Au@Pd Bimetallic Nanocatalyst for Carbon- Halogen Bond Cleavage

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Au@Pd Bimetallic Nanocatalyst for Carbon-Halogen Bond Cleavage: Old Story with New Insight into How the Activity of Pd is Influenced by Au Rui Liu, Huimin Chen, Liping Fang, Cuihong Xu, Zuoliang He, Yujian Lai, Huachao Zhao, Deribachew Bekana Hirpa, and Jing-fu Liu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05996 • Publication Date (Web): 16 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018

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Environmental Science & Technology

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Au@Pd Bimetallic Nanocatalyst for Carbon-

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Halogen Bond Cleavage: Old Story with New

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Insight into How the Activity of Pd is influenced by

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Au

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Rui Liu,a* Hui-min Chen,a,b Li-ping Fang,b Cuihong Xu,a,b Zuoliang He,a Yujian Laia, Huachao

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Zhaoa, Deribachew Bekanaa and Jing-fu Liua

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a

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Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

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b

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State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for

Department of Chemistry, Faculty of Material Sciences and Chemistry, China University of

Geosciences, Wuhan 430074, China TOC art:

12 13 ACS Paragon Plus Environment

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ABSTRACT.

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AuPd bimetallic nanocatalysts exhibit superior catalytic performance in the cleavage of carbon-

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halogen bonds (C-X) in the hazardous halogenated pollutants. A better understanding of how Au

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atoms promote the reactivity of Pd sites rather than vaguely interpreting as bimetallic effect, and

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determining which type of Pd sites are necessary for these reactions are crucial factors for the

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design of atomically precise nanocatalysts that make full use of both the Pd and Au atoms.

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Herein, we systematically manipulated the coordination number of Pd-Pd, d-orbital occupation

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state and Au-Pd interface of the Pd reactive centers, and studied the structure-activity

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relationship of Au-Pd in the catalyzed cleavage of C-X bonds. It is revealed that Au enhanced

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the activity of Pd atoms primarily by increasing the occupation state of Pd d-orbits. Meanwhile,

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among the Pd sites formed on the Au surface, 5-7 contiguous Pd atoms, 3-4 adjacent Pd atoms

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and isolated Pd atoms were found to be the most active in the cleavage of C-Cl, C-Br and C-I

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bonds, respectively. Besides, neighbouring Au atoms directly contribute to the weakening of the

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C-Br/C-I bond. This work provides new insight into the rational design of bimetallic metal

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catalysts with specific catalytic properties.

ACS Paragon Plus Environment

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INTRODUCTION

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The catalyzed cleavage of carbon-halogen (C-X) bonds has profound and permanent importance

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for many aspects of chemical engineering,1 including the safe treatment of hazardous

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halogenated organic compounds.2-3 Traditionally, Pd nanostructures are the most favourable

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catalysts for this process in terms of cost (comparable or even lower than that of other Pt-group

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metals), activity (much more active than Cu or Ni) and reusability (better than that of ligand-

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stabilized homogeneous Pd catalysts).4 Unfortunately, the sluggish reaction kinetics,5 catalyst

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deactivation by the formation of Pd halide/Pd black on the surface, and coking deposition or

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metal sintering reduce the cost effectiveness of Pd nanocatalysts are the main barriers hindering

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implementation of Pd catalysts for environmental improvement.2-3, 6 Tremendous efforts have

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been devoted to overcoming these disadvantages. One important strategy involves engineering

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the surface atomic arrangement of Pd crystals and maximizing the exposure of the low-

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coordinate corner, edge and step atoms.7-8 For example, the {730} facet enclosed concave Pd

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nanocrystals was reported to show an activity 3.5 times higher than that of Pd nanocubes in the

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Suzuki coupling between phenylboronic acid and iodobenzene.9-10 However, later research

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revealed that this high activity like originated from the increased dissolution of active molecular

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Pd,11 which is unfavourable for the long-time stability/recyclability of Pd nanocatalysts. This

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result at least partially showed that controlling the morphology of Pd nanocrystals alone cannot

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fully overcome the drawbacks associated with Pd nanocatalysts. At the same time, parallel

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studies revealed that a bimetallic Pd nanocatalyst can exhibit markedly enhanced performance

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over its monometallic counterparts.3, 12-15 The introduction of other metal atoms typically shifts

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the Pd d-orbital occupation state, which in turn alters the sorption energies of

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substrates/intermediates/products.12,

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In addition to the electronic and geometric effect, the


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synergetic effects, which refers to other metal(c.a., Au) atom forms additional M-C or M-X bond

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to weaken the C-X bond and facilitates its cleavage on Pd sites, are also believed to contribute to

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the enhanced activity of Pd atoms.15 From this perspective, Au plays an impressive role in

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promoting the catalytic performance of Pd atoms.3,

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depositing 1 to 1.5 monolayers (ML) Pd on Au nanoparticle surface to form core (Au)-shell (Pd)

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(denoted as Au@Pd) nanostructure, Pd based mass activity in catalyzed dehalogenation of

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trichloroethylene (TCE) increased100 times over that of Pd nanoparticles (NPs).3, 5, 17, 20 Zhang

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et al. diluted Pd atoms into Au nanoparticles to form a single-atom Pd catalyst with high activity

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and durability in the Ullmann Reaction.18 Dhital et al. showed that Au-Pd alloy nanoclusters

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(NCs) could activate and break C-Cl bonds at room temperature, a process usually catalyzed by

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nucleophilic ligands stabilized Pd species at harsh conditions.19 In addition to the catalyzed

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cleavage of C-X bonds, Au@Pd bimetallic catalysts have also shown promise for application in a

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variety of other reaction processes, including nitrite reduction,21 low-temperature oxidation of

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greenhouse gases,22 and synthesis of H2O223 for Fenton or electro-Fenton mineralization of

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pollutants.24-25

13, 17-19

Wong et al. reported that upon

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Despite the broadly observed promotion of Pd activity by Au in C-X cleavage reactions, a

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consensus is still lacking on how this process occurs at the atomic level. The proposed active

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centers and the exact role of Au atoms vary greatly from case to case. For example, small Pd

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clusters of 2-3 continuous atoms on a Au surface,17 and Au-Pd interfacial sites have been

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proposed as the active centers.19 In accordance with the different atomic structure of the active

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Pd centers, the exact role of Au also varies.10, 19, 26 In addition to facilitating the formation of Pd

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clusters (geometric effect)

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effect),18 Au has also been reported to play a role in the direct cleavage of C-X bonds by

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and altering the d-orbital occupation state of Pd atoms (electronic


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weakening the Pd-X bonds and facilitating the reductive removal of X ions (synergistic effect).19

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The full answer to the above question is very crucial for the design of atom-economical

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nanocatalysts with a high performance/cost ratio.28 A precondition for this design is the

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sophisticated generation of Pd centers with defined atomic geometries (with different Pd-Pd

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coordination number, CNPd-Pd, and coordination atom), and thus the different d-orbital

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occupation state on the Au host material. Unfortunately, for the weak interaction between Pd and

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Au (the bond dissociation energy is 142.7 KJ mol-1),29-30 Pd atoms on Au surface apt to form

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additional Pd-Pd bond, this makes controlling the atomic geometry of Pd sites in Au-Pd

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nanocatalysts,31-33 especially CNPd-Pd, is almost a formidable task.

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Very recently, we introduced a strategy for modifying the binding strength to tailor the atomic

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geometry of Pd on ultrathin Au nanowires (NWs)32 by pre-depositing a Ag ML, and successfully

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synthesized AuPd core-shell (Au@Pd) NWs dominated by continuous Pd atoms and AuPd core-

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shell NWs with a Ag monolayer (AgML) between Au core and Pd shell (denoted as

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Au@AgML@Pd NWs) featured with isolated Pd atoms. Therefore, these NWs provide an ideal

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platform for identifying the active center in the C-X cleavage step, including the answer to the

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question whether a single Pd atom is active during this process or not. In this work, based on the

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developed tactic in manipulating the structure of Pd atoms,32 we synthesized Pd coated Au

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(Au@Pd) and Au@AgML (Au@AgML@Pd) NWs with different Pd coverage, as well as Pd

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NWs/NCs as catalyst for the catalyzed dehalogenation of halogenated phenols. Through

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adjusting the Pd coverage, we also manipulated the relative abundance of the Au-Pd or Au-

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Ag/Pd interface and the d-orbital occupation state of the Pd atoms. The exact structures of these

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Pd sites and their relative abundance were characterized by surface enhanced Raman scattering

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spectroscopy (SERS), extended X-ray absorption fine structure spectroscopy (EXAFS, CNPd-Pd


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and coordination number), X-ray absorption near edge structure spectroscopy (XANES, d-orbital

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occupation state) and CO-stripping experiments (Au-Pd interfacial sites). Based on the Pd

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reactive centers with well-defined structures, we systematically studied how the geometric (the

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coordination number of Pd atoms), the electronic (d-orbital occupation state) and the synergistic

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effect influence the catalytic performance of Pd bimetallic catalysts in the dehalogenation of 4-

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chlorophenol/4-bromophenol/4-iodophenol (4-CP/4-BP/4-IP). This ultimately led us to propose a

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role for Au atoms in the Pd-catalyzed cleavage of C-X bonds at the atomic level.

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EXPERIMENTAL SECTION

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Pd catalyst synthesis. Au@Pd and Au@AgML@Pd NWs with different Pd coverages (1/16, 1/8,

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1/4, 1/2 and 1.0 ML) were prepared according to our recently developed protocol,32 and details

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for calculating the amount of Pd or Ag precursor needed for the formation of Pd ML/Ag ML, as

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well as experimental evidence for the presence of Pd ML has been presented in supporting

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information (Figure S1-5). In brief, ultrathin Au NWs was synthesized by quickly injecting 3.0

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mL 100 mM KBH4 solution into a septum-sealed 50 mL flask that containing 0.05 mmol of

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HAuCl4 and 25 mg of Triton X-114 (TX-114) in 47.0 mL ice-cooled water under vigorous

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stirring.34 For the coating of Pd shell on Au core, 0.19, 0.375, 0.75, 1.50, or 3.0 mL of ice-cold

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1.0 mM Na2PdCl4 solution was dropwise added into 10.0 mL dispersion of freshly synthesized

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Au NWs under stirring. In each of these cases, the Pd coverage is 1/16, 1/8, 1/4/, 1/2 or 1.0 ML,

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respectively. The synthesis of Au@AgML NWs is similar to that of Au@Pd NWs, but replaces

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Na2PdCl4 with 3.0 mL of 1.0 mM AgNO3 solution. In the case of the Au@AgML@Pd NWs, 3.0

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mL of 1.0 mM AgNO3 solution was added into a 10.0 mL dispersion of freshly synthesized Au

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NWs while stirring, followed by the addition of 0.19, 0.375, 0.75, 1.50, or 3.0 mL of ice-cold 1.0

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mM Na2PdCl4 solution.


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Pd NWs were synthesized using a similar protocol to that of Au NWs with Pd(NO3)2 as Pd

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precursor,34 the amount of injected 100 mM ice-cooled KBH4 solution was reduced to 1.0 mL.

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PVP stabilized Pd cluster was also synthesized by reduction of Na2PdCl4 with KBH4.26

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Specifically, 27.5 mg PVP and 0.05 mmol Na2PdCl4 were dissolved in 45 mL ultrapure water

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and stirring in ice bath for 30 min, followed by injecting 5.0 mL 100 mM KBH4 solution. The

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synthesized Pd NCs were purified by ultracentrifugation at 10, 000 rpm with a molecular cutoff

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of 10.0 kD to remove unbounded PVP and inorganic ions like Cl-, K+/Na+ and BO3-, and

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redispersed in ultrapure water. Furthermore, the synthesized nanocatalysts were supported on

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aminopropyl-triethoxysilane (APTS)-modified magnetic Fe3O4 nanospheres (NSs, ~400 nm in

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diameter) to increase the colloid stability and reusability of the catalysts.35

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Catalytic test. The catalyzed dehalogenation of 4-halogen phenol was employed as a model

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reaction for studying the catalytic performance of Pd in the cleavage of C-X bonds. This

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involves the dehalogenation of 1.0 mM 4-chlorophenol (4-CP), 4-bromophenol (4-BP) or 4-

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iodophenol (4-IP) dissolved in 1.5 mM KOH aqueous solution (10.0 mL, the initial pH is about

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11.1) under a continuous H2 flow (100.0 mLmin-1) and stirring at 400 rpm. The added Au@Pd

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and Au@AgML@Pd NWs catalysts contained 5.0 µM (0.53 mg L-1, for 4-CP and 4-IP) or 0.5 µM

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(0.05 mg L-1, for 4-BP) Pd.36 The amount of unreacted 4-halogen phenol was determined by

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sampling the reacting solution at selected time intervals, neutralized with HCl and analysed by

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high-performance liquid chromatography (HPLC). For the study of the catalytic performance of

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Pd catalysts at more environmentally realistic conditions, the concentration of 4-CP/4-BP/4-IP

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studied was lowered to 0.02 mM. The initial pH was adjusted to 7.0 ± 0.5 with KOH/HNO3, and

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the catalyst was lowered to 0.4 µM (0.04 mg L-1, for 4-CP and 4-IP) or 0.1 µM (0.01 mg L-1, for

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4-BP) Pd () in Au@Pd and Au@AgML@Pd NWs accordingly. To test the colloid stability and


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reusability of the Fe3O4 NSs-supported Au@Pd NWs catalyst, the nanocatalyst was recovered at

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the end of the reaction by an external magnetic field, washed with ethanol and water, and

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resuspended in the reactant solution. The catalytic performance in three successive runs was

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studied and compared. Data shown in the figures are mean values of at least three experiments.

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The experimental uncertainties, including instrumental errors, relative standard deviations and

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sorption were evaluated in the absence of catalyst, which showed that total uncertainty was less

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less than 1% for high concentration of halogenated phenols but increased to 3-5% with the

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decrease of their initial concentration to 0.02 mM.

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XAS study. The local coordination environment (coordination number and bond lengths) of

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the Pd atoms in the Au@Pd and Au@AgML@Pd NWs was probed by EXAFS/XANES. The Pd

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and Ag k-edge EXAFS and XANES spectra were acquired at beamline 14W1 of the Shanghai

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Synchrotron Radiation Facilities (SSRF). The X-ray was monochromatized by a double-crystal

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Si{311} monochromator. The storage ring of SSRF was operated at 3.5 GeV with a current of

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300 mA. The Au LIII-edge XAS spectra (1W1B beamline at the Beijing Synchrotron Radiation

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Facilities) were also acquired to build a full model of the Pd atoms. The acquired EXAFS data

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were processed according to the standard procedures using the WinXAS 3.1 program.37 The

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theoretical amplitudes and phase shift functions were calculated with the FEFF8.2 code using the

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crystal structure parameters of Ag and Pd foils.38

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CO-stripping experiment. A 50.0-µl aliquot of a Pd NWs (0.05 µmol Pd) or Au@Pd NWs

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dispersion containing 0.05 µmol Au was dropped on a glassy carbon electrode (5 mm id) as a

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supportless electrocatalyst. This was followed by adding 10 µl of a Nafion solution (0.5% w/v in

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ethanol, DuPont). The prepared electrode was activated in a N2-saturated 0.1 M KOH solution

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with cyclic voltammetry from −1.0 to 0.45 V, and Pt wire and a Ag/AgCl electrode were used as


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the counter electrode and reference electrode, respectively. After CO was chemisorbed on the Pd

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atoms, it was then stripped at 50 mV·s-1, and the Pd surface area (SAPd) was calculated by:

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SAPd=QCO-stripping/420 (mC cm−2)

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SERS experiment. Because of the δ-donation/π-back donation behaviour between the C≡N

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bond and its bounding metal atom, the SERS spectra of chemisorbed 2,6-DMPI is very sensitive

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to the bounding metal atom and adsorption configuration,32,

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characterize the atomic geometry of the Pd atoms and estimate the relative abundance of the

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different Pd sites and track the changes in the Pd site geometry during the reaction in this work.32

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32

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RESULTS AND DISCUSSION

39-40

and was employed to

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Characterization of the Pd catalysts. Figures 1a-d illustrate high-resolution transmission

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electron microscopy (HRTEM) images of the Pd nanostructures studied in this work. The sized

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of Pd NCs were in the range of 2 to 3 nm (Figure 1a), showing the effectiveness of polyvinyl

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pyrrolidone (PVP) in stabilizing the Pd NCs. Based on the magic cluster model,41 the surface Pd

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atoms made up ~50% of the total Pd atoms in the Pd NCs. The diameter of the Pd NWs was

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approximately 2 nm (Figure 1b), and about 50% of their Pd atoms were also located on the

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surface. On the other hand, although the epitaxially deposited Pd overlayer was difficult to

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distinguish based on the analysis of the morphology and lattice fingerprint (Figures 1c, d),

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energy-dispersive X-ray spectroscopy (EDS) mapping, SERS and EXAFS all supported the

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presence of Pd as an overlayer (Figures S6-9 and see below). Figure 1e depicts representative

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transmission electron microscopy (TEM) images of the Au@Pd NWs on Fe3O4 NSs (Figure

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S10), where the NWs are interlaced on the Fe3O4 NSs. Elemental analysis (Figure 1f) revealed

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that the loading efficiency, which referred to the ratio between nanostructures that adsorbed on


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Fe3O4 nanospheres and their total amount, was above 60% for both Au and Pd in the Au@Pd,

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Au@AgML@Pd and Pd NWs, signifying the effective immobilization of the NW catalyst on the

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Fe3O4 NSs. This is ascribed to the stabilization of the NWs by the weakly binding surfactant

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Triton X-114,34 and thus, the low steric hindrance to the formation of Metal-N bonds between

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the NWs and –NH2 groups.42 Moreover, the ratio between the capture efficiencies of Au and Pd

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in Au@Pd and Au@AgML@Pd was very close to 1.0, proving that Pd atoms were deposited on

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Au or the Au@AgML NWs and simultaneously adsorbed on the Fe3O4 NSs. Of note is for the

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ultralong nature of the NWs catalyst, only a small part of atoms (Au or Pd) take part in the

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formation of Metal-N bond, and therefore the change in the electronic structure of Pd reactive

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centers would be very small. In contrast, due to the strong binding of PVP, the associated steric

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hindrance limited the contact between the –NH2 groups and the surface Pd atoms, and thus, the

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loading efficiency for the PVP-stabilized Pd NCs was no more than 20.0%. This strong binding

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was also reflected in the inferior catalytic performance of the Pd NCs compared to that of the

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NWs (see below).

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Activities of the Pd catalysts in the dehalogenation of 4-CP. The catalytic performance of the

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Pd nanostructures in the cleavage of C-X bonds was initially evaluated through the

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dehalogenation of 4-CP. In the absence of the Pd catalyst, and also in the presence of Au or

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Au@AgML NWs, only a slight decrease (less than 5%) in the 4-CP concentrations was observed,

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even under a continuous flow of H2 for more than 2 hours, and the generated phenol was

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undetectable, suggesting that no dehalogenation of 4-CP occurred. In the reaction catalyzed by

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Au@Pd (1.0 ML) or Au@AgML@Pd (1.0 ML) NWs containing 5.0 µM Pd, the conversion

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reached 99% or above in 10 min (Figure 2a). This reveals the high activity of Au and

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Au@AgML-supported Pd atoms in the cleavage of the C-Cl bond. By comparison, the Pd NCs


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and NWs exhibited much lower conversions of 68.5 ± 9.0% and 88.2 ± 3.7%, respectively, even

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in the presence of a much higher dose of Pd atoms (20.0 µM) and elongated time of 20 min 20

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min. The higher activity of the Pd NWs over that of the Pd NCs is ascribed to the absence of the

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strongly binding PVP, which was negatively charged at the studied pH value and unfavourable

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for the adsorption of halogenated phenolate, and therefore the improved accessibility of the

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active sites. It’s deserved to point out that the influence of TX-114 molecule to the reaction

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process is neglectable (Figure S11-12). The discrepancy in the activities of the different Pd

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catalysts is more clearly illustrated in the kinetics curves (Figure 2b). The fitted rate constants for

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the Au@Pd, Au@Pd on Fe3O4 NSs and Au@AgML@Pd NWs reached at least 0.25 min-1, while

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this value was sharply reduced to 0.11 and 0.07 for the Pd NWs and Pd NCs. The Pd mass

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normalized rate constant were 53.1 ± 4.9, 53.8 ± 3.9, 46.5 ± 3.5, 5.30 ± 0.85 and 3.52 ± 0.27 min-

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1

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NWs (1.0 ML), Pd NWs and Pd NCs, respectively (Figure 2c). These values correspond to a 10-

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fold increase in mass activity of Pd after coating a monolayer of Pd on Au or Au@Ag NWs.

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Even taking into account the cost of Au, and the total mass (Au and Pd) normalized mass activity

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of Au@Pd and Au@AgML@Pd is still 2 times or higher than that of Pd NCs or NWs.

mM-1 for the Au@Pd NWs (1.0 ML), Au@Pd NWs (1.0 ML) on Fe3O4 NSs, Au@AgML@Pd

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Mass Transfer Limitation during the Catalytic Reaction. Since the catalyzed

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dehalogenation reaction is a three phase (solid−liquid−gas) process, the mass transfer process

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may influence the reaction kinetics. To ensure that the observed difference in catalytic

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performance of Pd catalysts stems from their intrinsic catalytic activity rather than the presence

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of mass transfer limitation, both external and internal mass transfer limitation was evaluated.43-47

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As shown in the Supporting Information, the external mass transfer limitation could be ignored

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in our experimental condition (bubbling the reaction solution with continuous H2 at a flow rate of


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100 mL·min-1 and stirring at 400 rpm), while the Weisz-Prater parameter down to 2.1×10-3,

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0.56×10-9 and 0.83×10-9 for Au@Pd on Fe3O4 nanospheres, Pd NCs and NWs suggests the

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internal mass transfer limitation is also negligible. Therefore, the different catalytic performance

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displayed in Figure 2, especially the superior activity of Au@Pd and Au@AgML@Pd over that of

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Pd NCs and NWs, is the direct result of their intrinsic activity.

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Enhanced stability of Pd atoms on Au or Au@AgML. In addition to the superior activity, the

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kinetics data also indicate an enhanced stability of the Pd atoms on the Au or Au@AgML surface

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(Figure 2b). Catalyzed by the Au@Pd and Au@AgML@Pd NWs, the change in the 4-CP

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concentration fits well to a pseudo-first-order kinetic model throughout the entire reaction. This

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indicates that the deactivation of Pd atoms was successfully suppressed. In contrast, gradual

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deactivation of the monometallic Pd (NWs and NCs) catalysts is indicated by the decrease in the

247

slope. In fact, for all tested Pd concentrations, i.e. 0.53 to 2.12 mg L-1, the activity diminished

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after 20-30 min (Figure S13). This enhanced stability of the Pd atoms on Au and Au@Ag is

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partially ascribed to the increased anti-oxidation capacity of the shell Pd atoms, since severe

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oxidation/dissolution was observed in Pd NWs/NCs as their deactivation (Table S1-2). Indeed,

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our XANES data (see below) confirmed that all the Pd atoms were in the metallic state after

252

being coated on the Au or Au@Ag surface, while up to 40% of the Pd atoms in the Pd NWs/NCs

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were in the form of PdII. These oxidized Pd atoms are apt to dissolve into the reaction solution

254

and either redeposit on the Pd NWs/NPs surface or deactivate the catalyst in the form of Pd black

255

over time.

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To further test the superior stability of the Au@Pd and Au@AgML@Pd, we studied the activity

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of the catalysts in the absence of base as well as the ability to recycle the used catalyst. Unlike

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the low activity and fast deactivation monometallic Pd NWs, the Au@Pd NWs effectively


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catalyzed the dehalogenation of 4-CP in the absence of base (Figure S14). Moreover, no

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perceptible loss of activity was observed after 3 successive runs (Figure S15). Both of these

261

results demonstrate the enhanced stability of Pd atoms on Au or Au@AgML NWs, and

262

meanwhile, the Pd atoms are more likely to play role in the heterogeneous pathway with Pd

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atoms rather than ions as the active center.48 In accordance with the well-preserved activity of the

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Au@Pd NWs catalyst, the similarity of the SERS spectrum of 2,6-DMPI chemisorbed on the as-

265

synthesized and spent Au@Pd and Au@AgML@Pd NWs catalyst also proved that the reaction

266

process did not influence the atomic geometry of the Pd atoms or the relative abundances of the

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Au and Pd atoms, as well as that of isolated and continuous Pd atoms (Figure S16). Notably, due

268

to the high sensitivity of the SERS spectra of 2,6-DMPI to the electronic structure of the metal

269

atoms underneath32, 40 as well as the high affinity of Cl- for transition metal atoms such as Pd, a

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~10 cm-1 red-shift occurred in the νNC, which is consistent with the enhanced occupation of the

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Au/Pd orbitals upon the adsorption of Cl- ions. However, leaching of Pd ions was observed for

272

the monometallic Pd catalyst (See discussion and Table S1-2, Figure S17).

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Pd coverage and substrate-dependent activity. In addition to displaying enhanced

274

activity/stability, the specific structure of the Pd atoms on the Au@Pd and Au@AgML@Pd NWs

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allowed us to ascertain the reactive centers in the reaction by studying the Pd coverage-activity

276

relationship.32 Figures 3a-c shows the changes in the catalytic performance of Au@Pd and

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Au@AgML@Pd with different Pd coverages in the dehalogenation of 4-CP. Obviously, all the

278

Au@Pd NWs were active in this reaction, even for a Pd coverage as low as 1/16 ML. Moreover,

279

as the Pd coverage increased to 1/2 ML, the activity of the Au@Pd NWs increased accordingly,

280

and the first-order rate constant reached 0.26 ± 0.02 min-1, which is comparable to that of the 1.0

281

ML-covered Au@Pd NWs. In sharp contrast, the Au@AgML@Pd NWs (1/4 ML) was completely


13


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282

inactive in this reaction, and the activity of Au@AgML@Pd (1/2 ML) was also very limited.

283

However, the activity of Au@AgML@Pd (1.0 ML) quickly caught up to that of Au@Pd (1.0 ML).

284

Based on these results and our previous knowledge of the changes in the structure of the Pd

285

atoms on the Au@AgML@Pd NWs with different Pd coverages, in particular, the totally inactive

286

of Au@AgML@Pd with the Pd coverage no more than 1/4ML, we hypothesize that the atomic

287

geometry of Pd sites strongly affects their activity in the cleavage of C-Cl.

288

The Au@Pd and Au@AgML@Pd NWs displayed even higher activities in the dehalogenation

289

of 4-BP than case of 4-CP. In the reaction catalyzed by Au@Pd NWs, the conversion approached

290

100% within 10 min for most of the Au@Pd catalysts and the whole process fits the first-order

291

model. Again, this result is ascribed to the enhanced stability of the Pd atoms on the Au NWs

292

surface (Figures 3d-f). In contrast, both the Pd NWs and NPs catalysts were rapidly deactivated

293

(within 10 min). This result is consistent with the Pd-related SERS band (Figure S16), which is

294

ascribed to the rapid dissolution of Pd induced by the high affinity of Br- ions for Pd

295

atoms/ions.49 Note that unlike the monotonic increase in activity with increasing Pd coverage

296

observed for the Au@Pd NWs in the dehalogenation of 4-CP, the activity of this catalyst in the

297

dehalogenation of 4-BP was the highest for a Pd coverage of 1/2 ML, in which the rate constant

298

reached 0.687 min-1, corresponding to an initial turnover of frequency (TOF) as high as 1254.8 ±

299

82.8 min-1. A similar trend was also observed for the Au@AgML@Pd NWs, in which

300

Au@AgML@Pd (1/2 ML, 0.49 ± 0.03 min-1) displayed a much higher activity than that of

301

Au@AgML@Pd (1/4 ML, 0.07 ± 0.01 min-1) and Au@AgML@Pd (1.0 ML, 0.39 ± 0.02 min-1)

302

(Figure 3f).

303

The higher activity in the dehalogenation of 4-BP over that of 4-CP is consistent with the

304

lower bond strength of C-Br (~250 kJ mol-1) compared to that of C-Cl (~350 kJ mol-1),30 and


14

Page 15 of 36


305

therefore the lower energy barrier/activation energy. To test whether the catalytic performance of

306

the Au@Pd and Au@AgML@Pd NWs in the cleavage of the C-X bonds was solely determined

307

by the bond strength, we further studied the reaction of C-I bonds (4-IP, Figures 3g-i). To our

308

surprise, although as much as 15-40% of the 4-IP was reduced within the first 2 min, all of the

309

catalysts, including the Au@Pd and Au@AgML@Pd NWs, were completely deactivated within

310

2-4 min. However, this deactivation of Au@Pd and Au@AgML@Pd did not necessarily result

311

from the total dissolution of Pd atoms from the Au and Au@Ag NWs. In fact, the majority of the

312

Pd atoms remained on the Au@AgML@Pd and Au@Pd NWs. The plausible explanation for this

313

is the strong binding of iodine ion on Pd sites inhibits the further adsorption of 4-IP molecule. On

314

the other hand, unlike in the case of 4-CP or 4-BP where the NWs with Pd coverages of 1/2 or

315

1.0 ML were much more active than their low-coverage counterparts, in the dehalogenation of 4-

316

IP, the activities of the Au@Pd and Au@AgML@Pd NWs decreased sharply with increasing Pd

317

coverage. Moreover, the Au@AgML@Pd NWs consistently displayed higher activities than the

318

Au@Pd NWs.

319

Changes in the structure of the Pd reactive center with changing Pd coverage. Based on

320

the data shown in Figure 3, the activity of the Pd atoms in the cleavage of C-X bond was highly

321

dependent on the Pd coverage. A similar trend was reported by Wong et al.5, 20 and is attributed

322

to the increased number of Pd-Au interfacial sites at which the formation of Au-Cl bond

323

facilitates the cleavage of the C-Cl bond. Sakurai et al. also proposed this mechanism as an

324

explanation for the unexpected activity of Au0.5Pd0.5 alloy NCs in the activation of C-Cl bonds at

325

mild condition.19 To elucidate the contribution of the Au-Pd interfacial sites to the cleavage of C-

326

X bonds, the relative abundances of the Au-Pd and Pd-Pd sites in the Au@Pd NWs were

327

estimated by CO-stripping experiments (the CO-stripping behaviour of the Au@AgML@Pd NWs


15


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328

is very complicated and beyond the scope of this work).50 According to the literature, CO

329

molecules bound to Pd atoms inside the Pd cluster can be stripped at low potential,50 while due to

330

the enhanced occupation of the d-orbitals and the strong back donation of π* electrons to the

331

adsorbed CO, Pd atoms located at Pd-Au interfaces bind CO more strongly and require a higher

332

potential to be stripped. Indeed, the CO-stripping curve for the Au@Pd NWs was split into two

333

peaks centerd at -0.08 and 0.11 V, and the current of the latter peak decreased with increasing Pd

334

coverage (Figure 4a). Therefore, the observed attenuation of the 0.11 V peak with increasing Pd

335

coverage reflects the growth of the Pd cluster and a decrease in the number of Au-Pd interfacial

336

sites. Quantitatively, with the increase in the Pd coverage on Au@Pd from 1/16 to 1.0 ML, the

337

relative abundance of the interfacial sites dropped from 84.8% to 31.9%. This decrease in the

338

number of Au-Pd sites is associated with the increase in activity, especially in the reactions with

339

C-Cl and C-Br, suggesting that such sites may play a less critical role in the cleavage of C-X

340

bonds (See below).

341

In addition to the change in the relative abundance of Au-Pd interfacial sites, a direct result of

342

the increase in the Pd coverage is a change in both the CNPd-Pd on the Au@Pd and

343

Au@AgML@Pd NWs and the d-orbital occupation state of the Pd atoms. These changes are

344

reflected in the Pd k-edge EXAFS spectra (Figures 4b-d) and the whiteline (WL) intensities of

345

the XANES spectra (Figures 4e-f), respectively. In the k3-weighted Fourier-transformed (FT) Pd

346

k-edge EXAFS spectra, the Au@Pd and Au@AgML@Pd NWs displayed a unique spectrum that

347

stemmed from the distinct coordination environments, which was unlike the case of the Pd NCs

348

and NWs, whose spectral features resembled those of Pd foil (Figures S18-20). More specifically,

349

all spectra of the Au@Pd NWs shared a nearly identical shape with only slight shift in the first

350

nearest coordination peaks in the R space compared to that of Pd foil/NCs/NWs. This result is


16

Page 17 of 36


351

consistent with the fact that the epitaxially grown Pd overlayer on the Au NWs was primarily

352

coordinated by Au atoms with Au-Pd bonds (~2.84 Å) that were longer than the Pd-Pd bonds in

353

Pd foil or the NCs/NWs (~2.74 Å). Meanwhile, the similarity in the spectra indicates that the

354

coordination environment did not change significantly. The detailed first-shell fitting revealed

355

that with the increase in the Pd coverage from 1/6 to 1.0 ML, the CNPd-Pd increased from 0.70 ±

356

0.27 to 1.31 ± 0.36, 1.55 ± 0.44, 2.42 ± 0.38 and 2.78 ± 0.53. The case for the Au@AgML@Pd

357

NWs is much more complicated than that of the Au@Pd NWs. The spectra of the

358

Au@AgML@Pd NWs with Pd coverages of 1/16, 1/8 and 1/4 ML possessed similar shapes, and

359

no Pd-Pd bonds were detected. With the increase in the Pd coverage to 1/2 and 1.0 ML, Pd-Pd

360

bonds were observed with CNs of 1.21 ± 0.25 and 2.34 ± 0.46, respectively, which are

361

comparatively smaller than those of the Au@Pd NWs. However, after accounting for the fact

362

that 49% and 29% of the isolated Pd atoms remained in the 1/2 and 1.0 ML-covered

363

Au@AgML@Pd NWs (Figure S6, Table S3-5), the CNPd-Pd reached 2.37 ± 0.25 and 3.30 ± 0.46,

364

respectively (see the discussion in the SI).

365

Meanwhile, as shown in the XANES spectra displayed in Figures 4e and f, the WL intensity of

366

the Pd NCs and NWs is between that of PdO and Pd foil. This results from the partial oxidation

367

of the surface Pd atoms, which is directly related to the low stability of the monometallic Pd

368

catalysts in the cleavage of C-X bonds. Upon epitaxial growth of Pd on the Au or Au@AgML

369

NWs, the Pd WL intensity decreased substantially due to the direct withdrawal of d-electrons

370

from the Au/Ag atoms.14 Naturally, a lower Pd coverage corresponded to a lower WL intensity.

371

Based on close inspection of the XANES spectra of the Au@AgML@Pd and Au@Pd NWs, the

372

WL intensity of the Au@AgML@Pd NWs was always lower than that of Au@Pd. This trend is

373

attributable to the fact that both the Au and Ag atoms donate electrons to the Pd atoms, and the


17


Page 18 of 36

374

Pd atoms on the Au@AgML@Pd NWs had a higher d-orbital occupation than the Au@Pd NWs.

375

Notably, both the Au@Pd and Au@AgML@Pd NWs exhibited a lower WL intensity than that of

376

Pd foil, signifying the more atom-like nature of Pd atoms on Au and Au@AgML, which is very

377

essential for the enhanced stability of these Pd catalysts.36, 51-52

378

Since all the above mentioned structure parameters (CNPd-Pd, WL intensity and Au-Pd

379

interfacial sites) of Au@Pd catalyst are relevant with other. Moreover, in most cases, the activity

380

of Pd centers is influenced by the above mentioned factors simultaneously. This makes the study

381

of how Au atoms work in the AuPd catalyst a very challenging work, and often vaguely

382

interpreted

383

tailoring/characterizing the structure of Pd sites on Au NWs surface, new door opened for

384

ranking the relative importance of the CNPd-Pd and d-orbital occupation state, and clarifying how

385

Au atoms influence the catalytic performance of AuPd in the cleavage of C-X bond preliminarily.

386

To this end, multivariable regression analysis was first performed for the association of the

387

activity of Au@Pd catalysts with the CNs and WL intensity of Pd center. As presented in Table

388

S6-8, the CNs (P < 0.005 for CNs in the case of 4-CP) and WL intensity (P < 0.005 for WL in

389

the case of 4-IP) are of significant influence. Besides, the high model R2 (> 0.9 in both cases)

390

reveals that it is reasonable to assume Au and Au@AgML promotes the activity of Pd atom in a

391

similar manner. On the other hand, it seems the third factor-the Au/Ag-Pd interfacial sites cannot

392

be ignored for 4-BP, as both the low model R2 and the high P-value (R2 = 0.48, P > 0.1 for CNs

393

and WL intensity) indicate that CNs and WL intensity alone are insufficient to explain the high

394

activity of Au@Pd.

as

the

bimetallic

effect.

Herein,

with

the

capacity

of

systematically

395

Critical number of Pd atoms in the reactive center for the cleavage of C-X bonds. Based

396

on the dependence of the activity on the Pd coverage shown in Figure 3, as well as the changes in


18

Page 19 of 36


397

the CNPd-Pd revealed by the EXAFS data and the coordination model (Figures 4b-d, S21), the

398

activity of the Pd atoms was strongly influenced by their atomic geometry, more specifically, the

399

CNPd-Pd, as summarized in Figures 5a-c. The observed inactivity of Au@AgML@Pd with Pd

400

coverages of 1/16 and 1/8 ML in the catalyzed dehalogenation of 4-CP and 4-BP indicates that a

401

single Pd atom is insufficient to activate/break the C-Cl and C-Br bonds. However, the opposite

402

situation was observed in the case of C-I, where the highest activity was obtained for the

403

Au@AgML@Pd and Au@Pd catalysts with the lowest Pd coverage, indicating that isolated Pd

404

atoms are highly active for the cleavage of C-I bonds (Figure 5a). Notably, the fast cleavage of

405

the C-I bond did not generate phenol, instead, 4,4-dihydroxybiphenyl was identified as the main

406

product. A plausible explanation for this is the strong adsorption of I- on Pd atoms hinders the

407

simultaneous activation of H2 and C-I bonds. Meanwhile, Au or Ag atoms facilitate the cleavage

408

of C-I bond and form Au-C/Ag-C intermediates, and finally the M-C intermediates (M=Au, Ag

409

or Pd) coupled into 4,4-dihydroxybiphenyl. On the other hand, satisfactory activity was observed

410

for Au@Pd and Au@AgML@Pd in the cleavage of C-Br catalyzed by with when the CNPd-Pd

411

reaches 2~2.5 (Figure 5b), indicates 3~4 continuous Pd atoms may form the active center in this

412

process. The case is even more apparent in the cleavage of C-Cl bonds, in which the activity was

413

linearly related to CNPd-Pd (Figure 5c) and independent of the metal atoms underneath (Au or

414

Ag), suggesting that 5-7 contiguous Pd atoms act as the active center in this reaction.

415

The cleavage of a C-X bond involves the adsorption of C-X onto a reactive center, activation

416

of C-X and H2, and finally, the breakage of C-X into C-H and X- (Figure 5f). As predicted by

417

Honkala et al.,12 only Pd atoms are favourable adsorption sites. Moreover, on the Au surface, Pd

418

sites with larger CNPd-Pd have higher adsorption energy. The observation that 5-7 contiguous Pd

419

atoms form the most active site for the cleavage of C-Cl bonds, matches well with the theoretical


19


Page 20 of 36

420

prediction that Pd4/Au is the most favourable site for the adsorption of C-Cl.12 Since the co-

421

adsorption of 4-CP and H2 (roughly needs 2 Pd atoms53) is necessary for the conversion of 4-CP

422

to phenol, it is reasonable to conclude that more Pd atoms, i.e., 5-7 atoms, are needed in this

423

process. As the affinity of Br and I for Au/Ag atoms increases, the number of Pd atoms in the

424

optimized adsorption site decreases accordingly, and only one Pd atom is sufficient for the

425

cleavage of C-I bonds.

426

The influence of d-orbital occupation state of Pd atoms. The CNPd-Pd or the size of the Pd

427

cluster is not the only factor that determines the adsorption energy, otherwise the Pd

428

monometallic catalysts would be more active than Au@Pd. In fact, the Au and Ag atoms

429

underneath the Pd layer also alter their d-orbital occupation state, and Pd atoms with higher d-

430

orbital occupation state display higher adsorption energies. Furthermore, the linear relationship

431

between the activity of the Au@Pd and Au@AgML@Pd NWs in the cleavage of C-I and the WL

432

intensities of the catalysts shown in Figure 5d demonstrates that Au or Au/Ag atoms enhanced

433

the activity of the Pd atoms through changing the d-orbital occupation state of Pd atoms. A

434

similar situation was observed in the reaction with C-Br bonds after excluding the contribution

435

from the ensemble effect. For example, Au@AgML@Pd (1/2 ML) and Au@Pd (1/2 ML)

436

possessed similar CNPd-Pd (2~2.5), but the former was much more active (Figure 5b). Note that

437

even though the highest activity in the cleavage of C-Cl bonds was observed for the Au@Pd and

438

Au@AgML@Pd NWs with the highest Pd coverage (highest WL intensity, or lowest d-orbital

439

occupation state), this does not mean that the d-orbital occupation state of Pd was less important

440

in this reaction. In fact, the 10-fold higher activity of the Au@Pd/Au@AgML@Pd NWs over that

441

of the Pd NWs/NCs demonstrated in Figure 2 stemmed from the increased d-orbital occupation

442

state.


20

Page 21 of 36


443

Mechanistically, the breakage of C-X bonds involves the oxidative addition of C-X on Pd

444

atoms to form Pd-X and Pd-C bonds through overlap between the lone pairs of electrons in the

445

X/C atoms and the unoccupied d-orbitals of the Pd atoms. This oxidative addition step is then

446

followed by the reductive removal of the halide ion and the formation of a C-C or C-H bond.19

447

The enhanced occupation of the d-orbit either strengthens the anti-bonding interaction or

448

weakens the Pd-X/Pd-C bonds, which is very essential for the subsequent removal of C/X from

449

the Pd atom and regeneration of the active site. Therefore, in addition to its contribution to the

450

determination of the adsorption energy, the d-orbital occupation state also directly influences the

451

activity of Pd atoms during the cleavage of C-X bonds.

452

The Au (Au@AgML)-Pd interfacial sites. In addition to the CNPd-Pd and d-orbital occupation

453

state, the Au/Ag-Pd interface sites also plays a vital role in the cleavage of C-X bonds. This is

454

especially important in the case of C-Br (Figures 5b, e, S25), where the Au@Pd (1/2 ML) and

455

Au@AgML@Pd (1/2 ML) were much more active than their counterparts with Pd coverages of

456

1.0 ML. Since the d-orbital occupation state of Au@Pd (1/2 ML) was similar to that of Au@Pd

457

(1.0 ML), the only explanation for this difference in reactivity is the decreased abundance of

458

Au/Ag-Pd interfaces. Cooperation between Pd and Au/Ag atoms also likely to occur in the

459

cleavage of C-I bonds, especially in the case of isolated Pd atoms in Au@AgML@Pd NWs,

460

where Pd atoms are coordinated by 4～5.5 Ag atoms (Table S3). This value is larger than the

461

theoretical one (3 for Pd ML on Au/Ag {111} facet), and indicates that parts of Pd atoms

462

diffused into the subsurface of Ag layer.54 Whether the location of Pd atoms on Ag overlayer

463

influence their catalytic performance, especially when Ag may involve in the cleavage of C-X

464

bond still needs further study. However, the role of Au/Ag-Pd interface sites is not very

465

significant in the case of C-Cl. We hypothesize that this difference in the importance of Au-Pd


21


Page 22 of 36

466

interface sites between C-Br and C-Cl arises from the difference in their binding affinities with

467

Au or Ag atoms,19 where the higher affinity between Br and Au or Ag atoms increases the

468

likelihood that these metal atoms participate in C-X cleavage, i.e., by forming a Br-Au/Ag bond

469

to weaken the C-Br bond. Remarkably, the involvement of Au or Ag atoms into the catalysis

470

process is not a necessary for the cleavage of the C-X bond. Only when the size of Pd ensemble

471

is not big enough for the adsorption/activation of the C-X molecule, the role of neighboring Au

472

or Ag atoms can be significant.

473

Removal of low concentration halogenated phenols at neutral pH. To evaluate whether the

474

mechanism proposed from the dehalogenation of high concentration pollutants in alkaline still

475

holds up for more environmentally realistic condition, i.e., with low concentration of pollutant

476

and neutral pH, where the catalyzed dehalogenation process would occur, the catalytic

477

performance of Au@Pd and Au@AgML@Pd NWs in the removal of 0.02 mM halogenated

478

phenols at pH 7.0 ± 0.5 were also studied. As presented in Table 1 and Figure S26-28, Au@Pd

479

and Au@AgML@Pd NWs showed high activity in catalyzed cleavage of the C-X bond at

480

environmental relevant condition. Moreover, a similar trend was observed in both experimental

481

conditions, i.e., high concentration at basic solution and low concentration at neutral pH.

482

Specifically, isolated Pd atoms (with Pd coverage no more than 1/4 ML in Au@AgML@Pd NWs)

483

showed extremely low activity in the case of 4-BP and 4-CP (effect of CNPd-Pd), while with the

484

highest activity in the hydrogenation of 4-IP (effect of d-orbital occupation state). On the other

485

hand, with the increase of Pd coverage, both Au@Pd and Au@AgML@Pd NWs showed

486

markedly increased activity in the removal of 4-CP. Meanwhile, the significant higher activity of

487

Au@Pd(1/2 ML) and Au@AgML@Pd(1/2 ML) over their counterparts covered with 1.0 ML Pd

488

in the removal of 4-BP, again infers the important role of Au/Ag-Pd interfacial sites.


22

Page 23 of 36


489

Environmental implications. The findings of this study highlight the potential for employing

490

rationally designed Pd nanoarchitectures with well-defined structures in the determination of

491

structure-activity relationships during environmental catalytic processes. Considering the

492

increasing importance of Pd and other Pt group metals based AuM bimetallic catalysts in various

493

environmental decontamination processes,21-25 and the demonstrated generality of coating an

494

additional metal on the surface of ultrathin Au/Au@Ag NWs to generate a highly effective

495

catalyst,32, 55 this study may create important opportunities for the atomically precise synthesis of

496

various supported bimetallic catalysts with core–shell structures for broad applications, in

497

particular for fundamental mechanistic studies. Meanwhile, previous work by Wong et al showed

498

that Au atoms also increased Pd atoms’ resistance to other deactivation factors like sulfur

499

poisoning, showing the potential of developed Au@Pd and Au@AgML@Pd NWs for water

500

treatment.56-57 Noteworthy is although C-I itself is easily cleaved in the presence of Pd atoms, the

501

high affinity of I atoms/ions for Pd atoms may hinder their further reaction. Therefore, iodinated

502

pollutants may be resistant to degradation by Pd catalysts and should be studied further.

503

ASSOCIATED CONTENT

504

Supporting Information.

505

Documentation of the experimental details; additional results of the study (HAADF-STEM-EDS

506

mapping of NWs, SERS spectra of chemisorbed 2,6-DMPI on Au@Pd and Au@AgML@Pd NWs,

507

catalytic data and fitted Pd k-edge EXAFS curve); additional discussion.

508

AUTHOR INFORMATION

509

Corresponding Author


23


510

* E-mail: [email protected].

511

Notes

512

The authors declare no competing financial interest.

513

ACKNOWLEDGEMENTS

Page 24 of 36

514

We acknowledge the editor and the anonymous reviewers for their constructive comments,

515

criticism and suggestions. This work was financially supported by the National Key R&D

516

Program of China (2016YFA0203102), the National Natural Science Foundation of China

517

(21577157 and 21777177), and the Strategic Priority Research Program of the CAS

518

(XDB14020101). The authors acknowledge the staff at the BL 14W1/1W1B beamline of the

519

SSRF/BSRF for their assistance during the XAS measurements. Liu R. acknowledges support

520

from the Youth Innovation Promotion Association of CAS.

521

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522

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Zhang, L. Y.; Wang, B. L.; Ding, Y. X.; Wen, G. D.; Abd Hamid, S. B.; Su, D. S.,

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Andersin, J.; Honkala, K., First principles investigations of Pd-on-Au nanostructures for

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30

Page 31 of 36


688

Figures

689

Figure 1. (a-d) Representative TEM images of the Pd catalysts in this study: (a) Pd NCs, (b) Pd

690

NWs, (c) Au@Pd NWs and (d) Au@AgML@Pd NWs. The corresponding HRTEM images and

691

fast Fourier transform images are shown as inserts. (e) TEM images of Au@Pd NWs loaded on

692

Fe3O4 NSs. (f) Immobilization efficiency of different Pd nanostructures on Fe3O4 NSs.

693


31


Page 32 of 36

694

Figure 2. Catalytic performance of the Pd nanostructures in the hydrodechlorination of 4-CP

695

(1.0 mM 4-CP+1.5 mM KOH). (a-c) 4-CP conversion, first-order kinetics data, and mass

696

normalized activity of Pd NWs (5.0, 10.0 and 20.0 µM ), Pd NCs (20.0 µM), Au@Pd NWs,

697

Au@AgML@Pd NWs (5.0 µM of Pd) and Au@Pd NWs on Fe3O4 NSs.

698 699


32

Page 33 of 36


700

Figure 3. Catalytic performance of Au@Pd and Au@AgML@Pd NWs with varying Pd coverage

701

in the catalyzed dehalogenation of (a-c) 4-CP (1.0 mM 4-CP + 1.5 mM KOH, 5.0 µM Pd catalyst)

702

and (d-f) 4-BP (1.0 mM 4-BP + 1.5 mM KOH, 0.5 µM Pdcatalyst). (g-i) Activity of Au@Pd (red

703

curves) and Au@AgML@Pd NWs (blue curves) in the dehalogenation of 4-IP (1.0 mM 4-IP + 1.5

704

mM KOH, 5.0 µM Pdcatalyst).

705 706


33


Page 34 of 36

707

Figure 4. a) CO-stripping curves of Au@Pd NWs. b-d) Fourier-transformed k3-weighted Pd k-

708

edge EXAFS spectra of Pd, Au@Pd, and Au@AgML@Pd NWs. e, f) Normalized Pd K-edge

709

XANES spectra of Au@Pd NWs and Au@AgML@Pd. The spectra of Pd foil, Pd NWs and Pd

710

NCs are also shown for comparison.

711 712


34

Page 35 of 36


713

Figure 5. Influence of the various primary factors on the activity of Au@Pd and Au@AgML@Pd

714

NWs in the cleavage of C-X bonds: a-c) geometric effect, d) electronic effect, and e) synergistic

715

effect. Schematic diagram of f) the size of the Pd ensemble on Au or Au@AgML increases with

716

the increase of coordination number (CN) and g) the mechanism by which Au atoms influence

717

the activity of Pd in the cleavage of C-X bonds

718 719


35


Page 36 of 36

720

Table 1. Catalytic performance of Au@Pd and Au@AgML@Pd NWs with varying Pd coverage

721

in the catalyzed dehalogenation of low concentration (0.02 mM) of 4-CP/4-BP/4-IP at neutral pH

722

(7.0± 0.5).

Catalyst

Au@Pd

Au@AgML@Pd

Pd coverage (monolayer, ML)

Initial TOF in (molphenol · molPd-1·min-1) 4-CP

4-BP

4-IP

1.0

9.10 ±2.19

55.6±9.8

0.75 ± 0.31

1/2

5.93±1.13

68.2 ± 12.3

1.06 ± 0.29

1/4

5.35±1.38

37.1 ± 5.7

2.50 ± 0.32

1/8

3.27±0.55

28.8 ± 4.5

3.02 ± 0.43

1/16

0.90±0.35

16.2 ± 3.3

4.41 ± 1.33

1.0

8.21±1.79

44.8 ± 10.3

0.95 ± 0.25

1/2

6.11±1.76

61.3 ± 17.7

2.02 ± 0.55

1/4

0.19±0.05

2.62 ± 0.53

3.11±0.90

1/8

-

3.63 ± 0.56

1/16

-

5.49 ±1.35

723


36

Au@Pd Bimetallic Nanocatalyst for Carbon- Halogen Bond Cleavage

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