CATALYTIC CONVERTER

ABSTRACTGrowth in population and growth in number of vehicles every year have combined together to give dramatic increase in both water and air pollution problems. The automotive vehicles have been a significant contributor to air pollution on total mass basis. According to one survey it has been shown that transportation contributes about 42% (by wt.) of the major pollutants. Motor vehicles alone contribute about 39%. Transportation contributes about 50% of the man made hydrocarbons (HC), a little over 60% of the total carbon mono-oxide (CO) emission and about 40% of the nitrogen oxide (NOx).

Major pollutants from automobiles are unburned hydrocarbon (UBHC), oxides of nitrogen, carbon monoxide, lead compounds, and oxides of sulphur. The air pollution due to these pollutants affects adversely materials, animals and human life. It reduces visibility to a large extent causing traffic hazards. The vegetarian and plants are affected badly by sulphur dioxide, photo chemical smog and lead.

POLUTANTS PORDUCED BY ENGINESIn order to reduce emissions, modern engines carefully control the amount of fuel they burn. They try to keep the air-to-fuel ratio very close to the stoichiometric point, which is the ideal ratio of air to fuel. Theoretically, at this ratio, all of the fuel will be burned using all of the oxygen in the air. For gasoline, the stoichiometric ratio is about 14.7:1, meaning that for each pound of gasoline, 14.7 pounds of air will be burned. The fuel mixture actually varies from the ideal ratio quite a bit during driving. Sometimes the mixture can be lean (an air-to-fuel ratio higher than 14.7), and other times the mixture can be rich (an air-to-fuel ratio lower than 14.7).AUTOMOBILE EMISSIONS

Tailpipe emissions Evaporative emissions Life cycle emissions LITERATURE REVIEW OF PREVIOUS WORK

Study of exhaust emissions and its control has been a matter of concern from a long period for the engineers. The operating variables affect the auto engine exhaust. The main variables for emitting pollution are HC, CO and NOx. Previously the auto engineers control the pollution emission by following ways. Controlling Air-Fuel ratio Controlling Engine Speed

Proper Spark Timing

Exhaust Back Pressure

Valve Overlap

Intake Manifold Pressure/ Vacuum

Combustion Chamber Deposit Build Up

Surface Temperature

Surface to Volume Ratio

Stroke to Bore Ratio

Compression Ratio

Effect Of Coolant TemperaturePROBLEM FORMULLATION (ADVANCE WORK) A lot of research work has been done since 1940s to check the menace of pollution by automobiles. There are many techniques to control the exhaust pollution through automobiles e.g. Exhaust Gas Recirculation Thermal Reactor Modification in Combustion System to Achieve Complete Combustion Injection System

Ignition System Carburetor After Burner Evaporative Emissions and Control

Crankcase Emissions and Control

Now a days use of catalytic converter dominates all other techniques. Catalytic converter is used in the tail pipe of the exhaust system so that the level of obnoxious emissions from the engine is reduced before they are let out to the atmosphere.

To evaluate the performance and design of catalytic converter is made which predicts the catalytic converter performance. We can optimize catalytic converter design by adjusting different parameters.

CHAPTER-1INTRODUCTION

Deterioration of air quality is a major environmental problem in many large urban centers in both developed and developing countries. Although urban air quality in developed countries has been controlled to some extent during the past two decades, in most of the developing countries it is worsening and becoming a major threat to the health and welfare of people and the environment. In our modern society, quality of life is greatly measured by the amount of consumption of electricity or by the use of car. Electricity generation and operation of vehicles mostly use fossil fuel. As these fuels are burnt, huge quantity of lethal chemicals and poisonous particulate matter are released as a part of emission into the surrounding atmosphere due to incomplete combustion causing serious air pollution, affecting public health. The contribution of motor vehicles to air pollution was first recognized by Prof. Hazen Smit who discovered that the two invisible vehicular emissions namely hydrocarbons and oxides of nitrogen were responsible for the famous Los Angeles photochemical smog. These gases interact with one another and other components of the atmosphere to generate several harmful compounds viz. carcinogens. The auto exhaust also affects our valuable cultural heritage, historical places/ monuments/ architecture and the environment. The number of vehicles in India has increased from 1.86 million in 1971 to 32 million in 1996 and about 53 million in 2000. Meteorological and topographical conditions affect dispersion and transport of these pollutants in ambient air. The growth in population and growth in energy consumption per person have combined together to give dramatic increase in both water and air pollution problems. The mankind can survive without food and water for few days, seldom the survival exists without air. Among several sources of pollution, vehicles used in transportation sectors have a substantial share in increasing atmospheric pollution.

It has been estimated that about 60 70% air pollution is from motor vehicles. Almost all these are powered from I.C. engines which use fossil fuel such as gasoline or diesel oil. The increases in two and three wheelers are aggregated the problem further. In an average motor vehicle with no emission control device (1, 3) the estimated values of annual production of these pollutants is about 770 kg. of carbon monoxide; 240 kg. of unburned hydrocarbons and 40 kg. of nitrogen oxides. This quantity increased proportionately with the no. of vehicles.1.1 EFFECT OF POLLUTANTS ON ENVIRONMENT Vehicles equipment with petrol or diesel engines emit considerable amount of carbon monoxide and unburned hydro carbons; Oxides of nitrogen, compounds of lead oxides of sulphur etc. The air pollution due to these pollutants effects adversely on materials, animals and human life .It reduces visibility to a large extent causing traffic hazards. The vegetation and plants are affected badly by sulphur dioxide, photo chemical smog and lead.1.2 EFFECTS OF AIR POLLUTANTS ON HUMAN HEALTH

Air pollutants emitted by motor vehicles have a number of adverse effects on human health and ecology. Diseases such as silicosis, pneumoconiosis, coniosis, allergy, asthma, inflammatory lung diseases, infections and cases of mycotoxicosis and neurological or vascular disorders have been associated with exposure to pollutants and bio aerosols (bacteria, fungi and their by-products). Exposure by inhalation directly affects respiratory, nervous and cardiovascular systems of humans, resulting in impaired pulmonary functions, sickness and even death. Pollution hampers the body normal immune system and consequently secondary diseases kill the subject. Pollution is a hidden enemy of civilization. There are no specific drugs for the diseases cause by the pollution. Pollutants thus give rise to enormous health costs, in addition to the general discomfort and poor quality of life in urban area.

Particulate matters cause breathing and pulmonary disorders like asthma, bronchitis, lowering of the general immune system, cancer and long term irreparable damage to the lungs. CO binds with hemoglobin to form carboxyhemoglobin (COHb) which reduces the bloods carrying capacity to transport O2 to the tissues. SO2 is known to cause decreased lung functions and a variety of respiratory diseases and increased risk of mortality and morbidity. Adverse effects include coughing, phlegm, chest discomfort and bronchitis. Nitrogen oxides-NO2 is linked with increased airway resistance in asthmatics and decreased pulmonary function. NO2 has been associated with respiratory illness in children (cough, runny nose, and sore throat). Occupational exposure to NO2 range from inflammation of the mucous membrane of the tracheobronchial tree to bronchitis, bronchopneumonia and acute pulmonary edema. Lead prevents hemoglobin synthesis in red blood cells in bone marrow, impairs liver and kidney function and causes neurological damage. PAH are known as carcinogenic. Ozone-adverse health effects include changes in pulmonary function, eye, nose and throat irritation, coughing, throat dryness, thoracic pain, increased mucous production, chest tightness, lassitude, malaise and nausea. Chlorofluorocarbons-exposure to increased UV-B radiation is suspected to increase the risk of skin cancer and eye illness especially cataract and to adversely affect the immune system. Benzene about 50% of inhaled benzene is adsorbed. Part of the absorbed benzene is exhaled by respiration and eliminated through the urinary tract. Benzene is accumulated in the fat tissue and bone marrow. Benzene has toxic and carcinogenic effects. The toxic effects are associated with central nervous system, hematological and immunological systems. Higher exposure can damage the respiratory tract, lung tissue and bone marrow and cause death. Carcinogenic effects include leukemia.

Polyaromatic Hydrocarbons PAH absorbed in the lungs and intestines and metabolized in the human body, are mutagenic and carcinogenic. Aldehydes-are absorbed in the respiratory and gastrointestinal tracts and metabolized. Adverse health effects of HCOH include eye and nose irritation, irritation of mucous membranes and alteration in respiration, coughing nausea and shortness of breath. Lead (Pb) - Tetraethyl lead was added to gasoline to increase the fuels octane number, which improves the antiknock characteristics of the fuel in spark-ignition engines.

1.3 POLLUTANTS EMISSIONS FROM MOTOR VEHICLES

Emission from vehicles especially automobiles contribute significantly two third of air pollution in the urban area. Main sources of emission from automobiles are:i) Volatile Organic Compounds (VOCs) / Evaporative Emissions- are HC vapors lost constantly and directly to the atmosphere due to volatile nature of petrol, mainly from the fuel lines, fuel tank and carburetor depending upon fuel composition, engine operating temperature and ambient temperature. Losses from the carburetor, called Hot Soak Emissions, occur when a hot engine is stopped. It should be noted that out of total emissions, which are much more in case of petrol than diesel, 20-32% of the total emissions are due to evaporation losses, of which the HCs happen to be the chief constituents. These gases are also known as hydrocarbons and defined as "volatile" because of their ability to evaporate quickly and easily into the air. They react with nitrogen oxides in the presence of sunlight to produce ground level ozone, a principle component of smog.

ii) Crankcase Emission (also called running loss emissions) are unburnt or partially burned fuel components that, under pressure, escape from the combustion chamber, pass the pistons and enter the crankcase. This mixture is called blow-by. The main constituent of blow-by emission is HCs. If uncontrolled, it may constitute 1325% of total emissions. Since, diesel engines compress only air, blow-by contain very low levels of pollutants.

iii) Exhaust Emission- Automotive exhaust is the major source constituting about 60% of the total emission. Automobile exhaust consists wide range of pollutants from simple to carcinogenic substances such as (1) Hydrocarbons (Unburnt), (2) Carbon monoxide, (3) Oxides of nitrogen (NOx), (4) Lead oxides, (5) Particulate matters e.g. lead, carbon, alkaline earth compounds, iron oxide, tar, oil, mist (6) Traces of aldehydes, esters, ethers, sulphur dioxide, peroxides, ketones benzene (C6H6), 1, 3 butadiene, Poly Aromatic Hydrocarbons (PAH), metal dust, asbestos fiber, dioxin, furan, ammonia, organic acids , chlorofluorocarbons (CFCs) etc. .

Hydrocarbons and CO appears in the exhaust gas products of incomplete combustion. Oxides of nitrogen result from the reaction of nitrogen and oxygen contained in the combustion air at high temperature prevailing during combustion. Further, many of these primary pollutants react with each other to form secondary pollutants. Chief among these are HC, CO, NOx when mixed with atmospheric water vapors in presence of sunlight form ozone and variety of complex organic gases and resultant particulates known as Photochemical Smog (Sharma, and Agnihotri, 1992). Particulate Matter (PM) includes particles of soot, ash, and dirt that are released from car exhaust. Particles are measured by their diameter and with respect to smog; two sizes are of the most concern - PM10 and PM2.5. Particulate matter and ground level ozone combine to make up smog. Sulphur Dioxide (SO2) gas contributes to smog formation, but is known better for combining with water molecules to form sulphuric acid and producing acid rain. A comparative sulphur content in diesel fuel in different countries and pollution arise from different parts of the automobiles are given in Table-1.

Table 1: Maximum Permitted Sulphur Content in Automotive Diesel FuelE.C. Countries% Mass

U.K.0.3

France0.3

Germany0.2

Italy0.3

Others

Canada0.5

Japan0.5

Switzerland0.2

South Korea1.0

USA0.5

India0.5

1.4 EMISSION FROM GASOLINE VEHICLEGasoline- powered engines are of two types 4 strokes and 2 strokes. The exhaust consists of CO, HC, NOx, SO2 and partial oxides of aldehydes, besides particulate matter, lead salts account for the larger chunk of all pollution from gasoline-run vehicles.

The 2 stroke engine requires 2-T oil for lubrication of engine. Since the burning quality of mineral based lubricating oil is very poor, major fraction either remain unburned or burns partially and comes out through exhaust and responsible for smoke emissions.TABLE-2 Emissions from Gasoline VehiclesS.N.SourceAmount of Emissions (%)

4-stroke2-stroke

1Crankcase blow by20-

2Evaporative Emissions203

3Exhaust Emissions6097

Table-3 Exhaust Emission from Indian Light Duty Gasoline VehicleThe average INDIAN light-duty gasoline vehicle annually emits approximately: 4500 kg of CO2

200 kg of CO

20 kg of VOCs

22 kg of NOX

1 kg of SOX

0.15 kg of PM10

0.15 kg of PM2.5

(based on data from the year 2000)

1.5 EMISSION FROM DIESEL VEHICLEDue to low volatility, evaporative emissions are non-significant. Though the concentration of CO and unburnt HC in the diesel exhaust are rather low, they are compensated by high concentration of NOx. There are smoke particles and oxygenated HC, including aldehydes and odour-producing compounds.

1.6 MAJOR POLLUTANTS Major pollutants from automobiles are: (a) Unburned hydro carbons. (UBHC).

(b) Oxides of nitrogen (NOx).

(c) Carbon mono oxides.

(d) Lead compounds.

(e) Oxides of sulphur.

(a) Hydro carbon exhaust emissions arise from three sources;

(i) Wall quenching,

(ii) Incomplete combustion of charge.

(iii) Exhaust scavenging as in a two cycle engines. (b) Formation of nitric oxide takes place within the combustion chamber at the peak combustion temperature and remains there during expansion and exhaust stroke in non equilibrium amount. When this Nitric oxide comes in contact with atmospheric oxygen, other compounds of nitrogen may be formed.

Carbon monoxide is generated from the automobiles largely by old and petrol vehicles, due to in complete combustion of organic matters. It has no smelt; inactivates hemoglobin and can be very harmful because it shows no physiological symptom or uncomforted. Human exposure to this for longer than eight hours at a concentration of about 120 PPM should not be allowed for more than one hour.

(d) Among anti knock additives TEL (Tetra ethyl lead (C2H5)4 Pb) is well known. The effect of adding TEL is to reduce a non volatile combustion product which tends to accumulate on the spark plug and cause the engine to misfire. Therefore TEL is always blended with ethylene dibromide and ethylene dichloride so that the lead compounds formed during combustion are sufficiently volatile and are discharged through exhaust to atmosphere.

(e) Sulphur is present in the crude oil itself. During refining large amount of sulphur is separated from the fuel. Still there remains some of its amount in the fuel. It forms oxide of sulphur during combustion. These Oxides of sulphur from exhaust go to atmosphere. This sulphur oxide may be combined with sulphuric acid (H2SO4); a very corrosive secondary pollutant.

CHAPTER-2

AUTOMOTIVE ENGINE TYPES & AUTOMOTIVE POLLUTIONSpark-ignition and Diesel engines are the two most common engines. Other types of engines are Rotary (Wankel) Engines, Gas-Turbine (Brayton) Engines, Steam (Rankine) Engines, Stirling) Engines, Electric and Hybrid Vehicles.

Spark-ignition gasoline engines have either a 2-Stroke (the cycle is completed in 2-strokes of the piston) or 4-Stroke design (the cycle is completed in 4-strokes, Suction Compression Expansion or Power and Exhaust Strokes of the piston). 2-stroke engines are cheaper, lighter and can produce greater power output per unit of displacement, so they are widely used in motorcycles, scooters and mopeds and small power equipment. It emits 20-50% of their fuel unburned in the exhaust, resulting in high emissions and poor fuel economy. All gasoline engines currently used in automobiles and larger vehicles use the 4-Stroke design. Advanced 2-Stroke engines under development would achieve lower emissions and fuel consumption than 4-Stroke engines.2.1 MODE OF VEHICLE OPERATION AND EMISSION RATES

The vehicle operation is divisible into 4 modes or driving cycles: (i) Idle / Start Mode - when the engine of vehicle has been started. It is yet stationary. At this stage there is high level of HC and CO and very low level of NOX, (ii) Acceleration- the emission of HC and CO come down with rise in NOX levels, (iii) Cruise/Steady Mode-steady speed produces relatively low concentration of HC and CO but high concentration of NOX and (iv) Deceleration (a) free and while (b) applying brake- slow speed contributes to more pollutants.

2.2 TYPES OF FUELS Pollutant emissions from motor vehicles are determined by the vehicles engine type and the fuel it uses. Ideal fuel must have certain physical, chemical and combustion properties, such as high energy density, good combustion qualities, high thermal stability, and low deposit forming tendencies, compatibility with engine hardware, good fire safety, low toxicity, low pollution, easy transferability and on-board vehicle storage.

Commonly use fuels are gasoline and diesel. Gasoline is a mixture of hydrocarbon compounds which have been distilled from petroleum. Sulphur is a constituent in oil; low sulfur oil is in great demand as a fuel because the SO2 emission is reduced. Low sulphur oil is called Sweet.Alternative fuels considered for vehicular use are natural gas (in compressed or liquefied form, Natural gas contains the lighter aliphatic compounds, largely methane, CH4. A typical gas would be 80-90% CH4 , 5 to 10 % ethane and the rest other compounds), liquefied petroleum gas (LPG), methanol (made from natural gas, coal or biomass), ethanol (made from grain or sugar) vegetable oils, hydrogen, synthetic liquid fuels derived from the hydrogenation of coal and various blends such as gasohol.

2.3 AIR POLLUTANTS FROM MOTOR VEHICLESThe major pollutants emitted from gasoline fueled vehicles are CO, HC, NOx and Pb while pollutants from diesel-fueled vehicles are particulate matter (including smoke), NOX, SO2, Polyaromatic Hydrocarbons PAH.The composition of automotive and diesel exhausts is characterized by greater amounts of carbon monoxide and hydrocarbons than that of emissions from other fuel burning processes. Factors for automotive and diesel exhaust emission, in pounds per 1,000 gallons of fuel consumed (Giver, 1972), are given in Table-4. Table-4: Emission Factors for Gasoline Engines and Diesel Engines (lb/1000 gal of Fuel)S. No.PollutantsGasoline EnginesDiesel Engines

1Particulates11110

2Oxides of Sulphur940

3Oxides of Nitrogen113222

4Carbon monoxide291060

5Hydrocarbons524180

6Aldehydes410

7Organic acids431

8Ammonia2-----

9Benzo(a) pyrene0.3 g/1000 gal0.4 g/1000 gal

Carbon monoxide (CO) - Colorless and odorless gas, slightly denser than air. Residence time and turbulence in the combustion chamber, flame temperature and excess O2 affect CO formation. Conversion of CO to CO2 in the atmosphere is slow and takes 2 to 5 months. In developing countries the transport sector account for 53% of CO emissions and the residential and commercial sectors, 46%.

Hydrocarbon Compounds (HC) - Compounds consisting of carbon and hydrogen and include a variety of other volatile organic compounds (VOCs). Most HCs are not directly harmful to health at concentrations found in the ambient air. Through chemical reactions in the troposphere, however they play an important role in forming NO2 and O3 which are health and environmental hazards. Among the various HC, methane (CH4) does not participate in these reactions. Remaining HC, non methane hydrocarbons (NMHC) are reactive in forming secondary air pollutants. NMHC are photo chemically reactive.

Benzene and Polyaromatic Hydrocarbons (PAH) - Motor vehicles emit toxic HC including benzene, aldehydes and polyaromatic hydrocarbons (PAH). About 85 to 90% benzene emissions come from exhaust and the remainder comes directly from gasoline evaporation and through distribution losses. Toluene and xylene HC compounds are present in the gasoline whereas aldehydes, 1, 3 butadiene are not present in gasoline, diesel fuel, ethanol or methanol but are present in their exhaust emissions as partial combustion products. Note: PAH are emitted at a higher rate in exhaust of diesel-fueled vehicles than gasoline fueled vehicles.

Nitrogen oxides (NOX) - includes nitric oxide (NO), nitrous oxide ( N2O), nitrogen dioxide (NO2), dinitrogen trioxide (N2O3) and nitrogen pentoxide (N2O5). NO and NO2 collectively represented as NOX, are the main nitrogen oxides emitted by vehicles. About 90% of these emissions are in the form of NO. NO is produced in the vehicle engine by combustion of nitrogen at high temperatures. NO2 formed by oxidation of NO, has a reddish brown color and pungent odour.

In the atmosphere, NO2 involved in a series of reactions in the presence of UV radiation that produce photochemical smog, reducing visibility. It may also react with moisture to form nitric acid (HNO3) aerosols. In the lower atmosphere (troposphere), NO2 forms O3 by reacting with HC. In the upper atmosphere, it reacts with chlorine monoxide to form chlorine nitrates. In developing countries, the transport sector accounts for 49% of NOX emissions and the power sector, 25%; the industrial sector, 11%; the residential and commercial sectors, 10% and other sources 5%.

Sulfur dioxide (SO2) - is a stable, nonflammable, nonexplosive, colorless gas. In the atmosphere, SOX may be converted to sulfur trioxide (SO3) by reacting with O2. SO2 and SO3 react with moisture in air to form sulfurous (H2SO3) and sulfuric (H2SO4) acids may precipitate to earth as acid rain. Sulphates may also be produced through reaction of these sulfur compounds with metals present in particulate matter.

Ozone (O3) - in the lower (troposphere) layer, ground level ozone (GLO) is formed by the reaction of VOCs and NOX with ambient O2 in the presence of sunlight and high temperatures. GLO is a major constituent of smog in urban areas and motor vehicles are the main emission source of its precursors. The reactions that form GLO also produce small quantities of other organic and inorganic compounds such as peroxyacetyl nitrate (PAN) and nitric acid. GLO concentrations depend on the absolute and relative concentrations of its precursors and the intensity of solar radiation, which exhibits diurnal and seasonal variations. Thermal inversions increase GLO concentrations.

Particulate matter (PM) - consists of fine solids and liquid droplets other than pure water that are dispersed in air. Total suspended particulates are particles with an aerodynamic diameter of >70 (m. PM with an aerodynamic diameter of ( 10 (m known as suspended inhalable particulate matter/ Respirable Suspended Particulate Matter (RSPM) or PM10, remains in the atmosphere for longer periods because of its low settling velocity. PM10 can penetrate deeply into the respiratory tract and cause respiratory illness in humans. PM with an aerodynamic diameter of 2.5-10 (m or less is defined as fine particles (PM2.5), while the larger PM is called coarse particles. Nearly all PM emitted by motor vehicles consists of fine particles and a large fraction of these particles has an aerodynamic diameter less than 1(m.

PM2.5 can also be formed in the atmosphere as aerosols from chemical reactions that involve gases such as SO2, NOX and VOC. Sulfates, which are commonly generated by conversion from primary sulfur emissions, make up the largest fraction of PM2.5 by mass. PM2.5 can also form as a result of solidification of volatile metals salts as crystals following cooling of hot exhaust gases from vehicles in ambient air. Gasoline fueled vehicles have lower PM emission rates than dieselfueled vehicles. PM emissions from gasoline fueled vehicles result from unburned lubricating oil and ash-forming fuel and oil additives. PM emitted by diesel-fueled vehicles consists of soot formed during combustion, heavy HC condensed or adsorbed on the soot and sulfates. These emissions contain PAH. With the advancement of emission control measures in engines, however, the contribution of soot has been reduced considerably.

Black smoke, associated with the soot portion of PM emitted by diesel-fueled vehicles, is caused by O2 deficiency during the full combustion or expansion phase. Blue, gray and white smokes are caused by the condensed HC in the exhaust of diesel-fueled vehicles. Blue or gray smoke- results from vaporized lubricating oil and white smoke occurs during engine start-up in cold weather. Diesel fuel additives such as Ba, Ca and Mg reduce smoke emissions but increase PM sulfate emissions. These additives may also increase PAH emissions..

Chlorofluorocarbons (CFCs) - The source of CFC emissions from motor vehicles is the Freon gases used in air conditioners. CFC emitted into the atmosphere rise to the stratosphere layer within 10 years and are estimated to remain there for 400 years. CFC molecules struck by UV radiation release chlorine atoms, which destroy O3 by forming chlorine monoxide. Furthermore, when a free O2 atom reacts with a chlorine molecule, an O2 molecule is formed and a chlorine atom is released to destroy more O3.

CHAPTER-3CONTROL TECHNOLOGIES FOR POLLUTION EMISSION(LITERATURE REVIEW OF PREVIOUS WORK)Study of exhaust emissions and its control has been a matter of concern from a long period for the engineers. The operating variables affect the auto engine exhaust.3.1 EFFECT OF OPERATING AND DESIGN VARIABLE ON HC & CO EMISSION3.1.1 Air-Fuel Ratio

For a fuel quality, concentrations of many of these pollutants are influenced by such factors as the air-fuel ratio in the cylinder at the time of combustion, ignition timing, combustion chamber geometry, engine parameters (e.g. Speed, load and engines temperatures) and use of emission control devices. Vehicles with electronic fuel injection engines maintain an air-fuel ratio of about 14.7: 1 (i.e. burning of 1 lb of fuel about 14.7 lbs of air is needed, which is the stoichiometric/ ideal ratio for the air-gasoline mixture) to achieve complete combustion.

Lean mixture (Higher Ratios) produces less HC & CO emissions while Rich mixture (Lower Ratios) produces more CO & HC and low values of NOx emissions from unburned or partially burned fuel. The air/fuel ratio is adjusted taking into consideration the emission and efficiency of an engine. It is seen that most of the gasoline operated engines are adjusted within the air/fuel ratio of 12:16. The air/fuel ratio and ignition timing are readily adjustable, both in design specifications and field tune up adjustments.

3.1.2 Engine Speed

Emission concentration is markedly reduced at higher engine speed. Primarily increase in engine speed improves the combustion process within the cylinder by increasing turbulent mixing and eddy diffusion. This promotes after oxidation of quenched layer. In addition increased exhaust port turbulence at higher speeds promote exhaust system oxidation reactions through deter mixing.3.1.3 Spark Timing

It has been found that at a constant power of 13 BHP and 1500 rpm, a retard of 10 from the manufacturers recommended setting of 30 BBDC reduced HC emissions by 100 PPM, but increased fuel consumption of 10%. The 100 PPM change for 10 retard suggests the importance of precise spark timing and minimum distributor tolerance.

3.1.4 Exhaust Back Pressure

Increasing exhaust gases in the cylinder at the end of the cycle. If this increase in residual does not increase the percentage dilution of the fresh charge to a level where the combustion process is adversely affected, the HC emission concentration will be lowered. The reduction arises from leaving the tail end of the exhaust, which is rich in unburned hydrocarbons in the cylinder. This tail is subsequently burned in next cycle.

3.1.5 Valve Overlap

It has similar effects as that of back pressure case. The charge is further diluted with the residual gases. Slight valve overlap (about 2) provided minimum emission. Thus a slight amount of residual tail did lower the average HC emission value. However a further increase in residual led to an incomplete combustion and a HC emission increase.

3.1.6 Intake Manifold Pressure

At a fixed mixture ratio and speed with best power timing there is no effect of engine horsepower on hydrocarbon or carbon mono oxide emission. However because carburetor and distribute settings are variable in a vehicle, there is a change in emission concentration as the throttle is varied at a constant speed.3.1.7 Combustion Chamber Deposit Build Up

Major source of combustion chamber deposit is TEL, fuel additive used to suppress the combustion knock. Deposits act to increases. Deposits also act as a sponge to trap raw fuel which remains unburned and adds to exhaust hydrocarbons. Deposits build up also increases compression ratio which in turn increases emission.

3.1.8 Surface Temperature

Combustion chamber surface temperature affects the UBHC emission by changing the thickness of the combustion chamber quench layer and the degree of after reaction. Went Worth (6) studied the effect of such changes on hydrocarbon emissions of 0.35 to 0.58 PPM hexane per one degree Fahrenheit (f) rise in combustion chamber surface temperature. In one test an increase of 100F decreased emissions by 37%

3.1.9 Surface to Volume Ratio

It is desirable to minimize the surface area of the combustion chamber because hydrocarbons emissions arise primarily from quenching at the chamber mixture, a thin layer of HC is left on the wall surface. Hence by decreasing the surface area for the same volume of the combustion chamber we can reduce this emission.3.1.10 Stroke to Bore Ratio

Engine with small bore born and large stroke have lower surface to volume ratio. But this modification is opposed due to increased frictional losses in case of long stroke and practice is for short stroke for reduced friction, increased power and economy.

3.1.11 Compression Ratio

A large reduction in surface to volume ratio can be affected by decreasing compression ratio. This increases the clearance volume greatly with little increase in surface area. However, reduced compression ratio results in lower thermal efficiency and reduced engine power. A decrease in compression ratio reduces hydrocarbon emissions a second way when C.R. reduces, thermal efficiency is lowered and as a re result exhaust gas temperature is increased which improves exhaust system after reactions.

3.2 EFFECT OF OPERATING AND DESIGN VARIABLE ON NOx EMISSION3.2.1 Equivalence Ratio

The equivalence ratio affects both the gas temperature and the available oxygen during combustion. Theoretically an increase in the equivalence ratio from 1 to 1.1 results in an increase in the maximum cycle temperature of 100F while o2 concentration occurs at an equivalence ratio of 0.9. The maximum cycle temperature with this lean mixture is lower that with a richer mixture, but the available oxygen concentration is much higher.

3.2.2 Spark timing

Increase in spark advance at any load and speed results in an increase in the no concentration.

3.2.3 Intake Manifold Vacuum

Increase in manifold vacuum decreases load and temperature and an increase the mass of the residual gases, due to this ignition delay is increased and flame speed is reduced. This results in increase in the time of combustion. If the spark timing is kept constant the increase in manifold vacuum would cause the greater part of the combustion process to occur during the expansion stroke. This would result in decrease of maximum temperature of cycle and a corresponding decrease in No concentration in exhaust.

3.2.4Engine speed

An increase in engine speed has a little effect on ignition delay, results in an increase in the flame speed due to turbulence and reduces heat losses/cycle. This tends to raise compress and combustion temperature and pressure. It spark timing is held constant a greater portion of this combustion tends to occur during expansion where temperature and pressure are comparatively low. This is slowest for burning mixture ratio 19:1. For richer mixture, which burn faster, the effect of reduced heat losses at higher speed predominates. This implies that an increase in the rate of no formation due to reduced heat losses opposed by a reduction in the rate of no formation due to late burning. For rich mixtures where combustion and NO formation are, the former effect predominates. For lean mixtures where combustion and no formation are rapid, the former effect predominates. For lean mixtures where combustion and no formation are flow the later effect predominates.

3.2.5 Effect of Coolant Temperature

An increase in coolant temperature results in a reduction in the heat lost to the cylinder walls and an increase in maximum gas temperature. This results in an increase in No concentration and vice versa.

3.2.6 Humidity

An increase in the mixture humidity is mainly due to the drop in maximum flame temperature which reduces No formation, Moore (7) has calculated that 1% (by weight) of water vapour reduced a hydrogen-air ethylene air flame temperature by 36F. This reduced the initial rate of No production by about 25%.CHAPTER-4CONTROL TECHNOLOGIES FOR POLLUTION EMISSION(ADVANCE WORK)

If we adjust above design and operating variables we can minimize

Exhaust emission pollution but we are not able to do so and to maintain the equilibrium along different variables. The following techniques have been used to reduce the exhaust emission:

4.1 Exhaust Gas Recirculation

A portion of the exhaust gas is recirculated to the cylinder intake charge. This reduces the peak combustion temperature, since the inert gases absorb a large quantity of heat. This also reduces the quantity of oxygen available for combustion. The exhaust gas for recirculation is directly taken from the stove area through an orifice; It passes through the butterfly control valve for regulation of the rate and ducted down to the throttle shaft by means of appropriate linkage and the amount of valve opening is recycle exhaust is normally shut off during full throttle acceleration to prevent loss of power when maximum performance is needed. There will be a little effect on Nox emission even if the above arrangement is not made because Non concentrations in idle and full throttle are already very low. Reduction of the peak combustion temperature by EGR reduces the formation of Nox.4.2 Thermal reactors

A thermal reactor is a chamber in the exhaust system designed to provide sufficient residence time to allow appreciable homogeneous oxidation of Co and hydrocarbon to occur. In order to improve co conversion efficiency, the exhaust temperature is increased by spark retard. This however, results in fuel economy loss. Thermal reactor consist of two enlarged exhaust manifold which allow greater residence time for burning of HC and CO with oxygen in the air, which is pumped. A cylindrical reactor with a tangential entry from the exhaust manifold is attached to the engine. Secondary air pumps inject fresh air into the reactor to keep a flame constantly burning and thereby assuring complete combustion.4.3 Modification in Combustion System to Achieve Complete Combustion

The undesirable exhaust emission of vehicle is formed mainly within the combustion process. Due to improper vaporization of fuel in inlet manifold incomplete combustion results which leads to liberation of Co and HC modification such as heating the inlet air(4), enhances the vaporization of liquid gasoline and improve mixture distribution among the cylinder resulting in a better. Combustion which reduce Hc and Co. 4.4 Injection System

The objective of the injection system is to atomize and distribute the fuel throughout the air in the cylinder while maintaining prescribed fuel-air rations. To accomplish these tasks a number of functional element might be required within the system.

(a) Pumping elements: to move the fuel from fuel tank to cylinder.

(b) Metering element: to measure and supply the fuel at the rate demanded by the speed and load.

(c) Metering control: to adjust the rate of metering elements for changes in load and speed of the engine.

(d) Mixture control: to adjust the ratio of fuel rate to air rate as demanded by the load and speed.

(e) Distributing elements: to divide the metered furl equally among the cylinders.

(f) Timing control to compensate for changes in temperature and pressure of either air or fuel or engine that affects the elements of the system.

(g) Ambient control: to compensate for changes in tem. And pressure of either air or fuel or engine that affects the element of system.

(h) Mixing Element: to atomize that fuel and mix with air to from a homogeneous mixture.

These changes ensure precise fuel metering in accordance with changing engine requirements at low and high loads.

4.5 Ignition System

Ignition system modification is carried out for burning leaner air fuel mixture. Lean mixtures have higher breakdown voltage than do slightly rich mixtures. Good ignition depends upon the following reasons.(a) Length of air gap of the spark plug the greater the gap, the larger is the required breakdown voltage.

(b) Geometry of the gap: Pointed electrodes require less breakdown voltage.

(c) The temperature of the electrodes and enclosed air fuel mixture: high temperature allow lower breakdown voltage

(d) The density of mixture: higher density requires high breakdown voltage. (e) The leakage resistance of the insulator: carbon and metallic oxides from electrically conductive coatings on the insulator which thus shunt the secondary winding and reduce the maximum voltage that the secondary can impress across the spark gap.

(f) The rate of increase of the voltage at the gap: if the ignition system builds up the voltage at a rapid rate, the effect leakage will be minimized and a Grater sparking voltage is available.

(g) The presence of ionized gases in the gap: Good ignition system also depends on following point. A combustible mixture must present between the electrodes. For this reason, a spark plug location near the intake valve is desirable, although opposed by the necessity to locate the plug near the hot exhaust valve to avoid knock.

A large gap increases the probability of regular firing, especially at parts loads, when stratification from exhaust gas dilution is present. A high mixture density allows a greater amount of energy to be liberated and probability of ignition is increased.

Ignition is best secured with slightly rich mixtures, since a greater release of energy is obtained.

The position of the plug and the position of the electrodes relative to the flow conditions in the chamber.

4.6 Carburetor

Main work of carburetor is to meter, atomize, vaporize and mix the fuel with air. If carburetor modifications are such that to ensure precise fuel metering in accordance with changing engine requirements at low and high loads.4.7 After Burner

The after burners designed to oxidize UBHC and CO; includes a pre-combustion chamber where secondary air and fuel are ignited by a spark to provide thermal energy for the reaction.

4.8 Evaporative Emissions and Control

Diurnal and Hot soak emissions have been controlled by venting the fuel tank to the atmosphere through a canister of activated carbon. The volatile nature of gasoline can be minimized by keeping gasoline Reid Vapor Pressure of 10 psi. Gasoline with an RVP of 11 psi will produce about twice the evaporative emissions of gasoline with an RVP of 8.7 psi.

4.9 Crankcase Emissions and Control

Crankcase emission controls involve closing the crankcase vent port and venting the crankcase to the air intake system via a check valve. In newer model crankcase blowby are controlled by recycling to the engine through the intake system.

4.10 Catalytic converter

The principal of catalytic converter is to control the emission levels of various pollutants by changing chemical characteristics of the exhaust gases.

Except catalytic converter above method require additional changes in the engine design. In the exhaust gas recirculation method, is basically to reduce the concentration of only one specie i.e. Nox. and may increase the concentration of other constituents. Thermal reactors need additional combustion chamber and spark ignition systems which make it more expensive. This may also increase the combustion of fuel.

In modern practice catalytic converter has been effectively used because it has some additional benefits e.g.

No additional fuel is necessary to initiate or sustain reaction.

Temperature inside the catalyst zone is lower, which reduces the problem of construction material. System is self initiating at exhaust gas temperature and does not need spark plug or other device. System operates even at low level of HC and CO.

CHAPTER-5CATALYTIC CONVERTER5.1 Historical Background

Air pollution problems became serious in 1940s and many nations took a serious view of it and some legislation was introduced since then in 1968 most of the automobile. Manufacturers started introducing some from of emission control devices, such as catalytic converters. There were studied extensively in 1957-64 and again appeared in 1990s when it was recognized to get large reductions in pollutants from the exhaust. Various materials such as platinum or its group metals have been used for reduction of pollutants. Besides of them oxides of base metals, such as copper, nickel, chromium and magnise were also used as oxidizing catalyst, which had shown significant reduction in pollutants.

Base metal alloys such as copper-nickel, and copper-Zink were successfully used as a catalyst. These alloys along with noble metals supported on alumina formed a dual catalyst system the reducing catalyst for reducing nitrogen oxides has been kept in first half of the converter and the second half is packed with oxidation catalyst. This would give out some oxygen for oxidation reactions in the second half, so as to improve the reduction mechanism.

Three way catalyst systems generally known as T.W.C. have been come into operation and are being widely used. A three way catalyst system is employed to promote the oxidation and reduction reactions simultaneously. The converter contains a catalytic element in a cylindrical block with a large number of channels forming a honey comb structure. The active material, platinum and pollodinum, platinum and rhodium in different combinations are dispersed over the outer surface of alumina. When the exhaust gases pass over the honey comb structure they would be in contact with the catalyst and would get oxidized and reduced simultaneously.

Gandhi Blum berg et alHansel Baruah Hammerle and many others evaluated the three way catalyst system under different operating conditions and the performance of a T.W.C. catalytic converter has been found very encouraging for reduction of pollutants, improved fuel economy and a negligible loss of engine power.

It has also been felt that duel catalyst system would be better than T.W.S. for certain operations. Whether it is a three way catalyst or a dual catalyst system the catalyst used are made of noble metals like platinum, palladium and rhodium which are very expensive and application to automobile industry in the country would mean additional cost.

Ching H.wu and Hammerle then presented a solution for the development of a low cost stable T.W.C, System. This consists of cheaper palladium in one half and platinum and rhodium in second half to give an improved performance of converter at a comparative cheaper cost. The operation of three way catalyst requires a very close control of air fuel ratio in the vicinity of the stoichiometric air- fuel ratio. This requires sophisticated electronically operated closed loop feed back control. T.W.C. system gets poisoned by lead, phosphorous and sulphur dioxide which are also present in exhaust gases. This deactivates the catalyst and requires a replacement cost. Therefore, the necessity of a low cost thermally stable catalyst preferably from indigenous sources for the reduction of the pollutants having same activation period as that of other noble metals. Base metal oxides such as copper oxide, iron oxide, chromium oxide, and magnese oxide can easily be used for reduction. Copper and Ni alloy known as Monel, is available in sheet form and could be easily which is indigenously available can also be used as a catalyst. This has got a tendency to promote oxidation and reduction of the pollutants. For an automobile industry, the primary consideration of selecting a catalyst is its cost effectiveness and availability of material. A newly (1993) searched compound Econogreen has been used as catalytic converter and it is given pollution control check certificate.

Galen et al. investigated the alternatives to the Rhodium. With World-wide growth of the automotive emissions control market, concerns about future cost and availability of catalytic metals, particularly Rhodium, have also grown. These factors have led to an increased interest in catalyst formulations which might allow reduced Rh usage or the complete removal of Rh from the catalyst without compromising the performance of the emission control system. They had tested a set of catalyst to examine Ru, Ir, and Pd as alternative to Rh, either alone or in combination with Pt. They found that addition of Pd (or Rh) always improves the activity of Pt/Pd system over Pt/Rh formulation.

Porter Doyle et al. have studied the performance characteristics of oxidation catalyst in heavy duty application and reviewed the optimization process required to match the system to the application. They reviewed the recent research by Svenska emissions Teknik A B and Recardo to investigate the potential and performance characteristics of catalysts for the application together with a critical examination of the matching process require to optimize the engine and catalyst as a system. They concluded a sound judgment to meet the 1994 standards of emissions from automobiles.

Suresh T. Gulati published a paper on the development and successful application of ceramic catalytic converters for controlling automotive exhaust emission. They designed the high surface area to meet both performances, durability requirement. They followed a step by step design process for each of the converter components. The initial design stage focuses on understanding automakers requirements and optimizing component design commensurate with them. The intermediate stage Involves laboratory testing of converter component in simulated environment and ensuring component compatibility from durability point of view. The final design stage addresses the critical tests on converter assembly to ensure performance and field durability. They also examined the necessary trade-offs and associated design modification and evaluates their impacts on warranty cost on system failure. They focused on integrated design approach for failure free operation of catalytic converter over the vehicles like them. Hurley et al. studied about electrically heated catalyst to meet stringent California and federal 1993/94 emission standards. They focused specific attention on the cold start characteristics of the vehicles emission system. Specially that of the catalyst. From test data it is evident that major portion of the total H C and Co emission occur within the first two minutes of the driving cycle. Of The use of an electrically heated catalyst (EHC) is shown to be advantageous in lowering cold start emissions during this portion of the drive cycle. They showed the effect on emissions from the stand point of EHC location, catalyst volume, and engine calibration in an overall emission improvements have been at a premium, i.e. lower fuel economy, additional vehicle weight, power consumption, and yet to be totally accessed EHC durability.

Martin j. Heimrich added some improvement in the Hurleys efforts for electrically heated catalyst. In his Study he injected air ahead of an electrically heated catalyst during cold start operation. He continuously recorded raw exhaust emissions. Analysis was used to determine air injection calibration and oxidation reduction trade offs. Improved control of non methane hydrocarbons (NMHC), benzene, and carbon mono oxide (co) emission control was maintained by the use of carefully controlled air injection flow rate and schedule. They determined that heating an automobile exhaust emission catalyst prior to cold start operation may not be sufficient in itself. Supplemented oxygen may be required for improved emission control.

Douglas J. ball and Robert Gattack gave a theory about the diesel exhaust catalysis using an oxidation flow-through type catalyst to reduce particulate emission. They discussed about converter design, catalyst support materials and the use of nodle metals for light and heavy duty applications. They performed experiments to determine the sulphur storage and release characteristics of alumina and silica catalyst support materials and the ability of platinum and palladium to oxidize so2 to sulfate particulate.

Mitsure et algave a technique to reduce Nox in diesel engine exhaust. Copier iron exchanged z sm-5 zeolite catalyst, which reduces nitrogen oxides (Nox) in the presence of oxygen and hydrocarbons, was applied to actual diesel engine exhaust. Copper ion exchanged zsm-5 zeolite effectively reduced Nox by 25% in normal engine operation and by 80% when hydrocarbons in the exhaust were increased. Water in the exhaust gas decreased the NOx reduction efficiency but oxygen and sulphur appeared to have only a small effect. Maximum NOx reduction was observed at 400c irrespective of hydrocarbon species.

Masaaki Takiguchi et al. gave a technique to reduce the NOx from diesel engines with NH3 as reducing agent NH3 is one of the most useful compounds that react with NOx selectively on a catalyst, such as v2o5- Ti o2, under oxygen containing exhaust gas. However ammonia can not be stored because of its toxicity for the small power generation in populated areas or for the diesel vehicles. This system is constructed from the hydrogen generation by fuel reformer, the NH3 synthesizer, SCR catalyst for NOx reduction and the gas injection system of reformed gas into the cylinder.

R. Beckmann et alexplained the working and use of a precious metal based, flow-through type diesel oxidation catalyst. He paid much attention to the durability of the diesel oxidation catalyst and especially to the influence of poisoning elements on the catalytic activity. Starting from 1984 an increasing number of diesel passenger cars in Europe have been equipped with this type of precious metal based catalyst. The main function of this flow-through type catalyst, containing precious metal, are the conversion of the gaseous pollutants carbon mono-oxide and hydrocarbon as well as the oxidation of hydro carbon components absorbed on the soot. Therefore it is possible to reduce the amount of particulates emitted by the diesel oxidation catalyst in such a way that it selectively catalyzes the oxidation of carbon containing components at the low exhaust gas temperature typical for diesel engine at partial load operation, and that it does not oxidize sulphur dioxide or nitrogen oxides in the range of high exhaust gas temperature occurring at full load.

Makoto Horiache studied the effects of flow-through type of oxidation catalysts on the particulates reduction of 1990s diesel engines. The reduction behavior of diesel particulate and so F by flow through type oxidation catalysts was investigated under steady and dynamic engine conditions using a current fuel (S.Content;0.38% by wt.). Each catalyst gave 40-90%, soluble organic fraction (SOF) reduction at exhaust gas temperature between 100C to 500C. SOF is absorbed on catalyst surface at lower temperature and is decomposed at the higher temperature PT only load catalyst which has high SO2 oxidation ability resulted in a low total particulate reduction due to high sulfate formation at higher temperature even when low sulphur fuel was used.

Paul Zeienka did research work on reduction of diesel exhaust emissions by using oxidation catalysts. Research describes the result work concerning the investigation and optimization of oxidation catalysts for diesel engines, especially for passenger cars and light duty trucks.

5.2 UNLEADED PETROL CAN NOT PLUG POLLUTION WITHIOUT CATALYTIC CONVERTERA Catalytic converter (CC) is placed inside the tailpipe through which deadly exhaust gases containing unburnt fuel, CO, NOx and air are emitted. The function of the CC is to convert these gases into CO2, water and N2. Unleaded petrol if used with catalytic converters could check pollution. Unleaded petrol has a low density and therefore, it evaporates fast.

Currently, it is compulsory for all automobiles plying on roads in US and Japan to have catalytic converters as they use unleaded petrol. In India, the government has made catalytic converters mandatory for registration of new cars. But what about old cars whose engine can get damaged due to prolonged use and two-wheelers and three wheelers which account for 60% of vehicular pollution?

5.3 PROBLEMS IN USE OF CATALYTIC CONVERTERNobel metals like platinum, rhodium and palladium required to make good catalytic converters is also not available in India. To increase the efficiency of fuel, benzene is added to petrol. In leaded petrol, the benzene content was 5% which have been brought down to 3% in unleaded petrol. Interestingly, the permissible euro limits are just 1% for benzene. 5.4 TYPES OF CATALYTIC CONVERTER

The catalytic converter is one of the most effective emission control devices available. Two types of catalytic converters are commonly used in automotive engines.

Two-way catalytic converters (Oxidation) - used in diesel- fueled vehicles can reduce CO emission by 80% and a large portion of HC present in particulate matter emissions.

Three-way Catalytic Converters (OxidationReduction)- installed on gasoline fueled vehicles can reduce CO and HC emissions by about 90% and NOX emission by 70% from uncontrolled levels.

Lean nitrogen-oxide Catalyst- is a new type of catalytic converter which reduces NOX emissions in lean conditions where a three-way catalyst is ineffective.

5.4.1 A TWO-WAY CATALYTIC CONVERTER HAS TWOSIMULTANEOUS TASK: 1. oxidationof carbon monoxide to carbon dioxide: 2CO + O2 2CO2

2. Oxidation of unburnt hydrocarbons (unburnt and partially-burnt fuel) to carbon dioxide and water: 2CxHy + (2x+y/2)O2 2xCO2 + yH2O

This type of catalytic converter is widely used on diesel engines to reduce hydrocarbon and carbon monoxide emissions. They were also used on spark ignition (gasoline) engines in USA market automobiles up until 1981, when they were replaced by three-way converters due to regulatory changes requiring reductions on NOx emissions.

Reduction of the NOx emissions requires an additional step. Platinum catalysis can be used. Instead of catalysis, a true reactant diesel fuel or ammonia pyrolyzed in situ from urea can be used to reduce the NOx into nitrogen.

Curiously, the regulations regarding hydrocarbons vary according to the engine regulated, as well as the jurisdiction. In some cases, "non-methane hydrocarbons" are regulated, while in other cases, "total hydrocarbons" are regulated. Technology for one application (to meet a non-methane hydrocarbon standard) may not be suitable for use in an application that has to meet a total hydrocarbon standard. Methane is not toxic, but is more difficult to break down in a catalytic converter, so in effect a "non-methane hydrocarbon" standard can be considered to be looser. Since methane is a greenhouse gas, interest is rising in how to eliminate emissions of it.

5.4.2THREE-WAY CATALYTIC CONVERTERS (OXIDATION- REDUCTION)

Ensuring good quality air is essential for the protection of public health. Governments worldwide have adopted a range of increasingly demanding measures to curb air pollution with a particular focus on the emissions from motor vehicles. An important part of this strategy has been the development of the three-way catalytic converter to remove exhaust pollutants such as carbon monoxide, unburnt hydrocarbons and nitrogen oxides. This unit takes an in-depth look at the construction of this converter for petrol-driven vehicles and investigates the catalytic chemistry taking place at the molecular level. It is assumed that you already have a scientific background.

The three-way catalytic converter

5.4.2.1 COMPOSITIONThe current three-way catalyst, shown schematically in Figure 1, is generally a multicomponent material, containing the precious metals rhodium, platinum and (to a lesser extent) palladium, ceria (CeO2), -alumina (Al2O3), and other metal oxides. It typically consists of a ceramic monolith of cordierite (2Mg.2Al2O3. 5SiO2) with strong porous walls enclosing an array of parallel channels. A typical monolith has 64 channel openings per cm2 (400 per in2), This design allows a high rate of flow of exhaust gases Cordierite is used because it can withstand the high temperatures in the exhaust, and the high rate of thermal expansion encountered when the engine first starts typically, the exhaust gas temperature can reach several hundred degrees in less than a minute. Metallic monoliths are also used, particularly for small converters, but these are more expensive.

Figure 1. Schematic diagram of the three-way catalytic converter. To achieve a large surface area for catalysis, the internal surfaces of the monolith are covered with a thin coating (3050 m) of a highly porous material, known as the washcoat (Figure 2). The total surface area is now equivalent to that of about two or three football pitches. The washcoat generally consists of alumina (7085%) with a large surface area, with oxides, such as BaO, added as structural promoters (stabilizers to maintain surface area) and others, for example CeO2, as chemical promoters. This system becomes the support for the precious metal components (Pt, Pd and Rh). These metals constitute only a small fraction (12%) of the total mass of the washcoat, but they are present in a highly dispersed form. They are generally applied by deposition from solution, although they may instead be introduced during formation of the washcoat itself. Exact catalyst formulations are, as one might expect, closely guarded secrets. Some compositions use all three metals; others use Rh together with only one of the other two, typically Pt, as in the present generation of Pt-Rh converters used in the UK, in which Pt constitutes 8090% of the total precious metal mass

Figure 2. Electron micrograph of a cross section of a ceramic monolith coated with an alumina washcoat 5.4.2.2 CATALYST PERFORMANCEFigure 3 shows the difference in the emission levels for CO, VOC and NOx for a vehicle, with and without a three-way catalytic converter. It is evident that the catalytic converter reduces the emissions of all three classes of pollutants quite dramatically over a wide range of speeds. Before we discuss the data in any detail, a few words about how they were obtained are in order.

Figure 3 Emission levels for CO, VOC and NOx for petrol-engine vehicles as a function of speed, with (green) and without (black) a three-way catalytic converter.

Federal and European Test Procedures are used to test emissions from a complete finished converter and engine together, to ensure that a new car model, for instance, will meet the current emissions legislation. Some sort of smaller-scale testing is obviously required in the laboratory. In the research and development of automotive catalysts, activity testing fulfils the function of screening and comparing novel and modified catalysts, and examining their performance under different conditions. The process of screening must provide a reliable means of identifying materials that will perform as active, selective and durable catalysts under automotive conditions. The approach usually taken is to measure conversion of the pollutants as a function of temperature, using a simulated exhaust-gas mixture flowing through a bed of powdered catalyst: the flow-rate has to be high enough to mimic the through-put or space velocity of a catalytic converter (typically a contact time for the gases with the catalyst of 72 milliseconds is used). The test is then repeated using a different simulated exhaust-gas to represent a different engine mode. Ageing studies are performed by exposing the catalyst to different and often extreme conditions, for varying lengths of time.

Figure 4 Activity of a three-way catalyst for the simultaneous conversion of CO (black), NOx (solid green) and the hydrocarbon propene (C3H6) (dotted green)Figure 4 shows a typical graph of catalytic performance over the normal range of operating temperature, 100600 C. Until the incoming gases have heated the catalyst to around 250300C, the activity of the catalyst is low. This temperature, at which the efficiency of the catalyst rapidly increases, is known as the light-off temperature. Until this temperature is reached, the catalyst is not working at full efficiency, and so CO, NOx and hydrocarbons will all be emitted from the exhaust pipe in significant amounts. This problem is known as cold start. Ideally the light-off temperature should be as low as possible. 5.4.2.3 Exhaust Emission Characteristics

Before we consider how the three-way catalyst functions in any detail, it is important to understand how the emissions of CO, HC and NOx, from the engine depend on the ratio of air (A) to fuel (F) the air/fuel ratio (or A/F ratio). The significance of this will become clear when we see that the ratio at which the three-way catalytic converter operates is crucial for its success.

Taking octane (C8H18) to be the only constituent of fuel, and assuming that air is 20% O2 by volume, estimate the stoichiometric A/F ratio (mass ratio) required for total combustion to occur. At this stage neglect the effect of NO as an oxidant. Comment on the difference between the value you obtain and the experimental value of 14.7:1 (Use the following relative atomic masses: C, 12.01; H, 1.01; O, 16.00; N, 14.01.)

The stoichiometric equation for the complete combustion of octane can be written as follows:

C8H18 + 121/2O2 = 8CO2 + 9H2O

So combustion of 1 mol of octane will require 12.5 mol of oxygen.

Assuming air to be approximately 20% O2 and 80% N2 (by volume), the mass of air required will be 12.5 (32.00 + 4 28.02) g = 1801 g.

The mass of 1 mol of octane is (8 12.01 + 18 1.01) g = 114.26 g.

Thus, the A/F mass ratio for complete combustion is:

A/F = 1801/114.26

= 15.8:1

Figure 5 The effect of changing air/fuel ratio on the levels of NOx (solid green), CO (black) and HC (dotted green) produced in the engine. The diagram also shows qualitatively how the engine power output changes with the A/F ratio. This is as close as you would expect to the experimental value of 14.7:1, because we have used a very simplified system. We had not included NO as an oxidant or the other hydrocarbons, CO or H2 as reductants, and we have used octane, not the real mix of hydrocarbons in petrol. A general relationship between levels of CO, HC and NOx released from the engine and the A/F ratio is shown in Figure 5. At A/F ratios somewhat above stoichiometric (14.7:1) that is, when the engine is operating under fuel-lean, net oxidizing conditions low levels of HC and CO are produced in the engine, and there is a peak in NOx concentration. At higher A/F values, NOx falls, but the hydrocarbon concentration increases as the engine begins to misfire.

Figure 6 Activity of a three-way catalyst for the simultaneous conversion of NOx (solid green), CO (black) and HC (dotted green) as a function of the air/fuel ratio. The shaded area defines the window for conversions of 80% and above for all three pollutants. (Note that, for clarity, the A/F ratios are expressed as the amount of air per unit of fuel, e.g. 14.7 instead of 14.7: 1. We shall use this notation for the rest of this unit.)

When the exhaust gas is close to its stoichiometrically balanced composition, at an A/F ratio of about 14.7:1, the concentrations of oxidizing gases (NO and O2) and reducing gases (HC and CO) are matched; in theory, it should then be possible to achieve complete conversion to produce only CO2, H2O and N2. This is, of course, the objective of the three-way catalytic converter, and so, ideally, it should be operated in a narrow band, or window, close to the stoichiometric ratio, within which it will promote simultaneously the nearly complete reduction of NOx to N2 and the nearly complete oxidation of CO and HC to CO2 and H2O. Figure 6 shows the catalyst conversion efficiency for all three classes of pollutants as a function of A/F ratio, with the dotted lines defining the window for conversions of 80% and above. Using the information given in Figures 5 and 6, explain the changes in conversion efficiency seen for all three pollutants when the A/F value is (a) greater than the window for optimum conversion, and (b) less than the window for optimum conversion.

(a) Figure 5 shows that over the narrow range of A/F ratios covered in Figure 6 the amounts of CO and HC emitted form the engine decrease as A/F increases. As there is a simultaneous increase in the total amount of oxidants (air + NOx), the overall conversion of CO and HC increases to approach effective completion at the stoichiometric ratio, and then remains constant in the net oxidizing conditions beyond that point. The sharp fall in NOx conversion for A/F values approaching and above stoichiometric is understandable in terms of the virtual elimination of reductions in this region. Because the system is unable to remove all of the NOx, we would expect to see an increase in NOx emissions from the exhaust. The three-way catalytic converter is therefore unsuitable for engines that run lean.

(b) At A/F ratios below the window value there is less NOx and more HC and CO present in the mixture expelled from the engine (Figure 5). All the NOx present will react over the catalyst, so the NOx conversion will still is high (as seen in Figure 6). However, we see a decrease in catalyst efficiency for destroying HC and CO, as there are insufficient oxidants present for complete conversion. We would therefore expect to see an increase in the HC and CO levels emitted from the exhaust.

Obviously in both cases, in the absence of the three-way converter the levels of CO, HC and NOx emitted for the exhaust would be much higher.Engine control systems have been developed to include an oxygen sensor (or lambda, , sensor as it is sometimes called), and an electronic module to regulate the A/F ratio, so that the exhaust composition is kept within the window for optimum conversion. However, because there are time delays in the A/F correction, the ratio cycles very rapidly between slightly fuel-rich and slightly fuel-lean, oscillating about the stoichiometrically balanced composition (14.7 0.3) at a typical frequency of 1 cycle per second. Minimizing the amplitude of the oscillation increases the effectiveness of the converter.

5.4.2.4 The Chemical ReactionsIntroduction

Since its development, the three-way catalyst has been exposed to the full spectrum of techniques available for the characterization of catalytic materials. The data provided can be correlated with the results of activity tests and kinetic measurements, which provide information on the performance of the catalyst. This reveals that although the catalyst functions as a composite material, it can be divided into distinct groups of catalytic centers that provide several different types of site, active for one or more of the many different reactions. The participation of a particular type of site at any given moment will depend on the conditions experienced by the catalyst; for example, whether the gases are a net reducing, stoichiometric, or oxidizing mixture.

Measurements of intrinsic kinetics are usually carried out on simple gas mixtures to allow activation energies and reaction orders to be calculated for specific reactions. The data can often contribute to an understanding of the mechanisms by which the surface reactions occur. They are also used to create reaction models that will predict the performance of the catalyst under various anticipated conditions.

The overall reaction scheme is complicated, with many contributing processes. The strategy of the three-way catalyst is to simultaneously remove CO, HC and NOx, and our treatment will accordingly be divided into three subsections. The desired reactions can be expressed in simple terms as follows: Removal of COCO oxidation:

Water-gas shift (WGS) reaction:

Removal of hydrocarbonsHydrocarbon oxidation:

For example:

Steam reforming:

Removal of NO (plus CO or HC (not shown))CO + NO redox reaction:

Or with hydrogen:

Any number of these reactions may be occurring simultaneously as the A/F ratio goes through its cycle about the stoichiometric composition. The following subsections will look more closely at the removal of each of the pollutants under various conditions, and will also examine the role of the catalyst components.

The supported commercial catalyst is the one most difficult to study because of its complexity, with a large number of different components Pt, Rh, Al2O3, CeO2, BaO, etc. present in one catalyst. It is therefore often simpler to study model systems, such as Pt/Al2O3 or Rh/CeO2, and if certain surface-science techniques are to be used, the catalyst under study has to be even simpler a particular face of a metal single crystal. These studies, often performed under ultrahigh vacuum (UHV), are far removed from the real catalyst system and the conditions it experiences. Hence, it cannot be assumed automatically that the results will be directly relevant to what is actually happening in a converter fitted to an operational vehicle.

Removal of CO

Under fuel-lean conditions (excess O2), the oxidation of CO has been studied over a very large range of single crystals and model noble metal catalysts, one of the most intensively investigated examples being the Pd(111) surface. Although this metal is not a component of the current three-way catalyst used in the UK, it is worth considering the results in some detail for a number of reasons. The reaction on metals such as Pt is in many ways similar to that on Pd and, in any case, palladium is already being incorporated into future generations of catalytic converter, particularly for the US market. Most notable, however, is the fact that this is one of the few cases in which surface-science techniques have successfully revealed the details of a real-world catalytic mechanism. Specifically, we will see how surface studies of the adsorption of CO and oxygen on Pd (111) both individually and together have led to the current understanding of the mechanism of CO oxidation.

LEED results for the adsorption of CO on Pd (111), obtained at room temperature and below, have been interpreted in terms of the structural models shown in Figure 7. One of the significant observations from this work is the readiness with which one arrangement of CO on the surface evolves into another. Thus at a surface fractional coverage of =1/3 (Figure 7a), the CO occupies a hollow site where it can bind to three Pd atoms. As is increased to 1/2 (Figure 7b), CO moves out of the hollow to a bridging site, where it binds to two Pd atoms. Finally, at =2/3 (Figure 7c), a hexagonal structure forms, in which half of the CO molecules reoccupy hollow sites, while the remainder bind to single Pd atoms at terminal sites. The readiness with which the CO molecules can reposition themselves suggests that the activation energy for surface migration in the chemisorbed state is low, and that CO is a highly mobile species under catalytic conditions.

Oxygen adsorbs dissociatively on Pd (111), and the O atoms are found to be less mobile on the surface than CO molecules. The structure of the chemisorbed layer at maximum coverage is shown in Figure 8.

Figure 7 Structural models for the adsorption of CO on Pd(111) at a surface fractional coverage of (a) =1/3; (b) =1/2; and (c) =2/3.

Identify the adsorbate structure shown in Figure 8 in terms of the (mn) notation, and determine the fractional surface coverage, , of oxygen atoms.

Figure 8 The surface structure of O atoms adsorbed on Pd(111) at maximum surface coverage

The unit meshes of the substrate (1 1) structure and of the adsorbate structure are shown in Figure 8A. Evidently, the latter is (2 2) in the (m n) notation, so the full description of this structure should be Pd (111) (2 2)O.

For adsorption on a single crystal surface, the fractional surface coverage is given by

= x/ (m n)

Where x is the number of adsorbate species within the (m n) adsorbate unit mesh. In this case (Figure 8A), O atoms occur only at the corners of the mesh, so the latter contains a total of (4 1/4) = 1 atom. Hence

= 1/ (2 2) = 1/4

Figure 8A the surface structure of O atoms adsorbed on Pd (111) at maximum surface coverage, showing the substrate unit mesh and the adsorbate unit meshes.

We might now assume that when CO and O2 are adsorbed together during the oxidation reaction, the properties of the system will be a simple combination of those of the two molecules adsorbed separately. The surface layer would then consist of mobile CO (maximum coverage, =2/3) within a fixed lattice of O atoms (maximum coverage, =1/4). The fact that this is not the case, as we shall see below, demonstrates an important point. Because of mutual interactions, the behavior of two (or more) co-adsorbed species very often differs from their behavior when adsorbed separately.

In the case of CO and O2, the order in which adsorption is carried out is significant. If CO is adsorbed first, to a coverage greater than one-third of a monolayer (=1/3), subsequent oxygen adsorption is completely blocked. With lower coverages of CO, dissociative oxygen adsorption does occur; but the two species form separate domains on the surface (Figure 9a). Oxidation will then take place only at the boundaries between domains, and so it will be relatively slow.

Figure 9 Schematic representation of domains of CO (ad) and O (ad) on Pd (111). (a) Separate domains (CO adsorbed first); (b) mixed domains (O2 adsorbed first).

When oxygen is adsorbed first to its maximum coverage, =1/4, subsequent CO adsorption occurs readily and compresses the O atoms into domains in which the local coverage reaches =1/3. At first, the adsorbed CO is found in separate areas (as when CO is adsorbed first), but as more is added mixed domains form, containing both CO and O, each at a local coverage of =1/2 (Figure 9b).

These mixed domains bring CO (ad) and O (ad) into intimate contact, with the O atoms at twice the surface concentration possible in the absence of CO. Thus, the stoichiometry is now that required for the oxidation reaction. Moreover, an electronegative O atom will withdraw charge from the surface. In turn, the surface will withdraw charge from neighboring PdCO bonds, weakening them and so making the CO more readily available for reaction. The net result is that the mixed domain is highly reactive and generates CO2 at temperatures far below room temperature.

Having thus established that, on Pd (111), rapid CO oxidation can occur by way of a LangmuirHinshelwood type process (a surface reaction between two adsorbed species), we are almost in a position to propose a detailed mechanism. First, however, we must consider the possibility of an alternative EleyRideal type mechanism, in which the rate-limiting step involves reaction between an adsorbed species and a molecule in the gas phase. In this case, there are two such possibilities:

We can make a decision about reaction 9 on the basis of the information provided in Figure 10. APd(111) surface presaturated with a quarter of a monolayer of O atoms was exposed to a beam of gaseous CO, and the surface coverages and the oxidation rate were monitored with time. Figure 10 shows that the rate became significant only after a population of CO had built up on the surface, and it reached a maximum when the coverages of O and CO were approximately equal. This is clear evidence for a LangmuirHinshelwood process. If reaction 9 had been operative, the rate would have been high initially, and would have fallen continuously as the oxygen layer was consumed by the reaction.

Figure 10 Changes in the rate of CO2 formation from CO and in the surface density of oxygen and CO on Pd(111). The surface was precovered with a quarter of a monolayer of oxygen atoms at time zero, and then exposed to a constant stream of CO at a pressure of 7.91011 atm.

Given the research effort that was involved, the mechanism finally proposed for CO oxidation on Pd(111) is deceptively simple:

Although the exact nature of the surface intermediates is still not known, the depth of understanding of the catalytic mechanism is quite an accomplishment. But just how relevant are these surface studies to the practical catalysis taking place in the converter?

Indeed, Figure 12 shows that in the case of rhodium there is excellent agreement between the rates of CO oxidation over a Rh(111) single crystal surface and over a Rh/Al2O3 catalyst.

Figure 12 Comparison of the rates for CO oxidation measured over Rh(111) (black) and over 0.01 mass % Rh/Al2O3 (green) at p(CO)=p(O2)=0.01 atm, as a function of temperature.

Although the sequence of elementary steps is quite simple, the overall kinetics of the CO oxidation reaction is not. The non-uniformity of the surface, and the segregation of the reactants in surface domains, complicates the detailed modelling of the kinetics. The exception is the special case of low surface coverages of CO and O atoms, when they are found to be randomly distributed over the surface and so satisfy one of the criteria for applicability of the Langmuir isotherm. Under these circumstances, LangmuirHinshelwood kinetics can be applied. Figure 13 shows the comparative performance of single-metal catalysts for the oxidation of CO at a fixed temperature. Evidently, all three of the platinum groups metals present in automotive catalysts are active for CO oxidation. In addition, results have shown that Rh may improve low-temperature activity. In the current (1996) three-way catalyst used in the UK, in which Pt constitutes 8090% of the noble metal composition and Rh the remainder, it is the Pt that is mainly responsible for CO oxidation. Under stoichiometric or slightly fuel-rich (reducing) conditions, where there is insufficient oxygen present to oxidise all of the CO, conversion can also occur by one of the following routes:

Figure 13 Comparison of catalytic activity for CO oxidation at 400 C for Pt, Pd and Rh at different A/F ratios

Via the CO + NO redox reaction (reaction 6). This will be discussed in detail in section 4.4,

via the water-gas shift reaction (equation 2), because H2O is present in the exhaust gases as a product of combustion:

The water-gas shift reaction is catalyzed by Pt and/or Rh, with ceria acting as an excellent promoter. Pt/CeO2Al2O3 and Pt-Rh/CeO2Al2O3 are particularly active combinations for the removal of CO under slightly fuel-rich conditions. The hydrogen produced in this reaction will react, in preference to CO, with any oxygen present. Hence, although the water-gas shift reaction removes CO, it also inhibits CO oxidation by producing hydrogen, which will remove any O2 present:

Removal of hydrocarbons

Figure 14 shows a comparative study for hydrocarbon oxidation over single-metal catalysts: it can be seen that Rh, Pd and Pt all give high conversions for A/F ratios at and above stoichiometric. Again (as in the case of CO), in the current (1996) UK three-way catalytic converter, Pt is the main component responsible for oxidation of the hydrocarbons. On noble metal surfaces, alkane adsorption is dissociative, whereas unsaturated and aromatic hydrocarbons adsorb either dissociatively or associatively as -complexes. The subsequent oxidation process is thought to be considerably more complicated than the oxidation of CO, and we shall not consider it in any detail. When the engine exhaust gas composition is reducing (fuel-rich), hydrocarbons compete effectively with CO for oxygen, and they can also react with water vapour to produce CO and H2 a reaction known as steam reforming:

Figure 14 Comparison of catalytic activity for HC oxidation at 400 C for Pt, Pd and Rh at different A/F ratios

This is catalyzed by Rh and/or Pt with ceria and, as in the case of the water-gas shift reaction; the combination PtRh/CeO2Al2O3 is particularly active. As we noted earlier, the H2 produced may react preferentially with any O2 present, thus reducing the amount of oxygen available to react with hydrocarbons and CO. In addition, the CO produced adds to the burden of carbon monoxide to be removed.

Removal of NOLaboratory experiments have shown that, under the conditions in the catalytic converter, the decomposition of NO to O2 and N2 over noble metal catalysts is too slow to be significant. When the A/F ratio is stoichiometric (or below stoichiometric), NO can be removed by reduction with CO and/or hydrocarbons. For simplicity we shall consider only reduction with CO, as with the oxidation reaction, the situation with hydrocarbons is considerably more complicated.

In principle, a variety of products can be formed, specifically:

In addition, H2 produced from the water-gas shift or steam reforming reactions can reduce NO to N2, N2O or NH3:

The NOx activities of Rh, Pt and Pd are shown in Figure 15. It is evident that Rh has the highest activity, particularly under net reducing conditions (low A/F). So why is Rh superior? To answer this question, we need to consider the mechanism of the reaction.

Figure 15 Comparison of catalytic activity for NOx reduction at 400 C over Rh, Pt and Pd at different A/F ratios.

The catalytic reduction of NO by CO and/or H2 over a variety of surfaces has been the subject of a great deal of research. Application of various surface-science techniques has provided some understanding of the elementary steps involved, but the exact mechanism is still controversial. One view is that the first step is the dissociative chemisorption of NO. (You should note that although we describe this as dissociative chemisorption strictly it does not meet the definition, as NO is in fact first adsorbed associatively and then dissociates on the surface.) The O atoms produced are then removed by the reducing agents CO or H2. The N atoms can combine to give N2, react with chemisorbed NO to give N2O (particularly important at low temperatures), or react with chemisorbed H atoms to form NH3. These and other processes that may be involved are listed below.

Adsorption

DissociationNO (ad) N(ad) + O(ad)

In the following steps we have assumed, for simplicity, that all products are desorbed as quickly as they are produced. You should recognize, however, that adsorbed species, no matter how transient, will be formed initially.

Surface reactions and desorption

Reactions with hydrogen

Figure 16 compares the rate of the NOCO reaction over an Rh (111) single crystal with that over a Rh/Al2O3-supported catalyst.

Figure 16 Comparison of the rates for the NOCO reaction measured over Rh(111) (black) and over 0.01 mass % Rh/Al2O3 (green) at p(CO)=p(NO)=0.01 atm, as a function of temperature

This seems to be the case, particularly at higher temperatures. At lower temperatures, the concentration of NO (ad) increases and reactions 22 and 23 would then be expected to contribute to N atom removal.

The elementary steps 2123 all require surface mobility of N atoms (to encounter either NO (ad) or other adsorbed N atoms). Although this process may occur on surfaces that are extensive on the atomic scale, such as those of single crystals or large supported crystallites, it has been argued that such mobility will be insignificant on or between the small highly dispersed particles of Rh in the automotive catalyst. Therefore, we might expect the rate-limiting steps and the observed kinetics in the cases of Rh(111) and supported Rh to be different. The Arrhenius-type plots in Figure 16 confirm that this is so: over Rh/Al2O3, the reaction has higher activation energy (the plot in Figure 16 has a larger gradient) and a lower rate (at a given temperature) than over Rh (111).

What then is the rate-limiting step with the supported catalyst? One suggestion is NO dissociation (reaction 19) but there is a more radical alternative, involving a different overall mechanism. Infrared spectra for NO adsorbed on Rh/Al2O3 (Figure 17) show bands at 1 743 cm1 and 1 825 cm1, which have been taken as evidence of a dinitrosyl species, O=NN=O, formed by reaction 26: This step provides a means, other than diffusion of N (ad), of accomplishing the most important task in the reduction of NO, namely the pairing of two nitrogen atoms on the surface. Once formed, the dinitrosyl species is thought to lose its two oxygen atoms by way of an N2O intermediate; for example:

Figure 17 Infrared spectra, recorded at 300 K, for NO adsorbed on Rh/Al2O3 as a function of NO coverage, increasing from spectrum A to spectrum D. The bands at 1 743 cm1 and 1 825 cm1 have been assigned to the dinitrosyl species O=NN=O.

To summaries Whichever mechanism is correct NO pairing to form a dinitrosyl species, or NO dissociation followed by N-atom combination and N2 desorption both require catalytic sites that can not only bind NO but also donate charge to the adsorbate. In the first case, this charge would be used to coordinate the two NO molecules. In the second case, it would be transferred into the partially vacant 2* antibonding orbital of NO (Figure 18), weakening the NO bond and hence facilitating dissociation.

On examining the electronic structures of the noble metals, that of rhodium is found to be particularly suitable for facilitating charge transfer to adsorbed NO, with the uppermost occupied electron levels of the metal at higher energy than the partially