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改进型钒钨钛NH3-SCR催化剂的构效机制研究
Alternative TitleStudy on Structure-activity Mechanism of Improved Vanadium-tungsten-titanium Catalysts for NH3-SCR Reaction
晏子頔
Subtype博士
Thesis Advisor贺泓
2020-06
Degree Grantor中国科学院生态环境研究中心
Place of Conferral北京
Degree Name理学博士
Degree Discipline环境科学
Keyword钒 钨钛 催化剂 ,氨选择性催化还原氮氧化物,热稳定性,反应路径 抗 碱 金属中毒 性能 vanadium-tungsten-titanium, Nh3-scr, Thermal Stability, Reaction Pathway, Alkali Resistance
Abstract

     以NH3为还原剂在催化剂的作用下选择性还原 NOx,即 NH3-SCR技术,是去除 柴油车尾气 NOx最为有效的技术之一。 钒基催化剂体系具有高NH3-SCR催化效率、经济与耐硫的特点,已经在柴油车NH3-SCR后处理系统 中得到广泛应用 。然而,钒基SCR催化剂在应用中也存在一些问题,如在柴油机高温尾气中热稳定性较差以及抗碱金属中毒性能有待提高等。 因此,针对钒基SCR催化剂在实际应用中出现的问题加以改进,开发高效稳定并可适应复杂工作条件的钒基SCR催化剂具有重要的现实意义。
     本论文采用浸渍法制备了一系列不同 V2O5含量( 2 wt%、 3.5 wt%与 5的 V2O5/WO3-TiO2 与 V2O5/SiO2-WO3-TiO2催化剂, 并考察了催化剂的 NH3-SCR活性与热稳定性,发现钒负载量与 SiO2的添加都会影响 V2O5/WO3-TiO2催化剂的热稳定性。增加钒负载量能提高新鲜催化剂的低温 SCR活性,但高钒负载量的催化剂在热老化后严重失活。当 V2O5含量较高( 3.5 wt%与 5 wt%)时V2O5/SiO2-WO3-TiO2催化剂具有比 V2O5/WO3-TiO2催化剂明显优异的热稳定性。
      通过 BET、 XRD、 SEM、 Raman、 XPS、 H2-TPR与 NH3-TPD表征手段,发现高钒负载量会加速催化剂载体 TiO2在高温中由锐钛矿型向金红石型的相变,导致比表面积降低与 SCR活性下降。 Si的添加能抑制钒基催化剂 在 热老化过程中VxTi1-xO2固溶体的形成,进而抑制载体 TiO2在高温中的相变,从而提高催化剂的热稳定性。
     采用暂态响应实验( TRM 定量测定了 NO、 NO2与 NH3在 V2O5/WO3-TiO2催化剂上的吸附量,结合 程序升温表面反应( TPSR)的方法研究了 反应气氛中无水 蒸汽 与有水 蒸汽 2 vol.% H2O)的条件下 V2O5/WO3-TiO2催化剂的活性位点与 NH3-SCR反应路径。研究表明 NO吸附在催化剂表面的 [V4+]-OH + [V5+]-O、[V5+]-OH与 Ti位点上,吸附量分别为 24、 28与 26 μmol g-1。 NH3吸附在催化剂表面的 [V4+]-OH + [V5+]-O、 [V5+]-OH、 Ti与 W位点上,吸附量分别为 24、 28、26与 274 μmol g-1。在无水的条件下, V2O5/WO3-TiO2催化剂在 150 oC时的标准与快速 SCR反应遵循“亚硝酸盐路径”:吸附 NH3与表面亚硝酸盐物种反应生成N2与 H2O。然而, NO与 NH4NO3(由表面硝酸盐与吸附 NH3反应生成)反应生成N2与 H2O,即 NH4NO3路径”,在无水 条件下 的催化剂上难以发生。在 反应气氛中 有水存在的条件下, NH4NO3路径”与“亚硝酸盐路径”在 V2O5/WO3-TiO2催化剂上都能发生,并且 150 oC时在催化剂表面硝酸盐与吸附 NH3的数量也有所增加。但水 的共存与 吸附抑制了 NO与吸附 NH3物种的反应,从而降低 NH3-SCR活性。
     研究了K中毒对 V2O5/WO3-TiO2催化剂与 Ce掺杂的 V2O5/WO3-TiO2催化剂的 SCR活性的影响,发现 Ce的掺杂显著提高了 V2O5/WO3-TiO2催化剂的抗 碱金属中毒 性能 。 当 K/V摩尔比为 4时, Ce掺杂的 V2O5/WO3-TiO2催化剂依然在350 oC以上区间具有 90%的 NOx转化率 ,而 V2O5/WO3-TiO2催化剂此时已完全失活。 通过 in situ DRIFTS、 NH3-TPD、 XPS与 H2-TPR研究了催化剂的碱金属中毒机制。研究表明 Ce的掺杂使得 K中毒后的 CeO2-V2O5/WO3-TiO2催化剂保留了更多的 氧化还原 位点,是其相较 V2O5/WO3-TiO2催化剂具有更优抗 碱 金属中毒 性能 的主要原因。

Other Abstract

      Selective catalytic reduction of NOx with NH3 (NH3-SCR) is one of the most efficient methods for the removal of NOx in exhaust gas from diesel engines. Vanadium-based catalysts, which are widely used as NH3-SCR catalysts for stationary sources, have also been applied for the abatement of NOx from heavy-duty diesel vehicles in Europe and China, due to their high catalytic efficiency, economy and sulfur resistance. However, some issues remain for V2O5/WO3-TiO2 catalysts in diesel vehicle applications, including poor thermal stability in high-temperature exhaust gas and alkali deactivation. Therefore, improvement and development of vanadium-based NH3-SCR catalysts to solve these application problems is of great importance.
       In this study, a series of V2O5/WO3-TiO2 and V2O5/SiO2-WO3-TiO2 catalysts containing 2wt%, 3.5wt% and 5wt% V2O5, respectively, were prepared by the impregnation method. The NH3-SCR activity and thermal stability of the catalysts were investigated. It was noted that both V2O5 loading and SiO2 modification could affect the thermal stability of the V2O5/WO3-TiO2 catalysts. Increasing V2O5 loading improved the low temperature SCR activity of the catalysts in the fresh state, but high V2O5 loading aggravated the deactivation of the catalysts upon aging. With relatively high V2O5 loading (3.5 wt% and 5 wt% V2O5), V2O5/SiO2-WO3-TiO2 showed remarkably improved thermal stability compared to V2O5/WO3-TiO2 NH3-SCR catalysts. The catalysts were characterized by BET, XRD, SEM, Raman, XPS, H2-TPR and NH3-TPD. The results indicated that high V2O5 loading on V2O5/WO3-TiO2 catalysts can accelerate the phase transition of anatase TiO2 to rutile TiO2 upon thermal aging, which results in a decrease in surface area and consequently deactivation, while the introduction of silica prevents the TiO2 phase transition from occurring, therefore enhancing the thermal stability of the catalysts.
       The adsorption amounts of NO, NO2, and NH3 on V2O5/WO3-TiO2 were quantitatively obtained using a transient response method (TRM). The reaction pathway for the selective catalytic reduction of nitrogen oxides (NOx) with NH3 (NH3-SCR) in the absence and presence of H2O and the active site distribution on V2O5/WO3-TiO2 were clarified quantitatively by TRM and temperature programmed surface reaction. NOx adsorption took place on [V4+]-OH + [V5+]-O, [V5+]-OH, and [Ti4+]2-O, with surface concentrations of 24, 28 and 26 μmol g-1 respectively, while NH3 adsorbed on [V4+]-OH + [V5+]-O, [V5+]-OH, [Ti4+]2-O and W species, with the concentrations of 24, 28, 26 and 274 μmol g-1, respectively. The “nitrite path”, i.e. the reaction between adsorbed NH3 and nitrite species to produce N2 and H2O, contributed to standard and fast SCR, but the “NH4NO3 path”, i.e. the reaction between NO and NH4NO3 formed from surface nitrates and adsorbed NH3 to form N2 and H2O, was absent at low temperatures without H2O. Both the “NH4NO3 path” and the “nitrite path” contributed to NH3-SCR with H2O in the gas feed. The presence of H2O increased the amounts of adsorbed nitrates and NH3 at 150 oC, but inhibited the reaction between NO and adsorbed NH3 species, decreasing the NOx activity during NH3-SCR.
      The deactivation effect of K on V2O5/WO3-TiO2 and Ce-doped V2O5 /WO3-TiO2 catalysts in the selective catalytic reduction (SCR) of NOx by NH3 was studied. Ce-doped V2O5/WO3-TiO2 showed significantly higher resistance to K deactivation than V2O5/WO3-TiO2. Ce-doped V2O5/WO3-TiO2 with K/V = 4 (molar ratio) showed 90% NOx conversion at 350oC, whereas under these conditions V2O5/WO3-TiO2 showed no activity. The fresh and K-poisoned V2O5/WO3-TiO2 and Ce-doped V2O5/WO3-TiO2 catalysts were investigated by means of in situ DRIFTS,NH3-TPD, XPS and H2-TPR. The effect of Ce doping on the improved K resistance of V2O5/WO3-TiO2 was discussed. The higher K resistance of CeO2-V2O5/WO3-TiO2 might be attributable to the increase in sites that are active for SCR reactions remaining after K doping in Ce-doped catalysts.

Pages109
Document Type学位论文
Identifierhttp://ir.rcees.ac.cn/handle/311016/43683
Collection大气环境与污染控制实验室
Recommended Citation
GB/T 7714
晏子頔. 改进型钒钨钛NH3-SCR催化剂的构效机制研究[D]. 北京. 中国科学院生态环境研究中心,2020.
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