High surface area carrier

Catalysts vary both in terms of compositional material and physical stmcture (18). The catalyst basically consists of the catalyst itself, which is a finely divided metal (14,17,19) a high surface area carrier and a support stmcture (see Catalysts, supported). Three types of conventional metal catalysts are used for oxidation reactions single- or mixed-metal oxides, noble (precious) metals, or a combination of the two (19). 

Carrier. The metal catalyst is generally dispersed on a high surface area carrier, ie, the carrier is given a washcoat of catalyst, such that very small (2—3 nm dia) precious metal crystaUites ate widely dispersed over the surface area, serving two basic functions. It maximizes the use of the cosdy precious metal, and provides a large surface area thereby increasing gas contact and associated catalytic reactions (18). 

Figure 4.1. Supported catalyst, consisting of small particles on a high surface area carrier such as silica or alumina, along with two simplified model systems, which in general offer much better opportunities for characterization at the molecular level.

Fig. 1. Steps in the formation of an olefin polymerization catalyst. Chromium is thought to bind the high-surface-area carrier by reaction with hydroxyl groups. Activation is accomplished by calcining the support at a temperature of 600° C or higher, which removes much of the excess hydroxyl group population.

Activation serves to oxidize the chromium to Cr(VI), to spread it out onto a compatible high-surface-area carrier such as silica, to anchor each Cr(VI) species individually to the carrier surface, and to dehydroxylate the... 

Precipitation into submicron-sized particles is another direct approach. Precipitation by pH shifting can be an effective approach for dyes that have weak acid functionality. A number of different families of such dyes have been dispersed by acidification of weakly alkaline dye solutions, in the presence of stabilizers such as surfactants and polymers. Alternatively, solvent shifting has been demonstrated to be an effective method of preparing absorber dye dispersions. Recent work by Brick et al. (14) has shown how such dyes can be very effectively precipitated from a variety of water-miscible organic solvents. Finally, another approach for incorporation of absorber dyes is to precipitate or condense them on the surface of a high-surface-area carrier species, such as colloidal silica. Such preparations can be prepared by pH- and solvent-shifting processes, in the presence of the carrier particles. 

There is a second procedure for increasing the surface area of the HPAs, and this consists in supporting them on a high surface area carrier. Suitable carriers should not have basic properties, otherwise the HPA will react with the surface and become deactivated. Best results have been obtained on silica, zirconia polymers and siliceous MCM-41 materials. When silica is used as the support, one optimum in the HPA content is observed for amounts above the monolayer. The optimum content... 

Studies to reduce the reaction of Rh with high surface area carriers such as stabilized y-Al203 have been conducted (119). At temperatures in excess of... 

Commercial Metal Oxide SCR Catalysts. In commercial metal oxide SCR catalysts, Ti02 (in the anatase form) is used as high surface area carrier to support the active components, that is, vanadium pentoxide and tungsten trioxide (or molybdenum trioxide). The choice of Ti02-anatase as the best support for SCR catalysts relies on two main reasons ... 

The abatement system is usually designed for maximum heat recovery, as shown in the diagram in Fig. 7.2. The igniter is needed only to pre-heat the pollutant-laden inlet gas to initiate the oxidation reaction. If the inlet gas temperature becomes sufficiently higher than that necessary for light-ofF no external heat source is needed to sustain catalytic oxidation. The catalyst shown is a precious metal(s) deposited on a high surface area carrier such as y-Al203 washcoated onto the walls of the monolith structure. 

The surface area of the support greatly affected the performances (VOC conversion and NOx selectivity) of the catalyst. The catalyst in which the precious metal was dispersed onto a high surface area carrier (150 m /g) showed complete conversion at around 225°C, while the catalyst with alow surface area (25 m /g) showed complete conversion at 300°C (Fig. 7.15). 

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