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高效负载型Pd催化剂的制备及其在CO低温氧化反应中的机理探究是近年来的研究热点.普遍认为,Pd催化剂上的CO氧化反应遵循Langmuir-Hinshelwood机理:首先,CO吸附于Pd物种表面;然后,CO与催化剂表面的晶格氧发生反应转化为CO_2,反应发生在金属-载体界面.另外,高分散的Pd活性物种有利于CO氧化反应.同时载体的形貌、暴露的晶面、氧空位以及孔结构等都是影响催化剂活性的重要因素.CeO_2纳米管具有独特的管状特征和较高的比表面积,是一种潜在的CO低温氧化催化剂载体.本文利用乙醇还原法,以CeO_2纳米管为载体,制备不同Pd含量的Pd/CeO_2-nanotube纳米催化剂,并利用N_2吸附脱附、X射线衍射(XRD)、透射电子显微镜(TEM)、CO程序升温脱附(CO-TPD)、X射线光电子能谱(XPS)等表征手段,探索纳米催化剂载体形貌对CO氧化反应活性的影响.氮气吸脱附结果表明,Pd/CeO_2-nanotube具有较高的比表面积(58.0 m~2/g),且存在介孔结构.XRD表征发现,Pd/CeO_2-nanotube的衍射峰对应立方萤石型结构的CeO_2的(111),(200),(220),(311)等晶面.TEM结果表明,Pd/CeO_2-nanotube具有均匀的纳米管形貌,其外径为40-60 nm,Pd纳米颗粒均匀分散在其表面.CO-TPD结果表明,Pd/CeO_2-nanotube在110℃附近具有很强的脱附峰,在370℃和600℃附近分别具有较宽和较弱的脱附峰,这表明该催化剂具有较多的吸附位,且具有很强的CO吸附能力;CO不可逆吸附量计算结果表明,该催化剂上的Pd具有很高的表面分散度(23.3%),Pd颗粒尺寸为7.3 nm.XPS表征显示,Pd以Pd~(2+)的形式分散于CeO_2纳米管的表面,且与载体发生相互作用,存在Pd-O-Ce键;同时该催化剂表面存在丰富的Ce~(3+),为反应提供更多的氧空位.0.9Pd/CeO_2-nanotube纳米催化剂在CO氧化反应中表现出优良的活性,能在100℃实现CO的完全转化;通过计算发现,该催化剂具有较高的TOF值(0.63s~(-1)),由Arrhenius曲线可得到该催化剂的活化能为26.5 kJ/mol.综上可见:金属活性组分的尺寸和分散度、载体的结构特征、CO吸附能力以及金属-载体间的相互作用决定催化剂的性能Pd/CeO_2-nanotube的高比表面积有利于Pd的分散;其强CO吸附能力有利于CO吸附于Pd物种表面;催化剂表面丰富的Ce~(3+)能为反应提供更多的氧空位,Pd-O-Ce键的形成能增强金属-载体间的相互作用,有利于CO与催化剂表面晶格氧发生反应.同时催化剂介孔结构有利于反应气体和产物气体的吸附和扩散,因此,Pd/CeO_2-nanotube纳米催化剂在CO氧化反应中表现出优良的活性.
It is generally believed that the CO oxidation reaction on Pd catalyst follows the Langmuir-Hinshelwood mechanism: first, CO is adsorbed on the surface of Pd species; Then, CO reacts with lattice oxygen on the surface of the catalyst to CO 2, and the reaction takes place at the metal-support interface. In addition, highly dispersed Pd active species is favorable for the CO oxidation reaction. Meanwhile, the morphology of the support, Vacancy and pore structure are all important factors affecting the activity of the catalyst.CeO_2 nanotube has unique tubular characteristics and high specific surface area and is a potential carrier for CO cryogenic oxidation catalyst.In this paper, As catalyst, Pd / CeO 2 -nanotube catalysts with different Pd contents were prepared and characterized by N 2 adsorption and desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), CO-TPD, Photoelectron spectroscopy (XPS) and other characterization methods to explore the nano-catalyst carrier morphology on the CO oxidation reaction activity.The results of nitrogen adsorption and desorption show that the Pd / CeO_2-nanotube has a higher specific surface (58.0 m 2 / g), and the existence of mesoporous structure.XRD results showed that the diffraction peaks of Pd / CeO 2 -nanube correspond to the (111), (200), (220), 311) .TEM results show that the Pd / CeO_2-nanotube has a uniform nanotube morphology with an outer diameter of 40-60 nm and Pd nanoparticles dispersed uniformly on the surface. The results of CO-TPD show that Pd / CeO_2 -nanotube has a strong desorption peak near 110 ° C and a broad and weak desorption peak near 370 ° C and 600 ° C, respectively, indicating that the catalyst has more adsorption sites and has a strong CO The results of XPS showed that Pd had a high surface area distribution (23.3%) and Pd particle size was 7.3 nm.XPS characterization showed that Pd was dispersed in the form of Pd ~ (2+) On the surface of CeO 2 nanotubes, Pd-O-Ce bond was found to exist on the surface of CeO 2 nanotubes, and abundant Ce 3+ existed on the surface of CeO 2 nanotubes, which provided more oxygen vacancies for the reaction.9.9Pd / CeO_2- The nanotube nanocatalyst showed excellent activity in the CO oxidation reaction and achieved complete conversion of CO at 100 ℃. The calculated TOF value (0.63s -1) Arrhenius curve obtained activation energy of the catalyst is 26.5 kJ / mol. In summary, the size and dispersion of the metal active component, the structure of the carrier, CO adsorption capacity and metal-carrier interaction determines the performance of the catalyst Pd / CeO_2-nanotube is favorable for the dispersion of Pd. The strong CO adsorption capacity favors the adsorption of CO to the surface of Pd species. The abundant Ce 3+ on the catalyst surface can provide more oxygen vacancies for the reaction and the Pd- The formation of O-Ce bonds can enhance the interaction between metal and support, which is favorable for the reaction between CO and the lattice oxygen on the catalyst surface. Meanwhile, the mesoporous structure of catalyst facilitates the adsorption and diffusion of reactant gases and product gases. Therefore, Pd / CeO2 Nanotube nanocatalysts show excellent activity in the CO oxidation reaction.