Removals of aqueous sulfur dioxide and hydrogen sulfide using CeO2-NiAl-LDHs coating activated carbon and its mix with carbon nano-tubes

Removals of aqueous sulfur dioxide and hydrogen sulfide using CeO2-NiAl-LDHs coating activated

carbon and its mix with carbon nano-tubes

Item Type Article

Authors Li, Jing;Chen, Fangping;Jin, Guanping;Feng, Xiaoshuang;Li, Xiaoxuan

Citation Li, J., Chen, F. P., Jin, G.-P., Feng, X.-S., & Li, X.-X. (2015).

Removals of aqueous sulfur dioxide and hydrogen sulfide using CeO2-NiAl-LDHs coating activated carbon and its mix with carbon nano-tubes. Colloids and Surfaces A:

Physicochemical and Engineering Aspects, 476, 90–97.

doi:10.1016/j.colsurfa.2015.03.026 DOI 10.1016/j.colsurfa.2015.03.026

Publisher Elsevier BV

Journal Colloids and Surfaces A: Physicochemical and Engineering Aspects

Download date 2023-12-03 17:31:58

Link to Item http://hdl.handle.net/10754/564189

ContentslistsavailableatScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects

jo u r n al ho me p ag e :w w w . e l s e v i e r . c o m / l o c a t e / c o l s u r f a

Removals of aqueous sulfur dioxide and hydrogen sulfide using CeO 2 -NiAl-LDHs coating activated carbon and its mix with carbon nano-tubes

Jing Li

^a

, Fang Ping Chen

^a

, Guan-Ping Jin

^a,∗

, Xiao-Shuang Feng

^b

, Xiao-Xuan Li

^a

aAnhuiKeyLabofControllableChemicalReaction&MaterialChemicalEngineering,SchoolofChemistryandChemicalEngineering, HefeiUniversityofTechnology,Hefei230009,China

bDivisionofPhysicalSciencesandEngineering,KingAbdullahUniversityofScienceandTechnology(KAUST),Thuwal23955-6900,SaudiArabia

h i g h l i g h t s

•Mixture of activated carbon and nano-tuabes support with good adsorptionandconduction.

•NiAl/layered double hydroxide catalyst-sorbent.

•CeO2isusedasapromoterinredox reactions.

•Ce-doped NiAl/layered dou- ble hydroxide/activated carbon catalyst-sorbents.

•Removalof aqueous sulfur dioxide andhydrogensulfideusingadsorp- tionandelectrochemicalmethod.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Articlehistory:

Received29December2014

Receivedinrevisedform14March2015 Accepted19March2015

Availableonline26March2015

Keywords:

Sulfurdioxide Hydrogensulfide Catalyst-sorbent Layereddoublehydroxides Activatedcarbon

a b s t r a c t

Ce-dopedNiAl/layereddoublehydroxidewascoatedatactivatedcarbonbyureahydrolysismethod (CeO2-NiAl-LDHs/AC)inonepot,whichwascharacterizedbyX-raydiffraction,infraredspectra,field emissionscanningelectronmicroscopeandelectrochemicaltechniques.CeO2-NiAl-LDHs/ACshowsgood uptakeforaqueoussulfurdioxide(483.09mg/g)andhydrogensulfide(181.15mg/g),respectivelyat 25^◦C.Meanwhile,theelectrochemicalremovalsofaqueoussulfurdioxideandhydrogensulfidewere respectivelyinvestigatedatthemixofCeO2-NiAl-LDHs/ACandcarbonnano-tubesmodifiedhomed paraffin-impregnatedelectrode.Bothsulfurdioxideandhydrogensulfidecouldbeeffectivelyoxidized tosulfuricacidat1.0Vinalkalineaqueoussolution.

©2015ElsevierB.V.Allrightsreserved.

∗Correspondingauthor.Tel.:+8655162901450;fax:+8655162901450.

E-mailaddress:jgp@hfut.edu.cn(G.-P.Jin).

1. Introduction

Hydrogen sulfide (H₂S)and sulfur dioxide(SO₂)oftencome fromprocessstreamsofindustryproduction,processingandrefin- ingoffossilfuels[1,2].Emissionsofthesecompoundscouldform http://dx.doi.org/10.1016/j.colsurfa.2015.03.026

0927-7757/©2015ElsevierB.V.Allrightsreserved.

acidrainanddestructtheozonelayer[3].Besides,H₂Sisacor- rosivegas towardspipelines and equipment,as wellas one of main poisons for many industry catalysts [4]. Therefore, their treatmentsaresignificantintermsofindustry,environmentand humanhealth.Amongvarioustreatmentmethodsincludingoxi- dation[2,4,5],reduction [6],adsorption[1,3,7]andprecipitation [8],boththeoxidationandadsorptionareeffectiveways.Many metalbasedoxidecatalystssuchastitanium[9],chromium[10], iron[11]andvanadium[12–14]wereproposedfortheselective oxidationofH₂Stoelementalsulfur.Butthedeactivationforthe catalystslosttreatmentefficiencyandeconomicadvantage.Mean- whileactivatedcarbon wasusedasadsorbent for H2Sand SO2

removals[15,16].Furthersomecatalyst-sorbentsformedbyacti- vatedcarbonandmetaloxidesincludingV[12],Mn[17],Fe,Co,Ni, CeandCu[18]weredevelopedtoenhancetheremovalsRecent reports showedthat layereddouble hydroxides(LDHs) suchas Ni/Al-LDHs[1,19],CeO₂/MgAl-LDHs[20] andMgFeAl-LDHs [21]

arebettercatalyst-sorbentsforSO2andH2Sremovals.Buttheir micronsizescouldresultindifficultiesinregenerationandsepara- tion.Moreover,ceriumoxide(CeO₂)isusedasapromoterinvarious redoxreactionsdue toitsreducibility andhighoxygenstorage capacity[16,22].Therefore,Ce-dopedNiAl-LDHs/activatedcarbon compositemaybeapromisingcatalyst-sorbentfortheremovals, buttherehasnotbeenanyreportuptonow.

On the other hand, electrochemical oxidation treatment offers an environmentally attractive method to remove SO₂ and H₂S [23–33,6,34,11,35–37]. Many researches have related the redox of SO2 and oxidation of H2S (sulfide) [23]. SO2

couldbeoxidized tosulfuric acid[24],andreduced toelemen- tal sulfur [23] or polysulfides [25]. The oxidation product of H2S (S²⁻) could be elemental sulfur [27–29,11], SO32− [30,33]

or a mix of S, S₂²⁻ and SO₄²⁻ [32]. The electrode materi- als involved in carbon [24], Pt [23,25], Au [31] Ti/Ta₂O₅-IrO₂ [32], V2O5 [27], B [29], [SbVO(CHL)2]Hex [30], ferrocyanide [6], cobalt pentacyanonitrosylferrate [34], Fe and Ti mix metal oxide[11],2,6-dichlorophenolindophenol [33],N,N-diphenyl-p- phenylenediamine[35],2-(4-fluorophenyl)indole-modifiedxero- gel [36] and hematoxylin [37]. Since the removals could be performedatatmospherictemperature,atmosphereandaqueous, thesemakeelectrochemicalmethodveryattractivecandidatesfor theremovalsofaqueoussulfurdioxideandhydrogensulfide.

In this work,Ce-dopedNiAl-LDHs/activated carboncompos- ite (CeO₂-NiAl-LDHs/AC) was prepared by ureamethod in one pot. The removals of aqueous SO2 and H2S were respectively investigatedbyadsorptionandelectro-oxidationmethodsusing CeO₂-NiAl-LDHs/AC and itsmix with carbon nano-tubesmodi- fiedparaffin-impregnatedelectrode(CeO2-NiAl-LDHs/NAC/WGE).

Here,carbonnano-tubes(CNTs)couldimprovetheconductivityof CeO₂-NiAl-LDHs/AC.

2. Experiment

2.1. Chemicalsandapparatus

Activated carbon (AC) was obtainedfrom Hengxin Environ- mentalProtectionMaterialCompanyofHuaibeiCity(HuaibeiCity, China).Carbonnanotubes(CNTs)werepurchasedfromSunNan- otech.Co.Ltd.ofChinaandweresynthesizedbycatalyticdecompo- sitionofCH4onaNiMgOcatalyst.Ni(NO3)2·6H2O,Al(NO3)3·9H2O, Ce(NO₃)₃·6H₂O andall otherchemicalswere ChemicalReagent CompanyofShanghaiproducts(China,Shanghai).Doublydistilled waterwasusedtoprepareallsolutions.

All electrochemical experiments were performed with a CH660Belectrochemicalworkstation(Chenhua,Shanghai,China).

Aconventionalthree-electrodeelectrochemicalsystemwasused

forallelectrochemicalexperiments,whichconsistedofahomed paraffin-impregnatedelectrode (WGE),atwisted platinumwire counter electrode, and a saturated calomel reference electrode (SCE).AllpotentialsreportedareversusSCE.Fieldemissionscan- ningelectron microscope(FE-SEM) imageswere obtainedona JSM-600 field emission scanning electron microanalyser (JEOL, Japan).X-raydiffraction(XRD)dataofthesampleswerecollected using a RigakuD/MAX-rB diffractometerwith Cu Karadiation.

Infraredspectra(IR)weremeasuredatIR200(NicoletAmerica).

2.2. PreparationofCeO2-Ni-Al/LDHs/AC

AC was firstly coated with tetraoxalyl ethylenediamine melamineresintoadheretheLDHs(labeledMFT/AC)[38].10g ACwasimpregnatedin 100mLof mixedacidsolutionof nitric acidandperchlorateacid(7:3).Themixedsolutionwasultrason- icallyagitatedfor7htoformcarboxyl-activatedcarbon;theAC waswashedwithdistilledwatertoneutralpH,anddriedinvac- uumat100^◦Cwith4h.In100mLflask,the5gACwerequickly addedin50mL1mMethylenediamine.Ethylenediaminemodified ACcouldbeobtainedwithamidationreactionoradsorption.Itwas centrifuged,washedwithacetoneanddistilledwater,anddried invacuumat40^◦Cin6h.5gethylenediaminemodifiedACwas quicklyputin50mLaqueoussolutionincluding1.5gtetraoxalyl ethylenediamineand1.8gmelamine,20mL15%formaldehydeand 0.1gsodiumdodecylsulfatewithdippingof4h,finallytetraoxalyl ethylenediaminemodifiedmelamineresincouldbeevenlycoated atthesurfaceofACunderultrasonicof1hat25^◦C. Itwascol- lected,washedthoroughlywithhotwater,coldwater,ethanoland acetone,anddriedinvacuum(signedMFT/AC).

CeO2-NiAl/LDHs/AC was prepared by urea hydrolysis method [1]. 50.0mL of a solution (M²⁺/M³⁺=2, Al³⁺/Ce³⁺=4, Ni²⁺+Al³⁺+Ce³⁺=0.06M,urea/M=4) wasprepared. 3gMFT/AC wasvacuumedin4htodecreaseinnerpressure,andquicklyputin thesolution.MFT/ACcouldbefullydippedinthesolutionwithless timeunderanaidofpressureatmospherein1h.Thenthemixture wasaddedintoathree-neckflaskandstirredat298K.Theflask wassoakedinanoilbathpreviouslyheatedat363Ktostartthe hydrolysisreaction.Thereactionwasstoppedbyquenchingitin acold-waterbathafter48hunderstirring.Theresultingmaterial waswashedseveraltimeswithdeionizedwater,anddriedat323K (labeledasCeO₂-NiAl-LDHs/AC).

2.3. SpecificsurfaceareaandporepropertiesofAC

Thespecificsurfaceareaandporestructureofthecarbonsam- plesweredeterminedbyN2 adsorption–desorptionisothermsat 77K(MicrometricsASAP2020system)afterbeingvacuum-dried at100^◦Covernight.Thespecificsurfaceareaswerecalculatedby theconventionalBET(Brunauer–Emmett–Teller)method.Thepore sizedistribution(PSD)plotswererecordedfromtheadsorption branchoftheisothermbasedontheBarrett–Joyner–Halenda(BJH) model.

2.4. Batchofadsorptionstudiesofaqueoussulfurdioxideand hydrogensulfide

Batchexperimentsincludedadsorptionequilibriumisotherm, adsorptionkineticandtemperaturewereperformed.Thepurpose wastoinvestigatetheuptakeofthecompositetowardsaqueous sulfurdioxideandhydrogensulfide.

Sulfurdioxide(hydrogensulfide)aqueoussolutionswerepre- paredbyaddingadequateamountsofsolidNa2SO3(Na2S)orby injectingacertainvolumeofanSO₂(H₂S)+0.5MHC1O₄solution intothetestsolution[23].Forthedeterminationoftheamount H2S(SO2)intheliquidsample,anexcessamountofiodinewas

Fig.1.SEMofAC(A,inset),CeO2-NiAl/LDHs/AC(A)and(B),mixofCeO2-NiAl/LDHs/ACwithcarbonnano-tubes(C).

addedtoreactwithH₂S(SO₂)inanacidsolution.Theamountof excessiodinewasdeterminedbytitrationusingsodiumthiosul- fate(starchasindicator)[2,37].Thedeterminationbasedfollowing reactions(1)–(3).

I₂+S²⁻↔S+2I⁻; (1)

2I2+4SO32−↔ 3SO42−+4I⁻; (2) I2+S2O32−↔ S4O62−+I⁻. (3) 2.5. Electrochemicaloxidationofaqueoussulfurdioxideand hydrogensulfide

AfterCeO₂-NiAl-LDHs/ACwasgroundintopowder,itwasmixed withcarbonnano-tubs(CNTs)toimprovetheconductivity.1gmix- turewasdispersedin25mLethanoland0.5wt.%nafionwithan aidofultrasonicagitationtogivetheblacksuspension.WGEwas polishedstep-by-steptoamirror-likefinishwithfinewetemery paper(grainsize400,800),followedbysonicationinethanoland waterfor 15min,respectively. Aftercleaning, 30␮L suspension wasdirectlycastatWGEandevaporatedinthesolventatroom temperature(labeledasCeO2-NiAl-LDHs/NAC/WGE).

Electrochemical oxidation of aqueous sulfur dioxide(hydro- gensulfide)wasinvestigatedusingcyclicvoltammograms(CVs) orpotentiostaticmethodatCeO2-NiAl-LDHs/NAC/WGE in0.1M NaOH.Alltheelectrochemical experimentswerecarried out at roomtemperatureinN₂atmosphere.

3. Resultanddiscussion

3.1. CharacterizationofCeO2-NiAl-LDHs/AC

Fig.1showsFE-SEMofAC(A,inset),CeO₂-NiAl/LDHs/AC(A)and (B)andmixofCeO2-NiAl-LDHs/ACwithcarbonnano-tubes(C).The originACgivesaroughandporoussurfaceinFig.1A,inset.After CeO₂-NiAl-LDHs/ACwascoatedatAC,itcouldbereadilyseenthat petal-likestructureswithmanycavitiesevenlydistributedatthe surfaceofACinFig.1AandB.MeanwhileCNTscouldbeobserved inmixofCeO₂-NiAl-LDHs/ACwithcarbonnano-tubesinFig.1C.

Fig.2showsXRDpatternsofAC(a)andCeO₂-NiAl-LDHs/AC(b).

Thecharacteristicpeakat23^◦matchestoACincurvea.Ahighly crystallineLDHphasecouldbeseenincurveb,the(003),(006), (009),(110)and(113)diffractionpeaks,whichareevidencesfor thelayeredstructure[34],appearat11.2^◦,23.8^◦,34.9^◦,60.3^◦and 62.2^◦,respectively.Furthermore,otherpeaks,locatedat2≈28^◦, 35^◦(overlapwiththeLDHs),48^◦and57^◦correspondtothe(111), (200),(220)and(311)reflectionofCeO2incubicfluoritestructure [41].Thus,ahighlycrystallineCeO₂-NiAl-LDHs/ACwasacquired.

Fig.3 showsFT-IRspectrumsofacidtreated AC(a),MFT/AC (b) and CeO2-NiAl-LDHs/AC (c). In curvea, the carbonyl(C O)

stretchat1630cm⁻¹ couldbeseenatacidtreatedAC.Incurve b, the peak at 1630cm⁻¹ matches to carbonyl (C O) stretch;

1389–1296cm⁻¹ and 871cm⁻¹ relate to melamine, which are caused by the framework vibration and the out-of-plane ring deformation;3317cm⁻¹,1630cm⁻¹,1430cm⁻¹,1156cm⁻¹ and 717cm⁻¹ could betraced back to tetraoxalyl ethylenediamine, whichareduetotheN–Hstretchattachedtoethylenebridgeand N-Hbendbridginginthesecondaryamine(3413cm⁻¹,1455cm⁻¹ and717cm⁻¹),N–C O(1156cm⁻¹)andtheC Oand–OHinCOOH (1630cm⁻¹,1430cm⁻¹)[38].Incurvec,abroadadsorptionband around3440cm⁻¹relatestotheO–Hstretchingvibrationsofinter- layerwatermoleculesandhydroxylgroupsofhydrotalcitelayers.

Theabsorptionbandat1380cm⁻¹ is attributedtotheantisym- metric stretching vibrationsof interlayer carbonategroup [39].

ThischaracteristicpeakindicatesthatanionsofLDHsconsistof carbonatebesidesthehydroxylions.Theabsorptionbandatnear 1630cm⁻¹isoriginatedbythebendingmodeofinterlayerwater moleculesand M–OHvibration inthebrucite-likelayers ofthe LDHs,respectively [40].In thelow-frequencyregion,thebands at975cm⁻¹,786cm⁻¹areascribedtothelatticevibrationmodes attributedtoM–OandO–M–Ovibrations[16,22,41].

60 40

20 0 600 1200 1800

*CeO2

∗ ∗ ∗

intensity (a.u.)

2θ (degree)

003 006 009 110 113∗

a b

Fig.2.XRDpatternsofAC(a)andCeO2-NiAl-LDHs/AC(b).

800 1600 2400 3200 54 60 66 72

78 c

b

Transmittance%

Wavenumbers (cm-1)

717

14301630

3317 1630 1156

3440 13801630 975 786

a

Fig.3.FT-IRspectrumsofacidtreatedAC(a),MFT/AC(b)andCeO2-NiAl/LDHs/AC (c).

Table1

SpecificsurfaceareaandporepropertiesofAC,MFT/ACandCeO2-NiAl/LDHs/AC.

Sample Specificsurface

area(m²/g)

Porevolume (cm³/g)

Averagepore diameter(nm)

aAC 743.19 0.063 4.65

MFT/AC 333.13 0.055 3.76

CeO2-NiAl/LDHs/AC 110.12 0.045 2.74

aAcidtreatedcoalactivecharcoal.

300 200 100 0

0 140 280 420

9 6 3 0 0 40 80 120

-1 q t / mg⋅g

t / h -1 q t / mg⋅g

t / min

Fig.4.EffectoftimeontheadsorptioncapacitiesofSO2andH2SatCeO2-NiAl- LDHs/ACfrominitialconcentration0.1M.

The physical and textural properties of CeO2-NiAl-LDHs/AC were furtherinvestigated and theresults weresummarized in Table1.Theaverageporesizesof27.38Aiscomparativetoori- ginAC and MFT/ACwithobvious decrease; thesurface areaof 110.12m²g⁻¹ for CeO₂-NiAl-LDHs/AC is significantly decreased compared tothat ofAC andMFT/AC. Theseillustrated thatthe CeO2-NiAl-LDHsprobablycoatedattheoutandinnersurfaceof AC.

3.2. Abatchadsorptionexperiments

Inordertoinvestigatetheabsorptionpropertyofthecompos- ite,batchadsorptionexperimentswereperformed.Theuptakesof aqueoussulfurdioxideandhydrogensulfidewereinvestigatedat CeO₂-NiAl-LDHs/ACwith0.1Minitialconcentration,respectively.

Adsorptionkinetics: AsshowninFig.4,theeffects ofcontact timeontheadsorptioncapacities ofaqueoussulfurdioxideand hydrogensulfide were illustrated atthe composite from initial concentration0.1Mat25^◦C.Theremovalsincreaserapidlyatthe initialstageofadsorption.Maximumadsorptioncapacitiesrespec- tively reachwithin 3hfor SO2 and 4hfor H2S. These are due tothedecreaseofadsorptionsitesatCeO₂-NiAl-LDHs/AC,which graduallyinteractedwiththem.In present experiments,SO₂ of 462.4mgg⁻¹andH2Sof116.8mgg⁻¹wereadsorbedatCeO2-NiAl- LDHs/ACin4h.

Twokinetic models such aspseudo-first-order and pseudo- second-orderareabletointerprettherateandmechanismofeach

adsorptionprocess.Thepseudo-first-ordermodelwasdescribed empiricallybyLagergrenEq.(4)andtheoreticallybyAzizianEq.

(5).

log(qe1−qt)=logqe1− k1t

2.303 (4)

t qt = 1

k2q²_e2+ t qe2

(5)

where qe is the amount adsorbed at equilibrium, qm is the maximum adsorption capacity of the adsorbent, t is adsorp- tion time, k₁ and k₁ are the pseudo-first-order constant and pseudo-second-order constant. The pseudo-first-order constant (k1), pseudo-second-order constant (k2), and linear correlation coefficient(R²)werelistedinTable2.Thepseudo-second-order modelseemstobestdescribetheadsorptionkineticsofSO₂and H2S.Whenthepseudo-second-ordermodelisthebestfitforthe experimentaldata,thesorptionmechanisminvolveschemisorp- tion.

Adsorptionisotherms:Theequilibriumdatawereanalyzedusing theLangmuir,Freundlich,Temkin,andDubinin–Radushkevichin Eqs.(6)–(9)equilibriummodelsinordertoobtainthebestfitting isotherm.

Ce

qe = 1 KLqm+ Ce

qm

(6)

lnqe=lnKF+1

n lnCe (7)

qe=RTlnkt

bt +RTlnCe

bt

(8)

lnqe=lnqD−2BDRTln(1+1/Ce) (9) whereK_ListheLangmuirisothermparameter,K_FistheFreundlich isothermparameter,bt andkt areTemkinisothermparameters, RisthegasconstantandTistheabsolutetemperature.q_eisthe amountadsorbedatequilibrium,qmisthemaximumadsorption capacityoftheadsorbent,andnistheheterogeneityparameterof theadsorbentsurface.B_Disrelatedtothefreeenergyofsorptionper moleofthesorbate,andq_DistheDubinin–Radushkevichisotherm constantrelatedtothedegreeofsorbatesorptionbythesorbent surface.TheadsorptionisothermsforSO2andH2Satdifferenttem- peraturesareshowninFig.5,andthecorrespondingparametersof adsorptionisothermsarepresentedinTable3.Theresultsindicate thattheLangmuirisothermsarewellfittodescribetheSO2and H₂SadsorptionequilibriaatCeO₂-NiAl-LDHs/AC.Inpresentresults, themaximumadsorptioncapacities(qm)are483.09–510.20mgg⁻¹ forSO2,and181.15–197.23mgg⁻¹forH2Srespectively,whilethe temperaturevariesfrom25^◦Cto45^◦C.Themaximumadsorbed capacitiesareincreasedastheadsorptionsystemtemperaturerose, whichmeanthattheincreaseinenergyfavoredtheadsorptionat

Table2

Parametersofthepseudofirst-orderandpseudosecond-orderfortheadsorptionsofaqueousSO2andH2SatCeO2-NiAl-LDHs/AC.

InitialC(mM) qe,exp(mg/g) Pseudo-first-order-constant Pseudo-second-order-constant

q_e1,cal(mg/g) k1(g/mgh) R² q_e2,cal(mg/g) K2(g/mgh) R²

SO32− 20 391.2 1229.1 0.0314 0.958 568.2 1.78E−05 0.973

40 405.6 1112.9 0.0307 0.972 552.5 2.25E−05 0.983

60 424.0 1038.7 0.0302 0.955 558.7 2.54E−05 0.988

80 442.4 1124.7 0.0308 0.973 568.2 2.78E−05 0.995

100 462.4 793.8 0.0284 0.981 581.4 2.99E−05 0.992

S²⁻ 20 104.64 274.7 0.932 0.965 146.4 0.00257 0.982

40 106.24 296.8 0.979 0.975 146.2 0.00273 0.984

60 108.8 282.8 0.967 0.943 142.2 0.00342 0.992

80 112.64 243.3 0.906 0.967 139.3 0.00442 0.997

100 116.8 210.5 0.905 0.981 156.9 0.00279 0.986

75 50 25

0 40 80 120 160 B

qe / mg/g

Ce / mM 75

50 25 0

0 150 300 450 A

qe/ mg/g

C / mM

Fig.5.AdsorptionisothermofSO2(A)andH2S(B)atdifferenttemperatures(25^◦C,•35^◦C,and45^◦C;–Langmuir,---Freundlich).

Table3

DifferentmodelparametersfortheadsorptionofaqueousSO2andH2SatCeO2-NiAl-LDHs/AC.

T(^◦C) SO2 H2S

qm(mg/g) KL(L/mM) R² qm(mg/g) KL(L/mM) R²

Langmuir 25 483.09 0.07 0.999 181.15 0.023 0.989

35 502.51 0.10 0.998 186.91 0.037 0.990

45 510.20 0.15 0.997 197.23 0.047 0.993

T(^◦C) SO2 H2S

1/n KF R² 1/n KF R²

Freundlich 25 0.44517 65.85 0.986 0.6159 8.61 0.982

35 0.38581 92.32 0.988 0.50175 16.16 0.994

45 0.31486 130.58 0.992 0.49485 19.46 0.993

T(^◦C) SO2 H2S

kt(L/mg) bt(kJ/mol) R² kt(L/mg) bt(kJ/mol) R²

Temkin 25 0.025 24.38 0.994 0.048 65.95 0.985

35 0.032 26.22 0.990 0.055 68.12 0.983

45 0.041 30.67 0.986 0.065 63.42 0.985

T(^◦C) SO2 H2S

qD(mg/g) BD(mol²/kJ²) R² qD(mg/g) BD(mol²/kJ²) R²

Dubinin–Radushkevich 25 407.04 0.001089 0.979 136.24 0.00304 0.989

35 426.75 0.000709 0.968 132.35 0.001396 0.953

45 435.48 0.000388 0.953 150.38 0.001228 0.959

thesurfaceofCeO₂-NiAl-LDHs/AC,andthesorptionprocesshasan endothermiccharacter.

Thermodynamicsofadsorption:TheAdsorptionthermodynamic parametersincludingGibbsfree energychange(G^◦),enthalpy change(H^◦)andentropychange(S^◦)werecalculatedtoeval- uatethethermodynamicfeasibilityand thespontaneousnature of the adsorption process. The thermodynamic constants were obtainedfromEq.(10).

lnKL=−G RT =−H

RT +S

R (10)

whereG^◦isthechangeinfreeenergy(kJ/mol),H^◦isthechange in enthalpy (kJ/mol), S^◦ is the change in entropy (J/(molK)), T is the absolute temperature in kelvin, R is the gas constant (8.314×10⁻³J/(molK)),andKisthethermodynamicequilibrium constant.ThevaluesofH^◦andS^◦werecalculatedfromtheslope andinterceptoftheplotoflnKversus1/T.Thethermodynamic parametersfortheadsorptionarepresentedinTable4.Thepositive valueofH^◦ confirmstheendothermicnatureoftheadsorption process,whilethepositiveS^◦valuesuggestsanincreaseinthe randomnessatthesolid/solutioninterfaceduringtheadsorptionof

SO₂andH₂SatCeO₂-NiAl-LDHs/AC.Thereforetheuptakeincreases withtheincreaseintemperature.

3.3. Electrochemicaloxidationofaqueoussulfurdioxideand hydrogensulfide

Fig.6shows theCVs of0.1Maqueous sulfurdioxideat rel- ativematerialsin0.1MNaOH.InFig.6A, apairofredoxpeaks (0.35/0.60V) correspond to the conversion between Ni(OOH) and Ni(OH)₂ at CeO₂-NiAl-LDHs/NAC/WGE in 0.1M NaOH (b)

Table4

ThermodynamicparametersofaqueousSO2 andH2SadsorptionatCeO2-NiAl- LDHs/AC.

T(K) G(kJ/mol) H(kJ/mol) S(Jmol⁻¹K⁻¹)

SO2 298 −10.53 29.99 135.91

308 −11.79 29.99 135.62

318 −13.25 29.99 135.93

H2S 298 −8.29 28.24 114.84

308 −9.55 28.24 118.80

318 −10.18 28.24 120.78

0.75 0.50 0.25 0.00 0.0 0.5 1.0 1.5

0.75 0.50 0.25 0.00 0.0 0.5 1.0 1.5

0.4 0.2

0.0 -0.2 0.4 0.5 0.6 0.7

10.0 7.5 5.0 2.5 0.4 0.5 0.6 0.7

i/ mA

E / V vs. SCE a b A c

i / mA

E / V vs. SCE ab

c B d

i/ mA

E / V vs. SCE C

i / mA

t / min D

Fig.6. (A)CVsofSO2atWGE(a)andCeO2-NiAl-LDHs/NAC/WGE(c).Curveb:CVofCeO2-NiAl-LDHs/NAC/WGEin0.1MNaOH.(B)CVsofSO2atWGE(a),CNTs/WGE (b),CeO2-NiAl-LDHs/WGE(c)andCeO2-NiAl-LDHs/NAC/WGE(d).(C)OxidationpeakcurrentsofSO2dependonconcentrationpotentialinconcentrationtime5min.(D) OxidationpeakcurrentsofSO2dependonconcentrationtimeat0.0V.System:0.1MNaOH.SO2concentration:0.1M.

[42]. An obvious oxidation peak (0.7V) could be observed at CeO2-NiAl-LDHs/NAC/WGE (c)compared tothat of WGE(a).In Fig. 6B, although the oxidation peak responses are obviously increasedatCNTs/WGE(b),CeO₂-NiAl-LDHs/WGE(c)andCeO₂- NiAl-LDHs/NAC/WGE(d)comparedtothatofWGE(a),itisnotable thattheoxidationpeakresponseatCeO₂-NiAl-LDHs/NAC/WGE(d) isthelargestamongthefourelectrodes.ItillustratesthatCeO₂- NiAl-LDHs/NAC/WGEshowsanexcellentelectrocatalysistowards the oxidation of aqueous sulfur dioxide. The reasons probably relatethecooperationofCeO₂-NiAl-LDHsandCNTs,theformerisa goodcatalyst-sorbent,andthelatterimprovetheconductivitywith goodadsorption.Meanwhile,theoptimumoxidationconditionsof

aqueoussulfurdioxidesuchasconcentrationpotentialandcon- centrationtime wereinvestigatedatCeO2-NiAl-LDHs/NAC/WGE in0.1MNaOH.Thepeakcurrentis increaseddependingonthe potentialfrom−0.2Vto0.0VinFig.6C.Thepeakcurrentobviously increasesfrom2minto5min,andreachestoaplatformafter6min inFig.6D,suggestingasaturationadsorption.Thus,0.0Vand5min couldbeselectedasoptimumconcentrationconditions.Further, theexhaustibleelectrochemicaloxidationof5mL0.1Maqueous sulfurdioxidewasinvestigatedatCeO₂-NiAl-LDHs/NAC/WGEby potentiostaticmethodat1.0Vwith3,9and12h.Beforetheelec- trochemicaloxidation,SO2 firstlyreactedwithNaOHwithform ofNaSO₃.SO₃^2-couldbedeterminedbythestandardiodometric

0.8 0.4 0.0 -0.4 0.0 0.7 1.4 2.1

0.90 0.45

0.00 -0.45

0.0 1.2 2.4 3.6

0.30 0.15 0.00 -0.15 0.3 0.4 0.5 0.6

8 6 4 1.0 2

1.2 1.4 1.6

i / mA

E / V vs. SCE

a

bc A

i / mA

E / V vs. SCE

a b c d B

i / mA

E / V vs. SCE C

i/ mA

t / min D

Fig.7. (A)CVsofH2SatWGE(a)andCeO2-NiAl-LDHs/NAC/WGE(c).Curveb:CVofCeO2-NiAl-LDHs/NAC/WGEin0.1MNaOH.(B)CVsofH2SatWGE(a),CNTs/WGE (b),CeO2-NiAl-LDHs/WGE(c)andCeO2-NiAl-LDHs/NAC/WGE(d).(C)OxidationpeakcurrentsofH2Sdependonconcentrationpotentialinconcentrationtime5min.(D) OxidationpeakcurrentsofH2Sdependonconcentrationtimeat0.0V.System:0.1MNaOH.H2Sconcentration:0.1M.

Table5

ComparisonsofSO2(H2S)atdifferentelectrodes.

R Material Desulfurizationefficiency(%)

[43] PbO2,ZnO/CFs 61.29%

[44] Pb/PbO2/CFs 55%

[45] ␤-PbO2/AC 87.09%

[46] CeO2/AC 83.87%

Thiswork CeO2-NiAl-LDHs/NAC/WGE 92.7%(SO2),84.1%(H2S)

method[2].Inpresentexperiment,52.4%,71.6%and92.7%ofSO32−

wasoxidizedtoSO42−.TheprocesscanbeexpressedinEqs.(11) and(12).

SO₂+H₂OSO₃²⁻+2H⁺ (11)

SO₃²⁻+2OH⁻→SO₄²⁻+H₂O+2e⁻ (12) Theelectrochemicaloxidationof0.1Maqueoushydrogensul- fide was followed the same performs in 0.1M NaOH. Fig. 7A showstheCVsofaqueoushydrogensulfideatWGE(a)andCeO2- NiAl-LDHs/NAC/WGE(c).Anobviousoxidationpeak(0.4V)could beseenatCeO₂-NiAl-LDHs/NAC/WGE (c)witha goodcatalysis.

Fig. 7B shows that the oxidation peak response at CeO2-NiAl- LDHs/NAC/WGE(d)isthelargestcomparedtothatof WGE(a), CNTs/WGE(b)andCeO₂-NiAl-LDHs/WGE(c),suggestinganexcel- lentelectrocatalysisandinlinewithFig.6.Fig.7Cshowsthatthe peakcurrentisincreasedwiththeadsorptionpotentialfrom−0.2V to0.0V.Fig.7Dshowsthatthecurrentresponseisthelargestat 5min.Therefore,theoptimalconcentrationconditionsare0.1V and5min.Theexhaustibleelectrochemicaloxidationschemeof 5mL0.1Maqueoushydrogensulfide wasinvestigated atCeO₂- NiAl-LDHs/NAC/WGEusingpotentiostaticmethodat1.0Vwith3, 9and12h.Beforetheelectrochemicaloxidation,H2Sfirstlyreacted withNaOHwithformofNa₂S.S²⁻ couldbedeterminedbythe standard iodometric method[2]. In present experiment, 43.8%, 71.9%and84.1%ofS²⁻wasoxidizedtoSO42−.Theprocesscanbe expressedinEqs.(13)and(14).

S²⁻+6OH⁻→SO32−+3H2O+6e⁻ (13) SO32−+2OH⁻→SO42−+H2O+2e⁻ (14) Moreover,theoxidationpeakcurrentsofaqueoussulfurdiox- ideandhydrogensulfidewereinvestigateddependingondifferent ratioCeO₂-NiAl-LDHs/NAC/WGE.WhentheratiosofCeO₂-NiAl- LDHsandCNTswere10:1,20:1and30:1,theresultsshowedthat thepeakcurrentsofaqueoussulfurdioxideandhydrogensulfide werelargerat20:1.Asacomparison,Table5summarizestheelec- trochemicaldesulfurization efficiencyusing differentelectrodes [43–46];wecanfindthatthedesulfurizationefficiencyatCeO2- NiAl-LDHs/ACiscomparablewiththeirwork.

4. Conclusions

CeO2-NiAl-layereddoublehydroxidewassuccessfullyprepared usingaureahydrolysismethodwhichwasuniformlydistributed onthesurfaceofACwithapetal-likestructure.CeO₂-NiAl-LDHs/AC compositewassuccessfullyusedfortheremovalsofaqueoussulfur dioxideandhydrogensulfidewithgooduptake.Theiradsorption processes were spontaneousand endothermic, which couldbe describedbypseudosecond-orderkineticsequationandLangmuir equation.ThemixofCeO2-NiAl-LDHs/ACand CNTs(m:m,20:1) showsexcellentelectrocatalysistowardstheoxidationofaqueous sulfurdioxideandhydrogensulfidewiththeformationofsulfuric acid.Thisworkprovidesanenvironmentallyattractivemethodto removetheaqueoussulfurdioxideandhydrogensulfide.

Acknowledgments

ThisworkwassupportedbytheNationalNaturalScienceFoun- dationof China(NSFC,U140710231,21076054and 21174001);

NaturalScienceImportantFoundationofEducationalCommission ofAnhuiProvince(2010AJZR-85,2011AJZR-87).StudyFoundation ofNewProductandTechnologyofAnhuiEconomicandInforma- tionTechnology Commission(2012AHST0797).Hefei University of Technology:Graduate TeachingReform Research Fund(106- 033049),NationalCollegeStudentInnovationFund(2014CXCY321 and2014CXCY349).

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