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Most of commercial diesel oxidation catalysts (DOC) have been optimized for the reduction of diesel particulate matter. A conventional DOC accomplishes that goal through the removal of the particulate matter’s heavy organic fraction or SOF. Certain emission regulations—for instance EU standards for diesel passenger cars—also establish stringent limits on gaseous emissions of HC and CO, which must be reduced over the catalyst as well, often with high efficiencies. Other regulations—for instance US standards for heavy-duty highway engines—set CO and HC limits that are easily met without a catalyst. Commercial diesel oxidation catalysts also reduce several unregulated emissions, such as aldehydes and PAHs, as well as the diesel odor.
Mechanisms of catalytic PM emission control were explained in the discussion on oxidation catalyst fundamentals. In short, SOF can be removed over the DOC with very high efficiency through catalytic oxidation and/or cracking. Storage and release of hydrocarbons in the catalyst washcoat also plays an important role and influences the apparent PM conversion efficiency. On the other hand, the carbonaceous fraction of diesel particulates—or dry soot—remains practically unaffected. Hence, the potential for total particulate matter (TPM) reduction depends on what fraction of TPM is composed of SOF (which, in turn, depends on the engine technology and the emission test cycle). For example, if SOF forms 40% of the total particulate matter emission, the maximum possible conversion of TPM in the catalyst is limited to about 40%. Certain specialized diesel oxidation catalysts also have the capacity to hold the solid soot particles that can be oxidized either concurrently or later when exhaust conditions are suitable. While sometimes simply referred to as diesel oxidation catalysts, these are more accurately described as particle oxidation catalysts.
At some point in time, catalyst makers strived to develop DOC formulations that would incorporate lean NOx reduction functionality. These catalysts—able to control all four regulated pollutants—are sometimes referred to as “four-way catalysts”. In spite of the considerable R&D effort, only limited success could be achieved—the NOx conversion efficiency of the catalysts that had been developed remained too low to deserve the “four-way” designation.
The scope of this paper is limited to applications of the DOC as a stand-alone device—a technology that appears to have entered a period of declining importance. Due to the limitations mentioned above, DOC technology alone has not been able to meet the stringent emission requirements that became effective in North America, Europe and Japan around 2005-2010. Diesel particulate filters (DPF) and SCR catalysts are examples of highly efficient PM and NOx (respectively) control technologies that were widely commercialized in that period. While the DOC plays an important role in many advanced diesel emission aftertreatment systems—examples include the warm-up catalyst in active DPFs or NO2 and NH3-slip catalysts in SCR systems—these specialized DOC applications are covered in the papers on the respective emission technologies.
We should note that—notwithstanding the diminishing role of the stand-alone DOC—the technology remains important for markets with less stringent emission regulations. The DOC has also entered certain new areas of application, such as large bore diesel engines, and will remain important for a number of years as a retrofit technology.
Commercial Status. The first commercial DOC application on a new vehicle dates back to 1989, when Volkswagen launched its diesel-powered Golf “Umwelt” model fitted with a catalyst. This application was, however, voluntary. The DOC was introduced on a wider scale only on Euro 2 (1996) cars, and became standard equipment on all Euro 3 (2000) and later vehicles. In contrast to the passenger car application, the use of DOC technology has been limited to certain markets and applications for heavy-duty diesel engines. A number of commercial, original equipment DOC applications on light- and heavy-duty engines are listed in Table 1.
|Emission Legislation||PM Limit||DOC Application|
|Euro 2 (1996)||PM = 0.08 g/km||DOCs introduced on larger size diesel cars.|
|Euro 3-4 (2000-2005)||PM = 0.05-0.025 g/km||The main aftertreatment strategy, used on most diesel passenger cars and light trucks.|
|Euro 5a (2009.09)||PM = 0.005 g/km||Technically, many cars could meet Euro 5a using a DOC, but DPFs became widely adopted for political reasons.|
|US 1994||PM = 0.10 g/bhp-hr||DOC introduced on many light and medium heavy-duty engine models, most with mechanical fuel injection systems. DOCs widely used on urban bus engines due to a more stringent PM limit of 0.07-0.05 g/bhp-hr.|
|US 1998||PM = 0.10 g/bhp-hr||DOC remained common in many light and medium heavy-duty engine models. In some cases, a DOC was no longer required as in-cylinder control was enabled by the replacement of remaining mechanical fuel injection systems with electronically controlled systems required for the lower NOx limits. Urban bus engines continued to rely heavily on DOCs.|
|US 2004||PM = 0.10 g/bhp-hr||The DOC continued to remain popular for light and medium heavy-duty engine models using EGR to comply with NOx limits. Used for all on-highway engines that did not use external EGR (e.g. Caterpillar ACERT engines). Continued to be used on urban bus engines.|
|Euro IV/V (2005/2008)||PM = 0.02 g/kWh||DOC technology used on some truck engines with EGR (without urea-SCR).|
|Nonroad Tier 4i/Stage IIIB (2011-2012)||PM = 0.02 g/kWh||DOC technology introduced on selected nonroad engine models (mostly those using EGR for NOx control).|
Even before new OE vehicle applications, DOCs were used as retrofit devices, mostly on heavy-duty diesel engines. One of the earliest applications was underground mining, where DOCs were introduced in the late 1980s to control diesel CO and HC emissions. A number of wider scale DOC retrofit programs were initiated in the 1990s and 2000s. Examples include the 1995 US Urban Bus Retrofit Rebuild (UBRR) Program, more recent retrofits under the US EPA National Clean Diesel Campaign, or Low Emission Zones (LEZ) in several European cities.
Catalysts and Configurations. Early diesel catalysts were generic, active oxidation formulations not optimized for the diesel application. The noble metal was platinum or a platinum/palladium blend of relatively high metal loading, usually between 30 and 50 g/ft3. Most catalysts utilized an alumina-based washcoat with rare earths or alkaline earths stabilizers (La, Ce, Ba, Sr, ...). The earliest diesel catalysts for retrofit applications utilized pelleted or beaded supports which were later replaced by monolithic substrates. To prevent clogging of the catalyst by diesel particulates from the high emission engines, large cells were used, typically of 200 cpsi cell density. Since neither the tailpipe emissions nor the catalyst durability were regulated, small catalyst volumes were used with space velocities up to 300,000 1/h. Monoliths (mostly metallic) of high diameter to length ratios were used to minimize the pressure drop at high exhaust gas velocities. Such catalyst configurations can still be found in some aftermarket products.
Diesel oxidation catalysts for regulated, OEM applications are larger, with space velocities often ranging from 50,000 to 150,000 1/h and with a catalyst volume approximately matching the engine displacement. Due to lower particulate emissions in more advanced engines, smaller cells can be used to enhance the catalyst performance. Cell densities of 300 and 400 cpsi are now common in diesel applications . Ceramic monolith substrates are more common than metallic substrates.
A number of catalyst formulations have been developed depending on the targeted emissions, duty cycle and light-of temperature requirements, as well as the sulfur content of the fuel. Many catalysts use Pt or Pt/Pd on an alumina/zeolite washcoat with platinum group metal (PGM) loadings from less than 10 g/ft3 (PM control) to as much as 80-100 g/ft3 (PM + CO control at low temperature). Base metal catalysts can also be used for PM control.
Catalyst washcoat loadings are on the order of 120 g/dm3 (2 g/in3) with a thickness typically less than about 40-80 microns in the thickest locations (corners) of the monolith cells. Substrates of higher cell density typically have higher washcoat loadings and lower washcoat thickness than larger cell substrates.
Catalyst substrates for diesel truck applications are usually about 180-250 mm (7-10") in diameter and 150-180 mm (6-7") in length. Smaller substrates are used for passenger car applications. The coated substrates are packaged into catalytic converters. Catalytic mufflers, with the catalyst installed inside a silencer, are also used, especially in heavy-duty engines.