Design,synthesis And Catalytic Properties Of Supported Ni-based Alloys For Hydrogen Generation From Hydrous Hydrazine

Hydrogen,as an efficient and clean energy carrier,provides an ideal solution to the global energy and environmental problems.However,the implementation of the hydrogen economy requires advanced technologies for hydrogen production,storage and use.Among these technologies,hydrogen storage is generally considered the most challenging"bottleneck",as it must meet demanding specifications on energy density,charge/discharge rate,operating temperature and safety.Recently,the emergence of chemical hydrogen storage technology provides an opportunity to break the"bottleneck"of hydrogen storage.Hydrous hydrazine(N₂H₄·H₂O),as a representative chemical hydride,has the advantages of high hydrogen density(8.0 wt%),relatively low material cost,satisfactory stability at ambient conditions.In particular,the decomposition reactions of N₂H₄ yield only gaseous products,which is highly beneficial for the design and practical application of H₂-source systems.The development of catalysts with high activity,high selectivity and enduring stability is a core issue to the catalytic decomposition of N₂H₄·H₂O for hydrogen production.After nearly20 years of unremitting efforts,considerable progresses had been achieved in developing N₂H₄·H₂O decomposition catalysts.By using the modification strategies of alloying,alkaline carrier loading and nano-structuring,a variety of supported Ni-based and Co-based alloys catalysts have successfully been developed which enabled complete decomposition of N₂H₄·H₂O to generate H₂ at mild conditions.From the field of application,the cost and performance of the reported nanocatalysts are still far from adequate for practical H₂ sources.Particularly,most of the developed catalysts are rich in precious metals,leading to excessively high material costs.In addition,the catalyst systems reported in the literature mostly use low-concentration N₂H₄·H₂O solutions,resulting in low hydrogen storage density.Based on this research status,the thesis focuses on the formation mechanism of Ni-based catalysts,the design and synthesis of efficient and low-cost catalysts,the construction of high-capacity hydrogen production system and other key basic scientific issues for hydrogen generation from hydrous hydrazine,the main progress achieved is as follows:(1)To investigate the formation mechanism of Ni-based catalyst,the representative Ni_0.5Pt_0.5/Ce O₂ was selected as the research object,and a combination of phase/structure/chemical state analyses were conducted systematically to study the effects of coprecipitation preparation conditions on the catalyst composition,phase,and microstructure and atomic distribution state.It was found that the aging time is a key event affecting the phase composition and microstructural characteristics of the catalysts.By prolonging the aging time appropriately,the Pt intermediate can be completely reduced to Pt atomic clusters/nanoparticles in the aging step,which ensures profound effects on the fine nanoparticle size,high dispersion distribution,homogeneous composition of the Ni-Pt alloy in the target catalyst and accordingly obtain excellent catalytic performance for hydrogen production from N₂H₄·H₂O decomposition.This work clearly describe the formation mechanism of Ni-Pt/Ce O₂ catalyst,which should be significant for the rational design and controlled synthesis of high-performance catalysts for chemical hydrogen storage/generation.(2)The cost-effective Ni-Pt/Ce O₂ supported alloy nanocatalyst was designed and synthesized by co-precipitation combined with electrostatic adsorption methods.This method is based on solid solution Ce Ni_0.1O_x carrier and introduces high dispersion noble metal Pt by electrostatic adsorption.Compared with the traditional surface engineering strategy(e.g.,impregnation and replacement),this method can improve the efficiency of noble metal utilization while enhance the catalytic activity and selectivity of hydrogen production from N₂H₄·H₂O decomposition.It should be attributing to the atomically dispersed Ni²⁺solute in Ce Ni_0.1O_x solid solution and the uniform dispersion of Pt clusters on the oxide surface render the resultant Ni-Pt NPs with homogeneous alloying and narrow size distribution,which enables more active sites and higher intrinsic catalytic activity simultaneously.In addition,the versatility of this approach was demonstrated by changing the solute metal of solid solution and/or metal precursor in the SEA operation.This work will yield a new generality method for cost-efficient and low-cost supported catalysts in the catalytic materials fields.(3)A series of noble-metal-free Ni-W/Ni-W-O nanocomposite catalyst was synthesized by a hydrothermal method in combination with reductive annealing treatment.Based on experimental and DFT calculations,the internal correlation of phase composition-structure-performance was explored deeply.It was found that thus-prepared Ni-based catalysts exhibit remarkably distinct catalytic properties toward N₂H₄·H₂O decomposition depending upon the annealing temperature,which stem from the formation of different Ni-W alloys with distinct intrinsic activity and distribution state.The optimized Ni₄W/WO₂/Ni WO₄ nanocomposite catalyst exhibits high TOF of 33 h^-1 at 50°C,nearly 100%selectivity,and excellent stability toward N₂H₄·H₂O decomposition for hydrogen production.Furthermore,a high-capacity HP system was successfully constructed using this noble-metal-free catalyst and the commercial high concentration N₂H₄·H₂O solution,which has a high hydrogen capacity of 6.28 wt%and satisfactory dynamic response property at room temperature.These results may lay the foundation for the practical applications of the N₂H₄·H₂O based H₂ source system.