Xian Mao,Fanglu Yuan,Anqi Zhou,Wenheng Jing*

State Key Laboratory of Materials-Oriented Chemical Engineering,National Engineering Research Center for Special Separation Membrane,College of Chemical Engineering,Nanjing Tech University,Nanjing,210009,China

A B S T R A C T Magnéli phases TinO2n-1have been demonstrated as promising environmentally friendly materials in advanced oxidation processes.In this study,Magnéli phases TinO2n-1have been used as catalysts for the ozonation of phenol in aqueous solution for the first time.The materials exhibited excellent catalytic ozonation activities both in phenol degradation and mineralization.When Ti4O7was added,the reaction rate was six-fold higher than that of with ozone alone,while the total organic carbon removal rate was substantially elevated from around 19.2%to 92%.By virtue of the good chemical stability of the materials,a low metal leaching of less than 0.15 mg·L-1could effectively avoid the secondary pollution by metal ions. Radical quenching tests revealed·O2-and1O2to be active oxygen species for phenol degradation at pH 5.As semiconductor catalysts,TinO2n-1materials show electronic transfer capability.Ozone adsorbed at B-acid sites of the catalyst surface can capture an electron from the conversion of Ti(III)to Ti(IV),and is thereby broken into the active oxygen species.It was interesting to observe that TinO2n-1exhibit better catalytic activity for phenol degradation and mineralization with lower n value.The difference in electrical conductivity can be considered as a major factor for the catalytic performances.More highly conductive catalysts show a faster electron-transfer rate and better catalytic activity.Thus,significant evidences have been obtained for a single-electron-transfer mechanism of catalytic ozonation with Magnéli phases TinO2n-1.

1.Introduction

Aqueous organic contaminants have been drawing increasing attention,particularly phenol compounds.The toxicity of phenol persists in the natural environment, and therefore it is imperative to seek degradation methods for the treatment of phenol-containing wastewater. As a strong oxidant, ozone is currently being widely applied in the treatment of phenol-containing wastewater[1–3].Unfortunately,it slow solubility and stability in water limit its applications[4].

Catalytic ozonation,combining the strong oxidizing ability of ozone with the adsorption and catalytic properties of catalyst,is considered to be a promising method for accelerating the reaction and improving the ozonation efficiency,especially in enhancing the mineralization rate of organic contaminants[5–7].Catalysts based on metaloxides,supported metals,activated carbon,and minerals have been investigated with regard to the ozonation process[8–13].Among them,manganese based and iron-based metal oxide catalysts have demonstrated excellent activities,because manganese and iron cations in catalysts behave as Lewis acid sites and increase the overall acidity,which contributes to higher catalytic activity[5].Moreover,manganese oxides have a redox cycle between oxidation states of+2 and+4,providing good redox reactions of Mn(II)/Mn(III)or Mn(III)/Mn(IV),and oxygen mobility in the oxide lattice[1,4].Nevertheless,secondary pollution resulting from the loss of metal cations poses a high risk to the environment[5,8].Therefore,some efforts have been made to develop novel leaching-free catalysts with high catalytic efficiency and non-toxicity.

As a biocompatible material,titanium dioxide has been applied in various contexts for its chemical stability and cost-effectiveness[14,15], especially in photocatalysis due to its semiconductor characteristics[16]. Moreover,TiO2can also be used as an active catalyst in the ozonation of organic molecules [17]. The adsorption and subsequent reaction of organics at catalytic sites are responsible for the enhancement of ozonation rate[18].Recently,Magnéli phases,having the general oxygen-deficient chemical formula TinO2n-1,where n is a number ranging from 4 to10,have also attracted much attention[19–23].Compared with TiO2,Magnéli phases TinO2n-1can be applied more effectively in the photocatalytic degradation of organic compounds,since their narrower band gaps facilitate the absorption of visible light.In addition,monolithic Magnéli phases TinO2n-1can also be used as electrodes for electrochemical oxidation in water treatment due to their robust characteristics[24,25].The catalytic activity of Magnéli

Keywords:TinO2n-1 Catalytic ozonation Conductivity B-acid sites Single-electron-transfer phases TinO2n-1depends on the oxidation states of the transition metals,the quantity of non-stoichiometric oxygen,and the presence of defect structures.Moreover,the materials must have high chemical and thermal stability.

However,to the best of our knowledge,Magnéli phase materials have not hitherto been applied as catalysts for ozonation in water treatment and so the mechanism of action is unknown.Herein,we report a facile method for the synthesis of Magnéli phases TinO2n-1with different n values.The catalytic ozonation activities of the obtained Magnéli phase materials were then evaluated in the degradation of phenol.The kinetics of degradation and mineralization of phenol in the catalytic ozonation process has been compared with that in a simple ozonation process.Competitive radical quenching experiments have been carried out to identify the reactive oxygen species.Furthermore,some evidences are provided for a single-electron-transfer catalytic ozonation mechanism.

2.Experimental Methods

2.1.Synthesis of Magnéli phases TinO2n-1

The raw materials for synthesis of the Magnéli phases TinO2n-1,namely rutile TiO2and glucose,were supplied by Macklin Co.,with purities＞99%.The pristine TiO2consisted of a single phase of rutile structure as primary particles,of size about 60 nm.TinO2n-1samples were prepared by calcining mixtures of TiO2and glucose under vacuum conditions.Powder mixtures with glucose content 40 wt%were heated at various temperatures ranging from 950to1075°C for 2h.The heating rate was set at 5 °C·min-1.Powder mixtures with glucose contents ranging from 10 to 40 wt%were heated at 1050°C for 2 h under the same conditions mentioned previously.

2.2.Characterization

Crystalline phases of samples were identified by X-ray diffraction(XRD)where using Cu Kαradiated with a tube voltage of 40 kV and 20 mA with 2θ ranges from 15°to 80°.The Brunauer–Emmett–Teller(BET)equation method was employed to calculate the specific surface area and pore volume.The surface morphology of the Magnéli phases was measured by using scanning electron microscopy(SEM-EDS).The structure size of Magnéli phase TinO2n-1was characterized by transmission electron microscopy(TEM)operated on a JEOL2010 with an acceleration voltage of 200 kV.Surface chemical states of the asprepared samples were evaluated by an X-ray photoelectron spectroscope(XPS)which was used under a voltage of 12 kV under vacuum condition(2× 10-5Pa)with an Al Kα.Binding energies were calibrated at the C1s peak which is at 284.6 eV[26].The total organic carbon(TOC)of the solution was determined with a Shimadzu TOC-vcph analyzer.The concentration of dissolved titanium ions in solution was measured by inductively coupled plasma emission spectrometry(ICP).The surface acidities of materials applied in catalytic ozonation were determined from the IR spectra of adsorbed pyridine(Py-IR).

2.3.Catalytic ozonation procedure

All catalytic ozonation experiments were conducted in a degradation reactor containing 1.0 L of phenol solution.Ozone generated in an ozone generator supplied with pure dry oxygen was used in the experiments.The reaction temperature was controlled at 25°C and the stirring speed was 150 r·min-1.Unless specified otherwise,the concentration of ozone was 113 mg·L-1,the inlet gas flow rate was 100ml·min-1,and0.3 g of catalyst was added to 1 L of 50 mg·L-1phenol solution.In adsorption experiments,0.3 g of catalyst was added to the phenol solution,and the mixture was stirred magnetically for 1 h to achieve adsorption–desorption equilibrium. At certain time intervals,a 1 ml aqueous sample was taken from the reactor and filtered through a 0.22 μm PTFE filter into a vial,which contained 0.5 ml of aqueous Na2S2O3solution to quench the reaction.The phenol concentrations of aqueous samples were analyzed with high-performance liquid chromatography on a C18 column.For stability tests,a vacuum filtration method was used to recover used samples,which were then washed three times with ultrapure water.

3.Results and Discussion

3.1.Characterization of synthesis Magnéli phases TinO2n-1

Magnéli phases TinO2n-1with different n values were prepared by reducing rutile TiO2with glucose.Fig.1(a)shows the XRD patterns of the samples produced at different temperatures from mixtures with a glucose content of 40 wt%.On heating the mixture to 950°C,only rutile structure TiO2was found,with virtually no Magnéli phase.On heating to 1000°C,the primary phase mainly contained Ti5O9,and Ti4O7was obtained at 1050 °C.When heated further to 1075 °C,these phases were transformed into the non-Magnéli phase Ti3O5.These experimental results confirmed that rutile TiO2can react with carbon produced from glucose to form Magnéli phases TinO2n-1,and that the optimal heating temperature is 1050 °C for obtaining a relatively pure Magnéli phase.Fig.1(b)shows the XRD patterns obtained after subjecting mixtures with different glucose mass contents(10%,20%,30%,40%)to heat treatment at 1050°C for 2 h.With increasing glucose content from 10 to 40 w.%,the n value of the resultant Magnéli phases tended to decrease from 9 to 4.

Fig.1.(a)XRD patterns of samples reduced at various temperatures.(b)XRD patterns of the samples reduced with different glucose/TiO2ratios.

BET surface areas and total pore volumes of the produced TinO2n-1were evaluated(Table 1).The BET surface area of the as-obtained raw TiO2decreased from 19.1 m2·g-1to 3.6 m2·g-1after heat treatment.Following heat treatment in the presence of glucose,the BET surface areas of the samples increased,more markedly with increasing glucose content, since the carbon coating suppressed sintering and grain growth of TinO2n-1[20].The carbon nanofibers on the Ti4O7can be seen in Fig.S1.Content of nano-carbon in the as-prepared Magnéli phases TinO2n-1is supplemented in Table S1.

Table 1Textural properties of TiO2and Magnéli phases

Fig.2 shows a high resolution transmission electron microscopy image of Magnéli phase Ti4O7.The labeled lattice distance is 0.338 nm,corresponding to the(-1 2 1)plane of the triclinic crystal.The XRD spectrum of Ti4O7at 2θ =26.3°showed the highest peak intensity among all the reflection peaks(Fig.1).Therefore,this crystalline plane was the easiest to discern in comparison to the other crystalline planes of Magnéli phase Ti4O7[26].

Fig.2.HRTEM image of the sample Ti4O7.

The Ti 2p XPS peaks were obtained for rutile TiO2and the product Ti4O7,to examine the changes in surface components before and after heat treatment(Fig.3).The peaks at around 459.0 and 464.5 eV can be assigned to Ti(IV)in rutile TiO2,and the peak at 460.9 and 456.8 eV can be ascribed to Ti(III)in Ti2O3.The valence states of Ti in Magnéli phases TinO2n-1are consistent with those reported by Tominaka et al.[27].The XPS pattern of the precursor before reduction,displays a main sharp peak located at 459.0 eV,corresponding to Ti(IV)in rutile(Fig.3(a)).After heat treatment,the Ti 2p doublet peaks have tails in the region of lower binding energy located at 456.8 eV for Ti 2p3/2,which could be attributed to Ti(III)(Fig.3(b)).

3.2.Catalytic ozonation of phenol with Magnéli phases TinO2n-1

The catalytic activities of three kinds of TinO2n-1(Ti4O7,Ti6O11,and Ti9O17)in the degradation of phenol were determined.Fig.4 shows plots of the degradation of phenol and the removal rates of TOC.For these three different TinO2n-1,the effect of adsorption on phenol removal was negligible.Phenol was completely degraded in 60 min by ozonation,indicating the oxidation capability of ozone.However,only 19.2%of the TOC was mineralized after 1 h,revealing that although ozone molecules are predisposed to break the bonds of phenol,mineralization of saturated organics is more difficult[28].When rutile TiO2was introduced as a catalyst,complete degradation of phenol was accomplished in less than 10 min in a single ozonation process.Meanwhile,the removal rate of TOC was only 7%more than that for ozonation without a catalyst.In comparison,when TinO2n-1samples were used as catalysts,degradation and mineralization efficiencies were significantly enhanced.The time required for the complete degradation of phenol over Ti9O17was 30 min,and TOC removal rates were substantially elevated from around 19.2%to 81%.Furthermore,the degradation rate of phenol was faster when Ti6O11was used as catalyst.Compared to Ti9O17and Ti6O11,Ti4O7showed the highest catalytic activity,the time for complete phenol degradation was shortened to 20 min,and the TOC mineralization rate reached 92%. The vast enhancement in catalytic performance indicated that the Magnéli phases Ti9O17,Ti6O11,and Ti4O7may convert ozone into reactive oxygen species for the degradation and mineralization of phenol.Moreover, at varying initial phenol concentrations,catalytic ozonation efficiency decreased with increasing initial phenol concentration.In terms of TOC removal,a higher initial concentration led to an inferior mineralization capability(Fig.S2).The effect of ozone concentration and flow rate on the catalytic efficiency was also investigated(Fig.S3).With increasing ozone loading,the degradation efficiency of phenol was improved,but the effect on TOC removal was very limited.In addition,the effect of carbon nanofibers on catalytic ozonation was investigated. In Fig. S4, the effect of adsorption on phenol removal was negligible.Meanwhile,when the prepared carbon nanofibers were used as catalyst,ozonation was not enhanced significantly.Fig.S5 shows the adsorption capacity and phenol degradation ability of amorphous TiO2to phenol.From this graph,when amorphous TiO2was added,the phenol removal rate changed little over time.Furthermore,adsorption of phenol by amorphous TiO2may be the reason for a slight increase in degradation efficiency.Therefore,amorphous TiO2has no catalytic activity in the reaction.

Fig.5 shows the reaction rate constants(k)for ozonation and catalytic ozonation using TiO2and TinO2n-1(Ti9O17,Ti6O11,and Ti4O7).The catalytic ozone reaction is evidently a pseudo- first-order process.Reaction rate constants were calculated as 0.0318,0.0409,0.1129,0.1293,and 0.188 min-1,and the five fitting equations were well fitted by a first-order kinetic model.As can be seen,the catalytic ozonation process with TinO2n-1gave higher reaction rate constants,and Ti4O7showed the best catalytic activity,its reaction rate constant being six times that with TiO2.

The catalytic stability and the recyclability of TinO2n-1were evaluated through successive reusability tests with Ti4O7.The catalyst was recovered by filtration,and the intermediate product adsorbed on its surface was removed by calcination at 450°C in a nitrogen atmosphere.Fig.6 shows the times required for complete phenol decomposition and the corresponding TOC removals in 1 h for eight cycles with Ti4O7.Compared with fresh Ti4O7,with each successive use of the catalyst,the time required for complete phenol decomposition increased.Meanwhile,the rate of TOC removal gradually decreased.After eight cycles of catalytic ozonation,the catalytic performance of the Magnéli Ti4O7was nevertheless still efficient.The time required for complete phenol decomposition increased from 20 to 30 min,and TOC removal decreased from 92.2%to 84.8%with Ti4O7.

Fig.3.(a)XPS spectra of Ti 2p for raw rutile TiO2;(b)XPS spectra of Ti 2p for product Ti4O7.

Fig.4.The degradation of phenol with various catalysts(a)and the corresponding TOC removal(b).Reaction conditions:[phenol]0=50 mg·L-1;catalyst=0.3 g·L-1;solution pH=5;ozone concentration:113 mg·L-1;gas flow rate:110 ml·min-1.

The reaction solutions were collected after each run,and catalyst leaching was assessed with ICP.It was found that the concentration of Ti ions in the reaction solutions was less than 0.15 mg·L-1,suggesting that negligible loss of catalytic activity occurred due to Ti ion leaching.The effect of adsorption of reaction intermediates on the surface of the catalyst could also be neglected,because heat treatment could completely remove these attached intermediates[29].

Fig.5.Kinetic study of catalytic ozonation.

Previous studies have indicated that free radicals participated in the catalytic ozonation process[30,31].It is known that hydroxyl radical(·OH),superoxide radical(·O2-),and singlet oxygen(1O2)are the free radicals that generated in the catalytic ozonation process,and the dominant species in the catalytic ozonation will vary depending on the catalyst[17].Competitive radical quenching experiments using tert-butanol(t-BA),p-benzoquinone(p-BQ),and sodium azide(NaN3)were carried out to determine the dominant free radicals for phenol degradation.t-BA is a hydroxyl radical quenching agent [12]. The contribution of superoxide radical for phenol degradation has widely been assessed using p-BQ as a scavenger[30].Singlet oxygen is quenched by NaN3[31].The dominant reactive species in the catalytic ozonation process for phenol degradation could thus be distinguished by adding t-BA,p-BQ,or NaN3into the reaction solution.

Fig.6.Stability tests on Ti4O7over eight consecutive phenol degradation runs:(a)complete phenol decomposition time;(b)the corresponding TOC removal in 1 h.

The results of competitive radical quenching experiments are presented in the form of reaction rate constants(k)(Fig.7).The difference between catalytic ozonation for phenol degradation without and with the addition of 10 mmol·L-1t-BA in the reaction process can be ignored,suggesting that the hydroxyl radical was not the dominant reactive specie responsible for phenol degradation.To further explore the major reactive species,10 mmol·L-1p-BQ was added to the reaction solution.The reaction rate decreased from 0.1881 to 0.0575 min-1,suggesting that·O2-was quenched in the solution by this additive.Similarly,when 10 mmol·L-1NaN3was added,the reaction rate decreased to 0.0616 min-1.Both p-BQ and NaN3were then applied as scavengers for further confirmation,and the reaction rate further decreased to 0.0349 min-1.Therefore,·O2-and1O2were recognized as the dominant reactive species in the catalytic ozonation system for phenol degradation using Ti4O7as catalyst.Hydroxyl radical is not considered to be a prominent reactive species since t-BA did not significantly decrease the reaction rate of catalytic ozonation.The main reaction pathway for phenol degradation process was shown in Fig.S6.At the beginning of this process,the O3and·O2-,1O2attacked the or tho and para positions on benzene which would lead to the formation of odihydroxybenzene and p-dihydroxybenzene.Then these intermediates were further oxided,following benzoquinone was produced.Ultimately,O3and·O2-,1O2would attack aromatic ring of benzoquinone,further form small molecule organic matter,such as carboxylic acid and aldehyde，and the CO2and H2O were the subsequent oxidation products.

Fig.7.Radical competition tests on the catalytic ozonation process with Ti4O7.Reaction conditions:[phenol]0=50 mg·L-1;catalyst=0.3 g·L-1;solution pH=5;ozone concentration:113 mg·L-1;gas flow rate:110 ml·min-1.TBA=10 mmol·L-1,p-BQ=10 mmol·L-1,NaN3=10 mmol·L-1.

Active oxygen species were generated by the cleavage of ozone following introduction of the catalyst Ti4O7.It has been reported that the enhancement of catalytic ozonation involved the adsorption of ozone or pollutant,or both of them,on the catalyst surface,leading to the formation of free radicals that react with non-adsorbed species in the bulk liquid[32,33].Therefore,the surface characteristics play an important role in the oxidation mechanism.Py-IR studies of rutile TiO2and Ti4O7were conducted to probe their surface acidities(Fig.8).IR bands at ν=1450,1490,and 1610 cm-1correspond to the adsorption of pyridine at Lewis(L)-acid sites.A peak atν=1540cm-1is due to interaction of bound pyridine with a Br?nsted(B)-acid site[34,35].For TiO2,the peaks at around ν=1446,1490,and 1600 cm-1clearly indicated the existence of L-acid sites.After heat treatment,the IR spectrum of pyridine adsorbed on the Ti4O7prepared from raw TiO2also featured a peak at ν=1540 cm-1,suggesting that Ti4O7possessed both L-and B-acid sites.The ratio of the number of B-acid sites to the number of L-acid sites for Magnéli phase Ti4O7was estimated as 9.5%.L-acid sites are generally considered as active sites for the adsorption of organic contaminants.Yang et al.[36]studied the ozonation of nitrobenzene on nano-TiO2in rutile form,and the adsorption of the substrate was found to play an important role in the catalytic ozonation process.Adsorption experiments indicated that Ti4O7has a lowphysical adsorption capacity for phenol at pH 5,and therefore the effect of L-acid sites in ozonation can be ignored.Ozone can be adsorbed at the negative centers or B-acid sites on the Ti4O7surface due to its electrophilic character.This promotes the generation of superoxide radicals,which initiate ozone decomposition,a significant factor determining activity[37].Ozone is adsorbed at active B-acid sites and subsequently broken down into active oxygen species.Hence,it is thought that these active sites provide the catalytic effect.

Fig.8.Py-IR spectra for different samples.

Most metal oxide catalysts are semiconductors,notably nonstoichiometric oxides.Magnéli phases TinO2n-1are n-type semiconductor catalysts with deficiency of oxygen anions.In these catalysts,singleelectron transfer associated with transition between different valence states of the same metal is responsible for catalytic reactions and the electron-transfer capacity has a direct bearing on catalytic activity[14,38].In the ozonation process,·O2-and1O2may be formed through valence transition of the Ti ion at B-acid sites at which ozone is adsorbed.To examine the chemical state change of Ti in the catalytic ozonation system,the Ti 2p XPS peaks for the fresh and used catalysts were investigated.It can be observed from Figs.3(b)and S7 that after catalytic ozonation reaction over eight cycles,the relative content of Ti(III)in Ti4O7decreased from12.3%to8.6%.Meanwhile,for Ti4O7,the characteristic peaks are located at the 2θ angles of 20.7°and31.7°.In Fig.9,the intensity of characteristic peaks gradually decreases with the increase of cycle times.As the cycle index increases,the value of n in the catalyst increases gradually which indicated that the valence state of part of titanium changed from trivalence to quadrivalence.In addition,it can be seen from Fig.9,the catalytic stability and the recyclability result are also consistent with the conclusion that worse catalytic activity resulted from higher n value.The results indicated that under the strongly oxidative environment of catalytic ozonation,part of the titanium ion with the+3 valence state was converted to the+4 valence state through electron transfer.As discussed earlier,this implies electron transfer from surface Ti(III)to Ti(IV)during the catalytic process.The electron transfer occurring on the catalyst surface is associated with the active sites for catalytic reaction.Moreover,this composition change might alsoaccount for the slight deactivation of the catalyst, thus explaining the results of the reusability experiments.

Fig.9.XRD patterns of Ti4O7,Ti4O7for 4 runs and Ti4O7for 8 runs.

Magnéli phases TinO2n-1contain Ti(III)and Ti(IV),and the chemical state change of Ti can be recognized as the critical factor for catalytic ozonation.Meanwhile,it is noteworthy that the catalytic activity decreased with increasing n value.Among Magnéli phases TinO2n-1(3＜n＜10),the electrical conductivity of Ti4O7is the highest which can draw a conclusion from Fig.S8[39].Meanwhile,as the n value increased,the conductivity of TinO2n-1decreases,revealing that electrical conductivity can be considered as a major factor for the catalytic performances of Magnéli phases TinO2n-1.The rate of electron transport can be greatly accelerated by a higher conductivity[8].A faster rate of electron transport is helpful to promote the decomposition of ozone into active oxygen components.This may be the reason for Ti4O7exhibiting better catalytic activity for phenol degradation and mineralization than other TinO2n-1materials with greater n value.

The proposed catalytic ozonation mechanism for phenol degradation is depicted in Fig.10.Firstly,in aqueous phenol solution,ozone is adsorbed at the B-acid sites on the surface of Magnéli phases TinO2n-1,at which it can be cleaved into1O2and·O2-by capturing an electron released from the valence transition of Ti(III)to Ti(IV).Finally,the active oxygens pecies·O2-and1O2attack the organic molecule and are degraded into H2O and CO2[Eqs.(1)and(2)].

4.Conclusions

Fig.10.Proposed degradation mechanism of catalytic ozonation for phenol degradation.

To conclude,Magnéli phases TinO2n-1have been synthesized by a facile thermal reduction method employing rutile and glucose as precursors.The obtained materials demonstrated superior capability in a catalytic ozonation process for phenol removal.Among them,activity showed a dependence on the n value,with Ti4O7being more active than Ti6O11and Ti9O17.Stability tests suggested that TinO2n-1showed good stability,even over eight runs.B-acid sites are suggested to be the main active sites,at which ozone is adsorbed.Mechanistic studies have revealed that phenol removal is dominated by·O2-and1O2,derived from cleavage of ozone due to the catalytic effect of TinO2n-1.These active oxygen species are generated by electron transfer between Ti(III)and Ti(IV)at the B-acid sites.This work provides insights into the roles of Magnéli phases TinO2n-1as catalysts for the decomposition of phenol in wastewater.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2018.01.011.