Study On Synthesis And CO Catalytic Oxidation Properties Of CeO₂-based Nanostructured Composites

Carbon monoxide(CO)is a kind of poisonous gas which does great harm to environment and life.It is so significant for our production and life to eliminate CO in the environment scientifically and effectively.The outermost electron configuration of Ce element in lanthanide rare earth elements is 4f~15d~16s~2,so it has a good valence change characteristic,and its oxide CeO2 has the function of oxygen storage and releasing,which has great application potential in catalysis and other fields.With the development of science and technology,low temperature catalysis has become the focus of current research.Although pure CeO2 has certain catalytic performance,the amount of active oxygen on the surface of the catalyst is small,so it is difficult to realize the catalytic oxidation of CO at low temperature.In order to solve the problem of low temperature catalysis,the catalytic temperature can be effectively reduced by nanocrystallization,doping or recombination of catalysts.In this paper,the transition metals(Mn,Co,Cu)and their oxides or precious metals(Pd,Ag,Au)in different proportions were combined with CeO2 matrix by hydrothermal method and combustion method to prepare particles,one-dimensional rod-shaped and three-dimensional porous nanocomposites,and their catalytic oxidation activity on CO was detected.Furthermore,the internal mechanism of CO oxidation catalyzed by the catalyst was revealed.The main research and conclusions are as follows:(1)Preparation,characterization and catalytic oxidation performance of transition metals(Mn,Co,Cu)and their oxides/CeO₂ nanomaterialsTransition metal M and its oxide composited with CeO2 nanomaterials with different proportions were prepared by hydrothermal method and combustion method,respectively.The phase,morphology,surface element valence state and material reduction ability of the prepared catalysts were characterized and discussed by XRD,SEM,XPS and H2-TPR.The results show that the nanomaterials prepared by hydrothermal method have better crystallinity and larger grain size,while the materials prepared by combustion method have smaller grain size and form a cheese-like porous porous structure.Low M^x+ions were successfully introduced into the surface of the composites prepared by the two methods,and the high Ce^x+ions acted synergistically with each other,which promoted the increasing of active oxygen content on the surface of the catalysts.Compared with pure CeO2,the catalytic performance of CeO2-based catalysts modified by transition metal M has been significantly improved.For example,the catalytic oxidation temperature(T50%)of pure CeO₂ prepared by combustion method is 238?,while the T_50%of 35%Cu-CeO₂ prepared by combustion method can be as low as 74?.(2)Preparation,characterization and catalytic oxidation performance of precious metal(Pd,Ag,Au)/CeO₂ nanomaterialsPrecious metal(Pd,Ag,Au)composited with CeO2 nanomaterials with different proportions were prepared by hydrothermal method and combustion method,respectively.The phase,morphology,surface element valence state and material reduction ability of the prepared materials were analyzed and discussed by XRD,TEM,XPS and H2-TPR characterization methods.The results show that the nanomaterials prepared by hydrothermal method have good crystallinity and large grain size,and noble metal nanoparticles are uniformly dispersed on the surface of catalyst materials.The material prepared by combustion method has a small grain size and a loose and porous structure like cheese.Because of the strong reductivity of hydrazine hydrate in the combustion method,precious metals are mixed with CeO2 in the form of elemental particles.The precious metal elemental substance can effectively increase the absorbed oxygen content,promote the ionization of oxygen molecules,and produce more electrons on the surface of the catalyst.Moreover,the surface of the composites prepared by the two methods was successfully introduced with the low valence state noble metal ions,which cooperated with the high-priced Ce^x+ions to promote the increase of active oxygen content on the surface of the catalysts.Compared with pure phase CeO2,the catalytic performance of CeO2-based nanomaterials modified by precious metals(Pd,Ag,Au)has been significantly improved.The T_50%of pure phase CeO₂ prepared by hydrothermal method is 208?,and the catalytic rate of 1%Au-CeO2 for CO can reach more than 70%at room temperature.