Ce~(4+),Zr~(4+)共掺杂提高V_2O_5-WO_3/TiO_2催化剂脱硝性能及抗K中毒能力（英文）
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摘要
近年来, NO_x的排放造成了严重的环境污染.氨选择性催化还原技术(NH3-SCR)是目前消除NO_x最有效的手段之一.V_2O_5-WO_3/TiO_2催化剂在300–400°C范围内表现出优异的脱硝性能,因此被广泛用于NH3-SCR反应.然而该催化剂抗碱(土)金属中毒性能较差,且碱(土)金属碱性越强对催化剂的毒害越大(即K> Na> Ca> Mg).已有研究显示,当K_2O质量分数达1%时,催化剂将完全失活,所以对传统的V_2O_5-WO_3/TiO_2催化剂进行改性以提高其抗K中毒性能具有十分重要的意义.最近, CeO_2由于具有优异的氧化还原性能和储/释氧能力,在NH3-SCR反应得到了广泛的关注.研究显示, CeO_2的改性可提高钒基催化剂脱硝活性及抗碱金属中毒性能,这主要是由于CeO_2的掺杂可以有效提高催化剂表面酸性及氧化还原能力. ZrO_2是一种酸碱两性氧化物,常被用作载体或者助剂.研究显示, ZrO_2的引入可以提高催化剂热稳定性,增大比表面积以及提高氧迁移能力.基于此,我们制备了一系列的V_2O_5-WO_3/TiO_2-ZrO_2, V_2O_5-WO_3/TiO_2-CeO_2以及V_2O_5-WO_3/TiO_2-CeO_2-ZrO_2催化剂,以期提高V_2O_5-WO_3/TiO_2催化剂脱硝性能及抗K中毒能力.研究发现, Ce~(4+), Zr~(4+)共掺杂可以有效提高V_2O_5-WO_3/TiO_2催化活性,拓宽反应温度窗口,增强抗K中毒能力.进一步借助X射线衍射、比表面积测定、氨气-程序升温脱附、氢气-程序升温还原和X射线光电子能谱等表征对催化剂进行全面分析.结果显示, Ce~(4+), Zr~(4+)共掺杂对V_2O_5-WO_3/TiO_2催化剂物理化学性质的影响与其脱硝性能及抗K中毒能力有着密不可分的关系.首先, Ce~(4+), Zr~(4+)可以掺杂进入TiO_2晶格,抑制TiO_2晶粒的生长,从而导致比表面积以及总孔体积的增加;比表面积的增加有利于活性物种的分散,而总孔体积的增加有利于反应物分子与催化剂充分接触.其次, Ce~(4+), Zr~(4+)共掺杂可以提高催化剂表面酸性和氧化还原性能,表面酸性的增加有利于催化剂吸附与活化反应物种NH_3,氧化还原性能的提高有利于NO氧化为NO_2,进而通过"快速NH3-SCR"反应提高催化剂活性;同时, Ce~(4+), Zr~(4+)共掺杂还可以有效降低K中毒对表面酸性和氧化还原性能的影响,这主要是由于Ce~(4+)可以与K原子结合形成Ce-O-K物种,而Zr~(4+)的引入可以增加Ce~(4+)的热稳定性,使得更多的Ce~(4+)与K结合,避免了K与活性钒物种结合形成V-O-K物种,使得活性V5+得到了有效的保护.原位红外实验揭示了V_2O_5-WO_3/TiO_2-CeO_2-ZrO_2催化反应遵循L-H机理,且K中毒并未改变其反应机理.最后,该催化剂在H_2O和SO_2存在的条件下仍具有最佳的脱硝性能,因而有望用于实际高K含量的燃煤烟气脱硝.
A series of V_2O_5-WO_3/TiO_2-ZrO_2, V2 O5-WO_3/TiO_2-CeO2, and V_2O_5-WO_3/TiO_2-CeO_2-ZrO_2 catalysts were synthesized to improve the selective catalytic reduction(SCR) performance and the K-poisoning resistance of a V_2O_5-WO_3/TiO_2 catalyst. The physicochemical properties were investigated by using XRD, BET, NH3-TPD, H2-TPR, and XPS, and the catalytic performance and K-poisoning resistance were evaluated via a NH3-SCR model reaction. Ce~(4+) and Zr~(4+) co-doping were found to enhance the conversion of NOx, and exhibit the best K-poisoning resistance owing to the largest BET-specific surface area, pore volume, and total acid site concentration, as well as the minimal effects on the surface acidity and redox ability from K poisoning. The V_2O_5-WO_3/TiO_2-CeO_2-ZrO_2 catalyst also presents outstanding H_2O + SO_2 tolerance. Finally, the in situ DRIFTS reveals that the NH3-SCR reaction over the V_2O_5-WO_3/TiO_2-CeO_2-ZrO_2 catalyst follows an L-H mechanism, and that K poisoning does not change the reaction mechanism.
A series of V_2O_5-WO_3/TiO_2-ZrO_2, V2 O5-WO_3/TiO_2-CeO2, and V_2O_5-WO_3/TiO_2-CeO_2-ZrO_2 catalysts were synthesized to improve the selective catalytic reduction(SCR) performance and the K-poisoning resistance of a V_2O_5-WO_3/TiO_2 catalyst. The physicochemical properties were investigated by using XRD, BET, NH3-TPD, H2-TPR, and XPS, and the catalytic performance and K-poisoning resistance were evaluated via a NH3-SCR model reaction. Ce~(4+) and Zr~(4+) co-doping were found to enhance the conversion of NOx, and exhibit the best K-poisoning resistance owing to the largest BET-specific surface area, pore volume, and total acid site concentration, as well as the minimal effects on the surface acidity and redox ability from K poisoning. The V_2O_5-WO_3/TiO_2-CeO_2-ZrO_2 catalyst also presents outstanding H_2O + SO_2 tolerance. Finally, the in situ DRIFTS reveals that the NH3-SCR reaction over the V_2O_5-WO_3/TiO_2-CeO_2-ZrO_2 catalyst follows an L-H mechanism, and that K poisoning does not change the reaction mechanism.
引文
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