Bifunctional Hydrogen Bonding of 2-Chloroethyl Ethyl Sulfide on TiO2

The cell windows are KBr single crystals mounted on concentric Viton O rings .... to the Cl and the S moiety of the 2-CEES molecule is presented in Fi...
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10560

J. Phys. Chem. B 2003, 107, 10560-10564

Bifunctional Hydrogen Bonding of 2-Chloroethyl Ethyl Sulfide on TiO2-SiO2 Powders D. Panayotov and J. T. Yates, Jr.* Department of Chemistry, Surface Science Center, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260 ReceiVed: April 11, 2003; In Final Form: July 10, 2003

The adsorption of 2-chloroethyl ethyl sulfide on a high area TiO2-SiO2 mixed oxide powder has been studied by transmission infrared spectroscopy. It has been found that hydrogen bonding of this molecule to Si-OH groups occurs through both the Cl and the S moieties and that characteristic Si-OH red-shifts are found for each of the hydrogen bonds produced. A similar pattern of hydrogen bonding is found for the interaction of 2-chloroethyl ethyl sulfide with OH groups on TiO2 surfaces.

I. Introduction Titania-silica mixed oxides have attracted considerable attention as catalytic and photocatalytic materials.1 The mixed oxide may generate new catalytically active sites.1,2 Highly dispersed nanosized TiO2 particles in or on high surface area SiO2 may possess high photooxidation as well as photoreduction capabilities.1 The increased band-gap, from ∼3.3 eV for TiO2 (anatase) up to ∼4.1 eV for titania-silica oxides,3 may be connected to the higher photooxidation capability of the mixed oxides. This capability is attributed to a combination of two effects: the quantum-size effect causing an increase in band gap and the interface interaction between oxide phases, a SiO2 matrix or SiO2 support effect.3,4 The interface interaction leads to a formation of Ti-O-Si bonds strongly modifying the electronic structure of the Ti atoms.1-4 This holds for both types of titania-silica oxides, mixed oxides prepared by the coprecipitation or the sol-gel method, and for supported oxides obtained by impregnation or chemical vapor deposition. The dispersion of the material is closely related to the concentration of hydroxyl groups on the silica surface and the preparation conditions.5,6 On the other hand, the most important effect of Ti on the macroscopic structure of these materials is the effect of the network-terminating/modifying O-H bond.7 At low pretreatment temperatures (up to 453 K), the surfaces of amorphous titania-silica binary oxides are covered by both Si-OH and Ti-OH hydroxyl groups, terminating the mixedoxide network.5-10 Both hydrogen-bonded and isolated Si-OH hydroxyls are present and may serve as reactive sites. During heat treatment, the OH groups are gradually lost. The Ti-OH groups begin to dehydroxylate at a temperature of 453 K,5 and may be removed by thermal activation in a vacuum at 7735873 K11 to leave the more strongly bound isolated Si-OH groups with an O-H stretching frequency of about 3745 cm-1.5-9 Binary titania-silica oxide is a catalyst for the photooxidation of organic molecules, exhibiting superior properties compared to pure TiO2.12,13 Such TiO2-based photooxidation processes are useful in environmental remediation.14 The 2-chloroethyl ethyl sulfide (2-CEES) molecule is a simulant for mustard gas15 and diethyl sulfide (DES) has also been used as a simulant for mustard gas.16 Mustard gas contains * Corresponding author. E-mail: [email protected].

two chloroethyl groups instead of the single chloroethyl group of 2-CEES. To understand the factors influencing the photooxidation of mustard gas, it is important to understand the bonding mode for the mustard gas molecule to the surface. II. Experimental Section The powdered samples were hydraulically pressed at 12000 lbs/in2 into a fine-tungsten support grid17 (0.0508 mm thick, with 0.22 mm2 square holes) as a circular spot 7 mm in diameter. The grid is about 80% transparent, so that infrared radiation can pass through the sample efficiently. The grid is held by nickel supports in the center of the infrared cell which are used for heating and cooling. Its temperature is measured with a type K thermocouple, spot welded to the top of the grid. Electrical heating and cooling with liquid nitrogen permits temperature control in the range 100-1500 K. Steady-state temperatures may be maintained electrically to about (2 K, using an electronic programmer18 that senses the output of the thermocouple. The cell windows are KBr single crystals mounted on concentric Viton O rings which are differentially pumped to prevent leaks. The cell is a part of an ultrahigh vacuum system, pumped by both a Pfeiffer Vacuum 60 L/s turbomolecular pump and a Varian 20 L/s ion pump, and equipped with a mass spectrometer (Dycor Electronics Inc.). The base pressure of the system is below 10-7 Torr. Gas pressures are measured using a Baratron capacitance manometer. A more complete description of the cell may be found in refs 19,20. The sample cell is mounted on a translation system (Newport Corporation). The translation system is computer-controlled and capable of moving the cell to (1 µm accuracy in the horizontal and vertical directions. This allows one to record spectra for two samples and for background (an empty position on the grid) in the same experiment. Thus, it is possible to employ two samples at different positions on the same grid for comparison of their surface behavior under identical conditions of temperature and gas exposure. The infrared spectrometer was a Mattson Research Series I FTIR, and all scans were made in the ratio mode with a resolution of 4 cm-1. Typically, 200 scans were accumulated in each spectrum. The TiO2-SiO2 samples were prepared by Professor K. Klabunde and co-workers, Kansas State University, using a modified aerogel procedure applied to other mixed oxides, as

10.1021/jp0304273 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/28/2003

Hydrogen Bonding of 2-CEES on TiO2-SiO2 reported previously.21 This procedure allows one to obtain intimately intermingled mixed oxides, essentially molecular in nature, and to gain advantages by obtaining higher surface areas of the more reactive component throughout the structure, in this case TiO2 (photoactive in nature) with SiO2 (which supports very high surface areas). One chooses an appropriate solvent, hydrolysis, and drying techniques. A rapid hydrolysis step and faster gelation ensure the production of a more “open” structure. Then, after supercritical drying of hydroxides and their dehydration, high surface area oxide samples with large pore volume are produced. Thus, the TiO2-SiO2 mixed oxide (50 mol % TiO2 and 50 mol % SiO2) has very high surface area 680 m2/g, large pore volume 2.97 cm3/g, and average pore diameter 175 Å. The TiO2 sample was prepared by the same procedure, and its texture characteristics were as follows: surface area 136 m2/ g, pore volume 0.58 cm3/g, and average pore diameter 171 Å. The SiO2 sample was Aerosil 200 obtained from Degussa. Its surface area was 200 m2/g. 2-Chloroethylethyl sulfide (98%) and diethyl sulfide (98%) used for this work was obtained from Aldrich. These liquids were transferred under nitrogen gas to glass bulbs and purified by five freeze-pump-thaw cycles. The vapor was transferred to the infrared cell from the glass bulbs attached to the stainless steel gas line. Ethyl chloride (EC), stored in a metal cylinder, was also obtained from Aldrich and was 99.7% pure. The oxygen used is obtained from VWSCO and was 99.8% pure. The oxide samples, typically weighing 1-1.5 mg (1.3-1.9 mg/cm2), were heated in a vacuum at 673 for 4 h and then treated with 6.2 Torr O2 for 30 min at the same temperature. This procedure removes traces of residual organic species which could come from remnant alkoxy groups retained by the mixed oxide after the preparation procedure. After evacuation, the samples were treated in a vacuum at 1026 K for 3 h, and then cooled to room temperature. At this point, reference spectra were taken. Before exposure to adsorbates, the samples were cooled in a vacuum to 200 K. Adsorption of CEES, DES, and EC as well as FTIR measurements were carried out at 200 K.

J. Phys. Chem. B, Vol. 107, No. 38, 2003 10561

Figure 1. IR spectra of TiO2-SiO2 mixed oxide and SiO2 for different stages of dehydroxylation at successively higher temperatures in a vacuum.

III. Experimental Results A. Dehydroxylation of TiO2-SiO2. The infrared spectrum of the TiO2-SiO2 mixed oxide is shown in Figure 1 for three stages of dehydroxylation at successively higher temperatures in a vacuum. Initially, after thermal treatment at 673 K for 5 h, a sharp -OH peak at 3735 cm-1 and a broad intense -OH band at 3690 cm-1 are observed. The broad band may arise from either: (1) hydrogen bonding among densely packed neighboring hydroxyl groups;22,23 or (2) Ti-OH stretching modes.8,23-25 The infrared spectrum of silica, treated at the same conditions, exhibit only a sharp band of the Si-OH mode at 3745 cm-1, assigned to isolated silanol groups, previously observed for the same type of silica.26 Pure titania has an insignificant hydroxyl population after the same treatment (spectra not shown here). Upon heating of TiO2-SiO2 to 947 K and then to 1026 K, the broad band at 3690 cm-1 decreases in intensity, leaving the sharp Si-OH mode at 3741 cm-1. The above data conclusively show that the broad band observed for TiO2-SiO2 at low pretreatment temperatures can be assigned to Ti-OH species which is removed at temperatures above 673 K. This assignment is in accordance of those of previous studies on the hydroxyl coverage of titania-silica binary oxides.8,23-25 The partially dehydroxylated TiO2-SiO2 was used for adsorption experiments to be described below. Based upon the literature,13,27,28 it is

Figure 2. Set of IR spectra in (A) the -OH region and (B) in the CHx stretching region for the sequential adsorption of the 2-CEES molecule, at increased exposure, on the partially dehydroxylated surface of TiO2SiO2.

believed that this thermal treatment leads to TiO2-SiO2 phase segregation. B. Adsorption of 2-CEES on TiO2-SiO2. Figure 2A shows a set of spectra in the -OH region for the sequential adsorption of the molecule, at increased exposure, on the partially dehydroxylated surface. A development of an O-H spectrum is seen where the sharp Si-OH mode of the isolated hydroxyl groups is converted into two broad OH modes at lower frequency, at 3606 and 3356 cm-1, respectively. These broad modes are characteristic of hydrogen bonding of OH groups to the 2-CEES molecule. This behavior is consistent with the weak interaction of the CEES molecule with the isolated Si-OH groups through a van der Waals interaction. Modes of this type are often seen for adsorption on hydroxylated surfaces.22,23,29-32 The simultaneous development of the infrared spectrum for the adsorbed 2-CEES molecule in the C-H stretching region

10562 J. Phys. Chem. B, Vol. 107, No. 38, 2003

Panayotov and Yates

Figure 4. IR spectra of OH groups hydrogen-bonded to S and Cl moieties in the diethyl sulfide and the ethyl chloride adsorbed on TiO2SiO2.

Figure 3. Correlation of peak absorbances of the two associated OH modes at 3361 and 3575 cm-1, and one of the C-H stretching modes at 2981 cm-1 during adsorption of 2-CEES on TiO2-SiO2.

is observed, and shown in Figure 2B. Four modes assigned to the CH stretching modes of both CH3 and CH2 moieties are seen. The bands at 2972 and 2933 cm-1 are assigned to the symmetric (CH2)s and (CH3)s modes, respectively.33,34 Some contribution to the high-frequency side of the band at 2972 cm-1 is from the antisymmetric (CH3)as mode (ν ) 2990 cm-1). The bands at 2876 and 2840 cm-1 are probably due to overtones of δ(CH3)a and δ(CH2)a modes, respectively.33 A detailed assignment of the infrared modes observed for the gas-phase 2-CEES has been discussed elsewhere.31 The parallel development in absorbance of the two associated OH modes at 3361 and 3575 cm-1, and one of the C-H stretching modes of 2-CEES at 2981 cm-1 has been measured, and in Figure 3, an almost linear correlation of the peak absorbencies is shown. Note that both curves for 3361 and 3575 cm-1 mode, respectively, pass thru the origin. C. Absorption of Diethyl Sulfide and Chloroethane on TiO2-SiO2. These two molecules were used to determine if the two associated OH modes at 3606 and 3356 cm-1 may be assigned to Cl‚‚‚HO-Si and to S‚‚‚HO-Si interactions. Figure 4 shows spectra for diethyl sulfide and for chloroethane adsorbates, respectively. In the case of chloroethane, an associated OH mode at about 3620 cm-1 is produced. In the case of diethyl sulfide, the observed associated OH mode is centered at about 3280 cm-1. These two behaviors are closely reminiscent of the parallel behavior of the two associated Si-OH modes observed during 2-CEES adsorption. D. Adsorption of 2-CEES on TiO2. TiO2 was dehydroxylated by heating in a vacuum at 1026 K, and the infrared spectrum was measured as shown in Figure 5. Upon the adsorption of 2-CEES, the hydroxyl groups previously observed at 3720 and 3680 cm-1 (not shown here) were converted to two associated Ti-OH groups exhibiting broad infrared bands at about 3530 and 3375 cm-1. It is seen that the hydrogen bonding of 2-CEES to the TiOH groups is similar to the bonding effects seen for 2-CEES as it interacts with the Si-OH groups of TiO2-SiO2.

Figure 5. IR spectra in the -OH region for the adsorption of the 2-CEES molecule on a dehydroxylated TiO2 surface.

IV. Discussion of Results A. Hydrogen Bonding through Cl and S Moieties. The highly dehydroxylated surfaces (1026 K) of TiO2-SiO2 mixed oxides with a Ti/Si ratio of 1:1 contains only freely vibrating, isolated Si-OH hydroxyls, as shown in Figure 1. These isolated silanol groups are weakly reactive sites capable of physical adsorption of molecules by hydrogen bonding. The Si-OH band in the IR spectrum attributed to isolated OH groups is perturbed.22,23 In their review on hydrogen bonding, Pimentel and McLellan35 noted that hydrogen bonding causes the OH-stretching mode to be shifted to lower frequencies; the half-width of the associated -OH band is broadened, and the integrated intensity of the associated -OH band is many times larger than that of unperturbed band. Our observations for the adsorption of the 2-CEES molecule on the highly dehydroxylated surface of TiO2-SiO2 mixed oxide fully coincide with this behavior.

Hydrogen Bonding of 2-CEES on TiO2-SiO2

J. Phys. Chem. B, Vol. 107, No. 38, 2003 10563 TABLE 1: Comparison of CH-Stretching Frequencies for 2-CEES and DES in Different States gas phase at 300 K freq. (cm-1)

adsorbed at 200 K freq. (cm-1)

∆ν (cm-1)

2-CEES

2978 2943 2887

2972 2933 2876

-6 -10 -11

DES

2977 2938 2887

2970 2936 2877

-7 -2 -10

molecule

Figure 6. Schematic diagram of the red-shifts of the OH-stretching mode associated with the hydrogen bonding of Si-OH groups on TiO2-SiO2 to the Cl and the S moiety of the 2-CEES molecule.

A schematic diagram of the red-shifts of OH-stretching mode (to lower frequencies) associated with the bonding of Si-OH groups on TiO2-SiO2 to the Cl and the S moiety of the 2-CEES molecule is presented in Figure 6. The diagram shows that the red-shift for Cl‚‚‚HO-Si is significantly smaller than that for S‚‚‚HO-Si. Two effects may be responsible for the relative magnitude of the shifts of the Si-OH groups for the two types of hydrogen bonding. One effect may be due to the different energetics of bonding of Cl and S moieties to the silanol group. A second effect has to do with the electronic character of the Cl and S bonds to the carbon atoms in the 2-CEES molecule. In the case of hydrogen bonding in solutions, correlations have been found between the observed hydroxyl frequency shift (∆ν) and various other parameters.35 Of particular interest is the approximately linear relationship between ∆ν and ∆H for hydrogen bond formation. Kiselev36 has proposed that the redshift is directly proportional to the heat of adsorption on hydroxyl groups. In the case of adsorption on silanol groups, attempts have also been made to correlate the observed frequency shift with physical parameters such as polarizability, quadrupole moment, ionization potential, etc., as discussed in ref 37. However, in the case of adsorption of molecules differing markedly in electronic structure, the relationship between ∆ν and ∆H is not completely valid.36,37 Hertl and Hair have found37 that for a given heat of adsorption, a low shift is observed when p-orbital electrons are available for hydrogen bonding, and a large shift is observed when hybrid orbital electrons are used for hydrogen bonding. This is because the degree of electrostatic interaction is related to the magnitude of the frequency shift and will be greater in the case of a hybrid orbital. When the donor adsorbate molecule has lone pair electrons located in a p orbital, having central symmetry, a charge-transfer interaction with the hydrogen 1s orbital causes the O-H stretching vibration frequency to be lowered. In this case the red-shift of OH frequency will depend on the ionization potential of the donor molecule, and the shift is inversely proportional to the ionization potential.37 In the case when lone pair electrons are located in a hybrid orbital of the donor molecule, the hybrid no longer possesses the central symmetry of its component s, p, ... orbitals, and “the center of a mean position of a hybrid may be at some distance from the nucleus”.38 Then the atomic dipole can be quite large for the donor atom37 which in turn will contribute to a large shift in the OH frequency. We suggest that the difference in the electronic character of the Cl and S bonds to carbon atoms in the 2-CEES molecule is the reason for the observed difference in the red-shifts observed for Cl‚‚‚HO-Si and S‚‚‚HO-Si bonding. The hybrid character

of the bond of S to carbon atoms in the thioethers (2-CEES and DES) causes a large shift in the frequency. The red-shift of 467 cm-1 for DES observed here is consistent with a red-shift of 460 cm-1, as found for diethyl ether.37 The low red-shift in the case of Cl‚‚‚HO-Si bonding for 2-CEES and for C2H5Cl is similar to that for CH3Cl and CH3I adsorption.37 For these molecules p-orbital electrons are involved in the H bonding to the halogen moieties. The observed redshifts for the Cl‚‚‚HO-Si bonding in the cases of 2-CEES and C2H5Cl adsorption are 166 and 127 cm-1, respectively. These values are of the range of those found in ref 37 for the adsorption of CH3Cl and CH3I on silica (25 and 125 cm-1, respectively), and for CH3I (∼180 cm-1)32 and C2H5I (∼130 cm-1)33 adsorption on TiO2. Figure 3 shows clearly that the absorbance of the red-shifted Si-OH modes develops in a linear fashion with respect to the absorbance (and coverage) of the 2-CEES molecule. The near linearity of the curves suggests that a single Si-OH group bonds to a single Cl or S atom in the 2-CEES molecule. In addition, since both curves pass thru the origin then the formation of Cl‚‚‚HO-Si and S‚‚‚HO-Si bonds is the only process causing the red-shifts of the hydroxyls during the adsorption of the 2-CEES molecule, and bonding of 2-CEES occurs exclusively to Si-OH groups. The observation of the two characteristic OH red-shifts for coupling to both Cl and to S moieties in the bifunctional molecule, 2-CEES, suggests that it will be bound to hydroxylated surfaces by both interactions. Therefore, the bonding of mustard gas should be even stronger than for 2-CEES due to the presence of 2 Cl moieties along with the single S moiety. B. C-H Mode Perturbation in 2-CEES Adsorption. Table 1 shows the frequencies associated with the various C-H stretching modes. We observe small C-H mode red-shifts when the 2-CEES molecule adsorbs on TiO2-SiO2 by hydrogen bonding to Cl and S moieties. The frequencies are compared to those measured for the gas phase. The magnitude of the red-shifts observed are similar to the general shifts observed for alkyl groups when they interact with nonspecific bonds to a surface.32-34 V. Summary of Results The following conclusions can be made regarding the adsorption of 2-chloethylethyl sulfide on a highly dehydroxylated TiO2-SiO2 surface: (1) The 2-chloethyl ethyl sulfide molecule adsorbs on the surface of TiO2-SiO2 mixed oxide through both the Cl and the S moieties. Each of the Cl and the S atoms is bonded to a single isolated Si-OH group via hydrogen bonding. (2) The relative magnitude of the red-shifts of the Si-OH groups for the Cl‚‚‚HO-Si and S‚‚‚HO-Si hydrogen bonding differ significantly. This difference is attributed to the different electronic structure of the Cl and S bonds to the carbon atoms in the 2-CEES molecule. The hybrid character of the bond of S to carbon atoms is related to a large shift in the frequency when

10564 J. Phys. Chem. B, Vol. 107, No. 38, 2003 the S‚‚‚HO-Si bond is formed. A similar relative magnitude of the red-shifts for Cl and S hydrogen bonding is found for the interaction of 2-chloroethyl ethyl sulfide with OH groups on TiO2 surfaces. (3) These results allow us to predict that the adsorption of mustard gas on the surface of TiO2-SiO2 mixed oxide should be even stronger than for 2-CEES due to the presence of two Cl moieties along with the single S moiety in mustard gas. Acknowledgment. We acknowledge with thanks the support of this work by the DoD Multidisciplinary University Research Initiative (MURI) program administered by the Army Research Office under Grant DAAD19-01-0-0619. We thank J. Q. Wang and K. J. Klabunde for preparing the TiO2-SiO2 sample. References and Notes (1) Gao, X.; Wachs, I. E. Catal. Today 1999, 51, 233. (2) Matthews, R. W. J. Catal. 1988, 113, 549. (3) Lassaletta, G.; Fernandez, A.; Espinos, J. P.; Gonzalez, A. R. J. Phys. Chem. 1995, 99, 1484. (4) Mejias, J. A.; Jimenez, V. M.; Lassaletta, G.; Fernandez, A.; Espinos, J. P.; Gonzalez, A. R. J. Phys. Chem. 1996, 100, 16255. (5) Haukka, S.; Lakomaa, E.; Root, A. J. Phys. Chem. 1993, 97, 5085. (6) Srinivasan, S.; Datye, A. K.; Smith, M. H.; Peden, C. H. F. J. Catal. 1994, 145, 565. (7) Walters, J. K.; Rigden, J. S.; Dirken, P. J.; Smith, M. E.; Howells, W. S.; Newport, R. J. Chem. Phys. Lett. 1997, 264, 539. (8) Liu, Z.; Tabora, J.; Davis, R. J. J. Catal. 1994, 149, 117. (9) Miller, J. B.; Johnston, S. T.; Ko, E. I. J. Catal. 1994, 150, 311. (10) Gao, X.; Bare, S. R.; Fierro, J. L. G.; Banares, N. A.; Wachs, I. E. J. Phys. Chem. B 1998, 102, 5653. (11) Smith, P. D.; Klendworth, D. D.; McDaniel, M. P. J. Catal. 1987, 105, 187. (12) Anderson, C.; Bard, A. J. J. Phys. Chem. 1995, 99, 9882. (13) Anderson, C.; Bard, A. J. J. Phys. Chem. B 1997, 101, 2611. (14) Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier: Amsterdam, 1993. (15) Yang, Y.-C.; Ward, J. R.; Luteran, T. J. Org. Chem. 1986, 51, 2756.

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