Langmuir 1995,11, 1184-1189
1184
Alteration of Primary Alcohol Reaction Pathways on Rh(111): Fluorination Blocks Oxametallacycle Formation Nicole F. Brown? and Mark A. Barteau" Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received August 10, 1994@ Temperature programmed desorption (TPD) and high-resolution electron energy loss spectroscopy
(HREELS)studies of 2,2,2-trifluoroethanol on Rh(ll1) were performed in order to determine if the
oxametallacycle-mediatedpathway for ethanol decarbonylation could be circumvented by replacement of H with F at the B-carbon. Reaction of 2,2,2-trifluoroethanol produced trifluoroacetyl fluoride, CO, HF, and Hz.Trifluoroacetaldehyde could also be desorbed as a product of trifluoroethanol decomposition under certain conditions; the observation of this product and of trifluoroacetyl fluoride indicates that aldehyde and acyl intermediates, rather than metallacycles, are formed in the course of this reaction. These results demonstrate that oxametallacycleformationfrom alcohols on Rh(ll1)can be blocked by complete substitution
at the ,%carbon.
Introduction Previous studies of the surface chemistry of alcohols on group VI11 metals have demonstrated a surprising dichotomy between the Pt-group metals (Ni, Pd, Pt) and rhodium. While initial 0 - H scission has been demonstrated for alcohols on (111)and other low index planes of all of these metals, the fate of the resulting alkoxide ligands is strikingly different. On the Pt-group metals, both kinetic and spectroscopic e v i d e n ~ e l -indicate ~ that these intermediates, e.g., ethoxides from ethanol, react via a series of C-H scission steps at the a-carbon, producing first an adsorbed aldehyde (acetaldehyde from ethanol) and then an adsorbed acyl (acetyl, CHsCO, in the case of ethanol decomposition). The net decarbonylation of surface acyls liberates CO plus surface hydrocarbons; these may hydrogenate, dehydrogenate, or couple depending upon the surface identity, adsorbate coverage, etc. Alcohols and aldehydes follow a common reaction pathway on these metals, and a common sequence of surface intermediates has been identified in their reactions on Pd(lll).415In contrast, primary alcoholsand aldehydes do not decarbonyIate via a common mechanism on Rh(l11).6-s Aldehydes decarbonylate via surface acyls, as for the Pt-group metals, and volatile hydrocarbons, e.g., CHI from CH3CH0, are produced in the process.6-s Decarbonylation of the corresponding alcohols, however, does not produce volatile hydrocarbons6-8 and both kinetic and spectroscopicevidence suggests that surface alkoxides formed from alcohols do not react via a C-H scission on the Rh(ll1) surface. We have previously suggested that alkoxide decarbonylation occurs by ,6 C-H scission to form surface oxametallacycle intermediates, e.g., -CHzCH20- from
* To whom correspondence should be addressed. +Present address: Merck & Co., Inc., P.O. Box 2000, R801, Rahway, NJ 07065-0900 Abstract published in Advance ACS Abstracts, April 1, 1995. (1)Gates, S. M.; Russell, J. N., Jr.; Yates, J. T., Jr. Surf. Sci. 1986, 171, 111. (2) Sexton,B. A.; Rendulic, K. D.; Hughes, A. E. Surf Sci. 1982,l'zl, 181. (3) Davis, J. L.; Barteau, M. A. Surf. Sci. 1987, 187, 387. (4) Davis, J. L.; Barteau, M. A. Surf. Sci. 1990, 235, 235. ( 5 ) Davis, J. L.; Barteau, M. A. J . Am. Chem. SOC.1989,111, 1782. (6) Houtman, C.; Barteau, M. A. J . Catal. 1991, 130, 528. (7) Brown, N. F.; Barteau, M. A. Langmuir 1992, 8, 862. (8) Brown, N. F.; Barteau, M. A. J . Am. Chem. SOC. 1992,114,4258. @
CH~CHZOH, on Rh(111).6-10This hypothesis is supported by a variety of circumstantial evidence, including the common chemistry of alcohols and epoxides (which can form oxametallacycles by ring opening) on this surface. However, spectroscopic confirmation of these intermediates has thus far proven elusive. An alternative strategy in building the case for oxametallacycles is to demonstrate that their formation (or the chemistry attributed to them) can be blocked by rational means. For example, if ,6 C-H scission is the critical step in oxametallacycle formation from surface alkoxides, primary alcohols that possess no ,6-hydrogens should be prevented from following this pathway, provided that the new substituents are less labile than hydrogen. We have investigated the use of both CH3 and F substituents to block the ,6 position in ethanol, in order to determine whether the chemistry of primary alcohols can be diverted from the postulated oxametallacycle pathway. In both cases oxametallacycle formation is blocked by complete substitution at the ,6 position, although this is more readily apparent from the volatile products observed from fluorinated alcohols. The present work reports results for the adsorption and reaction of 2,2,2-trifluoroethanol, CF~CHZOH, on the Rh(ll1) surface. These studies provide the first examples of desorption of aldehyde and acyl-derived products in the reaction of an alcohol on the Rh(ll1) surface.
Experimental Section The experimental apparatus and procedures in the present study were described previously.6-11 Surface structure and composition were verified by LEED and AES. TPD data were acquired with a UTI(100 C) quadrupole mass spectrometer, multiplexed with an IBM XT. The heating rate in all TPD experiments was 4 Ws. The HREEL spectrometer (McAllister Technical Services) was operated at a beam energy of 5 eV with a n elastic peak of ca. 2.0 x lo6 countsh with a fwhm of 70 cm-I from a clean Rh(l11) surface. The principal methods for restoring the clean surface between TPD experiments were oxygen adsorption and TPD to burn off surface carbon and high temperature (1350 K) annealing to remove deposited halogen atoms. 2,2,2-Trifluoroethanol (Johnson-Matthey 99+%) and 2,2,2trifluoroacetic acid (Johnson-Matthey, 99%) were stored in (9) Brown, N. F.; Barteau, M. A. In Selectiuity in Catalysis; Davis, M. E., Suib, S. L. Eds.;ACS Symposium Series 517;American Chemical Society: Washington, DC, 1993; p 345. (10)Brown, N. F.; Barteau, M. A. Surf. Sci. 1993,298, 6. (11)Houtman, C.; Barteau, M. A. Langmuir 1990, 6 , 1558.
0743-7463/95/2411-1184$09.00/0 0 1995 American Chemical Society
Alteration of Alcohol Reaction Pathways on Rh(ll1) 150 K I
Z.2.Z-aifluoroethanoI
1
yA
4,.
v)
x 100
mfa=ZO
l~
314K
100
H
I
I
xo l;
1
400 500 600 700 800 Temperature (K) Figure 1. TPD after an exposure of 0.45 langmuir of 2,2,2trifluoroethanol on the clean Rh(ll1) surface at 87K. 200
300
individual glass tubes attached to a stainless steel dosing line and purified by repeated freeze-pump-thaw cycles. These reagents were dosed onto the Rh(ll1) samplein ultrahigh vacuum (UHV) through a 1.5-mm stainless steel needle.
Results TF'D and HREELS studies of 2,2,2-trifluoroethanolwere performed in order to investigate the effect on oxametallacycle formationwhen the three /?-hydrogensin ethanol were completely substituted by fluorine. Activation of trifluoroethanol or its ethoxide at the /? position would be expected to be more difficult since the carbon-fluorine bond is stronger (107kcdmol) than the carbon-hydrogen bond (92 kcal/mol).12 It has been shown previously that ethanol decomposes unselectively on the clean Rh(ll1) surface, liberating CO and H2 but no volatile hydrocarbon products. 2,2,2-Trifluoroethanol, in contrast, produced trifluoroacetyl fluoride and hydrogen fluoride, as well as carbon monoxide, dihydrogen, and adsorbed carbon. Thus, TPD alone was sufficient to demonstrate that complete blocking with fluorine at the /?-position changes the characteristic alcohol decarbonylation route. TPD spectra for 2,2,2-trifluoroethanol decomposition are illustrated in Figure 1. The majority of the adsorbed 2,2,2-trifluoroethanol desorbed reversibly. The TPD peak at 150 K represents desorption of multilayers while the peak at 188 K is characteristic of monolayer desorption. These two states are consistent with those observed for alcohol desorption from group VI11 metal surfaces, as well as for 2,2,2trifluoroethanol on the clean Ag(l10)13 and Cu(lll)14 surfaces. The apportionment of the first layer of trifluoroethanol between desorption and decomposition channels in TPD appeared to be similar to that of ethanol (for which ca. 60% of chemisorbed species desorb at saturation coverages6). Dihydrogen from 2,2,24rifluoroethanol desorbed at 314 K. This temperature is slightly higher than that reported for desorption-limited evolution of Hzfrom non-halogenated The higher temperature is most likely a result of surface coverage effects as well as the delayed ~~
~
(12) Weast, R. C. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL,1989; p F213. (13)Dai, Q.;Gellman, A. J. J. Phys. Chem. 1991,95, 9443. (14)Gellman, A. J.; Dai, Q. J.Am. Chem. SOC.1993,115, 714.
Langmuir, Vol. 11, No. 4, 1995 1185
onset of reactions which resulted in the liberation of hydrogen atoms. From HREELS results presented below, it will be shown that trifluoroacetaldehyde intermediates were not formed from the dehydrogenation of alkoxide intermediates until about 300 K. Thus below 300 K, only one H atom was released from each molecule of the dissociatively adsorbed alcohol, and the increase of Tpat low coverage for second-order reactions is as expected.15 From Figure 1,it can be observed that HZdesorption was complete by ca. 350 K. This result would seem to imply that all H atoms released during the conversion of the alcohol to the aldehyde desorbed in the peak at 314 K. However, a hydrogen-containingspecies, hydrogen fluoride, was observed at higher temperatures. This observation is consistent with the conclusion that trifluoroacetaldehyde is highly stable on Rh(lll1, as discussed below. From Figure 1,it can be seen that hydrogen fluoride desorbed in a peak at 367 K. Formation of this product requires that some C-F bond scission occur in the course of trifluoroethanol decomposition to liberate fluorine atoms. These fluorine atoms subsequently react with hydrogen atoms to form HF. Some also add at the a-position of adsorbed C2 intermediates to form trifluoroacetyl fluoride. The TPD spectrum illustrated in Figure 1 shows two trifluoroacetyl fluoride peaks, at 395 K and at 500 K. The identification of trifluoroacetyl fluoride by mass spectrometry involved monitoring mlq = 69, 50, 51, 47, and 29. No peaks were observed at either 395 or 500 Kfor mlq = 51 (CFZH)or mlq = 29 (CHO). This is consistent with a molecule that contains no hydrogen atoms in either the methyl portion or the carbonyl portion. Peaks were detected for mlq = 69 (CF3),50 (CF21, and 47 (CFO). Peaks for mlq = 47 were not observed for desorption of either molecular alcohol or aldehyde species, as discussed below. The desorption of trifluoroacetyl fluoride at these two temperatures is indicative of two very different reaction processes, one of which commenced at about 350 K and the other which began at approximately 450 K. The identities of the surface intermediates and the details of their conversion are not apparent from TPD experiments with trifluoroethanol alone. These can be established however by comparison with the decomposition of trifluoroacetic acid and with experiments in which coadsorbed CO was used to block some ofthe reaction channels observed in trifluoroethanol decomposition. Studies of the decomposition behavior of trifluoroacetic acid on Rh(1111,illustrated in Figure 2, showed behavior similar to that observed for 2,2,2-trifluoroethanol on Rh(ll1) at temperatures around approximately 450 K. Decomposition of the trifluoroacetic acid produced a H2 peak at 278 K. This dihydrogen resulted from the conversion of the acid to the corresponding acetate. The trifluoroacetate decomposed to release trifluoroacetyl fluoride which appeared at 531 K, plus carbon monoxide which exhibited a desorption-limited peak at 496 K and a reaction-limited peak at 564 K. Carbon dioxide was also observed in this temperature range. The similarities between the TPD results for trifluoroethanol and trifluoroacetic acid suggest that a common trifluoroacetate species may be responsible for both above 450 K. As is evident at this point, the reaction network needed to explain the TPD results is quite complex. Scheme 1 presents a schematic sequence of 13 steps required, at minimum, to account for the observations from TPD experiments alone. In brief, this network involves sequential conversion of trifluoroethanol to the correspond(15) Redhead, P. A. Vacuum 1962,12, 203.
Brown and Barteau
1186 Langmuir, Vol. 11, No. 4, 1995
CF, CFO
xK
H,
x 100
~ q 4 7
CF3CFO
.u.-
100
200
500 600 Temperature (K)
300
400
700
800
Figure 2. TPD after an exposure of 0.21 langmuir of 2,2,2trifluoroacetic acid on the clean Rh(ll1) surface at 99 K.
Scheme 1
**,CF3CH@H(ad)
C F F H f l H (9) CF3CHflH (ad) CF3CHflH (ad)
150.250
C F W f l (ad) + H(ad)
300
2 H (ad)
-,K
CF3CO (ad) + CF3CHO (ad)
CF3CHflH (9)
",
