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

迈克尔·P·哈罗德目前面临的最大挑战之一是高效和公平地消耗化石燃料。需要节约有限的石油资源的后果之一是向更高效的柴油发动机和稀薄燃烧的汽油发动机过渡。与改用稀燃发动机相关的是，由于高度氧化的条件，降低NOx的难度增加，这导致需要改进传统的三效排气催化剂。氮氧化物储存减少(NSR)催化剂已经成为解决这一问题的最成功的方法。这些系统需要在两种不同的制度下运行。在氧化条件下，NO在贵金属成分(通常是铂)上反应生成NO2，然后NO2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，发生还原剂(例如H2、CO、C3H6)的喷射，由催化剂释放的NOx反应生成氮气。尽管丰田已经将这种方法商业化，但仍有几个关键的科学问题尚未解决，需要进一步改进。这项研究将研究开发包括双金属的新型多相催化剂的策略，以期改善低温(冷启动)NOx的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。氢反过来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种来再生存储部件。由于其他还原剂在NSR系统中比氢气更容易实现，因此从CO和H2O水煤气变换(WGS)反应或通过重整从碳氢化合物中高效地生产氢气对NSR催化剂的性能至关重要。因此，除了铂之外，还将使用第二种金属，如铜(促进WGS)或Ru(促进HC重整和低温NO氧化)。此外，Pd和Pd双金属合金将与铂和铂合金催化剂进行比较，因为最近的证据表明，Pd在低温氧化方面具有优势，并提高了对N_2的选择性(相对于N_2O)。智力上的价值这项工作的重要性是双重的。首先，这项工作的目的是从根本上了解非常复杂但可访问的双金属体系中涉及的催化机理。这样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，新的改进型催化剂的开发对更省油的稀燃发动机的环境性能至关重要。这项工作将在实验和理论之间的合作努力中进行，利用多种技术。将合成双金属合金催化剂，并用EXAFS、XPS和IR光谱对其进行广泛表征。在对催化剂进行表征后，将在实际操作条件下使用传统的微反应器和TAP反应器对催化剂进行测试，以确定重要的中间体，以加深对机理的理解，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景，并确定氧化和还原化学中的限速步骤。新的双金属催化剂将在迭代过程中开发，它结合了所需的特征，以满足在各种条件和环境下相互冲突的性能要求。更广泛的影响新型NSR催化剂的开发可以对稀燃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被用来培训未来的科学家和研究人员，他们拥有使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，因为他们不仅学习了超出他们自己实验室能力的各种技术，而且还将接触到其他机构的研究环境。在主要研究型大学中，这三家机构都是少数族裔招生的领先者，我们打算使用REU补充剂，以使少数族裔本科生接触到研究职业。这项研究将通过期刊出版物和会议记录进行传播。

Michael P. Harold0730824One 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.