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

The aim of the present work was the study of dissociation kinetics of molec ular 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 er ror of about 3%. Molecular fluorine dissociation on the walls of a heated n ickel tubes was measured. For process simulation and analysis of the experi mental 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 pressu re 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 dissociat ion constant on the surface of nickel foil is k(d) = (2.4 +/- 0.7) x 10(4) exp[-(19 000 +/- 500)/RT] cm/s (at 700-900 K), the probability of heterogen eous recombination on the nickel surface gamma = (2.2 +/- 0.5) x 10(-4), ra te constant of the three-body gas phase recombination on molecular fluorine as 'third' body at room temperature k(rec) = (4.7 +/- 1.2) x 10(-34) cm(6) /s. This last value coincides with literature value. Our measurements confi rmed the value of the equilibrium constant of the fluorine dissociation pro cess: K = 1.11 x 10(25) exp(-37 840/RT)cm(-3) (K is expressed in cal/mole). (C) 1999 Elsevier Science S.A. All rights reserved.