Which of the following is not an example of a heterogeneous catalytic reaction?

The combustion of coal is not a heterogeneous catalytic reaction. A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. Heterogeneous catalysis involves reactants and catalysts in different phases. In the combustion of coal, the reaction occurs between coal (solid) and oxygen (gas), and no catalyst is required for the reaction to proceed. Ostwald's process (synthesis of nitric acid), Haber's process (synthesis of ammonia), and hydrogenation of vegetable oils all involve heterogeneous catalysis because a solid catalyst is used in a gaseous or liquid phase reaction.