Preparation Of Co Catalysts From Hydrotalcite And Their Catalytic Performance For CH₄-CO₂ Reforming

Carbon dioxide reforming of methane to synthesis gas has become one of the research hotspots in recent decades because of its significant environmental benefits.It converts two major greenhouse gases(CH4+CO2)into a valuable feedstock(CO+H2),which can be used to produce clean liquid fuels and chemicals by Fischer-Tropsch synthesis and methanol synthesis.From an industrial viewpoint,non-precious metals such as Ni and Co are the most promising catalysts for CH4-CO2 reforming.However,Ni and Co catalysts easily suffer from deactivation by coke deposition,sintering of metal particles,and/or metal oxidation.Therefore,development of non-precious catalysts with high activity and stability is currently the focus of research.In this thesis,a series of supported Co catalysts were prepared by using hydrotalcite-like compounds(HTlcs)as precursors,and their catalytic activity,stability,and coke resistance property for CH4-CO2 reforming were investigated.The catalysts were characterized by various techniques including powder X-ray diffraction(XRD),temperature-programmed reduction(H2-TPR),temperature-programmed oxidation(O2-TPO),CO chemisorption,scanning electron microscopy(SEM),and scanning transmission electron microscopy(STEM).The effects of Co content,ZrO2 addition,and reaction conditions on the catalytic performance were stuided.The main results obtained are as follows:(1)Co-Mg-Al HTlcs with different Co contents are synthezed by co-precipitation method,all of which display a pure HTlcs phase.Upon calcination at 1073 K,Co-Mg-Al HTlcs are decomposed to Mg(Co,Al)O solid solution,where both Co2+ and Al3+ are incorporated into the MgO framework,and subsequent H2 reduction treatment at 1073 K produces Co metal nanoparticles with an average size of 9-10 nm.The catalytic activity,stability,and coke resistance of Co/Mg(Al)O catalysts are influenced by Co content,which increase with the increase of Co content from 6%to 15%.Under the reaction conditions of T= 1023 K,SV=60,000 mL h-1 g-1,and TOS?30h,the 12%-15%Co/Mg(Al)O catalysts showe high activity,stability,and good resistance against coke deposition and sintering of particles.This can be attributed to the high dispersion of Co particles and the strong metal-support interaction(SMSI)between Co particles and Mg(Al)O.Conclusively,a suitable Co content for Co/Mg(Al)O is 12-15%in terms of activity,stability,and coke resistance.(2)The catalyitc perforamnce of 12%Co/Mg(Al)O was further investigated under different reaction conditions including reacton temperature(T=773-1023 K),reaction time(TOS = 25-200 h),space velocity(SV = 60,000-120,000 mL h-1 g-1),and CH4/CO2 ratio(CH4/CO2/N2 = 25/25/50-50/25/25),and a comparison of catalytic performances of 12%Co/Mg(Al)O and 12%Ni/Mg(Al)O was made.It is significant to find that the Co/Mg(Al)O catalyst exhibits superior low-temperature stablility and coke resistance than the Ni/Mg(Al)O counterpart catalyst.Under the reaction conditions of T=773 K,SV=120,000 mL h-1 g-1,and TOS =25 h,the Ni/Mg(Al)O catalyst rapidly deactivates due to severe coke deposition,while the Co/Mg(Al)O catalyst maintains stable activity during the whole time on stream investigated.The initial coking rate on the Co catalyst is estimated to be about 1/20 times that on the Ni catalyst.The kinetic study suggests that the dissociative adsorption of CH4 is favored on the Ni catalyst,whereas the Co catalyst has high affinity for CO2 adsorption.The results of CH4-TPSR and direct CH4 decomposition experiments further reveal that the Co catalyst is much less active for carbon formation through CH4 decomposition.It is deduced that a combination of lower activity for CH4 dissociation/decomposition and higher affinity for CO2 adsorption as well as a stronger metal-support interaction is responsible for the higher coke resistance of the Co catalyst.(3)To further improve the coke resistance property of Co/Mg(Al)O,a Zr02-modified Co/Mg(Al)O catalyst was prepared by using Co-Mg-Al-Zr HTlcs as precursor.Under the reaction conditions of T= 873 K,SV = 60,000 mL h-1 g-1,and TOS = 25 h,all Co/Mg(Al)O-ZrO2 catalysts show good catalytic stability.It is found that the presence of a small amount of ZrO2(Zr/(Zr+Al)=0.025-0.05)can effectively suppress the coke deposition,which is reduced to be about 1/8 times that on Co/Mg(Al)O.The enhanced coke resistance by ZrO2 modification may be related to the increased CH4 activation energy and the interaction between Co metal and ZrO2.ZrO2 has the ability to rapidly adjust the two valence states of Zr4+/Zr3+,and it may provide oxygen atoms to react with CHx species on the Co metal surface,producing CO and H2;simultaneously,surface oxygen vacancies are formed,which may promote the dissociative adsorption of C02,thus inhibiting the formation of coke.