Auto Exhaust Catalysis

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Auto Exhaust Catalysis
Jude vimal michael.R

• Childrens club lecture series
• National Centre for Catalysis Research
• IIT-Madras
• 27April 2009

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What this picture suggest?
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Composition of Exhaust Gas
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Major Pollutants

• The pollutants present in auto exhaust gas are
• Sulphur dioxide, SO2 (primary pollutant)
• Nitrogen oxides NOx (primary or secondary
  pollutants)
• Particulate matter PM (primary and secondary
  pollutants)
• Carbon monoxide, CO, (primary pollutant)
• (volatile) organic compounds, HC (or VOCs)
  (primary and secondary pollutants), and
  photochemical oxidants,
• PAN -peroxyacetyl nitrate(secondary pollutants).

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Composition of diesel exhaust

• DE is a complex mixture of hundreds of
  constituents in either a gas or particle form.
• Gaseous components of DE include carbon dioxide,
  oxygen, nitrogen, water vapor, carbon monoxide,
  nitrogen compounds, sulfur compounds, and
  numerous low-molecular-weight hydrocarbons.
• Among the gaseous hydrocarbon components of DE
  that are individually known to be of
  toxicological relevance are the aldehydes (e.g.,
  formaldehyde, acetaldehyde, acrolein), benzene,
  1, 3-butadiene, and polycyclic aromatic
  hydrocarbons (PAHs) and nitro-PAHs.

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Contd..

• DPM consists of fine particles (fine particles
  have a diameter lt2.5µm), including a subgroup
  with a large number of ultrafine particles
  (ultrafine particles have a diameter lt0.1µm).
  Collectively, these particles have a large
  surface area which makes them an excellent medium
  for adsorbing organics.
• Their small size makes them highly respirable and
  able to reach the deep lung. A number of
  potentially 1-1 toxicologically relevant organic
  compounds are on the particles.
• The particles present in DE (i.e., diesel
  particulate matter DPM) are composed of a
  center core of elemental carbon and adsorbed
  organic compounds, as well as small amounts of
  sulfate, nitrate, metals, and other trace
  elements.

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Ill effects of Exhaust emission

• The hazards include acute exposure-related
  symptoms, chronic exposure related non-cancer
  respiratory effects, and lung cancer.
• The health hazard conclusions are based on
  exhaust emissions from diesel engines built prior
  to the mid-1990s. With current engine use
  including some new and many older engines
  (engines typically stay in service for a long
  time)
• The health hazard conclusions, in general, are
  applicable to engines currently in use. As new
  and cleaner diesel engines, together with
  different diesel fuels, replace a substantial
  number of existing engines, the general
  applicability of the health hazard conclusions
  will need to be reevaluated.

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Contd..

• According to the pollutants exposure the effects
  are classified as
• Acute (Short-Term Exposure) Effects
• Chronic (Long-Term Exposure) Non-cancer
  Respiratory Effects
• And Chronic (Long-Term Exposure) Carcinogenic
  Effects.

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Emission standards
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Emission Norms

• The Globally automotive pollution is controlled
  by various emission performance standards set by
  countries according to their population and
  economic considerations.
• The Emission Performance standard is the
  requirements that set specific limits to the
  amount of pollutants that can be released into
  the environment.
• Many emissions standards focus on regulating
  pollutants released by automobiles (motor cars)
  and other powered vehicles but they can also
  regulate emissions from industry, power plants,
  small equipment such as lawn mowers and diesel
  generators.
• Did you notice the Tamil nadu pollution control
  notice board in velachery gate-IITM

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Overview - Emission norms in India

• 1991 - Idle CO Limits for Gasoline Vehicles and
  Free Acceleration Smoke for Diesel Vehicles, Mass
  Emission Norms for Gasoline Vehicles.
• 1992 - Mass Emission Norms for Diesel Vehicles.
• 1996 - Revision of Mass Emission Norms for
  Gasoline and Diesel Vehicles, mandatory fitment
  of Catalytic Converter for Cars in Metros on
  Unleaded Gasoline.
• 1998 - Cold Start Norms Introduced.
• 2000 - India 2000 (Eq. to Euro I) Norms, Modified
  IDC (Indian Driving Cycle), Bharat Stage II Norms
  for Delhi.
• 2001 - Bharat Stage II (Eq. to Euro II) Norms for
  All Metros, Emission Norms for CNG LPG
  Vehicles.
• 2003 - Bharat Stage II (Eq. to Euro II) Norms for
  11 major cities.
• 2005 - From 1 April Bharat Stage III (Eq. to Euro
  III) Norms for 11 major cities.
• 2010 - Bharat Stage III Emission Norms for
  4-wheelers for entire country whereas Bharat
  Stage - IV (Eq. to Euro IV) for 11 major cities.
  Bharat Stage IV also has norms on OBD (similar to
  Euro III but diluted)

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What is that Euro Norms
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Contd..

• The European Union standards for emission limits
  of automotive exhaust gases (values in g km-1)
  are given in the Table previous slide.
• The typical concentration of the various
  pollutants are listed out and when our (Bharat
  stage IV) compared to the European standards we
  lag behind nearly 5 years.

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VARIOUS METHODS OF AUTO EMISSION ABATEMENT

• Modification of engine design (e.g., the fuel
  management system) and the engine calibration
  (e.g., the ignition timing) to decrease the
  engine output (also called raw emission)
• Aftertreatment of the engine exhaust by solid
  catalysts, to convert the engine raw emission
• A combination of engine exhaust after treatment
  by solid catalysts with engine design
  modification and/or controlled engine operation,
  to allow optimal functioning of the
  aftertreatment device.

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Particulate Filter
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Various Methods

• Retrofit
• Diesel retrofit involves the addition of an
  emission control device to remove emissions from
  the engine exhaust.
• Retrofits can be very effective at reducing
  emissions, eliminating up to 90 percent of
  pollutants in some cases.
• Some examples - Diesel oxidation catalysts,
  diesel particulate filters, NOx catalysts,
  selective catalytic reduction, and exhaust gas
  recirculation. Devices to control crankcase
  emissions also exist.
• Repower
• Repowering involves replacing an existing engine
  with a new engine. This strategy is most
  effective for use in diesel-powered equipment
  with a useful life longer than that of the
  engine.
• Rebuild
• All diesel equipment requires periodic
  maintenance. Routine maintenance and repairs help
  to ensure that engines operate at maximum
  performance and emission rates do not exceed the
  designed standard.

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Contd..

• Refuel
• A variety of alternative fuels can be used in
  diesel engines. Some require little or no
  modification to the engine while others require
  engine conversion or replacement.
• Some of the alternative fuels include emulsified
  diesel, biodiesel, natural gas, propane and
  ethanol. In addition to these fuels, use of
  diesel fuel with lower sulfur content can help to
  reduce emissions.
• Replace
• Replacement involves retiring higher polluting
  equipment from service before it would otherwise
  be retired. Newer equipment that meets more
  stringent emission standards is purchased to
  replace the retire equipment.

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Challenges in the development automotive catalyst

• Reducing the base emissions from the engine by
  improvements to the combustion process and fuel
  management, addition of air injection or exhaust
  gas recycle or by changes to the type of fuel or
  its composition.
• Decreasing the time taken for the catalytic
  converter to reach its full operating efficiency.
• Increasing the conversion efficiency of
  catalysts at their working temperature.
• Store pollutants during the cold start for the
  release when the catalyst is working.
• Device catalysts or strategies to destroy
  nitrogen oxides under lean (oxygen rich)
  operation.
• Devise reliable ways to regenerate particulate
  filters.
• Increase the operating lifetime during which
  autocatalysts and their supporting systems
  efficiently convert pollution.

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Location of catalytic converter
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Monolith
Typical automotive exhaust converters. The one on
the left has been cut open to reveal the
monolith. The insert shows a blow up of the upper
part of the monolith where a part has been
chipped off
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Reactions
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Concepts for catalytic aftertreatment

• Five basic catalytic concepts have been used in
  the development of catalytic emission control
  they are
• Closed loop control catalyst
• Open loop catalyst
• Dual bed catalyst
• Oxidation catalyst
• Lean oxidation catalyst

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Operating window concept
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(No Transcript)
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Closed loop control catalyst

• In the closed-loop-controlled three-way catalyst,
  one type of catalyst, which is placed in the
  exhaust gas stream, is able to promote all the
  main reactions that lead to the simultaneous
  removal of CO, HCs and NOx.
• To balance the extent of the oxidation and the
  reduction reactions, the composition of the
  engine-out exhaust gas is maintained at or around
  stoichiometry.
• This is achieved by a closed-loop engine
  operation control, in which the oxygen content of
  the engine-out exhaust gas is measured upstream
  of the catalyst with an electrochemical oxygen
  sensor, also called the lambda sensor.
• This component is used by the engine management
  system to regulate the amount of fuel fed into
  the engine, and so to regulate the engine
  operation around the stoichiometric A/F ratio.

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Various catalyst types
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Contd..

• The extent of the secondary reactions is minimal
  under these conditions. The feedback control of
  the engine causes a small cyclic variation of the
  engine exhaust gas composition.
• This variation occurs in a second, which means a
  frequency of 1 Hz, and with amplitude of 510 of
  the A/F set point. This transient operation of
  the catalyst, however, has a significant effect
  upon its performance, as will be described below.
  
• There exists a multitude of engine management
  systems with various degrees of complexity and
  refinement, affecting the speed and the amplitude
  range of control of the engine A/F ratio at each
  of the engine load and speed operation
  conditions.
• The refinement of the engine management system
  affects both the performance and the durability
  of the emission control catalyst.

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Open loop catalyst

• This concept is a simplification of the first, as
  again a multifunctional catalyst is used, that is
  able to promote all of the reactions that lead to
  the removal of CO, HCs and NOx.
• The composition of the exhaust gas is not
  controlled and therefore varies over a wide
  range. This wider operation range results in an
  overall lower simultaneous conversion of the
  three exhaust gas constituents
• This concept is used if the legislative limits
  can be reached with a conversion of about 50, or
  for the retrofitting of engines that were not
  designed to be equipped with catalytic emission
  control devices.

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Dual bed catalyst

• Two different types of catalyst are used. The
  first catalyst is either multifunctional, or is
  at least of capable of promoting NOx reduction
  reactions.
• The engine is calibrated so as to guarantee a net
  reducing exhaust gas composition. Under these
  conditions, the first catalyst will lead to an
  elimination of the nitrogen oxides. The second
  catalyst is an oxidation catalyst.
• Extra air is injected in front of the second
  catalyst to assist the removal of CO and HCs. The
  secondary air can be added either by mechanically
  or by electrically driven air pumps.
• The dual-bed concept allows for a wider engine
  A/F range and also maintains high conversion
  efficiency for the three exhaust gas constituents
  under these conditions. Therefore, a
  less-sophisticated engine management system can
  be used.

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Oxidation catalyst

• In this emission control concept, secondary air
  is added to the exhaust gas to ensure a lean
  composition, independent of the engine operation
  condition. The catalyst is designed to promote
  reactions between oxygen and both CO and HCs,
  which can be removed to a great extent, but NOx
  cannot be removed in this manner.

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Lean oxidation catalyst

• The fifth concept is also an oxidation catalyst,
  but it is applied to engines that operate under
  lean conditions, the so-called lean-burn engines.
  The A/F ratio of these engines reaches values up
  to 26, corresponding to a lambda value of about
  1.8.
• The function of the catalyst could be limited to
  converting CO and HCs. Because of the dilution
  effect in lean combustion, the exhaust gas is
  colder than for closed-loop controlled engines,
  and therefore special catalysts with good
  low-temperature activity for the oxidation
  reactions are needed.To date, however, this
  concept has not achieved widespread application.
• The latest generation of lean-burn gasoline
  engines applies the direct fuel injection
  principle, which enables different catalytic
  exhaust gas after treatment concepts to be used,
  such as the NOx-adsorber systems

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Three-way Catalyst

• The three-way catalyst, consisting of Pt and Rh
  particles supported on a ceramic monolith,
  represents a remarkably successful piece of
  catalytic technology. It enables the removal of
  the three pollutants CO, NO and hydrocarbons
• Additionally, NO is reduced by H2 and by
  hydrocarbons. To enable the three reactions to
  proceed simultaneously notice that the two
  first are oxidation reactions while the last is a
  reduction the composition of the exhaust gas
  needs to be properly adjusted to an air-to-fuel
  ratio of 14.7
• At higher oxygen content, the CO oxidation
  reaction consumes too much CO and hence NO
  conversion fails.

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Contd..
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Contd..
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Contd..

• If, however, the oxygen content is too low, the
  entire NO is converted, but hydrocarbons and CO
  are not completely oxidized.
• An oxygen sensor (?-probe) is mounted in front of
  the catalyst to ensure the proper balance of fuel
  and air via a microprocessor-controlled injection
  system.

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Oxygen sensor

• It is a simple oxygen sensor made in a similar
  manner to the solid oxide fuel cell. An oxide
  that allows oxygen ions to be transported is
  resistively heated to ensure sufficiently high
  mobility and a short response time (1 s.).
• The oxygen content in the exhaust is measured
  against a suitable reference, in this case
  atmospheric air.
• The response is given by the Nernst equation

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Oxygen sensor Contd..
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Oxygen sensor Contd..
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Contd..

• The ?-probe relies on the diffusion of atomic
  oxygen through a solid electrolyte and,
  therefore, it will have a certain response time.
• Reducing the thickness of the oxide membrane and
  increasing the temperature both shorten the
  response time, but a certain delay cannot be
  avoided.
• For example, if the driver suddenly steps on the
  gas pedal the exhaust becomes reducing.
  Consequently, sulfur deposited in the catalyst
  becomes hydrogenated to H2S, causing the
  characteristic rotten eggs smell (this smell
  sometimes arises during the startup of a cold
  engine).
• New types of sensors with faster response are
  therefore being explored to avoid these effects.
  Ideally these should be placed immediately after
  each cylinder and therefore they should be
  capable of withstanding high temperatures.

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Location of Gas sensor
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Performance of Three-way catalyst

• The Performance of the catalyst depends upon the
  various factors such as
• the chemistry of the catalyst (e.g., the wash
  coat, precious metals, age and preparation),
• the physics of the catalyst (e.g., support and
  converter design) and
• the chemical engineering aspects of the catalyst
  (e.g., reaction temperature, residence time, gas
  composition and dynamic conditions)

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Factors affecting performance of TWC
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Contd..
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What is that operating window

• The catalytic converters have three important
  layers. First is a wash coat, which increases the
  surface area that the catalysts are on a large
  surface area is essential for high-efficiency
  exhaust emission reductions.
• Next, a layer of noble metals like platinum and
  palladium are vaporized on to the wash coat
  these encourage carbon monoxide and hydrocarbons
  to react into water vapor and carbon dioxide.
• Then there is a third layer of platinum and
  rhodium that reduces nitrogen oxides (the third
  layer is what makes the converter 'three-way').
• These reactions seem contradictory the oxidation
  process is more efficient when large amounts of
  oxygen are present, but reduction happens more
  efficiently in a low oxygen environment. But
  there is a small window of exhaust stoichiometry,
  called the lambda window, which creates favorable
  conditions for both reactions to take place.
  Maintaining the air/fuel ratio to keep exhaust
  gasses in this window is extremely important,
  hence the requirement of oxygen sensor
  monitoring.

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Role of Ceria in the TWC

• Ceria is well known for his Oxygen storage
  capacity and redox properties and these
  properties are the key for the three-way catalyst
  development.
• The major problem for the catalyst to stay active
  is that to have adequate contact points between
  the pollutant soot and the catalyst.
• When these catalysts are used through fuel borne
  then this increases the contact points between
  the soot and the catalyst and in turn decreases
  the temperature of oxidation from 600 ?C to 400
  ?C.

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Interesting mechanism
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Contd..

• CeO2 is one of the extensively used catalytic
  components in many of the above described
  after-treatment technologies due to its high
  activity in the redox reactions
• . CeO2 is used in a well- known three-way
  catalyst for CO, HC, and NOx abatement, as a
  fuel-borne catalyst, and in the catalysed soot
  ?lters in elimination of the soot particulates.
• The fuel-borne ceria catalyst leads to the
  formation of the CeO2 nano-particles trapped
  within the soot particle.

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NOx assisted soot oxidation
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Future catalyst-Precious metals?

• Improving the cold-start performance
• HC adsorbers into the aftertreatment system-like
  activated carbon, zeolites etc.,
• usage of precious metals should be limited to
  achieve cost reduction
• future development will be the exchange of a
  substantial portion of the platinum by palladium
  in high performance oxidation catalyst
• In order to cope with the changing boundary
  conditions, and especially the further reduction
  in exhaust gas temperature, it is highly probable
  that at least a portion of the catalyst volume
  will be moved closer to the engine outlet.

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Catalysis can do this
Thanks