The catalyst in a catalytic converter is a homogeneous catalyst. Transition metals make good heterogeneous catalysts because of their 4s and 3d electrons. A catalyst speeds up a reaction because it decreases the enthalpy change for the reaction. Transition metal ions make good homogeneous catalysts because of their variable oxidation states. When nickel is the catalyst for the hydrogenation of ethene, the reactants are absorbed onto the surface. The catalyst in a catalytic converter is a homogeneous catalyst.