MnO2 nanosheet array supported on Ni foam: An advanced electrode for electrocatalytic hydrodechlorination reaction

Journal Pre-proofs Full Length Article Hierarchical Pd/MnO2 nanosheet array supported on Ni foam: An advanced electrode for electrocatalytic hydrodechlorination reaction Junxi Li, Yiyin Peng, Wendong Zhang, Xuelin Shi, Min Chen, Peng Wang, Xianming Zhang, Hailu Fu, Xiaoshu Lv, Fan Dong, Guangming Jiang PII: DOI: Reference:

S0169-4332(20)30125-2 https://doi.org/10.1016/j.apsusc.2020.145369 APSUSC 145369

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

21 October 2019 26 November 2019 10 January 2020

Please cite this article as: J. Li, Y. Peng, W. Zhang, X. Shi, M. Chen, P. Wang, X. Zhang, H. Fu, X. Lv, F. Dong, G. Jiang, Hierarchical Pd/MnO2 nanosheet array supported on Ni foam: An advanced electrode for electrocatalytic hydrodechlorination reaction, Applied Surface Science (2020), doi: https://doi.org/10.1016/ j.apsusc.2020.145369

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1

Hierarchical Pd/MnO2 nanosheet array supported on Ni

2

foam: An advanced electrode for electrocatalytic

3

hydrodechlorination reaction

4

Junxi Lia,#, Yiyin Penga,#, Wendong Zhangc, Xuelin Shia, Min Chena, Peng Wangb, Xianming

5

Zhanga, Hailu Fud, Xiaoshu Lva, Fan Donga, Guangming Jianga, *

6

a

Engineering Research Center for Waste Oil Recovery Technology and Equipment, Ministry of

7

Education, Chongqing Technology and Business University, Chongqing 400067, China

8

9

b

c

College of Architecture and Environment, Sichuan University. Chengdu, 610065, China

Chongqing Key Laboratory of Three Gorges Reservoir Area Surface Process and Environmental

10

Remote Sensing，Chongqing Key Laboratory of Inorganic Functional Materials, Chongqing

11

Normal University, Chongqing, 401331, China

12

13 14 15

d

Department of Environmental Engineering, China Jiliang University, Hangzhou 310018, China.

# These

two authors contribute equally to this work. *Corresponding

author:

E-mail: [email protected] (G. M. Jiang).

16 17

1

1

Abstract

2

Electrocatalytic hydrodechlorination (EHDC) is deemed as one promising approach

3

for efficient and safe detoxification of the trace halogenated organic pollutants in

4

water. Here we prepared one advanced Pd/MnO2-Ni foam electrode via the

5

construction of oxygen-deficient MnO2 nanosheet arrays on Ni foam skeleton, which

6

then served as the support and electron donator to capture and reduce Pd precursor to

7

nanoparticles. With the three-dimensional porous structure, hierarchical skeleton

8

surfaces and improved Pd dispersion, the Pd/MnO2-Ni foam electrode delivered an

9

unprecedented large mass activity (kobs) of 0.883 min-1 mmolPd-1 for EHDC of

10

2,4-dichlorophenol (2,4-DCP), in comparison to 0.081 min-1 mmolPd-1 of the Pd/Ni

11

foam electrode and those reported in literatures. The Pd/MnO2-Ni foam electrode also

12

displayed high durability during the repeated batch EHDC experiments without the

13

efficiency decay and the leaching of Mn/Ni/Pd, unless some reduced sulfur

14

compounds and nitrite were included. The mechanism study revealed the MnO2 in

15

electrode served as a mediator to transfer H* from Pd to 2,4-DCP, which extended the

16

reactive area beyond Pd and hindered the molecular hydrogen evolution, leading to

17

the enhanced reactions between H* and 2,4-DCP. The Pd/MnO2-Ni foam electrode

18

was also tested in a continuous-flow EHDC system, and displayed the potential and

19

retention time-dependent performances.

20

Keywords: Palladium; Electrocatalysis; Ni foam; Persistent organic pollutant;

21

Hydrodechlorination; Wastewater treatment

2

1

1. Introduction

2

Halogenated organic compounds, including halogenated chain and aromatic

3

hydrocarbons, are an important class of chemical and industrial feedstock with wide

4

applications in pharmaceutical, agricultural and polymer industries. The large-scale

5

use, however, increases their exposure and impact in ecotope [1]. As the leading

6

member of persistent organic pollutants, these halogenated compounds are stable in

7

chemical structure, and highly resistant to natural degradation. They can also be easily

8

accumulated in living bodies via food chain, exerting long-term harms to organs and

9

immune systems [2]. In this case, the technologies that can remove them in an

10

effective

11

hydrodechlorination (EHDC) represents one promising alternative by its high

12

efficiency, mild condition, green feature and low secondary pollution risk [9, 10]. In

13

EHDC, numerous atomic hydrogen (H*) were in situ produced from aqueous solution

14

at cathode via electrolysis of water, which served as the reductive agent to attack and

15

cleave C-Cl bond, converting halogenated organics to their nonhalogenated analogues

16

and chloride ions [11-13].

and

green

manner

are

highly

desired

[3-8].

Electrocatalytic

17

The metallic palladium (Pd) was one priority cathode catalyst due to its high

18

efficiency and durability in producing H* from aqueous solution at a wide pH range

19

[14, 15]. Additionally, it showed strong power in adsorption and activation of the

20

halogenated pollutants for sequent hydrodechlorination reaction [16]. However, Pd is

21

one precious metal, and its low earth-abundance forces us to maximize its mass

22

activity and reduce consumption. Engineering the particle into a nanoscale is one 3

1

efficient strategy, which enabled to raise the exposure of Pd atoms at particle surface,

2

making them accessible for the desired reactions [17-19]. To further improve the

3

performance, these NPs were supported on the metallic Ti, Cu or Ni foam substrate

4

that owned a self-supported three-dimensional (3D) porous structure facilitating the

5

pollutant mass diffusion [20-22]. Cheng reported the first Pd NP/Ti mesh electrode

6

for removal of 2,4-dichlorophenol [23]. Since then, various foam electrodes, such as

7

the Pd/Ni foam and Pd/Cu foam electrode, were developed [24-26]. However, as the

8

Pd NPs were grown on the foam via a simple spontaneous galvanic reaction between

9

the foam metal and a Pd salt, their dispersion and size were usually not

10

well-controlled, making their overall mass activity still unsatisfactory. On the other

11

hand, the intrinsic activity of Pd in these electrodes was actually not improved, due to

12

the little synergy between Pd and the support in EHDC.

13

In recent years, the researchers found that decoration of the foam electrode with

14

some other active species can significantly promote the mass activity of Pd. He ever

15

deposited an Ag or Cu layer between the Ni foam and Pd NPs, and found that the

16

presence of Ag improved the dispersion of Pd NPs and contributed to the adsorption

17

of pollutants on electrode [27, 28]. Instead, Mao decorated the Cu foam with N-doped

18

graphene (N-GR) before the loading of Pd, and identified that N-GR contributed to an

19

enhanced H* generation [29]. Sun introduced the conductive polymer in electrodes,

20

which was proved to promote NP dispersion and H* generation [30]. Xu ever

21

modified the Pd/Ni foam electrode with TiN or TiC NPs as both of these NPs could

22

contribute a promotional synergy for H* generation [31, 32]. The oxides with the 4

1

metal component of diverse valences (such as MnO2 and TiO2) represented another

2

important class of active additives. Lou ever deposited the Pd NPs on Ni foam with its

3

skeleton covered by layers of MnO2. Their experimental results confirmed that the

4

introduction of MnO2 could reduce the Pd NP size, and enhance H* generation at the

5

Pd-MnO2 interfaces, leading to a significant enhancement in mass activity [33, 34]. In

6

addition, the hydrophilic features of the oxide benefited the mass diffusion of

7

reactants around electrode.

8

In this work, we developed another more efficient Pd/MnO2-Ni foam electrode for

9

EHDC of 2,4-dichlorophenol (2,4-DCP, one typical halogenated organic pollutant). In

10

contrast to that in Lou’s work with a compact layer structure, the MnO2 in our work

11

displayed a uniform nanosheet array structure with much larger surface areas. Notably,

12

sequent Pd depositing was conducted by pre-constructing oxygen vacancies on MnO2

13

sheet by a reductive current, which served as the active sites to catch and reduce Pd2+

14

to Pd. By our binder-free approach, the formed Pd NPs are small in size (around 3.5

15

nm), well dispersed on MnO2 nanosheet array and form strong interactions with

16

MnO2. As expected, the Pd/MnO2-Ni foam electrode displayed an unprecedented high

17

EHDC performance and mass activity in batch experiments, in comparison to the

18

Pd/Ni foam and that reported in known literatures. The cathode potential and

19

coexisting anions effect on EHDC performance of Pd/MnO2-Ni foam electrode were

20

then investigated. Given the robust EHDC performance, the electrode was applied

21

into a continuous flow EHDC system to assess its feasibility in practical applications.

22

Finally, the real role of MnO2 played during the EHDC was explored. 5

1

2. Experimental

2

2.1. Materials

3

Ni foam substrate (Pore density: 110 PPI; Porosity: 98%; Surface density: 380 g

4

m-2) was obtained from Kunshan Tengerhui Electronic Technology Co., Ltd., China.

5

Analytical grade of anhydrous ethanol, 2,4-dichlorophenol (2,4-DCP), p-chlorophenol

6

(p-CP), o-chlorophenol (o-CP), phenol (P), sodium sulfate (Na2SO4), sodium chloride

7

(NaCl), sodium nitrate (NaNO3), sodium nitrite (NaNO2), sodium sulfide nonahydrate

8

(Na2S·9H2O), palladium chloride (PdCl2) and potassium permanganate (KMnO4), as

9

well as chromatographical grade of methanol were all obtained from the Sinopharm

10

Group Chemical Reagent Co., Ltd. China. Stock solutions of 2,4-DCP, p-CP, o-CP

11

and P were prepared by dissolving 5 g of them in 1.0 L methanol.

12

2.2. Preparation of Pd/MnO2-Ni foam electrode

13

The Pd/MnO2-Ni foam electrode was prepared via a multi-step process. Following

14

Huang’s work [35], in-situ growth of MnO2 nanosheet array on skeleton of Ni foam

15

was achieved through a simple hydrothermal process. Specifically, Ni foam with a

16

size of 30 × 30 × 0.5 mm was washed in sequence by HCl solution (3 M), ethanol and

17

deionized water to remove the surface oxide and organics. The clean Ni foam was

18

then transferred to a Teflon-lined stainless steel autoclave containing 50 mL of

19

KMnO4 aqueous solution (1.5 mM), which was sealed and kept at 160 oC for 24 h for

20

the growth of MnO2 nanosheets. The as-prepared MnO2-Ni foam was washed with

21

water for several times, and vacuum-dried at room temperature. Then it was subjected 6

1

to a reductive current (-8.0 mA) at cathode in a NaCl solution. The reduced MnO2-Ni

2

electrode was quickly immersed in a mixed solution (120 mL) of PdCl2 (1 mM) and

3

NaCl (30 mM) for 4 hours, leading to the Pd deposition. For comparison, Pd/Ni foam

4

electrode was prepared by immersing the cleaned Ni foam in a mixed solution (120

5

mL) of PdCl2 (1 mM) and NaCl (30 mM) for 4 hours under magnetic stirring.

6

2.3. Electrochemical hydrodechlorination experiments

7

The batch and continuous flow EHDCs of 2,4-DCP were both conducted in a

8

H-model electrochemical cell at room temperature with a Pd/MnO2-Ni foam working

9

electrode, a Ag/AgCl wire reference electrode (3.0 M KCl, 0.201 V vs. standard

10

hydrogen electrode at 25 oC) and a Pt foil counter electrode (See Figure 1). To

11

prevent the generated Cl- diffusing to anode surface, the H cell was separated to two

12

chambers by a cation exchange membrane. For each run of batch EHDC, 100 mL of

13

N2-saturated 50 mM Na2SO4 aqueous solution was added in both chambers, and 1 mL

14

2,4-DCP stock solution was added to the cathode chamber to obtain the concentration

15

of 50 mg L-1. Anaerobic environment was guaranteed by a continuous N2 bubbling in

16

anode chamber, where oxygen bubbles were generated on anode. The electrolyte

17

solution was also vigorously stirred during reaction to promote the mass diffusion of

18

reactants. During EHDC, 0.5 mL of the sample was taken from the cathode chamber

19

at certain intervals for analyses. The EHDC efficiency was calculated by

20

η(%)=(C0-Ct)/C0×100%, where C0 and Ct denoted the concentration of 2,4-DCP at the

21

beginning and the time of t, respectively. Current efficiency for EHDC was

22

determined following our previous work [36]. The batch EHDC reaction was also 7

1

described by the Pseudo-first-order kinetics model:

2

lnCt/C0 = -kobs×nPd×t

(1)

3

kobs (min-1 mmolPd-1) is the Pd amount (nPd)-normalized apparent reaction constant

4

that is obtained from a linear regression of lnCt/C0 versus t, and used to describe the

5

mass activity of the catalyst.

6

For the continuous-flow EHDC reaction, the cathode chamber was continuously

7

fed with the N2-saturated 50 mM Na2SO4 solution containing 50 mg L-1 of 2,4-DCP,

8

while the anode chamber was fed with the N2-saturated 50 mM of Na2SO4 solution.

9

The EHDC efficiency was determined by δ(%)=(Cin-Cout)/Cin ×100%, where the Cin

10

and Cout denoted the concentration of 2,4-DCP in the feeding and outlet flow from

11

cathode chamber, respectively.

12

2.4. Characterization

13

The surface morphology and element distribution of electrodes were observed

14

using a scanning electron microscope (SEM, Hitachi S-570, Hitachi) equipped with

15

an energy dispersive X-ray spectroscopy (EDS). The size, morphology and lattice

16

fringe of Pd NPs on electrode were examined by the transmission electron microscopy

17

(JEM-2010, JEOL). Electronic states of the component elements on surface were

18

investigated by X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry (CV)

19

analyses and the EHDC experiments were performed using an electrochemical

20

workstation (CHI660E, Chenhua). The Pd mass loading on electrode and the

21

concentrations of Ni/Mn/Pd leached in the electrolyte solution were analyzed by

22

inductively coupled plasma-atomic emission spectroscopy (ICP-AES, ICP2060t, 8

1

Tianrui). The concentrations of 2,4-DCP and the intermediate products were analyzed

2

by the high performance liquid chromatography (HPLC, Shimadzu 2010AT) with a

3

UV detector at 280 nm and an ODS-SP column (150 × 4.6 mm). The mobile phase

4

was composed of phosphoric acid, methanol and water (V:V:V=0.05:1:4) at a flow

5

rate of 1.0 mL min-1. The redox feature of the electrode was examined by the H2

6

temperature-programmed reduction technique (H2-TPR, Belcat-M, MicrotracBEL)

7

with a thermal conductivity detector (TCD). The concentrations of NO3-, NO2- and

8

NH4+ in the electrolyte solution were determined by the gas phase molecular

9

absorption spectrometry (GMA3370, Beiyu).

10

3. Results and discussion

11

3.1. Electrode characterization

12

The hierarchical 3D Pd/MnO2-Ni foam electrode was fabricated by a facile

13

multistep process, as schematically illustrated in Figure 2. At the first step, the MnO2

14

nanosheet array was grown on the skeleton of Ni foam (MnO2-Ni foam) via a

15

hydrothermal reaction. The formed MnO2-Ni foam was then subjected to a reductive

16

current, by which partial Mn (IV) was reduced to low valences and some oxygen

17

vacancies formed on MnO2 sheet [37, 38]. The resultant MnOx-Ni foam was quickly

18

immersed into a Pd2+ aqueous solution. The vacancies became the active sites to catch

19

and reduce Pd2+ to Pd0 by their excess electrons, leading to the formation of Pd NPs.

20

Obviously, directly growth of the MnO2 on Ni foam and the Pd on MnO2 can form

21

strong adhesions and electronic interactions between Pd, MnO2 and Ni foam,

9

1

effectively avoiding the active species falling off and accelerating the electron transfer

2

in electrocatalytic reaction.

3

Figure 3 compared the typical SEM images of the bare Ni foam, MnO2-Ni foam

4

and Pd/MnO2-Ni foam. Bare Ni foam in Figure 3a exhibited a self-supported 3D

5

porous structure with very clean and smooth skeleton surfaces. After the growth of

6

MnO2 nanosheet array, obviously increased surface roughness could be observed on

7

the skeleton in Figure 3b. The MnO2 nanosheets displayed rippled silk-like structures,

8

and also intercrossed with each other, leading to the formation of hierarchical porous

9

structures with large surface areas. The as-synthesized Pd/MnO2-Ni foam electrode in

10

Figure 3c clearly exhibited more compact but uniform surface. The porous

11

microstructure was preserved, but the MnO2 nanosheets became thicker and rougher.

12

Figure 3d displayed a magnified SEM image of the skeleton surface, which

13

evidenced the porous structure, and the uniform dispersion of very small Pd NPs on

14

MnO2 nanosheet. The elemental mapping results (Figure 3e) and the EDS spectrum

15

(Figure S1) further confirmed the uniform deposition of Pd on MnO2 nanosheets.

16

Figure 3f showed the one typical TEM image of the Pd/MnO2 composites those were

17

peeled off from the electrode by ultrasonication. It could be seen that the Pd NPs with

18

a uniform size of ~3.5 nm are evenly distributed on the MnO2 sheet. The HRTEM

19

image of several Pd NPs in Figure 3g displayed clear lattice fringes with a spacing of

20

~0.23 nm, corresponding to the (111) plane of metallic Pd phase. As a contrast, direct

21

Pd deposition on Ni foam resulted in irregular and aggregated Pd microparticles with

22

a mean diameter of 2-4 μm (Figure S1). The Pd loading amount was determined to be 10

1

3.70 mg (0.205 mg cm-2) on Pd/MnO2-Ni foam electrode and 9.72 mg (1.08 mg cm-2)

2

on Pd/Ni foam electrode by ICP-AES. All the above results clearly demonstrated that

3

our strategy could effectively improve the dispersion of Pd over electrode, which

4

should raise its mass activity and improve the utilization of Pd.

5

XPS analysis was performed to understand the surface composition and chemical

6

nature of our Pd/MnO2-Ni foam electrode. Since the EHDC reaction was conducted

7

under a very negative potential of -0.85 V, which may affect the electronic states of

8

surface atoms, the high-resolution Mn 2p and Pd 3d XPS spectra of the Pd/MnO2-Ni

9

foam electrode before and after the EHDC reaction were both analyzed. The Mn 2p

10

spectrum of the fresh Pd/MnO2-Ni foam electrode in Figure 4a shows a couple of

11

peaks at 642.15 and 653.60 eV, which can be assigned to the 2p3/2 and 2p1/2 levels of

12

Mn4+ in MnO2 [39]. These two peaks were well maintained, and no peaks for Mn

13

species in other valence states were detected after the EHDC reaction, revealing that

14

the MnO2 phase was preserved, though the valence of Mn might shuttle between +4

15

and lower values during reaction. In the Pd 3d spectrum (Figure 4b), the two main

16

peaks at 335.37 and 340.68 eV correspond to Pd0 3d5/2 and 3d3/2, while the satellite

17

peaks at 337.05 and 342.41 eV represent 3d5/2 and 3d3/2 of Pd(II) [40]. The presence

18

of Pd2+ in the as-synthesized electrode might result from the insufficient reduction of

19

Pd2+ by the oxygen vacancies during synthesis or the oxidation of formed Pd0 when

20

exposed in air. After EHDC reaction, the Pd0 and Pd2+ were found to coexist in

21

electrode. According to the redox potential of Pd0/Pd2+ in Na2SO4 solution, the Pd2+

22

should be totally reduced by the negative current with a potential of -0.85 V [41]. The 11

1

formation of Pd2+ could be ascribed to the small size of Pd, making its surface

2

sensitive to the oxygen oxidation. In spite of the XPS results, we still believed that the

3

metallic Pd was the active site serving in EHDC reaction. On the other hand, the Pd

4

3d5/2 and Pd 3d3/2 peaks of the Pd/MnO2-Ni foam both shifted to low binding energies

5

by 0.45 eV, in comparison to those of Pd/Ni foam (Figure S2). It indicated that the

6

MnO2 exported some electrons to Pd, and also evidenced the stronger interactions

7

between Pd and MnO2.

8

3.2. EHDC performance of Pd/MnO2-Ni foam electrode

9

Figure 5a presents the EHDC performances of bare Ni foam, MnO2-Ni foam, and

10

Pd/Ni foam and Pd/MnO2-Ni foam electrodes in removing 50 mg L-1 of 2,4-DCP at a

11

fixed potential of -0.85 V. It is clearly shown that only the Pd-involved electrodes are

12

active to remove 2,4-DCP, confirming that the Pd is the only active site. Pd/MnO2-Ni

13

foam performed more efficiently, and can totally eliminate the 2,4-DCP in 150 min.

14

Under identical conditions, the Pd/Ni foam only removed 63%. Figure 5b shows that

15

the C/C0 decay on both the Pd/Ni foam and Pd/MnO2-Ni foam electrodes obey the

16

pseudo-first-order reaction kinetics. As expected, the Pd/MnO2-Ni foam electrode

17

delivers an unprecedented large apparent rate constant (kobs) of 0.883 min-1 mmol-1Pd,

18

which is nearly ten times that of the Pd/Ni foam electrode (0.081 min-1 mmol-1Pd). We

19

also compared our kobs with that of the reported catalysts in known literatures. The

20

results in Table 1 show that the developed Pd/MnO2-Ni foam electrode was very

21

competitive, and outperformed all the reported catalysts under close operating

22

conditions. The durability of Pd/MnO2-Ni foam electrode was further investigated by 12

1

repeating the EHDC reaction on the electrode for five times. As clearly shown in

2

Figure 5c, the Pd/MnO2-Ni foam electrode maintained its high EHDC efficiency and

3

current efficiency in the 5 cycles. More importantly, little Ni, Mn and Pd were

4

detected in the electrolyte solution, indicating the high chemical stability of electrode

5

structure (Table S1). The displayed high durability of the electrode could be ascribed

6

to the strong metal-oxide interactions among Ni, MnO2 and Pd, as well as the less

7

corrosive operating conditions (a negative working potential and an alkaline

8

electrolyte solution for EHDC).

9

As known, the hydrogen evolution reaction is one inevitable side reaction on

10

cathodes, which competes with EHDC reaction for the consumption of H* and electric

11

energy, and thus exerts a significant negative effect on EHDC. Current efficiency for

12

EHDC is one effective descriptor of the competition relationship between EHDC and

13

HER, and a larger one denotes a larger proportion of H* utilized by EHDC. As shown

14

in Figure 5d, the current efficiencies of both the Pd/Ni foam and Pd/MnO2-Ni foam

15

electrodes in the EHDC reactions increased initially, reached the peaks at around 60

16

min and then decreased. The initial increase could be attributed to the gradually

17

increased amount of H* on catalyst surface along with the reaction, which raised the

18

reaction frequency between 2,4-DCP and H*. The sequent current efficiency drop

19

resulted from the decreased concentration of 2,4-DCP in solution and the repulsive

20

force between cathode and the deprotonated 2,4-DCP [47], both of which reduced the

21

pollutant coverage on catalyst surface. Figure 5d also shows that the current

22

efficiency of Pd/MnO2-Ni foam electrode is larger than the Pd/Ni foam electrode in 13

1

the whole reaction, which means that the Pd/MnO2-Ni foam offers more opportunities

2

for 2,4-DCP to react with H*. In spite of these, the peak current efficiency for both the

3

electrodes are still kept at a low level, indicating that only a small fraction of H* is

4

used by EHDC and most of them evolve into molecular H2. It was also suggested the

5

H* on the electrode was actually excess for EHDC, and the low current efficiency

6

might result from the low concentration of 2,4-DCP used in our system, leading to a

7

low coverage of them on catalysts.

8

3.3. Influences of the operation conditions

9

Figure 6a summarizes the kobs of the Pd/MnO2-Ni foam electrode under different

10

cathode potentials, in which a near volcano-shaped curve is observed with the

11

maximum kobs achieved at -0.80 V. As revealed by some literatures, the cathode

12

potential mainly influenced the H* generation rate and the diffusion of 2,4-DCP

13

around cathode surface [36]. A larger potential accelerated the H* production, and

14

provided more H* for EHDC, leading to an enhanced reaction. However, a larger

15

potential simultaneously hindered the adsorption of 2,4-DCP on electrode through

16

enlarging the repulsive force between them, resulting in an inefficient reaction [47].

17

The volcano-like dependence of kobs on cathode potential revealed that the positive

18

effect dominated at the less negative potential, while the negative effect took the

19

dominative place at more negative potentials. The cathode potential was optimized at

20

-0.80 V for our electrode. Effects of the initial 2,4-DCP concentration on the EHDC

21

performance of our Pd/MnO2-Ni foam electrode were also investigated. The 2,4-DCP

22

mass removal-reaction time profiles presented in Figure 6b clearly show that more 14

1

2,4-DCP are removed in the same period under an increasing 2,4-DCP concentration

2

from 25 to 150 mg L-1. These results well supported our current efficiency results,

3

which revealed that the EHDC rate was limited by the coverage of 2,4-DCP on

4

electrode. A larger 2,4-DCP concentration enabled to supply the electrode with

5

adequate 2,4-DCP once they were hydrodechlorinated, and raise the reaction

6

frequency between 2,4-DCP and H*, resulting in the higher EHDC rate.

7

For practical use, the influences of the anions those generally exist in water body,

8

including Cl-, NO3-, NO2-, S2- and SO32-, on EHDC performance of the Pd/MnO2-Ni

9

foam electrode should be assessed, due to their possible effects on poisoning the

10

active sites or competing the H* consumption [48-50]. The results in Figure 6c clearly

11

show that except for Cl- and NO3-, the NO2-, SO32- and S2- all pose negative effects on

12

electrode performances. The effect of S2- is the most significant, and 1 mM of them

13

can totally deactivate the electrode. In the presences of 1 mM SO32- and 2.5 mM NO2-,

14

the EHDC efficiency dropped from 100% to 48.3% and 49.6%, respectively. The

15

negative effects of S2- and SO32- on the Pd-based electrodes were also reported by

16

other groups, which was attributed to their poisoning effects on Pd sites by forming

17

the strong S-Pd bonds [51, 52]. This poisoning effect was also experimentally

18

evidenced by the suppressed reductive current after inclusion of S2- and SO32- during

19

EHDC (Figure 6d).

20

Figure 6d also shows that in the presence of NO2-, the cathode current is inversely

21

enlarged, which indicates that the Pd sites were activated instead of being poisoned.

22

In this case, we inferred the negative effect of NO2- might result from its competitive 15

1

role with EHDC in consuming H* by the reaction (2x+y) NO2- + (4x+6y) H* + (2x+y)

2

e-→xN2 + yNH4+ + (4x+2y) OH- [53, 54]. The N species in the solution before and

3

after the reaction were then examined. As expected, 0.75 mmol of NO2- was removed,

4

while 0.64 mmol of NH4+ was formed in cathode chamber, which substantially

5

confirmed that the NO2- was reduced by H* at cathode. These results also reminded us

6

that our Pd/MnO2-Ni foam might serve as one potential bifunctional electrode for

7

simultaneous removal of the trace 2,4-DCP and nitrite from water.

8

3.4. Mechanism

9

On basis of all above results, the robust EHDC performance of Pd/MnO2-Ni foam

10

electrode could be firstly attributed to both the 3D foam structure of the electrode and

11

the hierarchical surface structure on foam skeleton, which not only provided large

12

surface area for the well dispersion of small Pd NPs, but also facilitated the mass

13

diffusion during EHDC reaction. Figure 7a compares the CVs of commercial Pd-C,

14

Pd/Ni foam and Pd/MnO2-Pd foam electrode. An increasing capacitance can be

15

clearly observed in the order of Pd-C
16

larger electrochemical active surface area on Pd/MnO2-Ni foam electrode. Figure 7b

17

presents the reaction time-dependent current (normalized by the Pd mass) on both the

18

Pd/MnO2-Ni and Pd/Ni foam electrode during EHDC. The Pd/MnO2-Ni foam

19

electrode always delivered a much larger current, indicating the larger number of

20

accessible Pd sites, and thus a better dispersion of Pd atoms on MnO2-Ni foam. The

21

larger number of active Pd sites worked together to provide more H* for an enhanced

22

EHDC. 16

1

As revealed by some researchers, the oxide support with its metal component of

2

several valances could catch the H* flowed from the Pd surface (so-called the

3

hydrogen spillover), and help spread them over the whole catalyst surface for sequent

4

hydrogenation reaction [55-57]. In this case, the reaction sites were not limited at Pd

5

NP surface, and more H* thus preferred the EHDC reaction rather than evolving into

6

H2 even at the high H* production rate, both of which contribute to a higher EHDC

7

and current efficiency. The Mn has several valences (0, +2, +3, +4, +6 and +7), and

8

can also switch between each other easily via the acquisition/release of electrons.

9

Accordingly, we speculated that the MnO2 might participate in the hydrogen spillover

10

process, and serve as the mediator to transfer H* from Pd to 2,4-DCP. To verify the

11

speculation, H2-TRP analyses were performed on MnO2-Ni and the Pd/MnO2-Ni

12

foam electrode. As shown in Figure 8a, the H2-TPR spectrum of MnO2-Ni foam

13

displayed two reduction peaks at about 285 and 514 oC, corresponding to the

14

reduction of MnO2 to Mn3O4, and then to MnO [58, 59]. However for Pd/MnO2-Ni

15

foam, only one reduction peak was observed at a lower temperature of 203 oC. The

16

drop in the reduction peak temperature after the inclusion of Pd NPs demonstrated

17

that the MnO2 was indeed capable to accommodate the H* generated on Pd. As the

18

oxide at the surface would be reduced by H* during the hydrogen spillover process, it

19

was also speculated that the proportion of the surface hydroxyl group would be

20

enriched on electrode after the EHDC reaction. Accordingly, the O 1s XPS spectra of

21

Pd/MnO2-Ni foam electrode before and after the EHDC was examined. The results in

22

Figure 8b clearly show that the molar ratio of Mn-O-Mn (529.85 eV) to Mn-OH 17

1

(531.86 eV) decreases from initial 0.88 to 0.77, confirming that partial oxide was

2

reduced to hydroxide [60]. These results also provide the substantial evidences that

3

the H* generated by Pd migrated onto the MnO2 surface.

4

According to the above mechanism studies, the sketch of EHDC reaction on the

5

MnO2 decorated Pd/Ni foam was shown in Figure 9. As illustrated in Figure 9a, the

6

EHDC on the Pd NP initiated with the adsorption and activation of both the 2,4-DCP

7

and water, followed by the H* generation, hydrodechloriantion reaction and finally the

8

desorption of phenol. Depositing Pd NPs on MnO2 nanosheet array by our approach

9

minimized the particle size and improved their dispersion on electrode, which raised

10

the proportion of accessible Pd atoms in every Pd NP, leading to the higher EHDC

11

efficiency and mass activity (Figure 9b). On the other hand, the MnO2 enabled to

12

accommodate the H* that was produced continuously on Pd surface, and spread them

13

over the whole electrode for reaction. This unique feature extended the reaction area

14

beyond Pd, and alleviated the pressure of access H* on the small Pd surface, both of

15

which contributed to an enhanced EHDC performance (Figure 9b).

16

3.5 EHDC performance of Pd/MnO2-Ni foam electrode in a continuous flow

17

system

18

Given the robust performance of Pd/MnO2-Ni foam, the electrode was applied for

19

2,4-DCP removal in a continuous-flow system to assess the possibility for its real

20

application. The influence of hydraulic retention time on EHDC efficiency was firstly

21

investigated. The results in Figure 10a showed that the 2,4-DCP could be removed in

18

1

the continuous-flow system, though the efficiency was lower than that obtained in

2

batch experiments. The whole system was revealed to be steady in around 120 min,

3

reflecting the durability of our electrode. Figure 10a also shows that the EHDC

4

efficiency is very sensitive to the hydraulic retention time. With the retention time

5

decreasing from 66.6 to 10.5 min, the removal efficiency decreased from 97.8% to

6

71.4%. However as observed at Figure 10b, more 2,4-DCP were actually removed at

7

every minute when a shorter retention time was adopted. This could be attributed to

8

the fact that with the shorter retention time, the 2,4-DCP at the electrode surface could

9

be supplemented more quickly once consumed. This result was also consistent with

10

that presented in Figure 6b, where more 2,4-DCP was removed under a larger initial

11

2,4-DCP concentration. Influences of the cathode potential on EHDC performance in

12

this continuous flow system were also investigated (Figure 10c). Figure 10d revealed

13

a volcano-like dependence of EHDC efficiency on the cathode potential with a

14

maximum of 75.3% achieved at -0.8 V. As shown, the optimal potential was

15

consistent with that obtained in batch experiments.

16

4. Conclusions

17

This work developed one advanced Pd/MnO2-Ni foam composite electrode for

18

EHDC, which featured a self-supported 3D network structure, hierarchical skeleton

19

surface and improved Pd dispersion. With these merits, the electrode delivered an

20

unprecedented high mass activity (kobs) of 0.883 min-1 mmolPd-1 for EHDC of

21

2,4-DCP, which was nearly ten times that of the Pd/Ni foam electrode (0.081 min-1

22

mmolPd-1). The electrode also displayed robust durability in the repeated batch EHDC 19

1

experiments till some reduced sulfur compounds and nitrite were included in the

2

solutions. The MnO2 in the composite was revealed to play a critical role in the H*

3

spillover from Pd, which extended the reactive area beyond Pd and promoted the

4

reactions between H* and 2,4-DCP. The Pd/MnO2-Ni foam also displayed the

5

potential

6

continuous-flow EHDC system, reflecting the feasibility in practical use.

7

Acknowledgements

and

retention

time-dependent

performances

when

tested

in

a

8

The present work is financially supported by National Natural Science Foundation

9

of China (51878105), Venture & Innovation Support Program for Chongqing Overseas

10

Returnees (cx2017066), the Program for the Top Young Talents of Chongqing,

11

Scientific and Technological Research Program of Chongqing Municipal Education

12

Commission (KJQN201800829), Research Startup Foundation of Chongqing

13

Technology and Business University (2016-56-01 and 2016-56-02), Scientific

14

Platform Project, Ministry of Education (fykfx201908 and fykfx201914).

20

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30

spheres for enhanced performance of supercapacitors, J. Mater. Chem. A 7 (2019) 6686-6694. 26

Figure Captions Fig. 1. Schematical illustration of the whole reactor for the batch and continuous-flow EHDC reaction. Fig. 2. Schematical illustration of the preparation process for the Pd/MnO2-Ni foam electrode. Fig. 3. Representative SEM images of (a) Ni foam; (b) MnO2-Ni foam; (c-d) Pd/MnO2-Ni foam; (e) elemental mappings of Pd, Mn, Ni and O on Pd/MnO2-Ni foam; TEM (f) and HRTEM (g) images of the Pd NPs peeled off the Pd/MnO2-Ni foam electrode. Fig. 4. High-resolution XPS spectra of (a) Mn 2p and (b) Pd 3d for the Pd/MnO2-Ni foam electrode before and after EHDC reaction. Fig. 5. (a) EHDC performances of the Ni foam, MnO2-Ni foam, Pd-Ni foam and Pd/MnO2-Ni foam electrode; (b) Plots of the -ln(C/C0) versus electrolysis time for Pd/MnO2-Ni foam and Pd-Ni foam electrodes at -0.85 V; (c) Durability test for Pd/MnO2-Ni foam electrode; (d) the current efficiency of Pd-Ni foam and Pd/MnO2-Ni foam electrode. Fig. 6. Influences of (a) the cathode potential, (b) initial 2,4-DCP concentration and (c) coexisting anions on EHDC performances of the Pd/MnO2-Ni foam electrode; (d) the working current as a function of reaction time when some coexisting anions were involved in the solution. Fig. 7. (a) The CVs of Pd-C, Pd-Ni foam and Pd/MnO2-Ni foam electrodes; (b) the Pd mass-normalized current for EHDC reaction on the Pd-Ni foam and Pd/MnO2-Ni foam electrode, respectively. Fig. 8. (a) The H2-TPR curves for the MnO2-Ni foam and Pd/MnO2-Ni foam; (b)

High-resolution O 1s XPS spectra of the Pd/MnO2-Ni foam electrode before and after EHDC reaction. Fig. 9. Schematic illustration of (a) the EHDC mechanism on Pd-included catalyst, and (b) critical role of MnO2 in the H* spillover process. Fig. 10. Effects of (a-b) the hydraulic retention time and (c-d) cathode potential on the EHDC performances of the Pd/MnO2-Ni foam electrode in a continuous-flow system.

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

Fig. 10

Table

Table Captions Table 1. Comparison in the mass activity between our electrode and that reported in known literatures.

Table 1. No.

1

2

3

4

5

6

7

8

9

10

11

Cathode

Pd NWs

Ni/Pd foam

TiC-Pd/Ni foam

Pollutants

2,4-dichlorophenol (0.31 mM)

p-chloronitrobenzene (0.16 mM)

2,4-dichlorobenzoic acid (0.20 mM)

nTiN doped

2,4-dichlorophenoxya

Pd/Ni foam

cetic acid (0.23 mM)

Pd/MnO2/Ni

2,4-dichlorobenzoic

foam

acid (0.2 mM)

Pd/C

Pd/PPY(PTS)/Ni foam

Pd/Ni foam

Pd/Ni foam

Pd/TiN

NiPd/SDBS-C

2,4-dichlorophenol (0.31 mM)

2,4-dichloropheno ls (0.5 mM) 2-chlorobiphenyl (0.05 mM) 2,4-dichlorophenoxya cetic acid (0.19 mM) 2,4-dichlorophenol (0.31 mM) 2,4-dichlorophenol (0.62 mM)

Operating conditions

Mass activity / min-1 mmolPd-1

Ref.

-0.95 V; 50 mM Na2SO4;

0.541

[17]

0.097

[21]

0.16

[31]

0.791

[32]

0.301

[34]

0.16

[36]

0.201

[42]

0.499

[43]

0.050

[44]

0.506

[45]

0.340

[46]

298.15 K; pH = 7; 10 mA cm-2; 50 mM Na2SO4; 298.15K; pH = 7 -0.80 V; 10 mM Na2SO4; 289.3 K; pH = 4.0 1.67 mA cm-2; 10 mM Na2SO4; 298.15K; pH = 7 1.67 mA cm-2; 10 mM Na2SO4; 303.15 K;pH = 4 -0.85 V; 50 mM Na2SO4; 298.15 K; pH = 7.0 1.67 mA cm-2; 50 mM Na2SO4; 313.15 K; pH = 7 1 mA cm-2; 500 mM NaOH; 293K; pH = 7 1.67 mA cm-2; 34.5 mM NaCl; 298.15K; pH = 7 -0.80 V; 50 mM Na2SO4; 298.15 K; pH = 6.8 3.75 mA cm-2; 50 mM Na2SO4; 298.15 K; pH = 7 -0.85 V

12

Pd/MnO2-Ni

2,4-dichlorophenol

50 mM Na2SO4;

foam

(0.31 mM)

298.15 K; pH = 6.8

0.883

This work

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Author Contribution Section Junxi Li: Methodology, Investigation, Writing-original draft Yiyin Peng: Investigation, Writing-original draft Wendong Zhang: formal analysis Xuelin Shi: Investigation Min Chen: Project administration Peng Wang: Validation Xianming Zhang: Resources Hailu Fu: Software Xiaoshu Lv: Data Curation, visualization Fan Dong: Writing-reviewing&Editing Guangming Jiang: Conceptualization, Supervision, Funding acquisition