Catalysts · CatalystLab

A lower road

The surface cycle

The camera moves in on the iron surface. This is how one NH₃ molecule is “assembled” on it:

① Adsorption: N₂ drops into an empty site; H₂ splits into two H atoms as soon as it lands. ② Dissociation: N≡N breaks apart on the surface into two N atoms — the slowest step (the rate-determining step). ③ Hydrogenation: H atoms wander across the surface and add to N one by one: N → NH → NH₂ → NH₃. ④ Desorption: NH₃ leaves the surface and the empty site can take the next molecule.

Click “Track one N₂” and follow it around the cycle. Then raise the pressure: more molecules hit the surface, the empty sites fill up, and the rate stops rising with pressure.

Same destination

Two identical reactors with the same temperature, pressure and feed — only the right one has an iron catalyst. Look at the curves on the right: the reactor with the catalyst reaches equilibrium quickly, the one without is much slower — but both curves end up on the same dashed line.

A catalyst speeds up the forward and reverse reactions equally, so it only shortens the time to reach equilibrium; it doesn't change the NH₃ content at equilibrium.

Now the temperature: ammonia synthesis is exothermic, so heating lowers the equilibrium content while the rate rises. Industry picks 400–500 °C as a compromise between rate and yield; it is also where the iron catalyst works best.

The uncatalysed reactor here is only about a dozen times slower so that you can see it reach equilibrium too; in reality it is more than 10¹² times slower — you could never wait that long.

Selectivity

The feed is now carbon monoxide (black-and-red pairs) and hydrogen.

On nickel, the CO triple bond is pulled apart and C and O are hydrogenated separately: C becomes methane CH₄, O becomes water.

Switch to copper–zinc oxide (the blue-grey islands on the surface are zinc oxide): CO does not break, the whole molecule picks up H atoms one at a time and ends up as methanol CH₃OH.

Same reactants, a different catalyst, a different path and a different product — that is the selectivity of a catalyst. The “specificity” of enzymes is an extreme case of selectivity.

Catalyst poisoning

If the feed contains a little hydrogen sulfide (yellow), it breaks up as soon as it touches the iron, and the sulfur atom holds on to the site for good.

Poisoned sites can no longer adsorb N₂; fewer and fewer sites are usable and the NH₃ rate keeps falling — this is catalyst poisoning. In the chart on the right, the grey line is a fresh catalyst under the same conditions, for comparison.

Cleaning the feed afterwards doesn't bring the rate back: a poisoned catalyst has to be replaced or regenerated. That is why ammonia plants always remove sulfur from the feed gas before it enters the converter.

In-class testing