Page 181 - Introduction to chemical reaction engineering and kinetics

7.1 Simple Homogeneous Reactions 163

7.1.4 Photochemical Reactions
In the mechanism of a photochemical reaction, at least one step involves photons. The
most important such step is a reaction in which the absorption of light (ultraviolet or
visible) provides a reactive intermediate by activating a molecule or atom. The mecha-
nism is usually divided into primary photochemical steps and secondary processes that
are initiated by the primary steps.
Consider as an example the use of mercury vapor in a photoactivated hydrocarbon
process, and the following steps:
(1) Absorption of light to produce an energetically excited atom:

Hg+hv+Hg*

(2) Reaction of excited atom with a hydrocarbon molecule to produce a radical (de-
sired):

Hg* + RH 2 HgH + R’

(3) Parallel (competing) reaction(s) in which excitation energy is lost (undesired):
(3a) Re-emission of energy as light (fluorescence):

Hg**Hg + hv

(3b) Nonreactive energy transfer to another species (including reactant):

Hg* + M2Hg + M

(In a gas phase, the loss of energy requires collisions, whereas in a con-
densed phase, it can be considered a unimolecular process.)
The fraction of absorbed photons which results in the desired chemical step is called
the quantum yield, @. In this case,

@= b3-a (7.1-5)
k2CRH + ‘3 + k4cM
If all the re-emitted photons remain available to be reabsorbed (e.g., trapped by the use
of mirrors),

@= ~CRH (7.1-6)
k2cRH + k4cM
In this example, the Hg atom is the primary absorber of light. If the primary absorber
is regenerated, it can participate in subsequent cycles, and is called aphotosensitizer. In
other cases, the photoactive species yields the active species directly. Thus, chlorine
molecules can absorb light and dissociate into chlorine atomic radicals:

Cl, + hv ---f 2Cl’

The competing process which determines @ in this case is the recombination process:

2Cl’ + M -+ Cl, + M