Research on the selective reduction of NOx with hydrocarbons under lean-burn conditions using non-zeolitic oxides and platinum group metal (PGM) catalysts has been critically reviewed. Alumina and silver-promoted alumina catalysts have been described in detail with particular emphasis on an analysis of the various reaction mechanisms that have been put forward in the literature. The influence of the nature of the reducing agent, and the preparation and structure of the catalysts have also been discussed and rationalised for several other oxide systems. It is concluded for non-zeolitic oxides that species that are strongly adsorbed on the surface, such as nitrates/nitrites and acetates, could be key intermediates in the formation of various reduced and oxidised species of nitrogen, the further reaction of which leads eventually to the formation of molecular nitrogen. For the platinum group metal catalysts, the different mechanisms that have been proposed in the literature have been critically assessed. It is concluded that although there is indirect, mainly spectroscopic, evidence for various reaction intermediates on the catalyst surface, it is difficult to confirm that any of these are involved in a critical mechanistic step because of a lack of a direct quantitative correlation between infrared and kinetic measurements. A simple mechanism which involves the dissociation of NO on a reduced metal surface to give N(ads) and O(ads), with subsequent desorption of N2 and N2O and removal of O(ads) by the reductant can explain many of the results with the platinum group metal catalysts, although an additional contribution from organo-nitro-type species may contribute to the overall NOx reduction activity with these catalysts. It is concluded, after the investigation of hundreds of catalyst formulations, that many of the fundamental questions relating to lean deNOx reactions have been addressed and the main boundary conditions have been established. It seems clear that catalysts with sufficient activity, selectivity or stability to satisfy the demanding conditions that appertain in automotive applications are still far away. The rapidly growing interest in NOx storage systems reflects this situation, and it now seems to be the case that acceptable direct NOx reduction catalysts may be very difficult to find for lean-burn applications.

A review of the selective reduction of NOx, with hydrocarbons under lean-burn conditions with non-zeolitic oxide and platinum group metal 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

Trends in NOx abatement: A review

It has now been over 25 years since the introduction of the catalytic converter to reduce emissions from the internal combustion engine. It is considered one of the greatest environmental successes of the 20th century, however, new emission control technologies are still being developed to meet ever more stringent mobile source (gasoline and diesel) emissions. This short review will discuss the basis for improvements and highlight technology area, which will require further improvements in emissions and fuel economy. Some of the issues related to fuel cells which some believe may replace the internal combustion engines for automobile applications is also be briefly discussed.

Automobile exhaust catalysts

Catalytic NOx Abatement Systems for Mobile Sources: From Three-Way to Lean Burn after-Treatment Technologies Pascal Granger* and Vasile I. Parvulescu* Unit e de Catalyse et de Chimie du Solide, UMR CNRS 8181, University of Lille 1, 59655 Villeneuve d’Ascq, France Department of Organic Chemistry, Biochemistry and Catalysis, University of Bucharest, Romania, 412 Regina Elisabeta Boulevard, Bucharest 030016, Romania

Catalytic NOx Abatement Systems for Mobile Sources: From Three-Way to Lean Burn after-Treatment Technologies

Catalytic NOx reduction system with secondary fuel injection was studied not only in a laboratory scale, but also in a large bench scale. Diesel fuel was injected before the catalyst bed, and the activity and durability of the catalyst for the NOx reduction were tested. Using the fuel as a reductant, silver aluminate supported on alumina was selected as the most active and suitable catalyst. The resistance of the silver aluminate catalyst to SOx was enhanced by the addition of a small amount of WO3, MoO3 and Pt. It was found from the engine bench test that NOx conversion increased in proportion to the amount of aldehydes formed from the injected fuel in the exhaust gas. Thus, the secondary fuel injection into exhaust gas through a partial oxidation catalyst enhanced the NOx conversion. At 450°C, the engine bench test exhibited maximum NOx conversion of 75% and 45% at the SV of 15 000 and 70 000 h−1, respectively.

Catalytic reduction system of NOx in exhaust gases from diesel engines with secondary fuel injection

Catalytic NOx reduction system in the presence of excess oxygen using periodic two steps, an operation in oxidizing conditions and a relatively short operation in reducing conditions was investigated over various catalysts, comparing with HC-SCR in full lean conditions. Based on findings and studying the correlation between transient reaction profile from rich to lean excursions and reaction behaviors in both the NOx reductions, a highly effective and durable NOx reduction system in the presence of excess oxygen and sulfur oxides using the periodic two steps was designed. This system consisted of a short operation in oxidizing conditions and a far shorter operation in reducing conditions, and it effectively reduced NOx over Rh supported on alumina with no deterioration in the short-term durability test in the presence of 40 ppm SO2.

A highly durable catalytic NOx reduction in the presence of SOx using periodic two steps, an operation in oxidizing conditions and a relatively short operation in reducing conditions

Highly Durable NOx Reduction System and Catalysts for NOx Storage Reduction System

PURPOSE:To obtain a catalyst for catalytic reduction of NOx capable of efficient reduction of NOx in exhaust gas without using a large amt. of a reducing agent and excellent in durability even in the presence of moisture by carrying cerium oxide on a solid acid carrier. CONSTITUTION:Cerium oxide is carried on a solid acid carrier to obtain the objective catalyst for catalytic reduction of NOx with hydrocarbon or an oxygen-contg. org. compd. as a reducing agent. The solid acid carrier is a carrier exhibiting solid acidity in a temp. region in which the catalyst is used. A zeolite type or oxide type solid acid carrier may be used as the solid acid carrier and the pref. amt. of the cerium oxide carried on the solid acid carrier is 5-80wt.% of the total amt. of them. The catalyst itself can be formed into one of various shapes such as a honeycomb or sphere shape by a known forming method.

Catalyst for catalytic reduction of nox

PURPOSE:To provide a catalyst for efficiently reducing nitrogen oxide in a waste gas in a wide temp. range even in the coexistence of oxygen and moisture by providing an inside layer having a catalytic component expressed by a specific formula and a surface layer carrying a specific catalytic component on the inside layer. CONSTITUTION:The inside layer constituting a catalytic structural body on the base material contains a perovskite type multiple oxide expressed by the formula (A expresses La, Y, Ce, Pr, Nd or the like, B expresses Na, K, Bi, Th, Ba, Sr, Ca or the like, C expresses Mn, Co, Fe, Ni, Cr, Cu, V, Mo, W, Ta, Li or the like and 0<=X<=1) as the catalytic component. And the surface layer on the inside layer has the catalytic component made by carrying at least one kind of metals, the ions or the oxides of Ib group, IIa group, IIb group, IIIa group, IIIb group, IVa group, IVb group, Va group, VIIa group and VIII group in the Periodical Table on at least one kind of carrier selected from aluminum oxide, titanium dioxide, zirconium oxide and H type zeolite.

Ritsu Yasukawa

Papers

Catalytic reduction system of NOx in exhaust gases from diesel engines with secondary fuel injection

A highly durable catalytic NOx reduction in the presence of SOx using periodic two steps, an operation in oxidizing conditions and a relatively short operation in reducing conditions

Highly Durable NOx Reduction System and Catalysts for NOx Storage Reduction System

Catalyst for catalytic reduction of nox

Catalytic structure for nitrogen oxide catalytic reduction