水和氨分子在FCC鈷表面吸附和解離的理論研究
發(fā)布時間：2018-08-14 08:58
【摘要】：能源問題一直是人類關(guān)注的重點問題,費托合成反應(yīng)是生產(chǎn)清潔能源的一種重要工業(yè)途徑。過渡金屬鈷催化劑由于具有活性高、水煤氣轉(zhuǎn)化率低和高烴產(chǎn)物選擇性高等優(yōu)點,而被作為工業(yè)上費托合成反應(yīng)中的一種常用催化劑�，F(xiàn)有研究表明,費托合成反應(yīng)中的副產(chǎn)物水以及反應(yīng)合成氣中的一些雜質(zhì)氣體能夠使鈷催化劑中毒失活,然而它們致催化劑鈷失活的原因仍待探究。本文基于密度泛函理論的第一性原理方法,通過對水和氨分子在鈷金屬表面上的吸附性質(zhì)與解離過程的計算模擬,闡述了它們致鈷催化劑表面失活的微觀機(jī)理。通過對計算結(jié)果進(jìn)行分析,在以下方面獲得了重要的研究成果:1)水分子傾向于吸附在鈷表面的頂位,且吸附較弱容易從表面脫吸附。在水分子的兩步脫氫反應(yīng)中,水分子的解離勢壘低于羥基的解離勢壘。表面上氧原子的存在能增強(qiáng)水分子與鈷表面的吸附強(qiáng)度,并且能夠促進(jìn)水分子在鈷表面上的第一步脫氫反應(yīng),在Co(110)表面上這種促進(jìn)作用最為明顯。但是氧原子的存在對羥基在鈷表面上的解離并沒有促進(jìn)作用。在鈷表面上,羥基的存在對水分子的解離有輕微的抑制作用。水分子解離產(chǎn)生的羥基可能是使鈷催化劑失活的原因。計算結(jié)果也表明水分子與鈷表面的作用具有結(jié)構(gòu)敏感性。2)對于NH_x(x=0-3),隨著氫原子數(shù)目的逐漸減少,其與鈷表面上的吸附強(qiáng)度逐漸增大。鈷表面上氧原子的存在不改變NH_x(x=0-3)的最穩(wěn)定吸附位置,但是對吸附能有一定的影響。在不同的鈷表面上,氧原子的存在對氨氣分子的脫氫反應(yīng)有不同的影響。在Co(110)表面上,氧原子的存在對氨氣分子的每一步脫氫反應(yīng)都有極大的促進(jìn)作用,由此推測此表面上也許將有大量的N原子覆蓋。與此相比,在氧覆蓋的Co(100)和Co(111)表面上,NH的解離勢壘較高,是氨分子在這兩個表面上完全脫氫的限速步驟。因此,在氧覆蓋的鈷表面上,氨分子脫氫反應(yīng)的中間產(chǎn)物NH和N很可能占據(jù)表面的活性位從而導(dǎo)致鈷化劑鈍化失活。
[Abstract]:Energy problem has always been the focus of human attention. Fischer synthesis reaction is an important industrial way to produce clean energy. Because of its high activity, low conversion of water gas and high selectivity of hydrocarbon products, cobalt transition metal catalyst has been used as a common catalyst for Fischer-Tropsch synthesis in industry. It has been shown that the by-product water and some impurity gases in the reaction syngas can deactivate the cobalt catalyst. However, the reasons for the deactivation of cobalt catalyst are still to be explored. Based on the first-principle method of density functional theory (DFT), the microscopic mechanism of the surface deactivation of cobalt catalysts caused by water and ammonia molecules on the surface of cobalt was simulated by the calculation of the adsorption properties and dissociation process of water and ammonia molecules on the surface of cobalt metal. Through the analysis of the calculated results, the following important research results are obtained: 1) the water molecules tend to adsorb on the top of cobalt surface, and the adsorption is weak and easily desorbed from the surface. In the two-step dehydrogenation reaction of water molecules, the dissociation barrier of water molecules is lower than that of hydroxyl groups. The presence of oxygen atoms on the surface can enhance the adsorption strength of water molecules on cobalt surface and promote the first step dehydrogenation of water molecules on cobalt surface, especially on Co (110) surface. However, the presence of oxygen atoms does not promote the dissociation of hydroxyl groups on cobalt surfaces. On the surface of cobalt, the presence of hydroxyl groups has a slight inhibitory effect on the dissociation of water molecules. The hydroxyl group produced by the dissociation of water molecules may be the reason for the deactivation of cobalt catalyst. The results also show that the interaction between water molecule and cobalt surface is structurally sensitive. 2) for NH_x (x0-3), with the decrease of hydrogen atom number, the adsorption intensity on cobalt surface increases gradually. The existence of oxygen atoms on cobalt surface does not change the most stable adsorption site of NH_x (x0-3), but it has a certain effect on the adsorption energy. On different cobalt surfaces, the presence of oxygen atoms has different effects on the dehydrogenation of ammonia molecules. On the surface of Co (110), the presence of oxygen atoms can greatly promote the dehydrogenation of ammonia molecules in every step, so it is assumed that there will be a large number of N atoms on the surface. In contrast, the dissociation barrier of NH on the surface of Co (100) and Co (111) covered by oxygen is higher, which is the limit step for the complete dehydrogenation of ammonia on these two surfaces. Therefore, on the surface of cobalt covered by oxygen, NH and N, the intermediate products of ammonia dehydrogenation, may occupy the active sites of the surface and lead to the passivation inactivation of cobalt.
【學(xué)位授予單位】：河南師范大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：O643.36
本文編號：2182338
[Abstract]:Energy problem has always been the focus of human attention. Fischer synthesis reaction is an important industrial way to produce clean energy. Because of its high activity, low conversion of water gas and high selectivity of hydrocarbon products, cobalt transition metal catalyst has been used as a common catalyst for Fischer-Tropsch synthesis in industry. It has been shown that the by-product water and some impurity gases in the reaction syngas can deactivate the cobalt catalyst. However, the reasons for the deactivation of cobalt catalyst are still to be explored. Based on the first-principle method of density functional theory (DFT), the microscopic mechanism of the surface deactivation of cobalt catalysts caused by water and ammonia molecules on the surface of cobalt was simulated by the calculation of the adsorption properties and dissociation process of water and ammonia molecules on the surface of cobalt metal. Through the analysis of the calculated results, the following important research results are obtained: 1) the water molecules tend to adsorb on the top of cobalt surface, and the adsorption is weak and easily desorbed from the surface. In the two-step dehydrogenation reaction of water molecules, the dissociation barrier of water molecules is lower than that of hydroxyl groups. The presence of oxygen atoms on the surface can enhance the adsorption strength of water molecules on cobalt surface and promote the first step dehydrogenation of water molecules on cobalt surface, especially on Co (110) surface. However, the presence of oxygen atoms does not promote the dissociation of hydroxyl groups on cobalt surfaces. On the surface of cobalt, the presence of hydroxyl groups has a slight inhibitory effect on the dissociation of water molecules. The hydroxyl group produced by the dissociation of water molecules may be the reason for the deactivation of cobalt catalyst. The results also show that the interaction between water molecule and cobalt surface is structurally sensitive. 2) for NH_x (x0-3), with the decrease of hydrogen atom number, the adsorption intensity on cobalt surface increases gradually. The existence of oxygen atoms on cobalt surface does not change the most stable adsorption site of NH_x (x0-3), but it has a certain effect on the adsorption energy. On different cobalt surfaces, the presence of oxygen atoms has different effects on the dehydrogenation of ammonia molecules. On the surface of Co (110), the presence of oxygen atoms can greatly promote the dehydrogenation of ammonia molecules in every step, so it is assumed that there will be a large number of N atoms on the surface. In contrast, the dissociation barrier of NH on the surface of Co (100) and Co (111) covered by oxygen is higher, which is the limit step for the complete dehydrogenation of ammonia on these two surfaces. Therefore, on the surface of cobalt covered by oxygen, NH and N, the intermediate products of ammonia dehydrogenation, may occupy the active sites of the surface and lead to the passivation inactivation of cobalt.
【學(xué)位授予單位】：河南師范大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：O643.36
【參考文獻(xiàn)】
相關(guān)期刊論文 前1條
1 邵燕;姚楠;盧春山;呂德義;劉化章;李小年;;用于選擇性合成清潔液體燃料的鈷基F-T合成催化劑[J];化學(xué)進(jìn)展;2010年10期
，本文編號：2182338
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