Ammonia

Coordination chemistry insights into the role of alkali metal promoters in dinitrogen reduction

“…Building from these studies, industrial Mittasch catalysts for the H-B process are synthesized through the fusion of Fe 3 O 4 with various amounts of Al 2 O 3 , CaO, MgO, SiO 2 , and K 2 O. The first four are considered structural promoters that increase catalyst surface area, stabilize catalytically active Fe sites on the surface, and prevent sintering of the magnetite [29-32]. The potassium is generally considered to be an “electronic promoter;” its large ionic radius prevents incorporation into the magnetite during synthesis, and therefore it is localized at the surface [32].…”

“…The first four are considered structural promoters that increase catalyst surface area, stabilize catalytically active Fe sites on the surface, and prevent sintering of the magnetite [29-32]. The potassium is generally considered to be an “electronic promoter;” its large ionic radius prevents incorporation into the magnetite during synthesis, and therefore it is localized at the surface [32]. Catalytic activity reaches a maximum at a certain amount of added potassium, both on polycrystalline Fe foil [33] and in the technical H-B catalyst [31].…”

“…Ruthenium-based H-B catalysts are also promoted with alkali metals [32]. In general, Ru catalysts exhibit higher activity with larger alkali metals, but recently Li was found to be a competent promoter [6, 37-39].…”

“…In general, Ru catalysts exhibit higher activity with larger alkali metals, but recently Li was found to be a competent promoter [6, 37-39]. Analogous studies on alkali metal promoters for Fe catalysts do not always agree on the relative changes in activity when changing the alkali metal [28, 32, 34-36]. Understanding the precise role that these alkali metals play in improving the activity of structurally-promoted catalysts is vital for developing new ammonia synthesis catalysts with higher activity and robustness.…”

“…The specific role of potassium in promoting the technical H-B catalyst is a topic of great interest, considering the enhancement of catalytic activity upon addition of potash to the iron oxide precursor [32, 34]. Hypothetical explanations include potassium involvement in N 2 chemisorption to the Fe surface, hydrogenation of adsorbed N-atoms, and desorption of NH 3 [40] [33] [30] [31] [41] [28] [42] [11].…”