Study On Preparation Of Cobalt/Niobium-based Catalysts And Their Catalytic Properties Of Lightweight Hydrogen Storage Materials

Metal/ligand hydrides have the advantages of high hydrogen storage capacity and low cost,and are very promising solid-state hydrogen storage materials.Among them,the representative coordination/metal hydrides Na BH₄ and Mg H₂ have attracted a lot of attention.Mg H₂ and Na BH₄ have limited their practical application because of poor kinetic properties,high thermodynamic stability and low recyclability.It was found that catalysts can improve their hydrogen production and storage performance.In order to systematically investigate the effect of catalysts on the hydrogen production and storage performance of Na BH₄ and Mg H₂.In this thesis,Co-B-P/Graphene Oxide（GO）catalysts were firstly prepared by chemical in situ reduction method,which were used to catalyze the hydrogen production from Na BH₄ hydrolysis.Then,two catalysts,Ti₂Nb₁₀O₂₉（TNO）microspheres and Ni Nb₂O₆（NNO）nanoparticles,were prepared by solvent heat and heat treatment,and they were applied to the study of the hydrogen storage properties of Mg H₂.The main conclusions are as follows:（1）The graphene modified Co-B-P particles（Co-B-P/GO）catalysts were prepared by chemical in situ reduction method.The Co-B-P/GO catalysts for hydrogen production from Na BH₄ hydrolysis at different GO additions were compared.The results showed that the as-prepared catalyst with GO content of 75 mg（Co-B-P/75GO）exhibits an optimal catalytic activity with HGR of 12087.8 m L·min^-1·g^-1 at 25 ^oC,which was 3 times higher than that of Co-B-P catalyst(3872.9 m L·min^-1·g^-1).In addition,the Co-B-P/75GO-catalyzed Na BH₄ hydrolysis also exhibited an excellent activation energy of 28.6 k J·mol^-1 and a catalyst with 88.9%of the initial catalytic rate after 10cycles,which demonstrated the good cycling stability of the Co-B-P/75GO catalyst.（2）The TNO microsphere catalysts were synthesized by solvent heat and heat treatment,and were ball milled with Mg H₂ by mechanical ball milling method.The hydrogen release performance test showed that Mg H₂-7 wt%TNO microspheres had the best hydrogen storage performance with the system starting hydrogen release temperature of 189.1°C.The isothermal hydrogen release/absorption results showed that the system released up to 6.69 wt%H₂ in 20 min at 300°C with a maximum hydrogen release rate of 0.61 wt%·min^-1,and absorbed 6.38 wt%H₂ in 10 min at 200°C and 5 MPa H₂ pressure with a maximum hydrogenation rate of 1.43 wt%·min^-1.The activation energy of hydrogen release is 92.7±1.9 k J·mol^-1 and the activation energy of hydrogen adsorption was-26.0±1.4 k J·mol^-1.The enthalpy change(ΔH_des/ΔH_abs)of the system during desorption/adsorption is 69.6±2.3/-65.1±0.3 k J·mol^-1 H₂.The mechanism analysis reveals that the isothermal dehydrogenation process of the system follows a two-dimensional phase boundary pattern dominated by geometric contraction and two-dimensional phase boundary diffusion.The in situ generated low-valent Ti（i.e.,Ti（2+）and Ti（0））and Nb（i.e.,Nb（4+）and Nb（2+））as active catalytic centers act as electron transfer carriers,facilitating the electron e^-transfer between H^-/H（0）and Mg/Mg²⁺,and their synergistic effect enhances the hydrogen storage properties of Mg H₂.The cycling test showed that the system maintained a hydrogen release reversible capacity of 6.22 wt%H₂ after 15 cycles,which was equivalent to 93.8%of the hydrogen release capacity of the first cycle.The multivalent Ti/Nb active catalytic species formed in situ did not change during the cycling process,resulting in good cycling performance.（3）The Nano NNO catalyst was synthesized by a simple two-step hydrothermal and thermal treatment method.It was further doped into Mg H₂ by mechanical ball milling method.the Mg H₂-5 wt%Nano NNO system started to release hydrogen at205.3°C.The isothermal absorption/dehydrogenation experiments showed that the system could release up to 6.19 wt%H₂ in 20 min at 300°C and absorb 5.52 wt%H₂in 10 min at 200°C and 5 MPa hydrogen pressure.The activation energy of hydrogen release was 105.4±1.8 k J·mol^-1 and the activation energy of hydrogen adsorption was-27.2±1.4 k J·mol^-1.The enthalpy change(ΔH_des/ΔH_abs)of the system during desorption/adsorption was 72.8±1.8/-70.3±2.0 k J·mol^-1 H₂.Mechanistic analysis shows that the isothermal dehydrogenation process of the system follows a two-dimensional phase boundary model dominated by geometric contraction and two-dimensional phase boundary diffusion.The in situ generation of Nb O/Nb O₂ and Mg₂Ni/Mg₂Ni H₄ catalytically active species by the hydrogen release/absorption process can achieve both kinetic and thermodynamic performance modulation,and their synergistic catalysis improves the hydrogen storage performance.The cycling test showed that the hydrogen release capacity of the system can still be maintained at 5.49wt%H₂ after 10 cycles,which was equivalent to 84.3%of the capacity of the first cycle.And after 10 cycles Nb O/Nb O₂ and Mg₂Ni/Mg₂Ni H₄ catalytically active species were still present in the Mg H₂ matrix,which led to good cycling performance.