Increasing use of catalytic converters in cars to control air pollution and smog.
Pollution control technology in the automotive industry is caught in something of a double bind. The thrust is towards a "lean-burn" combustion engine, that is, one which is oxygen-rich and fuel-lean. Under these conditions the combustion process is very efficient; fuels burn more cleanly, and there is much less emission of pollutants such as carbon monoxide and hydrocarbons. However, this very same efficiency generates by-products of nitric oxide and carbon dioxide. Nitric oxide is formed by the oxidation of ambient nitrogen at high temperatures. It is a major contributor to acid rain, and react with the ozone layer leading to ozone depletion and increased exposure to harmful UV light. Carbon dioxide is a gas of global warming.
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.
Scientists are concerned that catalytic converters may be producing hydrogen sulphide, a highly toxic substance which blocks oxygen absorption by the blood. Laboratory studies also show that catalyst-equipped cars can reach 20 to 30 times the emission level of nitrous oxide than for other cars.