Over the last several years, nitrogen oxide(s) (NOx) storage/reduction (NSR) catalysts, also referred to as NOx adsorbers or lean NOx traps, have been developed as an aftertreatment technology to reduce NOx emissions from lean‐burn power sources. NSR operation is cyclic: during the lean part of the cycle, NOx are trapped on the catalyst; intermittent rich excursions are used to reduce the NOx to N2 and restore the original catalyst surface; and lean operation then resumes. This review will describe the work carried out in characterizing, developing, and understanding this catalyst technology for application in mobile exhaust‐gas aftertreatment. The discussion will first encompass the reaction process fundamentals, which include five general steps involved in NOx reduction to N2 on NSR catalysts; NO oxidation, NO2 and NO sorption leading to nitrite and nitrate species, reductant evolution, NOx release, and finally NOx reduction to N2. Major unresolved issues and questions are listed at the end of ...

Overview of the Fundamental Reactions and Degradation Mechanisms of NOx Storage/Reduction Catalysts

By the early 1970s increased use of cars in some major cities had resulted in serious concerns about urban air quality caused by engine exhaust gas emissions themselves, and by the more harmful species derived from them via photochemical reactions. The three main exhaust gas pollutants are hydrocarbons (including partially oxidised organic compounds), carbon monoxide and nitrogen oxides. Engine modifications alone were not sufficient to control them, and catalytic systems were introduced to do this. This catalytic chemistry involves activation of small pollutant molecules that is achieved particularly effectively over platinum group metal catalysts. Catalytic emissions control was introduced first in the form of platinum-based oxidation catalysts that lowered hydrocarbon and carbon monoxide emissions. Reduction of nitrogen oxides to nitrogen was initially done over a platinum/rhodium catalyst prior to oxidation, and subsequently simultaneous conversion of all three pollutants over a single three-way catalyst to harmless products became possible when the composition of the exhaust gas could be maintained close to the stoichiometric point. Today modern cars with three-way catalysts can achieve almost complete removal of all three exhaust pollutants over the life of the vehicle. There is now a high level of interest, especially in Europe, in improved fuel-efficient vehicles with reduced carbon dioxide emissions, and “lean-burn” engines, particularly diesels that can provide better fuel economy. Here oxidation of hydrocarbons and carbon monoxide is fairly straightforward, but direct reduction of NO x under lean conditions is practically impossible. Two very different approaches are being developed for lean-NO x control; these are NO x -trapping with periodic reductive regeneration, and selective catalytic reduction (SCR) with ammonia or hydrocarbon. Good progress has been made in developing these technologies and they are gradually being introduced into production. Because of the nature of the diesel engine combustion process they produce more particulate matter (PM) or soot than gasoline engines, and this gives rise to health concerns. The exhaust temperature of heavy-duty diesels is high enough (250–400 °C) for nitric oxide to be converted to nitrogen dioxide over an upstream platinum catalyst, and this smoothly oxidises retained soot in the filter. The exhaust temperature of passenger car diesels is too low for this to take place all the time, so trapped soot is periodically burnt in oxygen above 550 °C. Here a platinum catalyst is used to oxidise higher than normal amounts of hydrocarbon and carbon monoxide upstream of the filter to give sufficient temperature for soot combustion to take place with oxygen. Diesel PM control is discussed in terms of a range of vehicle applications, including very recent results from actual on-road measurements involving a mobile laboratory, and the technical challenges associated with developing ultra-clean diesel-powered cars are discussed.

Progress and future challenges in controlling automotive exhaust gas emissions

Catalysis for NOx abatement

A predominant challenge to both industrial and academic research institutions is the selective reduction of NOx during lean exhaust conditions. Despite a proliferation of publications, papers and reviews, a large number of catalysts fall short of a credible and consistent diesel application. Herein are reviewed systems designed specifically with the goal of assisting researchers in their assessment of catalytic performance. By way of conclusion, three principal types of catalysts active for HC-SCR are discussed.

The current state of research on automotive lean NOx catalysis

Magnesium aluminate (MgAl2O4) spinel (MAS) is a synthetic material with cubic crystal structure and excellent chemical, thermal, dielectric, mechanical and optical properties. These properties made MAS an indispensable material for optically transparent windows, domes and armours, and for certain refractory applications. High processing cost of dense MAS ceramics is in the main responsible for its limited usage in certain important applications despite its excellent performance in them. The volume expansion (∼8%) associated with MAS phase formation from alumina and magnesia does not allow obtaining dense MAS bodies in a single-stage reaction sintering process. Therefore, dense MAS bodies are made by following a double stage firing process, which is expensive. The existing literature suggests that the processing cost of dense MAS ceramics could be reduced to a great extent by decisive selection of starting raw materials, powder processing and densification conditions, and by understanding the under...

A review on magnesium aluminate (MgAl2O4) spinel: synthesis, processing and applications

There is an increasing interest in running passenger car engines lean of the stoichiometric air to fuel ratio (A/F) to improve fuel economy. One method being considered by vehicle manufacturers is to operate lean A/F (excess air) during cruising and return to stoichiometric operation when more power is required. The challenge for the catalyst manufacturer is to supply a product for this vehicle which will reduce NOx emissions from both lean and stoichiometric operating points to below probable EC tier III legislative limits. A technique has been developed where NOx is stored on an adsorbing catalyst while the engine runs lean. Upon switching to a short rich biased stoichiometric excursion the adsorbed NOx is released and reduced by precious metal components on the catalytic system. Selected adsorber catalysts were tested on both simulated exhaust gas and a standard bench engine with secondary air injection. The results suggest an adsorbing process whereby NO is oxidized to NO2 on Pt under lean conditions and the latter is subsequently adsorbed on an adsorbent material as a surface nitrate species. Upon return to a stoichiometric A/F ratio this species decomposes and NOx is released and subsequently reduced. The adsorber was also tested on a bench prototype lean tune engine. The results indicate an averaged NOx conversion of more than 90% over a test cycle in which the A/F ratio is cycled between lean operation and a rich biased stoichiometry.

Removal of nitrogen oxides from the exhaust of a lean-tune gasoline engine

The effects of in-cylinder water injection on a direct injection (DI) Diesel engine were studied using a computational fluid dynamics (CFD) program based on the Kiva-3v code. The spray model is validated against experimental bomb data with good agreement for vapor penetration as a function of time. It was found that liquid penetration increased approximately 35% with 23% of the fuel volume replaced by water, due mostly to the increase in latent heat of vaporization. Engine calculations were compared to experimental results and showed very good agreement with pressure, ignition delay and fuel consumption. Trends for emissions were accurately predicted for both 44% and 86% load conditions. Engine simulations showed that the vaporization of liquid water as well as a local increase in specific heat of the gas around the flame resulted in lower Nitrogen Oxide emissions (NOx) and soot formation rates. Using stratified fuel-water injection increases soot at 86% loads due in part to late injection. Because NOx decreased at all loads, the injection timing can be advanced to minimize fuel consumption and soot.

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F. Wirbeleit

Papers

Removal of nitrogen oxides from the exhaust of a lean-tune gasoline engine

Effects of Direct Water Injection on DI Diesel Engine Combustion

The Potential of a Combined Miller Cycle and Internal EGR Engine for Future Heavy Duty Truck Applications

Stratified Diesel Fuel-Water-Diesel Fuel Injection Combined with EGR-The Most Efficient In-Cylinder NOx and PM Reduction Technology

Strategic Analysis of Technologies for Future Truck Engines