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 elemental carbon,

heavy HCs and hydrated sulphuric acid

DPM contains a large portion of the polynuclear

aromatic hydrocarbons (PAH) found in diesel

exhaust

Non-gaseous diesel emissions are grouped into 3

categories : soluble organic fraction (SOF),

sulpate and soot

Solutions

Removal of liquid fraction of PM generally

achieved by an oxidation catalyst reduce HC, SOF content and CO

Removal of soot achieved by means of filtration

Conclusion

Development of automotive converters has

proceeded by a continuous improvement of the

catalytic performances and durability of

automotive catalysts.

TWC is a complex and intergrated system that

must be immediately effective and that its

lifetime must be equivalent to that of the car.