Adsorption, Desorption, and Displacement Kinetics of H₂O and CO₂ on Forsterite, Mg₂SiO₄(011)

We have examined the adsorbate–substrate interaction kinetics of CO₂ and H₂O on a natural forsterite crystal surface, Mg₂SiO₄(011), with 10–15% Fe²⁺ substituted for Mg²⁺. We used temperature-programmed desorption and molecular beam techniques to determine the adsorption, desorption, and displacement kinetics for H₂O and CO₂. Neither CO₂ nor H₂O has distinct submonolayer desorption peaks, but instead both have a broad continuous desorption feature that evolves smoothly into multilayer desorption. Inversion of the monolayer coverage spectra for both molecules reveals that the corresponding binding energies for H₂O are greater than those for CO₂ on all sites. The relative strength of these interactions is the dominant factor in the competitive adsorption and displacement kinetics. In experiments in which the two adsorbates are codosed, H₂O preferentially binds to the highest-energy binding sites available and displaces CO₂. The onset of significant CO₂ displacement by H₂O occurs between 65 and 75 K.