Ammonia Decomposition over CaNH-Supported Ni Catalysts via an NH2–-Vacancy-Mediated Mars–van Krevelen Mechanism

Nickel has been studied as an alternative catalyst to ruthenium for ammonia decomposition. However, high reaction temperatures are generally required to achieve good ammonia conversion because the weak interactions between nitrogen and the Ni surface reduce the frequency of the dehydrogenation reaction of ammonia. Here, we report Ni-supported CaNH as a highly efficient catalyst for ammonia decomposition through an NH2– vacancy-mediated Mars–van Krevelen mechanism. Ni/CaNH exhibited much greater ammonia decomposition activity than state-of-the-art Ni-based catalysts, and the operating temperature was 100 °C lower than those of the other Ni-based catalysts. Isotope-labeling experiments, NH3-pulse measurements, and density functional theory calculations suggested that two anionic electrons at the NH2– vacancy on the Ni/CaNH surface activated ammonia molecules to form intermediate NHx (x = 1, 2) species, even at 50 °C, and the supported Ni metal promoted the regeneration of NH2– vacancies on CaNH. These results demonstrate that NH2– vacancy sites on CaNH play an important role in overcoming the drawbacks of Ni-based catalysts in ammonia decomposition.