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. In modern diesel aftertreatment systems, an important function of the DOC is to increase the NO2 content of the exhaust gas, to support the performance of SCR catalysts and diesel particulate filters.

Catalytic Reactions

The diesel oxidation catalyst (DOC) owes its name to its ability to promote oxidation of exhaust gas components by oxygen, which is present in ample quantities in diesel exhaust. When passed over an oxidation catalyst, carbon monoxide (CO), gas phase hydrocarbons (HC), the organic fraction of diesel particulates (SOF), as well as non-regulated emissions such as aldehydes or PAHs can be oxidized to harmless products, and thus can be controlled using the DOC. In modern diesel aftertreatment systems, an important function of the DOC is to oxidize nitric oxide (NO) to nitrogen dioxide (NO2)—a gas needed to support the performance of diesel particulate filters and SCR catalysts used for NOx reduction. A comprehensive discussion of DOC reactions, reaction kinetics, and other aspects of the technology can be found in the literature [3829].

The reaction mechanism over the diesel oxidation catalyst 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.

The oxidation of hydrocarbons and CO in diesel emissions can be described by the following chemical reactions:

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

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

(2)2CO + O2 = 2CO2

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. The oxidation of HCs also results in a reduction of the diesel odor.

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.

The oxidation of NO to NO2 is essential for the operation of modern diesel emission control systems, where the DOC is an auxiliary catalyst supporting the performance of other types of catalysts—positioned downstream of the oxidation catalyst—that require an elevated NO2/NO ratio.

(5)2NO + O2 = 2NO2

Nitrogen dioxide is required to enhance the performance of several types of SCR catalysts, as well as to promote passive regeneration of diesel particulate filters (DPF). DOCs used in DPF/SCR applications are typically optimized for high NO2 production.

The increased NO2/NO ratios with oxidation catalysts—while essential for the operation of diesel aftertreatment systems—have also been a source of controversy. Among the two components of NOx emissions, NO2 shows a higher toxicity than NO. In some applications, the increased NO2 emissions can contribute to air quality problems. This potential adverse effect of the DOC was first discovered in underground mines [159]. This issue may also play a role in “street canyons” with high traffic intensity—even though the thermodynamic equilibrium of reaction (5) can be reached more quickly in the presence of sunlight, and NO can be oxidized rapidly by ozone.