自上向下的合成策略:最大程度提高贵金属的原子利用率（英文）

发布时间：2018-05-14 02:10

  本文选题：自上向下合成 + 单原子分散　； 参考：《催化学报》2017年09期

【摘要】：因为贵金属的价格比较高,并且很多催化反应主要发生在载体和金属接触的周围原子,所以减少贵金属的粒径对于提高金属原子利用率是非常可取的.原子利用率的最高极限就是形成单原子催化活性中心,然而合成稳定的单原子金属催化剂是一个巨大的挑战,因为单原子金属极易聚合成较大的金属颗粒.尽管存在着很大的困难,合成稳定的单原子金属还是可能的.研究表明,单原子金属容易镶嵌在表面能量最高的活性位上,以降低金属和载体的总能量,使之达到最稳定状态.随着金属的负载量增加,以此单原子金属为"晶种"将形成金属纳米粒子.根据这一原理,我们通过简单热扩散方法在HMO表面把Ag纳米粒子"拆分"成单个的Ag原子,并稳定地镶嵌在由HMO四个氧形成的空穴上(HMO的孔道口),使体系的能量降到最低.我们通过原位X射线衍射(XRD)、扩展X射线吸收精细结构光谱(EXAFS)和电子显微镜照片(TEM)详细证明了这种自上而下的合成过程,并通过X射线吸收近边结构光谱(XANES)、氢气程序升温还原(H_2-TPR)、CO吸附实验等表征手段和理论计算说明了诱导这一过程的原因.首先我们合成了具有高比表面积的Hollandite型二氧化锰(HMO)纳米颗粒,并且在上面负载纳米银颗粒.TEM数据表明经过焙烧纳米银颗粒消失,形成单原子分散在HMO表面.原位XRD的结果表明随着焙烧温度的升高,银颗粒的衍射峰强度逐渐降低,最后消失,说明纳米银颗粒随着温度的升高逐渐减少,最后达到银高分散的状态.通过对Ag(111)衍射峰强度进行分析,我们发现当温度低于150 ℃时,Ag(111)衍射峰强度基本保持不变,说明银颗粒没有变化.当温度高于150 ℃时,Ag(111)衍射峰强度开始减小,并且减小的程度随温度的升高而变大.当温度高于260 ℃时,Ag(111)衍射峰消失.为了更好的研究这个过程,我们分别在150,200,350 ℃焙烧银颗粒的样品,并测试了它们的EXAFS谱.结果表明随着焙烧温度的升高,银和银之间配位数减小,意味着银颗粒的减小.350 ℃焙烧样品的EXAFS谱在银原子散射的0.28 0.30 nm范围内没有吸收峰,说明银原子在HMO表面高度分散.然后我们通过XANES谱和理论计算证明了银和载体表面晶格氧的相互作用导致银的前线轨道的电子重新发生排布,从而诱导了整个自上向下的合成过程.最后活性测试表明,单原子银催化剂在甲醛催化氧化中表现出最好的催化活性,并简单研究了单原子催化氧化甲醛的机理.因此这种合成策略有两个重要的作用:(1)增加催化活性位的数量;(2)单原子催化剂的合成有利于催化反应机理的研究,比如甲醛催化氧化.
[Abstract]:Because the price of precious metals is relatively high and many catalytic reactions occur mainly in the atoms around the carrier and metal contact it is very desirable to reduce the particle size of precious metals in order to improve the utilization ratio of metal atoms. The highest limit of atom utilization rate is the formation of monoatomic catalytic active centers. However, the synthesis of stable monatomic metal catalysts is a great challenge, because monatomic metals are easily polymerized into large metal particles. In spite of the great difficulties, it is possible to synthesize stable monatomic metals. The results show that monatomic metals are easily embedded in the active sites with the highest surface energy in order to reduce the total energy of the metal and the carrier to the most stable state. With the increase of metal load, the monatomic metal will form metal nanoparticles. According to this principle, the Ag nanoparticles are "split" into a single Ag atom on the surface of HMO by a simple thermal diffusion method, and are stably embedded in the holes formed by the HMO four oxygen, so that the energy of the system is reduced to the minimum. We have demonstrated in detail this top-down synthesis process by in situ X-ray diffraction (XRD), extended X-ray absorption fine structure spectroscopy (EXAFS) and electron microscopy (TEM). The mechanism of inducing this process was explained by means of X ray absorption near edge structure spectra and hydrogen temperature programmed reduction H2-TPRN CO adsorption experiments. Firstly, Hollandite type manganese dioxide nanoparticles with high specific surface area were synthesized and loaded with silver nanoparticles. Tem data showed that the calcined silver nanoparticles disappeared to form monoatomic dispersion on the surface of HMO. The results of in-situ XRD show that with the increase of calcination temperature, the diffraction peak intensity of silver particles decreases gradually, and finally disappears, indicating that the silver nanoparticles decrease gradually with the increase of temperature, and finally reach the state of high silver dispersion. Through the analysis of the diffraction peak intensity of AgLi 111), we find that the diffraction peak intensity of Ag-111) remains basically unchanged when the temperature is lower than 150 鈩，

本文编号：1885844