Cu分隔开的Pd单原子用于乙炔选择加氢:Cu担载量的影响（英文）

发布时间：2018-07-01 18:38

  本文选题：铜 + 钯　； 参考：《催化学报》2017年09期

【摘要】：大量乙烯中少量乙炔的去除是化工生产中的重要过程之一,理想途径是将其选择加氢生成乙烯.负载型Pd催化剂因具有很高的乙炔转化率而被广泛用于该过程,但乙烯选择性很低,同时会使原料气中的乙烯被加氢,造成原料气的浪费.采用其它元素对Pd纳米粒子表面修饰,覆盖部分活性位,可以在一定程度上提高乙烯选择性,但是会大大降低Pd的利用率.因此,制备兼具高活性和高选择性且经济实用的催化剂,仍是这一过程亟待解决的主要问题之一.我们的前期工作中,将Pd与IB族金属(Au,Ag,Cu)分别结合制备得到了一系列含Pd的合金单原子催化剂(SAC),发现它们在大量乙烯存在条件下的乙炔选择加氢反应中表现出优异的催化性能.其中,Pd的用量仅为ppm级别,大大提高了Pd的利用率.作为IB族最为廉价的金属,Pd与Cu形成的合金SAC在提高Pd原子利用率的同时,能够进一步降低催化剂的经济成本.然而,当形成合金SAC时,Cu/Pd原子比例的极限值仍然不确定.本文通过固定Pd的担载量,采用简单的等体积共浸渍的方法,制备了一系列不同Cu/Pd原子比例的氧化硅负载的双金属催化剂.首先,我们采用程序升温还原(TPR)和X射线衍射(XRD)对催化剂的还原能力和双金属纳米粒子的尺寸进行了考察.进一步,采用X射线吸收光谱(XAS,包括EXAFS和XANES)对双金属催化剂中Pd的配位环境进行了分析.最后,结合它们在大量乙烯存在条件下的乙炔选择加氢反应中的催化性能,对形成合金SAC时Cu/Pd原子比例进行了讨论.TPR结果显示,Cu与Pd结合时会促进双金属纳米粒子的还原.XRD结果表明,随着Cu含量的降低,双金属纳米粒子的尺寸明显减小.XANES结果证实,当Pd与Cu结合时,Pd会带有部分负电荷,这也与Pd的电负性大于Cu相一致.通过对EXAFS拟合结果进行分析,我们发现当Cu/Pd的原子比例≥40/1时,Pd原子可以被Cu原子完全分隔开,形成含Pd的合金SAC,使其在大量乙烯存在条件下的乙炔选择加氢反应中表现出优异的催化性能.通过对还原温度的考察,我们发现还原温度由250 oC升高到400 oC时,对同一催化剂的催化性能影响不大;EXAFS拟合结果显示,对比分别经过250和400 oC还原后的催化剂,Pd的配位环境变化不明显,这可能是导致催化性能相似的主要原因.
[Abstract]:The removal of a small amount of acetylene from a large amount of ethylene is one of the important processes in chemical production. The ideal way is to produce ethylene by selective hydrogenation. The supported PD catalyst is widely used in this process because of its high conversion of acetylene, but the selectivity of ethylene is very low, and the ethylene in feedstock gas will be hydrogenated, resulting in waste of feedstock gas. Surface modification of PD nanoparticles with other elements covering some active sites can improve ethylene selectivity to a certain extent but greatly reduce PD utilization. Therefore, the preparation of high activity, high selectivity and economical and practical catalysts is still one of the main problems to be solved in this process. In our previous work, a series of alloy monoatomic catalysts (SAC) containing PD were prepared by combining PD with IB group metals (Au-Ag Cu). It was found that SAC exhibited excellent catalytic performance in the selective hydrogenation of acetylene in the presence of a large amount of ethylene. The dosage of Pd was only ppm level, which greatly improved the utilization rate of PD. The alloy SAC, which is the cheapest metal of IB group, formed by Cu and Pd, can further reduce the economic cost of catalyst while increasing the utilization ratio of PD atoms. However, the limit value of Cu / PD atomic ratio is still uncertain when SAC is formed. In this paper, a series of silicon oxide supported bimetallic catalysts with different Cu / PD atomic ratios were prepared by a simple equal volume co-impregnation method with fixed PD loading. Firstly, temperature programmed reduction (TPR) and X-ray diffraction (XRD) were used to investigate the reduction ability of the catalyst and the size of bimetallic nanoparticles. Furthermore, the coordination environment of PD in bimetallic catalysts was analyzed by X-ray absorption spectroscopy (XAS), including EXAFS and XANES. Finally, combined with their catalytic performance in the selective hydrogenation of acetylene in the presence of a large amount of ethylene, The ratio of Cu / PD atoms in the formation of alloy SAC was discussed. TPR results showed that Cu / PD could promote the reduction of bimetallic nanoparticles. XRD results showed that the size of bimetallic nanoparticles decreased obviously with the decrease of Cu content. When PD binds with Cu, Pd has partial negative charge, which is consistent with the fact that PD is more electronegativity than Cu. By analyzing the EXAFS fitting results, we find that when the atomic ratio of Cu / PD is 鈮，

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