Yuanfeng F Q Qi

Nihal Zhang

Mehmet A Oturan
email: mehmet.oturan@univ-paris-est.fr

Oturan

Jianan Gao

Bo Jiang

Congcong Ni

Yanqing Zhang

Nihal Oturan

electrocatalytic nitrate reduction: Preparation, performance and mechanism

Keywords: Nitrate reduction, Electrocatalysis, Co3O4-TiO2/Ti cathode, Co 3+ /Co 2+ redox couple, Non-precious catalyst

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

In water, nitrate (NO3 -) is considered an underlying hazardous material that is derived from various sources including the feedlot runoff, the excessive use of nitrogen fertilizer and natural degradation of pollutants [START_REF] Huo | Hydrogenation of aqueous nitrate and nitrite with ruthenium catalysts[END_REF]. The presence of NO3 -in drinking water can cause serious health problems such as the cancers, liver damage and blue baby syndrome due to its transformation product, i.e. nitrite anions (NO2 -) [START_REF] Hou | Novel Pd/GdCrO3 composite for photo-catalytical reduction of nitrate to N2 with high selectivity and activity[END_REF]. To diminish this potential health risk, the concentration limit for NO3 -in drinking water is set at 10 and 11.3 ppm by the United States EPA and the European Drinking Water Directive, respectively [START_REF] Song | Mechanism and optimization of electrochemical system for simultaneous removal of nitrate and ammonia[END_REF][START_REF] Gao | Highly active Pd-In/mesoporous alumina catalyst for nitrate reduction[END_REF]. In this regard, the removal technologies of NO3 -are imperious demands in populated regions.

Physicochemical treatments such as electrodialysis, ion exchange and reverse osmosis are the most common technologies for NO3 -removal from water [START_REF] Duca | Enhanced electrocatalytic nitrate reduction by preferentially-oriented (100) PtRh and PtIr alloys: the hidden treasures of the 'miscibility gap[END_REF], but the post-treatments are still needed to address the concentrated solutions produced in these methods. Although biological denitrification can gradually convert NO3 -to N2 [START_REF] Sharma | Nitrate removal from ground water: a review[END_REF], it is a time-consuming process and its performance is strongly impacted by the variable conditions, such as dissolved oxygen and temperature. Recently, zero valent metals (e.g. iron [START_REF] Lacasa | Removal of nitrates from groundwater by electrocoagulation[END_REF], aluminum [START_REF] Murphy | Chemical removal of nitrate from water[END_REF], and magnesium [START_REF] Kumar | Chemical denitrification of water by zero-valent magnesium powder[END_REF]) based stoichiometrically chemical reduction of NO3 -has been increasingly explored. Unfortunately, the flexibility of this strategy is greatly confined by the formation of inert oxide surface, deteriorating the electron transfer efficiency, and the inevitable production of secondary solid sludge. More recently, some catalytic hydrogenation processes with the combined noble/promoting metals (e.g., Pd/Cu and Pd/Fe) have been proposed for NO3 -reduction [START_REF] Kominami | Selective photocatalytic reduction of nitrate to nitrogen molecules in an aqueous suspension of metal-loaded titanium(IV) oxide particles[END_REF][START_REF] Yoshinaga | Hydrogenation of nitrate in water to nitrogen over Pd-Cu supported on active carbon[END_REF]. In these processes, the reduction of NO3 -is mainly initiated by promoter metal (Eq. ( 1)) with the formation of NO2 -, and the subsequent reduction process is gradually achieved by the noble metal activated hydrogen (Eq. ( 2)) [START_REF] Martinez | State-of-the-art and perspectives of the catalytic and electrocatalytic reduction of aqueous nitrates[END_REF]. However, catalytic hydrogenation reduction requires H2 as a reducing agent or electron donor, and there are safety hazards in the transport and use of H2, which greatly limits its industrial application potential [START_REF] Prüsse | Improving the catalytic nitrate reduction[END_REF].

NO3 -+ 2M[] → [M2O]surf + NO2 - (1)
H2 + 2Pd[] → 2Pd[H] (2)
Considering the advantages of self-produced hydrogen, environmental compatibility and ambient operating conditions, electrocatalysis has been regarded as a promising method for denitrification [START_REF] Dash | Electrochemical denitrificaton of simulated ground water[END_REF], in which the reduction of NO3 -can be obtained by either a direct or an indirect pathway at the cathode surface. Direct reduction of NO3 -is initiated by the electron tunneling or generation of chemisorbed complex between the NO3 -and cathode whereas in the indirect pathway, NO3 -is reduced by a surface-adsorbed atom H * , which is produced from the reduction of protons by electron via the Volmer process [START_REF] De Chialvo | Kinetics of hydrogen evolution reaction with Frumkin adsorption: re-examination of the Volmer-Heyrovsky and Volmer-Tafel routes[END_REF]. Specially, Pd-based catalysts have recently been distinguished mainly because of their potential to electrochemically reduce oxyanion contaminants in water in which the indirect reduction mediated by atomic H * plays a leading role compared to the direct electron transfer from the cathode to NO3 - [START_REF] Chaplin | Critical review of Pd-based catalytic treatment of priority contaminants in water[END_REF]. Nevertheless, considering the extremely expensive price of Pd, the research significance of the Pd-based catalyst is obviously theoretical but not practical. Additionally, despite the fact that the indirect reduction mechanism of NO3 -has been well explored in electrochemical noble metal-based process, the targeted strategies to develop some specific catalysts for the direct reduction of NO3 -is sparsely reported.

The cost-effective cobalt oxide (Co3O4) is a typical p-type metal oxide, which exhibits a normal spinel structure with Co 2+ ions in tetrahedral interstices and Co 3+ ions in octahedral interstices. During the recent decades, Co3O4 has been demonstrated to own strong catalytic reduction activity and thus extensively used in electrochemical hydrogen evolution reaction [START_REF] Zhou | Facet effect of Co3O4 nanocrystals on visible-light driven water oxidation[END_REF] and reduction of CO2 [START_REF] Gao | Ultrathin Co3O4 layers realizing optimized CO2 electroreduction to formate[END_REF] and NOx [START_REF] Meng | Selective catalytic reduction of nitrogen oxides by ammonia over Co3O4 nanocrystals with different shapes[END_REF]. Recently, Su et al. [START_REF] Su | Electrochemical nitrate reduction by using a novel Co3O4/Ti cathode[END_REF] reported that the pure Co3O4 loaded onto the titanium substrate by thermal sintering could also electrocatalytically reduce NO3 -in the water. However, the Co3O4-mediated electrochemical mechanism of NO3 -reduction has remained unclear. Moreover, it is known that the biologically toxic cobalt ion probably causes human carcinogen, and the discharge standard of cobalt ions in industrial wastewater is 1 mg L -1 based on GB/T 25467-2010 of China. Therefore, efforts should be made to improve the stability of the cobalt-based catalyst and prevent its leaching from the substrate and leaking into the water. To this end, we prepared a novel cathode, Co3O4-TiO2/Ti, by combining sol-gel and calcination methods, which seems to be promising for remarkably facilitate the electrocatalytic reduction of NO3 -. The effects of electrode preparation factors (calcination temperature and coating times) and operation parameters (pH, current density and chloride ions concentration) on NO3 -reduction were also investigated. In addition, based on the characterization of the prepared catalysts using XRD, SEM, HRTEM, EDS elemental mapping and XPS, we tried to relate the results to the catalytic activity.

Furthermore, electrochemical analyses, radical scavenging experiments and density functional theory calculations were utilized to inspect the possible NO3 -reduction mechanisms.

Experimental section

Preparation of the working cathode

The substrate of electrode was Ti plate with size of 2.5 × 5.0 cm. The Ti plate was first polished with 300-mesh emery paper and then boiled in 20% sulfuric acid for 40 min. Next, the Ti plate was cleaned by ultrapure water, sonicated for 15 min, and dried for use. To synthesize the typical Co3O4-TiO2/Ti cathode, 0.5 mL titanium butoxide, 0.1 mL hydrochloric acid and 0.15 mL ultrapure water were dissolved in 8.5 mL ethyl alcohol. Then, 0.5 g PVP was added and stirred vigorously for 0.5 h. Subsequently, 2.69 g Co(NO3)2 • 6H2O (1 M) was added and stirred to obtain a purplish red liquid. The prepared sol-gel was pasted to the both faces of the pretreated Ti plates using a brush. The painted Ti plate was dried at 100 • C for 10 min. Then it was sintered at 500 • C for 2 h in a muffle oven to remove all organic substances with the formation of the oxide. The above procedures were repeated 4 times and the electrode films were annealed for 6 h at 500

Electrocatalytic reduction of NO3 -

A DC potentiostat (GPS-3030D, Gwinstek, China) was utilized as the power supply for electrochemical experiments. The electrocatalytic NO3 -reduction experiments were carried out in a 150 mL electrolytic cell at room temperature (25 • C). If not specially mentioned, the volume of the working solution was 100 mL, NO3 -concentration was 50 mg L -1 , and 0.1 M Na2SO4 was used as supporting electrolyte to maintain the conductivity of the solution. The solution pH was adjusted using concentrated NaOH and H2SO4. The 12.5 cm 2 Co3O4-TiO2-PVP electrodes were used as cathode and 12.5 cm 2 IrO2-RuO2/Ti electrodes were used as anode. The distance between the electrodes was 2 cm. The typical electrochemical experiment was conducted at current density of 10 mA cm -2 . All experiments were repeated at least twice.

Physicochemical characterizations and DFT calculation

The crystalline nature of the surface of Co3O4-TiO2/Ti cathode was examined by X-ray diffraction (XRD) measurement and the 2θ values acquired within the range of 10 • to 80 • using a Bruker D2 PHASER X-ray diffractometer that uses Cu Kα radiation. The morphology of the cathode surface was depicted by a scanning electron microscopy (SEM, FEI QUANTA FEG250) and high-resolution transmission electron microscope (HRTEM, JEM-2100F). The X-ray photoelectron spectroscopy (XPS) was obtained using a Thermo Fisher ESCALAB 250Xi system with Al Kα as an X-ray source. C 1s peak at 284.8 eV from residual carbon was conducted to correct the binding energy of other elements. The details of DFT calculation were provided in the supplementary material.

Electrochemical characterizations

All electrochemical measurements were performed on an electrochemical station (PGSTAT 302N, Metrohm). A standard three-electrode system was used in which the working electrode was a home made Co3O4-TiO2-/Ti cathode, the counter electrode was a platinum plate, and the reference electrode was a saturated calomel electrode (SCE). The effective working area of the working and counter electrodes was 12.5 cm 2 . The electrolyte for linear sweep voltammetry (LSV) and cyclic voltammetry (CV) tests was composed of 0.1 M Na2SO4 solution with or without 50 mg L -1 NO3 -.

CV tests were recorded between 0.0 V and -1.5 V/SCE at a scan rate of 20 mV s -1 . Electrochemical impedance spectroscopy (EIS) measurements were operated in 0.1 M Na2SO4 solution using the frequency ranged from 10 5 to 10 -2 Hz and the amplitude of the potential of 10 mV.

Chemical analyses

The concentrations of NO3 -, NO2 -and NH4 + were determined with a UV-vis spectrophotometer as it has been depicted in detail in the previous literature [START_REF] Teng | Selective nitrate reduction to dinitrogen by electrocatalysis on nanoscale iron encapsulated in mesoporous carbon[END_REF]. The residual NO3 -ratio ( ), the generated NO2 -ratio ( ), the generated NH4 + ratio, ( ), the total nitrogen (TN) removal efficiency, ( ) and N2 selectivity ( , were calculated by the following equations:

(%) = × 100 (3)
(%) = × 100 (4)
(%) = × 100 (5) = 1 - - - (6)
(%) = × 100 [START_REF] Lacasa | Removal of nitrates from groundwater by electrocoagulation[END_REF] where [NO3 -]0 is the initial concentration of NO3 -and [NO3 -]t, [NO2 -]t and [NH4 + ]t are the concentrations of NO3 -, NO2 -and NH4 + at time t.

Results and discussion

Catalytic NO3 -reduction by different cathodes

The comparative NO3 -reduction process efficiency at different cathodes was depicted in Fig. 1.

As shown in Fig. 1(a), the NO3 -removal efficiency of TiO2/Ti cathode was only 45% at current density of 10 mA cm -2 after 120 min. However, as for Co3O4-TiO2/Ti (without PVP) cathode, NO3 - removal efficiency suddenly became twice that of TiO2/Ti cathode, reaching approximately 80%.

Moreover, with the introduction of the dispersant PVP, the removal efficiency of NO3 -was further increased to 89%. Although the addition of TiO2 resulted in a slight decrease (nearly 6%) in the removal efficiency of NO3 -compared to Co3O4/Ti cathode, the release of Co ions into the aqueous solution was remarkably inhibited, probably due to the stabilization effect of TiO2 (see Fig. 1(b)). In the case of Co3O4/Ti cathode, the leaching of Co ions (Co 2+ /Co 3+ ) reached 0.2 mg L -1 after 120 min, while Co ions were barely detected for both the Co3O4-TiO2/Ti and Co3O4-TiO2/Ti (without PVP) cathodes. Recently, many non-precious catalysts for NO3 -reduction have been developed as the alternatives to conventional noble Pd catalyst. As shown in Table S1, the Co3O4-TiO2/Ti cathode exhibits significantly superior NO3 -reduction activity over other non-precious electrodes.

<Fig. 1>

As shown in Fig. 1(c), the profiles of ammonium (NH4 + ) formation as a function of electrolysis time have similar trends with those of NO3 -removal in the above reaction systems. The amount of NH4 + formed in the case of Co3O4/Ti cathode was larger than those of Co3O4-TiO2/Ti and Co3O4-TiO2/Ti (without PVP) cathodes. In addition, among all the cathodes tested, the TiO2/Ti produced the lowest amount of NH4 + , due to its inferior performance for NO3 -reduction. During NO3 -reduction process, the toxic intermediate NO2 -was negligibly detected in the cases of Co3O4-TiO2/Ti, Co3O4-TiO2/Ti (without PVP) and Co3O4/Ti cathodes, while a significant amount (around 1%) of NO3 -was transformed into NO2 -when using TiO2/Ti as the cathode. Although NO3 - reduction efficiency of Co3O4-TiO2/Ti cathode was slightly lower than that of Co3O4/Ti cathode, the TN removal efficiency and N2 selectivity for the former were comparable to that of the latter as shown in Fig. S1. Besides, the TN removal efficiency and N2 selectivity of Co3O4-TiO2/Ti (without PVP) and TiO2/Ti cathodes were much lower than those of Co3O4-TiO2/Ti cathode. shows that the main components of the as-prepared cathode were the target catalyst Co3O4 and the dispersant carrier TiO2.

<Figs. 2>

SEM images (Fig. 3) illustrate the morphology of as-prepared catalysts.

Effect of coating times

The Co ions concentration of 0, 0.5, 1.0, and 2.0 M with 4 coating times was investigated. The NO3 -concentration decay kinetics and LSV curves (Fig. S2) demonstrate that 1.0 M was compatible.

In this section, the studied number of coating times was 2, 4, 6, and 8 with Co ions concentration of 1.0 M, which was represented as 2-Co3O4-TiO2/Ti, 4-Co3O4-TiO2/Ti, 6-Co3O4-TiO2/Ti and 8-Co3O4-TiO2/Ti cathodes, respectively. Although, as shown in Fig. S3, the coating times negligibly affected the morphology of the electrode surface, too many or too few coating times were not conducive to NO3 -reduction. The NO3 -removal efficiencies for 2-Co3O4-TiO2/Ti, 4-Co3O4-TiO2/Ti, 6-Co3O4-TiO2/Ti and 8-Co3O4-TiO2/Ti cathodes were 80%, 89%, 64% and 61%, respectively, within 120 min. Thus, the optimum number of coating times was 4. As illustrated in Fig. 5(b), 4-Co3O4-TiO2/Ti cathode has the largest response current intensity. And the trend of response current intensity at the same voltage was consistent with the NO3 -reduction trend of Fig. 5(a). The above experimental and LSV results can be explained by the fact that when the number of coating times was below 4, insufficient Co loadings may not afford enough active sites for NO3 -reduction. In contrast, when the number of coating times was more than 4, the catalyst coating on the Ti plate was thick, which probably deteriorated the electron transfer, and the increased potential would exacerbate the occurrence of hydrogen evolution side reactions, therefore led to a decrease in NO3 -reduction efficiency.

<Fig. 5>

Effect of calcination temperature

In this section, the effect of calcination temperatures on the performance of Co3O4-TiO2/Ti cathode was investigated. The as-prepared cathodes sintered at 300 °C, 400 °C, 500 °C, 600 °C and 700 °C were marked as 300-Co3O4-TiO2/Ti, 400-Co3O4-TiO2/Ti, 500-Co3O4-TiO2/Ti, 600-Co3O4-TiO2/Ti and 700-Co3O4-TiO2/Ti, respectively. Fig. 6(a) shows that there were significant differences in NO3 -reduction efficiency between the cathodes sintered at different temperatures. The optimum NO3 -removal efficiency was observed for the cathode calcination in the temperature range of 300-500 °C, e.g., approximately 89% for 500-Co3O4-TiO2/Ti cathode. A moderate NO3 -removal efficiency was obtained for 600-Co3O4-TiO2/Ti cathode (67%) while the 700-Co3O4-TiO2/Ti cathode had the marginal NO3 -removal efficiency, approximately 34%. The variation trend of response current intensity observed from the LSV measurements (Fig. 6(b)) was in good accordance with that of the cathodes for NO3 -removal, which confirmed the best performance of the 500-Co3O4-TiO2-PVP cathode.

<Fig. 6>

As shown in Fig. 2, the main characteristic peaks of Co3O4 in XRD spectra for Co3O4-TiO2/Ti cathode became narrower and sharper as the calcination temperature increased in the range of 300 -700 °C. According to the previous studies, complete conversion of Co3O4 can be achieved in the sol-gel and calcination process when the temperature exceeds 270 °C [START_REF] Su | Electrochemical nitrate reduction by using a novel Co3O4/Ti cathode[END_REF]. Better crystallinity structure of Co3O4 could be obtained at higher calcination temperatures. Nevertheless, Ti plate has some problems when being sintered at relatively high temperature. Specially, as demonstrated in the previous literature [START_REF] Su | Electrochemical nitrate reduction by using a novel Co3O4/Ti cathode[END_REF], the conductivity of Ti plate was greatly reduced when the calcination temperature reached 600 °C. Besides, the phase structure, composition, and crystallinity of TiO2 are of great influence on its electrocatalytic activity and electrochemical properties [START_REF] Yu | Fabrication and characterization of Ag-TiO2 multiphase nanocomposite thin films with enhanced photocatalytic activity[END_REF]. Therefore, XRD was also used to analyze the changes of TiO2 phase on the cathodes sintered at various temperatures.

It can be seen from the XRD patterns presented in Fig. 2 that the calcination temperature significantly impacted the crystallization and phase structures of surface catalysts on Co3O4-TiO2/Ti cathode. For Co3O4-TiO2/Ti cathodes sintered at the temperature of 300-500 °C, except the diffraction peaks of Ti substrate and Co3O4, two broad peaks at 2θ = 53.9 ° and 76.0 ° can be attributed to the (105) and (301) plane diffraction, respectively, of anatase TiO2. However, as for 600-Co3O4-TiO2/Ti cathode, small peaks at 2θ = 27.4 °, 36.1 °, 54.3 °, 56.6 °, 69.0 °, and 69.8 ° correspond to (110), (101), (211), ( 220), (301), and (112) plane diffraction, respectively, of rutile. Notably, the intensity of rutile diffraction peaks of Co3O4-TiO2/Ti cathode increases sharply at the calcination temperature of 700 °C. This suggests the occurrence of phase transformation of anatase to rutile when elevating the calcination temperature from 300 to 600 °C, which was even more pronounced for 700 °C. Furthermore, with the calcination temperature increase to 600 °C and 700 °C, the crystal turned to aggregation, which was probably responsible for the decrease of NO3 -reduction efficiency (Fig. S4). Based on these results, it can be concluded that the phase transformation of TiO2 began to take place at the calcination temperature above 500 °C. When calcination temperature reaches 600 °C, the rutile phase even became the prevalent phase of the composite. This was consistent with the results in the previous literature papers [START_REF] Yue | Efficiently electrochemical removal of nitrite contamination with stable RuO2-TiO2/Ti electrodes[END_REF].

Based on the EIS measurements, the Nyquist plots of various Co3O4-TiO2/Ti cathodes sintered at various temperatures are provided in Fig. 6(c). The radius of semicircle was related with the charge transfer resistance and the sample with smaller arc radius indicates faster interfacial electron transfer [START_REF] Hong | Efficient photoelectrochemical water splitting over Co3O4 and Co3O4/Ag composite structure[END_REF]. The calcination temperature exerts a significant effect on the electrochemical properties of the electrodes. In this study, when the calcination temperature was increased from 300 to 500 °C, the radius of the arc did not appear, indicating that the charge transfer resistance was small. However, when the calcination temperature rose to 600 °C, the arc radius appeared, suggesting the fact that increasing calcination temperature caused an increase in impedance for Co3O4-TiO2/Ti cathode. And the charge transfer resistance of Co3O4-TiO2/Ti cathode obtained at the calcination temperature of 700 °C was larger than that of 600 °C. The change of EIS results indicates that the phase transformation of anatase to rutile with elevating calcination temperature could inhibit the charge transfer within the cathode, which adverse to NO3 -reduction [START_REF] Yu | Effect of calcination temperature on morphology and photoelectrochemical properties of anodized titanium dioxide nanotube arrays[END_REF]. This can be ascribed to the higher electric resistance of rutile than that of anatase [START_REF] Yu | Effect of calcination temperature on morphology and photoelectrochemical properties of anodized titanium dioxide nanotube arrays[END_REF].

Therefore, the relatively low NO3 -reduction efficiencies for 600-Co3O4-TiO2/Ti and 700-Co3O4-TiO2/Ti cathodes could be explained by the increase in the charge transfer resistance of Co3O4-TiO2/Ti cathode prepared at a high calcination temperature. Consequently, appreciable Co3O4-TiO2/Ti cathodes could be obtained at the calcination temperature of 300-500 °C, in which cobalt precursor could be completely transformed to Co3O4 and the relatively low charge transfer resistance could be also ensured.

Effect of electrochemical reaction parameters

Effect of solution pH and current density

As illustrated in Fig. 7, the effect of solution pH (3.0-9.0) on NO3 -reduction was evaluated in this section. Figs. significantly increased after 120 min reaction. For example, after 2 h of nitrate reduction, the solution pH increased from 3.0, 5.0, 7.0 and 9.0 to 9.0, 10.6, 10.9 and 11.0, respectively. The explanation for this pH variation will be discussed in the following section 3.4.

<Fig. 7>

On the other hand, the reduction efficiency of NO3 -was significantly enhanced with the increase of current density from 2.5 to 10 mA cm -2 (Fig. 8(a)). However, when the current density was increased from 10 to 25 mA cm -2 , the removal efficiency of NO3 -was not greatly improved. Fig. S5 shows that the reduction of 50 mg L -1 NO3 -followed a zero-order kinetic trend at the current density of 2.5 and 5 mA cm -2 with the rate constant increased from 0.190 (R 2 = 0.99) to 0.440 (R 2 = 0.98) min -1 (indicating a charge controlled kinetic stage) while a first-order kinetic trend was observed at the current densities of 10 and 25 mA cm -2 (indicating a mass controlled kinetic stage)

with the rate constant of 0.018 (R 2 = 0.99) and 0.027 (R 2 = 0.99) min -1 , respectively. As the current density was increased from 10 to 25 mA cm -2 , more energy was used to generate hydrogen bubble, so that the increase in the NO3 -removal efficiency was not significant as expected. There is a good correspondence between the amount of NH4 + produced and the amount of NO3 -reduced as shown on Fig. 8(b). Low current density leads to lower energy consumption, but in actual use, it is necessary to seek the balance between the energy saving and the NO3 -treatment requirements to gain the highly appreciable treatment performance.

<Fig. 8>

Effect of chlorine concentration

As can be seen in Fig. S1, the main product of the electrocatalytic reduction of NO3 -was NH4 + , which is also a contaminant that needed to be removed. The removal of NH3 is primarily takes place through direct oxidation on the anode surface and indirect oxidation mediated by active chlorine in the solution bulk. In the direct oxidation process, the NH3 is first adsorbed on the surface of the anode and subsequently oxidized to N2, as shown in Eq. ( 8) [START_REF] Zhang | Active chlorine mediated ammonia oxidation revisited: Reaction mechanism, kinetic modelling and implications[END_REF]. However, NH4 + cannot be oxidized directly at the anode. Hence it is necessary to adjust the solution pH to reach the pKa of the NH4 + /NH3 acid-base pair (i.e., pH > 9.25). [START_REF] Qiang | Determination of monochloramine formation rate constants with stopped-flow spectrophotometry[END_REF] In addition to the direct oxidation of NH3 on the anode, the generated active chlorine species (Eqs. ( 9) and ( 10)) at the anode could also be involved in the oxidative transformation of NH4 + to nitrogen gas (Eq. ( 11)) or NO3 -(Eqs. ( 12)) as well as the oxidation of NO2 -to NO3 -(Eq. ( 13)) in the homogeneous solution, so-called indirect (or mediated) oxidation [START_REF] Zhang | Active chlorine mediated ammonia oxidation revisited: Reaction mechanism, kinetic modelling and implications[END_REF]. Furthermore, HOCl reacts also on NH3 to form chloramines (Eqs. ( 14)-( 16)).

Compared with direct ammonia oxidation, the chlorine-mediated ammonia oxidation is much more efficient and feasible due to the abundant presence of chloride ion in wastewaters.

2NH3 + 6OH

-→ N2 + 6H2O + 6e - (8) 2Cl -→ Cl2 + 2e - (9)
Cl2 + H2O → HOCl + H + + Cl -

2NH4 + + 3HOCl → N2 + 3H2O + 3Cl -+ 5H + (11)

NH4 + + 4HOCl → NO3 -+ H2O + 6H + + 4Cl - (12)
NO2 -+ HClO → NO3 -+ H2O + H + + Cl -

HOCl + NH3 → NH2Cl + H2O ( 14)

HOCl + NH2Cl → NHCl2 + H2O (15)
HOCl + NHCl2 → NCl3 + H2O [START_REF] Chaplin | Critical review of Pd-based catalytic treatment of priority contaminants in water[END_REF] Chloride ions are widely presented in drinking water and industrial water. For instance, the concentration of chloride ions in the bulk drug manufacturing industrial wastewater and zinc refinery wastewater reached 3119 and 10000 mg L -1 , respectively [START_REF] Garcia-Segura | Electrocatalytic reduction of nitrate: Fundamentals to full-scale water treatment applications[END_REF]. Considering the role of chloride ions in oxidation of NH4 + , various concentrations of chloride ions were added to the solution in order to study its effect. As shown in Fig. 9(a), the NO3 -removal efficiency was almost unaffected by the presence of chloride ions. But, as the concentration of chloride ions increased from 0 to 2000 mg L -1 , the formation of NH4 + progressively decreased (Fig. 9(b)). For instance, when the concentration of chloride ions increased to 2000 mg L -1 , the generated NH4 + ions ratio initially slightly increased to 7% within 40 min but then gradually decreased to approximately 0 after 100 min reaction time. Besides, it appears from Fig. 9(c) that the formation of NO2 -could be deteriorated with increasing chloride ions concentration from 0 to 2000 mg L -1 , with the highest NO2 -generation of 0.47%. It can also be seen from Fig. S6 that the TN removal efficiency and N2 selectivity of Co3O4-TiO2/Ti cathode increased with the increase of chloride ions concentration. When the chloride ions concentration reached 2000 mg L -1 , the TN removal efficiency and N2 selectivity reached 82% and 100%, respectively.

<Fig. 9>

NH4 + formation curves plotted in Fig. 9(d) clearly show that its amount formed at Co3O4/Ti cathode (49%) was about twice of that formed at Co3O4-TiO2/Ti cathode (24%) after 120 min reaction at the same chloride ions concentration of 1000 mg L -1 . Fig. S6 shows that the TN removal efficiency and N2 selectivity for Co3O4-TiO2/Ti cathode were 60% and 71%, respectively, while the corresponding values for Co3O4/Ti cathode were 42% and 46%, respectively. The difference in NT removal efficiency between Co3O4-TiO2/Ti and Co3O4/Ti cathodes in the presence of chloride ions was much larger than in the chloride-free cases as observed in Fig. S1. The indirect oxidation mechanism of NH4 + was similar to the intensively studied breakpoint chlorination [START_REF] Pressley | Ammonia-nitrogen removal by breakpoint chlorination[END_REF]. First, the NH4 + reacts with the active chlorine (HOCl or OCl -depending to the solution the pH) to produce intermediate products such as monochloramine, dichloramine and tri-chloramine, and were finally oxidized to N2 [START_REF] Qiang | Determination of monochloramine formation rate constants with stopped-flow spectrophotometry[END_REF]. It is worth noting that the oxidation process of the NH4 + began to occur only when active chlorine/ammonia mole ratio reaches 1.5 [START_REF] Li | Ammonia removal in electrochemical oxidation: mechanism and pseudo-kinetics[END_REF]. Specially, Ding et al. [START_REF] Ding | The eAND process: enabling simultaneous nitrogen-removal and disinfection for WWTP effluent[END_REF] found that higher active chlorine/ammonia mole ratio (>5) brought about 100% removal of NH4 + , while less efficient removal efficiencies (nearly 50%) were achieved with lower mole ratio (< 2). Thus, it can be inferred that electrochemical ammonia oxidation in these reaction systems was closely related with the active chlorine/ammonia mole ratio. As demonstrated in Fig. 1, the efficiency of NO3 - reduction to NH4 + with Co3O4/Ti cathode was higher than that with Co3O4-TiO2/Ti cathode. This would result in less mole ratio of active chlorine/ammonia in the former case compared with that in the latter case. Therefore, as shown in Fig. 9(d), the indirect oxidation of ammonia was more favorable for the Co3O4-TiO2/Ti cathode mediated electrochemical system.

The NO3

-removal mechanism

Direct reduction

Electrochemical reduction of NO3 -is a complex process involving a range of intermediates and broad valence changes (from +5 to -3). The process mainly includes direct reduction and indirect reduction. Direct reduction is mainly achieved by electrons and H + , and indirect reduction is mainly caused by atomic H* [START_REF] Garcia-Segura | Electrocatalytic reduction of nitrate: Fundamentals to full-scale water treatment applications[END_REF]. This mechanism was verified in the present electrochemical reaction system by CV tests as depicted in Fig. 10(a). When 50 mg L -1 NO3 -was added to the reaction solution, the peak current density of -2.4 and -4.0 mA cm -2 appeared at the potential of -0.9 and -1.2

V/SCE, respectively, at TiO2/Ti cathode. These reduction peaks can be attributed to the electron transfer from the cathode to NO3 -because no peak was observed in the absence of NO3 -. Thus, it is rational that approximately 45% NO3 -was reduced at TiO2/Ti cathode (Fig. 1(a)). According to the previous studies [START_REF] Hasnat | Copper-immobilized platinum electrocatalyst for the effective reduction of nitrate in a low conductive medium: Mechanism, adsorption thermodynamics and stability[END_REF], these two reduction peaks correspond to the reduction of NO3 -to NO2 -and the formation of NH4 + , respectively. Furthermore, single reduction peak does not mean single electron transfer [START_REF] Su | The electrochemical reduction of nitrate over micro-architectured metal electrodes with stainless steel scaffold[END_REF]. In contrast, the peaks were completely disappeared in the case of Co3O4-TiO2/Ti cathode, regardless of the presence or absence of NO3 -. Notably, the experimental results presented in Fig. 1 show that the NO3 -reduction efficiency was greatly increased in the presence of Co3O4.

Therefore, the disappearance of the reduction peak was not ascribed to the poor catalytic activity of the Co3O4 toward the electrocatalytic reduction of NO3 -. This peculiar phenomenon just became a breakthrough point to understand the mechanism of the NO3 -electrocatalytic reduction by Co3O4. It is known that two processes, i.e., heterogeneous charge transfer and diffusional mass transport, mainly determine the shape of the CV curve [START_REF] Heinze | Cyclic voltammetry-"electrochemical spectroscopy[END_REF]. The CV curve can reflect the heterogeneous charge transfer from the electrode to electroactive species. However, it fails to reflect the charge transfer and valence change inside the electrode. Co3O4 contains Co 2+ ions in tetrahedral interstices and Co 3+ ions in octahedral interstices. Thus, it can be inferred that NO3 -probably did not directly receive electrons from the cathode via the circuit, but from the "electron porter" of the structural Co 2+ ions. This In order to further confirm this, the electrochemical reduction of NO3 -at Co3O4-TiO2/Ti cathode was investigated at different cathode potentials, and the experimental results are shown in Fig. 10(b). Nearly no NO3 -was removed at the applied potential of -0.5 and -0.7 V/SCE within 120 min. The NO3 -removal efficiency of 36% and 40% were attained at -1.0 and -1.2 V/SCE, respectively. Besides, NO3 -removal efficiency finally raised to 98% after 120 min electrolysis when the applied potential was further decreased to -1.3 V/SCE. However, the NO3 -reduction efficiency was decreased to approximately 94% when further decreasing the cathode potential to -1.5 V/SCE.

This can be attributed to the enhancement of hydrogen evolution reaction at this potential and the hindrance of hydrogen bubbles to the electron transfer at the cathode surface [START_REF] Li | The electrocatalytic dechlorination of chloroacetic acids at electrodeposited Pd/Fe-modified carbon paper electrode[END_REF]. The strong dependence of NO3 -reduction on the cathode potential corresponding to reduction peaks of CV curve is in agreement with the literature [START_REF] Liu | Efficient electrochemical reduction of nitrobenzene by defect-engineered TiO2-x single crystals[END_REF]. This confirms the above conjecture that electrons were transferred from the power source to the cathode surface through a circuit, in which Co 3+ was reduced to Co 2+ via accepting an electron. Then nitrogen-containing contaminants, such as NO3 -, receive the electrons from the structural Co 2+ ions. This particular reaction process cannot be directly detected by CV test, explaining the disappearance of the reduction peak in Fig. 10(a) and the strong voltage-dependent phenomenon in Fig. 10(b).

<Fig. 10>

In the above process, the ratio Co 2+ / Co 3+ would definitely change. To further confirm this statement, XPS analysis was used. The XPS wide-survey scan of Co(2p) is illustrated in Fig. 10(c).

The peaks at 780 and 795 eV (binding energy) should be assigned to Co(2p3/2) and Co(2p1/2), respectively [START_REF] Liu | Sandwich-like Co3O4/MXene composite with enhanced catalytic performance for Bisphenol A degradation[END_REF]. The Co (2p3/2)/Co (2p1/2) ratio value of almost 2:1 was consistent with the previous literature report [START_REF] Qiu | Interfacial effects of the Cu2O nano-dots decorated Co3O4 nanorods array and its photocatalytic activity for cleaving organic molecules[END_REF]. The peaks of Co 2+ were located at 781.5 and 796.6 eV, while the peaks at 780 and 794.8 eV should be assigned to Co 3+ [START_REF] Kang | Catalytic carbon monoxide oxidation over CoOx/CeO2 composite catalysts[END_REF][START_REF] Lin | Efficient elimination of caffeine from water using Oxone activated by a magnetic and recyclable cobalt/carbon nanocomposite derived from ZIF-67[END_REF]. Besides, the peaks at 786.8 and 804 eV were attributed to the shake-up satellite peaks of Co 2+ [START_REF] Li | Effect of TiO2 crystal structure on the catalytic performance of Co3O4/TiO2 catalyst for low-temperature CO oxidation[END_REF]. Moreover, the Co 2+ /Co 3+ ratios of the pristine and used Co3O4-TiO2/Ti cathodes were calculated as 1.49 and 0.42, respectively, which indicated the transformation of Co 2+ to Co 3+ during the electrochemical NO3 -reduction process. This confirms the electrochemical redox cycle Co 2+ -Co 3+ -Co 2+ and the electron transfer from Co 2+ to NO3 -during the electrocatalytic NO3 -reduction process.

<Fig. 11>

Indirect reduction

In the indirect pathway, electrons from the cathode could produce surface-adsorbed atomic H * via the reduction of protons, which is known as Volmer process [START_REF] De Chialvo | Kinetics of hydrogen evolution reaction with Frumkin adsorption: re-examination of the Volmer-Heyrovsky and Volmer-Tafel routes[END_REF]. Here, we employed CV technique to qualitatively detect the formed H* during the electrochemical reaction process, which has been well introduced by Jiang et al. [START_REF] Jiang | Identification of active hydrogen species on palladium nanoparticles for an enhanced electrocatalytic hydrodechlorination of 2, 4-dichlorophenol in water[END_REF] and Liu et al. [START_REF] Liu | Defect sites in ultrathin Pd nanowires facilitate the highly efficient electrochemical hydrodechlorination of pollutants by H * ads[END_REF]. First, the cathode was repeatedly scanned for activation in a potential range of -0.20 to 0.90 V/SCE in 0.1 M Na2SO4 electrolyte at sweep rate of 100 mV s -1 . Electrochemical analysis did not start until a stable CV curve was obtained [START_REF] Jiang | Identification of active hydrogen species on palladium nanoparticles for an enhanced electrocatalytic hydrodechlorination of 2, 4-dichlorophenol in water[END_REF]. As shown in Fig. 10(d), the starting potential was set from -1.20 to -1.35 V/SCE and the termination potential was 0.45 V/SCE. As reported in the previous study involving C-Pd cathode for nitrate reduction [START_REF] Jiang | Identification of active hydrogen species on palladium nanoparticles for an enhanced electrocatalytic hydrodechlorination of 2, 4-dichlorophenol in water[END_REF]; the lower the starting potential, the stronger the corresponding oxidation peak.

And the oxidation peaks of molecular hydrogen and atomic H* were located at -0.80 to -0.60 V/SCE and -0.10 to -0.00 V/SCE, respectively. Fig. 10(d) shows that at different initial potentials, reduction peaks were observed at the potentials from -0.6 to -0.1 V/SCE, corresponding to the oxidation of molecular H2 and H*, respectively. After adding 10 mM TBA, the intensity of the oxidation peak corresponding to atomic H* immediately weakened. This markedly suppressed the availability of the formed H* for the NO3 -reduction.

Figs. 10(e) and (f) show LSV curves and NO3 -reduction results, respectively, after the addition of 10 mM TBA. As illustrated in Fig. 10(e), the current intensity was significantly enhanced after the addition of 5 and 50 mg L -1 NO3 -. However, the current intensity was decreased as TBA was added, which was more pronounced at higher TBA concentration. In addition, as shown in Fig. 10(f), increasing TBA concentration could gradually inhibit the NO3 -removal efficiency. When TBA concentration was 0.2 mM, the NO3 -removal efficiency was decreased by approximately 10%, and it was further decreased by 18% at TBA concentration of 1 mM. But negligible difference in the NO3 - reduction efficiency between 1 and 10 mM TBA was observed, suggesting nearly complete hindering of the contribution of the formed H* to NO3 -reduction when TBA concentration was 1 mM.

Therefore, we can reasonably speculate that approximately 18% NO3 -reduction proceeded via the indirect pathway mediated by the atomic H* during the electrocatalytic reduction of NO3 -process.

Atomic H* would accordingly be generated first by Volmer mechanism (Eq. ( 17)), which can explain the increase of solution pH observed for the Co3O4-TiO2-PVP cathode. In some noble metal catalysts (e.g., Pd and Ag)-based reaction systems, atomic H* would be favorably preserved for NO3 - reduction [START_REF] Martinez | State-of-the-art and perspectives of the catalytic and electrocatalytic reduction of aqueous nitrates[END_REF]. As a result, after the addition of 1 mM and 10 mM TBA in the Pd-mediated process, the NO3 -removal rate constant was reduced by 38% and 81%, respectively, which fully demonstrates the importance of atomic H* for NO3 -reduction. For Pd-based catalysts, the high efficiency of the use of protons to generate atomic H* at low overpotentials (Eq. ( 17)) and the strong adsorption of atomic H* to Pd atoms make Eqs. ( 18) and ( 19) difficult to occur [START_REF] Liu | Defect sites in ultrathin Pd nanowires facilitate the highly efficient electrochemical hydrodechlorination of pollutants by H * ads[END_REF][START_REF] Jiang | Reductive detoxification of Cr (VI) mediated by environmentally relevant carboxylic acids: State-of-the-art and perspectives[END_REF]. Even if atomic H* is converted to H2, Pd atoms can cleave H2 and regenerate atomic H* (this mechanism belongs to catalytic reduction) [START_REF] Martinez | State-of-the-art and perspectives of the catalytic and electrocatalytic reduction of aqueous nitrates[END_REF]. Through such electrocatalytic reduction processes, the indirect reduction of NO3 -by Pd-based catalysts is dominant. In contrast, in present Co3O4-based electrochemical system, the produced H* was inclined to transform to H2 by Heyrovsky and Tafel routes (Eqs. ( 17)-( 19)) [START_REF] De Chialvo | Kinetics of hydrogen evolution reaction with Frumkin adsorption: re-examination of the Volmer-Heyrovsky and Volmer-Tafel routes[END_REF].

Consequently, as for Co3O4-TiO2-PVP cathode, the indirect reduction mediated by atomic H* accounted for only 18% of NO3 -reduction. Thus, the role of atomic H* in the electrocatalytic reduction of NO3 -at the Co3O4-TiO2-PVP electrode was secondary, while the direct electron transfer was mainly responsible for NO3 -reduction. In view of this, future research should focus on improving the atomic H* adsorption capacity of non-precious metal catalysts.

H2O + e -→ H* + OH - (Volmer)

H2O + H* + e -→ H2(g) + OH - (Heyrovsky)

2H* → H2(g) (Tafel)

Proposed reaction pathway

It was observed from Fig. 1 that the NH4 + concentration gradually increased over the reaction time for all cathodes containing Co3O4 catalyst as it was the main product of electrocatalytic NO3 - reduction, while NO2 -was the minor intermediate product at a relatively low concentration.

According to the aforementioned discussion and the experimental results, there are two main pathways (i.e., Co-mediated direct reduction and atomic H*-mediated indirect reduction) responsible for the electrocatalytic reduction of NO3 -by the Co3O4 bearing cathodes investigated in this study.

It is known that the reduction of NO3 -to NO2 -(Eq. ( 20)) is the kinetically rate-determining step for the electrocatalytic reduction of NO3 -. This step actually consists of the reactions set presented through Eqs. ( 21)-( 23) which is known as an electrochemical-chemical-electrochemical path [START_REF] Garcia-Segura | Electrocatalytic reduction of nitrate: Fundamentals to full-scale water treatment applications[END_REF].

The elementary electron transfer reactions lead to the generation of NO # (%&) ' and NO ' (%&) , which are adsorbed short-lived intermediates. Then, NO ' (%&) converts to an unstable dianion radical, namely NO ' (%&) '

, by direct electron transfer (Eq. ( 24)), which would quickly hydrolyzes to form NO(ad) (Eq. ( 25)) [START_REF] Garcia-Segura | Electrocatalytic reduction of nitrate: Fundamentals to full-scale water treatment applications[END_REF]:

NO3 -+ 2H + + 2e -→ NO2 -+ H2O Eº = 0.01 V/SHE (20)
According to the previous studies, the stepwise reduction of NO(ad) eventually forms NH4 + , which is regarded as an electrochemical-electrochemical path, as depicted in Eqs. ( 26

The direct electron transfers in above reactions are mostly single electron transfer processes.

The above experimental results demonstrated that the redox process Co 2+ -Co 3+ -Co 2+ accelerates the electron transfer during NO3 -reduction. The source of electrons for NO3 -reduction is directly derived from Co 2+ instead of the cathode through the circuit. Co 2+ gives an electron to NO3 -or its daughter intermediates with the formation of Co 3+ , and then Co 3+ would transform to Co 2+ via obtaining an electron from the cathode surface. Thus, during the whole reaction, Co3O4 mainly acts as an electron shuttle rather than a reducing agent. Moreover, H + is consumed in many NO3 - reduction steps, which is consistent with the pH variation as shown in Fig. 7(b).

Fig. 2

 2 Fig.2shows the XRD pattern of Co3O4-TiO2/Ti cathode calcinated at the temperature of 500

  Figs. 3(d)-(h) provide the EDS maps of Co and Ti elements at the above three cathodes. Obviously,

  7 (a) and 7(b) show the reduction curves of NO3 -over time and the final solution pH for different initial pH values. Contrarily to the previous studies [29], Fig. 7(a) shows that different pH conditions did not play significant role on NO3 -reduction efficiency over the electrolysis time. This indicates that Co3O4-TiO2/Ti cathode prepared in this study has a wide working pH range and can be potentially used to treat the NO3 -containing wastewaters in a relatively large pH range. Besides, Fig. 7(b) shows that, regardless of the initial pH, the pH value was

  proposition can be validated by the density functional theory (DFT) calculation results, which can be seen in Fig.11. Co3O4 (222) was selected considering XRD detection and NO3 -reduction experiments. The DFT calculation results show that the O atom in NO3 -and the Co atom on the surface of Co3O4 (222) are thermodynamically favorably bonded with the required energy of -9.04 eV. Besides, the differential charge density distribution indicates that electrons can transfer from the Co atoms on the surface of Co3O4 (222) to NO3 -. Differential charge density analysis further showed that the O atoms in NO3 -obtained 0.41, 0.65, and 0.57 electrons, respectively, and the N atoms lost 0.82 electrons, and a total of 0.81 electrons were transferred from the Co atom to NO3 -.

  NO3(ad) -+ e -→ NO3(ad) 2- Eº = -0.89 V/SHE (21) NO3(ad) 2-+ H2O → NO2·(ad) + 2OH - k = 5.5 ×10 4 s -1 (22) NO2·(ad) + e -→ NO2(ad) -+ H2O Eº = 1.04 V/SHE (23) NO2(ad) -+ e -→ NO2(ad) 2- Eº = -0.47 V/SHE (24) NO2(ad) 2-+ H2O → NO(ad) + 2OH -

FIGURE CAPTIONS Figure 1 .

 CAPTIONS1 FIGURE CAPTIONS Figure 1. NO3 -concentration decay kinetics (a), and evolution of concentration of Co (b), NH4 + (c) and NO2 -(d) during electrolysis time. Experimental conditions: [NO3 -]0 = 50 mg L -1 , pH = 7.0, current density = 10 mA cm -2 .

Figure 2 .

 2 Figure 2. XRD patterns of Co3O4-TiO2/Ti cathodes calcined at 300 °C, 400 °C, 500 °C, 600 °C and 700 °C. The locations of the characteristic peaks for Co3O4 (C), Ti substrate (Ti), anatase (A), and rutile (R) are represented by the vertical dashed lines.

Figure 3 .

 3 Figure 3. SEM images of Co3O4-TiO2/Ti (a), (b) Co3O4-TiO2/Ti (without PVP) (b) and Co3O4/Ti (c) cathodes. Elemental mapping of Co3O4-TiO2/Ti (d,e), Co3O4-TiO2/Ti (without PVP) (f,g) Co3O4/Ti (h) cathodes. Elemental mapping: (d-h) correspond to Co, Ti, Co, Ti and Co maps, respectively.

Figure 4 .

 4 Figure 4. HRTEM images of Co3O4-TiO2/Ti.

Figure 5 .

 5 Figure 5. NO3 -concentration decay kinetics (a) and LSV curves (b) for different coating times. Experimental conditions: (a) [NO3 -]0 = 50 mg L -1 , pH = 7.0, current density = 10 mA cm -2 , (b) 0.1 M SO4 -+ 50 mg L -1 NO3 -solution, scan rate: 100 mV s -1 ).

Figure 6 .

 6 Figure 6. NO3 -concentration decay kinetics at different calcination temperature (a) under experimental conditions: [NO3 -]0 = 50 mg L -1 , pH = 7.0, current density = 10 mA cm -2 ), LSV curves at different calcination temperature (b) in 0.1 M SO4 -+ 50 mg L -1 NO3 -solution at a scan rate of 100 mV s -1 and (c) EIS Nyquist plots of the cathodes at different calcination temperature by applying an impedance amplitude of 10 mV with the frequency range from 10 5 to 10 -2 Hz.

Figure 7 .

 7 Figure 7. NO3 -concentration decay kinetics at different pH (a) and Final solution pH for various initial pH after 120 min reaction (b). Experimental conditions: [NO3 -]0 = 50 mg L -1 , current density = 10 mA cm -2 .

Figure 8 .

 8 Figure 8. NO3 -concentration decay kinetics (a) and NH4 + generation (b) at different current density. Experimental conditions: [NO3 -]0 = 50 mg L -1 , pH = 7.0.

Figure 9 .

 9 Figure 9. Effect of Cl -concentration on NO3 -decay kinetics (a), NH4 + generation (b), NO2 - generation (c) with Co3O4-TiO2/Ti cathode, and NH4 + generation with Co3O4-TiO2/Ti and Co3O4/Ti cathodes (d). Experimental conditions: [NO3 -]0 = 50 mg L -1 , pH = 7.0, current density = 10 mA cm -2 .

Figure 10 .

 10 Figure 10. Cyclic voltammograms of NO3 -reduction (a) at TiO2/Ti and Co3O4-TiO2/Ti cathodes (scan rate: 20 mV s -1 ); Effect of cathode potential on NO3 -concentration decay kinetics (b) at Co3O4-TiO2/Ti cathode (Experimental conditions: [NO3 -]0 = 50 mg L -1 , pH = 7.0, current density = 10 mA cm -2 ); XPS spectra of Co 2p for fresh and used Co3O4-TiO2/Ti (c); Cyclic voltammograms obtained with Co3O4-TiO2/Ti cathode (d) in 0.1 M SO4 -in the presence (dotted line) and absence (full line) of 10 mM TBA; LSV curves of different electrolyte (e); and concentration decay kinetics of NO3 -at Co3O4-TiO2/Ti cathode (f) with various TBA concentrations (Experimental conditions: [NO3 -]0 = 50 mg L -1 , pH = 7.0,current density = 10 mA cm -2 ).

Figure 11 .

 11 Figure 11. (a) Adsorption configuration of NO3 -on Co3O4 (222) surface and corresponding adsorption energy, (b) Differential charge density distribution of NO3 -on Co3O4 (222) surface. Electron accumulation and electron deletion are represented by yellow and blue, respectively.

Figure 1 NO3

 1 Figure 1 NO3 -concentration decay kinetics (a), and evolution of concentration of Co (b), NH4 + (c) and NO2 - (d) during electrolysis time. Experimental conditions: [NO3 -]0 = 50 mg L -1 , pH = 7.0, current density = 10 mA cm -2 .

Figure 3 SEM

 3 Figure 3 SEM images of Co3O4-TiO2/Ti (a), (b) Co3O4-TiO2/Ti (without PVP) (b) and Co3O4/Ti (c) cathodes. Elemental mapping of Co3O4-TiO2/Ti (d,e), Co3O4-TiO2/Ti (without PVP) (f,g) Co3O4/Ti (h) cathodes. Elemental mapping: (d-h) correspond to Co, Ti, Co, Ti and Co maps, respectively.

Figure 4 Figure 5 .

 45 Figure 4

Figure 6 NO3

 6 Figure 6 NO3 -concentration decay kinetics at different calcination temperature (a) under experimental conditions: [NO3 -]0 = 50 mg L -1 , pH = 7.0, current density = 10 mA cm -2 ), LSV curves at different calcination temperature (b) in 0.1 M SO4 -+ 50 mg L -1 NO3 -solution at a scan rate of 100 mV s -1 and (c) EIS Nyquist plots of the cathodes at different calcination temperature by applying an impedance amplitude of 10 mV with the frequency range from 10 5 to 10 -2 Hz.

Figure 8 NO3

 8 Figure 8 NO3 -concentration decay kinetics (a) and NH4 + generation (b) at different current density. Experimental conditions: [NO3 -]0 = 50 mg L -1 , pH = 7.0.

Figure 9 Figure 9 .

 99 Figure 9 Figure 9. Effect of Cl -concentration on NO3 -decay kinetics (a), NH4 + generation (b), NO2 - generation (c) with Co3O4-TiO2/Ti cathode, and NH4 + generation with Co3O4-TiO2/Ti and Co3O4/Ti cathodes (d). Experimental conditions: [NO3 -]0 = 50 mg L -1 , pH = 7.0, current density = 10 mA cm -2 .

Figure 10 Cyclic

 10 Figure 10 Cyclic voltammograms of NO3 -reduction (a) at TiO2/Ti and Co3O4-TiO2/Ti cathodes (scan rate: 20 mV s -1 ); Effect of cathode potential on NO3 -concentration decay kinetics (b) at Co3O4-TiO2/Ti cathode (Experimental conditions: [NO3 -]0 = 50 mg L -1 , pH = 7.0, current density = 10 mA cm -2 ); XPS spectra of Co 2p for fresh and used Co3O4-TiO2/Ti (c); Cyclic voltammograms obtained with Co3O4-TiO2/Ti cathode (d) in 0.1 M SO4 -in the presence (dotted line) and absence (full line) of 10 mM TBA; LSV curves of different electrolyte (e); and concentration decay kinetics of NO3 -at CO3O4-TiO2/Ti cathode (f) with various TBA concentrations (Experimental conditions: [NO3 -]0 = 50 mg L -1 , pH = 7.0,current density = 10 mA cm -2 ).

Acknowledgments

This work is financially supported by the National Natural Science Foundation of China (No. 51608284), National Major Science and Technology Program for Water Pollution Control and Treatment (No. 2017ZX07101-006), China Postdoctoral Science Foundation (No. 2017M610413), Shandong Province Postdoctoral Science Foundation (No. 201702041).

At the same time, due to the strong reducibility of atomic hydrogen (E°(H+/H) = -2.31 V/SHE) [START_REF] Vooys | The role of adsorbates in the electrochemical oxidation of ammonia on noble and transition metal electrodes[END_REF], the indirect reduction path of NO3 -can not be ignored. Atomic H* involved in Eqs. ( 32)- [START_REF] Su | The electrochemical reduction of nitrate over micro-architectured metal electrodes with stainless steel scaffold[END_REF].