Quasiequilibrium Treatment of Gas–Solid Reactions. I. Evaporation Rates of Volatile Species Formed in the Reaction of O2 with W, Mo, and C

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Abstract
A quasiequilibrium treatment of heterogeneous reactions is used to compute the rates of evaporation (desorption) of volatile species formed in the reaction of gaseous O2 with solid tungsten, molybdenum, and graphite under conditions of low pressure and high temperature. The results are compared with existing mass–spectrometric data for the O–W and O–Mo reactions, and the agreement is surprisingly close at the highest temperatures in view of the extreme simplicity of the theoretical model. Significantly better agreement is obtained over the entire temperature range (1400–3100°K) when an empirical method is introduced to account for the fact that not all of the molecules impinging upon the solid surface attain complete thermodynamic equilibrium with both the solid and adsorbate phase. It appears that the present treatment correctly predicts both the temperature dependence and the pressure dependence of the evaporation rate of each volatile species, as well as predicting that the proportions of the species will be those of an equilibrium mixture. The principal advantages of the quasiequilibrium treatment over existing kinetic treatments of gas–solid reactions are (1) the only empirical parameter is the equilibration or sticking probability, and (2) results are obtained without assuming microscopic models of the adsorbate phase and the reaction mechanisms.