Electronic Structure of CO Adsorbed on Electrodeposited Pt Thin

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Electronic Structure of CO Adsorbed on Electrodeposited Pt Thin Layers on Polycrystalline Au Electrodes Probed by Potential-dependent IR/ Visible Double Resonance Sum Frequency Generation Spectroscopy Shuo Yang, Hidenori Noguchi, and Kohei Uosaki J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10703 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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The Journal of Physical Chemistry

Electronic Structure of CO Adsorbed on Electrodeposited Pt Thin Layers on Polycrystalline Au Electrodes Probed by Potential-dependent IR/Visible Double Resonance Sum Frequency Generation Spectroscopy Shuo Yang,† Hidenori Noguchi,*,†,‡ and Kohei Uosaki*,† †

Global Research Center for Environment and Energy based on Nanomaterials Science

(GREEN), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan ‡

Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-

8628, Japan

ACS Paragon Plus Environment

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ABSTRACT: Potential-dependent double resonance sum frequency generation (DR-SFG) spectroscopy is used to probe a 5σa electronic state of adsorbed CO on Pt thin layer-modified Au surfaces in acid solution by showing an amplitude enhancement of atop CO peak due to a surface electronic resonance between visible light and the electronic transition from Fermi level of Pt to the 5σa anti-bonding state of CO. The energy level of 5σa state of adsorbed CO on the Pt layermodified Au electrodes is found to be different from that on bulk Pt electrodes and depend on the thickness of the Pt layer. The observed SFG results are tentatively explained by the shift in dband energy of the electrode substrates, which could also be responsible for their different electrochemical characteristics towards CO oxidation.


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INTRODUCTION In recent years, electrodes with secondary metal layers deposited on the surface of substrate metals have received increasing attention because these bimetallic electrode surfaces exhibit significantly different electrocatalytic characteristics than the pure metal surfaces.1–5 The unique electrocatalytic properties of the bimetallic electrodes are considered to result from modification of the surface electronic structure, in which a metal-metal bond is formed that alters the d-band vacancy and changes the electrocatalytic activities.6,7 Therefore, a deeper understanding of the underlying electronic structure at the surface of the bimetallic electrode in an electrochemical environment and its relationship with the reactivity of the electrode are important for fundamental studies on the surface processes in electrocatalytic reactions and to achieve rational improvements of electrocatalysts. Au is a widely used substrate material due to its catalytic inactivity. Overlayers of various metals on Au substrates have been obtained by electrochemical deposition8,9 and other modification methods,10,11 among which Pt modification has been investigated widely. Pt exhibits good stability and high catalytic activity for many important electrochemical reactions in fuel cells, such as electrocatalytic oxidation of small organic compounds12 and oxygen reduction reaction (ORR);13 however, Pt is expensive and relatively scarce, which limits the utilization of Pt in practical applications. Significant efforts have been made to minimize the Pt content in catalysts, while maintaining or increasing the catalytic activity as much as possible. Various forms, such as submonolayers, monolayers, and multilayers, of Pt on different forms of Au substrates, such as Au nanoparticles, single and polycrystalline Au, have been prepared,9,11,14,15 and many of these catalysts exhibit high electrocatalytic activity and stability.


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Extensive theoretical investigations on the electronic structure of Pt thin layers on Au electrodes have been conducted. Much research effort based on density functional theory (DFT) calculations have indicated that the d-band center for a Pt overlayer on a Au surface shifts up towards Fermi level with respect to that for bulk Pt.7,16,17 The origin of this d-band shift is most often explained by a tensile strain effect induced by the lattice mismatch between Pt and Au because the Au (4.065 Å) lattice constant for (111) surface is slightly larger than that of Pt (3.912 Å). This leads to an expansion of the Pt overlayer on Au compared with bare Pt, and narrowing of the Pt valence d-bands, which results in an increase of the lattice constant, so that the d-d coupling matrix elements become smaller. When the width of the Pt d-band is decreased, the dband center must move up in energy to maintain a roughly constant degree of band filling. Although the Pt d-band shift has not been experimentally measured directly, it can be inferred indirectly from measurements of the shift in the d-related electronic state of surface species. CO has been extensively used as a surface probe molecule to study interfacial electronic structures because it is a simple molecule that plays important roles in many electrochemical reactions.18,19 The electronic structure of adsorbed CO on a bare Pt surface has been clarified through many theoretical calculations20–23 and experiments in an ultrahigh vacuum (UHV) environment.24–28 These studies have demonstrated that when CO is chemisorbed on a Pt surface, the sharp states of CO observed in a vacuum are broadened and hybridized with the d-band of the Pt surface, which gives rise to 2π∗a and 5σa anti-bonding states above the Fermi level, and 2π∗b and 5σb bonding states below the Fermi level. Such electronic states of adsorbed CO are dband related, and can thus be good indicators of the shift of the Pt d-band. However, it is difficult to determine interfacial electronic information on adsorbed CO in an electrochemical environment due to a lack of suitable techniques. The presence of the electrolyte


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solution means that it is not possible to use techniques with electron probes, which are the most powerful techniques to determine electronic structures in a UHV environment. We have recently reported an interesting technique, potential-dependent infrared (IR)/visible double resonance sum frequency generation (DR-SFG) spectroscopy, which has both frequency-tunable IR and visible lights and allows probing surface electronic and vibrational transitions simultaneously by tuning the visible frequency over electronic transitions and tuning the IR frequency over vibrational transitions at the interface. The SFG amplitude enhancement of a vibrational mode of surface adsorbates in DR-SFG, as the visible frequency approaches a surface electronic state, suggests that the excited electronic resonance transition is vibronically coupled to the vibrational mode of the adsorbates, i.e., a double resonance effect. The potential of this technique for elucidation of the electronic structures of electrochemical interfaces has been demonstrated experimentally in our previous work that examined the effects of potential and visible light energy on the SFG amplitude for the CO/Pt(111) and CO/polycrystalline Pt model systems.29,30 Here, we further applied the potential-dependent DR-SFG spectroscopy to study more complicated electrochemical interfaces. Measurements were performed with CO adsorbed on a Pt monolayer-modified polycrystalline Au electrode in 0.5 M H2SO4 solution. The anomalous increase of the SFG amplitude for CO adsorbed on the Pt/Au electrode was observed when the resonance of visible light with the electronic transition from the Fermi level of Pt/Au to the 5σa anti-bonding state of adsorbed CO vibronically coupled the vibrational mode of the adsorbed CO (i.e., a double resonance effect), which is consistent with the results for CO/Pt(111) and CO/polycrystalline Pt.29,30 The dependence of the Pt layer thickness was also investigated and shows that the 5σa state of adsorbed CO shifts to lower energy levels with an increase in the thickness of the Pt layer, which is considered to be related to a d-band shift of the Pt thin layer


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deposited on Au. The d-band center energy of the Pt/Au electrode could also be responsible for its electrocatalytic activity towards CO oxidation.

EXPERIMENTAL Materials. H2SO4, H2O2, NaCl, HClO4 and H2PtCl6·6H2O were purchased from Wako Pure Chemicals Industries. All chemicals were analytical grade and used without further purification. Ultrapure water (TOC18 MΩ·cm) was obtained from a Milli-Q apparatus (WRX10, Yamato Scientific). Ultrapure Ar (>99.99%) and high purity CO (>99.95%) gases were purchased from Suzuki Shokan and Taiyo Nippon Sanso, respectively. A commercial polycrystalline Au disk (thickness=5 mm, ⌀=10 mm, 99.98%), a Pt(111) disk (thickness=5 mm, ⌀=10 mm, 99.98%), and a polycrystalline Pt disk (thickness=5 mm, ⌀=10 mm, 99.98%) were purchased from MaTeck GmbH. Pt wires (⌀=0.5 mm, 99.98%) were purchased from Nilaco. Alumina slurries (particle size of 1, 0.1 and 0.05 µm) were obtained from Baikowski International. Sapphire optical windows (⌀=30 mm) were obtained from Edmund Optics. Electrochemical Measurements. Electrochemical measurements were conducted in a standard three-compartment spectroelectrochemical cell. The configuration of the cell is presented in our previous article.29 A moveable Kel-F holder was employed when a polycrystalline disk was used as the working electrode, as shown in Figure 1, where the working electrode was embedded in one end of the holder, leaving only the polished surface exposed. The electrical contact between the electrode and the potentiostat was achieved with a stainless steel rod. All glassware used in these experiments were cleaned in freshly prepared piranha solution (2:1 concentrated H2SO4:H2O2) to minimize organic contaminants, followed by thorough rinsing with ultrapure


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water and with the electrolyte solution prior to use. Ag/AgCl (sat. NaCl) and a coiled Pt wire were used as a reference and counter electrodes, respectively, throughout all the experiments. The electrolyte solutions were deaerated by passing Ar gas through the electrolyte for at least 30 min before electrochemical measurements. The electrode potential was controlled with a potentiostat/galvanostat (HABF-501A, Hokuto Denko) and both potential and current were recorded with a data logger (GL900, Graphtec).

Figure 1. Schematic illustration of the Kel-F holder used for the polycrystalline working electrode experiments. Prior to Pt deposition, the polycrystalline Au disk as a working electrode was successively polished with 1, 0.1 and 0.05 µm alumina slurries on a polishing cloth, followed by 10 min sonication in ultrapure water. The Au electrode was then embedded into the Kel-F holder. Electrochemical cleaning of the electrode surface was performed by repeating oxidationreduction cycles between -200 and 1450 mV in Ar saturated 0.05 M H2SO4 aqueous solution with a scan rate of 100 mV s-1 until a reproducible cyclic voltammogram (CV) of the clean Au electrode, consistent with previous reports,31,32 was obtained.


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Pt thin layers were prepared using the reported electrochemical deposition method.9 The polycrystalline Au surface was contacted with deaerated 0.1 M HClO4 aqueous solution containing 0.05 mM H2PtCl6. The potential was started from an open circuit potential (OCP) of ca. 800 mV and slowly scanned in the negative direction at 2 mV s-1 until 500 mV, and then held at that potential for Pt deposition. The electrode was then rinsed with concentrated H2SO4 and ultrapure water to remove adsorbed PtCl62− ions. For comparison, Pt(111) and polycrystalline Pt disks were also used as working electrodes. The pretreatment methods and electrochemical characterizations of Pt(111) and polycrystalline Pt surfaces are introduced in the Supplementary data. After pretreatment, the working electrode was transferred to the spectroelectrochemical cell filled with deaerated 0.5 M H2SO4 aqueous solution, and CO gas was flowed through the electrolyte solution for 10 min at 0 mV, followed by purging with Ar gas for another 40 min to remove CO in the solution. All electrochemical measurements were conducted in a hanging meniscus configuration under an Ar-protected atmosphere. SFG Measurements. The details of the broadband femtosecond laser system employed in the present SFG study have been described elsewhere.29,30 Briefly, a fundamental pulse (2.5 mJ energy centered at 790 nm, 120 fs duration and 1 kHz repetition rate) was split and introduced into an optical parametric generation/optical parametric ampliﬁcation/difference frequency generation (OPG/OPA/DFG) system with AgGaSe2 crystal (TOPAS 8034, Light Conversion) to generate a broadband frequency tunable femtosecond IR probe (5 µJ pulse-1, 200 cm-1@5 µm), and into a second harmonic bandwidth compressor (SHBC) and another OPG/OPA system (TOPAS 400 WL, Light Conversion) to generate a narrowband frequency tunable picosecond


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visible pulse (10 µJ pulse-1,

Electronic Structure of CO Adsorbed on Electrodeposited Pt Thin

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