Promotion of Ternary Pt-Sn-Ag Catalysts toward Ethanol Oxidation

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Promotion of Ternary Pt-Sn-Ag Catalysts toward Ethanol Oxidation Reaction: Revealing Electronic and Structural Effect of Additive Metals Sheng Dai, Tzu-Hsi Huang, Xingxu Yan, Chao-Yu Yang, TsanYao Chen, Jeng-Han Wang, Xiaoqing Pan, and Kuan-Wen Wang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01632 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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ACS Energy Letters

Promotion of Ternary Pt-Sn-Ag Catalysts toward Ethanol Oxidation Reaction: Revealing Electronic and Structural Effects of Additive Metals

Sheng Dai1,∇, Tzu-Hsi Huang2,∇, Xingxu Yan1, Chao-Yu Yang2, Tsan-Yao Chen3,4, Jeng-Han Wang5,*, Xiaoqing Pan1,6, *, and Kuan-Wen Wang1,2,*

1

Department of Chemical Engineering and Materials Science, University of

California-Irvine, Irvine, California 92697, United States 2

Institute of Materials Science and Engineering, National Central University,

Taoyuan, 32001, Taiwan 3

Department of Engineering and System Science, National Tsing Hua University,

Hsinchu 30013, Taiwan. 4

Institute of Nuclear Engineering and Science, National Tsing Hua University,

Hsinchu 30013, Taiwan. 5

Department of Chemistry, National Taiwan Normal University, Taipei, 11677,

Taiwan 6

Department of Physics and Astronomy, University of California-Irvine, Irvine,

California 92697, United States

1

ACS Paragon Plus Environment

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ABSTRACT The use of computation-guided method and the discovered structure-property relationship would establish a rational strategy to aid the development of ethanol oxidation reaction (EOR) catalysts, for possible commercialization of direct ethanol fuel cells. Here, we investigate the promotion roles of additive metals in ternary Pt-Sn-Ag catalysts toward EOR via a combination of theoretical calculation and experimental evidence. By calculating the EOR energetics, the promotion roles of Sn and Ag were revealed from the viewpoints of electronic and structural effects, respectively: (1) Additions of Sn and Ag on Pt essentially reduce the reaction energy and activation barrier of the second 2-electron transfer process of EOR, facilitating the oxidation of acetaldehyde to acetic acid; (2) A homogeneous Pt-Sn-Ag surface configuration strengthens the adsorption energy of ethanol, thus improving the activity for ethanol oxidizing to acetaldehyde. Experimentally, various Pt-Sn-Ag nanorod catalysts with different surface configurations were synthesized, and their electrochemical performances demonstrate the two EOR promotion effects as predicted. Notably, our extended Pt6-Sn-Ag nanorod catalyst shows remarkably enhanced EOR activity and stability, highlighting a homogeneous Pt-Sn-Ag surface as an optimal structure for EOR catalysts.

2


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MAIN TEXT

Considering increasing environmental problems and possible depletion of fossil fuel, it is desirable to seek alternative energy resources to confront the global energy crisis. Over the past decades, great efforts have been dedicated to developing low-temperature fuel cells that can directly electro-oxidize small organic molecules for electricity with a high thermodynamic efficiency, thus providing an alternative power path. Regarding this point, ethanol is an attractive fuel resource due to its non-toxicity, renewability, and high energy density.1-3 More importantly, ethanol can be produced in large quantities by fermentation of sugar-containing and/or cellulose-containing raw materials, and thus it has been recognized as a substantial energy source in the future of "green" technology.3, 4 However, in spite of the above advantages, oxidation of ethanol generally shows sluggish kinetics at traditional Pt electrocatalysts, obstructing the commercialization of direct ethanol fuel cells (DEFCs). For example, oxidation of an ethanol molecule is often incomplete due to an involvement of C-C bond cleavage and 12-electron transfer.5 Various strongly adsorbed intermediates (e.g., CO and CHx) are produced during the ethanol oxidation reaction (EOR), and poison active sites on the surface of Pt catalysts.6, 7 To promote the practical application of DEFCs, efforts to develop highly active EOR electrocatalysts have, therefore, concentrated on the addition of co-catalysts to traditional Pt.8-13 It has been demonstrated that EOR activity can be enhanced in binary Pt-metal catalysts (e.g. Pt-Ru, Pt-Sn, and Pt-Ag), due to the increased number of d-band vacancies of Pt and a more favorable Pt-Pt inter-atomic distance.8-10 Recently, new Pt-based ternary catalysts have shown superb electrocatalytic activity toward EOR, superior to their binary counterparts.11-16 For example, the addition of Rh into Pt-Sn system is found to enhance the EOR activity;14, 15 the addition of Ni into 4


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Pt-Rh system improves the EOR stability.15 Although Pt-based ternary catalysts provide great promise for resolving the said challenges to develop DEFCs of practical significance, more detailed mechanisms of their improved EOR performances are still not well understood. Limited information about the roles of additive metals and the unrevealed structure-property relationships of Pt-based ternary catalysts hinders a rational design strategy for advanced EOR catalysts. Unanswered questions include: (1) how do the additive metals facilitate the EOR in Pt-based ternary catalysts? and (2) what is the optimum ternary structure for enhanced EOR performances? In this work, we here select Pt-Sn-Ag system as an example to investigate the promotion roles of additive metals toward enhanced EOR performances, via a combination of density function theory (DFT) calculation and experimental evidence. From the viewpoints of electronic and structural effects, it is firstly revealed by the calculation that EOR activity can be improved through co-addition of Ag and Sn on Pt and a homogenous ternary Pt-Sn-Ag surface configuration. Experimentally, various Pt-based nanorod catalysts with different surface configurations were designed and synthesized. Based on the comparison of their electrochemical performances, the computationally predicted promotion effects toward EOR are therefore demonstrated. A homogeneous Pt-Sn-Ag surface is highlighted as an optimal structure for EOR catalysts since it exhibits not only higher activity but also desirable stability. These results may prove useful for a better mechanistic understanding and a rational design strategy for Pt-based ternary EOR catalysts. DFT computation was firstly employed to explore the mechanism of EOR to elucidate the electronic and structural effects in the ternary Pt-Sn-Ag catalysts (more details about the computational method are provided in the Supporting Information). As schematically illustrated in Figure 1a, the major products of acetaldehyde and acetic acid in EOR17-19 are resulted from the 2- and 4-electron oxidation reactions, 5



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respectively. The reaction energy (∆E) and activation barrier (Ea) were computed and compared on Pt-Ag, Pt-Sn and Pt-Sn-Ag (as indicated by black, red and blue values, respectively, in Figure 1a) to clarify the electronic effect from the addition of Ag and Sn. The first 2-electron oxidation reaction is kinetically favored for the initial OH and subsequent CH dissociations17; the computed energetics in those two dehydrogenation steps are similar on all Pt-Ag (Ea ≤ 0.57 eV), Pt-Sn (Ea ≤ 0.34 eV) and Pt-Sn-Ag surfaces (Ea ≤ 0.51 eV), as Pt-Sn surface has somewhat lower energies. On the other hand, their small energetic difference indicates that Pt-Ag, Pt-Sn and Pt-Sn-Ag systems show similar activity on the first 2-electron oxidation reaction in EOR, much more active than pure Pt (Ea ≤ 0.99 eV) due to their surface oxygen containing species (OCS) from Ag and Sn.17 Of great significance, the ternary Pt-Sn-Ag system shows different energetics from binary systems of Pt-Ag and Pt-Sn in the later 2-electron oxidation reaction from acetaldehyde to acetic acid, which includes the dehydrogenation step of CH3CHO CH3CO + H and the OH association step of CH3CO + OH CH3COOH. The dehydrogenation step has similar energetics on both surfaces (Ea c.a. 0.60 eV); however, the OH association step is much different. Ternary Pt-Sn-Ag has a more exothermic ∆E (-1.16 eV) and a lower Ea (0.38 eV) than those on binary Pt-Ag (0.67 eV) and Pt-Sn (0.95 eV), indicating that the synergetic effect from additions of both Ag and Sn can effectively promote the oxidation reaction from acetaldehyde to acetic acid in EOR. The lowered energies in the OH association step on ternary Pt-Sn-Ag surface can be attributable to the fact that the surface OCS on Sn can better promote the dehydrogenation steps in EOR;17 additionally, that on the less oxophilic Ag has a denser charge and more localized DOS, in Figure S1 in the Supporting Information, will repulse neighboring OH to further assist its association step. As a result, the addition of Ag and Sn in the ternary Pt-Sn-Ag can essentially improve the oxidation 6


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of acetaldehyde to acetic acid in the viewpoint of electronic effect.

Figure 1. Energetic and synergetic properties of various Pt-based EOR catalysts. (a) Scheme of ethanol oxidizing to the major products of acetaldehyde and acetic acid in the first and later 2-electron oxidation reactions. The computed energetics of ∆E and Ea (unit: eV) on the binary Pt-Ag and Pt-Sn and ternary Pt-Sn-Ag surfaces are listed in black, red and blue numbers, respectively. (b) Adsorption sites of I, closest to surface Sn and Ag, II and III and the related Eads of ethanol on Pt-Sn-Ag surface in the insert. (c) DOS of O p band of adsorbed ethanol, which bonds to surface Pt on sites I, II and III in red, purple and blue lines, respectively.

Additionally, we investigate the structural effect of the adsorption energy (Eads) of ethanol, the initial reactant, on different sites of the ternary Pt-Sn-Ag surface. Three top sites, which are the energetically favored ones17 were examined and shown in Figure 1b. According to the energetic comparison, it is found that ethanol has a much stronger Eads (-0.88 eV) on site I, closest to surface Sn and Ag atoms, than those on other sites of II (-0.37 eV), III (-0.33 eV) and pure Pt (-0.23 eV), as shown in Figure 7



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1b. The strengthened Eads on site I can be rationalized from the DOS analysis (Figure 1c) that oxygen atom of the adsorbed ethanol has a much lower energetic p band, the bonding band; i.e. the lower energetic bonding band implies a more stable chemical bond. Also, the Bader charge analysis shows the bonded O is more negatively charged (more ionic) on site I (-1.62 |e|) than on sites II and III (-1.57 |e|). The strongest Eads of ethanol on site I can fully overcome its O-H bond dissociation Ea and effectively enhance the first 2-electron oxidation reaction from ethanol to acetaldehyde; however, the weaker Eads on the other sites can only overcome the Ea partially and are less active. Thus, the synthesized electrodes with more isolated Ag-Sn pairs on Pt surface can expose more site I to ethanol for better EOR activity in the viewpoint of structural effect. Concluded from the computational prediction, ternary Pt-Sn-Ag system in a homogeneous surface elemental distribution is expected to be an optimal catalyst for high EOR performances. To validate the DFT calculation output and the deduced correlation between the catalyst structure and EOR performance, various carbon-supported Pt-Sn-Ag ternary nanorod catalysts were then designed and prepared (more details of the synthesis are provided in the Supporting Information) for the following electrochemical test: (1) regular Pt6-Sn-Ag catalyst (with metal loading of 50 wt.% and Pt/Sn/Ag atomic ratio of 6/1/1), synthesized via a formic acid method; (2) dealloyed Pt-Sn-Ag catalyst, modified from the regular Pt6-Sn-Ag nanorods through an electrochemical dealloying treatment in order to leach some of the additive metals and expose more Pt sites; (3) extended Pt6-Sn-Ag catalyst, synthesized with an intention to extend the nanorod aspect ratios, for the aim to modify the surface elemental distribution. In addition, pure Pt and binary Pt-based (Pt6-Sn2 and Pt6-Ag2) catalysts were also synthesized for comparison. Aberration-corrected scanning transmission electron microscopy (AC-STEM) 8


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characterization was carried out to reveal the morphology and structural information of the as-synthesized Pt-based EOR catalysts. For example, Figures 2a, 2b and 2c show representative high angle annular dark field (HAADF)-STEM images of pure Pt, Pt6-Ag2 and Pt6-Sn2 nanorod catalysts, which are possessing similar diameters (about 4.0 nm) and aspect ratios (L/d, around 3.0). Particularly, as shown in the corresponding energy dispersive X-ray spectroscopy (EDS) elemental maps (see panels b1-b2 and c1-c2), it is confirmed that the additive metal Ag or Sn shows a homogeneous distribution on the binary Pt-based nanorod surface.

Figure 2. AC-STEM and EDS characterization of various Pt-based EOR catalysts. (a), (b), (c) (d) and (e) are representative HAADF-STEM images of the pure Pt nanorods, Pt6-Ag2 nanorods, Pt6-Sn2 nanorods, regular Pt6-Sn-Ag nanorods, dealloyed Pt6-Sn-Ag nanorods, and extended Pt6-Sn-Ag nanorods, respectively. 9



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Panels (b1-b2), (c1-c2), (d1-d3), (e1-e3), and (f1-f3) are the corresponding EDS elemental maps showing the elemental distribution of the binary and ternary Pt-based nanorod catalysts.

Figure 2d presents the AC-STEM result of regular Pt6-Sn-Ag (R-Pt6-Sn-Ag) catalysts. According to our STEM observation, diameter and aspect ratio of the R-Pt6-Sn-Ag nanorods are similar to those of the pure Pt and binary Pt-basednanorods. However, interestingly, the distribution of Sn is altered in the ternary R-Pt6-Sn-Ag catalysts. From the Z-contrast20 HAADF-STEM image (Figure 2d) and corresponding EDS elemental maps (panels d1-d3), it is clear that Sn agglomerates as small nanoparticles (indicated by green arrows in Figure 2d) while Ag distributes uniformly on the ternary catalyst surface. One possible reason for such Sn distribution behavior is that the attractive force of surface Pt to Sn atoms is weakened to some extent during the synthesis because of the addition of Ag, so that Sn only spreads on limited surface area of the regular ternary nanorods, and thus the aggregation takes place. In addition, Figures 2e and 2f show the characterization results of the modified Pt-Sn-Ag catalysts with either additional electrochemical treatment or extended aspect ratios. For the dealloyed Pt6-Sn-Ag catalyst (D-Pt6-Sn-Ag), it is obvious that EDS signal intensity of Ag is much weaker in panel e3, compared to that of R-Pt6-Sn-Ag catalyst in panel d3, indicating majority of Ag was removed through the dealloying treatment, thus exposing more Pt sites on the catalyst surface. Meanwhile, Sn is relatively stable on the dealloyed catalyst surface, retaining its agglomerated features. On the other hand, as shown in Figure 2f, the extended Pt6-Sn-Ag nanorod catalyst (Ex-Pt6-Sn-Ag) are found to have a larger aspect ratio of 7.2 in average (based on the observation of more than 50 nanorods) due to a lower reducing temperature of Pt precursor, while the diameter is still around 4.0 nm (more details about the 10


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dimensions of Pt-based catalysts can be found in Table S1 in the Supporting Information). It is interesting to find that both Sn and Ag show homogeneous distributions on the extended nanorod surface, as illustrated by the EDS elemental maps in panels f2 and f3. This observation suggests that the Sn distribution is directly related to the surface area of the nanorod: a homogeneous Sn distribution is realized in the ternary system only when the surface area (or aspect ratio) is large enough. Also, the lower-temperature synthesis in Ex-Pt6-Sn-Ag might contribute for the homogeneous distribution of Sn-Ag pairs; the adjacent Sn-Ag pair is slightly stable (see Figure S2 in the Supporting Information) and might appear at lower synthetic temperatures to avoid the Sn aggregation. Moreover, Figure S3 in the Supporting Information compares the X-ray diffraction (XRD) patterns of as-prepared Pt, Pt6-Ag2, Pt6-Sn2, R-Pt6-Sn-Ag, and Ex-Pt6-Sn-Ag catalysts, and their lattice parameters are calculated, as listed in Table S2 in the Supporting Information. All binary and ternary catalysts show the face-centered cubic Pt-based structures with slight expansion in lattice parameters due to the addition of Sn or/and Ag. Table 1.

XPS results showing the surface composition of Pt-based EOR catalysts. Surface species (at %)

Sample Pt

PtOx

Ag

Ag2O

SnO

SnO2

OCS

Pt

75.0

25.0

-

-

-

-

25.0

Pt6-Ag2

32.2

5.8

8.3

53.7

Pt6-Sn2

30.4

7.6

-

-

2.9

59.1

66.7

R- Pt6-Sn-Ag

28.4

7.6

3.3

26.7

1.0

33.0

67.3

D- Pt6-Sn-Ag

45.2

12.8

1.1

5.9

2.5

32.5

51.2

Ex- Pt6-Sn-Ag

27.3

7.7

4.1

26.9

1.4

32.6

67.2

59.5

11



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Furthermore, X-ray photoelectron spectroscopy (XPS, with a probing depth of only 1.5 nm) was utilized for a quantification of surface composition and species of the various Pt-based catalysts. Core level regions of Pt4f, Ag3d, and Sn3d can be clearly observed in the XPS survey spectra displayed in Figure S4 in the Supporting Information. As shown in the specific core level spectra (see Figures S5-S8 in the Supporting Information), it is evident that almost all Sn and Ag are in oxidizing states at every binary and ternary nanorod surface, thus forming abundant OCS, which are considered to promote the ethanol partial oxidation to acetaldehyde and acetic acid through a bifunctional mechanism17 while preventing Pt from dissolution21 during electrochemical reactions. The core level spectra of O1s (see Figure S8 in the Supporting Information) also confirm the adsorbed H2O, lattice O, and OH in the catalysts. Detailed quantitative results, showing the OCS proportions (including SnO, SnO2, Ag2O, and PtOx) of every EOR catalysts, are listed in Table 1. Notably, surface compositions of R-Pt6-Sn-Ag and Ex-Pt6-Sn-Ag catalysts are almost identical, showing an atomic ratio of Pt:Sn:Ag of approximately 1:1:1. According to our STEM observation and XPS result, Figure 3 illustrates surface configurations of our binary and ternary Pt-based catalysts, based on which electrochemical performances can be compared to explore the promotion effects toward EOR as predicted by our DFT calculation.

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Figure 3. Schematics showing surface configurations of various Pt-based EOR catalysts. Pt6-Ag2 catalysts: homogeneous distribution of Ag; Pt6-Sn2 catalysts: homogeneous distribution of Sn; R-Pt6-Sn-Ag catalysts: agglomerated Sn and homogeneously distributed Ag; D-Pt6-Sn-Ag catalysts: agglomerated Sn and homogeneously distributed Ag that leached out to some extent; Ex-Pt6-Sn-Ag catalysts: homogeneously distributed Sn and Ag, forming abundant Sn-Ag pairs. Oxygen atoms are not shown in the schematics. Blue, yellow and green balls represent as Pt, Ag, and Sn atoms, respectively.

For electrochemical testing, representative cyclic voltammetry (CV) scans of all the Pt-based EOR catalysts are presented in Figure 4a, where the information of electrochemical surface area (ECSA) can be revealed (specific ECSA result is provided in Table S3 in the Supporting Information). For the pure Pt catalyst (black solid

line),

the

expected

features

associated

with

not

only

hydrogen

adsorption/desorption peaks at the low potential region (0-0.4 V) but also surface oxide formation/reduction peaks in the higher potential region are clearly identifiable.11 For the binary Pt6-Sn2 catalysts, the CV result (orange dashed line) 13



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shows a decreased intensity of hydrogen adsorption/desorption peak and a strong characteristic peak located at 1.1 V, which is related to the existence of Sn-based OCS layers on the catalyst surface. Similar features are noted for the binary Pt6-Ag2 catalyst (magenta dashed line) with a characteristic Ag peak also located at 1.1 V. Notably, in the ternary R-Pt6-Sn-Ag and Ex-Pt6-Sn-Ag catalysts (see blue and red solid lines), the co-addition of Ag (Ag2O) and Sn (SnO, SnO2) results in even larger characteristic peaks at 1.1 V,9,10,22 while similar ECSA values and CV features confirm the almost identical surface compositions of these two catalysts. Moreover, the CV of D-Pt6-Sn-Ag catalyst is close to the pure Pt since more Pt sites were exposed on the surface due to the removal of majority of OCS layers. Corresponding EOR process was then studied on these Pt-based catalysts by CV in acidic electrolyte at the room temperature, and typical profiles are shown in Figure 4b, where collected currents are normalized by their respective ECSA. The order of onset potential (see the enlarged onset region in Figure 4c) of Ex-Pt6-Sn-Ag < R-Pt6-Sn-Ag < D-Pt6-Sn-Ag < Pt6-Sn2 < Pt6-Ag2 < Pt, corresponding to the stronger Eads of ethanol on the neighboring Sn-Ag pair (Figure 1b), indicates that EOR becomes easier in the ternary system (than the binary one). To be specific, EOR area specific activity at 0.62 V vs. NHE (J0.62) and at maximum current value (Jmax) are plotted in Figures 4d and 4e, respectively. Comparing the binary Pt6-Sn2 catalysts (orange column) and the R-Pt6-Sn-Ag catalysts (blue column), the area specific activity J0.62 and Jmax are remarkably enhanced by 185% and 295%, respectively, by replacing half of the Sn with Ag on the catalyst surface. Notably, the greater enhancement in Jmax (by 295%), reflecting a significant promotion of the additive Ag (in Pt-Sn) at the high potential region, is corresponding to the activation of H2O oxidized to the OH− species15. The surface hydroxyl (OHads) and the Ag2O OCS can facilitate the incomplete ethanol oxidation to acetic acid, demonstrating our 14


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computational prediction regarding the second 2-electron transfer process (see Figure 1a). In contrast, the D-Pt6-Sn-Ag catalyst, containing much less Ag2O OCS on the surface, only shows a slight improvement of EOR activity to the Pt6-Sn2 catalyst, but not comparable to that of the R-Pt6-Sn-Ag one, demonstrating the promotion of electronic effect toward EOR from the other side.

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Figure 4. Electrochemical results of various Pt-based EOR catalysts. (a) Representative CV curves of those Pt-based catalysts recorded in 0.5 M H2SO4. The y-axis is normalized to the geometric surface area of the glassy carbon electrode. (b) CV measurement for EOR. The measured current has been normalized to the ECSA. (c) Enlarged EOR onset region of (b) (d, e) Bar graph plots highlighting specific 16


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activity for those Pt-based catalysts at 0.62 V (vs. NHE) and at maximum current. (f) CA measurements of those Pt-based catalysts, obtained at a potential of 0.62 V (vs. NHE) for a period of 2 hours. Of great significance, the Ex-Pt6-Sn-Ag catalyst is found to possess the best EOR activity among the six catalysts. Comparing with the R-Pt6-Sn-Ag one, the Ex-Pt6-Sn-Ag catalyst, which is characteristic of a more homogeneous surface elemental distribution with more Sn-Ag pairs, exhibits further increments both in J0.62 and Jmax, as illustrated in Figures 4d and 4e. Particularly, a greater enhancement in J0.62 (by 122%) implies that the adsorption and initial oxidation of ethanol to acetaldehyde in EOR is greatly improved at the low potential region by the Ex-Pt6-Sn-Ag catalyst. This result is consistent with our calculation conclusion that more Sn-Ag pairs on the Pt surface is beneficial to initial ethanol adsorption in the first 2-electron oxidation reaction from ethanol to acetaldehyde. Our Ex-Pt6-Sn-Ag catalyst represents a comparable EOR activity to most typical Pt-based ternary catalysts14,23-27, as listed in Table S4 in the Supporting Information. Furthermore, the chronoamperometric (CA) measurement was performed to examine the stability of these Pt-based EOR catalysts. As a result (Figure 4f), the three ternary Pt-Sn-Ag catalysts are found to maintain higher steady state of current densities at a fixed potential of 0.62 V (vs. NHE) over the entire time range of 2 hours, due to an enhanced CO poisoning tolerance from Ag.9 After the aging test, the Ex-Pt6-Sn-Ag catalyst still exhibits a desirable EOR activity, up to 0.159 mA/cm2 at 0.62 V (vs. NHE), which may be attributed to the sufficient and homogeneous OCS layers, preventing the dissolution of Pt.28,29 Taking its better durability and higher activity into account, it is, thus, reasonable to highlight the importance of the structural effect on a homogeneous Pt-Sn-Ag surface structure in EOR catalysts. In summary, we systematically investigate the electronic and structural effects of 17



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the ternary Pt-Sn-Ag catalysts toward EOR via a combination of DFT calculation and experimental evidence. According to the calculated reaction energies and activation barriers, it is found that co-addition of Sn and Ag on Pt essentially facilitates the oxidation of acetaldehyde to acetic acid in EOR. Meanwhile, a Pt-Sn-Ag surface structure with a homogeneous elemental distribution is able to strengthen the adsorption energy of ethanol and promote the first 2-electron transfer process of EOR. Experimentally, various Pt-based nanorod catalysts showing different surface configurations were designed and synthesized for electrochemical testing in order to justify the DFT prediction. Along with detailed structure characterization results, electrochemical performances of these Pt-based catalysts indeed validate our DFT findings of the promotion roles from both electronic and structural effects on EOR: the activity is greatly enhanced in the ternary Pt-Sn-Ag system than the binary Pt-Sn and Pt-Ag catalysts (electronic effect), whereas it can be further improved by creating more Sn-Ag pairs on the Pt surface (structural effect). These results shed light on the mechanistic understanding of the Pt-based EOR ternary catalysts, and may broaden the design strategy for future electrocatalysts used in DEFCs.

ASSOCIATED CONTENT * Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX Detailed computational and experimental methods, tables showing comparisons of lattice parameters, dimensions, ECSAs and activity of Pt-based EOR catalysts, and figures showing detailed DOS analysis, XRD patterns and XPS spectra (PDF). 18


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AUTHOR INFORMATION ∇ These

authors contributed equally to this work.

* Corresponding authors: Jeng-Han Wang, [email protected] Xiaoqing Pan, [email protected] Kuan-Wen Wang, [email protected]

ACKNOWLEDGEMENTS This work was supported by the UC Irvine’s Office of Research, the National Science Foundation (grant number DMR-1506535) and the Ministry of Science and Technology, R.O.C. (MOST 104-2628-E-008-005-MY3 and 106-2113-M-003-003). CPU time at Taiwan’s National Center for High-performance Computing (NCHC) is greatly appreciated. The authors would like to thank National Synchrotron Radiation Research Center (NSRRC) for providing the beam time. Additional support was provided by Irvine Materials Research Institute (IMRI) through the use of TEM facilities.

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Promotion of Ternary Pt-Sn-Ag Catalysts toward Ethanol Oxidation

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