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 [1]. 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 [2]: 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 [5].
IR spectroscopy
Experimental Details
CH2CClF
CH2CBrF
Pre-treatment of TiO2
(a)
(x 10)
4
2
(x 10)
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
1
3
1
3
Residual surface hydroxyl groups
around 3700 cm-1
Transmittance
3050
The pre-treated surface contains two
surface Lewis acid sites which differ in
the electrophilicity
1360
1624
1118
1135
1186
1168
10%
5
(b)
(c)
3140
4
2
5
(b)
Transmittance
(a)
(c)
3036
3135
Adsorption spectra
Background
(TiO2 after the pre-treatment)
Introduction of the gas
(0.5 – 2.0 Torr)
20 scans at resolution of 4 cm-1
1112
1122
1632
1166
10%
1156
1654
3650
3400
3150
2900
1700
1643
Proposed adsorption model
1550
1400
1250
1100
-1
Wavenumber / cm
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.
Experimental vibrational frequencies
CH2CClFa(gas)
CH2CClF/TiO2
Vibration Approx. description Wavenumber Wavenumber
3140
CH2 asym stretching
3069
1
3050
CH2 sym stretching
3016
2
1654
C=C stretching
1656
3
1624
1360
CH2 bending
1383
4
1186; 1168 b
C-F stretching
1186
5
1135; 1118 b
Wavenumbers are given in cm-1.
a: from ref. [6]. b: the two frequencies refer to the presence of two
surface Lewis acid sites.
3650
F
Ti
O
Ti
O
O
1100
Wavenumbers are given in cm-1.
a: current work. b: the two frequencies refer to the presence of two
surface Lewis acid sites.
O
Structure II
Structure I
1250
CH2CBrFa(gas)
CH2CBrF/TiO2
Vibration Approx. description Wavenumber Wavenumber
CH2 asym stretching
3055
3135
1
CH2 sym stretching
3002
3036
2
1643
C=C stretching
1647
3
1632
CH2 bending
1369
1348
4
1166; 1156 b
C-F stretching
1166
5
1122; 1112 b
Cl
H
1400
Experimental vibrational frequencies
H
F
1550
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.
X = Cl, Br
H
2900 1700
Wavenumber / cm
Acid-base interaction between surface
Lewis acid site and a molecular basic
site (F atom or C=C bond)
Cl
3150
-1
No H-bond between CH2 group and
surface Lewis basic site (O2- or OH-)
H
3400
Quantum-mechanical calculations
Cl
Computational details
H1
C1
H1
Program
C2
F
Br
C1
CRYSTAL03 [7]
CRYSTAL06 [8]
H2
C2
F
H2
Method
CH2CClF molecule
Structure I
Eint = -20.19
Structure II
Eint = -15.41
CH2CClF molecule
DFT/B3LYP [9]
Structure I
Eint = -18.47
Structure II
Eint = -12.80
Basis set
Ti : DVAE (86-51G* ) [10]
O : TVAE (8-411G) [10]
CH2CFX : standard 6-31G** [11-13]
Calculated structural parameters
Free
C1-C2
C1-Cl
C1-F
C2-H1
C2-H2
Cl-C1-C2
F-C1-C2
H1-C2-C1
H2-C2-C1
F-Ti
C1-Ti
C2-Ti
Ti-F-C1
Ti-C1-C2
Ti-C2-C1
1.326
1.739
1.333
1.081
1.083
125.1
123.2
121.0
119.3
Adsorbeda
Structure I % Structure II
1.321
-0.4
1.331
1.728
-0.6
1.734
1.358
1.9
1.330
1.080
-0.1
1.082
1.080
-0.3
1.084
125.9
0.6
125.2
123.2
0.0
123.1
120.6
-0.3
120.3
120.9
1.3
118.6
2.557
3.763
3.237
148.2
57.0
102.8
%
0.4
-0.3
-0.2
0.1
0.1
0.1
-0.1
-0.6
-0.6
Lengths and angles are reported in Å and degrees,
respectively.
a: % refers to the isolated optimised molecule (Free).
Calculated structural parameters
Free
C1-C2
C1-Br
C1-F
C2-H1
C2-H2
Br-C1-C2
F-C1-C2
H1-C2-C1
H2-C2-C1
F-Ti
C1-Ti
C2-Ti
Ti-F-C1
Ti-C1-C2
Ti-C2-C1
Rutile (110) surface
O(2f)
Ti(6f) Ti(5f)
O(3f)
Bulk and surface parameters
Bulk
aB [Å] 4.572
cB [Å] 2.995
u
0.305
Vibration
1
2
3
4
5
6
7
8
9
10
11
12
Adsorbed
Structure I
Structure II
Approx.
Wavenumber Wavenumber Wavenumber
description
CH2 asym stretch
3179
3202
3077
CH2 sym stretch
3082
3110
2989
C=C stretch
1675
1668
1603
CH2 bend
1361
1350
1298
C-F stretch
1166
1120
1076
CH2 rock
927
897
862
C-Cl stretch
694
664
638
CClF bend
412
412
396
CClF rock
357
372
357
CH2 wag
819
824
791
torsion
658
652
626
CClF wag
510
479
460
Wavenumbers are given in cm-1.
The vibrational frequencies have been scaled by using a scaling factor equal to 0.961.
%
0.4
-0.3
-0.1
0.1
0.1
0.1
-0.2
-0.6
-0.5
Lengths and angles are reported in Å and degrees,
respectively.
a: % refers to the isolated optimised molecule (Free).
Surface
aS [Å] 2.995
bS [Å] 6.465
γ [°]
90.0
Calculated vibrational frequencies
Free
1.325
1.906
1.332
1.081
1.084
125.0
123.3
121.2
119.1
Adsorbeda
Structure I % Structure II
1.320
-0.4
1.330
1.893
-0.7
1.900
1.357
1.9
1.331
1.079
-0.2
1.082
1.080
-0.4
1.085
125.1
0.1
125.1
123.3
0.0
123.1
120.9
-0.2
120.5
120.7
1.3
118.5
2.568
3.857
3.290
145.2
55.4
105.1
Calculated vibrational frequencies
Free
References
[1] Linsebigler, A. L.; Lu, G.; Yates Jr., J. T. Chem. Rev. 1995, 95, 735.
[2] Scaranto, J.; Pietropolli Charmet, A.; Stoppa, P.; Giorgianni, S. J. Mol. Struct. 2005, 741, 213.
[3] Scaranto, J.; Mallia, G; Giorgianni, S.; Zicovich-Wilson, C. M.; Civalleri, B.; Harrison, N. M. Surf. Sci. 2006, 600,
305.
[4] Scaranto, J.; Giorgianni, S. J. Phys. Chem. C 2007, 111, 11039.
[5] Diebold, U. Surf. Sci. Rep. 2003, 48, 53.
[6] Mann, D. E.; Acquista, N; Plyler, E. K. J. Chem. Phys. 1955, 23, 2122.
[7] Saunders, V. R.; Dovesi, R.; Roetti, C.; Orlando, R.; Zicovich-Wilson C. M.; Harrison, N. M.; Doll, K.; Civalleri,
B.; Bush, I. J.; D’Arco, P.; Llunell, M. CRYSTAL03 User’s Manual, University of Torino (Torino, 2003).
[8] Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; Zicovich-Wilson C. M.; Pascale, F; Civalleri, B.; Doll, K.;
Harrison, N. M.; Bush, I. J.; D’Arco, P.; Llunell, M. CRYSTAL06 User’s Manual, University of Torino (Torino, 2006).
[9] Becke, A.D. J. Chem. Phys. 1993, 98, 5648.
[10] Muscat, J. PhD Thesis, University of Manchester, 1999.
[11] Hariharan, P. C.; Pople, J. A. Theoret. Chim. Acta 1973, 28, 213.
[12] Francl, M. M.; Petro, W. J.; Hehre W. J.; Binkley J. S.; Gordon, M. S.; DeFrees D. J.; Pople J. A. J. Chem. Phys.
1982, 77, 3654.
[13] Rassolov, V. A.; Ratner, M. A.; Pople, J. A.; Redfern, P. C.; Curtiss, L.A. J. Comp. Chem. 2001, 22, 976.
Vibration
1
2
3
4
5
6
7
8
9
10
11
12
Adsorbed
Structure I
Structure II
Approx.
Wavenumber Wavenumber Wavenumber
description
CH2 asym stretch
3175
3202
3177
CH2 sym stretch
3077
3112
3072
C=C stretch
1660
1662
1634
CH2 bend
1364
1345
1355
C-F stretch
1147
1099
1142
CH2 rock
934
896
936
C-Br stretch
581
581
584
CBrF bend
346
353
349
CBrF rock
310
328
313
CH2 wag
824
826
826
torsion
693
646
699
CBrF wag
473
432
486
Wavenumbers are given in cm-1.
The vibrational frequencies have been scaled by using a scaling factor equal to 0.961.