Core-Shell Engineering of Pd-Ag Bimetallic Catalysts for Efficient

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Core-Shell Engineering of Pd-Ag Bimetallic Catalysts for Efficient Hydrogen Production from Formic Acid Decomposition Bu-Seo Choi, Jaeeun Song, Minjin Song, Bon Seung Goo, Young Wook Lee, Yena Kim, Hyunwoo Yang, and Sang Woo Han ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04414 • Publication Date (Web): 14 Dec 2018 Downloaded from http://pubs.acs.org on December 14, 2018

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ACS Catalysis

Core-Shell Engineering of Pd-Ag Bimetallic Catalysts for Efficient Hydrogen Production from Formic Acid Decomposition Bu-Seo Choi,† Jaeeun Song,† Minjin Song,† Bon Seung Goo,† Young Wook Lee, Yena Kim, Hyunwoo Yang, and Sang Woo Han* Center for Nanotectonics, Department of Chemistry and KI for the NanoCentury, KAIST, Daejeon 34141, Korea ABSTRACT: To develop high-performance bimetallic catalysts, fine control over both the ligand and strain effects of secondary elements on the catalytic function of primary elements is crucial. Here we introduce an approach to produce Pd-Ag bimetallic coreshell nanocatalysts with synergistic regulation of the ligand and strain effects of Ag. Through precise core-shell engineering, (PdAg alloy core)@(ultrathin Pd shell) nanocrystals with controlled core compositions and shell thicknesses in addition to a well-defined octahedral morphology could be realized. The prepared octahedral PdAg@Pd core-shell nanocrystals exhibited pronounced catalytic performance toward hydrogen production from formic acid decomposition. The maximum catalytic activity was achieved with PdAg@Pd nanocrystals consisting of PdAg alloy cores with an average Pd/Ag atomic ratio of 3.5:1 and 1.1 atomic layer of Pd shells, which showed a record high turnover frequency of 21500 h-1 at 50 °C. This catalytic function could be attributed to the optimized combination of the electronic promotion and lattice strain effects of Ag on Pd. We envision that the present work can provide a rational guideline for the design of improved catalysts for various important chemical and electrochemical reactions. KEYWORDS: core-shell, Pd-Ag, bimetallic catalysts, hydrogen production, formic acid

INTRODUCTION Formic acid (FA) has attracted immense interest as a promising hydrogen storage/generation material due to its high gravimetric and volumetric hydrogen capacity, ease of handling, nontoxicity, stability at room temperature, and abundant supply from the conversion of biomass and CO2.1-4 Hydrogen stored in FA can readily be disengaged by the catalytic decomposition of FA through the dehydrogenation pathway.4-7 In this context, tremendous efforts have been devoted to the design and synthesis of efficient catalysts for FA dehydrogenation.1,3,8-28 Among a myriad of heterogeneous catalysts, Pd-based nanostructures with controlled morphologies and compositions have been of particular interest due to their remarkable catalytic performance toward FA dehydrogenation.8-20,29 To increase catalytic activity and to suppress the dehydration pathway that leads to the CO poisoning of Pd, as well as to improve tolerance toward poisoning intermediates, various secondary elements have been incorporated into Pd to form core-shell- or alloy-structured Pdbased catalysts.10-12,15,19,20,30,31 In particular, Tedsree et al. reported that Ag@Pd core-shell nanoparticles with 1-2 atomic layers of Pd shells are the most active catalyst for selective H2 production from FA decomposition among various monometallic (M) and bimetallic M@Pd core-shell nanoparticles (M = Ag, Rh, Ru, Au, Pt).10,11 The pronounced catalytic function of Ag@Pd nanoparticles has been attributed primarily to the optimized modification of the surface electronic structure of Pd by a charge transfer from Ag to Pd, the so-called “ligand effect”, which can lead to the optimization of the adsorption strength of a reactive intermediate, i.e., bridging

formate, on the Pd surface, resulting in acceleration of the H2 production rate. For the performance promotion of a catalytic material through the incorporation of a secondary element, the so-called the “strain effect” of the secondary element resulting from lattice mismatch between the secondary element and the catalytic material should also be considered because tensile or compressive strain on the lattice of the catalytic element imposed by the secondary element can also modify the binding affinity of reactive species on the surface of the catalytic material.32-35 In this regard, it is noteworthy that a recent density functional theory (DFT) study regarding FA decomposition on Ag@Pd core-shell catalysts revealed that tensile strain on Pd surfaces imposed by Ag cores due to the 5.3% larger lattice constant of Ag relative to that of Pd tends to increase the formation of CO species, while the ligand effect of Ag plays a significant role in selective H2 production from FA decomposition.34 Furthermore, another theoretical study demonstrated that in-plane compressive strain on Pd surfaces imposed by doped elements enhances H2 production activity by suppressing FA dehydration as a result of the increase in the activation barrier of the CO producing path.35 Accordingly, synergistic regulation of both the ligand and strain effects of secondary elements on the catalytic function of Pd is critical to design high-performance catalysts for FA dehydrogenation. In addition, control over the morphology of Pd-based catalysts is also important to achieve high FA dehydrogenation activity because the type of surface facets of catalysts associated with their morphology has a profound influence on catalytic function for FA decomposition.20,36-38

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Taking account of all these design principles, in the present work, we have developed a Pd-based nanostructured catalyst with controlled morphological and compositional structures for efficient H2 production from FA decomposition. Based on previous findings, we chose core-shell nanostructures with thin Pd shells as a catalyst for FA decomposition. As a core material, PdAg alloy was employed with an expectation that alloying Pd with Ag at the catalyst core could alleviate the tensile strain on Pd shells imposed by Ag, while maintaining the ligand effect of Ag. Furthermore, considering the high FA dehydrogenation activity of the Pd(111) surface,20 the morphology of the coreshell nanostructures was manipulated to have an octahedral shape, which is a representative polyhedral structure enclosed exclusively by {111} facets. The deposition of thin Pd shells on octahedral PdAg alloy nanocrystals (PdAg ONCs) with controlled compositions, which were prepared by a one-pot synthesis method, successfully yielded the desired octahedral PdAg@Pd core-shell NCs (PdAg@Pd ONCs) (Scheme 1). Through systematic studies on the effect of core composition and shell thickness on the catalytic activity of PdAg@Pd ONCs toward FA dehydrogenation in aqueous solution, we found that PdAg@Pd ONCs consisting of PdAg alloy cores with an average Pd/Ag atomic ratio of 3.5:1 and 1.1 atomic layer of Pd shells are an optimal catalyst. Notably, the turnover frequency (TOF) value of the optimal PdAg@Pd ONCs is the highest one yet reported for H2 production from the decomposition of FA in liquid phase with heterogeneous catalysts. This clearly indicates that the proposed strategy is highly promising for the development of efficient catalyst systems for hydrogen generation from FA.

RESULTS AND DISCUSSION Synthesis, characterization, and FA dehydrogenation activity of PdAg alloy cores. PdAg alloy NC cores with a distinct octahedral morphology and controlled compositions, PdAg ONCs, were readily prepared via a one-pot aqueous synthesis method, in which Pd and Ag precursors, namely, H2PdCl4 and AgNO3, were co-reduced by ascorbic acid (AA) in

the presence of cetyltrimethylammonium chloride (CTAC) (see Experimental section for synthesis details). Figure 1a shows a typical scanning electron microscopy (SEM) image of PdAg ONCs produced with an H2PdCl4/AgNO3 molar ratio of 7:3, demonstrating the good shape and size homogeneity of the prepared PdAg ONC cores. A representative transmission electron microscopy (TEM) image of the PdAg ONCs displayed in Figure 1b confirms their octahedral morphology. The average edge length of the prepared PdAg ONCs was estimated to be 34.2 ± 2.5 nm from their SEM images (inset of Figure 1a). The high-resolution TEM (HRTEM) image of a PdAg ONC obtained along the [110] zone axis clearly indicates the single crystalline nature of the prepared PdAg ONCs (Figure 1c). The d-spacing value of 2.00 Å between adjacent lattice fringes in the HRTEM image lies between those of the (200) planes of pure Pd (1.95 Å) and Ag (2.04 Å), verifying the PdAg alloy structure of the prepared PdAg ONCs. The corresponding fast Fourier transform (FFT) pattern further corroborates the single crystallinity of the PdAg ONCs (inset of Figure 1c). A high-angle annular dark field scanning TEM (HAADF-STEM) image and the corresponding energy dispersive X-ray spectroscopy (EDS) elemental mapping images and compositional line-scanning profiles of the PdAg ONCs reveal that Pd and Ag are distributed homogeneously throughout the PdAg ONCs (Figure 1d,e). The Pd/Ag molar ratio of the PdAg ONCs was determined to be 3.5 by inductively-coupled plasmamass spectrometry (ICP-MS). The X-ray diffraction (XRD) pattern of the PdAg ONCs exhibits characteristic diffraction peaks, which are located between those of pure face-centered cubic (fcc) Pd and Ag references (Figure 1f), also indicating the PdAg alloy nature of the PdAg ONCs. From the (111) diffraction peak position of the PdAg ONCs, 39.6°, the Pd/Ag molar ratio of the PdAg ONCs was calculated to be 3.2 based on Vegard’s law,39 which is similar to that obtained by ICP-MS. In addition, the average crystallite size of the PdAg ONCs calculated with their (111) diffraction peak using the Scherrer equation40 was 32.1 nm, which is comparable to the average size of the PdAg ONCs, further demonstrating their single crystallinity.

Scheme 1. PdAg@Pd ONCs can be produced through conformal overgrowth of Pd on PdAg ONCs prepared with a one-pot synthesis ACStheParagon Plus Environment method. PdAg ONC cores can allow the release of tensile strain on Pd shells imposed by Ag without significant attenuation of the electronic promotion effect of Ag.

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ACS Catalysis Notably, the composition of the PdAg ONCs could be controlled by adjusting the relative molar ratio of two metal precursors. PdAg ONCs with different Pd/Ag ratios of 3.4, 4, and 6.9, were yielded with H2PdCl4/AgNO3 ratios of 6:4, 8:2, and 9:1, respectively [Figure S1 in the Supporting Information (SI)]. Interestingly, all the prepared PdAg ONCs had similar average edge lengths despite their different compositions. When the relative amount of Ag precursor was larger than the above ratios in the synthesis, PdAg ONCs were not produced. Instead, nanostructures with ill-defined morphologies were formed. Hereafter, PdAg ONCs with Pd/Ag ratios of 3.4, 3.5, 4, and 6.9 are referred to as Pd3.4Ag, Pd3.5Ag, Pd4Ag, and Pd6.9Ag ONCs, respectively. To evaluate the ligand effect of Ag and to find the optimum composition of PdAg ONC cores for FA dehydrogenation, the H2 production activities of the PdAg ONCs with different compositions as well as pure Pd ONCs were examined in an additive-free aqueous solution of FA (1 M) at 30 °C (Figure 1g). Pd ONCs with an average edge length of 33.4 ± 2.1 nm were prepared using H2PdCl4 as a single metal precursor (Figure S2 in the SI). As shown in Figure 1g, the incorporation of Ag into

SI).30,41 Notably, the H2 production activity of the PdAg ONCs greatly depended on their composition. The highest activity was obtained with the Pd3.5Ag ONCs, of which the H2 production rate was 229.1 mmol g-1 h-1. PdAg ONCs with a Pd/Ag ratio either higher or lower than 3.5 exhibited lower H2 production activity than the Pd3.5Ag ONCs, which may be ascribed to a decrease in the extent of the ligand effect of Ag or an increase in the number of catalytically inactive Ag sites on the surfaces of the PdAg ONCs. Based on these results, we chose the Pd3.5Ag ONCs as optimal core materials to produce PdAg@Pd ONCs. Synthesis and characterization of PdAg@Pd core-shell ONCs. PdAg@Pd core-shell ONCs were synthesized by the deposition of thin Pd shells on Pd3.5Ag ONCs through the reduction of K2PdCl4 with NaBH4 in the presence of Pd3.5Ag ONCs (see Experimental section for synthesis details). The concentration of Pd precursors in the growth solution was kept low to suppress the self-nucleation of Pd. Notably, the thickness of the Pd shells could be tuned by controlling the amount of Pd precursors in the synthesis. PdAg@Pd ONCs with 0.8, 1.1, 2.0, 4.0, and 4.7 average atomic layers of Pd shells were yielded

Figure 1. (a) SEM and (b) TEM images of Pd3.5Ag ONCs. The edge length distribution of Pd3.5Ag ONCs is shown in the inset of a. (c) HRTEM image of a Pd3.5Ag ONC obtained along the [110] zone axis. Inset shows the corresponding FFT pattern. (d) HAADF-STEM image and corresponding EDS elemental mapping images of Pd3.5Ag ONCs (scale bar = 10 nm). (e) EDS compositional line-scanning profiles along the direction marked by an arrow in the HAADF-STEM image of Pd3.5Ag ONCs shown in inset (scale bar = 10 nm). (f) XRD pattern of Pd3.5Ag ONCs. The positions and intensities for pure Pd and Ag references were taken from the JCPDS database (Pd: 65-6174, Ag: 040783). (g) H2 production rates of ONCs with different compositions from FA dehydrogenation at 30 °C, which were normalized to the total mass of catalysts: 10.85, 61.51, 151.8, 229.1, and 206.5 mmol g-1 h-1 for Pd, Pd6.9Ag, Pd4Ag, Pd3.5Ag, and Pd3.4Ag ONCs, respectively.

Pd indeed boosted the H2 production activity of ONCs, which can be attributed to the ligand effect of Ag. In fact, the electronic promotion effect of Ag was evidenced by the red-shift of X-ray photoelectron spectroscopy (XPS) Pd 3d binding energy of PdAg ONCs compared to pure Pd ONCs (Figure S3 in the

with increasing amounts of Pd precursors. The average numbers of Pd layers were estimated by comparison of the ICP-MSdetermined Pd contents of the prepared PdAg@Pd ONCs with those calculated from model PdAg@Pd ONCs with Pd3.5Ag ONC cores and 1 to 5 layers of Pd shells (Table S1 in the SI).

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Hereafter, PdAg@Pd ONCs with 0.8, 1.1, 2.0, 4.0, and 4.7 average atomic layers of Pd shells are referred as [email protected], [email protected], [email protected], [email protected], and [email protected] ONCs, respectively. Figure 2a shows a representative TEM image of the prepared [email protected] ONCs, distinctly demonstrating that the octahedral morphology of NCs was well preserved during the deposition of Pd shells without the formation of any Pd islands. The high-magnification HAADF-STEM image of the near edge region of a [email protected] ONC further verifies the conformal overgrowth of Pd shells (Figure 2b). Since the atomic number difference between Pd and Ag is only 1, the core-shell structure of the [email protected] ONCs was difficult to determine with contrast variation in the HAADF-STEM image. However, the core-shell structure could be evidenced by different lattice spacings between the core and the shell region of the [email protected] ONCs. As denoted in Figure 2b, the lattice spacing of 2.32 Å, which is located between those of the (111) planes of pure Pd (2.24 Å) and Ag (2.36 Å), was measured at the core region of the [email protected] ONC, indicating its PdAg

and compositional line-scanning profiles of the [email protected] ONCs clearly confirm their PdAg@Pd core-shell structure (Figure 2c,d). The Pd shell thickness of the [email protected] ONCs was estimated to be 1.1 nm by EDS compositional linescanning profile analysis (Figure 2d), which corresponds to 4.9 atomic layers of Pd. This value is very similar to that calculated based on the ICP-MS data. In the XRD pattern of the [email protected] ONCs, no significant peak shift was observed in comparison to that of the Pd3.5Ag ONC cores due to the deposition of thin Pd layers on the cores (Figure S4 in the SI). Unlike the [email protected] ONCs, the core-shell structure of the PdAg@Pd ONCs with ultrathin Pd shells, like the [email protected] ONCs, could not be clearly identified by analysis of their STEM images (Figure 2e). To further verify the core-shell structure of the [email protected] ONCs, highmagnification HAADF-STEM-EDS and electron energy loss spectroscopy (EELS) elemental mapping analyses were performed (Figure 2f-m). Apparently, enriched Pd signals were detected at the outermost layer of the [email protected] ONCs, demonstrating their PdAg@Pd core-shell structure. The

Figure 2. (a) TEM image of [email protected] ONCs. (b) High-magnification HAADF-STEM image of the near edge region of a [email protected] ONC. (c) HAADF-STEM image and corresponding EDS elemental mapping images of [email protected] ONCs (scale bar = 10 nm). (d) EDS compositional line-scanning profiles along the direction marked by an arrow in the HAADF-STEM image of [email protected] ONCs shown in inset (scale bar = 20 nm). (e) High-magnification HAADF-STEM image of the near edge region of a [email protected] ONC. (f) HAADF-STEM image of the near edge region of a [email protected] ONC and the corresponding EDS mapping of (g) Pd, (h) Ag, and (i) Pd + Ag. The outermost layer is denoted by blue dotted-lines. (j) HAADF-STEM survey image of the corner region of a [email protected] ONC obtained for EELS analysis. 24 × 24 pixel EELS mapping of (k) Pd, (l) Ag, and (m) Pd + Ag for a square region in j.

alloy structure. In comparison, the lattice spacing at the outermost layer of the [email protected] ONC was 2.29 Å, which is much closer to that of Pd in comparison to the core region, verifying the formation of Pd shells as well as the release of the lattice strain imposed by Ag. Furthermore, an HAADF-STEM image and the corresponding EDS elemental mapping images

observation of some Ag signals at the outermost layer may be ascribed to the outward diffusion of Ag in the Pd3.5Ag ONC core resulting from the irradiation of a focused strong electron beam during the long-term imaging process.35 The presence of the Pd shells of the PdAg@Pd ONCs was further confirmed by cyclic voltammograms (CVs) obtained with the PdAg@Pd ONCs in

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ACS Catalysis 0.1 M KOH (Figure S5a in the SI). Although the Pd3.5Ag ONCs exhibited distinct two current peaks during the backward scan for the reduction of Pd as well as Ag oxides, which were formed during the forward scan, both the [email protected] and [email protected] ONCs showed only a single current peak associated with the reduction of surface Pd oxide during the backward scan, revealing the presence of complete Pd shells on the surfaces of the PdAg@Pd ONCs.30,42,43 Besides, the peak potentials of the reduction of surface Pd oxide for the PdAg@Pd ONCs were more negative than that of the Pd3.5Ag ONCs and the peak potential of the [email protected] ONCs shifted more toward that of the Pd ONCs compared to the [email protected] ONCs (Figure S5b in the SI). These results indicate that the surface of the PdAg@Pd ONCs had a more Pd-like nature as their Pd shell thickness increased due to the lessened electronic promotion effect of Ag.42 FA dehydrogenation with PdAg@Pd ONCs. To determine the optimal Pd shell thickness of the PdAg@Pd ONCs that can enable the synergistic regulation of both the ligand and strain

Pd shells on the surface of PdAg ONCs led to a significant increase in H2 production activity. Notably, among the different PdAg@Pd ONCs, the [email protected] ONCs showed the highest catalytic activity, of which the H2 production rate was 554.8 mmol g-1 h-1 (nPdAg/nFA = 2.00 × 10-4). This demonstrates that the deposition of approximately 1 atomic layer of Pd shells on the PdAg ONC cores can effectively optimize the ligand and strain effects of Ag. The inferior catalytic activity of the [email protected] ONCs in comparison to that of the [email protected] ONCs can be ascribed to their incomplete coverage of the Pd shells, which cannot effectively mitigate the strain effect of Ag. On the other hand, the decreased H2 production activity with an increasing numbers of Pd shell layers can be attributed to the decreased ligand effect of Ag, as evidenced by the CV measurements. The TOF of the [email protected] ONCs, the FA decomposition rate per surface Pd atom, was estimated to be 5143 h-1 (see Experimental section for calculation details). Notably, this value is significantly higher than those obtained with Ag@Pd nanoparticles as well as other Pd-based nanocatalysts (Table S2 in the SI),10-16,18 although it has been achieved at a mild temperature (30 °C) in

Figure 3. (a) H2 production rates of PdAg@Pd ONCs with different Pd shell thicknesses from FA dehydrogenation at 30 °C, which were normalized to the total mass of catalysts: 340.7, 554.8, 375.7, 231.6, and 171.0 mmol g-1 h-1 for [email protected], [email protected], [email protected], [email protected], and [email protected] ONCs, respectively. H2 production rates (bar) and corresponding TOF values (filled square) of [email protected] ONCs obtained (b) at 30 °C with various FA/SF ratios and (c) at various temperatures with an FA/SF ratio of 3:1. Arrhenius plot (ln TOF vs 1/T) is shown in the inset of c. (d) Gas chromatograms of evolved gas from FA dehydrogenation at 50°C catalyzed by [email protected] ONCs with an FA/SF ratio of 3:1 and air + H2 and air + CO reference gases.

effects of Ag on the catalytic function of Pd for FA dehydrogenation, the catalytic activities of the PdAg@Pd ONCs with different Pd shell thicknesses toward H2 production from FA decomposition were investigated under the experimental conditions identical to those employed for the PdAg ONCs. As shown in Figure 3a, the deposition of ultrathin

the absence of any additives, such as sodium formate (HCOONa, SF), which is commonly employed as a catalyst promotor in FA decomposition. The results of the catalysis experiments strongly corroborate our inference that PdAg@Pd core-shell ONCs with thin Pd shells could be a promising catalyst for FA dehydrogenation due to the release of tensile

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strain on Pd imposed by Ag without the attenuation of the ligand effect of Ag. Indeed, the conservation of the electronic promotion effect of Ag in the PdAg@Pd ONCs in comparison to the PdAg ONC cores was supported by very little difference between the XPS Pd 3d binding energies of the [email protected] and the Pd3.5Ag ONCs (Figure S3 in the SI). SF additive has been widely used in catalytic FA dehydrogenation to accelerate the reaction kinetics,13-18,20 because pre-adsorbed formate lowers the reaction barrier of the C-H bond cleavage step.44,45 In this regard, the FA dehydrogenation activity of the [email protected] ONCs in the presence of SF with various relative amounts to FA was investigated to further explore their enhanced catalytic capability. The H2 production rates and corresponding TOF values of the [email protected] ONCs obtained at 30 °C with various FA/SF ratios are summarized in Figure 3b. The kinetic data of the H2 production of the [email protected] ONCs is shown in Figure S6 in the SI. Evidently, the catalytic performance of the [email protected] ONCs was drastically promoted with the addition of SF. The highest H2 production rate was achieved with an FA/SF ratio of 3:1, although the TOF was slightly lower than that obtained with an FA/SF ratio of 1:1. This demonstrates that the effect of SF additive is maximized with this FA/SF ratio under the present catalysis conditions. The [email protected] ONCs retained their morphology and core-shell structure after the reaction without noticeable aggregation (Figures S7 and S8 in the SI). In addition, we further examined the dependence of the catalytic activity of the [email protected] ONCs on the reaction temperature with the optimal FA/SF ratio. As seen in Figure 3c, both the H2 production rate and corresponding TOF of the [email protected] ONCs were greatly enhanced as the temperature increased. Remarkably, the prepared [email protected] ONC catalysts exhibited a record high TOF of 21500 h-1 at 50 °C, which surpasses those of heterogeneous catalysts reported thus far (Table S2 in the SI). This clearly demonstrates that the proposed core-shell engineering approach is very effective to develop high-performance catalysts for H2 generation from FA. The selective production of H2 without CO contamination at this temperature was confirmed by gas chromatography (GC) measurements of evolved gas (Figure 3d and Figure S9 in the SI). The apparent activation energy (Ea) of the [email protected] ONCs for FA dehydrogenation was calculated to be 23.2 kJ mol-1 from the Arrhenius plot (ln TOF vs 1/T) shown in the inset of Figure 3c, which is similar to the lowest value reported in the literature, 22 ± 1 kJ mol-1.12

CONCLUSIONS In the present work, we have designed a Pd-based nanostructured catalyst for efficient hydrogen production from formic acid decomposition, namely, octahedral bimetallic coreshell nanocrystals consisting of PdAg alloy cores with controlled compositions and ultrathin Pd shells with manipulated thicknesses, and developed a facile synthetic route for the realization of the desired nanostructures. The generation of PdAg@Pd core-shell nanocrystals with controlled morphological and compositional structures was achieved in aqueous solution via the one-pot synthesis of octahedral PdAg alloy nanocrystal cores followed by the conformal overgrowth of Pd shells on the cores. Given that the prepared PdAg@Pd nanocrystals can allow the release of tensile strain on Pd imposed by Ag without significant attenuation of the ligand effect of Ag, they showed greatly improved catalytic activity

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for hydrogen production from formic acid decomposition. Systematic studies on the effect of core composition and Pd shell thickness on the catalytic performance of the PdAg@Pd core-shell nanocrystals revealed that a Pd/Ag atomic ratio of 3.5:1 and 1.1 atomic layer of Pd shells are optimal core composition and shell thickness for formic acid dehydrogenation, providing the highest turnover frequency ever reported for heterogeneous formic acid dehydrogenation in liquid phase. These results demonstrate that the proposed strategy can offer a new perspective for designing highperformance catalyst platforms for hydrogen production from formic acid, thus, it provides a promising opportunity to enhance the feasibility of using formic acid as a hydrogen storage/generation material.

EXPERIMENTAL SECTION Chemical and materials. PdCl2 (Aldrich, ≥99.9%), AgNO3 (Sigma-Aldrich, ≥99.0%), ascorbic acid (AA, DaeJung Chemicals & Metals, ≥99.5%), cetyltrimethylammonium chloride (CTAC, Aldrich, 25 wt% in H2O), HCl (Sigma-Aldrich, 37%), K2PdCl4 (Aldrich, 98%), NaBH4 (Aldrich, ≥98.0%), formic acid (FA, Sigma-Aldrich, ≥88.0%), and sodium formate (SF, Sigma-Aldrich, ≥99.0%) were used as received. Other chemicals, unless specified, were reagent grade and highly purified water with a resistivity of 18.3 MΩ·cm was used in the preparation of aqueous solutions. Synthesis of PdAg and Pd ONCs. In a typical synthesis of PdAg ONCs, an aqueous solution of AgNO3 (10 mM) was added to H2PdCl4 (10 mM), which was prepared by the dissolution of PdCl2 in 0.2 M HCl solution, and the resultant mixture was injected into a 10 mL vial containing water (4 mL) and CTAC (3.6 mL, 10 mM), which was pre-heated at 100 °C in an oven for 20 min. The volumes of Pd/Ag precursor solutions for the preparation of the Pd3.4Ag, Pd3.5Ag, Pd4Ag, and Pd6.9Ag ONCs were 1.2 mL/0.8 mL, 1.4 mL/0.6 mL, 1.6 mL/0.4 mL, and 1.8 mL/0.2 mL, respectively. Then, AA (0.3 mL, 30 mM) was injected into the reaction mixture and the mixture was heated at 100 °C for 2 h, and then cooled down to room temperature. The product was purified with water by centrifugation (9000 rpm for 10 min), and dispersed in 8 mL of water. Pd ONCs were synthesized with the identical procedure except the amount of AA (0.4 mL, 100 mM) using H2PdCl4 (2 mL, 10 mM) as a single metal precursor. Synthesis of PdAg@Pd ONCs. To prepare PdAg@Pd ONCs, 4 mL of Pd3.5Ag ONC solution was added to CTAC (100 mM, 5 mL) and then heated at 45 °C in a water bath for 10 s with vigorous stirring. Then, K2PdCl4 (1 or 5 mM) and equivalent amount of NaBH4 were injected into the reaction mixture. After 10 s, the reaction mixture was taken out of the water bath and stirred under ambient conditions for 5 min. The product was purified 2 times with water by centrifugation (9500 rpm for 5 min and 8500 rpm for 5 min). The concentrations and volumes of Pd precursor solution were 1 mM/25 μL, 1 mM/50 μL, 5 mM/25 μL, 5 mM/50 μL, and 5 mM/100 μL for the preparation of the [email protected], [email protected], [email protected], [email protected], and [email protected] ONCs, respectively. Characterizations. SEM images were obtained with a Phillips Model XL30 FEG field-emission scanning electron microscope. TEM measurements were conducted using a JEOL JEM-2010 transmission electron microscope operated at 200

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ACS Catalysis kV. HRTEM images were obtained with a Phillips Tecnai G2 F30 Super-Twin transmission electron microscope operated at 300 kV. TEM measurements were conducted after placing a drop of diluted NC solution on a carbon-coated Cu grid (300 mesh). HAADF-STEM images and corresponding EDS and EELS data were obtained using a double Cs-corrected transmission electron microscope (FEI Titan cubed G2 60-300). The electron acceleration voltages employed in EDS and EELS analyses were 80 and 120 kV, respectively. To minimize possible beam damage to the sample, a low beam current of 40 pA was used for imaging. Energy dispersion for the spectrometer of EELS was 0.05 eV. The samples for EDS and EELS analyses were placed on a lacey carbon grid (300 mesh) and kept under vacuum at 60 °C before measurements with a Rigaku ultradry to minimize contamination. ICP-MS measurements were conducted using an Agilent ICP-MS 7700S. XRD patterns were obtained using a Bruker AXS D8 DISCOVER diffractometer with Cu Kα radiation (0.1542nm). XPS data were obtained with a Thermo VG Scientific Sigma Probe spectrometer using Al Kα X-ray (1486.6 eV) as a light source, and calibrated with C 1s peak at 285.0 eV. Electrochemical measurements. CVs were obtained in a three-electrode cell using a CH Instruments Model 760D potentiostat. The drop-casting film of NCs (metal loading = 1.5 µg) on a glassy carbon electrode (diameter = 3 mm), Pt wire, and Ag/AgCl (in 3 M NaCl) served as the working, counter, and reference electrodes, respectively. Before CV measurements, 5 µg of Nafion solution (0.05 wt%) was dropped onto a working electrode and dried in a dry-keeper. Dried working electrode was washed thoroughly with ethanol and water, and then electrochemically cleaned by 30 potential cycles between -0.85 and 0.5 V vs Ag/AgCl at a scan rate of 50 mV s-1 in 0.1 M KOH in order to remove residual stabilizing agents on the surfaces of NCs. All CV measurements were conducted at room temperature and electrolyte solutions were purged with highpurity N2 gas for 1 h before use. FA dehydrogenation. Prior to FA dehydrogenation reaction, FA (5 mL, 1 M) or FA + SF mixture (5 mL, 1 M) with a predetermined FA/SF ratio was placed in a 50 mL round-bottom flask under ambient atmosphere. After the prepared catalysts, which were dispersed in 5 mL of water, were injected into the reaction flask, the neck of the reaction flask was immediately sealed with a septum and then transferred to a water bath. The reaction mixture was stirred at 400 rpm, and evolved gas was collected and analyzed by GC (Agilent 7890B GC System) after the 5 and 60 min of reaction. For the precise detection of CO, evolved gas was further analyzed using a gas chromatograph equipped with a flame ionization detector with a methanizer (YL6100 GC, YL Instrument), of which the detection limit for CO is below 10 ppm. The amount of catalysts in the reaction mixture was determined by ICP-MS. TOF of a PdAg@Pd ONC catalyst was calculated based on the amount of evolved H2 after the 5 min of FA dehydrogenation reaction with the catalyst and the total number of the surface Pd atoms of the catalyst, which was determined using the surface density of Pd atoms, 1.0 × 1019 m-2, and the electrochemically determined specific surface area of the catalyst (Figure S5 in the SI).

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions †These

ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program (2015R1A3A2033469) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT. We thank Prof. Hyunjoo Lee at KAIST for gas chromatography measurements.

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ASSOCIATED CONTENT Supporting Information. Additional data (Figures S1-9 and Tables S1 and S2). This material is available free of charge via the Internet at http://pubs.acs.org.

authors contributed equally to this work.

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