Surface characterization of ammonia synthesis catalysts

Composition and topography of the surfaces of unreduced and reduced industrial ammonia synthesis catalysts (BASF S6–10) were investigated by means of scanning Auger electron spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. The unreduced samples exhibit strong surface segregation and rather nonuniform lateral distribution of the promoter elements (Al, K, Ca); no pores with diameter ≲ 1000 Å are present. After reduction a network of particles and pores with diameters in the range between 100 and 500 Å can be observed which visualizes the pronounced increase of the surface area due to the presence of the structural promoter Al2O3 (+ CaO). Reduction transforms Fe completely into its metallic state. The iron particles are uniformly covered by a K + O adlayer which acts as “electronic” promoter. Previous reports in the literature on the surface properties of ammonia synthesis catalysts, which were based on more indirect methods (selective adsorption, etc.), are in full qualitative accordance with the present conclusions.