Rh-Pd-alloy catalyzed electrochemical hydrodefluorination of 4-fluorophenol in aqueous solutions

Article abstract of DOI:10.1016/j.electacta.2018.03.018

Hydrodefluorination (HDF) is the key step in fluoroaromatic pollutant (FAP) degradation, making the search for efficient HDF catalysts and the determination of the corresponding mechanism a matter of great importance. In this study, we developed an efficient HDF catalyst (a Rh-Pd alloy) and investigated the underlying catalytic reaction mechanism, using 4-fluorophenol (4-FP) as a model FAP. Specifically, the rapid aqueous-phase HDF of 4-FP at pH 3 was achieved on a Rh-Pd alloy-modified Ni foam electrode, which was followed by ring hydrogenation (RH) of the HDF product (phenol) to afford cyclohexanone and cyclohexanol. Inductively coupled plasma optical emission spectrometer (ICP-OES) and thermodynamic analyses showed that, in addition to those supplied by the applied current, the electrons involved in the HDF of 4-FP and the RH of phenol were largely provided by the oxidative dissolution of the Ni foam support. Hydrogenation and cyclic voltammetry experiments indicated that the HDF of 4-FP probably follows an indirect mechanism (featuring adsorbed H atoms as the direct reductant) and that the strength of 4-FP adsorption onto the surface of the catalyst significantly influences its HDF activity.

Full text of DOI:10.1016/j.electacta.2018.03.018

Accepted Manuscript
Rh-Pd-alloy catalyzed electrochemical hydrodefluorination of 4-fluorophenol in
aqueous solutions
Yinghua Xu, Tingjie Ge, Hongxing Ma, Xufen Ding, Xiaoyong Zhang, Qi Liu
PII:
S0013-4686(18)30506-1
10.1016/j.electacta.2018.03.018
EA 31384
DOI:
Reference:
To appear in: Electrochimica Acta
Received Date: 13 December 2017
Revised Date: 12 February 2018
Accepted Date: 3 March 2018
Please cite this article as: Y. Xu, T. Ge, H. Ma, X. Ding, X. Zhang, Q. Liu, Rh-Pd-alloy catalyzed
electrochemical hydrodefluorination of 4-fluorophenol in aqueous solutions, Electrochimica Acta (2018),
doi: 10.1016/j.electacta.2018.03.018.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Rh-Pd-alloy catalyzed electrochemical hydrodefluorination of
4
-fluorophenol in aqueous solutions
a,
a
a,b
a
a
c,
Yinghua Xu *, Tingjie Ge , Hongxing Ma , Xufen Ding , Xiaoyong Zhang , Qi Liu *
a
State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of
Chemical Engineering, Zhejiang University of Technology, Hangzhou City, Zhejiang 310032,
China
b
Guangdong Research Institute of Rare-Metal, Guangdong Academy of Science, Guangzhou City,
Guangdong 510650, China
c
College of Life and Environmental Science, Hangzhou Normal University, Hangzhou City,
Zhejiang 310036, China
*
Corresponding author. Tel & Fax: +86-0571-88320830. E-mail: xuyh@zjut.edu.cn (Y. Xu),
qiliu@hznu.edu.cn (Q. Liu)
ABSTRACT
Hydrodefluorination (HDF) is the key step in fluoroaromatic pollutant (FAP) degradation,
making the search for efficient HDF catalysts and the determination of the corresponding
mechanism a matter of great importance. In this study, we developed an efficient HDF catalyst (a
Rh-Pd alloy) and investigated the underlying catalytic reaction mechanism, using 4-fluorophenol
(4-FP) as a model FAP. Specifically, the rapid aqueous-phase HDF of 4-FP at pH 3 was achieved
on a Rh-Pd alloy-modified Ni foam electrode, which was followed by ring hydrogenation (RH) of
the HDF product (phenol) to afford cyclohexanone and cyclohexanol. Inductively coupled plasma
optical emission spectrometer (ICP-OES) and thermodynamic analyses showed that, in addition to
those supplied by the applied current, the electrons involved in the HDF of 4-FP and the RH of
phenol were largely provided by the oxidative dissolution of the Ni foam support. Hydrogenation
and cyclic voltammetry experiments indicated that the HDF of 4-FP probably follows an indirect
mechanism (featuring adsorbed H atoms as the direct reductant) and that the strength of 4-FP
adsorption onto the surface of the catalyst significantly influences its HDF activity.
Key words: Hydrodefluorination; Rh-Pd alloy; Oxidative dissolution of Ni; 4-Fluorophenol;
Ring hydrogenation
1

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1
. Introduction
Fluoroaromatics (FAs) are important fine chemical intermediates widely used to prepare
pesticides [1], pharmaceuticals [2,3], liquid crystals [4], etc., as exemplified by the use of
4
2
-fluorophenol,
2-fluorophenol,
3,5-difluroaniline,
2,6-difluorobenzonitrile,
,3,5,6-tetrafluorobenzyl alcohol, and 2,3,5,6-tetrafluoro-4-methylbenzyl alcohol for commercial
pesticide production [1]. From 2003 to 2011, 25 new pharmaceutical products containing FA
intermediates or FA fragments were marketed globally, which accounted for 16.2% of all
pharmaceutical products marketed during this period [2,3]. Additionally, 2,3-difluorophenol,
3
,5-difluorophenol, and 3,4,5-trifluorophenol are key intermediates in the synthesis of popular
colored liquid crystal materials [4]. Due to their excellent performance in the aforementioned
fields, the production of FA intermediates has been steadily growing, with the global FA market
expanding from 10,000 tons/year in 1994 to 35,000 tons/year in 2000 [5]. The output of FA, in
China, has reached approximately 50,000 tons/year in 2014 [6]. Since 2015, five new projects
have been established in China solely for the production of pentafluorophenols as intermediates
for peptide and liquid crystal syntheses [7-11]. The small radius and high electronegativity of
fluorine atoms account for the high energy of the C–F bond, which contributes to the high
biological activity, high fat solubility, high metabolic stability, high chemical stability, and other
characteristics of FAs [12]. On the one hand, these characteristics make FAs perfectly suited for
the production of pesticides, pharmaceuticals, liquid crystals, etc.; however, these properties are
also responsible for the environmental hazards posed by FAs [13,14]. As the global production of
FAs and their derivatives is currently on the rise, the amount of fluoroaromatic pollutants (FAPs)
entering the environment is expected to increase, which necessitates the development of efficient
FAP degradation methods.
Analogous to dechlorination being the key step of chlorinated organic pollutant degradation
[15-17], defluorination is the key step of FAP degradation. Due to the presence of fluorine atoms,
FAPs cannot be completely degraded in natural aerobic environments and are mostly likely
converted to more toxic end products [13]. Conversely, completely defluorinated products can be
treated using conventional approaches or be recycled as raw materials. A variety of FAP
defluorination methods have been developed, e.g., many fluoroaromatics can be defluorinated by
2

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aerobic bacterial degradation. However, such microbial degradation results in the formation of
numerous intermediates, some of which exhibit high toxicity [13]. FAPs can also be defluorinated
in acetonitrile or water employing an iron phthalocyanine–H O catalytic oxidation system, with
2
2
the maximum realized extent of defluorination being 89% [18]. As another example,
defluorination of 2-fluoroaniline can be achieved using a bioelectrochemically assisted microbial
system, with the corresponding maximum defluorination extent exceeding 60% [20].
Compared to the aforementioned oxidative defluorination methods, reductive
hydrodefluorination (HDF) exhibits the advantage of high reaction product controllability [21].
Unfortunately, FAPs cannot be efficiently hydrodefluorinated using conventional zero-valent
metal [20] and Pd-catalyzed H gas [22] reduction methods, despite these approaches being
2
efficient for the hydrodehalogenation of brominated and chlorinated aromatics. Recently, McNeill
et al. have proposed an HDF system comprising a Rh/Al O catalyst and hydrogen (as the
2
3
reducing agent), achieving efficient aqueous-phase HDF of various fluorobenzenes under ambient
conditions [23]. Chiu et al. investigated the HDF of vinyl fluoride over Rh/Al O in liquid and gas
2
3
phases, showing that HDF mainly occurred in the liquid phase, whereas double bond
–
hydrogenation mainly occurred in the gas phase [24]. In addition, a BH -promoted
4
electrochemical HDF system has been developed by Huang et al., allowing the selective HDF of
various FAs in organic solvents [25]. Inspired by these efforts, our research group has developed
an electrochemical HDF system featuring Rh/Ni foam electrodes and used it to efficiently and
completely hydrodefluorinate 18 representative FAs in water under ambient conditions without the
use of hydrogen gas [26].
The high cost of Rh makes the improvement of its catalytic activity crucial for the application
of Rh-catalyzed electrochemical HDF, hydrogen gas reductive HDF, and zero-valent metal
reductive HDF techniques. In this work, we developed a Rh-Pd alloy as an efficient HDF catalyst
utilizing catalyst loading, element proportion, and phase screening, employed 4-fluorophenol
(4-FP) as a model FAP, and investigated the underlying reaction mechanism. Electrodeposition
was utilized to fabricate Pd- and Rh-modified Ni foam electrodes with different noble metal
loadings, Rh/Pd and Pd/Rh composites, and Rh-Pd alloy-modified Ni foam electrodes, and the
HDF activities of these electrodes were subsequently compared in solution at optimized pH values.
Moreover, we also compared the degradation pathways of 4-FP on Rh- and Rh-Pd alloy-modified
3

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Ni (Rh-Pd/Ni) foam electrodes. Finally, the mechanism of 4-FP degradation on the Rh-Pd/Ni foam
electrode was investigated using ICP-OES analysis, thermodynamic analysis, hydrogenation with
H as a reducing agent, and cyclic voltammetry (CV). To the best of our knowledge, this paper is
2
the first report of Rh-Pd alloy-catalyzed HDF and the associated reaction mechanism.
2
2
. Material and methods
.1. Chemicals
Ni foam (1.0 mm thickness) and carbon (C) paper (TGP-H-060 and AvCarb EP40T) were
purchased from CEEE Co., China (www.hzcell.com). 4-Fluorophenol (4-FP) and its HDF and RH
products (phenol, cyclohexanone, and cyclohexanol), with purities of 98 wt%–99 wt%, and PdCl2
(
99.5 wt%), RhCl (99.5 wt%), NaF, H SO , NaOH, H PO , KH PO , ethanol and acetone,—all
3 2 4 3 4 2 4
with 97 wt%–99 wt% purity—were purchased from Aladdin Reagent Co., China. Rh/Al O with 5
2
3
wt% Rh loading was obtained from Sigma-Aldrich Corporation. Methanol and acetonitrile (HPLC
grade) used for HPLC analysis were obtain from National Medicines Co., Ltd., China. Water with
resistivity of 18 Mꢀ·cm obtained from a Millipore Milli-Q system was used to prepare all reaction
solutions.
2
.2. Electrode modification
All modified nickel foams were prepared by similar electrodepositions (Table 1). Prior to
electrodeposition, we degreased and cleaned the nickel foam supports in acetone under ultrasonic
vibration for 20 min and then etched them in a diluted H SO (0.8 M) solution for 300 sec to
2
4
remove surface native oxides. Electrodepositions (A constant current of 10 mA was passed
through an aqueous solution (40 mL) containing 1 M NaCl and 0.75 mM RhCl (or 0.75 mM
3
RhCl and 0.375 mM PdCl ) at pH 2 (adjusted using HCl) until the rose or orange-red color
3
2
disappeared) were used to prepare Rh/C paper and Rh-Pd/C paper. All electrodepositions were
carried out in air at 30 °C. We prepared a number of electrodes under exactly the same conditions
to ensure reproducibility. After modification, we kept the electrodes in water for later use without
drying.
2
.3. Apparatus
The surface morphology and elemental/crystal phase compositions of the modified nickel
foams were determined by scanning electron microscopy (SEM , Hitachi S-4700 II) equipped with
4

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energy dispersive spectrometry (EDS, Thermo NOANVANTAGE ESI) and X-ray diffraction
(XRD, Thermo ARLSCINTAG X’TRA, 45 kV and 40 mA, Cu Kα).
A potentiostat (PAR 283), a potentiostat (PAR 273), and a DC-regulated power supply
HZC-061) were used to perform cycle voltammetry (CV), chronoamperometry (CA),
(
chronopotentiometry (CP), and constant current electrolysis experiments, respectively. We used a
three-electrode cell composed of a Pt (2 cm × 2 cm) counter electrode, a Ag/AgCl reference
electrode, and a Ni, 0.5Pd/Ni, 1Rh/Ni, or 1Rh-0.5Pd/Ni disk (2 mm diameter) working electrode
to carry out the CV experiments. The 0.5Pd/Ni, 1Rh/Ni, and 1Rh-0.5Pd/Ni disks were prepared by
the same electrodeposition method as that used for the 0.5Pd/Ni, 1Rh/Ni, and 1Rh-0.5Pd/Ni foam,
respectively. A two-compartment H-cell (glass) composed of a Nafion-117 membrane, a Ag/AgCl
reference electrode, a graphite sheet (2 cm × 3 cm) counter electrode, and a Ni, 1Rh/Ni, or
2
1
Rh-0.5Pd/Ni foam (2 × 3 cm ) working electrode was used for the CA and CP experiments. The
electrode potentials in the CV, CA, and CP experiments were reported versus the
Ag/AgCl/(saturated KCl) reference. The constant current electrolysis experiments were performed
using a conventional two-compartment H-cell (glass) separated by a Nafion-117 membrane to
obtain the optimized HDF conditions. Ni or modified Ni foams (projected area: 2 cm × 3 cm) or
modified C paper (projected area: 2 cm × 2.5 cm) were served as the cathode and graphite sheet (2
cm × 3 cm) was served as the anode in the electrolysis experiments. 40 mL catholyte and 40 mL
anolyte were used in the electrolysis experiments, and the catholyte was maintained under
vigorous stirring during electrolysis. A 0.1 M H SO aqueous solution was used as the anolyte in
2
4
all CA, CP, and electrolysis experiments and all experiments were carried out at 30 °C.
A three-necked jacketed reactor (glass) equipped with a H2 supply was used for
hydrogenation experiments. A piece of modified foam or carbon paper or a certain amount of
catalyst (Rh/Al O ) was placed into the mixed solution of 40 or 100 mL solvent and under a
2
3
-1
continuous flow of H at 10 mL min . The reaction solution was maintained under vigorous
2
stirring for 45, 120, or 180 min at 30 ꢀ.
2
.4. Analytical methods
The concentrations of 4-FP and its degradation products (PhOH, phenol; CYC.one,
-
cyclohexanone; CYC.ol, cyclohexanol; F , fluoride ion) were determined by a Waters HPLC
system, an Agilent 7890 A gas chromatograph (GC), and a Dionex ICS-2100 ion chromatograph
5

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(IC), respectively. The Waters HPLC system mainly composed of a Waters 2996 photodiode array
detector (λ 210 nm), an injection valve fitted with a 20 ꢁL sample loop, and a symmetry column
(250 mm length × 4.6 mm i.d., 5 ꢁm particle size). A mobile phase of water/methanol/ acetonitrile
−
1
(
6:3:1 v/v) containing 30 mM H PO at a flow rate of 60 mL·hr was used to carry out isocratic
3
4
elution. The GC was equipped with an Agilent 7683B auto-sampler coupled to a flame ionization
detector. The analysis was performed on an HP-INNOWAX capillary column (30 m × 0.32 mm
i.d. × 0.5 ꢁm), and the column temperature program was raised from 50 °C to 250 °C at a rate of
−
1
1
5 °Cꢂmin . Nitrogen of high purity (99.999%) was used as the carrier gas at a flow rate of 6.5
−
1
mLꢂmin . The temperature of the injector was set at 250 °C. The organic product samples for GC
analysis were obtained through the following procedures: (1) extraction with CH Cl from the
2
2
catholytes, (2) desiccation over anhydrous sodium sulfate, (3) filtration using a sand core funnel,
and (4) appropriate dilution with CH Cl . The Dionex ICS-2100 IC was equipped with a Dionex
2
2
IonPac AS11-HC column. After appropriate dilution with water, the catholytes of the HDF
experiments were filtered with an injection filter head and then directly subjected to HPLC and IC.
Ni concentration in solutions was determined with an Inductively coupled plasma optical emission
spectrometer (ICP-OES, iCAP 7400, Thermo Scientific, USA). Three ICP-OES measurements
were performed for individual solution samples under exactly the same conditions to ensure
reproducibility.
The concentrations of 4-FP and its degradation products, and Ni were determined using
standard calibration curves. The precision of the mass balance in the measurement was
approximately 97–103% for HPLC and IC analysis, 95–115% for GC analysis, and 95–105% for
ICP-OES analysis. The conversion of 4-FP was defined as the molar ratio of 4-FP eliminated to
-
the initial amount of 4-FP. The yields of the organic product and F were defined as the molar ratio
of product produced to the initial 4-FP charged. Unless otherwise noted, faradic efficiency (FE, %)
in the degradation of 4-FP was determined using the following equation:
(n1
∆CPhOH
+
n2
∆
CCYC .o ne
+ n3∆CCYC .o l )FV
FE
=
I
× ∆t
where n n , and n are the electron transfer numbers per molecule of PhOH (n = 2), CYC.one (n
2
1
,
2
3
1
=
6), and CYC.ol (n = 8), respectively; △CPhOH, △CCYC.one, and △CCYC.ol are the concentration
2
−
1
differentials (molꢂL ) of PhOH, CYC.one, and CYC.ol during △t, respectively; F is the Faraday
6

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−
1
constant (96500 C·mol ); V is the volume of the total catholyte (0.04 L); and I is the applied
current (A).
3
3
. Results and discussion
.1 Electrode characterization
Due to the fact that the 1Rh-0.5Pd/Ni foam exhibited the best HDF performance among all
modified Ni foams, its morphology and elemental/crystal phase compositions were characterized
by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray
diffraction (XRD). As shown in the SEM images of Fig. 1, the surface of the above foam was
covered with a film containing five major elements (13.12 wt% C, 7.54 wt% O, 48.59 wt% Ni,
2
0.63 wt% Rh, and 10.12 wt% Pd), as determined by EDS. Notably, the Rh/Pd ratio of the film
was close to that of the solution used for electrodeposition, implying that these metals were
uniformly electrodeposited over the Ni foam surface. Fig. 2 shows the XRD patterns of fresh Ni,
1
Rh/Ni, 0.5Pd/Ni, and 1Rh-0.5Pd/Ni foams and that of used 1Rh-0.5Pd/Ni foam, with the
pronounced Rh-Pd alloy (111) diffraction peak [27] observed for the 1Rh-0.5Pd/Ni foam (Fig. 2B),
confirming the presence of a Rh-Pd alloy on the Ni foam surface and indicating that the Rh-Pd
alloy is the main HDF reaction site. It should be pointed out that Rh-Pd alloys of different
3
+
2+
compositions are easily produced by co-electrodeposition from solutions containing Rh and Pd
28]. Moreover, based on the XRD half-peak widths of fresh 1Rh/Ni, 0.5Pd/Ni, and 1Rh-0.5Pd/Ni
in the 2 range of 38–42°, the noble metal crystal size was suggested to decrease in the order Pd >
[
θ
Rh-Pd > Rh, which means that the specific surface areas of the three films on Ni foam decreased
in the same order. This conclusion was further confirmed by the results shown in Fig. 3A, which
revealed that the double-layer charging current of three modified Ni electrodes at 0 V decreased in
the order Rh/Ni > Rh-Pd/Ni > Pd/Ni. Furthermore, the similar heights and half-peak widths of the
(111) diffraction peak observed for both fresh and used 1Rh-0.5Pd/Ni foams suggested that the
Rh-Pd alloy film was relatively steadily held on the Ni foam during one HDF experiment.
3
.2 Optimization of HDF catalysts and other conditions
Based on our previous report [26], the efficiency of electrochemical HDF is determined by
the nature of the corresponding catalysts. Herein, catalyst optimization was performed using 4-FP
as the target compound, with the obtained results listed in Table 2 (entries 1–11). Screening of
7

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different types of catalysts, such as Pd, Rh, Rh/Pd composites, Pd/Rh composites, and Rh-Pd
–
alloys, revealed that the best HDF performance was observed for a Rh-Pd alloy. Notably, F was
not detected in the catholyte when Ni and Pd/Ni foams were used as electrodes, implying that Rh
was the HDF-active ingredient in the Rh-Pd alloy. With the optimized catalyst (1Rh-0.5Pd) in
hand, we then examined the effects of catholyte pH (entries 12–16), showing that the HDF
efficiency decreased with increasing pH, with the best performance observed at pH 3. In addition
to the abovementioned two parameters, we also found that the HDF system was immune to air
(entry 17), and that at a low concentration of 0.1 mM, 4-FP degraded rapidly with 100%
−
conversion and 98% F yield (entry 18), which suggested that the Rh-Pd alloy catalyst is
potentially suitable for the remediation of 4-FP-contaminated groundwater.
3
.3 Degradation pathway of 4-FP and FE of 4-FP degradation
With the optimal HDF conditions (catalyst = 1Rh-0.5Pd, catholyte pH = 3), the degradation
pathway of 4-FP were evaluated. As shown in Figs. 4A and B, concentration profiles of 4-FP and
its degradation products observed on the 1Rh-0.5Pd/Ni foam electrode is quite different from that
observed on the Rh/Ni foam electrode. The primary difference is the much higher concentration of
intermediates (PhOH and CYC.one) in the catholyte observed for the 1Rh-0.5Pd/Ni foam
electrode. As described elsewhere [29], the selectivity of CYC.one formation during the ring
hydrogenation (RH) of PhOH depends on the strength of CYC.one adsorption on catalysts, with
weaker adsorption strength resulting in higher selectivity. Therefore, we speculated that
the strength of PhOH and CYC.one adsorption on the 1Rh-0.5Pd/Ni foam was weaker than that on
the Rh/Ni foam. This is probably the reason why the former catalyst resulted in a higher HDF
performance. In addition, during HDF and RH over 1Rh-0.5Pd/Ni foam electrodes, the
concentration of the CYC.ol generated was very low during the 30-min period when the
concentrations of 4-FP and PhOH were high. Moreover, the rate of CYC.one concentration growth
was dependent only on the concentration of PhOH and not on that of 4-FP. This indicated that the
main degradation pathway of 4-FP can be expressed as 4-FP → PhOH → CYC.one → CYC.ol,
with competitive adsorption between 4-FP, PhOH, and CYC.one taking place on the 1Rh-0.5Pd/Ni
foam electrode and the adsorption of CYC.one thereon being much weaker than that of 4-FP and
PhOH.
FEs were calculated for PhOH, CYC.one, and CYC.ol as target products. The calculation
8

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revealed that the FE of 4-FP degradation exceeded 100% at both electrodes (Figs. 4C and D).
Since the maximum FE of an electrochemical reaction equals 100% according to Faraday's law,
we assumed that the degradation of 4-FP was accompanied by oxidative dissolution of nickel or
that a large number of hydrogen atoms were immobilized on Rh/Ni and 1Rh-0.5Pd/Ni foam
electrodes during their fabrication via adsorption and/or absorption. Thus, the oxidation of nickel
or hydrogen on electrodes can provide additional electrons, leading to FEs of greater than 100%
during degradation.
3
.4 Catalytic reaction mechanism
3
.4.1 Rationalization of overly high FE values
Theoretically, hydrogen atoms can be absorbed/adsorbed by Ni [30], Rh [31], and Rh-Pd
alloys [28] on 1Rh/Ni and 1Rh-0.5Pd/Ni foam electrodes during preparation, with the subsequent
oxidation of these hydrogen atoms providing electrons for 4-FP degradation and leading to FE
values above 100%. To verify this hypothesis, we performed CV, CA, and CP experiments. Figs.
3
A and C show the CV curves of Ni, Rh/Ni, and Rh-Pd/Ni electrodes, with Fig. 3A clearly
+
displaying the current responses corresponding to the reduction of H to Hads and the oxidation of
+
Hads to H on the above electrodes in the range of 0 to –0.4 V. For the wide potential range of 0.4
+
to –0.8 V in Fig. 3C, the current responses for the reduction of H to Habs and H , as well as the
ads
corresponding reverse reactions, were observed for all three electrodes, indicating that these
electrodes could absorb and adsorb hydrogen atoms. Fig. 3B shows the CA curves of freshly
prepared Rh/Ni and Rh-Pd/Ni electrodes, with their open-circuit potentials (OCPs) of ~0.3 and –
0
.05 V, respectively, implying that both electrodes exhibited no or low hydrogen storage capacities
during preparation. Figs. 3A and C indicate that the OCPs of these electrodes lay between the
hydrogen oxidation and reduction peak potentials (in the presence of oxidizable Hads or Habs on
Rh/Ni and Rh-Pd/Ni), i.e., these OCPs were expected to be seen around –0.25 V. Fig. 3D presents
the CP curves of Ni, Rh/Ni, and Rh-Pd/Ni electrodes, with the current plateau observed for Ni
foam during the first 2 min (corresponding to the oxidation of hydrogen) indicating that a certain
amount of hydrogen may have been stored in the above foam during pretreatment. Conversely, no
such plateaus were observed for Rh/Ni and Rh-Pd/Ni during the first 30 min, which indicated the
absence of hydrogen storage during fabrication [28]. Although a profound oxidation current
appeared within 30 min in the CP curves of Rh/Ni and Rh-Pd/Ni, it was significantly smaller than
9

ACCEPTED MANUSCRIPT
that observed for the Ni electrode, and was thus considered not to be related to the oxidation of
hydrogen. Based on the aforementioned results and discussion, it was concluded that FEs of over
1
00% observed for 4-FP degradation were not caused by hydrogen storage.
As described in Section 3.3, oxidative dissolution (OD) of the nickel support during 4-FP
degradation can also result in FEs exceeding 100%. To test this hypothesis, the 4-FP degradation
2
+
efficiency and Ni concentration were measured with and without an applied cathodic current.
2
+
Table 3 shows that high concentrations of Ni were observed in both cases when Rh/Ni and
Rh-Pd/Ni foam electrodes were used, demonstrating that 4-FP degradation was accompanied by
the OD of nickel in both materials. When the degradation FEs were recalculated considering the
above process, the obtained values were less than 100% in both cases, confirming that the OD of
nickel caused the FE to exceed 100%.
Additionally, it should be pointed out that the recalculated FE of Rh-Pd/Ni foam was
significantly higher than that of the Rh/Ni foam. Taking into account the conclusion of Section 3.1
that the surface area of the catalytic Rh-Pd/Ni foam layer was smaller than that of the Rh/Ni foam
layer, it was concluded that Rh-Pd led to a higher intrinsic catalytic activity for the degradation of
4
-FP than that offered by Rh. To further support this conclusion, 4-FP degradation experiments
were performed using Rh/C and Rh-Pd/C paper electrodes, with the obtained results (Table 3,
entries 5 and 6) confirming the above hypothesis.
3
.4.2 Direct/indirect HDF mechanism
Based on the previous work of McNeill [5] and our group [26], the HDF of 4-FP on
Rh-Pd/Ni foam is likely to follow an indirect mechanism:
4
2
-FP + Rh-Pd ↔ (4-FP) Rh-Pd
(1)
(2)
(3)
(4)
ads
+
-
H + 2 e + Rh-Pd ↔ 2 Rh-Pd-H
ads
(4-FP) Rh-Pd + 2 Rh-Pd-H → (PhOH) Rh-Pd + HF
ads ads ads
(PhOH) Rh-Pd → PhOH + Rh-Pd
ads
The first step of the above mechanism features the formation of adsorbed 4-FP [Eq. (1)] and
adsorbed hydrogen [Eq. (2)] on Rh-Pd, with the subsequent HDF proceeding [Eq. (3)] by the
hydrogenation of the adsorbed 4-FP with the adsorbed hydrogen, followed by desorption of PhOH
[Eq. (4)], similarly to the case of catalytic hydrogenation. Additionally, it is also possible that the
HDF of 4-FP proceed via a direct mechanism, as described by Eqs. (5) and (6) [25,32]:
-
(
4-FP) Rh-Pd + 2 e → [(R) Rh-Pd]*
(5)
ads
ads
1
0

ACCEPTED MANUSCRIPT
+
[
(R) Rh-Pd]* + 2 H → (PhOH) Rh-Pd + HF
(6)
ads
ads
The above pathway features the successive formation of series of adsorbed intermediates [Eq. (5)]
by electron transfer from the Rh-Pd/Ni electrode to 4-FP, which is followed by protonation of
these intermediates [Eq. (6)] and product desorption [Eq. (4)].
The HDF mechanism was investigated by performing CV experiments on Ni, Pd/Ni, Rh/Ni,
and Rh-Pd/Ni electrodes at different 4-FP concentrations. As indicated by Figs. 5A and B, 4-FP
did not react on the nickel electrode, with its adsorption being negligible. Similarly, 4-FP was only
weakly adsorbed on the Pd/Ni electrode, and almost no reaction occurred. These results were
consistent with those listed in Table 2 (entries 1 and 2). Moreover, it is worth noting that the
double-layer charging current of Pd/Ni was smaller than that of the Ni electrode in the range of
0
.4–0 V, indicating the presence of a very compact Pd coating on the Ni surface [33]. CV curves
of Rh-Ni and Rh-Pd/Ni (Figs. 5C and D, respectively) exhibited similar shapes and trends, i.e., the
intensity of reduction and oxidation peaks between 0 and –0.4 V gradually decreased with
increasing 4-FP concentration, with the same trend observed for the double layer charging current
at 0 V. Thus, the obtained results indicated that 4-FP was strongly adsorbed on both electrodes
[34]. Changes in the reduction and oxidation peak intensities and those in the double layer
charging current were more significant for Rh/Ni, implying that 4-FP was more strongly adsorbed
on Rh/Ni than on Rh-Pd/Ni. The above results, together with those depicted in Figs. 5A and B,
indicated that 4-FP was mainly adsorbed by Rh in Rh/Ni and Rh-Pd/Ni, with Pd tuning the
adsorption affinity of 4-FP to Rh and thus improving the HDF activity of these catalysts. In
addition, the decrease in the reduction peak intensities observed for Rh/Ni and Rh-Pd/Ni was
much less pronounced than that observed for the corresponding oxidation peak between 0 and –
0
.4 V after adding 1 mM 4-FP to a blank solution, which suggested that 4-FP was reduced on both
electrodes. This conclusion was consistent with the phenomenon demonstrated in Fig. 3B, i.e., the
stable potentials of both electrodes were within the range of –0.3 to –0.35 V during 4-FP
degradation. In addition, since the redox peaks of Rh/Ni and Rh-Pd/Ni corresponded to the
+
reversible oxidation-reduction between Hads and H at 0 to –0.4 V, the aforementioned reduction of
4
-FP was likely to follow the indirect mechanism (with Hads as the direct reducing agent).
To confirm the indirect mechanism of 4-FP reduction on Rh/Ni and Rh-Pd/Ni, we conducted
HDF experiments using hydrogen as a reducing agent, additionally employing self-made Rh/C
1
1

ACCEPTED MANUSCRIPT
and Rh-Pd/C papers and commercial Rh/Al O as catalysts to avoid possible direct HDF reactions
2
3
caused by the OD of Ni. As a result (Table 4), significant HDF of 4-FP was observed when Rh/Ni
and Rh-Pd/Ni foam were used as catalysts, with this reaction being accelerated by hydrogen.
Conversely, almost no HDF was observed in the case of Rh/C paper, whereas this reaction was
pronounced when Rh-Pd/C paper and Rh/Al O were used. Therefore, the HDF of 4-FP on Rh/Ni
2
3
and Rh-Pd/Ni was believed to either fully or partially occur via the indirect mechanism. In
addition, the Rh content of Rh/Al O was much lower than that of Rh/C paper, which indicated
2
3
that the HDF activity of the Rh catalyst was greatly influenced by its support.
3
.4.3 Thermodynamic Analysis
Based on the above results and discussion, the HDF of 4-FP was suggested to comprise the
following steps:
2
+
-
Ni ↔ Ni + 2 e
(7)
(8)
+
-
2
H + 2 e ↔ H
2
+
-
Ni + H + e ↔ Ni-H
(9)
Ni-H ↔ 2 Ni + H
(10)
(11)
(12)
(13)
(14)
2
+
-
Rh-Pd + H + e ↔ Rh-Pd-H
Rh-Pd-H ↔ 2 Rh-Pd + H2
4-FP) Rh-Pd + 2 Rh-Pd-H → Rh-Pd + PhOH + HF
2
(
(
ads
+
-
4-FP) Rh-Pd + 2 H + 2e → Rh-Pd + PhOH + HF
ads
Firstly, the possibility that the OD of nickel (Eq. (7)) provides electrons for indirect HDF (Eq. (13))
of 4-FP was analyzed. Based on the maximum nickel ion concentration (4.1 mM, Table 3), the
equilibrium potential of nickel OD was determined as –0.717 V (vs. Ag/AgCl) using the Nernst
equation (Eq. (15)).
2
+
RT
nF
[Ni ]
0
E
=
E
+
ln
ln
ꢁ15ꢂ
ꢁ16ꢂ
[
Ni]
+
RT
nF
[H
]
0
E
=
E
+
[
H 2]
+
Fig. 3A shows that the potential of H to Hads reduction on Ni and Rh-Pd/Ni electrodes varied
between –0.1 and –0.4 V, being more positive than the equilibrium potential of nickel OD. Thus,
+
the reduction of H to H (Eqs. (9) and (11)) was thermodynamically allowed. In addition, since
ads
the HDF reactions on Rh-Pd/Ni occurred via the indirect mechanism with Hads acting as the
1
2

ACCEPTED MANUSCRIPT
reducing agent (Section 3.4.2), the electrons generated by nickel OD could be thermodynamically
exploited in the indirect HDF of 4-FP.
Next, we analyzed whether the nickel OD provides electrons for the hydrogen evolution
reaction (HER) (Eq. (8)) and characterized HER-active sites. The HER equilibrium potential at an
+
H concentration of 1 mM (pH = 3) and a hydrogen pressure of ≤1 atm was calculated to exceed –
0
.527 (vs. Ag/AgCl) (Eq. (16)). Since the equilibrium potential of the nickel OD is more negative
than that of HER, the former reaction can provide electrons for the latter one in the studied system.
The reaction represented by Eq. (11) was thought to take precedence over that represented by Eq.
(9), since hydrogen atoms were more strongly adsorbed on Rh and Pd than on Ni [35], as
+
supported by the results in Fig. 3A, which showed that the current of H to H reduction on
ads
Rh-Pd/Ni significantly exceeded that on the Ni electrode. For the Ni electrode, Eq. (9) represents
the rate-determining step of HER, whereas HER on Rh and Pd is controlled by reactions
represented by both Eqs. (11) and (12), implying that the HER active sites mainly correspond to
Rh-Pd. In addition, the partial pressure of hydrogen at the degradation potential of 4-FP (–0.30 to
–
0.35 V) was calculated as 0.008–0.02 atm (Eq. (16)), which indicated that HER on Rh/Ni and
Rh-Pd/Ni electrodes should be slow. This result was consistent with the high FE efficiencies
observed for entries 1–4 in Table 3. Unfortunately, the thermodynamic feasibility of direct HDF
(Eq. (14)) could not be determined based on the available data.
4
. Conclusions
In this study, we developed an efficient HDF catalyst (Rh-Pd alloy) and established a
preliminary mechanism of HDF over a Rh-Pd alloy-modified Ni foam electrode. The above alloy
achieved more rapid HDF of 4-FP than the Rh catalyst, with the subsequent ring hydrogenation
(RH) of the HDF product (phenol) affording cyclohexanone and cyclohexanol. The HDF of 4-FP
over the Rh-Pd alloy probably follows an indirect hydrogenation mechanism (i.e., one featuring
adsorbed H as the direct reductant), and the strength of 4-FP and HDF product adsorption onto the
catalyst surface significantly affects the catalytic activity. In addition to those supplied by the
applied current, most of the electrons for the HDF of 4-FP and RH of phenol are provided by the
oxidative dissolution of Ni foam. The high HDF activity of the Rh-Pd alloy and its tendency to
afford less toxic end products make this catalyst potentially suitable for the remediation of
1
3

ACCEPTED MANUSCRIPT
4
-FP-contaminated groundwater. In addition, it should be noted that the Rh-Pd alloy can be
utilized not only as an electrochemical HDF catalyst, but also as a zero-valent metal reductive
HDF catalyst, in analogy to the efficient use of the zero-valent reduction method for the
remediation of chlorinated organics-contaminated groundwater [36].
Conflict of interest
There is no conflict of interest among authors related to this paper.
Acknowledgement
This research was supported by NSFC (Grant No. 21106133, 21576238), the Natural Science
Foundation of Zhejiang Province, China (Grant No. LY16B060012), the Hangzhou Science and
Technology Development Foundation of China (Grant No. 20170533B07), and the Special
Program on Science and Technology Development of Guangdong Province, China (Grant No.
2
017A070702012).
1
4

ACCEPTED MANUSCRIPT
References
[
1] H. Jiang, The promising fluoroaromatic intermediates of pesticides, Fine Chemical Materials and Its
Intermediates 8 (2011) 34-38.
2] Y. Zhang, The new medical products containing fluorine marketed from 2003 to 2011 (To be continued), Fine
Chemical Intermediates 43 (2013) 1-8.
3] Y. Zhang, The new medical products containing fluorine marketed from 2003 to 2011 (Finished), Fine
Chemical Intermediates 43 (2013) 1-7.
4] The presentation of liquid-crystal intermediates, Zhejiang Yongtai Technology co., LTD., Taizhou, 2017.
http://www.yongtaitech.com/prolist/zh/12/105.aspx.
5] R. Baumgartner, K. McNeill, Hydrodefluorination and hydrogenation of fluorobenzene under mild aqueous
conditions, Environ. Sci. Technol. 46 (2012) 10199-10205.
6] F.G. Jiao, Present situation and development analysis of fluorine-containing intermediates and fine chemicals,
Organo-Fluorine Industry, 2 (2017) 54-57.
7] Public announcement of environmental impact assessment information for the projects (to be approved),
Environmental Protection Agency, Fuxin, 2015.
http://fmx.fuxin.gov.cn/fmx/gzcy/myzj/content/4028e48a4ed35a4d014ed38d91900024.html.
8] Public announcement of pollution case information, Environmental Protection Agency, Weifang, 2016.
http://www.wfepb.gov.cn/hbyw/hjjc/xfgl/201604/t20160421_1629407.htm.
9] Public announcement of environmental impact assessment information for the projects (to be approved),
Environmental Protection Agency, Jingzhou, 2015. http://www.jzhbj.gov.cn/news_show.aspx?id=11825.
10] Public announcement of public participation on the environmental impact of the projects (to be approved),
Environmental Protection Agency, Xuzhou, 2015.
http://www.xzhb.gov.cn/hbj/tzgg/20150603/004002_57e13b04-22be-4ef6-9848-b3f2f0e9d8a7.htm.
11] Public announcement of the environmental impact report for the projects (to be approved), Environmental
Protection Agency, Yichun, 2016.
http://www.ycepb.gov.cn/zwgk/jsxhgjyxpj/ypzxmgs/201608/t20160823_478174.html.
12] J. Wang, M. Sánchez-Roselló, J. L. Aceña, C.D. Pozo, A. E. Sorochinsky, S. Fustero, V.A. Soloshonok, H. Liu,
Fluorine in pharmaceutical industry: fluorine-containing drugs introduced to the market in the last decade
[
[
[
[
[
[
[
[
[
[
[
(2001-2011), Chem. Rev. 114 (2014) 2432-2506.
[13] M. Kiel, K.H. Engesser, The biodegradation vs. biotransformation of fluorosubstituted aromatics, Appl.
Microbiol. Biotechnol. 99 (2015) 7433-7464.
14] X. Zeng, R. Qu, M. Feng, J. Chen, L. Wang, Z. Wang, Photodegradation of polyfluorinated dibenzo‑p‑
dioxins in organic solvents: experimental and theoretical studies, Environ. Sci. Technol. 50 (2016) 8128-8134.
15] Z. Sun, X.Wei, H. Shen, X. Hu, Preparation and evaluation of Pd/polymeric pyrrole-sodium lauryl
sulfonate/foam-Ni electrode for 2,4-dichlorophenol dechlorination in aqueous solution, Electrochim. Acta 129
[
[
(2014) 433-440.
[16] H. Ma, Y. Xu, X. Ding, Q. Liu, C.A. Ma, Electrocatalytic dechlorination of chloropicolinic acid mixtures by
using palladium-modified metal cathodes in aqueous solutions, Electrochim. Acta 210 (2016) 762-772.
17] Y.H. Xu, Q.Q. Cai, H.X. Ma, Y. He, H. Zhang, C.A. Ma, Optimisation of electrocatalytic dechlorination of
,4-dichlorophenoxyacetic acid on a roughened silver-palladium cathode, Electrochim. Acta 96 (2013) 90-96.
18] C. Colomban, E.V. Kudrik, P. Afanasiev, A.B. Sorokin, Catalytic defluorination of perfluorinated aromatics
under oxidative conditions using N-bridged diiron phthalocyanine, J. Am. Chem. Soc. 136 (2014) 11321-11330.
19] X. Zhang, H. Feng, D. Shan, J.L. Shentu, M. Wang, J. Yin, D. Shen, B. Huang, Y. Ding. The effect of
electricity on 2-fluoroaniline removal in a bioelectrochemically assisted microbial system (BEAMS), Electrochim.
[
2
[
[
1
5

ACCEPTED MANUSCRIPT
Acta 135 (2014) 439-446.
20] Y. Dai, Y. Song, S. Wang, Y. Yuan, Treatment of halogenated phenolic compounds by sequential tri-metal
reduction and laccase-catalytic oxidation, Water Res. 71 (2015) 64-73.
[
[21] J.W. Xie, CN Patent 105288927A, 2015.
[22] X. Ma, S. Liu, Y. Liu, G. Gu, C. Xia, Comparative study on catalytic hydrodehalogenation of halogenated
aromatic compounds over Pd/C and Raney Ni catalysts. Sci. Rep. 6 (2016) 1-10.
23] R. Baumgartner, G.K. Stieger, K. McNeill, Complete hydrodehalogenation of polyfluorinated and other
polyhalogenated benzenes under mild catalytic conditions, Environ. Sci. Technol. 47 (2013) 6545-6553.
24] Y.H. Yu, P.C. Chia, Kinetics and pathway of vinyl fluoride reduction over rhodium, Environ. Sci. Technol.
Lett. 1 (2014) 448-452.
25] W.B. Wu, M.L. Li, J.M. Huang, Electrochemical hydrodefluorination of fluoroaromatic compounds,
Tetrahedron Lett. 56 (2015) 1520-1523.
26] Y. Xu, H. Ma, T. Ge, Y. Chu, C.A. Ma, Rhodium-catalyzed electrochemical hydrodefluorination: A mild
approach for the degradation of fluoroaromatic pollutants, Electrochem. Commun. 66 (2016) 16-20.
27] E.R. Essinger-Hileman, D. DeCicco , J.F. Bondi, R.E. Schaak, Aqueous room-temperature synthesis of Au-Rh,
[
[
[
[
[
Au-Pt, Pt-Rh, and Pd-Rh alloy nanoparticles: fully tunable compositions within the miscibility gaps, J. Mater.
Chem. 21 (2011) 11599-11604.
[
28] U. Koss, M. Łukaszewski, K. Hubkowska, A. Czerwiński, Influence of rhodium additive on hydrogen
electrosorption in palladium-rich Pd–Rh alloys, J. Solid State Electrochem. 15 (2011) 2477-2487.
29] Y.Z. Xiang, L.N. Kong, P.Y. Xie, T.Y. Xu, J.G. Wang, X.N. Li, Carbon nanotubes and activated carbons
supported catalysts for phenol in situ hydrogenation: hydrophobic/hydrophilic effect, Ind. Eng. Chem. Res. 53
[
(2014) 2197-2203.
[30] S. Martinez, M. Metikos-Hukovic, L. Valek, Electrocatalytic properties of electrodeposited Ni-15Mo cathodes
for the HER in acid solutions: synergistic electronic effect, J. Mol. Catal. A-Chem. 245 (2006) 114-121.
31] B. Ren, X.F. Lin, J.W. Yan, B.W. Mao, Z.Q. Tian, Electrochemically roughened rhodium electrode as a
substrate for surface-enhanced raman spectroscopy. J. Phys. Chem. B 107 (2003) 899-902.
32] A.A. Isse, P.R. Mussini, A. Gennaro, New insights into electrocatalysis and dissociative electron transfer
mechanisms: The case of aromatic bromides, J. Phys. Chem. C 113 (2009) 14983–14992.
33] Y.H. Xu, H. Zhang, C.P. Chu, C.A. Ma, Dechlorination of chloroacetic acids by electrocatalytic reduction
using activated silver electrodes in aqueous solutions of different pH, J. Electroanal. Chem. 664 (2012) 39–45.
34] S. L. Yau, Y.G. Kim, K. Itaya, In Situ Scanning Tunneling Microscopy of Benzene Adsorbed on Rh(111) and
Pt(111) in HF Solution, J. Am. Chem. Soc., 118 (1996), 7795-7803.
[
[
[
[
[35] C.A. Ma, Introduction to organic electrochemical synthesis, Science Press, Beijing 2002, p. 51-52.
[36] S.F. O’Hannesin, R.W. Gillham, Long-term performance of an in situ “iron wall” for remediation of VOCs,
Ground Water 36 (1998) 164-170.
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6

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Table captions
Table 1. Electrodeposition conditions used to fabricate modified nickel foams.
Table 2. Optimization of HDF catalysts and catholyte pH using 4-FP as the target compound.
Table 3. Degradation of 4-FP in the presence/absence of applied current under N2.
Table 4. HDF of 1 mM 4-FP in 20 mM aqueous phosphate buffer with pH 3 performed at 30 °C
using H2.

ACCEPTED MANUSCRIPT
Table 1.
a
Entry
Modified nickel foams
Concentration of Rh (mM)
Concentration of Pd (mM)
1
2
3
4
5
6
0.5Pd/Ni foam
1Pd/Ni foam
0
0
0.5
1
1.5Pd/Ni foam
0
1.5
0
0.5Rh/Ni foam
0.5
1
1Rh/Ni foam
0
1.5 Rh/Ni foam
1Rh/0.5Pd/Ni foam
0.5Pd/1Rh/Ni foam
0.5Rh-1Pd/Ni foam
1Rh-0.5Pd/Ni foam
1.5
1
0
b
7
8
9
0.5
0.5
1
b
c
1
0.5
1
c
1
0
0.5
a
A constant current of 4 mA was passed through an aqueous solution (40 mL) containing 1 M NaCl and different
concentrations of RhCl and/or PdCl at pH 2 (adjusted using HCl) until the rose, yellow, or orange-red color
3
2
disappeared.
b
Rh and Pd were separately deposited in different solutions.
Rh and Pd were co-deposited in the same solution.
c
Table 2.
a
b
-
Entry
Cathodes
Concentration pH Atmosphere I mA/t
Conv.
(%)
2
Yield of F (%)
of FP
min
4/45
4/45
4/45
4/45
4/45
4/45
4/45
4/45
4/45
4/45
4/45
4/30
4/30
4/30
4/30
4/30
4/45
0.4/45
1
2
3
4
5
6
7
8
9
Ni foam
1.0 mM
1.0 mM
1.0 mM
1.0 mM
1.0 mM
1.0 mM
1.0 mM
1.0 mM
1.0 mM
1.0 mM
1.0 mM
1.0 mM
1.0 mM
1.0 mM
1.0 mM
1.0 mM
1.0 mM
0.1 mM
3
3
3
3
3
3
3
3
3
3
3
3
5
7
9
11
3
3
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
Air
N2
0
0
0.5Pd/Ni foam
1Pd/Ni foam
4.5
2.5
4
0
1.5Pd/Ni foam
0
0.5Rh/Ni foam
31
46
66
55
53
76
97
88
44
5
30
45
67
53
50
75
98
85
42
1
1Rh/Ni foam
1.5 Rh/Ni foam
1Rh/0.5Pd/Ni foam
0.5Pd/1Rh/Ni foam
0.5Rh-1Pd/Ni foam
1Rh-0.5Pd/Ni foam
1Rh-0.5Pd/Ni foam
1Rh-0.5Pd/Ni foam
1Rh-0.5Pd/Ni foam
1Rh-0.5Pd/Ni foam
1Rh-0.5Pd/Ni foam
1Rh-0.5Pd/Ni foam
1Rh-0.5Pd/Ni foam
1
0
1
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
2
0
1
0
99
100
98
98
a
Unless Specifically Noted Otherwise, 20 mM aqueous phosphate buffer (40 mL) containing 1.0 mM 4-FP served
as the catholyte, a current of 4 mA was applied for 45 min (i.e., 4 mA/45 min, the passed charge equaled 2.8 F: 2.8
2
mol of electrons per mol 4-FP), and the electrode area equaled 2 × 3 cm .
b
–2
–2
0
.5Pd corresponds to a Pd loading of 0.33 mmol dm and 1Rh corresponds to a Rh loading of 0.67 mmol dm .
For detailed preparation conditions, refer to Table 1.

ACCEPTED MANUSCRIPT
Table 3.
a
-
b
Entry
Cathodes
Project area
I / mA
(t / Min)
4 (45)
Conv.
(%)
67
Yield of F
(%)
Concentration of
FE
2
2+
/
cm
Ni
(%)
51
1
2
3
1.5 Rh/Ni foam
1.5 Rh/Ni foam
2×3
2×3
2×3
65
3.2 mM
4.1 mM
2.2 mM
0 (45)
4 (45)
48
95
47
95
41
92
1Rh-0.5Pd/Ni
foam
4
1Rh-0.5Pd/Ni
foam
2×3
0 (45)
81
82
3.1 mM
91
c
5
6
0.75 Rh/C paper
2×2.5
2×2.5
12 (120)
12 (120)
64
93
60
94
/
/
/
/
c
0.5Rh-0.25Pd/C
paper
a
Catholyte: 40 mL of 20 mM aqueous phosphate buffer with pH 3 containing 1 mM 4-FP.
Faradaic efficiency (FE, %) for the degradation of 4-FP was calculated as
b
n × F ×V × ∆C
1
4− FP
FE =
,
I × ∆t + n × F ×V × ∆C
Ni 2+
2
where n and n are the electron transfer numbers for the degradation of 4-FP (n ≈ 7) and the oxidative dissolution
1
2
1
2+
2+
−1
2+
of Ni (n = 2), respectively; ∆C
and ∆CNi are the concentration differentials (mol L ) of 4-FP and Ni
4-FP
2
−1
during time ∆t, respectively; F is the Faraday constant (96500 C mol ); V is the total volume of the catholyte (0.04
L); and I is the applied current.
c
The model number of the C paper is TGP-H-060.
Table 4.
Entry
Volume of
reaction solution
40 mL
Catalytic materials
Atmosphere
Reaction
time
Conv.
(%)
85
Yield of
-
F (%)
2
1
2
3
4
2×3 cm 1.5 Rh/Ni foam
H2
N2
H2
N2
H2
H2
45 min
45 min
45 min
45 min
120 min
120 min
83
47
100
80
0
2
40 mL
2×3 cm 1.5 Rh/Ni foam
48
2
40 mL
2×3 cm 1Rh-0.5Pd/Ni foam
100
81
2
40 mL
2×3 cm 1Rh-0.5Pd/Ni foam
a
2
5
40 mL
2×2.5 cm 0.75 Rh/C paper
3
a
2
6
40 mL
2×2.5 cm 0.5Rh-0.25Pd/C
7
3
paper
b
2
7
40 mL
2×2.5 cm 0.5Rh-0.25Pd/C
H2
H2
120 min
180 min
13
37
12
36
paper
8
100 mL
5 mg Rh/Al O3
2
a
The model number of the C paper is TGP-H-060.
The model number of the C paper is AvCarb EP40T.
b

ACCEPTED MANUSCRIPT
Figure captions
Fig. 1. SEM images of Ni and 1Rh-0.5Pd/Ni foams, and an EDS spectrum of the latter foam.
Fig. 2. XRD patterns of (A) Ni, 1Rh/Ni, 0.5Pd/Ni, fresh 1Rh-0.5Pd/Ni, and used 1Rh-0.5Pd/Ni
foams. (B) Magnified section of (A).
Fig. 3. ((A) and (C)) CV curves (third cycle) of Ni, 1Rh/Ni, and 1Rh-0.5Pd/Ni disks (diameter = 2
mm) recorded in 20 mM aqueous phosphate buffer with pH 3 (blank solution). (B) CA curves of
2
fresh 1Rh/Ni and 1Rh-0.5Pd/Ni foams (2 × 3 cm ) recorded in the blank solution containing 1 mM
2
4
-FP. (D) CP curves of fresh Ni, 1Rh/Ni, and 1Rh-0.5Pd/Ni foams (2 × 3 cm ) recorded in blank
solution under N2.
Fig. 4. Faradic efficiency and concentration profiles of 4-FP and its degradation products (PhOH =
phenol; CYC.one = cyclohexanone; CYC.ol = cyclohexanol) observed during HDF at an applied
current of 4 mA, under ambient conditions. Catholyte = 40 mL of 20 mM aqueous phosphate
buffer with pH 3 containing 1 mM 4-FP; electrode = ((A) and (C)) 1Rh/ Ni foam (projected area:
2
2
2
× 3 cm ), ((B) and (D)) 1Rh-0.5Pd/Ni foam (projected area: 2 × 3 cm ).
Fig. 5. CV curves of (A) Ni, (B) 0.5Pd/Ni, (C) 1Rh/Ni, and (D) 1Rh-0.5Pd/Ni disks (diameter = 2
mm) recorded in 20 mM aqueous phosphate buffer with pH 3 containing various concentrations of
–
1
4
-FP at a scan rate of 50 mV s under N .
2

ACCEPTED MANUSCRIPT
Fig. 1.
Fig. 2.
Fig. 3.

ACCEPTED MANUSCRIPT
Fig. 4.
Fig. 5.

ACCEPTED MANUSCRIPT
>
We developed an efficient Rh-Pd alloy catalyst for hydrodefluorination (HDF). >
The HDF of 4-fluorophenol (4-FP) over the catalyst follows the indirect
hydrogenation mechanism. > The catalyst could be used in electrochemical and
zero-valent metals reductive HDF. > The catalyst is potentially viable to remediate the
4
-FP-contaminated groundwater.