Q1: Iron is not used alone as a catalyst in ammonia synthesis, because

ANS:A - its activity declines rapidly, if heated to above 520°C.

Iron is not used alone as a catalyst in ammonia synthesis primarily because its activity declines rapidly if heated to above 520°C. At higher temperatures, iron becomes prone to sintering, which is the process where the catalyst particles agglomerate and lose their surface area and activity. This decline in activity can significantly reduce the efficiency of the ammonia synthesis reaction. Therefore, to prevent sintering and maintain catalytic activity, iron is typically used in combination with other promoters and stabilizers, such as alumina (aluminum oxide) or potassium oxide. These additives help to stabilize the iron catalyst and prevent sintering, allowing the catalyst to maintain its activity over prolonged periods of operation at the elevated temperatures required for the ammonia synthesis reaction. When iron is used as a catalyst in the synthesis of ammonia, its effectiveness is highly dependent on its surface area and structure. At temperatures above 520°C, iron particles are susceptible to a process called sintering. Sintering occurs when individual iron particles start to agglomerate or fuse together, leading to a reduction in the overall surface area of the catalyst. This reduction in surface area means that fewer active sites are available for the reactant molecules to adsorb and react, thereby decreasing the catalyst's activity. The decline in activity due to sintering can be quite rapid, especially at higher temperatures, where the mobility of iron atoms is increased, promoting particle movement and fusion. As a result, maintaining the temperature below 520°C helps to minimize sintering and preserve the catalytic activity of the iron catalyst, ensuring efficient production of ammonia in the synthesis process.