Structure Controlled Preparation And Catalytic Activity Of The New Metal-based Catalysts Hdc And Co Oxidation Studies

With the rapid society development and population growth, both the sourcedeficiency and environmental pollution have intimidated ecological equilibrium,human health and the sustainable development of society and economy. Chemicalindustry have played extremely important roles in creating the material civilization.However, at the same time, it also brings serious pollution, including air, water andsoil pollutions, among them, the air pollution has caused much more attention inenvironment cleaning since it is flexible and difficult for removing. Meanwhile, theair pollution is also an important source for inducing water and soil pollution. Themajor air pollutants include trace organic substances (POPs), inorganic oxides andheavy meatls. Aryl chlorides and carbon monoxide represent typical pollutants in air.On one hand, there are many technologies for removing aryl chlorides from air,among them, the catalytic hydrodechlorination (HDC) has received increasing interestowing to the advantages including rapid, safty and efficient pathway as well as theresource recovery from pollutants. On the other hand, catalytic oxidation of CO iscurrently a principal method for removing CO pollutant from air owing to the benefitsof easy operation, low cost and the absence of secondary pollution. Obviously, allthese reactions will proceed in the presence of metal catalysts. Accordingly, the metalcatalysts played key roles in those reactions and the enhancement of the efficiencyand lifetime of metal catalysts have become a hot topic in the current catalysis.With the development of nano science and technology, the nano effect in metalcatalysts including high surface area, quantum size effect, and volume effect etc. hasbecome more and more important. However, the tiny nanoparticles usually diplayhigh surface energy, which may induce particle agglomeration, leading to the catalystdeactivation. Meanwhile, the active phase is easily leached off during catalyticreactions, corresponding to the poor recyclability of catalysts. One of promising waysto emprove the photocatalytic performance and lifetime is the design of supportedmetal catalysts with the strong interaction between metal particles and supports.However, the supported metal catalysts obtained by traditional methods usually display large particle size and poor distribution of active phases and the weakinteraction between metals and supports. In this thesis, starting from the synthesis ofnanocatalysts, a series of efficiency and stable supported catalysts for HDC ofchlorobenzene and CO oxidation were designed by adjusting the active phases andsupports, optimizing the technology for anchoring active phases onto supports, andengineering the morphology, structure and surface chemistry, together with theexamination of the catalytic performances. Based on the detailed characterizationsand kinetic studies, the catalytic reaction mechanism and the nature of active centerstogether with the structural and electronic effects were tried to elucidate. The mainresearching work could be summarized into the following5parts.1. A novel Ni/TiO₂catalyst with Ni nanoparticles embedded into TiO₂mesoporous walls was synthesized through co-assembly between metallic ions andsurfactants, followed by reduction activation. In comparison with the Ni/TiO₂catalystobtained by tradition impregnation-reduction method, the as-prepared Ni/TiO₂catalyst exhibited enhanced efficiency and stability in gas-phase HDC ofchlorobenzene, which could be mainly attributed to the high dispersion degree of Niactive sites and the strong metal-support interaction. Moreover, the Ni nanoparticlesembedded in the TiO₂mesoporous walls also exhibited strong resistance abilityagainst HCl corrosion and Ni particle agglomeration.2. A series supported Pd catalysts including Pd/SiO₂, Pd/Al₂O₃, Pd/CeO₂andPd/TiO₂were designed by impregnation method and employed in catalytic oxidationof CO. The effects of reducibility and oxygen storing capacity (OSC) of varioussupports (SiO₂, γ-Al₂O₃, CeO₂and TiO₂) on the catalytic performance wereinvestigataed. Experimental results demonstrated that both the high reducibility andthe enhanced OSC promoted the catalytic activity.3. Silica encapsulated Pd nanoparticles stabilized with oleylamine weresynthesized by water-in-oil microemulsion technique, followed by removal of theoleylamine molecules by calcination, leading to the core-shell structured Pd@SiO₂catalyst. During CO oxidation, this catalyst exhibited much higher activity andstronger durability than the Pd catalyst deposited onto commercially available SiO₂, possibly owing to the high dispersion of Pd nanoparticles and protecting effect of theSiO₂outer shell on the Pd nanoparticles against agglomeration duringhigh-temperature calcinations and CO oxidation reaction.4. A novel Pd@SiO₂-CeO₂catalyst in core-shell morphology was prepared bydoping core-shell structured Pd@SiO₂with CeO₂by incipient wetness impregnationmethod. In comparison with the Pd@SiO₂, the as-prepared catalyst displayed muchlower igniting temperature and higher activity in CO oxidation. The main reason wasthat the CeO₂-modification could enhance the OSC and also increase the oxygenvacancies on the Pd–Ce interface, which might promote the oxygen adsorption on thecatalyst and thus, supply more active oxygen species for CO catalytic oxidation,corresponding to great enhancement in catalytic activity.5. A new Pd-Ag/CeO₂bimetallic catalyst was prepared by consectiveimpregnation-calcination methods. The effects of calcination temperature, metalloadings, and Pd/Ag ratio on the catalytic performance were explored in CO oxidation,based on which the catalyst was further optimized. In comparison with either thePd/CeO₂or the Ag/CeO₂catalyst, the as-prepared Pd-Ag/CeO₂bimetallic catalystdisplayed enhanced activity, obviously owing to the Ag-Pd synergetic effect. DuringCO oxidation, Ag and Pd could adsorb and activate O₂and CO, respectively. Theirmutual promotion resulted in the enhanced activity.