A catalyst is a substance that speeds up a chemical reaction, or lowers the temperature or pressure needed to start one, without itself being consumed during the reaction. Catalysis is the process of adding a catalyst to facilitate a reaction.

Metals like platinum and nickel make good catalysts because they adsorb strongly enough to hold and activate the reactants, but not so strongly that the products can't break away. The simplest example of this is the reaction between ethene and hydrogen in the presence of a nickel catalyst.

The catalytic converter is also an example of surface catalysis, where the reactant molecules are adsorbed onto a solid surface before they react with the catalyst to form the product. The rate of a surface-catalyzed reaction increases with the surface area of catalyst in contact with the reactants.

1 Criteria for catalyst selection When selecting a catalyst, the first step is to define the desired outcome of the reaction in terms of product yield, selectivity, purity, quality, reaction rate, temperature, and pressure. This will help narrow down the possible types of catalysts that can facilitate the reaction.