Rate constant for heterogeneous dissociation of fluorine in a temperature range of 700–900 K on a nickel surface

Abstract

The aim of the present work was the study of dissociation kinetics of molecular fluorine on the hot surface of a nickel catalyst. The concentration of fluorine atoms was measured by EPR spectroscopy using molecular oxygen as a reference. The fluorine atom concentration was measured with an random error of about 3%. Molecular fluorine dissociation on the walls of a heated nickel tubes was measured. For process simulation and analysis of the experimental curves a one-dimensional kinetic model of the reactor which did not take into account axial and radial diffusion of the gas in the reactor and radial gas speed distribution was used, but account was taken of the pressure drop in the reactor and tubes between the reactor and ESR spectrometer. In the experiments we measured the fluorine atom concentration dependencies on the gas flow velocity and fluorine pressure (10–30 Torr). The dissociation constant on the surface of nickel foil is k d =(2.4±0.7)×10 4 exp [−(19 000±500)/RT] cm/s (at 700–900 K), the probability of heterogeneous recombination on the nickel surface γ = (2.2 ± 0.5) × 10 −4, rate constant of the three-body gas phase recombination on molecular fluorine as `third' body at room temperature k rec = (4.7 ± 1.2) × 10 −34 cm 6/s. This last value coincides with literature value. Our measurements confirmed the value of the equilibrium constant of the fluorine dissociation process: K = 1.11 × 10 25 exp(−37 840/ RT)cm −3 ( K is expressed in cal/mole).