Mechanistic Insights into Electro-Oxidation of Solution CO on the

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Mechanistic Insights into Electro-Oxidation of Solution CO on the Polycrystalline Gold Surface as Seen by in Situ IR Spectroscopy De-Jun Chen,† Thomas C. Allison,‡ and YuYe J. Tong*,† †

Department of Chemistry, Georgetown University, 37th & O Streets, NW, Washington, D.C. 20057, United States Chemical Informatics Research Group, National Institute of Standards and Technology, 100 Bureau Drive, Stop 8320, Gaithersburg, Maryland 20899, United States

‡

S Supporting Information *

ABSTRACT: Carbon monoxide (CO) adsorption and electro-oxidation on a gold (Au) ultrathin film deposited onto a silicon prism infrared (IR) window in a CO-saturated (≈1 mM) 0.1 M HClO4 supporting electrolyte were investigated by in situ electrochemical attenuated-total-reflection (ATR) surface-enhanced IR reflection absorption spectroscopy (SEIRAS). By varying the reaction environment with sequential (CO and N2) purging of the supporting electrolyte and the Au surface morphology with CO annealing, we were able to assign adsorbed CO to terrace-like and step-like sites to deconvolute the corresponding time- and potential-dependent IR spectra. The results of these spectral deconvolutions suggest strongly that in the CO-saturated supporting electrolyte the weakly bound CO interacted mainly with the strongly adsorbed CO on the step-like sites and likely formed a dipolar-coupled weak interacting pair with the latter. Model ab initio density functional theory (DFT) calculations confirm the existence of the weakly bound CO only over a CO monolayer adsorbed on the step-like Au sites. The weakly bound CO was also identified as the active reaction intermediate for CO oxidation reaction (COR) in the CO-saturated acidic supporting electrolyte and therefore was proposed to be largely responsible for the high COR activity frequently observed on Au electrodes.

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positions, respectively. The ∼2130 cm−1 band on the Au(110) surface and ≈1965 cm−1 band on the Au(111) surface disappeared completely under Ar purging, which was speculated due to either irreversible CO oxidation or/and reversible CO desorption.5 However, even though a CO self-promoted electro-oxidation mechanism in both alkaline8,10,11 and acidic11 media was proposed recently, much still remains to be learned. For instance, although it has been consistently observed that COR of solution CO is more active in alkaline than in acidic media,11,14 some studies showed that the irreversible CO adsorption was only observed in alkaline media,15,16 but others reported the observation of rather strongly adsorbed CO in acidic media that could survive inert gas purging.5,6 A discrepancy also exists in rotating disk electrode data of COR on Au(111), Au(100), and Au(110),5,11 which might have to do with the detailed difference in the Au surface structures studied.11 Fundamentally, the most intriguing question still is why the almost universally inert bulk Au surface can be so active toward catalyzing COR of solution CO electrochemically, particularly about the nature and role of the weakly adsorbed CO in the process. Herein we report in situ

INTRODUCTION Carbon monoxide (CO) oxidation reaction (COR) on an Au surface has been extensively investigated because of the exceptional activity observed on oxide-supported Au nanoparticles at the solid/gas interface1−3 and on Au electrodes at the solid/liquid interface,4−11 particularly in alkaline media.8,10 Consequently, a great deal has been learned about it. For instance, in the case of COR of solution CO (COsol), the socalled weakly bound CO species on the Au surface (COW) were identified by in situ IR spectroscopy in as early as 1982: IR bands at 2120 cm−1 (assigned as linear-bound CO) and at 1930 cm−1 (assigned as bridge-bound CO) were both observed on a Au surface as compared to 2080 and 1860 cm−1 on Pt and 2030 and 1900 cm−1 on Rh, respectively.12 Higher vibrational frequencies of the adsorbed CO on the Au imply a weaker CO−metal bond on it than on the Pt-group metals.13 In addition to confirming the existence of the weakly bound CO whose IR bands largely disappeared after a noble gas purging of the solution, more detailed studies later4−6,14 also identified strongly/irreversibly adsorbed CO species on Au that remained on the surface after the noble gas purging, although the detailed bonding geometry and CO−Au interaction depended very sensitively on the surface orientations.4,5 By an analogy to the Pt single-crystal surfaces, Markovic et al.5 assigned the ≈2130 cm−1 (only observed on an Au(110) surface), ≈2040 cm−1 (on all Au(111), Au(100), and Au(110) surfaces), and ≈1965 cm−1 (on the Au(111) and Au(110) surfaces) IR bands to CO adsorbed at terminal, 2-fold bridging, and 3-fold bridging © XXXX American Chemical Society

Special Issue: Kohei Uosaki Festschrift Received: January 2, 2016 Revised: April 15, 2016

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gauze with large surface area and an Ag/AgCl (3M, BASi) were used as counter and reference electrodes, respectively. All electrode potentials reported herein are referred to the latter if not indicated otherwise. All EC SEIRAS data acquisitions were carried out on a Bruker Vector-22 Fourier transform IR spectrometer equipped with a liquid-nitrogen-cooled mercury− cadmium−telluride (MCT) detector. A homemade EC-IR cell combined with a triangular Si prism and an optical reflection accessory (incident angle of >60° enabling total attenuation reflection) was used for the in situ SEIRAS measurements. The spectral resolution was set to 4 cm−1. The obtained spectra were shown in the absorbance units defined as −log(I/I0) where I and I0 are the singe-beam spectral intensities at the measuring and reference potential, respectively. The potential interval was 50 mV, and 100 interferograms were collected and added at each potential. For time-dependent gas switching experiments, the CO bubbling was on from 0 to 720 s, which was followed by N2 bubbling from 720 to 1800 s, while the electrode potential was held at −0.2 V. CO annealing of the Au film was achieved by the repetitive 100 potential cyclings at 50 mV/s between −0.2 and 1.4 V in a CO-saturated 0.1 M HClO4. The cyclic voltammograms (CVs) for the initial and annealed Au surface were recorded in an N2-saturated 0.1 M HClO4. For potentialdependent COR measurements, CO bubbling was on during the entire potential steps from −0.2 to 1.0 V with a potential step of 0.05 V. The CO stripping experiments of the strongly adsorbed CO were conducted in N2-saturated 0.1 M HClO4 after the CO adsorption on the Au surface at −0.2 V was carried out for 1200 s in a CO-saturated solution that was subsequently purged by ultrapure N2 for another 1200 s. Density Functional Theory (DFT) Calculations. Ab initio DFT calculations of CO binding energies were performed on Au(110) and Au(111) surfaces. The former was modeled as a 2 × 2 × 5 and the latter as a 2 × 3 × 5 unit cell (4 or 6 surface atoms, five layers thick). During optimization, the bottom two layers of the gold surface were held at their bulk positions (lattice constant = 4.08 Å). Calculations were carried out using Quantum Espresso.19 The PBE density functional theory method20 was used with the van_ak pseudopotential21 for Au, C, and O atoms. Planewave cutoff energies of 60 and 480 Ry were used for the kinetic energy cutoffs for the wave function and for the charge density and potential, respectively. The Methfessel−Paxton smearing method22 was used with a smearing parameter of 0.02 Ry. A (shifted) k-point mesh of dimension 6 × 4 × 1 was used for all calculations. Convergence was tested for all parameters given above, and the parameters used were found to produce well-converged energies (in the case of the wave function cutoff energy, relative energies, not absolute energies, were found to be converged). Convergence in the size of the unit cell was tested in the z direction to ensure that the interlayer spacing was sufficient. Geometry optimizations were considered converged when successive energies differed by less than 0.0001 Eh and all forces were smaller than 0.001 Eh in magnitude. All unique arrangements of n CO molecules on the 2 × 2 or 2 × 3 Au surface were examined to n = [1, 2, 3, 4] or [1, 2, ..., 6]. The total energies of the system were found to depend strongly on the arrangement of surface-bound CO molecules in some cases. Binding energies were computed as the difference between the energy of the Au surface with bound n CO molecules and the sum of the bare (2 × 2) or (2 × 3) surface energy and the

attenuated-total-reflection (ATR) surface-enhanced IR reflection absorption spectroscopic (SEIRAS) studies of the COR of solution CO in acidic electrolyte on a Au thin-film deposition on a silicon prism used as the IR window that address the latter issue. Specifically, we observed the experimental evidence that suggests a (likely dipolar) coupling between the weakly linearand bridge-bound CO (COWL and COWB, respectively) and their respective strongly linear- and bridge-bound counterparts on the step-like sites (COLS and COBS, respectively) that enabled the CO vibrations of the former to screen those of the latter, similar to what has been observed previously.17,18 The screened vibrations of the COLS and COBS would re-emerge as the COsol being purged by inert gas that led to the desorption of the COWL and COWB. The existence of such weakly bound CO is further supported by model ab initio density functional theory (DFT) calculations. Moreover, the potential-dependent ATR-SEIRAS spectra of the COR in the presence of COsol and CO stripping in the absence of COsol on the same surface were compared, which shows that it was the oxidation of the weakly bound CO that was mainly responsible for the high activity of COR on the Au surface. These new insights as revealed by our in situ ATR-SEIRAS studies help clarify further the fundamentally important role of the weakly bonded CO on Au.

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EXPERIMENTAL METHODS Chemicals. The electrochemical (EC) characterization of the Au film was carried out in 0.1 M HClO4 (GFS chemicals, 70%, Cl < 0.1 ppm) supporting electrolyte blanketed by ultrahigh purity N2 (GTS-Welco, 99.999%, THC < 0.5 ppm). Ultrahigh purity CO gas (GTS-Welco, 99.9%) was used to saturate the dissolved CO solution. All electrode potentials in this paper were cited with respect to the Ag/AgCl reference electrode. All solutions were prepared using 18.2 MΩ cm MilliQ water with ultralow organic impurity (TOC < 2 ppb). In Situ EC ATR-SEIRAS. The working electrode of an ultrathin Au film (≈100 nm thick) was deposited in an electroless fashion onto a well-polished (equal edge) triangular Si prism (20 mm × 25 mm) optical window for ATR-SEIRAS. The EDS measurement shows no other metallic impurity (inset in Figure S1a of the Supporting Information-SI). The Si prism was first polished with successively finer grade alumina slurries down to 0.3 μm and cleaned by sonication in the Milli-Q water, followed by immersing one of the square surfaces in 5% NH4F solution for several minutes to etch the surface. The etching process generated a hydrophobic surface. The Au precursor solution was made by dropwise adding 0.5 mL of NaAuCl4 (15 mM) solution into the solution mixture of 0.5 mL of Na2SO3 (150 mM), Na2S2O3 (50 mM), and NH4Cl (50 mM). The solution so made should be colorless. It was then mixed with 0.5 mL of HF (0.4 mM) solution and quickly injected onto the Si surface that was preheated to 60 °C. After 90 s of plating, the plated surface was washed with copious ultrapure water and airdried by Ar gas, which constituted the as-freshly prepared Au film. In order to obtain the homogeneous Au film reproducibly, fresh plating solution has to be used each time on a wellcleaned, hydrophobically etched Si surface with precise temperature control within ±1 °C accuracy. The size of nanoparticles and surface morphology of the Au film electrodes were examined by field emission scanning electron microscopy (FE-SEM) (Zeiss SUPRA55-VP, 20 kV, Carl Zeiss Inc., Germany) as shown in Figure S1. All EC experiments were carried out in a conventional threeelectrode EC-IR cell using a CHI-660D potentiostat. A Pt B

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The Journal of Physical Chemistry C energy of n CO molecules or as the difference between the energy of the Au surface with n bound CO molecules and the sum of the energy of the Au surface with n − 1 bound CO molecules and the energy of a single CO molecule. In the former method for determining the binding energy, the quantity was divided by n to obtain the mean binding energy per CO molecule.

Table 1. Peak Assignments for the Adsorbed CO Species on the Au Surfaces

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RESULTS AND DISCUSSION Weakly vs Strongly Adsorbed CO on Au. Figure 1a presents the time-dependent ATR-SEIRAS spectra in the

COL/COB on terrace-like or step-like

weakly or strongly

peak/cm−1

COWL (Weak Linear) COLT (Linear Terrace) COLS (Linear Step) COWB (Weak Bridge) COBT (Bridge Terrace) COBS (Bridge Step)

COW COS

≈2106 ≈2036 ≈2016 ≈1912 ≈1846 ≈1803

COW COS

intensities of different species (vide inf ra) as a function of time are presented in Figure 1c and 1d with vertical dashed lines indicating the time of switching from CO to N2 bubbling. As the gas bubbling was switched from CO to N2 and then over the entire N2 bubbling period, the amplitudes of the bands at 2036 and 1845 cm−1 remained largely unchanged, although the frequency of the former showed a slight red while that of the latter a larger blue shift at 1800 s (Figure 1a). This indicates that they belong to strongly adsorbed species, and we tentatively identify them as strongly linear- and multi (2 or 3)-fold bridge-bound CO and named them as COL and COB, respectively. Notice that 1845 cm−1 is a much lower frequency than that observed for strongly adsorbed low-frequency species reported previously,4−7 which is most likely due to the specific surface structure of the ultrathin Au film on the Si prism that consisted of fractally connected ≈46 nm Au nanoparticles (see Figure S1).23 In contrast, as can be seen in Figure 1a and 1c (black triangles), the band at 2107 cm−1 disappeared completely at 1800 s (also see Figure S2) when the COsol was removed by N2 purging starting at 720 s. This is consistent with the behavior of the weakly bound linear CO on Au reported previously.4−6 Therefore, this peak is assigned as COWL. Interestingly, the band at 1906 cm−1 (see the vertical dashed line in Figure 1b and cyan triangles in Figure 1d) behaved exactly like that of the COWL. It also disappeared completely at 1800 s when the COsol was purged away from the electrolyte which suggests that this CO species should also be classified as a weakly bound one, and we tentatively assign it as weakly multifold bridge-bound CO (COWB). All the peak assignments are summarized in Table 1. Notice that a similarly weakly bound CO species vibration at 1944−1886 cm−1 on a reconstructed Au(111) surface was observed previously and assigned to 3-fold bridging CO.5 By close inspection of the IR spectra (see Figure S2) one can observe that both bands of the strongly adsorbed CO, i.e., the COL and COB, show a discernible shoulder/tail at the lowfrequency side, while those of the weakly adsorbed CO are largely symmetric. After subjecting the freshly prepared Au surface to a CO annealing, i.e., 100 potential cycles between −0.2 and 1.4 V at 50 mV/s in CO-saturated supporting electrolyte, a process that is known to eliminate surface irregularities and therefore to smooth the nanoparticle surface,24 the CVs in Figure 2a confirmed an increase of the terrace-like sites and a concomitant decrease of step-like sites.25,26 This was accompanied by an increase in the band intensities of the main peaks and a decrease in the intensities of the shoulders of the strongly adsorbed CO as observed in Figure 2b. This correlation suggests strongly that the main peaks and the shoulders can be reasonably associated with the terrace- and step-like sites, respectively, which led us to deconvolute the IR bands of the strongly adsorbed CO in Figure 1a into two Gaussians as shown in Figure S2 and assign

Figure 1. Time-dependent in situ EC ATR-SEIRAS spectra of adsorbed CO in the spectral region of 1700 cm−1 to 2150 cm−1 (a) and of 1700 cm−1 to 1950 cm−1 (b). The peaks of linear-bound COL and bridge-bound COB were deconvoluted into two subpeaks, one assigned to CO on the terrace-like sites (COLT or COBT) and the other to CO on the step-like sites (COLS or COBS). See the main text for details. The integrated peak intensities of the linear-bound CO (COLT and COLS) and that of the weakly linear-bound COWL are plotted as a function of time in (c): red stars for COLT, blue diamonds for COLS, and black triangles for COWL, respectively. Those for the bridge-bound CO are shown in (d): green solids dots for COBT, violet diamonds for COBS, and cyan triangles for COWB, respectively. The error bars were estimated from the noise amplitude of each individual spectrum by the difference between the actual spectrum and the fitted one. The vertical dashed lines in (c) and (d) indicate the time of 720 s at which the gas purging of the supporting electrolyte was switched from CO to N2.

1700−2200 cm−1 region of the CO adsorption on the Au film at −0.2 V with CO (from 0 to 720 s) and N2 (from 720 to 1800 s) bubbling into the supporting 0.1 M HClO4 electrolyte, respectively. Figure 1b amplifies the spectral region of 1700− 1950 cm−1 to make the very weak but observable band at ≈1906 cm−1 visible. A total of four bands located at ca. 2107, 2036, 1906, and 1845 cm−1, respectively, were observed on the Au film in the presence of the saturated CO in solution (COsol). The corresponding integrated (including deconvoluted; see Table 1 for detailed peak assignments) band C

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slight red shift of the overall COL band in Figure 1a. On the other hand, that the peak positions of the deconvoluted subpeaks of the COL remained constant after at least 600 s (Figure S2) implies strongly an overall constant dipole−dipole interaction among them, thus a constant amount of the COLT and COLS, respectively, despite the increased amplitude of the subpeak assigned to the latter, which is consistent with the vibrational screening and unscreening hypothesis. On the other hand, the blue shift of the COB band appeared to be caused by the band narrowing toward the high-frequency end as shown in Figure S4. This could be rationalized by an improved dipolar− dipolar interaction among the same type of adsorbed CO molecules as they were unscreened from the interaction with the weakly adsorbed CO, which would narrow the corresponding IR band and shift its frequency to higher value. To ensure that the above-observed phenomena are real and reproducible, we repeated the same experiments on a different Au thin film that was deposited onto a different Si IR window, the results of which are presented in Figure S5. Despite having a clearly different surface structure as expected but also as indicated by the much larger COWL band as compared to that in Figure 1a, the exact same screening−unscreening phenomena were observed. Moreover, we also carried out preliminary model ab initio DFT calculations of CO adsorbed on Au(110) as a representative for the step-like sites and on Au(111) as a representative for the terrace-like sites to see if the theoretical calculations can help better understand the experimental observations. For CO on the Au(110) surface, DFT calculations showed that stable CO adsorption can exist for all coverages up to one full monolayer, but the CO binding energy decreases as the CO coverage increases. For our purpose here, three different surface geometric configurations of CO binding as shown in Figure S6 were studied: (a) one full monolayer of CO on Au(110), i.e., 100% coverage; (b) a second 1/4 monolayer of CO on the top of the first full monolayer CO that sits right above the (110) grooves (i.e., between CO rows of the first monolayer); and (3) the CO of the second 1/4 monolayer down to the (110) grooves in direct contact with Au. The CO binding energies calculated are −0.44 eV, − 0.018 eV, and 0.58 eV per CO molecule for the CO in the first monolayer, CO in the second 1/4 monolayer, and CO down to the (110) grooves, respectively. As negative and positive values signify stable and unstable bindings, respectively, these calculations indicate clearly that the CO in the first layer binds quite strongly to the Au surface, i.e., the strongly bound CO; the CO in the second layer has much weaker but stable binding to the first CO monolayer, i.e., the weakly bound CO; and the CO down to the (110) grooves does not have stable binding. For CO on Au(111), DFT calculations showed that no stable CO binding can exist for CO coverage >2/3. For the highest stable CO coverage (2/3), the configuration shown in Figure S7a gives the highest CO binding energy, which is −0.094 eV. For the CO in a second layer of a configuration shown in Figure S7b, the binding energy is 0.11 eV, which indicates an unstable CO adsorption. That is, no stable weakly bound CO can be formed on the Au(111) surface. Summarizing the above DFT calculations thence, the results suggest that (1) CO can form a stronger bond to step-like sites than to terrace-like sites and (2) CO can form a weak but stable link with the CO bound to the step-like sites but not with CO bound to the terrace-like sites. These are exactly what our experimental data analysis suggested.

Figure 2. (a) Cyclic voltammograms of the freshly prepared gold film surface (blue) and after being CO-annealed (red) in 0.1 M HClO4; the inset shows the change of Au oxidation CV currents of different surface morphologies. (b) The corresponding ATR-SEIRAS spectra for the adsorbed CO on the freshly prepared (blue) and CO-annealed (red) Au film with (the upper spectra) and without (the lower spectra) the presence of saturated COsol at 0 V, respectively.

the high-frequency bands to the CO adsorbed on the terracelike sites (COLT and COBT, respectively) and the low-frequency bands to the CO adsorbed on the step-like sites (COLS and COBS, respectively) for both linear- and multifold bridge-bound species. Such assignments are consistent with the observed structural sensitivity of CO stretching frequency on the Au surfaces.5,27 Moreover, the decrease of the IR band intensity of the weakly bound CO species after the CO annealing (by ≈60% for the COWL, but the bands for the COWB were too weak to make meaningful comparison) as shown in Figure 2b (see Figure S3 and Table S1 for the deconvolution results) suggests strongly that these weakly bound CO species were likely associated with the step-like sites as well. The results of the peak deconvolutions are shown in Figure 1c and 1d (see Table 1 for the peak assignments). The most interesting observation is that the decreases in the amounts of the weakly bound CO species, COWL (black triangles in Figure 1c) and COWB (cyan triangles in Figure 1d), after the gas bubbling was switched from CO to N2 were accompanied by concomitant increases in the amounts of the strongly bound CO species on the step-like sites, i.e., the COLS (blue diamonds in Figure 1c) and COBS (purple diamonds in Figure 1d), while the amounts of the strongly bound CO species on the terrace-like sites (red stars in Figure 1c for the COLT and green diamonds for the COBT in Figure 1d) remained largely invariant. These correlations led us to the conjecture that the weakly bound CO interacted predominantly with the strongly bound CO on the step-like sites to form pairs of interacting CO molecules that had different vibrational frequencies. Notice that the SEIRAS spectrum at 30 s in Figure S2 indicates that the COLS formed first during the CO adsorption process and the intrinsically more open structure of a step-like site could also more easily accommodate an additional weakly bound CO than a terracelike site could. These pairs of interacting CO molecules satisfied the conditions that enabled the vibrational screening18 and/or intensity transfer17 between two coupled CO molecules of different vibrational frequencies to take place, and the CO of higher vibrational frequency (the weakly bound CO here) had its intensity enhanced and that of the lower frequency screened. As the COsol-containing solution was being purged by N2, the weakly bound CO would desorb so that the previously screened COLS or COBS would become unscreened and therefore IR visible, which would account for the increases in the intensities of the strongly bound CO species on the steplike sites as observed in Figure 1. The latter led in turn to the D

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saturated COsol (≈1 mM). As the COsol was being purged away by N2, the desorbing weakly bound CO would free the space for hydrogen-bound water to interact with the just unscreened strongly bound CO as illustrated by the schematic inset in Figure 3b, which could account for the appearance of the water band at ≈3215 cm−1. CO Electro-Oxidation on Au. Figure 4 presents the electrode potential-dependent SEIRAS spectra of COR on the Au film in the COsol-saturated supporting electrolyte (a) and the stripping of the strongly adsorbed CO in the COsol-free supporting electrolyte (b). As expected, the spectra in Figure 4a show the presence of both the weakly and strongly bound linear- and bridge-CO, while the spectra in Figure 4b show only the presence of the strongly bound linear- and bridge-CO. We deconvoluted, as done above, the IR bands of the strongly bound CO species into two sub-bands of the CO adsorbed on the terrace- and step-like sites, respectively (see Figure S8−S10 for the deconvoluted spectra). The potential-dependent integrated band intensities and peak positions so obtained are shown in Figure 5 for the linear-bound CO and in Figure 6 for the multifold bridge-bound CO. The dashed black curves are the CV currents of the COR (Figure S11). As can be seen in Figure 5a, the adsorption of the COWL (red triangles) was increased initially from −0.2 to 0.0 V before the onset of the COR in the presence of saturated COsol (≈1 mM) at ≈0.15 V (see Figure S11, also indicated by the leftmost vertical dashed line). The increase in COWL should lead to an increased vibrational screening of the COLS if the coupled COWL−COLS pair hypothesis is right (vide supra). Indeed, there was the concomitant decrease of the band intensity of the COLS (blue diamonds) until ≈0.15 V. As to the ensuing decrease of the COWL before the onset of the COR at ≈0.15 V, we would speculate that a displacement of the COWL by certain OHcontaining species, such as HClO4·5.5H2O clathrate(I)33 or/ and CO-adsorption-promoted OH adsorption,8,11 might have taken place. One piece of corroborating evidence is that these species would have relatively large polarizability so they could sustain further vibrational screening34 as implied by the continuous decrease of the band intensity of the COLS (blue circles in Figure 5a). Once the COR started at ≈0.15 V, the COWL and the OHcontaining species would be consumed assuming the former was the active reaction intermediate of the COR of COsol, i.e., COsol ⇆ COWL and COWL + OHads → CO2 + H+ + e−. The latter would lead to the unscreening of the previously screened

The time-dependent (as in Figure 1) IR spectra of the nearsurface water stretching bands presented in Figure 3 offer

Figure 3. (a) Time-dependent in situ EC ATR-SEIRAS spectra of the surface water in the 2800−3800 cm−1 spectral region on the Au film at −0.2 V as the supporting electrolyte was purged with CO (0−720 s) and N2 (720−1800 s). (b) The comparison of water bands at 30, 600, 990, and 1800 s, respectively; the inset is a schematic illustration of the interaction between the COLS and hydrogen-bound water after the desorption of the COWL.

further support to the formation of the weakly vs strongly adsorbed CO molecular pairs. Previous studies28−32 have identified three types of water at a solid−electrolyte interface: strongly hydrogen-bonded ice-like water (H2Oice‑like), disordered weakly hydrogen-bonded water (H2Oh), and isolated non-hydrogen-bonded free water (H2Oi) at ≈3000, ≈3400, and ≈3600 cm−1, respectively. As can be seen in Figure 3a and in agreement with the previous observations,28,29 the isolated water free of hydrogen bonding (H2Oi) at ≈3630 cm−1 was produced as CO was adsorbed onto the Au surface. The broad negative-going band at ≈3250 cm−1 could be reasonably assigned to the disordered weakly hydrogen-bonded water (H2Oh)32 and rationalized as a displacement of the near-surface H2O h by the adsorbed CO and near-surface CO sol.31 Interestingly, a positive-going band showed up at ≈3215 cm−1 gradually as the COsol was being purged away, as evidenced by the circled peak in the pink spectrum in Figure 3b. This is indicative of the reappearance of certain hydrogenbonded water and could be rationalized by the formation of the pairs of the weakly−strongly bound CO in the presence of

Figure 4. Potential-dependent in situ EC ATR-SEIRAS spectra (in the spectral region of 1700−2200 cm−1) of the adsorbed CO during the COR in the COsol-saturated supporting electrolyte (a) and in the COads-free electrolyte (0.1 M HClO4), i.e., the CO stripping reaction (b). E

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and therefore most responsible for the observed high COR activity on Au, at least in acidic media. What is somewhat puzzling, however, is that as the amount of the COWL continued to decrease and eventually become zero at ≈0.65 V (indicated by the right most vertical dashed line) at which the COR of the COsol reached the diffusion limit, that of the COLS remained almost constant until being oxidized beyond 0.65 V instead of increasing as expected by the vibrational unscreening process. We speculate that this might be because the rate of the COR became too fast to enable the pair of COWL and COLS to establish the necessary coherence needed for the vibrational screening. As to the COLT (linear CO bound to terrace-like sites), its initial band intensity increase up to 0.15 V could be simply rationalized by the electrode-potential-enhanced adsorption. After 0.15 V, the amount of the COLT remained largely constant until being oxidized beyond 0.65 V. The latter, together with the oxidation of the COLS, formed the sharp current peak at 0.78 V. It is also indicated (Figure 5c) that there existed a transition potential region for the Stark tuning effect, i.e., the electrodepotential-induced IR peak position shift, for both the COLS and COLT between 0.15 V, the onset potential of the COR of the saturated COsol, and 0.3 V, the onset potential for the COR acceleration. The transition potential region separates two distinguishably different adsorption regimes for the strongly adsorbed CO: the regime dominated by the vibrational screening interaction vs that dominated by the fast COR. However, this transition could also simply be an effect of the Au surface reconstruction/recovery,5,11,14 but we think that this may not be likely as no such transition was observed for CO stripping (Figure 5d). On the other hand, the single Starktuning rate observed for the COWL (red triangles in Figure 5c), even as it was being oxidized, suggests strongly that the pair interaction between a COWL and COLS was likely rather isolated (i.e., dipole−dipole interaction among COWL would be negligible), and the COWL was also likely not in direct contact with the Au surface as the DFT calculations alluded to. For the CO stripping of the strongly bound CO in the COsolfree supporting electrolyte (Figure 5b), the results of the peak deconvolution suggest that the observed large decrease in its band intensity even before the onset potential (≈0.55 V as indicated by the vertical dashed line) of the COR of the COL (black diamonds in Figure 5b) was mainly caused by the decrease in the band intensity of the COLS, i.e., CO bound to the step-like sites (blue diamonds). As there was no Faradaic reaction current associated with such decrease, it could only be rationalized by a physical displacement of the adsorbed CO by some other species, likely OH-containing species8,11,33 as enhanced by CO adsorption,8,11 which is consistent with the above discussion in which the special role of the step-like sites in defining the COR activity on Au was unraveled. On the other hand, the amount of the COLT was largely constant before being oxidized beyond 0.55 V. That single Stark tuning rates were observed for both the COLS and COLT (blue diamonds and pink stars, respectively, in Figure 5d) until they were oxidized beyond 0.55 V indicates that each had experienced similar chemistry, which was different from that in the presence of COsol (Figure 5c). Notice that the onset potential and the COR peak potential for the stripping of the strongly adsorbed CO were both about 0.1 V more negative than those of COR in the presence of the saturated COsol (Figure 5a), which is somewhat counterintuitive. This most likely has to do with that in the latter case,

Figure 5. In situ EC ATR-SEIRAS peaks of the strongly linear- and bridge-bound CO in Figure 4 were deconvoluted into two subpeaks (see Figures S8−S10) that were assigned to CO on the terrace-like and on step-like sites, respectively, as done above. The integrated peak intensities and peak positions of the different linear-bound CO species during the COR in the COsol-saturated supporting electrolyte are plotted as a function of the electrode potential in (a) and (c), respectively. The leftmost, middle, and rightmost vertical dashed lines indicate the onset potentials of the COR, the precipitous acceleration of the COR, and the COR of the strongly adsorbed CO species. The counterparts for CO stripping in the COsol-free supporting electrolyte are plotted in (b) and (d) with the vertical dashed line indicating the onset potential of the CO stripping reaction.

Figure 6. Same plots as those in Figure 5 but for the bridge-bound CO.

COLS, thus an increase in its band intensity would be expected. Again, this was what happened between 0.15 and 0.3 V (blue circles in Figure 5a). After 0.3 V (indicated by the middle vertical dashed line), the COR current rose precipitously, indicating a much accelerated COR rate. Yet, the most salient observation here is the unique direct correlation between the rising of the COR current (dashed black curve) and the decrease of the COWL band intensity (red triangles) in Figure 5a. This direct correlation suggests strongly that the COWL is most likely the active reaction intermediate for COR of COsol F

DOI: 10.1021/acs.jpcc.6b00024 J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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ACKNOWLEDGMENTS This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences Catalysis program under Award Number DE-FG0207ER15895. DJC also thanks the financial support from the Georgetown College.

where the oxidation of the strongly adsorbed CO had to compete with the fast oxidation of COsol for the OH-containing species. The bridge-bound CO (Figure 6) behaved quite similarly to what has been described for the linear-bound CO except that the bridge-bound CO species usually had larger Stark tuning rates and there was no large change in the amount of strongly adsorbed CO during the CO stripping in the COsol-free supporting electrolyte (Figure 6b). In other words, step-like bridge sites did not interact strongly with the OH-containing species. Also, the amount of the strongly adsorbed CO in the presence of the saturated COsol was about 60% less than that in the COsol-free supporting electrolyte, which indicates that the formation of the interacting COWL−COLS and COWB−COBS pairs suppressed the adsorption of the strongly bound CO species.

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CONCLUSIONS Built upon the previous work that had investigated COR on Au electrodes,4−8,11,12 the in situ EC ATR-SEIRAS studies of the COR on a Au thin film deposited on a silicon prism IR window reported herein have provided further mechanistic insights into the critical role of the weakly bound CO in determining the unusually high COR activity of a Au electrode. Specifically, in addition to confirm the previously observed weakly and strongly bound CO adsorbed on Au surfaces, we have presented both experimental and theoretical evidence that strongly suggests that (1) the weakly bound CO interacted mainly with the strongly bound CO on the step-like sites and formed reversible coupled pairs with the latter in the presence of the saturated COsol and (2) such weakly bound CO was the very active reaction intermediate responsible for high COR activity generally observed on Au electrodes, at least in acidic media. That is, COsol ⇆ COW@step-like sites and COW@steplike sites + OH@step-like sites → CO2 + H+ + e− where the adsorption of OH promoted by the CO adsorption as hypothesized in the CO self-promotion model8,11 could also be operational to enhancing the COR activity. It is expected that these new mechanistic insights will help stimulate further fundamental studies on electrochemical COR on noble metal surfaces in general and on the Au surface in particular. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b00024. SEM images, deconvoluted IR spectra, Table, model surfaces for DFT calculations, and cyclic voltammograms (PDF)

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*E-mail: [email protected]. Tel.: 202 6875872. Notes

Certain commercial equipment, instruments, or materials are identified in this paper. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the products identified are necessarily the best available for the purpose. The authors declare no competing financial interest. G

DOI: 10.1021/acs.jpcc.6b00024 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.6b00024 J. Phys. Chem. C XXXX, XXX, XXX−XXX