Highly Efficient Ni-based Catalysts For Hydrogen Production From Steam Reforming Of Glycerol

Hydrogen is one of the most competitive energy carriers in the future because of its unique advantages,such as environmental friendliness,high heat value,easy storage,various ways for application and so on.However,nowadays,more than 80%hydrogen in the world is produced from fossil fuels.It has been becoming a research focus worldwide to develop efficient and low-cost technologies for hydrogen production from renewable energy sources.Glycerol,a large by-product of biodiesel industry,has attracted much attention to be used as organic feedstock to produce hydrogen.Catalytic steam reforming is a promising process to convert glycerol to H₂-rich gases,nevertheless,research on the catalyst lifetime for long-term steam reforming reactions is still limited.Here in this work,we focus on developing low-cost Ni-based catalysts with long-term high activities for steam reforming of glycerol,by studying the promoter effects and optimizing the catalyst preparation methods.First,effect of promoters including La,Ce and Mg,on the catalytic performances of Al₂O₃ and ZrO₂ supported Ni catalysts?Ni/La₂O₃-Al₂O₃?Ni/CeO₂-Al₂O₃?Ni/MgO-Al₂O₃ and Ni/CeO₂-ZrO₂?were studied for the steam reforming of glycerol.The catalysts were prepared by step-impregnation method,and it was found that Ni/La₂O₃-Al₂O₃ with 10 wt.%La₂O₃ had the highest surface Ni contents and showed the highest catalytic activity.During the steam reforming of 5 wt.%glycerol at500?for 24 h,the Ni/La₂O₃-Al₂O₃ realized stable carbon gasification efficiency?CGE?of about 70%and hydrogen yield of about 4.2 mol/mol.The gas products mainly consisted of 70%H₂ and 30%CO₂.However,Ni/La₂O₃-Al₂O₃ encountered severe coking problem.Ni/CeO₂-ZrO₂ with 10 wt.%CeO₂ showed better catalytic activity stability and anti-coking ability than Ni/La₂O₃-Al₂O₃ catalyst,but its activity for the steam reforming reaction was low?CGE was about 30%?due to large nickel particles?³6 nm?over the bulk ZrO₂ with low specific surface area.Furthermore,catalyst preparation methods were evaluated and optimized to improve the anti-coking ability of Ni/La₂O₃-Al₂O₃ and the activity of Ni/CeO₂-ZrO₂for steam reforming of glycerol.For Ni/La₂O₃-Al₂O₃ catalyst,the step-impregnation preparation method exhibited higher surface Ni contents than those prepared by co-impregnation,sol-gel and co-precipitation methods.Thus,the step-impregnated Ni/La₂O₃-Al₂O₃ catalyst achieved the highest catalytic activity and the best stability for the steam reforming reactions.For Ni/CeO₂-ZrO₂ catalyst,the sol-gel sample obtained very stable and high CGE of⁹0%,3-to 4-fold of those catalyzed by the samples prepared by step-impregnation and co-precipitation methods,during the 24 h reaction.The catalyzed H₂ yield was about 5.5 mol/mol,79%of the theoretical maximum yield.The three components of sol-gel Ni/CeO₂-ZrO₂ catalyst prepared by the sol-gel method were well mixed,resulting in the formation of active Ni crystals of5-6 nm and Ce-Zr solid solution.The sol-gel Ni/CeO₂-ZrO₂ catalyst also showed high activity and excellent regeneration ability for steam reforming of 10 wt.%and 15 wt.%glycerol.Finally,doping amount of Ce to the Ni/ZrO₂ catalyst was optimized to further improve the activity of sol-gel Ni/CeO₂-ZrO₂ catalyst for steam reforming of glycerol with higher concentrations.CeO₂/ZrO₂ weight ratios in the studied series catalysts were set as 0/80,10/70,30/50,50/30 and 80/0.Results showed that the Ce promoter improved the dispersity of Ni crystals and promoted the specific surface area of Ni/ZrO₂ catalyst.When CeO₂/ZrO₂ was 30/50,the Ni/CeO₂-ZrO₂ catalyst exhibited the highest activity and the best stability,with the stable CGE of⁷0%and H₂ yield of 4.4 mol/mol for steam reforming of 15 wt.%glycerol at 500?.After 24 h reaction,the Ni/CeO₂-ZrO₂ catalyst with 30 wt.%CeO₂ also showed identical crystal structure and pore structure with the fresh catalyst,as well as excellent anti-coking ability,showing great potential as a catalyst for practical use.