To reduce the emission of nitric oxide it is necessary to burn fuel in oxygen-lean conditions, but this results in lower efficiency and increased hydrocarbon emissions. These hydrocarbons must then be neutralized by conversion to carbon dioxide and water, and it is for this purpose that the noble metals, such as platinum, are used as catalysts in post-combustion catalytic converters. Theses metals are not only very expensive, they will only work properly in oxygen-lean conditions. This leaves unresolved the problem of lowered fuel efficiency. Technological research, including catalysts made of zeolites (compounds of silica and aluminium), are attempting to address the problem.
2. Catalytic converters are supposed to convert 90% of the unburnt part of unleaded petrol into safer substances yet they only work when they reach 400Ã¸C (after about 8 kilometres). Furthermore their cleaning efficiency reduces to 60% after about 50,000 kilometres. This means that they are often offering no protection from the cancer-causing aromatics contained in unleaded fuel. (In the UK, 50% of journeys are less than 8 kilometres.) < 3. A complete switch to unleaded petrol will increase carbon dioxide emissions from EU countries by 17 million tonnes a year.
The U.S Environment Protection Agency is concerned that catalytic converters are becoming a significant and growing contribution to global warming. They rearrange the nitrogen oxygen compounds into nitrous oxide, a "greenhouse" gas that is 300 times more potent than Carbon dioxide. The US Environment Agency published results of it's study in 1998 that concluded that nitrous oxides comprised 7.2% of the gasses that cause global warming and it is rapidly increasing. The agency estimates that nitrous oxide may represent about one sixth of the global warming effect resulting from gasoline use. The rise stems from the increase in miles driven by cars with catalytic converters. Other sources of nitrous oxide include nitrogen based fertiliser and manure from farm animals.
The growing production of nitrous oxide is yet another reason to move away from gasoline powered cars.