Page 103 - 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

Robert J. Farrauto
Melvin C. Hobson
Engelhard Corporation

I. Physical Forms of Heterogeneous Catalysts
II. Physical Properties of Catalysts
III. Chemical Properties
IV. Complementary Techniques

GLOSSARY sible for its performance in a given reaction. Measure-
ments include composition, active sites, and the external
Active sites Microscopic locations of a catalyst where and internal structure of the solid material through which
chemisorption and reaction to products occur. reactants and products must be transported during catal-
Catalyst Substance that alters the rate of a chemical re- ysis. Within the catalyst matrix itself (when there is an
action without itself being consumed or generated. internal structure) five fundamental processes must take
Chemisorption Adsorption of reactants onto catalytic place (in order of occurence): (1) diffusion or transport
sites with comparable energetics and kinetics as a of reactant(s) to active sites through the pore structure
chemical reaction. of the catalyst, (2) chemisorption of reactant(s) onto ac-
Dispersion Measured number of catalytic sites available tive sites, (3) chemical reaction of chemisorbed species
to a probe gas compared with the total ideally present. to produce product(s), (4) desorption of product(s) from
Pore size Approximate diameter of the microchannels active sites, and (5) diffusion or transport of product(s)
within a porous material. through the pore structure. Figure 1 is an idealized pic-
Pore volume Amount of void volume within a material. ture of these five fundamental steps. These steps are
Surface area Internal surface of a material accessible to common to all catalysts with internal pore structures,
physically adsorbed gas. whereas nonporous materials perform catalysis only on
Washcoat High surface area oxide impregnated with cat- their external surface. These phenomena are referred to
alytic species and bound to the walls in the channel of frequently throughout this article since the catalyst prop-
a monolithic structure. erties to be characterized determine the efficiency of
the catalytic reaction. This review is divided into four
sections. Section I describes many physical forms of
THE CHARACTERIZATION of a heterogeneous cat- heterogeneous catalysts used commercially. Sections II
alyst is the quantitative measure of those physical and and III, physical and chemical properties, respectively, de-
chemical properties of the catalyst assumed to be respon- scribe those techniques most important in characterizing

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