摘要
用X射线粉末衍射 (XRD)、拉曼光谱 (LRS)、Hammett指示剂和微反测试等方法考察了负载型Nb2 O5/TiO2 催化剂表面铌氧 (NbOx)物种的分散状态、表面酸性和催化异丁烯 (IB)与异丁醛 (IBA)缩合生成 2 ,5 二甲基 2 ,4 己二烯 (DMHD)反应的催化性能 .实验测得Nb2 O5在TiO2 表面的分散容量为 0 94mmolNb/ 10 0m2 TiO2 ,与“嵌入模型”理论计算值相近 .当负载量低于分散容量的 1/ 3时 ,Nb2 O5主要以孤立的NbOx 物种通过Nb—O—Ti链与载体表面相连 ,这种孤立的NbOx 物种酸性很弱 ,催化活性很低 .随负载量的增加 ,孤立的NbOx 物种通过Nb—O—Nb键连接聚合 ,表面酸性增强 ,IB与IBA缩合生成DMHD的转化数 (TON)增加 .当Nb2 O5负载量超过分散容量时 ,表面NbOx 物种主要是通过Nb—O—Nb化学键相连形成多层的Nb2 O5,虽然催化剂的强酸中心数量有所增加 ,但NbOx 物种表面利用率下降 ,催化活性增加幅度趋缓 .
The dispersion state, surface acidity and catalytic properties of anatase-supported niobia species have been studied by means of X-ray powder diffraction (XRD), laser Raman spectroscopy (LRS), Hammett indicator method and the condensation reaction of isobutene (IB) with isobutyraldehyde (IBA) to 2,5-dimethyl-2,4-hexadiene (DMHD). The almost identical values of the experimental dispersion capacity of Nb2O5 on anatase (0.94 mmol Nb/100 m(2) TiO2) and the value derived theoretically by the incorporation model (0.98 mmol Nb/100 m2 TiO2) suggest that the highly dispersed niobium cations are bonded to the vacant sites on the preferentially exposed surface (001) of anatase. When the loading amount of Nb2O5 is below 1/3 of its dispersion capacity, the dispersed niobia species might mainly consist of isolated NbOx species bridging to the surface through Nb-O-Ti bonds. With the increase of Nb2O5 loading the isolated niobia species interact with their nearest neighbors through bridging Nb-O-Nb at the expenses of Nb-O-Ti bonds, resulting in the increase of the ratio of polymerized to isolated niobia species and the increase of surface acidic strength and catalytic activity for the condensation reaction. When the loading amount of Nb2O5 is higher than its dispersion capacity, the catalytic activity increases slightly with the increase of Nb2O5 loading due to the formation of bulk Nb2O5 in which Nb-O-Nb species are only partially exposed on the surface.
出处
《化学学报》
SCIE
CAS
CSCD
北大核心
2004年第14期1311-1317,J003,共8页
Acta Chimica Sinica
基金
国家重点基础研究发展规划 (No .G1 9990 2 2 4 0 0 )
南京大学分析测试基金项目资助
关键词
二氧化钛负载型铌酸催化剂
铌氧物种
分散状态
催化性能
表面酸性
niobium pentaoxide
titanium oxide
dispersion capacity
surface acidity
olefin-aldehyde condensation reaction
