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

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Highlights

  • Difluoromethane (HFC-32), atmospheric pollutant used in refrigerant mixtures.

  • Photo-catalysis over TiO2 surface, eco-sustainable method for removing pollutants.

  • Adsorption of HFC-32 on TiO2 studied by DRIFTS and periodic DFT simulations.

  • Interplay experiment–theory indicates that HFC-32 adsorbs mainly through one F atom.

  • Adsorption energetics and chemistry at different surface coverage.

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 8–12 μm 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 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.

Introduction

On the last decades, the role of many atmospheric trace compounds on the global environment and climate change has been the subject of a lot of investigations since apparently different phenomena like acid rains, photochemical air pollution and changes in stratospheric ozone layer are associated to their presence. During the past years halogen-containing molecules have attracted a great deal of attention because of their alarming connection with stratospheric ozone depletion and global warming [1], [2], [3]. Hydrohalofluorocarbons have been among the first candidates to replace the CFCs in many applications since they are degraded more efficiently in the lower atmosphere due to the attack by hydroxyl radicals [1], [2].

Difluoromethane (CH2F2, HFC-32) is a relatively non-toxic gas which has been proposed for CFC and HCFC (hydrochlorofluorocarbons) replacement in new refrigerant systems. Due to its atmospheric relevance, over the years this molecule has been the subject of several spectroscopic investigations, either experimental and theoretical (for an overview of the relevant literature see [4], [5], [6], [7], [8], [9], [10], [11], [12], [13] and references therein). Recently, the vibrational spectrum of this molecule has been thoroughly studied by coupling Fourier transform infrared (FTIR) spectroscopy to high level ab initio calculations [14]. The obtained potential energy surface and dipole moment surface, in conjunction with the accurate experimental data obtained from laboratory measurements, have led to a detailed modeling of the spectroscopic properties of CH2F2 concerning vibrational energy levels, integrated intensities and vibrational mixing.

Although HFC-32 has zero ozone depletion potential, it has a significant global worming potential [15] and for this reason its removal from the air should be desirable. Heterogeneous photocatalysis on semiconductors, such as on titanium dioxide (TiO2), represents an interesting approach since it leads to the decomposition of organic compounds into simpler molecules such as water and carbon dioxide [16], [17], [18]. The knowledge about the adsorbate–substrate interaction should allow an improvement of the catalytic performances since the adsorption of the compound is one of the key steps in the degradation process. The interaction with the surface may lead in fact to a variation of the molecular structure such as to the activation of some bonds through e.g. their weakening. Then, a prediction of the reaction mechanism pathways based on the variation of the structural parameters may help to develop more successful applications.

Infrared (IR) spectroscopy is a well established method to obtain experimental information related to the strengthen variation of molecular bonds through an analysis of the shift of the corresponding absorptions, while first-principles simulations allow to implement the experimental data through an investigation of the geometrical parameters, the involved energies and the comparison between computed and observed vibrational frequencies. Then, the coupling between IR spectroscopy and quantum–mechanical calculations is an important tool in the study of the adsorbate–substrate interaction.

There are different ways of applying IR spectroscopy to solids, the best known technique being the transmission measurement. Although widely and profitably used, this method suffers of some disadvantages for the investigation of chemical processes taking place on solid catalysts. These mainly arise from sample preparation, as the pressing procedure, employed for preparing pellets or self-supporting wafers, may let to structural modifications of the solid phase and, besides, gases can no longer flow freely through the catalyst thus limiting mass transport. Moreover, in the case of self-supporting wafer, the solid layer must be sufficiently thin in order to allow the radiation to be transmitted [19], [20].

Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy overcomes the drawbacks of transmission methods as the sample requires a minimal preparation and it can be directly employed in powdered form. In addition, a number of commercially affordable environmental reaction chambers is available, that allow a fine control of the experimental conditions. This technique is also more sensitive than transmission methods to species adsorbed on solids. As a consequence DRIFT spectroscopy is imposing itself as a powerful tool in surface chemistry and it is becoming the most effective technique for the study of processes taking place at the gas–solid interface (e.g. see Refs. [21], [22], [23], [24], [25]).

In a previous work the adsorption of CH2F2 on TiO2 was investigated through IR transmission spectroscopy and DFT calculations [26] and, based on the experimental and computational results, it was suggested that CH2F2 interacts with TiO2 through both one of the F atoms and one of the H atoms.

In order to gain more information related to the lateral effects among adsorbed molecules and to determine the energies in the limit of low coverage, in the present work we performed the calculation at different surface coverage and periodicities. One of the aims is to gain insights on the most probable ensemble configurations at the different surface coverage by comparing the binding and interaction energies for different periodicities corresponding to the same surface coverage. The other aim is to investigate the possible presence of different pathways for the different surface coverages or ensemble configurations. Finally, the adsorption of CH2F2 on titanium dioxide is also reinvestigated experimentally by DRIFT spectroscopy down to the limit of 1000 cm−1, in order to take advantage from the sensitivity of this technique for the study of surface processes, and critically examining its advantages and drawbacks for the study of the adsorption of this atmospheric organic pollutant on the TiO2 catalyst.

Section snippets

Experimental details

DRIFT experiments were performed by using the Harrick Scientific Praying Mantis diffuse reflectance accessory equipped with an high temperature reaction chamber (Harrick Scientific HVC-DRP-5). This reaction chamber is made of stainless steel and it is enclosed with a dome with three windows, two of which are made of KBr in order to allow the spectrometer radiation to enter and exit the chamber, while the third UV quartz window can be used for viewing or irradiating the sample. Within the

Computational details

All the calculations have been performed using the CRYSTAL program [28]. The study was performed within the same computational conditions employed for previous work on CH2F2 [26].

In particular, the calculations were performed at DFT/B3LYP level [29], [30] and an 86-51G and 8-411G contractions properly developed for the TiO2 surface were adopted for the O and Ti atoms, respectively [31]. For the molecule, the cc-pVTZ contraction was used [32].

In order to take into account the anharmonicity, the

DRIFT Spectra

The differential spectrum of CH2F2 adsorbed on TiO2 is presented in Fig. 1, together with the spectrum of the free molecule. In discussing the shifts of CH2F2 vibrational frequencies observed upon adsorption, which in turn mirror the structural modifications induced by the interaction with the semiconductor surface, it is convenient to start from the higher wavenumber region, around 3000 cm−1. In the spectrum of the free molecule, this region deals with the absorptions due to the ν1 (2948 cm−1)

Conclusions

The adsorption of CH2F2 over TiO2 has been investigated both experimentally and theoretically by coupling DRIFT spectroscopy to DFT periodic simulations. On the experimental side, the analysis of the DRIFT differential spectrum, has led to the assignment of the vibrational absorption bands of the adsorbed molecule, showing that CH2F2 interacts with the TiO2 surface by means of either fluorine and hydrogen atoms. The results have been compared to those previously obtained from transmission

Acknowledgments

This work has been supported by MIUR through PRIN 2012 funds for project STAR (Spectroscopic and computational Techniques for Astrophysical and atmospheric Research), and PRIN 2009 funds for project SPETTRAA (Molecular Spectroscopy for Atmospherical and Astrochemical Research: Experiment, Theory and Applications) and by University Ca’ Foscari Venezia (ADiR funds). N.T. thanks University Ca’ Foscari Venezia for his post-doctoral position.

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