Environmental catalysts atmospheric pollution

In a second example, we examine reactions that relate to the Ostwald oxidation process, where NH3 is converted to NO with high selectivity. The reaction is typically run at high temperatures of around 1100 K over Pt/Rh alloy catalysts. The NO that forms is subsequently converted into nitric acid via a series of consecutive reaction steps. At lower temperatures, ammonia reacts to form Ng and NgO instead. The low-temperature conversion of ammonia to N2 would be much more desirable in that it would lower energy costs and, in addition, replace NO, an atmospheric pollutant, with N2, which is environmentally benign. We will describe here the low-temperature catalytic conversion of ammonia to form N2. For a review of high-temperature oxidation, in which coupling with gas-phase radical chemistry plays an important role, we refer to Ref. [50]. 

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. 

Ethene and propene are produced as bulk feedstocks for the chemical (polymer) industry and therefore their purities are important parameters. In particular, H2S and COS are compounds which may not only cause corrosion problems in processing equipment, but also may have detrimental effects on the catalysts in use. Eurthermore, air pollution regulations issued by, among others, the US Environmental Protection Agency (EPA) require that most of the sulfur gases should be removed in order to minimize Sulfur emissions into the atmosphere. Therefore, these compounds have to be determined to the ppb level. 

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. 

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