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Support Effect in Single-Atom Platinum Catalyst for Electrochemical Oxygen Reduction Sungeun Yang, Young Joo Tak, Jiwhan Kim, Aloysius Soon, and Hyunjoo Lee ACS Catal., Just Accepted Manuscript • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 20, 2016
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ACS Catalysis
Support Effect in Single-Atom Platinum Catalyst for Electrochemical Oxygen Reduction Sungeun Yang,†,§ Young Joo Tak,‡,§ Jiwhan Kim,† Aloysius Soon,*,‡ and Hyunjoo Lee*,† †
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Yuseonggu, Daejeon 34141, Republic of Korea ‡ Department of Material Science and Engineering, Yonsei University, Seodaemun-gu, Seoul 03722, Republic of Korea
Supporting Information Placeholder
ABSTRACT: Single-atom catalysts (SACs) provide an ideal platform for reducing noble metal usage. SACs also exhibit unusual catalytic properties due to the absence of metal surface. The role of the support may have a significant impact on the catalytic properties similar to that of the ligand molecules in homogeneous catalysts. Here, the support effect was demonstrated by preparing the single-atom platinum catalyst on two different supports, titanium carbide (Pt1/TiC) and titanium nitride (Pt1/TiN). The formation of single-atom Pt was confirmed by STEM, EXAFS, and in-situ IR spectroscopy. Pt1/TiC showed higher activity, selectivity, and stability for electrochemical H2O2 production than Pt1/TiN. Density functional theory calculation presented that oxygen species have strong affinity into Pt1/TiN possibly acting as surface poisoning species, and Pt1/TiC preserves oxygenoxygen bond more with higher selectivity towards H2O2 production. This work clearly shows that the support in SACs actively participates in the surface reaction, not just acting as anchoring sites for single-atoms. Keywords: single-atom catalysts, titanium carbide, oxygen reduction reaction, support effect, platinum
INTRODUCTION Single-atom catalysts (SACs) are a new class of heterogeneous catalysts where every active metal atom is atomically dispersed and anchored on the support.1-3 Maximized, often 100%, metal utilization makes SACs efficient catalysts that reduce noble metal usage and obtain high mass activity for CO oxidation,4-5 preferential oxidation (PROX),4, 6 water-gasshift,7-8 selective hydrogenation,9-12 and electrochemical reactions.13-14 Together with high activity, high selectivity can be achieved in SACs due to the absence of ensemble sites in SACs.6, 10, 13-15 While SACs can be classified as heterogeneous catalysts, they also share similarities with homogeneous catalysts. The single-atom catalyst is anchored by or is coordinated with the surface atoms of the support. The surface atoms can be thought as ligand molecules in homogeneous catalysts that not only stabilize the metal atom but also participate in the catalytic reactions. Yet, current studies on SACs mostly focus on the anchoring effect of the supports but not on their effect on catalysis. Supports modulate catalytic properties by changing the morphology or electronic structure of the deposited metal nanoparticles.16-19 In some cases, supports actually participate in the catalytic reaction, e.g. by activating the water molecule in
CO oxidation and water-gas shift reaction.20, 21 Hundred percent metal utilization in SACs means that every metal atom is in direct contact with the support, while only a fraction of the metal atoms is in contact with the support in supported nanoparticles catalysts. Therefore, SACs should have maximized metal-support interaction. Ligand molecules play a key role in homogeneous catalysts. Ligands can change catalytic properties by modulating the electronic structure of the metal-atom centre,22-23 stabilizing or destabilizing reactants or intermediates,24-25 and causing steric hindrance.22, 24, 26 For homogeneous catalysts, the design of the ligands is often more important than changing the metal-atom centre. The ligands in homogeneous catalysts can be compared with the supports in heterogeneous catalysts, and the support in SAC may play a critical role in the control of the catalytic property. Pt single-atom was previously stabilized on the TiN supports (Pt1/TiN),14 well matching a theoretical study of Pt SAC on the TiN support.27 Pt1/TiN showed high selectivity towards electrochemical H2O2 production by the atomically dispersed active site with no Pt ensemble site. H2O2 is a main product of oxygen reduction reaction (ORR) when the O-O bond is preserved. If there are two continuing active atoms that can individually adsorb oxygen species, both O atoms of O2 will be adsorbed and O2 can be dissociated to produce H2O. There-
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fore, creating atomically dispersed active sites is important for the effective production of H2O2.15, 28-31 Here, we report the support effect in SACs by simply changing the TiN support to the TiC support with the same structural configuration. Atomically dispersed Pt catalysts deposited on TiN (Pt1/TiN) or TiC (Pt1/TiC) were prepared without Pt nanoparticles. The support in Pt1/TiN or Pt1/TiC may affect the electrochemical oxygen reduction, with different activity, selectivity, and stability. The reaction mechanism of H2O2 formation during ORR on both catalysts was investigated by density-functional theory (DFT) calculations.
RESULTS AND DISCUSSION Characterization of single-atom Pt catalysts. Firstly, the characteristics of the supports were measured in the absence of Pt. We purchased TiC and TiN nanoparticles from Nanostructured & Amorphous Materials, Inc. and treated them with hydrochloric acid to obtain clean surfaces. TiC and TiN share the same crystalline structure of cubic symmetry (Fm3m) with slightly different lattice parameters (Figure S1a. XRD). Based on the Scherer equation, the crystalline size of TiC was estimated as 25.4 nm, larger than 15.3 nm for TiN. TEM images (Figure S1c, S1d) also suggest the larger particle size of TiC. The surface structures of the two materials were different. Xray photoelectron spectroscopy (XPS) data of Ti 2p (Figure S1b) showed that the surface of TiC nanoparticle mostly exists as TiC (455 eV), and a small amount of oxide (459 eV) was mostly removed after the acid treatment. The TiN surface is much more easily oxidized and a significant amount of Ti species exist as oxynitride (457 eV) and oxide (459 eV) even after the acid treatment. These XPS data suggest that the TiN surface shows stronger interaction with oxygen species and is thus more easily oxidized. The oxygen reduction reaction (ORR) was conducted on both supports in the absence of Pt
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(Figure S1e-g). Both supports showed similarly low ORR activity. However, TiC showed higher selectivity towards H2O2, suggesting that TiC has a lower probability for the dissociation of oxygen molecules into H2O. Single-atom Pt was deposited on both supports by the incipient wetness impregnation method.14 The support powder was mixed with a Pt precursor solution, dried, and reduced in 10% H2 (N2 balance) at 100 °C for 1 h. The presence of the Pt single-atom was confirmed by three different methods to ensure the same structural configuration of Pt single-atom on both supports. HAADF-STEM is a powerful tool for distinguishing single-atom Pt from the support material. Pt atoms with a high atomic number are shown as bright dots (Figures 1a and 1b) with atomic dispersion. However, HAADF-STEM images only provide local information for the specific position. EXAFS data and the CO peak from in-situ IR spectra were used to confirm the absence of Pt nanoparticles and the presence of single atomic Pt in the entire samples. Examination of EXAFS data (Figure 1c) shows the local structure around absorber atom Pt. Pt-Pt bond appears at 2.6 Å in FT-EXAFS as in Pt foil (yellow line in Figure 1c). Pt-Pt peak almost disappears when the Pt single-atoms are deposited. A new peak at 1.9 Å arises at the single-atom catalyst that is due to the Pt-Cl interaction. The coordination number of 0.45 wt% Pt/TiC was 2.42 for Pt-Cl, 0.43 for Pt-Ti, and 1.72 for Pt-Pt. Coordination number of 0.35 wt% Pt/TiN was 3.04 for Pt-Cl, 0.57 for Pt-Ti, and 0.58 for Pt-Pt (Table 1). The coordination number of Pt-Pt for a typical nanoparticle is approximately 8 and is 12 for the Pt foil. Therefore, the small coordination number for Pt-Pt suggests that only a small fraction of Pt exists as a nanoparticle, and most of Pt exists as single-atoms. To further reduce the Pt-Pt contribution and increase the Pt single-atom fraction, the Pt weight percent was decreased to 0.2 wt%. However, at such a low weight percentage, EXAFS analysis was not possible. Nevertheless, IR spectroscopy results using diffuse reflectance FT-IR could be obtained for such low weight percentages. Table 1. Best fit values for the EXAFS analysis of various Pt samples. Sample
Path
Coorindation number
Interatomic distance (Å)
DebyeWaller factor 2
R-factor (%)
2
(࣌ /Å ) 0.45 wt% Pt1/TiC
Pt-Pt
1.720
2.783
0.003*
Pt-Cl
2.423
2.285
0.003
Pt-Ti
0.425
2.626
0.003*
0.35 wt% Pt1/TiN
Pt-Pt
0.583
2.871
0.003*
Pt-Cl
3.038
2.294
0.000
Pt-Ti
0.573
2.713
0.003*
Pt-Pt (1)
12*
2.767
0.005
Pt-Pt (2)
6*
3.912
0.007
Pt foil
Figure 1. Single-atom Pt catalysts characterized by various methods. HAADF-STEM image of (a) 0.2 wt% Pt1/TiC, and (b) 0.2 wt% Pt1/TiN. (c) Pt L3 edge k3-weighted FT-EXAFS spectra of 0.45 wt% Pt/TiC (black), 0.35 wt% Pt/TiN (blue), and Pt foil (yellow). Dots indicate experimental data, and lines indicate fitted results. (d) Diffuse reflectance FT-IR spectra of adsorbed CO on single-atom (line) and nanoparticle (dash) samples.
0.021
0.003
0.009
*These factors were fixed during the EXAFS fitting.
IR spectroscopy is another tool to observe the existence of single-atom Pt.4, 32-33 The peak position of COad changes according to the bonding configuration of CO to Pt (bridged or linear) and the electronic structure of Pt. Linearly bonded CO on ionic Pt appears at 2090 cm-1 as shown in Pt single-atom samples, while linearly bonded CO on metallic Pt appears at
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2045 cm-1 as shown in Pt nanoparticle samples (Figure 1d). The wide COad peak that appears at approximately 1820 cm-1 is due to bridge-bonded CO on Pt. The bridge-bonded CO peak is only observed on the Pt nanoparticles because bridge bonding is not possible on atomically dispersed Pt. The in-situ FT-IR measurements were repeated to confirm the existence of the COad peak. All attempts on single-atom Pt samples found a single COad peak located between approximately 2080 and 2100 cm-1 (Figure S2), while supports without Pt did not show any COad peak. Thus, HAADF-STEM, EXAFS, and FTIR characterizations all indicate that Pt exists as a single atom on both supports with the same structural configuration. For Pt1/TiC and Pt1/TiN, Pt exists as single-atom with chlorine ligands and anchored on support with small Pt-Ti interaction. Electrochemical Reactions. Oxygen reduction reaction was performed on both Pt1/TiC and Pt1/TiN. Oxygen reduction reaction on platinum nanoparticle typically produces H2O via a 4-electron pathway. However, the strong oxygen-oxygen double bond cannot be dissociated on atomically dispersed active sites.14-15, 28-31 Therefore, single-atom Pt produces H2O2 as a major product.14-15 Pt1/TiC and Pt1/TiN produced a significant amount of H2O2 resulting from the atomically dispersed active sites (Figure 2). Disk currents in Figure 2b indicate the total oxygen reduction currents producing both H2O and H2O2. The oxygen reduction current density of Pt1/TiC was almost twice larger than that of Pt1/TiN at all potential ranges. Ring currents in Figure 2a indicate the oxidation of H2O2 produced on the disk. Pt1/TiC showed a much higher oxidation current. H2O2 selectivity can be estimated from the disk and ring currents as shown in Figure 2c. The Pt1/TiC catalyst showed higher activity and also higher selectivity towards H2O2. At 0.2 V (vs RHE), 0.2 wt% Pt1/TiC showed ORR activity of -0.96 mA cm-2 and H2O2 selectivity of 68.0%, while 0.2 wt% Pt1/TiN showed -0.34 mA cm-2 and 53.1% for the ORR activity and H2O2 selectivity, respectively.
Figure 2. Oxygen reduction reaction polarization curves on single-atom Pt. (a) Ring currents measured concurrently during ORR with a potential held at 1.2 V. (b) ORR polarization curves of single-atom Pt in O2-saturated 0.1 M HClO4 solution with a scan rate of 0.01 V s-1. (c) H2O2 selectivity calculated from disk and ring current.
For comparison, Pt nanoparticles were deposited on two supports (5 wt% Pt/TiC and 5 wt% Pt/TiN). Supported nanoparticle catalysts are expected to show less support effect than the single-atom catalysts because the effect resulting from the interface between the metal atom and support may be blurred
by the reaction on the nanoparticle surface. Nanoparticle catalysts were prepared by the same incipient wetness impregnation method with the weight percentage of 5 wt%. Pt nanoparticles were observed in the TEM images (Figure S3). The mean particle size was 2.24 nm for 5 wt% Pt/TiC and 2.29 nm for 5 wt% Pt/TiN. Unlike single-atom catalysts, their activity and selectivity were very similar as shown in Figure 3. At 0.2 V (vs RHE), 5 wt% Pt1/TiC showed ORR activity of -3.03 mA cm-2 and H2O2 selectivity of 24.8%, while 5 wt% Pt1/TiN showed -3.40 mA cm-2 and 25.7% for the ORR activity and H2O2 selectivity, respectively. As expected, the supported nanoparticles demonstrated the negligible support effect compared to single-atom catalysts.
Figure 3. Oxygen reduction reaction polarization curves on Pt nanoparticles. (a) Ring currents measured concurrently during ORR with a potential held at 1.2 V. (b) ORR polarization curves of Pt nanoparticles in O2-saturated 0.1 M HClO4 solution with a scan rate of 0.01 V s-1. (c) H2O2 selectivity calculated from disk and ring current.
Density Functional Theory Calculation. To understand and elucidate the ORR chemistry on these single-atom catalysts, we calculated and compared the adsorption energies of OOH* intermediate using first-principles density-functional theory (DFT). The OOH* has been reported to be the key decisive intermediate for the formation of H2O2 via the 2-electron pathway. As shown in Figure S4a and S4b, the OOH* binds molecularly to the Pt/TiC(100) surface for both the Ti top site (–2.16 eV) and Ti-Ti bridge site (–2.34 eV), respectively. For Pt/TiN(100), OOH* energetically prefers to dissociate to O and OH (–7.46 eV, Figure S4d) compared to the molecularly intact state shown in Figure S4c (–4.14 eV). The oxygenoxygen bond would be dissociated more easily on Pt1/TiN than on Pt1/TiC. Turning now to the elementary reaction steps, we plot the ∆G profile using the calculated ZPE-corrected adsorption energies for the various ORR elementary steps for both the 2electron (denoted by the thick lines) and 4-electron (denoted by the thin lines) pathways for Pt/TiC(100) and Pt/TiN(100) under an applied potential of 0.2 V (vs RHE) (Figure 4). Different energy profiles were obtained for the 2-electron pathway on both catalysts. In the case of Pt/TiC (100) (Figure 4a), the entire energy profile for the 2-electron pathway is downhill. A different trend in the energy profile is shown for Pt/TiN(100) (Figure 4b). Formations of O2* and 2O* are more
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Figure 5. DFT energy profiles of O2* and OOH* dissociation as a function of the reaction coordinates: O2* dissociation on (a) Pt/TiC(100) and (c) Pt/TiN(100), OOH* dissociation on (b) Pt/TiC(100) and (d) Pt/TiN(100). Inset images: Top-view of the atomic structures for the various dissociation steps. Pt, N, Ti, C, O and H atoms are represented by orange, blue, white, black, red, and turquoise spheres, respectively.
In the presence of Cl, O-O bond was preserved more in both TiN and TiC cases as shown in Figure S5. The Cl ligand is helping H2O2 production with the similar effect on both TiN and TiC support. The adsorption energy of OOH is still much stronger on Pt1/TiN-Cl than Pt1/TiC-Cl.
Figure 4. Free-energy diagrams at 0.2 V for the ORR on (a) Pt/TiC(100) and (b) Pt/TiN(100). The 2-electron and 4-electron pathways are denoted in thick and thin lines, respectively, with the activation barriers denoted. (c) The schematic views of H2O2 formation reaction (upper) and O2 dissociation reactions (lower) are shown. Here, the O, H, Pt, N(C) and Ti atoms are denoted by red, turquoise, orange, gray, and white circles.
exothermic than on Pt/TiC(100), indicating a strong interaction of the oxygen species with the Pt/TiN(100) surface. After the formation of O2*, Pt/TiN(100) requires two successive energy uphill steps (0.52 eV and 0.94 eV) to produce H2O2, opposite to the Pt/TiC(100) at the same reaction coordinates. The energetically stable O2* or OOH* intermediates could behave as surface-poisoning species, resulting in the low activity of Pt1/TiN (Figure 2b). Dissociation of O2* to O* + O* and OOH* to O* + OH* species is the key step for the 4-electron pathway. The activation barriers were calculated for O2* dissociation and OOH* dissociation on both Pt/TiC(100) and Pt/TiN(100) from the most stable adsorption site of O2* and OOH*. Figure 5 shows DFT energy profiles of O2* and OOH* dissociation as a function of the reaction coordinates. For these two reaction steps on Pt/TiC(100), there exist non-negligible activation barriers of 0.19 eV and 0.51 eV. In contrast, no activation barriers were observed for these steps on Pt/TiN(100). The existence of activation barriers on Pt/TiC(100) implies that the formation of H2O2 on Pt/TiN(100) is predicted to be less probable than on Pt/TiC(100). Based on the DFT calculations, it can be concluded that different adsorption energies and energy profiles would give rise to high activity and selectivity for Pt1/TiC.
Stability of single-atom Pt catalysts. To estimate the stability of these Pt1/TiC and Pt1/TiN catalysts, a thousand cycles of cyclic voltammetry were conducted in O2-saturated solution between 0.2 V to 0.7 V with the scan rate of 0.1 V s-1 as shown in Figure 6. Reasonable stability was observed with Pt1/TiC. After the stability test, the activity decreased from 0.96 to -0.64 mA cm-2 with little change in selectivity at 0.2 V. In contrast, Pt1/TiN showed poor stability. After the stability test, the activity dropped from -0.34 to -0.09 mA cm-2 at 0.2 V. Pt1/TiC showed better stability than Pt1/TiN. Cyclic voltammograms in Ar-saturated solution before and after the stability tests were also different (Figure S6). Pt1/TiC showed almost the same cyclic voltammogram before and after the stability
Figure 6. Oxygen reduction polarization curves before and after stability test and corresponding H2O2 selectivity for (a) 0.2 wt% Pt1/TiC and (b) 0.2 wt% Pt1/TiN. A thousand cycles of cyclic voltammetry were conducted in O2-saturated solution between 0.2 V and 0.7 V with the scan rate of 0.1 V s-1.
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test. By contrast, Pt1/TiN showed degradation after the stability test, indicating the passivation of the surface.34 XPS measurement of Ti 2p electron after 5000 cycle CV test (Figure S7) showed that the TiC surface remained as TiC after the stability test whereas the TiN surface was completely oxidized to TiO2. The radical transformation of the TiN surface into TiO2 may result in the detachment of single-atom Pt and lower stability.
CONCLUSIONS The supports play a very important role in single-atom catalysts. A simple change of support from TiN to TiC significantly increased the catalytic activity, selectivity, and stability for the electrochemical H2O2 production. Energy profiles obtained from the DFT calculations indicate that differences of adsorption energies of oxygen species on TiC and TiN resulted in differences in activity and selectivity. Current studies on single-atom catalysts are mostly focused on the active metalatom. However, a careful selection of the support surface is indispensable for the development of better SACs.
EXPERIMENTAL SECTION Acid treatment on TiC and TiN nanoparticles. TiC nanoparticles (NanoAmor, 99%) and TiN nanoparticles (NanoAmor, 97+%) were acid-treated to remove oxidized surface species prior to Pt deposition. Five hundred milligrams of nanoparticles powder, TiC or TiN, were added to 20 ml of hydrochloric acid (35.0%, Samchun) followed by 10 min of sonication. The mixture was treated at 45 °C for 1 h under nitrogen atmosphere. The acid-treated nanoparticles were washed with deionized water (18.3 MΩ·cm, Human Power II+ Scholar) until pH of the solution became neutral, and dried in a vacuum oven at 50 °C for 4 h. TiC and TiN nanoparticles powder were ground well and stored in glove box prior to Pt deposition. Synthesis of single-atom Pt on TiC and TiN nanoparticles. Two different weight percentages of Pt were deposited onto the acid-treated TiC and TiN nanoparticles using incipient wetness impregnation (IWI) method.14 For single-atom catalyst, 0.2 wt% of Pt was deposited, and for nanoparticle catalyst, 5 wt% of Pt was deposited. Appropriate amounts of H2PtCl6·6H2O (Sigma–Aldrich) were dissolved in 32 µl of anhydrous ethanol (Sigma-Aldrich) for Pt/TiC, and 80 µl of anhydrous ethanol for Pt/TiN. The Pt precursor solution was finely mixed with 80 mg of acid-treated TiC or TiN nanoparticles, followed by drying in a vacuum oven at 50 °C for 4 h. The resulting powders were reduced at 100 °C for 1 h under 10 vol% H2 flow (with balance N2) at a flow rate of 200 sccm. The powders were stored in glove box before further characterization. The Pt content was not separately measured because there is no chance that Pt would be lost during the synthesis. When the Pt content was measured by ICP for three different batches, the content was 0.193, 0.211, 0.198 wt% for 0.2 wt% Pt/TiC samples. Because both supports of TiC and TiN are not dissolved completely in aqua regia solution, the difference between the target value and the measured value would be a measurement error. Therefore, instead of measuring the actual Pt content for every batches, we used the target value when the electrode was prepared. Electrochemical measurements. Conventional threeelectrode electrochemical cell with a bi-potentiostat (CHI 760E, CH Instruments, Inc.) was used for all the electrochemical measurements. A 3 M NaCl Ag/AgCl (RE-5B, BASi) elec-
trode was used as the reference electrode, and a platinum wire was used as the counter electrode. Temperature of the electrochemical cell was maintained at 25 °C. All the potentials reported in this study are versus reversible hydrogen electrode (RHE). The RHE was measured prior to each measurement by performing hydrogen oxidation and evolution reactions in H2purged electrolyte using a rotating Pt electrode. Five milligrams of the prepared catalyst and 2.5 mg of carbon black (XC72R, Vulcan) were dispersed in 5 ml of anhydrous ethanol (Sigma-Aldrich) containing 17 µl of a Nafion solution (5 wt%, Sigma-Aldrich), followed by 5 min of sonication. Fifteen micro liters of single-atom catalyst solution was deposited on the freshly polished glassy carbon disk of a rotating ring disk electrode (glassy carbon disk, platinum ring, PINE), which was used as the working electrode. For Pt nanoparticle catalyst, 1.05 µg of catalyst was deposited on the glassy carbon disk. The oxygen reduction reaction (ORR) was conducted by anodic linear scan voltammetry (LSV) in an O2-saturated 0.1 M HClO4 (Sigma–Aldrich) solution with a scan rate of 0.01 V s-1 and a rotating rate of 1600 rpm. The potential of the Pt ring electrode was maintained at 1.2 V during the ORR to measure H2O2 oxidation current. IR-compensation was performed using the CHI software. H2O2 selectivity was calculated by a following equation, H2O2 selectivity (%) = (2 * iR/N) / ((iR/N)+iD) * 100, where iR is the ring current, iD is the disk current, and N is the collection efficiency (0.3826 for the specific RRDE that we have used).35 Computational details. Spin-polarized density-functional theory calculations in this work were performed using the generalized gradient approximation (GGA) to the exchangecorrelation functional due to Perdew, Burke and Ernzerhof (PBE)36 and the projector augmented-wave (PAW) method as implemented in the Vienna Ab initio Simulations Package (VASP).37-39 A planewave kinetic energy cutoff of 500 eV and a vacuum region of 18 Å between repeating periodic images are used for the calculations of all surface slab models. A Monkhorst-Pack 4 x 4 x 1 k-point mesh is used for the p(3 x 3) TiN(100) and TiC(100) supercells. To assess the thermodynamic stability of various molecular fragments and intermediates on both Pt/TiN and Pt/TiC, we calculate the adsorption ୲ energy, Ead, for each adsorbate by Eୟୢ = E୲୭୲ − E୧ଡ଼ − E + Pt ∆ZPE where Etot and E TiX are the total energies of the optimized adsorbate/catalyst system and the specific Pt/TiX support (where Pt/TiX is either Pt/TiN or Pt/TiC), respectively. EM refers to the total energy of gas-phase species considered in this work, while the last term ∆ZPE is the change in the zero point energy of these gas-phase species upon adsorption. The vibrational frequencies of the gas-phase species are estimated within the harmonic approximation while the “frozen slab” approximation is applied to adsorbed species where the contribution of the slab atoms to the vibrational free energy is neglected. In passing, we note that all molecular species are taken as charge neutral.40 Aligning with previous theoretical studies on electrochemical reactions, the computational hydrogen electrode (CHE) model due to Nørskov et al. is also employed in this work to evaluate the electrochemical reaction pathways via free energy calculations of the ORR intermediates.40 The change in the free energy (∆G) is calculated using ∆G = ∆E + ∆ZPE − T∆S where ∆E, ∆ZPE and ∆S stands for the respective changes in the total DFT energy, the zero-point energy, and the entropy. Entropy values of the molecular species are obtained from literature.40 Here, the temperature (T) is taken as 298.15 K (i.e.
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under ambient conditions). Under the consideration of an experimental overpotential, we have also applied a numerical shift of -eU to the ∆G value where e is the elementary positive charge and U value for O2 reduction to H2O2 as 0.2591 V (vs SHE) following the experimental condition. For the dissociation of O2 and OOH*, the activation energy barrier is calculated using the climbing image nudged elastic band (CI-NEB) method, as is commonly used to locate the minimum energy paths (MEPs) and the corresponding transition states of adsorbate molecules on surfaces.41 Characterizations. Transmission electron microscopy (TEM) images were collected using a TF30 ST (Tencai). High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images were collected using Titan cubed G2 60-300 (FEI) with an accelerating voltage of 200 kV. Pretreatment of lacey-carbon TEM grid was crucial to obtain atomic resolution image by preventing carbon re-deposition. Lacey-carbon grid (300 mesh Cu, TED PELLA) was plasmacleaned using H2 and O2 for 2 min on a Solarus Advanced Plasma System 950 (Gatan). Plasma treated grid was then treated in a vacuum chamber at 120 °C for 12 h to remove residual carbonaceous species. Then the catalyst suspension was drop-casted and dried. X-ray absorption spectroscopy (XAS) was conducted using the 8C Nano XAFS beamline of the Pohang Light Source (PLS). The energy of the storage ring electron beam was 3.0 GeV with a ring current of ~300 mA. The incident X-ray was monochromatized by a Si (111) double-crystal monochromator and detuned by 30 % to minimize unwanted harmonics. The Pt L3 edge spectra were obtained in fluorescence mode using a passivated implanted planar silicon (PIPS) detector (Canberra). A reference Pt foil was concurrently measured to calibrate each sample. The XAS data were processed and fitted with the ATHENA and ARTEMIS software programs.42 X-ray absorption spectrum of some lower Pt weight percent samples were repeatedly collected over 5 times and then merged to reduce noise. A coordination number was derived by fixing the S02 value, which was obtained from fitting the reference Pt foil. The Debye-Waller factor (σ2) was fixed at a reasonable value in some cases when the number of independent points was limited. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) was conducted by using Nicolet iS50 (Thermo Scientific) FT-IR equipped with an in-situ DRIFT cell, DiffusIR (PIKE). Hg-Cd-Te (MCT) detector was used. Three milligrams of catalyst powder was mixed and ground with 100 mg of KBr powder. Resulting powder was put into a sample cup and located into DRIFT cell. Before CO adsorption, the sample was pretreated in 100 sccm of Ar flow at 100 °C for 2 hr. Background was collected after the sample was cooled down to room temperature. Then, 2 sccm CO + 98 sccm Ar was flowed for 10 min. DRIFT cell was purged by flowing 100 sccm Ar with vacuum pumping for 5 min. Sample spectra was collected afterward. X-ray photoelectron spectroscopy (XPS) characterizations were conducted using K-alpha (Thermo Scientific). X-ray diffraction (XRD) characterizations were conducted using D/MAX-2500 (RIGAKU).
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Author Contributions §
S.Y. and Y.J.T. contributed equally to this work.
Notes The authors declare no competing financial interests.
ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS publications website at DOI: characterization of supports, additional FT-IR spectra, Characterization of supported nanoparticle catalysts, CV and XPS before and after stability tests.
ACKNOWLEDGMENT This work was supported by Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA1601-05 and the Basic Science Research Program by the National Research Foundation of Korea (NRF) (2014R1A1A1003415). Computational resources have been provided by the KISTI (Korea Institute of Science and Technology Information) supercomputing center (KSC-2016-C3-0038).
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