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This work addresses the microstructure of perovskites LaBO3 (B = Fe, Co, Ni) and the in situ formation of ammonia (NH3) decomposition catalysts derived from perovskites ABO3 (A = La, Ca, Sr, and B = Fe, Co, Ni) via operando synchrotron X-ray diffraction experiments.
Bulk analysis by diffraction experiments and local structure analysis by total scattering methods with subsequent pair distribution function analysis were performed. Conducting these experimental methods with both neutron and synchrotron radiation, highly detailed insights into the microstructure were gained. While LaFeO3 did not show any deviation from its ideal crystal structure, there were deviations from the ideal crystal structure for LaCoO3 noticeable. Despite the complementary effect of all methods applied these could not be identified. In LaNiO3, Ruddlesden-Popper-type faults could be observed. A crystallographic model containing these stacking faults was set up and used in Rietveld refinements in order to evaluate the amount of stacking faults in LaNiO3. The gradual increase of the distortion of the ideal crystal structure from Fe via Co to Ni can be accounted to their individual electronic properties.
The reduction behavior of the perovskites during NH3 decomposition experiments, the formation of intermediate phases during activation, and the catalytic performance was studied in detail. In addition, microstructure properties of the active catalyst 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 mLg-1h-1) at 550°C.
