What distinguishes promoters in ruthenium-based and iron-based ammonia catalysts?

The difference between promoters in ruthenium-based and iron-based ammonia synthesis catalysts lies in their structural sensitivity and the predominant role of electronic factors. Ruthenium-based catalysts are less structurally sensitive, and electronic factors play a crucial role. Promoters for ruthenium-based catalysts can alter the surface electrostatic field of ruthenium, lower the electron work function, and enhance the electron transfer interaction with adsorbed nitrogen molecules, leading to a substantial increase in catalyst activity. Common promoters for ruthenium-based catalysts include alkali metal elements, alkali metal nitrates, alkaline earth metal nitrates, and rare earth nitrates.

Among these, alkali metal nitrates, such as CsNO3, serve as effective promoters. During the catalyst reduction process, alkali metal nitrates undergo reactions, ultimately forming hydroxides:

2CsNO3 + 8H2 → 2CsOH + 2NH3 + 5H2O (Equation 1)

2CsOH → Cs2O + 2Cs (Equation 2)

Cs2O + H2 → 2CsOH (Equation 3)

Cs + OH(a) → CsOH (Equation 4)

The formed hydroxides cover the surface or interface of the active ruthenium metal, influencing the surface electrostatic field and its ability to adsorb nitrogen molecules. The magnitude of the promoting effect is directly related to the basicity of the compound, with the order being CsOH > RbOH > KOH > NaOH. When alkaline earth metal nitrates are used as promoters, their activity is significantly lower compared to alkali metal nitrates.

In contrast to ruthenium-based catalysts, iron-based ammonia synthesis catalysts exhibit higher structural sensitivity, where the catalyst's performance is more influenced by its physical arrangement. Electronic factors are still essential but play a relatively less predominant role compared to ruthenium-based catalysts.

Promoters for iron-based catalysts function to modify the electronic properties and surface characteristics of the catalyst, enhancing its interaction with reactants. Common promoters for iron-based catalysts include alkali metal elements, alkali metal nitrates, alkaline earth metal nitrates, and rare earth nitrates.

The choice of promoters for iron-based catalysts aims to optimize the catalyst's electron transfer capabilities, ensuring efficient adsorption and desorption of nitrogen molecules during the ammonia synthesis process.

Unlike in ruthenium-based catalysts, where alkali metal nitrates, such as CsNO3, serve as effective promoters, the effectiveness of promoters in iron-based catalysts may vary. The impact of promoters depends on the specific characteristics of the iron catalyst and the nature of the catalytic reaction.

In summary, while both ruthenium-based and iron-based catalysts benefit from promoters, the choice of promoters and their effectiveness can differ based on the unique structural and electronic properties of each catalyst system.