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题名: Pd基催化剂上催化氧化邻-二甲苯研究
作者: 王亚飞
学位类别: 博士
答辩日期: 2014-05
授予单位: 中国科学院研究生院
授予地点: 北京
导师: 贺泓
关键词: 催化氧化,邻-二甲苯,钯基催化剂,有序介孔,铈锆复合氧化物,Catalytic oxidation, o-Xylene, Palladium catalysts, Ordered mesoporous, Zirconium-cerium mixed oxide
其他题名: Catalytic Oxidation of o-Xylene over Supported Pd Catalysts
学位专业: 环境科学
中文摘要:     挥发性有机物(VOCs)是大气污染物的重要成分,对大气环境和人体健康有严重危害,其中苯系物(苯、甲苯、二甲苯等)由于毒性大、难降解受到了广泛的关注。催化氧化是目前最具实用潜质的 VOCs控制技术,贵金属Pd由于具有较好的低温活性和选择性是最常用的催化剂活性组分之一。大量的研究关注了Pd物种在催化氧化VOCs中的作用,但何种价态的Pd是催化反应的活性中心一直存在争议,金属-载体间相互作用也尚不明确。
    本文中首先采用沉淀法和硬模板法分别制备了颗粒状 Co3O4(B)和三维有序介孔Co3O4(3D)载体,并采用后浸渍法制备了  Pd/Co3O4(BL)和Pd/Co3O4(3DL)催化剂,发现三维介孔结构的引入,促进了 PdO在载体表面的分散,提高了催化剂的性能,260˚C时可将邻-二甲苯完全转化.采用原位负载法制备了3D结构更为规整有序的Pd/Co3O4(3D)催化剂,发现其催化性能非常优异,200˚C时即可将邻-二甲苯完全转化。H2-TPR和XAFS表征结果表明,PdO在Pd/Co3O4(3D)催化剂上是以高度分散的状态存在。说明有序的3D结构对 Pd/Co3O4催化氧化性能有重要的促进作用。
    采用沉淀法和硬模板法分别制备了非介孔的 CeO2(B)和有序介孔的 CeO2(3D),发现 CeO2(3D)同样具有比 CeO2(B)催化剂更好的活性。但原位负载了 Pd后,Pd/CeO2(3D)的催化活性并未明显提高,H2-TPR和  XPS表征结果表明,Pd物种与 CeO2(3D)载体之间产生较强的相互作用,并且形成了不利于催化反应进行的含 Pd新物种。非介孔Pd/CeO2(B)的催化剂活性较  CeO2(B)有明显提高归因于载体表面高度分散的 Pd活性物种。
    采用浸渍法制备了一系列的 PdO/MOx(MOx=TiO2,Al2O3,SiO2,MnO2,CeO2,Co3O4)催化剂,并采用 H2和 NaBH4还原分别制备了载有还原态 Pd物种的Pd/MOx-H2和 Pd/MOx-NaBH4催化剂,发现170oC时,Pd/MOx-NaBH4催化剂对邻-二甲苯的去除率达到了 100%,而 PdO/MOx的去除率均低于 20%。Pd/MOx-H2催化剂中,MOx=TiO2,Al2O3,SiO2,CeO2时,其催化性能均较PdO/MOx有明显提高。而    Pd/MnO2-H2和Pd/Co3O4-H2催化剂,由于还原过程中氢溢流作用导致了Co3O4和MnO2载体的还原,出现了低价态的 Mn2O3、MnO、CoO和  Co单质等物种,破坏了催化剂的催化性能。因此,在还原预处理对载体没有产生影响的催化剂上,还原预处理制备的金属态Pd能够有效的提高 Pd基催化剂低温催化氧化邻二甲苯活性;NaBH4还原较为温和,是一种更适合制备高活性 Pd基催化剂的预处理方法。
    采用尿素均匀共沉淀法制备了不同 Zr添加量的Ce1-xZrxOy(x = 0.1, 0.3, 0.5)复合氧化物,并采用浸渍法和 NaBH4还原法制备了PdO/Ce1-xZrxO2和Pd/Ce1-xZrxO2-NaBH4催化剂。发现Pd/Ce1-xZrxO2-NaBH4的催化活性明显高于PdO/Ce1-xZrxO2催化剂,再次证明了还原预处理制备的金属态的  Pd是催化氧化反应的活性位点。Pd/Ce0.9Zr0.1O2-NaBH4(x= 0.1)催化剂具有最好的催化活性,150 oC时即将邻-二甲苯完全转化,归因于其较多的表面活性氧物种和 Pd的高度分散。

 
英文摘要:     BTX (Benzene, Toluene, and Xylene) form one of the major classes of VOCs in air on a large  scale. BTX  are of  particular concern due  to their  destruction of  environment and high toxic potential for human beings. The supported Pd catalysts have high activity for the catalytic oxidation of VOCs.  The catalytic oxidation of BTX  has also been widely studied over  supported   Pd  catalysts.  The   performance  of   Pd-based  catalysts  for   BTX  deep oxidation has  been reported  to be  dependent on many  factors. Although  the Pd  state has been investigated by several researchers, the effect of the state of  Pd on the catalytic VOCs still remains controversial. And the influence of the interaction between Pd and supports on catalytic oxidation was also not clear.
    In  this study,  ordered  mesoporous Co3O4 (3D)   and the  bulk  counterpart Co3O4 (B)were prepared by  a nanocasting route  and precipitation method, respectively.  Pd was next loaded  on   both  materials  by   an  impregnation  method (Namely  Pd/Co3O4(3DL) and Pd/Co3O4(BL)). In addition, Pd/Co3O4(3D) was prepared by an in situ nanocasting method. All  catalysts were  tested for  the  total oxidation  of  o-xylene. As  for Pd-loaded  samples, Pd/Co3O4(3D) had much higher activity than Pd/Co3O4(3DL)  and Pd/Co3O4(BL), with 100% conversion of  o-xylene by oxidation at  200 ˚C. The  characterization results show  that the Pd/Co3O4(3D) catalyst  synthesized by in situ  nanocasting presented much  more dispersed PdO species than  the Pd/Co3O4(3DL) and Pd/Co3O4(BL)  catalysts. The excellent catalytic performance of Pd/Co3O4(3D) is also attributed to its uniform mesostructure.
    Mesoporous CeO2(3D) and bulk CeO2(B)  supports were prepared by nanocasting and precipitation, respectively. The mesoporous CeO2(3D)  showed better activity than the bulk CeO2(B). Then Pd species was loaded  on the supports by two methods (in situ nanocasting and co-  precipitation).  Pd/CeO2(B) showed  much better  activity  than Pd/CeO2(3D).  The greater  quantity of  exposed  Pd species  on  bulk CeO2(B)  played  the major  role  in their better activity.  The loss of  active sites  led to worse  activity for  Pd/CeO2(3D), which was caused by the stronger interaction between Pd and CeO2(3D).
    We prepared  PdO/MOx catalysts with  PdO species by  an impregnation method.  The reduced Pd species on  different supports were prepared by two  methods (H2 reduction and NaBH4 reduction). The  performance of Pd supported  on a series of oxides  such as Co3O4,MnO2, CeO2, TiO2,  Al2O3 and SiO2 were  tested in the catalytic oxidation  of o-xylene. We found that reduced  Pd species supported on  these oxides serve as  an excellent catalyst for complete  oxidation of  o-xylene  in the  temperature  range 90-170   oC,  indicating that  the reduction  pretreatments are  effective  for  improvement of  catalytic  activity  of  Pd based catalysts at low  temperature. However, reduced Pd species  reduced by H2 on Pd/Co3O4-H2 and Pd/MnO2-H2 showed no better activity than  PdO species in the low temperature range,because the supports were partial reduced by  H2 spillover. The NaBH4 reduction method is more suitable for the preparation of the metallic Pd with high activity.
    CeZr mixed  metal oxides  were prepared by  coprecipitation. The  reduced Pd  species and PdO  species were  loaded on Ce-Zr  mixed oxides  by using NaBH4  reduction method and post-impregnation.  The reduced  Pd species  supported on these  oxides showed  better activity than  PdO, confirming that  the metallic  Pd species is  the major active  site for  the catalytic  oxidation of  o-xylene.  When  the ratio  of  Ce/Zr  is 9/1,  this  catalyst supported reduced  Pd  species  achieved  complete  oxidation of  o-xylene  at  150   oC.  This  may  be attributed to high Pd dispersion and more activated oxygen species.
内容类型: 学位论文
URI标识: http://ir.rcees.ac.cn/handle/311016/34440
Appears in Collections:大气污染控制中心_学位论文

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Recommended Citation:
王亚飞. Pd基催化剂上催化氧化邻-二甲苯研究[D]. 北京. 中国科学院研究生院. 2014.
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