Synthesis and characterization of Fe–Ni/ɣ-Al2O3 egg-shell catalyst for H2 generation by ammonia decomposition

Abstract

The Fe–Ni alloyed nanoparticles are a promising alternative to expensive ruthenium-based catalysts for a real-scale application of hydrogen generation by ammonia decomposition. In practical applications, millimeter-sized extrudates are used as catalyst supports, where the spatial distribution of the active phase should match with the type of reaction. In this work, a novel synthesis route was developed for the preparation of a Fe–Ni/ɣ-Al2O3 egg-shell catalyst. Egg-shell is a preferred profile considering the highly endothermic nature of ammonia decomposition reaction. The high viscosity of glycerol, used as a solvent during impregnation, prevents the fast migration of the Fe–Ni active phase solution toward the inner-core of ɣ-Al2O3, giving control over the large capillary pressures. The distribution profiles were analyzed at macroscopic scale through scanning electron microscopy mapping (SEM-EDX) and optical microscopy. A three-dimensional (3-D) reconstruction of the spherical-shaped ɣ-Al2O3 was achieved using X-ray micro tomography and the Fe–Ni egg-shell spatial distribution was inspected throughout the entire volume of the support body. Transmission electron microscopy (TEM) specimen preparation using focused ion bean (FIB) milling allowed to acquire high resolution images of the Ni and Fe nanoparticles on ɣ-Al2O3, which is particularly challenging due to the crystalline nature of this support. Distinct regions of the egg-shell catalyst were analyzed through scanning TEM (STEM) and TEM. The outer-shell region showed the presence of Fe and Ni alloyed nanoparticles with a size of approximately 5nm.. The egg-shell catalyst showed significant higher activity in ammonia decomposition by converting 3 times more ammonia to equilibrium conversion than either egg-white or catalyst with uniform distribution. Moreover, the egg-shell catalyst conversion only dropped 0.05% after 10h of reaction, for a space velocity of 475mlmin−1g−1.