Automotive Catalytic Converters

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Text of Automotive Catalytic Converters

  • Automotive Catalytic Converters:

    Current Status and Some Perspectives

  • OBJECTIVES

    To illustrate the technology for abatement of

    exhaust emissions by analysing the current

    understanding of three-way-catalysts (TWCs).

    To study the specific role of the various

    components, the achievements and the

    limitations.

    To discuss about the challenges in the

    development of new automotive catalysts, which

    can meet future highly demanding pollution

    abatement requirements.

  • INTRODUCTION

    Air pollution by mobile sources

    Increase of world vehicles fleet from 40 million

    vehicles to over 700 million in last 60 years.

    Non-perfect combustion cause exhaust contains

    significant amount of pollutants which need to

    transformed into harmless compound.

    Hence, the TWCs is important towards this

    matter.

  • Three Ways Catalysts (TWCs)

    used in rich-burn or stoichiometric engines for

    simultaneous conversion of oxides of nitrogen

    (NOx), carbon monoxide (CO), hydrocarbons (HC),

    formaldehyde (CH2O) and Environmental

    Protection Agency (EPA) classified Hazardous Air

    Pollutants (HAPs).

    effective in a wide variety of engine applications

    and fuels, including natural gas, propane and

    gasoline.

  • A three-way catalytic converter has three

    simultaneous tasks:

    Reduction of nitrogen oxide to nitrogen and oxygen:

    2NOx xO2 + N2

    Oxidation of carbon monoxide to carbon dioxide:

    2CO + O2 2CO2

    Oxidation of unburnt hydrocarbons (HC) to carbon

    dioxide and water: CxH2x+2 + 2xO2 xCO2 + 2xH2O

  • Three Ways Catalysts

  • Design of TWCs

    Basically, it is a stainless steel container which

    incorporates a honeycomb monolith made of

    cordierite (2MgO2Al2O35SiO2) or metal.

  • Traditionally, cordierite monoliths have been

    employed quite extensively, primarily due to

    their lower production cost.

    major advantages of the metal monoliths resides

    in;

    (i) their high thermal conductivity

    (ii) low heat capacity

    (iii) allow very fast heating of the CCCs during

    the phase-in of the engine

    (iv )minimising the light-off time.

  • The monolith is mounted in the container with a resilient matting material to ensure vibration resistance.

    The active catalysts is supported (washcoated) onto the monolith by dipping it into a slurry containing the catalyst precursors.

    common components, which represent the state-of-art of the washcoating composition:

    (i) Alumina, which is employed as a high

    surface area support.

    (ii) CeO2ZrO2 mixed oxides, principally added

    as oxygen storage promoters.

  • (iii) Noble metals (NM = Rh, Pt and Pd) as active

    phases.

    (iv) Barium and/or lanthana oxides as stabilisers of

    the alumina surface area

  • (i) Al2O3 as carrier

    due to its high surface area and relatively good

    thermal stability under the hydrothermal conditions

    of the exhausts

    necessity of increasing the surface area of the

    honeycomb monolith which is typically

    below 24m2 l1

    Suitable even for high temperature applications

    such as in close-coupled catalysts ( CCCs)

    stabilising agents are employed to improve the

    stability of the surface area.

    Not an important issue for next generation of

    TWCs.

  • (ii) CeO2ZrO2 mixed oxides

    The beneficial effects of CeO2-containing formulations of the

    TWC performances has long been recognised

    Many different promotional effects have been attributed to this

    component, such as the ability to:

    promote the noble metal dispersion;

    increase the thermal stability of the Al2O3 support;

    promote the water gas shift (WGS) and steam reforming

    reactions;

    favour catalytic activity at the interfacial metal support

    sites;

    promote CO removal trough oxidation employing a

    lattice oxygen;

    store and release oxygen under, respectively, lean

    and rich conditions.

  • Thermal stability of CeO2ZrO2 mixed oxides

    Several ways to enhance the thermal stability

    of the CeO2-based materials

    i) design of microstructure / textural properties

    by adopting an appropriate synthesis

    methodology.

    ii) appropriate doping of CeO2.

    iii) dispersing of CeO2 on a carrier.

    i) Design of microstructure / texture properties

    - sinter ability of any material related to its

    textural properties and its pore structure

    - pore structure depends on the synthesis

    conditions

  • Example , co-precipitation is typically employed

    to prepare mixed oxide catalysts

    When the precipitated cake is treated at 80 C in the presence of surfactants, extensive

    mesoporous texture develops in the CeO2ZrO2 mixed oxides, leading to remarkably high surface

    areas compared to the traditional co-precipitation

    route .

  • Oxygen storage of CeO2ZrO2 mixed oxides

    Easily remove bulk oxygen species at moderate

    temperature even in highly sintered samples

    the ability of ZrO2 to modify the oxygen sub-

    lattice in the CeO2ZrO2 mixed oxides, generating defective structures and highly mobile

    oxygen atoms in the lattice which can be released

    even at moderate temperatures

    Efficiency of the OSC property can be achieved by

    using CeO2ZrO2 mixed oxides instead of CeO2

  • if the sample sinters under the high temperature

    reaction conditions, it should be more effective

    then CeO2 due to:

    - high oxygen mobility in the bulk

    - lattice oxygen species could effectively

    participate in redox processes even under

    fluctuating exhaust feed-stream conditions

  • Noble metals

    NMs represent the key component of the TWC, as

    the catalytic activity occurs at the noble metal

    centre

    Rh, Pd and Pt have long been employed in the

    TWCs

    there is a general agreement about the specificity

    of Rh to promote NO dissociation, thus enhancing

    the NO removal

    Pt and Pd are considered as metal of choice to

    promote the oxidation reaction, even though Rh

    also has a good oxidation activity.

  • Pd has extensively been added to TWC

    formulations starting from mid-1990s due to its

    ability to promote HC oxidation

    The increase of the use of Pd in the TWC

    technology adversely affected Pd market price,

    which is now comparable to that of Pt due to the

    straightforward way to increase the efficiency of

    the TWCs at low temperatures is that of

    increasing the NM loading and the cheapest NM

    among the three employed

  • Deactivation of TWCs

    Sintering of NM, leading to decrease of active sites, is a

    major pathway for deactivation TWC.

    Poisoning of the catalyst may also attribute to their

    deactivation.

    There are other routes that contribute to deactivation;

    i) sintering of OSC promoter leading to loss of OSC

    ii) sintering of Al2O3

    iii) deactivation of Rh due to the migration of Rh3+ into

    alumina lattice

  • Limitations of TWCs

    TWCs represent quite a mature, highly effective technology for pollution abatement

    However, it has some inherent limitations which need further improvement and development

    These aspects are related to

    (i) Low activity at low temperatures (start-up of

    the engine)

    (ii) Use of stoichiometric A/F

    - large amounts of hydrocarbon are in fact

    emitted at rich A/F, which require and

    additional HC trap

  • Although catalytic converters are effective at

    removing hydrocarbons and other harmful

    emissions, most of exhaust gas leaving the engine

    through a catalytic converter is carbon dioxide

    (CO2) and nitrous oxide (N2O), ,one of the

    greenhouse gases indicated by the

    Intergovernmental Panel on Climate change

    (IPCC) to be a "most likely" cause of global

    warming.

    An alternative approach is that of developing new

    catalysts showing high conversion efficiency at

    low, nearly ambient, temperature

  • Alternative Catalysts

    Lean DeNOx

    Capable of reducing NOx even in excess O2

    Achieve significant fuel savings

  • Pt/Al2O3

    Maximum of NO conversion at relatively low

    temperature

    Comparable starting temperatures for NO

    reduction and HC conversion

    Significant NO2 formation at high temperatures

    when all the HC is burned out.

    Strong sensitivity of the NO conversion

    Poor selectivity towards de-nitrogen formation of

    the Pt catalyst, N2O being the major product at

    low temperatures

  • Other alternative catalysts

    Lean NOx traps

    Selective catalytic Nox reduction using urea

  • Particulate matter removal

    Related to diesel engines

    Diesel particulate matter (DPM) is the most

    complex of diesel emissions

    Basic fractions of DPM are ele