Thermodynamics of Carbon Monoxide Adsorption on Cu/SBA-15 Catalysts: Under Vacuum versus under Atmospheric Pressures
Tongxin Han, Ilkeun Lee, Yueqiang Cao, Xinggui Zhou, Francisco Zaera
Abstract
The thermodynamics of the adsorption of carbon monoxide on a copper-based catalyst were estimated by using data from in situ infrared absorption spectroscopy experiments. A direct comparison was performed between the energetics under a vacuum environment versus in the presence of CO gas. It was found that the magnitude of the enthalpy of adsorption is reduced by almost a factor of 4 in going from the first case to the second, from ΔH°ads,vacuum = −82 kJ/mol to ΔH°ads,CO-atm = −21 kJ/mol. Furthermore, isosteric analysis of the data indicated that the magnitude of the latter decreases in the low-coverage limit, to values below ΔH°ads,CO-atm = −18 kJ/mol, a trend opposite to what has been reported in other systems. These observations are explained in terms of the associated standard entropy of adsorption, which in the presence of gas-phase CO was estimated at ΔS°ads,CO-atm ∼ −24 J/(mol K), a value much smaller in magnitude than the standard entropy of CO condensation. The excess entropy of CO adsorption over that of CO condensation is here ascribed to excess entropy in the adsorbed state due to additional phenomena induced by the gas-phase molecules such as adsorbate displacement and adsorbate-assisted adsorption steps.