Design,Synthesis And Performance Study Of Nanocatalysts For Hydrogen Generation From Hydrazine Borane

Hydrogen has attracted an increasing level of attention as an important clean energy vector. Recently, hydrazine borane（N2H4BH3） has been proposed as an ideal hydrogen storage medium due to its high hydrogen content（15.4 wt%）, easy preparing and good stability. Suitable catalyst is essential for H2 generation from N2H4BH3 by hydrolysis of the BH3 group and selective decomposition of the N2H4 moiety to H2 and N2, which corresponds to a theoretical gravimetric hydrogen storage capacity（GHSC） of 10.0 wt% for the system of N2H4BH3-3H2 O. However, the N2H4 decomposition process is always sluggish and incomplete with undesired by-products of NH3. Thus, the key challenge of this issue is to design and synthesis of catalysts with high activity, 100% hydrogen selectivity and high stability for complete hydrogen generation from hydrazine borane. In this thesis, we have designed and synthesized a series of novel nanocatalysts for the complete hydrogen generation from hydrazine borane. The main research contents are described as follows:（1） A series of rare earth oxides（ReOx） doped Ni_0.9Pt_0.1-ReOx nanocomposites（NCs） were facilely synthesized via a co-reduction method. This research has demonstrated that the doping of amorphous CeO₂ can effectively transfers the morphology of Ni-Pt from crystalline to amorphous state. The amorphous Ni_0.9Pt_0.1-CeO₂ NCs was applied as catalyst for hydrogen generation from hydrazine borane. The hydrogen selectivity of Ni_0.9Pt_0.1-CeO₂ reached 93% and the total turnover frequency（TOF） was 234.0 h-1 at 323 K. However, the TOF value was only 77.5 h-1 for the crystalline Ni_0.9Pt_0.1 alloy nanoparticles（NPs） under the same reaction conditions.（2） In order to further improve the catalytic selectivity, the Ni_0.9Pt_0.1 NPs supported on reduced graphene oxide（rGO） has been synthesized by a facile co-reduction method. The characterized results showed that ultrafine NiPt NPs with a small size of around 2.3 nm were monodispersed on the graphene nanosheet. Compared to support-free Ni_0.9Pt_0.1 NPs, Ni_0.9Pt_0.1/rGO showed much higher catalytic performance with a 100% hydrogen selectivity and a total TOF value of 240.0 h-1. It was firstly found that the durability of the catalyst can be greatly enhanced by the addition of an excess amount of NaOH in this reaction.（3） Rh_0.8Ni_0.2@CeO_x/rGO NCs have been successfully synthesized via a surfactant aided co-reduction method. The research results demonstrated that the addition of CeOx can effectively decrease the crystallinity of RhNi, and rGO nanosheets served as a good dispersing agent and stable supporter for the metal NPs. Moreover, Ce Ox dopant and rGO nanosheets in the Rh_0.8Ni_0.2@CeO_x/rGO NCs both act as electron donors for atoms of Rh and Ni. Such electronic transport between RhNi, CeOx, and rGO in the Rh_0.8Ni_0.2@CeO_x/rGO NCs might enhance their catalytic activity. The synthesized RhNi@CeOx/rGO NCs displayed high activity（666.7 h-1） and 100% hydrogen selectivity in the hydrogen generation from hydrazine borane.（4） In order to improve the catalytic activity and stability, NiPt NPs were successfully immobilized by metal-organic frameworks MIL-101 with size and location control via a reductant dosage controlled reduction（RDCR） strategy. This research demonstrated that uniform ultrafine NiPt alloy NPs were encapsulated into the pore canals of MIL-101, when an excess dosage of reductant was applied. The as-synthesized NiPt/MIL-101 catalyst with only 10 mol% of Pt exerted so far the highest activity（1515.0 h-1） and remarkable durability for complete hydrogen generation from hydrazine borane.