DFT calculations of carbon monoxide adsorbed on anatase TiO2 (101) and (001)surfaces: correlation between the binding energy and the CO stretching frequency

The adsorption of carbon monoxide (CO) on anatase (101) and (001) surfaces was simulated using periodic density functional theory calculations. The surface Lewis acidity was evaluated by computing the binding energy and the adsorbed CO stretching frequency at surface coverages equal to 1 and 0.25 monolayer (ML). The obtained results, in agreement with the experimental data, indicate that the Ti cation of the (101) surface is more electrophilic than that of the (001) surface, corresponding to a larger surface Lewis acidity. A nearly linear correlation between the calculated binding energy and the CO stretching frequency was found for the first time at the computational level. The effects of slab relaxation on the two surfaces were also investigated and an opposite behaviour was found for the two parameters

Insights into the adsorption of CH2BrF on anatase TiO2(101) surface through DFT modelling

Bromofluoromethane (CH2BrF), considered a potential candidate to replace CFCs in many applications, generates serious problems about its effect on the ozone layer degradation and human effects. The adsorption of the compound on TiO2 is a key step for its decomposition through heterogeneous photocatalysis. Here, we investigated the energetics involved in the adsorption of CH2BrF on the anatase TiO2 (101) surface through detailed DFT analysis. Based on previous experimental results, the adsorbate-substrate geometry was modelled by simulating the acid-base interaction between the Br atom and the surface Ti ion and an H-bond between the CH2 group and the surface O ion. The adsorption was investigated at different surface coverages and periodicities in order to quantify and rationalise the lateral effects between co-adsorbed molecules and to estimate the interaction, distortion and binding energies in the limit of an isolated adsorbed molecule, i.e. in the limit of low coverage. The obtained constants indicate a strong repulsion due to the Br-Br interaction and a moderate attraction arising from the Br-H interaction. Then, at a given surface coverage, the most stable configuration involves the adsorption of the molecule through maximisation of the Br-Br distance and minimization of the Br-H distance. The lateral effects differ from those observed for chlorofluoromethane since the effects due to the Br-Br repulsion are stronger than those arising from the Cl-Cl repulsion. This behaviour suggests that the lateral effects cannot be generalised for a particular class of compounds, like as CH2XF, and a rigorous analysis should be always done in order to better rationalise the experimental data, to predict the most stable configuration under given experimental surface coverages and to provide the data for successive Monte Carlo simulations

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

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 8-12 mu m 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 (similar to 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 Ti4+ atoms as well as an H-bond between the CH2 group and an O2- 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. (C) 2014 Elsevier B.V. All rights reserved