Page 382 - Elements of Chemical Reaction Engineering Ebook

Sec. 7.2 Searching for a Mechanism 353

Smog Formation. Nitrogen and oxygen react to form nitric oxide in the cyl-
inder of automobile engines. The NO from automobile exhaust is oxidized to
NO2 in the presence of peroxide radicals.

Rod +NO -% RO + NO, (R1)

Nitrogen dioxide is then decomposed photochemically to give nascent oxygen,

NO,+hu k2_, NO+O

which reacts to form ozone:
o+02 5 03

The ozone then becomesjnvolved in a whole series of reactions with hydro-
carboiis in the atmosphere to form aldehydes, various free radicals, and other
interniediates, which react further to produce undesirable products in air
pollution:

ozone + olefin + aldehydes + free radicals

0, + RCH'CHR k5 RCHO + RO + HCO (R4)

R-tHCO (R5)
One specific example is the reaction of ozone with 1,3-butadiene to form
acrolein and formaldehyde, which are severe eye irritants.

5 03 + CH2=CHCH=CH, hv k4 + CH,=CHCHO+HCHO
Eye Mtants (R6)

By regenerating NO,, more ozone can be formed and the cycle continued. One
means by which this regeneration may be accomplished is through the reaction
R+02 -
of NO with the free radicals in the atmosphere (Rl). For example, the free: rad-
ical formed in Reaction (R4) can react with 0, to give the peroxy free radlical,

ROO
037)
The coupling of all the reactions above is shown schematically in Figure 7-2.
1We see that the cycle has been completed and that with a relatively small
amount of nitrogen oxides, a large amount of pollutants can be producedl. Of
course, many other reactions are taking place, so do not be misled by the brev-
ity of the preceding discussion; it does, however, serve to present, in rough
outline, the role of nitrogen oxides in air pollution.

Finding the Reaction Mechanism. Now that a rate law has been synthesized
from the experimental data, we shall try to propose a mechanism that is con-
sistent with this equation. The method of attack will be as given in Table 7-2.