Final answer:
Iron is a catalyst and molybdenum is a promoter used in the preparation of nitrogen gas, particularly in the ammonia synthesis process known as the Haber process.
Step-by-step explanation:
When preparing nitrogen gas, a common catalyst used is iron (Fe), while molybdenum (Mo) often serves as a promoter. Specifically, in the Haber process for synthesizing ammonia, iron acts as a catalyst to accelerate the combination of nitrogen (N₂) and hydrogen (H₂) gases. Molybdenum assists by increasing the effectiveness of the iron catalyst. For instance, the reaction conditions typically involve high pressures (approximately 150-250 atm) and moderate temperatures (400-500 °C) to ensure an efficient reaction rate while favoring the formation of ammonia (NH₃). Ammonia is continuously removed from the reaction mixture, shifting the equilibrium further towards product formation.