二维材料负载的过渡金属亚纳米结构的第一性原理研究
发布时间：2018-11-08 11:04
【摘要】：近些年来,随着纳米材料与纳米科技的发展,新型纳米材料的可控制备和应用成为当今世界最热门的研究领域之一。过渡金属纳米颗粒的一个最主要的应用就是用作化学反应的催化剂。过渡金属的催化活性由其d带位置决定,因此颗粒表面的配位不饱和原子是反应活性中心,而实验中观测到的尺寸效应主要是由颗粒大小不同而暴露的反应中心数量变化导致。减小纳米颗粒的大小,形成高分散、高活性、高选择性的过渡金属亚纳米结构是目前纳米催化研究的前沿。但是粒径的减小也伴随着表面能增大,稳定性下降。把纳米颗粒负载在载体材料上,通过两者之间的相互作用,不仅能提高纳米颗粒的稳定性,而且纳米颗粒与载体之间还能产生协同效应,进一步提高催化剂的催化性能。二维层状材料六方氮化硼和二硫化钼由于具有大的比表面积可用作过渡金属亚纳米结构的载体。本文通过第一性原理方法,重点研究了单个过渡金属原子(Cu, Pt)掺杂的六方氮化硼催化CO氧化反应的性能以及负载在单层二硫化钼上Pt纳米结构的生长规律,主要内容如下：首先,我们利用第一性原理方法研究了Cu掺杂的单层六方氮化硼的电子结构以及CO氧化反应机理。发现负载的Cu原子更倾向于直接与六方氮化硼上的B空位缺陷作用,这些缺陷位点可以捕获Cu原子阻止其进一步团聚。Cu原子与B空位缺陷的强相互作用使得Cu-d轨道向费米能级移动,能更好的活化反应物种,促进反应的进行。Cu掺杂的单层六方氮化硼对CO的催化氧化按照Langmuir-Hinshelwood机理进行：首先CO和02共吸附形成类似过氧化物的中间体,再进一步解离生成CO2分子和吸附的O原子；另一分子的CO与吸附的O原子反应生成物理吸附的CO2,CO2脱附实现活性中心的再生,完成一个催化循环。反应过程中过氧化物中间体的形成和解离以及催化剂再生的能垒分别为0.26 eV,0.11 eV,0.03 eV,说明Cu掺杂的六方氮化硼是性能优秀的CO低温氧化催化剂。其次,我们还研究了Pt掺杂的单层六方氮化硼的电子结构以及CO氧化反应机理。与Cu掺杂体系不同,引入Pt原子取代六方氮化硼的一个B原子后体系中有1个未配对电子,对催化性能有一定的促进作用。在Pt掺杂体系中,CO和02共吸附比两个CO共吸附稳定,不会有CO中毒的现象。结果表明,Pt掺杂的单层六方氮化硼上CO的氧化机理也为Langmuir-Hinshelwood机理,催化性能与Cu掺杂的六方氮化硼相当。最后,我们利用第一性原理方法研究了负载在单层二硫化钼上Pt纳米结构的生长模式和电子结构。结果表明,单个Pt原子最稳定吸附位点是Mo原子的顶位,其次是S原子的顶位；Pt2吸附时Mo原子的顶位仍为最稳定吸附位点；当Ptn (n≥3)吸附时,界面转变成Pt-S作用更稳定,此时Pt-Pt间的作用大于Pt-二硫化钼的作用,Pt纳米结构采取延性生长模式,更容易向上生长形成三维纳米颗粒。
[Abstract]:In recent years, with the development of nanomaterials and nanotechnology, the controllable preparation and application of new nanomaterials have become one of the hottest research fields in the world. One of the most important applications of transition metal nanoparticles is as a catalyst for chemical reactions. The catalytic activity of transition metals is determined by their d-band position, so the coordination unsaturated atoms on the particle surface are the reactive active centers, and the size effects observed in the experiments are mainly caused by the changes of the number of reaction centers exposed to different particle sizes. Reducing the size of nanoparticles to form highly dispersed, highly active and highly selective transition metal subnanostructures is the frontier of nanocatalytic research. However, the decrease of particle size is accompanied by the increase of surface energy and the decrease of stability. The stability of nanoparticles can be improved not only by the interaction of nanoparticles on the carrier material, but also by the synergistic effect between nanoparticles and the support, and the catalytic performance of the catalyst can be further improved. Two dimensional layered materials, hexagonal boron nitride and molybdenum disulfide, can be used as support for transition metal subnanostructures because of their large specific surface area. In this paper, the properties of CO oxidation catalyzed by single transition metal atom (Cu, Pt) doped hexagonal boron nitride and the growth rule of Pt nanostructures supported on monolayer molybdenum disulfide have been studied by first principle method. The main contents are as follows: firstly, the electronic structure of monolayer hexagonal boron nitride doped with Cu and the mechanism of CO oxidation have been studied by first principle method. It is found that the supported Cu atoms tend to interact directly with B vacancy defects on hexagonal boron nitride. These defect sites can trap Cu atoms to prevent further agglomeration. The strong interaction between Cu atoms and B vacancy defects causes the Cu-d orbital to move to Fermi level, thus enabling better activation of reactive species. The catalytic oxidation of CO by Cu doped monolayer boron nitride was carried out according to the mechanism of Langmuir-Hinshelwood. Firstly, CO and 02 co-adsorbed to form peroxide-like intermediates, and then dissociated to form CO2 molecules and O atoms. Another molecule of CO reacts with the adsorbed O atom to form a physically adsorbed CO2,CO2 desorption to regenerate the active center and complete a catalytic cycle. The energy barrier of peroxides intermediate formation and dissociation and catalyst regeneration were 0.26 eV,0.11 eV,0.03 eV, respectively. The results showed that Cu doped hexagonal boron nitride was an excellent low temperature oxidation catalyst for CO. Secondly, we also studied the electronic structure of Pt doped monolayer boron nitride and the mechanism of CO oxidation. Different from the Cu doped system, there is one unpaired electron in the system after the introduction of Pt atom to replace one B atom of hexagonal boron nitride, which can promote the catalytic performance to some extent. In Pt doped system, the co-adsorption of CO and 02 is more stable than that of two CO, and there is no phenomenon of CO poisoning. The results show that the oxidation mechanism of CO on monolayer hexagonal boron nitride doped with Pt is also the mechanism of Langmuir-Hinshelwood, and the catalytic performance is equivalent to that of Cu doped boron nitride. Finally, we studied the growth mode and electronic structure of Pt nanostructures loaded on monolayer molybdenum disulfide by first-principles method. The results show that the most stable adsorption site for a single Pt atom is the top position of the Mo atom, followed by the top position of the S atom, and the top site of the Mo atom is still the most stable site at the time of Pt2 adsorption. When Ptn (n 鈮，
本文编号：2318281
[Abstract]:In recent years, with the development of nanomaterials and nanotechnology, the controllable preparation and application of new nanomaterials have become one of the hottest research fields in the world. One of the most important applications of transition metal nanoparticles is as a catalyst for chemical reactions. The catalytic activity of transition metals is determined by their d-band position, so the coordination unsaturated atoms on the particle surface are the reactive active centers, and the size effects observed in the experiments are mainly caused by the changes of the number of reaction centers exposed to different particle sizes. Reducing the size of nanoparticles to form highly dispersed, highly active and highly selective transition metal subnanostructures is the frontier of nanocatalytic research. However, the decrease of particle size is accompanied by the increase of surface energy and the decrease of stability. The stability of nanoparticles can be improved not only by the interaction of nanoparticles on the carrier material, but also by the synergistic effect between nanoparticles and the support, and the catalytic performance of the catalyst can be further improved. Two dimensional layered materials, hexagonal boron nitride and molybdenum disulfide, can be used as support for transition metal subnanostructures because of their large specific surface area. In this paper, the properties of CO oxidation catalyzed by single transition metal atom (Cu, Pt) doped hexagonal boron nitride and the growth rule of Pt nanostructures supported on monolayer molybdenum disulfide have been studied by first principle method. The main contents are as follows: firstly, the electronic structure of monolayer hexagonal boron nitride doped with Cu and the mechanism of CO oxidation have been studied by first principle method. It is found that the supported Cu atoms tend to interact directly with B vacancy defects on hexagonal boron nitride. These defect sites can trap Cu atoms to prevent further agglomeration. The strong interaction between Cu atoms and B vacancy defects causes the Cu-d orbital to move to Fermi level, thus enabling better activation of reactive species. The catalytic oxidation of CO by Cu doped monolayer boron nitride was carried out according to the mechanism of Langmuir-Hinshelwood. Firstly, CO and 02 co-adsorbed to form peroxide-like intermediates, and then dissociated to form CO2 molecules and O atoms. Another molecule of CO reacts with the adsorbed O atom to form a physically adsorbed CO2,CO2 desorption to regenerate the active center and complete a catalytic cycle. The energy barrier of peroxides intermediate formation and dissociation and catalyst regeneration were 0.26 eV,0.11 eV,0.03 eV, respectively. The results showed that Cu doped hexagonal boron nitride was an excellent low temperature oxidation catalyst for CO. Secondly, we also studied the electronic structure of Pt doped monolayer boron nitride and the mechanism of CO oxidation. Different from the Cu doped system, there is one unpaired electron in the system after the introduction of Pt atom to replace one B atom of hexagonal boron nitride, which can promote the catalytic performance to some extent. In Pt doped system, the co-adsorption of CO and 02 is more stable than that of two CO, and there is no phenomenon of CO poisoning. The results show that the oxidation mechanism of CO on monolayer hexagonal boron nitride doped with Pt is also the mechanism of Langmuir-Hinshelwood, and the catalytic performance is equivalent to that of Cu doped boron nitride. Finally, we studied the growth mode and electronic structure of Pt nanostructures loaded on monolayer molybdenum disulfide by first-principles method. The results show that the most stable adsorption site for a single Pt atom is the top position of the Mo atom, followed by the top position of the S atom, and the top site of the Mo atom is still the most stable site at the time of Pt2 adsorption. When Ptn (n 鈮，
本文编号：2318281
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