Page 243 - Modeling of Chemical Kinetics and Reactor Design

Reaction Rate Expression 213

The activation energy − E = B = − 0 2988 × 10 5
.
R

×
.
= 0 2988 10 5 × 0 08206
.
l
,
= 2 452
mol
12
The value of the pre-exponential factor k = 3.378 × 10 l/mol • sec.
O
Example 3-12
For the synthesis of ammonia, N + 3H → 2NH , over an iron
2
2
3
catalyst, develop the rate expression for the following mechanism
k 1
H 2() + 2 S [ 2 HS (3-335)
g
k 1−

k 2
N 2() + 2 S  → 2 NS (3-336)
g
k 3
NS + 3 HS [ NH S + 3 S (3-337)
3
k −3

NH S[ NH + S (3-338)
k 4
3 k 4− g ()

Solution

Boudart (1972) introduced the assumption of the most abundant
surface intermediate (masi). This assumption suggests that the sites
occupied by all species except the most abundant surface intermediate
is regarded as negligible compared to those filled by the most abundant
intermediate and to those which are empty.
The rate expression for the above mechanism is based on the
following assumptions:

1. The most abundant surface intermediate is NS

that C = C + C . (3-339)
T s NS