Iron as Catalyst

The Haber Process combines nitrogen and hydrogen into ammonia. The nitrogen comes from the air and the hydrogen is obtained mainly from natural gas (methane). Iron is used as a catalyst.

The reaction between persulfate ions (peroxodisulfate ions), S₂O₈^2-, and iodide ions in solution can be catalyzed using either iron(II) or iron(III) ions. The overall equation for the reaction is:

For the sake of argument, we'll take the catalyst to be iron(II) ions. The reaction happens in two stages.

If you use iron(III) ions, the second of these reactions happens first. This is a good example of the use of transition metal compounds as catalysts because of their ability to change oxidation state.