Study On The Preparation Of Cobalt-based Catalysts And Performance For Hydrogen Production By Hydrolysis Of Sodium Borohydride

With ever-growing concerns over oil scarcity and greenhouse gas emissions,the development of new renewable green energy has attracted great attention.Hydrogen is considered as an environmentally friendly fuel and a good energy carrier because of its high energy density and renewability.Vigorously developing hydrogen energy can effectively reduce environmental pollution and dependence on fossil fuels,which is the focus of current energy research.However,efficient and safe hydrogen storage,transportation and supply are key technologies for the future development of hydrogen economy.Sodium borohydride?NaBH₄?is an ideal choice for chemical hydrogen storage because of its high bulk density,good storage performance and reaction controllability,low initial reaction temperature and environmentally benign hydrolysis products.For the hydrolysis of sodium borohydride to hydrogen,cobalt-based catalysts have attracted much attention due to their high activity and low cost,but they are easy to deactivate quickly.In order to explore the reasons,improve the stability of the catalyst and enhance the activity of the catalyst,Co@NC catalyst and CoxOy/BC catalyst were prepared and used in the hydrolysis of sodium borohydride to produce hydrogen.The structure and properties of the catalyst were characterized by XRD,TEM,SEM,BET and XPS.The effects of catalyst particle size,dispersion degree of active components,oxidation state and reaction conditions on the catalytic hydrolysis of sodium borohydride were also investigated.The main contents are as follows:1?Co@NC catalystsCobalt nanoparticles packaged into nitrogen-doped porous carbon were successfully prepared by pyrolysis of MOFs.The addition of Zn in MOFs as a“fence”expanded the distance of adjacent Co atoms in space and simultaneously,the leaving Zn²⁺sites generated free N sites during pyrolysis.These are beneficial to reduce the sizes of Co nanoparticles and enhance the dispersity of active sites.The modification method allows the cobalt nanoparticles to be uniformly and finely confined within the porous carbon.Contributed by highly dispersed Co nanoparticles and confinement effect,the Zn1Co1-Co@NC catalyst showed excellent catalytic activity with hydrogen production rate of1807 mL?H₂?·min^-1·g_Co^-1 and lower activation energy?26 kJ/mol?.It is also found that the catalytic activity of the sample increases first and then decreases with the increase of NaOH concentration.This indicates that the appropriate concentration of sodium hydroxide can promote the hydrolysis of sodium borohydride to produce hydrogen.When the concentration of sodium hydroxide is too high,the solubility of by-product sodium metaborate will be reduced,which will make NaBO₂ precipitate more easily on the surface of the catalyst,resulting in the reduction of the active sites on the catalyst surface,thus affecting the catalytic performance of sodium borohydride hydrolysis to produce hydrogen.2?CoxOy/BC catalystsB-?-Co?OH?₂ intermediate was prepared by using sodium tetraphenylborate as boron,carbon source and precipitator.B-?-Co?OH?₂ was used as precursor to prepare different cobalt oxide supported on boron-doped carbon carrier by pyrolysis at high temperature.Three-dimensional CoO+Co₃O₄/BC catalyst with like coral morphology was prepared by changing different preparation methods.The catalysts have the highest catalytic activity for hydrogen production from sodium borohydride hydrolysis due to oxygen-rich and the coordination of various metal oxides.The hydrogen production rate is 6478.78 mL?H₂?·min^-1·g_Co^-1,and the activation energy is41.14kJ/mol.Like Zn1Co1-Co@NC catalyst,the catalytic activity of the CoO+Co₃O₄/BC catalyst increases first and then decreases with the increase of NaOH concentration.3?Cycle Test of Zn1Co1-Co@NC and CoO+Co₃O₄/BC CatalystsThe cyclic stability of Zn1Co1-Co@NC and CoO+Co₃O₄/BC catalysts was tested.It was found that the cyclic stability of Zn1Co1-Co@NC catalyst was poor,and it almost lost its activity after five reactions.However,the CoO+Co₃O₄/BC catalyst maintained a stable hydrogen production rate after four reactions and would not be completely deactivated.It was found that the poor cyclic stability of the two catalysts for hydrogen production by hydrolysis of sodium borohydride was due to the strong deposition of borates on the surface of the catalysts after the reaction.However,the surface morphology of CoO+Co₃O₄/BC catalyst after reaction has porous structure,which makes the catalyst not completely deactivated after several reactions.