Surface areas gamma alumina

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. 

Five types of commercial SOx catalysts were tested for comparative ranking. Three of these commercial-type additives were well defined materials high surface area gamma alumina 10% Ce, as cerium oxide, on gamma alumina and 100 ppm Ft on gamma alumina. 

The most active catalysts are derived from supported platinum group metals, or copper chromite when the VOC contains chlorine. The catalyst supports are produced as high surface area gamma-alumina spheres, extmdates or cylindrical pellets that can be used in dust free conditions. Alternatively, when treating large volumes of effluent gas, honeycomb monohths made from cordierite or metal sheets are coated with a surface washcoat of the same alumina are particularly usefiil. The supports are selected to be stable at the operating temperature of the reaction. 

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. 

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. 

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... 

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. 

Materials. The sodium n—decylsulfate (CioS0 ) from Kodak and the sodium n-dodecylsulfate (CiaSO ) from Fisher were purified by recrystallization from water and from methanol, followed by drying under a vacuum. The alumina used was Aluminum Oxide C (Degussa Inc.), a primarily gamma alumina, with a surface area of lOO m /g. The NaCl was Fisher reagent grade and the water was distilled and deionized. 

The catalysts, both fresh and used, were characterized as to BET surface area, pore size distribution, elemental analysis, x-ray diffraction and XPS. Some BET and pore volume data are given in Table 1. The diffraction pattern of Catalyst B gave some indication of a gamma-alumina phase, not well resolved All other peaks were well-resolved, suggesting the absence of amorphous or highly-dispersed phases. 

The most common carrier is gamma alumina (y-Al203). It has an internal area of >200-300 m2/g. Its surface is highly hydrox-ylated (i.e., Al-O-2 H+). The H+ sites provide acidity required for many reactions and exchange sites for catalytic metal cations. 

In general, a catalyst consists of a support (alumina, silica, silica-alumina), an active catalytic metal and, in some cases, a promoter (] ). The support is usually a high-surface-area (up to several hundred square meters per gram) porous solid. Gamma alumina, for example, has a surface area of 100-300 m /g. The support is not necessarily inert and may play a significant role in chemisorption and oxidation state control. The active metal, which may be deposited by several techniques, is highly dispersed on the support. The promoter is an additive that can Increase the activity of the metal and/or maintain the physical characteristics of the support. 

Typically, a mono-metallic reforming catalyst contains 0.3-0.5 wt% Pt, in crystallites of the order of < 2 nm, dispersed on a defective transitional gamma-eta alumina whose surface area is of the order of 200m2/g. The purpose of the alumina is to provide a platform for Pt in high dispersion, and to provide acid sites with a narrow range of strengths in order to achieve this, the alumina must be chlorided to between 0.9 and 1.1 wt%. In the S/R reforming process both moisture and a source of chloride, typically propylene dichloride (PDC), are added and controlled continuously in the recycle gas in order to maintain an equilibrium level of acidity.26... 

These results demonstrate that zeolite catalysts perform well in honeycomb form. This is similar to the well established use of noble metals/gamma alumina washcoats on cordierite honeycombs for automotive emission control. This geometrical form offers the advantages of low pressure drop, high geometric surface area, and short diffusion distance in a fixed bed reactor. 

Gamma alumina extrusions (3 mm dia 10 mm length) were used as the support. Its BET surface area is 240 m /g and pore volume is 0.65 cc/g. All the catalysts were prepared by impregnating AI2O3 extrusions with H2PtCl6 solution. Wet impregnated extrusions were further processed to four different types of catalysts as follows ... 

Gamma alumina extrudates have been used in this work. This support has a BET surface area of 300 mVg and a total water pore volume of 0,8 cc/g of dry support. Trimethylmethoxysilane was purchased from Aldrich (99% purity, MW=104 g/mol). It was grafted on alumina in dried boiling toluene (VWR,... 

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