Novel Microfibrous-structured Ni-based Catalyst Composites For H₂ Production Via NH₃ Decomposition

A novel microfibrous composite bed reactor for high efficient production of hydrogen from ammonia decomposition was developed. The sintered microfibrous composite was prepared through wet layup papermaking method. We investigated the effect of supports and rare earth oxide promoters on performance of microfibrous enrapped supported-Ni catalysts for ammonia decomposition. Intensification effectiveness of microfbrous structure was also discussed.In the first part, the effect of supports on performance of microfibrous enrapped Ni-based catalysts was investigated. The catalytic activity of Ni supported on alumina oxide is higher than on silicon dioxide or AC. Effect of nickel loading was also studied in detail. Results indicated that nickel supported on alumina oxide with 10wt% loading exhibited good catalytic activity and stability.In the second part, microfibrous enrapped REO-promoted Ni/Al₂O₃ catalyst particulates were prepared and examined in use with ammonia decomposition. Results showed that microfibrous enrapped Ni/Al₂O₃ catalyst promoted with ceria oxide was the most active. This is attributed to the high dispersion of ceria oxide and nickel on alumina oxide. Ni/CeO₂-Al₂O₃ catalyst was prepared by stepwise incipient wetness impregnation (IWI) method, using their nitrate salts as precursors. The impregnated sample in each step was calcined in air at 450℃ and 250℃ for 2 h. The best loading of ceria oxide as well as nickel active component was 10wt% without including the mass of nickel microfibers. At 650℃ and a 145 sccm ammonia feed, roughly 217 sccm of hydrogen was produced with ammonia conversion of 99%, corresponding to an equivalent fuel cell power of ～22 W. The catalytic activity of rare earth oxide promoted catalyst was: Ni/CeO₂-Al₂O₃>Ni/La₂O₃-Al₂O₃ ～ Ni/Pr₆O₁₁-Al₂O₃ > Ni/Sm₂O₃-Al₂O₃ >Ni/Y₂O₃-Al₂O₃.This monolithic microfibrous composites reactor exhibited much higher activity for ammonia decomposition, compared to the bed packed with equivalent amount of catalyst diluted with quartz to 0.5 mL and bed which was sandwiched equivalent catalyst with bottom and top pieces of microfibrous mesh. In comparison with the bed packed with 2 mm catalyst pellets, at 90% conversion of a 36 sccm ammonia feed gas rate, the microfibrous composite bed provided a 4-fold reduction of catalytic bed volume and a 5-fold reduction of catalytic bed weight while leading to a reduction of reaction temperature up to 50℃, by taking the advantage of the use of both microfibrous structrue and small Particulate size.In the third part, a thin-sheet microfibrous structure consisting of 15vol% 8 μm diameter nickel fibers was built through regular wet-lay paper-making followed by subsequent sintering process, and its reactivity for ammonia decomposition was significantly improved through the chemical modifications with alumina and/or ceria. The chemical modification by immersing in a mixture solution with Al³⁺/Ce³⁺ (nitrite salts as precursors) ratio of 9 at 65℃ for 2 h led to 2-fold promotion of the reactivity of the microfibrous nickel catalyst for ammonia decomposition compared with the neat microfibrous nickel. Roughly 20 W power output hydrogen (～220 mL/min) could be achieved with an ammonia conversion of >99 % in a bed volume of 0.9 mL at 650℃. Chemical modifications provided a significant increase of the surface nickel atoms per gram catalyst but did not lead to the reduction of the activation energy of reaction for ammonia decomposition.