Page 106 - 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

504 Catalyst Characterization

suspension properties, related to particle size and den- are made porous by the selective leaching of an alloy con-
sity, are important. Pore size is critical for shape-selective stituent, usually aluminum. Combustible substances are
applications such as the dewaxing of lubricating oils or incorporated into ceramics, which, when burned out, cre-
synthesis of molecules of a particular size. ate pores in the host ceramic. Finally, during catalysis a
Massive metals themselves are used as unsupported material may become more porous by the volatilization or
fixed-bed catalysts; for example, Raney nickel is used in a recrystallization of certain components, the most common
variety of hydrogenation reactions. The synthesis of am- example being PtRh (or PtPdRh) alloys used for the oxi-
monia from N 2 and H 2 is carried out with reduced massive dation of ammonia to nitric acid, which becomes porous
iron containing minor amounts of promoters. by the volatization of platinum oxides during the reaction.
The size and number of pores determine the internal sur-
face area. It is usually advantageous to have high surface
C. Monolithic Catalysts
area (high density of small pore sizes) to maximize the
1. Catalyzed Washcoats on Monoliths dispersion of catalytic components; however, molecules
such as those present in heavy petroleum or coal-derived
A slurry of a high surface area oxide (Al 2 O 3 ,TiO 2 , SiO 2 ,
feedstocks may be so large that they are excluded from
etc.) is deposited as a thin layer onto the channels of a
small pores. The pore structure and surface area must be
ceramic or metal honeycomb. This washcoat is then made
optimized to provide maximum utilization of active cat-
active by impregnation with catalytic species. The honey-
alytic sites for a given feedstock.
combs vary in composition, cell density and shape, and
wall thickness (Fig. 2, label 5). They must have sufficient
surface porosity or roughness to allow the washcoat to
1. Gas Adsorption: Surface Area
adhere tightly. The overall geometry is dictated by the dy-
namics of the reaction of interest, but the most common The most common procedure for determining the inter-
use for honeycomb catalysts is for high-throughput gas re- nal surface area of a porous material, with surface areas
2
2
actions where pressure drop must be minimized such as in greater than 1 or 2 m /g and up to ∼1200 m /g, is based on
pollution abatement from both moving (auto exhaust) and the adsorption and condensation of N 2 at liquid N 2 tem-
stationary (chemical plants) sources. Other applications in perature. The partial pressure of N 2 above the sample is
the chemical industry are being pursued. gradually increased, and N 2 molecules are physically ad-
The catalyzed washcoat possesses internal structure sorbed onto the surface, approaching monolayer coverage
similar to those of catalyzed powders and particulates, and (first steep portion of isotherm shown in Fig. 3A). Each
hence the properties applicable to them are also important adsorbed molecule occupies an area of the surface compa-
˚ 2
to the washcoat. In addition, washcoat adhesion plays a rable to its cross-sectional area (∼16.2 A ). By measuring
critical role since gas throughputs are extremely high and the number of N 2 molecules adsorbed at monolayer cover-
exfoliation can be a common problem. age, one can calculate the internal surface area. In practice,
The most widespread use of monoliths is for catalytic coverage beyond a monolayer occurs, and at high relative
conversion of pollutants generated from the internal com- N 2 partial pressures, condensation of liquid N 2 in the pores
bustion engines of automobiles. Thus, the material must occurs. The Brunauer, Emmett, and Teller (BET) equation
be mechanically strong to resist vibration and rapid tem- describes the relationship between volume adsorbed at a
perature excursions. given partial pressure and the volume adsorbed at mono-
layer coverage:

P 1 (C − 1)P
II. PHYSICAL PROPERTIES = + .
OF CATALYSTS V (P 0 − P) V m C V m CP 0
Here, P is the partial pressure of N 2 , P 0 the saturation
A. Surface Area, Pore Size, and Pore Volume
pressure at the experimental temperature, V the volume
Surface area, pore size, and pore volume are among the adsorbed at P, V m the volume adsorbed at monolayer cov-
most fundamentally important properties in catalysis be- erage, and C a constant.
cause the active sites are present or dispersed throughout This equation can be linearized by plotting P/V (P 0 −
the internal surface through which reactants and products P) against P/P 0 , in which the slope is (C − 1)/V m C,
are transported. The pores are usually formed by drying or whereas the intercept is equal to 1/V m C (Fig. 3B). The
calcining precipitates of hydrous oxides; however, some sum of the slope and intercept yields the reciprocal of V m .
materials possess porosity naturally, as in the case of car- The most reliable results are obtained at relative pressures
bons, natural zeolites, and others. Raney nickel catalysts (P/P 0 ) of between 0.05 and 0.3.