Syntheis And Catalytic Activity Of Transition Metal Nitrides Supported On CNTs For Ammonia Decomposition

Proton exchange membrane fuel cell（PEMFC） is the most promising power source for electric cars as well as portable equipments due to its favorable advantages, such as high efficiency,low weight and less environmental impact. In recent years, ammonia decomposition has been attracted considerable attentions due to its use as a COx-free source of hydrogen for proton exchange membrane fuel cells. However, the activities of the present catalysts for ammonia decomposition are still low and could not meet the demand of on-board fuel cells. Therefore, the design and synthesis of highly efficient and stable catalysts for ammonia decomposition are of great significance.Transition metal nitrides are metallic interstitial compounds with unique physical and chemical properties because the element N fills into the metallic crystalline structure. They exhibit the catalytic characteristics similar to the precious metals and have shown excellent properties in a number of hydrogen-involved reactions such as ammonia synthesis and decomposition, HDN, HDS and F-T synthesis. A series of Co-Mo nitride catalysts supported on carbon nanotubes were synthesized through temperature programmed reaction. The physicochemical properties of the catalysts were characterized by XRD, BET, XRD, BET, SEM, TG/DSC, H₂-TPR/TPD, XPS techniques. In the meantime, the kinetic behavior of the ammonia decomposition reaction over high stable La-CoMoN_x/CNTs catalyst was investigated. A mechanism of the ammonia decomposition was proposed.Some details are as follows:1. A series of carbon nanotubes supported transition metal nitrides such as CoN_x/CNTs, FeN_x/CNTs, MoN_x/CNTs and NiN_x/CNTs were prepared by temperature programmed method in N₂-H₂mixture gases. Their surface composition and catalytic performance for ammonia decomposition were compared. Furthermore, the CoMoN_x/CNTscatalyst with Co content of2wt%and Mo content of15wt%was found to be the most suitable catalyst for ammonia decomposition. The XRD results indicated that the activity centers are not a single phase, but the various kinds of γ-Mo2N, Co3Mo3N and Co4N, even a certain quantity of MoO₃and CoMoO₄. The XPS results suggested that there were Mo⁶⁺, Mo⁴⁺ and Mo^δ+（0<δ <4） ions on the surface of the Co-Mo nitride catalysts.2. The effects of cationic promoters （K, Ba, La, Ce and Zr） on the catalytic performance and surface properties of the catalysts were investigated. It was found that doping appropriate amount of K, Ba, La, Ce and Zr into CoMoNx/CNTs catalysts improved the catalytic activity to some extent, and the La-CoMoNx/CNTs catalyst showed the highest ammonia conversion of99%at650℃. The results of XRD, BET, TG/DSC, H2-TPR/TPD and XPS indicated that that the addition of promoters changed the state of active sites and the content of low valence molybdenum species Moδ+（0<δ<4）, and the distribution of active species increased and the catalytic stability improved.3. The influence of the synthesis factors, such as including preparation methods, calcination temperature and type of oxide precursors, space velocity of nitriding gases, N2-H2ratio and nitriding temperature on catalytic activity of CoMoNx/CNTs was investigated in detail. The results showed the optimal nitriding conditions were as follows:the calcination temperature was500℃, calcination time was4h, the nitriding temperature was700℃, GHSV was10000h-1, the heating rate was1℃/min（300-500℃） and2.5℃/min（500-700℃）, while the composition of nitriding gases was V（H2）:V（N2）=5:1（300～500℃） and V（N2）:V（H2）=1:1（500～700℃）.4. The influence of reaction conditions including temperature, time, space velocity and pressure were studied. The results showed that temperature and space velocity have remarked influence on the catalytic activity of La-CoMoNx/CNTs for ammonia decomposition. The ammonia conversion increased with the increasing of reaction temperatures, reaching more than99%at650℃. It was also found that the ammonia conversion increased with the decreasing of GHSV of ammonia at the same temperature, and the content of ammonia in the raw materials had no effect on its activity. The optimal reaction conditions were as follows: the reaction temperature was about650℃and GHSV was10000h-15. Thermal stability experiment shown that La-CoMoNx/CNTs catalyst had a better thermal stability, and the ammonia conversion maintained above95%for30h at600℃and NH3space velocity of5000h-16. The reaction kinetics equation of ammonia decomposition on La-CoMoNx/CNTs was determined:The reaction mechanism was proposed. The reaction rate of ammonia decomposition over La-CoMoNx/CNTs was controlled by the combination desorption of nitrogen on the surface of the catalyst. The inovation of this dissertation are as follows:1. The La-CoMoNx/CNTs catalyst with good activity and stability for ammonia decomposition to produce hydrogen for fuel cells has been developed.2. The reaction kinetics equation of ammonia decomposition on La-CoMoNx/CNTs was determined:The reaction mechanism was proposed. The reaction rate of ammonia decomposition over La-CoMoNx/CNTs was controlled by the combination desorption of nitrogen on the surface of the catalyst.