Chemisorbed Oxygen-Species-Mediated Electrocatalytic Oxidation of

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Chemisorbed Oxygen-Species-Mediated Electrocatalytic Oxidation of Thiourea and Thiosulfate zhang wei, Changwei Pan, and Qingyu Gao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07540 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 9, 2018

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The Journal of Physical Chemistry

Chemisorbed Oxygen-Species-Mediated Electrocatalytic Oxidation of Thiourea and Thiosulfate Wei Zhang,1,2 Changwei Pan,*,1 Qingyu Gao*,1 1

College of Chemical Engineering, China University of Mining and Technology, Xuzhou 221116,

People’s Republic of China 2

College of Chemical Engineering and Material Science, Zaozhuang University, Zaozhuang

277160, People’s Republic of China Email: [email protected], [email protected]

ABSTRACT

The electrocatalytic oxidation of two sulfur (II) species (thiourea and thiosulfate) on a polycrystalline platinum electrode was studied by combining cyclic voltammetry (CV) with highperformance liquid chromatography (HPLC) and capillary electrophoresis (CE). The surface sulfur species distribution and the charge transfer peaks of the cyclic voltammetry curves depend on the formation of chemisorbed oxygen (such as OHads and Oads), indicating that those electrocatalytic reactions are mediated via the chemisorbed oxygen species. The formation of chemisorbed oxygen species play a dual role during the electrode processes as the poison for metal degradation and the bridge for the electrocatalytic reaction.

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1 INTRODUCTION During the past few decades, oxygen electrochemistry, such as the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), has been regarded as the critical challenge of proton exchange membrane fuel cells (PEMFCs)1 due to their high overpotential and kinetically sluggish process, particularly in acidic media at low temperature.2 The formation and dissolution of various surface metal-oxide films have been shown to reduce the ORR and OER activity and stability (dissolution or degradation,3,4 Ostwald ripening5 and agglomeration6) of the electrodes. Therefore, the design of efficient catalysts for OER and ORR becomes one of the key challenges in electrochemical energy conversion and storage.7 Pt is one of the most important components of the ORR/OER catalysts, and the oxygenated species or surface oxide that forms on the surface of Pt has attracted great attention and has been intensively studied during the past few decades.9-14 Cyclic voltammetry (CV) studies on Pt(111) have revealed that three characteristic potential regions, namely, the hydrogen underpotential deposition (Hups) region (0.05 V - 0.40 V vs RHE), the double-layer region (0.40 V - 0.60 V vs RHE) and the surface oxide region (> 0.7 V vs RHE), are distinguished during the scanning process of Pt electrodes.11 In the surface oxide region, two sharp peaks at 0.8 V vs RHE and 1.05 V vs RHE and a broad peak at 1.35 V vs RHE are presented on the Pt(111) electrode in HClO4.15 The first peak at 0.8 V vs RHE located in the “butterfly” region is related to the adsorption of OH from the electrochemical dissociation of water.16 The peak at approximately 1.05 V vs RHE has been assigned to formation of chemisorbed oxygen through a phase transition via a nucleation and growth mechanism.17 A series of in situ surface sensitive technologies (or strategies) and theoretical studies have been used to establish a detailed and systematic understanding of the oxidation of Pt at the atomic scale. Time-resolved hard X-ray diffraction and energy dispersive X-ray absorption spectroscopy studies on the oxidation behaviors of the surfaces of platinum nanoparticles at 1.4 V vs. RHE in 2


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0.5 M H2SO4 have revealed that platinum oxides are formed from PtOH, PtO, 2D α-PtO2 to quasi-3D β-PtO2.18X-ray crystal truncation rods19-21 and in situ surface X-ray diffraction (SXRD)23-25 can also be used for investigating the surface buckling and place exchange (PE) of Pt(111) surface oxide formation in the surface oxide region (> 0.6 V vs RHE) of platinum oxidation process in a HClO4 solution. In situ shell-isolated nanoparticle-enhanced Raman spectroscopic (SHINERS) measurements, a 2D (su)peroxide surface layer was identified around a relatively sharp voltammetric peak (1.05 V vs RHE) of the electrochemical oxidation of Pt(111),26 which was previously assigned as chemisorbed oxygen.16-17 Although a variety of surface analytical methods have been applied to study the oxygenated species evolution process during the electrooxidation of Pt, an unambiguous description of Pt-oxide distribution on the surface oxide region (> 0.7 V vs RHE) has not been provided due to the complex processes of Pt oxidation with different oxygen states and the competition among them.15 Ab initio theory (DFT)27calculations can describe the formation and structural changes in reactive oxygenated adsorbates such as H2Oads, OHads and Oads on a Pt(111) electrode surface.28The influence of the Pt adsorption state (surface water interactions, hydrogen coverage and monolayer of oxygen) on the formation/deactivation of reactive oxygen species can also be provided through DFT calculations, in which the three adsorption regimes of the Pt adsorption states are discernible in the cyclic voltammograms that span the appropriate potential range of Pt.11 Chemisorbed oxygen species produced on a Pt electrode could play a dual role during the electrode processes. On one hand, the Pt-oxide species can accelerate its dissolution,29-32 which reduces the long-term stability of the Pt electrode.33 On the other hand, the formation of active oxygen species on Pt are likely the source of electrocatalysis due to the high activities of chemisorbed Pt-OHads, Pt-Oads, etc. at the cyclic voltammetry peaks. It is further inferred that the electrocatalysis oxidation paths through chemisorbed oxygen-species (originated from the interaction between the solvent (i.e., water) and the metal on the surface) mediated indirect catalysis rather 3



than the direct adsorption and conversion of the reactants on the surface of the electrode.34-36 In this work, insights into the corresponding relationship between electrocatalysis and the surface Pt-adsorbed oxygenated species were studied during the electrooxidation of sulfur compounds. By combining our work on cyclic voltammetry with HPLC (or CE) and previous achievement on oxygen electrochemistry on low index planes of Pt. Our study indicated that electrocatalysis oxidation of thiosulfate and thiourea and on a platinum electrode is inferred to be water-mediated conversion, i. e., reactions between chemisorbed oxygen-containing intermediates and the reactant substrates. 2 EXPERIMENTAL SECTION 2.1 Reagents and Solutions Commercially available reagents such as thiosulfate, tetrathiosulfate, thiourea, formamidine disulfide (TU2) and aminoiminomethanesulfinic acid (TUO2, NH2NHCSO2H) were of the highest purity and were directly used without further purification. Potassium trithionate was prepared as described previously with slight modifications,37 and its purity was better than 98.5 %. Potassium pentathionate was prepared by following the procedure reported by Kelly and Wood,38 and its purity was checked by high-performance liquid chromatography (HPLC) or capillary electrophoresis (CE) and by titration with HgCl2 and was found to be 98.37%. Aminoiminomethanesulfonic acid (TUO3, NH2NHCSO3H) was prepared according to the literature.39, 40All solutions were prepared with Milli-Q distilled water with a specific conductivity of 18.2 Ω. Buffer solutions of KH2PO4-K2HPO4 (PBS) were used to maintain the pH (7.0) of the electrochemical system. 2.2 Instruments and Methods All experiments were performed in a conventional three-electrode electrochemical cell with a volume of 80 mL. A polycrystalline platinum disk with a diameter of 5.0 mm was used as the working electrode, and a large platinum sheet (15 mm × 15 mm) was used as a counter electrode. 4


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A saturated calomel electrode (SCE) was linked to the cell through a salt bridge applied as the reference electrode (RE). The potentials in all experiments were referred to the SCE scale. Before the experiments, the cell was deoxygenated with argon. All electrochemical experiments were performed at 25.0 oC by connecting with a computer-controlled electrochemical workstation (Bio-logic. VSP-300) at scan rates of 1 mV/s, 5 mV/s, 10 mV/s and 20 mV/s. A micron-sized sampling tip (diameter, 0.3 mm) was placed close (10 µm) to the surface of the working electrode (diameter, 5 mm) for rapid sampling41 (with a peristaltic pump, ISMATEC 78001-02) during CV scanning. The sampling rate of the CV instrument was 60 µL/min. Afterthe collection, all samples were analyzed by HPLC (Agilent 1260) with a Phenomenex Ginimi C18 column (250 × 4.6 mm, 5 µm) and a DAD detector (G4212B). The samples were acquired by taking 5 µL of the solution and injecting it into the HPLC equipment. The mobile phase was prepared by mixing acetonitrile and water according to an optimized ratio of 15:85 (Vacetonitrile/Vwater), and 5.0 mM TPAOH was added into the solution for the study of the thiosulfate system.42The chromatograms were monitored at 195 nm, 214 nm, 230 nm and 254 nm. The mobile phase was prepared by mixing methanol, acetonitrile and a 0.01 M HCl solution in a volumetric ratio of 6:26:68 (Vmethanol /Vacetonitrile/VHCl) for the study of the thiourea system.43-44 The chromatogram wavelengths were same as those of the thiosulfate system. The capillary electrophoresis (CE) analysis was performed on a P/ACE MDQ instrument (Beckman) equipped with a diode array detector (DAD).43, 45-46 A fused-silica capillary of 57.0 cm (50.0 cm to the detector) × 75 µm was used. The sample was injected into the capillary by overpressure. A negative voltage of 12.5 kV was applied for separation, and the spectrophotometric detection was determined at wavelength of 210.0 nm. 3. RESULTS AND DISCUSSION 3.1 Electrochemical Oxidation of Thiosulfate 5



3.2.1 The voltammetric analysis for the electrooxidation of thiosulfate The cyclic voltammograms of the Pt electrode in a 0.2 M phosphate buffer solution (PBS) with (solid line) and without 0.1 M thiosulfate (dashed line) at pH 7.0 is shown in Figure 1. In absence of thiosulfate, the current density experiencing a rapid increase step began with 0.85 V and an anodic current peak were observed at 1.43 V (see the enlarged curves in the rectangular parts of the CV curves in Figure 1). Since phosphate anions (HPO42-, H2PO4-) can only bring out their adsorption rather than charge-transfer reactions on Pt electrode surface, the anodic current step and peak on polycrystalline platinum electrode should not result from phosphate buffer. The current peak of Pt oxide can be obtained in PBS through comparing with valtammetric curves on Pt(111) electrode,15-17even adsorption of phosphate anions could change the shape and current value of valtammetric peak.47After the addition of 100 mM thiosulfate into the PBS, two broad anodic peaks at 0.81 V and 1.39 V in the positive scanning mode and an anodic peak at 0.63V in the negative scanning mode were also observed. So formation of active oxygen-species in absence of thiosulfate can catalyze the oxidation of thiosulfate, resulting two electrochemical reaction peaks at same applied potentials.

Figure 1.Cyclic voltammogram of the 0.2 M phosphate buffer (pH = 7.0) with (solid line) and without (dashed line) 0.1 M thiosulfate at a scan rate of 10 mV/s on the Pt electrode at T = 25.0 ± 0.1 °C. Inserts show the magnification of the rectangular parts of the CV curves in the phosphate buffer without thiosulfate at pH 7.0.

The effects of online sampling collection on the CV curves during the electrooxidation of thio6


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sulfate are present in Figure 2. As shown in Figure 2a and 2b respectively, the current density of the CVs from 0.7 V to 1.6 V is higher for the sample collection, and the high flow rate of sample collection is in favor of the increasing current density. When the flow rate is increased, more amount of low-active or inert intermediates such as S4O62-, S5O62- or other sulfur-containing species, which deposit on the electrode surface,48 were removed, resulting in exposing more active sites on the electrode surface and the enhanced transportation of thiosulfate for its oxidation.

Figure 2. Effect of sampling on the CV curves of thiosulfate oxidation. (a): CV curves with (solid line) and without (red line) sample collection: [S2O32-] = 0.1 M, the sampling rate = 60 µL/min, PBS = 0.2 M, the CV scan rate = 1 mV/s, and T = 25.0 ± 0.1 °C. (b): The effect of The sampling rates on the cyclic voltammetry of thiosulfate oxidation. The inserts show the magnification of the rectangular part. CV conditions: [S2O32-] = 0.1 M, PBS = 0.2 M, scan rate = 10 mV/s, pH = 7.0, and T = 25.0 ± 0.1 °C.

3.1.2 Species analysis on the surface of polycrystalline Pt Two products, tetrathionate and pentathionate, of the oxidation of thiosulfate on platinum 7



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are observed, as shown in Figure 3. These products are first detected at approximately 0.38 V and are consistent with the positive scanning CVs (Figure 3a). The first current peak at approximately 0.80 V in Figure 3a contains not only the main product tetrathionate but also contains another product pentathionate. This result means that tetrathionate resulting from initial electrochemical oxidation of thiosulfate reacts with thiosulfate in solution to produce pentathionate (R1). As the electrode surface has become passivated in the valley of potential sweep (red line) at potential around 1.2 V, then sustained electro-migration of the starting materials, i.e. thiosulfate, caused the increase of thiosulfate concentration, and a peak of the concentration was observed in Figure 3a. S2O32- + S4O62- → S5O62- + SO32-

(R1)

Figure 3. Sulfur species distributions of the thiosulfate electrooxidation reaction by HPLC corresponding to the related positive (a) and negative (b) sweep (red lines) of the Pt electrode.The insert plot is amplification part of rectangular region. Thiosulfate (), tetrathionate (◆) and pentathionate (○) are the detected species. [S2O32-] = 100.0 mM, PBS = 0.2 M, pH = 7.0, the scan rate was 1 mV/s, the sampling rate was 60 µL/min, and T = 25.0 ± 8


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0.1 °C.

For the negative CV scanning of thiosulfate solution (Figure 3b), a broad charge transfer peak is located at around 0.65 V(see the insert plot of Figure 3b), and both of the product concentrations have a low peak value at the same potential. The concentration of thiosulfate as a function of applied potential evolves in the opposite direction to the CV current and the product concentrations (such as tetrathionate and pentathionate) in both potential ranges of approximately 0.8 and 1.4 V. The compositions of the oxidized products during the potential sweeps were further determinedby capillary electrophoresis (CE), as shown in Figure 4. Both sulfate and trithionate could be detected after 0.3 V.

Figure 4. Species determination from the indirect method in the electrooxidation of thiosulfate as a function of applied potential on the Pt electrode. Separation conditions: 20 mM KNO3, 0.01 mM HDB, 10 mM phosphate buffer (pH = 7.0), scan rate = 1 mV/s, and the sample collection rate was 60 µL/min. The sample was diluted 20 times before injecting into the CE equipment.

The compositions of the oxidized products that emerged during the potentiostatic study of 0.1 M thiosulfate in 0.2 M PBS (pH = 7.0) solution at potentials of 0.80 V and 1.40 V were determined by CE, as shown inFigure5. In Figure 5a, the species detected during the process contains S2O32-, S3O62-, S4O62-, S5O62- and SO42- at a potential of 0.8 V; these species are same as the species detection at a potential of 1.4 V in Figure 5b. The solution became turbid because of the 9



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sulfur from the decomposition of pentathionate through reaction (R2).49 S3O62-was produced from the sulfitolysis of tetrathionate through reaction (R3). Sulfate can be produced through the electrochemical oxidation of sulfur-oxygen species and elemental S. S5O62- → S4O62- + S

(R2)

S4O62- + SO32-→ S3O62- + S2O32-

(R3)

Figure 5. Electropherogram of the sulfur species detected by the indirect CE method during the electrooxidation of thiosulfate on Pt. Applied potentials: 0.80 V (a) and 1.40 (b). Separation conditions: 20 mM KNO3, 0.01 mM HDB, 10 mM phosphate buffer (pH = 7.0), and electrolysis time = 20.0 h. The sample was diluted 20 times before analysis.

3.2 Electrooxidation of thiourea on Pt 3.2.1 The voltammetric analysis for the electrooxidation of thiourea 10


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The cyclic voltammograms of the Pt electrode in 0.1 M HClO4 with (solid line) and without (dashed line) 60 mM thiourea (TU) is shown in Figure 6. The CVs (dashed line) of the Pt electrode in 0.1 M HClO4exhibits an obvious peak at 1.43 V (the upper insert) and a distinct current increase starting from 0.80 V (the lower insert) in the positive potential sweep. Two anodic peaks at 0.85 V and 1.50 V were observed after the addition of 60 mM thiourea into the HClO4 solution, illustrated by the solid line. In the negative potential sweep, an anodic current peak appeared at 0.89 V. There is a reduction peak at approximately 0.45V in absence of thiourea, attributed to proton reduction. The pre-oxidized Pt electrodes were used to inject the thiourea to get the two valtammetric peaks at applied potentials of 0.75 V and 1.28 V, respectively (Figure S1 in Supporting Information), further indicating the peak currents caused from Pt oxide-catalyzed reactions.

Figure 6.Cyclic voltammogram of 0.1 M HClO4 with (solid line) and without (dashed line) 60 mM thiourea at a potential scan rate of 20 mV/s on the Pt electrode at T = 25.0 ± 0.1 °C. Inserts show the local magnification of the rectangular parts of the CV curves for 0.1 M HClO4.

11



Figure 7. Voltammograms of 60 mM thiourea at different scan rates on the Pt electrode. a) The positive scanning of applied potential. b) The negative scanning of applied potential. [HClO4] = 100.0 mM and T = 25.0 ± 0.1 °C.

The effect of the scanning rate of the potential sweep on the cyclic voltammograms of 60 mM thiourea in a 0.1 M HClO4 solution is shown in Figure 7. When the scan rate of potential is decreased, the anodic peak currents dropped and oscillation-like curves appeared, however, all curves almost overlapped during the cathodic sweep. This can be explained as follows: Pt-oxide species (Pt-OHads, Pt-Oads, etc.) act as the catalysts for electro-oxidation reaction, their formation and reduction can be either fast and slow, depending on the reaction conditions such as applied potential, substrate concentration, temperature and potential scan-rate. For example, due to unenough transportation of the reactant at the low scan rate, Pt-oxide species gradually form passive adlayer on the electrode/electrolyte interface, resulting in not only decrease of the anodic peak current, but also enhanced positive feedback of electrical double layer potential during 12


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adlayer formation and weakened negative feedback, i. e. reduction reaction of Pt-oxide, so electrochemical oscillations can occur.50 During the cathodic sweep, the dissolution of adlayer formation is beneficial when the applied potential is decreased continuously, so the cathodic scanrate of applied potential does not largely alter the voltammetric curves. The effect of sampling on the CV curves of the electrooxidation of TU in the 0.1 M HClO4 solution on Pt was presented in Figure 8. The black and red curves denote the sampling and nonsampling, respectively. The currents of the oxidation peaks at 0.80 V and 1.40 V in the positive potential sweep and 0.80 V in the negative potential sweep increase during sample collection. The diffusion of reactants and products on the surface of the electrode was accelerated by sampling, thus the mass transfer of reactants in the bulk solution to the surface of the electrode was enhanced. Therefore, the amount of the reactant substrate involved in the reaction increased on the surface of the electrode. Another reason is that sampling from the electrode surface will remove the low-active or poisonous species such as elementary sulfur near the electrode surface, which is the same reason as the situation for the oxidation of thiosulfate above.

Figure 8. Effect of sampling on the current during cyclic voltammetry on the Pt electrode with (solid line) and without (dashed line) the sampling rate of 60µL/min in 60 mM thioureasolution. The scan rate was 1 mV/s, [HClO4] =0.1 M and T = 25.0 ± 0.1 oC.

3.2.2 Species analysis on the surface of polycrystalline Pt The relationship between the sulfur species distributions detected by HPLC and the applied 13



potential is shown in Figure 9. The TU reactant and the products such as TU22+, NH2NHCSO2Hand NH2NHCSO3Hthat were detected by HPLC are plotted as a function of different applied potentials, while the potential-current curves are shown as a red color. In the positive potential sweep in Figure 9a, we observed two anodic peaks around 0.80 V and 1.40 V that correspond to the concentration peak of TU22+ and the concentration peak of TUO2, respectively. The first current peak results from the oxidation of TU to TU22+.The second current peak produces mainly NH2NHCSO2H. Then, NH2NHCSO3H (in Figure 9a) is further obtained with an increasing applied potential. The concentration maxima of NH2NHCSO2H and NH2NHCSO3H correspond to the consumptions (i.e., concentration minima) of TU22+ and NH2NHCSO2H, respectively. In the negative potential sweep in Figure 9b, the same corresponding relation between species distribution and applied potential was observed when compared with that of the positive potential sweep.

Figure 9. Sulfur species distributions vs. applied potential in the electrooxidation reaction of thiourea by HPLC. The red curves denote positive (a) and negative (b) potential sweeps. Sample collections of TU (■), TU22+ (●),

14


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NH2NHCSO2H(◆) and NH2NHCSO3H() are shown in the different curves (a) and (b). Conditions: [TU] = 60.0

mM, [HClO4] = 100.0 mM, scan rate was1 mV/s, sample collection rate was 60 µL/min, and T = 25.0 ± 0.1 °C.

The reaction solutions became turbid during the process of the potential sweeps, which means that elemental sulfur was produced during the reaction. The adsorbed TU22+ on the electrode continued to hydrolyze producing ammonia nitrile, elemental sulfur and TU through reaction (R4)5153

. [(NH2)2CSSC(NH2)2]2+ → (NH2)2CS + NH2CN + S + 2H+

(R4)

3.3 The analysis of electrochemical mechanism According to the analysis of charge transfer on Pt(111),10,16-17, 54-57 the so-called butterfly region exhibits a pair of two reversible peaks (a broad increase of the current that is followed a sharper peak at approximately 0.8 V vs. RHE) associated with OH adsorption/desorption processes (R5),54-55 and the sharp peak above 1.05 V vs. RHE in the positive scanning direction is attributed to a conversion from OHads to Oads (R6).16-17R5 and R6 were proposed by analysis of charge transfer during oxide formation on polycrystalline Pt.10,57The products (OHads and Oads) of R5 and R6 were identified and quantified by XPS,56 in situ surface-enhanced Raman spectroscopy.26,58 Also their individual evolutions of atomic-level structure on electrode interfaces were reported.14 The analysis of combined CV-HPLC on the electrooxidation products of thiourea and thiosulfate on the polycrystalline platinum surface indicates that two Pt-oxide peaks in blank solutions, two electrocatalytic current peaks and corresponding concentration extremum of products appeared in the same applied potentials (approximately 0.80 V and 1.35 V vs. SCE) for the two systems. Here a polycrystalline Pt rather than a low index Pt crystal was used, so there are observed potential difference for formation of Pt oxides. In summary, the results imply that the two active Pt-oxides (Pt-OHads, and Pt-Oads) formed in blank solutions act as catalysts for the electro15



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chemical reactions. Here, Pt-peroxo or -superoxo species, which were detected by in situ Raman spectroscopy26, could be ignored since Pt-OHads, and Pt-Oads can be converted into Pt-peroxo or -superoxo species without electron transfer (R7 and R8) according to scheme 1 of reference 7. H2O →OHads + H+ +e-

(R5)

OHads→ Oads+ H+ + e-

(R6)

OHads+ OHads⇌ OOH-ads + H+

(R7)

Oads+ OHads⇌OOHads

(R8)

The applied potential spanning from 0.40 -1.10 V is associated with the oxidation of thiosulfate in a typical oxygen-transfer reaction, in which the anodic discharge of water, as shown in reaction (R5), plays an important role because here thiosulfate is oxidized indirectly via the surfaceadsorbed hydroxide intermediates (OHads), as shown in reactions (R9) and (R10). The further oxidation of HOS2O3- by OHads can produce SO42- or S2O62-, as proposed in reference 46. Elemental sulfur from decomposition of pentathionate (R2) can also be oxidized to SO42- (R11). The anodic peak at 0.6 V in the negative scanning direction reproduces OHads (R5), whereas the oxidation of thiosulfate through a reaction with OHads produces tetrathionate, etc. 2OHads+ S2O32- → HOS2O3- + OH-

(R9)

S2O32- + HOS2O3-→ S4O62- + OH-

(R10)

6OHads+ S = SO42- + 2H2O + 4H+

(R11)

When increasing the applied potential over 1.10 V vs. SCE, chemisorbed oxygen (Oads in R6) acts as the catalyst for the electrochemical oxidation of thiosulfate in reactions such as R12-14. Oads+ S2O32- → OS2O32-

(R12)

S2O32- + OS2O32- + H2O → S4O62- + 2OH-

(R13)

3Oads + S + H2O = SO42- + 2H+

(R14)

16


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For thiourea oxidation, only TU22+can be directly produced (Figure 9) in an applied potential range from 0.40 V to 1.10 V vs. SCE, in which chemisorbed OHads acts as the bridge to catalyze the oxidation of thiourea (R15-16). In an applied potential range of more than 1.10 V vs. SCE, TU certainly can be oxidized by Oads(R17). However, NH2NHCSO2Hand NH2NHCSO3H come mainly from TU22+ and NH2NHCSO2H (R18-19), respectively, according to Figure 9. Also inevitably, SO42-would be produced by electrochemical oxidation of elemental sulfur (R4, R11 and R14) and NH2NHCSO3H(R20). The reaction scheme is shown in Figure 10. 2OHads + (NH2)2CS → (NH2)(NH)CSOH +H2O

(R15)

(NH2)(NH)CSOH +(NH2)2CS + 2H+⇌ [(NH2)2CSSC(NH2)2]2+ + H2O (R16) Oads+(NH2)2CS → (NH2)(NH)CSOH

(R17)

Oads+ (NH2)(NH)CSOH → (NH2)(NH)CSO2H

(R18)

Oads+ (NH2)(NH)CSO2H →(NH2)(NH)CSO3H

(R19)

Oads + (NH2)(NH)CSO3H + H2O → (NH2)2CO + H2SO4

(R20)

Figure 10. Mechanism scheme for the bridge role of the adsorbed OH or O on the platinum electrode surface during the electrooxidation of thiosulfate (a) and thiourea (b). 17



4 Conclusion From the combined electrochemistry-HPLC analysis, the oxidation product distribution of sulfur species depends on the electrode potential and stability of the reaction substrates. Even initial oxidation products such as S4O62- and TU22+ appear in all the voltammetry peaks; however, the high applied potential prefers higher oxidation state of sulfur. Furthermore, the voltammetry peaks and products of electrocatalytic oxidation coincide with those of the chemisorbed oxygen species (OHads and Oads), indicating that the reaction path for electrocatalytic oxidation of the sulfur species involves chemisorbed oxygen-mediated steps, which is similar to the oxidation by O2 on a metal.34-35 Here, our study on chemisorbed oxygen-species-mediated electrocatalytic oxidation is only carried out on a polycrystalline platinum electrode. For correspondence and consistence between experiments and mechanistic calculation, it is necessary to use single-crystal electrodes for the future experimental investigation, and the reaction scheme, that is composed of active oxygen formation and subsequent substrate oxidation, could obtain the product distributions and kinetic parameters (e.g., active energy, rate constants and rate-determining step) on the index planes of single-crystal Pt (or other metals) through density functional theory calculations and (or) Monte Carlo simulations. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 21773304), the Fundamental Research Funds for the Central Universities (Grant No. 2015XKZD09), the Natural Science Foundation of Jiangsu Province (Grant No. BK20160240), and the Research and Innovation Project for College Graduates of Jiangsu Province (No. KYLX16_0549) Supporting Information Supporting Information Available: Voltammogram of the 0.1 M HClO4 with the injection of 18


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thiourea at two different applied potentials on the polycrystalline Pt electrode. This material is available free of charge via the Internet at http://pubs.acs.org

References (1) Debe, M. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486, 43-51. (2) Lefèvre, M.; Proietti, E.; Jaouen, F.; Dodelet, J. Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells. Science 2009, 324, 71–74. (3) Topalov, A. A.; Katsounaros, I.; Auinger, M.;Cherevko, S.; Meier, J. C.; Klemm, K. O. Mayrhofer, K. J. J. Dissolution of Platinum: Limits for the Deployment of Electrochemical Energy Conversion? Angew. Chem. 2012, 124, 12782–12785; Angew. Chem. Int. Ed. 2012, 51, 12613–12615. (4) Inzelt, G.; Berkes, B.; Kriston, A. Temperature Dependence of Two Types of Dissolution of Platinum in Acid Media. An Electrochemical Nanogravimetric Study. Electrochem. Acta 2010, 55, 4742–4749. (5) Bett, J. A. S.; Kinoshita, K.; Stonehart, P. Crystallite Growth of Platinum Dispersed on Graphitized Carbon Black: II. Effect of Liquid Environment. J. Catal. 1976, 41, 124–133. (6) Meier, J. C.; Galeano, C.; Katsounaros, I.; Topalov, A. A.; Kostka, A.; Schuth, F.; Mayrhofer, K. J. J. Degradation Mechanisms of Pt/C Fuel Cell Catalysts under Simulated Start–Stop Conditions. ACS Catal. 2012, 2, 832–843. (7) Katsounaros, I.; Cherevko, S.; Zeradjanin, A. R.; Mayrhofer, K. J. Oxygen Electrochemistry as A Cornerstone for Sustainable Energy Conversion. Angew. Chem. Int. Ed. 2014, 53, 102–121 (8) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. Origin of the Overpotential for Oxygen Reduction at A Fuel-Cell Cathode. J. Phys. Chem. B, 2004, 108, 17886–17892. (9) Somorjai, G. A.; Li, Y. Introduction to Surface Chemistry and Catalysis, 2nd Ed.; John Wiley 19



& Sons: Hoboken, NJ, 2010. (10) Conway, B. E. Electrochemical Oxide Film Formation at Noble Metals as A SurfaceChemical Process. Prog. Surf. Sci. 1995, 49, 331−452. (11) Eslamibidgoli, M. J.; Eikerling, M. H. Electrochemical Formation of Reactive Oxygen Species at Pt (111)-A Density Functional Theory Study. ACS Catal. 2015, 5, 6090–6098. (12) Markovic, N. M.; Ross, P. N. Electrocatalysis at Well-Defined Surfaces. In Interfacial Electrochemistry: Theory, Experiment and Applications; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999. (13) Komanicky, V.; Iddir, H.; Chang, K.; Menzel, A.; Karapetrov, G.; Hennessy, D.; Zapol, P.; You, H. Shape-Dependent Activity of Platinum Array Catalyst. J. Am. Chem. Soc. 2009, 131, 5732–5735. (14) Drnec, J.; Harrington, D. A.; Magnussen, O. M. Electrooxidation of Pt(111) in Acid Solution. Curr. Opin. Electrochem.2017, 4, 69–75. (15) Gomez-Marin, A. M.; Feliu, J. M. Pt(111) Surface Disorder Kinetics in Perchloric Acid Solutions and the influence of Specific Anion Adsorption. Electrochim. Acta 2012, 82, 558–569. (16) Gómez-Marín, A. M.; Clavilier, J.; Feliu, J. M. Sequential Pt(111) Oxide Formation in Perchloric Acid: An Electrochemical Study of Surface Species Inter-Conversion. J. Electroanal. Chem. 2013, 688, 360–370. (17) Gómez-Marín, A. M.; Clavilier, J.; Feliu, J. M. Oxide Growth Dynamics at Pt(111) in absence of Specific Adsorption: A Mechanistic Study. Electrochim. Acta 2013, 104, 367–377. (18) Imai, H.; Izumi, K.; Matsumoto, M.; Kubo, Y.; Kato, K.; Imai, Y. In Situ and Real-Time Monitoring of Oxide Growth in A Few Monolayers at Surfaces of Platinum Nanoparticles in Aqueous Media. J. Am. Chem. Soc. 2009, 131, 6293–6300. (19) You, H.; Zurawski, D. J.; Nagy, Z.; Yonco, R. M. In-situ X-ray reflectivity Study of Incipient Oxidation of Pt(111) Surface in Electrolyte Solutions, J. Chem. Phys. 1994, 100, 4699–4702. (20) You, H. Nagy, Z. Oxidation-Reduction-Induced Roughening of Platinum (111) Surface, Phys. 20


Page 20 of 36

Page 21 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


B Condens. Matter 1994, 198, 187–194. (21) Nagy, Z.; You, H. Applications of Surface X-ray Scattering to Electrochemistry Problems, Electrochim. Acta 2002, 47, 3037–3055. (22) Liu, Y.; Barbour, A.; Komanicky, V.; You, H. X-ray Crystal Truncation Rod Studies of Surface Oxidation and Reduction on Pt(111), J. Phys. Chem. C 2016,120, 16174–16178. (23) Tidswell, I.M.; Markovic, N.M.;Ross, P. N., Jr Potential Dependent Surface Structure of the Pt(111) Electrolyte Interface, J. Electroanal. Chem. 1994, 376, 119–126. (24) Drnec, J.; Ruge, M.; Reikowski, F.; Rahn, B.; Carlà, F.; Felici, R.; Stettner, J.; Magnussen, O. M.; Harrington, D. N. Initial Stages of Pt(111) Electrooxidation: Dynamic and Structural Studies by Surface X-ray Diffraction, Electrochim. Acta 2017, 224, 220–227. (25) Drnec, J.; Ruge, M.; Reikowski, F.; Rahn, B.; Carlà, F.; Felici, R.; Stettner, J.; Magnussen, O. M.; Harrington, D. A. Pt Oxide and Oxygen Reduction at Pt(111) Studied by Surface X-ray Diffraction. Electrochim. Commun. 2017, 84, 50–52. (26) Huang, Y.; Kooyman, P. J.; Koper, M. T. M. Intermediate Stages of Electrochemical Oxidation of Single-Crystalline Platinum Revealed by in Situ Raman Spectroscopy. Nature Commun.2016, 7, 1–7. (27) Hansen, H. A.; Rossmeisl, J.; Nørskov, J. K. Surface Pourbaix Diagrams and Oxygen Reduction Activity of Pt, Ag and Ni(111) Surfaces Studied by DFT. Phys. Chem. Chem. Phys. 2008, 10, 3722–3730. (28) Chen, J.; Luo, S.; Liu, Y.; Chen, S. Theoretical Analysis of Electrochemical Formation and Phase Transition of Oxygenated Adsorbates on Pt(111). ACS Appl. Mater. Inter. 2016, 8, 20448– 20458. (29) Ahluwalia, R. K.; Papadias, D. D.; Kariuki, N. N.; Peng, J.; Wang, X. Tsai, Y.; Graczyk, D. G.; Myers, D. J. Potential Dependence of Pt and Co Dissolution from Platinum-Cobalt Alloy PEFC Catalysts Using Time-Resolved Measurements. J. Electrochem. Soc. 2018, 165, F3024– 3035. 21



(30) Cherevko, S.; Keeley, G. P.; Geiger, S.; Zeradjanin, A. R.; Hodnik, N.; Kulyk, N.; Mayrhofer, K. J. J. Dissolution of Platinum in The Operational Range of Fuel Cell. ChemElectroChem 2015, 2, 1471–1478. (31) Wakisaka, M.; Asizawa, S.; Uchida, H.; Watanabe, M. in Situ STM Observation of Morphological Changes of The Pt(111) Electrode Surface During Potential Cycling in 10 mM HF Solution. Phys. Chem. Chem. Phys. 2010, 12, 4184–4190. (32) Darling, R. M.; Meyer, J. Kinetics Model of Platinum Dissolution in PEMFCs. J. Electrochem. Soc. 2003, 150, A1523–A1527. (33) Darling, R. M.; Meyer, J. Methematical Model of Platinum Movement in PEM Fuel Cells. J. Electrochem. Soc. 2005, 152, A242–A247. (34) Zope, B. N.; Hibbitts, D. D.; Neurock, M.; Davis, R. J. Reactivity of the Gold/Water Interface During Selective Oxidation Catalysis. Science 2010, 330, 70–74. (35) Rodriguez, P.; Kwon, Y.; Koper, M. T. M. The Promoting Effect of Adsorbed Carbon Monoxide on the Oxidation of Alcohols on a Gold Catalyst. Nature Chem. 2011, 4, 177–182. (36) Kwon, Y.; Schouten, K. J. P.; Koper, M. T. M. Mechanism of the Catalytic Oxidation of Glycerol on Polycrystalline Gold and Platinum Electrodes. ChemCatChem. 2011, 3, 1679–1697. (37) Csekő, G.; Horvat́h, A. K. Non-Triiodide Autoinhibition by Iodide Ion in the Trithionate−Iodine Reaction. J. Phys. Chem. A 2010, 114, 6521−6526. (38) Kelly, D. P.; Wood, A. P. Synthesis and Determination of Thiosulfate and Polythionates. Methods Enzymol. 1994, 243, 34−61. (39) Kim, K.; Lin, Y.; Mosher, H. S. Monosubstituted Guanidines from Primary Amines and Aminoiminomethanesulfonic Acid. Tetrahedron Lett. 1988, 29, 3183–3186. (40) Gao, Q.; Wang, G.; Sun, Y.; Epstein, I. R. Simultaneous Tracking of Sulfur Species in the Oxidation of Thiourea by Hydrogen Peroxide. J. Phys. Chem. A 2008, 112, 5771–5773. (41) Kwon Y.; Koper, M. T. M. Combining Voltammetry with HPLC: Application to ElectroOxidation of Glycerol. Anal. Chem. 2010, 82, 5420–5424. 22


Page 22 of 36

Page 23 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(42) Feng, J.; Gao, Q.; Lv, X.; Epstein, I. R. Dynamic Complexity in the Electrochemical Oxidation of Thiourea. J. Phys. Chem. A2008, 112, 6578–6585. (43) Lu, Y.; Gao, Q.; Xu, L.; Zhao, Y.; Epstein, I. R. Oxygen-Sulfur Species Distribution and Kinetic Analysis in the Hydrogen Peroxide-Thiosulfate System. Inorg. Chem. 2010, 49, 6026–6034. (44) Xu, L.; Horváth, A. K.; Hu, Y.; Ji, C.; Zhao, Y.; Gao, Q. High Performance Liquid Chromatography Study on the Kinetics and Mechanism of Chlorite-Thiosulfate Reaction in Slightly Alkaline Medium. J. Phys. Chem. A 2011, 115, 1853–1860. (45) Bi, W.; He, Y.; Cabral, M. F.; Varela, H.; Yang, J.; Jiang, R.; Gao, Q. Oscillatory ElectroOxidation of Thiosulfate on Gold. Electrochim. Acta 2014, 133, 308–315. (46) Du, Z.; Gao, Q.; Feng, J.; Lu, Y.; Wang, J. Dynamic Instabilities and Mechanism of the Electrochemical Oxidation of Thiosulfate. J. Phys. Chem. B 2006, 110, 26098-26104. (47) Gisbert, R.; Garcia, G.; Koper, M. T. M. Adsorption of Phosphate Species on Poly-Oriented Pt and Pt(111) Electrodes over a Wide Range of pH. Electrochim. Acta 2010, 55, 7961–7968. (48) Santaslo-Aarnio, A.; Kwon, Y.; Ahlberg, E.; Kontturi, K.; Kallio, T.; Koper, M. T. M. Comparison of Methanol, Ethanol and Iso-Propanol Oxidation on Pt and Pd Electrodes in Alkaline Media Studied by HPLC. Electrochim. Commun. 2011, 13, 466–469. (49) Pan, C.; Wang, W.; Horvath, A. K.; Xie, J.; Lu, Y; Wang, Z.; Ji, C.; Gao, Q. Kinetics and Mechanism of Alkaline Decomposition of the Pentathionate Ion by the Simultaneous Tracking of Different Sulfur Species. Inorg. Chem. 2011, 50, 9670–9677. (50) Krischer, K. Nonlinear Dynamics in Electrochemical Systems. In Advances in Electrochemical Science and Engineering; Alkire, R. C.; Kolb, D. M. Eds.; Wiley-VCH: Weinheim, Germany, 2003; Vol. 8, 89−208. (51) Yan, M.; Liu, K.; Jiang, Z. Electrochemical Oxidation of Thiourea Studied by Use of in Situ FTIR Spectroscopy. Electroanal. Chem. 1996, 408, 225–229. (52) Hoffmann, M.; Edward, J. O. Kinetics and Mechanism of the Oxidation of Thiourea and N, N'-Dialkylthioureas by Hydrogen Peroxide. Inorg. Chem. 1978, 16, 3333–3338. 23



(53) Rio, L. G.; Munkley, C. G.; Stadman, G. Kinetic Study of the Stability of (NH2)2CSSC(NH2)22+. J. Chem. Soc. 1996, 2, 1947–1956. (54) Koper, M. T. M.; Lukkien, J. Modeling the Butterfly: the Voltammetry of (root 3 × root 3) R30 Degrees and p(2 × 2)Overlays on (111) Electrodes. J. Electroanal. Chem. 2000, 485, 161– 165. (55) Berná, A,; Climent, V.; Feliu, J. M. New Understanding of the Nature of OH Adsorption on Pt(111) Electrodes. Electrochem. Common.2007, 9, 2789–2794. (56) Wakissaka, M.; Suzuki, H,; Mitsui, S.; Uchida, H.; Watanabe, M. Identification and Quantification of Oxygen Species Adsorbed on Pt(11) Single-Crystal and Polycrystalline Pt Electrodes by Photoelectron Spectroscopy. Langmuir 2009, 25, 1897–1900. (57) Angerstein, H.; Conway, B. E.; Sharp, W. B. A. The Real Condition of Electrochemically Oxidized Platinum Surface. Electroanal. Chem. Interf. Electrochem. 1973, 43, 9–36. (58)Sugimura, F.; Sakai, N.; Nakamura, T.; Nakamura, M.; Ikeda, K.; Sakai, T; Hoshi, N. In Situ Observation of Pt Oxides on the Low Index Planes of Pt Using Surface Enhanced Raman Spectroscopy. Phys. Chem. Chem. Phys. 2017, 19, 27570–27579

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Chemisorbed Oxygen-Species-Mediated Electrocatalytic Oxidation of

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