FTIR Study of the Adsorption and Thermal Behavior of

As the surface is heated, decomposition of Al O Si(OC2H5)2CH CH2 occurs first via loss of the OC2H5 groups (starting at ∼520 K) and then with loss o...
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Langmuir 1997, 13, 5322-5328

FTIR Study of the Adsorption and Thermal Behavior of Vinyltriethoxysilane Chemisorbed on γ-Al2O3 Anya Kuznetsova, Edward A. Wovchko, and John T. Yates, Jr.* Department of Chemistry, Surface Science Center, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received March 4, 1997. In Final Form: July 18, 1997X The adsorption and thermal stability of vinyltriethoxysilane (VTES) was studied on a γ-Al2O3 surface. Adsorption occurs near room temperature with the production of ethanol from the reaction with isolated surface AlsOH groups, producing the surface species AlsOsSi(OC2H5)2CHdCH2. As the surface is heated, decomposition of AlsOsSi(OC2H5)2CHdCH2 occurs first via loss of the OC2H5 groups (starting at ∼520 K) and then with loss of the CHdCH2 groups (starting at ∼820 K). Below 375 K, ethanol is the sole gas phase product produced, and near 520 K, both ethanol gas and ethylene gas are produced. At higher temperatures, ethanol decomposes on the surface and ethylene is the only gas phase species present. The thermal stability of ethoxy groups on the VTES-treated Al2O3 surface decreases considerably under strong hydrolysis conditions in water vapor. There is also a decrease in the stability of the vinyl groups under strong hydrolysis conditions. Above ∼525 K, SisOH infrared bands are observed to develop as AlsOH groups disappear. Above ∼1100 K, organic ligands are no longer detected on the surface by FTIR spectroscopy and the surface remains covered with a silicate overlayer.

1. Introduction Organosilicon compounds are widely employed for surface modification. Their application as silane-coupling agents is based on their unique capability to form an interfacial layer involving organic and inorganic phases. This property is utilized for the improvement of the adhesion between the surfaces of metals and polymer coatings, protecting the metals against corrosion. Recent studies showed that in addition to improvement in adhesion, the alkoxysilanes, which have the structure RSi(OR)3, are themselves very effective for the formation of corrosion protection coatings on oxidized aluminum.1,2 As was shown by previous studies,2-5 silane monolayer adsorption produces alumosiloxane bonds (dAlsOsSi) and thereby inhibits hydration of Al2O3 and its transformation into aluminum hydroxide. The orientation of the negatively charged dipole of the chemisorbed silane species outward from the surface is postulated to cause repulsion of Cl- and SO42- anions from the surface in aqueous media. These anion species are known for inducing the corrosion of metals.1 The binding mechanism of alkoxysilanes upon adsorption from the gas phase to oxide-coated metal surfaces is governed by the hydrolysis reaction between the alkoxyfunctional group and the surface hydroxyls, which is accompanied by the elimination of ethanol. The hydrolysis of alkoxy groups is well-known to occur also in aqueous solutions.5-7 The main studies of the trialkoxysilane adsorption have investigated adsorption from solutions using various techniques.8,9 The mechanism of hydrolysis, condensation, and chemical stabilization of silanes on the surface X Abstract published in Advance ACS Abstracts, September 1, 1997.

(1) Petrunin, M. A.; Gil’dengorn, V. D.; Nazarov, A. P. Prot. Met. 1994, 30, 130. (2) Nazarov, A. P.; Stratmann, M. Prot. Met. 1994, 30, 52. (3) Boerio, F. J.; Gosselin, C. A. Infrared Spectra of Polymers and Coupling Agents Adsorbed Onto Oxidized Aluminium; Advances in Chemistry Series; American Chemical Society: Washington, DC, 1981; Vol. 203; p 541. (4) Danner, J. B.; Vohs, J. M. Appl. Surf. Sci. 1993, 72, 409. (5) Coast, R.; Pikus, M.; Henriksen, P. N.; Nitowski, G. A. J. Adhesion Sci. Technol. 1996, 19, 101. (6) Plueddemann, E. P. Silane Coupling Agents, 2nd ed.; Plenum Press: New York, 1991. (7) Kurth, D. G.; Bein, T. Langmuir 1995, 11, 3061.

S0743-7463(97)00240-0 CCC: $14.00

is still not completely understood for adsorption from the gas phase. To our knowledge there is no IR study of vinyltriethoxysilane (VTES) adsorption on Al2O3 from the gas phase. Vinyltriethoxysilane was reported to have a strong corrosion-inhibiting effect on aluminum and other metals covered with the oxide film10 in comparison with other alkoxysilanes. Here we investigate the adsorption from the gas phase of VTES on Al2O3, studying the participation of surface hydroxyl groups on the Al2O3. Our IR study has three objectives: (1) to explore the mechanism of VTES adsorption on the surface of alumina and the effect of Al-OH groups on adsorption in particular; (2) to examine the thermal stability of VTES-derived films and their mode of thermal degradation; and (3) to study the reaction of the adsorbed layer with water vapor. 2. Experimental Section The stainless steel ultrahigh vacuum IR cell described previously11 is equipped with KBr windows allowing IR measurements in the 400-4000 cm-1 spectral range. The KBr windows are sealed with differentially pumped Viton O-rings. The cell is attached to a bakeable all-metal gas-handling system equipped with a 60 L/s turbomolecular pump, a 30 L/s ion pump, a Baratron capacitance manometer, a Dycor M100M quadrupole mass spectrometer, and a Hewlett-Packard (5890 Series II) gas chromatograph. The base pressure in the system is typically 10-8 Torr, as estimated from the ion pump current. The high-area alumina is deposited by a spraying technique onto a tungsten grid, which is held rigidly by nickel clamps. The photoetched tungsten grid, 0.0254 mm thick, is highly uniform and contains square windows (0.22 mm × 0.22 mm) which transmit 70% of the incident IR radiation. Electrical heating power may be used to accurately control the grid temperature, during spraying and also under vacuum, using an electronic controller. The temperature of the grid and hence of the alumina is measured by a chromel-alumel thermocouple (0.003-in. diameter) spot-welded to the top central region of the grid. The temperature can be held constant and maintained to (2 K in the range 150-1500 K. (8) Koenig, J. L.; Shih, P. T. K. J. Colloid Interface Sci. 1971, 36, 247. (9) Woo, H.; Reucroft, P. J.; Jacob, R. J. J. Adhesion Sci. Technol. 1993, 7, 681. (10) Nazarov, A. P.; Petrunin, M. A.; Mikhailovski, Yu. N. Zashch. Met. 1992, 28, 564. (11) Basu, P.; Ballinger, T. H.; Yates, J. T., Jr. Rev. Sci. Instrum. 1988, 59, 1321.

© 1997 American Chemical Society

Vinyltriethoxysilane Chemisorbed on γ-Al2O3 Degussa γ-aluminum oxide C (surface area ) 101 m2/g) was used for the experiments. Alumina was prepared in a slurry (1 g of Al2O3 ,10 mL of H2O, and 90 mL of acetone, analytical reagent grade, Mallinckrodt), which was then sprayed onto the grid using a N2-pressurized atomizer. The grid was electrically heated to 330 K to flash evaporate the liquid phase. Vinyltriethoxysilane (Gelest, 98%) was transferred to a glass storage vessel under dry conditions and was then transferred to the alumina sample using normal gas handling procedures. After the Al2O3 was deposited onto the grid, the grid was mounted into the cell and outgassed at a pressure of 10-7 Torr and at a temperature of 475 K for 16-20 h. The infrared spectra were recorded at 300 K by means of a Mattson Fourier transform infrared spectrometer (Model RS-10000) purged with nitrogen. The spectrometer is equipped with a liquid nitrogen-cooled wideband HgCdTe (MCT) detector. The spectral resolution was 4 cm-1, and 1000 scans were used for data acquisition. The cell could be translated laterally so the IR beam could pass through the unsprayed portion of the grid to obtain the background spectra. Spectra of the surface species were obtained by ratioing single-beam spectra of the sample to the background singlebeam spectra. The experimental procedure for the adsorption of VTES on the surface of alumina consists of the preparation of the sample and exposure to different doses of the vapor adsorbate. In some experiments the sample was heated in vacuum at 300 K for 10 min after each dose of VTES. The saturation of the alumina surface with VTES was monitored by the absorbance of the characteristic IR modes of VTES. The saturation coverage was defined when no additional spectral changes were observed upon increasing exposure to VTES. All spectra were taken at a pressure of 10-7 Torr. For the study of the first stages of the reaction of adsorbed VTES with the Al2O3 surface, the sample was cooled down to 175 K and dosed with 5 Torr of VTES; the surface was then heated stepwise in vacuum to 250, 325, 350, and 375 K. The spectra were measured every time after cooling to 175 K. The reaction between the adsorbed silane and water vapor was performed by the exposure of the VTES-saturated surface to water vapor at a pressure of 1 Torr at various sample temperatures followed by evacuation to 10-7 Torr. For the analysis of the gas phase products of decomposition by IR spectroscopy, the cell with the heated sample was closed, keeping all produced gas inside the cell. The IR beam was then directed through the uncovered portion of the support grid to measure the infrared spectra of the gas phase only. The gaseous products of thermal decomposition were also collected in a metal cylinder cooled by liquid nitrogen. The condensed gas was warmed up to 300 K and analyzed with a gas chromatographer. For further analysis by mass spectrometer, the gaseous product of thermal decomposition was dosed through a leak valve into the continuously pumped quadrupole mass spectrometer. Since ethanol is a reaction product, parallel studies of ethanol (Aldrich, spectrophotometric grade) were conducted on the surface of alumina activated at 675 K. The ethanol adsorption was performed at a pressure of 1 Torr followed by evacuation to 10-7 Torr. The spectra of adsorbed ethanol were measured at 175 K after heating to various temperatures.

3. Results A. Adsorption of Vinyltriethoxysilane on Alumina at 300 K. The surface of aluminum oxide has been an object of extensive IR studies. In agreement with data reported before, our sample of γ-alumina is characterized by overlapping AlsOH adsorption bands at 3787, 3726, 3676, 3588, and 3500 cm-1. These features correspond to five types of AlsOH groups in accordance with the model developed by Peri in 196512 and refined by Kno¨zinger and Ratnasamy in 1978.13 These types of hydroxyls are assigned as isolated (3787, 3726, and 3676 cm-1) and associated hydroxyls. The latter can be removed by (12) Peri, J. B. J. Phys. Chem. 1965, 69, 220. (13) Kno¨zinger, H.; Ratnasamy, P. Catal. Rev.sSci. Eng. 1978, 17, 31.

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Figure 1. Dehydroxylation of Al2O3 surface. The sample is heated in vacuum to 475, 575, 675, 775, 875, and 1075 K. Table 1. Mode Assignment and IR Band Positions for VTES Adsorbed on Al2O3 wavenumber (cm-1)

assignment

3062 3059 2975 2933 2888 1600 1483 1443 1405 1392 1294 1274 1168 1105-1080

CH2 asym stretch (CHdCH2) CH stretch (-CHdCH2) CH3 stretch (OC2H5) CH3 stretch (OC2H5) CH3 stretch (OC2H5), CH2 stretch (CHdCH2) CdC stretch (CHdCH2) CH2 deformations (OC2H5) CH3 asym deformations (OC2H5) dCH2 in-plane deformations (CHdCH2) CH3 sym deformations (OC2H5) CH2 in-plane twist (OC2H5) CH in-plane bend (CHdCH2) CsC stretch (OC2H5) SisOsC asym stretch (OC2H5)

heating to 600-700 K, leaving isolated Al-OH groups, as shown in Figure 1. Heating to 1075 K removes almost all Al-OH groups. Silanes show a distinct absorption band pattern in their vibrational spectra which provides valuable information about the way these compounds adsorb and decompose on the surface. The characteristic bands and their assignments for VTES are summarized in Table 1. The CsH stretching modes of the ethoxy group in VTES slightly overlap with the CsH stretching vibrations of the vinyl group. Ethoxysilanes generally show a doublet for the SisOsC stretching vibrations at about 11001075 cm-1.14 The SisCHdCH2 group is characterized by vinyl vibrations at 1615-1590 cm-1 (CdC stretch), 14101390 cm-1 (CH2 deformation), 1020-1000 cm-1 (transCH wag), and 980-950 cm-1 (CH2 wag).15 In the case of vinyltriethoxysilane adsorbed on Al2O3, the bands due to the vinyl group at 1020-1000 and 980-950 cm-1 are overlapped by the strong modes of the ethoxy moiety and (14) Colthup, N. B. Introduction to Infrared and Raman Spectroscopy, 2nd ed.; Academic Press: New York, 1975. (15) Smith, A. L. The Analytical Chemistry of Silicones; John Wiley & Sons, Inc.: New York, 1991.

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Figure 2. Behavior of Al-OH stretch vibrations with adsorption of silane. The sample with high hydroxyl coverage has been heated in vacuum at 475 K. The sample with low hydroxyl coverage has been heated in vacuum to 675 K.

of alumina. In our experiments we therefore focus our attention on the strong CdC stretch mode at 1600 cm-1 and the CsH stretch at 3059 cm-1 (both characteristic of the vinyl fragment), which provide sufficient information about the behavior of this functional group. We also focus on the CH3 stretch at 2975 cm-1 (characteristic of the ethoxy fragment). During the adsorption of VTES on Al2O3 at 300 K, the features at 3787, 3726, and 3676 cm-1, which correspond to the isolated hydroxyls, disappear, indicating the involvement of the isolated hydroxyl groups in adsorption, as shown in Figure 2. At the same time intensification of the associated hydroxyl features is observed below 3600 cm-1. In the case of the partially dehydroxylated surface, on the right hand side of Figure 2, the two AlsOH features at 3787 and 3700 cm-1 shift to lower frequency as these AlsOH groups associate with VTES. The relation between the initial coverage of Al-OH groups and the saturated coverage of the adsorbed VTES (as judged by IR absorbance) is shown in Figure 3. The full coverage spectra of VTES on alumina and of the OH region prior to adsorption correspond to four Al2O3 samples activated in vacuum at different temperatures, as indicated, to produce a different initial distribution of surface Al-OH groups. Each sample received the same VTES exposure at 300 K. The sample activated at 1175 K shows almost no Al-OH groups on the surface of the Al2O3 prior to adsorption of VTES, and a small absorbance at 2975 cm-1 develops, corresponding to the CH3 mode of VTES. Figure 3 indicates that the maximum coverage of the silane occurs on Al2O3 activated at 675 K, which corresponds to an intermediate level of dehydroxylation. Small coverages of VTES are also produced on dehydroxylated Al2O3 surfaces. B. Initial VTES Reactive Adsorption Steps Production of Adsorbed Ethanol. Figure 4 shows the infrared spectral region from 1150 to 1000 cm-1, containing absorbances due to the Si-O-C asymmetric stretching modes of the ethoxy ligand in adsorbed VTES. In addition, a cross-hatched feature at 1052 cm-1 is due to the C-O

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Figure 3. Adsorption of VTES on alumina at different dehydroxylation levels of the sample. The samples were dehydroxylated at 475, 675, 875, and 1175 K, exposed to 5 Torr of silane at 300 K, and evacuated to 10-7 Torr.

Figure 4. Comparison of VTES and ethanol spectra during thermal decomposition on Al2O3. The ethanol C-O stretch in both spectra was fitted with a Gaussian function and deconvoluted from the largely overlapped spectral pattern in the 1000-1100 cm-1 region in the spectrum of adsorbed ethanol and VTES.

stretch in adsorbed ethanol molecules on Al2O3. The deconvoluted feature is of Gaussian peak shape, contributing to a shoulder at 1054 cm-1 in the spectra. For comparison, the 1052 cm-1 C-O stretch for pure adsorbed ethanol is shown on the right hand side of Figure 4. As the VTES-covered surface is heated from 175 to 375 K, the Si-O-C modes due to the ethoxy ligands in VTES slowly begin to disappear, whereas the 1052 cm-1 mode, due to ethanol produced from VTES, first increases

Vinyltriethoxysilane Chemisorbed on γ-Al2O3

Figure 5. Behavior of the ethoxy ligand and ethanol productsVTES decomposition/Al2O3. Comparison to pure ethanol behavior. The dashed line is added to guide the eye.

somewhat in absorbance and then above 325 K decreases rapidly in absorbance. The thermal behavior of the C-O stretch for chemisorbed ethanol alone is shown on the right hand side of Figure 4 for comparison. These results indicate that the spectral feature in the region 1105-1052 cm-1 is the result of the superposition of the spectrum of Si-OC2H5 groups and the spectrum of ethanol adsorbed on Al2O3. The frequency and absorbance behavior of the cross-hatched component shown on the left strongly resembles the behavior of pure ethanol adsorbed on Al2O3 (right spectra). The line shape changes on the left are due to the continued loss of Si-OC2H5 groups as temperature is increased combined with ethanol production from Si-OC2H5 groups, where the ethanol exhibits a different thermal history compared to the SiOC2H5 groups. Figure 5 shows a comparison of the normalized absorbances of the spectral features due to the ethoxy ligands in VTES (1105 cm-1) and to adsorbed ethanol produced by the reaction of the ethoxy ligand from adsorbed VTES (1052 cm-1). The behavior of pure ethanol is also shown (1052 cm-1 mode). Two important features may be seen in Figure 5: (1) The Si-O-C2H5 ligand begins to disappear at 175 K (or lower). (2) The 1052 cm-1 absorbance of ethanol initially increases when VTES reacts with AlOH groups and then decreases rapidly beginning at 350 K, in good agreement with the behavior of pure ethanol on Al2O3. The production of ethanol during the initial step of VTES adsorption on Al2O3 was confirmed by studies of the gas phase composition using FTIR, where only gaseous ethanol was observed below 375 K. These studies clearly show that, below 375 K, the reaction of VTES occurs through the ethoxy groups reacting with surface Al-OH groups and eliminating ethanol product. This result was confirmed by mass spectrometry, gas chromatography, and IR spectroscopy of the gas phase. C. Thermal Stability and Decomposition of Chemisorbed OsSi(OC2H5)2CHdCH2 Species Produced from VTES. The decomposition of adsorbed OsSi-

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(OC2H5)2CHdCH2 groups may be conveniently studied by observing the absorbance behavior of characteristic modes due to SisOC2H5 groups (2975 cm-1) and SisCHdCH2 groups (3059 cm-1). The results of this experiment, performed in vacuum, are shown in Figure 6. Four regions of chemical reactivity may be discerned in Figure 6. In the first region, below 375 K, SisOC2H5 groups react as ethanol is produced and OsSi(OC2H5)2CHdCH2 species chemisorb as a result of the reaction with surface AlsOH groups. In the second region from 375 K to about 520 K little change occurs in the adsorbed OsSi(OC2H5)2CHdCH2 species. In the temperature range 520-820 K, the ethoxy groups in the chemisorbed OsSi(OC2H5)2CHdCH2 species begin to undergo reaction. Above about 820 K, loss of vinyl groups occurs. The experiment shown in Figure 6 was repeated with the cell isolated from the vacuum system, and IR studies of the gas phase were performed. During the first chemisorption stage below 375 K, only gas phase ethanol was observed. At 520 K, the gas phase consisted of a mixture of ethanol and ethylene. Following the highest temperature decomposition (1100 K), only ethylene was observed in the gas phase. Representative gas phase spectra are shown in the insets to Figure 6. These results are indicative of three surface processes: (1) Ethoxy group reaction with AlsOH groups produces ethanol gas during chemisorption below 375 K and also during reaction of chemisorbed OsSi(OC2H5)2CHdCH2 with surface AlsOH groups at higher temperatures. (2) Gas phase ethanol reacts with Al2O3 above ∼520 K to produce ethylene. (3) Vinyl groups react with AlsOH groups to produce ethylene above ∼820 K. Detailed spectral changes may be seen in Figure 7 for various temperatures of decomposition. The sequential loss of OC2H5 and CHdCH2 functional groups from OsSi(OC2H5)2CHdCH2 species as the temperature is increased may be inferred from the observed spectral changes. Thus, in the CsH stretching region the symmetrical CH3 stretching mode (2975 cm-1) is strongly attenuated above 625 K before loss of the absorbance due to the vinyl CH2 stretch mode (3059 cm-1) begins to occur near 820 K. The same conclusions are reached for the behavior observed in the lower frequency region of Figure 7. D. SisOH Bond Formation at Elevated Decomposition Temperatures. Figure 8 shows the behavior of the AlsOH and SisOH stretching region as a function of the temperature of decomposition of the OsSi(OC2H5)2CHdCH2 surface species. As the absorbance of the 2975 cm-1 mode due to the CH3 stretch of the ethoxy groups decreases over the temperature range shown (475-775 K), the SisOH mode at 3731 cm-1 is observe to intensify. This mode is assigned to an isolated SisOH species on the basis of the observation of similar species at low coverages on SiO2 surfaces having frequencies in the range 3730-3750 cm-1.16,17 At 625 K, associated AlsOH modes at 3570 and 3470 cm-1 begin to decrease in intensity as dehydroxylation of the Al2O3 occurs, and the SisOH intensity continues to increase. This is in agreement with the known higher thermal stability of SisOH groups compared to AlsOH groups. The SisOH species is stable up to about 1000 K and disappears by about 1200 K (not shown), in agreement with ref 16. E. H2O Reaction with Surface OsSi(OC2H5)2CHd CH2 Species. Figure 9 shows the spectral developments which occur when a VTES/Al2O3 layer is treated with 1 (16) Wovchko, E. A.; Camp, J. C.; Glass, J. A.; Yates, J. T., Jr. Langmuir 1995, 11, 2592. (17) Van Cauwelaert, F. H.; Jacobs, P. A.; Uytterhoeven, J. B. J. Phys. Chem. 1972, 76, 1434.

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Figure 6. Thermal decomposition of ethoxy and vinyl groups of VTES on Al2O3. Here the Al2O3 is partially dehydroxylated at 675 K. Gas phase spectra are taken at sample temperatures of 375, 520, and 1100 K.

Figure 7. Thermal decomposition of VTES on Al2O3. The characteristic vibrational modes of VTES are shown for sample temperatures of 295, 375, 625, 875, 1075, and 1175 K.

Torr of H2O(g) at 300 K, and the behavior on heating. The following changes are observed: (1) Features due to Si-O-C2H5 groups diminish in intensity beginning at 300 K. This includes the symmetrical CH3 stretching mode, the C-C mode of O-C2H5, and the asymmetrical Si-O-C modes (Figure 9). (2) The feature at 3059 cm-1 (CHdCH2) is first attenu-

Figure 8. Si-OH bond formation at high temperature. The sample initially contains a coverage of surface hydroxyls (3570 and 3470 cm-1). The loss of both ethoxy and vinyl groups occurs at lower temperatures than shown in Figure 6. Here, higher initial hydroxyl coverage was present due to pretreatment in 1 Torr of H2O vapor.

ated at the temperature of ∼575 K in the course of hydrolysis (Figure 8). (3) The broad spectral band due to associated Al-OH groups increases in absorbance (Figure 9). (4) A feature due to an isolated SiO-H mode begins to develop at ∼575 K (Figure 8).

Vinyltriethoxysilane Chemisorbed on γ-Al2O3

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Figure 9. H2O adsorption on Al2O3 containing chemisorbed VTES. Hydrolysis was carried out under 1 Torr of H2O vapor with sequential heating to 300, 325, 375, and 475 K.

(5) A feature due to the C-O stretch in adsorbed C2H5OH develops at 300 K and then begins to disappear as the temperature is increased further (Figure 9). These results, compared to those of thermal decomposition experiments made in the absence of excess H2O, indicate that loss of the Si-OC2H5 group is strongly enhanced in the presence of H2O. Thus, in Figure 9, by 475 K, the spectral features due to Si-OC2H5 are almost completely extinguished (sym CH3 stretch; C-C stretch) whereas in Figure 7 (no added H2O) these modes are still very intense even at 625 K. In addition, comparison of the 3059 cm-1 mode (vinyl CH2 stretch) behavior in Figure 8 and Figure 7 shows that the vinyl group also is more reactive in the presence of excess H2O. Thus in excess H2O, at 775 K the vinyl group is strongly depleted (Figure 8), whereas extensive vinyl depletion only occurs above 875 K in the absence of H2O (Figure 7).

Figure 10. Adsorption and initial decomposition of VTES on Al2O3.

4. Discussion A. Preferential Reactivity of Isolated Hydroxyl Groups in VTES Adsorption. According to the model of the active sites on the alumina surface developed by Kno¨zinger and Ratnasamy,13 the surface of alumina is characterized by Al3+, O2-, and Al-OH groups of various types. The Al-OH groups may be distinguished from each other on the basis of their O-H stretch frequency, and the two higher frequency O-H modes are assigned to the isolated Al-OH groups which do not undergo hydrogen bonding with each other. The dehydroxylation process removes OH groups as water, consuming first the associated groups and producing isolated groups which themselves are then consumed as dehydroxylation proceeds. This process produces Al3+ and O2- ionic sites, which are Lewis acid and Lewis base sites, respectively. We observe that the capacity for chemisorption of VTES is enhanced under conditions of hydroxyl coverage in which the relative coverage of isolated Al-OH groups is maximized. Thus, in Figure 3, the Al2O3 surface, dehydroxylated at 675 K, exhibits the highest chemisorption capacity, judging from the intensity of the symmetrical CH3 stretching mode of the OC2H5 groups of VTES. The chemisorption of VTES on the isolated Al-OH groups is accompanied by a second effect in which the Al-OH groups shift down in wavenumber, indicative of hydrogen bonding of these groups. This hydrogen bonding of the surface Al-OH groups is postulated to occur with neighboring OC2H5 groups of chemisorbed VTES. A schematic view of the two routes to consumption of isolated Al-OH groups,

Figure 11. Continued thermal decomposition of VTES-derived surface species.

first by reaction and then by association, is shown in Figure 10. The preferential interaction of isolated Al-OH groups either by reaction or association through hydrogen bonding is a well-known phenomenon, and preferential Al-OH association with adsorbates has for example been seen for CH3Cl adsorption on isolated Al-OH groups on Al2O3.18 The reverse of this process, which accompanies VTES thermal decomposition above about 625 K, is observed to generate isolated OH groups, producing an Si-OH species, as shown in Figure 11. The Si-OH groups are known to be thermally more stable than Al-OH.16 At the same time, the ethoxy group absorbance decreases as reaction occurs between these groups and surface Al-OH groups, to produce C2H5OH(g), which was detected in the gas phase. B. Thermal Stability of Vinyl and Ethoxy Ligands. The vinyl ligand has a higher stability than the ethoxy ligand, as may be seen from Figure 6 and the spectra shown elsewhere. In addition, the thermal stability of the ethoxy ligand and the vinyl ligand may be decreased by the addition of water to the surface, indicative of a hydrolytic route for ethoxy destruction which then may (18) Beebe, T. P.; Crowell, J. E.; Yates, J. T., Jr. J. Phys. Chem. 1988, 92, 1296.

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stimulate loss of the vinyl ligand. The production of ethylene from the vinyl groups during thermal decomposition will likely involve a proton attack from hydroxyl groups on the alumina surface. In addition, ethylene is also produced from ethanol on the Al2O3 surface by a wellknown reaction in the temperature range 350-600 K.19 C. Final Thermal Products from Hydrolytic VTES Decomposition. The complete loss of both ethoxy and vinyl functionalities below about 1200 K suggests that the final thermal decomposition process will deposit a silicate on the Al2O3 surface. At the present time, the properties of this coating for passivation of Al2O3 films on aluminum surfaces are unknown and will be the subject of further investigation. 5. Summary The reaction of vinyltriethoxysilane (VTES) with an alumina surface has been investigated using infrared spectroscopy to monitor the behavior of both the surface (19) De Canio, E. C.; Nero, V. P.; Bruno, J. W. J. Catal. 1992, 135, 444.

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Al-OH functionalities and the functionalities of the chemisorbed VTES molecule. The following results have been found: (1) VTES reacts preferentially with isolated Al-OH groups and shows little evidence of reactivity with associated Al-OH groups. Some evidence is found for adsorption of VTES on sites other than Al-OH sites. (2) Hydrogen bonding of remaining isolated Al-OH groups with the chemisorbed product of VTES adsorption is observed. This interaction occurs through the ethoxy moieties on the chemisorbed species. (3) The thermal stability of SisOC2H5 groups is low relative to the stability of SisCHdCH2 ligands on the chemisorbed species derived from VTES adsorption. The SisOC2H5 group stability is decreased further with the addition of water to the surface. (4) The final stage of thermal decomposition of VTES on Al2O3 produces a silicate layer on the Al2O3 surface. Acknowledgment. We acknowledge, with thanks, the support of the Air Force Office of Scientific Research. LA9702401