水和氨分子在FCC钴表面吸附和解离的理论研究
发布时间：2018-08-14 08:58
【摘要】：能源问题一直是人类关注的重点问题,费托合成反应是生产清洁能源的一种重要工业途径。过渡金属钴催化剂由于具有活性高、水煤气转化率低和高烃产物选择性高等优点,而被作为工业上费托合成反应中的一种常用催化剂。现有研究表明,费托合成反应中的副产物水以及反应合成气中的一些杂质气体能够使钴催化剂中毒失活,然而它们致催化剂钴失活的原因仍待探究。本文基于密度泛函理论的第一性原理方法,通过对水和氨分子在钴金属表面上的吸附性质与解离过程的计算模拟,阐述了它们致钴催化剂表面失活的微观机理。通过对计算结果进行分析,在以下方面获得了重要的研究成果:1)水分子倾向于吸附在钴表面的顶位,且吸附较弱容易从表面脱吸附。在水分子的两步脱氢反应中,水分子的解离势垒低于羟基的解离势垒。表面上氧原子的存在能增强水分子与钴表面的吸附强度,并且能够促进水分子在钴表面上的第一步脱氢反应,在Co(110)表面上这种促进作用最为明显。但是氧原子的存在对羟基在钴表面上的解离并没有促进作用。在钴表面上,羟基的存在对水分子的解离有轻微的抑制作用。水分子解离产生的羟基可能是使钴催化剂失活的原因。计算结果也表明水分子与钴表面的作用具有结构敏感性。2)对于NH_x(x=0-3),随着氢原子数目的逐渐减少,其与钴表面上的吸附强度逐渐增大。钴表面上氧原子的存在不改变NH_x(x=0-3)的最稳定吸附位置,但是对吸附能有一定的影响。在不同的钴表面上,氧原子的存在对氨气分子的脱氢反应有不同的影响。在Co(110)表面上,氧原子的存在对氨气分子的每一步脱氢反应都有极大的促进作用,由此推测此表面上也许将有大量的N原子覆盖。与此相比,在氧覆盖的Co(100)和Co(111)表面上,NH的解离势垒较高,是氨分子在这两个表面上完全脱氢的限速步骤。因此,在氧覆盖的钴表面上,氨分子脱氢反应的中间产物NH和N很可能占据表面的活性位从而导致钴化剂钝化失活。
[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.
【学位授予单位】：河南师范大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O643.36
本文编号：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.
【学位授予单位】：河南师范大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O643.36
【参考文献】
相关期刊论文 前1条
1 邵燕;姚楠;卢春山;吕德义;刘化章;李小年;;用于选择性合成清洁液体燃料的钴基F-T合成催化剂[J];化学进展;2010年10期
，本文编号：2182338
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