| Methane（CH4）has stable chemical structure,higher ionization potential and low electron affinity characteristics,which make C-H bonds in CH4 molecules are hard to break under mild reaction conditions,hindering the conversion of CH4 to synthesis gas and affecting the production of high value-added chemicals.Cerium oxide（CeO2）and CeO2-containing materials have been widely used in the low temperature partial oxidation of CH4.The precious metal Pt is not strong in oxygen adsorption and is not easy to be oxidized.It is often used in selective catalytic oxidation reactions.Among noble metals of the same size,Pt clusters have the highest activity in cracking C-H bonds.As the catalytic active component,the dispersion and oxidation state of Pt affect the activation of CH4.Pt loading on CeO2 can regulate the catalytic activity of CeO2 by improving the Fermi level by integrating doping ions into the surface lattice of CeO2.Pt can activate adjacent oxygen atoms,change the oxygen vacancy formation energy,and affect the reactivity of CeO2 surface lattice oxygen atoms for C-H bond activation.However,due to the variable and complex structural characteristics of CeO2,it is difficult to study the structure-effect relationship between the structure and performance.In addition,the suitable oxidation species are essential for reducing the CH4 activation temperature in the partial oxidation of methane（POM）reaction.It is difficult to achieve low temperature partial oxidation of CH4,which puts forward high requirements for catalyst construction and reaction environment.Based on this,a series of CeO2-supported Pt catalysts with CeO2 or modified CeO2 as supports and Pt as active metals were prepared,and the physical and chemical properties of each catalyst were characterized and evaluated.The effects of catalyst structure features of oxidation state,surface oxygen vacancy,reducibility and lattice oxygen mobility on POM reaction were investigated,and the structure-effect relationship between catalyst structure and performance was revealed.The main research contents and results are as follows:（1）The CeO2-supported Pt catalyst was treated with steam.The dispersion and valence state of Pt,the formation of surface hydroxyl,the oxygen activities of CeO2 lattice and the formation of oxygen vacancy were characterized.The effects of steam treatment on physical structures and chemical properties of the catalysts were described by DFT calculation.The catalysts were applied to POM reaction to explore the effects of the changes of catalyst structures on catalytic activities.It had been found that steam dissociated on the catalyst surfaces to form hydroxyl groups,which could combine with Pt to form Pt-OH species to promote the dispersion of Pt.In addition,steam dissociation promoted the activation and removal of oxygen in the CeO2 lattices,resulting in the formation of abundant oxygen vacancy,which contributed to the activation of C-H bonds in CH4 molecules,showing excellent POM reaction performance.（2）A series of Pt/CeO2-based catalysts with different oxygen vacancy and Pt valence state were prepared by reducing Pt/CeO2 catalyst at different temperatures.The morphology,structure and composition of each reduced sample were analyzed,and the POM reaction performance was evaluated.Through the in situ DRIFS experiments under simulated reaction conditions,the roles of each catalyst component（Pt and CeO2）in the reaction were explained in detail,and the correlation between the surface structures of catalysts and catalytic reaction was revealed.It had been found that Pt/CeO2-H350 catalyst had the high efficiency in low temperature catalytic POM reaction.This is because the catalyst configuration achieved an optimal ratio between CeO2 oxygen vacancy and low valence Pt species.CH4 was activated on the low valence Pt species,and oxygen vacancy were mainly used for O2 activation.Metal-support synergetic system promoted CH4 reaction.（3）A series of CeO2-supported Pt catalysts doped with lanthanide ions(La3+,Nd3+,Er3+and Yb3+)were synthesized by electrospinning method,and the POM catalytic activity of each doped catalyst was studied.The relationship between catalytic configuration,such as the dispersion and valence state of loaded Pt,the oxygen vacancy of CeO2,the oxygen activation mobility or the oxygen vacancy formation capacity and oxygen storage capacity,and the catalytic performance was investigated.It had been found that Pt/CeLa catalyst had high catalytic activity and product selectivity at low temperature due to the low valence state of loaded Pt,the excellent oxygen migration and oxygen storage capacity of CeO2.（4）The surface chemical properties and catalytic processes of Pt/CeO2 catalyst at various POM reaction temperatures and atmospheres were monitored by in situ XPS and in situ DRIFTS characterization techniques.The chemical properties of catalyst surface,such as CeO2oxygen vacancy and Pt valence state,were correlated with the corresponding catalytic activity to reveal the dynamic change process of catalyst in the reaction and the influence of the dynamic change on the catalytic activity.It had been found that enrichment of the catalyst with oxygen vacancy and low valence Pt species prior to the initiation of the reaction is necessary for low temperature CH4 conversion,i.e.exposing the catalyst to anoxic environment prior to ignition temperature.In conclusion,in this study,coupled with CeO2 properties such as oxygen vacancy,lattice oxygen activity,oxygen migration and storage capacity,strong interaction between CeO2 and deposited Pt and metal properties such as dispersion and oxidation state of Pt,a metal-support cooperative configuration of catalyst was constructed,and the structure-effect relationship of low temperature partial oxidation of CH4 promoted by abundant oxygen vacancy,rapid oxygen activation and low-state Pt species was revealed. |
