Energetic Properties of Metal-oxide Surfaces

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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.