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Free keywords:
Ammonia; Perovskite phases; Supported catalysts; In situ X-ray diffraction; Exsolution
Abstract:
This work addresses the formation of ammonia (NH3) decomposition catalysts derived from perovskites ABO3 (A=La, Ca, Sr, and B=Fe, Co, Ni) precursors via operando synchrotron X-ray diffraction experiments. During the reaction in NH3, the perovskite precursors are decomposed and the transition metals are reduced. Depending on their reduction properties, active metallic catalysts are formed in situ on La2O3 as support. The reduction behavior of the perovskites, formation of intermediate phases during activation, and catalytic performance was studied in detail. In addition, microstructure properties such as crystallite sizes and particle morphology were analyzed. Co-/Ni-based perovskites decomposed completely during activation to Co0/Ni0 supported on La2O3 while Fe-based perovskites were fully stable but inactive in catalysis. This difference is due to varying electronic properties of the transition metals, e. g., decreasing electronegativity from Ni to Fe. With decreasing reducibility, the intermediate phases during activation formed more distinct. La3+ was partially substituted by Ca2+/Sr2+ in LaCoO3 to test for advantageous effects in NH3 decomposition. The best performance was observed using the precatalyst La0.8Sr0.2CoO3 with a conversion of 86 % (100 % NH3, 15000 mL g−1 h−1) at 550 °C.
