Hydrogen Production From Ammonia Decomposition Using Supported Bimetallic Catalysts

Ammonia decomposition to produce hydrogen is a promising technology for in situ hydrogen supply on fuel-cell vehicles.A large number of ammonia decomposition studies in heterogeneous catalytic method have shown that there is a volcanic curve between the nitrogen adsorption energy on the surface of the metal catalyst and the ammonia decomposition activity.The order of nitrogen adsorption energy on the surface of the monometallic catalysts of ammonia decomposition is Ni <Co <Ru <Fe <Mo.Ru catalyst has the most suitable metalnitrogen adsorption energy.Bimetallic catalysts with suitable nitrogen adsorption energy can be prepared by modulating the molar ratio of two non-noble metals on each sides of Ru,respectively.Dielectric barrier discharge plasma technology has been proved the ability of enhancing low temperature catalytic activity of non-noble metals.In this paper,a series of monometallic catalysts?Ni,Co,Fe and Mo?and bimetallic catalysts?Fe-Co,Fe-Ni,Mo-Co and Mo-Ni?were prepared using fumed-SiO₂ as supports and using Ni,Co,Fe and Mo as active components.The catalytic performance of these catalysts on ammonia decomposition have been investigated under both traditional heterogeneous catalytic condition and plasma catalytic condition,and the detailed physical and chemical properties were characterized.The results and conclusions obtained are shown as followings:1.Under the traditional heterogeneous catalytic condition,the Fe-Co bimetallic catalyst exhibited better ammonia decomposition activity than the other seven catalysts,and the apparent activation energy of NH3 decomposition reaction over the Fe-Co bimetallic catalyst was lower than those over the other seven catalysts.For Fe-Co bimetallic catalysts,the optimized Fe/Co molar ratio was 5/5,and the 5Fe-5Co/SiO₂ catalyst showed the best ammonia decomposition activity and a reliable stability.When the temperature of the 5Fe-5Co/SiO₂ bimetallic catalyst bed was 630 oC and the reaction space velocity was 14400h-1,ammonia conversion rate reached 100%.Combining with X-ray diffraction,H₂-programmed temperature reduction and high resolution transmission electron microscopy,it has been found that,in the bimetallic catalyst 5Fe-5Co/SiO₂,the interaction between Co₃O₄?311?crystal plane and Fe₂O₃?104?crystal plane resulted the formation of a spinel structure CoFe₂O₄,which is favorable for the formation of alloy CoFe15.7 during the reaction process,and is helpful to promote the dispersion of the active ingredient by avoiding the agglomeration of Fe and Co.This may be the reason why the 5Fe-5Co/SiO₂ catalyst exhibited the best heterogeneous atalytic activity of ammonia decomposition.2.Under the plasma catalytic condition,the Fe-Ni bimetallic catalyst exhibited better ammonia decomposition activity than the other seven catalysts,and the apparent activation energy of NH3 decomposition reaction over the Fe-Ni bimetallic catalyst was lower than those over the other seven catalysts.For Fe-Ni bimetallic catalysts,the optimized Fe/Ni molar ratio was 6/4,and the 6Fe-4Ni/SiO₂ catalyst showed the best ammonia decomposition activity and a reliable stability.Under the reaction conditions of 48 W discharge power,14400h-1 reaction space velocity and 550 oC catalystic bed temperature,the ammonia conversion reached 99.3% over the 6Fe-4Ni/SiO₂ catalyst.Combining the resulsts of X-ray diffraction,H₂-programmed temperature reduction and high resolution transmission electron microscopy,it has been found that in the 6Fe-4Ni/SiO₂ catalyst,the interaction between the NiO?200?crystal plane and the Fe₂O₃?104?crystal plane lead to the generation of a spinel structure NiFe₂O₄,which avoided the aggregation of Fe and Ni and easily maintained Fe and Ni in the metal state during the reaction process,resulting in easier desorption of adsorbed N atoms.This may be the reason why the 6Fe-4Ni/SiO₂ catalyst showed the best plasma-catalytic activity of ammonia decomposition.3.Through comparing the ammonia conversion under thermal catalytic condition,plasma condition and plasma-catalytic condition,respectively,it has been found that there was a significant synergistic effect between dielectric barrier discharge plasma and the prepared catalysts.Co/ SiO₂ catalyst showed a better synergistic ability than the other three monometallic catlaysts.While 5Fe-5Ni/ SiO₂ catalysts exhibited a best synergistic ability with plama for ammonia decomposition among the eight catalysts.The apparent activation energy of ammonia decomposition reaction over 5Fe-5Ni/SiO₂ catalyst was obviously reduced from 71.7 kJ/mol to 61.7 k J/mol,which could be the main reason why the 5Fe-5Ni/SiO₂ catalyst exhibited the best synergistic ability with plasma for ammonia decomposition.