Highly Active, Selective and Stable Direct H2O2 Generation by

Subscriber access provided by University of Massachusetts Amherst Libraries

Energy, Environmental, and Catalysis Applications

Highly Active, Selective and Stable Direct H2O2 Generation by Monodispersive Pd-Ag Nanoalloy Jin Zhang, Bolong Huang, Qi Shao, and Xiaoqing Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03756 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19 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

ACS Applied Materials & Interfaces

Highly Active, Selective and Stable Direct H2O2 Generation by Monodispersive Pd-Ag Nanoalloy Jin Zhang,† Bolong Huang,*# Qi Shao,† and Xiaoqing Huang*†

†

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu, 215123, China.

#

Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China.

Abstract: Hydrogen peroxide (H2O2), a green oxidant, has wide applications in various chemical syntheses, and is also a promising candidate to replace the traditional toxic oxidants. The direct synthesis of H2O2 from H2 and O2 is a potential approach as it is a green and atomically economic reaction. However, the most previous systems are notorious in complicated post-purification procedure, high energy cost as well as low selectivity due to the uncontrollable O-O bond cleavage. We have solved this challenge by tuning the chemical state of Pd with high H2O2 productivity of 80.4 mol kgcat-1 h-1 and high H2O2 selectivity of 82.1% via the design of Pd-Ag nanoalloy with flexibly tuned size and composition. The created Pd-Ag nanoalloy also exhibits excellent stability with limited performance decay over recycles. The X-ray photoelectron spectroscopy analysis confirms that the electron transfer from Ag to Pd, which generates more Pd0 and enables improved H2O2 productivity. The theoretical calculation shows that the incorporation of Ag into Pd is beneficial for the stabilization of O22- and the cleavage of H2 for the enhanced H2O2 generation. In addition, the enhanced H2O2 desorption on Pd-Ag nanoalloy is beneficial for releasing H2O2, which results in the increased H2O2 selectivity. Keywords: Bimetallic, Palladium, Silver, Electron Transfer, Hydrogen Peroxide

ACS Paragon Plus Environment

1

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

Page 2 of 19

INTRODUCTION Hydrogen peroxide (H2O2), listed as one of one hundred most important chemicals in the world, has drawn much research attention due to its high energy efficiency and the feature of clean.1-3 Until now, many synthesis processes of H2O2 have been investigated, such as: anthraquinone autoxidation,4,5 electrochemical synthesis6,7 and photocatalytic synthesis.8-10 While the anthraquinone process is the major industrial H2O2 production process, due to its safety and easy to continuous running, the energy waste is brought by the purification of the reaction substrate and the expensive operating cost, which become the major challenge to the large-scale application.11-13 Due to the distinct advantages of atom economy, lower energy consumption and only by-product of H2O, the direct synthesis of H2O2 has become a promising process to the production of H2O2.2,14,15 The formidable challenge of the direct H2O2 synthesis is to realize high H2O2 selectivity and prevent H2O formation from the excessive H2O2 hydrogenation simultaneously.16,17 Previous studies have indicated that Pd-based materials are the most active catalysts to the direct H2O2 synthesis.15,18-20 To further improve the H2O2 selectivity, a number of Pd-based catalysts have been investigated by adding extra acid and halide promoters, which however largely increased the difficulty to the purification of subsequent products.13,21 Another strategy is to improve performance by ligand modification of the catalysts, but the selectivity can be rapidly decreased due to the leaching of the organic ligand.15,22 In fact, several previous studies have shown that the introduction of the second metal to Pd-based catalyst would be a promising strategy to enhance the direct H2O2 synthesis.23-25 Considering that the oxidizable Ag can transfer electron to Pd to maintain the Pd0 species as the active sites, the well preserved the O-O bond guarantees the direct H2O2 synthesis changing from (O-O)0 (oxygen molecule) to (O-O)2- (peroxide ion) via migrating these charges from Pd0 to (O-O)* (the intermediate of oxygen


2

Page 3 of 19 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


adsorbed on the surface of the catalyst). Therefore, the bimetallic Pd-Ag nanoalloy can be a promising candidate for the direct H2O2 synthesis.26,27 Herein, we synthesized a class of monodisperse PdAg nanoalloys with high activity and favorable selectivity towards the direct H2O2 synthesis. With tuning the amounts of metal precursor and dodecyltrimethylammonium chloride (DTAC), three different components of Pd-Ag alloy NPs (denoted as: PdAg, Pd1.5Ag, and PdAg1.5, respectively) and the other three different sizes of Pd-Ag alloy NPs (the NPs sizes from small to medium and large were denoted as PdAg-S, PdAg-M, PdAg-L, respectively) were obtained. By tuning the catalytic parameters, the optimized PdAg-M NPs exhibited high H2O2 productivity of 80.4 mol kgcat-1 h-1, low H2O2 hydrogenation activity of 30.0 mol kgcat-1 h-1 and desirable selectivity of 82.1%, much better than those of Ag NPs, Pd NPs as well as the commercial Pd/C. The PdAg-M NPs also exhibited high stability with an acceptable productivity and selectivity degradations after six rounds. The theoretical calculation demonstrated that the PdAg alloy surface had the charge transfer from Ag to Pd and showed more energetically preference for the H2O2 synthesis than Pd surface, in line with the X-ray photoelectron spectroscopy (XPS) results. In addition, the enhanced H2O2 desorption on the PdAg alloy surface is beneficial for releasing the H2O2 molecules from the surface, which results in the increased H2O2 selectivity.

EXPERIMENTAL SECTION Chemicals. Silver (I) acetate (AgAc, 99%), palladium (II) acetylacetonate (Pd(acac)2, 97%), dodecyltrimethylammonium

chloride

(C12H25(CH3)3NCl,

DTAC,

>99.0%),

and

oleylamine

(CH3(CH2)7CH=CH(CH2)7CH2NH2, 70%) were all purchased from Sigma-Aldrich. Benzoin (C14H12O2, 98%) was purchased from Acros Organics (Taiwan, China). All the chemicals were used as received without further purification. The water (18 MΩ * cm) used in all experiments was prepared by passing


3


Page 4 of 19

through an ultra-pure purification system. Preparation of Pd-Ag nanoparticles (NPs). In a typical preparation of 8 nm PdAg NPs, 4.3 mg AgAc, 7.9 mg Pd(acac)2, 13 mg DTAC, 40 mg benzoin and 5 mL oleylamine were added into a 30 mL vial. After the vial had been capped, the mixture was ultrasonicated for around 10 min. The resulting homogeneous mixture was then heated at 140 ºC for 5 h in an oil bath, before it was cooled to room temperature. The resulting colloidal products were collected by centrifugation and washed three times with an ethanol/cyclohexane mixture. 4.5 nm PdAg NPs (PdAg-S) and 10 nm PdAg NPs (PdAg-L) were synthesized by changing the amount of DTAC form 13 mg to 0 mg and 39 mg, respectively. The preparations of Pd1.5Ag NPs and PdAg1.5 NPs were achieved by changing the amount of Pd(acac)2 from 7.9 mg to 11.8 mg and that of AgAc from 4.3 mg to 6.4 mg, respectively, while keeping the other parameters the same. Preparation of supported PdAg-M NPs. PdAg-M NPs and different supports (ZrO2 (Sinopharm Chemical Reagent Co. Ltd.), CeO2 (Inoke), Al2O3 (Alfa Aesar), TiO2 (Degussa) or SiO2 (Degussa)) were mixed in 5 mL chloroform and stirred for 3 h to load the PdAg-M NPs on different supports. The products were separated by centrifugation and washed with acetone for three times. The supported PdAg-M NPs were subjected to thermal annealing in 5% H2/N2 atmosphere at 200 ºC for 1 h. The resulting products were denoted as PdAg-M NPs/ZrO2, PdAg-M NPs/CeO2, PdAg-M NPs/Al2O3, PdAgM NPs/TiO2, and PdAg-M NPs/SiO2, respectively. Characterizations. TEM and HAADF-STEM were conducted on an FEI Tecnai F20 transmission electron microscope with acceleration voltage of 200 kV. The samples were prepared by dropping cyclohexane or ethanol dispersion of samples onto carbon-coated copper TEM grids using pipettes and dried under ambient condition. XRD patterns were collected on X’Pert-Pro MPD diffractometer (Netherlands PANalytical) with a Cu Kα X-ray source (λ = 1.540598 Å). Energy dispersive X-ray spectroscopy (EDS) was performed on a scanning electron microscope (Hitachi, S-4700). XPS was done


4

Page 5 of 19 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


with an SSI S-Probe XPS Spectrometer. The carbon peak at 284.6 eV was used as a reference to correct for charging effects. Catalytic tests. The direct synthesis of hydrogen peroxide (H2O2) was performed using a stainless steel autoclave with a volume of 60 mL and a maximum working pressure of 10 MPa. In the typical experiment, the catalyst (5 mg) was placed in the Teflon liner together with 10 mL solvent (7 mL methanol and 3 mL deionized water). After three purging steps with O2, the autoclave was pressurized with H2 (3.6 Mpa, 5 vol% H2/N2) and O2 (0.4 MPa). The reaction was performed at 0 ºC with magnetic stirring (1000 rpm) for 30 min. The H2O2 productivity was determined by titrating aliquots of the final reaction solution with acidified Ce(SO4)2 (0.01 M) in the presence of two drops of ferroin indicator. The productivities are reported as mol H2O2 kgcat-1 h-1. The Ce(SO4)2 solutions were standardized against (NH4)2Fe(SO4)2·6H2O using ferroin as indicator. Gas analysis was performed by gas chromatography (Shiweipx GC-7806) equipped with a GDX-502 column connected to a thermal conductivity detector. Conversion of H2 was calculated by gas analysis before and after reaction. H2O2 selectivity was calculated by the moles of H2O2 formed to the moles of H2 consumed. H2O2 hydrogenation tests were carried out as outlined above, expect for in the absence of 0.4 MPa O2 in the gas stream and with the presence of H2O2 in the solvent (solution composition: 7 mL methanol, 2 mL deionized water and 1 mL hydrogen peroxide ).

RESULTS AND DISCUSSION The PdAg alloy NPs were synthesized through a simple wet-chemical method, where silver (I) acetate (AgAc), palladium (II) acetylacetonate (Pd(acac)2), benzoin, DTAC and oleylamine (OAm) were added into a vial and immerged in an oil bath of 140 ºC for 5 h (see the supporting information for details). The high-angle annular dark-field scanning transmission electron microscopy images (HAADF-


5


Page 6 of 19

STEM, Figure 1a,b) showed that the obtained NPs had high uniformity and excellent dispersibility. The Pd/Ag atomic ratio was determined to be 49.2/50.8, as confirmed by scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDS, Figure 1c), which fairly closed the feeding ratio of metal precursors. The X-ray diffraction (XRD) pattern of PdAg-M NPs (Figure 1d) had a set of peaks at around 38.87º, 45.93º, 66.18º, and 79.15º, respectively, which were between the peaks of Ag (JCPDS no.04-0783) and Pd (JCPDS no.46-1043), indicating that the PdAg-M NPs had alloy phase.28,29 The high resolution TEM (HRTEM, Figure 1e) of the PdAg-M NPs was further carried out, where the 0.230 nm spacing between the d-spacing of Ag (111) crystal plane (0.236 nm) and the d-spacing of Pd (111) crystal plane (0.225 nm) was observed. The alloy feature of the obtained PdAg NPs was further demonstrated by STEM line scan analysis (Figure 1f) and HAADF-STEM image and corresponding element mappings analysis (Figure 1g), where the elements of Pd and Ag were uniformly distributed in the NPs.


6

Page 7 of 19 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


Figure 1. Morphological and structural analyses for PdAg-M NPs. Representative (a, b) HAADF-STEM images, (c) SEM-EDS spectrum, (d) XRD pattern, and (e) HRTEM image of the PdAg-M NPs. (f) STEM line scan analysis, and (g) HAADF-STEM image and corresponding element mappings of PdAg-M NPs.


7


Page 8 of 19

We have also obtained different sizes PdAg NPs by simply tuning the amounts of DTAC supplied. As shown in Figure S1, 4 ± 1 nm (PdAg-S NPs), 8 ± 1 nm (PdAg-M NPs), and 10 ± 1 nm (PdAg-L NPs) PdAg NPs were obtained when the dosages of DTAC were 0 mg, 13 mg, and 39 mg, respectively. XRD patterns and EDS patterns proved that these Pd-Ag NPs with different sizes maintained alloy phases (Figures S2&S3). As shown in Figure S4, HRTEM images clearly indicated the PdAg alloyed characteristic. In addition, with using different feeding ratios of the metal precursor, the regulation of component ratios to PdAg-M alloy NPs (PdAg-M NPs, Pd1.5Ag-M NPs, PdAg1.5-M NPs) can be realized. Corresponding characterizations were shown in Figures S5&S6, where EDS patterns indicated that both the component ratios of PdAg1.5-M NPs and Pd1.5Ag-M NPs were nearly close to the feeding ratios of metal precursors (Figure S5c,d). XRD patterns and HRTEM images proved that both PdAg1.5-M NPs and Pd1.5Ag-M NPs had alloy phase (Figures S5e&S6). Hence, we successfully obtained Pd-Ag alloy NPs with three different sizes and Pd-Ag alloy NPs with three different component ratios by simply regulating the amounts of DTAC and metal precursors introduced. To study the catalytic performance of the obtained Pd-Ag alloy NPs, the direct H2O2 synthesis was performed. The impact of supports on this reaction was initially investigated. To this end, we loaded catalysts on different supports, such as TiO2, Al2O3, SiO2, CeO2, and ZrO2, respectively (Figures S7&S8).30 Catalyst loaded on TiO2 revealed the best catalytic performance compared with those loaded on other supports. We further compared the catalytic performances of Pd-Ag alloy NPs with different sizes and different component ratios (Figures S9&S10). We found that the PdAg-M NPs had both the highest productivity (80.4 mol kgcat-1 h-1) and the lowest H2O2 hydrogenation activity (30.0 mol kgcat-1 h1

), while other four catalysts had inferior performances (Figure 2a,b&Table S1). Moreover, the PdAg-

M NPs also showed the highest selectivity to the direct H2O2 synthesis (82.1%), which was far better than other catalysts. We further investigated the impact from the loading amount (Figure 2c). As the Pd


8

Page 9 of 19 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


loading increasing from 1 wt% to 5 wt%, the productivity changed from 38.8 mol kgcat-1 h-1 to 80.4 mol kgcat-1 h-1 and the H2O2 hydrogenation activity changed from 225.5 mol kgcat-1 h-1 to 30.0 mol kgcat-1 h-1. Through the above study, we can conclude that the 5 wt% PdAg-M alloy NPs showed the best catalytic performance among all the catalysts investigated.

Figure 2. The direct synthesis H2O2 performance of the Pd-Ag NPs catalysts. The catalytic performances of (a) PdAg NPs with different sizes, (b) PdAg-M NPs with different component ratios, and (c) PdAg-M NPs/TiO2 with different loadings.

For further comparisons, we used the commercial Pd/C, Ag NPs, and Pd NPs as catalysts to perform the direct H2O2 synthesis reaction while kept the other parameters constant (Figure S11). We found that the commercial Pd/C and Ag NPs had rather poor catalytic performances (Table S1). The Pd NPs exhibited a relatively high productivity (66.2 mol kgcat-1 h-1), while the H2O2 selectivity of the Pd NPs was quite low (31.27%). Therefore, the PdAg-M alloy NPs showed much better catalytic activity than the commercial Pd/C, Ag NPs as well as Pd NPs. Finally, the stability of the PdAg-M alloy NPs was also investigated by recycling this reaction. After six rounds, the productivity can largely keep as high as 61.6 mol kgcat-1 h-1 (76.6% of the initial value). The H2O2 selectivity still kept as high as 71.7% (87.4% of the initial value) (Figure 3a). The PdAg-M alloy NPs showed excellent stability with an acceptable


9


Page 10 of 19

activity degradation after six rounds. TEM image shows that the catalysts can largely maintain their structure with the similar ratio of Pd/Ag (45.6/54.4) by the SEM-EDS analysis after six rounds (Figure 3b&Figure S12). Therefore, the PdAg-M alloy NPs showed both excellent activity and outstanding stability for the direct H2O2 synthesis.

Figure 3. The stability test for direct H2O2 generation. (a) The stability result of the PdAg-M for the direct H2O2 synthesis after six rounds. (b) TEM image of the PdAg-M NPs/TiO2 after six rounds.

To better understand the origin for the excellent catalytic performance of Pd-Ag NPs, XPS analysis was performed to investigate the valence states and electron transfer of the Pd-Ag NPs. Pd NPs and Ag NPs were also investigated for comparisons. As shown in Figure 4a, the Ag 3d binding energies of these five Pd-Ag alloy NPs were different compared with pure Ag NPs (Figure S13a). The binding energies of Ag 3d of these five PdAg alloy NPs were approximately 367.0 eV and 373.1 eV (corresponding to Ag+), while the binding energies of Ag 3d of pure Ag NPs were 367.5 eV and 373.6 eV (corresponding to Ag0),31 which fully indicated that electron had transferred from Ag to Pd in the PdAg alloy. More


10

Page 11 of 19 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


interesting, we found that the PdAg-M alloy NPs had a slightly decreased binding energy of Ag+ 3d (366.7 eV and 372.7 eV) compared with the other PdAg alloy NPs, suggesting the higher tendency of electron transfer (Figure 4c). In addition, we also explore the Pd/Ag ratios of NPs surface by XPS analysis. As shown in Table S2, the surface of PdAg-M NPs had the largest Ag/Pd ratio compared to other PdAg NPs except PdAg1.5 NPs, indicating the largest number of electrons transfer from Ag to Pd. This leads to the PdAg-M alloy NPs had the largest ratio of Pd0/Pd2+ (83.15/16.85, Figure 4b&4d), where the most Pd0 could ensure the best catalytic activity, since Pd0 was identified as the major active site.12,26

Figure 4. XPS analysis. XPS curves of (a) Ag 3d and (b) Pd 3d of different PdAg NPs. (c) The value of Ag 3d binding energy and (d) the ratio of Pd0/Pd2+ of different PdAg NPs.


11


Page 12 of 19

To support our experimental results, the theoretical calculation was further performed. We initially built fcc-PdAg with replacement of 50% of occupied Pd site by Ag within unit cell of fcc-Pd. The band structure showed the paths along (Z, A, M, Γ, Z, R, X, and Γ) in Figure 5a. The full path from A→M exhibited an evident forbidden band gap for transitions from occupied valence orbitals to the conducting states, which is the parallel direction along the nearest Ag→Ag. The other directions cross the Fermi level (EF) including the path along Ag→Pd. This indicated the charge transfer prefers Ag→Pd→Ag to Ag→Ag. The result was in accordance with the XPS analysis results. In this system, Pd site on the surface PdAg alloy can be more active than Ag because the Pd-4d orbital level was higher than the one of Ag-4d, closer to the EF (Figure 5b). The 4d-orbital real-space distribution on the surfaces within the energy level around EF (0 eV) (Figure 5c) implies the bonding and anti-bonding 4d-orbitals PdAg (100) were more directional than the ones seen in Pd (100). This contrast demonstrated the PdAg surface can have more selectivity than Pd surface.


12

Page 13 of 19 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


a

A

R M

X

Z

G

b

PdAg(100)

c

Ag Pd

Pd(100)

d

e

+ 2+

Figure 5. Theoretical calculation of PdAg NPs catalysts for the direct H2O2 synthesis. (a) The electronic band structure and total density of states (TDOS) of the primitive cell of fcc-PdAg model. The inserted figure is the reciprocal space of the fcc-PdAg. (b) The partial density of states (PDOS) of fcc-PdAg regarding the orbital level difference between Pd-4d and Ag-4d. (c) The 4d orbital real-spatial distribution on the PdAg (100) and Pd (100) with bonding and anti-bonding energy levels near EF (0 eV). (d) The energy profile of the interpreted reaction regarding the reaction free energy (∆G) simulated on the (100) surface of fcc-PdAg, formation energies of H2O2 on PdAg (100) and Pd (100), and the chemisorption energies of H2O2 on the PdAg (100) and Pd (100). (e) The local structures and bonding variations of H2, O2, and H2O2 in the simulation for interpreting the direct synthesis reaction (Pd=dark green, Ag=light blue, O=red, and H=white).

In general, the energetic interval between the reacting state and the thermoneutral line (∆G=0 eV) moderately determines the abilities of absorption and desorption of species molecules. The deeper means the stronger absorption but weakens desorption, vice versa. As illustrated in Figure 5d, the direct H2O2


13


Page 14 of 19

synthesis on the PdAg (∆G=-0.068 eV) won compared with the other system like Pd (∆G=-3.30 eV). This arises because of the energetic contrast for the formation energy between the surfaces and the H2O2 molecule. The PdAg (100) surface shows much easier to continuously yield the H2O2. On the other hand, there may further lead to a decomposition of H2O2 on Pd surface. We found that the Pd-Ag local squared lattice on PdAg (100) exhibited much higher activity than the Pd (100) surface (Figure 5e). Such squared local lattice could be treated as unit area to actively adsorb the H2 and O2 simultaneously, and further stabilized the O2 in the diatomic molecular form. The most stable configuration for O2 location on the PdAg (100) was the bridge-bonding with Pd and Ag for each O with the O-O bond along the diagonal line of Pd-Ag local square lattice. However, this configuration was rather unstable for H2. The H-H would be separated apart and individually formed Pd-H-Ag bonding at the bridge-site between PdAg bonding. The most stable location for the H was the hollow site on the PdAg (100) surface. Then, the two H would further intermediately evolve to the hollow site. From the Figure 5d, the spontaneous charge transfer for both H and O2 with PdAg (100) were much energetic favorable. With the Coulombic attraction, the 2H+ would have a fast combine with O22- and react into the H2O2 finally. Based on above results, the incorporation of Ag generates the new PdAg active sites, which enhances the charge transfer (Ag→Pd) and improves the adsorption-desorption ability of surface molecules (such as H2, O2, H2O2) compared with Pd NPs. Thus the H2O2 selectivity is increased via an energetic downhill.

CONCLUSIONS In summary, we have successfully synthesized PdAg alloy NPs with different sizes and different Pd/Ag ratios as highly efficient catalysts for the direct H2O2 synthesis. The optimized PdAg-M alloy NPs show the highest performance with H2O2 productivity of 80.4 mol kgcat-1 h-1, H2O2 hydrogenation of 30.0 mol kgcat-1 h-1, and selectivity of 82.1%. The catalyst also show outstanding durability with an acceptable


14

Page 15 of 19 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


degradation after six cycles. XPS analysis demonstrates that the electron transfers from Ag to Pd to generate more Pd0 species, which results in excellent catalytic activity. Theoretical calculation results indicate that the PdAg alloy can enhance the charge transfer from Ag to Pd and improve the adsorptiondesorption ability of the surface molecules. More importantly, the new PdAg active sites have the spontaneous combination process with H and O2 and much energetic preference to the direct H2O2 synthesis reaction. Our study reported here emphasize the importance of the bimetallic alloy design for the direct H2O2 synthesis with satisfactory efficiency.

ASSOCIATED CONTENT Supporting Information. Experimental details and data. Figure S1-S14&Table S1. This material is available free of charge via the Internet at http://pubs.acs.org. TEM images, statistics of particle size, XRD patterns, EDS patterns, and HRTEM images of the three different size PdAg NPs; TEM images, EDS patterns, XRD patterns, and HRTEM images of PdAg NPs with different ratios of Pd and Ag; TEM images of PdAg-M NPs loading on different supports; TEM images of the commercial Pd/C, Ag NPs, and Pd NPs; TEM image and EDS pattern of PdAg-M/TiO2 after six reaction rounds; XPS curves of Ag NPs and Pd NPs; Obtained values of Uout1 and Uout2 for 4d orbital of Pd on the (100) surface of PdAg within fcc lattice and 4d orbital of Ag on the (100) of fccPdAg; Data of the Productivity, hydrogenation, and selectivity of H2O2 of different catalysts; XPS data of the five different PdAg NPs. Corresponding Author [email protected]; [email protected] ACKNOWLEDGMENT This work was financially supported by the Ministry of Science and Technology (2016YFA0204100, 2017YFA0208200), the National Natural Science Foundation of China (21571135), Young Thousand Talented Program, the start-up supports from Soochow University, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).


15


Page 16 of 19

References 1. Zheng, Z.; Ng, Y. H.; Wang, D. W.; Amal, R. Epitaxial Growth of Au-Pt-Ni Nanorods for Direct High Selectivity H2O2 Production. Adv. Mater. 2016, 28, 9949-9955. 2. Edwards, J. K.; Solsona, B.; N, E. N.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Switching Off Hydrogen Peroxide Hydrogenation in the Direct Synthesis Process. Science 2009, 323, 10371041. 3. Siahrostami, S.; Verdaguer-Casadevall, A.; Karamad, M.; Deiana, D.; Malacrida, P.; Wickman, B.; Escudero-Escribano, M.; Paoli, E. A.; Frydendal, R.; Hansen, T. W.; Chorkendorff, I.; Stephens, I. E.; Rossmeisl, J. Enabling Direct H2O2 Production Through Rational Electrocatalyst Design. Nat. Mater. 2013, 12, 1137-1143. 4. Guo, Y.; Dai, C.; Lei, Z.; Chen, B.; Fang, X. Synthesis of Hydrogen Peroxide over Pd/SiO2/COR Monolith Catalysts by Anthraquinone Method. Catal. Today 2016, 276, 36-45. 5. Kamachi, T.; Ogata, T.; Mori, E.; Iura, K.; Okuda, N.; Nagata, M.; Yoshizawa, K. Computational Exploration of the Mechanism of the Hydrogenation Step of the Anthraquinone Process for Hydrogen Peroxide Production. J. Phys. Chem. C 2015, 119, 8748-8754. 6. Fellinger, T. P.; Hasche, F.; Strasser, P.; Antonietti, M. Mesoporous Nitrogen-Doped Carbon for the Electrocatalytic Synthesis of Hydrogen Peroxide. J. Am. Chem. Soc. 2012, 134, 4072-4075. 7. Jirkovsky, J. S.; Panas, I.; Ahlberg, E.; Halasa, M.; Romani, S.; Schiffrin, D. J. Single Atom Hot-Spots at Au-Pd Nanoalloys for Electrocatalytic H2O2 Production. J. Am. Chem. Soc. 2011, 133, 19432-19441. 8. Shiraishi, Y.; Kanazawa, S.; Kofuji, Y.; Sakamoto, H.; Ichikawa, S.; Tanaka, S.; Hirai, T. SunlightDriven Hydrogen Peroxide Production from Water and Molecular Oxygen by Metal-Free Photocatalysts. Angew. Chem. Int. Ed. 2014, 53, 13454-13459.


16

Page 17 of 19 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


9. Teranishi, M.; Naya, S.-i.; Tada, H. In Situ Liquid Phase Synthesis of Hydrogen Peroxide from Molecular Oxygen Using Gold Nanoparticle-Loaded Titanium(IV) Dioxide Photocatalyst. J. Am. Chem. Soc. 2010, 132, 7850-7851. 10. Tsukamoto, D.; Shiro, A.; Shiraishi, Y.; Sugano, Y.; Ichikawa, S.; Tanaka, S.; Hirai, T. Photocatalytic H2O2 Production from Ethanol/O2 System Using TiO2 Loaded with Au-Ag Bimetallic Alloy Nanoparticles. ACS Catal. 2012, 2, 599-603. 11. Pashkova, A.; Dittmeyer, R. Carbon Dioxide as an Alternative Solvent for the Direct Synthesis of Hydrogen Peroxide: A Review of Recent Activities. Catal. Today 2015, 248, 128-137. 12. Edwards, J. K.; Hutchings, G. J. Palladium and Gold-Palladium Catalysts for the Direct Synthesis of Hydrogen Peroxide. Angew. Chem. Int. Ed. 2008, 47, 9192-9198. 13. García-Serna, J.; Moreno, T.; Biasi, P.; Cocero, M. J.; Mikkola, J.-P.; Salmi, T. O. Engineering in Direct Synthesis of Hydrogen Peroxide: Targets, Reactors and Guidelines for Operational Conditions. Green Chem. 2014, 16, 2320-2343. 14. Kim, J.; Chung, Y.-M.; Kang, S.-M.; Choi, C.-H.; Kim, B.-Y.; Kwon, Y.-T.; Kim, T. J.; Oh, S.-H.; Lee, C.-S. Palladium Nanocatalysts Immobilized on Functionalized Resin for the Direct Synthesis of Hydrogen Peroxide from Hydrogen and Oxygen. ACS Catal. 2012, 2, 1042-1048. 15. Lari, G. M.; Puértolas, B.; Shahrokhi, M.; López, N.; Pérez-Ramírez, J. Hybrid Palladium Nanoparticles for Direct Hydrogen Peroxide Synthesis: The Key Role of the Ligand. Angew. Chem. Int. Ed. 2017, 129, 1801-1805. 16. Campos-Martin, J. M.; Blanco-Brieva, G.; Fierro, J. L. Hydrogen Peroxide Synthesis: An Outlook beyond the Anthraquinone Process. Angew. Chem. Int. Ed. 2006, 45, 6962-6984. 17. Torrente-Murciano, L.; He, Q.; Hutchings, G. J.; Kiely, C. J.; Chadwick, D. Enhanced Au-Pd Activity in the Direct Synthesis of Hydrogen Peroxide Using Nanostructured Titanate Nanotube Supports. ChemCatChem 2014, 6, 2531-2534.


17


Page 18 of 19

18. Menegazzo, F.; Manzoli, M.; Signoretto, M.; Pinna, F.; Strukul, G. H2O2 Direct Synthesis under Mild Conditions on Pd-Au Samples: Effect of the Morphology and of the Composition of the Metallic Phase. Catal. Today 2015, 248, 18-27. 19. Freakley, S. J.; He, Q.; Harrhy, J. H.; Lu, L.; Crole, D. A.; Morgan, D. J.; Ntainjua, E. N.; Edwards, J. K.; Carley, A. F.; Borisevich, A. Y. Palladium-Tin Catalysts for the Direct Synthesis of H2O2 with High Selectivity. Science 2016, 351, 965-968. 20. Wilson, N. M.; Flaherty, D. W. Mechanism for the Direct Synthesis of H2O2 on Pd Clusters: Heterolytic Reaction Pathways at the Liquid-Solid Interface. J. Am. Chem. Soc. 2016, 138, 574-586. 21. Freakley, S. J.; Piccinini, M.; Edwards, J. K.; Ntainjua, E. N.; Moulijn, J. A.; Hutchings, G. J. Effect of Reaction Conditions on the Direct Synthesis of Hydrogen Peroxide with a AuPd/TiO2 Catalyst in a Flow Reactor. ACS Catal. 2013, 3, 487-501. 22. Shibata, S.; Suenobu, T.; Fukuzumi, S. Direct Synthesis of Hydrogen Peroxide from Hydrogen and Oxygen by Using a Water-Soluble Iridium Complex and Flavin Mononucleotide. Angew. Chem. Int. Ed. 2013, 52, 12327-12331. 23. Edwards, J. K.; Ntainjua, E.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Direct Synthesis of H2O2 from H2 and O2 over Gold, Palladium, and Gold-Palladium Catalysts Supported on AcidPretreated TiO2. Angew. Chem. Int. Ed. 2009, 48, 8512-8515. 24. Sterchele, S.; Biasi, P.; Centomo, P.; Campestrini, S.; Shchukarev, A.; Rautio, A.-R.; Mikkola, J.-P.; Salmi, T.; Zecca, M. The Effect of the Metal Precursor-Reduction with Hydrogen on a Library of Bimetallic Pd-Au and Pd-Pt Catalysts for the Direct Synthesis of H2O2. Catal. Today 2015, 248, 40-47. 25. Maity, S.; Eswaramoorthy, M. Ni-Pd Bimetallic Catalysts for the Direct Synthesis of H2O2-Unusual Enhancement of Pd Activity in the Presence of Ni. J. Mater. Chem. A 2016, 4, 3233-3237. 26. Li, J.; Yoshizawa, K. Mechanistic Aspects in the Direct Synthesis of Hydrogen Peroxide on PdAu Catalyst from First Principles. Catal. Today 2015, 248, 142-148.


18

Page 19 of 19 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


27. Plauck, A.; Stangland, E. E.; Dumesic, J. A.; Mavrikakis, M. Active Sites and Mechanisms for H2O2 Decomposition over Pd Catalysts. Proc. Natl. Acad. Sci. USA 2016, 113, E1973-1982. 28. Luo, W. B.; Gao, X. W.; Chou, S. L.; Wang, J. Z.; Liu, H. K. Porous AgPd–Pd Composite Nanotubes as Highly Efficient Electrocatalysts for Lithium-Oxygen Batteries. Adv. Mater. 2015, 27, 6862-6869. 29. Zhang, S.; Metin, O.; Su, D.; Sun, S. Monodisperse AgPd Alloy Nanoparticles and Their Superior Catalysis for the Dehydrogenation of Formic Acid. Angew. Chem. Int. Ed. 2013, 52, 3681-3684. 30.

Zheng, N.; Stucky, G. D. A General Synthetic Strategy for Oxide-Supported Metal Nanoparticle

Catalysts. J. Am. Chem. Soc. 2006, 128, 14278-14280. 31. Zhang, J.; Li, Y.; Zhang, Y.; Chen, M.; Wang, L.; Zhang, C.; He, H. Effect of Support on the Activity of Ag-based Catalysts for Formaldehyde Oxidation. Sci. Rep. 2015, 5, 12950.

TOC


19

Highly Active, Selective and Stable Direct H2O2 Generation by

Recommend Documents