Page 41 - Modeling of Chemical Kinetics and Reactor Design

Reaction Mechanisms and Rate Expressions 11

Successive single reactions are:

CH + O → CH O + H O (1-46)
4 2 2 2
(Formaldehyde)

+
CH O O → CO H O 2 (1-47)
+
2
2
2
CO+ 1 O → CO 2 (1-48)
2
2
HO → HO + 1 O
2 2 2 2 (1-49)
2
In these reactions, the stable intermediates are CH O, CO, and H O .
2 2 2
ACTIVE CENTERS

Reactions that are catalyzed by solids occur on the surfaces of the
solids at points of high chemical activity. Therefore, the activity of a
catalytic surface is proportional to the number of active centers per
unit area. In many cases, the concentration of active centers is
relatively low. This is evident by the small quantities of poisons
present (material that retards the rate of a catalytic reaction) that are
sufficient to destroy the activity of a catalyst. Active centers depend
on the interatomic spacing of the solid structure, chemical constitution,
and lattice structure.
Generally, active centers are highly reactive intermediates present
in very small concentrations with short lifespans. For example, in the
case of

H + Br → 2 HBr (1-50)
2
2
*
H and Br *
and in the case of

CH CHO → CH 4 + CO (1-51)
3

∗ ∗ ∗
CH CHO CH CO
,
,
3 3