| one of the main atmospheric pollutants, can cause great damages to human beings. In more than ten years, the effective reduction of NOx in exhaust gases from diesel and lean-burn engines has become one of the greatest challenges in the fields of global environmental protection and catalysis. To improve the selectivity at low temperature in the selective catalytic reduction (SCR) of NOx, the performance of the H-ZSM-5 based catalysts prepared by some metal ions exchange (M-H-ZSM-5) and by zirconium impregnation (Zr/H-ZSM-5) was investigated for the SCR of NOx in the lean-burn conditions by using acetylene as reducing agent. The results were obtained as follows:When C2H2 was used as reducing agent, the activity of the M-H-ZSM-5 catalysts displayed the following orders: In-H-ZSM-5 , Mn-H-ZSM-5, H-ZSM-5 , Co-H-ZSM-5 , Ga-H-ZSM-5 , Fe-H-ZSM-5 , Zn-H-ZSM-5 , Cr-H-ZSM-5 , Cu-H-ZSM-5 , Ni-H-ZSM-5. This result indicates that the exchanged metal plays an important role for the performance of the H-ZSM-5 zeolite in the C2H2-SCR of NO. In the reaction condition of 325 ℃, 800 ppm C2H2, 1600 ppm NO, 9.95% O2 (He as balance gas) and GHSV=17000 h-1, the conversion of NOx (to N2) over the In-H-ZSM-5 catalyst reached to 76%, however, NO was hardly reduced to N2 on In-Na-ZSM-5 catalyst prepared form Na-ZSM-5 as the starting material. The results indicate that there is a synergjsm of indium species with the H-1in zeolite for catalyzing the SCR of NO reaction in the In-H-ZSM-5 catalyst when acetylene was used as reducing agent.On the 2%Zr/H-ZSM-5 catalyst that has the optimal zirconia loading, 89% of NO conversion to N2 was obtained at 350℃ using acetylene as reducing agent. However, when C3H6 or CH4 was used as reducing agent in the condition of containing the same amount of "C" as C2H2 over the same catalyst, only 27% or 45% of the maximum NO conversion to N2 was observed, respectively. The results indicate that C2H2 is much more effective than C3H6 and CH4 as a reductant. Moreover, compared with the literatures in which almost the same level of NO removal was achieved using C3H6 as reductant, only ca. 1/3 of the reductant in carbon moles was used in the case of C2H2-SCR, which means that the selectivity of C2H2-SCR we studied is about 3 times as high as that in the literatures. In addition, the time on stream test for 30h at 350 °C indicates that the 2%Zr/H-ZSM-5 catalyst has a very stable activity using C2H2 as reducing agent in the same normal reaction condition as mentioned above, and it also can resist to the temperature fluctuation between high (≤650℃) and low (200℃) temperatures.Compared with C2H2, when C3H6 was used as reducing agent, a rapid deactivation was observed for the H-ZSM-5 and Zr/H-ZSM-5 catalysts. The results of adsorption and desorptionof CsHeand C2H2 on these two catalysts show that the saturated adsorption amounts of C3H6 at 80 °C were 1.85 and 1.75 mmol-g"1, respectively. However, at the same condition the saturated adsorption amounts of CyHfewere only 0.08 and 0.11 rnmol-g'1, i.e. the saturated adsorption amount of C3H6 is 15 to 20 times as high as that of C2H2. The results gave a good explanation to the experimental facts that the catalysts fast lost their activity in the C3H6-SCR, while they were rather stable in the C2H2-SCR.By the characterization of XRD and UV-Vis, it is found that the ion-state zirconuim and the clustered zirconia in the Zr/H-ZSM-5 catalysts are the active species for C2H2-SCR of NO. These two zirconuim species accelerate the oxidation of NO to NO2 (the rate-determining step of the C2H2-SCR) by a synergistic effect with the protons in zeolites. Moreover, the results of the co-adsorption of NO with O2, and of NO2 adsorption and desorption on a series of catalysts show that the incorporation of the zirconuim to H-ZSM-5 zeolite promotes the strong adsorption of NO2 (corresponding to the desorbed NO2 at ca. 340 °C). This may be another reason of zirconium promoting effect for the C2H2-SCR of NO.
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