Adsorption of F2CCFCl on TiO2 nano-powder: Structures, energetics and vibrational properties from DRIFT spectroscopy and periodic quantum chemical calculations
Graphical abstract
Introduction
During the last years halogen-containing molecules have received a great deal of attention because of their alarming connection with stratospheric ozone depletion and global warming (e.g. [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]). Chlorotrifluoethene (F2CCFCl, halon 1113) is a molecule of industrial relevance mainly employed for the production of polymers and copolymers. Halon 1113 is also historically famous for being the first fluorinated ethene used for the preparation of commercial fluoro-plastics, PCTFE, that has been commercially available with the name ‘Kel-F’ in the early 1950s [11]. Despite their thermal stability, the thermolysis of chlorofluoropolymers releases several substances among which F2CCFCl can occur [12] and recently, this compound has been discovered in air samples [13]. This pollutant is a potential source of chlorine atoms, which in turn, enter in the catalytic destruction of stratospheric ozone. Very recently the vibrational properties of this molecule have been the subject of a detailed investigation carried out by coupling Fourier transform infrared (FTIR) spectroscopy to high level ab initio calculations employing the CCSD(T) method [14].
Eco-sustainable solutions to remove gas phase pollutants from the atmosphere are strongly required. A very attractive strategy is the heterogeneous photo-catalysis over semiconductor surfaces, such as titanium dioxide (TiO2). TiO2 is wide bandgap semiconductor, whose application in photo-catalytic processes (but also in photo-voltaic and water splitting devices) relies on its opto-electrical properties. Upon absorption of photons with energy larger than the band gap (about 3.0–3.2 eV), an electron–hole pair is created. The charge carriers might migrate to the surface, where they react with adsorbed species and ultimately lead to their complete decomposition into simpler molecules [15], [16], [17], [18], [19]. Application of this process has led to the development of self-cleaning cements and coating.
In order to improve the efficacy of a catalyst toward one molecule or a family of compounds, it is recommended to gain insights into the chemistry and the chemical physics of the complex processes underlying the reaction mechanism. Hence, the individual steps involved in the reaction pathway should be carefully investigated, the adsorption being the first one. In fact, the interaction of the target molecule with the surface may lead to a variation of its structure, with consequent activation of some bonds, e.g. through their weakening [20], [21], [22].
Infrared (IR) spectroscopy is a well-established technique to obtain experimental information about molecular structures and interactions, and DRIFT spectroscopy, in particular, is imposing itself as a prominent method to probe surface chemistry. During the last years, it has become the most effective technique for studying the processes taking place at the gas–solid interface [23], [24], [25], [26], [27], [28], [29]. Very recently, it has been shown that this technique can be efficiently employed to study the adsorption of fluorinated organic pollutants on titanium dioxide [30]. An even deeper knowledge about the structural, energetic and spectroscopic properties of molecules adsorbed on surfaces can be achieved by coupling experiments to molecular simulations. First-principles calculations complement the experimental data through the theoretical evaluation of geometrical parameters, interactions energies and vibrational frequencies, thus allowing the interpretation of the experimental results at an atomistic level. The interplay between experiments and quantum chemical simulations can be considered as the war horse for dissecting the chemical–physical properties underlying the adsorbate–substrate systems, and surface processes in general [31], [32], [33], [34], [35], [36], [37], [38].
Within this scenario, the aim of the present work is the study of the adsorption of F2CCFCl on titanium dioxide, in order to provide the chemical–physical picture (structures, energetic and vibrational features) of the adsorbate–substrate interaction at the microscopic scale. To this end the adsorption of F2CCFCl on TiO2 is investigated by coupling experimental DRIFT spectroscopy to periodic quantum chemical calculations rooted in density functional theory (DFT). The fundamental vibrations of the adsorbed molecule are assigned through the analysis of the DRIFT spectra, and the comparison with the vibrational frequencies of the gas phase molecule provides experimental indications about the interactions sites of the ad-structure. The most likely absorption configurations are simulated at B3LYP level in order to compute adsorption energies and vibrational frequencies of the adsorbed molecule. Theoretical predictions are then faced with the experimental evidences in order to obtain a thorough picture of the interaction between F2CCFCl and TiO2.
The work is structured as follow: experimental and computational details are described in Sections 2 Experimental details, 3 Molecular modeling, respectively; results are presented and discussed in Section 4 and finally conclusions are addressed in Section 5.
Section snippets
Experimental details
DRIFT spectra of F2CCFCl (UCAR, 99% purity) adsorbed on TiO2 nano-powder, were recorded in the medium IR region by using a Bruker Vertex 70 FTIR spectrometer equipped with a HgCdTe detector, a globar source and a KBr beamsplitter. The Harrick Scientific Praying Mantis diffuse reflectance accessory, which was mounted inside the sample compartment of the spectrometer, was fitted with a stainless steel high temperature reaction chamber (Harrick Scientific HVC-DRP-5). The chamber is enclosed by a
Molecular modeling
Periodic ab initio calculations were performed with the Crystal suite of programs [40], [41], [42], adopting DFT and specifically the B3LYP hybrid generalized gradient approximation (GGA) functional [43], [44]. The TiO2 substrate was modeled as the anatase (1 0 1) surface with a 12-atomic layers slab cut from the optimized bulk [45]. As usual, this surface was selected since it is the anatase most stable surface and hence it should be the most exposed surface available for the adsorption. The Ti
DRIFT spectra of F2CCFCl adsorbed on nano-powdered TiO2
The vibrational spectrum of an adsorbed molecule generally differs from that of the non-interacting species, the stronger the interaction the more pronounced the variation. Hence, IR spectroscopy can be exploited to obtain experimental information related to the strengthen variation of molecular bonds through an analysis of the shift of the corresponding absorptions, thus furnishing useful indications about the most likely anchor points.
From a spectroscopic point of view, chlorotrifluoroethene (
Conclusion
The present investigation dealt with a combined experimental and theoretical study on the adsorption of F2CCFCl (halon 1113), an atmospheric pollutant arising from the thermolysis of fluorinated polymers, over titanium dioxide. Experimentally, vibrational spectra of the adsorbed molecule have been recorded by means of DRIFT spectroscopy in conjunction with the use of an environmental chamber for controlling the reaction atmosphere over the surface. The analysis of the recorded spectra has led
Acknowledgments
The High Performance Computing department of the CINECA Supercomputer Centre and the SCSCF (”Sistema per il Calcolo Scientifico di Ca’ Foscari“) facility are gratefully acknowledged for the utilization of computer resources (grants n. HP10CVN2S9 and HP10CVEVP7). 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
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