【作者】 李珂；

【导师】 林赫；

【作者基本信息】 上海交通大学 ， 动力工程及工程热物理， 2020， 博士

【摘要】 甲烷是重要的温室气体,为实现温室气体减排目标,必须严格控制甲烷排放。在煤矿通风气和天然气发动机尾气中,存在着严重的甲烷泄漏问题,其排放的甲烷浓度远低于可燃极限,无法通过点燃的方式直接去除。因此,低浓度甲烷的低温去除成为当下研究的热点,具有深远的社会和环境意义。催化燃烧被认为是氧化低浓度甲烷的有效方法,但存在起燃温度高,贵金属用量大等问题。本文将静电场辅助催化技术应用到甲烷的催化氧化过程中,在大幅度降低贵金属催化剂用量的同时实现了低浓度（<1%）甲烷的低温（<300℃）起燃,并通过采用各种表征手法研究静电场对催化剂理化特性的影响,探索静电场协同催化甲烷的作用规律,通过原位实验和化学反应动力学计算的方法对传统反应中和静电场辅助催化系统中甲烷的催化氧化机理进行研究,最后从改进蜂窝陶瓷载体制备工艺和涂覆工艺入手开发了与静电场具有协同效应的整体式催化剂。具体研究内容如下:通过液相燃烧合成法制备了Pd/Co₃O₄系列催化剂,测试了其在传统催化反应与静电场辅助催化系统中的活性,验证了静电场辅助催化系统在低浓度甲烷低温氧化领域的可行性,研究了贵金属负载量,静电场输入参数以及CH₄浓度、O₂浓度、空速、H₂O含量等反应条件对催化剂与静电场协同效应的影响规律,获得了在保证高活性,高抗H₂O中毒能力和低成本下的最优输入电流和贵金属负载量。通过研究静电场对催化剂晶体结构,表面元素价态,微观形貌,还原能力的影响,发现了催化剂载体氧化物在静电场作用下的还原反应以及晶格氧O_L向表面氧O_A的转化过程。研究认为,催化剂载体的储氧性能是影响静电场与催化剂协同效应的关键因素,在此基础上本文通过Ce_mZr_nO₂载体取代Co₃O₄载体来进一步提高催化剂在静电场协同催化系统中的低温氧化能力。研究表明在相同的贵金属负载量下,Ce_mZr_nO₂载体由于比Co₃O₄载体拥有更大的储氧性能,在静电场作用下可以释放出更多的晶格氧,在催化剂表面形成更多的活性中心,从而进一步降低了甲烷的起燃温度。在充分了解静电场对贵金属催化剂的理化结构的影响的基础上,为了进一步降低催化剂成本,采用尖晶石类非贵金属氧化物代替Pd/Co₃O₄和Pd/Ce_mZr_nO₂中的贵金属,获得了在静电场作用下性能优于同等条件下的贵金属催化剂的Mn/Co₃O₄和Co/Ce_mZr_nO₂系列催化剂。通过对催化剂理化特性进行表征,研究了两种非贵金属元素中催化甲烷反应的活性离子,揭示了催化剂中元素比例对静电场协同催化作用的影响规律,获得了催化活性与抗H₂O中毒能力兼备的非贵金属催化剂配方。为了研究静电场协同催化氧化甲烷的反应机理,本文通过in-situ技术,对甲烷在Pd/Co₃O₄,Mn/Co₃O₄,Pd/Ce_mZr_nO₂和Co/Ce_mZr_nO₂催化剂表面的吸附和氧化反应路径进行了实验,分别提出传统催化系统与静电场协同催化系统中甲烷在催化剂表面的氧化反应路径。研究认为,甲烷在活性中心的化学吸附过程为甲烷催化氧化的速控步骤。在传统催化反应中,甲烷的吸附及氧化过程通过E-R机理实现,活性中心的形成与气相氧吸附有关,在静电场协同催化反应中,Mar-van Krevenlen（Mv K）机理主导了甲烷的低温吸附及氧化过程,催化剂体相中氧的活性影响活性中心的形成,使得静电场作用下甲烷的催化反应更容易在低温下进行。通过理论计算,量化了E-R机理与Mv K机理在甲烷催化反应中的贡献度,与in-situ实验得到的结论相互印证。为了进一步将研究成果推向实用化,本文将上述粉末催化剂涂覆于碳化硅蜂窝陶瓷载体上制成整体式催化剂,并通过对碳化硅载体进行配方优化和涂层设计来改善载体的导电性,进而提高静电场与整体式催化剂的协同效应。研究表明,相比于改变载体配方的方法,中间涂层对催化剂与载体协同效应的促进作用更大,且工艺更简单,更能满足实际应用的需求。更多还原

【Abstract】 CH₄ is listed as greenhouse gases and its emission must be controlled to achieve greenhouse gas reduction targets.In the field of natural gas-powered engines and coal mining activity,there is a serious leakage of lean CH₄.The CH₄ concentration is much lower than the flammability limit,making it difficult to be directly ignited.Therefore,the removal of lean methane at low temperature is a hot topic with great social and environmental significance.Catalytic combustion is considered to be an effective method for lean methane oxidation,but the light-off temperature is too high and more noble metal loading is demanded.In this paper,the electric field-assisted catalytic technology was applied on the catalytic oxidation process of CH₄.The introduction of electric field realized low temperature oxidation of lean CH₄and reduced the catalyst cost.Various characterization techniques were performed to investigate the effect of electric field on the physicochemical properties of the catalyst,and the synergetic effect between electric field and catalysis was explored.The in-situ experiment and reaction kinetics calculation were performed to study the catalytic oxidation mechanism in traditional reaction system and electric field-assisted catalytic system,and the oxidation reaction pathway of methane on catalyst surface were proposed in the two catalytic systems.In order to put our work into practical application,the monolithic catalyst that is highly effective in electric field was developed with improved honeycomb ceramic carrier preparation process and coating technology.The main results and conclusions in this work are summarized as follows:Pd/Co₃O₄ series catalysts were prepared by solution combustion synthesis,and their activities in traditional catalytic systems and electric field-assisted catalytic systems were tested.The feasibility of application of electric field assisted catalytic systems on lean methane oxidation at low temperature was investigated.The influence of noble metal loading,electric field input parameters,and reaction conditions such as CH₄ concentration,O₂concentration,space velocity and H₂O content on synergistic effect between catalyst and electric field were investigated,and the optimal input current and precious metal loading were obtained to ensured high activity,high H₂O resistance and low cost.The effect of electric field on crystal structure,surface element valence,micro-morphology and reducing ability of the catalyst were investigated.The reduction of catalyst support oxide under electric field and the conversion process of lattice oxygen O_L to surface oxygen O_A were proved.This research supposed that the oxygen storage capacity of the catalyst support is a key factor affecting the synergetic effect between electric field and catalyst.Based on this point,the Ce_mZr_nO₂ was used to replace Co₃O₄ support to further improve the low-temperature oxidation ability of catalyst in electric field assisted catalytic system.Studies have shown that under the same precious metal loading,Ce_mZr_nO₂ supports have greater oxygen storage capacity than Co₃O₄ supports,and can release more lattice oxygen under electric field,forming more active centers on the catalyst surface,thus further reduces the light-off temperature of methane.On the basis of fully understanding the influence of the electric field on the physical and chemical structure of precious metal catalyst,in order to further reduce the cost of catalyst,spinel-based non-precious metal oxides was used to replace the precious metal in Pd/Ce_mZr_nO₂ to obtain better performance under electric field.Mn/Co₃O₄ and Co/Ce_mZr_nO₂ catalysts which are as active as precious metal-based catalysts under the same conditions in electric field.By characterizing the physical and chemical properties of catalyst,the active element ions that catalyze the methane reaction on these two catalysts were studied,and the effect of the element ratio in the catalyst on the synergistic catalytic action of the electric field was revealed.Finally,the non-precious metal based catalyst with high catalytic activity and H₂O resistance ability were synthesized,which can meet application demands of lean methane oxidation.In order to study the methane oxidation mechanism under electric field,the in-situ technology was conducted to study the adsorption and oxidation reaction pathways of methane on Pd/Co₃O₄,Mn/Co₃O₄,Pd/Ce_mZr_nO₂ and Co/Ce_mZr_nO₂ catalyst surfaces.This research supposed that the chemisorption process of methane on active center is the rate-determining step for the catalytic oxidation of methane.In the traditional catalytic reaction,the low-temperature methane adsorption and oxidation proceeded according to E-R mechanism.And the formation of active center is mainly related to the adsorption of gas phase oxygen.In the electric field-assisted catalytic reaction system,the Mar-van Krevenlen（Mv K）mechanism dominates the methane adsorption and oxidation process at low temperature,and the activity of oxygen species in the catalyst bulk was involved in formation of active center.This is the key factor making the oxidation of methane occur at lower temperature under electric field.Based on reaction kinetics experiment,the contributions of E-R mechanism and Mv K mechanism to the methane catalytic reaction were quantified,and the conclusions obtained from the in-situ experiments were mutually confirmed.In order to put our work into practical application,the monolith catalyst was developed by coating catalyst powders on a Si C honeycomb ceramic support.Formulation optimization and coating design were applied to improve the conductivity of the Si C support,thereby improving the synergistic effect between electric field and monolith catalyst.The results show that compared with formulation optimization,the monolith catalyst with intermediate coating exhibit higher active,simpler preparation process and can better meet the needs of practical applications.更多还原