Thermal deactivation kinetics and thermodynamics of a silica gel using surface area data

Abstract

A new calculation method was presented using surface area data for the thermal analysis of adsorbents. Five parts from a silica gel (Hypersil) were heated at the temperatures of 500, 640, 700, 770, and 850 °C, respectively, for 16 h. The maximum adsorption capacity as liquid nitrogen volume (0.930 cm³ g⁻¹), monolayer capacity (0.093 cm³ g⁻¹), surface area (A_H = 245 m² g⁻¹), number of monolayer (10) in the multimolecular adsorption, and heat of the first layer (3300 J mol⁻¹) were evaluated from the nitrogen adsorption data obtained at − 196 °C. Surface area (A) of the preheated samples was determined similarly. The assumed parameters $k = - (\partial A/\partial T)_{\text{p}} / A$ and $K = \left( {1 - a} \right)/ a$ were calculated for each preheating temperature, where $a = A/A_{\text{H}}$ is the relative decrease in the surface area by the thermal deactivation, because the k and K supplying Arrhenius equations and van’t Hoff equation behave as reaction rate constant and equilibrium constant, respectively. The activation energy for the thermal deactivation of the silica gel was calculated as $E^{\# } = 27330$ J mol⁻¹ from the slope of a straight line which is plotted according to the Arrhenius equation. The enthalpy change $(\Delta H^{0} = 28936$ J mol⁻¹) and entropy change ($\Delta S^{0} = 47.42$ J mol⁻¹ K⁻¹) for the same case were, respectively, evaluated from the slope and intercept of a straight line which is plotted according to the van’t Hoff equation. Accordingly, temperature dependence of the Gibbs energy is written as $\Delta G^{0} = \Delta H^{0} - T\Delta S^{0} = 28936 - 47.42 T$ by the SI units. The spontaneous nature of the deactivation was discussed using the last relationship.