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

Abstract

A series of silver-doped cerium zirconium oxide(Ag-Ce _xZr) samples was synthesized successfully for selective catalytic reduction of nitric oxide(NO) with hydrogen and propene(H₂/C₃H₆-SCR) under excess oxygen condition. The catalytic activity test proved that Ag-Ce_0.4Zr exhibited the best C₃H₆-SCR activity. Hydrogen(H₂) 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 H₂ temperature programmed reduction (H₂-TPR). In situ FTIR results demonstrated that various types of nitrates and chelating nitrite were generated on Ag-Ce _xZr after introduction of NO. Besides, adding H₂ could increase the concentration of bidentate nitrate and chelated bidentate nitrate dramatically, especially for Ag-Ce_0.4Zr catalyst. Transient reaction between pre-adsorbing NO and C₃H₆/C₃H₆+H₂ illuminated that the most active intermediate was chelating nitrite,which was promoted significantly by H₂ participation. Furthermore, adding H₂ improved the formation of organo-nitro(R-NO₂), which was the key intermediate in C₃H₆-SCR. The reaction mechanism over Ag-Ce _xZr catalysts was proposed at 200 °C as follows: nitric oxide(NO)+propene(C₃H₆)+hydrogen(H₂)+oxygen(O₂)→chelating nitrite(NO₂ ⁻)+acrylate-type species(C _xH _yO _z)→organo-nitro(R-NO₂)→isocyanate(—NCO)+cyanide(—CN)→nitrogen(N₂).

Keywords

NO / H₂/C₃H₆-SCR / In situ Fourier transform infrared(FTIR) spectrometry

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