Construction Of Photocatalyst And Mechanism Study For Ammonia Borane Hydrolysis

The development and utilization of hydrogen energy is an important way to solve the fossil energy crisis and environmental pollution.Among them,the high-efficiency hydrogen release of hydrogen storage materials under mild conditions is the key to the utilization of hydrogen energy.Ammonia borane（AB）,as an important hydrogen storage material,has the advantages of high hydrogen content（19.6 wt%）,good stability,and non-toxicity.Its hydrolysis reaction is the most effective way to produce hydrogen.However,the efficiency of hydrogen production is still low due to the difficulty in activating AB and H₂O molecules.In recent years,the introduction of photocatalysis to enhance the hydrolysis of AB to achieve high-efficiency and green hydrogen production has received widespread attention.In this study,a variety of strategies were used to rationally design the catalyst structure and composition to improve the light utilization efficiency of the catalyst.The electron density of the active center of the catalyst is increased through light-driven effect,which promotes the adsorption and activation of AB and H₂O molecules,and improves the hydrogen production activity of the catalyst.Furthermore,the in-situ spectroscopy,density functional theory calculations were used to investigate the reaction mechanism of hydrogen production from the hydrolysis of ammonia borane.The main research contents include:Utilizing the alloy effect,the Mott-Schottky（MS）effect between metal and semiconductor,and the plasmon resonance（LSPR）effect of metal elements,the Ni Cualloy photocatalyst supported on carbon nitride nanosheets（Ni Cu/CNS）was designed and synthesized and used for AB hydrolysis.The results show that the NiCu/CNS catalyst exhibited a turnover frequency（TOF）value of 30.6 min^-1 under visible light irradiation,which is nearly 3.5 times higher than that in the dark.Photoelectric characterizations show that the alloy effect between Cuand Ni,the MS effect between the metal and the carrier g-C₃N₄,and the LSPR effect of metal Cucan effectively increase the electron density of the Ni active sites under visible light irradiation,which promoted the activation and breaking of O-H in H₂O and B-H in NH₃BH₃.In addition,the isotope labeling experiments,in situ mass spectrometry,infrared spectra were creatively applied to reveal the source of hydrogen and the reaction mechanism.The W₁₈O₄₉ photocatalyst with a sea urchin-like morphology(W₁₈O₄₉SU)was designed and synthesized by using the multiple light scattering effect and nano-tip effect of the hierarchical sea urchin-like structure.Subsequently,the effect of the catalyst morphology and structure on the photocatalytic AB hydrogen production was systematically studied.As a result,the W₁₈O₄₉SU exhibits significantly enhanced photocatalytic H₂ production with TOF of 53.1 min1,which is about 10.4 and 7.5 times higher than those of W₁₈O₄₉HS and W₁₈O₄₉NR,respectively.Control experiments and photoelectrochemical characterizations indicate that multiple light scattering effect of hierarchical sea urchin-like structure improves the visible light harvesting,the nano-tip effect facilitates the separation of electron-hole pairs,transfer and enrichment of electrons at the rod tip.Therefore,the local electron density of active sites was enriched and thus leading to improved photocatalytic hydrogen production.Furthermore,the significant advantages of this hierarchical structure have also been confirmed on the sea urchin-like Nb₂O₅ photocatalyst.Using the elemental doping,A series of（CuO）/（Co₃O₄）@CN catalysts were synthesized through one-step pyrolysis using the bimetallic CuCo-ZIFs as the precursors.Among them,（CuO）₁/（Co₃O₄）_8.4@CN-500 shows significantly improved hydrogen production performance.Under visible light irradiation,the TOF of hydrogen production is 32.0 min^-1,which is 2.6 times that under dark(12.3 min^-1).The results indicate that the N-doped carbon support promotes the activation and dissociation of H₂O through the strong interaction of the N-O covalent bond.Cu-doping enhances the local electron density of the active sites by optimizing the electronic structure at the interface of CuO and Co₃O₄.Furthermore,Visible light irradiation strengthens the effects of N doping and Cudoping,and the heterogeneous interface between CuO and Co₃O₄ promotes the separation of photogenerated carriers,leading to the enhanced electron density of the active site.Thus,the hydrogen production is significantly enhanced.Using the metal-support interaction,ultra-small Pt nanoclusters（～1.2 nm）embedded in the Co₃O₄ nanocage was designed and synthesized.As a result,the as-prepared Pt/Co₃O₄ NC exhibited extraordinary catalytic activity toward AB hydrolysis with a TOF of 721.0 min^-1,which was mainly attributed to the synergistic effect and metal-support interaction between Pt and Co₃O₄.Specifically,Co₃O₄ promotes the dissociation of H₂O and Pt promotes the dissociation of AB.The metal-support interaction promotes the transfer of electrons from Pt to Co₃O₄,enhances the electron density of the active site.After the introduction of light irradiation,the M-S effect between Pt and Co₃O₄ promotes the effective separation of photogenerated carriers and further increases the electron density of the active site.As a result,Pt/Co₃O₄ NC exhibits the high photocatalytic hydrogen production with a TOF of 1442.0 min^-1,which is 2.0 times than that of under dark.