Chemical of the Week 15 – Urea


So again this week in the news it’s chemicals that are getting big industry into trouble. This time the culprits are the car exhaust gases carbon dioxide and nitrogen dioxide. But behind the scenes it’s the chemical urea that has caused the problem. Cars need fuel, and using any hydrocarbon based fuel such as petrol or diesel will produce carbon dioxide. The nitrogen oxides are produced by the high temperature combustion of nitrogen gas present in air. Diesel engines are notoriously more ‘dirty’ than their petrol counterparts but of course everyone knows they give more miles to the gallon. The problem that faced car manufacturers was that consumers wanted diesel transport at accessible prices but the manufacturers needed to meet the environmental legislation with regards to emissions.

So the chemists came up with some clever design of the emission chamber, using selective chemical reduction (remember we want the nitrogen oxides to be converted to nitrogen which is a chemical reduction ) that was aimed at reducing nitrogen oxide emissions by 90%. This system uses ammonia or an associated compound such as urea as the reductant. Urea has the formula CO(NH₂)₂. The molecule has two —NH₂ groups joined by a carbonyl (C=O) functional group. It’s IUPAC name is amino methanamide and is also known as carbamide. It is more commonly known as a biological molecule as it is produced in the body by the breakdown of proteins. It has a neutral pH so it’s safer for the body than ammonia which would raise pH levels to dangerous levels. The kidneys then transfer urea from the blood to urine in order to allow it to be expelled from the body. It can also be made synthetically from ammonia and carbon dioxide and its main industrial use is in fertilisers.

Using urea is were the problems for Volkswagen started to occur. In large vehicles storage of a tank of urea was no problem but in smaller cars such as the Jetta, it was just not viable. So they turned to another technology LNT ( lean NOX trap) to store the NOX emissions. The problem with this technology is it could not reduce the emissions to the levels required by increasingly stringent environmental legislation and that’s when the idea of allegedly using software that circumvents emissions testing for nitrogen oxide gases. So how did Volkswagen finally get caught – it looks like the directors of a European agency, the International Council on Clean Transportation, became suspicious when comparing the emissions of European cars with American cars. They were working with West Virginia University’s Centre for Alternative Fuels Engines and Emissions to see why the European industry could not produce cars for sale in Europe with as clean emissions as those they sold in the US. Now it looks like 11 million cars have been caught up in this scandal and there is talk of $18 billion of fines – and reports now say there is to be a criminal investigation for fraud. The funny thing about all of this, it seems that the other car manufacturers must be peeing their pants laughing as it turns out they may escape blame as they stuck to injecting urine into their convertors !


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