KINETICS AND THERMODYNAMICS OF ADSORPTION–DESORPTION OF WATER VAPORS ON LAYERED MICROSIZED MOLYBDENUM DISULFIDE POWDERS

Abstract

Experimental kinetic data on the adsorption and desorption of water vapors on molybdenum disulfide in relative humidity ranges of the gas phase from zero to 100% are presented. Layered microsized molybdenum disulfide powders (manufactured by Climax Molybdenum Co., USA) were used in the experiment. The adsorption–desorption kinetics of water vapors was studied by the gravimetric method. In the continuous automatic mode, the change in the weight of the test sample and the rate of the process were recorded. Water vapor adsorption was carried out in an air stream with a relative humidity of 100%. Desorption was studied under decrease of partial pressure of air flow water vapor to 55% of the relative humidity in isothermal and nonisothermal modes (in temperature increase and decrease in the range 20–130 °C). The time dependence of the adsorptive process of water vapors in a flowing system with 100% humidity of the gas phase is close to parabolic. The dependence of adsorption on water vapor pressure (V = f(P_i) is S-shaped with an excess coordinate in the range of transition to the stage of condensation formation of the adsorptive volumes. The desorption rate exceeds the adsorption rate. The potential theory of Polyani, Dubinin, and Rudushkevich was used to assess the bond energy between the water molecules and nonuniform MoS₂ porous structure. The nature of change in the adsorptive forces, P_i/P_s ratio, depending on the change in volume of the adsorbate is presented. Adsorption isotherms for water vapors were plotted at 30 and 50 ºС. The method of graphical integration was used to determine the differential rate of forming the water adsorption volumes (dV_i) at water partial pressures 0 < P_i < 5 kPa. The kinetic features of the step process are shown to depend on the potential energy of interaction between the adsorbed volumes and adsorbent. The nature of the adsorption process does not change in conditions of temperature change. The coordinates of intensive formation of the adsorption volume changed towards increase of partial pressure of water vapors.