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Structural Dependence of Platinum Nanostructures on Catalytic Performance in Aromatic Azo Compound Reaction Investigated by X-ray Absorption Fine Structure Spectroscopy Duo Zhang, John A McLeod, Lei Hu, Shuanglong Lu, Yanyun Ma, Jun Zhong, Zheng Jiang, Hong-Wei Gu, and Xuhui Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04440 • Publication Date (Web): 20 Jun 2016 Downloaded from http://pubs.acs.org on June 23, 2016

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The Journal of Physical Chemistry

Structural Dependence of Platinum Nanostructures on Catalytic Performance in Aromatic Azo Compound Reaction Investigated by Xray Absorption Fine Structure Spectroscopy Duo Zhanga,c, John Mcleoda, Lei Hub, Shuanglong Lub, Yanyun Maa, Jun Zhonga, Zheng Jiangc*, Hongwei Gub*, Xuhui Suna* a

Soochow University-Western University Centre for Synchrotron Radiation Research,

Institute of Functional Nano and Soft Materials Laboratory (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu, 215123, P. R. China b

Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical

Engineering and Materials Science, Soochow University, Suzhou, 215123, P. R. China c

Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese

Academy of Sciences, Shanghai, 201204, P. R. China

Abstract Uniform one-dimensional Pt nanowires prepared by etching FePt nanowires precursor exhibit high conversion yields and selectivity in aromatic azo compounds reaction compared to the other Pt-based catalysts, such as Pt nanorods and Pt nanoparticles. The X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were employed to investigate the electronic structure and short-range local environment of the three Pt nanostructures, revealing that the high density of unsaturated coordinated atoms in the short-range local structure of Pt nanowires contribute to the superior catalytic performance of the nanowires. Furthermore, the quasi in-situ XANES was carried out to monitor the electronic structural evolution of the Pt nanowires during the different stages in the whole reaction process, which further clarify the Pt-OH involved catalytic reaction mechanism. This work delineates the correlation between catalytic performance and structural sensitivity of Pt-based catalysts investigated by X-ray absorption spectroscopy.

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1. Introduction The design and synthesis of catalysts with high activity and selectivity for the target products with long-term durability are critical for the practical industrial applications.1-4 Noble metal nanocrystals with controllable size, composition and shape are one important catalog for this goal. Many efforts have been devoted to develop synthetic methods to prepare the different composition, sizes and shapes of noble metal nanocrystals, which are related to the activity and selectivity of these nanocrystals.5 In the atomic scale, it is obviously that the activity and selectivity of the catalysts depend greatly on the surface structure factors including dangling bond, coordination number and bond distance, which are determined to offer the available active sites.6-8 However, it is still not clear that how the shape and size of nanocrystals can contribute to the selectivity and reactivity of the catalysts. Pt nanocrystals have been regarded as one of most promising catalysts in the hydrogenation and coupling reactions.9-12 Many studies have been dedicating to synthesize the Pt nanocrystals with various sizes, morphologies and composition in order to enhance the catalytic performance of Pt nanocrystals.13 Recently, we have successfully prepared the ultrathin Pt nanowires (denoted as Pt NWs) by etching method in the corresponding solution, which exhibit the excellent reactivity and selectivity performance in the hydrogenation synthesis of aromatic azo compounds (Aazos) (Scheme S1 in the supporting information) compared to that of Pt nanorods (denoted as Pt NRs) and Pt nanoparticles (denoted as Pt NPs).14 Typically, the reactivity and selectivity of Pt nanocrystals are attributed to the specific surface nanostructure and exposed facets of Pt nanostructures, which have high density of defects. However, in the atomic scale, the reactivity and selectivity are closely related to the electronic structure and short-range structure of Pt nanostructures. Due to the decreasing size, the surface area, the valence state, bond distance and even coordination number of Pt nanostructures are different from those in bulk Pt. Moreover, the bonding strength of absorbed reactant is also correlated with valence state and bond distance of Pt nanostructures, which determines the final product by virtue of the selectivity of Pt nanostructures. It is critical to understand the relationship between the catalytic performance and the electronic structure and local chemical environment induced by the different Pt nanostructures and elucidate the catalytic mechanism, which could facilitate the screening of the catalyst with superior selectivity and reactivity. In this work, the electronic structure and short-range environment of Pt NWs, Pt NRs and Pt NPs have been investigated by X-ray absorption fine spectroscopy (XAFS). The high reactivity of Pt NWs is mainly attributed to lower oxidation state and high density of


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dangling bonds, which can provide amount of available chemical active sites for the hydrogenation of nitrobenzene. Furthermore, the quasi in-situ XAFS have been employed to monitor the evolution of electronic structure of Pt NWs, which have unique capability to elucidate the relationship of adsorption, dissociation and desorption of Pt-H bonds and Pt-OH bonds during the reaction. It further directly confirms the proposed mechanism for the formation of Aazos on Pt NWs. XAFS containing XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure), is a powerful spectroscopic technique to explore the correlation

between

performance.

15-18

electronic

structure,

local chemical

structure,

and

catalytic

The excitations in the near edge region in the vicinity of the threshold arise

from the core to bound and quasi-bound unoccupied state dipole transitions. These features are the XANES, which probes densities of states and local symmetry. As the excitation energy increases, the photoelectron gains sufficient energy, leaving the absorbing atom. As the photoelectron travels away from the absorbing atom, it will be scattered by the neighboring atoms. Therefore, the constructive and destructive interference of these outgoing and backscattered waves produce oscillations in the absorption coefficient. These oscillations are the EXAFS, denoted χ(k), which contains information about the inter-atomic distance and the local dynamics of the system, χ(k) =φ(k)A(k), where φ(k) is the phase (containing bond length and phase information) and A(k) the amplitude (containing information about the backscattered atom, the bond length, the coordination number and the Debye-Waller factor thermal mean square displacement of the interatomic distance). EXAFS is only sensitive to short range order due to the short attenuation length of electrons. EXAFS can be readily analyzed using a Fourier transform technique, which separates the phase and the amplitude. XAFS offers a tool to understand the essence of nano-catalyst design, which will aid in the search for an ideal catalyst with accurate selectivity and high reactivity.

2. Experimental 2.1 Pt-based nanostructures synthesis The Pt NWs, Pt NRs and Pt NPs were prepared from acidic etching of the corresponding FePt nanomaterials, respectively.14,19 Firstly, the three different shapes (NWs, NRs and NPs) of FePt catalysts were prepared by the impregnation methods. And then bubbling oxygen gas and HCl/methanol mixed suspension was introduced into the as-prepared corresponding FePt precipitates at the temperature of 100 °C and then mixture suspension was kept at 60 °C for



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one hour. The final products were obtained after the centrifugation, washing with methanol three times and re-dispersing in hexane for further characterization and catalytic reaction.

2.2 Characterization Methods The morphology and size distribution of the Pt nanostructures were analyzed using a transmission electron microscope (FEI, Model Tecnai G2 F20). The surface area was analyzed by nitrogen gas adsorption using an ASAP 2050 system (ASAP 2050, Micromeritics). Prior to measurement, the degas temperature was set to 453 K to remove contamination (e.g. organic absorbers) and then the samples were outgassed to 10-3 Torr. XAFS experiments were carried out at the BL14W1 beamline at the Shanghai Synchrotron Radiation Facility (SSRF), China. The beamline was operated with a Si (111) double crystal monochromator and an uncoated glass mirror to reduce the higher harmonics. During the measurement, the synchrotron radiation ring was operating at 3.5 GeV and the current was maintained between 150 mA to 210mA. The Pt L3-edge X-ray absorption spectra were recorded at room temperature in the fluorescence mode with a Lytle detector filled with argon gas. The quasi in-situ XAFS was perform to track the electronic structure evolution of Pt NWs at the different stages of a nitrobenzene hydrogenation reaction catalyzed by Pt NWs. In a typical catalytic reaction, 0.05 mmol Pt NWs in hexane were added into a Schlenk tube and the hexane was evacuated using pressure reducing valves. 10 mmol Nitroaromatics, 2.5 mmol KOH, 1 mL tert-butylbenzene and 20 mL p-xylene as the solvent were added into the reaction tube and then sealed. The reaction tube was thrice evacuated and flushed with hydrogen which took place at 80°C under 1 bar hydrogen atmosphere for 3 hours. To perform the quasi in-situ XAFS experiment, the solution was taken at the different time throughout the reaction and analyzed by XAFS immediately. Pt foil was used as standard reference and measured in the transmission mode. Structural parameters were obtained by fitting the measured spectra in R space using FEFF6L program.20,21

3. Results and Discussion. The essential issue for the hydrogenation process in Aazos synthesis is the capacity of the Pt catalyst to adsorb and dissociate H2 to promote the selective hydrogenation of the double bond of nitro group of nitrobenzene, which has a close correlation to the chemical environment of the catalyst, based on the electronic structure22, metallic bonding23, charge distribution24, surface facets.25,26 Our previous work has demonstrated that ultra-thin Pt NWs


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with the average diameter of less than 2 nm showed excellent catalytic performance for the selective hydrogenation of nitrobenzene to the formation of Aazos compared to other Pt nanostructures such as NRs and NPs.14,27 The representative TEM images of Pt NPs, Pt NRs and Pt NWs are shown in Figure 1(A-C), respectively. The mean diameters of Pt NPs, Pt NRs and Pt NWs are approximately 3.7 nm, 2.5 nm and 1.5 nm, respectively. The lengths of the Pt NRs and Pt NWs were around 50-200 nm and more than 800 nm, respectively. The assynthesized samples of Pt nanostructures indicate a high degree of shape monodispersity (Figure 1). The high-resolution TEM (HRTEM) images of three Pt nanostructures are shown in Figure 1 (D-F), respectively. All three HRTEM images show the same lattice spacing of 0.23 nm, corresponding to the d-spacing of the (111) crystal plane of fcc Pt. The crystalline structures of Pt NPs, Pt NRs and Pt NWs were also characterized by powder X-ray diffraction, as shown in Figure 2. The main diffraction peaks can be indexed as (111), (200), (220) and (220). The higher intensity and sharper shape of peaks for Pt NPs are observed than those for Pt NWs and Pt NRs, indicating Pt NPs has the higher order crystalline structure than Pt NRs and NWs. Obviously Pt NWs has the lowest order crystalline structure among them. The calculated BET surface areas of Pt NWs, Pt NRs and Pt NPs are 7.88 m2/g, 13.99 m2/g 2

and 3.72 m /g, respectively (listed in Table 1 and the N2 adsorption/desorption isotherms of Pt

NWs, Pt NRs and Pt NPs are shown in Figure S1), which do not exhibit the big difference in the surface area among the three Pt nanostructures. It indicates that the different catalytic performance of the Pt nanostructures is not simply related to the BET surface area in this case. It is noted that surface-to-volume ratio of Pt NRs should be lower than that of Pt NWs according to the the average diameters and lengths of three Pt nanostructures but here the BET surface area of Pt NRs is larger than that of Pt NWs. During the degas procedure in the BET measurement, Pt NWs were aggregated seriously compared to Pt NRs due to the smaller diameter and the longer length of Pt NWs, which probably results in reducing the BET surface area of Pt NWs. To clarify the correlation between the electronic state of Pt nanostructures and their corresponding catalysis activity, we performed an XAFS study on the Pt nanostructures to offer supplementary information on the electronic structure, coordination number, and bonding distance of the Pt nanostructures. The normalized XANES spectra of three different Pt nanostructure as well as Pt foil as the standard reference are displayed in Figure 3. The XANES spectra at the Pt L3-edge of three Pt nanostructures exhibit similar features except the increase of the edge jump (whiteline) compared to the standard reference spectrum of Pt



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foil, which confirm the Pt nanostructures are fcc Pt in accord with the XRD results. The L3edge of Pt corresponds to transitions from the 2p3/2 core levels to empty states of 5d5/2 or 5d3/2 orbitals (diploe selection rule of X-ray transitions). Typically, the intensity of whitelines of the Pt nanostructures are attributed to larger activated empty states in the d orbital due to size effect of the Pt nanostructure, therefore the intensity of whiteline is proportional to oxidation state of Pt.28-30 The oxidation state of Pt NWs is slightly lower than those of Pt NRs and Pt NPs. It means that the surface electron concentration of Pt NWs lie in between those of two other Pt nanostructures and Pt foil, which makes the surface electron-rich of Pt NWs, favoring the adsorption of electron-deficient reaction and achieve the partial hydrogenation of nitrobenzene in order to generate the desired products.16 Pt nanostructures were prepared in methanol solution and methanol as electron donator could provide more electrons into Pt NWs due to high density of surface disorder and/or defects compared to Pt NRs and Pt NPs. Figure 4. show k3-weighted Fourier transforms (FT) of EXAFS of Pt NPs, Pt NRs and Pt NWs at the Pt L3-edge. The EXAFS spectra were analyzed using standard data analysis procedures. The EXAFS χ spectrum was first Fourier transformed from k space to R-space to obtain the radial structure function (RSF). The χ spectrum for the first coordination shells was then isolated by inverse transform of the RSF at the appropriate region and subsequently fitted using the single scattering EXAFS equation. The coordination numbers (N) and interatomic distances (R) for the first coordination shells of Pt nanostructures are obtained and listed in the Table 2. The prominent peaks located at around 2.62 Å are ascribed to the PtPt bond. All three EXAFS spectra in R-space are very similar rather than the different intensities. The Debye–Waller factor, representing disorder, will damp the amplitude noticeably, which is attributed to the lack of long-range order and surface disorder of the crystal. This can be seen in Figure S2 where the reduction in amplitude of the filtered Fourier back-transform in k-space of the first shell of Pt nanostructures compared to Pt foil is displayed. It should be noted that amplitude reduction can be seen even in perfect nanosized single crystals; this is because the coordination number decreases at the surface and interface of the crystal, which also introduces chemical inhomogeneity. All these effects are sometimes termed as disorder or nano effect. The fitting parameters of Pt nanostructures presented in Table 2 further clarify the fact. The Pt nanostructures with various shapes is significantly different from the bulk Pt in the coordination number and interatomic distance of the first shell Pt-Pt due to the nano size effect. Furthermore, the bond length of Pt NWs is slightly short than those of Pt NPs and Pt NRs. The Pt-Pt bonding distance of Pt NWs is 2.74 Å with


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average coordination number of 8.6, lower than those of Pt NRs (CN ≈ 10.6) and Pt NPs (CN ≈ 9.6). It indicates that the Pt NWs possess the most unsaturated Pt-Pt bonding with reduced interatomic distance, which could provide a large amount of active sites for the hydrogenation in the nitrobenzene reaction with respect to Pt NRs and Pt NPs.30 The higher selectivity and reactivity of Pt NWs are mainly attributed to the higher concentration of dangling Pt-Pt bonding with contracted Pt-Pt bonding distance compared with those of Pt NRs and Pt NPs. Our results demonstrate that the Pt NWs with lower coordination number and shrunk Pt-Pt bonding could offer a series of active sites rather than a simple physical BET surface area. The possible mechanism for the formation of Aazos in the partial hydrogenation of nitrobenzene is shown in Scheme 1. The hydrogenation process is correlated with the local environment and the catalyst in the reaction. When the hydrogen gas was introduced, hydrogen gas as reducing component is constantly adsorbed and dissociated under catalytic action of Pt NWs. In the acidic, these available H atoms adsorbed on Pt NWs surface area can attack the –NO2 group in the nitrobenzene to generate the nitrosobenzene and consequently hydrogenated to RNOH, which ultimately achieve full hydrogenation to obtain the aniline as final product. However, in the base, the different pathway toward full (a-b-c) or partial (a-bd-e) hydrogenation occur in the latter step, which two first steps (a and b steps) is in parallel to that in the acid. Then the RNOH could couple with RNO and consequently dehydrated into the Aazos as target product.27 1Therefore, in the base, the reduction process of nitrobenzene is inclined to partial hydrogenation compared with preference full hydrogenation in acid under catalysis action of Pt NWs. Moreover, it is noticeable that the –OH radical in the form of Pt-OH is key component for the selectivity of Pt catalyst in the nitrobenzene reduction process.14,31 To further clarify the Pt-OH involved catalytic reaction mechanism, quasi in-situ XANES measurement was carried out to monitor the electronic structure evolution of Pt NWs under the reaction condition. Figure 5 shows the Pt L3-edge XANES spectra for Pt NWs in solution at different reaction stages: 0 min, 50 min and 140 min, respectively. The shape resonance and intensity of the Pt L3 XANES spectra can be reflected to the oxidation state of platinum. The position of peak at about 11567 eV (Feature A, whiteline) is very sensitive to the degree of electron occupancy in the valence orbital. In general, with the increasing intensity of the peak, the density of unoccupied d state is higher and thus the oxidation state of Pt is higher.32 For Pt NWs in solution at 0 minute, there is no H2 absorption and the adsorbents on Pt NWs could be OH- groups in the solution. It is reported that the Pt-OH interaction could change



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the shape of Pt L3 edge XANES with an increase of the white line and a decrease of the shoulder (Feature B).32 Pt-OH interaction with the H2O adsorption on Pt had also shown spectral changes with an increase of feature A and a decrease of feature B.33 For Pt NWs in solution at 0 min (before H2 gas was introduced), a clear feature A stands for the increased oxidation state and a weak feature B can be observed, indicating the formation of Pt-OH radical. When H2 gas was introduced, the spectra for Pt NWs at 50 and 140 minutes show decreased feature A and increased feature B. For clarity, the differential spectra were obtained by subtracting the spectrum at 0 min from the spectra at 50 and 140 min, respectively. It is widely reported that the H2 adsorption on Pt will change the spectral shape with the reduction in the whiteline intensity and the growth of the adjacent shoulder.23,34,35 These spectral shape changes confirm the formation of Pt-H in the reaction process. The results verified the OHoccupation of active sites on Pt NWs mediating the competition relationship between OHand H absorption during the reaction.

4. Conclusions In summary, the electronic structure and short-range local environment of Pt nanostructures (Pt NWs, NRs and NPs) were investigated by XAFS. XANES result shows that the oxidation state of Pt NWs is slight lower than those of Pt NRs and Pt NPs, which makes the surface electron-rich of Pt NWs, favoring the adsorption of electron-deficient reaction and achieve the partial hydrogenation of nitrobenzene in order to generate the desired products. EXAFS data further reveal that Pt NWs with lower coordination number and shrunk Pt-Pt bonding could offer a series of active site rather than a simple physical BET surface area, contributing to the superior catalytic performance of the nanowires. The quasi in-situ XAFS results indicated the Pt NWs have unique capability to balance the relationship of adsorption, dissociation and desorption Pt-H bond and Pt-OH bond, which confirms the proposed mechanism for formation of Aazos compound. The work promotes the deep understanding of the relationship between the catalytic performance and the electronic structure and local chemical environment induced by the different Pt nanostructures and elucidate the catalytic mechanism, which could facilitate the screening of the catalyst with superior selectivity and reactivity.


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Figure 1. Representative TEM and HRTEM images of Pt NPs (A, D), Pt NRs (B, E) and Pt NWs (C, F), respectively, synthesized by acidic etching of the corresponding FePt bimetallic nanostructures.



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Figure 2. X-ray diffraction patterns of Pt NPs, Pt NRs and Pt NWs.


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Figure 3. The normalized XANES spectra at the Pt L3-edge of Pt NWs, Pt NRs and Pt NPs as well as Pt foil as a standard reference spectrum. These whiteline (label peak A) gradually decrease as the direction of the red arrow, indicating the oxidation state of Pt nanostructures slightly decrease.



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Figure 4. The k3-weighted Fourier transform EXAFS spectra as experimental and fitting from as-synthesized Pt NPs, Pt NRs, and Pt NWs. Pt foil as the reference spectra.


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Figure 5. The quasi in-situ XANES spectra of Pt L3-edge corresponding Pt NWs in solution during different reaction time. The ∆XANES spectra subtracted from the Pt L3-edge at reaction stage: 0 min.(Inset)



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Scheme 1. Proposed mechanism for the azobenzene formation in base solution and the aniline formation in acidic solution.


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Table 1. Physicochemical properties and the corresponding catalytic activity of Pt NWs, Pt NRs and Pt NPs. Pt sample

Diameter/nm

Length/nm

BET surface

Yield (%)

2

Area (m /g)

Azobenzene

Pt NWs

1.5

>800

7.88

92.7

Pt NRs

2.5

50-200

13.99

69.7

Pt NPs

3.7

-

3.72

44.3

The optimized reaction conditions: nitrobenzene (1.0 mmol), 0.25 equiv. Base (KOH) and pxylene (2mL) under 1 bar hydrogen Table 2. EXAFS parameters of as-synthesized Pt NPs, Pt NRs, and Pt NWs. Sample

Shell

N

R(Å)

σ2(Å-2)

∆E0(eV)

x 10-3

R factor x 10-2

Pt foil

Pt-Pt

12

2.76

Pt NRs

Pt-Pt

10.6

2.75

6.46

6.37

0.22

Pt NPs

Pt-Pt

9.6

2.75

6.38

6.23

0.24

Pt NWs

Pt-Pt

8.6

2.74

7.49

5.88

0.62

Here N is the coordination number; R is the distance between the absorber and backscatter atoms; σ2 is the Debye-Waller factor; ∆E0 is energy correction to the reference absorption onset. Error bounds that characterize the structural parameters obtained by EXAFS spectroscopy were estimated as: ±20% for N; ±1% for R; ±20% for σ2; ±20% for ∆E0. The Pt foil parameters were from data _41525-ICSD, the many body reduction factor (S02) was fixed to 0.80. R-space fit: ∆k=2.8 to 13.5 Å-1; ∆R =1.5 to 3.1 Å.



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ASSOCIATE CONTENT Author information *Corresponding Author *Xuhui Sun Email: [email protected]. Fax: 86-512-65880820. Tel: 86-512-65880943. *Hongwei Gu Email: [email protected]. Fax: 86-512-65880905. Tel: 86-512-65880905. *Zheng Jiang Email: [email protected]. Fax: 86-021-3393212. Tel: 86-021-3393212.

Notes The authors declare no competing financial interest.

Acknowledgements The authors thank beamline BL14W1 (Shanghai Synchrotron Radiation Facility) for providing the beam time. The work was supported by Natural Science Foundation of China (NSFC) (Grant No. 91333112, U1432249), the Priority Academic Program Development of Jiangsu Higher Education Institutions. This is also a project supported by Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices and Collaborative Innovation Center of Suzhou Nano Science & Technology, and sponsored by Qing Lan Project.

Supporting information Scheme S1. Azobenzene formation pathway from the nitrobenzene hydrogenation with the different byproducts under the mild condition. Table S1. The surface atom ratios and average bond length based on a spherical nanostructure model with different diameters range from 5.6 Å – 30.8 Å. Figure S1. N2 adsorption/desorption isotherms of Pt NWs, Pt NRs and Pt NPs. This material is available free of charge via the Internet at http://pubs.acs.org.

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