Diesel Catalysts [subscription]

DieselNet Technology Guide

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Diesel Catalysts
In-Depth Papers
Abstract: The first diesel catalysts, introduced in 1970s
for underground mining applications, were simple ● Diesel Oxidation
oxidation catalysts designed for conversion of CO and HC. Catalyst
These catalysts gradually evolved into the modern diesel
oxidation catalysts, optimized for PM emission reduction. ● Selective Catalytic
Future requirements for NOx emission reduction from Reduction
diesel engines call for new catalyst technologies, such as
SCR, lean NOx catalyst, and NOx adsorber systems. ● Lean NOx Catalyst

● NOx Adsorbers
● Catalyst Technologies
● Commercial Catalyst
● Diesel Oxidation Catalyst Technologies
● NOx Reduction Catalysts
● Deactivation of Diesel
Catalyst

Catalyst Technologies

Three-way catalyst (TWC) technology introduced in the 1980s became an integral part of the
spark-ignited engine. The TWC catalyst, operating on the principle of non-selective catalytic
reduction of NOx by CO and HC, requires that the engine is operated at a nearly stoichiometric air-
to-fuel (A/F) ratio [Eastwood 2000]. Modern catalyst systems for gasoline or natural gas engines
include an oxygen sensor in front of the catalyst and a closed loop electronic control system. The
electronic controller, based on a feedback system from the oxygen sensor, maintains the A/F ratio
within a narrow range around the stoichiometric point, to assure maximum catalyst efficiency. In
the presence of oxygen, the three-way catalyst becomes ineffective in reducing NOx.

For this reason, three-way catalysts cannot be employed for NOx control on diesel applications,
which, being lean burn engines, contain high concentrations of oxygen in their exhaust gases at all
operating conditions. Diesel emissions that can be controlled with high efficiency by today’s
catalyst technologies are CO and HC, including such HC material as the organic fraction of diesel
particulates (SOF) or polynuclear aromatic hydrocarbons (PAH). Catalyst systems for the
reduction of NOx from diesel engines are either still under development or at early stages of
commercialization, as summarized in Table 1.

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The design target of early diesel oxidation catalyst (DOC) formulations was high removal
efficiency of gas phase diesel pollutants, including carbon monoxide and hydrocarbons. At the
time of their introduction, in the 1970s, carbon monoxide and hydrocarbon emissions from diesel
engines were many times higher than they are in today’s engines. The first diesel catalysts were
used in confined space applications, such as in underground mining, where air quality was
extremely important.

Introduction of fuels with reduced sulfur content (< 500 ppm S) made it possible to achieve small
to moderate reductions of PM emissions with the DOC. In highway engine applications of modern
diesel catalysts, the PM reduction became an important—in some cases the only important—
catalyst function. Optimization of the PM performance of the diesel oxidation catalyst was the
main objective of catalyst research in the late 1980s and early 1990s, both in the USA and in EU.
These diesel catalysts have been commercialized on some heavy-duty diesel engines in the US
and on light-duty diesel engines in the EU.

Future expectations focus on the NOx reduction function of the diesel catalyst. Technologies for
the reduction of NOx from lean burn engines, both diesel and gasoline, have been researched for
many years by catalyst manufacturers and research institutes worldwide. That research has
produced some promising catalyst concepts, some of them being on the onset of
commercialization.

Future emission standards for both highway (US 2010, Euro V, light-duty US Tier 2) and nonroad
engines (US Tier 4, EU Stage IV) legislate NOx emission levels that will require highly efficient
NOx emission aftertreatment. For example, the US 2010 NOx limit of 0.2 g/bhp-hr represents an
approximate 90% reduction relative to the 2004 standard. The regulators expect that NOx
reduction catalysts will mature to provide this magnitude of NOx control on time. Indeed, the very
future of the diesel engine may depend on technological developments in the catalytic reduction of
NOx under lean conditions.

Table 1
Diesel Catalyst Technologies

Reduced
Catalyst Technology Reaction Type Commercial Status
Emissions
Diesel oxidation Oxidation CO, HC (incl. Established commercial
catalyst PAH), PM (SOF), technology
odor
SCR Selective catalytic NOx Commercial technology
reduction by ammonia/ for stationary and marine
urea engines, under
development for truck and
car engines

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DeNOx (lean NOx) Selective catalytic NOx, CO, HC, PM Some commercial
reduction by (SOF) oxidation catalysts
catalyst
hydrocarbons incorporate small NOx
reduction activity (passive
DeNOx)
NOx adsorbers Adsorption (trapping) NOx, CO, HC Technology under
of NOx from lean development,
exhaust, followed by commercialized on lean
release and catalytic burn gasoline engines
reduction under rich
conditions

Diesel Oxidation Catalyst

The diesel oxidation catalyst, historically the first type of diesel catalyst, is still the only catalyst
technology that demonstrated the required performance, robustness and durability and has been
commercially established in a number of light- and heavy-duty applications.

DOC promotes a range of oxidation reactions utilizing oxygen which is in ample supply in diesel
exhaust at all engine operating conditions. These reactions can convert virtually all emission
components of chemically reducing character, such as CO, HC, and HC derivatives, into CO2 and
water vapor. The DOC chemistry is schematically shown in Figure 1.

At sufficiently high exhaust temperatures, diesel oxidation catalysts can provide very effective
control of HC and CO emissions, with reduction efficiencies in excess of 90%. The DOC
hydrocarbon activity extends to compounds such as polynuclear aromatic hydrocarbons or the
SOF fraction of diesel particulates. The catalytic oxidation of SOF results in reduction of diesel
PM emission. The PM conversion efficiency can range widely, depending on the SOF contents of
the particulates and sulfur level in the fuel. In heavy-duty engines, it is usually between 15 and
30%. DOCs may be also effective in controlling diesel odor.

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Figure 1. Conversion of Pollutants in Diesel Oxidation Catalyst

Chemical reactions and DOC

High conversion efficiencies for CO and HC are
performance is discussed in Diesel
realized through the use of highly active, noble metal
Oxidation Catalyst.
(e.g., platinum) based catalysts. Many DOCs
developed in the 1990s for highway applications were optimized for PM emission reduction, as
opposed to the abatement of CO or HC. In order to suppress undesired oxidation of sulfur
compounds, these catalysts generally utilized formulations of reduced catalytic activity (e.g.,
through the use of base metals instead of platinum or the use of very low platinum loadings). It
needs to be emphasized that CO and gas-phase HC conversion in these PM-optimized catalysts
may be very low.

The diesel oxidation catalyst was introduced in the 1970s to protect the ambient air quality in
underground mines. The objective of the catalyst was to reduce carbon monoxide emissions and
odors from diesel-powered mining equipment. The use of the DOC extended into other enclosed
space applications including construction, tunneling, and material handling. In most cases, there
are no tailpipe emission regulations in these applications. Rather, the use of catalysts is driven by
occupational health and safety standards which protect the ambient air quality at the workplace.

Early diesel catalysts, used throughout the 1970s and 1980s, were simple oxidation formulations,
virtually the same as those developed for gasoline cars. Certain disadvantages of these catalysts,
such as increased sulfate PM emissions, were gradually discovered by diesel research. As a result,
the overall air quality benefit from using diesel oxidation catalysts in enclosed space applications
had been questioned [Mogan 1987]. The introduction of emission standards for highway diesel
engines in the 1990s stimulated catalyst research, which led to the development of next generation
diesel oxidation catalysts. These new catalysts, when used with low sulfur fuels (< 500 ppm S),
were able to control diesel particulate emissions in many diesel engine applications. Availability
of the improved diesel catalyst technologies reinstated an endorsement for the use of diesel
oxidation catalyst in underground mining and other enclosed space applications [MSHA 1998].

The first automotive application of diesel oxidation catalysts was the diesel-fueled Volkswagen
Golf “Umwelt” (German for “environment”) in 1989. The catalyst was designed to reduce CO and
hydrocarbons emissions. The use of a catalyst was not necessary to meet German or European
emission regulations at that time. Rather, it was introduced as a sign of an environmentally
responsible attitude of the car maker. Since the introduction of the Euro 2 emission standards in
the mid-1990s, the diesel oxidation catalyst has become a standard component in diesel fueled
cars in Europe. The task of the catalyst is to reduce the emissions of particulates, as well as to
provide some reduction in CO and HC emissions.

In the USA, the use of oxidation catalysts for highway heavy-duty engines was triggered in 1994,
when the US EPA introduced a particulate matter emission standard of 0.1 g/bhp-hr for trucks and
0.05 g/bhp-hr for buses. Diesel oxidation catalysts were introduced on several truck and bus
engines in order to meet that PM standard. However, the use of catalysts on US heavy-duty
engines had been steadily decreasing, as the 0.1 g/bhp-hr PM standard could be achieved in more
advanced, electronically controlled engines without the use of aftertreatment devices. The

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reduction of particulate matter emission was the only purpose of the DOC; no reductions of
gaseous HC or CO were necessary under the 1994 regulations. DOCs are being introduced again
on certain heavy-duty engines meeting the 2004 emission standards, such as on the Caterpillar
ACERT engines.

Oxidation catalysts have been also used in many diesel retrofit programs worldwide, mostly for
heavy-duty engines. The US Urban Bus Retrofit/Rebuild program, which became effective in
1995, triggered retrofitting of pre-1994 urban bus engines with emission control devices, mostly
diesel oxidation catalysts. DOCs were also used in the initial stage of the Swedish Environmental
Zones program, and in diesel retrofits in a number of cities including Hong Kong and Tokyo.

NOx Reduction Catalysts

Reduction of NOx in Lean Exhaust

Catalyst research has been increasingly focusing on catalysts for the reduction of oxides of
nitrogen from lean exhaust gases. There is a need to reduce NOx emissions from both heavy- and
light-duty diesel engines significantly below levels which are currently achieved by means of
engine design and calibration. Highly-efficient NOx reducing catalysts would also allow to
calibrate the diesel engine for maximum efficiency and fuel economy, which is currently being
sacrificed for the sake of in-cylinder NOx control.

NOx reducing catalysts are needed not only for diesels, but also for lean burn spark ignited
engines. Since the three-way catalyst remains the only commercially available, highly efficient
NOx control technology for mobile applications, today’s gasoline engines are designed to operate
at a stoichiometric air to fuel mixture. There is a significant fuel economy (and greenhouse gas
emission) penalty associated with use of SI engines operating with stoichiometric A/F ratio. An
impressive fuel economy improvement could be achieved by operating the gasoline engine at lean
mixture. Catalysts that could bring nitrogen oxides emissions from lean-burn gasoline engines to
the levels regulated in Europe and in North America would open the door for such engine
technology.

The following catalytic approaches have been investigated for the control of NOx in lean exhaust
gases:

● NO decomposition catalyst
● Selective catalytic reduction with ammonia (SCR)
● Selective catalytic reduction with hydrocarbons (DeNOx or lean NOx catalyst)
● NOx adsorber-catalyst systems.

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Decomposition of Nitric Oxide

Decomposition of nitric oxide to elements is described by the following equation:

2NO = N2 + O2 (1)

The decomposition of NO is thermodynamically favored under pressures and temperatures found

in diesel exhaust. However, the rate of the spontaneous reaction is practically zero. Catalysts have
been researched to accelerate that process but no significant progress has been achieved. One of
the best catalysts suggested for NO decomposition was a copper exchanged zeolite, Cu/ZSM-5.
Copper exchanged zeolites were previously known to exhibit three-way catalyst activity for
reducing NOx in stoichiometric exhaust gas compositions [Ritscher 1981]. In a series of papers
published in the late 1980s, Iwamoto and co-workers showed that Cu/ZSM-5, as well as other
transition metal-containing molecular sieves, were able to catalytically decompose NO [Iwamoto
1991]. Although initially promising, catalytic decomposition of NO has proven difficult to realize.
The decomposition of NO on Cu/ZSM-5 is subject to inhibition by water, is very sensitive to
poisoning by SO2, is effective only at low space velocities, and the catalyst activity and selectivity
are not satisfactory.

Ammonia SCR Catalysts

Selective catalytic reduction (SCR) of NOx can be realized if a reducing agent is injected into the
gas upstream of the catalyst bed. SCR processes utilizing nitrogen containing reductants such as
ammonia or urea are commercially available for stationary diesel engines and for industrial
sources. Figure 2 schematically illustrates the SCR performance using ammonia as the reductant.

Figure 2. NOx Reduction in SCR Catalyst

The primary selective reaction in this system is expressed by Equation (2). This reaction is
promoted by the catalyst over the competitive reaction of NH3 with oxygen, which is abundant in
the system.

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4NO + 4NH3 + O2 = 4N2 + 6H2O (2)

The main advantage of SCR, which contributed to the widespread use of SCR in industrial (non-
engine) applications, is its high NOx conversion efficiency, reaching 90%. Disadvantages of SCR
include high capital and operating cost, space requirements, generation of ammonia emissions
(ammonia slip), and fouling of equipment with ammonium sulfate (hazardous waste).

SCR technology for stationary and

SCR has been successfully adapted to control NOx
mobile diesel engines is presented in
from large stationary diesel engines. It has been used Selective Catalytic Reduction.
on diesel power generation and cogeneration
systems, primarily in Europe and Japan. SCR NOx control systems have been introduced on large
marine diesel engines.

Since the 1990s, there is ongoing interest in the application of SCR for mobile applications—
trucks and diesel cars [Held 1990][Havenith 1997]. Since mobile applications of SCR would require a
sophisticated, active catalyst system and the availability of a reductant (urea) fueling
infrastructure, that technology is not the most attractive option for diesel trucks and cars.
However, in the absence of more elegant NOx reduction catalyst systems, SCR is an option for
meeting future emission standards. Demonstration programs showed that NOx emissions from
existing heavy-duty Euro II engines (7 g/kWh) can be reduced by SCR to levels below 2 g/kWh, i.
e., the Euro V standard [Fritz 1999]. Most heavy-duty engine manufacturers in Europe selected urea-
SCR as the technology for meeting Euro IV/V emission standards. Stakeholders have been
working on the implementation of the distribution network for aqueous urea solution, known
under the trade name “AdBlue” [DMGK 2003]. SCR demonstration programs have been also
conducted in the USA [Miller 2000]. The technology remains an option for meeting US 2010 NOx
emission standards for heavy-duty engines.

Lean NOx Catalyst

One can substitute ammonia as the reducing agent in Equation (2) with hydrocarbons. By
arbitrarily choosing propane as a representative HC, the reaction can be re-written as follows:

10NO + C3H8 = 5N2 + 3CO2 + 4H2O (3)

Catalysts that promote the above reaction over the competitive, non-selective oxidation of
hydrocarbons by oxygen are commonly referred to as DeNOx or lean NOx catalysts (LNC). This
selective DeNOx chemistry can be realized using either hydrocarbons emitted by the engine (so
called “passive DeNOx” configuration) or else through the addition of extra hydrocarbons, such as
diesel fuel, upstream of the catalyst (“active DeNOx”). Active DeNOx systems have higher NOx
conversion efficiency at a cost of increased system complexity and a fuel economy penalty.

Selective catalytic reduction of NOx by

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It should be emphasized that the use of such terms as hydrocarbons is discussed in Lean
the “SCR” in reference to ammonia/urea based NOx Catalyst.
systems and “lean NOx” or “DeNOx” in reference to
HC based catalysts is purely customary. Considering the nature of the chemical reactions, both
processes represent selective catalytic reduction of NOx. Both types of systems can be also
described as lean NOx catalysts, as determined by their capability to reduce nitrogen oxides under
oxidizing, lean exhaust conditions. To complicate things even more, some authors use the term
“DeNOx” in reference to any NOx reducing catalyst technology, not necessarily limited to
selective reduction by hydrocarbons. These terms should be used with caution to avoid confusion.

A combination of the lean NOx catalyst with the diesel oxidation catalyst is sometimes called a
four-way catalyst. The four-way catalyst is capable of simultaneous reduction of four diesel
exhaust pollutants: CO, HC, PM, and NOx.

Two different classes of DeNOx catalysts have been developed: (1) platinum-based catalysts and
(2) base metal catalysts (Cu/ZSM-5). Each of these catalysts has its specific drawbacks, related
primarily to narrow temperature windows, insufficient thermal durability and/or sulfur tolerance.
More importantly, however, peak NOx conversions of known DeNOx catalysts are in the range of
40-60%. Combined with relatively narrow temperature activity windows, these numbers translate
to only about 10-20% NOx conversions of passive systems on regulatory test cycles, such as the
ECE/EUDC. Therefore, DeNOx catalysts are not currently seen as a technology capable of coping
with the stringent future emission targets.

The ongoing research aimed at improving DeNOx catalysts has resulted in commercialization of
this technology, primarily in the form of limited, passive NOx removal functionality in oxidation
catalysts. Several commercial diesel catalysts developed for the light-duty Euro 3 standard (year
2000) incorporate a passive DeNOx function and are capable of 5-15% NOx reduction in addition
to their main oxidation activity in respect to PM, CO, and HC [Kharas 1998].

NOx Adsorber Catalysts

NOx storage materials were first incorporated into DeNOx catalysts in an attempt to widen their
temperature window. These compounds were supposed to adsorb NOx during periods of low
exhaust gas temperature, when the catalyst activity was low, and to release it at temperatures
which would favor the selective reaction with hydrocarbons. This idea was then coupled with the
three-way catalyst, producing nitrogen oxide adsorber-catalyst systems, also known as NOx traps,
which represent the newest concept in lean NOx control research. First NOx adsorbers have been
commercialized on certain lean burn, DI gasoline engines in Japan and the EU. A huge amount of
development work is being conducted worldwide to adapt this technology to the diesel engine.
Laboratory experiments showed that NOx adsorbers can achieve a 90% NOx reduction efficiency

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over relatively wide temperature window.

Figure 3. Operation of NOx Adsorber

The application of NOx traps to diesel

These systems cycle through two stages of operation,
including (1) storage (chemisorption) of NOx in the engines is discussed in NOx

catalyst washcoat during lean operation, and (2) Adsorbers.

regeneration of the trap through desorption and non-
selective catalytic reduction of NOx during periods of rich operation. These phases of operation
are schematically represented by the top and bottom parts of Figure 3, respectively. NOx is stored
in the form of a metal nitrate, shown as MeNO3 in the schematic.

Regeneration of NOx adsorbers involves reaction mechanisms virtually identical to those found in
the gasoline three-way catalyst. A reductant, presumably carbon monoxide, reacts with NOx in the
absence of oxygen to form elemental nitrogen. Also the catalyst systems, such as the most
common platinum/rhodium formulation, are very similar to the three-way catalyst. Since the
regeneration requires periodic short pulses (say, a couple of seconds every minute) of rich A/F
mixture, this technology requires extremely tight integration with the engine control system and is
feasible only in conjunction with advanced electronic fuel injection systems. It also involves
certain fuel economy penalty, which depends on the catalyst system, the required NOx reduction,
and on the quality of the regeneration algorithm.

The major stumbling block in the development of diesel NOx adsorbers is their susceptibility to
sulfur poisoning. Ultra low sulfur diesel (ULSD) fuels are the necessary condition for the
commercialization of NOx adsorbers. But ULSD alone is still not sufficient to solve sulfur
deactivation and durability problems in NOx adsorbers. Even if ultra low sulfur fuel is used, sulfur
management strategies, including periodic desulfation, are required by this technology [Bailey
2000].

To conclude this overview, we should also mention a somewhat different NOx adsorber concept

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termed Selective NOx Recirculation (SNR) [Krutzsch 1998]. In the SNR technique, two NOx
adsorbers are installed in parallel in the engine exhaust system. Control valves allow to switch the
gas flow, so each of the adsorbers alternates between adsorption and desorption modes. While in
the desorption mode, the NOx carrying gas from the adsorber is recirculated to the engine intake
air. This way, the desorbed NOx can be reduced through in-cylinder reactions during regular
combustion, through processes that appear to have something in common with the mechanisms on
NOx reduction by exhaust gas recirculation (EGR). The SNR technique eliminates the need for
catalytic NOx reduction, requiring a simpler catalyst system. Most likely, there is also less demand
on the engine operating regime. However, SNR adsorber regeneration strategies still need to be
demonstrated. In experiments involving feeding NO/NO2 from bottles to the diesel engine air
intake port (i.e., not accounting for the adsorber performance), the SNR achieved a NOx reduction
efficiency of 60%—much less than the adsorber-catalyst system.

It is common that commercial diesel catalysts incorporate a number of technologies into one
catalytic device. Examples include oxidation and lean NOx catalysts that incorporate hydrocarbon
adsorbers/traps (see: Commercial Catalyst Technologies). Catalysts are also combined with non-
catalytic devices to enhance their performance, the most prominent example being plasma assisted
DeNOx catalyst (see: Plasma Exhaust Treatment).

Future Trends
As the emission standard deadlines approach, engine manufacturers must choose technologies for
their future diesel engines. In Europe, urea-SCR became the technology of choice for meeting
Euro IV and Euro V NOx limits. From today’s perspective, however, these standards—3.5 and 2.0
g/kWh, respectively—cannot be considered very stringent. Most likely, manufacturers will be able
to meet them (certainly the Euro IV limit) using in-cylinder technologies, such as EGR, without
the use of NOx reduction catalysts. It is believed that two engine technologies will exist in parallel
among Euro IV/V engines: urea-SCR and EGR.

More stringent NOx limits have been adopted in the USA. Meeting the 2010 limit for heavy-duty
engines of 0.2 g/bhp-hr or the light-duty Tier 2 (phase-in 2004-2009) Bin 5 limit of 0.07 g/mi is
widely believed to require the use of NOx aftertreatment. The same applies to the Tier 4 NOx limit
of 0.4 g/bhp-hr for nonroad engines, which becomes effective in 2014. The US EPA designed
these low NOx limits with the NOx adsorber technology in mind. Contrary to European regulators,
the US EPA has expressed reservations about urea-SCR technology, due to the urea infrastructure
and regulatory enforcement issues.

On the other hand, several engine manufacturers would choose urea-SCR rather than NOx
adsorbers for NOx control in future light- and heavy-duty diesel engines. NOx adsorbers are still
under development; their durability needs to be substantially improved before they become a

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commercial technology. NOx adsorbers are also believed to require larger and more expensive
(noble metals) catalysts than SCR. Finally, the SCR path would allow many manufacturers to use
the same or similar engine design in both European and North American markets.

A comparison of NOx reduction technologies available to engine manufacturers, considering the

state-of-the-art in 2003, was carried out by Ford [Lambert 2004]. The performance of urea-SCR,
NOx adsorber, and lean NOx (HC SCR) catalyst on a light-duty engine is shown in Figure 4. It is
clear that the urea-SCR catalyst provides an advantage over the NOx adsorber, especially when
tested over the FTP-75 cycle.

Figure 4. NOx Conversion Using Different Catalyst Technologies

Slightly aged catalysts. Shaded areas indicate typical temperature

ranges for underfloor converters over LD test cycles. SV = 30,000 1/h
(catalyst volume about equal to the engine displacement).

The study examined, based on literature review and experiment, a number of other aspects related
to the use of both technologies. It was concluded that urea-SCR can provide over 90% NOx
conversion, as required by future emission standards, with the following technical advantages over
NOx adsorbers:

● wider operating temperature window

● greater durability
● lower fuel economy penalty
● lower HC emissions
● lower greenhouse gas emissions

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● lower system cost

● lower usage cost

These advantages are balanced by the known drawbacks of urea-SCR: (1) the necessity to
establish and maintain urea infrastructure and (2) the long list of potential issues with urea
replenishment (vehicle operators have a financial incentive not to replenish urea) [EC 2004]. And
so, the choice of NOx reduction technology in future diesel engines remains an open question,
which is largely dependant on the progress in NOx adsorber technology in the coming years.

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