Promotion Effect Of Cerium Doping On Ni Core-shell Catalysts In Ethanol Steam Reforming For Hydrogen Produciton

This work focuses on Cerium's promotion effect on the catalytical performance of Ni-based and La-Ni ethanol steam reforming(ESR)core-shell catalysts.Cerium proves to be effective on improving activity and stability through increasing Ni species reducibility,Ni0 dispersion and stability of Ni0.Hydrogen produciton through steam reforming reaction is notably enhanced with Cerium partly replacing Lathanum in La-Ni perovskite-tpye oxide compounds,which is the main Nickel reservoir of LaNi@(Mg,AI)O core-shell catalyst.Oxygen storage capacity(OSC)of Cerium plays an important part in tuning surface properties.Carbon elimination is accelerated through Cerium addition.In chapter 2,Ni-based core-shell catalyst is devepoled by reconstruction method.Ni enrichment is achieved by reconstruction of Mg3AlO oxide compounds developed from hydrotalcite-type precursor.NiO is loosely dispersed on catalyst surface until its transformation into NiAl2O4 under severe calcination.Weak interaction is confirmed through the observation of a large amount of NiO remaining even after 800 ?calcination.NiO ? NiAl2O4 are the key Ni species of Ni@(Mg,Al)O catalysts,which show high YH2 and good CH4 conversing ability in 700 ? ESR.However,the Nickel particles reduced from NiO is prone to sintering during reduction section,thus limiting the initial activity of 800 ? calcinated Ni@(Mg,Al)O.Reduction of NiAl2O4 continues on 900 ? calcinated Ni@(Mg,Al)O catalyst through on-stream reduction,yet the fresh small nano-sized Nickel particles tend to agglomeration,thus the activity weaken and thick nano carbon fibers are produced on sintered Nickel particles.MSR activity of Ni@(Mg,Al)O catalysts is subjected to low surface OH-concentration and poor mobility.In chapter 3,reconstruction method is successful in introducing La into Ni@(Mg,Al)O catalyst and creating a LaNiO3 coating through 800 ? calcination.More LaNiO3 could be generated with 900 ? calcination at the cost of decreasing the reducibility and surface area of catalyst.The fine nano-sized Nickel particles reduced from LaNiO3 and its good anti-sintering stability are responsible for the significance improvement of water gas shift reaction(WGS)activity.La2O3 is not sufficient to neutralize the acidity of Mg3AlO support or to activate enough H2O molecules to react with CH,.C2H4 is produced in 700 ? ESR process.Carbon species are hard to remove on LaNi@(Mg,Al)O catalyst at low temperature.In chapter 4,Ni2+ induced reconstruciton is interferenced by Ce ions,which are added to Ni2+ nitrate solution used for synthesis core-shell catalyst.Ce02 is dispersed on catalyst surface through Ce ions loading on the surface of reconstructed hydrotalcite.CeAlO3 is generated through CeO2's interaction with support,thus facilitating Ni2+ entering surface ?-Al2O3 to form amorphous NiAl2O4.CeO2 promotes the reduction of various species,including NiO?(Ni,Mg,Al)O and(Ni,Mg)2Al4.WGS reactivity,as well as anti-coking ability are improved by the OSC of CeO2,especially by the oxygen vacancies at NiO/CeO2 interface.However,the bulk oxygen vacancies in CeO2 are only activated until 850 ?,thus low oxygen species mobility at lower temperature.Benefiting from good anti-coking ability of CeO2,Ce0.5Ni0.5-800 catalyst shows outstanding performace in 700 ? ESR process at a space velocity of 90,000 ml/g·h,achieving YH2 as high as 4.46 mol/mol EtOH and great maintenance during 200 h test,producing only a small nano carbon fiber formation of 0.00159 mg carbon/mg catalyst·h.Nickel phase is mostly preserved after stability test.CeO2 sinters easily in pre-reduction and ESR process,limiting its function as Ni reduction promoter?cocatalyst as well as coke eliminator.In chapter 5,Ce ions,La3+ and Ni2+ are introduced as shell constitutents by reconstruction method.Ce ions partly replace La3+ in perovskite La1-xCexNiO3.Both CeO2 and Ce ions in La1-xCexNiO3 facilitate Nickel reduction.La1-xCexNiO3 and(Ce,La)Ox solid solutions are produced and play critical parts in LaCeNi@(Mg,Al)O catalysts' catalytical performance in ESR process.Ce dispersion in Ni species,the key factor of the oxygen vacancy concentration of CeO2 and La1-xCexNiO3,is determined by Ce/La ratio in Ni2+ nitrate solution and calcination process.Cerium Is effect on improving activity,facilitating reduction and removing coke is optimized by maximizing Ce ions replacement rate in La1-xCexNiO3.Controlling Ce/La a low ratio helps to restrain Ce ions in perovskite oxide structure.A Catalyst synthesized by a Ce ions,La3+,Ni2+ nitrate solution with Ce/(Ce+La)=20%shows great anti-coking ability.Owing to enhanced WGS reaction catalyzed by CeO2 and fine NiO particles,catalysts synthesized by nitrate solutions with Ce/(Ce+La)=40%and Ce/(Ce+La)=80%show high YH2 as high as 4.17 mol/mol EtOH and 4.21 mol/mol EtOH,respectively.