Engineering 2D Mo-Based MXenes and Application in Ammonia Synthesis

Ammonia, which is one of the most important chemicals for synthesis of dyes, pharmaceuticals, and fertilizers, is conventionally produced by the reaction of molecular hydrogen with nitrogen, over an iron-based catalyst at 400-500 °C under pressure of over 200 bar, the Haber-Bosch process. While the H-B process is credited for providing a simple and cost-effective route for nitrogen-based fertilizer production, it is highly energy-intensive, accounting for ~ 2 % of the world's energy production, and as it relies on fossil-based resources, resulting in a staggering emission of 2.5% of global CO₂ annually. Decreasing the temperature and pressure of this century old, highly energy intensive process, would greatly decrease the energy consumption in the world and reduce carbon emissions. Fundamentally, producing ammonia at lower pressures is not thermodynamically prohibited, providing that the reaction takes place at temperatures below 300 °C. However, due to the high energy barrier for the activation/dissociation of the strong triple covalent NN bond (945 kJ mol^-1), most developed catalysts, including the industrial H-B catalyst, operate at temperatures over 400 °C. Consequently, high-pressure operation is required to increase ammonia yields to acceptable industrial production (15% conversion per cycle). In this work, for the first time in the literature, we engineered Co decorated Mo₂CT_x MXene multilayers as catalysts for ammonia synthesis under mild conditions. The MXene functionalized by Co showed catalytic activity for ammonia synthesis from H₂ and N₂ at temperatures as low as 250°C, without any pre-treatment. The developed catalyst was highly active for ammonia synthesis, demonstrating a high rate up to 9500 µmol g^-1_active_phase h^-1 at 400 °C under ambient pressure in steady state conditions, and did not suffer from any deactivation after 15 days of reaction. The apparent activation energy (<i>E</i>_a) was found to be in the range of 68 to 74 kJ mol^-1 which aligns with the value reported for highly active catalysts such as Li-MT, Mn₄N-BaH₂, Ru/C₁₂A₇:e^-, and BaTiO_2.5H_0.5. This improved catalyst may decrease the energy consumption in synthesis of ammonia and its derivatives, as well as facilitate the use of ammonia as a hydrogen carrier for renewable energy storage. Numerous characterization techniques were used to analyze the catalysts including, XRD, SEM, TEM, H2-TPR, XPS, EPR and ICP to understand their unique properties. Post-reaction analysis revealed partial substitution of lattice carbon with nitrogen, as well as partial reduction of Co²⁺ during ammonia synthesis. The catalytic activity profiles, supported by post-reaction catalysts characterization, suggest that active sites generated upon reaction might be cobalt decorated carbonitrides Co-Mo₂C_1-δN_δT<i>_x</i>. Upon reaching the steady state, the activity becomes stable upon cycling between low- and high-temperature conditions with no induction time. The results reported in this work demonstrate the appropriate modification of Mo₂CT<i>_x</i> resulted in a high-performance ammonia synthesis catalyst capable of operating under mild reaction conditions in the intermittent regime.