J. Phys. Chem. 1996, 100, 2269-2278
2269
Comparison of the Reactions of Branched Alcohols and Aldehydes on Rh(111) Nicole F. Brown† and Mark A. Barteau* Center of Catalytic Science and Technology, Department of Chemical Engineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: August 10, 1995X
Alcohols adsorbed on the Rh(111) surface have been suggested to decompose via unstable surface oxametallacycle intermediates rather than via aldehydes. The chemistry of alcohols and aldehydes containing multiple methyl groups at the β-position was examined in this study to determine whether metallacycle formation could be blocked. Temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) studies demonstrated that complete substitution of β-hydrogens with methyl groups did lead to common alcohol and aldehyde decomposition pathways. 2,2-Dimethyl-1-propanol and 2,2-dimethyl-1-propanal decarbonylated to deposit isobutene on the surface; the sequence of subsequent dehydrogenation steps was the same, whether adsorbed isobutene was generated from these oxygenates, from t-butanol, or by isobutene exposure. In contrast, partial substitution at the β-position did not produce a common path for alcohol and aldehyde decarbonylation. 2-Methyl-1-propanol decomposition resulted in fragmentation of the hydrocarbon backbone of the molecule, generating C1 and C2 fragments from the reaction of the oxametallacycle intermediate. 2-Methyl-1-propanal, however, decarbonylated cleanly to form surface propylidyne intermediates, analogous to the chemistry observed for other aldehydes on Rh(111). These results illustrate the importance of β-CH activation in producing the oxametallacycle-mediated reaction pathways characteristic of alcohols on the Rh(111) surface.
Introduction The chemistry of straight-chain alcohols and aldehydes on Rh(111) exhibits a surprising divergence. Although both types of oxygenates undergo decarbonylation, only aldehydes release volatile hydrocarbons as products of this reaction.1-3 In contrast, hydrocarbon tails of alcohols undergo complete fragmentation to carbon and hydrogen atoms on the surface in temperature-programmed desorption (TPD) experiments. This divergence of alcohol and aldehyde reaction pathways has been shown to hold for both saturated and unsaturated reagents.1-4 It suggests that alcohols do not react on this surface via the formation of the corresponding adsorbed aldehyde intermediates. We have previously proposed1-5 that on Rh(111) alcohols react instead via the formation of oxametallacycle intermediates:
These species are formed from alkoxide ligands (which have been spectroscopically identified1-2 by scission of a β-CH bond. Analogous intermediates have been proposed by Xu and Friend to explain various oxidation processes on the Rh(111) surface.6-9 However, in spite of the need for such intermediates to explain the surface chemistry of alcohols on Rh(111), they have yet to be isolated for spectroscopic examination. Other potential routes to these species, including ring opening of epoxides5 and halogen elimination from halohydrins,10 have failed to produce stable oxametallacycles on this surface. The epoxide chemistry does, * To whom correspondence should be addressed. † Present address: Merck & Co., Inc., P.O. Box 2000, R801, Rahway, NJ 07065-0900. X Abstract published in AdVance ACS Abstracts, January 1, 1996.
0022-3654/96/20100-2269$12.00/0
however, exhibit great similarity to that of the corresponding alcohols, suggesting once again that oxametallacycles are formed as transient intermediates in the course of the reactions of both classes of reagents. Since β-CH activation is the critical step in the formation of the hypothesized oxametallacycles from aliphatic alcohols, reactants in which this position is blocked might be expected to follow different reaction pathways (e.g., R-CH scission to form aldehyde intermediates). We have previously demonstrated that complete replacement of the β-hydrogens of ethanol by fluorine (e.g., CF3CH2OH) leads to the formation of trifluoroacetaldehyde (CF3CHO) and trifluoroacetyl fluoride (CF3CFO) on Rh(111),11 indicating that this “blocked” alcohol does indeed form a surface aldehyde. In the present work we consider the effect of partial and complete substitutions of the β-hydrogens with methyl groups, comparing the surface chemistry on Rh(111) of probe reagents such as 2-methyl-1-propanol ((CH3)2CHCH2OH), 2-methyl-1-propanal ((CH3)2CHCHO), 2,2dimethyl-1-propanol ((CH3)3CCH2OH), and 2,2-dimethyl-1propanal ((CH3)3CCHO). These results demonstrate that complete substitution at the β-position does lead to common alcohol and aldehyde reactions but that the partially blocked alcohol and aldehyde species still exhibit divergent pathways. Experimental Section The procedures and apparatus employed in this study were described previously.1-5 Surface composition and structure were examined by Auger electron spectroscopy (AES) and lowenergy electron diffraction (LEED). TPD data were obtained with a quadrupole mass spectrometer (UTI 100C) under the control of an IBM-XT. The heating rate was 4 K s-1 in all cases. The high-resolution electron energy loss (HREEL) spectrometer (McAllister Technical Services) was operated at a beam energy of 5 eV, producing an elastic peak of ca. 2 × 105 counts/s with a fwhm of 70 cm-1 from the clean surface. © 1996 American Chemical Society
2270 J. Phys. Chem., Vol. 100, No. 6, 1996
Figure 1. TPD after an exposure of 1.4 L of 2-methyl-1-propanol on the clean Rh(111) surface at 95 K.
The various oxygen-containing reagents (2-methyl-1-propanol, 99%; 2-methyl-1-propanal, 99%; 2,2-dimethyl-1-propanol, 99%; 2,2-dimethyl-1-propanal, 97%; tertiary butanol, 99%; all obtained from Aldrich) were stored in individual glass tubes attached to a stainless steel dosing line and purified by repeated freeze-pump-thaw cycles. Gaseous hydrocarbon reagents (propylene, 99%, and isobutene, 99%, both from Matheson) were attached to a separate manifold and used as received. All reagents were dosed onto the Rh(111) surface in ultrahigh vacuum (UHV) through 1.5 mm stainless steel dosing needles. The clean surface was restored between TPD experiments by oxygen adsorption and TPD to burn off surface carbon as CO and CO2. All TPD spectra and coverages were corrected for mass spectrometer sensitivity, calibrated separately for H2 and CO. The exposure values (in langmuirs) were not corrected for ionization gauge sensitivities. Surface coverages were calibrated against the TPD signal for an ordered (x3 × x3)R30° (1/3 monolayer) CO overlayer. Results and Discussion A. Partial Substitution at the β-Carbon. 2-Methyl-1propanol. TPD following adsorption of 1.4 langmuirs (L) of 2-methyl-1-propanol on the clean Rh(111) surface at 95 K demonstrated that the alcohol dehydrogenated completely, as shown in Figure 1. From this spectrum, it can be seen that the only volatile decomposition products were H2 and CO. No volatile hydrocarbon products were observed, consistent with the results for 1-propanol on Rh(111). Carbon monoxide desorbed in a desorption-limited peak at 470 K, while H2 desorbed over a broad temperature range (ca. 250-700 K). Maxima in the rate of H2 desorption occurred at 284, 320, 392, 507, and 613 K. The peak at 284 K represents desorptionlimited H2 evolution. The three highest temperature peaks represent the characteristic signature of the sequential decomposition of ethylidyne and acetylide species.12 The peak at 392 K is characteristic of ethylidyne decomposition to form acetylides, while the peaks above 450 K are assigned to acetylide decomposition. Other ethylidyne-forming reagents include ethylene,12 propanal,2 acrolein,3,4 and allyl alcohol,3,4 which give rise to essentially the same set of fingerprint H2 peaks on Rh(111). A comparison of the H2 TPD spectra for these reagents can be found in Figure 2. The remaining peak at 320 K in Figure 1 must therefore consist of hydrogen liberated in the formation of ethylidynes from 2-methyl-1-propanol or its
Brown and Barteau
Figure 2. Comparison of the H2 TPD spectra for propionaldehyde, acrolein, allyl alcohol, and ethylene adsorbed on the clean Rh(111) surface.
intermediate decomposition products. The area ratios of the hydrogen peaks at 284, 320, 392, and >500 K were roughly 4:3:2:1. Since the reactant contains 10 hydrogen atoms, these integers represent the number of hydrogen atoms per molecule released at each step. 2-Methyl-1-propanol most likely loses one hydrogen to form an alkoxide; the second hydrogen would then originate from the β-position to form the oxametallacycle, in keeping with the results for the other primary alcohols examined. Decarbonylation would release the remaining two R-hydrogens. The CO released in this step can be observed via high-resolution electron energy-loss spectroscopy (HREELS) at 252 K (shown below). Thus, this sequence would release four hydrogen atoms prior to the onset of H2 desorption. The hydrocarbon fragment initially formed in this scheme would be dimethylmethylene [(CH3)2C]. From a comparison of the decomposition behavior of acetaldehyde on Rh(111)1 and Pd(111),13 it has been determined that methyl, but not methylene, groups are hydrogenated to form methane on Rh(111), while methylene groups are hydrogenated to form methane on Pd(111). The observation of volatile methane on Rh(111) during acetaldehyde TPD experiments between 200 and 300 K results from a reaction-limited step since methane does not adsorb on transition metals above 150 K.14 Thus, if the dimethylmethylene ligand decomposed via initial C-C bond scission, ethylidynes and methyl groups would result and the methyl groups could then be hydrogenated to form methane. Since methane was not observed, C-H scission must precede C-C bond scission for the dimethylmethylenes. Decomposition of the dimethylmethylene fragment via C-H bond scission in one of the methyl groups followed by C-C bond scission would release adsorbed methylene and ethylidyne fragments. Methylene fragments are not hydrogenated on Rh(111) under UHV conditions but undergo complete decomposition.1,5 Thus, this sequence would account for the three hydrogen atoms in the 320 K peak. Conversion of ethylidynes to acetylides releases two hydrogen atoms at 392 K; the acetylide decomposes to surface carbon by releasing the final H atom. A reaction sequence consistent with the TPD results is summarized in Scheme 1. It will be shown below that the identification of ethylidynes and acetylides from their patterns of hydrogen release was supported by HREELS results. The total amount of H2 that desorbed from a saturated layer of 2-methyl-1-propanol was 0.43 monolayers (ML), while 0.083 ML of CO desorbed. Approximately 5 times as much H2
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J. Phys. Chem., Vol. 100, No. 6, 1996 2271
SCHEME 1
desorbed as CO; thus, the hydrogen stoichiometry of the 2-methyl-1-propanol was maintained. The desorption of 2-methyl-1-propanol occurred at two different temperatures. The peak at 168 K represents desorption from a condensed state, while the higher temperature peak at 216 K is assigned to the desorption of 2-methyl-1-propanol molecules adsorbed on the Rh(111) surface. The desorption temperatures for both of these states are similar to those observed for the corresponding states of 1-propanol on Rh(111).2 HREELS experiments were performed in order to identify the surface intermediates generated during the decomposition sequence. Figure 3 shows a HREEL spectrum obtained after exposing the Rh(111) surface to 1.31 L of 2-methyl-1-propanol at 93 K. Both adsorbed and condensed states of 2-methyl-1propanol would be expected in the 91 K spectrum at this exposure. The peaks observed in Figure 3 are assigned as follows based on comparisons of IR spectra for 2-methyl-1propanol on metallic Mn15 and of HREEL spectra for 1-propanol on Rh(111):2 CH3 rock at 2895 cm-1, δ(CH3) at 1440 cm-1, δ(COH) at 1200 cm-1, F(CH3) and ν(CCO) at 995 cm-1, and γ(CH2) at 785 cm-1. No major changes in the spectrum were observed until 228 K, after both molecular states were desorbed. The 228 K spectrum exhibits peaks at 690 and 1105 cm-1, not observed in the spectra of the adlayer at lower temperatures, along with a decrease in the 1200 cm-1 region. This spectrum represents saturation coverge of the 2-methyl-1-propoxide and shows more defined modes since the coverage is now reduced relative to that contributing to spectra at lower temperatures. By 252 K, decarbonylation had just begun, as is evident from the appearance of losses corresponding to the ν(CO) modes of linear (1965 cm-1) and bridge-bonded (1725 cm-1) molecular CO. Further heating to 273 K produced massive changes on the Rh(111) surface, as is evident from the HREEL spectrum shown in Figure 3. There were marked increases in the linear and bridging ν(CO) modes, while the modes at 695, 945, and 1445 cm-1 showed dramatic decreases in intensity with a mode at 1170 cm-1 becoming more prominent. By 304 K modes characteristic of adsorbed ethylidynes were evident. These modes include the ν(CC) mode at 1175 cm-1, the δ(CH3) mode at 1345 cm-1, and the ν(CH3) mode at 2910 cm-1. There are some differences in this spectrum when compared to other spectra characteristic of surface-bound ethylidyne species. First, the major portion of the adsorbed CO is usually bound in a bridging position when in the presence of ethylidynes.3,4,16 That is not true here since a larger portion appeared to be linearly
Figure 3. HREELS after an exposure of 1.3 L of 2-methyl-1-propanol on the Rh(111) surface at 93 K and HREELS after subsequent heating of the crystal to higher temperatures.
bonded. Next, the δ(CH3) mode has a lower intensity than is typical for ethylidynes. These observations suggest that only a small population of ethylidynes was formed. The spectrum of the mixed CO-ethylidyne adlayer persisted until 374 K, where the 1345 cm-1 mode began to decrease and a mode at 765 cm-1 began to develop. This is again consistent with the TPD results since the H2 peak resulting from ethylidyne decomposition was observed at 392 K. The spectrum of 374 K illustrates the onset of ethylidyne decomposition to form acetylides. The presence of acetylides is apparent from the weak γ(CH) mode at 765 cm-1. Further heating to 459 K resulted in desorption of some CO, while the characteristic spectrum of acetylides continued to develop. The peak at 480 cm-1 represents ν(Rh-CO) mode. The peak at 2005 cm-1 is characteristic of the ν(CO) mode of carbon monoxide. The γ(CH) mode at 805 cm-1 and the ν(CH) mode at 3000 cm-1 are due to the acetylides,2 while the weak peak at 1175 cm-1 represents the ν(CC) mode from the diminishing ethylidyne population. By 515 K only modes representative of acetylides remained, consistent with the TPD results that showed H2 desorption up to about 700 K. 2-Methyl-1-propanal and Propylene. TPD experiments after an exposure of 1.5 L of 2-methyl-1-propanal at 97 K showed that this molecule decarbonylated on Rh(111) to liberate volatile CO and H2 and adsorbed hydrocarbon species (which released hydrogen above 300 K). The results of these experiments can be found in Figure 4. In contrast, a C2 aldehyde (acetaldehyde)1 and two C3 aldehydes (propionaldehyde2 and acrolein3,4 reacted on Rh(111) to liberate volatile hydrocarbons one carbon shorter than the parent molecule. However, the yield of volatile hydrocarbons decreased with increasing number of carbon atoms in the parent molecule. This decrease in selectivity was also observed for hydrocarbons produced by oxygenate decarbonylation on Pd(111).13 On Pd(111) 1-butanol decomposed via surface-bound C3 hydrocarbon fragments,17 but no volatile C3 products were observed; shorter chain alcohols and aldehydes gave rise to desorbing hydrocarbons one unit shorter than the
2272 J. Phys. Chem., Vol. 100, No. 6, 1996
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Figure 4. TPD after an exposure of 1.5 L of 2-methyl-1-propanal on the clean Rh(111) surface at 97 K.
reactant.18 Although no evidence for propylene or propane desorption was observed in the TPD experiments of Figure 4, the participation of C3 hydrocarbon species in the decarbonylation of 2-methyl-1-propanal was inferred from the desorption behavior of H2. Propylene decomposition was also studied on Rh(111). These similarities will be discussed in more detail later through comparisons of TPD and HREEL spectra. Briefly, TPD of propylene on Rh(111) produced H2, which desorbed in peaks similar to those observed during 2-methyl-1-propanal decomposition. Also, surface-bound propylidynes (CH3CH2Ct) were observed by HREELS during propylene decomposition, as was the case for decomposition of 2-methyl-1propanal on Rh(111). Thus, these results suggested that decarbonylation of 2-methyl-1-propanal deposits propylene on the Rh(111) surface. Desorption-limited H2 resulting from the conversion of 2-methyl-1-propanal to surface C3 hydrocarbon intermediates at lower temperature was evolved at 282 K, while reaction-limited hydrogen was produced above 300 K. Decomposition of these intermediates to form ethylidynes (detected by HREELS; see below) liberated H2 at 335 K. H2, which desorbed at 395 K, resulted from ethylidyne decomposition to form acetylides; the latter species decomposed to liberate H2 in a broad spectrum between 425 and 700 K and to deposit carbon on the Rh(111) surface. The relative numbers of H atoms liberated in the peaks at 282, 335, 390, and above 425 K were 2.6, 2.2, 1.7, and 1.5, respectively, normalized to the stoichiometric value of 8 for the molecule. These values approximate the 3:2:2:1 distribution that would correspond to the sequential decomposition of 2-methyl-1-propanal through propylidynes, ethylidynes, and acetylides. A total of 0.44 ML of H2 desorbed during decomposition of this aldehyde, and 0.10 ML of desorption-limited CO was evolved in a peak at 452 K. The ratio of total H2/CO was equal to 4.4, which approximates the stoichiometric value of 4. This ratio and the absence of other products indicate that decarbonylation was the sole reaction path for this aldehyde on Rh(111). Molecular 2-methyl-1-propanal also desorbed from the Rh(111) surface. Condensed 2-methyl-1-propanal desorbed at 135 K, while monolayer coverages of the aldehyde desorbed at 171 K. 2-Methyl-1-propanal desorption peaks were also observed at 234 and 276 K. Similar aldehyde desorption peaks were observed for an exposure of 6.0 L of acetaldehyde on Rh(111). The resulting peaks at 234 and 272 K were attributed to the depolymerization of an acetaldehyde polymer.1 Thus, by the
Figure 5. HREELS after an exposure of 1.2 L of 2-methyl-1-propanal on the Rh(111) surface at 97 K and HREELS after subsequent heating of the crystal to higher temperatures.
TABLE 1: Mode Assignments for 2-Methyl-1-Propanal frequency, cm-1 modes
liquid phase Raman19
Rh(111)
ν(CH3) ν(CH) δ(CH3) δ(CHO) F(CH3) νs(CCC) F(CH3) νs(CCC) δ(CCO)
2973 2877 1467 1365 1175 1114 960 793 629
2945 2945 1440 1365 1160 1105 955 725 690
same reasoning, the peaks at 234 and 276 K are assigned to the depolymerization of 2-methyl-1-propanal polymeric chains on the surface. HREELS was used to track the sequence of stable reaction intermediates during decomposition of 2-methyl-1-propanal on Rh(111). Figure 5 illustrates HREEL spectra obtained after 1.2 L of 2-methyl-1-propanal was dosed onto the surface at 97 K. From the 97 K spectrum, it can be seen that molecular 2-methyl1-propanal was present. The vibrational frequencies observed in this spectrum were compared to those for 2-methyl-1-propanal in the liquid phase19 and are given in Table 1. Heating of the surface did not result in significant changes in the vibrational modes assigned to 2-methyl-1-propanal until 271 K. The 271 K spectrum shows modes indicative of propylidynes produced by decarbonylation of the aldehyde, along with modes for linear (2015 cm-1) and bridging (1780 cm-1) CO eliminated in this reaction. The assignment of peaks in this spectrum to adsorbed propylidynes was made by comparison to the 245 and 328 K spectra following propylene adsorption on Rh(111), shown in Figure 6. Bent et al.12 previously examined the decomposition behavior of C3 hydrocarbons, including propylene, on Rh(111). Mode assignments for propylidynes produced by propylene adsorption on Rh(111)12 and Pt(111)20 are compared to the
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J. Phys. Chem., Vol. 100, No. 6, 1996 2273
Figure 6. HREELS after an exposure of 2.1 L of propylene on the Rh(111) surface at 99 K and HREELS after subsequent heating of the crystal to higher temperatures.
vibrational frequencies for these species derived from propylene and 2-methyl-1-propanal in this work in Table 2. The propylidynes decomposed to ethylidynes upon heating to 328 K. The clearest indicators of ethylidynes are the F(CH3) mode at 985 cm-1, the ν(CC) mode at 1110 cm-1; the δ(CH3) mode at 1360 cm-1, and the ν(CH3) mode at 2940 cm-1. A key difference between the characteristic spectra of ethylidynes and propylidynes is the separation between the δ(CH3) and F(CH3) modes. The δ(CH3) mode from the ethylidynes is at 1360 cm-1, and the δ(CH3) mode from the propylidynes is at 1365 cm-1. The F(CH3) mode from the ethylidynes is at 985 cm-1, and the F(CH3) mode from the propylidynes is at 1110 cm-1. Thus, the distances between the F(CH3) and δ(CH3) modes for the ethylidynes and the propylidynes are 375 and 255 cm-1, respectively. The principal difference between the characteristic spectra of ethylidynes and propylidynes is a defined peak at 985 cm-1, representative of the F(CH3) mode, in the ethylidyne spectrum that is not in the propylidyne spectrum. Both alkylidynes force the majority of coadsorbed CO molecules into bridging positions as shown in the spectra of Figure 5. In the absence of ethylidynes the peak for the ν(CdO) mode of linear CO is typically more intense than that of bridging CO at these surface temperatures. The ethylidyne modes increased upon heating the aldehyde layer to 356 K, but by 387 K, both ethylidyne modes and that for bridge-bonded CO were significantly attenuated. At the same time a mode at 790 cm-1, characteristic of acetylide intermediates, began to emerge. The 424 K spectrum of Figure 5 corresponds to a Rh(111) surface covered with acetylides, some residual ethylidynes, and linearly bound CO. This spectrum is consistent with the TPD results that showed that CO and some reaction-limited H2 desorbed at temperatures above 424 K.
Figure 7. TPD after an exposure of 1.6 L of 2,2-dimethyl-1-propanol on the clean Rh(111) surface at 95 K.
The evolution of surface hydrocarbon ligands derived from propylene is illustrated in Figure 6. The 99 K spectrum represents adsorbed molecular propylene and is remarkably similar to those for propylene on Rh(111) presented by Bent et al.,12 for propylene on Pd(111),17 and for propylene in the gas phase.21 The assignments of these modes are as follows: 2940 cm-1 ) ν(CH3) + ν(CH2) + ν(CH); 1425 cm-1 ) δs(CH3) + CH2 scissor; 1185 cm-1 ) CH2 wag or δ(CH); 1010 cm-1 ) F(CH3); 930 cm-1 ) ν(CC) + F(CH2) + CH2 twist. The decrease in the 1425 and 930 cm-1 modes indicates that the 198 K spectrum represents a mix of both adsorbed propylene and propylidyne. By 245 K only propylidynes exist on the surface. The 328 K spectrum again illustrates carbon-carbon scission of the propylidynes to form adsorbed ethylidynes. By 354 K, the ethylidynes begin to decompose to acetylides, and by 392 K, mostly acetylides are present with some lingering ethylidynes. Thus, comparisons of Figures 5 and 6 show remarkable similarities after about 275 K since both propylene and 2-methyl-1-propanal decomposed via a common sequence of intermediates: propylidynes, ethylidynes, and acetylides. In contrast, propylidynes were not observed during the decomposition of 2-methyl-1-propanol. B. Complete Substitution at the β-Position. TemperatureProgrammed Desorption Studies. In the previous section it was shown that the presence of a lone β-hydrogen in an alcohol adsorbed on Rh(111) was sufficient to permit alcohol decomposition via a pathway circumventing aldehyde intermediates. In this section it will be shown that complete substitution of the two β-hydrogens of 1-propanol is sufficient to cause decomposition via an aldehyde intermediate. Figure 7 illustrates TPD results after an exposure of 1.6 L of 2,2-dimethyl-1propanol on Rh(111) at 95 K. Molecular 2,2-dimethyl-1propanol desorbed in two peaks. The first peak at 168 K represents the desorption of condensed 2,2-dimethyl-1-propanol,
TABLE 2: Propylidyne Mode Assignments frequency, cm-1 12
20
modes
C3H6/Rh(111) 240 K
C3H6/Pt(111) 300 K
C3H6/Rh(111) 245 K this work
C4H8O/Rh(111) 271 K this work
νa(CH3) νs(CH2) δa(CH3) + δs(CH2) δs(CH3) δw(CH2) + ν(CC) F(CH3) ν(CC) + F(CH3) + δw(CH2) ν(CC)
2985 n.r. 1445 1385 1290 1120 1055 950
2980 2920 1465 n.r. 1295 1115 1055 940
2940 2940 1450 1385 1285 1110 1030 950
2930 2930 1430 1365 1290 1110 1050 925
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Figure 8. TPD after an exposure of 1.7 L of 2,2-dimethyl-1-propanal on the clean Rh(111) surface at 97 K.
Figure 9. TPD after an exposure of 3.6 L of isobutene on the clean Rh(111) surface at 95 K.
while the peak at 235 K corresponds to desorption from the monolayer state. Again, the only volatile products were CO and H2. A total of 0.098 ML of CO desorbed at 467 K, while a total of 0.58 ML of H2 desorbed between 250 and 700 K. The experimentally determined ratio of H2/CO was found to be 5.9, in good agreement with the stoichiometric value of 6.0. This agreement supports the conclusion that no volatile hydrocarbon products were produced. H2 desorption was observed at 282, 330, and 439 K and in a broad peak between 500 and 700 K. The peak at 282 K most likely represents the H2 generated during decarbonylation since, as will be shown by HREELS, molecular CO was already present on the surface at this temperature. The area under the 282 K hydrogen peak corresponds to the removal of three H atoms per molecule reacted, suggesting that decarbonylation initially leaves the t-butyl group intact. The peak at 330 K represents the desorption of approximately five H atoms, which is in contrast to the three H atoms that desorbed at 320 K during 2-methyl1-propanol decomposition. The remaining four H atoms per molecule were removed in a series of peaks between 350 and 700 K. TPD results for a 1.7 L exposure of 2,2-dimethyl-1-propanal on Rh(111) are illustrated in Figure 8. From this figure it can be seen that molecular 2,2-dimethyl-1-propanal was observed in peaks at 141 and 291 K, which represent desorption of the aldehyde bound in multilayer and polymeric states, respectively. A peak representative of monolayer desorption of the aldehyde can be observed in the shoulder at 200 K. CO and H2 were the only volatile products produced during decomposition, and like the case of 2-methyl-1-propanal decomposition on Rh(111), no volatile hydrocarbon products were observed in TPD. A total of 0.11 ML of CO desorbed in a desorption-limited peak centered at 460 K. A total of 0.56 ML of H2 desorbed, with peaks at 295, 331, and 424 K and a broad peak between 500 and 700 K. These peaks are quite consistent with those observed during the decomposition of 2,2-dimethyl-1-propanol. The area under the peak at 295 K was found to correspond to approximately three H atoms per molecule reacted, and the peak at 331 K was also found to correspond to three H atoms. Between about 350 and 700 K, the area represented the liberation of four H atoms per molecule. As demonstrated for linear1,2 and singly branched aldehydes on Rh(111), these species react via initial R-CH scission to form acyls. Subsequent C-C bond scission releases an alkyl group, which may be hydrogenated to an alkane, (e.g., methane from acetaldehyde, ethane from propanal) or dehydrogenated as in
the case of the propylidynes from 2-methyl-1-propanal. The alkyl group eliminated by decarbonylation of 2,2-dimethyl-1propanol or 2,2-dimethyl-1,1-propanal would be a tertiary butyl group. Removal of a single hydrogen from this intermediate would produce adsorbed isobutene. This hypothesis was tested by comparison to the chemistry of isobutene on Rh(111) since, as noted above, C3+ hydrocarbon products are difficult to desorb. The sole decomposition product from isobutene TPD on Rh(111) was H2, and carbon was deposited on the metal surface. The TPD results are shown in Figure 9. Molecular isobutene desorbed at 121 K. H2 peaks were observed at the following temperatures: 282, 323, and 420 K and in a tail extending to 700 K. Again, these temperatures are consistent with those previously observed for the doubly branched alcohol and aldehyde, supporting the conclusion that these three reagents decompose via common surface hydrocarbon intermediates. Furthermore, the areas under the peaks were consistent with those noted for 2,2-dimethyl-1-propanol and 2,2-dimethyl-1propanal. The area under the peak at 282 K corresponded to approximately one H atom per isobutene molecule reacted, while the peak at 323 K represented the desorption of three H atoms, and the area between 350 and 700 K corresponded to the liberation of four H atoms. Friend et al.6 demonstrated that isobutene could be oxidized to t-butanol on Rh(111) and suggested that their observations for the decomposition of t-butanol on this surface could be explained in terms of the deoxygenation of this reactant to adsorbed isobutene. In a previous study by Friend et al.7 t-butanol on Rh(111) was observed to decompose to H2, CO, H2O, and surface carbon. Also molecular desorption of t-butanol was observed. These results were confirmed in this study, and the TPD data are presented in Figure 10. H2 desorbed in two peaks at 315 and 426 K and in a tail extending to 700 K. Water desorbed in a peak with its maximum at 301 K. This temperature corresponds to that for reaction of adsorbed H and O atoms,22 suggesting that oxygen atoms are released at lower temperatures by carbon-oxygen bond scission. Since the reaction-limited water began to desorb at approximately 250 K, carbon-oxygen bond scission must have occurred at or below this temperature to release t-butyl or isobutene intermediates. The desorption of t-butanol at 284 K was also observed, and this desorption is most likely a result of recombination of the t-butoxide with surface hydrogen. Friend7 also observed molecular desorption of t-butanol at 240 K along with trace amounts of the alcohol at 330 K. The desorption of the alcohol
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J. Phys. Chem., Vol. 100, No. 6, 1996 2275
Figure 10. TPD after an exposure of 1.3 L of t-butanol on the clean Rh(111) surface at 95 K.
Figure 12. HREELS after an exposure of 1.4 L of 2,2-dimethyl-1propanol on the Rh(111) surface at 93 K and HREELS after subsequent heating of the crystal to higher temperatures.
Figure 11. Comparison of the H2 TPD spectra for 2,2-dimethyl-1propanol, 2,2-dimethyl-1-propanal, isobutene, and t-butanol adsorbed on the clean Rh(111) surface.
at 240 K was attributed in that study to either reversible desorption of t-butanol or recombination of t-butoxide with surface hydrogen since only (CD3)3COH was detected during TPD experiments with (CD3)3COH. The small amount of t-butanol observed by Friend at 330 K was determined to result from decomposition of the alcohol since only (CD3)3COD was observed during TPD experiments using (CD3)3COH. Direct comparison of the H2 desorption spectra from the decomposition of 2,2-dimethyl-1-propanol, 2,2-dimethyl-1propanal, isobutene, and t-butanol clearly indicates the presence of a common sequence of surface hydrocarbon intermediates, as shown in Figure 11. It can be observed that all of these molecules show remarkable similarities in that they all give rise to an intense H2 peak at about 325 K. Above 350 K, however, the similarities are quite extraordinary. All exhibit a peak at about 430 K and a broad peak at about 600 K. The relative intensities of the 375, 430, and 600 K peaks are essentially the same for all four molecules. Only the desorption-limited peak at ca. 280 K, which represents the different amounts of hydrogen released from these different reactants to reach a common intermediate, exhibits significant variation. The identity of this common intermediate is explored further in the HREELS studies below. However, from integration of the peak areas under the H2 TPD spectra it was determined that, at about 350 K, the
surface-bound intermediates from all four of the reagents represented in Figure 11 have a stoichiometry corresponding to C4H4. High-Resolution Electron Energy Loss Spectra. Figure 12 shows HREEL spectra for 2,2-dimethyl-1-propanol adsorbed at 93 K and heated to progressively higher temperatures. Neither IR spectra nor HREEL spectra could be found in the literature for 2,2-dimethyl-1-propanol, but based on comparisons to 1-propanol and 2-methyl-1-propanol, the 93 K spectrum most likely represents condensed molecular 2,2-dimethyl-1-propanol. Between 93 and 249 K, the most obvious change was in the increasing definition of a peak at approximately 1190 cm-1. At 249 K modes indicative of molecular CO first appeared, signaling the onset of decarbonylation. Additional changes in the vibrational modes occurred because of decarbonylation between 249 and 273 K. At 273 K, modes indicative of bridged and linearly bonded CO were evident, as well as modes indicative of adsorbed hydrocarbon fragments. It will be shown below that the 273 K spectrum in this case bears a striking resemblance to that obtained upon heating to 273 K during the decomposition of 2,2-dimethyl-1-propanal on Rh(111). By 297 K, the bridged bonded CO in Figure 12 began to reconfigure to linearly bonded CO, and the mode at 955 cm-1 decreased. By 323 K, the 955 cm-1 mode had almost disappeared, and most of the CO was bonded linearly. Between 323 and 402 K, all of the CO shifted to the linear position, and the mode at 1400 cm-1 was attenuated. From the spectrum at 428 K, it can be observed that the mode at 835 cm-1 had begun to increase, most likely because of the formation of acetylides, which give rise to an intense peak at ca. 800 cm-1. In the spectrum at 512 K only the mode at 815 cm-1 remained, consistent with the TPD results that showed that volatile H2 continued to be released from hydrocarbon fragments up to 700 K.
2276 J. Phys. Chem., Vol. 100, No. 6, 1996
Figure 13. HREELS after an exposure of 1.6 L of 2,2-dimethyl-1propanal on the Rh(111) surface at 97 K and HREELS after subsequent heating of the crystal to higher temperatures.
Brown and Barteau
Figure 14. HREELS after an exposure of 1.4 L of isobutene on the Rh(111) surface at 95 K and HREELS after subsequent heating of the crystal to higher temperatures.
TABLE 3: Isobutene Mode Assignments frequency, cm-1
The results of temperature-programmed HREELS experiments for 2,2-dimethyl-1-propanal on Rh(111) are illustrated in Figure 13. From the 97 K spectrum it can be observed that the aldehyde bonded most likely in an η2-(C,O) position with its carbonyl group bound to the surface since the typical ν(CO) mode of η1-aldehydes at 1700 cm-1 was not observed. 2,2Dimethyl-1-propanal bound in an η2-(C,O) position would be expected to exhibit the ν(CO) frequency of η2-aldehydes at approximately 1400 cm-1. Only minor changes in the vibrational modes of the adsorbed aldehyde were observed below 273 K. At 273 K, decarbonylation began, as indicated by the appearance of modes at 1765 and 1980 cm-1 characteristic of bridged and linearly bonded molecular CO, respectively. As mentioned previously, the 273 K spectrum is quite similar to that observed at 273 K during 2,2-dimethyl-1-propanol decomposition. Both spectra showed peaks at approximately 960, 1180, 1410, 1765, 1980, and 2925 cm-1. Further changes occurred by 300 K, where the linearly bonded CO mode exhibited an increase in intensity. Again, this spectrum resembles that observed at 297 K during decomposition of the corresponding alcohol. Between 300 and 386 K the modes indicative of the adsorbed hydrocarbon fragments began to attenuate, and by 422 K, only CO and a mode at 780 cm-1, most likely representative of acetylides, remained. From comparisons of the spectra in Figures 12 and 13, it can be seen that the spectra were essentially indistinguishable once the respective adlayers had been heated to 273 K. These spectroscopic results are consistent with common intermediates as postulated from TPD experiments and thus provide additional evidence that the decarbonylation of alcohols without β-hydrogens occurs via the corresponding aldehyde. Characterization of adsorbed isobutene by HREELS provides an additional spectroscopic probe of these common intermediates. The HREELS results for adsorbed isobutene on Rh(111) are illustrated in Figure 14. The spectrum at 95 K represents adsorbed isobutene, most likely bound with the CdC bond parallel to the Rh(111) surface since the ν(CdC) mode at 1661 cm-1 does not appear. A comparison of assignments of the molecular modes for isobutene in the gas phase,23 adsorbed on Rh(111), and adsorbed on Pt(111)11 is given in Table 3. The modes in the other spectra shown in this figure are not well resolved. However, the modes in the spectrum at 272 K are quite similar to those observed in the 273 K spectra from both
modes
gas phase IR23
Rh(111) @95 K
ν(CH2) ν(CH3) ν(CdC) δa(CH3) δs(CH3) CH2 wag ν(CC) F(CH3) CH2 wag
3086, 2941 2941, 2893 1661 1470 1381
2923 2923
2930 2930
1445 1445
1282, 803a 1079, 1064 890
1175 1045 810
1470 1400 1240 1090, 800 1090, 800
a
Pt(111)20
From Raman spectroscopy.
2,2-dimethyl-1-propanol and 2,2-dimethyl-1-propanal decomposition. Studies by Avery and Sheppard20 for terminal olefins on Pt(111) also have proven useful. Their studies of propene, 1-butene, and 1-pentene provided evidence of adsorbed alkylidynes at about 300 K and thus provided a foundation for their identification of isobutylidynes from isobutene on Pt(111) at ca. 300 K. Isobutylidynes have the following molecular structure:
Thus, the spectra obtained on Rh(111) at 273 K from 2,2dimethyl-1-propanol and 2,2-dimethyl-1-propanal and at 272 K from isobutene most likely represent a combination of isobutylidynes and the initial reagent since, for the alcohol and the aldehyde, decarbonylation was not yet complete at this temperature. Comparison of the frequencies obtained from these spectra and their mode assignments is given in Table 4. Avery and Sheppard proposed that the isobutylidynes decomposed on Pt(111) to an intermediate with a stoichiometry of C4H4 at 420 K. This is consistent with results from the TPD experiments performed in this study, which showed that above 350 K all of the isobutylidyne-forming reagents decomposed via a common intermediate with a stoichiometry equivalent to C4H4. This
Branched Alcohols and Aldehydes on Rh(111)
J. Phys. Chem., Vol. 100, No. 6, 1996 2277
TABLE 4: Isobutylidyne HREELS Assignments frequency, cm-1 mode
isobutene @ 300 K on Pt(111)20
isobutene @ 272 K on Rh(111)
2,2-dimethyl-1-propanol @ 273 K on Rh(111)
2,2-dimethyl-1-propanal @ 273 K on Rh(111)
νa(CH3) νs(CH3) δa(CH3) δs(CH3) δ(CH) ν(CC), F(CH3)
2970 2880 1460 1400 1280 1080, 1010, 800
2950 2950 1430 1430 1140 1000
2920 2920 1425 1425 1185 955
2925 2925 1410 1410 1170 960
TABLE 5: Tertiary Butanol HREELS Assignments frequency, cm-1
Figure 15. HREELS after an exposure of 1.2 L of t-butanol on the Rh(111) surface at 91 K and HREELS after subsequent heating of the crystal to higher temperatures.
intermediate is proposed to be of the following form:
This species decomposed by ca. 450 K; the 452 K spectrum of the isobutene-derived layer shows only the mode at about 820 cm-1, consistent with the spectra obtained during decomposition of 2,2-dimethyl-1-propanol and 2,2-dimethyl-1-propanal. Since it appears that the isobutene decomposition pathway is important during decomposition of 2,2-dimethyl-1-propanol and 2,2dimethyl-1-propanal, HREELS experiments were performed on t-butanol since it has been shown previously that isobutene oxidation can yield t-butanol on the oxygen-covered Rh(111) surface.6 In Figure 15, HREEL spectra for adsorbed t-butanol adsorbed at 91 K and heated to progressively higher temperatures are illustrated. It can be seen that the alcohol is bonded nondissociatively at 91 K since the ν(OH) mode at 3160 cm-1 is quite evident. The modes in this spectrum were compared to those taken from infrared spectra for t-butanol in the gas phase.24 The assignments of the peaks observed in the 91 K spectrum of Figure 15 appear in Table 5. By 149 K, the only significant change in the vibrational modes was the attenuation of the mode at 745 cm-1. However, by 196 K, this mode no longer appeared, and the intensity of the ν(OH) mode also began to decrease. The mode at 1170 cm-1 also is more defined in this spectrum. Between 196 and 301 K, no noticeable changes were observed. Thus, a large portion of the t-butoxide
mode
liquid phase IR24
adsorbed on Rh(111)
ν(OH) νs(CH3) δa(CH3) νa(CCC) Fs(CH3) νs(CCC) δs(CCC)
3643 2913 1458 1205 911 750 420
3160 2895 1410 1205 910 745 460
intermediates is essentially stable up to about 300 K. This is also confirmed by the TPD results where it was observed that t-butanol desorbed at temperatures as high as 284 K. By 301 K, the onset of decarbonylation is evident from the energy loss at 1980 cm-1 for linearly bonded CO, and there are additional changes in the vibrational modes. The modes at 870 and 1420 cm-1 were attenuated and a mode at 985 cm-1 appeared. This spectrum at 301 K resembles that observed at 300 K during decomposition of the 2,2-dimethyl-1-propanal and at 297 K during decomposition of the 2,2-dimethyl-1-propanol. These similarities provide spectroscopic support for the formation of a common intermediate from these reactants by 300 K. Between 301 and 411 K these modes in the 301 K spectrum began to weaken and the mode at 810 cm-1 again began to increase. By 512 K, this acetylide mode was the most intense mode remaining. Again, this behavior has been observed for all the reagents compared in Table 4. As has been stated earlier, all of these molecules most likely decompose via isobutenes to isobutylidyne intermediates. The 2,2-dimethyl-1-propanol and the 2,2-dimethyl-1-propanal do so via t-butyl fragments, but the t-butanol most likely decomposes via an oxametallacycle since it has no R-hydrogens to cleave. This oxametallacycle derived from t-butanol would have the following form:
This oxametallacycle could then undergo C-O bond scission to release adsorbed isobutene species directly without going through a t-butyl intermediate. Friend6 has proposed in studies of the partial oxidation of isobutene to t-butanol on an oxygencovered Rh(111) surface that this same oxametallacycle is formed. Summary and Conclusions The TPD and HREELS results above demonstrate that, as long as one β-H exists in aliphatic alcohols, decomposition of the alkoxide via an oxametallacycle intermediate is the preferred mechanism on Rh(111). Decomposition of 2-methyl-1-propanol did not occur through a 2-methyl-1-propanal intermediate. 2-Methyl-1-propanal appears to have decomposed via the acyl
2278 J. Phys. Chem., Vol. 100, No. 6, 1996 and then undergone decarbonylation to release a propyl group, which then dehydrogenated to a propylidyne intermediate. It has also been shown in this work and in others that decomposition of propylene on Rh(111) occurred via propylidyne species that then decomposed through ethylidynes and acetylides to ultimately release H2 and deposit carbon on the metal surface. No propylidynes were observed during the decomposition of the 2-methyl-1-propanol, and even though some ethylidynes were observed, the vibrational intensities differed from those observed during decomposition of the aldehyde. Furthermore, when produced from the aldehyde, more bridge-bonded CO was detected in the presence of the ethylidynes. Thus, these somewhat subtle differences support the conclusion that 2-methyl-1-propanol decomposed via initial β-CH scission of the alkoxide to form an oxametallacycle, just like the linear primary alcohols previously examined. From TPD and HREELS experiments, it was shown that 2,2dimethyl-1-propanol, 2,2-dimethyl-1-propanal, t-butanol, and isobutene all gave rise to quite similar H2 desorption spectra and HREEL spectra at approximately 273 K and above. The agreement of these “fingerprints” demonstrates that, by 300 K, all of these molecules decompose along a common pathway involving the decomposition of isobutene. Thus, 2,2-dimethyl1-propanol decomposes through a 2,2-dimethyl-1-propanal intermediate to liberate isobutene intermediates either directly or via a t-butyl intermediate; isobutene is also produced during t-butanol decomposition. The isobutene then decomposes to isobutylidyne species, which decompose further to C4H4 intermediates. These in turn eventually decompose to release more H2 and deposit carbon on the surface. These reaction studies of alcohols in which all the β-hydrogens were completely substituted by less labile groups (CH3) or atoms (F)11 show that these alcohols can decompose via initial R-CH scission of the alkoxides on Rh(111). The absence of β-hydrogens allows the alcohols to cleave an R-CH bond. These observations imply that β-H bonds of primary alcohols are more reactive than the R-hydrogens on Rh(111), favoring formation of oxametallacycles. The Rh(111) surface appears to attack the β-CH bond in alcohols preferentially, in contrast to the behavior of Pt-group metal surfaces. These results provide further evidence that β-CH scission is responsible for the formation of oxametallacycles from alcohols on Rh(111). Ethanol and 1-propanol decompose via oxamet-
Brown and Barteau allacycles. When 1-propanol is completely substituted at the β-position by methyl groups to form 2,2-dimethyl-1-propanol, oxametallacycle formation is blocked. However, when the β-position is only partially blocked by methyl groups as in 2-methyl-1-propanol, the decomposition via an oxametallacycle pathway is again favored. These results imply that it is the ability of the Rh(111) surface to react with the β-hydrogens of alkoxides that produces divergent alcohol and aldehyde decomposition pathways. Acknowledgment. We are grateful for the financial support of the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences (Grant FG0284ER13290). References and Notes (1) Houtman, C. J.; Barteau, M. A. J. Catal. 1991, 130, 528. (2) Brown, N. F.; Barteau, M. A. Langmuir 1992, 8, 862. (3) Brown, N. F.; Barteau, M. A. In SelectiVity in Catalysis; Davis, M. E., Suib, S. L., Eds.; ACS Symposium Series 517; American Chemical Society: Washington DC, 1993; p 345. (4) Brown, N. F.; Barteau, M. A. J. Am. Chem. Soc. 1992, 114, 4258. (5) Brown, N. F.; Barteau, M. A. Surf. Sci. 1993, 298, 6. (6) Xu, X.; Friend, C. M. J. Phys. Chem. 1991, 95, 10753. (7) Xu, X.; Friend, C. M. Langmuir 1992, 8, 1103. (8) Xu, X.; Friend, C. M. J. Am. Chem. Soc. 1991, 113, 6779. (9) Xu, X.; Friend, C. M. Surf. Sci. 1992, 260, 14. (10) Brown, N. F.; Barteau, M. A. J. Phys. Chem. 1994, 98, 12737. (11) Brown, N. F.; Barteau, M. A. Langmuir 1995, 11, 1184. (12) Bent, B. E.; Mate, C. M.; Crowell, J. E.; Koel, B. E.; Somorjai, G. A. J. Phys. Chem. 1987, 91, 1493. (13) Davis, J. L.; Barteau, M. A. J. Am. Chem. Soc. 1989, 111, 1782. (14) Ceyer, S. T. Annu. ReV. Phys. Chem. 1988, 39, 479. (15) Blyholder, G.; Allen, M. C. J. Catal. 1970, 16, 189. (16) Blackman, G. S.; Kao, C. T.; Bent, B. E.; Mate, C. M.; Van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1988, 207, 66. (17) Davis, J. L. Ph.D. Dissertation, University of Delaware, Newark, DE, 1988. (18) Davis, J. L.; Barteau, M. A. Surf. Sci. 1987, 187, 387. (19) Lucazeau, G.; Novak, A. J. Chim. Phys. Phys.-Chim. Biol. 1971, 68, 252. (20) Avery, N. R.; Sheppard, N. Proc. R. Soc. London 1986, A405, 1. (21) Barnes, A. J.; Howells, J. D. R. J. Chem. Soc., Faraday Trans. 1973, 69, 532. (22) Thiel, P. A.; Yates, J. T., Jr.; Weinberg, W. H. Surf. Sci. 1979, 90, 121. (23) Pathak, C. M.; Fletcher, W. H. J. Mol. Spectrosc. 1969, 31, 32. (24) Korppi-Tommola, J. Spectrochim. Acta 1978, 34A, 1077.
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