Ammonia cracking for COx-free hydrogen production by plasma catalysis over Ni catalysts supported on different materials (Al2O3, CeO2): A comparative study combining experiments and kinetic modeling

As a carbon-free hydrogen carrier, ammonia has attracted considerable attention due to its high hydrogen storage capacity and convenient transportation. However, achieving efficient and economical hydrogen production from ammonia remains a substantial challenge. Plasma catalysis holds great promise for enabling low-temperature ammonia cracking. In this work, we combine experimental measurements with chemical kinetics modeling to comparatively analyze the mechanisms underlying ammonia cracking over Ni catalysts supported on Al2O3 and CeO2 under plasma activation. Experimental results show that packing Ni catalysts markedly enhances NH3 conversion, underscoring the plasma-catalyst synergy in promoting NH3 cracking. Notably, Ni/CeO2 achieves substantially higher conversion than Ni/Al2O3. Path flux analysis indicates that plasma-catalytic NH3 cracking on the catalyst surface primarily follows a stepwise dehydrogenation process (NH3 → NH2(s) → NH(s) → N(s) → N2). Nevertheless, the dominant consumption mechanisms of NH3, NH(s), and N(s) differ markedly between the Ni/Al2O3 and Ni/CeO2 catalysts. Analysis of H2 formation pathways reveals that, for Ni/Al2O3 catalyst, 86.2 % of H2 is produced through the electron impact reaction e + NH3 → e + NH + H2, whereas for Ni/CeO2 catalyst, 99.5 % of H2 is generated via the Langmuir-Hinshelwood (L-H) reaction H(s) + H(s) → H2 + 2Ni(s). Shifting the support from Al2O3 to CeO2, the dominant reaction mode for NH3 cracking and H2 formation transitions from a gas-phase driven mechanism to one predominantly mediated by the catalyst surface.