KINETICS AND PRESSURE DEPENDENCE OF SURFACE CONTROLLED METAL OXIDATION REACTIONS

The pressure dependence of surface controlled metal-oxygen oxidation reactions is discussed. It is concluded that if the concentration of electrons at the surface is virtually constant, the oxidation rate will be linear. The variation in reaction velocity which occurs when the pressure is changed is complex. Various simplifications of the rate expressions are possible. For example, at low pressures, the rate will be proportional to the pressure if the rate determining step is dissociation of the oxygen molecule, or is one of the steps prior to dissociation. In all other cases the rate at sufficiently low pressures is proportional to the square root of the oxygen pressure. The case where the electron concentration across the oxide is given by a Boltzmann distribution is also treated. It is found that for most of the possible rate determining steps, linear or parabolic kinetics will result. However if the ionization of a charged surface species governs the reaction rate, either cubic or logarithmic kinetics will be observed. The pressure dependence of these various rate laws is determined, and some literature results discussed. © 1969.