不同阳离子(NH4+,Na+,K+,Ca2+)溴化物对商用SCR催化剂化学中毒影响机制研究

燃煤飞灰中的碱金属和碱土金属对 NH3-SCR 催化剂的活性有显著的影响. 近年来, 研究者针对碱金属/碱土金属氧化物对 SCR 催化剂中毒作用开展了大量研究. 另一方面, 研究普遍认为, 含溴化合物对提高 SCR 催化剂汞氧化性能具有明显促进作用. 目前为止, 针对碱金属/碱土金属溴化物对 SCR 催化剂影响的系统研究较少. 我们课题组系统研究了不同阳离子的溴化物 (NH4Br, NaBr, KBr 和 CaBr2) 对商用 V2O5-WO3/TiO2催化剂性能的影响.与未中毒样品相比, KBr 中毒后的催化剂 (记为 L-KBr) 上 NOx转化率明显下降, 而 NaBr 和 CaBr2中毒的催化剂 (分别记为 L-NaBr 和 L-CaBr) 上的 SCR 活性也有一定程度的降低. 另外 L-NaBr, L-KBr 和 L-CaBr 催化剂的 N2选择性较差. XPS 结果显示, KBr 中毒后化学吸附氧 (Oα) 比例减小; 同时, KBr 中毒后还原性和表面酸度降低, 这些可能是导致 L-KBr催化剂的活性和 N2选择性变差的主要原因. 对于 L-CaBr 催化剂, 中毒后化学吸附氧 Oα比例有所增加, 这与 H2-TPR 结果显示可还原性增强一致. O2-TPO 结果显示, L-CaBr 催化剂可氧化性降低, 说明 CaBr2中毒还是影响到催化剂表面的氧化还原循环. 催化剂 CaBr2中毒后表面被覆盖减少了反应活性位数量, 但表面酸性的增强可能会抵消活性位点损失带来的负面影响. NH3氧化结果显示, NH3在 L-CaBr 催化剂表面发生过氧化反应, 特别是高温下生成较多 N2O, 降低 N2选择性, 这可能是高温下 L-CaBr 催化剂 SCR 活性和 N2选择性下降的重要原因. CO2-TPD 结果表明, L-KBr 和 L-CaBr 催化剂表面碱性强度增加, 可能有助于增加 NOx物种的吸附量. 基于以上活性评价和表征分析结果, 我们尝试建立了不同溴化物中毒的催化剂表面酸碱性、氧化还原和催化性能之间的关系.

The effect of cations(NH4+,Na+,K+,and Ca2+)on chemical deactivation of commercial SCR catalyst by bromides

Alkali and alkaline-earth metals from fly ash have a significant deactivation effect on catalysts used for selective catalytic reduction of NOxby NH3(NH3-SCR). Bromides are considered effective addi-tives to improve Hg0 oxidation on SCR catalysts. In this work, the effects of different bromides (NH4Br, NaBr, KBr, and CaBr2) on a commercial V2O5-WO3/TiO2catalyst were studied. NOxconver-sion decreased significantly over the KBr-poisoned catalyst (denoted as L-KBr), while that over NaBr- and CaBr2-poisoned catalysts (denoted as L-NaBr and L-CaBr, respectivity) decreased to a lesser extent compared with the fresh sample. Poor N2selectivity was observed over L-NaBr, L-KBr and L-CaBr catalysts. The decrease in the ratio of chemisorbed oxygen to total surface oxygen (Oα/(Oα+ Oβ+ Ow)), reducibility and surface acidity might contribute to the poor activity and N2 selectivity over L-KBr catalyst. The increased Oαratio was conducive to the enhanced reducibility of L-CaBr. Combined with enhanced surface acidity, this might offset the negative effect of the loss of active sites by CaBr2covering. The overoxidation of NH3and poor N2selectivity in NH3oxidation should retard the SCR activity at high temperatures over L-CaBr catalyst. The increased basicity might contribute to increased NOxadsorption on L-KBr and L-CaBr catalysts. A correlation between the acid-basic and redox properties of bromide-poisoned catalysts and their catalytic properties is established.