γ-Mo_2N和分子筛负载的钼氮化物的结构表征

Structure Characterization of γ-Mo_2N and Mo Nitrides Supported on Zeolites

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MoO_3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MoO_3中的比例大很多,表明分子筛负载的MoO_3具有更紧密的结构.氮化以后,Mo_2N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,Mo_2N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Mo_2N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Mo_2N具有更大的结构无序性.

Zeolites supported Mo nitrides were prepared via MoO_3＋NH_3 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 analysis of the extended X-ray absorption fine structure(EXAFS) of the Mo K-absorption edge,the structural informations of both oxidic and nitrided Mo catalysts were obtained.The supported MoO_3 samples show three peaks in the radial structure function(RSF).The first two peaks correspond to the nearest Mo－O coordination shell,but the ratio of the first peak to the second one is much larger than that of the MoO_3.This suggests that the MoO_3 supported on zeolites has more compact structure than the unsupported one.There are three peaks in the RSF of Mo_2N,representing one Mo－N and two Mo－Mo shells,which is in good agreement with the face-centered cubic model.Calculation from X-ray diffraction data and EXAFS spectra reveals that the N atoms in Mo_2N lengthen the distance between Mo atoms and weaken the Mo－Mo bond.Supported Mo_2N samples give out nearly the same RSFs as the Mo_2N,only the peak corresponding to Mo－N shell is weak,suggesting that the supported Mo_2N has more structural disorder than the unsupported one.Mo_2N supported on Na-ZSM-5 gives out shorter coordination distance than supported on NaY,suggesting stronger interaction between Mo_2N and Na-ZSM-5.