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The starting point is magnetite, Fe3O4,
to which promoters are added, typically:
- About 1% potassium as a chemical promoter (the added potassium covers about one third
of the catalyst surface)
- About 3% alumina as a structural
promoter
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| Question
Why are group 1 metals good chemical promoters
for the Haber process?
|
Answer
Single outer s electron is relatively
easy to donate to Fe. The iron can then more easily bind N2.
| The conditions of the reactor mean that magnetite
is reduced to give iron catalyst particles with many small pores.
The pores are important in exposing the surfaces of iron crystals.
Structural promoters prevent the iron
from changing, in the heat of the reactor, into a non-porous mass
with a much lower surface area, |
| Chemical
promoters increase the activity of the catalyst, in this
case by increasing the electron density of the iron, making it more
likely to bond to nitrogen. |
| Catalyst particles
can be up to 30 mm diameter, with many pores about 8 nm in diameter
The surface area is around 15m2/g.
|
| The faces of iron
crystals are not identical because the atoms can pack together in
different ways, and the most active face is 400 times more active
at 500oC than the least active face. But even this face has low
activity without the chemical promoters. |
| What Do These Promoters Do? |
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