Reaction Mechanism of H2-Assisted C3H6-SCR over Ag-CexZr Catalyst as Investigated by In situ FTIR

A series of silver-doped cerium zirconium oxide(Ag-CexZr) samples was synthesized successfully for selective catalytic reduction of nitric oxide(NO) with hydrogen and propene(H-2/C3H6-SCR) under excess oxygen condition. The catalytic activity test proved that Ag-Ce0.4Zr exhibited the best C3H6-SCR activity. Hydrogen(H-2) significantly enhanced NO conversion and widened the temperature window. Multi-technology characterizations were conducted to ascertain the properties of fabricated catalysts including X-ray diffraction(XRD), Fourier transform infrared spectrometry(FTIR), scanning electron microscopy(SEM) and H-2 temperature programmed reduction (H-2-TPR). In situ FTIR results demonstrated that various types of nitrates and chelating nitrite were generated on Ag-CexZr after introduction of NO. Besides, adding H-2 could increase the concentration of bidentate nitrate and chelated bidentate nitrate dramatically, especially for Ag-Ce0.4Zr catalyst. Transient reaction between pre-adsorbing NO and C3H6/C3H6+H-2 illuminated that the most active intermediate was chelating nitrite,which was promoted significantly by H-2 participation. Furthermore, adding H-2 improved the formation of organo-nitro(R-NO2), which was the key intermediate in C3H6-SCR. The reaction mechanism over Ag-CexZr catalysts was proposed at 200 degrees C as follows: nitric oxide(NO)+propene(C3H6)+hydrogen(H-2)+oxygen(O-2)-> chelating nitrite(NO2-)+acrylate-type species(CxHyOz)-> organo-nitro(R-NO2)-> isocyanate(-NCO)+cyanide(-CN)-> nitrogen(N-2).