An Introduction to Exhaust Catalysis Summer School in Energy and Environmental Catalysis University of Limerick, July 2005

C x H y + xsO 2  x CO 2 + y/2 H 2 O  H= -ve Problems Associated with Fossil Fuel combustion as a power source (1)Finite supply of fossil fuels (2)Release of CO 2 - 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 C x H y + O 2  CO 2 + H 2 O Combustion of a branched alkane to CO 2 and H 2 O 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 CO 2 Flame combustion is very rapid – but not rapid enough to combust all C x H y to CO 2 + H 2 O

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 SO 2 (from S in fuel) forms Acid rain, sulphate is a nucleation site for particulates, Carbonaceous Particulates sizes 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 N 2 and O 2 (radical mechanism) O 2  2O., O. +N 2  NO + N., N.. +O 2  NO + O. N 2 + O 2  2 NO Ln K = at 23 °C = at 1227 °C Therefore you need high temperatures – as are found in a flame combustion. NO x (NO + NO 2 ) 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

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 NO x Hydrocarbons 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/Al 2 O 3 ). High surface area Al 2 O 3 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 O 2 in the combustion chamber and thus O 2 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, CO 2, NO, H 2 O, O 2 CO 2, H 2 O, NO, O 2 1 st generation catalytic converters removed CO and unburned HC from tailpipe emissions [NO x ] emitted was also decreased as T comb decreases when the engine runs fuel lean However, as emission legislation grew stricter a method for removing the NO x pollutant was also needed DECOMPOSITION of NO (2NO  N 2 +O 2 ) is thermodynamically favoured. But is kinetically very slow  USE A CATALYST Pt

NO adsorbs and dissociates on “clean” metal surfaces NO (g)  NO ads  N ads + O ads N ads species combine on the surface forming N 2 ads which desorb into the gas phase as N 2. The O ads species on the surface are VERY stable. At moderate temperatures over time the “clean” metal surface becomes an oxidised metal surface - being saturated with O ads species. This surface no longer dissociates NO ads - the catalyst is poisoned.

This effect is worse in the presence of O 2(g) Very high temperatures are needed to get these to combine, form O 2 ads and eventually O 2 (g) These temperatures damage a catalyst (sintering / evaporation etc). If a reducing agent is added (CO, H 2, 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 2 nd 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 NO x reduction catalyst free of O ads species Fuel >1 Air <1 Engine Rhodium catalyst HC, CO, CO 2, NO, H 2 O HC, CO, CO 2, H 2 O, N 2 Rh is the most active metal for promoting the NO decomposition reaction in the presence of reducing agents (CO / H 2 / HC) However now there is no longer any O 2 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 H 2 O and CO 2. HC, CO, CO 2, H 2 O, N 2 Platinum catalyst Air / O 2 Intake O 2 + CO 2 + H 2 O + N 2 Problems with the 2 nd generation “Dual-Bed” catalytic converter. inefficient wrt fuel consumption greater costs (PGM) and bulkiness / weight. Over-Reduction of NO x to NH 3 in the presence of excess HC. This NH 3 subsequently reacts to form NO x on the Oxidation Catalyst or in the atmosphere.

Temperature decreases as  increases   [NO] decreases Low – not enough O 2 to combust fuel  low O 2 in exhaust. Higher excess [O 2 ] and  large [O 2 ] in exhaust Inverse reasoning for [CO] HC increases with because T decreases FUEL RICH FUEL LEAN stoichiometric At low  N 2 ] and [O 2 ] 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 O 2 ). 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. O2O2

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  [O 2 ] – through transport of O 2- 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 =1 Fuel 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 NO x conversion falls. < 1 (too reducing) NO conversion remains high but CO and HC are now converted

Fuel ~1 Air ~1 Engine TWC catalyst HC, CO, CO 2, NO, H 2 O, O 2 CO 2 H 2 O N 2 Sensor The three way catalyst is a mixture of catalysts supported on the same monolith Cordierite monolith, High surface area Al 2 O 3, Pt (for oxidation reactions), Rh (for NO x reduction reactions), Pd (for both oxid. and red. reactions). AND, CeO 2 as an additive.

Functions of CeO 2 Acts as a temporary buffer to changes in the lambda value through CeO 2  Ce 2 O 3 + O 2 when the mixture is fuel rich and the reverse reaction when the mixture is fuel lean. so 2CO+CeO 2  2CO 2 + Ce 2 O 3 when [O 2 ] is low and 2Ce 2 O 3 + O 2  2CeO 2 when [O 2 ] is high Catalyses the Water Gas Shift reaction H 2 O + CO  H 2 + CO 2 (removing CO and forming a “better” reductant H 2 ) 2CO+CeO 2  2CO 2 + Ce 2 O 3 Ce 2 O 3 + 2H 2 O  CeO 2 + 2H 2 Overall 2CO + 2H 2 O  2CO 2 + 2H 2 Stabilises both the metals and the Al 2 O 3 support against sintering

Problem with CeO 2 Can store SO 2(g) as SO 4 2- and reduce it under rich conditions to H 2 S (rotten egg smell) Possible Solutions Use Ni materials to store S during rich phases. Butnot permitted in Europe (fear of Ni(CO) 4 Sinter CeO 2 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 light off

Emissions Cumulative HC Emissions 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 SO 2 to H 2 S (on CeO 2 ) 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 N 2 O 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 O 2 or NO 2 ) Effective only for larger particles – can lead to large NO x emissions Proposed Treatment of Particulates

Bag1 Bag3 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