Diesel oxidation catalysts

Studies on the particulate distributions from compressed natural gas (CNG) or diesel-fuelled engines with diesel oxidation catalyst (DOC) or partial diesel particle filter (pDPF) have also been performed. The results obtained are used as data for the model, to study the particle penetration into the human respiratory tracts. As a result, the number distribution of particles in different parts of lungs can be modeled [99-101]. Understanding the particle formation and their effects and finding the methods to ehminate the formed particulates from exhaust gas contribute to a cleaner urban environment and thus to a better quality of life. 

Compliance with the EuroIII standards (2000) forced the fitting of Diesel oxidation catalysts (DOC) in the exhaust line [for the after-treatment of unburnt hydrocarbons (HC) and carbon monoxide (CO)]. Additionally, the exhaust gas recirculation (EGR) was adapted to reduce the engine-out emissions of nitrogen oxides (NOx). 

Compliance with EuroIII standards (applied in 2000) has led to the implementation of Diesel oxidation catalyst (DOC) in the exhaust line to convert the residual HC and CO released by the Diesel engine. 

The SCR technology is also considered for the control of NO emission in diesel vehicles. Here the SCR catalyst is typically placed after the diesel oxidation catalyst (DOC), which is used to oxidize CO and UHCs and to convert part of the NO to NO2. In this way, the SCR catalysts can take advantage of the fast SCR reaction to enhance significantly the de-NO efficiency at low temperature (Figure 13.4). The fast SCR reaction is based on the following stoichiometry ... 

Diesel oxidation catalysts (DOCs), which convert CO and hydrocarbons to more benign C02 and H20, and can also convert NO to N02 for possible downstream use or be used to generate higher temperatures from the... 

ESC and ETC diesel oxidation catalyst simulation results in Fig. 53 show that the N02/NOx ratio behind the DOC varies from 30% to 60% over the ESC and from 10% to 80% over the ETC for the configuration studied, with a mean value of approximately 40%. Using these simulated N02/NOx ratios behind the DOC as input for the SCR test cycle simulations, lower conversion efficiencies are obtained compared to the 50% N02 case. However, Fig. 54 and the values in Table VII indicate that there is still a significant increase in the total NOx conversion compared to the simulation without N02 in the inlet feed. This also confirms that the chosen DOC geometry and volume is quite well adapted to the specific application. 

As we proceed to the entire exhaust system scale we face the task of interfacing the DPF behavior to that of other emission control devices in the exhaust (e.g. diesel oxidation catalysts (DOC) and NOx reduction devices). An example of a coupled simulation of a DOC and a DPF in series is shown in Fig. 43. We observe how a hydrocarbon pulse injection upstream of the DOC raises the exhaust temperature and causes regeneration of the DPF. Such simulation tools are very useful for the development and optimization of postinjection strategies for DPF regeneration. 

Whilst NOx emissions from petrol vehicles can be controlled by catalytic reduction, this is not very effective under the oxygen-rich conditions of diesel combustion. A diesel oxidation catalyst (DOC) is similar to a TWC in terms of structure and configuration but is only capable of oxidation. As the exhaust gases pass through the catalyst CO, unbumt HC and volatile PM are oxidised. The conversion efficiency is a function of cell size, reactive surface, catalyst load and catalyst temperature, although emissions of CO and HC are typically reduced with an efficiency of more that 95%. 

Dyke, P.H., Sutton, M., et al. (2007) Investigations on the effect of chlorine in lubricating oil and the presence of a diesel oxidation catalyst on PCDD/F releases from an internal combustion engine. Chemosphere, 67(7) 1275-1286. 

CAES corporate average emission standard CIDDI compression ignition diesel direct injection CRDDI Common Rail diesel direct injection CVD chemical vapor deposition DOC diesel oxidation catalyst DPF diesel particulate filter... 

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.

A simpler way to achieve a considerable reduction in the mass of particulates emitted, is the use of special diesel oxidation catalysts (Section 1.4). They offer advantages over filtering devices as they are simple, cost effective and can also reduce the emissions of CO, gaseous hydrocarbons, aldehydes and polynuclear aromatic hydrocarbons. Their main disadvantage is that their particulate mass reduction efficiency is only about half that achieved by filtering devices. 

As described in Section 1.2.4.B, several technologies have been considered to reduce the amount of particulate matter in the tailpipe exhaust gas of diesel engines. Of these technologies, only the diesel oxidation catalyst has found widespread application. This technology will be described below. 

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. 

Figure 101. Operation principle of a diesel oxidation catalyst. Reprinted from ref. [13] with kind permission of Elsevier Science.

Figure 102. Emission of particulates (part) and gaseous hydrocarbons (HC) from a heavy duty diesel engine in the US transient engine test cycle, as a function of the catalyst volume (monolith catalyst with 46 cells cm" diesel oxidation catalyst formulation with platinum at a loading of 1.76 g 1 , fresh).

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.

A more important deactivation aspect is poisoning. A diesel oxidation catalyst is poisoned by the same elements that poison three-way catalysts, except for lead, which is absent from diesel fuel and the diesel fuel supply chain. Table 25 compares the amount of sulfur, phosphorus and zinc offered to a diesel catalyst and to a three-way catalyst during their lifetime. From this table, it is apparent that a diesel oxidation catalyst has to deal with a considerably higher amount of sulfur during its lifetime than is the case with a three-way catalyst. This is because the diesel fuel specification allows for a higher sulfur content than the gasoline specification. Table 26 gives an overview of the maximum sulfur content in the diesel fuel specification of some selected countries. 

Figure 108. Deactivation phenomena of diesel oxidation catalysts as a function of the catalyst temperature.

Table 25. Order of magnitude of lifetime effects comparison between a diesel oxidation catalyst and a three-way catalyst.

Figure 109. Conversion of carbon monoxide and gaseous hydrocarbons reached over a diesel oxidation catalyst at various settings of the exhaust gas temperature, for a model gas composition with and without SO2 (monolith catalyst with 62cells cm dedicated diesel washcoat formulations with platinum loading of 1.76gl" in the fresh state model gas light-off test at a space velocity of 50000Nir h model gas simulates the exhaust gas composition of an IDI passenger car diesel engine at medium load and speed).

Finally, an aged diesel oxidation catalyst always contains carbon, typically about... 

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