Development of iron-based catalysts for gas-phase ammonia decomposition under milder conditions

Ammonia decomposition is a promising route for hydrogen production in the context of ammonia usage as a carbon-free hydrogen carrier. This endothermic reaction is often performed in the range of 600–900 °C, depending on the catalyst and operating conditions. Therefore, developing catalysts that achieve high ammonia conversion at lower temperatures is important for reducing the energy demand of the process. To make this reaction more sustainable, catalysts play an important role by reducing the activation energy and, therefore, the energy input required for the reaction to occur. Although ruthenium-based catalysts offer high activity, iron offers a more abundant and cost-effective alternative. Hence, this master thesis investigates iron-based catalysts derived from layered double hydroxides (LDHs) for ammonia decomposition. Five Mg–Fe catalysts, Mg95Fe5, Mg88Fe12, Mg75Fe25, Mg67Fe33, and Mg50Fe50 were synthesized by coprecipitation followed by calcination and tested between 400 and 600 °C at a space velocity of 3200 h⁻¹. XRD, N2-physisorption, and H2-TPR were used for characterization. XRD confirmed LDH formation in the dried precursors, while the calcined catalysts mainly contained MgO and increasing amounts of MgFe₂Oₓ spinel-type phases at higher Fe contents. Mg75Fe25 showed 75% ammonia conversion at 500 °C and approximately 40% at 425 °C. Mg75Fe25 was selected for further optimization through partial Fe substitution with Ni. The Mg75Fe19Ni6 and Mg75Fe12.5Ni12.5 catalysts were prepared, together with Mg94Ni6 as a Fe-free reference. Mg75Fe19Ni6 achieved the highest conversion among the Ni-containing catalysts, reaching 83% at 500 °C and 61% at 425 °C. H2-TPR showed that Ni shifted the main reduction peak from approximately 425 °C to 345 °C, suggesting improved reducibility. Overall, partial substitution of Fe with a low amount of Ni improved catalytic performance, highlighting the potential of LDH-derived catalysts for ammonia decomposition at reduced temperatures.