Selective Reduction of NOx with Hydrocarbons over Ir Catalyst Supported
on MFI-type Metallosilicates
Shigeru NOJIMA*, Kouzou IIDA and Norihisa KOBAYASHI†
Hiroshima R & D Center, Mitsubishi Heavy Industries LTD; 4-6-22 Kanonshin-machi, Nishi-ku, Hiroshima-shi 733-8553 Japan
† Power Systems Headquarters, Mitsubishi Heavy Industries LTD; 3-3-1 Minatomirai, Nishi-ku, Yokohama-shi 220-0012 Japan
Iridium catalysts supported on MFI-type metallosilicates were prepared and their catalytic activity for the selective reduction of NOx with hydrocarbons under oxygen rich condition was investigated. The as-prepared catalyst hardly reduces NOx to N2 though it showed very high activity for the hydrocarbon conversion with oxygen. After subjecting the activation treatment in 10%H2O--N2 at 700 °C, on the other handCthe activated catalyst showed very high NOx reduction activity. The activated catalyst selectively and efficiently reduced NOx to N2 with olefinic and aromatic hydrocarbons at 300--500 °C in the stoichiometric to lean conditions.
The characterization by XRD and EXAFS revealed that the Ir species existed in the form of oxide (IrO2) and metal in the as-prepared and activated catalysts, respectively. In addition, H2-TPR measurement showed that the activated catalyst had active oxygen species on the Ir metal surface which was more reactive (reducible) than oxygen species on IrO2 and bulky Ir metal. It was inferred that the high NOx reduction activity of the activated catalyst resulted from the presence of the active oxygen species which might react with hydrocarbons to form reactive oxygenated intermediates.
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