【作者】 刘红艳；

【导师】 谢克昌； 王宝俊；

【作者基本信息】 太原理工大学 ， 化学工程与技术， 2012， 博士

【摘要】 Ni基催化剂作用下的CH4/CO2重整是煤化工过程中的一个重要反应。Ni催化剂由于低廉的价格、良好的活性和选择性成为应用和研究的重点,但也存在积碳严重的明显缺点。使用负载型Ni基双金属催化剂实现对反应机理的调控,是解决积碳问题的可行途径。本文主要采用量子化学DFT理论计算的方法,研究了积碳生成的微观动力学,在电子水平上描述了积碳问题的微观原因,描述了催化剂的成分和表面结构等主要因素与抗积碳性能的构效关系；建立了简化金属-载体相互作用的电子气模型,描述了载体对催化剂活性的影响；针对Ni基催化剂的积碳问题,本文阐述了解决Ni基催化剂上积碳问题的方法,为筛选、改性和设计新型双金属催化剂提供了一些较为可靠的理论线索。本文提出了从积碳的抑制、消除和阻止等方面多途径解决积碳问题的思路和方法,构建了一系列单金属(Fe,Co,Ni,Cu)及NiM(M=Fe,Co,Cu)双金属合金催化剂模型,基于这些模型研究了CH4/CO2重整反应体系的积碳问题,得到了与实验相吻合的结果。构建了物种在催化剂表面的迁移模型,详细描述了原子O和热解C在催化剂表面的迁移过程,明确了在Ni基催化剂中添加第二种金属以增加原子O的迁移能力,从而消除热解C的方法在理论上不具备可行性,也明确了在Ni催化剂中添加第二种金属以降低原子C的迁移能力,从而阻止热解C集聚的方法在理论上具备可行性。详细分析了催化剂上CH4解离路径,明确了各类催化剂上的速控步骤和活化能变化的规律。详细分析了催化剂表面热解C消除反应机理,明确了不同的催化剂表面C+O生成CO反应的微观机理。详细分析了热解C集聚的微观反应机理,明确了热解C的集聚反应在不同催化剂表面的难易程度。提出了基于金属-载体有相互作用的负载型催化剂的电子气模型,明确了通过使电子从金属流向载体的微观因素来调变金属-载体间的相互作用进而抑制积碳形成的基本原理。综合分析了各活性组分表面有关积碳问题的各步反应,明确了当CH4解离时决速步骤的活化能比在Ni表面解离的活化能(1.36eV)提高15-50%时,CH4在催化剂表面解离的速率得到了适度的抑制,可以实现对热解C生成的抑制进而抑制积碳的形成,通过比较得出Fe, Ni, NiFe, NiCo和均相的NiCu表面容易形成积碳,而偏聚的NiCu表面不容易产生积碳。综合分析并提出了抗积碳性能催化剂的理论线索,明确了催化剂金属d带中心远离Fermi能级,是抑制反应体系中热解C生成的微观因素,明确了通过调变载体结构使电子从金属流向载体,即可实现提高催化剂抗积碳性能的目的,明确了添加不同的金属可以实现阻止热解C在表而的迁移进而阻止其聚集生成积碳的方法。更多还原

【Abstract】 CH4/CO2reforming catalyzed by supported Ni-based catalysts is a key reaction in the process of coal chemical engineering. Because of its good initial activity and low cost, Ni-based catalysts have been widely applied and investigated. However, a fatal shortcoming exists for Ni catalyst, i.e., the serious carbon deposition. Therefore, to solve the problem of carbon deposition using supported Ni-based bimetal catalysts, it is necessary to systematically investigate the formation mechansim of carbon deposition. In this study, based on quantum chemical DFT calculations combined with experimental characterization methods, all possible mechanisms of carbon deposition formation are firstly proposed and disscussed in order to illustrate micro-dynamic of carbon deposition formation at the electronic level, the structure-activity relationship of the catalysts is elucidated, and the simplified electron-gas model of metal-support interaction is proposed to clarify the effect of support on the activity of catalyst. Focusing on the problem of carbon deposition on Ni-based catalysts, the methods of solving the carbon deposition are proposed, which can provide the basic theoretical clues for filtration, modification and design of the new supported catalysts. In this work, the ideas to solve carbon deposition from muti-path view, i.e., suppressing, eliminating and inhibiting carbon deposition, were proposed, and a series of models including of single metal(Fe,Co,Ni,Cu) and NiM(M=Fe,Co,Cu) bimetallic alloys were built. Based on the models, the carbon deposition were investigated in CH4/CO2reforming, and the results are in line with those from experimental observation. At the same time, the models of species migration on the surfaces also were built, on which the process of migration of C and O were investigated. The results show that elimination of carbon deposition by increasing the migration ability of O is found to be infeasible in theory. Whereas, that by decrease the migration ability of C is found to be feasible. Then, the dissociation pathway of CH4on catalysts are systematically investigated to illustrate the change rule of rate-determining and activation barrier, meanwhile, the mechanism of pyrolytic C elimination is analyzed to obtain the microscopic mechanism of C plus O reaction. Further, the mechanism of pyrolytic C accumulation are analyzed to clarify the reaction on different catalyst surfaces.On the basis of the metal-support interaction, the electron-gas model of supported catalyst are provided to adjust the metal-support interaction by making the electron transfer from metal to support, in which the basic principle of inhibiting the carbon deposition are obtained. The elementary reaction involving carbon deposition on different catalysts are synthetically analyzed, which show that when the activation barrier (1.36eV) of rate-determining step for CH4dissociation increase by15-50%in comparison with that on Ni surface, the formation of pyrolytic C can be inhibited in the expense of decreasing the rate of CH4dissociation to some extent, further the formation of carbon deposition is inhibited.Our results show that the carbon deposition is easily formed on Fe, Ni, NiFe, NiCo and uniform NiCu surfaces, whereas, the carbon deposition is hard to form on segregate NiCu surface. As a result, the theoretical clues for the design of anti-carbon deposition are provided, in which the d-band center away from Fermi level is the microscopic factor to inhibit the formation of pyrolytic C in reaction systems. Meanwhile, the electrons transfer from metal to support by adjusting the metal-support interaction can be used to realize the inhibitation of carbon deposition. Finally, adding different metal into Ni can inhibit the migration of pyrolytic C, further inhibit the accumulation of pyrolytic C to form carbon deposition.更多还原