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Pt Submonolayers on Au Nanoparticles: Coverage-Dependent Atomic Structures and Electrocatalytic Stability on Methanol Oxidation Lingyi Peng, Lin Gan, Yinping Wei, Hao Yang, Jia Li, Hongda Du, and Feiyu Kang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10445 • Publication Date (Web): 22 Nov 2016 Downloaded from http://pubs.acs.org on November 25, 2016

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The Journal of Physical Chemistry

Pt Submonolayers on Au Nanoparticles: CoverageDependent Atomic Structures and Electrocatalytic Stability on Methanol Oxidation Lingyi Peng, Lin Gan*, Yinping Wei, Hao Yang, Jia Li, Hongda Du, Feiyu Kang Division of Energy and Environment, Graduate School at Shenzhen, Tsinghua University, Shenzhen, 518055, China *Corresponding author: [email protected]

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ABSTRACT Deposition of platinum monolayers on Au substrate (denoted as Au@PtML) has been shown an efficient catalyst design strategy for the electrocatalysis of alcohol oxidation due to presumed 100% utilization of Pt atoms and substrate-enhanced catalytic activities. However, the atomic structure and stability of Pt (sub)monolayers on realistic nanoparticulate Au surface have still remained elusive. Here, we reveal coverage-dependent atomic structures and electrocatalytic stabilities of Pt submonolayers (sML) on Au nanoparticles on methanol oxidation reaction (MOR) by using high-resolution transmission electron microscopy combined with energy dispersive X-ray spectrum imaging and electrochemical techniques. At lower Pt coverages, the PtsML more resembled monoatomic-thick layers, whereas higher Pt coverages above 0.5 ML resulted in 3D subnanometer Pt nanoclusters leading to lower Pt utilization efficiencies. Moreover, the Au@PtsML catalysts with Pt coverage below 0.5 ML showed higher structural and electrocatalytic stability during MOR electrocatalysis. As a result, increasing the Pt coverage beyond 0.5 ML brought in no obvious gain in the overall catalytic performance. Our results suggest that the [email protected] catalyst appears to be a more reasonable MOR catalyst than previously reported [email protected] ML catalyst, providing more rational catalyst design for achieving high Pt utilization efficiency and high catalytic performance.


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1. Introduction Direct methanol fuel cells (DFMCs) are concerned as a promising portable energy conversion device due to high energy density and feasible transport/storage of the liquid fuel methanol.1 Exploring highly efficient electrocatalysts for the sluggish anodic methanol oxidation reaction (MOR) is one of the central tasks for DMFC development. Although Pt has been catalyst of choice for MOR among various pure metals, their scarcity and high price has greatly limited their large-scale application. To reduce the Pt usage and/or to increase the catalytic activity, various core-shell catalyst design strategies have been explored. One elegant approach is to construct a Pt monolayer (ML) on a second metal substrate (e.g. Pd, Ir and Au) by surface limited redox replacement of a underpotentially deposited (UPD) Cu ML2-10 or a self-terminating atomic layer electrodeposition technique.11-12 These PtML catalysts hold a presumable 100% utilization of Pt atoms and likely substrate-enhanced surface reactivity, leading to high Pt massnormalized activity.3-11 For MOR electrocatalysis, a 3-7 fold increase on the Pt-surface normalized activity has been reported on PtML deposited on Au (111) surfaces compared to pure Pt(111) surface, which has been attributed to substrate-induced tensile strain effect7 or ensemble effects11. Despite their promising catalytic activities, the realistic atomic structure and catalytic stability of the PtML particularly on practical nanoparticulate catalysts have still remained elusive. Earlier studies on Au single crystal surfaces claimed that the as-deposited Pt submonolayers (sML) prepared by galvanic replacement of CuML showed an ideal monoatomic thickness;2, 13 this argument was then extended to nanoparticulate PtML electrocatalysts. However, several recent studies argued that PtML on Au(111) surfaces was far from a perfect monoatomic thick monolayer but in reality a layer of 3 dimensional (3D) Pt cluster agglomerates;14-16 instead, monoatomic-



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thick PtML was only obtained and stabilized by surface adsorbents such as CO and hydrogen.14, 16 On practical Au nanoparticulate catalyst, microscopic images also indicated that the as-deposited PtML adopted a 3D nanocluster morphology.15 On the other hand, recent studies also revealed coverage-dependent morphology and catalytic activity of PtsML on single crystal Au(111) surface.13,

17-18

However, atomic insights into the influence of Pt coverage on the structure,

activity and stability of PtsML on nanoparticulate Au catalysts are still very limited. These structural uncertainties have greatly limited comprehensive understanding on the structureproperty relation of practical nanoscale Pt(s)ML catalysts and their further structural fine-tunings. In this contribution, we reveal coverage-dependent atomic structures and stabilities of PtsML on Au nanoparticles (denoted as Au@PtsML NPs) during MOR electrocatalysis by using highresolution transmission electron microscopy (TEM), scanning TEM (STEM) combined with energy dispersive X-ray (EDX) spectrum imaging, and electrochemical analysis. We show that at lower Pt coverages the PtsML on Au NPs more resembles monoatomic-thick layer, yet higher coverages above 0.5 ML led to the growth of 3D nanoclusters and thereby no further enhancement on the overall catalytic performance. Moreover, the PtsML with coverage below 0.5 ML exhibited higher structural and electrocatalytic stability during MOR electrocatalysis. Our results provide a clearer picture on the atomic structures of the Au-supported PtsML catalysts and propose more rational catalyst design for achieving high Pt utilization efficiency and high costperformance.


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2. Experimental 2.1 Synthesis and electrochemical measurement of Au and Au@PtsML nanocatalysts Gold nanoparticles supported on a high surface area XC-72 carbon support (denoted as Au/C, with Au content of 20 wt.%) were prepared by citrate-assisted reduction method. A mixed solution of 10 mg Vulcan XC carbon support, 10 mL isopropanol, 50 mL ultrapure water was sonicated for 15 min and then magnetically stirred in an ice bath. After that, 0.3 mL HAuCl4 (43.5 mM) and 1 mL aqueous solution containing 0.25 mg NaBH4 and 20 mg sodium citrate were successively added. After reacting for 15 min, the mixture was filtered and washed by ultrapure water and ethanol. The obtained Au/C catalyst was then dried overnight. To prepared Au@PtsML catalysts, sacrificial Au@CusML NPs were firstly prepared by UPD of CusML with different coverages on the Au/C catalyst. A thin film Au/C catalyst electrode was prepared on a 5 mm-in-diameter glassy carbon (0.196 cm2, containing 3.2 µg Au) rotating disk electrode (RDE, Pine Instrument) and then underwent 20 potential cycles between 0.06 V and 1.5 V/RHE at 100mV/s in 50 mM H2SO4 to ensure a clean NP surface. A certain amount of CuSO4·5H2O was subsequently added to the electrolyte, resulting in a mixed solution of 50mM H2SO4 and 50 mM CuSO4. The Au@CusML NPs at different Cu coverages were then prepared by linear sweep voltammetry (LSV) from 0.9V to different potentials at 5 mV/s, holding at this potential for 10min, and then emersed from the electrolyte under potential control. Secondly, the Au@CusML electrodes were immersed in 50 mM H2SO4 containing 50 mM K2PtCl4 solution for galvanic replacement, resulting in the Au@PtsML catalysts. The whole UPD and galvanic replacement processes were conducted under inert N2 atmosphere with the RDE rotated at 1000 rpm to prevent mass transport limitations. The obtained Au@PtsML catalyst was then rinsed by



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ultrapure water for several times and then used for subsequent electrochemical measurement and characterizations. The ECSA of Au@PtsML catalysts were measured by cyclic voltammetry between 0.06 and 1.0 V at 100 mV/s in N2-saturated 0.1M HClO4 solution. The MOR electrocatalytic activities were measured by firstly 20 cycles between 0.06 and 1.0 V at 100 mV/s and then 3 cycles at 10 mV/s in N2-saturated 0.5 M methanol and 0.1M HClO4. All electrochemical experiments were conducted in a three-compartment glass cell connected to a Biologic SP-200 potentiostat. A Ptmesh and a Hg/Hg2SO4 electrode (in saturated K2SO4) were used as the counter electrode and the reference electrode, respectively. All potentials reported in this paper were normalized with respect to reversible hydrogen electrode (RHE). 2.2 Microscopy and spectroscopy TEM, high resolution TEM (HRTEM) and STEM studies were conducted on a FEI Tecnai F30 field-emission transmission electron microscope operated at 300kV, which was equipped with a high angle annular dark field (HAADF) detector and an Oxford large window (80 mm2) silicon-drift detector (SDD) for EDX mapping. The Au@CusML TEM samples were prepared on amorphous carbon-coated nickel grids to avoid Cu signal from conventional Cu TEM grid. Elemental mapping of Cu, Au and Pt (using Cu Kα, Au Lα and Pt Lα line, respectively) was acquired in STEM mode (spot size 7) with auto drift-correction. 3. Results and Discussion Gold NPs were prepared by citrate assisted reduction method, showing particle sizes mainly between 2-10 nm with average size of 5 nm (Fig. S1). To construct Au@PtsML NPs with different Pt coverages, we firstly prepared sacrificial CusML on Au NPs (Au@CusML) with different Cu coverages by using the UPD method. Figure 1a shows the cyclic voltammogram


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(CV) of the Au NPs in 50 mM H2SO4 containing 50 mM CuSO4, showing characteristic Cu UPD features consisting two redox peaks at around 0.4V and 0.6V/RHE. These two UPD peaks have been previously observed on both Au (111) surface and polycrystalline Au surface and ascribed to two different deposition stages involving firstly co-adsorption of Cu and sulfate anion followed by a complete Cu adlayer deposition.19-20 From the charge associated with Cu UPD, the electrochemical surface area of the Au NPs (ECSAAu) was estimated to be around 1.2 cm2 , assuming a charge constant of 420 µC/cm2.21 By linear scanning voltammetry from 0.9 V to different potentials (as indicated in Fig. 1a), we obtained Au@CusML at different Cu coverages (s=0.25, 0.5, 0.75 and 1.0 ML, respectively). While the mono-atomic thickness of the UPD Cu has been generally accepted and verified on Au (111) single crystal surfaces,2 it has been seldom observed experimentally on nanoparticle surfaces. Insights into the structure of the CuML are crucial for understanding the subsequent galvanic replacement process. Price et al. has studied the Cu UPD process on 2 nm Au nanoparticle catalyst using X-ray adsorption spectroscopy, suggesting an incomplete Cu shell consisting of unevenly-distributed Cu nanoclusters.20 We characterized the [email protected] NPs as a showcase by HRTEM combined with STEM-EDX elemental mapping (Fig. 1 b and c, and Fig. S2 a). The low magnification TEM image of the [email protected] NPs in Fig. S2 a shows similarly smooth NP surface as the initial Au NPs (Fig. S1), indicating an ideal Cu ML structure without forming 3D nanoclusters. HRTEM of the NP surface (Fig. 1 b) shows atomic thin layer species with a larger interplanar spacing of 3.5 Å than that of the underlying Au {111} planes, which is highly suspected to be oxidized CuML due to inevitable exposure in air before TEM experiments. This monolayer-like Cu is further corroborated by STEM-EDX element mapping (Fig. 1 c),



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showing a uniformly distributed Cu skin on Au NPs. Our results suggest that the UPD CuML on Au NPs is most likely close to mono-atomic without forming 3D Cu nanoclusters.

Figure 1. (a) Cu UPD on Au/C nanocatalysts in N2-saturated 0.05 M H2SO4 + 0.05 M CuSO4 solution, the inset structural models illustrate the preparation of Au@CusML with controlled Cu coverages. (b) HRTEM images of Au@CuML and (c) STEM-HAADF image and EDX elemental mapping of Au@CuML, suggesting a near monoatomic-thick monolayer structure of the CuML.

The Au@CusML NPs were then subjected to galvanic replacement by K2PtCl4 in 50 mM H2SO4 solution under an N2 atmosphere to form the Au@PtsML catalysts. Following earlier reports by Brankovic et al., using the Pt2+ precursor and H2SO4 solution ensure a one-by-one galvanic replacement of Cu by Pt, whereas galvanic replacement in HClO4 solution resulted in the oxidation of Cu to Cu+ leading to 2:1 stoichiometry between sacrificial Cu and Pt.15, 17 To


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investigate the effect of nominal Pt coverage on the atomic structures of PtsML, the Au@PtsML catalyst were further characterized by HRTEM and STEM-EDX mapping. Both lowmagnification TEM (Fig. S2 b and c) and HRTEM images (Fig. 2 a and b) show that, at low Pt coverages (0.25 and 0.5 ML), the as-prepared [email protected] and [email protected] NPs manifest near spherical shape and similarly smooth NP surface as the original Au NPs, suggesting a high dispersion of Pt atoms deposited on the Au NP surface. Elemental mapping using STEM-EDX of the [email protected] NPs (Fig. 3c) clearly shows atomically thin Pt overlayer, indicating a likely monoatomic thickness. In contrast, the [email protected] catalyst demonstrates much higher surface roughness as judged from the low magnified TEM image (Fig. S2d) and HRTEM images (Fig. 2 d and e). Subnanometer-sized islands (Fig. 2d) and sharp corners (Fig. 2e) epitaxially grown on Au NPs can be easily found, leading to much more irregularly shaped NPs. Smaller interplanar distance of 2.26 Å in the sharp corner than that at the center part (2.34 Å) indicates the formation of 3D Pt nanoclusters on the Au NPs. STEM-EDX mapping results (Fig 2f) confirm that the thickness of the Pt1.0ML overlayers is quite inhomogeneous (as shown by the while arrows): part of the NP surface is covered by a thicker Pt shell, whereas the rest covered by thinner Pt shell or even free of Pt. The formed islands or sharp corners on the Au NPs are proved to be composed of deposited Pt (as indicated by the white circles). These results unequivocally show that, contrary to the Au@PtsML NPs with Pt coverage lower than 0.5 ML, the [email protected] NPs adopt a threedimensional Pt island/corner structure.



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Figure 2. Morphologies, atomic structures and compositional distributions of [email protected] (a-c) and [email protected] (d-f) catalysts. (a-b) HRTEM images of [email protected] NPs, showing a smooth NP surface; (c) STEM-EDX mapping of [email protected] NPs indicating an atomically thin Pt shell; (d-e) HRTEM images of [email protected] NPs showing much higher surface roughness and the emergent subnanometer Pt islands and corners; (f) STEM-EDX mapping showing an inhomogeneous and thicker Pt shell as indicated by the white arrows; white circles illustrate that the emergent sharp corners are composed of deposited Pt atoms. 10 ACS Paragon Plus Environment

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The revealed coverage-dependent atomic structures of the Au@PtsML catalysts indicates that the PtsML adopted a Volmer-Weber growth model (i.e., island growth) as developed in thin film growth.22 This means that monoatomic thick Pt layer can be only prepared at lower coverages. Likely, the higher surface energy of Pt than that of Au, as suggested by previous studies,14, 23 may contribute to the observed island growth mode through surface rearrangement. However, in a reversed system Zhang et al. showed that even the as-prepared AuML on Pt surface also adopted 3D cluster morphology after undergoing several potential sweeps to 1.2 V/RHE. 24 We therefore argue that the surface energy argument alone cannot account for the island growth mode. Instead, the weak Au-Pt interaction and hence high Au/Pt interface energy, inferred from the fact that Au and Pt are immiscible to form alloys in bulk despite their small lattice misfit (3.7%), may dominate the island growth mode of both PtML on Au and AuML on Pt. In contrast, the strong interaction and thereby low interface energy between Au and Cu, judged from the fact that Au-Cu are miscible in a broad composition range, enabled an ideal Cu ML structure on Au NPs despite even higher surface energy of Cu than that of Au as well as their large lattice misfit (11%).25 Due to the revealed island growth mode of Pt on Au NPs, the realistic utilization efficiency of Pt atoms (hereafter denoted as ηPt) at higher Pt coverages would no longer be 100% as previously presumed. To quantitatively measure ηPt of different Au@PtsML catalysts, we measured the electrochemical surface area of Pt (ECSAPt) using the characteristic hydrogen adsorption and desorption feature during cyclic voltammetry (CV) in 0.1 M HClO4 solution. The value of ηPt for different Au@PtsML catalysts can then be estimated as: ηPt = 100%·ECSAPt/(s·ECSAAu),



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where ECSAAu = 1.2 cm2, as evaluated from the Cu UPD; s is the nominal Pt coverage (s = 0.25, 0.5, 0.75 and 1.0 ML), and s·ECSAAu stands for the ideal surface area of the PtsML if it was an ideal monoatomic thick (sub)monolayer. A higher value of ηPt indicates that a higher portion of Pt atoms is exposed on the surface and the obtained PtsML is more similar to a monoatomic thick layer. Figure 3a and b present the initial 10 CV curves of the [email protected] and [email protected] catalyst (for [email protected] and [email protected], see Fig. S3) and the evolution of ECSAPt and ηPt (Fig.3 c and d), providing important implications on the structures and stabilities of the PtsML. Firstly, all the Au@PtsML catalysts show lower-than-100% ηPt; the larger the nominal Pt coverage s, the lower the ηPt. For instance, in the very beginning 1st cycle, the [email protected] catalyst shows much higher ηPt (55%) than the [email protected] (35%), meaning that the Pt0.5ML is closer to monoatomic thick layer. This is in good consistency with the HRTEM and STEM-EDX mapping results in Fig. 2. Secondly, all the Au@PtsML catalysts exhibit gradual decrease in the ECSAPt within the initial 10 potential cycles, suggesting an instability and surface rearrangement of the surface Pt atoms. This is also consistent with a recent study on PtML deposited on Au (111) surface by Ahn et al.11 Despite the reconstruction of PtsML during potential cycling, the conclusion that PtsML at larger Pt nominal coverages shows much lower ηPt still holds after the potential cycling. Figure 3e presents the steady CV curves of different Au@PtsML catalysts after 10 potential cycles, and a comparison of their ECSAPt and ηPt is shown in Fig. 3f. Clearly, the ECSAPt increases as the Pt nominal coverage increases from 0.25 ML to 0.5 ML yet almost achieves a plateau at Pt coverages above 0.5 ML, leading to a drastically reduced ηPt.


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Figure 3. (a, b) Initial CVs of [email protected] (a) and [email protected] (b) catalysts between 0.05 and 1.0 V/RHE in N2-saturated 0.1 M HClO4 solution at a scanning rate of 100 mV/s; (c, d) Evolution of the calculated ECSAPt (c) and ηPt (d) during the potential cycling; (e) Steady CV curves of different Au@PtsML catalysts after ten potential cycles in N2-saturated 0.1 M HClO4 solution at a scanning rate of 100 mV/s; (f) Comparison of the ECSAPt and ηPt.

To corroborate the likely surface rearrangement, the Au@PtsML catalysts after potential cycling were further characterized by TEM (Fig. 4 a-d). While the [email protected] catalyst shows no 13 ACS Paragon Plus Environment


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obvious surface roughening (Fig. 4a), surface faceting can be detected in some [email protected] catalyst particles, suggesting the rearrangement of Pt atoms into subnamometer islands (Fig. 4b). In comparison, the surface reconstruction became much more pronounced in the [email protected] catalyst, which shows the formation of ca. 2 nm Pt nanoparticles on the Au NPs (Fig. 4c and d). Our finding clearly supports the tendency of surface rearrangement of PtsML atoms into larger sized 3D nanoclusters/particles particularly at higher Pt coverages above 0.5ML. This also raises an important question regarding the structural stability of monolayer catalysts, for instance, those prepared by CO-stabilized monoatomic-thick Pt monolayer catalysts (considering CO will be inevitably removed during potential cycling). 14, 16

Figure 4. Morphologies of (a) [email protected] ML, (b) [email protected] and (c, d) [email protected] catalyst after 20 cycles between 0.06 -1.0V/RHE in 0.1 M HClO4 aqueous solution. White arrows in (c, d) indicate the formation of Pt nanoparticles at larger sizes on the Au NPs.


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In accompany with the surface rearrangement, the Au@PtsML catalysts also showed distinctly different voltammetric stability during the MOR electrocatalysis (Fig. 5 a-d). For [email protected] catalyst (Fig. 5a), it is interesting that the MOR activity gradually increases at the first 20 potential cycles despite the fact that its ECSAPt gradually decreases. This can be rationalized by the elimination of isolated Pt atoms most likely existing in the as-deposited [email protected] catalyst due to surface rearrangement during potential cycling. Following earlier reports, the efficient MOR electrocatalysis requires three-fold Pt assembly that are necessary for the dissociative adsorption of methanol, while isolated Pt atoms are inactive.26 Therefore, surface rearrangement of the isolated Pt atoms into larger Pt assembles during potential cycling may result in the enhanced catalytic activity. In contrast, at higher Pt coverages ([email protected] and [email protected] catalysts), there were few isolated Pt atoms and the significantly decreased ECSAPt resulted in the activity loss during the potential cycling (Fig. 5 c and d). Overall, the [email protected] catalyst appears to be most stable under the electrocatalytic MOR conditions (Fig. 5b). We emphasize that the voltammetric stability discussed here is a short-term stage stability and limited to the applied potential cycling range (between 0.05 and 1.0 V/RHE), reflecting their early-stage electrocatalytic behaviors. The effect of Pt coverage on the long-term stability associated with Pt electrochemical dissolution in Au@PtsML catalysts is not considered here and needs further investigations. 27



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Figure 5. (a-d) Initial 20 CVs of MOR on [email protected], [email protected], [email protected] and [email protected] catalyst, respectively, recorded in N2-saturated 0.1 M HClO4 and 0.5 M CH3OH solution at a scanning rate of 100 mV/s; insets illustrate the likely surface rearrangement of the surface Pt atoms in different Au@PtsML catalysts; (e) Comparison of the steady MOR polarization curves after 20 cycles recorded at a scanning rate of 10 mV/s. (f) Comparison of oxidative MOR currents at 0.8 V/RHE. Error bars are derived as the standard deviation from at least three independent measurements.


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Figure 5e presents the stable polarization curves of MOR after 20 potential cycling on different Au@PtsML catalysts (for clarify, only the positive scans are shown), from which the MOR oxidative currents at 0.8 V/RHE are compared in Fig. 5f. As the nominal Pt coverage increases from 0.25 ML to 0.5 ML, the MOR activity increases greatly; however, further increasing to 0.75 and 1.0 ML does not bring in significant enhancement in the overall MOR activity. This is fully consistent with the trend in the ECSAPt shown in Fig. 3f, indicating that the MOR performance was mainly controlled by the ECSAPt. Our results therefore suggest that the [email protected] catalyst appears to the most appropriate catalyst compared to previously reported [email protected] catalyst considering their comparable MOR catalytic performance yet higher stability and significantly decreased Pt usage in the former. We also found that the Pt surface normalized specific activity, however, shows no obvious difference for different Au@PtsML catalysts (Fig. S4), all of which demonstrate ca. 2-fold enhancement compared to commercial Pt/C catalyst (Johnson-Matthey, 20% Pt supported on Vulcan carbon black). This is at first look surprising but can be rationalized by a compromise between the intrinsic compressive strain within the PtsML due to finite size effect13 and the Ausubstrate-induced tensile strain effect caused by the lattice misfit. These two opposite effects were believed to control the overall strain-controlled catalytic activities. With increasing Pt coverage, the Pt nanocluster size increases and the intrinsic compressive strain arising from the finite size effect would decreases;13 however, it is on the other hand that substrate-induced tensile strain would also greatly relax due to the emergent 3D Pt island morphology. As a result, a similarly overall lattice strain within the PtsML surfaces can account for the similar specific MOR activities for different PtsML catalysts.



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The two-fold increase in the specific activity of the Au@PtsML NP catalysts compared to pure Pt catalyst is also lower than previously reported 7-fold enhancement on Au(111)@PtML single crystal surface over pure Pt(111) surface,7 yet can be supported by recent works.11 Ahn et al. reported an initial ca. 3-fold activity enhancement on PtML on Au (111) textured surface, which then exhibited continuous activity loss during potential cycling, fully consistent with our finding on the [email protected] catalyst. In addition, since nanoparticulate catalysts are generally enclosed by mixed (111) and (100) surfaces, their size and shape may affect the realistic activity enhancement in a complex way.

4. Conclusion We reveal a pronounced effect of Pt nominal coverage on the atomic structures and stability of Au@PtsML catalysts and as a consequence an unexpected nonlinear relationship between Pt coverage and electrocatalytic performance on MOR. While the sacrificial UPD CusML adopted a nearly ideal monoatomic thick layer, subsequent displacement of CusML led to coveragedependent atomic structures of the resulted PtsML: at lower Pt nominal coverages (s0.5 ML) resulted in the formation of 3D subnanometer Pt islands or corners on the Au NPs, leading to decreased Pt utilization efficiencies. Moreover, the Au@PtsML catalysts exhibited strong coverage-dependent catalytic stability; higher Pt coverages above 0.5 ML led to more significant structural instability and activity loss during MOR electrocatalysis. As a consequence, both the ECSAPt and the MOR performance almost reached a plateau as the Pt nominal coverage reached 0.5 ML. These results suggest that the [email protected] ML catalyst appears to be a more reasonable MOR catalyst than the previously reported [email protected]

ML

catalyst. Our


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results provide important insights into the atomic structures and stabilities of Pt monolayer and submonolayer catalysts and may aid further rational Pt monolayer catalyst design for achieving high Pt utilization efficiency and high catalytic performance. Supporting Information Supplementary figures (Fig. S1 to S4). Acknowledgment This work was supported by National Science Foundation of China (grant number 21573123 and 51622103) and Basic Research Programs of Shenzhen City, China (JCYJ20140902110354242). References 1.

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Replacement of Metal Adlayers on Electrode Surfaces. Surf. Sci. 2001, 474, L173-L179. 3.

Adzic, R. R.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Shao, M.; Wang, J. X.; Nilekar,

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