Structure-Performance Relationships Of Ni/MgAl₂O₄ Catalysts In Combined Steam And Carbon Dioxide Reforming Of Methane

Combined steam and carbon dioxide reforming of methane(CSCRM)is a novel technology for syngas production,which makes use of two types of greenhouse gases(CH4 and CO2)as well as steam.In this respect,CSCRM plays a critical role in reducing the emission of greenhouse gases and optimizing the methane utilization.Unfortunately,however,most Ni-based catalysts available for CSCRM are subject to metal sintering and coke deposition and thus,greatly restricting the application of CSCRM.To this end,this thesis aimes at developing an effective Ni-based catalyst with high anti-coking and anti-sintering properties.Both theoretical and experimental studies were conducted in this work.First,the reaction thermodynamic equilibrium involved in the CSCRM reaction networks was analyzed,focusing on the effects of temperature,pressure and feed composition on the equilibrium conversions of CH4 and CO2.H2/CO molar ratio in the product as well as the amount of coke deposition.The appropriate reaction condition for CSCRM was determined as follow:800?,0.1 MPa,CH4:H2O:CO2=2.5:2:1(molar ratio in the feedstock).Second,the interaction between Ni and five support materials(MgO,?-Al2O3,CeO2,ZrO2 and MgAl2O4,commonly used for CSCRM)was investigated by DFT calculations.The results showed that both the adsorption energy and the amount of charge transfer between Ni and support followed the order Ni/MgAl2O4(110)>Ni/ZrO2(110)>Ni/MgO(110)>Ni/CeO2(110)>Ni/y-Al2O3(110),indicating a strong interaction of Ni and support as well as an electron-rich state of Ni for the Ni/MgAl2O4 catalyst.Next,four types of MgAl2O4 and the Ni/MgAl2O4 counterparts with 10 wt%of Ni loading were prepared,with Ni/MgO and Ni/?-Al2O3 as reference.The structural properties of these catalysts were characterized in detail by different techniques,and their catalytic performance in CSCRM was evaluated,from which the structure-performance relationships of the catalysts were correlated.It was found that the Ni/MgAl2O4 supported on the sol-gel-derived MgAl2O4 had small Ni particle size,high Ni dispersion,large amounts of surface active sites and strong alkaline sites,which resulted in the best CSCRM performance among all the catalysts investigated.After 30 h of time on stream under a high gas hourly space velocity of 132000 mL/(gcat·h),this catalyst showed stable CH4 and CO2 conversions of 90%and 70%,respectively,with a H2/CO molar ratio of around 2.2 in the product.In addition,the spent catalyst had small Ni particles and low coke formation,demonstrating good anti-sintering and anti-coking properties.Finally,a series of sol-gel-derived Ni/MgAl2O4 with different Ni loadings varying from 5 wt%to 15 wt%were prepared,characterized and applied to CSCRM.The results showed that the Ni/MgAl2O4 with 10 wt%Ni loading exhibited the best activity and stability in CSCRM.