Synthesis And Performance Investigation Of Catalysts For Hydrogen Generation From Hydrous Hydrazine/Ammonia Borane

With the energy crisis caused by the increasing consumption of fossil fuels and theincreasingly environmental pollution due to their combustion products, development andutilization of new energy become the development direction of future energy society.Hydrogen energy, as an efficient, clean and abundant source of secondary energy, isconsidered to be one of the ideal energy of human beings in the future. When the hydrogenenergy is applied in the on-board proton exchange membrane fuel cells, safe and efficientstorage and transportation of hydrogen become a major problem for practical application,because of its low volumetric and weight energy densities. To solve this problem, lightweight small molecule compounds can be used as a hydrogen storage materials. Amongthem, hydrous hydrazine, which possesses high hydrogen content (8wt%), easy rechargingas a liquid, can be stored and transferred safely. Importantly, the production from completedecomposition of hydrous hydrazine is only hydrogen in addition to nitrogen, without anysolid byproduct. Ammonia borane is a non-toxic solid at room temperature with highhydrogen content of19.6wt%. Therefore, the above two kinds of hydrogen storage materialsboth become attractive candidates for chemical hydrogen storage applications.Whether hydrous hydrazine or ammonia borane, hydrogen generation efficiencydepends largely on the selection of catalysts. The previous research found that the noblemetal catalysts possess excellent catalytic performance, however, their high-costs andlimited resources become one of the main reasons for the invalidity of large-scalecommercialization of fuel cell. Therefore, development of the low-cost, high efficiencycatalysts to catalyze the hydrogen generation from hydrogen storage materials at mildconditions is the key target in the future. In this thesis, the main results are divided into threeparts as following:Firstly, materials in the amorphous states exist surface defects and high concentrations of unsaturated coordination sites, which makes the catalytic activities of amorphous catalystssuperior to those of crystalline ones. The catalytic performance of Ni0.9Pt0.1nanoparticlesprepared by the conventional method for the hydrogen generation from the decomposition ofhydrous hydrogen is not satisfactory. The amorphous Ni0.9Pt0.1/Ce2O3structure is obtainedby incorporation of Ce2O3into the crystalline Ni0.9Pt0.1one. As a result, the hydroushydrazine decomposition reaction catalyzed by the amorphous Ni0.9Pt0.1/Ce2O3catalystshows100%hydrogen selectivity, the initial turnover frequency (TOF) is28.1h-1, and underthe same conditions, the TOF value is only6.4h-1for the crystalline Ni0.9Pt0.1catalyst.Secondly, in order to further increase the catalytic activities, and at the same timereduce the costs of catalysts, we use the facile method to synthesize low-cost non-noblemetal catalyst without any surfactant, and acquire excellent catalytic performance upon thehydrogen generation reaction from the decomposition of hydrous hydrazine.(1) The Ni0.5Fe0.5CeOxnanocomposite supported on MIL-101has been synthesized bysolution infiltration method at room temperature. A strong anchoring effect between theNi0.5Fe0.5CeOxnanoparticles and MIL-101leads the nanoparticles are well dispersed onMIL-101, and no aggregation is observed. Thus, compared with support-free Ni0.5Fe0.5CeOxnanoparitcles, the Ni0.5Fe0.5CeOx/MIL-101shows superior catalytic activity, the TOF is68.2h-1at343K.(2) Ni0.6Fe0.4Mo catalyst has been prepared by co-reduction method without anysurfactant or support at room temperature. We found that although the absence of surfactantand support, Ni0.6Fe0.4Mo nanoparticles still possess good dispersion and small particle size.Moreover, for the Ni0.6Fe0.4Mo trimetallic alloy structure, Mo acts as an electron donor foratoms of Ni and Fe. For the above two reasons, hydrogen generation from the decompositionof hydrous hydrazine catalyzed by Ni0.6Fe0.4Mo exhibits excellent catalytic activity and100%hydrogen selectivity.Thirdly, We apply the non-noble metal catalysts on catalyzing hydrogen generationfrom ammonia borane.(1) We take advantage of the difference in reduction potentials of the three metal salts(Cu2+、Ni2+and Fe2+), but also benefit from the suitable reducing ability of ammonia borane to in-situ synthesize non-noble metal Cu0.4@Fe0.1Ni0.5core-shell nanoparticles via simpleand green one-step reducing method within2minutes at room temperature. As a result,Cu0.4@Fe0.1Ni0.5core-shell structure shows superior catalytic activity to the Cu0.4Fe0.1Ni0.5alloy structure and the physical mixture of Cu and FeNi. Furthermore, due to the constituentelements of shell are magnetic Fe and Ni, the Cu0.4@Fe0.1Ni0.5nanoparticles are easy torecycle. The cheap, efficient and easy recovery catalyst will further develop ammonia boraneas hydrogen storage materials in the new energy field.(2) Considering the heterogeneous catalysts for ammonia borane dehydrogenation aremainly consistent of noble metal, and their reaction rates, the reaction conditions and themaximum quantities of hydrogen need to be further improved. We prepare monodispersednon-noble metal Cu0.9Ni0.1alloy nanoparticles by organic synthesis method, and theas-synthesized Cu0.9Ni0.1exhibits high catalytic activity for the dehydrogenation of theammonia borane at room temperature. It owns the highest value of maximum quantity of H2(2.5equiv. of H2per ammonia borane) within18minutes and an initial turnover frequency(TOF) value of212.3h-1, both the reaction rate and the hydrogen yield are very excellent.