The Synthesis And Catalytic Applications Of Modified Metal Oxides

Metal oxides play key role in catalysis. On one hand, metal oxides are well-known as catalysts in some important industrial reations, such as ammonia synthesis and Fischer-Tropsch synthesis. On the other hand, due to their high surface areas, mechanical strength, good stability, recyclability and recoverability, metal oxides are widely used as supports. With the development of catalytic theory, the importance of metal oxides in catalysis is increasingly understood. Researchers proposed that the catalytic performances of metal oxides are closely related to the particle size, morphology, crystalline phase, exposed facet, acid-base property, electronic configuration as well as the supported active components. The development of advanced metal oxide catalysts motivate people to exploit novel synthetic strategies and to further understand the relationship between structure and catalytic performance. In light of this, the topic of this thesis focuses on the synthesis and catalytic applications of modified (i.e. metal oxides support metal nanoparticles/doping non-metallic oxides and surface modification) metal oxides. The main works in the thesis are as follows:(1) We reported a general synthesis of MNPs/MMOs composites via a one-step, versatile sol-gel process. The most distinguished feature of this strategy is that we can simultaneously control the composition and microstructure of metal oxide supports and supported metal nanoparticles. The anti-sintering and anti-agglomeration properties of the supported metal nanoparticles can be greatly improved by the confinement effect of mesoporous supports. Taking VOCs combustion as a probe reaction, we further investigate the structure-property relationship of MNPs/MMOs with Pt/m-TiO2 showing the best catalytic performance. Interestingly, the catalytic performance could be further optimized by tuning the acid-base properties of MMOs.(2) We successfully synthesized bimetallic nanoparticles supported metal oxides (BMNPs/MMOs) via an assembly process and a high temperature calcination process. This approach allows a facile control on compositional/structural parameters of both BMNPs and MMOs. In addition, the catalytic performances can be tuned by the BMNPs composition regulation. The alloying process could be easily controlled by adjusting the BMNPs loading amounts, which providing good platforms to investigate the structure-property relationship. Pt1Pd3/m-SiO2 is proven to be the best catalyst for nitrobenzene hydrogenation to aniline.(3) Mesoporous VOx-CeO2 composite oxides were prepared via a sol-gel process and were applied in low temperature gas-phase selective oxidation of benzyl alcohol. By adjusting the amount of the V precursor, the VOx with different polymeric states on CeO2 were obtained. VOX with different polymeric states show different catalytic activity and stability. The stable monomers can promote the activity while the unstable trimers decrease it as the temperature increases. The DFT calculation results prove that the specific monomeric VO3 species is electron-deficiency, just like a "hole" in photocatalysis. This unique "hole" structure lowers the barrier in C-H activation (rate-determining step), facilitates the capture of hydrogen and thus accelerates the reaction.(4) The deposition of non-metallic oxides (SiO2/GeO2) on the surface of Co3O4 can greatly improve the selectivity of Co3O4 in ethane dehydrogenation to ethylene. Experimental observations and DFT calculations both suggest that the SiO2/GeO2 deposition exerts an important role in Co3O4 redox properties. The presence of non-metallic oxides can decrease the dissociation of molecular O2 to atomic oxygen and suppress the oxidative ability of surface lattice oxygen on Co3O4.(5) We showed that Co3O4 calcined at different temperatures have different surface structures and therefore exhibit different selectivities in oxidative dehydrogenation of ethane to ethylene. Temperature programming reduction experiements suggest that the chemical environment of Co3O4 surface oxygen is varied by calcination temperatures. Moreover, the isotopic O18 exchange experiments show that the high calcination temperature of 800℃can decrease the ability of Co3O4 in dissociation of molecular O2 to atomic oxygen. The high temperature calcinations suppress the activity of surface lattice oxygen on Co3O4 surface, and thus improve the selectivity in ethane oxidative dehydrogenation to ethylene.