Manganese oxides and iron-manganese oxides supported on TiO2 were prepared by the sol-gel method and used for low-temperature selective catalytic reduction (SCR) of NO with NH3. Base on the previous study, Mn(0.4)/ TiO2 and Fe(0.1)-Mn(0.4)/TiO2 were then selected to carry out the in situ diffuse reflectance infrared transform spectroscopy (DRIFT) investigation for revealing the reaction mechanism. The DRIFT spectroscopy for the adsorption of NH3 indicated the presence of coordinated NH3 and NH4+ on both of the two catalysts. When NO was introduced, the coordinated NH3 on the catalyst surface was consumed rapidly, indicating these species could react with NO effectively. When NH3 was introduced into the sample preadsorbed with NO + O2, SCR reaction would not proceed on Mn(0.4)/TiO2. However, for Fe(0.1)-Mn(0.4)/ TiO2 the bands due to coordinated NH3 on Fe2O3 were formed. Simultaneously, the bidentate nitrates were transformed to monodentate nitrates and NH4+ was detected. And NO2 from the oxidation of NO on catalyst could react with NH4+ leading to the reduction of NO. Therefore, it was suggested that the SCR reaction on Fe(0.1)-Mn(0.4)/TiO2 could also take place in a different way from the reactions on Mn(0.4)/TiO2 proposed by other researchers. Furthermore, the SCR reaction steps for these two kinds of catalysts were proposed.

SO2 would deactivate the low-temperature SCR (selective catalytic reduction) catalysts and reduce NO removal. In this study, Fe(0.1)−Mn(0.4)/TiO2 prepared by sol−gel method was selected to carry out the in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) investigation for revealing the mechanism of the SO2 effect on the SCR reaction. The DRIFT spectroscopy showed that SO2 could be adsorbed on the surface of the catalyst as the bidentate mononuclear sulfate. This type of sulfate would retard the formation of NO complex on the surface of catalyst, resulting in the decrease of NO adsorption. For NH3 adsorption, the adsorption of SO2 had little effect on the coordinated NH3, but would increase the amount of NH4 + because of the formation of new Brønsted acid sites. Therefore, besides the deposition of ammonium sulfates, the competitive adsorption between SO2 and NO on the active sites of the catalysts also contributed to the poisoning effect of SO2 on the SCR reaction. When sulfate was formed on the catalyst, much less NO could be adsorbed and take part in the SCR reaction.

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Zr is added to the Fe−Mn/Ti catalyst to increase NO conversion and improve SO 2 tolerance. It is found that 0.03 is the optimal ratio for Zr/(Ti + Zr). With this ratio, the NO conversion below 150 °C increases, and the SO 2 poisoning is alleviated, while the further increase of Zr does not have a positive effect on NO conversion and SO 2 tolerance. With Zr additive, more manganese oxides are reduced in the form of MnO 2 and Mn 2 O 3 at lower temperature in H 2 -TPR, and the total amount of H 2 consumption rises, indicating better redox properties. It leads to the increase of NO complexes on the catalysts. Despite the small decrease of NH 3 adsorption, the reaction via the L−H way is promoted and NO conversion increases. Furthermore, more nitrates and NO 2 are formed in the reaction with SO 2 on Fe−Mn/Ti−Zr(0.03) compared to Fe−Mn/Ti, so the L−H reaction way is less inhibited by SO 2 with Zr additive, and the SO 2 tolerance of this catalyst is also improved.

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