Visibie-Light-Driven Hydrogen Generation From Ammonia Borane Over Non-Noble Metal Supported On Silicon-Based Semicondutors

Hydrogen is widely concerned because of its high calorific value,cleanness and water as the product after reaction.However,its preparation,transportation and storage are still facing great problems.Borane ammonia?NH₃BH₃?is considered as a promising chemical hydrogen-storage material due to its high hydrogen content?19.6wt%?and easy transportation and storage.In order to promote the rapid release of hydrogen from NH₃BH₃ at room temperature,the key is to design and synthesize cheap and efficient catalysts.Based on the above considerations and from the viewpoint of regulating the electronic density of metal nanoparticles to improve their photocatalytic hydrogen generation activity,three types of visible-light-responsive metal nanoparticles?NPs?supported on silicon-based semiconductors were prepared.Also,the relationship between their hydrogen generation performance and structures was studied.The main results are as follows:?1?A series of Co NPs supported on wide-spectrum-responsive Si,SiC and Si₃N₄were prepared and their photocatalytic hydrogen generation from NH₃BH₃ was studied.The results showed that the Co-based catalyst with Si had the optimal activity under visible light and the TOF value reached 154.7 min^-1.In order to further enhance the activity of catalysts,amorphous SiO₂ was added to Si to form composite supports.The catalytic results exhibited that when the amount of SiO₂ in the composite of Si and SiO₂ was 44 wt%,the activity of Co-based catalyst was the highest and the TOF value was 216.4 min^-1,which was higher than the values of reported non-noble metal catalysts.Compared with the catalysts containing single Si support,Co NPs can be better dispersed on the composite of Si and SiO₂ and then the utilization of visible light can be improved.This resulted in the remarkably enhanced photocatalytic activity of catalysts.?2?A series of Co and Ni NPs supported on semiconductors of composite SiC and TiO₂ were prepared and their photocatalytic hydrogen generation from NH₃BH₃ was studied.The results showed that compared with the catalyst supported on a single semiconductor,the catalytic activity of the catalysts with composite supports was significantly improved under visible light irradiation.When the amount of SiC in the composite of SiC and TiO₂ was 20 wt%,the Co-based catalyst had the highest activity with the TOF value of 288.5 min^-1,which was higher than the values of reported non-noble metal catalysts.A series of characterization results showed that with the increase of SiC content in the composite of SiC and TiO₂,the light responsive ability of the semiconductors was obviously enhanced and their band structures changed,which promoted the separation of photogenerated carriers.This led to that the electronic density on the surface of metal NPs was improved and then the hydrogen generation activity of catalysts was enhanded.?3?Based on the fact that the insulting layer in the metal-insulator-semiconductor?MIS?nanostructures can promote the transfer of photogenerated electrons from semiconductor to the surface of metal NPs,a series of thermal oxidation-induced SiC@SiO₂ species and supported Co and Ni nanocatalysts were synthesized.Then their photocatalytic hydrogen generation from NH₃BH₃ was studied.The results showed that the thickness of SiO₂ layer determined the hydrogen generation activity of catalysts.Particularly,the activity of Co catalyst with SiO₂ layer thickness of 3-4nm in Si@SiO₂ was the highest and the TOF value reached 314.2 min^-1,which was higher than the values of reported non-noble metal catalysts.