Page 37 - Modeling of Chemical Kinetics and Reactor Design

Reaction Mechanisms and Rate Expressions 7

4. NH ( ads) + H ( ads) → NH ( 2 ads) (1-17)
5. NH 2( ads) + H ( ads) → NH 3( ads) (1-18)

6. NH 3( ads) → NH 3() (1-19)
g
The “ads” denotes the adsorbed species.

AMMONIA OXIDATION
The overall stoichiometric reaction for the oxidation of ammonia
to nitric oxide is:

4NH + 5O → 4NO + 6H O (1-20)
3
2
2
This reaction is very rapid and has been difficult to study mechan-
istically. The direct oxidation of ammonia, NH , to nitric oxide, NO,
3
over platinum catalyst is one of the major steps in the manufacture
of nitric acid, HNO .
3
Mechanism

1. O → 2 O ∗ (1-21)
2
∗
2. NH + O → NH OH (1-22)
2
3
∗
3. NH OH → NH + H O (1-23)
2
2
∗
4. NH + O → HNO 2 (1-24)
2
5. HNO → NO + OH ∗ (1-25)
2
∗
6. 2OH → H O + O ∗ (1-26)
2
The oxygen is chemisorbed on the catalyst. This then reacts with
ammonia to produce a chemisorbed imide radical. The imide reacts
with a molecular oxygen to yield nitric oxide.
STEAM REFORMING

Steam reforming is an important process to generate hydrogen for
such uses as ammonia synthesis because of the high endothermic heat
reaction and its rapidity. High heat fluxes with a direct-fired furnace
are required. Although many steps of reactions are possible, the typical
reaction steps are as follows: