Kinetics and Mechanism of the Reaction of Powdered Chromium with Gaseous Phosphorus

Abstract

The phosphidation kinetics of powdered chromium has been studied under gaseous phosphorus atmosphere of 100-760 mmHg in a closed vessel at 600-700°C. As a sinter of phosphides formed, a-alumina powder was previously added to the chromium sample as a diluent at a weight ratio (Cr/Al₂O₃) of 2.
The reaction products were CrP and Cr₃P as a result of X-ray diffraction analysis, and CrP₂, a higher phosphide, was not found. In an earlier stage of the phosphidation the reaction proceeded linearly with time and, subsequently, parabolically. In an earlier stage, the ratedetermining step seemed to be a surface reaction; therefore the study was made by applying the Langmuir adsorption isotherm. As its result, it could be presumed that the phosphidation proceeded under the adsorption-dissociation of gaseous P₄, molecule into four phosphorus atoms. On the other hand, in an advanced stage of the phosphidation, the data showing the relation between the fractional phosphidation and time fitted approximately Jander's equation under the present reaction condition. Hence, the rate-determining step seemed to be a diffusion process.
It could be concluded that the rate-determining step of the phosphidation of chromium transferred from the surface reaction to the diffusion process.