A catalyst is a substance added to change the rate of a chemical reaction, generally to make it faster. Many different catalysts can be used for the Fischer–Tropsch process. The most common catalysts are the metals cobalt, iron, and ruthenium. These metals are all transition metals. The metal nickel can also be used, but generally with unwanted results. A nickel catalyst in the reaction usually produces a lot of methane, which is undesirable.