(95b) Hydrogen Production From Ammonia Decomposition: Study On the Synergetic Effect of DBD Plasma and Cheap Metal Catalysts

Hydrogen has been considered as an ideal candidate to solve the urgent energy and environmental issues, especially on-site H2 production for fuel-cell vehicles [1,2]. Ammonia decomposition is an appealing hydrogen production route for fuel cell application [3]. Since 2004, studies related to the in-site generation of COX-free hydrogen via the decomposition of ammonia have increased very rapidly, almost all of them used tradditional catalytic route which was characterized by using supported transitional metal catalysts and fixed-bed reactor. The emphases of these studies were given to the effects of the different active components, supports and promotors of the catalysts on the decomposition activity of ammonia. According to the published works, (1) noble metal Ru is the most active catalyst, carbon nanotubes (CNTs) are the most effective support, and the alkaline substances like KOH is the best promotor. At 350 °C, a kind of K-Ru/MgO-CNTs catalyst, the most active catalyst ever prepared, has reached a hydrogen yield of 585 ml/min.g-catal [4]; (2) recombinative desorption of surface-adsorbed nitrogen atoms is a rate-determining step in ammonia decomposition [5]. The recombinative desorption rates of surface-adsorbed nitrogen atoms of non-noble metals like Fe, Co and Ni are much lower than that of the noble metal Ru. As a result, the catalytic activities of the non-noble metals for ammonia decomposition are much slower than that of Ru; (3) although CNTs supported Ru catalysts already possess excellent catalytic activity, improvements must be made for them to overcome problems including the shortage of Ru resource, the high cost of CNTs, and the instability of the catalyst caused by the methanation of CNTs in H2 atmosphere during long-term usage [6].

Here, we would discuss a novel plasma-catalysis method for ammonia decomposition. Our motive is to apply non-equilibrium plasma to ammonia decopmposition catalyzed by non-noble metal catalysts, so as to accelerate the recombinative desorption rates of surface-adsorbed nitrogen atoms. Based on systematic explorative experiments, strong activity-enhancing synergetic effects have been seen between dielectric-barrier discharge (DBD) plasma and supported non-noble metal catalysts including Fe, Cu, Mo, Ni and Co.

By employing XRD, FT-IR, XRF, UV-Raman, XPS, TEM and TPD, the nature of the activity-enhancing synergetic effect between DBD plasma and supported metal catalysts was investigated. The promotion of the recombinative desorption rates of surface-adsorbed nitrogen atoms by DBD plasma has been confirmed by plasma desorption (PD) technique.

Results and Discussion

Based on systematic explorative experiments, strong activity-enhancing synergetic effects were seen between dielectric-barrier discharge (DBD) plasma and supported non-noble metal catalysts, including Fe, Cu, Mo, Ni and Co. (1) In the case of being used separately at 410℃, the conversion of NH3 obtained with Fe2-3N catalyst and DBD plasma was only 7.4% and 7.8%, respectively. However, in the same conditions the combination of the DBD plasma with Fe2-3N catalyst dramatically increased the conversion of NH3 to more than 99%. In other words, in the absence of DBD plasma, reaction temperature of 550 °C was needed for Fe2-3N catalyst, to reach more than 99% ammonia conversion. In the presence of DBD plasma, however, the reaction temperature for Fe2-3N catalyst to reach more than 99% ammonia conversion decreased by 140 °C. (2) Among transitional catalysts investigated, the activity-enhancing synergetic effect of DBD plasma and supported metal catalysts increased in Cu Acknowledgements

The authors acknowledge the financial supports from the Natural Science Foundation of China (NSFC: Grant No. 20473016 and 20673018)

Reference:

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