An automotive catalytic converter is one of the several elements of an exhaust system that reduces the emission of harmful pollutants, such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx). The catalytic function of the converter will activate certain oxidation and/or reduction reactions, which transform these harmful pollutants into carbon dioxide (CO2), water (H2O) and nitrogen (N2).
A converter typically consists of one or more honeycomb bricks. The term honeycomb describes a structure containing many fine channels. As such, it closely resembles a honeycomb, as we know them from bees in nature. The cross-section of such a honeycomb is most commonly little squares or, alternatively, triangles.
The substrate is protected from vibration and shock by a mineral or metallic 'mat' and then packaged into a stainless steel can. The use of a mat is only applicable to ceramic converters. Positioned between the ceramic substrate and the steel mantle, the mat helps to absorb temperature and mechanical shocks.
Current generation automotive catalyst material consists of a ceramic or metallic substrate coated by an aluminium oxide (Al2O3)-based wash coat. The most commonly found converters contain a ceramic substrate (cordierite: 2Al2O3.2SiO2.5MgO) coated with a precious metal containing washcoat. For certain applications (reducing light-off, reducing back-pressure, etc), a metallic substrate is preferred over a ceramic substrate, typically a specially rolled-up thin stainless steel foil coated with a thin washcoat. Most of these converters are easily recognizable by the circular/swirl movement, as can be seen in a cross section. However, other metallic substrate techniques exist as well. It is a commonly known fact that converters on a ceramic substrate cover about 95% of the total market.
This wash coat contains a combination of platinum group metals (Pt, Pd, Rh) with other rare earth oxides such as CeO2, ZrO2, etc. and provide for the catalytic function. PGM is a commonly used denominator for the precious metals platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru) and osmium (Os).
Today’s exhaust systems are characterized by an increased complexity, as one distinguishes oxidation catalysts and three-way catalysts for gasoline engines, NOx adsorbers for lean-burn engines, particulate filters and oxidation catalysts for diesel engines, aside from selective catalytic reduction units and NOx adsorbers for diesel and many others. A far cry from the muffler our parents were familiar with.
Oxidation catalysts for stoechiometric-operated gasoline engines
Oxidation catalysts convert carbon monoxide (CO) and hydrocarbons (HC) to carbon dioxide (CO2) and water (H2O), but have little effect on nitrogen oxides (NOx). The early versions of catalysts in the USA, installed in the mid-seventies, were oxidation catalysts, containing Pt and/or Pd but no Rh.
Three-way catalysts for stoechiometric-operated gasoline engines
This is the most common type of catalyst found on gasoline engines. The term ‘three-way’ underscores the catalysing of three different reactions (oxidation of CO into CO2, oxidation of HC into CO2 and H2O, reduction of NOx into N2, CO2 and H2O). Three-way catalysts operate in a closed-loop system including a lambda sensor (also called oxygen sensor) to regulate the air-fuel ratio.
Whereas the original three-way catalyst used the combination of Pt and Rh, typically in a 5:1 ratio, further development has led to a multitude of formulas, including Pt/Pd/Rh, Pd/Rh, Pd only, etc.
NOx adsorbers for lean-burn gasoline engines
Lean-burn engines are capable of reducing fuel consumption by 15 to 20%. They operate in lean condition, i.e. excess of air. This raises a challenge for emission control, since the catalyst can only function as an oxidation catalyst. Because of this, the reduction of NOx is no longer promoted.
NOx adsorber catalysts (typically one particular brick of the emission control system) is designed to adsorb NOx while operating in lean-burn condition. At regular intervals, the engine will be switched for a short time to rich conditions, which release the NOx (allowing the adsorber to regenerate) and reduce it to N2.
The adsorber function of such a catalyst is typically taken care of by particular rare earth oxides in the washcoat formulation, which allow storage of NO2 under lean engine operation conditions. The precious metal content of such an adsorber brick is not affected.
Umicore was the first catalyst manufacturer to supply the NOx adsorber catalyst technology for vehicles.
Diesel Particulate Filters (DPF)
Diesel Particulate Matter (often abbreviated as Diesel PM) refers to small solid particles, some of very small size, resulting from the burning of diesel fuel. These particles not only contain soot, but also hydrocarbons, ashes and sulphuric acid (resulting from the fuel’s sulphur). Although a diesel catalyst destroys most of the organic fraction of particulate matter, it does not affect the number of particles, so health issues related to ultra-fine particulates remain unresolved.
A diesel particulate filter (DPF) will act as a filter for these small particles. Ceramic wall flow filters (or traps) contain channels which are one-ended only, forcing the off-gas to find its way through the porous walls between the channels. PM will be stopped by this solid medium. On regular intervals, the filter has to be regenerated. Since most of the PM is carbon, the carbon needs to burn with air. This is initiated by lowering the temperature at which the carbon reacts with air: by using a variety of techniques such as a fuel additive (e.g. the EOLYS® from Rhodia) or through the presence of a precious metal in the diesel particulate filter (such as Pt; these DPFs are then called catalysed DPFs) or through electrical heating of the trap on or off the vehicle
HINT: Not all Diesel Particle Filters contain precious metals.
Diesel oxidation catalysts
A diesel engine always runs ‘lean’, i.e. with excess air. Therefore, a diesel catalyst operates as an oxidation catalyst only. Oxidation catalysts achieve significant reductions in CO, HC and destroy the organic fraction of the particulate matter (as it does the typical diesel smell). Diesel oxidation catalysts used to contain Pt only. In 2004 however, Umicore announced the development of diesel catalysts, whereby Pd has substituted some of the Pt.
HINT: Diesel oxidation catalysts contain no Rh. A large portion of diesel catalysts in a mixed load will have an adverse effect on the Rh assay
Selective Catalytic Reduction (SCR)
This technique, originally developed for stationary emission control from power plants, is aimed at reducing NOx (by conversion to nitrogen) in heavy-duty and light-duty diesel engines, by using ammonia or ammonia precursors as reductants in combination with excess oxygen.
HINT: SCR units never contain precious metals.
NOx adsorber for diesel engines
Particular washcoat formulations will allow for accumulation and storage of NOx during a particular period of time under which the engine operates under lean-burn / excess air conditions. Then, for a short period of time, either through the injection of excess hydrocarbons / fuel (for common-rail diesel engines) or excess fuel conditions (for lean-burn gasoline engines), the stored NOx will be reduced and, as such, eliminated.
Emission control catalysts are not only found on cars and trucks, but also on 2-wheelers and 3-wheelers as well as pollution abatement systems for stationary source emission control (engines and turbines for heat or power generation, off-gas in certain process industries, lawnmowers and other utilities, etc).
HINT: Most of the above usually contain precious metals and are therefore worthwhile to be recycled.
The muffler shouldn’t be confused with the catalytic converter. Its function is to reduce the noise from the exhaust gases flowing out of the exhaust manifold. In recent decades, the muffler has also become more complex (consisting of more than a single piece) and is tailor-made to produce a particular sound and resonance.
HINT: A muffler does not contain precious metals.