Selective catalytic reduction of nitrogen oxides with NH3 from diesel engine over VOx/TiO2 catalysts

Abstract

Nitrogen oxides (NOx) are pollutants promoting the photochemical smog, acid rain, ozone depletion and greenhouse effect. NOx are emitted from cars, trucks and buses, power plants, and off-road equipment. In addition, emission of N2O from mobile and off-road engine is now being currently regulated because of its 298 times larger greenhouse effect than CO2, thereby implying that N2O formation from the exhaust gas after-treatment system should be suppressed. Selective catalytic reduction (SCR) using vanadium supported TiO2 catalysts applied to reduce the emission of NOx from engines has been considered to be major source for N2O emission in the system. Recently, various promoters for commercial SCR catalysts are used to improve DeNOx activity at low temperature. Finding the optimum condition was aimed by changing promoters (W, Ce, Zr and Mn) in VOx/TiO2 catalyst, not only to improve SCR reactivity, but also to reduce N2O formation at high temperature. In addition, the order of impregnation between promoters and vanadium precursor on TiO2 support was changed to observe its effect on activity and N2O selectivity. It was found that W and Ce added VOx/TiO2 catalysts showed the most active DeNOx properties at low temperature. Additionally, the difference in the order of impregnation had an influence on the SCR activity. Advanced low temperature activity of the vanadium firstly added catalysts (W or Ce/V/TiO2) was attributed to the formation of more polymerized VOx on the sample. Based on the results described above, W and Ce were chosen as good promoters to improve selective catalytic reduction activity for VOx/TiO2 catalysts. Therefore, the optimum ratio and loading of W and Ce on VOx/TiO2 catalyst were investigated in order to improve SCR reactivity in low temperature region and to minimize N2O production in high temperature region. In addition, the order of impregnation between W and Ce precursors on VOx/TiO2 catalyst was changed during the preparation while observing its effect on SCR activity and N2 selectivity. Furthermore, it was found that W and Ce overloaded VOx/TiO2 catalyst such as W/Ce/V/TiO2 (15:15:1 wt%) showed the most remarkable DeNOx properties over the wide temperature region. Additionally, this catalyst significantly suppressed N2O formation during SCR reaction, especially at 350 – 400 oC. According to the characterization results, it was found that such promoted activity was originated from the improved reducibility and morphology of W and Ce species on VOx/TiO2 catalyst when they are incorporated together at high loading. Secondly, it was demonstrated that vanadium catalyst supported on microporous TiO2 obtained from the hydrothermal synthesis of anatase TiO2 in the presence of LiOH suppressed significantly N2O emission compared to conventional VOx/TiO2 catalysts. 5 wt% VOx/TiO2 catalysts supported on two types of TiO2 having distinctive pore structure, mesopore (DT-51) and micropore (microporous TiO2

micro) were applied to selective catalytic reduction of NOx with NH3 to investigate the effect of pore structure of TiO2 on sulfur poisoning. During the SCR reaction in the presence SO2 for 12 h, 5 wt% VT (DT-51) showed more drastic decrease in activity than 5 wt% VT (micro). Larger amount of SO2 was desorbed over the post-reaction 5 wt% VT (DT-51) sample during the temperature programmed decomposition which was consistent with the elemental analysis. Such larger amount of sulfate formation could be explained by the more active SO2 oxidation on the 5 wt% VT (DT-51) than 5 wt% VT (micro) because SO2 oxidation is the important step to generate sulfate species on the catalysts. It could be ascribed to the difference in the tendency of oxidation reaction affected by the vanadium species, since it was known that more V–O–V bonds existed on the surface of 5 wt% VT (DT-51) having bulk-like VOx species whereas V=O bonds were prevalent on 5 wt% VT (micro) having more dispersed VOx. In situ FT-IR results also provided the evidence about the formation of ammonium bisulfate through strong interaction between NH3 and SO3 on 5 wt% VT (DT-51), although 5 wt% VT (micro) did not. Consequently, the different vanadium species determined by the pore structure of TiO2 had a significant effect on the SO2 poisoning during SCR reaction. At last, sulfate solution was impregnated into two types of TiO2, P25 and microporous TiO2, to investigate the role of the bond of Ti–S in sulfur poisoning of VOx/TiO2. Among different kinds of sulfur sources, the introduction of 1.5 wt% of sulfur doped by H2SO4 solution showed the most significant effect on promoting SCR reactivity and N2O suppression. It was found that the improvement of NOx conversion was originated from the increase of acid sites of catalysts which was analyzed by NH3 TPD. In addition, the less decrease in NOx conversion was observed after being exposed to SO2 and H2O during SCR reaction for 12 h when TiO2 was pre-sulfated before VOx impregnation. According to the introduction of sulfur into TiO2, 1 wt% VOx supported by both P25 and microporous TiO2 showed minimized degradation of NOx conversion after sulfur poisoning by 0.19% and 0.68%, respectively. The formation of Ti–O–S bonds on VOx supported by sulfur doped TiO2 at the expense of Ti–OH bonds on the surface could explain the suppression of SO2 oxidation leading to the strong resistance to sulfur poisoning.