Surface (Electro)chemistry of CO2 on Pt Surface: An in Situ Surface

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Surface (Electro)Chemistry of CO on Pt Surface: An In Situ Surface-Enhanced Infrared Absorption Spectroscopy Study Yu Katayama, Livia Giordano, Reshma R. Rao, Jonathan Hwang, Hiroki Muroyama, Toshiaki Matsui, Koichi Eguchi, and Yang Shao-Horn J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03556 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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The Journal of Physical Chemistry

Surface (Electro)chemistry of CO2 on Pt surface: an In Situ Surface-Enhanced Infrared Absorption Spectroscopy Study

Yu Katayama1, 4, 5*, Livia Giordano3, Reshma R. Rao3, Jonathan Hwang2, Hiroki Muroyama4, Toshiaki Matsui4, Koichi Eguchi4, and Yang Shao-Horn1, 2, 3*

1

2

3

The Research Laboratory of Electronics,

Department of Materials Science and Engineering,

Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

4

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan

5

Department of Applied Chemistry, Graduate School of Sciences and Technology for Innovation, Yamaguchi University, Yamaguchi 755-8611, Japan

AUTHOR INFORMATION Corresponding Author * Yu Katayama ([email protected]) * Yang Shao-Horn ([email protected])

ACS Paragon Plus Environment

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ABSTRACT

Understanding the surface (electro)chemistry of CO2 and CO on Pt is needed to design active, selective catalysts for CO-tolerant fuel cell reactions and CO2 reduction. In this work, the surface reactivity of Pt in a CO2-saturated alkaline electrolyte was revealed by combining in situ surfaceenhanced infrared absorption spectroscopy (SEIRAS) with density functional theory (DFT) calculations. We show that during potential cycling in 1 M KHCO3 electrolyte, CO adsorbates (COad), more specifically, COad surrounded by OH adsorbates (OHad), with linear or bridged configuration, were produced through the reductive adsorption of HCO3– catalyzed by H adsorbates on Pt. These COad co-adsorbed with OHad was oxidized to COOHad at potentials as low as ~0.3 VRHE, which was further oxidized to CO2 at 0.9 VRHE and higher. Further analysis suggests that the proximity between COad and OHad is key to trigger the conversion reaction from COad to CO2 through forming COOHad intermediate at room temperature. The details about how Pt surface adsorbates change as a function of voltage in CO2-saturated alkaline electrolytes can provide strategies to design CO-tolerant catalysts for fuel cell applications and active and selective catalysts for the CO2 reduction reaction.


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TEXT 1. Introduction Platinum-based catalysts have been widely studied due to their technological importance for catalyzing a wide number of reactions including oxidation of small molecules such as hydrogen1 water 2, and methanol 3 as well as reduction of water1 (to make hydrogen), oxygen4 and CO2 5. The high catalytic activities of Pt stems from the facts that it can bind with reaction intermediates such as H6, O7, as well as OH7 strongly but not too strongly due to its unique electronic structure 8

, which dictate the energetic barriers for rate-limiting steps of reactions. For example, Pt can

bind atomic hydrogen as strongly as hydrogen bound in a hydrogen molecule9 10, which renders negligible energetic barriers oxidation/evolution reactions10

and then exhibits the highest activity for hydrogen 11

. On the other hand, it binds carbon monoxide (CO) more

strongly12 and this strong binding with CO can lead to Pt poisoning at room temperature and immediate deactivation of Pt-based catalysts in electrochemical processes including CO2 electrolyzers

13

and polymer electrolyte fuel cells (PEFCs) under the presence of trace amounts

of CO14. Alloying Pt with oxophilic metals such as Ru15 can greatly enhance the catalytic activity for CO oxidation, where the onset potential of CO electro-oxidation can be lowered from 0.6 V versus reversible hydrogen electrode (hereafter VRHE) to 0.35 VRHE

16 17

. This enhancement can

be attributed to the availability of OHad on Ru at low potentials, which can react with COad on Pt and oxidize to CO2

15 18

. Such mechanism to catalyze CO oxidation on Pt-based surfaces is

limited by hydroxide adsorption that provides reactive hydroxide adspecies16 17. Therefore, it is critical to understand and control the adsorption of oxy species on the surface that can compete with COad on Pt, at potentials lower than the onset potentials of CO oxidation 17.


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Unfortunately it is challenging to probe the interaction between CO/CO2 and the Pt surface in the presence of co-adsorbed species as introduction of even small quantities of CO in the KOH solution results in poisoning all active sites allowed at low potentials16 form adsorbed OH

20

19

12

, where simultaneous OH/H2O adsorption is not

. Increasing potential to ~0.6 VRHE results in water dissociation to

, which greatly enhance the kinetics of CO electro-oxidation17

20

. We

applied a recently reported method to form CO adsorbates on Pt in CO2-containing basic solutions (CO2 concentration of 300 ppm) at low potentials (< 0.5 VRHE) without introducing CO gas21. This method enables us to probe the interaction between CO/CO2 and the Pt surface in the presence of co-adsorbed species, which is critical to reveal the surface reaction mechanism for both electrocatalytic and catalytic reactions involving CO and CO2. Fundamental understanding of these reaction mechanisms have a significant influence on catalyst design for different electrochemical applications not only CO-tolerant catalyst for fuel cells and electrolyzers but also highly selective catalyst for CO2 reduction reaction22. In this study, the fundamental understanding of surface (electro)chemistry on Pt in KHCO3 solution is explored by tracking the surface intermediates using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) coupled with density functional theory (DFT) calculations. We propose that during potential cycling in 1 M KHCO3 electrolyte, HCO3– in the electrolyte is first reduced by underpotentially deposited hydrogen (Hupd) to form CO adsorbates (COad) coadsorbed with OH adsorbates (OHad). The proximity between COad and OHad is key to convert COad to COOH adsorbates, followed by the deprotonation reaction to produce CO2. The reaction mechanism as well as the reaction potential for this conversion from COad to CO2 is markedly different from conventional electrochemical pathway, suggesting this reaction could be a viable pathway for low-potential CO oxidation.


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2. Experimental 2.1. Pt Electrode Preparation For in situ SEIRA measurements, the Pt working electrode was composed of a thin (ca. 50 nm) Pt film deposited on a total reflecting plane of a hemispherical Si ATR prism (radius 22 mm, Pier optics) by an electroless deposition method23. First, the base plane of the Si prism was given the hydrophilic treatment by contacting with 40% NH4F solution for a minute. Then palladium seeds were deposited on the base plane with 1% HF–1 mM PdCl2 for 5 min at room temperature. After rinsing with water, platinum electroless deposition was carried out by contacting with the Pt plating solution at 50 °C for ca. 12 minutes. The Pt plating solution was prepared by mixing LECTROLESS Pt 100 basic solution (30 mL, Electroplating Engineering of Japan Ltd), LECTROLESS Pt 100 reducing solution (0.6 mL), 28% NH3 solution, and ultrapure water. Osawa et al. reported the morphology of a chemically deposited Pt electrode fabricated in a similar way, which has characteristic surface structure required to produce the surface enhancement effect24.

2.2. In situ SEIRA measurements A Pt-deposited Si prism was used for in situ SEIRA measurements and details of in situ SEIRA technique are described elsewhere25

26 27 28

. The prism was mounted in a spectro-

electrochemical three-electrode cell with an Ag/AgCl reference electrode and a platinum wire counter electrode. The SEIRA spectra were obtained using a FT-IR Vertex 70 (Bruker) equipped with an HgCdTe (MCT) detector. The optical path was fully replaced with N2 gas. The measurements were performed with 4 cm–1 resolution in the 3800–900 cm–1 spectral range; for a


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total of 32 times, unless otherwise noted. The SEIRA spectra were recorded using a single reflection ATR accessory (Pike Vee-Max II, Pike Technologies) with a Si ATR prism at an incident angle of 70 degree. In order to understand the physical origin of observed IR spectra, the strong band at 1400-1600 cm-1 was deconvoluted into several Voigt peaks with a linear background using OMNIC version 8.2 software. Spectra were fitted with full width at halfmaximum (FWHM) for each component constrained and the peak wavenumber allowed to vary within pre-defined limits. The electrolyte solution was prepared by mixing KOH (Sigma–Aldrich, >85 wt%) with KHCO3 (Sigma–Aldrich, 99.7 wt%) or K2CO3 (Sigma–Aldrich, 99.99 wt%) and ultrapure water (Milli-Q, 18.2 MΩ cm). For deuterium-substituted experiments, 40 wt% KOD in D2O (Aldrich, 98 atom% D) and D2O (Aldrich, 99.9 atom% D) was used. Before every experiment, Argon was bubbled through the electrolyte for 15 minutes in order to remove air from the solution. CO and CO2 were bubbled through the electrolyte for 2 hours before every experiment to saturate the solution with CO and CO2, and during the experiments the corresponding gas was kept flowing above the solution. After deoxygenation of the electrolyte solution by purging Ar, the prism surface was cleaned by cycling the potential between 0.05 and 0.90 V versus the reversible hydrogen electrode (RHE), unless otherwise noted. For electrochemical measurement, linear sweep voltammetry (LSV) was conducted at room temperature by using a HSV-110 potentiostat (Hokuto Denko). All potentials here on are reported versus RHE, unless otherwise noted (expressed as VRHE). The electrochemical surface area (ECSA) for Pt-based electrocatalyst was calculated from cyclic voltammograms recorded in the Ar-purged KOH electrolyte by integrating the charge in the hydrogen adsorption/desorption region, in the range of 0.05–0.45 VRHE. The current density was obtained by normalizing the current to the ECSA (expressed as


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µA cm–2). All spectra are shown in the absorbance units defined as log(I0/I), where I0 and I represent the spectra at reference and sample potentials, respectively. The reference spectrum I0 was measured at 0.05 VRHE in the blank KOH solution.

2.3. Computational methods The surface species adsorbed on the surface were studied by Density Functional Theory (DFT), with the PBE functional code.

30 31

29

as implemented in the Vienna Ab Initio Simulation Package (VASP)

Projector Augmented Wave (PAW) method

32

was employed and the plane wave

cutoff was set to 500 eV. A (3×3) supercell was used to model the Pt(111) surface with computed Pt lattice parameter of 3.975 Å. The slabs where separated by 15 Å vacuum and the dipole correction was applied. The reciprocal space was sampled with a (3×3×1) MonkhorstPack mesh. The coordinates of adsorbate and the surface Pt atoms where allowed to relax until the forces were below 0.01 eV/ Å, whereas the two bottom Pt layers were fixed at the bulk positions. The vibrational frequencies were computed within the harmonic approximation.


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3. Results and discussion 3.1. Reduction of bicarbonate ions to form COad co-adsorbed with OHad on Pt in 1 M KHCO3

Figure 1. In situ ATR-SEIRA measurement on a Pt electrode in 1 M KHCO3 (pH=6.8). (a) Linear sweep voltammogram in 1 M KHCO3 (orange) at scan rate of 10 mV s–1. Linear sweep voltammetry in 1 M KOH (blue) is also shown for comparison. In situ ATR-SEIRA spectra of (b) C≡O stretching region and (c) O-H librational region obtained during linear sweep voltammetry in a potential window from 0.05


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VRHE to 0.9 VRHE in 1 M KHCO3. (d) In situ ATR-SEIRA spectra of O-H librational region obtained during linear sweep voltammetry from 0.05 VRHE to 0.9 VRHE in CO-saturated 1 M KOH is shown for comparison. Cumulative number of (b) 37 and (c), (d) 64 was used at a 4 cm–1 resolution. Spectra were subtracted with respect to a reference spectrum obtained at 0.05 VRHE in 1 M KOH.

A linear sweep voltammogram (LSV) scanned from 0.05 to 0.9 VRHE and SEIRA spectra collected simultaneously from a Pt surface in 1 M KHCO3 (pH =6.8) are shown in Figure 1a and Figure 1b, respectively. Two bands at 1950–2000 cm–1 and 1755–1800 cm–1 (Figure 1b) were found to grow from 0.05 to 0.5 VRHE, which gradually reduced at potentials higher than 0.6 VRHE and completely disappeared at 0.9 VRHE. These two features cannot be assigned to the C=O stretching and/or C-O stretching mode of CO32– and HCO3– from the electrolyte (1600 cm–1 and below33), or the asymmetric O-C-O stretching mode of dissolved CO2 (ca. 2400 cm–1).33 As these two bands have similar wavenumbers to those found in CO-saturated KOH,34 35 they were assigned to the characteristic C≡O stretching mode of COad with linear and bridge configuration, respectively. The presence of COad on the Pt surface in the 1 M KHCO3 electrolyte is in agreement with smaller currents associated with desorption of underpotential deposited hydrogen atoms (Hupd) in the LSV in comparison to that collected in 1 M KOH shown in Figure 1a. We propose that bicarbonate/carbonate ions can be reduced by Hupd on the Pt surface to form COad in the range of 0.05 to 0.3 VRHE under neutral and alkaline conditions with similar pathway suggested in acidic conditions

36 37 38

, which is supported by the following arguments. First, no

signals of bicarbonate and/or carbonate adsorbates were detected before and after the formation of the COad (Figure S1). Second, the bicarbonate/carbonate ions (HCO3–/ CO32–) in the electrolyte39, having three oxygen atoms and one carbon atom per molecule, can give rise to one COad together with oxygen-containing species such as OHad (i.e. HCO3– + H2O => COad + 2OHad


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+ OH–). The hypothesis is supported by the detection of a librational Pt-OHad band at 1100–1175 cm–1

40 41

at potentials as low as 0.2 VRHE in 1 M KHCO3 (Figure 1c), which remained until 0.9

VRHE. In contrast, the detection of such Pt-OHad bands in both 1 M KOH42 40 43 and CO-saturated 1 M KOH (Figure 1d) was only possible at voltages greater than ~0.7 VRHE, where OH adsorption on Pt is known to accelerate4. Recently Niet et al. proposed a new model for water dissociation on Pt, suggesting the peak in LSV (Figure 1a) at so-called “hydrogen adsorption/desorption region (0.05–0.35 VRHE)” is not due to just adsorption/desorption of hydrogen, but to the replacement of H with O and/or OH 44. In our SEIRA spectra, however, we could not observe OHad band at such a low potential, possibly due to the detection limit of the measurement. Therefore, the OHad detected at low voltages such as 0.2 VRHE in 1 M KHCO3 cannot be attributed to OH adsorption associated with water dissociation on Pt but can be explained by the stabilization of OHad through attractive interactions between COad and OHad.45


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Figure 2. Potential dependence of the C≡O stretching wavenumber of (a) linear and (b) bridged COad species observed on Pt electrode under 1 M KHCO3 (orange line), CO-saturated 1 M KOH (blue line), and CO-saturated 1 M HClO4 (red line), with potentials scaled to standard hydrogen electrode (SHE). All of these measurements were performed using the same Pt electrode to avoid the effect of surface structures on the wavenumber of COad species.

To support the presence of COad and OHad interactions on the Pt surface in 1 M KHCO3, the COad band wavenumber and its potential dependence in 1 M KHCO3 was compared with that obtained in CO-saturated 1 M KOH (pH = 13.7) and 1 M HClO4 (pH = 0.1) (Figure 2). The linear mode COad band at >2000 cm-1 were found to exhibit a potential dependence of~50 cm–1 V–1 in Figure 2a, known as the Stark tuning slope46 47 48 while the bridge mode COad band at ca. 1850 cm-1 was shown to have the Stark tuning slope of ~30 cm–1 V–1 in Figure 2b. The Stark shift is caused by electrostatic interactions of the dipole moment of the adsorbates with the electric field between the charged surface and the outer Helmholtz plane

49

. When the electric


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field become larger at higher potentials, the interaction with COad become stronger and the bond distance between the oxygen atom and carbon atom is shortened 50. The shortening of C≡O bond distance causes blueshift of the band assigned to the C≡O stretching mode. In addition to the effect of electric field, other factors, such as CO adlayer compression/dissipation

48

and/or

difference in coverage51 might be the reason for the difference in the slope of linear and bridged COad. Experimentally, a large redshift was observed for COad in CO-saturated KOH at ~0.6 VRHE (~0.21 VSHE), which can be attributed to the decreased dipole-dipole interaction52

53

associated

with reduction in the COad coverage associated with CO electro-oxidation reaction54. Such COcoverage-dependent redshift was not visible for 1 M KHCO3 and CO-saturated 1 M HClO4 in Figure 2 as the potentials shown were not high enough for CO electro-oxidation. Moreover, the COad bands detected in KHCO3 were found to be consistently lower than those for CO-saturated 1 M KOH and 1 M HClO4 as well as reported values in CO-saturated KOH55, by ~40 and ~100 cm-1 for the linear and bridged COad at potentials lower than ~0.6 VRHE, respectively. Furthermore, the wavenumber for COad in KHCO3 is still lower than redshifted COad bands induced by reduced COad coverage at potentials greater than 0.6 VRHE in CO-saturated KOH. Therefore, the lower wavenumber of COad in KHCO3 cannot be accounted for the Stark effect associated with the potential-dependent electric field nor by the dipole-dipole interaction associated with COad coverage52 53. Therefore, it is proposed that the unusually low wavenumber for the COad bands found in KHCO3 can be attributed to the interactions with other adsorbates on the Pt surface such as OHad. This hypothesis is supported by previous findings that co-adsorbed H2Oad/OHad species cause the redshift of C≡O stretching mode of COad band, which include contributions from the dipolar field of the OHad56, nucleophilic interactions with H2Oad/OHad on the carbon of COad 57, and greater electron donors associated with lowered work function by


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H2Oad/OHad58 59. Further support came from calculated wavenumbers for C≡O stretching of COad on Pt(111) by DFT, which revealed that the addition of two OHad on neighboring site(s) could lower the wavenumber by 85 cm–1 (Table S1). Therefore, the COad band found in the KHCO3 electrolyte can be attributed to the COad co-adsorbed with OHad species. Such species was not observed in CO-saturated KOH or HClO4 electrolytes as Pt surface was poisoned by COad, limiting the number of vacant sites for OH adsorption. These results show that CO2 in alkaline electrolytes can interact with the Pt surface via carbonates and/or bicarbonate ions, which can dissociate to COad. and OHad. In situ ATR-SEIRA measurement performed in 1 M K2CO3 further revealed that CO32– might not be reduced to generate COad on the Pt surface. The K2CO3 electrolyte (pH = 11.5) has carbonate as the major anions (CO32– > 95%, HCO3–< 5%) instead of bicarbonate found in KHCO3 (pH = 6.8, HCO3– > 70%, CO2 < 30%). Although it has been shown that the pKa values may vary at the interface with respect to bulk solution, e.g. sulfate60 and phosphate

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systems,

due to the inductive effect and/or a direct effect of the electric field62, of significance is that there was no COad band observed in the spectra in 1 M K2CO3 (Figure S2), which indicates that bicarbonate but not carbonate ions were responsible for the presence of COad found in 1 M KHCO3 (Figure 1). Considering the coordination between HCO3– and water molecules to form hydrogen bonds63 from Ab initio molecular dynamics and the rapid equilibrium between CO2 and HCO3– in KHCO3 to form CO2–HCO3– equilibrium complexes (OCO–HOH–OCOOH),64 it is proposed that that COad and OHad detected on Pt in this study can be produced through the reductive adsorption of the HCO3– moiety of the complex by, overall, HCO3– + H2O => COad + 2OHad + OH– in Scheme 1(a)). This argument is supported by the fact that the formation of COad from HCO3- at 0.05 VRHE was not detected on the Pt surface that was not exposed to low


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potentials (Figure S3), which suggests that Had species on the Pt surface generated at low potentials (lower than ~0.3 VRHE 4) are needed for the formation of COad in 1 M KHCO3 (Scheme 1a). At voltages greater than 0.6 VRHE, two COad bands at 1950–2000 cm–1 and 1755– 1800 cm–1 (Figure 1b) were gradually reduced, suggesting the initiation of electrochemical CO oxidation reaction to form CO2 (Scheme 1c).

Scheme 1. Summary of the proposed reaction mechanism for the formation of COad and OHad, COOHad, and CO2(gas) during the potential scan between 0.05 VRHE and 1.4 VRHE on Pt surface in 1 M KHCO3. (a) Underpotential deposition of hydrogen followed by formation of the COad through the reductive adsorption of the HCO3– was observed in the potential range from 0.05-0.3 VRHE, (b) the COad species were converted to COOHad in the potential range from 0.3-0.9 VRHE. (c) Electrochemical CO oxidation reaction becomes dominant in the potential range from 0.65-0.9 VRHE due to the formation of reactive OHad on Pt surface. Pt, C, O, and H atoms are depicted as gray, dark-gray, red and blue spheres respectively. From the LSV and in situ SEIRA measurement, it was concluded that there are several chemical reactions involved in the entire process. These chemical reactions show potential dependency, which seems to correspond well with the electrochemical process on the Pt surface, e.g. hydrogen desorption and Pt oxidation. These electrochemical processes probably initiate the chemical reactions by helping the reactions such as reduction/dehydrogenation of the species.

3.2. Formation of COOHad from Oxidation of COad by coadsorbed OHad


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Figure 3. In situ ATR-SEIRA measurement on a Pt electrode in (a), (b) 1 M KHCO3 (pH=6.8) and (c), (d) 1 M K2CO3 (pH=11.5). (a), (c) Deconvolution of the band in 1400-1600 cm-1 region and (b), (d) spectra at 1000-1100 cm-1 region of selected in situ ATR-SEIRA spectra during linear sweep voltammetry from 0.05 VRHE to 0.9 VRHE. Cumulative number of 37 was used at a 4 cm–1 resolution. Spectra were subtracted with respect to a reference spectrum obtained at 0.05 VRHE in 1 M KOH. Inset shows a schematic description of the adsorbate being probed. Pt, C, O, and H atoms are depicted as gray, dark-gray, red and blue spheres respectively.

In addition to COad, COOHad was found between 0.3 and 0.9 VRHE in 1 M KHCO3 as indicated by a new potential-dependent band that emerged in the wavenumber range from 1400 to 1600 cm–1, as shown in Figure 3(a). This band was found to have a major, sharp peak at 1475–1560


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cm–1, which grows with increasing voltage from 0.3 to 0.9 VRHE and a minor, broad peak at 1430–1485 cm–1, which exhibits a local maximum intensity at 0.5 VRHE. DFT calculations suggest that the major and minor peaks can be assigned to C=O stretching in COOHad and C-O stretching of bidentate bicarbonate adsorbates, respectively, as shown in Table 1 (and Table S2). The adsorption of carbonate and bicarbonate on the Pt(111) surface were found to be only stable in the bidentate form, whereas the monodentate carbonate/bicarbonate adsorbates would spontaneously transform to the bidentate configuration and CO2 would not adsorb on the Pt surface (Figure S7). In addition, a band in the 1000–1100 cm–1 range was found to grow simultaneously with the COOHad band. The band can be attributed to the C–O stretching of COOHad from DFT calculations, further supporting the presence of COOHad (Figure 3(b)). Moreover, the COOHad peak intensity increases with increasing potential, suggesting the acceleration of the conversion reaction from COad to COOHad at relatively high potentials. The large blueshift (ca. 150 cm-1 / V) observed here can be explained by the Stark shift and dipoledipole interaction as well as the change in number of water molecules interacting with the COOHad. This hypothesis is supported by the calculated vibrational wavenumbers of adsorbed bidentate bicarbonate and COOHad, that shift by 30 to 90 cm–1 depending on the number of water molecules due to the formation of adsorbate-water hydrogen-bonds, as shown in Table 1.

Table 1 DFT computed infrared-active vibrational wavenumbers of possible adsorbed intermediates on Pt(111) and their schematic structures. Pt, C, O, and H atoms are depicted as gray, dark gray, red and blue spheres respectively.


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Species

Vibration mode

C=O stretching

Bidentate CO3,ad

C-O(H) stretching

Bidentate HCO3,ad

C* =O stretching

COOHad

Number of H2O molecules

Calculated wavenumber

0

1600 cm-1

1

1628 cm-1

2

1570 cm-1 (1)

0

1377 cm-1

1

1410 cm-1

2

1425 cm-1 (1)

0

1674 cm-1

1

1624 cm-1

2

1567 cm-1

(1) Averaged wavenumber value of two energetically degenerate water configurations.


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Figure 4. Comparison of the potential dependence of peak wavenumber of (a) C≡O stretching mode for the linear (black) and bridged (red) configuration. Inset shows the representative spectra of C≡O stretching band collected at 0.25 VRHE (b) C=O stretching mode obtained in 1 M KHCO3 and 1 M KDCO3-D2O electrolytes. Inset shows the C=O stretching band collected at 0.75 VRHE and 0.85 VRHE. All of these measurements were performed using the same Pt electrode to avoid the effect of change in surface structures and/or morphology.

In situ SEIRA measurements performed in a deuterium-substituted 1 M KDCO3–D2O electrolyte (having similar LSV and SEIRA spectra in Figure S4 to that without deuterium-substitution in Figure S1) further supported the presence of COOHad. The effect of deuterium substitution should result in slight redshift if the peaks correspond to bonds containing and/or interacting with H/D atoms. Deuterium substitution led to no visible change for the potential-dependent


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wavenumbers of COad but a measurable shift for C=O stretching of COOH adsorbates in the range from 5 to 15 cm–1, as shown in Figures 4a and 4b, respectively. This measurement was performed using the same Pt surface, and in both electrolytes, KHCO3 and KDCO3, COad band and COO(H/D)ad band had similar intensity throughout the potential scan (Figure 4a and 4b insets), indicating the effect of the coverage and morphology on the wavenumber was negligible. The small redshift (5–15 cm–1) of COOHad can be attributed to the replacement of hydrogen atoms with deuterium, which is also supported by DFT calculations (Table S4). In situ SEIRA spectra collected from 1 M K2CO3 electrolyte shown in (Figure 3(c), (d)) were analyzed and compared with those found in 1 M KHCO3, which revealed that COOHad was not formed in 1 M K2CO3. A clear potential-dependent band was observed at 1450–1480 cm–1 in the potential range between 0.3 and 0.65 VRHE, which can be ascribed to the O-C-O symmetric stretching of bidentate bicarbonate adsorbates,65

66

as shown in Figure 3(c). The observation

suggests that the adsorption of carbonate/bicarbonate ions starts from 0.3 VRHE until ca. 0.6 VRHE, at which point, the OHad gradually replaces the bidentate bicarbonate adsorbates. Unlike in the KHCO3, no peak was observed at 1475–1560 cm–1 or at 1000–1100 cm–1 (Figure 3d), indicating that COOHad was not formed in 1 M K2CO3. Therefore, it is proposed that the formation of COOHad can be attributed to (electro)chemical oxidation of co-adsorbed COad and OHad (i.e. COad + 2OHad => COOHad + OHad) on the Pt surface in the KHCO3 electrolyte (Scheme 1(b)). The proposed mechanism is a reverse reaction to the previously reported mechanism for CO2 reduction reaction on Pt67

22

from DFT, where COOHad is a major

intermediate species and is reduced continuously to COad. While the spectra shown in Figure 3 provided the direct experimental evidence for the presence of COOH adsorbates on the Pt


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surface, the potentials under which COOHad was observed (>0.3 VRHE), are much higher than the calculated potential (-0.35 VRHE) required for the protonation of CO2 to form COOHad 22 67.

3.3. Reaction mechanism discussion The potential dependence of the band intensity of COad, COOHad, and CO2(gas) is summarized in Figure 5b together with linear sweep voltammograms measured simultaneously in 1 M KHCO3 and 1 M KOH (Figure 5a).

Figure 5. In situ ATR-SEIRA measurement on a Pt electrode in 1 M KHCO3 electrolyte. (a) Corresponding linear sweep voltammogram in a 1 M KHCO3 (orange) at scanning rate of 10 mV s–1. Linear sweep voltammetry in a 1 M KOH (blue) is also shown for comparison. (b) Potential dependent band intensities for C≡O stretching mode (COad), C=O stretching mode (COOHad and CO2(gas)), and O-H


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librational mode (Pt-OHad). The band intensities for O-H librational mode of Pt-OHad was multiplied by ten.

From 0.05 to 0.30 VRHE, only linear and bridged COad stabilized by OHad/H2Oad were formed through the reductive adsorption of the HCO3– on the Pt surface (Scheme1 (a)). The formation of COOHad started at 0.3 VRHE and its intensity increased with increasing voltage while the band intensities of linear and bridged COad decreased, indicating that COad species co-adsorbed with OHad/H2Oad was replaced gradually by COOHad with increasing potential to 0.95 VRHE (Scheme 1(b)). The band intensity of the COad remains nearly constant between 0.55 VRHE and 0.65 VRHE, where reductive adsorption of the HCO3– to form COad might be balanced with the consumption of COad to form COOHad. At potentials above ca. 0.65 VRHE, the electrochemical COad oxidation commences as indicated by the small oxidation current observed between 0.65 and 0.9 VRHE. Note that CO2(gas) was not detected in the spectra probably due to the detection limit of the measurement. Finally, at potentials higher than 0.9 VRHE, the band intensity of the COOHad decreases with increasing CO2(gas), as shown in Figure S5 and S8. Therefore, we established a surface reaction mechanism for CO2-contained electrochemical system where the strong interaction between neighboring COad and OHad triggers chemical/electrochemical conversion of COad to COOHad, followed by the dehydrogenation of COOHad to form CO2. Our findings provide a new design strategy for CO-tolerant catalyst whereby the onset potential for CO oxidation reactions can be lowered by triggering the oxidation of adsorbed CO species coadsorbed with OHad, and for designing Pt-based catalysts for CO2 reduction reaction with improved activity/selectivity.


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4. Conclusions

In this work, surface reactions on Pt in CO2 containing alkaline solutions were revealed by combining in situ SEIRA measurements and DFT calculations. Scanning voltages of low voltages such as 0.05 VRHE led to the appearance of distinct vibrational bands at 1950–2000 cm–1 and 1755–1800 cm–1, which were assigned to linear and bridged COad co-adsorbed with OHad, respectively. The formation of CO adsorbates on Pt was attributed to the reductive adsorption of the HCO3– in the presence of H adsorbates on the Pt surface, which remained visible up to 0.9 VRHE. Increasing voltages greater than 0.3 VRHE, COad co-adsorbed with OHad was oxidized electrochemically to COOHad as evidenced by the appearance of bands at 1500–1550 cm–1, characteristic to the C=O stretching mode of COOHad. The intensity of COOHad band became greater with increasing voltages up to ~0.9 VRHE, at which CO2 gas was detected, and then decreased at greater voltages. Since there was no COOHad formation observed in CO-saturated 1 M KOH on Pt surface, we deduce that the vicinity between COad and OHad is key to trigger the conversion reaction from COad to CO2 in electrochemical systems via COOHad. These findings shed light on novel ways to reduce CO2 to generate and activate COad species on the Pt surface, which are known to exist as key intermediate species for the CO2 reduction reaction or poison fuel cell reactions, by tuning the interaction with co-adsorbed OH species. The interaction can be optimized either conventionally by designing the catalyst surface to have oxophilic site surrounding the Pt site or by tailor the electrolyte composition to stabilize OHad through electrostatic and/or non-covalent interactions.


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FIGURE CAPTIONS Figure 1. In situ ATR-SEIRA measurement on a Pt electrode in 1 M KHCO3 (pH=6.8). (a) Linear sweep voltammogram in 1 M KHCO3 (orange) at scan rate of 10 mV s–1. Linear sweep voltammetry in 1 M KOH (blue) is also shown for comparison. In situ ATR-SEIRA spectra of (b) C≡O stretching region and (c) O-H librational region obtained during linear sweep voltammetry in a potential window from 0.05 VRHE to 0.9 VRHE in 1 M KHCO3. (d) In situ ATR-SEIRA spectra of O-H librational region obtained during linear sweep voltammetry from 0.05 VRHE to 0.9 VRHE in CO-saturated 1 M KOH is shown for comparison. Cumulative number of (b) 37 and (c), (d) 64 was used at a 4 cm–1 resolution. Spectra were subtracted with respect to a reference spectrum obtained at 0.05 VRHE in 1 M KOH. Figure 2. Potential dependence of the C≡O stretching wavenumber of (a) linear and (b) bridged COad species observed on Pt electrode under 1 M KHCO3 (orange line), CO-saturated 1 M KOH (blue line), and CO-saturated 1 M HClO4 (red line), with potentials scaled to standard hydrogen electrode (SHE). All of these measurements were performed using the same Pt electrode to avoid the effect of surface structures on the wavenumber of COad species. Scheme 1. Summary of the proposed reaction mechanism for the formation of COad and OHad, COOHad, and CO2(gas) during the potential scan between 0.05 VRHE and 1.4 VRHE on Pt surface in 1 M KHCO3. (a) Underpotential deposition of hydrogen followed by formation of the COad through the reductive adsorption of the HCO3– was observed in the potential range from 0.05-0.3 VRHE, (b) the COad species were converted to COOHad in the potential range from 0.3-0.9 VRHE. (c) Electrochemical CO oxidation reaction becomes dominant in the potential range from 0.65-0.9 VRHE due to the formation of reactive OHad on Pt surface. Pt, C, O, and H atoms are depicted as gray, dark-gray, red and blue spheres respectively. From the LSV and in situ SEIRA measurement, it was concluded that there are several chemical reactions involved in the entire process. These chemical reactions show potential dependency, which seems to correspond well with the electrochemical process on the Pt surface, e.g. hydrogen desorption and Pt oxidation. These electrochemical processes probably initiate the chemical reactions by helping the reactions such as reduction/dehydrogenation of the species. Figure 3. In situ ATR-SEIRA measurement on a Pt electrode in (a), (b) 1 M KHCO3 (pH=6.8) and (c), (d) 1 M K2CO3 (pH=11.5). (a), (c) Deconvolution of the band in 1400-1600 cm-1 region and (b), (d) spectra at 1000-1100 cm-1 region of selected in situ ATR-SEIRA spectra during linear sweep voltammetry from 0.05 VRHE to 0.9 VRHE. Cumulative number of 37 was used at a 4 cm–1 resolution. Spectra were subtracted with respect to a reference spectrum obtained at 0.05 VRHE in 1 M KOH. Inset shows a schematic description of the adsorbate being probed. Pt, C, O, and H atoms are depicted as gray, dark-gray, red and blue spheres respectively. Table 1 DFT computed infrared-active vibrational wavenumbers of possible adsorbed intermediates on Pt(111) and their schematic structures. Pt, C, O, and H atoms are depicted as gray, dark gray, red and blue spheres respectively. Figure 4. Comparison of the potential dependence of peak wavenumber of (a) C≡O stretching mode for the linear (black) and bridged (red) configuration. Inset shows the representative spectra of C≡O stretching band collected at 0.25 VRHE (b) C=O stretching mode obtained in 1 M KHCO3 and 1 M KDCO3-D2O electrolytes. Inset shows the C=O stretching band collected at 0.75 VRHE and 0.85 VRHE. All of these measurements were performed using the same Pt electrode to avoid the effect of change in surface structures and/or morphology.


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Figure 5. In situ ATR-SEIRA measurement on a Pt electrode in 1 M KHCO3 electrolyte. (a) Corresponding linear sweep voltammogram in a 1 M KHCO3 (orange) at scanning rate of 10 mV s–1. Linear sweep voltammetry in a 1 M KOH (blue) is also shown for comparison. (b) Potential dependent band intensities for C≡O stretching mode (COad), C=O stretching mode (COOHad and CO2(gas)), and O-H librational mode (Pt-OHad). The band intensities for O-H librational mode of Pt-OHad was multiplied by ten.

ASSOCIATED CONTENT Supporting Information Available: In situ ATR-SEIRA spectra obtained during linear sweep voltammetry in 1 M KHCO3, 1 M K2CO3, 1 M KOD, 1 M KDCO3, and CO-saturated 1 M KOH. DFT computed infrared frequencies of bidentate CO3,ad, bidentate HCO3,ad, and COOHad intermediates on Pt with and without water molecules and their optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Corresponding author. Tel.: +1-617-253-2259(Y. S.-H.), +81-(0)836-85-9285(Y. K.) E-mail address: [email protected] (Y. S.-H.), [email protected] (Y. K.)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT


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Y.K. thanks the Japan Gateway Kyoto University Top Global Program for travel funds to Massachusetts Institute of Technology (MIT). Work at MIT was supported by Eni S.p.A.. Y. K. is supported by Grant-in-Aid for JSPS Research Fellow. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DEAC02-05CH11231. This work also used resources of the Extreme Science and Engineering Discovery Environment (XSEDE)

68

, which is supported by National Science Foundation grant

number ACI-1548562.

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H

O

C

0.05-0.3 VRHE

CO2-saturated KOH

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>0.9 VRHE

0.3-0.9 VRHE

TOC image


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Surface (Electro)chemistry of CO2 on Pt Surface: An in Situ Surface

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