Article pubs.acs.org/JPCC
Surface Oxidation of Au(111) Electrode in Alkaline Media Studied by Using X‑ray Diffraction and Infrared Spectroscopy: Effect of Alkali Metal Cation on the Alcohol Oxidation Reactions Masashi Nakamura,*,† Yo Nakajima,† Ken Kato,† Osami Sakata,‡ and Nagahiro Hoshi† †
Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan ‡ Synchrotron X-ray Station at SPring-8, National Institute for Materials Science, Kouto 1-1-1, Sayo-gun, Hyogo 679−5148, Japan ABSTRACT: The effects of alkali meal cation on the surface oxidation and alcohol oxidation reactions on Au(111) have been investigated using surface X-ray diffraction and infrared spectroscopy. It is known that alkali metal cations strongly affect the alcohol oxidation reactions on Pt(111); however, the oxidation reactions on Au(111) do not depend on alkali metal cations. Infrared spectroscopy reveals the formation of adsorbed OH in LiOH and CsOH solutions below the second anodic peak at 1.3 V. This result indicates that surface oxidation processes do not depend on alkali metal cations below 1.3 V. The interfacial structure, including the outer layer, of an Au(111) electrode has been determined using X-ray diffraction in LiOH and CsOH. During the surface oxidation, the Au(111) surface in CsOH gets roughened more remarkably than that in LiOH above 1.3 V. Li+ has a protective effect against surface roughening. Thus, the cationic effect is weaker in the potential region lower than the second anodic peak, which does not affect the lifting of the surface reconstruction and the alcohol oxidation reactions.
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INTRODUCTION The ionic species at the solid−liquid interface form the electrical double layer (EDL) that consists of the inner Helmholtz plane (IHP) and outer Helmholtz plane (OHP).1 The OHP species as well as the adsorbed species strongly affect the electrochemical reactions on various electrode surfaces.2−4 On Pt electrode, the activities for the alcohol oxidation and oxygen reduction reactions in solutions containing Li+ and Na+ are lower than those in K+ and Cs+ solutions.2,3 Recently, detailed structures of ionic species in the EDL have been investigated by X-ray diffraction method.5−12 We previously reported that Li+ located at the OHP inhibited the surface oxidation of Pt(111) due to the stabilization effect between Li+ and the surface oxygen species, whereas the high-order oxidation accompanied by surface roughness proceeded in solution containing Cs+.12 Thus, alkali cation effects on the surface oxidation result from the competitive interactions between the adsorption energy of oxygen-species and the binding energy of oxygen-species to cation. The surface oxidation state significantly affects the kinetics of the electrochemical reactions; it is important to identify the oxide species and to understand the dependence of the oxidation state on the cationic species. The surface oxidation of transition metals is complex because of the existence of multiple oxidation states, which depend on the surface structure, electrode potential, electrolyte ions, and pH. Under electrochemical conditions, determination of oxygen species on an electrode surface is difficult because soft X-ray and electron probing techniques are limited in solid− liquid interface. Vibration spectroscopy is one of the most © 2015 American Chemical Society
appropriate methods for the identification of surface adsorbed species in electrochemical environments. Recently, gap-mode surface enhanced Raman scattering using nanoparticles has been applied to in situ measurement using single crystal electrodes. On Au(111) in a neutral solution, the bending mode of AuOH and the stretching mode of AuO were observed at 800 and 600 cm−1, respectively.13 Infrared (IR) spectroscopy is also an important tool because it can detect the complementary vibrational modes of surface oxide species. However, observation of the low-frequency mode is restricted by infusible IR windows such as CaF2, which are generally used in the IR reflection absorption spectroscopy (IRAS). Recently, we reported a complementary IR method that can be used to avoid contamination by dissolved ions from window material.12,14 The fusible low-pass IR window can be applied to in situ measurements. Au electrodes in alkali solutions enhance the catalytic activity for the alcohol oxidation and the oxygen reduction reactions.15−18 These electrochemical reactions are affected by surface reconstruction as well as the surface oxidation.16 Au(111) surface is reconstructed to form a uniaxial compressed structure by thermal annealing. In electrolyte solution, the reconstruction of Au(111) electrode is lifted by the surface charge and by the formation of chemical bonds with adsorbed species.19 Oxidation of the Au surface is one of the driving factors in the lifting of Au(111) reconstruction. Although lifting by specifically adsorbed species has been widely investigated, its Received: August 13, 2015 Published: September 25, 2015 23586
DOI: 10.1021/acs.jpcc.5b07878 J. Phys. Chem. C 2015, 119, 23586−23591
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Figure 1. Voltammograms of (a) Au(111) and (b) Pt(111) in 0.1 M LiOH (blue lines) and 0.1 M CsOH (red lines). Scanning rate is 0.05 V s−1.
Figure 2. Voltammograms of (a) methanol and (b) ethanol oxidations of Au(111) in 0.1 M LiOH (blue lines) and 0.1 M CsOH (red lines). Methanol and ethanol concentrations are 2.5 and 0.2 M, respectively. Scanning rate is 0.05 V s−1.
were refined simultaneously using crystal truncation rods (CTRs). IRAS measurements were performed using a polymer-film covered ZnSe prism to detect the low frequency region. IR light was incident through a ZnSe trapezoidal prism at an angle of 15°. An IR electrochemical cell was attached to a Fouriertransform IR spectrometer (Bruker Vertex70v) with a wideband mercury cadmium telluride (MCT) detector. The prism was covered with a polypropylene film with thickness of 6 μm (Chemplex Industries) to prevent Au(111) from contamination of dissolved ions (Zn2+ or Se2−). Water was intercalated into the airspace between the polymer film and the prism to compensate for the difference between their refractive indices.12 All of the IR spectra were obtained using subtractively normalized interfacial Fourier transform IR spectroscopy (SNIFTIRS): 1024 spectra were collected with a resolution of 4 cm−1 in total.
dependence on OHP species has not been reported. Comprehensive investigation of the interface is necessary for the comprehension of the electrochemical reaction mechanism on Au(111). In this study, we have investigated the alcohol oxidation reactions and surface oxidation on Au(111) in alkaline solutions containing alkali metal cations. The results are compared with those of Pt(111). The surface oxidation species have been identified using IRAS, and the interfacial structure of Au(111) has been determined using surface X-ray diffraction method. We discuss the correlation between the activity for the alcohol oxidation reactions and the interfacial structure.
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EXPERIMENTAL SECTION Au(111) disk crystal (Surface Preparation Laboratory, The Netherlands) was used for X-ray diffraction and IR spectroscopy. The sample was annealed in a H2 flame and then cooled to room temperature in an Ar atmosphere. X-ray diffraction measurement was performed using a multiaxis diffractometer at BL4C (KEK/Photon Factory) with an X-ray photon energy of 11 keV. A hexagonal surface coordinate system was used for Au(111) crystal in which the reciprocal wave vector is Q = Ha* + Kb* + Lc*, where a* = b* = 4π/√3a, c* = 2π/√6a, a = 2.884 Å, and L is along the direction normal to the surface. Structural refinements were conducted using the least-squares method with the ANA-ROD program.20 The structural parameters, scale factor, surface fraction factor, roughness factor, Debye−Waller factor, and occupancy factor were optimized, and all of the parameters
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RESULTS AND DISCUSSION Figure 1a shows voltammograms of Au(111) in 0.1 M LiOH and 0.1 M CsOH. The weak anodic current is observed above 0.6 V and is followed by two clear anodic peaks at 1.1 and 1.3 V that are associated with surface oxidation.21,22 Although the potential and charge density of the first anodic peak at 1.1 V in LiOH solution are identical with those in CsOH, the charge density of the second anodic peak at 1.3 V in CsOH is larger than that in LiOH. This difference indicates that the oxidation at the second anodic peak is affected by the interaction between the alkali metal cation and the surface. Figure 2 shows 23587
DOI: 10.1021/acs.jpcc.5b07878 J. Phys. Chem. C 2015, 119, 23586−23591
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comparison with the reference potential at 0.1 V. The orientation of adsorbed water on Au surface depends on the applied potential.23,25 The abnormal IR bands around 1400 and 2800 cm−1 are attributed to the CH bending and CH stretching bands of the thin polymer film, respectively. The positive band at 940 cm−1 was observed in both solutions. This band can be assigned to the AuOH bending mode (δAuOH) of OHad. IRAS measurement was performed using deuterium water to confirm the band assignment. In deuterium water, this band was not observed because of the isotope shift of the vibrational mode including hydrogen. The δAuOH of Au(OH)2 molecule is observed at 885 cm−1.26 OHad on Pt(111) appears at 1130 and 1090 cm−1 in LiOH and HF, respectively.12,14 In general, the frequency of the bending mode is governed by the adsorption energy and hydrogen-bonding configuration. Surface enhanced Raman scattering yielded a band at 800 cm−1 in Na2SO4.13 The band frequency differences may be due to the existence of coadsorbed anions and difference of pH. The δAuOH at 940 cm−1 appears at 0.7 V, which corresponds to the onset potential of anodic current (Figure 1a). Thermodynamic analysis of the charge density data by Lipkowski et al. demonstrated that OHad is adsorbed on Au(111) above −0.4 V vs SCE (0.61 V vs RHE), which is consistent with this result.24 The decrease of the band intensity above 1.3 V corresponds to the second anodic peak at 1.3 V. Therefore, the second anodic peak at 1.3 V can be assigned to the further oxidation from AuOH to AuO. Similar behavior of the adsorbed OH is observed for Pt(111) in LiOH.12 Since the OHad on Au(111) appears in LiOH and CsOH, oxidation process in LiOH is similar to that in CsOH below 1.3 V. This result supports that the catalytic activity for the alcohol oxidation reactions does not depend on alkali metal cations. The adsorbed state of ions and molecules can be investigated using reconstruction of Au(111) as a measure because the strongly adsorbed species induce the lifting of reconstruction.27 The cationic effect of the reconstruction structure was investigated using surface X-ray diffraction. Figure 4 shows a reciprocal radial scan along the [11] direction, which is labeled as the vector q, around the (01) CTR of Au(111) at L = 0.3.28 The main diffraction peak at q = 0 and the satellite peak at q = 0.04 correspond to the (01) CTR and to the fractional-order rod from the compressed reconstructed structure, respectively.
voltammograms of alcohol oxidation on Au(111) in LiOH and CsOH. Typical anodic current peaks due to alcohol oxidation appear in methanol and ethanol containing solutions. Alkali media have previously been demonstrated to enhance these alcohol oxidation reactions on Au electrodes.15,16 The activities of alcohol oxidation on Au(111), specifically, the onset potential and peak current density, do not depend on the alkali metal cation. Since alcohol oxidation current decreases rapidly above 1.3 V, the second surface oxidation peak at 1.3 V is associated with the inhibition of the alcohol oxidation reactions. As shown in Figure 1b, on Pt(111), the potential and shape of surface oxidation peak in LiOH are different from those in CsOH. Alkali metal cations on Pt(111) strongly affect various electrochemical reactions.2,3 The activities for various electrochemical reactions in LiOH are significantly lower than those in CsOH; however, this is not the case with Au(111). We investigated the interfacial structure of Pt(111) electrode using IR spectroscopy and X-ray diffraction.12 The results suggest that the cationic effects originate from different composition of the oxidation state on Pt(111). Li+ cation interacts with oxygen lone pair of adsorbed OH (OHad) and protects the Pt surface from further oxidation as described below. The surface oxidation of Au(111) was investigated using spectroscopic and diffractive approaches to clarify the discrepancies between Au(111) with Pt(111). Vibrational spectroscopy is useful for the identification of oxygen species during surface oxidation. Raman scattering study reported that Au surface was oxidized to AuOH and AuO in a neutral solution.13 We measured Au oxidation species using IRAS in LiOH and CsOH. Figure 3 shows the IR spectra of
Figure 3. Potential dependences of IR spectra of Au(111) in (a) 0.1 M LiOH and (b) 0.1 M CsOH. Reference spectra were corrected at 0.1 V vs RHE. Resolution was 4 cm−1.
Au(111) in CsOH and LiOH. The reference spectra were collected at 0.1 V, at which the surface is not oxidized. The negative-going bands at 1620 and 3400 cm−1 are assigned to the HOH bending mode (δHOH) and the OH stretching mode (νOH) of adsorbed water, respectively.23−25 The negative-going band of the δHOH of adsorbed water indicates the decrease of the coverage or the orientation change of adsorbed water in
Figure 4. Potential dependence of in-plane radial scan around (01) CTR at L = 0.3 of Au(111) in (a) 0.1 M LiOH and (b) 0.1 M CsOH. 23588
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of Au(111) in LiOH and CsOH. Table 1 shows the structural parameters from the optimized structural model. The specular CTRs from reconstructed Au(111) show minimum intensities around L = 4 below 0.6 V. The CTR profiles are identical below 0.6 V in LiOH, the intensity of around L = 1 at 0.3 V in LiOH is weaker than that at 0.6 V in CsOH. Since the reconstruction structure of the Au surface does not change in this potential region, the potential dependence of the CTR intensity in CsOH can be attributed to structural change in the electrical double layer. Structural optimization indicates that the decrease of the CTR intensity around L = 1 is due to the interference of the Au surface layer and the OHP Cs. The OHP species at 3.5 Å from the surface is sensitive to the CTR intensity at L = 0.897 Å−1 (L ≈ 1). The layer formation of the Cs+ has previously been demonstrated to be reduced at CTR intensities below L = 1.5 on Pt(111).12 The CTR changes because of the layered structure formation have been investigated by the comparison of the CTRs in electrochemical conditions with those under ultrahigh vacuum.28 At the double layer region in CsOH, the occupancy factor, that is, coverage, of the OHP Cs decreases with potential increase. Cs coverage and distance from the surface are similar to those on Pt in CsOH.12 Above 1.15 V, the CTR intensities around L = 1.2 and L = 4.7 decrease with the potential increase in both solutions; that in CsOH is reduced significantly at 1.4 V. The decreases of the CTR intensities at L = 1.2 and 4.7 indicate that the Au surface is also oxidized roughly. The occupancy factor of the first Au layer decrease to 0.73 and 0.14 in LiOH and CsOH, respectively. The surface structure in CsOH is rougher than that in LiOH. The second anodic peak in CsOH is higher than that in LiOH, which indicates that the subsurface oxidation proceeds together with surface roughening. The surface area will also increase by surface roughening. We compare these results with those for Pt(111) in alkaline media. For the Pt(111) surface, two anodic CV peaks observed in the alkali metal hydroxide solutions are similar to those in the case of Au(111) (Figure 1b). However, the CV characteristics, such as peak potential, depend on the alkali metal cation, and the spectroscopic and X-ray diffraction results are obviously different between LiOH and CsOH.12 In LiOH, the δPtOH was observed and the surface structure of Pt(111) determined by X-
The satellite peak by the formation of superlattice structure shifts to negative q, and the peak intensity decreases above 0.65 V. The appearance potential of δAuOH (0.7 V) is consistent with that of the intensity decrease of the superlattice peak. This consistency indicates that specifically adsorbed OH induces the lifting of the reconstruction. There is no significant difference between LiOH and CsOH. This fact strongly supports the IR results. However, alkali metal cations affect the interfacial structure of Au(111) for this anodic peak above 1.3 V since the total charge density of the second anodic peak in CsOH is larger than that in LiOH (Figure 1a). The projected electron densities along the surface normal direction were determined by structural optimization using specular CTR scattering. The models of solid−liquid interface of Au(111) contains oxygen species such as H2O, OHad, or Oad, as well as the OHP Cs layer in the case of CsOH. These models were optimized using the least-squares method. For the roughened structural model at 1.4 V, the contribution of oxygen species was neglected because the higher-order oxides of Au have complex inhomogeneous structures in their subsurface layers.29 Figure 5 shows the specular CTR profiles
Figure 5. Specular CTR profiles of Au(111) in 0.1 M LiOH (blue circles and lines) and 0.1 M CsOH (red circles and lines). Solid lines are structure factors calculated from the optimized model.
Table 1. Vertical Layer Spacings and the Occupancy (Occ) Factors for the Optimized Model in LiOH and CsOH electrode potential E [V vs RHE] LiOH
0.3
0.6
0.9
1.15
1.25
2.2(1) 2.39(2) 2.35(2) 0.8(2) 0.99(2) 1.00(2)
2.2(1) 2.36(2) 2.35(2) 0.8(2) 0.94(3) 0.99(3)
dO−Au (Å) d1stAu‑2ndAu (Å) d2ndAu‑3rdAu (Å) OccO Occ1stAu Occ2ndAu
2.1(1) 2.46(2) 2.36(2) 0.6(2) 1.03(2) 1.00(2) 0.3
0.6
0.9
1.15
1.25
dCs−Au (Å) dO−Au (Å) d1stAu‑2ndAu (Å) d2ndAu‑3rdAu (Å) OccCs OccO Occ1stAu Occ2ndAu
3.5(1) 2.1(1) 2.46(2) 2.36(2) 0.16(4) 0.5(2) 1.03(2) 1.00(2)
3.6(1) 2.1(1) 2.46(2) 2.36(2) 0.08(4) 0.5(2) 1.02(2) 0.99(2)
3.3(1) 2.2(1) 2.40(2) 2.36(2) 0.06(4) 0.5(2) 1.04(2) 1.01(2)
3.4(1) 2.3(1) 2.36(2) 2.35(2) 0.03(5) 0.6(2) 0.99(2) 1.02(2)
3.4(1) 2.3(1) 2.35(2) 2.35(2) 0.10(4) 0.6(2) 0.86(3) 1.02(2)
CsOH
2.1(1) 2.1(1) 2.46(2) 2.42(2) 2.36(2) 2.35(2) 0.6(2) 0.7(2) 1.03(2) 1.03(2) 1.00(2) 1.00(2) electrode potential E [V vs RHE]
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1.4 2.35(2) 2.36(2) 0.73(3) 0.93(3) 1.4
1.42(2) 2.32(2)
0.14(4) 0.41(4)
DOI: 10.1021/acs.jpcc.5b07878 J. Phys. Chem. C 2015, 119, 23586−23591
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(5) Nakamura, M.; Sato, N.; Hoshi, N.; Sakata, O. Catalytically Active Structure of Bi Deposited Au(111) Electrode for Hydrogen Peroxide Reduction Reaction. Langmuir 2010, 26, 4590−4593. (6) Keller, H.; Saracino, M.; Nguyen, H. M. T.; Broekmann, P. Templating the Near-surface Liquid Electrolyte: in Situ Surface X-ray Diffraction Study on Anion/cation Interactions at Electrified Interfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 245425. (7) Nakamura, M.; Sato, N.; Hoshi, N.; Sakata, O. Outer Helmholtz Plane of the Electrical Double Layer Formed at the Solid-Liquid Interface. ChemPhysChem 2011, 12, 1430−1434. (8) Strmcnik, D.; Vliet, D. F.; Chang, K. C.; Komanicky, V.; Kodama, K.; You, H.; Stamenkovic, V. R.; Markovic, N. M. Effects of Li+, K+, and Ba2+ Cations on the ORR at Model and High Surface area Pt and Au Surfaces in Alkaline Solutions. J. Phys. Chem. Lett. 2011, 2, 2733− 2736. (9) Lucas, C. A.; Thompson, P.; Grunder, Y.; Markovic, N. M. The Structure of the Electrochemical Double Layer: Ag(111) in Alkaline Electrolyte. Electrochem. Commun. 2011, 13, 1205−1208. (10) Keller, H.; Saracino, M.; Nguyen, H. M. T.; Huynh, T. M. T.; Broekmann, P. Competitive Anion/water and Cation/water Interactions at Electrified Copper/electrolyte Interfaces Probed by in Situ X-ray Diffraction. J. Phys. Chem. C 2012, 116, 11068−11076. (11) Nakamura, M.; Nakajima, Y.; Sato, N.; Hoshi, N.; Sakata, O. Structure of the Electrical Double Layer on Ag(100): Promotive Effect of Cationic Species on Br Adlayer Formation. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 165433. (12) Nakamura, M.; Nakajima, Y.; Hoshi, N.; Tajiri, H.; Sakata, O. Effect of Non-specifically Adsorbed Ions on the Surface Oxidation of Pt(111). ChemPhysChem 2013, 14, 2426−2431. (13) Zhumaev, U.; Rudnev, A. V.; Li, J. F.; Kuzume, A.; Vu, T. H.; Wandlowski, T. Electro-oxidation of Au(111) in Contact with Aqueous Electrolytes: New Insight from In situ Vibration Spectroscopy. Electrochim. Acta 2013, 112, 853−863. (14) Tanaka, H.; Sugawara, S.; Shinohara, K.; Ueno, T.; Suzuki, S.; Hoshi, N.; Nakamura, M. Infrared Reflection Absorption Spectroscopy of OH Adsorption on the Low Index Planes of Pt. Electrocatalysis 2015, 6, 295−299. (15) Tremiliosi-Filho, G.; Gonzalez, E. R.; Motheo, A. J.; Belgsir, E. M.; Leger, J. M.; Lamy, C. Electro-oxidation of Ethanol on Gold: Analysis of the Reaction Products and Mechanism. J. Electroanal. Chem. 1998, 444, 31−39. (16) Rodriguez, P.; Koper, M. T. M. Electrocatalysis on Gold. Phys. Chem. Chem. Phys. 2014, 16, 13583−13594. (17) Adic, R. R.; Markovic, N. M.; Vesovic, V. B. Structural Effects in Electrocatalysis: Oxygen Reduction on Au(100) Single Crystal Electrode. J. Electroanal. Chem. Interfacial Electrochem. 1984, 165, 105−120. (18) Markovic, N. M.; Adic, R. R.; Vesovic, V. B. Oxygen Reduction on the Gold Single Crystal Electrodes with (110) and (111) Orientations. J. Electroanal. Chem. Interfacial Electrochem. 1984, 165, 121−133. (19) Wu, S.; Lipkowski, J.; Magnussen, O. M.; Ocko, B. M.; Wandlowski, T. The Driving Force for (p x √3) - (1 × 1) Phase Transition of Au(111) in the Presence of Organic Adsorption: A Combined Chronocoulometric and Surface X-ray Scattering Study. J. Electroanal. Chem. 1998, 446, 67−77. (20) Vlieg, E. ROD: A Program for Surface X-ray Crystallography. J. Appl. Crystallogr. 2000, 33, 401−405. (21) Hamelin, A.; Sottomayor, M. J.; Silva, F.; Chang, S. C.; Weaver, M. J. Cyclic Voltammetric Characterization of Oriented Monocrystalline Gold Surfaces in Aqueous Alkaline Solution. J. Electroanal. Chem. Interfacial Electrochem. 1990, 295, 291−300. (22) Strbac, S.; Hamelin, A.; Adzic, R. R. Electrochemical Indication of Surface Reconstruction of (100), (311), and (111) Gold Faces in Alkaline Solutions. J. Electroanal. Chem. 1993, 362, 47−53. (23) Ataka, K.; Yotsuyanagi, T.; Osawa, M. Potential-Dependent Reorientation of Water Molecules at an Electrode/Electrolyte Interface Studied by Surface-Enhanced Infrared Absorption Spectroscopy. J. Phys. Chem. 1996, 100, 10664−10672.
ray diffraction did not change at all between 0.1 and 1.2 V. These results indicate that Li+ strongly protects the surface against roughening caused by subsurface oxidation. Li+ stabilizes the OHad layer by the strong coordinative interaction between Li+ and the oxygen lone pair of OHad. When Cs+ is located near the Pt surface, the weakly protective effect of Cs+ causes irreversible surface roughening without the formation of OHad. We conclude that these different behaviors result from the different oxidation processes depending on alkali metal cations. Therefore, different surface oxidation states affect the activity for various electrochemical reactions. However, the IRAS of Au(111) reveal the presence of the OHad species in both LiOH and CsOH below 1.3 V. This indicates that initial oxidation processes on Au(111) do not depend on the alkali metal cations. The independence of alkali metal cations for the alcohol oxidation reactions on Au(111) can be explained by surface oxidation that proceeds via similar oxidation states in LiOH and CsOH. For further oxidation above the second anodic peak potential (1.3 V), alkali metal cations affect surface roughening of Au(111). Li+ protects the surface against roughening according to the mechanism similar to Pt(111).
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CONCLUSIONS The effects of the interfacial structure on the alcohol oxidation reaction have been investigated on Au(111) using X-ray diffraction and IR spectroscopy. The activities for the methanol and ethanol oxidation reactions on Au(111) do not depend on the alkali metal cations, which is different from the case of Pt(111). Although surface oxidation process of Pt(111) in CsOH differs from that in LiOH, the surface oxidation state of Au(111) does not depend on alkali metal cations. During surface oxidation, the surface in CsOH is rougher than that in LiOH. Li+ has a protective effect against surface roughening.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS X-ray measurements were supported by KEK/PF (2012G168 and 2012G693). This work was supported by the Asahi Glass Foundation and JSPS KAKENHI Grant Number 15H03763.
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REFERENCES
(1) Bockris, J. O.; Devanathan, M. A. V.; Muller, K. On the Structure of Charge Interfaces. Proc. R. Soc. London, Ser. A 1963, 274, 55−79. (2) Strmcnik, D.; Kodama, K.; Vliet, D.; Greely, J.; Stamenkovic, V. R.; Markovic, N. M. The Role of Non-covalent Interactions in Electrocatalytic Fuel-cell Reactions on Platinum. Nat. Chem. 2009, 1, 466−472. (3) Stoffelsma, C.; Rodriguez, P.; Garcia, G.; Garcia-Araz, N.; Strmcnik, D.; Markovic, N. M.; Koper, M. T. M. Promotion of the Oxidation of Carbon Monoxide at Stepped Platinum Single-crystal Electrodes in Alkaline Media by Lithium and Beryllium Cations. J. Am. Chem. Soc. 2010, 132, 16127−16133. (4) Ganassin, A.; Colic, V.; Tymoczko, J.; Bandarenka, A. S.; Schuhmann, W. Non-Covalent Interactions in Water Electrolysis: Influence on the Activity of Pt(111) and Iridium oxide catalysts in acidic media. Phys. Chem. Chem. Phys. 2015, 17, 8349−8355. 23590
DOI: 10.1021/acs.jpcc.5b07878 J. Phys. Chem. C 2015, 119, 23586−23591
Article
The Journal of Physical Chemistry C (24) Chen, A.; Lipkowski, J. Electrochemical and Spectroscopic Studies of Hydroxide Adsorption at the Au(111) Electrode. J. Phys. Chem. B 1999, 103, 682−691. (25) Nakamura, M.; Kato, H.; Hoshi, N. Infrared Spectroscopy of Water Adsorbed on M(111) (M = Pt, Pd, Rh, Au, Cu) Electrodes in Sulfuric Acid Solution. J. Phys. Chem. C 2008, 112, 9458−9463. (26) Wang, X.; Andrews, L. Infrared Spectrum and Structure of the Gold Dihydroxide Molecule. Chem. Commun. 2005, 4001−4003. (27) Kolb, D. M. Reconstruction Phenomena at Metal-electrolyte Interface. Prog. Surf. Sci. 1996, 51, 109−173. (28) Wang, J.; Ocko, B. M.; Davenport, A. J.; Isaacs, H. S. In Situ Xray Diffraction and − reflectivity Studies of the Au(111)/electrolyte Interface: Reconstruction and Anion Adsorption. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 46, 10321−10338. (29) Xia, S. J.; Birss, V. I. In-situ Quartz Crystal Microbalance Study of Au Oxide Growth in Alkaline Solutions. In Electrochemical Society Proceedings 2003−25, Surface Oxide Films, 2004; pp 171−180.
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