An Introduction to Exhaust Catalysis

Summer School in Energy and Environmental Catalysis

University of Limerick, July 2005

CxHy + xsO2 x CO2 + y/2 H2O

H= -ve

Problems Associated with Fossil Fuel combustion as a power source

(1)Finite supply of fossil fuels

(2)Release of CO2 - a greenhouse gas

(3)Release of Pollutants during combustion

A Pollutant can be thought of as any chemical that has detrimental consequences when added to the environment.

All pollutants MUST arise from the mixture of AIR and FUEL within the combustion chamber.

Combustion reaction

CxHy + O2 CO2 + H2O

Combustion of a branched alkane to CO2 and H2O takes place through a series of steps.

From the Hydrocarbon viewpoint each step involves a H abstraction or a C-C bond breaking

The final step is the oxidation of CO to CO2

Flame combustion is very rapid – but not rapid enough to combust all CxHy to CO2 + H2O

Thus pollutants include - Volatile Organic Compounds (VOC)Fragments of fuel, aromatics, carcinogens, smog formation

CO (from incomplete combustion)Colourless, odourless, toxic at >600 ppm, binds to haemoglobin. – found in all combustion exhausts

SO2 (from S in fuel) forms Acid rain,

sulphate is a nucleation site for particulates,

Carbonaceous Particulatessizes range from <0.1 m up to visible ~10 mm.Smaller particles damage lungs / carcinogens (surface pyrenes) larger particles contribute to global warming (glacial albedo)

Also

Formed from the high temperature reaction between N2 and O2 (radical mechanism)

O22O., O

.+N2NO + N

., N

..+O2NO + O

.

N2 + O2 2 NO Ln K = 15.11 at 23 °C = -2.48 at 1227 °C

Therefore you need high temperatures – as are found in a flame combustion.

NOx (NO + NO2)

from combustion of N-containing fuel

responsible for smog, acid rain, lung ailments

Recently found to be vital in the body (in very low concentrations) regulating all biological functions, breathing, muscle action, blood pressure etc. Also used by the body to attack infection

NOx (NO and NO2) is formed when fuel is burned at high temperatures and 50% of the

NOx produced in the world is from motor vehicles1.

It causes a wide variety of negative health and environmental impacts2, including:

Smog

Acid

Rain

Respiratory

ailments

Removed from gasoline engine exhausts using the TWC3

Mobile diesel engines operate at A/F ~ 21, more fuel economy lower CO2

(greenhouse gas) emissions.

BUT these have no NOx removal systems.

SMOG caused the first legislation

Removing any apex of this triangle prevents SMOG

Step 1 HYDROCARBONS

(A) Engineering modifications –

• increase level of air in the air/fuel mix

• re-circulate the exhaust gas through the engine

(B) Add a catalyst

NOxHydrocarbons

sunlight

SMOG

Monolith with channels placed in the car’s exhaust pipe close to the engine (in order to get to a working temperature rapidly)

The corderite - monolith (Ca/Mg/Al2O3).

High surface area Al2O3 wash-coat applied and doped with active metals (Pt).

Catalyst efficiently removes any unburned hydrocarbons from the exhaust

NB The car was operating under LEAN conditions – an excess of O2 in the combustion chamber and thus O2 remains in the exhaust combustion of the hydrocarbons (and, as a beneficial side reaction, CO) on the catalyst is made easier.

However, this catalyst rapidly became poisoned by lead which was added to gasoline to aid “knock-free” combustion. This Lead was removed from gasoline and replaced with oxygenated organic anti-knock agents - MTBE (persistent in ground water and later replaced by ETBE / Ethanol).

Fuel <1

Air >1

Engine monolith catalyst

re-circulate

HC, CO, CO2, NO, H2O, O2

CO2, H2O, NO, O2

1st generation catalytic converters removed CO and unburned HC from tailpipe emissions

[NOx] emitted was also decreased as Tcomb decreases when the engine runs fuel lean

However, as emission legislation grew stricter a method for removing the NOx pollutant was also

needed

DECOMPOSITION of NO (2NON2+O2) is thermodynamically favoured. But is kinetically very slow USE A CATALYST

Pt

NO

NO Oads + Nads

+ Nads N2 (g)

NO

O O O O O O O O O

O O O O O 800 °C + O2 (g)

OR

O O O O O CO/H2/HC

300°C

+ H2O CO2 (g)

NO

NO Oads + Nads

+ Nads N2 (g)

NO

O O O O O O O O O

O O O O O 800 °C + O2 (g)

OR

O O O O O CO/H2/HC

300°C

+ H2O CO2 (g)

NO adsorbs and dissociates on “clean” metal surfaces

NO (g) NO ads Nads + O ads

Nads species combine on the surface forming N2 ads which desorb into the gas phase as N2.

The Oads species on the surface are VERY stable.At moderate temperatures over time the “clean” metal surface becomes an oxidised metal surface - being saturated with Oads species.

This surface no longer dissociates NOads - the catalyst is poisoned.

NO

NO Oads + Nads

+ Nads N2 (g)

NO

O O O O O O O O O

O O O O O 800 °C + O2 (g)

OR

O O O O O CO/H2/HC

300°C

+ H2O CO2 (g)

NO

NO Oads + Nads

+ Nads N2 (g)

NO

O O O O O O O O O

O O O O O 800 °C + O2 (g)

OR

O O O O O CO/H2/HC

300°C

+ H2O CO2 (g)

This effect is worse in the presence of O2(g)

Very high temperatures are needed to get these to combine, form O2 ads and eventually O2 (g)

These temperatures damage a catalyst (sintering / evaporation etc).

NO

NO Oads + Nads

+ Nads N2 (g)

NO

O O O O O O O O O

O O O O O 800 °C + O2 (g)

OR

O O O O O CO/H2/HC

300°C

+ H2O CO2 (g)

NO

NO Oads + Nads

+ Nads N2 (g)

NO

O O O O O O O O O

O O O O O 800 °C + O2 (g)

OR

O O O O O CO/H2/HC

300°C

+ H2O CO2 (g)

NO

NO Oads + Nads

+ Nads N2 (g)

NO

O O O O O O O O O

O O O O O 800 °C + O2 (g)

OR

O O O O O CO/H2/HC

300°C

+ H2O CO2 (g)

NO

NO Oads + Nads

+ Nads N2 (g)

NO

O O O O O O O O O

O O O O O 800 °C + O2 (g)

OR

O O O O O CO/H2/HC

300°C

+ H2O CO2 (g)

If a reducing agent is added (CO, H2, Hydrocarbons) to remove the adsorbed O species we can regenerate the surface to one which will dissociate NO.

Now catalysing the NO + Reducing Agent reaction rather than the NO dissociation reaction

In the 2nd generation catalytic converter the engine is run fuel rich (an excess of fuel over air) in order for there to be enough CO and HC in the exhaust to keep the NOx reduction catalyst free of Oads species

Fuel >1

Air <1

EngineRhodium catalyst

HC, CO, CO2, NO, H2O

HC, CO, CO2, H2O, N2

Rh is the most active metal for promoting the NO decomposition reaction in the presence of reducing

agents (CO / H2 / HC)

However now there is no longer any O2 remaining to react with the CO and the HC.

EXCESS Air is drawn into the system and a second catalyst bed is added – an oxidation catalyst. HC and

CO oxidise – forming H2O and CO2.

HC, CO, CO2, H2O, N2

Platinum catalyst

Air / O2 Intake

O2 + CO2 + H2O + N2

Problems with the 2nd generation “Dual-Bed” catalytic converter.

• inefficient wrt fuel consumption

• greater costs (PGM) and bulkiness / weight.

• Over-Reduction of NOx to NH3 in the presence of excess HC. This NH3 subsequently reacts to form NOx on the Oxidation Catalyst or in the atmosphere.

Temperature decreases as increases [NO] decreases

Low – not enough O2 to combust fuel low O2 in exhaust.

Higher excess [O2] and large [O2] in exhaust

Inverse reasoning for [CO]

HC increases with because T decreases

FUELRICH

FUEL LEAN

stoichiometric

At low N2] and [O2] are low and effectiveness of the Zeldovich mechanism decreases.

At the stoichiometric point the concentration of species which have to be oxidised in the exhaust (CO and HC) equals the concentration of species that must

be (or can be) reduced (NO and O2).

Therefore at this point it is possible to remove ALL the pollutants using one catalyst bed.

This is the concept of the 3-way catalytic converter. It relies on a VERY accurate control of the air/fuel ratio

in the combustion process

The generation of these better engine management systems lead to the development of the single bed

Three -Way catalytic converter.

O2

The air/fuel ratio in the engine is controlled by the engine management system. This receives signals from a lambda () sensor positioned before the

catalyst bed.

The sensor is based on a Yttrium Stabilized Zirconium (YSZ) material (electrolyte in SOFC) that generates a voltage [O2] – through transport of O2-

This signal is fed back to the engine management system which adjusts the air/fuel ratio in whatever

direction is needed if it is outside the limits required

=1Fuel rich

Fuel lean

Given the correct value in the engine the concentrations of oxidants to reductants in the exhaust should be balanced. These can then react totally over

the twc

If the engine air/fuel ratio strays from a value of 1 then conversion of some of the pollutants drops.

> 1 (too oxidising) CO and HC effectively combusted over the catalyst but NOx conversion falls.

< 1 (too reducing) NO conversion remains high but CO and HC are now converted

Fuel ~1

Air ~1

Engine TWC catalystHC, CO, CO2, NO, H2O, O2

CO2 H2ON2

Sensor

The three way catalyst is a mixture of catalysts supported on the same monolith

Cordierite monolith, High surface area Al2O3,

Pt (for oxidation reactions), Rh (for NOx reduction reactions),

Pd (for both oxid. and red. reactions).

AND,

CeO2 as an additive.

Functions of CeO2

•Acts as a temporary buffer to changes in the lambda value through CeO2 Ce2O3 + O2 when the mixture is fuel rich and the reverse reaction when the mixture is fuel lean.

so 2CO+CeO22CO2+ Ce2O3 when [O2] is low

and 2Ce2O3 + O2 2CeO2 when [O2] is high

•Catalyses the Water Gas Shift reaction

H2O + CO H2 + CO2 (removing CO and forming a “better” reductant H2)

•2CO+CeO22CO2+ Ce2O3

•Ce2O3 + 2H2O CeO2 + 2H2

•Overall 2CO + 2H2O 2CO2 + 2H2

•Stabilises both the metals and the Al2O3 support against sintering

Problem with CeO2

•Can store SO2(g) as SO42- and reduce it under rich

conditions to H2S (rotten egg smell)

•Possible Solutions

•Use Ni materials to store S during rich phases.

•But not permitted in Europe (fear of Ni(CO)4

•Sinter CeO2 before adding to catalyst

•Decreases catalytic surface available for WGS reaction but does not affect Oxygen storage capacity

Generally the rate of a reaction doubles when the temperature is raised by 10 °C (from k =

Aexp(-Ea/RT)) This is not the case for the oxidation of HC and CO over a catalytic converter. Heat from

Exothermic reaction fed back to catalyst.

Therefore there is a period of time after the car has been switched on that the catalyst is not working. During this time all the pollutants generated in the

engine are emitted to the atmosphere

0

20

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0 200 400

Temperature / °C

% C

onve

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light off

Em

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Cum

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HC

Em

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Tackled using either electrical heating of catalyst before start or initial very exothermic combustion reaction (using CO from engine) to heat catalyst.

Problems with Three Way Catalysts.

•Susceptible to attrition and wear damage

•Poisoned by lead

•Can reduce any SO2 to H2S (on CeO2)

•Looses metals at high temp (via sintering, dissolution into the support bulk or evaporation)

•Need High T for activity – (10-20 min driving for full activity).

•Can produce N2O from incomplete NO reduction

•Cars cannot run at their most fuel efficient since must be in or around 1.

Alternate ends of monolith plugged

Exhaust flows through porous ceramic and larger particulates get trapped

Trapped carbonaceous species oxidised (by O2 or NO2)

Effective only for larger particles – can lead to large NOx emissions

Proposed Treatment of Particulates

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0 150 300 450 600 750 900

time / s

spee

d /

mph

Bag1Bag3

Bag2

Very simplified plot

Standard Testing of Motor Vehicle Emissions

Open to abuse through programming the engine management systems to recognise the driving trend.

i.e. vehicles are very fuel economic during normal driving and switch to a less economic – but cleaner – mode of power generation when the test cycle is recognised.

Older model Fiat Panda has a less steep climb to the motorway driving conditions

The TWC technology (due to its sensitivity to the lambda factor) only

operates on gasoline cars. The Air Fuel ratio in diesel vehicles and next

generation lean burn-gasoline engines is too high for effective operation.

Some diesel cars are fitted with oxidation catalysts to remove CO and HC.

For deNOx from these engines see lectures of Frederic Meunier and Jim

Anderson