Diesel Oxidation Catalyst - Theory

1/26/2015 Diesel Oxidation Catalyst

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Commercial DOC Technologies

Particle Oxidation Catalysts

Diesel Oxidation CatalystW. Addy Majewski

Abstract: Diesel oxidation catalysts promote chemicaloxidation of CO and HC as well as the SOF portion of dieselparticulates. They also oxidize sulfur dioxide which ispresent in diesel exhaust from the combustion of sulfurcontaining fuels. The oxidation of SO leads to thegeneration of sulfate particulates and may significantlyincrease total particulate emissions despite the decrease ofthe SOF fraction. Modern diesel oxidation catalysts aredesigned to be selective, i.e., to obtain a compromise between sufficiently high HC and SOF activity andacceptably low SO activity.

Catalytic Reactions

Catalyst Types and Functionality

CO & HC Performance

Nitrogen Oxides Performance

Particulate Matter Performance

Sulfate Formation & DOC Selectivity

Effect on Unregulated Emissions

Catalytic Reactions

The diesel oxidation catalyst (DOC) owes its name to its ability to promote oxidation of

several exhaust gas components by oxygen, which is present in ample quantities in diesel

exhaust. When passed over an oxidation catalyst, the following diesel pollutants can be

oxidized to harmless products, and thus can be controlled using the DOC:

carbon monoxide (CO),

gas phase hydrocarbons (HC),

organic fraction of diesel particulates (SOF).

Additional benefits of the DOC include oxidation of several nonregulated, HCderived

emissions, such as aldehydes or PAHs, as well as reduction or elimination of the odor of diesel

exhaust.

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(1)(1a)(2)

(3)(4)

(5)

The emission reductions in the DOC occur through chemical oxidation of pollutants occurring

over the active catalytic sites. These processes can be described by the following chemical

reactions.

[Hydrocarbons] + O = CO + H OC H + (n + m/2)O = nCO + mH OCO + 1/2O = CO

Hydrocarbons are oxidized to form carbon dioxide and water vapor, as described by reaction

(1) or—in a more stoichiometrically rigorous way—by reaction (1a). In fact, reactions (1) and

(1a) represent two processes: the oxidation of gas phase HC, as well as the oxidation of SOF

compounds. Reaction (2) describes the oxidation of carbon monoxide to carbon dioxide.

Since carbon dioxide and water vapor are considered harmless, the above reactions bring an

obvious emission benefit.

However, an oxidation catalyst will promote oxidation of all compounds of a reducing

character; some of the oxidation reactions can produce undesirable products and, in effect, be

counterproductive to the catalyst purpose. Oxidation of sulfur dioxide to sulfur trioxide with

the subsequent formation of sulfuric acid (H SO ), described by reactions (3) and (4), is

perhaps the most important of these processes.

2SO + O = 2SOSO + H O = H SO

When the exhaust gases are discharged from the tailpipe and mixed with air, either in the

environment or in the dilution tunnel which is used for particulate matter sampling, their

temperature decreases. Under such conditions the gaseous H SO combines with water

molecules and nucleates forming (liquid) particles composed of hydrated sulfuric acid. This

material, calledsulfate particulates, contributes to the total particulate matter emissions from

the engine. Catalytic formation of sulfates, especially in conjunction with high sulfur content

diesel fuel, can significantly increase the total PM emissions and, thus, become prohibitive for

the catalyst application.

Oxidation of NO to NO is another reaction which may be considered undesirable in some

applications:

NO + 1/2O = NO

Concerns have been raised that the catalytic generation of NO —which is more toxic than NO

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—can create air quality problems in some applications, for instance in underground

mines [Ambs 1993]. Due to the thermodynamic equilibrium of reaction (5), which is reached in

the atmosphere after some time regardless of the original composition of NOx, the oxidation

of NO is of less concern in most surface applications. Interestingly enough, nitrogen dioxide

can be effectively used to facilitate the regeneration of diesel particulate filters or to enhance

the performance of certain types of SCR catalysts. Increased NO levels may result from the

use of these devices.

The reaction mechanism in diesel oxidation catalysts is explained by the presence of active

catalytic sites on the surface of the catalyst carrier, that have the ability to adsorb oxygen. In

general, the catalytic oxidation reaction includes the following three stages:

i. oxygen is bonded to a catalytic site,

ii. reactants, such as CO and hydrocarbons, diffuse to the surface and react with the

bonded oxygen, and

iii. reaction products, such as CO and water vapor, desorb from the catalytic site and

diffuse to the bulk of the exhaust gas.

Catalyst Types and Functionality

The functionality of the diesel oxidation catalyst is not limited to catalytic oxidation.

Advanced DOCs include components that promote other reactions and processes, the most

important of which are summarized in Table 1. Different types of DOCs may show a different

activity in respect to various components of diesel exhaust. Therefore, the types of catalysts

and their functions should be introduced before we discuss DOC performance.

Table 1Diesel Oxidation Catalyst Functionality

Functionality CatalyticComponents Comments

Oxidation ofgases & SOF

Noble metals(Pt, Pt/Pd)

High Pt loadings required for CO & HC oxidation, especially at lowtemperatures.

Cracking and/oroxidation of SOF

Base metals(e.g., cerium)

Base metal catalysts (often including a low Pt loading) are effectivefor PM emission control, especially in high SOF engines.

Washcoat storageof HCs Zeolites Zeolite HC traps are commonly included in DOC washcoats to

improve cold start performance.

Lean NOx activity PlatinumSmall NOx reductions possible at low-to-moderate temperatures,but typically not required in commercial DOCs.

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In addition to catalytic oxidation, important processes that affect the catalyst performance

include catalytic cracking and/or partial oxidation of long chain hydrocarbons, as well as

storage and release of gas components—for instance HC or SO —from the washcoat.

Noble Metals. Catalytic oxidation of gases is promoted by precious metals, with platinum

on alumina washcoat (Pt/Al O ) being the most basic DOC formulation. Precious metal

formulations are used whenever the DOC is required to reduce CO & HC emissions. In

applications that require high CO conversions at low exhaust temperatures, high Pt loadings

are used on the order of 50100 g/ft and above.

Before the introduction of ultralow sulfur diesel (ULSD) fuels, Pt was practically the only

noble metal used in diesel oxidation catalysts. With ULSD fuels, Pt can be partially

substituted by palladium. The first mass produced Pt/Pd DOCs were launched in Europe on

Euro 4/5 passenger cars.

Other noble metals have also been suggested, for instance PdAu alloys [Kapur 2011]. These

formulations still remain at the research stage.

Base Metals. If the role of the DOC is to control PM emissions, and no reduction or only

moderate reductions of CO/HC emissions are necessary, a base metal catalyst can be used. A

number of base metal catalysts—including Ce, Fe, V, Cu, and others—can promote catalytic

cracking and partial oxidation of long chain hydrocarbons which constitute the SOF fraction

of diesel particulates. However, base metals usually have low activity for the oxidation of gas

phase hydrocarbons and CO. To provide some degree of control for CO/HC emissions and the

diesel odor, many base metal catalysts also include a low loading of Pt, typically on the order

of 110 g/ft .

As an alternative approach, catalysts for PM emission control can utilize lowtomedium

platinum loadings rather than base metals.

Washcoat Storage. Several components of the gas phase can be stored in the catalyst

washcoat, and then released back into the exhaust gas. The release may be triggered by a

change in temperature or other exhaust gas conditions. The storage and release phenomena

can have a significant impact on the DOC performance. Storage and release phenomena are

not always easy to detect—they are often overlooked during catalyst testing. The role of

storage and release may become apparent only after many repetitions of the test.

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Storage and release phenomena can be counterproductive or beneficial for the catalyst

performance. An example of a counterproductive process is the washcoat storage of sulfur

dioxide. Catalysts with an affinity for SO storage tend to produce more sulfate particulates

and, thus, be less effective (or even ineffective) in controlling diesel PM emissions.

Washcoat storage and release of hydrocarbons, on the other hand, can be utilized to enhance

cold start performance of diesel oxidation catalysts. Zeolites, optimized to effectively adsorb

diesel exhaust HC species, can be added to the washcoat formulation to trap HCs at low

temperatures, when the catalyst is still inactive. Once the temperature increases above the

catalyst lightoff, the stored HC species are released from the washcoat and catalytically

oxidized. An added benefit of this approach is the reduction of diesel exhaust odor

immediately after engine startup. Zeolitebased HC traps have become a common

component of DOC washcoat in both light and heavyduty applications. HC storage in the

washcoat also plays a role in SOF and NO/NO conversion, even with washcoats that do not

contain HC traps.

The effect of the various catalyst components (washcoat and precious metals) on catalyst

performance is discussed in the following sections. Further details on DOC formulations can

be found in the paper on commercial catalysts.

CO & HC Performance

The diesel oxidation catalyst is very effective in controlling carbon monoxide and

hydrocarbon emissions from diesel engines, including the PAH and hydrocarbon derivatives

such as aldehydes. Figure 1 illustrates catalyst performance as a function of temperature. The

catalyst shows no activity at low exhaust gas temperatures. As the temperature increases, so

does the oxidation rate of CO and HC. At high temperatures the catalyst performance

stabilizes to form the characteristic plateau on the lightoff curve. It should be noted that the

high CO & HC conversions and lightoff behavior in Figure 1 are typical for noble metal

catalysts, while base metal DOCs are generally less active. For most catalyst systems,

including the classic Pt/Al O , the conversion of carbon monoxide is higher than that of

hydrocarbons at any given temperature.

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Figure 1. Conversion of CO and HC in Diesel Oxidation Catalyst

The high temperature conversion rate for CO or HC, or the height of the plateau, depends on

the mass transfer conditions in the catalyst. Consequently, it can be increased or decreased by

changing factors which affect either the mass transfer coefficient or the mass transfer surface

area. For example, the maximum conversion of CO or HC will increase when a larger catalyst

or a higher cell density substrate is used. In fact, the high temperature conversion efficiency

can be brought near 100% by enlarging the catalyst size. If needed, the conversion efficiency

can be limited by using a smaller catalyst, or one of a lower cell density (larger cells).

The lightoff temperature, on the other hand, depends mainly on the chemical reaction

kinetics in the catalyst. Therefore, it is influenced primarily by the catalyst noble

metal/washcoat system. The overall performance of a catalyst system, as measured on

standard emission test cycles, is, thus, a combined result of the applied catalyst technology

and the substrate geometry.

Gas Composition Effects. In a given catalyst there is a competition between the oxidation

of CO and HC species, causing the conversion rates in a mixture of gases, such as in engine

exhaust, to be lower than laboratory conversions determined using pure CO gas in the

absence of HC, or vice versa. The four charts in Figure 2 illustrate the effect of CO and HC

concentrations on the catalytic conversion rates of both gases [Katare 2009].



Figure 2. Effect of CO & HC Concentration on Light-Off TemperatureSolid lines: data (laboratory flow gas reactor), dashed lines: model. Propylene as HC.

Catalyst: 400 cpsi, washcoat (containing zeolite) 1.75 g/in , Pt/Pd 105 g/ft , SV 30,000 h .(Courtesy of Ford Motor Company)

In the first chart, increasing HC concentration from 0 to 2000 ppm increased the CO lightoff

temperature by more than 40°C. Similarly, in the fourth chart, increased CO levels have a

detrimental effect on HC lightoff. It can be also noted that increased CO levels have an

adverse effect on CO conversion, and increased HCs have a negative impact on HC

conversion, as apparent from the remaining charts in Figure 2. While the effects are generally

nonlinear, models are being developed (dashed lines in Figure 2) that can predict conversion

rates in a mixture of gases.

In conventional diesel engines, CO and HC concentrations are usually well below their

respective maximum levels shown in Figure 2. The interest in DOC performance at high

CO/HC concentrations is driven by advanced combustion modes such as low temperature

combustion (LTC). During LTC combustion, relatively high CO and HC emissions may be

generated at low exhaust temperatures and low oxygen concentrations, making the DOC

application very challenging. Even at oxygen concentrations and temperatures that would be

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normally sufficient for the catalyst lightoff, catalysts may be deactivated by the high

concentrations of CO/HC [Jacobs 2007]. Catalyst manufacturers have been developing catalyst

formulations for LTC applications. One of the suggested approaches was to incorporate

oxygencarrying additives in the catalyst washcoat to improve the activity at low oxygen/high

CO conditions [Sumiya 2009].

Catalyst performance is also affected by the presence of other gases. This could be illustrated

by the example of hydrogen—another species present in the exhaust gases from LTC

combustion. A study of the impact of H on DOC performance has found that hydrogen could

actually improve the catalyst lightoff performance [Katare 2009]. Addition of H to the

feedgas at a molar CO/H ratio of about 3 in a laboratory flow reactor decreased the CO and

HC lightoff temperature by about 20°C, Figure 3.

Figure 3. Effect of Hydrogen on CO & HC Light-Off TemperatureGas composition (without/with H ): H 0/257 ppm, CO 750/770 ppm, C H 1200/1480 ppm,O 5/6%, NO 100/100 ppm, H O 3.5/3.6 %, CO 6/5.8%, N : balance. Catalyst: See note under

Figure 2.(Courtesy of Ford Motor Company)

Pt & Pd Formulations. Among the two common oxidation catalysts—platinum and

palladium—Pt is most active for the oxidation of CO and HC in diesel exhaust, as illustrated in

Figure 4 [Harayama 1992]. CO emissions with the Pt catalyst were less than a half of those with

the Pd catalyst. Also the hydrocarbon emissions were lower when the platinum catalyst was

used, although the performance difference between Pt and Pd was smaller.

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Figure 4. Platinum and Palladium Diesel Oxidation Catalyst6.925 liter DI turbocharged aftercooled diesel engine, 5.07 liter 400 cpsi catalyst

HD FTP Transient test, 400 ppm S fuel

Lower activity of Pd in diesel exhaust has been confirmed by a number of authors [Kawanami

1998][Majewski 1995]. One of the reasons for the low activity of Pd catalysts is the higher

susceptibility of palladium to sulfur poisoning. In applications with ultralow sulfur fuels, Pt

can be partially substituted by Pd, up to about 30% by mass.

Even with ultra low sulfur fuels, Pt/Pd formulations tend to show somewhat lower CO/HC

conversion compared to Pt only catalysts [Makino 2006]. However, the presence of Pd was

reported to improve the catalyst durability, with aged Pt/Pd catalysts performing equally or

better than Pt only catalysts. The durability improvement may be explained by better catalyst

dispersion and lower precious metal particle sizes in the Pt/Pd system. In the same study, the

Pt/Pd particle sizes were measured at 1030 nm (based on TEM imaging), compared to 3040

nm for Ptonly formulations[Makino 2006]. Due to the price difference between the metals,

substitution of Pt by Pd may produce a cost saving.

The lower activity of Pd in diesel applications is quite opposite to its activity in gasoline

exhaust. Palladium, due to superior low temperature hydrocarbon activity, is commonly used

in closecoupled lightoff catalysts in gasoline cars [Eastwood 2000]. The difference in activity

is explained by different chemical composition of gasoline and diesel exhaust hydrocarbons.

Gasoline HCs have short carbon chains and contain many unsaturated compounds, while

diesel hydrocarbons are characterized by long carbon chains and mostly saturated bonds.

The heat effect associated with the oxidation of carbon monoxide and hydrocarbons is also

different in the gasoline and diesel oxidation catalyst. The oxidation of CO and HC involves

exothermic reactions with significant release of heat. Of course, the heat of reaction for

particular chemical compounds is the same whether they originate from diesel or gasoline


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exhaust. The difference is in the concentrations of CO and HC, which are higher in gasoline

exhaust than in diesel exhaust. As a result, a temperature increase up to a few hundred

degrees Celsius may be observed in gasoline catalytic converters, while the temperature rise

across the diesel catalyst is rarely more than 1020°C.

Odor Control. The removal of hydrocarbons and their derivatives over the DOC also results

in reductions in the levels of odor in diesel exhaust [Zelenka 1994]. The effectiveness of

catalysts is difficult to quantify, because no universal methods exist to measure the intensity

of odors. Authors who worked on developing such methods reported decreases of threshold

value of diesel odor between 317% on a heavyduty engine with a 10 g/ft Pt catalyst, and

between 5262% on a lightduty engine with a 90 g/ft Pt catalyst [Hamm 1999].

Nitrogen Oxides Performance

Effect on Total NOx EmissionsIn most cases, the total nitrogen oxides emissions (NOx) are not changed over the diesel

oxidation catalyst. More complex catalyst technologies, such as SCR catalysts or NOx

adsorbers, are required to remove NOx from lean exhaust.

NOx Reduction. Small NOx reductions, usually not more than a few percent, are sometimes

measured over diesel oxidation catalysts. There are several possible explanations of such

activity, including:

lean NOx performance,

storage of nitrates in catalyst washcoat,

reactions with diesel PM or HCs.

Some catalyst systems, including Pt/Al O , exhibit lean NOx performance via selective

catalytic reduction of nitrogen oxides by hydrocarbons. Diesel oxidation formulations which

contain components with lean NOx activity may facilitate some NOx reduction, especially at

higher HC/NOx ratios. In the case of platinum/alumina catalysts, maximum lean NOx

activity occurs at low temperatures of approximately 200250°C. It should also be noted that

NOx is reduced mainly to nitrous oxide (N O) over Pt catalysts. N O—being a potent

greenhouse gas—is not a desirable reaction product.

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Storage of nitrates may occur on some catalysts, causing an apparent NOx reduction effect.

Catalyst materials capable of reacting with nitrogen oxides to form nitrates include barium, a

common alumina washcoat stabilizer in oxidation catalysts. Since barium nitrate is not

decomposing under typical conditions in diesel exhaust, repeated testing shows a declining

NOx activity in this type of catalyst. NOx adsorbers—a catalyst technology based on this type

of chemistry—require alternating adsorption and regeneration cycles.

NOx reductions may be also caused by reactions between NO and diesel particulates or HCs

(including HCs stored in the washcoat). As reactions with solid particulates require a long

residence time in the catalyst zone, they are more likely to occur in systems with catalyst

supports that can provide some soot holding capacity (e.g., ceramic foams).

NOx Increase. Small NOx increases over the DOC are occasionally reported [Yamamoto

2006], the mechanism of which remains unclear. It is generally believed that NOx cannot be

synthesized from nitrogen and oxygen under the low pressure and temperature conditions in

the exhaust system. One reaction pathway that has been suggested is via formation of nitrated

HC species, followed by their catalytic oxidation.

NO FormationWhile the total NOx remains essentially unchanged, nitrogen oxides do undergo chemical

changes over the diesel oxidation catalyst. Engineout NOx emissions are composed of nitric

oxide—the main NOx component—and nitrogen dioxide. Additional quantities of NO may be

generated via catalytic oxidation of NO according to Equation (5), resulting in increased

NO /NO ratios downstream of the DOC. Figure 5 shows example NO concentrations before

and after the catalyst, plotted versus exhaust gas temperature [Ambs 1993]. The data comes

from one of the first studies that identified the DOC as a source of undesirable NO emissions

in underground mines.

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Figure 5. Concentration of NO with Diesel Oxidation Catalyst7 liter Caterpillar 3304 PCNA engine rated 100 hp at 2200 rpm. High sulfur diesel fuel. 2.5 liter

diesel oxidation catalyst.

A large increase in NO concentration was observed at a temperature of about 300°C. The

nitrogen dioxide concentration increased from about 10 ppm in the raw exhaust to a

maximum of 120 ppm after the catalyst. When temperatures approached 400°C, the

NO concentration decreased to reach practically zero at about 460°C. That decrease in

NO is related to the thermodynamic equilibrium of the reaction. Since the concentration of

nitric oxide during the experiment was between 800 and 900 ppm, the observed maximum

conversion efficiency of NO to NO was approximately 12%.

The NO/NO chemistry over a DOC depends on a number of factors, including the catalyst

formulation and size, temperature, the sulfur level in the fuel and the composition of the

exhaust gas. Higher NO conversions are seen with ultra low sulfur fuels, as the catalytic

oxidation of nitric oxide is in part blocked by sulfur. NO oxidation is also inhibited by CO and

other reductants, including HC and soot [Crocoll 2005].

In the presence of reductants, NO in the feed gas is believed to be first reduced to NO. Only

after the reductants (CO, HC) are oxidized over the catalyst, the NO is reoxidized to NO . It

has been suggested that the NO reduction step takes place mostly at the catalyst inlet face

and is relatively independent of space velocity, while the following NO oxidation occurs

throughout the catalyst length and depends on the catalyst volume [Katare 2007]. Thus, if

reductants are present, the DOC may either increase or decrease NO emissions. As a general

trend, catalysts that are very active in oxidizing CO and HC are likely to increase

NO emissions, while catalyst of lower CO/HC activity may actually reduce NO .

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Examples of NO activity of catalysts on heavy and lightduty diesel engines are shown in the

following figures. The data in Figure 6 suggests that the NO/NO chemistry over a DOC can

be also influenced by washcoat storage phenomena [Strots 2008]. The upper chart shows the

NO /NOx ratio before and after the catalyst, measured over the duration of the FTP Transient

test. The lower chart represents the DOC inlet and outlet temperatures during the test. An

active catalyst formulation was used (designed to increase NO levels in an SCR system) but

in the beginning of the test, until approximately 250 seconds, the DOC actually reduced

NO levels. This reduction is most likely explained by reactions between the engineout

NO and hydrocarbons stored in catalyst washcoat, in the absence of NO conversion due to

low catalyst temperature. Only after the initial period, a net increase in NO concentration

was observed.

Figure 6. DOC Impact on NO /NOx Ratio Over FTP Transient Test(Courtesy of Navistar Inc.)

The data in Figure 7 shows that under certain conditions a DOC can be a net consumer of

NO [Katare 2007]. The chart depicts cumulative emissions (as well as the catalyst inlet

temperature) measured from a lightduty vehicle over the FTP test cycle. The catalyst—aged

to 120,000 miles and exposed to the relatively low FTP temperatures—provided a significant

nitrogen dioxide reduction, as apparent from the DOCin and DOCout NO lines. In the

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same study, a catalyst aged to 50,000 miles also reduced NO , but to a smaller degree, while a

fresh (degreened) catalyst increased NO emissions.

Figure 7. Impact of Aged DOC on Cumulative NO Emissions Over Light-Duty FTP Test

Literature data on catalyst formulations of suppressed NO/NO activity is scarce. There are

opinions that formulations developed for sulfatesuppressed catalysts, or at least some of

them, are also effective in preventing NO formation. Published data supports this notion in

the case of vanadium, the addition of which suppressed both the sulfate make and the

NO/NO shift [Layrer 1994].

NO Catalysts. In certain diesel emission control systems, increased nitrogen dioxide

concentrations are generated on purpose to facilitate particulate filter regeneration or SCR

reactions. Specially formulated oxidation catalysts of high activity are used to generate the

NO . To achieve high NO conversions, catalyst with Pt loadings of about 100 g/ft and above

are often used. Due to the high cost of these catalysts, a strong incentive exists to use Pt/Pd

formulations [Doumeki 2006][Makino 2006].

NO conversion efficiency as a function of temperature over an NO generating catalyst is

illustrated in Figure 8. The particular curves, generated in a laboratory flow reactor, illustrate

the effect of the feed gas composition and catalyst conditions on NO oxidation [Katare 2007a].

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Figure 8. Conversion of NO over Diesel Oxidation Catalyst

In all cases, beginning at around 275325°C the NO conversion efficiency declines due to

thermodynamic constrains. The reaction of NO oxidation, Equation (5), has an equilibrium

which shifts to the left side of the equation at higher temperatures, as indicated by the

“Equilibrium” curve in Figure 8 (the exact location of the curve depends on the

concentrations of reacting species in the exhaust gas).

The maximum NO conversion strongly depends on the composition of the gas. A conversion

in excess of 90% was reached with dry NO+O gas, while addition of water had an inhibiting

effect (solid lines in Figure 8). The reaction was further inhibited by CO and HC. A maximum

conversion of only about 50% was observed with 0.2% CO and 0.2% HC (and 4.5% H O).

While similar feed gas was used in all three experiments shown in dashed lines, a further drop

in NO2 activity was seen after aging the catalyst with phosphorus (the other catalysts were

aged with sulfur only) and after coating the catalyst with soot. In the latter case, NO was

likely consumed via reactions with the soot.

In real life diesel engine applications, aged NO catalysts typically have maximum efficiencies

of less than 50% [Makino 2006]. NO /NOx ratios of 3050% have been measured downstream

of various types of catalytic particulate filters [LeTavac 2002][Ayala 2001]. The

NO/NO reaction is further discussed in Diesel Filter Regeneration paper.

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Particulate Matter Performance

Transformation of PM in Oxidation CatalystThe total diesel particulate matter (TPM) emission is composed of three major fractions

including the carbonaceous particulates, the organic particulates (SOF), and sulfates (SO4).

Each of these fractions behaves differently over the diesel oxidation catalyst. In general, the

overall effect of the DOC on the total PM emission could be a decrease, as well as an increase.

Typical transformations of the three fractions and the resulting total PM emissions are

schematically illustrated in Figure 9 (for the sake of simplicity, the dependence of engineout

PM composition on exhaust temperature has been neglected). As apparent from the chart,

PM emissions can be reduced by the DOC through the removal of their organic fraction

(SOF). Under certain conditions, however, the SOF decrease can be more than offset by an

increase of sulfate PM, leading to an overall increase in TPM emission (if high sulfur fuels are

used, sulfate particulate emissions may be much higher than shown in Figure 9).

Figure 9. DOC Impact on PM Emissions with Sulfur Containing Fuels

While literature reports exist that show both small decreases and small increases of solid,

insoluble PM in the DOC, it is generally agreed that carbonaceous particulates pass virtually

unchanged through oxidation catalysts [Eastwood 2000]. Considering their large size in

comparison to gas molecules, carbon particles are not likely to come into contact with the

catalyst. Even if they do, both the residence time and the temperature in diesel exhaust are

insufficient for any significant oxidation rates.



The organic fraction (SOF) of diesel particulates, composed of heavy hydrocarbons, is very

effectively oxidized in the catalyst. SOF removal is the major effect contributing to a decrease

in the TPM emissions over the diesel oxidation catalyst. SOF oxidation is similar to the

oxidation of gas phase hydrocarbons, i.e., a certain temperature has to be reached for the

catalyst to lightoff (Figure 1). After the lightoff temperature is reached, the conversion of

SOF shows little change with further temperature increase.

The sulfate fraction of diesel particulates (SO4) is increased in the DOC due to the oxidation

of SO with subsequent formation of sulfuric acid, Equation (3) and Equation (4). This is a

counterproductive process causing an increase in TPM emissions. The intensity of sulfate

make increases with exhaust gas temperature and becomes critical at about 400°C. With high

sulfur fuels, the DOC is likely to increase TPM emissions due to sulfate formation. With low

sulfur fuels (approximately 300500 ppm S), special catalyst formulations are necessary to

suppress that process and, thus, to make the diesel oxidation catalyst a viable PM reduction

approach. Only ULSD fuels eliminate the sulfate problem, allowing for most flexibility in

diesel oxidation catalyst formulation.

The net effect of the DOC on diesel particulate matter is a combination of the SOF and SO4

reactivities, as it was shown in Figure 9. The catalytic oxidation of SOF contributes to the

decrease in TPM while the generation of sulfates increases particulate emissions. The desired

SOF effect prevails at low temperatures. In practice, if sulfur is present in the fuel, there is

always a temperature above which an increase of PM emissions will be observed.

Selective diesel catalysts specifically optimized for particulate matter control can tolerate

certain sulfur levels in the fuel and still control PM emissions. These selective formulations,

however, compromise the catalyst activity, resulting in less effective gas phase HC and CO

control. Only ultralow sulfur fuels can eliminate these constrains and maximize the benefits

of catalytic emission control technologies.

PM Emission ReductionThe potential of catalytic PM emission reduction critically depends on the composition of

engineout particulates. Only particulates of high SOF contents, so called “wet” particulates,

can be effectively controlled by catalysts. Due to different combustion system design, the

organic PM portion that is susceptible to catalytic oxidation varies between engines. In a

given engine, SOF content depends on the operating conditions, changing from well above

50%—in some cases approaching 90%—at low loads to just a few percent at high engine loads.

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Example data illustrating SOF content as a function of engine operating conditions was

presented in the paper on diesel particulate matter.

Figure 10 shows typical conversion curves for total PM and for SOF over a Pt and a Pd catalyst

using diesel fuel of 300 ppm S [Matsumoto 2003]. Both catalysts are effective in reducing PM

emissions at lower temperatures (with the Pt catalyst being active at lower temperatures than

the Pd formulation due to its better HC/SOF lightoff). As temperatures increase, the PM

conversion drops, despite the relatively steady SOF conversion. In the case of the Pt catalyst,

which is very active in the oxidation of SO , the drop in efficiency is mainly caused by the

formation of sulfates. The active Pt formulation in Figure 10 becomes a net PM producer from

just above 250°C. In the case of the Pd catalyst—with its low SO activity—the decreasing

efficiency is caused mainly by the decreasing SOF fraction, with sulfate make playing a certain

role at higher temperatures (450500°C).

Figure 10. PM and SOF Conversion over Pt and Pd Catalysts3.1 L IDI NA diesel engine @2000 rpm; 2.5 L catalyst; Pt: 2 g/L; Pd: 0.4 g/L; Fuel sulfur: 300 ppm

The average SOF content over an engine test cycle depends on its average load factor. Cold

test cycles of low engine loads will produce wet particulates, which can be effectively

controlled by the DOC (as long as the temperatures are sufficient for the catalyst to lightoff).

Hot, highly loaded test cycles, on the other hand, tend to produce dry particulates, which are

2

2

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not effectively controlled using the DOC. Wet particulates—collected over lightly loaded test

cycles—can have SOF contents of more than 50%, which can be almost entirely removed by

the catalyst. Indeed, PM removal efficiencies of 50% and more have been observed with diesel

cars tested on the relatively cold ECE+EUDC cycle using fuels of less than 500 ppm

sulfur [Kharas 1998].

In heavy duty applications (using the same 500 ppm sulfur fuel), the PM emission reduction

potential is in most cases limited to 2030%. In the US EPA Urban Bus Retrofit Rebuild

(UBRR) program, a number of catalysts were certified to achieve a 25% PM emission

reduction over the (medium load) FTP transient cycle on twostroke engines. Most of these

catalysts were not able to achieve the same 25% PM emission reduction on four stroke

engines, which produce PM emission of lower SOF content than the twostroke technology.

Even when tested on rather cold test cycles, such as the Japanese 13mode, PM emission from

heavyduty engines could be reduced by only about 20%, due to high sulfate production

during the high temperature modes [Mogi 1999]. The oxidation catalyst technology has not

been successful in controlling diesel particulates from very hot applications and test cycles,

such as the R49, ESC and ETC. Improved performance and higher PM emission reductions—

depending on the SOF levels—may be possible with ultra low sulfur diesel fuels.

Finally, it should be remembered that diesel particulates are sometimes defined in different

ways, which may have implications for catalytic control technologies. In some occupational

health regulations, diesel particulates are determined as elemental carbon (EC). In such case,

the DOC technology would be ineffective in controlling PM emissions. In certain other

regulations, diesel particulates are defined as total carbon (TC), i.e., including the SOF part,

but excluding sulfates. In this case, catalysts can be a very effective control approach, even

with sulfurcontaining fuels, as sulfate make is no longer an issue.

Oxidation of Solid Carbon (Soot)Reports on changes in PM carbon fraction over DOCs must be always approached with careful

scrutiny. The magnitude of change in many of such reports is on the threshold of detection.

Measurement error can be magnified due to the analytical methods that determine insolubles

only indirectly, by subtracting sulfate and SOF from total PM. If sulfuric acid reacts with

metals, such as with calcium from lube oil additives, the resulting sulfate salts may be

accounted for as insoluble material. Since storage of solid particles on the catalyst surface and

within its pore network—as discussed below—plays an important role in solid PM conversion,

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the catalyst performance may be also prone to particle blowoff, such as during engine

transients.

The marginal level of performance—or the lack thereof—of the conventional DOC to oxidize

and reduce emissions of the solid, carbonaceous PM fraction (a.k.a. soot) is attributed to the

insufficient residence time of the particles within the catalyst and to their poor contact with

the catalyst sites. Due to their larger mass and size, solid particles are less likely to enter the

pore network in the washcoat/substrate system than gas molecules. If they do, more time is

required for the catalytic reactions to complete.

Catalysts can be optimized for improved solid PM conversion through increasing their

capacity to trap and store (at least temporarily) solid particles. For instance, washcoat and

substrate optimization that would make more substrate porosity available to the exhaust gas

could result in more storage and oxidation of solid particles. Among the two catalysts in

Figure 11, catalyst B was a ‘conventional’ DOC, while the coating of catalyst C was optimized

for solid carbon conversion[Matsumoto 2003]. It is clear that the washcoat optimization indeed

increased the conversion of solid PM. It is also interesting to note that the solid carbon

conversion was improved with both catalysts in the presence of a higher SOF fraction (the

three SOF content points represent measurements on three different engines).

Figure 11. Conversion of Solid Carbon Fraction As a Function of SOF ContentJP 13-mode test using three engines of different SOF content. Two catalysts with the same

substrate (400 cpsi, 35% porosity) and noble metal (Pt = 2 g/L) but different washcoat loadingand formulation.

The results in Figure 11 represent measurements over the Japanese 13mode cycle, with

catalyst inlet temperatures increasing from approximately 100°C to 450°C during the test. In



the same study[Matsumoto 2003], steadystate measurements were performed at 500°C using

catalysts coated on different substrates, Table 2. At that temperature, the PM had a very low

SOF content of 5%. A correlation was found between the substrate porosity and conversion of

carbon particulates—solid PM conversion of nearly 30% was measured with substrate of the

highest, 65% porosity. Lower washcoat loadings had to be used on substrates of lower cell

density and higher porosity to avoid filling the porosity with washcoat. This might have

caused the tradeoff between the solid PM and the gas phase performance, the latter

illustrated by the HC conversion rates in the table.

Table 2Effect of Substrate Porosity on Solid PM Conversion at 500°C

Catalyst B Catalyst C Catalyst D Catalyst ECall Density, cpsi 400 400 300 300Porosity, % 35 35 59 65Mean Pore size, µm 12 12 24 22Solid PM Conversion 0% 0% 20% 29%HC Conversion 90% 83% - 80%

Note:Steady-state engine test @500°C.PM composition: 5% SOF, 5% sulfate (30 ppm S fuel).All catalysts 2 g/L Pt, but different washcoat loadings.Catalyst designations consistent with Figure 11.

The results in Figure 11 and in Table 2 implicate that solid particle trapping within the

catalyst may involve a combination of two mechanisms [Matsumoto 2003]. At lower

temperatures, SOF and heavier gas phase HCs may be condensing on the catalyst surface,

facilitating better adhesion of carbon particles, Figure 12. In addition, HC oxidation when the

temperature increases may cause a local temperature increase, enabling faster oxidation rates

of the carbon (via either the NO or O mechanisms). At high temperatures, the SOF content

is low and its effect is not significant, with particle trapping in the pores becoming the

dominant mechanism (Figure 12, right hand side).

2 2



Particle Oxidation Catalysts

Figure 12. Schematic Representation of Particle Trapping Mechanisms on Catalyst Surface

More sophisticated, special catalyst substrates have

been developed that can capture and store carbon

particles, which are oxidized by NO generated in an

upstream catalyst. These specialized diesel oxidation catalyst systems are referred to

as particle oxidation catalysts.

Mechanism of SOF RemovalTo analyze the mechanism of catalytic removal of SOF, one needs to recall again the definition

of diesel particulate matter. Diesel PM is defined as everything that is collected from diluted

diesel exhaust gas on an inline filter at a temperature not higher than 52°C. Temperatures in

the raw exhaust and in the catalyst are much higher, usually somewhere between 150 and

450°C. A significant proportion of the hydrocarbon species which contribute to the SOF

material are in the vapor phase under these conditions and can undergo catalytic oxidation in

exactly the same way as the gas phase HC. After the catalyst, when exhaust gases mix with air

in the dilution tunnel, heavy hydrocarbons condense and/or adsorb onto the carbon particles

to form the SOF.

While the above reasoning explains how SOF compounds can come into contact with the

catalyst, closer examination of the control of PM by catalysts revealed facts that were not

consistent with the oxidation theory, suggesting that other mechanisms of SOF removal must

be also taking place in the DOC. Once past the lightoff, emission control catalysts operate in

the mass transfer control regime. Reaction rates depend on mass transfer coefficients for the

particular reactants, which in turn depend on their diffusion coefficients. If so, hydrocarbons

of shorter carbon chains, characterized by higher diffusion coefficients, should have higher

conversion rates over the catalyst in comparison to heavier, longer carbon chain

hydrocarbons (provided all species are above their lightoff; short chain HCs are known to

have high lightoff temperatures). Experiments, however, were in disagreement with

theoretical modeling based on oxidation alone, showing higher conversion rates with

increasing HC molecular weight [Johnson 1994][Voss 1997]. This behavior is likely explained

by catalytic cracking of heavy HCs into lighter compounds, catalyzed by base metals

(washcoat oxides). Cracking appears to be very important, maybe even the dominant, SOF

removal mechanism over base metal catalysts, as reported with cerium oxide

2



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catalysts [Farrauto 1997] or zeolite catalysts (a rare earth metal exchanged zeolite oxidation

catalyst, not optimized for HC storage) [Standt 1995]. The cracked, shortchain HC products

are then measured as gas phase HCs, and not a part of SOF. This nonoxidation SOF removal

path seems to be also confirmed by the observation that while catalytic oxidation of CO and

gas phase HC increases with platinum loading, the conversion of SOF is relatively

independent from Pt load [Mogi 1999][Farrauto 1997].

The explanation of the fate of SOF over the diesel oxidation catalyst by a combination of

catalytic oxidation and catalytic cracking is still not entirely sufficient. Another important

mechanism that influences the DOC performance is HC storage and release in the washcoat.

HC storage effects are seen mostly at low catalyst temperatures. Results of laboratory

experiments on storage and release using feed gases with different HCs are illustrated in

Figure 13 [Adams 1996]. Adsorption of some gases in the washcoat, especially those of higher

boiling point such as decane or toluene, resulted in apparent conversion efficiency of as much

as 90% at very low temperatures. As temperature increased, the HCs were desorbed, but the

desorption temperatures varied among species. Toluene was almost entirely desorbed at

70°C, while the desorption of decane extended through 190°C. It was also observed that HCs

and water adsorbed in competition.

Figure 13. HC Storage in Diesel Oxidation CatalystTemperature ramp experiment (10 min. wait @ 59°C); Feed gas contains 3-4% water; SV =

33,333 hr

Since HCs in Figure 13 were released from the washcoat mostly below the lightoff

temperature, there was little or no emission benefit due to the storage effect. Catalyst

-1



manufacturers have developed materials that are able to hold the adsorbed HCs into higher

temperatures, say 250°C, in order to catalytically oxidize them once released from the

washcoat. These materials, which typically are various types of zeolites, are known

as hydrocarbon traps. Hydrocarbon traps eventually reach their saturation, thus losing their

effectiveness in a steadystate experiment, but may be very effective in enhancing PM

conversion and improving the apparent HC lightoff over transient test cycles. Many

commercial diesel catalysts introduced since the late 1990s for both light and heavyduty

applications include zeolites optimized for HC trapping [Sawyer 1998][Yavuz 2001].

Sulfate Formation & DOC Selectivity

Catalyst selectivity was the focus in the development of the diesel oxidation catalyst through

the 1990s. A selective catalyst would exhibit high activity towards the oxidation of

hydrocarbons but, at the same time, low activity in the oxidation of sulfur dioxide and low

sulfate particulate make. A lot of research had been conducted to understand the mechanisms

of catalytic sulfate formation on different catalyst systems and with fuels of different sulfur

contents. A number of commercial catalysts had been developed for the 500 ppm S fuel which

represented the certification fuel in the USA (1994), Europe (1996), and Japan (1996). The

selectivity of diesel catalysts and the sulfate emission problem became less important with the

introduction of ultra low sulfur diesel fuels of 1015 ppm S content. However, sulfate

suppressed catalyst formulations still must be used for markets with fuels of higher sulfur

content.

The generation of sulfates in the diesel oxidation catalyst depends on the following factors:

sulfur content in the fuel

catalyst temperature

catalyst formulation.

Sulfur dioxide, the precursor of sulfate particulates, originates from the fuel sulfur. The lower

the content of sulfur in the fuel, the less SO is present in the exhaust gas, and the less

sulfates are generated in the catalyst. If the catalytic conversion of SO into SO , Equation

(3), and the brakespecific fuel consumption are known, one can easily estimate the sulfate

particulate emissions. The following formulas can be used for the calculation. It is assumed

2

2 3



(6)

(7)

(8)

(9)

(10)

that the sulfate particulates include all of the sulfuric acid and the hydration water. The water

contribution depends on the assumed hydration ratio, as discussed in the “Diesel Particulate

Matter” paper. Thermodynamically favored ratio during sulfate formation is 2.67, while

typical hydration ratio in samples collected and preconditioned for TPM determination is

about 7. Equation (6) provides results in g/kWh, Equation (10) produces concentrations in

kg/m . The mass and volume (m, V) are brake energyspecific magnitudes (per kWh).

m = K m w M /M

M = M + xM

m = m (1 + R )

V = m /ρ

c = m /V

where: K fractional conversion of SO to SO m mass, kg/kWh w weight fraction of sulfur in fuel M molar mass, kg/kmole V volume, m /kWh R engine airfuel ratio, kg/kg ρ exhaust gas density, kg/m x hydration ratio of H SO in sulfate particulates c concentration in the exhaust gas, kg/m Indexes: F fuel; S sulfur; sulfate sulfate particulates; exh exhaust gas

A very illustrative experimental results on the dependence of sulfate formation on the fuel

quality are shown in Figure 14, which compares the total particulate matter emissions with a

platinumbased nonselective oxidation catalyst for three different diesel fuels containing

1500 ppm, 500 ppm, and 25 ppm of sulfur [Smedler 1995]. For the sake of clarity, the

composition of PM (carbon, sulfate, SOF) is not shown in the graph. The particulates are

composed mostly of SOF at the lowest, 130°C, temperature. At higher temperatures, the

particulates are composed primarily of sulfates. The carbon fraction is not more than 0.01

g/kWh at any catalyst temperature.

3

sulfate F S sulfate S

sulfate H SO2 4 H O2

exh F A/F

exh exh exh

sulfate sulfate exh

2 3

S

3

A/Fexh

3

2 4 3

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Figure 14. Particulate Emissions with an Oxidation Catalyst

A substantial increase in the TPM emissions was observed with the 1500 ppm S fuel.

Obviously, the nonselective catalyst was totally inappropriate for this application. When used

with the low sulfur (500 ppm S) fuel, the nonselective catalyst still increased the engineout

PM emissions several times, especially at higher temperatures, due to sulfate make. Only with

the 25 ppm S fuel the catalyst sulfate make was almost insignificant over the entire

temperature range. Hence, a nonselective oxidation catalyst can be used only in conjunction

with fuels of very low sulfur levels, without increasing PM emissions.

The generation of sulfates in Figure 14 increases with temperature up to about 450°C, where

it exhibits a maximum. This maximum can be explained by the thermodynamic equilibrium

of the oxidation of SO to SO , reaction Equation (3). At high temperatures the reaction

equilibrium shifts to the left side of the equation, i.e., the higher the temperature the less

SO can be oxidized and less SO is produced. An equilibrium curve, computed for conditions

typical for diesel exhaust, is presented in Figure 15 [Henk 1992].

2 3

2 3



(11)(12)

Figure 15. Thermodynamic Equilibrium for Oxidation of SO to SO

The equilibrium is expressed in terms of maximum (equilibrium) conversion of sulfur dioxide

as a function of temperature. The declining equilibrium conversion of SO is in agreement

with experimental data which typically show a gradual increase of PM emissions with

temperature up to about 400°C. At these temperatures the sulfate formation increases due to

the increased reaction kinetics at higher temperatures. After the temperature reaches 400

450°C, the reaction equilibrium limits the rate of sulfate formation and, despite very high

reaction rates, the overall rate of sulfate formation decreases.

In addition to the steadystate oxidation of SO with the following formation of sulfuric acid,

as described by equations (3) and (4), storage and release of sulfates from the washcoat can

occur in the diesel oxidation catalyst. Sulfur can be stored either as SO or as aluminum

sulfate on alumina, or other materials, used as the catalyst washcoat. At a certain

temperature, the stored material can be released and converted to sulfate. The sulfur release

can be described by the following reactions:

Al (SO ) = Al O + 3SOAl O ·SO = Al O + SO

Some studies measured the quantity of stored sulfur at up to around 2% of the catalyst

washcoat mass [Henk 1992]. Considering typical catalyst sizes and washcoat loads, the sulfur

storage can influence the sulfate response of the diesel catalyst.

Figure 16. Particulate Emissions over Repeated FTP Cycle

2 3

2

2

2

2 4 3 2 3 32 3 2 2 3 2



Figure 16 shows the results of 12 emission test repetitions of a catalyst equipped diesel engine

over the US heavyduty transient cycle [Harayama 1992]. The sulfur content in the test fuel was

400 ppm by weight. An increase of TPM emissions was recorded over the subsequent tests.

The particulate analysis indicated that the sulfate particulates were increasing with the test

repetitions. In the first test the catalyst was reducing the PM emissions by about 25%. In the

last test, the PM emissions were increased by about 30%, relative to the engine baseline.

Apparently, sulfates were stored in the catalyst washcoat but were not released under the test

conditions. During the first tests, sulfur oxides were adsorbed in the washcoat and, thus,

removed from the system. A net decrease of TPM emissions was observed. As the catalyst

washcoat gradually saturated, more sulfate particulates appeared after the catalyst, eventually

leading to an increase in the PM emissions.

The above example should be remembered whenever emission performance of fresh catalysts

is tested. It is important to precondition or “degreen” fresh catalysts before testing. If the

degreening is not properly done, the test results may be very misleading.

The sulfur storage phenomena were extensively studied in laboratories. A correlation was

found between the catalyst sulfur storage capacity and the formation of sulfate particulates.

The catalyst washcoat which stores the highest amount of sulfur is alumina. The Pt/alumina

catalyst is known as the most active oxidation catalyst, very effective for oxidation of SOF but

also generating the highest sulfate particulate emissions. The sulfur retention in the

Pt/alumina catalyst may be close to 2% of the washcoat mass. The alumina surface area is not

sufficient for a monolayer adsorption of that quantity of sulfur. The storage mechanism must

also involve some formation of bulk aluminum sulfates. Catalyst systems with lower sulfur

storage capacity also exhibit lower rates of sulfate particulate formation. Examples of such

systems are Pt on silica or Pd on silica with sulfur retention typically less than 0.05% of the

washcoat mass.

In spite of the correlation between sulfur storage and the catalyst sulfate make, it is not clear

if the sulfate release is an important contribution to sulfate emissions. It is difficult to set up

an experiment that would convincingly separate the contribution of the steady state

SO oxidation from that of the sulfate release from the washcoat. These two processes are also

interconnected by the equilibrium of the SO oxidation. If SO is released from the washcoat

at higher temperatures, it influences the equilibrium of reaction (3) preventing more

SO from being oxidized.

2

2 3

2



Selective Catalysts. Catalyst selectivity was the focus in the design of catalysts to control

particulate emissions from diesel engines operated with low sulfur fuels. There is always a

tradeoff between the catalyst activity and its selectivity. A catalyst with a suppressed sulfur

dioxide activity will also show lower HC and CO conversions. And vice versa, catalysts which

are most active in the oxidation of CO and HC (and SOF) generate most sulfate particulates.

The objective in the selective catalyst design was to minimize the CO/HC activity penalty for a

given sulfate suppression. The following approaches had been used to design selective diesel

catalysts:

special catalyst preparation methods such as precalcination

selection and loading of the noble metal

selection of washcoat materials

sulfate suppressing washcoat additives.

Thermal treatment (calcination) of the finished Pt/Al O catalyst improves its selectivity.

During that process some sintering of platinum occurs. Apparently, the finer dispersions of Pt

favor the oxidation of SO to SO , while larger Pt grains make the catalyst more selective.

Platinum catalysts are much more active in the oxidation of sulfur dioxide than palladium

based systems. The use of palladium rather than platinum, or Pt/Pd blends, had been

suggested as a means of sulfate suppression. The addition of rhodium was also found effective

in controlling sulfate formation. The activity of platinum towards SO oxidation depends on

the Pt loading to a much higher degree than other functions of the catalyst. Some selective

diesel oxidation catalysts used platinum at very low loadings of 0.52 g/ft .

Washcoat formulation can strongly influence the catalyst performance. Several nonsulfating,

chemically inert oxides, such as silica, titania, and zirconia, can be used to suppress the

sulfate. Some washcoat oxides, such as ceria, exhibit catalytic properties of their own and can

reduce SOF emission even without the presence of noble metals.

Various base metal oxides (V, Mo, Nb, ...) can be added to the catalyst washcoat to inhibit the

oxidation of SO . Vanadium oxide, which has been identified as one of the most effective

sulfate suppressing additives [Wyatt 1993], was used in some commercial diesel oxidation

catalysts (and still is in markets with higher sulfur fuels).

The addition of a sulfate suppressing agent may also have a negative impact on catalyst

2 3

2 3

2

3

2



durability. The exact mechanism varies from system to system, but acceleration of support

sintering, pore blockage, and formation of inactive surface compounds with the noble metal

component are possible.

Effect on Unregulated Emissions

Particle Number EmissionContradictory literature reports exist on the effect of diesel oxidation catalysts on particle

number emissions. Since diesel particle number emissions can be attributed primarily to

nuclei mode particles, which are composed mostly of hydrocarbon and sulfuric acid

condensates, one can easily explain the performance of the DOC by analyzing its effect on

nuclei mode particle precursors. If the catalyst removes hydrocarbons (gas phase and SOF), it

prevents their subsequent nucleation, thus reducing the particle number emission [Westphal

2012]. If, however, the catalyst produces sulfates—an effect more prominent with high sulfur

fuels and more active, noble metal catalysts—the particle numbers may be increased due to

sulfuric acid nucleation.

Experiments which attempt to quantify the impact of DOCs on particle numbers must be very

carefully designed. Catalysts can be a source of additional error, such as sample loss due to

thermophoretic forces or sample additions due to solid particle blowoff and/or release of

condensates from the washcoat.

Polynuclear Aromatic HydrocarbonsPAH emissions are divided between the gas and particulate phases. The heaviest PAHs

constitute part of the SOF fraction of diesel particulates, while the lighter species can be

identified as part of the gas phase HC emission. The HC and SOF effectiveness of the diesel

oxidation catalysts extends on the PAH class of compounds and other HC derivatives. Table 3

lists PAH results from two DOCs (“medium” (B) and “low” (D) activity formulations) tested

on a heavyduty diesel engine (test cycle: FTP Transient; engine: 1998 model 12.7 liter DDC

series 60 EUI engine, turbocharged and aftercooled, rated 400 hp @ 1800 rpm). The two

catalysts provided average PAH emission reductions of 68 and 54%, respectively [MECA

1999].

Table 3Catalytic Conversion of PAH Emissions

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CompoundEmissions Reduction

Baseline Cat B Cat D Cat B Cat Dµg/bhp-hr µg/bhp-hr µg/bhp-hr % %

Napthalene 295 159 182 46.1% 38.3%

2-Methylnapthalene 635 278 277 56.2% 56.4%Acenapthalene 40 13 13.6 67.5% 66.0%Acenapthene 46 25 24.4 45.7% 47.0%Fluorene 72 29 28.9 59.7% 59.9%Phenanthrene 169 54 56 68.0% 66.9%Anthracene 10 2.6 2.8 74.0% 72.0%Fluoranthene 7.7 2.6 4.9 66.2% 36.4%Pyrene 14 5 6.4 64.3% 54.3%Benzo(a)anthracene 0.22 0.05 0.18 77.3% 18.2%Chrysene 0.51 0.16 0.33 68.6% 35.3%Benzo(b)fluoranthene 0.26 0.09 0.12 65.4% 53.8%Benzo(k)fluoranthene 0.15 0.05 0.08 66.7% 46.7%Benzo(e)pyrene 0.26 0.08 0.14 69.2% 46.2%Perylene 0.01 0 0 100.0% 100.0%Indeno(123-cd)pyrene 0.13 0.04 0.07 69.2% 46.2%Dibenz(ah)anthracene 0.01 0 0 100.0% 100.0%Benzo(ghi)perylene 0.32 0.1 0.22 68.8% 31.3%Total 1290.57 568.77 597.14 55.9% 53.7%Average Reduction 68.5% 54.1%

Biological ActivityThe activity of untreated and catalysttreated diesel exhaust on living organisms can be

evaluated through a number of specialized biological tests. An example is the Ames test,

which was used by many studies, especially in the 1980s and 1990s, to evaluate the mutagenic

activity of diesel exhaust.

Several studies specifically examined the effect of the DOC on exhaust mutagenicity, but no

consensus was ever reached. Some studies reported an increase of the mutagenicity when

diesel catalysts were used [Hunter 1990]. In other studies, mutagenicity of diesel exhaust

extracts decreased with the use of diesel oxidation catalysts. For instance, McClure [McClure

1992] found that the mutagenicity of the SOF portion of diesel particulates decreased by 41%

and the mutagenicity of the TPM sample decreased by 51% with a DOC. In another study, a

DOC eliminated Ames mutagenicity in the gas phase, but only slightly reduced mutagenicity

of the particle phase [Westphal 2012].

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The biological activity of diesel exhaust can be reasonably anticipated to depend on a number

of factors, including fuel properties [Rasmussen 1990], engine and catalyst technology, as well

as engine operating conditions. In view of this complexity, more recent health studies tend to

focus on the effects of a given engine technology in its entirety [Khalek 2009], rather than

attempting to study the effects of the DOC as an isolated system component.

References

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Ambs, J.L., B.T. McClure, 1993. “The Influence of Oxidation Catalysts on NO2 in Diesel Exhaust”, SAETechnical Paper 932494, doi:10.4271/932494

Ayala, A., Kado, N., Okamoto, R., 2001. “ARB Study of Emissions from Latemodel Diesel and CNGHeavyduty Transit Buses”, California Air Resources Board

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Eastwood, P., 2000. “Critical Topics in Exhaust Gas Aftertreatment”, Research Studies Press, Baldock,Hertfordshire, England

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Jacobs, T.J., Assanis, D.N., 2007. “Characteristic response of a production diesel oxidation catalystexposed to lean and rich PCI exhaust”, Proceedings of the ASME ICE Division 2007 Fall technicalConference, Charleston, SC, ICEF20071733

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Kapur, N., J. Hyun, X. Hao, B. Shan and K. Cho, 2011. “Overcoming Hydrocarbon Inhibition on Pdbased Diesel Oxidation Catalysts with Rational Catalyst Design Approach”, 2011 Directions inEngineEfficiency and Emissions Research (DEER) Conference Presentations, October 36, 2011Detroit,Michigan,http://www1.eere.energy.gov/vehiclesandfuels/pdfs/deer_2011/wednesday/presentations/deer11_kapur.pdf

Katare, S., J.E. Patterson and P.M. Laing, 2007. “Aged DOC is a Net Consumer of NO2: Analyses ofVehicle, Enginedynamometer and Reactor Data”, SAE Technical Paper 2007013984, doi:10.4271/2007013984

Katare, S., J.E. Patterson and P.M. Laing, 2007a. “Diesel Aftertreatment Modeling: A Systems Approachto NOx Control”, Ind. Eng. Chem. Res., 46, 24452454

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