Catalyzed Diesel Filters

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Abstract: Most catalyzed diesel filters utilize monolithic wall-flow substrates coated with a catalyst. The catalyst lowers the soot combustion temperature, allowing the filter to self-regenerate during periods of high exhaust gas temperature. A number of diesel filter catalysts have been developed, including both noble and base metal formulations. Catalyzed ceramic filters exhibit very good PM filtration efficiencies, but are characterized by relatively high exhaust gas pressure drop.

Introduction

In the catalyzed diesel particulate filter (CDPF), a catalyst is applied onto the filter media to promote chemical reactions between components of the gas phase and the soot (carbon) collected in the filter. The main purpose of the catalyst is to facilitate passive regeneration of the filter by enabling the oxidation of diesel particulate matter under exhaust temperatures experienced during regular operation of the engine/vehicle, typically in the 300-400°C range. In the absence of the catalyst, particulates can be oxidized at appreciable rates only at temperatures around 550-650°C, which can occur only at full load conditions in the diesel engine and in most cases are rarely seen during real-life operation.

The use of fully passive filter systems is practically limited to retrofit applications. In most OEM applications, catalyzed filters are used with mixed passive-active regeneration, where the filter regenerates passively at high engine loads. Under light load conditions, the exhaust gas temperature is actively increased—typically to about 500°C—to trigger periodic regeneration whenever the maximum soot load is reached in the filter. Increased temperatures are realized through such means as engine management or injection of fuel into the exhaust gas, followed by HC oxidation over a warm-up catalyst. In these filter systems, the role of the catalyst—in addition to lowering the soot ignition temperature—is to accelerate the soot oxidation rate to minimize the fuel economy penalty. Shorter duration of the regeneration also minimizes the chances for unforeseen and unwanted interruptions (e.g., due to changed engine operating conditions where the engine management strategy to increase temperature can be no longer sustained). Finally, active oxidation catalysts may be able to minimize the high CO emissions that could otherwise occur during active regeneration, when large amounts of soot are burned during a short time period.

Application of a catalyst has been attempted with virtually all diesel filter media. Reported catalyst applications include wall-flow monoliths, wire mesh, ceramic foams, ceramic fibers, and other media. In the most common design, the CDPF utilizes a ceramic wall-flow monolith made of either cordierite or silicon carbide, packaged into a steel housing, as shown in Figure 1. The porous walls of the monolith are coated with the catalyst. As the diesel exhaust aerosol permeates through the walls, the soot particles are deposited within the wall pore network, as well as over the inlet channel surface. The catalyst, through a number of possible mechanisms discussed below, facilitates PM oxidation under the lean conditions in the diesel exhaust.

[schematic]

Figure 1. Catalyzed Diesel Particulate Filter

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