Characterization and reactivity of V2O5-MoO3/TiO2 De-NOx SCR catalysts
Characterization and reactivity of V2O5-MoO3/TiO2 De-NOx SCR catalysts
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DOI：
10.1006/jcat.1999.2603
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发表时间：
1999-10-25
影响因子：
7.3
通讯作者：
Bregani, F
中科院分区：
文献类型：
--
作者：
Lietti, L;Nova, I;Bregani, F
本研究以V2O5-MoO3负载的V2O5-MoO3催化剂为研究对象，其V和Mo负载量是商业SCR催化剂的代表。这些催化剂是由负载V和Mo活性组分的锐钛矿型二氧化钛组成的。MoO_3作为一种“结构性”促进剂，阻止了添加钒后的二氧化钛基质的烧结。催化剂表面的Mo和V氧化物分别以钼酸和钒的形式存在，不能排除聚合物MoxOy的存在。V和Mo表面氧化物物种的特征与V_2O_5/TiO_2和MoO_3/TiO_2催化剂上观察到的相似，从而表明V和Mo表面的钒和钼氧化物物种的振动独立性。然而，尽管它们的结构和振动是独立的，但在二氧化钛负载的钒和钼氧化物之间也存在明显的电子相互作用。这些相互作用可能通过二氧化钛载体进行，也可能涉及未被观察到的混合V-Mo表面氧化物物种。用NH3-TPD和FT-IR表征了催化剂表面的强酸性。氨被配位固定在Lewis酸中心上(与Ti、V和Mo表面阳离子有关)，并在Mo-OH或V-OH Bronsted中心上质子化为NH：离子。与二氧化钛相比，钼和钒的添加导致了Bronsted中心和更强的Lewis酸性中心的形成。V_2O_5-MoO_3/TiO_2催化剂在NH3还原NO反应中表现出很高的活性，表现出比相应的二元V_2O_5/TiO_2和MoO_3/TiO_2样品更高的反应活性。计算表明，V和/或Mo在三元催化剂中的反应活性高于V_2O_5/TiO_2和MoO_3/TiO_2在相同V和Mo负载量下的反应活性，这表明V和Mo表面氧化物物种之间的SCR反应存在协同作用。因此，在这些催化剂中，钼也是SCR反应的“化学”促进剂。根据表征数据，认为SCR反应中观察到的协同作用与V-Mo电子相互作用的存在有关。这幅图与类似的V2O5-WO_3/TiO_2体系的情况非常相似，表明WO_3和MoO_3的加入对V_2O_5/TiO_2的影响是相似的，这两种氧化物除了起“结构”作用外，还起到“化学”促进剂的作用。然而，V_2O_5-MoO_3/TiO_2样品在高温下表现出更高的N2O生成量和更低的NO转化率：这些差异可能与Mo与W的电子特性不同以及它们在氨氧化反应中具有更高的反应活性有关。结果表明，原料中加水改善了催化剂的性能，在高温下保持了高的NO转化率和高的钠选择性，这是因为它对与SCR反应同时发生的氨氧化反应有很强的抑制作用。(C)1999年学术出版社。
TiO2-supported V2O5-MoO3 catalysts, having V and Mo loadings representative of commercial SCR catalysts, are considered in this study. These catalysts are constituted by TiO2 anatase supporting the V and Mo active components. MoO3 acts as a "structural" promoter preventing the TiO2 matrix from sintering upon vanadia addition. The Mo and V oxide are present on the catalyst surface in the form of molybdenylic and vanadylic species, respectively, and the presence of polymeric MoxOy species cannot be excluded. The features of the V and Mo surface oxide species closely resemble those observed over the binary V2O5/TiO2 and MoO3/TiO2 catalysts, thus pointing out the vibrational independence of the V and Mo surface vanadyl and molybdenyl oxide species. However, in spite of their structural and vibrational independence, the presence of electronic interactions between the TiO2-supported V and Mo oxides is also apparent. These interactions may operate via the TiO2 support or may involve mixed V-Mo surface oxide species who were, however, not observed. The catalyst surface is characterized by strong acidity, probed by NH3-TPD and FT-IR. Ammonia is coordinatively held over Lewis acid sites (associated with Ti, V, and Mo surface cation species) and is protonated as NH: ions over Mo-OH or V-OH Bronsted sites. The addition of Mo and V causes the formation of Bronsted sites and of stronger Lewis acid sites, if compared to TiO2. The V2O5-MoO3/TiO2 catalysts are very active in the reduction of NO by NH3, and exhibit a higher reactivity with respect to the corresponding binary V2O5/TiO2 and MoO3/TiO2 samples. Calculations show that the reactivity of V and/or Mo in the ternary catalysts is higher than that measured over V2O5/TiO2 and MoO3/TiO2 having the same V and Mo loading: hence it is suggested that a synergism operates in the SCR reaction between the V and Mo surface oxide species. Accordingly in these catalysts molybdenum also acts as a "chemical" promoter for the SCR reaction. On the basis of the characterization data, it is suggested that the observed synergism in the SCR reaction is related to the existence of the V-Mo electronic interactions. This picture closely resembles that obtained in the case of the analogous V2O5-WO3/TiO2 system and indicates that the effects of the addition of WO3 and MoO3 to V2O5/TiO2 are similar, both oxides acting as "chemical" promoters besides playing a "structural" function as well. However the V2O5-MoO3/TiO2 samples show higher formation of N2O and lower NO conversions at high temperatures: these differences are possibly associated with the different electronic characteristics of Mo compared to W and to their higher reactivity in the ammonia oxidation reactions. It is found that water addition in the feed improves the catalyst performance in that it preserves high NO conversions and high Na selectivities at high temperatures: this is due to its strong inhibiting effect on the ammonia oxidation reactions occurring simultaneously with the SCR reactions. (C) 1999 Academic Press.