Ag Cluster Catalyst-Research of Reaction Mechanism and Development of Diesel deNOx Catalyst

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Reduction of NOx from diesel engine cars is serious problem from environmental points of view. We have found that Ag duster having the Size of nano to sub-nano scale is very effective for the selective reduction of NOx by unburned hydrocarbons RIC-SCR) in the excess of oxygen. The aim of this project is to reveal the dynamics and the reaction mechanism of HC-SCR over Ag duster by using in-situ spectroscopy and DET calculations in order to guide the design of practical catalysts. The following results have been obtained in the term of the project.1.Reactivity and reaction mechanism of Ag duster : It was hind by ESR spectrometer that the contact of gaseous oxygen and Ag duster formed by a slight reduction of Ag-alumina by hydrogen results in the formation of super-oxide species. The important role of Ag duster on the activation of oxygen was clarified. Furthermore, the role of hydrogen on the activation of oxygen and the formation of Ag cluster was suggested.2. Evaluation of reaction mechanism by DFP calculation: Formation of Ag duster by hydrogen and activation of oxygen on its surface was evaluated by DFP calculation. Ag 4-membered cluster (Ag_4^<2+>) is the most stable in zeolite cage, which is in harmony with the experimental results by EKAFS. The contact of hydrogenated Ag duster (H-H-Ag_4^<2+>) with oxygen results in the formation of hydrogen peroxide. The experimental results can be rationalized by this model, because the hydrogen peroxide is active for both oxidation of hydrocarbons ns to oxygenated species and NO to NO23. SOx tolerance of Ag duster in the presence of hydrogen: High SOx tolerance of Ag duster was achieved through optimization of the kind of reductants, temperature, and co-feeding of hydrogen. The high SIX tolerance is caused by migration of SOx from Ag surface to gas phase or alumina support.