Polyoxometallates Mediated Structural Properties Of Pt-C Nanocatalysts For Hydrogen Production

Hydrogen?H₂?is an ideal energy carrier owing to its high energy-efficiency and non-polluting nature.H₂-based PEMFCs have shown good potential in the post-petroleum era for an alternative solution to current energy problems.Hydrolytic dehydrogenation of ammonia borane?AB?has attracted considerable interests from academia and industry mainly because of the high hydrogen capacity?19.6 wt%?,mild reaction conditions and high impurity of hydrogen produced.In this thesis,we focus on the development of Pt-C nanocomposites as catalysts for hydrolytic dehydrogenation of ammonia borane.Using carbon nanotubes?CNT?as a model support,effects of polyoxometalates?POM?on the catalyst structural properties and hydrogen production performances are emphasizely studied,and the corresponding catalyst structure-performance relationship is established,aiming to guide the rational design and optimization of high-efficient and low-cost catalysts for hydrolytic dehydrogenation of ammonia borane.?1?The reaction behavior of Pt/CNT nanocomposite catalyzed hydrolytic dehydrogenation of ammonia borane is investigated,and multiple techniques such as N2 physisorption,H₂-TPR,HAADF-STEM and XPS are employed to characterize the structural properties of the catalyst.A comparison of hydrogen generation activities between the Pt/CNT catalyst and various Pt-based catalysts taken from literature is made.It is found that the CNT is a promising support for the Pt-based catalyst toward unique structural properties,leading to higher hydrogen generation activity.?2?The effects of polyoxometalates?POMs?on the performance of Pt/CNT nanocomposite catalyzed hydrolytic dehydrogenation of ammonia borane are probed.The addition of silicotungstic acid?STA?is found to significantly enhance the hydrogen generation activity and catalyst durability,while those of phosphotungstic acid?PTA?and molybdophosphoric aicd?PMA?to exhibit remarkable inhibition effects.Further kinetic analyses,isotopic experiments and structural characterization of fresh and used catalysts?e.g.,HAADF-STEM,EDX,XPS,Raman and ICP?show that compared to PTA and PMA,the addition of STA facilitates the increase in the binding energy of Pt catalysts,the stabilization of Pt nanoparticles,the decrease in the activation energy of the reaction and the promotion of H₂O reactant,which are dominant factors of STA-Pt/CNT nanocomposite catalysts with better catalytic performance.Thus,a relationship between the catalyst structure and the hydrogen generation performance is established.?3?The effects of STA and reduced one?i.e.,RSTA?on the performance of Pt/CNT nanocomposite catalyzed hydrolytic dehydrogenation of ammonia borane followed by their concentration effects are comparatively studied,and the characterization of the fresh and used catalysts and the solution?e.g.,UV-vis,HAADF-STEM,EDX,Raman,ICP and XPS?is carried out.Both the STA and RSTA appear to enhance the hydrogen generation performance of the catalysts.In particular,for the lower nW:npt,the pre-activation of AB by RSTA mainly contributes to the RSTA-Pt/CNT catalyst with the higher hydrogen generation activity,while for the higher nW:npt,the electronic structure of Pt nanoparciels mediated by STA is a dominat reason for the STA-Pt/CNT catalyst with the higher hydrogen generation activity.Finally,a plausible catalytic mechanism is proposed for the promotion of Pt/CNT nanocomposite catalyzed hydrolytic dehydrogenation of ammonia borane by STA.