Synthesis Of Hierarchical Porous Metal-Organic Framework Supported Transition Metal Catalysts And Their Catalytic Performance In Sodium Borohydride Hydrolysis For Hydrogen Generation

As fossil fuels dwindle,the global energy sector is increasingly focusing on clean and efficient hydrogen energy as a key competitive domain.Sodium borohydride（NaBH₄）,renowned for its exceptional attributes including high hydrogen storage capacity,convenient storage and transportation,safety,and non-toxicity,is widely recognized as an exemplary material for hydrogen storage and supply.However,the challenge lies in the inherent stability of NaBH₄ in water,which hinders the release of hydrogen through its self-hydrolysis.Recent research has focused on developing cost-effective and efficient catalysts that can achieve hydrolysis of NaBH₄ to produce hydrogen at ambient temperatures.The present study focuses on a series of cost-effective transition metal catalysts supported by hierarchical porous metal-organic framework materials（H-MOFs）.The correlation between the microstructure,surface elemental composition,and oxidation state of the catalysts is investigated with respect to their catalytic properties.The factors influencing the catalytic activity of these catalysts in the hydrolysis of NaBH₄ are also systematically analyzed.The main results are as follows:The catalysts consisting of ZIF-8 supported transition metals（Mn,Fe,Co,Ni and Cu）were prepared using an impregnation-reduction method.The crystal phase,micro-morphology,dispersion of the active phase and pore size of the catalysts were analyzed using XRD,SEM,TEM,EDS and N₂ adsorption/desorption tests.The results demonstrated the presence of numerous mesoporous during the metal loading process,causing the generation of a hierarchical porous structure within the support.This facilitated uniform deposition of ultra-fine transition metal powder onto the hierarchical porous ZIF-8,thereby enabling the catalysts to exhibit exceptional catalytic activity in the hydrolysis of NaBH₄ for hydrogen generation.The catalytic activity sequence for NaBH₄ hydrolysis was as follows:Co/HZIF-8>Ni/HZIF-8>Fe/HZIF-8>Mn/HZIF-8>Cu/HZIF-8.The catalytic activity of the Ni-Co double transition metal catalyst supported on hierarchical porous ZIF-8 was significantly superior to that of single metal catalyst.XPS analysis revealed a pronounced electronic synergistic effect between Co and Ni,thereby enhancing the overall catalytic performance.The catalytic performance reached its optimum when the molar ratio of Ni:Co was 6:4,exhibiting a TOF value of 41254 ml_H2·g_m^-1·min^-1,and an E_αvalue of 32.01 k J·mol^-1.This TOF value surpassed that of the majority of non-noble metal catalysts and even exceeded certain noble metal catalysts.The observation also revealed that the addition of HCl could facilitate the hydrolysis of NaBH₄ catalyzed by NiCo/HZIF-8,whereas the presence of NaOH could impede the hydrolysis process.Therefore,an"on-off"mode of hydrogen generation by NaBH₄ was achieved through the alternating addition of equal quantities of bases and acids to the reaction solution.The Ni/HZIF-67 catalyst was prepared using ZIF-67 with Co metal nodes as the support material,taking into consideration the superior catalytic activity of Co in NaBH₄ hydrolysis.The reduction of a portion of Co²⁺to Co⁰ by NaBH₄ in the hydrogen generation process was observed within ZIF-67,leading to an increase in active sites on the Ni/HZIF-67 catalyst and subsequent enhancement in catalytic activity.So,the catalytic performance of Ni/HZIF-67 significantly surpassed that of Ni/HZIF-8.In addition,the catalytic performance of Ni/HZIF-67 exhibited a significant enhancement with increasing NaBH₄concentration,while the quantity of catalyst had minimal impact on the catalytic hydrolysis of NaBH₄.Furthermore,the hierarchical porous Ni-Co/ZIF-67/rGO catalyst was prepared by utilizing reduced graphene oxide（rGO）and ZIF-67 as a composite support.The incorporation of rGO provided additional electron transport pathways for the support,leading to a significant enhancement in the catalytic performance of Ni-Co/ZIF-67/rGO.Additionally,NiCo/ZIF-67/rGO exhibited exceptional recycling performance,with a TOF value after recycling approximately 90%higher than that of the initial use.This enhancement could be attributed to the reduction of Co²⁺to Co⁰ at certain metal junctions in ZIF-67.The excellent recyclability offers a novel approach to enhance the reusability of the NaBH₄ hydrolysis catalyst.The last chapter focused on the catalytic performance of a Cu-Fe-Co-Ni four-metal catalyst supported by UiO-66,aiming to identify a catalyst that exhibits enhanced environmental adaptability and reduced cost.The orthogonal test method was employed to investigate the ratio of metals,and the range analysis revealed that the impact of four metals on catalytic performance followed the order of Co>Ni>Fe>Cu.Based on the orthogonal experimental findings,the optimal catalytic activity was achieved with Cu,Fe,Co,and Ni dosages of 0.02 mmol,0.08 mmol,0.1 mmol,and 0.08 mmol respectively.The TOF value reached 2140.24ml_H2·g_m^-1·min^-1,surpassing most reported Cu-and Fe-based catalysts.