Pt/BaO/CeO2 催化剂的NOx 储存还原性能及载体改性研究
文献类型：学位论文
| 作者 | 张燕 |
| 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 | 柴油车，NOx 储存还原，二氧化铈，铈锆氧化物，氧空穴 diesel engine, NOx storage reduction, CeO2, CeZrOx, oxygen vacancies |
| 其他题名 | Study on the Pt/BaO/CeO2 and support modification for NOx storage reduction |
| 学位专业 | 环境科学 |
| 中文摘要 | 柴油车污染物排放是造成大气灰霾、光化学烟雾等的重要原因。从源头上治理和削减柴油车NOx 排放对改善大气污染现状有着极其重要的意义。NOx 储存还原（NSR）技术是一种具有实用前景的稀燃发动机排气NOx 催化净化技术。传统的Pt/BaO/Al2O3 体系最佳活性温度窗口较窄，难以满足发动机排气温度范围内（100-500oC）NOx 的高效净化。因此，开发高效的NSR 催化剂对于削减柴油车尾气NOx 的排放有着非常重要的意义。 本论文选取Pt 为主催化活性组分、BaO 为NOx 储存组分，以CeO2、CeZrOx为载体，合成了Pt/BaO/CeO2 催化剂和Pt/BaO/CeZrOx 催化剂。通过X 射线衍射（XRD）、N2 物理吸附（BET）、透射电子显微镜（TEM）、X 射线光电子能谱（XPS）等技术表征催化剂的结构、各功能组分的存在状态及其分散；通过程序升温脱附（TPD）、程序升温还原（TPR）以及原位漫反射傅里叶变换红外光谱（in situ DRIFTS）深入研究了NSR 催化剂在稀燃条件下NO 的氧化、NOx的储存、以及硝酸盐的稳定性；浓燃条件下硝酸盐的分解、NOx 的还原；考察 了反应温度、稀燃-浓燃时间等对NSR 反应过程的影响。 本文首先考察了水热合成制备CeO2 纳米棒中沉淀剂NaOH 的添加浓度对NSR 催化剂催化活性的影响。当NaOH 浓度为7.5M 时， 制备的Pt/BaO/CeO2-7.5M 催化剂具有最高的NOx 储存量（NSC），尤其是在350 oC 时高达913.8 μmol/g，接近其理论储存量（937.4 μmol/g）；在200-400oC 的温度范围内，NOx 转化率基本接近100%。H2-TPR、XPS 等表征手段结果表明，该催化剂具有较强的氧化还原能力和较为丰富的表面氧空位，对NOx 具有较强的吸附存储能力，因而呈现出优异的NSR 反应活性。在稀燃时间90s、浓燃时间低至3s 时，该催化剂仍能维持较高NOx净化效率，有望保证柴油机的燃油经济性。 制备了以不同形貌CeO2 纳米材料为载体的NSR 催化剂，其催化性能存在明显的形貌效应：Pt/BaO/CeO2-NR（纳米棒） ＞ Pt/BaO/CeO2-NP（纳米颗粒）＞ Pt/BaO/CeO2-NC（纳米立方）。Pt/BaO/CeO2-NR 催化剂具有优异的储存性能、NOx 去除效率和较强的抗水、CO2 能力。HRTEM 表征结果显示，纳米棒样品优势暴露活性较高的（110）和（100）晶面，纳米颗粒和纳米立方样品分别暴露（100）+（111）和（100）晶面。相比于（100）和（111）晶面，氧空位更容易形成于（110）晶面。XPS 表征结果证明，Pt/BaO/CeO2-NR 催化剂表面氧空位含量高于Pt/BaO/CeO2-NP 和Pt/BaO/CeO2-NC 催化剂。关联XPS、H2-TPR及活性测试结果发现，三价铈浓度、催化剂表面活性氧含量均与催化剂的NOx储存量（NSC）线性相关，表明氧空穴在NO 存储过程中的关键作用。结合原位漫反射傅里叶变换红外光谱（in situ DRIFTS）结果，提出了氧空穴参与的铈基NSR 催化剂NO 储存机制：催化剂载体二氧化铈表面氧空位活化氧，表面活性氧物种促进了Pt 氧化NO 为NO2，氧空穴上的活性氧同时也促进硝酸盐的形成及随后向Ba 位的转移；这一转移过程释放了氧空位，保证了整个储存过程的持续进行。浓燃阶段，Pt/BaO/CeO2-NR 催化剂能有效利用H2 快速还原NOx，使得储存位点再生；同时，催化剂表面氧空穴上的活性氧促进NH3 氧化生成N2，减少了副产物NH3 的排放。 制备了系列Pt/BaO/CeZrOx 催化剂，较Pt/BaO/CeO2-NR 催化剂呈现出更为优异的低温活性。稀燃-浓燃动态循环实验显示，当Ce 与Zr 的摩尔比为1:9 时，催化剂具有最高的NOx 转化率，在150oC 时仍具有96.7%的NOx 去除率，大大拓宽了催化剂的活性温度窗口。 |
| 英文摘要 | The pollutants from diesel engine exhaust are major sources of haze and photochemical smog. Reducing the NOx emission from diesel engines is important for the improvement of air quality in the urban environment. NOx storage reduction (NSR) is one of the most promising techniques for the removal of NOx from lean burn engines. The optimum activity temperature window of the traditional Pt/BaO/Al2O3 system is narrow (300-350oC), so it is difficult to meet the requirements for NOx purification in the temperature range of engine exhaust gas (100-500oC). Thus, development of NSR catalysts with excellent performance is central to increase the NOx removal efficiency from diesel exhaust. In this study, several NSR catalysts were prepared, using BaO as the storage component supported on CeO2 and CeZrOx nanomaterials. Meanwhile, noble metal Pt was introduced to form the fully formulated NSR catalysts Pt/BaO/CeO2 and Pt/BaO/CeZrOx. To identify the state and dispersion of all the components in the catalysts, XRD, BET, TEM, and XPS experiments were performed. TPD, TPR and in situ DRIFTS were applied to measure the NO oxidation ability, nitrate stability and dynamic behavior of NOx storage reduction during lean-rich cycles. The effects of reaction temperature and lean-rich time were also tested during the NSR process. First, the influence of the support with different concentrations of precipitant on the NOx storage and reduction performance of Pt/BaO/CeO2 catalysts was investigated. The Pt/BaO/CeO2-7.5M catalyst exhibited the highest NOx storage capacity (NSC) of 913.8 μmol/g at 350oC, close to the theoretical storage capacity (937.4 μmol/g); the NOx conversion was close 100% in the temperature range of 200-400oC. H2-TPR and XPS results showed that the Pt/BaO/CeO2-7.5M catalyst had strong redox capacity and abundant oxygen vacancies, with strong NOx storage capacity, which resulted in excellent NSR activity. Under a long lean period (90s) and a short rich period (3s), the catalyst maintained high NOx removal efficiency, potentially ensuring diesel fuel economy. Pt/BaO/CeO2 catalysts derived from CeO2 nanomaterials with shapes of nanorods (NR), nanocubes (NC), and nanoparticles (NP) were investigated for NOx storage reduction. The catalytic activity ranked by CeO2 nanomaterials was as follows: Pt/BaO/CeO2-NR > Pt/BaO/CeO2-NP > Pt/BaO/CeO2-NC. The Pt/BaO/CeO2-NR catalyst possessed a superior NOx storage capacity (NSC), NOx removal efficiency and resistance to water and CO2. HRTEM images revealed that the CeO2 nanorodswere enclosed by (110) and (100) facets, while the nanoparticles and nanocubes mainly exposed (100)+(111) and (100), respectively. The formation of oxygen vacancies on the CeO2 (110) plane is much more favourable than on (100) and (111) planes. As corroborated by XPS results, it is reasonable that Pt/BaO/CeO2-NR catalyst exhibits a higher concentration of oxygen vacancies than the other two. Notably, a close linear correlation was drawn between the NOx storage capacity and the amount of oxygen vacancies of NSR catalysts. Over the NSR catalysts, oxygen vacancies play a crucial role in anchoring Pt. Meanwhile, H2-TPR results showed that the number of active surface oxygen species trapped in oxygen vacancies was closely related to the NSC value. This suggests that the oxygen vacancies on the NSR surface govern the NOx storage capacity by creating efficient sites or channels for the formation of nitrate. Under the rich period, H2 was sufficient to reduce all introduced NOx, regenerating the storage sites over Pt/BaO/CeO2-NR catalyst. The active surface oxygen species trapped in oxygen vacancies on Pt/BaO/CeO2-NR catalyst promoted the oxidation reaction of NH3 to N2, resulting in less NH3 emissions. A series of Pt/BaO/CeZrOx catalysts were prepared by doping Zr into the Pt/BaO/CeO2-NR catalyst, showing higher low-temperature activity than the undoped sample. The optimal Pt/BaO/Ce0.1Zr0.9O2 catalyst exhibited high NOx removal efficiency at low temperaures, giving 96.7% NOx conversion at the temperature of 150oC. |
| 源URL | [http://ir.rcees.ac.cn/handle/311016/37057]  |
| 专题 | 生态环境研究中心_大气污染控制中心 |
| 推荐引用方式 GB/T 7714 | 张燕. Pt/BaO/CeO2 催化剂的NOx 储存还原性能及载体改性研究[D]. 北京. 中国科学院研究生院. 2016. |
入库方式： OAI收割
来源：生态环境研究中心
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