A catalyst is a substance that increases the reaction rate without changing the change in the total standard Gibbs free energy of the reaction. This action is called catalysis, and reactions involving catalysts are called catalytic reactions
The composition, chemical properties and mass of the catalyst do not change before and after the reaction. The relationship between it and the reaction system is like the relationship between the lock and the key, which is highly selective (or specific). A catalyst does not catalyze all chemical reactions; Some chemical reactions do not have a single catalyst.