Langmuir 1992,8, 1103-1110
1103
Enhanced Kinetic Stability of 7-C-H Bonds in Surface Alkoxides: The Reactions of tert-Butyl Alcohol on Clean and Oxygen-Covered Rh( 11 1) Xueping Xu and C. M. Friend' The Department of Chemistry, Haruard University, Cambridge, Massachusetts 02138 Received June 14, 1991. I n Final Form: January 15, 1992 The reactions of tert-butyl alcohol (t-BuOH)on clean and oxygen-coveredRh(ll1) have been investigated under ultrahigh vacuum conditions using temperature-programmed reaction and X-ray photoelectron spectroscopies. Surface oxygen inhibits nonselective C-H, C-C, and C-O bond breaking. t-BuOH forms tert-butyl oxide (t-BuO) on the oxygen-covered surfaces below 300 K. On Rh(lll)-p(BXl)-O, 8, = 0.5, t-BuO remains intact up to 370 K where it decomposes to t-BuOH and butene. There is a substantial kinetic isotope effect for t-BuOH decomposition on Rh(lll)-p(2Xl)-O, suggesting that C-H bond breaking is the rate-limiting step. t-BuO is kinetically more stable than 2-propoxide on Rh(lll)-p(2xl)-O. This enhanced stability is attributed to the fact that t-BuOH has no C-H bond at the carbon adjacent to oxygen. The C-H bond adjacent to oxygen has been shown to be more labile in alkoxide reactions on Rh(ll1)~(2x1)-0as well as on late transition metals. In contrast to the Rh(lll)-p(BXl)-O surface, facile C-0 bond cleavage produces HzO and irreversibly bound hydrocarbon products below 300 K on clean Rh(ll1). For intermediate oxygen coverages, such as Rh(lll)-p(2X2)-0 which has an oxygen coverage of 0.25, no gaseous butene is evolved. This may be due to rapid dehydrogenation of butene on Rh(lll)-(2x2)-0. The relatively weak C-0 bond combined with the enhanced kinetic stability of t-BuOH opens pathways for C-0 bond cleavage on both clean and oxygen-covered Rh(ll1).
Introduction The reactions of alcohols on metal and metal oxide surfaces are of considerable interest because they provide models for many catalytic processes such as selective oxidation of alcohols and deoxygenation. Furthermore, they probe the microscopic reverse of alcohol synthesis reactions. While simple alcohols, methanol in particular, have been extensivelystudied on transition metal surfaces, longer-chain alcohols are less studied.' The reactivity of methanol on a wide range of metal surfaces correlates with the metal-oxygen bond strengths. Carbon-oxygen bond breaking is induced by early transition metals which have large metal-oxygen bond enthalpies, such as Ti(0001),2 W(100),3and M 0 ( l l 0 ) . ~In contrast, the C-0 bond remains intact on later transition metal surfaces which have intermediate or small metaloxygen bond strengths, including Fe, Rh, Ni, Pd, Pt, Cu, and Ag. The exceptions are Mo(~OO),~ which produces CO and H2, and highly currugated Pt(100), which leads to methane production.6 On the group I (Cu and Ag) metal surfaces, the C-0 bond mainly remains intact and the presence of the intact C-0 bond has been suggested to facilitate cleavage of the C-H bond that is 8 to the metal (ato the oxygen) in primary and secondary alkoxides (I). As a result, the first dehydrogenation step is suggested to be selective cleavage of the C-H bond j3 to the metal. Aldehydes and ketones are produced from longer-chain alcohols. The more facile elimination of the 8 hydrogen is attributed to the lower C-H bond energy due to the adjacent oxygen atom. Indeed, we have found similar trends in the facility of &elimination on Rh(lll)-p(2x1)-0.7 Isotopic labeling experiments have demonstrated that selective 8-C-H bond (1) Friend, C. M.; Xu, X. Annu. Reu. Phys. Chem. 1991,42, 251. (2) Hanson, D. M.; Stockbauer, R.; Madey, T. E. J.Chem. Phys. 1982, 77, 1569. (3) KO,E. I.; Benziger, J. B.; Madix, R. J. J. Cotal. 1980, 62, 264. (4) Serafin, J. G.;Friend, C. M. J. Am. Chem. SOC.1989, 111, 8967. (5) Miles, S. L.; Bernasek, S. L.; Gland, J. L. J.Phys. Chem. 1983,87, 1626. (6) Wang, J.; Masel, R. I. J. Catal. 1990, 126, 519.
I
breaking in 2-propanol is the primary pathway for acetone evolution on Rh(ll1)-p (2X 1)-0. This study was undertaken to better understand the role of 8-C-H bond breaking in determining reactivity. tert-Butyl alcohol (t-BuOH) does not have a C-H bond a t the 8-position, 11. Accordingly, dehydrogenation processes are expected to occur a t a slower rate than for
CH?
secondary or primary alcohols and new reaction channels may be opened involving C-0 and C-C bond breaking or re-formation of the O-H bond. For comparison, phenoxide, which likewise contains no 8-C-H bonds and reacts above 350 K on Rh(lll)-p(ZXl)-O, does not undergo C0 bond breaking: CO and C02 are the only carbon(7) Xu, X.; Friend, C. M. Surf. Sci., in press.
0743-7463/92/2408-1103$03.00/00 1992 American Chemical Society
Xu and Friend
1104 Langmuir, Vol. 8, No. 4,1992 containing products. However, the C-0 bond strength in t-BuOH 91 kcal/mol? 19 kcal/mol weaker than that in phenol. We are particularly interested in the reactivity of tBuOH on oxygen-covered rhodium surfaces because it serves as a model for the selective oxidation of isobutene to t-BuOH onoxygen-covered Rh(lll).S t-BuOH is formed from the partial oxidation of isobutene on oxygen-covered R h ( l l l ) , 0.3 < do < 0.5. In our previous work, we have found that oxygen chemisorbed on Rh(ll1) inhibits C-H and c-C bond breaking, both for alkenes and 2-propanol. For example, on clean Rh(lll), 2-propanol decomposes to CO and Hz, whereas it selectively eliminates the /3 hydrogen to produce acetone on Rh(lll)-p(2X1)-0.7 Similarly, oxygen inhibits C-H and C-C bond breaking in propene and isobutene, allowing for partial oxidation to acetone and t-BuOH, respectively.SJ0 In this work, the reactions of t-BuOH are more selective on Rh(lll)-p(BXl)-O than on clean Rh(ll1).
100
200
300 400 500 Tsaperaturs [ K )
BOO
700
Figure 1. Temperature programmed reaction spectra for a
saturation exposure of t-BuOH on Rh(lll)-p(2X1)-1s(3. The inset shows data for a 5-9 dose of t-BuOH.
Experimental Section All experiments were performed in a stainless steel ultrahigh butions of the oxygen layer and the Rh(ll1) substrate have been vacuum chamber with a base pressure of -1 X 10-10 Torr, subtracted from the data shown. No detectable differences in described in detail elsewhere.lOJ1It is equipped with X-ray phothe substrate peaks were induced by oxygen adsorption. toelectron and quadrupole mass spectrometers as well as electron optics for retarding field Auger electron spectroscopy and low Results energy electron diffraction."JJl Reactionsof t-BuOHonRh(lll)-p(2Xl)-O.Butene, The Rh(lll)-p(Bxl)-O was prepared by exposing clean Rht-BuOH, water, carbon monoxide, and carbon dioxide are (111)12 to dioxygen for 2 min at 300 K. The chamber background formed during temperature programmed reaction of a pressure was 5 X Torr and the local effective pressure in the vicinity of the crystal surface was 10-7 Torr during dosing. saturation exposure of t-BuOH on Rh(lll)-p(Oxl)-O As described elsewhere, the sharp (2x2) low-energy electron (Figure 1). The minimum exposure required for detection diffraction (LEED)pattern observed for oxygen coveragesof 0.5 of the 180 K t-BuOH peak is defined as saturation monolayer is ascribed t o three domains of (2x1) peri~dicity.~~J~exposure. This is also the exposure at which the yields of A (2x2) LEED pattern is also observed for an oxygen coverage all other products are maximized. The 180 K t-BuOH of 0.25 and is assigned to the true Rh(lll)-p(2x2)-0 surface. peak is attributed to multilayer sublimation because its The absolute coverage of the (2x1)-0 overlayer has been intensity increases indefinitely with exposure. Besides calibrated previously and is 0.5 monolayer.'O Other oxygen the 180 K peak, t-BuOH is evolved in peaks centered at coverages were determined by oxygen temperature programmed 210, 230, 270, and 370 K. The relative intensity of the desorption using the (2x1) overlayer as a standard. oxygen-containing fragments, mie 59,43, and 31, are the Dioxygen and dideuterium (research purity), obtained from Matheson, were used without further purification. t-BuOH same for all peaks, including the multilayer, suggesting (99.5%,Aldrich) and t-BuOH-d, (99% d, MSD Isotopes) were that no other oxygen-containing products are formed. After stored in glass bottles. The t-BuOH samples were subjected to temperature programmed reaction of t-BuOH to 750 K, several freeze-pump-thaw cycles and used without further no carbon, but a large amount of oxygen, is detected by purification. Samples were dosed through a leak valve with a X-ray photoelectron spectroscopy. The large amount of directed doser located -0.1 in. in front of the crystal; the residual oxygen reflects the small absolute amount of exposures are expressed in units of dosing times. reaction on the Rh(lll)-p(2Xl)-O surface. Reaction products were monitored with a computer-controlled The combustion products, CO and COZ,are both evolved quadrupole mass spectrometer.14 A broad search for products in the range of 400-600 K and are primarily derived from was performed by monitoring more than 100 masses during temreaction of surface oxygen and carbon from the t-BuOH. perature programmed reaction. Following identification of the products, a few masses were monitored in separate experiments Cl80 and Cl802 are the primary combustion products to achieve better temperature resolution and a signal-to-noise formed from reaction of t-BuOH on Rh( lll)-p(2xl)-l~O ratio. The heating rate during temperature programmed reaction (Figure 1). Only minor amounts of C160180, and no was approximately constant at 10 Kis. C160l6O and (2'60, are detected (data not shown). X-rayphotoelectronspectra were acquired with a Perkin-Elmer All water peaks are derived from reaction with surface PHI-5300 ESCA system with a resolution of 0.1 eV. Data were oxygen. Water is evolved at 245, 270, 360, and 480 K accumulated simultaneously for the Rh(3d) (1 min), C(1s) (10 (Figure 1). The molecular ion of l80water (mie 20) is the min),and O(1s)(10min)regions. All binding energiesare referred primary ion observed during temperature programmed to the Fermi level of the spectrometer calibrated against the reaction of t-BuOH on an '80-labeled Rh(lll)-p(BXl)-O bulk Rh(3dsp) peak at 307.1 eV. The spectra reported here are surface (Figure 1). There is also a rising background for the difference between those for the sample and those for a freshly prepared Rh(lll)-p(2xl)-O overlayer. Therefore, the contrithe H20+ ( m l e 18)as well as contributions to its intensity by t-BuOH and H2l80 fragmentation. As a result, the (8)CRC Handbook of Chemistry a n d Physics, 70th eds.; CRC Press, possibility that a minor amount of water formation from Inc.: Boca Raton, FL, 1990. hydrogenation of the oxygen originally in t-BuOH cannot (9) Xu,X.;Friend, C. M. J . Phys. Chem. 1991, 95, 10753. be completely ruled out, but it is, a t most, a minor pathway. (10) Xu,X.; Friend, C. M. J . Am. Chem. SOC.1991, 213, 6779. (11) Baldwin, E. K.; Friend, C. M. J. Phys. Chem. 1985, 89, 2576. Selective deuteration of the tert-butyl group demon(12)Xu,X.;Friend, C. M. J. Phys. Chem. 1989, 93, 8072. strates that the water formed in the 245 and 270 K peaks (13) Reimann, C. T.; El-Maazawi, M.; Walzl, K.; Garrison, B. J.: is derived from reaction of the alcoholic proton with surface Winograd, N.; Deaven, D. M. J. Chem. Phys. 1989, 90, 2027. (14) Liu,A. C.;Friend, C. M. Rev. Sci. Instrum. 1986, 57, 1519. oxygen, whereas water is produced from reaction of the
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7-C-HBonds in Surface Alkoxides
Langmuir, Vol. 8, No. 4, 1992 1105
Table I. Mars Spectra Data for t-BuOH Reaction on
Rh(l1 l)-D(ZX1)-0 fI\
relative intensity at mle molecule 59 57 56 55 43 41 A 360 K product for 5-11 33 15 100 expoeure B 360Kproductafor30-s 50 6 30 12 16 100 exposure C: afbrsubtractingt-BuOH 0 0 34 13 2 100 contribution from B* D t-BuOH sample 100 12 17 8 30 75 E isobutylene sample 34 15 100
40 39 31
0 All data were measured in our mass spectrometer. are rescaled to mle 41.
Intensities
I
16 60 14 60 32 15 65
10 38 67 16 63
methyl hydrogens and surface oxygen a t 360 and 480 K. HzO is produced a t 245 and 270 K and DzO a t 360 and 480 K during temperature programmed reaction of (CD313COH on Rh(lll)-p(2xl)-O. D P O , m / e 22, is only observed in peaka above 300 K for (CD3)3 COH reaction on a (2X1)-180 overlayer, demonstrating that the C-D bond does not break until -360 K. Careful examination of the mass spectral data shows that butene is also formed a t 360 K with kinetics similar to t-BuOH formation (Table I). It is apparent that a second product is formed a t 360 K after accounting for the fragmentation of t-BuOH using measured fragmentation patterns and assuming that all 59 m u intensity is due to t-BuOH. Table I compares mass spectral data for products evolved a t 360 K during temperature programmed reaction for two different initial t-BuOH exposures on Rh(lll)-(2Xl)-O. The fact that the relative intensity for the fragments a t 360 K depends on the tBuOH exposure unequivocally demonstrates that there is more than one product. Mass spectral data unambiguously show that the second product a t 360 K is an isomer of butene (Table I). No t-BuOH is produced a t 360 K for a 5-5 exposure of tBuOH (-1/6 saturation) on Rh(lll)-p(2Xl)-O, so that the second product can be unambiguously identified. No m/e 59, the major fragment of t-BuOH, is detected in the peak a t 360 K for a 5-5 exposure (Figure la). A single t-BuOH peak is observed a t 270 K for this exposure. There is quantitative agreement between the fragmentation pattern of the product and that measured for isobutene in our instrument. Furthermore, the mass spectral data for higher exposures can be quantitatively accounted for by a combination of t-BuOH and butene if all 59 amu intensity is assigned to t-BuOH. Unfortunately, the specific isomer of butene could not be determined since the fragmentation patterns of all isomers are essentially identical for a 70-eV ionization voltage. The overlap in fragmentation patterns of t-BuOH and butene precluded quantitative measurements a t lower ionization voltage where isomeric determination of a pure product is possible.l5 Isobutene is the most likely product because the least amount of structural rearrangement is required for its formation. Formation of other isomers would require migration of carbon and hydrogen. The yields of t-BuOH and butene in the 360 K peak as well as CO and COz are increased by exposing the t-BuOH to the Rh(lll)-p(Bxl)-O surface a t 300 K. These yields increase as a function of exposure even greater than 10 min a t 300 K. In contrast, the product yields reach a maximum for a 30-5 exposure a t 120 K with increasing t-BuOH exposure. At 300 K, gaseous water is presumably formed from reaction of the alcoholic proton and surface (15) Wiegand, B. C.; Urdal, P. C.; Friend, C. M. J . Phys. Chem., submitted for publication.
1
-. I
I
I
I
I
I
, I /
I
-I
537 535 533 531 529 527
1
290 288 288 284 282 280
B1ndtng Energy ( e V )
Figure 2. O(ld and C(1s) X-ray photoelectron data for t-BuOH exposed to Rh(lll)-p(2Xl)-O for 10 min at a crystal temperature of 300 K.
oxygen. Furthermore, molecular t-BuOH does not remain on the surface. As a result, here are more sites available for tert-butyl oxide (t-BuO) formation. The highest t-BuO coverage obtained is -0.1 monolayer. By stoichiometry, -0.45 monolayer of atomic oxygen remains on the surface when t-BuO and gaseous water are formed a t 300 K indicating that there is a minimal change in oxygen coverage even for long exposure times. The relative yield of butene vs t-BuOH a t 360 K decreases with increasing t-BuO coverage. For example, the yield of butene relative to t-BuOH is a factor of 4 lower for a 120-5exposure than for a 10-5t-BuOH exposure, even though the absolute yield for both butene and tBuOH is increased. The total CO and COz yield also increases by a factor of -2 for the longer exposure, indicating a higher yield and the excess of surface oxygen. In addition, the peak temperatures for butene and t-BuOH evolution increase with the coverage. X-ray photoelectron data are also consistent with the formation of t-BuO from t-BuOH reaction on Rh(ll1)p(2XlI-O below 300 K (Figure 2). Two C(1s) features are observed a t 284.9 and 283.4 eV following adsorption of t-BuOH a t 300 K on Rh(lll)-p(2Xl)-O, The data are best fit with these two peaks in aratio of 3:l and with peak widths of 1.2 eV (Figure 2). The 283.4-eV peak is assigned to the three methyl carbons and the 284.9-eV peak is attributed to the carbon bound to oxygen. These assignments are generally in agreement with other studies.l6 A similar 1.5-eV binding energy difference in C(ls) peaks has been observed for 2-propoxide on M0(ll0).'~ The O(1s) photoelectron data for t-BuOH on Rh(ll1)~(2x1)-0 are also consistent with the presence of t-BuO a t 300 K (Figure 2). The contribution of the Rh(ll1)p(2Xl)-O overlayer has been subtracted from the O(ls) data to augment the peak a t 530.5 eV. The negative peak a t 529.5 eV is due to depletion of atomic oxygen in the overlayer from the formation of water via the reaction of the alcoholic proton and surface oxygen. The binding energy of 530.5 eV is generally consistent with previous studies of alkoxides on transition metal surfaces. Adsorbed alkoxides, for example phenoxide, have an O(ls) binding energy of 530.5 eV on Rh(111),12and methoxide has a binding energy of 530.7 eV on Cu(110).'8 Atomic (16) Gelius, U.;Heden, P. F.; Hedman, J.; et al. Phys. Scr. 1970,2,70. (17) Wiegand, B. C.; Uvdal, P. E.; Serafin, J. G.;Friend, C. M. J . Am. Chem. SOC.1991,113,6686. (18) Bowker, M.; Madix, R. J. Surf. Sci. 1980, 95,190.
Xu and Friend
1106 Langmuir, Vol. 8, No. 4 , 1992
m c
Y-
300
350
400
300 Temperature ' K
350
400
450
1
Figure 3. Temperature programmed reaction of (a) (CH&COH (solid lines) and (CD&COH (dotted lines) in separate experiments and (b) a 1:l mixture of (CH&COH and (CDMOH on Rh(lll)-p(ZXl)-O. t-BuOH was exposed to the surface for 5 min at a crystal temperature of 300 K for all data shown.
oxygen has a binding energy of 529.5 eV, and molecular t-BuOH adsorbed a t 120 K on Rh(lll)-p(2Xl)-O has an O(1s) binding energy of 533.0 eV. Isotope exchange experiments demonstrate that butene is formed from a unimolecular decomposition of t-BuOH onRh(lll)-p(2Xl)-O. Only butene-doand -dsare formed during temperature programmed reaction of an equimolar mixture of t-BuOH-do and -dg on Rh(lll)-p(2xl)-O (Figure 3). Isobutene products are monitored at their parent molecular ions: mle 56 and 64 for isobutene-do and -d8, respectively. No intensity for mle 63 is observed, demonstrating that butene-dT is not formed. The parent ion for butene-dl (m/e 57) has the same masslcharge ratio as a minor fragment of t-BuOH; the intensity of mle 57 is completely accounted for on the basis of its intensity relative to 59 amu, the major fragment of t-BuOH. The t-BuOH evolution a t 360 K is derived from disproportionation of t-BuO, whereas all other t-BuOH peaks correspond to molecular desorption. Temperature programmed reaction of (CD&COH produces (CD3)3COD exclusively in the 360 K peak, indicating that neighboring perdeutero-tert-butyl oxide intermediates react to form the perdeutero-tert-butylalcohol.Conversely, only (CD3)3COH is evolved in the peaks in the range of 190-270 K (Figure lb).I9 There is a substantial kinetic isotope effect when the methyl groups of t-BuOH are deuterated providing strong evidence that there is a major component of C-H bond breaking in the rate-determining step for t-BuO decomposition to butene and t-BuOH on Rh(lll)-p(Zxl)-O. The peak temperatures for the perdeuterobutene and perdeutero-tert-butyl alcohol formed from t-BuOH-dg are 12 K higher than for the corresponding perhydrido products of t-BuO-do reaction under the same conditions (Figure 3). This is comparable to the temperature difference observed for decomposition of acetone-do on clean Rh(ll1) which was modeled as a primary kinetic isotope effect.20 A similar isotope effect was observed for acetone elimination from selective dehydrogenation of 2propanol on Rh(lll)-p(2X1)-0.7 The temperature differences for butene formation are smaller when a mixture of t-BuOH-do and -dg (Figure 3).
-
(19) (CDd30H is monitored a t m / e 65, (CD&C=OH+, and (CD?)30D monitored a t m / e 66, tCD&C=OD+. Only mie 66 is observed in the peak a t 370 K. (20)Houtman, C ; Barteau, M A. J . Phqs. Chem. 1991, 95, 3755 IS
In addition, the yield of the 0-D isotopes are w1/3 that of 0-H t-BuOH, and the peak temperatures for the 0-D isotopes are -4 K higher. While not understood in detail, these results are probably due to a convolution of the effects of the coverages of hydrogen and various t-BuO species. Since the alcoholic proton leaves the surface as water prior to re-formation of t-BuOH, C-H(D) bond breaking is necessary for 0-H(D) bond formation. Hence, the rates of the two processes are intertwined. These isotope exchange experiments further rule out reuersible C-H bond activation in the t-BuO prior to decomposition. Only (CH&COH, (CH3)3COD, (CD3)3COH, and (CD3)3COD are formed during the reaction of a mixture of (CH3)3CO- and (CD&CO-. Four fragments, mle 59,60,65, and 66, corresponding to t-BuOH isotopes which have lost a methyl group, are observed a t 380 K during temperature programmed reaction of an equal mixture of t-BuOH-do and -dg on Rh(lll)-p(2Xl)-O. The mle 59 and 66 fragments are unambiguously assigned as (CH&C=OH+ and (CD3)zC=OD+, demonstrating that both 0-H and 0-D bonds are formed in the t-BuOH products. No mle 61, corresponding to (CHs)(CHtD)C=OD+, is observed, demonstrating that there is no detectable H-D exchange in the t-Bu group. The m/e 60 fragment could be assigned as either (CH3)2C=OD+ or (CH3)(CHzD)C=OH+. Since no (CH3)2(CHzD)COD is formed, the mle 60 fragment is assigned entirely as (CH3)2C=OD+. Reactions of t-BuOH on Rh(lll)-p(2X2)-180. The (2x2)-0 overlayer has an oxygen coverage of 0.25 monolayer, the only other oxygen coverage for which an ordered structure is observed. At coverages between the Rh(ll1)(2X1)-0 and-(2X2)-0 surfaces, a (1x1)diffraction pattern is produced. By investigation of qualitative features of t-BuOH reactivity on this surface, the role of the high oxygen coverage in t-BuO decomposition on the Rh(ll1)(2x1)-0 surface can be inferred. Isotopes of water, carbon monoxide, and carbon dioxide are produced during reactions of t-BuOH on Rh(ll1)p(2x2)-ls0 (Figure 4). Dihydrogen is evolved in a peak a t 375 K. t-BuOH desorbs in four peaks a t 180,220,260, and 340 K. As for Rh(lll)-(2xl)-O, the 180 K peak is attributed to the sublimation of t-BuOH multilayers because its intensity increases infinitely with exposure. On the basis of X-ray photoelectron spectra and temperature programmed reaction data, all oxygen is consumed and a very small amount of carbon remains on the surface after temperature programmed reaction to 750 K for a saturation dose of t-BuOH on Rh(lll)-p(2x2)-0. Carbon-oxygen bond cleavage in t-BuOH commences at -300KonRh(lll)-p(2x2)-0, basedon theappearance of H2160 in temperature programmed reaction spectra. Surface oxygen reacts at lower temperature to form H2180 in the range of 200-300 K. Both H:!180 and H2160 are formed at 350 K. All C-0 bonds in t-BuOH are cleaved below 400 K on Rh(lll)-p(2xl)-O. All possible isotopes of the CO and COz combustion products are evolved in the range of 400600 K: Cl60,Cl80, Cl602,Cl60l8O,and ClsOz. There are no significant differences in the kinetics of formation of the different CO and CO:! isotopes, indicating that they all probably arise from reaction of surface carbon and oxygen, which is known to recombine in this temperature range (Figure 4). The water evolved below 300 K is attributed to reaction of the alcoholic proton with surface oxygen to yield t BuO. The kinetics for the 220 K water peak is probably desorption limited, whereas the rates of other water
y C - H Bonds in Surface Alkoxides
Langmuir, Vol. 8, No. 4, 1992 1107
carbon monoxide
C"O"0
de.46
1-butanol mle.59
XlO)
100
300
400
500
Temperature ( K
600
700
300
400 500 600 Temperature ( K )
700
800
Figure 5. Temperature programmed reaction spectra for t-BuOH on clean Rh(ll1). The initial crystal temperature was 120 K.
t-butanol d e 1 5 9
LOO
ZOO
8
I
I
Figure 4. Temperature programmed reaction spectra for t-BuOH on Rh(lll)-p(2X2)-180 (&, = 0.25). The t-BuOH exposure was 1.5 saturation and the initial crystal temperature was 120 K.
features are limited by reaction, based on their peak temperatures. Reaction of (CD3)sCOH on Rh(lll)-p(2X2)0 produce H2O between 200 and 300 K and DzO above 300 K. The t-BuOH evolution a t 340 K is derived from t-BuO rehydrogenation, whereas all other t-BuOH peaks are attributed to simple t-BuOH desorption. Temperature programmed reaction of (CD3)3COH produces (CD3)&OD in the 360 K peak, whereas only (CD3)3COH is evolved in peaks at 180-260 K. Similar to Rh(ll1) and Rh(ll1)p(2X1)-0, no reversible C-H(D) activation is observed in t-BuOH. No discernible butene evolution occurs during t-BuO decomposition on Rh(lll)-p(2x2)-0. The mle 41 signal, the major fragment ion of butene, is completely accounted for by t-BuOH fragmentation based on its intensity relative to mle 59. The mle 59:41 ratio was the same within 10% for all t-BuOH peaks, including the multilayer. Furthermore, the line shapes of the mle 59 and 41 peaks are essentially identical for all four of these peaks, indicating that there is a single product evolved. There is a slight excess of mle 41 relative to mle 59 which may indicate a minor amount of butene formation. It may also be due to a slight dependence of the t-BuOH fragmentation pattern on temperature. The lack of gaseous butene production does not preclude the possibility that it is formed. It may rapidly decompose on the surface. Reactions of t-BuOH on Clean Rh(ll1). t-BuOH decomposesto water, dihydrogen, CO, and surface carbon on clean Rh(ll1) (Figure 5). In addition, t-BuOH desorption is observed. No other products were detected in a broad search of the mass range of 2-80 amu.14 In particular, butene and carbon dioxide are not formed. t-BuOH is evolved at 180 and 240 K (Figure 5). Trace amounts of t-BuOH are also detected a t 330 K. The 180 K peak is attributed to t-BuOH multilayer sublimation because its intensity increases indefinitely with the exposure. The 240 K peak corresponds to either reversible desorption of intact t-BuOH or recombination of t-BuO with surface hydrogen, since only (CD&.COH is detected during temperature programmed reaction of (CD3)3COH.
The small amount of t-BuOH evolved a t 330 K is derived from t-BuO decomposition, since only (CD3)sCOD is produced a t this temperature during temperature programmed reaction of (CD3)3COH. No reversible H-D exchange is observed in t-BuOH, since only (CH&OH, (CH&OD, (CD3)30H,and (CD3130D are observed in the 330 K peak for a mixture of (CH3)30H and (CD3)30H (-1:l) on Rh(111).21 Carbon-oxygen bond scission produces water in the range of 250-350 K (Figure 5). Water itself desorbs from Rh(ll1) below 230 K,22and coadsorption of a small amount of D20 with t-BuOH produces DzO below 250 K, indicating that the rate of water evolution is limited by reaction, not desorption. Furthermore, the temperature for water evolution sets an upper bound of 250 K for the onset of C-0 bond scission in t-BuOH on Rh(ll1). Both the alcoholic and the methyl hydrogens are combined with oxygen to produce water. Temperature programmed reaction of (CD3)3COHon Rh(111)produces H20, HDO, and D20 a t -300 K. Dihydrogen is evolved in two peaks at 350 and 450 K and in a tail extending to 700 K. The leading edge of dihydrogen overlaps with the falling edge of water, suggestingthat hydrogen reacts preferentially with oxygen and/or OH to form water. H2, HD, and D2 are produced in peaks a t 360 and 460 K from the reactions of (CD3)3COH on R h ( l l l ) , suggesting that both the alcoholic and methyl hydrogens produce dihydrogen. A small amount of carbon monoxide is also evolved at 520 K. The peak is either due to CO desorption or due to reaction of carbon and oxygen to form CO which occurs between 500 and 700 K. At low CO coverages, carbon monoxide desorbs a t -520 K, whereas at high coverages CO desorbs in peaks a t 400 and 510 K. Temperature programmed reaction of t-BuOH in the presence of a small amount of 13C0 yields W O and '3CO with similar line shapes, strongly indicating that CO desorption is ratelimiting. Carbon-oxygen bond scission leading to water is the major reaction pathway for t-BuOHonRh(ll1). The O(1s) X-ray photoelectron intensity for a saturation dose of tBuOH annealed to 400 K is only -0.2 of those annealed (21) Products are monitored at m/e 59,60,65, and 66 for (CH&OH, (CH3)30D, (CD3)30H, and (CD3)30D, respectively. No m/e 61, (CHI)(CHzD)C=OD+ or ((CH3)(CHD2)C=OHf, is detected, for example, demonstrating that t-BuOH-dz is not formed. (22) Wagner, F. T.; Moylan, T. E. Surf. Sci. 1987, 191, 121.
Xu and Friend
1108 Langmuir, Vol. 8, No. 4 , 1992
I
0
H,O
cXHY
1
0
0 4 ‘f 450 K
OH
Figure 6. Proposed reaction scheme for t-BuOH on Rh(ll1)P(2Xl)-O.
to 250 K. Most of the oxygen in t-BuOH is, therefore, desorbed as water a t the -300 K. Some carbon but no oxygen remain on the surface following temperature programmed reaction to 700 K. No 02 is formed during heating up to 1400 K and no oxygen is detected in X-ray photoelectron or Auger experiments. The carbon coverage is estimated to be -0.26 based on X-ray photoelectron data.
Discussion We propose that t-BuOH reacts on Rh(lll)-p(BXl)-O according to the scheme in accordance with all spectroscopic and isotopic labeling experiments (Figure 6). tBuO is formed via selective 0-H bond cleavage on Rh(lll)-p(Sxl)-O below 300 K in competition with molecular desorption. The alcoholic proton of t-BuOH, which is released to the surface during t-BuO formation, reacts exclusively with surface oxygen to form water which is evolved below 300 K. t-BuO remains intact on the surface up to 360 K and subsequently decomposes to isobutene and t-BuOH in competition with nonselective decomposition to water and hydrocarbon fragments. The hydrocarbon fragments are oxidized to CO and COZabove 400 K. Based on the isotopic distribution of CO and COZproduced from reaction on Rh(lll)-p(2Xl)-1s0, most of the C-0 bonds in t-BuOH are broken prior to combustion. There is no reversible C-H bond breaking during tBuO reaction on Rh(lll)-p(Pxl)-O. Since no H-D exchange products are formed during reaction of a mixture of t-BuO-do and -dg, the butene must be formed from unimolecular decomposition, and the t-BuOH from reaction of hydrogen released in C-H bond breaking in the formation of butene with unreacted t-BuO. There is a substantial component of C-H bond breaking in the rate-limiting step for t-BuO decomposition on Rh(lll)-p(2Xl)-O based on kinetic isotope effects in the rates. t-BuO-dg decomposes more slowly than t-BuO-de; a -12 K difference in the butene-do and -de peaks is measured during separate temperature programmed reaction experiments for t-BuOH-d, and -do, respectively. Although we have not explicitly modeled this isotope effect, it is similar in magnitude to that measured for acetone decomposition on clean R h ( l l l ) ,which has been attributed to a primary isotope effect primarily involving C-H
-
stretching motion in the transition state.20 The kinetics for acetone evolution from 2-propanol dehydrogenation on Rh(lll)-(Bxl)-O were similarly affected by deutera t i ~ n .It~ is also substantially larger than the kinetic isotope effect for 2-propoxide decomposition to propene and adsorbed oxygen on Mo(ll0) for which C-0 bond breaking is proposed to be rate-limiting.” The kinetic isotope effects for butene and t-BuOH formation are smaller when a mixture of t-BuO-do and -dg react (Figure 3). While not understood in detail, these results are probably due to the interrelation of C-H bond cleavage, which serves as the only source of hydrogen, and 0-H bond formation, which requires hydrogen. When t-Bu0-do are present on the surface, the more rapid rate of C-H compared to C-D bond scission increases the rate of 0-H bond formation in the t-BuO-dg. If this is true, the selectivity for formation of 0-H isomers should be greater that 0-D isomers of t-BuOH. Experimentally, the yields of the 0-D isotopes are measured to be w 1 / 3 that of 0-H t-BuOH, in accordance with our model. The smaller differences in the peak temperatures for the deuterated isotopes of butene and t-BuOH can be rationalized on a similar basis. Since more of the t-BuO-dg is rehydrogenated to t-BuOH, less is available for dedeuteration to butene-de a t the temperature where C-D bond cleavage can occur. Hence, the butene-de formed during reaction of the mixture is similar to the leading edge of the butened8 peak when t-BuO-dg is reacted separately. The yield of butene-de relative to the combined yield of t-BuOH-dlo and -dg is significantly lower for the mixture, in accordance with this proposal. The dependence of the selectivity for butene vs t-BuOH formation on t-BuO coverage can also be rationalized on the basis of the interrelation of C-H bond scission and 0-H bond formation. On the basis of the kinetic isotope effect, the rate of 0-H bond re-formation must be determined, in part, by the amount of hydrogen available. At higher coverages, 0-H bond formation is competitive with initial C-H bond breaking in t-BuO on Rh(ll1)p(2X1)-0 whereas a t lower coverages, dehydrogenation should predominate. The selectivity for t-BuOH versus butene formation from t-BuO is thus expected to depend on the coverage of t-BuO since it is the source of hydrogen. As anticipated, butene formation is favored for low t-BuO coverages. For example, for a 5-9 t-BuOH exposure, the selectivity is 100% for butene over t-BuOH formation. Since the t-BuO coverage is small, the amount of hydrogen produced from C-H bond activation is also small. Hence, the probability for recombination to yield t-BuOH is low and C-H and C-0 bond breaking predominate. As the t-BuO coverage is increased, the amount of hydrogen produced from C-H bond breaking also increases and the yield of t-BuOH increases accordingly. The kinetic stability of the t-BuO intermediate is also a function of its coverage. At lower coverage, decomposition of t-BuO commences a t lower temperatures. At lower t-BuO coverages, the oxygen coverage is slightly lower. The change in the oxygen coverage on the surface is minimal, however, given that the maximum t-BuO coverage attained is -0.1 monolayer. Even though the local oxygen coverage may be lower for higher t-BuO coverages, it is most likely that the change in kinetics and product distributions is the result of interaction between t-BuO intermediates. A decrease in oxygen coverage should decrease the stability of the t-BuO. Furthermore, self-stabilization has been observed previously for phenoxide on Rh(111).12 The decomposition of t-BuO is proposed to proceed via
7-C-HBonds in Surface Alkoxides
Langmuir, Vol. 8, No. 4, 1992 1109
K for &dehydrogenation in 2-propanol. As a result, an oxametallacycle intermediate, 3, on Rh(lll)-p(2xl)reaction pathways other than simple dehydrogenation are 0, in analogy to Cu(ll0) and Ag(110).23p24Although oxyavailable for t-BuO, in particular, C-O bond breaking. metallacycle intermediates have been proposed previously, The C-0 bond strengths are essentially the same for tthey have not been identified spectroscopically on any BuOH and 2-propanol: 92 and 91 kcal/mol, respectively.* surface. There are numerous examples in the organoTherefore, the differences in reactivity cannot be due to metallic literaure, however, lending plausibility to our variation in the facility of C-O bond breaking. Apparently, pr0posal.2+~~ C-C bond breaking either is not competitive or does not Initial C-H bond breaking in t-BuO would form the yield volatile products. oxametallacycle and release hydrogen to the surface. The strength of the C-0 bond does play a role in Subsequent C-0 bond breaking would yield isobutene determining the mechanism for t-BuOH decomposition directly. Indeed, the reaction proposed is the microscopic reverse of isobutene oxidation to ~ - B U O HHydrogenation .~ on Rh(lll)-p(2Xl)-OI however, as is evident from a comparison to phenol reactivity. Like t-BuOH, phenol of the proposed oxametallacycle would be kinetically has no 8-C-H bonds. The C-0 bond strength of phenol, favorable only when there is a large amount of hydrogen however, is 110 kcal/mol,8 considerably higher than for present on the surface. Under the conditions of these t-BuOH. The C-0 bond of phenoxide remains intact experiments, the hydrogen coverage is relatively small so during temperature programmed reaction on both clean that no C-H bond formation occurs to re-form t-BuOH Rh(l11)12 and Rh(1ll)-p(2X 1)-0?0 Phenoxide decomfrom the oxymetallacycle. The reactivity of t-BuO on Rh(lll)-p(2Xl)-O is qualposes via C-C and C-H bond breaking steps at -450 K, itatively similar to oxygen-coveredCu(1lo), where butene yielding CO on clean Rh(ll1) and CO and C02 on Rhand t-BuOH are formed a t 600K.23 On Ag(ll0) an oxygen(111)-p(2x1)-Om The strong C-0 bond in phenoxide covered Ag(llO), isobutene oxide is also a product. favors C-C bond cleavage a t the expense of C-0 bond Furthermore, all three products are formed at 510 K on breaking, in contrast to t-BuO. ~g(1101.24 Oxygen chemisorbed on Rh(ll1) inhibits C-H, C-0, and C-C bond activation so that the more selective disCarbon-hydrogen bond cleavage has been proposed to the rate-determining step for t-BuO decompositionon both proportionation reaction can occur. Although the mechCu(ll0) and Ag( l10).23924Therefore, the product distrianistic details of the t-BuO reaction on clean Rh(ll1) and butions and kinetics for t-BuO decomposition are largely Rh(lll)-(2x2)-0 are not known, it is clear that nonsedetermined by the ability of these three surfaces to activate lective dehydrogenation is inhibited by oxygen based on C-H and C-0 bonds. The lower reaction temperature for comparison with the product distributions and reaction t-BuO on Rh(ll1)-(2x1)-0 compared to either Cu(ll0) temperatures for Rh(lll)-(2Xl)-O. Due to the fact that or Ag(ll0) covered with oxygen is generally consistent C-H, C-0, and C-C bond breaking all occur in a similar with the expected trends in reactivity toward C-H bond temperature range, the mechanistic pathways cannot be activation. The relative metal-oxygen bond strengths on deduced. Rh(111)and Ag(llO), 102 and 80 kcal/mol,respe~tively,~**~~ On clean R h ( l l l ) , t-BuOH decomposes to dihydrogen, suggest that this may also play a role in determining water, carbon monoxide, and surface carbon. Carbonhydrogen bond breaking commences below 300 K comselectivitya23 The fact that t-BuO has no C-H bonds a t the @-position pared to 360 K on the Rh(lll)-(2xl)-O surface. Gaseous is clearly important in determining the product distributene is not evolved from the clean surface, although it butions and reaction kinetics. The 8-C-H bond in admay be a transient intermediate. Butene and other alkenes facilely and irreversibly decompose on clean Rhsorbed alkoxides is typically more labile than other C-H bonds. For example, selective 8-C-H bond breaking occurs (111)below 200 K. for 2-propanol on Rh(lll)-p(2Xl)-O, affording acetone. Carbon-oxygen bond breaking commences a t 250 K Elimination at the 8-position is more favorable because andis theprimaryreactionpathoncleanRh(lll),yielding its homolytic C-H bond strength is reduced due to the water and dihydrogen. In contrast, C-0 bond breaking adjacent, electronegative oxygen. Since t-BuO has no @does not commence on Rh(lll)-(%Xl)-O until -325 K. C-H bond, the only possible dehydrogenation step is methOn the Rh(lll)-(2X2)-0 surface, C-0 bond breaking yl C-H bond cleavage, 111. Therefore, dehydrogenation commences at -300 K. While the kinetics for C-0 bond breaking may be intertwined with the rates of other steps such as dehydrogenation, it is clear that oxygen inhibits C-0 bond scission. Nonselective decomposition of t-BuOH also predomiL / \ nates on the 0.25 monolayer Rh(lll)-p(2X2)-0 surface, n indicating that adense layer of oxygen is required to inhibit nonselective reaction. Isotopic labeling experiments indicate that t-BuO is formed and remains intact up to 300 K on Rh(lll)-p(2X2)-OI suggesting that the rate of C-0 111 bond cleavage is comparable for the Rh(lll)-(2X2)-0 and -(2X1)-0 surfaces. occurs at a slower rate (higher temperature): -370 K for The temperature programmed reaction data suggest that methyl C-H bond breaking in t-BuO, compared to -250 the primary difference between the two oxygen overlayere is their ability to induce C-H bond activation. Non(23) Brainard, R. L.; Madix, R. J. Surf. Sci. 1989,224, 396. (24) Brainard, R. L.; Madix, R. J. J . Am. Chem. SOC.1989, 222,3826. selective dehydrogenation commences below 300 K on the (25) Day, V. W.; Klemperer, W. G.;Lockledge, S.P.;Main, D. J.J. Am. Rh(lll)-(2X2)-0 surface, based on the appearance of H2 Chem. SOC.1990, 222, 2031. in the temperature programmed reaction spectra. Fur(26) Sharplees, K. B.; Teranishi, A. Y.; Backvall, J. E. J. Am. Chem. SOC.1977, 99, 3120. thermore, no gaseous butene is detected from t-BuO (27) Collman, J. P.; Brauman, J. I.; Meunier, B.; Raybuck, S. A.; reaction on Rh(lll)-p(2X2)-0, although it may be a Kodadek, T. R o c . Natl. Acad. Sci. U S A . 1984,82, 3245.
-
(28) Root, T. W.;Schmidt, L. D.; Fisher, G. B.Surf. Sci. 1983,234,30. (29) Bowker, M.; Barteau, M. A.; Madix, R. J. Surf. Sci. 1980,92,528.
(30) Xu, X.; Friend, C. M. Unpublished results.
1110 Langmuir, Vol. 8, No. 4, 1992 transient intermediate. If formed, butene may rapidly decompose to Con, CO, and H20. Indeed, isobutene nonselectivity reacts on Rh(lll)-(2x2)-0 affording CO, CO2, and H2O. Theabsoluteamountof reactionon theRh(llW(2Xl)0 surface is small, raising the possibility that defects in the oxygen overlayer are necessary for t-BuO formation. Although the number of t-BuOH molecules that react cannot be quantified because of the complex product distributions, the amount of molecular desorption of tBuOH relative to the amount of reaction clearly increases as the oxygen coverage increases. The possible role of defects in the reactions of t-BuOH on Rh(lll)-(PXl)-O is the subject of on-going theoretical and experimental investigations.
Conclusions t-BuOH forms t-BuO below 300 K on Rh(lll)-p(2xl)0 and Rh(lll)-p(2X2)-0. On the (2X1)-0, t-BuO reacts to produce butene and t-BuOH at -370 K. Nonselective reaction affording HzO and hydrocarbon fragments and, ultimately, combustion products, CO and CO2, competes. Reversible desorption of t-BuOH also occurs below 300 K. Kinetic isotope effects indicate that there is a substantial amount of C-H bond breaking in the rate-limiting step for t-BuO decomposition. An oxametallacycle is postulated as the intermediate which further decomposes to
X u and Friend butene. Rapid 0-H bond formation may occur in unreacted t-BuO, leading to re-formation of t-BuOH if the hydrogen coverage is sufficiently high. Accordingly, the selectivity for butene versus t-BuOH depends on t-BuO coverage, which determines the amount of hydrogen on the surface: butene formation is favored at low coverage and a mixture is formed a t high coverage. Oxygen chemisorbed on Rh(lll)-p(%Xl)-O inhibits nonselective C-H, C-C, and C-0 bond breaking. t-BuOH decomposes on clean Rh(ll1) primarily via C-0 bond breaking at -270 K to yield water and hydrocarbon fragments that further decompose to dihydrogen and surface carbon. The reactivity of t-BuOH is largely determined by the absence of an activated 8-C-H bond and by the relatively weak C-0 bond. The absence of the 8-C-H bond precludes facile dehydrogenation and elimination, such as is observed for 2-propanol on Rh(lll)-p(PXl)-O. The enhanced kinetic stability allows for other reaction paths, in particular C-0 bond scission reactions, which are allowed due to the relatively weak C-0 bond.
Acknowledgment. We are pleased to acknowledge the generous support of the National Science Foundation, Grant No. CHE-90-06024. Registry No. t-BuOH, 75-65-0;t-BuO-, 16331-65-0;DB, 778239-0.
