Monolithic microfibrous nickel catalyst co-modified with ceria and alumina for miniature hydrogen production via ammonia decomposition

A non-woven microfibrous structure with 15 vol% 8 μm diameter nickel fibers was built using wet-lay papermaking and sintering processes. Surface of the sinter-locked nickel fibers was then chemically modified with Al₂O₃ and CeO₂, by immersing this novel microfibrous metallic media in a 65 °C aqueous solution containing each of Al(NO₃)₃·6H₂O and Ce(NO₃)₃·6H₂O or both of them for 2 or 4 h at a constant metal ion concentration of 0.5 mol/L. Chemical modifications provided a significant increase of the surface nickel atoms per gram catalyst but obviously suppressed the activity of the metallic nickel sites as indicated by the lowered TOF values. The chemical modification with a mixture solution with the optimal Al³⁺/Ce³⁺ ratio of 9 resulted in a 10-fold increase of the surface nickel atoms per gram catalyst but a 3-fold decrease of the TOF of ammonia, compared with the neat microfibrous nickel substrate. This chemically modified catalyst was capable of producing roughly 20 W power hydrogen with >99% ammonia conversion at 650 °C in a bed of 0.9 mL throughout a 100 h continuous test. Activation energies (E_a) for microfibrous nickel catalysts were all alike in range from 103 to 105 kJ/mol, suggesting that the active site nature was not changed by the chemical modification treatments.