Soot oxidation mobile catalysts

A number of ways was employed to make contact between soot and catalyst. It was shown tliat the contact between soot and catalyst is of prime importance for tlie reactivity of the catalyzed soot oxidation. Several catalysts were found to be unable enhancing the soot oxidation rate when the contact between soot and catalyst is poor. This explains why catalyst coated particulate filters have been found quite inactive under practical conditions. For catalytic systems wliich seem to be able to decrease soot combustion temperatures a possible explanation can be probably found in catalyst mobility. 

Numerous soot oxidation catalysts have been reported since the 1980s, because soot oxidation is fundamentally a simple complete oxidation reaction (carbonaceous compounds CO2 + H2O), so that sophisticated catalysts with high selectivity are not required. However, there is a critical problem in establishing contact and interaction, directly or indirectly, between the reactant (soot) and the catalyst, both of which are solid materials. Therefore, soot oxidation catalysts reported to date can be classified according to the assumed working mechanism that solves this problem. In this review the authors classify the catalysts into the four types shown in Fig. 2.5, based on the mediator for the oxidation reaction that connects the active sites of catalyst and soot surfaces mobile catalysts, mobile oxygen catalysts, NO2 mediating... 

Cerium oxide based catalysts have been widely studied during the last two decades. Main catalytic application was the elimination of automotive exhaust emissions (1,2). The catalytic properties of this oxide has often been related with the mobility of oxygen vacancies in the solid (3,4) and hence with its capacity to release stored oxygen under reducing conditions tests (5,6).Moreover, A.F. Ahlstrom and C.U.I. Odenbrand (7) reported the deactivation by sulphur dioxide of supported copper oxide during the oxidation of soot. 

It was observed that the molten salt catalysts are highly active compared to the solid single oxide catalysts, probably as a result of the increased contact area due to wetting of the soot by the mobile catalyst. The oxidation rate is strongly increased by the presence of NO in the gas phase. 

This chapter deals with the mechanism of soot oxidation for the regeneration of particulate filters. Nature of catalysts and effect of process parameters (temperature, NO2 partial pressure, intimacy of contact between soot and catalyst,. ..) are reviewed. The role of oxygen mobility and of nature of oxygen species are also discussed. 

However, the reaction mechanism proposed by Shimizu et al. for Ag/Ce02 is that Ag does not directly oxidize soot, but promotes the reduction of Ce " " to Ce " ", on which adsorbed oxygen is activated and oxidizes the soot. Clarification of the detailed mechanism of soot oxidation on Ce02, including the role of metal components, will make a significant contribution to the development of more active mobile oxygen type catalysts. 

Krishna et al. similarly reported that an unknown species was produced from K added to Pt/Al203, which was mobile on the catalyst surface and promoted soot oxidation, although the species could not be identified. ... 

Mazda has developed and commercialized a catalyst with Pt supported on a mixed oxide of Ce02 with Zr or Pr for light-duty diesel engine cars. Isotopic kinetic experiments using 02 revealed that formation of the mixed oxide increases the mobility of lattice oxygen and thereby soot oxidation. 

Evaluating the experimental results, the high activity of metal (oxy)chloride based catalysts in the oxidation of soot is induced by a high mobility or volatility of the metal chlorides. Chlorine modification of transition metal oxides might also induce beneficial oxygen activation properties and/or transfer of activated ojgfgen to the soot surface. 

All pressure drop curves in Figure 6 show a maximum except for the curve for the uncoated cordierite filter with 100 ppm copper (run 14). This maximum in pressure drop suggests that there is a build-up of activity on the filter. Two explanations can be given for this increased activity. A possible explanation is the formation of small copper particles that are stabilized by the soot. These particles have a high mobility and can be active [11]. A second explanation is also based on the formation of small copper particles. These particles can act as a catalyst for the oxidation of NO into NO2. The formed NO2 subsequently oxidizes the soot, forming NO. This catalytic cycle with NO as a key intermediate is described for platinum by Hawker [13] and for copper, chromium and molybdenum by Mul [10]. Although complete oxidation of diesel soot with only an NO-oxidation catalyst such as platinum may prove not to be possible, the cataljdic cycle with NO as intermediate should be considered crucial for every catalytic process for oxidation of diesel soot. 

This catalyst may also be referred to as the molten salt catalyst, as referred to by Idles et al, according to the catalyst materials of this type reported to date. It is well known that some of the transition metal oxides, alkaline, and alkaline-earth metal oxides promote carbon oxidation. These oxides are solid and immobile at room temperature but become mobile on the surfaces of soot and support materials on a micrometer scale above certain temperatures, the melting point, or so-called Tamman temperature. In such a mobile state, the catalyst can maintain contact with the soot while the soot surfaces are continually excavated by oxidation. 

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