Catalysis Communications 18 (2012) 72–75

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Catalysis Communications

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Short Communication

Catalytic combustion of chlorinated VOCs over VOx/TiO2 catalysts

Meng Wu, Kim Chol Ung, Qiguang Dai, Xingyi Wang ⁎Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, China

⁎ Corresponding author at: P.O. 396, East China Univer130 Meilonglu, Shanghai 200237, China. Tel.: +86 21 642

E-mail address: wangxy@ecust.edu.cn (X. Wang).

1566-7367/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.catcom.2011.11.028

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 October 2011Received in revised form 17 November 2011Accepted 21 November 2011Available online 28 November 2011

Keywords:CVOCCatalytic combustionVOx/TiO2

AnataseRutile

VOx/TiO2 catalystswere prepared bywetness of TiO2with anatase and rutile structures and tested in the catalyticcombustion of CB, DCB, DCM and TCE. The characterization by XRD, TPR and XPS shows that VOx catalyst sup-ported on anatase (VOx/TiO2-SG) possessesmoreV5+–O–V5+ and hence highermobility of oxygen than VOx cat-alyst supported on rutile (VOx/TiO2-HY). VOx/TiO2-SG presents lower T50 and T90 for CVOC. High activity of VOx/TiO2-SG is related to high mobility of oxygen which weakens the adsorption of Cl species, and enhances theremoval of Cl species from the surface of catalysts.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Chlorinated volatile organic compounds (CVOC) are hazardous pol-lutants that are considered among the most harmful organic contami-nants due to their acute toxicity and strong bioaccumulation potential[1], therefore, the safe disposal of CVOC has acquired great importancewith the ever increasing concern for environmental protections [2]. Sofar as we are concerned, the catalytic oxidation of CVOC to carbon diox-ide, HCl, and water is the best method for their degradation. The majoradvantage of the catalytic combustion is that oxidation can be efficientlyperformed at temperatures between 250 and 550 °C and very dilutepollutants which cannot be thermally combusted without additionalfuel can be treated efficiently. Therefore, a low-temperature catalyticcombustion process offers significant cost saving when comparedwith the conventional thermal process.

Of studies of the catalysts for CVOC catalytic combustion, most havebeen reported focusing on the three types of catalysts based on noblemetals [3,4], transition metals [5,6] or zeolites [7,8]. Metal catalystshave been widely used because of their ability to catalyze oxidation,but they are susceptible to the deactivation byHCl and Cl2. The activitiesof zeolite are related to their acid properties. Recently, VOx/TiO2-basedcatalysts are reported by several research groups as an effective catalystfor the removal of CVOC [6,9–11]. In these studies, VOx/TiO2-basedcatalysts were generally used for the catalytic oxidation of chlorinatedaromatics, as model compounds of PCDF/PCDFs. Only a few of litera-tures were available reporting the effect of TiO2 crystalline phase onthe activity of VOx/TiO2 catalysts for decomposition of CVOC, especially

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for the catalytic oxidation of chloralkanes and chlorinated alkylenes. Inthis work, TiO2 with anatase or rutile structure, as support, was synthe-sized from two different precursors to investigate the effect of TiO2

phase on VOx species. The synthesized VOx/TiO2 catalysts were evaluat-ed in the catalytic combustion of chlorobenzene (CB), dichlorobenzene(DCB), dichloromethane (DCM) and trichloroethylene (TCE).

2. Experimental

2.1. Catalyst preparation

TiO2-HY is prepared by hydrolysis method. 20 ml TiCl4 was added to200 ml deionized water under vigorously stirring, then ammonia isadded dropwise into the solution until pH=3. At that time, white stickysuspension is formed. The suspension is dried at 100 °C for 12 h, andthen calcined at 500 °C for 5 h. The TiO2-SG is prepared by sol–gelmeth-od. 80 ml Ti(OBu)4 and 4 ml acetic acid were added into 320 ml ethanolto form a homogenous solution. The mixture of 20 ml deionized water,2 ml concentrated hydrochloric acid and 160 ml ethanol was addeddropwise into the solution to form a yellowish gel, which subsequentlywas aged 12 h at 70 °C and then calcined at 480 °C for 5 h.

VOx/TiO2 catalysts were prepared by wetness of TiO2-HY and TiO2-SG. The aqueous solutions of NH4VO3 and oxalic acid (nNH4VO3:noxalic acid=1:2) were used as precursors. Two kinds of TiO2 supportswere impregnated with appropriate amounts of precursor aqueoussolution to incipient wetness. The impregnated solids were dried at110 °C for 12 h and calcined at 500 °C for 5 h in air. The final vanadialoading of VOx/TiO2 catalysts was determined by ICP measurementsto be 3.5 wt.%, considering VOx as vanadium pentoxide. VOx catalystssupported on TiO2-HY and TiO2-SG are denoted by VOx/TiO2-HY andVOx/TiO2-SG.

20 30 40 50 60 70 80

4

3

2

+ : Rutile# : Anatase

++

+

++

+++++#

#

#####

####

#

Inte

nsi

ty (

a.u

.)

2 Theta (o)

#

+

1

Fig. 1. XRD patterns of catalysts: 1. VOx/TiO2-SG; 2. TiO2-SG; 3. VOx/TiO2-HY; 4. TiO2-HY.

73M. Wu et al. / Catalysis Communications 18 (2012) 72–75

2.2. Characterization

The powder X-ray diffraction patterns (XRD) of the samples wererecorded on a Rigaku D/Max-rC powder diffractometer using Cu Kαradiation (40 kV and 100 mA). The diffractograms were recorded with-in the 2θ range of 10 to 80° with a 2θ step size of 0.02° and a step time of0.1 s. The nitrogen adsorption and desorption isotherms were mea-sured in staticmode at−196 °C on anASAP 2400 system. TheXPSmea-surements were made on a VG ESCALAB MK II spectrometer by usingMg Kα (1253.6 eV) radiation as the excitation source. H2-temperatureprogramming reduction (H2-TPR) was investigated by heating 100 mgsamples in H2 (5 vol.%)/Ar flow (30 ml/min) at a heating rate of 10 °C/min from 20 to 750 °C. Before H2-TPR analysis, the sample was heatedfor 60 min in Ar flow at 500 °C, and then treated in O2 at room temper-ature for 30 min.

2.3. Catalytic activity measurement

Catalytic combustion reactions were carried out at atmosphericpressure in a continuous flow micro-reactor made of a quartz tubeof 4 mm in inner diameter. 200 mg catalyst (grain size, 20–40mesh) was packed in the reactor bed. The feed stream to the reactorwas prepared by delivering liquid reactant with a syringe pump intodry air, which was metered by a mass flow controller. The injectionpoint was electrically heated to ensure the complete evaporation ofreactant. The feed flow through the reactor was set with the concen-tration of CVOC reactant 1000 ppm and the gas hourly space velocity(GHSV) at 15,000 h−1. The effluent gases were analyzed on-line at agiven temperature by using two gas chromatographs (GC), oneequipped with FID for organic chlorinated reactant, and the otherwith TCD for CO and CO2. The concentrations of Cl2 and HCl were an-alyzed by the effluent stream bubbling through a 0.0125 N NaOH so-lution, and chlorine concentration was then determined by thetitration with ferrous ammonium sulfate using N,N-diethyl-p-pheny-lenediamine as indicator [12]. The concentration of chloride ions inthe bubbled solution was determined by using a chloride ion selectiveelectrode [13].

3. Results and discussion

Wide angle XRD patterns of catalysts are shown in Fig. 1. For TiO2-SGand VOx/TiO2-SG, the diffraction peaks are due to the reflections fromanatase (PDF #21-1272), while for TiO2-HY and VOx/TiO2-HY, there ap-pear reflectionsmainly from rutile (PDF #65-0190) andwith small partfrom anatase. According to the Scherrer equation applied to 101 of an-atase and to 110 of rutile, the crystallite size of anatase in VOx/TiO2-SGand VOx/TiO2-HY are estimated to be 15.2 and 13.8 nm, respectively,and, the particle size of rutile in VOx/TiO2-HY, 33.4 nm (Table 1), indi-cating that anatase presents higher dispersion. For both catalysts, nodiffraction peaks from crystalline V2O5 are observed on XRD patterns,implying that the vanadia would be amorphous or highly dispersedon TiO2 phase. Due to smaller particle size of anatase VOx/TiO2-SG pre-sents higher BET surface area (46 m2/g) than VOx/TiO2-HY (23 m2/g).

H2-TPR analyses for VOx/TiO2 catalysts are shown in Fig. 2. Asknown, TiO2 is a reducible support showing a main reduction peak at540 °C or higher [9,14], and so within the range of experimental tem-perature, the peak at 700 °C corresponds to the reduction of TiO2 onthe profile of VOx/TiO2-SG, where there appears a strong reductionpeak at 479 °Cwith a shoulder around 453 °C, ascribed to the reductionof VOx species. H2 uptake was estimated to be 270 μmol/g.cat. For VOx/TiO2-HY, the reduction of VOx species mainly occurs at 453 °C with H2

uptake of 191 μmol/g.cat. Assuming that V2O3 is the final reductionstate from various V species in the initial state of VOx [15,16], it is rea-sonable to propose that the amount of H2 consumption can be ascribedto the removal of oxygen, that is, greatermobility of oxygen occurs overVOx/TiO2-SG. This difference in reduction can be attributed to VOx

species. XPS analyses show that V 2p3/2 level is at 517.6 and 516.6 eV(Fig. 2 insert), corresponding to V5+ and V4+ species, respectively[17]. Moreover, the value of V5+/V4+ ratio is estimated to be 6.6 and2.1 on VOx/TiO2-SG and VOx/TiO2-HY, respectively (Table 1), indicatingthat greater H consumption on TPR test on VOx/TiO2-SG can be theresult mainly from the reduction of more V5+–O–V5+ species.

In order to checkwhether or not some reactions under thermal com-bustion condition could take place, “blank test” (homogeneous reac-tion) was carried out, with 3 mm crushed quartz glass (40–60 mesh)packed in the reactor. As shown in Table 1, low conversion of CVOC re-actant (CB, DCB and TCE)without catalyst can only occurs above 500 °C.However, the conversion of DCM in homogeneous reaction reaches 90%at 520 °C. The great difference in activity for combustion of CB, DCB TCEand DCM is maybe related to the inductive and p–p resonance effectsbetween chlorine atom and p electron cloud of C_C double bond inchlorinated alkylenes and aromatics, which shortens the bond lengthand increases the bond energy of C\Cl.

VOx/TiO2 catalysts with loading of 3.5 wt.% (SI) exhibit high activityin the combustion of CVOC (Fig. 3). For VOx/TiO2-SG, the activity orderfor conversion of CVOC is as follows: CB>DCB>DCM>TCE. T50 andT90 (where 50% and 90% conversion are reached) of CB are 218 °C and262 °C, respectively, which is consistent with that reported previously[10]. As known, the reactivity of DCB is less than that of CB, because ad-ditional Cl substitution for H is electron-withdrawing from aromaticring. It can be expected that DCB conversion shifts slightly high temper-ature. It is very interesting to find high activity of VOx/TiO2-SG for DCBcombustion. T50 and T90 of DCM are 258 °C and 313 °C; temperaturesare much lower than that on acid zeolites [15,18] and even lower thanthat on Pt/Al2O3 [19,20]. TCE presents the worst reactivity, and at377 °C, 90% conversion can be reached. However, this T90 is reallylower than those over Pd and Pt catalysts [21,22], and acidic zeolites[23]. For VOx/TiO2-HY, the activity order for conversion of these CVOCis CB>DCM>DCB>TCE. Moreover, conversion curves of individual re-actants all shift with different degrees high temperatures as comparedwith that on VOx/TiO2-SG. For CB and DCB, T50 and T90 can increase by30–60 °C, especially for T90 of DCB. However, for DCM and TCE, the con-version at low temperature keeps almost unchanged, and the increasein T90 is below 16 °C. T50 and T90 of these CVOC reactants above TiO2-SG and TiO2-HY are higher than two corresponding VOx/TiO2 catalysts,except for those of DCM which increase to slight extent.

Within the performance limit of TCD and FID themselves, two VOx/TiO2 catalysts have more than 99.5% selectivity to carbon oxides(more than 98% CO2 and trace CO) and no other C-containing by-products are detected in the oxidation of CB, DCB, DCM and TCE. Theselectivity for Cl2 (Fig. 3) is below 2% during reaction, except for thatof TCE. According to stoichiometric ratio, the selectivity for Cl2 is 50%in TCE decomposition. Here, a small amount of water contained in

150 200 250 300 350 400 4500

20

40

60

80

100

0

20

40

60

80

100

Sel

ecti

vity

of

Cl 2

/ %

Co

nve

rsio

n /

%

Temperature / oC

VOx/TiO2-SG

100 100VOx/TiO2-HY

A)

B)

Table 1The properties of catalysts.

Catalysts SBET/m2/g

D110

(R)a/nm

D101

(A)b/nm

H2

uptake/μmol/g.cat

V5+/V4+/atom

V2p3/2

/eVT50/T90/°C

CB DCB DCM TCE

VOx/TiO2-SG 46 – 16.5 270 6.64 517.7 218/262 242/288 258/313 309/377VOx/TiO2-SG-U-1c 45 – 17.3 286 6.56 517.6 220/265 240/287 258/320 311/380VOx/TiO2-SG-U-2d 45 – 17.1 290 6.38 517.6 217/260 240/290 256/317 312/379VOx/TiO2-HY 23 33.7 16.4 191 2.15 517.5 250/310 267/354 262/327 311/393TiO2-HY 29 33.4 13.8 – – – 463/– 498/– 295/360 445/–TiO2-SG 49 – 15.2 16.6 – – 443/– 489/– 280/347 424/–Blank – – – – – – 500 e 510 e 400/520 510 e

aFrom the Scherrer equation, applied to the b110> reflection of the Rutil and bb101> reflection of the Anatase. cVOx/TiO2-SG-U-1: used in stability test of CB decomposition for 20 h.dVOx/TiO2-SG-U-2: used in stability test of DCM decomposition for 20 h. eThe temperature at which the conversion reaches 10%.

74 M. Wu et al. / Catalysis Communications 18 (2012) 72–75

feed (about 600–700 ppm) can promote the removal of Cl species asHCl. The selectivity for Cl2 increases with the increase of conversiondue to not enough H species to combine Cl, as shown in Fig. 3. It mustbe pointed that the HCl and Cl2 balance reaches only 80–85%. This isprobably ascribed to the error caused during analyses of Cl species.High selectivity for CO2 and HCl in the combustion of CVOC over VOx/TiO2 catalysts has been reported previously [24–26].

Generally, it is considered that the active sites contain vanadiumwith the highest oxidation states of 5+, and in oxidation reactions sur-face V sites undergo a redox cycle, V5+–O–V5+/V4+–O–V4+. The recenttheoretical work strongly suggests that the dynamic character of VOx

species during the redox cycle can radically influence the stability ofthe surface sites VO and V–O–V. Higher activity of VOx/TiO2-SG catalystfor decomposition of chlorinated VOC should be related to more V5+–

O–V5+ species and greater mobility of oxygen on its surface. However,the difference in VOx species influence the activity for DCM and TCE de-composition to a small extent, implying that there is another factoraffecting the reaction. As known, acidic materials, such as HY andZSM-5 zeolites, are effective for DCM and TCE decomposition [13,18].NH3-TPD tests showed that VOx/TiO2 catalysts and TiO2 under studypresent similar acidity (see SI) which is much weaker than acidic zeo-lites. A significant activity of TiO2-SG and TiO2-HY for DCM decomposi-tion (Table 1) should be related to acidity. However, this situation is notobserved for TCE decomposition (Table 1). It is probably that the acidityis too low to activate of TCE. On the other hand, the corporation of VOx

really promotes the conversion of TCE, indicating that combination ofmobility of oxygen due to V5+–O–V5+ species with acidity is responsi-ble for high activity for TCE decomposition.

The effect of inlet reactant concentration on the reaction ratewas in-vestigatedwhen reaction proceeded at the space velocity of 15,000 h−1

at 240 °C. Fig. 4 shows that reaction rate varies with the increase in theinlet CVOC reactant concentrations from500 to 1500 ppm. For all CVOC,

100 200 300 400 500 600 700

453oC

528 524 520 516

VOx/TiO

2-HY

Inte

nsity

(a.

u.)

Binding Energy / eV

517.6

516.6

V 2p

VOx/TiO

2-SG

453oC

Temperature (oC)

479oC

Inte

nsi

ty (

a.u

.)

VOx/TiO

2-SG

VOx/TiO

2-HY

Fig. 2. H2-TPR analysis of VOx/TiO2 catalysts; insert: XPS spectra of V2p.

dependence of reaction rate on reactant concentration is of zero-order.Hence, CVOC adsorption on the surface of VOx/TiO2-SG catalyst at240 °C must not be rate limiting. The observed zero-order kinetics sug-gests that the catalyst surface is saturated by the reacting species withinthe overall reaction time. The reaction is probably limited by one of theprocesses on the surface that follows adsorption, e.g., reaction withoxygen species and desorption of products, such as Cl species, whichhas an inhibition effect on the reaction [27–30]. In this context, the

Temperature / oC150 200 250 300 350 400 450

0

20

40

60

80

0

20

40

60

80

Co

nve

rsio

n /

%

Sel

ecti

vity

of

Cl 2

/ %

Fig. 3. The activity of VOx/TiO2-SG (A) and VOx/TiO2-HY (B) for catalytic combustion ofCVOC; ●/○: CB; ▲/△DCB; ★/☆: DCM; ■/□: TCE; gas composition: 1000 ppmreactant, 10% O2, N2 balance; GHSV=15,000 h−1.

400 600 800 1000 1200 1400 16000

50

100

150R

eact

ion

rat

e x1

03 (u

mo

l min

m2 )

Reactant concentration (ppm)

CB

TCE

DCM

Fig. 4. The reaction rate at different reactant concentration over VOx/TiO2-SG catalyst at240 °C; reaction gas composition: 10% O2, N2 balance; GHSV=15,000 h−1.

75M. Wu et al. / Catalysis Communications 18 (2012) 72–75

experiments over VOx/TiO2-SGwere carried outwith raisingO2 concen-tration from 10% up to 20%, maintaining 1000 ppm reactant in feed. Theresults show that no difference in rate is found. These phenomena canimply that desorption of Cl species can be a slow process. Consideringthe fact that the activity is related to the mobility of oxygen over VOx/TiO2 catalysts under study, it can be deduced that the increase in surfaceoxygen can weaken the adsorption of Cl species over VOx/TiO2-SG cata-lyst. On the other hand, when the reaction temperature is raised up tohigh values, the dependence of reaction rate on reactant concentrationbecomes stronger and stronger. It seems reasonable that the adsorptionof Cl species can be weakened at high temperature over VOx/TiO2-SGcatalyst.

For the catalytic combustion of CVOCs, catalyst deactivation is still ahurdle in commercial applications. The experiments for stability werecarried out by feeding the stream containing 1000 ppm reactant and10% O2 at 250 °C at space velocity 15,000 h−1. As shown in Fig. 5, stableactivity over VOx/TiO2-SG catalyst for the combustion of CB, DCB, DCMand TCE is observed without a substantial decrease in conversion.During the reaction within 20 h, no other C-containing by-products ex-cept for reactants are detected and the selectivity to CO2 of 100% and toHCl of 97–99% (50–55% during TCE decomposition) maintainsunchanged. The characterization of the used catalyst in stability testsof CB or DCM decomposition for 20 h showed in Table 1 that the struc-ture of VOx/TiO2-SG catalyst has not significantly changed. VOx/TiO2-SG

0 5 10 15 200

20

40

60

80

100

Co

nve

rsio

n /

%

Time (h)

CB

DCM

TCE

DCB

Fig. 5. The stability tests for CB, DCB, TCE andDCM catalytic combustion over VOx/TiO2-SGcatalysts at 270 °C for 20 h; reaction gas composition: 1000 ppm reactant, 10% O2, N2

balance; GHSV=15,000 h−1.

catalyst, because of high activity of CVOC combustion and high stability,becomes useful in the practice of removal of chlorinated VOCs, includ-ing chlorinated aromatics, chloralkanes and chlorinated alkylenes.

4. Conclusion

VOx/TiO2 catalyst supported on anatase prepared by wetness pre-sents high activity in the catalytic combustion of CB, DCB, DCM andTCE, which is related to high mobility of oxygen which weakens theadsorption of Cl species, and enhances the removal of Cl speciesfrom the surface of catalysts.

Acknowledgment

We would like to acknowledge the financial support from NationalBasic Research Program of China (No. 2010CB732300, 2011AA03A406)and National Natural Science Foundation of China (No. 20977029).

Appendix A. Supplementary data

Supplementary data to this article can be found online at doi:10.1016/j.catcom.2011.11.028.

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