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Physical Chemistry 3: — Chemical Kinetics — - Christian-Albrechts ...

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10.2 <strong>Kinetics</strong> of heterogeneous reactions (surface reactions) 226<br />

e) Example 2: Haber-Bosch NH 3 synthesis<br />

I Figure 10.8: Potential energy diagram for the catalytic reaction N 2 + 3H 2 → 2NH 3<br />

on a Fe 3 O 4 catalyst according to Ertl and co-workers (1982). Energies in kJ mol −1 .<br />

f) How can we control and accelerate heterogeneous reactions?<br />

The answer depends on which step is rate determining:<br />

• If diffusion to/from the surface is the rate determining step, we can accelerate<br />

the reaction by stirring<br />

• If the adsorption step is rate determining (because of an activation energy), we<br />

can accelerate the reaction by increasing and and, in particular, by increasing<br />

surface area.

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