A Scenario for a Carbon-Neutral Ammonia-Fueled Engine Mediated by Catalytic NH₃ Cracking and CO₂ Hydrogenation

Utilizing near zero-carbon NH₃ as fuel in engines is promising for carbon-neutrality. However, the application of NH₃ into the engine suffers from the intrinsic poor combustion characteristics of NH₃ and the emission of harmful NO_x exhausts. Herein, we proposed and successfully confirmed a novel scenario for converting a conventional "CH₄-fueled" engine to "NH₃-fueled" engine. Specifically, CH₄ was used to power the internal combustion engine and release CO₂ as the exhaust. Afterwards, we put forward two routes to convert the exhaust and NH₃ into N₂ and CH₄ for enclosing the carbon cycle. The first "spatially decoupled" route splits the exhaust treatment into NH₃ cracking over Ru clusters on the calcined Mg-Al hydrotalcite (Ru/MAO) and CO₂ methanation over a commercial Ni/Al₂O₃. Both NH₃ and CO₂ were almost completely converted into the target products under their respective optimal conditions. The second "spatially coupled" route refers to an one-pot reaction of NH₃ and CO₂ into N₂, CH₄, and H₂O. Due to the mismatch of reaction conditions and the competitive adsorption of reactants, the conversions of NH₃ and CO₂ were lowered to 80.1 % and 49.3 %, respectively, over Ru/MAO under 1 bar (CO₂:NH₃=3 : 8) at 550 °C.