Diesel NOx Catalytic Converter Development: A Review

Abstract

This paper summarizes the findings of several technical articles on diesel NOx catalytic converter technology. Simplified theoretical reactions for NOx removal are discussed. Currently, development of catalytic NOx control technology for diesel engines is focused on systems that incorporate fuel hydrocarbons as the chemical reducing agent. Copper-and zeolite-based catalysts have been the predominant systems studied to date, but now catalysts containing precious metals are being investigated. Observed NOx reduction efficiencies typically ranged from 10 to 30 percent on actual engine exhaust systems when exhaust hydrocarbon enrichment strategies were used. Effects of carbon monoxide, sulfur dioxide, and water on NOx reduction efficiencies are reviewed. Recommendations for future research include attempts to broaden the temperature range of efficient NOx reduction, improving hydrocarbon selectivity toward the NOx reduction reaction, and the development of a supplementary reductant delivery system suitable for transient diesel engine operation.

Catalytic control of automotive NOx: a review

“…Increased HC concentration tends to increase the NO V reduction efficiency while shifting the maximum NO V conversion point to slightly higher temperatures. Reduction of NO V is more efficient with olefinic HC than with paraffines (Heimrich, 1996). In presence of C H , Cu-exchanged MFI zeolite (Cu-MFI) can be a potential (Iwamoto and Mizuno, 1993).…”

“…This article highlights several technical studies reported in the literature on catalytic conversion technoogy to control pollution due to automotive exhausts with specific focus on NO V emission reduction. A recent review (Heimrich, 1996) has been reported specifically for diesel exhausts and only reviewed studies made on NO V control systems tested using diesel engine exhaust. Another recent (Koltsakis and Stamelos, 1997) article has reviewed catalytic automotive exhaust aftertreatment in a more general perspective with less emphasis on specific details of the technologies involved.…”

Partial reduction of NO to N2O on Cu{311}: role of intermediate N2O2

“…The relatively high cost of rhodium, however, implies a driving force towards the development of cheaper alternatives [7][8][9][10]. Further driving forces, and perhaps even more compelling ones, are that the exhaust streams of modern lean-burn engines typically constitute highly oxidising environments, whilst those of Diesel engines attain significantly lower temperatures than those associated with typical gasoline engines [6]. Both of these present challenges to the operation of traditional threeway catalysts and necessitate either cyclic control over exhaust conditions or the development of alternative catalytic strategies.…”

“…Undoubtedly the most successful of these alternative strategies is embodied in Selective Catalytic Reduction (SCR) by ammonia [11]. In a commercial automotive SCR system, ammonia is generated in situ from decomposition of urea, and then reacts with NO x over an oxide or zeolite catalyst to form N 2 and H 2 O [6,7,9,11]. It remains true, however, that the need for a dissolved urea additive has limited the uptake of this technology in most automotive contexts.…”

“…Indeed, previous work on silver and copper substrates indicates a low-temperature regime, in which N 2 O is the dominant gaseous product of NO adsorption, contrasting with a high-temperature regime, in which NO 2 is more prevalent [8,13]. From a mechanistic perspective, it is generally believed that NO 2 may be formed by the reaction of intact NO with surface O (the latter species derived from NO dissociation) [6,10,13]. On the other hand, N 2 O is thought to be generated via some kind of (NO) 2 intermediate that partially dissociates almost immediately upon formation [13,22,23].…”