NOx Trap Catalysts and Technologies

Fundamentals and Industrial Applications

Luca Lietti, Lidia Castoldi

• Publisher: Royal Society of Chemistry
• Year: 2018
• Edition: 1
• eBook ISBN:
  9781788014755
• Topic: Sciences physiques
• Subtopic: Chimie industrielle et technique

Vehicle exhaust emissions, particularly from diesel cars, are considered to be a significant problem for the environment and human health. Lean NOx Trap (LNT) or NOx Storage/Reduction (NSR) technology is one of the current techniques used in the abatement of NOx from lean exhausts. Researchers are constantly searching for new inexpensive catalysts with high efficiency at low temperatures and negligible fuel penalties, to meet the challenges of this field.

This book will be the first to comprehensively present the current research on this important area. Covering the technology used, from its development in the early 1990s up to the current state-of-the-art technologies and new legislation. Beginning with the fundamental aspects of the process, the discussion will cover the real application standard through to the detailed modelling of full scale catalysts.
Scientists, academic and industrial researchers, engineers working in the automotive sector and technicians working on emission control will find this book an invaluable resource.

CHAPTER 1

Review of deNO_x Technology for Mobile Applications

T. V. JOHNSON^* AND A. JOSHI

Corning Incorporated, HP-CB-2-1, One Museum Way, Corning, NY 14831, USA

^*Email: [email protected]

1.1 Introduction

NO_x is formed when air is heated to very high temperatures, and is thus emitted from combustion and engines. The most prevalent NO_x species from engines is NO. It will oxidize in the atmosphere to form NO₂, and also react with most hydrocarbons to form ozone; both ozone and NO₂ are strong oxidants and toxic. Thus NO_x is a criteria pollutant and is regulated.

NO_x is very-effectively controlled from gasoline engines with three-way catalysts (CO, hydrocarbons, NO_x), but they only operate under stoichiometric conditions. In the presence of excess oxygen, CO and hydrocarbon react with it rather than chemically reduce the NO_x. For lean conditions, selective catalytic reduction (SCR) is the leading method of remediation. The reductant, ammonia (NH₃), which generally needs to be added to the exhaust, selectively reduces the NO_x rather than being oxidized by the excess oxygen, like in the case of innate exhaust reductants, CO and hydrocarbons. NO_x trap catalysts^† (NTCs) can also be effective in reducing NO_x. During lean conditions the NO_x is stored as a nitrate in the catalyst, and then in periodic rich conditions the nitrate dissociates releasing the NO₂, with is reduced by CO or hydrocarbon reductant in the absence of oxygen. A subset of these is the passive NO_x adsorber (PNA), which generally stores NO_x as a nitrite at low temperatures (<200 °C) and releases the NO at higher temperatures.

In this introductory chapter a general outlook at the emission limits for both diesel and gasoline engines will be reported, along with general engine trends, and the actual aftertreatment technologies used for both types of engines. Future perspectives in the field of lean NO_x abatement might also be enlightening. This chapter is not intended to be all-encompassing and comprehensive. Representative papers and presentations were chosen to provide examples of new, key developments and direction. For a more detailed review of emissions and engine technologies, the reader is referred to the authors’ last five review papers.^1–5

1.2 Regulatory Overview

Although the first commercial lean deNO_x system was a NTC coated onto a diesel particulate filter (DPF) on the European Toyota Avensis diesel in the early 2000s, and then separate NTC on the US Dodge Ram diesel truck (Cummins engine) as a separate unit in 2007, the first wide-scale use of lean deNO_x was the implementation of SCR for heavy-duty (HD) truck applications in Europe in 2005 to meet the Euro IV HD regulations. The US Tier 2 and California LEVII (Low Emission Vehicle) light-duty regulations were the first to require lean NO_x control on light-duty (LD) applications, beginning in 2007, in which VW used only an NTC; BMW used SCR; and Mercedes used both an LTC and “passive” SCR (NH₃ generated by the upstream LTC during rich periods) in addition to SCR on some models. Mercedes is the only automaker to make a lean-burn gasoline vehicle, for the European market. It uses two NTCs in a series architecture. SCR did not make its way into non-road (NR) applications until 2011 in both the USA and Europe. NTCs were not widely used in Europe until the LD Euro 6 regulations started in 2014.

Following is a general overview of the LD and HD regulations pertinent to understanding the main drivers for NO_x control systems.

1.2.1 Light-duty Regulations

The leading light-duty diesel and gasoline non-methane hydrocarbon and NO_x regulations are graphically shown in Figure 1.1. Only the USA (Federal and California LEVIII) has the dynamometer test-cycle and limit value combination to force NO_x aftertreatment on diesel engines. All require a diesel particulate filter (regulations not shown). However, many of Euro 6 applications have NO_x aftertreatment to minimize NO₂ emissions and fuel consumption.

Figure 1.1 Leading light-duty diesel and gasoline NO_x and non-methane hydrocarbon emissions. China 6b begins in 2023 and US Tier 3 is phasing-in 2017 through 2025. All other regulations are currently being enforced.

Starting in September 2017 all new diesel platforms and in September 2019 all new diesel vehicles will need to meet an additional layer of NO_x tailpipe regulations called RDE (real driving emissions). As part of their type approval testing (and over the regulatory life of the vehicle), vehicles are equipped with PEMS (portable emissions measurement systems) and driven on routes that meet certain specifications. The data are analyzed using two different protocols.⁶ NO_x levels need to be below the RDE limits of 168 mg km⁻¹ (2.1× the dynamometer limit value). In January 2020 and 2021 (new platforms, and all new vehicles, respectively) the RDE NO_x limits drop to 120 mg mile⁻¹ (1.5× dynamometer limits). These two additional regulations require more NO_x control either through improved calibration or more hardware.

1.2.2 Heavy-duty Truck Regulations

Figure 1.2 shows a summary of the key heavy-duty (HD) truck regulations in the world, along with estimates of the best commercially-viable engine-out NO_x and PM capabilities as measured on the European Steady-state Cycle (ESC). The first vehicle regulation in the world that was attained with SCR systems was the Japan 2005 HD truck regulation in October 2004, shortly followed by Euro IV in January 2005. Although Euro IV was only a 30% NO_x tightening from Euro III (2000), the PM (particulate matter) regulation dropped ∼80%, and truck manufacturers generally elected to tune their engines for higher NO_x and lower PM and fuel consumption, and then use SCR to drop the tailpipe levels to within the NO_x (and PM) requirements. Interestingly, although the US2007 NO_x regulations were 60% lower than for Euro IV, and the PM regulations were about 35–55% tighter (steady state and transient testing, respectively), the US manufacturers chose to meet the NO_x regulations with engine technology (mainly exhaust gas recirculation, EGR), and the PM regulations with diesel particulate filters (DPFs). The Japan 2005 regulation is intermediate between Europe and the US for both NO_x and PM, and there was a split of approaches used in Japan, with trucks in high fuel consumption applications generally using a European SCR-only approach, and all others using a EGR+DPF approach.

Figure 1.2 Overview of key HD tailpipe regulations as measured on the European Steady-state Cycle (ESC). The dashed and solid lines represent an estimate of engine-out emissions for engines in 2007 and for meeting the US2010 and Euro VI regulations.

In the 2009+ timeframe, Japan 2009, US2010, and Euro VI (2013) all require both SCR and DPF solutions. These regulations range from 0.26 to 0.7 g NO_x kW-h⁻¹ and 0.010 to 0.013 g PM kW-h⁻¹. In 2016 India finalized their Bharat VI regulations, which are nearly identical to Euro VI, for implementation in April 2020. In late 2016 China proposed their China VI regulations, which deviate from Euro VI by requiring remote-transmission OBD (on-board diagnosis; signals sent to authorities) of key emissions components, higher altitude real-world testing, and some chassis dynamometer testing.

HD NO_x regulations are tightening further, with California taking the lead.^1,2 They are now evaluating the feasibility of cutting NO_x by about 90% from 0.260 g kWh⁻¹ by 2024.

1.3 Emission Control Technology for Diesel and Gasoline Engines

Emission control systems were for used on gasoline-fueled vehicles in 1975 to meet the US Clean Air Act requirements. These were simple oxidation catalysts by today's standards, and achieved about 70% conversion of CO and hydrocarbons. Forty years later gasoline emission control systems are achieving 99.5% conversion of hydrocarbons, CO, and NO_x. The first commercial vehicular SCR systems date to 2013, and fourteen years later are achieving 99% NO_x conversion under high-load conditions (T>250 °C). The first NTC systems in volume came in 2007, and achieved 70–75% conversion. Due to high temperature efficiency issues, the best accomplish perhaps 85–90% NO_x conversion over many driving conditions, but when combined with SCR the system can achieve 95% NO_x conversion over a range of conditions.

For particulate control, wall-flow filters are the norm. They started out uncoated for diesel applications, and quickly migrated to incorporating an oxidation c...

Table of contents

• Cover
• Title
• Copyright
• Preface
• Contents
• Chapter 1 Review of deNOx Technology for Mobile Applications
• Chapter 2 NSR Technology
• Chapter 3 NSR Catalytic Materials
• Chapter 4 Lean NOx Trap Performance Degradation – Reversible Sulfur Poisoning and Irreversible Thermally-induced Sintering
• Chapter 5 Mechanism and Kinetics of NOx Storage
• Chapter 6 Active Sites for NOx Uptake, and the Nature of Stored NOx Species
• Chapter 7 Mechanistic Aspects of the Reduction of the Stored NOx by H2 Investigated by Isotopic Labelling Experiments and FTIR Spectroscopy
• Chapter 8 NOx Storage and Reduction: Effects of Pt Dispersion, Reductant Type, and Cycle Timing
• Chapter 9 Reduction of Stored NOx with CO/H2 and Hydrocarbons: A Spatial Resolution Analysis1
• Chapter 10 Global Kinetic Modelling of the NSR Catalysts
• Chapter 11 Combined LNT–SCR Catalysts for NOx Reduction from Lean Exhaust Gas
• Chapter 12 LNT Catalysts for the Simultaneous Removal of NOx and Soot: The DPNR Concept
• Chapter 13 Non-thermal Plasma NOx Storage-reduction
• Chapter 14 New deNOx Concept by Fast Reductants Injection Pulse Upstream NSR
• Chapter 15 Development of Combined NSR DeNOx and DeSOx Model and Application for Control Strategy Pre-calibration of a Modern Diesel Engine
• Chapter 16 LNT Catalysis at Ford Motor Company – A Case History
• Subject Index