溶液燃烧合成Ni基催化剂结构调控及催化甲烷化性能研究

发布时间：2017-12-27 15:27

  本文关键词：溶液燃烧合成Ni基催化剂结构调控及催化甲烷化性能研究　出处：《太原理工大学》2017年博士论文　论文类型：学位论文

  更多相关文章： 煤制天然气 CO甲烷化 Ni基催化剂 溶液燃烧法 燃料

【摘要】：近年来,煤炭的直接燃烧利用排放了大量的SO_x、NO_x及VOC等污染物,由此引发了一系列严重的环境污染问题。可持续发展,环境友好型发展成为我国当下迫在眉睫、亟待解决的难题之一。相比之下,煤炭经过气化、净化、变换和甲烷化等过程转化为清洁燃料甲烷(煤制天然气),能够将污染物净化脱除,是煤炭清洁高效利用的重要途径之一。甲烷化反应是煤制天然气转化技术的核心工艺,其中甲烷化催化剂制备及其催化活性、选择性和寿命是该工艺过程的关键因素,决定了催化反应工艺,以及上游和下游相关工艺的配套和设计。目前,工业甲烷化技术通常使用固定床反应器,鉴于甲烷化反应强放热的特点,采用低温浆态床反应器更有利于反应热的传递,提高反应效率。然而,常规的浸渍法或沉淀法制备得到的Ni基催化剂具有颗粒尺度较大、分散不均等结构缺陷,在浆态床反应工艺中活性低,稳定性差,易于失活。溶液燃烧法是一种快速、高效的合成方法,可用于合成各种类型的纳米材料,其所制备的材料具有颗粒尺度小、形貌规则及分散均匀等优点,是制备催化材料较为新颖的技术,应用前景广泛。基于此,本文重点考察了微波辅助溶液燃烧合成Ni基催化剂过程中燃料种类、燃料加入量、燃料添加剂及可溶惰性盐对前驱体溶液、燃烧热、气体释放量等反应过程因素的影响,并和催化剂微观结构进行了有机关联。通过研究催化剂织构性质以及活性物种Ni表面形态、尺寸、化学结构等因素,建立了其对催化甲烷化反应的构效关系。具体研究结论如下:(1)在溶液燃烧合成Ni-Al_2O_3催化剂过程中考察了尿素、甘氨酸、乙二醇和柠檬酸四种燃料,研究发现燃料类型的不同对燃烧过程中产生的热量、气体量、燃烧强度以及持续时间具有显著的影响。其中,尿素和Ni(NO3)2以及Al(NO3)3在加热过程中几乎同时热分解,从而引发了温和、持续的燃烧反应,有利于热量均匀散逸进而避免Ni颗粒高温烧结。另外,以尿素为燃料的体系产生的热量最少,气体释放量最多。表征发现,以尿素为燃料制备的催化剂中,Ni颗粒高度分散且平均晶粒尺寸最小(9.2 nm),其BET比表面积和Ni金属比表面积分别达到了256.2 m2/g和34.6 m2/g,此外还形成了更多易于还原的Ni物种。其在浆态床CO甲烷化反应中,温度300°C,压力1.0 MPa,空速3000 m L/gcat·h条件下的CO转化率和CH4选择性分别达到了95.7%和96.2%,并且在较高空速8000 m L/gcat·h条件下反应300 h未失活,比其它燃料制备的催化剂及商业催化剂具有更好的活性和稳定性。(2)尿素有利于促进和稳定Ni~(2+)的分散,提高催化剂中Ni的分散。在尿素与硝酸盐形成的溶液中,尿素分子和Ni~(2+)离子络合形成镍胺络合物,形成原子分散的前驱体,促进了燃烧过程中高分散Ni物种的形成,且随着尿素加入量的增加,提高了络合程度,更有利于分散。在溶液燃烧过程中,燃烧产生的热量和释放的气体对催化剂性质的控制呈现为协同、竞争关系。具体而言,当RV/OV(还原价态/氧化价态)≤0.75时,释放气体产生的驱散作用对形成高比表面积来分散Ni纳米颗粒起了决定性的作用;而热效应随着RV/OV值的增加而增长,当RV/OV≥0.75时,较高的燃烧温度促进了Ni O迁移进入Al_2O_3体相中以至于生成了低活性前驱体Ni Al2O4尖晶石;当RV/OV=0.75时,释放气体和反应热促使燃烧过程形成了的催化剂具有最大的Ni金属比表面积(62.6 m2/g)和最小的Ni颗粒尺寸(10.8 nm),其CO转化率和CH4选择性在温度280°C,压力1.0 MPa及空速3000 m L/gcat·h条件下分别达到94.5%和91.3%。(3)燃料添加剂能够调节燃烧过程,影响催化剂的结构和催化活性。在尿素和硝酸盐以最优比例混合形成的燃烧溶液中,分别加入醋酸铵、淀粉和聚乙二醇(PEG4000)三种燃料添加剂。结果表明,醋酸铵较强的络合能力提高了Ni~(2+)离子的络合程度,进而促进了Ni物种在燃烧过程中分散。在前驱体溶液的燃烧过程中,含有 NH_2基团的醋酸铵比含有 OH基团的淀粉和PEG更具活性,加入醋酸铵的燃烧体系反应也更为迅速。另外,醋酸铵、尿素和硝酸铝在宽泛的温度区间内同时热分解产生气相产物,使得燃烧在理论上有无数多的起火点,使得燃烧的引发更为均匀。相比之下,在淀粉和PEG加入的体系中几乎不存在反应物同时分解的情况,所形成的非均相燃烧反应主要发生在气固相,少量的起火点使得火焰蔓延不均匀。另一方面,醋酸铵的分解是吸热反应,使得其可以像“灭火器”一样降低燃烧热。而淀粉和PEG在加热过程中放热,进一步加剧了燃烧的剧烈程度。表征发现,在加入醋酸铵的燃烧体系中形成的Ni O颗粒尺寸较小且高度分散,催化剂在浆态相CO甲烷化反应中表现出较好的活性和稳定性。而在淀粉和PEG加入的燃烧体系中,Ni O颗粒较大并高度团聚,且体系中较高的燃烧温度使得Ni O和Al_2O_3载体的相互作用力较强以致形成了Ni Al2O4尖晶石,导致其活性和稳定性均较差。(4)在尿素和硝酸盐混合而成的前驱体溶液中引入五种不参与反应的可溶惰性盐(Li Cl、Na Cl、KCl、Mg Cl2及Ca Cl2)。热力学研究表明,加热过程中Na Cl的融化效应吸收了大量的燃烧热,使得相应体系的燃烧温度最低。且由于Na Cl的熔点和体系的燃烧温度接近,因而熔融的Na Cl在同步生成的Ni O颗粒表面形成了包覆层,极大程度上抑制了Ni O颗粒的生长和团聚。另一方面,微波加热和传统电加热两种模式下加热环境的差别很大程度上影响了燃烧过程中Ni O颗粒的生长和迁移。采用微波加热且加入Na Cl惰性盐制备的催化剂具有最小的Ni晶粒尺寸(7.8 nm)、最大的BET比表面积和Ni金属比表面积(分别为416.5 m2/g和52.7 m2/g),其CO转化率在310°C、1.0 MPa和9200 m L/gcat·h的苛刻条件下达到87%,100h内未见失活。
[Abstract]:In recent years, a large number of pollutants such as SO_x, NO_x and VOC have been discharged from the direct combustion of coal, which has caused a series of serious environmental pollution problems. Sustainable development and environmental friendly development have become one of the most urgent and urgent problems to be solved in our country. In contrast, coal is transformed into clean fuels, such as methane, coal and natural gas through gasification, purification, transformation and methanation. It can remove pollutants and is an important way for clean and efficient utilization of coal. Methanation is the core technology of coal to natural gas conversion technology. Methanation catalyst preparation and its catalytic activity, selectivity and lifetime are the key factors in the process, which determines the catalytic reaction process and the matching and design of upstream and downstream related processes. At present, industrial methanation technology usually uses fixed bed reactor. In view of the strong exothermic characteristics of methanation reaction, the low temperature slurry bed reactor is more conducive to the transfer of reaction heat and the efficiency of reaction. However, the Ni based catalyst prepared by conventional impregnation or precipitation method has large particle size and uneven dispersion structure. It has low activity and poor stability in slurry reactor technology, and it is easy to deactivate. Solution combustion method is a fast and efficient synthetic method. It can be used for the synthesis of various kinds of nanomaterials. The prepared materials have the advantages of small particle size, regular morphology and uniform dispersion, which is a relatively new technology for preparing catalytic materials, and has wide application prospects. Based on this, this paper investigated the effect of microwave assisted solution combustion synthesis of Ni catalyst in the process of fuel type, fuel quantity, fuel additives and soluble inert salt of precursor solution, heat of combustion, emissions and other factors of the reaction process, and the microstructure of catalyst and organic association. The structure-activity relationship of catalytic methanation was studied by studying the texture characteristics of Ni and the surface morphology, size and chemical structure of active species. The specific conclusions are as follows: (1) in the solution combustion synthesis of Ni-Al_2O_3 catalyst was investigated in urea, glycine, ethylene glycol and citric acid of four kinds of fuel, found that the different types of fuel combustion heat, gas volume, combustion intensity and duration has significant effect. Among them, urea and Ni (NO3) 2 and Al (NO3) 3 in the process of heating at almost the same time thermal decomposition, causing a mild and sustained combustion reaction, is conducive to uniform heat dissipation and avoid high temperature sintering of Ni particles. In addition, the system that uses urea as the fuel produces the least heat and the largest amount of gas released. It was found that the Ni particles were highly dispersed and the average grain size was the smallest (9.2 nm), and the BET specific surface area and Ni Ni specific surface area reached 256.2 m2/g and 34.6 m2/g, respectively. The slurry bed CO methanation reaction, temperature 300 C, pressure 1 MPa, 3000 m L/gcat h velocity under the condition of CO conversion and CH4 selectivity reached 95.7% and 96.2%, and 300 h was not inactivated in response to higher space velocity of 8000 m under the condition of H L/gcat, the catalyst than other fuels preparation and commercial catalyst has better activity and stability. (2) urea is beneficial to promote and stabilize the dispersion of Ni~ (2+) and improve the dispersion of Ni in the catalyst. The solution formed in the urea and nitrate, urea molecules and Ni~ (2+) ion complex to form nickel amine complex, forming precursor atomic dispersion, promoted the formation of highly dispersed Ni species in the combustion process, and with the increase of the amount of urea, improves the degree of complexation, more conducive to the spread of. In the process of the combustion of the solution, the heat generated by the combustion and the release of the gas on the nature of the catalyst show a synergistic and competitive relationship. Specifically, when RV/OV (reduced valence / oxidation state is less than or equal to 0.75), the release of gas generated disperse effect on the formation of dispersed Ni nanoparticles plays a decisive role in high surface area and thermal effect; with the increase of RV/OV value and growth, when RV/OV is larger than 0.75, higher combustion temperature promoted Ni O migration into the Al_2O_3 body phase that generates low active precursor Ni Al2O4 spinel; when RV/OV=0.75, catalyst and reaction heat to release the gas combustion process has formed the largest surface area of the metal Ni (62.6 m2/g) and the minimum Ni grain size (10.8 nm), the conversion rate of CO and CH4 in the selective temperature 280 C, pressure 1 MPa and 3000 m L/gcat - H space velocity conditions were respectively 94.5% and 91.3%. (3) the fuel additive can regulate the combustion process and influence the structure and catalytic activity of the catalyst. Three kinds of fuel additives, ammonium acetate, starch and polyethylene glycol (PEG4000), were added in the mixture of urea and nitrate in the optimal proportion of combustion solutions. The results show that the stronger complexing ability of ammonium acetate improves the complexation degree of Ni~ (2+) ions, and thus promotes the dispersion of Ni species during the combustion process. In the combustion process of precursor solution, ammonium acetate containing NH_2 group is more active than starch and PEG containing OH group, and the combustion system adding ammonium acetate also has more rapid reaction. In addition, ammonium acetate, urea and aluminum nitrate are thermally decomposed at the same time in the wide temperature range to produce gaseous products, which makes combustion theoretically have numerous ignition points, making combustion more uniform. In contrast, in the system of starch and PEG addition, there is almost no simultaneous decomposition of reactants. The heterogeneous combustion reaction mainly occurs in gas-solid phase, and a small number of ignition points make the flame spread unevenly. On the other hand, the decomposition of ammonium acetate is a endothermic reaction, making it like a "fire extinguisher" to reduce the burning heat. When the starch and PEG are heated during the heating process, the intensity of the burning is further aggravated. surface
【学位授予单位】：太原理工大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O643.36;TQ546

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