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Applied Catalysis A: General 400 (2011) 104–110

Contents lists available at ScienceDirect

Applied Catalysis A: General

journa l homepage: www.e lsev ier .com/ locate /apcata

ffect of ceria on the MgO-�-Al2O3 supported CeO2/CuCl2/KCl catalysts for ethanexychlorination

hao Lia, Guangdong Zhoua,∗, Liping Wanga, Shuli Dongb, Jing Lia, Tiexin Chenga,∗

College of Chemistry, Jilin University, Changchun, 130021 ChinaChangchun No.11 high school, Changchun, 130062 China

r t i c l e i n f o

rticle history:eceived 26 November 2010eceived in revised form 6 April 2011ccepted 16 April 2011vailable online 22 April 2011

eywords:eria dispersionPRcidity

a b s t r a c t

A series of CeO2-doped CuCl2-KCl/MgO-�-Al2O3 catalysts were prepared and characterized by BET, XRD,H2-TPR, FTIR-pyridine adsorption, NH3-TPD and XPS techniques. XRD, BET and TPR results show that threetypes of ceria species exist on the surface of catalysts: dispersed ceria, small aggregated crystalline CeO2

species and large ceria particles. It was found that copper-based catalysts modified with small aggregatedcrystalline ceria species exhibited higher conversion of ethane and selectivity to vinyl chloride comparedto copper-based catalysts with dispersed ceria or large ceria particles. The promotional effects may beoriginated from the formation of large amount of surface capping oxygen species (O2

− or O−) due tostructural defects and electronic properties of nonstoichimetric ceria. Moreover, these surface cappingoxygen species accelerate oxidation of part of Cu+ to Cu2+, which are responsible for the increase of

thane oxychlorination intermediate Cl2 species in the process of ethane oxychlorination. NH3-TPD results show that the catalystsmodified with small aggregated crystalline ceria species have a large amount of weak acidic sites on thesurface, and these weak acidic sites benefit dehydrochlorination of dichloroethane. The activity testsrevealed that the copper-based catalyst with cerium content x = 5 wt.% exhibited the highest activity dueto the excellent coordination effect between ceria and copper species and the largest amount of weakacid sites for breaking C–H bonds and dehydrochlorination of dichloroethane.

. Introduction

Vinyl chloride (VC) has been used extensively as a type ofonomer in the production of poly-vinyl chloride (PVC). The oxy-

hlorination reaction of ethane is a promising way to produce vinylhloride, because ethane as a raw material is cheaper than ethylene1,2]. There are two main catalytic steps in the process consist-ng of oxidation of HCl and dehydrochlorination of dichloroethane3]. Since it was discovered that copper chloride can catalyze theonversion of hydrogen chloride to chlorine in the Deacon pro-ess, much work has been done on the oxychlorination reactionf methane and ethylene [4–7]. Previous studies [8–10] indicatedhat CuCl2/�-Al2O3 catalyst exhibited higher activity for ethanexychlorination. Many researchers reported that some promotersMgO, KCl, LaCl3) enhanced the stability of copper-based catalystbviously [10,11]. For example, KCl can decrease the interactionetween the active species CuCl2 and �-Al2O3 support and acceler-

te the release of chlorine from CuCl2; rare earth chlorides (LaCl3)ake CuCl2 highly dispersed and prevent effectively the catalyst

rom sintering and agglomeration. Chen et al. [12] reported that

∗ Corresponding authors. Tel.: +86 431 88499356; fax: +86 431 88949334.E-mail addresses: zhougd@jlu.edu.cn (G. Zhou), ctx@mail.jlu.edu.cn (T. Cheng).

926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apcata.2011.04.017

© 2011 Elsevier B.V. All rights reserved.

CeCl3 promoter enhanced the activity of ethane oxyclorination andstability of copper-based catalyst, and found that the formation ofinner phase with carrier and larger crystallite ceria during the reac-tion were responsible for the deactivation of catalyst. However, lessof study on the effect of ceria promoter on acid properties, and thedetails of redox properties of ceria on the support or the coordina-tion interaction between ceria and copper species on the support ofcatalyst has been reported for ethane oxychlorination. Moreover,these factors determine the catalytic activity actually.

Flid et al. [3] reported that dehydrochlorination ofdichloroethane can be affected by the acid sites of the sup-port; and Finocchio et al. [13] assumed that uncovered alumina isresponsible for the dehydrochlorination of dichloroethane. There-fore, the introduction of ceria promoter to copper-based catalystsis not only related to the high oxygen storage/release capacity, butalso to the change of acidic properties of catalysts aroused by ceriapromoter. The aim of this work is to investigate the effect of ceriumcontent on redox properties and acidic properties of copper-basedcatalyst, and to reveal the nature of the catalytic activity arousedby different cerium content.

In this work, copper-based catalysts with different cerium con-tent were prepared successfully by impregnation method andevaluated by ethane oxychlorination. It is found that cerium con-tent plays a key role for the surface properties (i.e. redox and

s A: General 400 (2011) 104–110 105

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cidic properties) of catalysts and catalytic properties. The experi-ental results showed that copper-based catalysts modified with

mall aggregated crystalline ceria species, within a limit rangecerium content: 3 wt.% ≤ x ≤ 7 wt.%), enhanced the catalytic activ-ties notably.

. Experimental

.1. Catalyst preparation

MgO-�-Al2O3 was prepared by conventional impregnation of-Al2O3 support with an aqueous solution of Mg(NO3)2·6H2O, fol-

owed by drying at 120 ◦C for 2 h and calcining at 900 ◦C for 5 h. Theontent of MgO in �-Al2O3 support is 10 wt.%.

A series of CuCl2-KCl-CeO2/MgO-�-Al2O3 catalysts with dif-erent cerium content denoted as CuKCex, where x representseight percent of cerium in MgO-�-Al2O3. (x = 0, 1, 3, 5, 7, 9 and

1%, respectively). The CuKCex catalysts were prepared by co-mpregnating 5 g MgO-�-Al2O3 support with an aqueous solutionontaining CuCl2·2H2O (0.67 g), KCl (0.58 g) and various content ofe(NO3)3·6H2O for 48 h, followed by drying at 120 ◦C for 2 h and cal-ining at 550 ◦C for 5 h. All catalysts have the same content of coppernd potassium (Cu 5 wt.% and Cu/K = 1:2 molar ratio). Pure CeO2as supplied by Jiang Xi Golden Century New Material Company,

nd its purity is 99.9%.

.2. Characterization

Surface areas (SBET) and pore Volumes (Vp) of the catalysts wereeasured by N2 adsorption at liquid nitrogen temperature usingicromeritics ASAP 2012 apparatus.X-ray powder diffraction spectra were recorded using a Shi-

adzu XRD-6000 X-ray diffractometer employing Ni-filtered andu K� radiation (40 kV, 30 mA), at scanning rate of 2 ◦/min for� = 5–80◦, 1 ◦/min for 2� = 25–35◦ and 2� = 55–58◦. Crystal size oferia in the samples can be estimated from the values of full-widtht half-maximal (fwhm) of its (3 1 1) diffraction peak by means ofcherrer equation [14]: (DXRD = 0.89�/ˇcos �), where � is the wave-ength of Cu K�1 radiation, ˇ is the corrected half-width of theiffraction peak and � is the Bragg diffraction angle.

The reducibility of catalyst was measured by hydrogenemperature-programmed reduction technique (H2-TPR). Beforeeduction, 0.1 g of sample was loaded in a quartz tube reactor andretreated in helium flow at 300 ◦C for 1 h to remove the adsorbedarbonates and hydrates, and then the reactor was heated from00 ◦C to 1200 ◦C at a heating rate of 8 ◦C/min in a 5% H2/Ar gasow of 30 ml/min. The consumption of hydrogen was monitoredsing a thermal conductivity detector (TCD).

FTIR-pyridine adsorption was conducted by using a FT-IR (Nico-et Impact 410) spectrometer. All the samples were pressed intoelf-supported disk (0.02 g) and pretreated at 250 ◦C under vacuum10−5 Torr) for 2 h. The wafers containing chemisorbed pyridineere subjected to thermal treatment at 50 ◦C and IR spectra were

ecorded.Temperature-programmed desorption of ammonia (NH3-TPD)

as employed to determine the acidic strength and acidic amountf catalysts. In each run 0.1 g sample was loaded in a quartz tubeeactor and pretreated in Ar flow at 500 ◦C for 1 h to remove thedsorbed carbonates and hydrates. Before the measurement, 20 mlH3 was injected to the high vacuum quartz tube reactor for 1 h

or copper-based catalysts to adsorb NH3, and then the sample

as treated in Ar flow at 100 ◦C for 1 h to eliminate the physical

dsorbed ammonia. NH3-TPD analysis was carried out at a heat-ng rate of 8 ◦C/min from 100 ◦C to 600 ◦C. Helium as a carrier gas22 ml/min) and a thermal conductivity detector (TCD) were used.

Fig. 1. XRD peak intensity ratio of CeO2 (1 1 1) to MgAl2O4 (3 1 1) versus CeO2 con-tent in CeO2/MgO-�-Al2O3 samples.

XPS characterization was recorded on a VG ESCA LAB M-II X-rayelectron spectrometer using Al K� radiation (15 kV and 15 mA).

2.3. Catalytic properties

The catalytic activity of ethane oxychlorination was evalu-ated in a fixed-bed quartz reactor under atmospheric pressure.Before reaction, the catalysts were activated under HCl/air mixture(28 ml/min) at 500 ◦C for 30 min. The optimal reaction conditionsare: reaction temperature range 450–550 ◦C; C2H6:HCl:air = 1:2:5,total flow rate of 32 ml/min, amount of catalysts 1.0 g (20–40 mesh).Products were analyzed using a Shimadzu GC-8A equipped with FIDdetector.

3. Results and discussion

3.1. XRD

XRD quantitative phase analysis [15,16] was applied to deter-mine the amount of residual crystalline CeO2 in the samples. In thecase of CeO2/MgO-�-Al2O3, the peak areas of reflections of (3 1 1)of MgAl2O4 and (1 1 1) of CeO2 were measured. The peak inten-sity ratio ICeO2 /IMgAl2O4

is plotted versus the total content of CeO2in the samples (Fig. 1). The resultant straight line gives an inter-cept with horizontal axis corresponding to the dispersion capacityof CeO2 on the surface of MgO-�-Al2O3. The intercept is 0.0148 gCeO2/MgO-�-Al2O3 (g), which is close to cerium content x = 1.2 wt.%in CeO2/MgO-�-Al2O3 (g) samples.

The theoretical dispersion capacity of CeO2 on MgO-�-Al2O3was calculated. The theoretical monolayer was ∼1.0 × 10−3 gCeO2/m2 [17] and total surface area of CuCl2-KCl-MgO-�-Al2O3 was57.76 m2/g obtained by BET measurement. The results shows thattheoretical dispersion capacity is 0.05776 g CeO2/CuCl2-KCl/MgO-�-Al2O3 (g), which is corresponding to 4.7 wt.% cerium content inCuKCex catalysts.

Notably, the theoretical value is larger than that obtained fromthe XRD results, which certifies that CeO2 does not disperse on thesurface of MgO-�-Al2O3 as a close-packed monolayer.

XRD patterns of CuKCex with different cerium loading after cal-cination at 550 ◦C are shown in Fig. 2. The characteristic diffractionpeaks of CeO2 fluorite structure at 2� = 28.5◦, 33.3◦, 47.5◦ and 56.4◦

are clearly observed for the CuKCex with cerium loading ≥1.4 wt.%,

and intensity of the peaks increases with the increase of ceriumloading. For CuKCe1.2 (Fig. 2b), that no clear diffraction peak of CeO2can be seen is ascribed to the high dispersion of ceria on the support,consistent with the critical dispersion capacity of CeO2 on the sup-

106 C. Li et al. / Applied Catalysis A: General 400 (2011) 104–110

Table 1Surface area, pore volume and DCeO2 of the CuKCex catalysts.

Catalysts Cerium (wt.%) CeO2 (wt.%) SBET (m2/g) Vp (cm3/g) DCeO2 (nm) (3 1 1)

CuKCe1 1 1.228 60.12 0.2551 n.d.CuKCe1.2 1.2 1.470 59.24 0.2491 n.d.CuKCe1.4 1.4 1.718 58.27 0.2440 7.1CuKCe3 3 3.684 55.81 0.2398 9.05CuKCe5 5 6.142 54.98 0.2291 10.75CuKCe7 7 8.596 58.23 0.2501 11.49

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CuKCe9 9 11.054CuKCe11 11 13.510CuKCe13 13 15.967

ort as shown in Fig. 1. Herein, it is worth to point out that CuCl2 andCl components nearly have no impact on the monolayer disper-ion of ceria promoter on the support. The crystal size of CeO2 (3 1 1)ace and the BET data of CuKCex are summarized in Table 1. It is clearhat the BET surface area decreases and the crystal size of CeO23 1 1) face increases with the increase of cerium content, respec-ively. However, the surface area decreases from 58.24 m2/g to3.16 m2/g and the crystal size increases from 11.49 nm to 15.74 nmapidly for CuKCe9, implying that large ceria particles present in thehase of the catalysts. Moreover, this evidence suggests that largeeria particles begin to form at cerium content x ≥ 9 wt.%. Thus, its clear that ceria is highly dispersed on the support without for-

ation of aggregated crystalline ceria species at cerium content< 1.4 wt.%; and large ceria particles begin to form at cerium load-

ng x ≥ 9 wt.% on the surface. The diffraction peaks of CuCl2 speciesannot be easily detected relative to KCl and MgAl2O4 diffractioneaks as shown in Fig. 2. This phenomenon is assigned to the lowerontent of CuCl2 in the catalysts or CuCl2 species are in a form ofanoclusters having particle size of about 2–3 nm as demonstratedy Leofanti et al. [7].

.2. TPR analysis

H2-TPR profiles of pure CeO2 and CuKCex are shown in Fig. 3.nsupported CeO2 exhibits mainly three reduction peaks, theiraximal temperatures are at about 450, 560 and 1050 ◦C, respec-

ively (Fig. 3 curve g), which are similar to that reported by Shyut al. [18]. The peak at 450 ◦C is ascribed to reduction of the surfaceapping oxygen (O2−, O−) in CeO2 [19], the peak at 560 ◦C is likelyo formation of nonstoichimetric ceria oxides (CeOx with x ranging

ig. 2. XRD patterns of CuKCex x = 1.0 wt.% (a); x = 1.2 wt.% (b); x = 1.4 wt.% (c);= 1.6 wt.% (d); x = 1.8 wt.% (e); x = 2.0 wt.%(f); x = 3.0 wt.% (g); x = 5.0 wt.% (h);= 7.0 wt.% (i); x = 9.0 wt.% (j).

53.16 0.2305 15.7444.85 0.1824 16.4039.39 0.1738 17.38

from 1.9 to 1.7, or its � phase) and the peak at 1050 ◦C is assignedto deep reduction of bulk ceria to Ce2O3 by elimination of O2− ionsof the lattice [20,21], respectively.

H2-TPR profiles of CuKCex catalysts with different cerium load-ing after calcination at 550 ◦C are shown in Fig. 3 (curves a–f).It can be seen that the CuKCex catalysts show mainly six peaksat around 316 ◦C(1), 400 ◦C(2), 420 ◦C(3), 615 ◦C(4), 776 ◦C(5) and908 ◦C(6), respectively. The peaks at 316 ◦C and 420 ◦C are assignedto a stepwise reduction of Cu2+ → Cu1+ → Cu0 [11]. Interestingly,with the increase of cerium content up to 7 wt.%, intensity of thesetwo peaks increase and their maxima shift to higher temperature,implying that the strong interaction exists between copper speciesand ceria promoter, and ceria promoter enhances the reductionof Cu2+ → Cu1+ → Cu0 due to the electronic properties of nonsto-ichimetric ceria. However, the peak at 420 ◦C shifts to 460 ◦C islarger than that at 316 ◦C shifted to 321 ◦C, meaning that Cu+ ishardly reduced to Cu0 relative to Cu2+ to Cu+ under the same elec-tronic environment. This fact suggests that surface capping oxygenspecies (O2

−, O−) accelerate the oxidation of Cu+ to Cu2+, which isresponsible for the higher activity and stability of copper-basedcatalyst. The peak at around 400 ◦C is ascribed to the reductionof surface capping oxygen (O2

−, O−) species and the peak shiftsfrom 450 ◦C to 400 ◦C due to the high dispersion of ceria on thecatalyst. Meanwhile, the intensity of this peak increases and a max-imum reaches with cerium loading x = 5 wt.% (Fig. 3curve d), whichindicates that cerium content plays a key role for the formationof surface capping oxygen species. The peak at about 615 ◦C isassigned to the formation of the nonstoichimetric ceria oxides and

◦ ◦

it shifts from 560 C to 615 C due to the difference of the particlesize of ceria [18]. Here, the intensity of this peak increases withan increase of cerium loading (1 wt.% < x ≤5 wt.%) and decreasessharply with cerium loading x ≥ 7 wt.% as shown in Fig. 3 (curves

Fig. 3. TPR study for the CuKCex catalyst with the different content of cerium a,0 wt.%; b, 1 wt.%; c, 3 wt.%; d, 5 wt.%; e, 7 wt.%; f, 9 wt.%; g, CeO2.

C. Li et al. / Applied Catalysis A: General 400 (2011) 104–110 107

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Fig. 4. FTIR spectra of pyridine adsorbed for a, CuK/MgO-�-Al2O3; b, CuKCe5.

–d and e–f). This phenomenon implies the relative amount ofurface crystalline ceria species that can be reduced to nonstoichi-etric ceria oxides of CuKCex catalysts, which presents the relative

mount of oxygen vacancies formed by elimination of lattice oxy-en ions during the H2-TPR. In other words, the peak intensity iselated to the probability of formation of oxygen vacancies of ceriaromoter during the reaction. The peak at around 776 ◦C is assignedo the reduction of MgO-�-Al2O3 support and it shifts to high tem-erature at 800 ◦C due to the reduction of some internal oxygenpecies (O2− ions of the lattice) of CeO2 particles to CeAlO3 phaseFig. 3curve e and f) [19–22], even though no reduction peak ofe2O3 (at 908 ◦C) is observed. This fact suggests that a small amountf large ceria particles exists on the surface for CuKCe7, and theeduction peak of the formed Ce2O3 is hardly detected comparedo CuKCe9 catalyst. In addition, it has been reported [23–25] thathe residual chloride originated form metal chloride precursors canubstitute to oxygen ions in the ceria lattice during the reductionnder H2 flow and to form CeOCl or Ce(OH)2Cl species. Based onhe above discussion of H2-TPR profiles, it is concluded that theopper-based catalyst modified with small aggregated crystallineeria species possesses higher amount of surface capping oxygenpecies and the larger probability of formation of oxygen vacanciesn the catalysts surface during the reaction (i.e. 3 wt.% ≤ x ≤ 5 wt.%),hich can adsorb O2 from gaseous mixture and convert it to higher

xidized oxygen species (O2− or O−) due to the defects of structure

nd electronic properties of nonstoichimetric ceria.

.3. Analysis of acidity

IR analysis of adsorbed pyridine is the most extensively usedethod for analysis of acidic types of the catalyst. The nature of

cidic sites can be discriminated by the different adsorption bandsf the pyridine and detected by IR spectroscopy, i.e. the bands at540 cm−1, 1490 cm−1 and 1449 cm−1 are used to determine Brön-ted sites, physical adsorbed pyridine and Lewis sites, respectively.hus, the catalysts possess mainly Lewis acid sites, since the adsorp-ion band of pyridine is detected at 1449 cm−1 in Fig. 4. Herein,

gO-�-Al2O3 support provides mainly Lewis acid centers, and thentroduction of MgO to �-Al2O3 support decreases the acid strengthotably, which has been reported in previous work [26], consistentith the observation by Penkova et al. [27].

The amount of acidic sites in CuKCex was determined by NH3-

PD technique. In the literature, it is defined that three types ofcidic sites exist on the catalyst, and the adsorbed ammonia on theatalyst desorbs in different temperature range: i.e. weak acidicites at 150–250 ◦C; intermediate acidic sites at 250–450 ◦C and

Fig. 5. Ammonia desorption on the CuKCex catalysts with cerium loading a, 0 wt.%;b, 1 wt.%; c, 3 wt.%; d, 5 wt.%; e, 7 wt.%; f, 9 wt.%; g, 11 wt.%.

strong acidic sites at 450–540 ◦C [28], respectively. Fig. 5 shows thatall curves display mainly two desorption peaks at around 200 ◦C and400 ◦C, meaning that the weak acidic sites and intermediate acidicsites exist in CuKCex catalysts. It is clear that the intensity of peakat around 200 ◦C increases and the peak shifts to the lower temper-ature with an increase of cerium content (i.e. 1 wt.% ≤ x ≤ 5 wt.%)and reaches a maximum with cerium content x = 5 wt.%. This factdemonstrates that both dispersed ceria species and small aggre-gated crystalline ceria species can decrease the strength of weakacidic sites on the catalyst. The promotional effect is that dispersedceria can forbid the combination of Al3+ with OH− or Ce3+ hasa weak force to combine OH− due to its larger radius, which isresponsible for the decrease of the amount of Lewis acidic sites[29,30]. However, the increase of the intensity of peak at 200 ◦C isattributed to the increase of the amount of surface capping oxygenspecies (O2

−, O−), which plays a key role for the formation of weakacidic sites on the catalyst. This fact is consistent with the obser-vation in literature [28]. Meanwhile, the peak shifts to the highertemperature for catalysts with cerium content x ≥ 7 wt.% and itsintensity decreases, which is ascribed to the decrease of the amountof surface capping oxygen species. Interestingly, the peak at 400 ◦Cshifts to higher temperature for catalysts with cerium content1 wt.% ≤ x ≤ 7 wt.%, and the peak intensity increases notably for cat-alysts with cerium content x = 9 wt.%, which indicates that the largeceria particles benefit the formation of intermediate acidic sites onthe surface. Besides, formation of acid–base pairs [CeOx]2+O2

2− onthe catalyst has been reported by Leofanti et al. [7], the pairs canattract H-atom from C2H6 by its polarization and break the C–Hbond to form C2H5

−, which can be easily chlorinated by Cl2 thanC2H6. In sum, large amount of weak acidic sites on the surface ben-efits the dehydrochlorination of dichloroethane and enhances theselectivity of vinyl chloride as expected.

3.4. XPS analysis

Further characterization of the state of ceria on the surface ofcatalysts was performed by XPS. The XPS spectra of Ce 3d are shownin Fig. 6, it is clear that six peaks donated as v, v′ ′, v′ ′ ′, u, u′ ′, u′ ′ ′

(assigned to Ce4+) and two peaks donated as v′ and u′ (assigned toCe3+) [31,32]. It is clear that the u′ ′ ′ peak is obvious due to higher

content of cerium for pure CeO2 catalyst compared to lower inten-sity of u′ ′ ′ peak in fresh CuKCe5. The u′ ′ ′ peak is usually selected asa rule to assess the oxidation state of ceria [33,34] and the appear-ance of u′ ′ ′ peak implies that a large amount of Ce4+ species exist

108 C. Li et al. / Applied Catalysis A: Ge

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Fig. 6. XPS spectra of Ce 3d: pure CeO2, CuKCe5 and used CuKCe5.

n the surface. Besides, TPR results show that no reduction peak ofeAlO3 is detected for lower ceria loading 1 wt.% ≤ x ≤ 5 wt.%, anduKCex catalysts show a stepwise reduction similar to pure CeO2,i) reduction of surface capping oxygen species; (ii), formation ofonstoichimetric ceria; (iii) formation of CeAlO3; (iv) formation ofe2O3. Based on the above discussion, we infer that small aggre-ated crystalline ceria species presented in the catalyst is in Ce4+

tate. However, comparing to fresh CuKCe5, the u′ ′ ′ peak disap-ears and the intensity of v′ and u′ peaks increase obviously in useduKCe5. This fact suggests that Ce3+ species exist on the surface ofsed catalyst.

Table 2 shows the BE values of O 1s and the ratio of peak intensityf Oads/Olat on the surface of CuKCex catalysts. It is clear that the BEalue and the ratio of peak intensity of Oads/Olat are sensitive to theontent of cerium in catalyst. In common, the component at bindingnergy of 531.4 eV can be associated to O 1s of Al2O3 support [35],hile O 1s spectra shows an asymmetric peak for CuKCex catalysts

specially for cerium content x ≥ 3 wt.%. With increasing of ceriumontent a shoulder on the high BE side of O 1s at ca. 532.3 ± 0.2 eV isbserved. This would be related to a contribution of adsorbed oxy-en [35] or oxygen ion (O2

− or O−) located near the oxygen vacancyites of CeO2 [36–38]. Moreover, the relative amount of oxygenacancies of ceria promoter can be evaluated by means of the cal-ulation the ratio of Oads/Olat. It is clear that the ratio of Oads/Olat

ncreases with an increase of cerium content 3 wt.% ≤ x ≤ 5 wt.%nd decreases clearly for catalysts with cerium content x ≥ 7 wt.%,eaning that the largest amount of oxygen vacancies exist on the

urface of CuKCe5, which is in good agreement with the H2-TPR

able 2PS data of O 1s for CuKCex catalysts.

Catalysts BE (eV) O 1s lattice BE (eV) O1s adsorb Ratio ofpeak areaOads/Olat

CuKCe1 531.46CuKCe3 531.24 532.56 0.271CuKCe5 531.14 532.28 0.412CuKCe7 531.13 532.50 0.240CuKCe9 531.08 532.40 0.191CuKCe11 530.76 (CeO2)

531.67 (Al2O3)532.91

CuKCe5 (used) 531.39 533.01CeO2 529.6 531.6

neral 400 (2011) 104–110

results. Herein, the BE value of lattice oxygen of Al2O3 shifted from531.46 to lower BE side at 531.08 eV is attributed to the oxygen fromCeO2 with the increase of ceria loading. Moreover, the BE value ofO 1s of CuKCe11 at ca. 530.76, 531.67 and 532.91 eV are ascribed tolattice oxygen of CeO2, Al2O3 and adsorb oxygen on the vacancies,respectively. This fact is attributed to the formation of large ceriaparticles on the surface, which can be explained by Charge PotentialModel as proposed by Siegbahn [39] and Fadley [40].

3.5. Activity test

The effect of cerium content on the catalytic activities of CuKCex

for ethane oxychlorination was investigated. As shown in Fig. 7(a),it is clear that the relatively high C2H6 conversion and selectivity toC2H3Cl can be achieved over copper-based catalysts doped withsmall aggregated crystalline ceria species. The C2H6 conversionranges from 95.2% to 98.6% and reaches a maximum (98.6%) withcerium content x = 5 wt.% and the selectivity to C2H3Cl shows thesimilar trend, and increases with an increase of cerium content andreaches a maximum (55.2%) with cerium content x = 5 wt.%. By con-trast, the copper-based catalysts modified with dispersed ceria orlarge ceria particles show the lower C2H6 conversion and selectivityto C2H3Cl. This fact certifies that the introduction of small aggre-gated crystalline ceria species to copper-based catalyst improvesthe catalytic activity notably, which is attributed to surface cappingoxygen species and a large amount of oxygen vacancies of ceria onthe surface, i.e. capping oxygen species can accelerate the conver-sion of redox group (Cu+/Cu2+); meanwhile, the surface cappingoxygen species (O2

−, O−) can be compensated form gas mixtureimmediately due to the large amount of oxygen vacancies on thesurface. As a result, the production rate of Cl2 intermediate canbe accelerated in the process of reaction and higher selectivity toC2H3Cl and stability of catalyst are achieved. In addition, the sur-face capping oxygen species can change the acidic properties ofcatalyst, and it benefits the formation of weak acidic sites on thecatalysts surface, which is advantageous to the rupture of C–H bondand dehydrochlorination of dichloroethane on the surface.

The reaction scheme is represented by following equations:

2CuCl2(s) → Cu2Cl2(s) + Cl2(g) (1)

Cu2Cl2(s) + 1/2O2(g) → CuO·CuCl2 (2)

CuO·CuCl2 + 2HCl(g) → 2CuCl2(s) + H2O(g) (3)

CeOx(x < 2) + O2 → [CeOx]+ O2− → [CeOx]2+O2

2− → [CeOx]2+

+ 2O− → CeOy(x < y ≤ 2) (4)

From this scheme it can be seen that CuCl2 is active centers ofthe reaction, and ceria as a promoter can improve the ability of oxy-gen storage/release capacity. Therefore, the copper-based catalystsmodified with small aggregated crystalline ceria species can adsorbO2 from the gaseous mixture and convert it to high oxidation state(O2

− or O−) species, which can accelerate the redox group Cu+/Cu2+

to some extent. In case of CuKCex, the process of Eq. (1) is acceler-

ated and more Cl2 is produced in the process of ethane chlorinationcompared to the unmodified CuCl2-KCl/MgO-�-Al2O3. As proposedby Flid et al. [3], the catalytic process of oxidative chlorination ofethane may be represented as follows:

C. Li et al. / Applied Catalysis A: General 400 (2011) 104–110 109

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[[[[[[

[

ig. 7. (a) Effect of cerium content on conversion of C2H6 and selectivity to C2H3Cl iith reaction time (a, d, e) for CuK-MgO-�-Al2O3 and (c, b, f) for CuKCe5.

CO+CO2

↑

C2H6 → C2H5Cl → C2H4Cl2

↓ ↓

C2H4 → C2H3Cl

↓

CO+CO2

It is clear that there is competition between formation of ethy-ene and vinyl chloride. Thus, the increase of production rate of Cl2s advantageous selectivity of vinyl chloride. On the other hand,arge amount of weak acidic sites also benefit dehydrochlorinationf dichloroethane on the catalyst surface. However, compared with1s BE value of fresh the CuKCex catalysts (Table 2), it is observed

hat the used catalyst shows the higher of BE (ca. 533.1 eV), whichndicates that the surface oxygen species [O2

−, O−] cannot be com-ensated rapidly by the gas phase oxygen due to the adsorption orlock of chlorine species on the catalyst surface [41]. In addition, itas been reported by Soria et al. [42] that the chlorine species modi-

ying adsorption centers interacts more weakly with O2 and is moreesistant towards oxidation. Therefore, it can be concluded that sur-ace oxygen species (O2

−, O−) play a vital role for the selectivity toinyl chloride, while large amount of chlorine species adsorbed onatalysts surface or the formation of CeOCl or CeAlO3 is responsi-le for the deactivation of copper-based catalyst. The stability ofuKCe5 catalyst was also investigated (Fig. 7b) and no deactivationf catalyst was detected after the reaction for 120 h. The conversionf C2H6 changes between 95% and 98%, and the sum of selectivityf C2H4 and C2H3Cl is higher than 71%.

. Conclusions

Based on the studies of surface properties (oxidation/reduction,cid property) and catalytic properties of copper-based catalystsith different cerium content, following conclusions are given:

hree types of ceria species exist on the surface of CuKCex withn increase of cerium loading: dispersed CeO2 species, aggregatedrystalline ceria species and large CeO2 particles. The activity testshowed that the copper-based catalyst modified with small aggre-

[[

[

Cex catalysts; (b) The changes of selectivity of C2H3Cl, C2H4 and conversion of C2H6

gated crystalline ceria species exhibited higher catalytic activityand achieved the highest conversion of C2H6 (98%) and selectivity ofC2H3Cl (55.4%) with cerium content x = 5 wt.%, and the promotionaleffect may be originated from a large amount of capping oxygenspecies and oxygen vacancies existed on the surface, which accel-erates the redox group (Cu+/Cu2+), provides more weak acidic sitesfor rupture C–H bond in C2H6 and C2H4 and dehydrochlorination ofdichloroethane; while a large amount of chlorine adsorbed on thecatalysts surface or the formation of CeOCl or CeAlO3 is responsiblefor the deactivation of copper-based catalyst.

Acknowledgments

The authors would like to thank Professor Zhou and ProfessorCheng for their instruction in the work.

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