CO Adsorption Isotherms on Ice by Fourier Transform Infrared Spectroscopy and New Insights of the Ice Surface from Quantum ab Initio Investigations

Various ice samples are prepared from highly concentrated H 2 O/Ar matrixes submitted to various annealing treatments. Each of these samples corresponds to a different stage in the solid organization from amorphous to nanocrystalline clusters. The type II isotherms are drawn from a 0.1 to a few monolayers surface coverage in the 43−48 K temperature range as a function of the integrated absorbance of the carbon monoxide (CO) vibrational mode, measured by FTIR spectroscopy, vs the CO equilibrium pressure. In the frame of the Brunauer, Emmett, and Teller (BET) model the mean enthalpy of adsorption of the first CO monolayer is evaluated to be about 10 kJ mol -1 and is independent of the history of the ice surfaces. The quantum modeling on a perfect ice surface leads to adsorption energies in good agreement with experimental results for an isolated admolecule as well as for the monolayer. The CO adsorbs perpendicularly to the surface plane, and the two orientations of CO or OC are energetically equivalent. Adsorption on a surface defect modeled as a hole indicates hydrogen bonding between the admolecule and the substrate. Comparing experimental and quantum results, we may conclude that the CO molecule sees the ice surface as covered by protons that make the adsorption dynamic process insensitive to the presence of surface defects.