【作者】 陈久岭；

【导师】 秦永宁； 李永丹；

【作者基本信息】 天津大学 ， 工业催化， 1999， 博士

【摘要】 本文根据碳纤维生长活性对催化剂的要求设计了催化剂及其制备条件。以多组份共沉淀制备的Feitknecht 化合物为母体,经过煅烧、还原制得准晶态、还原后金属颗粒大小为几到十几纳米的、高活性的镍基催化剂。对甲烷催化裂解制氢和生产碳纳米纤维两个过程耦合的可能性进行了研究。对碳纳米纤维的生长过程、形态、机理及其性质和初步应用进行了探索。采用XRD、DTA、TPR、化学吸附仪及TEM 等手段研究了Feitknecht 结构及其生成条件、煅烧后所得混合金属氧化物、以及还原后金属镍催化剂的结构及影响因素。研究表明,适当共沉淀条件下Feitknecht 结构可以在很宽的组成范围内生成。该结构煅烧得到氧化镍晶格为骨架的混合金属氧化物,Al₂O₃ 或CuO 均匀掺杂在氧化镍晶格中,不存在它们的结构特征,但造成NiO 晶格畸变。在合适还原条件下生成纳米级准晶态的镍颗粒,其结晶度、颗粒大小受母体结构、组成以及煅烧、还原等处理条件影响。以热力学为基础,对甲烷裂解制氢和生产碳纤维的过程进行了能量和物料衡算,对其可行性进行了分析。与甲烷水蒸汽重整制氢过程比较,本文提出的过程工艺简单,生产单位体积氢气所需能量比后者低40%以上,同时一步获得固态材料碳纳米纤维。采用管式反应器对甲烷裂解制氢和生长碳纤维两个过程的耦合进行了研究。实验表明,甲烷裂解过程中氢气是唯一气相产物,甲烷裂解转化率,碳纳米纤维的生产量与催化剂结构和反应条件有关,反应温度低时,催化剂寿命较长,但甲烷转化率低;反应温度越高,转化率越高,但催化剂寿命越短。773-873K 时,甲烷的转化率在20%左右,生成碳纳米纤维为颗粒状,单位重量镍上生成碳纤维的量一般在100-600gC/gNi,得到气相产品中氢气含量在33 vol%左右,1023 K 下,可以得到82%的氢气和191gC/gNi,过程耦合的很好。用热天平反应器对催化剂上碳纤维的生长过程进行了研究。结果显示,773K 左右,Ni/Al₂O₃和Ni-Cu/Al₂O₃催化剂上碳纤维生长有很高的活性,催化剂上碳纤维的生长速率与催化剂的组成和结构、反应温度以及反应气氛等因素有关。在甲烷与氮气混合气中,Ni/Al₂O₃催化剂上碳纤维的生长速率比Ni-Cu/Al2O3催化剂上生长速率高,但失活比后者快;同一类催化剂中金属的结晶度越低,催化剂颗粒越小,反应温度越高,碳纤维生长的速率越高,催化剂失活也越快。催化剂上碳纤维的生成量由生长速率和活性期两者决定。在甲烷中加入氢气能抑制碳纤维的生成,使生成温度升高,随着催化剂中铜含量的升高,碳纤维生成量得到改善。采用XRD、SEM、TEM 和HREM 对碳纤维的结构和形态进行了研究。结果表明,生成的碳纳米纤维有很高石墨化程度,碳纤维碳层间距在0.34nm 左右,和石墨晶体的层间距接近。每个催化剂颗粒构成一根碳纤维的顶端,催化其生长。碳纤维更多还原

【Abstract】 In the thesis, paracrystalline and highly active nickel-alumina and nickel-copper-alumina catalysts, in which the nickel particle size is between several and several tens nanometers, used for catalytic growth of carbon nanofibers from methane, were prepared from Feitknecht compound precursors by coprecipitation, and consequent calcination and reduction. The feasibility of simultaneous hydrogen and carbon nanofibers production from the catalytic decomposition of methane was discussed. Morphology, structure and their dependence on process conditions of carbon nanofibers were investigated. A mechanism of carbon nanofibers formation and growth on nickel catalyst is proposed. In the last section, the physical-chemical properties and the utilization of carbon nanofibers as an adsorbent were studied. The formation condition and structure of Feitknecht compound precursors during coprecipitation, the structure of mixed oxides after calcination and the nickel catalysts after reduction were investigated with XRD, DTA, TPR, BET and TEM. The results indicate that Feitknecht compound can be formed when M2+/M3+ ratio is in a range 2:1-19:1, however, when the ratio is higher than 3:1 the crystallinity of the coprecipitate becomes weak. After calcination, the mixed oxides resembles the structure of NiO, without showing structural information of Al2O3 or CuO, though these are uniformly solved in the lattice of NiO. A strong interaction between NiO and Al2O3, or CuO leads to a bad crystallinity of NiO lattice, and as a consequence makes the reduction temperature of nickel goes much higher than in other composites. During reduction of the mixed oxides, the existence of irreducible Al2O3 in NiO lattice enhances the formation of paracrystalline nickel particles. The better crystallinity of the Feitknecht compound precursors, the stronger the interaction between Al2O3 and NiO is, the more distortion of the nickel crystal in catalyst. Very high calcination or reduction temperatures may destroy this paracrystalline structure and make the crystallinity of nickel to be better. On the basis of thermodynamics calculation, the feasibility of simultaneous hydrogen and carbon nanofibers production from decomposition of methane was discussed. Compared to the steam reforming of methane, the energy saves amount up to over 40% for producing a unit volume of hydrogen. The solid material carbon nanofibers can be produced in a one step process. In a tubular reactor, simultaneous hydrogen and carbon nanofibers production was investigated experimentally. The results shows that hydrogen is the only gas phase product and the conversion of methane, the amount of carbon nanofibers produced on unit mass of catalyst and the productivity of hydrogen are related to the catalyst composition and reaction conditions. At 773-873 K, the conversion was about 20% and amount of carbon nanofibers produced was 100-600 gC/gNi and a gas with 33 vol% of hydrogen was obtained. With the increase of reaction temperature, the conversion of methane was increased, however, the catalyst was deactivated faster. The amount of carbon nanofibers produced depends on the conversion of methane and the stable time of the catalyst activity. At 1023 K, most of the catalysts were deactivated soon, however, with a catalyst containing 25% copper, a gas mixture with 82% hydrogen and 191 gC/gNi were obtained. In a thermal balance reactor, the process of carbon nanofibers growth was investigated. At temperatures around 773 K, the catalysts were highly active and a large amount of carbon was formed. The growth rate and the amount of carbon nanofibers produced with unit mass of catalyst were depended on the composition and structure of catalyst and reaction conditions. With methane and nitrogen as the feed, the growth of carbon on Ni/Al2O3 was faster than that on Ni-Cu/Al2O3, however, the stability of the former was lower than that of the latter. The total amount of carbon produced on a catalyst was determined by the growth rate and the stable time of its activity. Hydrogen in the feed prohibited the formation of carbon and raised the activation temperature for carbon formation and lowered the growth rate. The morphology and structure of the nanofibers were examined with XRD, SEM, TEM and HREM. The results show that the nanofibers are highly graphitized and the distance of the carbon layer of (002) is uniform and is about 0.34 nm, which is close to that of graphite. The shape and orientation of carbon layers, the morphology of catalyst particles at the tip of a fiber or encapsulated inside a fiber are complex and are related to the composition and structure of the catalyst, reaction conditions, and composition of the feed. Different morphologies of carbon nanofibers were found, such as fishbone-shaped, octopus-shaped, branch-shaped, stick-shaped, tubular, bamboo-shaped and helical and etc. The diameter of them was several nanometers to several tens of nanometers. Carbon nanotubes were found in a reaction condition. These results indicate the possibility of controlling the morphology and structure of nanocarbons by monitoring catalyst and reaction conditions. The mechanism for carbon formation on nickel catalysts was discussed. The growth of carbon naofibers was attributed to the surface diffusion of carbon and its morphology and orientation of carbon layers depended on surface composition and existent state of catalyst particle and reaction condition. The deactivation of catalyst was due to the bad matching of decomposition rate of methane and surface diffusion rate of carbon. The properties of catalytic grown carbon nanofibers were studied. The carbon nanofibers have a high mechanical strength. Their BET specific surface area is about 100m2/g. Stability in CO2 of the carbon nanofibers is better than that of active carbon. Benzene and phenol were used as adsorbates to characterize the adsorption ability of carbon nanofibers. The results show that the amounts of the adsorption of benzene from vapor phase and phenol from water solution on carbon nanofibers depended on their structure. These adsorbed amounts are much lower than that on active carbon, due to its small specific surface area.更多还原