五种二元过渡金属氧化物界面上的相互作用、非晶相结构及催化性能(Ⅱ)——DSC、半导体气敏特性、催化活性与亚单层分散模型

Interaction, Noncrystalline Structure and Catalysis Property on the Interface of Five Transition Metal Binary Oxides (Ⅱ) DSC, Semiconductor Gas-sensitivity, Catalysis Activity and Meta-monolayer Dispersion Model

应用DSC、半导体气敏特性、催化活性及亚单(分子)层分散模型共四项表征技术,进一步研究了五种二元氧化物的界面结构及其特性,DSC曲线的放热峰及吸热峰分别与界面化学反应、晶格畸变和瓦解、熔化、烧结以及固溶体的形成相关,导电性能的测试证明这些二元氧化物属于N-型半导体,对邻二甲苯具有气敏特性,其灵敏度在化学吸附的初期阶段与邻二甲苯蒸气浓度呈线性关系,催化选择性及转化率的测定证明V_2O_5-MnO_3及WO_3-MoO_3体系对邻二甲苯选择性氧化为苯酐具有催化活性,其非晶相MoO_3及V_2O_5的活性较为显著,尤其当二元氧化物的组成接近分散阈值Dt时,选择性最佳,为了解释大的分散阈值Dt与小的比表面积之间的关系,经计算机编程计算,在分子水平及纳米尺度上提出了球形八面体密置的亚单层分散模型并求得了模型的七个参数,通过讨论亚单层分散与非晶相结构之间的关系,提出了晶相损失的机理以及作为催化剂的非晶相结构对热的亚稳特性。

Continuing last paper, DSC results show that the exothermic and endothemic peaks could be attributed the formation of solid solution, lattice distortion or disintegration, melting, sintering and interface chemical reactions. Conductibility measurements prove these binary oxides are N-type semiconductors, their resistance decrease in o-dimethylbenzene atmosphere, i. e., they are sensitive to this vapor, and all the sensitivities depend linearly on the vapor concentration during the initial stage of chemical adsorption. Catalysis tests show V2O5-MoO3 and WO3-MoO3 are all catalytically active in the selective oxidation of o-dimethylbenzene to phthalic anhydride. Conversion and selectivity measurements indicate that non-crystalline dispersed MoO3 and V2O5 are obviously advantageous to the catalytic reaction, and selectivity is at its best when catalyst composition is close to the dispersed threshold value. In order to explain satisfactorily the relation of great dispersed threshold values with small specific area, the meta-monolayer dispersion model was suggested: each component oxide should be dispersed on other's surface with the unit of spherical octahedron MeO6 close-packed with each other by sharing O atoms to form a two-dimensional monolayer, and limited numbers of such monolayers are stacked up to form the meta-monolayer dispersed interfacial transition layer of a binary oxide. Seven parameters of the meta-monolayer model were calculated and listed:average mono- layer number (nL=1.6-8.5 layers), thickness per monolayer (tL=0.678-0.718nm), stacking-up thickness of each component oxide (tp=1.10-5.81nm), total thickness of interfacial transition layer (tI=5.43-8.78nm), octahedron radius (r0=0.199-0.219nm), close packed-monolayer capacity (C = 0.088-0.264g/100m2) and dispersed threshold value (Dt=0.33-1.43g/100m2). By discussing the relation of meta-monolayer dispersion with non-crystalline phase, the mechanism of crystalline phase loss, the non-crystalline structure and the thermal meta-stability of interfacial transition layer of binary oxide catalyst were described, respectively.