Pd (111) Surface Alloy with

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J. Phys. Chem. C 2008, 112, 3362-3372

Enantioselective Reactions on a Au/Pd(111) Surface Alloy with Coadsorbed Chiral 2-Butanol and Propylene Oxide Feng Gao, Yilin Wang, Zhenjun Li, Octavio Furlong, and W. T. Tysoe* Department of Chemistry and Biochemistry and Laboratory for Surface Studies, UniVersity of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53211 ReceiVed: October 24, 2007; In Final Form: December 5, 2007

The coadsorption of R- and S-propylene oxide with R-2-butanol was studied on a dilute Au/Pd(111) alloy containing ∼8% gold using temperature-programmed desorption and reflection absorption infrared spectroscopy. Alloy formation strongly affects the surface chemistry of these molecules as compared to clean Pd(111). A portion of the propylene oxide decomposes to form CO and ethylidyne species on the alloy, while on clean Pd(111), it adsorbs reversibly. 2-Butanol reacts to form a 2-butoxide species on the alloy that either rehydrogenates to form 2-butanol or undergoes a β-hydride elimination reaction to yield 2-butanone. Coadsorption of R-2-butanol with either R- or S-propylene oxide led to a larger 2-butanone yield in the presence of S-propylene oxide with an enantiomeric excess (ee) of ∼14%. This led to an enantioselective depletion of the 2-butoxide species such that the subsequent 2-butanol formation is highly enantioselective, with 100% ee.

1. Introduction

2. Experimental Procedures

It has been demonstrated previously that enantioselective chemisorption occurs on a Pd(111) surface in ultrahigh vacuum (UHV) when it is chirally modified by R- or S-2-butanol, where enhanced chemisorption of propylene oxide (PO) of the same chirality as the modifier is found over a narrow 2-butanol coverage range,1,2 and the effect is ascribed to hydrogen-bonding interactions between the O-H group of 2-butanol with propylene oxide.3 More recently, similar behavior was found on Pt(111).4 Since PO adsorbs and desorbs molecularly on Pd(111), this provided an ideal system for investigating the enantioselective chemisorption of R- and S-PO from the relative coverages and desorption temperature differences of R- and S-PO on chiral 2-butanol templated surfaces. Also, propylene oxide desorbs prior to extensive 2-butanol decomposition so that the coadsorption of PO does not significantly influence the 2-butanol chemistry. Thus, while this system enables chemisorptive enantioselectivity to be probed, it does not allow enantioselective reactions to be explored. Recently, the chemistry of small organic molecules has been studied on various Au/Pd(111) alloy films, where it was found that a small amount of gold in the alloy could significantly modify the surface chemistry.5,6 As will be shown below, the formation of such an alloy also affects the surface chemistry of both propylene oxide and 2-butanol as compared to clean Pd(111). Thus, while propylene oxide desorbs molecularly from clean Pd(111) without undergoing any reaction, on the Au/Pd(111) alloy, a significant decomposition is found. In addition, the 2-butanone yield from 2-butanol decomposition is also significantly enhanced on the alloy as compared to a clean Pd(111) surface. The much richer surface chemistry therefore allows us to investigate as to how reactivity is influenced by the presence of a chiral modifier.

The Pd(111) substrate (1 cm diameter, 0.5 mm thick) was cleaned using a standard procedure, which consisted of cycles of argon ion bombardment (2 kV, ∼1 µA/cm2) and annealing in 4 × 10-8 Torr of O2 at 1000 K as described previously.1 Gold was evaporated from a small alumina tube furnace as described elsewhere.7 Following deposition of ∼5 monolayers (ML) of gold, the sample was annealed gradually to 1100 K several times to allow the gold to diffuse into the palladium bulk to form a dilute Au/Pd(111) alloy where the near-surface composition was monitored using Auger spectroscopy. The structure of the outermost surface was determined by low-energy ion scattering (LEIS) experiments carried out in another UHV chamber as described elsewhere.8 Reflection adsorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) data were collected as described in detail elsewhere.1-3 Chiral propylene oxide and 2-butanol (Aldrich, research grade) were purified by several freeze-pump-thaw cycles before use. The purity was checked mass spectroscopically.

* Author to whom correspondence should be addressed. Tel.: (414) 2295222; fax: (414) 229-5036; e-mail: [email protected].

3. Results 3.1. Au/Pd(111) Alloy Formation. The dilute Au/Pd(111) alloy surface was generated by depositing ∼5 ML of gold onto a Pd(111) single-crystal sample held at 300 K and heated slowly to 1100 K to synthesize a dilute alloy. Following this process, the sample was heated in 4 × 10-8 Torr of O2 for 5 min at 800 K to remove any impurities that might have adsorbed on the sample during gold deposition, following which the sample was annealed to 1100 K to remove any residual oxygen. As has been shown previously, gold preferentially segregates to the surface in Pd alloys so that the gold concentration in the outmost layer is substantially larger than in the bulk.9,10 LEIS experiments of the alloy surface reveal that the gold concentration is 8 ( 2%.8 It has also been demonstrated that the gold and palladium are not randomly distributed on the alloy surface and that there is

10.1021/jp710285x CCC: $40.75 © 2008 American Chemical Society Published on Web 02/12/2008

Enantioselective Reactions on Au/Pd(111) Alloy

J. Phys. Chem. C, Vol. 112, No. 9, 2008 3363

Figure 1. Depiction of the gold palladium alloy surface used in this work where the gold and palladium distribution was calculated using Monte Carlo methods.

a small repulsive interaction between gold and palladium.11 The gold and palladium atom distribution in the alloy under the influence of this repulsive interaction was calculated using Monte Carlo methods as described elsewhere,8 and the resulting depiction of gold distribution on the surface of the alloy is shown in Figure 1. 3.2. Propylene Oxide Adsorption on the Au/Pd(111) Alloy. Figure 2 displays the reflection absorption infrared spectra (RAIRS) of propylene oxide adsorbed on the alloy surface, where Figure 2a shows the infrared spectra as a function of R-PO exposure at 80 K where peaks are found at 819, 942, 1032, 1263, 1408, and 1445 cm-1 following a 1.6 L (1 L ) 1 × 10-6 Torr s) exposure. At higher exposures, clear frequency shifts are evident in some of these features, and new peaks appear at 1252 and 1368 cm-1. The features detected following a 1.6 L exposure are assigned to the presence of a monolayer of propylene oxide and the peaks at higher exposures to the formation of a multilayer. Detailed assignments of the infrared modes are listed in Table 1. Figure 2b presents the spectra obtained following multilayer adsorption (a 9.6 L propylene oxide exposure) at 80 K and subsequently heating to higher temperatures, where the annealing temperatures are displayed adjacent to the corresponding spectrum. Most noticeably, the ring deformation mode (δoop(ring) ) ∼825 to 815 cm-1) shifts continually to lower frequencies during annealing, indicating the desorption of propylene oxide. The intensity of this feature remains constant and equal in intensity to the corresponding peak in Figure 2a due to the monolayer on heating to ∼190 K, indicating that the majority of the molecular propylene oxide is stable at this temperature, although a small amount of CO is detected at ∼1770 cm-1, suggesting that some decomposition has occurred. The δoop(ring) mode becomes very weak on heating to 245 K, indicating that the majority of adsorbed PO has either desorbed or reacted. This result is in accord with the detection of propylene oxide at this temperature in temperature-programmed desorption (TPD) (Figure 3). PO decomposition is also indicated by the features appearing in the CO stretching region (∼1800 cm-1) due to CO adsorbed exclusively in three-fold hollow sites.12 For annealing temperatures of 300 and 360 K, a weak but clear feature appears at 1316 cm-1, indicating the formation of ethylidyne.13 This suggests the presence of an intermediate on the surface between ∼245 and 300 K that finally thermally decomposes to form carbon monoxide and ethylidyne. A number of weak additional features may be present on the surface, but these are not clearly

Figure 2. Reflection absorption infrared spectra of (a) R-propylene oxide adsorbed on the Au/Pd(111) alloy at 80 K as a function of propylene oxide exposure, where the exposures are marked adjacent to the corresponding spectrum and (b) the effect of annealing a Au/ Pd(111) alloy exposed to 9.6 L propylene oxide at 80 K to various temperatures, where the annealing temperatures are displayed adjacent to the corresponding spectrum.

distinguishable from the background. This behavior is in stark contrast to that found for clean Pd(111), where propylene oxide

3364 J. Phys. Chem. C, Vol. 112, No. 9, 2008 TABLE 1: Vibrational Assignments of Frequencies of Propylene Oxide Adsorbed on a Au/Pd(111) Alloy assignmenta

monolayer

multilayer

δa(CH3) γ(CH2) δs(CH3) νa(COC) ω(CH3) F(CH3) δoop(ring)

1445 1408

1449 1411 1368 1265, 1252 1029 949 832

1263 1032 942 819

a δ: Deformation; γ: scissoring; ν: stretching; ω: wagging; F: rocking; oop: out-of-plane; s: symmetric; and a: asymmetric.

does not decompose,1,2 so that the presence of a small amount of gold in the surface of the alloy profoundly affects the surface chemistry. At all propylene oxide exposures, the H2 (2 amu) desorption profiles display three desorption states centered at ∼370, 430, and 520 K (Figure 3). These features are due to reaction ratelimited processes since they appear at higher temperatures than the desorption temperature of hydrogen from clean Pd(111).14 The 28 amu profiles (Figure 3) contain mass spectrometer ionizer fragments of propylene oxide below ∼400 K, and the peak at ∼500 K is due to carbon monoxide desorption, consistent with the detection of carbon monoxide by infrared spectroscopy (Figure 2). 3.3. 2-Butanol Adsorption on Au/Pd(111). Figure 4a displays the RAIR spectra collected as a function of R-2-butanol exposure at 80 K where exposures are marked adjacent to the corresponding spectra, and detailed spectral assignments are given in Table 2. Note especially the broad ν(OH) mode centered at ∼3280 cm-1 due to hydrogen-bonded OH species.4 This provides clear evidence for multilayer formation at 2-butanol exposures of ∼1 L and greater. Also note that for multilayer 2-butanol, the ν(CO) mode is located at 1107 cm-1, while in the monolayer, this mode shifts to 1089 cm-1, suggesting that the adsorption of 2-butanol takes place via the hydroxyl group. This conclusion is in accord with structural measurements for methanol on clean Pd(111) using low-energy electron diffraction (LEED) experiments where methanol was found to adsorb via the oxygen of the O-H group with the C-O bond slightly tilted with respect to the surface normal and the O-H bond close to parallel to the surface.15 In addition, in the monolayer region, the in-plane OH deformation mode (δip(OH) at ∼1160 cm-1 in the multilayer) is undetectable. Certainly, this mode would disappear when 2-butoxide is formed but would also be infrared forbidden if the O-H bond were oriented close to parallel to the surface. The feature detected at ∼1250 cm-1 in the multilayer region cannot be assigned to 2-butanol and is likely due to impurities. Figure 4b plots the RAIR spectra after annealing the multilayer film to higher temperatures where the annealing temperatures are marked adjacent to the corresponding spectrum. Note first the disappearance of a ν(OH) mode (at ∼3280 cm-1) upon heating to 170 K that suggests that the multilayer desorbs below this temperature, consistent with the desorption spectra (Figure 5a). It is difficult to unequivocally distinguish between 2-butoxide and 2-butanol on the surface using RAIRS, partially due to the fact that monolayer 2-butanol and 2-butoxide features overlap and partially due to the low signal intensities after annealing to 170 K and higher. In spite of this, some clues can still be obtained from the F(CH2) mode. At 190 K, this mode appears at 899 cm-1, while heating to 210 K causes this mode to shift to 887 cm-1. This rather large shift suggests that 2-butoxide is formed at slightly above 190 K.4

Gao et al. Figure 5a displays the TPD results following 2-butanol adsorption monitoring signals at 74 (2-butanol), 2 (H2), and 28 (CO) amu as a function of 2-butanol exposure. At exposures below 0.6 L, no 2-butanol desorption is found, suggesting complete decomposition of adsorbed 2-butanol, consistent with the detection of CO and hydrogen at these exposures. Note, however, that the integrated intensities of the CO and hydrogen desorption profiles remain reasonably constant at 2-butanol exposures greater than ∼0.6 L, indicating that 2-butanol completely decomposes at exposures of 0.4 L and lower but that any additional 2-butanol does not decompose. 2-Butanol desorption from the monolayer is observed at 255-265 K depending on exposure. This occurs at temperatures that are substantially higher than that at which 2-butoxide species are formed (∼190 K, Figure 4b), indicating that 2-butanol is formed by the hydrogenation of 2-butoxide species, although intact 2-butanol desorption cannot be completely excluded. Figure 5b shows plots of desorption at a number of masses following a 1.0 L 2-butanol exposure. Fragments at 31, 41, 44, 45, 55, and 59 amu display only one desorption peak at ∼250 K due to 2-butanol as determined by comparing the integrated intensities at these masses with the cracking pattern of pure 2-butanol measured using the same mass spectrometer. A desorption state is found at ∼213 K with fragments at 24-27, 29, 30, 42, 43, 57, and 72 amu assigned to 2-butanone desorption by comparing the relative signal intensities at various masses with the mass spectrum of 2-butanone.16 Previous studies have shown that this molecule is formed by β-hydride elimination of surface 2-butoxide species.1,2 This confirms that 2-butoxide species are indeed present on the surface at this temperature and corroborates the conclusion regarding the 2-butoxide formation temperature from RAIRS stated previously. 3.4. Propylene Oxide Adsorption on R-2-Butanol Modified Au/Pd(111) Surfaces. The adsorption of PO onto R-2-butanol templated surfaces was explored to investigate enantioselective adsorption/reaction. In these experiments, various coverages of R-2-butanol were first deposited onto the alloy surface at 150 K, following which the sample was cooled to 130 K and exposed to 3 L PO. These conditions were selected to avoid significant multilayer growth, and the PO exposure was just sufficient to saturate the (partially covered) surfaces.3 Figure 6a shows plots of the R-PO (58 amu) desorption profiles as a function of R-2butanol exposure, and Figure 6b shows the corresponding desorption of S-PO. In both cases, the propylene oxide yield decreases with increasing R-2-butanol exposure due to blocking of the surface by 2-butanol. There are differences in the propylene oxide yields as illustrated by the data in Figure 7, which plots the integrated R- (b) and S-propylene oxide (9) desorption yields as a function of 2-butanol exposure. There is a slight preference for S-propylene oxide desorption, although since propylene oxide thermally decomposes on the alloy (Figure 3), the desorption yield is not necessarily an accurate gauge of the propylene oxide coverage. To more directly measure adsorbate coverages, Figure 8 shows the RAIRS of a Au/Pd(111) alloy surface predosed with 0.3 L R-2-butanol at 150 K, followed by either 2.2 or 4.2 L Ror S-PO at 80 K. The peaks marked by asterisks are due to propylene oxide, while the unmarked features are due to 2-butanol. At both PO exposures, the PO coverage can be gauged by integrating the area of the δoop(ring) mode peak (∼830 cm-1). Note that this strategy has been used previously to monitor the amount of PO that adsorbs on the Pd(111)1,2 and Pt(111)4 surfaces. It was found that at both exposures, the integrated intensities of the R-PO δoop(ring) mode are within



Figure 3. Temperature-programmed desorption spectra following various exposures of R-propylene oxide to the Au/Pd(111) alloy using a heating rate of 6.5 K/s collected at 58 (propylene oxide), 28 (CO and propylene oxide), and 2 (hydrogen) amu, where the propylene oxide exposures are marked adjacent to the corresponding desorption profile.

(10% of those for S-PO. RAIRS data using a surface precovered with 0.9 L R-2-butanol also yielded similar results (data not shown). These data suggest that propylene oxide does not chemisorb strongly enantioselectively on the Au/Pd(111) alloy within the precision of infrared spectroscopy, while on clean Pd(111), enantioselective chemisorption was found.1-3 This implies that the slight preference for the desorption of S-propylene oxide noted in Figure 7 may be due not to enantioselective chemisorption but to enantiospecific decomposition on the surface. To explore this issue, TPD experiments were also performed to monitor reaction products. Figure 9 displays the 2 amu desorption H2 profiles using the experimental protocol used to obtain the data shown in Figure 6 (3 L propylene oxide). Figure 9a shows plots of the H2 desorption profile for an R-PO/R-2-butanol surface, while Figure 9b shows plots of the H2 profile for an S-PO/R-2-butanol sample. The features can be identified by comparison with the hydrogen desorption profiles of each of the components (Figures 3 and 5). Note first that the total hydrogen desorption yield from propylene oxide alone (Figure 3) is considerably lower than that from 2-butanol (Figure 5a). Thus, the 450 K desorption features in Figure 9(a,b) are clearly identified as being due to 2-butanol decomposition. The additional larger and broader feature centered at ∼380 K contains contributions from both 2-butanol (see Figure 5a) and propylene oxide (see Figure 3) decomposition, where this produces a shoulder at ∼400 K on the ∼380 K profile. There are several points to note for these desorption profiles. First, the 400 K shoulder due to R-PO decomposition is relatively well-resolved (see Figure 9a), while it is much less pronounced for S-PO adsorption (Figure 9b). That is, the lower R-PO yield from an R-2-butanol templated surface (Figure 7) is compensated for by the increase in the hydrogen yield due to propylene oxide decomposition (Figure 9) consistent with the approximately equal amounts of R- and S-propylene oxide detected on an R-2-butanol covered surface by RAIRS (Figure 8).

Note, however, in Figure 8, that in spite of there being identical initial coverages of 2-butanol on the surface in all cases, the relative intensities of the 2-butanol modes are substantially different. This implies that 2-butanol is interacting with the propylene oxide. Note finally that the evolution of the ∼450 K hydrogen desorption feature in the presence of both R- and S-propylene oxide with coverage (Figure 9) is identical to that for 2-butanol on the clean alloy (Figure 5a), where all the 2-butanol decomposes to produce hydrogen for exposures of ∼0.4 L or less, and above this exposure, the hydrogen yield remains constant, and 2-butanol either desorbs or forms 2-butanone. To explore the effects of R- and S-PO coadsorption on the surface chemistry of R-2-butanol, TPD experiments were carried out by monitoring desorption at 59 amu (due to 2-butanol) and 72 amu (due predominantly to 2-butanone) from surfaces first exposed to various amounts of R-2-butanol and then 3 L PO. Figure 10a displays the desorption of R-2-butanol when coadsorbed with R-PO, and Figure 10b displays the desorption of R-2-butanol when coadsorbed with with S-PO. Note that the peak at ∼100 K is due to desorption from the supports. It is clear that the R-2-butanol yield is substantially larger when coadsorbed with R-PO than when coadsorbed with S-PO for R-2-butanol exposures between 0.6 and 1.5 L. This is especially apparent at an R-2-butanol exposure of 0.9 L. In this case, a large amount of R-2-butanol desorbs when coadsorbed with R-PO, while with S-PO, the R-2-butanol signal is much weaker. The same is true for R-2-butanol exposures of 1.2 and 1.5 L, although the difference becomes smaller, and the signals are identical for 1.8 L R-2-butanol. Figure 11 shows plots of the corresponding desorption profiles of 2-butanone (72 amu). For R-2-butanol exposures of 0.6 L and below, the 2-butanone yield is slightly higher when coadsorbed with S-PO, while at higher R-2-butanol exposures, the 2-butanone yield when coadsorbed with S-PO is substantially higher. Note that for R-2-butanol exposures of 0.9 L and

3366 J. Phys. Chem. C, Vol. 112, No. 9, 2008

Gao et al. TABLE 2: Vibrational Assignments of RAIRS Signals for 2-Butanol Adsorbed on a Au/Pd(111) Alloy assignmenta ν(OH) νa(CH3) νa(CH2) νs(CH3), ν(CH) δa(CH3) γ(CH2) δs(CH3) δ(CH) δip(OH) ν(CC), F(CH3) ν(CO) ν(CC), F(CH3) F(CH3) F(CH2)

monolayer 2967, 2956 2880 1460 1408 1382 1333 1124 1089 1033 992 901

multilayer ∼3280 2969 2936, 2929 2880, 2868 1466, 1455 1407 1375 1341, 1317 1160 1128 1107 1034 993, 965 914

a ν: Stretching; δ: deformation; γ: scissoring; F: rocking; s: symmetric; a: asymmetric; and ip: in-plane.

Figure 4. Reflection absorption infrared spectra of (a) R-2-butanol adsorbed on the Au/Pd(111) alloy at 80 K as a function of 2-butanol exposure, where the exposures are marked adjacent to the corresponding spectrum and (b) the effect of annealing a Au/Pd(111) alloy exposed to 2.6 L 2-butanol at 80 K to various temperatures, where the annealing temperatures are displayed adjacent to the corresponding spectrum.

above, only the ∼220 K desorption state is due to 2-butanone, while the ∼260 K shoulder is due to fragmentation of R-2butanol. 4. Discussion Bimetallic surfaces have received considerable interest in recent years since they exhibit a variety of interesting physical and chemical properties different from those of the individual

components.17-21 Gold-palladium alloys provide rather ideal systems for fundamental study22-29 since gold and palladium are completely miscible in all proportions with only a slight lattice mismatch (∼4.5%). The alloy surface used for these experiments is depicted schematically in Figure 1. As noted previously, even for the relatively low gold coverages used in this work, which apparently leaves rather large patches of palladium that are not adjacent to gold atoms, the chemistry of both 2-butanol and propylene oxide is strongly affected by the presence of gold. In particular, substantial amounts of 2-butanone are formed, allowing enantioselective reactions rather than just chemisorption to be explored. The origin of this effect is not understood, although it must be due to a modification of the electronic structure of palladium by gold and is currently being explored. 4.1. Propylene Oxide Adsorption on the Au/Pd(111) Alloy. The data of Figure 2b clearly demonstrate that PO decomposes on the dilute Au/Pd(111) alloy as manifested by the fact that a small CO infrared signal appears at temperatures as low as 130 K, which must come from PO dissociation. The CO and H2 desorption profiles shown in Figure 3 also provide rather strong evidence for PO decomposition. The hydrogen desorption temperature is similar to that of ethylidyne species on palladium,30 and this is consistent with the infrared data in Figure 2b, showing the formation of ethylidyne (at 1316 cm-1). The infrared results (Figure 2) indicate that molecular propylene oxide is stable on the alloy to between 190 and 245 K, where a portion desorbs with the remainder forming an intermediate with vibrational mode intensities that are below our detection limit but that ultimately forms CO and ethylidyne species. It has been suggested that ethylene oxide forms from ethylene and adsorbed atomic oxygen on silver surfaces via an oxametallacycle,31-33 suggesting that the decomposition of propylene oxide on the alloy may occur by the reverse of this process. That is, between ∼190 and 250 K, propylene oxide both desorbs and undergoes C-O bond scission to form an oxametallacyle that finally decomposes to form CO and an ethylidyne species, where this process is complete by ∼300 K. 4.2. 2-Butanol Adsorption on the Au/Pd(111) Alloy. Substantial differences regarding the decomposition of 2-butanol are also found on the dilute Au/Pd(111) alloy as compared to pure Pd(111). The TPD results shown in Figure 5b reveal that a substantial amount of 2-butanone desorbs at ∼210 K, while 2-butanone is also generated on pure Pd(111) but with a much lower yield.3 It is clear, therefore, that on Pd(111) the majority of 2-butoxide completely decomposes, while on Au/Pd(111), a



Figure 5. (a) Temperature-programmed desorption spectra following various exposures of R-2-butanol to the Au/Pd(111) alloy using a heating rate of 6.5 K/s collected at 78 (2-butanol), 28 (CO), and 2 (hydrogen) amu, where the 2-butanol exposures are marked adjacent to the corresponding desorption profile and (b) desorption profiles collected at various masses following exposure of the Au/Pd(111) alloy to 1.0 L R-2-butanol at 80 K, where the detected masses are marked adjacent to the corresponding desorption trace.

larger portion undergoes β-hydride elimination to generate 2-butanone. On clean Pd(111), a sharp CdO vibrational feature is found at ∼1670 cm-1 due to 2-butanone adsorbed in an η1 conformation with the CdO bond oriented perpendicularly to the surface.1 On Au/Pd(111), although more 2-butanone is found in TPD, the 1670 cm-1 CdO stretching mode feature is not detected (Figure 4b). One possible explanation for this is that 2-butanone desorbs immediately following its formation. This notion is supported by the observation that 2-butoxide is detected at ∼190-210 K in RAIRS (Figure 4b) and that 2-butanone desorbs at approximately the same temperature in TPD (Figure 5b). An alternative explanation is that this effect could be due

to adsorption geometry differences between 2-butanone on these two surfaces. In cases where 2-butanone adopts an η2 conformation, the CdO bond will be close to parallel to the surface and therefore become infrared inactive. Previous studies of acetone on Pt(111)33 have revealed that the η1 species only adsorbs on the defect free portion of the surface, otherwise a slightly more energetically favored η2 geometry will be adopted. Thus, the picture that emerges for the chemistry on the alloy surfaces is that, for 2-butanol exposures of ∼0.4 L and lower, all the adsorbed 2-butanol fully decomposes to form hydrogen and carbon monoxide. The proportion of 2-butanol that completely decomposes is estimated by integrating the 2-butanol


Gao et al.

Figure 7. Plot of the integrated desorption yield of R- (b) and S-propylene oxide (9) vs R-2-butanol exposure taken from the data of Figure 6.

Figure 6. Temperature-programmed desorption spectra following (a) 3 L exposures of R-propylene oxide to the Au/Pd(111) alloy covered by various exposures of R-2-butanol using a heating rate of 6.5 K/s collected at 58 amu and (b) R-2-butanol covered surfaces exposed to S-propylene oxide where, in both cases, the 2-butanol exposure is marked adjacent to the corresponding desorption profile.

desorption yields from Figure 5a and extrapolating to zero coverage to estimate the amount that would have desorbed below an exposure of ∼0.6 L. This suggests that the first ∼0.27 ML of 2-butanol decomposes to yield hydrogen and CO. At higher coverages, 2-butoxide species are formed between 190 and 210 K (Figure 4b), a portion of which undergoes β-hydride elimina-

tion to form 2-butanone, which either desorbs immediately or is adsorbed with the CdO bond parallel to the surface. 2-Butanol is reformed by 2-butoxide hydrogenation at between 240 and 270 K. There is no evidence from infrared spectroscopy for the presence of 2-butanol on the surface over this temperature range, so this desorption temperature likely reflects 2-butoxide hydrogenation kinetics. The decrease in desorption temperature with increasing coverage is indeed consistent with a second-order desorption process, and this chemistry is summarized in Scheme 1. 4.3. Coadsorption on the Au/Pd(111) Alloy: Enantioselective Reactions. It is clear that alloying palladium with a small amount of gold profoundly affects the chemistry of both propylene oxide and 2-butanol as compared to the chemistry on clean Pd(111). This is also reflected in considerable differences in the enantioselective adsorption and reaction of 2-butanol modified alloy surfaces when coadsorbed with propylene oxide. Enantioselective adsorption of propylene oxide was found on Pd(111) surfaces modified by 2-butanol, where hydrogen-bonding interactions between propylene oxide and 2-butanol were identified as driving the enantiospecific interactions.3 In contrast, the results described previously indicate that the decomposition of propylene oxide on the surface appears to be enantiospecifically affected by the presence of 2-butanol, but the effect is small. The infrared spectra of 2-butanol coadsorbed at ∼150 K with propylene oxide on the Au/Pd(111) alloy surface are displayed in Figure 8. As noted previously, it is difficult from the infrared data to distinguish between 2-butanol and 2-butoxide species on the surface. However, on the alloy, the F(CH2) mode is between 891 and 894 cm-1, intermediate between that for 2-butanol (899 cm-1) and 2-butoxide (887 cm-1), and may suggest partial formation of 2-butoxide species. However, it is clear that the enantiose-



Figure 8. Reflection-absorption infrared spectra for R-propylene oxide (left panel) and S-propylene oxide (right panel) adsorbed at 80 K on a Au/Pd(111) alloy surface exposed to 0.3 L R-2-butanol. Peaks due to propylene oxide are marked by an asterisk.

Figure 9. Temperature-programmed desorption spectra following (a) 3 L exposures of R-propylene oxide to the Au/Pd(111) alloy covered by various exposures of R-2-butanol using a heating rate of 6.5 K/s collected at 2 amu (hydrogen) and (b) R-2-butanol covered surfaces exposed to S-propylene oxide where, in both cases, the 2-butanol exposure is marked adjacent to the corresponding desorption profile.

lective chemisorption of propylene oxide on 2-butanol templated surfaces found on Pd(111) is not observed on the alloy either because of extensive 2-butoxide formation that eliminates hydrogen bonding or due to some other effect caused by the alloy. As shown by the data in Figure 9, for coadsorbed propylene oxide and 2-butanol, the initial total thermal decomposition pathways of 2-butanol to form hydrogen and CO are unaffected by the presence of propylene oxide for 2-butanol exposures of ∼0.4 and below. Thus, the TPD data displayed in Figures 10 and 11 are for surfaces with identical coverages of R-PO (within

(10%), and the first ∼0.27 ML of adsorbed 2-butoxide species completely decomposes to yield CO and hydrogen as found on the clean alloy (Figure 5). Thus, it is argued that any difference in the enantiospecific reactivity that occurs on these surfaces is not due to differences in the extent of 2-butoxide decomposition or propylene oxide coverage. Comparing Figures 10a,b indicates that there are substantial differences in 2-butanol yield, which are assigned to enantioselective reactions. This is illustrated by the data shown in Figure 12, which plots the enantiomeric excess (ee) for 2-butanol (9) versus the 2-butanol coverage where this is estimated as indicated previously, from the integrated


Gao et al.

Figure 11. Temperature-programmed desorption spectra following (a) 3 L exposures of R-propylene oxide to the Au/Pd(111) alloy covered by various exposures of R-2-butanol using a heating rate of 6.5 K/s collected at 72 amu (2-butanone) and (b) R-2-butanol covered surfaces exposed to S-propylene oxide where, in both cases, the 2-butanol exposure is marked adjacent to the corresponding desorption profile. Figure 10. Temperature-programmed desorption spectra following (a) 3 L exposures of R-propylene oxide to the Au/Pd(111) alloy covered by various exposures of R-2-butanol using a heating rate of 6.5 K/s collected at 59 amu (2-butanol) and (b) R-2-butanol covered surfaces exposed to S-propylene oxide where, in both cases, the 2-butanol exposure is marked adjacent to the corresponding desorption profile.

2-butanol desorption yield (Figure 5), taking into account the initial amount that completely decomposes. The ee is calculated from

ee )

Y(R/R) - Y(R/S) Y(R/R) + Y(R/S)



SCHEME 1: Proposed Reaction Pathways for 2-Butanol on the Au/Pd(111) Alloy Surface

Figure 12. Plot of ee for the formation of 2-butanol (9) as a function of 2-butanol coverage taken from the data in Figure 10.

where Y(A/B) refers to the integrated desorption yield of 2-butanone or 2-butanol from a surface where A-2-butanol is coadsorbed with B-PO, where A and B are either R or S. This clearly shows an increase in ee for 2-butanol formation as a function of decreasing coverage to a maximum of 100%, where R-2-butanol formation is preferred when coadsorbed with R-PO. In contrast, S-2-butanone formation is slightly preferred when coadsorbed with R-PO with ee values of ∼14% (Figure 11), much lower than the enantioselectivity of 2-butanol. This presumably occurs since they both are derived from a common (2-butoxide) precursor (see Scheme 1), so that if more of one is formed, correspondingly less of the other will be produced. This clearly shows that enantioselective reactions can be probed in UHV and that the resulting ee is sensitive to coverage. Both 2-butanol and 2-butanone desorb between 200 and 300 K, while the infrared data of Figure 2 indicate that propylene oxide itself is no longer present on the surface at ∼245 K. However, as discussed previously, another intermediate, suggested to be an oxametallacycle, is proposed to be present between ∼190 and 240 K. This implies that the presence of a chiral oxametallacycle induces the enantioselective reactions. It is evident that 2-butanone forms at lower temperatures (∼210 K) than 2-butanol (∼250 K), while they are both produced from a common 2-butoxide intermediate (see Scheme 1). As emphasized previously, unlike clean Pd(111), there is no evidence in infrared spectroscopy for 2-butanone on the alloy surface since no CdO modes are detected. This implies that it

either desorbs as soon as it is formed or that it adsorbs in an η2 configuration and is not detected by infrared spectroscopy. Similarly, there is no evidence for the presence of 2-butanol on the surface so that the desorption profiles seen in Figure 10 reflect the rate of 2-butoxide hydrogenation. This suggests that the final enantioselectivity of 2-butanol is controlled by enantioselective β-hydride elimination of the 2-butoxide species to form 2-butanone, which is inhibited by the presence of the propylene oxide (or the resulting oxametallacycle) that leaves a preponderance of R-2-butoxy species on the surface that subsequently rehydrogenates to R-2-butanol, yielding the large enantiomeric excesses shown in Figure 12. Thus, the large ee values arise from a secondary reaction, the initial reaction being the preferential removal of S-2-butoxide species in the presence of R-PO. While this occurs with a relatively low ee, the total amount of 2-butanone desorbing from the surface, as estimated from the integrated desorption areas at 72 amu for 2-butanone and 59 amu for 2-butanol, corrected for their sensitivity factors for our mass spectrometer is significantly larger than the 2-butanol yield. This is particularly true at low coverages, where the largest ee values are measured (below 0.6 ML, Figure 12). Thus, for example, at a 2-butanol coverage of ∼0.6 ML, the amount of 2-butanone that is formed is at least 3 times larger than the 2-butanol yield. As indicated by the data in Figure 7, the adsorption of propylene oxide is blocked by the initially adsorbed 2-butanol. Thus, an increasing 2-butanol coverage in Figure 12 is accompanied by a decrease in propylene oxide coverage. Thus, the lack of enantioselectivity at large 2-butanol coverages merely reflects the lack of a coadsorbed propylene oxide modifier. 5. Conclusion The chemistry of both propylene oxide and 2-butanol is strongly affected by the formation of a dilute Au/Pd(111) alloy as compared to the chemistry on pure Pd(111). On clean Pd(111), propylene oxide desorbs without reacting, while on the alloy, it decomposes via a proposed oxametallacycle at ∼190 K, which finally decomposes at ∼250 K to yield adsorbed CO and ethylidyne species. 2-Butanol completely thermally decomposes on the alloy at exposures below ∼0.4 L to yield hydrogen and CO, while any additional adsorption produces 2-butanol by 2-butoxide rehydrogenation or 2-butanone via a β-hydride elimination reaction. Propylene oxide does not adsorb strongly enantiospecifically on a 2-butanol modified surface, and the extent of total decomposition of 2-butanol is not affected by the presence of propylene oxide. However, the reaction of the 2-butoxide species to form 2-butanone or 2-butanol is strongly enantioselective, where values of enantioselective excess of 100% toward the formation of R-2-butanol when coadsorbed with R-propylene

3372 J. Phys. Chem. C, Vol. 112, No. 9, 2008 oxide are found. It is suggested that this is controlled by the initial enantioselective decomposition of 2-butanone, which leaves a surface enriched with the R-2-butoxide species. Acknowledgment. We gratefully acknowledge support of this work by the U.S. Department of Energy, Division of Chemical Sciences, Office of Basic Energy Sciences, under Grant DE-FG02-03ER15474. References and Notes (1) Stacchiola, D.; Burkholder, L.; Tysoe, W. T. J. Am. Chem. Soc. 2002, 124, 8984. (2) Stacchiola, D.; Burkholder, L.; Tysoe, W. T. J. Mol. Catal. A: Chem. 2004, 216, 215. (3) Gao, F.; Wang, Y.; Burkholder, L.; Tysoe, W. T. J. Am. Chem. Soc. 2007, 129, 15240. (4) Lee, I.; Zaera, F. J. Phys. Chem. B 2005, 109, 12920. (5) Calaza, F.; Gao, F.; Li, Z.; Tysoe, W. T. Surf. Sci. 2007, 601, 714. (6) Li, Z.; Calaza, F.; Gao, F.; Tysoe, W. T. Surf. Sci. 2007, 601, 1351. (7) Li, Z.; Gao, F.; Wang, Y.; Calaza, F.; Burkholder, L.; Tysoe, W. T. Surf. Sci. 2007, 601, 1898. (8) Li, Z.; Furlong, O.; Calaza, F.; Burkholder, L.; Poon, H. K.; Saldin, D.; Tysoe, W. T. Surf. Sci., in press. (9) Anton, R.; Eggers, H.; Veletas, J. Thin Solid Films 1993, 226, 39. (10) Yi, C. W.; Luo, K.; Wei, T.; Goodman, D. W. J. Phys. Chem. B 2005, 109, 18535. (11) Boscoboinik, J. A.; Plaisance, C.; Neurock, M.; Tysoe, W. T. Phys. ReV. B: Condens. Matter Mater. Phys. 2008, 77, 045422. (12) Loffreda, D.; Simon, D.; Sautet, P. Surf. Sci. 1999, 425, 68. (13) Stacchiola, D.; Calaza, F.; Zheng, T.; Tysoe, W. T. J. Mol. Catal. A: Chem. 2005, 228, 35.

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Pd (111) Surface Alloy with

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