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

We have examined the adsorbate–substrate interaction kinetics of CO 2 and H 2 O on a natural forsterite crystal surface, Mg 2 SiO 4 (011), with 10–15% Fe 2+ substituted for Mg 2+ . We used temperature-programmed desorption and molecular beam techniques to determine the adsorption, desorption, and displacement kinetics for H 2 O and CO 2 . Neither CO 2 nor H 2 O 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 H 2 O are greater than those for CO 2 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, H 2 O preferentially binds to the highest-energy binding sites available and displaces CO 2 . The onset of significant CO 2 displacement by H 2 O occurs between 65 and 75 K.