Effect of calcination temperature on the structure and NOx adsorption-reduction performance of Fe modified hydrotalcite-based Pt/BaO/MgFeO catalyst
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摘要: 采用共沉淀法和浸渍法制备了贵金属负载均匀的镁铁水滑石基稀燃氮氧化物捕集（LNT）催化剂，并采用多种表征手段研究了焙烧温度对催化剂结构及NOx吸附-还原反应的影响。结果表明，500-700℃焙烧后催化剂晶相结构稳定，800℃焙烧后催化剂表面出现烧结，Pt颗粒发生团聚；随焙烧温度升高，催化剂的NOx脱附峰面积先减小后增大，但峰值温度变化不大；与500℃焙烧催化剂相比，800℃焙烧后催化剂表面NOx吸附物种及吸附路径发生改变，而还原反应产物中氨气/氮气体积比下降，NOx转化效率由91.7%降为85.2%。Abstract: In this paper, the iron-magnesium hydrotalcite-based catalysts with well dispersed noble metal for the lean NOx trap (LNT) were prepared by co-precipitation and impregnation methods. The effects of different calcination temperatures on the catalyst structure and the adsorption-reduction performance for NOx were investigated by analytical techniques. Results show that the crystalline structure of the catalysts remains stable when the calcination temperature is lower than 700℃. The sintering of the catalysts prepared and the conglobation of Pt particles appear on the catalyst surface after calcination at 800℃. With the increase in calcination temperature from 500 to 800℃, the desorption amounts of NOx increases after an initial decrease, while the corresponding peak temperature show a marginal change. DRIFTs results indicate that the adsorbed NOx species and the adsorption pathways over the catalyst after calcination at 800℃ differ from those at 500℃. The ratio of NH3/N2 released from the reduction of adsorbed NOx species over the catalyst calcinated at 800℃ decreases in comparison with those calcinated at lower temperatures, which is responsible for the decrease in the NOx conversion efficiency.
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Key words:
- hydrotalcite /
- LNT /
- NOx /
- calcination /
- diesel engine
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图 1 不同焙烧温度下Pt/BaO/MgFeO催化剂及Pt/BaO/Al2O3催化剂的NOx-TPD谱图
Figure 1 NOx-TPD profiles of Pt/BaO/MgFeO and Pt/BaO/Al2O3 catalysts calcinated at different temperatures
图 2 镁铁水滑石前驱体及不同焙烧温度下 Pt/BaO/MgFeO催化剂的XRD谱图
Figure 2 XRD patterns of MgFe-HTLC (a) and Pt/BaO/MgFeO catalysts (b-e) calcinated at different temperatures
a: MgFe-HTLC； b: 500℃； c: 600℃； d: 700℃； e: 800℃
图 3 500和800℃焙烧后 Pt/BaO/MgFeO催化剂的TEM照片
Figure 3 TEM images of Pt/BaO/MgFeO catalysts calcinated at different temperature (a)，(b)： 500℃； (c)，(d)： 800℃
图 4 500和800℃焙烧后Pt/BaO/MgFeO催化剂吸附NOx的原位红外光谱谱图
Figure 4 In situ FT-IR spectra of NOx on Pt/BaO/MgFeO catalysts calcinated at 500℃(a)and 800℃(b)
图 5 500和800℃焙烧后Pt/BaO/MgFeO催化剂上的H2还原过程
Figure 5 H2 reduction phase of Pt/BaO/MgFeO catalysts calcinated at 500 and 800℃
Table 1 Assignment of FT-IR spectra to species formed during adsorption of NOx
Species Wave number σ/cm-1 Bridged monodentate nitrate 1042 Bridged bidentate nitrite 1230 Monodentate nitrate 1327/1350 Ionic nitrate 1380 Bulk nitrate 1403 Chelating monodentate nitrate 1521/1538 Bridged bidentate nitrate 1596/1631 Adsorbed N2O4 1692/1770 
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表 2 500和800℃焙烧Pt/BaO/MgFeO催化剂在再生反应过程中所生成氨气与氮气的体积比
Table 2 Ratio of NH3/N2 on Pt/BaO/MgFeO catalysts calcinatedat 500 and 800℃
Temperature t/℃ NH3/N2 (volume ratio) 500 6.114 800 5.296
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