本文選題：自上向下合成 + 單原子分散　； 參考：《催化學報》2017年09期

【摘要】：因為貴金屬的價格比較高,并且很多催化反應主要發(fā)生在載體和金屬接觸的周圍原子,所以減少貴金屬的粒徑對于提高金屬原子利用率是非�？扇〉�.原子利用率的最高極限就是形成單原子催化活性中心,然而合成穩(wěn)定的單原子金屬催化劑是一個巨大的挑戰(zhàn),因為單原子金屬極易聚合成較大的金屬顆粒.盡管存在著很大的困難,合成穩(wěn)定的單原子金屬還是可能的.研究表明,單原子金屬容易鑲嵌在表面能量最高的活性位上,以降低金屬和載體的總能量,使之達到最穩(wěn)定狀態(tài).隨著金屬的負載量增加,以此單原子金屬為"晶種"將形成金屬納米粒子.根據(jù)這一原理,我們通過簡單熱擴散方法在HMO表面把Ag納米粒子"拆分"成單個的Ag原子,并穩(wěn)定地鑲嵌在由HMO四個氧形成的空穴上(HMO的孔道口),使體系的能量降到最低.我們通過原位X射線衍射(XRD)、擴展X射線吸收精細結構光譜(EXAFS)和電子顯微鏡照片(TEM)詳細證明了這種自上而下的合成過程,并通過X射線吸收近邊結構光譜(XANES)、氫氣程序升溫還原(H_2-TPR)、CO吸附實驗等表征手段和理論計算說明了誘導這一過程的原因.首先我們合成了具有高比表面積的Hollandite型二氧化錳(HMO)納米顆粒,并且在上面負載納米銀顆粒.TEM數(shù)據(jù)表明經(jīng)過焙燒納米銀顆粒消失,形成單原子分散在HMO表面.原位XRD的結果表明隨著焙燒溫度的升高,銀顆粒的衍射峰強度逐漸降低,最后消失,說明納米銀顆粒隨著溫度的升高逐漸減少,最后達到銀高分散的狀態(tài).通過對Ag(111)衍射峰強度進行分析,我們發(fā)現(xiàn)當溫度低于150 ℃時,Ag(111)衍射峰強度基本保持不變,說明銀顆粒沒有變化.當溫度高于150 ℃時,Ag(111)衍射峰強度開始減小,并且減小的程度隨溫度的升高而變大.當溫度高于260 ℃時,Ag(111)衍射峰消失.為了更好的研究這個過程,我們分別在150,200,350 ℃焙燒銀顆粒的樣品,并測試了它們的EXAFS譜.結果表明隨著焙燒溫度的升高,銀和銀之間配位數(shù)減小,意味著銀顆粒的減小.350 ℃焙燒樣品的EXAFS譜在銀原子散射的0.28 0.30 nm范圍內(nèi)沒有吸收峰,說明銀原子在HMO表面高度分散.然后我們通過XANES譜和理論計算證明了銀和載體表面晶格氧的相互作用導致銀的前線軌道的電子重新發(fā)生排布,從而誘導了整個自上向下的合成過程.最后活性測試表明,單原子銀催化劑在甲醛催化氧化中表現(xiàn)出最好的催化活性,并簡單研究了單原子催化氧化甲醛的機理.因此這種合成策略有兩個重要的作用:(1)增加催化活性位的數(shù)量;(2)單原子催化劑的合成有利于催化反應機理的研究,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，

本文編號：1885844