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Feb 5, 2016 - Attard , G. A.; Bennett , J. A.; Mikheenko , I.; Jenkins , P.; Guan , S.; Macaskie , L. E.; Wood , J.; Wain , A. J. Faraday Discuss. 201...
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Structure Sensitivity in Catalytic Hydrogenation at Platinum Surfaces Measured by Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy (SHINERS) Shaoliang Guan,† Oliver Donovan-Sheppard,† Christian Reece,† David J. Willock,† Andrew J. Wain,*,‡ and Gary A. Attard† †

School of Chemistry, Cardiff University, Park Place, Cardiff CF10 3AT, U.K. National Physical Laboratory, Hampton Road, Teddington TW11 0LW, U.K.

‡

S Supporting Information *

ABSTRACT: The in situ combination of electrochemistry and shellisolated nanoparticle enhanced Raman spectroscopy (SHINERS) has been used for the first time to investigate the surface structure sensitivity of asymmetric catalytic hydrogenation at single-crystal Pt electrodes. The adsorption and hydrogenation behavior of aqueous ethyl pyruvate (EP) at a range of modified and unmodified Pt{hkl} electrodes was measured both by cyclic voltammetry and by recording Raman spectra at hydrogen evolution potentials. Two primary surface intermediates were observed, including the previously reported halfhydrogenation state (HHS), formed by addition of a hydrogen atom to the keto carbonyl group, as well as a new species identified as intact chemisorbed EP bound in a μ2(C,O) configuration. The relative populations of these two species were sensitive to the Pt surface structure; whereas the μ2(C,O) EP adsorbate was dominant at pristine Pt{111} and Pt{100}, the HHS was only observed at these electrodes after the introduction of defects by electrochemical roughening. Intrinsically defective Pt{110} and kinked Pt{321} and Pt{721} surfaces exhibited behavior similar to that of electrochemically roughened basal surfaces, indicating the requirement for low coordination sites for observation of the HHS. Rationalization of the differing behaviors is given on the basis of density functional theory (DFT) calculations, which indicate that the μ2(C,O) EP adsorbate is considerably more stable on basal {111} than on {221} stepped surfaces. A mechanism is proposed in which the μ2(C,O)-bound species is a precursor to the HHS but the rate of the first hydrogen atom addition is slow, leading to a low steady-state population of the HHS at terrace sites. The implications of this in the context of enantioselective hydrogenation at chirally modified Pt are discussed. KEYWORDS: heterogeneous catalytic hydrogenation, ethyl pyruvate, structure sensitivity, platinum single crystal, DFT, surface vibrational spectroscopy, Orito reaction, electrochemistry

1. INTRODUCTION The concept of structure sensitivity in heterogeneous catalysis has been established for decades and is widely recognized as a potential path to controlling and optimizing surface reactivity and selectivity.1,2 Surface science studies have contributed immeasurably to our mechanistic understanding of catalytic processes at the molecular scale. However, the role of surface structure and the influence of active sites is often highly complex and poorly understood. A primary example is the asymmetric hydrogenation of prochiral substrates, which is of major technological importance for the synthesis of fine chemicals, pharmaceuticals, and fragrances.3 In particular, the enantioselective hydrogenation of α-keto esters on chirally modified Pt surfaces, first observed by Orito et al. in the late 1970s,4 has received considerable attention due to its notably high chiral efficiency.5−31 In the case of the substrate ethyl pyruvate (EP), hydrogenation to (R)-ethyl lactate (EL) over Pt catalysts modified with the alkaloid cinchonidine (CD) can be © XXXX American Chemical Society

achieved with exceptionally high enantiomeric excess (Scheme 1).32,33 However, in spite of the many studies of this reaction, there remain several unanswered questions about how EP interacts with the Pt surface, the effect of surface structure on the reaction, and ultimately the mechanism of chiral discrimination. Structure−activity−selectivity relationships for this reaction have been particularly difficult to establish due to conflicting data and the need to consider not only interactions between the α-keto ester substrate and the alkaloid modifier but also the binding of these two species with the Pt surface both together and in isolation. Catalyst treatments including exposure to hydrogen at elevated temperatures,34 sintering,35 and sonication36 have been shown to influence enantioselectivity, the Received: December 16, 2015 Revised: January 22, 2016

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technique enable the simulation of relevant hydrogenation conditions by evolution of hydrogen at catalytic electrodes for SERS analysis but also cyclic voltammetry (CV) can provide critical information on the surface population of various types of adsorption site (steps, terraces, kinks, etc.).47 Attard et al. used in situ SERS to study the Orito reaction at SERS-active Au@Pt core−shell particles and revealed evidence for a halfhydrogenated state intermediate of EP.48,49 However, these studies were limited to polycrystalline Pt surfaces and so were not able to reveal the detailed structure sensitivities that can be gleaned from single-crystal studies. The extension of SERS to encompass single-crystal analysis has recently been realized by Tian et al., who demonstrated the use of protected SERS-active Au@SiO2 particles to observe Raman scattering at weakly or nonenhancing surfaces.50−53 In this approach, termed shellisolated nanoparticle-enhanced Raman spectroscopy (SHINERS), core−shell nanoparticles (NPs) are deposited on the surface of the sample under investigation and the inert, dielectric SiO2 shell prevents the Raman-enhancing core from interfering chemically or electrically with any components of the system under study. SHINERS has since been employed by various groups to study adsorption processes at single-crystal surfaces.47,54−59 In this work, for the first time, SHINERS is applied to an investigation of the catalytic hydrogenation of EP on Pt singlecrystal electrodes. By application of Au@SiO2 NPs to various Pt{hkl} surfaces, the structure sensitivity of the reaction is clearly uncovered and the role of terrace and defect sites on the formation and stability of surface intermediates is investigated.

Scheme 1. Enantioselective Hydrogenation of EP to EL

behavior in each case being attributed to changes in the Pt surface. Attard et al. investigated the surface sensitivity of this reaction through the coadsorption of site-blocking inert adatoms and demonstrated that selective passivation of the step sites with bismuth enhanced the reaction rate but decreased the enantiomeric excess, while blocking the terraces with sulfur lowered the rate of reaction but increased enantioselectivity.22 It was concluded therefore that edge sites provide the highest enantiodifferentiation. More recently, Baiker and co-workers demonstrated a positive correlation between both reaction rate and enantiomeric excess and the ratio of Pt{111}/Pt{100} using shaped nanoparticles.31 In this case the difference in behavior between these two crystal facets was attributed to the different adsorption energies between the alkaloid modifier and the Pt. However, a challenge that is common to these investigations is the difficulty in isolating surface-structure effects from other variables such as electronic structure, defect density, and support effects. Single-crystal measurements offer a potential solution to the above problem, and several studies have focused on the use of molecular and electronic spectroscopy to probe the molecular binding modes of pyruvate esters on Pt surfaces.13,16,17,21,23,37,38 For example, Baiker et al. used in situ XANES to investigate the adsorption mode of EP on Pt{111} surfaces and demonstrated a strong influence of coadsorbed hydrogen.14 A perpendicular molecular orientation of EP was observed when hydrogen was absent, with bonding to the Pt surface achieved via the oxygen lone pairs, while a shift in tilt angle upon the introduction of hydrogen was purported to signify transition to a π-bonded species. McBreen and co-workers studied the adsorption of methyl pyruvate on pristine and modified Pt{111} using RAIRS and observed η1 and enediolate bonding modes, although these spectroscopic measurements were confined to 110 K.17,23 More recent RAIRS experiments by the same authors at higher temperatures revealed that methyl pyruvate can undergo keto− enol tautomerism on Pt{111} surfaces at sufficiently low coverages of hydrogen.38 Despite this ongoing attention, as well as more recent studies into analogous substrates,39−44 definitive structures for the active mode of EP binding during hydrogenation and the mechanism of ketone activation have remained elusive. Surface-enhanced Raman spectroscopy (SERS) is a powerful tool for the characterization of interfacial adsorbates that has significant advantages over many other molecular spectroscopies, most notably improved signal to noise and surface specificity.45 Its selectivity and sensitivity extends to a wide variety of applications, including in situ and ambient analysis of catalytic, biological, and organic systems. Furthermore, the combination of electrochemical analysis with in situ SERS presents a novel and unrivaled method of investigating surface reactions in the liquid phase.46 Not only does this coupled

2. EXPERIMENTAL SECTION 2.1. Chemicals. Solutions of HAuCl4 (Johnson Matthey, Assay 41.79%), sodium citrate dihydrate (Sigma-Aldrich, 99+ %), (3-aminopropyl)trimethoxysilane (APTMS, Alfa Aesar, 97%), sodium silicate (Sigma-Aldrich, reagent grade), H2SO4 (BDH, Aristar grade), HClO4 (Merck, Suprapur), ethyl pyruvate (EP, Fluka, > 97%), and (−)-cinchonidine (CD, Sigma-Aldrich, 96%) were used as received, and solutions were prepared using ultrapure water (Millipore) with a resistivity of not less than 18.2 MΩ cm. The relevant electrolyte solutions were degassed with argon (BOC, 99.998%) before measurements. 2.2. Materials Synthesis. Au seeds (55 nm) were grown by the following procedure:47 100 mL of 0.01% (by mass) HAuCl4 was measured and decanted into a 250 mL roundbottomed flask, and the solution was heated under reflux for 30 min at 112 °C in a silicone oil bath. An aqueous solution of sodium citrate (0.57 g in 50 mL of H2O) was prepared, and 600 μL of it was added directly to the HAuCl4 solution without touching the flask wall. The solution changed from yellow to black to dark red within seconds as Au was reduced. The resulting solution was refluxed for a further 30 min. To prepare Au@SiO2 NPs, the method developed by Tian et al.50 was employed after modification. Briefly, 0.4 mL of a freshly prepared 1.0 mM aqueous solution of (3-aminopropyl)trimethoxysilane (APTMS) was added to 30 mL of the gold seeds with vigorous stirring. Then, 3.2 mL of 0.54 wt % sodium silicate solution (pH ∼10.28) was added to the mixture. Subsequently, the mixture was kept at 70 °C for 13 min to form Au@SiO2 NPs with an ultrathin (2 nm) shell. 2.3. Raman Spectroscopic Measurements. Raman spectroscopy was performed using a LabRam Raman Microscope (Horiba JobinYvon Ltd.) fitted with a HeNe laser (λ 633 1823

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ACS Catalysis nm, output power 16 mW), with data recording and analysis performed using the proprietary LabSpec software (Horiba JobinYvon) run on a Dell Optiplex PC. 2.4. Electrochemical Measurements. The flow cell used for the electrochemical measurements has been described previously.49 Pt (GoodFellow, 99.999%) and Pd and Au (both from Advent Research Materials Ltd., 99.95%) were used as working, reference, and counter electrode materials, respectively. Pd was charged with H2 to form a saturated Pd/H reference electrode, against which all potentials were reported. Pt{hkl} single-crystal electrodes were prepared using the Clavilier bead method,60 and chiral kinked surfaces were all prepared with S stereochemistry. In order to investigate the EP adsorption behavior of Pt single crystals using cyclic voltammetry, they were exposed to neat liquid EP for 5 s followed by removal of excess liquid by gentle rinsing with ultrapure water and subsequent immersion in 0.1 M H2SO4. In order to obtain in situ SERS data, clean single-crystal electrodes were decorated with Au@SiO2 NPs by drop-casting 10 μL of the appropriate NP solution on the surface and allowing the solvent to evaporate under a flow of nitrogen. To create hydrogenation conditions, hydrogen gas was evolved at the working electrode surface by applying a potential of −0.1 V. By arrangement of the cell in an upward flow configuration and balance of the flow rate, any hydrogen gas bubbles that formed on the surface of the electrode, which might affect the detection of SERS signals, could be readily removed. To clean the surface of the electrode, electrochemical potential cycling was performed between −0.1 and 0.85 V at 100 mV s−1 in 0.1 M H2SO4 (or HClO4) without changing the surface atomic structure. Electrochemical roughening was also achieved by potential cycling, instead employing an anodic limit of 1.2 V. 2.5. DFT Calculations. Calculations were performed using the VASP (Vienna ab initio simulation package)61 using the Perdew−Burke- Ernzerhof (PBE)62 functional. A plane wave basis set was used with a three-dimensional periodic boundary, with a cutoff value of 600 eV for all calculations. The calculations included the DFT-D3 correction by Grimme63 for long-range dispersion forces, with a van der Waals radius of 10 Å. Vibrational frequencies were calculated using a numerical three-point approach, with atomic displacements of 0.02 Å and only the coordinates of the absorbate atoms included as degrees of freedom. The D3 correction was shown to have negligible impact on vibrational frequencies and so was turned off for these calculations.

Figure 1. CVs of Pt{hkl} electrodes in 0.1 M H2SO4 (sweep rate 50 mV s−1) before adsorption of EP (black curve), after exposure to EP for 5 s (red curve), and after EP adsorption followed by hydrogen evolution at −0.1 V (green curve): (a) Pt{111}; (b) Pt{100}; (c) Pt{110}.

distinctive “butterfly” region between 0.3 and 0.5 V. While this indicates disruption in the long-range order, the total electrochemical surface area was only diminished by approximately 10%. This small change is consistent with previous measurements65 and indicates that EP chemisorption on Pt{111} is relatively weak. Hydrogen evolution on the electrode by applying a potential of −0.1 V resulted in removal of the adsorbed EP, substantially cleaning the surface and recovering the {111} terrace sites. This is consistent with the catalytic hydrogenation of the remaining adsorbed EP under hydrogen-rich conditions. Equivalent experiments were carried out on Pt{100} and {110} single-crystal electrodes, and the resulting CVs are depicted in Figures 1b,c, respectively. In stark contrast to Pt{111}, strong EP adsorption to Pt{100} results in loss of approximately two-thirds of the electrochemical surface area after exposure to EP (Figure 1b). Also striking are the new electrosorption features revealed in the voltammetry after EP exposure, including a cathodic peak at 0.1 V, attributed to reductive EP desorption, and a significant anodic peak at 0.74 V, corresponding to electrooxidative stripping of adsorbed CO formed by EP decarbonylation.66 As in previous work, this CO feature only appears if the electrode potential is scanned into the hydrogen adsorption region, below 0.35 V.65 Potential cycling between 0 and 0.70 V resulted in the removal of some EP, as signified by a small recovery in electrosorption sites (not shown), but evolving hydrogen on the electrode surface at −0.1 V was significantly more effective (Figure 1b, green line), indicating efficient hydrogenative desorption of EP, to form EL.67 For Pt{110} surfaces the hydrogen adsorption peak was largely attenuated after EP dosing, although to a slightly lesser degree than at Pt{100} (Figure 1c), as reported previously.65 An increase in cathodic current density on the negative sweep near 0 V suggests hydrogenation of EP as the surface hydrogen

3. RESULTS AND DISCUSSION 3.1. Cyclic Voltammetry of EP Adsorption on Pt{hkl} Single-Crystal Electrodes. Cyclic voltammetry (CV) offers a unique means to determine the relative distribution of different atomic sites (terraces, steps, kinks) and to monitor site-selective adsorption processes at platinum surfaces in situ.60,64 Previous work using this technique has shown that the degree of EP coverage on Pt{hkl} single crystals after adsorption is highly structure sensitive.65 As a preliminary test, similar experiments were undertaken in the present work with additional measurements to investigate the ease of removal of adsorbed EP by hydrogen evolution. CVs of the Pt{111} single-crystal electrode, recorded in 0.1 M H2SO4, before and after adsorption of EP are shown in Figure 1a. The unmodified Pt{111} electrode exhibits the characteristic features of a clean surface,64 whereas dosing with EP results in some attenuation of the sharp peaks in the 1824

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ACS Catalysis coverage increases, and the subtle rise in anodic current at approximately 0.7 V suggests a small degree of CO electrooxidation. Remarkably, hydrogen evolution at −0.1 V resulted in almost complete recovery of the hydrogen adsorption sites. The above CV data indicate that although the strength of EP adsorption appears to vary with Pt surface structure, consistent with previous work,65 hydrogen evolution effectively results in rapid removal of EP by hydrogenative desorption. 3.2. SHINERS Studies of EP Adsorption and Hydrogenation on Basal Pt{hkl}. The adsorption and hydrogenation of EP on Pt surfaces have previously been investigated using Raman spectroscopy employing the SERS phenomenon.48,49 However, to date such studies have been limited to polycrystalline Pt. Here, we utilize the SHINERS technique to enable single-crystal studies of this system for the first time. We begin with analysis of the low-index basal Pt planes {111}, {110}, and {100} before examining higher index kinked surfaces (see Figure S1 in the Supporting Information for a schematic depiction of surfaces employed). Before performing spectroscopic experiments at Pt{hkl} surfaces, CVs were performed in order to verify that the Raman-enhancing SHINERS particles do not themselves affect the chemical or structural properties of the single-crystal electrodes. Example CVs are shown in Figure S2 in the Supporting Information, which confirm that, after deposition of Au@SiO2 particles, changes to the voltammetry are minimal, indicating that the SHINERS nanoparticles are truly inert and do not significantly affect the cleanliness of the Pt surface. Raman analysis of immobilized SHINERS particles also indicated the absence of any surface contamination. The behavior of adsorbed EP under hydrogenation conditions can be uniquely probed by immersing the Pt electrode in an acidic solution of EP and evolving molecular hydrogen at the surface by application of a cathodic bias (−0.1 V in this case), while simultaneously performing spectroscopic analysis. Figure 2a,b (line i) depicts the Raman spectra recorded at a pristine Pt{111} electrode decorated with Au@ SiO2 nanoparticles immersed in 0.1 M EP/0.1 M HClO4 under such electrochemical hydrogenation conditions. The broad band just below 3000 cm−1 is attributed to the C−H stretching vibrations of EP. Together with the C−C−O stretching band at 857 cm−1, the O−C−O scissoring band at 750 cm−1, and the CO rocking band at 649 cm−1, these peaks are assigned to bulk EP adsorbed on the Pt surface in accordance with previous work (see Figure S3 in the Supporting Information).48,49 The identity of the broad band at 1580 cm−1 is not completely clear, but it is likely associated with a carbonyl stretch of the EP interacting with the Pt surface. The sharp peak at 936 cm−1 arises from the symmetric Cl−O stretch of the perchlorate anion from the electrolyte solution, and this was used as a marker to normalize spectral intensities for comparison. The spectral features described above are in broad agreement with SER spectra reported for EP adsorption on polycrystalline Pt under the same electrochemical hydrogenation conditions.48,49 There are, however, some key differences that are important to highlight at this point. A SER spectrum of polycrystalline Pt is shown in Figure S4 in the Supporting Information for comparison. First of all, two intense peaks are present in Figure 2a,b at 418 and 965 cm−1 that were not observed or were very weak on polycrystalline Pt. Moreover, there are three important vibrational bands that are absent from the spectrum that were very strong on polycrystalline Pt. Most notably there is no band at 1050 cm−1 that was previously

Figure 2. SHINERS spectra of Pt{hkl} single crystals immersed in 0.1 M EP/0.1 M HClO4, recorded under hydrogen evolution conditions (−0.1 V): (i) pristine Pt{111}; (ii) electrochemically roughened Pt{111}; (iii) pristine Pt{100}; (iv) electrochemically roughened Pt{100}; (v) pristine Pt{110}; (vi) pristine Pt{100} in the presence of 20 μM CD. Part a shows full spectral range recorded, and part b shows the 300−1100 cm−1 region in detail. Signal intensities normalized to the Cl−O band of perchlorate and key bands are highlighted. (c) Schematic depiction of the half-hydrogenated state (HHS) intermediate.

attributed to the C−OH rocking mode/C−CH3 umbrella stretch of the half-hydrogenated state (HHS) intermediate, formed when the ketone oxygen of EP is hydrogenated but the carbon remains surface bound (depicted schematically in Figure 2c). The second absent band on Pt{111} is a CO stretch at 2000−2050 cm−1, expected due to adsorbed CO which is known to be formed from EP decarbonylation.66 The third missing band is a Pt−C stretch at 490 cm−1 associated with both the HHS as well as adsorbed CO. These substantial differences between Pt{111} and polycrystalline Pt indicate that surface structure has a dominant influence on EP adsorption and hydrogenation and suggest that defect sites could be a prerequisite to observing the HHS and CO bands. 1825

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with competitive CD adsorption. Similar behavior was observed for electrochemically roughened Pt{111} and Pt{100} surfaces, the only notable difference being that the CD ring stretch was particularly weak on Pt{111}, consistent with the weaker adsorption energy of CD on this surface.31 3.3. SHINERS Studies of EP Adsorption and Hydrogenation on Kinked Pt{hkl}. To investigate more thoroughly the influence of low-coordination sites on the adsorption and hydrogenation properties of EP, SHINERS measurements were next performed on kinked chiral surfaces Pt{321} and Pt{721}. The Pt{321} surface consists of two to three atom wide {111} terraces separated by short {110} and {100} steps, while on Pt{721} the terraces have {100} symmetry and are separated by long {111} and short {110} steps (see Figure S1 in the Supporting Information). SHINERS spectra are shown in Figure 3 (lines i and ii), and once again we observe the

In order to investigate this hypothesis, surface defects were introduced onto the Pt{111} electrode by continuous potential cycling in and out of the oxide formation/desorption region (upper potential limit of 1.2 V). The SHINERS spectrum observed on Pt{111} after such “electrochemical roughening” is shown in Figure 2 (line ii) in which a number of new Raman features emerge. For comparative purposes, the signal intensity scale was normalized to the bulk perchlorate Cl−O peak, which is expected to be constant. The peak appearing at 1050 cm−1 is ascribable to the HHS, as discussed above, and confirms the requirement of surface defects to observe this intermediate. A clear band also emerges at 2016 cm−1, corresponding to a CO stretch, and indicates that decarbonylation of EP to form adsorbed CO is facilitated by the newly generated defect sites. The most intense band appearing at 490 cm−1 is attributed to the Pt−C stretch common to both the HHS and adsorbed CO. It is also noteworthy that the unassigned bands at 418 and 965 cm−1 are slightly attenuated after electrochemical roughening, suggesting that these features are largely associated with EP adsorbed at defect-free terrace sites. The adsorption behavior of EP under hydrogenation conditions was next investigated at a Pt{100} electrode, and the SHINERS spectra recorded before and after electrochemical roughening are depicted in Figure 2 (lines iii and iv). The spectra are very similar to those recorded at Pt{111} surfaces, with bands at 1050 cm−1 and at 490 and 2016 cm−1, associated with the HHS and adsorbed CO, appearing only after the introduction of defects. Unassigned bands at 418 and 965 cm−1 were again observed prior to roughening, indicating that the responsible species is also stable on Pt{100}, although we note that these bands are weaker than those observed at pristine Pt{111} by approximately 25%. Further electrochemical roughening resulted in additional growth of the HHS and CO bands and concomitant loss of band intensity at 418 and 965 cm−1. Equivalent SHINERS spectra recorded at a pristine Pt{110} electrode were found to be in stark contrast to the data discussed above for Pt{111} and Pt{100} surfaces, as shown in Figure 2 (line v). In addition to the unassigned bands at 418 and 965 cm−1 that were present at Pt{111} and Pt{100}, there is clear evidence of strong HHS bands at 1050 and 490 cm−1 without the need for electrochemical roughening. A CO stretch is also observed at the native Pt{110} surface, although this band is comparatively weak. We attribute these observations to the intrinsic defects present in the Pt{110} surface. It is known that Pt{110} undergoes a surface reconstruction from a (1 × 1) arrangement to a “missing row” (1 × 2) atomic structure when it is clean and thermally equilibrated, wherein up to 50% of surface atoms undergo displacement.68,69 The extent of this reconstruction depends on the cooling conditions during electrode fabrication;70 thus, we expect a mixture of ordered phases, leading to an inherently defective surface which supports formation of the HHS and CO. The coadsorption of chiral modifiers has a significant effect on the adsorption and hydrogenation properties of EP, and this was investigated by performing in situ SHINERS measurements in the presence of 20 μM CD. An example spectrum is shown for the case of Pt{110} in Figure 2 (line vi), in which we observe attenuation of all bands associated with EP and its intermediates, including slight quenching of the CO and Pt−C bands, consistent with CD occupying surface sites. CD ring stretches observed in the 1600 cm−1 region were also consistent

Figure 3. SHINERS spectra of kinked Pt{hkl} single crystals immersed in 0.1 M EP/0.1 M HClO4, recorded under hydrogen evolution conditions (−0.1 V): (i) Pt{321}; (ii) Pt{721}; (iii) Pt{721} in the presence of 20 μM CD. Part a shows the full spectral range recorded, and part b shows the 300−1100 cm−1 region in detail. Signal intensities are normalized to the Cl−O band of perchlorate.

unassigned bands at 418 and 965 cm−1 for both surfaces as well as the HHS bands at 1050 and 490 cm−1, similar to the behavior of Pt{110} and electrochemically roughened Pt{111} and Pt{100}. We note the higher HHS band intensities in the case of Pt{721} in comparison to Pt{321}, which might suggest that the combination of a long {111} step adjacent to a {100} terrace stabilizes the HHS either by increasing its rate of formation or by suppressing subsequent hydrogen transfer. Some evidence of adsorbed CO bands can be seen at ∼2000 1826

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ACS Catalysis cm−1, although interestingly these are not as sharp as those observed at electrochemically roughened basal surfaces. Coadsorption of CD was found on the whole to have a quenching effect similar to that observed for Pt{110} and electrochemically roughened Pt{111} and Pt{100}. This is shown for Pt{721} in Figure 3 (line iii), in which the HHS band at 1050 cm−1 is completely attenuated by the presence of 20 μM CD. 3.4. Assignment of New Raman Bands. It has previously been shown that the HHS intermediate of EP is only stable at potentials where hydrogen evolution takes place.48 The present study has further verified that defect sites, as well as potentially terraces, are a prerequisite for observation of the HHS under these conditions, which has profound implications on our understanding of structure−activity relationships in such reactions. However, the question as to the assignment of the Raman bands at 418 and 965 cm−1 remains unanswered. These bands are common to all of the single-crystal spectra presented, but surprisingly they were absent on polycrystalline Pt nanoparticles under the same conditions (see Figure S4 in the Supporting Information).48,49 Furthermore, these bands appeared to be most intense at basal Pt{111} and Pt{100} surfaces, indicating an affiliation with terrace sites. Several assignments were considered for these two bands. First of all, the possibility that they might be due to contaminants introduced by the SHINERS particles was ruled out by undertaking measurements in the absence of EP, which resulted in predominantly featureless spectra. The band at 418 cm−1 might possibly be associated with the frequency-shifted Pt−C stretch of multicoordinated adsorbed CO, but this can be ruled out by the absence of a corresponding CO stretch in the region of 1750−1800 cm−1 on pristine Pt{111} and Pt{100} (Figure 2, lines i and iii). A more extreme interpretation is that EP adsorbs with full transfer of charge from the keto CO to the metal, with transfer of a single hydrogen atom to the carbon of the keto carbonyl. The resulting alkoxide species could exhibit a Pt−O stretch at 418 cm−1 and an alkoxide C−O stretch at 965 cm−1. However, this mechanism is very unlikely, since DFT calculations performed by Hammer and co-workers showed such addition of hydrogen to carbon to have a high activation barrier in comparison to hydrogen transfer to the oxygen atom of the keto carbonyl.71 The possibility was also considered that the band at 965 cm−1 may be associated with the vinyl carbon of EP adsorbed in its enolic form, as proposed by McBreen et al. for methyl pyruvate.38 Interestingly, the bands at 418 and 965 cm−1 do match those of the enol form of methyl pyruvate, as observed using HREELS.72 However, it has been reported by McBreen and co-workers that this enol on Pt{111} is completely destabilized in the presence of even a low coverage of hydrogen.38 Therefore, it is unlikely that such a species could populate the surface significantly while the electrode surface was generating hydrogen freely during electrolysis. Furthermore, control experiments with ethyl acrylate (an analogue of the EP enol tautomer) on Pt{111}, conducted in the presence and absence of H2, exhibited no evidence of bands at 418 or 965 cm−1 (see Figure S5 in the Supporting Information). The absence of other bands characteristic of the enol, such as a sharp CC stretch at 1580 cm−1, also supports our rejection of this interpretation. Given the difficulties in rationalizing our observations against previously reported Raman data, we propose that the unassigned bands are associated with an adsorbed state or

intermediate that was not previously observed by Raman spectroscopy. The fact that these peaks are most intense on the basal surfaces and are attenuated in the presence of defects suggests that they could relate to an intact adsorbed species of EP with a strong association with Pt terraces. A number of binding modes have been proposed for EP on basal Pt surfaces, including η1(O) (end-on, via the keto O), η2(C,O) (horizontal via the keto C and O), and enediolate (upright, via keto and ester carbonyl O) configurations (see Figure 4a).13,17,19,21,23

Figure 4. (a) Schematic depiction of η1(O), η2(C,O), enediolate, and μ2(C,O) EP configurations. (b, c) Calculated structure of EP adsorbed in (b) η1(O) and (c) μ2(C,O) configurations at Pt{111} from DFT modeling (color scheme: gray, C; red, O; white, H).

The enediolate binding mode was rejected due to its known instability at room temperature in the absence of a surface modifier,17,21 in addition to the complete lack of a CO stretch at ∼1740 cm−1 that would be expected for such an upright carbonyl configuration. DFT modeling by Baiker and co-workers indicated that, while both η1(O) and η2(C,O) binding modes are feasible for EP, η2(C,O) is the preferred state in the presence of coadsorbed hydrogen.19 The lack of a CO stretch is indeed consistent with this, since the horizontal molecular orientation could make this band Raman silent on the basis of surface selection rules. Furthermore, previous theoretical and experimental work suggests significant rehybridization of the CO bond upon η2(C,O) adsorption,14,19 favoring a di-σ configuration which is sensitive to the long-range order of the surface. Here we adopt the notation proposed by Willock et al.,73 in which such η2(C,O) binding across two Pt atoms is instead referred to as μ2(C,O). The prevalence of a μ2(C,O) configuration for EP on Pt{111} is in contrast to the adsorption behavior of simple ketones such as acetone, where the η1(O) configuration has been typically shown to prevail, albeit in the absence of hydrogen.73−75 As postulated by Kadodwala and co-workers, this disparity may be rationalized by the presence of the electron-withdrawing ester group changing the charge/back-donation properties of the molecule.76 In order to aid the assignment of the unknown spectral bands, DFT modeling was undertaken. Both η1(O) and μ2(C,O) binding configurations of adsorbed EP were explored (Figure 4b,c), and preliminary calculations indicated the adsorption energy of the μ2(C,O) state was considerably higher than that of the η1(O) configuration on Pt{111}. This is 1827

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965 cm−1, as already discussed. With a positive increase of electrode potential up to 0 V (Figure 5b) a slight increase in the intensity of both of these bands is observed. This is a striking difference in comparison to the behavior of the HHS, the Raman signature of which is almost completely attenuated at this potential.48 The slight increase in intensity observed may be related to a simple reorientation of the EP μ2(C,O) species toward the planar electrode surface, or it could indicate that the rate of hydrogenative desorption of this intermediate is slower at this potential such that the steady-state coverage increases slightly. As the potential is increased to 0.1 V (Figure 5c), both peaks associated with the μ2(C,O) state are reduced markedly in intensity. Further increase in the potential up to 0.4 V leads to almost complete attenuation of the μ2(C,O) bands (Figure 5d− f). This behavior cannot be explained in terms of relative rates of reduction/adsorption, since an increase in signal would be expected as the surface coverage of hydrogen decreases. One possible rationale for this behavior relates to the decarbonylation of EP, known only to occur at potentials in the hydrogen adsorption and evolution region.65 It is possible that the gradual attenuation of the μ2(C,O) EP bands reflects its slow conversion to CO (plus further SHINERS-silent or rapidly desorbed species). However, there is very little evidence of any CO adsorbed on Pt{111} under hydrogen evolution conditions and no concomitant increase in CO intensity with increasing potential, suggesting that this is unlikely. The most plausible explanation for the above observations is that the orientation of the adsorbed species changes as a function of potential (i.e., the crucial 418 and 965 cm−1 transitions become less SERS-active at more anodic potentials due to surface selection rule considerations). This is supported by the small shift of the peak at 965 cm−1 to higher wavenumbers with increasing potential, consistent with changes in the orientation of the intermediate leading to frequency damping. This is also verified by DFT-generated animations (see section S6 in the Supporting Information) which indicate that this frustrated rocking mode brings the methyl group close to the Pt surface, the extent of which would depend on the precise orientation of the EP molecule. The proposed variation of EP molecular orientation with electrode potential is consistent with in situ XANES data that indicated a change in tilt angle of adsorbed EP in the presence of coadsorbed hydrogen.14 As proposed by Baiker et al., such a change can be linked to the corresponding change in metal workfunction in the presence of hydrogen (or in this case a shift in the Pt Fermi level brought about by a change in applied electrode potential).19 It is possible that the absence of coadsorbed hydrogen promotes tilting toward the extreme case of an η1 state or that chains of EP in its enolic form assemble spontaneously,38 but the absence of a CO stretch or enol bands in the SHINERS spectra at positive potentials would not support these interpretations. 3.5. Calculation of Adsorption Energies. The SHINERS data presented suggest the coexistence of two intermediate EP species at Pt surfaces under hydrogenation conditions, namely the μ2(C,O)-bound adsorbate and the HHS, the relative coverages of which are highly surface structure dependent. Pristine Pt{111} and Pt{100} surfaces appear to strongly favor the μ2(C,O) species, while the HHS only reaches a measurable coverage on surfaces that are rich in lower coordination sites (i.e., electrochemically roughened surfaces, intrinsically defective Pt{110}, and kinked Pt{321} and Pt{721}). Polycrystalline

in contrast with equivalent calculations performed using acetone as a substrate which was shown to favor the η1(O) configuration,73 but substituent effects are not unexpected in the adsorption behavior of activated ketones.77 On the basis that the more stable μ2(C,O) EP adsorbate might be responsible for the unassigned Raman bands, further calculations were continued for this binding mode at Pt{111} in order to identify possible vibrational modes. Similar calculations were also performed at a Pt{221} surface, which we use in this work as a model stepped surface, as it consists of four-atom-wide {111} terraces separated by monatomic {110} steps (Figure S1 in the Supporting Information). These calculations revealed that the band observed at 965 cm−1 can be attributed to a methyl rocking mode of the μ2(C,O)-bound EP adsorbate (see section S6 in the Supporting Information). The band at 418 cm−1 was more challenging to identify unambiguously due to various coupled skeletal vibrations at low wavenumbers, but the DFT modeling did predict a band at 462 cm−1 for Pt{111} and 456 cm−1 for Pt{221} that can be likened to a Pt−O stretch. While absolute correlation between experimental and calculated spectra could not be achieved in this case, a discrepancy on the order of 40 cm−1 could easily be rationalized by a deficiency in the DFT model, which neglects to account for coadsorbed hydrogen, solvent effects, surface relaxation, or complex phenomena related to the plasmon enhancement of the SHINs. The interaction of the Pt-bound keto carbonyl oxygen atom with coadsorbed hydrogen is indeed quite poignant, as full H atom transfer to this oxygen atom would yield the HHS. Hence, it is reasonable to suggest that a Pt−H- - -OC interaction exists in the μ2(C,O)-bound EP, potentially as a precursor to full O−H bond formation, and that such an interaction could be strong enough to result in a red shift in the predicted Pt−O stretching frequency. A similar observation was reported by McBreen et al. for the CO stretch of methyl pyruvate on Pt{111} interacting with the H−C of coadsorbed aromatics.23 The behavior of the newly identified μ2(C,O) adsorbate was investigated further by monitoring the influence of electrode potential on the occurrence of these bands at a Pt{111} electrode. As shown in Figure 5a, the spectrum obtained for 0.1 M EP/0.1 M HClO4 at the hydrogen evolution potential of −0.1 V gives rise to the characteristic μ2(C,O) bands at 418 and

Figure 5. SHINERS spectra of 0.1 M EP adsorption in 0.1 M HClO4 on Pt{111} at different potentials (vs Pd/H): (a) − 0.1 V; (b) 0 V; (c) 0.1 V; (d) 0.2 V; (e) 0.3 V; (f) 0.4 V. 1828

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stepped Pt{221}, in keeping with its prevalence on pristine basal surfaces. Conversely, the binding energy of the HHS is a modest ∼8 kJ mol−1 higher on Pt{221} than on Pt{111}. Moreover, on stepped surfaces the HHS is much more stable than the μ2(C,O) adsorbate, while on terraces these two species are approximately equally favored. The fact that the HHS represents a thermodynamic well at Pt{221} is consistent with its observation as a long-lived intermediate in the SHINERS spectra measured at defective and kinked surfaces. However, given the similar adsorption energies for the two types of intermediates at terrace sites, one would expect to observe the HHS at Pt{111} and Pt{100}. The fact that the HHS is not observed experimentally on pristine basal surfaces can therefore only be rationalized by kinetics. We know from CV measurements (section 3.1) that hydrogenative desorption does occur at EP-modified Pt{111}; therefore, we propose that conversion between μ2(C,O) EP and HHS (directly or indirectly) is slow, while the second hydrogen transfer to the HHS to generate EL is relatively rapid. This is consistent with kinetic studies reported by Blaser and co-workers, who concluded that the first hydrogen transfer to EP is likely to be rate determining.78 As a result, on Pt{111} and Pt{100} surfaces the HHS coverage never reaches a measurable value, while the μ2(C,O)-bound EP species is allowed to accumulate. If the activation barrier for the first hydrogen atom transfer is lower at defect and step sites than at terraces (or conversely if that for the second hydrogen transfer is higher), this could allow the coverage of HHS to increase at the expense of μ2(C,O) EP such that both are observed in the SHINERS spectra at defective and kinked surfaces.

Pt presumably represents an extreme case where defect sites dominate over ordered terraces. DFT calculations of the thermodynamic stability of these species can help to rationalize the observed differences. Adsorption energies were calculated for μ2(C,O) EP, the HHS, and the product EL at the flat Pt{111} terraces and the model Pt{221} stepped surface. Calculated structures for μ2(C,O) EP and the HHS at stepped Pt{221} are shown in Figure 6.

Figure 6. Calculated structure of (a) EP adsorbed in the μ2(C,O) configuration and (b) HHS at Pt{221}.

The calculated adsorption energies are presented in Table 1 and represented schematically in Figure 7. While it remains to Table 1. Calculated Adsorption Energies for μ2(C,O)-Bound EP, the HHS, and the Ethyl Lactate Product on the Flat Pt{111} Surface and the Pt{221} Stepped Edgea adsorption energy/kJ mol−1

a

structure

Pt{111}

Pt{221}

μ2(C,O) ethyl pyruvate (μ2(C,O) EP) half-hydrogenated state (HHS) ethyl lactate (EL)

−136 −135 −104

−92 −143 −108

4. DISCUSSION It is clear that the adsorption behavior of EP on Pt surfaces under racemic hydrogenation conditions is sensitive to surface structure, with pristine basal surfaces kinetically favoring the accumulation of an μ2(C,O)-bound state and defective and kinked surfaces promoting the more thermodynamically stable HHS. If adsorption of the HHS is indeed more irreversible on defect sites, the issue of self-poisoning may play a role in the hydrogenation behavior of these surfaces, as proposed previously.49 The longevity of the HHS on defective and kinked surfaces would likely promote side reactions such as EP decarbonylation, which would explain the positive correlation between HHS and adsorbed CO Raman bands. Hence, the absence of any spectroscopic or electrochemical evidence of adsorbed CO during EP hydrogenation at the basal Pt{111} would then suggest that the HHS is too short-lived at this surface to enable appreciable decarbonylation to occur. It is interesting to note that, by comparison, Pt{100} appears to be a special case in terms of this decarbonylation side reaction. The CV data recorded at Pt{100} in the presence of EP indicate a very strong CO stripping peak at anodic potentials despite there being no spectroscopic evidence of adsorbed CO under hydrogen evolution conditions. Hence, although CO is clearly generated on Pt{100}, it must be efficiently removed from this surface under the sustained hydrogenative conditions employed for the SHINERS measurements. The fact that electrochemical roughening of this surface results in spectroscopic observation of adsorbed CO would suggest that at defect sites the rate of CO formation exceeds the rate of hydrogenative desorption. It is instructive to place our observations in the context of catalytic hydrogenation measurements at Pt nanoparticles. For example, on the basis of turnover frequency measurements at

Values are given to the nearest kJ mol−1.

Figure 7. Reaction scheme for EP hydrogenation at Pt{111} and Pt{221} based on calculated adsorption energies.

be seen if the μ2(C,O)-bound EP is a direct precursor to the HHS (further support for such a hypothesis would require rigorous simulation of potential surface reaction profiles), the mechanism of μ2(C,O) chemisorption, with concomitant CO rehybridization and bond elongation, followed by hydrogen transfer has been proposed for other activated ketones and is therefore not unreasonable.77 The DFT calculations show that the μ2(C,O) EP species adsorbs considerably more strongly on Pt{111} than on the 1829

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reconciliation of these apparently divergent interpretations might be that terraces facilitate binding both CD and the μ2(C,O) species in the correct orientation to favor high ee but that the chiral environment around the quinuclidine substituent vicinity of (e.g., overhanging) defect sites could provide an even greater chiral discrimination pathway. We note that the methyl group of the μ2(C,O) EP species sits in a slightly different position relative to the surface on terraces in comparison to steps (see Figures 4 and 6), which could aid in creating a chiral pocket. Such a center would actually be diastereomeric in nature, since defect sites such as kinks may be chiral and, combined with the quinuclidine of CD, would give rise to two chemically distinct docking positions for EP, namely +/+ and ±, where + signifies the handedness of CD and + or − the chirality of the kink site. This model would suggest that both terraces and defects are required for the highest levels of enantioselectivity.

shaped nanoparticles Baiker et al. concluded that racemic pyruvate hydrogenation on Pt is a structure-insensitive reaction.31 At first sight this might suggest that the different adsorption behaviors of EP observed in this work in the absence of CD have little impact on the mechanism or rate of hydrogenation. However, it is very important to note that the shaped nanoparticles used by Baiker and co-workers likely contained an appreciable number of defect sites, in comparison to the pristine basal single crystals used in this work. In this sense these shaped nanoparticles are more comparable to the electrochemically roughened basal crystals used here. Without performing operando measurement of turnover frequencies at pristine single crystals, it would therefore be difficult to assert with confidence if the complete absence of the HHS at pristine Pt{111} and Pt{100} surfaces is associated with an overall rate advantage. The discussion thus far has focused on the racemic hydrogenation of EP, but our observations may provide insights into the enantioselective reaction at chirally modified surfaces. SHINERS experiments revealed that the chiral modifier (CD) consistently competes for Pt surface sites and hence attenuates the μ2(C,O)-bound EP and the HHS signals. This, along with the concomitant decrease in adsorbed CO, is consistent with previous observations at polycrystalline Pt.49 Identifying the potential effect of this on the mechanism of rate acceleration in enantioselective hydrogenation, an issue that has been largely debated,79−81 remains challenging. However, we speculate that the different behaviors of terrace and step/defect sites play an important role. Enantiomeric induction at chirally modified Pt must occur before the HHS has formed because its orientation on the surface is already fixed by the Pt−C bond, which DFT calculations indicate is relatively strong. By the same logic, the μ2(C,O)-bound EP is itself adsorbed to the surface by its chiral center; thus again enantiodifferentiation is expected to take place before this adsorption state forms. However, if a dynamic equilibrium exists between (chiral) μ2(C,O)-bound and (prochiral) liquid-phase EP, we would expect reversible conversion between surface enantiomers. Given that the μ2(C,O) state is a longer-lived intermediate on terrace sites in comparison to steps and defects, we would therefore predict that terrace sites would be beneficial for enantioselectivity, since they favor formation of the most stable docking complex between EP and the chiral modifier. In section 3.2 we saw that the μ2(C,O) EP species had a measurably larger intensity at Pt{111} than at Pt{100}. Notwithstanding intrinsic differences in the Raman intensity at different crystal faces, which in this case should be minimal due to signal normalization relative to the perchlorate band, this suggests that the μ2(C,O) EP species is longer lived at Pt{111}; therefore, we would predict this surface to yield higher enantioselectivity when chirally modified. This is consistent with the shaped nanoparticle study cited above,31 which demonstrated that {111}-rich particles exhibit a higher ee and conversion rate than {100}-rich particles. The rationale offered by Baiker et al. was based on the adsorption energy of CD on these different facets, but we propose that this could also be affected by the lifetime of EP intermediates at different Pt sites as reported here. We note that, in previous studies by Attard et al.,22 the strategy of selectively decorating the defect sites of a supported 5% Pt on graphite catalyst demonstrated that the ee decreased if defect sites were blocked with Bi adatoms, which appears to contradict the findings of Baiker and co-workers.31 A possible

5. CONCLUSIONS We have demonstrated for the first time that the adsorption and hydrogenation behavior of EP at Pt surfaces under electrochemical conditions is structure sensitive. Using in situ electrochemical SHINERS measurements at Pt{hkl} surfaces, a distinct surface species has been identified that is assigned to an intact μ2 EP adsorbate, which we postulate exists as a precursor to the HHS observed previously. This μ2 EP species persists on basal Pt{111} and Pt{100} surfaces but is shorter-lived at defective and kinked surfaces due to the more favorable formation of the HHS. Unlike the HHS, the μ2 EP adsorbate can exist at potentials positive of the hydrogen evolution reaction and has been observed to change its molecular orientation according to the surface potential (and corresponding H coverage). At defective and kinked surfaces the longevity of the HHS results in the observation of adsorbed CO, a product of the known EP decarbonylation pathway. We propose that the observed differences in the relative population and lifetimes of the μ2 EP and HHS may play an important role in the enatioselective hydrogenation of EP at chirally modified Pt catalysts. We note that the electrochemical conditions employed in this work (i.e., aqueous solvent, applied electric field) differ from those typically employed in heterogeneous catalysis. While we cannot rule out the influence of these factors, it has been shown previously that the enantiomeric excess for the electrochemically induced Orito reaction on Pd is identical with that measured in the gas phase.67 Hence it is reasonable to assume that the hydrogenation reaction follows a similar mechanism under these conditions, suggesting that the mechanistic insights gleaned herein are transferrable to more classical catalytic conditions. In addition to progressing our understanding of EP hydrogenation, this work demonstrates the considerable value of the SHINERS technique in studies of catalysis. Identification of structure-sensitive phenomena is founded on interfacial measurements at single-crystal surfaces which until recently have not been possible using Raman spectroscopy. Our findings demonstrate that the powerful combination of SHINERS with in situ electrochemistry can give important mechanistic insights into catalytic reactions at the solid−liquid interface. 1830

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02872. Miller index planes, cyclic voltammetry of SHINs on Pt{hkl} single-crystal electrodes, Raman spectra of ethyl pyruvate and ethyl lactate, SER spectrum of EP hydrogenation on polycrystalline Pt, SHINERS spectrum of enol analogue, and DFT modeling (PDF) Key vibrational modes(AVI) Key vibrational modes(AVI)

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AUTHOR INFORMATION

Corresponding Author

*A.J.W.: e-mail, [email protected]; tel, +44 (0) 20 8943 6243. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank Peter Wells for helpful discussions throughout the preparation of the manuscript. We acknowledge financial support from the U.K. National Measurement System, part of the Department for Business, Innovation and Skills. O.D.-S. also thanks the EPSRC (grant number EP/L504749/1) for financial support. Via our membership of the U.K.’s HPC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202), this work made use of the facilities of HECToR and ARCHER. Computing resources were also provided by Advanced Research Computing at Cardiff (ARCCA) and the HPC-Wales supercomputer facilities.

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REFERENCES

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DOI: 10.1021/acscatal.5b02872 ACS Catal. 2016, 6, 1822−1832

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DOI: 10.1021/acscatal.5b02872 ACS Catal. 2016, 6, 1822−1832