Diesel makes a great fuel, but emissions can be a real problem, and treating the pollution caused by diesel fumes is difficult. Now, however, scientists have developed novel catalysts for the treatment of diesel exhaust. The new catalysts are cheaper to produce, and could potentially reduce hazardous emissions by promoting the oxidation of nitrogen oxide, a common diesel exhaust byproduct.
Diesel Combustion and Emissions of Pollutants
Combustion engines often employ diesel as a fuel, due to its higher efficiency in comparison to gasoline. Diesel’s exhaust emissions, however, contain hazardous chemicals and pollute the atmosphere. For example, if the exhaust contains an excess of oxygen (O2, lean exhaust), a reaction with atmospheric nitrogen (N2) can take place. As a result of this reaction, nitrogen oxides NO and NO2 may form. Both of these oxides are acidic and can contribute to acid rain; moreover, they can cause irritations to the human respiratory system.
The emissions can also contain black soot (carbon particles), due to the incomplete combustion of the fuel hydrocarbons. Soot in the atmosphere is harmful to human health and dangerous for the environment.
Treating Diesel Emissions
To avoid the emissions of these pollutants into the environment, it is necessary to treat the exhaust gases.
In the cases of nitrogen oxides, the treatment consists of converting the nitrogen oxides into molecular nitrogen.
This conversion is much more effective with NO2 than with NO, and converting more NO2 can also reduce the emission of black soot; in fact it can react with it, according to the following reactions:
2NO2 + C → CO2 + 2NO NO2 + C → CO + NO
For these reasons, oxidizing NO into NO2 in the diesel engine exhaust is an important step in the emissions treatment process.
Generally, a catalyst is employed to perform the NO>>NO2 conversion; at present, the most effective catalyst is based on platinum (Pt). The problem with this element, however, is that it is quite expensive (it costs about 1500 $/oz) and is not so commonly found.
Diesel Fuel Oxidization: Alternative Catalyst
Scientists have been working to develop alternative catalysts, made with cheaper and more available elements, but have been unsuccessful until recently.
Scientists at the University of Texas at Dallas (Departments of Materials Science and Engineering and Physics) were joined by researchers from the company Nanostellar Inc. and from the Universities of Kentucky (US), Huazong (China) and Seoul (Republic of Korea). They discovered a very successful catalyst that is based on a manganese (Mn) oxide in a form called mullite.
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