Langmuir 1996,11, 127-135
127
Surface Chemistry of Perfluoro Ethers: An Infrared Study of the Thermal Decomposition of (C2F5)20 on A l p 0 3 L. M. Ng* and E. Lyth Department of Chemistry, Cleveland State University, Cleveland, Ohio 441 15
M. V. Zeller and D. L. Boyd? NASA Lewis Research Center, Cleveland, Ohio 44135 Received November 18, 1993. In Final Form: November 8, 1994@ The adsorption and thermal decomposition of a model polyperfluorinated ether on high surface area A1203has been studied using transmission infrared spectroscopy. It has been found that (C2F&0 interacts with the isolated OH groups on the A1203 surface, forming an increasing number of associated OH groups from 150 to 600 K. Surface fluoroacetate and surface fluoroformate species are also formed from the thermal decompositionof the (CzF5)zOlayer. At temperatureshigher than 300 K, the surface fluoroacetate converts to surface fluoroformate. Oxidation of the (CzF5)20molecules also occurs on a surface preadsorbed with pyridine, indicating that Lewis acid A13+ sites, blocked by pyridine adsorption, are not involved in fluoroacetate and fluoroformate production. The absence of a chemical reaction between the (CzF5)zO adlayer and a (CH&SiCl-treated A1203 surface indicates that the presence of available surface hydroxyl groups is necessary for the oxidation of the perfluoro ether by A1203. A mechanism for the thermal decomposition of (CzF&O on A1203 is proposed.
Introduction Aluminum oxide, a typical ionic oxide, is commonly used as a catalyst and catalyst support. It also occurs as a "native" oxide on bearing and machinery surfaces made from aluminum alloys and on surfaces of amgnetic recording media. When hydrated alumina is calcined at moderate temperatures (473 K < T < 1173 K),OH groups with different reactivities appear on the surfacein addition to strong Lewis acid and base sites.'-3 The role of surface hydroxyl groups on A 1 2 0 3 supports in the oxidative degradation of metal catalysts has been recognized by Brenner et al.,4v5Solymosi et a l . , 6 3 ' and Basu et a L 8 p 9 Recent studies by Ng et al., Basu et al., and Chen et al. on the interaction of alkyl and partially fluorinated ethers with atomically clean A l ~ 0 3 ~as O well as high surface area A120311J2 have indicated that surface hydroxyl groups play a significant role in the thermal decomposition of fluorinated ethers. A thorough understanding of the reactivity of this class of organic molecules is of practical importance because perfluorinated polyethers are widely used as lubricants for machinery used in space, advanced gas turbine engines, and computer disks.
In this study, we investigate the interaction of perfluorinated diethyl ether with high surface area A 1 2 0 3 by transmission infrared spectroscopy. Our purpose is to gain further insight into the nature of reactive sites on A 1 2 0 3 and how these sites influence the decomposition of perfluorinated ethers. From the results of these studies, we will elucidate the reaction mechanism leading to the degradation of these fluorinated compounds.
Experimental Section The infrared spectra were measured using a purged Mattson Galaxy2020JTIR spectrophotometerwith the First data analysis package. Resolution was 2.0 cm-l with a data acquisition time of 14 s for five scans from 4000 to 1000 cm-'. Experimentation with increased number of scans and data acquisition time did not result in a better signal-to-noise ratio. Despite dry air purging, minor rotational structure due to absorption by atmospheric water was evident in the spectra. These features were removed by subtraction prior to any other data manipulation. Nine-point boxcar smoothing was applied in the region from 4000 to 3000 cm-l in all spectra. Spectra in the other regions were unsmoothed. The samplesfor the IR experimentswere prepared as follows. A slurry of approximately 2 g of Degussa aluminum oxide C (y-alumina,with a surface area of 100 m2g-l) in 100 mL of a 1:9 * Author to whom correspondenceshould be addressed. watedacetone mixture was prepared. The A 1 2 0 3 was fist t Permanent address: Technology Department, Kent State dispersed in 10 mL of water and 10 mL of acetone for 20 min University, Kent, OH 44242. using an ultrasonic bath. An 80 mL portion of acetone (spectral Abstractpublished inAdvance ACSAbstracts, January 1,1995. grade, 99.7%purity, J.T. Baker)was then added and the mixture (1)(a)Peri,J.B.J.Phys.Chem.1965,69,211. (b)Peri,J.B.;Hannan, was sonicated for another 30 min. The slurry was sprayed by R. B. J. Phys. Chem. 1960, 64, 1526. an atomizerusing Nz (99.998%purity)onto half of a 1in. diameter (2) Knozinger,H.; Ratnasamy,P. Cutul. Rev.-Sci.Eng. 1978,17(11, CaFz disk. The disk was maintained at 80-90 "C by a heat lamp 31. to flash evaporate the acetone. Sample weights ranged from 14 (3)Morrison, S. R. The Chemical Physics ofsurfaces, 2nd ed.; Plenum to 17 mg for the experiments with pyridine and 25 mg for the Press: New York, 1990. (4) Brenner, A.; Hucul, D. A. J . Cutul. 1980, 61, 216. perfluorodiethyl ether. A 1 2 0 3 samples were also prepared using (5) Hucul, D. A.; Brenner, A. J.Phy. Chem. 1981,85,496. pure water as the dispersing agent. Experiments performed on (6) Solymosi,F.; Pasztor, M. J.Phys. Chem. 1986, 89, 4789. these samplesproduced qualitativelyidenticalresults. The data (7) Solymosi, F.; Pasztor, M. J . Phys. Chem. 1986,90,5312. presented here are representative of our results to date. (8)Basu, P.; Panayotov,D.; Yates,J. T., Jr. J.Phys. Chem. 1987,91, The sample was mounted in a UHV stainless steel cell which 3133. has been described previ0us1y.l~Briefly, the main cell body (9) Basu, P.; Panayotov,D.; Yates, J. T., Jr. J.Am. Chem. SOC.1988, 110,2074. contains a copper/stainless steel ring which supports the CaFz (10) Ng, L.; Chen, J. G.; Basu, P.; Yates, J. T., Jr. Lungmuir 1987, disk. The temperature of the sample is controlled by passing 3, 1161. cooled and liquified Nz(g)or heated air through the support ring. (ll)Chen,J.G.;Basu,P.;Ballinger,T.H.;Yates,J.T.,Jr.LangmuirA chromel-alumel thermocouple attached to the ring monitors 1989, 5 , 352. (12) Basu, P.; Ballinger, T. H.; Yates, J. T., Jr. Langmuir 1989, 5, (13)Wang, H. P.; Yates, J. T., Jr. J.Phys. Chem. 1964, 88, 852. 502. @
0743-7463/95/2411-0127$09.00/00 1995 American Chemical Society
Ng et al.
128 Langmuir, Vol. 11, No. 1, 1995 sample temperature. The cell body is contained between two CaFz optical windowssealed in standard stainless steel flanges, permitting IR measurements in the 4000-1000 cm-l range. The IR cell is attached to a grease-free stainless steel gas handling system and is maintained at a base pressure P 5 1 x lo-' Torr by a Leybold TMP5O turbo pump backed by a D 1.5vane pump. Before the experiments, the sample was heated at 475-500 K for 100 h in vacuo. Four Torr of (CzF5)20 was adsorbed by increments onto the alumina surface at 100 K. Each dose was measured by closing the valve to the cell, charging the gas handling system of known bolume to the required pressure as measured by a MKS Baratron (Type 122A), and opening the cell valve once again. 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 (C2F&0 with Al2O3. After each dosage, the system was pumped to remove any residual gas phase ether before the IR spectrum was taken. During the experiments, the cell was translated in the I Rbeam path in order to acquire spectra on both the alumina and CaF2 sides of the disk. The effects of gas phase (CzF&O or (C2F&0 adsorption on the CaF2 can then be determined. The difference spectra presented here were obtained by subtraction of a clean alumina spectrum from the spectra of the adsorbate on alumina acquiredat the sametemperature. Following(C2F5)zOadsorption at 100 K, the sample was heated in situ to 600 Kin increments of 50 K. IR spectra were acquired at the temperatures t o which the sample was heated and maintained. The cooling apparatus was comprised of approximately 9 ft of I14 in. copper tubing immersed in liquid N2. The tubing and dewar were well insulated with Styrofoam, closed cell foam, and fiberglass. The tubing was attached to the cell with Swagelok fittings. Cooling to 100 K and heating to 300 K was regulated by adjusting the rate of Nz(g)and Nz(1)flowthrough the support ring. A heater was constructed using 6 ft of l/4 in. stainless steel tubing coiled in a 4 in. diameter spiral. The spiral was wrapped within a l18 in. thick stainless steel sheet. Two 4 ft heating tapes, rated to 750 "C,were wrapped around the steel sheet. The system was insulated using 1.5 in. thick fiberglass furnace insulation. A short section of more malleable copper tubing facilitated connection between the heater and the cell. The sample and the support ring were heated to various temperatures between 300 and 600 Kby controllingthe airflow through the heater. Previous experiments have indicated that there was excellent thermal contact between the CaF2 disk and its support ring and that the temperature Merencesbetweenthem werenegligiblein vacuum. X-rayphotoelectron spectroscopy(XPS)spectra were collected using a VG Clam 2 spectrometer. The samples were prepared by mounting the coated CaFz disk directly onto an XPS sample mount with conducting Cu tape. A Mg Ka X-ray source was operated with an emission current of 20 mA (300 W emission power)and an acceleratingvoltage of 15kV. The high-resolution data for C, F, and 0 were collected under constant analyzer energy and at a pass energy of 20 eV. For each spectral region, 200 scans were signal averaged and stored with no smoothing. The binding energies of the peaks were referenced to adventitious C at 284.7 eV. The reagents used in this study, (C2F&0 (90%purity, Strem), pyridine (>99.6%),and (CH3)3SiCl(Petrarch Systems Inc.) were transferred to glass storage flasksand purifiedby severalfreezepump-thaw cycles with liquid nitrogen. Water was treated by a Nanopure filter system. Results IR Study of (C2Fd20 Adsorption on A l a 0 3 at 100 K. IR spectra from the 4000-3000 cm-l region as a function of exposure of the A 2 0 3 surface to (CzF5)zO a t 100 K are shown in Figure 1. The spectrum of clean alumina is shown in Figure l a . Three IR absorption features are observed a t 3733,3678, and 3592 cm-l. Asmall shoulder is also apparent a t 3800 cm-l. The shoulders at 3733 and 3800 cm-l and the peak at 3678 cm-l are assigned to isolated surface hydroxyl groups on A 2 0 3 according to the model proposed by Peri.l The broad feature at 3592 cm-l is due to vibration of hydrogen-bonded (associated) OH groups. Both "free" (isolated) and associated hydroxyl
T = 100 K resolution = 2 cm-'
3878
(CgF&O Dose
--------(molecules) (g)
l.981lO1'
(f)
1.47x10"
(e)
8.8
(d)
4.9
XIO"
x10"
2.5 110" (b) 1.2 x10" (c)
(a)
0.0
T = 100 K
reeolution = 2 cm-'
1 = O.OO6A 1237
Dose ---------
(CIF&O
(molecules)
I
I
I
(g)
1.96~10"
(f)
1.47~10"
(e)
9.80 XIO"
(d)
4.90 x10"
(c)
2.45 x10"
(b)
1.23 x10"
(a)
0.0
I
groups are produced during the heating pretreatment. Surprisingly, no change was observed in the v(0H) region with increasing dosage of the (C2F5)zO as evidenced by Figure lb-g. In fact, no spectral change was observed a t frequencies above 1300cm-l due to adsorption of (CZF5)20. The spectral region from 1300 to 1000 cm-l is shown in Figure 2. Vibrational features are evidenced at 1250, 1237, 1147, and 1105 cm-l in this C-F, C - 0 stretching region. With increasing dosage a discernible shoulder appears a t -1202 cm-' in Figure 2e-g. Specific assignments of these features will be discussed below. In an effort to understand the IR absorption spectra at 100 K, the (CzF&O was assumed to be frozen on theAl203 surface at that temperature and the point of vaporization was determined by alternately heating and cooling the sample and monitoring the pressure increase in the cell. The temperature at which a pressure change was first measured, 147 K, was designated as the point of vaporization of (CzF&O. Effect of ThermalTreatment of (CaF5)zOAdsorbed on A l z O 3 . After 1.96 x 1019 molecules of (CzF5)zO are adsorbed on the alumina surface, the ether adlayer is heated from 100 to 600 K a t 50 K increments. The bulk
Surface Chemistry of Perfluoro Ethers
resolution=2cm-'
Langmuir, Vol. 11,No. 1, 1995 129
1 0.0s A
3041
T(K) ( 0 ) 600
(f) 600 (e) 400
. .
(d) 300
(a)
150
(c) 200
(a) 100
(b) 150 3bOO
3bOO
3f00
ab00
ab00
Si00
WAVENUMBERS Figure 3. Infrared difference spectra with A 1 2 0 3 background spectrum subtracted showing spectroscopicchanges in the OH stretching region following heatinf of the (CzF&O adsorbed layer.
1600
1boo
iboo
iboo
1400
1300
WAmNUMBERS
1
resolution=2cm-'
0.059
A
W
u
3m 0 v1
(b) 150 (0)
200
(d) 300 (e) 400
(f) 600 ( L ) 600
1300
lh50
lh00
I
1150
I
1100
I
1050
WAmNUMBERS Figure 4. Infrared spectra in the 1300-1050 cm-l region as a function of temperature showing vibrational modes of adsorbed (CzF&O and its thermal decomposition products. of the (CzF&O on the surface desorbs a t 150-200 K as confirmed by the rise in pressure in the cell and dramatic spectral changes. The spectra that depict the most obvious spectroscopicchanges are selected and difference spectra are calculated by subtracting a clean alumina spectrum from the absorbate spectrum. The difference spectra so obtained are shown in Figures 3-5. Negative absorption features indicate depletion of band intensities, and positive absorption features are due t o development of new or existing bands. Figure 3 shows the spectroscopic changes in the OH stretching region. Figure 3a is equivalent to the difference
Figure 5. Infrared spectra in the region 1800-1300 cm-' as a function of temperature showing characteristic vibrational modes of surface fluoroacetate and fluoroformate on A 1 2 0 3 . between parts g and a of Figure 1. Increased absorbance is apparent a t 150 K(Figure 3b) from -3711 to 3600 cm-'. The feature a t 3683 cm-' increases in intensity a t 150 K and then decreases in intensity. As the sample is heated, the vibrational features a t 3753 and 3683 cm-l are progressively depleted shown by increasing negative intensities. The feature at 3713 cm-l is slightly enhanced. The associated hydroxyl vibrational band a t 3592 cm-l is shifted to 3641 cm-l. Simultaneously, there is a broadening and an increase in intensity of this positive absorbance band. These spectral changes indicate that (a) at temperatures above 150 K, the perfluoro ether molecules interact with the isolated hydroxyl groups, converting them to associated OH groups, and (b) the interaction increases with increasing temperatures up to 600 K. The associated OH species are stable at 600 K and a t ambient temperature under ultrahigh vacuum for several days. There is no recovery of the isolated OH species even while the ether is being decomposed. Changes that occur upon heating in other spectral regions are shown in Figures 4 and 5. Figure 4 shows the C-F, C-0 spectral region between 1300 and 1050 cm-l. Figure 4a is identical to Figure 2g. When the adlayer is heated to 150 K (Figure 4b), the intensities of the IR features increase by 30-fold. Five vibrational modes are observed a t 1250,1237,1210, 1149, and 1105 cm-'. As will be discussed in detail later, these modes are identified as characteristicvibrationalmodes of physisorbed (CZFFJZO and its thermal decomposition products. After heating t o 200 K, the following spectral changes are observed: (1) the 1250 cm-l feature disappears, (2) the 1237 cm-' feature shifts to 1234 cm-l and decreases in intensity, (3) the 1149 and 1105 cm-' features also decrease in intensity, (4)several new peaks and shoulders appear at 1221,1215, 1163,and 1123cm-l. Between 300 and 500 K, the spectra remain almost identical with features a t 1268, 1215, -1163,1105 cm-l. The 1234cm-l feature becomes a weak shoulder and the -1215 cm-l feature increases slightly in intensity and is now the dominant spectroscopicpeak.
Ng et al.
130 Langmuir,Vol. 11,No.1, 1995 At 600 K, the dominant feature is at 1211 cm-I with other less intense features a t 1268, 1156, and 1105 cm-l. Corresponding IR spectra in the region 1800-1300 cm-' are shown in Figure 5. At 150 K, two distinguishable features are present at 1750 and 1358 cm-l. The 1358 cm-l mode disappears at T > 200 K As will be discussed later, we assign this feature to a vibrational mode related to the molecularly adsorbed (C2Fd20. The peak a t -1750 cm-l may be related to physisorbed (CzF&O or a n adsorbed unsaturated acryl fluoride,14 or it may be a n overtone of the combined Al-0, Al-C, Al-F vibrational feature identified by EELS at 870 cm-l in a previous study.1° A definitive assignment cannot be given a t the present time. Broad and weak features also appear a t -1600 and 1500 cm-l. When the perfluoro ether is heated to 200 K, dramatic spectral changes occur as noted in Figure 5. New features appear at 1650,1605, and 1485 cm-l and a weak shoulder appears a t -1384 cm-l. Increasing the temperature from 200 to 600 K has the following effects on the IR spectra: (1) the 1650 cm-' feature shifts to higher frequency, decreases in intensity, and eventually becomes a shoulder a t 1675 cm-l; (2) the 1605 cm-1 shoulder shifts to 1615 and continues to increase in intensity; (3)the intensity of the 1485 cm-1 feature decreases slightly; and (4) the 1384 cm-l shoulder becomes a broad feature and shifts to -1369 cm-'. The combined observations shown in Figures 4 and 5 clearly indicate that the perfluoro ether has undergone chemical reactions with the A1203 surface at temperatures as low as 150 K. However, the unambiguous assignment of these new IR features to appropriate vibrational modes presents some difficulties. (CzF&O is a molecule that contains atoms with similar masses (C, 12; 0 , 1 6 ; F, 19) as well as bonds with similar force constants (K, for C-F, C-0, and C-C are 5.9 x lo5,4.89 x lo5,4.5 x lo5 dyn/ cm, respectively). For such molecules, we would not expect vibrations that are independent and characteristic for each bond. For example, a predominantly C-F stretching mode also involves C-C stretching vibration. It is also difficult to distinguish pure bond-stretching or pure bond-bending vibrations. For some inorganic and organic pyramidal XY3 halogen compounds, Nakamoto stated that the symmetric stretching vibrational mode has a higher frequency than the asymmetric mode.14 We are aware of only one previous infrared and theoretical study of gaseous (C2F5)2, by Pacansky et al.15 Our assignment of the IR absorption features are based on the following criteria: (a) the gas phase IR absorption spectrum of (C2F5120 obtained in our laboratory, (b) assignments of gas phase (CzF5)zO absorption features in ref 15, (c) recognition of vibrational features that change intensity concomitantly as temperature changes in IR spectra reported in this study, (d) comparison with published IR spectra of fluorinated carboxylic acids and salts (Table 11, and (e) simulation of vibrational modes of (CzF&O, CF3COOAl, and FCOOAl by calculations using HyperChem software. It can be seen from Table 1that fluorination increases the frequency of the va,(OCO) vibration by -30 cm-' per F atom displacing a H atom. Fluorination induces partial positive charge on the C atom in the carboxylate ions. Greater electron density is concentrated to the C-0 bond, resulting in a stronger bond and a stretching vibrational mode a t higher frequency. We also estimate that v(CF3) modes of adsorbed species have frequencies greater than v(CF2) modes at > 1200 cm-l. Other vibrational modes of fluorinated species were (14)Nakamoto,K. In Infrared and Raman Spectra of Inorganic and Coordination Compounds, 3rd ed.; Wiley-Interscience: New York,1978. (15)Pacansky,J.;Miller,M.; Hatton, W.; Liu,B.; Scheiner,A. J.Am. Chem. SOC.1991,113,329.
Table 1. Vibrational Frequencies of Alkyl and Fluorinated Acetates and Formates compound vas(OCO) v,(OCO) CH&OONa(s)" 1578 1414 CFzHCOONa(s)b 1645 1445 CF3COONa(sY 1680 1430 CF3C0ONa(dd 1689 1446 C F ~ C O O A ~ ( S ) ~ 1660 1450 (CH~COO)&(SY 1570 1466 (CF3COO)&(sY 1680 1490 CH3COO/AlzO# 1572 1455 HCOONa(s)h 1567 1366 (HC00)3Al(s)' 1600 1377 HCOO/k&03i 1593 1382 HCOO/Al20+' 1587 1378 HCOO/&03' 1597 1377 f
v'(CF)
v"(CF)
1080-1120 1210 1213 1220
1130 1140 1140
121Q
1170
a See ref 14. See ref 44. See ref 45. See ref 47.e See ref 41. See ref 46.g See ref 42. See ref 43. See ref 12.
assigned in previous references. Gerhardt and Lagow reported v(CF) modes a t 1300-1100 cm-I for partially fluorinated ethers.16 Basu et al. reported v(CF) and v(C0) of (CF2H)zO/A12O3at -1025-1180 cm-l.12 The resolution of the IR spectra in the two studies does not allow the several peaks in this spectral region to be fully resolved. Pacansky et al. studied model compounds for polyperfluorinated ethers by ab initio SCF calculations and infrared spectroscopy of the ethers in argon matrixes and as neat films.15 For perfluorodiethyl ether the following theoretical features were reported: v(C-C) a t -1370 cm-l, v(CF3) a t -1290 cm-l, v(CF2), a t -1248 cm-l, v(C-0) at -1200 cm-l, and v(CF3 CF2) and v(CF3 CO) a t -1090 cm-l. The experimental IR spectrum of perfluorodiethyl ether in argon matrix was reported. The absorption features appeared to be all a t -50 cm-l lower than the theoretical frequencies, but exact frequencies were not assigned to the features. The IR absorption spectrum of gas phase (CzF5)20 obtained in our laboratory a t 300 K shows three principal bands a t 1250, 1155, and 1107 cm-l in the 2250-1050 cm-I range. These are readily assigned to v'(CF3), v,(COC), and v(CF3 CFZ GO) vibrational modes of (CzF&O. There are also very weak features a t -1360, -1280, and 1213 cm-'. The IR spectrum of physisorbed (CzF&O is expected to have similar absorption features. The IR features at 1250 and 1237 cm-1 which disappears by -200 K (Figure 4) are associated with the stretching mode ofthe CF3 moiety ofthe physisorbed perfluoro ether. Similarly, because of their diminished intensities and disappearance by 250 K, the vibrational features at 1221 and 1210 cm-' are assigned to stretching modes of the ether CF2 group. The feature a t 1149 cm-l is attributed to the v,(COC) mode. The feature a t 1105cm-l is assigned to a n interaction mode of CF3, CF2, and CO according to ref 15. Interestingly, upon continuous heating, this vibrational feature decreases in intensity but does not totally disappear even a t 600 K. In Figure 5, the 1650-1675 cm-l and the 1485 cm-I featuires are similar in frequencies to v,(OCO) and v,(OCO) modes offluorinated acetates (Table 1). The 1615 and 1369-1384 cm-I features are closed in frequencies to v,,(OCO) and v,(OCO) modes of a surface formate structure. Since we detected no evidence of C-H vibrational modes but have evidence of spectral features in the C-F region, we assign these carboxylate vibrational features to stretching modes of the OCO moieties of a surface fluorinated acetate and a surface fluorinated formate. The rate of decrease in intensity of the vibrational mode a t 1234 cm-l (Figure 4) is proportional to the intensity
+
+
+
+
(16)Gerhardt, G. E.; Lagow, R. J. J . Chem. Soc., Perkin Trans. 1 1981,5,1321.
Surface Chemistry of Perfluoro Ethers
Langmuir, Vol. 11, No. 1, 1995 131
Table 2. Assignment of Observed Vibrational Frequencies (cm-') of (CZFS)~O/A~SO~ at Different Temperatures (K) mode v(C-C) v'(CF3) v"(CF3) vYCFZ) v"(CF2) v,(COC) V,(COC) v(CF3fCFz) V,(OCO) VAOCO) v'(CF) v"(CF) v(CF CO)
+
VadOCO) V,(OCO) dCF) a
100 K
1250 1237 1147 1105
150 200 300 K K K Physisorbed (CzF&O 1358 1358 1250 (1250)" 1237 (1221) 1221 1210 (1210) 1149 1149 (1123) 1123 1105 1105 CF3COO/Al203 (1650) 1650 1675 (1485) 1485 1485 1234 1234 (1281) (1275) 1270 1105 FCOO/Alz03 1605 1615 (1384) 1384 (1215) 1215
400 K
500 K
A
GH$I Adsorption
I
600 K
lh
B 1675 1485 1234 1268 1105
1675 1485 1234 1268 1105
resolution= 2 cm-' T=300 K ring mode
I 0.02 A
1600 I
1)oo
lbo lboo
lh0
WAVENUMBERS
l h
FCOO( a)/Pyridine+AlrOa
T=500 K
(1675) 1485 (1234) 1268 1105
1615 1615 1615 1375 1370 1369 1214 1213 1211
Frequenciesin parentheses are estimates due to low intensity.
decrease ofthe v,(OCO) of the fluoroacetate a t 1650cm-' (Figure 5). We therefore assign the 1234cm-l IR feature to the v'(CF3) mode of the adsorbed fluoroacetate. At 600 K, the main oxidation product is a surface fluoroformate with v,,(OCO) at 1615 cm-l and v,(OCO) at 1369 cm-'. The strong feature remaining in the CF region is the peak at 1211 cm-l. This leads to the conclusion that the absorption peak a t 1211 cm-' must be due to the v(CF) mode of the fluoroformate species. Table 2 summarizes the assignments of observed vibrational features at different heating temperatures. While assignments of the vibrational modes of CF3COO-/Al203 and FCOO-/ A1203 are fairly straight forward, the stretching modes of (C2F&0 can only be considered as "predominantly stretching" modes of the bonds indicated and we have not assigned symmetry to these modes. The other parts of the molecules certainly vibrate when CF3 stretches! Adsorption and Heating of (C2Fd20 on A 1 2 0 3 Preadsorbed with Pyridine. The high-area alumina surface prepared in these studies contain various reactive sites: isolated hydroxyls of different reactivities determined by the number of nearest oxide neighbors, H-bonded associatedhydroxyl sites, coordinativelyunsaturated A13+ Lewis acid sites, and oxide ions.'S2 Other defective sites may also be present but have not been properly identified. In an effort to examine the possible role of A13+ sites in the thermal decomposition of (C2F5)20,pyridine was used as a nucleophilic reagent to block these sites. Pyridine is known to preferentially adsorb on the Lewis acid sites of alumina surfaces via the nitrogen lone pair electrons, forming distinctive ring vibrational modes in the 1450-1650 cm-l spectral Figure 6Ashows the IR spectrum of pyridine adsorption on A 1 2 0 3 in the ring vibrational modes region. The sample is exposed to 4 Torr of pyridine at 100 K and then heated to 500 K, the spectral features a t 1455 and 1600 cm-l in Figure 6A can be assigned to pyridine coordinated to the A13+ sites forming a Lewis acid adduct, in agreement with the assignments made by o t h e r ~ . l ~ - ~ ~ The sample that was pretreated with pyridine was then exposed to (CzF5)20 at 100 K and heated to 500 K as (17)Kline, C.H., Jr.; Turkevich, J. J . Phys. Chem. 1944,l.Z(7),300. (18)Gozinger, H.Adu. Catal. 1976,25,184. (19)Morterra, C.; Chiorino, A,; Ghiotti, G.; Gamone, E. J. Chem. SOC.,Faraday Trans. 1 1979,75 (2),271. (20)Parry, E.P.J.Catal. l963,2,371.
iboo
ih
ibw
iboo
ih
ih
lh
iiw
!boo
WAVENUMBERS Figure 6. (A) Infrared spectra of pyridine adsorption on A1203 in the ring vibrational modes region. Modes observed are characteristicof pyridine coordinated to AI3+Lewis acid surface sites. A 1 2 0 3 was exposed t o 4 Torr of pyridine at 100 K and heated to 500 K. (B)Infrared spectra showing the formation of surface fluoroformate at 500 K on &03 with preadsorbed pyridine (surface in A). The substrate was exposed to (CzF&O at 100 K and heated to 500 K.
described previously. Figure 6B shows the IR spectrum obtained a t 500 K from which the pyridine-treated A1203 has been subtracted. The split nature of the feature centered a t 1611 cm-' is a subtraction artifact because the 1600cm-' pyridine ring mode shiRed on heating with coadsorbed (C2Fs)zO. In the C-F region, peaks a t 1260, 1204, 1151, and 1096 cm-' are detected. Comparing Figure 6B with Figures 4f and 5f, we can see that except for slight shifts in peak frequencies in the C-F region, the spectra are identical. The two similar spectra indicate that (CzF5)zOreacts the same way with the clean alumina surface and the pyridine-treated alumina surface. The active sites responsible for the oxidation of (C2F5)ZO to acetate and formate are not blocked by pyridine adsorption. This is in agreement with previous studies on (CF2H)zO interaction with A l ~ 0 3 . ' ~ Adsorption and Heating of ( C Z F ~ Zon O AI203 Preadsorbedwith (C€&,)3SiCl. The reactivityof isolated hydroxyl groups on the high-area alumina surface can be diminished by dehydration a t elevated temperatures (T > 1173 K).3,6,21A more practical method is to passivate the hydroxylated alumina surface by reaction with a halosilane such as (CH3)3SiC1.3p22The reaction is as follows:
>Al-OH
+ C1Si(CH3),- Al-O-Si(CH,), + HC1
The reactive isolated OH groups are replaced by the relatively nonreactive methyl group. In an effort to determine the role of OH on the adsorption and thermal degradation of (CzF5)zOonAl~O3,the alumina surface is heated in 10 Torr of (CH3)3SiCl a t 450 K for about 8 h. The sample is then evacuated a t 450 K for 2 h to remove unreacted silane. The sample is further evacuated for 12h while cooling to ambient temperature. (21)Wong, C.;McCabe, R. W. J. Catal. 1989,119,47. (22)Paul, D.K.;Ballinger, T. H.; Yates, J. T., Jr. J . Phys. Chem. 1990,94,4617.
Ng et al.
132 Langmuir, Vol. 11, No. 1, 1995
1
resolution = 2 cm-'
(A)
f".-., I =
0.05 A
ab00
ab00
aloo
3bOO
I
2boo
2800
Figure 8. Infrared spectra in the 1800-1050 cm-l region showing the adsorption and heating of (C2Fd20on a silated A 1 2 0 3 surface. Each spectrum has the silated alumina surface subtracted from it.
""I loop
le00
1boo
1400
A2"" '
do5
d75
d86
do5
lP00
WAVENUMBERS
Figure 7. Infrared spectra of the A 1 2 0 3 surface preadsorbed with (CH3)3SiCl: (A) The disappearance of the surface OH modes, (B and C) the characteristic vibrational modes of the stable = Al-O-Si(CH& species. Spectrum a in each section pertains to the untreated alumina surface. Spectrumb in each section pertains to the silated alumina surface.
The IR spectrum of the silated A1203 surface is shown in Figure 7. Figure 7A depicts the disappearance of the isolated OH modes at 3729 and 3669 cm-l with concomitant formation of the stable =Al-O-Si(CH3)3 species as evidenced by the characteristic vas(CH3)a t 2964 cm-', v,(CH3) at 2908 cm-l, da,(CH3) a t 1469 and 1400 cm-l, and d,(CH3) a t 1261 cm-l (Figure 7B,C).22 The silated alumina surface is then cooled to 100 K, exposed to 1.96 x 1019molecules of (C2F5)20 and heated. IR spectra (1800-1050 cm-') ofthe adsorption and heating are shown in Figure 8. Each spectrum has the silated Also3 subtracted from it. In the v(CF), v(C0) region (1300-1050 cm-l) the spectra a t 150 and 200 K are similar to spectra b and c in Figure 4. The heating is stopped a t 300 K because (CzF5)zO completely desorbed by 300 K. There is no evidence of the formation of fluoroacetate or fluoroformate species, as indicated by the absence ofpeaks in the 1600-1400 cm-' region a t T > 150 K. There is also no evidence of the oxidation of the silated A1203 surface itself. The oxidation ofthe alumina siloxane species would have produced a HCOO(a) species with vibrational frequencies in the 1600-1300 cm-l region.23 Figure 8d a t 300 K shows some artifacts due to subtraction because the da,(CH3) and d,(CH3) vibrational features of the alumina siloxane species shifted slightly in frequency on heating. XPS Studies of the Thermally Treated (Ca&O Adlayer. Since the IR technique is less sensitive to monolayers and cannot determine elemental compositions, the authors felt it was important to collect the XPS data (23) Paul, D. K.;Yates, J. T.,Jr. J.Phys. Chem. 1991,95, 1699.
OXYGEN
"ooo~ 40000
o ' , , m d38
n 531.4
/\ I
da4
I
I
I
I
I
I
dao d26 BINDING ENERGY, eV
I
I
d22
Figure 9. XPS spectra of the thermally treated (C2F&0 adlayer on A 1 2 0 3 for C(ls), F(ls), and O(1s) regions. For assignments of peaks, see the text.
to verify the integrity of the catalytic surface. After the severe thermal cycle to which the cell was subjected to during these reaction studies, the XPS results, taken ex situ, confirmed that no major contaminants collected on the untreated and treated surfaces. Contaminants could contribute to the catalytic activity or interfere with the subsequent blocking experiments. For the thermally treated (C2F6)20/A1203)surface, the major elements contributed to the surface reactivity, as identified from an ex situ XPS survey scan (not shown), are Al, 0, F, and C. The XPS high-resolution results support several of the species identified by IR. Figure 9 shows the C(ls), F(ls), and O(ls) XPS spectra recorded for this surface. In the C(ls) region displayed in the top of Figure 9, the main peak at 284.7 eV is due to adventitious Cor decomposed fluoro ether. (No background subtraction for the clean unreacted A 1 2 0 3 surface was attempted. Only peaks a t 284.7 and 275.5 eV were observed in this region
Surface Chemistry of Perfluoro Ethers for the unreacted surface.) The C(1s) shoulder around 288.2 eV may be attributed to organic species such as 0-C-0 of the carboxylates or C-0 moieties. The peak a t 275.7 eV is an Ka3,r X-ray satellite peak and the peak at 298.3 eV is a ghost line from the Al anode of the dual anode source. This ghost line overlaps the C-F binding energy region and obscures any C-F species that may be present. Although the IR results indicate the presence of C-F species on the surface of the thermally treated (CzF&O/Al203 sample, these species are not detected in this C(1s) region, in part due to the ghost line. Another reason for the lack of an intense peak in the C-F region around 294-293 eV is the fact that a non-monochromatic Mg X-ray source is used in this study. Without a monochromatized source, which removes the damaging Brehmstrahlung radiation, X-ray beam damage results in a degradation of the fluoro ethers and produces hydrocarbon species and metal fluoride^.'^,^^ These hydrocarbon species contribute to the peak a t 284.7 eV and its higher energy shoulder. Even with these experimentally produced interferences in the C(1s) region, the presence of a carboxylate species supports the IR data. The spectral data from the F(ls) region of the thermally treated surface also confirms the presence of a F-C species within the top few monolayers. In agreement with the IR data, a small amount of fluoroformateis detected, as indicated in the F(ls) region recorded in the center section of Figure 9. The shoulder at 687.2 eV is assigned to this F-C species. The main peak in the fluorine region a t 685.5 eV is primarily due to decomposition products from the X-ray beam damage and are assigned to Al-F or Al-0-F species. Al-F may also be formed by the reaction of A1203 with F-containing fragments which are produced during the thermal degradation of (CZF5)20.Since the XPS sensitivity factor for the F(1s) photoelectron line is 4 times higher than the C(1s) line, 1and 0.25, respectively, the relative areas of the C, F, and 0 peaks observed in Figure 9 (1361,1502, and 22 349 counts, respectively) indicate that the concentration of Al-F species is low on the surface of this thermally treated (C2F&O/Al203 sample. The major component in the XPS survey spectrum (not shown) is alumina. X-ray and charged particle beams are known to readily break C-F bonds on the s u r f a ~ e l ~ and ,~~ contribute to the F( 1s) peak corresponding to fluorine in Al-F species (BE = 685.5 eV). Although the XPS equipment used in this study has limitations due to the lack of a monochromatic X-ray source, the XPS results, which show the presence of such species as the carboxylate and the F-C moiety, assist in confirming IR assignments. Since the decomposition products of fluorinated species, caused by radiation damage, are very detrimental to UHV systems, leaving behind F contamination, especially in the monochromator unit, in situ experiments are not recommended. The data obtained in this study are the most reliable with the minimum spectrometer damage.
Discussion Behavior of (C2F5)20Adsorbed on A l a 0 3 at 100 K. At 100 K, (CzF5)zO is condensed on the surface. The infrared spectra following adsorption of 1.96 x 1019 molecules of (CzF5)ZO show no adsorbate feature above 1300 cm-1. The 1300-1000 cm-l region exhibits some weak C-F, C-0 stretching features. At 150 K, these features show a 30-fold increase in intensity (Figure 5b). Clearly the fluoro ether is observable while frozen on the surface, yet the intensity is diminished. The low intensi(24)Herrera-Fierro, P.; Jones, W. R., Jr.; Petter, S. V. J. Vac.Sci. Technol. A 1993,11 (2), 354.
Langmuir, Vol. 11,No. 1, 1995 133 ties of these features may be due to two factors. The orientation ofvibrational modes with respect to the surface may be such that the effective dipole moment changes perpendicular to the surface are small. Alternatively, the majority of the ether molecules may interact associatiively with each other, forming a latticelike structure which vibrates a t a lower frequency than can be observed. The latter assertion is supported by the fact that a sample dosed with 4.5 x (1.96 x 1019)molecules of(CzF5)zOshowed no spectral features a t 100 K. Walczak and Thiel have reported formation of multilayer of perfluorodiethylether on Ru(OO1) a t temperatures below 130 K.25 Basu et al. also found associative interactions within an adlayer of (CF2H)zO on A l ~ 0 3 . l ~ No perturbation of the surface OH species by (C2Fd20 is observed a t 100 K (Figure 1).At this temperature the condensed (CzF&O forms a solid on the outer geometrical surface of the sample and does not interact with the majority of the hydroxyl groups that exist in the pores of the A1203 sample. When the sample is warmed, gaseous (CzF&O can diffuse into the pores and the interaction with OH groups begins. A similar phenomenon has been observed in Xe adsorption on Si02.26 Reactive Sites for (CJ?5)20 on A1203. Dehydroxylatedhigh surface areaAl203is well-known to contain both Lewis acid and Lewis base sites in addition to other less well-defined defect sites. Strong Lewis acid sites appear when the hydroxyl ions are removed from the surface. These sites are presumed to be on Al ions in regions where there are excess positive ions. In his studies of calcined alumina, Peri identified five different hydroxyl sites by infrared absorption spectroscopy.' In the sequence of increasing acidity, IR absorption modes at 3800, 3780, 3744, 3733, and 3700 indicate isolated hydroxyl groups with 4 , 3 , 2 , 1 , 0 oxide ions (and 0, 1 , 2 , 3 , 4 positive ions respectively) as nearest neighbors. Isolated hydroxyl groups have been found to be chemically more reactive than associated hydroxyl groups which are OH that are hydrogen bonded to each other. Knozinger and Ratnasamy proposed a model that attributed the different frequencies of the five isolated OH groups to the different coordinately unsaturated Al atom or atoms to which the OH is bonded.2 Isolated hydroxyl groups on either A1203 or Si02 supports act as oxidizingagents in the degradation of metal aggregates, changing the oxidation state of the metal.g Previous studies on partially fluorinated dimethyl ether found isolated surface hydroxyl groups to be involved in the adsorption of these molecules to high surface area Al~O3.l~ The importance of OH was not fully explored because the isolated hydroxyl groups were restored to their original composition a t 500 K. Our experimental results show that (C2F&O interacts with isolated hydroxyl groups with vibrational frequencies of 3753 and 3683 cm-' a t temperatures > 147 K. These are acidic hydroxyl groups that are postulated to be bonded to two octahedrally coordinated Al atoms and to three octahedrally coordinated Al atoms respectively by Knozinger and Ratnasamy.2 According to Peri's model, they are adjacent to positive ions.' These are the acidic isolated hydroxyl groups. Associated hydroxyl groups are formed as indicated by the enhancement of a broad band between 3650 and 3300 cm-l with the peak centering around 3641 cm-l (Figure 3). Hydrogen-bonded OH and HF both have broad vibrational features in this region. Therefore it is not possible to distinguish between OH groups that are (25)Walcrak, M.M.;Thiel, P. A. In Adhesion and Friction; Grunze, M., Krouzr, H. J., Ed.;Springer Series in Surface Sciences; SpringerVerlag: Berlin, 1986,Vol. 7, p 89. (26)Ballinger, T.H.; Basu, P.; Yates, J. T., Jr. J.Phys. Chem. 1989, 93,6758.
Ng et al.
134 Langmuir, VoE. 11, No. 1, 1995
hydrogen-bonded to a n 0 atom from OH groups that are bonded to a F atom. The interaction between surface hydroxyl groups and (CZF5)zO increases with increasing temperature and persists up to 600K. At low temperatures, the interaction may involve hydrogen bonding of the ether molecule to OH groups. However at higher temperatures it is most likely that stronger chemical interactions between the ether molecules or decomposition products of the perfluoro ether and isolated OH are present. Semiempirical MNDO calculations of the optimal geometry, electron densities, and orbitals of (CzF5)zO indicate that the ether oxygen is the site with the highest ‘d 160 8dO SdO 4& SdO SdO electron d e n ~ i t y . ~ Additionally, ~,’~ the highest occupied TEMPERATURE, K molecular orbital (HOMO) of (CzF5)zO is dominated by a p type orbital centered on the ether oxygen and located almost perpendicularly to the C-0-C bonding a x i ~ . ~ ’ , ~ ~ The high electronegativity of F atoms causes the C atoms to have a high positive charge and consequently induces a high negative charge on the 0 atom. The C-0 bonds are very polar and susceptible to attack by the acidic OH. We postulate that the ether molecule interacts with the isolated acidic surface hydroxyl groups through the ether oxygen with the C-0-C bonding axis parallel to the A 1 2 0 3 surface. In this bonding configuration, four of the six F atoms are brought close to the A 1 2 0 3 surface where they can hydrogen bond to adjacent isolated OH groups or chemically bond to neighboring A13+ Lewis acid sites, forming Al-F bond groups. This can explain why isolated acidic hydroxyl group (at 3753 and 3683)are preferentially 1400 lb60 lhl lb60 WAVENUMBERS depleted. Figure 10. (A) Plot of the total integrated intensity of the The absence of chemical reaction between the (CzF5)zO fluoroacetate and fluoroformate peaks versus sample heating adlayer and the silated A 1 2 0 3 surface indicates that the temperature. (B)For overlaid IR spectra taken at 300, 400, presence of available surface hydroxyl groups is necessary 500, and 600 K showing an isosbestic point at 1640 cm-’. for chemical reactions to occur between the perfluoro ether molecule and A1203 surface. The hydroxyl groups may serve some or all of the following functions: (1) to mode of the fluoroacetate at -1675 cm-l suggest that the chemically “anchor”the ether molecule on the surface until fluoroacetate species are being converted to fluoroformate the surface reaches the temperature necessary for chemispecies as the sample is heated. The total integrated cal reactions to occur, (2)to chemically anchor the ether intensity of the acetate and formate peaks from 1710 to molecule near the vicinity of a reactive site, and (3)to 1570 cm-’ remains practically constant after 300 K, take part actively in the oxidation process. The role of indicating that, at higher temperatures, fluoroacetate hydroxyl groups will be discussed in more detail below. converts to fluoroformate (Figure 10A). Figure 10B shows Oxidation of the (CzF5)zO molecules also occurs on the four overlaid infrared spectra taken at 300,400,500, and pyridine-coveredsurface. This clearly indicates that Lewis 600 K. An isosbestic pointz8 is present in the spectra at acid A13+ sites, blocked by pyridine adsorption, are not 1640 cm-l, indicative of the conversion process involving involved in fluoroacetate and fluoroformate production. the adsorbed fluoroacetate changing to adsorbed fluoroMechanism of the Decomposition and Oxidation formate. of (CzF&,O on A I 2 0 3 . When (CzF5)20/Al203layers are heated in vacuo, three thermally induced processes are Summarizing the discussion above, the following steps evident: (1)desorption of (CzF5)zO species a t 100-200 K, can be postulated for the adsorption and thermal decom(2)isolated hydroxyl species on A 1 2 0 3 are converted to positioddesorption mechanism for the (CzF5)~0/Al203 associated hydroxyl species, and (3) decomposition and system studied here: oxidation of (CzF&O to form surface fluoroacetate and surface fluoroformate species at 200-600 K. The thermal desorption process is indicated by the (C,F,),O(g) (CZF5),Oad,(multilayer, inclined, pressure increases during the heating experiments and or horizontal orientation) (1) by the decrease in intensity of the ether vibrational features from 150 to 200 K. Mass spectrometry data collected by a mass spectrometer placed in the vacuum line attached to the IR cell indicate that the ether desorbs molecularly from the A 1 2 0 3 surface at these temperatures. We estimate from the pressure increase in the system that only 1-2% of the physisorbed ether remains on the surface and is subsequently oxidized. At temperatures higher than 200 K, a trace of HF is detected among the gas phase products from the IR cell. The concommital intensity increase of the vae(OCO)mode of the fluoroformate a t -1615 cm-l and the decrease of the va,(OCO) (28) Ewing, G. W.Instrumental Methods of Chemical Analysis, 4th (27) Lyth, E.;Ng, L. M.; Yochum, S., to be submitted.
ed.; McGraw-Hill: New York, 1975, Chapter 3.
Surface Chemistry of Perfluoro Ethers
(C2F5)20ad, (monolayer) HF CF3C00ad,
200-300 K
FCOO,d,
Langmuir, Vol. 11, No. 1, 1995 135
+
+ A-F(?) + OH (assoc) (4)
300-600 K *FCooads
(5)
However, Basu et al. found that (CF2H)zOis oxidized by high surface areaAlz03to a surface alkyl formate species.lZ Our study has shown that (CzF5)zO decomposes on high surface area A 1 2 0 3 to form adsorbed fluoroacetate and fluoroformate species. Lewis acid Al sites are not responsible for the oxidation of (CzF5)2O, but isolated OH surface species are necessary for the thermal decomposition and oxidation of (CzF5)zO. Native A 1 2 0 3 and (OH),& are formed by exposure of Al metal or alloy to the atmosphere. At temperatures higher than 473 K (200"C),reactive isolated OH species appear. With the presence of the reactive OH species, oxidation of perfluoro ether occurs a t temperatures as low as 150 K (-123 "C). Our research results suggest that pretreatment of Al metdalloy surfaces that contain native A1203 and (OH),& species with a silane compound will retard the decomposition of perfluoro ether lubricants on these surfaces.
Since HF is being formed during the reaction process and isolated OH is not recovered, we propose that the isolated hydroxyl groups take part activelyin the oxidation process. The role of OH in the above mechanism is being confirmed by kinetic studies of the decomposition of (CzF5)zO on A1203 under 4 Torr pressure a t different temperatures. Implications for Polyperfluorinated Ether Tribology. Ordinarily it has been assumed that ether compounds are stable and unreactive and polyperfluorinated ethers are widely used as lubricants. However, many of the polyether lubricants are extensively degraded under mechanical stress and in the presence of heat or high-energy r a d i a t i ~ n . ~ The , ~ ~chemistry -~~ between the Conclusions polyether molecules and the surface is therefore an This report presents the followingresults of an infrared important issue. Studies that elucidate the decomposition spectroscopicinvestigation of the chemistry of perfluoromechanism can help identify ways to stabilize the ether diethyl ether on an A1203 surface. against decomposition. (1)(CzF&O adsorbs on A 1 2 0 3 surface a t >147 K via One mechanism proposed for the degradation of polyperinteraction with isolatedhydroxyl species. Acidic hydroxyl fluorinated lubricants involves a two step process: firstly, groups on coordinatively unsaturated Al atom or atoms the ether forms fluoride with the Lewis acidic metal ions or with positive ion neighbors are preferentially depleted ofthe surface. Secondly, the metal fluorides then catalyze as surface temperature increases from 150 to 600 K. the breaking of the polymer bonds a t temperatures below OH species are not recovered at temperatures up the intrinsic temperature of ether d e c o m p o s i t i ~ n . ~ ~ - ~Surface ~ to 600 K, indicating a strong interaction with the ether Studies with polyether lubricants have concentrated on or decomposition products of the ether. the mechanism of the second step.32-34On the contrary, there are few studies on the initial first step when metal (2)Most (CzF5)zO molecules desorb on heating to 200 fluorides are supposedly formed when the polyether K. Some species are oxidized on the surface at T > 150 contactsthe clean metal surface. To understand the initial K to form adsorbed fluoroacetate and adsorbed fluorofirst step, model ethers have to be used in combination formate a t 150-200 K. At 200-300 K, only fluoroformate with surface analytical techniques. Studies ofthe surface continues to form. At 300-600 K fluoroacetate converts chemistry of model monomeric ether on atomically clean to fluoroformate. There is the possibility that HF and single crystal metal surfaces have not been able to confirm Al-F are formed by F-containing fragments which are the formation of metal fluoride^.^^,^^ Studies on clean produced during the thermal degradation of (CzF&O. polycrystalline metal surfaces have yielded contradictory (3)Oxidation of (CzF5)zO also occurs on a pyridineresult^.^^-^^ For example, John and Liang found that covered AlzO3, indicating that A13+ sites, blocked by small perfluoro ether compounds are very stable when in pyridine adsorption, are not involved in fluoroacetate and contact with polycrystalline surfaces.38 Dekoven and fluoroformate production. Meyers found that iron fluoride is formed when iron is (4)There is no chemical reaction between the (CzF5)zO exposed to perfluorodiethyl ether.37 adlayer and the A 1 2 0 3 surface preadsorbed with (CH3)ZStudies of model fluoro ethers on oxide surfaces have SiC1. The surface hydroxyl species are necessary for also yielded contradictory results. Ng et al. found that chemical reactions to occur. (CF2H)zO does not decompose on A1203 films formed on (5) XPS indicates presence of organic C, C-F, and Al-F single crystal Al(ll1).lo Studies of perfluorinated monoor Al-0-F species. Organic C species are postulated to meric ethers and diethers on ZrOz thin films epitaxially be 0-C-0 and C=O moieties. C-F are due to fluorogrown on Pt(111)found that fluorination suppress strong formate. Al-F species are due to X-ray beam damage chemical interaction with both Pt and ZrOz surface^.^^^^^ and/or reaction products of A1203 and F-containing fragments produced during the degradation of (CzF5)zO. (29)Pacansky, J.; Waltman, R. J. J. Phys. Chem. 1991, 95, 1512. (30) Paciorek, K. J. L.; Kratzer, R. H.; Kaufman, J.; Nakahara, J. H. J. Appl. Polym. Sci. 1979,24, 1397. (31) Chandler, W. L.; Lloyd, L. B.; Farrow, M. M.; Burnham, R. K.; Eyring, E. M. Corrosion 1980, 36, 152. (32) Kasai, P. H.; Wheeler, P. Appl. Sug. Sci. 1991, 52, 91. (33) Kasai, P. H.; Tang, W. T.;Wheeler, P. Appl. Surf Sci. 1991,51, 2131.
(34) Zehe, M. S.; Faut, 0. D. Trib. Trans. 1990,33, 634. (35) Napier, M. E.; Stair, P. C. J. Vac. Sci. Technol. 1991, A9,649. (36) Mori, S.; Morales, W. Wear 1989,132, 111. (37) Dekoven, B. M.: Meyers, G. F. J. Vac. Sci. Technol. 1991, A9, 2570. (38) John, P. J.; Liang, J. J. Vac. Sci. Technol. 1994, A12, 199. (39) Maurice, V.; Takeuchi, K.; Salmeron, M.; Somojai, G. A. Suq. Sci. 1991,250, 99. (40) Takeuchi, K.; Salmeron, M.; Somojai, G. A. S U ? Sci. ~ 1992, 279, 328. (41) Greenler, R. G. J. Chem. Phys. 1962,37 (9), 2094. (42) Hertl, W.; Cuenca, A. M. J. Phys. Chem. 1975, 77, 1120. (43) lto, K.; Bernstein, H. 3. Can. J. Chem. 1966, 34, 170.
Acknowledgment. One of us (L.N.) would like to dedicate this paper to the memory of Pam Basu with whom she started the fluorinated ether studies. Pam's untimely, tragic death deprived us of a valued friend and colleague. The authors sincerelythankDr. J.T. Yates, Jr. (University of Pittsburgh) for the gifi of the perfluorodiethyl ether. LA9306701 (44)Sadtler Standard Infrared Prism Spectra, Sadtler Research Laboratories: Philadelphia, 1962, 1964, 1966. Spectra Nos.: 14892, 1348,24557. (45) Hauptachen,M.;Grosse, A.V. J.Am. Chem. SOC.1961,73,5139. (46) Weiblen, D. G. In Fluorine Chemistry; Simons, J. H., Ed.; Academic: New York, 1964; Vol. 11, p 449. (47) Spinner, E. J. Chem. SOC.1964, 4217.
