制备方法对Ni/CeO2-Al2O3催化剂甲烷部分氧化催化性能的影响

摘要

A series of Ni/CeO2-Al2O3 catalysts were prepared by the methods of impregnation-precipitation,hydrothermal synthesis,co-precipitation and citric acid complexation.The effects of preparation method on the physical structures and catalytic performance of the resultant catalysts in partial oxidation of methane (POM) were investigated.The fresh and spent catalysts were characterized by N2 physical adsorption,X-ray diffraction (XRD),temperature-programmed reduction by hydrogen (H2-TPR),temperature programmed desorption of ammonia (NH3-TPD) and thermogravimetry (TG) analysis.The results show that Ni/CeO2-Al2O3 prepared by impregnation-precipitation method exhibits the lowest activity and selectivity to H2 and CO; the highest catalytic performance is achieved by the catalyst prepared through citric acid complexation.The Ni/CeO2-Al2O3 catalyst prepared by citric acid complexation has the largest specific surface area and the smallest CeO2 crystals with good dispersion.Both Ni species and NiAl2O4 spinel in Ni/CeO2-Al2O3 prepared by citric acid complexation are prone to being reduced to metal Ni,which can produce more active sites for methane partial oxidation reaction.There is more acidic sites and deposited carbon over the catalyst prepared by citric acid complexation; however,taking its high activity into account,its stability is relatively high with low carbon deposition rate.%采用浸渍-沉淀法、水热合成法、共沉淀法和柠檬酸络合法制备了Ni/CeO2-Al2O3催化剂,考察了制备方法对该催化剂的物理结构和甲烷部分氧化(POM)催化性能的影响.利用N2物理吸附、X射线粉末衍射(XRD)、H2-程序升温还原(H2-TPR)、NH3-程序升温脱附(NH3-TPD)、热重(TG)分析等手段对反应前后催化剂的物理化学性质进行了表征.实验结果表明,浸渍-沉淀法制备催化剂的活性和产物H2和CO的选择性最低.而柠檬酸络合法制备的Ni/CeO2-Al2O3表现出最大的CH4转化率和最高的CO、H2选择性.BET和XRD表征结果表明,柠檬酸络合法制备的Ni/CeO2-Al2O3的比表面积最大,且CeO2晶粒粒径小、分散均匀；H2-TPR测试表明,该催化剂负载的Ni物种和Ni与Al2O3相互作用产生的尖晶石NiAl2O4都较容易被还原成金属Ni,产生更多的活性中心；NH3-TPD和TG分析表明,该催化剂具有较多酸性位点和表面积炭,但相比较于它的高反应活性,积炭速率较低、稳定性较高.