Defect Sites in Ultrathin Pd Nanowires Facilitate the Highly Efficient

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Remediation and Control Technologies

Defect Sites in Ultrathin Pd Nanowires Facilitate the Highly Efficient Electrochemical Hydrodechlorination of Pollutants by H

*ads

Rui Liu, Huachao Zhao, Xiaoyu Zhao, Zuoliang He, yujian Lai, Wanyu Shan, Deribachew Bekana, Gang Li, and Jing-fu Liu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02740 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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Environmental Science & Technology

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Defect Sites in Ultrathin Pd Nanowires Facilitate the

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Highly Efficient Electrochemical

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Hydrodechlorination of Pollutants by H*ads

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Rui Liu,*a Huachao Zhao,a,b Xiaoyu Zhao,b Zuoliang He,a,c Yujian Lai, a,c Wanyu Shan, a, c

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Deribachew Bekana, a, c Gang Li,a and Jingfu 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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Technology, Tianjin 300457, China

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c

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

College of Chemical Engineering and Materials Science, Tianjin University of Science and

University of Chinese Academy of Sciences, Beijing 100049, China

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ACS Paragon Plus Environment

1


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ABSTRACT

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Adsorbed atomic H (H*ads) facilitates indirect pathways play a major role in the

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electrochemical removal of various priority pollutants. It is crucial to identify the atomic sites

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responsible for the provision of H*ads. Herein, through a systematic study of the distribution of

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H*ads on Pd nanocatalysts with different sizes and, more importantly, deliberately controlled

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relative abundance of surface defects, we uncovered the central role of defects in the provision of

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H*ads. Specifically, the H*ads generated on Pd in an electrochemical process increased markedly

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upon introducing defect sites by changing the morphology to ultrathin polycrystalline Pd

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nanowires (NWs), while dramatically reduced upon decreasing the number of surface defects

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through an annealing treatment. Benefiting from a proportion of H*ads up to 40% of the total H*

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species, the Pd NWs showed an electrochemical active surface area normalized rate constant of

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13.8 ± 0.8 h-1 m-2, which is 8-9 times higher than its Pd/C counterparts. The pivotal role of

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defect sites for the generation of H*ads was further verified by blocking such sites with Rh and Pt

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atoms, while theoretical calculation also confirms that the adsorption energy of H*ads on these

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sites is much higher than that on the Pd{111} facet.

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KEYWORDS:

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Nanowire Catalyst, Hydrogen Evolution Reaction, Active Center, Indirect Pathway, Halogenated

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Organic Pollutants.

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INTRODUCTION

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The removal of environmental priority pollutants and disinfection byproducts with high

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efficiency under mild conditions through electrocatalytic reduction is an ever-growing area of

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research.1-11 Unlike the chemical or biological reduction process in which a reductive substance,

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e.g., H2, functions as the electron donator, the electrocatalytic reduction process is reagent free,

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and the electron itself reduces the target pollutant at the cathode surface through either a direct or

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an indirect pathway. Direct reduction occurs by electron tunneling or the formation of a

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chemisorption complex between the pollutant and cathode material, while in the indirect

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pathway, the electron primarily reduces a proton to form a surface-adsorbed atomic H (H*)

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species in a step known as the Volmer process.12 The indirect pathway usually occurs at a low

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overpotential, which is crucial for effectively suppressing side processes such as the hydrogen

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evolution reaction (HER), and, therefore, shows higher electron utilization and Faradaic

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efficiency over the direct pathway. Naturally, the electrochemical reduction process dominated

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or enhanced by the indirect pathway has drawn increasing research focus.4, 6, 8 In this regard, Pd

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is the most favorable electrocatalyst for indirect electrochemical reduction,13 not only because of

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its high efficiency in taking up protons to generate H* species at low overpotentials but also for

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its ability to retain the H* species via adsorption onto the Pd surface (H*ads) and absorption to the

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Pd atoms through the formation of Pd-H bonds (H*abs).14 However, in addition to the large

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portion of formed H* species that are converted to hydrogen molecules through the Heyrovsky

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(formation of H2 molecule by the reaction between electron, proton and H*ads) or Tafel

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(combination of two H*ads into one H2 molecule) step,12 which lowers the decontamination

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efficiency, recent work by Jiang et al. revealed that H*ads, which only accounts for less than 10%

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of the atomic H, is the sole active species for the reduction of halogenated pollutants.14


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Nevertheless, highly efficient indirect reduction of a pollutant can be achieved either by the

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judicious selection of the operating potential14 or by using other materials that show an improved

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surface/subsurface binding energy for H*ads.6 Thus, further identification of atomic Pd sites for

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the generation/retainment of H*ads15 and development of new materials/strategies that effectively

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remove hazardous pollutants electrochemically under mild conditions are highly desirable.

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Recently, two separate studies highlighted the influence of defect sites on the

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generation/retainment of atomic H* and hydrogen diffusion behavior.6, 16 Liu et al. reported that

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the formation of H2 was effectively suppressed by introducing defects into TiO2, and, as a result,

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highly efficient electrochemical reduction of nitrophenol was achieved.6 In addition, dislocations

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in Pd crystals were observed to effectively trap H atoms.16 Thus, we propose that defect sites are

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responsible for the stabilization of H*ads and that the performance of electrochemical

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decontamination mediated by the indirect pathway can be enhanced by increasing the density of

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such sites in Pd and related catalysts. To verify this hypothesis, we synthesized Pd/C catalysts

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with different sizes (from 2 to 6 nm) but similar degrees of defects and deliberately reduced the

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amount of defects by annealing Pd/C at elevated temperature. Moreover, as a strategy for

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maximizing the relative abundance of defects, the Pd nanoclusters were allowed to interconnect

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to form polycrystalline ultrathin nanowires (NWs), during which a large portion of the Pd facets

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were transformed into defect structures, such as stack faults (SFs, interruption of the normal

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stacking sequence of the atomic planes) and twin boundaries (TBs, atoms on either side of a

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plane are mirror images of each other with a 141° angle for the {111} facets).17-19 The cyclic

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voltammetric (CV) behavior of the synthesized Pd catalysts in Na2SO4 and 2,4-dichlorophenol

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(2,4-D) provided clear evidence that the relative abundance of defect sites in the Pd catalyst was

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the key factor that determines their ability to suppress the HER process and the amount of H*ads


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retained. Benefiting from the excellent H*ads generation/retainment capacity, the Pd NWs showed

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a higher 2,4-D removal efficiency with a six-times-lower Pd loading. Furthermore, we elegantly

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deposited Pt and Rh atoms, which exhibit high catalytic activity at defective sites but have low

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H*ads stabilizing capacity. The dramatically decreased amount of H*ads and the markedly reduced

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2,4-D removal efficiency unambiguously revealed the central role of the defect sites rather than

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the intrinsic catalytic activity of Pd NWs providing the primary contribution to the 2,4-D

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electrochemical reduction process. Finally, we calculated the adsorption energy diagram for H*ads

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on the Pd {111} facet and defect site and found that H*ads adsorbed on defect sites are much

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more stable over its counterparts on the Pd {111} facet in good agreement with our experiment

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data.

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EXPERIMENTAL SECTION

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Synthesis of Pd/C catalysts. Pd/C catalysts with different diameters were synthesized by

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following the reported method with necessary modification.20-21 Specifically, the 10 wt% Pd/C

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catalysts were synthesized in the presence of ethylenediaminetetraacetic acid (EDTA), 2-fold, 5-

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fold or no NaOH, and referred to as Pd/C-EDTA, Pd/C-NaOH-2, Pd/C-NaOH-5 and Pd/C-

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NaOH-0, respectively. Pd/C-NaOH-2 was annealed at 400°C for 2 hours under a continuous N2

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flow to increase the crystallinity, and the resulting material was denoted Pd/C-NaOH-2-400°C.

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For the preparation of Pd/C electrocatalyst ink, Pd/C catalyst was dispersed in a mixture of

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deionized water, isopropyl alcohol, and Nafion (v/v/v = 4/1/0.05) under sonication for 15 min,

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with a final Pd concentration of 5.35 mg mL−1.

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Synthesis of Pd, Pd@Rh and Pd@Pt NWs. Pd NWs with a diameter of ~2 nm were

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synthesized using our previously developed protocol, which referred to the reduction of

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Pd(NO3)3 by a proper amount of KBH4 with Triton X-114 as a stabilizer and structure director.17


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For the preparation of Pd@Rh or Pd@Pt core–shell NWs, 0.1, 0.2, 0.5, 1.0, or 2.0 mL of ice-cold

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(stored in ice bath for 30 min before use) 1.0 mM RhCl3 (or H2PtCl6) solution was added

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dropwise into 10.0 mL of freshly synthesized Pd NWs under stirring.22-23

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Electrode preparation. The working electrode was prepared by loading the desired amount

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of Pd catalyst onto a glassy carbon electrode (GCE, 5.0 mm inner diameter, id) or carbon paper

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(Toray 090, with a thickness of 280 µm and a porosity of 0.78, pretreated according to the

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reported method14). A 50 µl aliquot of a Pd NW (0.05 µmol Pd) or Pd@Pt/Pd@Rh NW

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dispersion containing 0.05 µmol Pd was drop cast onto a GCE as a supportless electrocatalyst.

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This was followed by adding 10 µl of a Nafion solution (0.5% w/v in ethanol, DuPont). For Pd/C

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electrocatalyst, a 10.0-µl aliquot of Pd/C ink was modified on the GCE. For electrochemical

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hydrodechlorination (EHDC) of 2,4-D, the Pd catalysts were modified on carbon paper, and the

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amount of Pd in Pd/C was increased to 2.0 mg. Whereas for Pd NWs, the amount of Pd loaded

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was 0.36 and 2.0 mg, with the Pd NWs-modified carbon paper washed three times with acetone

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to remove the residual Triton X-114 before the addition of Nafion. The prepared electrode was

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activated in a N2-saturated 0.1 M HClO4 solution under cyclic voltammetry at −0.25 to 1.0 V. Pt

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wire and a Ag/AgCl electrode were used as the counter and reference electrodes, respectively. To

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determine the mass content of the Pd catalyst, the catalyst dispersion was digested with aqua

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regia, filtered and then analyzed by inductively coupled plasma-mass spectrometry (ICP-MS,

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Agilent 7700).

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CO-stripping experiment. The electrochemical active surface area (EASA) for the Pd

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catalysts was estimated by a CO-stripping experiment, which was also utilized to demonstrate

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the presence of Pt/Rh atoms on Pd NWs. After CO was chemisorbed onto the surface metal

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atoms, it was then stripped at 50 mV·s-1, with the Pd surface area (SAPd) calculated by:21


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PdEASA=QCO-stripping/420 (mC cm−2)

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EHDC experiments. The EHDC of 2,4-D was performed in a two-compartment

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electrochemical cell separated by a proton-exchange membrane (Nafion-117). Ag/AgCl (3.0 M

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KCl) and Pt foil were utilized as the reference and counter electrodes, respectively. During the

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EHDC process, 50 mg·L−1 2,4-D (0.31 mM) in N2-saturated 50 mM Na2SO4 was electrolyzed at

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a constant potential, the residual 2,4-D and the generated 2-chlorophenol (2-CP)/4-chlorophenol

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(4-CP) and phenol were analyzed at specific time points by high-performance liquid

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chromatography (HPLC). The EHDC experiment was also performed in the presence of acetic

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acid/sodium acetate buffer with different initial pH to evaluate the effect of the solution pH on

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the EHDC process. To investigate the 2,4-D removal performance of the Pd NWs under more

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environmentally relevant conditions, 2.5 and 1.0 mg L-1 2,4-D were spiked into five real water

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samples collected from the Yellow Sea, the Olympic Green park, the Qunyu River, Qunming

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Lake and Beijing tap water.

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All reported data are mean values from at least three parallel experiments. The experimental

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uncertainties, including instrumental errors and relative standard deviations, and blank sorption

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were assessed in the absence of catalyst. The results showed that the total uncertainty was less

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than 3.0% for 2,4-D with high concentration, which slightly increased to 5~7% with the decrease

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of their initial concentration to 2.5 and 1.0 mg L-1.

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Simulation methodology. First-principles density functional theory calculations are carried

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out using the Vienna ab initio simulation package (VASP v.5.4.1) to examine the adsorption

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energy of the H atom on the free surface, vacancy, stack fault, and twin boundary. Throughout

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the calculation, the generalized gradient approximation (GGA) and the projector augmented

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wave (PAW) pseudopotentials with the exchange and correlation in the Perdew-Burke-Ernzerhof


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(PBE) are employed to calculate the total energy.24-25 A cut-off energy of 300 eV is used for the

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plane-wave basis. The bulk of Pd is first built and optimized using the Monkhorst-Pack 10 × 10

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× 10 k-pointing mesh. The lattice parameter for the optimized Pd unit cell is 3.94 Å. Based on

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the optimized Pd unit cell, the perfect plane, the plane with vacancy, the plane with stack fault,

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and the plane with twin boundary are built respectively using the slab model with a 2 nm thick

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vacuum layer added along the Z direction. During all calculations for the Pd planes, the

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Monkhorst-Pack 3 × 5 × 1 k-pointing mesh is used. The adsorption energy of H on the plane is

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calculated as:

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Eads = E(H) + EPd-site – EPd-site + H26

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Characterization. X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer

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with Cu Kα radiation in the 2ϴ range of 30-90°. Transmission electron microscopy (TEM) and

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high-resolution TEM (HRTEM) images were obtained on a JEM-2100F instrument, while

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spherical aberration corrected scanning transmission electron microscopy (Cs-STEM) and

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energy-dispersive X-ray spectroscopy (EDS) mapping were performed on a JEM-ARM200F

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system (JEOL, Japan). Electrochemistry experiments were carried out with an electrochemical

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workstation (CHI 852C, Chenhua Co., China).

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RESULTS AND DISCUSSION

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Pd catalyst characterization. Figure 1a displays TEM images of the Pd catalysts, which

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revealed varying sizes for the Pd NPs that are well-dispersed on the surface of carbon black and

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Pd NWs. Although we cannot quantitatively describe the amount of surface defects on the Pd/C

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catalysts, it is reasonable to assume that they have similar degrees of defects since they were

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synthesized using the same method. Note that although large NPs with diameters of up to 15 nm


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can be observed in Pd/C-NaOH-2-400°C, which is attributed to the sintering of neighboring Pd

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particles during the annealing process, the majority of NPs have diameters of less than 5 nm

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(Figure S1-6). The XRD patterns for the Pd samples shown in Figure 1b (marked with a red star)

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are consistent with the Pd standard (PDF#46-1043). The peak broadening analysis (based on the

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Scherrer equation) and the TEM observations show that the sizes of the Pd NWs, Pd/C-EDTA,

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Pd/C-NaOH-2, Pd/C-NaOH-5 and Pd/C-NaOH-0 samples are 2.38 ± 0.44, 2.24 ± 0.41, 2.51 ±

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0.37, 4.47 ± 0.90 and 6.27 ± 0.80 nm (Figure 1c), respectively, while the average size of Pd/C-

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NaOH-2-400°C is approximately 4.85 ± 1.48 nm. In accordance with the change in the size of

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the Pd catalysts, their surface area also changed. Estimated from CO-stripping, the EASA of

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Pd/C-EDTA, Pd/C-NaOH-2, Pd/C-NaOH-5 and Pd/C-NaOH-0 are 114.3, 93.9, 75.1 and 53.1 m2.

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g-1, respectively. For the partial sintering of Pd particles during the annealing process, the EASA

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of Pd/C-NaOH-2-400°C is largely reduced to 65.8 m2. g-1. However, this value is still 20% larger

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than that of Pd/C-NaOH-0, which is in good agreement with the smaller diameter of Pd/C-

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NaOH-2-400°C. On the other hand, the EASA of Pd NWs is also as large as 107.4 m2. g-1.

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The HRTEM image shown in Figure S1-6 displays a lattice spacing of ∼0.22 nm for the

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synthesized Pd nanocatalysts, which matches well with the {111} atomic planes of face-

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centered-cubic (fcc) Pd. Moreover, since the Pd/C catalysts were synthesized using chemical

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reduction in an ice bath, rich defect sites such as SFs and TBs were frequently observed in their

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HRTEM images (Figure S1-4). However, after annealing at 400°C for two hours, the majority of

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the defect sites were removed, and almost all of the Pd NPs became single crystalline (Figure S5).

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On the other hand, dense defect sites were distributed on the ultrathin Pd NWs, which formed by

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oriented attachment of nascent Pd clusters,17 with SFs (blue arrow), TBs (red arrow) as well as

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lattice defects (green arrow) observed across the whole NW (Figure 1d). Interestingly, the


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nonionic surfactant Triton X-114 utilized in the synthesis of the Pd NWs also facilitated the

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uniform distribution of the Pd NWs on carbon paper as a support-free electrocatalyst (Figure 1e).

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Enhanced H*ads provision capacity of Pd NWs. The H*ads provision capacities of the

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different Pd catalysts were first evaluated by CV in 50 mM Na2SO4 by varying starting potentials

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(from -0.65 to -1.15 V) but with a fixed ending potential of 0.45 V.14 As in the previous study,14

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upon negatively shifting the starting potential, broad peaks associated with the oxidation of

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molecular H2 and H*ads successively emerged at approximately -0.80 to -0.60 V and -0.10 to -

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0.00 V (Figure S8-13). For the Pd/C catalysts, when the starting potential was lowered to -0.95 V,

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the oxidation peak for H*abs emerged at ~-0.2 V, with its height rapidly increasing at the expense

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of the fast attenuation of the first two peaks. However, for the Pd NWs, besides the largely

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suppressed H2 oxidation peak, no apparent H*abs oxidation peak was observed at any potential

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except for a single broad oxidation peak at ~0 V, which is a direct result of the enhanced H*ads

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generation/storage capacity of the Pd NWs. The distribution of the different H* species on the Pd

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electrocatalysts at a starting potential of -1.05 V is highlighted in Figure 2a. Unlike the dominant

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oxidation peak for the H*ads observed for the Pd NWs at approximately 0 V, for all the Pd/C

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catalysts, regardless of their diameter, the main peak was located at ~-0.2 V with a small

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shoulder at ~0 V. In addition to the pronounced H*ads oxidation peak observed for the Pd NWs,

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the hydrogen evolution capacity of the Pd NWs was substantially weakened. Both the low

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cathodic current observed in Na2SO4 (Figure S14) and the high overpotential of -0.326 V (vs

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reversible hydrogen electrode, RHE) in 0.5 M H2SO4 (Figure S15-16) indicated an elevated

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activation energy for the HER process on the Pd NWs and the high binding affinity for H*ads on

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the Pd NWs. Moreover, the evident reduction peak in the backward scan reveals the reversibility

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of the H*ads reduction/oxidation cycle on defect sites.


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To verify that this observed H oxidation peak is associated with H*ads and that this H* species is

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active in the reduction of 2,4-D, the CV experiments were also performed in the presence of 50

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mg·L-1 2,4-D. As expected, the H*ads oxidation peak was completed quenched by 2,4-D, while

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the H2 and H*abs peaks remained nearly unchanged (Figure 2b). Moreover, from the backward

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scan data shown in Figure 2b and S12, the H* reduction peak was almost unaffected following

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the addition of 2,4-D. If the disappearance of H*ads is the direct result of coadsorption of 2,4-D on

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the H*ads adsorption site, we should have observed a significant decrease in the H* reduction peak.

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Otherwise, the unchanged H* reduction peak with the addition of 2,4-D rules out the possibility

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of 2,4-D coadsorption with H*ads on Pd sites, which is very crucial for Pd NWs to continuously

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and effectively provide H*ads for the electroreduction of 2,4-D.

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Based on the different oxidation behavior, the amount of the above-mentioned hydrogen

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species was estimated from the oxidation charge. Note the partial overlap of the H*ads and H*abs

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oxidation peak – the amount of H*abs is deduced from the oxidation peak between -0.4 to 0.2 V in

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the presence of 2,4-D, while that of H*ads is the difference in the oxidation peak in the presence

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and absence of 2,4-D.14 As is summarized in Figure 2c, the amount of H*ads increased with

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decreasing starting potential, with the largest amount generated at -1.05 V. Accordingly, the

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relative abundance of H*ads increased from approximately 10% at -0.7 V to 40% at -1.05 V but

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was lowered to ~30% with the further decrease of start potential to -1.10 V. In contrast, the

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relative abundance of H*ads for the Pd/C catalysts was in the range of 8% to 14% and further

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decreased to 5% for Pd/C-NaOH-2-400°C, strongly supporting the pronounced H* storage

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capacity of Pd NWs.

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Pronounced 2,4-D removal capacity of Pd NWs. The dramatically increased amount of H*ads

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on the Pd NWs is very meaningful for its usage as an advantageous electrocatalyst for the EHDC


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of 2,4-D. Although showed comparable activity with Pd/C catalysts in the reduction of 2,4-D

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with H2 as electron donor (Figure S17), Pd NWs displayed superior electrochemical

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decontamination performance in the constant-potential electrolysis of 50 mg· L-1 2,4-D.

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Catalyzed by 0.36 mg of Pd NWs for 6 hours, the removal ratio of 2,4-D increased stepwise from

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5.5 ± 1.4% at -0.65 V to 81.5 ± 4.1% at -1.05 V and largely decreased to 65.4 ± 3.6% with a

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further negative shift in the applied potential to -1.15 V (Figure 2d). Detailed analysis of the

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kinetics showed that the reduction was a pseudo-first-order process (Figure 2e), with the largest

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rate constant of 0.25 h-1 achieved at -1.05 V. Meanwhile, HPLC analysis revealed that phenol is

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the major product with a good mass balance of 94-108% (Figure S18), indicating the removal of

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2,4-D mainly occurs via the electrochemical reduction pathway. In contrast to the high 2,4-D

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removal efficiency of the Pd NWs, the activities of its Pd/C counterparts were much lower; even

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when the Pd loading was increased to ~2.0 mg, the highest removal ratio was only 66.6 ± 5.6%,

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which was achieved by the Pd/C-EDTA catalyst with the smallest diameter (Figure 2f and S19).

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With the gradually increasing size of the Pd NPs, this value decreased to 48.5 ± 4.5% for Pd/C-

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NaOH-0, which had the largest size of 6.0 nm. Importantly, although the EASA is 20% higher

253

than that of Pd/C-NaOH-0, the 2,4-D removal rate was further lowered to 43.5 ± 5.7% for Pd/C-

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NaOH-2-400°C.

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To highlight how the catalysis performance changes with Pd microstructure, the kinetics data

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for the Pd catalysts in the EHDC process were normalized with EASA (Figure 2g). Interestingly,

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for all the defect-containing Pd/C catalysts, the normalized rate constants are independent of the

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sizes of the Pd NPs, but almost remain constant in the range of 1.6~1.8 h-1·m-2. However, for

259

defect-free Pd/C-NaOH-2-400°C, this value drops to 1.03 ± 0.13 h-1·m-2. On the other hand, the

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normalized rate constant for Pd NWs is as high as 13.8 ± 0.8 h-1·m-2. This 8~9-fold increase in


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the activity of Pd NWs over defect containing Pd/C and ~14 times more catalytically active than

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defect-free Pd/C strongly support the superiority of defective sites in the EHDC process, as well

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as the effectiveness of introducing such sites by interconnecting Pd clusters into nanowires.

264

Meanwhile, besides showing the best EHDC performance, Pd NWs also showed a high current

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efficiency (CE) of 35.0 ± 2.9% during the whole EHDC process. In comparison, the CE for

266

Pd/C-EDTA, Pd/C-NaOH-2, Pd/C-NaOH-5 and Pd/C-NaOH-0 were decreased to 13.5 ± 1.2,

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14.9 ± 1.0, 16.6 ± 1.5 and 12.5 ± 1.2%, respectively, with this value further lowered to 8.5 ± 1.0%

268

for Pd/C-NaOH-2-400°C. Furthermore, the CEs from different Pd nanostructures displayed a

269

good linear dependence on their relative abundance of H*ads compared to the total amount of H

270

species (Figure S20), which supports the hypothesis that an H*ads mediated indirect pathway is

271

the main route for the EHDC process. Moreover, since the H*ads is generated from different Pd

272

electrocatalysts, the similarity in their reactivity infers that the studied Pd catalysts showed

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comparable activity in the EHDC of 2,4-D by H*ads.

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The critical role of H*ads in the EHDC of 2,4-D. To further demonstrate the critical role of

275

H*ads in the EHDC of 2,4-D, we first confirmed its presence by a 5,5-dimethyl-pyrroline-l-oxide

276

(DMPO) trapping and electron spin resonance (ESR) experiment (Figure 3a).8 Moreover, the

277

apparent linear correlation between the rate constant and the amount of H*ads generated (Figure

278

3a) verifies that H*ads (Figure 3b) is the essential reactive species in the removal of 2,4-D. In

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addition, the linear relationship also reflects the effective suppression of H2 formation, which

280

adversely affects the reduction of 2,4-D by inhibiting the mass transfer of 2,4-D to the reactive

281

sites.14 This was also verified by introducing tert-butanol (TBA), a previously reported effective

282

H*ads scavenger, into the reaction solution.8 With the addition of, approximately 5.0 mM TBA

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into the Na2SO4 electrolyte solution, an immediate quenching of the H*ads oxidation peak was


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observed (Figure 3c). This suppressed H*ads generation markedly changed the EHDC

285

performance, and the rate constant dropped by 38% and 81% in the presence of 1.0 and 10.0 mM

286

TBA, respectively (Figure 3d-e). Meanwhile, the presence of O2, which completes H*ads and

287

reactive sites with 2,4-D also has obvious influence on the EHDC performance of Pd NWs. The

288

EHDC efficiency dropped from 75.6 ± 1.7% for the N2 saturated solution to 35.2 ± 3.2% for the

289

air-saturated solution, and further decreased to 21.6 ± 2.0% when the solution is presaturated

290

with O2 (Figure 3d-e). The above results support the hypothesis that the H*ads-mediated indirect

291

pathway is the dominant mechanism during the EHDC of 2,4-D catalyzed by Pd NWs.

292

On the other hand, if we saturate the solution with H2, the 2,4-D removal rate, especially the

293

initial rate, can be largely increased, both in Pd NWs and Pd/C catalysts (Figure 3 f and g).

294

Specifically, on Pd NWs, the enhanced EHDC appears to be contributed by direct H2 reduction

295

because the 2,4-D removed by the EHDC process in the presence of H2 equals the amount of 2,4-

296

D removed by H2 reduction plus 2,4-D removed by the EHDC process in N2-saturated solution

297

(P> 0.05). Intriguingly, for catalyzed Pd/C-NaOH-2, the increased amount of 2,4-D removed

298

cannot be simply explained by H2 reduction, or the H2 molecule accelerated EHDC process itself

299

(P< 0.05), which again demonstrates that the EHDC of 2,4-D is hindered by the formation of H2

300

in Pd/C.

301

Notably, with the gradual uptake of H+ from the electrolysis solution, the solution pH was

302

quickly increased from 5.86 to more than 11 within 10 min of electrolysis (Figure 3h, i and S21).

303

Therefore, it is imperative to study how the solution pH influences the EHDC process, and more

304

importantly, whether the concentration of H+ is a crucial factor in determining the EHDC

305

kinetics.27 This was done by introducing an acetic acid/sodium acetate buffer into the electrolysis

306

solution. When the initial pH was adjusted to 3.72, 4.77, 6.09, and 7.10 by a buffer solution, the


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307

first-order rate constant for the Pd NW catalyzed EHDC process was slightly decreased from

308

0.30 ± 0.02 to 0.27 ± 0.01, 0.24 ± 0.02 and 0.18 ± 0.01 h-1, respectively. This weak pH

309

dependency reflects the fact that the EHDC of 2,4-D with Pd NWs can be performed under a

310

wide pH range. Moreover, this signifies an important factor that the stability of the generated H*

311

rather than the concentration of H+ is the rate-determining step in this process. In contrast,

312

catalyzed by Pd/C, i.e., Pd/C-NaOH-2, the rate constant decreased tenfold with an increase of the

313

initial pH from 3.72 to 7.10. This different pH-dependence of the EHDC kinetics on Pd NWs and

314

Pd/C is attributed to their different H+ or H* utilization efficiency. For the high H* utilization

315

efficiency of Pd NWs (up to 40%), less H* is needed compared to the case of Pd/C, where only

316

10% of H* take part in the EHDC process. It is worth pointing out that unlike the effective

317

control of the solution pH by the buffer, when the initial pH was adjusted to 7.10 with pure

318

sodium acetate, the solution pH also quickly increased to 11.3, similar to the case without buffer,

319

but showed much lower activity. We attributed this low activity to the acetate ion, which

320

competes with the binding site for 2,4-D on Pd nanostructures.

321

Blockage of Pd defect sites by deposition of Pt or Rh atoms. Moreover, to further highlight

322

the central role of defect sites in the EHDC process and to rule out the possibility that the

323

enhanced 2,4-D EHDC efficiency of Pd NWs stems from its intrinsic high catalytic

324

performance,28 we took advantage of the high surface energy of defective sites and selectively

325

deposited Pt and Rh atoms on them.22-23 As demonstrated in our previous works,22-23, 29 unlike the

326

formation of an atomic layer of Ag, Pd or Pt overlayer on Au NWs with a smooth surface and

327

low surface energy,29 the deposition of metal atoms on defect-rich Pd/Pt NWs initially occurs at

328

the high-energy sites, e.g., defective sites and the {100}/{110}facet,30 which results in the

329

formation of nanoislands (Figure S22-24).22 On the other hand, the deposited Pt/Rh atoms are


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330

highly active for the reductive cleavage of carbon-halogen bonds (Figure S17), including C-F

331

bonds with extraordinarily high energies,31 but are weak H*ads stabilizers, which can be

332

speculated from the low overpotential for the HER process on Pt and Rh electrocatalysts.26, 32-33

333

Therefore, if the enhanced 2,4-D EHDC performance of Pd NWs is a result of the superior

334

catalysis performance of Pd NWs, the deposited Pt/Rh atoms would increase the 2,4-D removal

335

rate, and vice versa, if the presence of highly catalytic active Rh/Pt results in the decreased

336

EHDC performance, the only explanation is that these atoms lowered the H*ads provision capacity.

337

As is shown in Figure 4a, EDX elemental mapping demonstrated that Pt atoms were enriched

338

on some parts of the Pd@Pt NWs, while benefiting from the large Z-contrast between Pd (Z=46)

339

and Pt atoms (Z=78), the atomic-resolution Cs-STEM image combined with intensity profile

340

analysis (Figure S25) revealed that the Pt atoms were enriched at the defect sites. Furthermore,

341

the appearance of a new CO stripping peak (Figure S26-28) at very low potential, approximately

342

0.25 V (vs Ag/AgCl) reveals the presence of highly catalytic active Pt/Rh atoms on the Pd NWs.

343

Since the CO-stripping peak from Pt NWs is located at ~0.70 V,22 the most plausible explanation

344

for the extremely high activity of Rh/Pt atoms on Pd NWs is that these atoms are located at

345

highly active sites such as defective sites.34-35 In addition, with the stepwise addition of Pt/Rh

346

adatoms, the peak associated with H*ads rapidly decreased and shifted to more negative values

347

(Figure 4b and c). At the same time, evident H2 and H*abs oxidation peaks were observed for

348

Pt/Rh deposition amounts of 5.0% or above. These results again demonstrated that these atoms

349

are primarily located at the defective sites and that their presence released the H*ads species and

350

promoted the HER process. In addition, since Pd@Pt and Pd@Rh NWs displayed the same

351

morphology and were stabilized by the same surfactant (TX-114) with Pd NWs, such large

352

negative-shifts of the oxidation peak upon the deposition of Pt/Rh atom on Pd NWs rule out the


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353

possibility that these factors, i.e., morphology or surfactant, are the main parameters influencing

354

the atomic H oxidation potential. Note that as a secondary contribution, the deposited Pt/Rh

355

atoms may also influence the electronic structure of the Pd atoms and change their affinity for

356

H*ads. In accordance with the decreased amount of H*ads, an immediate drop in the EHDC

357

efficiency was also observed. Importantly, as Pt atoms show better HER activity (or a lower

358

H*ads affinity) than Rh atoms, a much faster decrease in the EHDC efficiency was observed for

359

the Pt-modified catalyst (blue line in Figure 4d, e). When the amount of Pt/Rh relative to Pd

360

increased to 20%, the EHDC efficiency drops from 79.5 ± 7.1 to 23.8 ± 3.3 or 28.6 ± 4.1%, again

361

showing the importance of the Pd defect sites in the EHDC process. The presence of highly

362

catalytically active Pt/Rh but markedly decreased EHDC efficiency strongly supports the

363

hypothesis that the superior catalysis performance of Pd NWs is primarily associated with its

364

enhanced H*ads provision capacity.

365

Adsorption energy of H*ads (Eads) on Pd sites. The theory that defective sites possess

366

enhanced H*ads generation/retainment capacity was further supported by a density function

367

theory (DFT) calculation. Since the energy barrier for the Volmer process on Pd surface is very

368

low (down to 0.2 eV on Pd {111} facets)36, we ignore the difference in the amount of H*ads

369

generated on different Pd sites, and focus on the adsorption energy of this specie on Pd{111} and

370

three defective Pd sites, or the retainment capacity of H*ads on these sites. As shown in Figure 5a,

371

on the Pd{111}facet, both the atop- and fcc hollow-adsorption configuration are stable with an

372

Eads of 2.23 and 2.78 eV, while for lattice defect sites, where only the atop configuration is

373

permissible, the Eads increased to 2.45 eV, which is 0.22 eV higher than its counterpart on

374

Pd{111}. The situation is even more evident in the case of the TB and SF sites, where only the

375

fcc hollow-adsorption is stable with an Eads of 2.72 and 2.77 eV, respectively. Therefore,


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compared with high stability the H*ads on defective sites (especially on TB and SF site), only a

377

portion of H*ads on the Pd{111}, c.a., hollow-adsorbed H*ads, is stable enough and available for

378

the EHDC process.

379

Proposed mechanism. All the above results collectively point out that the defect sites in the Pd

380

NWs enhanced the stability of the generated H*ads, which in turn largely facilities its usage as an

381

electrocatalyst for the EHDC of 2,4-D. Polycrystalline Pd NWs have also been reported to

382

display superior H2 adsorption and dissociation/activation performance over that of Pd

383

cuboctahedra,28 which was attributed to the exposure of more Pd atoms at TB sites in the NWs.

384

In addition, these TB sites also have a high surface energy for the adsorption and catalytic

385

conversion of reactants. On the other hand, the defective-rich TiO2-x with abundant oxygen

386

vacancies or ≡Ti(III) was also shown to enhance electroreduction performance by increasing the

387

H* generation capacity. Meanwhile, these sites also increase the catalytic activity by ameliorating

388

the electron transfer kinetics and reducing electrode polarization.6 Herein, the enhanced EHDC

389

performance of Pd NWs neither result from its high activity (Figure 4, S16), nor the improved

390

electron transfer capacity of Pd NWs. In fact, our electrochemical impedance spectroscopy (EIS,

391

Figure S29) result showed that a Pd NW-modified electrode exhibits a much larger arc radius

392

than the Pd/C-modified electrode, revealing the increased interface impedance and the slowed

393

electron transfer in Pd NWs. Meanwhile, a four-point probe resistance measurement also

394

excluded possible interference from different resistances (Table S1). Therefore, the primary

395

result for the increased EHDC performance of Pd NWs is its superior H*ads generation/retainment

396

capacity.

397

We hypothesize that for the surface Pd atoms with “weak” binding energies, the H*ads generated

398

through the Volmer mechanism are highly mobile, i.e., the atop-adsorbed H*ads on Pd{111} sites


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399

quick diffuse into the Pd lattice to form a stable H*abs species or the formation of H2 (HER) by

400

the Tafel or Heyrovsky process. This results in a very low EHDC rate or even a long induction

401

time in which no EHDC occurs (Figure S30). Indeed, an induction time up to 20 min has been

402

observed by Sun et al. in a parallel study during which H*ads was converted into H*abs to saturate

403

the Pd crystalline rather than take part in the EHDC process,37 which results in a low current

404

efficiency of 3.7-6.7%. However, on the defect-rich Pd NWs, the high adsorption energy of H*ads

405

on such sites largely increased the stability of this crucial reactive specie for the effective

406

reduction of 2,4-D by suppressing the formation of H*abs (Figure 2a-c, 5b) and H2 (reflected in

407

the increased onset potential for the HER process(Figure S15).

408

Mechanistically, the reaction kinetic is controlled by both the binding energy of

409

reactants/intermediates/products on the reactive centers and the energy-barrier of the rate-

410

determining step. From this point, both the adsorption energy of the key reactant, e.g., H*ads and

411

the energy barrier of the rate-determining step for the cleavage of C-Cl bond, herein, influences

412

the EHDC performance. However, the presented data, especially the good linearity dependence

413

of the EC and amount of H*ads reveals that the amount of H*ads plays a pivotal role in determining

414

the EHDC process, with the effect of the adsorption energy on the reactivity of H*ads almost

415

insensible in the current study. This is because unlike the chemical reduction process, where a

416

sufficient number of electrons are supplied by H2 or other reductant, in the EHDC process, for

417

the insufficiency of the provided H*ads, the amount of H*ads becomes a crucial factor in

418

determining the reaction path. This is the reason why we focused our primary attention to the

419

stability of H*ads on different Pd atomic sites. From the viewpoint of the whole EHDC process, a

420

detailed calculation for how the adsorption configuration and adsorption energy of H*ads


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421

influences the reaction process is needed to obtain a more comprehensive conclusion on the

422

EHDC process on different Pd sites in the future.

423

Environmental implications. The defective Pd NWs are applicable to the decontamination of

424

2,4-D-polluted water through the EHDC process. This is demonstrated by the removal of 2.5 or

425

1.0 mg L-1 of 2,4-D from sea, river, lake or tap water. As presented in Table 1, up to 76.9% of

426

2,4-D in tap water was removed after six hours of electrochemical treatment. Even for the water

427

sample with a complex matrix, i.e., water sampled from the Beijing Olympic Green Park, which

428

is irrigated by recycled sewage, the EHDC efficiency was higher than 50%. It is worth noting

429

that compared with the satisfactory EHDC performance of Pd NWs in freshwater, the removal

430

rate of 2,4-D in sea water is much lower (

Defect Sites in Ultrathin Pd Nanowires Facilitate the Highly Efficient

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