Soluble organic fraction oxidation

Figure 6 Effect of platinum content (g ft ) in diesel oxidation catalysts on tailpipe emissions in the ECE R49 test carbon monoxide (a), hydrocarbon (b), soluble organic fraction (c), sulfur trioxide (d), and particulates (e). The fuel sulfur content was 500 ppm.

Since about 1991, diesel oxidation catalysts have been generally applied to passenger cars in the European Union and to some medium and heavy duty trucks in the USA. Their principle of operation is shown in Fig. 101. The amount of carbon monoxide, hydrocarbons and aldehydes is reduced by oxidation of these components to carbon dioxide and water. The mass of particulate matter emitted is reduced by the oxidation of the liquid hydrocarbons, which are adsorbed on the particulates. These liquid hydrocarbons originate both from the fuel and the lubricating oil, and are commonly denoted as the soluble organic fraction (SOF). The adsorbed polynuclear aromatic hydrocarbons are also removed by oxidation. 

The washcoat composition is quite different to that used for three-way catalysts. The washcoat oxides used are chosen so as to ensure a minimal adsorption both of the soluble organic fraction and of the sulfur oxides SO2 and SO3. The properties of three different oxides in these respects are shown in Figs. 103 and 104. The composition of the washcoat is, together with the choice of precious metal formulation. 

Figure 103. The amount of soluble organic fraction (SOF) adsorbed on an aged diesel oxidation catalyst, as a function of the washcoat formulation (monolith catalyst with 62cellscm", dedicated diesel washcoat formulations with platinum at a loading of l.76gl diesel engine bench aging for 50 h diesel fuel containing 0.15 wt. % sulfur). Reprinted with permission from ref. (68], C 1991 Society of Automotive Engineers, Inc.

The application of exhaust gas aftertreatment methods is currently mainly focussing on reducing the soluble organic fraction of particulates by means of oxidation catalysts. The exhaust gas aftertreatraent strategies that may be followed in the future are ... 

A laboratory method simulating the adsorption and subsequent catalytic combustion of the soluble organic fraction (SOF) on these newly developed diesel oxidation catalysts, is also described in this paper. This technique, based on thermal chromatography, allows characterization of the initiation of catalytic oxidation and analysis of the products giving insight into the mechanism of operation. 

Particulate matter consists basically of three fractions, the relative amounts of which depend strongly on fuel quality and engine conditions. The first is dry soot, which is a low-hydrogen-content, polyaromatic species that, regardless of the catalyst, cannot be oxidized under normal diesel exhaust conditions. The second component is the soluble organic fraction (SOF), derived from unbumt fuel and lube oil, and consisting mostly of HCs in the C20-C28 range. The SOF... 

A large variation of these parameters can be found in the literature and Fig. 11.2 shows how the values are distributed within the studies analysed in this work. The use of synthetic soot is preferred over real soot due to a better characterization and to a better reproducibility of results the specific composition of real soot depends upon several factors such as engine type, age, driving condition, speed, fuel, fuel and lube oil additives, and temperature. Moreover real soot is easier to oxidize than synthetic soot due to the soluble organic fraction that burns at a lower temperature, thus affecting in a conservative manner the results obtained. 

In addition to the dependence of sorption on the organic fraction of the sorbent, and the KQw of the sorbate, Chiou et al. (13) cite the following observations as support for the hypothesis that the sorptive mechanism is hydrophobic partitioning into the organic (humic) fraction of the sediments (1) the linearity of the isotherms as the concentration approaches solubility, (2) the small effect of temperature on sorption, and (3) the lack of competition between sorbates for the sorbent. These arguments also illustrate the applicability of the approach for modeling sorption on hydro-phobic compounds an approach which has been criticized when used in the context of adsorption of trace metals onto oxides (17). 

Chemical oxidation reactions and radical-induced hydrophobic-to-hydrophilic aging processes tend to increase the water solubility of OAs and, therefore, are thought to enhance the activity of atmospheric OAs as cloud condensation nuclei (CCN). As discussed by Gysel et al. (2004), at 75-90% of relative humidity (RH) the inorganic fraction dominates the water uptake (59-80%). Nevertheless, under the same conditions of RH, between 20% and 40% of total particulate water is associated with water-soluble organic matter. More data concerning the multiphase aerosol and cloud processes, as well as the chemical reactivity of carbonaceous aerosol components, have been compiled in the reviews of Jacobson et al. (2000), Kanakidou et al. (2005), and Poschl (2005) (and references therein). 

In both cases, however, the use of an oxidation catalyst for the conversion of the gaseous emissions HC and CO (as well as of the organic soluble particulate fraction) will remain indispensable. 

Reduction occurs less frequently in the organism than oxidation. Aromatic nitroderivatives (1) are reduced through hydroxylamine (2) to amines (3) by action of nitroreductases occurring in the microsomal and soluble fraction of the liver cells... 

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