received. Alumina HP14-150 is a gamma alumina with surface area around 150 m2 g which was supplied by Sasol and used as received. 

Perkins [18] carried out radium and radiobarium measurements in seawater by sorption and direct multidimensional gamma-ray spectrometry. The procedure described includes the removal of radium and barium from water samples on sorption beds of barium sulfate impregnated alumina (0.5-1 cm thick) and direct counting of these beds on a multidimensional y-ray spectrometer. The radioisotopes can be removed at Unear flow rates of sample of up to 1 m/min. 

Kuemmel, M. Grosso, D. Boissiere, U. Smarsly, B. Brezesinski, T. Albouy, P. A. Amenitsch, H. Sanchez, C. 2005. Thermally stable nanocrystalline gamma-alumina layers with highly ordered 3D mesoporosity. Angew. Chem. Int. Ed. 44 4589—4592. 

Borg, S. Storsaeter, S. Eri, H. Wigum, E. Rytter and A. Holmen, The effect of water on the activity and selectivity for gamma-alumina supported cobalt Fischer-Tropsch catalysts with different pore sizes, Catal. Lett., 2006, 107, 95-102. 

For small particles supported on thin films of amorphous or microcrystalline materials it is not easy to determine whether there is any consistent correlation between the particle orientation and the orientation of the adjacent locally ordered region of the substrate. For some samples of Pt and Pd on gamma-alumina, for example, nanodiffraction shows that the support films have regions of local ordering of extent 2 to 5 nm. Patterns from the metal particles often contain spots from the alumina which appear to be consistently related to the metal diffraction spots. 

Carbon deposition was also found on particles smaller than 1.0 nm supported on polycrystalline gamma alumina. In this case, the decay of TPD curves and the amount of carbon deposited was even more important than in the case discussed above of 1.7 nm particles on 000l a-A O. ... 

GAMMA-ALUMINA WITHOUT PVA pore model slit shaped... 

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