Fig. 8: Schematic representations of the morphological evolution of the promoter phases, the activated catalyst architecture, and the catalytic production of ammonia.
From: Decoding technical multi-promoted ammonia synthesis catalysts
a The chemical linking of Al, Si, Ca, and Fe oxidic phases produces a material similar to a cementitious phase. At early stages of hydration, this phase gives rise to nanofiber basic units. With ongoing aging (TOS) under decreasing water vapor the basic units assemble sequentially in platelets, needle-like aggregates, and densified/disordered material. b The activated catalyst exhibits a 3D nanodispersion of ammonia Fe crystallites structurally stabilized by a set of cementitious phases. Excess of the segregated promoter material accumulates at the external surface in the several forms depicted in (a). Ammonia K covers the ammonia Fe of the catalytic substrate at reaction conditions (500 °C), but preferentially accumulates on the external surface at low temperatures. c The dissociative chemisorption of N2 is enhanced by in situ formed ammonia K/ammonia Fe phases due to electronic transfer effects. The atomic N species reacts either sequentially with atomic H to give rise to the ammonia product, or with the Fe surface to produce nitrides. The presence of alkaline KOH increases the desorption of the ammonia product.
