Collaborative Research: Development of New Heterogeneous Catalysts for NOx Storage and Reduction (NSR)

当前最大的挑战之一是有效和公平地使用化石燃料。需要保护有限的石油资源的后果之一是向更高效的柴油发动机和低燃烧汽油发动机的过渡。由于高氧化性条件，与转向这些精益燃烧发动机相关的是氮氧化物减排的难度增加，这导致需要改进传统的三向排气催化剂。NOx存储还原（NSR）催化剂已经成为解决这一问题的最成功方法。这些系统需要在两种不同的制度下运作。在氧化条件下，NO与贵金属组分（通常是Pt）反应生成NO2， NO2随后可以吸附在储存剂（通常是氧化钡）上。然后，在第二个较短的循环中，注入还原剂（例如H2、CO、C3H6），催化剂释放的NOx反应生成N2。虽然这种方法已经被丰田公司商业化，但几个关键的科学问题仍未解决，需要进一步改进。这项研究将研究开发新型非均相催化剂的策略，包括双金属，有望改善低温（冷启动）氮氧化物储存，并更有效地利用还原剂。据推测，催化剂的主要作用之一是在不使用氢的情况下将还原剂转化为氢。反过来，氢可以有效地减少催化剂释放的NOx，更重要的是，通过减少任何积累的含硫表面物质来再生储存成分。由于其他还原剂比氢气更容易在NSR系统中使用，因此通过CO和H2O水气转换（WGS）反应或通过重整从碳氢化合物中高效生产氢气对NSR催化剂的性能至关重要。因此，除了Pt之外，还可以使用第二种金属，如Cu（促进WGS）或Ru（促进HC重整和低温NO氧化）。此外，Pd和Pd双金属合金将与Pt和Pt合金催化剂进行比较，因为最近的证据表明，Pd在低温氧化方面优于Pt和Pt合金催化剂，并且对N2（相对于N2O）的选择性也有所提高。这项工作的重要性有两方面。首先，这项工作的目的是获得对非常复杂但可获得的双金属系统所涉及的催化机制的基本理解。由此获得的信息可用于涉及双金属催化剂的各种其他应用。其次，新型改良催化剂的开发对于提高燃油效率的精益燃烧发动机的环保性能至关重要。这项工作将在实验和理论之间的协作努力中进行，利用多种技术。将合成双金属合金催化剂，并使用EXAFS， XPS和IR光谱对其进行广泛的表征。在对催化剂进行表征后，将在实际操作条件下使用传统微反应器和TAP反应器进行测试，以识别重要的中间体，从而提高对机理的理解，从而帮助设计新的催化剂。密度泛函理论计算将有助于提供反应机理的完整图像，并确定氧化和还原化学中的速率限制步骤。新的双金属催化剂将在一个迭代的过程中开发，该过程结合了所需的特征，以满足在广泛的条件和环境下对性能的相互冲突的要求。更广泛的影响新型NSR催化剂的开发可以对精益燃烧车辆的商业部署产生重大影响，具有明显的环境和社会效益。从教育的角度来看，这项研究的跨学科性质将用于培养未来的科学家和研究人员，使他们具备采用多方面方法进行催化剂设计所需的技能。这个项目将为参与的研究生提供一个独特的机会，因为他们不仅可以学习到自己实验室能力之外的各种技术，还可以接触到其他机构的研究环境。在主要研究型大学中，这三所机构在招收代表性不足的少数族裔方面处于领先地位，我们打算使用REU补充课程，目的是让少数族裔本科生从事研究工作。这项研究将通过期刊出版物和会议记录进行传播。

Randall Meyer0730937One of the great challenges of at this time is the efficient and equitable consumption of fossil fuels. One of the consequences of the need to conserve finite petroleum resources is a transition to more efficient diesel engines and lean burning gasoline engines. Associated with the switch to these lean burning engines is an increased difficulty of NOx abatement due to the highly oxidizing conditions which results in a need to improve conventional three-way exhaust catalysts. The NOx storage reduction (NSR) catalysts have emerged as the most successful approach to combat this problem. These systems are required to operate under two distinct regimes. Under oxidizing conditions, NO reacts over a noble metal component (usually Pt) to form NO2 which can subsequently adsorb on the storage agent (usually barium oxide). Then, during a second shorter cycle, an injection of a reducing agent (e.g. H2, CO, C3H6) takes place and NOx released by the catalyst reacts to produce N2. Although this approach has been already commercialized by Toyota, several key scientific issues remain unresolved and the need exists for further improvement. This research will examine strategies to develop new heterogeneous catalysts involving bimetallics, expected to improve the low temperature (cold start) NOx storage and to more efficiently use the reducing agent. It has been speculated that one of the primary roles of the catalyst is the conversion of the reducing agent to hydrogen in cases where hydrogen is not used. Hydrogen in turn can effectively reduce the NOx released by the catalyst and more importantly, regenerate the storage component by reducing any accumulated sulfur containing surface species. Since, other reducing agents are easier to implement in NSR systems then hydrogen, the efficient production of hydrogen either from CO and H2O Water-Gas Shift (WGS) reaction or from hydrocarbons by reforming is essential to the performance of the NSR catalyst. Therefore, the presence of a second metal such as Cu (to promote WGS) or Ru (to promote HC reforming and low termperature NO oxidation), in addition to Pt, will be used. Furthermore, Pd and Pd bimetallic alloys will be compared to Pt and Pt alloy catalysts as recent evidence shows that Pd is superior for low temperature oxidation as well as improved selectivity to N2 (as opposed to N2O). Intellectual Merit The importance of this work is two-fold. First, this work is aimed at obtaining a fundamental understanding of the catalytic mechanisms involved in very complex but accessible bimetallic systems. Information thus obtained may be used in a variety of other applications where bimetallic catalysts are involved. Second, the development of new improved catalysts is critical to the environmental performance of more fuel efficient lean burning engines. The work will be performed in a collaborative effort between experiment and theory utilizing multiple techniques. Bimetallic alloy catalysts will be synthesized, and extensively characterized using EXAFS, XPS, and IR spectroscopies. Following their characterization the catalyst will be tested under realistic operating conditions using both a traditional microreactor and a TAP reactor for identification of important intermediates so as to improve understanding of the mechanism and thereby aid in the design of new catalysts. Density functional theory calculations will aid in providing a complete picture of the reaction mechanism and identify the rate limiting steps in both the oxidation and reduction chemistry. New bimetallic catalysts will be developed in an iterative process which combine the desired features to meet the conflicting demands of performance over a wide range of conditions and environments. Broader Impact The development of novel NSR catalysts can have a substantial impact on the commercial deployment of lean-burn vehicles with apparent environmental and societal benefits. From an educational standpoint, the interdisciplinary nature of this research will be used to train future scientists and researchers with skills necessary to employ a multi-faceted approach to catalyst design. This project will provide a unique opportunity for the graduate students involved as they learn about not only a variety of techniques beyond their capabilities of their own lab but will have exposure to research environment of other institutions All three institutions are among the leaders in the enrollment of underrepresented minorities among major research universities and we intend to use REU supplements for the purpose of exposing minority undergraduates to research careers. This research will be disseminated through journal publications and conference proceedings.