352
Langmuir 1989, 5 , 352-356
in more extensive overlap of the 5a wave functions. As already mentioned, the crowding due to postdeposited Au atoms displaces CO molecules from their stable adsorption sites, which results in weakening of CO-metal bonds. This also contributes to the downward shift of the CO-induced
peaks, since weakening of adsorption bonds reduces the relaxation energy, i.e., the energy due to the screening of produced holes by metal electrons. Registry No. Au, 7440-57-5; CO, 630-08-0; Ru, 7440-18-8.
A Transmission Infrared Spectroscopic Investigation of the Reaction of Dimethyl Ether with Alumina Surfaces J. G. Chen, P. Basu, T. H. Ballinger, and J. T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received May 13, 1988. I n Final Form: November 14, 1988 The adsorption and reaction of (CHs),O and (CD3I2Owith high-area alumina (A1203)surfaces containing hydroxyl groups have been investigated by using transmission infrared spectroscopy in the temperature region 100-400 K. It has been found that the ether molecularly adsorbs on alumina surfaces, via hydrogen bonding to the surface hydroxyl groups, at 150 K. Heating the ether/alumina layers to T 250 K causes drsorption of the ether and reveals the formation of surface methoxy species. The results are also compared t c our previous EELS investigation of the interaction of ether molecules with well-characterized A1203 films grown on an Al(111) crystal surface.
Introduction A detailed understanding of the interaction of alkyl and fluoroalkyl ethers with aluminum oxide surfaces is of practical importance since fluorinated ethers are widely used as lubricants in high-technology applications. As a part of ongoing investigations of the nature of the interaction of ether molecules with various aluminum oxide surfaces, we report a n infrared spectroscopic study of the adsorption and reaction of dimethyl ether, (CH3120, molecules with high-area alumina surfaces, containing surface hydroxyl groups, in the temperature range 100-400 K. Compared to our earlier EELS study of the interaction of ether molecules with a well-characterized Al,O,/Al( 111) substrate,' a dramatic difference is observed in the reactivities of the ether/alumina system in which bulk A1,0, is the substrate. In this study, our results indicate that at 150 K dimethyl ether molecularly adsorbs on alumina surfaces via hydrogen bonding of the 0 atom of ether to the surface hydroxyl groups. After heating the ether/alumina layers to T 2 250 K, we observe the desorption and decomposition of ether molecules and the formation of a reaction intermediate, the adsorbed methoxy species. Thermal degradation experiments for both (CH3)20/aluminaand (CD,),O/alumina systems were studied. Experimental Section Infrared spectra were measured in the transmission mode by using a purged Perkin-Elmer Model PE-783infrared spectrometer. The spectrometer was coupled with a 3600 data acquisition system, which allowed one to carry out IR signal averaging in acquisition times from 3.9 to 208 s/cm-l and to perform spectral subtraction procedures for removing the contribution of the alumina background.' A spectrometer slit program yielding a maximum resolution (fwhm) of 5.4 cm-I was employed in the experiments. (1) Ng, L.; Chen, J. G.; Btsu, P.; Yates, J. T., Jr. Langmuir 1987, 3, 1161. (2) Beebe, T. P., Jr.; Crowell, J. E.; Yates, J. T., J r . J . Phys. Chem. 1988, 92, 1296.
0743-7463/89/2405-0352$01.50/0
The stainless steel infrared cell used for spectroscopic measurements has been described in detail previously? In brief, it consists of a main cell body containing a CaF, sample support plate attached to a copper support ring. The temperature of the CaF, plate, onto which the alumina was deposited, can be regulated by controlling the flow rate of cooled or heated N,(g) through the support ring. The IR cell, which permits infrared measurements between 4000 and 1000 cm-', was attached to a stainless steel gas handling system pumped by a 20 L/s ion pump and a liquid Torr). nitrogen-cooled sorption pump ( P 6 1 X Deposition of alumina (Degussa,Aluminum Oxide C, 100 m2.g-l) onto one-half of the CaF, sample plate was carried out by using a spraying procedure described previously? The weight of Al,03 on the 5.0 cm2 CaF, pIate was 27 mg. The CaF, plate with deposited alumina was then placed into the infrared cell. The alumina sample was then outgassed in vacuo at -475 K for 4 days; these treatment conditions produced alumina surfaces with a variety of "free" (isolated)and hydrogen-bonded hydroxyl groups! The reagents used in this study, (CH3),0 (99.8% purity, Matheson) and (CDJ'O (99% purity, SCM),were transferred from gas cylinders to glass storage flasks via a preparative glass vacuum line and purified with several freeze-pump-thaw cycles. The high purity of these gases was verified by mass spectrometry.
Results The infrared spectra presented here were obtained by using the following procedure: (1) about 2.5 X 1019molecules of (CH,),O or (CD3),0 were introduced into the IR cell at 100 K, ( 2 ) the adsorbed layer was heated under continuous pumping conditions to higher temperatures, (3) the IR spectra were then recorded under a vacuum of P < 1X Torr and at the indicated temperatures for T d 300 K and at 300 K after heating the layer to higher temperatures, and (4)the spectral contribution of the alumina background was removed by subtracting the IR spectrum of the clean alumina surface, measured a t the corresponding temperatures. (3) Wang, H. P.; Yates, J. T., Jr. J. Phys. Chem. 1984, 88, 852. (4) (a) Peri, J. B. J . Phys. Chem. 1965,69,211. (b) Peri, J. B.; Hannan, R. B. J . Phys. Chem. 1960, 64, 1526.
G 1989 American Chemical Society
Reaction of Dimethyl Ether with Alumina Surfaces
Langmuir, Vol. 5, No. 2, 1989 353
-+
;:
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0
c
aJ
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:
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a
9
(b) 200
a
(c)
250
(d)
300 (e) 350 1
1
1500
3100
3000
2900
2800
2700
Wavenumber (cm-' ) Figure 1. Infrared spectra of (CHJZO on alumina in the C-H stretching region recorded as a function of temperature. The (CH3)zO/aluminalayer was heated in vacuo to the indicated temperatures.
1
1
1400
1
1
/
1200
1
1
I100
1
1000
Wavenumber (cm-'1 Figure 2. Infrared spectra of (CHJ20/alumina in the C-H deformation and C-0 stretching regions recorded as a function of temperature.
367:
Interaction of (CH3)20 with Alumina Surfaces. Figure 1 shows the IR spectra of (CH,),O/alumina in the C-H stretching region recorded as a function of temperature. At 150 K (Figure la), five vibrational modes are observed a t 2984,2922,2890,2872, and 2821 cm-'. As will be discussed in detail in the next section, these modes are related to the characteristic vibrational features of the molecularly adsorbed (CH3)20 species. After the (CH3)20/aluminalayer is heated to 200 K (Figure lb), all vibrational features observed at 150 K decrease in intensity (notice the difference in the expansion factors), while the vibrational frequencies undergo a slight upward shift. Significant spectroscopic changes are observed after the layer is heated to 250 K (Figure IC). These changes are as follows: (1)a relatively strong feature becomes evident a t -2955 cm-', (2) the 2922-cm-' feature observed at 150 K now becomes a relative weak shoulder (at -2927 cm-l) on the new 2955-cm-' feature, and (3) the other features observed a t 150 K undergo rather large frequency shifts. The new feature observed a t 250 K (Figure IC)remains dominant in the 2955-2962-cm-' region a t higher temperatures (Figure I d and le), while the 2922-cm-l mode observed a t 150 K (Figure l a ) disappears. Finally, after the (CHJ20/alumina layer is heated to 350 K (Figure le), an IR spectrum with major vibrational modes a t 3036, 2962,2885, and 2848 cm-' is observed. Comparing Figure l e with Figure l a clearly shows after the (CHJ,O/alumina layer is heated to T 3 250 K, new surface species with different C-H vibrational modes are observed. Corresponding IR spectra in the region 1525-1025 cm-' (C-H deformation and C-0 stretching regions) are shown in Figure 2. The five vibrational modes observed a t 150 K (Figure 2a), 1477,1459, 1252,1166, and 1092 cm-l, are also indicative of the molecularly adsorbed (CH3)20 species. Significant spectroscopic changes are again observed after the (CH3)20/aluminalayer is heated to T I 250 K (Figure 2c-e). In addition to small frequency shifts, the most apparent changes are the onset of a new feature a t 1063-1059 cm-' and the disappearance of the 1092-cm-' mode observed a t 150 K. Figure 3 shows a typical set of IR spectra in the 0-H stretching region for the clean alumina substrate and for (CHJ,O/alumina layers heated to various temperatures. The spectra are shown in the upper portion of the figure, and difference spectra are presented in the lower portion
1
1300
3
7
T
0 07 A b 3
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2
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"
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-
fi
0
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n
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T(K1 150
3683
350
100
3600
3200
W a v e n u m b e r (cm-'1 Figure 3. Changes induced in the 0-H stretching frequency range
upon heating the (CH3)zO/aluminalayer. The top part of the figure shows the actual spectra, while the difference spectra, obtained by subtracting the IR spectra of alumina at the corresponding temperatures,are shown in the bottom portion of the
figure.
of the figure. It may be seen that upon adsorption of (CH3),0 and heating to 150 K, attenuation of the two high-frequency 0-H bands a t 3724 and 3683 cm-' (the isolated OH groups4) is observed in the difference spectra. In addition, an enhancement of the broad band centered a t -3457 cm-' (the hydrogen-bonded surface hydroxyl group^)^ is simultaneously observed. As the temperature of the surface is increased, all three 0-Hbands are gradually restored to their original shape and intensity. These observations suggest that the surface hydroxyl species are not involved in the irreversible surface changes seen a t T 6 250 K in Figures 1and 2; these changes will be discussed later. Experiments involving the heating of alumina in the presence of (CH3)20gas were also performed. The IR spectra obtained in these experiments (not shown) present
354 Langmuir, Vol. 5, No. 2, 1989
Chen et al. Table I. Vibrational Frequencies of Dimethyl Ether and Those Observed for (CHS)zO/Alumina at 150 K frequency, cm-'
(CH&O/aluminab
T(KI -
I
t
l
l
2400
!
I
l
1
2200
[a)
150
(b)
200
vn(CH3) 6(CH3)
2817 (2054) 1470-1456 (1062-1056)
(ci
250
r(CH3)
(d) (e) (f)
300
1244 (1033) 1179 (931) 1102 (1162-1146)
U,,(CO)
350
a See ref 6. This work. The frequencies given in parentheses are those of (CD3),0, which was dissolved in CCl, solution.
400
/
2000
Wavenumber ( c m - ' ) Figure 4. Infrared spectra of (CD,),O/alumina in the C-D stretching region recorded as a function of temperature. 7 "34
Resolullon
5 4 cm-l
T
1061
0 22Abs
E; 0
n
x 11 x 20 I
1300
\(fi ,
l
I
1
1200
,
I
,
l
1100
,
1
400 1
1
2984 (2250) 2922 (2200) 2890 (2136) 2872 2821 (2063) 1477 (1061) 1459 1252 (-) 1166 (-) 1092 (1134)
1
1000
Wavenumber (cm-'1 Figure 5. IR spectra of (CD3)20/aluminain the C-D deformation and C - 0 stretching regions recorded as a function of temperature.
vibrational features identical with those recorded by heating the adsorbed (CH3)20 layer in vacuo (Figures 1 and 2), with the only difference being that stronger intensities for all of the vibrational features are observed in the former case. Interaction of (CD3)20with Alumina Surfaces. Similar thermal degradation experiments were also performed for (CD,),O/alumina in the temperature region 150-400 K. IR spectra of (CD,),O/alumina are shown in Figure 4 (C-D stretching region) and Figure 5 (C-D deformation and C-0 stretching regions), respectively. In a manner similar to the behavior observed in Figures 1and 2, significant spectroscopic changes, i.e., the onset of new vibrational modes and large frequency shifts, are observed after the (CD,),O/alumina layer is heated to temperatures of 250 K or above. The observation of new surface species a t higher temperatures is again indicated by the marked differences between the IR spectra recorded a t 150 K (Figures 4a and 5a) and those obtained after heating the (CD,),O/alumina layer to T 2 250 K. Mass Spectral Studies of Desorbing Gases from (CH3)2O on Alumina. Mass spectrometric measurements of the gas-phase products upon the reaction of (CH3),0 with alumina were also carried out. In these experiments, 5 x 1 O I 8 molecules of (CH3)20 were introduced into the IR cell a t 350 K; after the IR cell temperature was maintained a t 350 K for 2 h, the gas-phase species were collected and analyzed. The mass spectra of these gas species were
then compared to those of pure (CH3),0. In order to detect the presence/absence of CHI, C,H,, and CH,OH in the gas-phase products, we have compared the intensity ratios 15 amu/45 amu, 27 amu/45 amu, and 31 amu/45 amu, where mass 45 amu is the major cracking pattern of pure (CH,),O species and masses 15, 27, and 31 amu are representative of CH,, C2Hs, and CH,OH species, re~pectively.~These ratios were found to be identical with those of the pure gas-phase (CH3)20,within *170 accuracy. These observations suggest that within our detection limit no CH,, C2H6, or CH30H species are produced upon the reaction of (CH3),0 with alumina.
Discussion Adsorption of Dimethyl Ether on Alumina at 150 K. A detailed assignment for the vibrational spectra of dimethyl ether in both gas and liquid phases has been given by Kanazawa and Nukada.6 Their assignment of the features, along with the vibrational modes observed for (CH3),0 adsorbed on A1203at 150 K in our work, are summarized in Table I. By comparison, one can assign all of the features observed a t 150 K to the vibrational motions of the molecularly adsorbed dimethyl ether species. Vibrational features observed in the C-H (C-D) stretching region (Figures l a and 4a) can be readily assigned as v,(CH,) at 2984 (2250 cm-') and 2922 cm-' (2200 cm-I), respectively, and vs(CH3)at 2821 cm-' (2063 cm-'). Other features observed in this region, the 2890- and 2872-cm-' modes of (CH,),O/alumina (Figure la) and the broad feature a t 2136 cm-' of (CD,),O/alumina (Figure 4a), can be assigned to the overtones of the CH3 (CD,) deformation modes by analogy to those of (CH,),O(g) and (CD,),O(l) molecules, as shown in Table I. A straightforward assignment for the features observed in the C-H deformation and C-0 stretching regions of (CH,),O/alumina (Figure 2a) can also be reached by comparing our spectra for the adsorbed species with the frequencies in Table I. They are CH3 deformation modes (6(CH,)) a t 1477 and 1459 cm-', CH, rocking modes (y(CH,)) a t 1252 and 1166 cm-', and an antisymmetric C-0 stretching mode, v,(CO), a t 1092 cm-'. In the corresponding spectroscopic regions of (CD3)20/aluminaa t 150 K (Figure 5a), only two vibrational features are observed a t 1134 and 1061 cm-l. Due to the fact that the behavior of the 1134-cm-I mode resembles that of the v,(CO) mode of (CH,),O/alumina (at 1092 cm-') upon heating, Le., it (5) Stenhagen, E.; Abrahamsson, S.; McLafferty, F. W. Registry of Mass Spectral Data; Wiley: New York, 1974; Vol 1. (6) Kanazawa, Y.; Nukada, K. Bull. Chem. SOC.Jpn. 1962, 35, 612.
Langmuir, Vol. 5, No. 2, 1989 355
Reaction of Dimethyl Ether with Alumina Surfaces Table 11. Vibrational Frequencies (cm-*)of Methoxy Species and Those Observed for the Dimethyl Ether/Alumina Layers above 350 K
u,(CH~) 2988 (2227)'
(2235) 2960 (2255) 2962 (2250)' 2970 u,(CH~) 2853 (2086) (2065) 2849 (2089) 2848 (2085) 6(CH3) 1457 (1140) 1475 (1080) 1475 (1119) 1477 (1067) 1420 (1086) 1454 y(CH3) 1200 (-) 1170 (-) ?lo81 (-) 1157 (-) v,(CO) 1019 (1000) 1025 (985) 1055 (1067) 1059 (1088) Osee ref 12. bSee ref 13. cSee ref 2. d350K. This work. OThe frequencies in parentheses are those of deuterated species. 'Those frequencies for (CD3)pO/alumina are observed after heating the layer t o 400 K.
initially decreases in intensity and then disappears (c.f. Figures 2 and 5), it can be assigned to the v,(CO) mode of the adsorbed (CD3),0 species. The other feature a t 1061 cm-' in Figure 5a can then be assigned to the CD, deformation mode on the basis of its characteristic frequency shift upon deuteration. The adsorption of ether molecules a t 150 K is via the hydrogen bonding between the oxygen atom in the ether molecule and the hydroxyl groups of alumina, as indicated by the IR spectra recorded in the 0-H stretching region (Figure 3). As is most commonly observed for the hydrogen-bonded species on alumina,' the adsorption of (CH3),0 a t 150 K gives rise to losses of v(OH) intensity a t 3724 and 3683 cm-' (consumption of the isolated hydroxyl groups), accompanied by the appearance of additional absorbance as a broad band centered at 3457 cm-' (development of the hydrogen-bonded hydroxyl groups). This phenomenon involving the special reactivity of isolated OH groups has been observed on Al,O, or SiO, surfaces in this laboratory for the weak adsorption of CO,s N2,9 CH,Cl,l0 and (CF,H),O on the hydroxylated surfaces. Observation of Surface Methoxy Species at T 2 250 K. The presence of new surface species upon heating the ether/alumina layers to higher temperatures is clearly indicated by the spectroscopic changes observed a t T 2 250 K; this new surface species is identified as the surface methoxy, CH,O(a), on the basis of its characteristic vibrational frequencies. Summarized in Table I1 is a comparison of the vibrational frequencies observed after heating the ether/alumina layers to 350 K with those of the methoxy group in an organometallic compound, [Al(CH3)20CH3],,'2and with the surface methoxy species adsorbed on Al(111) (formed upon reaction of methanol with an Al(111) surface)', and on alumina surfaces (produced by the reaction of CH,Cl with alumina)., On the basis of the comparison of the characteristic vibrational frequencies shown in Table 11, we can readily assign the major features, observed after heating adsorbed (CH3),0 ((CD3),0) on alumina to 350 K, to the following normal modes of the surface methoxy species: va5(CH3)a t 2962 cm-' (2250 cm-I), v,(CH,) at 2848 cm-' (2085 cm-'), 6(CH3)
-
''
(7)Kiselev, A. V.;Lygin, V. I. Infrared Spectra of Surface Compounds; Wiley: New York, 1975. (8)Beebe, T. P., Jr.; Gelin, P.; Yates, J. T., Jr. Surf. Sci. 1984,148,
-_-. 5%
(9)Beebe, T.P.,Jr.; Yates, J. T., Jr. Surf. Sci. 1985,159,369. (10)Crowell, J. E.;Beebe, T. P., Jr.; Yates, J. T., Jr. J.Chem. Phys. 1987,87,3668. (11) Basu. P.:Ballineer. T. H.: Yates. J. T.. Jr. LanPmuir. in mess. (12)Tarao, R. Bull. Ehem. Soc. Jpn.'1966,'39, 2126: (13)Chen, J. G.; Basu, P.; Ng, L.; Yates, J. T., Jr. Surf. Sci. 1988,194, 397. 1
at 1477 and 1454 cm-' (1067 cm-'), 7(CH3)a t 1157 cm-', and v,(CO) a t 1059 cm-l(l088 cm-'). The feature observed a t 2885 cm-' (2178 cm-') can be assigned to the overtone of the CH3 (CD,) deformation modes. In addition to those readily assigned major methoxy features, two relatively weak modes are also observed in the IR spectra a t high temperatures: a mode at 3036 cm-l (2289 cm-') (Figures 1 and 4) and a feature a t 1261 cm-' (Figure 2). In a model proposed by Morrow for methoxy species adsorbed on silica surface^,'^ he has pointed out that, since the bonding angle of silica-OCH, is not 180°, the methoxy species can no longer be treated as a C, point group; it must be treated as a C, point group. Thus, there should be two distinct sets of methoxy hydrogen^.'^ As a consequence, one should argue that there should be two v,(CH,) modes14 and two CH, rocking modes for the surface methoxy species. We thus tentatively assign the 3036-cm-' (2289-cm-') and the 1261-cm-' features to the other v,(CH3) and 7(CH3) modes of the adsorbed methoxy species, respectively. By knowing the origin of the IR spectra obtained a t 150 K (hydrogen-bonded (CHJ2O species) and those observed a t 350 K (surface methoxy species), we can conclude, on the basis of the spectra shown in Figures 1and 2, that there are two thermally induced processes for the (CH3),0/ alumina layers: (1) desorption of (CH3),0 species a t 150-200 K and (2) decomposition of (CH3),0 to form surface methoxy species. The thermal desorption process is indicated by the decrease in the vibrational intensities from 150 K to 200 K, as well as by the pressure rise during the heating experiments. A t 250 K (Figures ICand 2c), the decomposed (CH3),0 becomes observable and the IR spectra are representative of a surface layer with a mixture of (CHJ20 and CH,O, as indicated by the fact that the characteristic vibrational modes of (CH,),O, v,(CH,) a t -2927 cm-' and v,(CO) a t 1090 cm-l, still remain in the spectra. After the layer is heated to 300 and 350 K, all the features observed in the IR spectra can be assigned to those of surface methoxy species, as given in Table 11. . The reaction of (CH3),0 with alumina surfaces to form methoxy species is likely to occur via the bond scission of one of the C-0 bonds of (CH3),0 and the formation of two A1-OCH, species on the surface. This is supported by the mass spectrometric analysis of gas-phase species collected after the reaction of (CHJ20 with alumina a t 350 K for 2 h, which did not show any detectable amount of CH,OH or hydrocarbon species. These species would likely be pr.oduced if one (CH,),O molecule reacted to form one A1-OCH3 unit, since the remaining CH3 and/or CH,O radicals would be expected to recombine with H available on the surface or with itself to form gas-phase products such as CH,OH or CzHG. The adsorption of dimethyl ether on alumina surfaces in the temperature region 293-673 K was previously carried out by Yakerson e t al.15 using infrared spectroscopy; no methoxy species was reported by the authors. Upon the adsorption of (CH3)20a t 293-373 K, their IR spectra were very similar to those obtained in our experiments a t 300-350 K (Figure ld,e and Figure 2d,e). However, the authors assigned their IR spectra to the hydrogen-bonded (CH3),0 species. We do not agree with this assignment for the following reasons: (1)From the IR spectra shown in the 0-H stretching region (Figure 3), the hydrogen bonding occurs strongly a t 150 K; the extent of hydrogen bonding decreases substantially a t higher temperatures as
-
,
.
(14)Morrow, B. A. J.Chem. Soc., Faraday Trans 1 1974,70, 1527. (15)Yakerson, V. I.; Lafer, L. N.; Danyushevskii, V. Ya.; Rubinshtein, A. M. Izu. Akad Nauk SSSR, Ser. Khim. 1967,11,2246.
356 Langmuir, Vol. 5, No. 2, 1989
indicated by the spectroscopic behavior of the OH groups. (2) The dramatic spectroscopic changes, from the vibrational features of hydrogen-bonded (CH3),0 species to a new set of vibrational modes, occur at -250 K. All (CH3)2O adsorption experiments were carried out a t 293 K or above by Yakerson et al.,15and under these conditions they would not have observed significant H-bonded (CH3)20. (3) The previous experiments performed in this laboratory of the reaction of CH,Cl with alumina2J0J6have clearly detected the formation of methoxy species on alumina surfaces. The close match of the vibrational frequencies of our spectra to that of CH,O/alumina (as shown in Table 11)17indicates that surface methoxy species are indeed formed upon heating the dimethyl ether/alumina layers to T 2 250 K. (4) A surface ethoxy species, A1-OC2H5, was observed by Arai e t a1.18 upon the adsorption of (C2H5)ZO on alumina a t -373 K. We believe that (CH3),0, as a member of the same class of organic molecules, should have a similar chemistry on alumina surfaces. Comparison with the Reactivity of (CH3),0 on Well-Characterized Al,O,/Al( 111). In a previous study, we have carried out a vibrational EELS investigation of the interaction of (CH,),O with well-characterized Al,O,/Al( 111)films, prepared by oxidizing an atomically clean Al( 111)surface.l The ether molecules were chemically unreactive with A120,/A1( 111): (CH3)20only physisorbed on A1203,and it all molecularly desorbed a t T > 120 K (based on thermal desorption measurements). From the results presented here, the reactivity of ether molecules toward the high-area alumina surfaces is apparently different. There are two possible explanations for the different reactivities of ether molecules with Al,O,/Al( 111) and high-area alumina surfaces: (1)On high-area alumina surfaces, the availability of a large number of surface hydroxyl groups results in the hydrogen bonding between the ether molecules and the alumina substrate in the temperature region 150-250 K (Figure 3). Although the reversible behavior of the v(OH) (16) Beebe, T. P., Jr.; Crowell, J. E.; Yates, J. T.,Jr., to be submitted. (17) The question mark in Table I1 for the assignment of r(CHJ at 1081 cm-' was originally established by the authors of ref 2. (18) Arai, H.; Saito, Y.; Yoneda, Y. J. Catal. 1968, 10, 128. (19) Knozinger, H.; Patnasamy, P. Catal. Reu. Sci.-Eng. 1978, 17, 31.
Chen et al. bands a t high temperatures (Figure 3) syggests that the surface OH groups are not directly involved in the decomposition of (CH&O and subsequent formation of surface methoxy, the formation of stable hydrogen-bonded (CH3)20surface species in the temperature range 150-250 K would likely either activate the C-0 bond scission of the adsorbed (CH3),0 or simply cause the retention of the ether molecules on the surface to sufficiently high temperatures for the thermal activation, leading to decomposition of (CH,),O. (2) Another difference between the A1203/Al(lll)and the high-area alumina surfaces is the presence of coordinatively unsaturated cations (exposed A13+,anion vacancies) on the latter surfaces.18 These unsaturated A13+ cations could be the reactive sites for the decomposition of (CH,),O. Such unsaturated A13+ sites may not be available on the A1203/Al(lll)films prepared under the growth conditions used.l
Conclusions The following conclusions regarding the adsorption and reaction of dimethyl ether molecules on alumina surfaces have been reached: (1)Dimethyl ether molecules absorb on alumina surfaces a t 150 K, via hydrogen bonding of the oxygen atom of (CH3),0 to isolated surface hydroxyl groups. (2) Two thermally induced processes have been observed: desorption of (CH3)20a t 150-200 K and decomposition of the (CH3),0 species. It cannot be determined from the spectra whether the formation of the methoxy species occurs upon adsorption of (CH,),O a t 150 K or, upon heating, due to the possible overlap of the (CH,),O(a) spectrum with the less intense spectrum of CH,O(a). (3) The decomposition product of (CH3),0 has been identified as a surface methoxy species, A1-OCH,. (4)The surface hydroxyl groups of alumina may play a major role via hydrogen bonding in inducing the reaction between ether molecules and alumina substrates, while not undergoing direct reaction themselves. Acknowledgment. We gratefully acknowledge the support of this work by a research grant from IBM Research Laboratory, San Jose, CA. P. Basu acknowledges a fully supported educational leave from Alcoa. Registry No. Me,O, 115-10-6;A1,03, 1344-28-1.
