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Fingerprint Feature of Atomic Intermixing in Supported AuPd Nanocatalysts Probed by X-ray Absorption Fine Structure Chang-Hai Liu, Ning Chen, Jun Li, Xu Gao, Tsun-Kong Sham, and Sui-Dong Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10470 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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The Journal of Physical Chemistry

Fingerprint Feature of Atomic Intermixing in Supported AuPd Nanocatalysts Probed by X-ray Absorption Fine Structure Changhai Liua,b, Ning Chen*,c, Jun Lid, Xu Gaoa, Tsun-Kong Sham*,c,d, and Sui-Dong Wang*,a a

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory

for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, P. R. China; b

School of Materials Science & Engineering, Jiangsu Collaborative Innovation

Center of Photovoltaic Science and Engineering, Changzhou University, Changzhou, Jiangsu 213164, P. R. China; c

Canadian Light Source, University of Saskatchewan, Saskatoon, Saskatchewan S7N

0X4, Canada d

Department of Chemistry, Western University, London, Ontario N6A 5B7, Canada

*Corresponding email: Ning Chen: [email protected]; Tsun-Kong Sham: [email protected]; Sui-Dong Wang: [email protected]

ABSTRACT We report on a systematic study of AuPd bimetallic nanoparticles (NPs) prepared from two-step sputtering deposition in the suspension of graphene and ionic liquid using X-ray absorption fine structure (XAFS) spectroscopy. As-synthesized AuPd bimetallic catalysts possess well-enhanced activity and stability compared with monometallic (Au-only or Pd-only) catalysts, suggesting the synergistic effect upon alloying. A structural model with the introduction of Pd to Au cluster is provided to study the formation of bimetallic NPs and to offer detailed insights into the local

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structure information (interaction between Au and Pd). A novel and significant fingerprint is proposed to depict the intermixing structure of AuPd bimetallic NPs. The present XAFS study reveals a fingerprint feature in k-space, which can be an evaluation of the extent of the Au-Pd intermixing on the atomic scale, corresponding to the catalytic activity of bimetallic nanoparticles. The systematic XAFS-based methodology, demonstrated here on the AuPd bimetallic system, can easily be extended to investigate the local structure to develop many other types of bimetallic system for interesting applications. INTRODUCTION Due to the unique catalytic, electronic and magnetic properties, supported bimetallic nanoparticles (NPs) have attracted extensive attentions and been widely used in various important fields for many years, especially in some industrially important catalytic reactions such as hydrogenations,1,2 dehydrogenations,3,4 selective oxidation,5,6 and reactions in fuel cells.7-9 For many catalytic systems, bimetallic catalysts have been demonstrated to possess synergetic catalytic capability which is often superior to their monometallic counterparts. The superior performance of bimetallic nanomaterials is associated with the material morphology (size, shape, etc), chemical composition, diversity of their microscopic structure (such as random alloy, intermixed and core-shell structure or phase segregation), and electronic structure as well (charge transfer, orbital hybridization, etc).1,2,4,7 Among the various bimetallic systems, AuPd bimetallic alloy have been widely investigated, because of their illustriously catalytic performance. For example, Hutchings et al. have reported that carbon or TiO2 supported AuPd NPs are more active for the oxidation of alcohols and alkyl alcohols to aldehydes.10 Toshima et al. presented the ‘crown-jewel’ concept of apex gold atoms on palladium nanocluster, incorporating high catalytic activity for glucose oxidation depending on the sites of Au atoms in the crown-jewel nanoclusters.11 So it is eagerly desirable for many applications to seek for rational routes to develop heterogeneous supported nanocatalysts. For industrial applications, conventional synthetic procedures of supported NPs catalysts typically involve the deposition of metal precursors onto supports with high surface area, followed by ACS Paragon Plus 2 Environment

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various thermal activation steps.1,8,12-15 Inevitably, the size distribution of NPs and the degree of dispersion on the supports are influenced by many factors, such as pH value, concentration of the precursors solution, type of supports, and especially the calcination temperature and procedure.1-3,6,10 Consequently, bimetallic materials prepared by such methods with less controllability for the size and structures of bimetallic NPs are often complicated, leading to significant non-uniformity (chemistry, structure, or particle size/shape). In order to address the above issues and to gain a further understanding for the structure-property relations of bimetallic catalysts, there are two crucial sides which should be solved. On the one hand, it is the rational synthesis of AuPd NPs with controlled compositional ratio and high uniformity. On the other hand, due to the limitations of conventional characterization methods, advanced techniques for probing the local structure in AuPd NPs appears particularly important. Two key requirements are indispensable to elucidate the synergistic merits of AuPd NPs as high-performance nanocatalysts. In allusion to the two requirements, we firstly developed a simple and effective approach utilizing ionic liquid-assisted sputtering to prepare graphene supported AuPd bimetallic NP catalysts.16-18 The bimetallic NPs prepared via this method show excellent controllability for size, composition and structure. Particularly, this method is a physical process without reductants or stabilizers (such as surfactants, PVP, etc.) involved, which largely simplifies the synthetic procedure. Because in most chemical reduction methods, in order to produce the supported NPs catalysts with desirable functionality, the surfactant or polymer stabilizers must be removed without changing the size and compositional and structural integrity of the as-synthesized NPs, which is also a difficult issue.2,13,19 In addition, the Pd-to-Au ratio can be well controlled through regulating the relative amount of Pd and Au, respectively. Most importantly, the graphene-supported AuPd NPs show excellent catalytic performance for the reduction of nitro-compound. In order to reveal the synergistic interaction within the as-prepared bimetallic NPs, high resolution transmission electronic microscopy (HRTEM), energy dispersive spectroscopy (EDS) mapping, and X-ray absorption ACS Paragon Plus 3 Environment


spectroscopy (XAS) techniques were employed to get a comprehensive understanding about the morphology, composition, and electronic structure.9,12,16-25 A prototype benchmarked reaction, the reduction of 4-nitrophenol, was selected to evaluate the catalytic performance of supported bimetallic NPs.16,17,26,27 XAS, comprising both X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS) using synchrotron radiation, is the judicious method to study the electronic structure of metal NP. In XANES region (∼20 eV below and ∼50 eV above the absorption threshold), one can probe the local symmetry and unoccupied electronic states of the absorbing atom via the core-level electron transition. EXAFS (from ∼50 to ∼1000 eV above the absorption threshold), sensitive to interatomic distances and local disorder, is a powerful tool for the analysis of the local environment of absorbing atom with the interference from its neighboring atoms, hence providing the coordination number (CN) and bond length. EXAFS also can provide size information on low-dimensional materials like nanoclusters, i.e., lower CN associated with smaller-sized clusters corresponds to a lower R-space Fourier transform EXAFS peak intensity. EXAFS has been successfully utilized to resolve subtle structural details about NPs, especially bimetallic NPs.16-25,28-30 For example, Guo et al. detected the coordination between iron atom and silicon atom with Fe©SiO2 as highly active catalysts for conversion of methane to ethylene. Consequently, with the further understanding about the structure of fresh iron catalyst, in-situ and after catalytic reaction, a possible reaction mechanism was proposed.12 The previous works have shown that XAS can be a powerful technique for the characterization of bimetallic catalysts, since it can obtain in-depth structural information on those systems which are difficult to characterize by using other conventional material analysis methods. Therefore, systematic Feff modeling of EXAFS results to probe the structure of AuPd bimetallic NPs and a fringerprint peak was proposed to distinguish the different structure of AuPd, such as core-shell, random alloy or intermixing structure, etc. EXPERIMENTAL SECTION


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Synthesis of Graphene-supported AuPd Bimetallic NPs [BMIm][BF4] (purity > 99.9%) used in the procedure was purchased from Shanghai Cheng-Jie Chemical, and dried in vacuum for 24 h before using. Graphene powder prepared by thermal exfoliation reduction and hydrogen reduction was purchased from Nanjing XFNANO materials Tech.. Firstly, graphene powder was uniformly dispersed into RTIL of [BMIm][BF4] with a concentration of 2 mg/ml, with ultrasonication for 10 min to get a uniform black suspension. Secondly, 1.5 ml then the suspension was placed into a clean stainless steel pot, and then Pd was sputtered onto the suspension at different sputtering time (300 s, 200 s, 150 s, 100 s or 0 s) with a desktop sputtering system (Quorum Technology, Q 150 TS). The Ar working pressure and deposition rate were kept at 0.01 mbar and 0.2 Å/s, respectively. Thirdly, Au was sputtering onto the prepared Pd-graphene-RTIL suspension at different sputtering time (0 s, 100 s, 150 s and 300 s) to control Pd-to-Au ratio of deposited metal quantity. Eventually, the graphene-supported AuPd bimetallic NPs were separated from RTIL by high-speed centrifugation and decantation with acetone for about five times to completely remove the residual RTIL. The final products dried in black powder form. Characterization of Graphene-supported AuPd Bimetallic NPs The microscopic structures of the graphene-supported AuPd bimetallic NPs were characterized with HRTEM (FEI Quanta FRG 200F, operating at 200 kV) and HAADF-STEM. The XANES Au L3-edge measurements were performed at HXMA beamline of Canadian Light Source (CLS) and the BL14W1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF). The time-dependent UV-vis absorption measurements were carried out with UV-vis-NIR spectrophotometer (PerkinElmer, Lambda 750) using quartz cuvettes. The crystalline structures of graphene-supported AuPd NPs were characterized with X-ray diffraction (XRD, PANalytical Empyream) and the XRD data have been converted as Cu Kα radiation with wavelength of 1.54056 Å. The reaction rate of 4-nitrophenol reduction was evaluated by UV-vis absorption spectroscopy with a measuring interval of 2 min, where the scanning range was from ACS Paragon Plus 5 Environment


250 nm to 550 nm, and the data was recorded at room temperature without stirring. 20 µL catalysts aqueous solution (1 mg/mL) was added into a quartz cuvette containing 2 mL aqueous solution of NaBH4 (5 mM/L). Afterwards, 1 mL 4-nitrophenol aqueous solution (0.2 mM/L) was added into the mixed solution for the UV-vis absorption measurements, where deionized water was used as the reference. EXAFS Measurements and Fitting Methodologies Au L3-edge and Pd K-edge XAFS measurements of supported NPs were collected in transmission and fluorescence modes at the HXMA beamline of Canadian Light Source and 14W beamline of SSRF, respectively. Canberra 32 Ge detector was used for the fluorescence mode experiments at both facilities. Au metallic foil from EXAFS Materials has been used as reference material for monochromator energy calibration. The Au L3 edge data was in-step collected with each sample scan. XAFS data analysis software Athena was used for data reduction, and WinXAS for data R space curve fitting.31,32 The amplitude reduction factor (S02) was fixed at 0.9, which was determined using an Au foil reference and fixing the Au-Au CN at 12. A k-range of 2.77-13.75 Å-1 was used for EXAFS fitting. Uncertainties in EXAFS fitting results were computed from off-diagonal elements of the correlation matrix and weighted by the square root of the reduced chi-squared value obtained for the simulated fit. Several parameters describing electronic properties (e.g., correction to the photoelectron energy origin) and local structural environment (coordination numbers (CN), bond lengths (R), and their mean-squared relative deviations) absorbing atoms were varied in the fit. RESULTS AND DISCUSSION The typical reduction reaction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of excess NaBH4 was selected to evaluate the activity of the as-prepared graphene-supported catalysts.16,17,26,27 Upon mixing NaBH4 with 4-NP in aqueous solution, the UV-vis absorption spectrum is dominated by a characteristic absorption peak at about 400 nm arising from nitro compound. The reduction reaction which occurred immediately after the addition of catalysts into the mixed solution,


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was monitored by time-dependent UV-vis spectroscopy. The absorption peak at about 400 nm characteristic of 4-NP decreases dramatically and the correlations between absorption and reaction time for different as-prepared catalysts are shown in Figures 1a to 1e. It is apparent from Figure 1 that the reaction involving the use of the AuPd (1:1) catalysts (Figure 1c) takes the shortest time to complete, demonstrating that this catalyst possesses the highest catalytic efficiency comparing to the rest. The reaction kinetics of the four different NP catalysts is quantitatively compared in Figures 1a-1e in terms of a ln(At/A0) vs reaction time plot, where At and A0 are the time dependent and the initial (t = 0) absorbance of the 400-nm peak, respectively. The catalytic activity factor (κ), a value used to describe the activity, is obtained by κ=K/m where K is the reaction rate constant calculated from the slope of the linear fit of ln(At/A0), and m is the mass of catalysts.26,27 As displayed in Figure 1f, the relation between reaction time and reaction activity factor, i.e., κ is up to 130 s-1g-1 for AuPd (1:1). Meanwhile, the κ values for all bimetallic NPs show a great increase compared with the monometallic counterparts, implying that the synergistic effects are at work attributing to the bimetallic incorporation. Furthermore, in order to understand the synergistic effects and the size-structure-property relations for the AuPd bimetallic system, we have performed HRTEM, XRD, XANES and EXAFS to investigate the sample morphology, crystal structure, electronic structure and local structure of the as-prepared catalysts. It has been reported that the size of NPs can be controlled via the selection of desired ionic liquids and deposition parameters (working current or deposition time) through the ionic liquid assisted sputtering deposition procedure.33 The morphology and size distribution of the graphene-supported NPs has been characterized with HRTEM, as shown in Figure 2 and Figure S1 (see Supporting Information), revealing high uniformity, high coverage and crystal lattice of these supported NPs. Obviously, as shown in Figure 2d and S1, the average size of Au NPs (5.7±1.2 nm) is larger than that of Pd NPs (1.8±0.3 nm). Meanwhile, as the amount of Pd increases in AuPd NPs, the NP size decreases in the order of AuPd (2:1) (2.5±0.6 nm) > AuPd (1:1) (2.3±0.7 nm) > AuPd (1:2) (2.0±0.5nm) (Figure 2a to 2c).. This is attributed to the dilution of ACS Paragon Plus 7 Environment


between Au and Pd NPs, that is to say, when add fine Pd atoms/clusters to Au NPs in the formation of AuPd bimetallic NPs, the Au NPs will be fragment into small ensembles and the surface ratio of Au/Pd increases. Thus the size of adjacent Pd ensembles decreases, and eventually neighboring Au atoms are separated by Pd atoms into isolated Au clusters. With the decrease of ratio of Au/Pd, the size for bimetallic NPs will increase with the addition of Au, as shown in Figure 2a and S1. In addition, it is expected that the overall structure of AuPd alloy will progressively change in the diluting process. Much faster inter-diffusion of the atoms is expected in NPs than in the corresponding bulk, since the melting temperature (Tm) is much lower in the former compared with the latter.34 The XRD of as-prepared graphene-supported NPs are shown in Figure 3. Well-resolved peaks are indexed as (111), (200), (220), (311) and (222) from the Fm3m space group.14,16,17 As the amount of Au increases, the diffraction peaks gradually shifted to a lower 2θ angle for AuPd (1:2), AuPd (1:1) and AuPd (2:1) compared to the peak position of pure Pd, taking (111) for instance, indicating that the lattice of the Pd NPs expands due to the formation of AuPd alloy upon the deposition of the sputtered Pd into the ionic liquid contained Au NPs. While the crystal structure of Au/Pd alloy structures is verified by XRD, the distribution and the local structure of Pd and Au atoms inside the bimetallic NPs, and their significance to the catalytic performance of AuPd as catalysts remain inconclusive and need further investigation. Therefore, XAFS is performed to analyze the intra-atomic structure of as-prepared AuPd catalysts.20,21 For bimetallic NPs prepared in this work, it is important to understand the synergistic effect with the electronic structure arising from the size and alloying effects, manifesting in the changing of the Au L3-edge white line intensity. The white line (WL), the peak at the edge jump at about 11925 eV (Figure 4) in the Au L3-edge XANES, probes the electronic transition from Au 2p to unoccupied Au 5d states. Noted that although the d band in Au metal is nominally full, hybridization of the d band with the sp band produces some unoccupied d states above the Fermi level.22,24,28,35-37 Thus, the intensity of the WL corresponds to the density of ACS Paragon Plus 8 Environment

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unoccupied 5d states (i.e., d-hole). The experiment data reveal that the intensity of the white line gradually decreases further from Au foil to AuPd (1:2) with the reduction of Au concentration within the catalysts, in Figure 4a. This observation indicates that the d-hole of Au is filled upon alloying with Pd and gradually decreases with the increasing Pd concentration. When the Pd concentration increases to obtain AuPd (1:2), the white line is barely visible, indicating that Au atoms exists as very small clusters with a completely full d band, where there are more d electrons count of Au atoms in NPs than that of bulk due to less s-p-d hybridization with the size decreasing.24,28 This observation is in good agreement with the EXAFS results in Figure 4b to be discussed below, where a smaller value of Au coordination number (NAu-Au) in its first shell was found. Therefore, the above results indicate that a charge redistribution between Pd and Au occurs upon alloying, and electrons from Pd tend to fill the Au 5d band, which is consistent with the fact that gold is more electronegative than Pd.35,36 The Au L3-edge EXAFS are shown in Figure 4. As seen in Figure 4b, the magnitude of the Fourier transform of k3χ(k) of the AuPd NPs shows intense doublet peaks in the region of 1.8-3.4 Å, consisting of a lower-R peak (1.8-2.5 Å) and a high-R peak (2.5-3.4 Å). Both of them have been ascribed to the first shell metal-metal coordination as the Au ratio decreases, the peak in the lower R region becomes more intense due to the interference between the EXAFS backscattering of the Au-Au and AuPd scattering paths. This splitting of the first shell of the magnitude of Fourier transform is due to the difference in phase between the two types of the scattering (in both case, the phase and amplitude are nonlinear in k space). The Fourier Transform in R space with progressively changing magnitude and composition are displayed in Figure 4b. The doublet components vary accordingly with the increasing amount of Pd in AuPd NPs, indicating an increase of Pd atoms as the nearest neighboring atoms of Au in its first shell.23,24,28 Figure 5a shows the k3χ(k) EXAFS of the samples and reference of interest obtained from the back transform of their filtered first shell in R space with a window of 1.95-3.39 Å. The expanded region of interest (5.5-7.0 Å) is also shown in Figure 5b. ACS Paragon Plus 9 Environment


These results reveal the progressively changed EXAFS feature in line with the gradually varying in alloy composition in terms of Au/Pd ratio (Corresponding R space and k space curve fitting results for all samples are shown in Figure S2 and S3, see Supporting Information). By examining the Au L3-edge EXAFS in k-space at about 6.8 Å-1, the effect of alloying is apparent, of which the distinct phase shifts of the well-resolved k3χ(k) EXAFS features of various AuPd NPs relative to those of Au foil and Au NPs are due to the difference between the k-dependent backscattering phase and amplitude through the Au-Au and Au-Pd paths. As the Au ratio in the AuPd NPs decreases, the average number of neighboring Pd atoms surrounding the Au absorbers increase, resulting in the formation of more Pd nearest neighbors around Au per unit volume, which is also reflected in the FT-EXAFS plotted in the R (radial distribution) space (Figure 4b). Furthermore, from the selected k sub-range of k3χ(k) EXAFS shown in Figure 5b, an interesting feature at about 5.96 Å-1 is observed, where a gradually enhanced peak appears with the increase of Pd concentration. The appearance of this new peak can be correlated to the formation of AuPd bimetallic NPs. The structural parameters of the NPs have been obtained from the EXAFS fittings and are presented in Table 1. For a bulk fcc metal, the nearest-neighbor coordination number is 12. While for monometallic Au NPs, the coordination number is only 10.1, due to a relatively large fraction of atoms on the surface. There are two sets of coordination numbers for the Au L3-EXAFS of AuPd NPs, denoted as NAu-Au and NAu-Pd, corresponding to the average numbers of Au neighbor atoms and the number of Pd neighbor atoms around the Au absorber, respectively. The ratio of NAu-Pd to NAu-Au should be the same as the Au-to-Pd molar ratio in the NPs for a homogeneous AuPd alloy. While for the as-prepared bimetallic catalysts, the ratio of NAu-Pd to NAu-Au is apparently smaller than the Au/Pd molar ratio, indicating the formation of non-homogenous alloying structures. In order to give a more comprehensive understanding of the intermixed structure for the as-prepared bimetallic NPs, the Feff modeling for the as-prepared NPs starts with a Au19 cluster and the replacements of Au with different numbers of Pd atoms are ACS Paragon Plus 10 Environment

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performed for bimetallic AuPd NPs. Standard Au19 cluster with one Au central atom and 18 Au sites distributed on the 1st (CN=12) and the 2nd (CN=6) coordination shells (detailed parameters see Table S1). The modeled structures of NPs with different Au/Pd ratio, named after the different number of Au and Pd atoms surrounding the Au central atom, i.e., Pd0Au18 and Au1Pd1, are displayed in Figure 6. Accordingly, the Feff modeled k3χ(k) based on intermixing model 1 to 7 is displayed in Figure 6a to g, of which a similar feature appears at about 5.8 Å-1 with the Pd concentration increase for this cluster model. This result is consistent with the backward Fourier transform of the filtered k3χ(k) EXAFS data (Figure 5b), which supports the intermixed structure we proposed for the as-prepared bimetallic catalysts. To evaluate the EXAFS effect from the off-center Au, Feff modeling was performed for a Au19 cluster with a Pd (111) surface sheet. The structure model M-1 from the surface Au/Pd (111) modeling (Figure 6) was used. The thirteen Au atoms are numbered (Figure 7 and Table 2) based on the magnitude of their distance to Pd (111) facet, and used as the center absorbing Au, for Feff modeling. Feff modeling reveals that EXAFS analysis of the Au atoms within the cluster can be classed into 4 groups, shown in Figure 8a for k3χ(k) and Figure 8b for magnitude of the Fourier transform. As indicated in Figure 8a, the 5.8 Å-1 peak is the strongest for the group 1 Au (Au-22), which becomes weaker for the group 2 (Au-11, 12, 21, 23, 51, 52), then visible for group 3 (Au-32, 41, 42), and finally not resolved for group 4 (Au-31, 33, 61). Correspondingly, in R space in Figure 8b, FT of the group 2 is more close to the group 1, and FT of the group 3 is more close to the group 4. The black trace is the atom number weighted total EXAFS, which shows the EXAFS modeling by using the Au atom at the center of Au/Pd cluster is representative to an acceptable level. Therefore, the Feff modeling approach discussed in Figure 7 is qualitatively correct to represent the EXAFS data from the modeled Au/Pd intermixed structure. With the characterization using XAFS and the theoretical modeling analysis, we get a more in-depth understanding toward the structure of AuPd bimetallic NPs, especially confirming the formation of Au-Pd bond and distinguishing the different structure of AuPd bimetallic nanoparticles. As known, the activity of bimetallic ACS Paragon Plus 11 Environment


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catalytic depends not only on the “ligand effects” which encompass influences of bonding, charge transfer, and strain, but also strongly on the “ensemble effect” that describes

the

synergistic

behavior

of

different

components

in

specific

arrangements.38-39 The density functional theory (DFT) calculation provides a clear quantitative description of these two effects, of which the ensemble effect plays a dominant role for the enhancement of catalytic activity for the adsorption on catalysts surface.40-42 For the AuPd system in particular, the ensemble effect is usually a diluting effect where the catalytically more active component (Pd) is diluted by the less active component (Au).43 Sizes of contiguous Pd ensembles decrease and eventually all Pd atoms are separated by Au as isolated Pd monomers with the increase of the ratio of Au/Pd. Meanwhile, the interaction between adsorbates and AuPd bimetallic surfaces is weakened by the presence of Au at the adsorption site. CONCLUSIONS We successfully design and develop the graphene-supported AuPd bimetallic NPs with various controlled Au/Pd ratios via a sequential sputtering deposition. A systematic characterization with XAFS was conducted. The analysis of the EXAFS data and simulation reveals a novel and unique fingerprint for the intermixing structure of AuPd bimetallic NPs. For this structure, the increasing AuPd interaction, exhibiting increasing intensity of the fingerprint in the k-space of Au L3-edge EXAFS at about 5.8 Å-1, corresponds to the enhancement of catalytic activity for bimetallic NPs. It is expected that XAFS can give more in-depth structural information at atomic scale on other bimetallic systems, Supporting Information The Supporting Information is available free of charge on ACS Publications website at DOI:***. HADDF-STEM image for graphene-supported AuPd (1:1), EDX spectra, R space curve fitting result for Au L3 and Pd K edge EXAFS, EEFF modeling parameters for the Au19 cluster.


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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 61675143, 11661131002, 51702025), the Soochow University-Western University Joint Centre for Synchrotron Radiation Research, the Collaborative Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the National Science Foundation of Jiangsu Provence (No. BK20160277).



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Figure 1 Time-dependent UV-vis absorption spectroscopy for reduction of 4-NP using graphene-supported (a) Au, (b) AuPd (2:1), (c) AuPd (1:1), (d) AuPd (1:2) and (e) Pd NPs as the catalysts. (f) A comparison of reaction time and reaction constant for different nanocatalysts.



Figure 2 TEM images of (a) graphene sheets, and graphene-supported (b) Au, (c) AuPd (2:1), (d) AuPd (1:1), (e) AuPd (1:2) and (f) Pd NPs. Insets are corresponding HRTEM images of typical single NPs; and histograms show corresponding size distribution of the NPs. ACS Paragon Plus 18 Environment

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Figure 3 XRD pattern of as-prepared graphene-supported Au, AuPd (2:1), AuPd (1:1), AuPd (1:2) and Pd NPs.



Figure 4 Au L3 experimental data for the system under investigation. (a) Normalized Au L3 edge XANES, Au L3 data for Au metallic foil as reference and as-prepared catalysts with different Au-to-Pd ratio. (b) Magnitude of Fourier transform of k3χ(k) for Au L3 edge EXAFS data.


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Figure 5 Au L3 EXAFS data for Au metallic foil as reference and as-prepared catalysts with different Au-to-Pd ratio. (a) and (b) k3χ(k), (c) k3χ(k) from the backward Fourier transform within a R window of 1.95-3.39 Å, constrained only for the 1st nearest neighbor FT peak and (b) A selected sub-range k range of k3χ(k) from the back Fourier transform.



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Table 1 R space curve fitting for the Au L3-edge k3χ(k) for the supported NPs

Sampl

CN

e Au-A u Au foil Au-G AuPd 2:1 AuPd 1:1 AuPd 1:2

R(Å)

Au-Pd

Au-A u

DW(Å2)

Au-Pd

Au-Au

Au-Pd

Scattering Intensity (rela.) Au-A u

E0

CNtota l

Au-Pd

12

------

2.86

------

0.0081

------

100

------

3.4

12

10.1

------

2.86

------

0.0080

------

100

------

2.7

10.1

8.3

0.9

2.85

2.84

0.0076

0.0106

100

10.9

3.8

9.2

5.5

1.0

2.85

2.84

0.0062

0.0052

100

33.9

4.1

6.5

5.5

2.5

2.84

2.80

0.0067

0.0074

100

64.8

3.9

8


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Figure 6. AuPd clusters with Au/Pd random alloy model used for Feff modeling with Au/Pd ration progressively increased from Model (a) Pd0Au18, (b) Pd1Au17, (c) Pd1Au8, (c) Pd1Au5, (d) Pd1Au2, (e) Pd1Au1 to Model (7) Pd2Au1. Here yellow circles are Au and blue circle is Pd. (h) Feff modeled k3χ(k) based on intergrowth model-1 to 7 [Figures 6(a) to 6(f)]. k3χ(k) is color coded to the corresponding cluster models; and (i) a zoom-in view of the feature at ~5.7Å as shown in the gray area in (h).



Figure 7. Au19 cluster with 6 Pd atoms as the surface 111 sheet. The 13 Au atoms are numbered for the convenience of Feff modeling. Table 2 Parameters information for modeling Au19 cluster in Figure 7. Groupa Numberb Atom IDc Au-Pd(min)d Au-Pd(max)e 1 1 22 2.88 4.10 2 6 11,12,21,23,51,52 4.10 5.00 3 3 32,41,42 5.00 6.45 4 3 31,33,61 5.00 8.16 a. Group of Au; b. Number of Au atoms for the corresponding type of Au; c. ID for the Au atom, see Figure 7; d. Minimum Au-Pd interatomic distance for the corresponding type of Au; e. Maximum Au-Pd interatomic distance for the corresponding type of Au.


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Figure 8. Feff modeling of 4 groups of Au atoms within the Au19 cluster (a) kχ(k) and (b) FT of kχ(k) (FT window: 2.7-14.0 Å-1).



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