DOC Applications

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Abstract: In most stand-alone applications, the diesel oxidation catalyst (DOC) is used to control diesel particulate emissions through the control of their organic fraction (OF). In some cases, the catalyst is also required to control CO and HC emissions. Catalyst formulations have been optimized to minimize generation of sulfate particulates in applications with sulfur-containing fuels. In aftertreatment systems with SCR catalysts and particulate filters, the DOC is used to increase the NO₂/NOx ratio and to oxidize HCs and increase the exhaust gas temperature to support active DPF regeneration.

Introduction

The Function of DOC

The function of the diesel oxidation catalysts (DOC) in the diesel emission control system has evolved significantly. The DOC was first introduced as a stand-alone device to control gaseous emissions—CO and HC. Then, specialized DOC formulations were developed that could provide some degree of PM emission control, through the oxidation of the organic fraction (OF) of diesel particulates. However, the DOC alone was not sufficient to meet the increasingly more stringent diesel emission standards and new, more efficient technologies were developed such as diesel particulate filters (DPF) for PM control and SCR catalysts for NOx reduction. The DOC is used as an auxiliary catalyst in modern DPF/SCR aftertreatment systems, but it’s role has changed. The main functionality of the DOC in these emission systems is the generation of nitrogen dioxide (NO₂) to support the DPF and SCR operation.

Used as stand-alone emission control devices, most commercial DOCs were optimized for the reduction of diesel particulate matter emissions. The DOC accomplishes that goal through the removal of the particulate matter’s heavy organic fraction (OF). Certain emission regulations—for instance EU standards for diesel passenger cars—include stringent limits for gaseous HC and CO emissions, which must be reduced over the catalyst as well, often with high efficiencies. DOCs designed for PM/HC/CO control also reduce several unregulated emissions, such as aldehydes and PAHs, as well as the diesel odor.

In their auxiliary catalyst capacity in DPF/SCR systems, DOCs are used primarily as (1) NO₂ generating catalysts, and/or (2) “heat-up” catalysts for thermal management of DPF/SCR aftertreatment. An increased NO₂/NOx ratio promotes passive DPF regeneration (i.e., oxidation of soot collected in the filter), and can enhance NOx reduction efficiency in SCR catalysts. The role of the heat-up catalyst is to oxidize hydrocarbons—introduced via diesel fuel injection into the exhaust system or post-injection in the engine—to increase the exhaust gas temperature and facilitate active DPF regeneration. If both of these functions are required on a given engine, they are typically combined in one DOC by zone-coating of two DOC formulations optimized for the respective activity.

As many categories of diesel engines adopted DPF/SCR aftertreatment systems, the role of the stand-alone DOC technology has diminished. However, the technology remains important for markets with less stringent emission regulations. The stand-alone DOC has also entered certain new areas of application, such as large bore diesel engines, and will remain important for some time 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 stand-alone DOC technology in heavy-duty engines was limited to selected applications. A number of commercial, original equipment DOC applications on light- and heavy-duty engines are listed in Table 1.

**Table 1**
Commercial OE application of stand-alone DOC systems
Emission Legislation	PM Limit	DOC Application
Light-Duty Vehicles
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	Many cars could meet Euro 5a using a DOC, but DPFs became adopted in some markets (e.g., Germany) for political reasons.
Heavy-Duty Engines
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, retrofits under the US EPA National Clean Diesel Campaign, or Low Emission Zones (LEZ) in several European cities.

Beginning with the US EPA heavy-duty 2007/2010 and light-duty Tier 2 (2004-2009) regulations, followed by the Euro 5b (2011.09), Euro VI (2013) and other emission standards worldwide, diesel engines have been equipped with particulate filters and NOx reduction catalysts such as SCR. In most diesel engines meeting these and later emission standards, the DOC is used as a component of a more complex DPF/SCR emission control system.