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What is a Diesel Oxidation Catalyst?

By Jessica Reed
Updated May 23, 2024
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A diesel oxidation catalyst, or DOC, is a type of diesel particulate filter designed to remove harmful emissions from diesel exhaust. The interior filter is honeycomb-shaped and covered with a catalyst, such as platinum or palladium, which oxidizes the harmful exhaust emissions. This lowers exhaust pollution and causes less environmental damage. The device is fitted to the muffler of a diesel vehicle and typically lasts about ten years.

Installation of a diesel oxidation catalyst is simple. The device attaches to the vehicle’s muffler and rarely needs maintenance. Since the piece is added on to the vehicle and not part of the original design, it falls into a category known as diesel retrofit devices. Other diesel retrofit filtering devices include particulate filters (DPF) and exhaust gas recirculation (EGR).

Exhaust first passes through the muffler and through the catalyst layer. This is where the chemical reaction that breaks down the exhaust particles takes place. After passing through the catalyst layer, the exhaust exits the vehicle and is released into the air.

When exhaust passes through the diesel oxidation catalyst, a chemical reaction occurs. The harmful exhaust particles turn into a harmless mixture of carbon dioxide and water. Typically, the goal is to reduce harmful effects of carbon monoxide, SOF particulate matter, and gaseous hydrocarbons. Unfortunately, no option exists which can fully neutralize these particles. The amount of harmful material oxidized by the diesel oxidation catalyst depends on the vehicle and the filter, among other factors.

On average, the diesel oxidation catalyst can convert between 20% and 40% of harmful exhaust emissions. Hydrocarbons have the highest conversation rate while particulate matter has the lowest. The amount of exhaust efficiently filtered depends on the exhaust temperature. Larger amounts of harmful exhaust particles are eliminated as the exhaust temperature rises.

Diesel particulate filters are similar to the diesel oxidation catalysts, with the key difference being the diesel particulate filter uses other, non-catalyst filters to remove particulate matter. The particulate matter sticks to the filter as it passes through, thus preventing part of it from being released into the air. The filter needs a way to clean itself to ensure it is effective for more than a few uses. A catalyst may be used to clean the filter and keep it functioning.

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