过渡金属基复合材料的制备及其催化氨分解制氢性质研究

发布时间：2018-06-04 14:06

  本文选题：过渡金属 + 复合材料　； 参考：《山东大学》2016年博士论文

【摘要】：随着全球经济的高速发展,对化石燃料的过度开发使得传统燃料资源愈加显得捉襟见肘。在众多新能源中,氢被认为是有史以来最为清洁的能源,具有安全、高效、零污染等诸多优点。迄今为止,虽然人们在氢能的开发利用方面取得了一系列重要进展,但在氢燃料电池的应用方面还面临许多难题,其中,如何为燃料电池的发电装置提供高质量而廉价的氢气是主要问题之一。直接以氢为燃料无论在运输方面还是储存方面都存在诸多问题,而氨分解在线制氢技术可以有效的解决这两大难题。氨是一种富氢化合物,能量密度远远高于甲醇、汽油等燃料,常温下易于以液体形式存在,而液氨的储存安全可靠,成本低廉。此外,由于氨分子中不含碳元素,因而从源头上杜绝了COr等有毒气体的生成。将氨分解反应用于在线制氢领域是目前解决燃料电池氢能来源的有效途径。作为氨分解反应的研究内容之一,氨分解催化剂的设计和制备至关重要。目前对于氨分解催化剂的研究多数集中在负载型材料上,主要包含以钌为代表的贵金属型催化剂和以铁、镍等为代表的廉价过渡金属型催化剂。贵金属催化剂的催化氨分解反应活性一般较高,但其价格非常昂贵。而对于用于催化氨分解反应的负载型过渡金属催化剂,较低的活性组分含量与高温下易烧结的缺点是其存在的主要问题。此外,在已有的文献中,人们较多关注的是催化剂体系的选择和制备,而对催化剂在反应条件下活性物相的指认工作则涉及较少。本论文拟以过渡金属为主体,利用优良的热稳定性物质作为结构稳定组分,通过廉价、易行的合成方法,设计制备具有高活性组分含量的过渡金属基复合材料,研究其作为催化剂在氨分解反应中的催化行为,考察稳定剂和活性组分的相对含量、介观分布以及微观结构对催化剂结构稳定性和催化活性的影响。同时使用原位x射线粉末衍射表征技术追踪催化剂的活性组分在反应中的物相变化,揭示物相以及结构变化与其催化活性的内在联系,指认催化活性物相,从而进一步指导氨分解催化剂的设计合成,挖掘过渡金属基纳米复合材料在工业催化领域的应用价值。论文主要分为以下三部分工作：1.利用共沉淀方法合成了Fe、Co、Ni分别与氧化铝复合的三种高活性组分含量的过渡金属催化剂体系,通过XRD、氮气吸脱附、TEM、SEM等一系列手段表征了反应前后催化剂的物相和结构特点。研究表明,过渡金属的含量较低时,催化剂主要由大量的无定形的氧化铝和过渡金属氧化物组成,随着含量的提高,过渡金属以结晶态的氧化物形式存在。所得具有高活性组分含量的催化剂表现出优异的催化活性和稳定性,其中钴含量为90 at%的样品催化活性最高,在空速高达72000 cm3 goat-1 h-1的条件下,600℃时,可以实现88%的高转化率,并且在72小时的测试时间内没有任何降低的趋势。催化氨分解反应后,氧化铁被氨气氮化,生成氮化铁,而氧化钴和氧化镍均被还原成金属单质。相应的元素分布结果显示,掺入过渡金属化合物中的少量铝,分布在催化剂表面,有效抑制了活性组分在反应过程中的烧结现象,从而提高了氨分解反应的催化活性和稳定性。H2-TPR结合相应的原位XRD实验用于探究三种催化剂体系在氢气条件下的还原行为,结果表明,铝的含量越高,与过渡金属的相互作用越强,其中铝的加入使得氧化铁更容易被还原,而对于氧化钴和氧化镍来说,与氧化铝之间的强相互作用导致二者更难被还原。2.使用溶胶凝胶一步合成法,得到钴含量不等的Co-Al系列氨分解反应催化剂,并通过XRD、XAFS、、XPS,氮气吸脱附、TEM、SEM等一系列手段进行了表征。对样品的XRD衍射谱Rietveld结构精修结果证明,在含有铝的催化剂中,钴并不是以单纯的C0304相存在,而是Co和Al同时占据四面体和八面体配位位置的混合尖晶石相(Co,Al)(Co,Al)204。XAFS结果表明,反应前的样品中Co原子与O原子配位,反应后呈Co-Co配位。少量铝的加入,能够有效的阻止活性组分在反应过程中的聚集,从而大大提高钴催化剂的氨分解反应催化稳定性。在36000 cm3 gcat-1 h-1的高空速下,该系列催化剂表现出良好的催化氨解活性和稳定性,600℃时,钴含量为90 at%的催化剂产氢速率达到37 mmol gcat-1 min-1,并在120 h测试时间内没有丝毫降低。研究发现,影响Co-Al催化剂催化活性的因素有多种,包括钴的含量、相态、晶粒尺寸、比表面积和钴铝之间的相互作用等等。原位XRD用于追踪Co-Al催化剂在氨分解反应中的物相变化,结果证明,由混合尖晶石相(Co,Al)(Co,Al)2O4到CoO、CoO进一步到金属Co的还原过程受起始样品中钴含量的影响,钴的含量越高,(Co,Al)(Co,Al)2O4到CoO的还原温度越低。将原位XRD结果与催化活性数据结合在一起,首次指认了立方相的金属Co是钴基氨分解催化剂最可能的活性相,此外CoO也具有一定的催化活性。3.表面活性剂在溶液中可以组装成为有序分子聚集体,通过进一步的溶剂蒸发形成层状液晶,以层状液晶为模板,经过高温碳化、强碱剥离等过程,合成了碳纳米片基三元复合材料。通过TEM、SEM、XPS、氮气吸脱附、拉曼光谱、原位XRD等一系列手段对该碳纳米片基复合催化剂进行了研究,发现碳纳米片上同时分散着钴基化合物和无定形的铝,其中钴基化合物在碳纳米片上呈交联状分布。拉曼光谱显示,虽然碳化温度只有400℃,但是得到的碳纳米片具有非常高的石墨化结晶度。氨分解催化反应后,钴氧化物被还原成金属钴单质,而经过相变重结晶的过程,原先处于交联分布的钴物种变成了尺寸均一的钴颗粒,同时均匀而密集地嵌入在铝和碳的复合纳米片中,这样的结构特点可以有效的阻止钴颗粒在氨分解反应中的聚集,从而保证这种复合材料高效稳定的催化氨分解反应。催化测试结果显示,空速为12000 cm3 gcat-1 h-1时,钴与铝投料比为3/7的样品在500℃的低温下就可以实现氨的完全转化,即使将反应气体空速提高至76 000 cm3 gcat-1 h-1,600℃时,该催化剂仍然能达到96%的氨转化率。其催化稳定性更为突出,在长达144 h的稳定性测试中,催化剂没有发生任何失活现象。
[Abstract]:With the rapid development of the global economy, the overexploitation of fossil fuels has made traditional fuel resources more and more difficult. In many new energy sources, hydrogen is considered as the most clean energy in history, with many advantages, such as safety, efficiency, zero pollution and so on. So far, people have made a line in the development and utilization of hydrogen energy. However, there are many difficulties in the application of hydrogen fuel cells, among which, how to provide high quality and cheap hydrogen for the power plant of fuel cells is one of the main problems. There are many problems in both transport and storage for direct hydrogen as fuel, and the on-line hydrogen production technology of ammonia decomposition can be effective. To solve these two difficult problems. Ammonia is a kind of hydrogen rich compound. The energy density is far higher than that of methanol and gasoline. It is easy to exist in liquid form at normal temperature, while the storage of liquid ammonia is safe and reliable, and the cost is low. In addition, the formation of toxic gases such as COr is eliminated from the source because of no carbon element in the ammonia molecule. The field of on-line hydrogen production is an effective way to solve the hydrogen energy sources of fuel cells. As one of the research contents of the ammonia decomposition reaction, the design and preparation of the ammonia decomposition catalyst are very important. At present, most of the research on the ammonia decomposition catalyst is concentrated on the loaded materials, mainly including the precious metal catalysts and iron, represented by ruthenium. The catalytic ammonia decomposition reaction of the noble metal catalyst is generally high, but its price is very expensive. For the supported transition metal catalyst used to catalyze the ammonia decomposition reaction, the shortcomings of the lower active component content and the easy firing at high temperature are the main problems. In the existing literature, more attention is paid to the selection and preparation of the catalyst system, while the identification of the active phase of the catalyst is less than that of the catalyst. This paper is designed to use the transition metal as the main body, and use the excellent thermal stability material as the structural stability component, and design the preparation by the cheap and easy synthesis method. The transition metal matrix composite with high active component content was studied as a catalyst in the ammonia decomposition reaction, and the effect of the relative content of stabilizers and active components, mesoscopic distribution and microstructure on the structure stability and catalytic activity of the catalyst were investigated. At the same time, the technology of in situ X ray powder diffraction was used to characterize the technology. The active component of the tracer catalyst changes in the phase of the reaction, reveals the intrinsic relationship between the phase and the structural change and its catalytic activity, and identifies the catalytic active phase, thus further directing the design and synthesis of the ammonia decomposition catalyst and excavating the application value of the transition metal matrix nanocomposites in the industrial catalysis field. The thesis is mainly divided into the following The three part: 1. using coprecipitation method to synthesize the transition metal catalyst system with three kinds of high active component content of Fe, Co, Ni and alumina respectively. Through a series of means such as XRD, nitrogen desorption, TEM, SEM and so on, the phase and structure characteristics of the catalyst before and after the reaction are characterized. The catalyst is mainly composed of a large number of amorphous alumina and transition metal oxide. With the increase of the content, the transition metal exists in the form of crystalline oxide. The catalyst with high active component content shows excellent catalytic activity and stability. The catalytic activity of the sample with a cobalt content of 90 at% is the highest, and the velocity is high at the air velocity. Under the condition of 72000 cm3 goat-1 H-1, the high conversion rate of 88% can be achieved at 600 C, and there is no downward trend in the test time of 72 hours. After the catalytic ammonia decomposition reaction, the iron oxide is nitriding by ammonia to produce iron nitride, while cobalt oxide and nickel oxide are reduced to metal monomer. A small amount of aluminum in the metal compound is distributed on the surface of the catalyst, which effectively inhibits the sintering of the active components during the reaction process, thus improving the catalytic activity and stability of the ammonia decomposition reaction..H2-TPR binding in situ XRD experiment is used to explore the reduction behavior of the three catalyst systems under hydrogen conditions. The higher the content, the stronger the interaction with the transition metal, in which the addition of aluminum makes the iron oxide easier to be reduced, and for the cobalt oxide and the nickel oxide, the strong interaction with the alumina leads to the more difficult to be reduced by the reduced.2. using the sol-gel one step synthesis method, and the Co-Al series ammonia decomposition reaction with different cobalt content has been obtained. XRD, XAFS, XPS, XPS, nitrogen desorption, TEM, SEM and so on. The XRD diffraction spectrum Rietveld structure refinement results show that in the catalyst containing aluminum, cobalt is not a pure C0304 phase, but a mixed spinel phase (Co, Al) at the same time that Co and Al occupy the position of tetrahedron and eight sides. L) 204.XAFS results show that Co atoms are coordinated with O atoms in the samples before reaction and Co-Co coordination after reaction. The addition of a small amount of aluminum can effectively prevent the accumulation of active components during the reaction process, thus greatly improving the catalytic stability of the ammonia decomposition reaction of the cobalt catalyst. At the altitude of 36000 cm3 gcat-1 H-1, the series of catalyst tables The catalytic activity and stability of ammonia solution are obtained. At 600 C, the hydrogen production rate of the catalyst with a cobalt content of 90 at% reaches 37 mmol gcat-1 min-1 and has no reduction in the 120 h test time. It is found that there are a variety of factors affecting the catalytic activity of the Co-Al catalyst, including the content of cobalt, phase state, grain size, specific surface area and cobalt aluminum. In situ XRD is used to trace the phase change of the Co-Al catalyst in the ammonia decomposition reaction. The results show that the reduction process of the mixed spinel phase (Co, Al) (Co, Al) to CoO, CoO further to the reduction of metal Co is influenced by the cobalt content in the starting sample, the higher the cobalt content, the lower the reduction temperature of the (Co, Al). In situ XRD results are combined with catalytic activity data. For the first time, the metal Co of the cubic phase is the most likely active phase of the cobalt based ammonia decomposition catalyst. In addition, CoO has a certain catalytic activity of.3. surfactants which can be assembled into ordered molecular aggregates in the solution and through further solvent evaporation to form layered liquid crystals. A carbon nanoscale three element composite was synthesized by high temperature carbonization and strong alkali stripping. The carbon nanoscale composite catalyst was studied by means of TEM, SEM, XPS, nitrogen desorption, Raman spectroscopy, and in situ XRD. The Co based compound and amorphous carbon nanocomposite were simultaneously dispersed on the carbon nanoscale. The cobalt based compounds are crosslinked on carbon nanoscale. Raman spectra show that, although the carbonization temperature is only 400 C, the obtained carbon nanoscale has very high graphitization crystallinity. After the ammonia decomposition catalytic reaction, cobalt oxide is reduced to metal cobalt, and the process of phase change recrystallization is originally in the crosslinking distribution. The cobalt species becomes a homogeneous cobalt particle, which is uniformly and densely embedded in the composite nanoscale of aluminum and carbon. The structural characteristics can effectively prevent the aggregation of cobalt particles in the ammonia decomposition reaction, thus ensuring the efficient and stable catalytic ammonia decomposition reaction of the composite. The catalytic test results show that the velocity of air is 12000. At cm3 gcat-1 H-1, the total conversion of ammonia can be achieved at a low temperature of 500 C with the sample of CO and Al with a ratio of 3/7 to 3/7. Even if the reaction gas velocity is increased to 76000 cm3 gcat-1 h-1600, the catalyst can still reach 96% of the ammonia conversion rate. The catalytic stability is more prominent. In the stability test of up to 144 h, the catalyst has not been tested. Any inactivation occurs.
【学位授予单位】：山东大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O643.36;TQ116.2
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本文编号：1977639