Simultaneous utilization of electro-generated O2 and H2 for H2O2 production:An upgrade of the Pd-catalytic electro-Fenton process for pollutants degradation

Wei Zhou,Xiaoxiao Meng,Liang Xie,Junfeng Li,Yani Ding,Yanlin Su,Jihui Gao,Guangbo Zhao

School of Energy Science and Engineering,Harbin Institute of Technology,Harbin 150001,China

Keywords:Electro-Fenton Hydrogen peroxide Hydroxyl radicals Oxygen reduction reaction Organic pollutants

ABSTRACT The Electro-Fenton(EF)process is one of the promising advanced oxidation processes(AOPs)for environmental remediation.The H2O2 yield of EF process largely determines its performance on organic pollutants degradation.Conventional Pd-catalytic EF process generates H2O2 via the combination reaction of anodic O2 and cathodic H2.However,the relatively expensive catalyst limits its application.Herein,a hybrid Pd/activated carbon (Pd/AC)-stainless steel mesh (SS) cathode (PACSS) was proposed,which enables more efficient H2O2 generation.It utilizes AC,the support of Pd catalyst,as part of cathode for H2O2 generation via 2-electron anodic O2 reduction,and SS serve as a current distributor.Moreover,H2O2 could be catalytically decomposed upon AC to generate highly reactive ·OH,which avoids the use of Fe2+.Compared with conventional Pd catalyst,H2O2 concentration obtained by PACSS cathode is 248.2% higher,the O2 utilization efficiency was also increased from 3.2% to 10.8%.Within 50 min,26.3%,72.5%,and 94.0% H2O2 was decomposed by Pd,AC,and Pd/AC.Fluorescence detection results implied that Pd/AC is effective upon H2O2 activation for ·OH generation.Finally,iron-free EF process enabled by PACSS cathode was examined to be effective for reactive blue 19 (RB19) degradation.After continuous running for 10 cycles (500 min),the PACSS cathode was still stable for H2O2 generation,H2O2 activation,and RB19 degradation,showing its potential application for organic pollutants degradation without increase in the running cost.

1.Introduction

The electro-Fenton(EF)technology is based on the in-situ electrogeneration of H2O2from externally injected pure O2or air,along with the addition of Fe2+to produce highly reactive·OH for contaminants degradation.Its performance depends on cell configuration,the nature of the electrodes,applied potential or current,and solution conditions[1–6].Over the past decade,it has experienced a significant development showing great availability to treat nonbiodegradable or refractory organic compounds[7,8].Nonetheless,the O2utilization efficiency in many systems is lower than 0.1%[9,10],most of the oxygen is not effectively used for H2O2production.Moreover,as an important by-product from water electrolysis,H2is not used and thus,further increase the operating costs of EF process.

Pd-catalytic EF process,first reported by Yuanet al.in 2011[11],utilize cathodic H2and anodic O2from water electrolysis for H2O2production.Pd catalyst is supported on aluminum (Pd/Al2O3) or activated carbon (Pd/AC) and can be reused.It is recognized a promising EF system and has been well developed and applied to various organic contaminants in water matrix after its first report[12–16].However,even the catalyst can be reused,it is still expensive for full-scale application.It is,therefore,of great importance to propose a new strategy to lower the operating costs for Pdcatalytic EF process.

At present,various carbonaceous materials such as graphite[17],carbon felt [10,18],graphite felt [19,20],reticulated vitreous carbon foam (RVC foam) [5,21],carbon nanotube [22],activated carbon fiber [17,23,24] have been proved to be effective for H2O2generation.Therefore,it is theoretically reasonable that activated carbon,the support of Pd catalyst,can be used to electrogenerate H2O2via2-electron oxygen reduction reaction (2eORR) while Pd catalyze H2O2generation via reaction between H2and O2[11].Hopefully,if granular Pd/AC can be sandwiched by stainless steel mesh to an cathode(denoted as PACSS),H2O2could thus be generatedviatwo pathways,making the system more efficient and competitive.The electrolysis of one mole H2O generates one mole O2and two mole H2,whereas the synthesis of H2O2by Pd catalyst requires equimolar amount of O2and H2.However,the aqueous solubility of O2(1.25 mmol?L-1) is higher than H2(0.81 mmol?L-1)in water at room temperature[25],which means the excessive O2could be used for H2O2generation via 2-electron ORR.Moreover,in conventional EF process,Fe2+is required to catalyst H2O2decomposition.If PACSS cathode is used,Fe2+can be avoided as activated carbon can catalyze H2O2decomposition to produce ·OH [26,27],which could avoid the forming and handling of iron sludge [1].

In this study,a novel EF system using PACSS cathode,which enables the synergistic electrogeneration and activation of H2O2in iron-free solution,is proposed to upgrade the conventional Pdcatalytic EF system.The objectives are:(1) verify the feasibility of activated carbon,the support of Pd catalyst,as a cathode material for H2O2generation,(2)compare the H2O2yield by Pd catalyst and PACSS cathode,(3) evaluate the catalytic performance of PACSS on H2O2activation and·OH formation,and(4)test the effectiveness and long-term stability of the new system on model organic pollutants degradation.

2.Materials and Methods

2.1.Materials

All chemicals used in this study were of analytical grade.Sodium sulfate (anhydrous,≥99%) and titanium sulfate (99.9%)were supplied by Tianli Chemical Reagent Co.,Ltd.Ferrous sulfate(FeSO4?7H2O,≥99%),and Reative blue 19 (C22H16N2Na2O11S3,RB19)at 99.9%purity were purchased from Shuanghuan Chemical Reagent Co.,Ltd.Deionized water (18.2 MΩ?cm) obtained from a Millipore Milli-Q system was used in all the tests.

Pd supported on alumina pellets and activated carbon was used and denoted as Pd/Al2O30.5%(mass)on Al2O3,3.2 mm pellets)and Pd/AC (1% (mass) on 4–8 mesh activated carbon,average size of 4.75–2.36 mm).For comparison,AC without Pd catalyst but with same size and pore structure was also prepared.A 50×50 stainless steel mesh bag (SSM bag,2 cm×3 cm×3 mm,grade 304) was firstly prepared and used as current distributor.The AC or Pd/AC particle electrode was then fabricated by filling 1.0 g Pd/AC or AC to the SSM bag.The SSM bag was made tight so SSM have good contact with Pd/AC or AC,thus AC can conduct electricity and be part of electrode.Ti/mixed metal oxide (MMO,Baoji Longsheng Nonferrous Metal Co.,Ltd.) mesh was used as anode materials.The Ti/MMO electrode consists of IrO2and Ta2O5coatings(1.9 μm thickness)on Ti mesh with dimensions of 3.6 cm diameter by 1.8 mm thickness.The distance between two electrodes is 3.5 cm.Constant current was provided by CHI660E (Shanghai Chenhua Co.,Ltd) electrochemical workstation.

Na2SO4solution (50 mmol?L-1) was used as supporting electrolyte.No ferrous salts were used as AC can catalyze H2O2to generate ·OH.Sulfuric acid and sodium hydroxide (Fisher Scientific)were used to adjust the solution pH.H2O2was electrogenerated from anodic O2in a batch reactor(volume of 180 ml)without aeration.The degradation of RB19 was conducted in the same apparatus at initial concentration of 30 mg?L-1.

2.2.Analyses and calculations

H2O2was measured at 405 nm(λmax)on a UV–Vis spectrometer(T6 New Century,Beijing Analysis General Instrument Co.,Ltd.)after coloration with TiSO4.The concentration of RB19 was determined on the same spectrometer at 592 nm.The RB19 removal efficiency(η) was calculated using Eq.(1),whereC0andCtdenote the absorbance of the RB19 solution at time zero and timet,respectively.The faradic current efficiency(CE)of H2O2generation by 2eORR was calculated using the Eq.(2) [28],wherenis the number of electrons required for O2reduction to H2O2,Fis the Faraday constant (96485.3 C?mol-1),is the concentration of H2O2(mol?L-1),Vis the solution volume(L),Irepresents applied current intensity(A),andtis the time(s).For Pd catalytic H2O2generation,the current efficiency for H2O2accumulation were calculated by assuming that 1 mol H2O2resulted from 1 mol O2and 4 mol electrons [11].

The O2theoretical production (OTP) was calculated using Eq.(3),whereIis anode current (A),tis the time (s),Fis the Faraday constant,nis the electron umber of oxygen evolution reaction(n=4),Vtis molar gas volume at 25°C (24.5 L?mol-1) [29].

The O2utilization efficiency(OUE)was calculated using Eq.(4),wheren(O2,OTP) is the amount of O2theoretical production in moles,n(O2,2e-reduction) is the amount of O2that is used for H2O2production,which is the same value of H2O2production in moles [29].

3.Results and Discussion

3.1.H2O2 electrogeneration by ACSS cathode

ACSS electrode was firstly used as cathode to investigate the effectiveness of AC as cathode materials for H2O2productionviaO2reduction.Constant-current mode was applied in the tests.Results shown in Fig.1 shows that up to 13.5 mg?L-1H2O2(CE of 4.6%) was obtained at 100 mA after 30 min,which confirms our hypothesis that AC can be used as cathode material for O2reduction.Moreover,the applicability of AC on H2O2formation also implies that PACSS cathode can generate H2O2viatwo pathways(discussed in Section 3.2).As Fig.1 shows,compared with 100 mA,lower currents (50 mA and 75 mA) doesn’t enable effective H2O2production.However,H2O2concentration at 50 min under 150 mA was only 3.7 mg?L-1(CE of 1.3%),indicating higher current doesn’t necessarily improve the H2O2production.The decreased H2O2yield could be ascribed to the invalid decomposition and/or activation of H2O2by several pathways in higher currents (e.g.,disproportion (Eq.(5)) [2,30],cathodic reduction (Eq.(6))[3,4],and anodic oxidation(Eq.(7))[2]),which was systematically studied in our previous work [3].

Fig.1.H2O2 electrogeneration by ACSS cathode.(a)The H2O2 yield under different current intensity,(b)mechanism of H2O2 formation from anodic O2 and its decomposition pathways at high current intensity.Conditions:180 ml,1.0 g AC,350 r?min-1,0.05 mol?L-1 Na2SO4,initial pH of 7,room temperature.

3.2.Compare of H2O2 electrogeneration by Pd catalyst,ACSS cathode,and PACSS cathode

In conventional Pd-catalytic EF process,Pd catalyst is dispersed in solution [31,32].Here,Pd catalyst (Pd/AC) was fabricated to PACSS cathode,and the performance of Pd catalyst,ACSS cathode,and PACSS cathode on H2O2formation were compared(Fig.2a),the corresponding OUE was calculated and shown in Fig.2b.In conventional Pd-catalytic EF process,H2O2concentration reached 7.5 mg?L-1at 15 min and then decreased to 4.6 mg?L-1at 50 min.H2O2concentration generated by ACSS cathode is 12.2 mg?L-1(CE of 4.2%) at 50 min,which was 166.9% higher than Pd catalyst.Moreover,as we expected,PACSS cathode generated more H2O2than Pd catalyst or ACSS cathode (248.2% higher than Pd catalyst at 50 min).The OUE calculated based on Eqs.(3) and (4) was 3.2%,8.3%,and 10.8% in system with Pd catalyst,ACSS cathode,and PACSS cathode,respectively.This is because only a small proportion of O2from water electrolysis was used for H2O2formation in conventional Pd-catalytic EF process,where the excessive O2could be used for H2O2formationviaO2reduction by AC support.Therefore,it is obvious that by reinventing the utilization method of conventional Pd catalyst,a drastically enhanced H2O2concentration and higher OUE could be obtained,making the Pd-catalytic EF process more cost-effective and competitive.The OUE was significantly improved,especially compared with system whose O2was supplied by pure O2/air aeration (OUE<0.1%) [10,33].

Fig.2.H2O2 generation by Pd catalyst,ACSS cathode,and PACSS cathode.(a) The H2O2 yield by Pd catalyst,ACSS cathode,and PACSS cathode,(b) comparison of oxygen utilization efficiency(OUE),(c)different mechanisms of H2O2 generation in 3 processes(Pd catalyst:combination reaction of H2 and O2,ACSS:cathodic O2 reduction,PACSS:both the two pathways).Conditions:180 ml,1.0 g catalysts,350 r?min-1,100 mA,0.05 mol?L-1 Na2SO4,initial pH of 7,room temperature,Ti/MMO cathode(for Pd catalyst).

3.3.H2O2 activation and ·OH generation by PACSS

H2O2catalytic decomposition for·OH generation by AC has long been confirmed as an effective approach for H2O2heterogeneous activation[26,34,35].By applying PACSS cathode,the AC can serve as H2O2activator to replace Fe2+,which avoids the forming of iron sludge and its deactivation on Pd catalyst.Here,H2O2activation and ·OH generation in system with Pd/Al2O3,Pd/AC,and AC under neutral pH were evaluated,results were shown in Fig.3.At initial H2O2concentration of 0.5 mmol?L-1,26.3%,72.5%,and 94.0% H2O2was decomposed at 50 min by AC,Pd/Al2O3,and Pd/AC,respectively.Fluorescence detection results shown in Fig.3b demonstrated that Pd/Al2O3could catalyze H2O2to generate ·OH,however,its ability is obviously inferior than Pd/AC,which implied that the selective decomposition of H2O2into ·OH was majorly from the contribution of AC,but not Pd.In Fig.3b,it is obvious that the catalytic ability of Fe2+on transforming H2O2to ·OH is a lot higher than both Pd/AC and Pd/Al2O3.However,considering that the application of Pd/AC avoids the use of iron and its potential drawbacks(i.e.,iron sludge,deactivation to Pd),its catalytic ability towards H2O2for ·OH generation is acceptable.

3.4.Improved degradation on model pollutant RB19 and stability

In this part,EF process enabled by PACSS cathode was applied to examine its applicability on model organic pollutants RB19 degradation.Results of RB19 removal efficiency within 60 min are shown in Fig.4.Before running EF process,ACSS and PACSS electrode was pre-saturated with 100 mg?L-1RB19 solution (internal figure in Fig.4a),thus the removal of RB19 is not resulted from adsorption,but from EF process.As can be seen,EF process enabled by PACSS cathode and ACSS cathode performed a lot better than conventional Pd-catalytic EF process,where 71.2%,55.5%,and 21.8% RB19 was removed at 60 min using PACSS,ACSS,and Pd/Al2O3,respectively.This result was in accordance with the H2O2generation,H2O2activation,·OH generation shown in Figs.2 and 3.

Even Fe2+is superior upon H2O2activation for ·OH generation than AC and Pd,the low RB19 removal efficiency should be ascribed to the relatively low H2O2concentration in conventional Pd-catalytic EF process.Moreover,PACSS cathode was used for H2O2generation continuously for 450 min(9 cycles).Then,its performance on H2O2generation,H2O2activation,and RB19 removal was evaluated and shown in Fig.4b and Fig.4c.We observed that its performance on H2O2generation decreased,and its performance on H2O2activation kept stable with pristine electrode.These results led to a slightly decreased removal efficiency of RB19(inset Fig.4c),showing that the proposed EF process enabled by PACSS cathode is very promising for organic pollutants degradation under neutral pH.

Fig.3.H2O2 activation and ·OH generation by PACSS.(a)Catalytic H2O2 decomposition by AC,Pd/AC,and Pd/Al2O3,(b)detection of hydroxyl radicals by benzoic acid methods.Conditions:180 ml,1.0 g AC,1.0 g Pd/AC,2.0 g Pd/Al2O3,350 r?min-1,0.5 mmol?L-1 H2O2,10 mmol?L-1 benzoic acid (for (b)),10 mg?L-1 Fe2+ (for system with Pd/Al2O3 catalyst).

Fig.4.Assessment of EF process enabled by PACSS on RB19 removal.(a)Removal of RB19 by EF process with ACSS cathode,PACSS cathode,and Pd/Al2O3 catalyst,(b)stability of PACSS cathode for H2O2 electrogeneration,c stability of PACSS cathode for H2O2 activation and RB19 removal.Conditions:180 ml,1.0 g AC,1.0 g Pd/AC,2.0 g Pd/Al2O3,350 r?min-1,100 mA,0.05 mol?L-1 Na2SO4,initial pH of 7,30 mg?L-1 RB19,20 mg?L-1 Fe2+ (for Pd/Al2O3 catalysts),room temperature.

4.Conclusions

This work proposed a new electrochemical advanced oxidation process (EAOP) that upgrading the conventional Pd-catalytic EF process.Compared with the Pd-catalytic EF process,H2O2production by PACSS cathode proposed in this work is more efficient.H2O2was generated from H2and O2on Pd catalyst,as well as from O2reduction on AC.Furthermore,PACSS was confirmed to be effective upon H2O2decomposition towards ·OH formation.Finally,the EF process enabled by PACSS cathode was tested to be effective on RB19 removal.Long-term stability tests also show that the PACSS cathode can maintain its performance on both H2O2electrogeneration and catalytic activation.

CRediT Authorship Contribution Statement

Wei Zhou:Conceptualization,Investigation,Writing -original draft.Xiaoxiao Meng:Investigation,Visualization,Resources,Writing-review&editing.Liang Xie:Writing-original draft.Junfeng Li:Visualization,Validation.Yani Ding:Visualization,Validation.Yanlin Su:Visualization.Jihui Gao:Writing -review &editing.Guangbo Zhao:Writing -review &editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Nos.52006049,51776055) and the China Postdoctoral Science Foundation (Nos.2019M661293,2020T130149).The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Natural Science Foundation of China or the China Postdoctoral Science Foundation.