The Engineered By Design (ebd)

ebd Science Corner: - Doctor C's Take on Catalytic Converters;

Apparently, (if you believe the Government and Hippies) there is this problem called pollution, and the origin of pollution is anthropogenic activity. So in order to stop pollution we can either modify what we do, or wipe out the human race. Since death isn’t particularly appealing to most people then we modify the things we do. One of these modifications is the catalytic converter. Apparently the number one source of pollution in the world ever is the internal combustion engine. Internal combustion engines can be found in all types of machinery, but are most commonly found in cars. The internal combustion engine is a bad thing, it spews out all manner of filth including carbon monoxide (CO), nitrogen oxides (NOx), unburnt fuels (HC’s), and steam! Catalytic converters are used in car exhausts to convert these bad things (except steam) into less bad things;

If an engine runs with 100% efficiency (in a petrol engine, this requires a fuel/air ratio of 1/14.7) then the only products should be water vapour, carbon dioxide (CO2) and nitrogen gas (N2 – technically not a product, just something that has passed through the engine). In the real world engines don’t run at 100% efficiency, they either run lean, where there is too much air or they run rich where there is not enough air. Running lean can lead to the formation of nitrogen oxides, running rich can lead to the formation of carbon monoxide and also incompletely combusted hydrocarbons (HC’s). A three-way catalytic converter can convert all of these to less harmful substances (the alternative diesel catalytic converter is unable to deal with nitrogen oxides on its own).

The structure of a three-way catalytic converter is of a stainless steel or ceramic honeycomb support. This support is coated with an aluminium/silicon based washcoat. This washcoat forms a rough irregular surface which inherently has a high surface area. The washcoat also contains the catalyst (suspended in the washcoat before application). The catalyst, which here is a mixture of platinum (Pt), palladium (Pd) and rhodium (Rh) sits in active sites on the washcoat. To my knowledge other precious metals such as iridium (Ir) and ruthenium (Ru) may also be present along with some non-precious metals.

Catalysis occurs when the exhaust gases pass over the catalytic converter at an elevated temperature (so your car needs to warm up before the catalytic converter reaches full efficiency).

There are two processes which occur: Reduction & Oxidation

1) REDUCTION: The first stage of the catalytic conversion is the Pt/Rh catalysed reduction of nitrogen oxides:

Basically the nitrogen bonds to the surface of Pt/Rh and oxygen is released (as O2), two ‘free’ nitrogens can then combine to N2 and are released.

Process 2: Oxidation

2) OXIDATION: The second stage uses a Pt/Pd catalytic system to oxidise carbon monoxide to carbon dioxide and unburnt hydrocarbons to carbon dioxide and water vapour.

These reactions require an oxygen supply and use the remaining air in the exhaust gases, if there is not enough then a sensor tells the engine to pull more air through and typically an engine will alternate between lean and rich operation, dependent on the efficiency of the converter.

Precious Metals & Leaded Fuel...

Precious metal catalysts are sensitive and so cannot be used with leaded fuels (the lead coats the catalytic converter rendering it inactive). Also they are sensitive to sulphur, sulphur is a ‘class b’ ligand and since the heavy metals are also class b then they bind irreversibly to sulphur, and can no longer be used as cataysts. This compatibility with sulphur is why most heavy metal ores are found as sulphides. So it is necessary to use unleaded, low sulphur fuels. The removal of lead has prompted the addition of other compounds such as benzene as an ‘anti-knock’ agent. Benzene is a potent carcinogen (with a real preference for causing leukaemia) and is not dealt with by the converter.

Sulphur removal from chemical feedstocks is currently achieved using a method called hydrodesulphurisation (HDS), basically reaction with hydrogen over a cobalt-molybdenum catalyst at elevated temperatures. However this does not deal with sulphur containing aromatic species very well and other catalysts (including ironically platinum species) are being investigated for this purpose. I touch on this in my thesis and will post publication details at a later date, for now the reader can refer to R. J. Angelici Polyhedron, 16 (1997) 3073.

If you have any questions or comments then Email The Doctor!
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