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An Introduction to Exhaust Catalysis Summer School in Energy and Environmental Catalysis

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 D H= -ve. Problems Associated with Fossil Fuel combustion as a power source. Finite supply of fossil fuels

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An Introduction to Exhaust Catalysis Summer School in Energy and Environmental Catalysis

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

  2. CxHy + xsO2 x CO2 + y/2 H2O DH= -ve Problems Associated with Fossil Fuel combustion as a power source • Finite supply of fossil fuels • Release of CO2 - a greenhouse gas • Release of Pollutants during combustion

  3. 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.

  4. 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

  5. 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 Carbonaceous Particulates sizes range from <0.1 mm up to visible ~10 mm. Smaller particles damage lungs / carcinogens (surface pyrenes) larger particles contribute to global warming (glacial albedo) SO2 (from S in fuel) formsAcid rain, sulphate is a nucleation site for particulates,

  6. NOx (NO + NO2) from combustion of N-containing fuel Also Formed from the high temperature reaction between N2 and O2 (radical mechanism) O22O., O.+N2NO + N., N..+O2NO + 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. 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

  7. 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 sunlight NOx Hydrocarbons SMOG

  8. 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

  9. 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).

  10. Fuel <1 HC, CO, CO2, NO, H2O, O2 CO2, H2O, NO, O2 Engine monolith catalyst Air >1 re-circulate Pt 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 (2NON2+O2) is thermodynamically favoured. But is kinetically very slow  USE A CATALYST

  11. 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 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.

  12. 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. 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). Now catalysing the NO + Reducing Agent reaction rather than the NO dissociation reaction

  13. 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 HC, CO, CO2, NO, H2O HC, CO, CO2, H2O, N2 Rhodium catalyst Engine Air <1 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.

  14. 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 O2 + CO2 + H2O + N2 Platinum catalyst Air / O2 Intake • 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.

  15. At low l [N2] and [O2] are low and effectiveness of the Zeldovich mechanism decreases. Low l – not enough O2 to combust fuel  low O2 in exhaust. Higher l excess [O2] and  large [O2] in exhaust Inverse reasoning for [CO] Temperature decreases as l increases [NO] decreases stoichiometric FUELRICH FUEL LEAN HC increases with l because T decreases

  16. 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

  17. 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- l=1 Fuel rich Fuel lean 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

  18. 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. l> 1 (too oxidising) CO and HC effectively combusted over the catalyst but NOx conversion falls. l< 1 (too reducing) NO conversion remains high but CO and HC are now converted

  19. Fuel ~1 CO2 H2O N2 Engine TWC catalyst HC, CO, CO2, NO, H2O, O2 Air ~1  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.

  20. 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+CeO22CO2+ 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+CeO22CO2+ Ce2O3 • Ce2O3 + 2H2O  CeO2 + 2H2 • Overall 2CO + 2H2O  2CO2 + 2H2 • Stabilises both the metals and the Al2O3 support against sintering

  21. 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

  22. 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. light off 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

  23. 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.

  24. 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 l must be in or around 1.

  25. Proposed Treatment of Particulates 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

  26. Bag1 Bag2 Bag3 Standard Testing of Motor Vehicle Emissions Very simplified plot Older model Fiat Panda has a less steep climb to the motorway driving conditions 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.

  27. 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

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