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

Abstract

A series of NiCaAl layered double hydroxides (LDHs) compounds have been synthesized and used as precursors to prepare NiCaAl catalysts for ammonia decomposition. Various characterizations showed upon calcination LDHs were transformed into mixed oxides, and the reduction treatment leaded to metallic Ni particles. The Ca promoter could optimize the dispersion of Ni nanoparticles, and illustrated the electron accumulation at Ni in Ni4Ca2Al1 catalyst. Especially, Ni4Ca2Al1 catalyst exhibited the highest NH3 decomposition activity with an extremely high H2 yield of 5.51 mmolH2 gcat.-1min-1 at 550oC. Combined NH3 temperature-programmed surface reaction and DFT calculations indicated that the energy barrier of NH2 dehydrogenation was highest, making it the most difficult step, and was therefore the rate determining step on Ni4Al1 and Ni4Ca2Al1 surface. Moreover, the lower energy barrier could explain why Ni4Ca2Al1 catalyst had much higher ammonia decomposition activity than Ni4Al1 catalyst.