Tracking the Active Catalyst for Iron‐Based Ammonia Decomposition by In Situ Synchrotron Diffraction Studies

Abstract

Iron‐based catalysts for NH3 decomposition have been studied by a combination of catalytic tests and in situ synchrotron diffraction experiments performed in an inert sapphire plug‐flow cell. In contrast to steel‐based reaction cells, sapphire or quartz glass cells show no blind activity. Starting from iron oxide precursors, iron nitrides form during the activation cycle. Nitrides remain as main crystalline phases and govern the conversion of NH3 decomposition in the subsequent cycles. In this work structural and compositional changes of the nitrides were monitored in situ during heating and cooling cycles. The state of the catalyst under reaction conditions was analyzed by high resolution in situ synchrotron diffraction experiments. The analyses enable establishing reaction pathways and correlation of structural features with catalytic conversions. The most active phases are iron nitrides with high mobility and solubility for nitrogen atoms, such as Fe3Nx. Phase changes from Fe3Nx to γ‐FeNx were observed above 700 °C. The formation of γ‐FeNx seems to suppress the catalytic conversion. Moreover, the positive influence of a mesostructured support/catalyst composite on the catalytic conversion and catalyst stability were studied in detail.

Review of the Decomposition of Ammonia to Generate Hydrogen

“…Through in situ experiments, Tseng et al claimed that the active form of catalysts composed of Fe is Fe 3 N x , while at high temperatures (>675 °C) FeN x is formed, which has a negative influence on ammonia conversion. A Fe 2 O 3 catalyst supported on SBA-15 mesoporous silica shows better catalytic activity than the bare iron oxide (18 vs 4% at 500 °C) . Pelka et al observed that the catalytic decomposition reaction rate of ammonia is higher in the case of nanocrystalline iron compared to iron nitride Fe 4 N. The decrease of ammonia conversion in the presence of Fe 4 N has also been evidenced with a Fe catalyst promoted with Al 2 O 3 , CaO, and K 2 O with or without SiO 2 . − Temperature influences the degree of nitriding, being favored at temperatures above 400 °C .…”

“…297 The same behavior was obtained with a reference catalyst composed only of Fe 2 O 3 . 297 Through in situ experiments, Tseng et al 298 claimed that the active form of catalysts composed of Fe is Fe 3 N x , while at high temperatures (>675 °C) FeN x is formed, which has a negative influence on ammonia conversion. A Fe 2 O 3 catalyst supported on SBA-15 mesoporous silica shows better catalytic activity than the bare iron oxide (18 vs 4% at 500 °C).…”

Thermocatalytic Ammonia Decomposition – Status and Current Research Demands for a Carbon‐Free Hydrogen Fuel Technology

“…Considering the high reaction temperature and also technical/safety aspects related to ammonia may be quite challenging aspects for future investigations. Nevertheless, Tseng et al [77] were already able to observe the temperature-sensitive evolution of active nitrogenrich Fe 3 N x species during ammonia decomposition on iron catalysts via in situ X-ray diffraction. Moreover, Kirste et al [78] were able to observe different oxidation states and bimetallic interactions of CoÀ Re catalysts using in situ EXAFS and XANES experiments.…”

Micro‐/nanostructure evolution of C/SiFeO(N,C) polymer‐derived ceramic papers pyrolyzed in a reactive ammonia atmosphere

“…53 In consequence, diffusive NH 3 transport is expected to occur inside the fibers and along with the fiber-coating interface. Ammonia diffusing though the carbon fiber readily reacts with iron oxide precipitates along with the contact to the ceramic layer to either directly form Fe-based nitrides 55 or, if carbon is present, carbothermal reduction nitridation takes place. 56 N (g) + C (s) + Fe 𝑥 O (s) → Fe 𝑥 N (s) + CO (g)…”

“…At 1000 • C, the ε-Fe x N precipitates have not only withstood reduction but also undergone considerable growth compared to the sample prepared at 700 • C, while FeO enclosed in the ceramic layer has been reduced to α-iron by either CO or H 2 . According to Equation (1), ammonia decomposes due to both elevated temperature 43,44 and catalytic conversion at ε-Fe x N sites, 55 likely generating H 2 and N 2 inside the fiber. Hydrogen diffuses through the fiber, whereas N 2 is retained presumably resulting in a locally increased nitriding potential.…”