Automotive NO_x abatement using zeolite-based technologies

Rajamani Gounder

Rajamani Gounder received a BS in Chemical Engineering with a double major in Chemistry at the University of Wisconsin, a PhD in Chemical Engineering at the University of California, Berkeley, and completed postdoctoral research at the California Institute of Technology. In 2013, he started his independent scientific career at Purdue University, where he currently holds the title of Larry and Virginia Faith Associate Professor in the Davidson School of Chemical Engineering. He leads a research group in heterogeneous catalysis that is recognized for studying the kinetic and mechanistic details of catalytic reactions, for synthesizing and designing zeolites and porous materials with tailored site and surface properties, and for developing methods to characterize and titrate active sites in catalytic surfaces. His group studies the catalysis of energy and the environment, focusing on converting conventional and emerging carbon feedstocks to fuels and chemicals, and automotive pollution abatement. His group has published more than 40 papers on topics related to zeolite catalysis science and technology. His research has been recognized by the NSF Career Award, the DOE Early Career Award, and the Alfred P. Sloan Research Fellowship in Chemistry.

Ahmad Moini

Ahmad Moini is a Research Fellow at BASF Corporation in Iselin, NJ. He obtained his PhD in Chemistry from Texas A&M University, and held a postdoctoral appointment at Michigan State University. He started his career at Mobil Research & Development Corporation (now ExxonMobil), where he conducted research on microporous materials with focus on exploratory zeolite synthesis and the mechanism of zeolite crystallization. He joined Engelhard Corporation (now BASF) in 1996. Since then, his primary research interests have been in the area of materials synthesis, directed at a range of catalytic and functional applications. He applied high throughput methods for the synthesis and evaluation of catalytic materials, and he used these tools for the development of new products. A significant part of his work has been directed towards catalysts for environmental applications. These efforts, in collaboration with the extended BASF team, led to the discovery and development of Cu–CHA catalysts for selective catalytic reduction (SCR) of NOx from diesel vehicles. He holds 53 US patents relating to various aspects of materials and catalyst development. He was the recipient of the 2016 F.G. Ciapetta Lectureship from the North American Catalysis Society.

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The abatement of nitrogen oxide (NO_x) emissions from diesel vehicles presents a unique range of challenges. In contrast to three-way catalysis for gasoline vehicles, effective conversion of NO_x to N₂ must take place under lean (oxygen-rich) conditions and at relatively lower temperatures typical of diesel exhaust. In the meantime, the optimum catalyst has to function and remain stable at significantly higher temperatures, primarily due to the multi-component nature of the diesel emission system and its regeneration cycles. Effective catalyst design, therefore, requires the ultimate balance between two key features – catalytic activity over a wide temperature range, and durability towards extreme hydrothermal aging conditions. The use of zeolitic materials under such conditions is especially challenging due to the vulnerability of zeolites to steam aging under hydrothermal conditions.

The discovery of Cu–chabazite (Cu–CHA) zeolites was a significant recent breakthrough in automotive NO_x abatement catalysis, resulting in the selection of urea-selective catalytic reduction (SCR) as the technology of choice for removing NO_x from diesel engine exhaust. Since its commercial implementation one decade ago, research on the science and technology of urea-SCR on Cu- and Fe-exchanged zeolites has proliferated rapidly, and has now become a major topic of investigation in the catalysis and reaction engineering communities worldwide. This themed collection showcases the global impact of this problem of major environmental significance, and the diversity of chemistry, engineering, and modeling approaches that are being used to study this catalyst technology.

The articles in this themed collection highlight major themes in NO_x SCR science and technology, within the context of automotive emissions control using zeolite-based technologies. This collection brings together contributions among leading researchers from the academic community and the automotive industry. In addition to Persective and Review articles on major thematic topics, original research contributions range from microscopic and molecular-level descriptions of the catalytic materials and reaction mechanisms, to macroscopic descriptions of behavior and performance in engineered devices and catalyst configurations within after-treatment systems. The articles in this collection also highlight emerging trends in zeolite-based materials and after-treatment strategies to enable vehicles with advanced combustion strategies to meet future NO_x emissions standards.

In the cornerstone Perspective article (DOI: 10.1039/C8RE00284C) of this themed collection, Christine Lambert summarizes the technical challenges and design constraints of removing NO_x from the multicomponent mixtures that contain other criteria pollutants and poisons, and to do so under the highly transient nature of automotive exhaust. Lambert also discusses the design principles and choices involved in implementing a catalytic NO_x abatement technology for particular automotive applications, and provides her commentary on the current state of NO_x control and the future applicability of NO_x abatement technologies, which are prerequisite to the manufacture and operation of vehicles despite the fact that “people drive vehicles, not catalysts, and emission control is not a feature that is highlighted in a marketing advertisement”.

The catalytic performance of Cu–CHA zeolites used for urea-SCR depends on the synthesis and preparation methods used in their formulation, because of their effects on the bulk and atomic-scale compositions and structures of the active metal sites and the host zeolite support. A Review from Feng-Shou Xiao and co-workers summarizes recent advances in the synthesis of metal-exchanged zeolites for NO_x SCR, highlighting strategies for environmentally-friendly zeolite preparations that minimize the use of organic structure-directing agents and solvents (DOI: 10.1039/C8RE00214B). Recent mechanistic insights about the structure of Cu active sites in Cu–CHA zeolites using in situ and in operando experimental and computational interrogations have revealed that under low temperature (<250 °C) SCR reaction conditions, ammonia solvates Cu sites to form mobile Cu–amine coordination complexes that are precursors to the active sites in the SCR redox cycle. In a Minireview, Peirong Chen and co-workers discuss how in situ impedance-based spectroscopy (IS) can be used to directly monitor the motion of Cu ions during SCR reaction conditions (DOI: 10.1039/C8RE00283E). In an original research contribution, Elisa Borfecchia and co-workers describe how in situ X-ray absorption spectroscopy (XAS) and UV-vis-NIR spectroscopy can be used to monitor changes in the extent of ammonia-solvation of Cu active sites and Cu oxidation states with changes in temperature (DOI: 10.1039/C8RE00322J). An original research contribution from Jan-Dierk Grunwaldt and co-workers describes how in operando XAS and X-ray emission spectroscopy (XES) and in situ electron paramagnetic resonance spectroscopy measurements can be used to monitor the interconversion of monomeric and dimeric Cu complexes that are intermediates in the SCR catalytic cycle, and discusses the manifestation of loss in ammonia-coordination of Cu sites as the “seagull”-shaped NO conversion profile with temperature (DOI: 10.1039/C8RE00290H).

As a complement to these spectroscopic methods, various steady-state and transient kinetic techniques and reaction modeling approaches provide additional insight into SCR reaction kinetics and mechanisms on Cu–CHA. A Review from Enrico Tronconi and co-workers summarizes how transient chemical trapping techniques can be used to probe the nature of reaction intermediates formed on metal–zeolite SCR catalysts when placed in contact with NO_x storage materials, and potential applications of combined adsorber and catalyst systems for low temperature NO_x abatement (DOI: 10.1039/C9RE00012G). An original research article by Michael Harold and co-workers describes how modeling of transport and reaction processes within dual-layer Cu–CHA NO_x SCR catalysts and Pt–alumina ammonia slip catalysts can be used to model the performance of washcoated monoliths and provide strategies to optimize catalysts and reactor configurations for NO_x abatement (DOI: 10.1039/C8RE00325D). An original research article by Yuanzhou Xi and co-workers also highlights how mathematical models can be used to describe SCR catalyst performance, in this case study using hydrocarbon oxidation on a vanadia-based SCR catalyst (DOI: 10.1039/C8RE00291F).

In addition to catalyst performance, catalyst durability in the harsh automotive exhaust environment is paramount in the practical implementation of Cu–CHA zeolites in urea-SCR technologies. Two prominent deactivation mechanisms are the hydrothermal aging and degradation of the Cu–zeolite material experienced during high-temperature excursions in steam-containing exhaust streams, and the poisoning of Cu active sites by sulfur-containing compounds present in the exhaust stream. An original research article by Do Heui Kim and co-workers describes how Cu–CHA deactivation upon exposure to hydrothermal aging conditions reflects both intraparticle and interparticle migration of Cu species over much longer distances than previously recognized (DOI: 10.1039/C8RE00281A). In an original research article, Suk Bong Hong and co-workers report how the zeolite framework topology influences the hydrothermal stability of Fe-exchanged zeolites, identifying Fe–LTA zeolites as a particularly durable Fe-containing SCR catalyst (DOI: 10.1039/C9RE00007K). An original research article by Ton Janssens and co-workers reports the use of kinetic and spectroscopic probes to provide evidence for the different SO₂ poisoning and regeneration behavior of different types of Cu active sites in Cu–CHA zeolites (DOI: 10.1039/C8RE00275D). In an original research article, William Epling and co-workers describe the development of a multi-site kinetic model to predict the SO₂-poisoning and regeneration behavior of different Cu sites and Brønsted acid sites in Cu–CHA zeolites (DOI: 10.1039/C8RE00210J).

Although this themed collection is focused primarily on research on the preeminent Cu–CHA zeolite-based technology for urea-SCR in diesel exhaust after-treatment, it also includes discussion on future outlook and opportunities for other metal–zeolite-based technologies for automotive NO_x abatement. In addition to the Perspective from Lambert to open this themed collection, a Review article from Manuel Moliner and co-workers focuses on metal zeolites being investigated as passive NO_x adsorber (PNA) materials, which are capable of adsorbing NO_x compounds at low temperatures wherein SCR catalyst materials are ineffective, and desorbing NO_x at higher temperatures when SCR catalysts can complete NO_x reduction processes (DOI: 10.1039/C8RE00193F).

The contributions in this collection highlight the complexity in the science and technology of both metal-containing zeolites and NO_x transformations, as well as the rich chemistry that exists at the interface of these two fields. They also emphasize the continuing need for research focusing on reaction chemistry fundamentals and mechanisms, which in turn have a strong environmental impact through advancements in the design of catalyst materials and engineered systems for automotive pollution abatement.

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