Insights into the interaction between CH2F2 and titanium dioxide: DRIFT spectroscopy and DFT analysis of the adsorption energetics

Abstract

Difluoromethane (CH2F2, HFC-32) has been proposed as a valid replacement for both CFCs and HCFCs (in particular HCFC-22), and nowadays it is widely used in refrigerant mixtures. Due to its commercial use, in the last years, the atmospheric concentration of HFC-32 has increased significantly. However, this molecule presents strong absorptions within the 8-12μm atmospheric window, and hence it is a greenhouse gas which contributes to global warming. Heterogeneous photocatalysis over TiO2 surface is an interesting technology for removing atmospheric pollutants since it leads to the decomposition of organic compounds into simpler molecules. In the present work, the adsorbate-substrate interaction between CH2F2 and TiO2 is investigated by coupling experimental measurements using DRIFT spectroscopy to first-principle simulations at DFT/B3LYP level. The experimental results confirm that CH2F2 interacts with the TiO2 surface (∼80% rutile, 20% anatase) through both F and H atoms and show that the DRIFT technique is well suited to study the adsorption of halogenated methanes over semiconductor surfaces. DFT calculations are carried out by considering different periodicities and surface coverages, according to a structure involving an acid-base interaction between the F and Ti(4+) atoms as well as an H-bond between the CH2 group and an O(2-) ion. Lateral effects and energetics are analyzed in the limit of low coverage according to a procedure taking into account the binding, interaction, and distortion energies. The simulation at the different surface coverages and periodicities suggests similar decomposition pathways for the different investigated ensemble configurations.

Keywords: Adsorption; Density functional theory; Difluoromethane; Lateral effects; TiO(2).