Ni基催化剂上乙酸蒸汽重整反应机理研究

发布时间：2018-07-05 02:20

本文选题：Ni基催化剂 + 乙酸蒸汽重整　；参考：《太原理工大学》2017年硕士论文

【摘要】：可再生的生物质可以通过快速热解将其高效转化为高能量密度的生物油,产率高达80%。由于生物油具有含水量高、含氧量高、酸性较强等特点,其直接利用受到限制,故需要对生物油进行加工利用。生物油的利用方式主要包括生物油蒸汽重整和生物油加氢提质两大工艺。生物油蒸汽重整工艺可以充分利用水相组分中的水分制取氢气,并为油相组分加氢提质工艺提供氢源,是一项极具前景的制氢工艺。Ni基催化剂成本低廉,被广泛应用于生物油蒸汽重整研究中,然而,积碳导致催化剂失活是该工艺的主要问题。由于生物油组分极其复杂,通过实验研究很难对生物油蒸汽重整反应过程及积碳的本质有全面的认识,而选用模型化合物是深入研究复杂生物油蒸汽重整反应机理的有效途径,特别是积碳形成机理。生物油水相组分以有机酸类化合物为主,而乙酸是酸类化合物中的主要组分,同时,CH_3COOH分子包含丰富的化学键,包括C-C、C-O、C=O、C-H及O-H键。因此,本研究选用乙酸作为生物油水相组分的模型化合物。通常,乙酸蒸汽重整反应过程可以分为乙酸分解反应和水煤气变换反应。采用密度泛函理论的计算方法,充分考察了Ni(111)表面上乙酸解离过程中各类可能的反应路径,并对解离过程中重要中间体丙酮和乙醛的生成机理进行研究;探究了水的解离机理、水对积碳消除的影响和水煤气变换反应机理;此外,考虑到重整过程中水的参与,详细研究了水解离产生的氧物种(O*/OH*)和水自身的溶剂效应对乙酸蒸汽重整过程中关键脱氢反应的影响,选用H2O*、CH_3COOH*、trans-COOH*和CH_3CO*、CH3C*、CH_2C*的脱氢过程分别代表O-H键和C-H键断裂反应,在此基础上考察了乙酸的溶剂效应对O-H键断裂反应的影响。通过对以上工作的研究,阐明了乙酸蒸汽重整反应机理和速控步骤,明确了积碳形成的根本原因。主要结果和结论如下:1.乙酸在Ni(111)表面上的重整反应路径为CH_3COOH*→CH_3COO*→CH_3CO*→CH_2CO*→CH_2*+CO*→CH*→CHOH*→CHO*→CO*,进一步发生水煤气变换反应生成CO2和H2,其中,CH*是乙酸深度解离产生的重要积碳前驱体物种,水解离产生的主要组分OH*是消除CH*的活性物种,解离速控步骤为CH_3CO*脱氢生成CH_2CO*的反应,反应活化能为1.33 e V,而水煤气变换反应的速控步骤为CO*+OH*→cis-COOH*,反应活化能为1.85 e V,高于乙酸解离能垒,成为乙酸蒸汽重整反应的整体速控步骤。2.乙酸蒸汽重整过程中重要中间体丙酮的生成机理为间接生成路径:CH_3CO*+CH*→CH_3COCH*→CH_3COCH_2*→CH_3COCH3*,而非直接生成路径:CH_3CO*+CH3*→CH_3COCH3*;乙醛很容易由CH_3CO*加氢一步生成(Ea=0.49 e V),且在Ni表面吸附较弱而易脱附,而其易分解的性质是实验中不易检测到的主要原因。3.水解离产生的O*和OH*物种可参与到脱氢反应中分别生成OH*和H2O*,结果表明,O*和OH*均可以促进O-H键断裂反应,且O*的促进作用更为显著;而对于C-H键断裂反应,除Ni表面富集的OH*物种可以降低乙酸解离速控步骤(CH_3CO*的脱氢反应)外,O*和OH*均表现为抑制作用。4.水分子作为一种“溶剂”,其溶剂效应对乙酸蒸汽重整过程中的关键脱氢反应有重要影响。研究表明,预吸附的水分子并非真正参与到反应中,而是与含氧物种发生氢键相互作用,进而提高了反应物种的吸附稳定性,并降低了O-H键断裂反应能垒,而对C-H键断裂反应无明显影响。同时,乙酸的溶剂效应可以促进O-H键断裂反应。
[Abstract]:Renewable biomass can be converted into high energy density bio oil by rapid pyrolysis. The yield is up to 80%. due to the high water content, high oxygen content and strong acidity of bio oil. The direct utilization of bio oil is limited. Therefore, bio oil is mainly used for bio oil steam. The two process of reforming and bio oil hydrogenation can be used. The steam reforming process of bio oil can make full use of the water in the aqueous phase components to produce hydrogen and provide hydrogen source for the oil phase component hydrogenation process. It is a promising hydrogen production process with low cost and is widely used in the study of bio oil steam reforming. However, the product is widely used in the study of the steam reforming of bio oil. The deactivation of carbon catalyst is the main problem in this process. Because the bio oil component is extremely complex, it is difficult to understand the process of steam reforming reaction and the nature of carbon deposition through experimental research. The selection of model compounds is an effective way to further study the mechanism of the steam reforming reaction of complex bio oil, especially the formation of carbon. The main components of the bio oil and water phase are organic acids, and acetic acid is the main component of the acid compound. At the same time, the CH_3COOH molecule contains rich chemical bonds, including C-C, C-O, C=O, C-H and O-H bonds. Therefore, acetic acid is used as a model compound of the biologic oil phase component. It is divided into the acetic acid decomposition reaction and the water gas conversion reaction. Using the density functional theory, the possible reaction paths in the process of acetic acid dissociation on the surface of Ni (111) are fully investigated, and the formation mechanism of acetone and acetaldehyde, an important intermediate in the process of dissociation, is studied, and the dissociation mechanism of water is explored and the water is eliminated. In addition, considering the participation of water in the reforming process, the influence of the oxygen species (O*/OH*) and the solvent effect of water on the critical dehydrogenation of acetic acid during the steam reforming process is studied in detail. The dehydrogenation process of H2O*, CH_3COOH*, trans-COOH* and CH_3CO*, CH3C* and CH_2C* represents O, respectively. On the basis of the fracture reaction of -H and C-H bonds, the effect of the solvent effect of acetic acid on the fracture reaction of the O-H bond is investigated. Through the study of the above work, the mechanism and the speed control steps of the steam reforming of acetic acid are clarified, and the fundamental reasons for the formation of the carbon deposition are clarified. The main results and conclusions are as follows: 1. the reaction path of the reforming of acetic acid on the surface of Ni (111) The diameter is CH_3COOH*, CH_3COO*, CH_3CO*, CH_2CO*, CH_2*+CO*, CH*, CHOH*, CHO*, and CO*, and CO2 and H2 are generated by the water gas transformation reaction. In this case, CH* is an important carbon precursor species produced by the depth dissociation of acetic acid. The reaction of hydrogen generated CH_2CO*, the reaction activation energy is 1.33 e V, and the speed control step of the water gas conversion reaction is CO*+OH* to cis-COOH*, the activation energy is 1.85 e V, which is higher than the dissociation energy barrier of acetic acid, and it becomes the integral speed control step of the steam reforming reaction of acetic acid. The formation mechanism of acetone, an important intermediate in the steam reforming process of.2. acetic acid, is indirectly generated. The path: CH_3CO*+CH*, CH_3COCH*, CH_3COCH_2* to CH_3COCH3*, and not the direct generation path: CH_3CO*+CH3* to CH_3COCH3*; acetaldehyde is easily generated by CH_3CO* hydrogenation (Ea=0.49 e V), and is weak on Ni surface and easy to degenerate, and its easy decomposition is the main reason that is not easy to detect in the experiment. * species can participate in the generation of OH* and H2O* respectively in dehydrogenation. The results show that both O* and OH* can promote the O-H bond fracture reaction, and the promotion of O* is more significant. For the C-H bond fracture reaction, the OH* species enriched by the Ni surface can reduce the rate of acetic acid dissociation (CH_3CO* dehydrogenation reaction), and both O* and OH* are inhibited. As a "solvent", the solvent effect of 4. water molecules has an important effect on the key dehydrogenation in the process of steam reforming of acetic acid. The study shows that the pre adsorbed water molecules do not really participate in the reaction, but interact with the oxygen containing species in hydrogen bonds, thus improving the adsorption stability of the counter species and reducing the O-H bond fracture. The reaction energy barrier has no obvious effect on the C-H bond cleavage reaction. Meanwhile, the solvent effect of acetic acid can promote the O-H bond cleavage reaction.
【学位授予单位】：太原理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TE667;O621.251

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