ANS:C - K₂O

In catalytic ammonia synthesis reactions, K2O (potassium oxide) is often used as a catalyst promoter. Potassium oxide acts as a promoter in the Haber-Bosch process, which is the industrial method for the production of ammonia from nitrogen and hydrogen. While iron (Fe) is the primary catalyst in this process, potassium oxide is commonly added as a promoter to enhance the activity and selectivity of the catalyst. It helps to improve the performance of the iron catalyst by increasing its surface area and optimizing its interaction with the reactants. Additionally, potassium oxide can also influence the adsorption and desorption kinetics of nitrogen and hydrogen on the catalyst surface, thereby enhancing the overall efficiency of the ammonia synthesis reaction.