Decoding technical multi-promoted ammonia synthesis catalysts

Abstract

Abstract Ammonia is industrially produced by the Haber-Bosch process over a fused, multi-promoted iron-based catalyst. Current knowledge about the reaction has been derived from model systems of reduced structural complexity, impeding any clear-cut structure-activity correlation relevant for the industrial counterpart. Here, we unveil the structural evolution of complex, technical, multi-promoted ammonia synthesis catalysts by operando scanning electron microscopy and near-ambient pressure X-ray photoelectron spectroscopy. We highlight that the activation is the critical step in which the catalyst is formed and decode the pivotal role of the promoters. We discover that the active structure consists of a nanodispersion of Fe covered by mobile K-containing adsorbates, so called “ammonia K”. The porous catalyst is stabilized by mineral cementitious phases containing oxides of Al, Si, Ca, and Fe. The synergism between the different promoters contributes simultaneously to the structural stability, hierarchical architecture, catalytic activity, and poisoning resistance. The confluence of these aspects is the key for the superior performance of technical catalyst formulations.