Preparation Of Low-loaded Pd Catalysts And A Study Of Their Performance In The Complete Oxidation Of VOCs

Volatile organic compounds?VOCs?are one of the major components of air pollutants,and they are important precursors of PM2.5,ozone,and photochemical smog,causing serious harm to human health and environment.Catalytic oxidation technology is the most economical and effective for VOCs abatement.Noble metal catalysts have the advantages of low light-off temperature and high purification efficiency.Among them,Pd-based catalyst is considered to be one of the most effective catalysts for catalytic oxidation of VOCs due to the superior catalytic activity,high resistance to thermal and hydrothermal sintering.Decreasing the load of Pd-based catalysts can effectively reduce the cost,but at the same time,how to maximize the catalytic activitiy of Pd-based catalysts for VOCs oxidation is an urgent problem to be solved.In this dissertation,the effects of the preparation method of low-load Pd/?-Al₂O₃ catalysts and the preparation conditions of Pd nanoparticles in the self-assembled Pd/?-Al₂O₃ catalyst on the catalytic activity of toluene oxidation was studied,and the Pd/CeO₂/?-Al₂O₃ catalyst obtained from Ce modified self-assembled Pd/?-Al₂O₃ catalyst was tested in catalytic oxidation of VOCs.What's more,the influences of the pore structure parameters of catalysts,size,dispersion and oxidation state of Pd nanoparticles,and the interaction between the active species on the catalytic activity were investigated.In chapter 3,a series of 0.03 wt%Pd/?-Al₂O₃ catalysts were respectively prepared by impregnation,deposition-precipition and self-assembly methods.The self-assembled Pd/?-Al₂O₃ catalyst possessed the largest specific surface area and pore volume,the smallest average diameter of Pd nanopaticles.The main active PdO species were well-dispersed on the surface of the?-Al₂O₃ support,and there existed strong metal-support interaction between PdO and?-Al₂O₃.The temperature for conversion of 98%toluene(T₉₈)over self-assembled Pd/?-Al₂O₃ catalyst was 220?under the conditions of toluene concentration at 1000 ppm and space velocity at18000 mL·g^–1·h^–1,which was respectively 40 and 75?lower than those of Pd/?-Al₂O₃ catalysts prepared by deposition-precipition and impregnation methods.Besides,the preparation conditions of Pd nanoparticles on the self-assembled Pd/?-Al₂O₃ catalyst were studied.The increase of concentration of Pd nanopaticles in a certain range led to an increase in size of Pd nanoparticles and even caused agglomeration of Pd nanoparticles.The increase of preparation pressure was favor of the decomposition of the organometallic precursor,and could inhibited agglomeration of Pd nanoparticles to some extent.Appropriately raising preparation temperature was beneficial to the increase of dispersion of Pd nanoparticles,while,Pd nanoparticles were prone to agglomeration and growth when the temperature was too high.The increase of preparation time also facilitated the decomposition of the organometallic precursor and formation of uniform Pd nanoparticles.The better preparation conditions of Pd nanoparticles were as follows:concentration of 1.0 g·L^-1 Pd,preparation pressure of 4 MPa,preparation temperature of 40?,preparation time of3 h.Under these conditions,the self-assembled Pd/?-Al₂O₃ catalyst with low loading exhibited good activity for toluene oxidation.In chapter 4,to further improve the catalytic activity of the self-assembled Pd/?-Al₂O₃ catalyst with low loading,the metal oxides were added to modify it,and the Pd/CeO₂/?-Al₂O₃ catalyst selected had excellent activity.Compared with the self-assembled Pd/?-Al₂O₃ catalyst,the Pd/CeO₂/?-Al₂O₃ catalyst still maintained relatively high specific surface area,small average diameter and high dispersion of Pd nanoparticles.On the other hand,the presence of Ce³⁺in the Pd/CeO₂/?-Al₂O₃catalyst promoted the formation of surface oxygen species,and the relative content of PdO was higher,which was an important factor in the oxidation of VOCs with higher activity.Besides,the addition of Ce significantly decreased the reduced temperature of PdO,indicating that there was a strong interaction between non-stoichiometric cerium oxides and PdO,and the Pd/CeO₂/?-Al₂O₃ catalyst had excellent reduction ability.The T₉₈ of toluene,acetone and ethyl acetate over the Pd/CeO₂/?-Al₂O₃catalyst was 205,220 and 275?under the conditions of VOCs concentration at 1000ppm and space velocity at 18000 mL·g^-1·h^–1,which was correspondingly 15,15 and20?lower than those over the self-assembled Pd/?-Al₂O₃ catalyst.It possessed superior catalytic activity and stability,and it would be potentially promising for the practical application in VOCs oxidation.