TiC- and TiN-Supported Single-Atom Catalysts for Dramatic

Publication Date (Web): April 5, 2017. Copyright © 2017 ... The origin of an extraordinary catalytic activity of Ir@d-TiC is also explained. This wor...
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TiC- and TiN-Supported Single-Atom Catalysts for Dramatic Improvements in CO2 Electrochemical Reduction to CH4 Seoin Back and Yousung Jung* Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehakro, Daejeon 34141, Korea S Supporting Information *

ABSTRACT: CO2 electrochemical catalysis is limited by scaling relations due to a d-band theory of transition metals. As a means of breaking the scaling relation, it has recently been reported that hybridizing the d-orbitals of transition metal with p-orbitals of main group elements or using naturally hybridized materials such as metal carbides and nitrides is a promising strategy. In this Letter, by means of density functional theory calculations, we investigate the catalytic properties of TiC, TiN, and single-atom catalysts supported on them for CO2 electrochemical reduction. In particular, we found that when single transition-metal atoms are inserted into the surface defect sites of TiC, denoted as M@d-TiC (M = Ag, Au, Co, Cu, Fe, Ir, Ni, Os, Pd, Pt, Rh, or Ru), the iridium-doped TiC (Ir@d-TiC) is found to have a remarkably low overpotential of −0.09 V, the lowest value among any catalysts reported in the literature to selectively produce CH4 (−0.3 ∼ −1.0 V). It is also shown that possible surface protonation reactions on TiC as a side reaction can be ignored because the overpotential (−0.38 V) is significantly larger than that of the CO2 electrochemical reduction reaction on single-atom catalysts (e.g., −0.09 V). The origin of an extraordinary catalytic activity of Ir@d-TiC is also explained. This work clearly demonstrates the great potential of carbides and single-atom catalysts supported on TiC as active and selective CO2 reduction catalysts, and perhaps for other electrochemical applications as well. relations.10,11 At the same time, the scaling relation on transition metals also puts a significant limit on the developments of improved catalysts, because the optimal catalysts often should interact with one adsorbate strongly without affecting others; in other words, the usual scaling relation of transition metals must be overcome.6 For example, to develop a transition-metal catalyst with a low overpotential for the CRR to produce CH4 or CH3OH, the *CHO intermediate should be selectively stabilized over the *CO intermediate to reduce a free-energy change (ΔG) of the *CO protonation reaction to *CHO (*CO + H+ + e− → *CHO).6,12 As a means of breaking the scaling relation, it has been suggested that introducing a covalent character of main-group elements can be helpful to perturb the d-band of transition metals.13,14 For example, improved catalytic properties of single-atom catalysts (SACs) in a form dispersed on defective graphene have been suggested for efficient CO2 reduction.14 The authors showed that the binding energies of reaction intermediates on SACs noticeably deviate from the usual

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or a sustainable future and carbon-neutral production of hydrocarbon and alcohol fuels, CO2 electrochemical reduction reaction (CRR) is one of the promising and urgent technologies.1,2 However, various experiments of CRR over the past decades have demonstrated that CRR suffers from activity and selectivity issues.3,4 For example, although the equilibrium potentials for many CO2 reduction products are close to 0 V vs RHE (reversible hydrogen electrode), a reasonable current density for CO2 reduction is obtained only at potentials more negative than −1.0 V on Cu electrode, indicating a significant loss of the input electrical energy.5 Furthermore, similar equilibrium potentials needed for various undesired reduction products as well as kinetically favorable H2 evolution reactions are big hurdles to achieving a high faradaic efficiency for the target reaction product. Recent theoretical studies revealed that binding energies of CRR intermediates are strongly correlated with each other on transition-metal catalysts via the so-called “scaling relation”,6 whose origin is a pioneering d-band theory of Norskov and coworkers.7,8 Indeed, scaling relations have been observed in many catalytic reactions,7,9 and it has been demonstrated to be useful to predict and design new catalysts through firstprinciples calculations in conjunction with the scaling © 2017 American Chemical Society

Received: February 23, 2017 Accepted: April 5, 2017 Published: April 5, 2017 969

DOI: 10.1021/acsenergylett.7b00152 ACS Energy Lett. 2017, 2, 969−975

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Figure 1. (A) Top view of TiX(100) surface, where X is either C or N. Free-energy profiles for CO2 electroreduction reaction at 0 V vs RHE on (B) TiC(100) and (C) TiN(100). Free energies for two possible side reactions (*H adsorption and *OH poisoning) are also plotted. The lowest-energy reaction pathway is denoted as a dashed line.

metals,19 and on this basis, they proposed Mo2C for CO2 reduction catalyst to produce CH4 at low overpotential.23 Carbides and nitrides are also utilized as support materials for overlayer depositions and single-atom and nanoparticle catalysts because of their high electrical conductivity and stability under reaction conditions.25−30 For instance, Yang et al. recently reported that TiC and TiN can stabilize a Pt single atom, and these catalysts showed significantly different catalytic activity and product selectivity compared to Pt catalyst for electrochemical oxygen reduction reaction.27,31 The authors suggested that TiC and TiN not only stabilize highly dispersed Pt single atoms but also actively participate in the catalytic reactions. In this work, we theoretically investigated the catalytic properties of TiC, TiN, and single-atom catalysts supported on them for CO2 reduction reaction. Among other carbides and nitrides, TiC and TiN were chosen because they are electrically conductive and inexpensive and have already been shown to be experimentally viable as support materials for single Pt atom deposition, as described above. We find that TiC can effectively catalyze CO2 reduction, while TiN is poisoned by *OCHO or *OH because of an oxygen affinity that is too strong. Upon the incorporation of various single transition-metal dopants by using TiC as a support material, we find that the single iridium atom supported on TiC exhibits an exceptionally small

scaling relation of transition-metal (211) surfaces with a large reduction of the overpotential due to a partial electron transfer from a metal atom to a support material. In the same sense, various two-dimensional inorganic compounds which are composed of metal and nonmetal elements have been also proposed as potential CO2 reduction catalysts.15−18 Density functional theory (DFT) calculation by Chan et al. predicted that MoS2, MoSe2 and transition-metal atoms doped on them could effectively break the scaling relation to achieve low overpotential for CO2 reduction to CO or CH4.16 Later, it was indeed experimentally confirmed that MoS2 is capable of electrochemically reducing CO2 to CO with overpotentials of 54 mV.17 Nb doping on MoS2 further reduced the overpotentials to 31 mV.18 These theoretical and experimental results highlight an importance of the orbital hybridization between d-orbitals of transition metals and p-orbitals of maingroup elements. With regard to the orbital hybridization, metal carbides and nitrides are one of the natural and particularly interesting classes of materials because the d-orbitals of transition metals are already hybridized with p-orbitals of carbon or nitrogen.19,20 As such, carbides and nitrides have recently begun to receive attention as cost-effective and active catalysts for various reactions.20−24 Peterson and co-workers reported that metal carbide surfaces deviate from the scaling relation of transition 970

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Figure 2. Free-energy changes (ΔG) of the first protonation step of CO2 reduction reaction (CRR) and H2 evolution reaction (HER) at 0 V vs RHE. Black circles and blue diamonds indicate the formation of *COOH and *OCHO, respectively. Stoichiometric TiC and TiN are marked in red. Catalysts below the dotted parity line are expected to be CRR-selective.

overpotential of −0.09 V, the lowest overpotential to produce CH4 to date. The origin of such a large activity improvement lies in the change in *CO binding strength and site due to a unique electronic structure of a single Ir atom catalyst hybridized with the TiC support. CO2 Reduction on Bare TiC and TiN(100) Surfaces. We begin by examining the CO2 reduction activity of bare TiC and TiN (Figure 1A). Figure 1B,C shows free-energy profiles for CO2 reduction on TiC(100) and TiN(100) surfaces. On TiC(100), the formation of *COOH is more favorable than that of *OCHO or *H, indicating a preferred formation of *COOH. By following the lowest-energy pathway, the potentialdetermining step (PDS) of TiC(100) is a protonation of *CO to *CHO, and the corresponding UL is calculated to be −0.47 V. Because the lowest-energy pathway leads to CH4 production and it does not involve *OCH3 protonation, which was suggested to be a selectivity-determining step between CH4 and CH3OH,32,33 we expect that bare TiC reduces CO2 to selectively produce CH4. Thi value (−0.47 V) can also be compared with the calculated UL of Cu(211) surface of −0.8 V,33,34 making TiC a good candidate for use as a CH4producing CRR catalyst. However, on the basis of a recent report on the possibility of electrochemical surface protonation for N-containing catalysts as a side reaction of CRR,35,36 we also considered a possibility of CH4 production due to the protonations of the surface C of TiC. We find that the potential required for the latter surface protonation is −0.38 V, indeed less negative (more favorable) than that of CRR (−0.47 V) (Figure S1). Therefore, it appears that, thermodynamically, CH4 might be produced on the TiC surface primarily because of the surface protonations. As will be shown later, however, decoration of the TiC surface with single metal atoms lowers (making less negative) the thermodynamic overpotential of CRR significantly (from −0.47 V to −0.09 V), so that the surface protonation reactions can be ignored. TiN(100), on the other hand, has a substantially different catalytic behavior compared to TiC(100). On TiN(100), the formation of *OCHO is significantly more stable than that of *COOH. The further protonation of *OCHO yields only adsorbed *HCOOH, yet with a ΔG (1.72 eV) that is too large, indicating that the excessively O-affinitive TiN is not suitable for CRR. At the same time, the O-affinitive character also leads to *OH poisoning of TiN, requiring 0.92 eV to remove *OH species. It is noteworthy that binding strengths of *OCHO (or *OH) on TiC and TiN are significantly different (∼1 eV),

although binding configurations of *OCHO (or *OH) on both surfaces are identical (Figure S2). This difference shows that the electronic structure of the Ti atom is considerably modified by nearby C and N, an effect that we aim to control in the next section to maximize the catalytic activity and selectivity in single-atom catalysts. The latter difference is also evidenced by partial density of states of Ti d-orbital in TiC and TiN (Figure S3). We note that because of a large overpotential of the TiN surfaces for CRR, we did not consider surface protonations for this surface further. Single-Atom Catalysts Supported on TiC and TiN. In the previous section, we investigated the catalytic properties of bare TiC and TiN(100) surfaces and found that the catalytic activity of TiC is expected to be active and selective for CO2 reduction to CH4, while that of TiN is limited by an oxygen affinity that is too strong. In this section, we explore a potential of TiC and TiN as support materials for single-atom catalysts, where transition-metal atoms (M = Ag, Au, Co, Cu, Fe, Ir, Ni, Os, Pd, Pt, Rh, and Ru) are doped at a defect site of TiC and TiN. We first assessed the stability of a metal atom at two surface defect sites of TiC and TiN, where more stable configurations are termed M@d-TiC and M@d-TiN, respectively (Figure S4). For TiC, metal adsorption at a Ti vacancy is favorable in all cases of M with very strong binding energies (a few electronvolts). On the other hand, binding energies of a metal atom at the defect sites of TiN are relatively weaker (less than 1 eV), and only Ag, Au, Fe, Pd, Pt, and Rh are expected to be stable. We further analyzed the stability of metal atoms at subsurface sites (Figure S4). For metal doping on TiC, surface Ti defect sites are the most stable in all cases. Also, for a metal doping on TiN, the surface sites (Ti or N) were determined to be the most stable sites, except for Fe which is more stable when doped at a subsurface Ti site. Using the most stable geometries, we then calculated freeenergy changes (ΔG) of the first protonation step of CO2 reduction reaction (CO2 + H+ + e− → *COOH/*OCHO) and H2 evolution reaction (H+ + e− → *H) to compare a thermodynamic preference (Figure 2). This is because *H adsorption occurs competitively by consuming protons and electrons under the reducing conditions, and favorable *H adsorption is detrimental for a high efficiency of the target CO2 reduction reaction. We expect the reaction with smaller ΔG to be more selective. Figure 2 shows that only M@d-TiC (M = Co, Ir, Rh) prefers *COOH adsorption, while the rest of metal dopants prefer *H adsorption. M@d-TiN prefers *OCHO 971

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ACS Energy Letters adsorption in all cases similar to stoichiometric bare TiN. Notably, ΔG(*OCHO → *HCOOH) values for all M@d-TiN cases are greater than 1 eV; thus, SACs on the TiN support were not considered further and only the SACs on the TiC support are considered in the following analysis. For M@d-TiC, we considered further protonation steps following the lowest-energy pathway. Interestingly, *CHO and *CHOH are more stable than *COH and *CH2O in all cases, respectively, leading to selective CH4 production pathway over CH3OH, similarly to bare TiC. The PDS and corresponding UL are summarized in Figure 3. Two points are noteworthy. First,

Table 1. Theoretical Limiting Potentials (UL) and Experimental Onset Potentials of Various CO2 Electroreduction Catalysts for CH4 Productiona theory

limiting potential (V vs RHE)

Cu(211)33 LiFeAs15

−0.80 −0.55

Ru@dv-Gr, Os@ dv-Gr14 Ni-doped S edge of MoS216

−0.52

Mo2C(100)23 WC(0001)24 Ir@d-TiC (this work)

−0.56 −1.00 −0.09

−0.28

experiments Cu foil4 Cu2O-derived Cu40 bimetallic Cu− Pd41 graphene quantum dots42 Mo2C23 Ni5Ga343

onset potential (V vs RHE) −0.75 −0.65 −0.40 −0.48 −0.55 −0.50

a

We note that the experimental onset potentials are the values at which CH4 is first detected; therefore, the less negative onset potentials may not necessarily guarantee higher activity for CH4 production because of the selectivity issue.

Unlike *CO, whose binding site differs in TiC versus M@dTiC, *CHO and *COOH adsorb on the carbon of TiC for both TiC and M@d-TiC (Figure S2), making EB[*COOH] and EB[*CHO] vary significantly less compared to EB[*CO] for bare TiC versus M@d-TiC. We note in passing that a similar behavior was observed for pristine and transition-metal doped MoS2, where *CO binds to Mo while *COOH, *CHO, and *COH bind to S, resulting in a breaking of the scaling relation.46 For more detail, the free-energy profiles for bare TiC versus Ir@d-TiC are compared in Figure 5. For Ir@d-TiC, most of the reaction intermediates are destabilized relative to TiC, but noticeably, a considerable destabilization is observed for *CO (0.60 eV), causing the PDS to change from *CO → *CHO (ΔG = 0.47 eV) for bare TiC to *CHOH → *CH (ΔG = 0.09 eV) for Ir@d-TiC. As discussed above, this weakening is due to the *CO binding that occurs at the single Ir atom in Ir@d-TiC. In Figure 6, the density of states (DOS) is analyzed to understand the *CO bonding nature. For bare TiC binding with *CO (Figure 6A), there are two localized energy states arising from the interaction between C-pz of TiC and C-pz of *CO (i and iii) and one localized state arising from the interaction between C-px,py of TiC and C-px,py of *CO (ii). The same qualitative picture holds for *CO binding with pure Ir(111) (Figure 6B). By contrast, for Ir@d-TiC, there is only one localized energy state due to the interaction between Ir-d2z and C-pz of *CO, and another localized energy state due to the interaction between Ir-dxz,dyz and C-px,py of *CO (Figure 6C). If one visualizes these localized states in Figure 6D, it can be clearly seen that a lack of sigma-bonding type state (i) in Ir@dTiC is responsible for a weakening of *CO bonding. The weak binding strength of *CO with Ir@d-TiC (−1.18 eV) relative to that with TiC (−1.78 eV) and Ir(111) (−1.99 eV) and the elongated Ir-*CO bond length in Ir@d-TiC (1.88 Å) compared to the Ir(111) case (1.85 Å) are consistent with the latter interpretations. The DOS of *COOH and *CHO adsorbed on TiC and Ir@d-TiC (Figures S8 and S9), however, are very similar, perhaps to be expected from the similar bonding configurations. Therefore, the unique lowering of the overpotential for Ir@d-TiC is mainly due to the weakened *CO binding with the single Ir atom of Ir@d-TiC compared to bare TiC or bulk Ir(111) surfaces.

Figure 3. Theoretical limiting potential (UL) of M@d-TiC(100) for CO2 reduction reaction. The different potential-determining steps (PDS) are marked with different colors. The dotted horizontal line is the UL of TiC(100) surface, where the PDS is *CO + H+ + e− → *CHO.

the PDS of M@d-TiC is one of the following four steps: (i) *COOH + H+ + e− → *CO + H2O, (ii) *CHOH + H+ + e− → *CH + H2O, (iii) *CHO + H+ + e− → *CHOH, or (iv) *CH + H+ + e− → *CH2; for pure transition metals, the PDS is usually CO2 + H+ + e− → *COOH or *CO + H+ + e− → *CHO/*COH. Second, many M@d-TiC catalysts are predicted to be more active for CRR with less negative UL than bare TiC. Particularly, the estimated theoretical limiting potential of Ir@d-TiC for CH4 production is extremely small (−0.09 V vs RHE), the least negative of any CRR catalysts reported to date based on theoretical or experimental results (Table 1).12,14,16,33,37−39 For selected catalysts with very low overpotentials (Ir@d-TiC, Os@d-TiC, and Ru@d-TiC), we investigated the possibility of protonation of surface carbon to release CH4 and found that CO2 reduction to CH4 is preferred compared to the protonation of surface carbon in all cases (Figure S1).35,36 Origin of Remarkable CRR Activity of Ir@d-TiC. To understand the origin of high CRR catalytic activity of Ir@dTiC, we highlight a broken scaling relation of binding energies due to metal−support interactions in M@d-TiC. As can be seen in Figure 4, the binding energy of *COOH and *CHO is highly uncorrelated with the binding energy of *CO for TiCsupported SACs (blue circles), unlike the pure metals that show a conventional linear correlation (black circles and lines). We attribute this behavior mainly to the weakened binding of *CO for M@d-TiC due to a significantly different electronic structure compared to pure metals. A noticeable orbital overlap between the dopant metal and nearby TiC atoms is evident (Figure S6), and electron transfer from a dopant metal to nearby carbons (Figure S7 and Table S1) is also observed.44,45 As a result, the *CO binding site changes from C in the case of bare TiC to M or Ti in the case of M@d-TiC (Table S2). 972

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Figure 4. (A) *COOH and (B) *CHO binding energies plotted as a function of *CO binding energy for transition-metal (211) surfaces (black) and M@d-TiC (blue). Blue hollow circles indicate binding energies on the TiC(100) surface. We note that the binding energies on metal (211) surfaces were also calculated with the RPBE+vdW functional (see Figure S5 for the effects of vdW correction).

Figure 5. Free-energy profiles for CO2 reduction reaction at 0 V vs RHE on TiC (black) and Ir@d-TiC (blue). PDS and corresponding freeenergy change are shown.

Figure 6. Density of states (DOS) of (A) TiC, (B) Ir(111), and (C) Ir@d-TiC interacting with *CO. The *CO binds to the surface C for TiC but to Ir for Ir@d-TiC. (D) The electron density isosurfaces at the energy level as noted in panels A−C with isosurface level of 0.05 e/Å3. We note that (ii), (ii*), and (ii**) denote the equivalent bonding orbital, and similarly so for (iii), (iii*), and (iii**).

limiting potential to −0.09 V for the production of CH4, a theoretical overpotential substantially lower than that of any CO2 electrocatalyst performing eight-electron reduction reported to date. (iii) A significant deviation from the usual scaling relation of transition metals was observed for TiC-supported single metal atom catalysts. This breaking originates from the change in *CO binding site and strength due to the metal−support interactions in M@d-TiC. In particular, Ir@d-TiC lacked a sigma-type bonding interaction between *CO and a single Ir

In this Letter, we systematically investigated TiC, TiN, and single-atom catalysts supported on them for CO2 reduction reaction using DFT calculations. The key findings of this work are as follows: (i) Free-energy profiles for CO2 reduction on TiC and TiN indicated that bare TiC surface can catalyze CO2 reduction with the limiting potential of −0.47 V, whereas TiN is expected to be poisoned by O-containing species because of an O affinity that is too strong. (ii) Insertion of an Ir single atom on the TiC support (denoted as Ir@d-TiC) showed a significant further decrease of 973

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(11) Greeley, J.; Stephens, I.; Bondarenko, A.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T.; Rossmeisl, J.; Chorkendorff, I.; Nørskov, J. K. Alloys of platinum and early transition metals as oxygen reduction electrocatalysts. Nat. Chem. 2009, 1, 552−556. (12) Hansen, H.; Shi, C.; Lausche, A.; Peterson, A.; Nørskov, J. Bifunctional alloys for the electroreduction of CO2 and CO. Phys. Chem. Chem. Phys. 2016, 18, 9194−9201. (13) Lim, H.-K.; Shin, H.; Goddard, W. A., III; Hwang, Y. J.; Min, B. K.; Kim, H. Embedding covalency into metal catalysts for efficient electrochemical conversion of CO2. J. Am. Chem. Soc. 2014, 136, 11355−11361. (14) Back, S.; Lim, J.; Kim, N.-Y.; Kim, Y.-H.; Jung, Y. Single-atom catalysts for CO2 electroreduction with significant activity and selectivity improvements. Chem. Sci. 2017, 8, 1090−1096. (15) Shin, H.; Ha, Y.; Kim, H. 2D Covalent Metals: A New Materials Domain of Electrochemical CO2 Conversion with Broken Scaling Relationship. J. Phys. Chem. Lett. 2016, 7, 4124−4129. (16) Chan, K.; Tsai, C.; Hansen, H. A.; Nørskov, J. K. Molybdenum sulfides and selenides as possible electrocatalysts for CO2 reduction. ChemCatChem 2014, 6, 1899−1905. (17) Asadi, M.; Kumar, B.; Behranginia, A.; Rosen, B. A.; Baskin, A.; Repnin, N.; Pisasale, D.; Phillips, P.; Zhu, W.; Haasch, R.; et al. Robust carbon dioxide reduction on molybdenum disulphide edges. Nat. Commun. 2014, 5, 4470. (18) Abbasi, P.; Asadi, M.; Liu, C.; Sharifi-Asl, S.; Sayahpour, B.; Behranginia, A.; Zapol, P.; Shahbazian-Yassar, R.; Curtiss, L. A.; SalehiKhojin, A. Tailoring the Edge Structure of Molybdenum Disulfide towards Electrocatalytic Reduction of Carbon Dioxide. ACS Nano 2017, 11, 453−460. (19) Michalsky, R.; Zhang, Y.-J.; Medford, A. J.; Peterson, A. A. Departures from the adsorption energy scaling relations for metal carbide catalysts. J. Phys. Chem. C 2014, 118, 13026−13034. (20) Zhong, Y.; Xia, X.; Shi, F.; Zhan, J.; Tu, J.; Fan, H. J. Transition Metal Carbides and Nitrides in Energy Storage and Conversion. Adv. Sci. 2016, 3, 1500286. (21) Porosoff, M. D.; Kattel, S.; Li, W.; Liu, P.; Chen, J. G. Identifying trends and descriptors for selective CO 2 conversion to CO over transition metal carbides. Chem. Commun. 2015, 51, 6988− 6991. (22) Medford, A. J.; Vojvodic, A.; Studt, F.; Abild-Pedersen, F.; Nørskov, J. K. Elementary steps of syngas reactions on Mo2C (001): adsorption thermochemistry and bond dissociation. J. Catal. 2012, 290, 108−117. (23) Kim, S. K.; Zhang, Y.-J.; Bergstrom, H.; Michalsky, R.; Peterson, A. Understanding the Low-Overpotential Production of CH4 from CO2 on Mo2C Catalysts. ACS Catal. 2016, 6, 2003−2013. (24) Wannakao, S.; Artrith, N.; Limtrakul, J.; Kolpak, A. M. Engineering Transition-Metal-Coated Tungsten Carbides for Efficient and Selective Electrochemical Reduction of CO2 to Methane. ChemSusChem 2015, 8, 2745−2751. (25) Kimmel, Y. C.; Xu, X.; Yu, W.; Yang, X.; Chen, J. G. Trends in electrochemical stability of transition metal carbides and their potential use as supports for low-cost electrocatalysts. ACS Catal. 2014, 4, 1558−1562. (26) Regmi, Y. N.; Waetzig, G. R.; Duffee, K. D.; Schmuecker, S. M.; Thode, J. M.; Leonard, B. M. Carbides of group IVA, VA and VIA transition metals as alternative HER and ORR catalysts and support materials. J. Mater. Chem. A 2015, 3, 10085−10091. (27) Yang, S.; Kim, J.; Tak, Y. J.; Soon, A.; Lee, H. Single-Atom Catalyst of Platinum Supported on Titanium Nitride for Selective Electrochemical Reactions. Angew. Chem., Int. Ed. 2016, 55, 2058− 2062. (28) Cheng, N.; Norouzi Banis, M.; Liu, J.; Riese, A.; Mu, S.; Li, R.; Sham, T.-K.; Sun, X. Atomic scale enhancement of metal−support interactions between Pt and ZrC for highly stable electrocatalysts. Energy Environ. Sci. 2015, 8, 1450−1455. (29) Tian, X.; Luo, J.; Nan, H.; Zou, H.; Chen, R.; Shu, T.; Li, X.; Li, Y.; Song, H.; Liao, S.; et al. Transition metal nitride coated with atomic

atom compared to bare TiC or Ir(111) cases, which results in a considerable reduction in the limiting potential.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00152. Theoretical methods, free-energy correction values, calculated Bader charges, and the density of states of *CHO and *COOH binding on TiC and Ir@d-TiC (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Seoin Back: 0000-0003-4682-0621 Yousung Jung: 0000-0003-2615-8394 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support through the National Research Foundation of Korea from the Korean Government (NRF2015R1A2A1A15055539 & NRF-2017R1A2B3010176). S.B. acknowledges Global Ph.D. Fellowship Program through NRF funded by the Ministry of Education (NRF2014H1A2A1016055).

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REFERENCES

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DOI: 10.1021/acsenergylett.7b00152 ACS Energy Lett. 2017, 2, 969−975

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DOI: 10.1021/acsenergylett.7b00152 ACS Energy Lett. 2017, 2, 969−975