Promoting effects of Ca on NiAl catalysts derived from layered double hydroxides for efficient ammonia decomposition for hydrogen production

Abstract

A series of NiCaAl-layered double hydroxide (LDH) compounds have been synthesized and used as precursors to prepare NiCaAl catalysts for ammonia decomposition. Various characterization techniques showed that upon calcination LDHs were transformed into mixed oxides, and the reduction treatment led to metallic Ni particles. The Ca promoter optimized the dispersion of Ni nanoparticles and induced electron accumulation at Ni in the Ni₄4Ca2Al1 catalyst. In particular, the Ni4Ca2Al1 catalyst exhibited the highest NH₃ decomposition activity with an extremely high H₂ yield of 5.51 mmol H₂ g_cat.⁻¹ min⁻¹ at 550 °C. Combined NH₃ temperature-programmed surface reaction and DFT calculations indicated that the energy barrier for NH₂ dehydrogenation was the highest, making it the most difficult step and was therefore the rate-determining step on Ni4Al1 and Ni4Ca2Al1 surfaces. Moreover, the lower energy barrier could explain why the Ni4Ca2Al1 catalyst had much higher ammonia decomposition activity than the Ni4Al1 catalyst.