A method of surface analysis and its application to reduced nickel powder

Abstract

The possibility of analyzing the complex surfaces of reduced metals by comparing their adsorptive properties with the known behaviour of evaporated films has been explored by experiments on nickel powder. Nickel prepared by reduction of the oxide in hydrogen at 450°C for periods up to 12h has been shown by measurement of hydrogen and krypton adsorption at -183°C to have a surface containing up to 37% of clean nickel atoms. Traces of silica introduced during the preparation of the powder possibly account for the rest of the surface. Adsorption of hydrogen at -183°C is effectively complete within ½ min at about 10 -5 mm, and all gas is desorbed by evacuation at 450°C. Oxide is probably not a significant impurity, because the powder does not exhibit the slow adsorption at 20°C and above which is characteristic of incompletely reduced nickel. Such activated adsorption can be restored by the addition of oxygen and eliminated again by further reduction; its stoichiometry suggests that the process consists of hydroxyl formation. Reduction of the sulphide gives nickel of appreciable surface area but completely inert to hydrogen; presumably a stable layer of sulphide is retained. Some thermodynamic and kinetic problems involved in the preparation of metal powders with surfaces of known purity are briefly discussed in the light of these results.