How do heterolytic catalysts work?

What are catalysts? They are substances that speed up the rate of reaction by providing a favourable pathway, of lower activation energy, for the reaction to proceed. A heterogeneous catalyst is one that is of a different phase to the reactants. An example of this is Iron as a powdered solid in the Haber Process to manufacture gaseous ammonia (NH₃) from gaseous N₂ and H₂.  The reaction is: N_2(g) + 3H_2(g) <--> 2NH_{3 (g)} How do they work? Partially filled d orbitals in Iron form weak covalent bonds with H₂ and N₂. The molecules are said to be adsorbed. This reduces the stability of the molecule which means less energy is required to break the N₂ and H₂ bonds to form the 3 N-H bonds in Ammonia. Ammonia molecules then desorb from the surface and the process can be repeated to catalyse the next set of N2 and H2 molecules.