Atmospheric pollution catalytic reduction

Reduction of NO to N20 or N2 is important in biological denitrification where anaerobic organisms are involved and model studies using copper catalysts have been studied.42 Much study on other catalytic reductions of NO (and of N02) has been made in connection with atmospheric pollution.43... 

First NO is formed and then it is oxidized in the atmosphere to NOj. Since in combustion, the origin of nitrogen is not only from N-rich fuel but also from air supphed for oxidation, in the elimination of NO, postcombustion methods are important. So far the most effective technique has been selective catalytic reduction of NO . on various catalysts. When the activated carbons are used as removal media, the elimination process includes also adsorption combined either with oxidation or reduction. Oxidation usually leads to the formation of nitric acid whereas Nj is the product of NO, reduction. As in the cases of other pollutants addressed in this review, for NO, removal either unmodified or impregnated (caustics, catalytic metals) activated carbons have been used [101-114]. 

Flue gas purification (air pollution control, APC) refers to technologies used to condition a power plant s emissions after the combustion of fuel but before the release of gaseous and suspended particulate combustion by-products into the atmosphere. Each device or system corresponds to a given pollutant or category of pollutants to be removed from the flue gas stream. Reducing chemicals such as ammonia or urea, along with catalysts in the case of selective catalytic reduction (SCR) systems, are used to treat the exhaust to remove nitrogen oxides. 

Measures taken to control atmospheric pollution include (a) scrubbing industrial waste gases to remove SO2, and (b) reduction of NO in motor vehicle emissions. Explain how these are achieved and write balanced equations for relevant reactions. Are they catalytic processes ... 

Reduction of metal oxides with hydrogen is of interest in the metals refining industry (94,95) (see Metallurgy). Hydrogen is also used to reduce sulfites to sulfides in one step in the removal of SO2 pollutants (see Airpollution) (96). Hydrogen reacts directiy with SO2 under catalytic conditions to produce elemental sulfur and H2S (97—98). Under certain conditions, hydrogen reacts with nitric oxide, an atmospheric poUutant and contributor to photochemical smog, to produce N2 ... 

A reduction of the present levels of CO2 in the atmosphere has become a subject of increasing concern as regard the environmental pollution problem. Beside other choices currently under consideration its catalytic transformation at point sources, by hydrogenation into more valuable products (e.g. methanol), has been found particularly attractive. To this end, both commercial and novel (poison-resistant) types, among which supported catalysts based on Pd and other noble metals are gaining acceptance [1] and are now under scrutiny. 

Sulphites rarely occur in natural waters. They are chiefly of artificial origin (wastewaters from the production of sulphite cellulose and thermal processing of coal). They are washed out into atmospheric waters from urban and industrial air pollutants. In waters, sulphites are slowly oxidized into sulphates, consuming dissolved oxygen. Chemical oxidation is accelerated by catalytic effects of various metals, particularly the Co(II) compounds. In water treatment, sulphites are used for dechlorination, removal of oxygen from feed waters for steam boilers, and in the technology of wastewaters for reduction of Cr(VI) to Cr(III). 

The release of VOCs into the environment has widespread environmental imph-cations. Pollution by VOCs has been linked to the increase in photochemical smog and ozone depletion. In addition, many VOCs are themselves toxic and/or carcinogenic. The US Clean Air Act of 1990 was one of the first measures to call for a 90% reduction in the emissions of 189 toxic chemicals, with 70% of these classed as VOCs, by 1998. Hence, in recent years, the development of effective technologies for the removal of VOCs from the atmosphere has increased in importance with the introduction of legislation to control their release. Various methods have been proposed, and one of the best is heterogeneous catalytic oxidation. This has the advantage over the more common original thermal oxidation process, since it requires less supplementary fuel and is therefore a less expensive process. However, the characteristics of the catalyst selected for this process are of vital importance for successful operation, and potential problems such as lifetime and deactivation must be solved if catalytic oxidation is to be employed universally. Catalysts currently in use include noble metals, notably platinum and palladium, and those based on metal oxides, however, irrespective of the type of catalyst, the most important characteristics are activity and selectivity for total oxidation. 

---