Adsorption of CH 2 CClF and CH 2 CBrF on TiO 2 : infrared spectroscopy and quantum-mechanical calculations Jessica Scaranto and Santi Giorgianni Università Ca’ Foscari di Venezia – Dipartimento di Chimica Fisica, Calle Larga S. Marta 2137, I-30123 Venezia, Italy.
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Adsorption of CH2CClF and CH2CBrF on TiO2: infrared spectroscopy and quantum-mechanical calculations
Jessica Scaranto and Santi Giorgianni
Università Ca’ Foscari di Venezia – Dipartimento di Chimica Fisica, Calle Larga S. Marta 2137, I-30123 Venezia, Italy
The toxicity of the halogenated ethenes, which are compounds widely employed in the industrial field, represents a serious problem for the human health. Heterogeneous photocatalysis on TiO2 represents a promising approach for removing these compounds from the air . Since the decomposition occurs after the adsorption, a study on the nature of the adsorbate-substrate interaction can lead to useful information for a complete understanding of the reaction mechanisms and then, for the develop of successful applications. In a recent work, the adsorption of vinyl halides at room temperature was investigated by IR spectroscopy : according to the results it has been concluded that these molecules adsorb by an acid-base interaction through the halogen atom and the surface Lewis acid site (Ti4+), and an H-bond through the CH2 group and a surface Lewis basic site (O2- or OH-). This adsorbate-substrate model was successively studied by periodic quantum-mechanical calculations [3,4].
The aim of the present work is to formulate an adsorption model of the 1-chloro-1-fluoroethene (CH2CClF) and 1-bromo-1-fluoroethene (CH2CBrF) on TiO2 at room temperature through the analysis of the FTIR spectra of the adsorbed molecules. The attention has been focused on the adsorbate absorptions above 1000 cm-1 and in particular on the bands related to the C-H, C=C and C-F stretching modes. The approximate description of the vibrations of the adsorbates has been carried out by comparing the related absorptions with those of the compounds in the gas-phase. In order to obtain information on the variation of the molecular structural parameters, a periodic quantum-mechanical study according to the formulated model has been performed; the calculations have been carried out by considering the rutile (110) which represents the most stable surface of TiO2 .
Pre-treatment of TiO2
TiO2 powder (Degussa P25)
[pellet of 20 mg.cm-2]
T = 723 K, P ~ 10-4 Torr, t = 5 h
re-oxidation with mix N2/O2
Residual surface hydroxyl groups
The pre-treated surface contains two surface Lewis acid sites which differ in the electrophilicity
(TiO2 after the pre-treatment)
Introduction of the gas
(0.5 – 2.0 Torr)
20 scans at resolution of 4 cm-1
Proposed adsorption model
No H-bond between CH2 group and surface Lewis basic site (O2- or OH-)
Acid-base interaction between surface Lewis acid site and a molecular basic site (F atom or C=C bond)
IR spectra of CH2CClF in gas-phase and adsorbed on TiO2. (a) Room temperature, P ~ 1.0 Torr, 16 cm cell; the spectrum in the region 3800-2850 cm-1 has been multiplied by a factor of 10. Infrared spectrum of TiO2 taken after being in contact with ~ 0.6 (b) and ~ 1.2 (c) Torr of CH2CClF at room temperature.
IR spectra of CH2CBrF in gas-phase and adsorbed on TiO2. (a) Room temperature, P ~ 1.0 Torr, 16 cm cell; the spectrum in the region 3800-2850 cm-1 has been multiplied by a factor of 10. Infrared spectrum of TiO2 taken after being in contact with ~ 0.6 (b) and ~ 1.2 (c) Torr of CH2CBrF at room temperature.
Ti : DVAE (86-51G* ) 
O : TVAE (8-411G) 
CH2CFX : standard 6-31G** [11-13]
CH2CClF molecule Structure I Structure II
Eint = -20.19 Eint = -15.41
CH2CClF molecule Structure I Structure II
Eint = -18.47 Eint = -12.80
Rutile (110) surface
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