【摘要】 分别采用柠檬酸络合法、改性共沉淀法和湿浸渍法制备了掺Ba纳米ZrO2材料,负载Ru后用于催化氨合成反应.采用X射线衍射、CO2程序升温脱附(CO2-TPD)、N2物理低温吸附、H2程序升温还原技术(H2-TPR)、扫描电镜(SEM)、透射电镜(TEM)、X射线光电子能谱(XPS)和CO化学吸附对载体材料和催化剂进行了表征.结果表明,不同方法制备载体的物相结构和织构性能均有明显差别,负载Ru后催化剂的氨合成性能差别也较大.其中,以柠檬酸络合法制备的载体材料中Ba以BaZrO3的形式存在,钙钛矿型BaZrO3具有较强的供电子能力,电子可以通过Ru与载体间强相互作用传递到Ru表面,有效地促进N≡N的断裂,使催化剂的低温活性显著提高.在425℃,3 MPa,空速为10000 h-1条件下,出口氨浓度为5.72%.其氨合成活性分别是改性共沉淀法和湿浸渍法制备催化剂的3.8倍和14.3倍.更多还原

【Abstract】 The Ba-doped ZrO2 materials were prepared by three methods and used as support for Ru catalysts for ammonia synthesis,i.e.,citric acid sol-gel method(SG),modified co-precipitation(CP) and impregnation method(IP).The certain amount of analytical-grade Zr(NO3)4 5H2O and Ba(NO3)2 were dissolved in deionized water to form a transparent mixed nitrate solution.The citric acid was slowly added into the mixture to form a transparent solution and then heated to 80 ℃ under vigorous stirring until all the water evaporated and a viscous material was obtained.After calcination at 750 ℃ for 5 h and a puffy white powder was obtained.This was BZ-SG.The solution of NH3 H2O and K2C2O4 H2O was added dropwise to the mixture solution of Zr(NO3)4 5H2O and Ba(NO3)2 with vigorous stirring,and the obtained white suspension was aged at 60 ℃ for 60 min.The resulting precipitate was centrifuged and washed with distilled water for several times,and then calcined at 750 ℃ for 5 h.The obtained white solid was named as BZ-CP.The Zr(OH)4was prepared by adding the KOH into the Zr(NO3)4 5H2O solution.Then the obtained Zr(OH)4was baked at 300 ℃ for 3 h and impregnated with aqueous solution of Ba(NO3)2.After dried at 85 ℃ for 12 h,the sample was heated at 750 ℃ for 5 h and obtained the BZ-IP sample.Ruthenium catalysts were prepared by impregnating the supports directly with K2RuO4solution.After reduction with ethyl alcohol,then was dried at 120 ℃ for 12 h.The samples with 4 wt% Ru were labeled as RBZ-X(X=SG,CP and IP).The molar ratio of Ba to Zr in all the samples is 1∶9.The composites materials and catalysts were characterized by X-ray diffraction(XRD),temperature programmed reduction of H2(H2-TPR),temperature programmed desorption of CO2(CO2-TPD),N2adsorptiondesorption isotherms,scanning electron microscopy(SEM),transmission electron microscopy(TEM) and CO chemisorption.The results displayed that the RBZ-SG catalyst showed the highest activity for ammonia synthesis compared to those of RBZ-CP and RBZ-IP.The optimum ammonia concentration over RBZ-SG catalyst is 5.72% under the conditions of 3 MPa,10000 h-1 and 425 ℃.This activity is 3.8 and 14.3 times of that of RBZ-CP and RBZ-IP,respectively.Such high activity is mainly resulted from the presence of BaZrO3,which has high electron-donating ability.Mobile electrons would be transferred from BaZrO3 to the Ru metal surface by means of the strong metal-support interaction existed between Ru and reduction BaZrO3,which can facilitate the cleavage of N≡N and enhance the activity for ammonia synthesis sufficiently.更多还原