【作者】 吴壮志；

【导师】 王德志； Kevin J. Smith；

【作者基本信息】 中南大学 ， 材料加工工程， 2012， 博士

【摘要】 纳米结构二硫化钼(钨)除了在润滑和催化领域具有广泛应用之外,在锂电池、储氢、光电学、机械、化工等领域也有巨大的应用前景。由于其结构的各向异性,不同形貌结构对性能的影响十分显著,因此,通过改进现有方法与工艺,同时不断开发新方法和新工艺,进一步拓展纳米结构二硫化钼(钨)的制备技术,实现微结构的调控制备,可大幅提升其应用性能,使之在更广泛的领域得到应用,具有重要的理论和实践意义。本文主要致力于纳米结构二硫化钼(钨)新技术的开发,并围绕所得产物的形貌结构,研究其形成机理,同时拓展其潜在的应用价值。采用表面活性剂促助法,分别选用不同类型的表面活性剂,包括阴离子型的SDBS,非离子型的PEG和阳离子型的CTAC,用于制备各种不同形貌的二硫化钼纳米结构,同时研究了各种活性剂对最终产物形貌的影响；采用液相还原法,阴离子型的SDBS可以促助生成粒度约100nm的二硫化钼六方颗粒,而非离子型的PEG通过构建脚手架,促助形成二硫化钼空心微球,阳离子型的CTAC则直接参与化学反应,得到具有中空结构的二硫化钼纳米球,而阴离子型的SDBS和非离子型的PEG作为复配活性剂时,产物为长约200-400nm的二硫化钼纳米杆；采用高浓度的PEG作为包覆剂,借助“空间位阻效应”,得到前驱体三硫化钼纳米球,再经后续高温氢还原,可以制备出直径约为100nm的实心纳米球。首次采用机械化学法,基于机械球磨过程中的瞬时高温高压效应,以三硫化钼为前驱体,成功制备出二硫化钼纳米颗粒,并研究了转速和球磨时间的影响；同时,研究了机械球磨预处理对普通微米颗粒及其加氢脱氧催化性能的影响。转速为400rpm以上,且球磨时间超过24h才能使前驱体完全分解；机械球磨预处理会碾碎原先的大块微米颗粒,创造大量多空配位键,使之具有很高的反应频率因子,从而极大提高催化活性,但球磨预处理并未改变其反应路径选择性。首次采用固相微区反应法,借助机械球磨实现微米粒子的超细匀态分布,成功制备出二硫化钼纳米片,系统研究了硫化温度,硫化时间和机械球磨预处理对产物的影响,讨论其形成机制,并探讨其在高温煅烧下的固相自组装过程。在600℃下硫化时间超过10min即可完成反应,而硫化温度则在400-700℃为宜；机械预处理实现了超细粒子的匀态分布,创造出大量“反应微区”,有利于最终纳米片的形成；二硫化钼纳米片通过粘结和堆垛,在高温煅烧下可以形成尺寸更大的规则六方纳米板结构。采用四球摩擦实验机,综合对比了二硫化钼纳米片与商用二硫化钼超细颗粒的润滑性能,同时研究了添加剂含量和分散剂对摩擦性能的影响。二硫化钼纳米片由于表面活性较高,能形成牢固的润滑膜,表现出较低的摩擦系数,更好的减磨和抗负荷能力,其最佳添加量为1.5wt%,而分散剂Span-80能有效改善添加剂在基础油中的分散,增强抗磨能力。通过改变硫化温度获得具有不同堆垛层数的纳米片结构,然后研究硫化温度对二硫化钼纳米片催化剂的影响及其堆垛结构随加氢脱硫反应性能之间的变化关系。随着硫化温度的升高,无助剂催化剂的比表面积和金属分散性均不断下降；而其堆垛结构-催化性能变化关系,则遵循"Rim-Edge"模型,但是在临界值(7.7nm),其变化趋势发生逆转。系统研究了镍和钻助剂的添加对二硫化钼纳米片催化剂的影响,包括结构及其催化性能。镍和钴助剂的添加都会明显减小孔径,但会提高金属分散性；镍助剂会促进二硫化钼的晶体生长,而钴助剂则会在较低温度下抑制其生长；镍和钴的添加,都会提高活性,且倾向于DDS反应路径,但随着硫化温度的升高,两者对催化活性和选择性的影响却大不一样；NiMo催化剂的活性不断下降而其HYD选择性则不断升高；CoMo催化剂的活性先是不断增加,直到700℃达到最大值,而后才会因烧结相的形成而急剧下降,其HYD选择性则呈现与之相反的变化趋势；首次采用固相微区反应法,成功制备出单分散二硫化钨纳米片和无机富勒烯结构,考察了硫化温度,硫化时间及机械球磨对产物形成的影响,并测试了其制氢反应催化性能。只有在600-700℃下硫化,才能制得二硫化钨纳米片,而在800℃硫化,可以制得二硫化钨无机富勒烯结构；在600℃下硫化,反应时间必须持续30min以上,才能保证反应完全；机械球磨预处理的机械活化效应不仅使整个反应过程更加直接快速,没有任何中间反应产物,还促使氧化钨外层迅速硫化,起到模板导向作用,引导纳米片和富勒烯结构的形成；基于纳米片结构较为疏松的堆垛结构,其制氢催化活性是二硫化钼催化剂的8.6倍,其制氢反应遵循Volmer-Heyrovsky机制,速控步为电化学脱附过程。更多还原

【Abstract】 Besides in the field of lubrications and catalysts, molybdenum/tungsten disulfide nanomaterials were also widely used in lithium batteries, hydrogen storage, photoelectricity, engine and chemical industry. Due to the structural anisotropy, the differences of morphology and structure will affect the performances significantly. Therefore, it is necessary to improve the current preparation technologies and explore novel methods, which can control the preparation of microstructures, enhancing the final performances. And then, they can be used in more fields, which is very important for developments of theory and applications. The current thesis is focused on exploring novel methods to prepare molybdenum/tungsten disulfide nanomaterials, discussing the formation mechanism of the products’morphologies and structures, and exploring their potential applications.Molybdenum disulfide nanostructures with different morphologies were prepared by a surfactant-assisted method, adopting surfactants with different types, including anionic SDBS, non-ionic PEG and cationic CTAC. And the influences of various surfactants on the morphologies of final products were studied. It was shown that molybdenum disulfide hexagonal nanoparticles with a size of100nm was prepared via a liquid phase reduction method, adopting anionic SDBS, and hollow molybdenum disulfide microspheres were obtained using non-ionic PEG as a template. However, the cationic CTAC would attend the chemical reaction and promote the formation of hollow molybdenum disulfide nanospheres. If anionic SDBS and non-ionic PEG were used together, the final products were molybdenum disulfide nanorods with a length of200-400nm. Moreover, molybdenum disulfide solid nanosperes with a diameter of100nm could be synthesized by a hydrogen reduction process adopting PEG as the covering agent, in which molybdenum trisulfide nanopsheres were used as precursors and then reduced in an atmosphere of hydrogen at high temperatures.Based on the effect of instantaneous high-temperature and high-pressure during ball milling process, molybdenum disulfide nanoparticles were successfully prepared by a mechanochemical method, adopting molybdenum trisulfide as precursors, and the effects of ball-milling speed and time were also studied. Moreover, the influence of the ball milling pretreatment on general microparticles and their hydrodeoxygenation performances was discussed. It was shown that the precursors could be decomposed completely only at a rotation speed of over400rpm for more than24h, and the pretreatment of ball milling would crash the previous huge microparticles, creating a lot of multiple coordinatively unsaturated vacancies, which would rise the TOFs (turnover frequencies) and enhance the final catalytic activity. However, the pretreatment of ball milling did not change the selectivity of reaction route at all.Molybdenum disulfide nanosheets were successfully prepared by a solid micro-domain method, in which microparticles could be crashed into ultrafine particles and distributed homogeneously. The effects of annealing temperature, annealing time and the pretreatment of ball milling on the final products were also discussed. The formation mechanism of nanosheets and their solid self-assembly process at high temperatures were also studied. It was found that the nanosheets could be obtained at400-700℃, and it took only10min to finish the reaction at600℃. The pretreatment of ball milling helped distribute the ultrafine particles homogeneously and create lots of "micro-domains", which promoted the formation of the final nanosheets. Moreover, the molybdenum disulfide nanosheets would transform into larger regular hexagonal nanoplates through cohering and stacking at high temperatures.The lubricating properties of molybdenum disulfide nanosheets were tested by a four-ball tribometer, and compared with that of commercial ultrafine particles. The influences of dispersant and additive content on tribological performances were also investigated. The results showed that the molybdenum disulfide nanosheets could form stable lubricating films due to high surface activity, and showed lower friction factor, better anti-attrition and load-carrying capacity. The optimized content of additive was1.5wt%and the dispersant of Span-80could improve the dispersion of additives in basal oils, enhancing their antiwear capacity.Nanosheets with various stacking heights were obtained by adjusting the annealing temperatures. Then, the effect of annealing temperatures on molybdenum disulfide nanosheet catalysts and the relation bwteen the stacking height and hydrodesulfurization function were studied. It was found that the surface area and metal dispersion would decrease with the increasing of annealing temperatures and the relation between stacking structures and catalytic function could be explained by "Rim-Edge" model with a slight modification that there was a crucial value (7.7nm) reversing the trend.The influences of assistants (Ni and Co) on molybdenum disulfide nanosheet catalysts were investigated systematically, including their structures and catalytic functions. The results showed that the addition of Ni and Co would reduce the pore diameter significantly but improve their metal dispersion, and the addition of Ni would promote the crystal growth of molybdenum disulfide but Co would suspend it at relatively low temperatures. Both of them enhanced the final activity and preferred the reaction route of DDS. However, the effects of them on catalytic activity and selectivity were quite different with increasing annealing temperatures:the activity of NiMo catalysts would be decreased and their HYD selectivity would be increased; While, the activity of CoMo catalysts was increased consistantly and reached the maximum value at700℃, then decreased sharply due to the formation of segregations, but the HYD selectivity showed the reverse trend.Tungsten disulfide monodisperse nanosheets and inorganic fullerene structures were successfully synthesized by a solid micro-domain reaction method, and the effects of annealing temperatures, annealing time and the pretreatment of ball milling on the formation of final products were investigated. And their catalytic function for hydrogen evolution reaction (HER) was also tested. It was found that the tungsten disulfide nanosheets could only be obtained at600-700℃, and inorganic fullerene structures was prepared at800℃. The reaction must be kept for more than30min at600℃, or the final products could not be sulfurized completely. The pretreatment of ball milling not only made the whole reaction process faster and more direct, but also promoted the sulfurization of outer layers of tungsten oxide, inducing the formation of nanosheets and inorganic fullerene structures as a template. Due to the loosely stacking structures of nanosheets, their catalytic activity for hydrogen evolution reaction was much better (8.6times) than that of common molybdenum disulfide catalysts. The rate-limiting step was electrochemical desorption and the Volmer-Heyrovsky HER mechanism was operative in the HER catalyzed by the tungsten disulfide nanosheets.更多还原