Reaction Mechanism of H 2 -Assisted C 3 H 6 -SCR over Ag-Ce x Zr Catalyst as Investigated by In situ FTIR
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doi: 10.1007/s40242-020-0026-1
Article
Reaction Mechanism of H2-Assisted C3H6-SCR over Ag-CexZr Catalyst as Investigated by In situ FTIR DUAN Jun, ZHAO Ling *, GAO Shengjun and ZHANG Yu School of Ecology and Environment, Inner Mongolia University, Hohhot 010021, P. R. China Abstract 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(H2/C3H6-SCR) under excess oxygen condition. The catalytic activity test proved that Ag-Ce0.4Zr exhibited the best C 3H6-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 (H2-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+H2 illuminated that the most active intermediate was chelating nitrite，which was promoted significantly by H2 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 °C as follows: nitric oxide(NO)+propene(C 3H6)+hydrogen(H2)+oxygen(O2)→chelating nitrite(NO 2–)+acrylate-type species(CxHyOz)→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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Introduction
Compared to conventional stoichiometric engines, the competitiveness of lean-burn engines is increasing on account of its low fuel consumption under excess oxygen[1] and its advantages about carbon dioxide(CO2) emissions[2]. Nevertheless, a major issue of nitrogen oxides(NOx) release is that the three-way catalyst cannot reduce the NOx under excess oxygen[1] at low temperature[3]. In response to increasingly stringent emissions standards, several aftertreatment technologies, such as the selective catalytic reduction of NOx by ammonia (NH3-SCR) and lean-NOx traps(LNT) have been measured. However, these techniques have some notable drawbacks[4,5]. Apart from this, the selective catalytic reduction of NOx by hydrocarbon(HC-SCR), hydrogen(H2-SCR) or carbon monoxide(CO-SCR) technologies, which use HC, H2 or CO as reducing agents attract much attention, since these technologies would exclude the shortcomings related to the storage and slip of NH3[6]. More critically, in the absence of exogenous reducing agents, the use of HC, H2 or CO, which directly present in the exhaust gas, has become more and more competitive[2].
Hitherto, HC-SCR has been intensively st
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