Catalyst (in industrial processes)

A catalyst is a substance that increases the rate of a chemical reaction without itself being used up and without changing the yield or the position of equilibrium. Malay: mangkin · Chinese: 催化剂.

In an industrial process this definition matters twice over. The catalyst lets the plant reach a useful rate at a lower temperature than would otherwise be needed, which saves energy and cost, and it does this while being recovered unchanged, so a fixed charge of catalyst keeps working for a long time. What a catalyst does not do is shift the equilibrium: it speeds the forward and backward reactions equally, so the final amount of product, the yield, is set by temperature and pressure alone. It works by providing an alternative reaction path with a lower activation energy, so a larger fraction of collisions becomes effective.

Example. In the Contact process the catalyst is vanadium(V) oxide, V₂O₅, which speeds the conversion of sulfur dioxide to sulfur trioxide. In the Haber process the catalyst is iron, which speeds the combination of nitrogen and hydrogen into ammonia. In both, the catalyst is chosen for the specific reaction and is not consumed.

Confusion to avoid. Do not say a catalyst increases the yield. A catalyst increases only the rate; the yield of these reversible reactions depends on temperature and pressure. Keep the catalyst separate from the reactants, which are used up.

A neat way to prove a catalyst is unchanged is to recover it at the end, dry it and weigh it, and show the mass is the same as at the start. Note too that a catalyst is usually specific, meaning V₂O₅ works for the Contact reaction and iron for the Haber reaction rather than one catalyst serving both.

Understanding that the catalyst changes rate but not yield is exactly the distinction the Contact and Haber questions test in SPM Chemistry.