Study on the mechanism of K-poisoning in Mn/TiO2 low temperature SCR catalysts
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摘要: Mn/TiO2具有良好的低温NH3选择性催化还原NOx（SCR）的活性。烟气中存在的碱金属会从物理和化学上毒害催化剂导致Mn/TiO2催化剂中毒失活。论文以暴露{101}面TiO2为载体制备Mn/TiO2催化剂，采用浸渍法制备K中毒催化剂，研究了Mn/TiO2低温SCR催化剂钾中毒机理。实验发现，Mn/TiO2催化剂脱硝效率随K中毒浓度增加而减少；新鲜Mn/TiO2催化剂表面NH3-SCR反应由E-R和L-H机理共同控制；K吸附会导致催化剂比表面积降低，催化剂表面Mn4 + 、化学吸附氧比例降低，表面酸性位点数量减少，导致脱硝活性降低；同时K更易吸附在Mn顶位以及桥接O位附近，导致NO的吸附活化受到严重遏制，同时削弱NH3的吸附，使得L-H机理受到阻断，只能以E-R机理控制为主。Abstract: Mn/TiO2 has good low temperature NH3 selective catalytic reduction (SCR) activity for NOx. The presence of alkali metals in the flue gas can physically and chemically poison the catalyst leading to toxic deactivation of the Mn/TiO2 catalyst. This study investigated the mechanism of K-poisoning in Mn/TiO2 low temperature SCR catalysts by preparing K-poisoning Mn/TiO2 catalysts using exposed {101} surface TiO2 as a carrier. It was found that the denitrification efficiency of the Mn/TiO2 catalyst decreased with increasing K-poisoning concentration. Experimental characterisation and DFT calculations showed that the NH3-SCR reaction on the surface of the fresh Mn/TiO2 catalyst was controlled by both E-R and L-H mechanisms. K adsorption led to a reduction in the catalyst specific surface area, a decrease in the ratio of Mn4 + and chemisorbed oxygen on the catalyst surface and a decrease in the number of acidic sites on the surface, resulting in a decrease in denitrification activity; at the same time, K was more likely to adsorb near the Mn top site as well as the bridging O site, resulting in the activation of NO adsorption was severely curtailed and the adsorption of NH3 was weakened, making the L-H mechanism blocked and the E-R mechanism the main control.
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Key words:
- low-temperatureSCR /
- Mn/TiO2 /
- K-poisoning /
- catalyst
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图 1 催化剂活性评价装置示意图
Figure 1 Schematic diagram of the catalyst activity evaluation device
(1)：N2；(2)：O2；(3)：NO；(4)：NH3；(5-8)：Mass Flow Meters；(9)：Gas mixer；(10)：Temperature controller； (11)：Reaction tube；(12)：Reaction Furnace；(13)：Flue gas analyser；(14)：Computer；(15)：Exhaust gas treatment units
图 3 锐钛矿{101}晶面的（a）SEM和（b）TEM照片
Figure 3 (a) SEM and (b) TEM images of anatase {101} crystal faces
图 4 Mn/TiO2催化剂的SEM和EDS照片
Figure 4 SEM and EDS images of Mn/TiO2 catalyst samples (Ti: cyan O: red Mn: purple K: green)
图 6 Mn/TiO2催化剂的Mn 2p和O 1s XPS谱图
Figure 6 Mn 2p and O 1s XPS results for Mn/TiO2 catalysts
图 7 200 ℃下催化剂表面NO和预先吸附NH3的反应光谱
Figure 7 Reaction spectra of NO and pre-adsorbed NH3 on the surface of the catalysts at 200 ℃
图 8 200 ℃下催化剂表面NH3和预先吸附NO的反应光谱
Figure 8 Reaction spectra of NH3 and pre-adsorbed NO on the catalyst surface at 200 ℃
图 9 Mn/TiO2及其K中毒催化剂优化构型
Figure 9 Optimised configuration of Mn/TiO2 and its K-poisoning catalyst
图 10 NH3、NO在Mn/TiO2表面吸附构型
Figure 10 Adsorption configuration of NH3 and NO on Mn/TiO2 surface
表 1 催化剂表面元素浓度（EDS）
Table 1 Analysis of elemental concentrations on the surface of catalyst samples (EDS)
Sample K/% Mn/% O/% Ti Mn/TiO2 - 12.03 51.25 36.72 0.01K- Mn/TiO2 0.25 11.81 55.37 32.57 0.02K- Mn/TiO2 0.41 10.77 56.95 31.87 0.045K- Mn/TiO2 1.23 9.95 56.72 32.10 0.09K- Mn/TiO2 2.28 9.82 57.12 30.78 
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表 2 催化剂的BET参数
Table 2 BET parameters for catalyst samples
Catalyst BET surf
area
/(m2·g−1)Pore volume
×10−2/(cm3·g−1)Average pore
diameterd
/nmMn/TiO2 52.7 12.1 9.2 0.01K- Mn/TiO2 44.1 13.0 11.8 0.02K- Mn/TiO2 44.4 12.9 11.7 0.045K- Mn/TiO2 45.0 11.6 9.1 0.09K- Mn/TiO2 42.3 11.3 9.1
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表 3 催化剂表面元素浓度分析（XPS）
Table 3 Analysis of elemental concentrations on the surface of catalyst samples (XPS)
Sample K/% Mn/% O/% Mn2 + Mn3 + + Mn4 + Oβ Oα Mn/TiO2 - 32.6 67.4 49.2 50.8 0.01K- Mn/TiO2 0.55 35.1 64.9 65.1 34.9 0.02K- Mn/TiO2 0.53 38.7 61.3 65.2 34.8 0.045K- Mn/TiO2 2.13 41.7 58.3 72.4 27.6 0.09K- Mn/TiO2 2.58 47.8 52.2 72.8 27.2
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