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NO and NO2 (i.e. NOx) are typical pollutants emitted from internal combustion engines. NO is the most prevalent NOx 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 NOx emissions in stoichiometric gasoline engines, different technologies must be used for lean-burn engines (e.g. diesel engines) as TWCs are unable to convert NOx in the presence of oxygen. Current technologies for lean NOx abatement are based on the NH3 selective catalytic reduction (SCR) reaction, or on the lean NOx trap (LNT) technology, also referred as NOx-storage reduction (NSR). In the NH3–SCR technique, the NOx 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 NOx 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 NOx adsorption/reduction cycles: NOx are stored on the catalyst during the lean operations of the engine, then the stored NOx are reduced to N2 (and other by-products like N2O and NH3) during periodic rich purges. Accordingly, a LNT catalyst consists of sites for NOx sorption (alkali metal or alkaline-earth metal compounds like K and/or Ba, or Ce) and sites for NOx 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 NOx 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 DeNOx system. In addition, a passive NOx adsorber (PNA) may be present to reduce cold-start NOx 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 deNOx–deSoot devices able to accomplish the simultaneous removal of soot and NOx. 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 NOx 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, deNOx–deSoot systems). New trends in NSR applications are also discussed, like the recent Di-Air concept for the reduction of NOx 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 NOx.

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

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