Oxidative Decomposition of Dioxins and VOCs

Dioxins are a family of heterocyclic hydrocarbons, mostly poisonous chemical by-products of the manufacture of certain herbicides and bactericides. The most toxic are polychlorinated dibenzo-p-dioxins (CDDs) and dibenzo-furans (CDFs, also called dioxins) [514,515]. Amongst the most dangerous are the isomers of the 2,3,7,8-tetrachlorodibenzo-para-dioxin (TCDD) and 2,3,7,8-tetrachlorodibenzo-furan (TCDF). The former occurs in small amounts in some herbicides and defoliants, including the so-called Agent Orange (a highly toxic herbicide sprayed as a defoliant in chemical warfare) [466]. 

Consequently, the control of dioxin emissions from incinerators is one of the most urgent needs in environmental protection today [466, 516]. There are several treatment techniques presently in use, including the raising of the waste gas temperature to cause catalytic oxidative decomposition [516]. However, when the initial investment and mechanical complexity is taken into account, this method is not practicable for small-scale incinerators. A novel solution for this task would be to decompose dioxin and its derivatives at the dust filter using catal3dic oxidation, preferably below 473 K. This particular temperature is relevant since the waste gases are usually reduced to this temperature at the dust filter after efficient heat recovery. 

Haruta and co-workers have developed multi-component catalysts containing various supported noble metal systems [516]. These catalysts were found to interact and work in S3mergy for several catalytic reactions. Although Ir/La203 exhibits low activity in the decomposition of dioxin derivatives. 

Incinerator exhaust gases do, however, contain a variety of pollutants -not only dioxin derivatives but also other compounds such as odours, volatile organic compounds (VOCs) and the reaction intermediates of dioxins [516], VOCs are a major contributor to air pollution because of their toxic and malodorous nature and their contribution to ozone and smog formation [131], They are emitted from a wide range of industrial processes and transportation activities [131,162,519-521], Some representative VOCs are methanol, ethanol, 2-propanol, acetone and toluene. 

Catalytic combustion is the most promising technology for the destruction of VOCs [131,162,519-522], This approach is preferred to a thermal one due to the lower temperature required and its greater selectivity, which implies a considerable saving of energy. Other advantages are its definitive character and the fact that it can operate with dilute effluent streams ( 1% VOCs) [131,162,519-522], For this reason, supported noble metals (Pt, Pd, Rh) or metal oxides (Cu, Cr, Mn) were the conventional catalysts used [523-529], 

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