Mechansim Understanding of NOx storage, release and reduction on Pt/doped ceria catalysts

Mechansim 了解 Pt/掺杂二氧化铈催化剂上 NOx 的储存、释放和还原

• 批准号: 2280885
• 金额: --
• 依托单位: Queen's University Belfast
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2019
• 资助国家: 英国
• 起止时间: 2019 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2280885
• 关键词: Mechansim; Understanding; NOx; storage; release

Due to the strengthening of emission legislation both in Europe and North America, there is a need for further optimisation of existing emission after-treatment catalytic converters for automotive applications. High surface area ceria is successfully employed as excellent support of metals (Pd, Ru, Pt, etc.) in commercial catalytic systems for the oxidations of CO and propane and automotive emission control.Ceria is a unique material with a rich and complex chemistry which shows marked structure sensitive properties than can be assessed through shape-controlled synthesis. It possesses the high oxygen storage capacity (OSC), a unique redox property by the cycle of Ce4+/Ce3+ redox pairs and it can increase the migration of lattice oxygen. The catalytic activity of ceria can be further enhanced by the use of dopants.Among all, Pt doped ceria catalysts show an enhanced NOx storage at low temperature, together with an improved CO/HC light off. It is not clear whether this is due to the presence of higher number of active sites (dopants create a more favourable environment for NOx storage), same number of sites but intrinsically more active, or enhanced NO oxidation activity (rate determining step).It has been proposed that the dopants increase the concentration of surface vacancies which affect the ionic conductivity, oxygen mobility and oxygen storage capacity of the CeO2. It can be speculated that all these properties are responsible for the enhanced oxidation activity (CO, HC, NO) by promoting oxygen diffusion and the formation of more "reactive oxygen" species. Moreover, the oxygen vacancies could play a role in the mechanism of the reaction, favoring the NOx storage.It has also been found that the purge efficiency of doped systems is lower compared to the un-doped catalyst. This is related to the different surface intermediates species observed during preliminary NOx storage IR experiments. However, it is unknown how this relates to the presence of dopants (modification of the rate of the redox reaction, different electronic environment which allows stronger NOx adsorption, ...).Additionally, presence of dopants modifies the Pt reducibility and Pt - CeO2 interaction, allowing more readily activation during rich purge and increased stability to lean deactivation. This is related to:Improved reducibility after a rich activation treatmentPt is not fully re-oxidised after the lean treatment and can be easily re-activated when exposed to the reductant present in the gas feed againFurther characterisation is necessary to probe the enhanced promoting effect, especially after the reducing pre-treatment and greater stability to oxidising condition.Aim of project/expected outcome:1. Improve the understanding of Pt doped ceria NOx storage mechanism together with the mechanism of rich purge.2. Gain more knowledge of the rich activation/lean deactivation mechanisms.3. Determine the structure of the active sites under reaction conditions and differentiate between active species and spectators by transient methods.4. Development of a global kinetic model.

由于欧洲和北美排放法规的加强，需要进一步优化现有的汽车排放后处理催化转化器。高比表面积的氧化铈被成功地用作金属（Pd、Ru、Pt等）的优良载体。氧化铈是一种独特的材料，具有丰富而复杂的化学性质，与通过形状控制合成评估的材料相比，它显示出显著的结构敏感性。它具有高储氧能力（OSC），通过Ce 4 +/Ce 3+氧化还原对的循环而具有独特的氧化还原性能，可以增加晶格氧的迁移。掺杂可以进一步提高CeO 2的催化活性，其中Pt掺杂CeO 2催化剂在低温下表现出更好的NOx储存和CO/HC起燃性能。目前尚不清楚这是否是由于存在更多的活性位点（掺杂剂创造了更有利的NOx储存环境），相同数量的位点但本质上更活性，或增强的NO氧化活性（速率决定步骤）。可以推测，所有这些性质是负责增强氧化活性（CO，HC，NO），通过促进氧扩散和形成更多的“活性氧”物种。此外，氧空位可能在反应机理中发挥作用，有利于NOx的储存。还发现掺杂体系的净化效率低于未掺杂催化剂。这与在初步NOx存储IR实验期间观察到的不同表面中间体物种有关。然而，尚不清楚这与掺杂剂的存在如何相关（氧化还原反应速率的改变、允许更强的NOx吸附的不同电子环境等）。此外，掺杂剂的存在改变了Pt还原性和Pt-CeO 2相互作用，允许在富燃吹扫期间更容易活化，并增加了对贫燃失活的稳定性。这与以下方面有关：在富活化处理后改善的还原性在贫活化处理后Pt没有完全再氧化，当再次暴露于气体进料中存在的还原剂时，可以很容易地再活化需要进一步表征以探测增强的促进作用，特别是在还原预处理后和对氧化条件的更大稳定性。项目目的/预期结果：1.提高了对Pt掺杂氧化铈NOx存储机理以及富气净化机理的认识.获得更多关于富启动/贫停用机制的知识。3.确定反应条件下活性中心的结构，并通过瞬态方法区分活性物种和旁观者.建立全球动力学模型。