High‐Temperature Stable, Iron‐Based Core–Shell Catalysts for Ammonia Decomposition

High-temperature, stable core-shell catalysts for ammonia de- composition have been synthesized. The highly active catalysts, which were found to be also excellent model sys- tems for fundamental studies, are based on a-Fe2O3 nanoparticles coated by porous silica shells. In a bottom-up approach, hematite nanoparticles were firstly obtained from the hydrothermal reaction of ferric chlorides, l-lysine, and water with adjustable average sizes of 35, 47, and 75 nm. Secondly, parti- cles of each size could be coated by a porous silica shell by means of the base-catalyzed hydrolysis of tetraethyl- A (TEOS) with cetyltetra- methylammonium bromide (CTABr) as porogen. After calcination, TEM, high-resolution scanning electron mi- croscopy (HR-SEM), energy-dispersive X-ray (EDX), XRD, and nitrogen sorption studies confirmed the success- ful encapsulation of hematite nanopar- ticles inside porous silica shells with a thickness of 20 nm, thereby leading to composites with surface areas of ap- proximately 380 m 2 g 1 and iron con- tents between 10.5 and 12.2 wt %. The obtained catalysts were tested in am- monia decomposition. The influence of temperature, iron oxide core size, pos- sible diffusion limitations, and dilution effects of the reagent gas stream with noble gases were studied. The catalysts are highly stable at 7508C with a space velocity of 120 000 cm 3 gcat 1 h 1 and maintained conversions of around 80 % for the testing period time of 33 h. On the basis of the excellent stability under reaction conditions up to 8008C, the system was investigated by in situ XRD, in which body-centered iron was determined, in addition to FeNx, as the crystalline phase under reaction condi- tions above 6508C.