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Automotive Catalyst

朱信Hsin ChuProfessorDept. of Environmental Eng. National Cheng Kung University

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1. Emissions and Regulations

GasolineA mixture of paraffins and aromatic hydrocarbons for spark-ignited combustion engineThe year 2000: over 500 million passenger cars in use worldwide with an annual production of new cars approaching 60 million.

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Incomplete combustion products of CO and unburned hydrocarbons (UHCs), thermal and fuel NOX

CO range: 1~2%UHCs range: 500~1000 ppmNOX range: 100~3000 ppmThe 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

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CO, HC ↑ while richNOX ↓ 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.

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US Clean Air Act1975/76 federal requirements:HC: 1.5 g/mileCO: 15 g/mileNOX: 3.1 g/mile

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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.

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Typical precontrolled vehicle emissions in the total FTP cycle:CO: 83~90 g/mileHC: 13~16 g/mileNOX: 3.5~7.0 g/mileTherefore, the catalyst was required to obtain >90% conversion of CO and HC by 1976 and to maintain performance for 50,000 miles.

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US Clean Air Act Amendment of 1990The catalyst would be required to last 100,000 miles for new automobiles after 1996.Emissions requirements by 2004:NMHC (nonmethane hydrocarbon): 0.125 g/mileCO: 1.7 g/mileNOX: 0.2 g/mile

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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.

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The current summary of the California emission standards for passenger cars of 2000:

Category DurabilityBasis (miles)

NMOG(g/mile)

CO(g/mile)

NOX

(g/mile)

TLEV 50,000120,000

0.1250.156

3.44.2

0.40.6

LEV 50,000120,000

0.0750.09

3.44.2

0.050.07

ULEV 50,000120,000

0.040.055

1.72.1

0.050.07

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

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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

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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 nC H O yCO H O

CO O CO

CO H O CO H

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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( )

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( )2

(2 ) ( ) (1 )2 4 2y n

NO orNO CO N CO

NO orNO H N H O

n n nNO orNO C H N yCO H O

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Lightoff TemperatureA 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.

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3. The Physical Structure of the Catalytic Converter

Both beaded (or particulate) and monolithic catalyst have been used for passenger vehicles.

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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.

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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.

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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

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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.

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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%))

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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 Si‧ 2O3 2‧Al2O3) satifies the needs.

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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.

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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.

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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.

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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

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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

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The progress of the automotive catalyst (Detailed in following)(1) Oxidation Catalyst

Bead and monolith supportHC and CO emissions onlyPt-based catalyst Stabilized alumina

(2) Three-way CatalystHC, CO, and NOX emissionsPt/Rh-based catalystCe oxygen storage

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(3) High-temperature Three-Way CatalystApproaching 950℃Stabilized Ce with ZrPt/Rh, Pd/Rh, and Pt/Rh/Pd

(4) All-Palladium Three-way CatalystLayered coatingStabilized Ce with Zr

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(5) Low-emission VehiclesHigh temperature, with/without Ce, close- co

upled catalyst Approaching 1050℃With underfloor catalyst

(6) Ultra-low-emission VehiclesHigh temperature, with/without Ce, close- co

upled catalyst Approaching 1050℃Increased volume underfloor, higher preciou

s- metal loading Optional trap

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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.

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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

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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.

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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).

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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%)

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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.

,900oair CPt or Pd Pb PtPb or PdPb

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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!

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Sintering of carriersNa and K acted as fluxes, accelerating the sintering process of washcoat (γ-Al2O3). Thus, preparations had to exclude these elements.

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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.

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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

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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)

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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 vo℃latilize by forming RuO2.This was dropped from further consideration.

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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.

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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

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Narrow operating window for TWCThis 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.

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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

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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

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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.

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This was accomplished by the development of the O2 storage component, which liberates or adsorbs O

2 during the air:fuel perturbations. CeO2 was found to have the proper redox (reductio

n-oxidation) response and is the most commonly used O2 storage component in modern three-way catalytic converters.

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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

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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

CeOx y

CeOX

CO H O H CO

yC H H O H x CO

NO x H N x H O

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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 La2

O3 and/or BaO 400 cells per square inch honeycomb

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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.

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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.

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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.

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At temperature in excess of 800-900 , in an oxidizin℃g 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( )

oC lean

rich

Rh Al O RhAl O

RhAl O H or CO Rh Al O

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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

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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.

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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.

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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.

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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.

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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

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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.

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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.

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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

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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).

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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.

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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.

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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.

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Typical composition of the HCs during cold start:

Hydrocarbon Type Sampling Time (seconds after cold start)HC composition (%)

3 s 30 s

ParaffinsOlefinsAromatics, C6, C7Aromatics, >C8

20452015

35202025

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.

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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

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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 s℃hown 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.

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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 tem℃p.) → sintering

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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.

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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).

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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

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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.

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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 show℃n in Fig. 6.34 (next slide). Currently, the lightoff of the catalyst is too late for cleanup of hydrocarbons released from hydrocarbon trap.

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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

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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

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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.

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(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.

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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

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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.

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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

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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.

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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.

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The effect of sulfur continues to affect the modern catalyst technologies. The sulfur affects mostly the lightoff characteristics of the TWC catalyst.

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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.

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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.

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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.

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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.

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The exhaust piping was redesigned to minimize heat loss during the critical cold start with fabrication of the low heat capacity piping.

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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.

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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.

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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.

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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.

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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.

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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.

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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

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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.

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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.

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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).

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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

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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 .℃

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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.

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(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.

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(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:NO

X ratios).

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(4) Narrow Temperature Window:

A combination of technologies was required for operation over the range of operating temperatures for normal engine operation.

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A list of the materials investigated for the lean NOX reduction:Cu/ZSM-5Pt/ZSM-5Fe/ZSM-5Co/ZSM-5Ir/ZSM-5Protonated zeolites, H-ZSM-5, H-Y zeolitesNoble metalsPerovskites

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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.

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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.

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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 2PtNO O NO

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At temperatures above ~500 , NO℃ 2 is not thermodynamically favored; however, because the trap continuously removes the NO2 from the gas stream, the equilibrium is shifted towards more NO2.

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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.

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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

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The trap function will be finally saturated with the adsorbed NOX, so the trap function will have to be regenerated and the NOX reduced.

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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 2RhBaO NO H BaO N H O

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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

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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: f℃uel ratio for a short time to remove the adsorbed sulfur oxides.

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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.

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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.

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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.

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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.

Ref.: Time, March 19, 2007

Ref.: Time, March 19, 2007

Ref.: Time, March 19, 2007