Synthesis Of Complex Oxides Derived From Co/Mn Hvdrotalcites Compounds And Their Catalytic Performance For VOCs Oxidation

The situation of environmental pollution is more and more serious,solving environmental problems is imminent.Volatile organic compounds(VOCs)are one of the main air pollutants,which are not only harmful to the environment but also hazardous to human health.Catalytic oxidation is regarded as one of the high-efficiency way for VOCs degradation and less harmful reaction by-products.The key issue is the availability of a low-cost and stabilized catalyst with high activity in catalytic oxidation technology.The mixed metal oxides(MMO)derived from hydrotalcite-like offer a number of advantages such as pore structure,high specific surface area,synergistic effect,acid-alkali double function and grain high dispersion,which exhibit a favorable potential application in environmental field and are the components of the main function of the catalytic reactor,especially monolithic membrane materials.In this thesis,The multi-mixed metal oxides catalysts obtained by calcination of layered double hydroxides containing Co/Mn to explore the different morphology,structure,chemical properties.The relationship between the structure and catalytic performances of the catalysts was revealed.Meanwhile,the monolithic film catalysts with the ab plane in a perpendicular orientation to the substrate,were prepared with the in-situ growth method on a pure metal substrate.The main contents and results are as follows:(1)Synthesis of Co-A1 composite oxides and their catalytic performances for total VOCs oxidationThe synthesis of Co-Al LDHs with different morphology nanostructures was constructed by simply controlling reaction solvents(water,methanol and ethanol)and ammonia release agents(ammonia,methenamine and urea),including frizzy platelets,imperfect hexagonal platelets and coral-like.The catalytic performances for total VOCs oxidation over all oxides were investigated.The results indicated that the coral-like Co-Al catalyst obtained under ehtanol and urea displayed the highest catalytic activity(T99 =230?)for benzene combustion.(2)Structure defects of composite oxides and their catalytic performances for total VOCs oxidationThe Co?Co? LDHs with mixed valence states were prepared by the oxidation of Co2+(3d7)to Co3+(3d6)under a dynamic oxygen atmosphere.The structure defects of the samples including surface acid sites,oxygen vacancy,low-temperature reducibility and surface elemental compositions,are characterized by numerous analytical techniques,such as Raman spectroscopy,H2-TPR,TPD and XPS.It is concluded that the catalyst prepared by in-situ oxidation method displayed the higher catalytic activity(T99(benzene)= 210?and T99(toluene)= 220?),for which possessed the most rich surface Co3+ inducing excellent low-temperature reducibility,surface Lewis acid sites and surface active oxygen species(Oads).(3)Synergistic effects of composite components and catalytic performances relationshipA series of ternary LDHs were fabricated with the method of the Co and Ce substitution in the brucite layer of MnAl LDHs,respectively,which were completely transformed to oxide phases after calcination in air.The results indicated that the metal species and ratio of LDHs had a influence on the crystal structure,specific surface area,low-temperature reducibility,surface elemental compositions of the composite oxides.The influence of calcined temperature on the structure of composite oxides catalysts were studied.Among the Co-Mn-Al oxides,when the mole ratio of Mn/Co is 2.0,the obtained sample exhibited the highest surface area(128.04m2 g-1),the narrowest average pore size distributions and the smallest grain size of crystallites,which displayed superior catalytic activity with T90 at 238?.Among CeMnAl samples,the replacement of Mn by Ce in the brucite layer of Mn-Al LDHs should cause the formation of a solid solution,which can obviously improve the catalytic activities due to the synergistic effects between CeO2 and MnOx,the abundant Mn4+/Mn3+ ratios and surface lattice oxygen(Oiatt).(4)The catalytic performances of the monolithic film catalysts for total VOCs oxidationBased on the results mentioned above,the CoMnAl LDHs film with the ab plane in a perpendicular orientation to the substrate,was prepared with the in-situ growth method on a pure A1 substrate,which was used as the precursors for fabricating the monolithic film catalysts.The catalytic performances of the monolithic film catalysts for total VOCs oxidation were investigated.It is concluded that the optimal catalyst of the monolithic CoMnAlO film catalysts exhibited the higher catalytic performance with T90 about 240?,which were 25? and 60? lower than those of the binary Mn-Al and Co-Al film,respectively.The structure of the monolithic catalysts was more complicared than that of powder samples,and the monolithic catalysts exhibited a higer reaction rate,because the Oads/Olatt molar ratio of monolithic catalyst(0.916)is higher than that of powder sample(0.55).The monolithic film catalyst exhibited high catalytic efficiency in total oxidation of VOCs,which provides great potential for their practical application.