liquid interface

    Molecular electrocatalysis of oxygen reduction by iron(II) phthalocyanine at the liquid/liquid interface Yaofang Xuan, Lisiqi Xie, Xiao Huang, Bin Su PII: DOI: Reference:

S1572-6657(16)30029-7 doi: 10.1016/j.jelechem.2016.01.028 JEAC 2469

To appear in:

Journal of Electroanalytical Chemistry

Received date: Revised date: Accepted date:

26 November 2015 22 January 2016 25 January 2016

Please cite this article as: Yaofang Xuan, Lisiqi Xie, Xiao Huang, Bin Su, Molecular electrocatalysis of oxygen reduction by iron(II) phthalocyanine at the liquid/liquid interface, Journal of Electroanalytical Chemistry (2016), doi: 10.1016/j.jelechem.2016.01.028

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ACCEPTED MANUSCRIPT Molecular electrocatalysis of oxygen reduction by iron(II) phthalocyanine at the liquid/liquid interface

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Yaofang Xuan, Lisiqi Xie, Xiao Huang and Bin Su*

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Institute of Microanalytical Systems, Department of Chemistry, Zhejiang University, Hangzhou 310058, China

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Keywords: Iron(II) Phthalocyanine, O2 Reduction, Liquid/Liquid Interface, Catalysis,

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Biphasic Reaction

Abstract Liquid/liquid interface electrochemistry has manifested itself as a good

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approach to study oxygen reduction reaction (ORR) and ORR catalyzed by various

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catalysts. In this work, we investigated the ORR catalyzed by iron(II) phthalocyanine

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(FePc), which has a structure similarity to the heme group of O2-binding proteins and reducing enzymes, at the polarized water/1,2-dichloroethane interface. Using the

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four-electrode cyclic voltammetry and the shake-flask biphasic reaction under chemically controlled polarization, it was demonstrated that FePc could catalyze the molecular oxygen reduction by lipophilic electron donors, such as 1,1’-dimethylferrocene (DFc) or tetrathiafulvalene (TTF), at the heterogeneous phase boundary. The overall process essentially can be equivalent to an interfacial proton transfer coupled ORR, which proceeds preferentially via a four electron reduction pathway to produce mainly H2O with only minority of H2O2 (less than 5%). The catalytic activity of FePc was compared with all previously studied porphyrins and phthalocyanines. The reaction mechanism was also analyzed, in which a hydroperoxo intermediate was probably involved. 1

ACCEPTED MANUSCRIPT 1. Introduction Investigation of oxygen reduction reaction (ORR) is of significance from both

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technological and fundamental standpoints [1]. It is the cathodic reaction in fuel cells that

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convert chemical energy to electricity in an environmentally friendly way. In biology, respiratory ORR occurs at biomembranes to power the synthesis of adenosine

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triphosphate to sustain life activities. However, reduction of molecular oxygen in its ground state is spin-forbidden and kinetically inert, thus requires the catalytic activation

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of appropriate catalysts. In nature, terminal oxidases, such as cytochrome c oxidases (CsOs), have evolved to catalyze the biological ORR. The catalytic unit comprises a heme

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site, to which O2 binds and is reduced to H2O rapidly, efficiently and selectively without

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releasing partially reduced yet cytotoxic oxygen species (such as peroxide and superoxide)

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[1]. Inspired by this nature choice, a variety of metallic macrocyclic complexes, for examples porphyrins and phthalocyanines, have been synthesized and studied to unravel

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the catalytic function of CcOs and to reproduce this reactivity practically in fuel cells [2, 3]. The major intention for the latter purpose is to replace precious and scarce platinum electrocatalyst. Iron porphyrins and phthalocyanines were among the earliest synthetic non-precious metal catalysts for the ORR, given their structure similarity to the heme group of O2-binding proteins and reducing enzymes [4, 5]. Over decades, electrochemical reduction of O2 by iron porphyrins/phthalocyanines immobilized on electrodes has been extensively investigated, while there remains a substantial debate in the literature regarding their electrocatalytic activities, e.g., the potential dependent selectivity of O2 2

ACCEPTED MANUSCRIPT reduction [6]. The four electron reduction to H2O at high potentials has been ascribed to the generation of ferrous-peroxo intermediates that favor the O-O bond cleavage more

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strongly than ferric-peroxo ones [1, 7]. Another possibility proposed is associated with

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the multilayer film structure of catalysts confined to electrode surface, whereby the O2 reduction by individual catalyst molecules produces H2O2 and subsequently H2O2 is reduced or disproportionated to H2O by surrounding catalysts before its release [6, 7]. It

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means that the catalyst concentration and steric morphology on electrode surfaces affect

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significantly the reactivity. So it is highly desired to study the catalytic properties of these compounds under conditions where they exist in their individual, isolated state.

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Electrochemistry at the liquid/liquid interface has recently manifested itself as an

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alternative approach to study the ORR and ORR catalyzed by various catalysts, such as

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nanoparticles [8-10], metalated porphyrin [11-15] or free-base porphyrins [16, 17], metallic phthalocyanines [18, 19] and enzymes [20, 21]. In this approach, all reactants and

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catalyst molecules involved in the ORR can exist in the form of isolated state. Interaction of catalysts with the substrate electrodes can be also avoided at this interface. In addition, the liquid/liquid phase boundary can provide a physical separation of reactants and products, which is in particular interesting to study proton-coupled electron transfer (PCET) reactions [11, 12, 22], such as ORR [8, 9, 11-14, 16-19, 22], carbon dioxide reduction [23] and hydrogen evolution reaction [24-26]. Herein we study the electrocatalytic reduction of O2 by iron(II) phthalocyanine (FePc) at the polarized water/1,2-dichloroethane (DCE) interface. As studied previously, FePc adsorbed on pyrolytic graphite electrodes can catalyze four-electron reduction of O2 to H2O [5]. 3

ACCEPTED MANUSCRIPT Furthermore, in comparison with cobalt phthalocyanines, FePc exhibited a stronger catalytic activity [27, 28]. In this work, we demonstrated by voltammetry at the

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water/DCE interface and biphasic reaction under chemically controlled polarization that

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FePc could efficiently catalyze O2 reduction by molecular electron donors, such as 1,1’-dimethylferrocene (DFc) and tetrathiafulvalene (TTF). Since little amount of H2O2 (less than 5%) was detected in the products, it was concluded that the catalyzed ORR

2. Experimental Section

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2.1. Chemicals and reagents

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proceeded mainly via the four-electron reduction pathway.

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All chemicals and reagents were used as received without further purification. The

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aqueous solutions were all prepared with ultrapure water (Millipore Milli-Q, resistivity 18.2 Mcm). Iron(II) phthalocyanine (FePc, 95%) was ordered from J&K Chemical.

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1,1’-Dimethylferrocene (DFc) and potassium tatrakis(4-chlorophenyl)borate (KTPBCl, 98%) were purchased from TCI. Tetrathiafulvalene (TTF), tetramethylammonium chloride (TMACl) and 1,2-dichloroethane (DCE, HPLC grade) were bought from Aladdin. Bis(triphenylphosphoranylidene)ammonium chloride (BACl, 97%) was received from Alfa Aesar. Lithium tetrakis(pentafluorophenyl)borate (LiTB) diethyl etherate was ordered from Boulder Scientific. Lithium chloride anhydrous (LiCl, 97%) was ordered from Hengxin Chemicals. Hydrochloric acid (HCl, 37%) and anhydrous methanol were obtained from Sinopharm. Sodium iodide (NaI) was bought from Hichi Chemicals. Bis(triphenylphosphoranylidene) ammonium tatrakis(4-chlorophenyl)borate (BATPBCl) 4

ACCEPTED MANUSCRIPT was prepared by metathesis of 1:1 mixture of BACl and KTPBCl in methanol/water (v:v

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Insert here Scheme 1 and Scheme 2

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= 2:1), followed by recrystallization from acetone [29].

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2.2．Electrochemical measurements

Cyclic voltammetry (CV) measurements at the water/DCE interface were performed on

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a home-made four-electrode system connected to a Stanford Research System DS335

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synthesized function generator at ambient temperature (23 ± 2 C). A threecompartment glass cell with a cylindrical vessel and two capillary arms was used, as

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illustrated in Scheme 1. The water/DCE interface formed had a geometric area of 2.01

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cm2. Two platinum counter electrodes positioned in the aqueous and DCE phases were

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used to supply the current flow. Two silver/silver chloride (Ag/AgCl) reference electrodes were used for imposition of the external potential, which were connected to

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the aqueous and DCE phases, respectively, by means of a Luggin capillary. The electrochemical cell composition is schematically illustrated in Scheme 2. The Galvani potential difference (  ow ) was estimated by assuming the formal ion transfer potential of tetramethylammonium cation (TMA+) to be 0.16 V [30]. The differential capacitance was calculated from the admittance measurement using a Stanford Research System SR830 lock-in amplifier at a frequency of 6 Hz and amplitude of 4 mV rms. Insert here Scheme 3

2.3．Biphasic reactions with chemically controlled polarization 5

ACCEPTED MANUSCRIPT Biphasic reactions with the interface chemically polarized by the distribution of electrolyte ions were performed in small glass flasks. 1.5 mL of DCE solution containing

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reactants (FePc, DFc/TTF or both) was added first to the glass flask, followed by the

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addition of an equal volume of water containing 10 mM HCl and 5 mM LiTB. The detailed solution composition is shown by the cell in Scheme 3. After stirring for a certain period and waiting for clear phase separation, the aqueous and organic solutions

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were isolated from each other. The organic phase was directly subject to UV-visible

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spectroscopic analysis in order to monitor the formation of 1,1’-dimethylferrocenium (DFc+) or TTF radical cation (TTF•+). The aqueous solution was first treated with excess

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NaI (equivalent to 0.1 M) and then analyzed by the UV-visible spectroscopic

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measurement to quantify the generated H2O2 (H2O2 can oxidize I to I3, which displays

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two absorption bands centered at 287 nm and 352 nm, respectively, in the UV-visible spectrum) [31]. All the UV-visible absorption spectroscopic measurements were carried

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out on a SP-756PC spectrophotometer (Shanghai Spectrum).

3. Results and Discussions 3.1. Oxygen reduction catalyzed by FePc Fig. 1a displays the representative CV of the water/DCE interface in the absence and presence of 50 M FePc in DCE phase. The background CV (black curve) shows a classical potential window determined by the transfer of H+/Li+ and Cl at the positive and negative extremes, respectively. Addition of FePc to DCE phase yielded an almost identical CV (red curve), suggesting that FePc was neither involved in any Faradaic 6

ACCEPTED MANUSCRIPT charge transfer nor adsorbed at the water/DCE interface. Indeed, the differential capacitance curve in the presence of FePc (red curve) matches well with that in the

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absence of FePc (see Fig. 1b), confirming that FePc has no infinity for the interface. In

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the presence of DFc alone in DCE (see the blue curve in Fig. 2a), only a very small wave was observed at 0.05 V, corresponding to the transfer of DFc+ present in the stock solution [13]. In contrast, when both FePc and DFc were added to DCE phase (the

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green curve in Fig. 2a), an irreversible positive current wave (i.e. a wave without a signal

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on the reverse scan) was displayed in the positive potential regime. In addition, the current wave due to DFc+ transfer turned much more pronounced, indicating more

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DFc+ was produced in DCE. Upon repetitive cycling the potential scan (see the purple

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curve in Fig. 2a), a steady increase of this ion transfer wave is indicative of the

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continuous generation of DFc+. Furthermore, a control experiment under anaerobic condition was performed with the solutions pre-purged by argon stream for 20 min. As

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shown in Fig. 2b, the DFc+ transfer current wave is apparently much smaller than that under the aerobic condition and did not increase with potential cycling, proving that O2 was involved to the production of DFc+. The current observed at the positive potential limit under anaerobic condition is most probably associated with the heterogeneous formation of DFc-hydride and/or proton reduction [25, 32]. Insert here Fig. 1 and Fig. 2 All these voltammetric data indicate that the irreversible positive current wave at the positive potentials roots in the combined presence of DFc, FePc and O2. It most likely arises from an interfacial O2 reduction reaction catalyzed by FePc, i.e., a proton coupled 7

ACCEPTED MANUSCRIPT electron transfer (PCET) process. Indeed, this current signal is phenomenologically similar to that observed previously with lipophilic cobalt(II) porphyrins or cobalt(II)

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phthalocyanines as catalysts [11-14, 18, 19]. The remarkable irreversible positive current

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in the presence of FePc indicates the strong catalytic activity of FePc on the O2 reduction by DFc, given DFc itself nearly does not react with O2 on the time scale of CV.

supporting information).

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3.2. Shake-flask biphasic reactions

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Similar CV was also obtained with another electron donor, TTF (see Fig. S1, in the

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Biphasic reactions (namely so-called shake flask experiments), where the interface was

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polarized chemically by the ion partition [31], were performed to investigate the catalytic

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activity of FePc and to identify the ORR products. The detailed solution composition is shown in Scheme 3, which fixes the initial Galvani potential difference across the interface at a positive potential (namely +0.58 V) in terms of the partition of all ions and

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the electroneutrality (see Table 1 and the detailed calculation in the Supporting Information). This value falls within the potential regime of the PCET current wave, as shown in Fig. 2. Insert here Table 1 and Fig. 3 Fig. 3 displays the photographs of three flasks in the course of a biphasic reaction. DCE solutions containing only 5 mM DFc, both 50 M FePc and 5 mM DFc, only 50 M FePc were added to the flask 1, 2 and 3, respectively, which appeared yellow, light green and pale blue (as seen in Fig. 3a). Shown in Fig. 3b and 3c are photographs 8

ACCEPTED MANUSCRIPT immediately after adding the acidic water solution on the top and subsequently stirring for 30 min. Clearly, the color of DCE solution in the flask 1 did not change, while that

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of flask 2 changed its color from light green to dark green obviously, corresponding to

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the mixed color of DFc+ and FePc. In the case of the flask 3, the DCE phase turned from pale blue to violet.

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Insert here Fig. 4

After separating the water and DCE phases from each other, the UV-visible spectra

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of both phases were analyzed. As shown in Fig. 4a, when only DFc was dissolved in the DCE phase (flask 1), the absorption band at 652 nm due to DFc+ was pretty small (red

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curve), indicating only little amount of DFc+ was generated. This coincides with the fact that DFc reacts slowly with O2 on its own [13]. In contrast, when both FePc and DFc

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were added to the DCE phase (flask 2), a much stronger absorption band at 652 nm was displayed (blue curve). In the case of flask 3 where only FePc was dissolved in DCE, its

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UV-visible spectrum featured with two Q bands at 650 nm and 764 nm were replaced with two new sharper bands at 532 and 658 nm, as shown in Fig. 4b. This variation most probably arises from the proton-facilitated oxygenation of FePc to form an adduct, namely (Fe-O2)Pc or (Fe-O2H)Pc+. In the case of the top aqueous solutions, they were first treated with sodium iodide (NaI, 0.1 M) and then diluted twice prior to the UV-visible spectroscopic measurement. The pretreatment with NaI was carried out to determine if and how much H 2O2 was produced. H2O2 can oxidize I to triiodide (I3), which displays a yellow color and two absorption bands (max= 287 and 352 nm) in the UV-visible spectroscopy [31]. For the 9

ACCEPTED MANUSCRIPT aqueous solution separated from flask 2, as shown by the blue curve in Fig. 4c, two absorption bands at 287 and 352 were observed, indicating the formation of H2O2. In

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contrast, when only FePc or DFc was dissolved in DCE phase (see the green and red

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curves in Fig. 4c), no I3 was detected, proving the catalytic role of FePc on the ORR by DFc. Similar phenomenon has also been observed with another electron donor, namely

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TTF (see Fig. S2 and Fig. S3, in the supporting information). Insert here Fig. 5

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Fig. 5 displays the time profiles for the formation of DFc+ and H2O2 in the absence (black curve) and presence (red curve) of FePc in the course of a biphasic reaction with

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the Galvani potential difference fixed at +0.58 V. The presence of FePc apparently led to a much faster production of DFc+ and H2O2, confirming the catalytic role of FePc in the

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reduction of O2 by DFc. Furthermore, the quantity of DFc+ increased sharply at shorter time but slowly after 10 min, suggesting that most of DFc was consumed and oxidized

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to DFc+ (see Fig. 5a). The amount of H2O2 also decreased at longer time (after 45 min, see Fig. 5b), which is most likely due to heterogeneous catalytic activity of DFc towards the H2O2 decomposition [14]. If assuming that DFc was completely oxidized after 90 min, the percentage of DFc used for H2O2 produced at 30 min was 2.92% and the total number of transferred electrons per molecular oxygen reduction in the biphasic reaction is 3.94. Similar values, 4.22% and 3.92, were also obtained when using TTF as the electron donor (previous study has proved that TTF itself does not react with H2O2 [33]) in the biphasic reaction. Apparently, the former value is smaller whereas the latter one larger than those obtained with cobalt(II) phthalocyanines and momomeric porphyrins, 10

ACCEPTED MANUSCRIPT but both are comparable to those obtained with dimeric porphyrins, as illustrated in Table 2. However, it is in agreement with the previous study that FePc catalyzes the

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four-electron reduction of O2 to H2O at the modified electrode [5].

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Insert here Table 2 and Table 3

If assuming a first order reaction with respect to the DFc concentration [14, 34], the

d[DFc  ]  kapp [DFc] dt

(1)

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v

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reaction rate can be expressed as,

where kapp is the apparent rate constant of the reaction . The integrated rate law is given

a ax

(2)

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kappt  ln

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by,

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where a is the initial concentration of DFc and x is the concentration of DFc+. As shown in Fig. 5c, a straight line fitting in terms of eq. 2 yielded a slope of 0.173 (min1)

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as the apparent rate constant. In comparison with the oxygen reduction by DFc alone (kapp = 2.25  104 min1)，the apparent reaction rate was enhanced by approximately three orders. As compared in Table 3, this value is smaller than those obtained with cobalt porphyrins.

3.3. Mechanism analysis The mechanism of oxygen reduction with DFc catalyzed by FePc is proposed as illustrated in Scheme 4, which is similar to that proposed previously for CoPc and CoFPc catalysts at the water/DCE interface [18, 19]. The proton transfer (PT) across the 11

ACCEPTED MANUSCRIPT water/DCE interface and the electron transfer (ET) from DFc to the superoxide adduct, (FeII-O2)Pc or (FeIII-O2•)Pc, are coupled at the water/DCE interface. The formation of

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(3)

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FeII Pc DCE  O2 DCE    FeIII -O2  Pc DCE

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the superoxide adduct in DCE can be expressed as,

The catalyst-O2 adduct can be converted to the ferric-hydroperoxo intermediate at the

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-O2  Pc DCE  DFc DCE  H W   FeIII -O2H  Pc DCE  DFc DCE

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 Fe

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interface by capture of an electron from DFc and one proton from water,

(4)

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Subsequently, the formed ferric-hydroperoxo may undergo the O-O bond cleavage to

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-O2 H  Pc DCE  H W   H2O W    FeIV =O  Pc+ DCE 

(5)

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 Fe

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generate H2O and high-valence oxoferryl cation radical by,

The oxoferryl cation radical can then capture two electrons and two protons to produce

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H2O eventually by,

 Fe

IV

=O  Pc + DCE   2DFc DCE   2H   W   H 2O  W   2DFc   DCE   FeIII Pc+ DCE 

(6)

This reaction pathway competes with the reactions for the formation of H2O2. H2O2 can generate from the O-O hemolysis of the ferric-hydroperoxo intermediate by,

 Fe

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-O2 H  Pc DCE  H W  H2O2 W  FeIII Pc+ DCE 

(7)

Alternatively, the ferrous-hydroperoxo intermediate may be involved to generate H2O2 by the following two steps,

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ACCEPTED MANUSCRIPT  Fe

III

-O2 H  Pc DCE  DFc DCE    FeII -O2 H  Pc DCE   DFc DCE 

(8)

 Fe

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-O2 H  Pc DCE  H+ W  FeII Pc DCE   H 2O2 W 

(9)

(10)

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FeIII Pc+ DCE  DFc DCE  FeII Pc DCE  DFc+ DCE

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Another step possibly involved is the regeneration of FeIIPc by DFc in DCE,

These reaction steps are summarized by a reaction diagram shown in Scheme 4. For

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simplicity, the ferric-hydroperoxo, ferrous-hydroperoxo and high-valence oxoferryl

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intermediate species are not shown. As shown in Fig. 6, cyclic voltammetry and square-wave voltammetry reveal that the FePc+/FePc couple has a redox potential of

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+0.98 V with respect to the standard hydrogen electrode (SHE) in DCE by using Fc as

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the internal reference (Fc+/Fc couple, +0.64 V). Therefore, the intermolecular electron

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transfer from DFc to FeIIIPc+, namely eq. 10, is thermodynamically favorable, given the standard redox potential of DFc+/DFc is +0.55 V [35].

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Insert here Scheme 4 and Fig. 6

4. Conclusions The catalytic activation of FePc on the oxygen reduction by DFc and TTF has been investigated at the polarized water/DCE interface. Cyclic voltammetry measurements at the interface has revealed that the ORR catalyzed by FePc proceeds via a proton transfer coupled electron transfer pathway, with protons provided by aqueous phase and electron by donors in the DCE phase. By biphasic reactions controlled by chemical polarization, it was found that the selectivity of FePc towards four-electron reduction of O2 by DFc and 13

ACCEPTED MANUSCRIPT TTF was more than 95%. This value is much higher than that obtained recently with microperoxidase-11, a biomimetic analogue to CsOs containing a heme group bonded to

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an undecapeptide chain, which strongly adsorbs at the water/DCE interface and catalyze

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the ORR to yield significant amount of H2O2 [21]. This difference indicates that the molecular structure and reaction microenvironment have significant effect on the ORR

properties of iron macrocyclic complexes.

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Acknowledgement

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pathway. Thus, more in-depth investigations are necessary to fully unravel the catalytic

The work is supported by the National Nature Science Foundation of China (21335001)

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and the Zhejiang Provincial Nature Science Foundation (LR14B050001).

[1]

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Y. Li, S. Wu and B. Su, Proton‐coupled O2 reduction reaction catalysed by cobalt phthalocyanine at liquid/liquid interfaces, Chem. Eur. J. 18 (2012) 7372-7376. I. H. Patir, Fluorinated-cobalt phthalocyanine catalyzed oxygen reduction at liquid/liquid

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interfaces, Electrochim. Acta 87 (2013) 788-793. [20] T. Sugihara, H. Hotta and T. Osakai, Electrochemical control of glucose oxidase-catalyzed redox reaction using an oil/water interface, Phys. Chem. Chem. Phys. 6 (2004) 3563-3568. [21]

Y. Xuan, X. Huang and B. Su, Biomimetic oxygen reduction reaction catalyzed by microperoxidase-11 at liquid/liquid interfaces, J. Phys. Chem. C 119 (2015) 11685-11693.

[22]

M. A. Méndez, R. Partovi-Nia, I. Hatay, B. Su, P. Ge, A. Olaya, N. Younan, M. Hojeij and H. H. Girault, Molecular electrocatalysis at soft interfaces, Phys. Chem. Chem. Phys. 12 (2010) 15163-15171.

[23]

M. A. Méndez, P. Voyame and H. H. Girault, Interfacial Photoreduction of Supercritical CO2 by an Aqueous Catalyst, Angew. Chem. Int. Ed. 50 (2011) 7391-7394.

[24]

I. Hatay, P. Y. Ge, H. Vrubel, X. L. Hu and H. H. Girault, Hydrogen evolution at polarised liquid/liquid interfaces catalyzed by molybdenum disulfide, Energy Environ. Sci. 4 (2011) 4246-4251.

[25]

I. Hatay, B. Su, F. Li, R. Partovi‐Nia, H. Vrubel, X. Hu, M. Ersoz and H. H. Girault, Hydrogen Evolution at Liquid–Liquid Interfaces, Angew. Chem. Int. Ed. 48 (2009) 5139-5142.

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J. J. Nieminen, I. Hatay, P. Y. Ge, M. A. Mendez, L. Murtomaki and H. H. Girault, Hydrogen 15

ACCEPTED MANUSCRIPT evolution catalyzed by electrodeposited nanoparticles at the liquid/liquid interface, Chem. Commun. 47 (2011) 5548-5550. [27]

J. H. Zagal, Metallophthalocyanines as catalysts in electrochemical reactions, Coord. Chem. Rev. 119 (1992) 89-136. J.-P. Dodelet, Oxygen Reduction in PEM Fuel Cell Conditions: Heat-Treated Non-Precious

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[28]

Metal-N4 Macrocycles and Beyond, in: J. H. Zagal, F. Bedioui and J.-P. Dodelet (Eds.), [29]

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N4-macrocycles metal complexes, Springer, New York, 2006, vol. 1, pp. 83-147. B. Su, J. P. Abid, D. J. Fermin, H. H. Girault, H. Hoffmannova, P. Krtil and Z. Samec,

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Reversible voltage-induced assembly of Au nanoparticles at liquid vertical bar liquid interfaces, J. Am. Chem. Soc. 126 (2004) 915-919.

[30] T. Wandlowski, V. Marecek and Z. Samec, Galvani potential scales for water-nitrobenzene and water/1,2-dichloroethane interfaces, Electrochim. Acta 35 (1990) 1173-1175. B. Su, R. P. Nia, F. Li, M. Hojeij, M. Prudent, C. Corminboeuf, Z. Samec and H. H. Girault,

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[31]

H2O2 generation by decamethylferrocene at a liquid| liquid interface, Angew. Chem. Int. Ed. 47 (2008) 4675-4678.

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[32] T. J. Stockmann, H. Q. Deng, P. Peljo, K. Kontturi, M. Opallo and H. H. Girault, Mechanism of oxygen reduction by metallocenes near liquid/liquid interfaces, J. Electroanal. Chem. 729 (2014) 43-52. [33]

A. J. Olaya, P. Ge, J. F. Gonthier, P. Pechy, C. Corminboeuf and H. H. Girault, Four-electron

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oxygen reduction by tetrathiafulvalene, J. Am. Chem. Soc. 133 (2011) 12115-12123.

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[34] P. Y. Ge, M. D. Scanlon, P. Peljo, X. J. Bian, H. Vubrel, A. O'Neill, J. N. Coleman, M. Cantoni, X. L. Hu, K. Kontturi, B. H. Liu and H. H. Girault, Hydrogen evolution across nano-Schottky junctions at carbon supported MoS2 catalysts in biphasic liquid systems, Chem. Commun. 48 N. Eugster, D. J. Fermín and H. H. Girault, Photoinduced electron transfer at liquid/liquid interfaces. Part VI. On the thermodynamic driving force dependence of the phenomenological electron-transfer rate constant, J. Phys. Chem. B 106 (2002) 3428-3433.

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[35]

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(2012) 6484-6486.

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Scheme 1. The four electrode glass cell with the DCE phase shown shaded

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Scheme 2. Composition of the electrochemical cell

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Scheme 3. The cell used for biphasic reactions

Scheme 4. Interfacial PCET mechanism. PT = proton transfer, ET = electron transfer

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ACCEPTED MANUSCRIPT Table 1. Calculated equilibrium concentrations after contacting 5 mM DFc/1 mM TTF in DCE with 10 mM HCl and 5 mM LiTB in water. cLi+/mM

cTB/mM

cCl/mM

Water

5.75

4.33

0.07

10.00

DCE

4.25

0.67

4.93

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cH+/mM

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7.33  1018

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The calculated Galvani potential difference is approximately +0.58 V.

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ACCEPTED MANUSCRIPT Table 2. Percentage of DFc used for hydrogen peroxide production, and total number of transferred electron per molecular oxygen reduction in two-phase experiment [14, 21] .

System

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NaI method

Co2(DPOx)+DFca,1 min

12

Co2(DPX)+DFca,1 min

7

CoTPP+DFca,1 min

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Co2(DPO)+DFca,1 min

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rH2O2, %

DFca,10 min

n

3.7 3.8 3.9 3.6

37

3.2

11

3.8

41

3.2

24

3.5

19

3.6

32

3.4

27

3.5

36

3.3

FePc+DFcb,30 min

2.92

3.94

FePc+TTFb,15 min

4.22

3.92

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Co2(DPOx)+DFcb,1 min Co2(DPOx)+TTFa,1 min

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Co2(DPOx)+TTFa,10 min

MP-11+DFcb,45 min

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MP-11+TTFb,5 min

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MP-11+DFcb,5 min

a

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MP-11+TTFb,180 min

DCB as a solvent. bDCE as a solvent.

Co2(DPO): cobalt(II) 4,6-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)] dibenzofuran Co2(DPOx): cobalt(II) 2,2’-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)] diphenylether Co2(DPX) : cobalt(II) 4,5-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]-9,9-dimethylxanthene CoTPP: cobalt(II) 5,10,15,20-meso-tetraphenylporphyrin cobalt MP-11: microperoxidase-11 DCB: 1,2-dichlorobenzene

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ACCEPTED MANUSCRIPT Table 3. The estimated rate constants of oxygen reduction reaction catalyzed by

Co2(DPOx) a

0.2

Co2(DPX) a

0.20.08

CoTPPa

0.20.08

DFc alone

0.38  105

TTF alone

1.0  105

CoFPcb

0.006

FePca

0.0029

a

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0.2

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Co2(DPO) a

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kapp, s1

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catalyst

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different catalysts [14, 19, 33].

DFc as electron donor;. bTTF as electron donor.

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CoFPc: cobalt(II) 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadecafluoro-29H,31H-phthalocyanine

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1. (a) Cyclic voltammograms (CVs) at a scan rate of 50 mV s1 and (b) differential

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capacitance versus potential curves in the absence (x = 0, black curve) and presence (x =

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50, red curve) of FePc using the cell shown in Scheme 2: y = 0, z = 2.

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Fig. 2. (a) CVs of the polarized water/DCE interface using the electrochemical cell shown in Scheme 2: x = 0 and y = 0 (black curve); x = 50 and y = 0 (red curve); x = 0 and y = 5 (blue curve); x = 50 and y = 5 (green curve) at pH 2 (z = 2). The purple curves

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correspond to the 20 scan cycles of CVs in the presence of both FeFc and DFc (x = 50 and y = 5). (b) 20 cycles of CVs in the presence of both 50 M FePc and 5 mM DFc

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under aerobic (red curve) and anaerobic (black curve) using the cell illustrated in Scheme 2: x = 50, y = 5, z = 2. The scan rate was 50 mV s1 in all cases.

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Fig. 3. Photographic illustrations of the two-phase reaction flasks before (a) and after (b)

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adding top aqueous solutions and stirring 30 min (c) using the cell shown in Scheme 3: the top aqueous phase containing 5 mM LiTB + 10 mM HCl was the same for all three

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flasks; the bottom DCE solutions contained 1) DFc (x = 0, y = 5), 2) FePc and DFc (x = 50, y = 5) and 3) FePc (x = 50, y = 0). Fig. 4. (a) UV-visible spectra of the DCE phases separated from the flask 1 (only DFc,

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red curve), flask 2 (both FePc and DFc, blue curve), after the two-phase reaction; that of freshly prepared DFc (black curve) are also given for comparison. (b) UV-visible spectra of the DCE solution in flask 3 before (black curve) and after (red curve) the two-phase reaction. (c) UV-visible spectra of the aqueous phases separated from the flask 1 (only DFc, red curve), flask 2 (both FePc and DFc, blue curve), flask 3 (only FePc, green curve), treated by excess NaI for 30 min and then diluted twice. Fig. 5. Time profile of the formation of DFc+ (a) and H2O2 (b) in the absence (black) and presence (red) of 50 M FePc in the course of a biphasic reaction with the solution composition illustrated by the cell shown in Scheme 3. All aqueous solutions were diluted twice before measurements. (c) Plot of the integrated rate law for the evolution of rate constant. a is the initial concentration of DFc and x is the concentration of 21

ACCEPTED MANUSCRIPT DFc+. Fig. 6. (a) CVs at a scan rate of 0.05 V s1 of 0.1 mM FePc in DCE containing 0.2 M

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TBAClO4 as the supporting electrolyte in the absence (black curve) and presence (red

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curve) of Fc as the internal electrode. Electrochemical measurements were performed in a three-electrode configuration by a glass carbon electrode, a platinum wire and a silver

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wire were employed as the working, counter and reference electrodes, respectively. (b) The corresponding square-wave voltammogram for determination of the redox potential

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value of [FeIIIPc]+/[FeIIPc].

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Fig. 1. Su et al.

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Figure 2. Su et al.

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ACCEPTED MANUSCRIPT Figure 3. Su et al.

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b)

c)

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Figure 4. Su et al.

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Figure 5. Su et al.

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Figure 6. Su et al.

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Graphic Abstract

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ACCEPTED MANUSCRIPT Highlights ORR catalyzed by iron(II) phthalocyanine was studied at the liquid/liquid interface.



The catalytic O2 reduction mainly produces H2O with only minority of H2O2 (less

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The reaction possibly involves a hydroperoxo intermediate.

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than 5%).

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