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Selective Surface Engineering of Heterogeneous Nanostructures: In-Situ Unravelling the Catalytic Mechanism on Pt-Au Catalyst Fanpeng Kong, Chunyu Du, Jinyu Ye, Guangyu Chen, Lei Du, and Geping Yin ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01901 • Publication Date (Web): 11 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017
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Selective Surface Engineering of Heterogeneous Nanostructures: In-Situ Unravelling the Catalytic Mechanism on Pt-Au Catalyst Fanpeng Kong1, Chunyu Du1,*, Jinyu Ye2, Guangyu Chen1, Lei Du1, Geping Yin1,3,* 1
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and
Storage, Harbin Institute of Technology, 150001, China 2
State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry,
Xiamen University, 361005, China 3
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of
Technology, 150001, China
ABSTRACT. Heterogeneous nanostructures hold substantial promise in many fields including the energy conversion and catalysis, and their functionality is largely determined by the interactive surface features of respective architectural components. Herein, we report the facile preparation of a Pt-Au heterogeneous nanostructure, which as a model exhibits not only the enhanced electrocatalysis for oxygen reduction reaction (ORR) and ethanol oxidation reaction (EOR), but also an unprecedented activity for formic acid oxidation (FAO). Further, selective under potential deposition coupled with CO adsorption is employed to probe the electrocatalytic mechanism of the Pt-Au heterogeneous nanostructure. The electronic effect is clarified for the ORR and EOR mechanisms on the Pt-Au nanostructure. A synergetic mechanism that Pt ruptures the C-H bond of HCOOH and Au converts the resulted –COOH into CO2 is revealed for FAO.
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This strategy provides a versatile and powerful tool for the selective surface engineering and indepth analysis of functionalizing mechanism of heterogeneous nanostructures.
KEYWORDS: Selective under potential deposition; Electrocatalysis; Nanostructure; Synergy effect; Formic acid oxidation 1. Introduction Heterogeneous nanostructures with variable sizes, shapes and compositions have recently received substantial attention, 1-4 because the combination of multiple components into a hybrid nanostructure gives rise to new properties different from each component. These nanostructures hold great promise for the potential applications in many technological fields such as nanoelectronics, sensors, energy conversion, catalysis, drug delivery and biomedicine.
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Especially, heterogeneous nanostructures play an increasing role in the electrochemical energy conversion devices, such as fuel cells and metal-air batteries,8 which is among the most promising technologies for addressing the mounting threats from environmental pollution and energy shortage. Nowadays, the anodic oxidation of formic acid and ethanol, which are the representative small organic molecules, are attracting great attention for fuel cells due to their high theoretical energy density, nontoxicity, easy accessibility, and convenient storage and transport.9 Meanwhile, the cathodic oxygen reduction reaction (ORR) is a universal and indispensable reaction for both fuel cells and metal-air batteries.10 Unfortunately, these reactions are sluggish and irreversible, which significantly impedes the development of fuel cells and metal-air batteries. The functionalities of heterogeneous nanostructures are closely related to the surface properties of respective architectural component and their interactions. 11 Clearly understanding
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the functionalizing mechanisms of heterogeneous nanostructures at atomic levels is critical to rationally designing and tuning their properties. However, this understanding remains a great challenge because the architectural components are closely coupled together and their in-situ reaction information is lacking. For example, the Pt-Au nanostructures have exhibited high catalytic activity and are believed to be promising catalysts for formic acid oxidation (FAO), ethanol oxidation and oxygen reduction reactions. 12-14 However, it is still unclear whether their outstanding formic acid oxidation activity is attributed to the modified electronic structure or the ensemble effect of Pt. 15, 16 Also, it is not answered why Au clusters decorated Pt exhibits the same activity for oxygen reduction reaction as Pt/C, even if one third surface area of Pt is covered by Au. 17 Herein, we report the facile synthesis of a Pt-Au heterogeneous nanostructure and demonstrate its selective surface engineering as a model to probe the catalytic mechanism of FAO, ORR and ethanol oxidation reaction (EOR) for the first time. This selectivity is delicately achieved by the combination of selective CO absorption and its induced under potential deposition (UPD). The key to this selectivity is the complete and independent coverage of Pt by CO, due to the big difference in the CO affinity on Pt and Au, 18 followed by the selective UPD of Cu on Au. By this means, the in-operando electrochemical behaviors of individual Pt or Au component are investigated. It is revealed that the modified electronic structure of Pt accounts for the increased performance of ORR and EOR. Nevertheless, the co-existence of Pt and Au is indispensable for the outstanding FAO activity, and a new synergetic mechanism of FAO is proposed based on the in-situ electrochemical Fourier transform infrared spectroscopy (FTIR) results. 2. Materials and Methods
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Materials: Chloroplatinic acid (H2PtCl6, Sinopharm Chemical Reagent), chloroauric acid (HAuCl4·3H2O, Sinopharm Chemical Reagent), sodium chloropalladite (Na2PdCl4, Aladdin), sodium borohydride (NaBH4, Sinopharm Chemical Reagent), sodium citrate (Na3C6H5O7·2H2O, Sinopharm Chemical Reagent), carbon powder (Cabot Corporation), perchloric acid (HClO4, Aladdin), sulfuric acid (H2SO4, Tianjin Kemiou Chemical Reagent) and copper sulfate (CuSO4·5H2O, Aladdin) were used without any purification. Deionized water (18.2 MΩ, Mill-Q Corporation) was used for preparing the solutions. Nafion solution (5.0 wt. %) was purchased from Dupont to prepare the thin film electrode. Preparation of Pt1-Au1/C and Pd1-Au1/C: The Pt and Au nanoparticle colloids were firstly synthesized. For Pt colloid, 1.267 mL H2PtCl6 solution (40.46 mmol/L) and 75 mg sodium citrate were mixed in 100 mL deionized water and stirred for 10 min. Subsequently, 6 mg NaBH4 was directly added into the mixture to reduce the Pt precursor and form Pt nanoparticles. The preparation of Pd and Au colloids was similar to that of Pt nanoparticles. In order to synthesize the Pt1-Au1/C or Pd1-Au1/C catalyst, the prepared Pt and Au colloids or Pd and Au colloids were uniformly mixed, and carbon black dispersed in water was subsequently added. Afterwards, 5 g NaOH was added to break the electric double layer and deposit Pt and Au nanoparticles or Pd and Au nanoparticles on carbon. After stirring for 24 h, the obtained mixture was filtered, washed and dried in vacuum at 353 K. Preparation of Pt1Au1/C: The PtAu alloy nanoparticles were firstly synthesized. In brief, 0.634 mL H2PtCl6 solution (40.46 mmol/L), 0.98 mL HAuCl4 (25.62 mmol/L) and 75 mg sodium citrate were mixed in 100 mL deionized water. Subsequently, 8 mg NaBH4 was added to prepare the PtAu colloid. The PtAu colloid was deposited on carbon support by the same procedure with Pt1-Au1/C to prepare the Pt1Au1/C.
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Physical characterization: Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) were carried out on an FEI Tecnai G2F30 with an acceleration voltage of 300 kV. Scanning transmission electron microscopy (STEM) images and energy dispersive spectroscopy (EDS) were collected on a FEI Titan G2 80-200 ChemiSTEM with four probe technique and drift correction function. The size of electron beam is 0.08 nm, the energies of Pt and Au Lα1 edges are 9.435 keV and 9.704 keV, and the grid is made of Mo, which are used during all the EDS analyses. X-ray photoelectron spectroscopy (XPS) was performed on a PHI 5700 ECSA System, which is an electron energy analyzer with half spherical precision, using Al Kα radiation (1486.6 eV). The pass energy used is 187.85 eV for wide scan and 5.85 eV for narrow scan. In-situ FTIR was conducted on a glass carbon electrode using a Nicolet Nexus 870 FTIR spectrometer with a liquid-nitrogen-cooled MCT-A detector. A thin-layer electrochemical cell with a transparent CaF2 window was utilized to perform the FTIR measurement. The reference spectrum was recorded at 0 mV and the sample spectra were collected at the potentials from 50 mV to 950 mV with the successive potential step of 50 mV. Each FTIR spectrum was tested for 400 scans to obtain better resolution. The spectra were presented as the relative change in reflectivity (∆R/R) using the following equation: △R R(E S ) − R(E R ) = R R(E R )
where R(ES) and R(ER) are the single-beam spectra collected at the sample potential (ES) and reference potential (ER). Inductively coupled plasma (ICP) was performed on a PerkinElmer Optima 5300DV to determine the composition of the catalysts. Electrochemical testing: 8 mg catalyst was dispersed in a solution of 6 mL water, 2 mL isopropanol and 8 µL Nafion by sonicating for 15 min. 10 µL of the catalyst ink was dropped on
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a rotating disk glass carbon electrode with a diameter of 5 mm, which was polished by 0.3 µm alumina powder in advance. The electrode was dried at ambient temperature in the argon atmosphere. A Pt foil and a Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The electrochemical performance was examined by cyclic voltammetry (CV), CO stripping voltammetry and chronoamperometry (CA). The CV curves were obtained in an argon saturated 0.1 M perchloric acid solution at a scan rate of 50 mV s-1. The adsorption of monolayer CO on Pt was achieved by immersing the working electrode in CO-saturated 0.1 M perchloric acid electrolyte for 20 min, followed by purging the electrolyte with argon and holding the working electrode at 0.4 V for 30 min. The adsorbed CO was subsequently stripped from Pt surface by a potential sweep at a rate of 10 mV s-1. The backgrounds of all the CO stripping curves were corrected by the double-layer capacity and adsorption process. The CA curves were measured for 3 hour at a fixed potential of 0.4 V in argon saturated 0.5 M formic acid and 0.1 M perchloric acid solution. The formic acid oxidation kinetics was measured by the CV in argon saturated 0.5 M formic acid and 0.1 M perchloric acid mixture. The oxygen reduction performance was tested by the linear sweep voltammogram using the rotation disk electrode in oxygen saturated 0.1 M perchloric acid, and the ethanol oxidation activity was obtained by the CV in argon saturated 0.5 M ethanol and 0.1 M perchloric acid mixture. The under potential deposition of Cu monolayer was performed by the CV in 50 mM CuSO4 + 50 mM H2SO4 solution. The Pt monolayer was prepared via a galvanic displacement reaction between Cu and PtCl4- by transferring the Cu monolayer covered electrode into 1 mM K2PtCl4 + 50 mM H2SO4 solution for 3 min. All the electrochemical measurements were conducted on an EG&G 263A
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potentiostat at room temperature, and the potentials were reported with respect to the reversible hydrogen electrode (RHE). 3. Results and Discussion X-ray diffraction (XRD) patterns of carbon supported Pt-Au nanostructure (Pt1-Au1/C) and PtAu alloy (Pt1Au1/C) are shown in Figure 1a, which present the typical peaks of face-centered cubic metals. The diffraction peaks of Pt1Au1/C are located between the standard peaks of Pt and Au, indicating the formation of PtAu alloy. The diffraction peaks of Pt1-Au1/C at 38.18°, 44.37°, 64.56°, 77.74° and at 67.53°, 81.34° correspond to Au and Pt, respectively, demonstrating that the lattice parameters of Pt and Au do not change. It is noteworthy that the (111) and (100) peaks for Pt are almost buried in Au peaks, probably due to the smaller size of Pt NPs in comparison with Au and their negligible lattice mismatch (
