Spatially and Size Selective Synthesis of Fe-Based Nanoparticles on Ordered Mesoporous Supports as Highly Active and Stable Catalysts for Ammonia Decomposition

摘要

Uniform and highly dispersed γ-Fe₂O₃ nanoparticles with a diameter of 6 nm supported on CMK-5 carbons and C/SBA-15 composites were prepared via simple impregnation and thermal treatment. The nanostructures of these materials were characterized by XRD, Mössbauer spectroscopy, XPS, SEM, TEM, and nitrogen sorption. Due to the confinement effect of the mesoporous ordered matrices, γ-Fe₂O₃ nanoparticles were fully immobilized within the channels of the supports. Even at high Fe-loadings (up to about 12 wt %) on CMK-5 carbon no iron species were detected on the external surface of the carbon support by XPS analysis and electron microscopy. Fe₂O₃/CMK-5 showed the highest ammonia decomposition activity of all previously described Fe-based catalysts in this reaction. Complete ammonia decomposition was achieved at 700 °C and space velocities as high as 60 000 cm3 g_cat−1 h−1. At a space velocity of 7500 cm3 g_cat−1 h−1, complete ammonia conversion was maintained at 600 °C for 20 h. After the reaction, the immobilized γ-Fe₂O₃ nanoparticles were found to be converted to much smaller nanoparticles (γ-Fe₂O₃ and a small fraction of nitride), which were still embedded within the carbon matrix. The Fe₂O₃/CMK-5 catalyst is much more active than the benchmark NiO/Al₂O₃ catalyst at high space velocity, due to its highly developed mesoporosity. γ-Fe₂O₃ nanoparticles supported on carbon-silica composites are structurally much more stable over extended periods of time but less active than those supported on carbon. TEM observation reveals that iron-based nanoparticles penetrate through the carbon layer and then are anchored on the silica walls, thus preventing them from moving and sintering. In this way, the stability of the carbon-silica catalyst is improved. Comparison with the silica supported iron oxide catalyst reveals that the presence of a thin layer of carbon is essential for increased catalytic activity.