Energetic Properties of Metal-oxide Surfaces

固体的表面是化学键断裂的地方。处于细颗粒聚集状态的粉末具有大量这样的表面，因此其表面具有非常活跃的能量。粉末表面的能量性质可以用固体表面与被吸附分子的相互作用能，即吸附热来评价。本文在实验的基础上，用自制的吸附量热仪直接测量了水蒸气在几种金属氧化物上的吸附热，并通过计算微分吸附热来研究金属氧化物表面的能量均质性或非均质性。结果表明，由吸附热与吸附水量作图得到的吸附热曲线大致可分为两大类：一类是随吸附量的增加而单调递减的差热曲线，这在非均质表面是典型的；另一类是吸附热呈恒定值，在吸附热曲线上出现一个平台。TiO_2、SiO_2、Al_2O_3和ZrO_2属于前者。在这些情况下，化学吸附和物理吸附之间的吸附能差别很小。后者包括氧化锌、二氧化锡和三氧化二铬。这些氧化物的热曲线具有能量均一的特点，化学吸附和物理吸附的能量相差很大。氧化镁位于这两组之间。从不同条件下处理的γ-Fe_2O_3和NiO的吸附热数据可以看出，能量的均匀性与金属氧化物粉末的表面结构密切相关。

The surface of a solid is a place where the chemical bonds are broken. A powder which is in the state of aggregation of fine particles has a large quantity of such surfaces, and hence its surface is very active in energy. The energetic properties of powder surfaces can be evaluated by the interaction energy of solid surface with adsorbed molecules, namely, the heat of adsorption.In the present study, the heat of adsorption of water vapor on several kinds of metal oxides were measured directly by using an adsorption calorimeter which had been made by us on an experimental basis, and the energetic homogeneity or heterogeneity of metal-oxide surfaces were investigated by evaluating the differential heat of adsorption. It was found that the heat-of-adsorption curves obtained by plotting the differential heat of adsorption against the amount of adsorbed water are roughly divided into two main groups : one is that the differential heat of adsorption decreases monotonously with increasing amount of adsorption, which is typical for the heterogeneous surface, and the other is that the heat of adsorption shows a constant value, giving rise to a plateau in the heat curve. TiO_2, SiO_2, Al_2O_3, and ZrO_2 belong to the former group. In these cases, the difference in the adsorption energy between chemisorption and physisorption is small. On the other hand, the latter group includes ZnO, SnO_2, and Cr_2O_3. The heat curves for these oxides are characteristic of energetic homogeneity and the difference in energy between chemisorption and physisorption is significantly large. MgO was situated in the intermediate between these two groups. From the heat-of-adsorption data for gamma-Fe_2O_3 and NiO pretreated under various conditions, the energetic homogeneity was found to be closely related to the surface structure of metaloxide powders.