Not only is the reaction rate enhanced, the reaction path is also altered. In the intermediate steps, adsorption on the catalyst surface plays an important role.
Figure 30.6 Adsorbed H2 and ethylene on a catalyst surface.
A C2H4 molecule colliding with an adsorbed H2 can combine with it with much smaller activation energy and then the product is desorbed.
Figure 30.7 Potential energy diagram in heterogeneous catalysis
Fig 30.7 shows the reduced activation energy in heterogeneous catalysis. The activation energy for the decomposition of ammonia is 350 kJ / mol. The value of this activation energy is reduced when the reaction is catalyzed by tungsten (160 kJ/mol), molybdenum (140 kJ /mol) and osmium (200 kJ/mol). The need for cheaper and easily renewable catalysts can not be overemphasized.
Common examples of homogeneous (one phase) catalysis are acid- base catalysis, enzyme catalysis, gas phase catalysis (e.g., conversion of SO2 to SO3 catalysised by NO) and catalsis by metal ions. The mechanism in the catalysis of 2 H2O2 2 H2 O + O2 by Fe2+ ions which reduce the activation energy from 70 kJ / mol to 42 kJ / mol is shown below.
2 H2 O + O2 by Fe2+ ions which reduce the activation energy from 70 kJ / mol to 42 kJ / mol is shown below. 
