Studies on catalytic activity of hydrogen peroxide generation

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Energy, Environmental, and Catalysis Applications

Studies on catalytic activity of hydrogen peroxide generation according to Au shell thickness of Pd/Au nanocubes Inho Kim, Myung-gi Seo, Changhyeok Choi, Jin Soo Kim, Euiyoung Jung, Geun-Ho Han, JaeChul Lee, Sang Soo Han, Jae-Pyoung Ahn, Yousung Jung, Kwan-Young Lee, and Taekyung Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14166 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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ACS Applied Materials & Interfaces

Studies on Catalytic Activity of Hydrogen Peroxide Generation according to Au Shell Thickness of Pd/Au Nanocubes Inho Kim,∇,† Myung-gi Seo,∇,‡ Changhyeok Choi,∇,∥ Jin Soo Kim,⊥ Euiyoung Jung,† Geun-Ho Han,‡ Jae-Chul Lee,⊥ Sang Soo Han,# Jae-Pyoung Ahn,*,○ Yousung Jung,*,∥ Kwan-Young Lee,*,‡ Taekyung Yu*,†

†Department

of Chemical Engineering, College of Engineering, Kyung Hee University, Yongin 17140, Korea

‡Department

of Chemical and Biological Engineering, Korea University, Seoul 02841, Korea

∥Graduate

School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

⊥Department

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of Materials Science and Engineering, Korea University, Seoul 02841, Korea

#Computational

Science Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Korea

○Advanced

Analysis Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Korea

KEYWORDS: Pd/Au core/shell nanocubes, thin Au layer, lattice strain, calculation, H2O2 synthesis

ABSTRACT: The catalytic properties of materials are determined by their electronic structures, which are based on the arrangement of atoms. Using precise calculations, synthesis, analysis, and catalytic activity studies, we demonstrate that changing the lattice constant of a material can modify its electronic structure and therefore its catalytic activity. Pd/Au core/shell nanocubes with a thin Au shell thickness of 1 nm exhibit high H2O2


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production rates due to their improved oxygen binding energy (∆EO) and hydrogen binding energy (∆EH), as well as their reduced activation barriers for key reactions.

1. INTRODUCTION

With an exciting progress in modern understanding of metals and their chemistry, imparting the properties of a solid A onto a different solid B, a form of an alchemy, is becoming increasingly possible. The properties of metals are determined by their electronic structures based on the arrangement of atoms.1−5 The distance and strain between atoms depend on crystal structure and affect orbital binding to produce an electron configuration.6−9 If we control atomic distance and strain using external factors, it is possible to control the physicochemical properties of a metal.6,7 Previous studies have shown that catalytic activity can be improved by adjusting metal strain via mixing other metals with a metal with good activity, such as platinum.10−20 Building on this research, our goal was to test if a substance with no activity could acquire catalytic activity by controlling strain.


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The most common catalyst for hydrogen peroxide (H2O2) generation is palladium (Pd), while Au is a very poor catalyst.20−24 If the interatomic distance of Au is adjusted to that of Pd, will it exhibit good catalytic activity for H2O2 generation? Because the number of electrons and the energy of the orbitals in Au are different than in Pd, Au will not be the same as Pd, but the catalytic activity is still expected to change. To adjust the lattice constant of Au to that of Pd, we coated a very thin layer of Au on Pd.25 A previous study found that the lattice constant of Au matches that of platinum (Pt) when one or two atomic Au layers were coated on the surface of Pt.26 We calculated the electronic structure of a Au layer on Pd using density function theory (DFT) assuming that Au deposited on Pd has the same lattice constant as Pd. The calculated electronic structure of thin Au film on Pd was significantly different from that of pure metallic Au, and it was expected to exhibit better catalytic activity than Pd for H2O2 generation. To confirm calculations, Pd/Au core/shell nanocubes bound by {100} facets were prepared, and the interatomic distance of the thin Au shell was confirmed to be similar to Pd. As predicted, the thin Au shell deposited on Pd exhibited a high H2O2 generation product rate.


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2. EXPERIMENTAL SECTION Calculation details: All calculations were performed using spin-polarized density functional theory (DFT) calculations implemented in the Vienna Ab initio Simulation Package (VASP) with the projector-augmented wave (PAW) pseudopotential.39−41 We used the revised Perdew–Burke– Ernzerhof (RPBE) exchange function.42,43 The cut-off energy for the plane wave basis set was set to 400 eV, and k-points were sampled using 16x16x16 for bulk unit cells. Geometric optimizations were performed until the residual force on each atom was less than 0.05 eV/Å. Calculated lattice parameters for Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au were 3.55, 3.67, 3.85, 3.98, 4.20, 3.89, 3.99, and 4.20 Å, respectively, which agreed with experimental values (3.52, 3.61, 3.80, 3.89, 4.09, 3.84, 3.92, and 4.08 Å).44 Activation barriers for each surface reaction step were calculated by a climbing image nudged elastic band (CI-NEB) method, with eight intermediate images.45 M/Au (core/shell) was constructed by three layers of M (core metal) and additional Au shell layers with calculated bulk lattice parameters of M. We first relaxed the M/Au structure at the bottom, two core metal layers were fixed at the bulk position, and the other upper layers were allowed to relax. Adsorption energies on pure metal surfaces were calculated using four-layered slab models. Adsorbates and the top two layers were allowed to relax, while the bottom two layers were fixed at their optimized bulk positions. We used various supercells of different sizes for core metal screening and elementary reactions for H2O2 synthesis on surfaces. A (2x2) supercell with a 6x6x1 k-point mesh was used to calculate ∆EO and ∆EH for core metal screening.46 A larger cell of a (3x3) supercell with a 4x4x1 k-point mesh was used to calculate reaction energies and activation barriers for H2O2 synthesis on Pd(100) and Pd/3Au(100). All surface slab models include 15 Å of vacuum along the z-axis. Gas molecules (O2, H2, and H2O2) were calculated over 15 x 15 x 15 Å with gamma point sampling.


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Synthesis of Pd nanocubes: Pd nanocubes were synthesized by dissolving poly(vinyl pyrrolidone) (PVP, MW = 55,000, 105 mg, Aldrich), L-ascorbic acid (60 mg, Aldrich), and potassium bromide (KBr, 300 mg, Aldrich) in 8 mL of deionized water in a 20 mL vial (stirred with a Teflon-coated magnetic stirring bar) heated to 80 °C. Then, 3 mL of an aqueous sodium tetrachloropalladate (Na2PdCl4, 57 mg, Aldrich) solution was pipetted into the vial. The reaction mixture was heated at 80 °C in air for 3 h. The product was collected by centrifugation and washed sequentially with acetone, ethanol, and a mixture of ethanol and water. The synthesized Pd nanocubes were then re-dispersed in 8 mL of water. Synthesis of Pd/Au core/shell nanocubes: Pd/Au core/shell nanocubes were synthesized by adding 1 mL of an aqueous suspension of Pd nanocubes to 8 mL of an aqueous solution containing PVP (5 mg) and L-ascorbic acid (6 mg). The mixture was then heated to 95 °C in air while stirring. The Au shell thickness was controlled by varying the amount of chloroauric acid (HAuCl4, Aldrich), which was dissolved in 2 mL of deionized water. When 1.09 mg of HAuCl4 was used, Pd/Au core/shell nanocubes with thin Au shell thicknesses (1 nm) were synthesized. By increasing the amount of HAuCl4 to 2.45 mg and 5.30 mg, Pd/Cu core/shell nanocubes with Au shell thickness of 3 nm and 5 nm were synthesized, respectively. The HAuCl4 solution was injected into the mixture solution using a pipette. The reaction mixture was heated at 95 °C in air for 15 min and then cooled to room temperature.22 Catalyst impregnation on SiO2: Synthesized nanoparticles (Pd nanocubes and Pd/Au core/shell nanocubes) were dispersed in DI water solution containing an appropriate amount of silica gel (Sigma-Aldrich, Davisil Grade 633, pore size = 60 Å, 200-425 mesh particle size), and the mixture was stirred at room temperature for 24 h. After impregnation, the product was collected by centrifugation and re-dispersed in a 30 vol% acetic acid solution, which was stirred at room


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temperature for 8 h to remove PVP from the surface of the nanoparticles. The recovered catalysts were centrifuged and dried at 60 °C overnight. Before activity tests, the catalysts were reduced at 40 °C for more than 3 h under 50 mL/min diluted H2 gas (10 vol% = H2/N2). Structural analysis: The size and distribution of core-shell nanoparticles were observed using a FEI Talos TEM F200X microscope operating at 200 kV. The high resolution transmission electron microcopy (HRTEM) images provided information on deformation while directly measuring lattice constants of Pd, Pd/3Au(100), Pd/9Au(100), and Pd/15Au(100). However, an issue reduced precision in the ultrafine region of the atomic unit. The first step for measuring precise d-spacing by HRTEM is aligning the zone axis of the nanoparticles exactly ([001] direction in this work). However, it is difficult and inefficient to match the zone axis of nanoparticles below 10 nm in size, and in the process, the electron beam can damage the nanoparticles. Using precession nano-beam diffraction (NBD) to compensate for the disadvantages of HRTEM, the exact strain was calculated by interpreting the diffraction pattern even without the exact zone-axis. The quantitative composition of Pd and Au in the core and shell was determined using line profile curves and elemental mapping images in scanning TEM (STEM) mode working with a Super-X EDS system. X-ray diffraction (XRD) was used to analyze the crystal phase and strain of the epitaxial grown Pd/Au core/shell nanocubes, using a Bruker D8 advanced diffractometer with a Cu Kα source (λ = 1.5418 Å). The intensity of XRD spectra was normalized using a constant weight of 30 mg particles with a scan rate of 30 seconds per 0.02° step over a range of 30 < 2 < 90. Deformation at each position within the core-shell particle was measured very precisely from precession NBD strain mapping images acquired on a FEI Tecnai F20 G2 microscope operated with a Nanomegas Topspin system. A precession angle of 0.13° was used to acquire NBD patterns. Pd/Au(100) strain maps were acquired at a spot size of 1 nm and a camera length of 185 mm. The


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acquisition time of each Pd/Au(100) strain map (50 x 37 points) was 35 min, and analysis was performed using F-Strain software. Laser-assisted atomic probe tomography (APT) of Pd/Au core/shell nanocubes was performed using a CAMECA LEAP 4000X HR with a 355 nm ultraviolet laser. A laser pulse energy of 30 pJ at a base temperature of 30 K and evaporation rate of 0.2 % was used, and 3D reconstruction was done with IVAS 3.6.14 software. Sample preparation for APT measurements was conducted using a focused ion beam (FIB) Helios with an electroplated Ni plate for 200 s at a constant current of 100 mA. The resulting electrodeposited Ni film was about 3 μm thick and was suitable for the standard lift-out and annular ion milling processes using FIB. Elemental analysis was performed via inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (Shimadzu). Catalytic performance: Direct synthesis of H2O2 was performed in a double-jacket glass reactor with a reaction medium of 150 mL of DI water solution containing 0.03 M phosphoric acid (H3PO4, Sigma-Aldrich, 85 wt% in H2O, ≥ 99.99%) and 20 vol% ethyl alcohol (Sigma-Aldrich, ≥ 99.5%). The total volumetric flow rate of the reactant gas was 22 mL/min, with 2 mL/min of H2 and 20 mL/min of O2 flow (H2:O2 volume ratio = 1:10). Each catalyst (0.2 g) was then added into the reactor. The reaction took place at ambient conditions (293 K, 1 atm) for 3 h. After the reaction, H2O2 concentration was measured by iodometric titration. H2 conversion was calculated with H2 concentration data measured by gas chromatography (Younglin). H2 conversion, H2O2 selectivity, and H2O2 production rate were defined as below. The production rate of H2O2 (mmol/g-metal·h) is defined as the amount of H2O2 (mmol) generated per hour (h) per gram of metal (metal-g) and is proportional to the H2O2 yield (H2 conversion ｘ H2O2 selectivity).


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H2 conversion (%) =

H2O2 selectivity (%) =

𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝐻2 𝑟𝑒𝑎𝑐𝑡𝑒𝑑 𝑑𝑢𝑟𝑖𝑛𝑔 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒 𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝐻2 𝑠𝑢𝑝𝑝𝑙𝑖𝑒𝑑 𝑑𝑢𝑟𝑖𝑛𝑔 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒

× 100

𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝐻2𝑂2 𝑓𝑜𝑟𝑚𝑒𝑑 𝑑𝑢𝑟𝑖𝑛𝑔 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒 𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝐻2 𝑟𝑒𝑎𝑐𝑡𝑒𝑑 𝑑𝑢𝑟𝑖𝑛𝑔 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒

H2O2 production rate (mmol/𝑔 ―𝑚𝑒𝑡𝑎𝑙 ∙ ℎ) =

× 100

𝑚𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝐻2𝑂2 𝑓𝑜𝑟𝑚𝑒𝑑 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑚𝑒𝑡𝑎𝑙 𝑖𝑛 𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 × 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒

3. RESULTS AND DISCUSSION Previous studies have shown that the binding energies of oxygen (∆EO) and hydrogen (∆EH) on the surface of catalysts are key factors in determining catalytic activity and selectivity for H2O2 generation.27,28 To find the optimal M/Au (core/shell) catalyst for H2O2 generation, we first calculated ∆EO and ∆EH adsorption energies of the Au shell catalysts with different core metals using DFT. This calculation assumed that the lattice constant of Au was the same as that of the core metal (Figure S1). The reaction network for H2O2 generation indicates that enhanced H2 dissociation and suppressed O-O bond dissociation are important in improving catalytic performance (Figure 1a). More negative ∆EH values can facilitate H2 dissociation and provide surface H adatom (*H), which is important reactant for H2O2 generation. However, more negative ∆EO values can boost


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O-O bond dissociation, leading to side reactions (e.g. surface oxides and H2O formation). For example, Pd is known to exhibit a high catalytic activity but a low selectivity for H2O2 generation due to very negative ∆EO and ∆EH values (Figure 1b). Contrary to Pd, Au has good selectivity but low catalytic activity due to weak *O and *H binding. To obtain both higher catalytic activity and high selectivity than those of Au, a ∆EO more positive than Au(111) and a ∆EH more negative than Au(111) would be required (Figure 1b). Transition metals (Rh, Pd, Ag, Ir, and Pt) with the same face-centered cubic (fcc) crystal structures as Au were considered for use as core metals to enhance the activity of Au while maintaining selectivity. Several M/Au catalysts were identified as promising core/shell materials for H2O2 generation (Figure 1b, Figure 1c, and Table S1). The (100) surface of M/Au with Pd, Pt, Ag, and Ir core metals showed the desired behavior, with more positive ∆EO values and more negative ∆EH values compared to Au(111). Monolayers of Au on Pt (Pt/1Au) and Ir (Ir/1Au), as well as three layers of Au on Pd (Pd/3Au), gave especially promising results. We should note that the (111) surfaces of the same M/Au (M = Pt, Ir, Pd) materials exhibited more positive ∆EO and ∆EH values than Au(111), indicating poor catalytic activity. Among these potential core materials, we


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chose Pd/Au core/shell nanocubes bound by (100) planes for further testing due to their ease of synthesis. To understand the origin of increased activity and selectivity of Au coated on a Pd core, we investigated ligand and strain effects by comparing ∆EO and ∆EH values for four cases: Pd/3Au(100), Pd(100), Au(100), and Au(100) with a constant Pd lattice (Au(100) compressed) (Figure 1d). Compared with Au(100), Pd/3Au(100) had a more positive ∆EO value, indicating better selectivity. Because of the small size of the Pd atom, we believe that strain from the Pd core decreased the *O binding strength on the Au shell and increased H2O2 selectivity. The ∆EO value of Au(100) when compressed is 0.18 eV larger than that of Au(100), supporting the hypothesis that strain from the Pd core increases H2O2 selectivity of Pd/3Au(100). The ∆EH values for Pd/3Au(100) and Au(100) were similar, demonstrating that the strain effect for *H binding was small. In addition to strain, the presence of Pd in the Pd/Au core/shell nanocubes could affect binding because the electron structure of Pd might alter ∆EO and ∆EH. However, differences in ∆EO and ∆EH values between Pd/3Au(100) and Au(100) compressed were small (< 0.1 eV), indicating that strain was more important than ligand effects. Strain induced by the Pd core


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decreased *O binding, resulting in high H2O2 productivity of Pd/3Au(100) by increasing selectivity. We also found that the Pd core changes the electronic structure of Au shell in Pd/3Au(100). The density of states (DOS) at the top layer of Pd/3Au(100) is clearly different from that of Au(100), especially at -5 eV < E – Ef < -1 eV (Figure S2) Thus, we expect that the Pd/3Au(100) can show different binding strength for adsorbents and catalytic activity compared to those of pure Au(100). H2O2 generation consists of a chain of reactions (Figure 1a). We calculated the reaction energies and activation barriers for each of the primary and side reaction steps of H2O2 synthesis on Pd(100) and Pd/3Au(100) (Figure 1e, Figure 1f, and Table S2). Activation barriers for the side reactions on Pd (*O2, *OOH, and *H2O2 dissociation) were lower than those for the primary reactions, indicating that a low H2O2 selectivity was expected on Pd nanocubes due to facile dissociation of O-O bonds on Pd(100) (Figure 1e). Contrary to Pd(100), Pd/3Au(100) exhibited lower activation barriers for the primary reactions than the undesired side reactions (Figure 1f). Activation barriers for dissociation of *O2 (0.70 eV), *OOH (0.56 eV), and *H2O2 (0.64 eV) on Pd/3Au(100) were significantly higher than on Pd(100) (Table S2, 0.12, 0.01, and 0.11 eV, respectively). In addition, activation


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barriers for hydrogenation of *O2 (0.24 eV) and *OOH (0.18 eV) on Pd/3Au(100) were significantly lower than on Pd(100) (0.98 and 0.77 eV, respectively). These calculations demonstrated that Pd/3Au(100) has higher selectivity for H2O2 generation. The activation barrier of the rate-determining step (RDS) for Pd/3Au(100) was 1.04 eV for H2 dissociation, and the activation barrier of the RDS for Pd(100) was 0.98 eV for hydrogenation of *O2, indicating that Pd/3Au(100) should have a lower reaction rate than Pd(100). In spite of the high RDS activation barrier, H2O2 productivity by Pd/3Au(100) was expected to be higher than that of Pd(100) due to the higher activation barriers for side reactions on Pd/3Au(100) than on Pd(100). This is illustrated in Figure 1a, which shows that side reactions can dissociate reaction intermediates for primary reactions, decreasing H2O2 productivity. Thus, in addition to lowering the activation barrier for the primary reaction (catalytic activity), increasing the activation barriers for side reactions (H2O2 selectivity) is also important for improvement of H2O2 production.29 Pd and Pd/Au core/shell nanoparticles were prepared by modifying a previously published method.25,30 Transmission electron microscope (TEM) images of samples showed the formation of Pd cubes and Pd/Au core/shell nanocubes with Au shell


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thicknesses of 1 nm and truncated cubic Pd/Au core/shell nanoparticles with Au shell thickness of 3 and 5 nm, respectively (Figure 2). Since one atomic layer of Au on Pd corresponds to a Au shell thickness of around 0.33 nm, Pd/Au core/shell nanocubes with thickness of 1, 3, and 5 nm were designated Pd/3Au(100), Pd/9Au(100), and Pd/15Au(100), respectively. The aim of this study was to investigate the change in catalytic activity of strained Au shells. Therefore, it was very important to confirm that Pd was not exposed, and that the Au shell completely coated the surface of Pd/3Au(100). Atomic distributions in Pd/Au particles were measured by 3-dimensional atomic probe tomography (3D APT) (Figure S3). Pd and Au were intermixed at the interface, but were located at the inside and shell of the cube, respectively. Pd/3Au(100) therefore had a perfect core/shell structure surrounded by a thin Au shell. Pd nanocubes were around 10 nm and were perfectly bound by a (100) plane of fcc Pd (Figure 3a). High-resolution TEM (HRTEM) images and energy dispersive spectrometry (EDS) mapping data of Pd/3Au(100) nanocubes showed epitaxial overgrowth of 3-4 atomic Au layers on the surface of Pd nanocubes (Figure 3b and S4a-e). When the Au layer was more than 9 layers (3 nm), corners and edges of cubes were truncated, and


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the exposure ratio of (110) and (111) surface increased (Figure 3c and S4f-j).31 When the Au layer was heavily loaded, the driving force for lowering the surface energy was activated, and the morphology of core/shell nanocubes was transformed into polyhedrons with a large portion of (110) and (111) facets, while maintaining the morphology of Pd nanocubes in the core (Figure 3d and Figure S4).25 The Au lattice constant on the surface of Pd/3Au(100) was the most important concern of the present study, as it characterized catalytic activity based on engineering the strain. The lattice constants of Pd and Au in the (100) plane were 3.89 and 4.08 Å, respectively.32 X-ray diffraction (XRD) measured the average strain of the lattice constant of Pd/3Au(100). XRD spectra showed that diffraction peaks were shifted slightly to the left compared to Pd cubes, and no Au metal peak was observed (Figure S5). The Au shell epitaxial therefore grew on the surface of Pd cubes, decreasing the lattice constant of Au compared with pure Au.11 In addition, Pd/3Au(100) exhibited asymmetric Gaussian peaks, while pure Pd and Au had symmetric Gaussian peaks, meaning that the lattice constant of the Au shell gradually increased with distance from the Pd core.14,33 As the Au layer became thicker, the distance between the Au atoms returned to its original value, and


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deformation decreased (Figure 3i). HRTEM analysis and TOP SPIN using nano-beam electron diffraction (NBD, 1 nm diameter spot size) yielded more precise lattice strain measurements on the surface of Pd/3Au(100).34−36 Figure 3, e to h show the strain map from NBD in a specific direction on core/shell nanoparticles. NBD strain mapping was performed on the [001] zone axis to confirm the surface strain of the Pd/3Au(100) surface exposed to the (100) plane. Since the strain reference was set to Pd, and the difference between the Pd and Au lattice constants is 4.8%, the shell with the Au lattice should have a strain of 4% to 5%. Pd/3Au(100) was cubic, so a strain map of the (001) zone axis was measured to determine the strain of the exposed (100) plane (Figure 3e). The results confirmed that the Au lattice was present with the compressive strain in the x or z directions on the surface of Pd cube ((010) plane), but strain was not observed in the y direction ([010] strain). The same strain distribution was also observed on the (010) plane of the Pd/3Au(100) (Figure 3f), indicating that the Au lattice of Pd/3Au(100) had a tetragonal structure (4.08 Å) in the y direction and a Pd lattice (3.89 Å) in the x direction, not a cubic structure. In Pd/9Au(100) and Pd/15Au(100), strain maps were obtained at the (011) zone axis because the (111) and (110) planes were exposed as the Au shell


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layer became thicker (Figure 3g and 3h). As the shell thickness increased, the decreased lattice constant that was similar to that of Pd in the x direction returned to the value of the original Au lattice.13,37 The Pd/Au core/shell nanocubes had asymmetric d-spacing on the Pd/Au {100} interface due to mismatch of the lattice constant between Pd and Au, resulting in maximum strain of εx = -4% and εy = 0% at the interface (Figure 3i). We therefore concluded that Au in the shell of Pd/3Au(100) had the same lattice constant as Pd in the region parallel to the cube plane, as calculations indicated. The H2O2 generation reaction, which is a direct synthesis of H2O2 from a mixture of hydrogen (H2) and oxygen (O2) gases in the absence of halide additives, was used to confirm the catalytic activities of Pd/3Au(100) (Figure S6).38 Pd/3Au(100), Pd/9Au(100), and Pd nanocubes were first immobilized on silica particles. Nanoparticles were welldispersed on the surface of the silica particles and maintained their morphology after immobilization (Figure S7, a to c). TEM showed that tested catalyst nanocubes maintained their size and morphology after the reaction, indicating their high stability (Figure S7, d to f). H2O2 production rate was calculated as the amount of H2O2 (mmol) generated per hour (h) per gram of metal (metal-g), which is proportional to the H2O2 yield


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(H2 conversion  H2O2 selectivity).40 The production rate of Pd/3Au(100) was 246 mmol/g-metal·h, which was around 2 times faster than that of Pd nanocubes (123 mmol/gmetal·h,

Figure 4a). When Pd/9Au(100) was used as a catalyst, the production rate (100

mmol/g-metal·h) was less than that of Pd nanocubes. H2 conversion was 18.2 % for Pd nanocubes, 15.9 % for Pd/3Au(100), and 7.56 % for Pd/9Au(100), respectively, indicating that catalytic activity decreased as Au shell size increased (Figure 4b and Table S3). Although H2 conversion was highest (18.2%) with Pd nanocubes, H2O2 selectivity was low (26.2%), resulting in a lower production rate. In contrast, Pd/3Au(100) had the highest production rate due to its high H2O2 selectivity (59.7%). As mentioned above, the high selectivity of the Pd/3Au(100) catalyst for H2O2 generation agreed with predictions based on DFT. H2O2 selectivity is determined by the difference in energy levels between the reaction pathway that maintains O–O bonds in *O2 and intermediate (*OOH) and the reaction pathway in which O–O bonds break. DFT calculations of hydrogenation energies of *O2 (0.24 eV, Figure 1f, TS1) and *OOH (0.18 eV, Figure 1f, TS2) compared with dissociation energies of *O2 (0.7 eV Figure 1f TS1’) and *OOH (0.56 eV, Figure 1f, TS2’) indicated that Pd/3Au(100) favors hydrogenation


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over O2 dissociation, while Pd(100) favors O2 dissociation over hydrogenation. Pd/9Au(100) exhibited high H2O2 selectivity, but H2O2 productivity decreased (Figure 4a). Pd/9Au(100) had a large proportion of (111) planes, and the change in catalytic performance might be associated with the Pd/Au(111) surface. Both ∆EO and ∆EH on Pd/3Au(111) were positive than those on Au(111) (Figure 1c), decreasing H2 conversion and thereby leading to lower production rates.

4. CONCLUSION Most of the previous researches that utilize lattice strain in catalysis field have been mainly to further improve existing catalysts by alloying with inexpensive alternatives, rather than creating a new catalytic activity of an otherwise inactive material. In this study, we demonstrated that, by combining electronic structure calculations for prescreening of possible candidates, novel synthesis, detailed characterizations, and catalytic activity measurements, it is possible to catalytically activate Au towards the production of H2O2. Although it was not possible to directly compare calculated values due to challenges associated with calculating core/shell nanoparticles with thick shells and experimental


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limitations, it is meaningful to discover that the change of electronic structure due to strain may affect the actual catalytic activity. Future work will focus on better explaining experimental results through theoretical calculations.


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Figure 1. Calculation of Pd/Au core/shell nanocubes for H2O2 synthesis. (a) Schematic illustration of elementary reaction steps for H2O2 synthesis from H2 and O2. (b and c) Binding energies of H and O on the (100) surface of M/Au (b) and the (111) surface of M/Au (c). For all M/Au, 1-3 layers of Au were added on the core metal (M/1-3Au). Cu/Au and Ni/Au are not shown because these two core/shell nanocubes were unstable due to large lattice constant mismatch. (d) ∆EO and ∆EH on Pd(100), Au(100), Pd/3Au(100), and Au(100) compressed to a Pd lattice in the xy-direction. (e and f) Energy diagram for H2O2 synthesis on (e) Pd(100) and (f) Pd/3Au(100). Black and red lines indicate reaction


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energies and intermediates of primary reactions and side reactions, respectively. The reference energies of ∆EO and ∆EH are O2(g) and H2(g) energies, respectively.


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Figure 2. Characterization of Pd and Pd/Au core/shell nanocubes. TEM images of (a) Pd and (b to d) Pd/Au core/shell nanocubes. The Au shell thicknesses of Pd/Au core/shell nanocubes were (b) 1 nm, (c) 3 nm, and (d) 5 nm.


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Figure 3. Characterization of Pd/Au core/shell nanocubes with different shell layers. HRTEM images of (a) Pd nanocube, (b) Pd/3Au(100) nanocube, (c) Pd/9Au(100) nanocube, and (d) Pd/15Au nanocube. (e to h) Precession nano-beam diffraction strain map showing the compressive strain along the Au surface. Precession NBD strain map of (e) Pd/3Au(100) nanocube at the [001] zone-axis, (f) Pd/3Au(001) nanocube, (g) Pd/9Au(100) nanocube, and (h) Pd/15Au(100) nanocube at the [011] zone-axis. (i) Schematic distribution image of Au shell layers.


25


Figure

4.

Catalytic

performance

of

Pd(100)/SiO2,

Page 26 of 39

Pd/3Au(100)/SiO2,

and

Pd/9Au(100)/SiO2 for H2O2 synthesis. (a) H2O2 production rate and (b) H2 conversion and H2O2 selectivity


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ASSOCIATED CONTENT

Supporting Information.

Pd/3Au(100) model (Figure S1), d-DOS and d-band center (Figure S2), Atom probe tomograph (ATP) and line profile (Figure S3), HAADF-STEM images (Figure S4), XRD patterns (Figure S5), Schematic of the reaction system for H2O2 synthesis (Figure S6), TEM images of catalysts immobilized on silica (Figure S7), ∆EO and ∆EH on (100) and (111) surfaces of Pd, Au, Au (compressed), M/1Au, M/2Au, and M/3Au (Table S1), Reaction energies (∆E, eV) and activation barrier (∆Ea, eV) for the elementary steps in H2O2 synthesis on Pd/3Au and Pd(100) (Table S2), Mass fraction of nanoparticle-impregnated SiO2 catalysts and catalytic activity of H2O2 synthesis (Table S3) (PDF)

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected], Jae-Pyoung Ahn

*E-mail: [email protected], Yousung Jung


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*E-mail: [email protected], Kwan-Young Lee

*E-mail: [email protected], Taekyung Yu

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ∇I.K., M.S. and C.C. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

T.Y. acknowledges financial support by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, & Future Planning (2014R1A5A1009799). We acknowledge financial support from the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (NRF-2016M3D1A1021140).

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Table of Contents Graphic


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Studies on catalytic activity of hydrogen peroxide generation

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