Insights into Compositional and Structural Effects of Bimetallic Hollow

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Insights into Compositional and Structural Effects of Bimetallic Hollow Mesoporous Nanospheres toward Ethanol Oxidation Electrolcatalysis Hao Lv, Lizhi Sun, Aaron Lopes, Dongdong Xu, and Ben Liu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b02218 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 28, 2019

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The Journal of Physical Chemistry Letters

Insights into Compositional and Structural Effects of Bimetallic Hollow Mesoporous Nanospheres toward Ethanol Oxidation Electrolcatalysis Hao Lv,† Lizhi Sun,† Aaron Lopes,‡ Dongdong Xu,† and Ben Liu†,* †Jiangsu

Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Center of Biomedical Functional

Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, China ‡Department

of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139,

United States Abstract: A one-pot soft-templating method is reported to fabricate nanosized bimetallic PdAg hollow mesoporous nanospheres (HMSs) for electrocatalytic ethanol oxidation reaction (EOR). The synthesis relies on the “dual-template” surfactant of dioctadecyldimethylammonium chloride that drives in-situ growth of mesoporous frameworks on the surface of vesicles into the HMSs with radially opened mesochannels. The synthetic protocol is extendable to engineer elemental compositions and hierarchical nanostructures of PdAg nanoalloys. This system thus provides a direct yet solid platform to understand catalytic add-in synergies toward electrochemical EOR. By evaluating compositional and structural features separately, bimetallic Pd65Ag35 HMSs display the highest EOR activity with a mass activity of 4.61 A mgPd-1. Mechanism studies indicate synergistically electronic and bifunctional effects as well as structural advantages of Pd65Ag35 HMSs kinetically optimize the removal of poisoning carbonaceous intermediates and accelerate the diffusion processes (the rate-determining step), thus promoting the EOR performance accordingly. TOC Graphics:

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In the past two decades, noble metal (NM)-based nanocatalysts have received increasing attention in the electrocatalysis.1-8 A notable example is palladium (Pd), the most important electrocatalyst element toward electrochemical ethanol oxidation reaction (EOR) that is the anode reaction in direct ethanol fuel cell (DEFC).9,10 Unfortunately, metallic Pd generally suffers extensive poison by carbonaceous intermediates and results in the decrease of catalytic activity quickly.11,12 Meanwhile, relatively high price and notoriously low abundance of precise Pd also hamper practical applications. One direct solution is alloying secondary inexpensive metals to form bimetallic PdM alloys.13-15 This not only brings compositional synergies to boost catalytic activity, but also decreases the usage of precious Pd. For example, bimetallic PdM alloys consisting of a more oxophilic metal (Ag, Au, Co, et al.) exhibit interesting electronic and bifunctional effects that accelerate the removal of reaction intermediates and improved electrochemical EOR activity significantly.16-21 Catalytic performances strongly relate to their compositional ratios of PdM alloys.22,23 Therefore, rationally engineering the composition ratios of bimetallic PdM alloys is important to better understand compositional synergies and maximize catalytic performance. Apart from the compositional engineering, precise control over the nanostructures represents another important route to enhance catalytic activity of NM-based nanoalloys.7,24-30 Among various nanostructures, hierarchically hollow and porous nanomaterials are of great importance, because they can effectively enlarge the catalytic sites and the utilization efficiency of precious NMs, and also facilitate the electron and mass transfer during the catalysis.31-37 Theoretically, when downsizing hollow mesoporous nanomaterials into sub-100 nm nanospheres with opened nanochannels, catalytic performances can be further improved. Although various synthetic strategies have been developed, soft-templating method is currently one of the most reliable synthetic approach to fabricate hollow mesoporous nanospheres (HMSs), including silica, carbon and other compounds.38-43 Precisely engineering the synthesis of the HMSs, practically for bimetallic alloys with two different elemental compositions and uniform crystalline phases, remains technologically challenging.32,44 This is likely because the varied nucleation kinetics of two metal precursors disrupts the assembled micelle structures of the templates. This limitation greatly restricts the fundamental understanding of compositional and structural features of bimetallic HMSs toward various chemical transformations. Recently, our group developed a facile one-pot surfactant-templating synthesis of multimetallic HMSs by using amphiphilic dioctadecyldimethylammonium chloride (DODAC) as the template.45 Considering the strong chemical hydrophobicity of DODAC and structural adjustability of assembled micelles simultaneously,46,47 this synthetic protocol is expected to rationally engineer elemental compositions and hierarchical nanostructures of bimetallic PdM HMSs for fully understanding their catalytic add-in synergies toward the electrocatalysis. In the current contribution, we extend the synthetic capacity of DODAC-directing soft-templating method for fabricating bimetallic PdAg HMSs with radially opened mesoporous nanochannels. We chose Ag as the secondary alloyed metal, since Ag is a more oxophilic metal (for Pd) that exhibited great potential for optimizing the EOR performance of Pd.21,23 Bimetallic PdAg HMSs with molar ratios from 100/0 to 35/65 are precisely adjusted, without destroying the HMS structure under optimum synthetic conditions. Structural engineering of PdAg nanoalloys, ACS Paragon Plus Environment

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including HMSs, mesoporous nanospheres (MSs), hollow nanospheres (HSs) and nanoparticles (NPs), is also carried out by changing initial micelle structures of DODAC. By separately comparing the EOR activities of PdAg HMSs with different compositional ratios and PdAg alloys with different nanostructures, the insights of compositional and structural add-in synergies are revealed by studying their surface electronic states, catalytic kinetics and CO antipoisoning abilities. Such add-in synergies of bimetallic HMSs is also appreciable to other electrocatalytic reactions. In a typical synthesis of bimetallic PdAg HMSs, amphiphilic surfactant of DODAC was first dissolved in deionized H2O to obtain a homogeneous solution. Then, metal precursors of H2PdCl4 and AgNO3 under predominant molar ratios were carefully injected into above solution, driving the self-assembly into “dual” templates of vesicle and cylinder micelles.45,48 After starting the reduction with ascorbic acid (AA), some smaller nuclei of PdAg alloys were first grown in situ on the surface of vesicle templates. Then, further nucleation and growth of PdAg mesoporous framework with cylinder micelles along initial nuclei on vesicle templates resulted in the formation of hierarchical HMSs [see Figure S1 in Supporting Information (SI)]. As-resultant products were finally cleaned with acetic acid, ethanol and H2O, to remove surfactant template and expose catalytically active surface of the HMSs (Figure 1a). Amphiphilic feature of DODAC that chemically composes of two hydrophobic C18 tails covalently bonded to a hydrophilic quaternary ammonium head is chemically critical to drive the self-assembly of “dual-template” micelles and nanoconfine the formation of the HMSs. When the similar surfactants with shorter hydrophobic tails were used to template the synthesis, the HMSs with broader size distributions and/or disordered mesoporous frameworks were obtained (Figure S2). Furthermore, amphiphilic surfactants with only one hydrophobic tail templated the synthesis of mesoporous nanospheres (MSs) without any interior hollow cavity (Figure S3). The ineffectiveness of such surfactants further indicated critical structural features of DODAC in “dual-template”-directed synthesis of bimetallic PdAg HMSs. Hierarchically porous structures and bimetallic elemental compositions of bimetallic PdAg HMSs were thoroughly studied by various characterization techniques. Bimetallic PdAg HMSs with a compositional ratio of 65/35 were first investigated as a typical example, since it exhibited the best EOR activity (see below). The compositional ratio was determined by inductively coupled plasma mass spectrometry (ICP-MS). A typical low-magnification transmission electron microscopy (TEM) image in Figure 1b displays uniform and high-purity hollow nanospheres with a distinct mesoporous shell. The average diameter of PdAg HMSs is 77 nm with an average interior hollow cavity size of 37 nm and shell thickness of 20 nm. The high-magnification TEM image distinguishes hierarchically hollow and mesoporous nanostructures (Figure 1c). The mesoporous nanochannels of PdAg HMSs are cylindrically and radially opened. The average mesopore diameter is 2.7 nm, and corresponding wall thickness is 3.9 nm (Figure 1d). The clearer observations were also carried out with high-angle annular dark-field scanning TEM (HAADF-STEM). High contrasts of HAADF-STEM images clearly reveal the interior hollow cavity and radially opened mesoporous shell (Figures 1g,h and S4), further indicating successful formation of hierarchically hollow and mesoporous nanospheres. ACS Paragon Plus Environment


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Both interior hollow cavity and radially opened mesoporous shell of the HMSs would kinetically facilitate mass transfers of the reactants, (poisoning) intermediates and products during the (electro)catalysis.45,49 Wide-angle X-ray diffraction (XRD) pattern of bimetallic PdAg HMSs exhibits five typical diffraction peaks in the 2θ range of 35-90o, that can be exactly attributed to (111), (200), (220), (311) and (222) planes of a crystalline-pure face-centered cubic (fcc) structure (Figure S5). All the diffraction peaks locate in between pure Pd and Ag crystals (Pd: 46-1043 and Ag: 04-0783), indicating bimetallic alloy compositions.21,23 The results are in good agreement with the ones obtained from selected-area electron diffraction (SAED, Figure 1f). The high-resolution TEM image depicts clear lattice fringes with a constant d-spacing of 0.227 nm (Figure 1e). Considering slightly smaller lattice spacing of 0.225 nm for pure Pd,50 the results further indicate the formation of bimetallic PdAg alloys. To reveal bimetallic compositions more clearly, STEM energy dispersive X-ray spectroscopy (EDS) was further used to map the elemental compositions and distributions of the HMSs. As shown in Figure 1i, homogeneous mixing of Pd and Ag thoroughly the whole HMS definitely suggests bimetallic and crystalline-pure alloy features. The atomic ratio is 62 : 38 by EDS analysis, almost same to the ratio obtained from ICP-MS (65 : 35). In addition to STEM EDS line scan analysis that reveals the hollow nanospheres with a well-alloyed PdAg shell (Figure 1j), our results suggest successful fabrication of bimetallic PdAg HMSs with hierarchically hollow and mesoporous structures. Elemental composition ratios of bimetallic alloys are strongly responsible to their (electro)catalytic performance, based on compositionally synergetic effects. We thus extended this surfactant-templating synthetic protocol to engineer elemental compositions of bimetallic PdAg HMSs by changing initial feed ratios of H2PdCl4 and AgNO3. As depicted in Figure 2a, the products with Ag atomic percentage ranging from 0 to 64 at. % are nearly spherical with distinct hollow and mesoporous nanostructures. With increasing Ag content, the average size of HMSs slightly decreases to 63 nm; the shell thickness also decreases from 29 nm to 16 nm (Figure 2d). In contrast, the interior hollow cavity first enlarges in the lower Ag content (< 28 at. %), and then slightly decreases with further increasing Ag contents. Such differences of the HMSs with different Ag contents are possibly originated from different reduction and crystallization rates of Pd and Ag.21,23 In this synthetic solution, the reduction potential of Ag+/Ag is 0.36 V negative than that of PdCl42-/Pd (Figure S6), indicating Ag+ is favorable to be reduced into metallic Ag than PdCl42(and/or Pd2+) under the same conditions. In the absence of Ag+ or in the presence of a lower amount of Ag+, the crystallization and nucleation rate of Pd (or PdAg) is slower, causing the HMSs with thermodynamically more stable nanostructure (for example, smaller hollow cavity, thicker shell thickness and larger HMS size). In contrast, in the higher Ag content, a faster reduction rate would enlarge initial nuclei. This would enlarge the amounts and decrease the sizes of the HMSs. However, mesoporous shell disappears when Ag content is higher than 77 at. % (Figure S7). Wide-angle XRD further indicates compositional control of bimetallic PdAg alloy HMSs (Figure 2b). All the HMSs exhibit five typical diffraction peaks, indicating an fcc crystalline phase of PdAg HMSs with different Ag contents. No other set of XRD impurity peaks further confirm that alloying Ag cannot change intrinsic crystalline structure of Pd HMSs (no phase-separated products). When zoom-in of (111) plane of XRD patterns, it is obvious to note that the ACS Paragon Plus Environment

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increase of Ag content in bimetallic PdAg HMSs results in a peak shift from 40.1o to 38.9o (Figure 2c). It implies that alloying more Ag with a larger atomic radius crystallographically enlarges interatomic distance of Pd gradually.22,51 The results, again, confirm our synthetic method is capable to precisely engineer compositional ratios of bimetallic PdAg HMSs. The successful synthesis of bimetallic PdAg HMSs is on the basis of in situ crystallization and growth of PdAg alloys with “dual” templates of vesicle and cylinder micelles assembled by amphiphilic DODAC. Therefore, we expect that the nanostructures of PdAg alloys can be rationally engineered by controlling initial assembled structures of surfactant template. The pH of the reaction solution was first studied to tailor the micelle structures of DODAC by changing the surfactant packing parameters,46,50,52 that could be used to engineer resultant PdAg nanostructures (Figure 3a). In a very low pH of 2.3, DODAC only self-assemble into spherical vesicle, resulting in the formation of hollow PdAg nanospheres with a nearly solid shell. This is possibly because repulsive force between positively charged quaternary ammonium head of DODAC is increased in a lower pH, thus destroying the rod-like cylinder micelles. With the increase of pH to 3.6, “dual” templates of DODAC appear simultaneously. However, due to inappropriate reduction kinetics, mesoporous shell of resultant PdAg HMSs is still disordered partially. Perfect HMSs can only be created at an optimal pH value. In contrast, in a very high pH of > 11.2, spherical vesicles of DODAC are destroyed, remaining cylinder micelles only. This results in the formation of the MSs without interior hollow cavity, as synthesized by the surfactant with one hydrophobic tails. Other synthetic parameters were also investigated to engineer the nanostructures of bimetallic PdAg alloys. Reaction temperature is sensitive to the reduction rates of metal precursors and assembled micelle structures of DODAC. As shown in Figure 3b, with increasing the reaction temperature from 0 oC to 35 oC, the mesoporous shell thickness of PdAg HMSs correspondingly increases. In contrast, the interior hollow cavity gradually disappears under a high temperature of 50 oC, likely because the vesicle templates of DODAC are destroyed. Both the “dual-template” micelles of DODAC can even be disrupted in a higher reaction temperature (75 oC and 90 oC), resulting in the formation of nanoparticle and nanowire mixtures (Figure S8). Besides, the interior hollow cavity and mesoporous shell of the PdAg HMSs can be also tailored by changing metal precursor concentrations (Figure S9). These results definitely correlate the capacity of our synthetic approach to precisely engineer elemental compositions and hierarchically porous nanostructures of bimetallic PdAg HMSs. Successful synthesis of bimetallic PdAg HMSs provides a direct possibility to solidly understand compositional and structural features toward electrochemical EOR. To differentiate catalytic add-in synergies, compositional and structural effects of bimetallic PdAg HMSs are studied separately. Compositional effect of bimetallic PdAg HMSs (samples shown in Figure 2) is first investigated, and commercial Pd/C (3-7 nm Pd NPs, Figure S10a,b) is also compared as a benchmark catalyst. Before electrocatalytic EOR studies, the samples are first tested in N2-saturated 1.0 M KOH, to reveal the electrochemically active surface areas (ECSAs). All the samples exhibit a cathode signal in the potential range between -0.5 V and -0.1 V, indicating that oxidized Pd2+ (for example, PdO) was electrochemically ACS Paragon Plus Environment


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reduced to metallic Pd (Figure 4a).21,23,45 The peak intensities and corresponding peak areas are strongly related to the elemental compositions of bimetallic PdAg HMSs, corresponding to distinctly different ECSAs. Specifically, the ECSAs of monometallic Pd, bimetallic Pd84Ag16, Pd72Ag28, Pd65Ag35, Pd51Ag49, and Pd35Ag65 are 22.6, 23.9, 34.1, 45.7, 51.2 and 45.8 m2 gPd-1, respectively (Figure S11). As expected, alloying Ag enlarges the ECSAs of bimetallic PdAg HMSs. In contrast, the ECSA of the benchmark Pd/C is 23.4 m2 gPd-1. Electrocatalytic EOR performance of bimetallic PdAg HMSs with different Ag contents is evaluated in 1.0 M KOH and 1.0 M ethanol solution. Cyclic voltammetry (CV) curves are totally changed after the addition of ethanol (Figure 4b), in comparison to the ones collected in 1.0 M KOH. Two new signals appear, where the one in the forward scan is ascribed to electrooxidation of ethanol into the intermediates and the other in the backward sweep is associated with further oxidation and/or removal of above intermediates.21,22 It indicates bimetallic PdAg HMSs are electrocatalytically active toward the EOR. The peak intensities are strongly related to Ag contents of PdAg HMSs, confirming remarkably different EOR activities. For quantitative comparisons, mass activities of bimetallic PdAg HMSs are normalized to the Pd mass based on their peak current densities (Figure 4c). Overall, our catalysts show significantly higher activity, when compared with the activities of commercial Pd/C and previously reported Pd-based

EOR nanocatalysts (Table S1). Among all the catalysts, Pd65Ag35 HMSs display the highest mass activity of 4.61 A mgPd1,

which is 1.80, 1.41, 1.19, 1.06, and 1.21 times higher than the activity of Pd (2.57 A mgPd-1), Pd86Ag14 (3.27 A mgPd-

1), Pd

72Ag28 (3.86 A mgPd

-1), Pd

51Ag49

(4.33 A mgPd-1) and Pd35Ag65 (3.80 A mgPd-1), respectively. Alloying Ag in bimetallic

PdAg HMSs obviously results in a “volcano”-type mass activity, indicating strong compositional synergies toward electrochemical EOR. In contrast, commercial Pd/C achieves the lowest mass activity of 1.49 A mgPd-1. Similarly, the lowest onset potential is achieved by Pd65Ag35 HMSs, implying the easier ethanol oxidation ability and faster catalytic kinetics (Figure 4d). To fully reveal the origin of improved mass activities in electrocatalytic EOR, surface electronic features of bimetallic PdAg HMSs with different compositional ratios are carefully studied by X-ray photoelectron spectra (XPS). Figure 5a displays the high-resolution XPS Pd 3d spectra of bimetallic PdAg HMSs with increasing Pd contents (from bottom to up). Two typical Pd 3d peaks at the binding energies of 341.0 eV (3d3/2) and 335.7 eV (3d5/2) are observed for metallic Pd HMSs, comparable to the values of standard Pd. After alloying Ag into PdAg HMSs, the peak positions of both Pd 3d negatively shift toward the lower binding energies. Specifically, the binding energy of Pd 3d5/2 gradually decreases from 335.7 eV to 334.9 eV with decreasing Pd content from 100 at. % to 65 at. %, and then keeps this constant value (334.9 eV) when continuously decreasing Pd content to 35 at. % (Figure 5b). These results indicate the downshifted Fermi levels of Pd that would decrease the adsorption interaction of O-containing species on Pd surface.22,51,53 Correspondingly, decreasing Pd contents enlarges the Ag amounts in bimetallic PdAg HMSs, causing the positive shifts in binding energy of XPS Ag 3d (Figure 5c). As summarized in Figure 5d, the binding energy of Ag 3d5/2 lineally increases from 367.56 eV to 367.85 eV, suggesting the upshift of Fermi levels and the enhancement of adsorption interaction of O-containing species on Ag. Considering opposite surface electronic effects of PdAg alloys, ACS Paragon Plus Environment

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rationally engineering the PdAg HMSs with a suitable compositional ratio would optimize the electronic states and the adsorption features of catalytic intermediates, and thus boost catalytic performance. In alkaline solution, electrochemical EOR proceeded on bimetallic PdM nanocatalysts generally follows a fourstep reaction mechanism:9,17,45

The rate-determining step of the EOR is the removal (or further oxidation) of carbonaceous intermediates on Pd [for example, Pd-(CH3CO)ads and further decomposed Pd-(CO)ads] (Step 3), since strong adsorption affinity of the carbonaceous intermediates poisons and inactivates the catalysts. In comparison to Pd, Ag is more oxophilic that favors the formation of Ag-OHads. Thus, alloying more oxophilic Ag compositionally facilitates direct oxidation reaction of Ag-OHads with Pd-(CH3CO)ads and/or Pd-(CO)ads, and kinetically accelerates the removal of (CH3CO)ads and/or (CO)ads on catalytically active Pd (Step 3). Meanwhile, the increase of Ag contents in bimetallic PdAg alloys also weakens the adsorption affinity of (CH3CO)ads and/or (CO)ads on Pd. These results are in good agreement with the ones observed from XPS analysis. Kinetically accelerating the removal of carbonaceous intermediates on Pd thus remarkably improves the EOR performace of bimetallic PdAg HMSs. However, the higher Ag contents slightly decrease the activity of PdAg alloys, because excess Ag would cover exposed active sites of the Pd in PdAg HMSs. We thus deduce that optimum compositional ratio of bimetallic PdAg HMSs with the highest EOR activity should balance the removal of poisoning intermediates and the exposure of catalytically active Pd sites. Compositional synergies of bimetallic PdAg HMSs toward the removal of poisoning carbonaceous intermediates are directly mimicked by CO anti-poisoning tests. To perform the experiments, CO that imitates the poisoning intermediates during the EOR was totally adsorbed on the catalysts at a fixed potential of 0.15 V, and then swept CVs in 1.0 M KOH under the potential range between -0.9 and 0.2 V (see SI for experimental details). As depicted in Figure 5e, adsorbed CO on the catalysts is almost eliminated by two consecutive CV scans. In comparison to monometallic Pd HMSs, alloying Ag into PdAg alloys decreases the onset potential from -0.22 V to -0.39 V, indicating PdAg HMSs have an easier CO removal ability. Similarly, larger active areas of CO oxidation/removal peaks are achieved by bimetallic PdAg HMSs. Among all the PdAg HMSs with different compositional ratios, Pd65Ag35 HMSs exhibit the largest active areas. The result is strongly related to the EOR activity, further indicating that the removal ability of poisoning CO is the rate-determining step of bimetallic PdAg HMS. Structural effects of bimetallic PdAg HMSs toward electrocatalytic EOR were also evaluated. To highlight structurally hollow and mesoporous features of PdAg HMSs, the results were compared with PdAg MSs and PdAg NPs having the same compositional ratio (65:35). Bimetallic PdAg MSs were synthesized with H2O/ethanol as the cosolvents (Figure S10c, d),54 while PdAg NPs were obtained using cetrimonium chloride as the template and sodium ACS Paragon Plus Environment


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borohydrite (NaBH4) as reducing agent (Figure S10e, f). In comparison to the counterpart structures of MSs and NPs, hierarchically hollow and mesoporous nanostructures of PdAg HMSs expose more catalytic sites with a higher ECSA of 45.7 m2 gPd-1 (Figure S12). Meanwhile, structural merits of PdAg HMSs also facilitates mass transfers of the reactants, intermediates and products, thus resulting in improved EOR activity. CV curves and corresponding mass activities of bimetallic PdAg HMSs, PdAg MSs, PdAg NPs and Pd/C are depicted in Figure 6a and b. Obviously, PdAg HMSs achieve the highest EOR mass activity of 4.61 A mgPd-1. In contrast, the mass activities of PdAg MSs, PdAg NPs and Pd/C are 2.89, 2.29, and 1.49 m2 gPd-1, respectively. Meanwhile, PdAg HMSs also exhibit the lowest onset potential (Figure 6c). To deeply understand the origin of the higher EOR activity for PdAg HMSs, electrochemical kinetics was systemically investigated. Kinetic transport processes of electrocatalytic EOR for PdAg alloys with different structures (HMSs, MSs, and NPs) were studied under the scan rates from 30 to 200 mV s-1. As shown in Figure S13, mass activities of all the samples are enhanced with the increase of scan rates. When plotting mass activity against square root of scan rate (v1/2), a linear relationship is found (Figure 6d), implying a diffusion-controlled process for all the samples toward electrocatalytic EOR.11,23 Among them, PdAg HMSs show the highest slope value, indicating the enhanced catalytic kinetics. The results are strongly related to structurally hollow and mesoporous features of the HMSs that kinetically promote the electron/mass transfers. Meanwhile, PdAg HMSs also exhibit the improved CO anti-poisoning performance, comparable to their counterpart catalysts of PdAg MSs, PdAg NPs and commercial Pd/C (Figure 6e). The results discussed above correlate that compositional and structural add-in synergies of bimetallic PdAg HMSs kinetically accelerate the removal of carbonaceous intermediates, and thus boost the EOR activity accordingly. In addition to HMS structure that alleviates the ripening and dissolution processes,45 such add-in synergies would also enhance electrocatalytic stability of bimetallic PdAg HMSs. Two methods were used to evaluate the stability of PdAg HMSs toward electrochemical EOR. First, cycling stability was assessed by continuously sweeping CVs in the potential range between -0.9 and 0.2 V (Figure 7a). Gradually decreased peak intensities in both forward and backward scans indicate the loss of mass activity with increasing the cycling numbers. Mass activities of PdAg HMSs during the cycling sweeps are further normalized (Figure 7b). Initial mass activity of 4.61 A mgPd-1 gradually decreases to 2.75 A mgPd-1 after 2000 cycles. In contrast, commercial Pd/C only retains a mass activity of 0.43 A mgPd-1 after sweeping CVs for 2000 cycles. Second, current-time (i-t) chronoamperometry measurements of bimetallic PdAg HMSs and Pd/C were carried out at a fixed potential of -0.2 V (Figure 7c). In comparison to commercial Pd/C, a remarkably higher mass activity of 0.79 A mgPd-1 for PdAg HMSs is retained after 5000 s, further indicating a good stability. TEM images of the catalysts after stability tests were also characterized and compared (Figure 7d). Hollow mesoporous structure of PdAg HMSs is almost intact after stability tests, although mesoporous framework is partially destroyed. In contrast, commercial Pd/C displays a remarkable size increase to 5-14 nm. ACS Paragon Plus Environment

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To summarize, we have reported a facile soft-templating method for the formation of uniform and nanosized bimetallic PdAg alloy HMSs with radially opened mesoporous nanochannels. Thanks to the strong nanoconfinement effect of DODAC and structural adjustability of assembled micelles, this synthetic protocol enables precise controllability of bimetallic elemental compositions and hierarchically hollow/mesoporous structures by optimizing the synthetic parameters (ratios of metal precursors, pH, temperature, et al.). Benefiting from the synthesis capacities, compositional and structural features of bimetallic PdAg HMSs are systematically evaluated toward electrocatalytic EOR. Compared to its counterpart catalysts, optimized Pd65Ag35 HMSs remarkably enhance the EOR activity with a highest mass activity of 4.61 A mgPd-1. The analysis on surface electronic states, catalytic kinetics and CO anti-poisoning experiments indicate that catalytic add-in synergies of PdAg HMSs kinetically accelerate the removal of poisoning carbonaceous intermediates (the rate-determining step) and thus boost the EOR activity in alkaline media. The use of bimetallic PdAg HMSs as a model catalyst shed solidly light on compositional and structural effects in the electrocatalytic EOR. Considering the diversities of element chemistries and hierarchical structures, our study could provide a general guideline for rational design of highly efficient synergic electrocatalysts and fundamental understanding of (electro)catalytic behaviors toward various reactions. Supporting Information Synthesis and characterization details of bimetallic PdAg HMSs and their counterparts, control experiments, and electrocatalytic tests. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author Email: [email protected] Notes The authors declare no competing financial interest. Acknowledgements The authors thank the financial supports from Jiangsu Specially Appointed Professor Plan, National Natural Science Foundation of China (No. 21501095), Natural Science Foundation of Jiangsu Province (No. BK20180723), the research fund from Priority Academic Program Development of Jiangsu Higher Education Institutions, National and Local Joint Engineering Research Center of Biomedical Functional Materials. References (1) Xia, Z.; Guo, S. Strain Engineering of Metal-Based Nanomaterials for Energy Electrocatalysis. Chem. Soc. Rev. 2019, 48, 3265-3278. (2) Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486, 43-51. ACS Paragon Plus Environment


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Figures

Figure 1. Synthesis and structural characterizations of bimetallic PdAg HMSs. (a) Schematic illustration of a “dualtemplate”-directed synthesis of PdAg HMSs. (b-d) TEM and (e) high-resolution TEM images, (f) SAED pattern, (g, h) HAADFSTEM images, (i) STEM EDS elemental mappings and (j) corresponding line scan analysis of bimetallic PdAg HMSs.



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Figure 2. Compositional control of bimetallic PdAg HMSs. (a) TEM images, (b) wide-angle XRD patterns, and (c) zoom-in wide-angle XRD patterns in the range of 37.5o and 42.5o of bimetallic PdAg HMSs with different Ag contents. (d) Summarized relationships between sizes (hollow cavity, shell thickness, and entire HMS) and Ag contents of PdAg HMSs. The compositional ratios of bimetallic PdAg HMSs were obtained by ICP-MS.


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Figure 3. Structural engineering of bimetallic PdAg HMSs. TEM images of bimetallic PdAg HMSs synthesized under (a) different pH and (b) reaction temperatures.



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Figure 4. Electrocatalytic EOR activities of bimetallic PdAg HMSs with different Ag contents. (a) CVs collected in N2saturated 1.0 M KOH solution. (b) CVs, (c) summarized mass activities, and (d) zoom-in LSV curves of bimetallic PdAg HMSs with different Ag contents obtained in N2-saturated 1.0 M KOH and 1.0 M ethanol.


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Figure 5. XPS characterizations and CO anti-poisoning studies of bimetallic PdAg HMSs with different Ag contents. (a) High-resolution XPS spectra and (b) summarized binding energies of Pd 3d, (c) high-resolution XPS spectra and (d) summarized binding energies of Ag 3d. (e) CO-stripping voltammograms of PdAg HMSs with different Ag contents.



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Figure 6. Electrocatalytic EOR activities of bimetallic Pd65Ag35 alloys with different nanostructures. (a) CVs, (b) summarized mass activities, (c) zoom-in LSV curves, and (d) catalytic kinetics of bimetallic PdAg HMSs, PdAg MSs, PdAg NPs and commercial Pd/C obtained in N2-saturated 1.0 M KOH and 1.0 M ethanol. (e) CO-stripping voltammograms of PdAg alloys with different nanostructures. The molar ratios of Pg/Ag for all the PdAg samples are fixed to be 65/35.


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Figure 7. Electrocatalytic EOR stability of bimetallic Pd65Ag35 HMSs. (a) CVs and (b) summarized mass activities of Pd65Ag35 HMSs and commercial Pd/C during CV scans for 2000 cycles. (c) EOR i-t chronoamperometric curves of Pd65Ag35 HMSs and Pd/C. (d) TEM images of Pd65Ag35 HMSs and Pd/C after the stability test.


Insights into Compositional and Structural Effects of Bimetallic Hollow

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