Applied Surface Science, Vol.469, 82-89, 2019
TOF-SIMS study of morphology and chemical composition of wustite-based precursor and iron catalyst for ammonia synthesis
This study focuses on the application of time-of-flight secondary ion mass spectrometry (TOF-SIMS) for analysis of the morphology and chemical composition of a wustite-based precursor and iron catalyst for ammonia synthesis. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used as supplementary methods. TOF-SIMS imaging was capable of visualizing the micrometer-scale spatial distribution of elements on the surface of the catalyst precursor, which was composed of wustite crystallites separated by spaces, with high concentrations of K, Al and Ca used as promoters. Moreover, aluminum and calcium were detected in the wustite crystallites. Potassium, despite having a vanishingly low concentration in wustite phase, was able effectively to wet the wustite surfaces upon annealing at 350 degrees C. The potassium layer that formed on the surface of the wustite precursor had a different composition than that on the surface of the iron nanocrystallites of the iron catalyst. The most important finding concerns the composition of the surface of the iron catalyst. TOF-SIMS analysis showed a layer containing oxygen in the subsurface region of the iron catalyst obtained from the wustite-based precursor. This structure on the surface of the iron catalyst enabled the formation of active sites for the adsorption of nitrogen on iron atoms during ammonia synthesis.