Study On Hydrogen Production Mechanism Of Methane Cracking Enhanced By Activated Carbon Modified By Carbon Black

With the proposal of China’s dual carbon goal of "carbon peaking and carbon neutralization",finding sustainable,clean and efficient energy is the key to solve the world’s energy problems.At present,as an efficient clean energy,hydrogen energy has greater development potential compared with other energy sources because of its advantages of wide source,high calorific value and no pollution.In recent years,methane cracking,an efficient hydrogen production method,has attracted extensive attention because its products are only solid carbon and hydrogen without carbon dioxide emission.However,the reaction requires the use of catalysts to reduce the reaction temperature to achieve appropriate hydrogen yield.At present,the types of catalysts used mainly include carbon based-catalysts and metal catalysts.Metal catalysts have high activity,but they are easy to inactivate and have high cost.Carbon based catalysts have been widely studied because of their low cost and high temperature resistance.Among them,activated carbon(AC)has rich pore structure,large specific surface area,good gas adsorption and high initial conversion,but it is easy to be inactivated due to carbon deposition.In contrast,carbon black has good stability although its initial activity is low.Therefore,this thesis proposes to load carbon black into the pore structure of activated carbon to make it have higher initial activity and better stability,so as to delay the deactivation effect.The mechanism of hydrogen production from chemical chain methane cracking enhanced by carbon black modified activated carbon is studied through reasonable preparation process,experimental design,characterization analysis and molecular simulation.Main research contents are as follows:Firstly,three concentration gradient samples of AC/CB-1.0,AC/CB-0.5 and AC/CB-0.25 were prepared by ultrasonic vibration and mechanical stirring.The results show that there is no aggregation of carbon black on the surface of AC/CB-1.0,and the carbon black particles are fine,the dispersion is the highest,but the load is the lowest,only 16%;In the AC/CB-0.5 sample with 40% load,a large amount of carbon black is attached to the surface of activated carbon,but carbon black agglomeration occurs;A small amount of carbon black loading and large agglomerates were observed on the surface of AC/CB-0.25.The pore structure of AC/CB-0.25 and AC/CB-0.5 activated carbon is damaged and cracks are generated on the surface of some activated carbon particles.In contrast,the pore structure of AC/CB-1.0 surface has greater integrity with its prominent structure.Secondly,the above three catalysts were tested for hydrogen production by methane cracking at the reaction temperatures of 900 ℃,950 ℃ and 1000 ℃,respectively.Through the comparative experiment,it was found that the overall methane conversion was AC/CB-1.0 > AC/CB-0.25 > AC/CB-0.5 > AC.Among them,AC/CB-1.0 sample has the best catalytic performance,which not only greatly improves the initial methane conversion,but also prolongs the deactivation time.Through the comparative analysis of its surface physical and chemical properties by SEM,XPS,XRD,bet and other characterization methods,it can be seen that the carbon black loading increases the micropore volume of activated carbon,increases its crystallization degree,strengthens the order of chemical structure,and the types and concentrations of oxygen-containing functional groups change significantly before and after the sample reaction.Therefore,the changes of micropore volume and surface hydroxyl,carbonyl and carboxyl functional groups of activated carbon are closely related to its catalytic activity.Finally,the growth mechanism of carbon deposited on its surface and the effects of pore size and functional group factors on the adsorption of methane on activated carbon were studied by HRTEM characterization and molecular simulation,and the mechanism of carbon black modified activated carbon enhanced chemical chain methane cracking to produce hydrogen was explored.The results show that carbon black loading increases the micropore volume of activated carbon,which is conducive to improve the initial adsorption performance of methane molecules.With the progress of the reaction,loaded carbon black induces activated carbon to form carbon filaments,which cover its surface and block the pores;At the same time,the continuous consumption of carboxyl functional groups is not conducive to the adsorption of methane molecules by activated carbon,but the deposited carbon generated by carbon black increases with the progress of the reaction,providing it with new active sites,and the reaction rate is directly proportional to the number of active sites,which promotes the progress of the reaction;However,the active sites will soon be covered by the network deposited carbon,and the generation rate of the active sites is far less than the disappearance rate of the active sites caused by the coverage.The active sites can not maintain balance,resulting in the deactivation of the catalyst.