γ-Mo2N和分子筛负载的钼氮化物的结构表征

采用程序升温氮化的方法制备了分子筛负载的钼氮化物催化剂,并用EXAFS方法研究了 氮化前后Mo原子的局域配位情况.氮化前负载MoO3样品的径向结构函数中有三个峰,其中前两 个峰对应着最近的Mo-O配位壳层,但是第一个峰与第二个峰的比例比晶体MoO3中的比例大很 多,表明分子筛负载的MoO3具有更紧密的结构.氮化以后,Mo2N样品的径向结构函数中有三个 峰,对应于一个Mo-N和两个Mo-Mo配位壳层,与面心立方模型符合得很好.根据XRD和EXAFS谱 的计算表明,Mo2N中的N原子使Mo-Mo键拉长并削弱.分子筛负载的Mo2N样品具有与非负载Mo2 N样品近似相同的径向结构函数,只是对应于Mo-N壳层的峰较弱,表明负载的Mo2N具有更大的 结构无序性.

Structure Characterization of

Zeolites supported Mo nitrides were prepared via MoO3＋NH3 reaction in temperature programmed mannar.The XRD data show that the frame structure of NaY and Na-ZSM-5 zeolites can still be maintained after nitridation at 973K.By an alysis of the extended X-ray absorption fine structure(EXAFS) of the Mo K-abso rption edge,the structural informations of both oxidic and nitrided Mo catalysts were obtained.The supported MoO3 samples show three peaks in the radial structu re function(RSF).The first two peaks correspond to the nearest Mo-O coordinatio n shell,but the ratio of the first peak to the second one is much larger than th at of the MoO3.This suggests that the MoO3 supported on zeolites has more compac t structure than the unsupported one.There are three peaks in the RSF of Mo2N,re presenting one Mo-N and two Mo-Mo shells,which is in good agreement with the f ace-centered cubic model.Calculation from X-ray diffraction data and EXAFS spe ctra reveals that the N atoms in Mo2N lengthen the distance between Mo atoms and weaken the Mo-Mo bond.Supported Mo2N samples give out nearly the same RSFs as the Mo2N,only the peak corresponding to Mo-N shell is weak,suggesting that the supported Mo2N has more structural disorder than the unsupported one.Mo2N suppor ted on Na-ZSM-5 gives out shorter coordination distance than supported on NaY, suggesting stronger interaction between Mo2N and Na-ZSM-5.