1. This document discusses the history and development of automotive catalysts used to reduce vehicle emissions. It describes the regulations that led to the development of catalytic converters and the different generations of converters used.
2. Early oxidation catalysts from 1976-1979 used platinum and palladium on bead or honeycomb supports to reduce carbon monoxide and hydrocarbons from gasoline engines. Subsequent three-way catalysts introduced from 1979-1986 added rhodium to also reduce nitrogen oxides through precise fuel control enabled by oxygen sensors.
3. More recent generations of catalysts focus on improving durability, temperature operation, and meeting increasingly stringent emissions standards through substrate and washcoat modifications as well as alternative precious metal formulations.
2. 2
1. Emissions and Regulations
Gasoline
A mixture of paraffins and aromatic
hydrocarbons for spark-ignited combustion
engine
The year 2000: over 500 million passenger
cars in use worldwide with an annual
production of new cars approaching 60
million.
3. 3
Incomplete combustion products of CO and
unburned hydrocarbons (UHCs), thermal and
fuel NOX
CO range: 1~2%
UHCs range: 500~1000 ppm
NOX range: 100~3000 ppm
The exhaust also contains approximately 0.3
moles of H2 per mole of CO.
Next slide (Fig. 6.1)
Gasoline engine emissions as a function of air:
fuel ratio
4.
5. 5
CO, HC ↑ while rich
NOX ↓ while rich
CO is a direct poison to human.
HC and NOX undergo photochemical
reactions in the sunlight leading to the
generation of smog and ozone.
6. 6
US Clean Air Act
1975/76 federal requirements:
HC: 1.5 g/mile
CO: 15 g/mile
NOX: 3.1 g/mile
7. 7
USEPA established the Federal Test
Procedure (FTP) simulating the average
driving conditions:
(1) cold start, after the engine was idle for
8 h
(2) hot start
(3) a combination of urban and highway
driving conditions called FTP cycle.
8. 8
Typical precontrolled vehicle emissions in
the total FTP cycle:
CO: 83~90 g/mile
HC: 13~16 g/mile
NOX: 3.5~7.0 g/mile
Therefore, the catalyst was required to
obtain >90% conversion of CO and HC by
1976 and to maintain performance for
50,000 miles.
9. 9
US Clean Air Act Amendment of 1990
The catalyst would be required to last
100,000 miles for new automobiles after
1996.
Emissions requirements by 2004:
NMHC (nonmethane hydrocarbon): 0.125
g/mile
CO: 1.7 g/mile
NOX: 0.2 g/mile
10. 10
California sets even more stringent
regulations:
NMHC: 0.075 g/mile by 2000 for 96% of all
passenger cars.
By 2003, 10% of these must have emissions
no greater than 0.04 g/mile, and 10% must
emit no NMHCs at all.
11. 11
The current summary of the California emission standards for passenger
cars of 2000:
Category Durability
Basis (miles)
NMOG
(g/mile)
CO
(g/mile)
NOX
(g/mile)
TLEV 50,000
120,000
0.125
0.156
3.4
4.2
0.4
0.6
LEV 50,000
120,000
0.075
0.09
3.4
4.2
0.05
0.07
ULEV 50,000
120,000
0.04
0.055
1.7
2.1
0.05
0.07
SULEV 120,000 0.010 1.0 0.02
ZEV 0 0 0 0
Where LEV: low-emission vehicle, T: transitional, U: ultra, S: super,
ZEV: zero-emission vehicle, NMOG: nonmethane organics
12. 12
The European Standards for light duty gasoline engine passenger
cars
Category Stage 3 (g/km)
(2000)
Stage 4 (g/km)
(2005)
CO 2.3 1.0
UHC 0.2 0.1
NOX 0.15 0.08
Where 1 g/mile: 0.62 g/km
13. 13
2. The Catalytic Reactions for Pollution Abatement
Oxidation of CO and HC to CO2 and H2O:
2 2 2
2 2
2 2 2
(1 )
4 2
1
2
y n
n n
C H O yCO H O
CO O CO
CO H O CO H
14. 14
Reduction of NO/NO2 to N2:
Next slide (Fig. 6.2)
Automobile catalytic converter
2 2 2
2 2 2 2
2 2 2 2
1
( )
2
1
( )
2
(2 ) ( ) (1 )
2 4 2
y n
NO orNO CO N CO
NO orNO H N H O
n n n
NO orNO C H N yCO H O
15.
16. 16
Lightoff Temperature
A temperature high enough to initiate the
catalytic reactions
The rate of reaction is kinetically controlled.
Typically, the CO (and H2) reaction begins
first, followed by the HC and NOX reactions.
When the vehicle exhaust is hot, the chemical
reaction rates are fast, and pore diffusion
and/or bulk mass transfer controls the
reactions.
17. 17
3. The Physical Structure of the Catalytic Converter
Both beaded (or particulate) and
monolithic catalyst have been used for
passenger vehicles.
18. 18
Engineering issues:
(1) How much back pressure would the catalytic
reactor contribute?
(2) Would the catalyst be able to maintain its
physical integrity and shape in the extreme
temperature and corrosive environment of
the exhaust?
(3) How much weight of the catalyst would be
added?
(4) What would be the effect on fuel economy?
(5) The vehicle exhaust catalyst operation is in a
continuously transient mode, in contrast to
normal stationary catalyst operation.
19. 19
3.1 The Beaded Catalyst
The most traditional way:
Spherical particulate γ-Al2O3 particles, anywhere
from 1/8 to 1/4 in. in diameter, into which the
stabilizers and active catalytic components (i.e.,
precious metals) would be incorporated.
20. 20
Since the engine exhaust gas was deficient in
oxygen, air was added into the exhaust using
an air pump.
Next slide (Fig. 6.3)
A bead bed reactor design for the early
oxidation catalysts
21.
22. 22
The precious metal salts are impregnated into the
bead, then, dried at typically 120℃, and calcined
to about 500℃ to their finished state.
The finished catalyst usually had about 0.05 wt%
precious metal with a Pt:Pd weight ratio of 2.5:1.
After 1979 the need for NOX reduction in the US
required the introduction of small amount of Rh
into the second-generation catalysts.
23. 23
3.2 The Honeycomb Catalyst
In the mid-1960s, Engelhard began
investigating the use of monolithic
structures for reducing emissions from
forklift trucks, mining vehicles,
stationary engines, and so on.
Advantages:
Low pressure drop (high open frontal
area (~70%))
24. 24
The ceramic companies continued to
modify the materials and structures to
provide sufficient strength and resistance
to cracking under thermal shock
conditions experienced during rapid
acceleration and deceleration.
A low-thermal-expansion ceramic
material called cordierite (2 MgO‧5
Si2O3‧2Al2O3) satifies the needs.
25. 25
The first honeycomb catalyst of large
quantity to be used in automobile exhaust had
300 cells per squar inch (cpsi), with wall
thickness of about 0.012 in., and open frontal
area of about 63%.
Later developments in extrusion technology
resulted in a 400 cpsi honeycomb with a wall
thickness of 0.006 in. (150μm) and open
frontal area of 71%.
This increased the geometric surface area for
the mass-transfer-controlled reactions.
26. 26
The washcoat thickness could be kept at a
minimum to decrease pore diffusion effects
while allowing sufficient thickness for
anticipated aging due to deposition of
contaminants.
The washcoat is about 20 and 60 μm on the
walls and corners (fillets), respectively.
27. 27
Typically, the catalyst contains about
0.1~0.15% precious metals. For the oxidation
catalysts of the first generation, the weight
ratio of Pt to Pd was 2.5:1, whereas the
second generation contained a weight ratio of
5:1 Pt:Rh.
The honeycomb catalyst is mounted in a steel
container with a resilient matting material
wrapped around it to ensure vibration
resistance and retention.
28. 28
Positive experience with honeycomb
technologies has resulted in increased use of
these structures over that of the beads, due to
size and weight benefits. (open surface)
Next slide (Fi.g 6.4)
Honeycomb-supported catalysts
29.
30. 30
Although the early honeycombs were ceramic,
recently metal substrates have been finding
use because they can be made with thinner
walls and have open frontal areas of close to
90%, allowing lower pressure drop.
Next slide (Fig. 6.5)
Typical auto catalyst detailed design
31.
32. 32
The progress of the automotive catalyst (Detailed
in following)
(1) Oxidation Catalyst
Bead and monolith support
HC and CO emissions only
Pt-based catalyst
Stabilized alumina
(2) Three-way Catalyst
HC, CO, and NOX emissions
Pt/Rh-based catalyst
Ce oxygen storage
33. 33
(3) High-temperature Three-Way Catalyst
Approaching 950℃
Stabilized Ce with Zr
Pt/Rh, Pd/Rh, and Pt/Rh/Pd
(4) All-Palladium Three-way Catalyst
Layered coating
Stabilized Ce with Zr
34. 34
(5) Low-emission Vehicles
High temperature, with/without Ce, close-
coupled catalyst
Approaching 1050℃
With underfloor catalyst
(6) Ultra-low-emission Vehicles
High temperature, with/without Ce, close-
coupled catalyst
Approaching 1050℃
Increased volume underfloor, higher precious-
metal loading
Optional trap
35. 35
4. First-Generation Converters: Oxidation Catalyst
(1976-1979)
Only required for CO and HC (early Clean Air
Act)
The NOX standard was relaxed so engine
manufacturers used exhaust gas recycle (EGR)
to meet the NOX standards.
The engine was operated just rich of
stoichiometric to further reduce the formation
of NOX, and secondary air was pumped into
the exhaust gas to provide sufficient O2 for the
catalytic oxidation of CO and HC on the
catalyst.
36. 36
Precious metals, Pt and Pd, were
excellent oxidation catalysts. Base metals,
such as Cu, Cr, Ni and Mn, were less
active but substantially cheaper.
Next slide (Table 6.1)
Relative activities of precious-metal and
base metal catalysts
37.
38. 38
The base metal oxides would require
larger reactor volumes. This would be a
problem in the engine exhaust underfloor
piping where space is at a premium.
The base metal oxides are very susceptible
to sulfur poisoning.
39. 39
Therefore, the first-generation oxidation
catalysts were a combination of Pt and Pd and
operated in the temperature range of 250~600℃,
with space velocities varying during vehicle
operation from 10,000 to 100,000 h-1, depending
on the engine size and mode of driving cycle (i.e.,
idle, cruise, or acceleration).
Typical catalyst compositions were Pt and Pd in
a 2.5:1 or 5:1 ratio ranging from 0.05 to 0.1 troy
oz/car (a troy oz is ~31g).
40. 40
4.1 Deactivation
The oxidation catalyst was negatively affected by the
exhaust impurities of sulfur oxides and tetraethyl
lead from the octane booster, both present in the
gasoline, and phosphorus and zinc from engine
lubricating oil.
Next slide (Fig. 6.6)
Effect of lead, sulfur, and thermal aging on CO
(Pt + Pd = 0.05 wt%)
Second slide (Fig. 6.7)
Effect of lead, sulfur, and thermal aging on
propylene (Pt + Pd = 0.05 wt%)
41.
42.
43. 43
The Pb present as an octane booster continued to
deactivate most severely all the catalytic materials.
Poisoning of Pt and Pd by traces of Pb (~3-4 mg/g
as of Pb were in gasoline) was caused by formation
of a low-activity alloy.
,900o
air C
Pt or Pd Pb PtPb or PdPb
44. 44
From Figs. 6.6 & 6.7, the Pt was more
tolerant than Pd to Pb poisoning, so
prepration processes were developed that
permitted the deposition of the Pt slightly
below the surface, while the Pd had a deeper,
subsurface penetration.
Unleaded gasoline now!
45. 45
Sintering of carriers
Na and K acted as fluxes, accelerating the
sintering process of washcoat (γ-Al2O3). Thus,
preparations had to exclude these elements.
46. 46
Small amount (1-3%) of La2O3, BaO, or SiO2,
if properly incorporated into the preparation
process, had a stabilizing effect on the γ-
Al2O3 and significantly reduced its sintering
rate.
Next slide (Fig. 6.8) (TWC: Three-Way
Catalysis)
Thermal stabilization of aluminas after
1200℃ aging surface areas of 150-175 m2/g
are typical for the aluminas in modern
automotive catalysts.
47.
48. 48
Agglomeration or sintering of the Pt and Pd
hydrogen chemisorption and XRD studies
revealed that the Pt and Pd, initially well
dispersed on stabilized γ-Al2O3, had
undergone significant crystallization after
high-temperature treatment.
Next slide (Fig. 6.9)
Effect of thermal aging on Pt and Pd
49.
50. 50
5. NOX, CO, and HC Reduction: the Second Generation
(1979-1986)
NOX reduction is most-effective in the
absence of O2, while the abatement of CO
and HC requires O2.
exhaust: rich (NOX) → lean (CO, HC) (two
stages)
51. 51
A primary catalyst for the reduction reaction
was Ru.
However, on an occasion when the engine
exhaust might be oxidizing and the
temperature exceeded about 700℃, it was
found to volatilize by forming RuO2.
This was dropped from further
consideration.
52. 52
When Pt or Pd was used instead of Ru, the
NOX was reduced to NH3 and not N2. The
NH3 would then enter the oxidation catalyst
and be converted to NOX.
Finally, Rh has been shown to be an excellent
NOX reduction catalyst. It had less NH3
formation than Pt or Pd.
53. 53
If the engine exhaust could be operated close
to the stoichiometric air:fuel ratio, then all
three pollutants (in theory) could be
simultaneously converted.
Next slide (Fig. 6.10)
Conversion of HC, CO, and NOX for TWC
54.
55. 55
Narrow operating window for TWC
This was made possible by the development
of the O2 sensor.
The O2 sensor was composed of an anionic
conductive solid electrolyte of stabilized
zirconia (ZrO2) with electrodes of high-
surface-area Pt.
56. 56
The voltage generated across the sensor was
strongly dependent on the O2 content. The
voltage signal generated is fed back to the
carburetor or to the fuel injection control
device, which adjusts the air:fuel ratio.
Next slide (Fig. 6.11)
Response profile for the O2 sensor
57.
58. 58
Modern O2 sensors have been modified to be
more poison tolerant to P and Si found in the
engine exhaust. Also to improve the operating
range of the O2 sensor in cold start the heated
O2 sensor was developed.
Next slide (Fig. 6.12)
The automotive feed back control system
59.
60. 60
The primary precious metals to convert all
three pollutants were Pt and Rh; the latter
were most responsible for reduction of NOX
(although it also contributes to CO oxidation
along with the Pt).
When operating rich, there was a need to
provide a small amount of O2 to consume the
unreacted CO and HC. Conversely when the
exhaust goes slightly oxidizing, the excess O2
needs to be consumed.
61. 61
This was accomplished by the development of the
O2 storage component, which liberates or adsorbs
O2 during the air:fuel perturbations.
CeO2 was found to have the proper redox
(reduction-oxidation) response and is the most
commonly used O2 storage component in modern
three-way catalytic converters.
62. 62
The O2 storage reactions:
2 2 3 2
2 3 2 2
:
1
:
2
Rich CeO CO Ce O CO
Lean Ce O O CeO
63. 63
Another benefit of CeO2:
It is a good steam-reforming catalyst and thus
catalyzes the reactions of CO and HC with H2O in
the rich mode. The H2 formed then reduced a portion
of the NOX to N2:
(Shift Reaction)
Other O2 storage components:
NiO/Ni and Fe2O3/FeO
2
2
2
2 2 2
2 2 2
2 2 2
2 (2 )
2
1
2
CeO
CeO
x y
CeO
X
CO H O H CO
y
C H H O H x CO
NO x H N x H O
64. 64
The modern three-way catalysts:
0.1~0.15% precious metals at a Pt:Rh ratio of
5:1 High concentrations of bulk high surface
area CeO2 (10-20%)
γ-Al2O3 washcoat stabilized with 1-2% of
La2O3 and/or BaO
400 cells per square inch honeycomb
65. 65
The washcoat loading is about 1.5-2.0 g/in3
or about 15% of the weight of the finished
honeycomb catalyst.
The size and shape of the final catalyst
configuration varies with each automobile
company but, typically, they are about 5-6 in.
in diameter and 3-6 in. long.
66. 66
6. NOX, CO and HC Reduction: the Third Generation
(1986-1992)
Fuel economy was important, yet operating
speeds were higher in this period. This
situation resulted in higher exposure
temperatures to the TWC catalyst.
67. 67
Higher fuel economy was met by introducing
a driving strategy whereby fuel is shut off
during deceleration. The catalyst, therefore, is
exposed to a highly oxidizing atmosphere that
results in deactivation of the Rh function by
reaction with the γ-alumina, forming an
inactive rhodium-aluminate species.
Next slide (Figs. 6.21 & 6.22)
Fuel-cut aging temperature and oxygen
concentration negatively affects total FTP
(Federal Test Procedure) performance.
68.
69.
70. 70
At temperature in excess of 800-900℃, in an
oxidizing mode, the Rh reacts with the Al2O3,
forming the inactive aluminate.
Fortunately, this reaction is partially reversible:
Next slide (Fig. 6.23)
The effect of rich and lean treatment cycles on the
performance of a TWC catalyst
800 ,
2 3 2 3
2 3 2 2 3
( )
o
C lean
rich
Rh Al O RhAl O
RhAl O H or CO Rh Al O
71.
72. 72
A promising route to minimize the Rh deactivation
appears to be to deposit the Rh on a less reactive
carrier such as ZrO2.
Another observation with regard to Rh stabilization
is its possible interaction with CeO2, the oxygen
storage component.
Therefore, segregating the Rh is suggested as a way
to improve tolerance to high-temperature lean
excursions.
Next slide (Fig. 6.24)
(b) double layers of washcoats with the Rh and CeO2
in different layers
73.
74. 74
Catalyst deactivation and reaction inhibition due
to P and S, respectively, are still concerns in
modern TWC catalysts.
The phosphorous present in the lubricating oil as
zinc dialkyldithiophosphate (ZDDP) deposits on
the catalyst and results in deactivation.
It usually deposits as a P2O5 film or polymeric
glaze on the outer surface of the Al2O3 carrier,
causing pore blockage and masking.
75. 75
Some studies have also considered the
effect of silicon from various lubricants on
catalyst performance.
Gasoline averages anywhere from 200 to
500 ppmw and can contain up to 1200
ppmw organosulfur compounds, which
convert to SO2 and SO3 during combustion.
76. 76
The SO2 adsorbs onto the precious-metal sites at
temperature below about 300℃ and inhibits the
catalytic conversions of CO, NOX, and HC.
At higher temperatures, the SO2 is converted to
SO3, which either passes through the catalyst bed
or can react with the Al2O3 forming Al2(SO4)3.
The latter is a large volume, low-density material
that alters the Al2O3 high surface area leading to
catalyst deactivation.
77. 77
In addition, the SO3 can react with Ce and
other rare earths.
Next slide (Fig. 6.25)
Sulfur in gasoline negatively affects
performance of TWC.
Future gasoline may contain 40-10 ppmw
S only.
78.
79. 79
Next slide (Fig. 6.26)
Penetration of S, P, and Zn into the washcoat
at inlet section of vehicle-aged catalyst
Second slide (Fig. 6.27)
Penetration of S, P, and Zn into to washcoat
at outlet section of a vehicle-aged catalyst
80.
81.
82. 82
Summary of Figs. 6.26 & 6.27:
(a) The concentrations of S, P, and Zn are much
greater in the inlet than the outlet section,
indicating that the former serves as a filter.
(b) The sulfur is uniformly present throughout the
washcoat, suggesting an interaction between it
and the Al2O3. The drop in poison concentration
at ~20μm is at the washcoat/monolith interface.
(c) The P and Zn are concentrated near the outer
periphery of the washcoat, but only in the inlet
section.
83. 83
7.Palladium TWC Catalyst: The Fourth Generation
(MID-1990s)
The use of Pd as a replacement for Pt and/or
Rh has been desirable because it is
considerably less expensive than either.
Pd/Rh and Pt/Pd catalysts in the early 1990s
This period, the catalysts were being placed
closer to the manifold, giving faster heatup of
the catalyst and higher steady-state operating
temperatures. This diminished the adsorption
of impurities such as sulfur and phosphorous.
84. 84
First commercial installations of all Pd
catalysts were in the 1995 model year for
Ford.
Next slide (Fig. 6.28)
Pd performance ≈ Pt/Rh performance
85.
86. 86
In geographic locations where Pb continues
to be in the gasoline source, Pd-only catalysts
are susceptible to Pb poisoning.
Lead was found on the aged catalysts and
was on the surface of the washcoat coatings
and did penetrate within the washcoat, and
was more predominant in the inlet section of
the catalyzed monolith (next slide , Fig. 6.29).
87.
88. 88
The impact of the Pb was mainly on NOX
performance.
Adding Rh to the Pd catalyst improved the
resistance to Pb and the catalyst performance
especially for NOX conversion.
89. 89
At the end of the twentieth century, the shift
to a higher price of Pd combined with the
short supply from the mine source resulted in
a reevaluation of the use of Pd.
Pt began to be substituted for Pd particularly
in underfloor locations.
90. 90
8. Low-Emission Catalyst Technologies
CARB (California Air Resources Board) ULEV
and SULEV
The emphasis: reduction of HCs in the exhaust
A majority of hydrocarbon emissions (60-80% of
the total emitted) are produced in the cold-start
portion of the automobile, this is, in the first 2 min .
of operation.
91. 91
Typical composition of the HCs during cold
start:
Hydrocarbon Type Sampling Time (seconds after cold start)
HC composition (%)
3 s 30 s
Paraffins
Olefins
Aromatics, C6, C7
Aromatics, >C8
20
45
20
15
35
20
20
25
Next slide (Fig. 6.30)
The emission control device must be functional in 50 s (for
ULEV) to 80 s (for LEV) to meet the standards.
92.
93. 93
Methods to control cold-start hydrocarbons included
both catalytic and some unique system approaches:
(1) Close-coupled Catalyst
(2) Electrically heated catalyzed metal monolith
(3) Hydrocarbon trap
(4) Chemically heated catalyst
(5) Exhaust gas ignition
(6) Preheat burners
(7) Cold-start spark retard or postmanifold
combustion
(8) Variable valve combustion chamber
(9) Double-walled exhaust pipe
94. 94
8.1 Close-Coupled Catalyst (The leading technology)
To use a catalyst near the engine manifold or in the
vicinity of the vehicle firewall to reduce the heatup
time.
A shift in the technology for close-coupled catalyst
occurred when a close-coupled catalyst capable of
sustained performance after 1050℃aging was
developed and shown to give LEV performance in
combination with an underfloor catalyst.
The close-coupled catalyst was designed mainly for
HC removal, while the underfloor catalyst removed the
remaining CO and NOX.
95. 95
The characteristic of the close-coupled
technologies is that Ce is removed.
Ce is an excellent CO oxidation catalyst and
also stores oxygen, which then can react with
CO during the rich transient driving excursions.
This causes a localized exotherm, resulting in
very high catalyst surface temperatures. (Every
percent of CO oxidized gives 90℃ rise in temp.)
→ sintering
96. 96
The early lightoff of the close-coupled
catalyst can be accomplished by a number of
methods related to the engine control
technology during cold start.
97. 97
One of the initial methods was to control the
ignition spark retard, which would allow
unburned gases to escape the engine
combustion chamber and continue to burn in
the exhaust manifold, thus providing heat to
the catalytic converter.
In all of these control strategies it is important
to have oxygen present in the exhaust gas for
early catalyst lightoff as shown in Fig. 6.32
(next slide).
98.
99. 99
8.2 Hydrocarbon Traps
Another approach was the hydrocarbon
adsorption trap in which the cold-start HCs
are adsorbed and retained, on an adsorbent,
until the catalyst reaches the lightoff
temperature.
Next slide (Fig. 6.33)
A hydrocarbon trap stores cold-start unburned
HCs
100.
101. 101
Hydrocarbon trap materials considered to
date have been mainly various types of
zeolite (silicalite, mordenite, Y-type, ZSM-5
and beta zeolite) with some studies on
carbon-based material.
102. 102
For an inline hydrocarbon trap system to
work, the hydrocarbons must be eluted from
the trap at the exact time the underfloor
catalyst reaches a reaction temperature
>250℃ as shown in Fig. 6.34 (next slide).
Currently, the lightoff of the catalyst is too
late for cleanup of hydrocarbons released
from hydrocarbon trap.
103.
104. 104
8.3 Electrically Heated Catalyst (EHC)
Studies began prior to 1990 to develop an
electrically heated monolith capable of
providing in situ heat to the cold exhaust gas.
Next slide (Fig. 6.35)
The cold-start performance of an EHC
105.
106. 106
An underfloor catalyst that is much larger in
volume supplies the reaction efficiency
during the remainder of the driving cycle
after the cold start.
The base material of EHC is ferritic steel with
varying amounts of Cr/Al/Fe with additives
of rare earths.
Next slide (Fig. 6.36)
An electrically heated catalyst
107.
108. 108
8.4 Noncatalytic Approaches
(1) The preheat burner uses the gasoline fuel in
a small burner placed in front of the
catalyst. The burner is turned on during
cold start.
(2) The exhaust gas igniter involves placing an
ignition source (e.g., glow plug) in
between two catalysts. During cold start,
some of the cylinders of the engine are run
rich to produce concentrations of CO and
H2 in the exhaust to make a flammable
mixture.
109. 109
(3) The chemically heated catalyst uses highly
reactive species, usually H2, which is
generated in a device onboard the
vehicle. Since this reacts at room
temperature over the catalyst, the heat
of reaction warms up the catalyst to react
during cold start (similar to the H2 sensor
in petroleum plants).
These approaches are complex and expensive.
None of them are presently being used in the
new low-emission vehicles.
110. 110
9. Modern TWC Technologies For the 2000s
The major components in a modern TWC are
as follows:
(1) Active component-precious metal
(2) Oxygen storage component (OSC)
(3) Base metal oxide stabilizers
(4) Moderator or scavenger for H2S
(5) Layered structure
(6) Segregated washcoat
111. 111
The Ce is now made as a Ce/Zr/X mixture
(where X is a proprietary component), which
stabilizes the OSC component for high-
temperature operations.
112. 112
Ce is now added to the catalyst in various
forms for a number of reasons:
(1) Oxygen storage
(2) Improved precious-metal dispersion
(3) Improved precious-metal reduction
(4) Catalyst for water-gas shift reaction,
steam reforming, and NO reduction
113. 113
Additionally, the similar study looked at
stabilizing the Zr with different components
of Al, Ba, Ca, Co, Cr, Cu, Mg, La, and Y.
One study showed improved surface area
stability by adding 15% SiO2 or 6% La2O3 to
a 30/70 (percent) CeO2/ZrO2 system.
114. 114
The precious metals are segregated in the
washcoat and are often prepared associated
with a specific compound such as Rh/Ce/Zr
and Pt/Al.
NOx conversion was sharply improved by
ceria, especially in combination with rhodium.
However, under certain conditions, ceria,
because of its ability to store and release
sulfur, can be shown to increase the negative
impact of sulfur.
115. 115
The effect of sulfur continues to affect the
modern catalyst technologies. The sulfur
affects mostly the lightoff characteristics of
the TWC catalyst.
116. 116
The P and Zn in the lubricating continue to be
an issue.
A study conclued that the P and Zn deposit
could be removed using the chemical wash
procedure, and once removed, the lightoff
performance and conversion of the TWC
catalyst improved.
117. 117
10. Toward a Zero-Emission Stoichiometric
Spark-Ignited Vehicle
The ULEV performance requirement for a 4-
cylinder vehicle, which may range from a
hydrocarbon engine-out emissions of 1.5-2.0
g/mile, is around 98% hydrocarbon
conversion.
118. 118
A SULEV vehicle is greater than 99%
hydrocarbon conversion.
The tailpipe HC emission from a SULEV
vehicle may be less than 5 ppmv HC, while
the background level of ambient HCs is in the
same range of 1-5 ppmv, so the measurement
of these low emission vehicles presents
another challenge.
119. 119
Because of these high emission reduction
efficiencies and hence a requirement for more
geometric surface area, monolith suppliers
began to make higher cell density substrates
approaching 1200 cpsi.
120. 120
The exhaust piping was redesigned to
minimize heat loss during the critical cold
start with fabrication of the low heat capacity
piping.
121. 121
A new sensor was developed, based on the
operating principles of the oxygen sensor but
with more sophisticated design and
electronics to give a gradual response curve
to changes in A/F ratio or oxygen content in
the engine exhaust.
122. 122
This universal exhaust gas sensor (UEGO)
minimizes the perturbation effects on the
TWC operation compared to the HEGO
(heated EGO) as shown in Fig. 6.37 (next
slide).
Second slide (Fig. 6.38)
With UEGO the operating window for the
TWC is narrowed.
This gives better overall HC, CO, and NOX
conversion over the TWC.
123.
124.
125. 125
LEV vehicles became common in the late
90s and ULEV vehicles were supplied to
the California market in 1998.
In 1999, a ZLEV (zero-level emission
vehicle) vehicle was demonstrated after
100,000-mile aging.
126. 126
The key features regarding catalyst
performance are the use of an engine
designed as lean cold-start and fuel
management to supply oxygen for the
catalytic oxidation reactions and the
reduction of heat loss during cold start.
127. 127
Honda:
The first ULEV underfloor catalyst is a
600-cpsi Pd catalyst designed for high-
temperature operation, and the remaining
underfloor catalyst accommodates
emissions during normal operation.
128. 128
Honda:
the ZLEV vehicle utilizes spark retard
during cold start to aid in catalyst heatup
and lightoff. Also, the Pd close-coupled
catalyst is 1200 cpsi followed by an
underfloor catalyst system having a separate
TWC and a trap-catalyst hybrid to manage
the hydrocarbons during the first 10 s
during cold start.
129. 129
Nissan:
The partial zero-emission vehicle (PZEV) not
only meets the SULEV tailpipe emissioms but
also has a zero evaporative emissions system.
The engine emission control technology consists
of a close-coupled catalyst followed by a series
of trap-catalyst combinations to further reduce
cold-start emissions.
Next slide (Fig. 6.39)
The excellent benefits of the increased cell
density
130.
131. 131
The critical effect of fuel properties on the
near-zero-emissions levels for the advanced
technologies has been studied. Fuels were
prepared with sulfur < 1 ppm and up to 600
ppm for tests.
The fuel sulfur affects the HC and NOX
emissions most dramatically for the SULEV
vehicles. The USEPA was targeting a sulfur
standard at 30 ppm for 2004.
132. 132
11. Lean Burn Spark-Ignited Gasoline Engine
A requirement for automotive three-way
catalysis is that the A/F combustion ratio be
at the stoichiometric point, which for gasoline
engine is about 14.6 on a weight basis.
133. 133
Leaner ratios greater than 14.6 would result
in a decrease in fuel consumption and
consequently less generation of CO2, but the
TWC cannot reduce NOX in excess air (fuel
efficiencies 20-30% higher).
Thus, the challenge is clear: develop a
catalytic system for a lean-burn engine that
will reduce all three pollutants (HC, CO,
NOX).
134. 134
11.1 NOX Reduction
The reduction of NOX in lean environments is
a technology still currently under
investigation. The dominant reaction is as
follows:
HC + NOX + O2 → N2 + CO2 + H2O
135. 135
A lean NOX reduction system must be
integrated with the engine so the exhaust
stream will have the type and amount of
hydrocarbons needed to reduce these oxides
at the optimum temperature for the particular
hydrocarbons.
Propane is effective at 500℃ with Cu/ZSM-5
(a zeolite structure), but is ineffective at
lower temperature. In contrast, ethylene
reduces NOX at 160-200℃.
136. 136
In the 1990s, scientist tried to come up with a
lean NOX catalyst technology but have failed
to date because:
(1) Hydrothermal Aging:
In the presence of water vapor, the
catalytic materials lost activity through a
sintering mechanism or lost selectivity
through competitive adsorption.
137. 137
(2) Sulfur Deactivation:
Most of the catalytic materials were
sensitive to sulfur and lost activity in the
presence of even very small amounts of
sulfur in the gasoline.
138. 138
(3) Poor Selectivity:
The hydrocarbon reductant had to be added to
the exhaust stream for the NOX reduction since
none were present from the combustion
process under lean engine operation.
Only certain species of HCs would work, and
the amount of HCs added was well in excess
of that needed for the stoichiometric reduction
of NOX (anywhere from 5:1 to 10:1 HC:NOX
ratios).
139. 139
(4) Narrow Temperature Window:
A combination of technologies was
required for operation over the range of
operating temperatures for normal
engine operation.
140. 140
A list of the materials investigated for the
lean NOX reduction:
Cu/ZSM-5
Pt/ZSM-5
Fe/ZSM-5
Co/ZSM-5
Ir/ZSM-5
Protonated zeolites, H-ZSM-5, H-Y zeolites
Noble metals
Perovskites
141. 141
Different HCs have also been tried as
reductants ranging from CO to low-
molecular-weight parrafins to partially
oxygenated hydrocarbons.
Next slide (Fig. 6.40)
Performance of typical lean NOX catalysts
These initial catalysts had in use durability
issues and are no longer being used.
142.
143. 143
11.2 NOX Traps for Direct-Injected Gasoline Engines
The TWC/trap appears to be the most
promising solution for NOX reduction for
gasoline direct-injected gasoline lean burn
engines.
144. 144
An alkaline metal oxide trap adsorbs the
NOX in the lean mode during the lean-burn
operation. The NO must first be converted
to NO2 over the Pt in the three-way catalyst:
2 2
Pt
NO O NO
145. 145
At temperatures above ~500℃, NO2 is not
thermodynamically favored; however,
because the trap continuously removes the
NO2 from the gas stream, the equilibrium is
shifted towards more NO2.
146. 146
Two kinds of Pt sites seem to operate, the
sites closer to the BaO crystallites are active
in barium nitrate formation while the other
sites are responsible for NO2 formation.
147. 147
The NO2 is trapped and stored on an alkaline
metal oxide such as BaO or K2CO3, which is
incorporated within the precious-metal-
containing washcoat of the three-way catalyst:
NO2 + BaO → BaO–NO2
Next slide (Fig. 6.41)
The trapping function of the lean NOX trap
148.
149. 149
The trap function will be finally saturated with the
adsorbed NOX, so the trap function will have to be
regenerated and the NOX reduced.
150. 150
The engine will typically operate in the fuel
economy lean mode for up to about 60 s, after which
time the engine is commanded into a fuel-rich mode
for less than 1 s, where the adsorbed NO2 is
desorbed and reduced on the Rh in the three-way
catalyst:
This is the so-called partial lean-burn engine
operation.
Next slide (Fig. 6.42)
A typical partial lean-burn operating cycle
2 2 2 2
Rh
BaO NO H BaO N H O
151.
152. 152
Sulfur oxides derived from the fuel form
alkali compounds more stable than the
nitrates and are not removed during the rich
excursion.
Therefore, the trap progressively becomes
less effective for NO2 adsorption due to
poisoning by the SOX:
BaO + SOX → BaO–SOX
BaO–SOX + H2 → no reaction
153. 153
Complicated engine control strategies are
being developed to desulfate the poisoned
trap by operating the engine at a high
temperature (> 650 ℃) and the conditions
rich of the air: fuel ratio for a short time to
remove the adsorbed sulfur oxides.
154. 154
The air: fuel ratio must be controlled to
prevent H2S from forming at excessive rich
conditions.
Reductions in NOX up to 90% are possible
provided the gasoline has less than 10 ppm
sulfur.
155. 155
One study looked at reducing the retention SOX on
the catalyst surface by changing the washcoat from
γ-alumina to a mixture of γ-alumina and TiO2 and
various washcoat dopants.
They found that a Li-doped γ-alumina had the lowest
SOX desorption temperature. The final catalyst
formulation contained a combination of 33 mol%
TiO2 and 67 mol% Li-doped γ-alumina to maintain
the amount of NOX storage and to minimize the
amount of SOX deposit.
156. 156
Alternative fuels are another area of active
study.
Fuels such as compressed natural gas, liquid
petroleum gas, and alcohols are attractive
alternatives to gasoline because they are
potentially less polluting.
157. 157
One drawback to the use of alcohol fuels is
the potential aldehyde emissions.
Studies have shown that these aldehyde
emissions can be abated by using small
starter catalyst located near the engine.
