Page 149 - Materials Science and Engineering An Introduction

4.6 Interfacial Defects • 121

M A T E R I A L S O F I M P O R T A N C E

Catalysts (and Surface Defects)

catalyst is a substance that speeds up the rate of carbon dioxide (CO 2 ). Furthermore, any unburned
A a chemical reaction without participating in the hydrocarbons are also oxidized to CO 2 and H 2 O.
reaction itself (i.e., it is not consumed). One type of One of the materials used as a catalyst in this
catalyst exists as a solid; reactant molecules in a gas or application is (Ce 0.5 Zr 0.5 )O 2 . Figure 4.12 is a high-
9
liquid phase are adsorbed onto the catalytic surface, resolution transmission electron micrograph that
at which point some type of interaction occurs that shows several single crystals of this material. In-
promotes an increase in their chemical reactivity rate. dividual atoms are resolved in this micrograph as
Adsorption sites on a catalyst are normally sur- well as some of the defects presented in Figure 4.11.
face defects associated with planes of atoms; an in- These surface defects act as adsorption sites for the
teratomic/intermolecular bond is formed between a atomic and molecular species noted in the previous
defect site and an adsorbed molecular species. The paragraph. Consequently, dissociation, combination,
several types of surface defects, represented schemati- and oxidation reactions involving these species are
cally in Figure 4.11, include ledges, kinks, terraces, va- facilitated, such that the content of pollutant spe-
cancies, and individual adatoms (i.e., atoms adsorbed cies (CO, NO x , and unburned hydrocarbons) in the
on the surface). exhaust gas stream is reduced significantly.
One important use of catalysts is in catalytic con-
verters on automobiles, which reduce the emission
of exhaust gas pollutants such as carbon monoxide
(CO), nitrogen oxides (NO x , where x is variable), and
unburned hydrocarbons. (See the chapter-opening
diagrams and photograph for this chapter.) Air is in-
troduced into the exhaust emissions from the auto-
mobile engine; this mixture of gases then passes over
the catalyst, which on its surface adsorbs molecules
of CO, NO x , and O 2 . The NO x dissociates into N and
O atoms, whereas the O 2 dissociates into its atomic
species. Pairs of nitrogen atoms combine to form N 2
molecules, and carbon monoxide is oxidized to form

Figure 4.12 High-resolution transmission electron
micrograph that shows single crystals of (Ce 0.5 Zr 0.5 )O 2 ;
this material is used in catalytic converters for auto-
mobiles. Surface defects represented schematically in
Figure 4.11 are noted on the crystals.
[From W. J. Stark, L. Mädler, M. Maciejewski, S. E. Pratsinis,
and A. Baiker, “Flame-Synthesis of Nanocrystalline Ceria/
Figure 4.11 Schematic representations of surface Zirconia: Effect of Carrier Liquid,” Chem. Comm., 588–589
defects that are potential adsorption sites for catalysis. (2003). Reproduced by permission of The Royal Society
Individual atom sites are represented as cubes. of Chemistry.]

9 Adsorption is the adhesion of molecules of a gas or liquid to a solid surface. It should not be confused with
absorption, which is the assimilation of molecules into a solid or liquid.