Soot oxidation oxygen mobility

Contact between the soot particles and the active phase is achieved through the mobility of alkali-metal ions. These ions act as oxygen transfer agents, which convert soot to carbon dioxide, before themselves being re-oxidised by gas phase oxidants (oxygen, water and carbon dioxide). 

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. 

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... 

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. 

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. 

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. 

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. 

---