What is a catalyst?

A catalyst is a substance that speeds up a chemical reaction without being used up. It opens an easier route, one that needs less energy to get started. Speed is the only thing it changes. It cannot alter what forms, or how much of it forms. But a reaction that would take a thousand years is a reaction that never happens, so speed decides a great deal.

Hydrogen peroxide sitting in a bottle turns into water and oxygen over months. Add a little manganese dioxide and the same change finishes in seconds. The manganese dioxide is still sitting there afterwards, ready to do it again. Molecules need a certain amount of energy before they can react at all, which is called the activation energy. A catalyst offers a route where that requirement is lower, so far more molecules clear it every second. Why not just heat the mixture, which also speeds reactions? Because heat speeds up everything in the vessel, including the reactions ruining your product, and heating industrial equipment is expensive. What a catalyst cannot do is change the outcome, since the same products form in the same amounts. A reaction too slow to observe is still a reaction that does not happen, which is why the large majority of manufactured chemicals pass through one.

Two towns sit either side of a mountain. The only route climbs over the summit, so almost nobody makes the trip. Dig a tunnel and the traffic increases enormously. The destination is the same town, and travellers still walk there themselves. What the tunnel cannot do is put a town where there was never one.