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题名: 改性Pd/TiO2 催化剂氧化室内甲醛研究
作者: 李要彬
学位类别: 博士
答辩日期: 2016-05
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: 甲醛,Pd/TiO2,室温,碱金属,还原温度 ; formaldehyde, Pd/TiO2, room temperature, alkali metals, reduction temperature
其他题名: Catalytic Oxidaiton of Formaldehyde over modified Pd/TiO2 catalyst at room temperature
学位专业: 环境科学
中文摘要:       甲醛是室内空气的主要污染物之一,长期接触甲醛会严重影响人体健康,因此开发高效的净化甲醛技术或材料具有重要的环境和健康意义。催化氧化法是有效净化室内甲醛的技术之一。已知的甲醛氧化催化剂主要包括负载贵金属(Pt、Au、Pd、Rh 等)催化剂和金属氧化物催化剂(如MnO2、Co3O4、Ag2O等)。负载贵金属Pt 和Au 催化剂具有较好的低温活性,但成本较高,金属氧化物催化剂虽然成本低,但低温活性较差。相比之下,Pd 催化剂价格较Pt 或Au催化剂便宜且具有一定的低温活性。研究发现,碱金属添加可提高Pt 的分散并
促进了H2O 和O2 的活化形成表面羟基,从而使Pt/TiO2 催化剂氧化甲醛的活性得到显著地提高。另外,TiO2 是一种具有可还原性的氧化物,在还原的过程中可被部分还原形成表面氧空位,进而有利于H2O 的吸附与活化形成表面羟基。鉴于此,本论文旨在通过添加碱金属助剂或高温还原预处理提高Pd/TiO2 催化剂氧化甲醛的低温活性,以期开发一种低成本且具高效的低温活性的甲醛净化材料。论文考察了碱金属添加量、碱金属种类、还原温度等对Pd/TiO2 催化剂氧化甲醛的性能的影响,阐释了碱金属添加对Pd/TiO2 催化剂氧化甲醛性能的促进机制,明确了甲醛在低温或高温还原处理的Pd/TiO2 催化剂上的氧化反应途径。论文主要取得了以下研究成果:
      首先,用浸渍法制备了一系列Pd/TiO2 催化剂,考察了碱金属Na 的添加对Pd/TiO2 催化剂氧化甲醛的性能的影响。结果表明,碱金属Na 的添加显著提高了Pd/TiO2 催化剂氧化甲醛的性能,而最优添加量为2 wt.%。在室温、空速为95000 h-1 和相对湿度为35%的反应条件下,2Na-Pd/TiO2-R 催化剂可将140 ppm的甲醛完全催化分解为H2O 和CO2。多种表征结果发现,还原态的Pd 是Pd/TiO2催化剂氧化甲醛的活性中心;碱金属Na 的添加使Na 物种与Pd 物种之间形成强相互作用,这种相互作用提高了Pd 在载体上的分散,使Pd 周围的电子密度
增加,同时促进了催化剂对H2O 和O2 的吸附与活化并形成表面羟基,从而显著提高了Pd/TiO2 催化剂氧化甲醛的性能。然而,过量的Na(> 4 wt.%)会堵塞催化剂的孔道并覆盖部分活性位点,影响其催化氧化甲醛的性能。
      其次,对比了不同碱金属(Li、Na、K、Cs)对Pd/TiO2 催化剂氧化甲醛的影响,结果表明,碱金属添加对Pd/TiO2 催化剂氧化甲醛性能的促进作用具有普遍性,其促进顺序为K > Cs > Na > Li。通过表征发现,碱金属物种与Pd 物种之间普遍存在强相互作用,使Pd 分散度和表面羟基的量不同程度提高并且Pd 粒径也有不同程度的降低。Pd 分散度的大小以及表面羟基的量与其相应的催化剂活性顺序具有高度一致性。由此可知,Pd 的分散度和表面羟基的量是影响Pd/TiO2 催化剂氧化甲醛性能的两个至关重要的因素,而3.4K-Pd/TiO2-R 催化剂具有最高的Pd 分散度(活性位点最多)和表面羟基浓度,从而表现出最优的甲醛催化氧化活性。
      最后,对比了低温(300 oC)和高温(450 oC)还原对Pd/TiO2 催化剂氧化甲醛的性能的影响,发现高温还原的Pd/TiO2 催化剂氧化甲醛的活性显著提高,可在室温、空速为95000 h-1 和相对湿度为35%的条件下将140 ppm 甲醛完全降解。表征结果发现,高温还原的Pd/TiO2 催化剂载体TiO2 对Pd 颗粒形成包埋作用、氧化还原能力提高、Pd0 周围电子增加、表面存在大量羟基等因素使Pd/TiO2-450R 催化剂表现出优异的甲醛催化氧化活性。甲醛在两种催化剂上经历不同的反应途径: 在Pd/TiO2-300R 催化剂上,中间产物甲酸盐先分解成CO,最终被表面化学吸附氧氧化成CO2;而在Pd/TiO2-450R 催化剂上,中间产物甲酸盐直接被表面羟基氧化为H2O 和CO2。
英文摘要:       Formaldehyde (HCHO) is a major indoor air pollutant in airtight houses.Long-term exposure to indoor air containing even very low concentrations of HCHO may be detrimental to human health. Therefore, technologies or materials to improve indoor air quality and reduce public health risk with high efficiency and low cost are urgently needed. Catalytic oxidation, which can selectively decompose HCHO to harmLess CO2 and H2O at low temperature without any secondary pollution, is regarded as one of most effective methods for HCHO removal. The conventional catalysts for HCHO oxidation include supported noble metal (Pt, Au, Pd and Rh etc.) and metal oxide (MnO2, Co3O4 and Ag2O etc.) catalysts. Though the supported Pt and Au catalysts exhibit excellent activity for catalytic oxidation of HCHO at low and even room temperature, their high cost restricts their wide application. In
contrast, the metal oxide catalysts have low cost but poor performance at low temperature. Compared with supported Pt or Au catalysts, supported Pd catalysts have relatively low cost and good performance at low temperature. According to the previous works, alkali metal addition was able to improve the Pt dispersion and enhance the activation of H2O and O2 to form surface hydroxyl groups, leading to significant promotion of the performance of Pt/TiO2 catalysts for HCHO oxidation at room temperature. Therefore, in this work, it was anticipated that the performance of supported Pd catalysts for HCHO oxidation at room temperature could be enhanced through addition of alkali metals or high temperature pre-reduction, so that materials with low cost and high performance for HCHO catalytic oxidation at low temperature could be developed. In this work, the influence of the reduction
temperature and the amount and type of alkali metal added on the performance of Pd/TiO2 catalysts for HCHO oxidation were investigated. The mechanism of the promotion effect of alkali metals was elucidated, and the proposed reaction pathways of HCHO catalytic oxidation on Pd/TiO2 catalysts pre-reduced at low or high temperature were also clarified. The results are summarized as follows:
      First, a series of Pd/TiO2 catalysts were prepared by co-impregnation and the effect of Na addition on Pd/TiO2 catalysts for HCHO oxidation was investigated. The results showed that the performance of Pd/TiO2 catalysts for HCHO oxidation was obviously improved by Na addition, while the optimal Na addition amount was 2 wt.%. Nearly 100% HCHO conversion could be achieved over the 2Na−Pd/TiO2-R catalyst at a GHSV of 95000 h−1 with HCHO inlet concentration of 140 ppm at 25 °C. The characterization results showed that the active sites of the Pd/TiO2 catalyst for HCHO oxidation were metallic Pd species. A strong interaction between Na species and Pd species took place, which induced and further stabilized negatively charged and well-dispersed Pd species. These then facilitated the activation of H2O and chemisorbed oxygen to form surface hydroxyl groups, therefore resulting in excellent performance for the 2Na-Pd/TiO2-R catalyst in ambient HCHO destruction. However, excess Na addition ( > 4 wt.% ) possibly blocked small pores and covered a portion of the active Pd sites, leading to a decrease of their activity in HCHO catalytic oxidation.
      Second, a series of alkali metal (Li, Na, K, Cs) doped Pd/TiO2 catalysts were prepared and tested for ambient temperature HCHO oxidation. The results showed that the alkali metal dopants have a common promotion effect on the performance of the Pd/TiO2 catalyst in HCHO oxidation at ambient temperature, and the promotion effect followed the order K > Cs > Na >Li. The results of characterization showed that there was a common interaction between alkali species and Pd species, which led to an increase in the Pd dispersion and concentration of surface hydroxyl groups,and a decrease in Pd particle size. The Pd dispersion, concentration of surface hydroxyls and Pd particle size had a high correlation with the performance of the alkali metal doped Pd/TiO2 catalysts for HCHO oxidation. Accordingly, it is clear that both the Pd dispersion and the concentration of surface hydroxyl groups play important roles in HCHO catalytic oxidation on Pd/TiO2 catalysts. The K-Pd/TiO2-R catalyst in particular exhibited the highest Pd dispersion and concentration of surface hydroxyls, and thus the best activity for HCHO oxidation at ambient temperature.
      Finally, Pd/TiO2 catalysts pre-reduced at low temperature (300 oC) or high temperature (450 oC) were tested for HCHO oxidation. The Pd/TiO2-450R catalyst showed much better performance than the Pd/TiO2-300R catalyst. The Pd/TiO2-450R catalyst showed 100% HCHO conversion at a GHSV of 95000 h-1 with HCHO inlet concentration of 140 ppm at room temperature. The characterization results indicated that partial encapsulation of Pd particles by TiO2, good reducibility, electron-rich metallic Pd particles and abundant surface OH groups were probably responsible for the high activity of the Pd/TiO2-450R catalyst. Meanwhile, it was found that there were two different reaction pathways of HCHO catalytic oxidation on the two catalysts, that is, on the Pd/TiO2-300R catalyst, the intermediate formate first decomposed to surface CO followed by oxidation by surface O2 to form CO2, while the formate was directly oxidized by surface hydroxyl to form H2O and CO2 on the
Pd/TiO2-450R catalyst.
内容类型: 学位论文
URI标识: http://ir.rcees.ac.cn/handle/311016/36883
Appears in Collections:大气污染控制中心_学位论文

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Recommended Citation:
李要彬. 改性Pd/TiO2 催化剂氧化室内甲醛研究[D]. 北京. 中国科学院研究生院. 2016.
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