Page 105 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: ZBU 2nd Revised Pages
Encyclopedia of Physical Science and Technology EN002G-87 May 19, 2001 20:3

Catalyst Characterization 503

FIGURE 2 Some physical forms of heterogeneous catalysts. 1, Particulates; 2, extrudates, 3, powders; 4, rings; 5,
monoliths; 6, tablets; 7, spheres; 8, carbon powders and particulates.

B. Unsupported Catalysts the oxidation of methanol is sometimes carried out with
screens or gauzes of silver.
1. Powders
These catalysts are manufactured as smooth wires with
Powders possessing relatively high surface area and ac- nointernalporesandthenwovenintogauzepads.Mechan-
tive sites can be intrinsically catalytically active them- icalrigidityisimportantsincethereactorsareusuallylarge
selves. Powders of nickel, platinum, palladium, and cop- in diameter (i.e., 4–12 ft) and are used in the reactor with
per chromites find broad use in various hydrogenation minimum physical support. Furthermore, the conditions
reactions, whereas zeolites and metal oxide powders are of operation are quite severe with respect to temperature
used primarily for cracking and isomerization. All of and corrosion, and thus metallurgical integrity must be
the properties important for supported powdered catalysts maximized. The most important properties are the purity
such as particle size, resistance to attrition, pore size, and of composition, wire diameter, and mesh size as well as
surface area are likewise important for unsupported cat- mechanical strength.
alysts. Since no additional catalytic species are added, it
is difficult to control active site location; however, intu-
itively it is advantageous to maximize the area of active 3. Unsupported Particulate Catalysts
sites within the matrix. This parameter can be influenced
Unsupported particulates, like their powder counterparts,
by preparative procedures.
contain active sites without the addition of other catalytic
species. Synthetic zeolites and SiO 2 –Al 2 O 3 catalysts used
for cracking heavy oils to gasolines are catalytic due to
2. Gauzes
their acid sites. They are produced by chemical reac-
Metal wires and screens are used as fixed-bed catalysts tions between the various components but can be found
in which reactants are passed through the openings in the in nature. These materials are often modified by chemical
gauze, the size of which is defined by the mesh and wire techniques such as ion exchange; however, the impregna-
diameter (see Fig. 10A). Gauzes composed of an alloy of tion techniques typical of dispersed catalysts are not used.
platinum and rhodium catalyze the air oxidation of am- Promoters can be added to enhance performance.
monia to nitric oxide, which is subsequently converted to These materials are usually used in moving- or
nitric acid, and the production of hydrogen cyanide from fluidized-bed reactors and thus are prone to severe at-
ammonia, air, and methane. Formaldehyde production by trition. Furthermore, because they are fluidized their