Physical Chemistry Third Edition

566 13 Chemical Reaction Mechanisms II: Catalysis and Miscellaneous Topics

13.1 Catalysis

Jons Jakob Berzelius, 1779–1848,
was a great Swedish chemist who
dominated the field of chemistry for
several decades and who invented the
present system of chemical symbols for
the elements.

A substance that increases the rate of a chemical reaction but does not appear in its

stoichiometric equation is called acatalyst. This term was coined in 1836 by Berzelius

from the Greek wordskata(“wholly”) andlyein(“to loosen”).^1 Catalysis can be divided

into three classes: Inhomogeneous catalysisall substances involved in the reaction,

including the catalyst, occur in the same phase. Inheterogeneous catalysisthe catalyzed

reaction occurs at the boundary between two phases (usually on the surface of a solid

catalyst).Enzyme catalysisis a special case of homogeneous catalysis, which we will

discuss separately. A catalyst generally provides an alternative mechanism involving

the catalyst that competes with the uncatalyzed mechanism. If the catalyzed mechanism

is faster than the uncatalyzed mechanism, the observed rate of the reaction is primarily

due to the catalyzed mechanism, although the reaction is also still proceeding by the

uncatalyzed mechanism.

Exercise 13.1

Some elementary chemistry textbooks include the statement “A catalyst lowers the activation

energy of a reaction.” Write a critique of this misleading statement.

Heterogeneous Catalysis

This type of catalysis occurs at a surface, usually a solid surface. Recent developments

in nanotechnology have produced nanometer-size solid particles that act as efficient

catalysts.^2 A solid catalyst acts by adsorbing molecules from a gas or liquid phase onto

its surface, where they react. Consider a unimolecular process in the gas phase,

A(gas)−→products (13.1-1)

and a catalyzed mechanism,

(1) A(gas)+surface site
A(adsorbed) (13.1-2)

(2) A(adsorbed)−→products (13.1-3)

If the second step is rate-limiting, the first step can be assumed to be at equilibrium.

We now present a simple equilibrium theory of adsorption in order to discuss the

equilibrium of the first step of this mechanism.

The Langmuir Theory of Adsorption

This theory assumes the process

A+surface site
A(adsorbed) (13.1-4)

We assume that only a single layer of molecules (amonolayer) can be adsorbed on the

surface sites. These sites might include all of the atoms of the solid surface or might be

surface imperfections such as a “step” between two layers of atoms, as schematically

depicted in Figure 13.1.

Figure 13.1 Idealized Sites on a
Solid Surface.

(^1) K. J. Laidler,Chemical Kinetics, Harper and Row, New York, 1987, p. 229.
(^2) N. Tian, Z.-Y. Zhou, S.-G. Sun, Y. Ding, and Z. L. Want,Science, 316 , 732 (2007).