Self-Restraining Electroless Deposition for Shell ... - ACS Publications

Subscriber access provided by UNIV OF DURHAM

C: Energy Conversion and Storage; Energy and Charge Transport

Self Restraining Electroless Deposition for Shell@Core Particles and Influence of Lattice Parameter on the ORR Activity of Pt(shell)@Pd(core)/C Electrocatalyst Ijjada Mahesh, and Arindam Sarkar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12353 • Publication Date (Web): 06 Apr 2018 Downloaded from http://pubs.acs.org on April 7, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Self Restraining Electroless Deposition for Shell@Core Particles and Influence of Lattice Parameter on the ORR Activity of Pt(shell)@Pd(core)/C Electrocatalyst

Ijjada Mahesh and A. Sarkar* Department of Chemical Engineering, Indian Institute of Technology Bombay, Mumbai, India *

E-mail: [email protected]

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Ultrathin metal layer coated particles have potential applications in various fields, especially in electrocatalysis where catalytic activity can be increased by shell@core design. In this article, a synthetic method is introduced to synthesize shell@core nanoparticles, in which, the selected reducing agent can be electrochemically oxidized preferentially on the core particle but not on the shell metal. Once the shell metal is deposited on the core metal, the oxidation of reducing agent is inhibited, subsequently forming a shell@core particle with an ultrathin shell. By this method, carbon supported shell(Pt)@core(Pd) nanoparticles with sub-monolayer Pt shell were synthesized using formic acid as reducing agent. Spectroscopic characterizations, XPS and EDS confirmed the Pt deposition. Shell@core structure of Pt@Pd was corroborated by STEM analysis. This Pt(shell)@Pd(core) electrocatalyst was tested for electrochemical reduction of oxygen. Further, the influence of lattice parameter on the catalytic activity for ORR was examined by varying the lattice parameter of Pt@Pd nanoparticles.


2

Page 2 of 32

Page 3 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Thin film deposited substrates have lot of applications in catalysis, electronics and optical sectors.1-8 The physical and chemical properties of the material can be modified by the thin film deposition.9,10 Generally, ultra high vacuum (UHV) deposition and electrochemical techniques are used to deposit an ultrathin layer on a substrate. When compared to UHV, electrochemical technique is less expensive and can be controlled easily.11 In last few years, several electrodeposition and electroless deposition methods have been developed to deposit thin films and these have been extended to a monolayer of shell on a core metal. Prominently, these methods include underpotential deposition (UPD) - galvanic replacement,12 kinetically controlled autocatalytic deposition,13 polyol process14 and self terminated electrodeposition15,16. Carbon supported shell@core (monolayer shell) nanoparticles have been synthesized by the some of the aforementioned methods in bulk scale and have been used as catalysts for several electrochemical reactions. It has been observed that the electro catalytic activity of the reactions can be tuned by the composition of shell or core, and coverage and thickness of the shell.9,17-22 This change in activity is attributed to the change in d-band centre of the shell metal atoms by ligand and structural effects.23,24 These catalysts have potential applications due to the advantage of minimizing the cost of the catalyst while simultaneously increasing the activity. In this article, we describe a method to synthesize shell@core nanoparticles having an ultrathin shell by utilizing the difference in kinetics of the oxidation of a reducing agent on core and shell metals. This method is inspired by kinetically controlled autocatalytic deposition method proposed by Taufany et al.13, in which, Cu monolayer was deposited on a Pd core using H2 as reducing agent followed by galvanic replacement of Cu by Pt leading to Pt(shell)@Pd(core) nanoparticles. It may be worth mentioning here that the kinetics of H2 bond


3


dissociation on Cu is poorer by several orders of magnitude compared to than on palladium, thus inhibiting multilayer deposition of Cu atoms on Pd.13 In the present work, this method was modified by making shell@core particles in a single step by avoiding sacrificial metal deposition step (such as Cu deposition) altogether, importantly, by not using H2 as reducing agent, since it can alter the physico-chemical properties of the some metals by absorbing into their lattice.25-27 Here, the reducing agent, which does not affect the core metal physico-chemically, is carefully chosen such that it has a very high kinetic resistance for its electro-oxidation on shell metal compared to than on the core metal. Effectively, the reducing agent is oxidized electrochemically on the core metal and the generated electrons are utilized to reduce the shell metal ions to corresponding metal atoms on the surface of the core metal nanoparticles. Once, the core metal surface is covered by the shell metal atoms, further oxidation of reducing agent is hindered due to its high kinetic resistance on the shell metal. This leads to a shell@core nanoparticle having an ultrathin layer shell, which can be restrained to be a monolayer. In this work, this method was demonstrated by synthesizing carbon supported Pt(shell)@Pd(core) nanoparticles. The system, Pt@Pd, was chosen because of two reasons, (i) it shows high activity than pure Pt for oxygen reduction reaction (ORR), which was attributed to the change in the d-band centre of Pt shell23 and (ii) to comprehend the effect of lattice parameter on ORR activity, since it can be easily altered in this Pt@Pd system by incorporating H atoms into its core particle before Pt deposition.28 The d-band centre of an electrocatalyst influences the catalytic activity for ORR, in fact, for any electrochemical reaction. By tuning the d-band centre, binding energies of the reaction intermediates on the catalyst can be manipulated and it can be achieved by using either alloy or shell@core designed catalysts. In shell@core catalyst, shell metal is an active catalyst for the


4

Page 4 of 32

Page 5 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reaction, whose activity is manipulated by core metal. Effect of d-band centre shift on ORR activity was well studied on shell@core catalysts by several research groups.23,29,30 The enhancement of ORR activity on Pt@Pd was ascribed to the decrease in d-band centre of Pt by the combined effect of compressive strain on Pt shell due to the difference in lattice constants of Pt and Pd metals (structural or strain effect), and the induced charge on the Pt atoms in the shell due to difference in electronegativity of Pt and Pd metals (ligand or electronic effect). In this work, lattice parameter effect on the XPS binding energy of Pt, correspondingly, on the d-band centre and ORR activity was studied at two different lattice parameter values of Pt@Pd catalyst. 2. EXPERIMENTAL METHODS This section will discuss about the synthesis procedure of carbon supported Pd nanoparticles with two different lattice parameter values, microgram scale synthesis procedure of Pt@Pd/C and its electrochemical characterization, and large scale synthesis procedure of Pt@Pd/C nanoparticles

with

two

different

lattice

parameter

values,

their

physico-chemical

characterizations and ORR activity measurements. 2.1 Synthesis of Pd/C nanoparticles Palladium nanoparticles supported on carbon (nominally 20 wt %) were prepared by the reduction of PdCl4-2 ions by the reducing agents, sodium borohydride (NaBH4, Sigma Aldrich, USA) and formic acid (HCOOH, Merck, India) separately. Detailed procedure of synthesis is given in the supporting information. In this article, carbon supported palladium nanoparticles are referred as Pd/C. Specifically, sodium borohydride and formic acid (FA) synthesised carbon supported palladium nanoparticles are referred as PdS/C and PdF/C respectively. 2.2 Synthesis of Pt(shell)@Pd(core)/C nanoparticles in microgram scale on GC electrode


5


Initially, Pt@Pd/C nanoparticles were synthesized on glassy carbon (GC) electrode, so that it can be electrochemically characterized as soon as it is formed. This catalyst is designated as Pt@Pd/C/GC in this article. Prior to Pt deposition, Pd/C drop casted GC electrode was electrochemically cleaned to remove any surface oxides/hydroxides and adsorbed/absorbed H from the Pd, the procedures for making catalyst ink and electrochemical cleaning of the electrode are given in section 2.5.1. This Pd/C/GC electrode was immersed in a solution of Ar saturated 5.0 mM H2SO4 + 0.21 M HCOOH solution without any potential control (open circuit). After 5 minutes, 70 µl of 77 mM H2PtCl6.6H2O was added while the solution was under forced convection by vigorous Ar purging. The Pt@Pd/C/GC electrode was taken out after 2 minutes and rinsed in Ar saturated deionized water for further electrochemical characterization. 2.3 Synthesis of Pt(shell)@Pd(core)/C nanoparticles in large scale 50 mg of freshly prepared Pd/C was uniformly dispersed in 40 ml of Ar saturated solution of 5.0 mM H2SO4 + 0.21 M HCOOH by sonication for 1 hour. The solution was then kept under magnetic stirring while being continuously purging with Ar gas. Thereafter, 300 µl of Ar saturated 77 mM H2PtCl6.6H2O solution was added to the above solution. After ~ 3 minutes, Pt@Pd/C particles were separated by filtration. These are designated as Pt@Pd/C, specifically as Pt@PdS/C and Pt@PdF/C synthesised by PdS/C and PdF/C seed particles respectively. On the other hand, to verify whether Pt is deposited on carbon without Pd, 50 mg of Vulcan carbon (XC-72R) was subjected to the same procedure so as to synthesize Pt@Pd/C nanoparticles and the obtained powder was further characterised. 2.4 Physical characterization of the catalysts Phase characterization of the catalysts was done by X-Ray diffraction (XRD) technique. A


6

Page 6 of 32

Page 7 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PANalytical Empyrean diffractometer (Netherlands) with Cu (Kα) X-ray source was used for XRD measurements at 0.154 nm wavelength from 10o to 100o. Lattice parameter values of the particles were measured from the unit cell refinement of X ray diffractograms by X’Pert HighScore Plus software (version 2.1.0). X-Ray photoelectron spectroscopy (XPS) technique was used for the surface characterization of the catalysts by an AXIS Supra model Kratos Analytical X-ray photoelectron spectrometer (UK) with Al (Kα) X-ray source with photon energy of 1486.6 eV. The analysis chamber was maintained at a pressure below 2×10-7 Pa. High resolution scanning was taken with a resolution of 0.5 eV at 20 eV pass energy. Angle between X-ray source and the analyzer was 54.7o and the take off angle was 90o. XPS data was analyzed by XPS peak 4.1 software. After back ground correction by Shirley’s method, peaks were deconvoluted and fitted to 20% Lorentzian – 80% Gaussian distribution functions. Binding energies (BE) were corrected by taking the BE of C 1s (Graphite) as 284.3 eV.31,32 Elemental composition of the catalyst was measured by the JEOL JSM-7600F model FEG-SEM with an OXFORD instruments XmaxN-80 mm2 energy dispersive spectroscopy (EDS) attachment. High resolution imaging studies of nanoparticles, elemental mapping and line scan were taken on a JEOL’s FEG-TEM model JEM-2100F operating at 200 kV with an EDS attachment. For TEM analysis, sample was prepared by depositing ethanol dispersed catalyst on a copper grid followed by evaporation of ethanol. 2.5 Electrochemical characterization and measurements 2.5.1 Electrochemical characterization Catalyst ink was prepared by dispersing 5 mg of catalyst powder in 5 ml of deionized water and 50 µl of 5 wt % Nafion solution (Sigma Aldrich, USA) by sonication for 15 minutes. Twenty microlitre of the catalyst ink was drop casted on a GC electrode (5 mm diameter, Pine


7


instruments, USA) and dried under infrared lamp (IR) for 20 minutes. Before drop casting, the GC electrode was polished successively with 1.0, 0.3 and 0.05 µm alumina paste (Buehler, USA). The electrochemical characterization of the catalysts was done on a Gamry instruments Interface 1000 potentiostat (USA) in an electrochemical cell with Hg/Hg2SO4 as the reference electrode, Pt mesh as the counter electrode, and drop casted catalyst on GC as the working electrode. The Hg/Hg2SO4 reference electrode was calibrated against a reversible hydrogen electrode (RHE) and the calibration value (0.7 V) was used to convert the potentials with respect to RHE and reported accordingly in this article. Besides, prior to deposition of Pt on Pd/C drop casted GC electrode, it was electrochemically cleaned in Ar purged 0.5 M H2SO4 solution by cyclic voltammetry at the scan rate of 50 mV.s-1, in the potential window of 0.02 to 1.00 VRHE for 15 cycles and the final potential was kept at 0.45 VRHE (double layer capacitive region), so that Pd/C would be free from any absorbent and adsorbents such as O, H and OH. The freshly made Pt@Pd/C/GC (made in microgram scale), Pt@Pd/C (made in large scale) and Pd/C catalysts were electrochemically characterized in Ar purged 0.5 M H2SO4 solution by cyclic voltammetry technique from the potential 0.02 to 1.00 VRHE with the scan rate of 50 mV.s-1. 2.5.2 Formic acid oxidation overpotential measurements Electrode potential of electrochemically cleaned Pt/C (Alfa Aesar, HISPEC 3000) and Pd/C (homemade) drop casted GC electrodes, separately, was measured in Ar purged 0.21 M HCOOH + 0.5 M H2SO4 solution at the constant current of 135 µA by chronopotentiometry technique. This current value was chosen based on the current of the pre plateau region of the positive cycle in the cyclic voltammogram (CV) of FA oxidation (Fig. S1) on Pt/C (commercial) drop casted GC electrode at 50 mV s-1 scan rate in Ar purged 0.21 M HCOOH + 0.5 M H2SO4 solution. Overpotential of FA oxidation on both of these catalysts was calculated by subtracting the open


8

Page 8 of 32

Page 9 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


circuit potential (OCP), from the potentials obtained by chronopotentiometry, of the corresponding catalysts, which were attained in the same solution (see Fig. S1). 2.5.3 Oxygen reduction reaction activity measurements Electrochemical activities of the catalysts were tested for ORR on a rotating disk electrode using cyclic voltammetry technique at 1600 RPM in O2 saturated 0.1 M HClO4 solution in a homemade cell of 500 ml capacity, which was similar in design as outlined by Brett et al.33 with Pt mesh, reversible hydrogen electrode (RHE) and catalyst drop casted rotating disk electrode as counter, reference and working electrodes respectively. The polarization curves were obtained in the potential range from 0.02 to 1.00 VRHE with 10 mV.s-1 scan rate. The current measurements were taken from the stable 5th cycle at 0.9 VRHE and activities are calculated per unit electrochemical surface area (ECSA) of the catalyst. The ECSA of the catalysts was measured from the double layer capacitive compensated charge obtained by either H or Cu UPD, based on the catalyst. Comprehensively, ECSA of Pt in commercial Pt/C was obtained from HUPD charge of a CV, which was taken in Ar purged 0.5 M H2SO4 solution. The adsorption charge for a monolayer of H atoms on 1 cm2 of Pt surface was taken as 210 μC.34 On the other hand, for Pd/C samples, H atoms not only adsorbed on Pd surface but also absorbs into its lattice, consequently, HUPD cannot be used for ECSA calculations of this catalysts. In addition, it will be known in the following discussion that in Pt@Pd/C catalysts, Pd core is not fully covered by Pt shell. Therefore, CuUPD was used to calculate ECSA of both Pd/C and Pt@Pd/C catalysts, and the CV was obtained in Ar purged 50 mM CuSO4 + 0.5 M H2SO4 solution at 50 mV.s-1 scan rate. It was assumed that monolayer of Cu adsorption on 1 cm2 of Pd surface consumes 2×240 μC of charge.35 For Pt@Pd/C catalysts also, same number was taken by assuming the epitaxial deposition of Pt on Pd surface.


9


3. RESULTS AND DISCUSSIONS 3.1 Synthesis and characterization of Pd/C nanoparticles Palladium with normal lattice parameter was synthesized by reducing PdCl4-2 ions to PdF by the reducing agent, formic acid. Whereas, the expanded lattice palladium nanoparticles (PdS) were obtained by reducing PdCl4-2 ions by sodium borohydride. In later case, the role of NaBH4 was twofold, (i) reduction of PdCl4-2 ions to Pd and (ii) catalytic evolution of H2 gas on Pd catalyst, which was absorbed into Pd lattice and transformed it to palladium hydride.28,36,37 Figure S2 shows the XRD diffractograms of PdS/C and PdF/C. Lattice parameter values of these Pd nanoparticles were obtained from the XRD measurements. For PdF, lattice parameter was measured as 3.8919 Å, which is in well agreement with the previous reports of normal palladium.38,39 In case of PdS, lattice structure was found to be expanded due to hydrogen absorption into its lattice (4.0190 Å). This value indicates that PdS existed as PdH0.37 in the α-β intermediate region of the phase diagram.40 Hydride formation of palladium was further confirmed from the XPS analysis, where the characteristic 3d core level BE of Pd in palladium hydride was found to be more than in normal palladium.26,27,41,42 As shown in the Fig. 1, BE of Pd 3d in PdS is 0.23 eV more than the BE in PdF. Morphological characterization by FEG-TEM reveals that PdS nanoparticles were mostly spherical or ellipsoidal with an average particle size of 8.12 ± 3.29 nm, whereas, the PdF nanoparticles consisted of several shapes ranging from spherical to cubic, and they had a very wide particle size distribution with average particle size of 16.97 nm having a standard deviation of 16.09 nm. High resolution TEM images and particle size distributions of both PdS and PdF nanoparticles are shown in the Fig. S3. The (111) dspacing values of PdF and PdS, measured from the high resolution TEM images are almost equal to those obtained by XRD measurements. This result further confirms the expansion of Pd lattice


10

Page 10 of 32

Page 11 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in PdS nanoparticles.

Figure 1. XPS spectra of Pd 3d in PdF/C and PdS/C nanoparticles. 3.2 Synthesis of Pt@Pd/C on GC electrode in microgram scale To demonstrate the making of shell@core nanoparticles by utilizing the difference in the oxidation kinetics of a reducing agent, carbon supported Pt(shell)@Pd(core) nanoparticles were synthesized by using FA as the reducing agent, which reduces Pt+4 to Pt on the surface of Pd nanoparticles. The reason to select FA is that it has high oxidation overpotential on Pt (≈ 0.18 V)


11


compared to that on Pd (≈ 0.07 V). Overpotential values for the oxidation of FA on Pt and Pd were estimated as explained in section 2.5.2. Moreover, it is already known that when Pt is exposed to the FA solution at open circuit potential, it is covered by COads.43 Effectively, the oxidation of FA on deposited Pt atoms is inhibited, while that on Pd, it proceeds by releasing electrons which can be further utilized for reducing platinum ions. Thus, when Pd nanoparticles are exposed to FA, they can catalyse the reaction of FA oxidation and reduce Pt+4 ions to Pt on Pd surface, while simultaneously the carbon monoxide is adsorbed on Pt shell and further deposition of platinum atoms on top of the Pt shell is hindered. A succession of this may lead to monolayer coverage of Pt on the Pd nanoparticle. Since, the electrochemical surface area (ECSA) per unit mass for PdF is very low (ECSA = 2.24 m2.g-1), PdS (ECSA = 45.0 m2.g-1) was chosen to demonstrate the effectiveness of the method and making of Pt@Pd/C on GC electrode, which was in microgram scale. The synthesized Pt@Pd/C/GC electrode was electrochemically characterized by cyclic voltammetry in Ar saturated 0.5 M H2SO4 solution at the scan rate of 50 mV.s-1, the first cycle shows the characteristic current peak in the positive scan at ca. 0.75 VRHE due to oxidation of adsorbed CO, which confirms the presence of CO on Pt, which incidentally is absent in the successive scan (Fig. 2).


12

Page 12 of 32

Page 13 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. CV of Pd/C, and 1st and 2nd cycle CVs of Pt@Pd/C/GC in Ar saturated 0.5 M H2SO4 solution at 50 mV.s-1 scan rate. Inset: deconvolution of CO oxidation peak (after back ground correction) by Gaussian function using Fityk software.44

The presence of HUPD desorption charge in the first cycle of the CV of Pt@Pd/C/GC (Fig. 2) indicates that Pd core particle is not fully covered by CO adsorbed Pt. The partial coverage of Pd core by Pt atoms can be due to the following reason: the kinetics of FA oxidation on Pd depends on its structure as well as on electrolyte medium.45,46 In HClO4 medium, FA oxidation kinetics on different sites follows the order as Pd(110) < Pd(111) < Pd(100) whereas, in H2SO4 medium, it follows the sequence as Pd(111) < Pd(110) < Pd(100). The lower activity on (111) sites than (110) in H2SO4 medium was attributed to the greater adsorption of sulphate ions on the former sites than the later.45 Therefore, in the present electrolyte medium (H2SO4), deposition of Pt on Pd (111) sites is less probable than on other sites during the short reaction time of 2


13


minutes (longer reaction time is avoided to minimize the Pt deposition on carbon support by direct FA oxidation). Figure 2 also shows that the CV of Pd/C has two sharp peaks in HUPD region, peak I and peak II, apparently corresponding to Pd (111) and Pd (100), while, HUPD happens on Pd (110) in a wide potential range from 0.02 to 0.3 VRHE.47,48 Interestingly, in the first sweep cycle of Pt@Pd/C/GC, the disappearance of peak II (see Fig. 2) coincided with the appearance of current peak due to the oxidation of adsorbed CO. Thus, it can be understood that Pd (100) sites are covered by CO adsorbed Pt. More information on the specific site deposition can be obtained from the deconvolution of the CO oxidation peak and matching it with the literature reported peak potential values for the oxidation of CO on (100), (110) and (111) sites of Pt. Urchaga et al. reported that CO is oxidized on (111), (110) and (100) sites of Pt at the potentials of 0.68-0.70, 0.70-0.72 and 0.76 VRHE respectively.49 Vidal-Iglesias et al. also reported similar value (~ 0.775 VRHE) for (100) sites.50 Indeed, the deconvolution of CO oxidation peak and determination of the peak positions (obtained from the first and second derivative of the current vs potential curve of CO oxidation (Fig. S4)) on Pt@Pd/C/GC as shown in the inset of Fig. 2 indicates that CO blocks (100) sites (CO oxidation peak potential at 0.779 VRHE) and (110) sites (CO oxidation peak potential at 0.728 VRHE). This result suggests that Pt has been deposited on Pd (100) and (110) sites only, consistent with the kinetics of FA oxidation on Pd as mentioned earlier, which leads to an ultrathin layer of Pt on Pd core. 3.3 Synthesis of Pt@Pd/C in large scale and characterization To make Pt(shell)@Pd(core)/C nanoparticles in bulk scale, freshly synthesized PdS/C and PdF/C nanoparticles, separately, were used as seed particles to deposit Pt atoms on Pd core using FA as the reducing agent to self restrain the Pt deposition (see section 2.3 in experimental methods). Cyclic voltammograms of Pd/C and Pt@Pd/C are shown in the Fig. S5 and the essential features


14

Page 14 of 32

Page 15 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


remain identical to those obtained on a microgram scale on GC. Line scan and elemental mapping by scanning transmission electron microscopy (STEM) on Pt@Pd/C nanoparticles confirm the shell@core structure of Pt@Pd nanoparticle as shown in the Figs. 3 and S6. Elemental compositions of Pt and Pd in Pt@Pd/C were determined by SEM-EDS, and Fig. S7 shows SEM-EDS spectra with Pt and Pd compositions. X-Ray diffractograms were used to get the information about phase change of the particles before and after Pt deposition by measuring the lattice parameter values. Figure 4 shows the X-ray diffractograms of Pd/C and Pt@Pd/C nanoparticles along with commercial 20 wt % Pt/C. Interestingly, the X-Ray diffraction reflections corresponding for different planes for the Pt@PdS/C sample were shifted to higher angles compared to the PdS/C sample suggesting a lower lattice parameter. The lattice parameter values suggest that Pt deposition on PdS compressed the core by 2.7 %. However, even after the compression, its lattice parameter did not attain the value corresponding to that of PdF, which indicates that the Pt@PdS sample still has some H atoms absorbed inside its core. The lattice parameter values of all the samples, measured by unit cell refinement, are listed in the Table 1. Table 1. Lattice Parameter Values of Pd/C, Pt@Pd/C and Commercial Pt/C

sample

PdS/C

Pt@PdS/C

PdF/C

Pt@PdF/C

commercial Pt/C

lattice parameter (Å)

4.019a

3.910b

3.891b

3.886b

3.907c

For unit cell refinement, a Pd (H-loaded) (01-087-0637), b Pd (00-046-1043), and c Pt (00-0040802) were taken as references


15


Figure 3. (a) Line scan and (b) Elemental mapping of Pt@PdS/C sample by STEM.


16

Page 16 of 32

Page 17 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. XRD spectra of Pd/C, Pt@Pd/C and commercial Pt/C. 3.4 Investigating Pt deposition on Carbon support without any Pd seed particles The standard electrode potential of FA oxidation (HCOOH ↔ CO2 + 2H+ + 2e-, -0.200 VSHE) is lower than the standard potential for reduction of Pt ions (PtCl6-2 + 4 e- ↔ Pt + 6 Cl-, 0.705 VSHE).51 Thus, there is a plausibility that Pt+4 ions can be reduced to Pt by FA oxidation even in the absence of Pd seed particles, nonetheless, the short reaction time of less than 3 minutes was maintained. To check this possibility, only Vulcan carbon was used instead of Pd/C by following


17


the same procedure as to make Pt@Pd/C and this sample has been denoted as Pt/C. Indeed line scan and elemental mapping analysis of Pt@PdS/C sample confirmed the presence of Pt nanoparticles on carbon (Fig. 3) by direct reduction of Pt ions by FA. However the amount of Pt deposited on carbon is very small (≈ 5 wt %) as confirmed by thermogravimetric analysis of the Pt/C samples. Thermogravimetric analysis curve, CV, XPS and SEM-EDS spectrum of Pt/C are shown in Fig. S8. 3.5 Correlation between ORR activity, lattice parameter and XPS binding energy The Pt@Pd/C catalysts having two different lattice parameter values were evaluated for ORR in oxygen saturated 0.1 M HClO4 solution and their activities were compared with both Pd/C (homemade) and the commercial 20 wt % Pt/C. The ORR polarization curves after correcting for the double layer capacitance are shown in Fig. 5. Since, ECSA of PdF/C is very low when compared to PdS/C and commercial Pt/C, it is reasonable to compare ORR activities per unit ECSA instead of per unit mass. The activities based on kinetic current at 0.9 VRHE (see supporting information for calculations) are shown in the inset of Fig. 5. The results clearly elucidate that formation of a Pt shell on Pd core increased its ORR activity. This is irrespective of the core composition as the increase was observed for both PdF and PdS based nanoparticles. Importantly, the surface area specific activity of Pt@PdF/C catalyst (lattice parameter = 3.886 Å) was higher than that of commercial Pt/C but for the Pt@PdS/C catalyst (lattice parameter = 3.910 Å), it was lower. This behaviour of changing catalytic activity with lattice parameter of Pt@Pd particles can be explained from the volcano plot of ORR activity and oxygen binding energy on different metals as reported by Norskov et al.,.52 The plot indicates that ORR activity on Pt can be increased by decreasing oxygen binding energy on the Pt surface. Along with oxygen, OH binding energy also plays vital role in ORR. Since, they are correlated with each other almost


18

Page 18 of 32

Page 19 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


linearly on elemental surfaces,52 only O binding energy is referred to in the present discussion. The O binding energy can be manipulated by a shell@core design, which changes the electronic state of Pt by ligand and structural effects. This change in electronic state can be monitored by the change in d-band centre position of Pt from its pure state after it is deposited on different metal substrates. Even though, XPS analysis doesn’t give the exact d-band centre values, according to recent work by M.A. El-Sayed group, it can give the comparative position of the dband centre of different Pt based samples.53 Accordingly, the XPS BE and the d-band centre energy of Pt are related as the increase in the XPS BE is the result of decrease in the d-band centre value, which leads to less oxygen binding energy on Pt surface. X-Ray photoelectron spectroscopic BE values of Pt and Pd metals in Pd/C and Pt@Pd/C are given in the Table 2 and their XPS spectra are shown in Fig. S9. Since, intensity of XPS peaks of Pt in Pt/C is very low (due to very low Pt loading on C) when compared to Pt intensity of Pt@Pd/C samples (see Figs. S8 and S9), it is assumed that Pt nanoparticles made by direct FA oxidation do not have significant contribution to the XPS intensity of Pt in Pt@Pd/C samples as well as for ORR activity. X-Ray photoelectron spectroscopic BE values of Pt(0) 4f7/2 in Pt@PdF/C and Pt@PdS/C samples were 71.32 and 71.11 eV, respectively. This indicates that the d-band centre of Pt is lower in Pt@PdF/C than Pt@PdS/C. As a consequence, oxygen binding energy during ORR on Pt is lower on Pt@PdF/C and higher on Pt@PdS/C, correspondingly, ORR activity is more on Pt@PdF/C than Pt@PdS/C. Figure 6 shows the correlation of ORR activity (kinetic current per unit ECSA at 0.9 VRHE) with XPS BE of Pt for different lattice parameter values of Pt@Pd/C, which suggests that ORR activity was inversely proportional to the lattice parameter and directly proportional to the XPS BE of Pt in Pt@Pd particles. For the sake of comparison, data of commercial Pt/C is also shown in this figure.


19


Page 20 of 32

Table 2. XPS BEs (in eV) of Pt and Pd Elements in Pd/C, Pt@Pd/C and Commercial Pt/C commercial Pt/C

PdF/C

Pt@PdF/C

PdS/C

Pt@PdS/C

Pt (0) 4f7/2

71.23

--

71.32

--

71.11

Pd (0) 3d5/2

--

335.48

335.23

335.71

335.02


20

Page 21 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. ORR polarization curves of (a) Pt@PdS/C and PdS/C, (b) Pt@PdF/C and PdF/C along with commercial Pt/C in O2 saturated 0.1 M HClO4 solution at the scan rate of 10 mV.s-1 at 1600 RPM. Inset: Comparison of ORR specific kinetic activities at 0.9 VRHE.


21


Figure 6. Plot of lattice parameter, XPS BE of Pt 4f7/2 and kinetic current density of Pt@Pd/C samples along with commercial Pt/C.

It is widely known that in aqueous solutions, oxygen is electrochemically reduced either by four electron reduction to water or by two electron reduction to hydrogen peroxide.54 In this regard, it would be interesting to know whether the shell(Pt)@core(Pd)/C nanoparticles follow the same four electron oxygen reduction mechanism as on Pt and Pd metals, and does the change in lattice parameter of Pt@Pd nanoparticles affect the four electron reduction mechanism? Levich analysis can provide answers to these questions. The number of electrons transferred


22

Page 22 of 32

Page 23 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


during ORR on Pd/C, Pt@Pd/C and commercial Pt/C as obtained from the Levich plots (at four different rotation rates (Fig. S10)) was found to be ≈ 4. Therefore, it can be concluded that similar to the individual Pt and Pd metal catalysts, the shell(Pt)@core(PdF)/C catalyst also reduces the oxygen with four electrons, and the expansion of the lattice of the catalyst (Pt@PdS/C) doesn’t alter the four electron oxygen reduction mechanism. 3. CONCLUSION The difference in the kinetic resistance of an electrochemical reaction on two different metals was used to make shell@core nanoparticles having ultrathin shells. To make carbon supported Pt(shell)@Pd(core) nanoparticles, formic acid was chosen as reducing agent, since, it has high oxidation overpotential on Pt than that on Pd. Surface characterizations, XPS and SEM-EDS confirmed the Pt deposition. Electrochemical characterization showed that Pd core is covered by CO adsorbed Pt shell, which inhibits further Pt deposition on the Pt shell. Line scan and elemental mapping analysis by STEM confirmed the shell@core structure of Pt@Pd nanoparticles. These shell(Pt)@core(Pd) catalysts were studied for ORR at two different lattice parameter values. It was observed that stretching of Pt atoms on Pd core decreased the ORR activity, conversely, compression of Pt atoms increased the activity.

Supporting Information Description Synthesis procedural of Pd/C nanoparticles; Electrochemical analysis of formic acid oxidation on Pt/C and Pd/C catalysts (CVs, OCPs and chronopotentiometry plots); XRD and Morphology characterizations of carbon supported normal and H-loaded Pd samples; First and second derivative plots of the COads oxidation peak; Electrochemical characterization of Pd/C and Pt@Pd/C catalysts (CVs); STEM analysis of the Pt@PdF nanoparticles; SEM-EDS analysis of


23


Pt@Pd/C catalysts (spectra with composition values); Physico-chemical characterization of the Pt/C sample, which was made by direct formic acid oxidation (TGA plot, CV, XPS and SEMEDS spectra); XPS spectra of Pt and Pd elements of Pd/C and Pt@Pd/C and commercial Pt/C samples; Levich plots; ORR specific kinetic current calculations of Pd/C, Pt@Pd/C and commercial Pt/C catalysts.

Note Part of this work was presented in the conference of International symposium on electrocatalysis: A key to sustainable society held at Kanagawa, Japan from September 11 to 14, 2016, however, any part of this work is not published in the proceedings or anywhere else.

ACKNOWLEDGEMENTS Financial support by the Department of Science and Technology, Government of India under Ramanujan fellowship is gratefully acknowledged. We are thankful to Department of Metallurgical Engineering and Materials Science, Central Surface Analytical Facility and Sophisticated Analytical Instruments Facility at IIT Bombay for the XRD, XPS, SEM and TEM data.


24

Page 24 of 32

Page 25 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES 1. Sarkar, A.; Manthiram, A. Synthesis of Pt@Cu Core-Shell Nanoparticles by Galvanic Displacement of Cu by Pt4+ Ions and Their Application as Electrocatalysts for Oxygen Reduction Reaction in Fuel Cells. J. Phys. Chem. C 2010, 114, 4725–4732. 2. Vukmirovic, M. B.; Zhang, J.; Sasaki, K.; Nilekar, A. U.; Uribe, F.; Mavrikakis, M.; Adzic, R. R. Platinum Monolayer Electrocatalysts for Oxygen Reduction. Electrochim. Acta 2007, 52, 2257–2263. 3. Pupo, M. M. S.; López-Suárez, F. E.; Bueno-López, A.; Meneses, C. T.; Eguiluz, K. I. B.; Salazar-Banda, G. R. Sn@Pt and Rh@Pt Core-Shell Nanoparticles Synthesis for Glycerol Oxidation. J. Appl. Electrochem. 2015, 45, 139–150. 4. Das, S.; Gulotty, R.; Sumant, A. V.; Roelofs, A. All Two-Dimensional, Flexible, Transparent, and Thinnest Thin Film Transistor. Nano letters 2014, 14, 2861–2866. 5. Jeon, S.; Park, S.; Song, I.; Hur, J.-H.; Park, J.; Kim, H.; Kim, S.; Kim, S.; Yin, H.; Chung, U.-I. et al. Nanometer-Scale Oxide Thin Film Transistor with Potential for HighDensity Image Sensor Applications. ACS Appl. Mater. Interfaces 2011, 3, 1–6. 6. Sheng, J.; Lee, H.-J.; Oh, S.; Park, J.-S. Flexible and High-Performance Amorphous Indium Zinc Oxide Thin-Film Transistor Using Low-Temperature Atomic Layer Deposition. ACS Appl. Mater. Interfaces 2016, 8, 33821–33828. 7. Chander, S.; Dhaka, M. S. Enhanced Structural, Electrical and Optical Properties of Evaporated CdZnTe Thin Films Deposited on Different Substrates. Mater. Lett. 2017, 186, 45–48. 8. Ganesh, V.; Yahia, I. S.; AlFaify, S.; Shkir, M. Sn-Doped ZnO Nanocrystalline Thin Films with Enhanced Linear and Nonlinear Optical Properties for Optoelectronic Application. J. Phys. Chem. Solids 2017, 100, 115–125. 9. Ahn, S. H.; Liu, Y.; Moffat, T. P. Ultrathin Platinum Films for Methanol and Formic Acid Oxidation: Activity as a Function of Film Thickness and Coverage. ACS Catal. 2015, 5, 2124–2136. 10. Muller, K.; Bugnicourt, E.; Latorre, M.; Jorda, M.; Sanz, Y. E.; Lagaron, J. M.; Miesbauer, O.; Bianchin, A.; Hankin, S.; Bolz, U. et al. Review on the Processing and Properties of Polymer Nanocomposites and Nanocoatings and their Applications in the Packaging, Automotive and Solar Energy Fields. Nanomaterials 2017, 7, 74–120.


25


11. Dimitrov, N. Recent Advances in the Growth of Metals, Alloys, and Multilayers by Surface Limited Redox Replacement (SLRR) Based Approaches. Electrochim. Acta 2016, 209, 599–622. 12. Brankovic, S.R.; Wang, J.X.; Adzic, R.R. Metal Monolayer Deposition by Replacement of Metal Adlayers on Electrode Surfaces. Surface Science 2001, 474, L173–L179. 13. Taufany, F.; Pan, C.-J.; Rick, J.; Chou, H.-L.; Tsai, M.-C.; Hwang, B.-J.; Liu, D.-G.; Lee, J.-F.; Tang, M.-T.; Lee,Y.-C. et al. Kinetically Controlled Autocatalytic Chemical Process for Bulk Production of Bimetallic Core-Shell Structured Nanoparticles. ACS Nano 2011, 5, 9370–9381. 14. Hsieh, Y.-C.; Zhang, Y.; Su, D.; Volkov, V.; Si, R.; Wu, L.; Zhu, Y.; An, W.; Liu, P.; He, P. et al. Ordered Bilayer Ruthenium-Platinum Core-Shell Nanoparticles as Carbon Monoxide-Tolerant Fuel Cell Catalysts. Nat. Commun. 2013, 4, 2466–2474. 15. Liu, Y.; Gokcen, D.; Bertocci, U.; Moffat, T. P. Self-Terminating Growth of Platinum Films by Electrochemical Deposition. Science 2012, 338, 1327–1330. 16. Brimaud, S.; Behm, R. J. Electrodeposition of a Pt Monolayer Film: Using Kinetic Limitations for Atomic Layer Epitaxy. J. Am. Chem. Soc. 2013, 135, 11716–11719. 17. Zhang, J.; Vukmirovic, M. B.; Sasaki, K.; Nilekar, A. U.; Mavrikakis, M.; Adzic, R. R. Mixed-Metal Pt Monolayer Electrocatalysts for Enhanced Oxygen Reduction Kinetics. J. Am. Chem. Soc. 2005, 127, 12480–12481. 18. Sasaki, K.; Naohara, H.; Cai, Y.; Choi, Y. M.; Liu, P.; Vukmirovic, M. B.; Wang, J.X.; Adzic, R. R. Core-Protected Platinum Monolayer Shell High-Stability Electrocatalysts for Fuel-Cell Cathodes. Angew. Chem. Int. Ed. 2010, 49, 8602 –8607. 19. Zhang, J.; Lima, F. H. B.; Shao, M. H.; Sasaki, K.; Wang, J. X.; Hanson, J.; Adzic, R. R. Platinum Monolayer on Nonnoble Metal-Noble Metal Core-Shell Nanoparticle Electrocatalysts for O2 Reduction. J. Phys. Chem. B 2005, 109, 22701–22704. 20. Mahesh, I.; Sarkar, A. Electrochemical Study of Oxygen Reduction on a CarbonSupported Core-Shell Platinum-Gold Electrocatalyst with Tuneable Gold Surface Composition. ChemElectroChem 2016, 3, 836–845.


26

Page 26 of 32

Page 27 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21. Peng, L.; Gan, L.; Wei, Y.; Yang, H.; Li, J.; Du, H.; Kang, F. Pt Submonolayers on Au Nanoparticles: Coverage-Dependent Atomic Structures and Electrocatalytic Stability on Methanol Oxidation. J. Phys. Chem. C 2016, 120, 28664–28671. 22. Amri, Z. A.; Mercer, M. P.; Vasiljevic, N. Surface Limited Redox Replacement Deposition of Platinum Ultrathin Films on Gold: Thickness and Structure Dependent Activity towards the Carbon Monoxide and Formic Acid Oxidation reactions. Electrochim. Acta 2016, 210, 520–529. 23. Adzic, R. R.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Shao, M.; Wang, J. X.; Nilekar, A. U.; Mavrikakis, M.; Valerio, J. A.; Uribe, F. Platinum Monolayer Fuel Cell Electrocatalysts. Top.Catal. 2007, 46, 249–262. 24. Kitchin, J. R.; Norskov, J. K.; Barteau, M. A.; Chen, J. G. Role of Strain and Ligand Effects in the Modification of the Electronic and Chemical Properties of Bimetallic Surfaces. Phys. Rev. Lett. 2004, 93, 156801–156804. 25. Zlotea, C.; Cuevas, F.; Paul-Boncour, V.; Leroy, E.; Dibandjo, P.; Gadiou, R.; VixGuterl, C.; Latroche, M. Size-Dependent Hydrogen Sorption in Ultrasmall Pd Clusters Embedded in a Mesoporous Carbon Template. J. Am. Chem. Soc. 2010, 132, 7720–7729. 26. Bennett, P. A.; Fuggle, J. C. Electronic Structure and Surface Kinetics of Palladium Hydride Studied with X-ray Photoelectron Spectroscopy and Electron-Energy-Loss Spectroscopy. Phys. Rev. B 1982, 26, 6030–6039. 27. Teschner, D.; Pestryakov, A.; Kleimenov, E.; Havecker, M.; Bluhm, H.; Sauer, H.; KnopGericke, A.; Schlogl, R. High-Pressure X-ray Photoelectron Spectroscopy of Palladium Model Hydrogenation Catalysts.: Part 1: Effect of Gas Ambient and Temperature. J. Catal. 2005, 230, 186–194. 28. Singh, R. K.; Rahul, R.; Neergat, M. Stability Issues in Pd-Based Catalysts: The Role of Surface Pt in Improving the Stability and Oxygen Reduction Reaction (ORR) Activity. Phys. Chem. Chem. Phys. 2013, 15, 13044–13051. 29. Peles, A.; Shao, M.; Protsailo, L. Pt Monolayer Electrocatalyst for Oxygen Reduction Reaction on Pd-Cu Alloy: First-Principles Investigation. Catalysts 2015, 5, 1193–1201. 30. Jennings, P. C.; Aleksandrov, H. A.; Neyman, K. M.; Johnston, R. L. O2 Dissociation on M@Pt Core Shell Particles for 3d, 4d, and 5d Transition Metals. J. Phys. Chem. C 2015, 119, 11031–11041.


27


31. Tom, J.; Andreas, H. A. The Influence of Carbon-Oxygen Surface Functional Groups of Carbon Electrodes on the Electrochemical Reduction of Hemoglobin. Carbon 2017, 112, 230–237. 32. O’Connor, D. J.; Sexton, B. A.; Smart, R. S. C. Surface Analysis Methods in Materials Science; Springer-Verlag Berlin Heidelberg: New York, 2013. 33. Brett, C. M. A.; Brett, A. M. O. Electrochemistry Principles, Methods, and Applications; Oxford University Press: Oxford, 1993. 34. Trassatti, S.; Petrii, O. A.; Real Surface Area Measurements in Electrochemistry. J. Electroanal. Chem. 1992, 327, 353–376. 35. Gabrielli, C.; Grand, P. P.; Lasia, A.; Perrot, H. Investigation of Hydrogen Adsorption and Absorption in Palladium Thin Films II. Cyclic Voltammetry. J. Electrochem. Soc. 2004, 151, A1937–A1942. 36. Phan, T.-H.; Schaak, R. E. Polyol Synthesis of Palladium Hydride: Bulk Powders vs. Nanocrystals. Chem. Commun. 2009, 0, 3026–3028. 37. Nalajala, N.; Saleha, W. F. G.; Ladewig, B. P.; Neergat, M. Sodium Borohydride Treatment: A Simple and Effective Process for the Removal of Stabilizer and Capping Agents from Shape-Controlled Palladium Nanoparticles. Chem. Commun. 2014, 50, 9365–9368. 38. Teranishi, T.; Miyake, M. Size Control of Palladium Nanoparticles and their Crystal Structures. Chem. Mater. 1998, 10, 594-600. 39. Dutta, B. N.; Dayal, B. Lattice Constants and Thermal Expansion of Palladium and Tungsten up to 878oC by X-Ray Method. Phys. Stat. Sol. 1963, 3, 2253–2259. 40. Bugaev, A. L.; Guda, A. A.; Lomachenko, K. A.; Shapovalov, V. V.; Lazzarini, A.; Vitillo, J. G.; Bugaev, L. A.; Groppo, E.; Pellegrini, R.; Soldatov, A. V. et al. Core-Shell Structure of Palladium Hydride Nanoparticles Revealed by Combined X-ray Absorption Spectroscopy and X-ray Diffraction. J. Phys. Chem. C 2017, 121, 18202–18213. 41. Sinha, S.; Badrinarayanan, S.; Sinha, A. P. B. The Pd-H System Revisited: An XPS and UPS Study. J. Phys. F: Met. Phys. 1986, 16, L229–L232.


28

Page 28 of 32

Page 29 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


42. Schlapbach, L.; Burger, J. P. A New XPS/UPS Study of the Electronic Structure of PdH0.6. J. Phys. Lett-Paris 1982, 43, L273–L276. 43. Luque, G. C.; de Chialvo, M. R. G.; Chialvo, A. C. Kinetic Study of the Formic Acid Oxidation on Steady State Using a Flow Cell. J. Electrochem. Soc. 2017, 164, H748– H754. 44. Wojdyr, M. Fityk: A General-Purpose Peak Fitting Program. J. Appl. Cryst. 2010, 43, 1126–1128. 45. Hoshi, N.; Kida, K.; Nakamura, M.; Nakada, M.; Osada, K. Structural Effects of Electrochemical Oxidation of Formic Acid on Single Crystal Electrodes of Palladium. J. Phys. Chem. B 2006, 110, 12480–12484. 46. Baldauf, M.; Kolb, D. M. Formic Acid Oxidation on Ultrathin Pd Films on Au(hkl) and Pt(hkl) Electrodes. J. Phys. Chem. 1996, 100, 11375-11381. 47. Hara, M.; Linke, U.; Wandlowski, T. Preparation and Electrochemical Characterization of Palladium Single Crystal Electrodes in 0.1 M H2SO4 and HClO4: Part I. Low-Index Phases. Electrochim. Acta 2007, 52, 5733–5748. 48. Hoshi, N.; Nakamura, M.; Haneishi, H. Structural Effects on Methanol Oxidation on Single Crystal Electrodes of Palladium. Electrochemistry 2017, 85, 634–636. 49. Urchaga, P.; Baranton, S.; Coutanceau, C.; Jerkiewicz, G. Electro-oxidation of COchem on Pt Nanosurfaces: Solution of the Peak Multiplicity Puzzle. Langmuir 2012, 28, 3658– 3663. 50. Vidal-Iglesias, F. J.; Solla-Gullón, J.; Campina, J. M.; Herrero, E.; Aldaz, A.; Feliu, J. M. CO Monolayer Oxidation on Stepped Pt(S) [(n-1)(100)x(110)] Surfaces. Electrochim. Acta 2009, 54, 4459–4466. 51. Antelman, M. The Encyclopedia of Chemical Electrode Potentials; Plenum Press: New York, 1982. 52. Norskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108, 17886–17892.


29


53. Bordley, J. A.; El-Sayed, M. A. Enhanced Electrocatalytic Activity toward the Oxygen Reduction Reaction through Alloy Formation: Platinum-Silver Alloy Nanocages. J. Phys. Chem. C 2016, 120, 14643–14651. 54. Song, C.; Zhang, J. In PEM Fuel Cell Electrocatalysts and Catalyst Layers Fundamentals and Applications; Zhang, J., Eds.; Springer: London, 2008; pp 89-134.


30

Page 30 of 32

Page 31 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic


31



Page 32 of 32

Self-Restraining Electroless Deposition for Shell ... - ACS Publications

Recommend Documents