【摘要】：近些年來,隨著納米材料與納米科技的發(fā)展,新型納米材料的可控制備和應(yīng)用成為當(dāng)今世界最熱門的研究領(lǐng)域之一。過渡金屬納米顆粒的一個最主要的應(yīng)用就是用作化學(xué)反應(yīng)的催化劑。過渡金屬的催化活性由其d帶位置決定,因此顆粒表面的配位不飽和原子是反應(yīng)活性中心,而實(shí)驗(yàn)中觀測到的尺寸效應(yīng)主要是由顆粒大小不同而暴露的反應(yīng)中心數(shù)量變化導(dǎo)致。減小納米顆粒的大小,形成高分散、高活性、高選擇性的過渡金屬亞納米結(jié)構(gòu)是目前納米催化研究的前沿。但是粒徑的減小也伴隨著表面能增大,穩(wěn)定性下降。把納米顆粒負(fù)載在載體材料上,通過兩者之間的相互作用,不僅能提高納米顆粒的穩(wěn)定性,而且納米顆粒與載體之間還能產(chǎn)生協(xié)同效應(yīng),進(jìn)一步提高催化劑的催化性能。二維層狀材料六方氮化硼和二硫化鉬由于具有大的比表面積可用作過渡金屬亞納米結(jié)構(gòu)的載體。本文通過第一性原理方法,重點(diǎn)研究了單個過渡金屬原子(Cu, Pt)摻雜的六方氮化硼催化CO氧化反應(yīng)的性能以及負(fù)載在單層二硫化鉬上Pt納米結(jié)構(gòu)的生長規(guī)律,主要內(nèi)容如下：首先,我們利用第一性原理方法研究了Cu摻雜的單層六方氮化硼的電子結(jié)構(gòu)以及CO氧化反應(yīng)機(jī)理。發(fā)現(xiàn)負(fù)載的Cu原子更傾向于直接與六方氮化硼上的B空位缺陷作用,這些缺陷位點(diǎn)可以捕獲Cu原子阻止其進(jìn)一步團(tuán)聚。Cu原子與B空位缺陷的強(qiáng)相互作用使得Cu-d軌道向費(fèi)米能級移動,能更好的活化反應(yīng)物種,促進(jìn)反應(yīng)的進(jìn)行。Cu摻雜的單層六方氮化硼對CO的催化氧化按照Langmuir-Hinshelwood機(jī)理進(jìn)行：首先CO和02共吸附形成類似過氧化物的中間體,再進(jìn)一步解離生成CO2分子和吸附的O原子；另一分子的CO與吸附的O原子反應(yīng)生成物理吸附的CO2,CO2脫附實(shí)現(xiàn)活性中心的再生,完成一個催化循環(huán)。反應(yīng)過程中過氧化物中間體的形成和解離以及催化劑再生的能壘分別為0.26 eV,0.11 eV,0.03 eV,說明Cu摻雜的六方氮化硼是性能優(yōu)秀的CO低溫氧化催化劑。其次,我們還研究了Pt摻雜的單層六方氮化硼的電子結(jié)構(gòu)以及CO氧化反應(yīng)機(jī)理。與Cu摻雜體系不同,引入Pt原子取代六方氮化硼的一個B原子后體系中有1個未配對電子,對催化性能有一定的促進(jìn)作用。在Pt摻雜體系中,CO和02共吸附比兩個CO共吸附穩(wěn)定,不會有CO中毒的現(xiàn)象。結(jié)果表明,Pt摻雜的單層六方氮化硼上CO的氧化機(jī)理也為Langmuir-Hinshelwood機(jī)理,催化性能與Cu摻雜的六方氮化硼相當(dāng)。最后,我們利用第一性原理方法研究了負(fù)載在單層二硫化鉬上Pt納米結(jié)構(gòu)的生長模式和電子結(jié)構(gòu)。結(jié)果表明,單個Pt原子最穩(wěn)定吸附位點(diǎn)是Mo原子的頂位,其次是S原子的頂位；Pt2吸附時Mo原子的頂位仍為最穩(wěn)定吸附位點(diǎn)；當(dāng)Ptn (n≥3)吸附時,界面轉(zhuǎn)變成Pt-S作用更穩(wěn)定,此時Pt-Pt間的作用大于Pt-二硫化鉬的作用,Pt納米結(jié)構(gòu)采取延性生長模式,更容易向上生長形成三維納米顆粒。
[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，

本文編號：2318281