| Reduction of NOx emitted from diesel vehicle and lean-burn gasoline vehicle remains challenging in the environmental catalysis research. Selective catalytic reduction （SCR） method is one of the most efficient methods for the NOx removal under learn burn conditions. In this work the H2-SCR performances of Pd-based catalysts under learn burn conditions were studied. Firstly, the effects of a series of supports and modifiers on H2-SCR performances of Pd-based catalysts were examined. Then the promoting effect of Mn modifier was evaluated and investigated from the point of altering the dominant surface acid sites and the existence form of Pd. Besides, the H2-SCR performance of Au modified Pd-based catalyst was also examined by the formation of PdAu alloy. Moreover, Pd-nano catalysts prepared by liquid phase reduction method were also studied. The distinct difference of H2-SCR activity of Pd-nano/Al2O3 from that of Pd/Al2O3 prepared by incipient wetness impregnation method was demonstrated. Several characterizations methods were carried out to reveal the relationships between the catalytic performances and the structure. In order to get the insight of H2-SCR reaction mechanism, in-situ DRIFTS was also studied. The main contents and conclusions of the present work were described as followings:1. Supports and modifiers are closely related to the activity and N2 selectivity of Pd-based catalysts in H2-SCR reaction. It is found that compound oxide support is superior to the single-component support, and the best H2-SCR activity was obtained on TiO2-Al2O3 support prepared by mechanical blending method. Moreover, it showed that the low-temperature activity of all Pd-based catalysts were obviously increased with the increasing of Pd loading, which exerts different influence on N2 selectivity due to using different support. In addition, the activity and temperature window of Pd/TiO2-Al2O3 catalyst were greatly influenced by pre-treatment atmosphere:comparing with the catalyst without pretreatment, the catalyst pre-treated in H2 showed higher activity at temperature below 150℃ and maintained stable N2 selectivity, the catalyst pre-treated in O2 atmosphere exhibited higher high-temperature activity but lower N2 selectivity above 200℃; the catalyst pretreated in O2-H2 atmosphere showed higher activity and broader operation temperature window. Sn and Ni modifiers greatly improved the ability to adsorb and oxidize NO to NO2, as well as the interaction between Pd and the promoters, thus favoring the NOX conversion and N2 selectivity. While the appropriate NO adsorption and activation ability of Sn modified Pd-based catalyst showed the highest NOx conversion and N2 selectivity.2. Not only NOx conversion but also N2 selectivity was significantly promoted above 200℃ over 0.5%Pd-2%Mn/TiO2-Al2O3 catalyst after Mn modification. It is found that the promoted NOx conversion is correlated with the only existence form of PdO and the formation of more and new active intermediates in H2-SCR reaction. Moreover, H2-TPR, in-situ DRIFTS, XPS and NOX-TPD results revealed that the synergistic effect between Mn and Pd increased its redox property and oxygen vacants, lowered the bonding energy of nitrogen oxides with the surface of catalyst, and thus accelerating NO+O2+H2 reaction. More importantly, the dominant surface acid sites was transformed from Br（?）nsted acid to Lewis acid, which greatly contributed to NH3 generation in H2+O2+NO reaction, thus leading to high N2 selectivity at high temperature by changing the reaction route to the well-established NH3-SCR pathway.3. Since the operation temperature window of Pd-Mn catalyst is not broad enough to satisfy the demand of dynamic temperature of diesel vehicle; and inspired by the synergistic effect between Mn and Pd, Pd alloy catalyst was prepared and investigated in this work. Over 1%Pd-0.5%Au/TiO2 catalyst, more than 75% NOx conversion and N2 selectivity were achieved in the considerably wide temperature range of 100～400℃. The catalysts were characterized by XRD, SEM, HRTEM, H2-TPR and CO adsorption and the results revealed that the broadened temperature window resulted from the presence of PdAu alloy and metallic Pd0, both of which inhibited the formation of PdO and the aggregation of the active sites during the H2-SCR reaction and led to smaller particle size as well as the more uniform particle dispersion compared with 1%Pd/TiO2. On the other hand, compared with 0.5%Pd/TiO2 catalyst, more active intermediates were formed and the band intensity of NH3 species adsorbed on Lewis acid sites and NH4+ species adsorbed on Br（?）nsted acid sites became stronger. What’s more these active intermediates were more stable in the whole temperature region of 100～400℃ over 1%Pd-0.5%Au/TiO2 catalyst. The tested results showed that the catalyst with 1%Pd and 0.5%Au supported on TiO2 and calcined at 500℃ displayed superior H2-SCR performance.4. Pd-nano catalysts supported on Al2O3 support prepared by liquid phase reduction method （Pd-LPR） showed the highest NOx conversion and broadest temperature window compared with those catalysts prepared by co-precipitation, hydrothermal, micro-emulsion and incipient wetness impregnation （Pd-IM） methods. The desirable reductant used in LPR method was PEG-400. Based on the H2-SCR performances of above catalysts, we put the emphasis on the comparison between Pd-LPR and Pd-IM catalysts. It is found that Pd-LPR catalyst, which maintained high N2 selectivity, exhibited higher NOx conversion than that of Pd-IM catalyst. BET and SEM results showed that Pd-LPR catalyst showed larger BET surface area and smaller particle size, narrower particle size distribution and higher particle dispersion than that of Pd-IM catalyst. XRD results showed that metallic Pd0 was the only existence form of Pd on Pd-LPR catalyst. Moreover, in-situ DRIFTS results showed that the band intensity of the essential Pd0-NO active intermediates species was strong; furthermore, the other adsorbed active species was thermodynamically stable even at high temperature; more NH4+ species adsorbed on Br（?）nsted acid sites in NO+O2+H2 reaction were observed. It is well worth noting that the used Pd-LPR catalyst still exhibited much higher NOX conversion than that of the used Pd-IM catalysts in H2-SCR reaction.The developed Pd-Mn/TiO2-Al2O3 and Pd-Au/TiO2 catalysts in this work exhibited significantly high NOx conversion and N2 selectivity in H2-SCR of NOx. More importantly, the operation temperature window was greatly broadened. The present research showed light on the design of novel de-NOx catalyst with high H2-SCR performance. |
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