NO and NO₂ (i.e. NO_x) are typical pollutants emitted from internal combustion engines. NO is the most prevalent NO_x species emitted, but it is oxidized in the atmosphere to form NO₂, which has a significant impact on human health, particularly in urban areas. This has considerable economic impacts cutting lives short, increasing medical costs and reducing productivity through working days lost across the economy.

While three-way catalysts (TWCs) effectively control NO_x emissions in stoichiometric gasoline engines, different technologies must be used for lean-burn engines (e.g. diesel engines) as TWCs are unable to convert NO_x in the presence of oxygen. Current technologies for lean NO_x abatement are based on the NH₃ selective catalytic reduction (SCR) reaction, or on the lean NO_x trap (LNT) technology, also referred as NO_x-storage reduction (NSR). In the NH₃–SCR technique, the NO_x reduction is based on the use of Cu/ or Fe/zeolite catalysts coupled with an urea injection apparatus. This is an effective approach for treating the NO_x emissions from diesel engines once the exhaust temperature is above 200 °C, to guarantee the complete hydrolysis of urea. In contrast, LNTs were introduced in the 1990s by Toyota Motor Company. This catalytic technology is based on sequential NO_x adsorption/reduction cycles: NO_x are stored on the catalyst during the lean operations of the engine, then the stored NO_x are reduced to N₂ (and other by-products like N₂O and NH₃) during periodic rich purges. Accordingly, a LNT catalyst consists of sites for NO_x sorption (alkali metal or alkaline-earth metal compounds like K and/or Ba, or Ce) and sites for NO_x oxidation/reduction, generally noble metals like Pt and/or Rh, Pd. Notably, since NH₃ may be formed during the operations of NSR catalysts, combined NSR+SCR catalysts have also been proposed. In these systems the SCR catalyst makes use of the ammonia slip from the LNT system to increase the overall NO_x reduction efficiency.

SCR is nowadays preferred for large cars and trucks, whereas LNTs are used for small engines, or coupled with SCR systems and used as in situ ammonia generator. Since its introduction in the 1990s, driven by the increasingly strict legislation limits, NSR catalysis has undergone tremendous advances, although new challenges are currently on the way. High efficiency and selectivity at low temperatures with negligible fuel penalty, noble metal-free catalysts with high activity and durability is what is sought by technicians and scientist nowadays, but this requires noteworthy improvements of the state-of-the-art catalytic systems. Besides, current aftertreatment systems have a complex design consisting of several emission control devices, e.g. diesel oxidation catalysts (DOC) and diesel particulate filters (DPF) besides the DeNO_x system. In addition, a passive NO_x adsorber (PNA) may be present to reduce cold-start NO_x emissions, when the exhaust temperature is below 180–200 °C. Different architecture and strategies can be used in the aftertreatment systems, and combined one-pot systems also exist, like deNO_x–deSoot devices able to accomplish the simultaneous removal of soot and NO_x. It is therefore mandatory to consider both the NSR functionality and durability when determining its placement within the exhaust treatment system.

The aim of this book is to provide a comprehensive and complete overview on the catalytic reduction of NO_x by the NSR technique. Outstanding experts in the field have been selected and asked to provide an up-to-date survey on various aspects of the NSR technology, from the current legislation and market situation to fundamental aspects of the technology (catalytic materials, chemistry, mechanisms and kinetics); from real application standards to the detailed modeling of full-scale systems; and from durability aspects to integration with other aftertreatment devices, including combined systems (e.g. hybrid NSR–SCR, deNO_x–deSoot systems). New trends in NSR applications are also discussed, like the recent Di-Air concept for the reduction of NO_x under fast lean/rich cycling conditions. The book also includes case histories presented by leading companies which have greatly contributed to the application of the NSR technology to the automotive industry.

This book is primarily devoted to scientists and researchers working both in the industry and in the academia on exhaust gas aftertreatment systems. It may also be used in teaching courses related to catalytic processes for environmental protection, or on the catalytic reduction of NO_x.

The editors of this book are deeply indebted to the authors of all chapters, experts and scientists from industry and academia. All of them made substantial contributions to the book: this volume was possible only thanks to their efforts, time and knowledge.

Luca Lietti and Lidia Castoldi