A catalyst is a substance that increases the rate of a chemical reaction.  A catalyst is a substance that increases the rate of a chemical reaction. The presence of a catalyst is one of several factors that influence the rate of chemical reactions. (Other factors include the temperature, concentration, and surface area of reactants.) A catalyst isn’t a reactant in the chemical reaction it speeds up. As a result, it isn’t changed or used up in the reaction, so it can go on to catalyze many more reactions.
Matt borrowed his fathers  pan to use on a camping trip. When he got home he sat it on his back porch, meaning to return the next day but forgot about it. A week later he saw the pan sitting on the porch with a puddle of rain water in it and rust on the bottom.
What caused the rust.