Nitrogen Oxides Reactions in Diesel Oxidation Catalyst

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ABSTRACT

Two catalyst formulations were tested OII a diesel engine. A Fourier Transform Infrared (FTIR) analyzer was used to measure concentrations of nitric oxide (NO) and nitrogen dioxide (NO2). Other exhaust gas components were measuredby conventional analyzers. The results indicate that nitrogen oxides (NO,) undergo

complex chemical reactions in the diesel oxidation catalyst (DOC). The increase of NO2 occurring at a certain temperature range is accompanied by a decrease of the total NO, emission. NO2 is probably an important intermediate product in the lean NO, catalyst reaction chain

 

INTRODUCTION

Catalytic reduction of NO, under lean conditions became the research focus for both diesel and automotive applications. Surprisingly, very little data is published on the role of NO2 in the nitrogen oxides catalysis. Majority of experimental work is limited to the total NO, measurements only and attempts are made to interpret the results in terms of NO reactions.  On the other hand, it is a known fact that precious metal catalysts can oxidize NO to NO2 in diesel exhaust [9,10]. As a result a significant fraction of NOx may be present as NO2. Because NOz exhibits high chemical activity, it may deserve more attention in the NO, catalysis models.  From the underground mining perspective the oxidation of nitric oxide to nitrogen dioxide may be a counterproductive process. Occupational Health and Safety standards legislating the ambient air quality regulate each nitrogen oxide separately. Exposure limits of NO2 are about ten times lower than those for NO. The Mine Safety and Health Administration has proposed permissible exposure limits f 3 pp111 for NO2 and 25 ppm for NO for mines in the U.S.[8/. The purpose of this paper was to study the influence of different catalyst formulations and engine conditions on the transformations of NO, and generation of NO2 in diesel exhaust gas. To interpret the experimental data a discussion of several possible nitrogen oxides reactions was necessary. Some of the reactions are those occurring in the “lean NO, catalyst”.

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