Design Of Oxygen Carriers And Study On The Intrinsic Oxygen Release Kinetics For Chemical Looping Partial Oxidation Of Methane

With the exhaustion of oil and coal resource,methane as the main component of natural and shale gas plays an increasingly important role in the global energy supply and chemical production.Unlike conventional methane partial oxidation approach,chemical looping methane partial oxidation temporally or spatially separates the feed of CH₄ and O₂ via using a metal oxide（Me O_x）as oxygen carrier（OC）.Such design provides a novel route to directly produce syngas with minimal security risks and energy penalty by using the lattice oxygen of OC as the resource instead of molecular oxygen.This thesis focuses on the design of oxygen carriers and study on the intrinsic lattice oxygen release kinetics for chemical looping methane partial oxidation.Considering the weak release kinetics of lattice oxygen in La Fe O₃,a typical perovskite oxygen carrier,we investigated the distortion of Fe O₆ octahedra in La_1-xCe_xFe O₃（x=0,0.25,0.5,0.75,1）orthorhombic perovskite oxygen carriers for the promotion of lattice oxygen activation.By combined electrical conductivity relaxation（ECR）measurements and density functional theory（DFT）calculations studies,we demonstrated that the enhancement of Fe O₆ octahedral distortion in La_1-xCe_xFe O₃promoted their surface oxygen exchange capability and bulk oxygen mobility.Consequently,La_0.5Ce_0.5Fe O₃ with the highest Fe O₆ distortion achieves the best syngas yield in chemical looping methane partial oxidation step.Given the high capability for methane over-oxidizing and too strong oxygen diffusion ability of the lattice oxygen in La₂Co Mn O₆ double perovskite,we developed a new triple perovskite,La₃Co Mn Al O₉,as oxygen carrier by inserting Al O₆ into the structure of La₂Co Mn O₆.ECR measurements proved that the surface oxygen exchange constant(K_chem)and bulk oxygen diffusion coefficient(D_chem)were decreased by Al O₆insertion,which results in the inhibition of lattice oxygen for the overoxidation of methane.As a result,the triple perovskite exhibits higher syngas selectivity and hydrogen yield than that of double perovskite.In addition,in situ XRD combined with time-resolved XPS revealed that the reduction of Co³⁺-O to Co²⁺-O is principally contribute to complete oxidation of methane,while the rduction of Co³⁺-O and Mn³⁺-O execute the partial oxidation of methane.The corresponding oxidation processes of Co⁰→Co²⁺-O and Mn²⁺-O→Mn³⁺-O are responsible for hydrogen production in H₂O splitting step.On the basis of the former two parts of study,we studied quantitatively the effect of matching degree of surface oxygen exchange capability and bulk oxygen diffusion ability on the“surface oxygen coverage"and products distribution of oxygen carriers in chemical looping methane patial oxidation by constructing x Ni O/Ce Al O_4-δ（x=0,0.2,0.5,1,2.5,5）oxygen carriers.It is revaled by thermogravimetric relaxation combined with XPS that with the kinetics of oxygen release varying from the surface exchange regime to bulk duffision regime,the surface oxygen coverage decreased,which results in the decreased CO₂ selectivity,the crescendo-decrescendo CO selectivity,and increasd methane cracking reaction.Consequently,1Ni O/Ce Al O_4-δwith moderate surface oxygen coverage achieves the best syngas yield in chemical looping methane partial oxidation.Furthermore,from the perspective of the whole and individual oxygen carriers,the promoting effect of surface oxygen exchange capability on the differences in bulk lattice oxygen mobility was evaluated by in-situ XRD and visualized by in-situ Raman Mapping,respectively.This thesis reveals the feasibility of improving the reaction performance of oxygen carriers in chemical looping methane partial oxidation by modulating the release kinetics of lattice oxygen,thereby paving the way for the design of oxygen carrier with high performance for chemical looping processes.