One in which the reactants are in different phases is called

Heterogeneous

Explanation: Heterogeneous catalysts are catalysts that are in a different phase than the reactants. For example, the catalyst might be in the solid phase while the reactants are in a liquid or gas phase. A catalyst that is in the same phase as the reactants is called a homogeneous catalyst. Enzymes are an example of homogeneous catalysts, and acids can also be homogeneous catalysts.