Pt-Rh/CeZrYLa+LaAl催化剂用于理论空燃比天然气汽车尾气净化

采用共沉淀法制备了CeZrYLa+LaAl纳米复合载体,以三种方法制备了一系列Pt-Rh/CeZrYLa+LaAl催化剂.对所制样品进行了N2吸附-脱附、粉末X射线衍射、X射线光电子能谱和H2程序升温还原的表征.并考察了三种方法所制得催化剂的理论空燃比天然气汽车尾气净化性能.结果表明,三个催化剂的活性顺序为Cat3≈ Cat2> Cat1,其中Cat3具有最低的CO和NO起燃温度(T50),分别为114oC和149 oC,最低的CH4和CO完全转化温度(T90),分别为398 oC和179 oC,以及最佳的CH4和CO温度特性,ΔT (T90–T50)值分别为34 oC和65 oC. Cat2具有最低的CH4起燃温度(342°C)和最低的NO完全转化温度(174°C). Cat1具有最差的转化活性,说明物理混合法制备的催化剂(Cat3和Cat2)性能优于共浸渍法制备的催化剂(Cat1).这是由于物理混合法制备的催化剂, Pt和Rh均匀分散在载体表面,两者物理接触共同参与CH4/CO/NO三种污染物的转化.相反,共浸渍法制备的催化剂, Pt和Rh之间存在较强的相互作用,改变了Pt的电子状态,而且形成了表面Pt富集的Pt-Rh双金属颗粒覆盖了Rh活性位,从而降低催化活性；同时,对于通过物理混合法并进一步添加助剂所制备的Cat3, XRD结果显示助剂Zr4+进入了铈锆固溶体晶格,产生晶格缺陷； XPS结果显示Cat3具有最高的Ce3+/Ce比例.这些都有利于提高催化剂的氧流动性,从而提高催化剂活性并拓宽空燃比窗口.

Catalytic performance of Pt-Rh/CeZrYLa+LaAl with stoichiometric natural gas vehicles emissions

The composite support CeZrYLa+LaAl was prepared by a co‐precipitation method, and Pt–Rh bime‐tallic catalysts were fabricated on this support using different preparation procedures. The catalytic activities of these materials were tested in a gas mixture simulating the exhaust from a stoichio‐metric natural gas vehicle. The as‐prepared catalysts were also characterized by X‐ray photoelec‐tron spectroscopy, X‐ray diffraction, N2 adsorption‐desorption and H2‐temperature‐programmed reduction. It was found that the order of activities for CH4, CO and NO conversion was Cat3≈Cat2>Cat1, where Cat3 had the lowest light‐off temperature (T50) for CO (114 °C) and NO (149 °C), the lowest complete conversion temperature (T90) for CH4 (398 °C) and CO (179 °C), and the lowestΔT (T90–T50) for CH4 (34 °C) and CO (65 °C). Cat2 showed the lowest T50 for CH4 (342 °C), the lowest T90 for NO (174 °C), and the lowestΔT for NO (17 °C). Cat1 had the highest T50 and T90 and the largestΔT out of all three catalysts. Indicating that Pt–Rh bimetallic catalysts (Cat2 and Cat3) prepared by physically mixing Pt and Rh powders exhibited much better catalytic activity than those (Cat1) prepared by co‐impregnation, since homogeneous Pt and Rh sites made a significant contribution to CH4/CO/NO conversions. In contrast, strong Pt–Rh interactions in the co‐impregnation materials affected the oxidation states of Pt, and the Pt‐enriched surface blocked active Rh sites. Moreover, Cat3 was prepared by adding additives (La3+, Zr4+and Ba2+) into the physically mixed Pt–Rh cata‐lysts. XRD results demonstrated that the additive cation (Zr4+) was incorporated into the CeO2–ZrO2 lattice, thus creating a higher concentration of defects and improving the O2‐mobility. XPS results showed that the Cat3 had the highest Ce3+/Ce ratio, suggesting the presence of a significant quantity of oxygen vacancies and cerium in the Ce3+state. All of these further promoted the three‐way cata‐lytic activity and widened the air‐to‐fuel working‐window.