The in situ formation of ammonia decomposition catalysts derived from perovskites ABO₃ (A=La,Ca,Sr, and B=Fe,Co,Ni) was examined via operando X-ray diffraction experiments. The reduction behavior of the perovskites, the intermediate phases during activation, the catalysts’ crystallite size distribution, their morphology, and their catalytic activity were analyzed.

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 La_0.8Sr_0.2CoO₃ with a conversion of 86 % (100 % NH₃, 15000 mL g⁻¹ h⁻¹) at 550 °C.