Diesel Oxidation Catalyst

W. Addy Majewski

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Abstract: Diesel oxidation catalysts promote chemical oxidation of CO and HC as well as the SOF portion of diesel particulates. They also oxidize sulfur dioxide which is present in diesel exhaust from the combustion of sulfur containing fuels. The oxidation of SO2 leads to the generation of sulfate particulates and may significantly increase total particulate emissions despite the decrease of the SOF fraction. Modern diesel oxidation catalysts are designed to be selective, i.e., to obtain a compromise between sufficiently high HC and SOF activity and acceptably low SO2 activity.

Catalytic Reactions

The diesel oxidation catalyst (DOC) owes its name to its ability to promote oxidation of several exhaust gas components by oxygen, which is present in ample quantities in diesel exhaust. When passed over an oxidation catalyst, the following diesel pollutants can be oxidized to harmless products, and thus can be controlled using the DOC:

Additional benefits of the DOC include oxidation of several non-regulated, HC-derived emissions, such as aldehydes or PAHs, as well as reduction or elimination of the odor of diesel exhaust.

The emission reductions in the DOC occur through chemical oxidation of pollutants occurring over the active catalytic sites. These processes can be described by the following chemical reactions.

(1)[Hydrocarbons] + O2 = CO2 + H2O

(1a)CnH2m + (n + m/2)O2 = nCO2 + mH2O

(2)CO + 1/2O2 = CO2

Hydrocarbons are oxidized to form carbon dioxide and water vapor, as described by reaction (1) or—in a more stoichiometrically rigorous way—by reaction (1a). In fact, reactions (1) and (1a) represent two processes: the oxidation of gas phase HC, as well as the oxidation of SOF compounds. Reaction (2) describes the oxidation of carbon monoxide to carbon dioxide. Since carbon dioxide and water vapor are considered harmless, the above reactions bring an obvious emission benefit.

However, an oxidation catalyst will promote oxidation of all compounds of a reducing character; some of the oxidation reactions can produce undesirable products and, in effect, be counterproductive to the catalyst purpose. Oxidation of sulfur dioxide to sulfur trioxide with the subsequent formation of sulfuric acid (H2SO4), described by reactions (3) and (4), is perhaps the most important of these processes.

(3)2SO2 + O2 = 2SO3

(4)SO3 + H2O = H2SO4

When the exhaust gases are discharged from the tailpipe and mixed with air, either in the environment or in the dilution tunnel which is used for particulate matter sampling, their temperature decreases. Under such conditions the gaseous H2SO4 combines with water molecules and nucleates forming (liquid) particles composed of hydrated sulfuric acid. This material, called sulfate particulates, contributes to the total particulate matter emissions from the engine. Catalytic formation of sulfates, especially in conjunction with high sulfur content diesel fuel, can significantly increase the total PM emissions and, thus, become prohibitive for the catalyst application.

Oxidation of NO to NO2 is another important reaction that is considered undesirable in some DOC applications, but is essential for the operation of many modern diesel emission control systems:

(5)NO + 1/2O2 = NO2

Concerns have been raised that the catalytic generation of NO2—which is more toxic than NO—can create air quality problems in some applications, for instance in underground mines [159]. Due to the thermodynamic equilibrium of reaction (5), which is reached in the atmosphere after some time regardless of the original composition of NOx, the oxidation of NO tends to be of less concern in most surface applications. Nitrogen dioxide can be effectively used for the regeneration of diesel particulate filters (DPF) or to enhance the performance of several types of SCR catalysts. In modern diesel emission control systems, DOCs are often specifically designed for high NO2 production to support the operation of DPF/SCR systems. Increased NO2 levels may result from the use of these devices.

The reaction mechanism in diesel oxidation catalysts is explained by the presence of active catalytic sites on the surface of the catalyst carrier, that have the ability to adsorb oxygen. In general, the catalytic oxidation reaction includes the following three stages:

  1. oxygen is bonded to a catalytic site,
  2. reactants, such as CO and hydrocarbons, diffuse to the surface and react with the bonded oxygen, and
  3. reaction products, such as CO2 and water vapor, desorb from the catalytic site and diffuse to the bulk of the exhaust gas.