Alternative Catalytic Approach for Reduction of Cold-Start Hydrocarbon Emissions 

Introduction

Cold-start hydrocarbon (HC) emissions remain one of the most significant roadblocks to achieving ultra-low emissions from automobiles. Many strategies have been developed to reduce cold-start HC emissions, including secondary air pumps to lean out rich exhaust gases, hydrocarbon adsorbers, electrically heated catalysts to accelerate catalyst warm up, heat storage devices, etc. However, all of these methods add significant complexity and cost to the emissions control system. Traditional close-coupled and manifold-mounted catalysts can heat up quickly, but are not very effective under rich exhaust conditions. Current technology fuel system controls often maintain a lean or a stoichiometric air-fuel ratio during the cold-start for optimal emissions performance. However, drivability can be compromized, especially in cold climates. The POx catalyst concept addresses all these concerns.
 

POx Catalyst Used to Reduce Cold-Start HC Emissions

Background

In 1998, SwRI® conceived the use of a high space velocity partial oxidation (PO_x) catalyst to reduce cold-start HC emissions, and received a patent on the technology in 2001. Partial oxidation is the process whereby HCs are converted to carbon monoxide (CO) and hydrogen (H₂) as compared to full oxidation where the products are carbon dioxide (CO₂) and water (H₂O). The reaction preferentially occurs at high-space velocity (e.g., 500,000 hr-1) and rich (oxygen deficient) conditions.

HC + O₂ → CO + H₂
 

1993 Dodge Ram 250 Pickup Truck

Approach

The program utilized two test vehicles. The first vehicle was a California certified 1993 Dodge Ram 250 pick-up truck. The vehicle was equipped with a 5.9L, V8 engine, and had 40,400 miles (65,000 km) of on-road service at time of use. The original OEM catalyst system was used for all testing. The second vehicle was a Canadian specification 1999 Chrysler Cirrus passenger car with a 2.4L, 4-cylinder engine, and had 20,200 miles (32,500 km) of on-road service at time of use. A proprietary catalyst system was used for all testing on the Cirrus. This two-brick catalyst system had a total catalyst volume of 2.5L. The catalyst system was aged using a proprietary two-mode bench aging cycle for 124 hours, with a peak catalyst bed temperature of 1000°C.
 

Improvement in Hydrocarbon Conversion Efficiency During FTP Cold-Start

Results

On the Dodge Ram test vehicle, baseline emissions data showed that 79 percent of the HC emissions occurred during the cold-start light-off bag of the FTP. About 62 percent of the HC emissions occurred during the first two minutes. When compared to the OEM catalyst alone, a 1.24 gL^-1 PO_x/OEM catalyst system reduced tailpipe emissions by 34, 48, -7, and 40 percent for THC, NMHC, CO, and NO_x, respectively.

A 1.24 gL^-1 rhodium/zirconia catalyst was tested in the exhaust of the Chrysler Cirrus test vehicle. When compared to the main catalyst alone, this PO_x/Catalyst system reduced tailpipe emissions by 37, 39, 44, and 33 percent for THC, HMHC, CO, and NO_x, respectively.

The small size of the partial oxidation catalysts used and levels of rhodium tested suggest that an effective catalyst could be employed with rhodium loading in the region of 40 to 125mg, which correlates to a price of $1.00 to $3.00 at a rhodium price of $750.00 per troy ounce.

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