ABSTRACT

Alkali and alkaline earth metal impurities found in diesel

fuels are potential poisons for diesel exhaust catalysts. Using

an accelerated aging procedure, a set of production exhaust

systems from a 2011 Ford F250 equipped with a 6.7L diesel

engine have been aged to an equivalent of 150,000 miles of

thermal aging and metal exposure. These exhaust systems

included a diesel oxidation catalyst (DOC), selective catalytic

reduction (SCR) catalyst, and diesel particulate filter (DPF).

Four separate exhaust systems were aged, each with a

different fuel: ULSD containing no measureable metals, B20

containing sodium, B20 containing potassium and B20

containing calcium. Metals levels were selected to simulate

the maximum allowable levels in B100 according to the

ASTM D6751 standard. Analysis of the aged catalysts

included Federal Test Procedure emissions testing with the

systems installed on a Ford F250 pickup, bench flow reactor

testing of catalyst cores, and electron probe microanalysis

(EPMA). The thermo-mechanical properties of the aged

DPFs were also measured.

EPMA imaging of aged catalyst parts found that both the Na

and K penetrated into the washcoat of the DOC and SCR

catalysts, while Ca remained on the surface of the washcoat.

Bench flow reactor experiments were used to measure the

standard NOx conversion, NH3

storage and NH3 oxidation

for each of the aged SCR catalysts. Flow reactor results

showed that the first inch of the SCR catalysts exposed to Na

and K had reduced NOx conversion through a range of

temperatures and also had reduced NH3 storage capacity. The

SCR catalyst exposed to Ca had similar NOx conversion and

NH3 storage performance compared to the catalyst aged with

ULSD. Using a chassis dynamometer, vehicle emissions tests

were conducted with each of the aged catalyst systems

installed onto a Ford F250 pickup. Regardless of the evidence

of catalyst deactivation seen in flow reactor experiments and

EPMA imaging, the vehicle successfully passed the 0.2 gram/

mile NOx emission standard with each of the four aged

exhaust systems. This indicates that total catalyst volume is

adequate to accommodate the catalyst activity loss observed

in the flow reactor experiments.

INTRODUCTION

Biodiesel can be made from various renewable sources such

as vegetable oil, animal fat, or waste cooking oil. It is

produced through transesterification of the oil or fat with

methanol, resulting in mono-alkyl esters and the co-product

glycerin. The process occurs in the presence of a catalyst,

Impact of Fuel Metal Impurities on the Durability of

a Light-Duty Diesel Aftertreatment System

2013-01-0513

Published

04/08/2013

Aaron Williams, Jonathan Burton and Robert L. McCormick

National Renewable Energy Laboratory

Todd Toops, Andrew A. Wereszczak, Ethan E. Fox and Michael J. Lance

Oak Ridge National Laboratory

Giovanni Cavataio, Douglas Dobson and Jim Warner

Ford Motor Co

Rasto Brezny

Manufacturers of Emission Controls Assoc

K. Nguyen and D. William Brookshear

University of Tennessee - Knoxville

doi:10.4271/2013-01-0513

NREL/CP-5400-57585. Posted with permission.

Presented at the SAE 2013 World Congress and 

Exhibition
typically sodium hydroxide or potassium hydroxide [1].

Following transesterification and separation of the glycerin,

the biodiesel must be purified by extraction of the impurities

into water or other means. In this process, residual amounts

of sodium (Na) or potassium (K) from the catalyst can be left

behind. In addition, small amounts of calcium (Ca) or

magnesium (Mg) can be added to the fuel from the

purification process [2,3]. Na and Ca may also be present in

conventional diesel fuel and have been implicated in internal

injector deposits [4,5]. Regardless of the source, these

metallic fuel contaminants are converted to oxides, sulfates,

hydroxides or carbonates in the combustion process to form

an inorganic ash that can be deposited onto the exhaust

emission control devices found in modern diesel engines.

Alkali metals are well known poisons for catalysts and have

been shown to negatively impact the mechanical properties of

ceramic substrates [6,7]. Furthermore, alkali metal

hydroxides such as Na and K are volatilized in the presence

of steam and can, therefore, penetrate the catalyst washcoat

or substrate. Early results showed no negative short-term

impact of biodiesel on the performance of emission control

system components [8,9,10,11]. Here, we examine long-term

impacts with a focus on the effects of residual metals.

Phase I Study

In 2011, a study led by the National Renewable Energy

Laboratory (NREL) investigated the impact of these alkali

and alkaline earth metals on the durability of catalysts in a

heavy-duty diesel application [12]. This study used an

accelerated aging method to expose catalysts to the

equivalent of 435,000 miles of fuel metal and thermal

exposure. The aftertreatment system consisted of a diesel

oxidation catalyst (DOC), diesel particulate filter (DPF), and

a selective catalytic reduction (SCR) catalyst placed at the

end. Several key findings in this study created the motivation

for further research, which is the purpose of the work

described in this paper. This previous study found that a

cordierite DPF exposed to the equivalent of 435,000 miles

through accelerated aging with Na, K and Ca in the fuel

experienced a 69% decrease in thermal shock resistance,

relative to a system aged with no metals in the fuel. This

study also found that the DOC exposed to the equivalent of

150,000 miles with fuel containing metals experienced a loss

of catalytic activity for hydrocarbons (HC) and NO oxidation

when compared with a catalyst aged with no metals. The

study showed that an SCR catalyst aged to 435,000 miles

with fuel containing metals showed a slight loss in NOx

conversion when compared with one aged with no metals.

This first phase of work studied a typical heavy-duty

configuration where the SCR catalyst is placed downstream

of the DPF. However, a light-duty system typically places the

SCR catalyst upstream of the DPF where it would experience

more exposure to metals and high DPF regeneration

temperatures. While the U.S. Environmental Protection

Agency's “full useful life” requirement is much shorter for a

light-duty application, the system configuration could

potentially represent a worst-case scenario for durability. The

purpose of the work presented in this paper is to investigate

this possibility.

TEST METHODS

Catalyst Aging

The exhaust catalysts selected for accelerated aging studies

were from a production 2011 Ford F250 pickup equipped

with a 6.7L diesel engine. These systems included a DOC,

SCR, and a DPF. The DOC consisted of two bricks mounted

in series with a total volume of 4.35 L. The SCR system

included a pair of Cu/zeolite bricks mounted in series with a

total volume of 9.88 L. The DPF was a catalyzed SiC

substrate and had a total volume of 8.2 L. Further catalyst

details are listed in Table 1. Four OEM exhaust systems were

purchased from a local dealership to be used for accelerated

aging. Each system was instrumented for temperature and

pressure measurements and for exhaust sampling. These

systems, as purchased from the dealership, were welded from

end to end with no access to the individual catalyst bricks.

Therefore, the DPF section was cut and flanged from the rest

of the exhaust to allow for weighing of the DPF to quantify

ash loading. A schematic of the exhaust system is shown in

Figure 1.

Table 1. Catalyst Specification

Figure 1. Schematic of exhaust system

For accelerated aging, these full exhaust systems were placed

onto an accelerated aging platform that was described in the

Phase 1 study [12]. The accelerated aging platform consisted

of a Caterpillar 2008 C9 ACERT engine. This is an 8.8L,

300-hp engine calibrated to meet the Tier III off-road

emissions limit. The engine is turbocharged and direct

injected and does not use an exhaust catalyst in its base

configuration. By retrofitting the engine with exhaust

emission controls, it served as a platform for exposing

various catalysts and filters to biodiesel exhaust gas. The
exhaust was retrofitted with a fuel injector placed 10 cm

downstream of the turbo flange and 183 cm upstream of a

DOC. This fuel injector was used to simulate hot exhaust

temperatures typical of a DPF regeneration event. A threemode test cycle was developed for the Caterpillar engine in

order to achieve catalyst temperature and space velocity

conditions consistent with operating parameters in an F250

pickup. The three modes, along with conditions for the SCR

catalyst, are shown in Table 2. An illustration of a 10-hour

segment of the test cycle is shown in Figure 2. Mode 1

represents typical low-temperature operation for this system

and allows for some level of soot accumulation on the DPF.

Mode 2 represents typical high-temperature operation without

the use of active DPF regeneration. Mode 3 represents

temperatures typical of an active DPF regeneration event.

This three-mode cycle is one hour in length and is repeated

100 times. Operating this cycle for 100 hours results in

approximately 50 hours of DPF regeneration conditions.

Based on the estimations below, this is equivalent to 150,000

miles of thermal aging for the exhaust system.

Thermal aging exposure estimate

Peak temperatures of a DPF regeneration last 10 minutes

and occurs every 500 miles

150,000 miles / 500 miles × 10 min = 50 hours

SCR inlet temperatures commonly reach 700°C in a DOC

+SCR+DPF configuration.

Table 2. Catalyst Aging Conditions

Figure 2. 10 hour example of the aging cycle

Using this accelerated aging platform, four identical exhaust

systems were aged, each with a different fuel: petroleum

diesel containing no detectable metals, B20 doped with Na,

B20 doped with K, and B20 doped with Ca. The petroleum

diesel was an ultra low sulfur diesel (ULSD) and contained

no measureable quantities of Na, K or Ca at a detection limit

of 1 ppm. Each of the biodiesel fuels was a blend of this same

ULSD with 20vol% soy-based biodiesel. The soy-based

biodiesel also contained no measureable quantities of Na, K

or Ca. The B20 was blended in 250 gallon tote quantities and

then doped with metal impurities to achieve precisely

controlled levels. Laboratory experiments were conducted to

determine compounds suitable for doping the B20 based on

solubility at room temperatures. The dopants used to add Na,

K and Ca were dioxtyl sulfonate sodium salt, potassium

dodecylbenzene and calcium napthenate, respectively. Once

the B20 batch was blended, it was doped with the desired

metal impurity and allowed to mix overnight with a paddle

wheel agitator. A sample was then collected for analysis.

Samples were collected from the top and bottom of the tote to

ensure a homogeneous mixture of the dopant.

Fuel dopant levels were selected to achieve 150,000 miles

worth of metal exposure in the 100 hour aging period. The

dopants were added to the fuel at 14ppm. This level was

chosen to achieve 150,000 miles worth of metal exposure,

assuming normal vehicle operation with a B20 blended from

B100 at the ASTM International's specification limit of 5

ppm metals. This would correspond to a B20 fuel containing

1 ppm of the impurity. The calculations and assumptions used

for this estimate are shown below.

Biodiesel metal exposure estimate

Fuel economy of a Ford F-series pickup = 18 mpg

0.00015% of B20 becomes oxide ash at current spec limits

(Assuming 1ppm Na in B20 forming Na2O ash)

150,000 miles / 18 mpg × 0.00015% × 3.1 kg/gal = 39 grams

Accelerated ash loading calculation

Test length = 100 hours

Caterpillar engine fuel consumption over 3-mode test cycle =

19.6 kg/hr

0.0021% of B20 becomes ash (14× metal in fuel)

100 hrs × 19.6 kg/hr × 0.0021% = 41 grams

Emissions Testing of Aged Catalysts

The 100 hours of accelerated aging was interrupted at 10-

hour intervals for DPF weighing and emissions testing. At the

10- hour interval, the exhaust system was removed from the

accelerated aging test bed. The DPF was separated from the

rest of the exhaust and weighed. Then the full exhaust system

was installed onto the Ford F250 pickup truck for emissions

testing. The vehicle was tested on a chassis dynamometer

over the Federal Test Procedure (FTP) cycle with

measurement of regulated emissions following U.S. EPA
certification test methods [13]. The purpose of intermittent

emissions evaluation was to identify the onset of catalyst

failure due to metal exposure. Prior to each emission test, the

system was prepped to ensure that the catalysts were at the

same state. The catalyst prep procedure consisted of (1)

conducting a forced DPF regeneration event while driving the

Highway Fuel Economy Test (HWFET) cycle, followed by

(2) conducting two repeats of the LA4 test cycle, and finally,

(3) a minimum 7-hour soak period. This system conditioning

ensured that the vehicle did not initiate a DPF regeneration

event during the FTP and that the SCR catalysts had the same

opportunity for ammonia (NH3) adsorption prior to the FTP

emissions test.

Emissions concentrations were measured at multiple

locations in the exhaust, as indicated in Figure 1, to allow for

calculation of percent conversion. Dilute bag emissions were

measured for all three phases of the FTP and a final

calculation was made in grams/mile of NOx, carbon

monoxide (CO), total HC, and carbon dioxide (CO2). All

system prep work and emissions testing was conducted with a

standard ULSD, regardless of the fuel type used to age the

catalyst.

Postmortem Analysis of Aged Catalysts

Figure 3. Sample location for postmortem analysis

Following the end of simulated aging and the final emissions

tests on the F250 pickup truck, each of the four exhaust

systems was sent to Oak Ridge National Laboratory (ORNL)

and Ford for further postmortem analysis. In preparation for

postmortem analysis core sections were harvested from each

of the catalyst bricks. One inch diameter cores were taken

down the length of each of the DOC and SCR bricks. These

cores were taken close to the radial center of each brick.

Electron probe microanalysis (EPMA) was used to determine

the location and concentration of the metal species and ash.

Bench flow reactor experiments were used to investigate

catalytic activity for the aged parts. The relative locations for

samples harvested for each of these analyses are shown in

Figure 3. Further description of methods for postmortem

analysis is included in the Results section of this paper.

RESULTS

Thermal and Metal Exposure

Following every 10 hours of catalyst aging, the DPF was

removed from the exhaust system and weighed to determine

total ash loading. The weighing took place following 30

minutes of operation with temperatures around 700°C (Mode

3). This ensured that minimal soot was present on the filter at

the time of weighing. The filter was above 200°C at the time

of weighing to prevent water adsorption. Table 3 shows the

total amount of ash collected by the end of the 100 hours of

simulated aging. The system aged with ULSD collected 28

grams of ash, which would primarily come from the engine's

lubricant. As expected, each of the systems aged with B20

doped with metals collected higher levels of ash. It is

expected that a portion of this ash on the B20 aged filters

would also come from the lubricant oil with the rest coming

from the metals doped into the fuel. The system aged with

B20 containing 14 ppm of Ca collected the most ash, at 82

grams. The DPF was the final component in the exhaust

system; thus, the DOCs and SCRs ahead of the filter would

have been exposed to these levels of ash at a minimum. Table

3 also shows thermal exposure data for each of the four

systems. This table shows the time where the temperature is

greater than 600°C, generally considered to be DPF

regeneration conditions, and the average temperature

measured at the inlet to the SCR catalyst. As can be seen

from this data, the thermal aging is very similar for each of

the four systems. Thus, the primary difference between these

systems is the amount and type of metal ash to which they

were exposed.

Table 3. Thermal and Ash Exposure

Vehicle Emissions

Following the DPF weighing procedure, the exhaust system

was reassembled and installed onto a Ford F250 pickup truck

for emissions testing. This occurred at 10-hour intervals

throughout the accelerated aging and was always conducted

utilizing ULSD fuel without biodiesel or doped metals. The

first emissions test was conducted after 1 hour of aging to

allow for some catalyst de-greening. Figure 4 shows the NOx

emissions from the vehicle, measured in grams/mile,

throughout the accelerated aging process. The final test
conducted at 100 hours represents 150,000 miles of thermal

aging and metal exposure. As illustrated in the figure, the

NOx emission certification level for this vehicle is 0.2 g/mile.

At the end of the 100-hour test, all four of the exhaust

systems met the NOx emission standard and were within 0.03

g/mile of each other. Only a single FTP emissions test was

run at each 10-hour interval throughout the aging process,

therefore we cannot determine the test-to-test variability.

Based on the absence of replicate data and the nearly

identical result in the final emission test, it is difficult to draw

any conclusions about catalyst deactivation as a result of

metal exposure. Although it may be within the measurement

uncertainty, the system aged with B20 containing Na failed

the 0.2 g/mile NOx emission standard at 60 hours of aging

and the system aged with B20 containing K failed the

standard at 80 hours and again at 90 hours. Figure 5 shows

similar results for HC emissions where 0.2 g/mile is the

emission certification level. All four systems consistently

pass the HC emission standard by a wide margin, and there

are no apparent trends in the data throughout the 100 hours of

aging to indicate deactivation of the DOC.

Figure 4. NOx emissions test results

Figure 5. HC emissions test results

SCR Postmortem Analysis

Vehicle emissions tests give a measure of how the entire

aftertreatment system is performing throughout the catalyst

aging process. However, bench flow reactor experiments

allow for more detailed characterization of catalyst

performance, which could reveal signs of catalyst

deactivation not seen in full system testing. For each of the

aged SCR catalysts, 1 inch diameter by 1 inch long cores

were harvested from each of the bricks, as shown in Figure 3.

Because there are two bricks in each system, this allowed for

analysis of the first inch and the seventh inch (first inch of the

second brick) of the full SCR catalyst length. The conditions

for the NOx conversion and NH3 oxidation experiments are

given in Table 4.

Figure 6 shows the NOx conversion performance over the

first inch of the SCR catalyst for each of the four aged

systems. This figure shows conversion performance for a

range of temperatures from 125°C to 650°C. The samples

harvested from the catalyst aged with ULSD containing no

metals and B20 containing Ca have nearly the same

performance. Both of these aged samples achieve better than

90% NOx conversion between roughly 200°C and 400°C.

Table 4. Bench Flow Reactor Conditions
The first inch of the SCR catalysts aged with B20 containing

Na, and B20 containing K, clearly have reduced performance

over the majority of the tested temperature range. At 200°C,

the Na and K exposed catalysts only achieve about 70% NOx

conversion. The K exposed catalyst never achieved greater

than 90% conversion throughout the temperature range.

Figure 6. Standard SCR reaction for first inch of aged

SCR catalysts

Figure 7 shows the NOx conversion achieved over the

seventh inch (first inch of the second brick) of the aged SCR

catalysts. Because the exhaust system has two 6 inch bricks

mounted in series, this would represent the midpoint of the

full SCR system. In this case all four of the systems

performed identically at temperatures between roughly 125°C

and 500°C. The samples exposed to Na and K did show

decreased performance above 500°C, but this is outside of the

normal operating range for these catalysts.

To illustrate the relevant operating range for this system in an

F250 pickup, the inlet temperature of the SCR catalyst was

measured during FTP and US06 drive cycles. Data from these

driving events showed that the SCR catalyst primarily

operates between 200°C and 300°C during an FTP emissions

test and reaches peak temperatures of 385°C during the US06

cycle. However, the most challenging period of operation is

the first two minutes of a cold start FTP where the catalyst

operates below 200°C, thus highlighting the importance of

the lost performance at low temperatures for the Na and K

aged samples.

Figure 8 shows the NH3 oxidation activity over the first inch

of the SCR catalyst for each of the four aged systems. As was

the case for NOx conversion performance, the systems aged

with ULSD and B20 containing Ca are similar. The samples

exposed to Na and K show very different results, with Na

exposure causing NH3 oxidation to occur at lower

temperatures and K exposure causing NH3 oxidation at

higher temperatures. These changes in temperature for the

onset of NH3 oxidation represent two different stages of

catalyst degradation and has been demonstrated and

described in previous work by Cavataio and coworkers [7].

The state of the Cu within the Cu/zeolite SCR catalyst has a

direct impact on NH3 oxidation and the observed changes in

NH3 oxidation are likely a result of Cu becoming freed-up

from the zeolite structure.

Figure 7. Standard SCR reaction for seventh inch of

aged SCR catalysts

Figure 8. NH3 oxidation over first inch of aged SCR

catalysts

Figure 9 shows the NH3 oxidation over the seventh inch of

the aged SCR catalysts. In this case the performance is much

more similar for each sample, however the samples exposed

to Na and K have slightly higher NH3 oxidation. At 450°C

NH3 oxidation is roughly 10% higher for the Na and K

exposed samples than for the systems aged with ULSD and

B20 containing Ca.

Using the bench flow reactor, experiments were also

conducted to determine the NH3 storage capacity for each of

the aged SCR samples. For this experiment the catalyst

sample was maintained at 200°C for 3,600 seconds with 350

ppm of NH3 and 0 ppm of NOx in the exhaust stream. This

allowed the catalyst sample to adsorb NH3. By measuring the
NH3 downstream of the SCR catalyst it is possible to

determine how much NH3 the catalyst is able to store. This

experiment is illustrated in Figure 10, which shows the NH3

profiles measured both upstream and downstream of the

catalyst. The difference between the two profiles is used to

calculate the total NH3 storage capacity. This total storage

capacity is further divided into a strongly held portion and a

weakly held portion. The strongly held portion represents

chemical adsorption of the NH3 to the acid sites within the

zeolite, whereas the weakly held portion represents physical

adsorption or condensation of NH3. The differentiation

between the two types of storage is marked by the onset of

NH3 breakthrough measured downstream of the catalyst.

Figure 9. NH3 oxidation over seventh inch of aged SCR

catalysts

Figure 10. Example of NH3 adsorption experiment

Figure 11 shows the strongly held NH3 as a percentage of the

total adsorption capacity for the first inch of the aged SCR

catalysts. The adsorption experiment was run at three

separate temperatures; 200°C, 250°C and 300°C. In all three

cases, the samples exposed to K and Na clearly have

diminished capacity for strongly held NH3 storage. Figure 12

shows the strongly held NH3 as a percentage of total capacity

for the seventh inch of the aged SCR catalysts. In this case all

of the systems exposed to fuel metals have slightly lower

capacity for strongly held NH3, however the impact is much

less than what is seen in the first inch.

Figure 11. NH3 storage in first inch of the aged SCR

catalysts

Figure 12. NH3 storage in seventh inch of the aged SCR

catalysts

Figures 13, 14, 15, 16 show EPMA micrographs for a quarter

channel cross section of each of the aged SCR catalysts. The

images highlight relative concentrations and distributions of

Ca, Na, K, and P in each of the samples. The color legend to

the right of each image represents the spectrum of relative

concentration for the elements, with red being the highest and

dark blue representing background levels. Phosphorus was

selected because it is a common additive in lubricating oils.

As shown in Figure 3, these samples were taken from the

front SCR, approximately one inch into the brick. These cross

section images in Figures 13, 14, 15, 16 show the lower left

hand corner of a single channel of the flow through the SCR

catalyst, including both the washcoat and substrate. The

substrate is in the lower left corner of each of the four images

and shows visible porosity. The washcoat fills the corner of

the substrate channel with the surface exposed to the exhaust

flow located in the upper right hand corner. The exhaust flow

direction is perpendicular to the plane of the image. Naturally
occurring cracks divide the washcoat into sections which we

designate as “corner” and “side” for the purpose of

discussion. This is illustrated in Figure 13. In each of the four

aged samples there is a thin layer of P deposited directly on

top of the washcoat. In the case of the sample aged with Ca, a

thin, highly concentrated layer of Ca is deposited directly on

top of the washcoat as well. There is no evidence of either the

P or Ca penetrating into the washcoat. In the case of the Na

exposed sample, the EPMA image shows an elevated signal

for Na in the washcoat of the catalyst. A similar result is seen

for the K-exposed sample with a high concentration of K in

the washcoat. The alkali elements have diffused into the

porous washcoat to the substrate surface rather than being

isolated to the washcoat external surface like the Ca and P.

Also of note is that the side portion of the washcoat tends to

show higher concentrations for the Na and K than the corner

portions.

Figure 13. EPMA of first inch of ULSD aged SCR

Previous studies have employed similar accelerated

techniques to probe the question of Na and K impact on SCR

catalysis [7]. In particular, Brookshear et al. [14] investigated

both chemical characterization with EPMA and performance

impact with the key differences being the use of an older

generation zeolite framework (Beta versus Chabazite) and

exposure to much higher levels of Na (to mimic heavy duty

exposure of 435,000 miles). Similar results were observed in

the EPMA data, as Na was observed throughout the washcoat

with higher concentrations in the side position. The

performance deactivation was more significant in this

investigation, but this is to be expected since the zeolite is

more susceptible to thermal aging and the total Na exposure

was nearly 3 times higher. The similarities in these findings

suggest that the impact of Na is similar across all

commercially relevant Cu-exchanged zeolite. Extrapolation

to Fe-zeolites cannot be made at this time. The proposed

mechanism of substitution of the exchanged metal with Na or

K is expected to still be relevant; however, the relative

exchange strength would need to be probed in a similar

manner as was done here.

Figure 14. EPMA of first inch of Ca aged SCR

Figure 15. EPMA of first inch of Na aged SCR
Figure 16. EPMA of first inch of K aged SCR

DOC Postmortem Analysis

For each of the aged DOC catalysts, 1 inch diameter by 1

inch long cores were harvested from the first brick and the

second brick, as shown in Figure 3. Front and rear samples of

the first brick and a front sample of the rear brick were

analyzed in a bench flow reactor for CO light-off, HC lightoff, and NO oxidation, and for HC slip during simulated DPF

regenerations. CO, HC, and NO oxidation experiments were

carried out in a pulsator laboratory flow reactor where the

combustion of iso-octane served to generate HC, NOx and

CO levels. Table 5 shows the conditions for these DOC

evaluation experiments. The gas temperature measured at the

inlet of the DOC sample was ramped from 100°C to 650°C at

a 10°C/min ramp rate. CO and HC profiles were used to

determine the temperature required for 50% (T50), 80%

(T80), and 90% (T90) conversion. The nitrogen dioxide

(NO2) formation test consisted of determining the peak

percentage of NO2 relative to total NOx and the

corresponding temperature.

Figures 17 and 18 show the results for the HC and CO lightoff for each of the aged DOC samples. Light-off temperatures

are shown for 50% (T50), 80% (T80), and 90% (T90)

conversion. The T50 and T80 HC light-off temperatures were

similar for all four of the aged DOCs. However, the T90 HC

light-off temperatures were significantly higher for the K and

Na exposed parts. In the case of the CO light-off, the T50

temperatures were also similar for all four aged systems.

However, the T90 light-off temperature was higher for the

Ca, K, and Na exposed parts. This effect was mostly

observed further down the length of the catalyst and was most

prominent for the DOCs exposed to Na.

Table 5. Flow Reactor Conditions for HC, CO and NO

Oxidation Experiments

Figure 17. HC light-off temperatures

Figure 18. CO light-off temperatures

Figure 19 shows the peak NO2:NOx ratio and the

corresponding temperature for each of the aged DOCs. The

only significant finding is the small loss of NO oxidation

function for the DOC exposed to K.
Figure 19. NO oxidation

Experiments to evaluate the DOC for HC slip during a DPF

regeneration were carried out in a flow reactor designed for

air-assisted diesel fuel injection upstream of a DOC sample.

The inlet gas temperature was held constant at 300°C and a

sufficient amount of diesel fuel was injected to generate an

outlet temperature of 650°C. The steady-state HC and

nonmethane hydrocarbon breakthrough was measured for

each of the aged DOC samples. Figure 20 shows the results

for the HC slip during a simulated DPF regeneration. This

plot shows the HC slip for the three different DOC cores. In

every position, the samples exposed to K showed higher

levels of HC slip compared to the samples aged with ULSD

and B20 containing Ca. The samples exposed to Na showed

higher levels of HC slip further down the length of the

catalyst.

Figure 20. HC slip during regeneration

Figures 21, 22, 23, 24 show EPMA micrographs obtained for

each of the aged DOCs. The substrate and catalyst washcoat

positions are the same as described for Figures 13, 14, 15, 16,

i.e., from the front DOC approximately one inch into the

brick, as shown in Figure 3. These images show the relative

concentration and distribution of Na, K, Ca, and P in each of

the samples. Unlike the SCR samples the channels of the

DOC contain two separate layers. The corners appear to

contain an inert filler and a thin layer of active catalyst is

washcoated onto the substrate; this last layer will be referred

to as the primary washcoat. In each case a thin layer of P can

be seen not only at the surface of the DOC washcoat, but it

also has penetrated throughout the thin primary washcoat

layer. In the case of the Ca exposed sample, a layer of Ca can

also be seen on the surface of the washcoat. In the case of the

Na and K exposed samples, large amounts of the alkali

metals have diffused throughout the primary washcoat and

penetrated the underlying filler. This indicates a high level of

interaction between these metals and the catalyst materials,

which are even reactive enough to penetrate the underlying

nonporous material including the cordierite.

Figure 21. EPMA of first inch of ULSD aged DOC

Figure 22. EPMA of first inch of Ca aged DOC
Figure 23. EPMA of first inch of Na aged DOC

Figure 24. EPMA of first inch of K aged DOC

DPF Postmortem Analysis

Measurements of fracture strength, elastic modulus, and

coefficient of thermal expansion (CTE) were taken for each

of the four aged DPFs as well as a fresh DPF. A critical

parameter for estimating DPF durability is the thermal shock

resistance (RS):

(1)

where K is the thermal conductivity, σf

 is the tensile fracture

strength, ν is the Poisson's ratio, α is the CTE, and E is the

elastic modulus. It is desirable to maximize RS. Failure is

likely to occur during regeneration when the thermal

gradients are largest, so any increase in the elastic modulus

and CTE or decrease in the fracture strength and thermal

conductivity will be detrimental to the DPF durability.

For measurement of material strength, approximately 70 tiles,

each 30 × 30 mm square, were extracted down the length of

the DPF. One-half of the tiles were taken from the radial

center of the DPF while the other half were taken closer to

the radial edge. Each individual tile was subject to a ring-onring biaxial flexure test as described in Reference [15]. This

test measured the force and displacement at failure from

which the elastic modulus and tensile fracture strength were

estimated. This was done by modeling the same loading

scenario and DPF strut structure using finite element analysis

and then adjusting the elastic modulus in the model until it

matched the load and displacement measured at the point of

fracture after the subtraction of the compliance introduced by

the testing equipment. Once the elastic modulus was

determined, the fracture strength was calculated. The

estimated elastic modulus and tensile fracture strength is

shown in Figure 25 alongside the average specimen thickness

for both the center and edge locations. This data is shown for

each of the aged samples as well as for a fresh sample.

Both the elastic modulus and the fracture strength increase

with the ULSD, B20+K, and B20+Na samples, but they

decline slightly with the B20+Ca samples relative to the fresh

sample. However, the specimen thickness also follows the

same trend which resulted from difficulties in machining the

SiC material. It cannot be ruled out that the different response

in mechanical properties was largely caused by the change in

specimen thickness and not from exposure as the thicker

specimens will have higher contact stresses causing either the

loading rings to crush the specimen or the specimen to flatten

the rings thereby erroneously increasing the measured failure

force. Nevertheless, because the estimated elastic modulus

correlates with fracture strength in Figure 25, and these two

properties are, respectively, in the denominator and

numerator of Equation 1, the effect on the thermal shock

resistance will be negligible. Moreover, resonant ultrasound

measurements were conducted on samples from all the

conditions and locations in Figure 25 but on specimens with

identical dimensions and no statistically significant change in

elastic modulus was observed, thereby suggesting that the

changes measured in Figure 25 are related to specimen

thickness and not engine testing.

Measurement of the CTE was conducted using a dual-rod

dilatometer. The CTE specimens were 3 by 3 DPF channels

square and one inch in length. A small spring load was

applied along the axial direction of the specimen and the

expansion with temperature was measured. Figure 26 shows

the CTE for each of the aged samples as well as a fresh DPF.

The CTE for all samples is approximately 4×10−6/°C at

200°C and rises to approximately 5.5×10−6/°C at 800°C.

There is no apparent difference in the CTE responses
between any of the aged parts and the fresh sample. Some

samples had broad peaks in the thermal expansion curve

above 800°C which may be due to phase transformations in

the ash that is present between the sample and the loading

rod.

Figure 25. The estimated tensile fracture strength,

estimated elastic modulus and thickness of the DPF

samples

Figure 26. CTE versus temperature for the aged and

fresh DPF samples

The aged and fresh samples were mounted in epoxy and the

microstructure was observed both optically and with a

scanning electron microscope; no evidence of cracking or

corrosion was found. Energy-dispersive X-ray spectra were

also collected from the SiC in each condition and no evidence

of diffusion of metal species into the SiC structure or grain

boundaries was detected. These observations coupled with

the thermo-mechanical measurements show that there was no

measureable degradation of the SiC DPF caused by exposure

to the fuel metal impurities.

SUMMARY / CONCLUSIONS

An earlier study showed that alkali and alkaline earth metals

could have a significant impact on the catalytic activity of a

DOC and the thermo-mechanical durability of a DPF in a

heavy-duty application [12]. The purpose of the work

presented in this paper was to investigate the impact of these

metal impurities in a light-duty application, with the

additional concern that the SCR catalyst is located in front of

the DPF where it will see more thermal and metal exposure.

Bench flow reactor experiments conducted on samples taken

from each of the aged SCR catalysts showed that parts

exposed to Na and K had reduced catalytic activity. However,

this reduced performance was only seen in the first inch of

the catalyst and was no longer observed further down the

length of the catalyst. The cause of this deactivation is further

explained by EPMA analysis of the SCR, which show that

the Na and K have migrated into the catalyst washcoat,

whereas Ca and lube oil derived Phosphorous remain on the

washcoat surface. The overall magnitude of this SCR

deactivation seen in the first inch of the catalyst is put into

perspective by conducting FTP emissions tests of the full

aftertreatment system. Emissions tests with a Ford F250

pickup showed that all of the systems continued to meet the

0.2 g/mile NOx emission standard after the equivalent of

150,000 miles of accelerated aging. Bench flow reactor and

EPMA studies of the aged DOCs showed similar signs of

deactivation for parts exposed to Na and K. However, the

vehicle also continued to meet the HC emission standards

with each of the aged systems. The vehicle emission testing

and postmortem results taken together indicate that the alkali

metals, Na, and K are volatile at temperatures typically seen

in diesel exhaust, which allows them to migrate into the

catalyst washcoat leading to catalyst deactivation. However,

after the simulated equivalent of 150,000 miles of exposure,

there remains enough unaltered catalyst volume that impacts

on tailpipe vehicle emissions are not observed. In addition the

thermo-mechanical properties of the aged SiC DPFs were

unchanged relative to each other and to a fresh DPF.

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CONTACT INFORMATION

Aaron Williams

National Renewable Energy Lab

15013 Denver West Parkway

Golden, CO 80401

(303) 275-3153

aaron.williams@nrel.gov

ACKNOWLEDGMENTS

Support from the National Biodiesel Board, the United

Soybean Board, the U.S. Department of Energy, Office of

Vehicle Technologies, Fuels Technologies Program and

Propulsion Materials Programs is gratefully acknowledged.

The authors would like to acknowledge the support of Roger

Gault of the Truck and Engine Manufacturers Association

and an industry technical steering committee comprised of

engine and emission control experts for their technical input

used in developing the accelerated durability test protocol.

We would also like to acknowledge the contributions of

Shirley Waters in the collection of the CTE data.

DEFINITIONS/ABBREVIATIONS

B20 - 20 vol% biodiesel blended into petroleum diesel

Ca - Calcium

CO - Carbon Monoxide

CO2 - Carbon Dioxide

CPSI - Cells per Square Inch

CTE - Coefficient of Thermal Expansion

Cu-Cha - Copper Chabazite

DOC - Diesel Oxidation Catalyst

DPF - Diesel Particulate Filter

EPA - U.S. Environmental Protection Agency

EPMA - Electron Probe Microanalysis

FTP - Federal Test Procedure

HC - Hydrocarbons

K - Potassium

Mg - Magnesium

Na - Sodium

NH3 - Ammonia

NO - Nitrous Oxide

NO2 - Nitrogen Dioxide

NOx - Oxides of Nitrogen

NREL - National Renewable Energy Laboratory

P - Phosphorous

ORNL - Oak Ridge National Laboratory

ppm - Parts per Million
Rs - Thermal Shock Resistance Parameter

SCR - Selective Catalytic Reduction Catalyst

SV - Space Velocity

ULSD - Ultra Low Sulfur Diesel (<15ppm Sulfur)

The Engineering Meetings Board has approved this paper for publication. It has

successfully completed SAE's peer review process under the supervision of the session

organizer. This process requires a minimum of three (3) reviews by industry experts.

ISSN 0148-7191

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