J . Phys. Chem. 1988, 92, 1296-1301
1296
Reaction of Methyl Chtoi'ide wlth Alumina Surfaces: Study of the Nlethoxy Surface Specles by TransmMon Infrared Spectroscopy Thomas P. Beebe, Jr., J. E. Crowell: and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: August 10, 1987)
We have used the transmission infrared spectroscopy technique to study the interaction of CH3C1and CD3CI with alumina surfaces containing hydroxyl groups, for the temperature range where significant physical adsorption does not occur. At temperatures near 400 K,surface hydroxyl groups are consumed and surface methoxy species are observed on the surface. This reaction of CH3Cl with hydroxyl groups leads to the production of HCl(g), which we have been able to detect mass spectrometrically. These data therefore implicate, for the first time, surface -OH groups in the surface methoxy species formation reaction. Physisorption experiments at lower temperatures on methoxylated surfaces display a reduced capacity for CD3CI physisorption, a process that has been shown to occur exclusively on -OH groups; this indicates that adsorbed -OCD3 has consumed and/or sterically blocked -OH sites (through CD3CIreaction with -OH groups) which would otherwise be available for CD,CI physisorption.
1. introduction
The methoxy surface species, -OCH3, has been successfully formed and studied on a variety of single-crystalline metallic and semiconductor surfaces1-I2 and high-area oxidic surface^.'^-^^ These studies have all employed the oxygenated species methanol as the reactant that leads to surface methoxy species formation. As a result, routes to formation of the methoxy species did not need to include interactions with the surface hydroxyl groups which are commonly found on high-area oxidic materials. Indeed, evidence for such a mechanism involving the reaction of methanol with surface hydroxyl groups has been lackingzs due to the limited spectroscopic regions investigated in many studies to date. More importantly, it has been shown that the hydroxyl group need not be present for formation of surface methoxy species from CH30H. This is known from three classes of experiment: (1) methoxy species formation on clean single-crystal metal and semiconductor surfaces,'-lZ (2) methoxy species formation on oxidic surfaces devoid of hydroxyl group^,'^^'^ and (3) the experimentally determined inverse relationship between surface hydroxyl group coverage and the capacity to form methoxy groups.I6 This last observation suggests a siteblocking relationship between surface hydroxyl and methoxy groups. The mechanism proposed in the literature to date for surface methoxy species formation from C H 3 0 H on oxidic surfaces such as alumina involves breakage of the aluminum-oxygenaluminum bridges (as shown in reaction 1) and not the direct esterification
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of surface hydroxyl groups.1517 The asterisk on the surface oxygen schematically illustiates the origin of the oxygen in the methoxy species. It is thought that these strained A1-O-A1 bridges formed in the high-temperature dehydroxylation of alumina provide the reactive sites for methoxy species formation from methan01.'~~'~ In methoxy species formation on single-crystalline metal and semiconductor surfaces, a direct substrat-xygen bond is formed, as well as a substrate-hydrogen bond to account for the hydrogen.'-'* In this work, we present spectroscopic data that clearly demonstrate the involvement of hydroxyl-groups in the irreversible methoxy species formation reaction between methyl chloride and alumina surfaces containing hydroxyl groups. This work is an extension of a recent study by usz6in which the low-temperature physical adsorption and Fermi resonance phenomena for CH3Cl and CD3CI adsorbed on AI203 were investigated. We have shown Permanent address: Department of Chemistry, B-014, University of California, San Diego, La Jolla, CA 92093. 'Author to whom correspondence should be addressed.
0022-3654/88/2092-1296$01.50/0
in the present work that, at temperatures above the physisorption regime, the formation of surface methoxy species proceeds together with the spectroscopically observed consumption of surface hy-
(1) Demuth, J. E.; Ibach, H. Chem. Phys. Lett. 1979,60,395 [Ni(lll)].
(2) Sexton, B. Surf.Sci. 1979, 88, 299 [Cu(IOO)]. (3) Ryberg, R. Chem. Phys. Letr. 1981, 83, 423 [Cu(IOO)]. (4) Sexton, B. Surf.Sci. 1981, 102, 271 [Pt( 11I)]. ( 5 ) Christmann, K.; Demuth, J. E. J . Chem. Phys. 1982, 76, 6308; J . Chem. Phys. 1982, 76, 6318 [Pd(100)]. (6) Dubois, L. H.; Nuzzo, R. G. J . Am. Chem. SOC.1983, 105, 365 [NiSi2(lll)]. (7) Miles, S.L.; Bernasek, S.L.; Gland, J. L. J. Phys. Chem. 1983, 87, 1626 [Mo(100)]. (8) McBreen, P. H.; Erley, W.; Ibach, H. Surf. Sci. 1983, 133, 437 [Fe(1lO)l. (9) Hrbek, J.; DePaola, R. A.; Hoffmann, F. M. J . Vac. Sci. Technol., A 1983, 1, 1222; J . Chem. Phys. 1984,81, 2818 [Ru(001)]. (10) Bare, S. R.; Stroscio, J. A.; Ho, W. Surf. Sci. 1985, 150, 399; Surf. Sci. 1985, 155, L281 [Ni(llO)]. (11) Strmcio,J.A.;Bare,S.R.;Ho,W.Surf.Sci. 1985,154,35 [Si(lll)]. (12) Ryberg, R. J. Chem. Phys. 1985,82, 567 [Cu(lOO)]. (13) Greenler, R. G. J . Chem. Phys. 1962, 37, 2094 (aluminum oxide). (14) Tench, A. J.; Giles, D.; Kibblewhite, J. F. J . Trans. Faraday SOC. 1967, 67, 854 (mangesium oxide). (15) Kagel, R. 0. J. Phys. Chem. 1967, 71, 844 (aluminum oxide). (16) Borello, E.; Zecchina, A.; Morterra, C. J . Phys. Chem. 1967, 71, 2938; J . Phys. Chem. 1967, 71, 2945 (silicon oxide). (17) McManus, J. C.; Matsushita, K.-I.; Low, M. J. D. Can. J . Chem. 1969, 47, 1077 (germanium oxide). (18) Morrow, B. A.; Thomson, L. W.; Wetmore, R. W. J . Caral. 1973, 28, 332 (silicon oxide). (19) Takeiawa, N.; Kobayashi, H. J . Caral. 1973,28,335 (metal oxides). (20) Morrow, B. A. J. Chem. SOC.,Faraday Trans. 1 1974, 70, 1527 (silicon oxide). (21) Morrow, B. A. J. Phys. Chem. 1977, 81, 2663 (silicon oxide). (22) Groff, R. P. J. Caral. 1984, 86, 215 (molybdenum trioxide). (23) Fukushima, T.; Arakawa, H.; Ichikawa, M. J . Phys. Chem. 1985,89, 4440 (silicon oxide and silicon oxide supported Rh-Fe). (24) Busca, G.; Rossi, P. F.; Lorenzelli, V.; Benaissa, M.; Travert, J.; Lavalley, J.-C. J . Phys. Chem. 1985, 89, 5433 (aluminum oxide). (25) Hair, M. L. Infrared Spectroscopy in Surface Chemistry; Dekker: New York, 1967; pp 155-157. (26) Crowell, J. E.; b b e , T. P., Jr.; Yates, J. T., Jr. J . Chem. Phys. 1987, 87, 3668. This work describes the physical adsorption of CH$I and CD$I onto these same alumina surfaces. All observations indicate that the CI end of the molecule forms a hydrogen bond to surface hydroxyl groups, leaving the methyl end of the molecule largely unperturbed, relative to the gas phase. The observed CHJCl (CD3C1) frequencies (in cm-') for the physically adsorbed molecule as v,(CHJ), 3044 (2292); vs(CH3),2965 (2159); 6 3 C H 3 ) , 2861 (2095); 6,(CH3), 1446 (1057); and 6,(CH3), 1352 (1022). Note, from comparison of these frequencies with those in Table I for the species produced in this work at high temperature, that the frequencies are significantly different. It is therefore unlikely that the high-temperature species is a methyl chloride species and very likely that it is a methoxy species, based on similarities in Table I.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 5, 1988
Reaction of Methyl Chloride with Alumina Surfaces I
'
2960
1297
'
W 0
z
a m c
am a
3100
3000
2900
2800
a 380 K 2700
I
Wavenumber (cm-')
Figure 1. Surface methoxy species formation in the reaction of CH,C1 with alumina showing the C-H stretching frequency range. This surface (also applies to Figures 2-4) had a mass of 3.68 X lo-* g alumina and was baked under vacuum at T = 475 K for -246 h.
I
1600
I300
I400
1500
Wavenumber (cm-') Figure 2. Surface methoxy species formation in the reaction of CH,C1 with alumina showing the C-H deformation frequency range. I " " I " "
droxyl groups and the elimination of HCl(g). Hydroxyl group consumption by reaction with CH3Cl is verified by subsequent physisorption studies on the methoxylated alumina surfaces indicating that OH sites for CH3C1 physisorption have been consumed in forming the methoxy groups.
1055
10.01 A
A
.I ~ F W H M Tscan= 280 K ,,,P, = Torr
2. Experimental Methods By use of a spraying procedure described el~ewhere,~' alumina (Degussa; aluminum oxide C, 100 m2-g-') was deposited in a thin g - ~ m - ~onto ] an inert CaF, sample layer [(1.11-1.45) X support plate. The CaF, support plate was then installed into a stainless steel IR cell previously described. The A1203was then outgassed in vacuo at temperatures of -540 or -480 K for several days prior to exposure to methyl chloride (the exact pretreatment conditions for the various samples employed in these studies are indicated in the appropriate figure captions). These treatment conditions will produce surfaces containing a variety of "free" and hydrogen-bonded hydroxyl groups. In addition, a variety of strongly hydrogen-bonded forms of water are present (although low in concentration as evidenced by relatively weak intensity for H-0-H bending vibrations in the 1600-cm-' region). The heating treatment was certainly nor harsh enough to produce the strong Lewis acid sites responsible for the rather unique chemistry of alumina, which has been heated to much higher temperatures (>900 K). The CH3C1 (Matheson; 99.5% minimum purity) and CD3Cl (MSD Isotopes; 99.5 atom % D) reagents were transferred from high-pressure cylinders to glass bulbs by using standard highvacuum procedures. Their high purity was verified by gas-phase I R spectroscopy and mass spectrometry. Infrared spectra were acquired in the transmission mode by using a Perkin-Elmer Model PE-783 infrared spectrometer and 3600 data acquisition system. The latter allowed signal averaging ranging from 3.9 to 208 s/cm-l, as well as spectral subtraction so that all spectra presented (except where otherwise noted) are difference spectra in which the alumina background has been removed.,* In addition, contributions of gas-phase methyl
-
(27) Beebe, T. P., Jr.; Gelin, P.;Yates, J. T., Jr. Surf.Sci. 1984,148, 526. (28) The subtraction of the background spectrum of alumina generally results only in the removal of a small slope from the base line. In cases where the alumina background is changing rapidly, for example, near 1000 cm-I (Figures 3 and 6). the subtraction results in the ability to see weak features on a large sloping background. Since, in addition, the throughput of radiation at 1000 cm-' is low, the resulting signal to noise in the difference spectrum, as well as in the raw spectra, is low in these figures. It is our experience in using the background subtraction procedure in the past that artifacts resulting from the subtraction are very sensitive to the multiplication factor,f, used for the background spectrum [foreground -f(background)], whereas true spectral features are insensitive to small changes in this factor. We always employ this "test" for features in spectral regions with quickly changing backgrounds, and in the case of Figures 3 and 6, we believe these to be real adsorbate spectral features. A11 background subtractions in the present work usef = 1.000 k 0.005.
475
K
380 430
K
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a I , I100
,
,
,
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,
,
,
IC
IO
Wavenumber (cm?
Figure 3. Surface methoxy species formation in the reaction of CH3C1 with alumina showing the C-0 stretching and CH, rocking frequency ranges.
chloride, when present (Figure 7a-c), have been removed by direct subtraction of the gas-phase spectrum measured through a blank section of the CaF, plate. A spectrometer slit program yielding a maximum resolution of 5.4 cm-' was employed; this resolution is indicated in the figures by the symbol "FWHM". Mass spectrometric measurements were made in our stainless steel vacuum system with the Spectrum Scientific SMIOOOD quadrupole mass spectrometer, capable of amu resolution over the 1-100 amu range. Detection was by a Faraday cup, and the gas of interest was leaked into the continuously pumped quadrupole region by a Granville-Phillips leak valve. The IR cellz7 allows spectra to be acquired in the 40001000-cm-' range, a t temperatures ranging from -80 to 600 K. Temperature control is achieved through use of a flow controller driven by a thermocouple attached to the copper ring supporting the CaF2 plate and alumina layer, through which hot or cold gases are passed. Details of these and other procedures are found else~here.~~,~~ 3. Results 3.1. IR Studies of the Reaction CH3Cl(CD3Cl)(g) OH(a) -0CH3 (-OCD3)(a) + HCl(g). Methyl chloride was reacted with alumina surfaces by the following procedure: (1) the IR cell was filled to a pressure of 10.00 Torr of CH3Cl at 280 K; (2) the surface was heated to the desired temperature, where it was held for 1.0 h; (3) the cell was allowed to cool to 280 K, the pressure was recorded (always 610.00 Torr), and the IR spectrum was
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( 2 9 ) Beebe, T. P., Jr.; Albert, M. R.; Yates,
1.
+
J. T., Jr. J . Card. 1985, 96,
1298 The Journal of Physical Chemistry, Vol. 92, No. 5, 1988
Beebe et al.
dft FWHM ,,T ,,
=280 K
4
KFWHM
Tsc,,,=280 K
1119
540
520
3800
3700
3600
3500
3300
3400
475 380 K a
3200
1
1
1
1
I100
Wavenumber (cm-')
Figure 4. Changes induced in the 0-H stretching frequency range upon surface methoxy species formation in the reaction of CH3CI with alumina. Spectrum a is a raw spectrum, whereas spectra W are difference spectra in which spectrum a has been subtracted from the raw spectrum obtained for the indicated conditions.
1
1
1
1
1
1050
1
,
*
1
1000
Wavenumber (cm-l) Figure 6. Surface methoxy species formation in the reaction of CD3Cl
with alumina showing the C-D deformation and C-O stretching frequency ranges.
I 2089
10.007 A
TsC,,=28O K w
0
z a
m
a
s:m
100
U
' 430
I
K a 380 K I
2350
2250
2150
2050
Wavenumber (cm")
Figure 5. Surface methoxy species formation in the reaction of CD3CI
with alumina showing the C-D stretching frequency range. This surface (also applies to Figures 6 and 7) had a mass of 2.80 X g alumina and was baked under vacuum at T = 540 K for -288 h. From the pressure change following reaction, the following numbers of CD3Clhave reacted: (a) -0.0 X lOI7, (b) 2.0 X lo", (c) 4.9 X lo", (d) 8.3 X lOI7, and (e) 13.6 X loT7.See section 4.6 for more details. measured; and (4) the cell was evacuated ( P
