【作者】 司知蠢；

【导师】 翁端；

【作者基本信息】 清华大学 ， 材料科学与工程， 2010， 博士

【摘要】 本论文通过共沉淀法合成WO_x-ZrO₂固溶体作为载体材料,采用等体积浸渍法在载体上负载组分CuO;以NH₃-SCR活性测试、NO/NH₃的程序升温氧化、XRD、Raman光谱、紫外可见光漫反射光谱、H₂程序升温还原和NO/NH₃原位吸附与脱附测试等手段研究了催化剂组分、结构与性能的构效关系,建立了NH₃-SCR反应机理模型,进而通过动力学方法探讨了机理模型的合理性。CuO、WO₃和ZrO₂重量百分比为1:1:8,且焙烧温度为500°C时,催化剂的催化性能最好,在210-310°C NO转化率和N₂选择性均超过了80%,具有比商业V₂O₅-WO₃-TiO₂催化剂更好的低温NH₃-SCR活性。催化剂的NO氧化能力越强,其低温NH₃-SCR活性就越高。催化剂的NH₃吸附性能越高且对NH₃的氧化能力越弱,其高温NH₃-SCR活性越好。以WO_x-ZrO₂固溶体作为载体材料,一方面部分未固溶的钨组分在载体表面形成高分散的WO_x基团,不但可以提高催化剂表面的Lewis酸位和Br（?）nsted酸位浓度促进NH₃和NO在催化剂表面的吸附,而且可以抑制NH₃在催化剂表面的氧化从使催化剂的高温活性得到提高;另一方面,可以提高载体表面铜的分散性,促使WO_x与CuO_x发生结构相互作用生成含Cuⁿ⁺-O₂--Wⁿ⁺基团的界面可以促进两者之间的电子相互作用,有利于NO氧化为亚硝酸根提高催化剂的低温活性。NH₃-SCR反应机理研究表明:CuO_x为主要的NO吸附位,WO_x为主要的NH₃吸附位。“NH₃-NO₂机理”为低温（<200°C）NH₃+NO+O₂反应的主要途径,即NO吸附于Cu位并被氧化为NO₂,然后与吸附于W位的NH₃反应生成N₂和H₂O;“NH₂-NO₂/NO机理”为高温（>250°C）NH₃+NO+O₂反应的主要途径,即NH₃在W位上被活化为NH₂,然后与吸附于Cu位的NO₂/NO反应生成N₂和H₂O。NH₃在W位上吸附进而活化为NH₂是该温度条件下NH₃-SCR反应的控速步骤。提高催化剂的低温NO氧化活性和抑制催化剂的高温NH₃氧化活性是拓宽其温度窗口的关键。更多还原

【Abstract】 WO_x-ZrO₂ solid solution support was synthesized by a coprecipitation method before loading CuO. The catalysts were characterized by NH₃-SCR, temperature-programmed oxidation of NO/NH₃, XRD, Raman, UV-vis, H₂-TPR and NO/NH₃ adsorption, to study the relationships among the catalyst composition, structure, adsorption property and NH₃-SCR activity. The NH₃-SCR reaction mechanisms at different temperature intervals were established and validated via kinetics methods.The highest activity is achived over the optimized catalyst with the CuO: WO₃:ZrO₂ weight ratio of 1:1:8 and calcined at 500°C. Above 80% NO conversion and 80% N₂ selectivity ishas been obtained on this catalyst at 210-310°C, which is even better than commercial V₂O₅-WO₃-TiO₂ catalyst. The optimized catalyst present higher low-temperature NH₃-SCR activity and N₂ selectivity thanthe commercial V₂O₅-WO₃-TiO₂ catalyst. High NO oxidation activity facilitates the high NH₃-SCR activity of the catalysts in low-temperature range. The larger NH₃ adsorption capacity and lower NH₃ oxidation ability of the catalysts lead to the higher NH₃-SCR activity of catalysts in high-temperatures range. O_xidation treatment of catalyst results in an increase of NH₃-SCR activity.Copper oxides are well dispersed on WO_x-ZrO₂ supports due to the increased surface area by the incorporation of tungsten, and the activity for NO to NO₂ on copper sites is thereby improved. Highly dispersed WO_x clusters form on the support when the tungsten content exceeds its solubility in zirconia. Therefore, the concentration of Lewis and Br（?）nsted acid sites on the catalysts increases, facilitating the adsorption of NO and NH₃ and inhibiting the ammonia oxidation on the catalysts. Furthermore, these highly dispersed WO_x clusters strongly interact with the highly dispersed CuO_x, generating abundant Cuⁿ⁺-O₂--Wⁿ⁺ interfaces. The electronic interaction arising from the oxide interfaces leads to NO oxidation to nitro compounds rather than nitrates. These factors improve the activity and selectivity to nitrogen of the catalysts in NH₃-SCR reaction.“Ammonia-NO₂ route”at low temperatures （<200°C） and“amine -NO/NO₂”at high temperatures （>250°C） are presented for NH₃-SCR reaction over this catalyst. Copper oxides are considered as the active sites for NO adsorption and its oxidation to NO₂. The adsorption of ammonia and activation to amine is the rate-determing step for the high-temperature SCR reaction, which is effectively improved by tungsten modification. These effects improve the high-temperature NH₃-SCR activity and N₂ selectivity of catalyst. According to the above mechanism, the key factor to enlarge the temperature window of the SCR catalyst is to enhance the NO oxidation activity at low temperatures and to suppress the NH₃ oxidation activity at high temperatures.更多还原