More than you ever wanted to know about Catalytic Converters

I have seen a few posts about catalytic converters so I thought I would post what I know about them.

First, recycling catalytic converters is hard work, just like any other type of scrapping. It's not something you are going to get rich on fast, but you can make a very good living scraping them if you know what you are doing.

I am going to start with the different types of catalytic converters, and try to cover most of the information pertaining to the different types.

Oxidation catalysts

Oxidation catalysts convert carbon monoxide and hydrocarbons to carbon dioxide and water, but have but have almost no affect on nitrogen oxides. The early versions of catalysts in the USA, installed in the mid-seventies, were oxidation catalysts, containing Pt and/or Pd but no Rh.

Here is a picture of an oxidation catalyst



Three-way catalysts

This is the most common type of catalyst found on gasoline engines. The term ‘three-way’ means that it uses three different methods to reduce emissions (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 oxygen sensor to regulate the air-fuel ratio.

The original three-way catalyst used the combination of Pt and Rh, typically in a 5:1 ratio, Newer three-way converters use different formulas, including Pt/Pd/Rh, Pd/Rh, Pd only, etc.

Here is a picture of a typical Three-Way Catalyst, notice the three different honeycomb structures, this is how you can tell it's a three way, three different catalytic honeycombs that each do a different function:





NOx adsorber catalysts 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.

Here is a picture of a NOx adsorber, this one for a Cummings:



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.

Here is a picture of a Diesel Particle Filter:



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\

Here is a picture of a Diesel oxidation catalysts:



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.

Here is a picture of a Selective Catalytic Reduction, this one is actually mine and scrubs far more than just NOx:



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.
Other

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).

Here is a diagram of a NOx adsorber for diesel engines:



HINT: Most of the above usually contain precious metals and are therefore worthwhile to be recycled. Also, I have recently seen a few assays where the catalytic material also contained Ruthenium, if you are assaying and having the catalytic material processed yourself, this might be something to look for, it makes the value of the material worth more.

SOURCE: http://www.autocatalyst-recycling.um...yticConverter/