Controllable Modification of the Electronic Structure of Carbon

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Controllable Modification of the Electronic Structure of CarbonSupported Core–Shell Cu@Pd Catalysts for Formic-Acid Oxidation Mingjun Ren, Yi Zhou, Feifei Tao, Zhiqing Zou, Daniel L. Akins, and Hui Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5033417 • Publication Date (Web): 13 May 2014 Downloaded from http://pubs.acs.org on May 26, 2014

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The Journal of Physical Chemistry

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Controllable Modification of the Electronic Structure of Carbon-Supported

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Core–Shell Cu@Pd Catalysts for Formic Acid Oxidation

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Mingjun Ren,†,‡ Yi Zhou,† Feifei Tao,† Zhiqing Zou,† Daniel L. Akins,§and Hui Yang,†,*

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† Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China

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‡ University of Chinese Academy of Sciences (CAS), Beijing 100039, China

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§CASI, The City College of The City University of New York, NY 10031, USA

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ABSTRACT: This study analyzes the synthesis of carbon-supported core–shell structured Cu@Pd

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catalysts (Cu@Pd/C) through a galvanic replacement reaction to be utilized in the electrocatalytic

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oxidation of formic acid. The strategy used in this study explores the relationship among lattice

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strain, electronic structure, and catalytic performance. X-ray diffraction and X-ray photoelectron

11

spectroscopy indicate that the inclusion of Cu in the nanocatalyst increases lattice strain and results

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in a downshift of the d-band of palladium. Electrochemical tests show that Cu@Pd/C catalysts

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exhibit weaker adsorption strength for CO with increased Cu content, which can be attributed to

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the downshift of the electronic d-band. For the synthesized materials, the Cu@Pd/C catalyst with a

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Cu:Pd atomic ratio of 27:73 is found to have the highest activity for formic acid oxidation. A peak-

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like plot between activity and atomic composition is acquired and reveals the relationship among

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lattice strain, electronic structure, and catalytic performance.

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Keywords: Electrocatalysis, d-band center, lattice strain, galvanic replacement reaction

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ACS Paragon Plus Environment

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1. Introduction

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Research aimed at creating high-performance heterogeneous bimetallic/trimetallic catalysts to

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overcome commercial viability issues regarding the wide utility of fuel cells has been conducted

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for several decades.1–5 The main reason for the focus on bimetallic/trimetallic catalysts is the

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compositional diversity inherent in using such catalysts, which provides a number of opportunities

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to control the properties of a material with resultant performance enhancements, e.g., catalytic

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activity and stability through geometrical or ligand effects.5,6 Progress has been made in the

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preparation of core–shell Cu@Pt and hollow-structured Pt nanoscale materials, which have

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suggested that lattice contraction at the surface plays a role in performance enhancement for the

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oxygen reduction reaction.7,8 Kibler and coworkers deposited Pd monolayer atoms onto a series of

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metallic substrates and clarified the relationship among lattice strain (geometric effect), ligand

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effect, and catalytic performance for formic acid oxidation.9 Generally, such properties are

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influenced by the d-band of the host metal, which is known to affect the performance of catalysts

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significantly.9–11 However, significantly more mechanistic studies of related properties should be

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conducted, such as the characterization of how lattice strain affects the electronic structure of a

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metal, the catalytic performance, and the design of high-performance catalysts.

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In the last decade, direct formic acid fuel cells (DFAFCs) have drawn increasing attention for

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transportation and portable electronic device applications. Compared with methanol, formic acid

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has numerous advantages as a fuel. These advantages include higher kinetic activity, lower

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crossover through the Nafion membrane, high practical energy density, ease of integration into a

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power system, nontoxicity, and others.1,2,12,13 However, the performances of DFAFCs are still

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negatively affected by several problems.14–18 In particular, Pd has been shown to perform better

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than Pt for formic acid oxidation,1,18 and the high initial catalytic activity of Pd compared with Pt


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results in its preferred use for formic acid oxidation.1,18 However, state-of-the-art Pd-based

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catalysts suffer from lower stability and activity than Pt-based catalysts because of poisoning of

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the electrode by CO adsorbate (COad).14,15,17 As a result, the development of high-performance Pd-

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based catalyst for use in DFAFCs continues to be an active research focus area. For example,

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porous Cu–Pd alloy and Pd-enriched Cu–Pd nanoparticles have been recently prepared, and the

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authors have intimated that the enhancement might be the result of variation of electronic structure

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through a synergistic effect that involves Cu.19–21 Furthermore, recent investigations that use X-ray

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photoelectron spectroscopy (XPS) have detected electronic structure variation in Cu–Pd catalysts

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and have indicated that enhanced performance for formic acid correlates with the downshift of the

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d-band center.22,23 However, such findings are insufficient to account for the level of improved

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performance attained.

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In the present study, Cu@Pd/C catalysts are prepared with different atomic compositions

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through galvanic replacement reactions. The lattice strain increase associated with the lattice

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contraction in the crystal and its relationship to the electronic structure and catalytic performance

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for formic acid oxidation are discussed.

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2. Experimental Section

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2.1 Preparation of Catalysts

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Palladium chloride (PdCl2 AR), copper sulfate (CuSO4·5H2O, AR), ethylene glycol (EG, AR),

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sodium chloride (AR), formic acid (AR), sodium hydroxide (AR), and sulfuric acid (AR) were

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obtained from Sinopharm Chemical Reagent Co. Ltd (SCRC). VXC-72 carbon and 5 wt.% Nafion

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solution were purchased from Cabot Co. and Sigma–Aldrich, respectively.

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The Cu@Pd/C catalysts were synthesized by a two-step process. Briefly, a selected amount of

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copper sulfate was added to a flask that contained 20 mL of 0.25 M sodium hydroxide in EG. The


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mixture was continuously stirred at 80 °C under high-purity N2 flow for 1 h. Then, the mixture

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was refluxed at 160 °C under N2 gas atmosphere for 12 h. The resultant reddish brown mixture

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was cooled in ice-cold water. Next, 0.7 mL of concentrated sulfuric acid and 12.5 mL of 40 mM

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Na2PdCl4 were added to the mixture under ultrasonic dispersion for 1 h. Finally, 80 mg VXC-72

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carbon dispersed in 200 mL EG was introduced to support the metallic nanoparticles. Precipitates

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were collected by filtration and washed three times with ultrapure water.

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Pd/C catalyst was prepared using ethylene glycol at 140 °C as the dispersant and reductant.

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Typically, a selected amount of Na2PdCl4 in EG was mixed with 20 ml of 0.25 M sodium

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hydroxide in EG, followed by the addition of 80 mg VXC-72 carbon in EG. The resultant 200 mL

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mixture was heated at 140 °C for 12 h under a N2 gas atmosphere. Lastly, the precipitates were

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collected by filtration and washed three times with ultrapure water.

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2.2 Physical Characterization

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X-ray diffraction (XRD) measurements were conducted using a Bruke D8 Advanced XRD, with

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Cu Kα1 radiation (λ = 1.54 Å). The voltage of the tube was maintained at 40 kV and the current at

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100 mA. Diffraction patterns in the range between 20° and 90° were collected at a scanning rate of

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2° min-1 with a step size of 0.02°; in the ranges of 30° to 45° and 61° to 76°, the scanning rate was

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1° min-1.

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Evaluation of the electronic structure of the catalysts was conducted with an XPS (Kratos Axis

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Ultra DLD, Britain) using Al Kα radiation. The binding energy (BE) was calibrated using the C1s at

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284.45 eV.

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Transmission electron microscopy (TEM) characterizations were conducted on a JEOL JEM-

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2100F TEM. The samples were prepared by ultrasonically suspending catalyst powder in ethanol.

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A drop of suspension was placed onto a holey copper grid and dried under air. The atomic


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composition of the catalysts was determined with an IRIS advantage inductively coupled plasma

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atomic emission spectroscopy (ICP-AES).

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2.3 Electrochemical Evaluation of Catalysts

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Electrochemical experiments were performed using a Solartron Electrochemical Interface

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SI1287 with a standard three-electrode cell. Exactly, 10 mg of the catalyst, 0.5 mL of 5 wt.%

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Nafion solution, and 2.5 mL of ultrapure water were ultrasonically mixed to form the catalyst ink.

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Subsequently, 3 µL of such a mixture was transferred onto a glassy carbon (GC; 3 mm in diameter)

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electrode using a microsyringe. The electrolyte used was 0.5 M H2SO4 or 0.5 M H2SO4 + 0.5 M

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HCOOH. High-purity N2 was used to deaerate the solutions. To determine the real electrochemical

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surface area (ECSA) of the catalyst, cyclic voltammograms (CVs) and CO-stripping

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voltammograms were recorded. For the CO-stripping voltammograms, CO was pre-adsorbed at an

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open potential for 30 min by bubbling CO into 0.5 M H2SO4 solution. Then, dissolved CO was

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subsequently removed by purging for 30 min with high-purity N2. In all cases, electrochemical

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measurements were conducted at a temperature of 25 ± 1 °C.

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The ECSA of catalysts can be obtained by calculating the charge accumulated during the CO desorption and/or H desorption: ECSA = 100×Q/(m×C)

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where Q(µC) is the charge caused by the H desorption or CO desorption; C is the electrical charge

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associated with monolayer adsorption of H (210 µC cm-2) or CO (420 µC cm-2); and m is the

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catalyst loading on the GC electrode (10 µg).

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3. Results and Discussion

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3.1 Composition and Morphology


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The average bulk compositions of the catalysts were determined using ICP-AES analysis by

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atomic compositions of the Cu15@Pd85/C, Cu21@Pd79/C, Cu27@Pd73/C, Cu35@Pd65/C, and

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Cu44@Pd56/C catalysts. The Pd loadings were 38.5, 36.9, 36.73, 36.04, 35.63, and 34.5 wt.% for

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the

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Cu44@Pd56/C, respectively; whereas the Cu loadings exhibited a reverse ordering of 0, 4.0, 5.92,

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7.77, 10.53, and 15.74 wt.%. For clear comparisons, the as-prepared Pd/C, Cu15@Pd85/C,

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Cu21@Pd79/C, Cu27@Pd73/C, Cu35@Pd65/C, and Cu44@Pd56/C catalysts will be correspondingly

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labelled using letters a to f in some of the following figures. Given that the Cu is inactive in the

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formic acid oxidation, comparing only the Pd loading levels to analyze the performance of the

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various catalyst compositions would seem valid.21,22

as-prepared

Pd/C,

Cu15@Pd85/C,

Cu21@Pd79/C,

Cu27@Pd73/C,

Cu35@Pd65/C,

and

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Figure 1. TEM images (scale bar: 20 nm) of the as-prepared Pd/C (A), Cu15@Pd85/C (B),

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Cu21@Pd79/C (C), Cu27@Pd73/C (D), Cu35@Pd65/C (E), and Cu44@Pd56/C (F).


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The morphologies of the as-prepared Pd/C and Cu@Pd/C were determined using TEM. Figures

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1A–1F shows that all the metallic NPs are well dispersed on the surface of VXC-72 carbon. The

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corresponding particle sizes of the catalysts have been determined by measurement of more than

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100 NPs. The measured mean particle diameters obtained are ca. 3.9 to 4.0 nm for the Cu@Pd/C

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catalysts and 3.8 nm for the as-prepared Pd/C catalyst. Hence, a comparison of catalytic

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performance in this work is appropriate.

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3.2 Crystalline and Electronic Structure

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Figure 2 presents the XRD patterns of the Cu@Pd/C and Pd/C catalysts. Figure 2A shows that

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the first peak located at ca. 25° is attributable to VXC-72 carbon .24 All the catalysts also exhibit

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five characteristic peaks indexed with planes (111), (200), (220), (311), and (222), demonstrating

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that all the bimetallic catalysts are single-phase disordered Pd face-centered-cubic structures

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(JCPDF #46-1043). Compared with the reflections in the as-prepared Pd/C, the diffraction peaks

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of the Cu@Pd/C catalysts shift to higher 2θ with increase in Cu content. At a slower scan rate, the

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Pd (111) and (220) facets exhibit the same trends, as shown in Figures 2B and 2C. The lattice

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spacings for the catalysts, determined by fitting the XRD intensity profiles, are provided in Table

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1 .24,25 The lattice spacings obtained for the (111) and (220) planes decrease with increased Cu

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content relative to that of bulk Pd (JCPDF #46-1043). The observed lattice contraction can be

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ascribed to Vegard’s law, which involves metals with different lattice spacings.25 Specifically, the

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addition of Cu, whose lattice spacings for the (111) and (220) facets are 2.09 and 1.28 Å (JCPDF

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#04-0836), which are lower than that of the corresponding Pd facets 2.25 and 1.38 Å (JCPDF #46-

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1043), respectively, would markedly decrease the mean lattice distance of Pd.

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To further understand the lattice contraction, lattice strain was calculated using the following

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formula: ε = (a-a0)/a0. For the (111) and (220) facets, with a0 = 2.25 and 1.38 Å, respectively,7,8


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the lattice strains are provided in Table 1. The average lattice strains for the (111) and (220) facets

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of the Cu@Pd/C catalysts have values ranging from 0% to –2.67% and –0.72% to –3.62%,

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respectively. Obviously, the addition of Cu accounts for the lattice strain in the Cu@Pd/C. This

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finding is similar to that reported in the investigation of Cu@Pt catalysts by Strasser et al. .7

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Figure 2. XRD patterns of the catalysts at a scan rate of 2° min-1 (A) and at a lower scan rate of 1°

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min-1 of the Pd (111) (B) and Pd (220) facets (C). Traces a, b, c, d, e, and f are corresponding to

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the Pd/C, Cu15@Pd85/C, Cu21@Pd79/C, Cu27@Pd73/C, Cu35@Pd65/C, and Cu44@Pd56/C catalysts.

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Table 1. Lattice distance, lattice strain (ε1 and ε2), and εd results of the catalysts. ε2 / % b

εd / eV

0

–0.72

2.79

1.37

–0.89

–1.72

2.92

2.22

1.36

–1.33

–1.45

3.10

Cu27@Pd73/C

2.20

1.35

–2.22

–2.17

3.26

Cu35@Pd65/C

2.20

1.34

–2.22

–2.90

3.53

Cu44@Pd56/C

2.19

1.33

–2.67

–3.62

3.84

Sample

Pd (111)/Å

Pd (220)/Å

Pd/C

2.25

1.37

Cu15@Pd85/C

2.23

Cu21@Pd79/C

ε1 / % a

157

a

a0 = 2.25 Å for the Pd (111) planes (JCPDF #46-1043).

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b

a0 = 1.38 Å for the Pd (220) planes (JCPDF #46-1043).

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Historically, the variation of the lattice distance has been considered to be related to the

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electronic structure of the nanocrystal.6,7,9,26,27 Such a determination, generally from XRD

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measurements, necessitates the exploration of the electronic properties of the Cu@Pd/C catalysts.

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Hence, the samples were analyzed using XPS. Figure 3A shows XPS Pd 3d spectra for the Pd 3d5/2

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peak with BE of 335.29, 335.37, 335.44, 335.60, 335.68, and 335.80 eV, as well as for the Pd 3d3/2

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peak with BE of 340.59, 340.67, 340.83, 340.91, 341.02, and 341.10 eV for as-prepared Pd/C,

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Cu15@Pd85/C, Cu21@Pd79/C, Cu27@Pd73/C, Cu35@Pd65/C, and Cu44@Pd56/C, respectively.18,23

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The BEs of the Pd 3d peaks positively shift with increase in Cu content, indicating electron lose

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for surface Pd atoms. Such a result is consistent with the finding by Hu et al..23 No oxide species is

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formed in the catalysts, and no broad trend of the reflection peaks can be observed. Thus, the

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positive surface core-level shift of the Pd 3d can be attributed to the electron transfer from the

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surface Pd atoms to their adjacent sub-layer atoms. This latter finding suggests an enhanced Pd–Pd

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strength and a decreased bond length, which might explain the lattice contraction.28


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Figure 3. XPS spectra in the binding energy ranges for Pd 3d (A), and XPS spectra in the valence

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band region for the catalysts (B). Traces a, b, c, d, e, and f are corresponding to the Pd/C,

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Cu15@Pd85/C, Cu21@Pd79/C, Cu27@Pd73/C, Cu35@Pd65/C, and Cu44@Pd56/C catalysts.

178

Pd valence band spectra were also acquired in the present study, as shown in Figure 3B. The

179

spectra of the Cu@Pd/C catalysts show broader d-bands with increase in Cu content. The d-band

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center (εd) is found to shift downward relative to the Fermi level, as shown in Table 1. Obviously,

181

the broadening of the d-band may be related to the compressive strain associated with lattice

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contraction, which means increased electronic state overlap between the metal atoms. To keep the

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d-occupancy constant, a downward shift of the d-band center occurs.9,29 Such an observation is


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also reported by Hu.23 The positive core-level shift of the 3d spectrum and the downshift of the d-

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band center occur together owing to electron loss.23,30,31 Generally, the downshift of the d-band

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center is significant because it suggests a weaker adsorption strength between the catalysis surface

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and the adsorption species, which is a result of decreased electron back-donation from the metallic

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surface to the anti-bonding level of adsorbed molecule.7,10,32,33 The increase in lattice strain and the

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downshift of the d-band center explain the performance improvement for formic acid oxidation.

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The deduction in this study is based on substantial syntheses efforts to form the Pd-based catalysts,

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as well as important characterization studies using XRD and/or XPS.3,9,22,34–38

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3.3 Electrocatalytic activity for formic acid oxidation

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To verify the conclusions in this study, the electrochemical performance of the Cu@Pd/C was

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tested by depositing the catalysts onto a GC electrode. Figure 4 shows the CVs of the as-prepared

195

Pd/C and Cu@Pd/C catalysts at a scan rate of 20 mV s-1 in 0.5 M H2SO4 solution at room

196

temperature. All the catalysts possess typical H adsorption/desorption regions at the lower

197

potential, which can be assigned to the behavior of Pd. The ECSA, acquired by integration of the

198

H desorption region, was obtained and is provided in Table 2. The ECSAs of the catalysts exhibit

199

an optimized maximum value of 12.33 m2 g-1 for the Cu27@Pd73/C. A similar trend is also

200

observed for the oxide reduction peaks at ca. 0.5 V. Identical peaks of the Cu redox are indicated

201

by arrows in the figure, revealing the presence of Cu atoms in the catalysts.19,22

202


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Figure 4. CVs of the catalysts in 0.5 M H2SO4 solution at a scan rate of 20 mV s-1. Traces a, b, c,

205

d, e, and f are corresponding to the Pd/C, Cu15@Pd85/C, Cu21@Pd79/C, Cu27@Pd73/C,

206

Cu35@Pd65/C, and Cu44@Pd56/C catalysts.

207 208

Table 2. Electrochemical characteristics of the catalysts. Sample

ECSA / m2g-1 a

ECSA / m2g-1 b

[email protected] / mA cm-2 c

Pd/C

7.16

6.31

49.85

Cu15@Pd85/C

8.28

7.98

53.27

Cu21@Pd79/C

12.04

12.62

61.26

Cu27@Pd73/C

12.33

13.92

82.56

Cu35@Pd65/C

7.73

7.40

39.96

Cu44@Pd56/C

4.34

6.99

22.09

209

a

Results obtained from CVs.

210

b

Results obtained from CO-stripping voltammograms.

211

c

Results obtained with the CVs measured in electrolyte that contains formic acid.

212


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Figure 5. CO-stripping voltammograms of the catalysts in 0.5 M H2SO4 solution at a scan rate of

215

20 mV s-1.


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To gain more insight into the electrochemical performance of the catalysts, CO-stripping

217

voltammograms of the catalysts were obtained, as shown in Figure 5. The figure shows that the

218

oxidation peaks of the adsorbed CO on the Cu21@Pd79/C and Cu27@Pd73/C have the maximum

219

current density, indicating that the two catalysts might have the most catalytic activity sites for

220

adsorbing CO. The ECSAs of the catalysts are also calculated using the CO oxidation peaks and

221

are provided in Table 2. The results clearly demonstrate that Cu27@Pd73/C has the highest ECSA

222

among the catalysts, which qualitatively agree with the data obtained from CVs. More

223

interestingly, the Cu@Pd/C catalysts display improved CO tolerance, as indicated by the negative

224

shift of the onset oxidation potential associated with the vertical line in the figure. The downshift

225

of the d-band center would decrease the back-donation of d electrons to the 2π* level of the COad.

226

Hence, the adsorption strength between the metallic surface and COad onto Cu@Pd/C would be

227

much weaker than that for the as-prepared Pd/C.33,39 As a result, the COad on Cu@Pd/C with

228

higher Cu content can be more easily removed. Given that COad is inevitably formed during the

229

electrooxidation of formic acid, and certainly would poison the Pd-based catalysts, the increased

230

resistance to the COad poisoning effect could be a meaningful avenue to improve

231

performance.14,15,17

232

Figure 6 is a comparison of formic acid oxidation on the Pd/C and Cu@Pd/C catalysts. All

233

measurements were conducted at 50 mV s-1 in 0.5 M H2SO4 + 0.5 M formic acid. Compared with

234

the as-prepared Pd/C, only one formic acid oxidation peak exists for each of the Cu@Pd/C

235

catalysts, suggesting different oxidation mechanisms, as shown in Figure 6A. The current density

236

for formic acid oxidation on the catalysts varies with atomic compositions of the catalysts in the

237

order Cu27@Pd73/C > Cu21@Pd79/C > Cu15@Pd85/C > Pd/C > Cu35@Pd65/C > Cu44@Pd56/C,

238

demonstrating enhanced formic acid catalytic activity attributable to a synergistic effect of copper

239

atoms.


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Figure 6. Cyclic voltammograms of the catalysts in 0.5 M H2SO4 + 0.5 M formic acid solution at

242

a scan rate of 50 mV s-1 (A) and a relationship between current density at 0.16 V (vs. SCE) and the

243

composition of the catalysts (B). Traces a, b, c, d, e, and f are corresponding to the Pd/C,

244


245

To further understand the formic acid oxidation mechanism on the Cu@Pd/C catalysts, the

246

current density of formic acid oxidation at 0.16 V (vs. SCE) is plotted as a function of the

247

compositions of the catalysts, as shown in Figure 6B. A peak-like profile is revealed, as frequently

248

occurs in catalysis sciences. Generally, the shape of this plot could be a result of several

249

phenomena, such as increased particle size and electronic structure change.30,40,41 For the

250

Cu@Pd/C system, the performance variation is clearly dependent on the lattice strain associated

251

with lattice contraction and also the formation of core–shell structure. The key ingredient to the

252

peak–like profile could be the tuning of the reactive species adsorption change. In another study

253

involving chronoamperometry in 0.5 M H2SO4 + 0.5 M HCOOH, the electrodes were held at 0.16

254

V (vs. SCE) for 1 h to obtain i–t curves, as shown in Figure 7. The Cu27@Pd73/C catalyst exhibits

255

the highest initial current density compared with the other catalysts: the initial current densities

256

decreased in the order Cu27@Pd73/C > Cu21@Pd79/C > Cu15@Pd85/C > Cu35@Pd65/C > Pd/C >


15


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Cu44@Pd56/C, a slight difference from that shown in Figure 6A at 0.16 V. Despite the enhanced

258

catalytic activity for formic acid oxidation of the Cu15@Pd85/C, Cu21@Pd79/C, and Cu27@Pd73/C

259

catalysts, compared with that of the as-prepared Pd/C, no improved stability toward formic acid

260

oxidation was observed. By contrast, the decreased catalytic activity for the Cu35@Pd65/C and

261

Cu44@Pd56/C catalysts indicates the greater stability of the latter than that of the as-prepared Pd/C.

262 263

Figure 7. I–t curves of the catalysts in 0.5 M H2SO4 + 0.5 M formic acid solution at a given

264

potential of 0.16 V (vs. SCE). Traces a, b, c, d, e, and f are corresponding to the Pd/C,

265


266

The downshift of the d-band center promotes improved performance of the Pd-based catalysts

267

for formic acid oxidation.9 Although Cu27@Pd73/C can produce the highest catalytic activity, a

268

compromise between the oxidation activity of formic acid and the poisoning effect of the catalysts

269

(by COad and/or (COOH)ad) is important.23,30,40,42 By contrast, the downshift of the d-band relative

270

to the Fermi level implies weaker adsorption strength of the formic acid molecule on the metal

271

surface, which makes breaking bonds harder, resulting in a diminution of the catalytic activity

272

toward the formic acid.10,41 The downshift of the d-band center would also lead to weaker bonds


16

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273

between the metal surface and the poisoning species that is produced during the oxidation of

274

formic acid, indicating that the poisoning species can be more easily removed. Surface active sites

275

occupied by the poisoning species may be freed more quickly in this latter case.

276

As reported, the galvanic replacement reaction is an efficient method to synthesize core-shell

277

metallic nanoparticles with improved catalytic activity.8 The gradually enhanced electrochemical

278

activity on the Cu15@Pd85/C, Cu21@Pd79/C and Cu35@Pd65/C catalysts may be due to the

279

formation of core-shell structure, which would result in an increased ratio of coordination

280

unsaturated Pd. However, with the further increase in Cu content within the catalysts, the thickness

281

of the Pd shell will gradually decrease, and the lattice strain in the nanocrystals is raised, thus

282

leading to a down-shift of the d-band center. Hence, the adsorption strength between the adsorbate

283

and the active site on the catalysts would be decreased with the increase in Cu content. Therefore,

284

an excessive increase of Cu would lead to a decrease of the catalytic activity and to an

285

improvement in durability, as verified by using the Cu35@Pd65/C, and Cu44@Pd56/C catalysts.

286

4. Conclusions

287

The Cu@Pd/C catalysts with different atomic compositions have been successfully prepared.

288

The lattice strain–induced electronic structure variation and modulation of catalytic performance

289

toward the formic acid oxidation have been clarified. Lattice contraction increases with Cu content

290

and leads to lattice strain associated with surface Pd atoms, which, in turn, directly affects the

291

electronic structure of the catalysts. Specifically, increased lattice strain leads to a downshift of the

292

d-band center. The catalytic performance of the catalysts exhibits a peak-like profile when plotted

293

versus the composition of the catalysts, with the maximum value found for Cu27@Pd73/C.

294

Catalytic stabilities are improved for the Cu35@Pd65/C and Cu44@Pd56/C catalysts.


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295

AUTHOR INFORMATION

296

Corresponding Author

297

Tel. & Fax: +86 21 20321112

298

E-mail: [email protected]

299

Notes

300

The authors declare no competing financial interest.

301

ACKNOWLEDGMENT

Page 18 of 26

302

We are grateful for the financial support from the National Basic Research Program of China

303

(973 Program) (2012CB932800), the National Natural Science Foundation of China (21073219),

304

the Shanghai Science and Technology Committee (11DZ1200400), the Knowledge Innovation

305

Engineering of the CAS (12406, 124091231), and the Scientific and Technological Innovation

306

Fund for Graduate Students of the CAS. DLA also thanks the U.S. NSF under Cooperative

307

Agreement number HRD-0833180 for supporting this work.

308 309

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Table of Content

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24

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Revised Figures Figure 4

Figure 4

Figure 6B



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TOC


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Controllable Modification of the Electronic Structure of Carbon

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