Chemistry of Perfluorodiethyl Ether and Diethyl Ether on the TiO2

fully fluorinated ether, (C2F5)2O, physisorbed onto the TiO2 surface. As the ... reaction, indicating interaction of the ether with the surface hydrox...
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Langmuir 1996, 12, 739-745

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Chemistry of Perfluorodiethyl Ether and Diethyl Ether on the TiO2 Surface Debra A. Munro and Lily M. Ng* Department of Chemistry, Cleveland State University, Cleveland, Ohio 44115 Received April 13, 1995X The adsorption and thermal decomposition of a perfluorinated ether and an alkyl ether have been studied on the TiO2 surface using Fourier transform infrared spectroscopy. It has been found that the fully fluorinated ether, (C2F5)2O, physisorbed onto the TiO2 surface. As the surface was heated to 500 K, it was found that the (C2F5)2O thermally decomposed to a surface fluoroacetate species at temperatures greater than 200 K. The more acidic surface hydroxyl groups are permanently perturbed by the oxidation reaction, indicating interaction of the ether with the surface hydroxyl groups. The fully hydrogenated ether, (C2H5)2O, decomposed on the TiO2 surface to form a surface ethoxide at temperatures greater than 150 K. The more acidic surface hydroxyl groups are involved in this chemical reaction and do not recover by 500 K.

Introduction This study is part of our systematic investigation of the adsorption and thermal decomposition of fluorinated and hydrogenated ethers on oxide (Al2O3, TiO2, and SiO2) surfaces. Fluorinated polyethers have been widely used as lubricants in the computer and aerospace industry.1-3 While the decomposition of these fluorinated ethers is significantly less than their hydrogenated counterparts,1 the fluorinated ethers do eventually decompose.2,4 Simple fluorinated ethers, such as perfluorodiethyl ether, have been used as “model” ethers, and have been studied on other oxide surfaces, especially the Al2O3 surface.1,5 Studies of the mechanism of the fluorinated ethers’ thermal decomposition on different surfaces can give insight to factors that affect their thermal stability, and help to design more thermally stable lubricants. To fully understand the interactions of these fluorinated ethers with different oxide surfaces, the study of the adsorption and thermal decomposition of fluorinated ethers on the TiO2 surface seems a natural progression. The TiO2 surface has been the subject of many studies because it is chemically stable.6 It is now known that the TiO2 surface is quite reactive. TiO2 has been widely used as a catalyst, or a catalytic support, and is a good material for photocatalytic systems.7 The TiO2 surface studied here is anatase in character8 and differs greatly from the Al2O3 surface in both acidity and availability of hydroxyl groups on the surface. The surface of TiO2 is known to be less acidic than Al2O3.9 The TiO2 surface has been found to have acid-base properties.8 The acidic sites on the TiO2 surface are Lewis acid sites7,8 and are few in number.10 * Author to whom correspondence should be addressed. (1) Walczak, M. M.; Leavitt, P. K.; Thiel, P. A. Mat. Res. Soc. Symp. Proc. 1989, 140, 417. (2) Chandler, W. L.; Lloyd, L. B.; Farrow, M. M.; Burnham, R. K.; Eyring, E. M. Corrosion 1980, 36 (3), 152. (3) Gerhardt, G. E.; Lagow, R. J. J. Chem. Soc., Perkins Trans. 1 1981, 1321. (4) Pacansky, J.; Waltman, R. J. Phys. Chem. 1991, 95, 1512. (5) (a) Ng, L. M.; Lyth, E.; Zeller, M. V.; Boyd, D. L. Langmuir 1995, 11, 127. (b) Ng, L. M.; Li, P. J. Phys. Chem. 1995, 99, 17615. (6) Yates, D. J. C. J. Phys. Chem. 1961, 65, 746. (7) Tanaka, K.; White J. M. J. Phys. Chem. 1982, 86, 4708. (8) Hussein, G. A. M.; Sheppard, N.; Zaki, M. I.; Fahim. R. B. J Chem. Soc., Faraday Trans. 1 1989, 85 (7), 1723. (9) Morrison, S. R. The Chemical Physics of Surfaces, 2nd ed.; Plenum: New York, p 41C. (10) Morterra, C. J. Chem. Soc., Faraday Trans. 1 1988, 84 (5), 1617.

0743-7463/96/2412-0739$12.00/0

Two Lewis acid sites are known to exist on the TiO2 surface.7,10-12 One Lewis acid site is formed by the removal of a water molecule, and the other, stronger Lewis acid site is created by the removal of an isolated hydroxyl group.11 The hydroxyl groups on the TiO2 surface are amphoteric in character.7,8,11 There are two kinds of hydroxyl groups on the surface of TiO2, isolated and associated.6,8,13-15 The reason for this, according to Tsyganenko, is that each oxygen atom is surrounded by three Ti4+ atoms located at the apices of an isosceles triangle, and the number of hydroxyl group types is one unit less than the coordination number.14 Isolated hydroxyl groups are those free of hydrogen bonding. The oxygen atom of a terminal isolated hydroxyl group is bound to one Ti4+ atom, and the oxygen atom of a bridged, isolated hydroxyl group is bound to two Ti4+ atoms.14 On the clean TiO2 surface, two bridged hydroxyl groups are seen, owing to the fact that there are two positions in the titania lattice that the Ti4+ atom can occupy. In this study, perfluorodiethyl ether, (C2F5)2O, is used as a model monomer of the fluorinated polyether lubricants. Previous studies of (C2F5)2O on Al2O35a and SiO25b surfaces have produced drastically different results. While (C2F5)2O is thermally oxidized to the fluorinated and alkyl acetate and formate on the Al2O3 surface, there is no reaction on the SiO2 surface. The difference was partly attributed to the Lewis acidity of the Al2O3 surface as compared to that of SiO2. It is therefore logical to study the thermal chemistry of (C2F5)2O on a TiO2 surface as an “intermediate” surface. To the best of our knowledge, the activity of fluorinated ethers has not been studied on the titanium dioxide surface. For comparison purposes, the adsorption and thermal decomposition of (C2H5)2O is also studied on the same surface. The thermal decomposition of the fluorinated versus the alkyl ether and the role of the surface hydroxyl groups in the decomposition mechanism are investigated. (11) Primet, M.; Pichat, P.; Mathieu, M. J. J. Phys. Chem. 1971, 75, 1221. (12) Morterra, C.; Ghiotti, G.; Garrone, E.; Fisicaro, E. J. Chem. Soc., Faraday Trans. 1 1983, 79, 2343. (13) Primet, M.; Pichar, P.; Mathieu, M. J. Phys. Chem. 1971, 75, 1216. (14) Tsyganenko, A. A.; Filimonov, V. N. J. Mol. Struct. 1973, 19, 579. (15) Kiselev, A. V.; Uvarov, A. V. Surf. Sci. 1967, 6, 399.

© 1996 American Chemical Society

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Experimental Section The infrared spectra were obtained using a Mattson Galaxy 2020 FTIR spectrophotometer with the First Data analysis package. Resolution was 2.0 cm-1 with a data aquisition time of 14 s for five scans. Transmission infrared spectra were obtained from 4000 to 1000 cm-1. Thirteen-point boxcar smoothing was applied in all spectra. The spot size of the IR beam seen on the TiO2 sample was approximately 2 mm in diameter. Details of the experimental method have been previously described.5a The samples for the experiments were prepared by dispersing approximately 1.2 g of Degussa titanium dioxide P25 (with a surface area of 50 m2 g-1) in 10 mL of nanopure water with an ultrasonic bath for 20 min. Next, 80 mL of acetone (spectral grade, 99.7% purity, J. T. Baker) was added, and the mixture was sonicated for an additional 30 min. The resulting slurry was sprayed by an atomizer using N2 (99.998% purity) onto half of a CaF2 disk measuring 1 in. in diameter. Using a heat lamp, the disk was maintained at 80-90 °C to flash evaporate the acetone. Sample weights were 23.5 and 19.0 mg for the adsorption of (C2F5)2O and (C2H5)2O, respectively. The sample was mounted in a ultra high vacuum stainless steel cell which has been described previously.5a,16 The main cell body contains a stainless steel/copper ring which supports the sample. The temperature of the sample is controlled by passing cooled N2 gas or heated air through the support ring. A thermocouple attached to the support ring monitors the temperature of the sample. The cell body is contained between two CaF2 optical windows allowing for IR measurements in the 40001000 cm-1 range. The IR cell is attached to a grease-free stainless steel gas handling system which is maintained at a base pressure of e1 × 10-7 Torr by a Leybold TMP50 turbo pump backed by a D 1.5 Vane pump. Each sample was heated at 500 K for 20 h in vacuo prior to the experiment. Following heating, the sample was outgassed for 24 h. A total of 0.5 Torr of perfluorodiethyl ether was adsorbed onto the titania surface in 0.1 Torr increments at 100 K. For the diethyl ether, a total of 4 Torr was adsorbed onto the surface in 1 Torr increments. The total number of molecules adsorbed is calculated from the known volume of the gas line and the pressure difference before and after equilibration of the ether with the TiO2 surface. After each dose was adsorbed onto the surface, the cell body was pumped for at least 1 min to remove any residual gas phase ether before the IR spectrum were taken. During the experiments, the cell was translated in the IR beam path in order to acquire spectra on both the titania and CaF2 sides of the disk. The effects of the gas phase molecules or molecules adsorbed on the CaF2 disk can then be determined. The (C2F5)2O (90% minimum purity, Strem) and (C2H5)2O (99% purity, J. T. Baker) were transferred to glass storage flasks and purifid by several freeze-pump-thaw cycles. (C2F5)2O was certified to be completely fluorinated by Strem. The gas phase IR spectrum of the transferred (C2F5)2O shows only C-F and C-O vibrational modes. The mass spectrum of the gas phase ether shows no 14, 15, 18, 29, or 44 amu peaks.

Figure 1. (a) Surface hydroxyl groups on TiO2 at 100 K. (b-f) Difference spectra with (a) subtracted for various doses of (C2F5)2O at 100 K.

Results Study of (C2F5)2O Adsorption on TiO2 at 100 K. The effect of dosing the TiO2 surface with (C2F5)2O in the OH stretching region is shown in Figure 1. The spectrum of the clean TiO2 surface is shown in Figure 1a. Four sharp features are observed at 3733, 3702, 3679, and 3647 cm-1 and are assigned to isolated hydroxyl groups.8,13,14 The features at 3733 and 3702 cm-1 are assigned to terminal hydroxyl groups, and the features at 3679 and 3647 cm-1 are assigned to bridged hydroxyl groups. The broad feature centering at 3420 cm-1 is assigned to associated hydroxyl groups. The intensities of these peaks are quite small, signifying a small number of hydroxyl groups on the surface. Parts b-f of Figure 1 are difference spectra. The spectrum of the clean surface has been subtracted from the spectrum with the indicated dosage. (16) Wang, H. P.; Yates, J. T., Jr. J. Phys. Chem. 1984, 88, 852. (17) Pacansky, J.; Miller, M.; Halton, W.; Liu, B.; Scheiner, A. J. Am. Chem. Soc. 1991, 113, 329.

Figure 2. As for Figure 1, in the 1300-1000 cm-1 region of the spectrum.

No spectral change was detected in this region as the dosage of (C2F5)2O was increased. Figure 2 shows the spectral region from 1300 to 1050 cm-1 as a function of dosing. The spectra shown are difference spectra. Sharp features are observed at 1219, 1194, 1140, and 1100 cm-1. There is also a shoulder at 1269 cm-1. These peaks correspond to the perfluorodiethyl

Chemistry of Perfluoroethers

Figure 3. Difference spectra with TiO2 background spectrum subtracted showing spectroscopic changes in the OH stretching region following heating of the (C2F5)2O adsorbed layer.

ether and line up very nicely, increasing in intensity as the dosage of the ether increases. Identification of these features is discussed below. Assignment of these peaks will be discussed later. Effect of Thermal Treatment of (C2F5)2O Adsorbed on TiO2. After 0.5 Torr of (C2F5)2O was adsorbed onto the titania surface, the sample was heated from 100 to 500 K, and spectra were collected at 50 K increments. The effect of heating on the 3800-3200 cm-1 region is shown in Figure 3. The spectrum of the clean titania surface, 3a, has been subtracted from spectra 3b-g. Positive going features indicate an increase in vibrational intensity while negative going features indicate a decrease in intensity. As the heating continued, the bridged, isolated OH groups shift slightly and there is a small increase in intensity. Small peaks are seen at 3663 and 3634 cm-1 from 200 to 500 K as a result. The effect of heating on the 1300-1050 cm-1 region is shown in Figure 4. Spectrum 4a is identical to spectrum 2f. At 150 K, the intensity of the peaks increases substantially, as seen in spectrum 4b. This increase in intensity is expected. At 150 K, the bulk of the (C2F5)2O has desorbed from the surface, corresponding with the melting point of 147 K for (C2F5)2O.5a Figure 4b represents the spectrum of physisorbed and chemisorbed (C2F5)2O. Figure 4c shows the surface at 200 K. The monolayer of the perfluorodiethyl ether can now be seen. Identification of the peaks is discussed below. At 250 K, a new feature is seen at 1086 cm-1. This feature is still evident at 500 K. Also present at 500 K are broad, weak features at approximately 1247, 1205, and 1143 cm-1, indicating species remaining from 250 K. The effect of heating in the 1800-1350 cm-1 range is shown in Figure 5. The features at 1430, 1420, and 1359 cm-1 are associated with the adsorbed (C2F5)2O. At 200 K, features are seen corresponding to the chemisorbed monolayer, and new features appear at 1441 and 1590 cm-1. While the feature at 1590 cm-1 is very weak, the

Langmuir, Vol. 12, No. 3, 1996 741

Figure 4. As for Figure 3, in the 1300-1000 cm-1 region of the spectrum.

Figure 5. As for Figure 3, in the 1800-1350 cm-1 region of the spectrum.

feature at 1441 cm-1 continues to increase in intensity as the feature at 1359 cm-1 decreases in intensity. At 300 K, the the vibrational feature at 1359 cm-1 has disappeared and those at 1441 and 1590 cm-1 persist to 500 K. Study of (C2H5)2O Adsorption on TiO2 at 100 K. A total of 4 Torr of (C2H5)2O was adsorbed onto the TiO2 surface without affecting the OH stretching region of the

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Munro and Ng Table 1. Assignment of Vibrational Modes (cm-1) of Gas Phase and Physisorbed (C2H5)2Oa methyl νas(CH) methyl νas(CH) methylene νs(CH) methyl (HCH) bendas methyl (HCH) bendas methylene wag, bend methyl (HCH) bends methyl (HCH) bends methylene twist methyl(ene) rock ν(C-C), CH3 rock νas(COC) ν(C-C), CH3 rock

A

B

C

D

E

2958 2896 2700 1494 1455 1428 1390 1355 1283

2975 2940 2865 1472 1450

2985 2930 2870 1470 1460

1391 1337 1286 1195 1154 1119 1094

1380

2980 2835 2868 1456 1443 1414 1383 1351 1279 1163 1153 1120 1043

2971 2933 2869 1460 1443 1413 1371 1348 1283 1174 1155 1116 1045

1135 1093 1053

1280 1180 1140 1080

a

A: gas phase, see ref 25. B: see ref 19. C: see ref 20. D: see ref 18. E: this work.

Figure 6. Difference spectra with TiO2 background spectrum subtracted showing the 3000-2600 cm-1 region of the spectrum for various doses of (C2H5)2O at 100 K.

Figure 8. Difference spectra showing spectroscopic changes in the OH stretching region following heating of the (C2F5)2O adsorbed layer.

Figure 7. As for Figure 6, in the 1500-1000 cm-1 region of the spectrum.

spectrum, indicating no involvement of the surface hydroxyl groups. The effect of dosing the titania surface on the rest of the spectrum is seen in Figures 6 and 7. At 100 K, (C2H5)2O is physisorbed onto the titania surface. A multitude of features are seen in the CH stretching and bending regions and the C-O, C-C stretching region of the spectrum.

Due to the complexity of the adsorption bands, assignment of all the features is difficult. We have attempted to identify these features on the basis of the work by Weiser.18 This data is presented in Table 1. Effect of Thermal Treatment of (C2H5)2O Adsorbed on TiO2. After 4 Torr of (C2H5)2O was adsorbed onto the clean titania surface, the sample was heated from 100 to 500 K. The OH stretching region of the spectrum is seen in Figure 8 as the surface was heated. The clean titania surface has been subtracted from the spectra. At 150 K, a feature at 3216 cm-1 appears while negative features begin to be seen at 3672 and 3645 cm-1, indicating isolated surface hydroxyl groups are being converted to associated surface hydroxyl groups. At 250 K, the feature assigned to the associated hydroxyl groups, at 3216 cm-1, is no longer seen. At 500 K, the negative features at 3672 and 3645 cm-1 are still seen, indicating depletion of the bridged isolated hydroxyl groups. (18) Wieser, H.; Laidlaw, W. G.; Krueger, P. J.; Fuhrer, H. Spectrochim. Acta 1967, 24A, 1055.

Chemistry of Perfluoroethers

Langmuir, Vol. 12, No. 3, 1996 743 Table 2. Assignment of Vibrational Modes (cm-1) for Surface Ethoxide Species modea

Ab

Bb

Cb

Db

Eb

νas(CH3) νas(CH2) νs(CH3) δas(CH2) δas(CH3) δs(CH3) δs(CH2) (CH2)wag ν(C-C)/ν(CO)t ν(C-C)/ν(C-O)t ν(C-C)/ν(C-O)b ν(C-C)/ν(C-O)b

2970 2934 2875 1472 1449 1389

2965 2930 2865 1490 1445 1387

2980 2940 2885 1472 1448 1384 1356

2970

1286 1190 1149 1090 1043

1287 1172 1115 1070

2975 2930 2875 1470 1440 1380 1360 1270 1150 1110 1070

1196 1104 1056

2860 1480 1450 1390 1170 1120 1070 1050

a

Assignment of vibrational modes is for this work. See text for clarification of assignments. t denotes terminal; b denotes bridged. A: this work. B: see ref 19. C: see ref 20. D: see ref 27. E: see ref 26.

b

Table 3. Assignment of Vibrational Modes (cm-1) of Gas Phase and Physisorbed (C2F5)2O (C2F5)2O (C2F5)2O/ gas theo- (C2F5)2O/ (C2F5)2O/ TiO2 phase retical17 Al2O35a SiO25b (this work)

Figure 9. As for Figure 8, in the 1800-1000 the spectrum.

cm-1

region of

ν(C-C) ν′(CF3) ν′′(CF3) ν(CF2) ν(COC) ν(CF3 + CF2)

1250 1213 1155 1107

1370 1307 1289 1248 1200 1098

1358 1250 1237 1210 1147 1105

1359 1250 1223 1143 1102

1359 1269 1219 1194 1140 1100

cm-1 increases in intensity and broadens, indicating a mixture of different C-H stretching modes. At 250 K, a dramatically different spectrum appears. Features corresponding to the diethyl ether are gone, and a new species is seen. Features appear at 2988, 2970, 2934, 2875, 1472, 1449, 1389, 1286, 1190, 1149, 1090, and 1043 cm-1 indicating formation of the epoxide. The identification of the ethoxide species is presented in Table 2. Discussion of the peak assignments is presented later. By 500 K, features due to the ethoxide are of very little intensity, and no new peaks are seen. Discussion

Figure 10. As for Figure 8, in the 3000-2500 cm-1 region of the spectrum.

In Figures 9 and 10, the 3000-1000 cm-1 region of the spectrum is seen as the dosed TiO2 surface was heated from 100 to 500 K. Spectral changes can be seen to begin on the surface by 150 and 200 K. Shoulders appeared at approximately 1090 and 1190 cm-1. The feature at 1116 cm-1 broadens and shifts to a higher wavenumber. The band at 1371 cm-1 increases in intensity and shifts to higher energy. The C-H stretching band around 2900

Behavior of (C2F5)2O Adsorbed on TiO2 at 100 K. At 100 K, (C2F5)2O is physisorbed onto the titania surface. The region from 1300 to 1050 cm-1 exhibits strong features due to the C-O and C-F stretching frequencies of the ether. These features increase in intensity as the dosing of the (C2F5)2O increases. After the majority of the physisorbed ether has desorbed from the surface at 150 K, the (C2F5)2O monolayer is seen. The assignations of these features are based upon (1) the spectrum of gas phase (C2F5)2O, (2) calculated vibrational frequencies of the CO-CF stretching region done by Pacansky,17 (3) spectra of (C2F5)2O adsorbed onto the alumina surface,5a and (4) spectra of (C2F5)2O adsorbed onto the silica surface.5b These assignments are presented in Table 3. The region from 3800 to 3200 cm-1 is not affected by the adsorption of the perfluorodiethy ether, indicating no involvement of the surface hydroxyl groups. (C2F5)2O constitutes a solid on the surface of the TiO2 at 100 K and does not interact with the surface hydroxyl groups. Thermal Decomposition of (C2F5)2O on TiO2. When the (C2F5)2O/TiO2 surface is heated from 100 to 500 K, the majority of the (C2F5)2O desorbs at 150 K. It can be seen in Figures 4 and 5, beginning at 250 K, that the features associated with the perfluorodiethyl ether at 1430, 1359, 1269, 1219, 1194, 1140, and 1100 cm-1 decrease in intensity. As the temperature increases, features that

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Table 4. Comparison of Assignments of Vibrational Modes (cm-1) of Surface Acetates (CH3COO)3Al(s)5a (CF3COO)3Al(s)29 CF3COO-/Al2O35a CH3COO-/TiO220 CF3COO-/TiO230 CF3COO-/TiO224 CF3COO-/TiO2 (this work)

νas(OCO)

νs(OCO)

1570 1680 1675 1540 1734 1572 1590

1466 1490 1485 1440 1470 1482 1441

can be assigned to a surface fluoroacetate group are seen at 1590, 1441, 1247, 1205, 1143, and 1086 cm-1. Assignment of the vibrational modes of fluoroacetate is made in Table 4. These assignments require further discussion. It is well-known that the νas(OCO) vibrational frequencies for salts of fluorocarbon acids are in the range of 16951615 cm-1.29 For fluorinated carboxylic acids, ν(CdO) IR absorption frequencies are at about 1820-1890 cm-1 for the monomeric and about 1770-1790 cm-1 for the associated forms.29 The frequency of the νas(OCO) mode increases with the electron-withdrawing effects of substituants while the νs(OCO) frequency changes very little. Each additional fluorine atom increases νas(OCO) by approximately 30 cm-1. In Table 4, we listed for comparison purposes a study of CH3COO-/TiO2,20 a study of CF3COO-/Al2O3,5a and two studies of CF3COO-/TiO2.30 The two studies of CF3COO-/TiO2 give vastly different vibrational frequencies for the νas(OCO). Our own results correlate much better with the results given in ref 24. The νas(OCO) frequency assigned in ref 30 appears to be too high in comparison with other νas(OCO) modes of fluorinated carboxylates, solid, or surface-adsorbed fluorinated carboxylic ions (see ref 5a and references within). As stated by the authors, 1734 cm-1 is closer to the frequency of ν(CdO) of associated fluorinated carboxylic acids.30 In Griffith and Rochester’s study24 of hexafluoroacetone adsorption on a partially deuterated rutile surface, they originally detected IR absorption bands at 1640, 1615, 1579, 1572 (most intense), and 1482 cm-1. On evacuation at 473 K, the intensities of the bands at 1572 and 1482 cm-1 (assigned to asymmetric and symetric stretching vibrations of trifluoroacetate ions) increased at the expense of the bands at 1670 and 1643 cm-1. They also noted that the position of the asymmetric (COO) stretching vibration is primarily influenced by the extent of hydration of the rutile surface. Assuming the νas(OCO) vibrational frequency of CH3COO-/TiO2 as 1540 cm-1 as given in ref 20, and assuring the effect of addition fluorines on νas(OCO) frequency given by Spinner,31 CH2FCOOads species on TiO2 will have an asymmetric stretching mode at 1570 cm-1, exactly the frequency found by Griffiths and Rochester. A CH2FCOOads species can also be expected to be affected by hydration of the surface. Following the same reasoning, the fluroinated acetate identified in this study is probably (19) Arai, Saito, Y.; Yoneda, Y. J. Catal. 1968, 10, 128. (20) Hussein, G. A. M.; Sheppard, N.; Zaki, M. I.; Fahim, R. B. J. Chem. Soc., Faraday Trans. 1 1991, 87 (16), 2661. (21) Lyth, L.; Ng, L.; Yochum, S. Unpublished results. (22) Ito, K.; Bernstein, H. J. Can. J. Chem. 1956, 34, 170. (23) Gonzalez, F.; Munuera, G.; Prieto, J. J. Chem. Soc., Faraday Trans. 1 1978, 74, 1517. (24) Griffiths, D.; Rochester, C. J. Chem. Soc., Faraday Trans. 1 1977, 73, 1913. (25) Sadtler Standard Spectra; Sadtler Research Laboratories: Philadelphia, 1964; Spectrum No. 1335. (26) Greenler, R. J. Chem. Phys. 1962, 37, 2094. (27) Jackson, P.; Parfitt, G. J. Chem. Soc., Faraday Trans. 1 1972, 68, 1443. (28) Carrizosa, I.; Munuera, G. J. Catal. 1977, 49, 174. (29) Weiblen, D. G. Fluorine Chemistry; Simon, J. H., Ed.; Academic Press: New York, 1954; Vol. II, p 449. (30) Fan, J.; Yates, J. T., Jr. J. Phys. Chem. 1994, 98, 10621. (31) Spinner, E. J. J. Chem. Soc. 1964, 4217.

ν′(CF)

ν′′(CF)

ν(CF + CO)

1210 1268

1170 1234

1105

1258 1247 1247

1124 1201 1205

1143

ν(CF + CO)

1086

a CHF2COOads species, and not a CF3COOads species. The hydrogen in the CHF2COOads species can only come from surface hydroxyl groups. In Figure 3, the slight shifting of features assigned to the bridged isolated hydroxyl groups indicates the hydroxyl groups are being perturbed by the decomposition of the perfluorodiethyl ether. Thermal Decomposition of (C2H5)2O on TiO2. When the (C2H5)2O/TiO2 surface is heated, new features begin to appear at 150 K, indicating the formation of a new species. A dramatic spectral change is observed at 250 K. At this temperature, the features associated with (C2H5)2O are no longer seen, and features associated with the adsorbed epoxide are seen. This result was not unexpected. (C2H5)2O has been found to thermally decompose to a surface ethoxide on alumina19 and also on titania.20,27 The spectra obtained here are clearer, and present more distinct features. In Table 2, we have combined assignments given by Hussein et al.20 and Jackson and Parfitt.27 These assignments merit further discussion. Hussein et al. and Jackson and Parfitt differed in their assignments of several vibrational features. Hussein et al. assigned the modes at 1470 cm-1 as δs(CH2); 1380 cm-1 as δs(CH3), ethanol; 1360 cm-1 as δs(CH3), ethoxide; and 1270 cm-1 as δ(OH), ethanol. Jackson and Parfitt assigned the modes at 1472 cm-1 as δas(CH2); 1384 cm-1 as δs(CH3); 1356 cm-1 as δs(CH2); and 1196 cm-1 as δ(OH), ethanol. Both studies investigated the adsorption of ethanol on TiO2, anatase in Hussein’s study and rutile in Jackson’s study. Diethyl ether was formed as an intermediate on TiO2 in Hussein’s studies, and further decomposed to the ethoxide. Since our adsorbate was (C2H5)2O, we did not expect any vibrational features due to ethanol. Therefore, we have assigned the features at 1389 and 1472 cm-1 as δs(CH3) and δas(CH2), ethoxide, respectively, in agreement with Jackson and Parfitt. The feature at 1190 cm-1 was assigned as a ν(C-C)/ν(C-O) combination mode of the ethoxide, in agreement with Hussein et al. The feature at 1286 cm-1 was assigned as a (CH2) wag. As seen in Figure 10, at 150 K the surface hydroxyl groups become involved in the decomposition of the diethyl ether to the ethoxide. At 150 K, the peak at 3216 cm-1 is evidence that the hydroxyl groups are beginning to interact with the ether via hydrogen bonding. As the bridged hydroxyl groups are being converted to associated hydroxyl groups, negative peaks are seen at 3672 and 3645 cm-1. By 250 K, the feature at 3216 cm-1 is no longer seen; however, the negative peaks at 3672 and 3645 cm-1 persist as the surface is heated to 500 K. A possible explanation for this is offered. The diethyl ether decomposes to yield two ethoxide molecules. One of the ethoxide molecules must utilize an oxygen atom from the surface. The more acidic bridged hydroxyl groups12 supply this oxygen atom. At 500 K, the negative peaks at 3672 and 3645 cm-1 are still in evidence. The surface hydroxyl groups do not recover, even as the ethoxide desorbs from the surface. Comparison of Thermal Treatment of (C2F5)2O and (C2H5)2O on TiO2. At 100 K, both (C2F5)2O and (C2H5)2O

Chemistry of Perfluoroethers

are seen to be frozen on the TiO2 surface. As the surface is heated, both ethers are seen to thermally decompose, the (C2F5)2O to fluoroacetate, and the (C2H5)2O to an ethoxide. The extent of the decomposition reaction is lesser in the case of the (C2F5)2O and is directly related to the fluorination of the ether molecule. One reason is the stability of the fluorinated ether. The (C2F5)2O molecule is more stable than the (C2H5)2O molecule.1 Another factor is the effect fluorination has on the hydrogen bonding between the ether molecule and the surface. Both ethers were seen to interact with the surface via hydrogen bonding between the bridged, isolated hydroxyl groups and the oxygen atom of the ether molecule. The oxygen atom has the highest electron density in the ether molecule.22,23 The fluorine atoms on the (C2F5)2O decrease the extent of the hydrogen bonding. Hydrogen bonding between the (C2F5)2O and the surface hydroxyl groups was barely evident, as seen in Figure 3. The surface hydroxyl groups appear only to be perturbed. The fluorine atoms are more electronegative than the oxygen atom and withdraw electron density from the oxygen atom. This renders the oxygen atom less eager to become involved in hydrogen bonding with the surface hydroxyl groups. Another factor is the size of the flourine atom. Steric hindrance prevents efficient overlap of the oxygen lone pair orbital with surface atoms. The highly stable (C2F5)2O is loosely held to the surface, and the majority of the ether thermally desorbs before the surface can reach the temperature needed for the decomposition reaction to occur. On Al2O3, it has been shown that (C2F5)2O also decomposed to a surface fluoroacetate species. The fluoroacetate species converts to a surface fluoroformate species as the surface undergoes continued heating.5a (C2F5)2O thermally decomposed to a surface fluoroacetate species on the TiO2 surface to a much lesser extent, and the formation of the fluoroformate species is not seen. TiO2 is less acidic and contains fewer hydroxyl groups than the Al2O3 surface. Ng et al. concluded that isolated hydroxyl groups play a vital role in the adsorption of (C2F5)2O on the alumina surface and in the oxidation

Langmuir, Vol. 12, No. 3, 1996 745

reaction of (C2F5)2O to fluoroacetate and fluoroformate.5a The surface hydroxyl groups on titania appear unaffected by the adsorption of (C2F5)2O onto the surface at 100 K and are permanently perturbed, but not depleted, by the oxidation reaction of the (C2F5)2O to a surface fluoroacetate species at higher temperatures. (C2H5)2O is not as stable a molecule as (C2F5)2O, and the oxygen to surface bond is stronger than in the case of the (C2F5)2O. As the surface is heated, C-O bond scission occurs and the (C2H5)2O decomposes to form adsorbed ethoxide. On the titania surface, the surface hydroxyl groups remain unaffected by the adsorption of (C2H5)2O at 100 K, but are seen to be involved in the decomposition of the diethyl ether to the surface ethoxide at higher temperatures. Conclusions (C2F5)2O has been found to physisorb onto the TiO2 surface at 100 K. By 250 K, the majority of the adsorbed (C2F5)2O thermally desorbs. The few molecules remaining are thermally oxidized to a surface fluoroacetate species. The surface hydroxyl groups are not affected by the adsorption of (C2F5)2O onto the TiO2 surface, but are permanently perturbed by the thermal decomposition of the (C2F5)2O to the surface fluoroacetate. (C2H5)2O has also been found to physisorb onto the TiO2 surface at 100 K without involvement of the surface hydroxyl groups. At 150 K, the (C2H5)2O begins to thermally decompose to the surface ethoxide. The more acidic bridged hydroxyl groups are involved in the formation of the ethoxide. As the surface is heated to 500 K, the ethoxide desorbs from the surface. No new surface species are seen. The bridged, isolated surface hydroxyl groups on the TiO2 surface are seen to be necessary for the decomposition reactions to occur. The ether is initially held to the surface via hydrogen bonding between the oxygen atom of the ether and the bridged, isolated surface hydroxyl groups, allowing the decomposition reaction to take place. LA9502958