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Abstract

A new method to analyze microcalorimetry data was employed to study the adsorption energies and sticking probabilities of D₂O and CO₂ on CaO(001) at several temperatures. This method deconvolutes the line shapes of the heat detector response into an instrument response function and exponential decay functions, which correspond to the desorption of distinct surface species. This allows for a thorough analysis of the adsorption, dissociation, and desorption processes that occur during our microcalorimetry experiments. Our microcalorimetry results, show that D₂O adsorbs initially with an adsorption energy of 85–90 kJ/mol at temperatures ranging from 120 to 300 K, consistent with prior spectroscopic studies that indicate dissociation. This adsorption energy decreases with increasing coverage until either D₂O multilayers are formed at low temperatures (120 K) or the surface is saturated (150 K). Artificially producing defects on the surface by sputtering prior to dosing D₂O sharply increases this adsorption energy, but these defects may be healed after annealing the surface to 1300 K. CO₂ adsorbs on CaO(001) with an initial adsorption energy of ~ 125 kJ/mol, and decreases until the saturation coverage is reached, which is a function of surface temperature. The results showed that pre-adsorbed water blocks adsorption sites, lowers the saturation coverage, and lowers the measured adsorption energy of CO₂. The calorimetry data further adds to our understanding of D₂O and CO₂ adsorption on oxide surfaces.