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.