Effect of Surface Ni on Oxygen Reduction Reaction in Dealloyed

Subscriber access provided by Iowa State University | Library

Kinetics, Catalysis, and Reaction Engineering

Effect Of Surface Ni On Oxygen Reduction Reaction In Dealloyed Nanoporous Pt-Ni Venkataramana Imandi, and Abhijit Chatterjee Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05204 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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

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.

Subscriber access provided by Iowa State University | Library

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 38 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

Industrial & Engineering Chemistry Research

Nanoporosity evolution in Pt-Ni nanoparticles

ACS Paragon Plus Environment

ORR mechanism

Industrial & Engineering Chemistry Research 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

Effect Of Surface Ni On Oxygen Reduction Reaction In Dealloyed Nanoporous Pt-Ni Venkataramana Imandi and Abhijit Chatterjee* Department of Chemical Engineering, Indian Institute of Technology Bombay, Mumbai 400076 India *Email: [email protected]

1 ACS Paragon Plus Environment

Page 2 of 38

Page 3 of 38 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 The high specific-surface area nanoporous Pt-Ni synthesized via selective dissolution of Ni exhibits a high catalytic activity towards oxygen reduction reaction (ORR). Using kinetic Monte Carlo simulations, we show that the elemental distribution in nanoporous Pt-Ni is different from the well-understood Pt-skin Pt-Ni catalyst. In nanoporous Pt-Ni both surface and sub-surface layer are rich in Pt. Crucially, a small amount of Ni is present in the surface layer. Using density functional theory we show that the presence of Ni in the surface layer in nanoporous Pt-Ni can speed-up ORR by orders of magnitude. Different Pt-Ni atom arrangements in the surface layer are considered. Our simulations show that the OH formation step is the rate limiting step for ORR is same for Pt-Ni(111) surface. The low concentration of surface Ni explains why nanoporous Pt-Ni provides only 10 times improvement in ORR rates over the pure Pt catalyst.

Submitted for the Special Issue of I&EC Research, Vinay Juvekar Festschrift



1. INTRODUCTION The oxygen reduction reaction (ORR) is of importance to low temperature fuel cell applications1. The highest ORR activity is reported with a (111) Pt-skin layer on an extended Pt-Ni alloy surface, which possesses an activity about 90 times higher than that of pure Pt catalyst2. The possibility of a low cost, high activity Pt-Ni catalyst in comparison to pure Pt has provided in the past a strong motivation for understanding the mechanistic details of the ORR at the Pt(111) and Pt-Ni(111) surface2–13. In this work, we are interested in nanoporous Pt-Ni. Snyder et al. demonstrated that nanoporous 15-20 nm sized Pt-Ni nanoparticles (NP) can provide a specific ORR activity that is about five times higher than that of Pt14. Given the 10-times loss in ORR activity going from extended Pt-Ni(111) surfaces to alloy nanoparticles15, a NP with Pt-skin layer would be roughly about 9 times more active than pure Pt. It implies that the activities of nanoporous Pt-Ni and NPs with Pt-skin are comparable12. The origin of the high ORR activity in nanoporous Pt-Ni is relatively less understood compared to the Pt-skin layer. The goal of this work is to study the ORR mechanism on nanoporous Pt-Ni. Most explanations for the high activity in Pt-skin layer originate from computational density functional theory (DFT) calculations. Here the surface layer contains Pt, sub-surface (second) layer is 100% Ni or in some cases 50-100% Ni16. The main argument is that at thermodynamic equilibrium Pt is known to preferentially segregate to the surface in Pt3Ni alloys17. Due to compositional oscillations at the Pt-Ni surface, it is reasonable to assume Ni in the subsurface layer. To simplify the problem it is common to assume that 100% Ni is present in layers further below. Based on this assumed elemental Pt/Ni distribution, DFT studies have focused on nanoparticles11,12,18–23 and slab models of Pt(111), (100) and (110)2,4,13,24. The computed activation barriers for Pt(111), unreconstructed Pt(100) and their Pt-Ni analogs shows that the water 3 ACS Paragon Plus Environment

Page 4 of 38

Page 5 of 38 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


formation step is the rate determining step (RDS) on both Pt(100) and Pt/Ni(100), whereas OHformation is the RDS on Pt(111) and O2 dissociation is the RDS on Pt/Ni(111) surface25. Both ligand effect and strain induced by subsurface and bulk Ni play an important role in the binding of adsorbates on the alloy surface and significantly influence the activity. A brief review of Pt-alloy nanostructures catalytic activity and stability at fuel cell operating conditions is provided in Ref. 21.

Nanoporous Pt-Ni is different. Nanoporous Pt-Ni is synthesized far-from-equilibrium via selective dissolution of Ni from a starting Pt-Ni alloy material that is rich in Ni. As the Ni is dissolved from the alloy a Pt-rich nanoporous material26 is eventually obtained that contains a residual amount of Ni. The surface and sub-surface is rich in Pt. Since dissolution proceeds at under-coordinated sites such as at step edges, some of the residual Ni could possibly be present at the terrace sites in the surface where the Ni can remain for long periods of time. One would expect that the presence of few Ni atoms in the surface layer in an overall Pt rich material should not affect the catalytic activity, but this needs to be confirmed using DFT. The influence of Ni-content in the surface layer of nanoporous Pt catalysts on ORR are lacking. To understand the origin of the high activity in nanoporous Pt-Ni, we determine i) the elemental distribution within the nanoporous material and ii) the effect of this distribution on the ORR mechanism. First, the elemental distribution in the nanoporous structure is studied as a function of time using the kinetic Monte Carlo (KMC) method27. The KMC method is an ideal tool for our purpose because it enables the study of diffusion and reaction processes during dissolution while reaching experimentally long timescales. In particular, we follow the morphological evolution of an 18 nm Pt-Ni nanoparticle at constant overpotential and temperature. The resulting nanoporous structure contains a high density of steps, and {100} and {111}-oriented facets. An important finding in our 4 ACS Paragon Plus Environment


KMC simulations is the presence of a small amount of Ni in the surface layer. We perform a large number of (combinatorial) DFT calculations based on different Pt-Ni atomic arrangements determined from the KMC simulations. The DFT calculations confirm that the presence of surface Ni, even though at low concentration, can explain the higher activity of nanoporous Pt-Ni compared to Pt catalyst. The paper is divided into the following sections. Details of the KMC and the DFT calculations are presented in Sec. 2. The results obtained from these calculations are discussed in Sec. 3. Finally, the conclusions from this work are presented in Sec. 4.

2. METHODS

Figure 1. Nanoporosity evolution in an 18 nm Pt-Ni nanoparticle in terms of the extent of dissolution (EOD). Pink and green atoms denote Ni and Pt, respectively.


Page 6 of 38

Page 7 of 38 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


2.1 Kinetic Monte Carlo calculations An 18 nm truncated octahedron nanoparticle (NP) consisting of a random alloy 𝑃𝑡0.2𝑁𝑖0.8 is considered (see top-left panel in Figure 1). The truncated octahedron was chosen because of its lower surface energy compared to other nanoparticle shapes. The Pt-Ni alloy adopts a facecentered cubic lattice structure. It is assumed that no mass transfer limitations are present at the electrolyte side. The selective dissolution process is simulated using a standard KMC model at a constant temperature of 80 °C. In the model, diffusion proceeds via hopping of an atom to its nearest neighbor vacant site. Both electroactive (EA), i.e., Ni, and electrochemically-noble (EN), i.e., Pt, atoms participate in hopping events. The rate constant for hopping is given by

(

𝑘𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 = ν𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 exp ―

𝜖0 + 𝑁𝜖 𝑘𝐵𝑇

)

.

(1)

Here 𝜈𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 is the diffusion pre-exponential factor, 𝑁 is the number of nearest neighbors, 𝜖 is the bond energy, 𝑘𝐵 is the Boltzmann constant and 𝑇 is the absolute temperature. 𝜈𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 is 1013 𝑠 ―1. The interaction between the hopping atom with the surface that are not at the first nearestneighbor positions are contained within 𝜖0 = ―0.56 𝑒𝑉28,29. The bond energy 𝜖 is assumed to be identical for Pt and Ni and given by 0.25 eV. Only EA atoms are allowed to be selectively dissolved. The dissolution rate constant is given by

(

𝑘𝑑𝑖𝑠𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 = 𝜈𝑑𝑖𝑠𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛exp ―

)

𝑁𝜖 ― 𝜙 . 𝑘𝐵𝑇

(2)

The dissolution pre-exponential factor 𝜈𝑑𝑖𝑠𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 is taken as 109 𝑠 ―1 and the overpotential 𝜙 is 1.75 eV. The parameters are chosen based on our earlier modeling studies on synthesis of



nanoporous materials via selective dissolution30. Under-coordinated atoms, which possess smaller values of 𝑁, can undergo rapid diffusion/dissolution.

2.2 Density function theory calculations Ab Initio simulations within density functional theory (DFT) framework were performed with the VASP (Vienna Ab Initio Simulation Package) software package31. The pseudopotential of H, O, Ni and Pt were ultrasoft type32. The ORR is assumed to comprise of three elementary steps as discussed later. The DFT calculations reveal the RDS for the ORR as well as effect of surface Ni on the reaction rates. We assume that species like OH can bind strongly to the low coordination surface sites33–35, e.g., at steps, as a result of which these sites are inaccessible for adsorption of molecular O2 and further steps of the ORR. Therefore, we employ slab calculations. Pt(111) surface was simulated using 3x2 slab with a calculated lattice constant of 3.98 Å. The experimental lattice constant of Pt is 3.92 Å. The slab contained four layers with each layer having 12 Pt atoms. The simulation box size was 8.443 Å x 9.749 Å x 9.192 Å. A 10 Å vacuum space was introduced along z-direction to avoid interaction between periodic images. The generalized gradient approximation method was invoked for the exchange-correlation functional, in which PW91 functional36 was used. The planewave cutoff energy of 350 eV was found to be sufficient for our calculations. Methfessel-Paxton smearing of order 1 with 0.2 eV was utilized. The fourth layer was constrained to crystal lattice positions, while the top three are allowed to relax. Ionic optimization was carried out by quasi-Newton algorithm. The tolerance of 0.001 eV/Å was used for the force throughout the calculation. A Davidson algorithm was employed for electronic self-consistent calculation, and its tolerance was 10-5. Brillouin zone was sampled with


Page 8 of 38

Page 9 of 38 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


2x2x1 k-point mesh including Gamma point for all calculations. The minimum energy path is found by NEB method37 with 7 images. The highest energy image in that path was assumed to be close to the saddle location. 5.0 eV/Å2 spring constant was used to prevent images from sliding. To study the effect of water on the ORR mechanism in the presence of surface Ni, we used the approach of Ref.

38.

Pt(111) surface was simulated using 3x2x1 slab, in which each layer

contains 12 atoms. A single layer of water, i.e., 2x2x1 units, is created on top of Pt(111) surface. The water layer contains 8 water molecules. Every alternate water molecule (O-H part) is oriented towards metal surface (H-down). The O-H bonds in the remaining water molecules are parallel to the surface. An O2-molecule is placed on the Ni-Pt-surface by removing 2 water molecules. Similarly, OH and H2O is placed on the top of Pt-surface by removing one water molecule. To limit our computational requirement, Pt surfaces containing either zero or one Ni atom at various positions were studied. Previous experiments39,40 and theoretical41 studies suggest that Pt(100) surface reconstructs to hex phase upon adsorption of oxygen. We perform the study only for the {111} surfaces which is larger in area than the {100} surface and believed to be more active. The ORR rates would depend on surface Pt-Ni atom arrangements. The effect of the surface Pt-Ni atom arrangements on the reaction rates is investigated to ascertain whether presence of Pt-Ni regions can synergistically speed-up the ORR rates. This effect where catalytic properties of an ensemble of atoms at the surface change as the chemical composition of the ensemble is altered is known as the ensemble effect42.

3. RESULTS



3.1 Nanoporosity evolution The nanoporosity evolution of the 18 nm Pt-Ni nanoparticle is followed in terms of the extent of dissolution (EOD), i.e., fraction of electrochemically active Ni atoms that have dissolved in the KMC simulation. Figure 1 shows the evolution from the truncated octahedral nanoparticle to a nanoporous structure. The extent of dissolution in nanoporous Pt synthesized in most experiments can reach up to 98%. The dissolution process is characterized by three different stages. The large availability of Ni at short timescales allows for rapid dissolution at the initial stage. Ni at low coordination number sites such as step edges dissolve more easily than Ni present at the terrace (see Eq. (2)). A porous shell with surface sites passivated with Pt and a core containing unreacted Ni is formed. Ligaments start appearing at nearly 40% EOD. During the intermediate stage the core shrinks and ultimately a fully porous structure is formed at 80 % EOD. At longer timescales (EOD>0.9), the rate-limiting step of the dealloying process is the dissolution of Ni from high-coordination sites like terrace and in the bulk. Coarsening of ligament is also observed at these timescales.


Page 10 of 38

Page 11 of 38 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. Fraction of atoms in surface (blue line) and sub-surface (green line) being Pt are shown as a function of the extent of dissolution. Solid and dashed lines show fraction for the {111} and {100} facets, respectively. Top-left and bottom-right insets show the porous nanoparticle at 90 and 98% EOD, respectively. Green and black atoms denote Pt and Ni.

Figure 2 shows the surface and sub-surface layer composition as a function of EOD. Initially, both layers contain 20% Pt. Compositions shown in Figure 2 are averaged values over 20 stochastic realizations. The error bars are small and not shown in Figure 2. Rapid dissolution of Ni at the {111} and {100} facets results in a high fraction of surface atoms being Pt (blue solid and dashed line in Figure 2 for {111} and {100} facets, respectively). At longer timescales, the Pt fraction at {111} surface varies between 0.991-0.998 for 90-98% EOD. Although the Ni:Pt ratio 10 ACS Paragon Plus Environment


in the surface can be as low 1:500 the Ni sites could be important provided the reaction rates are at least four orders of magnitude larger than on the pure Pt surface. This number is obtained based on the site-specific rates for a surface containing 𝑛𝑁𝑖 and 𝑛𝑃𝑡 sites, such that 𝑛𝑃𝑡 ≫ 𝑛𝑁𝑖. Suppose the overall ORR rate is 𝑟𝑁𝑖 and 𝑟𝑃𝑡 at the Ni- and Pt-containing sites, the activity obtained with PtNi and Pt catalysts will scale approximately as (𝑛𝑃𝑡𝑟𝑃𝑡 + 𝑛𝑁𝑖𝑟𝑁𝑖) 𝑛𝑃𝑡𝑟𝑃𝑡 ≈ 𝑛𝑁𝑖𝑟𝑁𝑖 𝑛𝑃𝑡𝑟𝑃𝑡 when 𝑛𝑁𝑖 𝑟𝑁𝑖 ≫ 𝑛𝑃𝑡𝑟𝑃𝑡. Requiring 𝑛𝑁𝑖𝑟𝑁𝑖 𝑛𝑃𝑡𝑟𝑃𝑡 > 10 we conclude that 𝑟𝑁𝑖 𝑟𝑃𝑡 > 10𝑛𝑃𝑡 𝑛𝑁𝑖. When 𝑛𝑃𝑡 𝑛𝑁𝑖 = 1000, we require 𝑟𝑁𝑖 𝑟𝑃𝑡 > 104. In contrast, the Pt sub-surface composition remains close to the initial value (green solid and dashed line in Figure 2 for {111} and {100} facets, respectively) till 70% EOD. The rearrangement of the atoms from within the nanoporous structure allows the Ni from the subsurface to appear at the surface and be dissolved. The Pt fraction in the sub-surface can increase to values as high as 0.89 at 98% EOD. The common assumption used in the literature of a Pt-skin over a Ni sub-surface layer is clearly not valid in case of nanoporous Pt-Ni.


Page 12 of 38

Page 13 of 38 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 3. a) Time as a function of the extent of dissolution (EOD) in the porous nanoparticle. b) Average and largest number of atoms in {111} and {100} facet. c) Fraction of atoms of each coordination number. Symbols for coordination numbers are shown in legend in panel c.



Figure 3a shows that the extent of dissolution (EOD) varies nonlinearly with time. After rapidly reaching 80% EOD, it takes three orders of magnitude longer time to dissolve 98% of the original Ni. As the dissolution events become rare (during EOD=0.8-0.98), the evolution is characterized by the coarsening process, which results in a significant increase in the average and largest number of atoms in {111} and {100} facet (Figure 3b). The final nanoporosity is achieved in minutes to hours timescales. The largest {111} facet contains up to 230 atoms. It is reasonable to employ a slab model for such facet sizes. The coordination number of Pt/Ni atoms can lie between 0-12. Bulk atoms have a coordination number of 12. The fraction of atoms with a particular coordination number is calculated on the basis of initial number of atoms in Figure 3c. The fraction of atoms in the bulk remains practically unchanged from 90-98% EOD (Figure 3c). The {111} surface (coordination number 9) is approximately twice that of the {100} surface (coordination number 8), highlighting the greater importance of studying the {111} surfaces. The timescales, material structure and EOD for the nanoporous Pt-Ni observed in our simulations are in good agreement with experiments14. For instance, the ligament diameters from Figure 1 are close to the approximately 2 nm diameters for 15 nm Pt-Ni nanoparticle observed experimentally.


Page 14 of 38

Page 15 of 38 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. Different views of nanoporous Pt-Ni particle at 90 and 98% EOD: a-b) Composite view showing {111} (blue) and {100} (orange) planes, c-d) {111} surface, e-f) {100} surface. Panels a, c and e correspond to 90% EOD, while b, d and f correspond to 98% EOD. Green and pink atoms in panels c-f denote Pt and Ni atoms, respectively.

Figure 4 shows atoms in the {111} and {100} surface. In Figure 4a-b, we can see composite views with {111} atoms shown in blue and {100} atoms in orange. A {111} atom is found on the basis of six nearest neighbor atoms lying on the {111} plane being present. Similarly, a {100} atom is found on the basis of four nearest neighbor atoms lying on the {100} plane being present. As a result, edge atoms are not included in (see Figure 4a-b). Large facets can be witnessed once 0.98 EOD is reached after coarsening. That the {111} facets are typically much larger than the {100} ones is clearly visible in Figure 4 c-f. The {111} and {100} facets contain 17.2 and 5.9 atoms on an average (Figure 4b) at 98 % EOD. An important observation is that few Ni atoms can be found at the {111} surface. The black atom in the insets of Figure 2 denotes surface Ni.



Figure 5. Different views of the nanoporous Pt-Ni particle at 90 and 98% EOD: a-b) atoms lying in the sub-surface below {111} surface and c-d) atoms lying in the sub-surface below {100} surface. Green and pink atoms denote Pt and Ni atoms, respectively.

Figure 5 shows the Pt and Ni arrangement in the sub-surface layer in the {111} and {100} facet. Although the sub-surface is predominantly rich in Pt, several Ni clusters within the subsurface layer are observed (see Figure 5 a and b). This implies that as far as DFT calculations are concerned, one can investigate the effect of Pt- and Ni-rich sub-surface separately. As mentioned in the introduction, previous studies with a Pt-skin layer and 100% Ni in subsurface and layers 15 ACS Paragon Plus Environment

Page 16 of 38

Page 17 of 38 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


have already shown that the ligand effect and strain induced by subsurface and bulk Ni plays an important role in the binding of adsorbates on the alloy surface and it enhances the ORR activity. For e.g., the rate determining step (OH formation) for Pt-skin on Pt3Ni alloy can be 15 times faster in solution and 300 times faster in the gas phase at 300 K than pure Pt43. Considering the large number of different Ni arrangements in the surface region (as performed in this work) itself is a computationally arduous task. To simplify the problem we have focused only on the effect of surface Pt-Ni atomic arrangement on the ORR with the more commonly found pure Pt sub-surface layer. In other words, the direct role of only the surface Ni in the catalytic activity is probed in our DFT calculations.

Figure 6. a) Various binding sites on Pt-Ni surface. Ensemble of Pt-Ni atoms considered b) S1-S4 for O2 dissociation and c) S1-S5 for OH formation step is shown by the shaded polygon. Atoms other than these are Pt. 16 ACS Paragon Plus Environment


Page 18 of 38

3.2 Binding energies of intermediate species The ORR mechanism involves the following reaction steps: 𝑂2(𝑔) + (𝑠)⇌𝑂2(𝑎𝑑𝑠)

(3)

𝑂2(𝑎𝑑𝑠)⇌2𝑂(𝑎𝑑𝑠)

(4)

𝑂(𝑎𝑑𝑠) + 𝐻(𝑎𝑑𝑠)⇌𝑂𝐻(𝑎𝑑𝑠)

(5)

𝑂𝐻(𝑎𝑑𝑠) + 𝐻(𝑎𝑑𝑠)⇌𝐻2𝑂(𝑎𝑑𝑠)

(6)

𝐻2𝑂(𝑎𝑑𝑠)⇌𝐻2𝑂(𝑔)

(7)

We begin by determining the preferred binding sites for 𝑂(𝑎𝑑𝑠), 𝐻(𝑎𝑑𝑠), 𝑂𝐻(𝑎𝑑𝑠) and 𝐻2 𝑂(𝑎𝑑𝑠). It is assumed that the main reactions will involve the preferred sites. To investigate the effect of the Ni-Pt surface arrangement on the binding energies, we place 0-2 Ni atoms at the surface and calculate the binding energies for top, bridge and hollow sites. Some of these sites are shown in Figure 6a. When two Ni atoms were placed, the arrangement was given by Figure 6b. The notation used to indicate metal arrangement around the three-fold hollow site involves specifying the three surface metal atoms. For instance, a Pt-Pt-Ni fcc site implies that 2 Pt and 1 Ni form the site. The Pt-Pt-Pt fcc site in the Pt+1Ni surface is away from the surface Ni atom. Similar notations are used for the bridge and top sites. 20 binding energy calculations were performed for 𝑂(𝑎𝑑𝑠), 𝐻(𝑎𝑑𝑠), 𝑂𝐻(𝑎𝑑𝑠) and 𝐻2𝑂(𝑎𝑑𝑠) to account for the various combinations. Table S1 in Supporting Information shows the calculated binding energies. Binding energies were calculated as 𝐸𝑎𝑑𝑠 = 𝐸𝑠𝑙𝑎𝑏 ― 𝑎𝑑𝑠𝑜𝑟𝑏𝑎𝑡𝑒 ― 𝐸𝑠𝑙𝑎𝑏 ― 𝐸𝑎𝑑𝑠𝑜𝑟𝑏𝑎𝑡𝑒, where 𝐸𝑠𝑙𝑎𝑏 ― 𝑎𝑑𝑠𝑜𝑟𝑏𝑎𝑡𝑒 is the energy of


Page 19 of 38 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 slab-adsorbate super-system, 𝐸𝑠𝑙𝑎𝑏 is the energy of the slab and 𝐸𝑎𝑑𝑠𝑜𝑟𝑏𝑎𝑡𝑒 is the energy of the adsorbate in vacuum.

Figure 7. Strongest binding sites for adsorbed a) H, b) O, c) OH and d) H2O.

H binding



The most stable binding site for H is the fcc site (Figure 7a). Binding energy on fcc, hcp, bridge and top in pure Pt(111) is -0.30, -0.25, -0.24 and -0.18 eV, respectively. H adsorbs slightly more strongly in the vicinity of Ni on both Pt+1Ni and Pt+2Ni surface. For e.g., binding of H at fcc(PtPt-Ni) site (-0.38 eV) is more favorable than fcc(Pt-Pt-Pt) site (-0.33 eV) in Pt+1Ni. Similarly, the binding energies at Pt-Pt-Ni and Pt-Ni-Ni fcc sites are -0.37, and -0.44 eV, respectively.

O binding Upon dissociating, O2 on Pt(111) surface results in two atomic O. O adsorbs at fcc site of Pt(111) with a binding energy of -1.98 eV followed by hcp, bridge and top sites (see Table S1 in Supporting Information). O2 binds more strongly on Pt+1Ni and Pt+2Ni (see Figure 7b). For instance, a binding energy of -2.47 and -2.99 eV was obtained with Pt-Pt-Ni and Pt-Ni-Ni fcc sites for Pt+1Ni. The Pt-Pt-Pt fcc site in Pt+1Ni is found to have a binding energy of -2.09 eV, which is marginally higher than that with pure Pt indicating that the effect of Ni rapidly diminishes with distance from the site. Similar trend is observed for remaining sites, i.e., hcp (Pt-Pt-Ni, Pt-Ni-Ni, Pt-Pt-Pt), bridge (Ni-Ni, Pt-Ni, Pt-Pt) and top(Pt top, Ni top) sites.

OH binding On pure Pt, OH binds most favorably at the bridge site (binding energy -3.08 eV). This is followed by the top site (-2.95 eV) whereas binding at the hcp site is much weaker (-2.20 eV). OH was unstable at the fcc site of pure Pt. However, OH is stable at both Pt+1Ni and Pt+2Ni fcc sites. As with pure Pt, OH binds strongly on Pt-Ni bridge site for Pt+1Ni and Pt-Ni/Ni-Ni bridge site for Pt+2Ni. The next stable sites are Pt- and Ni-top positions. It is followed by hcp and fcc sites. 19 ACS Paragon Plus Environment

Page 20 of 38

Page 21 of 38 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


H2O binding On pure Pt, H2O binds strongly at Pt top site with a binding energy -0.33 eV. Other sites were not found to be stable. On Pt+1Ni surface, water adsorbs strongly at Ni top site with a binding energy of -0.54 eV compared to the binding energy of -0.36 eV at Pt top site. On Pt+2Ni surface, water adsorbs at Ni top site with a binding energy of -0.57 eV. In conclusion, H and O prefer fcc sites, OH prefers the bridge site and H2O prefers top sites. In all cases, the presence of Ni results in stronger adsorbate binding.

3.2 Activation barriers for reaction steps Activation barriers calculated for the reaction steps in Eqs. (4)-(6) are next discussed.



Figure 8. Molecular precursor state and product of 𝑶𝟐(𝒂𝒅𝒔) dissociation on pure Pt and PtNi (111) surfaces: green (Pt), blue (Ni) and red (oxygen). Barriers in eV for forward and reverse are indicated above and below the arrows. S1-S4 metal atoms of Figure 6b are shown. Barriers in presence of water are shown in boldface.

O2 dissociation An O2 molecule chemisorbs on the fcc site of Pt(111) and Pt-Ni(111) surfaces with an axis parallel to the surface with a slight tilt. This configuration is termed as the molecular precursor state (MPS) (see Figure 8). The O-O bond length on Pt(111) surface in the MPS, transition state and product 21 ACS Paragon Plus Environment

Page 22 of 38

Page 23 of 38 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


form are 1.47, 2.16 and 3.39 Å, respectively. Two atomic O in the product state are considered to be positioned at the top-hcp sites (see Figure 8). For pure Pt, the dissociation energy barrier with respect to the MPS is 0.40 eV, and its reverse barrier is 0.26 eV. The energy barrier is determined by the relative effect of Ni on the binding energies of the MPS and the chemisorbed O atoms. The binding energies of chemisorbed two O atoms at these positions calculated using 𝐸 = 𝐸(𝑠 + 2𝑂 ∗ ) ―𝐸(𝑠) ―𝐸(𝑂2) are reported in Table S2 in Supporting Information with respect to O2 molecule in vacuum. The binding energy of the two atomic O at a distance of 3.39 Å on Pt(111) was -2.13 eV. The binding energy for the product configuration P1, P2 and P3 is -2.40, -2.81 and -2.63 eV, respectively, with atomic O-O distance being 3.20, 3.22 and 3.64 Å. Stronger binding of O at the hcp site to Ni (P3) results in the lower activation barrier for O2 dissociation (see Figure 8). Similar behavior is observed with two Ni atoms (P6, P7 and P8).

On the basis of above results, we conclude that substitution of Pt with Ni at the (111) surface (Pt-Ni alloys) shows significant activity for O2 dissociation, especially when Ni is kept close to the chemisorbed O atom at the hcp site. The speed-up can be as much as 400 times at 300 K (configuration R3-P3 vs R1-P1) assuming similar pre-exponential factors for the two configurations. Greater increase in reaction rate is possible with surface Pt-Ni atom arrangements involving 2 or more Ni atoms. Geometric distances pertaining to O2 MPS, TS and product are provided in Table S3 in Supporting Information.



Figure 9. Reactant (atomic O with atomic H) and product (OH) on pure Pt and Pt-Ni surface: green (Pt), blue(Ni), red(O) and white(H). Barriers for forward and reverse reactions are indicated above and below the arrows. S1-S5 atoms of Figure 6c are shown. Barriers in presence of water are shown in boldface.

OH formation The most stable sites for O and H binding, i.e., the fcc positions (see Table S1 in Supporting Information), were considered for the reactant state of Eq. (5). The product OH was kept at the bridge site. Presence of Ni at 𝑆5 position is expected to result in stronger binding of H and consequently a higher activation barrier. In order to keep the number of combinations in check we considered Ni at positions 𝑆1 ― 𝑆4 of Figure 6c while Pt has been kept at position 𝑆5 so that the 23 ACS Paragon Plus Environment

Page 24 of 38

Page 25 of 38 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


binding energy of the H to the surface in the reactant state is low. Figure 9 shows the optimized reactant and product states for the OH formation reaction. The computed barriers for OH formation on pure Pt along the forward and reverse directions are 0.87 and 0.91 eV, respectively. Presence of Ni (R2-P2 and R3-P3) causes both forwards and reverse barriers to decrease. The calculated forward and reverse barriers for the Pt3Ni1 arrangements in Figure 9 shows that for some arrangements the OH formation reaction is endothermic. When two Pt atoms are replaced with Ni, the calculated forward barriers are generally higher than the one for pure Pt. Similar observations are made with Pt-Ni3 and Ni4 arrangements. Table S4 in Supporting Information shows optimized reactant, transition state and product states of various paths.



Figure 10. Reactant (adsorbed OH and H) and product (adsorbed H2O) on pure Pt and PtNi composition is shown. Atom colors: green (Pt), blue (Ni), red (oxygen) and white (hydrogen). Barriers for forward and reverse reactions are indicated alongside arrows. Atoms other than S1-S5 included in the DFT calculation are shown in Figure 6c. Barriers in presence of water are shown in boldface.

H2O formation For H2O formation we place chemisorbed OH at a bridge site and a chemisorbed H species at the hollow fcc site. The product adsorbed H2O is placed at the top position of either Ni or Pt depending on the arrangement studied in Figure 10. Table S5 in Supporting Information shows the optimized distance of nearest Pt/Ni-O/H and O with H in the reactant, transition state and product 25 ACS Paragon Plus Environment

Page 26 of 38

Page 27 of 38 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


configurations. On pure Pt, forward and reverse reaction barriers are 0.24 and 0.93 eV. Once Pt is replaced with Ni at the surface the forward barriers generally tend to increase and the exothermicity of the reaction decreases. The reaction is endothermic when 3 or more Ni atoms are present. This is mainly due to the stronger binding of the OH species when Ni is present in the vicinity. Hence, presence of Ni at the surface may not result in speed-up of the 𝐻2𝑂 formation reaction. As in the case of OH formation, the presence of Ni at S5 in Figure 6c would result in a stronger binding of H with the surface that would result in higher activation barriers. Therefore, we have ignored Ni at the S5 position in Figure 10. After comparing the barriers calculated in Figs. 8-10 we conclude that the rate determining step on the Pt-Ni(111) surface is the OH-formation step. Addition of Ni (configuration R3-P3 in Figure 9) can help accelerate the OH-formation reaction by nearly 100 times over the pure Pt catalyst at 300 K.

Table 1. Equilibrium constants calculated in vacuum at 300 K using the forward and reverse barriers in Figures 8-10. Surface Pt-Ni arrangements highlighted in boldface are relevant to the dynamics based on the abundance of the arrangement, rate constant and equilibrium constant. Values obtained in presence of water are given in square brackets.

R1-P1 R2-P2

Equilibrium constant for Eq. (4) 0.004 1.3x10-5

R3-P3 R4-P4 R5-P5 R6-P6

716 3.48x105 0.2 1.6x104

Equilibrium constant for Eq. (5) 4.7 [152] 1 [1.4] 1.4x10-4 [3x104] 3.2 [224] 9x10-7 1.3x10-5 26

ACS Paragon Plus Environment

Equilibrium constant for Eq. (6) 4x1011 4x108 5.04x104 2x109 716 2x105


R7-P7 R8-P8 R9-P9 R10-P10 R11-P11 R12-P12

8x105 1.2x1011 152 6x1012 2x107 109

2x10-6 1.37x10-4 2x10-6 9x10-6 8.7x10-9 4x10-9

Page 28 of 38

3 152 10-5 0.01 0.001 2x10-8

In Table 1 we calculate the equilibrium constant for the reactions. Assuming equal preexponential factors for the forward and reverse directions we determine the equilibrium constant 𝐾𝑒𝑞 associated with the 12 arrangements as 𝐾𝑒𝑞 = 𝑘𝑓/𝑘𝑏, where 𝑘𝑓 and 𝑘𝑏 denote the forwards and reverse rates. Equilibrium constants greater than one are preferred as it implies a thermodynamic driving force for the forward reaction. For most Pt-Ni surface arrangements the equilibrium constant has a value much smaller than one (Table 1). This suggests that the reverse reaction will be thermodynamically preferred over the forward one, however, the kinetic rates are also important to determine the relevant pathways. Based on the forward barrier and the equilibrium constant we conclude that only pure Pt or Pt surfaces with one Ni atom are relevant to our study of the ORR. In practical applications, the Pt-Ni catalyst is used in presence of aqueous solution. To understand the effect of water we performed additional calculations in the presence of water as described in Sec. 2.2. The activation barriers obtained in these cases are shown in boldface in Figs. 8-10. OH formation continues to be the rate determining step on Pt surface in presence of water. Focusing on the rate-limiting OH formation step (Eq. (5)), R2-P2 provides a reasonable value of equilibrium constant and small activation barriers. Such configurations actively result in OH formation. The activation barrier for the configuration R2-P2 in Figure 9 is 0.16 eV compared to 0.73 for pure Pt catalysts. On the other hand, there are other configurations where the equilibrium constant is smaller than one (undesirable), however, these configurations are associated with large activation barriers. Such configurations do not yield significant amount of OH formation or OH 27 ACS Paragon Plus Environment

Page 29 of 38 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


dissociation, and are not kinetically relevant to the ORR. The R2-P2 configuration in a speed-up of nine orders of magnitude for the OH formation step over the R1-P1 configuration. As noted earlier for Ni:Pt ratio in the surface as low as 1:500 it is required that the rate limiting step be at least four orders of magnitude faster than the pure Pt surface, which is satisfied by the R2-P2 configuration. The resulting equilibrium constant for the R2-P2 configuration is also greater than 1 implying that the formation of OH is thermodynamically preferred. Implications of the reaction kinetics on the surface coverage of the reactant intermediates will need to be assessed separately either using microkinetic or kinetic Monte Carlo models. However, this aspect lies beyond the scope of this work. Recently, enhancement in experimental ORR activity in nanoporous Pt-Ni by a factor of 50 times over commercial Pt/C catalyst was reported in Ref. 44. The authors mention the presence of a large surface stress and under-coordinated sites as the reason for higher activity. Our KMC simulations indeed show the presence of a large number of under-coordinated sites. However, such sites are expected to be bound by species like OH33–35 rendering them inaccessible for adsorption of molecular O2 and further steps of the ORR. Moreover, the trace amount of subsurface Ni present in nanoporous Pt-Ni cannot induce enough surface stress to have such a large effect on the ORR activity. The surface stress could arise from relaxation at the nanoporous surfaces, finite size effects, or presence of surface oxides, which cannot be assessed using the lattice KMC simulations and DFT simulations with extended surfaces performed in this work. Nonetheless, this work shows that although too much of Ni in the surface layer can be detrimental to the ORR activity, trace amounts of surface Ni (e.g., surface fraction around 0.001) can significantly enhance the ORR activity. Finally, using DFT45 and classical interatomic potentials46,47 it has been previously



established that Pt preferentially segregates to the surface. Figure S6 in Supporting Information provides a comparison of the Pt segregation from the two approaches in terms of the distribution coefficient48. The two approaches suggest a low fraction of surface Ni (0.0007-0.0134) at equilibrium with 98% Pt. However, the nanoporous Pt-Ni morphological evolution is achieved far-from-equilibrium. Our kinetic model establishes the presence of trace amount (0.002-0.009 fraction) of surface Ni resulting from simultaneous dissolution and coarsening mechanisms, which surprisingly is within the expected limits at equilibrium.

4. CONCLUSIONS We have studied the oxygen reduction reaction (ORR) reaction on nanoporous Pt-Ni. Two aspects of the reaction were considered, namely, (i) the elemental distribution of Pt/Ni in nanoporous PtNi and (ii) ORR activity due to the resulting elemental distribution. Formation of the nanoporous structure as obtained via selective dissolution of Ni from a starting Ni0.8Pt0.2 alloy NP was considered. Our KMC simulations show that overall the surface is rich in Pt but few Ni atoms can be present at the surface as well. The Ni surface fraction is expected to be less than 10-3. Additionally, the sub-surface layer is rich in Pt. A large number of DFT calculations performed for ORR on both pure Pt(111) and Ni-covered Pt(111) surface suggest that surface Pt3-Ni atom arrangements can outperform pure Pt surface. The rate-limiting step is 𝑂𝐻 formation for both types of surfaces. Although the fraction of surface Ni atoms is very low, the rates of 𝑂𝐻 formation achieved at surface Ni can be nine orders of magnitude higher than that achieved by pure Pt surface. This may explain the higher ORR activity of the nanoporous Pt-Ni. Such low fractions of surface Ni cannot be easily detected by many experimental characterization techniques like EDX


Page 30 of 38

Page 31 of 38 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


and X-ray photoelectron spectroscopy. Understanding the origin of the higher activity can be useful for optimizing the nanoporous Pt-Ni catalyst.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Binding energies of intermediates H, O, OH and H2O at various sites on Pt-Ni(111) surface, optimized distances in the reactant, transition and product configurations, images of nanoporous Pt-Ni, distribution coefficient for Pt-Ni using DFT and classical interatomic potentials in literature (PDF).

ACKNOWLEDGEMENTS AC acknowledges support from Science and Engineering Research Board, Department of Science and Technology Grant No. EMR/2017/001520 and Indian National Science Academy Grant No. SP/YSP/120/2015/307.

REFERENCES (1)

Rabis, A.; Rodriguez, P.; Schmidt, T. J. Electrocatalysis for Polymer Electrolyte Fuel Cells: Recent Achievements and Future Challenges. ACS Catal. 2012, 2 (5), 864–890.

(2)

Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Marković, N. M. Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability. Science 2007, 315 (5811), 493–497. 30 ACS Paragon Plus Environment


(3)

Bogicevic, A.; Stromquist, J.; Lundqvist, B. I. First-Principles Diffusion-Barrier Calculation for Atomic Oxygen on Pt(111). Phys. Rev. B 1998, 57, R4289.

(4)

Cao, L.; Mueller, T. Rational Design of Pt 3 Ni Surface Structures for the Oxygen Reduction Reaction. J. Phys. Chem. C 2015, 119 (31), 17735–17747.

(5)

Sementa, L.; Andreussi, O.; Goddard, W. A.; Fortunelli, A. Catalytic Activity of Pt38 in the Oxygen Reduction Reaction from First-Principles Simulations. Catal. Sci. Technol. 2016, 6 (18).

(6)

Feibelman, P.; Esch, S.; Michely, T. O Binding Sites on Stepped Pt(111) Surfaces. Phys. Rev. Lett. 1996, 77 (111), 2257–2260.

(7)

Eichler, A.; Hafner, J. Molecular Precursors in the Dissociative Adsorption of O2 on Pt(111). Phys. Rev. Lett. 1997, 79, 4481–4484.

(8)

Eichler, A.; Mittendorfer, F.; Hafner, J. Precursor-Mediated Adsorption of Oxygen on the (111) Surfaces of Platinum-Group Metals. Phys. Rev. B 2000, 62 (7), 4744–4755.

(9)

Gambardella, P.; Šljivančanin, Ž.; Hammer, B.; Blanc, M.; Kuhnke, K.; Kern, K. Oxygen Dissociation at Pt Steps. Phys. Rev. Lett. 2001, 87 (5), 056103.

(10)

Viswanathan, V.; Hansen, H. A.; Rossmeisl, J.; Nørskov, J. K. Universality in Oxygen Reduction Electrocatalysis on Metal Surfaces. ACS Catal. 2012, 2 (8), 1654–1660.

(11)

Stamenkovic, V. R. V. R.; Mun, B. S. B. S.; Arenz, M.; Mayrhofer, K. J. J. K. J. J.; Lucas, C. a C. A.; Wang, G. F. G.; Ross, P. N. P. N.; Markovic, N. M. N. M. Trends in Electrocatalysis on Extended and Nanoscale Pt-Bimetallic Alloy Surfaces. Nat. Mater. 2007, 6 (3), 241–247. 31 ACS Paragon Plus Environment

Page 32 of 38

Page 33 of 38 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


(12)

Cui, C.; Gan, L.; Li, H. H.; Yu, S. H.; Heggen, M.; Strasser, P. Octahedral PtNi Nanoparticle Catalysts: Exceptional Oxygen Reduction Activity by Tuning the Alloy Particle Surface Composition. Nano Lett. 2012, 12 (11), 5885–5889.

(13)

Chou, S. W.; Lai, Y. R.; Yang, Y. Y.; Tang, C. Y.; Hayashi, M.; Chen, H. C.; Chen, H. L.; Chou, P. T. Uniform Size and Composition Tuning of PtNi Octahedra for Systematic Studies of Oxygen Reduction Reactions. J. Catal. 2014, 309, 343–350.

(14)

Snyder, J.; McCue, I.; Livi, K.; Erlebacher, J. Structure/Processing/Properties Relationships in Nanoporous Nanoparticles As Applied to Catalysis of the Cathodic Oxygen Reduction Reaction. J. Am. Chem. Soc. 2012, 134 (20), 8633–8645.

(15)

Activity Benchmarks and Requirements for Pt, Pt-Alloy, and Non-Pt Oxygen Reduction Catalysts for PEMFCs. Appl. Catal. B Environ. 2005, 56 (1–2), 9–35.

(16)

Matanović, I.; Garzon, F. H.; Henson, N. J. Theoretical Study of Electrochemical Processes on Pt–Ni Alloys. J. Phys. Chem. C 2011, 115 (21), 10640–10650.

(17)

Wang, G.; Van Hove, M. A. A.; Ross, P. N. N.; Baskes, M. I. I. Monte Carlo Simulations of Segregation in Pt-Ni Catalyst Nanoparticles. 2005, 122 (2), 024706.

(18)

Fortunelli, A.; Goddard, W. A.; Sementa, L.; Barcaro, G.; Negreiros, F. R.; JaramilloBotero, A. The Atomistic Origin of the Extraordinary Oxygen Reduction Activity of Pt3Ni7 Fuel Cell Catalysts. Chem. Sci. 2015, 6 (7), 3915–3925.

(19)

Asara, G. G.; Paz-Borbón, L. O.; Baletto, F. “Get in Touch and Keep in Contact”: Interface Effect on the Oxygen Reduction Reaction (ORR) Activity for Supported PtNi Nanoparticles. ACS Catal. 2016, 6 (7), 4388–4393.



(20)

Greeley, J.; Rossmeisl, J. Theoretical Trends in Particle Size Effects for the Oxygen Reduction Reaction. Zeitschrift für Phys. Chemie 2007, 221 (9–10), 1209–1220.

(21)

Bing, Y.; Liu, H.; Zhang, L.; Ghosh, D.; Zhang, J. Nanostructured Pt-Alloy Electrocatalysts for PEM Fuel Cell Oxygen Reduction Reaction. Chem. Soc. Rev. 2010, 39 (6), 2184–2202.

(22)

Shao, M.; Chang, Q.; Dodelet, J.-P.; Chenitz, R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116 (6), 3594–3657.

(23)

Cui, C.; Gan, L.; Heggen, M.; Rudi, S.; Strasser, P. Compositional Segregation in Shaped Pt Alloy Nanoparticles and Their Structural Behaviour during Electrocatalysis. Nat. Mater. 2013, 12 (8), 765–771.

(24)

Greeley, J.; Nørskov, J. K. Combinatorial Density Functional Theory-Based Screening of Surface Alloys for the Oxygen Reduction Reaction. J. Phys. Chem. C 2009, 113 (12), 4932–4939.

(25)

Duan, Z.; Wang, G. Comparison of Reaction Energetics for Oxygen Reduction Reactions on Pt(100), Pt(111), Pt/Ni(100), and Pt/Ni(111) Surfaces: A First-Principles Study. J. Phys. Chem. C 2013, 117 (12), 6284–6292.

(26)

Haldar, P.; Chatterjee, A. Connectivity-List Based Characterization of 3D Nanoporous Structures Formed via Selective Dissolution. Acta Mater. 2017, 127, 379–388.

(27)

Chatterjee, A.; Vlachos, D. G. An Overview of Spatial Microscopic and Accelerated Kinetic Monte Carlo Methods. J. Comput. Mater. Des. 2007, 14 (2), 253–308.

(28)

Rehman, T.; Jaipal, M.; Chatterjee, A. A Cluster Expansion Model for Predicting the 33 ACS Paragon Plus Environment

Page 34 of 38

Page 35 of 38 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


Activation Barrier of Atomic Processes. J. Comp. Phys. 2013, 243, 244–259. (29)

Verma, S.; Rehman, T.; Chatterjee, A. A Cluster Expansion Model For Rate Constants Of Surface Diffusion Processes On Ag, Al, Cu, Ni, Pd And Pt (100) Surfaces. Surf. Sci. 2013, 613, 114–125.

(30)

Haldar, P.; Chatterjee, A. Extent Of Dissolution Determines The Structural Similarity Between Dealloyed Nanoporous Materials Synthesized At Unrelated Dissolution Conditions. Acta Mater. 2018, https://doi.org/10.1016/j.actamat.2018.04.020.

(31)

Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54 (16), 11169–11186.

(32)

Vanderbilt, D. Soft Self-Consistent Pseudopotentials in a Generalized Eigenvalue Formalism. Phys. Rev. B 1990, 41 (11), 7892–7895.

(33)

Han, B. C.; Miranda, C. R.; Ceder, G. Effect of Particle Size and Surface Structure on Adsorption of O and OH on Platinum Nanoparticles: A First-Principles Study. Phys. Rev. B 2008, 77 (7), 075410.

(34)

Strmcnik, D. S.; Tripkovic, D. V.; van der Vliet, D.; Chang, K.-C.; Komanicky, V.; You, H.; Karapetrov, G.; Greeley, J. P.; Stamenkovic, V. R.; Marković, N. M. Unique Activity of Platinum Adislands in the CO Electrooxidation Reaction. J. Am. Chem. Soc. 2008, 130 (46), 15332–15339.

(35)

Wang, C.; Chi, M.; Li, D.; Strmcnik, D.; van der Vliet, D.; Wang, G.; Komanicky, V.; Chang, K.-C.; Paulikas, A. P.; Tripkovic, D.; et al. Design and Synthesis of Bimetallic Electrocatalyst with Multilayered Pt-Skin Surfaces. J. Am. Chem. Soc. 2011, 133 (36),



14396–14403. (36)

Perdew, J.; Chevary, J.; Vosko, S.; Jackson, K.; Pederson, M.; Singh, D.; Fiolhais, C. Atoms, Molecules, Solids, and Surfaces: Applications of the Generalized Gradient Approximation for Exchange and Correlation. Phys. Rev. B 1992, 46 (11), 6671–6687.

(37)

Henkelman, G.; J??nsson, H. Improved Tangent Estimate in the Nudged Elastic Band Method for Finding Minimum Energy Paths and Saddle Points. J. Chem. Phys. 2000, 113 (22), 9978–9985.

(38)

Ogasawara, H.; Brena, B.; Nordlund, D.; Nyberg, M.; Pelmenschikov, A.; Pettersson, L. G. M.; Nilsson, A. Structure and Bonding of Water on Pt(111). Phys. Rev. Lett. 2002, 89, 276102.

(39)

Borg, A.; Hilmen, A. M.; Bergene, E. STM Studies of Clean, CO- and O2-Exposed Pt(100)-Hex-R0.7°. Surf. Sci. 1994, 306 (1–2), 10–20.

(40)

Van Hove, M. A.; Koestner, R. J.; Stair, P. C.; Bibérian, J. P.; Kesmodel, L. L.; Bartoš, I.; Somorjai, G. A. The Surface Reconstructions of the (100) Crystal Faces of Iridium, Platinum and Gold. II. Structural Determination by LEED Intensity Analysis. Surf. Sci. 1981, 103 (1), 218–238.

(41)

Havu, P.; Blum, V.; Havu, V.; Rinke, P.; Scheffler, M. Large-Scale Surface Reconstruction Energetics of Pt(100) and Au(100) by All-Electron Density Functional Theory. Phys. Rev. B - Condens. Matter Mater. Phys. 2010, 82 (16).

(42)

Schneider, U.; Busse, H.; Linke, R.; Castro, G. R.; Wandelt, K. Interaction Properties of Molecules with Binary Alloy Surfaces. J. Vac. Sci. Tech. A 1994, 12, 2069.


Page 36 of 38

Page 37 of 38 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


(43)

Sha, Y.; Yu, T. H.; Merinov, B. V.; Shirvanian, P.; Goddard, W. a. Mechanism for Oxygen Reduction Reaction on Pt 3 Ni Alloy Fuel Cell Cathode. J. Phys. Chem. C 2012, 116 (40), 21334–21342.

(44)

Li, M.; Zhao, Z.; Cheng, T.; Fortunelli, A.; Chen, C.-Y.; Yu, R.; Zhang, Q.; Gu, L.; Merinov, B. V.; Lin, Z.; et al. Ultrafine Jagged Platinum Nanowires Enable Ultrahigh Mass Activity for the Oxygen Reduction Reaction. Science (80-. ). 2016, 354 (6318), 1414–1419.

(45)

Modak, S.; Gangopadhyay, S. Surface Segregation of Pt-Ni Alloys. Solid State Commun. 1991, 78 (5), 429–432.

(46)

Divi, S.; Agrahari, G.; Kadulkar, S.; Kumar, S.; Chatterjee, A. Improved Prediction Of Heat Of Mixing And Segregation In Metallic Alloys Using Tunable Mixing Rule For Embedded Atom Method. Model. Simul. Mater. Sci. Eng. 2017, 25 (8), 085011.

(47)

Divi, S.; Chatterjee, A. Understanding Segregation Behavior in AuPt, NiPt, and AgAu Bimetallic Nanoparticles Using Distribution Coefficients. J. Phys. Chem. C 2016, 120 (48), 27296–27306.

(48)

Divi, S.; Chatterjee, A. Generalized Nano-Thermodynamic Model for Capturing SizeDependent Surface Segregation in Multi-Metal Alloy Nanoparticles. RSC Adv. 2018, 8 (10409).



TOC Image:


Page 38 of 38

Effect of Surface Ni on Oxygen Reduction Reaction in Dealloyed

Recommend Documents