From “Single Sites” to Stable Nanoparticles Derived from Spray-Flame Synthesized Solid Solutions of Cobalt in MgO for Ammonia Decomposition

Abstract

Chemical energy storage, in particular, the storage of hydrogen in carbon-free molecules such as ammonia, is being considered as an essential element in the transformation of our future energy system. In the present work, cobalt catalysts were investigated as alternatives to Ru- and Ni-based materials for the decomposition of ammonia. Spray-flame synthesis (SFS) was used to prepare metastable, phase-pure solid solutions Mg_1–xCo_xO, 0.03 ≤ x ≤ 0.47, as catalyst precursors composed of oxide nanoparticles with a particle size of 7–8 nm. In situ and operando XRD, H₂-TPR studies, electron microscopy, Raman spectroscopy, and FTIR spectroscopy of adsorbed CO were applied to characterize the nanostructure of the host oxides and catalysts generated by exsolution of Co under reductive conditions. Calcination at 600 °C causes partial segregation of a Co₃O₄ spinel phase at a higher Co content (x ≥ 0.24). Small Co clusters are formed by exsolution from the solid solution, while the reduction of the spinel component yields Co nanoparticles of 3.3–8.5 nm. Very high space-time yields of 19 mmol_H2·g_cat^–1·min^–1 at 500 °C can be achieved due to the high dispersion of metallic cobalt. The turnover frequency (TOF) for metallic nanoparticles in the range between 1 and 5 nm remains constant at about 0.2 s^–1, suggesting a structure insensitivity of the reaction in this size range at industrially relevant reaction conditions (500 °C, 100% NH₃, WHSV of 36,000 N mL·g^–1·h^–1, 1 atm) caused by structural dynamics. Only atomically dispersed cobalt shows a TOF of about 0.5 s^–1. The Co/Mg_1–xCo_xO catalysts exhibit very good stability even at high Co contents under the harsh conditions of ammonia decomposition due to strong metal–support interaction, making continuous SFS, which is potentially scalable, an attractive method for preparing cost-effective and resource-saving catalysts for ammonia decomposition.